Handbook of Combinatorial Chemistry: Drugs, Catalysts, Materials (2-Vol. Set)

Handbook of Combinatorial Chemistry Drugs, Catalysts, Materials Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Han...

Author: K. C. Nicolaou | Rudolf Hanko | Wolfgang Hartwig

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Handbook of Combinatorial Chemistry Drugs, Catalysts, Materials Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig

Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

Further Titles of Interest W. Bannwarth, E. Felder (Eds.)

Combinatorial Chemistry 2000, ISBN 3-527-30186-0

G. Jung (Ed.)

Combinatorial Chemistry Synthesis, Analysis, Screening 1999, ISBN 3-527-29869-X

F. Zaragoza D€ orwald

Organic Synthesis on Solid Phase Supports, Linkers, Reactions 2000, ISBN 3-527-29950-5

A. Beck-Sickinger, P. Weber

Combinatorial Strategies in Biology and Chemistry 2002, ISBN 0-471-49726-6

Handbook of Combinatorial Chemistry Drugs, Catalysts, Materials

Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig

Editors Prof. Dr. K. C. Nicolaou Department of Chemistry and the Skaggs Institute for Chemical Biology The Scripps Research Institute 10550 North Torrey Pines Road La Jolla, CA 92037 USA Dr. R. Hanko Bayer AG Chemical Division Head of BUFine Chemicals 51368 Leverkusen Germany Prof. Dr. W. Hartwig Bayer AG Gescha¨ftsbereich Pharma Leiter Forschung International 42096 Wuppertal Germany

9 This book was carefully produced. Nevertheless, editors, authors and publisher do not warrant the information contained therein to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for A catalogue record for this book is available from the British Library. Die Deutsche Bibliothek – CIP Cataloguing-in-Publication Data A catalogue record for this publication is available from Die Deutsche Bibliothek ( WILEY-VCH Verlag GmbH, D-69469 Weinheim (Federal Republic of Germany). 2002 All rights reserved (including those of translation in other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into machine language without written permission from the publisher. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Printed in the Federal Republic of Germany. Printed on acid-free paper. Typesetting Asco Typesetters, Hong Kong Printing Strauss Offsetdruck GmbH, 69503 Mo¨rlenbach Bookbinding J. Scha¨ffer GmbH & Co., KG, 67269 Grunstadt ISBN 3-527-30509-2

v

Contents Volume 1 Part I

General Aspects

1

Combinatorial Chemistry in Perspective 3 K. C. Nicolaou, R. Hanko, and W. Hartwig Introduction 3

1.1 1.2 1.3 1.4

1

Brief History of Combinatorial Chemistry Applications of Combinatorial Chemistry Outline of the Book 7 Acknowledgments 8 References 8

4 6

2

Introduction to Combinatorial Chemistry David L. Coffen and Joachim E. A. Luithle

2.1 2.2 2.3 2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.1.4 2.3.1.5 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.4

Combinatorial Chemistry in Drug Discovery – a Perspective 10 Key Issues 11 Combinatorial Synthesis 15 Solid-phase Combinatorial Synthesis 16 Reagents and Conditions 16 Automation 17 Split and Combine 17 Cost 19 The Products from Solid-phase Chemistry 19 Solution-phase Combinatorial Synthesis 19 Reagents and Conditions 19 Scavenger Resins, Polymer-supported Reagents and Fluorous Tags Equipment and Costs 22 Conclusion 22 References 22 Valuable Internet Links 23

3

Solid Phase and Soluble Polymers for Combinatorial Synthesis Rainer Haag, Andre´ Hebel, and Jean-Francois Stumbe´ Introduction 24

3.1

10

24

20

vi

Contents

3.2 3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.2 3.2.2.1 3.2.2.2 3.2.3 3.2.3.1 3.2.3.2 3.2.4 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.3.5 3.4

Solid-phase Supports 25 Polystyrene-based Resins 25 General Aspects 25 Macroporous Resins 28 Microporous Resins 31 Polystyrene Hybrid Supports 35 PEGylated Resins 35 High-loading PS Hybrid Supports 36 Other Crosslinked Polymeric Supports 41 Crosslinked Acrylamides 41 Crosslinked PEGs 44 Inorganic Supports 46 Soluble Polymeric Supports 46 Separation Techniques for Soluble Polymeric Supports 47 Terminal Functionalized Linear Polymeric Supports 49 Polyfunctional Linear Polymeric Supports 49 Dendritic Polymeric Supports 51 Microgels 54 Conclusions 54 References 54

4

Linkers for Solid-phase Synthesis Stefan Bra¨se and Stefan Dahmen Introduction 59 General Linker Structures 60

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 4.3.7 4.3.7.1 4.3.7.2 4.3.7.3 4.3.8 4.3.9 4.4 4.4.1 4.4.2 4.4.3 4.4.4

59

Immobilization of Molecules 60 Spacers 61 Functionalized Linkers as Analytical Constructs 62 Linker Families 62 Benzyl-type Linkers Including Trityl and Benzhydryl Linkers Allyl-based Linkers 68 Ketal/Acetal-based Linkers 70 Ester-, Amide-, and Carbamate-based Linkers 71 Silyl Linkers 73 Boronate Linkers 76 Sulfur, Stannane- and Selenium-based Linkers 77 Sulfur-based Linkers 77 Stannane-based Linkers 81 Selenium-based Linkers 81 Triazene-based Linkers 83 Orthogonality Between Linkers 88 Cleavage 88 Electrophilic and Nucleophilic Cleavage 88 Oxidative/Reductive Methods 90 Photocleavable Linkers 91 Metal-assisted Cleavage 93

63

Contents

4.4.4.1 4.4.4.2 4.4.5 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.5.5 4.5.6 4.5.7 4.6 4.6.1 4.6.1.1 4.6.1.2 4.6.1.3 4.6.1.4 4.6.1.5 4.6.1.6 4.6.1.7 4.6.1.8 4.6.1.9 4.6.1.10 4.6.1.11 4.6.2 4.6.2.1 4.6.2.2 4.6.2.3 4.6.2.4 4.6.2.5 4.6.2.6 4.6.3 4.6.4 4.6.4.1 4.6.4.2 4.6.5 4.6.5.1 4.6.5.2 4.6.5.3 4.6.5.4 4.6.5.5 4.6.6 4.6.6.1 4.6.6.2 4.6.6.3

Cleavage with Ensuing Allylic Substitution or Cross-coupling Reactions 94 Cleavage via Alkene Metathesis 97 Unusual Cleavage Methods 98 Linker and Cleavage Strategies 98 Safety-catch Linkers 99 Cyclative Cleavage (Cyclorelease Strategy) 103 Cleavage-cyclization Cases 105 Fragmentation Strategies 107 Traceless Linkers 110 Multifunctional Cleavage 117 Linkers for Asymmetric Synthesis 120 Linkers for Functional Groups 121 Linkers for Nitrogen Functionalities 122 Linkers for Amines 123 Linkers for Primary Amines 123 Linkers for Secondary Amines 125 Linkers for Tertiary Amines 125 Linkers for Hydrazines, Hydrazones and Hydroxylamines 128 Linkers for Diazonium Salts 128 Linkers for Azides 129 Linkers for Nitro Compounds 129 Linkers for Azo Compounds 129 Linkers for Nitriles 129 Linkers for N-Heterocycles 129 Linkers for Carbonyl Functionalities 133 Linkers for Carboxylic Acids 133 Linkers for Carboxylic Esters, Anhydrides and Lactones 134 Linkers for Thiocarboxylic Acids and Esters 135 Linkers for Carboxamides and Related Structures 136 Linkers for Hydrazides and Semicarbazones 142 Linkers for Cyclic Amides and Related Structures 143 Linkers for Ketones and Aldehydes 144 Linkers for Alcohols, Phenols, Ethers, and Ketals 144 Linkers for Alcohols 144 Linkers for Phenols 146 Linkers for Sulfur Compounds 146 Linkers for Thiols and Thioethers 146 Linkers for Sulfonamides 147 Linkers for Sulfonic Acids 147 Linkers for Sulfones and Sulfoxides 148 Linkers for Sulfoximines 148 Linkers for Hydrocarbons 148 Linkers for Alkanes 149 Linkers for Arenes and Heteroarenes 149 Linkers for Alkenes 149

vii

viii

Contents

4.6.6.4 4.6.7 4.6.8 4.6.9 4.6.10 4.6.10.1 4.6.10.2 4.6.10.3 4.7 4.8

Linkers for Alkynes 150 Linkers for Aryl and Alkyl Halides 150 Linkers for Heterocycles 151 Linkers for Reactive Intermediates 151 Linkers for Other Functional Groups 152 Linkers for Phosphonates 152 Linkers for Boronates 152 Linkers for Silanes and Silanols 152 Overview for Linkers for Functional Groups Conclusion, Summary and Outlook 152 References 153

5

Encoding Technologies 170 Thomas Kra¨mer, Valery V. Antonenko, Reza Mortezaei, Nicolay V. Kulikov Introduction 170 Chemical-encoding Methods 171 Oligonucleotide Tags 171 Peptide Tags 173 Haloaromatic Binary Coding 173 Secondary Amine Binary Coding 175 Mass Encoding 178 Nonchemical Encoding Methods 180 Positional Encoding 180 Light-directed Synthesis 180 Microtiter Plate-based Positional Encoding 181 Nonpositional Encoding 183 Tea-bag Approach 183 Cellulose and Laminar Supports 183 Radiofrequency Tags 183 Laser Encoding 186 Conclusion 186 References 187

5.1 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.4

152

6

Instrumentation for Combinatorial Chemistry Marcus Bauser and Hubertus Stakemeier

6.1 6.1.1 6.1.2 6.1.2.1 6.1.2.2 6.1.2.3 6.2 6.2.1 6.2.2 6.2.3 6.2.3.1

Automation in Combinatorial Synthesis 190 General Remarks 190 Fully Automated Systems for Solid- and Solution-phase Synthesis Robot-arm-based Systems 190 Fully Automated Workstation Systems 201 Modular Systems 207 Purification of Combinatorial Libraries 209 Automated Liquid–Liquid Extraction 210 Solid-phase Extraction 211 Normal Phase Chromatography 212 CombiFlashTM from Isco 212

190

190

Contents

6.2.3.2 6.2.3.3 6.2.4 6.2.4.1 6.2.4.2 6.2.4.3 6.2.4.4 6.2.4.5 6.2.5 6.2.5.1 6.2.5.2 6.2.5.3 6.2.5.4 6.2.5.5 6.3 6.3.1 6.3.2 6.3.3 6.4 6.4.1 6.4.2

Quad3TM from Biotage 212 FlashMaster TM from Jones Chromatography 213 Reversed Phase Chromatography 213 Biotage 214 Gilson 214 Merck 215 Varian 215 Shimadzu 215 Preparative HPLC-MS 215 PE Sciex 215 Waters Micromass 216 Merck-Hitachi 216 Shimadzu 216 Gilson ThermoQuest 216 Analysis of Combinatorial Libraries 216 Purity of Combinatorial Libraries 217 Identity of Combinatorial Libraries 218 Quantification of Combinatorial Libraries 219 Automated Sample Processing 219 Sample Logistics 219 Evaporation 221 References 222

Part II

Synthetic Chemistry

7

Radical Reactions in Combinatorial Chemistry A. Ganesan and Mukund P. Sibi Introduction 227

7.1 7.2 7.3 7.4 7.5 7.5.1 7.5.2 7.5.3 7.5.4 7.6

8

8.1 8.2 8.2.1 8.2.2 8.2.3

225 227

Intramolecular Radical Additions to sp 2 and sp Carbon 228 Intermolecular Radical Additions 230 Functional Group Removal 237 Polymer-supported Reagents for Radical Chemistry 238 Polymer-supported Tinhydrides 239 Polymer-supported Allyl Stannane 242 Polymer-supported Reagents for Atom-transfer Reactions 242 Photochemical Generation of Radicals 243 Summary 244 References 244 Nucleophilic Substitution in Combinatorial and Solid-phase Synthesis Jan-Gerd Hansel and Stephan Jordan Introduction 247 Nucleophilic Substitution at Aliphatic Carbons 247 General Remarks 247 Halogen Nucleophiles 248 Oxygen Nucleophiles 249

247

ix

x

Contents

8.2.4 8.2.5 8.2.6 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5

Sulfur Nucleophiles 251 Nitrogen Nucleophiles 251 Ring-closing Reactions 254 Nucleophilic Substitution at Aromatic Carbons General Remarks 254 Nitrogen Nucleophiles 255 Oxygen Nucleophiles 262 Sulfur Nucleophiles 264 Macrocyclization Reactions 265 References 266

9

Electrophilic Substitution in Combinatorial and Solid-phase Synthesis Jan-Gerd Hansel and Stephan Jordan Introduction 270 Electrophilic Substitution at Aliphatic Carbons 271 Halogen Electrophiles 271 Nitrogen Electrophiles 271 Carbon Electrophiles 272 Electrophilic Substitution at Aromatic Carbons 272 General Remarks 272 Halogen Electrophiles 273 Nitrogen Electrophiles 274 Carbon Electrophiles 275 References 277

9.1 9.2 9.2.1 9.2.2 9.2.3 9.3 9.3.1 9.3.2 9.3.3 9.3.4

10

10.1 10.2 10.2.1 10.2.1.1 10.2.1.2 10.2.1.3 10.2.1.4 10.2.2 10.2.3 10.2.4 10.2.5 10.2.6 10.2.7 10.2.7.1 10.2.7.2 10.3 10.3.1

254

270

Elimination Chemistry in the Solution- and Solid-phase Synthesis of Combinatorial Libraries 279 Demosthenes Fokas and Carmen Baldino Introduction 279 b-Eliminations in Combinatorial Chemistry 279

The Hofmann Elimination Solid-phase Synthesis of Tertiary Amines Via a Regenerated Michael Acceptor (REM) Resin 280 Via a Safety-catch Resin 282 Via a Hydroxylamine Resin 283 Alternative Cleavage Techniques 284 b-Elimination on Selenyl Resins 286 b-Elimination on Sulfone Resins 288 b-Elimination on Silyl Resins 289 b-Elimination on Fluorenyl Resins 291 b-Elimination on 2-(2-Nitrophenyl)ethyl Resins 291 Radical-based b-Eliminations 292 b-C,O Bond Scission 292 b-C,Se Bond Scission-release of Olefins 293 Conjugate Eliminations 293 1,6-Conjugate Eliminations 293

280

Contents

10.3.2 10.4 10.4.1 10.4.1.1 10.4.1.2 10.4.1.3 10.4.2 10.4.2.1 10.4.2.2 10.4.2.3 10.4.2.4 10.5

1,4-Conjugate Eliminations 295 Addition–Elimination Reactions 296 Addition–Elimination on Vinylogous Systems 296 Entry to Aminomethyleneoxazolones 296 Entry to Benzopyrones 297 2,3-Dihydro-4-pyridone Libraries 298 Cycloreversions 299 Pyrrole Libraries 299 Imidazole Libraries 300 Traceless Solid-phase Synthesis of Furans 300 1,2-Diazines 301 Summary 302 References 302

11

Addition to CC Multiple Bonds (Except for CC Bond Formation) Adrian L. Smith Introduction 305 Addition to CbC Double Bonds 306 Epoxidation and Subsequent Epoxide Opening 306 Dihydroxylation 309 Oxidative Cleavage 311 Electrophilic Addition of AaX 313 Hydrogenation 315 Hydrometallation 316 1,4-Addition to a,b-Unsaturated Carbonyl Systems 317 Addition to CcC Triple Bonds 318 References 319

11.1 11.2 11.2.1 11.2.2 11.2.3 11.2.4 11.2.5 11.2.6 11.2.7 11.3

12

12.1 12.2 12.2.1 12.2.2 12.2.2.1 12.2.2.2 12.2.2.3 12.2.2.4 12.2.2.5 12.2.2.6 12.2.2.7 12.2.3 12.2.4 12.2.5

305

Addition to Carbon–Hetero Multiple Bonds 322 Philipp Grosche, Jo¨rg Rademann, and Gu¨nther Jung Introduction 322

Additions to CN Double Bonds in sp2 Systems 322 Attack by Hydride (Reductive Alkylation) 323 Addition of Carbon Nucleophiles 324 Imino Aldol Reaction 324 Reaction with Boronic Acids 326 Addition of Allylsilanes (Imino-Sakurai Reaction) 327 Reaction with Grignard Reagents, Lithium Organyles, and Zinc Organyles 328 Addition of Copper Alkynes 329 Addition of Electron-rich Aromatic and Heteroaromatic Cycles 329 Radical Reactions 330 Addition of Nitrogen Nucleophiles 331 Addition of Phosphorus Nucleophiles 332 Reactions with Oxygen Nucleophiles 333

xi

xii

Contents

12.2.6 12.3 12.3.1 12.3.2 12.3.2.1 12.3.2.2 12.3.2.3 12.3.3 12.3.4 12.4 12.4.1 12.4.2 12.4.3

Addition of Sulfur Nucleophiles 333 Additions to CN Double Bonds in sp-Systems 334 Additions to Carbodiimides 335 Reaction of Isocyanates and Isothiocyanates 335 Addition of Carbon Nucleophiles 336 Addition of Nitrogen Nucleophiles 336 Addition of Oxygen Nucleophiles 337 Addition to CS Double Bonds in sp2 Systems 338 Reaction of CS Double Bonds in sp Systems 339 Additions to CN Triple Bonds (Cyanides, not Isocyanides) Addition of Carbon Nucleophiles 339 Addition of Nitrogen Nucleophiles 340 Addition of Sulfur Nucleophiles 342 References 342

13

Chemistry of the Carbonyl Group Tobias Wunberg Introduction 346

13.1 13.2 13.3 13.3.1 13.3.1.1 13.3.1.2 13.3.2 13.3.2.1 13.3.2.2 13.3.2.3 13.3.2.4 13.4 13.4.1 13.4.1.1 13.4.2 13.4.2.1 13.4.2.2 13.4.2.3 13.4.2.4

14

14.1 14.2 14.2.1 14.2.2

339

346

Chemistry of the Carbonyl Group and Combinatorial Chemistry 346 Chemistry of Carboxylic Acids 347 C(O)aX Bond-forming Reactions: General Remarks 347 Amides and Ureas 347 Esters and Urethanes 353 Transformation of Carboxylic Acids into Other Functional Groups 355 Formation of Ketones 355 Formation of Amines: Curtius Degradation 356 Tebbe Olefination 356 Formation of Thioamides 357 Reactions of Aldehydes and Ketones 357 Reactions of Carbonyl Groups with CaH Acidic Compounds 357 Wittig and Horner–Emmons Olefinations 357 Reductive Amination 360 General Aspects 360 Formation of Imines 360 Reduction of Imines/Enamines 361 Applications 362 References 365 Oxidation Except CC Double Bonds Henning Steinhagen Introduction 369

369

Oxidation of Alcohols to Aldehydes and Ketones 369 Examples of the Oxidation of Polymer-bound Primary Alcohols to Aldehydes 371 Examples of the Oxidation of Polymer-bound Secondary Alcohols to Ketones 372

Contents

14.2.3 14.3 14.4 14.4.1 14.5 14.5.1 14.6 14.7 14.7.1 14.7.2

Examples of the Oxidation of Alcohols by Polymer-bound Reagents 373 Oxidation of Polymer-bound Aldehydes to Carboxylic Acids 375 Oxidation of Sulfur-containing Compounds 376 Examples of the Oxidation of Polymer-bound Sulfides to Sulfoxides and Sulfones 376 Oxidation of Selenium- and Phosphorus-containing Compounds 378 Examples of the Oxidation (Cleavage) of Selenides to Selenoxides on Solid Support 378 Oxidative Formation of Heterocycles on Solid Support 379 Oxidative Coupling and Cleavage Reactions on Solid Support 380 Examples of Oxidative Coupling Reactions on Solid Support 381 Examples of Oxidative Cleavage Reactions on Solid Support 382 References 382

15

Reductions in Combinatorial Synthesis 387 Christopher P. Corrette and Conrad W. Hummel 15.1 Introduction 387 15.2 Solid-phase Reductions 387 15.2.1 Aldehyde Reductions 387 15.2.2 Ketone Reductions 388 15.2.3 Ester Reductions 390 15.2.4 Mixed Anhydride Reductions 392 15.2.5 Thioester Reductions 393 15.2.6 Weinreb Amide Reductions 394 15.2.7 Sulfur Reductions 396 15.2.8 Selenium Reductions 397 15.2.9 Quinone Reductions 399 15.2.10 Amide Reductions 399 15.2.11 Carbamate Reductions 400 15.2.12 Reductive Amination 401 15.2.12.1 General Considerations 401 15.2.12.2 Imine Formation 402

15.2.12.3 15.2.12.4 15.2.12.5 15.2.13 15.2.13.1 15.2.13.2 15.2.13.3 15.2.13.4 15.2.14 15.2.14.1 15.2.14.2 15.2.14.3 15.2.14.4

Reducing Agents for Reductive Amination 403 Reductive Aminations as the Entry Point for Library Preparation Recent Examples of Reductive Amination on Resin 405 Azide Reductions 406 General Considerations 406 Azide Reductions in Glycopeptide Preparations 406 Small Molecule Libraries Incorporating Azide Reduction 407 Recent Examples of Azide Reduction on Resin 408 Nitro Group Reductions 409 General Considerations 409 Tin-mediated Nitro Reductions 409 Nitro Reductions with Alternative Reagents 411 Recent Examples of Nitro Reduction on Resin 411

403

xiii

xiv

Contents

15.2.15 15.2.16 15.2.17 15.2.18 15.3 15.3.1 15.3.1.1 15.3.1.2 15.3.2 15.3.2.1 15.3.2.2 15.3.3 15.3.3.1 15.3.3.2 15.3.4 15.4

Imine Reductions (not Reductive Amination) 411 Nitrile Reduction 413 NaN and NaO Bond Reductions 413 Miscellaneous Reductions 414 Solution-phase Reductions 414 Supported Reagents 414 Asymmetric Reagents 414 Non-asymmetric Reagents 415 Supported Catalysts 422 Asymmetric Catalysis 422 Non-asymmetric Catalysis 428 Unsupported Reagents Using Catch-and-release Purification Reductive Amination 430 Amide Reductions 430 Fluorous Chemistry 430 Conclusions 431 References 431

16

Cycloadditions in Combinatorial and Solid-phase Synthesis Markus Albers and Thorsten Meyer Introduction 440 ½4 þ 2 Cycloadditions 441 Diels–Alder Reaction with Resin-bound Dienes 441

16.1 16.2 16.2.1 16.2.2 16.2.3 16.2.4 16.2.5 16.3 16.3.1 16.3.2 16.3.3 16.3.4 16.4 16.5 16.6

17

17.1 17.2 17.3 17.3.1 17.3.2 17.3.3

430

440

Diels–Alder Reaction with Resin-bound Dienophiles 445 Intramolecular Diels–Alder Reaction on Solid Support 448 Hetero-Diels–Alder Reaction on Solid Support 450 Diels–Alder Reaction in Solution Phase 451 ½3 þ 2 Cycloadditions 453 Formation of Isoxazoles, Isoxazolines, and Isoxazolidines 453 Formation of Pyrrolidines 458 Formation of Furans 460 Formation of Imidazoles, Pyrroles, Pyrazoles, and Other Nitrogencontaining Heterocycles 461 ½2 þ 2 Cycloadditions 463 ½6 þ 3 Cycloadditions on Solid Support 464 Rearrangements 465 References 467 Main Group Organometallics Christopher Kallus Introduction 470

470

Reactions of Metalated Aromatics 471 1,2-Additions to CbX Groups 474 Reactions with Aldehydes 474 Reaction with Ketones 477 Reaction with Imines 479

Contents

17.3.4 17.4 17.5 17.6 17.6.1 17.6.2 17.7

18

18.1 18.2 18.2.1 18.2.2 18.2.3 18.2.4 18.2.4.1 18.2.4.2 18.3 18.4 18.4.1 18.4.2 18.4.3 18.4.4 18.5 18.6 18.7 18.8 18.8.1 18.8.2 18.9 18.10 18.11

19

19.1 19.2

Reaction with Enolates 481 Conjugate Addition to a,b-Unsaturated Carbonyls and Related Systems 482 Nucleophilic Substitutions 483 Reactions on Carboxylic Acid Derivatives and Related Systems Reaction with Esters 484 Reactions with Weinreb Amides and Related Systems 485 Aminolysis of Esters 489 References 490

484

Enolates and Related Species in Combinatorial and Solid-phase Synthesis 492 Jochen Kru¨ger Introduction 492 Aldol Reactions 492 General Aspects 492 Li, Na, K, and Zn Enolates in Aldol Reactions 493 Boron Enolates in Aldol Reactions 495 The Mukaiyama Aldol Reaction 498

Solution-phase Protocols Using Polymer-bound Reagents 498 Solid-phase Protocols 502 1,4-Addition of Enolates to Michael Acceptors 503 Alkylation of Enolates 506 a-Alkylation of Carbonyl Compounds 506 a- and g-Alkylation of 1,3-Dicarbonyl Compounds 507 Stereoselective Alkylations of Enolates 510 Alkylation of Protected Glycines 513 Claisen-type Condensations 515 Dieckmann Condensations 516 Knoevenagel Condensations 518 Addition of Enolates to Imines 520 Synthesis of b-Amino Esters and Alcohols via Enolate Addition to Imines 520 Solid-phase Synthesis of b-Lactams via Enolate Additions to Imines 522 Nitro-aldol Reactions 522 The Baylis–Hillman Reaction 524 Miscellaneous 525 References 527 Solid-phase Palladium Catalysis for High-throughput Organic Synthesis 531 Yasuhiro Uozumi and Tamio Hayashi Introduction 531

Carbon–Carbon and Carbon–Nitrogen Bond-forming Reactions of Aryl and Alkenyl Halides 531

xv

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Contents

19.2.1 19.2.1.1 19.2.1.2 19.2.1.3 19.2.1.4 19.2.2 19.2.3 19.2.4 19.2.4.1 19.2.4.2 19.2.4.3 19.3 19.3.1 19.3.2 19.3.3 19.4 19.4.1 19.4.2

20

20.1 20.2 20.2.1 20.2.2 20.2.3 20.2.4 20.3 20.3.1 20.3.1.1 20.3.1.2 20.3.2 20.3.2.1 20.3.2.2 20.3.3 20.3.3.1 20.3.3.2 20.4

Cross-coupling Reactions 531 Reactions of Aryl and Alkenyl Halides with Organoboron Reagents 532 Reactions of Aryl and Alkenyl Halides with Organotin Reagents 543 Reactions of Aryl Halides with Terminal Alkynes 544 Solid-phase Palladium-catalyzed Cross-coupling Using Aryl and Benzylzinc Reagents 553 Palladium-catalyzed Arylation and Alkenylation of Olefins 555 Amination of Aryl Halides 561 Miscellaneous Reactions 565 Heteroannulation 565 Insertion Cross-coupling Sequence (Dialkylation of Tropene) 566 Coupling Reactions on Various Solid Supports 567 Solid-phase Reactions by Way of p-Allylpalladium Intermediates 568 Cleavage of Allyl Ester Linkers 568 N-Allylation via p-Allylpalladium Intermediates 571 Insertion–p-Allylic Substitution System 571 Palladium Catalysis with Solid-supported Complexes 573 Preparation of Solid-supported Palladium Complexes and Their Use in Palladium Catalysis 574 Solid-supported Chiral Palladium Catalysts 579 References 581 Olefin Metathesis and Related Processes for CC Multiple Bond Formation 585 Florencio Zaragoza Introduction 585 Olefin Metathesis in Solution 588 Scope and Limitations of Olefin Metathesis in Solution 588

Examples of Library Preparation by Cross-metathesis in Solution 589 Examples of Library Preparation by Ring-closing Metathesis in Solution 592 Examples of Library Preparation by Ring-opening Metathesis Polymerization in Solution 595 Olefin Metathesis on Solid Phase 595 Cleavage from the Support by Olefin Metathesis 597 Scope and Limitations 597 Examples of Cleavage from the Support by Olefin Metathesis 599 Ring-closing Metathesis on Solid Phase 600 Scope and Limitations 600 Examples of Ring-closing Metathesis on Solid Phase 601 Cross- and Self-metathesis on Solid Phase 603 Scope and Limitations 603 Examples of Cross- and Self-metathesis on Solid Phase 603 Conclusion 606 References 606

Contents

Volume 2 Part III

Special Synthetic Topics

21

Solid-phase Synthesis of Natural Products and Natural Product-like Libraries 613 K. C. Nicolaou and Jeffrey A. Pfefferkorn Introduction 613

21.1 21.2 21.2.1 21.2.2 21.2.3 21.2.4 21.2.5 21.3 21.3.1 21.3.2 21.3.3 21.3.4 21.3.5 21.3.6 21.4 21.5 21.6

22

611

Solid-phase Derivatization of Natural Product Scaffolds – Combinatorial Semisynthesis 614 Solid-phase Semisynthesis of Rauwolfa Alkaloids 615 Solid-phase Synthesis of Purine Derivatives 617 Solid-phase Semisynthesis of a Taxoid Library 618 Solid-phase Semisynthesis of Sarcodictyns A and B and Libraries Thereof 620 Solid-phase Semisynthesis of Vancomycin 623 Solid-phase Total Synthesis of Natural Products – Combinatorial Total Synthesis 626 Solid-phase Synthesis of Prostaglandins and Libraries Thereof 626 Solid-phase Synthesis of Epothilone A and Libraries Thereof 628 Solid-phase Synthesis of (S)-Zearalenone 630 Solid-phase Synthesis of (DL)-Muscone and Libraries Thereof 632 Solid-phase Synthesis of the Vitamin D3 System 633 Solid-phase Synthesis of Carpanone-like Molecules 634 Combinatorial Solid-phase Synthesis of Natural Product-like Libraries 634 Conclusion 639 Addendum 639 References 640 Solid-phase Synthesis of Heterocyclic Systems (Heterocycles Containing One Heteroatom) 643 Roland E. Dolle

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22.1 22.2 22.2.1 22.2.2 22.2.3 22.2.4 22.2.5 22.2.6 22.2.7 22.2.8 22.2.9 22.2.10 22.2.11 22.2.12 22.2.13 22.2.14 22.3 22.3.1 22.3.2 22.3.3 22.3.4 22.4 22.5

23

23.1 23.2 23.3 23.4 23.5 23.6 23.7 23.7.1 23.7.2 23.7.3 23.7.4 23.7.5 23.7.6 23.7.7

Introduction 643 Solid-phase Synthesis of Heterocycles Containing One Nitrogen Atom 643 Aziridines 643 b-Lactams 649 Pyrrolidines and Derivatives 651 Tetramic Acids 653 Pyrroles 655 Piperidine and Derivatives 655 Dihydropyridines 659 Pyridines 661 Azepanes, Benzazepines, and Derivatives 662 Indoles 665 Tetrahydroquinolines 668 Quinolinones 671 Quinolines 672 Tetrahydroisoquinolines 674 Solid-phase Synthesis of Heterocycles Containing One Oxygen Atom 675 Tetrahydrofurans and g-Butyrolactams 675 Furans 676 Benzofurans 677 Pyrans, Benzopyrans, and Derivatives 678 Solid-phase Synthesis of Thiophenes 678 Summary 679 References 680 Multicomponent Reactions 685 Arounarith Tuch and Stefan Walle´ Introduction 685 Mannich Reaction 685 Hantzsch Reaction 689 Baylis–Hillman Reaction 690

Grieco Three-component Reaction 691 Biginelli Reaction 692 Multicomponent Reactions with Isocyanides 693 History of Isocyanides 693 Isocyanide Chemistry 694 Isocyanides on Solid Phase 695 Passerini Reaction 696 The Ugi Reaction 697 The Ugi Reaction on Solid Phase 699 Other Multicomponent Reactions with Isocyanides References 703

701

Contents

24

24.1 24.2 24.3 24.4 24.5

Strategies for Creating the Diversity of Oligosaccharides Pamela Sears and Chi-Huey Wong Introduction 706 Chemical Synthesis of Oligosaccharides 707 Enzymatic Synthesis of Oligosaccharides 711 Programmable One-pot Synthesis 717 Conclusions 720 References 721

706

Part IV

Molecular Design and Combinatorial Compound Libraries

25

Design Criteria 725 Josef Pernerstorfer Introduction 725

25.1 25.2 25.3 25.4 25.4.1 25.4.2 25.4.3 25.4.4 25.5 25.5.1 25.5.2 25.5.3 25.6

26

26.1 26.2 26.3 26.3.1 26.3.2 26.3.3 26.4 26.4.1 26.4.2 26.4.3

27

723

Properties of Combinatorial Libraries for Drug Development 725 Differentiation of Drug-like and Nondrug-like Compounds 728 Diversity in Combinatorial Chemistry for Drug Development 730 Introduction 730 Descriptors 731 Selection Algorithms 732 Diversity Assessment 734 Privileged Structures 735 Introduction 735 Further Examples of Privileged Structural Motifs 736 Substructure Analysis of Drugs 738 Conclusion 740 References 740 Estimation of Physicochemical and ADME Parameters Michael W. Ha¨rter, Jo¨rg Keldenich, and Walter Schmitt Introduction 743

743

ADME/PK Considerations in Combinatorial Library Design 744 Estimation of ADME/PK from Physicochemical Parameters 745 Models for Permeation Through Membranes (Absorption) 746 Models for Distribution in the Body 749 Models for Clearance and Metabolism 752 Estimation of Physicochemical Parameters 753 Lipophilicity 754 Solubility Including pK a Effects 755 Plasma Protein Binding 758 References 758 Virtual Compound Libraries and Molecular Modeling Roger M. Brunne, Gerhard Hessler, and Ingo Muegge

761

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Contents

27.1 27.2 27.2.1 27.2.2 27.3 27.3.1 27.3.2 27.3.3 27.3.4 27.4 27.4.1 27.4.2 27.4.2.1 27.4.2.2 27.4.2.3 27.5

Introduction 761 Lead-finding Libraries 762 Diversity Assessment of Library Compounds 762 ‘‘Drug-likeness’’ of Library Compounds 763 Focused Libraries 765 Targeting Protein Families 766 Privileged Structures 767 Similarity 770 Docking 772 Methods for Library Optimization 774 Genetic Algorithm 775 Fitness Function 775 Potency 775 Diversity 776 Physicochemical Properties 776 Conclusion 778 References 781

28

Erythropoietin Sensitizer – A Case Study 784 Berthold Hinzen, Gabriele Braünlich, Christoph Gerdes, Thomas Kra¨mer, Klemens Lustig, Ulrich Nielsch, Michael Sperzel, Josef Pernerstorfer Introduction 784 Results 785 High-throughput Screening and Biological Evaluations 785 Concept for Chemical Optimization 787 4-Fluoro-3-nitroaniline as Central Core 787 Libraries Around Single Heterocycles 789 Hydantoins 789 Pyrazoles 790 The Candidate for Preclinical Development 801 Combinatorial Chemistry in Drug Discovery 802 Acknowledgements 802 References 803

28.1 28.2 28.2.1 28.2.2 28.2.3 28.2.4 28.2.4.1 28.2.4.2 28.2.5 28.3

29

29.1 29.1.1 29.1.2 29.1.3 29.2 29.2.1

Estimation of Stability and Shelf Life for Compounds, Libraries, and Repositories in Combination with Systematic Discovery of New Rearrangement Pathways 806 Ferenc Darvas, Gyo¨rgy Dormań, Tama´s Karancsi, Tama´s Nagy, and Istvań Ba´gyi Introduction 806 Stability and Shelf Life Characterization: the Need 806 Stability Characterization: Empirical Studies 807 Stability and Shelf Life Estimation: Model-based Approaches 807 Methods and Tools for Combinatorial Stability Assessment 808

Modeling Intrinsic and Extrinsic Factors Influencing the Stability of Individual Compounds 808

Contents

29.2.2 29.2.3 29.2.4 29.2.5 29.3 29.3.1 29.3.2 29.3.3 29.4 29.4.1 29.4.2 29.5 29.6 29.7 29.7.1 29.7.2 29.7.3 29.8 29.9

Modeling: from Compounds to Libraries 809 Modeling: from Libraries to Repositories 809 Realization of Shelf Life Estimation for Individual Compounds and Combinatorial Libraries 810 Instrumentation 812 Validation Studies for Combinatorial Stability Assessment 812 General Experimental Conditions 812 Stability Study for an Indole Library 812 Combinatorial Stability Investigation for a Small Repository 815 Stability Investigations in Combinatorial Drug Discovery 816 Pilot Design Phase 816 Libraries 817 A Way Towards Systematic Discovery of New Rearrangement Pathways 818 Summary 823 Appendix I: Stability Testing of Drug Substances 823 Stress Stability Testing 823 Accelerated Stability Testing 824 Long-term Stability Testing 824 Appendix II: The Arrhenius Model 824 Appendix III: Model Realization – the Stabex TM System 825 Acknowledgements 827 References 827

Part V

Novel Applications of Combinatorial Chemistry

30

Concepts of Combinatorial Chemistry in Process Development Markus Eckert and Ulrich Notheis Introduction 831 General 831 Subject of this Chapter 831 Literature 832 Process Development 834 Overview and Definition 834 Combinatorial Chemistry in Process Development 835 Demands on Process Development 835 Process Development for Different Applications 837 Parallelization in Process Development 838

30.1 30.1.1 30.1.2 30.1.3 30.2 30.2.1 30.2.2 30.2.3 30.2.4 30.3 30.3.1 30.3.2 30.3.2.1 30.3.2.2 30.3.2.3 30.3.2.4

829

Number of Experiments Compared with Scale of Experiment Requirements and Equipment for Parallelization in Different Development Phases 840 Route Scouting 840 Process Screening 841 Process Optimization 847 Process Characterization and Validation 849

831

838

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30.3.3 30.4 30.4.1 30.4.1.1 30.4.2 30.4.2.1 30.4.2.2 30.4.2.3 30.5 30.6

31

31.1 31.1.1 31.1.2 31.1.3 31.2 31.2.1 31.2.2 31.2.3 31.3

32

32.1 32.1.1 32.1.2 32.2 32.2.1 32.2.1.1 32.2.1.2 32.2.1.3 32.2.2 32.2.3

Requirements for Analytical Instruments 850 Planning of Parallel Process Development 851 Screening Strategies 851 Experimental Design 852 Strategies for the Selection of the Equipment 853 Automation versus Manual Work? 853 What is the Right Distribution and Number of Pieces of Apparatus for Different Stages? 855 Buying Commercially Available Systems or Carrying out In-house Development? 856 Case Studies 857 Summary 861 References 861 High-throughput Screening Applied to Process Development 864 Oliver Bru¨mmer, Bernd Jandeleit, Tetsuo Uno, and W. Henry Weinberg Introduction 864 General 864 Automation and Experimental Design 865 High-throughput Process Development 866 Case Studies 867 High-throughput Combinatorial Process Discovery 867 Route Scouting, Screening, Optimization, and Validation 869 Miscellaneous 880 Summary and Outlook 882 Acknowledgments 883 References 883 Combinatorial Methods in Catalysis 885 Bill Archibald, Oliver Bru¨mmer, Martin Devenney, Sasha Gorer, Bernd Jandeleit, Tetsuo Uno, W. Henry Weinberg, and Thomas Weskamp Introduction 885 Combinatorial Catalysis 885 Combinatorial Organic and Organometallic Catalysis 887

Metal-binding Ligands, Polymeric Enzyme Mimetics, and Metal Complexes as Enzyme Mimetics 888 Combinatorial Approaches to Metal-binding Ligands 888 Combinatorial Functionalization of Metal-binding Core Structures 888 Combinatorial Synthesis of Metal-binding Ligands from Building Blocks with Metal-coordinating Functionalities 892 Miscellaneous 895 Combinatorial Libraries of Polymeric Catalysts as Enzyme Mimetics 899 Combinatorial Synthesis Enzyme Mimetics 903

Contents

32.2.3.1 32.2.3.2 32.2.3.3 32.3 32.3.1 32.3.2 32.3.3 32.3.4 32.3.5 32.3.6 32.4 32.4.1 32.4.2 32.5 32.6 32.7 32.7.1 32.7.2 32.8 32.8.1 32.8.2 32.8.2.1 32.8.2.2 32.8.2.3 32.9 32.9.1 32.9.2 32.10 32.10.1 32.10.1.1 32.10.1.2 32.10.2 32.10.2.1 32.10.2.2 32.10.2.3 32.10.2.4 32.10.3 32.10.3.1

Hydrolytically Active Metal Complexes 903 Evolutionary Solid-phase Synthesis of Oxygenase Mimics 904 Libraries of Organic Acylation Catalysts 905 Combinatorial Catalysis in Asymmetric Synthesis 906 Combinatorial Catalyst Libraries in Enantioselective Additions of Dialkyl Zinc Reagents 906 Ligands for the Lewis Acid-catalyzed Asymmetric Aza-Diels–Alder Reaction 908 Divergent Ligand Synthesis for Enantioselective Alkylations 909 Chiral Phosphine Ligands for Asymmetric Hydrogenation 912 Asymmetric Reactions Catalyzed by Schiff Base-type Ligands – the Positional Scanning Approach 913 Identification of Novel Catalysts for the Asymmetric Epoxidations via the Positional Scanning Approach 916 Multidimensional Combinatorial Screening 917 Catalyst Discovery and Optimization Using Catalyst Arrays 919 Parallel Array Screening for Catalyst Optimization Using Discovery and Focused Ligand Libraries 925 One-pot, Multisubstrate Screening 927 Combinatorial Approaches to Olefin Polymerization Catalysts 930 Combinatorial Inorganic Catalysis 936 Combinatorial Libraries of Homogeneous Polyoxometalate-based Catalysts 936 Combinatorial Libraries and High-throughput Screening of Heterogeneous Polyoxometalate Catalysts 938 Combinatorial Heterogeneous Catalysis 939 Introduction 939 Case Studies 944 Oxidative Dehydrogenation of Ethane 944 Oxidative Dehydrogenation of Propane 945 Catalytic Oxidation of CO and the Reduction of NO 947 Combinatorial Electrocatalysis 947 Electrocatalysts for Fuel Cells 947 Combinatorial Electrosynthesis 951 Novel High-throughput Screening Tools 952 Infrared Screening Tools 953 Infrared Thermography 953 High-throughput Infrared Spectroscopy 956 Optical High-throughput Screening Techniques 957 Colorimetric Assays 958 Resonance-enhanced Multiphoton Ionization (REMPI) 963 Photothermal Deflection 965 Enantiomeric Excess by Circular Dichroism 966 High-throughput Screening Using Mass Spectrometry 966 Scanning Mass Spectrometry 966

xxiii

xxiv

Contents

32.10.3.2 32.10.4 32.10.4.1 32.10.4.2 32.10.4.3 32.10.5 32.10.5.1 32.10.5.2 32.10.5.3 32.10.6 32.11

‘‘Mass Tags’’ as Chirality Probes 968 Electronic High-throughput Methods 969 Electrochemical Analysis with Electrode Arrays Solid-state Temperature Sensors 970 Solid-state Gas Sensors 971 Array Reactors 973 Array Microreactors 973 Micromachined Array Reactors 976 Catalysis on a Chip 977 Capillary Array Electrophoresis 977 Summary and Outlook 978 Acknowledgements 979 References 979

33

Diversity-Based Identification of Efficient Homochiral Organometallic Catalysts for Enantioselective Synthesis 991 Amir H. Hoveyda Introduction 991

33.1 33.2 33.3 33.3.1 33.3.2 33.3.3 33.3.4 33.3.5 33.4

34

34.1 34.2 34.2.1 34.2.2 34.2.3 34.3 34.3.1

969

Factors Critical to the Success of Diversity-based Reaction Development 992 Peptidic Schiff Bases as Chiral Ligands 994 Ti-Catalyzed Enantioselective Addition of Cyanide to Meso Epoxides 995 Ti-Catalyzed Enantioselective Addition of Cyanide to Imines 1000 Zr-Catalyzed Enantioselective Addition of Dialkyl Zincs to Imines 1004 Cu-Catalyzed Enantioselective Addition of Dialkyl Zincs to Allylic Phosphates: Pyridyl Dipeptides as Chiral Ligands 1006 Cu-Catalyzed Enantioselective Conjugate Addition of Dialkyl Zincs to Unsaturated Ketones: Peptidic Phosphines as Chiral Ligands 1009 Conclusions and Outlook 1012 Acknowledgments 1013 Endnotes and References 1013 Combinatorial Aspects of Materials Science 1017 Bill Archibald, Oliver Bru¨mmer, Martin Devenney, Daniel M. Giaquinta, Bernd Jandeleit, W. Henry Weinberg, and Thomas Weskamp Abstract 1017 Introduction 1018 Combinatorial Solid-state Materials Science 1020 Materials Library Synthesis 1022 Vapor Deposition Techniques 1022 Alternative Library Synthesis Techniques 1026 High-throughput Screening 1030 Optical Screening 1030

Contents

34.3.2 34.4 34.4.1 34.4.2 34.4.3 34.4.4 34.4.5 34.5 34.5.1 34.5.2 34.6 34.6.1 34.6.2 34.6.3 34.6.4 34.6.5 34.7

X-Ray Characterization 1031 Applications 1032 Superconductivity 1032 Ferromagnetic Semiconductors 1033 Magnetoresistant Materials 1034 Dielectric and Ferroelectric Materials 1035 Luminescent Materials 1038 Case Studies 1041 Materials Discovery 1041 Device Optimization 1042 Organic Materials and Polymers 1045 Schiff Bases for Nonlinear Optical (NLO) Materials 1045 Artificial Receptors for Small Organic Molecules 1046 New Materials for the Separation of Enantiomers 1046 Molecular Imprinting 1050 Polymers with Novel Topologies and Functionalization 1050 Summary and Outlook 1056 Acknowledgments 1057 References 1057

35

Reprogramming Combinatorial Biology for Combinatorial Chemistry Sean V. Taylor Introduction 1063

35.1 35.2 35.2.1 35.2.1.1 35.2.1.2 35.2.2 35.2.3 35.2.4 35.3 35.3.1 35.3.2 35.3.3 35.4

1063

Combinatorial Biosynthesis: Creation of Novel Small-molecule Natural Products 1064 Polyketide Combinatorial Biosynthesis 1066 Combinatorial Biosynthesis from Type II PKSs 1068 Combinatorial Biosynthesis from Type I Modular PKSs 1070 Combinatorial Biosynthesis of Nonribosomal Peptide Products 1074 Combinatorial Biosynthesis from Hybrid PKS/NRPS Systems 1075 Combinatorial Biosynthesis of Carbohydrates 1077 Other Combinatorial Aspects of Biology 1081 Combinatorial Libraries of Random DNA and RNA 1082 Peptide Combinatorial Libraries 1084 Protein Combinatorial Libraries 1085 Perspectives 1090 Acknowledgments 1092 References 1092 Index

1099

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List of Authors Markus Albers Bayer AG Central Research D-51368 Leverkusen Germany Valery V. Antonenko Affymax Research Institute 3410 Central Expressway Santa Clara CA 95051 USA Bill Archibald H3-Technologies Inc. 1021 Grebe Street Foster City CA 94404 USA Istvań Ba´gyi ComGenex Inc. Bem rkp. 33-34 H-1024 Budapest Hungary Carmen Baldino Department of Chemistry ArQule Inc. 19 Presidential Way Woburn MA 01801 USA Marcus Bauser Bayer AG Pharma Research PH-R-CR VII D-42096 Wuppertal Germany

Stefan Bra¨se Kekule´-Institut fu¨r Organische Chemie und Biochemie Universita¨t Bonn Gerhard-Domagk-Straße 1 D-53121 Bonn Germany Gabriele Braünlich Bayer AG Pharma BPA Leverkusen Germany O. Bru¨mmer Symyx Technologies Inc. 3100 Central Expressway Santa Clara CA 95051 USA Roger M. Brunne Bayer AG Pharma Research Centre 42096 Wuppertal Germany David L. Coffen Discovery Partners International San Diego CA 92121 USA Christopher P. Corrette Array Biopharma Inc. 1885 33rd Street Boulder CO 80301 USA

xxviii

List of Authors Stefan Dahmen Kekule´-Institut fu¨r Organische Chemie und Biochemie Universita¨t Bonn Gerhard-Domagk-Straße 1 D-53121 Bonn Germany Ferenc Darvas ComGenex Inc. Bem rkp. 33-34 H-1024 Budapest Hungary M. Devenney Symyx Technologies Inc. 3100 Central Expressway Santa Clara CA 95051 USA Roland E. Dolle Department of Chemistry Adolor Corporation 371 Phoenixville Pike Malvern PA 19355 USA Gyo¨rgy Dormań ComGenex Inc. Bem rkp. 33-34 H-1024 Budapest Hungary Markus Eckert Bayer AG Chemicals Division Business Unit Fine Chemicals Research and Development D-51368 Leverkusen Germany Demosthenes Fokas Department of Chemistry ArQule Inc. 19 Presidential Way Woburn MA 01801 USA A. Ganesan Department of Chemistry University of Southampton Highfield Southampton SO17 1BJ UK

Christoph Gerdes Bayer AG Pharma Research Pharmacology 42096 Wuppertal Germany D. Giaquinta Symyx Technologies Inc. 3100 Central Expressway Santa Clara CA 95051 USA S. Gorer Symyx Technologies Inc. 3100 Central Expressway Santa Clara CA 95051 USA Philipp Grosche Institute for Organic Chemistry University of Tu¨bingen Auf der Morgenstelle 18 D-72076 Tu¨bingen Germany Rainer Haag Freiburger Materialforschungszentrum und Institut fu¨r Makromolekulare Chemie Albert-Ludwigs-Universita¨t Freiburg Stefan-Meier-Straße 21 D-79104 Freiburg Germany R. Hanko Bayer AG Chemical Division Head of BUFine Chemicals D-51368 Leverkusen Germany Jan-Gerd Hansel Bayer AG Central Reseach D-51368 Leverkusen Germany Michael W. Ha¨rter Business Group Pharma Building 470 D-42096 Wuppertal Germany

List of Authors Wolfgang Hartwig Bayer AG Gescha¨ftsbereich Pharma Leiter Forschung International D-42096 Wuppertal

Stephan Jordan Bayer AG Central Reseach D-51368 Leverkusen Germany

Tamio Hayashi Department of Chemistry Graduate School of Science Kyoto University Sakayo, Kyoto 606-8502 Japan

Gu¨nther Jung Institute for Organic Chemistry University of Tu¨bingen Auf der Morgenstelle 18 D-72076 Tu¨bingen Germany

Andre´ Hebel Freiburger Materialforschungszentrum und Institut fu¨r Makromolekulare Chemie Albert-Ludwigs-Universita¨t Freiburg Stefan-Meier-Str. 21 D-79104 Freiburg Germany

Christopher Kallus Bayer AG Central Research D-51368 Leverkusen Germany

Gerhard Hessler Bayer AG Central Research D-51368 Leverkusen Germany Berthold Hinzen Bayer AG Pharma Research Medicinal Chemistry D-42096 Wuppertal Germany Amir H. Hoveyda Department of Chemistry Merkert Chemistry Center Boston College 2609 Beacon Street Chestnut Hill MA 02467 USA Conrad Hummel Array Biopharma Inc. 1885, 33rd Street Boulder CO 80301 USA Bernd Jandeleit XenoPort Inc. 2631 Hanover Street Palo Alto CA 94304 USA

Tama´s Karancsi ComGenex Inc. Bem rkp. 33-34 H-1024 Budapest Hungary Jo¨rg Keldenich Business Group Pharma Building 470 D-42096 Wuppertal Germany Thomas Kra¨mer Bayer AG Pharma Research Medicinal Chemistry D-42096 Wuppertal Germany Jochen Kru¨ger Bayer AG Business Group Pharma PH-R CR-MC III D-42096 Wuppertal Germany Nicolay V. Kulikov Affymax Research Institut 3410 Central Expressway Santa Clara, CA 95091 USA Joachim E. A. Luithle Bayer AG Business Group Pharma Medicinal Chemistry D-42096 Wuppertal Germany

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List of Authors Klemens Lustig Bayer AG Pharma Research Pharmakokinetics 42096 Wuppertal Germany Thorsten Meyer Bayer AG Central Research D-51368 Leverkusen Germany Reza Mortezaei Affymax Research Institut 3410 Central Expressway Santa Clara, CA 95091 USA Ingo Muegge Bayer Research Center 400 Morgan Lane West Haven CT 06516 USA Tama´s Nagy ComGenex Inc. Bem rkp. 33-34 H-1024 Budapest Hungary K. C. Nicolaou Department of Chemistry and the Skaggs Institute for Chemical Biology The Scripps Research Institute 10550 North Torrey Pines Road La Jolla CA 92037 USA and Department of Chemistry and Biochemistry University of California San Diego 9500 Gilman Dr La Jolla, CA 92093 USA Ulrich Nielsch Bayer AG Pharma Research Pharmacology 42096 Wuppertal Germany

Ulrich Notheis Bayer AG Chemicals Division Business Unit Fine Chemicals Research and Development D-51368 Leverkuten Germany Josef Pernerstorfer Bayer AG Pharma Research Medicinal Chemistry D-42096 Wuppertal Germany Jeffrey A. Pfefferkorn Pharmacia Corp. Combinatorial and Medicinal Chemistry 7000 Portage Rd Kalamazoo, MI 49001 USA Jo¨rg Rademann Institute for Organic Chemistry University of Tu¨bingen Auf der Morgenstelle 18 D-72076 Tu¨bingen Germany Walter Schmitt Business Group Pharma Building 470 D-42096 Wuppertal Germany Pamela Sears Department of Chemistry The Scripps Research Institute 10550 N. Torrey Pines Rd La Jolla CA 92037 USA Mukund P. Sibi Department of Chemistry North Dakota State University Fargo ND 58105-5516 USA Adrian L. Smith Amgen Inc. One Amgen Center Drive Thousand Oaks, CA 91320 USA

List of Authors Hubertus Stakemeier Bayer AG Central Research ZF-LSc D-51368 Leverkusen Germany

Stefan Walle´ Bayer AG Central Research ZF-LSc-KC D-51368 Leverkusen Germany

Henning Steinhagen Bayer AG Business Group Pharma PH-R CR-MC II D-42096 Wuppertal Germany

W. H. Weinberg Symyx Technologies Inc. 3100 Central Expressway Santa Clara CA 95051 USA

Jean-François Stumbe´ Freiburger Materialforschungszentrum und Institut fu¨r Makromolekulare Chemie Albert-Ludwigs-Universita¨t Freiburg Stefan-Meier-Str. 21 D-79104 Freiburg Germany

T. Weskamp Symyx Technologies Inc. 3100 Central Expressway Santa Clara CA 95051 USA

Sean V. Taylor Laboratorium fu¨r Organische Chemie ETH Ho¨nggerberg-HCI CH-8093 Zu¨rich Switzerland Arounarith Tuch Bayer AG Central Research ZF-LSc-KC D-51368 Leverkusen Germany Tetsuo Uno Genomics Institute of the Novartis Research Foundation 3115 Merryfield Row Suite 200 San Diego CA 92121 USA Yasuhiro Uozumi Laboratory of Complex Catalysis Institute for Molecular Science Nishigonaka 38 Myodaiji Okazaki 444-8585 Japan

Chi-Huey Wong Department of Chemistry and Skaggs Institute for Chemical Biology The Scripps Research Institute 10550 N. Torrey Pines Rd La Jolla CA 92037 USA Tobias Wunberg Bayer AG Business Group Pharma PH-R CR-MC II D-42096 Wuppertal Germany Florencio Zaragoza Novo Nordisk A/S Novo Nordisk Park DK-2760 Ma˚løv Denmark

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Part I

General Aspects

Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

3

1

Combinatorial Chemistry in Perspective K. C. Nicolaou, R. Hanko, and W. Hartwig 1.1

Introduction

A fundamental aspect of organic chemistry is its ability to create new carboncontaining substances. This endeavor, called organic synthesis, made huge contributions to society by delivering a myriad of synthetic materials. Until recently the science of organic synthesis and its branches, such as medicinal chemistry, practiced the construction of organic molecules by targeting one molecule at a time. During the last decade of the twentieth century, however, a new concept took hold by which a collection of molecules is targeted, simultaneously producing a library of compounds instead of a single product. This strategy of concurrently synthesizing large numbers of compounds is called combinatorial chemistry or combinatorial synthesis. Although the roots of combinatorial chemistry were already in place, this philosophy took on a special meaning and assumed high priority in the 1990s because of pressures within the pharmaceutical industry to speed up the drug discovery process and because of the advent of high-throughput screening. Although combinatorial chemistry was initially met with resistance from some medicinal and other chemists, it rapidly became mainstream, and its practice today is widespread in academia and industry. It is utilized to synthesize libraries of compounds either as mixtures or as single compounds. The latter approach is preferred by most practitioners, and as new synthetic technologies, high-speed purification techniques and characterization strategies are developed, combinatorial chemistry will assume an increasingly prominent position within the armamentarium of the synthetic chemist. Combinatorial chemistry is currently being applied, in addition to natural product synthesis and drug discovery, to agricultural chemistry, chemical biology, catalyst discovery and material science. Combinatorial synthesis may be performed either in solution or on solid support. Each having their own advantages and disadvantages, both methods have been widely used in the construction of various compound libraries. Although solid-phase chemistry is currently not as well developed as solution-phase chemistry, particularly with regard to small organic molecules, the former method has Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim ISBN: 3-527-30509-2

4

1 Combinatorial Chemistry in Perspective

distinct advantages over the latter. First, in solid-phase synthesis, large excesses of reagents can be used to accelerate reactions and to drive them to completion; these reagents can then conveniently be removed at the end by filtration and washing. Furthermore, and because of easy separation, solid-phase chemistry can be automated through robotics to a higher degree than solution chemistry can. Most importantly, solid-phase chemistry can be applied to the elegant and powerful ‘‘splitand-pool’’ synthesis strategy for combinatorial chemistry. Despite its dramatic contribution to increasing efficiency over traditional methods, high-throughput parallel synthesis remains a laborious task. Thus, combinatorial chemists quickly recognized the benefits of automation as a crucial component of combinatorial chemistry. Besides the synthetic sequence, purification and characterization of compounds are also important aspects of combinatorial chemistry. The development of high-throughput chromatography methods to support automated parallel synthesis and of high-performance liquid chromatography–mass spectrometry (HPLC/MS) systems to direct the collection of the desired products are only the beginning of such high-throughput methods. Progress in this area is moving quickly and will be indispensable to any serious combinatorial chemistry effort.

1.2

Brief History of Combinatorial Chemistry

While combinatorial synthesis is a relatively new field of chemistry, Nature has been utilizing the same principles since the beginning. Although biologists had previously recognized the power of combinatorial chemistry, its application to problems relating to chemistry did not emerge until recently. Chemists’ confidence in rational design has previously kept them away from systematic explorations in chemical synthesis. The rather dramatic developments in molecular biology and high-throughput screening increased the demand for large numbers of small organic molecules to be screened against the ever-increasing biological targets. A solution to these challenges came from the peptide and oligonucleotide chemists, who could conveniently implement combinatorial chemistry strategies given the ease with which the amide and phosphate bonds could be constructed from the readily available building block libraries of amino acids and nucleotides respectively. Solid-phase chemistry was pioneered by Merrifield [1] and applied to the peptide and oligonucleotide fields quite effectively. In the early 1970s developments had already occurred in solid-phase synthesis of nonpeptide and nonoligonucleotide molecules. For example, the groups of Leznoff [2], Frećhet [3], Camps [4], Patchornik [5] and Rapoport [6] all reported early results on solid-phase synthesis. Camps et al. even applied solid-phase synthesis to the pharmaceutically relevant benzodiazepine system [7]. In Germany in the 1980s Frank and coworkers synthesized collections of oligonucleotides and, later, peptides on circles of cellulose paper [8]. Geysen et al. in

1.2 Brief History of Combinatorial Chemistry

Australia prepared a library of peptides [9] on functionalized polypropylene pins by immersing them sequentially into various solutions of activated amino acids held in the wells of a microtiter plate. Houghten at The Scripps Research Institute in La Jolla synthesized a library of 260 peptides [10] in polypropylene mesh containers encapsulating polystyrene resin, a process that came to be known as the ‘‘tea-bag’’ strategy. Both the pin and the tea-bag techniques went on to gain wide popularity and led to new generations of improved technologies for combinatorial chemistry. Researchers at Affymax reported very large spatially addressable libraries on glass chips using photolithographic techniques in conjunction with photolabile protecting group chemistry [11]. In parallel with the chemical approaches to peptide diversity, phages were being exploited to display very large libraries of peptides [12]. In 1992, Bunin and Ellman reported another synthesis of a benzodiazepine library [13] using the ‘‘multi-pin’’ technology pioneered by Geysen. At about the same time, a group of scientists at Parke–Davis reported the construction of hydantoins and benzodiazepines using a semiautomated robotic synthesizer [14]. In addition, a Chiron group reported the synthesis of a library of peptoids [oligo(Nsubstituted glycine)] and a robotic synthesizer of such compounds [15]. In the meantime, an elegant and ingenious strategy for combinatorial synthesis was proposed and demonstrated. This strategy called ‘‘split synthesis’’ or ‘‘split and pool’’ was introduced by Furka and coworkers at two European symposia in 1988; this work was published in 1991 [16]. The groups of Lam [17] and Houghten [18] independently developed the same technique and also published their results in 1991. These strategies led to the concept of ‘‘one bead–one compound’’ and promised the delivery of millions of compounds synthesized simultaneously on beads and with unprecedented rapidity. As elegant as it is, this method left much to be desired in terms of structure deconvolution and quantity of material produced. To solve the first problem, a number of encoding strategies were developed based on technologies ranging from DNA sequences to polychlorinated aromatics as well as nonchemical encoding methods such as radiofrequency tagging and two-dimensional (2D) bar-coding (for further discussion of library encoding, see Chapter 5). From the early 1990s onwards, the chemical literature exploded with reports addressing all aspects of combinatorial synthesis, including solid-phase chemistry, encoding strategies and molecular diversity. In the late 1990s alternative strategies were investigated, and an interesting compromise between solid-phase and solution-phase chemistry was found with polymers which are soluble in certain solvents but can be precipitated efficiently in others [19]. Thus the reactions on such polymers are carried out in homogeneous solution while the convenience of purification via a simple filtration is maintained. In a highly efficient extension of this principle, Curran and coworkers [20] have developed a number of fluorous tags which allow extraction of tagged compounds into a three-phase separation system (aqueous, organic, and fluorinated). Today, many well-known solution-phase reactions have been demonstrated to perform equally well on solid phase [21] and a plethora of reagents have been im-

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mobilized on solid supports [22]. Such techniques lead to high-speed purification procedures and often to higher yields of targeted products, which in turn lead to an increase in efficiency and productivity. While the peptide and oligonucleotide chemists may have opened the field of combinatorial chemistry, it was left to those chemists concerned with small organic molecules to make the methods widely applicable to more ‘‘lead-like’’ and ‘‘drug-like’’ structures. Of particular interest were new solid-phase synthetic strategies, new linkers for solid-phase chemistry [23], and new polymer-bound reagents [24].

1.3

Applications of Combinatorial Chemistry

With the advent of combinatorial chemistry, the traditional one molecule at a time approach to drug discovery was severely shaken. The initial euphoria of the early 1990s, however, was based to a considerable extent, on faulty grounds. The idea of synthesizing a myriad of compounds randomly, often as mixtures, sounded like a dream to many chemists of biotechnology and pharmaceutical companies. Soon thereafter, however, the principles of combinatorial chemistry for small organic molecules crystallized on a more pragmatic platform. The prevailing approach today is that based on both solution-phase and solid-phase chemistry applied in parallel or split-and-pool formats and directed at discrete and high-purity compounds [25, 26]. Initial-phase combinatorial chemistry is applied to discover lead compounds rapidly which are then subjected to lead optimization to produce drug candidates. The last part of the process is the domain of the medicinal chemists, who may also practice combinatorial strategies to achieve their goals. Thus, smaller focused libraries are carefully designed and synthesized, either in parallel or by the split-and-pool strategy using solution- or solid-phase chemistry. Combinatorial chemistry has, therefore, penetrated the laboratories of medicinal chemists who recognized its power in delivering the targeted compounds in a much faster way, and in acceptable quantities and purities. In similar ways, academic laboratories have adopted and refined combinatorial techniques in their quest for libraries of compounds needed for chemical biology studies [27]. The capability of combinatorial chemistry to produce large numbers of compounds rapidly is a powerful tool not only for chemical biology and drug discovery but also for a host of other research endeavors. Indeed, this philosophy and these combinatorial processes have been successfully applied to reaction optimization, the discovery of new materials [28] and the development of new catalysts [29]. In pioneering work, Fuchs and coworkers [30] reported in 1984 the use of automation to optimize reaction conditions with multiple variables. Reddington and Sapienza [31] reported in 1998 results from a highly parallel, optical screening method to discover novel electrocatalysts. Such practices are currently gaining wide popularity in industry for the optimization of process chemistry. The first

1.4 Outline of the Book

report of a combinatorial approach to new high-technology materials came from Schultz and coworkers [32], who prepared a spatially addressable array of potential superconducting materials. Similar techniques were then applied to a number of studies including ferroelectric materials [33] and phosphorescent materials [34]. Combinatorial techniques have also been applied to the development of chiral separation methods [35, 36] and optimization of protein catalysts through DNAshuffling techniques [37]. Most significantly, combinatorial chemistry has proven itself to be useful in the discovery of new catalytic systems. Early examples include Liu and Ellman’s synthesis of 2-pyrrolidinemethanol ligands intended for enantioselective additions [38], Burgess and coworkers’ diazo-compound library for a CaH insertion [39], Hoveyda and coworkers’ library of dipeptide Schiff base ligands for enantioselective addition of trimethylsilylcyanide to epoxides [40], and Sigman and Jacobsen’s Schiff base catalysts for the Strecker reaction [41]. The payoff of combinatorial chemistry to drug discovery is already becoming obvious to the industry in terms of a significant increase in the number of drug candidates and of decreases in time from target identification to drug candidates and manpower employed per drug candidate. Similar benefits are beginning to emerge in process chemistry, catalyst discovery and material science, where combinatorial chemistry techniques have also been implemented.

1.4

Outline of the Book

The following chapters in this series will address the various aspects of combinatorial chemistry in order to facilitate further advances in the field as well as to aid the reader in his or her practice of combinatorial chemistry. The book is divided into three parts. The first chapters (1–5) serve as the introductory part and build the foundation on which the next chapters are based. The following part (Chapters 5–20) deals with basic reaction mechanisms. The aim is to describe thoroughly the repertoire of combinatorial chemistry in solution and on solid support and to provide the reader with a critical overview of the best methods and conditions found so far. As an introduction, each chapter briefly discusses the applicability of the reactions dealt with for library synthesis in general and for solution-phase or solid-phase chemistry in particular. This entry is followed by the detailed reflection of each reaction type including the most recent developments/achievements. Based on mechanistic considerations, emphasis is put on the suitability of a given reaction for library synthesis using either solidphase or solution-phase chemistry. Generally, synthetic examples from all techniques are presented (solution-phase parallel synthesis, solid-phase chemistry, solid-supported reagents). In most cases examples originate from syntheses of large libraries. The last part of the book (Chapters 21–35) describes special topics such as applications, design and instrumentation as well as aspects beyond pure synthetic

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organic chemistry. In addition, since combinatorial chemistry is a tool aiming at specific applications, for example medicinal chemistry, material research and catalysis, selected examples of breakthroughs in these applications are discussed.

Acknowledgments

This book would not have been possible without the dedicated work of the editing team, especially Berthold Hinzen and Tobias Wunberg. We would also like to thank P. Go¨litz and his team for their conceptual contributions and their support to ensure the ambitious scheduling of this book.

References 1 R. B. Merrifield, J. Am. Chem. Soc. 2

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1963, 85, 2149–2154. a) C. C. Leznoff, J. Y. Wong, Can. J. Chem. 1972, 50, 2892–2893; b) C. C. Leznoff, Acc. Chem. Res. 1978, 11, 327. J. M. J. Frećhet, Tetrahedron 1981, 37, 663–683. F. Camps, J. Castells, M. J. Ferrando, J. Font, Tetrahedron Lett. 1971, 20, 713–1714. A. Patchornik, M. A. Kraus, J. Am. Chem. Soc. 1970, 92, 7587–7589. a) J. I. Crowley, H. Rapoport, J. Am. Chem. Soc. 1970, 92, 6363–6365; b) J. I. Crowley, H. Rapoport, Acc. Chem. Res. 1976, 9, 135–144. F. Camps, J. Castells, J. Pi, An. Quim. 1974, 70, 848–849. R. Frank, W. Heikens, G. Heisterberg-Moutsis, H. Blo¨cker, Nucl. Acids Res. 1983, 11, 4365–4377. H. M. Geysen, R. H. Meloen, S. J. Barteling, Proc. Natl. Acad. Sci. USA 1984, 81, 3998–4002. R. A. Houghten, Proc. Natl. Acad. Sci. USA 1985, 82, 5131–5135. a) S. P. A. Fodor, R. J. Leighton, M. C. Pirrung, L. Stryer, A. T. Lu, D. Solas, Science 1991, 251, 767– 773;ˇb) C. Y. Cho, E. J. Moran, S. R. Cherry, J. C. Stephans, S. P. A. Fodor, C. L. Adams, A. Sundaram, J. W. Jacobs, P. G. Schultz, Science 1993, 261, 1303– 1305.

12 J. K. Scott, G. P. Smith, Science

1990, 249, 386–390. 13 B. A. Bunin, J. A. Ellman, J. Am.

Chem. Soc. 1992, 114, 10997–10998. 14 S. H. DeWitt, J. S. Kiely, C. J.

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Stankovic, M. C. Schroeder, D. M. R. Cody, M. R. Pavia, Proc. Natl. Acad. Sci. USA 1993, 90, 6909–6913. R. N. Zuckermann, J. M. Kerr, M. A. Siani, S. C. Banville, Int. J. Pept. Protein Res. 1992, 40, 497–506. A´. Furka, F. Sebestyeń, M. Asgedom, G. Dibo´, Highlights of Modern Biochemistry, Proceedings of the 14th International Congress of Biochemistry, Prague, Czechoslovakia, 1988, VSP, Ultrecht, The Netherlands, ´ . Furka, F. 1988, 13, 47; b) A Sebestyeń, M. Asgedom, G. Dibo´, Int. J. Peptide Prot. Res. 1991, 37, 487– 493. K. S. Lam, S. E. Salmon, E. M. Hersh, V. J. Hruby, W. M. Kazmierski, R. J. Knapp, Nature 1991, 354, 82–84. R. A. Houghten, C. Pinilla, S. E. Blondelle, J. R. Appel, C. T. Dooley, J. H. Cuervo, Nature 1991, 354, 84– 86. D. J. Gravert, K. D. Janda, Chem. Rev. 1997, 97, 489–510. A. Studer, S. Hadida, R. Ferritto, S.-Y. Kim, P. Jeger, P. Wipf, D. Curran, Science 1997, 275, 823–826. a) P. H. H. Hermkens, H. C. J. Ottenheijm, D. Rees, Tetrahedron

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1996, 52, 4527–4554; b) P. H. H. Hermkens, H. C. J. Ottenheijm, D. Rees, Tetrahedron 1997, 53, 5643– 5678. S. V. Ley, J. Chem. Soc. Perkin Trans 1, 2000, 1235. For a review of linkers, see: I. W. James, Tetrahedron 1999, 55, 4855– 4946 For a recent review, see: S. Bhattacharyya, Comb. Chem. High Throughput Screening 2000, 3, 65–92. H. N. Weller, M. G. Young, S. J. Michalczyk, G. H. Reitnauer, R. S. Cooley, P. C. Rahn, D. J. Loyd, D. Fiore, S. J. Fishman, Mol Diversity 1997, 3, 61–70 L. Zeng, L. Burton, K. Yung, B. Shushan, D. B. Kassel, J. Chromat. A 1998, 794, 3–13. For examples, see: a) S. Feng, J. K. Chen, H. Yu, J. A. Simon, S. L. Schreiber, Science 1994, 266, 1241– 1247; b) K. C. Nicolaou, N. Winssinger, J. Pastor, S. Ninkovic, F. Sarabia, Y. He, D. Vourloumis, Z. Yang, T. Li, P. Giannakakou, E. Hamel, Nature 1997, 387, 268–272; c) N. S. Gray, L. Wodicka, A.-M. W. H. Thunnissen, T. C. Norman, S. Kwon, F. H. Espinoza, D. O. Morgan, G. Barnes, S. LeClerc, L. Meijer, S.-H. Kim, D. J. Lockhart, P. G. Schultz, Science 1998, 281, 533–538. a) B. Jandeleit, D. J. Schaefer, T. S. Powers, H. W. Turner, W. H. Weinberg, Angew. Chem. Int. Ed. 1998, 38, 2494–2532; b) P. G. Schultz, X.-D. Xiang, Curr. Opin. Solid State Mater. Sci. 1998, 3, 153– 158.

29 For recent reviews, see: a) K. D.

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Shimizu, M. L. Snapper, A. H. Hoveyda, Chem. Eur. J. 1998, 4, 1885–1889; b) T. Bein, Angew. Chem. Int. Ed. 1999, 38, 323–326. A. R. Fisbee, M. H. Nantz, G. W. Kramer, P. L. Fuchs, J. Am. Chem. Soc. 1984, 106, 7143–7145. E. Reddington, A. Sapienza, Science 1998, 280, 1735. X.-D. Xiang, X.-D. Sun, G. Briceno, Y. Lou, K. A. Wang, H. Chang, W. G. Wallace-Freedman, S.-W. Chen, P. G. Schultz, Science 1995, 268, 1738–1740. H. Chang, C. Gao, I. Takeuchi, Y. Yoo, J. Wang, P. G. Schultz, X.-D. Xiang, R. P. Sharma, M. Downes, T. Venkatesan, Appl. Phys. Lett. 1998, 72, 2185–2187. E. Danielson, J. H. Golden, E. W. McFarland, C. M. Reaves, W. H. Weinberg, X. D. Wu, Nature 1997, 389, 944–948. K. Sada, K. Yoshikawa, M. Miyata, Chem. Commun. 1998, 1763–1764. P. Murer, K. Lewandoski, F. Svec, J. M. J. Frećhet, Chem. Commun. 1998, 2559–2560. W. P. C. Stemmer, Nature 1994, 370, 389–391. G. C. Liu, J. A. Ellman, J. Org. Chem. 1995, 60, 7712–7713. K. Burgess, H.-J. Lim, A. M. Porte, G. A. Sulikowski, Angew. Chem. Int. Ed. Engl. 1996, 35, 220–222. B. M. Cole, K. D. Shimizu, C. A. Krueger, J. P. A. Harrity, M. L. Snapper, A. H. Hoveyda, Angew. Chem. Int. Ed. Engl. 1996, 35, 1668–1671. M. S. Sigman, E. N. Jacobsen, J. Am. Chem. Soc. 1998, 120, 4901–4902.

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Introduction to Combinatorial Chemistry David L. Coffen and Joachim E. A. Luithle 2.1

Combinatorial Chemistry in Drug Discovery – a Perspective

The categorical imperatives of modern drug discovery are to produce better clinical candidates that are less prone to failure at a late-stage, and to do this more rapidly than the industry performance standards of the past two decades would predict and at a cost that is responsive to social and political pressures on drug prices. Combinatorial chemistry is nothing less than a cornerstone technology in the realization of these imperatives. Its function is basically that of a high-output engine, providing very large numbers of well-designed, well-made, high-quality compounds for high-throughput evaluation as potential drug candidates. In fact, this ability to leverage productivity has prompted researchers in other fields, such as catalysis and material science, to adopt and adapt this engine to their needs as well. The purpose of this chapter is to provide an overview of the basic principles of combinatorial chemistry and to outline the operating principles associated with its most widely practiced forms. Subsequent chapters provide more detailed accounts of how the various types of synthetic chemistry are utilized, of specific combinatorial chemistry technologies, and of specific applications. By way of background one can compare how a typical medicinal chemist of 1975 would set about making a compound for testing with the way that the same chemist would approach the task today. First of all, each decision about what compound to make was typically embedded in the program category in which the chemist was a participant. The program description frequently included a structural component, such as ‘muscarinic cholinergics’, ‘cephalosporin antibiotics’, ‘benzodiazepine anxiolytics’, or ‘tricyclics’, and the chemist would have had considerable expertise in the synthetic aspects, structure–activity relationships, the literature and patents, and, to some extent at least, the pharmacology of the particular compound class. Each new hypothesis was based on a blend of past experience, recent papers and/or patents in the field, and on the latest biological testing results, and was then refracted through knowledge of how to make the compounds of interest and what was available to make them from. If the synthesis worked, a Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

2.2 Key Issues

1–5-g sample (huge by today’s standards) would be made and rigorously purified by column chromatography or recrystallization. Meticulous analyses using, as a minimum, thin-layer chromatography (TLC), mass spectrometry (MS), infrared (IR), nuclear magnetic resonance (NMR), ultraviolet (UV), and combustion analysis determined if the sample was suitable for registration and biological testing. A compound data sheet that included analytical results, melting point, solubility data, and other information would typically be prepared, reviewed, approved, and filed before a sample could be sent for testing. Using this process, a chemist with one or two assistants was deemed productive if 50–200 new compounds a year were forthcoming, depending on the complexity of the structural class. In contrast, the drug designer of year 2000 is most likely a hybrid medicinal, combinatorial, computational, and analytical chemist – with a working knowledge of molecular and cellular biology. He or she spends little time in the library, lots of time at a computer, and much of the remaining time meeting with the project team to review results and plan activities. Also, in comparison with the handful of laboratory appliances considered essential 25 years ago, today’s chemistry laboratory is replete with complex (and expensive) productivity-enhancing equipment, making it possible for the 200 compounds of yesteryear to be made in an automated synthesizer during the hour spent in a meeting. The decision as to which compound to make next has been replaced by a design process regarding which set (or library) of compounds to make next. Routine synthesis of single compounds now occurs mainly in the later stages of lead optimization. The design process flows not from a compound class associated with a particular drug discovery program, but from the biomolecular targets associated with the program. This fact underlies the critical relationship between modern medicinal chemistry and structural biology/computational chemistry. As an illustration, a 1970s’ program on influenza drugs might be centered on aminoadamantane derivatives (antiviral compound class), whereas a modern program might be focused on inhibitors of neuraminidase (viral protein target). The discovery phase of the program is driven by the target and by knowledge of its structure and natural ligand in the case of a receptor or by its mechanism of catalysis and natural substrate in the case of an enzyme. Overall, it can be stated that the integration of biomolecular target-driven computational design methods, combinatorial techniques for compound production, and high-throughput biological screening technologies has, in principle, resulted in a huge increase in productivity in medicinal chemistry. The role of combinatorial chemistry in modern pharmaceutical research and development (R&D) is perhaps best shown in Figs. 2.1 and 2.2, which depict as triads the major components of both the overall R&D process and its discovery phase. 2.2

Key Issues

The basic principles of combinatorial chemistry are often tied to, and sometimes confused with, the origins of combinatorial chemistry. The founding (but flawed)

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2 Introduction to Combinatorial Chemistry

Fig. 2.1.

The essential elements of modern pharmaceutical research and development.

principle was that, given a sufficiently large and diverse set of compounds to test, the discovery of an ideal drug for any given disease state would be statistically unavoidable. This principle was frequently popularized in metaphorical terms, such as the likelihood that any lock could be opened if a sufficiently large number of keys are tried, or that the pharmacological richness of natural products could be easily accessed and expanded through the laboratory equivalent of a rain forest. The anticipated surge in overall pharmaceutical R&D productivity has not materialized and this optimistic view of combinatorial chemistry has largely been abandoned. The contemporary view is more pragmatic and generally conforms to the principles outlined below. 1 The synthesis of many compounds simultaneously is more efficient than the synthesis of a single compound. In terms of simple time utilization, this principle was

Fig. 2.2.

The essential elements of the discovery phase.

2.2 Key Issues

intuitive to generations of chemists who set up two or more reactions in a hood each night. In the combinatorial context it has far greater significance as the number of compounds made from combinations of reagent sets increases geometrically with the number of reagents in these sets. 2 Any synthesis scheme can be executed in a manner that affords multiple products if individual reagents are replaced by complementary reagent sets used in different combinations. This perception is eponymous with combinatorial chemistry and lies at its heart. It can be readily illustrated with a Diels–Alder reaction (Fig. 2.3).

Fig. 2.3. Combinatorial chemistry illustrated with a Diels–Alder reaction.

A chemist could easily execute this synthesis in a single workday. However within this same day, that chemist could weigh and prepare stock solutions of ten dienes and ten dienophiles. Then by making equimolar combinations of each diene with each of the ten dienophiles and running the corresponding Diels–Alder reactions, a total of 100 Diels–Alder adducts could be synthesized, thus achieving a 100-fold productivity gain. When a synthesis scheme provides for three reagent sets (incorporated into the products), the use of ten members in each set affords 1000 products. Similarly the use of four sets of ten reagents distributed over one (e.g. Ugi reaction) or several stages of a synthesis affords 10,000 possible combinations. The number of products increases geometrically (A B C) whereas the number of reagents increases arithmetically (A þ B þ C) and in this simple reality lies the true power of combinatorial chemistry. 3 Rigorous design of products and synthesis is critical and large numbers cannot compensate for poor design. Combinatorial chemistry, often viewed and applied solely as a tool to leverage serendipity, frequently fails to meet expectations. However when used to leverage the output of sound experimental designs based on synthetic and medicinal chemistry knowledge, the outcome is generally more satisfactory. There is probably a more fundamental principle behind this empirically derived view and it relates to the fact that the pharmacological effects of bioactive small molecules are derived from their ability to bind to and modulate the function of macromolecular targets, usually proteins. Proteins are built from a bounded set of amino acids. Their biological and biochemical functions have structural determinants derived from a bounded set of secondary structure motifs and these are usually associated with small molecule binding. Thus the number and types of small molecules that are likely to exhibit biological activity also represent bounded sets and combinatorial chemistry applied within this domain is far more likely to succeed than a totally random approach.

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4 While synthetic organic chemistry evolved primarily in terms of serial processing, its productivity can be greatly enhanced by introducing parallel processing. Strong emphasis needs to be put on the importance of parallel processing as an explicit component of experimental design. The synthesis plan must comprehend the fact that hundreds or thousands of compounds are being synthesized simultaneously using hundreds or thousands of reagent combinations in each synthetic step. Each unit operation and each operating parameter – such as solvent used, temperature, and level of agitation – should encompass the conditions needed to bring each reagent combination to complete reaction while minimizing side-reactions and product decomposition. Sample tracking and inprocess control data must also be managed at very high volume. 5 Laboratory automation, robotics, and mechanical devices enabling the simultaneous performance of multiple tasks are essential to combinatorial chemistry. The range of enabling tools goes from simple devices such as multichannel hand pipettes to high-performance, fully integrated systems such as the Irori NanoKan system. This issue will be revisited, but the basic principle is that the successful practice of combinatorial chemistry requires some level of commitment to and investment in laboratory automation. 6 Robust process chemistry is required to assure the desired outcome. In the pharmaceutical world, the importance of process chemistry lies at the two extremes. For single compound synthesis in a medicinal chemistry laboratory, a good process may be on the ‘‘nice to have’’ list, but as long as the synthesis scheme affords some of the desired compounds, intermediate and endstage purifications will overcome deficiencies in the process. For bulk pharmaceutical production, the process is everything: it must be extremely efficient, fully validated, and conform to regulated manufacturing practices. The importance of process chemistry in combinatorial synthesis is closer to the bulk production context. For large compound libraries, the opportunity for product purification is very limited (although technologies for doing this are evolving nicely). Library members with poor-quality or outright failures are highly undesirable as they waste time and resources in biological testing laboratories. The best resolution of this dilemma is to develop the synthesis scheme to be used for production of a library into a robust, well-defined process. This requires careful optimization of reaction conditions, validating the individual members of the reagent sets, developing reliable analytical methods, and defining the process in something equivalent to a ‘standard operating procedure’. 7 Electronic tracking and control systems are critical components of combinatorial chemistry. Every chemist has been exposed to the tedium of labeling samples by hand, filling out analytical request forms, and completing compound data sheets. Preserving the identity of samples and information about them by linkage to laboratory notebook pages is a cumbersome system that works when a few hundred samples per year are involved but it cannot function when individual experiments are producing thousands of individual compounds. The data management aspect of combinatorial chemistry requires access to electronic tracking and control systems for sorting and labeling samples (e.g. with

2.3 Combinatorial Synthesis

machine-readable barcodes), for generating structure lists (SD files) that link sample codes to compound structures, and for collecting and processing analytical data and linking these data to sample codes and structure files. In addition to these sample and data management applications, electronic control systems embedded in the operating software of automated synthesis systems are necessary to ensure that each in-process material is in the right place at the right time for each step in order to ensure that the library plan is faithfully executed. 8 Analysis and quality control procedures are just as important in combinatorial chemistry as in other forms of synthetic chemistry. The shortcomings of early expressions of the combinatorial concept included the misperception that quality was not important. In fact many libraries were deliberately prepared as compound mixtures. It was taken for granted that biologically active samples could be separated, deconvoluted, or in some way dealt with after hits were detected. Two things resulted. The fact that synthesis without analysis is a prescription for poor science was reaffirmed and confidence in combinatorial chemistry as a productive discovery tool developed rather slowly. At its current state of development and acceptance, combinatorial chemistry is expected to provide samples of individual compounds with a purity level [by high-performance liquid chromatography (HPLC)] of at least 80%. The identity of compounds is routinely verified by mass spectrometry, using electronic comparison of probable molecular ion peaks with calculated molecular weights. Combined liquid chromatography/mass spectrometry (LC/MS) is preferable as it will confirm or deny that the major peak in an HPLC trace corresponds to the design intent for that particular library member. An aspect of quality control which needs further development is the application of in-process control procedures (IPCs) which are basic and routine in multistep synthesis of single compounds. 9 Since the purpose of combinatorial chemistry is to facilitate the discovery of useful compounds, combinatorial syntheses must be reproducible and scalable. This is not to say that the reaction sequence and mode of synthesis (solid or solution phase) must eventually serve to provide multikilogram supplies, but the compounds in a library have little value if interesting compounds cannot be conveniently resynthesized in small amounts for closer examination. 10 As a final basic principle it should be understood that combinatorial chemistry is a productivity-enhancing tool for chemists engaged in pharmaceutical research, agrochemicals, catalysis, and materials science – any field where the preparation and testing of new compositions of matter are the essential elements of discovery. However it does not displace or supplant established fields of synthetic chemistry such as medicinal chemistry – its best use is to leverage the productivity of existing fields.

2.3

Combinatorial Synthesis

As mentioned before libraries of mixtures and of single compounds are accessible by combinatorial chemistry. Furthermore the syntheses can either be performed in

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solution or on solid support. In the following, both methods will be discussed in detail and the underlying principles will be described. To be able to compare the advantages and disadvantages of these technologies the following aspects will be emphasized: reaction conditions, equipment, and possible degree of automation. 2.3.1

Solid-phase Combinatorial Synthesis

Solid-phase combinatorial chemistry relies on the fact that the molecule under construction is attached via a linker to a polymeric carrier (bead) (Fig. 2.4). This immobilization allows a simple separation of intermediates and finally of the product from reagents and soluble byproducts. To guarantee the efficacy of this principle, the bead and linker have to be stable under the reaction conditions. Furthermore, the bond between the target product and the linker has to be cleaved selectively under mild conditions without destroying the product. Thus, after simple filtration, nothing but the liberated pure product AB is obtained.

Fig. 2.4. Solid-phase chemistry. After reaction the product is liberated and filtered off.

2.3.1.1 Reagents and Conditions

A key feature of solid-phase synthesis is the possibility of driving reactions to completion with excess reagent. In normal solution-phase chemistry such forcing conditions would have devastating effects on product quality, whereas in solidphase chemistry the excess reagents are simply removed by filtration. Another important effect is the pseudo dilution caused by spatial separation of reactive sites. Difunctional compounds such as diamines or diacids react selectively in a monofunctional manner with only one of the two groups resulting in a highly effective desymmetrization. The spatial separation of the functional groups also renders the macrocyclization reaction a comparatively effective processe. Solution-phase chemistry, on the other hand, often affords statistical mixtures of products under these conditions. The positive features mentioned before are opposed by some drawbacks: adapting a standard solution-phase reaction to solid phase frequently entails problems associated with finding suitable, robust and versatile linkers – points of attachment in the starting material. These inherent features limit the choice of possible reaction conditions. Furthermore, the selection of usable solvents can be quite restricted. While crosslinked polystyrene resin beads may be insoluble, polyethylene or polypropylene used in containers, pins, etc. will deform or dissolve in many solvents at elevated temperatures. Another restriction stems from the swelling/ shrinking characteristics of crosslinked polystyrene beads. Dichloromethane, tetrahydrofuran (THF), dimethylformamide (DMF), toluene, and (marginally) dimethyl sulfoxide (DMSO) are all suitable in this respect, but ether, methanol, ethyl acetate, acetonitrile, and water are not. Tentagel resins are compatible with more solvents


but introduce several more restrictions. Also the temperature that can be applied is restricted: þ100 C is generally at or beyond the upper limit for most solid-phase systems – room temperature to 80 C being the ‘comfort zone’. It should be noted, however, that very low temperature operations involving, for example, reactive enolates generated at 78 C are more easily handled in a directed split-and-pool solid-phase mode (fewer batches). Chemistry involving heterogeneous catalysts such as palladium on charcoal or solid reagents such as manganese dioxide are intrinsically incompatible with solid-phase synthesis methods. Furthermore, the solid-phase chemistry literature is still limited, and only a relatively small proportion of the synthetic solution-phase repertoire has been adapted to solid phase. Therefore the development of a suitable methodology for the synthesis of large libraries can be time-consuming (several months). Another drawback is the difficulty to analyze the outcome of a given reaction. On-bead analysis is still not satisfying, although NMR and IR as well as MS techniques have been improved. In most cases, reaction products have to be cleaved from the support and are analyzed by normal methods (HPLC, NMR). Automation As pointed out before, the filtration of the reaction mixture, e.g. the separation of solid-supported products from starting materials and reagents, is the key process in solid-phase chemistry. Because of its simplicity, this process can easily be automated and run in parallel. Thus, the first robotic synthesizers were those for solidphase chemistry: many reactions were set up in parallel by automated dispensing stations and the normally very tedious work-up procedure was reduced to simple filtrations. This worked fine as long as the library size was not too large ( 1000 compounds). Only a few reactions lead to quantitative conversion using equimolar amounts of reagents. Solution-phase methods encourage parsimony in the use of reagents as any excess can become a product impurity. The use of excess reagents in solution phase can only be contemplated when facile removal of the excess is possible. However, purification and isolation procedures were tremendously improved in the last year, allowing the purification of hundreds of compounds per day by preparative HPLC methods. Furthermore, more recently, a number of technologies have emerged making combinatorial chemistry in solution phase competitive to solid-phase reactions. 2.3.2.1 Reagents and Conditions

In principle, any chemistry and any reaction can be employed in solution-phase methods, including complex organometallic reagents, biocatalysis, etc. Few, if any, of the reactions and procedures documented in compendia of ‘organic syntheses’ and ‘organic reactions’ could not be adapted to solution-phase combinatorial chemistry if suitable time and effort were applied. Adapting viable reactions to solutionphase combinatorial chemistry entails a set of problems which can be routinely solved in a month or two. The problems are fairly standard and address aspects

19

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such as control of stoichiometry, capturing a broad range of reactivity within a single set of reaction conditions, and product quality assurance. Complex molecule synthesis relies heavily on protecting groups. However their use in solution-phase library synthesis is restricted to situations where their removal gives volatile byproducts, e.g. Boc or Cbz groups. Certain synthetic transformations that produce nonvolatile coproducts are also a problem in solution. For example, Mitsunobu reactions, which produce phosphine oxide and hydrazide coproducts, work very well in solid phase but require major adaptation of procedures for use in solution. Multistep synthesis conducted in normal solution-phase parallel synthesis leads to rapid deterioration of product quality because of incomplete reactions and accumulating byproducts. Compatibility with automated solution-handling devices is the only solvent and reagent restriction for solution-phase methods. Problems that may exist, such as loss of accuracy in dispensing small volumes of highly volatile solutions in ether or dichloromethane, can usually be overcome with less volatile alternatives such as dioxane or tetrachloroethylene. Solution-phase systems equipped to prevent condensation of water vapor (or icing up) at the low end, or equipped to condense/reflux solvent vapor at the high end, can easily operate in the 20 C to þ150 C range. 2.3.2.2 Scavenger Resins, Polymer-supported Reagents and Fluorous Tags

Solution-phase combinatorial chemistry suffers from one main disadvantage. To drive reactions to completion, more than one equivalent of reagent is frequently necessary, but the use of excess reagents is often prohibitive because their removal causes in most cases severe problems. Two new techniques were established to overcome these difficulties: polymer-supported reagents and scavenger resins. The latter are functional group-specific, reactive resins, e.g. polystyrene-bound isocyanates which react selectively with primary and secondary amines. Thus excess reagents are first used to drive reactions to completion. Subsequently, scavenger resins are added to the reaction mixture. After selective coupling of the resin to the excess starting material the insoluble material is removed by filtration and the product remains in pure form (Fig. 2.6). Also the second approach to simplify solution-phase chemistry relies on the separation of insoluble material from the reaction mixture and combines the benefits of solid- and solution-phase chemistry (Fig. 2.7): instead of immobilizing the starting material on the support, only the employed reagents are polymer supported. Again, the work-up and purification are reduced to a simple filtration. Furthermore, different polymer-supported reagents do not interfere with each other, making one-pot reactions possible that would not work in classic solution

Fig. 2.6. Excess reagents can easily be removed using scavengers.


Fig. 2.7.

The principle of polymer-supported reagents.

Fig. 2.8.

The total synthesis of (G)-oxomaritidine and (G)-epimaritidine.

chemistry (Fig. 2.8). The versatility of this approach has been proved by Ley et al. in the total synthesis of two natural products, (G)-oxomaritidine and (G)epimaritidine. Another possibility to deal with the problem of excess reagents in solution-phase combinatorial chemistry is the use of fluorous tagging and extraction with fluorinated solvents (Fig. 2.9). Substrate A is attached to a moiety which is polyfluorinated. Because of the fluorous tag the product of the reaction is exclusively soluble in the fluorous solvents. After reaction with excess quantities of reagent B an extraction with a polyfluorinated solvent is performed. Reagent B is more solu-

Fig. 2.9. Fluorous tagging – a powerful tool for solution-phase chemistry.

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ble in the organic phase and can be separated from the product. Finally the fluorous tag is cleaved off, removed by another extraction and the pure product can be isolated. 2.3.2.3 Equipment and Costs

Automation in solution-phase chemistry is more difficult than in solid-phase chemistry. Especially the purification of products requires large systems for preparatory HPLC. For highly automated solution-phase parallel synthesis on the scale of >100,000 compounds per year, an investment of several million dollars may be required to build and equip a suitable facility.

2.4

Conclusion

Solid-phase chemistry has made tremendous progress during recent years, and today highly complex natural products can be prepared on solid supports (see Chapter 3). Nevertheless, method development is slow and often fails to produce the compounds needed, e.g. in medicinal chemistry optimization programs. Thus, careful validation of the possible outcome of solid-phase chemistry has to be given the highest priority before the development of methodology is started. However, for the preparation of large libraries with only minor time constraints, solid-phase chemistry remains the method of choice. On the other hand, solution-phase chemistry is much more flexible and quicker. However, the production of large libraries is very tedious owing to the difficult purification of the products. Polymer-supported reagents and scavengers represent a versatile addition to solid-phase organic synthesis and parallel solution-phase chemistry. The combination of these reagents offers exciting possibilities. The methods described in this chapter are interesting and fascinating but, so far, they do not enable the chemist to exploit all the synthetic routes that he or she might think of. Thus, there is a need for more solid-supported reagents, linkers, scavengers, and, especially, solidphase methodology.

Bibliography 1 J. N. Abelson, Combinatorial

4 A. W. Czarnik, S. H. DeWitt, A

Chemistry, Academic Press, San Diego, 1996. 2 G. Jung, Combinatorial Peptide and Nonpeptide Libraries: A Handbook, VCH, Weinheim, 1996. 3 I. M. Chaiken, K. D. Janada, Molecular Diversity and Combinatorial Chemistry: Libraries and Drug Discovery, ACS, Washington, DC, 1996.

Practical Guide to Combinatorial Chemistry, American Chemical Society, Washington, DC, 1997. 5 G. B. Fields, Solid-Phase Peptide Synthesis, Academic Press, San Diego, 1997. 6 B. A. Bunin, The Combinatorial Index, Academic Press, San Diego, 1998. 7 E. M. Gordon, J. F. Kerwin, Combinatorial Chemistry and Molecular

Bibliography

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9

10

11

12

13

14 15

Diversity in Drug Discovery, John Wiley & Sons, New York, 1998. K. Burgess, Solid-Phase Organic Synthesis, John Wiley & Sons, New York, 2000. D. Obrecht, Solid-Supported Combinatorial and Parallel Synthesis of Small-Molecular-Weight Compound, Pergamon Press, Oxford, 1998. W. Bannwarth, E. Felder, Combinatorial Chemistry. A Practical Approach, Wiley-VCH, Weinheim, New York, 2000. M. R. Pavia, T. K. Sawyer, W. H. Moos, Bioorg. Med. Chem. Lett. 1993, 3, 387–396. W. H. Moos, G. D. Green, M. R. Pavia, Annu. Rep. Med. Chem. 1993, 28, 315–324. K. S. Lam, M. Lebl, Methods: A Companion to Methods in Enzymology 1994, 6, 372–380. K. D. Janda, Proc. Natl. Acad. Sci. USA 1994, 91, 10779–10785. M. A. Gallop, R. W. Barrett, W. J. Dower, S. P. A. Fodor, E. M.

Valuable Internet Links http://www.5z.com/divinfo/ http://www.combinatorial.com/

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Gordon, J. Med. Chem. 1994, 37, 1233–1251. E. M. Gordon, R. W. Barrett, W. J. Dower, S. P. A. Fodor, M. A. Gallop, J. Med. Chem. 1994, 37, 1385–1401. G. Lowe, Chem. Soc. Rev. 1995, 24, 309–382. Dedicated issue, Account. Chem. Res. 1996, 29, 111–170. F. Balkenhohl, C. von dem ¨nnefeld, A. Lansky, C. Bussche-Hu Zechel, Angew. Chem. Int. Ed. Engl. 1996, 35, 2288–2337. P. H. H. Hermkens, H. C. J. Ottenheijm, D. Rees, Tetrahedron 1996, 52, 4527–4554. R. E. Dolle, Mol. Diversity 1997, 2, 223–236. P. H. H. Hermkens, H. C. J. Ottenheijm, D. Rees, Tetrahedron 1997, 53, 5643–5678. R. E. Dolle, Mol. Diversity 1998, 3, 199–233. D. P. Curran, Angew. Chem. Int. Ed. 1998, 37, 1175–1196. R. E. Dolle, K. H. Nelson, Jr, J. Comb. Chem. 1999, 1, 235–282.

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3

Solid Phase and Soluble Polymers for Combinatorial Synthesis Rainer Haag, Andre´ Hebel, and Jean-Francois Stumbe´ 3.1

Introduction

Polymeric supports have revolutionized organic synthesis in the past decade and have become a major driving force for laboratory automation and combinatorial chemistry in general. While polystyrene (PS)-based ion exchange resins have been known since the 1950s, the first solid-phase peptide synthesis on modified PS microbeads was reported by Merrifield in 1963 [1]. Until the late 1980s, when the first nonpeptidic molecule libraries were reported [2], polymeric supports were mainly used for special applications, such as peptide and oligonucleotide synthesis. In these cases the polymeric support had to be stable only toward two repetitive reaction conditions (coupling and deprotection). Therefore, some of these original supports are of limited use for parallel multistep synthesis of small organic molecules, generally known as combinatorial chemistry. In the last decade a rapidly increasing number of new polymeric supports, crosslinked (insoluble) [3– 5] and noncrosslinked (soluble) polymers [6–10], have been described and used for combinatorial synthesis. Although solid-phase synthesis on PS-based resins exhibits a number of problems due to the heterogeneous nature of reactions and the low concentration of functional groups (typically a1.5 mmol substrate/g polymer) [11], no other polymeric support has yet found the same broad application for combinatorial synthesis [12]. However, the reader should be aware of the fact that there is no polymeric support for general application in organic synthesis. Every polymer has its drawbacks (e.g. chemical stability, polarity) and hence is stable only within a certain range of reaction conditions. In this chapter, we will describe the structure and the properties of polymeric supports as well as the effects of different spacers (Fig. 3.1). Spacer molecules, compared with linker molecules, are used to provide more accessible linker functionalities and to modify the properties of the polymer backbone (e.g. polarity, swelling characteristics). Linker functionalities, which also influence the materials’ properties, will be the subject of the next chapter. In the following sections the two major classes of polymeric supports – solid and soluble polymers – will be further classified by their chemical structures and the polymer topologies. A special focus Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

3.2 Solid-phase Supports

Fig. 3.1. General structure of a polymeric support.

will be put on the polymer properties, such as stability, swelling property, reactive site distribution, loading capacity and the range of applications in organic synthesis.

3.2

Solid-phase Supports 3.2.1

Polystyrene-based Resins

Functionalized polystyrenes are available as linear noncrosslinked and as crosslinked polymers. The latter, generally referred to as PS resins, are among the most commonly used solid supports for solid-phase organic synthesis (SPOS), solidphase-supported peptide (SPPS) synthesis and nucleotide synthesis. Nowadays, numerous types of polystyrene resins are commercially available and a nonexhaustive list of the most frequently used resins is given in Table 3.1. They are available in different sizes, with different loading capacities (amount of functionalization) and degrees of crosslinking. Macroporous and microporous polystyrene resins are typically prepared by suspension polymerization. The basis of this process is the dispersion of an organic phase (consisting of a monomer, a radical initiator, a crosslinking agent and potentially a comonomer) into an aqueous phase [13, 14]. The size of the initial droplets is adjusted by emulsifying the organic phase under stirring in the presence of a polymeric surfactant, which governs the final size and the final size distribution of the beads after polymerization. The different bead sizes are then separated by a multiple sieving process. This is why the size of the beads is usually given in mesh (number of sieve holes per inch); however, for most chemists the mm size of the final bead is more relevant for handling (Fig. 3.2). Resin beads used for combinatorial synthesis are spherical particles typically in the range 50–500 mm which can be easily handled (weighting, filtering and drying). 3.2.1.1 General Aspects

Crosslinkers In order to obtain insoluble resins, given amounts of crosslinking agent have to be used for the synthesis of the beads by suspension polymerization (see also Sect. 3.2.1.2 and Sect. 3.2.1.3). A crosslinking agent is generally a bi- or multifunctional molecule that can be incorporated into two or more growing

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3 Solid Phase and Soluble Polymers for Combinatorial Synthesis Tab. 3.1. Features and applications of frequently used solid-phase resins.

Type

Common features

Representatives

Applications a

Loading [mmol gC1 ]

Gel (microporous)

High capacity

1% or 2% crosslinked PS (Merrifield resin)

A, B, D, E

0.2–5.0

PEGA, PAP POEPOP SPOCC POEPS-3 CLEAR

A, F

A, F

0.4 0.7 0.4–0.7 0.4–1.2 0.2–0.3 0.2–0.3

Different solvent swelling Reagent access by diffusion Uniform sites

Many unsuitable for continuous flow synthesis

A

Hybrid

High capacity Different solvent swelling (but less than microporous gels)

ROMP Rasta silanes

D D

3.0 1.6–3.8

Encapsulated gel

Suitable for continuous flow synthesis

PolyHIPE (acrylamide resin encapsulated in PS ‘‘shell’’) Pepsyn-K (similar resin encapsulated in inorganic scaffold)

F

1.0–5.0

A, F

–

PEG-PS Tentagel9

D, E A, B, D

1.2–2.9 0.25–0.6

Argogel9 NovaGel9

A, B, D A, B, D

0.4–0.5 0.5–0.7

Highly crosslinked PS

E, F

5%) crosslinked polystyrene microbeads [12]. The term ‘‘macroporous’’ refers to their inner skeleton, which is made up of a permanent porous structure even in the dry state (see Scheme 3.1c). Historically, functionalized macroporous resins have mainly been used for ion exchange. Nowadays, many new applications, especially in the field of polymer-supported reagents [26, 27], have been developed (see also Section end). Macroporous resins are prepared by suspension polymerization of monomers such as styrene, vinyl pyridine, acrylamide, or glycidyl methacrylate with a porogen agent (Scheme 3.1) [28, 29]. Thus, a mixture of monomer with potentially a comonomer and a crosslinking agent are copolymerized after dispersion in an aqueous medium in the presence of the porogen, which, remains within the beads during the polymerization and acts as a template for the formation of the permanent internal porous structure of the final resin. Porogen agents can be of different natures (e.g. solvents, noncrosslinked polymers). After completion of the polymerization the porogens are removed by processes dependent on their characteristics and a hard opaque bead with a rough surface remains. The opacity of the macroporous resins, compared with the glassy appearance of the microporous beads, is due to their heterogeneous structure, which is made up of highly crosslinked polymeric microdomains and pores that are devoid of polymer [20, 30].

Scheme 3.1. Synthesis and structure of

macroporous resins: (a) polymer network forming; (b) porogen phase acts as pore template; (c) dry macroporous resin with large

interconnected pores. (1) Porogen and network start to phase separate; (2) porogen phase removed to yield pores (hatched area, crosslinked polymer; dots, porogen phase).

As mentioned above the generation of pores can take place in two ways: noncrosslinked polymers as well as organic solvents [28, 31]. For instance, if linear


Fig. 3.3. Different chemical species present in the technical grade DVB mixture.

and noncrosslinked macromolecules are dissolved in the starting organic phase, they can be removed from the final polymerized beads by Soxhlet extraction. In this process, these linear soluble macromolecules act as templates by creating permanent pores after their removal [32, 33]. The size and the distribution of the pores can be controlled by the characteristics of the linear polymer (i.e. molecular weight, concentration) [29]. Another possibility is suspension polymerization in the presence of an organic solvent that is a poor solvent, or even a nonsolvent, of the final polymer. Phase separation that occurs within the beads during the polymerization process leads to the formation of globular entities that are free of polymer with characteristics that depend on the nature and on the concentration of the porogen [20]. The amount of crosslinking agent plays a major role in the formation of the macroporous structure and has to be carefully considered. For example, during polymerization conducted with relatively large amounts of crosslinkers (up to 20–25 mol% compared to the monomer), the formation of highly DVBcrosslinked microdomains ensures a rapid phase separation [20, 32]. The most commonly used crosslinking agents for the preparation of macroporous resins are divinylbenzenes (DVB) (1, 2). The technical grade DVB typically used is a mixture of three chemical species: 27% m-DVB (1), 53% p-DVB (2), and 20% ethylvinylbenzenes (3) (Fig. 3.3). Other frequently used crosslinkers for macroporous resins include ethyleneglycol dimethacrylate (EGDMA) (4) and methylene bisacrylamide (6) (Fig. 3.4). They are selected depending on the nature and reactivity of the monomer engaged in the polymerization. Structure and physical properties of macroporous resins As a consequence of their mode of preparation, macroporous microbeads consist, on one side, of a permanent macroporous internal structure and, on the other side, of highly crosslinked areas (Scheme 3.1c). The porous areas are made up of numerous interconnected cavities of different sizes leading to a large internal surface available for functionalization, whereas, the crosslinked areas provide the rigidity for such structures [20, 28]. This high internal surface area – typically ranging from 50 to 1000 m 2 g1 (determined by N2 BET-isotherm) – is accessible even in the dry state [20]. In general macroporous resins show very low swelling in organic solvents because of the very highly crosslinked areas. For this reason, macroporous beads remain unaffected by changes in the direct environment, even in the presence of good solvents. Another consequence is that the pores can accommodate a large variety of solvents, including polar solvents such as water and low-molecular-

29

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3 Solid Phase and Soluble Polymers for Combinatorial Synthesis

Fig. 3.4. Different crosslinking agents for PS beads.

weight alcohols. The pore size and the presence of channels interconnecting these cavities allow solvents to diffuse quickly in and out of these pores. One drawback of these heterogeneous structures is the very low accessibility of the solvents and reagents to the highly crosslinked areas, leading to limited loading capacities with a typical range of 0.8–1.0 mmol g1 [34, 35]. However, some commercial ion exchange resins, also used for organic reagents, have loading capacities up to 4.5 mmol g1 [26]. Generally, macroporous resins display lower reactivities than microporous swollen beads. In contrast to microporous resins, they show high resistance toward osmotic shock (see Fig. 3.5b) [20], but they can be brittle when not manipulated carefully [13]. Applications of macroporous resins The most extensively used macroporous resins are polystyrene-based ionic exchange resins. They are made of poly(styreneco-divinylbenzene) copolymers and are subsequently modified to arylsulfonic acids, quarternary ammonium salts or other derivatives mainly located on the internal surface of the pores [28, 36]. This renders them accessible to numerous organic solvents, including water and alcohols. Recently, these ion exchange resins have had a revival for the immobilization of ionic reagents in automated synthesis [26, 27]. Macroporous beads have also been used for the immobilization of cata-


Fig. 3.5. ESEM pictures demonstrating the mechanical stability of 2% crosslinked Merrifield resins under different conditions: (a) after swelling in THF and drying; (b) after osmotic shock (swelling in THF, washing with MeOH and drying); (c) after magnetic stirring

of a swollen resin (24 h, 300 rpm, 20 C, DMF); (d) after mechanical shaking of a swollen bead (7 days, 60 rpm, 20 C, DMF). The ESEM pictures were taken by Dr. Rüdiger Landers, Materials Research Center, University of Freiburg, Germany.

lysts [28] and as adsorbents [37]. Other interesting applications involve their use as chromatographic materials and for continuous flow synthesis in columns [28, 38, 39]. 3.2.1.3 Microporous Resins

Microporous beads are weakly crosslinked resins obtained by suspension polymerization of undissolved styrene and divinylbenzene in the absence of any porogen agent. This process leads to the formation of a homogeneous network evidenced by a glassy and transparent appearance. The size of the beads is controlled by the stirring conditions and by the nature and the amount of the stabilizing agent, e.g.

31

32


a partially hydrolyzed polyvinylalcohol (PVA) (85–87%) [40]. The most commonly used supports for solid-phase organic synthesis are styrene–divinylbenzene copolymers crosslinked with 1–2% DVB. Numerous derivatives of these copolymers are commercially available. Three synthetic routes are used for the preparation of functionalized polystyrene microbead derivatives (i.e. Merrifield resins): 1 chemical transformation of the unfunctionalized poly(styrene-co-divinylbenzene) [1, 41, 42]; 2 copolymerization of a mixture of styrene, divinylbenzene with 4-(chloromethyl)styrene [43, 44]; 3 copolymerization of the mixture styrene, divinylbenzene with a-methylstyrene and its consecutive transformation by chlorination [45, 46]. The loading capacity is controlled by the yield of the electrophilic aromatic substitution in method 1 or by the amount of 4-(chloromethyl)-styrene incorporated in the starting organic phase for copolymerization in method 2. Typical loading values are between 0.2 and 4.0 mmol g1 , and a loading capacity of 1.5 mmol g1 (for the most commonly used Merrifield resins) corresponds approximately to 20% substituted aromatic groups. Higher loading Merrifield resins (4 mmol g1 ) have also been used in organic synthesis [47]; however, the highest possible loading 6.55 mmol g1 , which corresponds to 100% chloromethylstyrene, would not be useful in practice [12]. For microporous resins the exact degree of crosslinking and the nature of the crosslinker are even more important than for macroporous resins, owing to the severe effect on the swelling properties. The most common microporous resins are 1–2% crosslinked, but resins with less crosslinkage have also been studied [48]. They are mechanically weak and consequently easily subject to damage. However, increased reaction rates have been observed for these more flexible polymer networks. It is important to keep in mind that the DVB used for crosslinking is usually a technical grade product with a composition that can vary from one batch to another (see Fig. 3.3), which influences the properties of the beads. The consequences are that varying amounts of crosslinking agents are incorporated in resins depending on the different polymerization batches, which generally leads to relatively high error values (G0.5%) in the degree of crosslinking indicated in the catalogues of many suppliers. Other crosslinkers are ethyleneglycol dimethacrylate (EGDMA) (4), N,Nmethylene bisacrylamide (6) (MBA), trimethylolpropane trimethacrylate (8) (TRIM), and, more recently, novel crosslinkers have been introduced such as 1,4bis(vinylphenoxy)-butane (5) [49], and bis(vinylphenoxy)-polyethylene glycol (PEG) (7) [50, 51] which present the advantage of having a strong influence on the swelling properties due to the increased flexibility between the two crosslinking units (Fig. 3.4). In addition, styryl-terminated dendrimers have been introduced as novel polymer crosslinkers (9). They consist of 8–16 peripheral styryl units attached to aryl end branches of dendritic TADDOL or BINAP ligands and these were copolymerized


with styrene by suspension polymerization [52]. However, it is not yet clear whether such highly functional crosslinkers are of any advantage for practical use. Physical properties of microporous resins In contrast to macroporous resins, microporous beads have a low internal surface area in the dry state of less than 10 m 2 g1 (determined by N2 BET-isotherm) [20], owing to their more homogeneous structure that does not allow the diffusion of gases or ‘‘bad’’ solvents into the polymeric network. As polystyrene is hydrophobic and nonpolar, swelling of microporous polystyrene resins will occur in nonprotic solvents such as dioxane, dichoromethane, dimethylformamide (DMF), tetrahydrofuran (THF), or toluene, but not in polar protic solvents (e.g. water, alcohols) and apolar aprotic solvents (e.g. alkanes) [53]. However, the swelling of a resin depends not only on the nature of the solvent but also on the degree of crosslinking and the spacer molecules [48, 54]. It is indeed obvious that the percentage of swelling is inversely proportional to the crosslinker–monomer ratio. Thus, 1% DVB polystyrene swells 4–6 times its volume in dichloromethane, whereas in contrast 2% DVB polystyrene swells 2–4 times its volume in dichloromethane. The transformation of Merrifield microbeads by the conversion of the chloromethyl groups or the grafting of spacer groups can lead to dramatic changes in the swelling properties, as illustrated in Table 3.2. In order to anticipate the swelling properties of a modified resin, the chemist can compare the Hildebrand solubility parameters of solvents and of the polymer [55]. Thus, the calculation of the solubility parameter for crosslinked polystyrene gives values of 9.1 (Cal mL1 ) 0:5 and this value can then be compared with the solubility parameters of different solvents [49]. It is generally accepted that polymers and solvents are miscible when the difference of solubility parameters is not higher than 1. For example, the comparison of the values for the solubility parameter of polystyrene and of different solvents allow for the selection of ‘‘good’’ solvents for swelling the resin beads such as chloroform, DMF, dioxane, toluene, or THF, which was confirmed experimentally (Table 3.2) [49]. Mechanical and chemical stability of microporous resins Care has to be taken when working with microporous resins in different solvents. Osmotic shock can occur when a preswollen bead (in a ‘‘good’’ solvent) is introduced into a ‘‘bad’’ solvent. The beads start to shrink rapidly under expulsion of the ‘‘good’’ solvent and are subjected to stress. This leads to mechanical damage or at least to non-negligible modifications of the structure, as shown in Fig. 3.5b (compare with the surface of the original Merrifield resin in Fig. 3.5a). Another problem that has to be encountered when conducting a reaction on swollen microporous resins is that they can break if agitation conditions are too vigorous. Figure 3.5c shows an example of beads after a solid-phase reaction conducted with a magnetic stirrer. The scanning electron microscope (SEM) picture reveals that most of the beads are broken compared with the beads collected after a reaction conducted on a mechanical shaker (Fig. 3.5d). The consequence for broken resin beads is the clogging of the filters used during their purification.

33

Merrifield resins (1% crosslinked) – – 1.6 1.7 – 5.6 – 3.2 – 8.8 7.8 – 8.3 – 4.0 8.5 –

Solvent

CF3 CH2 OH Water MeOH EtOH 2-Propanol DMF DMSO MeCN EtOAc THF Dioxane Acetone CH2 Cl2 CHCl3 Et2 O Toluene Hexane

– 3.6 3.6 2.9 – 4.7 – 4.2 – 5.0 5.4 – 6.3 – 1.9 4.8 –

Tentagel S

– – 1.0 – – 1.8 1.3 1.5 – 2.2 – – 2.2 – 1.5 – –

ROMP spheres

Tab. 3.2. Swelling of selected solid phase resins (mL g1 ).

– 3.2 – 4.0 – 4.0 5.4 – – 3.2 2.8 – 3.7 4.0 – 3.0 2.4

Polyamide dendrimer

PS denrimers

– 3.5 3.3 – – 5.8 – – – 4.7 – – 4.6 – – – –

Arylether dendrimer 14.1 10.5 9.5 8.1 7.3 6.4 – – 1.9 – – 1.5 6.4 6.8 – – –

PAP

12.0 8.0 7.0 – – 8.0 – 6.5 5.0 6.5 – – 10.0 – – 5.0 3.0

CLEAR

SPOCC

– 14.0 – – – 15.5 – 12.0 – 12.5 – – 22.8 – – – –

– 10.5 – – – 11.0 – 7.9 – 8.1 – – 15.5 – – – –

Mn (PEG)F1500 g mol C1

POEPOP

– 8.0 – – – 9.5 – 6.0 – 7.0 – – 12.0 – – – –

POEPS-3

34



Concerning the chemical stability of Merrifield resins and their derivatives, it has been shown that they are relatively stable toward weak oxidants, strong bases, and acids. In fact, reactions that are known to proceed on alkyl substituted aromatic compounds, especially electrophilic substitutions, will also occur on crosslinked polystyrene [12]. Strong oxidants at elevated temperatures and electrophilic reagents should therefore be avoided [12, 13]. 3.2.2

Polystyrene Hybrid Supports 3.2.2.1 PEGylated Resins

Merrifield resins and their derivatives are still the most commonly used resins for the synthesis of small molecules, but one of their limitations is the poor swelling in polar protic solvents. For instance, Merrifield resins cannot be applied for synthesis of polar compounds in water or alcohols. This problem, however, can be overcome by designing ‘‘amphiphilic’’ resins made of a 1% crosslinked polystyrene matrix onto which polyethylene oxide chains are grafted [56, 57]. These resins are commercially available as TentaGel9 (Fig. 3.6). Typically, their composition is 70 wt% of polyethylene glycol (PEG) grafts (average molecular weight of 3000 g mol1 ) and 30 wt% of PS. These resins can be prepared by two different pathways: 1 grafting of PEG chains onto Merrifield resins [58, 59]; 2 anionic ring opening polymerization of oxiranes initiated from the active sites of a hydroxy-terminated polystyrene resin [56, 57, 60].

Fig. 3.6. Picture of Tentagel9 resin and chemical structures of TentaGel9, ArgoGel9 and NovaGel9 resins.

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36


The main drawback of the first technique is the possibility of additional crosslinking due to the fact that PEG macromolecules are bifunctional and consequently that the two end chains can both react with the resin [56]. This, however, can be overcome if monoprotected PEGs are used. PEGylated supports are far more polar than Merrifield resins and hence they swell in a broad range of solvents from apolar aprotic to polar protic solvents (Table 3.2) [53]. However, the loading of TentaGel9 resins is relatively low (loading values range from 0.15 to 0.40 mmol g1 for PEG35 chains) compared with the higher loading capacities that can be reached for Merrifield resins. A similar type of PEGylated resin known as ArgoGel9 has slightly higher loading capacities (up to 0.5 mmol g1 for PEG35 chains) because it has two PEG chains that are grafted onto every active site [61, 62]. PS–PEG hybrid resins opened many new possibilities in combinatorial synthesis by allowing the use of protic solvents. Even on bead, screening is possible with these resins. However, the user should be aware of some drawbacks, such as bleeding, which is defined as the loss and the release of PEG grafts when treated, for example, with strong acids [63, 64]. It has been reported for instance that trifluoroacetic acid (TFA), which is commonly used for cleavage of protecting groups, can also cleave the benzylic–CH2 O–PEG bond [65]. This problem can be overcome by using a TentaGel9 resin with the PEG spacer attached to the polystyrene backbone via an alkyl linkage. This hybrid resin is not sensitive to acids or bases [34, 57, 62, 64]. Other drawbacks of such PS–PEG hybrid resins originate from the nature of the PEG polymer, which is hygroscopic. Hence, the presence of large amounts of PEG (up to 70%) render the beads more sticky and more difficult to manipulate in some cases than regular Merrifield resins [20]. In order to overcome the decrease of loading capacity due to bleeding of the PEG spacers, a similar type of resin has been designed (e.g. NovaGel9) with active sites located on the PS backbone [64] rather than at the end of the PEG-grafted chains as in TentaGel9 and ArgoGel9 resins. This results not only in a good swelling behavior in both apolar and polar solvents due to the PEG chains (PEG content @ 50%) but also in a slightly higher loading capacity (0.7 mmol g1 ). In addition, eventual bleeding of the PEG grafts does not decrease the loading capacity and only a slight modification of the swelling properties will occur. However, the advantage of improved kinetics and better on-bead analytics (e.g. nuclear magnetic resonance) due to the highly flexible polyether spacers are lost in this approach. It should be pointed out again that all PEGylated resins are sensitive toward oxidation and hence bleeding will occur if not properly handled. 3.2.2.2 High-loading PS Hybrid Supports

A general problem in solid-phase combinatorial chemistry is low loading capacity of the commonly used resins. Often, large quantities of resin are required in order to obtain substantial amounts of products. Ring-opening-metathesis-polymer hybrids: ROMP spheres A recent approach toward high-loading solid-phase supports uses cross metathesis between vinyl poly-


styrene beads and norbornene derivatives [66]. In a first step the immobilization of the ruthenium catalyst (Grubbs catalyst) on vinyl polystyrene (11) can be achieved via insertion of ruthenium into the styryl double bond (Scheme 3.2). The PSsupported ruthenium alkylidene (12) shows good stability under normal atmospheric conditions when dried. In the presence of solvents (e.g. dichloromethane), however, the catalyst becomes inactive within a period of 5 h. Treatment of the PSsupported catalyst (12) with an excess of a norbornene derivative (13) yields a ROMP-based polymer (14), so-called ROMP spheres (Scheme 3.2).

Scheme 3.2. Cross metathesis of the supported Grubbs catalyst and a norbornene derivate to yield ROMP spheres (14).

This resin shows high loading capacities (up to 3.0 mmol g1 ), but the swelling properties are lower than for conventional resins, ‘‘good’’ solvents include THF and DCM (see Table 3.2). The utility of these high-loading supports was exemplified by a palladium-catalyzed coupling reaction between a resin-bound bromobenzoate and aryl zincates in a THF/MeOH (4:1) mixture. Also, crosslinked ROMP-based polymers have been used as supports for catalysts [67] and highloading amine scavengers [68]. Rasta silanes Silyl linkers (see Chapter 4.3.5) offer a broad range of mild and chemospecific cleavage conditions and are frequently used in SPOS. A highloading silyl-functionalized PS hybrid resin, so-called ‘Rasta silane’, has recently been introduced for solid-phase chemistry [69]. The required silyl substituted styrene monomer (16) can be prepared by lithium–halogen exchange of pbromostyrene (15) and subsequent quenching with the appropriate dialkylchlorosilane. A living free radical polymerization initiated by heating TEMPO-methyl resin (17) with the silyl styrene (16) under solvent-free conditions at 130 C leads to the formation of Rasta silanes (18) (Scheme 3.3).

Scheme 3.3. Formation of high-loading Rasta silane resins (18) by living free radical polymerization.

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38


Rasta resins are characterized by high loading levels between 1.6 and 3.8 mmol g1 with regard to the silicon linker. These values are significantly higher than for conventional silicon-linker resins, which show loading capacities between 0.5 and 1.6 mmol g1 . Easy conversion of the silyl units to reactive silyl chloride or triflate by standard synthetic methods allow the immobilization of alcohols and phenols for solid-phase synthesis. It has been shown that these Rasta silanes allow fast diffusion of reagents throughout the resin beads [70]. Polystyrene dendrimer hybrids Another approach to high-loading resins are polystyrene dendrimer hybrids [71]. Dendrimers are highly branched macromolecules built by a stepwise approach from a central core. The dendritic spacer molecules provide a rapid and efficient method of increasing the loading capacity. For example, the naturally occurring amino acid lysine has been used as a building block in creating an inert dendrimeric scaffold [72]. The tris-Boc-protected amino acid (20) can be attached to a preswollen aminomethylated PS resin (19) (Scheme 3.4) and thus creates the first generation of this dendrimer (21).

Scheme 3.4. Synthesis of dendritic polyamide

PS hybrid resins by stepwise attachment of a tris-Boc amino acid (20) to an aminomethylated PS resin (19) (0.8 mmol g1 ) via

the DIPCDI-HOAt-mediated amide coupling leads to the first generation [G1] resin (22) (Fmoc loading capacity 0.9 mmol g1 ) and [G2] 23 (Fmoc loading capacity 1.2 mmol g1 ).

After coupling of a Rink amide linker [73] onto resin (21), the derivatized PS dendrimer resin (22) with a loading capacity of 0.9 mmol g1 , with regard to the Fmoc (9-fluorenylmethoxycarbonyl) amino groups, is obtained. Repeating this procedure yields the second generation [G2] (23). This exhibits a slightly higher loading capacity of 1.17 mmol g1 . It is important to mention that the loading per gram of resin does not increase substantially because of the mass of the attached linker. However, the loading per bead increases geometrically from one generation


Fig. 3.7. [G2] of a triple branched symmetrical PS dendrimer

resin (24) (loading capacity 0.8 mmol g1 , 30.6 nmol per bead).

to the next. These PS dendrimer resins have successfully been tested in the synthesis of small peptides [72]. The synthesis of symmetrical tri-branching dendrimers on aminomethyl polystyrene macrobeads has also been described [74]. These [G2] dendritic resins (Fig. 3.7) offer loadings of 30.6 nmol per macrobead (0.8 mmol g1 ). A split-and-mix synthesis (e.g. SPPS, Suzuki-coupling) on a gram of these resin beads (@ 27,000 beads g1 ) provides the number of compounds and the amounts necessary for a single bead screening approach (including 1 H-NMR!). The obtained beads, especially the PS–[G2] dendrons (24), show higher swelling values in polar solvents such as methanol (MeOH) and dimethyl sulfoxide (DMSO) than the originally used aminomethyl resin (see Table 3.2). Some aryl ether dendrimers can be synthesized directly on hydroxymethyl polystyrene (25). Mitsunobu reaction with 3,5-bis(acetoxymethyl)-phenol (26) gives the branched precursor (27) (Scheme 3.5) [75]. After deprotection and repetition of the sequence a [G3] dendrimer resin was obtained with a loading capacity of 2.85 mmol g1 (3 nmol per bead). In unpolar solvents such as DCM and THF the dendrimer resin (28) swells better than in MeOH or water. The development of high-loading immobilized catalysts is another interesting application for PS dendrimer resins. An example is given by the rhodium complex

Scheme 3.5. Synthesis of PS–arylether dendrimer hybrid (28)

(loading capacity 2.85 mmol g1 ).

39

40


Fig. 3.8. PS–[G2] dendrimer hybrid (29) supporting a tetravalent catalyst for heterogeneous hydroformylation reactions.

with dendritic phosphine ligands that are anchored onto a PS resin (Fig. 3.8) [76]. Different dendrimer generations have been prepared and tested as catalysts for the hydroformylation reaction of several olefins. The PS–[G2] dendrimer (29) shows higher reactivities than the first generation and does not decrease for up to five cycles. The origin of this increased reactivity might be due to the higher density of ligands on the outer core and cooperative effects. High internal phase emulsion polymers: PolyHIPE PolyHIPE structures are formed by polymerization of a high internal phase emulsion with styrene and divinylbenzene [77]. The resulting material consists of an extremely porous and rigid matrix, which provides a scaffold onto which functional polymers (e.g. polyacrylamide gels) can be chemically bound (Scheme 3.6). The polyacrylamide gel is prepared

Scheme 3.6. Preparation of highly porous ‘PolyHIPE’ PS–acrylamide composite resins.


from N,N 0 -dimethylacrylamide (31), acryloylsarcosine methyl ester (32), a crosslinker and a radical initiator that is grafted onto the highly porous support (30). The polymer matrix (30) can be obtained from aminomethylated PS and acrylic chloride. Although these polyacrylamide grafts are of limited use for SPOS, very high loading capacities (up to 5 mmol g1 ) can be achieved by this approach [78]. The high internal surface area and the limited swelling allow the usage of these materials in column reactors for automated synthesis. Applications in low-pressure continuous-flow solid-phase peptide synthesis have already been described [78]. In principle these highly porous PS structures (see Scheme 3.6) should also be suitable as supports for reagents or catalysts in SPOS. 3.2.3

Other Crosslinked Polymeric Supports 3.2.3.1 Crosslinked Acrylamides

Solid-phase crosslinked acrylic amides possess a more polar character than the frequently used polystyrene supports. Thus, they are rather compatible in respect to solvation properties with a growing peptide chain. A wide range of polar and protic solvents can freely permeate and swell these materials. This class of resins, however, has been specifically developed for peptide and oligonucleotide synthesis. The limited chemical stability of the amide bonds causes these resins to be less suitable for broad application in SPOS with only some limited specific applications. Pepsyn-K resins Crosslinked and functionalized polydimethylacrylamide gels which are kept inside the pores of an inert, rigid and macroporous matrix are well suited for continuous flow applications. Compared with PolyHIPE materials, in which crosslinked polystyrene is used as matrix material, Pepsyn-K resins contain a highly porous kieselguhr matrix as the scaffold [79]. A mixture of dimethylacrylamide (31), ethylene bisacrylamide (33), acryloylsarcosine methyl ester (32) in DMF and water is soaked into kieselguhr particles and then polymerized at room temperature (Scheme 3.7). This support is a hybrid between an inorganic and an organic support. So far it has been used only for peptide synthesis.

Scheme 3.7. Synthesis of Pepsyn-K resins from

dimethylacrylamide (31), ethylene bisacrylamide (33), and acryloylsarcosine methyl ester (32) on a highly porous kieselguhr matrix.

41

42


Poly(ethylene glycol)-copoly(N,N 0 -dimethylacrylamide) (PEGA) Another flow-stable, highly polar solid support is poly(ethylene glycol)-copoly(N,N 0 -dimethylacrylamide) (PEGA). This resin is accessible by an inverse suspension copolymerization of N,N 0 -dimethyl acrylamide (31), acryloylsarcosine ethyl ester (34) and bis-2-acrylamidoprop-1-yl-PEG1900 (35) in silicon oil as a dispersing medium (Scheme 3.8) [80, 81].

Scheme 3.8. Synthesis and structure of the high-capacity PEGA

resin (36), suitable for continuous flow synthesis.

The obtained PEGA resin (36) is characterized by good mechanical stability as well as by high swelling properties in both organic solvents and aqueous buffers. The loading capacity is 0.4 mmol g1 . Furthermore, it has been shown that larger peptides and other macromolecules can easily diffuse through the PEGA polymer matrix [82]. It is, therefore, suitable for solid-phase synthesis using both the batch method and the continuous flow synthesis [83]. In addition, it was demonstrated that biological screening is possible on the resin-bound substrate without further cleavage [84]. Poly(N-acrylylpyrrolidine) (PAP) It has been reported that acrylamide polymers swell very well in polar solvents such as water, DMF, and certain alcohols but less well with CH2 Cl2 and CHCl3 [85]. Development of poly(N-acrylylpyrrolidine) (PAP) allows peptide-coupling reactions in organic media of high and low polarity as well as in aqueous solution. The synthesis of PAP (39) can be achieved by inverse suspension polymerization. An aqueous solution made up of the monomer N-acrylpyrrolidine (37), the functionalization agent N-acryl-1,6-diaminohexane hydrochloride (38) and the crosslinker N,N 0 -bis(acrylyl)-1,2-diaminoethane (33) is dispersed in a mixture of hexane and carbon tetrachloride. The polymerization is then initiated with a redox initiator system consisting of ammonium peroxydisulfate and N,N,N 0,N 0 -tetramethyl-1,2-diaminoethane (Scheme 3.9) [86].


Scheme 3.9. Preparation and structure of poly(N-acrylyl-

pyrrolidine) (PAP) resins (39).

The obtained PAP resin (39) has a loading capacity of 0.7 mmol g1 (amino groups), which can be used for SPPS. In either its protonated or acylated form, this resin exhibits high swelling values in water, methanol, CH2 Cl2 and CHCl3 . Several applications of PEGA resins have been reported for SPPS [87]. Crosslinked-ethoxylate acrylate resins (CLEAR) Another class of highly crosslinked polymeric supports for SPPS are crosslinked ethoxylate acrylate resins (CLEARs) [88]. The CLEAR supports are prepared from branched crosslinker (8, 40) that is used in high molar ratios for copolymerization with different monomers (41–44) (Fig. 3.9). The resulting polymers show a polyethylene glycol-like character, although individual chains are quite short compared with chains of already known PEG-containing resins such as PEGA or TentaGel9. Both the oxyethylene chains and the ester functionalities are responsible for the hydrophilic character of this class of resins. In contrast to many other resins, amino groups can be introduced directly onto the resin. It does not require additional transformations of other functional groups or deprotection of a protected amino polymer. CLEAR supports have good swelling properties, in both hydrophilic and hydrophobic solvents (see Table 3.2). This magnifies the range of chemistry that could be carried out using these resins. The loading capacities are typically between 0.20 and 0.29 mmol NH2 g1 , which is significantly lower than those of PAP (39), POEPOP (51), and SPOCC (52) (see below). CLEAR supports have been successfully tested in batchwise and continuous-flow peptide synthesis

43

44


Fig. 3.9. CLEAR supports consist of these monomers which can be combined in different ways, yielding resins that are suitable for batchwise and continuous-flow modes.

but they should also be suitable for limited application in SPOS, especially when protic solvents are required. Crosslinked PEGs This class of PEG-based supports is chemically more stable than the polyacrylamide-based supports. However, their applications in SPOS have not yet been extensively demonstrated. Interesting features of these resins include the magnitude of swelling in all kinds of solvents and a relatively high loading capacity when compared with regular PS–PEG hybrid resins (see Sect. 3.2.2.1). 3.2.3.2

Polyoxyethylene-polyoxypropylene (POEPOP) Polyethylene glycol derivatives [e.g. polyethylene glycol polyacrylamide (PEGA 36)], as a result of their inert character, offer a wide range of applications in polymer chemistry and biomedical sciences. Endgroup-modified PEGs (see Section 3.2.2.1) are often used as polymeric supports in peptide and combinatorial synthesis [89]. In the case of PEGA, a limiting factor is its amide backbone. An exchange of the amide linker in PEGA for the more stable ether linkages offers a larger diversity for application in organic reactions. These properties are represented in polyoxyethylene-polyoxypropylene (POEPOP 51), which can be obtained by anionic polymerization between endmodified PEG macromonomers (46, 47) and epichlorhydrin (48) (Scheme 3.10) [90]. The resulting POEPOP (51) shows an optimized balance between hydrophilic and hydrophobic properties and has a high mechanical and chemical stability [91]. However, due to the physical properties of the PEG chains these beads might be sticky in some cases [20]. One prerequisite for a successful polymeric support is that good swelling properties are available both in organic solvents and in enzyme buffers. The POEPOP resins swell best in CH2 Cl2 , less in DMF and water, and much less in THF and MeOH (see Table 3.2) [90]. However, swelling is strongly influenced by the length of the PEG chains. The longer the chain the higher the swelling, but at the same


Scheme 3.10. Preparation of PEG macromonomers and

polymerization to provide the hydrophilic POEPOP (51), SPOCC (52) and POEPS-3 (53) resins.

time the loading capacity drops. Typical loading capacities for POEPOP resins are between 0.4 and 0.7 mmol g1 . In comparison with typical polystyrene-based TentaGel9 resins (see Table 3.1), this is still a relatively high value [90]. POEPOP supports have been successfully employed for a solid-phase synthesis of peptide isoesters by nucleophilic reactions [91]. Polyoxyethylene-polyoxethane (SPOCC) As a result of the secondary ether bonds that are formed during the polymerization reaction, POEPOP (51) is unstable under extremely strong acidic or basic conditions. Cationic polymerization of end-modified PEG macromonomers (46, 47) with 3-methyl-oxetan-3-yl-4-methyl toluenesulfonate (49) leads to polyoxyethylene-polyoxetanes (SPOCC 52; Scheme 3.10) [89]. It contains only primary ethers and alcohols and exhibits, therefore, an excellent chemical stability. No changes in the SPOCC structure were observed by treatment with 37% HCl or neat anhydrous hydrogen fluoride. In addition, heating with a large excess of thionyl chloride in toluene did not change the resin structure. The same conditions, however, dissolve POEPOP within a few minutes. Loading capacities of SPOCC are typically in the range 0.4–1.2 mmol g1 . The swelling behavior is similar to that of POEPOP in the same solvents, but SPOCC resins swell slightly more than POEPOP resins with equal PEG chain length [90]. Easy conversion of the primary alcohols into amine or thiol functionalities is possible and therefore allows different types of chemistry. Solid-phase glycosylation of peptides as well as enzymatic reactions have been performed on this resin [89]. Polyoxyethylene-polystyrene (POEPS-3) Another class of crosslinked polyethylene glycols are polyoxyethylene-polystyrene resins (POEPS-3 53). These supports can

45

46


be obtained via radical polymerization of an end-modified PEG macromonomer (46, 47) with chloro-propyl-styrene (50) (Scheme 3.10). In order to obtain beads this reaction is performed by inverse suspension polymerization [90]. Loading capacities are similar to TentaGelSOH and are in the range 0.2–0.3 mmol g1 . POEPS-3 (53) shows similar swelling properties to POEPOP and SPOCC. With regard to the chemical stability of these compounds, POEPS-3 is less stable than SPOCC but more stable than PEGA due to its polyether backbone. As shown by fluorescence quenching diffusion experiments in DMF, diffusion processes are slower than in TentaGel and ArgoGel. In water, however, all three crosslinked PEG-based resins show faster diffusion rates than the PS–PEG hybrid resins [90]. So far POEPS-3 resins have been used exclusively for SPPS [92]. 3.2.4

Inorganic Supports

In the following section, we give a short overview of some inorganic supports that are suitable for SPOS. A good summary was published recently [4, 26]. Different forms of silica gel are very often used for SPOS. They offer a number of advantages and are now commercially available in various derivatized forms. These supports are rigid and do not swell or contract in the solvents employed in synthetic cycles. Reagents and products can easily diffuse into and out of the pores. Typical pore sizes are between 2 and 10 nm, and the surface area is between 200 and 800 m 2 g1 . Although higher coupling rates in peptide synthesis were obtained on silica than on polystyrene [93], silica gel did not become an established support for solid-phase organic synthesis. In the meantime, silica gel has been partially replaced by controlled pore glass (CPG) as an inorganic support [38, 94–98]. The main advantages of CPG, compared with silica gel, are its more regular particle size and shape, and greater mechanical stability. Functionalization with long-chain alkylamine groups leads to exceptional properties: rigid, nonswelling, mechanical stability within a series of polar solvents, and chemical stability over the whole pH range. Furthermore, CPG is unreactive to a broad variety of nucleophiles and electrophilic reagents. Its stability during heating and its excellent solvation and flow properties make it the support of choice for automated solid-phase synthesis. Typical loading capacities, however, are rather low (0.1 mmol g1 ).

3.3

Soluble Polymeric Supports

In contrast to solid supports, soluble noncrosslinked polymers give rise to homogeneous reaction conditions [6–8, 10]. Many of the problems of crosslinked polymers arising from heterogeneous reaction conditions, such as nonlinear kinetic behavior, reactive site distribution and solvation problems, can be overcome by using soluble polymeric supports. Unlike solid-phase supports, soluble supports enable the use of insoluble reagents or catalysts and also allow for the characteriza-

3.3 Soluble Polymeric Supports

tion of intermediates by standard analytical techniques (e.g. NMR, UV), in the homogeneous phase. On the other hand, the isolation and separation of these soluble materials is not as straightforward as for solid-phase resins. Although soluble polymeric supports have been used in organic synthesis since the 1970s, it is only recently that several groups have started to investigate automated separation techniques that enable the use of these soluble supports for parallel synthesis. Soluble polymeric supports can be classified according to their topology, which strongly influences the physical properties of these materials. Linear polymers with one or two functional endgroups, polyfunctional linear structures with many functional groups along the chain and highly branched dendritic polymers will be discussed in the following sections. 3.3.1

Separation Techniques for Soluble Polymeric Supports

It is generally believed that soluble polymeric supports, neglecting their advantages, are difficult to separate from the reaction mixture. For many years the technique of precipitation and subsequent filtration seemed to be the most promising method for separating soluble polymers from low-molecular-weight species. However, for automation several other separation techniques appear to be more promising [10]. Especially in the field of homogeneous catalysis, membrane filtration techniques have found application and show great potential for industrial use [99]. Therefore it seems appropriate to summarize briefly the multiple commercially available techniques for the separation of soluble polymers from the reaction mixture, i.e. low-molecular-weight compounds (Table 3.3). There are several methods which have been used for the separation of macromolecules by size, i.e. dialysis [100, 101], membrane filtration [102], preparative size exclusion chromatography (SEC) [103] and filtration through a silica cartridge [68]. All of them are suitable for automation. However, little effort has been undertaken to develop multiparallel automation in this area compared with the progress achieved in solid-phase synthesis. For efficient and fast separations by size, soluble polymeric supports should have medium molecular weights (5000–10,000) and narrow molecular weight distributions (< 1.5). In addition, macromolecules with a more globular branched structure are preferable to a linear polymer structure. Other separation techniques for soluble polymers, such as precipitation/ filtration and liquid–liquid phase separation rely on polymer properties rather than on their hydrodynamic volume. Precipitation is frequently used in polymer chemistry to purify the respective polymer from low-molecular-weight impurities. This method works especially well when the polymer is crystalline and the Tg is above ambient temperature. It has also been used in the separation of functionalized PEG supports [6–8, 10]. Precipitation, however, is unsuitable for multistep syntheses because impurities often remain trapped in the polymer. In addition, large solvent volumes are required to perform quantitative precipitations and hence automation of the process is difficult. Another relatively simple separation technique, which is suitable for the separation of organic molecules from water-

47

>1000 g mol1

>1000 g mol1 10 mL to 1 L Yes Yes No Unsuitable for final cleavage step

Minimum MW of polymer

Typical sample volume

Commercially available

Suitable for automation

Suitable for high throughput

Limitations

–

Yes

Yes

Yes

1–100 mL

Hydrodynamic volume

Hydrodynamic volume

Separation by

Ultrafiltration

Dialysis

Parameter

Tab. 3.3. Separation techniques for soluble polymeric supports.

–

Yes

Yes

Suitable only for final cleavage step or removal of polymeric reagents

Yes

Yes

Yes

0.1–10 mL

3000 g mol1

Solubility

Precipitation/ filtration

Different solubilities required

No

Yes

Yes

10 mL to 1 L

–

Phase distribution coefficients

Liquid–liquid phase separation

48



soluble polymers, is based on liquid–liquid phase separation between an organic phase which contains the cleaved organic product and an aqueous phase which contains the water-soluble polymer [104]. This separation technique, however, is limited to systems with different distribution coefficients of the components in the two phases. Yet another separation technique that is used for soluble polymeric reagents is filtration through a silica cartridge [68]. This technique has already been established for automated synthesis in many laboratories and does not require additional equipment. However, it can only serve for purification of the product after the final cleavage step or the removal of a polymeric reagent. 3.3.2

Terminal Functionalized Linear Polymeric Supports

Linear polymers that carry functionalities only on their chain ends (e.g. PEGs), have been frequently used as soluble polymeric supports and are listed in Table 3.4 (Entries 1 and 2) [6–10]. Because of the limited number of functional groups (one or two per chain) these materials have rather poor loading capacities (typically 2). Hyperbranched polymers (Fig. 3.10) have recently been introduced as soluble supports in organic synthesis [103, 104]. A commercial polyester support (Boltorn 54) containing 1,3-diols as terminal units was used for the synthesis of disaccharides (Scheme 3.6) [103]. Theoretically, these polyesters have a relatively high loading capacity (8.8 mmol OH g1 ), but the experimentally achieved loading with monosaccharides attached to a photo-labile linker was 0.8 mmol g1 , additionally reduced by the weight of the linker functionality. The general use of this hyperbranched polymer (54) as a soluble polymeric support, however, is limited as a result of the chemical sensitivity and the low molecular weights of the polyester backbone [129]. This is especially problematic for the separation by dialysis or ultrafiltration with a minimum molecular weight cut-off (MWCO) of 1000 g mol1 (see Table 3.3). Recently, the controlled synthesis of well-defined hyperbranched polyglycerols (55) (R ¼ H) has been achieved, by using both racemic and enantiomerically pure glycidol monomers [130, 131]. These polyether polyols are conveniently prepared in a one-step synthesis on a kilogram scale [132], and possess molecular weights (Mn ) up to 30.000 g mol1 with molecular weight distributions typically below 1.5.


Fig. 3.10. Dendritic polyester (54) and polyether (55) (R ¼ H, alkyl, glyceryl) as high-loading soluble polymeric supports.

The dendrimer-like structure of the hyperbranched polyglycerol (55) is characterized by exactly one core unit with multiple hydroxyl groups randomly incorporated as linear (OH groups) and terminal groups (1,2-diols). The total density of functional groups in polymer 55 (R ¼ H) is 13.5 mmol OH g1 polymer, of which approximately 60% (8.2 mmol OH g1 ) are terminal 1,2-diols. These terminal diols can be used directly as linker functionalities for many applications in organic synthesis [10]. The complete derivatization of the terminal diols in polyglycerol (55) (R ¼ H), with, for example, acetals, leaves about 40% of the OH groups unaffected (see Scheme 3.3). These remaining OH groups might limit the scope of this new polymeric support for some synthetic applications. For the preparation of a chemically inert polyether support (55) (R ¼ alkyl) these residual OH groups can be selectively alkylated by using phase transfer conditions to obtain dendritic polymers with exclusively diol linkers [133]. This approach also permits the introduction of a second type of functional group and tunes the solubility of the polymer in various organic solvents. In order to increase the loading capacity of the dendritic polyglycerol support further, the linear glycerol units can also be converted into terminal 1,2-diols [124]. This strategy increases the capacity of the terminal 1,2-diol units from 4.1 mmol g1 for the hyperbranched polymer (55) (R ¼ H) to 7.1 mmol g1 for the dendritic structure (55) (R ¼ glyceryl) and preserves all advantages of the polyether scaffold. For a more general application of polyglycerol (55) in organic synthesis and in order to increase the scope of possible reactions on this support, the conversion of the hydroxyl groups into various other linker functionalities by postsynthetic transformations has been explored [134]. In one or two synthetic steps after the polymerization several reactive linker functionalities, such as aldehydes, alkenes, amines, carbonic acids and esters, are accessible in good yields and with high loading [135]. Dendritic polyglycerols (55) are readily soluble in many organic solvents and can easily be separated from low-molecular-weight compounds by dialysis or ultrafiltration with a typical MWCO of 1000 g mol1 . Because of the high

53

54


flexibility of the branches, the terminal 1,2-diol groups show excellent accessibility and can be used directly as linker groups for various applications in organic synthesis [104]. In addition the characterization of these dendritic polyethers by standard analytical techniques (e.g. NMR) is much more rapid than the monofunctionalized PEG derivatives owing to the high loading capacity of functional groups [104]. 3.3.5

Microgels

Recently, Wulff et al. introduced ‘‘microgels’’ as a new type of polymeric supports for use in parallel synthesis [136]. Microgels are intramolecularly crosslinked polymer molecules that build stable solutions in suitable solvents [137]. Oxazaborolidine functionalized PS-microgels have been prepared and used as catalysts in enantioselective reduction of prochiral ketones [136]. In contrast to gel-type polymer supports (see Section 3.2.1.3), microgels offer the advantage of higher loading capacities and homogeneous reaction conditions. Like dendritic polymeric supports (see Section 3.3.4), they have a very low solution viscosity, which simplifies the handling of the reaction mixture. Also, separation can be performed by ultrafiltration and the process can be performed in an automated or continuous mode.

3.4

Conclusions

After more than 10 years of intensive research on the preparation and the application of new polymeric supports in combinatorial chemistry many new polymeric materials have been developed and evaluated. However, there is still no polymeric support for general application in combinatorial chemistry. Every polymer has its drawbacks (e.g. chemical stability, polarity, loading capacity). Therefore, a polymeric support has to be carefully selected for the synthetic problem that needs to be solved. In solid-phase organic synthesis several new supports with better swelling properties in a wider range of organic solvents and higher loading capacities have been introduced. Among these are several new PS hybrid resins as well as a new family of crosslinked PEGs. In terms of material stability, inorganic supports, i.e. controlled pore glass, cannot be beaten. However, the width of synthetic applications is very limited so far and will depend on the chemical stability of the spacer and linker molecules used. In addition, these supports suffer a heterogenic nature and rather low loading capacities. Soluble polymeric supports, like solid supports, have had a similar revival over the past decade. In terms of stability, aliphatic polyethers and noncrosslinked polystyrene are among the most promising candidates. Dendritic and linear polyfunctional soluble polymers have by far the highest loading capacities and show great potential as supports for reagents and catalysts in combinatorial synthesis because of their homogeneous reaction conditions. The use of these soluble polymers for

References

SPOS, however, requires further progress in automation of solution-phase separation techniques. Nevertheless, the current trend in the pharmaceutical industry to move back to combinatorial solution-phase chemistry renders these high-loading soluble polymers as potential homogeneous supports for solution-phase synthesis.


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4

Linkers for Solid-phase Synthesis Stefan Bra¨se and Stefan Dahmen 4.1

Introduction

Solid-phase organic chemistry is one of the key tools in combinatorial chemistry [1–6] used to synthesize large compound libraries of potential new drugs and other biologically active compounds, especially in automated synthesis. With the aid of linkers that are capable of attaching building blocks or intermediates onto solid support as well as facilitating their ultimate release into solution, the synthetic gap between solid and liquid phase is diminished [7–10]. Linkers and their associated synthesis strategies, therefore, play a pivotal role in the successful implementation of solid-phase organic synthesis and its application to combinatorial chemistry [11]. 9 Linker: bifunctional chemical moiety attaching a compound to a solid support or soluble support which can be cleaved to release compounds from the support. A careful choice of the linker allows cleavage to be performed under appropriate conditions compatible with the stability of the compound and assay method [12].

In solid-phase synthesis, a starting material is attached reversibly to a linker, which is bound again directly or over a spacer (Sect. 4.2.2, for example a polyethylene glycol chain such as that shown in Fig. 4.1) to the actual resin (usually with divinylbenzene crosslinked polystyrene, see Chapter 3). Anchoring groups for the use of soluble polymers [MeO-polyethylene glycol (PEG)] are in principle the same as for the use of insoluble polymers. Therefore, throughout this chapter they will not be distinguished. While the group attached to the solid support is in general unchanged upon cleavage, the anchoring bond to the compound is sensitive to certain conditions, which then lead to bond breakage and release of the final compounds. Traditionally, linkers were designed to release one functional group and hence acted more or less as bulky protecting groups (see Sect. 4.3). Therefore, the release of carboxHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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4 Linkers for Solid-phase Synthesis

Fig. 4.1. Linker construct.

ylic acids and amines, which are essential for peptide synthesis, has been studied extensively. However, the synthesis of nonoligomeric compound libraries requires more versatile linkers (see Sect. 4.5) [13]. A particular linker should resist the entire synthesis sequence without bias to the diversity or structure of the target compound library and without limiting the chemical methodology. None of the reaction conditions used for the construction of the building blocks should lead to a premature cleavage (orthogonality principle; Sect. 4.3.8). Beyond that, the linker must be cleavable off the resin as mildly as possible to insure that the product will not be affected. In this chapter, linker types classified according to their structures, and cleavage strategies are presented. In general, linkers for all kinds of building blocks (peptides, oligosaccharides, nucleic acids and small molecules) are covered.

4.2

General Linker Structures 4.2.1

Immobilization of Molecules

In general, the anchoring of molecules to a resin can be realized by two different strategies. Direct loading is clearly the most straightforward technique for the set-up of a solid-phase organic synthesis. A molecule with a reactive or potentially reactive functional group is coupled directly to the preformed linker. This strategy is useful if the linker and the building block can be coupled efficiently. Coupling rates greater than 90% are essential at this point. Successful examples are, for instance, the formation of an amide bond, reductive aminations, alkylation reactions (including Mitsunobu reactions), and olefin metathesis. Since the building blocks can be used without any additional purification step, this method is especially suitable for the anchoring of libraries of starting materials and/or automated synthesis. The attachment of molecules to a particular resin is strongly dependent on the nature of the linker and will be discussed for each linker separately. While in

4.2 General Linker Structures

Fig. 4.2.

The unloaded Knorr linker (2) as a handle.

some cases a simple and rapid mixing of reagents is sufficient to drive the attachment to completion, in other cases tedious monitoring is necessary. Additionally, in some cases the attachment proceeds under similar conditions as the detachment, for example the formation of ketals, where excess reagents are required to drive the reaction to completion. In a second general method for attachment, the building block can be coupled to give the preformed linker in solution. The fragment thus formed, which is called the handle (see for example Fig. 4.2), can then be activated for attachment onto the resin using the coupling strategies outlined above. This method requires an additional step in solution, which can be advantageously used for an increase of purity. Especially in cases where the activated linker tends to decompose (e.g. silyl chlorides) or can be formed only in moderate yields and purities, this handle approach can be favorable. However, in automated synthesis, the required solutionphase step is clearly a drawback. 4.2.2

Spacers

The linker either can be attached directly to the resin or can be located further from the polymeric backbone using spacers. These bifunctional constructs either can be built sequentially or can be attached via a handle approach. The spacer acts as a connection to give the building block more mobility and hence the kinetics for a given reaction are superior to the corresponding resin without spacers [14]. Furthermore, larger spacers considerably change the physical properties, such as swelling behaviors. This has been demonstrated with Tentagel resin [15]. However, one must take into account both that the use of spacers requires an additional synthetic step usually connected with a decrease of loading capacity and that the spacer has to be as robust as the linker toward the reaction conditions performed on the bead; Tentagel resin, in particular, leads to bleeding. For the characterization of compounds attached to a polymeric support by nuclear magnetic resonance (NMR) spectroscopy, however, long spacers are an advantage, because they increase the mobility of the substrate and reduce the line broadening usually observed in NMR spectra of polymers. In most cases, a clear distinction between linker and spacer is not easy. The linker is the minimal part of the resin required for the functional cleavage (for the

61

62


silyl linkers this is the silyl group, for the trityl linkers this is the triphenylmethyl moiety, for the triazene linker this is the 1-aryltriazenyl group, and so on). The spacer therefore is the part between the linker and the resin as depicted in Fig. 4.1 and described in Chapter 3 of this book. 4.2.3

Functionalized Linkers as Analytical Constructs

Functionalized spacers or linkers can play an important role for the determination of loading and reaction conversion. The NMR-active fluorinated linkers are especially suitable in this regard [16].

4.3

Linker Families

Usually, linkers are categorized according to the kind of functional group or substrate class they are able to selectively immobilize (linkers for carboxylic acids, alcohols, amines, and so on). As there are various types of linkers available for solid-phase synthesis, many of them belong to certain well-established classes of protecting groups (Table 4.1) and therefore can be grouped into families. The members of each family have certain reactivity patterns in common. While a linker presents the chemical structure essential for the loading and cleavage of a particular functional group, a linker system provides the whole protocol for the attachment to and cleavage from the resin. However, because of an increased demand for the flexible anchoring of molecules, other families of linkers such as sulfur linkers, triazenes linkers, among others, have emerged. Nevertheless, the largest number of linkers developed so far are based on benzylic-type groups.

Tab. 4.1. Common protecting groups and the analogous linker families.

Protecting group in liquid phase

Functional group protected

Linkers or linker families

Benzyl

Alcohols, esters

Allyl Cbz (Z)

Amines Amines

Alloc Boc Silyl ether

Amines Alcohols Alcohols

Alkyl ester SEM (trimethylsilylethoxymethyl)

Carboxylic esters Alcohols

Benzyl-type linkers: Sect. 4.3.1, Table 4.2 Allyl-type linkers: Sect. 4.3.2 Carbamate-based linkers: Sect. 4.3.4 [17] Allyl-type linkers: Sect. 4.3.2 Boc-type linker [18] Silyl-type linkers: Table 4.6. [19, 20] Sect. 4.3.4 SEM linker (99) [21]

4.3 Linker Families

4.3.1

Benzyl-type Linkers Including Trityl and Benzhydryl Linkers

Benzyl-type linkers are the most common anchoring groups for various kinds of functionalities. In particular, esters, amides, amines, alcohols, and thiols can be immobilized by this linker family. As the pioneering works of Merrifield [22] and Wang [23] are based on this linker type, they represent the starting point of modern linker development. Benzylic linkers are typically cleaved by strong acids (e.g. trifluoroacetic acid, TFA), which causes a protonation and subsequent elimination. A nucleophilic scavenger usually quenches the resonance-stabilized cation thus formed. An increase of acid lability can thus be achieved by stabilization of this intermediate by, for example, oand p-substitution of methoxy groups onto the ring [24]. This has been demonstrated in the development of the SASRIN resin (11; super acid-sensitive resin: 1% TFA cleavable) [25, 26] with one additional alkoxy group related to the Wang resin and the HAL linker (13; hypersensitive acid labile: 0.1% TFA cleavable) [27] having two additional alkoxy groups. In addition, benzyl-type linkers might be cleaved by ammonolysis [28], light (Sect. 4.4.3; see Table 4.15), metal salts [29] (see Scheme 4.8), and oxidation reagents (e.g. Wang resin with H2 O2 [30] or dichlorodicyanobenzoquinone (DDQ) [31]). The introduction of nitro groups onto benzyl-type linkers leads to photolabile systems (see Sect. 4.4.3). The prototype of a functional group with an appropriate breakable bond is the Wang resin (6), which contains a 4-hydroxybenzyl alcohol linker moiety. The benzyl alcohol hydroxyl group can be functionalized using either electrophilic or nucleophilic substrates (Scheme 4.1) to give a benzylic linkage. It is very stable in a whole set of reactions, but can be cleaved by acids such as trifluoroacetic acid or HF. Acids, alcohols, esters, and amides can be obtained as products after cleavage.

Scheme 4.1. Loading of the Wang resin (6). DEAD, diethyl

azodicarboxylate. DMAP, 4-dimethylaminopyridine.

The Rink resin is particularly useful for the attachment of various functionalities such as primary amines (see for example [82]; Scheme 4.2) and is commercially available from various sources. The loading can be achieved via the Rink chloride (29) or triflate (30) (Table 4.2). A linker that is particularly suitable for peptide amides and cyclopeptides is the PAL linker and the backbone amide linker (BAL) concept [83] (Scheme 4.3).

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64


Scheme 4.2.

The Rink resin as a linker for primary amines according to Garigipati [82].

Scheme 4.3. Backbone amide linkage using the PAL linker by Barany and coworkers [83]. Pg, Protecting Group.

Trityl resins (38–43) (Table 4.2) are especially suitable for the immobilization of nucleophilic substrates such as acids, alcohols, thiols, and amines. They are quite acid sensitive and can be cleaved with acetic acid, for example; this feature is useful in cases where acid-labile protecting groups are used. The stability of trityl resin can be tailored using substituted arene rings, as demonstrated with the chlorotrityl resin, which furnishes a more stable linker than the trityl resin itself. Furthermore, the steric hindrance prohibits the formation of diketopiperazines during the synthesis of peptides. The orthogonality towards allyl-based protective groups was demonstrated in the reverse solid-phase peptide synthesis of oligopeptides [84] (Scheme 4.4).

Scheme 4.4. Reverse (N ! C) directed solid-phase peptide synthesis with the 2-chlorotrityl resin according to Thieriet et al. [84].

4.3 Linker Families

65

Tab. 4.2. Structures and properties of benzyl-type linkers including trityl and benzhydryl linkers (excluding photolabile linkers: see Sect. 4.4.3, Table 4.15).

Generic name of the resin Merrifield resin (X ¼ Cl: 3) [22]; AM PS (Aminomethyl polystyrene) (X ¼ NH2 : 4); hydroxymethyl polystyrene (X ¼ OH: 5) Wang linker (X ¼ OH: 6) [23], Boba resin (X ¼ NH2 : 7) [32]

PAM (8) ( phenylacetamidomethyl) [35–37]

HMPA (9) (hydroxymethylphenoxyacetic acid) [38]

HMPP (10) (hydroxymethylphenoxypropionic acid) [40]

SASRIN (11) (super acidsensitive resin) [25] R ¼ Me: AMEBA (12) (acidsensitive methoxybenzaldehyde) [41, 42]; MALDRE (a-methoxybenzaldehyde resin) [43]; [44]; R ¼ H: AHB linker [45] HAL (13) (hypersensitive acid labile) [27]

Structure

Cleavage

Comments

HF

Standard resin

Cleavage for thiols: HF [33]; for esters: CF3 SO3 H [34]; 95% TFA [23]; H2 O2 [30] Cleavage for esters: 90% HF, anisole [35]; stable in 50% TFA, CH2 Cl2 [35] Cleavage for esters: 95% TFA [28, 39]; 20% TFA, CH2 Cl2 (incomplete) [40] Cleavage for esters: 20% TFA, CH2 Cl2 [40]

Standard resin; very robust; for Boc strategy Very robust

Cleavage for esters: 1% TFA, CH2 Cl2 , 20 C, 5 min [25] Precursor of the SASRIN linker (R ¼ Me)

Very sensitive

Cleavage for esters: 0.1% TFA (CH2 Cl2 ), 25 C, 1 h

Very sensitive

PAL (peptide amide linker) (14) [46, 47]

Cleavage for amides: TFA [48]; TFA or HF [49] (quinoxalinone)

BAL (backbone amide linker) (15) [50]

Cleavage for esters: [14]

66


Tab. 4.2. (continued)

Generic name of the resin

Structure

Cleavage

Extended HAL linker (16) [51]

Cleavage for amides: 25% TFA, CH2 Cl2 [51]

Ramage linker (17) [52, 53]

Cleavage for esters: nBu 4 NF

HMB (18) (Sheppard) [28]

Cleavage for amides: NH3 [28]

19 [54]


SAL linker (20) [55] (silyl amide linker)

Cleavage for amides: 90% TFA, scavenger [55]

SAC linker (21) [56] (silyl acid linker)

Cleavage for acids: TBAF [56]

Comments

Starting resin for various linkers

BHA (22) [57] (benzylhydryl amin)

MBHA (23) (methylbenzylhydryl amin) [58]

Cleavage for amides: HF; CF3 SO3 H [58], HF/PhOMe (sec. amide) [59]

MAMP linker (24) (Merrifield, a-methoxybenzyl) [60, 61]

Cleavage for amides: TFA/CH2 Cl2 /H2 O (9:90:1) [60]

p-Acyloxy BHA (25) [62]

Cleavage for amides: HF, cresol (9:1) [62, 63]

4.3 Linker Families

67



Structure

Cleavage

SCAL (26) (safety-catch acidsensitive linker) (Sect. 4.5.1) [64] (cf. [65])

Cleavage for amides: TFA/(EtO)2 (PS)SH [64]; Me3 SiBr/ PhSMe/TFA [64]

Rink acid (X ¼ OH: 27); Rink amide (X ¼ NH2 : 28) [66, 67]; Rink chloride (X ¼ Cl: 29); Rink triflate (X ¼ OTf: 30)

Cleavage for esters: 10% AcOH, CH2 Cl2 , 20 C, 1.5 h; 1% TFA [66]; cleavage for amides: TFA [66]; 20% TFA, CH2 Cl2 [68]

Comments

Widely used anchor for amines

Similar to the Rink resin above

Rink amide AM (31) (RAM) (rink amide aminomethyl)

32 [69, 67]

Cleavage for esters: TBAF or Cs2 CO3 [69]

Sieber XAN (33) [70–72]

Cleavage for amides: [70]

2-XAL (34), 3-XAL (35) (xanthyl acid-labile linker)

Cleavage for amides: TFA [71]

CHA (36), CHE (37) (dibenzo[a,d]cyclohepta(e)ne) [73]


More sensitive than Rink or PAL [73]

Trityl (38) [74, 75]

Cleavage for esters: 1% TFA/AcOH [76, 77]

Quite unstable for esters

2-Chlorotrityl (39)

Cleavage for esters: (CF3 )2 CHOH (HFIP) (20%), CH2 Cl2 [48]; AcOH/CF3 CH2 OH/ CH2 Cl2

Increased stability compared to trityl linker

68




Structure

Cleavage

Comments

4-Methyltrityl (40)

Cleavage for esters: AcOH/CF3 CH2 OH/ CH2 Cl2 (1:1:8)

Increased stability compared to 2-chlorotrityl linker

4-Methoxytrityl (41)

Cleavage for esters: AcOH/CF3 CH2 OH/ CH2 Cl2 [78]

Increased stability compared to 4-methyltrityl linker

4-Fluoro(chloro)-tritylcarboxyl linker (42)

Cleavage for esters: 0.1% TFA [79], AcOH/CF3 CH2 OH/ CH2 Cl2 (1:1:8)

4-Cyanotrityl (43)

Cleavage for esters: [80]

(44) [81]

Cleavage for esters: TFA [81]

Besides benzyl-type linkers, other arylmethyl moieties can serve as linkers. Recently, a new backbone amide linker has also been devised using indole chemistry [85] (Scheme 4.5).

Scheme 4.5.

The indole linker according to Estep et al. [85].

4.3.2

Allyl-based Linkers

A second, to the benzyl linkers related class is the family of the allyl-based linkers (Table 4.3). They have been used for the attachment of carboxylic acids, which can

4.3 Linker Families Tab. 4.3. Overview of allylic linkers.

Structure

Reference [86]

[86]

[90, 91, 98]

[93]

[94]

be detached using metal catalysis. The advantages of linker cleavage under palladium catalysis are the mild reaction conditions and the orthogonality (Sect. 4.3.8) to various protecting groups. Kunz et al. [86, 87] developed the first and simplest linker to use the p-allyl detachment strategy. Starting from 2-bromocrotonic acid, attachment to an amino group on a resin and further reaction with the cesium salt of an appropriate protected amino acid or peptidic structure yields the HYCRAM (hydroxycrotonylamide) resin [88]. The allylic cleavage proceeds with Pd(PPh3 )4 and morpholine or hydroxybenzotriazole [89]. The readily available (hydroxycrotyloligoethyleneglycol-n-alkanoyl) HYCRON linker [90–92] is based on a similar concept; however, in this case, a handle comprising an amino acid and a preformed linker has been used to minimize the risk of racemization upon cleavage. A higher stability towards unwanted nucleophilic cleavage was achieved than that observed with the HYCRAM linker. The incorporation of b-alanine facilitates easier monitoring of the reaction. Several other similar constructs have been used for comparable purposes [93–96]. Recently, the semisynthesis of vancomycin on solid support was accomplished using an allylic linker [97]. An example for the use of the allyl linkers has been provided in the synthesis of peptide nucleic acids (PNAs) [91] (Scheme 4.6).

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Scheme 4.6. Synthesis of PNAs using the HYCRON linker according to Seitz [91].

4.3.3

Ketal/acetal-based Linkers

Ketals and their corresponding sulfur analogs are well-established protecting groups in solution-phase synthesis. However, only a few constructs have been used in solid-phase organic synthesis (SPOS) as linkers (Table 4.4). The most versatile ketal linker is the tetrahydropyranyl THP linker developed by Thompson and Ellman [99]. This linker allows the attachment of alcohols, phenols, and nitrogen functionalities in the presence of pyridinium toluene sulfonate. Both the loading and the detachment proceed under acidic conditions. Similarly, other acetal linkers have also been used for the attachment of alcohols [100, 106]. All these linkers are stable toward bases and organometallic reagents. However, a drawback is certainly the formation of diastereomers as a result of the chirality of these linkers. Tab. 4.4. Overview of acetal/ketal-based linkers.

Structure

Reference

Comments

THP resin [99]

Most common linker for alcohols

[100]

New linker for alcohols

[101–104]

Common linker for ketones and aldehydes

[105]

Linker for sterically hindered ketones

4.3 Linker Families

An example of the synthesis of complex molecules using THP-type linkers was provided by Chen and Janda [107] in their synthesis of prostaglandins on noncrosslinked polystyrene (Scheme 4.7). Further successful examples demonstrate the generality of the THP ketal-type linkers [108–114].

Scheme 4.7. Prostaglandin synthesis on solid phase according to Chen and Janda [107]. TBS, tert-butyldimethylsilyl.

Another class of linkers incorporating a ketal moiety are the Leznoff diol-linked aldehydes [101, 102] and ketones [103, 104]. A similar linker using dithianes is also suitable for ketones [105] (Sect. 4.3.7.1). 4.3.4

Ester-, Amide-, and Carbamate-based Linkers

Ester and amide moieties are, apart from being used in the benzyl and allyl linker types, also suitable for attachment. Various examples have demonstrated the useful applications of these functional groups (Table 4.5). Basically, two different strategies can be used (Fig. 4.3). For example, May et al. [115] recently introduced thioesters as linkers for alcohols, ketones, and lactones.

Fig. 4.3. Ester linkers: general structures.

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4 Linkers for Solid-phase Synthesis Tab. 4.5. Overview of some ester- and amide-based linkers (see also Table 4.16).

Structure

Reference

Possible structures achievable

e.g. [116]

Esters: cleavage with methoxide; alcohols: cleavage by Grignard reagents [117] Alcohols: cleavage with methoxide

[118]

Alcohols: cleavage with enzymes

[115]

Ketones, amides: cleavage with reducing reagents

[17]

Amines: cleavage with Pd(OAc)2 , H2 (45 psi), DMF, rt, 16 h

[119]

Amides, esters: cleavage with Cu(II) salts

[120]

Methyl arenes: cleavage under photolytic conditions

Benzyl carbamates are also useful linkers for the synthesis of amines since they are readily cleavable by palladium salts [17] (Scheme 4.8). Carboxylic esters have been released by the action of alkoxides on ester resins. In most cases, the cleavage has been performed using methoxide in methanol (e.g. [116]; Scheme 4.9). Drawbacks to this method are the need to remove excess metal salts and/or the aqueous work-up. Alternatively, after cleavage from solid support, postsynthesis of methyl esters with diazomethane is also possible [121] (Scheme 4.10).

Scheme 4.8. Detachment of peptides from polymeric benzyl-type protecting groups [17].

4.3 Linker Families

Scheme 4.9.

The use of esters as linkers for benzoic acid derivative by Kondo et al. [116].

Scheme 4.10. Synthesis of macrocycles by Soucy et al. [121].

Arylhydrazides can serve as safety-catch linkers for C-terminal carboxylic acid, amide, or ester functionalities. The cleavage proceeds via an oxidation with copper(II) and subsequent cleavage of the diazenyl moiety by means of a nucleophile (Scheme 4.11) [119].

Scheme 4.11. Detachment of peptides from hydrazide resins by Millington et al. [119].

4.3.5

Silyl Linkers

The electronic and steric properties of silicon compounds have been used in many applications for the design and use of new linker types (Table 4.6). The different applications can be divided as follows: 1 Direct attachment of building blocks on silylated resins: linker for alcohols [19, 20], traceless linkers for arenes (Sect. 4.5.5). 2 Use of the b-silicon effect for elimination reactions: e.g. the SEM (2-trimethylsilylethoxymethyl) linker [21]. 3 Silylated benzhydryl linkers: [69, 122].

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Tab. 4.6. Overview of silyl linkers.

Structure

Reference


[123]

Alcohols; cleavage with (Tetrabutylammonium fluoride) TBAF Alcohols

[21]

[124]

Traceless linking of arenes (Scheme 4.60)

[125]

Traceless linking of arenes

[126]

Traceless linking of arenes

[127–129]

Traceless linking of arenes; cleavage with fluoride

[55]

Amides; cleavage with 90% TFA, scavenger

[56]

Acids; cleavage by TBAF [56]

[122]

Carboxylic acids; cleavage with TBAF

[52, 53]

Carboxylic acids; cleavage with TBAF

[21]

Alcohols; cleavage with TBAF

[67, 69]

Estes; cleavage with TBAF or Cs2 CO3 [69]

4.3 Linker Families

The robustness of silicon linkers against basic and organometallic reagents makes them especially suitable for solid-phase organic synthesis. Cleavage can be affected by electrophiles such as protons (trifluoroacetic acid; TFA). A special feature of silyl linkers is their sensitivity to fluoride ions, which makes them ideally orthogonal (Sect. 4.3.8) to various other functionalities present in the molecule. The fine-tuning of electronic and steric properties is possible by using different substituents on the silicon atom (trimethylsilyl vs. tert-butyldimethylsilyl). The first traceless linkers (Sect. 4.5.5) for arenes were described independently by Ellman and coworkers [125, 130] and Chenera et al. [124] (see Scheme 4.60) in the 1990s using silyl linkers. This linker type was used in Ellman’s synthesis of a benzodiazepine library, which is a milestone in the solid-phase synthesis of small organic molecules (Scheme 4.12). The synthesis of the linker involves a lithium– halogen exchange and, after chlorosilane attachment, coupling with an aryl halide. As the silyl arene might be cleaved in the unwanted direction to give silylated arenes, further improvement led to the development of a germanium linker [131], which gives rise to the formation of pure material (Sect. 4.5.5, see Scheme 4.61). In addition, cleavage of these linkers can be accomplished by electrophiles other than protons to yield halogenated residues, thus rendering this linker into a multiple cleavage linker system [see 379] (Sect. 4.5.6). The range of electrophiles is limited, since only small, reactive electrophiles (e.g. iodine, bromine, and chlorine) react to give the desired products.

Scheme 4.12. Synthesis of a benzodiazepine library using silyl linker [131].

The silicon-based linkers, which are commercially available [132], were developed further by several groups [133] and have been used in the synthesis of various systems for the traceless detachment of various arenes and heteroarenes [126– 128, 133–143]. They are also suitable as traceless linkers for allyl silanes to give alkenes [144] (Sect. 4.5.5). A recent traceless application was demonstrated in the synthesis of chromenones (104) [145]. In this case, a mild cyclization method was used to circumvent a premature cleavage for the support (Scheme 4.13). Similarly, heteroarenes are also accessible [128] (Scheme 4.14). Other linkers having silyl fragments are the silyl acid linker (SAC linker 21) [56], the silyl amide linker (SAL linker 20) [55], the Pbs linker (‘‘silico Wang linker’’) (98) [122], the Ramage silyl linker (15) [52], the SEM linker (99) [21], and silylated benzhydryl linkers (32) [69].

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Scheme 4.13. Silicon linker for the synthesis of chromenones 104 by Harikrishnan and Showalter [145].

Scheme 4.14. Synthesis of oligo 3-arylthiophenes 107 by Briehn et al. [128].

4.3.6

Boronate Linkers

Boronates have been used in various linker types either as linkers for diols [146] or as precursors for metal-mediated cleavage (Table 4.7). A boronic acid ester, which contains an aryl iodide moiety attached by an appropriate tether, can act as an intramolecular arylation agent. Thus, Li and Burgess [148] developed a polymer-bound precursor, the ensuing cleavage of which furnished a macrocyclic constrained b-turn peptide mimic via biaryl coupling (Scheme 4.15). Tab. 4.7. Overview of boronate linkers.

Structure

Reference


[147]

Arenes; cleavage by Agþ

[148]

Biaryls by Suzuki crosscoupling

[146]

Diols

4.3 Linker Families

Scheme 4.15. Intramolecular cleavage Suzuki coupling by Li and Burgess [148].

Immobilized aryl boronic esters (113) can be cleaved to the corresponding hydrogen-substituted products (115) (traceless cleavage, Sect. 4.5.5) using aqueous silver nitrate solution [147] (Scheme 4.16).

Scheme 4.16.

Traceless cleavage of boronic acid derivatives according to Pourbaix et al. [147].

4.3.7

Sulfur, Stannane- and Selenium-based Linkers

A set of modern linkers (Table 4.8) based on sulfur, stannane, and selenium chemistry can be found in the literature. Their popularity obviously stems from the fact that these elements can favorably be tailored for use as fragile points of attachment. 4.3.7.1 Sulfur-based Linkers

Sulfur has been used in linkers as thioethers, sulfoxides, sulfones, sulfonic acids, and their corresponding derivatives. The relatively weak carbon–sulfur bond can be cleaved under reductive conditions (see Scheme 4.62) [155, 156], photolytic conditions, or in the presence of strong bases [160]. Since thiols can be oxidized to the corresponding sulfoxides and/or sulfones, various safety-catch linkers [155, 156] (Sect. 4.5.1) have benefited from this fact.

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4 Linkers for Solid-phase Synthesis Tab. 4.8. Overview of sulfur, stannane and selenium linkers.

Structure

Reference


[149] (Scheme 4.42)

Amides

[150] (Scheme 4.43)

Carboxylic acids

[151] (Scheme 4.20)

Amines

[115]

Ketones, amides; cleavage with reducing reagents

[105]

Linker for sterically hindered ketones

[152, 153]

a) R ¼ Aryl: Arenes (Scheme 4.17); cleavage with palladium/formate; b) R ¼ a-carbonylalkyl: heterocycles Diaryl methanes (Scheme 4.18); cleavage with alkylation/Suzuki coupling

[154]

[155–159]

Alkanes (Scheme 4.62); cleavage with Na/Hg

[160]

Benzofurans (Scheme 4.56); cleavage with bases

[161]

Methylarenes; photolytic cleavage

4.3 Linker Families Tab. 4.8. (continued)

Structure

Reference


[162]

Methylarenes; cleavage under photolysis

[120]


[163]

Alkenes (Scheme 4.35, Scheme 4.36); cleavage with oxidants

[164, 165]

Alkanes, alkenes

Aryl sulfonates can be used as linkers for arenes, as shown by Jin et al. [152]. These can be cleaved under reducing conditions to give the corresponding hydrocarbons (130). In addition, this linker might be suitable as a multifunctional anchor, as proposed in a patent [166] (Scheme 4.17).

Scheme 4.17. Cleavage-hydrogenation reaction with a sulfonate

linker (129) according to Jin et al. [152].

Arylmethyl(homobenzyl)ethylsulfonium salts are also appropriate substrates for Suzuki-type coupling reactions. In this reaction performed on a polymer-bound sulfonium tetrafluoroborate, the benzyl fragment on the sulfur is transferred to the boronic acid residue. The sulfonium salt was prepared from an alkylthiol resin by alkylation with a substituted benzyl halide to give thioether (131) and subsequent alkylation with triethyloxonium tetrafluoroborate. Reaction with a boronic acid derivative yielded a diaryl methane (132) [154] (Scheme 4.18). The cleavage of sulfonamides on substituted thiophenes deposited on platinum electrodes can be conducted by an electrochemical cleaving step [167] (Scheme 4.19).

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80


Scheme 4.18. Cleavage Suzuki coupling approach using sulfonium salts by Vanier et al. [154].

Scheme 4.19.

The use of an electrocleavable anchoring by Marchand et al. [167].

Photolabile sulfur linkers are based either on the relatively weak carbon–sulfur bond in thioethers [161] or on the photolytic decarboxylation of thiohydroxamic acids to give methylindoles [120]. Also, a safety-catch linker (Sect. 4.5.1) for amines is based on 2-(thiobenzyl)ethylcarbamates [151, 168]. The linkage is performed with preformed handles containing ethenyloxycarbonyl-protected amines (138). Attachment to thiomethylated polystyrene (139) was performed under conditions involving radicals. The cleavage was carried out with an oxidizing agent, which forms the retro-Michael substrate [151] (Scheme 4.20). The utility of a thioacetal-based anchor (68) as a chemically robust linker for the immobilization of ketones employed the commercially available (G)-alphalipoic acid. The products were easily cleaved from solid support by treatment with [bis(trifluoroacetoxy)iodo]-benzene [105] (Table 4.4). Recently, Nicolaou et al. [153] have shown that the reaction of alkenes in the presence of dimethyldioxirane could be used for loading onto polystyrene sulfonic acid resin. Subsequent cleavage with nucleophiles proceeded smoothly to give a vast array of heterocycles.

4.3 Linker Families

Scheme 4.20. Thiobenzylethylcarbamates as linkers for amines according to Timar and Gallagher [151]. AIBN, Azobisisobutyronitrile.

4.3.7.2 Stannane-based Linkers

Stannanes have become prominent members in the area of multifunctional anchoring groups. A polymer-bound tin hydride (142) has been used to hydrostannylate alkynes under palladium catalysis to give polymer-bound alkenylstannanes (143). Alternatively, the latter can be prepared from a polymer-bound tin chloride and an alkenyl lithium or magnesium halide reagent [163] (Scheme 4.21). These alkenyl stannanes were employed in intermolecular [169] as well intramolecular Stille reactions. Alkenylstannanes can also undergo protonation to give alkenes (145) in a traceless fashion (Sect. 4.5.5). Therefore, this linker is able to operate in a multifunctional mode (see Fig. 4.5) (Sect. 4.5.6).

Scheme 4.21.

The stannane linker for Stille reactions according to Nicolaou et al. [163].

4.3.7.3 Selenium-based Linkers

The selenium–carbon bond is, because of its weakness (E ¼ 217 kJ mol1 ), prone to undergo homolytic cleavage, thus producing radicals. This fact was first recognized and used for solid-phase synthesis by Michels et al. in 1976 [170]. More recently, Nicolaou et al. [164] and Ruhland et al. [165] independently developed

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more efficient methods for the preparation of selenium-containing supports in their development of traceless linkers (Sect. 4.5.5). Starting from polystyrene, various steps including selenation with selenium powder or MeSeSeMe give rise to the formation of selenium resins, which can then be alkylated to give selenoethers (148). The traceless cleavage that yields alkanes (149) can be conducted by reduction with tributyltin hydride, while the formation of alkenes (150) can be observed after mild oxidation (Scheme 4.22). This linker holds promise for numerous applicability since the starting materials (alkenes, alkyl halides) are readily available, although the toxicity of the reagents and starting materials has to be considered.

Scheme 4.22. The selenium linker for alkanes and alkenes according to Nicolaou et al. [164]. TBDPS, tert-butyldiphenylsilyl.

Based on a similar concept, a selenium linker has been described that is loaded via a preformed handle [171]. Oxidation and thermal elimination give rise to the formation of alkenes (153) (Scheme 4.23). Interestingly, the selenoxides (152) decompose at room temperature, whereby the corresponding sulfoxides fragment at 100 C.

Scheme 4.23.

The selenium linker according to Russell et al. [171].

The selenium bromide linker (154) (Scheme 4.24) is also the starting point for a library of natural products and analogs of the benzopyran type through reaction of prenylphenols [172–174] (Scheme 4.24). Furthermore, this linker enables the synthesis of medicinally interesting molecules [175]. Besides phenols, allyl anilines can be used in this sequence to produce indolines. Depending on the substitution pattern, either traceless (! 162) (Sect. 4.5.5)

4.3 Linker Families

Scheme 4.24.

The selenium linker in chromene synthesis according to Nicolaou et al. [172].

or cyclative (! 160) cleavage can be realized [176] (Scheme 4.25). The seleno linker has also been used in the synthesis of 2-deoxy glycosides, orthoesters, and allyl orthoesters [177]. Finally, the SEM linker for the attachment and detachment of alcohols takes advantage of the substitution of phenylselenyl groups at seleno acetal resins [21].

Scheme 4.25.

The selenium linker in the indoline synthesis by Nicolaou et al. [176].

4.3.8

Triazene-based Linkers

Inspired by the use of triazenes in the total synthesis of vancomycin [178] and the pioneering work of Moore and coworkers [179, 180] and Tour and coworkers [181] in the synthesis of triazenes on a solid support and the final detachment to give iodoarenes (180-I), a whole set of triazene-based linkers (Table 4.9) has been developed [182]. The chemistry of diazonium salts provides tremendous opportunities for the construction of a wide range of aromatic compounds. Triazenes, which have been used as traceless linkers for arenes as shown in Sect. 4.5.5, provide both interesting new possibilities for activation of the ortho-position of the arenes and are ideal synthons for diazonium salts. Triazenes are stable toward light, moisture, and bases; however, they are cleaved by Brønsted acids and certain Lewis acids to give diazonium salts and amines.

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4 Linkers for Solid-phase Synthesis Tab. 4.9. Overview of triazene linkers.

Structure

Possible structures achievable Iodoarenes [179]

T1 resin: ‘‘traceless’’ linker [182–185]; synthesis of phenols [186], biaryls, alkyl arenes [187, 188], azides [189], aromatic hydrazines, halides [190, cf. 180, 181], ester, azo compounds, cinnolines [191], benzotriazoles [192] As above

T2 resin: synthesis of substituted amines [193], amides (peptides) [194], (thio)ureas [194, 195], hydrazines, alcohols, esters [183, 196, 197], guanidines [195], alkyl halides [183, 196, 197], sulfoximines T2 resin: as above [198]; scavenger for amines and phenols

Two linkers based on triazene chemistry have been developed. While the T1 linker system consists of 3,3-dialkyl-1-aryl triazene bound to a support via the alkyl chain (Scheme 4.26), the T2 linker family is based on immobilized aryl diazonium salts.

Scheme 4.26. Concept of the T1 linker [183].

4.3 Linker Families

The triazene T1 linker has been successfully used as a linker for arenes (see also Sect. 4.5.5). Until now, approximately 100 different anilines (168) have been immobilized. Functionalization on the bead has been demonstrated extensively. These immobilized diazonium derivatives are stable towards various reaction conditions, such as alkyl lithium reagents, reducing agents, and oxidizing reagents. However, acids cleave the triazenes to give the amine resin (170) and the modified aryl diazonium salts (173). The latter can be transformed into various different products giving modified arenes in high yields and purities [purities > 90–95% according to gas chromatography (GC), NMR, and high-performance liquid chromatography (HPLC)] directly at the cleavage step [183] (Scheme 4.27).

Scheme 4.27. Possibilities of the T1 triazene linker [183].

The diazonium salts, for example, can be reduced to the hydrocarbon (180-H) in THF with the aid of ultrasound [182]. The latter facilitates this reduction due to a radical pathway. A new reagent for this reduction was found to be trichlorosilane [184]. This is not only a source of traces of hydrochloric acid, which cleaves the

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triazene moiety, but, as a hydride donor, it is also able to reduce diazonium ions cleanly (Scheme 4.27 and see Scheme 4.64). As already shown by Moore and coworkers [180] and Tour and coworkers [181], addition of methyl iodide to a triazene resin at elevated temperature (110 C) gives rise to aryliodides (180-I) (Nu ¼ I) in excellent yields. Furthermore, aryl halides (180-X) (X ¼ Cl, Br, I) are readily available by the action of lithium halides in the presence of an acidic ion exchange resin or with the corresponding trimethylsilyl halide at room temperature [190]. A mixture of acetic anhydride and acetic acid produces phenol acetates (180-OAc) [186]. Although quite flexible in the range of possible electrophiles that may be employed, the most striking feature was the development of a cleavage cross-coupling strategy [187]. Starting from modified triazene resins, a one-pot cleavage cross-coupling reaction was conducted with two equivalents of trifluoroacetic acid in MeOH at 0 C to give a diazonium ion. In situ coupling with various alkenes (182) in the presence of catalytic amounts (5 mol%) palladium(II) acetate furnished the corresponding products (183) in excellent yield and purities. Using palladium on charcoal as the catalyst has the advantage of decreasing palladium contamination, as well as providing a subsequent hydrogenation option [187]. Multicomponent Heck reactions (domino Heck Diels–Alder reaction) are possible in this context and lead to further diversification [187]. In the examples above, the diazonium group, upon cleavage from the resin, is lost as dinitrogen. However, a suitable nucleophilic ortho-substituent favors cyclization to give heterocyclic structures. Benzotriazoles, for example, are accessible from o-aminoaryl-substituted triazenes [192]. Other heterocyclic systems such as cinnolines (178) are available by a cleavage Richter reaction strategy, which starts from o-alkynylaryl triazenes. Cleavage was conducted with aqueous hydrogen chloride or hydrogen bromide in acetone or dioxane at room temperature to produce various cinnolines (178) in a library format with up to 95% yield and with a range of purities between 60% and 95% [191]. In addition to these methods, reduction of diazonium salts gives rise to the formation of hydrazines (177) [199], which are important building blocks for the synthesis of other heterocyclic compounds. Whereas the T1 linker involves the immobilization of a diazonium salt on an amine resin, the T2 linker represents the reverse of this concept. Thus, an immobilized diazonium salt (190) was prepared from Merrifield resin (3) in two steps, and subsequent additions of primary and secondary amines generated triazenes (191). In addition, attachment of hydroxylamines, hydrazines, sulfoximines, and phenols (to give azo coupling products) proceeds equally well (Scheme 4.28). Secondary amines can be cleaved directly [193] or after modification from the resin. Primary amines can be derivatized on the free NaH functionality and can therefore be modified to an array of products [194, 195, 198]. Thus, ureas (198) [194], thioureas (197) [195], guanidines (196) [195], and carboxamides (199) [194] were prepared in excellent yields (Scheme 4.29). While the cleavage of trisubstituted triazenes gives rise to the formation of secondary amines in excellent yields [193], the cleavage of disubstituted triazene (194) gives rise to aliphatic diazonium salts [183, 196, 197]. The diazonium ion thus formed undergoes substitution with the nucleophile present in the reaction mix-

4.3 Linker Families

Scheme 4.28. Synthesis of the T2 linker [193].

Scheme 4.29. Possibilities with the T2 linker [183].

ture. Therefore, alkyl halides (202), alcohols (201-OH), and alkyl esters (201OCOR) can be formed by cleavage with trimethylsilyl halides (X ¼ I, Br, Cl) or carboxylic acids (X ¼ for example OAc, OTfa) [200] [183, 196, 197]. The regioselectivity of the cleavage can be explained by the presence of one tautomer of the triazene in which the hydrogen atom is next to the arene ring. Overall, this reaction sequence provides a substitution of an amino group with oxygen or a halogen (Cl, Br, I). Various immobilized diazonium salts have been prepared [201], thermo-analytically characterized [202], and used for the linkage of nucleophiles. The structure of the salts clearly influences the stability of the diazonium moiety. The thermally

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4 Linkers for Solid-phase Synthesis Tab. 4.10. Orthogonality of linker families.a

Cleavage reagents

Benzyltype linkers

Ketal/ acetal linkers

Esters/ amide linkers

Silyl linkers

Triazene linkers

Selenium/ sulfur/ stannyl linkers

Electrophiles Nucleophiles Fluoride hn Oxidative conditions Reductive conditions

þþ 0 0 (þþ) 0 0

þþ 0 0 0 0 0

þþ þþ 0 0 0 þ

þþ 0 þþ 0 0 0

þþ 0 0 þ 0/þ 0/þ

þþ 0 0 0 þþ þþ

a

(þþ): specially designed linker; þþ: cleavage; þ: partial cleavage; 0: no cleavage.

stable diazonium ion (194a) (Z ¼ Cl, Y ¼ CH2 O) [t1=2 (25 C) > 100 days] is also capable of scavenging various nucleophiles (amines, phenols, and anilines) [198]. 4.3.9

Orthogonality Between Linkers

The orthogonality of linkers is important for the design and execution of both simple and complex reaction sequences performed on a solid support. Recently, an entire set of innovative linkers and cleavage strategies has been disclosed, which enables the full set of orthogonality to be produced. As discussed above, each linker family is sensitive toward a certain spectrum of cleavage conditions and is therefore stable to dissimilar conditions. Since most of the linkers are based on well-established protecting groups, Table 4.10 can be used for the determination of orthogonality. For example, benzyl-type linkers, which are mostly cleaved by electrophiles and are stable toward nucleophiles, can be combined with ester-based protective groups.

4.4

Cleavage

In this section, various methods and reagents for the cleavage of linkers are presented. In most cases, cleavage of linkers is conducted with protons. However, other electrophiles, photones, oxidizing and reducing reagents, and nucleophiles can be used in many cases. 4.4.1

Electrophilic and Nucleophilic Cleavage

Cleavage of linkers can be conducted with various kinds of electrophiles and nucleophiles (Tables 4.11 and 4.12). The most popular cleaving reagent is trifluoro-

4.4 Cleavage Tab. 4.11.

Typical electrophiles and Lewis acids used for detachment.

Electrophiles (concentration)

Solvent

Additive

HF HF HF HF

Anisole Cresol

CF3 SO3 H TFA (0.1% to neat) TFA TFA TFA TFA (25%) TFA TFA TFA TFA HCl CF3 SO2 OSiMe3 HBr/Ac2 O AcOH AlCl3 Et2 AlCl Me3 SiCl

Tab. 4.12.

CH2 Cl2 p-Cresol p-Cresol, Me2 S Anisole (PhOMe) Et3 SiH Ethanedithiol (EDT) iPr3 SiH Et3 SiH PhSMe Dioxane

CH2 Cl2 CH2 Cl2 /MeNO2

[203–205] Wang linker [33] PAM resin [35] p-Acyloxy BHA resin [62] [206] Various linkers [205, 207, 208] [205]

Product

Thiols Carboxylic acids Amides

[209] [210] [211] [212, 213] [214] Ketals [103] [215] [216, 217] [66] [218] [219] [194]

Ketones

Typical nucleophiles used for detachment.

Nucleophile (concentration) RMgX RMgX R2 CuLi NaOH KOH NaOMe F

Example for a suitable linker

Solvent

Additive

Suitable linker

Products(s)

Ketones, alcohols Alcohols [115] Ketones [115] Carboxylic acids Isoquinolines [351] THF/H2 O Esters Hydrocarbons, alcohols (Sect. 4.3.5) DMF NMM, AcOH Dinitroaryl linker Thiols [229]

THF THF

HSCH2 CH2 OH (2-mercaptoethanol) DMF nPrNH2 (10%) DMF N2 H4 (vapor) NH3

Carboxyl linker Thioester Thioester Carboxyl linker Reissert complex Carboxyl linker Silyl linkers

Dde [228] Dde [226]

Primary amines Primary amines [230]

89

90


Fig. 4.4.

The REM and the Dde linker.

acetic acid in various solvents and concentrations. Because of its low boiling point, removal is readily achieved. Besides this reagent, various other acids have been used. Anhydrous HF, quite a toxic reagent, or triflic acid are required for more stable linkers. A mild reagent is trimethylsilyl chloride, which solvolyzes slowly to HCl and hexamethyldisiloxane. Typical nucleophilically cleavable linkers are the REM (regenerated Michael acceptor) linker (203) [220–225] and Dde (Dimethyldioxocyclohexylidene)ethyl group (204) (ADCC anchor) [226–228], which are linkers for tertiary and primary amines, respectively (Fig. 4.4). 4.4.2

Oxidative/Reductive Methods

Besides electrophiles and nucleophiles, several linkers are designed to be cleaved by oxidative or reductive methods (Tables 4.13 and 4.14). Besides the feature of Tab. 4.13. Some reducing agents used for detachment.

Reducing agent

Solvent

Suitable linker

Product

nBu3 SnH NaBH4 LiBH4 Na/Hg Phosphines

Toluene THF/H2 O THF MeOH

Selenium linker Amide Thioester Sulfon linker Disulfide linker

Hydrocarbons (Scheme 4.25) [176] Alcohols [231] Alcohols [115] Alkanes [155] (Scheme 4.62) Thiols [232–235]

Tab. 4.14. Some oxidizing agents used for detachment.

Oxidant

Solvent

Suitable linker

mCPBA

CH2 Cl2

Thiol-based safety-catch Secondary amines [151] linkers Selenium linkers Alkenes [177] Thioketal-based linker Ketones [105] (Sect. 4.3.7.1)

mCPBA CH2 Cl2 [Bis(trifluoroacetoxy)iodo] benzene DDQ Wang resin Wang resin H2 O2 Pyr, MeCN Hydrazide linker Cu(OAc)2 Hydrazide linker Cu(OAc)2

Product

Alcohols [31] Carboxylic acids [30] Amides (Scheme 4.48) [236] Arenes [237]

4.4 Cleavage

orthogonality with other cleaving methods, a drawback for the use of oxidative or reductive reagents is the necessity to remove excess reagents or byproducts. 4.4.3

Photocleavable Linkers

Light-induced cleavage offers new possibilities for orthogonal use of linkers (Sect. 4.3.8) and acid- or base-labile protecting groups (Table 4.15). The first photolabile linker (215), which was based on the o-nitrobenzyl protecting group, was developed by Rich and Gurwara [238, 247] for the synthesis of protected peptides (Scheme 4.30). This linker was developed further because in the synthesis of the original linker, a nitration of Merrifield resin was involved, thus leading to nitration of excess phenyl rings. Therefore, mostly preformed handles have been used. In all cases, upon ultraviolet (UV) photolysis the photo byproduct, a nitrosobenzaldehyde, is also photoactive and causes a reduction in cleavage yield from the support. To circumvent this problem, an additional methyl group was introduced to give linkers, which lead to the photoreactive nitrosoactophenone system [248]. Moreover, introduction of methoxy groups para to the nitro groups (vanilline-type linker) [249] improves cleavage properties and the compounds are typically released within 3 h in >90% yield and >95% purity under neutral conditions [241, 250, 251]. Therefore, these linkers are biocompatible and are suitable for Fmoc solid-phase peptide synthesis (Fmoc-SPPS) (however, see [252]). Various modifications and improvements [239, 241, 248, 253–258] and applications [259] of this linker type have been reported [260–263].

Scheme 4.30.

The prototype of a photolabile linker according to Rich and Gurwara [238].

Benzoins [264] and related b-keto systems (phenacyl esters) [245, 246, 265] have been used as linkers for over 25 years now. A novel safety-catch linker (217) (Sect. 4.5.1) based on the benzoin-protecting group has been utilized to anchor carboxylic acids. The use of a dithiane-protected 3-alkoxy-benzoin allows for elaboration of molecules, linked as esters to the secondary hydroxy function of the benzoin, prior to deprotection of the dithiane and photolytic cleavage [244, 266] (Scheme 4.31). A new photolabile linker (126) based on a thiohydroxamic acid has been shown to be an efficient ‘‘traceless’’ linker yielding an aliphatic CH bond upon photolysis

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4 Linkers for Solid-phase Synthesis Tab. 4.15. Overview of photolabile linkers.

Structure

Reference


[238]

Carboxylic acids

[239]

Carboxylic acids

[240]

Amines

[241]

Amines

[242]

Carboxylic acids

[243]

Alcohols

[244]

Carboxylic acids

[161]

Methylarenes

4.4 Cleavage Tab. 4.15. (continued)

Structure

Reference


[162]

Methylarenes

[120]

Methylarenes

[245, 246]

Carboxylic acids

Scheme 4.31. A benzoin safety-catch photolabile linker according to Balasubramanian and coworkers [244, 266].

at 350 nm [120]. Alternatively, the Methoxynitrophenyldithiooxopropylphenylacetamide NPSSMPact handle is suitable for the detachment of benzyl-type structures (124, 125) for the traceless synthesis of methylarenes [161, 162] (see Scheme 4.59). Besides these structures, various other linkers, such as those (209, 210) introduced by Giese and coworkers [242, 243] that are cleavable by CaC fragmentation reaction, and other systems have been developed [267–272]. 4.4.4

Metal-assisted Cleavage

Cleavage mediated or catalyzed by (transition) metals is particularly interesting for several reasons. First, this type of cleavage is in most cases orthogonal (Sect. 4.3.8) to other procedures, thus enabling various types of transformations. Second, reactive intermediate organometallics can be suitable for further transformations. In particular, the cleavage of substrates from a solid support using palladium-

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promoted or -catalyzed reactions has some advantages over other cleavage methods. Since most protecting groups and functionalities are resistant towards palladium complexes, a selective surgical removal is frequently possible. In addition, intermediate p-allyl– and s-aryl–palladium complexes can in principle be used for further derivatization with the use of appropriate linker types. Boronates can be reduced to the corresponding hydrocarbons using silver salts, making this linker as a traceless linker (Sect. 4.5.5) for arenes [147]. Similarly, aryl sulfonates are reduced by the action of palladium or a reducing medium [166]. 4.4.4.1 Cleavage with Ensuing Allylic Substitution or Cross-coupling Reactions

The detachment of molecules with a concomitant cross-coupling or allylic substitution is an elegant method for the increase of diversity upon cleavage (see also Chapter 19). A common drawback of most methods is the contamination with transition metal catalysts and organometallic byproducts. However, various methods are available for the sequestering of transition metals from the products. The same holds true the removal of other byproducts. Allylic substitution reactions The cleavage of polymer-bound allyl esters with palladium catalysts provides a general access to p-allyl complexes, which can react with various nucleophiles. This approach has been used in the development of pallyl-based linkers (Sect. 4.3.2). Schu¨rer and Blechert [273] reported on an ene–yne cross metathesis (see also Sect. 4.4.4.2) and a subsequent cleavage in the presence of various nucleophiles to yield the corresponding functionalized dienes (224) (Scheme 4.32).

Scheme 4.32. Cleavage via formation of p-allyl intermediates according to Sch€ urer and Blechert [273].

Similarly, polymer-bound 1-alkenylcyclobutylsulfones (225) can be reacted with suitable nucleophiles (226) and palladium catalysts to give the corresponding cyclobutylidene derivatives (227) [274] (Scheme 4.33). The latter linker employed in an allylic substitution reaction can be regarded as a multifunctional linker (Sect. 4.32).

4.4 Cleavage

Scheme 4.33. Cleavage of allylsulfones according to Cheng et al. [274].

Heck reactions Cleavage by an ensuing Heck reaction was developed utilizing the T1 triazene linker [187]. Upon cleavage with trifluoroacetic acid, a diazonium ion is formed which can couple to an alkene under palladium catalysis (Scheme 4.34). The coupling proceeds well with simple terminal alkenes, styrenes, as well as diand even trisubstituted alkenes. The coupling with 1,3-cyclohexadiene eventually yields a biaryl, apparently by a facile dehydrogenation of the primary coupling product. The advantage of this process is clearly the possibility of using volatile alkenes (and alkynes) without contamination of any salt or other less volatile byproducts, particularly with the use of palladium on charcoal as the catalyst. In this case, a subsequent hydrogenation is also possible [187].

Scheme 4.34. Cleavage with ensuing Heck coupling using the

triazene linker by Br€ase and Schroen [187].

Stille couplings The intermolecular Stille reaction of aryl halides with immobilized stannanes (Scheme 4.21) provide coupling products in good yields, as demonstrated by Kuhn and Neumann [169]. In addition, the stannylated resin produced in the cleavage coupling can be recycled. Although the products obtained were not contaminated by any stannane, they were separated from an excess of the reactive electrophiles that had to be applied in the cleavage-coupling step. The intramolecular variant, which was used by Nicolaou et al. [163] to produce macrocyclic ring systems such as the natural product (S)-zearalenone (231), does not have this drawback (Schemes 4.35 and 4.36). Suzuki couplings Suzuki couplings following a cleavage reaction are potentially applicable in a multifunctional sense (Sect. 4.5.6). However, owing to the tendency

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Scheme 4.35. A cleavage Stille strategy using a stannane linker for the synthesis of zearalenone (231) by Nicolaou et al. [163]. Mem, methoxyethoxymethyl.

Scheme 4.36. Schematic reaction for the Stille cleavage reaction.

of the boronic acid derivative to give homocoupling products, the need to apply additional ligands, and the low volatility of the boronic acid derivative, a more or less tedious work-up is required after these types of transformations. A few studies have proven that certain functionalities, when generated during cleavage, may act as leaving groups for a subsequent Suzuki reaction. One of these is the diazonium group, which can be generated by cleavage of the triazene T1 linker. While the Heck-type coupling with alkenes gives good yields of the desired products [187] (Scheme 4.34), the analogous reaction with phenylboronic acid appears to be difficult because of work-up problems [187] (Scheme 4.37). Arylmethyl(homobenzyl)ethylsulfonium salts [154] and aryl boronates [148] have been used as precursors for a cleavage/Suzuki approach (Schemes 4.15 and 4.20).

Scheme 4.37. Cleavage with subsequent Suzuki coupling according to Br€ ase and Schroen [187].

4.4 Cleavage

Sonogashira-type couplings The coupling of alkynes with diazonium salts has been reported in the context of the T1 linker. Here, the product (240) was isolated in moderate yields, and it had to be separated by chromatography from alkyne homodi- and trimers [187] (Scheme 4.38).

Scheme 4.38. Sonogashira coupling associated with the cleavage according to Br€ase and Schroen [187].

4.4.4.2 Cleavage via Alkene Metathesis

The cleavage via alkene metathesis is particularly useful since a clean and selective scissoring of molecules is possible (see also Chapter 20). The cleavage by metathesis (Scheme 4.39) can be performed by cyclization during cleavage [275–281] (ring-closing metathesis, RCM), by intermolecular metathesis [144, 281] (crossmetathesis) (Sect. 4.5.6), or by intramolecular metathesis [275]. Successful examples for this cleavage/cyclization method are the synthesis of epothilone A [277] and medium-sized heterocycles [278–282].

Scheme 4.39. The concept of metathetic cleavage by Blechert and coworkers [275, 276], Nicolaou et al. [277], Piscopio et al. [278–280], and Van Maarseveen et al. [281].

The intermolecular cross-metathesis of alkenes and alkynes provides a general access to 1,3-dienes [273] (see also Sect. 4.4.4, Scheme 4.32). A new anchoring group for the solid-phase synthesis of oligosaccharides has been described by Melean et al. [283]. Alkenyl units are not suitable for glycosidation reactions in the

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presence of strong electrophilic activators. However, using the 4,5-dibromooctane (DBOD) anchor, an iodide-mediated elimination reaction provides the active linker, which can be cleaved under metathesis conditions [283] (Scheme 4.40).

Scheme 4.40. Oligosaccharide synthesis on solid support

using a linker cleavable by metathesis according to Melean et al. [283].

4.4.5

Unusual Cleavage Methods

While most linkers are cleavable with electrophilic or nucleophilic reagents, or under photolytic conditions, some more or less unusual cleaving conditions have occasionally been used. An enzyme-labile safety-catch linker (Sect. 4.5.1) was reported by Grether and Waldmann [118] (Scheme 4.41). They used an acyl-protected amine, which was deprotected by an acylase that triggered the release of the ester-bound substrate. A number of other enzyme-labile linkers have also been reported [284, 285].

4.5

Linker and Cleavage Strategies

Apart from simple monofunctional cleavage, various different linker strategies have been developed in recent years. In particular, new concepts based on safety-

4.5 Linker and Cleavage Strategies

Scheme 4.41.

An enzyme-labile linker by Grether and Waldmann [118].

catch linkers (Sect. 4.5.1), cyclative cleavage strategies, and fragmentation reactions have been presented. Cleavage of linkers might be monofunctional or with functionalization of the linking site, whatever is required (Fig. 4.5). In the latter case, which is also known as the multifunctional cleavage strategy (see Sect. 4.5.6), the membership of library compounds is multiplied by the number of building blocks or functional groups that can be incorporated into the cleavage step. Hence, an anchoring group capable of functionalization and traceless linking is a versatile tool for enhancing diversity in a given system. 4.5.1

Safety-catch Linkers 9 Safety-catch linker: a linker which is cleaved by performing two different reactions instead of the normal single step, thus providing greater control over the timing of compound release [12].

The ‘safety-catch’ consists of a linker that, during synthesis, is inert towards the cleavage conditions and has to be activated (Table 4.16). Ellman and coworkers [286–288] have used this strategy in various applications, such as in the sulfonamide linker proposed by Kenner et al. [289]. Since ‘safety-catch’ means the activa-

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100


Fig. 4.5. Linker types for solid-phase synthesis [197].

tion of the linker before cleavage, such a system can be applied to monodirectional linkers, such as traceless linkers (Sect. 4.5.5) or to multifunctional linkers (Sect. 4.5.6), as well as to cleavage-cyclization strategies. Recent applications of the Kenner linker have been shown in the synthesis of vinylsulfones [149] (Scheme 4.42) or in the synthesis of amides [294, 295]. The oxidation of sulfides to sulfones [150] or the reduction in the opposite direction [64, 65] are popular methods for the design of a safety-catch protocol. A resin-bound thioether can be activated by oxidation to insure the nucleophilic cleavage of a phenoxide moiety [150] (Scheme 4.43). A safety-catch p-allyl-cleavable linker was developed for the synthesis of DNA on solid support. Starting from a resin carrying an Alloc-protected amino group fragment, conventional phosphoramidite chemistry was carried out to build up the desired nucleotide (267). Removal of the Allyloxycarbonyl (Alloc) group under palla-


101

Tab. 4.16. Overview of safety-catch linkers.

Structure

Reference


[289]

Amides, carboxylic acids, hydrazides

[149] (Scheme 4.42)

Amides

[290]

Amides

[150] (Scheme 4.43)

Carboxylic acids

[151] (Scheme 4.20)

Amines

[64] (cf. [65])

Amines; cleavage with TFA/ (EtO)2 (PS)SH [64]; Me3 SiBr/PhS Me/TFA [64]

[65]

Esters

[296]

Nucleic acids

102



Structure

Reference


[291, 292]

Amides

[118]

Alcohols; cleavage with enzymes

[293]

Amides

[236]

Diketopiperazines

[160]

Benzofurans (Scheme 4.56); cleavage with bases

dium catalysis and neutral conditions produces a polymer-bound intermediate (268) with a free amino group that can intramolecularly attack the activated phosphonates and liberate the nucleotide (269) from the solid support [296] (Scheme 4.44). Other safety-catch linkers have been developed using the selenium (Scheme 4.22), amide [293] and ester linkers [297], the Phenyloxycarbonyl (Phoc) linker [291, 292], intramolecular catalysis by an imidazole residue [298], hydrazide linkers [236], sulfone linkers [156], and other structures [290, 299–303].


Scheme 4.42.

The Kenner safety-catch linker according to Overkleeft et al. [149].

Scheme 4.43. Sulfide safety-catch linker by Marshall and Liener [150].

Scheme 4.44.

A safety-catch palladium activated linker by Lyttle et al. [296]. Nuc ¼ nucleotide.

4.5.2

Cyclative Cleavage (Cyclorelease Strategy) 9 Cyclative cleavage: cleavage resulting from intramolecular reaction at the linker which results in a cyclized product. The cleavage may also act as a purification if resin-bound sideproducts are incapable of cyclizing, and thus remain attached to the solid support on release of the desired material [12].

The cyclization-cleavage strategy (cyclative cleavage or cyclorelease strategy) is a typical example of the reaction for the synthesis of cyclic structures on solid support. It uses the characteristics of quasi-high-dilution kinetics on solid support and thus offers advantages that are not found in solution-phase chemistry.

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104


In general, the starting material for cyclative cleavage is anchored to the resin via a leaving group (Scheme 4.45). An internal nucleophile provides the ring closure by displacement of this leaving group either directly or after activation. Apart from nucleophilic attack, cyclative cleavage can be achieved, for example, by Stille (Scheme 4.35) or Wittig–Horner reactions [304, 305].

Scheme 4.45. General scheme for the cyclative cleavage.

Since the intramolecular reaction is by far faster than any intermolecular step, this strategy provides an additional purification step since only the cyclized structures are detached from the bead. Incomplete building blocks will remain on the solid support. Cyclative cleavage has to be distinguished from cases in which the cyclization occurs in solution after cleavage because unsuccessful cyclization precursors remain in the liquid phase (Sect. 4.5.3). In most cases, the precursor for the cyclization cleavage is linked via an ester bond to solid support whereby the nucleophile is based on an amine functionality. The product thus formed is therefore a cyclic amide or analog. Indeed, one of the first examples of this type was the pioneering benzodiazepine synthesis by Camps and Castells in 1974 [306]. In this case, the benzodiazepine ring formation proceeded by simultaneous cleavage from the bead. A vast list of examples has appeared since that suggests various kinds of heterocyclic systems. Examples are lactams, hydantoins [307–319], thiohydantoins [315], oxazolidinones [320, 321] (Scheme 4.46), diketopiperazines [322–331], benzodiazepines and benzodiazepinones [306, 318, 332–334], pyrazolones [335, 336], diketomorpholines [323], tetramic acids [337–340], quinazolinediones [341], dihydropyrimidine-2,4-diones [342], quinolinones [343], tetrahydrocarbolines [326], thiazoles [317], perhydrodiazepinones [327], sulfahydantoins [344], and benzimidazoles [345].

Scheme 4.46. Synthesis of oxazolidines (276) by cyclative cleavage according to Buchstaller [320].


In addition to nitrogen functionalities, oxygen nucleophiles can also act effectively. This concept was demonstrated by an approach to 3,5-disubstituted 1,3oxazolidinones (276) via a ring-opening cyclization-cleavage step [320] (Scheme 4.46). Lactones are also accessible in a similar way [346]. The intramolecular Wittig reaction provides elegant access to cyclic alkenes. Here, an intermediate ylide is formed which interacts with an internal carbonyl functionality. Release from the bead is achieved via final elimination or cycloreversion. Thus, starting from the appropriate substituted phosphonium salts that have regional amide functionality, treatment with a base provides indoles when this reaction is performed under anhydrous conditions [364] (Scheme 4.47). Even phosphonates can undergo cyclative cleavage, as shown by Nicolaou et al. [304] in their synthesis of a; b-unsaturated macrolactones using the Horner–Wadsworth alkene synthesis.

Scheme 4.47.

The phosphonium linker for the synthesis of indoles by Hughes [363].

A new safety-catch linker (Sect. 4.5.1), which is based on the lability of the diazenyl carbonyl derivative, was developed for the synthesis of monoketopiperazines (285) (piperazinones). After elaboration of the anchoring system, the dimethylbenzyl group was removed by TFA and the hydrazide thus formed was oxidized by copper(II) acetate furnishing a diazenyl group. This motif was attacked intramolecularly by the primary amine functionality, which resulted in cleavage of the product from the resin [236] (Scheme 4.48). The potential of this linker class for the synthesis of heterocyclic amidic structures is discussed in Chapter 22. A cyclative approach for the synthesis of thiazoles (288) begins with the acylated Rink amide resin (286) which has been transformed into the thioamide using Lawesson reagent. The S-alkylation with a-bromo ketones (287) proceeds with concomitant cleavage from the resin [347] (Scheme 4.49). The intramolecular Stille reaction proceeds similarly under cyclorelease conditions (see above; see also Schemes 4.35 and 4.36). 4.5.3

Cleavage-cyclization Cases

The cleavage-cyclization reaction has to be distinguished from cases where a cyclization proceeds after cleavage (and not at the same time). This is true for most acidic cleavage conditions. In these cases, larger quantities of non-cyclized bypro-

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Scheme 4.48. A safety-catch linker for the cyclative cleavage by Murray and coworkers [236]. TBAD, tert-butyl azodicarboxylate. DiPEA, diisopropylethylamine.

Scheme 4.49. Cyclorelease strategy for the synthesis of

thiazoles by Brookfield and coworkers [347].

ducts can be obtained, e.g. the synthesis of benzofurans. The alkylidenylation of esters with thioacetals (290) and titanium complexes by the Takeda process proceeds without any problems [348]. The resulting enol ethers can be cleaved from the solid support, resulting in the formation of benzofurans (291) (Scheme 4.50).


Scheme 4.50. Alkylidenylation and subsequent cleavagecyclization reaction for the synthesis of benzofurans (291) by Hartley and coworkers [348].

Another example is the synthesis of imidazoquinoxalinones (293) (Scheme 4.51) [209, 349]. Likewise, diketopiperazines are formed using the T2 linker and immobilized glycinamide (294) [350] (Scheme 4.52). Finally, the synthesis of certain heterocycles belongs to this strategy (Chapter 22; Sect. 4.6.1.11).

Scheme 4.51. Cleavage and subsequent cyclization in the

synthesis of heteroanellated benzimidazoles (293) by Mazurov [209].

Scheme 4.52. Cleavage of diketopiperazines (295) from

triazene T2 resin by Br€ase and Lazny [350].

4.5.4

Fragmentation Strategies

The fragmentation strategy is related to the traceless anchoring groups (defined in Sect. 4.5.5) and also contains strategies which can be considered as retro-cycloaddition cleavage, cycloelimination, or cyclofragmentation reactions. Here, a double

107

108


or triple bond results from 1,n-elimination processes. Occasionally, a retro-cycloaddition should also be considered as a fragmentation cleavage. The only example so far of an attachment of heteroarenes via an addition/elimination strategy has been described by Kurth and coworkers [351, 352]. While arenes are more or less resistant toward addition, heteroaromatic systems such as isoquinolines (296) are prone to the addition of nucleophiles. Subsequent reactions with the addition of electrophiles results in the so-called Reissert compounds (298). These are stable compounds, which for example can be alkylated. In the case of solid-phase synthesis, the electrophile chosen was a polymer-based acid chloride. The detachment can be carried out by simple addition of hydroxide ions (Scheme 4.53).

Scheme 4.53.

The Reissert complex strategy used by Kurth and coworkers [351].

Recently, Gibson et al. demonstrated that arenes can be attached to a solid support using chromium arene complexes (301) [353; cf. 354] (Scheme 4.54). This method even allows, at least from a theoretical point of view, the complete variation of the arene backbone; however, modification of the arene ring system might be limited to a certain extent. It has already been shown that various other metals or unsaturated molecules (e.g. alkynes with dicobaltoctacarbonyl fragments) can lead to interesting examples for the design of new linkers [355]. Linkers based on arene complexes have been patented [356, 357].

Scheme 4.54.

The p-complexation for the linkage of arenes by Gibson et al. [353].

The fragmentation-cleavage strategy has been used for the synthesis of pyrazoles (308) [358]. An a-silylated N-nitrosamide (304) has been rearranged via a thermal silyl shift to an azomethine ylide (306), which then reacted with the dipolarophile (305); subsequent fragmentation yielded the target pyrazoles (308) (Scheme 4.55).


Scheme 4.55.

The synthesis of pyrazole (308) according to Komatsu and coworkers [358].

A recent synthesis of benzofurans was based on cyclofragmentation. An appropriately substituted sulfone was used as a nucleophile in an intramolecular ring opening of an epoxide, wherein the resulting molecule lost a sulfinate and formaldehyde. By immobilization of the sulfone precursor to a resin, this sequence can be used for the cleavage of benzofurans from solid support, as demonstrated by Nicolaou et al. [160] (Scheme 4.56).

Scheme 4.56. Benzofuran synthesis according to Nicolaou et al. [160].

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110


Further examples of the fragmentation strategy have also been published [359– 362]. The semisynthesis of vancomycin clearly demonstrates the advantage of this kind of linker strategy [97]. 4.5.5

Traceless Linkers 9 Traceless linker: type of linker which leaves no residue on the compound after cleavage, i.e. replaced by hydrogen [12].

The term ‘‘traceless linker’’ has led to ambiguous interpretations in the past. Many authors have claimed their linkers to be traceless because the term has, in the past, been quite fashionable even when the reported linker was used to immobilize and release amines (which upon cleavage carry a hydrogen). ‘‘Traceless linking’’ is nowadays considered to be ‘‘leaving no functionality,’’ meaning for arenes and alkanes that only a CaH bond remains at the original position of attachment (Scheme 4.57). A broadening of this definition to OH or NH groups is not useful, because otherwise every linker derived from polymeric protecting groups would have to be regarded a traceless linker.

Scheme 4.57. Principle of a traceless linker.

When designing a traceless linker, one has to start from a heteroatom–carbon bond, which is labile toward protogenolytic, hydrogenolytic, or hydridolytic cleavage. Since most heteroatom–carbon single bonds are less stable than a carbon– carbon bond, traceless linkers can be synthesized based on nearly all heteroatoms. However, the enthalpies of CaX bonds are only relevant for homolytic bond scission. Many linkers are cleaved heterolytically, and the kinetic stability toward heterolytic bond cleavage is decisive in these cases. The first traceless linker was developed by Kamogawa and coworkers as early as 1983 [367]. Starting from a polymer-bound sulfonylhydrazine, formation of a sulfonylhydrazone resin (325) was achieved by reaction with ketones or aldehydes. The cleavage step was conducted either by reduction with borohydride or alanate to yield alkanes (326), or by treatment with a base to give the corresponding alkenes (327) (Bamford–Stevens reaction) (Scheme 4.58). In addition, one of the first papers dealing with traceless linkers was published in 1994 by Sucholeiki [161] and describes the use of thioethers (328) that are attached via an aromatic core that enhances the photolytic cleavage (Sect. 4.4.3). Irradiation at 350 nm gives rise to the formation of hydrocarbon 329 [161, 162]


Scheme 4.58.

The first traceless linker by Kamogawa et al. [367].

(Scheme 4.59). So far, this linker has not been fully explored and is limited in its range of functionalized arenes, since a phenyl substitution instead of biphenyl results in the formation of disulfides.

Scheme 4.59. Photolabile traceless linkage by Sucholeiki [161].

However, the most prominent anchors for traceless linkage for arenes (Table 4.17) are based on silyl linkers [124, 125, 369, 370] (Scheme 4.60) (Sect. 4.3.5). The generation of a diverse benzodiazepine library by Plunkett and Ellman [131] has shown clearly the advantages of this type of detachment since no additional functionalities were retained in the final molecules, which might bias the library. Starting from an immobilized stannane, palladium-catalyzed coupling with acid chlorides, deprotection of the aniline protecting group, acylation of the aniline with a series of Fmoc amino acids, Fmoc deprotection, and cyclization afforded resin-bound benzodiazepines, which were cleaved from the support using trifluoroacetic acid. Improvements in the chemoselectivity of the cleavage step – the silyl linker produces a substantial amount of the silyl arene upon cleavage – were accomplished using a germanium linker, which is more labile towards acids [131,

Scheme 4.60.

The traceless silicon linker according to Veber and coworkers [124].

111

112

4 Linkers for Solid-phase Synthesis Tab. 4.17. Overview of traceless linkers for arenes.

Structure

Reference

Cleavage

[124]

Scheme 4.60; cleavage with fluoride

[125]

Cleavage with TFA

[126]

[127, 128]

Cleavage with TBAF

[131, 363]

Scheme 4.61

[152]

Scheme 4.17; cleavage with palladium/formate

[182–185]

T1 resin: cleavage with HCl/THF or HSiCl3

[182]

T1 resin: cleavage with HCl/THF or HSiCl3

[237]

Scheme 4.65

[147]

Arenes; cleavage by Agþ (Scheme 4.16)


363] (Scheme 4.61). For example, the synthesis of the silyl linker has been optimized using preloaded handles to assist the coupling of the product to the resin. Other arylsilyl linkers have also been used (Table 4.6) to facilitate loading, synthesis, and/or detachment from the support [126–128, 130, 134–136].

Scheme 4.61. Synthesis of a benzodiazepine library with the aid of a germanium linker by Plunkett and Ellman [131].

Furthermore, silicon linkers can be used for the attachment for allylsilanes, which can be cleaved to alkenes in a traceless fashion [144]. The use of sulfones as suitable anchoring groups for alkanes in soluble polymer chemistry has been reported previously [155, 156] (Table 4.18). After oxidation of a sulfide to a sulfone (338), treatment of the latter with sodium/mercury gives rise to the formation of the parent hydrocarbon (339) in high yields. However, aqueous work-up is necessary to provide the pure product (339) [155] (Scheme 4.62). Another traceless linker type was independently developed by the groups of Nicolaou et al. [164] and Ruhland et al. [165]. Starting from lithiated selenium polystyrene (146), readily available from metallated polystyrene and selenium reagents, reaction with iodoalkanes led to the smooth formation of alkylated compounds. The cleavage can be conducted in such a way as to give alkenes (150)

113

114

4 Linkers for Solid-phase Synthesis Tab. 4.18. Overview of traceless linkers for alkenes and alkanes.

Structure

Scheme 4.62.

Reference


[161]


[120]


[364, 365]

Scheme 4.63

[366]

Scheme 4.67

[156, 155]

Alkanes; cleavage with Na/Hg

[164, 165]

Alkanes

[163]

Alkenes

[367]

Alkenes and alkanes; cleavage by reducing media or bases (Scheme 4.58)

[368]

Ketones

The sulfone traceless linker for alkanes according to Janda and coworkers [156].


upon treatment with hydrogen peroxide. Alternatively, tin hydride reduction leads to the generation of alkanes (149) (Scheme 4.22). Electron-poor aryl sulfonates are suitable candidates for oxidative palladium insertion. Hence, immobilized phenol sulfonates (129) have been employed in a palladium-catalyzed reductive cleavage using formic acid to yield arenes (130) with overall traceless cleavage [152] (Scheme 4.17). It might be anticipated that this type of linker is also suitable for functionalization (see below). The phosphorus–carbon bond in phosphonium salts is readily cleavable by the aid of a base in the absence of an aldehyde. Hence, the polymer-bound phosphonium salt (340) gives direct access to methylarenes (341). An interesting feature of this linker is the fact that carbonyl compounds can be olefinated, which leads to a cleavage-olefination linker system [364, 365] (Scheme 4.63).

Scheme 4.63.

The phosphonium linker for methylarenes according to Hughes [364].

Tin hydride reagents are versatile tools for the functionalization of alkenes and alkynes. Based on this concept, Nicolaou and coworkers [163] developed a polymer-bound tin hydride (142), which reacts in a hydrostannylation reaction with alkynes to give alkenylstannanes (143). After further transformation, the latter undergo proteolytic traceless cleavage to yield unsubstituted alkenes (145) [163] (Scheme 4.21). The decarboxylation of appropriately substituted arenes [371] and alkanes [343, 368, 372–375] has been used to generate the parent hydrocarbons. Since the neighboring group effect is essential, limitation to special substrates is required. One possible method for converting functionalized arenes into the corresponding hydrocarbons is the reduction of diazonium compounds [182]. Hence, the synthetic utility of the triazene linker as a traceless anchor for arenes has been demonstrated by Bra¨se et al. [182] using short reaction sequences. Thus, cinnamic esters were synthesized in a sequence starting from the iodoarene resin (343). Heck coupling with acrylates using palladium catalysis affords an immobilized cinnamate. This can be detached either directly or by a sequence of transformations yielding to an allyl amine in a traceless fashion either using trichlorosilane [184] or a HCl/THF mixture [182], to give the products 342 and 344, respectively, in high yields and without further purification or aqueous work-up (Scheme 4.64). Acyl aryl diazenes are known to fragment upon treatment with nucleophiles, a strategy which was used for a linker for carboxylic acid derivatives [119] (Scheme 4.11). Waldmann and coworkers recently developed a traceless linker for arenes based on this methodology [237]. Starting from a hydrazide resin (345), which

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116


Scheme 4.64.

The T1 linker for traceless cleavage [182, 184].

is converted into an activated species by oxidation, detachment of the molecule is carried out by the addition of nucleophiles (Scheme 4.65). This safety-catch principle – the activated linker is generated before cleavage (Sect. 4.5.1) – shows promising stability in palladium-catalyzed reactions. Furthermore, arylboronic acids can be used for the traceless synthesis of arenes [147] (Scheme 4.16).

Scheme 4.65.

The hydrazide linker according to Waldmann and coworkers [237].

A new traceless photolabile linker has been published using a hydroxamic acid derivative (83) [120]. The cleavage of specially designed polymeric benzyl-type protecting groups has been achieved using heterogeneous palladium black. In these cases, the catalytic hydrogenation furnishes methyl-substituted arenes as side products or targets. An early example takes advantage of the properties of the MeO-PEG-type support for the synthesis of di- and higher oligosaccharides (Scheme 4.66). It is interesting to note that the DOX linker enables the cleavage of the PEG structure, leaving the pmethylbenzyl group attached under certain conditions [376] (but see [29]) (Scheme 4.66). Similarly, cleavage from polystyrene resins was achieved using homogeneous palladium catalysis (palladium acetate) either with formate reduction [366] (Scheme 4.67) or under an atmosphere of hydrogen [377] to yield methyl arenes. The chemistry of traceless linkers is a fast-emerging field in the intensively investigated area of solid-phase organic synthesis [ for reviews, see 183, 197, 378]. Although some confusion about the definition or classification has been related to


Scheme 4.66. Syntheses of methylarenes on polymeric support [376].

Scheme 4.67. Syntheses of methylarenes (351) on solid support [366].

this linker type, and therefore a careful designation has to be made, it is now clear that this anchoring mode will play an important role in the design and syntheses of drug-like molecules. 4.5.6

Multifunctional Cleavage

Traceless cleavage (Sect. 4.5.5) provides an efficient access to hydrocarbon-like molecules. However, monofunctional linkers (Table 4.19) provide only one type of compound in a library. The so-called multifunctional cleavage [379] offers an important opportunity to incorporate additional diversity upon cleavage (Scheme 4.68). Hence, the number of new functionalities (Fig. 4.5) can multiply the number of produced compounds. If the linker is amenable to various types of building blocks (e.g. nucleophile [A] and electrophiles [B]) incorporated during cleavage, a substantial library of novel molecules can be prepared from one immobilized compound [380]. However, when considering using a multifunctional linker, one must take into account the nature of the cleavage reagent and the cleavage step. A cleavage consisting of, for example, addition of a Grignard reagent to an ester with a huge excess of the organometallic component requires an aqueous work-up and hence potential annihilation of valuable material. Thus, the excess reagents need to be easily removable (volatile, low or very high solubility in certain solvents, easy to

117

118

4 Linkers for Solid-phase Synthesis Tab. 4.19. Overview of multifunctional linkers.

Structure

Reference


[273]

Scheme 4.32

[364]

Alkenes by Wittig reaction

[163]

Alkanes by crosscoupling (Scheme 4.35) T1 resin: ‘‘traceless’’ linker [182– 184]; synthesis of phenols [186], biaryls, alkyl arenes [187, 188], azides, aromatic hydrazines, halides [190, cf. 181, 180], ester, azo compounds; cinnolines [191], benzotriazoles [192] T2 resin: synthesis of alcohols, esters [196, 197, 183], alkyl halides [196, 197, 183] T2 -resin: as above [198]

[381] (Scheme 4.69)

Amines, thiols [381], halides [382], azides [382], acetates [382] by nucleophilic displacement

[274] (Scheme 4.32)

Alkenes by palladium-catalyzed cleavage

[154] (Scheme 4.18)

Diaryl methanes by activation and subsequent palladium-catalyzed cleavage

[153]

Heterocycles by cleavage with nucleophiles

[277, 154] (Scheme 4.70)

Alkenes by cleavage through metathesis


Scheme 4.68.

Traceless vs. multifunctional cleavage.

eliminate or to be removed by for example scavenger resins, etc.) and should not interfere with the functionalities of the library compounds. The addition of nucleophiles such as amines, alkoxides, thiolates, and carbon nucleophiles to carbonyl groups leading to modified carbonyl moieties has been used widely for multifunctional cleavage. Hence, anchoring a Weinreb amide to the resin and cleaving with Grignard reagents leads to the formation of ketones [383]. Alternatively, thioesters provide access to amides or ketones, whereas sulfonate esters such as 359 provide access to amines and thiols (360) [381] (Scheme 4.69), halides [382], azides [382], or acetates [382].

Scheme 4.69. Synthesis of tertiary amines, thioethers or imidazoles using a nucleophilic substitution cleavage [381].

Aromatic sulfones, synthesized by oxidation of thiopyrimidines (safety-catch linker, Sect. 4.5.1), have been used for the synthesis of aminopyrimidines by displacement with amines [384]. Silyl linker 96, described previously [126], may be used for a smooth multifunctional cleavage. Hard electrophiles such as chlorine, bromine, iodine, or nitrosyl replace the silicon at the ipso-position of the silyl arene, leading to a variety of arenes upon cleavage. The same strategy has been demonstrated with the germanium linker. Cleavage by metathesis has been used by various groups to accomplish either detachment by cyclization (ring-closing metathesis, RCM) [281], intermolecular metathesis (cross-metathesis) [385] (Scheme 4.70), or intramolecular metathesis [275] (see also Sect. 4.4.4.2). One advantage of cleavage by metathesis is the possibility of introducing fragments with additional functionalities. The successful examples include the total synthesis of epothilone by this cleavage strategy [277]. The nucleophilic substitution of allylic fragments provides a general access by multifunctional anchoring. Staring from dienes, prepared by en-yne cross-

119

120


Scheme 4.70.

The concept of cleavage by cross-metathesis.

metathesis, nucleophilic substitution catalyzed by palladium via p-allyl complexes provides access to functionalized dienes [273] (Scheme 4.33). Similarly, 1-alkenylcyclobutylsulfones can be used to yield cyclobutylidene derivatives [274] (Scheme 4.32). Besides the possibility of conducting traceless cleavage (Sect. 4.5.5) from triazene linkers, a cleavage cross-coupling can be achieved using palladium catalysis [187, 188] (Scheme 4.34). In addition, functionalization of triazene resins upon cleavage leads to the formation of aryl halides (180-Nu, Nu ¼ halogen) [191], phenols, and aryl ethers (180-Nu, Nu ¼ OAc), azo compounds (176), biaryls (185) by a Gomberg–Bachmann reaction, Meerwein alkylation products (187), benzotriazoles (177) [192], and Richter products such as cinnolines (178) [191] (Scheme 4.27). Disubstituted triazenes (194) prepared on the T2 resin undergo cleavage to yield aliphatic diazonium ions, which in turn solvolyze in the presence of nucleophiles to give alkyl halides (202), alcohols (201-OH) and alkyl esters (201-OCOR) [see 200] [183, 196, 197] (Scheme 4.29). It is possible, but not fully established, that silyl linkers can be cleared in a multifunctional fashion to enable the synthesis of arenes and aryl halides (see Scheme 4.61). A flexible tool for the synthesis of heterocycles is the cleavage of a-sulfonylated ketones attached to a solid support [153]. Since the loading proceeds smoothly from alkenes or epoxides onto polystyrene sulfonic acid resin, this method provides ample possibilities for the functionalization of simple compounds. 4.5.7

Linkers for Asymmetric Synthesis

The use of enantiomerically pure drugs has increased within the pharmaceutical industry over recent years. It is assumed that the market was well over US$120 billion in the year 2000 [386, 387], and approximately 60% of all marketed drugs are enantiomerically pure. Chiral building blocks have been used extensively in solid-phase synthesis (Fig. 4.6) and have been incorporated into modern smallmolecule compound libraries. However, asymmetric synthesis based on linking strategies, with either diastereoselective or enantioselective methods, has been more or less neglected. The obvious advantage of a linker, which induces stereoselectivity, is the ease with which the auxiliary can be removed and recycled [388]. Pioneering work on asymmetric synthesis on an insoluble support was described in 1972 when Kawana and Emoto [389, 390] reported the synthesis of an atrolactic acid on a polymer containing the sugar 1,2-o-cyclohexylidene-a-d-xylofuranose (365). Shortly after this, Leznoff and coworkers [391, 392] demonstrated the synthesis of a-chiral cyclohexanone derivatives using polymer-bound imines (366).

4.6 Linkers for Functional Groups

Fig. 4.6. Overview of chiral auxiliaries on solid support.

These early examples have demonstrated that polymer-bound chiral auxiliaries are suitable both for anchoring of organic molecules and for the induction of asymmetry. Furthermore, the recycling of chiral auxiliaries can be confirmed. In recent years, a series of auxiliaries has been immobilized on solid support, including oxazolidinones (372) according to Evans [231, 393–398], oxazoles (367) [399], mono- (368) [400] and bisalkoxymethylpyrrolidines (369) [401], as well as SMP/SAMP auxiliaries (370, 371) by Enders and coworkers [402, 403]. Alternatively, polymer-bound chiral sulfinamides [404], sulfoximines by Hachtel and Gais [405], imines [406], and amines (as galactosylamine) (Scheme 4.71) have been used.

Scheme 4.71. Stereoselective Ugi-3CC according to Kunz and coworkers [407].

121

122


A recent example is the diastereoselective Ugi reaction for the synthesis of aamino acids by Kunz et al. [407]. Starting from a galactosylamine (374), a threecomponent reaction of aldehydes and isocyanides produced amides (375) in good diasteroselectivities (Scheme 4.71) after removal of the auxiliary. 4.6

Linkers for Functional Groups

The requirement for diverse compound libraries by means of solid-phase synthesis has led to the development of linkers for most functional groups found in organic synthesis. The number of linkers developed for a specific group also reflects the distribution of pharmacophoric groups (Fig. 4.7) present in natural products and other bioactive compounds. In this section, linkers for functional groups are highlighted. In all cases, the functional group that is attached to the solid support, whether it remains unchanged upon cleavage or is formed during cleavage, acts as the guide through this section. For example, a benzodiazepine attached via the aromatic core would be found in the section for the linking of arenes (Sect. 4.6.7.2), whereas attachment via the nitrogen atom (amide functionality) will be described in Sect. 4.6.2.4.

Fig. 4.7. Distribution of pharmacophoric groups of com-

pounds in three different databases according to Henkel et al. [408]. DNP, Dictionary of Natural Products; drugs, internal database of the Bayer company; synthetika, screening pool of the Bayer company.

4.6.1

Linkers for Nitrogen Functionalities

Nitrogen-containing structures are mandatory structural units in biologically active compounds. Therefore, it is not surprising that a vast number of linkers for various nitrogen functionalities has been developed.


4.6.1.1 Linkers for Amines

The solid-phase synthesis of primary amines is of great importance to various fields of organic and medicinal chemistry, since these compounds are both valuable synthetic intermediates and often interesting target molecules. The generation of libraries of commercially unavailable amines has received considerable interest within combinatorial chemistry as a large number of primary amines show biological activity and the primary amine moiety is incorporated into various molecules encountered in medicinal chemistry. Moreover, these amines can serve as building blocks in the preparation of further libraries. Consequently, linkers for amines have been used excessively. Among these are the Rink linker, carbamate linkers [301], and Rink carbamates [409]. 4.6.1.2 Linkers for Primary Amines

For the solid-phase synthesis of primary amines miscellaneous linker types have been developed [85, 94, 316, 410–425] which are resistant to various reaction conditions. The standard linker family suitable for the detachment of amines is the benzyl-type linkers (Sect. 4.3.1) [32, 82, 426–438], such as the Rink linker [82] (Scheme 4.72), the trityl linkers [76], the indole linker [85], the BAL linker [14], and others [226, 439]. Carbamates are suitable linkers for primary amines, which can be released by diluted TFA [94, 268, 440–446].

Scheme 4.72.

The Rink resin as a linker for primary amines [82].

Alternatively, benzyl carbamates can be attached to the solid support, and hydrogenolytic cleavage can be used to detach the molecules, which are then usually left with a nitrogen functionality (cleavage of CaO bond) [29, 447]. The polymers in these cases are formally immobilized Benzyloxycarbonyl Cbz (Z) groups. Interestingly, TentaGel and polystyrene give the products in similar yields under identical conditions [17] (Scheme 4.73). Benzylic linkers can also be used advantageously in the presence of other benzylic protecting groups, since they can be removed in the same step [29].

Scheme 4.73. Detachment of peptides from polymeric benzyl-type protecting groups [17].

123

124


Fig. 4.8. Overview of amine linkers.

An anchor for primary amines, which is cleavable under basic conditions (Table 4.12), is the dimedone-based Dde group (204) [226, 228, 425] (Fig. 4.8). Anilines are released from the PAL linker, the Rink linker [82], the PhFl linkers [437, 448], the p-benzyloxybenzylamine (BOBA) resin [32], and carbamate linkers. The latter type has also been used as a safety-catch version [168] (Sect. 4.5.1). A novel dialkylhydrazine linker (381), which is stable toward organometallic reagents, has recently been reported. A series of a-branched primary amines was synthesized by attachment of various aldehydes, the resulting hydrazones (384) were modified via 1,2-addition of organolithium reagents furnishing trisubstituted hydrazines (385) which in turn were cleaved from the solid support to yield acylated a-branched primary amines [449] (Scheme 4.74).

Scheme 4.74. Use of a hydrazine linker (381) according to

Enders and coworkers [449] for the synthesis of primary amines.


The cleavage of primary sulfonamides to give primary amines can be performed with electrochemical cleavage, when carried out on a conducting polymer [167] (Scheme 4.19). 4.6.1.3 Linkers for Secondary Amines

Secondary amines have been detached from solid support using the trityl, chlorotrityl, and other linkers [69, 193, 340, 416, 418, 450] (Fig. 4.9). Secondary amines can be detached from benzyl-type resins if they are activated to the corresponding a-chlorocarbamates 376 [417].

Fig. 4.9. Overview of linkers for secondary amines.

Carbamates are suitable linkers for secondary amines, which can be released by diluted TFA [17, 345, 451, 452], as demonstrated in a Hantzsch dihydropyridine synthesis by Breitenbucher and Figliozzi [409] (Scheme 4.75).

Scheme 4.75. Carbamates as linkers for amines by Breitenbucher et al. [409].

125

126


The triazene T2 linker is capable of clean detachment of secondary amines. Since disubstituted triazenes can be alkylated with electrophiles in the presence of a strong base, this method provides a clean procedure for the conversion of primary to secondary amines [198] (Sect. 4.3.8) (Scheme 4.76).

Scheme 4.76.

The T2 linker for the synthesis of secondary amines on solid support [198].

Other linkers for secondary amines are the thiobenzylethylcarbamate linker [151] (Scheme 4.20), PAL-type linkers [453] (dihydropyridines) [453] (Scheme 4.51) [454], and other systems [455–457]. 4.6.1.4 Linkers for Tertiary Amines

The cleavage of tertiary amines from solid support is somehow different from the analogous primary and secondary amines, since no apparent linking site is available. However, the ease of formation of the tetravalent nitrogen moiety can be favorably used for the linking of tertiary amines (Fig. 4.10). The prototype is the REM resin, which is based on a Michael addition/alkylation and elimination sequence on an immobilized acrylate [220–225] (Scheme 4.77). Besides acrylates, vinyl sulfones might serve the same purpose [458]. The cleavage of dialkylaryl triazenes with methyl iodide was investigated by Moore and coworkers [180] and Tour and coworkers [181] during their synthesis of aryl iodides on solid support (Sect. 4.14.3.8). Using the piperazine resin (394), cleavage with methyl iodide in dichloromethane yielded the tertiary amines with good purities. A quarternization of the nitrogen was not detected [459] (Scheme 4.78). Similarly, the hydroxylamine linker (396) can be used for the synthesis of tertiary amines [460], as demonstrated in the synthesis of the analgetic (G)Tramadol9 (401) on solid support [461] (Scheme 4.79). Carbamates can be cleaved by means of lithium aluminum hydride in THF to give N-methylalkylamines with high purities [446, 462, 463]. While the latter


Fig. 4.10. Overview of linkers for tertiary amines.

Scheme 4.77.

The REM linker for the synthesis of tertiary amines [223].

Scheme 4.78.

Alkylative cleavage of triazenes [459].

linker provides methylamines, the reaction of secondary amines with electrophiles consisting of polymer-bound leaving groups enables the synthesis of higher substituted amines. However, the removal of the excess of secondary amine can be a drawback to this method [153, 381] (Scheme 4.80). Besides aliphatic electrophiles, certain heteroaromatic structures can also be

127

128


Scheme 4.79.

Tramadol9 synthesis according to Grigg and coworkers [461].

Scheme 4.80. Synthesis of tertiary amines using a sulfonate linker [464].

used [384]. Furthermore, various other linkers are suitable for tertiary amines [224, 225, 465]. 4.6.1.5 Linkers for Hydrazines, Hydrazones and Hydroxylamines

While hydrazines can be released from trityl linkers, the detachment of disubstituted hydrazones (418) is possible with the triazene linker [466] (Scheme 4.81). The hydrazones (417) are readily available from polymer-bound hydrazines and subsequent condensation with aldehydes. 4.6.1.6 Linkers for Diazonium Salts

The triazene T1 linker is a suitable anchoring group for the detachment of diazonium salts. Upon cleavage with dilute acids, the diazonium salts can be obtained

Scheme 4.81. Synthesis of hydrazones [466].


in high yields. However, owing to their instability, the diazonium ion is mostly transformed in situ to a new functional group [467] (Scheme 4.82) (see also Scheme 4.27).

Scheme 4.82. In situ cleavage of triazenes for the synthesis of diazonium salts [467].

4.6.1.7 Linkers for Azides

Aromatic and aliphatic azides are interesting targets and intermediates in solidphase organic synthesis. Aryl diazonium salts react with azide ions without a catalyst to give aryl azides. In contrast to the classical Sandmeyer reaction, this transformation proceeds without cleavage of the carbon–nitrogen bond. This reaction can be used for the synthesis of libraries of aryl azides. Starting from the triazene T1 resin, cleavage with trifluoroacetic acid in the presence of trimethylsilyl azide furnished, after simple removal of the solvent, the aryl azides in good yields and excellent purities [189] (Scheme 4.27). This synthesis can also be applied to the synthesis of heterocycles such as benzofuroxanes [189]. Aliphatic azides have been released from solid support using a sulfonate linker [382, 465, 468]. 4.6.1.8 Linkers for Nitro Compounds

The reaction of arenes attached to silyl-type linkers with nitronium ions might lead to the formation of nitroarenes, as proposed in [128]. 4.6.1.9 Linkers for Azo Compounds

Although important structures for the synthesis of dyes, the synthesis of azo compounds on solid support has scarcely being described. The transfer of diazonium salts to nucleophiles such as phenols can be used for the synthesis of azo compounds. This approach has been demonstrated either with diazonium salts immobilized on an ion exchange resin [469] (Scheme 4.83) or starting from triazene T1 resins [199] (Scheme 4.84). 4.6.1.10 Linkers for Nitriles

Polymeric sulfonylhydrazines react readily with aldehydes and ketones to form hydrazones, which can release the corresponding nitriles [367]. 4.6.1.11 Linkers for N-Heterocycles

Various heterocyclic nitrogen compounds can be detached from solid support (for a review, see [470]). Basically, two options are conceivable. The removal of heterocycles bound via the nitrogen atom to solid support provides a general entry to this

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Scheme 4.83.

Azo coupling through immobilized diazonium salts according to Das et al. [469].

Scheme 4.84. Cleavage of a triazene with concomitant azo coupling [199].

class of compounds. Otherwise, a cleavage-cyclization technique can be considered to build up the nitrogen heterocycle as the very last step. Both strategies have been used extensively. Tetrazoles, for example, can be attached to a THP linker. Cleavage is best achieved either with 3% HCl or with TFA [112] (Scheme 4.85).

Scheme 4.85. Cleavage of tetrazoles from a THP-type linker [112].

The THP (Table 4.4) [99] and other acetal linkers [100] are also suitable for the attachment of indoles. Alternatively, heterocyclic amines with a free NaH functionality can be linked via an acetal, which can be cleaved first under acidic and then under basic conditions [456] (Scheme 4.86).


Scheme 4.86. Synthesis of indoles (434) using a diol linker by Gmeiner and coworkers [456].

Indole synthesis can be realized on solid support via a sulfone linker. Advantageously, the heterocyclic core was installed in this case via a palladium-catalyzed process [302] (Scheme 4.87). Similarly, benzimidazoles can be released via a retroMichael addition from sulfonate resins [471] (Scheme 4.88).

Scheme 4.87. Cleavage of indoles from sulfonate resins [302].

Scheme 4.88. Cleavage of benzimidazoles [471].

Other examples for the synthesis of aromatic nitrogen heterocycles include the synthesis of pyridines and tetrahydropyridines [452], benzimidazoles [471], isoxazolines [472], isoquinolines via Reissert complexes (Scheme 4.53) [351, 352], quinoxalinones [49], benzisoxazoles [473], and imidazoles on trityl linkers [78]. The second strategy is dedicated to cleavage conditions, which result in the installation of the heterocyclic core; this strategy obviously belongs to the cleavage-

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cyclization technique (Sect. 4.5.3). An example of this detachment uses diazonium salts, which can be cyclized when a nucleophilic ortho-substituent is present (Scheme 4.89). This reaction yields cinnolines [191] (Scheme 4.90), benzotriazoles (Scheme 4.91) [192], benzothiadiazoles, indazoles, benzo[c]cinnolines, and other structures. Suitable precursors are the triazene T1 resins (Sect. 4.14.3.8), which are cleavable by mild acids. Azides are also precursors for a similar cyclization. This process has been demonstrated in a synthesis of benzofuroxanes on solid support [189] (Scheme 4.92). Finally, certain arylsulfonyl hydrazones cyclized to the corresponding thiadiazoles (453) when treated with thionyl chloride [474] (Scheme 4.93).

Scheme 4.89. General synthesis of heterocycles via cyclization

of diazonium salts generated from the T1 linker [191].

Scheme 4.90. Synthesis of cinnolines (445) [191].

Scheme 4.91. Synthesis of benzotriazoles (448) [192].

Scheme 4.92. Synthesis of benzofuroxanes (438) [189].


Scheme 4.93. Synthesis of thiadiazoles [474].

4.6.2

Linkers for Carbonyl Functionalities 4.6.2.1 Linkers for Carboxylic Acids

The prominent number of linkers is certainly dedicated to the anchoring and detachment of carboxylic acid. This is due to the widespread application of solidphase peptide synthesis. However, a large number of pharmacologically active, lowmolecular-weight compounds contain the carboxylic acid functionality and therefore new and innovative linkers have been developed [475]. In general, most syntheses rely on the Wang resin [476] or related benzyl, trityl, and benzhydryl systems, which are cleavable by trifluoroacetic acid [23, 34, 48, 66, 77] (see Table 4.2) or HF/ anisole [35]. Other examples include acetal resins, sulfones [477], b-thioethylesters [168], fluoride-labile linkers [52, 69, 122], Kaiser oxime resin [478], p-allyl linkers, photo linkers [238, 241, 244, 245, 260, 266, 479], the safety-catch ketal linker [293], and other linkers [18, 442, 443, 480, 481]. A new anchoring group based on phenanthridines has been developed [482], which can be acylated by conventional methods. It shows stability toward acidic, basic, and reductive conditions; however, in the presence of a suitable oxidant, detachment of the carboxylate takes place (Scheme 4.94).

Scheme 4.94.

Anchoring and detachment of carboxylic acids by Li et al. [482].

The well-optimized and established BMPSE (b-dimethylphenylsilylethyl) protocol for esters has been adapted by Kurth and coworkers [483] for the immobilization of carboxylates, which can be cleaved under mild conditions (Scheme 4.95).

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Scheme 4.95.

The solid-phase BMPSE group by Kurth and coworkers [483].

Other reports describe the use of arylhydrazides as linkers [119, 484] and also other linkers [372, 373, 485–489]. 4.6.2.2 Linkers for Carboxylic Esters, Anhydrides and Lactones

Carboxylic esters have been released mostly by the action of alkoxides onto ester resins. In most cases, the cleavage has been performed using methoxide in methanol (Scheme 4.96). A drawback is certainly the necessity to remove excess metal salts.

Scheme 4.96.

The use of esters as linkers for benzoic acid derivative by Kondo et al. [116].

A new safety-catch linker based on the activation of an amide bond by cyclization to an indolyl carboxamide is suitable for the release of methyl esters [293]. Similarly, the tert-butoxy group of polymer-bound 2-(tert-butoxy)phenyl carboxylates may be removed with trifluoroacetic acid, and subsequently the ester cleaved by primary or secondary amines [121]. Alternatively, postsynthesis with diazomethane is also possible [121] (Scheme 4.97). Polymer-bound arylhydrazides have been used in the synthesis of peptide carboxylic esters in a safety-catch strategy (Sect. 4.5.1) [119, 484]. A direct synthesis of carboxylic esters based on the cleavage of triazene T1 resins in the presence of a palladium catalyst under carbon monoxide atmosphere (1 bar) to yield methyl benzoate (466) in 87% yield (92% purity) [187] (Scheme 4.98). Arylalkyl triazenes with a free NH moiety can also be used for the synthesis of carboxylic esters. Starting from amines attached to the T2 linker [183, 196, 197] or the analogous para-substituted resin [200], cleavage with different carboxylic acids proceeds smoothly to give esters (Scheme 4.99).


Scheme 4.97. Synthesis of macrocycles by Deslongchamps and coworkers [121].

Scheme 4.98. Cleavage and carbonylation of the triazene T1 resin (465) [187].

Scheme 4.99. Synthesis of carboxylic esters via triazene T2 resins [183, 196, 197].

Lactones are accessible via thioesters [115], esters [346, 400], or amides [490] with a suitably positioned internal nucleophile or electrophile. In addition, alkyl acetates are also accessible from sulfonates upon cleavage with sodium acetate [382]. 4.6.2.3 Linkers for Thiocarboxylic Acids and Esters

A benzhydryl linker has been used to attach thioesters, while the cleavage can be conducted with HF [491]. Alternatively, treatment of an oxime linker with bistriethylthioether results in the fission of the product to give the thio carboxylic acid [478, 492].

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Fig. 4.11. Strategies for amide linkers.

4.6.2.4 Linkers for Carboxamides and Related Structures

The solid-phase synthesis of amide-containing structures is important to various fields of organic and bioorganic chemistry. The solid-phase peptide synthesis (SPPS) has provided the chemical community with various solutions toward linking, reaction, and detachment of amide structures. In general, these protocols involve the attachment of amine derivatives by their carbon backbone or, in case of amino acids, by their carboxy functionality. The coupling with carboxyl derivatives proceeds via the free amine, usually using Fmoc-protected amino acids. Basically, two strategies have been used for the synthesis on solid support (Fig. 4.11). In most cases, an amine has been attached to solid support and then acylated on the bead to give secondary amides (470). The latter can be cleaved under acidic conditions. Alternatively, carboxyl functionalities (471) can be cleaved with ammonia or with primary or secondary amines. A synthesis of secondary and tertiary amides is based on Lewis acid-mediated cleavage of the ester resin [493]. Safety-catch approaches (Sect. 4.5.1) in the synthesis of peptide carboxamides use polymer-bound arylhydrazides [119, 236, 484] (see Scheme 4.48) or indolyl carboxamides [293]. Linkers for primary amides Primary amides can be synthesized on solid support using, for example, the Rink resin [68, 494]. This support is suitable for various reaction conditions, as outlined in Schemes 4.100 and 4.101.

Scheme 4.100. Detachment of primary amides from a Rink resin by Wang et al. [494].


Scheme 4.101. Synthesis of Nenitzescu products according to Wilson and coworkers [495].

Linkers for secondary amides Linking by the NaH of a secondary amide bond has been developed, hence leading to so-called backbone amide linkers (BAL). Originally designed for the NaH protection of amide bonds to circumvent b-turns and other problems during peptide synthesis, the Hydroxymethylbenzoic acid Hmb group [496, 497] or the Silylmethylmethoxyhydroxybenzyl SiMB group [498] can also serve as linkers for SPPS. Although important in the synthesis of cyclopeptides, they also play an important role in the synthesis of small molecule libraries containing amide moieties. Barany and coworkers have described an application of a backbone amide linker for the synthesis of oligopeptides [40, 83, 424, 499, 500] (Scheme 4.102), glycopep-

Scheme 4.102. Backbone amide linkage using the PAL linker by Barany and coworkers [83].

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tides, and peptide aldehydes [501] based on the peptide amide linker (PAL) concept. Attachment of amines furnishes alkylated products (482), which in turn can be acylated under peptide-coupling reaction conditions to give amidic structures (484). The latter can be cleaved from the resin using acidic media to give amides (485). This concept was used for the synthesis of benzodiazepine libraries by Ellman and coworkers [47] (Scheme 4.103), hapalosin mimetics [502], and further refined to yield an alternative, TFA-stable benzylamine linker [503]. Benzylaminebased linkers have encountered some problems during cleavage which also caused the fragmentation of the whole linker. Furthermore, the reactivity is decreased because of steric hindrance [14, 241].

Scheme 4.103. Synthesis of benzodiazepines according to Ellman and coworkers [47].

Recently, it has been shown that the BAL linker is also suitable as a safety-catch linker (Sect. 4.5.1) for the synthesis oligosaccharides [454] (Scheme 4.104). Starting from an aldehyde, reductive amination of a protected glucosamine and subse-

Scheme 4.104. Oligosaccharide synthesis via the BAL structure according to Jensen and coworkers [454].


quent glycosidation with a glucopyranosyltrichloroacetimidate yielded the corresponding disaccharide (492). The cleavage was performed after acylation of the amine functionality, which is required to suppress premature decomposition. In addition, the p-benzyloxybenzylamine (BOBA) resin (7) developed by Kobayashi and Aoki [32, 504] shows interesting possibilities, such as reductive amination and acylation. Furthermore, it can be cleaved under oxidative conditions to yield the amidic structures. Recently, a new backbone amide linker has been devised using indole chemistry [85] (Scheme 4.105). In addition, various other related benzyl-type linkers [503, 505, 506] have been used for the synthesis of secondary amides. The Rink linker (28) [507], the Merrifield a-methoxybenzyl (MAMP) linker (24) [61], methylbenzylhydryl amine (MBHA) resin (23) (cleavable with HF/anisole) [59], or AHB (4-alkoxy-2-hydroxybenzaldehyde) linker (12) (R ¼ H) [45], a reductively aminated acetophenone linker [54], and SASRIN-related systems [44, 51] are also suitable.

Scheme 4.105.

The indole linker according to Estep et al. [85].

The T2 linker has recently been shown to be a versatile backbone amide anchor [194]. Immobilized disubstituted triazenes were acylated with carboxylic acid anhydrides or chlorides to give amidic structures (482) [194] (Scheme 4.106).

Scheme 4.106. Synthesis of amides using the triazene T2 linker [194].

A new backbone linker for b-lactams and secondary amides was presented using a benzyloxyaniline linker, which is cleavable by ceric ammonium nitrate [508]. Secondary amides and ureas can be synthesized using supported hydroxylamine, which can be cleaved by samarium(II) iodide [509]. Linkers for tertiary amides Linkers for tertiary amides are the ester anchors cleavable with secondary amines. In addition, the triazene T2 linker is also a precursor for amides when cleaved by acid chlorides [193, 510].

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Linkers for carbamates Both the SASRIN and the indole linker are suitable for the synthesis of carbamates, however the SASRIN linker can be cleaved with 5% TFA in dichloromethane [41], whereas 50% TFA in dichloromethane is required for the indole linker [85]. Linkers for ureas Urea derivatives, which are important biologically active compounds and building blocks for organic syntheses, have been synthesized on solid support using various strategies. A useful linker for ureas is the phoxime resin (498) (phosgenated oxime resin) [511, 512], which is cleavable at elevated temperature with amines to yield ureas (Scheme 4.107). Other linkers for ureas are the SASRIN linker [41, 44] and other systems [422, 513].

Scheme 4.107.

The phoxime resin for the synthesis of urea derivatives [511].

The T2 linker has also being used for the attachment and modification of primary amines, yielding urea derivatives after mild cleavage (Scheme 4.108). Aminosulfonylureas have been released from carbamate linkers using primary and secondary amines (multifunctional: Sect. 4.5.6) [420]. The samarium(II) iodide-promoted cleavage of acylated polymer-bound hydroxylamines gives general access to urea derivatives [509]. Linkers for thioureas (Scheme 4.109).

Thioureas have been synthesized using the T2 linker [194]

Linkers for isothioureas Isothioureas have been detached from solid support using the T2 linker under mild conditions [514] (Scheme 4.110). Linkers for guanidines Guanidines are basic molecules with the capacity of forming H-bonding interactions. They are therefore a promising class of potentially use-


Scheme 4.108.

The backbone amide-anchoring mode of the T2 linker [194].

Scheme 4.109. Synthesis of thioureas using the T2 linker [194].

Scheme 4.110. Synthesis of isothioureas using the T2 linker [514].

ful pharmacologically active compounds and the synthesis of guanidines in liquid phase has found widespread application in organic chemistry [515]. The T2 linker [193, 194] and the improved T2*-linker [183, 198] offer a unique possibility to immobilize and modify amine derivatives on solid support. Starting from immobilized amines, a three-step sequence based on coupling with isothiocyanates, conversion to guanidines with amines, and subsequent cleavage presents an approach to the formation of guanidines in which all three substituents can be varied to a wide extent (Scheme 4.111) [195]. In addition, various other linkers for guanidines are available [85, 427, 516–524].

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Scheme 4.111. Synthesis of functionalized guanidines [195].

Linkers for amidines Several techniques enable the synthesis of amidines on solid support. In most cases, benzyl-type linkers such as the indole linker [85], the Wang linker [525], or a carbamate system on the Wang linker [525] have been used. Linkers for hydroxamic acids Hydroxamic acids are important building blocks in metalloproteinase inhibitors. Therefore, various linkers have been developed to satisfy these requirements. The Wang linker is, for example, suitable for the detachment of hydroxamic acids as demonstrated in a cascade carbopalladation reaction [526] (Scheme 4.112).

Scheme 4.112. Carbonylation cascade on solid support toward hydroxamic acids according to Grigg et al. [526].

Alternatively, the THP, the Wang resin [527, 528], the Rink linker [529], trityl resin [530], PAL resin [528, 531], oxime resins [532], and others [285, 437, 533– 537] are suitable linkers for hydroxamic acids. 4.6.2.5 Linkers for Hydrazides and Semicarbazones

Only a few linkers have been used for the synthesis of hydrazides and semicarbazones, including a phthalamide linker cleavable with hydrazines [538] and the


T2 linker (Sect. 4.14.3.8) [466] (Scheme 4.113). Hydrazides can be cleaved from solid support using the trityl linkers, for example.

Scheme 4.113. Cleavage of semicarbazones from triazene resins [466].

4.6.2.6 Linkers for Cyclic Amides and Related Structures

Cyclic amide structures have found widespread application in the synthesis of biologically active compounds. Most of the linkers mentioned above for tertiary and secondary amide structures, ureas, and so forth, are suitable for the synthesis of cyclic amidic moieties; however, the most widespread applications are dedicated to the cyclative cleavage from the bead (Sect. 4.5.2). Linkers for lactams The synthesis of b-lactams has been achieved using the hydroxyaniline linker [508] (Scheme 4.114).

Scheme 4.114. Linker for b-lactams and secondary amides [508].

Linkers for other heterocycle-containing amidic structures Various techniques have been used for the synthesis of hydantoins [307, 308–313, 319], thiohydantoins [315], oxazolidinones [320, 321], diketopiperazines, frequently the ‘‘byproducts’’ of peptide synthesis [322–331], benzodiazepines and benzodiazepinones [306, 318, 332, 333], pyrazolones [335, 336], diketomorpholines [323], tetramic acids [337– 340], quinazolinediones [341], dihydropyrimidine-2,4-diones [342], quinolinones [343], tetrahydrocarbolines [326], thiazoles [317], and perhydrodiazepinones [327]. Sulfahydantoins (1,2,5-thiadiazolidin-3-one 1,1-dioxides) were prepared from esterbound amino acids, which were first reductively alkylated, then reacted with sulfamoyl chloride, and finally cleaved from the resin using 1,8-diazabicyclo[5.4.0]undecene-7 (DBU) [344].

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4.6.3

Linkers for Ketones and Aldehydes

The anchoring of carbonyl compounds is generally based on established protecting groups. For example, alkyl hydrazones (Scheme 4.74) [449], sulfonyl hydrazones [539], and semicarbazones [540] can be used as linkers for aldehydes and ketones. Cleavage is conducted with acids, preferably in the presence of formaldehyde. In addition, aldehydes and ketones can be synthesized using photo-labile linkers, reduction of or nucleophilic additions to Weinreb amides [541–543], nucleophilic addition to thioesters [115, 544], oxidative cleavage (ozonolysis) of alkenes [545, 546], hydrolysis of enamines [419], and using various other methods [547]. Since their introduction some 20 years ago by Leznoff and coworkers [101, 102], acetal linkers have been used in the solid-phase synthesis of ketones and aldehydes [104, 548] (Scheme 4.115). Recently, it was demonstrated that even sterically hindered ketones could be attached to this support using scandium triflate and trimethylorthoformate by acetal exchange reaction [104]. Similarly, thioketal structures are also suitable linkers for ketones [105] (Sect. 4.3.7.1).

Scheme 4.115. Cleavage of an acetal resin by Snieckus and coworkers [548].

4.6.4

Linkers for Alcohols, Phenols, Ethers, and Ketals 4.6.4.1 Linkers for Alcohols

Various linkers are suitable for the anchoring and release of alcohols; in particular, silyl ethers have been frequently used [20]. An early example of this type was provided by Farrall and Frechet [549]. Furthermore, these linkers are suitable for glycopeptides [550], oligosaccharides [20], and prostaglandins (511) [123] (Scheme 4.116). A direct loading of alcohols onto a silyl resin is possible using a hydridosilane [133], which might be better than the procedure described using silyl chloride [135]. A SEM (2-trimethylsilylethoxymethyl) linker is also suitable for the attachment and detachment of sterically encumbered alcohols. In this case, cleavage is conducted by tetrabutylammonium fluoride [21]. In addition to the silyl linkers, various ketal-based methods have been reported recently [23, 49, 69, 543, 551–553] (for a review, see [554]). The method of choice for various applications are the THP-type linkers [99,


Scheme 4.116. Synthesis of prostaglandin by Ellman and coworkers [123].

108–114] (Sect. 4.3.3), as demonstrated in the synthesis of indolactams on solid support [555] (Scheme 4.117). However, other acetal linkers [100, 106] (Sect. 4.3.3) (Scheme 4.118) have also been used in this context. In addition, alcohols can be attached to a Wang or trityl resin, and can be cleaved in turn by the action of mild acids [533]. Alternatively, the benzyl group can be attached to the solid support, and hydrogenolytic cleavage has been used to detach the molecules which are then usu-

Scheme 4.117. Synthesis of indolactam analogs according to Waldmann and coworkers [555].

Scheme 4.118. Synthesis of benzyl alcohols on solid support [100].

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ally left with an oxygen or nitrogen functionality (cleavage of CaO Bn and CaN bond respectively) [29, 447]. The polymers in these cases are formally immobilized Z or Cbz groups. Interestingly, TentaGel and polystyrene give similar yields under identical conditions. Benzylic linkers can also be used advantageously in the presence of other benzylic protecting groups, since they can be removed in the same step [29] (Scheme 4.119).

Scheme 4.119. Detachment of saccharides from polymeric benzyl-type protecting groups [29].

Finally, alcohols can be released from ester [117, 556], thioester [115], and amide resins using reductive methods (Sect. 4.4.2, Table 4.13). Furthermore, the reaction of Grignard reagents with thioesters has been reported to give tertiary alcohols [115]. Other anchoring groups are enzyme-labile linkers [285] and fluoridesensitive linkers [557]. Cyclic ethers (tetrahydrofurans) are accessible via an iodolactonization approach [558, 559]. 4.6.4.2 Linkers for Phenols

Similar to alcohols, phenols have been linked to anchors such as Wang [560], Rink [82], trityl [561], 2-chlorotrityl [562], and modified trityl [563] linkers. However, since these linkers are more acid stable, simple hydroxymethyl polystyrene can serve as a linker when cleaved by triflic acid [549, 557, 564, 565] (Scheme 4.120). A direct method for the synthesis of phenol acetates has been demonstrated with the triazene T1 linker. An in situ cleavage and acylation proceeds with good yields when cleaved by acetic acid/acetic anhydride (Scheme 4.121) [186]. 4.6.5

Linkers for Sulfur Compounds 4.6.5.1 Linkers for Thiols and Thioethers

Thiols can be synthesized using disulfides as remarkably stable linkers. The detachment proceeds in the presence of certain phosphines [232–235] or by exchange


Scheme 4.120. Synthesis of quinolones from flavilylium salts according to Sato et al. [564].

Scheme 4.121. Synthesis of phenol acetates (541) [186].

with thiols [566]. However, other suitable linkers are the Wang linker, cleavable by HF [33], Rink-type linkers [82, 567], thiocarbamates, cleavable by bases [568], dinitroaryl linker, cleavable by nucleophiles [229]; and others [569, 570]. Because thiols are prone to oxidation to give symmetrical disulfides, the latter were frequently found during cleavage if oxygen had not been strictly excluded. 4.6.5.2 Linkers for Sulfonamides

The synthesis of sulfonamides is similar to the strategies used for carboxamides. Therefore, primary sulfonamides were synthesized using, for example, the Rink resin (Scheme 4.122) [67]. The indole resin [85], the SASRIN linker [41, 44], and a reductively aminated acetophenone linker [54] are also suitable for the detachment for sulfonamides [24, 41, 67, 415, 571–574]. 4.6.5.3 Linkers for Sulfonic Acids

Aryl and alkyl sulfonic acids have been detached from both Wang resin and SASRIN-type resins under mild conditions (20% TFA in CH2 Cl2 ) after being at-

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Scheme 4.122. Detachment of sulfonamides from the Rink resin [67].

tached to a solid support using the corresponding benzyl alcohol resins and sulfonyl chlorides [575]. 4.6.5.4 Linkers for Sulfones and Sulfoxides

Only one example has been presented so far for the synthesis of sulfones [405]. Starting from Merrifield resin, attachment of enantiopure sulfoximines and subsequent aldol-type coupling gives access to highly substituted sulfoximines. Cleavage proceeds under oxidative conditions using meta-chloroperbenzoic acid (mCPBA) to give sulfones (Scheme 4.123) [405].

Scheme 4.123.

A linker for sulfones according to Gais and Hachtel [405].

4.6.5.5 Linkers for Sulfoximines

A linker for sulfoximines is the triazene T2 linker. The cleavage can be conducted under mild condition using 10% trimethylchlorosilane solution in dichloromethane with retention of configuration [466]. 4.6.6

Linkers for Hydrocarbons

Linkers for hydrocarbons are important tools in combinatorial chemistry for the synthesis of the lipophilic compounds required for modern drug research. As described above, access to hydrocarbons can proceed via the formation of either a CaC bond or a CaH bond. The latter strategy has been discussed in detail in Sect. 4.5.5 (traceless linkers), since the introduction of a hydrogen atom clearly is the prototype for this kind of linker.


4.6.6.1 Linkers for Alkanes

Alkanes have been synthesized on solid support, mostly by reduction of a CaX bond to a CaH functionality. This concept has been demonstrated using the selenium [165], sulfur [120], and stannane linkers. The formation of CaC bonds for the synthesis of alkanes has been described by Schiemann and Showalter [576]. Beginning with an aryl fragment attached to an immobilized benzotriazole, cleavage and subsequent CaC bond formation are achieved using organomagnesium compounds. 4.6.6.2 Linkers for Arenes and Heteroarenes

Alternatively, Caaryl bonds were formed using various cross-coupling methods, including stannanes [163, 169, 577], triazenes [187, 188] (Scheme 4.34), and boronates [148] as precursors. Naphthalene derivatives are accessible via an electrocyclic ring opening of benzocyclobutane derivatives and a subsequent Diels–Alder reaction with dienophiles [578]. Hydrogenolytic removal of substrates from the solid support is important as it cleaves the substrate to form a C–H bond at the former binding site of the polymer. These types of linkers are also called traceless linkers (Sect. 4.5.5) [183]. The detachment of substituted arylsulfonates in the presence of a reducing agent such as formic acid provides a traceless cleavage. In this case, it is important that the arene core is substituted with electron-withdrawing substituents to enhance the yields [152]. This approach has been described previously (without experimental details) quite early in a patent and includes the possible derivatization of the intermediate s-aryl palladium aryl complex [579]. 4.6.6.3 Linkers for Alkenes

Most of the linkers for alkenes are traceless linkers, such as those described in Sect. 4.5.5. Besides these, classical double-bond-forming reactions, such as the Wittig–Horner–Emmons [304, 305] or the Wittig reaction, can be used for the formation of CbC bonds [364]. Syntheses via metathesis (Sect. 4.4.4.2), for example the ring-closing metathesis of olefins [277, 580], have been used for the preparation of alkenes on solid support. In addition, multifunctional cleavage (Sect. 4.5.6) can be achieved using cross-metathesis. Allylic groups can be attached to solid support via a sulfone, which is prepared by lithiation of polystyrene and subsequent treatment with sulfur dioxides and an allylbromide. After modification on the bead, cleavage proceeds with the action of a Grignard reagent in the presence of copper iodide. This overall SN 2 0 alkylation provides a route to substituted alkenes [581] (Scheme 4.124). The b-elimination generating alkenes has been used in the chemistry of the sulfone and selenium linkers [164, 582, 583] (Scheme 4.22). Similarly, polymerbound 1-alkenylcyclobutylsulfones [274] (Scheme 4.33) or pentadienol carboxylates [273] (Scheme 4.32) were cleaved from a resin in the presence of suitable nucleophiles and palladium catalysts to give substituted cyclobutylidene derivatives or dienes, respectively.

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Scheme 4.124. Nucleophilic substitution of an allylic sulfone [581].

4.6.6.4 Linkers for Alkynes

Two linkers have been used for the detachment of alkynes from solid support. Gibson and coworkers [355] have described the immobilization of alkynes onto polymer-bound triphenyl phosphine via a dicobaltoctacarbonyl arm. The detachment was conducted using air as the final oxidant. Alkynes were obtained by the cleavage cross-coupling strategy of the T1 triazene resin (Scheme 4.26). In contrast to the Heck cleavage, these cleavage conditions give rise to di- and trimerization, thus making a chromatographic separation necessary [187] (Scheme 4.38). 4.6.7

Linkers for Aryl and Alkyl Halides

Aryl iodides have been synthesized by Moore et al. [179], starting from triazene resin by the action of methyl iodide (Sect. 4.14.3.8) (Scheme 4.125). Aryl iodides, bromides, and chlorides are also accessible from the triazene T1 linker using the corresponding trimethylsilyl halide (Scheme 4.126) [190].

The use of triazene anchoring groups in the synthesis of iodo arenes by Moore and coworkers [179].

Scheme 4.125.

The cleavage of triazene T2-linked primary amines with trimethylsilyl chloride, bromide, or iodide proceeds smoothly to give alkyl halides. This reaction proceeds presumably via the aliphatic diazonium ion. In some cases, a rearrangement was observed (Scheme 4.127). Arylbromides and -iodides are accessible from silicon or germanium-linked arene fragments (Sect. 4.3.5). The released can be conducted with either bromine/ pyridine [126, 131] or iodochloride [126, 138].


Scheme 4.126. Synthesis of aryl halides via the T1 triazene resin [190].

Scheme 4.127. Synthesis of alkyl halides via the triazene T2 linker [183, 196, 197].

The nucleophilic substitution of alkylsulfonates was used for the synthesis of alkyl iodides. Starting from the corresponding alcohols, attachment to a sulfonyl chloride and subsequent release from the bead was performed using sodium iodide [382, 465]. Allyl bromides can be released from a trityl linker if cleaved with hydrobromic acid in acetic acid [74]. Other methods start from with trityl linkers [74, 163, 382, 559]. 4.6.8

Linkers for Heterocycles

Various methods are applicable in the synthesis of heterocycles [361, 362, 384, 584]. The cyclofragmentation of a certain class of sulfones leads to 3-arylbenzofurans [160]. 4.6.9

Linkers for Reactive Intermediates

Reactive intermediates cleaved from a solid support can be used for a subsequent functionalization. Thus, radicals, carbanions, and carbocations might then react with additional building blocks. This multifunctional cleavage mode (Sect. 4.14.5.6) has, for example, been used with the stannane, selenium (Sect. 4.14.3.7), and triazene linkers (Sect. 4.14.3.8).

151

152


4.6.10

Linkers for Other Functional Groups 4.6.10.1 Linkers for Phosphonates

Phosphonates can be released from the resin using trimethylsilyl iodide (Scheme 4.128) [585]. They have been attached to a solid support using the Wang linker [586, 587, 588].

Scheme 4.128. Synthesis of phosphonates using

noncrosslinked polystyrene (NCPS) as support [585].

4.6.10.2 Linkers for Boronates

Diols are suitable anchors for boronic acids, as shown in the synthesis of hepatitis C virus proteinases [589]. 4.6.10.3 Linkers for Silanes and Silanols

Silanols are accessible from silyl ether linker [127]. The silicon–oxygen bond can be cleaved with TFA via a protio-ipsodesilylation.

4.7

Overview for Linkers for Functional Groups

Table 4.20 gives a short overview of the different linker families, as described in Chapter X.

4.8

Conclusion, Summary and Outlook

In recent years, various new types of linkers have emerged. The design of a new anchoring group can be essential for the success of a synthesis, especially for small molecules on a solid support. Linker, cleavage conditions, and functional groups are associated with each other. Therefore, the decision to use one specific linker type has to be balanced with the requirements of the library to be synthesized. Although the ‘‘perfect’’ or ‘‘universal’’ linker has not yet been developed, and will prove unattainable, interesting new developments increase the flexibility of solid-phase synthesis by traceless (Sect. 4.5.5) and multifunctional cleavage (Sect.

References Tab. 4.20. Short overview for various linker types.

Functional group

R3 N ROH R2 NCOR RH (traceless) (Sect. 4.5.5) RCO2 H Heterocycles BAL RX Safety-catch option Multifunctional cleavage Photo cleavage

Benzyltype linkers

Ketal/ acetal linkers

Esters/ amide linkers

p p p p p p p

p p

p p p p p

Silane linkers

Triazene linkers

Selenium/ sulfur/ stannyl linkers

p

p

p

p p

p

p

p p p

p

p

p

p

p

p

p p

p

4.5.6). While traceless linkers provide access to unsubstituted compounds with ‘‘no memory’’ of solid-phase synthesis, multifunctional cleavage allows the introduction of various new functionalities during cleavage from the resin. Backbone amide linkers present new opportunities for solid-phase synthesis of small amidic structures, and cyclization-release strategies provide an opportunity to create novel carbo- and heterocyclic structures upon cleavage. An anchor for traceless linking can also be a safety-catch linker (Sect. 4.5.1), or it can be suitable for multifunctional cleavage. Linker systems allow the introduction of certain atoms or molecule fragments and will play an important role in the development of diverse organic substance libraries. It is important to point out that the final diversification is achieved in the cleavage step and not in an additional solution-phase reaction step after the cleavage. However, only a few linker systems that are applicable to a wider range of substrates have been developed so far. As these linker systems offer the widest possibilities for the final diversification of a synthesized library, they will be the subject of increasing attention in the future.

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Encoding Technologies Thomas Kra¨mer, Valery V. Antonenko, Reza Mortezaei, Nicolay V. Kulikov 5.1

Introduction

With the application of combinatorial chemistry methods large collections of individual chemical entities (chemical libraries) have been synthesized that require some sort of organization to be used and handled efficiently. Conventional and convenient tracking methods such as writing codes on reaction flasks and vials with waterproof pens were no longer appropriate. Assuming that the solid- and solution-phase reactions used to synthesize a combinatorial library member are successful, then knowledge of the specific reaction sequence is equivalent to knowing the member’s chemical identity. Because the determination of chemical identity is typically not automatable and requires a substantial amount of material, schemes that encode a member’s reaction history onto the synthesis platform are of value [1, 2]. One of the most obvious methods for the encoding of chemical compounds is spatial (or positional) encoding. The structure of a compound or its chemical history is encoded by the position of the corresponding reaction vessel in a spatially fixed, two-dimensional matrix. However, there are a number of options for encoding a chemical structure: graphical encoding methods; the use of chemical tags consisting of peptides, oligonucleotides or aromatic compounds; spectrometric encoding; or radiofrequency encoding, which is the most advanced technique. Encoding is a prerequisite for the efficient handling of compound libraries and offers two major advantages: establishing a relational nomenclature and providing automated compound and data handling (if the tags were machine-readable). Readability of tags by technical devices is of major importance since manual handling of a large number of chemical entities is either very tedious or simply not possible. Post-synthesis encoding is successfully being applied in the compound repositories of major pharmaceutical companies where hundreds of thousands of archived test samples are stored in bar-coded vials. In this case the bar code maintains the relation between the vial and the corresponding compound data [structure, molecular weight (MW), amount, etc.] and offers robotic vial handling. HowHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

5.2 Chemical-encoding Methods

ever, for combinatorial library syntheses before or during synthesis encoding (i.e. encoding compound libraries at the time of their synthesis, rather than afterwards) is far more efficient. Again, it is the synthetic history of a compound that is encoded rather than its chemical identity. Furthermore, a major requirement of encoding during synthesis is that the tag must invariably be connected to the compound itself (like a protecting group) or to the polymer on which the compound is being synthesized. Consequently, the tags need to be chemically inert under the reaction conditions used to synthesize the library compounds, i.e. tag chemistry and library chemistry have to be orthogonal. Since some compounds are screened for biological activity in an on-bead screening assay (with the tag still attached), the tags should also not interfere with the biological properties of the library compound. Therefore, it is essential that tags have to be chemically and biologically inert. In the following sections a review of the existing methods for encoding combinatorial libraries will be given that divide encoding strategies into two categories: chemical encoding and non-chemical (physical) encoding.

5.2

Chemical Encoding Methods

One of the solutions to the structure elucidation problem utilizes a number of chemical tags that can unambiguously identify the chemical entities with which they are associated. The tags should be incorporated on the same bead on which the compound they encode is synthesized. Therefore, the tagging process should not interfere with the synthesis. The tags should not occupy much of the bead capacity. It should be possible to cleave the synthesized compounds from the bead selectively in the presence of the encoding elements. The decoding process should be quick and reliable, while the chemical nature of the tags should permit their rapid determination in small quantities using conventional analytical technologies. Time-consuming methods may defeat the purpose of the encoding. 5.2.1

Oligonucleotide Tags

In 1992, Brenner and Lerner [3] were the first to suggest in the literature the concept of chemical encoding when they proposed a method for producing an oligonucleotide-encoded peptide library. Addition of each amino acid to the polymeric bead is followed by the attachment of two preselected oligonucleotides to a different ‘‘tag site’’ on the same bead. Each base pair encodes one, and only one, amino acid. In the ‘‘split-and-pool’’ strategy, the oligonucleotide chain grows in parallel with the peptide chain; thus, each unique peptide sequence is encoded by a unique oligonucleotide. The decoding process starts by amplifying the encoding oligonucleotide using the polymerase chain reaction (PCR). To reveal the primary

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structure, the encoding oligonucleotide is sequenced, unambiguously unveiling the amino acid sequence of the satellite peptide. Oligonucleotide encoding was reported by Nielsen and colleagues [4], who used the construct presented in Scheme 5.1. a-Amino functionality of the serine residue was used as an attachment point for the growing peptide chain, while the b-hydroxyl of the side-chain was used for the encoding oligonucleotide assembly. Therefore, the tag-to-peptide ratio used in this work was 1:1. Synthesis was carried out on controlled porous glass (CPG) beads.

Scheme 5.1. Nielsen construct for oligonucleotide encoding [4].

Gallop and coworkers from the Affymax Research Institute used the oligonucleotide tagging strategy for the encoding of a library containing 823,543 heptapeptides [5]. The synthesis was carried out on 10-mm-diameter monodisperse beads. The beads were used in sufficient number to synthesize 200 copies of each sequence in the library. The beads were made of polystyrene crosslinked with divinylbenzene and derivated with a 1,12-diaminododecane linker. The capacity of the resin was 10 mmol g1 , corresponding to 10 fmol per bead. The resin was further functionalized with two chemical linkers: a mixture of 9-fluorenylmethoxycarbonyl (Fmoc)–Thr(t-Bu)–OBt and succinimidyl activated ester of 4-O-(dimethoxytrityl) oxobutyrate. The former is introduced to support the growing peptide chain; the latter to support the synthesis of the encoding oligonucleotide. The resulting construct contained a 20:1 ratio of peptides to oligonucleotides on the bead. The structure of the construct is shown in Scheme 5.2.

Scheme 5.2.

Affymax’s construct for oligonucleotide encoding [5].

The peptides were synthesized using Fmoc-protecting groups for a-amino functionalities and t-butyl-type protection for the side-chains of amino acids. Oligonucleotides were assembled using dimethoxytrityl-protected 3 0 -O-methyl-N,Ndiisopropyl phosphorimidates. To protect the encoded oligonucleotides from the depurination side reaction upon trifluoracetic acid (TFA)-mediated side-chain deprotection of t-butyl-type protecting groups, the susceptible 2 0 -deoxyguanosine (dG) unit was not used. 2 0 -Deoxyadenosine (dA) was substituted with 7-deaza-2 deoxyadenosine (c7dA), which is stable to TFA treatment. To make the encoding


compatible with the peptide synthesis, phosphorimidites carried 3 0 -O-methyl protection, which is, contrary to the b-cyanomethyl protective group, stable to piperidine used for Fmoc-deprotection. The library was built to construct 7 7 analogs of the C-terminal heptapeptide RQFKVVT of opioid peptide dynorphin B. Amino acids Arg, Gln, Phe, Lys, Val, Thr and d-Val were used in a seven-step ‘‘split-and-pool’’ synthesis. Each amino acid was encoded by a unique dinucleotide. Thus, each heptapeptide was synthesized with the corresponding sequence of 14 nucleotide bases. At the end of the synthesis, the encoding sequences were framed by the degenerate DNA PCR primer sequence (55 bases). The library was screened against the fluorescently labeled anti-dynorphin B antibody, D32.39, with the fully deprotected peptides attached to the beads. A sample containing a sufficient number of beads to represent all synthesized sequences with high statistical certainty was analyzed. The beads containing binding sequences became fluorescent. The top 0.17% of the beads with the most intense fluorescence was collected using a fluorescence-activated cell-sorting (FACS) instrument. The collected beads were subjected to PCR amplification and subsequent sequencing of the encoding oligonucleotides. The sequences of the binding peptides were revealed from their primary structures. Oligonucleotide tagging was developed and used successfully for the synthesis of peptides; many methodological issues, related mostly to the chemical compatibility of the approach, were solved. However, the compatibility of the encoding strategy with the synthesis of other classes of organic molecules remains limited. 5.2.2

Peptide Tags

The technology for automated Edman degradation of peptides to determine their sequence is well developed. Therefore, peptides can be used similarly to oligonucleotides as encoding molecules. Several groups have reported using peptide encoding [6, 7]. Unfortunately, applications of the peptide encoding are very limited because of severe restrictions imposed on the scope of chemical methods that are compatible with this approach. With the development of alternative encoding strategies, peptide encoding has very limited applications in the modern approaches to the synthesis of small organic molecules. 5.2.3

Haloaromatic Binary Coding

Several laboratories have worked on the development of robust encoding strategies that can be used for combinatorial synthesis of pharmacologically appealing chemical libraries. In 1993, Still and colleagues were the first to report binary encoding [8] using chromatographically resolvable haloaromatic reagents as tagging molecules. The tags are attached to the beads during library synthesis in a binary coding strategy intended to keep a molecular record of all chemical transformations to which the beads are subjected. The tags are incorporated via amide bond forma-

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Scheme 5.3. Haloaromatic tags on a photolinker [8].

tion as part of a construct with a photolabile linker (Scheme 5.3) at the expense of the ligand synthesis sites. Each compound in the library is encoded by a limited set of tags. The presence, as well as the absence, of each member of the set carries information about the specific encoded structure. In a binary code, a set of n tags can encode 2 n 1 different structures. For example, 20 tags can encode 20 20 1 ¼ 1,048,575 different library members. After being released from the beads by photolysis, the tags are detected by capillary gas chromatography using electron capture detection (ECGC), a detection method that is particularly sensitive to heavily chlorinated aromatics. The haloaromatic compounds were selected to ensure reliable and reproducible separation by ECGC, which is capable of detecting subpicomolar amounts of the tags. Consequently, the beads can be tagged at only 0.5– 1% of the resin loading (0.5–1 pmol per bead) without detectable interference with the library synthesis. Later, Still and coworkers modified the original strategy by developing a new type of tagging reagent TnC [9], presented in Scheme 5.4. A derivative of vanillic acid (3-methoxy-4-hydroxybenzoic acid) was chosen as a linker. Synthesis of the tagging reagent TnC begins with a Mitsunobu reaction of a tag alcohol Tn with methyl vanillate, followed by LiOH hydrolysis of the methyl ester. This produces free acid TnA, which is converted to acid chloride TnB. Excess of diazomethane converts acid chloride TnB into the tagging diazoketone TnC. In the presence of rhodium reagents Rh2 (OAc)4 or Rh2 (O2 CCF3 )4 TnC forms an acylcarbene, which rapidly and cleanly reacts with benzene, forming a derivative of cycloheptatriene Tn1. Benzene was used as a soluble analog of polystyrene resin. Derivatives of TnC containing different numbers of methylene groups n have been prepared. The researchers found that diazoketones TnC are stable solids and can be stored at room temperature for months. They react easily with polystyrene resin, providing a means for binary encoding. At the decoding stage, the tags are oxidatively cleaved by ceric ammonium nitrate and analyzed by ECGC. The acylcarbene tagging strategy does not require any specific functional group for tag attachment, and the tags and linkers are generally compatible with a wide


Scheme 5.4.

Acylcarbene-generating tagging reagents [9].

range of chemical reactions. The acylcarbenes can unselectively add to some of the synthesized compounds. However, the bulk of the library is represented by the polymeric support, which accepts the major portion of the tag molecules. Because tags are added at molar levels corresponding to 1% or less of the library members, interference with the synthesis is minimal [10]. 5.2.4

Secondary Amine Binary Coding

In 1996, Gallop and coworkers developed another robust encoding strategy [11, 12], based on secondary amine tags, which are incorporated into a polyamide backbone. The secondary amine binary coding scheme utilizes an amine-based polymeric resin that is differentially functionalized with sites for both ligand synthesis and tag addition (Scheme 5.5). The ligand synthesis site is derivatized with a NFmoc-protected photocleavable linker group [13]. This linker allows for the release of the ligand from the resin by exposure to ultraviolet (UV) light. The amino group of the tag site is protected with orthogonal to the Fmoc group functionalities (e.g. Boc or Alloc). The tag site occupies only 10% of the total number of amino groups on the resin. Each 130-mm-diameter bead of the TentaGel S resin, which is recommended by the authors of this methodology, contains about 300 pmol of amino groups. Therefore, the theoretical yield of the ligand is 270 pmol per bead. Such

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Scheme 5.5. Resin construct for secondary amine binary coding [11, 12].

quantities being photo-released in 100 ml of a solvent would be adequate for the concentration of the ligand in most biological assays. The tags are a set of relatively hydrophobic amines, such as HN(Et)(Bu), HNMe(C6 H13 ), HNBu2 , HNMe(C7 H15 ), and HN[CH2 CH(Et)C4 H9 ]2 . The set is selected to ensure reliable separation of the dansyl derivatives of the amines by reversed-phase high-performance liquid chromatography (HPLC). The tagging monomer units are synthesized by reaction of an N-protected iminodiacetic anhydride with a secondary amine from the set (Scheme 5.6).

Scheme 5.6. Preparation of tag monomers for secondary amine coding.

The resulting N-protected N-[(dialkylcarbamoyl)methyl]glycines are assembled into binary mixtures, which are incorporated into the tag sites of the resin beads by using HATU or other peptide-coupling reagents (Scheme 5.7). Addition of each new building block at the ligand synthesis site in the course of a ‘‘split-and-pool’’ combinatorial synthesis is accompanied by the incorporation of the preselected mixture of the monomer units at the tag addition site. Selection of the protecting groups allows for the addition of the tags either before or after the addition of the building block to the ligand. The ability to choose different protecting groups for the N-protected tag monomers helps to resolve potential chemical compatibility issues.


Scheme 5.7. Coupling of secondary amine tag constructs to solid support.

Upon completion of the library synthesis, each bead is distributed into a separate well of a microtiter plate. The ligands are released from the beads into the assay medium by exposure to UV light at 365 nm. The tag residues remain covalently attached to the beads. After screening, beads from the wells containing active compounds are collected for decoding. The decoding process is shown schematically in Scheme 5.8, and begins with the acid hydrolysis of the beads in 6 N hydrochloric acid. Under these conditions all amide bonds are hydrolyzed, releasing free secondary amine tags into the solution. After evaporation of the HCl, the amines are converted into the corresponding dansyl derivatives by treatment with dansyl chloride. Analysis of the resulting mixture of dansylated tags is carried out

Scheme 5.8.

The decoding process for secondary amine binary coding.

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by reversed-phase HPLC on a microbore column. Fluorescence detection allows for the reliable analysis of 20–30 fmol of a dansylated tag. Only 2–5% of the entire dansylated hydrolyzate from a single bead is sufficient to obtain unambiguous results. The secondary amine encoding method was used in the synthesis of a library of pyrrolidine-based inhibitors of angiotensin converting enzyme (ACE) via [2 þ 3] cycloaddition [14]. 5.2.5

Mass Encoding

As mentioned previously, conventional methods of characterization of chemical compounds cannot be applied to most combinatorial technologies owing to the insufficient quantities of the analytes. One fortunate exception is mass spectroscopy. Matrix-assisted laser desorption and electrospray ionization mass spectroscopy (MS) have been used [15, 16] to identify compounds synthesized on a single bead. For beads with diameters larger than 100 mm, a mass spectrum can be acquired in less than 1 min. In 1996, Geysen and colleagues from Glaxo-Wellcome proposed a novel encoding method [17], which takes advantage of modern MS techniques. In general, when recording the chemical history of a compound synthesized on a bead mass encoding incorporates stable isotopes that give distinct isotopic patterns in mass spectra. In one of the proposed strategies, the resin is derivatized with a linker (Scheme 5.9) to which an MS code is attached. The mass of the coding block is designed to appear in a convenient region of the mass spectrum. As an example, the code can be a dipeptide built from the combinations of the natural amino acids glycine (Gly) and alanine (Ala), and their 13 C-labeled derivatives: [ 13 C]Gly, [ 13 C]2 Gly, [ 13 C]Ala, and [ 13 C]4 Ala. As an example, the authors present mass spectra recorded in the 295–330 MW range for the following ten mass codes: Gly–Gly, Gly–[ 13 C]Gly, [ 13 C]Gly–[ 13 C]Gly, Gly–[ 13 C]2 Gly, Gly–Ala, Gly–[ 13 C]Ala, [ 13 C]2 Gly–Ala, [ 13 C]2 Gly–[ 13 C]Ala, and[ 13 C]2 Gly–[ 13 C]4 Ala. After the code (Scheme 5.10), an additional linker is introduced; this should be orthogonal to the first one and is used for the release of the synthesized compound ABC. In a ‘‘split-and-pool’’ synthesis, the code defines the identity of the first building block A. The third building block C can be known from the final pool in which its addition was carried out. The identity of the building block B can be calculated from the molecular weight of the ligand and the molecular weight of monomers A and C. For unambiguous results, the set of building blocks used at the second step of the library synthesis should not contain any compounds with the same mass as building block B.

Scheme 5.9. Resin construct for mass encoding.


Scheme 5.10. Principle of light-directed, spatially addressable synthesis.

The code block can also consist of equimolar mixtures built from a set of isotopically labeled dipeptides: Gly–Gly, Gly–[ 13 C]Gly, Gly–[ 13 C]2 Gly, [ 13 C]Gly–[ 13 C]Gly, [ 13 C]Gly–[ 13 C]2 Gly, and [ 13 C]Gly–[ 13 C]2 Gly. These dipeptides incorporate equal to their content in the mixture, because isotopic labeling does not affect chemical reactivity. The resulting mass spectra of these mixture codes serve as ‘‘bar codes’’. In the ratio-encoding strategy, the encoded information is derived from just two peaks in the mass spectrum. A mixture of a reagent, common to all members of the library, is prepared from different ratios of the isotopic isomers of the reagent. The resulting mass spectrum will reveal two distinct peaks corresponding

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to each of the isomers. The relative heights of the peaks provides the encoded information. The mass encoding strategy was used by Wagner and coworkers [18] for the synthesis of a peptoid library. A set of 20 14 N: 15 N ratio-encoded imidazoles was synthesized ‘‘to investigate pharmacokinetic applications of isotopic labeling’’.

5.3

Non-Chemical Encoding Methods

Chemical methods of encoding combinatorial libraries can be used for synthesis on very small elements of solid supports (i.e. 10-mm-diameter beads). However, none of the procedures developed so far is totally compatible with all chemical transformations that might be necessary for the implementation of successful drug discovery programs. In principle, physical methods of encoding are fully compatible. Unfortunately, none of the conventionally used labeling techniques (bar codes, labels, etc.) is small enough to be used with resin beads. It is often necessary to obtain compounds via combinatorial chemistry approaches in substantially larger quantities than the single-bead approach is capable of. As an alternative, macroscopic pieces of a solid support can be used, the larger physical dimensions allowing for the application of more space-demanding encoding strategies. 5.3.1

Positional Encoding

Positional, or spatial, encoding directly links the reaction history of a compound to the particular location of the corresponding synthesis compartment (reactor) in a spatially arrayed, fixed matrix of reactors, i.e. the 8 12 two-dimensional (2D) matrix in a 96-well plate or special chip formats. 5.3.1.1 Light-directed Synthesis

In 1991, Fodor and colleagues combined photolithography, photochemistry, and solid-phase synthesis in a new technology [19–22] called light-directed, spatially addressable parallel synthesis or VLSIPS (very-large-scale immobilized polymer synthesis). The principal points of the technology are illustrated in Scheme 5.10. The synthesis occurs on a flat glass surface modified with an appropriate linker (e.g. 3-aminopropyl-triethoxysilane) to allow for the covalent attachment of protected amino acids. The entire synthesis area of the slide is derivatized with a photolabile protecting group (PG). At the first step of the synthesis, selected sites of the synthesis area (typically three squares per slide, 1.28 cm 1.28 cm each) are exposed to UV light through photolithographic mask A. The variety of patterns available for photolithography is essentially unlimited. The exposure to light causes removal of the photolabile groups, thus elaborating amino functionality. At the next step, the synthesis area is treated with the reagents necessary for the elongation of the peptide chain. Only the sites that were previously photodeprotected will

5.3 Nonchemical Encoding Methods

participate in a coupling reaction; the rest of the synthesis area remains protected and intact. Synthesis continues by illuminating another part of the surface through photolithographic mask B, followed by the next chain elongation reaction. By repeating the photodeprotection and coupling steps, highly dense arrays, each consisting of thousands of peptides, can by synthesized. Importantly, the primary structure of each peptide in the array is sufficiently defined by the sequence of coupling and photolysis steps, and by photolithography mask patterns. Therefore, the structure can be easily deduced from the (x, y) coordinates of the peptide on the slide. This eliminates the need for encoding–decoding procedures required by some other combinatorial technologies. After completion of the synthesis, the synthesis area is exposed to reagents necessary for the elimination of side-chain protecting groups. To assess the binding properties of all synthesized peptides, the entire array is incubated with a fluorescently labeled target molecule and scanned using a stage-scanning confocal fluorescence microscope. Sites, containing peptides that bind to the target, become fluorescent. Affinity data on all peptides in the entire array are obtained in one step. The consumption of chemical reagents required for the synthesis of thousands of peptides composing the array, together with the biological reagents necessary for bioassay, is very small, because the capacity of the flat glass surface is only 5– 20 pmol cm2 . Biological reagents used in this technology are recoverable and can be reused. Moreover, after performing an assay with one target molecule, the bound target can be easily dissociated from the array (e.g. by treating it with 6 M guanidine hydrochloride), making the array available for subsequent screening with other targets. These arrays are reusable for at least 6 months. With special (orthogonal) masking strategies the number of synthetic regions on the glass surface can be increased until the limit of photolithographic resolution is achieved (10–20 mm). With this resolution, 250,000–1,000,000 compounds can be synthesized in 1 cm 2 . Routinely, 50-mm resolution is practiced and allows for the production of 40,000 compounds in the same area. Light-directed, spatially addressable synthesis is a powerful technology for generating chemical diversity. Unfortunately, the technique is limited to peptides, oligonucleotides and other linear oligomeric structures.

5.3.1.2 Microtiter Plate-based Positional Encoding

The 96-well microtiter format has been a common platform for high throughput screening (HTS) for many years, and also offers many advantages as a platform for parallel synthesis. There are multiple liquid-handling devices (multichannel pipettes, robotic systems, etc.) that significantly simplify and accelerate the process of the reagent delivery to the wells on the plate. Moreover, all 96 compounds can be handled as one synthesis entity. Their structures are spatially encoded by their location on the plate. Contemporary analytical instrumentation, such as mass spectrometers and HPLC systems, can also easily handle compounds in the 96-well microtiter format. Combined with the commonly used 96-well-based systems for screening, this eliminates the need for time-consuming redistributing and relabeling of a large number of synthesized compounds for analytical and biological characterization.

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Synthesis on multipins In 1984, Geysen and coworkers [23] introduced an alternative to the synthesis on polymeric beads. As a solid support, they proposed the use of ‘‘pins’’; these are reusable polyethylene rods, which have a diameter of 4 mm and a length of 40 mm, and are grafted with polyacrylic acid. The pins are attached to a supporting block arranged in an 8 12 array to fit into a 96-well microtiter plate. The 96-well plate is used as 96 separate reaction vessels. Each well can be used for the attachment of a different building block. Washing and deprotection steps can be carried out in a separate common reaction vessel. After the synthesis is completed and side-chain protecting groups are removed, peptides or small organic molecules can be assayed while attached to the pins [24, 25]. If a cleavable linker was used, individual peptides are separated from the solid support in a 96-well microtiter plate containing appropriate reagents [26], or by exposing the array to vapor-phase ammonolysis [27, 28]. Originally, the capacity of each pin was 10–100 nmol. Later, a much greater variety of multipins of different capacities with a wide selection of linkers became commercially available from Chiron Technologies Pty. Ltd. In the latest development, the pins can be attached to the radiofrequency tags to allow for encoding in the ‘‘split-and-pool’’ mode. HiTOPS system Over time, original approaches have been developed by many companies to perform the synthesis of organic compounds on the footprint of a 96-well plate. The HiTOPS (high-throughput organic parallel synthesis) system [29] (Scheme 5.11) utilizes a variety of 96-deep-well filtration microtiter plates available from Polyfiltronics/Whatman. The plates are made of polypropylene and other polymers, and are available with a selection of different filters. The volume of each well is 2 mL and allows the use of up to 50 mg of resin. For larger scale syntheses, several wells or even the entire row or the entire column can be used for the preparation of the same compound. Reactants are retained in wells by the positive pressure of an inert gas.

Scheme 5.11. HiTOPS system. Synthesis device (left) and cleavage device (right).


5.3.2

Non-Positional Encoding 5.3.2.1 Tea-Bag Approach

In 1985, Houghten [30] reported on peptide synthesis carried out on a resin sealed in porous polypropylene packets. The pore size of the polypropylene mesh (74 mm) allows the free access of chemicals to and from the contained resin. Each packet or ‘‘tea-bag’’ can be individually labeled to identify the peptide synthesized on the entrapped polymer. Many ‘‘tea-bags’’ can be combined in the same reaction to carry out common synthetic steps, such as washing and deprotection. The packets are sorted in separate reaction vessels according to the specific amino acid that will be coupled next. Cleavage, depending on the amount of used resin, can be carried out in separate vessels or in a 96-well microtiter plate with a 2-mL well volume. In the original paper, 248 different tridecapeptides were synthesized in 10- to 20-mg quantities and characterized in less than 4 weeks. The ‘‘tea-bag’’ method is very practical as it does not require any special tools, except a sealing device to make the ‘‘tea-bags’’. A number of mesh materials of different porosity are available form Spectrum Medical Industries. Almost all commercially available resins can be used, as long as the size of the beads is larger than the mesh size; in addition, the mesh should be stable to reaction conditions. The scale of the synthesis is easy to control by the size of the packet chosen to contain the desired amount of a polymer support. Multiple synthesis can be carried out manually or on a synthesizer. The ‘‘tea-bags’’ can be mechanically labeled, or radiofrequency tags can be used for this purpose. 5.3.2.2 Cellulose and Laminar Supports

Similarly to the ‘‘tea-bag’’ approach, pieces of any solid-phase support suitable for solid-phase synthesis can be used for parallel peptide preparation. One of the first materials used in this manner was paper. Frank used paper disks [31, 32] (Whatman 3MM, 1.5 cm diameter) packed into columns of a multicolumn continuous flow synthesizer. Prior to the synthesis, the paper was derivatized with palkoxybenzyl linkage to allow for the cleavage with trifluoroacetic acid. The disks can easily be labeled with a pencil, sorted, and combined depending on the common amino acid to be coupled at the next step. Cotton has mechanical properties superior to those of paper, and was used in a similar way by Lebl and colleagues [33, 34]. Researchers from the Rockefeller University used polystyrene-grafted polyethylene film [35, 36], whereas scientists from Pfizer used material prepared from two sheets of polypropylene mesh with resin sandwiched in between [37]. The ‘‘sandwich’’ was fused together with a polymer with a low melting point. 5.3.2.3 Radiofrequency Tags

One of the most successful automated solutions for the synthesis of encoded chemical libraries is utilizing electronic identification devices. Scheme 5.12 illustrates one such device – the radiofrequency (RF) identification (ID) tag. Similar

183

184


Scheme 5.12. Schematic of an RF memory chip useful for encoded synthesis.

devices have been used for years to tag laboratory mice subcutaneously, and more recently these have been used to provide security for automobile ignition keys, to secure building entrance identification, and for a variety of other functions. The application of RF tags to combinatorial syntheses was reported in 1995 by two groups, one working at IRORI [38] and the other at Ontogen [39]. The RF ID tag is about the size of a ‘‘flea’’ stirbar and is encased in a thick-walled glass shell. As shown in Scheme 5.12, the essential components are an antenna (the largest component) and a microelectronic chip. Each chip has a unique, nonvolatile 40-bit ID code laser-etched into it. With 40 bits available, a total of 2 40 (over 1 trillion) unique ID codes is possible. The virtually inexhaustible range allows one to guarantee the uniqueness of all present and future RF tag ID codes. Additional bits are used to perform extensive digital error detection, which prevents incorrect reporting. (Bar codes are a more common graphical embodiment of encoding, but of course one that is substantially less information-rich.) A transceiver controlled by a computer is used to interrogate and receive the ID code of each RF tag. The transceiver antenna transmits a specially modulated, 125kHz electromagnetic field. This field is of very low energy and is not harmful. When an RF tag is held within about 1 cm from the transceiver’s antenna, energy is picked up by the RF tag’s antenna. A rectifier in the chip converts this energy to microwatt levels of direct current (DC) power, which is enough to power-up the logic circuitry on the chip. In a very real sense, the RF tag is similar to a crystal radio (which does not require an external power source), except that the device serves as both receiver and transmitter. It is ‘‘self-contained’’, in that the chip used no internal batteries and has no external metallic connections. A synchronization signal modulated onto the transceiver’s signal allows the chip to respond with its ID code (a serial sequence of ID bits) and error-checking bits. The time elapsed between placing the chip on the transceiver and seeing the ID code on the interfaced computer screen is about 0.5 s. If one could physically associate this chip with a compound undergoing syn-


thetic transformations, then the ID code on the chip would permit one to pick up a sample at any point of the combinatorial synthesis and know which reaction(s) it had already been through. Of course, just knowing the reaction history of a sample is not equivalent to knowing the chemical concept of a sample, but it is much better than nothing. If each reactor (‘‘compound-carrying unit’’) must be present in a series of reaction flasks containing lots of other reactors, then having them tagged makes it possible to put them into the right flask for each step of the synthesis. Moreover, if that tag can be read easily, then that movement of reactors can be automated. Electronic tagging permits all of these benefits. How this works in practice is detailed as follows. After a compound has been identified for which several hundred to several thousand derivatives would be of value, a synthetic route is chosen that: (i) permits linkage to a solid-phase support; (ii) utilizes reaction steps that appear possible to optimize to b90% yield; and (iii) affords reagents in each step for which desirable variants can be purchased (or, less optimally, can be made trivially). In the synthesis itself, one of the significant advantages of the ‘‘microreactor’’ approach becomes evident: one can use standard laboratory glassware and equipment to accomplish the library synthesis. There is no need for the automation of liquid-handling steps, and indeed there is no need for automation at all until rather large libraries are desired. IRORI offers several miniature reactors – MicroKans9 and MicroTubes9 (Scheme 5.13). MicroKans9 is a small cylindrical container with mesh walls; the internal volume of the container is 330 ml. In addition to the RF tag, the container hold up to 30 mg of any commercially available resin.

Scheme 5.13. MicroKans9 and MicroTubes9.

Manual sorting of microreactors can become tedious when a large number (more than 1000–2000) of them is used. IRORI designed the AutoSort TM – 10-K microreactor sorting system (Scheme 5.14) to solve this problem. The system accommodates up to 10,000 microreactors that can be distributed between chemical steps into 48 different containers. The distribution rate is 1000 microreactors per hour. The device is also very useful for sorting MicroKrans9 and MicroTubes9 into microreactor carriers for further cleavage, which takes place on the Accu-Cleave-96

185

186


Scheme 5.14. IRORI AutoSort TM – 10-K microreactor sorting system.

cleavage station. Twelve 8 12 microreactor carriers can be used simultaneously on the sorting system. One of the first applications of the RF tagging technology was demonstrated by the synthesis of a 432-member library (18 8 3 array) of tyrphostins (Scheme 5.15) [40]. 5.3.2.4 Laser Encoding

Another example of encoding that does not interfere with chemistry was suggested in 1997 by Xiao et al. [41]. The coding structure is an inert ceramic plate with a two-dimensional, laser-etched bar code. The encoded plate, which is a 3 mm 3 mm square, is placed in the center of a laser optical synthesis chip (LOSC). The chip is a 1 cm 1 cm square made of polypropylene grafted with polystyrene. The smallest possible size of the encoding ceramic plate is 0.5 mm 0.5 mm. Unfortunately, the bar code cannot be modified during the course of the synthesis.

5.4

Conclusion

The development of various encoding methods for combinatorial chemistry played a crucial role in the chemist’s ability to synthesize large numbers of organic mol-

References

Scheme 5.15. A combinatorial synthesis of 432 tyrphostins (prepared using 29 reaction vessels) [40].

ecules rapidly in distinct formats. If the focus in library synthesis is on single compounds, milligram quantities, automated handling, flexibility and efficiency radiofrequency encoding being broadly applied in pharmaceutical companies is considered to be the most promising technology.

References 1 a) V. V. Antonenko, N. V. Kulikov,

R. Mortezaei, F. Gualtieri (eds) Methods and Priniples in Medicinal Chemistry: New Trends in Medicinal Chemistry, Wiley-VCH, Weinheim 2000, pp. 39–80; b) A. W. Czarnik, F. Gualtieri (eds) Methods and Priniples in Medicinal Chemistry: New Trends in Medicinal Chemistry, Wiley-VCH, Weinheim 2000, pp. 81–96. 2 A. W. Czarnik, Curr. Opin. Chem. Biol. 1997, 1, 60–66.

3 S. Brenner, R. A. Lerner, Proc. Natl.

Acad. Sci USA 1992, 89, 5381–5383. 4 J. Nielsen, S. Brenner, K. D. Janda,

J. Am. Chem. Soc. 1993, 115, 9812– 9813. 5 M. C. Needels, D. G. Jones, E. H. Tate, G. L. Heinkel, L. M. Kochersperger, W. J. Dower, R. W. Barrett, M. A. Gallop, Proc. Natl. Acad. Sci. USA 1993, 90, 10700–10704. 6 V. Nikolaev, A. Stierandova, V. Krchnak, B. Seligmann, K. S. Lam,

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S. E. Salmon, M. Lebl, Pept. Res. 1993, 6, 161–170. J. M. Kerr, S. C. Banville, R. N. Zuckermann, J. Am. Chem. Soc. 1993, 115, 2529–2531. M. H. J. Ohlmeyer, R. N. Swanson, L. W. Dillard, J. C. Reader, G. Asouline, R. Kobayashi, M. Wigler, W. C. Still, Proc. Natl. Sci. USA 1993, 90, 10922–10926. H. P. Nestler, P. A. Bartlett, W. C. Still, J. Org. Chem. 1994, 59, 4723– 4724. W. C. Still, Acc. Chem. Res. 1996, 29, 155–163. Z.-J. Ni, D. Maclean, C. P. Holmes, M. M. Murphy, B. Ruhland, J. W. Jacobs, E. M. Gordon, M. M. Gallop, J. Med. Chem. 1996, 39, 1601–1608. Z.-J. Ni, D. Maclean, C. P. Holmes, M. M. Gallop in: Methods in Enzymology, vol. 267, Combinatorial Chemistry. Abelson, J. N. (ed.). Academic Press, San Diego 1996, pp. 261–272. C. P. Holmes, D. G. Jones, J. Org. Chem. 1995, 60, 2318–2319. D. Maclean, J. R. Schullek, M. M. Murphy, Z.-J. Ni, E. M. Gordon, M. A. Gallop, Proc. Natl. Acad. Sci. USA 1997, 94, 2805–2810. M. Stankova, O. Issakova, N. F. Sepetov, V. Krchnak, K. S. Lam, M. Lebl, Drug. Dev. Res. 1994, 33, 146– 156. R. A. Zambias, D. A. Boulton, P. R. Griffin, Tetrahedron Lett. 1994, 35, 4283–4286. H. M. Geysen, C. D. Wagner, W. M. Bodnar, C. J. Markworth, G. J. Parke, F. J. Schoenen, D. S. Wagner, D. S. Kinder, Chem. Biol. 1996, 3, 679–688. D. S. Wagner, C. J. Markworth, C. D. Wagner, F. J. Schoenen, C. E. Rewerts, B. K. Kay, H. M. Geysen, Comb. Chem. High Throughput Screening 1998, 1, 143–153. S. P. A. Fodor, L. J. Read, M. C. Pirrung, L. Stryer, A. T. Lu, D. Solas, Science 1991, 251, 767–773. M. C. Pirrung, J. L. Read, S. P. A.

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Fodor, L. Stryer, US Patent No. 5,143,854, 1992. J. W. Jacobs, S. P. A. Fodor, Trends. Biotechnol. 1994, 12, 19–26. M. C. Pirrung, Chem. Rev. 1997, 97, 473–488. H. M. Geysen, R. H. Meloen, S. J. Barteling, Proc. Natl. Acad. Sci. USA 1984, 81, 3998–4002. H. M. Geysen, S. J. Rodda, T. J. Mason, Mol. Immunol. 1986, 23, 709– 715. H. M. Geysen, S. J. Rodda, T. J. Mason, G. Tribbick, P. G. Scoofs, J. Immunol. Methods 1987, 102, 259– 274. A. M. Bray, N. J. Maeji, R. M. Valerio, R. A. Campbell, H. M. Geysen, J. Org. Chem. 1991, 56, 6659–6666. A. M. Bray, N. J. Maeji, A. G. Jhingran, R. M. Valerio, Tetrahedron Lett. 1991, 32, 6163–6166. A. M. Bray, A. G. Jhingran, R. M. Valerio, N. J. Maeji, J. Org. Chem. 1994, 59, 2197–2203. V. V. Antonenko in: Combinatorial Chemistry and Combinatorial Technologies Methods and Applications. Miertus, S., Fassina, G. (eds), Marcel Dekker 1998, pp. 205–232. R. A. Houghten, Proc. Natl. Acad. Sci. USA 1985, 82, 5131–5135. R. Frank, R. Doring, Tetrahedron 1988, 44, 6031–6040. B. Blankenmayer-Menge, R. Frank, Tetrahedron Lett, 1988, 29, 5871– 5874. M. Lebl, V. Gut, J. Eichler in: Peptides 1990. Giralt, E., Andreu, D. (eds), ESCOM, Leiden 1991, pp. 1059–1060. M. Lebl, J. Eichler, Peptide Res. 1989, 2, 297–300. R. H. Berg, K. Almdal, W. Watsberg Pedersen, A. Holm, J. P. Tam, R. B. Merrifield, J. Am. Chem. Soc. 1989, 111, 8024–8026. R. H. Berg, K. Almdal, W. Watsberg Pedersen, A. Holm, J. P. Tam, R. B. Merrifield in: Innovation and Perspectives in Solid Phase Synthesis:

References Peptides, Polypeptides and Oligonucleotides, Macro-Organic Reagents and Catalysts-1990. Epton, R. (ed.), SPCC, Birmingham 1990, pp. 453– 459. 37 N. K. Terrett, M. Gardner, D. W. Gordon, R. J. Kobylecki, J. Steele, Chem.-Eur. J. 1997, 3, 1917–1920. 38 K. C. Nicolaou, X. Y. Xiao, Z. Parandoosh, A. Senyei, M. Nova,

Angew. Chem. Int. Ed. 1995, 3, 2289– 2291. 39 E. J. Moran, S. Sarshar, J. F. Cargill, M. M. Shabaz, A. Lio, A. M. M. Mjalli, R. W. Armstrong, J. Am. Chem. Soc. 1995, 117, 10787–10788. 40 T. Czarnik, M. Nova, Chem. Britain 1997, 33, 39–41. 41 X. Y. Xiao, C. Zhao, H. Potash, M. P. Nova, Angew. Chem. Int. Ed. Engl. 1997, 36, 780–782.

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Instrumentation for Combinatorial Chemistry Marcus Bauser and Hubertus Stakemeier 6.1

Automation in Combinatorial Synthesis 6.1.1

General Remarks

Economic pressure to speed up the drug discovery process has had a huge impact on all fields of medicinal chemistry [1], therefore automation has increasingly become one of the main strategies to fulfill this demand [2]. While automation was successful in high-throughput screening (HTS) [3] and peptide synthesis, it played a minor role in mainstream organic synthesis. Automated systems are recommended for procedures that are highly predictable and repetitive [4]. However, organic chemistry is seldom defined in this way. The successful synthesis of organic molecules depends strongly on the chemical properties of the reagents and reactants. Within the library production process solubility and reactivity of synthons can be highly different, therefore it is very difficult to find one protocol that works for every building block. Since multiple parallel synthesis began, there has been a wide range of different approaches and concepts for the design of automated systems to overcome these problems [5–7]. The first attempts at automation were the simple parallelization of commercially available reaction vessels. Secondly, reaction blocks were designed and used in combination with existing liquid-handling systems. Another approach uses stand alone systems that mimic the typical action of a chemist. Today, modern automated systems are modular workstation approaches. An overview is given in Table 6.1. 6.1.2

Fully Automated Systems for Solid- and Solution-phase Synthesis 6.1.2.1 Robot-arm-based Systems

The main advantage of fully automated robot-arm-based systems is that they offer customized solutions similar to HTS equipment [8]. The individual user needs to define the architecture, the layout, and the processes of the robotic system. Within Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

Filtration Evaporation in rv Agitation Temperature range Pressure Inert atmosphere

Reaction procedures Reflux Bottom – Magnetic levitation stirring 80 to 160 C – In rv (rv ¼ reaction vessel)

þ

Polypropylene or glass syringes with frit or customized blocks

20/5 ml

Reaction vessels Number/volume

Material

20 (4 5) Up to 5 RBs Fixed during synthesis

Reaction block (RB) Array Number of possible RBs Type

Unit Cylindric arm Solid phase Solution phase

Accelab Acrosyn98

Bottom – Orbital shaking 70 to 150 C – In rv

þ (condenser module)

Solid phase: 96/3.5 ml; 40/9 ml; 16/15 ml; 8/30 ml Solution phase: 96/6 ml; 40/14.5 ml Multiple-well teflon reactor block with frits for solid phase

Monomer rack 8-, 16-, 40-, 96-well reactor Up to 4 RBs (Omega 384) Fixed during synthesis

Unit Two multiprobe XYZ arms Solid phase Solution phase

ACT BenchMark Omega Series

Automated systems for solid- and solution-phase chemistry.

Automation Type Arm Chemistry

Tab. 6.1.

Completely sealed reaction block Bottom – Orbital shaking 70 to 150 C Up to 150 psi In rv

Teflon block with fritted wells

384/0.5 ml; 96/2 ml; 40/6 ml

Ares reactor 40, 96, 384 1 Fixed during synthesis


ACT Vantage

Up to 150 psi In rv

–

Completely sealed reaction block

Teflon block with fritted wells

384/0.5 ml; 96/2 ml; 40/6 ml

40, 96, 384 Multiple Fixed during synthesis


ACT Venture

6.1 Automation in Combinatorial Synthesis 191

Number of possible RBs Type

Reaction block (RB) Array (wells/vials)


Special features

Calypso Reaction Block Microplate footprint e.g. 96 (8 12) Up to 4 RBs Fixed during synthesis

Trident Reaction Cassette TM 48 (6 8) Up to 4 RBs Flexible during synthesis

Unit Two XYZ arms Solid phase Solution phase

Modular XYZ arm Solid phase Solution phase

Charybdis Technologies Illiad PS 2

7 Fixed during synthesis

16 (8 2)

Unit XYZ arm (Gilson 222) Solid phase Solution phase

Chemspeed ASW200

–

Argonaut Trident

–

– –

ACT Vantage

. Heating above boiling point is possible . Multichannel solvent/ reagent delivery . Fully enclosed

HPLC upgradable

Analysis

– –

ACT BenchMark Omega Series

. Fully enclosed robot-arm- . Automatic on-board based system (SCARA) cleavage of all products . Multiple segregation of with balance, vortexer, reactor waste for hazard vacuum centrifuge . Flexible control software classification or reagent recovery . Fully enclosed

þ þ

Accelab Acrosyn98

Work-up Liquid–liquid extraction Solid-phase extraction


– –

–

Solid phase

Modular

Irori

–

. Conductometric detection of phase boundry . Up to 10,000 parallel reactions . Fully enclosed

þ –

ACT Venture

192

6 Instrumentation for Combinatorial Chemistry

.

and format

. Modular bench layout

. Fully enclosed . Temperature and

Special features agitation speed flexible for each reaction cassette Rvs sealed with rotatable Teflon valve

–

–

–

Bottom – Orbital shaking – – In rv

– (Sealed vessels)

Multiple-well teflon reactor block with frits

6/50 ml; 12/25 ml; 24/10 ml; 48/5 ml; 96/2 ml

HPLC upgradable

þ (Trident Processing Station) þ (Trident Processing Station)

Top – Orbital shaking 40 to 150 C – In rv

– (Sealed vessels)

Glass vessels

48/5 m, 24/14 ml

Analysis

Solid-phase extraction

Work-up Liquid–liquid extraction

Filtration Evaporation in rv (online) Agitation Temperature range Pressure Inert atmosphere

Reaction procedures Reflux

Material

Reaction vessels Number/volume

. .

.

shaking or stirring and heating or cooling Closure of the reactors by a ceramic valve allowing for efficient evaporation (DMSO, DMF) Fully enclosed Handling of slurries and suspensions

. Reagent additions while

.

automatically sorts microreactors between reaction steps Sort 1000 microreactors per hour

. Modular, expandable system . AutoSortTM10K

–

–

þ Online TLC; online HPLC possible valve preinstalled

–

þ þ þ 60 to 120 C þ In rv

þ (standard glassware)

þ (by volume)

Reflux condensers for each vial Parallel vessel to vessel þ Orbital shaking 70 to 150 C þ (upgradable up to 10 bar) In rv

Individual glass vials

112/13 ml; 64/27 ml; 32/75 ml; 16/100 ml

KANTM Kans for 30 mg, 60 mg and 300 mg resin capacity are available Polypropylene


40 to 150 C – In rv

Temperature range Pressure Inert atmosphere

Closed reactors

Reaction procedures Reflux þ þ Magnetic stirring

50/35 ml Glass vessels

Reaction vessels Number/volume Material

Filtration Evaporation in rv (online) Agitation

– – – –

Solution phase

Unit Articulated arm

ISRA

Reaction block (RB) Array (wells/vials) Number of possible RBs Type

Chemistry

Automation Type Arm


Closed rvs with overpressure value Bottom – Magnetic stirring, gas bubbling 60 to 150 C – In rv

12/10 ml Glass vessels

Minitray 12 (2 6) Multiple Flexible during synthesis

Modular Transfer of RBs by a conveyor belt Solid phase Solution phase

Mettler Toledo Myriad Core system

60 to 150 C – In rv

Bottom – Magnetic levitation stirring

þ

96/2 ml; 60/5 ml, 40/10 ml Glass or polypropylene syringes

96; 60; 40 1 Fixed during synthesis

Solid phase

Unit Two XYZ arms

MultiSyn Tech Syro II

30 to 150 C – In rv

Top – Orbital shaking

þ (reflux channel)

48/10 ml Single glass reactors

48 (6 8) Up to 4 RBs Fixed during synthesis

Solid phase Solution phase

Unit XYZ arm (TECAN)

Perkin-Elmer Solaris 530

194


Stem Block 98 1 Fixed during synthesis 96/10 ml Glass vessels

Reaction vessels Number/volume Material

Unit XYZ arm Solid phase (Solution phase)

Zenyx Magellan

.

boundary via a camera system Online error detection

Reaction block (RB) Array (wells/vials) Number of possible RBs Type


Special features

Online HPLC

Analysis

. Detection of phase

þ þ


and simultaneous module operation Compatibility with the Myriad Discoverer series Septumless rvs with twist cap

Customized Glass vessels

Customized Customized Fixed during synthesis

Unit Cylindrical arm Solid phase Solution phase

Zymark

. .

–

. Modular, expandable system . Automated scheduling

þ (Allex TM ) –

addition under agitation possible

96/2 ml; 60/5 ml, 40/10 ml Teflon, glass vessels

Fixed during synthesis

Unit XYZ arm Solid phase

Zinsser Sophas

–

. Fully enclosed . Reagent and solvent

– –

.

–

critical instrument functions Fully enclosed

. Offline incubator available . Seven sensors to monitor

þ –


–

Analysis

Optional

þ þ

Closed reactors þ Optional Orbital shaking Ambient to 150 C – In rv

Zymark

. Offline incubation . Modular, expandable possible system . 96-channel filtration head . Fully enclosed

þ þ


Special features

Closed reactors Top – Orbital shaking Ambient to 150 C – –

Zenyx Magellan

Reaction procedures Reflux Filtration Evaporation in rv (online) Agitation Temperature range Pressure Inert atmosphere


between functional unit on workbench

. Offline incubation possible . Reaction vessels are moved

þ

– þ

Orbital shaking 80 to 150 C – In rv

Optional Top

Zinsser Sophas

196


6.1 Automation in Combinatorial Synthesis

the typical workflow of an organic synthesis every step can be fully automated via specialized devices that work totally independently but are at the same time connected and controlled through an intelligent and easy-to-use software program. Therefore, robotic systems are one of the most flexible solutions to laboratory automation, and established systems can be further developed or redesigned if necessary. Once a robotic system is established it is subject to constant optimization, which is in contrast to workstation approaches where fixed hardware and software subroutines are usually used. In customized robotic systems, the control software is used with a wide range of devices from different suppliers, and subroutines for the specialized stations can be easily redefined. With flexible control software, automated error identification mechanisms and correction tasks are possible that allow an unattended ‘‘round-the-clock’’ operation. But maximum versatility comes at a price, which is one of the main disadvantages of customized robotic systems. The size of the instrument requires custom-built ventilation cabinets and safety installations such as a fire extinguisher system, therefore the timeline for the successful implementation of an automated ‘‘robotlab’’ is often a critical point for pharmaceutical companies. The reliability of the equipment is difficult to predict and can only be tested during installation, which can be a time-intensive process. The key components of robot-arm systems are industrial robots that are used extensively in many areas of industry, such as the car industry, for a wide range of different tasks. Some of the main suppliers of robots are CRS [9], Beckman Coulter [10], Mitsubishi [11], and Zymark [12] (robot-arm features are shown in Table 6.2). Isra system A one-arm-based system for solution-phase synthesis was manufactured by Isra (Darmstadt, Germany). This system was developed in cooperation with different companies, among them Bayer [13], at the beginning of 1997. The system was designed for the synthesis, work-up, and analysis of arrays of 50 compounds per run in amounts of up to 2 g. In this system, the CRS robot arm is used for transport functions. It can be easily programmed via access tasks by using getand-put commands. Synthesis planning and navigation of the robotic system as

Tab. 6.2. Robot arm features.

Physical characteristics Travel length Height to enclosure Height to saddle mounting surface Weight approximately Performance specifications Speeds Acceleration Repeatability

Up to 5 m 138.7 mm 201 mm 50 kg/m 0.01–0.9 m s1 3 m s2 0.08 mm

197

198


Fig. 6.1. Layout of the Isra robot-arm-based system. Implemented at Bayer’s central research center.

well as documentation of the chemistry undertaken are also very important features of the control software. Therefore, a software concept was developed that offers complete and legally admissible documentation of all substances which are to be prepared by the robotic system. The data generated can easily be transferred into widely used electronic laboratory journals. The whole system has a footprint of 5 m long by 2 m wide and is divided into three functional parts for the synthesis, work-up, and analysis; the layout is outlined in Fig. 6.1. The individual components for the synthesis are a needle XYZ-pipetting robot, heating and cooling reaction blocks with magnetic stirrers, a rack for reagents, and a rack for starting materials. These devices have been successfully used in many different syntheses under various conditions. The largest part of the machine is used for storage and work-up procedures. The main devices for work-up are a filter station, a drying station, a balance, a centrifuge, a solvent evaporator, and a phase separation and a phase boundary recognition station. The analytic part of the robot is represented by a Shimadzu high-performance liquid chromatography/ultraviolet (HPLC-UV) system. Via an interface it can be used for online analytical characterization of the synthesized products. The main features of the system are:

. liquid capability per vial: 35 ml; . all manipulations under argon;


Fig. 6.2. Phase separation.

. classical laboratory procedures: .

– mixing, vortexing, and filtration, – liquid–liquid extraction, centrifugation, – drying, evaporation; online analysis of purity and yield.

The system contains a specially designed phase-separation station. An integrated camera (Fig. 6.2) system is able to measure the z-height; the registered value is used as the input command for the phase-separation device. Zymark One of the most widely used robot arms in fully automated systems for process development and organic synthesis is the Zymate XP arm [14], which has been commercially available since the early 1980s. The example shown uses a cylindrical Zymark robot with interchangeable effectors for performing different manipulations (Fig. 6.3). This system was developed as a result of collaboration between Zeneca and Zymark (Runcorn, UK) [15]. Depending on the individual components, the main features of the system are:

. synthesis of 40–50 reactions at a time in gram scale; . classical laboratory procedures: .

– heating, cooling, mixing, vortexing, – filtration, liquid–liquid extraction; sample preparation for analytical purposes [16].

199

200


Fig. 6.3. Layout of a Zymark robot-arm-based system.

The customized synthesizer has an open architecture with considerable flexibility. Based on a Zymark synthesizer, SmithKline Beecham developed a system for synthesis and online purification via column chromatography [17]. Accelab Arcosyn98 The Arcosyn98 manufactured by Accelab [18] is a fully automated system for solid- and solution-phase synthesis. A fixed Scara [19] robot arm is equipped with a gripper change system, enabling it to change its effector tool. Based on this technology, the Scara arm is in charge of all vessel transfers. In addition, other functions such as pipetting and vial opening/closing mechanisms can be performed without an in-built peripheral station (Fig. 6.4). To achieve high throughput, the system includes devices for heating and cooling, evaporation, dissolution, liquid–liquid extractions, yield determination, and sample preparation for analytical purposes, as shown. The Arcosyn98 can be used with five reaction blocks at a time, therefore 100 parallel reactions are possible in one run (Fig. 6.5). The whole system can be controlled and programmed with a software package


Fig. 6.4. Layout of Accelab’s Arcosyn98 robot system.

using one master computer. The multifunctional software can also be used for planning, scheduling, and administration of data entries and results. 6.1.2.2 Fully Automated Workstation Systems

Chemspeed ASW2000 system Chemspeed [20] was founded in 1997 by a team of chemists who had previously worked in a medicinal chemistry laboratory at Roche.

Fig. 6.5. Reaction block Arcosysn98.

201

202


Fig. 6.6.

ASW2000 layout.

Their main objective was to develop an unattended automated system that allowed parallel preparation of compounds in glassware reactors with online purification and analysis in a compact workstation approach (Fig. 6.6). Chemspeed delivers the system on a trolley (footprint 1.4 m 0.8 m) covered with a hood. Because of this compact design, the very flexible system can be used in nearly every laboratory. Within one run, 80 parallel reactions (with an option of 112) can be performed that have the following features:

. liquid capability of 13, 27, 75 or 100 ml per vial; . fully inert environment inside and outside the reactors; . reagent additions while shaking or stirring and while heating or cooling; . classical laboratory procedures: – mixing, vortexing, filtration, – liquid–liquid extraction, – drying, evaporation, cold finger refluxing;


. solid-phase extraction (SPE); . temperature range from 70 to þ150 C, measured and controlled in the reaction mixture;

. online analysis of purity and yield by thin layer chromatography (TLC) or analytical HPLC (optional);

. rheodyne valves for preparative HPLC. The whole system can be upgraded into the ASW2000P workstation, which allows pressurized reactions that integrate work-up and analysis procedures during the synthesis. The pressure capabilities of the system are:

. up to five reactor blocks (80 parallel reactions at maximum); . up to 12 bar; . pressure sensor; . autosecurity system to provide optimum seal performance. Advanced ChemTech Since 1985, Advanced ChemTech (ACT) [21] has supplied systems for organic synthesis, which in the beginning were especially for peptide synthesis. Today ACT offers a broad ‘‘family’’ of manual, semiautomated, and automated synthesizers for solution- and solid-phase synthesis (Table 6.1). The Venture 576 (launched in May 1999) is designed to be a platform for fully automated high-throughput synthesis (Fig. 6.7). The heart of the Venture 576 is a special reaction block combined with two multiprobe XYZ arms. The microplate reactor block contains 96 wells and is constructed of glass-impregnated polytetrafluoroethylene (PTFE) constructions. The usable reaction well volume is approximately 3 ml compared with a total volume of 6 ml and allows parallel reactions in milligram scale even under a reactive gas atmosphere because reaction vessels can be pressurized up to 150 psi. The system is capable of up to 10,000 simultaneous reactions that are determined by the customer. The following description summarizes the main features of the system:

. inert atmosphere for reactor, reactants, and the whole platform; . classical laboratory procedures: – vortexing, filtration, liquid–liquid extraction;

. reactor block with integrated condenser module; . temperature range from 70 to þ150 C, electric resistive heating, cooling by nitrogen gas generated from liquid nitrogen. The floor-standing cabinet is equipped with connections for a ventilation system. Because the system is capable of performing up to 10,000 reactions per run, it is recommended for combinatorial laboratories producing large numbers libraries. MultiSyn Tech Syro II MultiSyn Tech [22] offers manual, semiautomated, and fully automated systems for combinatorial chemistry. The fully automated Syro system

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Fig. 6.7.

ACT Venture 576.

(Fig. 6.8) consists of two independent XYZ robot arms and a specially designed reaction block. The system can be equipped with different reaction vessels. For solid-phase synthesis the removable reaction vessels (number/volume: 96/2 ml; 60/5 ml; 40/10 ml) in glass or polypropylene with glass or PTFE frits are rec-

Fig. 6.8. Syro by MultiSyn Tech.


Fig. 6.9. Levitation mechanism.

ommended. Owing to the reaction block design all manipulations can be performed under inert gas atmosphere using reaction temperatures between 60 and þ150 C. The system has a specially designed agitation mechanism. Each reaction tube is circumvented by electric coils that are used to generate a magnetic field. The coils are placed in a movable levitation plate, the center of the magnetic field is about 6 mm above the frit of the reaction tubes. This special set-up ensures that, during agitation, the resin is not crushed between the stirring bar and the frit (Fig. 6.9). Zenyx Magellan synthesizer In 1996, Zeneca Pharmaceuticals entered into a collaboration with Zenyx Scientific to design and build an instrument for multiple parallel synthesis [23]. The new system was intended to fulfill the following main objectives:

. access to libraries containing up to 1000 single compounds; . scale: up to 30 mg for primary screening and repository; . microplates should be used as transfer racks. The Magellan system is a fully automated synthesizer controlled by a computer. An Excel interface within the control software allows substance data to be imported for the reprogramming of standard protocols. With this system 96 reactions can be performed within one run, using a Stem [24] reaction block. The set-up is very flexible because the layout of the workstation can easily be changed. The XYZ robot arm is in charge of all pipetting jobs. After cleavage from the resin, the compounds can be delivered directly into disposable vials using a special 96-filter station with 96 individual needles with a filter frit at the end. Perkin-Elmer Solaris 530 The Solaris TM 530 [25] organic synthesizer for automated combinatorial chemistry was introduced in late 1998 by PE Biosystems, a division of the PE Corporation (Fig. 6.10). The system is able to synthesize 48 discrete molecules per run in parallel. The portable synthesis module contains an array of 8 6 reaction vessels with a volume of 10 ml per vial (Fig. 6.11). A reflux channel is built in the chemically inert module to provide reflux conditions. The dual septa secures an inert atmosphere.

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Fig. 6.10. Solaris.

Key features of the system include:

. eight liquid-handling tips; . seven sensors to monitor critical instrument functions; . completely enclosed system with flow-through ventilation; . multiple instrument sensors; . offline workstation for heating, mixing, and cooling. An offline incubation workstation enables multiuser access and staggered synthesis runs, which increases throughput. The system is designed for the automated lead optimization using solid-phase synthesis techniques. Zinsser Sophas In 1998 Zinsser [26] launched the Sophas system, which is specially designed for solid-phase synthesis. The synthesizer uses a robotic XYZ arm with four independent probes that are manufactured by Rosys [27]. All liquidhandling tasks are controlled via a personal computer. The easy-to-use software is very flexible, can import data from any database, and allows customized layouts to


Fig. 6.11. Glass vials.

be defined. The automated workbench offers the opportunity to choose from a set of movable reactors that range from 96-well plates to 25-ml reaction vessels. Charybdis Technologies Illiad PS 2 Charybdis Technologies [28] was founded in 1996, and provides solutions in the areas of high-throughput organic synthesis, laboratory automation, and chemical discovery. The Iliad PS 2 Series are multitasking robotic workstations for solid- and solution-phase chemistry. The two independent XYZ robotic arms work in parallel and are controlled via the OASys software. The heart of all Charybdis synthesizers is the Calypso reaction block system, which consists of a top frame with a specially designed top cover plate and a base frame with a base cover plate. The reaction block accepts any array with the standard footprint of a microplate with well volumes of 50, 25, 10, 5, and 2 ml. The Iliad systems can be upgraded to provide online filtration and agitation. 6.1.2.3 Modular Systems

Argonaut Trident Argonaut Technologies [29] was founded in 1994, and provides systems for parallel solution- and solid-phase chemistry. The Trident [30] family consists of a reaction cassette, a library synthesizer, a workstation, and a processing station. The core of the system is the Trident Reaction Cassette TM that contains 48 5-ml vessels or 24 14-ml vessels which are made from glass. The reaction cassette fits into every member of the Trident family and can be easily removed during synthesis for spot checks on the reactions. With one run, the fully automated Trident library synthesizer can prepare up to 192 reactions in parallel. The whole system is controlled via the Trident software, which controls, for example, the temperature, solvent deliveries, or product collection. The synthesizer can handle up to four different reaction cassettes, and each cassette position can be set to a different temperature and agitation speed.

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The Trident workstation is a straightforward supplement to the synthesizer, because it can be used for the manual delivery of solvent and reagent, so it performs parallel resin washing and sample collection in a very effective way. The workstation is ideal for development chemistry, because the methods are developed and adopted in the same cassette as that used for library production. The Trident Processing Station completes the family. It is a multipurpose liquid handler with a special interface to the reaction cassette. The key features of the station are:

. liquid–liquid extraction; . solid-phase extraction; . clean up with scavenger resins; . reverse filtration; . dry solid loading to open vessels; . reformatting (e.g. from tubes to microplates). Mettler Toledo Myriad Core System The Myrid TM Core System (MCS) [31] was developed by a consortium of pharmaceutical companies (SmithKline Beecham, Pfizer, BASF, Novartis, Merck, Takeda, and Chiroscience) and The Technology Partnership [32]. In December 1998 the MCS was sold to Mettler Toledo [33]. The MCS was created as a synthesis system based on a series of robotic processing modules combined with a unique reaction vessel design, fulfilling the following objectives:

. solid- and solution-phase chemistry should be possible; . synthesis of large numbers of compounds for high-throughput screening; . multistep directed synthesis of pure compounds; . method development should be possible; . modular, expandable design; . automatic transfer of vessels between processing modules; . automated scheduling and simultaneous module operation. The reaction block consists of an array of 12 reaction vessels with twist caps. The whole blocks can be transferred to the different units (incubator and processing module) by a conveyor belt. The system is controlled by the chemist using software that allows the processing of four completely independent batches (48 reactions). Irori In 1996 Irori [34] introduced the AccuTag TM -100 Combinatorial Chemistry System. With this system, large numbers of discrete compounds can be produced using the ‘‘directed sorting’’ split-and-pool technique [35]. The reactions are performed in single microreactors which are identified using miniature electronic tags. The Irori Kan TM reactor family is specially designed for solid-phase synthesis. Three different Kans – the MicroKan TM , the MiniKan TM , and the MacroKan TM – with a resin capacity of up to 30, 60, and 300 mg are available, therefore compounds can be synthesized in approximately 10-, 20- and 100-mg batches (Fig.


Fig. 6.12. Irori Kans.

6.12). Another big advantage of this approach is that standard glassware can be used for performing the library synthesis. To achieve high throughput, the Kans are used together with the AutoSort-10K. This workstation has been designed to extend the directed sorting technique for use with libraries in the range of 1000 to 10,000 compounds. The AutoSort TM -10K Microreactor Sorting System (Fig. 6.13) automatically sorts microreactors between reaction steps. The key features of the system are:

. accommodates up to 10,000 microreactors; . sorts 1000 microreactors per hour; . automatically sorts for cleavage into the AccuCleave-96; . standard glassware applicable. 6.2

Purification of Combinatorial Libraries

In the early days of combinatorial chemistry [36], mixtures derived from solidphase split-and-mix libraries [37] as well as nonpurified compounds from solution-

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Fig. 6.13.

Autosort.

phase synthesis played an important role as test compounds for biological screening, basically because it was a very easy way to produce the promised numbers of compounds. It was soon recognized that those compounds often led to falsepositive test results, and that the deconvolution of mixtures and extraction of biologically active molecule in an HTS mode are difficult tasks [38]. This problem resulted in the synthesis of single compounds fulfilling properties such as diversity, drug likeness, and a high degree of purity [39]. Classical purification procedures such as liquid–liquid extraction and chromatography were automated, solid-phase extractions with ion exchangers were adopted, and scavenger reagents for trapping excess starting material or reagents were developed. Automated preparative reversed phase HPLC systems were set up to address high-throughput purification issues. 6.2.1

Automated Liquid–Liquid Extraction

Liquid–liquid extraction is one of the standard methods for removing hydrophilic byproducts, reactants, or reagents. Several examples of the parallel usage of this technique for purification of combinatorial libraries have been described [40]. This

6.2 Purification of Combinatorial Libraries

method is only appropriate for very simple separation problems and can be performed in an automated fashion by using a liquid handler. The method totally fails when polar compounds have to be purified because emulsions can appear or products remain in the aqueous phase. Coupled to a solid-phase extraction system, automated liquid–liquid extraction can be performed together with a 96-needle pipetting system such as Quadra 96 [41]. Separation problems, for example removal of amines from a combinatorial library, were solved by applying several extraction cycles. Dichloromethane and dilute HCl were mixed and separated through filtration using solid-phase diatomaceous earth. The phase separation was achieved via vacuum filtration through a 96-well filterplate carrying a hydrophobic membrane which held the aqueous phase back. Liquid–liquid extraction can be easily automated by using pipetting workstations. Reaction mixtures are mixed with buffers and the upper or lower phase, depending on the density, is removed. Efficient mixing can be achieved through redispensing, which can be repeated several times. The use of a capacity sensor represents a more sophisticated approach. This sensor type is available for several liquid handlers (Zinsser, Tecan). A drawback of this method is when phase separation is not complete or undetectable, erratic results are obtained. 6.2.2

Solid-phase Extraction

Solid-phase extraction is widely used in bio- or environmental analyses in order to enrich or discover organic molecules from complex aqueous matrices. It can also be very useful for the purification of combinatorial libraries. There are rather simple desktop instruments available which are designed for this application – Zymark Benchmate TM , Gilson Aspec TM , and Hamilton Microlab TM – in addition to integrated devices in automated systems (e.g. Chemspeed); simple self-constructed systems have also been reported [42]. One of the big advantages of solid-phase extraction is the availability of various solid phases for the separation of diverse molecules, which range from hard and soft anion or cation exchangers to reversed phase silica columns which are all available in prepacked formats. One disadvantage of the system is that the separation problem has to be evaluated very carefully to achieve optimal results. This becomes a problem when the library is diverse in terms of biophysical behavior and limits the universality of this methodology. An example of the successful application of solid-phase extraction is the synthesis of 225 basic amines which were adsorbed on an acidic ion exchanger. Neutral side products were eluted, and after several washing steps the products were cleaved using ammonia in methanol [43]. Another example is the separation of a library containing neutral compounds from basic and acidic side products via inline solidphase extraction using ion exchangers. The neutral products were not affected by the solid phases and only side products were absorbed. Excess isocyanates were also successfully removed by trapping them as ureas using an excess of a basic amine. This side product was then efficiently absorbed to an acidic solid-phase ab-

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sorber [44]. An enormous increase in the versatility of this methodology can be achieved with scavenging reagents. Scavenging reagents offer the opportunity to use chemical reactivity as a mode of irreversible absorption to a solid support. This allows the selective and efficient removal of many excess reagents. Examples are the removal of excess electrophiles such as acid chlorides, sulfonic chlorides, and isocyanates as well as the trapping of excess nucleophiles such as amines [45]. Despite these simple separation problems rather complex multistep transformations have been performed using this separation technique [46]. Several of these polymer-supported reagents are commercially available [47]. Furthermore, solidphase reagents prepacked in columns or microplates would be a great advantage in performing parallel reactions. 6.2.3

Normal Phase Chromatography

Together with crystallization and liquid–liquid extraction, normal phase chromatography is one of the most important purification operations in organic synthesis. For automation and parallelization normal phase chromatography is also appropriate and several systems are commercially available, e.g. CombiFlash TM from Isco [48], Quad3 TM from Biotage [49], and FlashMaster TM from Jones Chromatography [50]. Parallel flash chromatography ranging from 4 to 12 single-use columns is widely used. The Isco and Biotage systems provide time-triggered fractionation; the system from Jones chromatography collects fractions via UV detection, which minimizes the number of fractions. Prepacked columns are available and have to be exchanged after each run, therefore only batchwise working is possible. If salts and other polar byproducts are carefully removed before chromatography, the columns can be used several times. The commercially available columns can also be self-packed with very simple equipment. The loading of these columns is usually very high: a 20-g cartridge can be used for the purification of up to gram quantities of product. 6.2.3.1 CombiFlashTM from Isco [48]

This instrument is designed for parallel separation with ten columns and is equipped with an injection system in which dissolved samples or samples absorbed on silica gel can be loaded. Besides the commercially available Isco RedSep TM columns all other columns with luer fittings can be used. Fractionation is time guided, and a maximum of 40 fractions per run can be collected. An advantage of this system is the very small hood space of 50 cm which is required. Quad3TM from Biotage [49] With the Quad3 TM 12 columns can be used in parallel together with a fraction collector which is able to handle different vial types as well as microplates. The microplates simplify the evaporation process because vacuum centrifuges can be used. A step gradient can be applied, and an injection system for the delivery of 6.2.3.2

6.2 Purification of Combinatorial Libraries Tab. 6.3. Common stationary phases for reversed phase preparative HPLC.

Material

Application

– C18 endcapped – C18 hydropylic endcapped

Drug-like compounds Drug-like compounds especially for polar compounds injected in DMSO Drug-like compounds, amines, lipids Polar compounds

– C8 – CN

dissolved samples or samples absorbed on silica gel is available. In addition to this system a simple desktop version, the MultiElute TM system for 12 columns with fraction collector, is also available. 6.2.3.3 FlashMasterTM from Jones Chromatography [50]

FlashMaster TM is a computer-controlled desktop system for automated flash chromatography of ten columns, which allows gradient elution. With an integrated UV detector, UV-triggered fractionation minimizes identification logistics. Together with postrun reporting of chromatograms, product identification can be easily realized. In addition to this system, Jones Chromatography also offers simple solutions for parallel flash chromatography. 6.2.4

Reversed Phase Chromatography

Reversed phase chromatography (RP-HPLC) is widely used in analyses and purification of combinatorial libraries (Tables 6.3 and 6.4). The method is very universal in terms of polarity of the analytes and a wide range of compound types can be purified with RP-HPLC. This makes RP-HPLC a favorable method for purifying combinatorial libraries, mostly because no method development is necessary. The available equipment is very rugged and has been developed for high throughput. Columns can be used up to 1000 times and continuous working is applicable, enabling fairly high throughput which compensates for the serial nature of HPLC. In theory, HPLC can be used 24 h per day, but normally this does not happen because of solvent delivery and removal, sample supply, and fraction removal. Limited throughput because of the serial nature of the system is compensated by very

Tab. 6.4. HPLC columns, dimensions, and applicable loading.

Dimension (mm)/material/particle size (mm)

Loading (mmol)

Flow rate (ml min1 )

8 75/C18/5 20 50/C18/5 30 125/C18/5

10 50 75–100

8–14 25–40 50–70

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fast chromatography, which allows the semipreparative separation of one sample within minutes and a daily throughput of approximately 200 samples is possible. Fractions are usually triggered via UV and only compound-containing fractions are collected. Difficulties can be encountered in the identification of the target product-containing fraction, which is usually done offline with FIA-MS (FIA ¼ flow injection analysis) or HPLC-MS. Column switching is desirable for speeding up cycle times. One column is used for the chromatography while the other column is equilibrated to starting conditions, saving about 10–20% of the cycle time. There are systems on the market which are able to run two columns. This doubles throughput and also the price of the system because further pumps, a detector, and fraction collector must be used. The advantage of running two columns is small compared with running two individual systems. Because of the complexity of the whole set-up – consisting of HPLC plus downstream processing – large efforts are required for the implementation of this technology in a laboratory workflow in order to run the system in an optimal fashion. Depending on the separation problem, and on the number of byproducts, up to ten fractions can easily be obtained per injection. For identification of the target product, each fraction must be analyzed with FIA-MS. This issue consequently demands very close interaction with the analytical group performing these experiments. RP-HPLC is complementary to RP-HPLC-MS and the information from analytical HPLC-MS can be directly used for identification of the desired product fraction without any FIA-MS [51]. For every kind of set-up the logistics are complex, and adoption or development of useful software solutions are desirable for tracking the large amounts of data and to prevent bottlenecks. A reduction in the amount of solvent used may be achieved through supercritical fluid chromatography (SFC). This method offers the opportunity to use a supercritical gas, for example CO2 , as a mobile phase. CO2 is cheap, it has high flow rates because of low backpressure, and it ‘‘evaporates’’ when external pressure is removed [52]. 6.2.4.1 Biotage [49]

The Parallex TM System from Biotage is a four times parallel HPLC-UV workstation. The system has four channels with four individual fraction collectors, detectors, and pump heads. Throughput lies in the range of 40 injections per hour and the injection system can handle up to 768 samples per run. Loading can be from 1 mg up to 200 mg depending on the column used. The system is fully software controlled and a detailed reporting software package is available. 6.2.4.2 Gilson [53]

Gilson offers a very rugged preparative HPLC system named Nebula Series TM . The pumps especially are outstanding. Flow rates of 200 ml min1 and pressure limits of 400 bar can be achieved. The fraction collector 215 TM has a capacity of up to 12 microplates and completes the high-throughput system. The system is totally software controlled and detailed reporting allowing fraction tracking is also included.

6.2 Purification of Combinatorial Libraries

6.2.4.3 Merck [54]

Merck offers an automated preparative HPLC for parallel chromatography of two columns named the High Throughput Purifier TM (HTP). Four columns are integrated: two are conditioned while the other pair is used for chromatography. The optionally available Merck-Hitachi Ion Trap MS offers online or offline product identification. The MS is fully integrated into the workflow and offers very simple reporting to Excel, which is essential for integration into a laboratory workflow. Varian [55] Varian distributes an HPLC equipped with Dynamax HPLC Pumps. Flow rates of up to 200 ml min1 with a pressure limit of 410 bar are applicable. High throughput is not possible because the system lacks a useful fraction collector that can handle microplates. In addition to this, the Apple software is hard to integrate with Microsoft-based laboratory environment. 6.2.4.4

6.2.4.5 Shimadzu [56]

Shimadzu is one of the leading companies in preparative HPLC. The systems offered are very robust and the equipment, such as the UV detector and pumps, can be very easily controlled. A disadvantage is the fraction collector, which cannot handle microplates – this feature minimizes throughput and capacity. 6.2.5

Preparative HPLC-MS

The search for more powerful techniques for the purification of combinatorial libraries using HPLC led to collaboration between researchers from the pharmaceutical and analytical industries, namely PE Sciex and Micromass, with the goal of developing preparative HPLC-MS [57, 58]. The advantage of this technique is that the molecular mass of the target molecule triggers the fraction collector. This allows the collection of the desired compounds and the online identification of the target molecules. Furthermore, this method offers logistical benefits. If just the target molecules are collected, fraction collector capacity is not critical and downstream processing can be performed very efficiently. This enables the technique to work very efficiently in terms of throughput. For setting up such a system integration of the mass spectrometer and a reliable software platform which ensures fraction collecting and sample tracking are necessary. This application is now offered from more vendors than the pioneers Micromass and PE Sciex, e.g. MerckHitachi, Gilson in collaboration with Thermoquest and Shimadzu. 6.2.5.1 PE Sciex [59]

PE Sciex, the first vendor with this application type on the market, offers a preparative HPLC-MS/UV system with Shimadzu pumps and a Gilson 215 as a fraction collector. The hardware components, especially the API 150EX mass spectrometer, are very rugged, and from this point of view the system is very useful for high

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throughput. One of the main disadvantages is the Apple-based software and the unstable fraction collector script used for tracking fractions. Besides lacking software stability, it is also not easy to integrate an Apple-based system into a usually Microsoft-based laboratory environment. Waters Micromass [60] Micromass merged with Waters and was the second vendor offering this application. As HPLC components, Waters devices can be integrated with equipment from other HPLC providers. The Gilson 215 is used as a fraction collector device that is fully integrated in the software platform. The Masslynx TM software is fairly well adapted to high throughput, making it easy to integrate these systems into laboratory workflow. 6.2.5.2

6.2.5.3 Merck-Hitachi

As already mentioned above, Merck-Hitachi offers a system that is designed for this application. By using a special valve as an interface, it is possible to switch between two flows running into the MS inlet. Therefore two preparative separations at a time can be performed and analyzed, thus doubling the throughput and reducing hardware costs. Merck-Hitachi devices together with Gilson fraction collectors are compatible HPLC components. Shimadzu Shimadzu offers a preparative HPLC-MS/UV system with a Gilson 215 fraction collector. Besides being good HPLC equipment, a reliable and flexible software package is available. In addition to the software package operating the instrument, a Shimadzu LIMS (LIMS ¼ laboratory information management system) system is available which can also be used for documentation of data other than those from the HPLC-MS. 6.2.5.4

6.2.5.5 Gilson ThermoQuest

Gilson, one of the leading experts in HPLC, has established a preparative HPLCMS system in collaboration with Thermoquest. The HPLC part of the system, supplied by Gilson, is controlled by Uni Point software (Gilson); the ThermoQuest MS however is controlled by Excalibur software (Thermoquest). Both software applications are controlled via special interface software, which has the consequence that the MS detector is only marginally controlled by the HPLC part of the equipment, by pulsing externally, and the two independent software packages always have to run on a single PC.

6.3

Analysis of Combinatorial Libraries

Combinatorial syntheses need fast and reliable analyses in order to determine the identity of products and intermediates in each step of a parallel reaction sequence.

6.3 Analysis of Combinatorial Libraries Tab. 6.5.

Analytical methods and their applications in combinatorial chemistry.

Method

Application

FT-IR

Characterization of resin-bound intermediates in reflection (ATR (ATR ¼ attenuated total reflectance) DRIFTS (DRIFTS ¼ diffuse reflectance infrared fourier transform spectroscopy)) Characterization and quantification of combinatorial libraries with fast FIA or sample exchanger devices Characterization of libraries in coupling to HPLC or CE. Identification of fractions from preparative HPLC in FIA mode

NMR ESI-MS

Consequently, analyses were adopted that operate at the same speed as combinatorial methodologies when these methods became popular for synthesis. The analysis of the final products from solid- and solution-phase synthesis – usually compounds for biological testing – are the same because similar purity criteria are applicable. Because of the nature of solid-phase synthesis, the analysis of polymerbound intermediates is rather difficult. Special nuclear magnetic resonance (NMR) and FT-IR methods (Table 6.5) have been developed for the purpose of characterizing polymer-bound intermediates. In the case of FT-IR techniques, reflection measurements are usually used, e.g. ATR and DRIFTS [61]. These methods can be performed directly on bead and no KBr solids have to be made, as is the case in transmission experiments. In the case of NMR, magic angle spinning can be used to suppress signals from the polymer matrix [62]. All these special analytical techniques are usually used for developing the method owing to the fact that they are very time-consuming and therefore have a fairly low throughput. The minimal requirements for combinatorial libraries are usually purity, identity, and quantity. 6.3.1

Purity of Combinatorial Libraries

Purity assessment is historically carried out using chromatographic methodologies. Thin layer chromatography (TLC), reversed phase (RP)-TLC, and HPLC are therefore widely used; these methods may also be used for spot tests in parallel synthesis. For a purity check of a whole library, methods with a higher throughput such as automated HPLC, supercritical fluid chromatography (SFC) [63], and capillary electrophoresis (CE) are applicable. In addition to chromatographic separation, detection plays an important role. Standard detection is UV absorption at different wavelengths. In addition, another optical detector – the light-scattering detector (ELSD) – can be used [64]. Light-scattering detectors have a more linear signal to mass response and are more likely to be quantitative. For use in quantitative analysis of combinatorial libraries, a nitrogen chemoluminescence detector is even more appropriate because the nitrogen signal is proportional to the amount of nitrogen present in the sample [65]. HPLC-MS coupling is the most widely used method of purity assessment in parallel synthesis, mainly because separation and

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identification of the target molecules is carried out in a parallel fashion. Mild ionization techniques such as ESI and APCI are used. They almost always show the molecular ion peak, which can be directly taken from the synthesis protocol. Cycle times are highly optimized. It is possible to perform 400 HPLC-MS runs per day [66]. The resulting data have to be stored in an appropriate LIMS system. Tools are available from hardware suppliers for the semiautomatic processing of these data. Using these tools, further steps such as adaptation to the laboratory workflow are additional issues, and efforts to optimize these interfaces are necessary [67]. The widespread use of HPLC-MS for purity evaluation gives a limited insight into the actual purity of a product because only UV-absorbing side products and those on RP-absorbable compounds can be detected. Salts and polymer fragments are usually not detected. One should take this feature into account when assessing the purity of combinatorial libraries if they are not purified chromatographically before biological testing. Many suppliers sell fast HPLC-MS applications especially designed for library purity checks, and all have semiautomated software packages available for data processing. 6.3.2

Identity of Combinatorial Libraries

Besides purity, determination of the identity of the individual library member is one of the important analytical tasks in combinatorial chemistry. Usually, classical methods such as NMR and mass spectrometry are used. NMR has a disadvantage in that it is possible to measure hundreds of NMR spectra daily but the automatic processing of data is still a bottleneck. Therefore, NMR is mostly used during method development, especially when stereochemical aspects have to be addressed. As already mentioned ESI (ESI ¼ electro spray ionisation) and APCI (APCI ¼ atmospheric pressure chemical ionisation) mass spectrometry are widely used ionization methods. The advantages of these ionization techniques are that detection from a liquid spray is possible and that an FIA system or an HPLC system can be directly coupled. Additionally almost always a molecular peak is obtained which makes the semiautomatic processing of the data very easy. Classical ionization methods such as EI (EI ¼ electron ionisation) and CI (CI ¼ chemical ionisation) are not applicable for polar molecules in the mass range of 300 to 500. MALDI is also very popular, especially for peptides and proteins in bioanalyses [68]. Critical sample preparation procedures and the impossibility of coupling to an HPLC system limit the methodology. For mass analyses, the highly robust quadruple technique is widely used, as are TOF [69] and FT-ICR [70]. The advantage of TOF and especially FT-ICR is the very good mass resolution of 5 ppm in the case of TOF (TOF ¼ time of flight) and better than 1 ppm in the case of FT-ICR (FT-ICR ¼ fourier transform ion cyclotron resonance spectroscopy). Owing to this high resolution, the molecular composition can be obtained directly from the mass signal. For fast analysis of combinatorial libraries and for identification of products in fractions coming from preparative HPLC, FIA-ESI-MS is the method of choice [71,

6.4 Automated Sample Processing

72]. Cycle times below 10 s can be achieved even with standard equipment. Problems can arise because of the vast amounts of data, but software packages are available from all suppliers of ESI mass spectrometers for automated processing. The implementation of these data into the workflow of combinatorial laboratories is of course another issue and is only achievable with programming resources which can be limited to automated reformatting of Excel tables. FIA-ESI-MS does not give any results concerning purity, and if mixtures are obtained ionization yields can vary dramatically and lead to false results if the product is suppressed, e.g. by basic impurities. In addition to FIA-ESI-MS, MALDI-MS (MALDI ¼ matrix aided laser desorption ionisation) in an offline FIA mode can also be performed. Through an automated process MALDI targets have been coated and prepared for measurement [73]. This application is rather complex because it is impossible to work directly from solution. The advantage however is that it is the only method which gives a molecular peak from larger molecules such as proteins. 6.3.3

Quantification of Combinatorial Libraries

In addition to purity and identity, quantification of combinatorial libraries is one of the major issues in combinatorial analyses because it is essential to know the amount of compound in order to quantify the biological effects. Unfortunately, there is no simple way to automate quantification. The best way to quantify the system, but also the worst way to automate it, is by weighing. Besides weighing, it is also possible to use a nitrogen detector, but accuracy can be very low for diverse sets of compounds. Therefore weighing as the method of choice must be automated. This can be achieved with liquid handlers carrying a small robotic arm. These systems, which are commercially available from Zinsser Calli TM and Mettler Toledo Bodan TM , can handle single vials. The bar-coded vials are weighed empty and the weight is recorded in a database. After weighing all the vials, the dispenser transfers solutions of the synthesized compounds into the vials. After evaporation of the solvent in a vacuum centrifuge, the robot arm again weighs the vials and the quantity of compound is determined.

6.4

Automated Sample Processing 6.4.1

Sample Logistics

Parallel high-throughput synthesis is a mass production process. The process consists of several stages that must operate in an interlocking way. It is very important to identify and remove bottlenecks in order to obtain optimal throughput. A very important role that is often regarded as a bottleneck is played by the sample logistics (Fig. 6.14).

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Fig. 6.14. Flow chart of an automated processes in parallel synthesis.

After evaluation of a synthesis that is considered useful for library production, the production process begins with weighing of the starting materials. The weighing process cannot be performed in an automated fashion because of the diverse physical nature of common starting materials, which can be liquids, solids, or oils. After weighing the starting materials, they can be dissolved in an appropriate solvent and then transferred very efficiently to the reaction vessels by using liquid handlers; some four- or eight-needle systems are described in Table 6.6. When the reaction is complete, the work-up protocols described in Chapter 6.2 are used for purification of the library. For liquid–liquid extractions or solid-phase extractions, the samples are produced in ratios of 1:1, which simplifies the sample logistics for reformatting and analysis. Samples can be taken directly with pipetting robots and then diluted or filtered for analysis. After analysis, those samples that are to be processed further to determine their biological activity are selected. This decision is based on the analytic results, usually those from HPLC-ESI-MS. Qualitatively acceptable compounds are taken through a quantification and reformatting step. Sometimes further analytical and quality control steps are necessary. When HPLC or HPLC-MS are used for purification of the library, the logistics are much more difficult than for the 1:1 purification strategies. The raw products from synthesis have to be filtered and dissolved in an HPLC-compatible solvent. Additionally, all of them have to be reprocessed into an HPLC-suitable format, usually a microplate. After chromatographic purification a large number of fractions have to be identified and evaporated. A pooling step is necessary for those fractions that give

Tab. 6.6. Liquid handlers suitable for sample processing in parallel synthesis.

Liquid handler

Application

Tecan

Four- and eight-needle systems suitable for reformatting and synthesis in MTPs. A robot arm can also be adopted for use in vial handling, weighing, etc. Further equipment for evaporating of filter plates, etc. available Four-needle system with robot arm specially designed for weighing, SPE Four-needle system for reformatting and synthesis. Robot arm and further equipment for weighing, SPE and synthesis available Eight-needle system, second arm with disposable tips, evaporation manifold available

Myriad Mettler Toledo Zinsser Lissy Packard

MTP, microtiter plate; SPE, solid-phase extraction.

6.4 Automated Sample Processing

Fig. 6.15. Flow chart of sample logistics if preparative HPLC is used for purification.

a positive result. This step can be performed before or after evaporation of the fractions. Reformatting before evaporation has the advantage that only the product-containing fractions are evaporated, thus saving capacity in the vacuum centrifuges. After evaporation, the fractions are pooled and analyzed and normally redissolved and processed into vials or microplates (MTPs), from where biological testing can be performed. The whole process is described in Fig. 6.15. The HPLC system distributes the fractions from one injection into different cavities of the collecting MTP. After identification of the products a software tool, usually an Excel macro, produces a run file for the liquid handler, which pools the fractions back into a 96-well MTP. From there, further analysis and reformatting steps are performed after the Excel macro has produced the second run file pooling the quality-controlled samples into the desired final formats. 6.4.2

Evaporation

Solvent removal is one of the most important downstream processes in parallel synthesis. Several attempts to address this issue have been made, and vacuum centrifuges and freeze-drying for water-containing mixtures are the most appropriate methods. Besides these techniques, shakers using infrared (IR) radiation [74] or stirrers in combination with air or nitrogen streams [75] are also in use. The main advantages of freeze-drying are that no thermal stress is applied to the sample and the loading of liquid can be fairly high. Also, carrier formats are not limited and almost every type of carrier, for example microplates, racks, and single vials, can be loaded into the freeze-dryer. The disadvantages are that the evaporation time can be long and that the methodology is limited to aqueous solutions.

221

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6 Instrumentation for Combinatorial Chemistry Tab. 6.7.

Vacuum centrifuges commonly used in parallel synthesis.

Centrifuge

Advantages/disadvantages

Genevac Christ Savant

Robust in terms of imbalance, high capacity, fast evaporation/price, stability Price, fast evaporation/capacity, unstable in case of imbalance Vapor stability/capacity

Vacuum centrifuges have the advantage that almost every solvent can be evaporated, even those with very high boiling points such as dimethylsulfoxide (DMSO). Evaporation times are usually fast and the samples are concentrated and centrifuged at once, which results in all the material ending up at the bottom of the vial. Disadvantages are the thermal stress, which can lead to degradation of the products, and the possibility of imbalance due to the different concentrations of solvent mixtures. Several suppliers sell vacuum centrifuges for evaporation of HPLC fractions or cleaving solutions from solid-phase chemistry. Usually, carrier formats are fixed or limited. The microplate footprint is the most common, which makes it necessary to optimize the whole workflow to this footprint. Advantages and disadvantages of several commercially available instruments are shown in Table 6.7.

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1998, 3, 343–349; b) J. Drews, S. Ryser, Drug Discovery Today 1997, 2, 365–372. a) A. T. Merrit, Drug Discovery Today 1998, 3, 505–510; b) J. S. Lindsey, Lab. Information Manage. 1992, 17, 15–45; c) J. F. Cargill, M. Lebl, Curr. Opin. Chem. Biol. 1997, 1, 67–71. a) D. Harding, M. Banks, S. Fogarty, A. Binnie, Drug Discovery Today 1997, 9, 385–390; b) J. R. Broach, J. Thorner, Nature 1996, 7, 14–16. N. W. Hird, Drug Discovery Today 1999, 4, 265–274. S. H. DeWitt, A. W. Czarnik, Curr. Opin. Biotechnol. 1995, 6, 640–645. J. H. Hardin, F. R. Smietana in: High Througput Screening. Devli, J. P. (ed.), New York, Basel, Decker 1997, pp. 251–261. A. M. M. Mjali, B. E. Toyonaga in: High Througput Screening. Devli,

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39 H. N. Weller, Mol. Diversity 1999, 4,

47–52. 40 S. Cheng, D. D. Corner, J. P.

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Williams, P. L. Meyers, D. L. Boger, J. Am. Chem. Soc. 1996, 118, 2567– 2573 S. X. Peng, C. Henson, M. J. Strojnowsky, A. Golebiowski, S. R. Klopfstein, Anal. Chem. 2000, 72, 261–266. B. Kaye, W. J. Herron, P. V. Macrae, S. Robinson, D. S. Stopher, R. F. Venn, W. Wild, Anal. Chem. 1996, 68, 1658–1660. R. M. Lawrence, S. A. Biller, O. M. Fryszman, M. A. Poss, Synthesis 1997, 553–558. M. G. Siegel, P. J. Hahn, B. A. Dressman, J. E. Fritz, J. R. Grunwell, S. W. Kaldor, Tetrahedron Lett. 1997, 38, 3357–3360. R. J. Booth, J. C. Hodges, J. Am. Chem. Soc. 1997, 119, 4882. S. V. Ley, I. R. Baxendale, R. N. Bream, P. S. Jackson, A. G. Leach, D. A. Longbottom, M. Nesi, J. S. Scott, R. I. Storer, S. J. Taylor, J. Chem. Soc. Perkin Trans. 1 2000, 3815–4195. Calbiochem-Novabiochem AG, Weidenmattweg 4, CH-4448 Laüflingen. http://www.isco.com http//www.biotage.com http://www.joneschrom.com I. Hughes, J. Assoc. Lab. Autom. 2000, 5, 69–71. W. C. Ripka, G. Barker, J. Krakover, Drug Discovery Today 2001, 6, 471– 477. http//www.gilson.com http//www.hii-hitachi.com http://www.varianinc.com http://www.shimadzu.com L. Zeng, L. Burton, K. Yung, B. Shushan, D. B. Kassel, J. Chromatogr. A 1998, 794, 3–13. J. P. Kiplinger, R. S. Ware, E. Roskamp, P. Dorf, R. G. Cole, S. Robinson, A. Brailsford, R. Raso, C. Frey in: Pittcon’97 Conference, Atlanta 1997. http://www.appliedbiosystems.com http://www.waters.com

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68 R. J. Cotter, Anal. Chem. 1999, 71,

445A–451A. 69 U. Selditz, S. Nilsson, D. Barnidge,

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Part II

Synthetic Chemistry


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7

Radical Reactions in Combinatorial Chemistry A. Ganesan and Mukund P. Sibi 7.1

Introduction

Organic synthesis is dominated by polar transformations, in which an electronrich center reacts with an electron-deficient center. By contrast, homolytic processes involving organic radicals were largely unexploited for many years. The general belief was that such ‘‘free radicals’’ would be undisciplined in their reactions, and prone to undesirable pathways such as premature radical–radical recombination or hydrogen atom abstraction from the solvent. Even under successful chain-propagating conditions, the chemistry seemed best suited to polymerization. More recently, a deeper understanding of the kinetics of radical reactions has enabled [1] the orchestration of a complicated series of elementary steps, and there is now a vast number of synthetically useful radical reactions which are complementary to traditional polar processes. For example, the reaction conditions are relatively mild, avoiding strongly acidic or basic reagents, and many functional groups are tolerated without requiring protection. Additionally, the product of a radical abstraction reaction or a radical addition to an unsaturated center is a new radical that can participate in further tandem reactions. Such reaction cascades can represent strategically powerful disconnections, as a relatively complex product is derived from a much simpler starting material in one step. Combinatorial chemistry is also heavily dominated by polar reactions. Early efforts were predominantly based around peptides and nucleotides, as their synthesis was adaptable to the preparation of large libraries and compatible with existing automated equipment. Solid-phase synthesis techniques for such oligomers were already highly advanced [2], with nearly quantitative coupling yields at each cycle. However, the emphasis in combinatorial libraries rapidly shifted toward drug-like ‘‘small molecules’’. The resultant need to assemble diverse nonoligomeric carbocyclic and heterocyclic scaffolds has led to an intense effort to devise combinatorial versions of more sophisticated organic reactions, including those in which carbon–carbon bonds [3] are made. In recent years, the potential of radical reactions for combinatorial purposes has become apparent. The majority of radical reactions explored for combinatorial applications have Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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7 Radical Reactions in Combinatorial Chemistry

been reported on solid phase. While this does not change the actual chemistry compared with traditional solution-phase radical reactions, there are contexts in which the solid-phase environment may be advantageous. Even in the swollen gel phase, reactions are kinetically slower than in homogeneous solution-phase conditions, which may be helpful in determining the relative partitioning of a radical intermediate into various pathways. A second unique feature is the loading of polymeric resins, typically a1 mmol g1 of beads, thus effectively placing an upper limit on the maximum concentration attainable with solid-phase reactions. This enforced dilution can be useful for radical reactions, although gel-phase polymer chains do exhibit significant freedom of motion and certainly do not approach ‘‘infinite dilution’’. A further variable is the spacer length between the substrate and the matrix. Many reactions, including Merrifield’s original peptide synthesis, were carried out with the substrate attached directly to the polystyrene matrix. Today, it is more common to include a ‘‘linker’’ [4] between the polymer and the substrate. The linker can profoundly influence chain mobility as well as the polymer microenvironment where the reaction is taking place. Linkers are often used for solid-phase radical reactions, although the reasons for selecting a particular linker are seldom described. Finally, the ability to filter off reagents and byproducts can certainly be a bonus for solid-phase radical reactions, especially those mediated by tin reagents, whose removal is not always trivial in solution phase (see below).

7.2

Intramolecular Radical Additions to sp 2 and sp Carbon

The first carbon–carbon bond-forming radical reactions reported on solid phase were intramolecular aryl radical 5-exo cyclizations, which have ample solutionphase precedents [5]. Routledge et al. [6] investigated the formation of dihydrobenzofuran (2) from an aryl halide precursor (Scheme 7.1). The efficiency of the reaction was found to depend on the resin: with polystyrene, more than 1 equivalent of AIBN was required as radical initiator, whereas the reaction was complete within 20 h using 6 mol% of AIBN on TentaGel resin. The intermediate alkyl radical underwent two different types of reactions: a b-elimination or a H-atom abstraction from tributyltinhydride. The b-elimination process could be suppressed

Scheme 7.1.

Aryl halide cyclizations by Routledge et al. [6].

7.2 Intramolecular Radical Additions to sp 2 and sp Carbon

by the addition of t-butanol. An attempt to make the b-elimination the major pathway by switching to thiyl linker (3) only yielded a small amount of 4. 5-Exo cyclizations of an alkyl radical onto an acetylene group leading to exomethylene furans were also reported in this study. Du and Armstrong [7] reported similar cyclizations of aryl iodides attached to polystyrene–Rink resin using SmI2 for radical generation (Scheme 7.2). The reactions can be carried out under mild conditions without the solvent degassing. However, a large excess of HMPA was found to be necessary for efficient reaction. Use of a TentaGel-type resin allowed polymer swelling in aqueous solvents, enabling Sm(III) impurities in the beads to be removed by saturated NaHCO3 prior to resin cleavage. The feasibility of radical–polar crossover reactions by anionic capture of the intermediary Sm(III) species by electrophiles was attempted [8]. The reaction was unsuccessful when the carboxylic acid was immobilized on polystyrene–Rink resin as an amide, possibly due to quenching of the anion by the amide proton. On the other hand, when substrate 7 on TentaGel– Wang resin was treated with HMPA, 3-pentanone, and SmI2 , the crossover product 8 was obtained in moderate yield after resin cleavage. Reaction efficiency with the TentaGel–Wang-supported substrate (33% yield) was similar to that in solution phase (40% yield), although perhaps still too low for reliable generation of libraries.

Scheme 7.2.

Aryl iodide cyclizations by Du and Armstrong [7, 8]. THF, tetrahydrofuran.

De Mesmaeker and coworkers have reported a series of aryl radical cyclizations [9], and compared them with Pd-mediated Heck cyclization of the same substrates. Radical cyclization of iodo alkenes immobilized on polystyrene resin through a Wang-like linker (9) using tributyltinhydride (Scheme 7.3) gave dihydrobenzofurans (10) [10]. A tandem cyclization using allyltributyltin gave the allylated products (11) in low to moderate yields. Formation of isomeric dihydrobenzofurans (13) could be accomplished by radical cyclization onto b-alkoxy esters [11]. For best results, the tributyltinhydride and AIBN were added portionwise every 5–8 h. The impressive 95% yield was in fact higher than that for the solid-phase Heck cyclization of 12.

229

230


Scheme 7.3.

Aryl iodide cyclizations by the Novartis group [10, 11].

The Novartis group has also studied [12] the radical cyclization of cyclohexenediols, immobilized by a ketal linkage on polystyrene (Scheme 7.4). The reaction of 14 gave the desired dihydrobenzofuran (15) and the uncyclized product of direct reduction (16). Jia et al. [13] have reported a related cyclization with allylamine (17) immobilized on polystyrene–Wang resin. The reaction was monitored by acetylation and cleavage to yield 18, as a mixture of free and Boc-protected amines. This solid-phase synthesis of seco-CBI (18, R ¼ H), related to the pharmacophore of the CC-1065 and duocarmycin class of cyclopropylindole antitumor antibiotics, has potential for the preparation of analog libraries, and an example of further conversion of resin-bound 18 to a polyamide has been presented. A series of bromoacetals (19) (Scheme 7.5) linked to polystyrene was cyclized to the corresponding acetals by Watanabe et al. [14]. Oxidative cleavage of the resin using Jones reagent gave easy access to g-butyrolactones (20).

7.3

Intermolecular Radical Additions

The carbon–carbon bond-forming steps of intramolecular reactions are facilitated by entropic acceleration. Intermolecular reactions offer a more stringent test of the

7.3 Intermolecular Radical Additions

Scheme 7.4.

Aryl and vinyl iodide cyclizations by Berteina et al. and Jia et al. [12, 13].

Scheme 7.5. Intramolecular cyclizations by Watanabe et al. [14].

feasibility of solid-phase radical reactions. Sibi and Chandramouli [15] reported the first examples of intermolecular radical allylations. The polymer-bound electrophilic C-radicals generated from a-bromo esters (21) gave g; d-unsaturated acids (22) (Scheme 7.6). Large excesses of allylstannane and AIBN were required for good yields. Radical initiation at room temperature using Et3 B/O2 was less efficient. The yields were similar to those for solution-phase reactions, while reduction of 21 with tributyltin deuteride gave @93% deuterium incorporation, implying 95% diastereoselectivity [23]. This was better than the analogous solution-phase reaction, suggesting that the immobilized oxime is less reactive. The addition of isopropyl

7.3 Intermolecular Radical Additions

Scheme 7.11. Intermolecular alkene additions [21].

Scheme 7.12. Intermolecular oxime ether additions by Miyabe et al. [22, 23].

and cyclohexyl radicals by atom transfer from the corresponding iodide, to furnish 48b,c, has also been demonstrated. In these cases, the product was contaminated by the competing addition of initiating ethyl radical, and this was avoided by using a large excess of the alkyl iodide. Pyrrolidines have been synthesized on solid phase by a combination of an intermolecular radical addition followed by an intramolecular oxime ether cyclization, as exemplified in the preparation of 50 [24] and 52 [25] (Scheme 7.13). The solidphase reactions were sluggish with triethylborane as an initiator at room temperature, while the analogous solution-phase process was kinetically much faster. Rad-

235

236


Scheme 7.13. Intermolecular oxime ether additions by Miyabe et al. [24, 25].

ical addition to chiral substrate 53 using triethylborane as an initiator gave mostly the ethyl addition product 54 (R ¼ Et). Successful incorporation of the alkyl radical generated from the alkyl iodide required the reaction to be performed almost neat in the latter, and proceeded with decent diastereoselectivity. Radical additions to the phenylsulfonyl oxime ether 55 (Scheme 7.14) have been reported by Jeon et al. [26]. Yields were better with primary and secondary alkyl

Scheme 7.14. Intermolecular oxime ether additions by Jeon et al. [26].

7.4 Functional Group Removal

iodides, and the tandem cyclization sequence with iodide (57) to afford bicyclic 58 has been accomplished, albeit in modest yield.

7.4

Functional Group Removal

Solid-phase synthesis necessarily requires attachment to the polymer matrix by a functional group, which will be unmasked at the end of the synthetic sequence upon cleavage from the resin. In peptide and nucleotide synthesis, this respectively reveals the C-terminus carboxylic acid and the 5 0 alcohol. As these groups are inherently part of the final biopolymer, this does not pose a problem. For small-molecule synthesis, however, it is not always desirable to have a dangling functional group whose sole purpose had been to enable immobilization. One possible solution is to devise ‘‘traceless’’ linkers [27] that produce CaH bonds upon cleavage, as realized by Plunkett and Ellman [28] with an arylsilane cleaved by protodesilylation. Homolytic carbon–heteroatom bond fission followed by quenching of the organic radical represents an alternative means of achieving a ‘‘traceless’’ solid-phase synthesis. A pioneering effort by Sucholeiki [29] involved the photochemical irradiation of resin-bound thioethers (Scheme 7.15) attached via two different linkers to TentaGel resin. The isolated yields of biphenyl 60 were reportedly low owing to product volatility, and degassed solvent was crucial to minimize the formation of byproduct aldehyde 61.

Scheme 7.15.

Thioether cleavage by Sucholeiki [29].

More recently, several groups have explored the homolytic cleavage of resinbound selenides as a route to ‘‘traceless’’ synthesis. Nicolaou et al. [30] started with lithiated polystyrene, which was trapped by dimethyl diselenide to furnish the alkylselenide resin. Reaction with bromine generated selenylbromide resin (63) (Scheme 7.16), which could be reduced to the selenide anion (64). This resin can

237

238


Scheme 7.16.

Traceless selenide cleavage by Nicolaou et al. and Ruhland et al. [30, 31].

then be reacted with alkyl halides, followed by ‘‘traceless’’ reductive cleavage with tributyltinhydride. A similar dealkylation was simultaneously reported by Ruhland et al. [31], in which a chloro- and a bromoalcohol were immobilized by the selenide resin (70). The free alcohol was then functionalized by a Mitsunobu reaction prior to ‘‘traceless’’ cleavage. Nicolaou also described the use of resin 63 in promoting the glycosidations of sugar glycals, resulting in a polymer-bound selenide whose radical cleavage yields 2-deoxyglycosides. The Nicolaou group has since reported a number of further applications with these selenide resins. In the previous sections, many solid-phase radical reactions were described that had an extended linker between the substrate and the polymer matrix, unlike the Nicolaou and Ruhland examples. Fujita et al. were the first to report [32] a ‘‘traceless’’ selenide cleavage with a linker. However, the yield from 73 (Scheme 7.17) was poor, and the authors suggest that the tinhydride reagent is reacting with the linker itself. A more robust ether-based selenide linker (76) has recently been described by Li et al. [33], and an example of homolytic cleavage proceeded with good yields.

7.5

Polymer-supported Reagents for Radical Chemistry

As was illustrated in the above sections, radical reactions are feasible on solid support and are relatively new entrants to the field. In contrast, the use of polymer-

7.5 Polymer-supported Reagents for Radical Chemistry

Scheme 7.17.

Traceless selenide cleavage by Fujita et al. and Li et al. [32, 33].

supported organic reagents has a much longer history in radical chemistry. Developments in this area can be attributed to the difficulty in removal of organotin byproducts, since tin-derived compounds are used extensively for radical chemistry. Use of polymer-supported reagents alleviates most of the difficulties associated with product purification. The tin-containing polymer can be easily removed by simple washing. 7.5.1

Polymer-supported Tinhydrides

Most of the activity in polymer-supported reagents has been in the preparation of tinhydride derivatives. Early work in this area was carried out by Neumann and coworkers [34]. A polystyrene-derived reagent (80), which mimics tributyltinhydride, was prepared and evaluated as a reducing and chain transfer reagent. Representative examples are shown in Scheme 7.18. Similar chemical efficiency in the cyclization of 81 and in the reductions of 82–84 demonstrated that the polymeric tinhydride has similar reaction characteristics to tributyltinhydride. Dumartin and coworkers have evaluated several polystyrene-supported tinhydrides prepared from Amberlite XE305 in reductions [35]. These reagents (85–86) contain different spacers between the tin atom and the phenyl group. The amount of residual tin was determined by ICP-MS for the reduction of 87 (Scheme 7.19). The polymer hydride produced 45 ppm of residual tin compared with 7000 ppm

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240


Scheme 7.18. Reductions using polystyrene-supported tinhydride [34].

Scheme 7.19. Reductions using polystyrene-supported tinhydride [35].

from tributyltinhydride. In situ generation of the polymer-supported reagent from the corresponding halide and sodium borohydride and reactions without mechanical stirring significantly reduced the tin pollution level. The Dumartin group has also reported Barton–McCombie deoxygenation of secondary alcohols using catalytic amounts of polymer-supported tinhydride [36]. Deluze and coworkers [37] have prepared a new type of macroporous polymersupported tinhydride using suspension copolymerization (Scheme 7.20). The authors have determined the swelling characteristics of the resin in different solvents as well as the specific surface areas. The organotin chloride-functionalized beads showed good stability and reactivity in reductions using sodium borohydride as the co-reductant. Recently, Enholm and Schulte [38] prepared a soluble tinhydride on a noncrosslinked polystyrene support (89) and demonstrated its utility in reductions


Scheme 7.20.

A macroporous polymer-supported tinhydride precursor [37].

(Scheme 7.21). The reductions use only catalytic amounts of the polymeric tinhydride and sodium borohydride as the co-reductant. Several examples (90–94) of chemoselective reductions have been reported. The tin contents in the products were also determined and shown to be in the ppm range.

Scheme 7.21. Reductions with polymer-supported tinhydride [38].

Curran and coworkers [39] have developed novel alternatives for polymersupported reagents. They have successfully demonstrated the use of very versatile fluorous reagents (95) for free radical chemistry [39]. The fluorous tinhydrides (Scheme 7.22) are commercially available and have the advantages of ease of product purification by simple separation and high reaction rates since the reactions are carried out in the solution phase.

241

242


Scheme 7.22. Fluorous tinhydride [39].

7.5.2

Polymer-supported Allyl Stannane

Enholm and coworkers [40] have prepared a soluble noncrosslinked polystyrenesupported allyltin reagent (96) (Scheme 7.23). The reagent is quite successful in transferring an allyl group to a variety of substrates in moderate to good yields. The products (98–101) obtained from allylation show reasonable structural diversity. The selective allyl transfer to the more electrophilic site in 101 is noteworthy. Ryu, Curran, and coworkers [41] have demonstrated the utility of fluorous allyltin reagents in organic synthesis.

Scheme 7.23. Polymer-supported allylating agent [40].

7.5.3

Polymer-supported Reagents for Atom-transfer Reactions

Atom- and group-transfer radical reactions have enjoyed a lot of popularity in recent years. Clark and coworkers [42] have prepared solid-supported catalysts for atom-transfer radical cyclizations (Scheme 7.24). Functionalized aminopropyl silicagel was coupled to pyridine 2-carboxaldehye to provide an orange solid (102). The catalyst (103) was prepared by stirring a mixture of 102 and copper halide in acetonitrile. Atom transfer cyclization of several N-allyl amides using 30 mol% of the catalyst gave the product in high yields. The catalyst could be recovered by simple filtration. Recently, a soluble copper catalyst supported on poly(ethylene)block-poly(ethylene glycol)-polymer (108) has been effectively used for atomtransfer radical polymerizations [43]. Marsh and coworkers [44] have reported Cu(I)-mediated radical polymerization of nucleoside monomers on silica support.


The preparation of polystyrene-block-polymethylmethacrylate films by a sequential carbocationic polymerization of styrene followed by radical polymerization of methylmethacrylate has been reported previously [45].

Scheme 7.24. Atom-transfer radical cyclizations and polymerizations with polymer-supported catalysts [42, 43].

7.5.4

Photochemical Generation of Radicals

O-Acyl and O-alkylthiohydroxamates are convenient precursors for C- and Ocentered radicals. Product purification at times is difficult because of the sulfur byproducts. De Luca and coworkers [46] have reported a solution to this problem. The polymer support with appropriate functionality was prepared from N-hydroxythiazole 2(3)-thione and functionalized Wang resin (109). A Hunsdicker reaction of 110 furnished the corresponding bromide (111) cleanly. Similarly, the cyclization of the O-centered radical under reductive conditions furnished ether 113. In both cases, product purification was simple (Scheme 7.25).

Scheme 7.25. Photochemical generation of radicals [46].

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7 Radical Reactions in Combinatorial Chemistry Tab. 7.1. Methods used in solid-phase radical reactions.

Radical precursor

Method of radical generation

Alkyl/aryl halide RBr or RI

AIBN, Bu3 SnH Me3 SnSnMe3 , hn SmI2 Et3 B, O2 Et2 Zn, O2 NaBH4 hn AIBN AIBN AIBN, Bu3 SnH hn

Organomercurial RHgCl Thioether RSR Thiol RSH Sulfonyl bromide ArSO2 Br Alkyl selenide RSeAr Barton ester RCO2 aNAr

7.6

Summary

Radical reactions have yet to be applied to the synthesis of compound libraries, whether in combinatorial fashion or as parallel arrays. This is perhaps not surprising, as it is only in the last few years that the feasibility of carrying out radical reactions at all on solid phase has been demonstrated. The examples in this chapter convincingly illustrate that there is no intrinsic limitation to such processes. Now that the gestation period is over, one can predict that radical reactions will be part and parcel of the combinatorial chemist’s toolkit of organic reactions. The various precursors to radicals used in solid-phase reactions, and the methods of generation, are summarized in Table 7.1. The early preponderance of solid-phase reactions in combinatorial chemistry has also evolved to a more equal weighting with solution-phase synthesis. Here also, the field of radical reactions has not been left out. A number of polymer-supported reagents and scavenger resins are now available that facilitate radical reactions in parallel, with the potential for considerably simplified work-up and purification compared with the original procedures. Further advances in combinatorial radical chemistry, both solution and solid phase, can certainly be anticipated.

References 1 P. Renaud, M. P. Sibi (eds) in:

Radicals in Organic Synthesis, WileyVCH, Weinheim, 2001. 2 For an account of the early history of solid-phase synthesis, see: a) D. Hudson, J. Comb. Chem. 1999, 1, 333–360. b) D. Hudson, J. Comb. Chem. 1999, 1, 403–457. 3 For reviews, see: a) B. A. Lorsbach,

M. J. Kurth, Chem. Rev. 1999, 99, 1549–1581; b) R. E. Sammelson, M. J. Kurth, Chem. Rev. 2001, 101, 137– 202. 4 For reviews, see: a) I. W. James, Tetrahedron 1999, 55, 4855–4946; b) F. Guillier, D. Orain, M. Bradley, Chem. Rev. 2000, 100, 2091– 2157.

References 5 For a review, see: B. K. Banik, Curr. 6

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Org. Chem. 1999, 3, 469–496. A. Routledge, C. Abell, S. Balasubramanian, Synlett 1997, 61– 62. X. Du, R. W. Armstrong, J. Org. Chem. 1997, 62, 5678–5679. X. Du, R. W. Armstrong, Tetrahedron Lett. 1998, 39, 2281–2284. For a review, see: S. Wendeborn, A. De Mesmaeker, W. K. D. Brill, S. Berteina, Acc. Chem. Res. 2000, 33, 215–224. S. Berteina, A. De Mesmaeker, Tetrahedron Lett. 1998, 39, 5759–5762. S. Berteina, S. Wendeborn, A. De Mesmaeker, Synlett 1998, 1231–1233. S. Berteina, A. De Mesmaeker, S. Wendeborn, Synlett 1999, 1121– 1123. G. Jia, H. Iida, J. W. Lown, Synlett 2000, 603–606. Y. Watanabe, S. Ishikawa, G. Takao, T. Toru, Tetrahedron Lett. 1999, 40, 3411–3414. M. P. Sibi, S. V. Chandramouli, Tetrahedron Lett. 1997, 38, 8929–8932. E. J. Enholm, M. E. Gallagher, S. Jiang, W. A. Batson, Org. Lett. 2000, 2, 3355–3357. X. Zhu, A. Ganesan, J. Comb. Chem. 1999, 1, 157–162. M. E. Attardi, M. Taddei, Tetrahedron Lett. 2001, 42, 3519–3522. A.-M. Yim, Y. Vidal, P. Viallefont, J. Martinez, Tetrahedron Lett. 1999, 40, 4535–4538. S. Caddick, D. Hamza, S. N. Wadman, Tetrahedron Lett. 1999, 40, 7285–7288. R. Plourde Jr, L. L. Johnson Jr, R. K. Longo, Synlett 2001, 439–441. H. Miyabe, Y. Fujishima, T. Naito, J. Org. Chem. 1999, 64, 2174–2175. H. Miyabe, C. Konishi, T. Naito, Org. Lett. 2000, 2, 1443–1445. H. Miyabe, H. Tanaka, T. Naito, Tetrahedron Lett. 1999, 40, 8387–8390. H. Miyabe, K. Fujii, H. Tanaka, T. Naito, Chem. Commun. 2001, 831– 832. G.-H. Jeon, J.-Y. Yoon, S. Kim, S. S. Kim, Synlett 2000, 128–130. For a review, see: S. Bra¨se, S.

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Dahmen, Chem. Eur. J. 2000, 6, 1899– 1905. M. J. Plunkett, J. A. Ellman, J. Org. Chem. 1995, 60, 6006–6007. I. Sucholeiki, Tetrahedron Lett. 1994, 35, 7307–7310. K. C. Nicolaou, J. Pastor, S. Barluenga, N. Winssinger, Chem. Commun. 1998, 1947–1948. T. Ruhland, K. Anderson, H. Pedersen, J. Org. Chem. 1998, 63, 9204–9211. K. Fujita, K. Watanabe, A. Oishi, Y. Ikeda, Y. Taguchi, Y. Synlett 1999, 1760–1762. Z. Li, B. A. Kulkarni, A. Ganesan, Biotechnol. Bioeng. 2001, 71, 104–106. a) U. Gerigk, M. Gerlach, W. P. Neumann, R. Vieler, V. Weintritt, Synthesis 1990, 458–452; b) M. Gerlach, F. Jordens, H. Kuhn, W. P. Neumann, M. Peerseim, J. Org. Chem. 1991, 56, 5971–5972; c) C. Bokelmann, W. P. Neumann, M. Peterseim, J. Chem. Soc., Perkin Trans. 1 1992, 3165. a) G. Dumartin, M. Pourcel, B. Delmond, O. Donard, M. Pereyre, Tetrahedron Lett. 1998, 39, 4663–4666; b) G. Dumartin, G. Ruel, J. Kharboutli, B. Delmond, M.-F. Connil, B. Jousseaume, M. Pereyre, Synlett 1994, 952–954; c) G. Ruel, N. K. The, G. Dumartin, B. Delmond, M. Pereyre, J. Organomet. Chem. 1993, 444, C18–C20. P. Boussaguet, B. Delmond, G. Dumartin, M. Pereyre, Tetrahedron Lett. 2000, 41, 3377–3380. a) A. Chemin, H. Deleuze, B. Maillard, Eur. Polym. J. 1998, 34, 1395; b) A. Chemin, H. Deleuze, B. Mailard, J. Chem. Soc., Perkin Trans. 1, 1999, 137–142. E. J. Enholm, J. P. Schulte, II, Org. Lett. 1999, 1, 1275–1277. a) D. P. Curran, S. Hadida, S.-Y. Kim, Z. Luo, J. Am. Chem. Soc. 1999, 121, 6607–6615; b) D. P. Curran, S. Hadida, J. Am. Chem. Soc. 1996, 118, 2531–2532; c) I. Ryu, T. Niguma, S. Minakata, M. Komatsu, S. Hadida, D. P. Curran, Tetrahedron Lett. 1997, 38, 7883–7886.

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K. M. Moran, J. S. Lombardi, J. P. Schulte, II, Org. Lett. 1999, 1, 689– 691. 41 a) I. Ryu, T. Niguma, S. Minakata, M. Komatsu, Z. Luo, D. P. Curran, Tetrahedron Lett. 1999, 40, 2367–2370; b) see also: D. P. Curran, S. Hadida, M. He, J. Am. Chem. Soc. 1997, 119, 6714–6715. 42 A. J. Clark, R. P. Filik, D. M. Haddleton, A. Radigue, C. J. Sanders, G. H. Thomas, M. E.

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Smith, J. Org. Chem. 1999, 64, 8954– 8957. Y. Shen, S. Zhu, R. Pelton, Macromolecules 2001, 34, 3182– 3185. A. Marsh, A. Khan, M. Garcia, D. M. Haddleton, Chem. Commun. 2000, 2083–2084. B. Zhao, W. J. Brittain, J. Am. Chem. Soc. 1999, 121, 3557–3558. L. De Luca, G. Giacomelli, G. Porcu, M. Taddei, Org. Lett. 2001, 3, 855–857.

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Nucleophilic Substitution in Combinatorial and Solid-phase Synthesis Jan-Gerd Hansel and Stephan Jordan 8.1

Introduction

Nucleophilic substitution (SN ) reactions are widely used in combinatorial chemistry. Two reasons account for this prevalence: first the potential to combine building blocks by SN and second the availability of building blocks that can participate in SN reactions. In addition, SN reactions are typically very reliable transformations, robust, and thus ideally suited for high-throughput synthesis of organic compounds. In this chapter the application of SN reactions to combinatorial chemistry using both solution- and solid-phase formats will be discussed. The sections on aliphatic and aromatic substrates are subdivided by the classes of nucleophiles that participate in the reaction. Reactions used to form medium-sized or large rings are treated separately owing to their significance in macrocyclic ring chemistry. This chapter will not cover nucleophilic substitution reactions at acyl carbons, reactions of organometallic reagents, reactions of carbon nucleophiles, and transition metalcatalyzed substitution reactions of nucleophiles (see other Chapters).

8.2

Nucleophilic Substitution at Aliphatic Carbons 8.2.1

General Remarks

Nucleophilic substitutions (SN ) at aliphatic carbons play an important role in the combinatorial synthesis of diverse classes of compounds. The reactions have had great impact right from the beginning of combinatorial chemistry. R. B. Merrifield used SN of chloromethyl-polystyrene with cesium carboxylates as a means to attach amino acids to a solid support (see Scheme 8.1) [1]. Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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8 Nucleophilic Substitution in Combinatorial and Solid-phase Synthesis

Scheme 8.1. Immobilization of amino acids by Merrifield.

A broad range of electrophilic substrates can be used in SN reactions. Alkyl chlorides and bromides are fairly common whereas alkyl iodides are used less often [2]. Alkyl chlorides are usually activated with catalytic amounts of iodide salts such as potassium iodide or tetrabutylammonium iodide (TBAI). Alkyl esters of sulfonic acids such as alkyl tosylates or alkyl mesylates are also common electrophiles. They are prepared from a sulfonyl halide and the corresponding alcohol. The alcohol itself can be employed as an electrophilic substrate when activated in situ. The Mitsunobu reaction [3] and related transformations are frequently utilized for this purpose. Recently, the activation of alcohols as trichloroacetimidates has been applied to parallel synthesis [4]. Epoxides and cyclic sulfates [5] are synthetically valuable substrates for library generation since the reaction with a nucleophilic building block liberates another hydroxyl function ready for further derivatization. 8.2.2

Halogen Nucleophiles

Benzylic alcohols are converted into the corresponding benzyl chlorides or bromides by reaction with phosphorus trihalides or under milder conditions with triphenylphosphine (TPP) and tetrahalogenomethanes. The reaction has found widespread application in the activation of benzyl alcohol-type linkers for subsequent attachment of nucleophilic substrates to a solid support [6]. The reaction was also applied to one of the masterpieces of multistep solutionphase synthesis involving polymer-supported reagents and scavengers. During Ley’s pyrrole synthesis, all six reaction steps to the highly diverse pyrroles used these reagents. In those cases where the benzylic halides are not commercially available, the alkyl halides used for N-alkylation are prepared from the corresponding alcohols. In the bromination step, a reagent combination of polymersupported TPP and tetrabromomethane is used (see Scheme 8.2) [7].

Scheme 8.2. Solution-phase benzyl bromide synthesis.

8.2 Nucleophilic Substitution at Aliphatic Carbons

8.2.3

Oxygen Nucleophiles

Carboxylate alkylation by alkyl halides can be used both on solid phase (see above) and in solution [8] to give alkyl carboxylic acid esters. For optimum results, cesium salts are employed. Alternatively, carboxylate alkylation can be performed under Mitsunobu conditions using alcohols as electrophiles. For solid-phase applications the common reagents TPP and diethyl azodicarboxylate (DEAD) can be used. In solution-phase parallel synthesis, it is advantageous to use reagents that do not require a chromatographic product isolation. A combination of polymer-bound TPP [9] and di-tert-butyl azodicarboxylate has been found to be particularly useful (see Scheme 8.3) [10]. While the former reagent is filtered off easily, the latter (and its byproduct di-tert-butylhydrazodicarboxylate) is readily converted into volatile compounds by treatment with trifluoroacetic acid (TFA).

Scheme 8.3. Solution-phase Mitsunobu carboxylate alkylation.

Cesium carbonate in the presence of an alcohol and carbon dioxide can be alkylated by Merrifield resin to give solid-supported carbonates (see Scheme 8.4). Adding an amine instead of an alcohol gives rise to carbamate formation [11].

Scheme 8.4. Solid-phase carbonate formation.

Alcohols are transformed into ethers by reaction with an alkylating agent in the presence of a very strong base such as sodium hydride. The reaction, known as the Williamson ether synthesis, is frequently used for attaching alcoholic substrates to Merrifield-type resins (see Scheme 8.5) [12].

Scheme 8.5. Solid-phase Williamson ether synthesis.

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In a similar reaction, Wang resin is converted into a trichloroacetimidate derivative and used as a polymer-bound benzylating agent. Attachment of alcohols to this resin is achieved under acid catalysis [4]. Ether-forming reactions involving phenolate nucleophiles proceed much more readily. All types of electrophiles have been used in both solution- and solid-phase chemistry. Usually, the phenolates are generated in situ from phenols and bases such as 1,8-diazabicyclo[5.4.0]undecene-7 (DBU) or potassium carbonate (see Scheme 8.6) [13].

Scheme 8.6. Solid-phase phenolate alkylation.

The solution-phase synthesis of alkyl aryl ethers is simplified by using ionexchange resins. The reaction of various phenolates loaded on Amberlite IRA-900 with alkyl bromides gives alkyl aryl ethers in solution. Thereby the ammonium bromide byproduct remains on the Amberlite resin [14]. Aryl alkyl ethers are also obtained by Mitsunobu reaction of phenols and alcohols. For solid-phase applications, the standard reagents TPP and DEAD have been used successfully. In some cases, Castro’s sulfonamide betaine 1 (see Scheme 8.7) [15] offers cleaner reactions and is easier to handle especially with automated equipment. Since the betaine decomposes upon exposure to air into inactive components without a change of appearance, care should be taken to ensure pure material is used.

Scheme 8.7. Castro and Matassa’s sulfonamide betaine (1) [15].

The solid-phase Mitsunobu reaction has been employed for linker attachment [16] as well as diversity generation [17]. In solution-phase chemistry, specially designed reagents are used to facilitate product separation (see above). Thus, polymer-bound TPP and DEAD derivatives that are easily filtered off are commercially available. Also, ionophoric side-chains have been attached to both TPP and DEAD to make them (and the corresponding byproducts) easily separable using ion-exchange resins [18]. This technique is an example of a general concept called phase tagging or phase labeling [19]. The solution-phase reaction of phenols with epoxides is catalyzed by polymersupported Co(salen) complexes [20]. Using chiral catalysts the corresponding 1aryloxy-2-alcohols are obtained in high yields, purities, and enantiomeric excesses.


8.2.4

Sulfur Nucleophiles

The reactions of sulfur nucleophiles show similarities to those of oxygen nucleophiles. The greater nucleophilicity of sulfur compounds, however, makes a range of unique additional transformations possible. A convenient way to prepare the thiol function involves thioacetate alkylation. This method has been adapted to polymer-supported solution-phase synthesis. Displacement of halides or tosylates with a polymer-supported thioacetate reagent affords an intermediate thioester, which can be reduced to the thiol by the addition of borohydride exchange resin [21]. As expected, alkane thiols [22] and thiophenols [23] are readily alkylated by alkyl halides. The substitution proceeds at room temperature in the presence of a base (DIEA for aromatic and DBU for aliphatic thiols). Thioether formation can also be used for synthesis of heterocycles, as shown in the preparation of 2,4,5-trisubstituted thiomorpholin-3-ones [24]. Dithiocarbamates, which are formed in situ from the reaction of carbon disulfide with amines, are alkylated by Merrifield resin to obtain resin-bound dithiocarbamates (see Scheme 8.8). These compounds are intermediates in a reaction sequence leading to guanidines carrying three different side-chains [25].

Scheme 8.8. Solid-phase dithiocarbamate alkylation.

Similarly, thiourea is attached to Merrifield resin by S-alkylation to give a S-alkylisothiouronium salt, which is further transformed into guanidine derivatives [26] and pyrimidines (see Sect. 8.3.2). Thiourea alkylation is also involved in the classical Hantzsch thiazole synthesis. N-Substituted thioureas are alkylated with a-bromoketones, and after immediate cyclization aminothiazoles are obtained (see Scheme 8.9). The reaction has been adapted to a solution-phase library synthesis by simply mixing the components as dimethylformamide (DMF) solutions and heating the mixtures to 70 C [27].

Scheme 8.9. Solution-phase Hantzsch thiazole synthesis.

8.2.5

Nitrogen Nucleophiles

The direct N-alkylation of primary amines is in general not the method of choice for the synthesis of secondary amines because of potential of overalkylation. There-

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by a reductive alkylation with a carbonyl compound is to be preferred. Most often the primary amine is reacted with an aldehyde in the presence of a water-removing agent such as trimethyl orthoformate (TMOF) or molecular sieves to form an imine intermediate. This is reduced to the secondary amine with a suitable borohydride agent, preferably tris(acetoxy)sodiumborohydride. A different situation occurs in solid-phase synthesis when the alkylating agent is attached to the solid support. Upon reaction with a primary amine – usually applied in great excess – a clean conversion to the immobilized secondary amine is observed. The reaction can be used for the attachment of amines to the resin [28]. The most important application of this reaction is in the solid-phase synthesis of N-substituted glycine (NSG) oligomers, so-called peptoids [29]. These oligomers resemble peptides. Their side-chains are not attached at the a-carbon as in peptides but rather at the nitrogen atom. Peptoids are synthesized by a repeated sequence of acylation with bromoacetic acid followed by SN reaction with a primary amine (see Scheme 8.10). Iodoacetic acid can also be used, but chloroacetic acid gives inferior results.

Scheme 8.10. Solid-phase peptoid synthesis.

The straightforward synthesis has mostly been developed by Chiron researchers. Exploiting the great number of available amines, they were able to identify rapidly nanomolar ligands for a1-adrenergic and opiate receptors out of huge peptoid libraries. Peptoids offer a good example of the evolution of methods that mimic natural oligomers with easy-to-make unnatural compounds. A recent similar development has been peptide nucleic acids (PNAs). PNAs resemble DNA with the phosphodiester backbone of the DNA being replaced by an oligo-[N-(2-aminoethyl)glycine] motif. They have been investigated for diagnostic and antisense purposes [30]. The SN reaction of alkylating agents with secondary amines can be controlled to give tertiary amines selectively. Thus, reaction of polymer-bound alkyl sulfonates with secondary amines gives immobilized tertiary amines [31]. In solution-phase synthesis, selective monoalkylation of secondary amines has been achieved mostly with monosubstituted piperazine substrates (see Scheme 8.11) [32]. The reaction can be performed with an excess of piperazine or with an additional base. To facilitate product isolation in solution-phase chemistry, either a water-soluble base in combination with an aqueous work-up [33] or a polymer-bound base [34] can be employed.

Scheme 8.11. Solution-phase piperazine alkylation.


Intramolecular amine alkylation does not usually bear the risk of overalkylation and is an excellent way to close heterocyclic rings. An Epibatidine synthesis using polymer-supported reagents provides an example that involves a mesylate substitution [35]. Even strained rings such as aziridines can be formed (Gabriel– Cromwell reaction) [36]. Epoxide opening with amine nucleophiles is frequently used in combinatorial chemistry since it leads to the attractive aminoalcohol substructure. When catalyzed with Lewis acids, for example lithium perchlorate, the reaction proceeds smoothly with a range of alkyl amines, anilines, and heteroaromatic amines. The reaction is useful in solution-phase synthesis (see Scheme 8.12) [37] as well as in solid-phase synthesis [38].

Scheme 8.12. Solution-phase epoxide opening by amines.

The nucleophilic attack of an epoxide by amines can be followed by reactions involving the newly generated hydroxyl function. In Scheme 8.13 a carbamate group at a suitable distance is attacked by the hydroxyl group that is exposed during epoxide opening. The resulting oxazoline formation occurs with concomitant cleavage off the resin [39].

Scheme 8.13. Solid-phase oxazolidinone synthesis.

Other nucleophiles that are readily alkylated on the solid phase by alkyl halides or sulfonates include hydroxyl amine [40], O-benzyl hydroxylamine [41], hydrazine derivatives [42], azide ion [43], and sulfonamides [44]. Nitrogen compounds bearing hydrogens of sufficient acidity can also be alkylated under Mitsunobu conditions (see above). Thus, a polymer-bound imidodicarbonate can be alkylated with primary and secondary alcohols using the standard reagents TPP and DEAD. After cleavage off the resin, primary amines are obtained (see Scheme 8.14) [45].

Scheme 8.14. Solid-phase Mitsunobu reaction of imidodicarbonate.

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8.2.6

Ring-closing Reactions

The formation of medium-sized or large rings in combinatorial synthesis is frequently accomplished with SN reactions. Especially S-alkylation reactions are used in the ring-closing step. Representative examples involve the syntheses of b-turn mimetics containing nine- or ten-member rings [46] and a cyclic oligocarbamate consisting of a 26-membered ring [47]. An unusual case of macrocyclization is observed in solid-phase synthesis of [Arg-8]-vasopressin. The thiol groups of two cysteines of the peptide can be linked by a methylene unit to form a macrocyclic methylenedithioether [48]. The transformation is achieved by simply treating the resin with tetrabutylammonium fluoride in dichloromethane.

8.3

Nucleophilic Substitution at Aromatic Carbons 8.3.1

General Remarks

Nucleophilic aromatic substitution (SN Ar) is an attractive approach to the functionalization of electron-deficient aromatic systems, in solution phase as well as on solid support. It has become an invaluable part of combinatorial transformations, particularly for the installation of nitrogen- or oxygen-linked substituents. A wide range of readily accessible nucleophiles can be introduced, making SN Ar reactions as popular as amide formations and outstanding for the synthesis of combinatorial libraries. The large majority of these reactions is based on two classes of reactive scaffolds. Di- and trihalogenated heterocycles with two or more heteroatoms in the heterocyclic ring (mostly nitrogen atoms) have been used extensively. In these molecules the halogen atoms can be replaced selectively and sequentially with various nucleophiles, making the reaction ideal for combinatorial library production. On the other hand, there is a large number of examples for attractive and practical methods to cleanly and efficiently prepare novel libraries using substituted halogeno-nitrobenzenes as templates. First, the nitro group activates the aromatic system for nucleophilic attack. After halogen substitution the nitro group can be reduced easily and used in an excellent way for further diversification of the libraries. If the chloro derivatives prove not to be reactive enough in the SN Ar reactions, the chloro substrates can be transformed into the fluoro analogs by halex reaction (which is a SN Ar reaction in itself ). Fluoroaromatics are significantly more reactive toward nucleophiles [49].

8.3 Nucleophilic Substitution at Aromatic Carbons

8.3.2

Nitrogen Nucleophiles

From a historical point of view, the first template for SN Ar reactions was cyanuric chloride (2,4,6-trichloro-1,3,5-triazine), which is commercially available and inexpensive [50]. Compounds containing this core element have shown biological activity, e.g. as herbicides (Atrazin). A research group at ArQule showed that the selective and sequential derivatization of cyanuric chloride could be achieved by simply controlling the reaction temperature. The generality of the research group’s method has been proven by the solution-phase synthesis of a large combinatorial library of over 40,000 individual compounds (see Scheme 8.15) [51]. The first chloride substitution proceeds at 20 C using N,N-diisopropylethylamine (DIPEA) as the base and acetonitrile as the solvent. Even anilines react with the very reactive cyanuric chloride in the proposed way. Usually, the arylation of the primary amine reduces its nucleophilicity strongly. Therefore, no bis-arylation occurs at this position and the second chlorine atom can be substituted at room temperature. Owing to the relatively low reactivity of the third position, heating is required for the last exchange and only strong nucleophiles such as secondary amines give pure products and high conversions. Besides amines and anilines, also carbohydrates, dipeptides, amidines, and a-ketoamides can be incorporated, giving access to more complex structures. Upon screening of these compounds, a series of hits in the cardiovascular area has emerged.

Scheme 8.15. Sequential displacement of the chlorines of cyanuric chloride.

Using the cyanuric chloride template for solid-phase synthesis has become a very efficient method for the production of large combinatorial libraries. This was also demonstrated by the synthesis of a 12,000-compound library by Stankova´ and Lebl [52]. The first chlorine atom was selectively substituted by coupling the tem-

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plate to a resin loaded with amino acids (see Scheme 8.16). Taking advantage of the decreasing reactivity of tri-, di-, and monochlorotriazines, various nucleophiles (amines, anilines, hydrazines) were introduced at different temperatures [53]. Likewise, an 8000-membered library on a cellulose-based polymeric membrane has been synthesized [54].

Scheme 8.16. Solid-phase library based on the triazine template.

The reaction sequence can be extended to related starting materials such as 2,6dichloropurines, although the reaction conditions need to be harsher [55]. In a representative example, the dichloropurine was treated with a primary amine at elevated temperature. For the second substitution, reflux conditions and five equivalents of amine were necessary. Excess amine was removed by the use of formylpolystyrene beads. The compounds could be benzylated at the N-9 position by an alkylation protocol or by using the Mitsunobu reaction (see Scheme 8.17) [56].

Scheme 8.17. Liquid-phase synthesis of a purine-based library.

SN Ar reactions on perhalogenated heterocyclic systems are well established in solution phase and these reactions have been well adapted to solid-phase synthesis. They have been shown to be useful – indeed, more advantageous in many cases – than their solution-phase counterparts. Special attention in this area should be given to purine templates as purines are involved in signal pathways and metabolic processes in all living organisms (see Scheme 8.18). For example, the discov-


Scheme 8.18. Natural products containing the purine core.

ery of the biologically active natural product olomoucine stimulated attempts to generate diverse analogs based on the adenosine template on solid support [57]. First an aldehyde-functionalized resin preloaded with benzylamines was reacted with the purine scaffold. The authors chose N-9-SEM (SEM: trimethylsilylethoxymethylene)-protected 2-fluoro-6-chloropurine as the starting material because of the activating potential of the SEM group in the amination reaction. After removal of this group using tetrabutylammonium fluoride in tetrahydrofuran (THF), a Mitsunobu reaction introduces the isopropyl moiety. A second nucleophilic substitution and final cleavage from the resin led to the desired purine analogs (see Scheme 8.19). Since the molecules were attached to the resin via the amino group at position 6, only primary amines could be introduced into the reaction sequence [58]. As a result, myoseverin, a novel microtubule-binding molecule, was identified upon screening these libraries [59].

Scheme 8.19. Synthesis of myoseverin on solid support.

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The production of new compound libraries from polyhalogenated heterocycles is very common in combinatorial chemistry – for example reactions involving nucleophilic amines. Besides cyanuric chloride and chloropurines, many other templates have been used as starting materials (see Scheme 8.20) [50b] [60]. For example, for the synthesis of a library of 160 pyrimidine carboxamides, Suto et al. [61b] took advantage of the difference in reactivity between the two reactive sites of the substituted 2-chloropyrimidine-5-carboxylic acid chloride core (see Scheme 8.21). After amidation of the carboxylic function, some of the products were treated with amines to increase the polarity [61].

Scheme 8.20. Useful templates for SN Ar reactions.

Scheme 8.21. Liquid-phase synthesis of a pyrimidine-based library.

Alternatively, structurally diverse pyrimidines can be obtained by a de novo synthesis. The synthesis commencing with isothiouronium salts (R1 ¼ Ar-CH2 or resin-CH2 , see Section 2.3) is amenable to solution as well as to solid-phase applications. When condensing the isothiouronium salts with ketene derivatives, a pyrimidine skeleton with versatile functional groups is obtained. Oxidation of the alkylthio-linkage with m-chloroperbenzoic acid (m-CPBA) activates the molecule for SN Ar derivatization (see Scheme 8.22). The corresponding sulfinyl or sulfonyl compounds are then easily substituted with various amines [62]. In a very similar approach, the solution- and solid-phase synthesis of libraries of trisubstituted 1,3,5-triazines has been described previously [63]. Arylpiperazines have been identified as a privileged structural element in various biologically active compounds. Besides strategies involving the cyclization of a


Scheme 8.22. De novo synthesis of pyrimidines.

substituted aniline with bis-(2-chloroethyl)amine, a synthetic route based on nucleophilic aromatic substitutions was also required. Different fluorobenzenes with a nitro group either at the ortho or para position underwent SN Ar reactions with N-Boc-piperazine (see Scheme 8.23). After removal of the protection group, acylation under Schotten–Baumann conditions with a set of eight carboxylic acid chlorides gave 48 N-alkyl-N 0 -acylpiperazines [64]. This methodology has been well adapted to solid-phase chemistry, as reflected in recent reviews.

Scheme 8.23. Synthesis of an arylpiperazine library.

Resin-bound 4-fluoro-3-nitrobenzoic acid is also an outstanding template for nucleophilic aromatic substitution reactions with nitrogen nucleophiles. An enormous number of publications report the syntheses of benzodiazepin-2-ones, benzimidazoles, and related structures. Not surprisingly, only a short selection of examples can be described here. Owing to the importance of benzodiazepines in many therapeutic areas, fundamental work in the area of solid-phase synthesis has been carried out using this structural element [65]. The 4-fluoro-3-nitrobenzoic acid has been immobilized via the acid group and reacted with a variety of a- and/or b-substituted b-amino esters in DMF in the presence of DIEA. The reduction of the arylic nitro compound to the aniline was carried out using standard conditions [2 M SnCl2 H2 O in DMF, room temperature (rt)] (see Scheme 8.24). After hydrolysis of the ester with a mixture of 1 N NaOH/

Scheme 8.24. Benzodiazepine synthesis on solid support.

259

260


THF, the resulting compound was cyclized with DIC and HOBt and the 1,5benzodiazepin-2-one was obtained. Alternatively, selective alkylation at the N-5 position adds further diversity to the library [66]. Using a-amino acids in the place of the b-amino acids, the [6,6]-fused ring system of quinoxalin-2-ones has been accessed [67]. The synthetic strategy is an adaptation of TenBrinks and coworkers’ solution-phase synthesis on solid phase [68]. Variably substituted tetrahydroquinoxalin-2-ones can also be prepared based on 4-fluoro-3-nitrobenzoic acid. After substitution of the fluorine with primary aliphatic amines at room temperature and reduction of the nitro group, double acylation with chloroacetic anhydride has been shown to be the key step in the synthesis [69]. Further ring contraction to [6,5]-fused systems such as benzimidazoles has been the aim of other synthetic efforts – in solution and in solid-phase synthesis. Again, 4-fluoro-3-nitroarenes were linked to a solid support via an ether linkage [70] or via a carboxylic acid [71]. Commonly, both strategies use a SN Ar displacement reaction of the fluorine atom by an amine with subsequent reduction of the nitro group. Whereas Phillips and Wie [70] achieved immediate cyclization by condensation with benzimidates, a research group at Affymax acylated the intermediate with an activated bromoacetic acetic acid first (see Scheme 8.25). After displacement of the bromide groups by nucleophiles, cyclization occurred upon cleavage with a concomitant dehydration [72].

Scheme 8.25. Solid-phase synthesis of benzimidazoles.

Finally, benzimidazolones can also be prepared from solid-supported 4-fluoro3-nitrobenzoic acid. The key step in the synthesis was again the displacement of fluorine by a nitrogen nucleophile. The nitro group was reduced as described above and the resulting molecules underwent cyclization with the phosgene equivalent disuccinimidocarbonate [73]. An efficient liquid-phase synthesis of substituted benzimidazolones has also been described using a soluble polymer support [MeO-PEG, molecular weight (MW) 5000] [74]. This polymer support dissolves in many organic solvents (e.g.


DMF, THF) and precipitates in particular solvents (e.g. diethyl ether) (see Scheme 8.26). Again, 4-fluoro-3-nitrobenzoic acid was loaded onto the support and was then allowed to react with a variety of amines. After reduction of the nitro group, cyclization was achieved with trichlorophosgene.

Scheme 8.26. Benzimidazolones via solid-phase chemistry.

Recently, a synthetic route to substituted 7-azabenzimidazoles was published. As a key template the highly reactive 6-chloro-5-nitro-nicotinyl chloride was used. The sequential alkylation with different amines by replacement of the strongly activated chloro atoms proceeds easily at room temperature [75]. Another scaffold well suited to the generation of huge libraries by combinatorial methods is 1,5-difluoro-2,4-dinitrobenzene. The two fluoro groups in the ortho positions of two aromatic nitro groups can be sequentially substituted with two amines. As the scaffold is planar and symmetrical, no problems with regioselectivity occur. Lam and coworkers [76] demonstrated the viability of this concept by the production of a 2485-membered library designed for screening for antibacterial activity (see Scheme 8.27) – whereas the first substitution takes place within hours, the second runs overnight [76]. The authors then used the same concept for a solid-phase synthesis on 2-chloro-trityl resin, demonstrating that the two methods are complementary.

Scheme 8.27. 1,5-Difluoro-2,4-dinitrobenzene as a scaffold for combinatorial libraries.

The displacement of activated halides with nucleophilic amines such as piperazines is also a key step in the synthesis of antiviral quinolones and other pharmaceutically relevant compounds [77]. The starting materials, suitably substituted 3-oxo-3-phenyl-propanoates, are converted into enamines and cyclized to the quinolone core via an intramolecular SN Ar reaction (see Scheme 8.28). After ester hydrolysis, a wide range of amines can be introduced by displacement of an activated halogen, e.g. a fluorine atom. A library of related compounds was synthesized and screened for human immunodeficiency virus (HIV) suppression [78]. A solid-phase approach to quinolones was published using the same cycloarylation procedure (see Scheme 8.29). A resin-bound b-keto ester was transferred

261

262


Scheme 8.28. Synthesis of quinolones.

Scheme 8.29. Solid-phase approach to Ciprofloxacin.

into the enamine and cyclized using tetramethylguanidine (TMG). Many amines can be incorporated by displacement of the fluorine atom at C-7 before the products are cleaved off the resin [79]. 8.3.3

Oxygen Nucleophiles

Reactive arenes such as fluoro-nitroarenes, halopyridines, halopyrimidines, and halotriazines are preferred for reactions with oxygen nucleophiles [80]. Diarylethers can be prepared by simply reacting the well-known 4-fluoro-3-nitrobenzoic acid template with a wide range of functionalized phenols in a solid-phase reaction (see Scheme 8.30) [81].

Scheme 8.30. Diaryl ether via SN Ar reaction on solid support.


In solution-phase chemistry, diarylethers can be produced utilizing a polymersupported guanidine base. The reaction requires an excess of phenol to achieve complete conversion. The polymer-supported base deprotonates the phenol and also traps unreacted starting material (see Scheme 8.31). Since numerous phenols are commercially available, the method is well suited to library synthesis in an automated and parallel manner [82].

Scheme 8.31. Diaryl ether synthesis using polymer-supported reagents.

The seven- and eight-member ring systems of dibenzoxazepins and dibenzoxazocines, respectively, have been well investigated in combinatorial chemistry using solution- and solid-phase synthesis [83]. The target heterocycles are efficiently assembled via intramolecular aromatic substitution of the fluorine in 2fluoro-5-nitroarenes with the OH function of various phenols (see Scheme 8.32). For the solid-phase approach, the cyclization step was achieved using a 5% solution of DBU in DMF. DBU was found to give superior results when compared with TMG or N-methylmorpholine. The authors preferred a solid-phase approach rather than a solution-phase approach because yields were generally better while purities were identical.

Scheme 8.32. Solution- and solid-phase synthesis of medium-sized rings.

The value of trichlorotriazine as a template for library synthesis by sequential substitution of the chloro atoms is discussed in Section 8.3.2. Besides this, trichlorotriazine can react with a soluble polymer support (MeO-PEG-OH, MW 5000) to give PEG-bound dichlorotriazine, a new soluble electrophilic scavenger (see Scheme 8.33). Because of the high reactivity of the scavenger toward nucleophiles, it was used to remove alcohols at the end of ester or silyl ether-forming reactions [84].

263

264


Scheme 8.33. Ester synthesis using dichlorotriazine scavenger.

8.3.4

Sulfur Nucleophiles

The fluoro-nitroarene motif is also the most preferred template for SN Ar for sulfur nucleophiles. In many applications, a suitably protected form of cysteine as a b-mercapto acid is reacted with 4-fluoro-3-nitrobenzoic acid to form 1,5-benzothiazepin-4-ones, an important class of drugs in the treatment of cardiovascular disorders. For example, 4-fluoro-3-nitrobenzoic acid was treated with 1.5 equivalents of 9-fluorenylmethoxycarbonyl (Fmoc)-l-cysteine in DMF to be converted to the 2nitro-thioether (see Scheme 8.34) [85]. After the reduction of the nitro group and a subsequent reductive alkylation (difficult because of the poor nucleophilicity of some anilines), the resulting secondary anilines were cyclized to form the sevenmember thiazepine ring.

Scheme 8.34. Solid-phase synthesis of 1,5-benzothiazepin-4-ones.

Fmoc-homocysteine reacts with similar effectiveness to a nucleophile in the same reaction sequence. Shortening the side-chain from b-mercapto acid to a-mercapto acids, the SN Ar reaction is also reliable in this case. However, the synthetic utility is limited owing to the fact that only a very small number of a-mercapto acids are commercially available. Using a reversed synthetic strategy, a great variety of other fused heterocycles is accessible [86]. Wang resin loaded with cysteine via a carbamate linker can be reacted with numerous halo-nitroarenes bearing diverse functional groups.


Recently, immobilized 4-fluoro-3-nitrobenzoic acid was transformed into 2amino-4-carboxythiophenol as an intermediate for the synthesis of several classes of heterocyclic compounds such as benzothiazoles or 3,4-dihydro-1,4-benzothiazines (see Scheme 8.35) [87]. The resin-bound 2-amino-4-carboxythiophenol was prepared by displacement of the fluorine atom with triphenylmethylmercaptan, reduction of the nitro group, and removal of the sulfur-protecting group.

Scheme 8.35. Solid-phase synthesis of 2-amino-4-carboxythiophenol.

8.3.5

Macrocyclization Reactions

In this section the reaction for closing large rings by nucleophilic substitution of activated arenes will be discussed. Cyclorelease reactions such as those used in the well-established syntheses of hydantoins [88] or diketopiperazines [89] will not be covered. At first sight, the closure of medium-sized or large rings by nucleophilic displacement does not meet the demands of combinatorial synthesis. No new substituents are incorporated and therefore no diversification is achieved. On closer inspection, the great importance of this reaction becomes obvious in view of the immense difference between the three-dimensional structure of a linear oligopeptide and the corresponding cyclic analog. So SN Ar macrocyclizations have become an important part of solid-phase organic synthesis, especially for the preparation of libraries of b-turn mimics. Based on the experiences presented earlier in this chapter, suitably substituted fluoro-nitrobenzoic acids are the substrates of choice for intramolecular SN Ar reactions [90]. Small libraries of 14-membered macrocyclic diaryl ethers and thioethers can be produced using a very similar procedure. Precursors are synthesized by the acylation reaction of solid-supported peptides with 3-fluoro-4-nitro benzoic acid. The substrates undergo cyclization by displacement of the fluorine with the phenolic oxygen of a tyrosine derivative [91] or with the thiol group of cysteines [92] under exceptionally mild conditions (see Scheme 8.36). Diversity could be increased via postmodification reactions of the nitro group. Recently, Burgess and coworkers reported in detail on their research on libraries of peptide turn mimetics [93]. The effects on nucleophilicity, product ring size, resins, and other reaction conditions were examined. Optimized procedures to produce 13- to 16-membered ring systems are described [93]. Larger rings are synthesized by the replacement of the fluorine atom in three regioisomeric fluoro-nitrobenzoic acids to prepare analogs of tocinoic acid (see Scheme 8.37) [94]. In this synthesis the amino group of lysine serves as an internal nucleophile for the closure of the macrocycle.

265

266


Scheme 8.36. Macrocyclizations using SN Ar reactions.

Scheme 8.37. Synthesis of large-ring systems.


3 4 5 6

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Kiselyov, Tetrahedron 1999, 55, 5827– 5830; b) X. Ouyang, A. S. Kiselyov, Tetrahedron 1999, 55, 8295–8302; c) X. Ouyang, Z. Chen, L. Longbin, C. Dominguez, A. S. Kiselyov, Tetrahedron 2000, 56, 2369–2377. a) A. Falchi, M. Taddei, Org. Lett. 2000, 2, 3429–3431; b) H. Deleuze, D. C. Sherrington, J. Chem. Soc., Perkin Trans. 2 1995, 2217–2221; c) C. R. Johnson, B. Zhang, P. Fantauzzi, M. Hocker, K. M. Yager, Tetrahedron 1998, 54, 4097–4106. a) B. Yan, G. Kumaravel, Tetrahedron 1996, 52, 843–848; b) M. K. Schwarz, D. Tumelty, M. A. Gallop, J. Org. Chem. 1999, 64, 2219–2231. a) G. C. Morton, J. S. M. Salvino, R. F. Labaudinie`re, T. F. Herpin, Tetrahedron Lett. 2000, 41, 3029–3030; b) A. Nefzi, N. A. Ong, M. A. Giulianotti, J. M. Ostresh, R. A. Houghten, Tetrahedron Lett. 1999, 40, 4939–4942. T. S. Yokum, J. Alsina, G. Barany, J. Comb. Chem. 2000, 2, 282–292. a) S. Hobbs-DeWitt, J. S. Kiely, C. J. Stankowic, M. C. Schroeder, D. M. Reynolds-Cody, M. R. Pavia, Proc. Natl. Acad. Sci. USA 1993, 90, 6909– 6913; b) S. Hobbs-DeWitt, A. W. Czarnik, Acc. Chem. Res. 1996, 29, 114–122. A. K. Szardenings, T. S. Burkoth, H. H. Lu, D. W. Tien, D. A. Campbell, Tetrahedron 1997, 53, 6573–6593. E. A. Jefferson, E. E. Swayze, Tetrahedron Lett. 1999, 40, 7757–7760. A. S. Kiselyov, S. Eisenberg, Y. Luo, Tetrahedron 1998, 54, 10635–10640. A. S. Kiselyov, S. Eisenberg, Y. Luo, Tetrahedron Lett. 1999, 40, 2465–2468. a) Y. Feng, K. Burgess, Chem. Eur. J. 1999, 5, 3261–3272; b) Z. Wang, S. Jin, Y. Feng, K. Burgess, Chem. Eur. J. 1999, 5, 3273–3278; c) A. J. Zhang, S. Khare, K. Gokulan, D. S. Linthicum, K. Burgess, Bioorg. Med. Chem. Lett. 2001, 11, 207–210. C. Fotsch, G. Kumaravel, S. K. Sharma, A. D. Wu, J. S. Gounarides, N. R. Nirmala, R. C. Petter, Biorg. Med. Chem. Lett. 1999, 9, 2125–2130.

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Electrophilic Substitution in Combinatorial and Solid-phase Synthesis Jan-Gerd Hansel and Stephan Jordan 9.1

Introduction

Reactions in which an electron-deficient reagent attacks a substrate and an electron-deficient leaving group is displaced are called electrophilic substitutions (SE ). The most common leaving group is the proton. While the reaction is typical for aromatic systems, aliphatic substrates only react when hydrogens of sufficient acidity are available. This chapter focuses on the application of SE in the generation of chemical diversity. Compared with nucleophilic substitution reactions (see Chapter 8) the use of SE in the context of combinatorial chemistry is rare. Much still remains to be done in this area of research. Although a number of functional group transformations involving SE mechanisms have been applied to solution- and solid-phase synthesis, the potential of the reaction to link building blocks in a combinatorial sense is restricted to very special cases. The wide application of acid-labile linkers in solidphase synthesis often prohibits the use of electrophilic reagents since the linkers are electron-rich aromatics and thus highly susceptible to aromatic SE . Reactions involving SE mechanisms are frequently used in building-block synthesis. Despite its importance in the overall workflow of combinatorial chemistry, this aspect is beyond the scope of this chapter and will not be covered. Reference will only be made to the use of SE in the preparation of linkers and solid supports (see also Chapter 4). SE reactions involving organometallic reagents or carbon nucleophiles will be dealt with in the appropriate chapters. Traditional synthetic methods involving SE have been supplemented by modern transition metal-catalyzed substitution reactions. Please refer to other Chapters for their applications in combinatorial chemistry.


9.2 Electrophilic Substitution at Aliphatic Carbons

9.2

Electrophilic Substitution at Aliphatic Carbons 9.2.1

Halogen Electrophiles

Halogenated carbonyl compounds are important synthetic intermediates, especially in the formation of heterocycles. Suitable reaction conditions have been devised for the transformation of enolizable carbonyl compounds into the corresponding a-bromo derivatives. The reagent of choice is pyridinium perbromide, which can be employed in both solid-phase [1] and solution-phase synthesis. In the latter case the commercially available polymer-bound version of the reagent has been frequently used [2]. As an example of this transformation the solution-phase bromination of acetophenones to a-bromo-acetophenones will be discussed. The transformation has been incorporated in an oxidation, bromination, and nucleophilic substitution reaction sequence (see Scheme 9.1). Notably, this sequence is performed by adding all the necessary polymer-supported reagents to the starting phenylethanol at the same time in a single reaction vessel. The final yield in this multistep/onechamber solution-phase synthesis is higher than the combined yields of the three steps performed sequentially [3].

Scheme 9.1. Solution-phase bromination of acetophenones.

9.2.2

Nitrogen Electrophiles

The formation of hydrazones by the reaction of diazonium salts with activated methylene compounds can be adapted to parallel synthesis by linking either of the reactants to a polymer support. Polymer-supported aryl diazonium cations have been treated with the potassium salt of Meldrum’s acid at 25 C to give the corresponding 5-phenylhydrazone derivatives of Meldrum’s acid in good yield [4]. Alternatively, polymer-supported Meldrum’s acid anion reacts with various aryl diazonium fluoroborates at 25 C in acetonitrile, yielding the same products [5]. Aliphatic diazo compounds are obtained on solid support from the reaction of tosyl or mesyl azide with immobilized activated methylene compounds. The substrates used are b-ketoesters [6], b-ketoamides [7] (see Scheme 9.2), or malonic acid derivatives [8]. Typically, the reaction is carried out at room temperature using an excess of the azide and an even larger excess of triethylamine or diisopropylethylamine.

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Scheme 9.2. Solid-phase diazotation.

9.2.3

Carbon Electrophiles

Enamines react with a number of electrophiles such as electron-poor olefins. The reaction is used in heterocycle chemistry, as shown in a solid-phase synthesis of dihydropyridines (see Scheme 9.3) [9].

Scheme 9.3. Solid-phase enamine substitution and cyclization.

9.3

Electrophilic Substitution at Aromatic Carbons 9.3.1

General Remarks

Almost all kinds of SE reactions involving arene substrates are used in solutionphase chemistry as a powerful synthetic tool. In solid-phase chemistry the use of this reaction is limited owing to its incompatibility with electron-rich linkers. With the exception of a few examples mentioned below, aromatic SE is restricted to the functionalization of polystyrene-based supports [10]. Polystyrene can be brominated, nitrated, and acylated or alkylated applying Friedel–Crafts conditions in solvents such as carbon tetrachloride or nitrobenzene. For example, in 1988 Ajayaghosh and Pillai [11] demonstrated the preparation of a photosensitive resin using SE reactions (see Scheme 9.4). Commercially available

Scheme 9.4. Solid-phase Friedel–Crafts acylation.

9.3 Electrophilic Substitution at Aromatic Carbons

polystyrene (crosslinked with 1% divinylbenzene) was first acylated with acetyl chloride under typical Friedel–Crafts conditions. The resulting ketone was reduced to the corresponding alcohol and then halogenated. Using a second SE reaction, a nitro group was introduced with fuming nitric acid at low temperature. Higher reaction temperatures applied during acylation of solid-supported material may lead to side-reactions such as partial dealkylation of phenyl groups and hence to soluble polymers. Although Friedel–Crafts alkylations on polystyrene are possible under harsh conditions (strong acids), there are more suitable methods for this kind of CaC bond formation. Reaction of an immobilized organometallic compound (most commonly lithiated by a halogen–lithium exchange reaction) [12] with alkyl halides can be considered as the method of choice. 9.3.2

Halogen Electrophiles

In accordance with their inherent preference for electrophilic substitutions, many heterocycles are easily halogenated. For example, a simple bromination can serve as a starting point for further diversification when followed by palladium-catalyzed CaC bond formation (see Scheme 9.5). In addition to Suzuki couplings, the 6bromonalidixic acid derivative obtained in the bromination step can also undergo Heck reaction [13].

Scheme 9.5. Solution-phase arene bromination and subsequent Suzuki coupling.

There are only a few examples of arene brominations on solid support in the literature. Using N-bromosuccinimide in dimethyl formamide at room temperature, electron-rich arenes such as thiophenes can be brominated (see Scheme 9.6). Combination with a Stille coupling and reiteration of the reaction sequence leads to oligothiophenes – new materials with interesting optical and electronic properties [14].

Scheme 9.6. Solid-phase arene bromination and subsequent Stille coupling.

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A rare example of direct halogenation on a solid support has been reported for phenols. The phenolic moiety of tyrosine undergoes iodination when treated with bis-(pyridine) iodonium(I) tetrafluoroborate (Ipy2 BF4 ) for no more than 10 min (see Scheme 9.7) [15].

Scheme 9.7. Solid-phase iodination.

The only example in this chapter in which the leaving group in an SE reaction is not a proton involves germanium-based linkers. These linkers have been developed for solid-phase synthesis as a means of traceless linkage (see Chapter 4). However, reaction of germanium-linked substrates with bromine rapidly releases the corresponding aryl bromides by ipso substitution of the germanium by bromine (see Scheme 9.8). Aryl iodides can be prepared by the same method using iodomonochloride [16].

Scheme 9.8. Solid-phase ipso substitution of germanium by bromine.

9.3.3

Nitrogen Electrophiles

Very recently the diazotization of aromatic compounds has found applications in combinatorial chemistry. The formation of diazonium salts and the coupling to electron-rich aromatics to give azo dyes can be performed using polymer-supported reagents (see Scheme 9.9) [17]. Polymer-bound aryl diazonium salts also play a pivotal role in the chemistry of triazene linkers.

9.3 Electrophilic Substitution at Aromatic Carbons

Scheme 9.9. Solution-phase diazotation and azo coupling.

9.3.4

Carbon Electrophiles

Friedel–Crafts chemistry is rarely used to generate diversity. One of the few examples is the superacid-induced solution-phase synthesis of a small library of 3,3-diaryloxindoles (see Scheme 9.10) [18]. The reaction proceeds smoothly in pure triflic acid at room temperature.

Scheme 9.10. Solution-phase Friedel–Crafts alkylation.

The Pictet–Spengler reaction is a very well-established method for the synthesis of tetrahydroisoquinolines and tetrahydro-b-carbolines both in solution and on solid support [19]. The molecules are easily prepared by intramolecular reaction of an iminium ion with an arene usually under acidic conditions. The imines are typically formed by condensing amines with (aromatic) aldehydes (see Scheme 9.11), cyclohexanones, or aryl methyl ketones [20].

Scheme 9.11. Solution-phase Pictet–Spengler reaction.

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Using tryptophan as the starting material, the synthesis of tricyclic carbolines has been reported in numerous examples in the literature. Whereas reactions with aldehydes are normally complete within hours, less reactive ketones require up to several days to reach complete conversion (see Scheme 9.12) [21].

Scheme 9.12. Solid-phase Pictet–Spengler reaction.

A Pictet–Spengler reaction has been used as the key step during the synthesis of indolyl diketopiperazine-based libraries of Fumitremorgin C analogs (see Scheme 9.13). This natural product, isolated by fermentation of the fungi Aspergillus fumigatus, appears to be of interest in the area of central nervous system (CNS) and cancer research and has therefore resulted in the preparation of some solid-phase combinatorial libraries [22].

Scheme 9.13. Solid-phase synthesis of Fumitremorgin C analogs.

The Pictet–Spengler reaction is not limited to the synthesis of six-member ring systems. Also the class of seven-member rings present in the pharmaceutically important diazepine class of compounds is easily accessed by condensing aldehydes with suitably substituted aminomethylfurans (see Scheme 9.14) [23].

Scheme 9.14. Solution-phase Pictet–Spengler reaction yielding seven-member rings.

References

In the closely related Bischler–Napieralski reaction dihydroisoquinolines are formed. The intramolecular condensation of an acylated amine with an arene is mediated by the action of a strong dehydrating agent, usually phosphoryl chloride. It leads to dihydroisoquinolines, which can be transformed either into isoquinolines by oxidation or into tetrahydroisoquinolines by reduction (see Scheme 9.15). The harsh reaction conditions are not generally amenable to solid-phase reactions since most linkers are not stable to a large excess of phosphoryl chloride in toluene at 80 C [24].

Scheme 9.15. Solid-phase Bischler–Napieralski reaction.

The reaction has also been applied to solution-phase synthesis, but has found little application in library production to date [25]. In a Reissert-type reaction involving pyrrole derivatives, CaC bond formation is achieved by aromatic SE on solid support using N-oxides (see Scheme 9.16). The reaction also works with indoles and with enamines [26].

Scheme 9.16. Solid-phase Reissert reaction.

References 1 A. Barco, S. Benetti, C. De Risi,

C. Marchetti, C. P. Pollini, V. Zanirato, Tetrahedron Lett. 1998, 39, 7591–7594. 2 a) J. Habermann, S. V. Ley, J. S. Scott, J. Chem. Soc. Perkin Trans. 1 1998, 3127–3130; b) J. Habermann, S. V. Ley, R. Smits, J. Chem. Soc. Perkin Trans. 1 1999, 2421–2423; c) J. Habermann, S. V. Ley, J. J. Scicinski, J. S. Scott, R. Smits, A. W.

3 4

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Thomas, J. Chem. Soc. Perkin Trans. 1 1999, 2425–2427. J. J. Parlow, Tetrahedron Lett. 1995, 36, 1395–1396. B. P. Bandgar, J. V. Totare, J. N. Nigal, Ind. J. Heterocycle Chem. 1998, 8, 77–78. B. P. Bandgar, A. M. Tavhare, S. S. Pandit, Ind. J. Chem., Sect. B 2000, 38, 721–723. M. Cano, F. Camps, J. Joglar,

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Tetrahedron Lett. 1998, 39, 9819– 9822. F. Zaragoza, S. V. Petersen, Tetrahedron 1996, 52, 5999–6002. a) D. L. Whitehouse, K. H. Nelson Jr, S. N. Savinov, D. J. Austin, Tetrahedron Lett. 1997, 38, 7139–7142; b) M. R. Gowravaram, M. A. Gallop, Tetrahedron Lett. 1997, 38, 6973–6976; c) D. L. Whitehouse, K. H. Nelson Jr, S. N. Savinov, R. S. Lo¨we, D. J. Austin, Bioorg. Med. Chem. 1998, 6, 1273–1282. M. F. Gordeev, D. V. Patel, J. Wu, E. M. Gordon, Tetrahedron Lett. 1996, 37, 4643–4646. a) J. H. Adams, R. M. Cook, D. Hudson, V. Jammalamadaka, M. H. Lyttle, M. F. Songster, J. Org. Chem. 1998, 63, 3706–3716; b) G. Orosz, L. P. Kiss, Tetrahedron Lett. 1998, 39, 3241–3242. A. Ajayaghosh, V. N. R. Pillai, Tetrahedron Lett. 1988, 21, 6661–6666. a) S. Havez, M. Begtrup, P. Vedso, J. Org. Chem. 1998, 63, 7418–7420; b) Z. Li, A. Ganesan, Synlett 1998, 405–406. C. Plisson, J. Chenault, Heterocycles 1999, 51, 2627–2637. P. R. L. Malenfant, J. M. Frećhet, Chem. Commun. 1998, 2657–2658. ã, G. G. Arsequell, G. Espun Valencia, J. Barluenga, R. P. Carloń, J. M. Gonza´lez, Tetrahedron Lett. 1998, 39, 7393–7396. a) A. C. Spivey, M. C. Diaper, H. Adams, J. Org. Chem. 2000, 65, 5253–5263; b) M. J. Plunkett, J. A. Ellman, J. Org. Chem. 1997, 62, 2885– 2893. M. Caldarelli, I. R. Baxendale, S. V. Ley, Green Chem. 2000, 43–45. D. A. Klumpp, K. Y. Yeung, G. K. S. Prakash, G. A. Olah, J. Org. Chem. 1998, 63, 4481–4484.

19 P. P. Fantauzzi, K. M. Yager,

20

21

22

23

24

25

26

Tetrahedron Lett. 1998, 39, 1291– 1294. M. G. Siegel, M. O. Chaney, R. F. Bruns, M. P. Clay, D. A. Schober, A. M. Van Abbema, D. W. Johnson, B. E. Cantrell, P. J. Hahn, D. C. Hunden, D. R. Gehlert, H. Zarrinmayeh, P. L. Ornstein, D. M. Zimmerman, G. A. Koppel, Tetrahedron 1999, 55, 11619–11639. a) J. P. Mayer, D. Bankaitis-Davis, J. Zhang, G. Beaton, K. Bjergarde, C. M. Andersen, B. A. Goodman, C. J. Herrera, Tetrahedron Lett. 1996, 37, 5633–5637; b) L. Yang, L. Guo, Tetrahedron Lett. 1996, 37, 5041–5044; c) L. Yang, Tetrahedron Lett. 2000, 41, 6981–6984; d) X. Li, L. Zhang, W. Zhang, S. E. Hall, J. P. Tam, Org. Lett. 2000, 2, 3075–3078. a) A. van Loevezijn, J. H. van Maarseveen, K. Stegman, G. M. Visser, G.-J. Koomen, Tetrahedron Lett. 1998, 39, 4737–4740; b) H. Wang, A. Ganesan, Org. Lett. 1999, 1, 1647–1649. a) X. Feng, J. C. Lancelot, A. C. Gillard, H. Landelle, S. Rault, J. Heterocycle Chem. 1998, 35, 1313– 1316; b) S. Vega, M. S. Gil, V. Darias, C. C. Sanchez Mateo, M. A. Exposito, Pharmazie 1995, 50, 27–33. ¨nzer, a) K. Ro¨lfing, M. Thiel, H. Ku Synlett 1996, 1036–1038; b) W. D. F. Meutermans, P. F. Alewood, Tetrahedron Lett. 1995, 36, 7709–7712. a) V. Jullian, J. C. Quirion, H. P. Husson, Eur. J. Org. Chem. 2000, 7, 1319–1325; b) S. Deprets, G. Kirsch, Eur. J. Org. Chem. 2000, 7, 1353– 1357. M. Z. Hoemann, M. MelikianBadalian, G. Kumaravel, J. R. Hauske, Tetrahedron Lett. 1998, 39, 4749–4752.

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Elimination Chemistry in the Solution- and Solid-phase Synthesis of Combinatorial Libraries Demosthenes Fokas and Carmen Baldino 10.1

Introduction

One of the challenges for organic chemists involved in the burgeoning field of combinatorial chemistry is to rediscover new uses and applications of old reactions and subsequently adapt them to either solid- or solution-phase chemistry. Elimination reactions, which have been studied thoroughly and used extensively by the chemistry community in the synthesis of several complex molecules, fall into this category [1]. Among them, b-eliminations prevail in organic synthesis with numerous applications in the preparation of olefins. Although olefins are versatile and useful intermediates for combinatorial chemistry, the synthesis of libraries of olefinic substrates by adaptation of the classical b-elimination reaction has not received much attention so far, presumably because of the lack of convergence in the synthesis and diversity of the final products. Instead, b-eliminations along with other elimination reactions are gaining favor in solid-phase synthesis as a release strategy of the desired products from solid support. In this chapter we will address elimination reactions from a mechanistic rational including: (1) b-eliminations, (2) conjugate eliminations, and (3) addition– elimination reactions. We will also discuss the utility of these transformations in combinatorial chemistry. Olefination reactions such as the Wittig, Horner– Emmons, and ring-closing olefin metathesis, which could fall into the addition– elimination category, will not be discussed herein since they will be addressed in different chapters.

10.2

b-Eliminations in Combinatorial Chemistry

b-Elimination has been used for peptide synthesis since 1967. However, broader applications have been limited, presumably because the cleavage conditions as described needed to be tightly controlled [2]. The expanded utility of b-elimination reactions in combinatorial chemistry and solid-phase synthesis has been realized Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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very recently with the advent of new linkers [3]. The design of traceless linkers that leave no residue on the cleaved product enabled chemists to envision b-elimination reactions as an effective release strategy of the desired products from solid support. The term traceless usually defines linkers that leave no obvious residue on the cleaved molecule. A traceless linker is defined as one where a new CaH or CaC bond is formed at the linkage site of the cleaved molecule. However, this definition has been expanded to linkers that include other cleavage reactions. Although there are several transformations in combinatorial chemistry that involve an intermediate b-elimination step, we will focus on release-based b-eliminations and their application to combinatorial chemistry. 10.2.1

The Hofmann Elimination Solid-phase Synthesis of Tertiary Amines 10.2.1.1 Via a Regenerated Michael Acceptor (REM) Resin

The Hofmann elimination can be traced back to 1851 when Hofmann first reported the elimination of quaternary ammonium compounds [4]. The reaction is usually considered to be a useful method for the synthesis of alkenes and indeed has been previously used by Blettner and Bradley to synthesize dehydroalanine derivatives on resin [5]. The utility of this reaction in combinatorial chemistry was not realized until 1996 when Rees and coworkers introduced this reaction as part of a traceless linker strategy for the solid-phase synthesis of libraries of tertiary amines (Scheme 10.1) [6].

Scheme 10.1. Synthesis of tertiary amines on a REM resin.

This strategy utilizes a hydroxymethylpolystyrene resin (1) derivatized as the acrylate ester (2) which upon a Michael addition of a primary or secondary amine gives the secondary or tertiary resin-bound amine (3) respectively. In the case where a primary amine is added, the resulting resin-bound secondary amine can be converted into tertiary amine (3) by a reductive alkylation, thus introducing a new element of diversity. Quaternization of the tertiary amine with an alkyl halide

10.2 b-Eliminations in Combinatorial Chemistry

to give ammonium salt (4) introduces another site of diversity and activates the linker for cleavage by a facile Hofmann elimination reaction. Then, NiPr2 Et at room temperature liberates the tertiary amine (5) into solution and regenerates the acrylate resin (2). Similarly, tertiary amines with the general structure 5 and with three sites of diversity can result from a resin-bound equivalent of ammonia (6), which can be derived from the coupling of 9-fluorenylmethoxycarbonyl (Fmoc)-balanine to hydroxymethylpolystyrene resin followed by Fmoc deprotection. Since the resin linker 2 is regenerated after elimination of the product and is functionalized via a Michael reaction, this resin was referred to as a REM (regenerated Michael acceptor) resin. The REM resin system can be used in the monoalkylation of diamines without the use of protecting groups (Scheme 10.2). For example, piperazine can be added to resin 2 to give the monoalkylated derivative 7, which can then be acylated or alkylated cleanly at the second nitrogen [7]. Treatment of resin-bound piperazine (7) with isocyanates or alkyl halides can generate compounds of the general structure 8 or 9 respectively. Quaternization and elimination provides disubstituted piperazine adducts 10 and 11 in high purity. A library of 125 piperazines with the general structure 11 was prepared and a few of the compounds were found to be active against d-opioid receptors [8]. The high purity of substrate 11 demonstrates that although quaternization at the undesired nitrogen in piperazine (9) is possible, it does not reduce the purity of the final product since only the desired product can be cleaved from the resin.

Scheme 10.2. Immobilization and derivatization of diamines on a REM resin.

The overall yield for the three-step sequence ranges from 40% to 80% and is substrate dependent. The purity of the isolated products is good because only the desired tertiary resin-bound amine will be susceptible to the elimination conditions. The method works well when the quaternization step can be conducted at ambient temperature using reactive alkyl halides (i.e. allyl, benzyl, and methyl). The quaternization step only requires heat when less reactive alkylating agents are

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used. However, quaternization at elevated temperatures can cause the Hofmann elimination to occur prematurely, releasing the tertiary amine into solution where it is susceptible to quaternization by a second equivalent of alkylating agent [7]. Therefore, a more thermally stable linker needs to be developed in order to prevent premature cleavage when less reactive alkyl halides are used. Quaternization can also be problematic with sterically hindered alkylating agents or when sterically hindered resin-bound amines are involved. Although the Michael addition of primary and secondary alkyl amines to resin 2 works well, there are no examples describing the addition of anilines to resin 2 and, consequently, the synthesis of tertiary anilines. Addition of anilines would presumably require more vigorous reaction conditions. The REM linker is stable to both mildly acidic and basic conditions, making possible Fmoc protection of amines and tert-butyl ester protection of carboxylic acids. Boc protection of amines is also likely to be compatible with the resin, since the cleavage conditions are likely to be similar to those used for deprotecting tertbutyl esters. The amount of racemization occurring during the synthesis of Nalkylated a-amino ester derivatives on REM resin is likely to be minimal. Furthermore, the REM resin 2 can be recycled and used successfully with no substantial loss of its reactivity. No decrease in yield or purity of products was observed even after five synthesis cycles [7]. However, the ester linkage is not compatible with Grignard reagents, metal hydride reducing agents, or transesterification conditions, which limits the scope of reactions that can be performed on solid phase. However, this problem can be addressed through the use of the more stable sulfone REM resins 12–14, which have been successfully used in the synthesis of amine libraries. These sulfone REM resins provide enhanced chemical stability and compatibility with a wider range of chemical reagents and reaction conditions [9]. Recently, the amide REM resin derivatives have emerged and two of these resins (15, 16) are currently under evaluation for the automated preparation of amine libraries (Scheme 10.3) [10].

Scheme 10.3. Other REM resins used in the synthesis of tertiary amines.

Via a Safety-catch Resin A small library of 13 G-coupled protein receptor agonists and antagonists was produced by Wade et al. using the sulfide safety-catch resin 17 (Scheme 10.4) [11]. Quaternization of resin 17 followed by oxidation gave resin-bound sulfone 18, which resembles those derived from the corresponding vinyl sulfone REM resins via a Michael addition–quaternization sequence [9]. Sulfone 18 underwent a Hof10.2.1.2


Scheme 10.4. Hofmann elimination on a safety-catch resin.

mann elimination, upon treatment with Me 2 NH, to furnish vinyl sulfone resin 20 and release in solution the desired tertiary amine with general structure 19 in a 25% average yield for the six-step sequence. However, all of the final compounds had to be purified by reversed phase high-performance liquid chromatography (HPLC) owing to a minor 3-chlorobenzoic acid impurity. This problem can be circumvented by extended washing of the activated resin with 2 N HCl in tetrahydrofuran (THF), presumably exchanging the anions of the ammonium salt for chloride. The advantage of the sulfide safety-catch resin is the decreased sensitivity of the alkylated product to b-elimination, which could be utilized to increase the yield or extend the scope of alkylating agents to less activated systems. However, oxidation of the sulfide resin prior to amine synthesis would be necessary for more oxidation-sensitive systems. 10.2.1.3 Via a Hydroxylamine Resin

A Hofmann-type elimination has also been utilized independently by both Grigg (Scheme 10.5, route a) and Andersson (Scheme 10.5, route b) in the synthesis of tertiary methylamines using an extremely robust and versatile traceless linker [12]. Polystyrene resin (21) with a hydroxylamine linker attached can be converted to resin-bound tertiary hydroxylamine 22. Quaternization of 22 with MeOTf resulted in the alkoxyammonium intermediate 23, which upon treatment with NEt3 in CH2 Cl2 furnished the resin-bound aldehyde 25 and released into solution the tertiary methylamine with the general structure 24. An alternative route involving the exposure of 23 to much milder reagents such as lithium iodide (in dioxane or ace-

Scheme 10.5. Hofmann elimination on a hydroxylamine resin.

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tonitrile) or samarium iodide (in THF) also resulted in highly efficient cleavage delivering tertiary methylamines of high purity in good overall yields. The quaternization works only with methyl triflate as the alkylating reagent, thus limiting the diversity of the produced library. Although other triflates prepared in situ via AgOTf/alkyl halide exchange gave the corresponding quaternary salts in solution, this approach is unsuitable for the solid-phase sequence because of interference of reaction kinetics from the silver halide precipitate. Both routes are amenable to solution-phase parallel synthesis by careful selection of reaction conditions and work-up methods starting from O-benzylhydroxylamine. Although this hydroxylamine resin (22) is more stable than the ester REM resin (2) toward a variety of nucleophilic reagents, it limits a library design to tertiary methylamines. Furthermore, this resin cannot be recycled once it is used – an advantage of the REM resins. Alternative Cleavage Techniques A major challenge associated with the cleavage of amines from resins is the removal of any excess cleavage reagents and byproducts from the reaction mixture. The process generally requires extraction or chromatography, which could render the synthesis of large libraries cumbersome. The pursuit of other cleavage strategies that would allow for the direct isolation of pure compounds has become necessary. Murphy and coworkers introduced a novel two-resin system in which the resin-bound quaternary ammonium compounds were treated with an excess of a second resin-bound amine such as the weakly basic ion-exchange resin Amberlite IRA-95 in dimethylformamide (DMF) and a catalytic amount of Et3 N (Scheme 10.6) [13]. Under these conditions, highly pure products were recovered with good yields after filtration and evaporation of the solvent. 10.2.1.4

Scheme 10.6. Proposed mechanism for the two-resin system-promoted Hofmann elimination.

Similar results were obtained by treating the resin-bound quaternary ammonium compounds with Amberlite weakly basic ion-exchange resin in DMF in the absence of any additional base. The basic resin is sufficient to achieve cleavage and avoid the need for an aqueous work-up. These surprising two-resin results may be explained by a thermal elimination of the amine from the resin as the HBr salt.


The basic resin then desalts the amine to catalyze the b-elimination. Alternatively, it may also be due to trace amounts of base, present either from previous steps or from trace dimethylamine in the DMF. It was found that yield was a function of reaction time with an optimal length of 18 h for a 14-member library production. Identical results generating highly pure products were obtained using a deprotected Rink amide resin, albeit at lower yield [13]. A novel Wang resin-bound piperazine base (26) that resembles Murphy’s tworesin system was introduced by Yamamoto et al. (Scheme 10.7) [14]. It was used successfully (2 equiv., loading 1.62 mmol g1 ), in the absence of any other external base, to cleave N-aryl-N 0 -benzylpiperazines from the resin by treating the quaternary ammonium compounds with resin 26 in CH2 Cl2 for 16 h at room temperature. However, the caveat of this reagent and the ion-exchange resin is that they complicate the reusability of the REM resin since at the end of the sequence both resins are mixed together.

Scheme 10.7. Polymer-supported bases used in Hofmann elimination.

A soluble noncrosslinked polystyrene-bound basic reagent (NCPS-NEt2 ) (27) has been developed recently by Janda and coworkers (Scheme 10.7) [15]. Use of base 27 (3 equiv., loading 0.85 mmol g1 ) in CH2 Cl2 eliminates the need for purification and allows the direct isolation of a library of pure tertiary amines through simple filtration and concentration operations. The advantage of this method over the ion-exchange resin and the polymer-supported base methods is that it allows for the recycling of the REM resin by taking advantage of the insolubility of cleavage reagent 27 in methanol. Once the cleavage is complete, filtration of the reaction mixture separates the REM resin from the tertiary amine and the soluble reagent 27. Concentration of the filtrate followed by trituration with cold MeOH results in precipitation of 27. Filtration of the resulting slurry effectively separates the noncrosslinked polystyrene reagent and evaporation of the filtrate leaves behind the tertiary amine in good yields and high purity. However, a large amount of MeOH might be required to triturate the soluble reagent 27, which could render the synthesis of large libraries cumbersome. Alternatively, Brown has reported a vapor-phase elimination approach as a rapid method for the cleavage of tertiary amines from REM resins 2 and 12 in Irori

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MacroKans TM [16]. The MacroKans TM were placed in a glass peptide vessel, which was then sealed under a slight positive pressure of ammonia gas. Products were isolated cleanly in good yields after evaporation, resin sorting, and washing with CH3 CN or dimethylsulfoxide (DMSO). This is a particularly suitable method of parallel processing for the synthesis of large libraries, thus minimizing or eliminating the impurities due to the cleavage reagent. 10.2.2

b-Elimination on Selenyl Resins

The oxidation of selenides to selenoxides and their thermal elimination to alkenes has been studied extensively and has found numerous applications in synthesis [17]. The chemistry was first adapted to solid phase in 1976 when Heitz and coworkers prepared a polymer-supported selenide and oxidatively eliminated it to release an a; b-unsaturated ketone [18]. However, its application in solid-phase synthesis as a cleavage method was realized with the advent of the polymer-bound selenium reagents used in cyclization reactions to construct resin-bound carbocyclic scaffolds [19]. Recently, Nicolaou et al. utilized it in a strategy to generate a 10,000-compound benzopyran library by a solid-phase split-and-pool technique using Irori’s NanoKan TM technology [20]. This novel strategy involves immobilization of an o-prenylated phenol (28) through cycloloading with a polystyrene-based selenyl bromide resin [21] to give resin-bound benzopyran scaffold 29 via a precedented 6-endo-trig cyclization (Scheme 10.8) [22]. Further elaboration of 29 to 30 and subsequent cleavage from solid support via oxidation and spontaneous syn-elimination of the selenoxide tether provides benzopyran 31. This is an example of a traceless release from solid support where functionality is generated at the released molecule instead of any linker residue being incorporated at the cleavage site.

Scheme 10.8. Release of benzopyrans via a resin-bound selenoxide elimination.


Indeed, the newly formed double bond can serve as a starting point for the generation of secondary libraries or more focused libraries, thus introducing additional elements of diversity. For example, epoxidation of the released benzopyran 31 followed by ring opening of the intermediate epoxide 32 with a variety of nucleophiles provides access to a new series of benzopyran derivatives with the general structure 33. Additionally, benzopyrans with the general structure 34 can result from further elaboration of the secondary hydroxyl group of 33 with a series of electrophiles [20c]. Taking into consideration the current advances in asymmetric epoxidation of olefinic substrates, this sequence could provide entry to chiral benzopyran libraries starting from chiral benzopyran epoxides [23]. The loading to the selenium solid support is compatible with a great variety of prenylated phenols except for substrates with electron-withdrawing groups adjacent to the prenyl group or adjacent to the phenol hydroxyl group participating in the cyclization step. Therefore, these scaffolds have to be loaded with the electronwithdrawing groups masked. The elimination of the selenoxide resin tether proceeds smoothly at room temperature and seems to be independent of the substitution pattern of the benzopyran scaffold. A similar resin-bound selenoxide elimination on 2-seleno carbohydrates (35) was utilized by Nicolaou et al. in the synthesis of a small library of carbohydrate orthoesters representing novel regions of the potent antibiotic everninomicin (Scheme 10.9) [24]. Oxidation of glycoside 35 with meta-chloroperbenzoic acid (mCPBA) gave the corresponding resin-bound selenoxide, which underwent a thermal syn-elimination. This thermal selenoxide elimination to the intermediate ketene acetal 36 introduces the desired functionality for the formation of orthoesters 37 and 38 and can release the desired products in a traceless manner from solid support. Orthoester 37 is formed from glycoside 35a and 2,3-allyl orthoester 38 is formed from deprotected glycoside 35b. However, unlike the solution-phase selenoxides, it was observed that the resin-bound selenoxide was more prone to eliminate at room temperature and therefore necessitated the use of lower temperatures in the oxidation step. Thus, treatment of selenide 35 with m-CPBA in CH2 Cl2 at 78 C, followed by rapid filtration and transfer to a sealed tube, was found to give the best results. Although the chemistry has been developed in both solution- and solid-phase chemistry, it is well suited to the solid-phase synthesis of novel semisynthetic everninomicins and other carbohydrate libraries.

Scheme 10.9. Carbohydrate orthoesters via a resin-bound selenoxide elimination.

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10.2.3

b-Elimination on Sulfone Resins

Sulfone elimination has the potential to be a preferred strategy for solid-phase compound cleavage. While no linker and cleavage strategy can be stable to the full range of conditions available to the synthetic chemist, the oxidative activation– elimination strategy promises to increase substantially the variety of options. Schwyzer et al. first described a 2-(4-carboxyphenylsulfonyl)ethanol linker for the synthesis of peptides and oligonucleotides [25]. However, the application and utility of a sulfone-type linker in peptide synthesis was demonstrated in 1992 by Katti et al. with the introduction of a new and readily available linker in the solidphase synthesis of C-terminal peptides (Scheme 10.10) [26]. For example, Leuenkephalin (40) was released from resin 39 in 54% or 60% overall yields, using either Boc or Fmoc chemistries respectively, after cleavage from the solid support with dioxane/MeOH/4 N NaOH followed by re-acidification.

Scheme 10.10. Release of C-terminal peptides via a b-elimination from a sulfone resin.

Apart from being used successfully in the area of peptide chemistry, sulfone linkers have gained favor in the solid-phase synthesis of small molecules. For example, 4-aminobenzenesulfonamides with the general structure 44 were prepared from 2-mercaptoethanol resin 42 (Scheme 10.11) [27]. Resin 42 was converted to sulfone 43, which underwent a facile b-elimination to release the desired

Scheme 10.11. Release of arylsulfonamides via a b-elimination from a sulfone resin.


4-aminobenzenesulfonamides, in good yields and purities, upon treatment with aqueous NH4 OH. A small library of seven dehydroalanine derivatives has been prepared by a belimination of a sulfinate resin (Scheme 10.12). Anchoring of cysteine onto Merrifield resin through the side-chain thiol group gave a resin-bound sulfide (46). Modification of both C- and N-termini and oxidation of the sulfide with m-CPBA followed by a b-elimination of the sulfinate resin 48 furnished the dehydroalanine derivatives 47 in 31–86% yields with high purities after aqueous work-up [28].

Scheme 10.12. Dehydroalanine derivatives via a sulfone elimination.

Elimination of a sulfinate resin has also been utilized in heterocyclization chemistry for the synthesis of a few 2-substituted-4-piperidone derivatives (50) from resin-bound sulfone 49, which serves as a divinyl ketone synthon (Scheme 10.13). The amine reagent can act both as a nucleophile and as a base, thus promoting a Michael addition to resin 49 and inducing elimination of the sulfinate resin followed by a second Michael addition to the newly formed enone [29].

Scheme 10.13. 2-Substituted-4-piperidones via a sulfone elimination.

10.2.4

b-Elimination on Silyl Resins

b-Elimination was utilized in the cleavage of several silyl amide linkers (SAL) and trimethylsilylethyl ester linkers for the facile release of peptide fragments from the solid support (Scheme 10.14). Stabilization of a carbocation by a b-trialkylsilyl group, as shown in intermediate 52, seems to facilitate the release of C-terminal

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Scheme 10.14. Release of peptides via a b-silyl elimination on silyl amide linkers.

amides (53) from silyl resin 51 [30]. b-Elimination of the trialkylsilyl group neutralizes the transient carbocation to give a stable styrene derivative (54). These silyl amide linkers gave improved yields of C-terminal tryptophan amides over conventional linkers since an irreversible alkylation of the tryptophan indole nucleus by such carbocations is suppressed. However, acid scavengers (1,2-ethanedithiol/ phenol/thioanisole, 5:3:2) were needed as the styrene moiety is sensitive to protonation. Therefore, purification of the final product is required in order to remove the scavenger byproducts. Similarly, linker 55 was designed to be cleaved by a b-elimination mechanism based on the 2-(trimethylsilyl)ethylester protecting group (Scheme 10.15) [31]. Fluoridolysis or dilute acid cleavage enabled the preparation of protected peptide fragments such as 56. C-Terminal tryptophans or prolines could be successfully anchored with this linker and no undesired alkylation or diketopiperazine formation was observed upon cleavage.

Scheme 10.15. Release of peptides via b-silyl elimination on 2-(trimethylsilyl)ethyl ester linkers.

A b-elimination mechanism is also involved in the release of olefins 60 from resin-bound allyl silanes such as 59, the product of a solid-phase cross-olefin metathesis between allylsilane 58 and an olefin (Scheme 10.16) [32].


Scheme 10.16. Release of olefins from an allyl silane resin via a b-silyl elimination.

10.2.5

b-Elimination on Fluorenyl Resins

b-Elimination was also implemented in the cleavage of several fluorene-based linkers for the facile release of peptide fragments from solid support (Scheme 10.17) [33]. Quantitative cleavage of the Merrifield peptide 62 and peptide 63, which corresponds to the sequence 31–38 of uteroglobin, was achieved from the fluorene resin 61 in good yields and high purities with 20% morpholine in DMF or 10% piperidine in DMF. Resin 61 proved to be superior to other fluorenederived resins where incomplete removal of the protected peptide from the resin has been described [33b]. Also, slight lability to N,N-diisopropylethylamine, which was used at the neutralization step after Boc deprotection in peptide synthesis, and basic amino groups of the growing peptide has been detected occasionally in other fluorene-derived resins [33c]. The fluorene nucleus in resin 61 has been conveniently substituted with an electron-donating N-amide group to fine-tune its base lability in order to prevent any premature cleavage of the growing peptide chain.

Scheme 10.17. Release of C-terminal peptides via a b-elimination from a fluorenyl resin.

10.2.6

b-Elimination on 2-(2-Nitrophenyl)ethyl Resins

b-Elimination was also utilized with 2-(2-nitrophenyl)ethyl (NPE) linkers. Release of 3 0 -hydroxy-and 3 0 -phosphateoligonucleotides 66 and 67 from CPG (controlled pore glass) support was achieved through carbonate and phosphate linkers 64 and 65 respectively (Scheme 10.18) [34]. The conditions used were either 0.5 M 1,8diazabicyclo[5.4.0]undecene-7 (DBU) in dioxane, pyridine for 1 h, and ammonia for 5 h at 55 C or 20% piperidine in DMF for 3 h. The linkage was found to be resistant to 40% Et3 N in pyridine for 16 h, conditions commonly used to remove the 2-cyanoethylphosphate protecting group. An important application of 2-(2nitrophenyl)ethyl linkages is that these supports can be used together with p-

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nitrophenylethyl-protected nucleoside 2-cyanoethylphosphoramidites for the preparation of oligonucleotides without using ammonia during the final deblocking, because all protecting groups will be cleaved by DBU. This strategy will be of interest for the preparation of oligonucleotides containing ammonia-sensitive compounds such as base analogs, fluorescent compounds, and so on.

O

Scheme 10.18. Release of oligonucleotides via a b-elimination.

10.2.7

Radical-based b-Eliminations 10.2.7.1 b-C,O Bond Scission

Peukert and Giese devised the original photolabile linker (68) based on the radicalinduced b-C,O bond scission of a 2-pivaloylglycerol group for the release of immobilized acids (Scheme 10.19) [35]. Upon irradiation, an a-hydroxyalkyl radical intermediate (69) is generated via a Norrish type I reaction with release of carbon monoxide and a t-butylradical that leads to isobutene. Elimination then takes place where the glycerol radical is converted into an enolate radical (70) and a carboxylic acid (71) is released. The reaction is not solvent dependent but selection of the irradiation wavelength is crucial. The pivaloyl linker 68 was found to cleave aromatic

Scheme 10.19. Release of carboxylic acids–peptides by a radical b-C,O scission.

10.3 Conjugate Eliminations

carboxylic acids and peptides with high yields (65–93%) and purities in various solvents such as THF, CH2 Cl2 , dioxane, and DMSO by irradiation with light above 320 nm. The photo byproducts are either volatile (CO and isobutene/isobutane) or inert resin-bound acetone. The linker proved to be stable upon treatment with acids and bases and to be compatible with many reagents and reaction conditions, such as palladium-catalyzed cross-coupling and epoxidation, with broad applicability in combinatorial chemistry. b-C,Se Bond Scission-Release of Olefins Release of olefins from solid support can also be achieved under mild reductive conditions. Olefins can be loaded onto polymer support by treatment with the polymer-bound selenium bromide resin and released reductively under the influence of Bu3 SnH-AIBN (cat). A radical b-elimination is responsible for producing the release. For example, once olefin 72 is loaded onto solid support as the resinbound bromoselenide (73), reductive debromination of 73 followed by a homolytic cleavage of the CaSe bond and subsequent elimination of resin-bound selenol (74) will regenerate the starting olefin (72) (Scheme 10.20) [21]. However, purification would be required to isolate the released olefin from tin byproducts. Alternatively, Curran and coworkers’ fluorous hydrides in fluorinated solvents could be utilized as well in order to isolate the released olefin in pure form from tin byproducts through a liquid–liquid extraction [36]. In addition, Barton et al.’s buffered Nethylpiperidine hypophosphite reagent in conjuction with Et3 B as the radical initiator instead of AIBN could provide a viable alternative for this reductive release [37]. Although this radical-based release strategy has not been implemented for library synthesis, it is well suited to the solid-phase synthesis of small molecules. 10.2.7.2

Scheme 10.20. Release of olefins by a radical b-C,Se scission.

10.3

Conjugate Eliminations 10.3.1

1,6-Conjugate Eliminations

Conjugate eliminations were also envisioned as a release strategy of the desired products from solid support, thus leading to the development of new linkers.

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Waldmann and coworkers developed an enzyme-labile 4-acyloxybenzyloxy linker system (75), which was hydrolyzed with lipase RB 001–05 on TentaGel resin (Scheme 10.21) [38]. The linker is attached through a carboxyl group as an amide to the solid support. The resulting resin contains an acyl group, such as acetate, which can be cleaved by lipases or esterases. A phenolate is thus generated which fragments to give a quinone methide (76) and releases compound 77, the desired product. The quinone methide remains bound to the solid support and is quenched by water or an additional nucleophile.

Scheme 10.21. Enzymatic release based on a conjugate 1,6-elimination.

In this way, amines (bound as urethanes), alcohols (bound as carbonates), and carboxylic acids (bound as esters) can be detached from the polymeric carrier under very mild conditions (pH 5–7, room temperature (rt)) and with complete selectivity. The substrate specificity of the enzyme guarantees that only the intended ester is cleaved, and the mild conditions of the biocatalyzed transformations ensure that the compounds constructed on the solid support remain intact during cleavage. The applicability of the enzyme-labile group to multistep synthesis on solid support was proven by the synthesis of tetrahydro-b-carboline carboxylic acids (79) by means of the Pictet–Spengler reaction and their subsequent enzyme-mediated release from resin 78. A similar 1,6-conjugate elimination mechanism is also involved in the release of carboxylic acids, peptides, amines, and alcohols in good yields by the fluorideinduced cleavage of several labile silyl linkers, such as 80–82 (Scheme 10.22) [39]. Apart from the desired carboxylic acid, linker 82 also releases a quinone methide, which is scavenged by thiophenol. Consequently, the final products have to be purified to remove the thiophenol byproduct.

10.3 Conjugate Eliminations

Scheme 10.22. Fluoride-induced release based on a conjugate elimination.

10.3.2

1,4-Conjugate Eliminations

A conjugate 1,4-elimination of a phenylsulfinate anion yielding functionalized 3arylbenzofurans in a traceless manner was observed in a novel cyclofragmentation pathway involving epoxides (85) (Scheme 10.23) [40]. Sulfone anion 85a undergoes a 5-exo-trig cyclization to alkoxide 85b, which next collapses to benzofuran 86 by a concomitant expulsion of both formaldehyde and a phenylsulfinate anion. This cyclofragmentation-release pathway has been developed in both solution and solid phase. It is well suited to solid-phase synthesis and can lead to the generation of a diverse family of 3-arylbenzofurans starting from commercially available or proprietary 2-hydroxybenzophenones.

Scheme 10.23. 3-Arylbenzofurans by a 1,4-conjugate cyclofragmentation-release assay.

Greater structural diversity can be introduced by starting from a series of 2hydroxybenzophenones (84) or made directly from the addition of arylmagnesium bromides to resin-bound salicaldehydes (83), followed by an oxidation of the inter-

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mediate carbinol. All cleavage products are remarkably clean since the mechanism only allows for the release of products generated through intermediate 85. The limitation of this novel fragmentation pathway lies in the necessity for epoxides derived from 2-hydroxybenzophenones, since both aryl groups are required for the regioselective epoxide opening.

10.4

Addition–Elimination Reactions

Addition–elimination reactions in which a CaC or CaX bond (X ¼ heteroatom) is formed by the combination of two or more reactive centers, in an intra- or intermolecular fashion, followed by an elimination or extrusion of a small molecule can be useful in combinatorial chemistry for the convergent synthesis of diverse libraries. The structural diversity, which originates directly from the starting materials or building blocks, is retained through the library synthesis. If a polymer support is directly linked to the eliminated molecule, then a traceless release of the desired compounds from solid support can be achieved. Although there are several mechanistically different addition–elimination transformations, here we will identify those utilized in the synthesis of libraries where the addition–elimination step is the final process that completes the synthesis of the desired scaffold. 10.4.1

Addition–Elimination on Vinylogous Systems 10.4.1.1 Entry to Aminomethyleneoxazolones

Aminomethyleneoxazolones are recognized as a class of serine protease inhibitors, and therefore have been the focus of library synthesis [41]. The synthesis involves the addition of a primary or secondary amine to an ethoxymethyleneoxazolone (EMO) (87) followed by elimination of a molecule of ethanol (Scheme 10.24). The convergent nature of this scheme greatly facilitates its execution starting from either commercially available or proprietary amines and ethoxymethyleneoxazolones. Baldino et al. reported the automated parallel synthesis of 1600 amino-

Scheme 10.24. Solution-phase synthesis of aminomethyleneoxazolones.

10.4 Addition--Elimination Reactions

methyleneoxazolones (89) starting from 20 EMOs and 80 secondary amino aketoamide intermediates (88), prepared in situ from ten a-ketoesters and eight diamines under equimolar reagent combinations [42]. The chemistry works well in a wide range of solvents such as CH3 CN, DMF, THF, and dioxane. Ultimately, the choice of the appropriate solvent depends mainly on the solubility of the starting oxazolones. The corresponding products were isolated as E,Z isomers, in good yields and high purities, after evaporation of the solvent. Although there are no examples illustrating the solid-phase implementation of this chemistry, it could be amenable to solid phase as well (i.e. via a resin-bound EMO). However, mild cleavage conditions would be required, since strong acidic or basic conditions might disrupt the oxazolone ring system. 10.4.1.2 Entry to Benzopyrones

Substituted benzopyrones encompass an important class of molecules that possess a wide range of interesting biological activities [43, 44]. Although there are numerous literature methods for the synthesis of the benzopyrone ring system, they are not ideally suited for combinatorial approaches owing to harsh reaction conditions, poor yields, and limited substituent tolerance. A method that is amenable to solution-phase parallel synthesis was developed by Brueggemeier and coworkers for the preparation of a seven-member benzopyrone library (Scheme 10.25) [44]. This sequence utilizes chlorination of a bis-silylated salicylic acid (90) with oxalyl chloride to generate the corresponding acid chloride (91) followed by a Sonogashira coupling with a terminal alkyne to give cleanly alkynone 92. Treatment of alkynone 92 with a secondary amine such as diethylamine in refluxing ethanol provides an intermediate enaminone (94a) which undergoes a disilylation and an intramolecular Michael addition followed by elimination of volatile diethylamine to give the benzopyrone 95. Removal of silyl impurities along with the palladium and copper catalysts and any triphenylphosphine requires purification.

Scheme 10.25. Entry to benzopyrones via an addition–elimination sequence.

The problems identified above were circumvented by a resin capture strategy. Indeed, capturing alkynone 92 with a fivefold excess of piperazinyl Merrifield resin

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(93) (loading 0.7 mmol g1 ) eliminates the need for any further purification [45]. Thus, the resin-bound enaminone 94b could be easily separated from the excess reagents and byproducts of the reaction mixture by a simple filtration. On-resin cyclization of the enaminone 94b released the benzopyrone 95 in good yields and good purities and regenerated the piperazinyl resin. This resin capture would facilitate a one-pot conversion of silylated salicylic acids to benzopyrones without requiring any intermediate purification steps – an attractive feature for the synthesis of large libraries. Furthermore, this method can be applied to the synthesis of benzopyrones with no residual functionalities required for linkage to the solid support. The solid-supported capture reagent can be regenerated and recycled for additional rounds of resin capture. 10.4.1.3 2,3-Dihydro-4-pyridone Libraries

The reaction of Danishefsky’s diene with imines leading to 2,3-dihydro-4pyridones has been studied extensively by the chemistry community and has numerous applications in the synthesis of piperidine alkaloids [46]. The reaction is postulated to proceed via a Mannich reaction followed by an intramolecular Michael reaction with a subsequent elimination of a molecule of methanol [46a–c]. The large number of readily available aromatic aldehydes and amines prompted chemists to adapt this reaction to both solid- and solution-phase automated synthesis of large libraries of 2,3-dihydro-4-pyridones. Application of this chemistry to solution-phase parallel synthesis was demonstrated recently by Yu et al. in the construction of a 4320-member library (Scheme 10.26) [47a]. Condensation of benzaldehydes with anilines in the presence of a Yb resin [48] gave the intermediate imine, which upon immediate treatment with Danishefsky’s diene gave the corresponding dihydropyridones (96) in good yields and purities after filtration of the resin-bound catalyst.

Scheme 10.26. Solution-phase synthesis of a 2,3-dihydro-4-pyridone library.

Adaptation of this chemistry to solid phase was demonstrated by Wang and Wilson starting from Wang resin-bound benzaldehyde and anilines or aliphatic amines [47b]. Several different Lewis acids were found to be compatible with


the reaction conditions, including ZnCl2 , AlCl3 , Et2 AlCl, TiCl 4 , BF3 Et2 O, and Yb(OTf )3 . However, the water-tolerant Yb(OTf )3 gave the highest yield. The final products were obtained after cleavage with trifluoroacetic acid (TFA)/CH2 Cl2 (1:1) in relatively high yields and purities. 10.4.2

Cycloreversions 10.4.2.1 Pyrrole Libraries

A cycloreversion reaction with loss of CO2 is involved in the pyrrole synthesis via a 1,3-dipolar cycloaddition of alkynes to mu¨nchnones. Mjalli and coworkers and Armstrong and coworkers reported independently the solid-phase synthesis of pyrroles where the mu¨nchnone precursors – N-acyl-N-alkyl-a-amino amides – were generated in a single step via an Ugi four-component condensation (U-4CC) instead of relying upon individual acylated amino acids (Scheme 10.27) [49]. Treatment of the N-acyl-N-alkyl-a-amino acid 97a with Ac2 O or treatment of Armstrong’s cyclohexenamide precursor 97b with HCl in the presence of an acetylenic dipolarophile resulted in the bicyclo intermediates 98a and 98b, which rapidly aromatized to pyrroles 99a and 99b, respectively, with loss of CO2 .

Scheme 10.27. Entry to pyrroles via a dipolar cycloaddition–cycloreversion pathway.

Pyrroles 99a were isolated in 40% overall yield with high purities, according to Mjalli and coworkers’ protocol, after release from solid support. Armstrong and coworkers’ four-step protocol, although not optimized, gave pyrroles 99b with 4– 17% yields. Although this chemistry is amenable to solution-phase synthesis, low yields of the tetra-substituted pyrroles are usually observed [50]. This may be attributed to the substitution pattern of the acetylenic dipolarophile and to the tendency of mu¨nchnones to self-condense. Furthermore, it would require the use of the corresponding N-acyl-N-alkyl-a-amino acids as building blocks in order to increase the purity of the final products.

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10.4.2.2 Imidazole Libraries

Application of the same cycloaddition–cycloreversion strategy described above resulted in the solid-phase synthesis of a 12-member library of 2,4,5triarylimidazoles by employing an aryltosylimine as the dipolarophile (Scheme 10.28) [51]. Treatment of the resin-bound acid 100 with N 0 -(3-dimethylaminopropyl)-N-ethylene carbodiimide (EDC) in CH2 Cl2 at ambient temperature for 24– 48 h followed by a cycloaddition of the intermediate mu¨nchnone 101 with a tosylimine gave bicyclo compound 102. Cycloreversion of 102 with elimination of toluenesulfinic acid and CO2 provided the polymer-linked imidazole 103, which was next washed with TFA, without any observed cleavage, to remove any unreacted starting materials. Release from resin upon treatment with AcOH at 100 C gave the free imidazole 104. All reagent combinations provided the desired products in good yields and high purities with a 73% average yield over the six-step sequence. However, low yields of imidazoles were observed when this chemistry was carried out in solution phase [52b]. This is at least partly the result of the potential for mu¨nchnones to self-condense, which can be suppressed in a solid-phase approach [52].

Scheme 10.28. Entry to imidazoles via a dipolar cycloaddition–cycloreversion pathway.

Traceless Solid-phase Synthesis of Furans Isomu¨nchnones readily undergo a [3 þ 2] cycloaddition with acetylenes to give bicyclo intermediates, which can lead to furans after a cycloreversion and elimination of isocyanate [53]. This strategy was applied to the solid-phase synthesis of furan libraries independently by Gowravaram and Gallop and by Austin and coworkers (Scheme 10.29) [54]. a-Diazocarbonyl 107 can react with Rh(II) catalysts to form a highly reactive rhodium carbenoid that collapses to a mesoionic dipole intermediate – the isomu¨nchnone 108 – which in the presence of an acetylenic dipolarophile can lead to a bicyclo intermediate (109). Cycloreversion of 109 and extrusion of the resin-bound isocyanate liberates the tetrasubstituted furan 110 into solution, in good yields and high purities, and leaves no obvious remnant of polymer tethering in the desired product. 10.4.2.3


Scheme 10.29.

Tetrasubstituted furans via a cycloaddition–cycloreversion pathway.

The cleavage rate from solid support, at similar temperatures, is highest in polar protic solvents, which could allow for the cleavage to be carried out directly in aqueous media. The chemistry could be amenable to solution-phase parallel synthesis since furans were recovered in good yields but would require a special workup or purification to remove the rhodium catalyst. The released isocyanate could be effectively scavenged with a resin-bound amine. 10.4.2.4 1,2-Diazines

Access to a small library of functionalized 1,2-diazines can be provided by the inverse electron demand Diels–Alder reactions of 3,6-substituted-1,2,4,5-tetrazines on solid phase (Scheme 10.30) [55a]. Treatment of an immobilized azadiene (111) with a variety of electron-rich olefins in dioxane at room or elevated temperatures gave bicyclo intermediate 112, which underwent a cycloreversion with loss of N2 and concomitant loss of HX to give resin-bound diazine 113. Removal of the Boc group first, followed by cleavage from solid support under basic conditions, gave the corresponding 3-amino-6-thiomethyl-1,2-pyridazines (114) in good to moderate yields. The chemistry can be extended with the azadienes bearing a sulfone group in the 6-position. The sulfone substrate is more reactive than the corresponding thiomethyl substrate in a Diels–Alder reaction as more efficient conversion is generally achieved with the less reactive alkynes. A wide range of electron-rich dienophiles can be used which permits the introduction of two diversity elements on

Scheme 10.30. Entry to diazines via a [4 þ 2] cycloaddition–cycloreversion pathway.

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heteroaromatic scaffolds. Subsequent nucleophilic aromatic substitution of the C-6 methylsulfide/sulfone and acylation/alkylation of the C-3 amine will introduce the third and fourth diversity elements. This chemistry could be amenable to solution phase although library purification might be required for the less reactive enamines or enol ethers derived from acetophenones [55b].

10.5

Summary

b-Elimination has been successfully used in solid-phase synthesis as a release strategy of the desired products from solid support. However, it has not been broadly applied in the synthesis of diverse libraries as the final step that determines the synthesis of the desired scaffold. This is presumably because of the lack of convergence in the synthesis and diversity of the final products. On the contrary, addition–elimination reactions, apart from also being used as a release method of the final products from solid support, can be utilized in the convergent synthesis of diverse libraries. However, b-elimination could be very well utilized in the synthesis of functionalized intermediates where the generated functionality (i.e. double bond) could serve as a handle for the introduction of additional diversity elements. Although elimination reactions can be very useful to the highthroughput synthesis of small molecules by either solid- or solution-phase parallel synthesis, their potential has not been fully realized yet.

References 1 J. March in: Advanced Organic

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3

4 5

Chemistry, 4th edn, Wiley-Interscience, New York 1992, pp. 982–1050. a) G. I. Tesser, J. T. W. A. R. M. Buis, E. T. M. Wolters, E. G. A. M. Bothe´Helmes, Tetrahedron 1976, 32, 1069– 1072; b) G. I. Tesser, B. W. J. Ellenbroek in: Eighth European Peptide Symposium. Beyerman, H. C., Linde, A. v. d., Brink, W. M. v. d. (eds), Amsterdam 1967. For a review on linkers, see: a) F. Guillier, D. Orain, M. Bradley, Chem. Rev. 2000, 100, 2091–2157; b) I. W. James, Tetrahedron 1999, 55, 4855– 4946. A. W. Hofmann, Annalen 1851, 78, 253–286. C. Blettner, M. Bradley, Tetrahedron Lett. 1994, 35, 467–470.

6 J. R. Morphy, Z. Rankovic, D. C. Rees,

Tetrahedron Lett. 1996, 37, 3209–3212. 7 A. R. Brown, D. C. Rees, Z.

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Rankovic, J. R. Morphy, J. Am. Chem. Soc. 1997, 119, 3288–3295. J. Cottney, Z. Rankovic, J. R. Morphy, Bioorg. Med. Chem. Lett. 1999, 9, 1323–1328. For synthesis of tertiary amines via vinyl sulfone REM resins, see: a) F. E. K. Kroll, R. Morphy, D. Rees, D. Gani, Tetrahedron Lett. 1997, 38, 8573–8576; b) P. Heinonen, H. Lo¨nnberg, Tetrahedron Lett. 1997, 38, 8569–8572. M. J. Plater, A. M. Murdoch, J. R. Morphy, Z. Rankovic, D. C. Rees, J. Comb. Chem. 2000, 2, 508–512. W. S. Wade, F. Yang, T. J. Sowin, J. Comb. Chem. 2000, 2, 266–275.

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M. Thoroughgood, Tetrahedron Lett. 2000, 41, 6635–6638; b) M. Gustafsson, R. Olsson, C.-M. Andersson, Tetrahedron Lett. 2001, 42, 133–136. X. Ouyang, R. W. Armstrong, M. M. Murphy, J. Org. Chem. 1998, 63, 1027–1032. Y. Yamamoto, K. Tanabe, T. Okonogi, Chem. Lett. 1999, 103–104. P. H. Toy, T. S. Reger, K. D. Janda, Org. Lett. 2000, 2, 2205–2207. A. R. Brown, J. Comb. Chem. 1999, 1, 283–285. Y. Nishibayashi, S. Uemura in: Topics in Current Chemistry. Wirth, T. (ed.), Springer Verlag, Berlin 2000, vol. 208, pp. 201–214. R. Michels, M. Kato, W. Heitz, Makromol. Chem. 1976, 177, 2311– 2320. For cyclizations mediated by resinbound selenium reagents, see: a) K. Fujita, K. Watanabe, A. Oishi, Y. Ikeda, Y. Taguchi, Synlett 1999, 1760–1762; b) K. C. Nicolaou, J. A. Pfefferkorn, G.-Q. Cao, S. Kim, J. Kessabi, Org. Lett. 1999, 1, 807– 810. a) K. C. Nicolaou, J. A. Pfefferkorn, A. J. Roecker, G.-Q. Cao, S. Barluenga, H. J. Mitchell, J. Am. Chem. Soc. 2000, 122, 9939–9953; b) K. C. Nicolaou, J. A. Pfefferkorn, H. J. Mitchell, A. J. Roecker, S. Barluenga, G.-Q. Cao, R. L. Affleck, J. E. Lillig, J. Am. Chem. Soc. 2000, 122, 9954–9967; c) K. C. Nicolaou, J. A. Pfefferkorn, S. Barluenga, H. J. Mitchell, A. J. Roecker, G.-Q. Cao, J. Am. Chem. Soc. 2000, 122, 9968– 9976. K. C. Nicolaou, J. Pastor, S. Barluenga, N. Winssinger, Chem. Commun. 1998, 1947–1948. For a solution-phase precedent of selenium-mediated 6-exo-trig cyclizations of ortho-prenylated phenols and related systems, see: a) D. L. J. Clive, G. Chittattu, N. J. Curtis, W. A. Kiel, C. K. Wong, J. Chem. Soc., Chem. Commun. 1977, 725–727; b) P. B. Anzeveno, J. Org. Chem. 1979, 44, 2578–2580; c) K. C.

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27 28

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32 33

34

35 36

Nicolaou, Z. Lysenko, J. Am. Chem. Soc. 1977, 99, 3185–3187. For asymmetric epoxidation of benzopyran systems with a polymerbound chiral salen manganese (III) complex, see: C. E. Song, E. J. Roh, B. M. Yu, D. Y. Hi, S. C. Kim, K.-J. Lee, Chem. Commun. 2000, 615–616. K. C. Nicolaou, H. J. Mitchell, K. C. Fylaktakidou, H. Suzuki, R. M. Rodrı´guez, Angew. Chem. Int. Ed. 2000, 39, 1089–1093. R. Schwyzer, E. Felder, P. Faili, Helv. Chim. Acta 1984, 67, 1316– 1326. S. B. Katti, P. K. Misra, W. Haq, K. B. Mathur, J. Chem. Soc., Chem. Commun. 1992, 843–844. C. G. Echeverrıá, Tetrahedron Lett. 1997, 38, 8933–8934. M. Yamada, T. Miyajima, H. Horikawa, Tetrahedron Lett. 1998, 39, 289–292. A. Barco, S. Benetti, C. De Risi, P. Marchetti, G. P. Pollini, V. Zanirato, Tetrahedron Lett. 1998, 39, 7591–7594. H.-G. Chao, M. S. Bernatowicz, G. R. Matsueda, J. Org. Chem. 1993, 58, 2640–2644. H.-G. Chao, M. S. Bernatowicz, P. D. Reiss, C. E. Klimas, G. R. Matsueda, J. Am. Chem. Soc. 1994, 116, 1746–1752. M. Schuster, N. Lucas, S. Blechert, Chem. Commun. 1997, 823–824. a) F. Rabanal, E. Giralt, F. Albericio, Tetrahedron Lett. 1992, 33, 1775–1778; b) M. Mutter, D. Bellof, Helv. Chim. Acta 1984, 67, 2009–2016; c) Y. Z. Liu, S. H. Ding, J. Y. Chu, A. M. Felix, Int. J. Pept. Prot. Res. 1990, 35, 95–98. a) R. Eritja, J. Robles, D. Fernandez-Forner, F. Albericio, E. Giralt, E. Pedroso, Tetrahedron Lett. 1991, 32, 1511–1514; b) F. Albericio, E. Giralt, R. Eritja, Tetrahedron Lett. 1991, 32, 1515–1518. S. Peukert, B. Giese, J. Org. Chem. 1998, 63, 9045–9051. D. P. Curran, S. Hadida, S.-Y. Kim, Z. Luo, J. Am. Chem. Soc. 1999, 121, 6607–6615.

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38

39

40

41

42

43

44

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Jaszberenyi, J. Org. Chem. 1993, 58, 6838–6842. B. Sauerbrei, V. Jungmann, H. Waldmann, Angew. Chem. Int. Ed. 1998, 37, 1143–1146. a) R. Ramage, C. A. Barron, S. Bielecki, D. W. Thomas, Tetrahedron Lett. 1987, 28, 4105–4108; b) A. Routledge, H. T. Stock, S. L. Flitsch, N. J. Turner, Tetrahedron Lett. 1997, 38, 8287–8290; c) D. G. Mullen, G. Barany, J. Org. Chem. 1988, 53, 5240–5248. K. C. Nicolaou, S. A. Snyder, A. Bigot, J. A. Pfefferkorn, Angew. Chem. Int. Ed. 2000, 39, 1093–1096. N. L. Benoiton, F. Hudesz, F. M. F. Chen, Int. J. Pept. Prot. Res. 1995, 45, 266–271. C. M. Baldino, D. S. Casebier, J. Caserta, G. Slobodkin, C. Tu, D. L. Coffen, Synlett 1997, 488–490. E. S. C. Wu, J. T. Loch, B. H. Toder, A. R. Borrelli, D. Gawlak, L. A. Radov, N. P. Gensmantel, J. Med. Chem. 1992, 35, 3519–3525. A. S. Bhat, J. L. Whetstone, R. W. Brueggemeier, Tetrahedron Lett. 1999, 40, 2469–2472. A. S. Bhat, J. L. Whetstone, R. W. Brueggemeier, J. Comb. Chem. 2000, 2, 597–599. a) H. Kunz, W. Pfrengle, Angew. Chem. Int. Ed. 1989, 28, 1067–1068; b) H. Waldmann, M. Braun, M. Drager, Angew. Chem. Int. Ed. 1990, 29, 1468–1471; c) H. Waldmann, M. Braun, J. Org. Chem. 1992, 57, 4444– 4451; d) H. Yamamoto, K. Hattori, Tetrahedron 1993, 49, 1749–1760;

47

48 49

50 51 52

53 54

55

e) S. Kobayashi, H. Ishitami, S. Nagayama, Synthesis 1995, 1195–1202. a) L. Yu, C. M. Baldino, M. S. Harris, E. Marler, B. Carr, J. Troth, M. Kearny, J. Mills, J. Brochu, J. Gordon, D. L. Coffen, paper presented at the 217th ACS National Meeting, Anaheim, March 1999, Organic Division, abstract no. 335; b) Y. Wang, S. R. Wilson, Tetrahedron Lett. 1997, 38, 4021–4024. L. Yu, D. Chen, J. Li, P. G. Wang, J. Org. Chem. 1997, 62, 3575–3581. a) A. M. M. Mjalli, S. Sarshar, T. J. Baiga, Tetrahedron Lett. 1996, 37, 2943–2946; b) A. M. Strocker, T. A. Keating, P. A. Tempest, R. W. Armstrong, Tetrahedron Lett. 1996, 37, 1149–1152. T. A. Keating, R. W. Armstrong, J. Am. Chem. Soc. 1996, 118, 2574–2583. M. T. Bilodeau, A. M. Cunningham, J. Org. Chem. 1998, 63, 2800–2801. a) K. T. Potts in: 1,3-Dipolar Cycloaddition Chemistry. Padwa, A. (ed.), Wiley-Interscience, New York 1984, vol. 2, pp. 1–82; b) R. Consonni, P. D. Croce, R. Ferraccioli, C. La Rosa, J. Chem. Res., Synop. 1991, 188–189. M. H. Osterhout, W. R. Nadler, A. Padwa, Synthesis 1994, 123–141. a) M. R. Gowravaram, M. A. Gallop, Tetrahedron Lett. 1997, 38, 6973–6976; b) D. L. Whitehouse, K. H. Nelson, Jr, S. N. Savinov, D. J. Austin, Tetrahedron Lett. 1997, 38, 7139–7142. a) J. S. Panek, B. Zhu, Tetrahedron Lett. 1996, 37, 8151–8154; b) S. M. Sakya, K. K. Groskopf, D. L. Boger, Tetrahedron Lett. 1997, 38, 3805–3808.

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11

Addition to CC Multiple Bonds (Except for CC Bond Formation) Adrian L. Smith 11.1

Introduction

The last decade has witnessed the birth and maturation of combinatorial chemistry as a technique for synthesizing large numbers of compounds. Much of the early emphasis was on synthesizing large (mixtures) libraries for screening and lead generation, and this was almost exclusively based upon solid-phase chemistry. However, the technologies developed during this work were also applicable to the more traditional medicinal chemistry lead optimization process, and this has led to a rethink in the ways in which the medicinal chemist can most efficiently optimize a lead series, both in vitro and in vivo. Whilst this initially resulted in many people developing solid-phase synthetic routes for the parallel synthesis of arrays of single compounds (whether it be to make 10 or 10,000 compounds), the co-development of postsynthesis sample-handling techniques (particularly in the area of purification) has significantly broadened the scope of chemistries which can be used. In particular, the initial advantages of solid-phase chemistry (primarily related to purification) are now less compelling in many cases where solution-phase chemistry will require less chemistry development time and where crude synthetic products can be purified in an automated fashion. Today, the term ‘‘combinatorial chemistry’’ is loosely used to describe a very broad range of techniques including solid- and solution-phase chemistry and the synthesis of discrete (single) compound arrays and mixtures. Whilst the different techniques each have their pros and cons, their unifying factor when properly applied is the ability to address a particular problem (e.g. a medicinal chemistry problem) by making and testing a larger and more diverse set of compounds than would have been the case by more traditional methods. Therefore, although a large number of publications have appeared in the area of solid-phase chemistry, the purpose of this chapter is not to act as a comprehensive review of solid-phase chemistry per se, but rather to highlight key areas in which chemists may practically utilize these techniques in chemical transformations involving overall addition to carbon–carbon multiple bonds. It should be noted that certain transformations such as carbon–carbon bond formation, heterocycle formation, cycloHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

306

11 Addition to CC Multiple Bonds (Except for CC Bond Formation)

addition reactions, and transition metal-catalyzed formation of single bonds are covered in other chapters and will not be duplicated here.

11.2

Addition to C=C Double Bonds

Additions to CbC double bonds fall into two broad main categories dependent upon the electronic nature of the double bond. The main focus of this section will deal with electrophilic addition to isolated double bonds, whilst nucleophilic 1,4addition to conjugated a; b-unsaturated systems will be briefly touched upon at the end. The reactivity of the isolated CbC double bond arises from the nucleophilic nature of the p-bond, with the majority of reactions involving some form of electrophilic addition. This may result in formal oxidation (e.g. epoxidation) or reduction (e.g. hydrogenation). It may be expected that more electron-rich double bonds will generally exhibit greater reactivity than electron-deficient double bonds in the absence of overriding steric factors. Whilst the CbC double bond is also frequently encountered in radical reactions, this is usually implicated in CaC bond formation and is outside the scope of this chapter. This section aims to summarize some of the key transformations which can be effected by addition to the CbC double bond, emphasizing applicability to combinatorial chemistry. 11.2.1

Epoxidation and Subsequent Epoxide Opening

The epoxidation of an olefin represents one of the more versatile transformations available to the aspiring combinatorial chemist, generating a reactive epoxide intermediate which can be opened by a range of nucleophiles (Scheme 11.1). In particular, opening with amines gives rise to the (hydroxyethyl)amine isostere (3), which mimics the tetrahedral intermediate for amide hydrolysis [1], making this a very powerful two-step transformation of an olefin.

Scheme 11.1. Epoxidation and subsequent epoxide opening of an olefin.

Epoxidation is most usually carried out with meta-chloroperbenzoic acid (mCPBA) in a solvent such as dichloromethane and, being a very efficient and mild reaction in the absence of competing functionalities, generally gives clean and full

11.2 Addition to C=C Double Bonds

conversion with a minimal excess of reagent. As such, it is suitable for use in both solid- and solution-phase chemistry. It should be noted that there is often little diastereoselectivity with this reagent. Scheme 11.2 shows an example of an epoxidation carried out on solid phase [2]. Here, the author experienced instability of the urethane linker to the generated 3-chlorobenzoic acid byproduct and buffered the reaction with NaHCO3 . Another side-reaction sometimes observed is opening of the epoxide by the generated 3-chlorobenzoic acid, and in these cases more success may be possible with reagents such as dimethyldioxirane [3]. It is also possible to use a range of solid-supported reagents for epoxidation (see Table 11.1) [4].

Scheme 11.2. Epoxidation on solid phase.

The two-step processes shown in Scheme 11.1 are very well suited to solutionphase parallel synthesis. Generally, the intermediate (2) is prepared in bulk, purified, and then split into individual reactors for parallel ring opening by a range of nucleophiles. For reactions with amines to give 3, we find a small excess of the amine (typically 1.5 equiv.) in isopropyl alcohol heated at 65 C for 16 h usually gives clean conversion, even with relatively non-nucleophilic amines such as aniline. It is important that no epoxide remains in final products, since this can give misleading data in biological assays by acting as a suicide inhibitor. It is therefore generally preferred to use the epoxide as the limiting reagent. For reactions with secondary amines to give tertiary amine products, it may be possible to scavenge excess amine with an appropriate resin [9]. Alternatively, we find automated preparative high-performance liquid chromatography (HPLC) to be a convenient method for purification. Examples of larger solution-phase combinatorial mixtures libraries are known utilizing a similar approach. A library of over 6000 b-amino alcohols was prepared using LiClO4 in acetonitrile and 1.2 equiv. of amine [10]. In such cases, care is needed to validate the reactivity of reagents properly in order to avoid misleading bioassay data since purification is more difficult. A solid-phase mixtures library of

307

308

11 Addition to CC Multiple Bonds (Except for CC Bond Formation) Tab. 11.1.

Polymer-supported epoxidation reagents.

Reagents

Reaction Time

Yield

Comments

Ref.

48 h

13–97%

TentaGel resin (2 equiv.). 5 equiv. Oxone8. Generates dioxirane resin in situ

5

60 h

70–90%

2 equiv. of dioxirane resin used. Resin is prepared by treatment of polystyrene 2oxoalkyl resin with Oxone8 and NaHCO3 in THF/H2 O and isolated prior to use. Shelf stability of dioxirane resin not reported

6

4h

80%

AG 50W-X8 ion exchange resin (BioRad), converted to persulfonic acid with potassium persulfate

7

4h

50–95%

Resin prepared by treatment of 1% crosslinked carboxypolystyrene with 85% H2 O2 in MeSO3 H for 16 h. Peracid resin stable to storage at 20 C. THF is vastly superior to CH2 Cl2 as solvent

8

Oxone8/NaHCO3/THF/H2 O

CHCl3

THF/D

THF/40 C

5800 phenoxypropanolamines, prepared via a split-and-mix strategy, has also been reported [11]. Here, a resin-bound epoxide was opened with excess amine in acetonitrile at 80 C for 18 h. Opening of epoxides on solid phase with alternative nucleophiles to amines is illustrated in Scheme 11.3 [12]. In this example, which includes experimental procedures, azides (10) were prepared using buffered sodium azide in dimethylformamide (DMF) at 100 C for 2 h, whilst thiols (11) were prepared by reaction with sodium thiophenoxide in DMF at 0 C. In these systems, lactonization occurred under the cleavage conditions.


Scheme 11.3. Epoxidation, epoxide opening, and lactonization during cleavage from resin.

An elegant and potentially very versatile extension to the utility of epoxides is demonstrated in Scheme 11.4 [13]. Here, a range of olefins (14) was converted in a one-pot procedure to the immobilized a-sulfonated ketones (17) by epoxidation with dimethyldioxirane, epoxide opening with a sulfonic acid resin, and subsequent oxidation with Dess–Martin periodinane. The activated sulfonyloxy moiety is an excellent leaving group, and 20 efficient functionalizing cleavage options were demonstrated for resin 17.

Scheme 11.4. One-pot epoxidation and conversion to a-sulfonated ketones.

11.2.2

Dihydroxylation

The dihydroxylation of an olefin is most efficiently carried out using osmium tetroxide, giving rise to vicinal diols (Scheme 11.5) [14]. The catalytic variant of the reaction is usually employed owing to the toxic and volatile nature of the reagent,

309

310


and the facile regeneration of the catalyst with oxidants such as N-methylmorpholine N-oxide (NMO), potassium ferricyanide, hydrogen peroxide, or tert-butyl hydroperoxide. Osmium(III) chloride is sometimes used as the osmium source in the catalytic reaction, being less volatile than the tetroxide and therefore easier to weigh out safely. It is converted in situ to the tetroxide by the oxidant. High levels of enantioselectivity can be achieved in the reaction using catalytic amounts of osmium tetroxide in the presence of cinchona alkaloid derivatives [15].

Scheme 11.5. Dihydroxylation of an olefin.

Osmium tetroxide is a very mild reagent and compatible with many functional groups, making it suitable for combinatorial synthesis, at least in principle. There have, however, been few reported uses in library synthesis. For solution-phase library synthesis, perhaps the most practical methods will involve the use of solidsupported osmium tetroxide [4] since this will minimize handling problems for the toxic osmium reagent, allowing recovery by simple filtration. One such example is osmium tetroxide microencapsulated in polystyrene [16]. This work highlighted one potential practical problem associated with such reagents. The dihydroxylation is usually carried out in acetone–water or tert-butanol–water mixed solvent systems. Not only does this raise the possibility of solubility issues for reactants, but also solvent/reactant access within the solid-supported reagent may be restricted since polymers such as polystyrene are poorly solvated and not swelled well by these solvents. In this work, acetonitrile was added as a cosolvent in order to achieve good conversion. Good results have been obtained in the area of supported asymmetric dihydroxylations. The OsO4 copolymer of 20 [an analog of (DHQ)2 -PHAL] behaves well under standard conditions [t-BuOH/H2 O (1:1) at 10 C using K3 Fe(CN)6 -K2 CO3 (3 equiv.) as secondary oxidant], giving high yields and enantioselectivities (Scheme 11.6) [17]. The copolymer is easily recovered by filtration.

Scheme 11.6. Poly((QN)2 -PHAL-co-MMA) used in asymmetric dihydroxylation of olefins.


An example of solid-phase parallel synthesis employing asymmetric dihydroxylation is shown in Scheme 11.7 [18]. For this work, ArgoGel Wang resin was employed for the synthesis because of its compatibility with the solvent system, allowing standard Sharpless asymmetric dihydroxylation reaction conditions to be used which would have been less compatible with polystyrene Wang resin as discussed above. A second example demonstrating the use of osmylation in solidphase chemistry is shown in Scheme 11.8 [19]. Here, a polystyrene-based resin was used and the solvent conditions were modified to tetrahydrofuran (THF)/H2 O (5:1) in order to promote resin swelling.

Scheme 11.7. Synthesis of khellactone derivatives.

Scheme 11.8. Solid-phase dihydroxylation using T2 triazine linker.

11.2.3

Oxidative Cleavage

In situ oxidative cleavage of olefins to carbonyl compounds can be efficiently achieved by inclusion of sodium periodate as the oxidant in the osmium tetroxide dihydroxylation of olefins (the Lemieux–Johnson reagent) [20]. There have, however, been few if any reports on the use of this reagent combination in combinato-

311

312


rial synthesis, owing to either environmental/handling concerns or possible compatibility problems with resin systems. More use has been made of ozonolysis (Scheme 11.9) [21], primarily for the solid-phase synthesis of peptide aldehydes (Scheme 11.10) [22–24]. A simple alkenyl ester linkage to Merrifield hydroxy resin may conveniently be used for tethering small molecules [21]; an a; b-unsaturated g-amino acid linker to polystyrene [22, 24] or direct vinyl linkage to polystyrene (prepared by Wittig reaction) [23] has been used in the preparation of peptide aldehydes.

Scheme 11.9. Oxidative cleavage of olefins by ozonolysis.

Scheme 11.10. Solid-phase synthesis of peptide aldehydes.

The procedure usually involves bubbling a stream of ozone through a dichloromethane solution of the olefin at 78 C until a blue coloration remains, followed by quenching of the ozonide with a reducing agent such as dimethylsulfide, triphenylphosphine, or thiourea to give the carbonyl compound. Quenching with a reducing agent such as sodium borohydride gives rise to the corresponding alcohol [21], and a variation on this approach has been used for the preparation of a lactone (Scheme 11.11) [25]. There are certain logistical questions to be addressed in carrying out a large number of such reactions in parallel which may limit the overall utility of the reaction for combinatorial chemistry in the absence of specialist equipment. Additionally, there is a general tendency for swollen polystyrenebased resins to collapse when cooled to the low temperatures usually utilized in this reaction. This may limit reagent diffusion into the resin and will need to be borne in mind.

Scheme 11.11. Lactone formation via ozonolysis.


11.2.4

Electrophilic Addition of A--X

The examples above represent electrophilic addition of oxygen to a carbon–carbon double bond. Additionally, other powerful electrophiles – most notably sources of Iþ , Brþ, ArSeþ , and Hþ – are also able to add (Scheme 11.12). The formation of the intermediate ‘‘onium’’ species such as 35 can then be followed by nucleophilic attack of the counterion X , by addition of some external nucleophile (e.g. water), or by an intramolecular process such as lactonization. Examples of such reactions are reported for both solution-phase and solid-phase chemistry and can be very efficient, although their reported use in library synthesis is somewhat limited.

Scheme 11.12. Electrophilic addition of AaX to olefins.

An example of solid-phase iodolactonization and concomitant resin release is shown in Scheme 11.13 [26]. In this case, a modest degree of diastereofacial control (2:1 mixture of enantiomers) was achieved in the iodolactone 38. Similarly, Scheme 11.14 demonstrates an example of iodoetherification on solid phase with concomitant oxidative cleavage to give the 2,5-disubstituted tetrahydrofuran derivative 40 [27]. Both of these examples provide functional groups which could, in principle, be used for postcleavage derivatization in solution phase.

Scheme 11.13. Solid-phase intramolecular iodolactonization.

Scheme 11.14. Solid-phase intramolecular iodoetherification.

Perhaps most use has been made of organoselenium reagents in this area of research, providing mild and extremely versatile reactivity [28, 29]. A resin-bound

313

314


version of phenylselenyl bromide (43) has been developed which provides a convenient and odorless way of handling this toxic and smelly reagent (Scheme 11.15) [30]. This reagent can also be converted to polymer-supported phenylselenyl phthalimide (44), which provides a convenient method for hydration of olefins (Scheme 11.16) [30]. A polymer-supported version of phenylselenyl cyanide is readily prepared from Merrifield resin and potassium selenocyanate [31].

Scheme 11.15. Synthesis of polymer-supported phenylselenyl bromide.

Scheme 11.16. Synthesis and use of polymer-supported phenylselenyl phthalimide.

The polymer-supported phenylselenyl bromide 43 efficiently adds to olefins in the manner depicted in Scheme 11.12 [30]. However, most use has been made of the way in which these selenium reagents facilitate intramolecular cyclizations such as cyclic ether formation (Scheme 11.17) [30], lactonization (Scheme 11.18) [32], and cyclic amine formation (Scheme 11.19) [33]. Reductive hydrodeselenation can be achieved with tributyltinhydride or, alternatively, oxidation results in the formation of an olefin via selenoxide elimination. The use of such reactions in the formation of benzopyran libraries (Scheme 11.20) [34–36] and polycyclic indoline libraries 53 [33] is described in more detail in Chapter X.

Scheme 11.17. Intramolecular etherification/selenoxide elimination using polymer-supported PhSeBr.


Scheme 11.18. Intramolecular lactonization using polymer-supported PhSeCN.

Scheme 11.19. Intramolecular selenoamination of o-allylamines in the presence of SnCl4 .

Scheme 11.20. Polymer-supported synthesis of benzopyrans.

11.2.5

Hydrogenation

The catalytic hydrogenation of olefins is, in the absence of sensitive functionalities and with the use of an appropriate catalyst, a very efficient transformation. Catalysts such as 10% Pd on activated carbon were in widespread use long before polymer-supported reagents became fashionable, and are easily removed from reaction mixtures by simple filtration. The parallel processing of multiple samples introduces certain complexities such as addition of hydrogen gas which may be overcome with specialist equipment [37] or, alternatively, catalytic transfer hydrogenation from reagents such as ammonium formate [38] may be considered. Carrying out hydrogenation during a solid-phase synthesis adds yet another layer of complexity since, by necessity, the catalyst must be soluble in order to gain access to the polymer-bound reactant [39]. Additionally, the reaction conditions must be compatible with the resin and linker, which often contain functionalities such as benzyl ethers that are themselves sensitive to hydrogenation. Consequently, this is an underutilized reaction in combinatorial chemistry. Diimide offers an alternative

315

316


for reduction of alkenes (and alkynes) on solid phase, and excellent conversions have been observed using benzene sulfonylhydrazide in DMF at 100 C as diimide source [40]. 11.2.6

Hydrometallation

Of the hydrometallation processes available, the hydroboration of an olefin represents one of the more useful, not only allowing access to alkylboranes but also the formal hydration of the double bond to an alcohol through a subsequent oxidation step. The utility of in situ hydroborated alkenes as coupling partners in the Suzuki reaction for library generation was demonstrated through the parallel synthesis of an array of 26 prostaglandin analogs on solid phase (Scheme 11.21) [41]. In this work, terminal olefins (57) were hydroborated with 9-BBN dimer in vials and then directly transferred to resin 59 contained within the reaction vessels of a parallel synthesizer, efficiently converting it to resin 60 under standard Suzuki conditions. Subsequent elaboration provided the prostaglandin E1 analogs (61) in 18–56% overall yield.

Scheme 11.21. Parallel synthesis of prostaglandin E1 analogs, utilizing hydroborated alkenes as coupling partners in a Suzuki reaction.

The hydroboration/oxidation of alkenes to alcohols is a potentially useful transformation for the combinatorial chemist, particularly on solid phase. This reaction effectively allows the use of a terminal alkene as a protecting group for the hydroxyethyl moiety, which can then be further functionalized. This was demonstrated during the course of some synthetic studies toward mniopetals (Scheme 11.22) [42]. The triene 62 was selectively hydroborated with 9-BBN to give the pri-


mary alcohol 63. IBX oxidation and a subsequent Bayliss–Hillman reaction using PhSeLi as nucleophile provided the key intermediate 64 which, upon oxidation to the corresponding ketone, underwent an intramolecular Diels–Alder reaction. Cleavage from the resin then gave the polycyclic compound 65 in 35% overall yield.

Scheme 11.22. Solid-phase hydroboration/oxidation of a

terminal alkene and subsequent elaboration.

11.2.7

1,4-Addition to a; b-Unsaturated Carbonyl Systems

The reactions described above all make use of the electron-rich nature of isolated carbon–carbon double bonds and their consequent reactivity toward electrophiles. By contrast, when the carbon–carbon double bond is conjugated to a carbonyl or sulfonyl group, the electronic character of the double bond changes, making it susceptible to 1,4-nucleophilic addition (also known as Michael addition in the case of carbon nucleophiles). Pioneering work in this area of combinatorial chemistry is shown in Scheme 11.23, in which secondary amines 68 add to resinbound acrylate (67 ! 69), are quaternized with alkyl halides, and then undergo a Hofmann-type b-elimination in the presence of triethylamine to give tertiary amines 72 [43]. When primary amines 68 are used in this sequence (R2 ¼ H), a reductive amination step has been used prior to quaternization to prepare terti-

317

318


ary amine libraries (72) with three points of diversity [43, 44]. Subsequent work has shown that replacement of triethylamine with Amberlite weakly basic ionexchange resin in DMF simplifies work-up procedures since it overcomes the necessity to remove triethylamine salts from cleaved products, and this procedure has been used to generate large libraries of trisubstituted amines (> 10,000 compounds) [44]. A similar approach has been employed utilizing a vinylsulfone resin in place of the acrylate resin 67 [45].

Scheme 11.23. REM linker – 1,4-addition of amines to acrylate, quaternization, and b-elimination to give tertiary amines. DIPEA, N,N-diisopropylethylamine.

11.3

Addition to C ––– C Triple Bonds

The acetylenic group displays a versatile range of reactivities for use in combinatorial chemistry. Its use in cycloaddition reactions, palladium-mediated reactions, and radical reactions is well documented and covered here in the relevant chapters. Prominent examples on solid phase include their use in the palladiummediated syntheses of heterocyclic templates such as indoles [46–49] and benzofurans [50]. The discussion above (Sect. 11.2.5) on hydrogenation of olefins applies equally to the hydrogenation of acetylenes – the diimide reduction of an acetylene on solid phase is one of the few reported examples [40]. Hydrometallation is another area

References

where addition to acetylenes could be a very powerful transformation for further functionalization, although there are few reported examples. One such example is the hydrostannylation of a terminal acetylene using a polymer-supported tinhydride to give a vinyl stannane (74 ! 75, Scheme 11.24) [51]. The vinyl stannane 75 was obtained as a mixture of E:Z isomers and was taken through to compound 76 in order to carry out a cyclorelease palladium-mediated Stille coupling to give 77. There is, however, relatively little to report of relevance to this chapter at present, although there is scope for future work in this area.

Scheme 11.24. Solid-phase hydrostannylation of an acetylene

and subsequent synthesis of a macrocyclic system.

References 1 E. K. Kick, D. C. Roe, A. G.

2 3

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Skillman, G. Liu, T. J. A. Ewing, Y. Sun, I. D. Kuntz, J. A. Ellman, Chem. Biol. 1997, 4, 297–307. D. P. Rotella, J. Am. Chem. Soc. 1996, 118, 12246–12247. H. M. C. Ferraz, R. M. Muzzi, T. de O. Vieira, H. Viertler, Tetrahedron Lett. 2000, 41, 5021–5023. S. V. Ley, I. R. Baxendale, R. N. Bream, P. S. Jackson, A. G. Leach, D. A. Longbottom, M. Nesi, J. S. Scott, R. I. Storer, S. J. Taylor, J. Chem. Soc., Perkin Trans. I 2000, 3815–4196. T. R. Boehlow, P. C. Buxton, E. L. Grocock, B. A. Marples, V. L.

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Waddington, Tetrahedron Lett. 1998, 39, 1839–1842. A. Shiney, P. K. Rajan, K. Sreekumar, Polym. Int. 1996, 41, 377–381. C. S. Pande, N. Jain, Synth. Commun. 1989, 19, 1271–1279. C. R. Harrison, P. Hodge, J. Chem. Soc., Perkin I 1976, 605–609. G. M. Coppola, Tetrahedron Lett. 1998, 39, 8233–8236. B. L. Chng, A. Ganesan, Bioorg. Med. Chem. Lett. 1997, 7, 1511–1514. W. M. Bryan, W. F. Huffman, P. K. Bhatnagar, Tetrahedron Lett. 2000, 41, 6997–7000. C. L. Hetet, M. David, F. Carreaux,

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B. Carboni, A. Sauleau, Tetrahedron Lett. 1997, 38, 5153–5156. K. C. Nicolaou, P. S. Baran, Y.-L. Zhong, J. Am. Chem. Soc. 2000, 122, 10246–10248. M. Schro¨der, Chem. Rev. 1980, 80, 187–213. R. A. Johnson, K. B. Sharpless in: Catalytic Asymmetric Synthesis. Ojima, I. (ed.), VCH, New York 1993, pp. 227–272. S. Nagayama, M. Edo, S. Kobayashi, J. Org. Chem. 1998, 63, 6094–6095. C. E. Song, J. W. Yang, H. J. Ha, S. Lee, Tetrahedron: Asymmetry 1996, 7, 645–648. Y. Xia, Z.-Y. Yang, A. Brossi, K.-H. Lee, Org. Lett. 1999, 1, 2113– 2115. S. Bra¨se, S. Dahmen, M. Pfefferkorn, J. Comb. Chem. 2000, 2, 710– 715. R. Pappo, D. S. Allen, R. U. Lemieux, W. S. Johnson, J. Org. Chem. 1956, 21, 478–479. C. Sylvain, A. Wagner, C. Mioskowski, Tetrahedron Lett. 1997, 38, 1043–1044. C. Pothion, M. Paris, A. Heitz, L. Rocheblave, F. Rouch, J.-A. Fehrentz, J. Martinez, Tetrahedron Lett. 1997, 38, 7749–7752. B. J. Hall, J. D. Sutherland, Tetrahedron Lett. 1998, 39, 6593– 6596. M. Paris, A. Heitz, V. Guerlavais, M. Cristau, J.-A. Fehrentz, J. Martinez, Tetrahedron Lett. 1998, 39, 7287–7290. S. Hanessian, F. Xie, Tetrahedron Lett. 1998, 39, 737–740. H. S. Moon, N. E. Schore, M. J. Kurth J. Org. Chem. 1992, 57, 6088– 6089. X. Beebe, N. E. Schore, M. J. Kurth, J. Org. Chem. 1992, 57, 10061– 10062. K. C. Nicolaou, N. A. Petasis, Selenium in Natural Product Synthesis. CIS Inc., Philadelphia, PA, 1984. D. Liotta, Organoselenium Chemistry, Wiley, New York 1986. K. C. Nicolaou, J. Pastor, S. Barluenga, N. Winssinger, J. Chem.

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Soc., Chem. Commun. 1998, 1947– 1948. K. Fujita, K. Watanabe, A. Oishi, Y. Ikeda, Y. Taguchi, Synlett 1999, 1760–1762. K.-i. Fujita, H. Taka, A. Oishi, Y. Ikeda, Y. Taguchi, K. Fujie, T. Saeki, M. Sakuma, Synlett 2000, 1509–1511. K. C. Nicolaou, A. J. Roecker, J. A. Pfefferkorn, G.-Q. Cao, J. Am. Chem. Soc. 2000, 122, 2966– 2967. K. C. Nicolaou, J. A. Pfefferkorn, G.-Q. Cao, Angew. Chem., Int. Ed. Engl. 2000, 39, 734–739. K. C. Nicolaou, G.-Q. Cao, J. A. Pfefferkorn, Angew. Chem., Int. Ed. Engl. 2000, 39, 739–743. K. C. Nicolaou, J. A. Pfefferkorn, A. J. Roecker, G.-Q. Cao, S. Barluenga, H. J. Mitchell, J. Am. Chem. Soc 2000, 122, 9939– 9953. T. Bruckdorfer, H. Linnertz, GIT Labor-Fachz 2000, 44, 58. B. C. Ranu, A. Sarkar, S. K. Guchhait, K. Ghosh, J. Indian Chem. Soc. 1998, 75, 690–694. I. Ojima, C.-Y. Tsai, Z. Zhang, Tetrahedron Lett. 1994, 35, 5785– 5788. P. Lacombe, B. Castagner, Y. Gareau, R. Ruel, Tetrahedron Lett. 1998, 39, 6785–6786. D. R. Dragoli, L. A. Thompson, J. O’Brien, J. A. Ellman, J. Comb. Chem. 1999, 1, 534–539. U. Reiser, J. Jauch, Synlett 2001, 90– 92. A. R. Brown, D. C. Rees, Z. Rankovic, J. R. Morphy, J. Am. Chem. Soc. 1997, 119, 3288–3295. X. Ouyang, R. W. Armstrong, M. M. Murphy, J. Org. Chem. 1998, 63, 1027–1032. F. E. K. Kroll, R. Morphy, D. Rees, D. Gani, Tetrahedron Lett. 1997, 38, 8573–8576. A. L. Smith, G. I. Stevenson, C. J. Swain, J. L. Castro, Tetrahedron Lett. 1998, 39, 8317–8320. H.-C. Zhang, B. E. Maryanoff, J. Org. Chem. 1997, 62, 1804–1809.

References 48 H.-C. Zhang, K. K. Brumfield, B. E.

50 D. Fancelli, M. C. Fagnola, D.

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Severino, A. Bedeschi, Tetrahedron Lett. 1997, 38, 2311–2314. 51 K. C. Nicolaou, N. Winssinger, J. Pastor, F. Murphy, Angew. Chem., Int. Ed. Engl. 1998, 37, 2534– 2537.

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12

Addition to Carbon–Hetero Multiple Bonds Philipp Grosche, Jo¨rg Rademann, and Gu¨nther Jung 12.1

Introduction

Carbon–hetero multiple bonds play a central role in synthetic organic chemistry. They possess a polar character and react with various nucleophiles as well as with electrophiles in numerous addition reactions. They form valuable substrates for the synthesis of structurally diverse and pharmacologically important compounds, such as tetrahydro-b-carbolines, ureas, and guanidines. Many of the addition reactions to CX multiple bonds have found wide application in combinatorial chemistry such as the synthesis of ureas via isocyanates, the Pictet–Spengler reaction, and several Mannich-type reactions. In the literature, most articles in the field of combinatorial chemistry deal with solid-phase reactions, however, solution-phase protocols are in many instances also suited to parallel synthesis. In this chapter addition reactions to CN double bonds, CS double bonds, and CN triple bonds are discussed. The chapter will not include cycloaddition reactions of CX multiple bond systems and reactions of isocyanides typically employed in multiple component reactions. This chapter reviews those transformations of the CX multiple bonds that are suitable for combinatorial chemistry, either in solution or on solid phase. To assure clarity and readability, the reactions are grouped according to the number of bonds between the C atom and the heteroatom, the hybridization of the carbon center, and the nature of the attacking agent.

12.2

Additions to CN Double Bonds in sp2 Systems

CN double bonds are the structural motif of various functional groups. This section refers to CN double bonds in sp2 systems such as imines, iminium ions, acyliminium ions, and N-oxides of nitrogen heterocycles (pyridine, quinoline, isoquinoline). The reacting CN double bond can be isolated; however, the reactions are often performed as a three-component reaction consisting of an amine, a carHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

12.2 Additions to CN Double Bonds in sp2 Systems

bonyl compound and the attacking nucleophile, with in situ formation of the reactive CN double bond.

12.2.1

Attack by Hydride (Reductive Alkylation)

Reductive alkylation, or reductive amination, is one of the most commonly used reactions in combinatorial chemistry. It is a convenient method for the preparation of secondary and tertiary amines and is one of the easiest ways to generate diversity, as usually the substrate tolerance is very broad. The monoalkylation of primary or secondary amines is affected by condensation with aldehydes or ketones and the subsequent reduction of the imine species. In the case of primary amines, the reaction can be performed either in a two-step procedure by first synthesizing and isolating the imine followed by reduction, or in a one-pot procedure by adding the carbonyl partner together with the reducing agent. Secondary amines are typically converted in a one-pot synthesis because of the low stability of the intermediary iminium ion. The use of primary amines in a one-pot procedure carries the risk of dialkylation as the formed secondary amine can react with another carbonyl, thus forming the tertiary amine. As a reducing agent, NaBH3 CN, NaBH(OAc)3 , Me4 NBH4 , LiBH4 , NaBH4 , BH3 py, and BH3 THF can be employed. In solution phase, both the one-pot synthesis [1] and the reduction of preformed imine species [2] were used. Anilines were, for example, converted upon treatment with aldehydes and NaBH(OAc)3 in dichloromethane at room temperature (rt) to give the respective secondary amines [3]. The degree of dialkylation in the one-pot procedure increases with less-hindered amines and aldehydes and with more electronrich amines. To avoid double alkylation an excess of amine is often used [4]. Polymer-bound reducing agents have found wide application in solution-phase combinatorial synthesis mostly in the form of ion-exchange resins loaded with borohydride or cyanoborohydride (Scheme 12.1). Polymer-bound ammonium cyanoborohydride was used for the reductive alkylation of primary and secondary amines with aromatic aldehydes in MeOH at rt (Scheme 12.1) [5, 6], or for the reduction of preformed imines in toluene/MeOH under reflux conditions [7]. Polymer-supported borohydride was utilized for the reduction of preformed imines in MeOH at rt. An excess of amine relative to the aldehyde was applied in order to drive the imine formation to completion and to suppress dialkylation. Excess amine was readily scavenged from the desired secondary amine by selective imine formation using a polymer-supported aldehyde [8]. Tertiary amines were formed by treating aldehydes with an excess of secondary amine and polymer-supported

Scheme 12.1

323

324

12 Addition to Carbon--Hetero Multiple Bonds

cyanoborohydride in acetic acid/dichloromethane. In this case excess secondary amine was removed using polymer-supported benzoyl chloride [8]. In solid-phase chemistry, both of the components forming the imine species can be linked to the solid phase. Both approaches, the reduction of isolated imines (Scheme 12.2) [9, 10] and the one-pot synthesis [11], can be used. Polymer-bound primary amines can be partly overalkylated using the one-pot strategy, as reported for solution-phase chemistry. Reactions using secondary amines are always carried out as a one-pot procedure. Imines can easily be hydrolyzed, especially under acidic conditions. Thus, with isolated imines the condensation and reduction step can be repeated to achieve complete conversion [12].

Scheme 12.2

Usually NaBH3 CN in AcOH/dimethylformamide (DMF) is used as the reducing agent. However, several other reagents have also been used in solid-phase chemistry: NaBH(OAc)3 [13], Me4 NBH4 [14], LiBH4 [15], NaBH4 [10], BH3 py [16], and BH3 THF [17]. The borane–pyridine complex was found to be an excellent reagent for the in situ reduction of iminium ions on solid phase [16]. The reaction was performed in DMF/EtOH (3:1) using an excess of aldehyde or ketone respectively. However, this reagent is not suitable for primary amines as it leads to dialkylation. In contrast to imines, oximes required harsher conditions for efficient reduction. In a recent example, resin-bound oximes were reduced using BH3 THF in 4 M HCl in dioxane, tetrahydrofuran (THF)/MeOH at rt [17]. 12.2.2

Addition of Carbon Nucleophiles

A broad range of C-nucleophiles could be added to CN double bonds, namely enolates, ketene acetals, C,H-acidic compounds, electron-rich heterocycles, organometallic compounds, boronic acids, and silanes. The reactions are sorted according to the nature of the nucleophile. In several cases there are only a few examples of solution-phase combinatorial chemistry, which does not necessarily reflect the suitability of solution-phase protocols to library synthesis. 12.2.2.1 Imino Aldol Reaction

Imines react readily with silyl enolates, silyl ketene acetals, and silyl thioketene acetals to afford N-substituted b-amino ketones, b-amino esters, and b-amino thioesters. The reaction is catalyzed typically by Lewis acids, preferably with the lanthanide triflates Ln(OTf )3 , Sc(OTf )3 , and Yb(OTf )3 . It can also be performed as a three-component reaction – between amine, aldehyde or ketone, and the silyl


component – where the imine is formed in situ. In this case, a dehydration agent such as trimethyl orthoformate is normally added to support imine formation and to prevent degradation of the ketene silyl acetals by H2 O. This Mannich-type reaction has attracted a lot of interest in solution-phase as well as in solid-phase chemistry. A typical procedure in solution applies equimolar (or nearly equimolar) amounts of imine (or amine and aldehyde) and silyl component and 0.05–0.3 mol% Ln(OTf )3 . The reaction is performed in dichloromethane (DCM), acetonitrile (ACN), or mixtures of both solvents at rt. The suitability of this reaction for library synthesis was exemplified by the generation of a small library of 40 2,3-dihydro-4pyridones using Danishefsky’s diene as the silyl component and Yb(OTf )3 as the Lewis acid catalyst [18]. Byproducts and unreacted aldehyde were removed with polyamine resin and the products were obtained in good to excellent purities and yields. Although this reaction was often described as an aza-Diels–Alder reaction in the literature, other authors favored a tandem Mannich–Michael mechanism [19]. Another solution-phase approach utilizes a polymer-bound scandium catalyst (polyallyl-scandium-triflylamide ditriflate) (Scheme 12.3). The reaction is conducted as a three-component reaction, with silyl enolates, ketene silyl acetals, and cyanotrimethyl silane as nucleophiles [20]. Treatment with cyanotrimethyl silane as the nucleophile affords a-aminonitriles in a Lewis acid-catalyzed variation of the classical Strecker synthesis. This reaction is performed in dichloromethane/ acetonitrile (2:1) at rt for 19 h. When ketene silyl acetal is used, MgSO4 is added to prevent decomposition of the ketene silyl acetal by traces of water. A cationexchange resin has also been used for the promotion of the imino-aldol reaction [21]. In the imino-aldol reaction at least one asymmetric C atom is generated. Thus, a procedure for enantioselective addition is of special interest. Catalysis of the reaction with polymer-supported palladium BINAP m-hydroxo complex (BINAP ¼ 2,2 0 bis(diphenylphorphino)-1,10 -binaphthyl) yields an ee of 81% [22].

Scheme 12.3

This Mannich-type reaction is very suitable for combinatorial chemistry and has also been adapted to solid-phase chemistry, thus pursuing both possible strategies: the immobilization of the imine and the immobilization of the silyl component. Polymer-supported silyl thioketene acetals obtained by silylation of immobilized thioesters were converted to b-aminoesters by treatment with imines in the presence of Sc(OTf )3 using dichloromethane as solvent [23, 24]. Alternatively, a threecomponent protocol can be employed (Scheme 12.4) [25]. In both cases reductive cleavage of b-amino acid thioesters with LiBH4 yields b-amino alcohols.

325

326


Scheme 12.4

In the second approach, a polymer-supported imine was treated with silyl enol ethers or silyl ketene acetals in the presence of Yb(OTf )3 in dichloromethane to yield b-amino ketones and esters respectively [26, 27]. Instead of imines, polymersupported acyl hydrazones can also be used as substrates [28]. 2,3-Dihydro-4-pyridones were synthesized by tandem Mannich–Michael reaction in good yields. Resin-supported imines were reacted with Danishefsky’s diene in THF using Yb(OTf )3 (Scheme 12.5) [29] or ZnCl2 [30] as catalyst.

Scheme 12.5

A more classical example of Mannich chemistry is the solid-phase variant of the Robinson tropanone synthesis. Starting from immobilized primary amines, tropanone derivatives are formed upon treatment with 1,3-acetonedicarboxylic acid and succinic aldehyde under acidic conditions at rt [31]. 12.2.2.2 Reaction with Boronic Acids

The Petasis reaction is a three-component condensation reaction between an aldehyde, an amine (usually secondary amine), and a boronic acid. It is also referred to as a boronic acid Mannich reaction. The reaction mechanism is not yet completely elaborated, however, a mechanism involving iminium species is likely and therefore this reaction is discussed here. A wide range of building blocks is accepted in this reaction, but the limited number of suitable aldehydes is remarkable. Besides glyoxylic acid, only a few aldehydes react satisfactorily, e.g. a-hydroxyaldehydes and aldehydes bearing an a-heteroatom. In solution-phase chemistry the condensation is performed using equimolar amounts of the components in THF or acetonitrile at rt. By using 1,2-diamines, glyoxylic acid, and boronic acids, piperazinones and benzopiperazinones are accessible (Scheme 12.6) [32]. The Petasis reaction has also been adapted to solid-phase chemistry. Each of the required components can


Scheme 12.6

be linked to the solid support. Typically, the components in solution are employed in large excess. Polymer-bound N-substituted amino acids have been treated with glyoxylic acid and boronic acids in dichloromethane at rt for 18 h, and the reaction was repeated for additional 60 h. The carboxylic acids obtained were further modified by coupling with amines. After cleavage from the resin, products were obtained in good purity and yield [33, 34]. This reaction was also carried out at 50 C for 1–2 days in DMF/1,2-dichloroethane (DCE), and these reaction conditions have also been successfully applied to immobilized phenyl boronic acid, glyoxylic acid (Scheme 12.7), and proline or piperazine [35].

Scheme 12.7

Addition of Allylsilanes (Imino-Sakurai Reaction) Allylsilanes add to CN double bonds in an imino-Sakurai reaction. Both the imine species and the allylsilane can be the immobilized component. Polymer-bound acyl imines have been generated in situ by reaction of immobilized carbamate with aromatic and heteroaromatic aldehydes under Lewis acid conditions. These intermediates reacted directly with allylsilanes to yield homoallylic amines. The reaction was performed as a one-pot procedure in acetonitrile using BF3 OEt2 as Lewis acid (Scheme 12.8) [36]. A variation employing cyclic N-acyliminium ion intermediates has also been performed [37]. 12.2.2.3

Scheme 12.8

In another approach immobilized allylsilanes were treated with Boc-protected aldimines in dichloromethane using BF3 OEt2 as Lewis acid. This reaction yielded Boc-protected homoallylic amines [38].

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328


12.2.2.4 Reaction with Grignard Reagents, Lithium Organyles, and Zinc Organyles

Imines and iminium species are converted to amines upon treatment with Grignard reagents, lithium reagents, and zinc organyles. This reaction is often used in solution-phase chemistry and has been adapted to solid-phase chemistry. Resin-immobilized aldimines, derived from condensation of Rink amide polystyrene resin with aldehydes, reacted with Grignard reagents at 60 C or lithium reagents at 78 C to 20 C to yield after cleavage primary amines in good to excellent purity (Scheme 12.9) [15]. Since the aldimines of ammonia are unstable, similar approaches to primary amines in solution phase need protecting groups at the N atom to improve the stability. In this example, the function is fulfilled by the resin. If the imines are not immobilized via the N atom, secondary amines are obtained after cleavage [39, 40]. Enantiopure immobilized aldimines have been converted diastereoselectively to homoallylamines upon treatment with allyl zinc reagents in THF. The latter were obtained from allylbromide and zinc using CeCl3 7H2 O as an additive [41].

Scheme 12.9

Another kind of CN double bond-containing substrate are the N-acyl-pyridinium or N-acyl-quinoline intermediates. These are generated by the reaction of the heterocycle with acylchlorides or chloroformates. Activation of 4-methoxypyridine can be achieved by reaction with a resin-bound chloroformate. Addition of the Grignard reagent was performed at 20 C in THF and yielded 2,3-dihydro-4pyridones in high purity following basic cleavage [42, 43]. Another possibility to generate acyl-pyridinium intermediates is the immobilization of hydroxypyridine and subsequent acylation. Conversion with a Grignard reagent and acidic cleavage affords N-acyl-2,3-dihydro-4-pyridones (Scheme 12.10) [44, 45]. In a similar approach, quinolines were converted to 2-substituted Nacyl-dihydroquinolin-4-ones [46]. A related reaction is the solid-phase variant of the Reissert reaction, which is an efficient activation method for introducing substituents at the C-1 position of isoquinolines. Polymer-bound N-acyl-isoquinolinium intermediates generated in situ from immobilized benzoyl chloride react with trimethylsilyl cyanide to form N-acyl-1-cyanodihydroisoquinolines. These Reissert intermediates possess increased acidity at the C-1 position and are smoothly alkylated [47, 48].

Scheme 12.10


Addition of Copper Alkynes Aldehydes, amines, and alkynes react in a Mannich-type reaction yielding propargylic amines. As in other examples the reactive imine species is formed in situ. The reaction is catalyzed by CuCl and proceeds presumably via copper alkynes. Each of the three components can be linked to the solid support. Propargylamine was immobilized on 2-chlorotrityl chloride resin and was reacted with secondary amines and paraformaldehyde in the presence of CuCl to give the aminomethylated products in high purity. The reaction was performed in dioxane at 70–75 C for 3 h [49]. Solid-supported alkynes can also be prepared via the Sonogashira reaction [50] and have been employed in the addition of alkyne cuprates on solid phase. Besides the amino alkynes, resin-bound piperazine and benzaldehydes have served as substrates for this reaction (Scheme 12.11) [51]. 12.2.2.5

Scheme 12.11

Addition of Electron-rich Aromatic and Heteroaromatic Cycles Electron-rich heterocycles can add to CN double bonds. The most famous example in this field is the Pictet–Spengler reaction. In this reaction 3-(20 -aminoethyl)indoles are condensed with aldehydes or ketones and subsequently cyclized to yield 1,2,3,4-tetrahydro-b-carbolines. This reaction has often been used in solid-phase chemistry. Tryptophan derivatives serve as substrates and are immobilized via the carboxy group. Condensation with the carbonyl compound and cyclization is performed in one pot. Aliphatic and aromatic aldehydes as well as ketones have been used. Neutral or acidic conditions are suitable for the Pictet–Spengler reaction. However, the acid-catalyzed route using 1–25% trifluoroacetic acid (TFA) in dichloromethane is most often used, and under neutral conditions the reaction can be conducted at 50–110 C. A typical procedure employs tryptophan immobilized on Merrifield resin, with a tenfold excess of aldehyde in 10% TFA/dichloromethane at rt for 16 h (Scheme 12.12). The tetrahydro-b-carbolines were obtained with high purity after cleavage via aminolysis with ethylamine [52]. N-Substituted tryptophan derivatives can also be used for the Pictet–Spengler reaction [53]. In an alternative approach the imine obtained from the condensation of various aldehydes with tryptophan immobilized on Wang-polystyrene resin was isolated. Treatment with fluorenylmethoxycarbonyl (Fmoc)-protected amino acid chloride results in the formation of an N-acyliminium intermediate which undergoes Pictet–Spengler condensation. Fmoc deprotection leads to resin cleavage through cyclization to yield diketopiperazines in high purity (Scheme 12.13) [54]. 12.2.2.6

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330


Scheme 12.12

Scheme 12.13

Instead of tryptamine derivatives, electron-rich m-tyramine and histamine derivatives can be linked to the support. Condensation and cyclization proceeds well with aliphatic, aromatic, and heteroaromatic aldehydes in pyridine at 100 C for 14 h. Tetrahydroisoquinolines and tetrahydroimidazopyridines are obtained with high purity after cleavage (Scheme 12.14).

Scheme 12.14

Another reaction of this type is the aminomethylation of resin-supported indoles with formaldehyde and secondary amines, affording 3-aminomethylindole in high yield and purity. The reaction is conducted as a three-component reaction in AcOH/1,4-dioxane (1:4) at rt for 1.5 h. The iminium intermediate is formed in situ and is trapped by the indole [55]. Immobilized heterocyclic N-oxides, such as quinoline-N-oxide or isoquinolineN-oxide react with various nucleophiles, for example indoles, pyrroles, and enamines, in the presence of benzoylchloride to afford 2-substituted quinolines or 1substituted isoquinolines, respectively [118]. The reaction proceeds via an addition to the CN double bond followed by cleavage of benzoic acid. Radical Reactions CN double bonds can participate in radical reactions. Radical reactions are covered in detail in Chapter 7, however, some typical examples are also discussed here. 12.2.2.7


In solid-phase chemistry the intermolecular and intramolecular carbon radical additions to CN double bonds have been established. In the intermolecular reaction, radicals generated from alkyl iodides in the presence of Et3 B and Bu3 SnH have been added to resin-bound glyoxylic oxime ethers in dichloromethane at rt to yield a-hydroxylamino acid derivatives in moderate to good yields (Scheme 12.15) [56]. Another approach uses phenylsulfonyl oxime ethers that react with alkyl iodides and hexamethylditin in benzene under irradiation at 300 nm at rt to afford alkylated a-oxime ethers in moderate to good yields [57].

Scheme 12.15

Resin-bound allylamino acetaldoxime and propargylamino acetaldoxime were employed as substrates for an intramolecular radical reaction using Et3 B and Bu3 SnH in toluene at 80 C. Functionalized pyrrolidines were formed via a carbon– carbon bond-forming process [58]. 12.2.3

Addition of Nitrogen Nucleophiles

The addition of N-nucleophiles to CN double bonds is typically conducted as a three-component reaction between amines, carbonyl compounds, and Nnucleophiles. The active imine species is formed in situ and trapped by the Nnucleophile. Benzotriazoles add readily to imines. This approach was adapted to solid-phase chemistry as a linker strategy using polymer-bound benzotriazole and excess aldehyde and amine in THF/trimethyl orthoformate (TMOF) at 20–60 C (Scheme 12.16) [59, 60]. The adducts can be released in solution with nucleophiles such as hydride ions, Grignard reagents, and organo zinc compounds.

Scheme 12.16

In a second group of addition reactions used in heterocycle synthesis, the reactions proceed presumably via CN double bond intermediates. Phenylenediamines obtained from immobilized nitroanilines via reduction condense with aldehydes to form benzimidazoles. The reaction is performed at 50 C [61] or with

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2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) at rt [62]. The reaction can also be conducted in a one-pot procedure treating nitroaniline with aldehyde and SnCl2 2H2 O [63]. Polymer-bound 2-aminobenzamides (immobilization via the amide nitrogen) react with aldehydes in N,N-dimethylacetamide (DMA) in the presence of 5% acetic acid at 100 C to form dihydroquinazolinones via an imine intermediate. The relatively harsh reaction conditions are required because of the low activity of the N-nucleophile [64]. 12.2.4

Addition of Phosphorus Nucleophiles

Various P-nucleophiles react with imines or iminium ions in Mannich-type reactions. Protocols have been developed for solution-phase combinatorial as well as for solid-phase chemistry. Condensation of a secondary phosphine with an aromatic or heteroaromatic aldehyde and an amine proceeds in THF at rt to yield aminomethylphosphines in good yields (Scheme 12.17). Owing to the air sensitivity of the phosphines the reaction has to be conducted under inert conditions. The protocol was utilized for the synthesis of a 96-member library [65].

Scheme 12.17

The addition of dialkyl phosphite to imines can be effectively promoted with polymer-supported TBD (1,5,7-triazabicyclo[4.4.0]dec-1-ene) as base to afford aaminophosphonates [66]. A solid-phase approach to a-amino phosphonates and phosphonic acids starts from polymer-bound H-phosphonates, which were obtained in a three-step procedure. Wang-PS resin was treated with 2-chloro-4H-1,3,2-benzodioxaphosphorin4-one followed by hydrolysis and esterification. Addition of imines using either sonification or Yb(OTf )3 catalysis followed by cleavage afforded the corresponding a-amino phosphonates in high yield and purity [67]. Cleavage of the p-nitrophenylethyl group using 1,8-diazabicyclo[5.4.0]undecene-7 (DBU) followed by TFA cleavage from the resin yielded a-amino phosphonic acids. Aminophosphonites were synthesized by nucleophilic addition of bis-(trimethylsilyl)phosphonite (BTSP) to polymer-supported imine in dichloromethane/ DMF. The silyl groups are cleaved upon treatment with methanol to produce amino-phosphinic acid (Scheme 12.18). N-Fluorenylmethoxy-carbonyl-9-aminoxanthen-3-yloxymethyl polystyrene was used as support and cleavage was performed using TFA/CH2 Cl2 /triisopropylsilane. Aromatic, heteroaromatic, aliphatic, and sterically hindered aldehydes can be used for imine formation [68].


Scheme 12.18

12.2.5

Reactions with Oxygen Nucleophiles

There are only few addition reactions of O-nucleophiles to CN double bonds reported in solid-phase chemistry. 1,2-Aminoalcohols were cyclized with aldehydes to form oxazolidines via an imine intermediate. This reaction has been exploited for the immobilization of aldehydes. Polymer-bound serine or threonine was treated with an aldehyde in 1% N,N-diisopropylethylamine (DIPEA)/MeOH for 2 h at 60 C. This oxazolidine linker is stable to the conditions of Fmoc peptide synthesis and is cleaved by heating with 5% AcOH/water for 30 min at 60 C [69]. A similar approach was chosen for the synthesis of disubstituted 1,3oxazolidines [70]. 1,2-Aminoalcohols linked to the polymeric support were condensed with aldehydes in TMOF at rt to yield the corresponding imines that were converted to N-acyl-1,3-oxazolidines by treatment with acyl chlorides, isocyanates, or isothiocyanates (Scheme 12.19). Cleavage of the Wang-PS resin was performed using DDQ and afforded 1,3-substituted 4-(40 -formylphenoxymethyl)oxazolidines in good yields. In this example, the cleavage using 1% TFA led to the decomposition of the heterocycles. Thus, an oxidative method was required for smooth isolation of the products.

Scheme 12.19

12.2.6

Addition of Sulfur Nucleophiles

Thiols can readily add to imines, which is exploited in combinatorial chemistry for the formation of several heterocycles, namely thiazolidinones, metathiazanones,

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thiazolines, and benzothiazoles. Imines react with mercapto acids under addition of the thiol followed by acylation of the resulting N,S-acetal. a-Mercaptocarboxylic acids are used for the synthesis of thiazolidinones; b-mercaptocarboxylic acids for the synthesis of metathiazanones. The latter reaction is most conveniently performed as a three-component condensation of a primary amine, an aldehyde, and a mercapto carboxylic acid, but a stepwise approach is also possible. Solid-phase procedures of this reaction [71, 72] start from immobilized amines that are treated with excess aldehyde and mercapto carboxylic acids in THF at 70 C for 2 h (Scheme 12.20). Molecular sieves or TMOF are used to remove H2 O that formed during the condensation as well as during the cyclization step [72]. Whereas thiazolidinones are obtained in high purities, the formation of metathiazanones is more problematic. Besides immobilized amines, immobilized glyoxylic acid has also been used as a substrate [73].

Scheme 12.20

The solid-phase synthesis of thiazolines and benzothiazoles begins from related compounds. 2-Substituted thiazole-4-carboxylic acids were obtained by reaction of aldehydes with unprotected cysteine attached to the polymeric support via an ester bond. The condensation and cyclization step is performed under acidic conditions (toluene/acetonitrile/AcOH; 45:45:10). The thiazolidines formed can be acylated and obtained in good yield after cleavage from the resin [74]. If immobilized 3-amino-4-mercaptobenzoic acid is used instead of cysteine, the reaction with aldehydes in refluxing ethanol for 4 h affords benzothiazoles in good to high purities and similar yields. The reaction proceeds presumably via an imine intermediate, which is subsequently attacked by the mercapto group followed by oxidation. This pathway is analogous to that of the benzimidazole formation from phenylenediamines [75].

12.3

Additions to CN Double Bonds in sp-Systems

In this section, transformations of carbodiimides, isocyanates, and isothiocyanates are discussed. These functional groups have a common feature in that the C atom is highly reactive against nucleophiles because of the two neighboring electronwithdrawing heteroatoms.

12.3 Additions to CN Double Bonds in sp-Systems

12.3.1

Additions to Carbodiimides

Besides their importance as coupling reagents in amide and ester formation, (Chapter 13.3.1) carbodiimides are important synthetic intermediates. They are easily attacked by primary and secondary amines to form guanidines. The carbodiimides are generated commonly by eliminating water or H2 S from ureas or thioureas using suitable dehydrating agents such as Mukaiyama’s reagent or ptoluenesulfonyl chloride together with Et3 N. Alternatively, the aza-Wittig reaction of iminophosphoranes with isocyanates or isothiocyanates is employed. Often, the carbodiimides are not isolated. In the solid-phase synthesis of guanidines, the carbodiimide is most often the immobilized component. There are several examples of carbodiimide formation from iminophosphoranes with either isocyanates [76] or with isothiocyanates [77, 78] via an aza-Wittig reaction. Another approach to carbodiimides is the elimination of H2 S from thioureas using Mukaiyama’s reagent [79]. The addition of primary and secondary amines to carbodiimides is typically performed at rt. Higher temperatures are used if the formed guanidines are to react further as nucleophiles. This strategy has been exploited for the syntheses of several heterocycles employing a cyclization-release approach. Polymer-supported carbodiimides that are obtained by the reaction of immobilized a-amino acids with isothiocyanates followed by treatment with Mukaiyama’s reagent are converted to guanidines upon treatment with amines. These guanidines cyclize to release 2-aminoimidazolinones [79]. A similar cyclization-cleavage approach has been used for the synthesis of 3Hquinazolin-4-ones using immobilized 2-azido-benzoic acid as substrate (Scheme 12.21) [80]. If polymer-supported 20 -amino-cinnamic acid is employed, guanidine formation is followed by a Michael addition to yield 3,4-dihydroquinazolines [76].

Scheme 12.21

12.3.2

Reaction of Isocyanates and Isothiocyanates

The chemistry of these two groups is very similar. There is a vast number of articles describing the addition of nucleophiles to isocyanates and isothiocyanates.

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12.3.2.1 Addition of Carbon Nucleophiles

CH acidic compounds react with isothiocyanates to form thioamides. This reaction has been adapted to solid-phase chemistry. Both components required for this synthesis can be linked to the solid support. Resin-bound cyanoacetamide was reacted with aliphatic and aromatic isothiocyanates in DMF at rt using DBU as base to yield thioamides [81]. Also resin-bound cyanoacetic acid has been used as a substrate, employing DIPEA as a base (Scheme 12.22) [82].

Scheme 12.22

Resin-bound isothiocyanates are alkylated with acceptor-substituted acetonitriles, such as malononitrile and methanesulfonylacetonitrile, under basic conditions to yield the respective thioamides [83]. Addition of Nitrogen Nucleophiles Amines readily react with isocyanates or isothiocyanates to form ureas or thioureas. Both ureas and thioureas are of interest as final products and as precious synthetic intermediates which, among other reactions can be further converted to carbodiimides, guanidines, hydantoins, and quinazoline-2,4-diones. Typically, heterocycle formation proceeds via an intramolecular acylation of the urea. In accordance with their importance, these reactions were soon adapted to combinatorial solution- and solid-phase chemistry. In solution-phase chemistry, each of the two components may be used in excess. Treatment of the reaction mixture with an appropriate scavenger resin yields pure products (Scheme 12.23). When there is an excess of the amine, a scavenger resin functionalized with isocyanate, aldehyde, or carboxy groups is used. An excess of isocyanate or isothiocyanate has best been removed with amino resins [4, 6]. In solid-phase chemistry either of the components has been linked to the resin; most often, resin-bound primary and secondary aliphatic amines or anilines are used. Typically an excess of the solution component is used in dichloromethane, THF, or DMF at rt. Polymer-bound isocyanates have to be generated on solid support. They are accessible by reaction of immobilized Fmoc-protected amino acids with methyltrichlorosilane [84] or by Curtius rearrangement of acyl azides [85]. The 12.3.2.2

Scheme 12.23

12.3 Additions to CN Double Bonds in sp-Systems

reaction of resin-bound amines with p-nitrophenyl chloroformate under basic conditions yields isocyanates via the active carbamate. Ureas are often formed as intermediates in reaction sequences leading to heterocycles. Several syntheses of this kind have been established on solid phase, namely the syntheses of hydantoins, thiohydantoins, and quinazoline-2,4-diones. Often, a cyclizing cleavage strategy is pursued which guarantees high purities. For example, quinazoline-2,4-diones are obtained in >90% purities if the route depicted in Scheme 12.24 is followed [85].

Scheme 12.24

DPPA ¼ Diphenylphosphorylazide; TEA ¼ Triethyl amine.

12.3.2.3 Addition of Oxygen Nucleophiles

Isocyanates form carbamates when treated with alcohols. Alcohols linked to a solid support or to soluble polymers have been reacted with isocyanates at rt using Et3 N as a base (Scheme 12.25) [86, 87]. Highly activated isocyanates have been reacted with hydroxy resins without base at 0 C [88, 89]. The carbamates obtained using this approach are cleaved either under acidic conditions to yield the amines via the carbamic acid or by nucleophiles. Carbamates have also been accessed from immobilized secondary alcohols using isocyanates in DMF in the presence of catalytic amounts of CuCl at rt. This procedure was embedded in a multistep library synthesis, in which the products were obtained in good to excellent purity [90].

Scheme 12.25

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12.3.3

Addition to CS Double Bonds in sp2 Systems

Thioamides and thioureas are the most frequently used synthons bearing CS double bonds. Owing to its high nucleophilicity, the sulfur is readily alkylated. The alkylated species is often used as an intermediate to facilitate subsequent substitution with amines to afford amidines and guanidines. Furthermore, the alkylated species can be exploited for the synthesis of heterocycles such as thiazoles from thioamides and a-halo ketones. These reactions are well established in solution and have also been employed successfully on solid phase. The alkylation of benzimidazol-2-thiones with various alkyl halides was performed using MeO-PEG-OH as soluble polymeric support. The reaction proceeds in dichloromethane using Et3 N as a base and yields the 2-alkylthiobenzimidazoles in good to high purity [91]. S-Alkylation has been conducted on solid phase using, for example, immobilized benzimidazolthiones and quinazoline-2-thioxo-4-ones as substrates. The reaction is typically performed in DMF or N-methyl-2-pyrrolidone using a tertiary base and various alkyl or aryl halides as alkylating agents [92, 93]. As already mentioned, S-alkylations can be a key step in heterocycle syntheses. Thiazoles are formed by the Hantzsch synthesis by reaction of resin-bound thioamides with a-halo ketones [94]. 2,4-Diaminothiophenes are obtained by alkylation of immobilized a-cyano thioamides with a-halo ketones followed by cylization (see Sect. 12.4.1). Formation of guanidines from thioureas via S-alkylation and substitution with amines has been performed using methyl iodide, Mukaiyama’s reagent, or carbodiimides as alkylating agents. Merrifield resin can also act as an alkylation reagent and yields immobilized S-alkyl isothioureas. These are typically cleaved as guanidines when treated with amines [95, 96]. For carbodiimides, the alkylated species is not isolated but is directly substituted with amines [97, 98], whereas the alkylation products of methyl iodide [99, 100] or Mukaiyama’s reagent are isolated (Scheme 12.26) [101].

Scheme 12.26

Treatment of thioureas with Mukaiyama’s reagent, Et3 N in dichloromethane, or MeCN afforded carbodiimides [79, 102]. Thioamides react in a similar way to thioureas with N 0 -(3-dimethylaminopropyl)-N-ethylene carbodiimide (EDC) and amines to yield amidine derivatives (Scheme 12.22) [82].

12.4 Additions to CN Triple Bonds (Cyanides, not Isocyanides)

12.3.4

Reaction of CS Double Bonds in sp Systems

CS double bonds occur as part of sp systems in isothiocyanates and in carbon disulfide. Additions of isothiocyanates have been discussed previously together with additions of isocyanates. Carbon disulfide reacts with amines and Merrifield resin in a three-component reaction to give resin-bound dithiocarbamates. In the first step, the amine adds to carbon disulfide to form dithiocarbamates which are subsequently trapped by the Merrifield resin (Scheme 12.27). The reaction proceeds at rt using DIPEA as the base. The dithiocarbamates obtained with primary amines and anilines can be cleaved with primary and secondary amines in toluene at 60 C to yield thioureas in good to excellent yield [103].

Scheme 12.27

A similar approach to dithiocarbamates uses resin-bound amines. Carbon disulfide was added to the amine in 1,4-dioxane using aqueous KOH as a base. This dithiocarbamate was converted upon treatment with formaldehyde and amino acids to yield tetrahydro-2H-1,3,5-thiadiazin-2-thiones in good to excellent purity and yield [104].

12.4

Additions to CN Triple Bonds (Cyanides, not Isocyanides)

Cyanides and isocyanides participate in numerous inter- and intramolecular addition reactions. Most reactions in which cyanides are transformed lead to aminosubstituted heterocycles. Isonitriles are typically converted in multicomponent reactions that are discussed in Chapter 23 and will not be discussed here. 12.4.1

Addition of Carbon Nucleophiles

Nitriles can be attacked by CH acidic compounds. This feature was exploited in the synthesis of aminothienopyridine libraries in a solution-phase approach [105, 106]. Treatment of 3-cyanopyridine-2-thiones with acceptor-substituted bromomethylene compounds and aqueous KOH in DMF at rt resulted in S-alkylation. Cyclization was carried out again with aqueous base to yield aminothienopyridines (Scheme 12.28) [106].

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Scheme 12.28

A similar approach in solid-phase chemistry uses a-cyanothioamides as starting material for the formation of aminothiophenes. The a-cyanothioamides are readily obtainable from isothiocyanates and acceptor-substituted acetonitriles. Both of the synthons can be linked to the support. Treatment of the thioamides with abromoketones results in S-alkylation. Under basic conditions (DBU in DMF), this intermediate cyclizes with the nitrile and forms 3-aminothiophenes in good purity (Scheme 12.29) [81, 83].

Scheme 12.29

Cyanopyridines are formed upon treatment of resin-bound chalcones with acetonitrile and t-BuOK at rt using ultrasonic irradiation. The first step is a Thorpe reaction to yield 3-aminocrotononitriles which react in the subsequent step with the chalcone [107]. 12.4.2

Addition of Nitrogen Nucleophiles

Several additions of N-nucleophiles to nitriles were established on solid phase. One of the earliest examples of this concept was the reaction of polymer-bound phthalonitrile with ammonia and NaOMe to yield 1,3-diiminoisoindolines that were further converted to phthalocyanines [108]. The reaction of b-oxo-nitriles with hydrazines afforded 5-aminopyrazoles regioselectively (Scheme 12.30). Both polymer-bound b-keto nitriles [109] and a-formyl nitriles [110] were used as substrates. With the former, a preformed combination between a linker and the b-keto nitrile

Scheme 12.30

12.4 Additions to CN Triple Bonds (Cyanides, not Isocyanides)

was attached to the support. a-Formyl nitriles are accessible by reaction of immobilized benzylnitrile with Bredereck’s reagent [bis-(dimethylamino)-t-butyloxymethane]. Cyclization occurs upon treatment with hydrazines and heating at 70 C in 10% AcOH/EtOH for 5 h. The 5-aminopyrazoles were obtained in high purity. In a similar approach, a; b-unsaturated nitriles were cyclized with hydrazines using NaOEt as a base to yield 3-amino-2-pyrazolines in high purity. Heating at 70 C in ethanol for 24 h was necessary for complete conversion [111]. Kaiser’s oxime resin was used for the introduction of a hydroxylamine group to cyanofluorobenzenes. Cleavage of the hydroxylamine and cyclization was performed in a one-step procedure using TFA/5 N HCl to yield 3-aminobenzisoxazoles in high purity (Scheme 12.31) [112, 113]. An example with the in situ generation of the hydroxylamine nucleophile is the reduction of a polymer-bound 2-nitro-benzylnitrile with tin(II) chloride dihydrate. Hydroxylamine, an intermediate of the reduction, is trapped by cyclization with the nitrile to afford 2-amino-1hydroxyindoles [114].

Scheme 12.31

Guanidines can also act as the attacking N-nucleophile. This feature has been exploited for the solid-phase synthesis of 2,4-diaminoquinazolines. The guanidines were synthesized using acylisothiocyanate resin and 2-aminobenzonitriles as building blocks. Cleavage and cyclization occurs during heating with TFA/H2 O (95:5) at 80 C for 16 h. The cleavage step is repeated once. Following this procedure, diaminoquinazolines were obtained in high purity and good yield [115]. There are only a few examples of nucleophilic conversions of nitriles that occur without cyclization. One example is the aminolysis of polymer-bound nitriles with hydroxylamine hydrochloride and DIPEA in 2-methoxyethanol at 85 C for 16 h to provide amide oximes with a quantitative yield (Scheme 12.32). The products obtained were used in the synthesis of oxadiazoles [116]. Another example is the reaction of a-(benzotriazol-1-yl)acetonitril with amines in 2-methoxyethanol at 75–80 C. The formed amidines were not isolated but were directly treated with resinbound chalcones to yield aminopyridines that had high purity after cleavage.

Scheme 12.32

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12.4.3

Addition of Sulfur Nucleophiles

Polymer-bound nitriles can be converted to thioamides using dithiophosphoric acid O,O-diethylester in THF/H2 O at 70 C (Scheme 12.33) [117]. The thioamides have been used for the synthesis of thiazoles (see Sect. 12.3.3) [94].

Scheme 12.33

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Curtis, J. W. Zhang, Tetrahedron Lett. 1997, 38, 8445–8448. A. M. LaPointe, J. Comb. Chem. 1999, 1, 101–104. D. Simoni, R. Rondanin, M. Morini, R. Baruchello, F. P. Invidiata, Tetrahedron Lett. 2000, 41, 1607–1610. C. Z. Zhang, A. M. M. Mjalli, Tetrahedron Lett. 1996, 37, 5457–5460. E. A. Boyd, W. C. Chan, V. M. Loh, Tetrahedron Lett. 1996, 37, 1647–1650. N. J. Ede, A. M. Bray, Tetrahedron Lett. 1997, 38, 7119–7122. H. S. Oh, H. G. Hahn, S. H. Cheon, D. C. Ha, Tetrahedron Lett. 2000, 41, 5069–5072. M. C. Munson, A. W. Cook, J. A. Josey, C. Rao, Tetrahedron Lett. 1998, 39, 7223–7226. C. P. Holmes, J. P. Chinn, G. C. Look, E. M. Gordon, M. A. Gallop, J. Org. Chem. 1995, 60, 7328–7333. N. Schlienger, M. R. Bryce, T. K. Hansen, Tetrahedron Lett. 2000, 41, 5147–5150. M. Patek, B. Drake, M. Lebl, Tetrahedron Lett. 1996, 36, 2227–2230. C. L. Lee, Y. L. Lam, S. Y. Lee, Tetrahedron Lett. 2001, 42, 109–111. F. J. Wang, J. R. Hauske, Tetrahedron Lett. 1997, 38, 8651–8654. D. H. Drewry, S. W. Gerritz, J. A. Linn, Tetrahedron Lett. 1997, 38, 3377–3380. C. P. Lopez, P. Molina, E. Aller, A. Lorenzo, Synlett 2000, 1411–1414. D. H. Drewry, C. Ghiron, Tetrahedron Lett. 2000, 41, 6989–6992. J. M. Villalgordo, D. Obrecht, A. Chucholowsky, Synlett 1998, 1405– 1407. F. Zaragoza, Tetrahedron Lett. 1996, 37, 6213–6216. F. Zaragoza, Tetrahedron Lett. 1997, 38, 7291–7294. H. Stephensen, F. Zaragoza, J. Org. Chem. 1997, 62, 6096–6097. P. Y. Chong, P. A. Petillo, Tetrahedron Lett. 1999, 40, 4501–4504. H. Shao, M. Colucci, S. J. Tong, H. S. Zhang, A. L. Castelhano, Tetrahedron Lett. 1998, 39, 7235–7238.

86 H. P. Buchstaller, Tetrahedron 1998,

54, 3465–3470. 87 J. Y. Yoon, C. W. Cho, H. Han, K. D.

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Janda, J. Chem. Soc. Chem. Commun. 1998, 2703–2704. C. Subramanyam, Tetrahedron Lett. 2000, 41, 6537–6540. L. J. Fitzpatrick, R. A. Rivero, Tetrahedron Lett. 1997, 38, 7479– 7482. M. J. Sofia, R. Hunter, T. Y. Chan, A. Vaughan, R. Dulina, H. M. Wang, D. Gange, J. Org. Chem. 1998, 63, 2802–2803. C. M. Yeh, C. M. Sun, Tetrahedron Lett. 1999, 40, 7247–7250. J. Lee, D. Gauthier, R. A. Rivero, Tetrahedron Lett. 1998, 39, 201–204. S. Makino, N. Suzuki, E. Nakanishi, T. Tsuji, Tetrahedron Lett. 2000, 41, 8333–8337. D. Goff, J. Fernandez, Tetrahedron Lett. 1999, 40, 423–426. D. S. Dodd, O. B. Wallace, Tetrahedron Lett. 1998, 39, 5701–5704. R. Y. Yang, A. Kaplan, Tetrahedron Lett. 2000, 41, 7005–7008. L. J. Wilson, S. R. Klopfenstein, M. Li, Tetrahedron Lett. 1999, 40, 3999– 4002. J. A. Josey, C. A. Tarlton, C. E. Payne, Tetrahedron Lett. 1998, 39, 5899–5902. P. C. Kearney, M. Fernandez, J. A. Flygare, Tetrahedron Lett. 1998, 39, 2663–2666. A. Gopalsamy, H. Yang, J. Comb. Chem. 2000, 2, 378–381. S. E. Schneider, P. A. Bishop, M. A. Salazar, O. A. Bishop, E. V. Anslyn, Tetrahedron 1998, 54, 15063–15086. J. Chen, M. Pattarawarapan, A. J. Zhang, K. Burgess, J. Comb. Chem. 2000, 2, 276–281. L. Gomez, F. Gellibert, A. Wagner, C. Mioskowski, J. Comb. Chem. 2000, 2, 75–79. R. Perez, O. Reyes, M. Suarez, H. E. Garay, L. J. Cruz, H. Rodriguez, M. D. Molero-Vilchez, C. Ochoa, Tetrahedron Lett. 2000, 41, 613–616. S. J. Shuttleworth, M. Quimpere, N. Lee, J. DeLuca, Mol. Diversity 1998, 4, 183–185.

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112 S. D. Lepore, M. R. Wiley, J. Org.

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13

Chemistry of the Carbonyl Group Tobias Wunberg 13.1

Introduction

The carbonyl group is one of the most important functional groups in organic chemistry. It is found in various structural classes, each one having its own characteristic and fascinating chemistry. The wealth of chemical transformations involving carbonyl groups covers virtually all forms of organic reactions, thus making the carbonyl group an unreplacable tool in synthetic organic chemistry. Additionally, this structural unit is extremely important in medicinal chemistry. Seventy per cent of known drugs (CMC database vers. 94.1) contain carbonyl groups in various manifestations [1]. This chapter deals with the application of carbonyl group chemistry to the generation of libraries. It will not include CaC single bond-forming reactions, enolate chemistry, or reductions and oxidations involving carbonyl groups. These reactions are dealt with in other chapters.

13.2

Chemistry of the Carbonyl Group and Combinatorial Chemistry

Since this chapter deals with mechanistically different chemical transformations, a general statement about the application of carbonyl group chemistry for library synthesis cannot be made. Some of the described reactions such as the formation of amides or reductive aminations play a key role in combinatorial chemistry. Others are frequently used, for example Wittig-type olefinations, and some have only rarely been used, e.g. Curtius degradation. Basically, they are all suited for both solution-phase and solid-phase chemistry even though the choice between these two formats has consequences for the reaction conditions. These aspects will be discussed in more detail for each individual reaction. For some reactions there is a standard procedure which is generally suited to a first test. But this does not guarantee a satisfactory result and does not suggest that other methods may not give better yields and purities. Each reaction may have to be optimized by parameters such as base, solvent, time, temperature, and reagents Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

13.3 Chemistry of Carboxylic Acids

in order to determine reaction conditions that are applicable to as broad a range of reactions as possible.

13.3

Chemistry of Carboxylic Acids 13.3.1

C(O)--X Bond-forming Reactions: General Remarks

From a mechanistic point of view, the transformation of carboxylic acids into amides or esters might not be the most spectacular reaction, however nicely they demonstrate the fundamental principles and mechanisms of carbonyl group chemistry. For his pioneering work in solid-phase chemistry, Bruce Merrifield chose previously established procedures for the formation of amides (more precisely, peptides) [2]. Since then, the formation of amides has become one of the best-elaborated reactions in solid-phase chemistry [3]. Since peptides usually have pharmacokinetically unfavorable properties, the focus of combinatorial chemistry in medicinal chemistry has recently shifted toward the synthesis of nonpeptidic, more drug-like small molecules (see Chapter 22). Nevertheless, C(O)aN and C(O)aO bond-forming reactions still play a key role in combinatorial chemistry both in solution phase and on solid phase. The reliability and large number of commercially available building blocks explain why this reaction type has maintained its importance. 13.3.1.1 Amides and Ureas

Formation of amides There is a mechanistic requirement for the transformation of carboxylic acids into amides: the introduction of a suitable leaving group instead of the OH group prior to the reaction with a nucleophile (amine). Two principal ways of activation are feasible: replacement of the OH group with better leaving groups (e.g. acid halides) or transformation of the OH moiety into a suitable leaving group (e.g. active esters, anhydrides). While planning the synthesis of a library one should keep in mind that the choice between solution and solid phase may cause limitations which have their origin in the combination of the specific demands of resin-based chemistry, the efficacy of coupling reagents, and the formation of acid chlorides. A plethora of methods and conditions has been described for the formation of acid chlorides in solution phase and most of them are applicable to parallel synthesis and library production. Consequently, the reaction of acid chlorides with aliphatic amines is a standard reaction and has been used in numerous preparations of libraries in solution. In our hands, CH2 Cl2 or tetrahydrofuran (THF) as solvent and NEt3 or DIEA as base are the standard methods. Even aromatic amines can be smoothly converted into amides under these conditions. Furthermore, this protocol is also suitable for the formation of amides on solid phase with

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13 Chemistry of the Carbonyl Group

immobilized aliphatic and aromatic amines. For resin-bound carboxylic acids, however, there is an intrinsic problem with the formation of the corresponding acid chloride. Acidic reagents such as SOCl2 or POCl3 are incompatible with acidsensitive linkers as well as with the resin itself. A few approaches promise a solution to this problem, e.g. Ghosez’s reagent [4], Appel’s PPh3 /CCl 4 combination [5], oxalyl chloride [6], or alternatively formation of acid fluorides (Scheme 13.1) using cyanuric fluoride [7], DAST [8], or TFFH [9]. However, none of these methods has so far been described for the synthesis of larger libraries.

Scheme 13.1.

Transformation of aromatic carboxylic acids into acid fluorides.

In addition to the use of acid chlorides, chemists have been successfully transforming carboxylic acids into amides using coupling reagents. A large variety of reagents has been developed which provide chemists with a tool-box for the synthesis of large and complex structures [10]. Numerous examples demonstrate the efficacy of these methods for library synthesis in solution phase as well as in solid phase. In the latter case, both starting materials can be bound to the resin: activation of a polymer-bound acid followed by addition of the amine, or the addition of an excess acid plus coupling reagent to a polymer-bound amine. The reaction of active esters with aliphatic amines generally leads to the formation of the desired amide in high yield and purity. However, the formation of anilides using active esters as acylating agents poses difficulties and only the most reactive coupling reagents may give satisfactory results (see Table 13.1). Therefore, the use of acid chlorides is necessary when anilines are used as nucleophiles. The best conditions for a particular acylation are highly dependent on the steric and electronic nature of both the amine and carboxylic acid. There is no general rule as to which reagent is the best for a particular reaction and each case has to be optimized with respect to coupling reagent, base, solvent, etc. Nevertheless, there are some general guidelines for the choice of an appropriate coupling reagent:

. Carbodiimide-mediated . . .

couplings without additional reagents (HOBt) are a source for racemization. Therefore they are not recommended for fluorenylmethoxycarbonyl (Fmoc) amino acids [18]. DIC/HONSu-mediated couplings occur under slightly acidic conditions, thus avoiding formation of N-acyl ureas. HOBt-based active esters suppress racemization of amino acids. This type of activation usually requires activating bases (usually DIEA or NMM). Pyrrolidino derivatives of both phosphonium and uronium salts derived from HOBt are slightly more reactive and less toxic than the dimethylamino derivatives (e.g. PyBOP vs. BOP).

13.3 Chemistry of Carboxylic Acids Tab. 13.1. Coupling reagents for amide synthesis.

Building blocks Acids

Coupling reagent

Comment

Reference

DCC or DIC

Potential racemization 11 during coupling of aamino acids. DIC is preferred for solid-phase chemistry (soluble urea) Slightly acidic conditions 12 suitable for strongly basic amines (e.g. hydrazine)

Amines

Aliphatic and Aliphatic aromatic (primary) (sterically undemanding)

DIC/HONSu

Aliphatic and aromatic amino acids

Aliphatic (primary)

HOBt/DIC/DIEA Intermediate formation of HOBt esters avoids racemization


Aliphatic HBTU or TBTU/ (primary and DIEA secondary) PyBOP/DIEA


Aliphatic and aromatic

13

14 Preformed HOBt/DIC combination (HBTU: PF 6 salt; TBTU: BF4 salt) Fast coupling makes it 15 suitable for base-sensitive substrates

HOAt/DIC/DIEA Developed for cyclization of peptides, useful for coupling of hindered amino acids. HATU/DIEA Preformed HOAt/DIC combination (PF 6 salt)

16

17

. Uronium salt-mediated couplings may cause side-reactions such as the transfor. .

mation of the N-terminus into a guanidinium residue [19]. Phosphonium reagents do not take part in this reaction [20]. HBTU and TBTU are very popular for peptide synthesis. HATU or HOAt/DIC most likely yield the best results for difficult couplings, e.g. secondary amines [21], cyclizations, or anilines. Unfortunately, compared with other reagents they are rather expensive.

In the early days of combinatorial chemistry, these methods were used for the construction of peptide libraries on solid phase leading to potent, bioactive lead structures or elucidation of binding motifs [22]. Today, the application of coupling reagents for amide synthesis remains an essential part of combinatorial chemistry. Amino acids have gained popularity as readily available, multifunctional templates with a high degree of diversity and biological importance, and they are often used as valuable building blocks for the library synthesis of small molecules [23]. For parallel amide synthesis in solution phase, polymer-bound variations of the well-known coupling reagents have been shown to be powerful tools for avoiding

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tedious work-up procedures. The first report of modified DIC on a crosslinked polystyrene (PS) resin appeared in the early 1970s [24]. More recently, an N 0 (3-dimethylaminopropyl)-N-ethylene carbodiimide (EDC)-based resin (P-EDC) was developed that couples amines and carboxylic acids more efficiently (Scheme 13.2) [25].

Scheme 13.2. Use of P-EDC in amide bond formation.

Additionally, carboxylic acids can be activated by polymer-bound HOBt [26] or 4hydroxy-3-nitrobenzophenone. The latter reagent has been used for the synthesis of a library of 8000 amides and esters from which a compound with considerable herbicidal activity was identified (Scheme 13.3) [27].

Scheme 13.3. Synthesis of amides using polymer-bound 4-hydroxy-3-nitrobenzophenone.

Shortcomings of these methods include the limited scope of reactivity of Nnucleophiles toward these reagents and problems in determining the absolute loading of the activated resin. The use of polymer-supported tetrafluorophenol promises a solution to both of these problems [28]. In analogy to well-known solution-phase transformations [29], carboxyl and sulfonyl activated esters can be prepared that yield amides and sulfonamides, respectively, with a wide range of Nnucleophiles (Scheme 13.4). Loading of the resin may be quantitatively determined by 19 F-nuclear magnetic resonance (NMR) spectroscopy.

Scheme 13.4. Synthesis of amides using polymer-bound 4-hydroxy-3-nitrobenzophenone.


This methodology is routinely used for the synthesis of large libraries (10,000 members) for lead discovery or for targeted libraries, e.g. optimization of factor Xa inhibitors [30]. Formation of ureas – general aspects There are two standard procedures that are used in the formation of ureas: The classical reaction of isocyanates with amines, and the treatment of amines with carbonyl insertion compounds such as p-NPCF [31] or CDI [32] followed by addition of another amine. Both procedures are suitable for library synthesis in solution phase or on solid support [33]. Formation of ureas using isocyanates. The solvent of choice for the reaction of isocyanates with amines is CH2 Cl2 . The reaction usually is completed within 1–2 h (Scheme 13.5). Prolonged reaction times have been reported to lead to lower product yields [34].

Scheme 13.5. Formation of ureas from amines and isocyanates.

In solid-phase chemistry, either amines or isocyanates can be immobilized. The generation of resin-bound isocyanates may be achieved by treating amines with phosgene or triphosgene [35]. Furthermore, the direct conversion of Fmocprotected amines into isocyanates has been described previously (Scheme 13.6) [36].

Scheme 13.6. Formation of isocyanates from Fmoc-protected amines on solid phase.

Trapping immobilized isocyanates with excess of amine proceeds rapidly (30 min). Using polymer-bound isocyanates as scavenging reagents, the time required for the complete removal of amines (generally nucleophiles) from the reaction mixture depends on the character of the amine (Scheme 13.7, Table 13.2) [37].

Scheme 13.7. Scavenging of amines using polymer-supported isocyanate.

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13 Chemistry of the Carbonyl Group Tab. 13.2. Comparative scavenging of nucleophiles in DCM (20 C) or DCE (60 C).a

Nucleophile

Piperidine Benzyl amine Aniline Aniline 2-Aminobenzophenone 4-Methoxyphenyl-1-butanol 4-Methoxyphenyl-1-butanol a

PS-isocyanate (equiv.) (1% crosslinked)

T ( C)

3.0 3.0 2.0 3.0 3.0 2.0 3.0

20 20 20 60 60 20 60

% Scavenged 1h

16 h

100 100 19 – – 0 0

– – 89 99 81 68 29

Brochure by Argonaut Technologies: Polymer Reagents and Scavengers.

Whereas aliphatic amines are completely removed within 1 h, aromatic amines generally require longer reaction times and elevated temperatures. Formation of ureas using carbonyl insertion compounds Since a larger number of amines than isocyanates is commercially available, the use of the carbonyl insertion method provides access to a larger and more diverse number of compounds than the reaction of isocyanates (Scheme 13.8).

Scheme 13.8.

p-NPCF-mediated formation of ureas.

This method is limited to the use of electron-rich and sterically undemanding amines as nucleophiles. Even though anilines have also been used, ureas derived from anilines are preferably prepared via transforming them into corresponding isocyanates prior to coupling with the second amine. The above-mentioned methods for the activation of amines proceed via in situ formation of the corresponding isocyanate (Schemes 13.6 and 13.8). Therefore, they are limited to primary amines. However, secondary amines may also be activated via formation of the corresponding carbamoyl chloride using phosgene or triphosgene (Scheme 13.9) [38] or by the CDI analog 1,1 0 -carbonylbisbenzotriazole [39].


Scheme 13.9.

Activation of secondary amines with COCl2 for solid-phase synthesis of ureas.

In summary, the formation of ureas using either isocyanates or carbonyl insertion compounds is a straightforward synthetic transformation suitable for library synthesis both in solution and on solid phase. Formation of ureas – recent applications Ureas have been used as precursors of the benzoxazine moiety. Anthranilic acids were treated with isocyanates in solution and the obtained ureas were cyclized with polymeric EDC (Scheme 13.10) [40]. Simultaneously, unreacted excess of anthranilic acid was bound to the carbodiimide and could be filtered off from the reaction mixture yielding products in 80–97% purity.

Scheme 13.10. Solution-phase library using ureas as precursors of benzoxazines.

13.3.1.2 Esters and Urethanes

General remarks For both solution- and solid-phase chemistry, esters and urethanes commonly serve as protecting groups for carboxylic acids and amines, respectively [41]. Additionally, they are one of the most common ways to attach a molecule to a polymeric support. In the context of library synthesis, esters and urethanes may of course occur as part of the targeted molecule [42]. Formation of esters A wealth of methods exist for the esterification of carboxylic acids. Almost all of them are suitable for parallel synthesis. Among these methods, DIC/DMAP (Steglich esterification) [43], 2,4,6-trichlorobenzoyl chloride (Yamaguchi esterification) [44], or 2,6-dichlorobenzoyl chloride [45] and the reaction of alcohols with acid chlorides or anhydrides are the most prominent ones in solution phase. In particular, the Steglich procedure seems to be suitable for automation and parallelization since polymeric carbodiimides are readily available (Scheme 13.11) [46]. Conversion of polymer-bound alcohols using these methods is as efficient as in solution phase. Owing to the lack of methods for acid chloride formation on solid phase (see Section 13.3.1), esterification of immobilized carboxylic acids is

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Scheme 13.11. Parallel esterification using polymeric carbodiimides: taxol analogs.

mostly realized using either the Steglich or the Yamaguchi procedure. Nevertheless, the condensation with primary or secondary alcohols using DIC/DMAP or 2,4,6-trichlorobenzoyl chloride has been well described. Additionally, the combination MSNT/NMI has been reported to be superior for the coupling of immobilized aromatic carboxylic acids to primary and secondary alcohols compared with the Steglich or Yamaguchi esterification (Scheme 13.12) [47].

Scheme 13.12. Esterification of aromatic carboxylic acids using MSNT/NMI.

Hydrolysis of esters In solution, esters are normally hydrolyzed using hydroxides such as LiOH, NaOH, or KOH in aqueous (50%) MeOH or THF [41]. These procedures are suitable for parallelization. Adaptation of these methods to solid phase sometimes requires slight modification of experimental details. Because of the extremely poor swelling properties of PS-based resins in water and alcohols, hydrolysis requires addition of solvents with better swelling properties – usually THF. NaOH or LiOH in THF/H2 O (4:1 v/v) or THF/MeOH/H2 O (3:1:1 v/v) at room temperature (rt) or at 50 C generally give good conversions (Scheme 13.13) [48].

Scheme 13.13. Saponification of esters during library synthesis on solid phase.

Formation of carbamates There are two standard ways for the formation of carbamates: first, from amines and chloroformates (Scheme 13.13); second, from alcohols and isocyanates or carbonyl insertion compounds. In contrast to the reaction of amines with isocyanates,


the reaction of alcohols and isocyanates requires an activating base in order to achieve fast conversion. Usually, NEt3 or DIEA is used (Scheme 13.14).

Scheme 13.14. Formation of carbamates from alcohols and isocyanates.

Besides this small modification, experimental details, scope, and limitations as well as applications of this reaction in solution or on solid phase are analogous to the formation of amides and ureas respectively (Section 13.3.1) [49]. 13.3.2

Transformation of Carboxylic Acids into Other Functional Groups

The transformation of carboxylic acids into other functional groups is a powerful tool for the generation of new functionality. However, the reactions covered in this chapter have been described mostly as useful transformations but have not been used for library synthesis. The synthesis of larger libraries using these concepts has yet to be realized. 13.3.2.1 Formation of Ketones

Carboxylic acids are often used as precursors in the synthesis of aldehydes and ketones. The standard way to transform a carboxylic acid into a ketone is the two-step procedure of forming the corresponding Weinreb amide and subsequent addition of metallorganic reagents (see Chapter 16). The transformation of a carboxylic acid into the corresponding a-halomethyl ketone can be achieved by adding a diazomethane solution to the previously activated (via a mixed anhydride) carboxylic acid. The resulting diazomethylketone can be transformed into the a-halo ketone by treating it with tetrabutyl ammonium salts, (n-Bu 4 Nþ X ) [50]. Using appropriately protected aspartic acid, this reaction sequence was used for the solid-phase synthesis of a highly functionalized template bearing four different functional groups (Scheme 13.15). Substitution of the chlorine followed by further

Scheme 13.15. Synthesis of amino acid-derived a-halomethyl ketones on solid phase.

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356


derivatization completed the synthesis of (acyloxy)methylketones as irreversible inhibitors of the cysteine protease ICE. 13.3.2.2 Formation of Amines: Curtius Degradation

The Curtius degradation transforms carboxylic acids via thermal degradation of their corresponding azides into isocyanates. In the presence of alcohols, the reaction yields carbamates, whereas in the presence of water the intermediate isocyanate is directly degraded into an amino group. The standard procedure for solution-phase conversion of the carboxylic acid into the rearranged isocyanate is the two-step procedure forming the acid azide with DPPA in the presence of base (usually NEt3 ) followed by heating the bound azide to 90 C in toluene or xylene [51]. This well-established procedure has been adapted to solid-phase chemistry using polymer-bound aromatic carboxylic acids (Scheme 13.16) [52]. In contrast to solution phase, the presence of water gave impure products, whereas in the presence of alcohols the reaction cleanly yielded the carbamates. However, the use of 9fluorenylmethanol (Fm-OH) yielded the Fmoc-protected amines from which the unprotected amine can easily be obtained.

Scheme 13.16. Curtius degradation of carboxylic acids leading to Fmoc-protected amines.

13.3.2.3 Tebbe Olefination

The Tebbe olefination converts esters into the corresponding enol ethers that are the starting points for a variety of chemical transformations leading to such different structures as ketones, amines, thiazoles, and cyclohexanones (Scheme 13.17) [53].

Scheme 13.17.

Tebbe olefination of polymer-bound esters.

13.4 Reactions of Aldehydes and Ketones

Despite its synthetic potential, the Tebbe olefination has only rarely been used for parallel synthesis. The starting point for a library of thiazoles on solid phase was the conversion of polymer-bound esters into the corresponding enol ethers using Tebbe reagents in toluene/THF at room temperature. For the following onresin functionalization, the authors elegantly combined the principles of solid- and solution-phase chemistry (Scheme 13.18): in a first step, the enol ether is brominated on the resin, yielding an a-halo ketone equivalent. Formation of thiazoles via Hantzsch synthesis then simultaneously led to cleavage of the heterocycles from the resin. Removal of excess thiourea was accomplished by applying a polymeric ahalo ketone as a scavenger resin.

Scheme 13.18. Modification of enol ethers yielding thiazoles.

13.3.2.4 Formation of Thioamides

Thioamides are valuable precursors for heterocycle synthesis, a prevalent structural element in marketed drugs. Formation of thioamides on solid support can be achieved from readily synthesized primary amides of aliphatic and aromatic carboxylic acids by treatment with Lawesson reagent (Scheme 13.19) [54].

Scheme 13.19. Formation of thioamides on solid support.

13.4

Reactions of Aldehydes and Ketones 13.4.1

Reactions of Carbonyl Groups with CaH Acidic Compounds 13.4.1.1 Wittig and Horner–Emmons Olefinations

General aspects The Wittig and the Horner–Emmons olefinations are well-established synthetic transformations for the generation of double bonds. Parallelization of these extensively used reactions should be straightforward. However, in the literature there are hardly any descriptions of libraries synthesized in solution. Probably, reactions

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have been carried out in a parallel fashion but have not specifically been reported as libraries. Some reports deal with ways to simplify the work-up, e.g. by using polymeric triphenylphosphine [55]. The situation is different for solid-phase synthesis. The reaction of aldehydes or ketones with phosphonium ylides yielding CaC double bonds was among the first reactions to be used for the synthesis of nonpeptidic molecules on solid phase [56]. Numerous examples prove the value of Wittig and the Horner–Emmons olefinations for solid-phase synthesis. Double bonds are versatile starting points for diversification and are, thus, interesting building blocks for combinatorial chemistry. The double bonds obtained by the Wittig reaction have been used for various purposes, e.g. in olefin metathesis reactions [57] or cycloadditions [58] (Scheme 13.20).

Scheme 13.20.

Wittig reaction: diene formation as a precursor for a cycloaddition.

For solid-phase synthesis, two strategies have been pursued: generation of the ylide in solution and adding it to a polymer-bound aldehyde, as well as the opposite direction – generation of the ylide on solid phase and subsequent addition of the aldehyde to the resin. It is important to note that the choice between these two possibilities has consequences for the reaction conditions. If the ylide is generated on solid phase, KOtBu/THF has been reported to be better than NaHMDS/THF for phosphonium salt-derived ylides [58a]. Additionally, the reaction time for the generation of immobilized ylides is shorter (5–60 min) than in solution phase (up to 3 h). Hydrolysis of these sensitive intermediates can be a significant problem unless the resin is rigorously dried prior to use, e.g. azeotroping it with benzene. Owing to the highly basic conditions of the reaction, the choice of the linker is also crucial. Even though the Wang linker has been employed previously [see 65], most examples use more base-stable linkers, e.g. amide linkage [59], trityl-ethers [56], and carbamates [60]. To avoid ester hydrolysis of substrates bound to Wang resin, the following procedure has been recommended (Scheme 13.21): generation of the phosphonate anion, removal of excess base under inert conditions, and addition of the aldehyde dissolved in 60% cyclohexane in THF [61].

Scheme 13.21. Horner–Emmons reaction on Wang resin.


The reasoning behind the cyclohexane/THF mixture was that the less-polar solvent mixture should suppress ester hydrolysis, while allowing the Horner– Emmons condensation to proceed. The authors successfully synthesized acrylic acid derivatives from aldehydes containing aliphatic, aromatic, and basic functionalities. The reaction generally was completed within 2 days. The undesired cleavage from the resin during ylide generation can also be avoided by attaching the ylide to the resin via the phosphonate moiety. In this case, the Horner–Emmons reaction has been used for cleaving the molecule from the resin (Scheme 13.22) [62]. This approach was used for the library synthesis of cyclic ketones such as (dl)-muscone: a polymer-bound methylphosphonate was deprotonated with n-BuLi and quenched with an alkenyl ester. The double bond was used for subsequent introduction of an aldehyde moiety. The following intramolecular olefination (‘‘cyclorelease’’) led to the cyclic enones which were further derivatized in solution phase.

Scheme 13.22. Intramolecular Horner–Emmons reaction.

Recent examples The use of commercially available polymer-bound PPh3 for Wittig reactions combines ideas from solid-phase synthesis and the application of polymer-supported reagents (Scheme 13.23) [63]. On the one hand (as a polymer-supported reagent) it offers the advantage that the byproduct, triphenylphosphine oxide, remains at-

Scheme 13.23.

Wittig reaction using polymer-supported triphenylphosphine.

359

360


tached to the resin. On the other hand (similar to the function of a linker in solidphase synthesis) it immobilizes benzyl halides. Starting from 2-nitrobenzylbromide, after immobilization and chemical modifications, the products may be cleaved off from the support by an intra- or intermolecular Wittig reaction. The intramolecular Wittig reaction yields indoles, whereas the intermolecular pathway gives access to stilbenes [64]. Double bonds have also been used for the synthesis of peptidomimetics. In this context, they serve as nonhydrolyzable rigid mimetics of the three-dimensional structure of the amide bond. The Horner–Emmons reaction has been employed as part of a two-step procedure for solid-phase synthesis of b; g-unsaturated d-amino acids that mimic dipeptides (Scheme 13.24) [65]. This concept combines both the use of the double bond as an amide bond mimetic as well as the use of the double bond for diversification.

Scheme 13.24. Horner–Emmons reaction: solid-phase derivatization of the double bond.

13.4.2

Reductive Amination 13.4.2.1 General Aspects

Reductive amination is one of the most frequently used reactions for library generation in solution phase as well as on solid phase. Starting materials are diverse, inexpensive, and readily available from commercial sources. Additionally, the secondary amines obtained may be widely applied to the generation of novel structures, e.g. heterocycles, sulfonamides, and amides [66]. Last but not least, the reaction is well suited to automated synthesis. Not surprisingly, a large number of methods has been elaborated for the multitude of electronic and steric demands of the building blocks. 13.4.2.2 Formation of Imines

Imines are formed by the condensation of an amine with a ketone or an aldehyde. Classical methods in solution phase shift the equilibrium toward the imine by removing the water generated during the reaction, e.g. via azeotropic distillation or applying drying reagents such as molecular sieves or inorganic sulfates. These methods have also been used for solid-phase chemistry [67], but one should bear in mind that the distribution of these reagents into a parallel array is not very


convenient. Therefore, trimethylorthoformate (TMOF) has gained popularity as a universal and resin-compatible desiccant (Scheme 13.25) [68]. Additional methods for imine formation include the simple use of a large excess of amine [69] or using ultrasound in the presence of Na2 SO4 [70].

Scheme 13.25. Imine formation on solid support using TMOF as dehydrating reagent.

Imines formed in the presence of TMOF may be isolated and hence can be used for various purposes besides reduction to amines, e.g. for cycloadditions [71]. In cases where the solubility of the amine component in neat TMOF is low, cosolvents such as THF or dimethylformamide (DMF) can be used. As a rule of thumb, the reaction is slower with polymer-bound aldehydes and the amine in solution than with the opposite situation. A given combination of building blocks has consequences for the reaction conditions depending on the reactivity of the amine and carbonyl components (Table 13.3). The reaction of aliphatic amines and aromatic aldehydes reliably yields the desired imine within short reaction times at room temperature. Other scenarios may result in side-reactions or incomplete conversions. Aromatic amines also form imines, but, depending on their electronic nature, this reaction may require elevated temperatures and elongated reaction times. Imines derived from aliphatic aldehydes tend to form the tautomeric enamines. The reactivity of ketones is strongly influenced by their structure. Cyclic ketones react nicely whereas acyclic ketones often show hardly any conversion [72]. 13.4.2.3 Reduction of Imines/Enamines

A number of reducing agents has been used for the reduction of imines to secondary amines. The most frequently used reagents on solid phase are: NaCNBH3 in 1% HOAc/DMF [73], freshly prepared NaBH(OAc)3 or (NBu 4 )BH(OAc)3 [74], and BH3 py [69a, 75]. They all reduce imines and enamines (e.g. derived from secondary amines and aldehydes) equally well. Generally, the reducing agent is added in situ without isolation of the imines or the enamines.

Tab. 13.3. Imine formation depending on amine and carbonyl component.

Carbonyl

Amine

Comment

Aromatic aldehyde Aliphatic aldehyde Aromatic aldehyde

Aliphatic Aliphatic Aromatic

Cyclic ketone Acyclic ketone

Aliphatic Aliphatic

Clean reaction (3 h, rt) Equilibrium with the tautomeric enamine Requires elevated temperatures and longer reaction times (e.g. 50 C, 16 h) Mostly a clean reaction Low conversion

361

362

13 Chemistry of the Carbonyl Group Tab. 13.4. Optimized reaction conditions for reductive aminations using NaCNBH3 .

Aldehyde/Aminea

Format

Experimental detail

Aliphatic aldehyde/aliphatic amine

Solution phase

Aliphatic aldehyde/aliphatic aminepb

Solid phase (resin)

Sterically hindered aliphatic aldehydes/aliphatic amine

Solution phase

Sterically hindered aliphatic aldehydes/aliphatic aminepb

Solid phase (resin)

Aromatic aldehyde/ aliphatic amine

Solution phase

Aromatic aldehyde/ aliphatic aminepb

Solid phase (resin)

Aliphatic amine/ketonepb

Solid phase (pin) Solid phase (pin)

Simply requires NaCNBH3 in TMOF, large excess of aldehyde, no additional proton source Simply requires NaCNBH3 in TMOF, large excess of aldehyde, no additional proton source Requires premixing (30 min) of amine and aldehyde prior to addition of NaCNBH3 , no additional proton source Requires premixing (30 min) of amine and aldehyde prior to addition of NaCNBH3 , requires additional proton source (MeOH !!!, HOAc leads to overalkylation) Premixing with the amine in TMOF followed by the addition of NaCNBH3 in the presence of 1% HOAc Premixing with the amine in TMOF followed by the addition of NaCNBH3 in the presence of 1% HOAc pH ¼ 5 in MeOH (HOAc)

Aromatic amine/ketonepb a

pH ¼ 7 in MeOH (HOAc, NMM)

p.b., (¼ polymer bound) – indicates which part is on solid phase.

Reaction conditions have been investigated for all reagents in order to achieve maximum conversion and to suppress overalkylation. The classical reagent for reduction, NaCNBH3 [76], has been optimized with respect to aldehydes and amines for both solution- and solid-phase chemistry (Table 13.4) [77]. 13.4.2.4

Applications

Reductive aminations using polymeric reagents Reductive aminations in solution phase sometimes require the tedious removal of boron-containing by-products generated from the reduction step. Solid-supported borohydrides circumvent this problem. For the synthesis of secondary amines from primary aliphatic amines an excess of primary amines relative to the aldehyde component has been used. After reduction of the corresponding imine with


resin-bound borohydride, excess primary amine was removed by selective imine formation using a polymer-supported aldehyde (Scheme 13.26) [78].

Scheme 13.26. Reductive amination in solution phase using polymeric borohydride.

Favorable relative kinetics of the competing reactions allowed simultaneous addition of both resins to the reaction mixture, thus simplifying the experimental procedure. Reductive aminations as a starting point for library synthesis For library synthesis on solid phase, a major application of reductive aminations is their use as a starting point for library synthesis. During the first step, large diversity has already been introduced from a multitude of commercially available amines as building blocks. Subsequent acylation or sulfonylation of the generated secondary amines is the next step during the synthesis of the desired compounds. Most commonly, secondary amines are generated from a polymer-bound, electron-rich aromatic aldehyde (Scheme 13.27). The choice of the appropriate reducing agent does not seem to be crucial, e.g. 2 equiv. RNH2 /NaBH(OAc)3 and 3 equiv. RNH2 /CH(OMe)3 /BH3 py have been successfully applied [79].

Scheme 13.27. Reductive amination as starting point for library synthesis.

Rink’s amide linker has been used as a polymer-bound source of amines (Scheme 13.28) [80]. It became obvious that clean conversion of the sterically demanding benzhydrylamine was not straightforward and that the choice of the

Scheme 13.28. Reductive amination of Rink’s amide linker.

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reaction conditions for reducing the imine was crucial. Standard methods failed (e.g. 1–10 equiv. R-CHO/NaCNBH3/1% HOAc in DMF). However, adjusting the solvent system from anhydrous DMF to aqueous THF gave clean monoalkylation with aliphatic and aromatic aldehydes as well as with ketones. Reductive amination of ketones: benzopyran library Even though reductive amination of ketones is often difficult, thorough optimization of the reaction conditions can still lead to suitable synthetic protocols, as demonstrated by the conversion of benzopyranones (Scheme 13.29). For the reduction of imines derived from these relatively unreactive ketones, combinations of complex boron hydrides with standard proton sources such as HOAc or MeOH (Table 13.4) failed. Furthermore, heating or sonication led to significant cleavage from the resin. However, the use of Ti(OiPr)4 in combination with NaBH(OAc)3 resulted in complete conversion to the desired amine.

Scheme 13.29. Reductive amination of ketones using Ti(OiPr)4 /NaBH(OAc)3 .

The presence of Ti(OiPr)4 requires that the reaction is carefully run under inert conditions in order to avoid precipitation of TiO2 . The feasibility of this protocol was demonstrated by the synthesis of a 8448-member library of benzopyrans [81]. Serendipitous reactions as an opportunity for affinity breakthroughs Neuropeptide Y is believed to be involved in the regulation of feeding, energy metabolism, vascular tone, learning and memory, and the release of pituitary hormones. Researchers have discovered that benzimidazoles act as potent antagonists of the NPY-1 receptor. Further optimization of the side-chain has been addressed by a solution-phase library. In order to optimize interaction with the receptor, the distal piperidine has been modified via reductive amination using polymeric reagents and scavenger resins. Only one compound from this library appeared to be more active than the lead for the library. However, upon resynthesis it became clear that the anticipated product was not consistent with the spectroscopic data and that the intermediate imine underwent a spontaneous Pictet–Spengler cyclization, even in the absence of any reducing agent (Scheme 13.30) [82].

References

Scheme 13.30.

Unexpected side-reaction yielding biologically active compound.

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Oxidation Except CC Double Bonds Henning Steinhagen 14.1

Introduction

Oxidations belong to the very fundamental set of transformations in organic chemistry. Comparing the rich repertoire of modern oxidation reactions in solution [1], relatively few reaction types and principles have been transferred to solid phase and consequently to combinatorial chemistry. The main reason for this fact is that oxidations usually do not add diversity in the construction of compound libraries. Also, manipulations of the oxidation stage in combinatorial (multistep) synthesis are often unwanted and can be avoided by choosing the right building blocks of correct oxidation states. Nevertheless, oxidations on solid support are sometimes very useful and reliable transformations. By far the most applied transformation is the oxidation of primary and secondary alcohol functions to aldehydes and ketones. Another important class of oxidation reactions are the formation of sulfoxides and sulfones from sulfides. But also more complex oxidation types such as heterocyclic synthesis and oxidative coupling reactions are becoming more popular. The aim of this chapter is to give an overview of useful oxidation reactions on solid phase. The reactions can be principally divided into two classes: (1) polymer-bound substrate and (2) polymer-bound oxidant. Being applied to combinatorial chemistry these two classes often correspond to solid-phase (class 1) and to solution-phase (class 2) combinatorial chemistry. Since the field of oxidation reactions on solid support [2] and especially the use of polymer-bound reagents [3] has been reviewed and published extensively, the focus of this chapter is more to present some interesting examples rather than giving a complete literature survey of all published procedures. Nevertheless, it has been an aim to supply the reader with the most relevant literature in the references. 14.2

Oxidation of Alcohols to Aldehydes and Ketones

Conversion of alcohols are among the most used and reliable oxidation reactions on solid phase. In general, many protocols from the classic repertoire Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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of solution-phase oxidation reactions have been transferred successfully to solid phase. By choosing the appropriate reagent, synthetic limitations of the substrate (e.g. temperature instability, chemoselectivity, solvent) and also of the linker/ polymer support system should be carefully considered. The most common and reliable reagents for oxidation of polymer-bound alcohols include SO3/pyridine (Scheme 14.1) [4], Swern conditions (Scheme 14.2) [5], Dess–Martin periodinane (DMP; Scheme 14.3) [6], perruthenate (TPAP) [7] and Cr reagents (PDC [8], CrO2 Cl2 [9], CrO2 (Ot-Bu)2 [9b]; Scheme 14.4). Other published methods involve NCS [10] and IBX [11] as oxidants. There have also been some reports for the oxidation of polymer-bound phenols by various conditions mostly for coupling reactions [12, see 57, 59, 60a] (Scheme 14.14). In comparison, there are many polymer-

Scheme 14.1. Oxidation step in the synthesis of peptide aldehydes of the (S)-MAPI type [4f ].

Scheme 14.2. Oxidation using the aldol strategy in the solid-phase synthesis of epothilone A [5].

Scheme 14.3. Domino oxidation/Diels–Alder pathway to mniopetals [6e].

14.2 Oxidation of Alcohols to Aldehydes and Ketones

Scheme 14.4. Oxidation–cyclization strategy for the solidphase synthesis of carboxypyrrolinones [9a].

bound oxidants available that are suitable for oxidation of alcohols in solution to the corresponding aldehydes and ketones. Poly(vinyl pyridinium dichromate) (PVPDC) is an inexpensive, easy-to-use, recyclable reagent for the oxidation of different alcohols, including primary, secondary and allylic alcohols [13]. The latter can be also oxidized using polymer-supported ammonium perchromate, although the reagent fails with saturated alcohols [14]. Silica-based chromium reagents have also been reported as easy to prepare and use for the oxidation of several alcohols [15]. Many other convenient-to-use reagents such as KMnO4 on kieselguhr [16] and ammonium chlorochromate on silica [17] have been reported to oxidize a variety of alcohols. Ammonium chlorochromate proved especially useful for the oxidation of benzoins to benzils. Nevertheless, the main problems of chromium-based reagents remain selectivity and toxicity issues and the mostly stoichiometric use. This draw back could be overcome by a very useful reagent for the oxidation of several alcohols, including primary, benzylic and secondary alcohols: polymersupported perruthenate (PSP) [18]. The reagent can be used stoichiometrically or in catalytic amounts with molecular oxygen or an N-oxide as cooxidant. Using molecular oxygen as a cooxidant also avoids the need for conventional work-up (Scheme 14.5) [18b]. Another recently introduced useful oxidant is a TEMPOderived oxammonium resin [19]. This powerful oxidant cleanly converts primary alcohols to aldehydes and also secondary alcohols to ketones (Scheme 14.6). 14.2.1

Examples of the Oxidation of Polymer-bound Primary Alcohols to Aldehydes

Bradley and coworkers [4f ] have described an interesting example using SO3/ pyridine for the oxidation of polymer-bound primary alcohols in the synthesis of peptide aldehydes based on the human immunodeficiency virus (HIV) protease inhibitor (S)-MAPI. Peptide aldehydes are an important class of transition-state analogs of different proteases and have been of considerable interest since their initial discovery in natural products [20]. The efficiency and reliability of the oxidation step was dramatically improved by the incorporation of a small polyethylene glycol (PEG) spacer between the linker and the solid support (polystyrene resin).

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Scheme 14.5. Examples of oxidation using PSP [18].

In the course of the synthesis, the polymer-bound primary alcohol 1 was oxidized by SO3/pyridine to the peptide aldehyde 2, which could be readily cleaved using trifluoroacetic acid (TFA). An interesting example of the Swern oxidation of a primary alcohol in a multistep sequence on solid support (Merrifield resin) was demonstrated by Nicolaou et al. [5] in the synthesis of the anticancer agents epothilones A and B. In the course of the synthesis, the TBS-protected alcohol 3 was deprotected with HF/pyridine and subsequently oxidized to the aldehyde 4 (> 95% yield). The aldehyde was essential for further conversion to 5 in a stereoselective aldol reaction. After three additional steps [1, esterification; 2, metathesis upon cleavage; 3, epoxidation with methyl-(trifluoro-methyl)dioxirane], epothilone A was obtained. 14.2.2

Examples of the Oxidation of Polymer-bound Secondary Alcohols to Ketones

An interesting example using the oxidation of a secondary alcohol was demonstrated in the synthesis of a building block for a library of analogs of mniopetal [6e]. The Dess–Martin oxidation was used as an electronic activation of the dienophile 6 for the domino oxidation/inverse intramolecular Diels–Alder reaction attached to Wang resin. The final product 7 was cleaved under oxidative conditions using 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) [21].

14.2 Oxidation of Alcohols to Aldehydes and Ketones

Scheme 14.6. Examples of oxidation with oxammonium resin [19].

Miller at al. presented a simple and efficient way for the preparation of carboxypyrrolinones on solid phase [9a]. The pyrrole system is an interesting potentially bioactive motif for both agrochemicals and pharmaceutical drugs. In this approach, Wang resin-bound malonic acid [22] 8 was coupled with an amino alcohol in the presence of 1-hydroxybenzotriazole hydrate (HOBt)/DIC, providing the hydroxyamide 9 in high yield. For the oxidation step to 10, several oxidizing reagents were tested. Under Swern conditions the conversion was not complete because the Swern reagent is unstable at temperatures above 10 C. SO3/pyridine is stable at room temperature, but the reagent was not effective for all of the examples shown, especially N-aryl-substituted compounds. The most broadly applicable reagent was CrO2 (Ot-Bu)2 generated in situ according to Leznoff et al. [9b]. The oxidation of 9 to 10 proceeded successfully for most substrates, and after ring closure with lithium diisopropylamide (LDA)/ZnCl2 and subsequent cleavage with TFA, the carboxypyrrolinones 11 could be obtained. 14.2.3

Examples of the Oxidation of Alcohols by Polymer-bound Reagents

Ley and Hinzen introduced polymer-supported perruthenate (PSP), which can be easily prepared by washing an anion exchange resin with an aqueous solution of potassium perruthenate [18]. The obtained reagent can be used stoichiometrically

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or in catalytic amounts using molecular oxygen or N-oxides [N-methylmorpholine N-oxide (NMO), TMAO] as cooxidant. Especially in the catalytic version (0.1 equiv. PSP, O2 as cooxidant), this versatile reagent is very useful. PSP cleanly oxidizes a,b-unsaturated (e.g. 12) and benzylic alcohols (e.g. 13) in 1 h to the corresponding ketones. Also nonactivated primary alcohols (e.g. 14) and epoxy alcohols (e.g. 15) are converted, but reaction times are somewhat longer. Secondary alcohols are unreactive, providing chemoselectivity with this reagent. Another useful polymer-bound oxidant is the oxammonium TEMPO resin that was introduced by Rademann and coworkers [19]. In contrast to PSP, this strong oxidant reacts with both primary alcohols to aldehydes (e.g. 16, 18) and secondary alcohols (e.g. 17) to the corresponding ketones. In substrates such as 19, where the initial product is easy enolizable, further oxidation directly leads to the 1,2-diketo compounds. The mechanism of oxidation (Scheme 14.7) can be rationalized by the intermediate formation of an alkoxyTEMPO salt 22 that cleaves to 23 and the corresponding oxidized compound 24. Prior to the oxidation, the TEMPO polystyrene 20 needs to be activated by NCS, Br2 or Cl2 , yielding the oxammonium salt 21.

Scheme 14.7. Proposed mechanism for the oxidation of alcohols using oxammonium resin [19].

de Frutos and Curren presented a solution-phase synthesis of libraries of polycyclic natural product analogs by cascade radical annulation (Scheme 14.8) [23]. The aim of the study was to explore the structure–activity relationship (SAR) of mappicine ketone, which is an antiviral (HCMV, HSV) lead structure in the low micromolar range [24]. A 48-member library was prepared from three building blocks and the library was purified at the stage of the mappicine analogs. Therefore, it was anticipated that a clean and easily removable oxidant would be obtained with the goal of having sufficiently clean products for biological testing without

14.3 Oxidation of Polymer-bound Aldehydes to Carboxylic Acids

Scheme 14.8. Screening of polymer-bound oxidants in the synthesis of mappicine ketone analogs [23].

further chromatography. From the screened oxidants, only the polymer-supported perruthenate (PSP) and chromic acid proved to be useful, whereas other reagents such as Dess–Martin periodinane (DMP), polymer-bound PDC, and permanganate oxidation did not give good results.

14.3

Oxidation of Polymer-bound Aldehydes to Carboxylic Acids

Compared with the oxidation of alcohols to aldehydes and ketones, there are relatively few reports of the direct oxidation of aldehydes to carboxylic acids under solid-phase conditions. This is because the corresponding alcohols serve as more stable and easily protectable functional groups in both solid and liquid phase. Nevertheless, there have been some reports of the oxidation of polymer-bound aldehydes to carboxylic acids including the use of NaClO2 [25] and MCPBA [26] as oxidants. An impressive example was demonstrated by Nicolaou et al. in the solidphase synthesis of a combinatorial library of sarcodictyins (Scheme 14.9) [25a]. Sarcodictyins were discovered in 1987 in a Mediterranean coral [27a] and their potent antitumor activity (Taxol-like mode of action) was recognized by Ciomei et al. in 1997 [27b]. In the example given, the tricyclic core system 25 was constructed via multistep synthesis on Merrifield resin. After deprotection of 25 using TBAF, the primary alcohol was oxidized stepwise first to the unsaturated aldehyde 26 with DMP. After subsequent oxidation to the corresponding carboxylic acid with NaClO2 , esterification using MeOH/DCC led to the methyl ester 27. The stepwise procedure gave optimal chemical yields (@95%).

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Scheme 14.9. Oxidation sequence in the solid-phase synthesis of sarcodictyins [25a].

14.4

Oxidation of Sulfur-containing Compounds

Oxidation of sulfur-containing compounds on solid phase is a quite common and usually high-yielding reaction. Most typically sulfides are converted to their corresponding sulfoxides or sulfones. Typical oxidants for polymer-bound sulfurcontaining compounds are MCPBA [28, 29] and H2 O2 [30, 31], but also less common reagents such as NaBrO2 [32], OsO4/NMO [33], and ozone [29e] have been reported. In the case of MCPBA, the oxidation can usually be controlled by the number of equivalents used (see Scheme 14.11) [28a]. Besides the oxidation of sulfides and sulfoxides, polymer-bound sulfinamides have also been oxidized to sulfonamides using different conditions [34]. Other oxidation procedures of polymer-bound sulfur-containing compounds include thioacetals, which have been oxidatively cleaved from solid support using PhI(Tfa)2 [35] or H5 IO6 [36]. 14.4.1

Examples of the Oxidation of Polymer-bound Sulfides to Sulfoxides and Sulfones

There are in general two reasons for the oxidation of a sulfur-containing compound on solid phase. Either the oxidized form displays some interesting biological or chemical activity, or the oxidation procedure is part of a cleavage strategy in which, for example, the sulfoxide function is cleaved by elimination or the sulfone is directly displaced with a nucleophile. A typical example of a cleavage procedure using a sulfone was presented by Gayo and Suto in the synthesis of substituted aminopyrimidines (Scheme 14.10) [29b]. The solid-supported (TG Thiol resin)

14.4 Oxidation of Sulfur-containing Compounds

Scheme 14.10. Oxidation–cleavage strategy for the synthesis of aminopyrimidines [29b].

pyrimidine 28 was oxidized to the corresponding sulfone 29 with MCPBA and subsequently cleaved through nucleophilic displacement by an amine, yielding the substituted 2-amino pyrimidine 30. An interesting example for the oxidation of sulfides to either sulfoxides or sulfones with MCPBA was demonstrated by Mata in the synthetic approach to blactams on solid support (Merrifield and Wang resin) (Scheme 14.11) [28a]. The carboxylic acid 31 was loaded onto Merrifield resin yielding 32, and then either oxidized to the sulfoxide 33 with 1.4 equiv. MCPBA at 0 C or to the sulfone 34 using a fivefold excess of MCPBA at room temperature. Eventually, the b-lactams could be cleaved using AlCl3 .

Scheme 14.11. Oxidation sequence of b-lactams to sulfoxides and sulfones [28a].

Grimstrup and Zaragoza presented an interesting synthetic route to highly functionalized benzamides on solid phase (Wang resin) by a facile nucleophilic substitution pathway (Scheme 14.12) [29c]. The thiol 35, which was derived from the corresponding fluoro compound and RSH, was first oxidized to the sulfone 36. This highly electron-deficient aryl fluoride was now easily substituted by a secondary amine R 0 R 00 NH, yielding 37, which could be readily cleaved to 38 from solid support upon treatment with TFA. The order of the oxidation step and the displacement step were in this case very important, since MCPBA also oxidizes the anilide amine if the nucleophilic displacement with the amine was performed directly on 35 followed by MCPBA oxidation.

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Scheme 14.12. Nucleophilic substitution strategy to highly

functionalized benzamides on solid support [29c].

14.5

Oxidation of Selenium- and Phosphorus-containing Compounds

Compared with oxidations of sulfur-containing compounds on solid support, there have been fewer examples of oxidation of selenium- or phosphorus-containing compounds. Nevertheless, selenium compounds have attracted some attention mainly because of their use as linker systems which can be easily cleaved under oxidative conditions (see Chapter 4.3.7.3). Compared with nonpolymer-bound selenium compounds [37], which are usually problematic because of their odor and toxicity, the solid-phase-bound analogs are both totally odorless and convenient to use. The oxidation of selenides to the corresponding selenoxides is usually performed using H2 O2 [38], but also with other reagents such as NaIO4 [39], have been reported. Oxidation of phosphorus-containing compounds is relatively rare and often linked to the synthesis of oligonucleotides [40]. Phosphites have been oxidized to phosphates using NMO [41] and iodine [42]. Phosphonates have also been oxidized to phosphates using iodine [40].

14.5.1

Examples of the Oxidation (Cleavage) of Selenides to Selenoxides on Solid Support

A useful and versatile application of selenium-based solid-phase synthesis was presented by Nicolaou et al. in the combinatorial synthesis of benzopyran-based natural products (Scheme 14.13) [38c,d]. Various substituted ortho-prenylated phenols 39 were directly loaded onto the selenium-based solid support (polystyrene) leading to formation of the 2,2-dimethylbenzopyran system 40. This very mild and selective procedure exhibited a broad tolerance toward a wide range of polyfunctionalized aromatic compounds. In all cases, the resin could be quantitatively

14.6 Oxidative Formation of Heterocycles on Solid Support

Scheme 14.13. Combinatorial synthesis of benzopyran-based

natural products using a selenium linker system [38c,d].

loaded using a threefold excess of 39. The further strategy in the course of constructing a combinatorial library contained various chemical manipulations (in general transformation of R2 to R5). These transformations included annulations, glycosidations, aldol condensations, and various coupling reactions, yielding the products represented by formula 41. The selenium oxidation step readily cleaved the products, yielding 42 in high purity and efficiency with the introduction of a double bond.

14.6

Oxidative Formation of Heterocycles on Solid Support

Solid-phase synthesis of heterocycles has become a central part of research in the field of combinatorial chemistry mainly because of the great structural variety and potential biological activity. Familiar examples include quinolones, b-lactams and dihydropyridines. There are many methods for the synthesis of heterocycles using oxidations as important synthesis steps (compare Chapter 22). In general, the main synthetic approaches can be divided into three categories represented by: A, oxidative aromatization; B, oxidative cyclization or condensation; and C, formation of heterocycles upon oxidative cleavage. Examples of A include the synthesis of pyridines [43], quinazolinones [44] and isoquinolines [45]. Examples of B include the synthesis of indolizines [46], pyrimidines [47], pyridazines [48], isoxazolines [49], benzodiazepines [50], indazoles [51] and benzimidazoles [52]. Polymer-bound quinolines have been activated for coupling by oxidation to the corresponding Noxides using MCPBA [53]. Examples of C include the synthesis of isoxazolines [54], g-butyrolactones [55] and phenanthridines [56].

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14.7

Oxidative Coupling and Cleavage Reactions on Solid Support

There are relatively few reports of oxidative coupling reactions on solid support. Nevertheless, some quite complex reaction sequences have been successfully performed. Phenols have been coupled to cyclic biarylethers using phenolic oxidation employing thallium(III) salts [57]. Biaryls have been formed by oxidative coupling using 1,3-dinitrobenzene (see Scheme 14.15) [58] and vanadium-based oxidants [59]. Oxidative homocouplings of ortho-hydroxystyrenes yielding carpanones have been performed using PhI(OAc)2 , as described in Scheme 14.14 [60a].

Scheme 14.14. Oxidative coupling strategy for the synthesis of carpanones [60].

In contrast to oxidative couplings, there have been many reports of cleavage reactions under oxidative conditions (compare Section 14.7.2). Most of these reports directly refer to linker cleavage strategies (see Chapter 4.5). An interesting application is the preparation of peptide aldehydes using the cleavage of amino alcohols [61] and diols [62] with NaIO4 (see Scheme 14.16). Other applications of oxidative cleavage procedures include arylethers which have been cleaved using cerium ammonium nitrate (CAN) as an oxidant [63]. Benzylethers have been cleaved from solid support (Wang resin) using DDQ oxidation [64] and benzylamines which have been cleaved from Merrifield p-benzyloxybenzylamine (BOBA) resin by DDQ [65]. Aryl hydrazides have been used as linker systems and can be oxidatively cleaved using different conditions including Cu(OAc)2 and NBS [66].

14.7 Oxidative Coupling and Cleavage Reactions on Solid Support

14.7.1

Examples of Oxidative Coupling Reactions on Solid Support

Shair and coworkers presented a biomimetic approach to carpanone-like molecules through oxidative heterocoupling on solid support (Scheme 14.14, PS-DES resin, silyl and trityl linkers) [60a] based on the previous synthesis in solution phase by Chapman et al. [67]. In a first step, phenols 43 and 44 were oxidatively coupled with PhI(OAc)2 and directly transformed to the Diels–Alder adduct 45. The choice of the oxidant was crucial to this step, since the reacting phenols had to be electronically differentiated and coupled in the anticipated way. The inverse electron demand Diels–Alder reaction proceeded via an electronically matched transition state. Cleavage from solid support with HF/pyridine yielded the carpanone-like products 46. Experiments are under way to generate a split-andpool synthesis of a 100,000-member library and high-throughput biological screens [60b]. Schreiber and coworkers presented a stereoselective synthesis of a biarylcontaining medium ring system using the oxidation of biaryl cuprates on solid support (polystyrene resin, silicon linker) (Scheme 14.15) [58a].

Scheme 14.15. Intramolecular biaryl synthesis using oxidation of cuprates [58a].

In the first step, the attached biarylic compound 47 was converted to the intermediary cyclic cuprate 48 using lithiation and treatment with copper cyanide. Subsequently, the biaryl ten-membered ring 49 was formed upon exposure to the oxidant 1,3-dinitrobenzene (1,3-DNB). After cleavage from solid support using HF/ pyridine the product 50 could be isolated in a diastereomeric ratio of 6:1 (P/M). A disadvantage of the solid-phase approach was the decreased diastereomeric ratio, which was better (20:1) in the analog reaction in solution phase.

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14.7.2

Examples of Oxidative Cleavage Reactions on Solid Support

A useful route to the synthesis of C-terminal a-oxo aldehydes on solid support was presented by Malnyk and coworkers [62]. As already described in Section 14.2.1, peptide aldehydes are an important class of potentially bioactive compounds [20]. Since peptide aldehydes are prone to epimerization, considerable attention has been focused on the construction of a new appropriate linker system to allow mild reaction conditions. The strategy that was chosen (Scheme 14.16) contained a new linker system as a key element that could be removed in a two-step procedure using TFA and NaIO4 as oxidants. The peptide to be converted into the peptide aldehyde was first loaded onto the linker moiety 51 using standard solid-phase peptide synthesis (SPPS), yielding 52, and then subsequently transferred to diol 53. Mild cleavage of the diol using NaIO4 yielded the desired peptide aldehyde 54.

Scheme 14.16. Synthesis of a-oxo aldehydes using formyl transfer from the linker system [62].

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Perkin 1 1997, 1907; b) B. Hinzen, R. Lenz, S. V. Ley, Synthesis 1998, 977. S. Weik, G. Nicholson, G. Jung, J. Rademann, Angew. Chem., Int. Ed. Engl. 2001, 40, 1436. a) T. Aoyagi, T. Takeuchi, A. Matsuzaki, M. Kawamura, M. Kondo, M. Hamada, K. Maeda, H. Umezawa, J. Antibiot. 1969, 22, 283; b) J. O. Westerik, R. Wolfenden, J. Biol. Chem. 1972, 247, 8195. a) S. Kobayashi, R. Akiyama, Tetrahedron Lett. 1998, 39, 9211; b) T. L. Deegan, O. W. Gooding, S. Baudart, J. A. Proco, Tetrahedron Lett. 1997, 38, 4973. B. C. Hamper, S. A. Kolodziej, A. M. Scates, Tetrahedron Lett. 1998, 39, 2047. O. de Frutos, D. P. Curran, J. Comb. Chem. 2000, 2, 639. a) T. R. Govindachari, K. R. Ravidranath, N. Viswanathan, J. Chem. Soc., Perkin Trans 1 1974, 1215; b) I. Pendrak, R. Wittrock, W. D. Kingsbury, J. Org. Chem. 1995, 60, 2912. a) K. C. Nicolaou, N. Winssinger, D. Vourloumis, T. Ohshima, S. Kim, J. Pfefferkorn, T. Li, J. Am. Chem. Soc. 1998, 120, 10814; b) J. C. H. M. Wijkmans, A. J. Culshaw, A. D. Baxter, Mol. Diversity 1998, 3, 117. a) X. Beebe, N. E. Schore, M. J. Kurth, J. Am. Chem. Soc. 1992, 114, 10061; b) X. Beebe, C. L. Chiappari, M. J. Kurth, N. E. Schore, J. Org. Chem. 1993, 58, 7320; c) J. T. Ayres, C. K. Mann, J. Polym. Sci., Polym. Lett. Ed. 1965, 3, 505. a) M. D’Ambrosio, A. Guerriero, F. Pietra, Helv. Chim. Acta 1987, 70, 2019. b) M. Ciomei, C. Albanese, W. Pastori, M. Grandi, F. Pietra, M. D’Ambrosio, A. Guerriero, C. Battistini, Proc. Am. Assoc. Cancer Res. 1997, 38, 5, Abstract 30. Oxidation to sulfoxides: a) E. G. Mata, Tetrahedron Lett. 1998, 39, 5287; b) T. Masquelin, D. Sprenger, R. Baer, F. Gerber, Y. Mercadal, Helv. Chim. Acta 1998, 81, 646; c) M. Patek, B. Drake, M. Lebl, Tetrahedron Lett.

29

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1995, 36, 2227; d) C. P. Holmes, J. P. Chinn, G. C. Look, E. M. Gordon, M. A. Gallop, J. Org. Chem. 1995, 60, 7328. Oxidation to sulfones: a) J. S. Patek, B. Zhu, Tetrahedron Lett. 1996, 37, 8151; b) L. M. Gayo, M. J. Suto, Tetrahedron Lett. 1997, 38, 211; c) M. Grimstrup, F. Zaragoza, Eur. J. Org. Chem. 2001, 3233; d) U. Grabowska, A. Rizzo, K. Farnell, M. Quibell, J. Comb. Chem. 2000, 2, 475; e) F. E. K. Kroll, R. Morphy, D. Rees, D. Gani, Tetrahedron Lett. 1997, 38, 8573; f ) M. Yamada, T. Miyajima, H. Horikawa, Tetrahedron Lett. 1998, 39, 289; g) B. A. Kulkarni, A. Ganesan, Tetrahedron Lett. 1999, 40, 5633; h) D. Obrecht, C. Abrecht, A. Grieder, J. M. Villalgordo, Helv. Chim. Acta 1997, 80, 65; i) C. GarciaEcheverria, Tetrahedron Lett. 1997, 38, 8933; k) A. Barco, S. Benneti, C. De Risi, P. Machetti, G. P. Pollini, V. Zanirato, Tetrahedron Lett. 1998, 39, 7591; l) C. J. Burns, D. Robert, L. M. Salvino, G. McGeehan, S. M. Condon, R. Morris, M. Morrissette, R. Mathew, S. Darnbrough, K. Neuenschwander, A. Scotese, S. W. Djuric, J. Ullrich, R. Labaudiniere, Angew. Chem., Int. Ed. Engl. 1998, 37, 2848; m) F. Gosselin, M. Di Renzo, T. H. Ellis, W. B. Lubell, J. Org. Chem. 1996, 61, 7980; n) E. Flanigan, G. R. Marshal, Tetrahedron Lett. 1970, 11, 2403; PEG-bound thioethers: o) X. Y. Zhao, K. W. Jung, K. D. Janda, Tetrahedron Lett. 1997, 38, 977; p) X. Y. Zhao, K. D. Janda, Tetrahedron Lett. 1997, 38, 5437. q) K. C. Nicolaou, S. A. Snyder, A. Bigot, J. A. Pfefferkorn, Angew. Chem. 2000, 112, 1135. Angew Chem. Int. Ed. Engl. 2000, 39, 1036. Oxidation to sulfoxides: H. R. Russel, R. W. A. Luke, M. Bradley, Tetrahedron Lett. 2000, 41, 5287. Oxidation to sulfones: D. L. Marshall, I. E. Liener, J. Org. Chem. 1970, 35, 867. Oxidation to sulfoxides: D. M. R. Cody, S. H. DeVitt, J. C. Hodges,

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J. S. Kiely, W. H. Moss, M. R. Pavia, B. D. Roth, M. C. Schroeder, C. J. Stankovic, US patent no. 5324483, 1994. Oxidation to sulfones: H. S. Han, K. D. Janda, Tetrahedron Lett. 1997, 38, 1527. D. B. D. De Bont, W. J. Moree, R. M. J. Liskamp, Bioorg. Med. Chem. 1996, 4, 667. ¨nzer, Tetrahedron C. M. Huwe, H. Ku Lett. 1999, 40, 683. a) H. B. Lee, S. Balasubramanian, J. Org. Chem. 1999, 64, 3454; b) V. Bertini, F. Lucchesini, M. Pocci, A. De Munno, Tetrahedron Lett. 1998, 39, 9263. a) K. C. Nicolaou, N. A. Petasis, Selenium in Natural Product Synthesis. CIS, Philadelphia, PA 1984; b) D. Liotta, Organoselenium Chemistry. Wiley, New York 1986. a) K. C. Nicolaou, J. Pastor, S. Barluenga, N. Winssinger, Chem. Comm. 1998, 1974; b) T. Ruhland, K. Anderson, H. Pedersen, J. Org. Chem. 1998, 63, 9204; c) K. C. Nicolaou, J. A. Pfefferkorn, G.-Q. Cao, Angew. Chem. 2000, 112, 750; d) K. C. Nicolaou, G.-Q. Cao, J. A. Pfefferkorn, Angew. Chem. 2000, 112, 755; e) R. Michels, M. Kato, W. Heitz, Makromol. Chem. 1976, 177, 2311; f ) H. Russell, R. W. A. Luke, M. Bradley, Tetrahedron Lett. 2000, 41, 5287. M. J. Kurth, L. A. Ahlberg Randall, K. Takenouchi, J. Org. Chem. 1996, 61, 8755. F. Eckstein, Oligonucleotides and Analogues; A Practical Approach. Oxford University Press, Oxford 1991. C. A. Metcalf III, C. B. Vu, R. Sundaramoorthi, V. A. Jacobsen, E. A. Laborde, J. Green, Y. Green, K. J. Macek, T. J. Merry, S. G. Pradeepan, M. Uesugi, V. M. Varkhedkar, D. A. Holt, Tetrahedron Lett. 1998, 39, 3435. a) K. K. Ogelvie, M. J. Nemer, Tetrahedron Lett. 1980, 21, 4159; b) M. D. Matteucci, M. H. Caruthers, Tetrahedron Lett. 1980, 21, 3185. a) M. F. Gordeev, D. V. Patel, J. Wu,

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E. M. Gordon, Tetrahedron Lett. 1996, 37, 4643; b) S. Tadasse, A. Bhandari, M. A. Gallop, J. Comb. Chem. 1999, 1, 184. J. P. Mayer, G. S. Lewis, M. J. Curtis, J. W. Zhang, Tetrahedron Lett. 1997, 38, 8445. S. Berteina, A. De Maesmaeker, Tetrahedron Lett. 1998, 39, 5759. D. A. Goff, Tetrahedron Lett. 1999, 40, 8741. a) A. L. Marzinzik, E. R. Felder, J. Org. Chem. 1998, 63, 723; b) B. C. Hamper, K. Z. Gan, T. J. Owen, Tetrahedron Lett. 1999, 40, 4973. a) A. M. Boldi, C. R. Johnson, H. O. Eissa, Tetrahedron Lett. 1999, 40, 619; b) C. O. Ogbu, M. N. Qabar, P. D. Boatman, J. Urban, J. P. Meara, M. D. Ferguson, J. Tulinsky, C. Lum, S. Babu, M. A. Blaskovich, H. Nakanishi, F. Q. Ruan, B. L. Cao, R. Minarik, T. Little, S. Nelson, M. Nguyen, A. Gall, M. Kahn, Bioorg. Med. Chem. Lett. 1998, 8, 2321. a) R. C. Johnson, B. Zhang, PCT WO 0116116 A1; b) J.-F. Cheng, A. M. M. Mjalli, Tetrahedron Lett. 1998, 39, 939; c) B. B. Shankar, D. Y. Yang, S. Girton, A. K. Ganguly, Tetrahedron Lett. 1998, 39, 2447. J. M. Berry, P. W. Howard, D. E. Thurston, Tetrahedron Lett. 2000, 41, 6171. B. Yan, H. Gstach, Tetrahedron Lett. 1996, 37, 8325. D. Tumelty, M. K. Schwarz, K. Cao, M. C. Needels, Tetrahedron Lett. 1999, 40, 6185. M. Z. Hoemann, A. MelikianBadalian, G. Kumaravel, J. R. Hauske, Tetrahedron Lett. 1998, 39, 4749. S. Kobayashi, R. Akiyama, Tetrahedron Lett. 1998, 39, 9211. Y. Watanabe, S. Ishikawa, G. Takao, T. Toru, Tetrahedron Lett. 1999, 40, 3411. W.-R. Li, N.-M. Hsu, H.-H. Chou, S. T. Lin, Y.-S. Lin, Chem. Comm. 2000, 401. a) K. Nakamura, H. Nishiya, S. Nishiyama, Tetrahedron Lett. 2001, 42, 6311; b) S. Yamamura, S. Nishiyama, J. Synth. Org. Chem. Jpn. 1997, 55, 1029.

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15

Reductions in Combinatorial Synthesis Christopher P. Corrette and Conrad W. Hummel 15.1

Introduction

Reductions are one of the most fundamental and widely used transformations in organic chemistry. The reduction of a wide variety of functional groups has been exploited in both solid- and solution-phase combinatorial library synthesis. There are a number of reviews that cover combinatorial chemistry, including reductions [1]. This chapter is divided into two main sections. The first section covers reductions where the substrate is support bound. The second section covers reductions where the substrate remains in solution (resin-bound reagents and catalysts, catchand-release purification, and fluorous chemistry). In general, the combinatorial chemistry of peptides has been omitted in our coverage.

15.2

Solid-phase Reductions 15.2.1

Aldehyde Reductions

The reduction of solid-supported aldehydes to alcohols is a straightforward reaction. As is the case with solution-phase chemistry, the popular borohydride reagent NaBH4 chemoselectively reduces aldehydes in the presence of other reducible functionalities, including esters [2], acetals [3], and nitro groups [4]. Typical conditions use excess NaBH4 in tetrahydrofuran (THF) at ambient temperatures. The use of an alcoholic co-solvent such as ethanol or methanol is often required. In most cases, upon completion of the reaction, a dilute acid wash (AcOH or HCl) is needed to completely remove the unreacted reagent. The progress of these reactions, as well as other carbonyl reductions, can be followed by monitoring the disappearance of the carbonyl band (about 1700 cm1 ) in the FTIR spectrum of the resin [5]. Aldehyde reductions have been performed on a number of resins, including Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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15 Reductions in Combinatorial Synthesis

the Merrifield resin [2a], the Wang resin [2b], and an arylsulfonate ester resin [6]. They have also been used as a key step in the preparation of a variety of other resins. The preparation of the acid-labile resins 2 [7] and 4 [8] each required NaBH4 -mediated aldehyde reductions (Scheme 15.1). The improved synthesis of SASRIN resin, described by Katritzky et al. [9], and the synthesis of a photolabile 6-nitrovanillin-based resin, described by Zehavi and Patchornik [4], also required aldehyde reductions. The use of an acid wash to remove excess reagent is inappropriate in the case of the acid-labile resin preparations. In some of these cases, an alcoholic solvent wash suffices.

Scheme 15.1. Examples of aldehyde reductions in linker preparations.

15.2.2

Ketone Reductions

As with aldehyde reductions, the reagent of choice for simple ketone reductions on a solid support is NaBH4 [10]. However, various other reducing agents have been described in the literature to carry out this transformation. These include the borohydride reducing agents diisoamylborane [11] and LiBH4 [12]. The latter was used in the preparation of an acid-labile xanthone-based resin described by Sieber. NaBH4 in combination with diethyl methoxyborane has also been used to reduce the diketone 5 [13] to diol 6 (Scheme 15.2). Following boron oxidation and removal, this diol was cyclized with dilute acid to d-lactone 7.

Scheme 15.2. Diketone reduction on solid phase.

15.2 Solid-phase Reductions

Asymmetric reductions of ketones bound to resin are also possible. The silane reagents diphenylsilane and a-naphthylphenylsilane in the presence of the catalyst prepared from m-dichlorotetraethylenedirhodium(I) and (þ) or ()-DIOP asymmetrically reduced a polymer-bound ketone [14]. The resulting secondary alcohol, of unknown optical purity, was part of a resin-bound asymmetric rhodium-containing hydrogenation catalyst that was used to reduce the double bond of a-N-acylaminoacrylic acids. However, the optical purities (@ 70% ee) of the resulting amino acids were comparable to those obtained from the analogous homogeneous catalyst. Diastereoselective ketone reductions can be achieved by a variety of methods. An example of a diastereoselective ketone reduction is found in Paterson’s library synthesis of polyketides [15]. To increase the degree of diversity in this library, the reduction of b-hydroxy ketone intermediates employed two different sets of conditions, allowing access to both syn- and anti-1,3-diols. The former was obtained with 95% diastereoselectivity (ds). Treatment of the b-hydroxy ketone with (c-Hex)2 BCl and triethylamine delivered a boron aldolate, which could be reduced with LiBH4 affording the syn derivative with 95% ds. The 1,3-anti-diol was generated with 97% ds via the anti-selective Evans–Tishchenko reduction protocol, using stoichiometric quantities of SmI2 and propanal, followed by a LiBH4 -mediated ester cleavage. Paterson also points out that attempts to effect the anti reduction with Me4 NBH(OAc)3 resulted in lower diastereoselectivity (75%). Other examples of both chelation- and nonchelation-controlled diastereoselective ketone reductions were described by Ellman in his synthesis of an aspartyl protease inhibitor library [16]. To achieve chelation-controlled reduction, the a,bdialkoxy ketone 8 was treated with Zn(BH4 )2 in diethyl ether and THF at 20 C (Scheme 15.3). Following deprotection and resin cleavage, the triol product was peracetylated. Analysis by gas chromatography (GC) showed that the syn product had been delivered in 60–80% ds. Following separation of this mixture by preparative high-performance liquid chromatography (HPLC), the major diastereomer was found to have a 90% ee, which demonstrated that only a small degree of racemization had occurred during an eight-step sequence that included reduction. The anti compound was obtained with 74% ds, utilizing the nonchelation-controlled conditions of l-selectride in THF at 75 C. In another example of substratecontrolled reduction, Schlessinger was able to reduce ketone 10 stereoselectively with NaBH4 (Scheme 15.4) [17].

Scheme 15.3. Diastereoselective reduction of an a,b-dialkoxy ketone.

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Scheme 15.4. Diastereoselective reduction of an oxabicyclo ketone.

15.2.3

Ester Reductions

A number of hydride-based reagents can reduce solid-supported esters. For all practical purposes, alcohols have been the usual targets of these reductions. Despite the ability to stop the reduction at the aldehyde oxidation state with the use of stoichiometric reagents such as DIBAL in solution, this transformation has not been reported for solid-phase-supported esters. The difficulty of this transformation is not surprising, as chemists usually use excess reagents to drive reactions to completion on solid phase and the use of stoichiometric reagents is very difficult to achieve. The transformation from a carboxylic acid oxidation state to an aldehyde has been successfully addressed on solid phase by utilizing Weinreb amides instead of esters. These amides are known reliably to produce aldehydes upon reduction (see Section 15.2.6). Of the reducing reagents used to carry out ester reductions to the corresponding alcohol, DIBAL [2b, 18] and LAH [4, 13, 19] have been used most frequently. An example of a solid-phase DIBAL reduction is shown in Scheme 15.5, where the resin-bound a,b-unsaturated tert-butyl ester is reduced to the allylic alcohol 13. DIBAL reductions have been run in various solvents, including THF, Et2 O, CH2 Cl2 , and toluene, from 78 C to ambient temperature, while LAH reductions typically utilize THF or ether as the solvent at ambient to reflux reaction temperatures. The quench of the excess reagent is usually carried out using ethyl acetate/ water or dilute aqueous HCl. Merrifield, Wang, and trityl resins have all proven to be stable to this strong reducing agent. Examples of LAH reductions are shown in Scheme 15.6. Example 15.6A shows that an amide is concomitantly reduced with an ester of a resin-bound amino acid to give the amino alcohol 15 [13]. LiBH4 has also been used to reduce resin-bound esters, as shown in Scheme 15.7 [20]. A

Scheme 15.5. DIBAL-mediated ester reduction of resin-bound substrate.


Scheme 15.6. LAH reductions on solid support.

Scheme 15.7. Ester reduction on solid phase with LiBH4 .

NaBH4 -mediated ester reduction in ethanol and water has also been demonstrated; however, this protocol may not be general as esters are typically inert to this reagent. Esters are common points of attachment for substrates to solid supports. Cleavage can be effected by saponification or by reduction, affording acids or alcohols, respectively, as the final products. For reductions, both DIBAL [21] and LAH [22] have been described in the literature. Tietze and coworkers have demonstrated the use of DIBAL in the two examples shown in Scheme 15.8, where concomitant reduction of a diester [21c] and a b-ketoester [21d] have led to diols 21 and 23, respectively. Kurth and coworkers have also used this reagent to reduce the resinbound b-hydroxy ester 24, thus providing diol 25 [21a], which is a representative of a 27-compound library (Scheme 15.9). Kuster and Scheeren have reductively cleaved

Scheme 15.8. Reductive cleavage of ester-linked substrates.

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Scheme 15.9. Kurth’s DIBAL-mediated reduction of a b-hydroxy ester.

the a-amino ester 26 from Wang resin to obtain the primary amino alcohol 27 using LAH (Scheme 15.10) [22].

Scheme 15.10. Reductive cleavage of an amino ester with LAH.

Resin-bound acid chlorides have also been reduced to alcohols. Goldwasser and Leznoff have shown that diacid chlorides can be loaded onto 2% crosslinked polystyrene resin to give ester-linked acid chlorides. The resins can then be treated with amines or hydride reducing agents to give amides or alcohols, respectively [23]. In the latter case, the most successful reducing reagents were NaBH4 and NaCNBH3 (4 equiv., in THF at ambient temperature), which delivered the desired products in 67% and 65% yields, respectively. However, following basic cleavage and esterification, it was found that these products were contaminated with the dimethylester byproduct arising from incomplete reduction.

15.2.4

Mixed Anhydride Reductions

A mild method employed to obtain alcohols from carboxylic acids consists of using a mixed anhydride intermediate. In their search for an optimized phosphomannose isomerase inhibitor, researchers at Affymax coupled ten different symmetrical anhydrides and diacids to an immobilized 2-aminoindane-2-carboxamide to deliver acids of the general structure 29 (Scheme 15.11). This set of acids was treated with isobutylchloroformate and triethylamine in THF, followed by NaBH4 in water, to deliver the primary alcohols 30 [24]. Further diversification led to a library of 600 analogs (60 pools of ten compounds). Chemists at Signal have also utilized this reduction protocol to deliver pyrimidine 33 following Mitsunobu alkylation and subsequent cleavage (Scheme 15.12) [25].


Scheme 15.11. Solid-phase reduction of acid via mixed anhydride.

Scheme 15.12. Pyrimidine synthesis using mixed anhydride reduction.

15.2.5

Thioester Reductions

In 1996, Kobayashi reported the first synthesis of polymer-supported silyl enol ethers (thioketene silyl acetals) [26]. These reactive intermediates were converted to small libraries of b-amino alcohols [26a,c] and 1,3-diols [26b] according to Scheme 15.13. Treatment of compound 34 with either an imine or an aldehyde in the presence of catalytic amounts of Sc(OTf )3 gave the resin-bound b-amino thioester 35 or b-hydroxy thioester 37, respectively. Following reductive cleavage of the thioester with LiBH4 , the desired b-amino alcohols and 1,3-diols were obtained in good yields. However, a chromatography step was required to obtain pure compounds in both cases. The authors also demonstrated that the resin-bound b-

Scheme 15.13. Reductive cleavage of thioesters.

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hydroxy thioester could be reduced to the b-hydroxy aldehyde 38 with DIBAL in CH2 Cl2 , at 78 C [26b]. Thioester reduction on solid phase has also been successfully carried out using LAH in THF. Scheme 15.14 shows the reduction of dithioester 40 to the propane1,3-dithiol derivative 41, which was used to produce ketones via dithiane chemistry [27].

Scheme 15.14. LAH reduction of 1,3-dithioester.

In their Evans aldol approach to polyketide libraries, Reggelin et al. have also utilized thioesters and their reduction as part of the synthetic strategy [28]. They first converted resin-bound oxazolidinones 42 to thioesters 43 via a three-step protocol involving hydrolysis, thioester formation, and alcohol protection. They next reduced the thioester with LiBH4 and reoxidized the resulting alcohol to aldehyde 45 with Dess–Martin reagent (Scheme 15.15). This aldehyde could then be used in a subsequent Evans aldol reaction. Unlike the examples given above, this route was designed to leave the substrate on the resin following thioester reduction.

Scheme 15.15. Resin-bound thioester reduction with LiBH4 .

15.2.6

Weinreb Amide Reductions

As mentioned in Section 15.2.3, the partial reduction of an ester to an aldehyde is a difficult transformation on solid phase, since excess reagents are typically used to drive reactions to completion. If reactions fail to go to completion and purifications are required, then one of the major advantages of solid-phase synthesis is forfeited. In the case of ester reductions, alcohols are normally obtained owing to the use


of excess reagents. Reggelin et al. solved this problem in the solid phase by making use of Weinreb amides, which deliver aldehydes upon reduction in standard solution-phase chemistry. In their approach to polyketide libraries, they demonstrated that the imides 46 can be converted to Weinreb amides under standard conditions (Scheme 15.16). Following alcohol protection, these amides can be selectively reduced with DIBAL to deliver the support-bound aldehydes 48 [29]. In 1998, Reggelin et al. replaced this sequence with another (see Section 15.2.5) that goes through a thioester instead of the Weinreb amide. However, this change was due to difficulties arising from steps other than reduction [28].

Scheme 15.16.

Weinreb amide approach to support-bound aldehyde.

Two other groups have developed Weinreb amide-based linkers that give aldehydes in solution upon reductive cleavage [30]. Salvino et al. modified a resin that had been used for the synthesis of hydroxamic acids to give an O-linked Weinreb amide linker [30d], whereas Martinez and coworkers synthesized an N-linked version [30a]. The synthesis of C-terminal amino acid aldehydes has been successfully demonstrated by both groups, providing clean products in low to moderate yields using LAH as the reductant. Schemes 15.17 and 15.18 show the synthesis of BocPhe-H using both linkers. In the case of the Martinez linker, this support has also been applied to the synthesis of C-terminal peptide aldehydes [30b] and side-chain aldehydes [30c]. The peptide analogs were produced without noticeable epimeri-

Scheme 15.17. Synthesis of C-terminal amino acid aldehyde

using an O-linked Weinreb amide resin.

Scheme 15.18. Synthesis of C-terminal amino acid aldehyde

using an N-linked Weinreb amide resin.

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zation and the side-chains were reduced in high yield with LiAl(OtBu)3 H. The O-linked resin was also employed to deliver ethyl ketones by treating intermediate Weinreb amides with ethyl Grignard [30d]. Dinh and Armstrong also introduced an N-linked Weinreb amide resin, which delivered ketones in low to moderate yields upon treatment with Grignard reagents [31]. However, attempts to reductively cleave these amides with LAH produced aldehydes in very low yields (< 20%). 15.2.7

Sulfur Reductions

There have been a number of different solid-phase applications involving disulfide and sulfone reductions. These include examples of simple functional group manipulation, linker preparation, and ‘‘traceless’’ linker cleavage. Hummel and Hindsgaul have shown that thio-oligosaccharides can be synthesized from polymersupported sugar derivatives functionalized with an anomeric disulfide (Scheme 15.19) [32]. Reduction of the disulfide with dithiothreitol (DTT) and subsequent sodium thiolate formation leads to a thio-sugar capable of reacting with triflated glycosides. Following deprotection and resin cleavage, thio-oligosaccharides 57 were formed. In another example, Ellman and coworkers synthesized a series of cyclic 9- and 10-member thioethers, utilizing a PBu3 -mediated reduction of a tertbutyl-protected disulfide as a key step [33]. They later modified this approach by linking the substrate through a disulfide bond, as shown in Scheme 15.20 [34]. In this case, the disulfide bond was cleaved with tris-(2-carboxyethyl)phosphine (TCEP), which gave the free thiol in solution along with excess TCEP and the phosphine oxide byproduct. The contaminants were of little consequence, as supportbound N,N,N 0 ,N 0 -tetramethylguanidine (TMG) was not only able to induce cyclization to the thioether, but was also able to scavenge these byproducts. In a recent

Scheme 15.19.

Anomeric disulfide reduction in the preparation of thio-oligosaccharides.

Scheme 15.20.

TCEP-mediated disulfide reduction leading to a b-turn mimetic library.


application, Lam and coworkers used NaBH4 to reduce a disulfide bond in their solid-phase approach to 1,4-benzothiazin-3(4H)-one derivatives [35]. In an example of linker preparation, Sucholeiki and coworkers have utilized a b-mercaptoethanol-mediated reduction to synthesize the photolabile resin 62 (Scheme 15.21) [36]. Zhao and Janda also utilized a disulfide reduction in the synthesis of the thiol linker shown in Scheme 15.22 [37]. Treatment of a mixture of disulfides generated in a previous alkylation step with DTT in water led to the desired resin 64 in excellent yield. Following construction of the targeted substrate on solid phase, oxidation of the thiol with KHSO5 in water gave a sulfone that was cleaved with 5% Na/Hg in methanol and DMF to yield 66 [38]. The solvent mixture of methanol and DMF (1:8) was crucial for this successful reductive cleavage [39]. The use of additional methanol or THF as a replacement for DMF gave lower yields and required the use of larger quantities of Na/Hg. Zhao and Janda have also pointed out that, for reactions using soluble polyethylene glycol (PEG)-based polymers, isopropanol is an excellent choice for polymer precipitation, and leading to products of higher purity [38].

Scheme 15.21. Linker preparation utilizing b-mercaptoethanol-mediated disulfide reduction.

Scheme 15.22. Preparation and use of Janda’s soluble polymeric thiol linker.

15.2.8

Selenium Reductions

The ease with which selenium can be reduced and oxidized is well understood, however the toxicity and odor of selenium compounds has limited their use. Nicolaou et al. recognized that by attaching selenium to a solid support these draw-

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backs would be mitigated, and the positive attributes of selenium reagents and substrates could then be exploited [40]. They have shown that supported selenium reagents make excellent ‘‘traceless’’ linkers because loading of substrates and subsequent cleavage to simple alkyl groups is carried out with equal ease. As shown in Scheme 15.23, reduction of the selenium bromide 67 with LiBH4 delivers lithium selenide 68, which then reacts with substrates containing an alkyl halide. Following substrate modification (not shown), the carbon–selenium bond can be reduced with Bu3 SnH to give an alkyl group with no vestige of the linker remaining. Alternatively, Nicolaou et al. have shown that these selenides can be treated with oxidizing reagents to give an alkene.

Scheme 15.23. Preparation and use of Nicolaou’s selenium linker.

In another example of a selenium-based traceless linking strategy, Ruhland et al. þ prepared and reacted PS-SeB(OEt) 3 Na with hydroxy-containing alkyl halides. The resulting intermediates were then alkylated with phenols under Mitsunobu conditions and cleaved with Bu3 SnH to give alkylated phenols [41]. In an example that does not involve selenium linkage to a solid support, Pearson and Clark used a vinyl selenium reagent as an anionophile in a [3 þ 2] cycloaddition with a solid-supported 2-azaallyl anion (Scheme 15.24) [42]. Following condensation, the phenylselenide group was then reduced with Bu3 SnH giving a 1,2-disubstituted pyrrolidine.

Scheme 15.24. Use of phenylvinylselenium as an ‘‘anionophile’’ and selenium removal with n-Bu3 SnH.


15.2.9

Quinone Reductions

The redox chemistry of quinones has been used to prepare solid-phase linkers for peptide synthesis [43]. The quinone can be easily reduced with NaBH4 or Na2 S2 O3 to provide the corresponding dihydroquinone. Treatment of the reduced product with TBAF results in phenoxide displacement at the C-terminus of the attached peptide, thus releasing the molecule from the resin. In initial efforts, the phenolic oxygen forms a lactone and releases the peptide functionalized at the C-terminus (Scheme 15.25A) [43a]. An improved approach is shown in Scheme 15.25B, in which the peptide is provided without functionalization at the C-terminus via SN 2 displacement by the phenol [43b].

Scheme 15.25. Peptide release via quinone reduction and

phenol or phenoxide-mediated cleavage.

15.2.10

Amide Reductions

The reduction of amides to amines is a useful technique for the generation of diversity in combinatorial synthesis. Either separately, or in combination with the complementary reductive amination techniques of aldehydes described in Section 15.2.12.4, an enormous number of amines and polyamines can be accessed from the diverse set of amides and polyamides that can be generated via a combinatorial approach. Further utility arises from the ability to increase diversity by modification of the product amines through any number of common approaches. The reducing agents most routinely employed are BH3 THF [44], BH3 pyr [45], or BH3 SMe 2 [46], although Red-Al [47] and LiBH4 [48] have also been used. Borane reductions are easily applied to resin-bound amides since any excess reducing agent and resulting inorganic byproducts are easily washed away from the polymer support. An important factor for such reductions is the cleavage of the borane–

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amine adducts. Acidic cleavage is incompatible with a variety of polymer supports, therefore heating with piperidine has been used to remove boron from the desired products [44b]. An iodine-mediated cleavage of the borane–amine adduct has also been reported (Scheme 15.26) [45b, 49]. This method is compatible with acidsensitive solid supports such as Wang or Rink resins.

Scheme 15.26. Iodine work-up for borane reduction of amides on solid phase.

Several articles have been published on the use of amide reductions for the generation of combinatorial libraries [44, 45a,c]. Dipeptides are typically constructed on resin, acylated at the N-terminus, and exhaustively reduced with BH3 THF or BH3 pyr to give polyamines. This methodology has been used to generate hydantoins and cyclic ureas, thus generating ‘‘libraries from libraries’’ [44a]. This sequence has also been used to construct a 100,000-compound library of polyamines and bicyclic guanidines (Scheme 15.27) [44b].

Scheme 15.27. Synthesis of polyamines and bicyclic

guanadines via borane reduction of dipeptides.

15.2.11

Carbamate Reductions

There are several examples of carbamate reductions wherein the carbonyl is exhaustively reduced to an N-methylamine. All of these examples use aluminum hy-


dride reagents. For example, Liu and Ellman used Red-Al-mediated carbamate reduction to prepare chiral N-methyl-2-pyrrolidinemethanol ligands [47]. Carbamate linkers have been used as latent methylamines in solid-phase synthesis. Formation of the carbamate on resin followed by LAH reduction generated the desired primary and secondary amines [50]. A carbamate linker has also been used to generate substituted N-methylpyrrolidines via reductive cleavage with LAH (Scheme 15.28) [51]. Xiao et al. used Na2 S2 O4 to reduce an O-piperidinyl carbamate linker, thus revealing a secondary amine that underwent a cyclizative cleavage with a pendant ester to release a desired alcohol [52].

Scheme 15.28. Synthesis of subsituted N-methylpyrrolidines via reductive cleavage.

15.2.12

Reductive Amination 15.2.12.1 General Considerations

Reductive amination is one of the most widely applied techniques in combinatorial chemistry and is an excellent method for generating diversity (Scheme 15.29) [53]. Amines, aldehydes, and ketones as inputs for reductive amination are plentiful, commercially available, and typically inexpensive. The technique has been well established in routine organic chemistry [54], and has therefore provided a strong foundation for explorations in both solid- and solution-phase library synthesis. There is a wide variety of protocols for both imine formation and reduction, thus providing a large set of choices for reaction conditions and reagents that can be screened for a given set of inputs (e.g. primary vs. secondary amines).

Scheme 15.29. Diversity from secondary amines prepared by reductive amination.

Generally carbonyl reactivity follows the order aldehyde > cyclic ketone g acyclic ketone. Steric considerations are important in reductive amination reactions, thus hindered inputs may require forcing conditions for adequate results. For example, hindered amino acid esters such as valine [53a] have been reported to be poor

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inputs for reductive amination reactions as have methyl amines [55]. Secondary amines can require repetitive cycles [56] of reductive amination, or very long reaction times [56, 57] to obtain good conversion. Electronic factors can also be important in reductive amination reactions, as evidenced by the higher reactivity of alkyl amines versus anilines. The more reactive nature of alkyl amines and aldehydes can render them prone to bis-alkylation [44b, 58]. Reductive aminations can also be sensitive to solvent choice. Brown and coworkers found that the use of aqueous THF was crucial for the successful reductive amination of a Rink amide resin [59]. Racemization of amino acid esters as amine inputs during reductive amination is another concern for library synthesis [60]. Racemization is expected to occur by equilibration during imine formation [61]. In order to suppress racemization, imines can be formed in the presence of a reducing agent, thus avoiding equilibration [58a, 61]. There are several methods for monitoring the progress of solid-phase reductive amination. Qualitative nitrogen analyses such as a (Kaiser) ninhydrin test can quickly and easily assess reaction progress [62]. Infrared spectroscopy can also be used to detect consumption of the carbonyl and/or imine components in a reductive amination process by their characteristic CbO (1740–1720 cm1 ) and CbN (@1650 cm1 ) stretches [63]. 15.2.12.2 Imine Formation

Imines are formed by condensation of amines and aldehydes or ketones with loss of water. Typically, aliphatic aldehyde-derived imines tautomerize to their corresponding enamines. The classic methods for water removal such as azeotrope formation or trapping with molecular sieves are neither practical nor economical for application in a solid-phase library format [64]. A number of different approaches have therefore been developed for imine formation in combinatorial chemistry to supplant these techniques. The most widely used reagent for imine formation in solid-phase organic synthesis has been trimethylorthoformate (TMOF) [53b, 65]. TMOF is used as the reaction solvent, or in conjunction with other solvents as solubility dictates, and reacts with water as it is formed, thus driving the equilibrium toward imine formation. TMOF is compatible with most resins and common reducing agents such as NaBH3 CN, NaBH(OAc)3 , or BH3 pyridine (BAP). The imine equilibrium is often shifted by using an excess of the input not loaded on resin (anywhere from 2 to 20 equiv.). Imines can be formed in situ in the presence of a reducing agent with or without added acids. The typical additive for imine formation is 1–10% acetic acid (by volume) but p-TsOH [66] and PPTS [67] have also been used to catalyze the condensation. Anhydrous inorganic salts such as Na2 SO4 (with ultrasound) have been used to drive formation of imines in solidphase applications [68]. Titanium reagents such as Ti(OiPr)4 have been used to facilitate reductive aminations via carbonyl activation and amine addition [69]. Use of Ti(OiPr)4 made possible the reductive amination of a relatively unreactive ketone 97, as shown in Scheme 15.30 [69a].


Scheme 15.30. Reductive amination with Ti(OiPR)4 .

15.2.12.3 Reducing Agents for Reductive Amination

There is a myriad of reducing agents that can be used for reductive aminations in solid-phase applications. The most common are NaBH3 CN (Borch reduction) [53, 56, 70], NaBH(OAc)3 [63a, 21b, 71], and BAP [57, 72], but Me4 NBH(OAc)3 [73], NaBH4 [74], and LiBH4 [75] have also been used. Me4 NBH(OAc)3 and NaBH3 CN have been used sequentially in a reductive amination protocol in which the individual reductants provided incomplete or impure reaction products respectively [76]. The borohydride reagents listed above typically reduce both imines and tautomeric enamines (formed from aliphatic aldehydes) and can be used in large excess as they are easily washed from the resin [77]. NaBH(OAc)3 has become a favored reducing agent as it is not toxic and displays excellent chemoselectivity, but NaBH3 CN is still frequently used. Borohydride reagents such as NaBH4 and LiBH4 can be problematic as they may reduce starting aldehydes and ketones to alcohols. The usual solution in such cases is to preform the imine before application of the reductant in a two-step process. 15.2.12.4 Reductive Aminations as the Entry Point for Library Preparation

Frequently, reductive amination is used to load a variety of inputs onto a resin, thus introducing diversity in the first step of a library synthesis. A typical approach is to react primary amines (e.g. amino acid esters) with electron-rich, resin-bound aldehydes (Scheme 15.31) [63a, 78]. The acetophenone-containing resin 103 has been loaded with amines via reductive amination for entry into parallel amide and sulfonamide preparations (Scheme 15.32) [79].

Scheme 15.31. Loading of amino acid esters onto AMEBA-type resin.

Scheme 15.32. Reductive amination on an acetophenone-containing resin.

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The combination of primary amines and electron-rich, resin-bound aldehydes usually works well under a number of conditions, but hindered aldehydes have occasionally been problematic. For example, a 3,5-dimethoxy-4-benzaldehydecontaining resin was shown to be less reactive toward reductive amination than the monomethoxy analog [80]. Schwarz et al. also observed that di-ortho (e.g. dimethyl)-substituted aldehydes can be poor substrates for reductive amination [81]. However, Ellman and coworkers reported reductive amination conditions that work well with a polymer-supported 3,5-dimethoxy-4-benzaldehyde (Scheme 15.33) [78a].

Scheme 15.33. Ellman’s 1,4-benzodiazepine-2,5-dione synthesis.

Polymer-supported amines have also been used for reductive aminations with aldehydes. There are several examples where Rink amide resin has been reductively alkylated [59b, 62b, 75a]. Katritzky et al. formed a variety of imines (both electron-rich and -poor aromatic aldehydes) with TMOF and reduced them with LiBH4 [75a]. Rivero and coworkers also used a two-step reductive amination procedure to prepare a library of 500 macrocycles (Scheme 15.34) [62b]. Benzyloxy-

Scheme 15.34. Macrocycle synthesis incorporating reductive amination as a diversity element.

15.2 Solid-phase Reductions Tab. 15.1. Recent libraries using reductive amination (since January 2000).

Library

Reactantsa

Reducing agent

Method b

Reference

Secondary amides Macrolides

Aniline/aldehydes Aldehyde/primary amines Secondary amines/aldehydes Hydroxylamine/aldehydes Aldehydes/amino alcohol

NaBH4 NaBH3 CN

B B

74e 58b

BAP NaBH(OAc)3

B A

67 78f

NaBH(OAc)3 NaBH(OAc)3 NaBH3 CN

A B A

83 62b, 85 84

NaBH(OAc)3 NaBH3 CN NaBH3 CN LiBH4

A A A B

86 87 88 75b

Aldehydes/primary amines Primary amine/ cinnamaldehydes Aldehydes/primary amines Hydroxylamine/aldehydes

BAP NaBH(OAc)3

B A

72c 89

BAP NaBH(OAc)3

B A

72b 90

Primary amines/aldehydes

NaBH(OAc)3

B

91

Amino acids/b-glycoside aldehydes Amino acids/aldehydes

NaBH3 CN

A

92

NaBH(OAc)3

B

93

Tertiary amines Tetrahydroquinoxalines Peptide aldehydes Macrocycles 3,4-Dihydro-1,4benzothiazines Quinoxalinones Pyrrolidines 2-Carboxyindoles b-Ketoamides/ imidazoles Amines/amides Tricyclics Piperidines Tertiary methylamines Phenolic amino acids Neoglycopeptides Lysine/glutamic acid derivatives a b

Aldehydes/primary amines Primary amine/aldehydes Anilines/ketones (intramolecular) Aldehydes/amino acid Primary amine/aldehydes 5-Aminoindole/aldehydes Aldehydes/primary amines

First entry is resin-bound component. A, one-step procedure, in situ imine formation/reduction; B, twostep procedure, imine formation then reduction.

aniline and p-benzyloxybenzylamine (BOBA) resins have also been reductively alkylated with aldehydes and NaBH4 using two-step procedures [74c,e]. 15.2.12.5 Recent Examples of Reductive Amination on Resin

There is a large number of libraries that have incorporated a reductive amination step. This fact highlights the power and reliability of reductive amination for library synthesis. Recently, a number of libraries have been prepared using reductive amination chemistry. These library syntheses are listed in Table 15.1. Specific examples include a diketopiperizine library effort at Affymax that relies on reductive amination for a key diversity step (Scheme 15.35) [65a, 82]. Groth and Meldal reported a combinatorial approach to N-terminal peptide aldehydes and diketopiperazines using reductive amination [83]. They found that NaBH(OAc)3 in dimethyl sulfoxide (DMSO)/CH2 Cl2 /AcOH (50:50:1) was optimal after an exten-

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Scheme 15.35.

Affymax’s solid-phase approach to diketopiperazines.

sive study of conditions, thus emphasizing the importance of varying reaction conditions for successful reductive aminations. Barany and coworkers prepared a set of 3,4-dihydro-1,4-benzothiazines using a one-pot alkylation–intramolecular reductive amination to prepare the thiazine core [84]. A group at Abbott has prepared an antibiotic screening library of 70,000 macrolides [58b]. The synthesis involved three reductive amination steps (aldehydes with a primary amine and two secondary amines) to introduce diversity and began with a preconstructed macrolide core. 15.2.13

Azide Reductions 15.2.13.1 General Considerations

Reductions of aromatic azides provide anilines that are handles for diversification and may be incorporated into benzo-fused heterocycles. Tin (SnCl2 ), phosphine, or sulfur reagents are commonly used to carry out this transformation. Under some tin reduction conditions, side-reactions, such as azide displacement or N-acetylation, can be problematic. In these cases, phosphine- or sulfur-mediated azide reductions can be used as replacements. Azide Reductions in Glycopeptide Preparations Solid-phase azide reductions are heavily used in glycopeptide preparations. The azide is frequently used as a point of attachment for the peptide, but may also be a handle for diversification. The most commonly used reductants are sulfur based, such as DTT or 1,3-propanedithiol, although phosphines have been used in several examples. Peters et al. [94] in an early example, and later Rademann and Schmidt [95], obtained N-acetates from azide reductions using thioacetic acid and pyridine in solidphase glycopeptide preparations. Danishefsky and coworkers used both thiophenol and 1,3-propanedithiol with Hu¨nigs base in THF to effect azide reduction in solidsupported trisaccharide- and disaccharide-containing glycopeptides, thus avoiding N-acetate formation [96]. Glycopeptides have also been prepared on solid support via azide reduction using DTT and 1,8-diazabicyclo[5.4.0]undecene-7 (DBU) in 15.2.13.2


DMF [97]. A modified Staudinger reaction has been applied to the preparation of amides using a solid-supported glycoazide in a one-pot procedure (Scheme 15.36) [98].

Scheme 15.36. Modified Staudinger reaction for the preparation of glycopeptides.

15.2.13.3 Small Molecule Libraries Incorporating Azide Reduction

A variety of small molecule libraries has been synthesized that incorporate azide reductions using primarily SnCl2 and triarylphosphines. Ellman’s group has prepared several small molecule arrays by generating diversity at the amine prepared by a tin-mediated azide reduction (Scheme 15.37) [16, 99]. Kim and coworkers have also prepared a group of oligoureas using a tin-mediated azide reduction [100].

Scheme 15.37. Examples of small-molecule synthesis using azide reductions.

Another example from Chiron is the preparation of a small group of 1,4-benzodiazepine-2,5-diones by a PBu3 -mediated azide reduction [101]. Reaction of the resulting aniline with a pendant ester formed the diazepine heterocycle. Trifluoroacetic acid cleavage provided 21 benzodiazepines in good yields (Scheme 15.38). Kahne and coworkers also used aqueous PMe3 to reduce azides on a TentaGelsupported carbohydrate [102]. It has been reported by Zhou and coworkers that the reduction of a primary azide with SnCl2 and thiophenol provided significant amounts of an azide displacement product (resulting from attack by thiophenol) [103]. To circumvent this

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Scheme 15.38. Synthesis of 1,4-benzodiazepine-2,5-diones using an azide reduction.

problem, aqueous PPh3 provided the amine in good yield without any azide displacement and allowed the synthesis of an array of hydroxybisamides. Nicolaou et al. also used an aqueous PPh3 -mediated azide reduction methodology to provide an amine for diversification in the preparation of a library of 50 sarcodictyins [104]. 15.2.13.4 Recent Examples of Azide Reduction on Resin

A variety of recent libraries has incorporated azide reductions in their synthetic approach and are summarized in Table 15.2. For example, a library of phenolic steroids has been prepared by Poirier and coworkers using both tin- and phosphinemediated azide reductions on various solid supports [105]. The phosphine reduction was employed when an o-nitrobenzyl ether linkage was used to avoid reduction of the nitro group on the linker [105b]. A library of substituted oxazoles has been prepared via derivatization of an amine generated from an azide reduction with DTT and Hu¨nigs base (Scheme 15.39) [106]. A small library of 1,3-oxazolidines has been prepared using a tin-mediated azide reduction on solid support [107]. An azide on solid support has also been reduced with TMSI. This acidic protocol was used to avoid base-induced formation of a lactam side-product when using DTT/ DBU [108]. A library of 1300 disaccharides has been prepared on solid phase via azide reduction with aqueous PMe3 followed by amine derivatization with isocyanates and Tab. 15.2. Recent libraries utilizing azide reduction (since January 1999).

Library

Reducing agent

Reference

Glycopeptides

PhSH or 1,3-propanedithiol, DIEA DTT, DBU PBu3 DTT, DBU or TMSI Aqueous PPh3 Aqueous PPh3 SnCl2 , PhSH, TEA or aqueous PPh3 DTT, DIEA SnCl2 , PhSH, TEA Aqueous PMe3 SnCl2 , PhSH, TEA Aqueous PMe3

96b 97b 98 108 103 105a 105b 106 107 109 78e 110

Hydroxybisamides Estradiols Oxazoles 1,3-Oxazolidines Disaccharides 1,3-Bis(acylamino)-2-butanones Phenylglycinnamides


Scheme 15.39. Synthesis of oxazoles incorporating an azide reduction.

acids (Scheme 15.40) [109]. An array of 18 1,3-bis-(acylamino)-2-butanones has been prepared in which diversity was generated at an amine prepared by a SnCl2 azide reduction [78e]. Notably, a dimethyl ketal survived the conditions of this tinmediated reduction. A library of phenylglycinnamides has been prepared by generating a galactosylamine by means of an azide reduction with 1,3-propanedithiol [110]. The galactosylamine was used in a series of Ugi reactions and cleaved from the resin to generate eight different phenylglycinnamides.

Scheme 15.40. Disaccharide derivatization at an amine generated by an azide reduction.

15.2.14

Nitro Group Reductions 15.2.14.1 General Considerations

The reduction of aromatic nitro groups to anilines is an often-used transformation in combinatorial chemistry. The aromatic nitro group serves two important functions: it facilitates SN Ar reactions and provides an amine for further manipulation following reduction. A frequent use of the resulting aniline in library synthesis has been in the preparation of various benzo-fused heterocyclic compounds (Scheme 15.41) [111]. There are a variety of protocols that have been developed for the reduction of nitro groups, and each offers different advantages and disadvantages. Some experimentation may be required to find suitable conditions for the system under study since the most frequently used reducing reagent (SnCl2 ), while quite reliable, does not always provide consistent results [112]. 15.2.14.2 Tin-mediated Nitro Reductions

The reduction of aromatic nitro groups is often carried out using a tin reagent (usually an aqueous solution of SnCl2 in DMF) [113, 114]. Acidic conditions typi-

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Scheme 15.41. Synthesis of a benzimidazole library via tin-mediated nitro reduction.

cally accompany nitro reduction with SnCl2 , which presents a potential problem with acid-sensitive polymer supports. Addition of a small amount of buffer such as sodium acetate often remedies this situation [35, 115]. DMF is the solvent of choice for tin reductions, but N-formylation of o-diaminobenzenes generated from 2-amino-substituted nitrobenzenes and subsequent cyclization to benzimidazoles has been observed as a side-reaction [116]. This result can be avoided by employing other solvents such as NMP or N-methylmorpholine (NMM). Tin reductions often require heating and can benefit from exclusion of oxygen [117]. Tin reductions have been used in a number of library syntheses. In an early example, the tin-mediated reduction of solid-supported substituted nitrobenzenes led to anilines that were derivatized to provide a small library of phenols following cleavage (Scheme 15.42) [118]. The synthesis of a library of 3,4,5-substituted 1,5benzodiazepin-2-ones began with a tin-mediated nitro reduction on polymer support (Scheme 15.43) [119]. Sequential hydrolysis and intramolecular amide coupling provided the benzodiazepine core.

Scheme 15.42. Meyer’s approach to a phenolic library via a tin-mediated nitro reduction.

Scheme 15.43. Synthesis of a substituted benzodiazepin-2-one

library via a tin-mediated nitro reduction.


15.2.14.3 Nitro Reductions with Alternative Reagents

Tin-mediated reductions of nitro groups can occasionally give inconsistent results or suffer from incomplete reactions [112, 120], a serious problem in library synthesis where reliability and purity are essential. Furthermore, tin impurities are known to be problematic in many drug-screening assays, especially cellular assays [121]. Therefore, a variety of reagents and conditions has been developed as alternatives to tin-mediated nitro reductions. Sodium borohydride with Cu(acac)2 was used in the preparation of a benzimidazole library in which SnCl2 gave inconsistent results (Scheme 15.44) [112]. A comparative study of nitro reductions with Na2 S2 O4 versus SnCl2 on a set of 74 compounds has been performed [121]. The results show that Na2 S2 O4 is as effective as SnCl2 in nitro reductions, although resins compatible with aqueous solutions must be used with Na2 S2 O4 . A set of sixteen different conditions for the solidphase reduction of a nitropyrimidine has also been explored [120]. In this study, SnCl2 gave only 50–65% conversion to the aminopyrimidine, while the best results were obtained with LAH/AlCl3 , although the final products were contaminated with aluminum salts. Aromatic nitro groups have been successfully reduced with CrCl2 at room temperature [122]. Other metal-mediated nitro reductions have also been applied to library synthesis in both solid phase (Zn, NH4 Cl [123]) and solution phase (Fe, HCl [124]).

Scheme 15.44. Preparation of a benzimidazole library via a

NaBH4 -Cu(acac)2 -mediated nitro reduction.

15.2.14.4 Recent Examples of Nitro Reduction on Resin

A diverse set of structural motifs has been realized which incorporate a nitro reduction into the synthetic scheme of the library, typically mediated by tin. Table 15.3 lists recent libraries synthesized with incorporation of a nitro reduction. Included in this set are libraries of 1,4-benzoxa- and benzothiazin-3(4H)-ones as well as benzimidazoles. A library of 56 1,4-benzoxa- and benzothiazin-3(4H)-ones was prepared via a reduction, cyclization, and derivatization approach (Scheme 15.45) [35]. A traceless solid-phase approach to a diverse group of substituted benzimidazoles incorporated a tin-mediated nitro reduction (Scheme 15.46) [125]. 15.2.15

Imine Reductions (not Reductive Amination)

There are relatively few examples of imine reductions in combinatorial chemistry that do not involve imines formed from carbonyls and amines. For the reduction of imines generated from carbonyl compounds, see Sections 15.2.12 and 15.3.1.2 (reductive amination). For an example of imine formation via an aza-Wittig reac-

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412

15 Reductions in Combinatorial Synthesis Tab. 15.3. Recent libraries utilizing nitro reduction (since January 1999).

Library

Reducing agent

Reference

2-Alkylthioimidazoles Benzimidazoles

Zn, NH4 Cl, methanol SnCl2 , NMP SnCl2 , NMP SnCl2 , NMM Zn, NH4 Cl, methanol Zn, NH4 Cl, methanol SnCl2 , DMF SnCl2 , DMF SnCl2 SnCl2 , DMF SnCl2 , DMF SnCl2 , DMF, NaOAc SnCl2 , DMF, NaOAc SnCl2 , DMF Fe, HCl SnCl2 , DMF SnCl2 , DMF, NaOAc SnCl2 , DMF SnCl2 , DMF SnCl2 , DMF SnCl2 , NMP

123b 125 126 116b 123c 123a 127 119 128 129 84 115 35 88 124 130 115 81 131 131 132

Benzimidazolones Benzo[c]isoxazoles 1,5-Benzodiazepin-2-ones 1,4-Benzothiazepin-5-ones 1,5-Benzothiazepin-4-ones Benzothiazines Benzothiazoles 1,4-Benzoxa/thiazin-3(4H)-ones 2-Carboxyindoles Diaminobenzamides Dibenzo[b, f ]oxazocines 2,3-Dihydro-[1,5]-benzothiazepines 2,3-Dihydro-[1,5]-benzothiazepine-4(5H)-ones 3,4-Dihydro-2(1H)-quinazolinones 3,4-Dihydro-1H-quinazolin-2-thiones 1,2,3,4-Tetrahydroquinoxalin-2-ones

Scheme 15.45. Synthesis of 1,4-benzoxa- and thiazin-3(4H)-ones via a tin-mediated reduction.

Scheme 15.46. Synthesis of benzimidazoles via a traceless

linker and tin-mediated nitro reduction.


tion and subsequent reduction, see Section 15.3.1.2. Imine reductions have been used in the synthesis of compounds on solid support as well as in linker activation prior to cleavage (see below). Bischler–Napieralski cyclization products have been prepared on solid phase, and the resulting cyclic imines were reduced with NaBH3 CN to provide tetrahydroisoquinolines (Scheme 15.47) [133]. When NaBH4 was used in this application, the dihydroisoquinoline was cleaved at the ester–resin linkage. An indolenine intermediate, generated via a Fischer indole reaction, was reduced with NaBH4 in the synthesis of a small library of spiroindolines [134]. Resin-bound imines of amino acids, prepared from transimination with N-H ketimines, have been reduced with NaBH3 CN in an approach to a library of hydantoins [135]. In a new linker application, a phenanthridine was reduced with NaBH4/BH3 THF [136]. The desired acid was subsequently released via oxidative cleavage.

Scheme 15.47. Synthesis of tetrahydroisoquinolines involving an imine reduction.

15.2.16

Nitrile Reduction

Conti and coworkers have reported the reduction of a nitrile on solid support [137]. An aromatic nitrile was reduced with BH3 SMe 2 in diglyme at 80 C to provide a benzylamine. The resulting molecule was then released from the resin using an a-chloroethyl chloroformate methanol activation-cleavage strategy. 15.2.17

N–N and N–O Bond Reductions

Samarium diiodide has been used to cleave NaO bonds in a hydroxylamine traceless linker application [138]. Recently, a report was published that described both nitrosamine and hydrazine reductions on solid phase for the preparation of an array of a-substituted primary amines (Scheme 15.48) [139]. DIBAL reduction of the nitrosamine to the corresponding hydrazine followed by addition of an aldehyde gave the resin-bound hydrazone. Nucleophilic addition and subsequent borane reduction of the resulting derivatized hydrazine provided the target amines in mod-

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414


Scheme 15.48. Preparation of a-substituted primary amines via hydrazine reductive cleavage.

est yields. This approach has also been used to prepare chiral hydrazones and the corresponding chiral amine products with modest enantioselectivity (50–86% ee) [140]. 15.2.18

Miscellaneous Reductions

There are a number of reductions performed on solid-supported functional groups for which there are relatively few examples. These reductions can be categorized as those in which the substrate remains attached to the resin, and those where it is released. Wustrow and coworkers have used a reductive cleavage performed with Pd(OAc)2 and formic acid to provide benzoate esters and benzamides from aryl sulfonates [141]. Reductions where substrates remain attached are listed in Table 15.4.

15.3

Solution-phase Reductions 15.3.1

Supported Reagents 15.3.1.1 Asymmetric Reagents

A number of different research groups have shown that polymer-bound amino alcohols can act as chiral ligands in asymmetric hydride reductions of various funcTab. 15.4. Miscellaneous reductions on solid-supported substrates.

Reduction

Reducing agent

Reference

Ozonide to alcohol Ozonide to aldehyde Epoxide to alcohol Peroxide to alcohol Acetal to hydroxyether Lactone to diol Alkyl chloride to alkane Isoxazole to aldehyde Hydroxybenzotriazole to benzotriazole Tin chloride to tin hydride Phosphine sulfide to phosphine

NaBH4 , sonication PPh3 , sonication LiBH4 (EtCO2 )3 P DIBAL LAH NaI then Bu3 SnH LAH PCl3 or SmI2 LiBH4 TfOMe then HMPT

142 142 143 144 145 43b 146 147 148 149 150

15.3 Solution-phase Reductions

Fig. 15.1. Itsuno’s asymmetric reduction ligands.

tional groups. Itsuno and coworkers attached optically active prolinol to polystyrene to give 152 and treated this product with BH3 THF to derive an enantioselective reducing agent (Figure 15.1) [151]. This reagent reduced prochiral ketones to secondary alcohols in good optical purity. The highest optical yield (80%) was obtained with a 1% crosslinked reagent (with 50% functionalization), which was 20% higher than that obtained by the solution-phase control. Following hydrolysis of the reaction mixture with 2 M HCl, the polymer was collected via filtration. Borane regeneration allowed this reagent to be used two more times. Itsuno and coworkers also attached amino alcohols to a polymer through a pendant aromatic group (Figure 15.1) [152]. An acetophenone oxime was reduced with the reagent derived from this polymer-bound amino alcohol and NaBH4/ZrCl 4 or BH3 THF [153]. The optical purity of the product was only 35% ee; however, the reagent could be recycled. Adjidjonou and Caze have also synthesized polymer-bound amino alcohols that were combined with NaBH4 to reduce acetophenone [154]. These reagents delivered the product with modest enantioselectivity (up to 75% ee), which was much more enantioselective than the product obtained from a solution-phase control experiment (12% ee). Frechet et al. derived ligands from ephedrine and polystyrene resin and utilized them in the LAH-mediated reduction of acetophenone [155]. The enantiomeric excess of the product was 79% when a lightly loaded insoluble polymer species was utilized in the presence of an achiral phenol. The minimally substituted resin allows the chiral amino alcohols to act independently from one another and allows the hydride to access all of these units fully, thus providing higher enantioselectivity. 15.3.1.2 Non-asymmetric Reagents

Borane-based reagents In 1977, Gibson and Bailey introduced the first solid-supported borohydride exchange resin (BER) [156]. It should can be noted that, following use, this reagent can be collected by filtration and regenerated. Early studies with this reagent focused on carbonyl reductions and related chemoselectivities, which were found to be better than those produced by NaBH4 in solution [157]. It was understood that this difference in selectivity was due in part to the slower reaction kinetics of the support-bound reagent.

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Fig. 15.2. Macroporous polystyrene-supported borohydride and cyanoborohydride.

Despite the recognized benefits of BER, improvements have been continually sought. As has been demonstrated with solution-phase NaBH4 reductions, the addition of catalytic quantities of transition metal salts (CuSO4 [158], Ni 2 B [159], and Ni(OAc)2 [160]) enhances reactivity and provides the ability to reduce a broader spectrum of functional groups. This area of research has also seen the introduction of zinc [161] and zirconium [162] borohydride polymers. Reductive amination. Commercially available solid-supported reducing agents such as BER and NaBH3 CN on exchange resin (PS-BH3 CN) are useful for solutionphase reductive aminations [163]. Recently macroporous polystyrene versions of NaBH4 (MP-BH4 ) and NaBH3 CN (MP-BH3 CN) have also become commercially available (Figure 15.2) [164]. All of these reagents have the same advantage: they are easily removed from the reaction mixture via filtration. PS-BH3 CN and MPBH3 CN have the added advantages of avoiding contamination of final products with cyanide and providing enhanced chemoselectivities (relative to BER and MP-BH4 ). Typically, reductive aminations with BER and MP-BH4 are two-step procedures, usually performed in methanol (Scheme 15.49). The imine is preformed with 3-A˚ molecular sieves followed by addition of the reducing agent [165]. Any unreacted amine can be scavenged with an appropriate polymer-supported scavenger (e.g. Wang resin or PS-carboxaldehyde) [166].

Scheme 15.49.

Two-step solution-phase reductive amination with BER.

Ley et al. have contributed a number of papers on the subject of polymersupported reagents, including reductive amination with BER [165c, 167] and PSBH3 CN in conjunction with scavenger resins [168]. Ley et al. recently described the reductive amination of substituted bicyclo[2.2.2]octanes with BER and amine scavenging with Wang resin (Scheme 15.50) [165c]. Kaldor et al. also reported the use of BER and scavenger resins in the parallel preparation of small molecules [166a]. They used PS-NCO, PS-CHO, and PS-COCl to scavenge excess primary and secondary amines from crude reaction mixtures and isolated products with purities exceeding 90% (HPLC).


Scheme 15.50. Ley ’s reductive amination/amine scavenging

approach to subsituted bicyclo[2.2.2]octanes.

Aldehyde and ketone reductions. In 1983, Yoon et al. studied the chemoselectivity of carbonyl reductions in a series of competitive reduction experiments with BER (no additives) [157a]. Their results showed that aldehydes were reduced in preference to ketones. More interesting were their observations that there was selectivity between aldehydes and between ketones. Aromatic aldehydes were preferentially reduced in the presence of aliphatic aldehydes. Benzaldehydes with parasubstituted electron-withdrawing groups were reduced preferentially to those with para electron-donating groups. It was also shown that unhindered ketones were reduced in preference to hindered ketones. In a separate study by Yoon et al., it was also shown that the addition of CuSO4 to BER increased the diastereoselectivity of the reduction of norcamphor to norborneol (endo/exo ¼ 94:6 vs. 82:18) [158]. The reduction of ketones and aldehydes has also been carried out using zinc [161] and zirconium [162] borohydride reagents immobilized on polyvinylpyridine. The zinc-based reagent is completely inert toward ketones; however, addition of FeCl3 gives low to moderate yields of ketone reduction products. The solid-supported zirconium borohydride reduces both aldehydes and ketones in the absence of an additive. Further, it has been shown that the BER-Ni(OAc)2 system fully reduces aromatic aldehydes to toluene derivatives in high yield regardless of aromatic substitution [160b]. A hindered equivalent of BER, which diastereoselectively reduces ketones to secondary alcohols, has recently been introduced by Smith et al. [169]. Studies on the reduction of a,b-unsaturated aldehydes and ketones have also been carried out using these reducing agents. BER selectively adds hydride in a 1,2-fashion to these substrates, delivering allylic alcohols in high yield [157b]. The same properties are exhibited by the zirconium reagent [162]; however, the zinc reagent [161] shows chemoselectivity in that it reduces aldehydes without affecting ketones. Sim and Yoon showed that addition of 0.1 equiv. of CuSO4 to BER under standard conditions (5 equiv. BER, methanol, room temperature) fully reduced a,bunsaturated systems to saturated alcohols [158]. However, if the amount of BER was reduced to 2 equiv., the saturated ketone was isolated [158]. Despite these results, Ley et al. recently published a report describing the isolation of the allylic alcohol from a BER-CuSO4 -mediated a,b-unsaturated ketone reduction [167a]. In their synthesis of (G)-epimaritidine, Ley et al. successfully utilized BER-CuSO4 and BER-NiCl2 to carry out the 1,2-reduction of an a,b-unsaturated ketone [167a]. It should be pointed out that the structural complexity of the substrate in the Ley synthesis is much greater than that of Sim and Yoon. In the report of the synthesis

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of (G)-epibatidine, Ley and coworkers also used the parent BER to carry out ketone reductions in high yield [170]. Ester and acid chloride reductions. The reduction of fully oxidized carbons has also been studied, but to a much lesser extent. Esters, for example, seem to be inert to these exchange resins even when transition metal salts are employed. Acid chlorides, on the other hand, have been reduced to both aldehydes and alcohols depending on the resin used. Simple long-chain acid chlorides have been selectively reduced to aldehydes in high yield by passage through a column of BER [171]. Depending on the reaction conditions, Tamami and Goudarzian have shown that polymeric Zn(BH4 )2 can deliver either the alcohol or the aldehyde, however the products are not obtained cleanly [161b]. For example, if phenylacetyl chloride is treated with Zn(BH4 )2 in hot THF, a 70:20 mixture of the alcohol and aldehyde is recovered. If the reaction is run at room temperature in CH2 Cl2 , a 25:65 mixture is obtained. Tamani and Lakouraj have also demonstrated that high yields of clean alcohol can be obtained by using another polymeric zinc borohydride, poly-h(pyrazine)zinc borohydride, in THF at ambient temperature [172]. In Ley and coworkers’ synthesis of (G)-epibatidine, the first step involved an aromatic acid chloride reduction to an alcohol mediated by BER [170]. Epoxide reductions. The reduction of epoxides has also been studied. BER with CuSO4 does not react with aliphatic epoxides, yet cleanly reduces styrene oxide to ethylbenzene [158]. Despite requiring additional quantities of reagents (10 equiv. BER and 0.5 equiv. CuSO4 ), a-methylstyrene oxide and b-methylstyrene oxide also gave the fully saturated alkylphenyl derivatives upon reduction. Supported Zn(BH4 )2 was capable of reducing both aliphatic and styrenyl derivatives, however this reagent did not give fully reduced products. Instead, a mixture of the more and less substituted alcohols was obtained, with the former predominating [161b]. The poly-pyrazine zinc reagent was inert toward both types of epoxides [172]. Halide reductions. Sim and Yoon looked at the reduction of alkyl and aryl halides in detail. BER-CuSO4 was found to be inert toward simple alkyl and aryl chlorides, while readily reducing primary and secondary alkyl bromides as well as aryl bromides and iodides [158]. It should be noted that activated chlorides (benzylic or a to an ester) can be reduced by this system. These chemoselectivities were demonstrated by performing competition experiments. For instance, 1-bromo-4-chlorobutane was readily reduced to 1-chlorobutane (95%) and p-bromochlorobenzene was cleanly reduced to chlorobenzene (99%). Since aryl bromides required heat to be effectively reduced, while aryl iodides did not, it was possible to selectively reduce p-bromoiodobenzene to bromobenzene at ambient temperature with a 97% yield. Yoon et al. have also shown that BER-Ni(OAc)2 has nearly the same selectivity profile as BER-CuSO4 , and that this nickel-based system can be used to reduce 1-octyl tosylate to octane in 95% yield provided that NaI is present [160a].


Disulfide reductions. Attempts to reduce disulfides with polymer-supported reagents has given variable results. BER-CuSO4 quantitatively reduces diphenyl disulfide, yet fails to convert n-butyl disulfide to n-butylthiol [158]. On the other hand, polymeric Zn(BH4 )2 has been successful in reducing both substrates (100% and 40% respectively), as well as others [161b]. The parent BER quantitatively reduces diphenyl disulfide [173]. Azide reductions. BERs and combinations with nickel or copper salts are effective at reducing alkyl and aryl azides [158, 174]. In an early application of BERs, both aryl and arylsulfonyl azides were reduced in methanol to amines and sulfonamides [175]. BER-Ni(OAc)2 has been used to reduce a variety of azides [174]. Tamami and Lakouraj’s piperazine-based zinc reagent can reduce both aryl and alkyl azides to amines [172]. Tamami and Goudarzian’s pyridine-based version reduces aryl and arylsulfonyl azides but does not react with alkyl azides [161b]. Nitro reductions. A number of support-bound borohydride reagents has been used to reduce nitro groups [176]. BER-Ni(OAc)2 reduces aromatic and aliphatic nitro groups and can be easily removed via filtration in a solution-phase approach [177]. The BER-CuSO4 reagent couple also reduces aromatic and aliphatic nitro groups [158]. BER-NiCl2 was used by Ley and coworkers to reduce a nitro group in their synthesis of epibatidine [170]. Reductive cyclizations. The reductive addition of alkyl iodides to electron-deficient alkenes has been demonstrated utilizing the BER-Ni 2 B system [159a]. Examples of radical additions to a,b-unsaturated esters, nitriles, and ketones have been shown to occur in high yields. It has been demonstrated that the same reagent affects aliphatic alkene and vinyl ether reactions with a-bromo esters [159b]. Miscellaneous reductions. BER-Ni(OAc)2 also has been reported to reduce aldehyde oximes to amines [178]. Tin-based reagents Polymeric tin hydrides are capable of reducing a number of functional groups, including carbonyls, alkyl halides, and alcohols [179]. The last are reduced through the intermediacy of a phenylthionocarbonate, according to the methodology set forth by Barton [179b,c]. The main advantage of these reagents over tributyltinhydride (TBTH) is in the work-up. Separations to remove toxic tin byproducts are avoided as the tin species can be easily removed by filtration. In 1975, Crosby and coworkers introduced the first of the supported tin reagents, a polystyrene-based n-butyldihydridotin species [179a]. This reagent directly links a tin atom to the phenyl ring of the polystyrene backbone. In 1993, Neumann and Petersheim published an optimized preparation for a polystyrenebased monohydridotin reagent that utilizes a two-carbon linker between the tin and aromatic backbone of the polymer [180]. Since aromatic tin bonds can be

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labile, this aliphatic carbon-linked tin reagent was believed to be more stable than Crosby’s reagent. Dumartin et al. introduced tin reagents with 3- and 4-carbon linkers that more closely resemble the structure and reactivity of TBTH [179d]. These tin reagents have been used to carry out carbonyl reductions in high yield, including the reduction of both aliphatic and aromatic aldehydes and ketones. It has also been shown that chemoselectivity can be achieved with these reagents, as alkyl halides have been reduced in the presence of ketones [179a]. Neumann and coworkers demonstrated the feasibility of alcohol deoxygenation by utilizing the Barton protocol. This methodology required the conversion of an alcohol to a phenylthionocarbonate, which was then reduced with a solid-supported tin reagent to give the saturated alkyl compound [179b,c]. Neumann and coworkers have also applied this reagent to the reductive cyclization of o-alkenyl halides [181]. Trialkylsilane-mediated reduction of carbonyls A polymer-supported trialkylsilane has been used to hydrosilylate carbonyl aldehydes and ketones [182]. Treatment of the carbonyl compounds with the trialkylsilane and Wilkinson’s catalyst generated resin-bound alkoxysilanes (Scheme 15.51). Cleavage of the resulting alkoxysilane with HF provided the desired alcohols in fair to good yields.

Scheme 15.51. Reduction of aldehydes and ketones via hydrosilylation.

Polymer-supported dihydrolipoic acid-mediated reduction of disulfides Disulfides of cystamine, cysteine, 2-hydroxyethyl disulfide, and oxidized glutathione have been reduced with polymer-bound dihydrolipoic acid [183]. The polymer is prepared via NaBH4 reduction of lipoic acid on polymer (Scheme 15.52). The best results for disulfide reduction were obtained with a polyacrylamide solid support in a pH range of 7.5–8.5.

Scheme 15.52. Preparation of polymer-supported dihydrolipoic acid.

Polymer-supported dihydropyridine-mediated reductions Polymer-supported 1,4-dihydropyridines (PS-DHPs) have been used as NADH-type reducing agents [184]. A divalent cation, typically magnesium, is required for reducing activity and the reactions can be run in either organic or aqueous systems.


Bourguignon and coworkers used 1,4-dihydronicotinamide on Merrifield resin to reduce CbO, CbN, CbS, and CbC double bonds [184a]. Obika and coworkers have developed a chiral sulfinyl-containing DHP on Merrifield resin that was used to reduce methyl benzoylformate to the corresponding hydroxy ester (Scheme 15.53) [184c]. Quantitative chemical yields and high optical yields (96% ee) were obtained when the reaction was run in acetonitrile–benzene (1:1) with 2.5 equiv. of supported DHP and Mg(ClO4 )2 , respectively. The oxidized supported DHP could be regenerated by treatment with propyl-1,4-dihydronicotinamide (PNAH).

Scheme 15.53. Polymer-supported chiral NADH model ketoester reduction.

Polymer-supported sulfide reductions of ozonides Ozonide reductions have been performed with solid-supported triphenylphosphine [185] and sulfides [186]. Appell and coworkers have prepared 3,3 0 -thiodipropionic acid and its sodium salts as supported analogues of dimethylsulfide for reductive quenching of ozonides [186]. The best results were obtained in ozonolysis reactions with the monosodium salt; as such, a polymer-supported version 172 was prepared. The corresponding dialdehyde of ethyl 3-cyclopentenecarboxylate was generated in a 92% yield after quenching the ozonide with this polymer-supported reagent (Scheme 15.54).

Scheme 15.54. Polymer-supported sulfide for reductive ozonolysis work-up.

Polymer-supported triphenylphosphines for the reduction of azides Polymer-supported triphenylphosphine (PS-PPh2 ) is similar to unsupported triphenylphosphine in solution-phase azide reductions. An added advantage of PS-PPh2 is that the phosphine oxide generated is left on the polymer and is easily removed from the product by filtration. Polystyrene-supported triphenylphosphine has been used to reduce azides in a series of azido nucleosides [187]. Yields were nearly quantitative and were similar to those obtained with unsupported triphenylphosphine. Polyethylene glycolsupported triphenylphosphine (PEG-PPh2 ) has been successfully applied to azide reductions, providing amines in shorter reaction times than with PS-PPh2 [188].

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Reaction of PS-PPh2 and azides provides iminophosphoranes that in turn can react with aldehydes to provide imines (aza-Wittig reaction). This approach has been used to generate a set of 20 imines which were reduced with PS-BH3 CN or NaBH3 CN to give amines in good to excellent yields (Scheme 15.55) [189]. Imines have also been prepared in a similar fashion using 1 equiv. of a noncrosslinked polystyrene-supported triphenylphosphine [190]. This resin-bound phosphine has a higher loading (1 mmol g1 ) than PEG-PPh2 (0.5 mmol g1 ) and can be used in stoichiometric quantities (PS-PPh2 is typically used in excess).

Scheme 15.55. Synthesis of amines via aza-Wittig reaction and imine reduction.

15.3.2

Supported Catalysts 15.3.2.1 Asymmetric Catalysis

Homogeneous asymmetric catalysis has been widely studied in both academic and industrial settings. A subset of this research involves the reduction of prochiral ketones to chirally enriched secondary alcohols. Two of the more efficient methods of carrying out this transformation have been described by the research groups of Noyori [191] and Corey [192]. Despite the advantages of the catalyst systems introduced by these groups, the cost of catalyst preparation is high, thereby making reuse desirable. The recovery and purification can be a difficult process; therefore, a number of research groups have pursued the preparation and use of heterogeneous analogs of these catalysts. By attaching these compounds to a solid support, it is believed that the ease with which a catalyst could be recovered and reused would be increased. However, catalyst-recycling improvements cannot come about at the expense of catalyst activity and stereoselectivity. Polymeric catalyst design has therefore taken into account the issues of active site symmetry, accessibility, and flexibility. The three major areas of research in this field include hydrogenations, transfer hydrogenations, and borane-mediated reductions. Hydrogenations Of the homogeneous asymmetric catalysts designed to carry out the reduction of prochiral ketones with molecular hydrogen, perhaps none has garnered more attention than the BINAP-Ru catalyst designed by Noyori [191b]. It should not be surprising therefore that this catalyst system has been chosen for exploitation by a number of research groups interested in heterogeneous catalysis. At least two


Fig. 15.3. Polymeric BINAP ligands.

different approaches have been used to incorporate the BINAP structure into a polymer. An approach chosen by a group from Oxford Asymmetry involved attaching this C-2-symmetric ligand to an existing polymer with the attachment point distal from the active site phosphine atoms [193]. This goal was accomplished by monofunctionalizing the ligand at the 6-position with an alkyl carboxylic acid and then coupling this group to aminomethyl polystyrene resin. The resulting non-C-2symmetric resin-bound ligand 179 (Figure 15.3) was then treated with a ruthenium(II) complex and methanolic HBr in acetone to give the active hydrogenation catalyst. The catalyst (1.7 mol%) was added to a methanol/THF solution of the substrate, which was then treated with 10 atm of hydrogen and heated to 70 C. Reduction of the b-ketoester, methyl propionylacetate, was complete in 18 h with an enantioselectivity of 97%. This heterogeneous catalyst was similar in activity and selectivity to the parent homogeneous BINAP-Ru catalyst. Further, these data show that the loss of C-2 symmetry is not detrimental to the parent catalyst’s selectivity. Perhaps more important is that this catalyst was easily recovered and reused two more times with only minimal losses in yield and enantioselectivity. Another approach that has been used to incorporate BINAP into a polymer was carried out by the Lemaire group [194]. This approach involved copolymerization of a 6,6 0 -dimethylamine BINAP ligand with 2,6-tolylene diisocyanate to give a C-2symmetric BINAP polymer 180 (Figure 15.3). This noncrosslinked polymer was soluble in DMF and DMSO, yet insoluble in the typical hydrogenation solvent – methanol. Utilizing conditions similar to those described above, Lemaire and coworkers were able to completely reduce methyl propionylacetate in 14 h (0.1 mol% catalyst, 40 atm., 50 C) to the desired b-hydroxyester in 98% ee. This catalyst was also recovered and reused up to four times without any loss in activity or selectivity. Lemaire and coworkers utilized the same polymer in the presence of chiral diamines [191c] to reduce ‘‘simple’’ ketones (lacking proximal heteroatoms), such as substituted acetophenones to alcohols [195]. However, the enantiomeric excesses of the products varied between 58% and 96%. It was also shown that the absolute configuration of the added diamine is crucial to retain good enantioselectivity.

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Chan and coworkers described another example of catalytic asymmetric hydrogenation in 1999 [196]. Although the prepared catalyst was used in an olefin reduction, which is beyond the scope of this chapter, it is worthy of note. The polymer formation was conceptually similar to that described by Lemaire, in that the C-2 symmetry was retained by copolymerizing either enantiomer of a 5,5 0 difunctionalized BINAP with (2S,4S)-pentanediol and terephthaloyl chloride. These polymers contained a polyester backbone, which imparted solubility in the reaction solvent mixture of methanol and toluene (2:3, v/v). It was also possible to precipitate these catalysts with excess methanol following reaction completion. Utilization of either polymer to reduce 2-(6 0 -methoxy-2 0 -naphthyl)acrylic acid was complete within 18 h, giving nearly equal and opposite enantiomeric excesses (about 93% ee) of naproxen. These polymers were recycled up to ten times without any loss of activity or selectivity. Transfer hydrogenations The replacement of molecular hydrogen by hydrogen donors is an issue of practical importance in the field of catalytic asymmetric reduction (because of safety concerns). As was the case for standard homogeneous hydrogenations, the Noyori laboratory has made some of the most significant contributions in this area. Noyori and coworkers introduced the (1S,2S)- and (1R,2R)-N-( p-toluenesulfonyl)1,2-diphenylethylenediamine (TsDPEN) ligands, which carry out hydride transfermediated ketone reductions in high yields and enantioselectivities when complexed with ruthenium [191d]. Both the Oxford Asymmetry [197] and Lemaire [198] groups have incorporated this ligand into polymers, using handles on the aromatic sulfonyl portion of the ligand as the linkage point to the resin (Figure 15.4). Each group adopted a strategy similar to the one they took in forming the BINAP polymers, described above. The Oxford group attached the ligand via an amide bond to preformed polymers (PS and PEG-PS) whereas the Lemaire group took a copolymerization approach. The Lemaire group did not concern itself with producing a linear C-2-symmetric polymer as they had previously, because the parent TsDPEN ligand is not C-2 symmetric. They copolymerized styrene and a TsDPEN ligand, equipped with a vinyl group, in both the presence and absence of divinylbenzene, thus producing both a crosslinked and a linear polymer. Both groups studied the reduction of acetophenone; however, each group took their own approach to optimize the reaction conditions. The Oxford group focused on the variation of the hydride source, polymer, and solvent, while keeping the

Fig. 15.4. Polymeric TsDPEN ligands.


transition metal constant [197]. The Lemaire group varied the polymer and transition metal, while keeping the hydride source and solvent constant [198]. Regardless of which polymeric ligand (PS or PEG-PS) was used in the catalyst preparation with [RuCl2 ( p-cymene)]2 , the Oxford group encountered difficulties with isopropanol as the hydrogen donor. In the case of ligand 181 (Figure 15.4), the activity of the catalyst and the optical purity of the products were acceptable; however, catalyst recycling failed. In the case of ligand 182, both the conversion and the enantioselectivity observed were low with the initial use of the catalyst. To circumvent these problems a switch was made from isopropanol to a mixture of formic acid and triethylamine (5:2). This combination led to successful reductions using either ligand. The catalyst formed from ligand 182, in neat HCO2 H:Et3 N, gave product in 97% ee with 95% conversion in 28 h and could be reused once without any loss in ee. The catalyst formed from ligand 181 required a cosolvent to deliver favorable results. Addition of either DMF or CH2 Cl2 resulted in enantiomer excesses of 94% or better with a reasonable degree of conversion (> 60% at 18 h). This catalyst was also successfully subjected to recycling. Although the Lemaire group varied both the transition metal and the polymer in their efforts to find a heterogeneous transfer hydrogenation catalyst, there was little difference in activity and selectivity between their crosslinked and noncrosslinked polymers. From these results, they chose to focus on the significance of the transition metal [198]. Both Ir(I) and Ru(II) complexes were used in the preparation of the catalysts. The iridium catalyst was prepared by combination of the polymeric TsDPEN ligand 183 and [Ir(I)(COD)Cl]2 in an isopropanolic solution of KOH, whereas the preparation of two ruthenium catalysts (from either [Ru(benzene)Cl2 ]2 or [Ru( p-cymene)Cl2 ]2 ) required heat (70 C) and the replacement of KOH with triethylamine. Of these, the best results were found utilizing the iridium-based catalyst, which gave 96% conversion to the S-alcohol with 94% ee after 72 h. Unfortunately, the reuse of this catalyst led to poor results in terms of activity and selectivity. The ruthenium-based catalysts, on the other hand, were much less selective (31–64% ee), but were able to be reused up to four times. For comparative purposes it is interesting to note that when both groups employed their ligand with [RuCl2 ( p-cymene)]2 , a crosslinked polymer, and isopropanol (as solvent and hydride source), the optical purity of the alcohol produced was similar (84% ee for Lemaire and 90.5% ee for Oxford); however, the activities were quite different. After 48 h, the former group saw just 23% conversion while the latter group saw 88% conversion after 18 h. It must be noted that the catalyst load (2.5% vs. 1% respectively) and usage of KOH (presence vs. absence respectively) were different. In another effort to identify a heterogeneous catalyst system capable of carrying out asymmetric reductions, the Lemaire group has copolymerized dialdimine ligands 184, 185 (Figure 15.5) with varying amounts of polystyrene and/or DVB [199]. The iridium-based catalysts formed from the resulting ligands were used in the isopropanol-mediated transfer hydrogenation of acetophenone. Although the level of activity for these catalysts was high, the enantiomeric excess of the products obtained were never greater than 70%. Catalyst recycling suffered losses in

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Fig. 15.5. Dialdimines and diamine used in the preparation of transfer hydrogenation catalysts.

both activity and selectivity. It is interesting to note that ruthenium and cobalt failed to catalyze the reduction, and that a 71% crosslinked polymer gave higher optical purity than both 15% and 100% crosslinked polymers. In another example from the Lemaire group, diamine 186 (Figure 15.5) was copolymerized with both a diacid chloride and a diisocyanate to give a poly(amide) and a poly(urea), respectively [200]. Utilizing the rhodium-based catalysts prepared from these ligands, the reduction results were less than optimal. The poly(amide) gave product in only 28% ee and the poly(urea) resulted in a product of only 60% ee. The latter catalyst could be recycled at least twice. Also worthy of mention are the Lemaire group’s efforts directed toward catalyst formation using ‘‘molecular imprinting’’ [201]. In an application of this methodology, this group copolymerized a preformed diamine–rhodium complex with diisocyanate in the presence of the compound to be imprinted (the alcohol product) – optically pure 1-(S)-phenylethanol. Once the polymer was formed and the alcohol was washed away, an ‘‘imprint’’ of the product was left in the catalytic site, which allowed for binding of acetophenone (or a similar substrate) and ‘‘biased’’ reduction to the desired products. In practice, the ‘‘imprinting’’ effect was found to be real, yet small. The enantiomeric excess of the product from acetophenone reduction increased modestly, from 33% to 43%, from the polymer catalyst formed in the absence of the template to the one formed in the presence of the template. Both of these optical purities were lower than those obtained using the diamine in a homogeneous control reaction (55%). A drawback to this method is that it does not allow for the reduction of a diverse set of ketones as the substrates must have a similar structure to the imprinted molecule. Borane-based reductions A third major area of research directed at the heterogeneous asymmetric catalysis of prochiral ketone reductions is focused on borane-based catalysts. Successful solution-phase asymmetric reductions using chiral oxazaborolidines, described by Itsuno et al. [151–153] and Corey et al. [192], have prompted much of this research. Some of the early work carried out by Itsuno et al. involved covalent attachment


Fig. 15.6. Oxazaborolidine catalysts.

of an amino alcohol to a polystyrene backbone followed by carbonyl reduction with the amino alcohol based-borane reagent to give products with moderate enantioselectivity [151–153]. Although this work allowed for the recovery of the ligand, it did not allow for the recovery of the intact boron catalyst for reuse. Some of the more recent work has addressed this issue by covalently linking the boron atom of the catalyst directly to the aromatic ring of polystyrene. In an effort to capture some of the success of Corey and coworkers’ CBS catalyst and apply it to the solid phase, a group from Sandoz derived a catalyst from (S)a,a-diphenyl-2-pyrrolidinemethanol and a crosslinked polystyrene boronic acid [202]. Once in hand, this catalyst 187 was used to reduce acetophenone and cyclohexylmethyl ketone (Figure 15.6). In both cases, 10 mol% of the catalyst was sufficient when used in combination with a stoichiometric reductant in THF at 40 C. In the case of the reduction of the aromatic ketone acetophenone, an excellent enantioselectivity of 98% was obtained for the product when BH3 SMe 2 was used as the stoichiometric reductant and care was taken to add the ketone slowly. This result was in line with that obtained when the monomeric catalyst was employed. The reduction of cyclohexylmethyl ketone also gave product with the same selectivity (about 80% ee) as was obtained by the monomer catalyst. This reduction required the use of either BH3 SMe 2 or BH3 1,4-oxathiane as the stoichiometric reductant. Following a methanol quench, it was shown that at least one round of recycling was possible with this catalyst. In a conceptually similar approach, Caze et al. have prepared catalysts from (1R,2S)-()-norephedrine and two polystyrene boronic acids with differing degrees of crosslinking [203]. The optimized reduction conditions involved premixing 30 mol% of BH3 SMe 2 (the stoichiometric reductant) with 30 mol% of the lesser crosslinked catalyst 188 in THF at 20 C, and after 30 min gradually adding all of the ketone and the remainder of the catalyst. This procedure delivered the product of propiophenone reduction in 89% ee and in high yield. Recycling of this catalyst was carried out up to three times. The more highly crosslinked catalyst, as well as a thiophene-linked catalyst [204], gave inferior results to those obtained by the less crosslinked polymer mentioned above. More recently, Wandrey and coworkers attached a modified CBS ligand to a siloxane-based copolymer via Pt-catalyzed hydrosilylation [205]. The resulting soluble polymer is similar to the original Itsuno polymers in that the chiral amino

427

428


Fig. 15.7.

Wandrey ’s oxazaborolidine catalyst.

alcohol, not the boron atom, acts as the point of attachment to the polymer. The catalyst 189 is formed by combination of the polymer amino alcohol with BH3 SMe 2 in THF (Figure 15.7). Aryl ketone reduction is then carried out by treatment with the catalyst and stoichiometric quantities of BH3 SMe 2 in THF. The resulting secondary alcohols are obtained in enantiomeric excesses ranging from 89% to 98%, which compares favorably with nonpolymeric results. Unfortunately, the products still have to be purified by distillation or chromatography. In early 2001, Zhao and coworkers reported the preparation of catalyst 190 (Figure 15.8) [206]. Unlike the amino alcohol described above, the nitrogen of this ligand is attached to the resin via a sulfonamide bond. Product ee values were good to excellent for the reduction of aromatic ketones and moderate for alkyl ketones when this catalyst was employed. The combination of NaBH4 and Me3 SiCl (or BF3 OEt2 ) was used as the stoichiometric reductant. Although the catalyst could be reused up to three times, a regeneration step was required. The asymmetric reduction of ketones has also been carried out using zinc complexes of polynaphthyl ligands. These catalysts have been shown by the Pu group to mediate the catecholborane reduction of prochiral ketones [207]. Although the reduction of arylmethyl ketones gave products in good yield with ee values as high as 80%, the reductions of alkyl and branched methyl ketones were much less successful. After quenching the reaction, the homogeneous polymer was precipitated by addition of methanol. Reuse of this catalyst also required a regeneration step. 15.3.2.2 Non-Asymmetric Catalysis

One of the main drawbacks of tributyltinhydride-mediated reductive dehalogenations is the tin waste stream that is created. Utilization of polymeric tin reagents reduces the difficulties associated with their removal. A further improvement has

Fig. 15.8. Zhao’s polymer-supported sulfonamide.


Fig. 15.9. Enholm’s tin catalyst.

been introduced which uses these tin reagents in catalytic quantities in the presence of stoichiometric amounts of sodium borohydride. Enholm and Schulte developed a noncrosslinked polymer (191) that is soluble in a number of organic solvents (Figure 15.9) [208]. This reagent can easily be removed from a reaction mixture by precipitation with methanol. Alkyl and aryl halide reductions have been carried out in N,N-dimethylacetamide (DMA) with 1.5 equiv. of NaBH4 , 0.1 equiv. of 191, and AIBN, as initiator. Because the reactions are homogeneous, the reaction rates are faster than those found with insoluble polymeric catalysts, with completion typically observed in just a few hours at 80 C. Dumartin and coworkers demonstrated the utility of a polymeric tin iodide (192) and compared it with the reducing capabilities of Neumann’s tin chloride reagent (193) (Figure 15.10, see Section 15.3.1.2) [181, 209]. In the comparative analysis of 1-bromoadamantane reduction, 0.05–0.9 equiv. of polymer 192 or 193 was combined with NaBH4 (2.5 equiv.), AIBN (0.1 equiv.), and substrate in ethanol and heated to 65 C for 12 h. When 0.2 equiv. of the catalyst 192 was used, adamantane was obtained in 93% GC yield, while 0.5 equiv. of Neumann’s reagent gave only 40% of the same product. Dumartin’s group also showed that catalyst 192 produces very low levels of tin pollution and can be reused. Bergbreiter and Walker introduced a catalytic tin halide polymer that reduced alkyl and aryl bromides and iodides when combined with NaBH4 and catalytic quantities of a crown ether [210]. Blanton and Salley extended this methodology by attachment of both the crown ether and tin chloride to the same lightly crosslinked polymer [211]. Although this polymeric co-catalyst showed lower activity than the soluble catalyst controls, it showed a marked increase in activity (48%) over controls with one catalyst supported and the other in solution. It appears that this lightly crosslinked polymer was sufficiently mobile to allow the two catalysts to interact. More recently, Deleuze and coworkers introduced an insoluble maleimidebased polymer for catalytic tin reductions [212]. The reduction of 1-bromoadamantane was successfully demonstrated, but the high reaction temperature required (95 C) caused significant leaching of tin.

Fig. 15.10.

The tin catalysts of Dumartin and Neumann.

429

430


15.3.3

Unsupported Reagents Using Catch-and-release Purification 15.3.3.1 Reductive Amination

A catch-and-release approach has been used for purification of reductive aminations on acid-containing products [213]. The reductive amination was performed with NaBH4 and the crude reaction was mixed with DOWEX 1 8–400 formate resin. The solution was drained and the resin treated with TFA to provide the clean product. A catch-and-release strategy has also been used to prepare small groups of ureas and amides via solution-phase reductive amination with Ti(OiPr)4 / NaBH4 (Scheme 15.56) [69c].

Scheme 15.56. Capture and release for rapid purification of a

solution-phase reductive amination reaction.

Amide Reductions Bussolari and coworkers have also reported a resin quench-capture method for the work-up of solution-phase amide reductions with BH3 THF [214]. Borane–amine adducts were quenched by acidic AG 50W-X2 resin and boron-containing salts were washed away while the desired amine was captured by the resin. Subsequent treatment of the resin with ammonia released the desired amine products with excellent purities (> 95% by LCMS). This approach was used to prepare a 300member library of 2-alkoxy- and 2-acyloxyphenylpropyl amines. 15.3.3.2

15.3.4

Fluorous Chemistry

All of the reductions described above required that either the substrate or the reagent be attached to a polymer support; however, a new method is emerging that allows both reactants to remain in solution which takes advantage of the fact that highly fluorinated reagents are immiscible in both standard organic and aqueous phases at ambient temperature, yet are miscible in organic solvents at elevated temperatures. This solubility profile simplifies product isolation and purification by making it possible to separate products from byproducts by straightforward extractive work-ups. Curran and coworkers has shown that ‘‘fluorous’’ chemistry is ideally suited to carry out tin-based reductions [215]. They demonstrated that perfluorinated tin reagents can reduce a number of functional groups including selenides, alkyl halides, nitro groups, xanthates, and aldehydes. Alkyl halides have been reduced with both stoichiometric and catalytic quantities of the tin hydride reagent using NaCNBH3 as the stoichiometric reductant in the latter case. Curran

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15.4

Conclusions

Reductions have been of enormous synthetic utility in both supported and unsupported combinatorial applications. While solid-phase organic synthesis has provided many examples of reductions over the last few decades, the area of solutionphase combinatorial synthesis has emerged and has grown rapidly more recently. As the introduction of new solid-supported reagents and catalysts continues, the ability of those involved in the drug discovery process to both generate and optimize lead compounds should increase.

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C. J. Lock, J. Ma, J. Chem. Soc., Perkin Trans. 1 1995, 345–349. C. Caze, N. El Moualij, P. Hodge, C. J. Lock, Polymer 1995, 36, 621–629. M. Felder, G. Giffels, C. Wandrey, Tetrahedron: Asym. 1997, 8, 1975–1977. J.-B. Hu, G. Zhao, Z.-D. Ding, Angew. Chem. Int. Ed. Engl. 2001, 40, 1109–1111. a) L. Pu, Chem. Rev. 1998, 98, 2405– 2494; b) W.-S. Huang, Q.-S. Hu, L. Pu, J. Org. Chem. 1999, 64, 7940– 7956. E. J. Enholm, J. P. Schulte II, Org. Lett. 1999, 8, 1275–1277. G. Dumartin, M. Pourcel, B. Delmond, O. Donard, M. Pereyre, Tetrahedron Lett. 1998, 39, 4663–4666. D. E. Bergbreiter, S. A. Walker, J. Org. Chem. 1989, 54, 5138–5141.

211 J. R. Blanton, J. M. Salley, J. Org.

Chem. 1991, 56, 490–491. 212 A. Chemin, H. Deleuze, B.

Maillard, J. Chem. Soc., Perkin Trans. 1 1999, 137–142. 213 B. C. Bookser, S. Zhu, J. Comb. Chem. 2001, 3, 205–215. 214 J. C. Bussolari, D. C. Rehborn, D. W. Combs, Tetrahedron Lett. 1999, 40, 1241–1244. 215 a) D. P. Curran, S. Hadida, J. Am. Chem. Soc. 1996, 118, 2531–2532; b) K. Olofsson, S.-Y. Kim, M. Larhed, D. P. Curran, A. Hallberg, J. Org. Chem. 1999, 64, 4539–4541; c) D. P. Curran, S. Hadida, M. Hoshino, A. Studer, P. Wipf, P. Jeger, S.-Y. Kim, R. Ferritto, US Patent No. 4,859,247, January 12, 1999; d) D. P. Curran, S. Hadida, S.-Y. Kim, Z. Luo, J. Am. Chem. Soc. 1999, 121, 6607–6615.

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16

Cycloadditions in Combinatorial and Solid-phase Synthesis Markus Albers and Thorsten Meyer 16.1

Introduction

Cycloadditions are one of the most efficient reactions for the synthesis of isocyclic and heterocyclic compounds in organic chemistry. The two most widespread are the Diels–Alder reaction ([4 þ 2]) and the 1,3-dipolar cycloaddition ([3 þ 2]). The Diels–Alder reaction is not only suitable for synthesis of carbocyclic but also for N- or O-heterocyclic six-member rings (hetero-Diels–Alder). The 1,3-dipolar cycloadditions are often used to synthesize five-member aza- or azoxaheterocycles. In addition to these, there is a whole string of other, less common cycloadditions such as the [2 þ 2] cycloaddition for the formation of four-member rings or the [6 þ 3] cycloaddition. The reactions usually proceed smoothly and only in some cases is a moderate application of heat required. Owing to the pericyclic mechanism two bonds are formed simultaneously in a usually stereo- and regiospecific way, which can be promoted by Lewis acid catalysis [1]. Therefore, cycloadditions are extremely valuable for the generation of stereogenic centers, especially during natural product synthesis. In this chapter the application of cycloadditions to combinatorial chemistry is discussed. Almost all concepts and strategies are based on solid phase, with only a few based on solution-phase chemistry. One reason behind this is the requirement for full conversions. Transformations on solid support have significant advantages over those in solution. Most important is the simple removal of nonresin-bound byproducts and excess of reagents, which is necessary to allow completion of the cycloaddition within a reasonable time, by simply washing the resin with an appropriate solvent. In this way the products have high purity without a time-consuming and expensive chromatographic purification. Solution-phase combinatorial chemistry is only applicable if the excess component is volatile, which allows its removal in vacuo, or it carries an additional functional group, such as an amine, alcohol, ketone, or aldehyde, so that scavengers can be used for its capture. Many types of cycloadditions have been applied to solid phase, including the use of a plethora of different dienes, dienophiles, dipoles, dipolarophiles, or olefins. These are either resin-bound or used as reagents, and are inter- or intramolecular Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

16.2 [4 þ 2] Cycloadditions

variants. Normal or inverse electron demand, and ordinary or hetero-Diels–Alder reactions, complete the set of combinatorial methods available to chemistry [2]. In this chapter, the [4 þ 2], [3 þ 2], [2 þ 2], and [6 þ 3] cycloadditions are described in more detail; additionally, sigmatropic rearrangements as pericyclic reactions are described.

16.2

[4B2] Cycloadditions

The Diels–Alder reaction is the most synthetically efficient method for the preparation of six-member rings and is a well-established reaction in combinatorial chemistry. The typical [4 þ 2] cycloaddition is a Diels–Alder reaction with normal electron demand between an electron-rich diene and an electron-poor dienophile. There have been many examples of this reaction, especially in solid-phase synthesis [2]. 16.2.1

Diels–Alder Reaction with Resin-bound Dienes

A variation on the typical reaction is to start with resin-bound dienes. One of the most reactive, because it is electron rich, and well-known dienes is the Danishefsky diene. Scheme 16.1 shows a direct route for the generation of a polymersupported version. For this purpose a polystyrene diethylsilane resin reacts first with trifluoromethanesulfonic acid, forming a silyl triflate resin. Subsequent treatment with diverse a,b-unsaturated ketones and aldehydes produces silyl enol ether dienes very easily. These reactive dienes are trapped with dienophiles such as Nphenylmaleimide, yielding the bicyclic cycloadducts. Cleavage off the resin with

Scheme 16.1. Diels–Alder reaction with resin-bound siloxy dienes.

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16 Cycloadditions in Combinatorial and Solid-phase Synthesis

trifluoroacetic acid forms the corresponding ketones and alcohols in 62–100% yield and 81–98% purity [3]. Another possible route for the synthesis of cyclohexanone derivatives can be realized by the attachment of a,b-unsaturated ketones to a piperazine resin as enamines (Scheme 16.2) [4]. The electron-rich dienes react with electron-poor E-nitrostyrene derivatives to the nitro-substituted cyclohexanones with moderate yields but high purities. However, an almost equal mixture of diastereomers is obtained, which may be explained by a nonconcerted cycloaddition mechanism reflecting the enamine character of the diene used. It is known from solution phase that [4 þ 2] cycloaddition with aminobutadienes can proceed via a step-wise process [5]. This conclusion is supported by the observation of small amounts of an open-chain enamine addition product.

Scheme 16.2. Diels–Alder reaction with resin-bound amino

dienes. THF, tetrahydrofuran; TFA, trifluoroacetic acid.

Using different substituted resin-bound cyclic dienamines, as depicted in Scheme 16.3, the cycloaddition with a set of dienophiles (maleimides, nitro styrenes, diazo derivatives) occurs in a stereospecific way [6]. The endo but racemic adduct is obtained by an a-face attack of phenylmaleimide giving 61% yield and purity > 85%. The cyclic dienamine is built up by acylation of resin-bound 4hydroxypyridine followed by treatment with a Grignard reagent.

Scheme 16.3. Solid-phase-supported Diels–Alder reaction with cyclic amino dienes.


Tebbe olefination of supported a,b-unsaturated esters is another elegant method used to build up dienyl ethers (Scheme 16.4). The cycloaddition to diverse dienophiles in toluene produces cyclohexenes, which are converted to the corresponding cyclohexanone library after cleavage with trifluoroacetic acid. High endoselectivity is observed at reaction temperatures of 80–100 C, except for Nmethylmaleimide, for which room temperature is found to be sufficient [7].

Scheme 16.4. Resin-bound oxydienes by Tebbe olefination. (dichloromethane (DCM))

An interesting intramolecular ruthenium-catalyzed olefin/alkyne metathesis reaction is used to produce different cyclic dienes suitable for Diels–Alder reactions on solid support (Scheme 16.5) [8]. The synthesis starts with resin-bound allylic amides, which are deprotonated with lithium tert-butoxide, followed by reaction with acetylene methanesulfonate derivatives in dimethylsulfoxide. The alkynylated product obtained undergoes metathesis reaction with Grubb’s ruthenium catalyst and the diene thus formed is treated with maleimide in refluxing toluene to yield the desired cycloadduct as a single diastereomer. The reaction sequence is used in a ‘‘split-and-mix’’ fashion to prepare a 10 4 5 16-member isoindoline combinatorial library.

Scheme 16.5. Formation of resin-bound dienes by metathesis.

Scheme 16.6 gives a very attractive example of a completely stereoselective synthesis of a tricyclic core starting from optically pure 3-bromo-3,5-cyclohexadiene1,2-diol linked to solid support via a ketal [9]. Subsequent epoxidation with dimethyldioxirane takes place with complete facial selectivity, and epoxide opening is achieved with amines followed by acylation of the intermediate alcohol. The use of

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Scheme 16.6. Stereoselective Diels–Alder reaction via a chiral cyclic diene.

a Stille coupling with vinyl stannanes generates the diene for the Diels–Alder reaction with different dienophiles, which proceeds with complete facial selectivity and endoselectivity. The high facial selectivity is rationalized by the sterically demanding ketal group which is efficiently shielding the b-face of the molecule. In sum, 16 cycloadducts are synthesized in high yields and purities. Another example of an enantioselective cycloaddition is the reaction between polymer-supported chiral amino furans and a variety of dienophiles, which can be applied to the formation of sugar derivatives in natural product synthesis (Scheme 16.7) [10]. In order to generate the resin-bound furans, silylchloride resin [11] is treated with the potassium enolate of the optically pure amino furanone. Reaction with methyl acrylate provides the oxabicycloheptene adduct in a regio- and stereoselective way and, because of the steric demand of the pyrrolidine residue, the dienophile reacts at the less hampered a-face. Cleavage off the resin with tetrabutylammoniumfluoride resumes the synthesis of a-substituted cyclohexenones.

Scheme 16.7. Enantioselective cycloaddition via optically pure furan as diene.

A synthetically interesting methodology is the Diels–Alder/retro-Diels–Alder reaction as a safety-catch procedure. A resin-bound diene temporarily catches a dienophile by Diels–Alder reaction, this is then modified further on solid support


[12]. Finally, a retro-Diels–Alder reaction releases the transformed dienophiles from the resin by regeneration of the resin-bound diene. Obviously, a major advantage of this method lies in the high chemical and stereochemical purity of the released compounds. Scheme 16.8 displays this procedure with a resin-bound furan in a cycloaddition reaction to an electron-deficient alkyne. The resulting resin-bound bicyclic Michael system undergoes stereoselective addition of thiophenol, while the desired olefinic product is released from the support by a retroDiels–Alder reaction. It is noteworthy that the Michael addition carried out in solution shows an E/Z ratio of 81:19.

Scheme 16.8. Solid-phase Diels–Alder/retro-Diels–Alder as a safety-catch procedure.

16.2.2

Diels–Alder Reaction with Resin-bound Dienophiles

In contrast to the polymer-supported dienes, examples with resin-bound dienophiles are not so well represented. This may be due to the fact that more dienophiles than dienes are commercially available. Therefore, in solid-phase Diels– Alder reactions dienophiles are usually used as excess reagents. Nevertheless, few reports with resin-bound dienophiles are published. One of the first Diels–Alder cycloadditions applied to solid-phase combinatorial chemistry is the reaction between polymer-supported acrylates and butadiene derivatives (Scheme 16.9) [13]. Acrylic acids are first attached to a polystyrene resin via esterification, and the subsequent cycloaddition reactions take place in hot toluene or xylene. After cleavage off the resin with tetrabutylammonium hydroxide and esterification with diazomethane, mixtures of cis/trans isomers are obtained

Scheme 16.9. Diels–Alder reaction with resin-bound acrylates.

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with high regioselectivities. The observed ortho/meta ratio is in accordance with the results obtained in solution phase. It is well documented that 1-substituted-1,3butadienes react with 1-substituted alkenes containing an electron-withdrawing group, forming predominantly the ortho adduct. Polymer-supported dehydroalanine derivatives are used as dienophiles in Diels– Alder reactions (Scheme 16.10) [14]. The dehydroalanines are generated by coupling of N- and S-protected cysteines to the resin, oxidation of the sulfides to the sulfones, and elimination to the desired olefins. After cycloaddition with cyclopentadiene at 80 C and cleavage with 20% trifluoroacetic acid in dichloromethane, mixtures of endo/exo isomers (1:2 to 1:4, 51–81% yield) are detected (determined by 1 H-NMR). The selectivities are similar to those reported in solution phase.

Scheme 16.10. Diels–Alder reaction with resin-bound dehydroalanine derivatives.

The stereoselectivity of Diels–Alder reactions can be increased by use of chiral auxiliaries, as shown with Evans oxazolidinone. In order to introduce the stereochemical information, enantiomerically pure Boc-l-tyrosine methylester is coupled to hydroxymethyl Merrifield resin using Mitsunobu methodology (Scheme 16.11) [15]. Reduction of the ester and treatment with thionylchloride gives the chiral oxazolidinone, which is then acylated with trans-crotonic anhydride, triethylamine, and dimethylaminopyridine, thus forming the desired resin-bound dienophile. The subsequent cycloaddition with cyclopentadiene is catalyzed by diethylaluminum chloride as Lewis acid and the cycloadduct is cleaved from the resin by lithium benzyloxide. The endo/exo ratio of 21:1 (86% ee) compares nicely with the results obtained in solution phase.

Scheme 16.11. Optically pure oxazolidinones as chiral auxiliaries in cycloaddition.


Besides Lewis acids, the application of high pressure also facilitates the cycloaddition reaction. Especially highly substituted and unreactive starting materials, which show no reaction under normal conditions, can be forced to react. In a microwave-assisted Knoevenagel reaction between resin-bound nitroalkenes and aldehydes, E/Z mixtures of trisubstituted dienophiles are generated (Scheme 16.12) [16]. Treatment with 2,3-dimethylbutadiene under high pressure conditions (15 kbar, 25 C) yields the cycloadducts, whereas stereoselective reduction with lithium aluminumhydride gives the cyclic amines via a traceless linker strategy. The stereoselective formation of one diastereomer from a diastereomeric mixture of two cycloadducts (from a E/Z mixture of nitroalkenes) is rationalized by an aci-nitro intermediate.

Scheme 16.12. Diels–Alder reaction under high-pressure conditions.

During the synthesis of oligomers via tandem Diels–Alder reactions, solid-phase methodology is superior to solution-phase chemistry (Scheme 16.13) [17], whereas under homogeneous reaction conditions, the reaction of an acrylate and a bisdiene would lead to mixtures of oligomers and polymers. Oligomerization can be pre-

Scheme 16.13. Oligomerization via iterative Diels–Alder reaction on solid support.

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cisely controlled on solid support. Starting with a resin-bound acrylate derivative, the Diels–Alder reaction with an excess of bisdiene gives only one cycloadduct. The second cycloaddition is performed with the bisdienophile divinyl ketone in the presence of 15 equiv. of ZnCl2 and the final cycloaddition is achieved by capping the resin-bound dienophile with butadiene. Treatment with Triton B= and iodomethane then reveals the tricyclic ester. All cycloadditions reported proceed in a regioselective way and, after aromatization with palladium/carbon in boiling dichlorobenzene, the desired acetophenones are obtained. 16.2.3

Intramolecular Diels–Alder Reaction on Solid Support

In cycloaddition chemistry the intramolecular strategy is an elegant method of reaction management. By tethering both components it is possible to bring the reactive centers spatially together, so that the reaction proceeds under milder conditions and the turnovers are also improved. The intramolecular Diels–Alder reaction (IMDA) with furan as the diene partner has been widely used to prepare rigid oxygenated tricyclic compounds. A resinbound furan can easily be prepared through reductive alkylation of resin-bound glycine and a set of furaldehydes (Scheme 16.14) [18]. Introduction of an activated dienophile is then realized by acylation with different acrylic acids and cleavage with trifluoroacetic acid provides the tricyclic lactams in high yields and purities (> 90%). Owing to the pericyclic and intramolecular reaction pathway just the exo isomers are obtained. In order to overcome the limitation of the commercial availability of activated dienophiles, maleic anhydride is used, which can be hydrolyzed and further reacted with amines.

Scheme 16.14. Intramolecular Diels–Alder reaction with furan derivatives. TMOF, trimethylorthoformate. (PFPTFA, trifluoroacetic acid pentafluorophenylester).

In order to increase the stereoselectivity, novel unsaturated amino acids are used as dienophiles (Scheme 16.15) [19]. Phosphonoacetyl Wang resin [20] is hence treated with an optically pure fluorenylmethoxycarbonyl (Fmoc)-protected amino acid aldehyde, forming the electron-deficient dienophile with the intention of in-


Scheme 16.15. Enantioselective cycloaddition with unsaturated amino acid derivatives.

troducing the required stereochemical information into the cycloaddition process. Deprotection of the amine allows the introduction of the diene component using different methods. The first example shows the acylation with 2,4-hexadionic acid using isobutyl chloroformate as an activator. The following reductive alkylation with benzaldehyde gives the benzylated precursor for the intramolecular Diels– Alder reaction, which is complete within 30 h at room temperature. Cleavage from the resin provides a major diastereomer (> 90%) derived from the endo transition state, which is influenced by the 1,3-allylic interaction of the dienophile and steric effect of the substituent R1. An alternative transformation of the primary unsaturated amines is to connect them directly to a diene by reductive alkylation with 2,4-hexadienal [19] (Scheme 16.16). Coupling to the resin-bound amine is realized by reductive alkylation of the aldehyde or by acylation of the corresponding furanacrylic acids. Again, during the cycloaddition only one stereoisomer is formed but after prolonged reaction times of up to 2 days. The side-chain double bond of the vinylfuran acts as a part of the diene participating in the Diels–Alder reaction and the aromaticity of the furan ring is restored through final rearrangement.

Scheme 16.16. Formation of tricyclic products by intramolecular cycloaddition.

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16.2.4

Hetero-Diels–Alder Reaction on Solid Support

The hetero-Diels–Alder reaction is a well-established method for the synthesis of six-member heterocycles. An efficient way to access dihydropyrans lies in the reaction of a,b-unsaturated ketones with enols. Starting with resin-bound acetoacetate, the unsaturated ketones are formed by Knoevenagel reactions with different aliphatic aldehydes (Scheme 16.17) [21]. Treatment with a variety of enol ethers at 60 C for 3 days sets up a hetero-Diels–Alder reaction with inverse electron demand. Final cleavage with sodium methanolate provides the cycloadducts with excellent purities of 90%. However, the diastereoselectivities are not very satisfying, displaying a ratio of 1:1 to 5:1 for the endo and exo products.

Scheme 16.17. Formation of dihydropyrans by hetero-Diels–Alder reaction.

Diastereoselectivity can be increased by the use of chiral Lewis acids. Scheme 16.18 shows another cycloaddition to dihydropyrans with inverse electron demand. The synthesis commences with resin-bound benzylidenepyruvic acid [22] and the hetero-Diels–Alder reactions with electron-rich enol ethers are catalyzed by Eu(fod)3 . Cleavage off the resin is achieved by reduction with lithium aluminumhydride and the corresponding alcohols are formed in high yields with an endo/exo selectivity of up to >97:3. The results are similar to those obtained under homogeneous liquid-phase conditions.

Scheme 16.18. Chiral Lewis acids as catalyst in a hetero-Diels–Alder reaction.

The aza-Diels–Alder reaction allows one of the most convenient and versatile approaches to nitrogen-containing six-member heterocycles. In a typical reaction imines act as dienophiles and react with dienes to the precious cycloadducts. In most cases the imines can be prepared in situ from amines and aldehydes, and even simply mixing all three components (amine, aldehyde, and olefin) together


gives good results using Lewis acids for catalysis. A very short synthesis is the onepot reaction between amino-methylated polystyrene resin, aldehydes, and diene catalyzed by ytterbium triflate (Scheme 16.19) [23]. The cycloaddition proceeds smoothly at room temperature over 12–48 h and yields and purities are >90%. The cycloadducts are released from the resin utilizing a tertiary amine N-dealkylation method which involves chloroethyl chloroformate treatment and methanolic decomposition of the resulting carbamates [24]. Only the desired [4 þ 2] products are released from the solid support, enabling a clean resin cleavage.

Scheme 16.19.

Aza-Diels–Alder reaction using Lewis acids for catalysis.

In order to generate triazolopyridazines, urazines as electron-poor diaza dienophiles are used in a hetero-Diels–Alder reaction with dienes [25]. Therefore, diethylphosphonoacetic acid is coupled to resin-bound amino acids and submitted to Horner–Wadsworth–Emmons reaction conditions with different a,b-unsaturated aldehydes (Scheme 16.20) [26]. The dienes thus obtained then react with different urazines, generated in situ from urazoles and iodobenzene diacetate.

Scheme 16.20. Hetero-Diels–Alder reaction with in situgenerated urazines. (PyBOP, benzotriazole-1-yl-oxy-tispyrrolidino-phosphonium hexafluorophosphate, NMM, N-methylmorpholine).

16.2.5

Diels–Alder Reaction in Solution Phase

It is true that transformations on solid support have some advantages over those in solution, such as the use of an excess of reagents and the ease of removal of nonresin-bound byproducts; in contrast, solution-phase chemistry often requires minimal investment of time during method development, has feasible scale-up, has easy reaction monitoring, and no attachment points are required. A variety of imag-

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inative techniques have been developed to rapidly purify the many reactions of a solution-phase library in a parallel fashion. Some of these techniques include acid/ base extraction [27], fluorous-phase extraction [28], polymer-supported reagents [29] and catalysts [30], solid-phase extraction [31], or polymer-supported quench/ scavenging reagents [32]. The reaction with imines and dienes is well suited to solution-phase combinatorial chemistry using the scavenger methodology. Scheme 16.21 outlines the cycloaddition to 2,3-dihydro-4-pyridones under Lewis acid catalysis [33]. An equimolar mixture of aldehydes and primary amines in trimethylorthoformate reacts to form the imines. After evaporation of the solvent the cycloadducts are formed using an excess of Danishefsky diene under ytterbium triflate catalysis, which finally hydrolyzes both the cycloadducts to the desired pyridones and the excess of the diene to the corresponding ketone. The diene decomposition product and, if the reaction does not go to completion, any unreacted imine are removed with a polyamine resin. After simple filtration followed by an acidic aqueous work-up dihydropyridone products are obtained with good yields (up to 90%) and high purities (80– 90%). A variety of different imines, derived from alkyl, alkylaryl, pyridyl amines, and from substituted anilines, undergo the cyclization.

Scheme 16.21. Diels–Alder reaction using polymer-supported scavengers.

Even whole natural products are synthesized using an organized array of polymer-supported reagents. Scheme 16.22 shows the synthesis of epibatidine with a purity > 90%, avoiding the use of chromatographic purification steps [34]. A key step in the synthesis is a cycloaddition reaction between a nitro alkene derivative and an excess of a silyl-protected 2-oxadiene. Beginning with chloronicotinic acid, chloride – the dienophile – is obtained by a reduction/oxidation sequence to the aldehyde and addition of nitromethane with a subsequent elimination step. Treatment with an excess of the volatile silyl-protected 2-oxadiene at 120 C provides the cycloadduct in a quantitative yield. Hydrolysis to the corresponding ketone, reduction to the alcohol, mesylation, and reduction of the nitro group to the amine with final cyclization forms the endo isomer of epibatidine.


Scheme 16.22. Synthesis of epibatidine by Diels–Alder reaction.

16.3


The most widely studied cycloadditions in solid-phase combinatorial synthesis are [3 þ 2] cycloadditions, which have been shown to comprise a wide range of dipoles (nitrones, nitrile oxides, pyridinium salts, azomethine ylides, etc.) and dipolarophiles (alkenes, dienes, and alkynes). Depending on the nature of the 1,3-dipoles employed in the transformations, various heterocycles such as isoxozazoles, isoxazolines, pyrrolidines, indolizines, and pyrrazoles are obtained [35]. These fivemember ring systems represent a branch of unique pharmacophores and some are also versatile synthetic intermediates in further functional group interconversions. As mentioned in the introduction to this chapter, most applications of these transformations are aimed at solid-phase combinatorial chemistry, while only one solution-phase example has been reported to date [36]. 16.3.1

Formation of Isoxazoles, Isoxazolines, and Isoxazolidines

Isoxazoles and isoxazolines are obtained by [3 þ 2] cycloaddition of nitrile oxides to alkynes or alkenes [37], while isoxazolidines are formed through reactions of

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nitrones and olefins (Scheme 16.23). As nitrile oxides often suffer from decomposition and dimerization in solution [38], these transformations should be carried out on solid phase on which either the dipolarophile or the 1,3-dipole can be immobilized.

Scheme 16.23. Synthesis strategies for the preparation of

isoxazoles, isoxazolines, and isoxazolidines.

In a representative example of the preparation of isoxazoles and oxazolines [39], a polymer-bound olefin or alkyne is treated with nitrile oxides (Scheme 16.24), which are typically generated in situ either by using Mukaiyama’s method utilizing phenyl isocyanate and triethylamine [40] or by oxidizing oximes with sodium hyporchloride [41]. The conversions observed are generally high, although in some cases the cycloaddition step has to be repeated up to three times when less stable nitrile oxides are used.

Scheme 16.24. Formation of isoxazoles from resin-bound alkynes.

Intramolecular modifications of the above-mentioned [3 þ 2] process have also been well established on solid-phase [42]. In a generic example, polymer-supported nitro olefins undergo 1,3-dipolar cycloadditions, giving three stereogenic centers in


the resulting tetrahydrofuroisoxazolines (Scheme 16.25). This highly stereoselective process proceeded after obtaining the nitro olefins from Michael additions of dienol alkoxides to b-nitrostyrene.

Scheme 16.25. Intramolecular addition of a polymer-bound nitrile oxide to an olefin.

During the synthesis of a natural product-like library (see also Chapter 21), an intramolecular cycloaddition is used as the key step in building up a polycyclic template [43]. This product is formed by a Tamura tandem reaction [44] of a polymer-bound epoxycyclohexanol and a set of nitrone carboxylic acids (Scheme 16.26). After initial coupling of the 1,3-dipoles to a shikimic acid-derived alcohol, the subsequent [3 þ 2] cycloaddition proceeds with high stereo- and regioselectivity. A variety of reagents and conditions have been screened for further manipulations of the tetracyclic core thus formed and a split-and-mix library of more than 2 million compounds has been synthesized.

Scheme 16.26. Syntheses of tetracyclic cores by tandem

transesterification cycloadditions of epoxycyclohexanols and nitrone carboxylic acids.

On the other hand, when nitrones are prepared in situ from 2-bromobenzaldehyde and methyl hydroxylamine and consequently reacted with polymer-bound acrylates through a nontethered transition state (Scheme 16.27), they were found to be less suitable for solid-phase combinatorial synthesis [45]. The yields recorded are in the range of 24–45% and even boosting the excess of reagent up to 40 equiv. and extending the reaction times does not improve the results. In the latter case, cleavage of the acids from the 2-chlorotrityl resin is observed owing to the prolonged exposure to heat.

Scheme 16.27. Syntheses of isoxazolidines by reactions of polymer-bound olefins and nitrones.

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More success is encountered by the same research group when polymer-bound hydroxylamines are reacted with aldehydes and electron-deficient olefins such as vinylsulfones and N-substituted maleimides (Scheme 16.28). Moreover, it is worth mentioning that the electronic nature of the aldehydes employed has little impact on the synthesis of isoxazolidines, whereas nitrones derived from ketones do not react at all. The superiority of this alternative approach over the route involving immobilized olefins is demonstrated by the synthesis of a small split-and-mix library comprising ten compounds.

Scheme 16.28. Syntheses of isoxazolidines by reactions of polymer-bound nitrones and olefins.

In order to avoid elevated temperatures, ytterbium triflate was successfully introduced to [3 þ 2] cycloaddition reactions, which were then found to proceed at room temperature (Scheme 16.29) [46]. Acrylates of 1,3-oxazolin-2-ones are the best olefinic reaction partners, which can be attributed to their favorable electronics. When other dipolarophiles such as methyl vinyl ketones or substituted acetylenes are used, reduced yields are observed, which can be attributed to the reduced ability of Lewis acid coordination of the unsaturated systems screened.

Scheme 16.29.

Yb(Otf )3 -catalyzed 1,3-dipolar cycloadditions of polymer-bound nitrones.

The isoxazolidines thus obtained are then derivatized further and consequently converted to their corresponding isoxazolines by oxidative cleavage using 2,3dichloro-5,6-dicyanobenzoquinone (DDQ). During the initial work-up, ascorbic


acid is added in order to reduce the amount of remaining oxidant, but chromatography on silica gel is still found to be necessary. Isoxazolidines can also be prepared from immobilized nitrile oxides, which are easily generated through oxidation of polymer-bound aldoximes. In one case, Nchlorosuccinimide (NCS) was used as the oxidant and the corresponding hydroximoyl chlorides were converted to the reactive species by the dropwise addition of triethylamine (Scheme 16.30) [47]. Immediate trapping with an excess of olefins gave the desired heterocycles with yields of 60–80% and purities of >90%. Isoxazoles can also be prepared using this methodology.

Scheme 16.30. Syntheses of isoxazolines by oxidation of a

polymer-bound oxime with NCS and subsequent olefinic trapping.

In another example, the use of an additional amine base can be avoided when a polymer-bound aldoxime is oxidized with commercially available household bleach. After elimination of hydrogen chloride, the corresponding nitrile oxides are obtained [48]. The generality and ease of this protocol is demonstrated when the inverse strategy is pursued and resin-bound acrylates are successfully converted to isoxazolines. Solid-supported reagents can also be used for the in situ preparation of nitrones with regard to the solution-phase synthesis of isoxazolidines (Scheme 16.31) [49]. This process has been carried out using polymer-supported perruthenate (PPS) as the oxidant, but this procedure is limited to symmetrical hydroxylamines only. In order to circumvent this limitation, aldehydes are normally condensed with primary hydroxylamines in the presence of solid-supported acetate. After removal of the polymer-bound reagent and transfer of the crude nitrone to a solution of methyl acrylate, the desired cycloaddition product is isolated with an 81% yield.

Scheme 16.31. Synthesis of isoxazolidines using polymer-supported reagents.

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16.3.2

Formation of Pyrrolidines

Pyrrolidines are typically formed by [3 þ 2] cycloaddition reactions of stabilized azomethine ylides and alkenes – a well-documented process using solid support (Scheme 16.32) [50]. Generation of the reactive dipoles can be achieved thermally by Lewis acid activation or under basic conditions. The neighboring effects of a stabilizing electron-withdrawing group are thereby required and, for that reason, amino acids are the preferred building blocks as they are commercially available in large numbers. Strategically, either the azomethine ylide or the dipolarophile can be immobilized on solid support and both strategies have been used successfully. Less commonly, ylides have been formed through transmetalation processes.

Scheme 16.32. Formation of pyrrolidines by [3 þ 2] cycloaddition of azomethine ylides to olefins.

In a representative example of a heat-induced cycloaddition, a polymer-bound amino acid was first condensed with aldehydes and then reacted with N-substituted maleimides (Scheme 16.33) [51]. The resulting prolines were obtained with high diastereoselectivities and with satisfactory yields and purities of >72%. Alternatively, the process can be carried out as a multicomponent procedure [52] when amino acids and maleimides are reacted together with polymer-bound aldehydes.

Scheme 16.33. Solid-phase synthesis of substituted prolines.

Pursuing the inverse strategy, resin-bound dipolarophiles can also be reacted with azomethine ylides [53], but the introduction of a base and a Lewis acid is vital for the success of the transformations. After condensation of aromatic aldehydes to 3-hydroxyacetophenone attached to Wang resin, the resulting a,b-unsaturated ketones are treated with N-metalated azomethine ylides in the presence of 1,8diazabicyclo[5.4.0]undecene-7 (DBU) and LiBr (Scheme 16.34). Highly substituted pyrrolidines are obtained with satisfying regio- and diastereoselectivities, but chalcones derived from sterically demanding aldehydes, for example 2,6-dichlorobenz-


Scheme 16.34. Formation of pyrrolidines from polymersupported chalcones. rt, room temperature.

aldehyde, do not yield any products. There has been no success using silver(I) acetate as an additive – the catalyst most often used in imine cycloaddition reactions [54]. Other examples that make use of silver salts include the silver acetate-induced cycloaddition of tryptophan-derived imines to polymer-bound acrylates (Scheme 16.35) [55] and synthetic efforts toward a split-and-mix library of mercaptoacyl proline-based inhibitors of angiotensin-converting enzyme (ACE) [56]. Silver nitrate has also been used in the synthesis of fully substituted prolines derived from histidine precursors [57], while intramolecular cycloadditions have yielded polycyclic cores when both the imine and the enone are immobilized on solid support [58]. It is worth mentioning that, during the syntheses of hydantoin-containing

Scheme 16.35. Silver(I) salt-induced formation of substituted prolines.

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heterocycles, the insertion of 1,3-propanediol as a spacer moiety between the polymeric backbone and the glycinate generally facilitates the cycloaddition (Scheme 16.35). The desired tetracyclic cores are released from the resin after treatment with isocyanates and base, whereas the latter reagent epimerizes the stereogenic center at C7a. Another example of the tandem azomethine ylide cycloaddition and carbanilide cyclization strategy uses zinc acetate and DBU as the catalytic system [59]. Other conventionally used Lewis acids such as cesium fluoride [60], silver triflate [61], or cobalt dichloride [62] have not yet been adapted to solid-phase combinatorial synthesis. More reactive dipoles can be generated by transmetalation of 2-aza-allylstannanes and butyl lithium (Scheme 16.36) [63], and the resulting unstabilized anions are able to undergo [2 þ 3] cycloadditions with electron-rich alkenes. Although mixtures of regio- and stereoisomers are generally obtained, this protocol complements the related azomethine ylide transformations which usually require electron-poor olefins.

Scheme 16.36. Synthesis of pyrrolidines via aza-allyl anion cycloadditions.

16.3.3

Formation of Furans

Efficient traceless solid-phase syntheses of furans derived from polymer-supported isomu¨nchnones have been reported [64]. The highly reactive 1,3-dipolar intermediates which participate in the cycloaddition reactions with electron-deficient acetylenes are generated in situ by the decomposition of diazoesters with Rh(II) catalysts. Upon heating, the intermediate bicyclic cycloadduct rearranges to the desired furans and leaves polymer-supported isocyanate behind (Scheme 16.37).

Traceless synthesis of furans via 1,3-dipolar cycloaddition reactions of isomu €nchnones.

Scheme 16.37.


16.3.4

Formation of Imidazoles, Pyrroles, Pyrazoles, and Other Nitrogen-containing Heterocycles

Imidazoles have also been synthesized on solid support utilizing a mu¨nchnone [3 þ 2] cycloaddition reaction with aryltosylimines as the key bond-forming step (Scheme 16.38) [65]. This methodology has been successfully executed in solution phase before, but the reaction yields are reduced by the tendency of mu¨nchnones to undergo self-condensation [66]. This problem can be readily circumvented by attaching the dipoles to AgroGel TM -MB-CHO.

Scheme 16.38. Formation of imidazoles via mu €nchnone intermediates.

Mu¨nchnones are prepared by reaction of an acylated polymer-bound amino acid and N 0 -(3-dimethylaminopropyl)-N-ethylene carbodiimide (EDC) and should immediately be reacted with tosylimines in one pot. It is difficult to cleave the immobilized imidazoles thus obtained from the polymeric support but their release can be achieved by boiling the resins in neat acetic acid, which takes advantage of the robustness of the polymer-bound heterocycles, and unreacted starting materials or nonimidazole byproducts are removed through simple washing with trifluoroacetic acid (TFA) prior to the cleavage step. This new linking strategy has allowed the preparation of an exploratory library containing 12 heterocycles (Scheme 16.38). When mu¨nchnones are combined with electron-deficient acetylenes, pyrroles are obtained (Scheme 16.39) [67]. The precursors for the 1,3-dipolar cycloaddition are available through the Ugi four-component condensation (4UCC) (see Chapter 23.7.5) and undergo an acid-catalyzed cycloelimination step. The resulting

Scheme 16.39. Formation of pyrroles via mu €nchnone intermediates.

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1,3-oxazolinium-2-ones are then trapped with dimethyl acetylenedicarboxylate (DMAD) or other electron-deficient acetylenes [68] and yield pyrroles after aromatization and loss of carbon dioxide. In another example of a [2 þ 3] cycloaddition reaction involving electrondeficient acetylenes, DMAD reacts with polymer-bound azomethine imines, forming pyrazoles (Scheme 16.40) [69]. The 1,3-dipoles employed are generated from asilylnitrosoamides by a 1,4-silatropic shift and give heterocyclic Michael adducts in up to 70% yield. The ratio of the regioisomers obtained is highly dependent on the size of the adjacent substituents, whereas in the case of R ¼ H only one isomer can be detected. Another interesting aspect of this strategy is the cyclizationrelease methodology, avoiding the need for the cleavage operation. However, purification by silica gel chromatography was still found to be necessary.

Scheme 16.40.

Traceless synthesis of pyrazoles.

Indolizines have also been synthesized on solid support by [3 þ 2] cycloaddition reactions of pyridinium ylides with electron-deficient olefins [70]. After alkylation of polymer-bound isonicotinic acid with 2-bromoacetophenones, the resulting pyridinium salts are treated with a,b-unsaturated ketones at elevated temperatures (Scheme 16.41). However, the resulting tetrahydroindolizines rearrange upon

Scheme 16.41. Solid-phase synthesis of indolizines.


acidic cleavage with TFA, a phenomenon also observed during the transfer of the Tsuge reaction to solid-phase chemistry [71]. The formation of the open-chain pyridinium salts is suppressed through oxidation of the bicyclic core with the bimetallic complex TPCD [Co(pyridine)4 (HCrO4 )2 ]. After treatment with TFA, aromatic indolizines are obtained and an exploratory library of nine members has been prepared.

16.4


The [2 þ 2] cycloaddition reaction is one of the most synthetically efficient methods used for the preparation of four-member rings. However, only a limited number of protocols have been adapted to solid-phase combinatorial chemistry, while particular focus has been turned toward the synthesis of mono-cyclic b-lactams via the venerable Staudinger reaction [72]. In a representative example (Scheme 16.42), the cycloaddition reaction is initiated through the slow addition of acid chlorides to a suspension of the imine resins in the presence of triethylamine [73]. Owing to the high reactivity and the accompanying tendency to undergo polymerization reactions, the use of a multifold excess of the reagent is required. However, even cycloadditions to imines derived from highly hindered amino acids usually give satisfying results and the scope of the reaction can be extended to amino, Oprotected and vinyl ketenes.

Scheme 16.42. b-Lactams through [2 þ 2] cycloaddition reactions of ketenes to resin-bound imines.

The thus formed highly functionalized four-member ring heterocycles are also valuable precursors for further chemical manipulations, particularly, when the blactam strain is used to facilitate ring-opening reactions. A striking example of blactams as versatile intermediates was given en route to a split-and-mix library of 4140 dihydroquinolinones (Scheme 16.43) [74]. Here, the nitro group of a [2 þ 2] cycloadduct is reduced and used as an internal nucleophile for the ring expansion reactions.

Scheme 16.43. Dihydroquinolinones via polymer-supported b-lactam intermediates.

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Another method for the solid-phase preparation of b-lactams from imines involves titanium ester enolates derived from 2-pyridinethiols (Scheme 16.44) [75].

Scheme 16.44. Solid-phase synthesis of b-sultams.

Moreover, when sulfenes are used in the cycloaddition reactions to polymersupported imines, structurally analogous b-sultams are obtained [76]. Both reactive species are generated in situ and smoothly react at 78 C. While the imines are prepared by the condensation of aldehydes to immobilized amino acids, the sulfenes are formed by the addition of pyridine to methylchlorosulfonyl acetates. In agreement with the solution-phase synthesis of sultams, two trans diastereomers are obtained [77], but the success of the reaction is reduced when sterically more demanding amino acids such as aspartic acid tert-butyl ester are used. On the other hand, the utility of this method is indisputably high – the entire reaction sequence can be carried out with an acid-labile and a photolabile linker. It nicely allows for the tiered release of compounds from polymeric beads onto live cells during high-throughput screening (HTS). Further chemical modifications of the thiazetidine core also extend the scope of this strategy. A [2 þ 2] keteneiminium cycloaddition reaction has been used to prepare cyclobutanones on solid support (Scheme 16.45) [78]. The alkene resins are thereby added to a fivefold excess of the keteneiminium salts generated in situ from N,Ndialkylamides according to the method of Ghosez and coworkers [79]. The resulting iminium salts are then hydrolyzed to the corresponding ketones with aqueous sodium bicarbonate solution and further converted to g-lactams and g-lactones. This solid-phase protocol is superior to the analogous chemistry carried out in solution, as generally higher conversions are obtained and the purification of the cyclobutanone iminium salts is facilitated by the immobilization on solid phase.

Scheme 16.45. Solid-phase synthesis of cyclobutanes.

16.5

[6B3] Cycloadditions on Solid Support

The [6 þ 3] cycloaddition is an example of a more exotic reaction in combinatorial solid-phase chemistry. One example of a [6 þ 3] cycloaddition is the reaction be-

16.6 Rearrangements

tween fulvenes and benzoquinones forming heterosteroid frameworks (Scheme 16.46) [80]. In order to build up the resin-bound fulvene derivatives, different acids are attached to polystyrene amino resin employing standard conditions (dicyclohexyl carbodiimide (DCC), hydroxybenzotriazole (HOBt), dimethylaminopyridine (DMAP)). Treatment with Meerwein salt and different sodium cyclopentadienides provides the desired resin-bound fulvenes. Through cycloaddition with benzoquinones the tricyclic adduct is released in a traceless fashion from the resin, which can be recovered and used again. After purification by filtration through a short pad of silica gel, the products are isolated in good yields and purity. In addition to benzoquinones, iminobenzoquinones are also used in this type of cycloaddition.

Scheme 16.46.

[6 þ 3] cycloaddition on solid support.

16.6

Rearrangements

Among cycloadditions, sigmatropic rearrangements also belong to the group of pericyclic reactions. New carbon–carbon bonds are formed and, owing to the pericyclic mechanism, there is the possibility of building up stereogenic centers in a stereoselective fashion using chiral induction. Until now only Claisen rearrangements have been applied to combinatorial chemistry. A typical example is the polymer-supported Ireland–Claisen rearrangement (Scheme 16.47) [81]. In this solid-phase synthesis, a trialkylsilane linker is used that is first converted to the more reactive silyl triflate. Treatment with different enolizable allylic esters provides the resin-bound silyl enol ethers as the reactive precursors for rearrange-

Scheme 16.47. Polymer-supported Ireland–Claisen rearrangement.

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ment. After completion of the reaction at 50 C in tetrahydrofuran, cleavage from the resin is realized by methanolysis of the resin-bound silyl esters. The products are isolated in good yields and high purity. Not only polystyrene-based resins but also silica gel or mesoporous molecular sieves have been used as solid support. Their thermal resistance at high temperatures and the opportunity of using polar solvents such as methanol or water make these materials superior to traditional resins. Several different silica gels and molecular sieves, which are capped with aminopropyltriethoxysilane and which vary in their average mean pore size, are employed in a Claisen rearrangement (Scheme 16.48) [82]. Attachment of hydroxymethylbenzoic acid with diisopropylcarbodiimide gives the hydroxy-methylated support, which is coupled to further acids bearing allylic side-chains. The Claisen rearrangement is then performed at 225 C without using any solvent and the products are cleaved as their methyl esters after treatment with methanolic sodium methanolate. When silica gel is used as the solid support, three major products have been isolated by column chromatography that have been identified as two Claisen products (ratio 1.6:1) and a phenol. In contrast, using the mesoporous molecular sieves gave only the desired Claisen product. It is therefore concluded that higher selectivity correlates with the greater distance between the molecules attached to the mesoporous molecular sieves.

Scheme 16.48. Claisen rearrangement on silica gel and mesoporous molecular sieves.

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Main Group Organometallics Christopher Kallus 17.1

Introduction

Main group organometallics represent a class of powerful carbon nucleophiles that allow the construction of CaC single bonds. Moreover, they can act as strong bases and metalating agents, but also possess chelating and Lewis acid characteristics. They are indispensable tools for the construction of organic molecular frameworks which receive strong interest in classical syntheses of physiologically active substances. In contrast, the use of main group organometallics in combinatorial chemistry is underrepresented. This situation may be due to two major factors. First, the number of commercially available compounds is limited, as is their ‘‘diversity’’ in terms of structural variety. Second, the organometallic species are rather moisturesensitive and decompose rapidly in the presence of air. On the other hand, almost no combinatorial equipment provides fully inert reaction conditions, making many combinatorial syntheses involving organometallic reagents difficult to carry out. Reagents carrying more sophisticated residues have to be freshly prepared, but not every desirable chemical functionality is compatible with the high reactivity of this reagent class. As a consequence, only robust anchoring groups such as Ellman’s tetrahydropyranyl linker or Wang ethers can be applied in solid-phase chemistry, whereas standard linkers such as Wang esters or Rink amides are mostly excluded. The resins used have to be dried carefully and the reactions need to be carried out in dry glassware where deep cooling can be applied. In practice, batches of resins for library synthesis have been prepared simultaneously prior to further diversification by easy reaction steps carried out in common parallel synthesis equipment. Novel synthetic technologies such as Chemspeed TM or the Irori TM system may lead to new trends. All these factors in a very small number of combinatorial protocols in the area in question. Moreover, not a single large library has been prepared with main group organometallics, but only small collections of single compounds. In the context of combinatorial chemistry, organometallic reagents have also been used for the synthesis of novel resin–anchor conjugates. Generally, the published protocols describe solid-phase syntheses. However, an increasing number of interesting Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

17.2 Reactions of Metalated Aromatics

solutions to this enormously challenging field have been presented and will be reviewed in this chapter. C–C bonds are formed by the attack of metalated carbon nucleophiles onto electrophiles such as alkyl and aryl halides, carbonyl groups such as aldehydes, ketones, and their heteroanalogs, as well as carbonic acid derivatives. Among metalated species, organomagnesium and -lithium reagents are most commonly used in combinatorial chemistry, whereas cuprates have drawn less attention. Only occasionally and in special cases have organoaluminum, -boron, or -indium species been employed. Since the chemistry of the different metalated species is quite often similar, the structure of this chapter is not guided by the various main group elements. Instead, it is organized by the type of chemical transformation, thus avoiding double citations of the literature examples, where different organometallics are used for the same transformation. Rather than dividing the subject into a strictly mechanistic sense, the order is completely practical, following the synthetic intention focused on a diversifiable substrate (organonucleophile or electrophile) which can be modified in a combinatorial sense. For instance, metalated aromatics are used to make structurally diverse aromatic compounds by reaction with different types of electrophiles while resin-bound ketones give diverse alcohols when reacted with organometallics.

17.2

Reactions of Metalated Aromatics

Aryl lithium species are versatile precursors in the preparation of substituted aromatics. Major applications include the syntheses of different functionalized resins in polymer-assisted synthesis and of ‘‘mini-libraries’’ of substituted heterocycles. Metalated aromatics are generally prepared from the corresponding aryl halides with n-BuLi in tetrahydrofuran (THF) at low temperatures. Their reactions with electrophiles are among the most frequently applied in combinatorial synthesis owing to the fact that not only a broad range of electrophiles can be applied under formation of different functional motifs, but also a large number of aryl bromides as direct precursors of aryl lithiums are commercially available or can be obtained by bromination during the combinatorial synthesis. Additionally, some heterocyclic systems can be directly lithiated, which makes this type of reaction even more interesting in a combinatorial sense. This feature represents an elegant alternative to the functionalization of aromatics by Suzuki reactions, avoiding the preparation of boronic acid building blocks. Metalation of aromatic rings is one of the most fundamental reactions in solidphase synthesis. It has become the standard way of functionalizing simple polystyrene in order to couple handles, linkers, and reagents for further modifications. A typical reaction sequence consists of bromination of polystyrene in the presence of Tl(OAc)3 or with ferric(III) chloride as a catalyst, and subsequent lithiation using n-BuLi. Direct lithiation can be performed using n-BuLi and tetramethyl-

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17 Main Group Organometallics

ethylene diamine or triethylene diamine. Lithiated polystyrene can serve as the starting material for the preparation of polymer-bound carboxylic acids, thiols, sulfides, boronic acids, amides, silyl chlorides, phosphines, alkyl bromides, aldehydes, alcohols, or trityl functional groups for applications in polymer-assisted syntheses using the corresponding electrophiles (Scheme 17.1) [1–6]. It also serves as the starting material for sodium tris-ethoxyborane on selenylpolystyrene, one of the first selenium-based linker systems that allows a ‘‘traceless’’ cleavage [7]. The exchange was achieved by treating the lithiated polystyrene with selenium powder in dry THF. This material also reacts with SO2 in THF to give polymer-bound lithium phenyl sulfinate, a well-established linker for the synthesis of trisubstituted olefins (see below) [8].

Scheme 17.1. Reactions of polymer-supported aryl lithiums.

A direct lithiation-substitution sequence on solid support has been described with several five-member ring heterocycles. Substituted hydroxyimidazoles are obtained from O-imidazolyl-hydroxypolystyrene and n-BuLi followed by reactions with various electrophiles such as alkyl halides, amides, aldehydes, carbon tetrachloride, disulfides, or acid chlorides (Scheme 17.2). Interestingly, in contrast to the solution-phase procedure, solid-phase lithiation acylation occurs without any formation of tertiary alcohols. The compounds thus synthesized have been purified by crystallization or chromatography [9]. In a similar manner, 2- and 2,5functionalizations of 3-polystyrenyl-O-trityl-hydroxymethylfurans and -thiophenes can be achieved. The first substitution takes place at the least hindered a-position, presumably because of the steric bulk of the trityl linker. A subsequent lithiation

17.2 Reactions of Metalated Aromatics

Scheme 17.2. Lithiation-substitution sequences of resin-bound heteroaromatics.

provides access to an attack on the position between the heteroatom and the hydroxymethyl group [10]. Solid-supported phenyl lithium and thiophenyl lithium serve as versatile precursors to the preparation of the corresponding resin-bound aryl isopropylsquarene by reaction with diisopropyl squarate. The reactive intermediates provide a reaction platform to generate several completely different cores, the so-called Multiple Core Structure Libraries (MCSLs). From various possible compound shapes, quinones, hydroquinones, and vinylogous amines derived from arylsquarenes have been realized in small libraries. Further transformations at the squarene carbonyl group are also possible (see below) [11]. In a similar fashion, the replacement of bromine with Grignard reagents leads to magnesiated heterocycles as useful intermediates for further diversification. This procedure has successfully been applied to the synthesis of several functionalized thiophenes (Scheme 17.3). In contrast to reactions with the very polar organolithiums, the ester linkage and other functional groups are stable below 20 C in the presence of an excess of Grignard reagent. Moreover, selective exchanges on thiophene dibromides can be achieved at low temperatures. It should be mentioned that, in contrast to the alkoxymethyl thiophenyl lithium species, the first exchange and reaction with an electrophile takes place at the sterically more hindered position between the heteroatom and the linking ester group. The directing effect of the anchoring group is based on a complexation between the magnesium and the ester group. The reaction is typically performed in the presence of CuCN2LiCl in THF [12, 13]. Furthermore, organozinc bromides serve as building blocks for the solid-phase synthesis of substituted aromatics. Since this reaction is catalyzed by Pd, it will be discussed in Chapter 19.

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Scheme 17.3. Halogen exchange with Grignard reagents and

subsequent reactions with electrophiles.

17.3

1,2-Additions to C – X Groups 17.3.1

Reactions with Aldehydes

The reaction of resin-bound aldehydes with Grignard reagents is one of the most commonly performed main group organometallic transformations on solid support. The reaction has been applied to the synthesis of a fluoride-labile linker system which is suitable for immobilization of substrates such as esters, carbamates, and carbonates. Polystyrene methoxybenzaldehyde reacts smoothly with pTMSCH2 C6 H4 MgBr, forming the secondary diphenyl alcohol [14] (Scheme 17.4). A Grignard reaction with an aldehyde has also been applied during the solidphase synthesis of (S)-zearalenone [15]. A stannane linker derived from polymersupported tin chloride in a reaction carries the aldehyde component which reacted with a chiral TBS-protected organomagnesium alcohol leading to both diastereomers of the secondary alcohol (Scheme 17.5). In another example, polymer-supported fluorenyl aminoaldehydes were treated with various aryl magnesium reagents (Scheme 17.6). The norephedrines were obtained after trifluoroacetic acid (TFA)-induced cleavage of the fluorenyl linker [16]. One of the few examples dealing with resin-bound alkyl Grignard reagents is the synthesis of 2,5-dihydrofurans and 1,3-dihydroisobenzofurans [17]. Hydroxy aryl or alkenyl iodides were coupled on Wang resin as p-alkoxybenzyl ethers. The

17.3 1,2-Additions to CbX Groups

Scheme 17.4. Solid-phase synthesis of a secondary alcohol as fluorine-labile linker via addition of a Grignard reagent to an immobilized benzaldehyde derivative.

Scheme 17.5. Addition of a TBS-protected Grignard reagent to a resin-bound aldehyde during synthesis of (S)-zearalenone.

Scheme 17.6. Formation of secondary alcohols from amino

acid derivatives in the synthesis of norephedrines.

subsequent iodine–magnesium exchange with i-PrMgBr gave the corresponding Grignard reagent on beads, which reacts with several aldehydes to form secondary alcohols (Scheme 17.7). Finally, cyclization during cleavage from the resin with TFA occurs, yielding the desired furans. Secondary alcohols can also be prepared by reacting organolithium species with aldehydes. The reaction of polymer-supported m-alkoxy benzaldehyde with deprotonated 2-phenyl-1,3-dithiane formed a dithiane-protected 3-alkoxy benzoin as the key step in the synthesis of a photolabile safety-catch linker [18]. Since the dithiane group is easily removed by either bis-((trifluoroacetoxy)iodo)benzene, mercury(II) perchlorate, or periodic acid, this example may serve as a general protocol for the preparation of a-hydroxy ketones. The success of the reaction is controlled

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Scheme 17.7. Formation of resin-bound Grignard reagents and

addition of aldehydes providing secondary alcohols as precursors to dihydrofurans.

by elemental analysis of sulfur and characteristic gel-phase dithiane methylenes (Scheme 17.8).

13

C-NMR signals of the

Scheme 17.8. a-Hydroxy ketones by reaction of polymer-bound aldehydes with lithiated dithiane.

Similarly, secondary amines can be lithiated when immobilized as amidines and transformed into secondary a-amino alcohols by reaction with aromatic aldehydes [19]. Subsequent Williamson ether synthesis with benzyl halides and cleavage from the resin by treatment with hydrazine/acetic acid provides a small library of aminoethers (Scheme 17.9). Allyl indium and allyl boronic acid pinacolates have been proven to be very mild reagents for the nucleophilic attack of aldehydes. They are compatible even with base and photolabile nitrobenzylic linker groups and are, in some cases, superior to Grignard reagents or organolithiums [20]. In recent examples, resin-bound aryl or amino acid aldehydes have been transformed into their homoallylic alcohols (Scheme 17.10). The organoindium species have been prepared in situ from allyl


Scheme 17.9. Preparation of secondary amino alcohols from lithiated amidines.

Scheme 17.10. Formation of homoallylic alcohols on solid support from allyl indium or allyl boronic acid pinacolate and aldehydes.

bromide and indium powder in aqueous THF under ultrasonication. Alternatively, the transformation can be carried out with boronic acid pinacolates in dichloromethane under anhydrous conditions. The reactions were successfully conducted on both Tentagel= resin and polystyrene resins. 17.3.2

Reaction with Ketones

The reaction of ketones as electrophiles with organometallic reagents leads to tertiary alcohols. As in the preceding chapters, the solid-phase application of this reaction has been used in the synthesis of linkers and mini-libraries. In a representative example, a novel acid-labile linker for solid-phase synthesis has been prepared by the reaction of MeMgBr with resin-bound methyl phenylethyl ketone [21]. Amines can then be immobilized on solid support by a functional group equivalent to the Boc group, when coupled with p-nitrophenyl chloroformate onto this resin (Scheme 17.11).

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Scheme 17.11. Solid-phase synthesis of tertiary alcohols for the

preparation of t-alkyloxycarbonyl-linked amines.

3-Substituted 2-cyclohexenones can be synthesized by 1,2-addition of Grignard reagents to polymer-bound alkoxy cyclohexenones, which were primarily prepared from hydroxymethyl resin and 1,3-cyclohexandione [22]. During cleavage from the resin with TFA, cyclohexenones were formed by elimination of water (Scheme 17.12). Organolithium compounds are also feasible but give somewhat lower yields, while the reaction with more hindered carbon nucleophiles generally suffers from low yields and unsatisfactory purities.

Scheme 17.12. Synthesis of substituted cyclohexenones.

For the preparation of tertiary alcohols from aryl squarenes in MCSLs, two strategies using phenyl lithium reagents have been considered [23]. One involves the use of a polymer-bound aryl lithium species, generated from an aryl bromide with n-BuLi, which was reacted with alkyl 4-isopropoxysquarenes to yield benzofused quinones after thermal cyclization and cleavage from the resin (Scheme 17.13, see also Section 17.2). The other pathway starts with a resin-bound squarene which is then converted to a tertiary alcohol with phenyl lithium. If the hydroxyl


Scheme 17.13. MCSLs by addition of alkyl lithiums on squarenic acid derivatives.

group is protected, the subsequent cyclization affords aryl-substituted benzohydroquinones. Treatment of resin-bound 2-substituted dihydropyridones with Grignard species in the presence of CeCl3 led to 1,2-addition and the formation of tertiary alcohols [24]. TFA-induced cleavage of the compounds from the resin produced 2,4disubstituted pyridines under oxidative conditions (O2 or air) or 2,4-disubstituted tetrahydropyridines under reductive conditions (triethylsilane) (Scheme 17.14). The starting material for this reaction sequence was obtained by the resin activation/capture strategy (REACAP, see below).

Scheme 17.14. Reaction of Grignard reagents with piperidinones to tertiary alcohols during the preparation of pyridines and tetrahydropyridines.

17.3.3

Reaction with Imines

The conversion of imines with organometallic reagents is slightly harder because their reactivity is lower than the carbonyl groups. While organolithium compounds undergo complete conversions at 78 C, Grignard reagents do react with imines

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at elevated temperatures. The scope of the solid-phase version of this reaction has been demonstrated on aldimines prepared from the amino group of Rink resin and aldehydes making use of the tea-bag method [25] (Scheme 17.15). In principle, aldimines derived from aromatic aldehydes work well, independent of their electronic nature. The polymer support functions as a suitable NH-protecting group, while other ‘‘masked’’ imines such as sulfenimines, sulfonimines, and TMSprotected imines were not applicable in solid-phase chemistry. The cleavage of the products thus formed from the resin yielded primary amines. Similarly, Schiff bases from polymer-bound benzaldehydes were reacted with allyl magnesium bromide, providing secondary a-allyl amines [26].

Scheme 17.15. Substituted primary amines from solid-phase

synthesis of aldimines with organonucleophiles.

A convenient approach toward 2-substituted dihydropyridones that is an alternative to the tandem Mannich–Michael reaction with Danishefsky’s diene uses 4-methoxypyridine or 4-hydroxypyridine attached to polystyrene-hydroxymethyl chloroformate resin [27, 28]. The activated resin reacts with 4-methoxypyridine giving the acylpyridinium species, which is now accessible to a nucleophilic attack by the Grignard reagent, a methodology which has been named ‘‘resin activation/ capture approach’’ (REACAP) by the authors. The reaction can principally be considered as an addition of a Grignard reagent to an activated CbN double bond. It leads to the direct formation of the resin-bound dihydropyridinone, which can be cleaved off with sodium hydroxide in methanol. The same reaction type was applied after linking 4-hydroxypyridine to hydroxymethyl polystyrene under Mitsunobu conditions. Activation of the pyridine by acylation with acid chlorides allowed the addition of Grignard reagents. The desired dihydropyridone was formed during the cleavage with TFA from the resin (Scheme 17.16).

Scheme 17.16. 2-Substituted dihydropyridones formed by attack of organometallics on acyl pyridinium species.


Any unreacted acylpyridinium intermediate remained on the solid support, forming polymer-bound pyridine during the cleavage step. The carboxylic acid thus released as a byproduct was removed by a scavenger resin. The reactivity of terminal alkynes in the presence of catalytic amounts of copper(I) chloride has been demonstrated by reaction of aldimines in Mannich reactions [29]. The solid-phase three-component synthesis of a-substituted propargylic amines is an application of this reaction type carried out with resin-bound alkynes, with secondary amines, or with aldehydes in hot dioxane (Scheme 17.17). In very recent examples, the milder and more flexible Petasis variant of the solidsupported Mannich reaction has been presented [30–32]. Immobilized secondary amines have been reacted with aldehydes and boronic acids to give the Mannich products in a very simple manner.

Scheme 17.17. Solid-supported Mannich reactions with

metalated species. DMF, dimethylformamide.

17.3.4

Reaction with Enolates

a-Substituted ketones can be easily prepared by reaction of polymer-bound TMS enolethers with triflates after the addition of methyl lithium, as demonstrated during the synthesis of prostaglandin E2 methyl esters on noncrosslinked polystyrene (Scheme 17.18) [33]. Presumably, the organonucleophile cleaves the relatively stable OaTMS bond, forming the highly reactive lithium enolate and tetramethylsilane. This reaction has not yet been applied to the synthesis of a series of compounds or libraries.

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Scheme 17.18.

a-Alkylated ketones by reaction of TMS enolates with alkynyl lithium reagent.

17.4

Conjugate Addition to a,b-Unsaturated Carbonyls and Related Systems

Because of their particularly soft character, cuprates are the reagents of choice for conjugate additions, but only a few examples of this reaction type in solid-phase syntheses or combinatorial chemistry have yet been described. An application has been demonstrated by the 1,4-addition to resin-bound dihydropyridones (obtained from a Grignard reaction, see above) [27]. Cuprates have been prepared in a classical manner by the reaction of copper(I) iodide and Grignard reagents in the presence of borontrifluoride etherate (Scheme 17.19).

Scheme 17.19. Synthesis of 3-substituted piperinones by

reaction of cuprates with dihydropyridones.

Alkenyl-substituted ketones can be prepared by the reaction of a,b-unsaturated ketones, as demonstrated during the synthesis of prostaglandins either by use of solid or soluble supports (Scheme 17.20) [33, 34]. The vinyl cuprates employed in

Scheme 17.20. 1,4-Additions of unsaturated metalated species to cyclopentenones.

17.5 Nucleophilic Substitutions

this reaction can be prepared in situ from readily accessible terminal alkynes by hydrozirconation or stannylation followed by transmetalation using Lipshutz’s or Babiak’s protocol [35, 36]. The synthesis of b-substituted esters can be performed in a similar manner. High enantioselectivities have been obtained in addition reactions of Me 2 CuLi or Ph2 CuMgBr to chiral unsaturated esters immobilized on Wang Resin [37]. The resulting trisubstituted hydroxy ester cyclizes upon cleavage from the resin forming d-lactones in excellent yields and enantiopurities (Scheme 17.21).

Scheme 17.21. 1,4-Addition of cuprates on immobilized unsaturated esters.

17.5

Nucleophilic Substitutions

N-Nosylated alkenylaziridines are suitable substrates for SN 2 0 substitutions, where the nosylate serves as a leaving group, expelled initially by the attack of cuprates on the double bond [38]. Typical products are (E )-alkene amino acids, which are useful isosteric nonhydrolyzable peptide bond replacements. In selected examples, copper reagents prepared from organolithium or Grignard reagents were used for the preparation of these a-substituted dipeptide isosteres on Wang resin (Scheme 17.22).

Scheme 17.22.

Addition of cuprates to vinyl nosylate aziridines.

Grignard reagents can also serve as agents for functionalizing cleavage products synthesized on the benzotriazole linker [39]. In an extremely short reaction sequence, consisting of resin capture of aldimines in a Mannich reaction and subse-

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quent cleavage by nucleophilic substitution with Grignard reagents in refluxing toluene, primary and secondary amines have been obtained in variable yields and excellent purities. As in all similar cleavage reactions, an aqueous extraction step is necessary (Scheme 17.23).

Scheme 17.23. Cleavage of highly substituted amines from benzotriazole linker with Grignard reagents.

Finally, the application of cuprates as cleavage reagents has been demonstrated in related solid-phase reactions [8]. Trisubstituted olefins were synthesized by the reaction of terminal allyl species immobilized on a phenylsulfone linker with cuprates prepared in situ from a range of Grignard reagents and copper(I) iodide (Scheme 17.24). Purification by column chromatography of the products thus obtained was found to be necessary.

Scheme 17.24.

Trisubstituted olefins from additions of cuprates on vinyl sulfones.

17.6

Reactions on Carboxylic Acid Derivatives and Related Systems

From a mechanistic point of view, reaction of Grignard compounds with carboxylic acid derivatives is quite similar to the 1,2-additions mentioned previously. The initial step is again the nucleophilic attack on the CbO double bond, leading to a tetrahedral intermediate. The subsequent restitution of the carbonyl group is accompanied by the simultaneous ejection of a leaving group. 17.6.1

Reaction with Esters

The solid-phase synthesis of a,a-dialkyl-substituted alcohols has been accomplished by the reaction of resin-bound esters with Grignard reagents. After two successive additions, tertiary alcohols are obtained. In contrast to the previously mentioned additions to ketones, only symmetrically disubstituted products are accessible. This type of reaction has been carried out with methyl-2-hydroxy-3phenyl-propionate immobilized on Wang resin by an ether linkage giving the corresponding dimethyl alcohol [40]. It has also been used for the preparation of a

17.6 Reactions on Carboxylic Acid Derivatives and Related Systems

small library of N-alkyl-2-pyrrolidine-dialkyl-methanol ligands as potential catalysts in diethylzinc additions from hydroxy proline, attached by the THP linker onto solid support [41] (Scheme 17.25).

Scheme 17.25. Formation of a,a-distributed alcohols by the

addition of Grignard reagents to resin-bound esters.

In many examples, Grignard reagents have been applied for the release of products from the solid support. During the solid-phase synthesis of a PDE4 inhibitor, MeMgBr was used to cleave the ester linkage leading to a dimethylphenyl alcohol after treatment with ammonium chloride in ethyl acetate (Scheme 17.26) [42].

Scheme 17.26.

Tertiary alcohols by cleavage of ester linkage with Grignard species.

17.6.2

Reactions with Weinreb Amides and Related Systems

One of the most popular transformations with main group organometallics on solid supports is the reaction of Weinreb amides with kryptobases. These Nmethoxy-N-methylamides react with Grignard species to give ketones and with hydrides to give aldehydes, which contain desirable functional groups either for further diversification (reductive amination) or as recognizing moieties in libraries of serine/cysteine protease inhibitors. Little or no overalkylation occurs, presumably because of the formation of a chelate of the metal ion with the carbonyl oxygen and the N-methoxy oxygen. This stable intermediate decomposes readily to give a carbonyl group upon treatment with acid (Scheme 17.27).

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Scheme 17.27. Formation and breakdown of the tetrahedral bidentate during reaction of Grignard reagents with Weinreb amides.

For the synthesis of ketone or aldehyde libraries, the use of Weinreb amides as anchoring groups is preferable. Although additional efforts during the work-up are necessary, this strategy is advantageous because the reactive keto group is released only at the stage of the cleavage step. In this context, the N-alkyoxyamide function of b-alanin Weinreb amide on aminomethyl polystyrene has been used as a linker group [43]. Since preformation of the linker conjugate with the substrate has to be carried out in solution phase, this methodology appears to be somewhat laborious (Scheme 17.28).

Scheme 17.28. Syntheses of ketones by cleavage of Weinreb-type linkers.

More conveniently, solid-supported Wang O-hydroxyl-N-alkyl amine, obtained by reaction of N-hydroxyphthalimide with Wang resin and subsequent cleavage of the phthalimide with methylamine, has been used in the immobilization of substrates

17.6 Reactions on Carboxylic Acid Derivatives and Related Systems

by a Weinreb amide linker [44]. The cleavage was carried out using EtMgBr to provide ethyl ketones. Another successful method for the preparation of ketone libraries on solid phase uses mercaptoacetylamide on polystyrene resin, which acts in the same way as a Weinreb amide [45]. The starting carboxylic acid building blocks were coupled as thioamides onto this linker. After treatment with Grignard reagents, analytically pure products were released from the resin. The combination of a Weinreb amide and an isonitrile functionality in one single synthetic building block allows the preparation of a dipeptide methyl ketone library by application of methyl Grignard reagent after the Ugi 4CC reaction (Scheme 17.29). A 96-member library around a known anticonvulsant structure has been synthesized according to this strategy [46].

Scheme 17.29. Synthesis of dipeptide alkyl ketones.

Another solid-phase protocol using the Weinreb methodology leads to a-hydroxyketones, which have been shown to be efficient inhibitors of the aspartate protease renin [47]. Since the hydroxy group is the recognizing motif within this enzyme family, fluorenylmethoxycarbonyl (Fmoc)-hydroxy-b-amino acids are suitable templates for this purpose, preferably attached via the hydroxy group to the solid support. The immobilized amino acids have been converted into ketones by formation of the Weinreb amide with HClNH(OMe)Me and dropwise addition of Grignard reagent in a one-pot procedure (Scheme 17.30). It is noteworthy that an acetic acid/dichloromethane washing step is of crucial importance, presumably because magnesium salts from the bisdentate complex have to be removed prior to cleavage from the polystyrene resin. This method worked very well in the case of primary unhindered Grignard reagents, while attempts with organolithium species resulted in complex product mixtures.

Scheme 17.30. Solid-phase synthesis of an a-hydroxy ketone library.

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A highly sophisticated application of the Weinreb amide transformation was part of the synthesis of aspartyl protease inhibitor libraries targeting cathepsin D [48, 49]. 2,3-Dihydroxy propionic acid served as a suitable template, which was attached to Wang resin via its a-hydroxy group, as in the previous example. The carboxylic acid had to be transformed into a ketone for further chemical modifications. The standard Weinreb methodology that was chosen initially to accomplish this task led to unwanted side-products for two reasons. First, overalkylation occurred to a remarkable degree since the excess Grignard reagent was not completely destroyed, while the breakdown of the tetrahedral intermediate had already delivered the free ketone susceptible to further attack. Second, a competitive NaO bond cleavage leads to N-methyl products in substantial amounts, depending on the nature of the Grignard reagent used. While the problem of overalkylation has been countered by addition of acetone to the reaction mixture to quench excess Grignard, the problem of side-products occurring as a result of NaO bond cleavage has remained unsolved. Therefore, the N-methoxy-N-methyl amide was replaced by a simple pyrrolidine amide. The Grignard addition to this substrate provided the desired ketones with no detectable overalkylation product (Scheme 17.31). Herein, the a-alkoxy group serves as a Lewis base to stabilize the tetrahedral intermediate instead of the N-methoxy group of the standard Weinreb amide. For the library synthesis, a diverse set of commercially unavailable Grignard reagents has been prepared from activated magnesium turnings (alkyl Grignards) or Mg(anthracene)(THF)3 complex (activated benzyl-type Grignards). Using the aforementioned chemistry, a library of more than 1000 compounds has been synthesized, delivering several highly potent cathepsin D inhibitors. Later, the same concept was applied to the synthesis of libraries targeting plasmepsin II, which also yielded nanomolar inhibitors [50].

Scheme 17.31. Side-reactions of standard Weinreb amides and

synthesis of polymer-bound ketones from a-alkoxypyrrolidine amides.

The solution-phase preparation of ketones from Weinreb amides is also feasible using polymer-bound sulfonylhydrazide as a resin-capture reagent to isolate the desired products [51]. In a short reaction sequence, this strategy was applied to synthesize a small library of 1,2,3-thiadiazoles, released by treating the resin-bound

17.7 Aminolysis of Esters

hydrazones with SOCl2 . The tetrahedral intermediates from the Grignard reaction were decomposed by addition of macroporous polystyrene-sulfonic acid resin (MPTsOH) and the ketones thus produced were captured by gentle heating with PSTsNHNH2 in acetic acid/THF (Scheme 17.32).

Solution phase synthesis of ketones from Weinreb amides during preparation of 1,2,3-thiadiazoles.

Scheme 17.32.

17.7

Aminolysis of Esters

Aminolysis of esters has been applied in combinatorial chemistry in order to generate the overwhelming diversity of amides in a single step. Organoaluminums are the reagents of choice for inducing this reaction owing to their strong Lewis acidity. This protocol has mostly been used for the derivatizing cleavage from hydroxyethyl polystyrene resin. Published examples include the synthesis of tetrahydrochinoline amides by Pictet–Spengler reaction [52] and the formation of 3hydroxypropionamides utilizing the Baylis–Hillman reaction [53] (Scheme 17.33).

Scheme 17.33.

Aminolysis of ester bonds with organoaluminium reagents on solid support.

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The filtrates must be carefully quenched and additional work-up was found to be necessary. This reaction type has also been used for the simultaneous lactone opening and amidation on solid support [37].

References 1 T. M. Fyles, C. C. Leznoff, Can. J. 2

3 4 5 6

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10 11 12

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Chem. 1976, 54, 935. F. Camps, J. Castells, M. J. Ferrando, J. Font, Tetrahedron Lett. 1971, 1713–1714. W. Heitz, R. Michels, Makromol. Chem. 1971, 148, 9. M. J. Farall, J. M. Frećhet, J. Org. Chem. 1976, 41, 3877–3882. F. G. Bordwell, B. M. Pitt, J. Am. Chem. Soc. 1955, 77, 572–577. G. A. Crosby, N. M. Weinshenker, H.-S. Uh, J. Am. Chem. Soc. 1975, 97, 2232–2235. T. Ruhland, K. Andersen, H. Pedersen, J. Org. Chem. 1998, 63, 9204–9211. C. Halm, J. Evarts, M. J. Kurth, Tetrahedron Lett. 1997, 38, 7709–7712. S. Havez, M. Begtrup, K. Andersen, T. Ruhland, J. Org. Chem. 1998, 63, 7418–7420. Z. Li, A. Ganesan, Synlett 1998, 405– 406. P. A. Tempest, R. W. Armstrong, J. Am. Chem. Soc. 1997, 119, 7607–7608. L. Boymond, M. Rottla¨nder, G. Cahiez, P. Knochel, Angew. Chem. Int. Ed. Engl. 1998, 37, 1701–1703. M. Abarbi, J. Thibonnet, L. Be´rillon, F. Dehmel, M. Rottla¨nder, P. Knochel, J. Org. Chem. 2000, 65, 4618–4634. A. Routledge, H. T. Stock, S. L. Flitsch, N. J. Turner, Tetrahedron Lett. 1997, 38, 8287–8290. K. C. Nicolaou, N. Winssinger, J. Pastor, F. Murphy, Angew. Chem. Int. Ed. Engl. 1998, 37, 2534–2537. F. Gosselin, J. v. Betsbrugge, M. Hatam, W. D. Lubell, J. Org. Chem. 1999, 64, 2486–2493. M. Rottla¨nder, P. Knochel, J. Comb. Chem. 1999, 1, 181–183. A. Routledge, C. Abell, S.

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Balasubramanian, Tetrahedron Lett. 1997, 38, 1227–1230. P. S. Furth, M. S. Reitman, R. Gentles, A. F. Cook, Tetrahedron Lett. 1997, 38, 6643–6646. C. L. Cavallaro, T. Herpin, B. F. McGuiness, Y. C. Shimshock, R. E. Dolle, Tetrahedron Lett. 1999, 40, 2711–2714. R. Le´ger, R. Yen, M. W. She, V. J. Lee, S. C. Hecker, Tetrahedron Lett. 1998, 39, 4171–4174. M. E. Fraley, R. S. Rubino, Tetrahedron Lett. 1997, 38, 3365–3368. P. A. Tempest, R. W. Armstrong, J. Am. Chem. Soc. 1997, 119, 7607–7608. C. Chen, B. Munoz, Tetrahedron Lett. 1998, 39, 3401–3404. A. R. Katritzky, L. Xie, G. Zhang, Tetrahedron Lett. 1997, 38, 7011–7014. B. Chenera, J. A. Finkelstein, D. F. Veber, J. Am. Chem. Soc. 1995, 117, 11999–12000. C. Chen, I. A. McDonald, B. Munoz, Tetrahedron Lett. 1998, 39, 217–220. C. Chen, B. Munoz, Tetrahedron Lett. 1998, 39, 6781–6784. J. J. McNally, M. A. Youngman, S. L. Dax, Tetrahedron Lett. 1998, 39, 967– 970. N. Schlienger, M. R. Bryce, T. K. Hansen, Tetrahedron 2000, 56, 10023– 10030. S. R. Klopfenstein, J. J. Chen, A. Golebiowski, M. Li, S. X. Peng, X. Shao, Tetrahedron Lett. 2000, 41, 4835–4839. A. Golebiowski, S. R. Klopfenstein, J. J. Chen, X. Shao, Tetrahedron Lett. 2000, 41, 4841–4844. S. Chen, K. D. Janda, J. Am. Chem. Soc. 1997, 119, 8724–8725. L. A. Thompson, F. L. Moore, Y.-C. Moon, J. A. Ellman, J. Org. Chem. 1998, 63, 2066–2067.

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Am. Chem. Soc. 1990, 112, 7440. K. A. Babiak, J. R. Behling, J. H. Dygos, K. T. McLaughlin, J. S. Ng, V. J. Kalish, S. W. Kramer, R. L. Shone, J. Am. Chem. Soc. 1990, 112, 7441. S. Hanessian, J. Ma, W. Wang, Tetrahedron Lett. 1999, 4631–4634. P. Wipf, T. C. Henninger, J. Org. Chem. 1997, 62, 1586–1587. K. Schiemann, H. D. H. Showalter, J. Org. Chem. 1999, 64, 4972–4975. S. Hanessian, F. Xie, Tetrahedron Lett. 1998, 39, 737–740. G. Liu, J. A. Ellman, J. Org. Chem. 1995, 60, 7712–7713. Y. Han, A. Giroux, C. Le´pine, F. Laliberte´, Z. Huang, H. Perrier, C. I. Bayly, R. N. Young, Tetrahedron 1999, 55, 11669–11685. T. Q. Dinh, R. W. Armstrong, Tetrahedron Lett. 1996, 37, 1161–1164. J. M. Salvino, M. Mervic, H. J. Mason, T. Kiesow, D. Teager, J. Airey, R. Labudiniere, J. Comb. Chem. 1999, 1, 134–139.

45 I. Vlattas, J. Dellureficio, R. Dunn,

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I. I. Sytwu, J. Stanton, Tetrahedron Lett. 1997, 38, 7321–7324. S. W. Kim, S. M. Bauer, R. W. Armstrong, Tetrahedron Lett. 1998, 39, 6993–6996. O. B. Wallace, Tetrahedron Lett. 1997, 4939–4942. E. K. Kick, D. C. Roe, A. G. Skillman, G. Liu, T. J. A. Ewing, Y. Sun, I. D. Kuntz, J. A. Ellman, Chem. Biol. 1997, 4, 297–307. C. E. Lee, E. K. Kick, J. A. Ellman, J. Am. Chem. Soc. 1998, 120, 9735– 9747. T. S. Haque, A. G. Skillman, C. E. Lee, H. Habashita, I. Y. Gluzman, T. J. A. Ewing, D. E. Goldberg, I. D. Kuntz, J. A. Ellman, J. Med. Chem. 1999, 42, 1428–1440. Y. Hu, S. Baudart, J. A. Porco, Jr. J. Org. Chem. 1999, 64, 1049– 1051. ¨ nzer, K. Ro¨lfing, M. Thiel, H. Ku Synlett 1996, 1036–1038. O. Prien, K. Ro¨lfing, M. Thiel, ¨nzer, Synlett 1997, 325–326. H. Ku

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Enolates and Related Species in Combinatorial and Solid-phase Synthesis Jochen Kru¨ger 18.1

Introduction

The formation of carbon–carbon bonds utilizing enolates and related species is among the most prominent and useful processes in organic synthesis. Accordingly, the extension of the established solution-phase repertoire to combinatorial synthesis onto liquid phase and solid phase represents both an ambitious as well as a rewarding goal. Comprehensive compilations of the latest achievements in solid- and liquidphase enolate chemistry have been published recently [1]. In this chapter, we give a detailed account of the use of enolates and related species in combinatorial chemistry. In addition to the available literature, we provide an extensive comparison of solution- and solid-phase strategies in each section. Moreover, we focus on the practical aspects and assess the suitability of a given method for automation and library synthesis. We also discuss the scope and limitations of each method, as well as its convenience for practical use.

18.2

Aldol Reactions 18.2.1

General Aspects

The aldol reaction between an aldehyde and an enolate or an enolate equivalent delivers b-hydroxy carbonyls or, after dehydration, a,b-unsaturated carbonyls. Both structural motifs represent valuable functionalities in natural product and heterocycle synthesis. Thus, it seems conceivable that well-established methods for classical solution-phase aldol chemistry could be adapted to the requirements of combinatorial chemistry. However, the emphasis in this field was clearly placed on solid-phase strategies. Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

18.2 Aldol Reactions

The direct condensation between an aldehyde and a CaH acidic ketone was successfully employed in solid-phase synthesis and library construction (see Section 18.2.2). In particular, Na and Zn enolates have played a dominant role in this field. Although the basic reaction conditions entail limitations arising from the base sensitivity of some polymeric linkers or starting materials, this method presents a powerful tool to perform aldol reactions on solid support. For stereoselective aldol protocols, we discuss recent progress in the application of boron enolates in solid-phase chemistry (see Section 18.2.3). In general, the efficiency of a boron aldol process on solid support is highly dependent on the nature of the polymer, the polymeric linker, and the reactivity of the enolate. Thus, for a given synthetic problem in solid-phase chemistry, the boron aldol protocol has to be adjusted to the requirements of the desired transformation. To the best of our knowledge, multicomponent library synthesis utilizing stereoselective boron aldol reactions has not yet been reported. In Section 18.2.4 we detail the latest achievements in the field of liquid- and solid-phase Mukaiyama aldol chemistry. This mild version of the aldol reaction can serve as an attractive alternative for the direct aldol condensation employing enolizable carbonyl compounds. 18.2.2

Li, Na, K, and Zn Enolates in Aldol Reactions

The synthesis of b-hydroxy carbonyls via a crossed aldol reaction between an aldehyde and an enolizable carbonyl compound has been adopted to solid phase, whereas liquid-phase protocols in this field are rather scarce. Ruhland and Ku¨nzer described the formation of aldol intermediates (2) upon reaction of resin-bound aldehyde 1 with acetophenone in the presence of K2 CO3 at elevated temperature (Scheme 18.1) [2]. Six aromatic and heteroaromatic methylketones were reported to be suitable substrates for this transformation, however, yields and purities were not given. The intermediates (2) were used in the synthesis of a diverse quinoline library of 12 products. Thereby, the effectiveness of the aldol transformation as a key step in this sequence was demonstrated.

Scheme 18.1.

Aldol reactions on solid phase employing K enolates.

A major obstacle in aldol chemistry on solid support is incomplete or sluggish reactions due to variable extents of retro-aldol processes. In this regard, the use of

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18 Enolates and Related Species in Combinatorial and Solid-phase Synthesis

Zn enolates has proved to be advantageous, since the amounts of retro-aldol products being formed during the process were minimized. As Kurth et al. [3] demonstrated, immobilized Zn enolates react smoothly with aromatic aldehydes and give the desired b-hydroxy ketones in good yields and purities. Accordingly, ester 3 loaded on Merrifield resin was deprotonated at 78 C using an excess of lithium diisopropylamide (LDA) followed by the addition of anhydrous ZnCl2 to trigger the transmetalation to the corresponding Zn enolate. Finally, 27 aromatic aldehydes were added and the aldol reaction proceeded smoothly at 0 C within 30 min (Scheme 18.2) [3]. Cleavage from the resin was accomplished by reduction of the ester linkage with diisobutylaluminium-hydride (DIBAL-H) to yield the crude diols (4) as mixtures of syn and anti products which were purified by preparative thin layer chromatography (TLC). In the case of p-methoxybenzaldehyde as electrophile in this reaction, an overall isolated yield of 26% was reported.

Scheme 18.2.

Aldol reactions on solid phase employing Zn enolates.

A related strategy was applied by Nicolaou et al. for the synthesis of an epothilone library [4]. A key step in this route leading to diversity with concomitant construction of the CaC skeleton was an aldol reaction mediated by in situ-generated Zn enolates (see Scheme 21.14). Both contributions demonstrate that even more sophisticated reaction conditions including inert gas techniques and lowtemperature protocols are routinely applied in solid-phase synthesis today. The only drawback in terms of manipulative convenience arose from the fact that the products were purified by preparative TLC after cleavage. This aspect might be difficult to realize in an automated library synthesis process. The synthesis of a,b-unsaturated ketones using a crossed aldol strategy on solid support is well established and has successfully been applied to solid-phase organic synthesis. Either the aldehyde moiety or the CaH acidic ketone can be linked to the polymer via an ether or an amide linkage, whereas linkage via an ester proved to be incompatible with the strongly basic reaction conditions. In general, a solution of NaOMe as a solution in methanol is routinely used as a standard base to promote the desired condensation event. A typical example of this reaction is illustrated in Scheme 18.3. Here, the immobilized acetophenone (5) was treated with 12 equiv. of NaOMe (0.5 M solution in MeOH) and 12 equiv. of aromatic aldehydes at room temperature to yield enones (6) (five examples) [5]. The reaction was conducted in tetrahydrofuran (THF) to effect sufficient swelling of the polymer. Alternatively, trimethylorthoformate (TMOF), which facilitates the dehydration step, was used as the solvent instead [6]. This type of aldol condensa-


Scheme 18.3.

Aldol condensations employing Na enolates.

tion of ketones has also been carried out employing Li enolates which were generated in situ using an excess of LiOHH2 O (20 equiv.) in dioxane [7] or dimethoxyethane (DME) [8] as solvent. In conclusion, reliable aldol protocols for the solid-phase synthesis of a,b-unsaturated ketones are available; however, these processes are primarily restricted to the condensation of methyl-arylketones with aromatic aldehydes, thus limiting the options for diversification in library synthesis. 18.2.3

Boron Enolates in Aldol Reactions

The introduction of boron enolates has provided a significant impetus in aldol chemistry, especially with regard to stereoselective protocols. Therefore, the adaptation of solution-phase boron aldol strategies to solid-phase chemistry has been an intense field of investigation. Gennari et al. reported an enantioselective aldol addition on solid support mediated by a chiral boron enolate (Scheme 18.4) [9]. Accordingly, 4-(hydroxymethyl)benzaldehyde was bound to trityl resin and the resulting polymer (7) was treated three times with an excess (@ 3 equiv.) of chiral boron enolate (8) in dichloromethane at 78 C. The resulting product was cleaved from the resin and purified by chromatography to yield the corresponding b-hydroxythiolester (9) in 60% yield with 88% ee.

Scheme 18.4. Enantioselective aldol reactions using chiral B enolates.

Paterson and coworkers demonstrated a powerful methodology that permits the construction of polyketide libraries employing stereoselective propionate additions to aldehydes (Scheme 18.5) [9, 10]. Thus, aldehyde 10 was reacted twice with an excess of preformed (E )-propionate (11) in an argon atmosphere at low temperature to yield the corresponding aldol products on the polymer (Scheme 18.5, path

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A). The resulting target molecules were cleaved from the resin using HF-pyridine to yield the anti-configurated diols (12) after flash chromatography with high levels of diastereoselectivity (five examples: 90–97% de) and good yields. Complementary to this, the syn-configurated product (14) could be accessed using the (Z)-enolate (13) (Scheme 18.5. path B). To achieve complete conversion in this reaction, aldehyde 10 was successively incubated three times, each time with an excess of enolate 13. The aldol product was cleaved from the resin and the resulting diol was isolated after chromatography with 99% yield with 95% de in favor of the syn configuration.

Scheme 18.5. Stereoselective synthesis of anti and syn aldols using chiral B enolates.

Eventually, this concept of stereoregulated aldol chemistry was extended to the construction of more complex polyketide sequences on solid support [10]. Evans oxazolidinone presumably represents the most prominent chiral auxiliary in aldol chemistry. Accordingly, two strategies have been described to transfer this concept to solid-phase chemistry. In the initial report by Reggelin and Brennig, aldehyde 16, immobilized on Wang resin, was reacted with preformed boron enolate (15) (Scheme 18.6, path A) [11]. The aldol addition proceeded as expected and delivered the syn product 17 after cleavage from the resin (BCl3 ) in 74% de (the authors could demonstrate that slight epimerization occurred under the harsh cleavage conditions; no yield was reported). However, further manipulations of the resin-bound aldol product (e.g. OH protection) proved difficult. These problems were attributed to the Wang linker present in 16. Accordingly, a more flexible Si-based linker was introduced for this reaction (Scheme 18.6, path B) [12]. With 10 in place, the aldol reaction as well as further manipulations proceeded without complications, even though lactone 18, which


Scheme 18.6. Stereoselective addition of chiral B enolates to resin-bound aldehydes.

formed upon cleavage from the resin was obtained in poor yield. Moreover, isolation of the final product required chromatography since the crude reaction mixture was contaminated by oxazolidinone (19). Optimal results for the solid-supported Evans aldol protocol were elaborated for soluble polyethylene glycol (PEG)-bound aldehyde 20. This scenario comes closest to solution-like conditions having a favorable impact on reaction kinetics (Scheme 18.6, path C) [13]. The polymeric aldol adducts were precipitated from solution to obtain 21 (six examples, 56–88% yield) and 1 H-NMR measurements revealed that each product was formed as a pure stereoisomer. The three examples depicted in Scheme 18.6 clearly demonstrate that the choice of the polymeric linker plays a crucial role in this solid-supported aldol process. Furthermore, the scope of these reactions is limited to aldehydes bearing additional functional groups which allow the electrophile to be anchored to the polymer. After cleavage, these functional groups reside as integral parts in the target molecules. An alternative strategy is based on resin-bound oxazolidinone 22, which was synthesized on polymer support using l-tyrosine as the source of chirality [14]. As depicted in Scheme 18.7 the (Z )-configurated boron enolate was generated by reaction of 22 with an access of Bu2 BOTf in the presence of a tertiary amine such as triethylamine using methylene chloride as the solvent of choice. The excess of Bu2 BOTf had to be drained away prior to the addition of the aldehyde since deleterious effects with respect to the stereochemical outcome were reported if additional Bu2 BOTf was present in the reaction mixture [15]. After completion of the aldol reaction, the resin was subjected to an aqueous LiOH solution

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Scheme 18.7. Stereoselective addition of polymer-bound chiral B enolate 22 to benzaldehyde.

in THF to effect cleavage of carboxylic acid 23. Alternatively, NaOMe was used to form the corresponding methyl ester instead. Finally, product 23 was obtained as the optically pure syn diastereomer with a chemical purity of 94%, as concluded from high-performance liquid chromatography (HPLC). In summary, a number of solution-phase protocols for the addition of boron enolates to aldehydes have been successfully adapted to solid-phase synthesis. These reactions proceed with similar efficiency to the well-established solutionphase reactions. However, a relatively small number of examples have been reported to date, rendering an assessment of the scope of solid-supported boron aldol reactions exceedingly difficult. Moreover, product isolation is routinely achieved by aqueous work-up in combination with chromatographic methods. This aspect could certainly hamper the application of such a protocol for automated synthesis and the construction of multicomponent libraries still remains a challenging goal. 18.2.4

The Mukaiyama Aldol Reaction

The Mukaiyama protocol describes the Lewis acid-mediated reaction between an aldehyde and a silyl enol ether or ketene silyl acetal to yield aldol adducts. Much effort has been expended in adapting this reaction to the needs of combinatorial chemistry. The research activities in this area can be divided into two major categories: (1) the solution-phase approach employing immobilized catalysts and (2) the solid-phase strategy comprising soluble catalysts and polymer-bound aldehydes and silyl enol ethers. 18.2.4.1 Solution-phase Protocols Using Polymer-bound Reagents

As outlined in Table 18.1, a variety of polymeric catalysts have been reported to effect the Mukaiyama aldol reaction in solution. For instance, immobilized trityl cation 24 can function as a Lewis acid to activate dimethylacetals or benzaldehyde toward addition of a range of silyl enol ethers (Table 18.1, entries 1 and 2). The corresponding aldol products were isolated after aqueous work-up and preparative TLC in good yields.

Yb-Amberlyst 15

Al-Montmorillonite

Montmorillonite K10

CH2 O

R3aCHO R3 ¼ aromatic and aliphatic


3

4

5

Catalyst

24

R4 ¼ aromatic and aliphatic

Electrophile

PhaCHO

Silyl enol ether

2

1

Entry

Tab. 18.1. Polymer-supported reagents promoting the Mukaiyama reaction.

Product

15 examples; 62–87% yield; reactions carried out in water; all products chromatographed

11 examples; 68–97% yield; products isolated by filtration and Kugelrohr destillation

Reaction carried out in THF/H2 O 4:1; 82% yield; product chromatographed

19

18

17

16

16

5 examples; 72–95% yield; all products purified by TLC

6 examples; 72–96% yield; products isolated by aqueous work-up and preparative TLC

Reference

Conditions and comments

18.2 Aldol Reactions 499

22

23

13 examples; 70–90% yield; sonication necessary; reagents used neat; all products chromatographed 19 examples; 70–90% yield; sonication essential; products isolated by aqueous work-up and chromatography 1 example; reaction in water; 98% yield

10 examples; CH2 Cl2 at 78 C; 8–85% yield; pure products obtained by filtration

Neutral Al2 O3

Al2 O3 aZnCl2


R4aCHO R4 ¼ aromatic and aliphatic PhaCHO

R3aCHO R3 ¼ aromatic

8

9

10

21

20

19

7

5 examples; 52–89% yield; water as solvent

Montmorillonite K10

Reference


Conditions and comments

6

Product

Catalyst

Silyl enol ether

Electrophile

Entry


500


1 example stoichiometric amount of BH3 was used; at 78 C: 28% yield; 90% ee; at 10 C: 70% yield; 69% ee; best solvent: THF; product chromatographed 1 example 32% conversion after 36 h, 26% ee; 2 mol% polymer used; catalyst removed by precipitation from pentane product obtained by filtration

1 example; 5 mol% catalyst in DMF at rt; 94% yield; 74% ee

PhaCHO

PhaCHO

PhaCHO

11

12

14

26

25

24

18.2 Aldol Reactions 501

502


Commercially available ion-exchange resins such as amberlysts or montmorillonite were shown to promote the Mukaiyama reaction in aqueous solvent mixtures (Table 18.1, entries 3–6). The products were obtained after filtration and distillation or chromatography. Neutral Al2 O3 or ZnCl2 -doped Al2 O3 (Table 18.1, entries 7 and 8) have also been described as catalysts for the aldol addition, however, tedious reaction conditions, including sonication, render these methods less attractive for automation and library synthesis. A polymeric Sc(OTf )3 (25) was introduced by Kobayashi et al. [27] which cleanly effected the synthesis of aldol adducts, especially in aqueous solution. A major advantage of polymer-supported reagents consists in simplification of the work-up procedure. Under optimal conditions, the product isolation is reduced to a filtration and washing process, thus avoiding tedious chromatography. In this regard, Ley and coworkers reported the formation of a,b-unsaturated ketones via a Mukaiyama protocol (Table 18.1, entry 10). Accordingly, the reaction of aromatic aldehydes and silyl enol ethers was catalyzed by immobilized trimethylsilyltriflate (TMSOTf ) (26) and the desired aldol products were isolated in excellent yields with high purities by mere filtration. Finally, enantioselective protocols were elaborated for the Mukaiyama reaction (Table 18.1, entries 11–14). The authors describe the synthesis of polymeric Lewis acids and their application in asymmetric aldol reactions. In general, only benzaldehyde was used as a model system for these studies and moderate levels of enantioselectivities with variable yields were observed. 18.2.4.2 Solid-phase Protocols

Kobayashi et al. [27] reported the synthesis of immobilized thioketene silyl acetals and their use in Mukaiyama reactions mediated by rare earth metal salts. Thus, silylated propionate 27 was reacted with benzaldehyde in the presence of 20 mol% Sc(OTf )3 to effect conversion to the corresponding aldol adduct (Scheme 18.8) [27]. The latter was cleaved from the resin upon reduction with LiBH4 to yield diol 28, which was isolated after aqueous work-up and preparative TLC in 82% yield. Moreover, heteroaromatic and aliphatic aldehydes as well as dimethylacetals work equally well as substrates for this sequence (in total, 12 examples). However, if unsubstituted trimethylsilyl ketene acetals derived from acetic acid were employed, the aldol adducts were isolated in poor yields. Better results were obtained if the TMS group was replaced by t-butyldimethylsilyl (TBDMS). Accordingly, TBDMS ketene silyl acetal (29) reacted smoothly with aldehyde 30 under BF3 catalysis (Scheme 18.9) [28]. The resulting aldol intermediate was subjected to a tetrabutyl-

Scheme 18.8. Mukaiyama reactions on solid support using Sc(Otf )3 as catalyst.

18.3 1,4-Addition of Enolates to Michael Acceptors

ammonium-fluoride (TBAF) solution in THF buffered by acetic acid to effect removal of the TBDMS group. The deprotected alcohol induced cyclative cleavage to give lactone 31, which served as a precursor for the synthesis of 2-deoxy-l-glucose.

Scheme 18.9. Synthesis of 2-deoxy-l-glucose precursor using a solid-phase Mukaiyama reaction.

The Mukaiyama protocol has also been successfully conducted with immobilized aldehydes. As depicted in Scheme 18.10, cyclic TMS enolether 33 was reacted with aldehyde 32 mediated by Yb(OTf )3 to yield a,b-unsaturated ketone 34 after cleavage in 91% yield [29]. The salient features of this process include the use of a MeCN/H2 O mixture as the solvent for this reaction. Since conventional polystyrene resins were not compatible with these hydrophilic conditions, a resin based on a polar polyoxyethylene/polyoxypropylene matrix was introduced for this reaction.

Scheme 18.10. Solid-phase Mukaiyama reactions using immobilized aldehydes.

In summary, a broad range of liquid- and solution-phase strategies are available for the Mukaiyama aldol reaction in a parallel fashion. However, the limited commercial availability of silyl enol ethers or ketene silyl acetals should be taken into account for library design. Additionally, for both solution- and solid-phase strategies, it has to be evaluated whether the work-up and purification protocols suit the individual requirements for automation and library synthesis.

18.3

1,4-Addition of Enolates to Michael Acceptors

The 1,4-addition of enolates to Michael acceptors can be effected by base catalysis. Consequently, method development for solution-phase parallel synthesis focused on the use of immobilized bases to promote the addition step and simplify catalyst separation and product isolation. As early as 1958, Bergmann and Corett reported the use of basic exchange resins as catalysts for the Michael addition of enolates to

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a,b-unsaturated carbonyls and acrylonitriles (Scheme 18.11) [30]. The reaction conditions were elaborated for a variety of Michael donors such as ketones and a-branched aldehydes (15 examples in total). To date, a range of novel polymeric catalysts has been introduced. Of particular interest are immobilized quaternary ammonium salts that proved to be effective for the addition of soft enolate nucleophiles to Michael acceptors [31]. Along these lines, asymmetric Michael additions have been reported based on immobilized optically active alkaloids [32]. However, applications of these methods for the synthesis of versatile libraries in solution phase have not yet been demonstrated.

Scheme 18.11. Enolate additions to Michael acceptors mediated by a basic exchange resin.

More recently, Ley and Massi reported a tandem Michael protocol for solutionphase combinatorial synthesis leading to the bicyclo[2.2.2]octanone skeleton 40 that was further elaborated in subsequent amination reactions [33]. The bicyclic scaffold was constructed upon reaction of 2 equiv. of conjugated dianions (39) with tert-butyl acrylates (38). As outlined in Scheme 18.12, the reaction mixture was quenched with Amberlyst 15 (path A) and a simple filtration process provided pure endo products (40) (with R2 ¼ OEt or OiBu), whereas the crude product (with R2 ¼ methyl) was contaminated by hydrolyzed 3-methyl-2-cyclohexen-1-one. Therefore, this mixture was treated with polymer-supported thiophenol and diisopropylethylamine (DIPEA) to scavenge the excess of hydrolyzed starting material (39c) followed by neutralization of ammonium salts using MP carbonate (path B). After filtration the desired products (40) were finally isolated in pure form. These carefully orchestrated reaction and work-up conditions formed the basis for a rapid

Scheme 18.12.

Tandem Michael reactions using polymer-supported reagents.

18.3 1,4-Addition of Enolates to Michael Acceptors

synthesis of a larger library, since the reaction protocol can easily be automated and avoids tedious chromatography. The synthesis of the bicyclo[2.2.2]octanone framework via a tandem Michael reaction has also been adapted to solid phase [34]. The 1,4-addition of enolates to Michael acceptors in solid-phase chemistry has been examined by Domıńguez et al. [35]. These authors investigated the addition of resin-bound glycinates (41) to various a,b-unsaturated acceptors (Scheme 18.13). The standard conditions for typical Michael acceptors such as acrylates, include 3 equiv. of Schwesinger-type base BEMP and 5 equiv. of acceptor 42 in NMP at room temperature (rt) for 16 h to yield the Michael adducts 43 in high purity. For more reactive acceptors (e.g. acrylonitriles) only 1.5 equiv. of 42 were used to avoid double addition, while less reactive electrophiles 42 (e.g. substituted acrylates) required additional base and acceptor 42 (each 10 equiv.). In total, 15 examples are given composing a diverse set of enolate additions to a,b-unsaturated Michael acceptors. Additionally, the resin-bound intermediates 42 were further modified on the polymer, including deprotection of the imine, acylation of the amino group with quinaldic acid, and cleavage using trifluoroacetic acid (TFA). The final products were obtained in good yields (61–88%) and purities (74–87%), indicating that the Michael addition as depicted in Scheme 18.13 proceeded on the resin without incident and with high efficiency.

Scheme 18.13. 1,4-Addition of resin-bound glycinates.

Enantioselective Michael additions on solid phase utilizing immobilized Evans oxazolidinone has also been reported; however, only one model system was described, lacking options for diversity in library synthesis [14b]. In contrast, the addition of silyl enol ethers and silyl ketene acetals to a,bunsaturated carbonyls has attracted more attention in solid-phase chemistry. As depicted in Scheme 18.14, solid-supported thiosilyl ketene acetal 44 reacted smoothly with a,b-unsaturated ketones in the presence of 20 mol% Sc(OTf )3 to yield the corresponding adducts on solid phase. The products were cleaved by the

Scheme 18.14. 1,4-Additions of silyl ketene acetals on solid phase.

505

506


action of NaOMe and were subjected to chromatography to yield 1,5-dicarbonyls (46) in good to excellent yields (48–91%, nine examples) [17a]. An alternative approach was based on resin-bound Michael acceptors which were reacted with soluble silyl enol ethers. This concept was realized by Ellingboe and coworkers, who reported the synthesis of immobilized 1,5-diketones employing polymeric enone 47 and 4 equiv. of TMS enol ether (48). The Michael addition was effected by CsF in dimethyl sulfoxide (DMSO) to yield 49. The best results were obtained if R1 was an aromatic or nonenolizable aliphatic residue, while R2 should preferably be aromatic or heteroaromatic. The target molecules 49 represented important intermediates for the construction of a pyridine library (Scheme 18.15) [36].

Scheme 18.15. Michael addition of silyl enol ether to polymer-

supported a,b-unsaturated ketones.

In summary, a diverse repertoire of combinatorial methods for Michael additions is available. The application of these methods to library synthesis has demonstrated that 1,4-additions of enolates are useful transformations for creating diversity with concomitant formation of CaC bonds.

18.4

Alkylation of Enolates

The alkylation of CaH acidic carbonyls represents a valuable transformation to gain an additional diversity step for a given library. Thus, extensive efforts have been devoted to optimizing alkylation protocols especially for solid-phase synthesis. 18.4.1

a-Alkylation of Carbonyl Compounds

The alkylation of ketone-derived enolates has been realized in solution-phase chemistry employing solid-supported reagents. In this regard, Montanari and coworkers reported the reaction of benzylbromide with benzylmethylketone using an immobilized quaternary phosphonium salt as a phase transfer catalyst [37]. The product could be obtained in excellent yield (95%) after filtration and extraction. However, the scope of this reaction has not yet been extended to additional examples or library synthesis. Silyl enol ethers have also been described to function as suitable substrates for the alkylation process. Accordingly, 56 was reacted with prenylbromide (51) in the

18.4 Alkylation of Enolates

presence of ZnCl2 -doped acidic alumina to give ketone 52 in 72% yield (Scheme 18.16) [38]. This reaction worked well with allylic, benzylic, and tertiary halides, leading to the alkylated products after a simple extraction step with CH2 Cl2 in good yields (60–72%).

Scheme 18.16.

Alkylation of silyl enol ethers using ZnCl2 -doped alumina.

The elegant studies of Ellman and coworkers toward the synthesis of arylacetic acid libraries represented classical solid-phase synthesis [39, 40]. This concept called for a reliable enolate-alkylation protocol that proved to be feasible for the construction of a diverse set of target molecules (Scheme 18.17). Accordingly, Kenner et al.’s sulfonamide linker [41] was chosen to bind the aryl acetic acid precursor to solid phase to yield polymer 53. The resin was subsequently treated with 15 equiv. of LDA to generate the corresponding trianion, which was trapped by quenching with an excess of alkylhalide to selectively yield the monoalkylated product 54. This alkylation reaction works equally well for a range of halides, including benzylic and aliphatic halides (five examples). The intermediates 54 were further manipulated to give a diverse set of arylacetic acids which were isolated in excellent yields (88–100%) and high purities after cleavage. From these results, it could be concluded that the alkylation event further upstream in this sequence proceeded smoothly and with high efficiency.

Scheme 18.17. a-Alkylation on solid support using Kenner’s sulfonamide linker.

The alkylation of amide and ester enolates derived from deprotonation of the corresponding a-acidic precursor has been reported in solid-phase synthesis using LiHMDS [42] and KHMDS [43], respectively. 18.4.2

a- and g-Alkylation of 1,3-Dicarbonyl Compounds

The 1,3-dicarbonyl moiety represents a versatile template for the synthesis of heterocycle libraries, especially if alkylation protocols can provide an additional an-

507

508


chor for diversity. Accordingly, the development of methods which selectively target the mono- or dialkylation of a,b-dicarbonyls has attracted recent attention. In this context, a variety of solution-phase strategies has been elaborated. Most strategies are based on the use of polymer-supported bases such as Amberlite anion-exchange resins [44] or immobilized fluoride sources [45]. A typical example is illustrated in Scheme 18.18 [46]. Celite coated with KF cleanly effected the monoalkylation of acetylacetone to yield 56 in 96% yield. In contrast, the system Al2 O3 aKF predominantly delivered the corresponding disubstituted product under otherwise identical conditions.

Scheme 18.18.

Alkylation of b-dicarbonyls using immobilized KF.

In general, the scope of the solution-phase strategies is limited to a small number of examples, and to the best of our knowledge no application in library synthesis has been published to date. In contrast, methods that were developed for the alkylation of 1,3-dicarbonyls on solid support have had a significant impact on library synthesis. The optimal conditions for the a-alkylation of b-diketones and b-ketoesters include the use of excess TBAF and alkyl halide in THF at rt (Table 18.2, entries 1 and 2). Under these conditions, the oxygen atoms in 57 and 58 are shielded by complexation with the sterically demanding tetrabutylammonium counterion, thereby preventing O-alkylations with concomitant nucleophilic activation toward a-alkylation. Importantly, this protocol required strictly anhydrous conditions, otherwise the yields were poor. This methodology has successfully been applied in the synthesis of pyrazole and isoxazole libraries. However, a major drawback arises from the incompatibility of this process with the presence of N-heteroaryl or furylfunctionalities, and, furthermore, the use of benzylbromide as alkylating agent gave only sluggish results. As an alternative to TBAF, other bases such as 1,8diazabicyclo[5.4.0]undecene-7 (DBU) were used to effect the desired a-alkylation of CaH acidic b-dicarbonyl compounds (Table 18.2, entry 3). The latter example also illustrates that electrophiles bound to solid phase can be efficiently employed in these alkylation reactions. Similar processes involved the use of Merrifield resin or the more reactive (bromo-methyl)-polystyrene/divinylbenzene (PS/DVB) resin as polymeric electrophile to covalently bind 1,3-dicarbonyls. The requisite enolates were generated in solution by utilizing NaH [51], Na in ethanol [52], or KOH under phase transfer conditions as base [53]. Contributions in this field mainly focused on the development of new linkers and will not be discussed in detail. A slightly different concept relies on the well-established Pd-catalyzed allylic alkylation of CaH acidic 1,3-dicarbonyls with allylic acetates and carbonates. This

Z ¼ electron-withdrawing group.

50

3 examples 10% DBU/DMF 20 C, 16 h

3

48 49

5 examples conditions incompatible with R ¼ N-heteroaryl or furyl

47

Reference

10 equiv. TBAF 44 equiv. R 0 X THF, 2 h, rt

6 examples no g- or Oalkylation observed

Comment

R 0 ¼ allyl, Et, CH2 Z R 0 0 benzyl X ¼ Br, I

Conditions

2

Product 20 equiv. TBAF 36 equiv. R 0 X THF, 3 h, rt

R 0 X/electrophile R 0 ¼ aliphatic X ¼ Br, I

b-Dicarbonyl

Alkylation of resin-bound b-dicarbonyls.

1

Entry

Tab. 18.2.

18.4 Alkylation of Enolates 509

510


chemistry has been adapted to solid phase and it has been shown that resin-bound malonates could be cleanly a-dialkylated by this method [54]. An important modification of 1,3-dicarbonyls is the g-alkylation. This has been accomplished on solid phase commencing from resin-bound b-ketoester (63), which was deprotonated with an excess of LDA to generate the corresponding dienolate (Scheme 18.19) [55].

Scheme 18.19. g-Alkylation of resin-bound b-ketoesters.

Subsequently, the excess LDA was drained away and 2–5 equiv. of alkyl halide (R1X) were added to effect mono-g-alkylation. The resulting monoenolate could serve as starting material for a second alkylation step. Thus, treatment of the enolate intermediate with n-BuLi regenerated the dianion which was quenched with R2X to yield dialkylated products (64) in good yields. The g-alkylation worked well with aliphatic, benzylic, and allylic halides (X ¼ Br or I; nine examples). 18.4.3

Stereoselective Alkylations of Enolates

The diastereoselective alkylation of enolates controlled by chiral auxiliaries is one of the most reliable processes in asymmetric solution-phase chemistry. The transfer of this concept to solid phase would offer manipulative convenience since excess reagents could be removed by simple washing procedures, and, finally, the auxiliary could be recovered by filtration. Table 18.3 summarizes the progress in this field that has been achieved to date. Leznoff and coworkers [56, 57] reported the application of immobilized amino alcohols as chiral auxiliaries to effect stereoselective alkylation of ketones (Table 18.3, entry 1). The levels of asymmetric induction were found to be in the same range as in solution. The stereoselective alkylation of resin-bound amide 67 was described by Kurth and coworkers (Table 18.3, entry 2) [58, 59]. The alkylation occurred with good diastereomeric excess and cleavage from the resin was accomplished by an iodo-lactonization protocol. The adoption of the Meyers oxazoline methodology to solid phase (Table 18.3, entry 3) was accompanied by drawbacks, mainly arising from low chemical yields due to incomplete hydrolysis of the final product. Allin [61] and Burgess [62] independently reported their endeavors toward a solid-phase version of an Evans oxazolidinone auxiliary (Table 18.3, entry 4). Both groups could demonstrate that strong electrophilic halides react readily with pregenerated oxazolidinone enolates to yield the corresponding alkylated products with medium to high levels of optical purity. Cleavage from the resin was accomplished by hy-

60

1 example; 56% ee; 43–45% yield

4 examples; 76–96% ee; 20–66% yield

1. n-BuLi THF 2. 650 equiv. BnCl 3. Cleavage (H2 SO4 , EtOH)

1. 2–3 equiv. LDA, THF, 0 C 2. 2–5 equiv. BnBr 3. LiOH or LiBH4 (cleavage)

BnCl

BnBr BnOCH2 Cl allyl-Br

3

4

61 62

58 59

2 examples; >87% de

1. 2 equiv. LDA, 0 C 2. 3 equiv. R 0 X, 0 C ! rt 24 h

Mel, allyl-l

56 57

Reference

2

Remarks

Conditions 2 examples; 60–95% ee; 80–87% yield

Product 1. LDA, THF, 0 C 2. R 0 X, rt 3. Cleavage (Hþ )

R 0X Mel, iPrl

Resin

1

Entry

Tab. 18.3. Stereoselective enolate alkylations on solid phase.

18.4 Alkylation of Enolates 511

512


drolysis (LiOH) to liberate the carboxylic acid or, alternatively, by reduction (LiBH4 ) leading to the corresponding alcohols. For all examples depicted in Table 18.3, convenient recycling of the chiral auxiliary has been demonstrated and the polymers could be reused for further applications. However, the majority of protocols required rather drastic reaction conditions for the cleavage of the final product, which implies tedious work-up and purification procedures rendering this chemistry less amenable for automation. Diastereoselective alkylations have also been successfully applied in the synthesis of more complex molecules on solid support. For instance, Hanessian et al. showed that polymeric ester 73 could be deprotonated by the action of KHMDS followed by the addition of reactive electrophiles such as allylic iodides (three examples) to yield the corresponding a-alkylated product on solid support (Scheme 18.20) [63].

Scheme 18.20. Diastereoselective alkylation of esters on solid support.

This resin was treated with TFA to effect cleavage and lactonization leading to enantiopure lactones (74) which were isolated in 75% yield (using two steps). The stereochemical outcome of this reaction was rationalized via a Felkin-type transition state model. In Chen and Janda’s elegant studies toward a resin-supported synthesis of prostaglandin F2a , the a-alkylation of the central cyclopentanone core represented a key step [64]. As depicted in Scheme 18.21, the requisite enolate was generated by lithiation of the TMS enol ether (75) using excess methyl lithium. Upon addition of triflate 76 the desired product 77 was isolated in pure form. For these studies a soluble noncrosslinked polystyrene was selected as polymer support, since this choice guaranteed solution-like kinetics for each step of this sequence. Additionally, the soluble

Scheme 18.21. Enolate alkylation as a key step in solid-phase synthesis of prostaglandin F2a .

18.4 Alkylation of Enolates

polymer allowed the monitoring of reaction intermediates by nuclear magnetic resonance (NMR). 18.4.4

Alkylation of Protected Glycines

Suitably protected glycine esters are readily alkylated to yield protected a-amino acids. Accordingly, this route offers convenient access to tailor-made unnatural amino acids which are valuable templates in organic synthesis. Benzophenone imines derived from glycine esters serve as the starting materials of choice for the alkylation of glycine. This is the result of the convenient protocols that are available for imine formation as well as the high selectivity of these imines toward monoalkylation. Polymer-supported reagents have been developed to produce the abovementioned transformation. For instance, Palacios et al. reported the use of immobilized P-ylides as strong non-nucleophilic bases [65]. Moreover, a stereoselective alkylation procedure has been elaborated previously [66]. As depicted in Scheme 18.22, protected glycine 78 was reacted with an excess of reactive halides, such as benzyl or allylic bromide under phase transfer conditions (seven examples). As chiral catalyst, the immobilized tertiary ammonium salt 80 was used, and for a range of products medium to good levels of induction were observed. The products 79 were isolated after filtration and extraction into the organic layer without the need for chromatographic purification.

Scheme 18.22. Stereoselective alkylation of glycine imine 78.

The alkylation of glycine Schiff bases has also been established in solid-phase chemistry (Table 18.4). The most frequently reported conditions include the use of Schwesinger-type bases in combination with an excess of alkyl halides in polar solvents. For instance, imine 81 immobilized on Wang resin was treated with BEMP and alkyl halide to selectively yield the monoalkylated products 82 (Table 18.4, entry 1). These products were further modified on solid support by cleavage of the benzophenone imine followed by acylation of the amino group. Finally, the resulting peptides were cleaved from the resin with TFA to deliver the corre-

513

2 equiv. BEMP 3 equiv. R 0 X

1. 5 equiv. BEMP, 5 equiv. R 0 X, DCM 1 equiv.

R 0 ¼ benzylic, allylic X ¼ Br

R 0 ¼ benzyl, allyl, aliphatic X ¼ Br, I

3

4

2. NH2 OH 3. TFA

MeCN, excess K2 CO3 1.5 equiv. R 0 X, reflux

R 0 ¼ benzyl, allyl, aliphatic X ¼ Br

Conditions

2

Product 2–10 equiv. R 0 X 20 equiv. BEMP NMP, rt, 24 h

R 0X R 0 ¼ aliphtic, benzylic X ¼ Br, I

Polymer

Alkylation of glycine derivatives on solid support.

1

Entry

Tab. 18.4.

17 examples; 61–100% yield; 51– 89% ee

36 examples; for R ¼ iPr no complete reaction

5 examples; products obtained by precipitation from Et2 O

10 examples; exclusively monoalkylation

Remarks

71

70

69

67 68

Reference

514


18.5 Claisen-type Condensations

sponding products in excellent yields (77–100%). These results indicate that the alkylation event at the outset of the sequence must have proceeded with high efficiency. A related process was described for PEG-bound glycine 83 (Table 18.4, entry 2). The authors showed that the PEG polymer not only simplified product isolation, but also accelerated the reaction significantly. This catalytic effect was attributed to the close relationship of the PEG polymers with crown ethers, which are well known to be beneficial in phase transfer reactions. Dialkylated amino acids were accessed starting from natural a-amino acids which were transformed to aldimines of type 85, and subsequent alkylation with halides (Table 18.4, entry 3) led to the expected products 86. This concept has been validated for a broad range of amino acids (36 examples). Finally, a stereoselective alkylation of protected glycine on polymer support has been reported by O’Donnell et al. (Table 18.4, entry 4) [71]. Optimal results were obtained if Corey’s quaternary ammonium catalyst 89 was used as the source of chirality. The immobilized intermediates were deprotected (NH4 OH) and cleaved from the resin (TFA) to yield the free amino acids (88) in good chemical and optical purity.

18.5

Claisen-type Condensations

The reactions between enolates and esters deliver 1,3-dicarbonyl compounds which are valuable intermediates in heterocyclic chemistry. An early study by Cohen et al. demonstrated that this kind of transformation could be realized utilizing polymer reagents. Accordingly, benzophenone was deprotonated with immobilized trityl lithium (91) and the resulting enolate was trapped with polymeric o-nitrophenyl benzoate (90) to yield b-diketone (92) with a 96% yield after filtration (Scheme 18.23) [72]. The yield by far exceeded the results obtained for the corresponding reaction in solution phase (a 50%). This result is due to the fact that immobilized trityl lithium could be used in great excess to drive the enolate addition to completion. More importantly, the presence of the alkyl lithium base proved to be compatible with the o-nitrophenyl benzoate (90) since both reactive sites were shielded by a polymeric matrix, thus avoiding deleterious interactions. This ‘‘wolf– lamb’’ strategy has clearly shown that polymer reagents not only simplify work-up but can also provide an innovative effect in optimizing reaction conditions.

Scheme 18.23. ‘‘Wolf–lamb’’ reaction for the synthesis of b-ketoesters.

515

516


The synthesis of b-diketones has also been reported in solid-phase chemistry. In a standard protocol NaH is used as the base of choice to generate the requisite ketone enolate, which reacts with an excess of ester to yield the desired products. A typical example is illustrated in Scheme 18.24. Here, methylketone (93) was attached to the polymer by a Rink amide linker which proved stable toward the strongly basic reaction conditions. Upon reaction with aromatic esters in the presence of NaH, the b-diketones (95) were obtained in excellent yields [48, 49, 73]. However, the scope of the reaction was limited to a narrow set of aromatic and heteroaromatic esters.

Scheme 18.24. 1,3-b-Diketone synthesis on solid support.

A more recent contribution in this field reported a novel selenium linker which was used to immobilize methylketone (96). Enolate generation was effected by LiHMDS followed by addition of acylcyanides as electrophiles to produce 1,3diketones (97) in good yields (Scheme 18.25) [74]. Alternatively, the use of Weinreb amides [75] or b-ketoester [76] as electrophiles in this Claisen-type reaction have been described.

Scheme 18.25. Solid-phase synthesis of 1,3-diketones using acylcyanides.

In conclusion, the construction of b-dicarbonyls, especially in solid-phase synthesis, is a well-established process. However, the scope of this reaction is primarily restricted to nonenolizable electrophiles such as aromatic esters, acylcyanides, or Weinreb amides.

18.6

Dieckmann Condensations

The Dieckmann reaction as an intramolecular version of the Claisen condensation offers a valuable means of construction of ring systems with the concomitant for-

18.6 Dieckmann Condensations

mation of a CaC bond. If nonsymmetric dicarbonyl precursors are used in this reaction, fine-tuning of the conditions is often needed to selectively obtain the desired product. For instance, amino acid derivatives (98) have been used in solutionphase chemistry as open-chain cyclization precursors. Chemoselective deprotonation of the CaH acidic amide was achieved by preferably allowing electron withdrawing groups for R3 (Scheme 18.26) [77, 78]. Amberlyst A26 (OH form) was selected as base for this reaction and the resulting tetramic acids (99) were first obtained as salts immobilized on the basic exchange resin. Subsequently, the resin was washed to remove byproducts and excess reagents followed by incubation with TFA to liberate the final products (99) as free acids. In total, this concept of solid-phase resin capture delivered a range of tetramic acids in good yields and high purities (ten examples), including a convenient work-up protocol which should be suitable for automation.

Scheme 18.26. Parallel synthesis of tetramic acids in solution phase.

Following the seminal contributions of Rapoport in the field of solid-phasesupported unidirectional Dieckmann reactions [79], the vast majority of more recent communications have dealt with the solid-phase synthesis of tetramic acids. Accordingly, OH-functionalized polymers such as Wang resin loaded with amino acid derivatives were exposed to basic reaction conditions, as outlined in Scheme 18.27. A variety of bases were employed for this reaction, including NaOEt [80], KOH [81], DIPEA [82], or Bu 4 NOH [83]. The cyclization occurred as expected with concomitant release of the anionic form of the tetramic acid into solution. The free acid (101) was finally isolated by treatment with a strong acid or acidic ionexchange resin. More importantly, the cyclative cleavage strategy employed in these examples guaranteed high purities for the products since the latter were only released into solution if the desired cyclization event took place first.

Scheme 18.27. Solid-phase synthesis of tetramic acids.

517

518


Owing to the convenient reaction protocols both the liquid- and solid-phase synthesis of tetramic acids appears to be an attractive tool for multicomponent library synthesis.

18.7

Knoevenagel Condensations

The condensation of aldehydes with carbonyl compounds bearing an additional electron withdrawing group (e.g. b-ketoesters or malonates) is usually designated as a Knoevenagel condensation. The resulting alkylidene products serve as ideal precursors for the synthesis of a range of heterocycles such as dihydropyrimidines, pyrimidines, pyrimidinones, dihydropyridines, pyrazoles, dihydropyranes, or coumarines. Libraries prepared in solution phase as well as on solid phase with a broad range of templates underscore the value of this reaction for combinatorial purposes. Owing to the high acidity of the activated carbonyl moiety, weak bases such as ammonia, ammonium salts, primary or secondary amines, and salts thereof suffice to promote the condensation. Hence, polymer-supported Bro¨nsted bases have been applied in solution-phase Knoevenagel reactions. A typical example is shown in Scheme 18.28. A range of malonates, malonitriles, cyanoacetates, and oxoesters (102) were condensed with nonenolizable aromatic aldehydes in the presence of polymeric amine (103) under continuous flow conditions. The alkylidene products (104) were isolated after removal of the solvent and purification by chromatography or recrystallization in acceptable to excellent yields (29–98%) [84]. Other polymeric reagents reported for the Knoevenagel process include the use of ionexchange resins [85], immobilized tetraalkylammonium hydroxides [86], Al2 O3 [87], Al2 O3 aKF [88], or molecular sieves [89]. However, these solution-phase methods have not yet been applied in library synthesis.

Scheme 18.28. Knoevenagel condensations using polymer reagents.

In contrast, solid-phase protocols have been far more popular for Knoevenagel reactions since the condensation products are often important intermediates in multistep sequences which are best supported by solid-phase chemistry. Depending on the nature of the linker, the CaH acidity of the substrate, and the structure of the aldehyde, different conditions have been employed. Piperidine or piperidine acetate have been successfully used as catalysts for the Knoevenagel reaction and

18.7 Knoevenagel Condensations

seem to be the methods of choice. For instance, Gallop, Bhandari, and coworkers reported that malonate (105) immobilized on TentaGel resin cleanly reacts with aromatic aldehydes (eight examples) under the conditions depicted in Scheme 18.29 [90, 91]. The Knoevenagel products (106) were further transformed to pyrrolopyridines utilizing a Hantzsch protocol for construction of the pyridine heterocycle. The final products in this sequence were obtained in decent yields (20–50%) with excellent purities (> 90%), reflecting almost quantitative conversion for each step of the synthesis. Additionally, this methodology was extended to a library synthesis of 4800 compounds, thereby demonstrating the broad scope of the Knoevenagel reaction. Instead of TMOF, molecular sieves were also used to remove water from the reaction mixture [92]. Under similar reaction conditions, salicylaldehydes were effectively condensed with resin-bound malonates to yield the coumarin skeleton on solid support [93].

Scheme 18.29. Knoevenagel condensations on solid support.

If the basic piperidine is buffered by acetic acid, even milder conditions for the Knoevenagel condensation are obtained. As illustrated in Scheme 18.30, malonates (107) loaded onto Wang resin were reacted with a range of aromatic and aliphatic aldehydes to yield the corresponding Knoevenagel products on solid support. After cleavage, the carboxylic acids (108) were obtained as mixtures of E and Z isomers in good yields (45–98%) and high purities (conversion 92–98%). This protocol was further applied in a library synthesis comprising 96 products of type 108 and the majority of products were obtained with purities > 75% as determined by HPLC [94]. Moreover, this procedure was especially suited for the use of enolizable aliphatic aldehydes in the Knoevenagel condensation on solid support [95]. In this context, it was also reported that ethylenediamine diacetate (EDDA) very effectively catalyzed the condensation of both aromatic and aliphatic aldehydes with resinbound malonates [5c].

Scheme 18.30. Knoevenagel condensations on solid support.

519

520


18.8

Addition of Enolates to Imines 18.8.1

Synthesis of b-Amino Esters and Alcohols via Enolate Addition to Imines

Mannich-type reactions include the addition of enolates to imines and deliver valuable b-amino acid derivatives. Mild reaction conditions were reported employing silyl enol ether or ketene silyl acetals which add to preformed or in situ-generated aldimines in the presence of catalytic amounts of Lewis acids. In this regard, a variety of polymeric rare earth metal salts were reported to promote the aforementioned process efficiently [96]. For instance, Kobayashi et al. introduced a microencapsulated scandium triflate which exhibited high catalytic activity in the Mannich reaction [22a]. As outlined in Scheme 18.31, the imino-aldol reaction between aldimine 109 and propiophenone-derived silyl enol ether 110 proceeded smoothly to produce b-aminoketone 111 in excellent yield. Furthermore, the catalyst was recycled and reused up to seven times in the Mannich process without there being any deleterious effects on the yield. These reactions work best with aldimines originating from aromatic or heteroaromatic aldehydes.

Scheme 18.31. Mannich-type reactions promoted by microencapsulated Sc(Otf )3 .

Complementary, polymer-supported catalysts such as p-acidic dicyanoketeneacetals [97] or optically active palladium–BINAP complexes [26] have also been used to effect the addition of silyl enol ethers to imines. The use of polymeric catalysts for the Mannich reaction has dramatically simplified the isolation and recycling of the catalysts by mere filtration and washing operations, however, the isolation of the reaction products, such as 111, was routinely achieved by aqueous work-up procedures followed by chromatography. Therefore, the application of this methodology to the synthesis of libraries would require the work-up and purification protocols to be adapted to automation. An interesting multicomponent Mannich reaction has been recently reported by Prabhakaran and Iqbal (Scheme 18.32) [98]. Ketones or b-ketoesters (112) were condensed with aromatic aldehydes (113) and acetylchloride in acetonitrile in the presence of a polyaniline-supported cobalt catalyst. The resulting N-acetylb-amino acid derivatives (114) (eight examples given) were isolated by a simple work-up protocol comprising a filtration and a washing step to deliver 114 in acceptable yields (46–68%) and excellent HPLC purities (95–100%). Here, the anti-

18.8 Addition of Enolates to Imines

configurated products were formed predominantly. The mechanism of this reaction presumably involves the addition of in situ-generated cobalt enolates to a cationic iminium species [99].

Scheme 18.32. Multicomponent Mannich reactions using a polymer-supported cobalt catalyst.

Mannich reactions have also been established in solid-phase chemistry. Kobayashi et al. introduced resin-bound thiosilyl ketene acetal 115 and elaborated an efficient Mannich protocol on solid support (Scheme 18.33) [100]. Thus, reaction of 115 with imines 116 in the presence of 10 mol% Sc(OTf )3 delivered the corresponding b-amino thioesters which were subsequently cleaved from the resin upon reduction with LiBH4 to yield the b-amino alcohols 117 in good yields (42– 78%; 19 examples) after aqueous work-up and preparative TLC. A broad range of substrates were used in this reaction with R1 being hydrogen, methyl, or benzyloxy, and R2 and R3 preferably being aromatic or heteroaromatic groups. This reaction can also be conducted as a one-pot, three-component condensation starting from amine, aldehyde, and silyl enol ether comprising in situ generation of the requisite aldimine [101]. Furthermore, the scope of this methodology has been extended by using the Sc(OTf )3 -mediated Mannich reaction as a key step in the solid-phase synthesis of amino sugar derivatives [28].

Scheme 18.33. Mannich reactions on solid support to yield b-amino alcohols.

The solid-phase synthesis of novel a-amino acid derivatives (119) has also been elaborated for soluble silyl enol ethers in combination with immobilized imines [102]. A recent report described the use of polymer-supported imino acetates (118) as suitable substrates for the Mannich reaction (Scheme 18.34) [103]. Accordingly, the Sc(OTf )3 -catalyzed process delivered a set of unnatural aspartic acid derivatives (119) which were cleaved from the resin by transesterification using NaOMe. Under similar conditions polymer-supported acylhydrazones reacted with ketene silyl acetals and the corresponding adducts underwent cyclative cleavage upon treatment with NaOMe to yield substituted pyrazolones [104].

521

522


Scheme 18.34. Solid-phase synthesis of aspartic acid derivatives.

18.8.2

Solid-phase Synthesis of b-Lactams via Enolate Additions to Imines

The synthesis of b-lactams is conveniently achieved by [2 þ 2] cycloaddition of ketenes with imines [105], or, alternatively, by addition of enolates to imines with subsequent ring closure [106]. The latter strategy has been adapted to solid-phase synthesis by Enders and coworkers, who utilized the T1-triazene linker system [107]. The resin was loaded with alanine methylester to give 120 (Scheme 18.35) [108]. The precursor 120 was then transferred into its dianion using an excess of LiHMDS and subsequently treated with imines derived from aniline and aromatic aldehydes. Traceless cleavage of the resulting b-lactams was accomplished in a twostep process including formation of the diazonium salt followed by decomposition of the diazonium intermediate to yield the target molecules 121. With R ¼ aryl or heteroaryl, a range of products (eight examples) was obtained in good yields (53– 71%) with high purities (88–98%) and excellent diastereoselectivity in favor of the trans-configurated product (90 to b96% de).

Scheme 18.35. Solid-phase synthesis of b-lactams employing a traceless linker strategy.

A related process was reported involving in situ-generated Ti enolates and imines immobilized on soluble PEG resins [109].

18.9

Nitro-aldol Reactions

The nitro-aldol reaction (Henry reaction) represents a useful tool for the introduction of a nitrogen functionality onto an organic scaffold. The base-promoted addi-

18.9 Nitro-aldol Reactions

tion of CaH acidic nitroalkanes to aldehydes has been realized in solution phase by utilizing polymer-supported reagents. For instance, immobilized 1,5,7-triazabizyclo[4.4.0]dec-1-ene (TBD) has been reported to catalyze the aforementioned process [110]. The use of polymeric TBD dramatically simplified work-up, and product isolation was reduced to a simple filtration process. However, only a limited number of examples were reported that lacked data on experimental details and product purity. Other polymeric bases which catalyze the Henry reaction are polymeric ylides [65], ion-exchange resins such as Amberlyst A21 [111], or basic La 3þ polymers [112]. Again, the scope of these methods is rather limited to a small set of examples, and an assessment regarding the suitability of these reagents for library synthesis is therefore exceedingly difficult. Polymer-supported reagents were also successfully used to promote the 1,4addition of nitroalkanes to Michael acceptors. Accordingly, KF on basic alumina was found to catalyze the addition of CaH acidic nitro-compounds to a range of a,b-unsaturated ketones and esters (Scheme 18.36) [113, 114]. The reactions were driven to completion by using a large excess of the nitroalkane. Finally, the adducts 124 were isolated after filtration and evaporation of the excess nitroalkane in good yields (39–100% for 14 examples). The best results were obtained if the Michael acceptors were unsubstituted in the b-position.

Scheme 18.36. Michael addition of nitroalkanes mediated by KF on alumina.

Other groups introduced basic alumina [115] or Amberlyst ion-exchange resins [116] as catalysts for the Michael addition of nitroalkanes. In solid-phase chemistry the addition of nitromethane to immobilized aromatic aldehydes belongs to a standard procedure. A typical example of the Henry reaction on solid support is illustrated in Scheme 18.37 [117]. The isoxazole aldehyde 125 loaded onto the polystyrene resin via a Cl-trityl linker was treated with a large excess of nitromethane and triethylamine in a THF/EtOH solvent mixture. The addition proceeded at rt within 3 h

Scheme 18.37. Henry reaction on solid support.

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and the resulting Henry adduct was cleaved from the resin to produce phenol 126 with a 96% yield and with 95% purity. Practically identical conditions were reported for other polymeric substrates [118].

18.10

The Baylis–Hillman Reaction

The Baylis–Hillman reaction comprises the addition of aldehydes to activated alkenes mediated by tertiary amines such as 1,4-diazabicyclo[2.2.2]octane (DABCO) or 3-quinuclidinol (3-QDL). The resulting allylic alcohols display a dense array of functional groups and thereby serve as versatile templates for further modifications. Several protocols for the Baylis–Hillman reaction on solid phase have been described. In a typical example reported by Jung and coworkers, acryloin-loaded Cltrityl resin was treated with a large excess of p-trifluoromethylbenzaldehyde and 10 equiv. DABCO in a 1:1 mixture of DMSO/CHCl3 (Scheme 18.38) [119].

Scheme 18.38. Baylis–Hillman reaction on solid support.

The mixture was maintained at rt for 2 days and the procedure was repeated to achieve complete conversion. After acidolysis with TFA, the Baylis–Hillman product 128 was obtained with a crude purity of 97% and with an 85% yield after preparative HPLC. Similar protocols have been disclosed by other groups. Ku¨nzer and coworkers used dimethylformamide (DMF) as solvent and 3-QDL as base to promote the Baylis–Hillman addition [120], while Kulkarni and Ganesan reported that a 3:1 DMF/MeCN solvent mixture in combination with DABCO and catalytic amounts of La(OTf )3 were beneficial to the process [121]. The major limitations for the solid-phase versions of the Baylis–Hillman transformation arise from incomplete reactions and a narrow range of aldehydes that are able to react smoothly under the aforementioned conditions. Large excesses of the reagents or prolonged reaction times with repeated incubations did not dramatically improve the situation. Preferably, aromatic aldehydes with electron-withdrawing substituents work well in these reactions while less reactive aromatic or aliphatic aldehydes gave sluggish reactions. A slightly different protocol for the polymer-supported Baylis–Hillman reaction was described by Reiser and coworkers [122]. Herein, acrolein immobilized on a soluble PEG resin via an ester linkage was reacted with a range of alde-

18.11 Miscellaneous

hydes using ethanol as solvent and 3-QDL as base. The reaction with a diverse set of aldehydes (six examples), including aliphatic aldehydes, could be driven to completion since the soluble polymer guaranteed solution-like kinetics. However, it was also reported that the ester linkage to the PEG polymer was partly cleaved under the reaction conditions, leading to a lower yield of the desired product. In conclusion, the Baylis–Hillman reaction on solid phase is an established process; however, owing to the inherent limitations, a careful selection of the polymeric linker as well as the aldehyde set used in this reaction should be addressed prior to multicomponent library synthesis. Finally, an interesting three-component version of the Baylis–Hillman reaction was reported on solid support. Thus, resin 127 was treated with aldehydes and sulfonamides in the presence of DABCO in dioxane at 70 C. This cocktail gave rise to the formation of functionalized sulfonamides 129 which represent useful intermediates for further manipulations (Scheme 18.39) [123]. The versatility of this chemistry has been shown for aromatic and heteroaromatic aldehydes in combination with arylsulfonamides. The best results were obtained if electrondeficient aldehydes were employed in this process.

Scheme 18.39.

Three-component Baylis–Hillman reaction on solid support.

18.11

Miscellaneous

A solid-phase version of the Ireland–Claisen rearrangement has been reported. A key feature of this concept was the formation of silyl ketene acetals on solid support employing polymeric silyl triflate 132 (Scheme 18.40) [124]. The resulting enolate intermediate (133) cleanly underwent an Ireland–Claisen rearrangement

Scheme 18.40. Ireland–Claisen rearrangement on solid support.

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in THF at 50 C to yield the corresponding silyl esters which were cleaved from the resin by transesterification. The reaction worked well for a range of allylic esters, however, esters derived from acetic acid failed to give the desired rearrangement product (134). This result is presumably due – to a large extent – to C-silylation in the enolization–immobilization event. An elegant approach to substituted phenols via a cyclative cleavage strategy was reported by Katritzky and coworkers (Scheme 18.41) [125]. Merrifield resin loaded with 3-hydroxypyridine was alkylated with bromoacetone to give pyridinium salt (135). The latter was reacted with a,b-unsaturated ketones under basic conditions to yield Robinson anellation product 136, which aromatized with concomitant cleavage from the resin. A range of chalcones (12 examples) were successfully utilized in this reaction and the phenols 137 were isolated after aqueous work-up and column chromatography in good yields with high purities.

Scheme 18.41. Solid-phase synthesis of substituted phenols.

Another example demonstrates that functionalized organometallics can conveniently be handled under anhydrous conditions in solid-phase organic synthesis (Scheme 18.42) [126]. The amino acid derivative 138 loaded on Wang resin was treated with an excess of LDA under a nitrogen atmosphere to generate the corresponding lithium enolate. Subsequent transmetalation with ZnBr2 resulted in the formation of the Zn enolate, which underwent intramolecular carbozincation to yield organozinc intermediate 139. The latter was quenched with reactive electrophiles (I2 or Hþ ) and cleavage from the resin led to the formation of proline derivatives (140). As concluded from liquid chromatography/mass spectrometry (LC/ MS), the desired products have been formed virtually quantitatively (no yields were reported) as single diastereomers.

Scheme 18.42. Solid-phase synthesis of substituted proline derivatives.

References

This carbozincation protocol (depicted in Scheme 18.42) offers an innovative approach to substituted proline derivatives; however, the scope of this process is limited with respect to diversification. Thus, the extension of this methodology to a broader range of substrates as well as the adaptation of the reaction protocol to automation clearly remains a challenge.

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Solid-phase Palladium Catalysis for Highthroughput Organic Synthesis Yasuhiro Uozumi and Tamio Hayashi 19.1

Introduction

Palladium-mediated organic transformations have emerged as a powerful tool in the domain of synthetic organic chemistry [1]. Recently, high-throughput organic synthesis by solid-phase chemistry has been gaining in popularity owing to the ease of purification of the products [2]. Palladium-catalyzed reactions have also found widespread utility in the preparation of small molecule libraries, especially for medicinal screening purposes. This chapter surveys solid-phase synthetic reactions [3] by palladium catalysis. 19.2

Carbon–Carbon and Carbon–Nitrogen Bond-forming Reactions of Aryl and Alkenyl Halides 19.2.1

Cross-coupling Reactions

Palladium complexes catalyze the reaction of organometallic reagents (R-m) with aryl or alkenyl halides and related compounds (R 0 –X) to give cross-coupling products (R–R 0 ) (Scheme 19.1). It is generally accepted that the catalytic cycle of the reaction proceeds via oxidative addition of R 0 –X to palladium(0) and subsequent ligand exchange of the resulting LnPd(R 0 )X with R-m to give, as the key intermediate, the unsymmetrical diorganometal LnPd(II)(R)R 0 . From this intermediate, the product R–R 0 is released by reductive elimination to leave a LnPd(0) species that undergoes the next catalytic cycle. Transfer of an alkyl group from R-m to LnPd(R 0 )X, the so-called transmetalation step, takes place with organometallic compounds of Mg, Zn, B, Al, Sn, Si, Zr, etc. Among the many organometallic reagents used for cross-coupling, organoboron reagents and organotin reagents have been extensively examined for solid-phase applications. Palladium-catalyzed coupling of aryl halides with a terminal alkyne in the presence of a copper salt has also been studied on solid phase. Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

532

19 Solid-phase Palladium Catalysis for High-throughput Organic Synthesis

Scheme 19.1. Reaction pathway of cross-coupling reactions.

19.2.1.1 Reactions of Aryl and Alkenyl Halides with Organoboron Reagents

The palladium-catalyzed cross-coupling of aryl halides with organoboron reagents (the so-called Suzuki–Miyaura coupling) has become an indispensable carbon– carbon bond-forming reaction in fine organic synthesis [4]. One of the earliest examples of its solid-phase application was using the aryl bromide 1 supported on polystyrene resin with a sulfonamide linker (Scheme 19.2) [5]. The Suzuki– Miyaura coupling of the bromide 1 with aryl boronic acids or alkyl 9-BBN in aqueous tetrahydrofuran (THF) took place in the presence of Pd(Ph3 P)4 with Na2 CO3 as the base at 65 C. Nucleophilic cleavage of the resulting resin-bound products 2 and 3 gave the corresponding coupling products in excellent yields. Couplings of various halogenobenzoates (4–9), the m-iodobenzyl ester 10, and the p-, m-, o-bromobenzaldehyde acetals 12–14 bound to the polystyrene (PS) matrix resin are summarized in Tables 19.1, 19.2, and 19.3, respectively. Reactions of the resin-supported p-iodobenzoate 4 with arylboronic acids were catalyzed by Pd2 (dba)3 (dba ¼ dibenzylideneacetone) at room temperature in aqueous dimethylformamide (DMF) to give good to excellent yields of the corresponding biaryl carboxylic acids after hydrolysis (Table 19.1, entries 1–3). The iodobenzoate 4 also reacted with pinacol alkenylborates and tributylborane in the presence of Pd(Ph3 P)4 (Table 19.1, entries 4 and 5) [6]. Coupling of the p-bromobenzoate 5 with various electron-rich as well as electron-deficient arylboronic acids was examined with Pd(Ph3 P)4 in aqueous dimethoxyethane (DME) (Table 19.1, entries 6– 10) [7]. Base-mediated methanolysis of the resulting resin gave methyl 4-(4 0 -

19.2 Carbon--Carbon and Carbon--Nitrogen Bond-forming Reactions of Aryl and Alkenyl Halides

Scheme 19.2. Solid-phase Suzuki–Miyaura coupling: preparation of substituted arylacetic acid derivatives.

substituted aryl)benzoates in excellent yields. The bromobenzoates 7–9 having ortho substituents underwent the Suzuki–Miyaura coupling under the same reaction conditions (Table 19.1, entries 12–14). The iodobenzene 10 connected to the PS resin by benzyl benzoate linkage reacted with a wide variety of arylboronic acids, including heteroaromatic reagents in aqueous dioxane at 100 C (Table 19.2) [8]. Palladium acetate catalyzed the coupling to give the m-arylbenzyl alcohols (11) after methanolysis in satisfactory yields. Palladium-catalyzed cross-coupling of the cyclic acetals of o-, m-, and p-bromobenzaldehydes bound to the PS resin (12–14) was also examined with various arylboron reagents in aqueous DME in the presence of Na2 CO3 to produce various biaryl carboxaldehydes after acid hydrolysis of the acetal linker (Table 19.3) [9]. It is worth noting that the biaryl coupling products having substituents at their 2,6- as well as 2,2 0 -positions were readily obtained despite their steric hindrance (Table 19.3, entries 3 and 12). Thus, the coupling of mesitylboronic acid with the p-bromobenzene 12 and o-carbamoylphenylboronic acid with the o-bromobenzene 14 gave 4-mesitylbenzaldehyde and 2-(2-carbamoylphenyl)benzaldehyde, respectively.

533

534

19 Solid-phase Palladium Catalysis for High-throughput Organic Synthesis Tab. 19.1.

Entry

Coupling of resin-bound aryl iodide with boron reagents.

Aryl halide

Boron reagent

Product

Yield (%)

19.2 Carbon--Carbon and Carbon--Nitrogen Bond-forming Reactions of Aryl and Alkenyl Halides Tab. 19.1. (continued)

Entry

Aryl halide

Boron reagent

Product

Entries 1–5, ref. 6; entries 6–14, ref. 7. Conditions: Pd2 (dba)3 (5–10 mol%), K2 CO3 (2 equiv.), DMF, rt, then CF3 COOH/CH2 Cl2 (for entries 1–3); Pd(Ph3 P)4 (5–10 mol%), K2 CO3 (2 equiv.), DMF, rt, then CF3 COOH/CH2 Cl2 (for entries 4 and 5); Pd(Ph3 P)4 (5 mol%), 2 M Na2 CO3 , DME, reflux, then NaOMe, MeOH (for entries 6–14).

Yield (%)

535

536


Entry

Solid-phase Suzuki–Miyaura coupling with base labile linker.

Ar–B(OH)2

Conditions

Yield (%)


Entry

Ar–B(OH)2

Conditions

Ref. 8: conditions: A, AraB(OH)2 (4 equiv.), K2 CO3 (9 equiv.), Pd(OAc)2 (10 mol%), dioxane/H2 O ¼ 6/1, 100 C, 24 h; B, AraB(OH)2 (8 equiv.), K2 CO3 (18 equiv.), Pd(OAc)2 (20 mol%), dioxane/H2 O ¼ 6/1, 100 C, 24 h.

Yield (%)

537

538


Entry

Coupling of resin-bound aryl iodide with boron reagents.

Aryl halide

Boron reagent

Product

Yield (%)


Entry

Aryl halide

Boron reagent

Product

Ref. 9: conditions: boron reagent (3 equiv.), Pd(Ph3 P)4 (5 mol%), 2 M Na2 CO3 (8 equiv.), DME, reflux, 24 h; then dioxane, 3 M HCl, 80 C, 24 h.

Yield (%)

539

540


Scheme 19.3. Sequential transformation of polymer-supported haloamides into boronates and biaryls by use of Miyaura’s arylboronate formation and Suzuki–Miyaura coupling.

The Suzuki–Miyaura coupling using resin-supported boron reagents has also been investigated (Scheme 19.3 and Table 19.4) [10]. The resin-supported arylboron reagents (17) were prepared by palladium-catalyzed introduction of pinacol borate on solid support. Thus, p-, m-, and o-iodobenzene Rink amide resins (16) were treated with 2 equiv. of bis-(pinacolato)diboron in DMF in the presence of PdCl2 (dppf ) to give 17 in high yields. The coupling reactions of the resulting borates with an excess of aryl halides (Ar–X in Table 19.4) were catalyzed by Pd(Ph3 P)4 in DMF upon heating to give excellent yields of the corresponding biaryl carboxamides 18, which were readily converted to 19 (Table 19.4). One advantage of solid-phase coupling was demonstrated in the preparation of tetrasubstituted alkenes (Scheme 19.4) [11]. The Z-bisborate 20 prepared by palladium-catalyzed diboration was treated with an alkyl halide R 0 –X in the presence of catalytic PdCl2 (Ph3 P)2 and KOH as base in aqueous DMF to give a mixture of the mono- and dialkylated olefins (21 and 22). The reaction mixture was then taken onto the coupling with a Rink p-iodobenzamide without further addition of the palladium catalyst. The monoalkylated intermediate bearing the unreacted borate group underwent solid-phase coupling to form 23, leaving the dialkylated alkene 22 in the solution phase. After removal of the dialkylated alkene 22 by filtration, the resulting resin-bound tetrasubstituted alkene 23 was detached from the resin to give the p-alkenylbenzamide 24 in excellent purity. Solid-phase cross-coupling of the alkyl 9-BBN was used to introduce the silyl traceless linker on solid support (Scheme 19.5) [12]. Palladium-catalyzed solidphase coupling of the alkyl 9-BBN 26 having an anisyl-(dimethyl)silyl group gave the PS-alkyl-(anisyl)dimethylsilane 27, which was readily converted into the chlorosilane 28 by treatment with HCl. The alkylchlorosilane resin reacted with the lithiated pyridine 29 to give 30, the intermediate for the pyridine-fused benzazepines 31 being released in traceless fashion on subsequent treatment with a fluoride reagent.

19.2 Carbon--Carbon and Carbon--Nitrogen Bond-forming Reactions of Aryl and Alkenyl Halides Tab. 19.4.

Starting haloamide

541

Transformation of haloamides into polymer-supported boronates and biaryls. Boronate

Yield (%)

Ar–X

Product

Ref. 10: yields are based on incorporation of halobenzoyl group on the resin.

Yield (%)

542


Scheme 19.4. Synthesis of tetrasubstituted ethylenes on solid support via resin capture.

Scheme 19.5. Introduction of a traceless silicon linker onto polymer support using Suzuki–Miyaura coupling.


The solid-phase synthesis of the prostaglandin E (PGE) and prostaglandin F (PGF) families was carried out via palladium-catalyzed coupling of alkyl 9-BBN as the key step (Scheme 19.6) [13]. The solid-supported bromocyclopentene 34 and the (bromopropenyl)cyclopentene 38 were substituted with alkyl groups by coupling with butyl or pentyl 9-BBN to give the key intermediates of PGE and PGF derivatives (for example 36), respectively.

Scheme 19.6. Solid-phase synthesis of PGE and -F series.

Another interesting application of the Suzuki–Miyaura coupling is the solidsupported pinacolate boron linker (Scheme 19.7) [14]. The boron pinacolate prepared from the arylboronic acid 39 and the pinacol carboxylic acid 40 was connected to polystyrene-poly(ethylene glycol) resin (TentaGel) to give 41. After condensation with 42, the resulting aryl iodide–aryl borate 43 underwent intramolecular cross-coupling to release the biaryl macrocyclic peptide 44. 19.2.1.2 Reactions of Aryl and Alkenyl Halides with Organotin Reagents

In addition to the Suzuki–Miyaura coupling, another widely investigated coupling reaction is palladium-catalyzed cross-coupling of aryl and alkenyl halides with organotin reagents (the so-called Stille coupling) [15]. Scope and limitation studies on the coupling of halogenoaryl carboxylic amides supported on Rink, Wang, and PS resin are summarized in Table 19.5 [16, 17]. The Stille coupling of o-, m-, and p-halogenobenzamides with alkenyl and aryl-(tributyl)stannanes (including heteroaromatics) was carried out in aprotic solvents using palladium complexes of triarylphosphine or arsenic ligands to give good to excellent yields of the corresponding cross-coupling products (Table 19.5).

543

544


Scheme 19.7. Solid-phase synthesis of macrocyclic b-turn mimics with boronate linker.

The resin-supported arylstannane 45 was also examined for Stille coupling with bromobenzenes (Table 19.6) [18]. Thus, the reaction of the PS resin-supported ptributylstannylbenzoate 45 with 3 equiv. of aryl bromide took place in 1-methyl-2pyrrolidinone (NMP) at 90 C to give the coupling products on solid support. A palladacycle catalyst 46 was used for the coupling in the presence of LiCl to promote the reaction more efficiently. Methanolysis of the resulting resin gave the corresponding methyl p-arylbenzoates in 80–95% yields. The solid-phase synthesis of the library of the highly functionalized fused ring system shown in Scheme 19.8 was achieved via the Stille coupling of an alkenyl bromide with a series of alkenylstannanes, resulting in conjugate dienes and a subsequent Diels–Alder reaction (Scheme 19.8) [19]. 19.2.1.3 Reactions of Aryl Halides with Terminal Alkynes

The palladium-catalyzed coupling of an aryl halide with a terminal alkyne promoted by CuI and base, the so-called Sonogashira reaction [20], is recognized as the most powerful method for the preparation of aryl acetylene derivatives. The solid-phase Sonogashira reaction was examined for the coupling of the resinsupported m-iodobenzylbenzoate 54 and (m-iodobenzyloxy)acetamide 55 with various terminal alkynes, as shown in Table 19.7 [21]. The coupling was carried out with an excess of alkynes using a PdCl2 (Ph3 P)2 /CuI/Et3 N system to give the corre-


Coupling of polymer-supported aryl iodides with vinyl/aryl stannanes. AraX þ (alkyl)3 SnR (R ¼ vinyl or aryl) ! AraR (R ¼ vinyl or aryl)

Entry Aryl iodide

Stannane

Product a

Yield (%)b

545

546

19 Solid-phase Palladium Catalysis for High-throughput Organic Synthesis Tab. 19.5. (continued)

Entry Aryl iodide

a

b

Stannane

Producta

Conditions: entries 1–7 (ref. 16): Pd2 (dba)3 (5 mol%), Ph3 As (20 mol%), NMP, 45 C; entries 8, 9, 11–15, 17–24, and 26 (ref. 17): stannane (3 equiv.), Pd(Ph3 P)4 (5 mol%), DMF, 60 C, 24 h; entries 10, 16, 25, and 27: Pd2 (dba)3 (5 mol% Pd), (2-furyl)3 P, DMF, 60 C, 48 h. Entries 1–5: product cleaved from the resin with 5% TFA/DCM (entries 1–5), 90% TFA (entries 6–7), or aq. LiOH THF-MeOHH2 O (entries 8–27). Ar 1 ¼ 4-H2 NCOC6 H4 , Ar 2 ¼ 4HOCOCH(CH3 )NH-COC6 H4 , Ar 3 ¼ 2-HOCOC6 H4 , Ar 4 ¼ 3HOCOC6 H4 , Ar 5 ¼ 4-HOCOC6 H4 , Ar 6 ¼ (3-hydroxycarbonyl)-5pyridyl, Ar 7 ¼ (2-hydroxycarbonyl)-5-furyl. Entries 1–24: isolated yield; entries 25–27: determined by HPLC and 1 H NMR.

Yield (%)b


Entry

Coupling of polymer-supported aryl stannane with aryl halides.

Aryl halide

Yield (%)

Ref. 18

sponding ethynylbenzenes in nearly quantitative yields. The ethynylbenzyl alcohols 56 or the (ethynylbenzyloxy)acetamide 57 were released from the resulting resin by methanolysis of the benzoate linker or acid hydrolysis of the Rink acetamide linker, respectively. The Sonogashira reaction has often been used as a key step for the solid-phase preparation of indole derivatives. Thus, for example, the solid-phase coupling of o-iodoacetanilide 58 with phenylacetylene using PdCl2 (Ph3 P)2 , CuI, and base, followed by alkaline hydrolysis of the ester linkage, gave the indole 5-carboxylic acid

547

548


Scheme 19.8. Solid-phase synthesis of highly functionalized fused ring systems.

59 in 72% yield in one pot, thereby showing that the Sonogashira coupling and subsequent intramolecular cyclization to form the indole ring proceeds on solid phase (Scheme 19.9) [22]. The Rink resin-supported o-iodo(N-methanesulfonyl)anilide 60 gave the 2substituted indoles 61 which underwent, after deprotection of the N-mesyl group, a solid-phase Mannich reaction to afford the (2-alkyl-3-aminomethyl)indole 5carboxamides 64 (Scheme 19.10) [23].


Palladium-catalyzed coupling of solid-supported aryl iodides with terminal

acetylenes.

Entry

Aryl iodide

Acetylene

Conditions

Yield (%)

549

550


Entry

Aryl iodide

Acetylene

Conditions

Ref. 21 conditions: A, acetylene (4.0 equiv.), PdCl2 (Ph3 P)2 (10 mol%), Cul (20 mol%), Et3 N/dioxane ¼ 1/2, rt, 24 h; B, acetylene (8.0 equiv.), PdCl2 (Ph3 P)2 (20 mol%), Cul (40 mol%), Et3 N/dioxane ¼ 1/2, rt, 24 h; C, acetylene (4.0 equiv.), PdCl2 (Ph3 P)2 (10 mol%), Cul (20 mol%), Et3 N/dioxane ¼ 1/2, 50 C, 24 h.

Scheme 19.9. Solid-phase synthesis of indoles using Pd-catalyzed acetylene coupling.

Yield (%)


Scheme 19.10. Solid-phase Mannich substitution of indoles.

Similarly, the benzofurans 66 as well as the (2,3,6-substituted)indoles 70 were also synthesized on solid phase (Schemes 19.11 and 19.12) [24, 25].

Scheme 19.11. Solid-phase synthesis of benzofurans using Pd-catalyzed acetylene coupling.

An unsubstituted terminal acetylene group was introduced on a resin-supported aromatic ring by Sonogashira coupling of (trimethylsilyl)acetylene followed by carbon–silicon bond cleavage with tetrabutylammonium fluoride (Scheme 19.13) [26]. The resulting arylacetylene 73 was subjected to the copper-mediated Mannich reaction on solid support with various aldehydes and piperazines to afford a library of arylpropynylamines 74 in excellent chemical yields and purities.

551

552


Scheme 19.12. Solid-phase synthesis of trisubstituted indoles.

Scheme 19.13. Combination of Sonogashira and Mannich reactions.


A solid-supported terminal alkyne was also examined for the Sonogashira coupling (Scheme 19.14) [27]. Thus, propiolic acid was connected to PS oxyamino resin prepared from the phthalimide 75 via the Gabriel synthesis to give the solidsupported propiolic amide 76. The palladium-catalyzed coupling of the supported propiolic amide 76 with the halogenonucleoside 77 proceeded under the standard Sonogashira conditions to give the nucleoside hydroxamic acid 79.

Scheme 19.14. Solid-phase synthesis of nucleoside hydroxamic acids.

Solid-phase Palladium-catalyzed Cross-coupling Using Aryl and Benzylzinc Reagents It has been well established that cross-coupling of aryl halides (pseudo-halides) with aryl and benzylzinc reagents are also catalyzed by palladium–phosphine complexes, as are those with organoboron and organostannane reagents. Solid-phase cross-coupling of arylzinc reagents with supported aryl halides was catalyzed by a palladium complex of bis-(diphenylphosphino)ferrocene (dppf ) in THF to give biaryls in good to excellent yields. Representative results are summarized in Table 19.8 [28]. Reactions of supported p-bromobenzoate with 2 equiv. of o-, m-, and psubstituted arylzinc reagents bearing an electron-withdrawing or electron-donating group took place at room temperature to give the corresponding biaryls in good chemical yields (Table 19.8, entries 1–7). Meta- and o-bromobenzoate also underwent coupling on resin support under the same reaction conditions to give the 19.2.1.4

553

554


Coupling of polymer-supported aryl bromides with arylzinc reagents.

Entry Aryl halide

Ref. 28

Aryl zinc

Product

Yield (%)


biaryl 2-carboxylic ester and the 3-carboxylic ester, respectively (Table 19.8, entries 9 and 10). Heteroaromatic biaryl derivatives were obtained from solid-supported bromopyridine or thiophenylzinc (Table 19.8, entries 8 and 11). It is noteworthy that zinc-mediated cross-coupling was used for the preparation of diarylmethanes (Scheme 19.15) [29]. Thus, the coupling of the Rink amide of the o-, m-, and p-iodobenzoyl substrates with 10 equiv. of 4-cyanobenzylzinc and 2,6-dichlorobenzylzinc reagents was catalyzed by a catalyst generated from Pd2 (dba)3 and tri(2-furyl)phosphine to give the corresponding benzylphenylcarboxamides 80 and 81 in high yields. The synthetic sequences shown in Scheme 19.16, where the zinc-mediated aryl–aryl and aryl–benzyl couplings were performed on resin support, gave AraAraCH2 aAr products in high purity.

Scheme 19.15. Solid-phase coupling of aryl iodides with benzylzinc reagents.

19.2.2

Palladium-catalyzed Arylation and Alkenylation of Olefins

Palladium-catalyzed arylation and alkenylation of olefins (the so-called Heck reaction) [30] are versatile means for making a carbon–carbon bond. Aryl and alkenyl halides (pseudo-halides) (R–X) are employed for the Heck reaction as alkylation agents for the olefinic substrates.

555

556


Scheme 19.16. Multiple coupling process of aryl- and benzylzinc reagents with supported aryl halides.

The Heck reaction is generally thought to proceed via the reaction pathway shown in Scheme 19.17. Oxidative addition of R–X to palladium(0) gives LnPd(R)X. Coordination of an olefin substrate to LnPd(R)X gives Pd(II) (h 2 -olefin)R. Insertion of the h 2 -olefin ligand into the Pd–R bond gives a s-alkylpalladium intermediate which subsequently undergoes b-hydride elimination to give the Heck product and LnPd(H)X. The intermolecular Heck reaction of resin-supported aryl iodides with olefin substrates was examined on various polymer supports. Representative results are


Scheme 19.17. Pathway of Heck reaction.

summarized in Table 19.9, where conjugated olefins were used to exhibit high reactivity [31, 32]. The Heck reaction of the Wang resin-supported 4-iodobenzoate with ethyl acrylate was catalyzed by Pd(OAc)2 in DMF to give the ethyl cinnamate product in 91% yield (Table 19.9, entry 1). The unsymmetrical stilbene was obtained from methoxystyrene in 90% yield (Table 19.9, entry 2). Alkenylation of iodobenzoates supported on PS–polyethylene glycol (PEG) resin and peptide amide linker (PAL) resin took place at 37 C in the presence of Pd(OAc)2 -PPh3 in aqueous DMF (Table 19.9, entries 4–7). The Heck reaction of a supported styrene with solution-phase aryl iodides was also examined under essentially the same conditions (Table 19.10). Thus, the Wang resin-supported styrene carboxylate reacted with iodobenzene, bromonaphthalene, bromothiophene, and bromopyridine in DMF on heating to give the corresponding b-arylstyrene-4-carboxylic acids upon release from the resin support. A palladium complex generated in situ by mixing Pd2 (dba)3 and tri-(2-furyl)phosphine (Table 19.9, conditions B) exhibited high catalytic activity for the reaction with aryl bromides (Table 19.9, entries 2, 4, and 5). Aryl halides having allylamine or acrylamide substituents at their ortho positions have been well documented to undergo an intramolecular Heck reaction to form indole ring systems. The intramolecular indole ring construction has been applied to the solid-phase synthesis of libraries of indole derivatives. Thus, the 2bromoaniline 85, which was connected at its 5-position to PS–PEG resin by a Wang-type linker, was converted to the N-acyl-N-allylanilines 86 using the standard fluorenylmethoxycarbonyl (Fmoc) method. The supported substrates 86 were sub-

557

558


Entry

Solid-phase Heck reaction of supported aryl halides.

Ar–X

Alkene (alkyne)

Product

Yield (%)

Entries 1–3, ref. 31; entries 4–7, ref. 32. Conditions: entries 1 and 3, Pd(OAc)2 , Et3 N, Bu 4 NCl, DMF, 80–90 C, 16 h; entry 2, Pd2 (dba)3 , (o-tol)3 P, DMF, 100 C, 20 h; entries 4–7, Pd(OAc)2 , PPh3 , Et3 N, Bu 4 NCl, DMF/H2 O (9:1), 37 C, 4 h.

jected to the palladium-catalyzed Heck reaction. The resulting resin was treated with CF3 COOH to give N-acyl-3-alkyl-6-hydroxyindoles (87) in good to excellent yields and purities (Scheme 19.18) [33]. An acrylamide group showed good reactivity for the intramolecular Heck reaction (Scheme 19.19) [34]. Thus, cyclization of Rink resin-supported (2-iodo)acrylanilides (90) was catalyzed by Pd(OAc)2 -PPh3


Entry Ar–X

Solid-phase Heck reaction of supported styrene with aryl halides.

Conditions

Product

Ref. 31: condition A, Pd(OAc)2 , Et3 N, Bu 4 NCl, DMF, 80–90 C, 16 h; B, Pd2 (dba)3 , (o-tol)3 P, DMF, 100 C, 20 h.

Scheme 19.18. Solid-phase synthesis of indoles.

Yield (%)

559

560


Scheme 19.19. Solid-phase synthesis of oxindoles.

in DMF to give the oxindoles 91. N-Allyl-2-iodoanilines (94) which were supported on Rink amide resin by their N-alkyl side-chains also underwent Heck cyclization in the presence of Pd-PPh3 species in aqueous DMF at 80 C to afford 3-(resinconnected alkyl)indoles (95) (Scheme 19.20) [35]. Cleavage of the resin support under acidic conditions gave high yields of the indoles 96, which bear 3-(aminocarbonyl)methyl and N-benzyl groups. Preparation of an oligopeptide library at the 5-position of the indole skeleton was achieved by use of resin-supported 5-carboxy indole 95 as a scaffold (Scheme 19.20, bottom). A macrocyclic peptide was synthesized on solid support via Heck cyclization (Scheme 19.21) [36]. An oligopeptide chain (98) bearing aryl iodide and acrylamide groups was prepared by the standard Fmoc method on PS resin. Heck reaction of the oligopeptide 98 took place at 37 C in aqueous DMF in the presence of the Pd(OAc)2 -PPh3 catalyst to give, after deprotection and release from resin support,


Scheme 19.20. Solid-phase synthesis of indoles (2).

the macrocyclic peptide 100 in 30% overall yield based on the loading of the starting resin. A benzazepine skeleton was constructed on solid support by intramolecular Heck reaction (Scheme 19.22) [37]. The Wang resin-supported (N-butenyl)2-iodobenzamides 101 (R ¼ CH3 or CH2 Ph) underwent Heck cyclization with Pd(OAc)2 -PPh3 to give 102. Acidic cleavage of the Wang ester, followed by treatment with diazomethane, gave the benzazepines 103 in high yields. 19.2.3

Amination of Aryl Halides

Palladium-catalyzed amination of aromatic halides [38] has become a powerful tool in solid-phase organic synthesis. Thus, various Rink resin-supported aryl bromides were coupled with aniline derivatives to give N-aryl anilines in quantitative yields, as seen in Table 19.11, entries 1, 2, 4, and 5 [39]. The aromatic ami-

561

562


Scheme 19.21. Palladium-catalyzed formation of macrocyclic peptide on solid support.

Scheme 19.22. Solid-phase synthesis of benzazepines via intermolecular Heck reaction.


Palladium-catalyzed amination of resin-bound aryl bromide.

Entry Aryl halide

Amine

Condition

Product

Yield (%)

563

564


Entry Aryl halide

Amine

Condition

Product

Yield (%)

Entries 1–5, ref. 39; entries 6–10, ref. 40. Conditions: A, amine (3 equiv.), Pd2 (dba)3 (5 mol%), (o-tol)3 P, NaOBu-t (10–20 equiv.), toluene, 100 C; B, amine (10 equiv.), Pd2 (dba)3 (20 mol%), (o-tol)3 P (80 mol%), NaOt-Bu (10–20 equiv.), toluene, 100 C; C, amine (10 equiv.), Pd2 (dba)3 (20 mol%), BINAP (80 mol% P), NaOBu-t (10–20 equiv.), toluene, 100 C

nations were carried out with the Pd2 (dba)3 /tri-(o-tolyl)phosphine catalyst system and NaOt-Bu in toluene at 100 C to give complete conversion of the substrates. Resin-bound o-bromides showed little activity, presumably owing to their steric hindrance (Table 19.11, entry 3). PS–PEG Rink amide (TG RAM) resin-bound pbromobenzamide was also examined for coupling with piperidine and pyrrolidine to give the N-arylpiperidine and N-arylpyrrolidine in 81% and 49% yields, respectively, under essentially the same conditions (Table 19.11, entries 6 and 7) [40]. It has been documented that primary and secondary aliphatic amines result in significant reduction of the bromide using (o-tol)3 P and that the improved conditions with 2,2 0 -bis(diphenylphosphino)-1,1 0 -binaphthyl (BINAP) (Table 19.11, conditions C) decrease this side reaction. The yield of N-arylpyrrolidine (49%) increased to 93% with BINAP (Table 19.11, compare entry 7 with 8). The use of BINAP as a ligand also allowed for the successful coupling of primary amines. Thus, benzylamine reacted with the TG RAM-supported p- and m-bromobenzamide with the Pd/BINAP catalyst to give 99% and 89% yields, respectively, of the p- and m(benzylamino)benzamide (Table 19.11, entries 9 and 10).


19.2.4

Miscellaneous Reactions [41] 19.2.4.1 Heteroannulation

It has been reported that annulation of a 2-iodoaniline with an internal alkyne takes place in the presence of a palladium catalyst to give a 2,3-disubstituted indole in one step (Larock annulation) [42]. The annulation of 4-carboxamide-2-iodoanilines (104) supported on Rink resin with an excess amount of disubstituted alkynes was catalyzed by Pd(OAc)2 -PPh3 to give the indoles 105 (Table 19.12, route A) [43]. Cleavage of the resin moiety from 105 by trifluoroacetic acid gave the 2,3,5-trisubstituted indoles 108 in excellent yields (Table 19.12, entries 1–5). The 2iodoaniline 106 bound to the resin support at its N1 position by the THP linker reacted with alkynes under palladium-catalyzed conditions to give N-resin-bound indole 107 (Table 19.12, route B). Acidic cleavage of the N–THP linkage gave high

Solid-phase Larock heteroannulation.

Tab. 19.12.

Entry

Route

Condition

X

R

R1

R2

Yield (%)

1 2 3 4 5 6 7

A A A A A B B

i i i ii i iii iii

CONH2 CONH2 CONH2 CONH2 CONH2 H H

H H H COCH3 COCH(CH3 )2 H H

Pr Me Me Pr Me Ph Me

Pr t-Bu Ph Pr t-Bu SiMe3 t-Bu

91 87 86 95 75 73 55

Entries 1–5, ref. 41; entries 6–7, ref. 42. Condition i, alkyne (10–15 equiv.), Pd(OAc)2 (10 mol%), Ph3 P (20 mol%), LiCl (1 equiv.), K2 CO3 (5 equiv.), DMF, 80 C; ii, alkyne (10–15 equiv.), Pd(OAc)2 (10 mol%), Ph3 P (20 mol%), Bu 4 NCl (1 equiv.), KOAc (5 equiv.), DMF, 80 C; iii, alkyne (excess), PdCl2 (Ph3 P)2 (20 mol%), tetramethylguanidine (10 equiv.), DMF, 110 C.

565

566


Scheme 19.23. Solid-phase synthesis of tropane derivatives via

palladium-mediated three-component coupling.

yields of the 2,3-disubstituted indole 108 (X ¼ H) (Table 19.12, entries 6 and 7) [44]. 19.2.4.2 Insertion Cross-coupling Sequence (Dialkylation of Tropene)

Three-component couplings of the resin-supported tropene 109, an aryl bromide, and an arylboronic acid or phenylacetylene were promoted by palladium(0) to give vicinal disubstituted tropanes (111) (Scheme 19.23) [45]. Thus, the reaction of 109 with an aryl bromide took place in THF with palladium(0) to give the s-alkylpalladium intermediate 110, which coupled with an arylboronic acid or phenylacetylene successively to give 111 (R ¼ Ar or CCPh). Tropane 112 or 113 was obtained from 111 through deprotection, reductive N-alkylation, and acidic cleavage of resin. The monosubstituted tropane 114 was obtained similarly by reductive cleavage of the salkylpalladium bond of the intermediate 110 with formic acid.


567

Coupling Reactions on Various Solid Supports The Heck reaction, the Suzuki–Miyaura, Sonogashira, and Stille couplings with aryl iodide were examined on various resin supports (Table 19.13) [46]. Thus, aryl 19.2.4.3

Tab. 19.13.

Resin

Palladium-catalyzed various coupling with a traceless linker.

Coupling conditions

Cleavage conditions

Substrate (6 equiv.) Pd(OAc)2 (20 mol%) NaOAc (3 equiv.) Bu 4 NBr (1 equiv.) DMA, 100 C, 24 h

A

83

B

85

B

96

Substrate (10 equiv.) Pd(Ph3 P)4 (2 mol%) K3 PO4 (2 equiv.) aq. DMA, 80 C, 24 h

A

74

B

86

TentaGel

C

93

Polystyrene

C

67

ArgoPore

A

60

C

50

C

92

C

86

C

90

C

79

C

80

TentaGel Polystyrene ArgoPore TentaGel TentaGel

TentaGel Polystyrene

Substrate

Substrate (6 equiv.) PdCl2 (Ph3 P)2 (10 mol%) Cul (20 mol%) Dioxane, Et3 N, rt, 24 h

ArgoPore TentaGel Polystyrene ArgoPore

Substrate (5 equiv.) Pd2 (dba)3 (10 mol%) Ph3 As (40 mol%) Dioxane, 60 C, 24 h

Ref. 46: conditions A, Cu(OAc)2 (0.5 equiv.), MeOH, pyridine (10 equiv.), rt, 2 h; B, Cu(OAc)2 (0.5 equiv.), n-propylamine, rt, 2 h; C, NBS (2 equiv.), CH2 Cl2 , rt, 45 min.

Product

Yield (%)

568


iodide was connected to PS–PEG resin (TentaGel), standard PS resin, and macroporous PS resin (ArgoPore) with a hydrazine linker. The supported aryl iodides (115) were subjected to the coupling reactions under the conditions listed in Table 19.13 to give 116. The resulting resins (116) were subsequently subjected to linker cleavage conditions A, B, or C (see Table 19.13) to give the substituted aromatics in a traceless fashion.

19.3

Solid-phase Reactions by Way of p-Allylpalladium Intermediates

Substitution reactions of allylic substrates with nucleophiles have been shown to be catalyzed by certain palladium complexes. The catalytic cycle of the reactions involves p-allylpalladium as the key intermediate (Scheme 19.24). Oxidative addition of the allylic substrate to a palladium(0) species forms a p-allylpalladium(II) complex, which undergoes attack of a nucleophile on the p-allyl moiety to give an allylic substitution product.

Scheme 19.24. Reaction pathway of allylic substitution via a p-allylpalladium intermediate.

19.3.1

Cleavage of Allyl Ester Linkers

A carboxylic acid moiety connected to a polymer resin by an allyl ester linker was released under palladium-catalyzed allylic substitution conditions. Thus, an allyl ester group of the PS resin-supported tripeptide 117 was cleaved reductively by tin hydride in the presence of a palladium-PPh3 catalyst to release the peptide in high yield. Carbon–oxygen bonds of supported allyl esters 118–120 were also readily cleaved by morpholine by way of p-allylpalladium intermediates (Scheme 19.25) [47].

19.3 Solid-phase Reactions by Way of p-Allylpalladium Intermediates

Scheme 19.25. Palladium-catalyzed cleavage of allylic anchoring groups.

An allyl ester of a resin-bound carboxylic acid was activated with palladium(0) to form the Pd(h 3 -allyl)(OC(O)-resin) species 124 which readily undergoes attack by a nucleophile to provide functionalization and release of the allyl moiety 123 in one step (Table 19.14) [48]. Thus, the resin carboxylate ester 122 bearing a conjugated diene moiety prepared by solid-phase ruthenium-mediated metathesis reacted with an active methylene compound (Table 19.14, entries 1–4) or morpholine (Table

569

570

19 Solid-phase Palladium Catalysis for High-throughput Organic Synthesis Preparation of conjugated dienes via Ru-catalyzed cross-metathesis and Pdcatalyzed allylic substitution on solid support.

Tab. 19.14.

Entry

NuH or NuNa

Product

Yield (%)

Ref. 48.

19.14, entry 5) in the presence of a palladium-phosphine catalyst to give the diene 123 in high yield. Treatment of compound 125 bearing an amino group at the homoallylic position with palladium-dppe catalyst gave the exo-methylenepyrrole 126 via formation of a p-allylpalladium intermediate and subsequent intramolecular nucleophilic attack of an amino group (Scheme 19.26) [49].

19.3 Solid-phase Reactions by Way of p-Allylpalladium Intermediates

Scheme 19.26. Solid-phase synthesis of pyrrolidines via

palladium-catalyzed cyclization cleavage.

19.3.2

N-Allylation via p-Allylpalladium Intermediates

The reaction of an allyl ester with a nitrogen nucleophile bound to the PS–PEG resin gave the N-allylation product (Scheme 19.27) [50]. Thus, the reaction of 2methoxycarbonylmethyl-2-propen-1-ol (128) with the TentaGel-bound benzylamine 127 in the presence of Pd(PPh3 )4 gave the N-allylation product 129. After esterification of the allylic alcohol of 129, the resulting allyl acetate 130 was subjected to palladium-catalyzed allylic substitution, again with various nitrogen nucleophiles. A resin-supported p-allylpalladium intermediate generated in situ underwent nucleophilic attack by primary, secondary, tertiary, and cyclic amines to give the corresponding allylic amines (131) on solid support. The N-(2-aminomethyl-2propenyl)-N-benzylglycine derivatives 132 were released from the resin 131 by alkaline hydrolysis in moderate to high yields. 19.3.3

Insertion–p-Allylic Substitution System

Solid-phase synthesis of the (2-alkenyl)indoline derivatives 134 has been achieved in one pot by the reaction of the immobilized aryl halides 133 and conjugated dienes which proceeded through a palladium-catalyzed insertion–p-allylic substitution sequence (Scheme 19.28) [51]. Thus, the Rink resin-supported aryl iodide 133 was added to palladium(0) oxidatively to form the arylpalladium intermediate 135. The arylpalladium intermediate 135 reacted with the diene to give the p-allylpalla-

571

572


Scheme 19.27. Solid-phase preparation of N-benzylglycine derivatives.

Scheme 19.28. Synthesis of indoles via palladium-catalyzed annulation.

19.4 Palladium Catalysis with Solid-supported Complexes

dium 137 via the alkylpalladium 136. The p-allylpalladium should readily undergo intramolecular nucleophilic attack of nitrogen atom at the ortho position to form the 2-(alkenyl)indoline 134. Three-component coupling of an aryl halide, 1,5-hexadiene, and the Rink-supported piperidine 138 was catalyzed by palladium to give the N-(6-aryl-2-hexenyl)piperidine 141 via the insertion–p-allylic substitution pathway (Scheme 19.29) [52]. The alkylpalladium intermediate 140 generated in solution phase underwent a belimination–insertion process which was terminated by the formation of thermodynamically stable p-allylpalladium 143. The resulting p-allylpalladium complex 143 reacted with piperidine on the resin supports to give the N-alkylated piperidines 139 in high yield.

Scheme 19.29. Palladium-catalyzed three-component coupling.

19.4

Palladium Catalysis with Solid-supported Complexes

Homogeneous transition metal catalysts are widely used for a variety of organic transformations. High-throughput synthesis by solution-phase catalysis has also

573

574


been recognized as a useful methodology with the advent of efficient methods for compound purification. One approach employs supported catalysts that can be readily removed by filtration. Several reviews have covered the synthetic use of solid-supported reagents, including transition metal complexes [53]. A number of support-bound palladium complexes, in particular palladium–phosphine complexes, have been designed and prepared to combine the advantages of both homogeneous and heterogeneous catalysts in one system [54]. This class of resinbound palladium catalysts would solve the basic problems of homogeneous catalysts, namely the separation and recycling of the catalysts. These palladium complex catalysts are also advantageous in that contamination of the ligand residue in the products is avoided. 19.4.1

Preparation of Solid-supported Palladium Complexes and Their Use in Palladium Catalysis

Standard procedures for the preparation of polymer-supported catalysts usually entail surface modification of commercially available polymer resins, e.g. polystyrene–divinylbenzene (PS–DVB) or chloromethylated PS–DVB resin. Thus, the reaction of chloromethylated polystyrene with an excess of lithium diphenylphosphide gave the (diphenylphosphino)methylated polystyrene 145 in quantitative yield (Scheme 19.30). The palladium(0) complex 146 was obtained by the treatment of 145 with Pd(PPh3 )4 . The reaction of 145 with PdCl2 (or PdCl2 (cod)) gave the resin-bound palladium(II) complex 147 which was readily converted to 146 by reduction with hydrazine in the presence of PPh3 . The physical properties of the resin matrix and the loading value of the phosphine residue are dependent on the crosslinking value (DVB, %) and the yield of the chloromethylation step, respec-

Scheme 19.30. Preparation of phosphinylated polystyrene–palladium complexes.


tively. The resin-bound palladium–phosphine complex 146 catalyzed nucleophilic allylic substitution via p-allylpalladium intermediates [55], telomerization of dienes [56], the Heck reaction [57], the Suzuki–Miyaura coupling [58], etc. The bisphosphines 148 and 150 bearing alkyl substituents on their phosphorus atoms were supported on PS resin by the nucleophilic substitution of the chloromethyl groups on the resin to give 149 and 151, respectively (Scheme 19.31) [59]. A palladium complex of 149 showed moderate catalytic activity to promote the Heck reaction of iodobenzene with methyl acrylate.

Scheme 19.31.

Various ligands bound to polystyrene support.

The biarylphosphines 152 also reacted with the chloromethylated PS resin under basic conditions to give the PS-supported biarylphosphines 153 (Scheme 19.32) [60]. The resin-bound biaryl-(dialkyl)phosphines 153 were the ligands designed for use in the palladium-catalyzed amination and Suzuki–Miyaura coupling of aryl halides, especially those of aryl chlorides, whereas the use of electron-rich phosphine ligands allowed for an increase in the scope of the aryl halide substrate [61].

Scheme 19.32.

Various ligands bound to polystyrene support (2).

The polymer-supported carbene complexes of palladium 155 were prepared by the nucleophilic substitution of the bromomethylated Wang resin with 154 under basic reaction conditions (Scheme 19.33) [62]. The catalytic activity of 155 for the Heck reaction of aryl bromides with acrylates or styrene was found to exhibit high

575

576


turnover numbers (TON) up to 5000. The supported carbene complexes 155 were air-stable and recyclable catalysts.

Scheme 19.33.

Various ligands bound to polystyrene support (3).

Polymerization of ligand monomers is a useful tool for preparing polymersupported ligands. The crosslinked polystyrene-bound ferrocenyl bisphosphine ligand 157 was prepared by the copolymerization of styrene, divinylbenzene, and 1,1 0 -bis-(diphenylphosphino)-2-vinylferrocene (156) (Scheme 19.34) [63]. The loading density of the catalyst on the support was readily controlled by the ratio of the monomers used.

Scheme 19.34. Preparation of polymer-bound ferrocenylphosphine.

Carbonylative intramolecular Stille coupling to form macrocyclic molecules was investigated with a palladium complex of the polymer-bound ferrocenyl phosphine 157 (Scheme 19.35). One of the major problems encountered in the intramolecular macrocyclization is the formation of linear oligomers via an intermolecular pathway. ‘‘Site isolation’’ of the catalytic sites on a polymer backbone has been achieved with relatively low loading density of the catalyst to suppress the in-


termolecular reactions. Thus, ester 158 bearing an alkenylstannane and an alkenyl triflate gave high yields of the corresponding keto lactone 159 with the Pd(0)/157 complex and LiCl under carbon monoxide, whereas only moderate yields of the macrocycles were obtained under homogeneous conditions using Pd(PPh3 )4 or PdCl2 (dppf ).

Scheme 19.35. Palladium-catalyzed macrocyclization.

The polypyrrole-bound mono- and bisphosphines 162 and 163 were prepared as their P-borane complexes from the corresponding monomers 160 and 161 via FeCl3 -induced or electrochemical polymerization conditions (Scheme 19.36) [64]. These phosphine–borane complexes reacted with palladium(II) without predecomplexation to give the polypyrrole-bound palladium(0)–phosphine complexes, where the borane on the phosphorus atom served as a reducing agent of palladium(II). The resulting immobilized polypyrrole palladium(0)–phosphine complexes catalyzed the Heck reaction and the p-allylic substitution of allyl acetates.

Scheme 19.36. Preparation of polypyrrole-bound phosphine–borane.

Ring-opening methathesis polymerization of the norbornene monomer 164 having a 2-endo-N,N-di-(2-pyridyl)carbamide group was carried out via a ‘‘living polymerization’’ using the Schrock catalyst (Scheme 19.37) [65]. The resulting living polymer chains were crosslinked using 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo-endodimethanonaphthalene (165) to give the bispyridyl ligand 166. Its palladium com-

577

578


plex 167 generated by treatment of 166 with H2 PdCl 4 catalyzed the Heck reaction of aryl bromides and even aryl chlorides. Thus, the reaction of chlorobenzene with styrene in the presence of 0.003 mol% of the palladium species 167 and tetrabutylammonium bromide in dimethylacetamide at 140 C gave an 89% isolated yield of trans-stilbene where the TON observed reached 23,600.

Scheme 19.37. Ring-opening metathesis polymerization (ROMP) of monomer ligands.

Polyaminoamide (PAMAM) dendrimers of generation 0–4 on silica [66] and carbosilane dendrimers [67] were used as solid support for immobilization of the palladium catalysts. Thus, for example, (diphenylphosphino)methyl groups were introduced on the terminal nitrogen of PAMAM chains by treatment of 168 with paraformaldehyde and diphenylphosphine (Scheme 19.38). Treatment of the resulting dendrimer bearing diphenylphosphino groups with PdMe 2 (tmeda) gave the chelate complex 169, which showed good catalytic activity in the Heck reaction. The triarylphosphine moiety was incorporated into the PS–PEG resin by a solidphase amide-forming reaction (Scheme 19.39) [68]. Thus, a mixture of the PEG– PS amino resin, 2 equiv. of 4-(diphenylphosphino)benzoic acid, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDCI), and 1-hydroxybenzotriazole hydrate (HOBt) in DMF was agitated to give the PS–PEG resin-supported phosphine 171. The complete consumption of the amino residue of the PEG chain was conveniently monitored by the Kaiser test. Formation of the palladium–phosphine complex 172 on the resin was performed by mixing [PdCl(p-C3 H5 )]2 and 171 in an appropriate organic solvent at ambient temperature for 10 min. The PS–PEG resin-supported complex 172 exhibited high catalytic activity in water due to its amphiphilic property. Allylic substitution [68], Heck reaction [69], carbonylation [70], and Suzuki–Miyaura [70] coupling took place in a single aqueous medium at room temperature by use of 172.


Scheme 19.38. Dendrimer-bound palladium–phosphine complex.

Amphiphilic polymer-supported phosphine ligands were also prepared on poly(N-isopropyl)acrylamide (PNIPAM) resin (Scheme 19.40) [71]. The palladium complex of the PNIPAM–phosphine, formed from reaction of 174 or 175 with Pd(dba)2 , showed high catalytic activity both in organic solvents and in water to promote p-allylic substitution of allyl carbonates and the Sonogashira reaction of aryl iodides. 19.4.2

Solid-supported Chiral Palladium Catalysts

Asymmetric reactions catalyzed by transition metal complexes containing optically active ligands have attracted great interest because of their synthetic utility. A vast

579

580


Scheme 19.39. PS–PEG resin-supported amphiphilic palladium–phosphine complexes.

Scheme 19.40. PNIPAM-supported amphiphilic phosphine ligands.

amount of research has been reported to date on asymmetric reactions using homogeneous catalyst systems in which activity and stereoselectivity can be tuned by varying the ligand structure. Recently, immobilization of the enantioselective catalysts has been recognized as one of the most promising strategies for achieving highly stereoselective catalysis under heterogeneous conditions [72]. Several examples of chiral ligands supported on polymer resin, which have found utility in asymmetric palladium catalysis, are shown in Scheme 19.41. Palladium complexes of the resinsupported 2-diphenylphosphino-2 0 -substituted-1,1 0 -binaphthyl (MOP) ligand 176 [73] and pyridinoxazoline 179 [74] catalyzed allylic substitution with good to high stereoselectivity. The PS-supported BINAP 177 [75] was applied to a palladium(II)catalyzed aldol reaction of a silyl enolate [76]. A novel chiral ligand, (3R,9aS)-(2aryl-3-(2-diphenylphosphino)-phenyl)-tetrahydro-1H-imidazo[1,5-a]indole-1-one was designed, prepared, and immobilized on an amphiphilic polystyrene-poly(ethylene

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Scheme 19.41. Resin-supported chiral ligands.

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D. Nasturica, Synthesis 2000, 1035– 1074. Development of nano-particles of palladium crystallites encapsulated inside polymer resin have become a major challenge; their synthetic use is, however, still limited to hydrogenation of unsaturated compounds. For examples, see: a) D. E. Bergbreiter, B. Chen, T. J. Lynch, J. Org. Chem. 1983, 48, 4179–4186; b) M. Zecca, R. Fisera, G. Palma, S. Lora, M. Hronec, M. Kralik, Chem. Eur. J. 2000, 6, 1980–1986. B. M. Trost, E. Keinan, J. Am. Chem. Soc. 1978, 100, 7779–7781. K. Kaneda, H. Kurosaki, M. Terasawa, T. Imanaka, S. Teranishi, J. Org. Chem. 1981, 46, 2356–2362. C.-M. Andersson, K. Karabelas, A. Hallberg, J. Org. Chem. 1985, 50, 3891–3895. a) S.-B. Jang, Tetrahedron Lett. 1997, 38, 1793–1796; b) I. Fenger, C. L. Drian, Tetrahedron Lett. 1998, 39, 4287–4290; c) K. Inada, N. Miyaura, Tetrahedron 2000, 56, 8661–8664. a) M. A. Fox, D. A. Chandler, P.-W. Wang, Macromolecules 1991, 24, 4626– 4636; b) P.-W. Wang, M. A. Fox, J. Org. Chem. 1994, 59, 5358–5364. C. A. Parrish, S. L. Buchwald, J. Org. Chem. 2001, 66, 3820–3827. A. F. Littke, C. Dai, G. C. Fu, J. Am. Chem. Soc. 2000, 122, 4020. J. Schwarz, V. P. W. Bo¨hm, M. G. Gardiner, M. Grosche, W. A. Herrmann, W. Hieringer, G. Raudaschl-Sieber, Chem. Eur. J. 2000, 6, 1773–1780. J. K. Stille, H. Su, D. H. Hill, P. Schneider, M. Tanaka, D. L. Morrison, L. S. Hegedus, Organometallics 1991, 10, 1993–2000. N. Riegel, C. Darcel, O. Ste´phan, S. Juge´, J. Organomet. Chem. 1998, 567, 219–233. M. R. Buchmeiser, K. Wurst, J. Am. Chem. Soc. 1999, 121, 11101–11107. a) H. Alper, P. Arya, S. C. Bourque, G. R. Jefferson, L. E. Manzer, Can. J. Chem. 2000, 78, 920–924; b) similar dendrimer-bound phosphine ligands

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anchored on PS resin have been prepared for rhodium-catalyzed reactions, see: P. Arya, G. Panda, N. V. Rao, H. Alper, S. C. Bourque, L. E. Manzer, J. Am. Chem. Soc. 2001, 123, 2889–2890. E. B. Eggeling, N. J. Hovestad, J. T. B. H. Jastrzebski, D. Vog, G. V. Koten, J. Org. Chem. 2000, 65, 8857– 8865. a) Y. Uozumi, H. Danjo, T. Hayashi, Tetrahedron Lett. 1997, 38, 3557; b) H. Danjo, D. Tanaka, T. Hayashi, Y. Uozumi, Tetrahedron 1999, 55, 14341. Y. Uozumi, T. Watanabe, J. Org. Chem. 1999, 64, 6921. Y. Uozumi, H. Danjo, T. Hayashi, J. Org. Chem. 1999, 64, 3384. D. E. Bergbreiter, Y.-S. Liu, Tetrahedron Lett. 1997, 38, 7843. For a review, see: D. E. De Vos, I. F. J.

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Vankelecom, P. A. Jacobs (eds) Chiral Catalyst Immobilization and Recycling. Wiley-VCH, Weinheim 2000. Y. Uozumi, H. Danjo, T. Hayashi, Tetrahedron Lett. 1998, 39, 8303. K. Hallman, E. Macedo, K. ¨ m, C. Moberg, Tetrahedron Nordstro Asymmetry 1999, 10, 4037–4046. a) D. J. Bayston, J. L. Fraser, M. R. Ashton, A. D. Baxter, M. E. Polywka, E. Moses, J. Org. Chem. 1998, 63, 3137–3140; b) D. J. Bayston, J. L. Fraser, M. R. Ashton, A. D. Baxter, M. E. Polywka, E. Moses, Speciality Chemicals 1998, 18, 224– 226. A. Fujii, M. Sodeoka, Tetrahedron Lett. 1999, 40, 8011–8014. Y. Uozumi, K. Shibatomi, J. Am. Chem. Soc. 2001, 123, 2919–2920.

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Olefin Metathesis and Related Processes for CC Multiple Bond Formation Florencio Zaragoza 20.1

Introduction

Olefin metathesis refers to a reaction in which two alkenes exchange their alkylidene fragments (Scheme 20.1) [1–5]. This reaction has been applied to the preparation of compound libraries in solution and has also been used for solid-phase synthesis, thus enabling its application to automated parallel synthesis. Examples of metathesis on solid phase include the chemical transformation of resin-bound intermediates as well as the cleavage of final products from the support.

Scheme 20.1. The mechanism of olefin metathesis, dissociative mechanism for ruthenacyclobutane formation [27], and some useful ruthenium-based catalysts. Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 1. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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20 Olefin Metathesis and Related Processes for CC Multiple Bond Formation

Olefin metathesis can be catalyzed both by heterogeneous catalysts (mainly supported transition metal oxides) and by soluble transition metal complexes (mainly tungsten, molybdenum, or ruthenium carbene complexes); the process has been known and exploited industrially since the early 1960s [6, 7]. However, it was not until soluble and highly effective catalysts were discovered by Schrock, Grubbs, and others that this reaction could be performed under sufficiently mild conditions to enable its systematic application to organic synthesis. Ruthenium carbene complex 1 [8], and the newer, more effective complexes 2 [9–12] and 3 [13–18] (Scheme 20.1) are particularly well suited for organic synthesis because of their high stability and chemoselectivity. These and related complexes tolerate a broad selection of functional groups and mediate olefin metathesis even in the presence of air and protic solvents, including water [19–21]. New catalysts with improved properties are continuously being developed [22–26]. The mechanism by which carbene complexes catalyze olefin metathesis is shown in Scheme 20.1. A reversible 2 þ 2 cycloaddition of the carbene complex to the alkene yields a metallacyclobutane, which upon cycloreversion can yield either the products of olefin metathesis or the starting materials. Because all of these transformations are reversible, equilibrium toward the desired products must be shifted either by continuous removal of one of the products (e.g. ethylene) or by using substrates for which olefin metathesis cannot be reversed (e.g. strained cycloalkenes). In the case of ruthenium carbene complexes such as 1, one of the phosphine ligands dissociates from the complex during catalysis [27]. It is still being debated whether this dissociation takes place before or after coordination to the alkene. In the case of complex 3, it has been shown that olefin metathesis is initiated by a dissociative mechanism, as shown in Scheme 20.1 [27]. Although the currently available catalysts are exceedingly useful for many applications, a series of drawbacks still limits their scope, and there is ample room for improvement. Hopefully, some of these weaknesses will be overcome as our understanding of the precise mechanism of carbene complex-mediated olefin metathesis deepens and new types of catalysts emerge. One of the problems of current metathesis catalysts is that they generally yield internal alkenes as mixtures of Eand Z-isomers. Moreover, because the required carbene complexes can also react with soft nucleophiles other than alkenes, care must be taken to remove all traces of amines, pyridines, imidazoles, or other potential ligands from the solvent and the reactants in order to avoid deactivation of the catalyst. For the same reason, olefin metathesis proceeds best in solvents of low nucleophilicity [CH2 Cl2 > toluene > tetrahydrofuran (THF)]. Electron-rich alkenes (enamines, enol ethers) do not usually undergo metathesis because donor-substituted carbene complexes are formed as intermediates, which are no longer electrophilic enough to act as metathesis catalysts. With some of the most recently developed catalysts (such as 3, Scheme 20.1), however, even enol ethers may undergo metathesis [17]. The reaction rate of olefin metathesis sharply decreases in the series terminal alkene > internal, disubstituted alkene > trisubstituted alkene [2, 14], and 1,2disubstituted cis-alkenes are usually more reactive than the corresponding transalkenes.

20.1 Introduction

Most of the currently used catalysts (e.g. 1, 2, and 3) are ruthenium benzylidene complexes. One of the reasons for choosing the benzylidene ligand is that styrene does not generally undergo cross-metathesis efficiently when using ruthenium carbene complexes as catalysts [28]. For this reason, only small amounts of product resulting from a cross-metathesis with styrene will result, even if large amounts of catalyst are used (see, for example, Scheme 20.20). Olefin metathesis reactions have been grouped into different categories (Scheme 20.2). These include self-metathesis (reaction of one alkene with itself ), crossmetathesis (reaction of two different alkenes with each other), ring-opening metathesis polymerization (ROMP, polymerization of a strained, cyclic alkene), and ring-closing metathesis (RCM, cyclization of a diene). All these types of olefin metathesis can be conducted on insoluble supports, and can thus be adapted to automated, parallel solid-phase synthesis. For combinatorial chemistry in solution, mainly cross-metathesis and ROMP have been used.

Scheme 20.2. Categories of olefin metathesis reactions [2].

Alkynes are also suitable substrates for catalytic olefin metathesis (Scheme 20.3). Treatment of a mixture of an alkene and an alkyne with a metathesis catalyst can lead to the clean formation of dienes [29, 30]. Because alkynes usually react faster than alkenes with carbene complexes, the formation of dienes from alkenes and

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alkynes is assumed to proceed via the initial addition of the catalyst to the alkyne, followed by cycloreversion and reaction of the resulting vinylcarbene complex with the alkene (Scheme 20.3). Treatment of alkynes with carbyne complexes can bring about alkyne metathesis [1, 31], which presumably also proceeds via reversible 2 þ 2 cycloadditions (Scheme 20.3). Some of these intriguing transformations have also been performed on insoluble supports, and might be suitable for parallel synthesis.

Scheme 20.3. Metathesis of alkynes.

20.2

Olefin Metathesis in Solution

For the preparation of compound libraries in solution using olefin metathesis, mainly two strategies have been employed. These are the cross-metathesis of mixtures of terminal alkenes, to yield mixtures of internal, disubstituted alkenes, and the oligomerization by ring-opening metathesis of strained, cyclic alkenes (Scheme 20.4). Ring-closing metathesis in solution has been used mainly for the preparation of small arrays of compounds or of single compounds.

Scheme 20.4. Strategies for the preparation of compound mixtures using olefin metathesis.

20.2.1

Scope and Limitations of Olefin Metathesis in Solution

Cross-metathesis of two different terminal alkenes in solution only rarely gives high yields of one product (for recent advances in selective cross-metathesis, see

20.2 Olefin Metathesis in Solution

[14, 28, 32–34]). Usually, mixtures of the products of cross-metathesis and of selfmetathesis are obtained, each of them as mixtures of E- and Z-isomers. Unfortunately, some alkenes show a high tendency to undergo self-metathesis (to form symmetric ‘‘dimers’’; see, for example, [35, 36]), whereas other alkenes (acrylonitrile, styrenes) undergo self-metathesis only slowly or not at all. For this reason, cross-metathesis of mixtures of different olefins will not always yield the statistically expected amounts of internal olefins. This feature can cause problems during the deconvolution of such compound libraries because potent ligands formed only in low quantities are usually difficult to identify by deconvolution. A further problem of cross-metathesis in solution is that a purification step will usually be required to remove the catalyst. With the recent development of immobilized catalysts (e.g. 4–6) [37–41], however, this problem has been reduced. Unfortunately, all of the immobilized ruthenium carbene complexes described so far (Scheme 20.5) lose activity rather quickly; this fact might be due to the inherent instability of these complexes and to the fact that during catalysis detachment of the metal from the support can readily occur.

Scheme 20.5. Support-bound ruthenium carbene complexes, useful as insoluble metathesis catalysts [24, 38–41]. PS, crosslinked polystyrene; PEG, poly(ethylene glycol).

All known metathesis catalysts, being essentially electrophilic reagents, react with nucleophiles such as amines, nitrogen-containing heterocycles, and thiols. Accordingly, alkenes containing these functional groups (which are often important for the interaction of small molecules with proteins) cannot be used as building blocks for library preparation, unless these functional groups are effectively masked. 20.2.2

Examples of Library Preparation by Cross-metathesis in Solution

One of the first examples of the preparation of compound libraries by crossmetathesis was reported by Boger and coworkers [42–44], who dimerized mixtures of alkenoyl iminodiacetamides by cross-metathesis in solution (Scheme 20.6). The aim of this work was to identify new agonists or antagonists for biochemical signal transduction processes which involve the dimerization or oligomerization of pro-

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Scheme 20.6. Preparation of libraries of iminodiacetamides by cross-metathesis.

teins [e.g. tyrosin/serine/threonine kinase receptors, cytokine receptors, tumor necrosis factor (TNF) receptors]. During optimization of the chemistry they found that 3-butenamides (n ¼ 1; Scheme 20.6) did not undergo metathesis at all, and 4-pentenamides ðn ¼ 2Þ only reacted sluggishly under the conditions of cross-metathesis. Longer o-alkenoyl amides, however, cleanly yielded the expected internal alkenes [44]. The libraries were usually purified by column chromatography. Similarly, Benner and coworkers [45] prepared mixtures of internal alkenes by cross-metathesis of mixtures of terminal olefins. The resulting libraries of alkenes were oxidized to the corresponding diols or epoxides. The mixtures of diols were the starting monomers for ‘‘receptor-assisted combinatorial synthesis’’, in which these diols were to be dimerized reversibly to borate esters in the presence of a receptor. Under conditions of dynamic equilibrium, enhanced concentrations of those borate esters with highest affinity to the receptor are to be expected [45]. The authors observed during the optimization of the metathesis reaction that certain alkenes (Scheme 20.7) failed to undergo cross-metathesis and others only reacted sluggishly, depending on the functional groups present in these alkenes. In particular, nitrogen-containing alkenes did not undergo metathesis – this might be due to complexation with the catalyst. A further example of target-accelerated combinatorial synthesis has been reported by Nicolaou et al. [46]. With the aim of finding new vancomycin dimers with improved antibiotic activity, various alkenylated derivatives of vancomycin were subjected to conditions of olefin metathesis in the presence of derivatives of l-Lys-d-Ala-d-Ala, the peptide to which vancomycin strongly binds and thereby inhibits the cell wall growth of bacteria (Scheme 20.8). Cross-metathesis was performed in aqueous solution at 23 C in the presence of a phase-transfer catalyst (C12 H25 NMe3 Br) and with (Cy3 P)2 Cl2 RubCHPh (0.2 equiv.) (1) as metathesis catalyst. In this instance, it was observed that addition of the target peptide in fact led to increased concentrations of those dimers which were also the more potent antibiotics.


Scheme 20.7. Suitability of alkenes for cross-metathesis [45].

Bra¨ndli and Ward [47] prepared mixtures of internal, disubstituted alkenes by equilibration of internal olefins (oleic acid derivatives). Their synthesis was performed either in dichloromethane or without any solvent, and proceeded satisfactorily with as little as 0.1% of (Cy3 P)2 Cl2 RubCHPh (1) if no solvent was used. With gas chromatography–mass spectrometry (GC/MS) and 13 C-NMR (nuclear magnetic resonance) spectroscopy the authors were able to identify all ten expected products (each as E/Z mixture) of the equilibration of two different, unsymmetrical alkenes (Scheme 20.9).

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Scheme 20.8. Vancomycin dimers prepared by crossmetathesis (m ¼ 1, 2, 3, 7; R ¼ H, b-Ala, l-Asn, d-LeuNMe, gAbu, l-Arg, l-Phe [46]).

Scheme 20.9. Equilibration of internal alkenes by cross-metathesis [47].

20.2.3

Examples of Library Preparation by Ring-closing Metathesis in Solution

Ring-closing metathesis is increasingly being used for the preparation of conformationally constrained analogs of peptides. Most of the examples reported, however, only describe the synthesis of single compounds or of small arrays of compounds. These syntheses are only rarely based on easily available dienes, and are therefore not always suitable for the preparation of large compound libraries. Moreover, unlike cross-metathesis, ring-closing metathesis is an intramolecular reaction which does not increase the number of products or their diversity. Hence, ring-closing metathesis only allows the conversion of one library into another, without changing the total number of products within this library. Cyclic peptides are an important tool for the identification of turns within a peptide which are critical for its biological activity. In analogy to the ‘‘Ala-scan’’, in which all the amino acids of a peptide are sequentially replaced by alanine to identify those amino acids which are crucial for biological activity, a ‘‘loop scan’’ (Scheme 20.10 [48]) may be used to locate possible turns within a peptide and to identify conformationally constrained analogs (7–10) of the original peptide (11).


Scheme 20.10. Illustrative example of ‘‘loop scan’’. Four cyclic analogs (7–10) of the original peptide are prepared and their biological activity is compared with the activity of the original peptide (11). X ¼ variable spacer.

Some new strategies for the preparation of cyclic peptides by ring-closing metathesis are presented below to illustrate the scope of these cyclizations. Liskamp and coworkers have investigated the cyclization of N-alkenylated peptides (12) by ring-closing metathesis (Scheme 20.11) [48–50]. The peptides were prepared by standard solid-phase synthesis, and the N-alkenylation was effected during the assembly of the peptide by N-sulfonylation with 2-nitrobenzenesulfonyl chloride, followed by N-alkenylation under Mitsunobu conditions and sulfonamide cleavage by treatment with mercaptoethanol/1,8-diazabicyclo[5.4.0]undec-7ene (DBU). Ring-closing metathesis could be performed either in solution or on solid phase, but in solution higher yields were usually obtained [50]. Cyclization experiments showed that the length of the N-alkenyl group was crucial for ring

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closure. N-Allyl peptides (12) could only be cyclized to yield eight-member rings (13). Larger ring sizes required the use of N-homoallyl or N-(4-penten-1-yl) peptides. The cyclization of tripeptides (to form a 15-member ring, e.g. (14)) was particularly difficult, and only proceeded satisfactorily with N-(4-penten-1-yl) substitution (Scheme 20.11).

Scheme 20.11. Preparation of cyclic peptides by ring-closing metathesis in solution, and lengths of the N-alkenyl substituent required for ring formation [49].

Other recent examples of the preparation of cyclic peptide analogs by ringclosing metathesis in solution include the cyclic sulfamides 15 [51], cyclic sulfonamides 16 [37], and siloxanes 17 [52] (Scheme 20.12). The last were synthetic intermediates for the preparation of diols such as 18, which were used as building blocks for the solid-phase synthesis of peptide analogs [52]. Further examples of

Scheme 20.12. Peptidomimetics prepared by ring-closing metathesis in solution.

20.3 Olefin Metathesis on Solid Phase

solution-phase synthesis of peptide mimetics by ring-closing metathesis have been reported [2, 53–56]. 20.2.4

Examples of Library Preparation by Ring-opening Metathesis Polymerization in Solution

Functionalized oligomers and polymers are of interest for a variety of applications. These include their use in chromatography as the stationary phase [57] for the separation of metals [58] or soluble receptors, and as carriers for the controlled release of drugs [59]. Oligomers functionalized with biologically relevant molecules such as amino acids or carbohydrates can also be used to mimic various biopolymers (proteins, DNA) or the surface of a cell. Such biopolymer mimetics are useful tools for studying the interaction of cell surfaces with biopolymers. Ring-opening metathesis polymerization (ROMP), in which a strained, cyclic alkene is polymerized with the aid of a metathesis catalyst, offers several features which make this reaction particularly attractive for the preparation of functionalized oligomers [58, 60–62]. ROMP can be conducted as a living polymerization because the rate of initiation can be faster than the rate of propagation. This feature enables the preparation of oligomers with well-defined length and narrow molecular weight distribution. Because the oligomers persist as active carbene complexes even when one monomer has been consumed, ROMP also enables the preparation of block copolymers, in which various different monomers are polymerized sequentially. Kiessling and coworkers have used ROMP for the preparation of carbohydratefunctionalized oligomers, which were used as ligands for various carbohydratebinding proteins (concanavalin A [63], P-selectin [64], L-selectin [65]). Initially, ROMP was performed with norbornenes that were already covalently linked to a carbohydrate. However, better results were later, obtained by preparing activated oligomers by ROMP, which were then derivatized with the carbohydrate (Scheme 20.13). Maynard et al. [67] used ROMP of exo-5-norbornene-2-carboxylic acid derivatives for the preparation of oligomers displaying the peptide sequences GlyaArga GlyaAsp and SeraArgaAsn, which play an important role in the binding of extracellular matrix proteins to cell-surface integrins. Both homopolymers and copolymers were prepared and characterized (Scheme 20.14). Polymers substituted with these peptides are being considered for use in the treatment of cancer [67]. 20.3

Olefin Metathesis on Solid Phase

In solid-phase synthesis, the metathesis of alkenes has been used both for the chemical transformation of support-bound intermediates as well as for the cleavage of products from the support. Although these techniques have not yet been

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Scheme 20.13. Strategies for the preparation of carbohydrate-

functionalized oligomers by ROMP [65, 66]. DCE, 1,2dichloroethane.

Scheme 20.14. Preparation of peptide-functionalized oligomers by ROMP [67].

extensively used for the preparation of large libraries by parallel synthesis, solidphase chemistry is generally well suited for this purpose, and some of the reactions described below can probably be used for the preparation of compound libraries.


20.3.1

Cleavage from the Support by Olefin Metathesis 20.3.1.1 Scope and Limitations

With the discovery of highly efficient and robust soluble catalysts which mediate olefin metathesis under mild reaction conditions even in the presence of water and air, the use of alkenes as linkers for solid-phase synthesis became a realistic option. The use of alkenes as linkers is an attractive alternative to other types of linkers because alkenes are inert toward a broad range of reaction conditions, and because they provide for a reliable fixation of intermediates to the support. Various strategies for the cleavage of compounds from insoluble supports by olefin metathesis have been described (Scheme 20.15). Support-bound dienes can yield either terminal alkenes or cycloalkenes, depending on how the diene is bound to the resin. Terminal alkenes can also be prepared by cross-metathesis of resin-bound internal alkenes with ethylene [68].

Scheme 20.15. Strategies for the cleavage of alkenes from

insoluble supports by olefin metathesis.

Occasionally, carbene complex-mediated cleavage reactions give only low yields. When an additional olefin was added to the reaction mixture, however, better yields could be obtained [69]. This effect was attributed to the irreversible fixation of the carbene complex to the support when little or no amounts of terminal alkenes were present in the reaction mixture (Scheme 20.16). Later studies [71] suggest that the irreversible fixation of the catalyst to the support is not necessarily detrimental to the yield of the cleavage reaction if spacers of sufficient flexibility are used. Thus, diene 19 (Scheme 20.17) could not be cleaved from the support, and even in the presence of 1-octene only traces of the desired product were obtained. The more flexible diene 20, on the other hand, underwent smoothly RCM smoothly in the absence of any additional alkene, to give the expected cyclic product (21) in high yield. The fact that the support was colored after cleavage and catalyzed olefin metathesis suggests that carbene complexes were indeed covalently bound to the support. The flexibility of the spacer enables the metal fragment to migrate from one attachment point to the next, so that catalytic

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Scheme 20.16. Mechanism of the cleavage of dienes from supports by RCM [70].

Scheme 20.17. Dependence of cleavage yields on the flexibility of spacers and on the type of alkene used as linker [71].

amounts of the carbene complex are sufficient to achieve complete metathesis of all attachment sites. Another reason for the resistance of 19 toward carbene complex-mediated cleavage may be the fact that 19 is a styrene derivative. Styrenes usually react more slowly with ruthenium carbene complexes than unconjugated, internal cis-alkenes. One problem which is inherent to olefin metathesis-induced cleavage is the elution of catalyst-derived byproducts together with the final product. The currently known metathesis catalysts (mainly ruthenium carbene complexes) decompose slowly during the metathesis reaction to yield various ruthenium complexes, which do not remain attached to the support. These impurities have to be removed by chromatographic purification of the products. However, large libraries of compounds for direct biological screening cannot always be purified, and cleavage by


olefin metathesis will only be of limited use for the preparation of such libraries unless more stable metathesis catalysts or selective scavengers for metal-containing byproducts become available. 20.3.1.2 Examples of Cleavage from the Support by Olefin Metathesis

Knerr and Schmidt [72, 73] have used a metathesis-based cleavage strategy for the solid-phase synthesis of oligosaccharides (Scheme 20.18). Cleavage by treatment with Grubbs’ catalyst yielded O-allyl glycosides (22), which represent versatile, protected intermediates for further synthetic manipulations [73].

Scheme 20.18. Synthesis of allyl glycosides by RCM-mediated

cleavage from a polymeric support [72, 73].

A similar strategy has been described by Peters and Blechert [74], in which RCM of a support-bound diene was used to release styrenes from a polystyrene-based, insoluble support. Linkers of this type can also be cleaved by cross-metathesis with ethylene [68]. Several groups have investigated the preparation of cyclic compounds by RCM with simultaneous cleavage from the support [71, 75, 76]. One recent example, reported by Piscopio et al. [77], is shown in Scheme 20.19. The substrate (23) for olefin metathesis was prepared in one step by an Ugi reaction. The product (24), a Freidinger lactam, was designed to mimic b-turns, which play a pivotal role in the

Scheme 20.19. Solid-phase synthesis of b-turn mimetics by

RCM with simultaneous cleavage from the support [77].

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interaction of proteins. Because styrene derivative 25 was chosen as linker, cleavage required prolonged heating for a long time. An unconjugated cis-alkene would probably allow milder cleavage conditions (cf. Schemes 20.17 and 20.18). Nicolaou et al. [78, 79] have used ring-closing metathesis with simultaneous cleavage from the support in an elegant solid-phase synthesis of epothilone analogs (Scheme 20.20). Epothilones are a group of natural products which promote the polymerization of a- and b-tubulin subunits, and which show higher cytotoxicity than taxol [80]. These interesting biological properties have prompted several research groups to develop syntheses for these compounds and analogs thereof [80, 81].

Scheme 20.20. Solid-phase synthesis of epothilone analogs [78, 79].

In Nicolaou’s solid-phase synthesis of epothilone analogs, a Merrifield resin (PSaCH2 Cl) with low loading (0.3 mmol g1 ) was used. After release from the support the products were purified by preparative thin layer chromatography. More than 100 epothilone analogs have been prepared using this methodology, and their biological evaluation gave detailed insight into the structure–activity relationship of this family of natural products [79]. 20.3.2

Ring-closing Metathesis on Solid Phase 20.3.2.1 Scope and Limitations

Ring-closing metathesis (RCM), being a reversible process, is best suited to the preparation of unstrained cyclic compounds. In most of the reported examples of RCM on solid phase [2], five- or six-member rings were generated. Other ring sizes are also accessible, but careful optimization of the reaction conditions are often necessary. Macrocyclizations, for instance, usually require the use of supports with low loading to avoid self-metathesis (Scheme 20.21).

Scheme 20.21. Ring-closing metathesis on solid phase, and self-metathesis as a potential side reaction.


20.3.2.2 Examples of Ring-closing Metathesis on Solid Phase

Several examples of the solid-phase synthesis of nitrogen-containing heterocycles have been reported (Scheme 20.22) (for an example performed on soluble poly(ethylene glycol), see [82]). Heating and substantial amounts of ruthenium carbene complex are usually required to attain complete conversion of the starting diene (e.g. 25). Eneyne 28 is an interesting example of an intramolecular ene–yne coupling, which gives ready access to highly substituted dienes (29), which in turn are suitable starting materials for Diels–Alder reactions [83].

Scheme 20.22. Examples of the preparation of nitrogencontaining heterocycles by RCM on solid phase [83–85]. PS, crosslinked polystyrene.

Various groups have investigated the preparation of cyclic peptides by RCM on solid phase. In Section 20.2.3, the work of Liskamp and coworkers concerning the cyclization of peptide-derived dienes was presented. These cyclizations generally give higher yields in solution than on solid phase [48–50]. Another example on crosslinked polystyrene is shown in Scheme 20.23. With the aim of finding efficient routes to structurally complex, polycyclic compounds, Lee et al. [86] recently developed the synthesis shown in Scheme 20.24. With an Ugi reaction followed by an intramolecular Diels–Alder reaction and an

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Scheme 20.23. Preparation of cyclic peptides by RCM on solid phase [55]. TG, Tentagel.

Scheme 20.24. Synthesis of polycyclic structures using a ring-

opening ring-closing metathesis cascade [86].

allylation, strained triene (30) was generated, which upon treatment with a metathesis catalysts underwent a ring-opening/ring-closing cascade to yield highly substituted, tetracyclic compounds (31). A valuable feature of this synthesis is the ready availability of some of the four building blocks. One drawback of this reac-


tion sequence is the low selectivity of the allylation reaction, which necessitates the protection of all nucleophilic functional groups. 20.3.3

Cross- and Self-metathesis on Solid Phase 20.3.3.1 Scope and Limitations

Cross-metathesis should in principle enable the efficient preparation of unsymmetrical, acyclic alkenes on solid phase. Unfortunately, this reaction does not always proceed as expected, mainly because only few types of terminal alkenes smoothly undergo cross-metathesis (see Scheme 20.7). Alkenes bearing ‘‘interesting’’ functional groups (hydrogen bond donors and acceptors) sometimes react only sluggishly or not at all, leading either to complete consumption of the catalyst (formation of unreactive carbene complexes) and/or to formation of large amounts of the products of self-metathesis (e.g. 33) (Scheme 20.25).

Scheme 20.25. Strategies for performing cross-metathesis on solid phase.

An interesting variant of cross-metathesis is the so-called ring-opening crossmetathesis. Strained, cyclic alkenes (e.g. norbornene (35), cyclobutene) react rapidly and irreversibly with metathesis catalysts to yield a new carbene complex, which can react with a second alkene to yield the product of cross-metathesis (36). This reaction has also been performed successfully on solid phase (Scheme 20.25). 20.3.3.2 Examples of Cross- and Self-metathesis on Solid Phase

Some illustrative preparations of internal alkenes by cross-metathesis on solid phase are shown in Scheme 20.26. Allylsilanes (37) appear to be well suited for this reaction [33, 87], and substituted allylsilanes (38), which are valuable synthetic intermediates, can be easily prepared by cross-metathesis (Scheme 20.26). Nicolaou et al. used cross-metathesis of support-bound, alkenyl-substituted ketophospho-

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nates (39) with o-alkenols (40) in a solid-phase synthesis of muscone analogs [88] (Scheme 20.26). Unsaturated a-amino acid derivatives (42) have also been prepared on solid phase by cross-metathesis [35] (Scheme 20.26). The low loading of the starting resin (0.07 mmol g1 ) was required to suppress self-metathesis.

Scheme 20.26. Cross-metathesis on solid phase [35, 87, 89]. PS, crosslinked polystyrene; 1, (Cy3 P)2 Cl2 RubCHPh.

The combination of ring-opening with cross-metathesis (‘‘ring-opening crossmetathesis’’) has been realized on insoluble supports by Cuny and coworkers [90– 92]. Support-bound norbornene derivative 43 was treated with styrenes 44 (which do not undergo self-metathesis efficiently) and a ruthenium carbene complex, to yield regioisomeric mixtures of highly substituted cyclopentenes 45 (Scheme 20.27). Substituents on styrene which were tolerated included tert-butyl, alkoxy, acyloxy, and hydroxy.

Scheme 20.27. Ring-opening cross metathesis on solid phase [92]. (PS), crosslinked polystyrene with spacer; 1, (Cy3 P)2 Cl2 RubCHPh.

The carbene complex-mediated coupling of alkynes with alkenes to yield 1,3dienes [13, 16, 93] is one of the most surprising metathesis reactions. Despite a number of potential side reactions (polymerization of the alkyne, self-metathesis of


the alkene), high yields of dienes can be obtained. This reaction can also be conducted on insoluble supports, with either the alkene or the alkyne attached to the resin (Scheme 20.28). The resulting dienes can be further transformed by 2 þ 4 cycloaddition with suitable dienophiles to yield substituted cyclohexenes [94].

Scheme 20.28. Examples of alkene/alkyne cross-metathesis on solid phase [29, 95]. PS, crosslinked polystyrene; 1, (Cy3 P)2 Cl2 RubCHPh.

Self-metathesis of support-bound N-alkenoylated peptides was used by CondeFriboes et al. [96] for the preparation of symmetric peptidomimetics (Scheme 20.29). Peptides were prepared on crosslinked polystyrene by standard fluorenylmethoxycarbonyl (Fmoc) chemistry and then acylated with an o-alkenoic acid. In accordance with similar results of Boger and Chai [44] (Section 20.2.2), neither 3-butenamides nor 4-pentenamides underwent self-metathesis efficiently. With longer alkenoic acids, however, the peptide dimers 46 were formed in high yield and purity as mixtures of E/Z isomers.

Scheme 20.29. Self-metathesis of support-bound, N-alkenoyl peptides [96].

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20.4

Conclusion

With the development of highly efficient and selective catalysts for olefin metathesis in recent years, this reaction has become a valuable tool for organic chemists. Cross-metathesis and ROMP in solution can now be performed with functionalized alkenes, and can offer interesting new possibilities for the preparation of compound libraries. In particular, the selective cross-metathesis of different alkenes, for which the underlying principles are now slowly emerging, has huge potential and could become a process with an impact similar to those of the Wittig or the Diels–Alder reactions if its scope and limitations are clearly understood. Solid-phase synthesis has also greatly benefited from these new catalysts, and new cleavage strategies and other methodologies based on carbene complex-mediated olefin metathesis on solid phase have been developed successfully. The properties of the currently available catalysts are, however, far from ideal. Because of their limited stability, large amounts of catalyst are often required to drive reactions to completion. This feature can lead to significant amounts of metal-derived impurities in the crude products. Moreover, most carbene complexes suitable for olefin metathesis are also highly sensitive toward amines, azoles, and other nucleophiles, which severely limits the choice of functional groups tolerated in the starting materials. This facet is particularly problematic for the preparation of libraries of biologically active compounds because nucleophilic functional groups are often of crucial importance for biological activity. Future research should aim to overcome these limitations of current catalysts by further enhancing their selectivity and stability. References ¨rstner, Angew. Chem. Int. Ed. 1 A. Fu 2

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Part III

Special Synthetic Topics


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21

Solid-phase Synthesis of Natural Products and Natural Product-like Libraries K. C. Nicolaou and Jeffrey A. Pfefferkorn 21.1

Introduction

Natural products have always played, and continue to play, important roles in both drug discovery and chemical biology. In fact, from 1989 to 1995, 60% of all approved drugs and new drug application (NDA) candidates were derived from natural sources [1]. Furthermore, natural products have been used extensively for studying complex biological processes such as signal transduction and cell cycle regulation [2]. Yet, in spite of their importance to both biology and medicine, it was only recently that solid-phase chemistry has been applied to the synthesis of natural products and their analogs. Initial attempts were hampered by the limited scope of early solid-phase reactions for the construction of nonoligomeric structures as well as the lack of appropriate solid-phase analytical techniques [3]. However, given the remarkable advances in solid-phase chemistry over the last decade, it is not surprising that numerous research groups are now actively engaged in the construction of structurally complex, natural product-based libraries using solid-phase combinatorial synthesis techniques [3, 4]. Figure 21.1 shows a diverse collection of naturally occurring structures which have been synthesized using solid-phase technologies. Within this chapter, we seek to highlight these accomplishments, focusing particular attention on the novel strategies developed to solve each problem and on how the synthetically demanding complexity of natural products has spurred the development of new linking and cleavage devices and protocols, as well as on the chemical biology contributions made possible by these technologies. The discussion will be divided into three sections paralleling, chronologically, the trends which have emerged in the solid-phase synthesis of natural products. These include: (1) combinatorial derivatization of immobilized natural product skeletons; (2) solid-phase target-oriented total synthesis of natural products; and (3) construction of diversity-oriented natural product-like libraries for chemical biology and drug discovery purposes.


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21 Solid-phase Synthesis of Natural Products and Natural Product-like Libraries

Fig. 21.1. Selected natural products synthesized and/or diversified through solid-phase chemistry.

21.2

Solid-phase Derivatization of Natural Product Scaffolds – Combinatorial Semisynthesis

The earliest examples of the solid-phase library construction of natural product analogs focused on the immobilization of natural product skeletons on solid supports so as to facilitate installation of diversity elements in either a parallel or splitand-pool format. This strategy has been used both to create focused libraries around a particular natural product for the development of structure–activity data as well as to create diversity-based libraries for high-throughput screening in drug discovery programs. Regardless of the final application, the success of such combinatorial chemistry efforts is contingent upon several key factors which include: (1) the ready availability of natural product scaffolds from natural sources, degradation chemistry, or solution-phase synthesis; (2) the identification of appropriate sites on the scaffold for attachment onto solid support so as to facilitate reliable

21.2 Solid-phase Derivatization of Natural Product Scaffolds -- Combinatorial Semisynthesis

loading/release, as well as installation of maximum structural diversity; and (3) the development or application of an appropriate encoding technology in order to facilitate rapid structural identification of individual library members. In order to illustrate better both the applicability of this approach and the experimental procedures involved in this scaffold-based solid-phase strategy, we will discuss below several studies which employed a variety of natural product skeletons, including yohimbine, purine, Taxol TM , sarcodictyns A and B, and vancomycin. 21.2.1

Solid-phase Semisynthesis of Rauwolfa Alkaloids

In an early example of this scaffold-based approach, researchers at the Affymax Research Institute enlisted the pentacyclic framework of the Rauwolfia alkaloids as a template for combinatorial derivatization to create a 792-member library [5]. The indole alkaloid yohimbinic acid (14) was selected as a scaffold by virtue of its rigid framework and rich functionality for elaboration and derivative formation. Although this natural product exhibits a wide range of biological activities, the primary objective of these investigations was not to optimize the activity toward a specific target, rather, the researchers sought to generate a diverse library of compounds to be used in high-throughput screening studies against various therapeutic targets. Preliminary efforts focused on the development of a linking and diversification strategy, which when tested resulted in the construction of an unencoded 792-member library, as outlined in Scheme 21.1 wherein the final compounds were obtained in 22 pools, each containing 36 compounds. As shown, TentaGel-SNH2 , functionalized with a trifluoroacetic acid (TFA)-labile Rink amide linker 13,

Scheme 21.1. Non-encoded solid-phase synthesis of alkaloids.

DIEA, diisopropylethylamine; 4-DAMP, 4-dimethyl amino pyridine; HATU, O-(7-azabenzotriazol-1-yl)-N,N,N 0 ,N 0 tetramethyluronium hexafluorophosphate; TFA, trifluoroacetic acid [5].

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was deprotected with piperidine to furnish free amine 15 which was then coupled to a series of N-fluorenylmethoxycarbonyl (Fmoc)-protected amino acids (16) via HATU activation. These 36 amino acids represented the first element of diversity in the library. After piperidine-mediated deprotection of the newly coupled amino acid 17, the liberated amine was coupled to the free C1 carboxylic acid of yohimbinic acid (14) to effect loading of the natural product skeleton onto the solid support. A second element of diversity was then installed at the C2 hydroxyl position of yohimbinic acid through reaction with the symmetrical anhydrides of a variety of carboxylic acids [(R2CO)2 O] to afford esters of type 20. After acidic cleavage, 22 pools each containing 36 compounds were obtained with the structures of selected pools confirmed by flow injection mass spectroscopy (MS) analysis. While validating the overall strategy, this first approach required extensive deconvolution efforts to identify the active component(s) of any of the pools. As such, the authors then elaborated on the above approach by cosynthesizing ‘‘surrogate analytes,’’ or tags, onto each bead which could be cleaved under conditions orthogonal to the chemistry of the actual library member on the same bead. In this way, if during biological assay the subanalytical amount of cleavage product from a given bead demonstrated activity, the tag on that bead could be subsequently cleaved and identified by MS, thereby revealing the structure of the active compound. The success of this strategy hinged upon construction of orthogonally differentiated beads; this was accomplished by treatment of TentaGel-S-NH2 with a 9:1 mixture of FmocCl and allyl chloroformate, giving a resin with a @5:1 ratio of Fmoc/Alloc sites. The N-Fmoc-protecting group was then cleaved with piperidine and the generated amine was acetylated with the N-Fmoc Rink amide (RAM) linker to give resin 22. The spacer for the amine tag was installed by removal of the Alloc protecting group [Pd(PPh3 )4 /TMSN3/TBAF] and reaction with N-Alloc iminodiacetic anhydride, leading to carboxylic acid 25. The free carboxylic acid of 25 was subsequently coupled with 36 secondary amines ( n Ry NH) through the intermediacy of an activated pentafluorophenolate ester. These 36 amines (each with a distinct molecular weight) served as the tags, and each of them encoded for a different amino acid to be coupled in the next step. After deprotection of 26 using piperidine, each of the 36 N-Fmoc-protected amino acids (16) were coupled to their respective resins using HATU activation to provide amides of type 28. Upon completion of this operation, the resins were pooled and then split into 22 new reaction vessels where the C2 hydroxyl group was acylated using a symmetrical acid anhydride as described above. This final acylation did not require tagging since the beads were already spatially segregated for the reactions. The products of this final reaction were then maintained in the 22 separate pools for screening. Since the synthesis was encoded, if any of the products released from a particular bead proved biologically active the surrogate amine tag from that bead could then be cleaved to provide identification, as illustrated at the bottom of Scheme 21.2. Treatment of the residual resin with HCl followed by Li 2 CO3 and subsequent labeling of the released amine with dansyl chloride would provide conjugate 32, which could be readily analyzed by MS to determine the structure of the active library member.


Scheme 21.2. Encoded solid-phase synthesis of alkaloids [5].

21.2.2

Solid-phase Synthesis of Purine Derivatives

In a second example, the Schultz group employed two complementary solid-phase strategies to construct libraries of purine derivatives [6, 7]. Since the purine ring system is a key structural element of many endogenous ligands which act as regulatory and signal transduction-mediating molecules, it was envisaged that libraries based on this scaffold might facilitate the discovery of novel inhibitors of these biological processes. These libraries were specifically designed for the purpose of identifying small molecule inhibitors of key cell cycle kinases which could potentially find applications as both biological probes and therapeutic leads. In order to retain spatial separation of library members, and thereby avoid the need for deconvolution, this solid-phase chemistry was conducted using the Geysen pin apparatus [8]. For the construction of the first library (Scheme 21.3a), the purine scaffold was linked to the Rink resin through a glycinamide moiety, furnishing structure 35. The initial diversity element was installed by acylation of the C2 exocyclic amine with various acid chlorides (R1COCl). The second diversity element was then introduced by nucleophilic displacement of the C6 chloride with various primary amines (R2NH2 ). Upon completion, these purine derivatives were released from the solid support by treatment with TFA to afford structures of type 38. To enable greater diversification at the C2 amino group, a complementary linking strategy was also developed as outlined in Scheme 21.3b. The tetrahydropyranyl-derivatized purine 40 was linked to the Rink resin via an intermediary – an activated pentafluorophenolate ester – to furnish 41. Elaboration of the C2 exocyclic amino group commenced with acylation using trifluoroacetic anhydride,

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Scheme 21.3. Solid-phase synthesis of 2-(acylamino)-6aminopurines and 2,6-diaminopurines. DIEA, diisopropylethylamine; PFP, pentafluorophenolate; PNP, p-nitrophenolate [6].

followed by alkylation, under Mitsunobu conditions with a variety of alcohols (R1OH) to give structures of type 42. As before, the chloride at C6 was then displaced with a series of primary amines (R2NH2 ) with concomitant cleavage of the trifluoroacetate group from the C2 amino group. Once complete, library members were then released from solid support by treatment with TFA to provide purine analogs of type 44. The combined application of the strategies in Schemes 21.3 and 21.4 resulted initially in the construction of 362 purine derivatives while subsequent efforts expanded these libraries. More importantly, through the screening of these libraries, several potent protein kinases inhibitors were identified and used to probe the role of protein kinases in biochemical processes [6, 7, 9]. 21.2.3

Solid-phase Semisynthesis of a Taxoid Library

In a third example of combinatorial natural product diversification, researchers at Irori Quantum Chemistry, now a division of Discovery Partners International, employed radiofrequency (RF) encoding to facilitate the combinatorial split-and-


Scheme 21.4. Radiofrequency (RF)-encoded solid-phase split-

and-pool synthesis of taxoid library [10].

pool construction of a 400-member focused library based on paclitaxel (4, Fig. 21.1), also known as Taxol TM [10]. Taxol TM (Bristol-Myers Squibb) was originally isolated from the bark of the Pacific yew (Taxus brevifolia) and was approved in 1992 by the Food and Drug Administration (FDA) as a drug for the treatment of refractory ovarian cancer [11]. Since then its indications have been expanded and it is often the drug of choice for treating ovarian, breast, nonsmall cell lung, bladder, esophagus, head, and neck carcinomas. Despite its impressive record, however, the poor water solubility of Taxol TM and its susceptibility to drug resistance prompted numerous efforts to construct analogs with improved pharmacological properties. The Irori group utilized RF-encoding technology in its synthesis of the taxoid library. Developed in 1995, this technology employs microreactors – polypropylenemade vessels with porous side panels capable of encapsulating both a solid-phase

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resin and a radiofrequency-addressable semiconductor tagging device [12]. These semiconductor tagging devices are capable of receiving, storing, and emitting RF signals to a suitably interfaced computer programmed with the synthetic algorithm [12]. The starting scaffold for these efforts was baccatin III (45, Scheme 21.4), a diterpene obtained from the European yew tree (Taxus baccata) and used for the semisynthesis of taxol. In order to attach this scaffold to the resin, a glutamate spacer unit was first installed on a Troc-protected variant of baccatin III by a PyBOP-mediated coupling with N-Fmoc-O-(trichloroethyl)-l-glutamic acid 47 to provide ester 48. After simultaneous removal of both the Troc and trichloroethyl groups with zinc dust, carboxylic acid 49 was reacted with excess 2-chlorotrityl resin in the presence of diisopropylethylamine to afford resin-bound ester 50. Resin 50 was then divided into 400 microreactors, each containing a radiofrequency tag. In a pooled operation, the N-Fmoc protecting group of 51 was removed by treatment with piperidine, after which the reactors were sorted into 20 pools and each set coupled with carboxylic acids (R2CO2 H) using PyBOP activation conditions to effect N-acylation. After recombining and then splitting the microreactors into 20 new pools, each set was then coupled to a second carboxylic acid under DIC-mediated coupling conditions to induce O-acylation at both the side-chain and C7 sites. The resin in the individual microreactors was then subjected to cleavage using standard acid conditions (AcOH) to release library members of type 55 in 2to 4-mg quantities with purities ranging from 50% to 100% as determined by thin layer chromatography (TLC) and high-performance liquid chromatography (HPLC) analysis. The structures of randomly selected members were also confirmed by 1 H-NMR (nuclear magnetic resonance) and ES/MS techniques. 21.2.4

Solid-phase Semisynthesis of Sarcodictyns A and B and Libraries Thereof

A fourth scaffold-driven natural products solid-phase synthesis from the Nicolaou group focused on sarcodictyns A and B (5 and 6, Fig. 21.1): two members of a family of marine-derived diterpenoids with potent cytotoxicities against various tumor cell lines [13]. Preliminary investigations revealed that these compounds, like Taxol TM , exert their cytotoxic effects through induction of tubulin polymerization and microtubule stabilization. At the time of isolation, little was known about which structural features of these marine natural products were required for activity, and their relative scarcity significantly hampered further investigations [14]. As a consequence, several groups undertook the challenge of their total synthesis and, today, two syntheses have been reported: one from our laboratory [15] and the other from Professor Danishesfky’s group [16]. With the completion of these syntheses came an opportunity to explore the structure–activity relationships of this family of marine natural products through the generation of analog libraries. Given the efficiency that solid-phase-based combinatorial chemistry could bring to such a task, the Nicolaou group embarked on a strategy employing this tool in the construction of sarcodictyn libraries. In contrast to the previous two examples, the parent natural product was only available in trace quantities from natural sources;


therefore, the operation of library construction had to rely on the solution-phase total synthesis to produce a key advanced intermediate which could be attached to a solid support and subsequently diversified. The first step of this combinatorial approach was to develop a linking strategy wherein such an advanced intermediate from the synthesis could be tethered so as to allow for maximum installation of diversity elements while maintaining a reliable and efficient cleavage protocol [13]. It became evident that tricyclic hemiketal 57 could be employed as the library scaffold and that this intermediate could be linked through a mixed ketal moiety at C(4), as outlined in Scheme 21.5. Hence, compound 57, obtained from ðþÞ-carvone (56) in 22 synthetic steps, was first peracetylated to give diacetate 58, and the latter compound was then treated with 1,6hexanediol and PPTS to provide 59. In preparation for resin loading, the terminal hydroxy group of 59 was oxidized to the corresponding aldehyde 60, which upon treatment with a resin ylide underwent smooth loading via olefination to give the resin-bound sarcodictyn scaffold 61. To confirm the viability and fidelity of this linking strategy, the total syntheses of both sarcodictyns A (5) and B (6) were completed from intermediate 61 in a fashion analogous to the solution-phase synthesis. Briefly, treatment of 61 with NaOMe revealed the free C(8) hydroxyl group which was esterified with the (E )-N-(6 0 )-methylurocanic acid side-chain. This was followed by deprotection and oxidation of the C(15) alcohol leading to carboxylic acid 65. DCC-mediated esterification of 65 with either methanol or ethanol gave 66 or 67, respectively, which were independently cleaved from the resin via treatment with CSA to provide sarcodictyns A (5) and B (6).

Scheme 21.5. Solid-phase total synthesis of sarcodictyns A and B [13].

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With the linker-cleavage protocol validated, the avenue toward library construction was then open, as illustrated in Scheme 21.6. Inspection of the sarcodictyn structure suggested the following three sites for diversification; (1) the side-chain appended to C(8), (2) the hemiketal at C(4) and (3) the C(15) ester moiety. As outlined in Scheme 21.7, each of these sites was modified in a sequential fashion. Thus, the previously linked scaffold 61 was first esterified at the C(8) hydroxyl moiety employing five distinct (R1COX) acyl donors to give 68. After desilylation of

Scheme 21.6. Radiofrequency (RF)-encoded solid-phase split-

and-pool synthesis of sarcodictyn analog library [13].

Scheme 21.7. Synthesis of novel polystyrene-based selenyl bromide resin (82) and selenoether resin (84) [17].


the C(15) hydroxy group, intermediate 69 was split along three pathways. The first entailed acylation (R2COX) of the C(15) hydroxyl group followed by transketalization release with a series of alkyl alcohols (R3OH), ultimately yielding structures of the general type 75. The second pathway involved oxidation of the C(15) hydroxyl group of 69 to the corresponding carboxylic acid, which was converted to either an amide or an ester (74) by coupling with a series of amines (R4NH2 ) or alcohols (R4OH) respectively. These derivatives were then released by transketalization as before, giving rise to 76. The final path involved conversion of alcohol 69 to the corresponding amine (71), which was subsequently acylated (R5COX) giving structures of the general type 74. Release of the latter structures by transketalization provided 77. This approach yielded a 66-member library which was evaluated using the colorimetric tubulin polymerization assay. Active compounds were also tested in cytotoxicity assays employing ovarian cancer cells (1A9) and two Taxol TM -resistant cell lines (1A9PTX10 and 1A9PTX22), leading to a number of promising antitumor agents with higher potencies than those exhibited by the naturally occurring substances themselves [14]. 21.2.5

Solid-phase Semisynthesis of Vancomycin

Our final example in this section involves the solid-phase semisynthesis of the glycopeptide antibiotic vancomycin (12, Fig. 21.1) [17]. This solid-phase construction provides an example of how the multitude of functionality commonly found in complex natural products can necessitate the development of novel linking and release strategies. Vancomycin, renowned for its activity against methicillin-resistant Staphylococcus auerus (MRSA), has been used for the past 40 years to treat Gram-positive bacterial infections. The emergence of vancomycin-resistant enterococci strains (VRE) has raised serious health concerns and prompted a renewed vigor in the field of glycopetide antibiotics [18]. Researchers from Eli Lilly have demonstrated that modification of the oligosaccharide portion of vancomycin can enhance its activity against VRE to a clinically significant level [19]. However, more in-depth investigations of the role of the oligosaccharide in the glycopeptide’s biological activity have been hampered by the synthetic inaccessibility of analogs. With completion of the total synthesis of vancomycin in these laboratories, the stage was set for studies directed toward a solid-phase semisynthesis of vancomycin libraries for biological screening purposes. The carboxylic acid group of vancomycin was identified as the most suitable site for linkage to a solid support since it is the only singularly present handle on vancomycin and, therefore, does not require some form of selective derivatization. The criteria for the selection of a linker included acid and base stability, as well as mild loading and cleavage conditions compatible with silyl protecting groups. After numerous unsatisfactory attempts with conventional linkers, attention was turned to our recently reported preparation of polystyrene-based selenenyl bromide [20]. We reasoned that the facile oxidation/elimination of the polymer-bound phenylseleno group could be used to mask an allylic functionality, thereby serving as precursor

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to an Alloc protecting group in a safety-catch fashion. Thus, selenenyl bromide resin 82 (Scheme 21.7) was prepared from commercial polystyrene 78 via lithiation followed by treatment with dimethyl diselenide to give methyl selenide 80, a conjugate whose subsequent oxidation with bromine led to adduct 81 which, upon heating in ethanol, afforded resin 82. Treatment of 82 with LiBH4 then furnished 83, which was quenched with 1,3-diiodopropane to provide the requisite linker 84. The essential feature of this new safety-catch linker is outlined in Scheme 21.8. Carboxylic acids are loaded onto the resin under basic conditions, and can then be liberated in a two-step procedure in which the desired compound is initially released from the resin by oxidation of the selenide to the corresponding selenoxide, which undergoes a facile syn-elimination to afford the O-Alloc derivative, and then liberation of the carboxylic acid upon treatment with n-Bu3 SnH and catalytic amounts of Pd(PPh3 )4 . The application of this strategy to the loading and release of vancomycin is outlined in Scheme 21.9. Thus, the free carboxylic acid of persilylated vancomycin 90 was reacted with resin 84 in the presence of CsHCO3 to afford resin-bound species 91. As before, release was accomplished by oxidation with hydrogen peroxide to afford the Alloc-protected derivative, which was then deprotected.

Scheme 21.8. Use of selenoether 84 as a safety-catch linker for carboxylic acids [17].

Scheme 21.9. Loading and cleavage of vancomycin scaffold

using a selenium-base safety-catch linker [17].

With the resin-bound vancomycin 91 in hand, the solid-phase construction of vancomycin from the corresponding aglycone, obtained from the plentiful natural product via deglycosidation, became feasible. As shown in Scheme 21.10, after loading, the disaccharide moiety was hydrolyzed from 91 under acidic conditions


Scheme 21.10. Solid-phase synthesis of vancomycin (12) using

a selenium-based safety-catch linker [17].

to provide polymer-bound phenol 92, which was then glycosylated with the trichloroacetimidate 93 to afford monosaccharide 94. Removal of the C2 Alloc group from glucose using palladium(0)-mediated allyl transfer conditions afforded compound 95 in excellent yield. The polymer-bound conjugate 95 was then subjected

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to a second glycosidation reaction with glycosyl fluoride 96 to deliver fully protected polymer-bound vancomycin derivative 97, which was then cleaved with H2 O2 thus releasing allyl derivative 98. Finally, vancomycin (12) was obtained from 98 via a deprotection sequence involving deacetylation (K2 CO3 -MeOH), desilylation (CsF), deprotection of the carboxylic acid group [Pd(PPh3 )4 -n-Bu3 SnH] and Cbz cleavage (10% Pd/C, NH4 HCO3 ) via intermediate 99. The developed technology was combined with parallel solution-phase chemistry to produce an extensive library whose biological screening revealed a number of interesting and highly potent vancomycin analogs.

21.3

Solid-phase Total Synthesis of Natural Products – Combinatorial Total Synthesis

The excitement generated in the 1990s by combinatorial chemistry did not escape the attention of synthetic chemists involved in the total synthesis of natural products. The attractiveness of solid-phase chemistry and the lure of the possibility of using the split-and-pool strategy for library construction prompted several studies directed toward diversity-oriented total synthesis on solid support. In a typical case, the natural product itself is first targeted for synthesis on solid phase – an endeavor that provides the opportunity to develop the necessary chemistry for loading, elaborating, and cleaving the final target. Once the technology is developed, the practitioner then employs a pool of building blocks to construct a library of small to medium size, depending on the complexity of the target molecules involved. Below, we discuss several such solid-phase syntheses, paying particular attention to the novel linking and cleavage strategies developed during these programs and as a consequence of the challenges posed by each natural product. 21.3.1

Solid-phase Synthesis of Prostaglandins and Libraries Thereof

One of the early examples of total synthesis on polymeric support was reported by Chen and Janda in which the construction of prostaglandin E2 (PGE2 ) methyl ester was carried out using a soluble polystyrene polymer; although, strictly speaking, this is not a solid-phase synthesis, it is included here for its elegance and for historical reasons [21]. The practical features of soluble polymeric supports have been well known for some time and include solubility in most organic solvents [tetrahydrofuran (THF), CH2 Cl2 , CHCl3 ], thus allowing reactions to be carried out under standard solution-phase conditions and, more importantly, allowing standard analytical techniques, especially NMR, to be used to monitor reaction progress [22]. When necessary, however, the polymer support can be precipitated out of the reaction mixture since it is insoluble in solvents such as methanol and water so that the benefits of easy purification typically associated with solid-phase chemistry can still be realized [22].

21.3 Solid-phase Total Synthesis of Natural Products -- Combinatorial Total Synthesis

In their prostaglandin synthesis (Scheme 21.11), the Janda group employed an acid-labile pyran linker 101 constructed by the treatment of a noncrosslinked chloromethylated polystyrene resin with hydroxymethyl pyran derivative 100 in the presence of NaH. The requisite prostaglandin core, hydroxycyclopentenone, was then loaded under nonaqueous acid catalysis to afford resin-bound enone 102, which was subsequently subjected to a 1,4-cuprate conjugate addition and the resulting enolate was trapped as the corresponding silyl enol ether 103. Installation of the second appendage was accomplished by cleavage of the silyl enol ether with MeLi, followed by quenching with an alkynyl triflate to provide alkyne 104. Controlled hydrogenation of 104 followed by treatment of the resulting conjugate 105 with HF resulted in release of PGE2 methyl ester (106) in 37% overall yield, while reduction of the carbonyl group with l-selectride, followed by saponification of the methyl ester, furnished the polymer-bound PGFa 108. Cleavage via exposure to HF then provided PGFa (2) in 30% overall yield. The sequential installation of key structural elements made this synthesis particularly amenable to the construction of prostaglandin-like libraries through the use of structurally diverse pools of building blocks, and, in fact, the Janda group subsequently reported the construction of the prostanoid library shown in Scheme 21.12 [23]. Resin-bound enone 102 was reacted with four different cuprates to afford structures of type 109. Pools containing a portion of these various silyl enol ethers were then derivatized with a set of

Scheme 21.11. Soluble polymer-supported synthesis of prostaglandins [21].

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Scheme 21.12. Synthesis of a prostanoid library on soluble polymeric support [23].

four different a-chains to furnish structures of the general type 110. After reduction of the alkyne of the a-chain to the alkene moiety of the natural prostaglandins, the pools were cleaved with aqueous HF to afford four new pools of prostanoids, each consisting of four compounds. These mixtures were then assayed for inhibition of cytomegalovirus (CMV) – a member of the herpesvirus family – and deconvolution of an active pool led to the identification of a lead compound. A similar solid-phase synthesis of a related series of prostaglandins was reported by Ellman and coworkers, who employed a traditional, crosslinked polystyrenebased resin [24]. The Ellman approach relied on tethering the cyclopentene building block 113 (Scheme 21.13) through its hydroxyl group to a suitably derivatized resin. Thus, treatment of the polystyrene-derived di-n-butylsilyl chloride resin with 113 in the presence of imidazole afforded conjugate 114. The tri-(4-methoxyphenyl)methyl (TMT) protecting group of 114 was then removed from the molecule by treatment with formic acid and the a-side-chain was installed through a Suzuki coupling with a borane derivative to provide structure 116. Dess–Martin oxidation of 116 followed by attachment of the lower side-chain via a conjugate addition provided 118. The sulfonamide moiety of the a-side-chain was then activated with bromoacetonitrile and coupled with various alcohols (R1OH) and amines (R1NH2 ). Cleavage of the silyl ether bridge with HFpyridine resulted in the formation of 120, whereas l-selectride reduction of 119 followed by a similar cleavage resulted in the formation of prostaglandins of type 122. As with the Janda approach, the strategy developed by Ellman allowed the construction of a small library demonstrating its usefulness in combinatorial chemistry. 21.3.2

Solid-phase Synthesis of Epothilone A and Libraries Thereof

Another early example of the solid-phase synthesis of natural products is the olefin metathesis-based total synthesis of epothilone A emanating from these laboratories in 1997 [24]. Epothilones A (7, Fig. 21.1) and B were originally isolated from a series of myxobacteria by Ho¨fle, Reichenbach, and coworkers at the Gesell-


Scheme 21.13. Solid-phase synthesis of prostaglandins [24].

shaft fu¨r Biotechnologische Fonschung in Germany, and subsequently by scientists at Merck, who also determined their tubulin-binding properties. Once their Taxol TM -like mechanism of action and their potent activities against Taxol TM resistant tumor cells became known, interest in these natural products soared [26]. Following our solution-phase chemical synthesis of these natural products, we became interested in a solid-phase total synthesis which could allow us to apply the split-and-pool strategy to construct combinatorial libraries of such substances. Our olefin metathesis approach to epothilone A, developed previously, appeared to be ideally suited to such a solid-phase approach. To this end, a novel cyclorelease strategy was devised and executed as shown in Scheme 21.14 [27]. In addition to its inherent efficiency, the cyclorelease strategy is also advantageous in that it requires a compound to have successfully undergone all the requisite reaction processes for it to be released, thereby assuring against contamination of the cleavage products. The essential features of the epothilone synthesis include convergent assembly of building blocks, a ring-closing metathesis reaction which releases the generated macrocycle, and a final deprotection/epoxidation sequence in solution. As shown in Scheme 21.14, a resin-bound ylide (129) is used to load the first building block giving rise to 130, which is then deprotected and oxidized to afford aldehyde 132. Subsequent aldol condensation provided acid 133, which was coupled to the heterocycle side-chain as shown to furnish metathesis precursor 134. Cyclorelease via ring-closing metathesis then afforded olefins 135–139, which could be further elaborated. The advantage of this approach is enhanced by the three somewhat controllable steps which can be coached to deliver optimum

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Scheme 21.14. General cyclorelease strategy (a) and solidphase synthesis of epithilone A (7) via an olefin metathesis [25].

diversity: aldol reaction (syn/anti-stereoisomers); metathesis reaction (cis/transolefins); and epoxidation (a/b-epoxides). These advantages have been exploited in the expedient construction of a combinatorial library, as summarized in Scheme 21.15. In addition, these compounds have been added to a series of other analogs obtained via solution-phase methods and screened for their tubulin polymerization and cytotoxicity properties, leading to a useful set of structure–activity relationships within the epothilone field. 21.3.3

Solid-phase Synthesis of (S)-Zearalenone

Continuing the theme of cyclorelease (see Scheme 21.14a), the Nicolaou group reported the solid-phase synthesis of (S)-zearalenone (10) using a Stille coupling reaction to form and simultaneously release the macrocyclic product from the resin


Scheme 21.15. Radiofrequency (RF)-encoded split-and-pool solid-phase synthesis of epithilone library [25].

(Scheme 21.16) [28]. (S)-Zearalenone (10) is a naturally occurring substance possessing anabolic, estrogenic, and antibacterial properties and whose multitude of solution-phase syntheses include the one developed by Hegedus and coworkers which utilizes a Stille coupling reaction for the formation of the large ring [29]. Relying on the reported methodology, the Nicolaou group demonstrated a highly efficient and convenient solid-phase synthesis of this natural product, as shown in Scheme 21.17. Thus, the sequence began with oxidation of Merrifield resin to afford styrene 150, which was then stannylated to provide resin 151. Treatment of 151 with a vinyl lithium species furnished vinyl stannane 152, the terminal alcohol of which was oxidized to afford aldehyde 154. Grignard addition followed by oxidation provided access to structure 156, which was deprotected and coupled with an iodocarboxylic acid to provide 158 as the cyclorelease precursor. In the final palladium-catalyzed step, the zearalenone derivative 159 was released and then deprotected to afford (S)-zearalenone (10) in high overall yield and purity. A key advantage of this strategy was that all tin residues remained bound to the solid support.

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Scheme 21.16. Solid-phase synthesis of (S)-zearalenone (160)

via a palladium-mediated cyclorelease strategy [28]. DMSO, dimethyl sulfoxide.

Scheme 21.17. Solid-phase total synthesis of (dl)-muscone via a Horner–Wadsworth–Emmons-mediated cyclorelease strategy [30].

21.3.4

Solid-phase Synthesis of (DL)-Muscone and Libraries Thereof

In an effort to develop a resin suitable for the solid-phase version of the powerful Horner–Wadsworth–Emmons reaction, we targeted (dl)-muscone and a library thereof [30]. The adopted strategy represents yet another example of the cyclorelease strategy (see Scheme 21.15, top) in which the cyclic product is simulta-


neously formed and released, leaving the troublesome phosphorous residues on the resin. As illustrated in Scheme 21.17, Merrifield resin was derivatized with 1,4butanediol in the presence of NaH to afford resin 160, which was then reacted with MeP(O)(OMe)Cl and Et3 N to give resin-bound methylphosphonate 161. This versatile resin was then deprotonated with n-BuLi and quenched with an alkenyl ester leading to terminal olefin conjugate 162, whose cross-metathesis reaction with a hydroxy alkene resulted in the new resin 163 (cis/trans mixture). Oxidation to the aldehyde 164 was then followed by smooth cyclorelease (K2 CO3/18-crown-6), furnishing enone 165, which underwent 1,4-addition with LiCuMe 2 to afford 166 and, hence, (dl)-muscone (11) upon hydrogenation. Application of the developed chemistry in a split-and-pool fashion using the radiofrequency-encoding system also allowed the synthesis of a small muscone library. 21.3.5

Solid-phase Synthesis of the Vitamin D3 System

Among the most recent examples of the solid-phase synthesis of natural products and their analogs is one reported by Takahashi and coworkers around the vitamin D3 skeleton [31]. As shown in Scheme 21.18, the Takahashi group employed the trialkylsilane linker 168, which was alkylated with the presynthesized bicyclic upper unit of vitamin D3 to furnish intermediate 169. The ketone of 169 was then subjected to a Horner–Wittig condensation with the presynthesized lower unit in the presence of n-BuLi to afford triene 170. The side-chain was then installed by a Grignard reaction in which the tosylate moiety of 170 was displaced to furnish compound 171, which was ultimately cleaved to provide the 1a,25-(OH)2 -vitamin D3 analog 172.

Scheme 21.18. Solid-phase synthesis of vitamin D3 system

173. DCDMH, 1,3-dichloro-5,5-dimethylhydantoin [31].

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21.3.6

Solid-phase Synthesis of Carpanone-like Molecules

The Shair group has described the synthesis of a small library of carpanone-like molecules employing a solid-phase variant of the biomimetic carpanone synthesis first reported by Chapman et al. [33]. The strategy, as outlined in Scheme 21.19, relied on initial tethering of a series of electron-rich o-hydroxystyrenes to a solid support to generate adducts of type 173. These styrenes were then treated with a series of electron-poor o-hydroxystyrenes in the presence of PhI(OAc)2 to effect an oxidative coupling furnishing 174, which underwent a spontaneous inverse electron-demand Diels–Alder reaction to afford a small library of benzoxanthenones (175) representing the carpanone skeleton. Cleavage with HF led to the release of carpanone-like compounds 176. The researchers minimized oxidative homocoupling of the resin-bound styrenes species through electronic considerations, low resin loading, and linker optimization.

Scheme 21.19. Biomimetic solid-phase synthesis of carpanone-like compounds (176) [32].

21.4

Combinatorial Solid-phase Synthesis of Natural Product-like Libraries

Over the past centuries, nature has served as a most generous source supplier of cures and, in more recent times, leads for drug discovery purposes. Indeed, many of the clinically used drugs of the twentieth century have their origins in nature, whose library of compounds exceeds our wildest imagination in terms of both numbers and molecular diversity. However, the tremendous strides made by biology and high-throughput screening technologies in recent times demand a much faster pace of isolation and characterization of new natural products than currently possible [34]. This problematic phenomenon is compounded by the pressure to discover new drugs and to fulfill the promise of the human genome project, the fruits of which will soon be available for further biomedical advances [35]. The advent of combinatorial chemistry provided a powerful alternative to nature, for now chemists can design and rapidly synthesize large libraries of natural product analogs or even libraries of natural product-like molecules. The value of such endeavors should be measured by various factors, such as novelty of the chemistry developed, the molecular diversity generated, the practicality of library construction, and the relevance of the designed structures to biology. The first examples of combinatorial synthesis of natural product analog libraries are those already discussed above. More recently, several research groups have

21.4 Combinatorial Solid-phase Synthesis of Natural Product-like Libraries

described the solid-phase combinatorial syntheses of larger libraries of natural product-like compounds. Many of these libraries attempt to emulate the structural characteristics observed in natural products, but, at the same time, provide more rapid access to larger collections of products that possess greater diversity and incorporate optimized physical and pharmacological properties into their structures. Several strategies for the construction of such libraries have been described. Below, we discuss a number of representative examples from the Schreiber and Nicolaou laboratories. To achieve the construction of larger diversity-oriented libraries, Schreiber and coworkers have described several combinatorial solid-phase synthetic strategies used to create libraries of complex structures for screening in various chemical genetics applications [36–39]. One of these strategies, shown in Scheme 21.20, targeted a 2.18 million-compound library [36]. In this effort, researchers converted both enantiomers of shikimic acid (177) to epoxycyclohexenol carboxylic acid 178, which was then coupled via a photolabile linker to solid support to give conjugate 179. Subsequent treatment of enone 179 with a series of nitrone carboxylic acids

Scheme 21.20. Solid-phase synthesis of a natural product-like library [36].

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under esterification conditions furnished polycyclic compounds of type 180 to be used as library scaffolds. The rigid, highly functionalized octahydrobenzisoxazole core 180 was further functionalized through a variety of reactions, as shown at the bottom of Scheme 21.20. These reactions included palladium couplings, epoxide openings, lactone openings, acylations, and others. The reaction pathways and building blocks were validated through a number of testing protocols, and a test library of 456 compounds was constructed and analyzed by LC/MS to establish the validity of the library strategy. Upon completion of the full-sized library, it was employed in several miniaturized cell-based assays to screen for cell-permeable, protein-binding ligands [36]. More recently, our group has introduced a strategy for the construction of natural product-like libraries using naturally occurring privileged structures as templates [40–42]. The concept of privileged structures was first introduced by a Merck group to describe select structural types (originally benzodiazepines and benzazepines) that bind to multiple, unrelated classes of protein receptors as high affinity ligands [43]. These privileged structures are typically rigid, polycyclic heteroatomic systems capable of orienting varied substituent patterns in a well-defined threedimensional space. The tendencies of derivatives of these privileged structures to exhibit binding affinity toward various receptors and enzymes has made them attractive scaffolds for discovery libraries, particularly in cases where there is only limited structural information available about the target. Given the success of privileged structures in medicinal chemistry, we envisaged a similarly advantageous application of this principle to the construction of natural product-like libraries, especially since the principal use of such libraries is to discover ligands for either unknown or recently discovered biological targets. We were particularly intrigued by the possibility that using scaffolds of natural origin, which presumably have undergone evolutionary selection, might confer favorable bioactivities and bioavailabilities to library members. Thus, we sought to identify privileged structural subunits found in biologically active natural products and to develop novel solidphase chemistries to allow these structures to be employed as templates for the construction of versatile compound libraries. These efforts have resulted in the development of a number of unique solid-phase cycloloading strategies (shown in Scheme 21.21) which capitalized on the electrophilicity of the selenenyl bromide resin described previously [20]. Thus, as shown, various alkenyl systems containing an internal nucleophile can be treated with the selenium bromide resin to effect an electrophilic cyclization reaction concomitant with loading. Once loaded, the stability of the resulting selenoether tether enables further functionalization of the polycyclic scaffolds, and, upon completion, the target molecules can then be released either through oxidation elimination or through radical hydride transfer conditions. To date, this process has been demonstrated for both heterocyclic and carbocylic ring systems. As illustrated at the bottom of Scheme 21.21, benzopyrans can be constructed by the cycloloading of o-prenylated phenols (194) followed by functionalization and oxidative cleavage to give 2,2-dimethylbenzopyrans of type 197 [44]. Notably, this benzopyran motif and its derivatives have been found in over 12,000 natural and designed structures with wide-ranging biological activities,

21.4 Combinatorial Solid-phase Synthesis of Natural Product-like Libraries

Scheme 21.21. General strategy for solid-phase cycloloading reactions using an electrophilic selenyl bromide resin (a) and representative hetero- and carbocyclic resin-bound scaffolds useful in the construction of natural product-like libraries (b) [44–46].

thereby making it an optimal template for construction of a natural product-like library (see below). The cycloloading of o-allylanilines (198) affords resin-bound indolines 199 which can be further functionalized and then cleaved under radical hydride conditions [45]. Interestingly, if a conjugated radical acceptor is placed in a suitably proximal position to the generated carbon-centered radical, complex polycyclic indolines reminiscent of alkaloid-type natural products can be obtained. Lastly, cycloloading of alkenyl-substituted b-ketoesters 202 provided access to a series of resin-bound bicyclic scaffolds which could be derivatized and cleaved to afford highly functionalized carbocycles of type 205. The last compounds resemble a family of natural products from the Guttiferae classification with promising biological activities [46]. As an illustration of the utility of such cycloloading strategies in the construction of large natural product-like libraries, the benzopyran strategy was recently employed in the construction a 10,000-member natural product-like library, as outlined in Scheme 21.22 [40–42]. This library was constructed by directed split-andpool techniques employing the NanoKan TM optical encoding platform recently developed by Irori, a Discovery Partners International Company [47]. Synthesis of this diverse library commenced with a series of resin-bound benzaldehydes (207) which were subjected to various reactions, including organometallic additions, reductive aminations, condensations, acylations, Mitsunobu inversions, and glyco-

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Scheme 21.22. Optically encoded split-and-pool solid-phase synthesis of 10,000-member natural product-like library based on 2,2-dimethylbenzopyran template [40–42].

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21.6 Addendum

sidations. An automated cleavage protocol employing H2 O2 furnished 2- to 3-mg quantities of each library member in high purity (often > 90%) with the entire library formatted in 96-well microtiter plates for use in various chemical biology studies [48].

21.5

Conclusion

Merrifield’s pioneering work on solid-phase chemistry revolutionalized the field of peptide synthesis. It subsequently influenced profoundly the field of oligonucleotide synthesis where similar concepts were utilized to construct much-needed DNA sequences for the genetic engineering revolution. The same philosophy of solid-phase chemistry is now being implemented in the latest revolution in organic synthesis to deliver small organic molecules urgently needed to confront the human genome via biological screening. Indeed, in the 1990s, we have seen a strong push in research directed at the discovery and invention of new methods for solid-phase synthesis and the application of the latter for the construction of compound libraries for, among others, biological screening purposes. Many of these efforts focused on developing new resins and linkers for convenient loading and cleavage, others focused on suitable synthetic strategies to construct specific libraries, and yet others focused on designing and synthesizing novel molecular diversities. Given their molecular complexities, natural products are playing a crucial role in this process by serving as challenging targets for solid-phase synthesis, frequently demanding the development of new linking and synthesis strategies. When successful, such endeavors can deliver, in addition to the natural substances, highly valuable analog libraries of these compounds for biological investigations. In contrast to the repetitive nature of peptide and oligonucleotide synthesis, the construction of small nonoligomeric natural products demands a mastery of a much wider range of synthetic chemistry. For this reason, chemists will remain busy for some time inventing, discovering, and developing enabling technologies that will enhance the power of solid-phase synthesis, and, in turn, sharpen the tools of combinatorial chemistry. The benefits of such efforts to biology and medicine are already becoming apparent and are bound to increase in the future.

21.6

Addendum

Since the submission of this chapter for publication, an important contribution to the field of natural product-based combinatorial chemistry has been reported. Specifically, the laboratory of Shair and coworkers described the biosynthetically inspired construction of a 2527-membered library using the natural product galanthamine as a scaffold. The galanthamine skeleton was functionalized at four sites using solid phase split-and-pool chemistry. While the natural product itself is

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a potent acetylcholinesterase inhibitor, the authors were interested in creating a non-focused, diversity oriented library. The resulting library was screened and a molecule capable of interfering with protein trafficking was identified for use as a chemical probe. The interested reader is referred to a full account of this work: H. E. Pelish, N. J. Westwood, Y. Feng, T. Kirchhausen, M. D. Shair, J. Am. Chem. Soc. 2001, 123, 6740–6741.

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Snader, J. Nat. Prod. 1997, 60, 52–60; b) Y.-Z. Shu, J. Nat. Prod. 1998, 61, 1053–1071. a) K. Hinterding, D. Alonso-Dıáz, H. Waldmann, Angew. Chem. Int. Ed. 1998, 37, 688–749; b) D. T. Hung, T. F. Jamison, S. L. Schreiber, Chem. Biol. 1996, 3, 623–639. C. Watson, Angew. Chem. Int. Ed. 1999, 38, 1903–1908. S. Bertels, S. Frormann, G. Jas, K. U. Bindseil, Drug Discovery Nat. 1999, 72–105. A. Atuegbu, D. Maclean, C. Nguyen, E. M. Gordon, J. W. Jacobs, Bioorg. Med. Chem. 1996, 4, 1097– 1106. T. C. Norman, N. S. Gray, J. T. Koh, P. G. Schultz, J. Am. Chem. Soc. 1996, 118, 7430–7431. N. S. Gray, L. Wodicka, A.-M. W. H. Thunnissen, T. C. Norman, S. Kwon, F. H. Espinoza, D. O. Morgan, G. Barnes, S. LeClerc, L. Meijer, S.-H. Kim, D. J. Lockhart, P. G. Schultz, Science 1998, 281, 533–538. H. M. Geysen, S. J. Rodda, T. J. Mason, G. Tribbick, P. G. Schoofs, J. Immunol. Methods 1987, 102, 259– 274. G. R. Rosania, J. Merlie, Jr, N. Gray, Y.-T. Chang, P. G. Schultz, R. Heald, Proc. Natl. Acad. Sci. USA 1999, 96, 4797–4802. X.-Y. Xiao, Z. Parandoosh, M. P. Nova, J. Org. Chem. 1997, 62, 6029– 6033. For a review, see: K. C. Nicolaou, W.-M. Dai, R. K. Guy, Angew. Chem. Int. Ed. Engl. 1994, 33, 15–44.

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Parandoosh, A. Senyei, M. P. Nova, Angew. Chem. Int. Ed. Engl. 1995, 107, 2476–2479; b) E. J. Moran, S. Sarshar, J. F. Cargill, M. J. M. Shahbaz, A. Lio, A. M. M. Mjalli, R. W. Armstrong, J. Am. Chem. Soc. 1995, 117, 10787–10788. K. C. Nicolaou, N. Winssinger, D. Vourloumis, T. Ohshima, S. Kim, J. Pfefferkorn, J. Y. Xu, T. Li, J. Am. Chem. Soc. 1998, 120, 10814–10826. For a review, see: K. C. Nicolaou, J. Pfefferkorn, J. Xu, N. Winssinger, T. Ohshima, S. Kim, S. Hosokawa, D. Vourloumis, F. van Delft, T. Li, Chem. Pharm. Bull. 1999, 47, 1199– 1213. K. C. Nicolaou, J. Xu, S. Kim, J. Pfefferkorn, T. Ohshima, D. Vourloumis, S. Hosokawa, J. Am. Chem. Soc. 1998, 120, 8661–8673. X.-T. Chen, B. Zhou, S. K. Bhattacharya, C. E. Gutteridge, T. R. Pettus, S. J. Danishefsky, Angew. Chem. Int. Ed. 1998, 37, 789–792. K. C. Nicolaou, N. Winssinger, R. Hughes, C. Smethurst, S. Y. Cho, Angew. Chem. Int. Ed. 2000, 39, 1984– 1989. For reviews, see: a) K. C. Nicolaou, C. N. C. Boddy, S. Bra¨se, N. Winssinger, Angew. Chem. Int. Ed. 1999, 111, 2230–2287; b) D. H. Williams, B. Bardsley, Angew. Chem. Int. Ed. 1999, 38, 1172–1193. R. Nagarajan, J. Antibiot. 1993, 46, 1181–1195. a) K. C. Nicolaou, J. Pastor, S. Barluenga, N. Winssinger, Chem. Commun. 1998, 1947–1948; for preparation of related selenium-based

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resins, see: b) T. Ruhland, K. Andersen, H. Pedersen, J. Org. Chem. 1998, 63, 9204–9211; c) K. Fujita, K. Watanabe, A. Oishi, Y. Ikeda, Y. Taguchi, Synlett 1999, 11, 1760–1761. S. Chen, K. D. Janda, J. Am. Chem. Soc. 1997, 119, 8724–8725. For an early use of soluble polymers in peptide synthesis, see: a) M. Narita, M. Hirat, K. Kusano, S.-I. Itsuno, M. Ue, M. Okawara in: Peptide Chemistry. Yonehara, J. (ed.), Protein Research Foundation, Osaka 1979, vol. 99, pp. 107–112; b) M. Narita, Bull. Chem. Soc. Jpn. 1978, 51, 1477; c) E. Bayer, M. Mutter, Nature 1972, 237, 512–513; for a review, see: d) C. W. Harwig, D. J. Gravert, K. D. Janda, Chemtracts 1999, 12, 1–26. K. J. Lee, A. Angulo, P. Ghazal, K. D. Janda, Org. Lett. 1999, 1, 1859– 1862. a) L. A. Thompson, F. L. Moore, Y.-C. Moon, J. A. Ellman, J. Org. Chem. 1998, 63, 2066–2067; b) D. R. Dragoli, L. A. Thompson, J. O’Brien, J. A. Ellman, J. Comb. Chem. 1999, 1, 534–539. a) K. C. Nicolaou, N. Winssinger, J. Pastor, S. Ninkovic, F. Sarabia Y. He, D. Vourloumis, Z. Yang, T. Li, P. Giannakakou, E. Hamel, Nature 1997, 387, 268–272; b) K. C. Nicolaou, D. Vourloumis, T. Li, J. Pastor, N. Winssinger, Y. He, S. Ninkovic, F. Sarabia, H. Vallberg, F. Roschangar, N. P. King, M. R. V. Finlay, P. Giannakakou, P. VerdierPinard, E. Hamel, Angew. Chem. Int. Ed. Engl. 1997, 36, 2097–2103. For a review, see: a) K. C. Nicolaou, F. Roschangar, D. Vourloumis, Angew. Chem. Int. Ed. 1998, 37, 2014– 2045. For a review, see: J. H. van Maarseveen, Comb. Chem. High Throughput Screening 1998, 1, 185–214. K. C. Nicolaou, N. Winssinger, J. Pastor, F. Murphy, Angew. Chem. Int. Ed. 1998, 37, 2534–2537. K. Kalivretenos, J. K. Stille, L. S. Hegedus, J. Org. Chem. 1991, 56, 2883–2894.

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Winssinger, F. Murphy, J. Am. Chem. Soc. 1998, 120, 5132–5133. T. Doi, I. Hijikuro, T. Takahashi, J. Am. Chem. Soc. 1999, 121, 6749– 6750. C. W. Lindsley, L. K. Chan, B. C. Goess, R. Joseph, M. D. Shair, J. Am. Chem. Soc. 2000, 122, 422–423. O. L. Chapman, M. R. Engel, J. P. Springer, J. C. Clardy, J. Am. Chem. Soc. 1971, 93, 6696. S. L. Schreiber, Bioorg. Med. Chem. 1998, 6, 1127–1152. a) G. R. Lenz, H. M. Nash, S. Jindal, Drug Discovery Today 2000, 5, 145– 156; b) E. S. Razvi, L. J. Leytes, Mod. Drug Discovery 2000, 41–42; c) J. Rosamond, A. Allsop, Science 2000, 287, 1973–1976; d) M. D. Garrett, P. Workman, Eur. J. Cancer 1999, 35, 2010–2030; e) C. E. Barry, R. A. Slayden, A. E. Sampson, R. E. Lee, Biochem. Pharmacol. 1999, 59, 221– 231. D. S. Tan, M. A. Foley, B. R. Stockwell, M. D. Shair, S. L. Schreiber, J. Am. Chem. Soc. 1999, 121, 9073–9087. D. Lee, J. K. Sello, S. L. Schreiber, J. Am. Chem. Soc. 1999, 121, 10648– 10649. D. Lee, J. K. Sello, S. L. Schreiber, Org. Lett. 2000, 2, 709–712. D. R. Spring, S. Krishnan, S. L. Schreiber, J. Am. Chem. Soc. 2000, 122, 5656–5657. K. C. Nicolaou, J. A. Pfefferkorn, G.-Q. Cao, A. J. Roecker, S. Barluenga, H. J. Mitchell, J. Am. Chem. Soc. in press. K. C. Nicolaou, J. A. Pfefferkorn, H. J. Mitchell, A. J. Roecker, S. Barluenga, G.-Q. Cao, R. L. Affleck, J. E. Lillig, J. Am. Chem. Soc. in press. K. C. Nicolaou, J. A. Pfefferkorn, S. Barluenga, H. J. Mitchell, A. J. Roecker, G.-Q. Cao, J. Am. Chem. Soc. in press. B. E. Evans, K. E. Rittle, M. G. Bock, R. M. DiPardo, R. M. Freidinger, W. L. Whitter, G. F. Lundell, D. F. Veber, P. S. Anderson, R. S. L.

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21 Solid-phase Synthesis of Natural Products and Natural Product-like Libraries Chang, V. J. Lotti, D. J. Cerino, T. B. Chen, P. J. Kling, K. A. Kunkel, J. P. Springer, J. Hirshfield, J. Med. Chem. 1988, 31, 2235–2246. 44 a) K. C. Nicolaou, J. A. Pfefferkorn, G.-Q. Cao, Angew. Chem. Int. Ed. 2000, 39, 734–739; b) K. C. Nicolaou, G.-Q. Cao, J. A. Pfefferkorn, Angew. Chem. Int. Ed. 2000, 39, 739–743. 45 K. C. Nicolaou, A. J. Roecker, J. A. Pfefferkorn, G.-Q. Cao, J. Am. Chem. Soc. 2000, 122, 2966–2967.

46 a) K. C. Nicolaou, J. A. Pfeffer-

korn, G.-Q. Cao, S. Kim, J. Kessabi, Org. Lett. 1999, 5, 807–810; b) K. C. Nicolaou, J. A. Pfefferkorn, S. Kim, H. X. Wei, J. Am. Chem. Soc. 1999, 121, 4724–4725. 47 X.-Y. Xiao, C. Zhoa, H. Potash, M. P. Nova, Angew. Chem. Int. Ed. Engl. 1997, 36, 780–782. 48 K. C. Nicolaou, J. A. Pfefferkorn, F. Schuler, A. J. Roecker, G.-Q. Cao, J. E. Casida, J. Chem. Biol. in press.

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22

Solid-phase Synthesis of Heterocyclic Systems (Heterocycles Containing One Heteroatom) Roland E. Dolle 22.1

Introduction

The synthesis of chemical libraries is now widely practiced in academic and industrial laboratories. The first examples of small-molecule libraries were collections of heterocycles, e.g. Ellman’s 1,4-benzodiazepines and b-lactams from Affymax. Of the 1250 libraries published since 1992, over 80% are based on a heterocyclic theme [1]. The impetus for the development and application of high-speed synthesis is tied to the pharmaceutical industry’s quest for discovering new biologically active agents; heterocycles enjoy a rich history of providing leads for this multibillion dollar industry. This chapter presents a systematic examination of the solid-phase synthesis of heterocycles containing a single heteroatom. This treatise is organized by atom type (nitrogen, oxygen, sulfur), ring size, and degree of unsaturation. Table 22.1A–H lists the generic structures of all solid- and solutionphase heterocyclic libraries containing a single heteroatom that were published during 1992–2000.

22.2

Solid-phase Synthesis of Heterocycles Containing One Nitrogen Atom 22.2.1

Aziridines

The Gabrial–Cromwell reaction involving the conjugate addition of an amine to ahaloacrylate and intramolecular ring closure is a classical method for preparing aziridine-2-carboxylates. This reaction sequence was adapted to solid phase by Filigheddu and coworkers [2] in a construction of aziridine-containing peptidomimetics (Scheme 22.1). Wang resin esterified with amino acids 1 serves as resinbound amine substrates. A direct synthesis of the heterocyclic unit was carried out upon the Michael addition of a-bromoacrylates 2 or a-bromoacrylamides 3–5 in tetrahydrofuran (THF) in the presence of triethylamine (25 C, 12 h). Cleavage Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

644

22 Solid-phase Synthesis of Heterocyclic Systems (Heterocycles Containing One Heteroatom)

Tab. 22.1. Generic structures of heterocyclic libraries containing a single heteroatom: 1992– 2000. *Solid-phase attachment point.

22.2 Solid-phase Synthesis of Heterocycles Containing One Nitrogen Atom Tab. 22.1. (continued)

[85]

[84]

645

646



[38]

[39]

[40]

[37]

[36]

22.2 Solid-phase Synthesis of Heterocycles Containing One Nitrogen Atom Tab. 22.1. (continued)

I. 10-membered nitrogen heterocycles

647

648



22.2 Solid-phase Synthesis of Heterocycles Containing One Nitrogen Atom

Scheme 22.1. Gabriel–Cromwell aziridine synthesis on solid phase.

of the aziridine carboxylic acids 6 with trifluoroacetic acid (TFA) affords products in >70% yield and in high purity. A second complementary method for aziridine synthesis was described in which substrates 1 are acylated with activated 2,3dibromopropane 7 to yield resin-bound a-bromoacrylamides 8. The use of 7 – required as an alternative three-step acylation of 1 with acrylic acid chloride, bromine addition, and HBr elimination – gave a low yield of 8 owing to premature cleavage. Treatment of Michael substrates 8 with a range of primary amines and amino acid esters, followed by TFA cleavage, affords aziridines 10 in high purity. 22.2.2

b-Lactams

2-Azetidinones are important constituents of b-lactam antibiotics and are useful intermediates in the synthesis of chiral b-amino acids. The [2 þ 2] cycloaddition reaction of ketones with resin-bound imines to yield b-lactams was one of the first successful examples of cycloaddition chemistry on solid phase. The solid-phase

649

650


version of the Staudinger reaction, initially described by researchers at Affymax (Scheme 22.2A,B) [3, 4], proceeds via condensation of amino acid-esterified Sasrin resin 1 with alkyl, aromatic, or a,b-unsaturated aldehydes to furnish quantitatively resin-bound imines 11. Cycloaddition is efficiently carried out by the slow addition

Scheme 22.2.

b-Lactam solid-phase syntheses.


Scheme 22.2. (continued)

of acid chlorides to a methylene chloride suspension of imine resins 11 in the presence of triethylamine. Large molar excess of acid chloride is used to convert hindered imines to b-lactams. Sasrin resin is preferred over Wang resin because of Sasrin’s increased sensitivity to acid, resulting in mild cleavage of the b-lactams from solid support. Product yields are uniformly high (>80%) with the expected cis disposition of 3- and 4-substituents on the b-lactam ring. Diastereomeric ratios are modest and range from 1:1 to 3:1 for chiral amine substrates. Analogous chemistry was employed in the synthesis of N-unsubstituted b-lactams using photolysis as the cleavage method (Scheme 22.2B). In a reverse strategy, Singh and Nuss showed that the Staudinger reaction can be carried out using immobilized imines 18 generated from resin-bound aryl aldehydes 17 (Scheme 22.2C) [5]. In their sequence, resins 18 are reacted with acetoxyacetyl chloride [triethylamine (TEA), 0 C] to give cis-azetidinones 19 which may be further elaborated to carbamates 22. b-Lactams (relative stereochemistry not defined) are also prepared by condensation of resin-bound imines 24 with titanium enolates derived from b-substituted propionic acid pyridinylthioesters (Scheme 22.2D) [6]. 22.2.3

Pyrrolidines and Derivatives

Another early example of heterocyclic synthesis on solid support is the 1,3-dipolar cycloaddition reaction of resin-bound azomethine ylides with olefins to furnish functionalized pyrrolidines (Scheme 22.3A). This work was carried out by pioneers at Affymax in 1995 [8, 9]. In situ ylide formation occurs upon treatment of resin-

651

652


Scheme 22.3. Pyrrolidines via cycloaddition on solid phase.

bound amino acid imines (11) with AgNO3 and triethylamine in acetonitrile. Reaction of these silver azomethine ylides with electron-poor olefins (28) furnishes proline derivatives after resin cleavage. The synthetic scheme was optimized using gel-phase 13 C-NMR (nuclear magnetic resonance) analysis. Product yields range from 50% to 80% and diastereoselectivities from 2.5:1 to >10:1. Because the pyr-


rolidine ring is a structural component of angiotensin-converting enzyme (ACE) inhibitors, intermediate prolines 29 were acylated with mercaptoacyl chlorides yielding mercaptoacyl prolines 31 after thioester deprotection and resin cleavage. Biological evaluation of a split-and-pool library prepared from 240 building blocks led to the identification of an exceptionally potent series of ACE inhibitors (Ki values < 1 nM). This same library was subsequently encoded using oligomeric amine tags, providing more detailed structure–activity relationship (SAR) information [9]. The reverse strategy for pyrrolidine synthesis is also known, in which electrondeficient olefins – prepared on solid support via the condensation of immobilized 3-hydroxyacetophenone with aryl aldehydes – are reacted with metalated azomethine ylide (derived from PhCHbNCH2 CO2 Me) [10]. In this synthesis, high regioand diastereoselectivity is observed. A solid-phase synthesis of highly substituted pyrrolidines was conducted via [2ps þ 4ps] cycloaddition of 2-azaallyl anions with alkenes (Scheme 22.3B) [11]. Hydroxyaldehydes are loaded onto resin via a dihydropyridine (DHP) linker to give bound aldehydes 33 and are condensed with stannyl amine 35. Resin-bound 2(azaallyl) stannanes 34 so obtained are transmetalated (n-BuLi), in the presence of electron-rich alkenes, to furnish pyrrolidine anions 37 which may be alkylated upon subsequent addition of an electrophile. Six fully substituted pyrrolidines (38, 39) were prepared in 32–50% overall isolated yield. Other reports of solid-phase methods for generating pyrrolidines include application of N-acyliminium ion chemistry [12], the imino-Sakurai and Pd-catalyzed cyclization cleavage of allylic amine systems [13], radical cyclization of oxime ethers [14], and intramolecular amino-zinc-enolate cyclization [15]. Cyclic imides (2,5-dioxopyrrolidines) form another class of compound that often possesses interesting biological properties. Solid-phase strategies for the synthesis of cyclic imides include a variant of the classical amine/cyclic anhydride condensation [16] and Diels–Alder reaction of N-substituted maleimide with resin-bound dienes [17, 18]. In the latter example, Barn and Morphy at Organon [16] optimized the coupling of succinic and phthalic anhydrides to hydroxymethyl polystyrene resin using dimethylaminopyridine (DMAP) in dimethylformamide (DMF) (Scheme 22.4). Acids 41 are converted to amides 42 using a primary amine in the presence of dicyclohexylcarbodiimide (DCC) and hydroxybenzotriazole (HOBT). Heating the resulting resin 42 in 5% acetic acid/DMF at 130 C for 18 h promotes cyclative release of the cyclic imide 43 from resin. Although the yields for nine examples range from 20% to 71%, the purity of the products are excellent owing to the fact the resin cleavage can only occur upon imide formation. The new chemistry was used to construct a @40-member d-opioid optimization library of cyclic imides. 22.2.4

Tetramic Acids

Three publications appeared in 1998 from three independent laboratories describing the synthesis of tetramic acids in high yield and purity [19–21]. Each method

653

654


Scheme 22.4. Solid-phase synthesis of cyclic imides.

relies on some variation of the following three-step sequence: (1) reductive alkylation of resin-bound a-amino acids with aldehydes (44); (2) acylation of the secondary amine (44 to 48–50); and (3) base-promoted Claisen-type intracyclative cleavage (48–50 to 51–53; Scheme 22.5). Acylating reagents include Meldrums’s acid, ma-

Scheme 22.5.

Tetramic acid synthesis on solid support.


lonic half esters, aryl acetic acids, and acetic acids bearing electron-withdrawing groups, with the last two reagents providing convenient access to diversity at position C3 on the heterocycle. 22.2.5

Pyrroles

Tetra- and penta-substituted pyrroles were first prepared by Mjalli and coworkers via the 1,3-dipolar cycloaddition of alkynes to polymer-bound mu¨nchnones (Scheme 22.6A) [22]. Resin-bound amino acids 54 are subjected to a fourcomponent condensation with aldehydes, carboxylic acids, and either phenyl or pyridinylisocyanates to yield Ugi products (55, 56). Hydrolysis of the terminal amide bond in 55 and 56 to 57 is carried out after Boc anhydride activation. The selection of either phenyl or pyridinylisocyanate stems from the necessity to hydrolyze the amide bond in 55 and 56 under conditions compatible with resin linkage. Originally, benzylisocyanate was used in this process, but the rate of hydrolysis of the corresponding amide is too slow (t1=2 > 7 days) to be of practical utility. The reaction of 57 with electron-deficient alkynes either in neat acetic anhydride as solvent or in a toluene solution of Et3 N and isobutyl chloroformate at 100 C (24– 48 h) leads to sequential in situ formation of mu¨nchnones 58 and 1,3-dipolar cycloaddition. Some ten pyrroles were obtained in 26–76% overall yield using this protocol (TFA cleavage). The purity of all the products was exceptional. A slight variant of this reaction was reported by Armstrong wherein resin-bound succinate 61 is used in the Ugi condensation employing 1-isocyanocyclohexene as a convertible isocyanide (Scheme 22.6B) [23]. Mu¨nchnone precursors 62 are sufficiently reactive such that heating 62 in toluene containing 10 equiv. HCl and 25 equiv. dimethyl acetylenedicarboxylate (DMAD) affords the pyrrole ring, negating the earlier requirement for intermediate amide hydrolysis (55, 56 to 57; Scheme 22.6A). Resin-bound enaminones 69 are particularly versatile intermediates with respect to pyrrole synthesis (Scheme 22.7). They are prepared from the condensation of resin derivative 68 and primary amines [trimethylorthoformate (TMOF)] as the dehydrating reagent in DMF. The intramolecular net reaction of 69 with a-alkylsubstituted nitroalkenes (neat or prepared in situ from nitroalkanes and aldehydes) affords pyrroles in 46–90% yields [24]. Enaminones 69 readily react with a-bromoketones in the classical Hantzch pyrrole synthesis [25]. The key to the success of this reaction is the use of the non-nucleophilic Lewis base 2,6,-di-t-butyl-pyridine and DMF as solvent. 22.2.6

Piperidine and Derivatives

The aza-Diels–Alder reaction provides convenient access to a range of nitrogencontaining heterocycles. In an operationally simple one-pot solid-phase synthesis of 3,4-dihydropiperidines 76 [26], a solution of aminomethylated polystyrene resin

655

656


Scheme 22.6. Pyrroles via immobilized mu €nchnones.

71, an aldehyde (72), a diene (73), and ytterbium triflate (10 mol%) in methylene chloride is shaken at room temperature for @24 h (Scheme 22.8). Traceless cleavage of adducts 75 is performed using ACE chloride (N-debenzylation) to give collections of 76. Electron-rich dienes in combination with ethyl glyoxylate, phenylglyoxal, and 37% aqueous formaldehyde are the preferred reagents.


Scheme 22.7. Pyrroles from enaminones.

Scheme 22.8. Multicomponent condensation to dihydropiperidines.

The construction of b-aryl-2,3-dihydro-4-pyridones 80 on solid support has been described by Wang and Wilson (Scheme 22.9) [27]. Resin-bound imines 78, prepared from resin-bound aldehyde 77 (Mitsunobu reaction of 4-hydroxybenzaldehyde and Wang resin) and primary amines, undergo a tandem Mannich– Michael reaction with Danishefsky’s diene 78 catalyzed by Yb(OTf )3 in dry THF. A wide range of amines may be employed in the chemistry, including aliphatic and arylalkyl amines, as well anilines. Product purities average over 85% after TFA cleavage. A resin-bound divinyl ketone equivalent (82) was utilized in the construction of 2-substituted-piperidine-4-ones (Scheme 22.10) [28]. Reagent 82 is generated via a six-step sequence: (1) reaction of Merrifield’s chloromethyl resin with potassium thioacetate; (2) LiBH4 -mediated reduction of the thioester to methylmercaptan; (3)

657

658


Scheme 22.9. Efficient solid-phase synthesis of piperidinones.

Scheme 22.10. Piperidinone synthesis via cyclorelease strategy.

Michael addition with 3-butenone in EtOH; (4) oxidation to the sulfide to sulfone 81; (5) bromination; and (6) reaction with triphenylphosphine. Phosphonium salt 82 undergoes Wittig condensation to 83 in high yield with a variety of aldehydes using NaOMe as base. Treatment of vinyl ketones 83 with an excess of benzyl-


amine for 3 days at room temperature provides N-benzyl-2-subsititued-piperidin-4ones 86 in 50–75% yield. Benzylamine serves as both nucleophile (Michael addition; 83 to 84) and base (1,4-elimation; 84 to 85) in this ring construction. A second example of a traceless linker strategy utilizing Ru-catalyzed ringclosing metathesis (RCM) was reported by Rutjes and coworkers (Scheme 22.11) [29]. Substrates 89, readily prepared from the Mitsunobu coupling of 87 with allylic sulfonamide 88, undergo RCM in >90% yield at 50 C in toluene.

Scheme 22.11. Metathesis release method for the synthesis of N-heterocycles.

22.2.7

Dihydropyridines

The dihydropyridine (DHP) pharmacophore, well known for its affinity for calcium channels, is regarded as a privileged scaffold possessing broad-based biological activity. The synthesis of DHPs has been largely explored by Gordeev and coworkers at Affymax [30, 31]. Their synthesis is based on the multicomponent cyclocondensation of resin-bound enamino esters 92 with either 2-benzylidine bketo esters 93 or b-keto esters and aldehydes (94, 95) followed by TFA cleavage from solid support (Scheme 22.12A). The success of the cyclocondensation relies upon the use of pyridine as solvent, which facilitates the formation of the thermodynamically favored enamine 97 from imine 96 (isomerization of the p-bond). An elegant application of 13 C-labeled substrates and 13 C-NMR was used to define the reaction mechanism. Acyclic adducts 97 are cleaved from resin prior to cyclization. The solid-phase protocol is sufficiently robust for the construction of a 300member DHP library (Scheme 22.12B). Rink amine resin was split into ten portions and condensed with one of ten b-keto esters. The enamines were then combined and portioned into 30 reaction vessels. Each enamine lot was treated with one of three 1,3-dicarbonyl building blocks and ten aldehydes in pyridine at 45 C. Each resin lot was cleaved with 3% TFA in CH2 Cl2 to yield 30 pools of ten DHPs per pool. All 30 pools were evaluated for Ca 2þ channel binding (rat brain membranes). Two of the pools displayed binding affinities at @10 nM. All 20 com-

659

660


Scheme 22.12. Dihydropiperidine library synthesis.


pounds from the two pools were resynthesized as discrete analogs from which 3,5dicarboxymethyl-2,6-dimethyl-4-(2-fluorophenyl) DHP 105 was identified with an IC50 ¼ 14 nM. 22.2.8

Pyridines

The facile oxidation of dihydropyridines to pyridines prompted the development of a solid-phase pyridine synthesis exploiting this transformation (Schemes 22.13 and 22.14) [32]. Immobilized b-keto esters 106 are reacted with aliphatic or aryl aldehydes 95 to furnish Knoevenagel derivatives 107 (Scheme 22.13). These products then undergo Hantzsch-type condensation with g-oxo-enamines 108 to yield DHPs that in turn are oxidized to pyridines 110 with cerium ammonium nitrate (CAN). TFA-mediated cleavage affords 111. This chemistry also serves as the basis for the solid-phase synthesis of 2,2 0 -bipyridines [33].

Scheme 22.13. Solid-phase synthesis of pyridines.

Ellingboe and coworkers [35] generated 1,5-diketones (114) on resin from immobilized hydroxyacetophenones via Claisen–Schmidt reaction with an aromatic aldehyde followed by Michael addition of trimethylsilyl enol ether (112 ! 113 ! 114; Scheme 22.14). Heterocycle formation 116 occurs upon heating 114 with NH4 OAc in HOAc/DMF at 100 C in an atmosphere of air. Presumably, DHPs 115 initially form and spontaneously oxidize to the corresponding aromatic pyridines. The yield and purity of ten examples (117) range from 19% to 62% and 21% to 70%, respectively. Jung [34] condensed the same enone substrates 113 with 1-(2-oxo-2-arylethyl) pyridinium salts (118) and ammonium acetate in a Krohnke-type pyridine synthesis

661

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Scheme 22.14. Pyridine-derived biaryls.

(Scheme 22.14). DMF containing glacial acetic acid (1:1) is the preferred solvent. Conducting the reaction in the absence of acetic acid gives a side product arising from the addition of pyridine to 113. 22.2.9

Azepanes, Benzazepines, and Derivatives

To date, the solid-phase synthesis of seven- and eight-member ring nitrogen heterocycles has relied primarily on the use of the ring-closing metathesis (RCM) reaction (Schemes 22.15–22.17) [36–38]. Metathesis reactions generally proceed in good yield with minimal intermolecular cross-linking. The challenge is devising a synthetic scheme that permits the introduction of diversity elements in the requisite bis-olefin substrates. The traceless linker strategies reported by Piscopio introduce up to three points of diversity in azepinone structures (Schemes 22.15 and 22.16). In one example, bis-olefin substrates 123 with two points of diversity are prepared by sequential Fukuyama–Mitsunobu and acylation reactions [39]. Ugi four-component coupling of resin-bound cinnamyl amine 125, aldehyde 128, isocyanate 127, and olefin-bearing carboxylic acid 126 conveniently introduces three diversity parameters in a single reaction (Scheme 22.16) [36]. Elevated temperature is generally required for successful RCM. For example, treatment of substrate 129 with Grubb’s catalyst (5–10 mol%) in CH2 Cl2 at 25 C fails to generate metathesis products. However, conducting the RCM reaction in dichloroethane at 80 C for 24 h gives the corresponding lactams as a 1:1 mixture of diastereomers in good yield.


Scheme 22.15. Solid-phase azepine synthesis.

Scheme 22.16.

Azepines via multicomponent condensation and metathesis cyclorelease.

Immobilized diene substrates for azepane synthesis may also be prepared by the addition of allyl lithium to tethered imine ethers (132 to 133 to 134; Scheme 22.17) [37]. Polyhydroxylated azepane scaffold 137 was obtained upon the reaction of Rink linker 71 with chiral l-iditol bis-epoxide 135 (Scheme 22.18) [40]. The intramolecular ring closure required heating for 5 days at 80 C in DMF. Azepanes 137 are obtained following acylation of diol 136 and resin cleavage. Bolten and Hodges [41] at Parke-Davis Pharmaceuticals (now Pfizer) developed a solid-phase synthesis of substituted benzazepines (147) via intramolecular Heck reaction of allyl or propargyl glycines bearing a 2-iodophenyl group (140, 144)

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Scheme 22.17. Metathesis approach to azepines.

Scheme 22.18. Solid-phase azepine synthesis via a chiral pool.

(Scheme 22.19A,B). Resin-bound N-(3-nitro)sulfonyl allylglycine is methylated with MeI in the presence of 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) to give the key cyclization precursor 140, after N-deprotection and acylation with o-iodobenzoyl chloride. Heating 140 in DMF with catalytic Pd(0) cleanly provides the benzazepinone 142 in @65% overall yield following TFA-mediated resin cleavage and treatment with diazomethane. The propargyl substrates 144 give the analogous Heck products 146 following the same protocol. The latter sequence was further exemplified via the synthesis of several benzazepinones (148–153). Yields for the multistep sequence ranged from 40% to 70%. Octahydrobenzazepinones [42] were synthesized via intracyclative ring closure of resin-bound intermediate 156 (Scheme 22.20). Intermediate 156 is efficiently prepared via sequential yne–ene cross-metathesis (154 and an alkyne) and Diels– Alder cycloaddition reactions (155, 156).


Scheme 22.19. Intramolecular Heck reaction to produce benzazepinones.

22.2.10

Indoles

The synthesis of the indole system has received more attention from combinatorial chemists than any other heterocycle. This is a reflection of the broad-based biological activity ascribed to indole-containing compounds. Some 11 reports from eight laboratories describe the solid-phase methodology used to synthesize substituted

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Scheme 22.20.

Tetrahydroazepinones on solid phase.

indoles [43–52; see also 141, 144]. With the exception of the classical Fischer indole synthesis by Merck [51], all of the examples employ Pd catalysis as a central theme. The requisite o-iodoanilines are generally tethered to solid support through an appended carboxylate function. The immobilized substrates are then subjected to the Sonogashira protocol for terminal alkyne coupling followed by Pd-catalyzed cyclization. The overall yield and efficiency of the reaction sequence are highly dependent on the structure of the alkyne and the base used in the heteroannulation step (Schemes 22.21 and 22.22) [44]. Because of its ability to provide homogeneous reaction conditions, tetramethylguanidine (TMG) is the preferred base in many of the reported protocols. Indoles with three points of diversity were generated by Collini and Ellingboe (Scheme 22.23) [45] by coupling trifluoracetylated alkyne intermediates 168 with vinyl triflates 170 using catalytic Pd(PPh3 )4 and potassium carbonate as base. Removal of the trifluoroacetyl protecting group followed by N-alkylation gave a set of functionalized indoles (173). Using an intramolecular Heck reaction, o-iodoanilines 176, bearing a N-allyl substituent, may be cyclized to give indoles 177 (Scheme 22.24) [43, 46]. Several variations on this theme have been published, including the synthesis of 2-oxindoles [52] (178 to 179; 180 to 181). Solid-phase reaction conditions have been optimized for Fischer indole synthesis (Scheme 22.25) [51]. In a single step, resin-bound arylketones 182 are treated with a solution of an arylhydrazine (183) and ZnCl2 in glacial acetic acid at 70 C for 18–20 h. Methanolysis of the resin generates indole esters 186. Because acid catal-


Scheme 22.21. Indoles via intramolecular Heck cyclization.

Scheme 22.22.

Application of tetrahydropyran linkage to indole synthesis.

ysis is required for the reaction, the acid-stable hydroxymethylbenzoic acid (HMB) is the preferred linker (ester linkage). A survey of resins revealed that polystyrene was superior to polyethylene glycol (PEG) polystyrene (PEG-PS), minimizing resin-based impurities in the indole products. Electron-rich, electron-deficient, and even sterically demanding 2,5-disubstituted hydrazines can be used in this solidphase synthesis.

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Scheme 22.23. Multicomponent indole synthesis on solid phase.

22.2.11

Tetrahydroquinolines

In a series of three publications, Kiselyov and coworkers described the synthesis of diverse arrays of tetrahydroquinolines via a three-component condensation reaction (Schemes 22.26 and 22.27) [53–55]. Approximately a decade earlier, Grieco reported this multicomponent reaction in solution based on the condensation of substituted anilines, electron-rich olefins, and aldehydes in the presence of TFA in acetonitrile. In his initial disclosure, Kiselyov anchored 4-nitrobenzoic acid (191) to Wang resin (Scheme 22.26) [53]. The nitro group is selectively reduced to the aniline 192 by SnCl2 in DMF/water. Aniline 121 is then suspended in a 1M solution of benzaldehyde (189) and cyclopentadiene (188) with a catalytic amount of TFA. After 12 h at room temperature, the resin is filtered, washed, and treated with 15% TFA to give tetrahydroquinoline 190 in 76% yield, free of any byproducts. A library of 80 members was generated using aniline (192), five alkenes (187, 194–197), and eight aldehydes (189, 198–204). With the exception of 189, aliphatic aldehydes do not participate in the reaction. Thus, the inputs are largely limited to aryl aldehydes. The yield of library compounds ranges from 53% to 92% with purities in excess of 90%. The highest yields were obtained using electron-deficient aryl aldehydes.


Scheme 22.24. Indoles via intramolecular Heck reaction.

Scheme 22.25. Fisher indole synthesis on solid phase.

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Scheme 22.26. Multicomponent condensation yielding a tetrahydroisoquinoline library.

In further studies, it was observed that, with certain synthons, substantial amounts of nonreacted Schiff base was present in the cleaved products [54], which prompted a survey of Lewis acids to replace TFA as catalysts. Ultimately, a 0.1% solution of Yb(OTf )3 in MeCN/CH2 Cl2 (1:1) was identified as a superior catalyst system for the three-component condensation. The scope of the reaction was then expanded to resin-bound phenolamine substrate 209 and the synthesis of other tricyclic tetrahydroquinolines (Scheme 22.27) [55].


Scheme 22.27.

Variations on the preparation of tetrahydroisoquinolines.

22.2.12

Quinolinones

The synthesis of a 4140-member library of 3,4-dihydro-2-(1H)-quinolinones was achieved through rearrangement of resin-bound b-lactams [56]. Boc-protected amino acids are attached to resin via standard peptide coupling conditions in either dichloromethane (DCM) or DMF depending on amino acid solubility and the deprotected (Gly resin 211 shown; Scheme 22.28). Condensation of 211 with onitrobenzaldehyde in DCM using Na2 SO4 as a water scavenger furnishes imine 213. Imine 213 is then subjected to [2 þ 2] cycloaddition with the ketene derived in situ from phenoxyacetyl chloride and TEA at 78 C. The aryl nitro-containing cisb-lactam (structure confirmation by cleavage and 1 H-NMR analysis) is reduced with SnCl2 to the anilino species 215. This intermediate spontaneously rearranges to the desired aminoquinolinone 216 in high yield and in >85% purity. In the model studies, methylbenzylhydryl amine (MBHA) resin was identified as the resin most compatible with the reaction scheme. Interestingly, when Wang resin was used, the alternative bicyclic system 217 (Scheme 22.28, reaction b) became a

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Scheme 22.28. Solid phase of quinolinones via b-lactams.

significant contaminant. A split-and-pool protocol using tea-bags created a 4140 (2070 pairs of enantiomers)-member library: 69 amino acids (R1) 6 o-nitrobenzaldehydes (R2) 5 acid chlorides (R3). In the actual production run, the R1 position was in mixture format and R2 and R3 were formatted in a two-dimensional array. Other quinolinone heterocycles synthesized on solid support include substituted quinolin-2-(1H)-one-3-carboxylic acids 222 [57] and the antibacterial quinolones [58]. The former heterocycle 222 can be synthesized in three steps: (1) acylation of the malonic esterified Wang resin 219 with o-aminophenones 220; (2) intramolecular Knoevenagel condensation (220 to 221); and (3) resin cleavage (221 to 222; Scheme 22.29). Aminophenones containing electron-donating and electronwithdrawing substituents are equally tolerated. Quantitative yields were obtained in some examples. 22.2.13

Quinolines

A four-step solid-phase synthesis of 2-aryl-6-alkoxyquinolines 232 was reported by the Schering group (Scheme 22.30) [59]. Reaction conditions were initially developed in solution, and the chemistry was transferred to solid phase. Esters 224 and 225, derived from 5-hydroxy-2-nitrobenzaldehyde 223, were the starting materials for the solution- and solid-phase synthesis, respectively.


Scheme 22.29. Synthesis of tetrahydroisoquinolones from resin-bound malonic acid ester.

Scheme 22.30.

Quinoline synthesis on solid support.

The first step in both sequences is an aldol condensation of 224 (225) with an aryl ketone. Optimal reaction conditions in solution employ K2 CO3 in CH3 CN at room temperature. Optimal reaction conditions for the solid-phase synthesis require CH2 Cl2 /THF (1:1) and K2 CO3 at reflux temperature for 2 days. Dichloromethane was used as a cosolvent to ensure resin swelling (polystyrene hydroxy-

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methyl resin). In solution, the nitro reduction to quinoline N-oxide (226 to 227) was effected with SnCl2 in refluxing EtOH for 2 h. This same transformation on solid phase requires a 4-h reaction time, and again preswelling the resin with CH2 Cl2 is essential. The solution- and solid-phase reaction conditions for the remaining two-steps, deoxygenation (TiCl3 ) and aminolysis (amine, AlMe3 ), are virtually identical. Solid-phase reaction monitoring was conveniently achieved via fourier transform infrared FT-IR spectroscopy. A small parallel array of 12 quinolines was synthesized and the products were purified by silica gel chromatography (yields 20–65%). The solid-phase synthesis of quinolines based on the Doebner quinoline synthesis (multicomponent condensation) has been described by Gopalsamy and Pallai [60]. 22.2.14

Tetrahydroisoquinolines

The tetrahydroisoquinoline nucleus is a common structured feature in many biologically active alkaloids. The reaction of an imine with a cyclic anhydride yields a 4-carboxy-substituted tetrahydroisoquinoline. This transformation was a key step in the construction of a 43,000-tetrahydroisoquinoline library (236; Scheme 22.31) [61–63]. Sets of aldehydes, cyclic anhydrides, and amines were sources of diversity in the library. MBHA resin was superior to TentaGel resin with respect to purer products obtained upon release of the library compounds with HF.

Scheme 22.31.

Addition of cyclic anhydrides to imines affording tetrahydroisoquinolones.

Researchers at Merck developed a general synthetic strategy for the preparation of tetrahydroisoquinolines via thermal cyclization of resin-bound imines under mild conditions (Scheme 22.32) [64]. Heating resin-bound m-tyramine 237 with aldehydes (pyridine, 100 C) leads to cyclocondensation, and standard cleavage conditions give the target compounds 238. The reaction accommodates a variety of substituted m-tyramine derivatives. Two reports describe the solid-phase synthesis of tetrahydroisoquinolines using the Bischler–Napieralski reaction [65, 66]. The key step in the synthesis is the in-

22.3 Solid-phase Synthesis of Heterocycles Containing One Oxygen Atom

Scheme 22.32.

Tetrahydroisoquinolines via solid-phase imine cyclization.

tramolecular cyclization of m-phenoxyphenethylacetamides with POCl3 , substrates, which survive the rather harsh reaction conditions, attached through ester-type linkages to Merrifield resin.

22.3

Solid-phase Synthesis of Heterocycles Containing One Oxygen Atom 22.3.1

Tetrahydrofurans and g-Butyrolactams

Intramolecular radical cyclizations of unsaturated carbon–carbon bonds to access heterocyclic systems have been known for over 20 years. Two publications explored the synthesis of tetrahydrofurans using such methodology [67, 68]. In research carried out by Balasubramanian and coworkers [67], radical cyclizations to form tetrahydrofurans (243) were achieved upon treatment of substrates 241 with Bu3 SuH and catalytic 2,2 0 -azobisisobutyronitrile (AIBU) (Scheme 22.33). The synthesis was initially attempted on polystyrene support, but the yields were poor. Use of TentaGel resin results in substantially improved yields of >90%. It is thought that the failure of the reaction on polystyrene is due to competing hydrogen abstraction from

Scheme 22.33.

Tetrahydrofurans via intramolecular radical cyclization.

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the benzylic position of the polymeric backbone. The polyethylene spacing units in TentaGel minimize the formation of backbone radicals. No byproducts are produced, and tin residues are conveniently washed out of the resin to give pure cyclized products. This chemistry is also useful for constructing dihydrobenzofurans. As further confirmation of the observations made by Balasubramanian’s group, Toru and coworkers [68] found that linear spacing groups on polystyrene support were necessary to enable high-yielding radical cyclization of substrates 245 en route to g-butyrolactones 247 (Scheme 22.34). The traceless linker solid-phase synthesis begins with the preparation of b-bromoacetals (245). Intramolecular radical cyclization furnishes cyclic actetals (246), whereupon g-butyrolactones are isolated after Jones oxidation. Six g-butyrolactones were prepared using this methodology, with yields ranging from 47% to 93%. The same six examples based on Merrifield resin with linear spacing groups gave reduced yields on the order of 23–43%.

Scheme 22.34. Radical cyclization and oxidative cleavage to butyrolactones.

Several other g-butyrolactone syntheses have been described using nonradical chemistry [69–74]. 22.3.2

Furans

A general method for the construction of substituted furans on solid support is described by Austin and coworkers [75] and by Gororavram and Gallop [76]. Furans are assembled in aqueous medium via the 1,3-dipolar cycloaddition reaction of acetylenes with resin-bound isomu¨nchnones. Upon heating, the transient cycloadducts experience a facile retro-Diels–Alder reaction (cycloreversion) to release tetra-substituted furans in solution while isocyanate is retained on resin. Model solid-phase studies began with amide 248 – Wang resin esterified with N-acyl-protected amino caproic acid (Scheme 22.35) [75]. Acylation of 249 with methylmalonyl chloride and diazo transfer yields the isomu¨nchnone precursor 250. Rhodium-mediated decomposition of 250 in the presence of DMAD at room temperature affords adduct 251. The reaction is easily monitored by infrared (IR)

22.3 Solid-phase Synthesis of Heterocycles Containing One Oxygen Atom

Scheme 22.35. Cycloaddition–cycloreversion approach to furans.

spectroscopy. Furans 252 are obtained in 70% yield and in 98% purity upon thermolytic cleavage of 251 in benzene at 70 C. The solid-phase chemistry was applied to a split-and-pool synthesis of a small furan library (18 members). Both aryl and alkyl groups may be incorporated into the furan ring using a rhodium(II) perfluorobutyramidate catalyst. The chemistry appears to be limited to acetylenes possessing electron-withdrawing groups. 22.3.3

Benzofurans

Given the structural similarity of benzopyrans and indoles, many of the solidphase approaches developed for indoles have been applied to benzofuran synthesis. Solid-phase strategies to this class of compounds utilize Pd-catalyzed heteroannulation of terminal acetylenes in the presence of o-hydroxyaryl iodides [77, 78]. Fancelli and coworkers prepared 254 by esterifying TentaGel S-OH resin (Mitsunobu reaction) with acetate-protected 3-iodo-4-hydroxy benzoic acid (Scheme 22.36) [77]. Heteroannulation with terminal acetylenes is carried out using Pd(0)

Scheme 22.36. Intramolecular Heck cyclization to furnish benzofurans.

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catalyst, CuI, and tetramethylguanidine as a soluble base. Saponification of the products gives essentially pure benzofuran carboxylic acids in 40–70% yield. The methodology is exemplified using ten different terminal acetylenes possessing amino- and hydroxyalkyl substituents and is suitable for automated synthesis in 96-well format. Other methods for preparing benzofurans and derivatives on solid support include SmI2 -mediated radical cyclization/anionic capture of aryl iodides [79, 80] and intramolecular condensation of aryl keto-b-alkoxyesters [81]. 22.3.4

Pyrans, Benzopyrans, and Derivatives

In contrast to furans, there are few reports on the synthesis of pyrans and their derivatives. Solid-phase chemistries for the construction of dihydropyrans (241) have been reported that utilize a hetero-Diels–Alder reaction of oxabutadienes (239) with enol ethers (Scheme 22.37) [82]. Baldwin and coworkers at Pharmacopeia, have described the solid-phase synthesis of a dihydrobenzopyran library using molecular tagging technology [83]. In this chemistry, resin-bound o-hydroxyaryl methylketones are attached to resin and condensed with a variety of cyclic and acyclic ketones. When bifunctional ketones were used, four points of diversity were achieved (Scheme 22.38).

Scheme 22.37. Dihydropyrans via hetero-Diels–Alder reaction on solid phase.

Nicolaou and coworkers have prepared natural product-like libraries based on a benzopyran template; this process is reviewed in Chapter 21.

22.4

Solid-phase Synthesis of Thiophenes

3-Aminothiophenes have been synthesized on solid phase by Zaragoza and colleagues at Novo Nordisk [84, 85]. The reaction sequence is an adaptation of solu-

22.5 Summary

Scheme 22.38. Encoded benzopyran library.

tion chemistry originally described by Laliberte. Resin-bound cyanoacetamide 247 is reacted with aliphatic or aromatic isothiocyanates in DMF/1,8-diazabicyclo[5.4.0]undecene-7 (DBU) (Scheme 22.39). Subsequent S-alkylation of intermediates 248 with a set of aryl a-haloketones and acidolytic cleavage furnishes 3-aminothiophenes in high yield. The reaction sequence precludes the use of strongly electrondonating isothiocyanates or aliphatic a-haloketones. In subsequent studies, resinbound isothiocyanates were found to be useful in this synthesis.

Scheme 22.39. Solid-phase thiophene synthesis.

22.5

Summary

The solid-phase syntheses of all the major classes of heterocycles containing a single heteroatom have been described in the literature. Much of the solid-phase heterocyclic chemistry published thus far is synthetic methodology that is typically

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exemplified by fewer than a dozen compounds. Relatively few libraries containing thousands of members are known. In general, methodology developed on solid support is a direct adaptation of solution chemistry. Solid-phase chemistries require extended reaction times and excess reagents to drive them to completion. In this regard, extensive reaction development and synthon profiling must be engaged to optimize solid-phase chemistry. Yields of solid-phase synthesis are generally equal to or lower than that observed in solution, although the ease of purification is greatly facilitated by solid support. Several examples are cited in which the type of resin is critically important for selected chemistries. A case in point is the failure (or greatly reduced yields) of radical-mediated cyclizations on Merrifield versus TentaGel resin.

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Alewood, Tetrahedron Lett. 1995, 36, 7709–7712. K. Rolfing, M. Thiel, H. Kunzer, Letters, 1996, 1036. A. Routledge, C. Abell, S. Balasubramanian, Synlett 1997, 61. Y. Watanabe, S. Ishikawa, G. Takao, T. Toru, Tetrahedron Lett. 1999, 40, 3411–3414. M. Rottlander, P. Knochel, J. Comb. Chem. 1999, 1, 181–183. D. H. Ko, D. J. Kim, C. S. Lyu, I. K. Min, H. Moon, Tetrahedron Lett. 1998, 39, 297–300. S. Kobayashi, T. Wakabayashi, M. Yasuda, J. Org. Chem. 1998, 63, 4868– 4869. C. Le Hetet, M. David, F. Carreaux, B. Carboni, A. Sauleau, Tetrahedron Lett. 1997, 38, 5153–5156. X. Beebe, C. L. Chiappari, M. M. Olmstead, M. J. Kurth, N. E. Schore, J. Org. Chem. 1995, 60, 4204– 4212. X. Beebe, N. E. Schore, M. J. Kurth, J. Org. Chem. 1995, 60, 4196–4203. D. L. Whitehouse, K. H. Nelson, S. N. Savinov, R. S. Lowe, D. J. Austin, Bioorg. Med. Chem. Lett. 1998, 6, 1273– 1282. M. R. Gowravaram, M. A. Gallop, Tetrahedron Lett. 1997, 38, 6973–6976. D. Fancelli, M. C. Fagnola, D. Severino, A. Bedeschi, Tetrahedron Lett. 1997, 38, 2311–2314. Y. Wang, T. Huang, Tetrahedron Lett. 1998, 39, 9605–9608. X. Du, R. W. Armstrong, Tetrahedron Lett. 1998, 39, 2281–2284. X. Du, R. W. Armstrong, J. Org. Chem. 1997, 62, 5678–5679. T. L. Boehm, H. D. H. Showalter, J. Org. Chem. 1996, 61, 6498–6499. L. F. Tietze, T. Hippe, A. Steinmetz, Synlett 1996, 1043. J. J. Baldwin, Mol. Diversity 1996, 2, 81–88. H. Stephensen, F. Zaragoza, J. Org. Chem. 1997, 62, 6090–6097. F. Zaragoza, Tetrahedron Lett. 1996, 37, 6213–6216. B. T. Watson, G. E. Christiansen, Tetrahedron Lett. 1998, 39, 6087–6090.

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Multicomponent Reactions Arounarith Tuch and Stefan Walle´ 23.1

Introduction

Multicomponent reactions (MCRs) are usually condensation reactions involving three or more educts; they are fundamentally different from two-component reactions. During the past decades, this type of reaction has been recognized as a very efficient method that can be used in all fields of organic synthesis. Especially in both liquid- and solid-phase combinatorial chemistry, these reactions have been accepted as an indispensable tool for library construction. This chapter describes the most important multicomponent reactions and their applications to combinatorial chemistry; examples of the synthesis of combinatorial libraries on solid support as well as in solution phase will be given.

23.2

Mannich Reaction

The Mannich reaction is a classic three-component reaction in which a component containing at least one hydrogen atom of pronounced reactivity reacts with an imine species formed from the condensation of an amine and an aldehyde. Although Mannich chemistry in traditional solution-phase organic synthesis is of general utility [1], its application to resin-bound substrates as well as to parallel synthesis in solution has not been so widespread. Independently, and almost simultaneously, the Katritzky [2] (Scheme 23.1) and Showalter groups [3] (Scheme 23.2) described the synthesis of polymer-supported benzotriazole and its application to the generation of Mannich adducts. Secondary [2] and tertiary [2, 3] amines (1, 3) and tosylamides [2] formed by Mannich reaction were cleaved from the resin with Grignard [2, 3] and organozinc reagents [2]. The reactions were performed with a large excess of aldehyde (5 mol equiv. [2], 10 mol equiv. [3]) and amine (5 mol equiv. [2], 10 mol equiv. [3]) in tetrahydrofuran (THF)/methoxyethanol (1:1) at 60 C for 8–12 h [2] or THF/EtOH at reflux or in dimethylformamide (DMF) at 70 C for @17 h [3]. These solvents were chosen Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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23 Multicomponent Reactions

Scheme 23.1.

Tertiary amines and tosylamide [2].

Scheme 23.2. Mannich-type adducts from polymer-supported benzotriazoles [3].

because they led to better swelling of the polymer. The aldehydes tested by Katritzky were of diverse reactivity – HCHO, PhCHO, and iso-BuCHO – and only secondary amines were used, whereas Showalter also used primary amines. In the case of the less reactive benzaldehyde, harsher conditions were necessary to carry out the reaction: toluene, reflux, and a Dean–Stark trap for 12 h [2]. These Mannich adducts (1, 3) were then cleaved from the polymer-supported benzotriazole with Grignard (4 mol equiv., THF, reflux, 4 h) [2], (5 mol equiv., toluene, reflux, overnight) [3], or organozinc reagents (5 mol equiv., THF, 60 C, 12 h) [2]. After work-up, the amines (2, 3) were isolated in good overall yields [2, 3]. In their synthesis of tertiary methylamines (8), Blaney et al. [4] (Scheme 23.3) also used benzotriazole, but this component was not polymer supported. In this alternative strategy, the reaction conditions are milder than those in the previous two strategies [2, 3]. The resin-bound secondary hydroxylamine 5 reacted with an excess of aliphatic or aromatic aldehyde (10 mol equiv.) in the presence of an excess of benzotriazole (10 mol equiv.) in CH2 Cl2 at room temperature (rt) for 18 h

Scheme 23.3.

Tertiary methylamines via resin-bound oxyiminium ions [4].

23.2 Mannich Reaction

to form the resin-bound benzotriazole Mannich adduct 6. The latter was then displaced with Grignard reagents (10 mol equiv., THF, rt, 16 h). Quaternization of 7 (5 mol equiv. MeOTf, CH2 Cl2 , rt, 16 h) followed by base-induced cleavage (5 mol equiv. NEt3 , CH2 Cl2 , rt, 16 h) gave very pure amines (8). 3-Aminomethylindoles [5] (Scheme 23.4), which are interesting because of their biological activity, were prepared from polymer-bound indoles (10) by Mannich reaction. The resin-supported 2-substituted indoles 10 obtained in three steps from Rink amide resin and 4-amino-3-iodobenzoic acid [5b] were subjected to Mannich reaction conditions with an excess of formaldehyde (2–8 mol equiv.) and secondary amines (2–8 mol equiv.) at rt for 1.5 h in 1,4-dioxane/HOAc (4:1). The reactions were reported to proceed more effectively in this mixture than in only HOAc. After acidic cleavage, the 2-substituted 3-aminomethylindoles 11 were isolated in good yields and purities.

Scheme 23.4. Resin-bound indoles [5].

Any Mannich partner can be immobilized on an appropriate solid support. Researchers at the R. W. Johnson Pharmaceutical Research Institute [6] elegantly demonstrated this versatility. Terminal alkynes can also serve as components in the Mannich reaction. In such cases, a copper(I) salt is necessary to promote the aminomethylation [6]. Youngman and Dax [6a] (Scheme 23.5) reported the first Mannich reaction with resin-bound alkynes. After binding propargylamine to 2chlorotrityl resin, the Mannich chemistry was performed with secondary amines (5 mol equiv.), cuprous chloride (1 mol equiv.), and paraformaldehyde (10 mol equiv.) in 1,4-dioxane at 70–75 C for 3 h. The overall isolated yields and purities were reported to be >90% and >95%, respectively. The reaction seemed to be a general one, as evidenced by the diversity of the compounds synthesized.

Scheme 23.5. Resin-bound terminal alkynes [6a].

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However, the number of commercially available acetylenes is limited. This problem was clearly overcome by a combination of the Sonogashira and Mannich reactions [6b] (Scheme 23.6). The Sonogashira reaction was run prior to Mannich chemistry, thus generating diverse building blocks (15) for the Mannich reaction. The overall yields and purities of 17 were reported to be good. The Mannich partner amine was then bound to solid support (Scheme 23.7), e.g. piperazine was bound to 2-chlorotrityl resin [6c]. The Mannich reaction was carried out under the reaction conditions described above. The Mannich adducts 19 were isolated as bistrifluoroacetic acid (TFA) salts with high purity.

Scheme 23.6. Resin-bound terminal alkynes [6b].

Scheme 23.7. Resin-bound terminal alkynes [6c].

Finally, a Wang resin-supported aldehyde [6c] (Scheme 23.7) was subjected to the same reaction conditions as described above. The Mannich adducts 21 were isolated with lower yields: the incomplete iminium formation was suspected to be the cause. Compounds of the Mannich reaction having a hydrogen atom of pronounced reactivity can be successfully substituted by silyl enol ethers. The Kobayashi group

23.3 Hantzsch Reaction

[7] efficiently applied this Mannich-type reaction to solid phase to prepare successfully a 48-amino alcohol library [7b] (Scheme 23.8). Aldehyde (1.2 mol equiv.), primary amine (1.2 mol equiv.), Drierite, and scandium triflate (0.1 mol equiv.) were stirred in CH2 Cl2 at rt for 1 h prior to the addition of the polymer-supported silyl enol ether 22. The mixture was then stirred at rt for 20 h and, after completion of the reaction, the desired amino alcohols (24) were reductively cleaved from the support in good overall yields.

Scheme 23.8. Mannich-type reaction [7b].

Furthermore, this method worked in solution phase as well as on solid support [7c] (Scheme 23.9). b-Amino esters 25 were thus efficiently prepared in parallel in large quantities, with high yields and purities. The ease of the work-up and purification procedure could render this method amenable to automation.

Scheme 23.9. Mannich-type reaction [7c].

23.3

Hantzsch Reaction

The Hantzsch reaction is a condensation involving 2 mol of b-ketoester or b-ketoamide, 1 mol of aldehyde, and 1 mol of ammonia. The product of the condensation – 1,4-dihydropyridine – can then be oxidized to the corresponding pyridine derivative. Patel et al. [8] (Scheme 23.10) used Rink resin-bound enamino esters for the synthesis of a ‘‘300-member’’ dihydropyridine library via the Hantzsch condensation. The reaction was performed with resin-supported enamino esters (26), an aldehyde (2 mol equiv.), and a b-dicarbonyl derivative (2.3 mol equiv.) in dry pyridine in the presence of 4A molecular sieves at 45 C for 24 h. Pyridine was found to be the most effective solvent in this heterocyclization. An acidic cleavage with 3% TFA in CH2 Cl2 gave the desired 1,4-dihydropyridines (28), whose yields and purities were not reported. Breitenbucher and Figliozzi [9] (Scheme 23.11) reported a small ‘‘272-member’’ 4-aryl-1,4-dihydropyridine library using a slightly different approach. Resin-bound

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690


Scheme 23.10. Hantzsch reaction [8].

Scheme 23.11. Hantzsch reaction [9].

b-acetoacetate (29) was formed upon treatment of diverse amino alcohol linkers with 2,2,6-trimethyl-1,3-dioxanone. The Hantzsch reaction was then carried out with aminocrotonate derivatives and aryl aldehydes in DMF at 80 C. The desired 4-aryl-1,4,-dihydropyridines (30) were obtained along with the byproduct (31) (15%). This Knoevenagel product (31) was selectively removed by washing the resin with hydrazine. After acidic cleavage and supported liquid extraction, the 4aryl-1,4,-dihydropyridines 30 were obtained in modest to good overall yields (44– 88%) and in good purities (77–90%).

23.4

Baylis–Hillman Reaction

To date, there is only one publication on a multicomponent version of the Baylis– Hillman reaction, reported by the Jung group [10] (Scheme 23.12). The threecomponent reaction was performed with 2-chlorotrityl resin-bound acrylic acid (32), aryl aldehydes (16 mol equiv.), sulfonamides (16–20 mol equiv.), and 1,4-dia-

23.5 Grieco Three-component Reaction

Scheme 23.12. Baylis–Hillman reaction [10].

zabicyclo[2.2.2]octane (DABCO) (1.6 mol equiv.) as catalyst in dioxane at 70 C for 20 h. The substituted sulfonamides 33 were obtained in good purities (53–90%), but overall yields were not reported. 23.5

Grieco Three-component Reaction

The Grieco three-component reaction [11a] is a condensation involving anilines, aldehydes, and electron-rich olefins catalyzed by TFA [11b,c], Yb(OTf )3 [11d], or Sc(OTf )3 [11e] as Lewis acid catalysts to form nitrogen-containing six-member heterocycles. Kiselyov and Armstrong reported the first version of this reaction on solid phase [11b] (Scheme 23.13). Furthermore, they elegantly immobilized every Grieco partner on appropriate solid support [11b–d] (Scheme 23.14), thus demonstrating the

Scheme 23.13. Grieco three-component reaction [11b].

Scheme 23.14. Grieco three-component reaction [11c].

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versatility of this multicomponent reaction. The typical reaction conditions were an excess of aldehydes (1.5–8 mol equiv.), anilines (1.5 mol equiv.), and alkenes (2.5–5 mol equiv.) catalyzed by TFA [11b,c] or Yb(OTf )3 [11d] in MeCN [11b,c] or MeCN/CH2 Cl2 (2:1) [11d] at rt for 12–24 h. Other catalysts, namely FeCl3 [11b], LiBF4 [11c], BF3 Et2 O, TiCl 4 , and InCl3 [11d], afforded lower yields of the products. Several other solvents such as dioxane, DMF, MeOH, and THF were found to be inferior to MeCN [11b,d]. The yields and purities of the synthesized tetrahydroquinoline derivative (34–36) were reported to be good [11b–d]. This three-component condensation was successfully carried out in solution by the Kobayashi group [11e] (Scheme 23.15) using a polymer-supported scandium catalyst. The quinoline derivatives (37) were reported to be obtained in high yields (65–100%) and quantity. Given the number of commercially available Grieco partners and the effectiveness of this method, a quinoline library of more than 1 million members could be prepared.

Scheme 23.15. Grieco three-component reaction [11d].

23.6

Biginelli Reaction

In 1893, Biginelli first reported the synthesis of 3,4-dihydropyrimidine-2-(1H)-ones of type 38 by a very simple one-pot condensation reaction of acetoacetate, benzaldehyde, and urea in ethanolic solution [12] (Scheme 23.16).

Scheme 23.16. ‘‘Biginelli reaction’’: synthesis of dihydropyrimidinones.

A major drawback of this so-called Biginelli reaction is the moderate yield associated with it – only 20–60% of the expected product is usually obtained. Since 4aryldihydropyrimidinones have emerged as an integral backbone of several drugs such as calcium channel blockers or antihypertensive agents during the past few

23.7 Multicomponent Reactions with Isocyanides

decades, some improved procedures for their preparation have been reported, including various solid-phase modifications suitable for combinatorial chemistry [13]. Recently, the synthesis of ‘‘Biginelli compounds’’ involving the use of BF3 OEt2 / CuCl and polyphosphate ester leading to high yields of dihydropyrimidinones was published [14]. The use of ferric chloride hexahydrate as a mild catalyst in ethanol as the solvent consistently produced yields of 80–90% of the desired dihydropyrimidinones [15].

23.7

Multicomponent Reactions with Isocyanides 23.7.1

History of Isocyanides

A large and important class of multicomponent reactions involve isocyanides. Isocyanides, also known as isonitriles, are compounds with remarkable reactivity. Their unusual valence structure and reactivity have been discussed for more than a century, and they belong to the only class of stable organic compounds with a formally divalent carbon (carbon monoxide also contains a divalent carbon). In exothermic reactions, the C(II) of isocyanides is oxidized to C(IV); this feature was originally noted by Nef in 1892 [16]. Hundreds of natural products containing an isocyano function have been isolated over the years, mostly from marine organisms (xanthocillin was commercially available from Arzneimittelwerk Dresden) [17]. Many of these natural isocyanides show antibiotic and fungicidal effects as well as the potential to be used in crop protection [18]. In addition to the isolated isocyanides, many natural products have been found as N-formamides. These can be regarded as products of the hydrolysis of isocyanides or even as their precursors, meaning that probably more isocyanides occur naturally than is generally assumed. Table 23.1 shows a few examples of naturally occurring isocyanides [19]. Isocyanide chemistry began in the middle of the nineteenth century. In 1859, Lieke succeeded in synthesizing allyl isocyanide by reacting allyl iodide and silver cyanide; however, he did not recognize the synthesized species to be an isocyanide, believing it instead to be a nitrile [20]. His attempts to transform the compound into the corresponding carboxylic acid failed, leading instead to the formamide. It took another 8 years for isocyanides to become generally available. In 1867, Hoffmann published [21] a new method of preparation of isocyanides involving the reaction of primary amines, chloroform, and potash. Despite that synthetic success, the chemistry of isocyanides stagnated for almost a century since they could be obtained only in low yields. All preparative methods known at that time were complicated and were poorly generalizable, and, in addition, the desired compounds were known to have a very unpleasant smell, which forced chemists to work outside [22]. During the first 100 years of isocyanide chemistry, only 12 of

693

694

23 Multicomponent Reactions Tab. 23.1. Examples of naturally occurring isocyanides.

Xanthocillin

Leptocillin

4-((E )-2-carboxyvinyl)-3,4-epoxy1-cyclopentenyl isocyanide

Acenthellin 1

Aerocyanidine

these compounds were known; by 1971, more than 300 isocyanides had been described in the literature. The synthesis of tetrazoles by Olivieri-Mandala and Malagna in 1910 was an important and remarkable event that took place during the period when isocyanide chemistry was not very popular [23]. In the 1920s Passerini worked on the synthesis of a-acyloxy carbonamides [22, 24, 25], inventing a new type of threecomponent reaction utilizing carbonyl compounds, carboxylic acids, and isocyanides. Again, the lack of a general method to synthesize isocyanides in good yields postponed the general use of the Passerini reaction by another 30 years. In 1958, a breakthrough in isocyanide chemistry occurred: a high number of various isocyanides could be synthesized in good yields by the dehydration of N-formyl amines. Even though dozens of methods for the preparation have been described [26], the reaction of N-formyl amines with phosgene, triphosgene, or other inorganic dehydratants and matching bases is still the method of choice regarding costs and yields of the desired products [27, 28]. Various organic bases such as triethylamine, pyridine, quinoline, diisopropylamine, or DABCO have been used. Depending on further functionalities, many alternative isocyanide synthesis methods have been developed, some of which are summarized in Table 23.2 [19]. 23.7.2

Isocyanide Chemistry

The chemistry of isocyanides is characterized by three properties: a-addition, aacidity, and the formation of radicals. The synthetically most important property is

23.7 Multicomponent Reactions with Isocyanides Tab. 23.2.

Various synthetic routes to isocyanides. Reference 20 29

30

31

32

33

34

the reaction of the isocyanide carbon with electrophiles and nucleophiles. (Most other functional groups in organic chemistry react with nucleophiles and electrophiles at different centers, only carbenes and carbon monoxide share this behavior with isocyanides.) A very important field of chemistry to which isocyanides contribute is the synthesis of heterocyclic compounds. Among others, useful imidazoline, oxazoline [35], thiazoline, pyrrole [36], imidazole [37], oxazole [38], and thiazole [39] syntheses have been described [40]. 23.7.3

Isocyanides on Solid Phase [41]

Isocyanide-bearing polymers are generally prepared by dehydration of the corresponding formamido resins. Among the various dehydrating agents [42, 43] for the preparation of low-molecular-weight isocyanides, diphosgene and toluenesulfonyl chloride have been found to be the most effective. However, the use of TsCl in pyridine is certainly the most convenient method: a concentrated solution of TsCl in pyridine is used and the reaction is carried out at 0–20 C. Under these conditions, various resins swell adequately and the reaction is complete within minutes.

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23.7.4

Passerini Reaction

In 1921, the reaction between carboxylic acids, oxo components, and isocyanides was described for the first time by the Italian chemist Passerini. The reaction leads to a-acyloxycarboxamides, which are produced in one step [24] (Scheme 23.17).

Scheme 23.17. Passerini reaction.

Different mechanisms have been discussed for the Passerini reaction [44, 45]; since the reaction is accelerated and usually carried out in aprotic solvents, a nonionic mechanism is indicated [46]. Most likely, the first step of the reaction leads to a hydrogen-bound adduct of the carbonyl compound and the carboxylic acid 39; in the second step, the a-addition of the electrophilic carbonyl carbon and the nucleophilic oxygen atom of the carboxylic acid to the isocyanide carbon forms the cyclic transition state (40) involving all three parent compounds. The a-adduct cannot be isolated and rearranges to the stable a-acyloxycarboxamide (41) (Scheme 23.18).

Scheme 23.18. Passerini reaction mechanism.

The solvent of choice for the Passerini reaction is CH2 Cl2 , and the reaction is usually carried out at rt with a high concentration of the three components. There are almost no limitations in the choice of the oxo component used: both aldehydes and ketones can be utilized. Some sterically hindered and a,b-unsaturated ketones as well as bicyclic extremely hindered ketones such as camphor do not react with isocyanides in a Passerini reaction [47]. Besides ‘‘normal’’ C-isocyanides,


Me3 SiCN, which is in an equilibrium with Me3 SiNC, also reacts with mineral acids in the presence of ZnI2 to give 2-hydroxy carboxylic acids and amides [48]. The treatment of ketones such as 42 with two equivalents of the isocyanide in the presence of BF3 OEt3 in nonpolar solvents leads to the b,g-unsaturated aoxocarboxylic amide 43 (Scheme 23.19).

Scheme 23.19. Reaction of ketone 42 with tert-butyl isocyanide.

The a-acyloxy carboxamide group is a frequently recurring motif in many natural products. For that reason, the Passerini reaction is a suitable route to numerous depsipeptides [49]. Lewis and Brønsted acids catalyze the formation of a-hydroxy tetrazoles from isocyanides, HN3 , and oxo compounds. a-Hydroxytetrazoles were described in 1931 – as a cyclic version of the Passerini reaction – as products of the reaction of hydrazoic acid, oxo compounds, and isocyanides [50]. The Passerini reaction has had a renaissance in the last few years, and there are numerous examples of this reaction that lead to interesting structures [19]. 23.7.5

The Ugi Reaction

The Ugi four-component condensation was discovered in 1959. Basically, a carboxylic acid reacts with a ketone or an aldehyde, an amine, and an isocyanide [51, 52] (Scheme 23.20).

Scheme 23.20. Ugi reaction.

In a simplified mechanism, the oxo component and the amine react in the first step to the imine (Scheme 23.21). Then, the acid component protonates the nitrogen atom of the imine, increasing the electrophilicity of the CbN bond. In the third step, the electrophilic iminium cation as well as the nucleophilic acid anion add to the isocyanide carbon atom. An intramolecular acylation of the nitrogen atom of the former amine and the subsequent rearrangement to an a-acylaminoamide follow, forming the stable Ugi product. All the steps of the reaction sequence are equilibria; however, the sequence for the last step lies completely on the product side. In this respect, this type of MCR is different from other MCRs. The

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Scheme 23.21. Ugi reaction, proposed mechanism.

driving force for the reaction sequence is the oxidation of the isocyanide C(II) atom to the amide C(IV) atom. Changes in the nucleophiles and electrophiles of the components of the Ugi condensation can be observed during the reaction sequence: the reactive centers of the acid component and the imine change the sign of their reactivity several times. First, the CbN bond of the imine behaves like a base toward the acid component. Then, the protonated Schiff base functions as the electrophilic and the acid anion as the nucleophilic component of the a-addition. In the cycloaddition and elimination that follows the addition, the reactive centers change their signs once again. In the course of the Ugi reaction, one CaC bond and several heteroatom–C bonds are newly formed. Reactivity tables for isocyanides have been generated; the formation of the product with different isocyanides has been examined as a function of solvent and concentration [53]. The reactivity is mainly influenced by inductive, mesomeric, and steric effects – the last of which play the least important role; in addition, the concentration of the reactants has a significant influence on the reactivity and the products of the reaction! The Ugi reaction can be carried out in solution as well as on solid phase. In solution phase, low-molecular-weight alcohols, such as methanol, ethanol, or trifluoroethanol, are used as typical solvents. Aprotic polar solvents such as DMF, chloroform, dichloromethane, THF, or dioxane have also been described in many cases. However, the choice of the solvent has a decisive influence on the product of the reaction: benzoic acid, benzyl amine, dichloroacetic aldehyde, and tert-butyl isocyanide react in methanol at 0 C almost quantitatively to form the Ugi product; in methylene chloride the Passerini product of benzoic acid, dichloroacetic aldehyde and tert-butyl isocyanide is formed [54]. The Ugi condensation is an exothermic reaction and usually proceeds at a fast rate at room temperature or below; for large batches, an external cooling of the reaction mixture is recommended. If the reactants are present in high concen-


trations, the reaction proceeds better – this is typical behavior for many multicomponent reactions. 23.7.6

The Ugi Reaction on Solid Phase

The Ugi reaction has also been described on solid phase. Basically, each of the reaction’s components can be attached to resin; which component is best attached to the solid phase certainly depends on the structure and the electronic properties of each component and varies from case to case. In 1997, the synthesis of small-molecule combinatorial libraries on solid phase by the Ugi reaction was published [56]. Besides pyrroles [57, 58] and imidazoles [58], the solid-phase preparations of small ring lactams [59], a-(dialkylamino)amides, hydantoin-4-imides, 2-thiohydantoin-4-imides, and 5-(1-aminoalkyl)tetrazoles have been described. The last are prepared by replacing the carboxylic acid with HN3 (Scheme 23.22).

Scheme 23.22. Synthesis of tetrazoles 44 on solid phase via Ugi reaction.

The reaction on solid phase has been carried out using NaN3 ; thus, the exposure of various isocyanides to a range of amines, aldehydes, and sodium azide with pyridine hydrochloride in a methanol/dichloromethane/water (3:3:1) mixture for 4 days led to the Ugi product [60]. The resin was washed with methanol and dichloromethane (DCM), then agitated with 20% TFA in DCM. After removing the solvent, the residues were purified by preparative thin layer chromatography (TLC). With the isocyanides immobilized on Wang resin, 45 was obtained by stirring the isocyanides with aldehydes, amines, and in situ-generated HOCN [HXCN can be conveniently generated in situ by the addition of pyridineHCl to form KXCN (X ¼ O, S)] for 24 h in methanol/chloroform/water (5:5:1). The TFA/CH2 Cl2 cleavage provided the desired hydantoin-4-imides 46 in yields up to 80% (Scheme 23.23). The synthesis of the a-(acylamino)amides 47 was also carried out by an Ugi reaction, starting from pyrimidine carboxylic acids [44] (Scheme 23.24).

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Scheme 23.23. Synthesis of hydantoin-4-imides on solid phase via Ugi reaction.

Scheme 23.24. Synthesis of a-(acylamino)amides on solid phase via Ugi reaction.

A mixture of a primary amine and an aldehyde was shaken at rt in dry dioxane for 3 h, then the isocyanide and the polymer-bound pyrimidine carboxylic acid were added. After a reaction time of 48 h at 55 C, the a-(acylamino)amides were obtained.


23.7.7

Other Multicomponent Reactions with Isocyanides

During the last few years, several new multicomponent reactions have been discovered, mostly involving isocyanides. In 1998, Bienayme´ and Bouzid [61], Blackburn et al. [62], and Groebke et al. [63] described a synthesis of fused 3aminoimidazoles 48. Heteroaromatic amidines, such as 2-aminopyridine or pyrimidine, react with isocyanides and aldehydes in the presence of a catalytic amount of protic acids in a very efficient three-component reaction to form the corresponding imidazo-pyridines and -pyrimidines respectively (Scheme 23.25). Many heteroaromatic amidines have been tested in this reaction with good results; only electron-poor amidines tend to react slowly, with the frequent appearance of side-products.

Scheme 23.25. Synthesis of aminoimidazo-pyridines (XbC) and

aminoimidazo-pyrimidines (XbN) respectively via three component condensation.

A probable mechanism involves [4 þ 1] cycloaddition between the protonated Schiff base of the 2-aminopyridines holding the electrophilic and the nucleophilic centers and the isocyanide that behaves as a vinylidene carbenoid. A subsequent prototropic shift gives the final aromatic fused 3-aminoimidazole 48. The above-described reaction has also been carried out on solid phase. A resinbound aldehyde, prepared by anchoring 3-carboxybenzaldehyde to Rink amide resin in the presence of HATU [O-(7-azabenzotriazol-1-yl)-N,N,N 0 ,N 0 -tetramethyluronium hexafluorophosphate], was converted to an imine by reaction with 2aminopyridine in a solution of CH2 Cl2 /CH3 OH (3:1) with Sc(OTf )3 as a catalyst and allowed to react with benzyl isocyanide. After 48 h, the TFA-induced cleavage afforded the condensation product in a similar yield to that obtained in solution phase (Scheme 23.26).

Scheme 23.26.

Aminoimidazoles on solid phase: amine bound to the polymer.

Another attractive strategy for the solid-phase synthesis of the aminoimidazoles is to anchor the least readily available compound to the resin, namely the iso-

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cyanide. N-Fluorenylmethoxycarbonyl (Fmoc)/g-amino butyric acid (GABA) was attached to Rink amide resin and deprotected with piperidine/DMF. The amine was then formulated by treatment with 2,4,5-trichlorophenylformiate [64]. The dehydration to the isocyanide was accomplished using PPh3 , CCl 4 , and NEt3 . The imine formation of 2-aminopyridine and benzaldehyde was followed by the addition to the resin-bound isocyanide. After a reaction time of 48 h and a TFAinduced cleavage, the product was obtained in low yield but good purity (Scheme 23.27).

Scheme 23.27.

Aminoimidazoles on solid phase: isocyanide bound to the polymer.

In conclusion, the three-component reaction of amidines, aldehydes, and isocyanides can be carried out on solid phase with any of the three components anchored to the solid support, affording moderate to good yields of high-purity imidazoles for most amine inputs. The very interesting three-component reaction of a phenylhydrazine, cyclohexanone, and an isocyanide, leading to a 4-amino-dihydrocinnoline 49, was described in 1996 for the first time by the Weber group [65, 66] (Scheme 23.28).

Scheme 23.28. Synthesis of dihydrocinnolines via three-component condensation.

The four-component reaction of arylglyoxals, primary amines, carboxylic acids, and isocyanides was reported by Zhang et al. in 1996 [58]. First carried out in solution, the reaction of phenylglyoxal, isobutylamine, benzoic acid, and n-butylisocyanide gave amide 50 and was cyclized to the imidazole 51 (16 h at 100 C) with an overall yield of 43% (Scheme 23.29). The solid-phase synthesis began with the formulation of aliphatic amino acids attached to Wang resin with HCOOEt or HCOOH/Ac2 O followed by the dehydration with PPh3 /NEt3 /CCl 4 to provide the resin-bound isocyanides. The reaction with phenylglyoxal, isobutylamine, and benzoic acid in CHCl3 /CH3 OH/pyridine (1:1:1) at 65 C for 3 days led to the tetrasubstituted imidazole. The construction of

References

Scheme 23.29. Synthesis of the imidazole 51 on solid phase via four-component condensation.

the imidazole nucleus using the above-described methodology increases the overall diversity and size of an imidazole library.

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303; b) J. H. Brewster, E. L. Eliel, Org. React. 1953, VIII, 99–197. A. R. Katritzky, S. A. Belyakov, D. O. Tymoshenko, J. Comb. Chem. 1999, 1, 173–176. K. Schiemann, H. D. H. Showalter, J. Org. Chem. 1999, 64, 4972–4975. P. Blaney, R. Grigg, Z. Rankovic, M. Thoroughgood, Tetrahedron Lett. 2000, 41, 6639–6642. a) H.-C. Zhang, K. K. Brumfield, L. Jaroskova, B. E. Maryanoff, Tetrahedron Lett. 1998, 39, 4449–4452; b) H.-C. Zhang, K. K. Brumfield, B. E. Maryanoff, Tetrahedron Lett. 1997, 38, 2439–2442. a) M. A. Youngman, S. L. Dax, Tetrahedron Lett. 1997, 38, 6347–6350; b) A. B. Dyatkin, R. A. Rivero, Tetrahedron Lett. 1998, 39, 3647–3650; c) J. J. McNally, M. A. Youngman, S. L. Dax, Tetrahedron Lett. 1998, 39, 967–970. a) S. Kobayashi, I. Hachiya, S. Suzuki, M. Morikaki, Tetrahedron Lett. 1996, 37, 2809–2812; b) S. Kobayashi, M. Morikaki, R. Akiyama, S. Suzuki, I. Hachiya, Tetrahedron Lett. 1996, 37, 7783–7786; c) S. Kobayashi, S. Komiyama, H. Ishitani, Biotechnol Bioeng (Comb Chem) 1998, 61, 23–31.

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England, E. M. Gordon, Mol. Diversity Combin. Chem: Lib. Drug Discovery 1996, 58–69; b) M. F. Gordeev, D. V. Patel, B. P. England, S. Joannalagadda, J. D. Combs, E. M. Gordon, Bioorg. Med. Chem. 1998, 6, 833–889. J. G. Breitenbucher, G. Figliozzi, Tetrahedron Lett. 2000, 41, 4311–4315. H. Richter, G. Jung, Tetrahedron Lett. 1998, 3, 2729–2730. a) P. A. Grieco, A. Bahsas, Tetrahedron Lett. 1988, 29, 5855–5858; b) A. S. Kiselyov, R. W. Armstrong Tetrahedron Lett. 1997, 38, 6163–6166; c) S. Kiselyov, L. Smith II, A. Virgilio, R. W. Armstrong, Tetrahedron 1998, 54, 7987–7996; d) S. Kiselyov, L. Smith II, R. W. Armstrong, Tetrahedron 1998, 54, 5089–5096; e) S. Kobayashi, S. Nagayama, J. Org. Chem. Soc. 1996, 118, 8977–8978. a) P. Biginelli, Gazz. Chim. Ital. 1893, 23, 360; b) for a review, see: C. O. Kappe, Tetrahedron 1993, 49, 6937; c) C. O. Kappe, Molecules, 1998, 3, 1. a) B. C. O’Reilly, K. S. Atwal, Heterocycles 1987, 26, 1185; b) B. C. O’Reilly, K. S. Atwal, J. Z. Gougoutas, F. M. Malley, Heterocycles 1987, 26, 1189; c) K. S.

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Atwal, G. C. Rovnyak, B. C. O’Reilly, J. Schwartz, J. Org. Chem. 1989, 54, 5898; d) R. Gupta, A. K. Gupta, S. Paul, P. L. Kachroo, Ind. J. Chem. 1995, 34B, 151. a) P. Wipf, A. Cunningham, Tetrahedron Lett. 1995, 36, 7819; b) A. Studer, S. Hadida, R. Ferritto, S.-Y. Kim, P. Jeger, P. Wipf, D. P. Curran, Science 1997, 275, 823; c) A. Studer, P. Jeger, D. P. Curran, J. Org. Chem. 1997, 62, 2917. J. Lu, H. Ma, Synlett 2000, 1, 63–65. a) J. U. Nef, Justus Liebigs Ann. Chem. 1892, 270, 267; b) J. U. Nef, Justus Liebigs Ann. Chem. 1899, 309, 126. a) P. J. Scheuer, Acc. Chem. Res. 1992, 25, 433–439; b) M. S. Edenborough, R. B. Herbert, Nat. Prod. Rep. 1988, 5, 229–245. a) I. Ugi, U. Fetzer, W. Unterstenho¨fer, P. E. Behrenz, P. E. Frohberger, H. Schinpflug, G. Unterstenho¨fer (Bayer AG), DE-B 1,209,789 1962/1966; b) U. Fetzer, I. Ugi, G. Unterstenho¨fer (Bayer AG), DE-B 1,235,298 1964/1967; c) U. Eholzer, U. Fetzer, I. Ugi, I. Hammann, G. Unterstenho¨fer (Bayer AG), DE-B 1,215,141, 1964/ 1966; d) H. L. Yale AS-A 1970.000098106, 1970. A. Do¨mling, I. Ugi, Angew. Chem. Int. Ed. 2000, 39, 3168–3210. W. Lieke, Justus Liebigs Ann. Chem. 1859, 112, 316. A. W. Hoffmann, C. R. Acad. Sci. 1867, 65, 484. I. Ugi, Isonitrile Chemistry, Vol. 20. Academic Press, New York 1971. E. Olivieri-Mandala, E. B. Malagna, Gazz. Chim. Ital. 1910, 40 (II), 441. a) M. Passerini, Gazz. Chim. Ital. 1921, 51, 126; b) M. Passerini, Gazz. Chim. Ital. 1921, 51, 181. I. Ugi, S. Lohberger, R. Karl in: Comprehensive Organic Chemistry for Synthesis Efficiency, Vol. 2. Trost, B. M., Heathcock, C. H. (eds), Pergamon, Oxford 1991, pp. 1083–1109. For reviews of isocyanides, see: a) C. Grundman, Methoden Org. Chem, 1952–, 4th edn. Houben-Weyl 1985, Vol. E5, p. 1611; b) I. A. O’Neil in:

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Comprehensive Organic Functional Group Transformations, Vol. 3. Katritzky, A. R., Meth-Cohn, D. C., Rees W. (eds), Pergamon, Oxford 1995, p. 963; c) for a review of fluorinated isocyanides: D. Lentz, Angew. Chem. 1994, 106, 1377; Angew. Chem. Int. Ed. Engl. 1994, 33, 1315; d) P. Hoffmann, G. Gokel, D. Marquarding, I. Ugi in: Isonitrile Chemistry. Ugi, I. (ed.), Academic Press, New York 1971, p. 9. a) G. Skorna, I. Ugi, Angew. Chem. 1977, 89, 267; Angew. Chem. Int. Ed. Engl. 1977, 16, 259; b) H. Eckert, B. Forster, Angew. Chem. 1987, 99, 927; Angew. Chem. Int. Ed. Engl. 1987, 26, 1221. R. Obrecht, R. Herrmann, I. Ugi, Synthesis 1985, 400. a) E. Meyer, J. Prakt. Chem. 1866, 147; b) P. Boullanger, G. Descotes, Tetrahedron Lett. 1976, 38, 4327. A. W. Hofmann, Justus Liebigs Ann. Chem. 1867, 144, 114. A. W. Hofmann, Ber. Dtsch. Chem. Ges. 1870, 3, 766. a) I. Ugi, R. Meyr, Angew. Chem. 1958, 70, 702; b) I. Ugi, R. Meyr, Chem. Ber. 1960, 93, 239. ¨ck, K. D. Ziehn, R. Appel, R. Kleistu Angew. Chem. 1971, 83, 143; Angew. Chem. Int. Ed. Engl. 1971, 10, 132. D. H. R. Barton, T. Bowles, S. Husinec, J. E. Forbes, A. Llobera, A. E. A. Porter, S. Z. Zard, Tetrahedron Lett. 1988, 29, 3343. A. M. van Leusen, Synthesis 1991, 531. A. M. van Leusen, H. Siderius, B. E. Hoogenboom, D. van Leusen, Tetrahedron Lett. 1972, 13, 5337. A. M. van Leusen, J. Wildeman, O. H. Oldenziel, J. Org. Chem. 1977, 42, 1153. R. Schro¨der, U. Scho¨llkopf, E. Blume, I. Hoppe, Liebigs Ann. Chem. 1975, 533 ff. U. Scho¨llkopf, P. H. Porsch, E. Blume, Liebigs Ann. Chem. 1976, 2122 ff. S. Marcaccini, T. Torroba, Org. Prep. Proced. Int. 1993, 25, 141. a) G. Skorna, I. Ugi, Chem. Ber. 1978, 111, 3965; b) R. Ashady, I. Ugi, Z.

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Naturforsch. B 1981, 36, 1202; c) R. Ashady, I. Ugi, Angew. Chem. 1982, 94, 367; Angew. Chem. Int. Ed. Engl. 1982, 21, 374; Angew. Chem. Suppl. 1982, 761; d) R. Ashady, I. Ugi, Talanta 1984, 31, 842; e) for a review, see: R. Ashady, I. Ugi, Polymer 1990, 31, 1164. U. Casellato, B. Corain, R. Craziani, R. Michelin, M. Mozzon, M. Zecca, in press. R. Obrecht, R. Hermann, I. Ugi, Synthesis, 1985, 400. a) M. Passerini, Gazz. Chim. Ital. 1922, 52, 432; b) M. I. S. Dewar, Electronic Theory of Organic Chemistry. Clarendon, Oxford 1949, p. 116; c) R. H. Baker, D. Stanonis, J. Am. Chem. Soc. 1951, 73, 699; d) I. Hagedorn, U. Eberholz, H. D. Winkelmann, Angew. Chem. 1994, 76, 583; Angew. Chem. Int. Ed. Engl. 1964, 3, 647; e) T. Carfiglio, P. G. Cozzi, C. Floriani, A. Chiesi-Villa, C. Rizzoli, Organometallics 1993, 12, 2726; f ) D. Seebach, G. Adam, T. Gees, M. Schiess, W. Weigand, Chem. Ber. 1988, 121, 507. a) I. Ugi, R. Meyr, Chem. Ber. 1961, 94, 2229; b) I. Hagedorn, U. Eholzer, Chem. Ber. 1965, 98, 936. I. Ugi, R. Meyr, Angew. Chem. 1962, 74, 9; Angew. Chem. Int. Ed. Engl. 1962, 1, 8. ¨ller, B. Zech, Liebigs Ann. E. Mu Chem. 1966, 696, 72. G. L. Grunewald, W. J. Brouillette, J. A. Finney, Tetrahedron Lett. 1980, 21, 1219. a) U. Fetzer, I. Ugi, Justus Liebigs Ann. Chem. 1962, 659, 184; b) J. Rachon, Chimia 1983, 37, 299. M. Passerini, G. Ragni, Gazz. Chim. Ital. 1931, 61, 826. a) M. Passerini, Gazz. Chim. Ital. ¨ller, B. Zeeh, 1924, 54, 529; b) E. Mu Justus Liebigs Ann. Chem. 1966, 696,

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¨ller, Justus 72; c) B. Zeeh, E. Mu Liebigs Ann. Chem. 1968, 715, 47. J. W. McFarland, J. Org. Chem. 1963, 28, 2179. a) I. Ugi, H. Aigner, M. L. V. Arnaez, G. Glahsl, P. Lemmen, R. Sto¨cklein-Schneiderwind, M. Balla-Tamasi in: Vortra¨ge anla¨sslich der Ko¨nigsteiner ChromatographieTage. Aigner, H. (ed.), Waters, Eschborn 1984, p. 1; b) H. Aigner, PhD dissertation, Technische Universita¨t Mu¨nchen, 1984. I. Ugi, D. Marquarding, R. Urban, Chemistry and Biochemistry of Amino Acids, Peptides and Proteins, Bd. 6. Marcel Decker, New York 1982, pp. 245. K. M. Short, B. W. Ching, A. M. M. Mjalli, Tetrahedron 1997, 53, 6653– 6679. A. M. Strocker, T. A. Keating, P. A. Tempest, R. W. Armstrong, Tetrahedron Lett. 1996, 37, 1149. A. M. M. Mjalli, S. Sarshar, T. J. Baiga, Tetrahedron Lett. 1996, 37, 2934. C. Zhang, E. J. Moran, T. F. Woiwode, K. M. Short, A. M. M. Mjalli, Tetrahedron Lett. 1996, 37, 751. K. M. Short, A. M. M. Mjalli, Tetrahedron Lett. 1996, 37, 751–754. K. M. Short, B. W. Ching, A. M. M. Mjalli, Tetrahedron 1997, 42, 7489– 7492. H. Bienayme´, K. Bouzid, Angew. Chem. Int. Ed. 1998, 37, 16. C. Blackburn, B. Guan, P. Fleming, K. Shiosaki, S. Tsai, Tetrahedron Lett. 1998, 39, 3635–3638. K. Groebke, L. Weber, F. Mehlin, Synlett 1998, 661–663. J. Martinez, J. Laur, Synthesis 1982, 979. O. Lack, L. Weber, Chimica 1996, 50, 9. L. Weber, K. Illgen, M. Almstetter, Synlett 1999, 3, 366–374.

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24

Strategies for Creating the Diversity of Oligosaccharides Pamela Sears and Chi-Huey Wong 24.1

Introduction

Carbohydrates are essential for many important biological functions [1–4]. When conjugated to protein to form glycoproteins, they can alter protein structure and function. As components of glycolipids, they can play pivotal roles in intercellular recognition and signaling. The extracellular matrix contains proteoglycans that not only modify the physicochemical properties of the matrix, but also are involved in a variety of recognition processes such as cell adhesion in response to inflammatory factors and cancer metastasis. Although numerous carbohydrate structures occur in nature, in general the role of saccharide structure in function at the molecular level has been minimally studied. This longstanding problem can be attributed mainly to the difficulty of synthesizing saccharides, especially when compared to proteins and nucleic. Nucleic acids can now be easily made via chemical and biological synthetic techniques, and proteins, which are encoded by DNA, can therefore be easily produced and manipulated through recombinant DNA technology. In addition, automatic synthesizers are available for the synthesis of polypeptides and oligonucleotides. Saccharides, however, are made in nature with a diverse set of enzymes competing to produce very diverse products [1]. There is no information carrier that ‘‘encodes’’ a particular saccharide structure, and so creating libraries of saccharides with methods analogous to protein mutagenesis is not possible. Furthermore, unlike proteins and nucleic acids, saccharides are more difficult to synthesize chemically because (1) oligosaccharides are typically branched rather than linear; (2) the monosaccharide units can be connected by a- or blinkages; and (3) oligosaccharide synthesis requires multiple selective protection and deprotection steps, a process called protecting group manipulation. This last requirement is quite formidable, and currently there is no general route for combinatorial saccharide synthesis. In a glycosidation reaction, both donors (monosaccharides activated for reaction) and acceptors (which receive the activated monosaccharide) contain many hydroxyl and other functional groups that must be differentially and selectively protected. The product must then be selectively deprotected for the next round of reactions. The complexity of protecting Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

24.2 Chemical Synthesis of Oligosaccharides

group manipulation increases with each additional glycosidic linkage. Development of stepwise solid-phase synthesis can simplify the intermediate work-up and purification steps, but the complexity of protecting group manipulation remains the same. Because of this problem, there is currently no single stepwise method that is applicable to the synthesis of all oligosaccharides or even just the >15 million possible tetrasaccharides that can be assembled from the nine common monosaccharides found in humans. In contrast, solid-phase synthesis of peptides and oligonucleotides involves only one protecting group manipulation in the iterative process. In the last few decades, however, the work of many research groups has started to open up new paths to saccharide and glycoconjugate synthesis. Coupling techniques with better yields and stereoselectivity have been worked out, and new protecting group chemistries have also become available. The possibility of constructing libraries of saccharides, which was considered at one time to be a hopeless prospect, is now starting to appear feasible. The next step in making oligosaccharides widely accessible will be the automation of saccharide synthesis. This chapter will focus on the current state of the subject and emphasize the developments with potential application to automated combinatorial synthesis of saccharides, glycopeptides, and glycoproteins.

24.2

Chemical Synthesis of Oligosaccharides

Several practical approaches have been taken with success for the chemical synthesis of oligosaccharides (Fig. 24.1) [5–24]. Most involve the activation of the anomeric leaving group with a Lewis acid and then displacement of that leaving group by the free hydroxyl of the acceptor sugar. The Koenigs–Knorr method of coupling glycosyl halides, one of the first techniques to gain widespread use, is still employed [5] and most other glycosidation reagents used to date proceed by the same basic mechanism. The relative instability of the sugar halide necessitates the construction of the saccharide from the reducing end, and in fact, many of the most successful approaches are those that minimize side-reactions of the activated sugar. New leaving groups have been further developed to improve the stability of the glycosyl donors and their reactivity. Trichloroacetimidates [6], prepared by the reaction of free sugars with trichloroacetonitrile and base, are used most frequently for coupling, as are glycosyl sulfoxides [7], phosphites [8, 9], phosphates [10], thio-glycosides [11], and pentenyl-glycosides [12]. Another scheme for glycoside synthesis is to build the saccharide from the nonreducing to the reducing end using glycals [13], which can be activated through epoxidation for either direct attack of the epoxide with the aglycon or intermediate formation of, for example, the thioacetal or phosphate (Fig. 24.2). The control of anomeric configuration of the product can be complicated, especially because the reaction can occur readily via either an SN 1- or an SN 2-type process. The anomeric configuration of the activated sugar, therefore, does not ensure

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24 Strategies for Creating the Diversity of Oligosaccharides

(A) Common mechanisms for glycosidation. (B) Commonly used glycosidation reagents and their activators (in parentheses). Some of these glycosidation reagents can be used orthogonally. For example, the activator for glycosyl fluorides or

Fig. 24.1.

phosphites will not activate thioglycosides or pentenyl glycosides. Tf, triflate; TMSOTf, trimethylsilyl triflate; TfOH, triflic acid; NIS, N-iodosuccinimide; Et, ethyl; DMDO, 3,3dimethyldioxirane; DMTST, dimethylthiosulfonium triflate.

the anomeric configuration of the product. Furthermore, which products form can be heavily influenced by the protecting groups used. Acyl protecting groups at C2 can strongly direct the trans configuration at C1 by forming an intermediate dioxocarbenium ion (Fig. 24.1A). In general, a-1,2-cis-glycosides such as a-d-glucosides and a-d-galactosides can be formed either by taking advantage of the kinetic anomeric effect [14] in the displacement of glycosyl halides and thioglycosides or by direct displacement of b-trichloroacetimidates under conditions that favor inversion (with no participating substituent at C2 and a nonpolar solvent) [15]. b-1,2trans-Glycosides such as b-d-glucosides and -galactosides can be obtained by using neighboring group effects mediated by the 2-O-acyl protecting group or polar media to favor SN 1 displacement and formation of the dioxocarbenium species. Glu-

24.2 Chemical Synthesis of Oligosaccharides

Fig. 24.2. Glycal approach to the solid-phase synthesis of oligosaccharides from the nonreducing to the reducing end.

cosyl and galactosyl phosphates have, in all cases explored, produced the b-1,2trans-glycoside, regardless of the anomeric configuration of the phosphate [10], and glycal chemistry also produces mainly the b-anomer. a-1,2-trans-Glycosides, such as a-d-mannosides, are simple to obtain as they are favored both by the kinetic anomeric effect and by the presence of participating groups at C2, but b-1,2-cisglycosides are still quite difficult to construct. Preparation of the b-d-glucoside followed by inversion at C2 has been one common method, and recent attempts to direct the attack of the incoming sugar by tethering it in a position that allowed only b-attack have met with success [16–19]. In general, control of anomeric stereochemistry is still a problem, especially when neighboring group participation is lacking. Also, there are certain chemistries that do not work well with some sugars. In nature, only a-sialic acid linkages are observed, but sulfoxide and trichloroacetimidate chemistries only give the b-anomer, a problem that can be solved by using other activating groups such as phosphites [8, 9], thioglycosides [20], and 2-xanthates [21]. In automating oligosaccharide synthesis, it is convenient for the reactions to be performed on solid phase. This approach allows rapid removal of reactants, relatively easy purification, and (in the case of library construction) the encoding of the product either by position (as in a two-dimensional array ‘‘chip’’ format) or, for ‘‘mix-and-split’’ type library construction, by an accessory encoding reaction [7] in which labels are added to the solid support as the chain is extended or by radiofrequency-encoded combinatorial chemistry technology [22]. Most of the saccharide synthetic techniques outlined above have been applied to solid-phase synthetic strategies on a variety of supports [7, 13, 22–26]. Polystyrene-based resins such as the Merrifield resin are commonly used [6, 24], although these do not necessarily have the optimal characteristics for synthesis of sugars with regard to swelling properties and reactant accessibility, particularly in hydrophilic media [26]. More hydrophilic supports such as polyethylene glycol-based resins have been used with good success [26], as have ‘‘hybrid’’ resins such as TentaGel that have a polysty-

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rene core coated in polyethylene. To a lesser extent, soluble supports such as polyethylene glycols and derivatives as well as thermoresponsive polyacrylamide derivatives [27] have been used in oligosaccharide synthesis. However, there are many disadvantages to using a solid support earlier. As mentioned, protecting group manipulation on a solid support is extremely difficult, and with the protecting group chemistry known to date, it is impossible to create true diversity based on this stepwise solid-phase method. In addition, oligosaccharides and glycopeptides are sterically hindered compounds. Blocking one side of the molecule further with a solid support is likely to drop yields dramatically. Long, flexible linkers can be used to alleviate this problem somewhat, but such linkers must be both cleavable and yet still compatible with the coupling and protection–deprotection reactions (e.g. photo- or enzyme-sensitive linkers or linkers which can be cleaved by Pd(0) or by olefin cross-metathesis). Monitoring reaction progress on solid phase is also not trivial. In addition, protecting group manipulation on resins is extremely difficult, as non-soluble reagents are generally not amenable to solid-phase synthesis. Palladium nanoparticles, however, have been found to be useful in the debenzylation of sugars attached to a polyethylene glycol–acrylamide (PEGA) resin [28]. The most challenging task, however, is the selection of orthogonal protecting groups and their selective manipulation during synthesis. Commonly used protecting groups include benzyl or silyl ethers and derivatives, as well as acid- or base-sensitive protecting groups [15, 23, 29] (Fig. 24.3). Although conditions have been developed for their selective deprotection, in general their application to the synthesis of oligosaccharide libraries with great diversity has not been demonstrated. To date, the largest oligosaccharide made by solid-phase synthesis is that reported by Nicolaou et al. (Fig. 24.4) [25] and Seeberger and coworkers (Fig. 24.5) [30]. Both groups synthesized the same branched dodecasaccharide on solid phase using phenyl thioglycosides [25] or glycosyl phosphates and imidates [30], and the products were released from the support with photolysis [25] or olefin cross-metathesis [30]. In the Nicolaou group’s synthesis, trisaccharide blocks were coupled successively, with typically 50–60% yields on the coupling steps, while Seeberger’s group alternated mono- and disaccharide couplings to obtain the repeating trisaccharide unit of the phytoalexin elicitor, and the process has been automated using a modified peptide synthesizer. Although the individual coupling yields were not tabulated, making direct comparison of the two strategies difficult, the overall yield of Seeberger’s synthesis was very good, in excess of 50%. The approaches are similar in principle. The difficulty of these approaches is the generality of the methods. In both cases, building blocks were tailor-made to fit the synthesis of this particular compound. The blocks used by the Seeberger group are more general only in that they are less complex, but the pattern of protecting and activating groups still pigeonholes them into the synthesis of a certain class of compounds, namely b1,2/6-linked polymers. The use of either type of scheme for the general synthesis of many different polysaccharides will require the maintenance of a very large stock of building blocks that are appropriate for the construction of different types of links.

24.3 Enzymatic Synthesis of Oligosaccharides

Fig. 24.3. Commonly used protecting groups and their removal conditions (in parentheses). See references 5–23 and citations therein. Ar, aryl; DDQ, 2,3-dichloro-5,6-dicyano-1,4benzoquinone; TBDMS, tert-butyl-dimethylsilyl; TBDPS, tert-butyl-diphenylsilyl; TsOH, p-

toluenesulfonic acid; MsOH, methanesulfonic acid; TFA, trifluoroacetic acid; Tr, trityl; All, allyl; DMAP, 4,-N,N-dimethylaminopyridine; Lev, levulinoyl; Piv, pivaloyl; Bn, benzyl; Phth, phthalimidyl; Ph, phenyl; Troc, trichloroethoxycarbonyl.

24.3

Enzymatic Synthesis of Oligosaccharides

Over the past few decades, enzymatic approaches have been gaining popularity for the synthesis of saccharides and glycopeptides [31, 32]. Enzymes feature exquisite stereo- and regioselectivity and catalyze the reaction under very mild conditions. Extensive protection–deprotection schemes are thus unnecessary, and the control of anomeric configuration is simple. Both glycosyltransferases, the enzymes which are naturally used to synthesize saccharides, and glycosidases, enzymes normally used to hydrolyze glycosidic bonds, have been used. Drawbacks to an enzymatic approach are the availability and cost of the catalysts and substrates, which can be high. The enzymes themselves are in many cases only just becoming available, particularly in the case of glycosyltransferases. The substrates, which for glycosyl-

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Fig. 24.4. The Nicolaou group’s solid-phase synthesis of a dodecasaccharide phytoalexin elicitor. Trisaccharide blocks were added in succession to provide the final dodecasaccharide. DMTST, (dimethylthio) methylsulfonium triflate; TMU, tetramethylurea.

transferases are the nucleotide-activated sugars, are relatively expensive, but can be prepared from sugars or sugar phosphates through enzymatic or biological methods that have been worked out [31, 33]. Glycosidases, which use cheaper substrates such as sugar halides and p-nitrophenyl glycosides, can be used but the yields have typically been lower. However, the Withers group recently found that mutagenesis of glycosidases to remove one of the two catalytic carboxylates in the active site produces an enzyme, coined a ‘‘glycosynthase,’’ that can catalyze the synthesis of a saccharide from a fluorosugar donor but cannot catalyze hydrolysis of the re-


Fig. 24.5. The Seeberger group’s solid-phase synthesis of the dodecasaccharide phytoalexin elicitor. A modified peptide synthesizer was used to couple mono- and disaccharide phosphate donors alternately, providing the repeating trimer of the structure.

sulting product [34] (Fig. 24.6). Whether this approach will be applicable to other exo-glycosidases remains to be investigated. Another drawback of the enzymatic approach is that while enzymes are excellent at catalyzing the synthesis of natural products, their ability to accept novel saccharides with unusual or unnatural sugars as substrates may be poor; at best, it will be unknown. Models for the substrate preferences of glycosyltransferases are currently unavailable, and alteration of their specificity using protein engineering has experienced limited success. Prediction of reaction products with novel substrates will become easier as the enzymes begin to enjoy more widespread use and their substrate specificities become better characterized. Since the preparative scale enzymatic synthesis of N-acetyllactosamine involving sugar nucleotide regeneration in the 1980s [35], enzymatic and chemoenzymatic approaches have been used in the synthesis of a great number of oligosaccharides and glycoconjugates [32]. For example, the synthesis of sialyl trimeric Lewis X [36] was accomplished through

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Fig. 24.6. Synthesis of an oligosaccharide with glycosynthases. In principle, exo-glycosidases can be genetically altered to accept glycosyl fluorides as donors to perform glycosidation.

the transfer of sialic acid and fucose to a chemically synthesized trimeric LacNAc acceptor. Further improvement in the area with the multiple enzymes required for sugar nucleotide regeneration immobilized on beads has been developed (Fig. 24.7) [37]. The four enzymes required for the (re)generation of UDP-galactose from uridine diphosphate (UDP), galactose, and phosphoenol pyruvate [with catalytic amounts of glucose-1-phosphate and adenosine diphosphate (ADP)] are coimmobilized on a bead, which can be added to the reaction medium to allow in situ generation of the galactosyltransferase substrate, UDP-galactose. This process both facilitates the reaction by removing the product, UDP (a galactosyltransferase inhibitor), and can reduce the cost of the reaction by allowing the use of cheaper substrates, assuming that the immobilized enzymes are stable enough to be reused multiple times. However, if one or more of the immobilized enzymes is inactivated, replacement of that enzyme will be difficult. Application of enzymes to an automated scheme is possible. The logic of such a reaction scheme is conceptually simple, as it is determined by the enzymes’ preferred reaction: the saccharide must be built stepwise, in a linear fashion, from the reducing end (Fig. 24.8). Conducting the reaction on solid phase requires supplying the enzymes in solution, from which they must be either recovered for recycling or discarded. Recovery can be achieved via a variety of techniques such as affinity-based capture (of affinity-tagged enzymes), passage through a microfilter, or enzyme precipitation. Enzymes are large molecules, and thus care must be taken in choosing the support for solid-phase synthesis. The support, if porous, should have pores large enough to accommodate these macromolecules and


Fig. 24.7. Preparation of ‘‘superbeads’’ for the facile regeneration of UDP-galactose. Galactosyltransferase (GalT), an enzyme that transfers galactose from the UDP-galactose to an alcohol donor, releases UDP, which inhibits the enzyme. (A) In order to prevent inhibition of the enzyme and to limit the use of the expensive UDP-galactose substrate, a regeneration scheme using pyruvate

kinase (PyrK), galactose-1-phosphate uridylyltransferase (GalPUT), glucose-1phosphate uridylyltransferase (GalU), and galactose kinase (GalK) is used. Gal, galactose; Glc, glucose; Gal-1-P, galactose-1-phosphate; Glc-1-P, glucose-1-phosphate. (B) These enzymes can be produced with a polyhistidine tag that allows them to be purified and immobilized onto a nickel resin.

should be hydrophilic to allow good swelling in water, or the support should be rigid so that the enzyme will not become entrapped [31]. Use of long cleavable tethers to attach the growing saccharide may also help the substrate to enter the enzyme’s active site. Many resins have been used, including polysaccharide-based resins such as Sepharose, polyethylene-based resins such as SPOCC (polyoxyethylene-polyoxethane), and polyacrylamide supports [23, 26]. However, more standard solid-phase supports such as derivatized silica and polystyrene have also been used with success [38, 39]. Solution-phase synthesis, while solving the problem of enzymatic accessibility, adds problems of product recovery, which may be substantial, given the frequent complexity of the reaction buffer required for enzymatic reactions. A good approach may be to couple the substrate to a water-soluble polymer, which can be easily removed from solution either by precipitation of the polymer or by affinity-based capture (if an affinity label is attached to the support). Water-soluble supports such as uncrosslinked polyacrylamide have been used in the enzymatic synthesis of saccharides and glycoconjugates such as pseudo-GM3 (Fig. 24.9) [40]. Other water-soluble polymers such as polyethylene glycol [41] and

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Fig. 24.8. Approaches to automated enzymatic saccharide synthesis. In (A), the enzymes are left in solution and the growing saccharide is immobilized on the solid phase. This approach simplifies purification, but requires an enzyme recovery step to avoid losing the expensive catalyst. In (B), the growing saccharide is attached to a water-

soluble polymer, which is passed across columns of immobilized enzymes. Product recovery at the end can be accomplished by precipitating the polymer or by affinity techniques if the polymer is tagged with an affinity ligand (such as biotin). Alternatively, both enzymes and substrates can be used in free form.

thermoresponsive polyacrylamide [27] may find use in enzymatic oligosaccharide synthesis. One can thus envision a scheme in which glycosyltransferases (and, if necessary, the enzymes required for the regeneration of their substrates) are immobilized onto a resin and packed into different columns. The substrate, free or bound to a water-soluble resin with an affinity tag, is passed through the columns in sequence depending on the glycosyltransferases desired. Intermediate isolation, if necessary, can be achieved via capture of the substrate using, for example, affinity capture. Attachment of enzymes and substrates to a support is, however, not trivial. In the end, given the high yields observed with glycosyltransferase-catalyzed glycosidations and the simplicity of product isolation, the choice of the reactor configuration is probably not critical, and both solution and solid-phase methods can be used for automated synthesis.

24.4 Programmable One-pot Synthesis

Fig. 24.9. Enzymatic synthesis of the

ganglioside pseudo-GM3 on a polymeric support. Ceramide glycanase-catalyzed transglycosidation can be used to remove the

ganglioside from the support. Polymer-bound versions of GM3 have also been shown to be useful as picomolar inhibitors of influenza hemagglutinin [48].

24.4

Programmable One-pot Synthesis

A recent approach that shows promise for automation is the use of one-pot reaction schemes that use the reactivity profile of different protected sugars [42, 43] to determine the outcome. The reactivity of a sugar is highly dependent on the protecting groups and the anomeric activating group used. By adding substrates in sequence from the most to least reactive, one can assure the predominance of a desired target compound (Fig. 24.10). Compared with the stepwise solid-phase method, the one-pot method involves no protecting group manipulation in the iterative process. The key to this approach is to have extensive quantitative data regarding the relative reactivities of differentially protected sugar building blocks. A large amount of reactivity data for more than 100 protected p-methylphenyl thioglycosides (Fig. 24.11) were recently generated and used as the basis of a computer program, termed OptiMer, which selects the best reactants for the one-pot synthesis

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Fig. 24.10. (A) Traditional step-wise solidphase synthesis requires on-resin protecting group manipulation, which can become very complicated as the number of glycosidic linkages increases. (B) OptiMer’s one-pot approach. OptiMer is a program which predicts the optimal type and order of addition of partially protected sugars, based on a

database of relative reactivities. This approach requires preparation of a number of building blocks with their glycosidation reactivities quantitatively measured. A reactivity difference greater than 1000 between the building blocks will give a high-yield in coupling. No protecting group manipulation and intermediate isolation are required during the one-pot synthesis.

of a target compound [32, 43]. p-Methylphenyl thioglycosides were chosen as they are applicable to most monosaccharides and are more reactive toward thiophilic activators such as N-iodosuccinimide (NIS) and dimethylthiosulfonium triflate (DMTST) than other thioglycosides [11, 20, 44, 45] which have been used in practical synthesis. This approach has been used with success in the synthesis of a large number of oligosaccharides including the cancer antigen Globo-H hexasaccharide (Fig. 24.12) [46]. In practice, the sequence of Globo-H was entered into the computer program OptiMer, three building blocks and their corresponding relative reactivities were shown. One simply mixes the building blocks and NIS in sequence, and after a few minutes the desired product is generated in protected form. After deprotection and purification, the target is obtained. If necessary, the trisaccharide building blocks can be prepared separately by the one-pot approach using the same procedure. While the one-pot strategy is quite effective, further work is needed to design a complete set of building blocks (probably @500 or so are needed) for use in the synthesis of most bioactive saccharides. So far, branchpoints have been incorporated by using the thioglycosides of disaccharides as reactants in the linear

24.4 Programmable One-pot Synthesis

Fig. 24.11. Building blocks and their relative reactivities (in parentheses) used for OptiMer’s one-pot approach.

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Fig. 24.12. Synthesis of the cancer antigen

Globo-H using OptiMer technology. In brief, the sequence of Globo-H is entered into the computer, which predicts the best building blocks to be used. These building blocks are then mixed in sequence, starting with the most

reactive one, in the presence of an activator. The product obtained is then purified and deprotected to give the target. Ac, acetyl; Bn, benzyl; Bz, benzoyl; Tol, tolyl; Troc, trichloroethoxycarbonyl.

scheme. These reactions are typically performed in solution, but, in order to facilitate removal of reactants at the end, the final acceptor may be attached to solid support or other aglycons. Future development in this approach is to expand the building-block repertoire and to ensure its applicability in programmable one-pot synthesis. Compared with stepwise solid-phase synthesis, the one-pot approach requires protecting group manipulation only at the stage of building-block synthesis, and thus holds greater potential for automation and for a greater diversity of oligosaccharide structures. Other solution-phase syntheses of oligosaccharide libraries have been reported, especially the method for the synthesis of mixtures [47], but the methods have not been demonstrated to create great diversity. In addition, characterization of the mixture represents a difficult problem.

24.5

Conclusions

Recent advances in synthetic carbohydrate chemistry have provided new strategies for creating the diversity of oligosaccharides needed to tackle many interesting problems in glycobiology. Many technical problems that hinder the development of combinatorial carbohydrate chemistry still exist and remain to be solved. Of particular importance is development of convenient and effective automated systems

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Part IV

Molecular Design and Combinatorial Compound Libraries


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Design Criteria Josef Pernerstorfer 25.1

Introduction

The original goal of combinatorial synthesis was to synthesize huge numbers of organic compounds. The simple belief that a certain number of test compounds had to be evaluated to identify a new drug led to the assumption that the numbers game of combinatorial chemistry could speed up the drug development process. It was several years before scientists realized that it was not only the number of compounds but also their quality that was essential for medicinal chemistry projects. This chapter deals with the design of reasonable combinatorial libraries for drug development. First, some general aspects of drug-like compounds will be considered, followed by diversity concepts and the synthesis of ‘‘diverse’’ compound libraries. A discussion of preferred structural motifs in medicinal chemistry will complete the chapter.

25.2

Properties of Combinatorial Libraries for Drug Development

Generally, the drug development process aims to synthesize chemical entities which are, apart from their biological effects on specified targets, orally bioavailable. A major criterion for good absorption of these compounds is that certain physiological properties will allow them to be reasonably absorbed into the gastrointestinal system. In a landmark paper, Lipinski and coworkers [1] from Pfizer were looking for factors which influence absorption and permeability of drugs and which could be considered even at the stage of the drug discovery process. They studied a database of 2245 drug compounds which had passed clinical phase I and had entered clinical phase II. Lipinski and coworkers assumed that these compounds had good absorption properties since insoluble and poorly permeable substances had been eliminated from further research at an earlier stage of the drug development process. The properties which were taken into account had to be easily calculable and could be expected to have a pronounced effect on permeabilHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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25 Design Criteria

ity. These authors identified: (1) molecular weight, (2) lipophilicity, (3) number of hydrogen bond donor groups (i.e. number of NH and OH bonds), and (4) number of hydrogen bond acceptors (i.e. number of N þ O) as the key properties which have an essential effect on the permeation through lipid bilayers and therefore on the intestinal absorption process. Properties 1, 3, and 4 are obviously easily calculable from the structural formula. As a measure of lipophilicity, Lipinski and coworkers used the partition coefficient of the substance between n-octanol and water, P, which can be approximated by increment systems. The authors used two such systems. The first was the Pomona College Medicinal Chemistry program which calculated log P values (C log P) from structural fragments and gave very accurate results, but failed in many instances because of fragments in the molecule which were not assigned in the program. The other system they used had been described by Moriguchi et al. [2]. This system gives less but still reasonably accurate results and allows the calculation of M log P directly from the structural formula [2]. Lipinski et al. determined the distribution of these four properties among the drug database and found cut-off levels for each parameter such that @90% of the drugs were within these levels. From these, they stated that poor absorption and permeation are more likely when: 1 2 3 4 5

the molecular weight (MW) is >500; C log P is >5 (or M log P is >4.15); the number of N þ O is >10 (H-bond acceptors); the number of NH þ OH is >5 (H-bond donors); compound classes which are substrates for biological carriers are exceptions to the rule.

These became known as ‘‘Lipinski’s rule of 5’’. Lipinski and coworkers found that no more than 10% of drugs obeyed any combination of two of these rules. For example, only 1% of the drugs had both MW and log P outside the cut-off levels. Orally bioavailable substances which violate the first four rules mainly belong to a few therapeutic categories, i.e. antibiotics, antifungals, vitamins, and cardiac glycosides. These substances are assumed to be substrates for naturally occurring transporters. Using these results, Lipinski and coworkers examined the lead generation process at Pfizer, which is representative of that used by many large pharmaceutical companies. The advent of high-throughput screening (HTS) allowed testing of huge numbers of chemical compounds. In contrast to earlier techniques, it was not necessary to obtain thermodynamically stable aqueous solutions of the compounds for biological testing: the compounds were delivered as dimethyl sulfoxide (DMSO) stock solutions, which allowed for testing of very lipophilic compounds. At the same time, combinatorial chemistry enabled the synthesis of vast numbers of test compounds for in vitro HTS screening. Lipinski and coworkers compared the properties of test compounds which were synthesized 1986 and 1994 (pre- and post-HTS era) (Table 25.1). They found that test compounds were becoming much

25.2 Properties of Combinatorial Libraries for Drug Development Tab. 25.1. Distribution of M log P and molecular weight in the pre- and post-HTS era [1].

Percentile

90 75 50

M log P

MW

1986

1994

1986

1994

4.30 3.48 2.60

4.76 3.90 2.86

514 415 352

726 535 412

more lipophilic and heavier. Both these factors led to the discovery of a number of highly active compounds in vitro but caused severe problems in developing orally bioavailable drugs owing to insufficient solubility and permeability in vivo. Obviously, the strategy in combinatorial chemistry of taking a multifunctional central building block and decorating it with organic residues runs the risk of generating very high MW compounds which are furthermore very lipophilic since all polar groups (acids, amines, alcohols, and phenols) would be masked by combinatorial variation (acid amides, ureas, ethers, esters). Although this can be useful during the late stages of the optimization process, it is not a good procedure for generating lead compound libraries. In accordance with these results, Teague et al. [3] studied the drug optimization process at Astra Zeneca. They found that only very few lead compounds have such desirable properties and that it would be very unlikely for one of them to pass on to clinical development directly. Generally, potency and pharmacokinetic profile have to be further optimized – this goal is accomplished by the addition of suitable groups and side-chains and causes an increase in MW and lipophilicity of the drug compared with the initial lead. Teague et al. divided the properties of lead structures identified by HTS into three groups with respect to affinity, MW, and log P: 1 Intermediate affinity (> 0.1 mM), low MW (< 350), and low C log P (< 3). These are ‘‘classical’’ lead structures which can be optimized by introducing lipophilic groups to increase potency and improve pharmacokinetic properties. 2 High affinity (f 0.1 mM), high MW (g 350), and low C log P (< 3). These leads are often derived from natural products and can be optimized by derivatization to improve pharmacokinetics while potency is retained. 3 Intermediate affinity (> 0.1 mM), high MW (g 350), and high C log P (> 3). These leads are often generated in HTS assays of combinatorial libraries. An optimization of these drug-like leads normally proves difficult, since the affinity results from many nonoptimized interactions between lead and target and a further optimization produces very lipophilic, poorly soluble compounds. Teague et al. concluded that combinatorial libraries for lead identification should be fundamentally different from those for lead optimization. Lead structure libraries have to produce compounds with low MW (100–350) and low C log P (1.0– 3.0). Focused libraries based on such leads can quickly improve drug properties.

727

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25 Design Criteria

Fig. 25.1. Molecular weight distribution of drugs that have

good and poor oral bioavailability [4].

Although there is general agreement in the literature on the influence of lipophilicity and H-bond donor and H-bond acceptor properties on the bioavailability of drugs, the influence of MW is controversial. A set of 286 marketed drugs was examined for the relationship between MW and oral bioavailability and a boundary of 40% blood levels after oral application was chosen, above which no bioavailability problems should be expected [4]. Of the total set of 286 drugs, 168 lay above the 40% level and 118 lay below. As expected, about 50% of drugs that perform well had a MW between 250 and 350 and only very few had a MW > 500. Surprisingly, however, the drugs with low bioavailability had a quite similar distribution, with an average MW of between 350 and 400, and only a few compounds had MW > 500 (Fig. 25.1). When those drugs with obvious metabolic instabilities (such as b-lactams), with very high lipophilicities (which cause poor water solubility), and with quaternary amines were subtracted from the 116 inferior performers, 97 substances remained whose correlation between MW and oral availability did not significantly differ from the distribution of the well-bioavailable compounds. This indicates that MW is not a primary cause of low bioavailability but that high MW is very often correlated with factors such as high lipophilicity and poor solubility and with a high number of heteroatoms.

25.3

Differentiation of Drug-like and Nondrug-like Compounds

Since an accessible virtual library is much larger than one that is actually synthesized, the first criterion when choosing compounds for synthesis should be that they have reasonable drug-like properties. For example, toxic and reactive substructures which can react with proteins can cause false-positive read-outs in screening assays and should, therefore, be eliminated (Fig. 25.2) [5]. Several authors have tried to differentiate drugs from nondrugs by looking at the

25.3 Differentiation of Drug-like and Nondrug-like Compounds

Fig. 25.2. Reactive functional groups leading to in vitro false positives [5].

general structural properties using computational methods [6]. Generally, databases of known drug compounds are used as examples of drug-like substances [e.g. World Drug Index (WDI)] and databases of nondrugs [e.g. Available Chemicals Directory (ACD)] are used for the studies. Some authors operate with neural networks trained on drug and nondrug databases [7, 8]. These networks use large sets of approximately 80–90 descriptors for each structural parameter. After training the network on part of a database, these authors classified 80–90% of the members of each database correctly. A much smaller set of only 15 molecular descriptors (XaH aromatic, XaH nonaromatic, CaH (any), CbO, C sp 2 conj, bNa, N nonaromatic, NbC, O nonaromatic, O s 2 (any), P s 2 , F any, Cl any, number of atoms, total charge) was used to gain essentially equally good results in differentiating between drugs and nondrugs [9]. However, the authors had difficulties in explaining the decision processes of such trained ‘‘black box’’ neuronal networks. A similar quality of differentiation between drug-like and nondrug-like compounds was obtained using a fairly simple decision tree which differentiates on the presence/ absence of functional groups. By simply checking for the presence of hydroxyl, tertiary/secondary amino, carboxyl, phenol, and enol groups, 75% of the drugs were correctly recognized, whereas nondrugs were shown to have an aromatic nature and a low content of functional groups except for halogens. Further refinement of the decision tree allowed a prediction quality of around 80% [10]. Another

729

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25 Design Criteria

approach used the deviation of 25 structural parameters from a global drug-like cluster center which was calculated from a drug database in a 25-dimensional parametric space. This deviation produced a ‘‘drug-like index’’ (DLI) value between 0 and 1. It could be shown that compounds from drug databases have a significantly higher DLI than compounds from nondrug databases [11]. Furthermore, complexity parameters such as numbers of ring systems (RNG), rotatable bonds (RTB), and rigid bonds (RGB) were examined to differentiate drugs from nondrugs. It was found that drugs are more likely to have properties RNG b 3/ RGB b 18/RTB b 6, whereas nondrugs are more likely to have RNG a 2/ RGB a 17/RTB a 5) [12]. This finding means that more complex molecules are more likely to be drug-like. Although the statistical significance of the last two approaches is without doubt, it remains to be determined whether these findings are sufficiently exact to differentiate drugs from nondrugs in practice.

25.4

Diversity in Combinatorial Chemistry for Drug Development 25.4.1

Introduction

The number of compounds theoretically accessible from all permutations of chemical building blocks with an appropriate molecular weight goes well into the billions [13] and by far exceeds the number of compounds which can be realistically synthesized. Therefore, considering the diversity of compounds actually prepared can further exploit the potential of combinatorial synthesis [14–18]. In medicinal chemistry, a sine qua non for every description of chemical diversity is some kind of neighborhood behavior. As such, similar compounds with respect to a reasonable descriptor system should have similar biological properties [19]. This feature allows compounds that are dissimilar to a certain degree to be synthesized and screened without losing relevant information about their biological effects. Thus, a much larger chemical and biological ‘‘space’’ can be evaluated using an acceptable amount of effort. The diversity of combinatorial libraries is also dependent on the actual scope of the library and on the information which is available for library design. One can differentiate focused libraries, targeted libraries, and primary screening libraries [20]. Focused libraries are designed to evaluate a given lead structure in more detail. The information content for the library design is very high and the chemical space which has to be evaluated is rather small since key pharmacophoric elements are already known. Targeted or biased libraries are directed toward a particular biological target that belongs to a specific protein class such as a kinase or a G-protein-coupled receptor. Information about key structural features addressing the target class can be derived from literature, but is much less than for focused libraries. Therefore, the structural space which the library has to evaluate is much larger. Finally, primary or general screening libraries are designed for screening

25.4 Diversity in Combinatorial Chemistry for Drug Development

against new pharmaceutical targets. Generally, there is no information about the type of molecule that will interact with a biological target. The only limitation which can be made to these libraries is to generate drug-like structures, but no further limitation can be made initially.

25.4.2

Descriptors

To evaluate the diversity of compounds and of combinatorial libraries, there first needs to be some kind of description system for each substance in the form of a number or a set of numbers which allows a quantitative comparison of the structural differences between two entities. Instead of a set of numbers, a bitstring is often used which displays properties just in terms of ‘‘absent’’ (i.e. 0) or ‘‘present’’ (i.e. 1). This system allows a fast comparison of two bitstrings. Descriptors can be differentiated according to their dimensionality [21–23]:

. One-dimensional

.

.

(1D) descriptors describe the molecular properties of molecules holistically with a unique number such as log P, molecular weight, or number of hydrogen bond donors and acceptors. These values have a large impact on pharmacokinetic parameters and are often used as filters in library design rather than as actual descriptors for molecular similarity and diversity [20]. Two-dimensional (2D) descriptors are based on the 2-dimensional representation of molecules, i.e. the structural formula. They normally use bitstrings to describe the presence or absence of structural fragments (e.g. carboxylic acids or certain ring systems) or of certain atom patterns of 2–7 atoms’ length (molecular fingerprints) [24]. For the use of structural fragments, the MACCS (Molecule Accessing System) structural fragment keys [25] are often used, which were originally developed for substructure searching in chemical databases. These fragment keys suffer from their lack of generality, since not all possible structural fragments can be described by a reasonable set of fragment keys. Molecular fingerprints generate a bitstring from indexing all possible paths of defined lengths through a molecule. Since the number of all possible paths is too high to assign every different path to unique bits, the fingerprints are hashed, meaning that bits are associated with several paths. This hashing causes a lack of accuracy in the description. Three-dimensional (3D) descriptors take the spatial relationships of chemical features into account. Since distances and angles between functional groups can adopt continuous values, a distance or angle range (e.g. 2–10 A˚ or 0–180 ) is defined which is subdivided into a number of bins of certain bin width. A set of bits is then assigned to these bins to encode all possible conformations of predefined pairs of features between which the distances or the angles are determined. These descriptors do not consider high-energy conformations but treat all possible conformations which descend from the rotation around CaC bonds as being equal. The computational effort used for these flexible descriptors is

731

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25 Design Criteria

very high. Therefore, sometimes rigid descriptors are used, which only consider one or a few minimum energy conformations of the molecule. Pharmacophores are also used as 3D descriptors. These pharmacophores are specified by three interaction centers of seven-center types which are responsible for the interaction with molecular receptor sites: these are hydrogen bond donors, hydrogen bond acceptors, hydrogen bond donors and acceptors, aromatic rings, hydrophobic regions, acidic sites, and basic sites. While the arrangement of three pharmacophores does not allow the differentiation of enantiomeric compounds, a further refinement of this concept uses four pharmacophoric points, which also consider the chirality of the centers. A major drawback to the use of three- or fourpoint pharmacophores is the need for rather long calculation times (about 1 min per compound in the case of four-point pharmacophores) [26], which can be a limiting criterion in the evaluation of whole virtual libraries containing numerous compounds. 25.4.3

Selection Algorithms

After diversity has been assessed using a descriptor system, compounds can be subdivided into classes which cover the available chemical space with representative molecules. A widely used system to evaluate the similarity between the bitstrings of two compounds is the Tanimoto coefficient. If two molecules have A and B bits set in their fragment bitstrings and C of these in common [27], then: Tanimoto ¼ C=ðA þ B CÞ The Tanimoto coefficient can adopt values between 1 (identical substances with respect to the descriptor system) and 0 (no similarity). Using this similarity metric, the set of compounds can be subdivided into clusters of similar compounds which are different from the members of other clusters. There are two categories of clustering: hierarchical and nonhierarchical. Hierarchical clustering can be divided into agglomerative and divisive approaches. The agglomerative approach begins with all the molecules as singleton clusters and combines these gradually into larger clusters by successively joining the most similar compounds. Divisive clustering begins from one large cluster containing all entities and splits this cluster successively into smaller clusters of dissimilar compounds. Both methods continue until the desired number of clusters is reached. An example of a nonhierarchical clustering algorithm is the Jarvis– Patrick method. For this approach, in the first stage, the K nearest neighbors for each member are calculated, according to the Tanimoto coefficient. In the next step, clustering is performed. Two structures are assigned to the same cluster, when the following three conditions are met: 1 A is in the top K nearest neighbor list of B. 2 B is in the top K nearest neighbor list of A. 3 A and B have at least M of their top nearest neighbors in common (M < K).

25.4 Diversity in Combinatorial Chemistry for Drug Development

Fig. 25.3. Clustering may produce diverse sets of compounds, in which diversity occurs along one dimension.

Depending on the values of K and M, the Jarvis–Patrick procedure tends to produce either very large clusters (if M and K are rather low) or many singleton clusters (if M and K are high), but it is a relatively fast algorithm compared with the hierarchical clustering methods. A major drawback to clustering is that it gives no information about the chemical space which is covered. The clusters may appear to be very widespread, but if diversity occurs just along one or a few dimensions of chemical space, the other dimensions still may not be covered at all. In Fig. 25.3, the compound clusters seem to be diverse, but they are almost exclusively spread along the x-axis. There is hardly any diversity along the y-axis. Another approach which circumvents this problem is a partition-based selection. In this method, chemical space is divided into a number of segments along each axis. These segments generate a number of smaller volume elements (‘‘bins’’) in the chemical space envisaged. A diverse set of compounds can then be chosen to maximize the number of different bins filled with representatives. The same set of compounds as in Fig. 25.3 shows in Fig. 25.4 that the chemical space along the x-axis is covered very uniformly, whereas the space along the y-axis is hardly covered at all.

Fig. 25.4. Partitioning of chemical space shows that diversity only appears along the x-axis (filled squares).

733

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25 Design Criteria

25.4.4

Diversity Assessment

There is much discussion in the literature about the relevance of different descriptors and diversity selection processes [28–35]. Although 3D descriptors can encode more information on molecules, it was shown that 2D descriptors perform better in separating biologically active molecules from inactive ones. Hierarchical clustering in combination with MACCS 2D descriptors performed best in several different datasets of enzyme assays, high-throughput screening results, and activities in an monoamine oxidase assay. Jarvis–Patrick clustering was shown to be rather ineffective since it generated either singleton clusters of actives or very large clusters containing the actives together with many inactives. Using hierarchical clustering, it was possible to separate active clusters containing 30–50% actives on average whereas only 0.82% of the whole dataset was active. It appeared that a reasonable cluster size is around five molecules per cluster, which means that about 20% of a dataset still has to be sampled in order to obtain valid results [21]. The MACCS structural keys also performed best in predicting hydrophobicity, electrostatics, sterics, dispersion interaction, and hydrogen bonding of molecules. These factors are relevant for ligand–receptor interaction and molecular recognition. This result is the reason why this descriptor system performs well in biological clustering [36]. Another statistical evaluation of random selection methods and of designed compound subsets for biological screening systems proved that the latter were, indeed, superior. About 3.5 times more compounds have to be sampled randomly to obtain results equivalent to a rational design approach. For initial screening, a Tanimoto similarity radius of 0.7 between similar compounds in the subset is expected to be sufficient to cover relevant biological effects, whereas, for lead refinement, a Tanimoto similarity radius of 0.85 is recommended [37]. The question of which level of diversity of combinatorial libraries should be assessed has also been widely discussed in the literature. Three main strategies have been proposed [38]: reactant-based design means that the diversity of the reagents is evaluated and these diverse reactants are assumed to give a diverse product library. The number of reagents is orders of magnitude lower than the number of products, therefore this design process is very fast. However, it is not guaranteed that a maximum diverse set of reagents also provides a maximum diverse set of products. Therefore, product-based design uses fully enumerated combinatorial libraries and picks the most diverse products thereof. During this process, the combinatorial nature of the products gets lost and, therefore, the process loses efficiency. A compromise between these two approaches is a reactant-biased/productbased algorithm. This means maintaining the combinatorial nature of chemistry at the reactant level, but optimizing the diversity at the product level by use of mathematical models such as genetic algorithms and simulated annealing [39– 41]. A general problem with these product-based approaches is that diversity has to be calculated for the whole virtual library, which needs very large computational

25.5 Privileged Structures

resources. Which of these methods is finally more efficient remains unclear. On one hand, there are reports which show that diversity design at the product level results in more diverse libraries than if design is performed at the reagent level [42]. On the other hand, it could be shown that efficiency of selection is equivalent both at the product level and at the reagent level if the number of building blocks is sufficiently high. Just with a very few building blocks (e.g. 85% by HPLC) and approximately 600 compounds were tested for biological activity. Some of these compounds showed weak activity, e.g. 25 (Fig. 28.4), which stimulated erythropoiesis approximately 1.5-fold. However, these hits were not further pursued since the activity was too weak and, based on the already existing structural information, there was no potential for further improvement. 28.2.4.2 Pyrazoles

Following the concepts outlined above, the replacement and modification of the polar heterocyclic head group in lead structure 1 by a pyrazolyl group was the next aim. A solid-phase synthesis of pyrazoles 26 based on 1,3-dicarbonyl compounds 27 as the key intermediates has been described by Marzinzik and Felder (Fig. 28.5) [17]. The diketone 27 is prepared in a Claisen reaction from acetophenone 28 and ester building block 29 [18]. Intermediate 27 is then condensed with a hydrazine building block to give the corresponding pyrazole [19]. Following this retrosynthetic concept, two routes to the pyrazole analogs (30) of lead 1 were investigated (Fig. 28.6). The first route (a) proceeds via immobilized

Fig. 28.5. Literature route to pyrazoles.

791

792

28 Erythropoietin Sensitizer -- A Case Study

Fig. 28.6. Retrosynthetic routes to the target structures.

secondary amide-substituted pyrazole (31), which is alkylated at the amide nitrogen atom with a variety of alkylating agents to give the final products [20]. Since this last step was thought to be critical (with respect to overalkylation and the reactivity of the alkylating agents/diversity of the reagent pool) another route was investigated first (b): starting from a formyl resin [21] the first step of the synthesis is a reductive alkylation with amines to form immobilized secondary amine 32. This intermediate is acylated with acetyl benzoic acid 28 to give diketo intermediate 27, which can be transformed to the product 30 (Fig. 28.6). The reductive alkylation of the formyl resin 3 was performed with benzyl amines 34 in a two-step sequence using trimethylorthoformate as solvent and dehydrating agent (Scheme 28.3). The imine was then reduced under acidic conditions using

Scheme 28.3. The Claisen condensation of resin-bound acetophenone derivatives was reported for secondary amide-substituted acetophenone derivatives. However, the reaction failed with resin-bound tertiary amide-

substituted acetophenones. This result might be due to the fact that the immobilized enolate is a much better nucleophile when bound to the support as secondary amide than as tertiary amide.

28.2 Results

tetrabutylammonium borohydride in DMF/acetic acid (100:1). Subsequently, the second building block, 4-acetyl benzoic acid (28), was coupled to the resin under standard acylation conditions using TBTU as the activating agent to give 35. Finally, attempts were made to synthesize the 1,3-dicarbonyl compound 36 first by a Claisen condensation using the conditions described in the literature for monosubstituted amides [20 equiv. NaH, N,N-dimethylacetamide (DMA), 90 C, 1 h]. However, these conditions and a variety of others employing different solvents and bases failed to give the product. In all cases, the starting material was recovered. In contrast to the first result using disubstituted amides, the dicarbonyl compound 37 (carrying only one substituent at the amide nitrogen) was accessible via a Claisen condensation from 38 (Scheme 28.3). It is probably the case that the enolate 39 formed under the strongly basic Claisen conditions from 38 is a better nucleophile than enolate 40 (from 35) since the secondary amide can also be deprotonated. This event is not possible when the enolate is bound as a tertiary amide to the resin (Scheme 28.4).

Scheme 28.4. The immobilized enolate is a much better nucleophile when bound to the support as a secondary amide than as a tertiary amide.

The failure of route (b) (Fig. 28.6) prompted further investigation of the second route (a): TentaGel resin TM 41 [22] was employed as support instead of Rink amide resin [23] as used by Marzinzik and Felder since better and more reliable results were obtained with the more solution-phase-like resin 41. Immobilization of 4-acetyl benzoic acid (28) was performed under standard conditions. The subsequent Claisen condensation to 42 was achieved using benzoic acid esters in DMA with NaH as base at 90 C (1 h). However, the reaction remained unreliable and no general trends describing the reactivity of the ester components were observed. Furthermore, reaction conditions could not be improved by using other bases (sodium methoxide [24], NaHMDS [25], sodium amide, potassium tert-butoxide [26]), temperatures, reaction times, or solvents. In solution phase, closely related reactions gave improved yields when 18-crown6 (18-C-6) was added [27]. This modification was also tested on solid support. Interestingly, in some cases the addition of 18-C-6 gave improved yields, but in many

793

794


other cases the reaction still did not work. The Claisen condensation remained a critical step in the sequence, and for library production each ester building block had to be tested prior to use. Furthermore, only esters which had no (acidic) aproton could be used. For the first library, only aromatic esters were employed (Scheme 28.5).

Scheme 28.5. Solid-phase synthesis of primary and secondary carboxamides containing the pyrazole moiety as the key fragment. Cyclization of the dicarbonyl

intermediate with hydrazines gave in all cases near equimolar mixtures of the two possible regioisomers. For clarity, only one of the regioisomers is shown.

The cyclization to pyrazoles 43 was cleanly achieved using a variety of hydrazines 44 in ethanol after stirring overnight at 70 C. No limitations concerning the hydrazine building blocks were observed: hydrazine, and aliphatic and aromatic derivatives thereof, gave consistently good yields and purities of the corresponding pyrazoles (43). Isoxazoles were obtained from the reaction with hydroxylamine. In all cases, roughly equimolar mixtures of the two regioisomers were obtained. For initial biological testing these compounds were not separated. However, the separation of the two regioisomers was in most cases possible by chromatography. For clarity, only the active regioisomer is shown in the schemes. The last step of the synthesis – the alkylation of amide 43 to give, after cleavage from the resin, the secondary amides 46 – was the most critical one and required the longest time for optimization. The deprotonation of solid-supported amides with sodium hydride in DMSO [28] or with lithiated oxazolidinone in DMF [29] is well described in the literature, but both bases gave unsatisfying results. Similar results were obtained with lithium tert-butoxide in tetrahydrofuran (THF) [30] or toluene. Best results were achieved using sodium hexamethyldisilylamide as base in THF [31] under carefully controlled conditions (exclusion of water). The first library (Fig. 28.7) was prepared using a fully automated synthesizer (Syro, MultiSyn Tech). Twenty-nine products (out of 120 theoretically accessible compounds) of sufficient purity (> 85% HPLC/UV detection) for biological testing

28.2 Results

The first libraries of pyrazoles. Within these libraries the first highly active compounds were identified and a SAR for the hydrazine building block was established.

Fig. 28.7.

were obtained. This low success rate was due to the last modification step, which required rigorous exclusion of any traces of water. This requirement is not readily achieved using fully automated synthesizers. The library was strongly influenced by the SAR of the original lead structure 1, and only a limited diversity was achieved. In vitro activity was not observed for this library. However, since only a limited area of the available and accessible diversity space was exploited, the biological activity of the entire virtual library was by far not verified by this first set of products. Therefore, a second library of 96 compounds was prepared from 288 theoretically accessible compounds (Fig. 28.7). To exploit further the chemical space accessible by the described chemistry, not only the secondary amides 46 but also the primary amides 45 were submitted to testing. Furthermore, a more diverse set of hydrazine building blocks was employed. Instead of using a fully automated synthesizer, the radiofrequency-encoding technology from Irori TM (see Chapter 6) was used. This technique allows the preparation of larger libraries as it is not restricted to spatial encoding of the products. Furthermore, reaction conditions can be much better controlled and inert conditions can be ensured more easily. Within this second library, the first biologically highly active compounds were identified and a major breakthrough in the project was realized. Interestingly, it was not the secondary amides (46) that were the active compounds but rather the corresponding synthetic precursors (45). Furthermore, a first SAR of the hydrazine building block (R2) was identified: the N-methyl-substituted pyrazoles were the

795

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28 Erythropoietin Sensitizer -- A Case Study Tab. 28.1.

SAR of pyrazole substituent.

Entry

1 2 3 4 5 6 7 8

Maximum stimulation (%) at [0.1 mM]

R ¼ H (50) R ¼ Me (47) R ¼ Et (48) R ¼ i-Pr (51) R ¼ t-Bu (52) R ¼ Ph (49) R ¼ Bn (53) Isoxazole 54

156 410 219 100 100 100 100 314

most active compounds, e.g. 47, but already N-Et (48) and N-Bn (49) were significantly less active. The unsubstituted pyrazole 50 displayed some weak activity (Table 28.1). These initial results were confirmed by follow-up libraries and isoxazoles, e.g. 54 (Table 28.1), were identified as being almost as potent as N-methyl pyrazoles. Results for the R1 group were less clear and a variety of libraries were prepared using a large set of ester building blocks. Based on the already established SAR the number of hydrazine building blocks was kept small within these libraries. Biological testing finally established an SAR for the R1 building block (Table 28.2). This residue was the most variable part of the molecule and many para-substituted phenyl derivatives showed good activity; meta-Substituted phenyl derivatives were less active and ortho-substituted compounds were inactive. Furthermore, a number of heterocyclic ester building blocks led to active compounds (Table 28.2). Until this stage, three out of the four building blocks employed [acetophenone, ester (R1), hydrazine (R2), and alkylating agent (R3)] had been modified. From this optimization process involving around 600 compounds, pyrazole 55 was one of the most active molecules in vitro (entry 1, Table 28.2). Compound 55 displayed a 4.5fold stimulation of erythropoiesis at rather high concentrations (0.1 mM). Based on this result, in vivo experiments were initiated, but, disappointingly, no increase in the hematocrit was observed. This result was most probably due to the poor pharmacokinetic profile of 55: Cmax ¼ 0.16 kg L1 and area under the curve (AUC) ¼ 0.11 kg h L1 after being given intraperitoneally to mice (Table 28.3). Mono- and dialkylation of the primary amide with small alkyl groups increased the metabolic stability [dimethylamide analog of 55 and 71 (for the synthesis, see below): Cmax ¼ 0.193 kg L1 and AUC ¼ 0.416 kg h L1 after intraperitoneal application to mice; Table 28.3], but was also associated with a loss of in vitro activity [3.4-fold stimulation of erythropoiesis at a high concentration (10 mM)]. The first result indicated the primary amide functionality as an important point in the met-

28.2 Results Tab. 28.2.

SAR of arylester building block.

Entry

Maximum stimulation (%) [ mM]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

4-Trifluoromethylphenyl (55) 3-Trifluoromethylphenyl (56) 4-Fluorophenyl (57) 4-Chlorophenyl (58) 3-Chlorophenyl (59) 2-Chlorophenyl (60) 3,4-Dichlorophenyl (61) 4-Bromophenyl (62) 4-Methylphenyl (63) 4-Methoxy phenyl (64) 2-Pyridinyl (65) 2-Thiophenyl (66) 2-Benzofuryl (67) 2-(5-Ethyl)furyl (68) 4-Nitrophenyl (69) 4-Dimethylaminophenyl (70)

450 [0.1] 281 [10] 390 [0.1] 332 [0.36] 294 [1] 100 [0.1] 156 [0.7] 400 [0.3] 390 [0.1] 350 [0.1] 360 [0.3] 400 [0.03] 330 [1] 370 [0.16] 330 [2] 320 [0.12]

abolic degradation of 55 and probably also for the other hits within this class. To address this problem and to obtain a thorough picture of the structure–activity space around 55 and of the pyrazoles in general, the following issues were next examined: 1 How can the primary amide functionality in hits 56–70 be replaced to improve metabolic stability? Tab. 28.3.

Pharmokokinetic profile of key compounds.

C (max) AUC Maximum stimulation AUC, area under the curve.

0.16 kg/L 0.11 kg h/L 450 (0.5 mM)

0.19 kg/L 0.42 kg h/L 340 (10 mM)

797

798


Fig. 28.8. Modifications of the primary amide functionality in 55 to increase metabolic stability.

2 Is the pyrazole moiety the optimal central core? 3 If the pyrazole is the optimal core, does the acetophenone building block need to be optimized? Answering the first question was a chemically demanding task since it required a change in the link to the solid support and, as mentioned above, the Claisen condensation was sensitive to substitution at the nitrogen. As potential replacements for the primary amide, five functionalities were considered: (1) carboxylic acids prepared by hydrolysis of the primary amides, (2) (benzylic) amines as products of reduction of the corresponding amides, and (3) ureas, (4) sulfonamides, and (5) inverse amides as derivatives of the benzylic amines obtained by reduction (Fig. 28.8). Some methods for the reduction of polymer-bound amides to the corresponding benzylic amines have been described in the literature [32, 33]. However, we have not obtained positive results for the reduction and subsequent acylation of pyrazoles on solid support, e.g. the sequence of reduction of 43 to 72, subsequent acylation, and cleavage to give 73 failed (Scheme 28.6). Since literature precedent is only given for easily accessible amide functionalities, this failure was thought be due to steric hindrance around the amide functionality. This problem was solved by solution-phase parallel synthesis: after cleavage of the products from solid phase the primary amides 45 were reduced using borane in THF at room temperature overnight to give 74 [34]. The borane reduction proved to be superior to reductions with lithium aluminum hydride since the work-up of the latter reaction in a parallel fashion was tedious [35]. The resulting amines (74) were submitted to testing and, in addition, derivatized using acylating agents and isocyanates to give analogs 73a and 73b (Scheme 28.6). However, the activity of these libraries was very low. Therefore, these classes were abandoned. To prepare the carboxylic acid analogs of 55 and other active pyrazoles, the linker to the resin was redesigned. Instead of the amine-functionalized TentaGel SAM resin TM 39 employed so far, it was thought that the corresponding Wang resin [36]

28.2 Results

Amines and derivatives thereof were prepared as analogs of 55 by a combination of solid- and solution-phase chemistry.

Scheme 28.6.

would allow the preparation of acetophenone derivative 75 (Scheme 28.7), which would then serve as the starting material for the Claisen condensation. The final products of this route would have been esters 76 and after cleavage the desired carboxylic acids 77. However, the Claisen condensation of the ester-functionalized acetophenone derivatives failed to yield the 1,3-dicarbonyl compounds. This problem could be caused by the immobilized ester reacting as an electrophile in the Claisen reaction, and thereby being released from the support. However, the acetophenone starting material was cleanly isolated after the failed reaction. Therefore, this failure was probably the same as that observed for the condensation with im-

Analogs of pyrazole 55 carrying tertiary amides and carboxyl groups instead of the primary amide functionality were prepared by a combination of solid- and solution-phase chemistry.

Scheme 28.7.

799

800


Fig. 28.9. Carboxylic acid 79 – the compound selected for preclinical development.

mobilized tertiary amide-functionalized acetophenone derivatives (Scheme 28.4) – a lack of nucleophilicity of the corresponding enolate. The acid analogs of 55 were finally prepared by a combination of solid- and solution-phase chemistry (Scheme 28.7). Primary amides 45 were synthesized and cleaved from the resin, as described in Scheme 28.5, and were subsequently hydrolyzed in solution to the carboxylic acids 77. This conversion was cleanly achieved using in situ-prepared hydrochloric acid from TiCl 4 in a mixture of dioxane and water at 100 C [37]. Other, less sophisticated conditions (aq. HCl in THF or alcohols; KOH in alcohols or in DMSO [38]) were also examined, but the results were not satisfactory since the purity of the resulting products 77 was low. The carboxylic acids 77 were tested and were also used as precursors for the preparation of tertiary amides 78, which are not accessible by solid-phase chemistry (Scheme 28.7). The latter displayed only very low biological activity and were consequently abandoned. On the other hand, the carboxylic acids 77 were as active as the corresponding primary amides in vitro and the same SAR was observed for the R1 and R2 groups. Consequently, 79 (Fig. 28.9) was tested for its in vivo activity and an increased hematocrit was observed. Although carboxylic acid 79 displayed an excellent biological profile, two features were still open to question: 1 Is the pyrazole moiety the optimal central core? 2 If the pyrazole is the optimal core, does the acetophenone building block need to be optimized? The first remaining question was answered by conventional chemistry, since the development of a solid-phase methodology for the preparation of heterocyclic analogs of 78 was envisaged to be too time-consuming with respect to the time constraints at this stage of the project. Keeping the size of the heterocyclic cores constant, the position, type, and number of heteroatoms were varied (Table 28.4). Although the resulting structural changes were in some cases very small, e.g. 80 and 81, these molecules displayed a different SAR and were less active than the corresponding pyrazoles and isoxazoles. The second question of whether the para-acetophenone building block optimal was resolved using established solid-phase chemistry: phenyl derivatives were the most active class and para substitution was essential. Carboxylic acid 79 remained the most active compound. Some pyridine analogs still displayed some activity, whereas pyrimidine derivatives were found to be inactive.

28.2 Results Tab. 28.4.

Entry

1 2 3 4 5 7 8 9 10

Maximum stimulation (%) [ mM] 3,5-N-Methylpyrazolyl (82) 3,5-N-H-pyrazolyl (83) 3,5-N-Ethylpyrazolyl (84) 3,5-Isoxazolyl (85) 3,5-Oxadiazolyl (86) 3,5-Oxadiazolyl (87) 2,4-Triazolyl (88) 2,4-(5-Methyl)triazolyl (81) 3,5-(2-Methyl)-triazolyl (80)

370 [0.1] 220 [3] 100 410 [0.4] 380 [1] 330 [3] 370 [1] 330 [0.1] 300 [3]

The combination of the structure–activity relationship discussed above and the biological properties which are described in the following paragraph rendered carboxylic acid 78 the most promising compound in the project. The compound was promoted into preclinical development. 28.2.5

The Candidate for Preclinical Development

Carboxylic acid 79 has no PDE3 activity and dose-dependently increases the effects of EPO (10 mU mL1 ). The EC50 is approximately 30 nM and the maximal response is an approximately fourfold stimulation of the effects of 10 mU mL1 EPO. Thus, the sensitizing effects are displayed at the relevant EPO concentration encountered in renal anemia. Even at high EPO concentrations, EPO-mediated effects were increased. In a mouse model, 79 (3 and 10 mg kg1 day1 orally, once daily, 5 days of treatment) increased the hematocrit on day 8 by 1.9% points and 2.8% points respectively. However, the effects of 79 appear to be rather modest compared with treatment with rhEPO, which gives an increased hematocrit of at least 10% points after 5 days of treatment with 400 U kg day1 following the same protocol. The lack of a more pronounced increase, a fast return to baseline, and the total absence of activity in rats were surprising. To exclude metabolic instability as a potential explanation for these disappointing results, the kinetic parameters of 79 were determined in several species. In all species tested (mouse, rat, dog), no pharmocokinetic problems were seen after peroral administration and very high expositions could be achieved in mice and dogs: Cmax ¼ 0.847 kg L1 , AUC ¼ 9.9 kg h L1 and a 100% bioavailability after p.o. application to mice. These data excluded unfavorable pharmacokinetics as the reason for the modest in vivo effects. However, a possible explanation for the limitation of the effects of EPO-sensitizing agents can be given by the very tight regulation of the red blood cell mass in mice and rats. Further studies concerning the biological activity of 79 are ongoing.

801

802


28.3

Combinatorial Chemistry in Drug Discovery

Over a period of 7 months approximately 700 pyrazoles and isoxazoles were prepared within nine libraries with 2.5 Full-time-employees (FTEs). The libraries varied according to size, the synthetic techniques employed (automated synthesizer, radiofrequency tagged, manual synthesis), and the diversity of the building blocks used. The first libraries were mostly a verification of the method development and the selection of building blocks was mostly ruled by the pre-existing SAR of the lead structure – dihydropyridazinone 1 – and its close analogs. Still, these libraries helped to identify the primary and not the secondary amides as relevant compounds. The sizes and diversities of the next libraries were increased. These libraries further validated the SAR for the primary amides but did not help to solve the lack of in vivo potency. However, based on the SAR, small and highly focused libraries were prepared, in most cases manually, to verify the medicinal chemistry hypotheses. By using parallel methods, this objective was carried out more quickly and more thoroughly than would have been possible by conventional methods. Combinatorial chemistry in solution and on solid support has been shown to be a very successful tool in supporting drug discovery. To realize this potential, some strategic considerations have been important:

. combinatorial chemistry joined the project early . targeted libraries were prepared quickly (biological results were obtained within 2–3 weeks)

. learning cycles involving method development, production, and biological testing have been short has been close interaction with conventional chemistry (building block synthesis, establishment of SAR)

. there

The preparation of libraries of compounds not covered by the range of known SARs is time-consuming and very risky. But once active compounds have been identified within one of those libraries, the full potential of solid-phase chemistry can be realized very quickly and with many advantages over conventional chemistry. Thus, taking the risk of preparing hundreds of inactive compounds (which will then serve as feedstock for future HTS) is clearly outweighed by the prospect of rapid success in medicinal chemistry programs if the project has been selected properly.

Acknowledgements

The identification of 78 as a candidate for preclinical development was a tremendous joint effort from various disciplines. However, the outline and the context of this chapter does not allow for the presentation of all the results obtained within the project, nor do they give sufficient credit to all the people who participated

References

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37 L. E. Fisher, J. M. Caroon, S. R.

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29

Estimation of Stability and Shelf Life for Compounds, Libraries, and Repositories in Combination with Systematic Discovery of New Rearrangement Pathways Ferenc Darvas, Gyo¨rgy Dormań, Tama´s Karancsi, Tama´s Nagy, and Istvań Ba´gyi 29.1

Introduction 29.1.1

Stability and Shelf Life Characterization: the Need

With the emergence of high-throughput screening (HTS) and combinatorial chemistry the most suitable drug candidates are selected from millions of new chemical entities [1, 2]. Today, millions of compounds are synthesized and stored every year with the aim of testing them in biological assays either immediately or after a certain storage period in the hope of identifying new biologically active hit compounds [3, 4]. It is one of the paradoxes of combinatorial chemistry that the stability of the compounds is not considered to be as important as it is for other chemical products. Marketed drugs, bulk, and fine chemicals normally possess an expiry date stated explicitly by the manufacturers. Today, practically every pharmaceutical corporation and a growing number of academic research units with discovery research are involved in archiving discovery compounds in solid state or in a frozen dimethyl sulfoxide (DMSO). Questions as to how the purity of the substances changes as a result of prolonged storage – possibly combined with repeated melting and freezing of the DMSO solution – are largely unanswered, despite their serious implications for the new lead research. The lack of stability of the compounds, which is due to their chemical structure, the nature of impurities, as well as the less than optimal storage and handling, have a primary impact on discovery research in two ways: through inventory losses, and, which is more important, via the increased frequency of false-positive results. False positives resulting from the lack of stability of library members probably cost the pharmaceutical industry $600–900 million in 2000 (assuming 200 million HTS assays worldwide with a 0.1% hit rate, a cost of $30,000 per false positive, and 10–15% decomposed members among the tested libraries). In general, drug canHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

29.1 Introduction

didates that fail to exhibit sufficient stability should be stopped before investing millions of dollars [5]. 29.1.2

Stability Characterization: Empirical Studies

Giving an estimation about the purity degradation of a stored library over a time period, such as a year, is principally a simple procedure: quality sampling before and periodically during the storage can lead to a quantitative description about the purity deterioration. These data can be utilized to develop a qualitative picture of the library or repository stability. Such empirical studies of the library stability are easy to perform and might be useful as a first guess regarding the expected stability issues. To the best of our knowledge, the first open account of an empirical stability study was presented by Ortner in 1995 [6], in which she stated that approximately 20% of a collection of several thousand organic compounds (combinatorial library members and individually synthesized samples) stored in ethanol degraded over a 1-year period. At Merck [7], @10,000 compounds were stored at 20 C, 0 C and at room temperature in DMSO for a year. The decomposition ratio, which was measured by high-performance liquid chromatography/mass spectrometry (HPLC/MS), suggested that compounds fall into two distinct classes: stable and unstable samples. Compounds belonging to the unstable class had already decomposed to a large extent within 6 months. Storing the samples at 20 C did not significantly improve the stability of the compounds compared with storage at 0 C. The European Union is also sponsoring an empirical study involving regular periodic purity checking of 10,000 compounds by LC/MS and nuclear magnetic resonance [1 Hnuclear magnetic resonance (NMR)] for a period of 3 years [8]. 29.1.3

Stability and Shelf Life Estimation: Model-based Approaches

Empirical studies unfortunately suffer from the problem that they do not deliver an exact forecast about the future quality of the samples studied. Also, it is difficult and subjective to extrapolate conclusions drawn from these studies to other samples. They generally do not allow conclusions to be drawn on a number of important stability issues, such as structure–stability relationships, the influence of different storage conditions on the stability, or the implications of the quantity and nature of the impurities for the stability of the compounds. Three years ago, the authors concluded that a more qualitative stability description was needed in the field of combinatorial libraries and repositories, and, as a result, developed a model that extends beyond empirical description. As a starting point, models introduced at stability expiry date determination of drug substances are applied (see Section 29.7).

807

808

29 Estimation of Stability

The fundamental assumption is that long-term degradations are simply thermal decomposition with a retarded rate at lower temperatures. It is assumed that degradation processes are speeded up by elevating the degradation temperature. The degradation rate at high temperature is characteristic of the structure and it can be used to estimate the shelf life under storage conditions at a particular temperature. The shelf life of a chemical species (compound, library, collection, or repository), according to our definition (and essentially according to the ICH guidelines, see Section 29.7), is a time interval that the species is expected to remain within an approved purity specification provided that it is stored under the conditions prescribed by the respective stability enforcing protocol. The concept of expiry timing in combinatorial chemistry was first mentioned by Fitch and coworkers in 1998 [9], while a model and algorithm for the calculation of expiry date/shelf life [stability expert system (Stabex TM ) approach] was suggested by Darvas and coworkers in 1999 [10], and published in 2000 [11]. The present chapter describes the different elements of a new area of HT stability studies, which we call ‘‘combinatorial stability assessment.’’ Combinatorial stability assessment deploys the computational armory (diversity, analog search, and clustering) and experimental methodology of combinatorial chemistry in order to obtain fast measurement-supported predictions for the stability characteristics of libraries or repositories. This chapter introduces the authors’ approach and validation studies. In the course of developing this assessment, the authors found that the experimental method used for stability studies can be utilized in the systematic discovery of new scaffold structures via thermal rearrangements of heat-stresschallenged library members, as described in Section 29.5. Throughout the chapter, ‘‘stability’’ is used to refer to chemical stability (as opposed to metabolic stability).

29.2

Methods and Tools for Combinatorial Stability Assessment 29.2.1

Modeling Intrinsic and Extrinsic Factors Influencing the Stability of Individual Compounds

The major factors of combinatorial library stability can be classified into two groups: intrinsic and extrinsic factors [12]. Intrinsic factors are the properties that are inherent to a compound with absolute purity and depend solely on the chemical structure. From our point of view, the decomposition rate of the pure compound at different temperatures is the most important property. Less relevant factors of the pure compound are sensitivity to light, sensitivity to exposure to air or oxygen, and sensitivity to humidity. Extrinsic factors of compound stability are the conditions that are beyond the chemical structure of the compounds being studied. Such factors are the purity of the library members, the chemical nature and the distribution of the impurities, such as the accumulated side products and reagent residues, and the nature of the

29.2 Methods and Tools for Combinatorial Stability Assessment

solvent(s) used for storage of the compounds. External factors, such as the temperature of storage, the speed of the repeated freeze/thaw cycles, or the chemical nature of the container are also included in this group. The authors have proposed that the effect of the intrinsic factors should be modeled by the Arrhenius equation [11], as described in Section 29.8. This model enables a qualitative shelf life time to be predicted (and, therefore, the expiry date) for solid compounds in a pure state or in solution for a single solvent system under fixed storage conditions, if we assume that no structure-dependent interaction exists between the compounds and the impurities or the solvents respectively. These assumptions can be investigated by statistical significance testing calculations. However, the model cannot be used to give a forecast of how the shelf life will be changed if essential storage conditions are altered, for instance whether the substances are stored in DMSO instead of as a solid or in plastic instead of glass vials, since the listed extrinsic factors are not part of the Arrhenius model. For treating such effects, a dimensional analysis approach has been suggested by the authors [10], but it is not detailed here. 29.2.2

Modeling: from Compounds to Libraries

Theoretically, the best stability estimation can be gained when both Arrhenius parameters (a and b) are determined for each individual compound in the library. But, this would require vast numbers of tests to be conducted. For fast estimation of the entire compound library composed of structurally similar compounds, a single slope of the Arrhenius line (b) can be used in theory. A good approximation is to apply an average b, which is calculated from the previously determined b values of the individual compounds or cluster representatives. According to this assumption, a slope transposition can be used within clusters to determine the interception of the Arrhenius line (a) for the compounds within the rest of the library cluster. Stability estimation of combinatorial libraries by experimental determination of the Arrhenius parameters for individual compounds is time-consuming work. It would be advantageous to reduce the number of experiments by identifying correlation between structural similarity and stability. The correlation can be determined by performing a homogeneity test, which is a statistical parallelism test [13] of the Arrhenius lines for library members (Fig. 29.1). If no significant difference between the slopes of the regression equation can be proven, a joint regression line can be calculated for the entire library, which will be used to estimate a shelf life for each individual compound in the library. 29.2.3

Modeling: from Libraries to Repositories

If a repository consists of sets of libraries, and, furthermore, the homogeneity test is successful for these libraries, a simplified stability test can be attempted on the

809

810


Fig. 29.1. Parallelism test of the Arrhenius plots.

entire repository. The stability test includes a homogeneity test for each library in the repository followed by a decrease in the number of homogeneous libraries using cluster analysis [14], and an experimental stability test for individual compounds that represent the reduced homogeneous library clusters. There is another way to model stability: if the collection of compounds can be divided into several, discrete structure-related stability classes, the number of experiments can be reduced by using simplified experimental techniques, plus statistical calculation, which ensures a robust classification of the products into the stability classes. 29.2.4

Realization of Shelf Life Estimation for Individual Compounds and Combinatorial Libraries

In a typical combinatorial library, compounds are designed around specific core structures and the cores are surrounded by a standard substituent set. Based on structural similarity with regard to those library elements, the compounds can be clustered into structurally related subsets. Then, a representative sample set for each cluster (series 1) is determined in a given library. Each compound in series 1 is stored at multiple (e.g. ten) elevated temperatures for a given time, which results in a measurable degradation in the thermal analysis module (Fig. 29.2). The compounds are analyzed using HPLC before and after the incubation to determine the thermal decomposition of compounds. For structurally unrelated individual compounds in a compound repository, a similar thermal analysis procedure is followed. The measured data are processed in the calculation module (Fig. 29.3) for the estimation of the shelf life. (For calculating the results of the validation studies, calculation modules written in Cþþ and in Visual Basic language of the central data-

29.2 Methods and Tools for Combinatorial Stability Assessment

Fig. 29.2. HT thermal analysis module.

Fig. 29.3. Calculation module of the shelf life for (a) individual compounds and (b) combinatorial libraries.

base management system at ComGenex, Inc. are utilized. Part of the information system is an expert system, which is still under development. It is intended to provide a virtual stability screening during the evaluation phase of the library design and to establish structure–stability relationships.) The decomposition rate constants for each elevated temperature are calculated from the measured degradation

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according to the decomposition kinetic equation (Section 29.8, Eq. 1). The rate constants are used to generate Arrhenius parameters by linear regression (Section 29.8, Eq. 2). If the Arrhenius line is extrapolated to lower temperatures (e.g. room temperature), k25 and the shelf life can be calculated according to Eq. 2 (now T refers to 25 C) and Eq. 3 (Section 29.8) respectively. The minimum acceptable purity limit or percentage of acceptable purity loss should be defined before calculation. As noted above, in the case of structurally unrelated individual compounds, each of them should be tested at multiple temperatures. However, for structurally clustered compounds the shelf life estimation for the rest of the library (series 2) is based on the degradation at one, optimally selected temperature, assuming that their decomposition kinetics (the temperature dependence of the rate constants) are similar. 29.2.5

Instrumentation

For thermal decomposition analysis, test compounds are distributed to wells and incubated in various media at different temperatures in time- and temperaturecontrolled multiwell heating blocks, e.g. MultiAger TM (see Fig. 29.12, [15]). The decomposition of compounds is monitored by a HT HPLC system equipped with ultraviolet (UV) and integrated with a two-arm robotic sample processor that injects samples and collects fractions. For identification of degradation products online LC/MS experiments are performed. Data acquisition and overall system operation software are integrated to a central database management system.

29.3

Validation Studies for Combinatorial Stability Assessment 29.3.1

General Experimental Conditions

Initial purities of the individual compounds were 70% or better (determined by HPLC at 254 nm). Stability investigations were performed using dry materials, composed of about 20% crystalline materials and 80% dry films. For the storage and for the thermal decomposition experiments, polypropylene wells were used. Owing to the recognition limit for degradation monitoring, an upper limit of the prediction of shelf life of 15 years was selected. 29.3.2

Stability Study for an Indole Library

For validation studies, from an indole library [11] of 1385 members (Fig. 29.4), ten compounds were selected randomly as a ‘‘learning’’ set (to determine the model

29.3 Validation Studies for Combinatorial Stability Assessment

Fig. 29.4.

The indole core-based library investigated in the stability validation study.

parameters). Another set of 16 compounds was chosen from the same library for the validation of the stability assessment model. In order to check the linearity and parallelism of the correlation between log kT and 1=T, the learning set of ten compounds was stored at ten different temperatures (115–190 C) for two different durations (in time intervals of 1 h and 17 h). Two rate constants for each temperature were calculated from the measured degradation (Section 29.8, Eq. 1). The average values of the rate constants were used for the generation of the Arrhenius parameters (Section 29.8, Eq. 2). The model was validated at two levels. First, using the Arrhenius equation, the rate constants of the compounds were calculated and extrapolated to 75 C and 100 C. The rate constants at 75 C and 100 C were used to estimate the decreased purity of the compounds after storage for a predetermined time. The decomposition was monitored experimentally by HPLC. The estimated and the measured purity values were compared for six compounds (shown in Table 29.1; the remaining four compounds did not show significant degradation). Two examples of linear regression calculated for the learning set are shown in Fig. 29.5. Since they are structurally related, their b values are similar. They confirm the expected correlation between the log kT and 1=T values. Based on the temperature dependence, kT was estimated at lower temperatures in order to estimate the shelf life of the compounds under storage conditions. To further validate the model, b values determined experimentally from the learning set were used to predict the purity of the structurally similar set of 16 com-

Tab. 29.1. Comparison of the predicted and detected values of purity for the learning set of compounds.

Entry

1 2 3 4 5 6

Initial purity (HPLC) (%)

Experiment 1 (100 C, 40 h)


Purity (HPLC) (%)

Prediction (%)

Difference (%)

Purity (HPLC) (%)

Prediction (%)

Difference (%)

99 92 97 93 97 98

82 80 78 92 87 93

77 85 86 92 90 92

5 5 8 0 3 1

96 90 95 91 92 97

81 88 90 92 93 96

15 2 5 1 1 1

813

814


Fig. 29.5. Linearity checking of the temperature dependence of the decomposition rate constants (examples of Arrhenius plots for two members of the learning set between 115 C and 190 C).

pounds. The a and kT values for the validation set were determined from a single thermal decomposition experiment at 150 C according to the Arrhenius law (Section 29.8, Eq. 2). Then, a and b were used to calculate the decomposition rate constants at two temperatures: 105 C and 115 C. The purity loss, estimated for a particular length of time at 105 C and 115 C, was compared with the experimental results for the ten compounds as shown in Table 29.2 (the remaining six compounds did not show significant degradation). The majority of the predictions (80%) fit the experimental data well. However, in some cases, the decomposition was overestimated or underestimated, possibly as a result of different degradation pathways (Table 29.1, entry 1, experiment 2; Table 29.2, entries 7, 9, and 11). At these high temperatures, the overall degradation mechanism may be different from that at storage temperature. The predicted HPLC

29.3 Validation Studies for Combinatorial Stability Assessment Tab. 29.2. Comparison of the predicted and detected values of purity for the validation set of compounds.

Entry

7 8 9 10 11 12 13 14 15 16

Initial purity (HPLC) (%)



Purity (HPLC) (%)

Prediction (%)

Difference (%)

Purity (HPLC) (%)

Prediction (%)

Difference (%)

97 95 95 99 97 97 95 99 98 97

55 95 80 97 75 93 95 99 97 82

71 95 66 97 10 91 88 99 98 85

16 0 14 0 65 2 7 0 1 3

72 95 85 98 96 95 92 99 98 95

88 95 85 99 47 95 93 99 98 93

16 0 0 1 49 0 1 0 0 2

results of the remaining test compounds were in good agreement with the measured ones. In some cases (Table 29.2, entries 8 and 14), the initial purity and the purity after the thermal decomposition test were identical, i.e. these compounds did not show thermal decomposition – they had a long shelf life. Although monitoring thermal decomposition is faster at higher temperatures, this results in a loss of reliability of the predictions because of the changes in the overall kinetics along with the increased temperature. 29.3.3

Combinatorial Stability Investigation for a Small Repository

In these investigations, a small repository of 3000 compounds was considered that had significant diversity. The repository is composed of 300 libraries; each library has ten members. The initial study to establish the optimal temperature and time ranges valid for the whole repository was performed with a representative set of 10% of the repository. Library members were selected and incubated at eight different temperatures (60–95 C) for 192 h. During the large-scale test performed for the remaining repository compounds, treatment at 75 C for 192 h was selected as the experimental conditions. The rate constants at higher and lower temperatures as well as the Arrhenius parameters and the shelf life of compounds at 25 C were determined. For the practical purposes of these investigations, a relative shelf life is defined as the time period in years needed for a 10% purity loss (measured by HPLC). The results obtained for the shelf life of 1178 compounds are shown in Fig. 29.6. Interestingly, a particular pattern was observed for the distribution of the shelf life of compounds, with three maxima at 3, 6, and 11 years.

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Fig. 29.6. Shelf life distribution of compounds with detectable degradation.

29.4

Stability Investigations in Combinatorial Drug Discovery

The investigation of the stability of combinatorial libraries can be useful at different stages of the drug discovery process, e.g. the pilot design phase (as exemplified in the following lead optimization program based on cyclopentapyrroles), the library evaluation, and the hit validation. 29.4.1

Pilot Design Phase

An early hit of a cyclopenta[b]pyrrole derivative showed very promising 5lipoxygenase and SRS-A antagonist activity [16]. To check the synthetic feasibility, a core scaffold of a cyclopenta[b]pyrrole derivative (Fig. 29.7) was synthesized. Unfortunately, the compound faced serious stability problems, with an unusually short chemical lifetime when exposed to air. In order to stabilize the heterocyclic core structure, electron-withdrawing groups (R3) had to be introduced to reduce the electron density of the ring system. Alternatively, bulky substituents resulted in the same effect by increasing the steric hindrance in the vicinity of the pyrrol ring. These structural modifications incorporated into the synthetic scheme led to a series of stable analogs [17]. For example, introducing a NO2 group into the phenyl ring at position 4 increased the stability fivefold, when R1 and R2 were hydroxyl groups. Furthermore, if the

29.4 Stability Investigations in Combinatorial Drug Discovery

Fig. 29.7. Stabilization of the cyclopenta[b]pyrrole core.

hydroxyl groups were protected with bulky pivaloyl groups, sixfold higher stability was detected. Proton NMR studies based on the downfield effect of the chemical shift of the pyrrole proton confirmed that in the first case the electron density was significantly reduced while in the second case no changes were observed, indicating that the stabilizing factors are purely steric effects. To avoid similar problems, a two-stage stability checking procedure is advisable. First, empirical knowledge in combination with an expert system should be used to determine structural features which may lead to unstable library structures that are still in the design phase. Second, during the pilot chemistry feasibility studies, the pilot matrix representing the major reactivity classes of substituents can be used in a combinatorial stability study. The matrix can show how the substituents affect the chemical stability and which substituents should preferably be eliminated from the full library. Initial stage stability investigations should be extended to the key intermediates. Since the last intermediates can often be the main source of impurities of the final library, knowledge about their stability is a key factor in ensuring that library products not only show high quality immediately after the production but also retain their quality for a longer period of time. Poor chemical stability often prevents the synthetic realization of a promising new library. On the other hand, changing the substitution pattern in the library design phase can avoid the stability problems in a number of cases. 29.4.2

Libraries

All library members reflect their synthetic history through their ‘‘impurity profile.’’ Properly designed combinatorial stability assessment of the library members could reveal interactions between the impurities and the major product library member, which result in a deterioration of its long-term stability. Poor stability often prevents the optimization of a lead structure or extra effort is required to ‘‘stabilize’’ the molecule at later stages of drug development. (In one of the HIV entry inhibitor hit

817

818


Fig. 29.8. Cluster representative (entry 17) of 2-(a-aminoalkyl)-3H-quinazolin-4-one-based cores.

compounds, identified from a ComGenex library, analytical retesting revealed side products during the hit validation procedure that were most possibly due to decomposition [18].) Such a stability analysis can guide purification, which is designed to maintain and ensure long-term stability for the library and its members.

29.5

A Way Towards Systematic Discovery of New Rearrangement Pathways

Combinatorial stability assessment and routine investigations can be combined with systematic discovery of new thermally induced chemical transformations or rearrangements, such as in the case of 2-(a-aminoalkyl)-3H-quinazolin-4-one-based cores [17] Fig. 29.8. An almost clean thermally induced chemical transformation was detected [19] using a special discovery module (Fig. 29.9). The HPLC/UV trace was significantly different for the parent and thermally treated entities (Fig. 29.10); however, the molecular masses were identical in both cases (523 M þ 1 pseudomolecular ion for the sample shown in Fig. 29.10). After analyzing several compounds, a similarly unique fragmentation pattern was identified for the parent and daughter peaks, indicating a consistent transformation pathway. One of these spectra is shown in Fig. 29.11. Based on the mass spectrometry (MS) data, three possibilities were considered: a dihydroquinolinone core (I), a spiro fused tetrahydroquinazoline–imidazolidine ring system (II), and a ring-opened structural variant (III) (Scheme 29.1). More detailed structural determination together with further chemical and mechanistic considerations seemed to support the third pathway. The corresponding fragmentation pattern is shown in Scheme 29.2. The transformation identified here is a ringchain tautomerism, consistent with literature data [20]. These results can be extrapolated to the entire cluster, searching for the same rearrangement ‘‘fingerprint.’’

Thermal rearrangement discovery module.

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Fig. 29.9.

29.5 A Way Towards Systematic Discovery of New Rearrangement Pathways

Fig. 29.10. HPLC/MS monitoring of thermal rearrangement (entry 17).

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Fig. 29.11. Fragmented mass spectra of the thermally treated (upper) and the original (lower) sample (entry 17).

29.5 A Way Towards Systematic Discovery of New Rearrangement Pathways 821

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Scheme 29.1. Possible rearrangement pathways.

Scheme 29.2. Fragmentation pattern.

Selected library members have already been used for the discovery and characterization of thermally induced transformations by combining MS fragmentation with standard structural analytical methods [21, 22]. To detect thermal rearrangement, routine process-specific MS fragmentation patterns (rearrangement ‘‘fingerprints’’) need to be identified and subsequently recognized by the appropriate software tools. The MS fingerprint specifies a unique fragmentation pattern that

29.7 Appendix I: Stability Testing of Drug Substances

indicates the formation of a new sublibrary via thermal rearrangement and will rapidly speed up the identification of such transformations. Such rearrangements can be transferred to a synthetic preparative protocol. This approach can be considered as an equivalent of the Houghten library-from-library approach [23] via thermal rearrangements.

29.6

Summary

The approach presented in this chapter for predicting shelf life and expiry date for compounds, libraries, and repositories is based on modeling reaction kinetics, modeling, completed with thermal decomposition experiments in which a series of purity measurements of the compounds are involved. To save experimental time and substances, extrapolation techniques of the experimental values for sublibraries, libraries, and repositories are proposed. Algorithms, software and expert system solutions, instrumentation for thermal decomposition, and high throughput analyses are integrated with a chemical database management system to a uniform, high-performance stability estimation solution – the Stabex TM system. Stabex TM is capable of estimating the stability of several tens of thousands of library members per month in the library design and synthesis phases, or, later, during the systematic, long-term stability investigation of the whole repository. Stability investigations of the key intermediates promise the synthesis of compounds which may remain pure for a long time, thus contributing to a future total quality discovery management system. As a side product of the high throughput analytical procedures associated with the stability investigation, a systematic search and discovery of new thermal rearrangements is proposed and has been demonstrated in this chapter.

29.7

Appendix I: Stability Testing of Drug Substances

Stability expiry date determination of drug substances is well regulated and controlled by standardized international guidelines. Besides the photochemical- and media-initiated structural changes, the most important factor that influences chemical degradation is the temperature. Guidelines for stability testing are determined by the International Conference on Harmonization (ICH), which define three levels of stability testing: stress, accelerated, and long-term (real) stability testing. All three have different roles in stability analysis [24, 25]. 29.7.1

Stress Stability Testing

Stress stability testing is undertaken to elucidate intrinsic stability characteristics of the drug substance and to provide data on forced decomposition products and

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decomposition mechanisms (degradation pathways). Such testing is normally carried out under more severe conditions than those used for the (normally later) accelerated tests. Degradation products are collected in quantities necessary for their chemical identification. Reference products are also collected for accelerated and long-term studies, an expected impurity profile for the decomposition products has also been established. 29.7.2

Accelerated Stability Testing

Accelerated stability testing is designed to increase the rate of chemical degradation or the physical change of an active drug substance or drug product using exaggerated storage conditions as part of the formal, definitive, storage program. These data, in addition to long-term stability studies, may also be used to assess longer term chemical effects at nonaccelerated conditions. 29.7.3

Long-term Stability Testing

Long-term stability testing is carried out to evaluate the stability of the physical, chemical, biological, and microbiological characteristics of a drug product and a drug substance, covering the expected duration of the shelf life and retest period, which are claimed in the submission and will appear on the labeling. The shelf life, according to the ICH guidelines, is a time interval that a drug product is expected to remain within an approved specification provided that it is stored under the conditions defined on the label in the proposed containers and with the correct closure. A related term is the expiry/expiration date, which is a date placed on the container/labels of a drug product designating the time during which a batch of the product is expected to remain within the approved shelf life specification if stored under defined conditions and after which it must not be used [24, 25].

29.8

Appendix II: The Arrhenius Model

Thermal activation is a frequently used empirical model simulating long-term decomposition kinetics at the storage temperature [26, 27]. Forced decomposition under thermal stress conditions (at multiple temperatures) allows Arrhenius parameters to be extracted, which are needed for extrapolation to any storage temperature. This accelerated stability profiling provides a sufficient approximation of selected structural representatives of a cluster. The defined Arrhenius parameters can be transferred to structurally related compounds to estimate the shelf life from a single stress stability experiment. The algorithm used for calculating shelf life involves three kinetic equations.

29.9 Appendix III: Model Realization -- the Stabex TM System

From the decrease in purity at a given temperature, which is determined using HPLC, the rate constant of thermal decomposition of compounds (kT ; l/s) can be calculated according to Eq. 1, kT ¼

ln C0 =C t

ð1Þ

where C0 is the initial purity (v/v, %); C is the measured concentration of the compound of interest in the untreated sample (v/v, %); Ccrit is the minimum purity requirement of compounds at a given storage temperature (v/v, %); and t is the duration of the thermal burden treatment (h). Having measured the rate constant at different elevated temperatures, a (intercept of the Arrhenius line) and b (slope of the Arrhenius line) parameters can be determined according to the Arrhenius law (Eq. 2). Using a and b, the rate constant can be extrapolated to lower (including ambient) temperatures: log kT ¼ a

b T

ð2Þ

where T is the temperature (K). From the extrapolated kT value, the shelf life is calculated according to Eq. 3: t shl ¼

ln Ccrit =C0 kT

ð3Þ

where t shl is the shelf life (years). If the t shl value is low for a cluster, it can be eliminated from the initial library.

29.9

Appendix III: Model Realization – the Stabex TM System

In the traditional drug development process, stability testing is a low throughput process, which is designed individually for each drug candidate based upon a detailed plan. Performing these tasks for large sets of compounds is a rather complex procedure, which would benefit from an integrated system. The Stabex TM system has been developed to solve the following combined tasks: 1 2 3 4

Evaluating the shelf life of libraries under initial study before production. Evaluating the shelf life of library members after production. Retrospective evaluation of the shelf life of the repository already available. Development and methodological studies of combinatorial stability issues.

Stabex TM has specific links to the elements of the CMT library technology [28], which integrates library design, synthetic plan evaluation, synthesis and purification, HT analyses, storage, and data management. The overall flowchart for Stabex TM is shown in Fig. 29.12.

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Fig. 29.12. Combinatorial stability assessment system overview.

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References

Acknowledgements

Thanks to Pe´ter Sle´gel (EGIS Pharmaceuticals, Budapest Hungary) for his contribution to this work and helpful discussions. The help given by Ro´bert Ferenczi and Ga´bor Po˝cze (ComGenex) is much appreciated.

References 1 A. Furka, Drug Dev. Res. 1995, 36, 2

3

4 5

6

7

8 9

10

1–12. R. A. Houghten, C. Pinilla, S. E. Blondelle, J. R. Appel, C. T. Dooley, J. H. Cuervo, Nature 1985, 354, 84–86. J. N. Kyranos, J. C. Hogan, Jr, Modern Drug Discovery 1999, July/ August, 73–81. R. Wedin, Modern Drug Discovery 1999, January/February, 47–53. A. Persidis in: Combinatorial Chemistry and Technology/Principles, Methods and Applications. Miertus, S. and Fassina, G. (eds), Marcel Dekker, New York 1999, pp. 405–422. M. J. Ortner, Proceedings of the CHI Conference on Studies on the Impact of Solvent and Storage Conditions on the Stability of Different Chemical Groups within High Throughput Screening Libraries, Exploiting Molecular Diversity, Small Molecule Libraries for Drug Discovery, CHI Conference, Wendy Warr & Associates, Holmes Chapel UK, 1995, pp. 37–38. M. Valenciano, Proceedings of the SBS CSS Discussion Group on Substances, SBS 5th Annual Conference and Exhibition, Edinburgh, UK 1999. H. Herklots, Modern Drug Discovery 2000, March, 46–50. W. L. Fitch, G. C. Look, G. Detre in: Combinatorial Chemistry and Molecular Diversity in Drug Discovery. Gordon, E. M., Kervin, Jr, J. F. (eds), Wiley-Liss, New York 1998, pp. 349– 368. F. Darvas, Proceedings of the SBS CSS Discussion Group on Substances, SBS 5th Annual Conference and Exhibition, Edinburgh, UK 1999, p. 127.

11 F. Darvas, T. Karancsi, P. Sle´gel, G.

12

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17 18 19

20

21 22

23

Dormań, Gen. Eng. News, 2000, 20, 30–34. F. Darvas, T. Karancsi, P. Sle´gel, G. Dormań, A Stability Prediction System for Estimating Library Expiry Time. Proceedings of the CHI Conference – Drug Discovery, Tokyo, Japan, 2000, poster presentation 8. H. Spa¨th, Cluster Dissection and Analysis. Ellis Horwood, Chichester 1985. H. Mager, Moderne Regressionsanalyse. Otto Salle Verlag, Frankfurt am Main 1982. Allen P. Mills, European Patent, EP– 1115120. Z. Kapui, G. Dormań, J. Ivanics, G. Galambos, I. Stadler, Pharm. Res. Comm. 1988, 20, 195–196. G. Dormań, unpublished data. A. K. Debnath, L. Radigan, S. Jiang, J. Med. Chem. 1999, 42, 3203–3209. F. Darvas, T. Nagy, T. Karancsi, P. Sle´gel, G. Dormań, Combinatorial Discovery of New Thermal Rearrangement Pathways. Proceedings of IKCOC-8, 2000. ¨lo¨p, D. R. E. Valters, F. Fu Korbonits, Adv. Heterocyclic Chem. 1996, 66, 1–71. D. B. Kassel, Chem. Rev. 2001, 101, 255–267. S. Lane in: Separation Methods in Drug Synthesis and Purification, Handbook of Analytical Separations, vol. 1. Valko´, K. (ed.), Elsevier Science B. V., Amsterdam 2000, pp. 160–161. J. M. Ostresh, B. Do¨rner, S. E. Blondelle, R. A. Houghten in: Combinatorial Chemistry – Synthesis and Application. Wilson, S. R.,

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29 Estimation of Stability Czarnik, A. W. (eds), John Wiley and Sons, New York 1997, pp. 225– 240. 24 Stability Testing of New Drug Substances and Products, Q1A, ICH Harmonized Tripartite Guideline, ICH Steering Committee 1993. 25 Stability Testing of New Drug Substances and Products (Revised Guideline), Q1A(R), ICH Harmonized

Tripartite Guideline, ICH Steering Committee 2000. 26 D. Dollimore, Analyt. Chem. 1996, 68, 63R–71R. 27 S. Vyazovkin, C. A. Wight, Annu. Rev. Phys. Chem. 1997, 48, 125–149. 28 F. Darvas, L. Kovaćs in: HighThroughput Screening. Devlin, J. P. (ed.), Marcell Dekker, New York 1997, pp. 223–242.

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Part V

Novel Applications of Combinatorial Chemistry


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30

Concepts of Combinatorial Chemistry in Process Development Markus Eckert and Ulrich Notheis 30.1

Introduction 30.1.1

General

Automation, high-throughput screening, and combinatorial chemistry have already revolutionized biological high-throughput screening and laboratory-scale preparative chemistry. The combinatorial synthesis of large libraries of small drug-like molecules became a standard tool in medicinal chemistry. Over the last few years, these methods have been finding their way ‘‘downstream’’ to larger scale chemistry, catalyst research, and process development. Here, shorter development times and increasing numbers of clinical development candidates have increased the pressure to identify and optimize efficient large-scale syntheses for life science intermediates and products quickly. The principles of combinatorial chemistry seem to be appropriate to fulfill these demands, not only for the synthesis of life science products but also for the process development of basic and bulk chemicals. 30.1.2

Subject of this Chapter

In this chapter, we want to give a description of the possibilities and challenges for combinatorial chemistry in the different phases of process development with the focus on fine chemicals. Therefore, we review some principal issues of process development which have had an influence on the use of combinatorial methods in process development. Furthermore, strategies and equipment for the implementation of parallel screening methods in the laboratory will be discussed.


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30 Concepts of Combinatorial Chemistry in Process Development

30.1.3

Literature

Although the real revolution in high-throughput screening happened only one decade ago, some of the underlying ideas and strategies are as old as the science of chemistry itself. For example, parallelization can be traced back to the sixteenth century, when G. Agricola, in the first book on industrial chemistry in modern times, describes how 700 pairs of pots were placed in an ‘‘array’’ of rows and were used for the preparation of mercury from its ores (see Fig. 30.1) [1].

Fig. 30.1. Parallel working in the sixteenth century [1]. A, The burning hearth; B, the timbers; C, the unlit hearth, into which the pots are put; D, the rocks; E, the rows of the pots; F, the upper pots; G, the lower pots. We thank the Agricola foundation for kind permission to reprint this figure.

30.1 Introduction

The pioneering work on spot test plates was performed in the 1920s and 1930s by F. Feigl [2], who began preparing materials on spot test plates to analyze specific properties afterwards. Impressive examples of fast quantitative analysis of heavy metals are given in the journals of that time [3]. Also, parallel catalyst screening was used by Mittasch in the development of the ammonia process during the first 20 years of the twentieth century [4]. The use of parallel reactors for the testing of heterogeneous catalysts was described in the 1950s [5]. However, parallelization was limited, as, for example, Dowden and Bridger stated in 1957: ‘‘a test apparatus . . . can advantageously be multiplied to give a greater rate of testing, providing the temptation to overtax both observer and supervisor is avoided’’ [6]. Obviously, this limit could not be overcome until the era of computers and process automation. Therefore, just recently, numerous articles and books have been published covering the topic of automated synthesis in process development. The main emphasis has been on chemical process development in the pharmaceutical industry. An excellent review was published in 1999 by Harre et al. describing automated chemistry in organic process research and development [7]. Describing the pioneering work that started during the 1980s, they revealed that most published examples were based on a Zymark robot system. Second, owing to the lack of other commercially available solutions some companies launched programs with inhouse developments such as the DART (development automated reaction toolkit) system of Glaxo Wellcome [8], or the ATLAS (assessing technologies for laboratory automation in synthesis) initiative by SmithKline Beecham [9]. Nowadays, the user has the choice between several commercially available systems with different degrees of automation and features. Harre et al. presented a table with a dozen systems available as at summer 1999 [7]. In mid-2001, the boom for parallelization in process development is continuing, leading to a greater variety of apparatus available on the market (see Tables 30.1 and 30.2). Since 1996, numerous examples of the optimization of organic reactions by parallel process development have covered almost the whole area of organic chemistry. Recommended literature in this respect is by Hird [10], Orita et al. [11, 12], Zhang et al. [13], Gooding et al. [14], Kirchhoff et al. [15], and Emiabata-Smith et al. [8]. In addition, there are two books covering special aspects of parallel process development. The book Automated Synthetic Methods for Specialty Chemicals, edited by William Hoyle, includes a number of papers presented at a Royal Society of Chemistry symposium in 1999 [16]. For process development especially, the contributions by Evens [17] and by Armitage and Smith [18] are relevant, describing their experience with the Anachem SK233 workstation and the HEL AutoMate system. Owen and Dewitt wrote about ‘‘Laboratory automation in chemical development’’ in Process Chemistry in the Pharmaceutical Industry, edited by Gadamasetti [19]. They predict a shift of paradigm, where simply the emulation of manual methods with automation is not enough, but the chemists must develop techniques that are suited to automation, e.g. statistical experimental strategies.

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30.2

Process Development 30.2.1

Overview and Definition

The task of process development is to make a defined product available to a given specification, on a given scale, in a defined time-frame. In life sciences, process development is usually divided into several phases. Frequently, expressions such as ‘‘route scouting,’’ ‘‘process screening,’’ ‘‘process optimization,’’ and ‘‘process characterization and validation’’ are used. Process development is usually carried out by interdisciplinary teams of chemists and chemical engineers, with the emphasis shifted from chemists to engineers as the development proceeds. A schematic representation of the development team’s role in the whole development workflow has been described by Carpenter for the example of an agrochemical product (Fig. 30.2) [20, 21]. In particular, the important input of the route selection step onto process development is elaborated as well as the two targets of process development in agrochemical production: 1 optimizing the process and 2 delivering first quantities for field trials. For basic chemicals, process development is carried out along similar lines, however the ‘‘route scouting,’’ the choice of different starting materials and routes, is usually very limited and much more emphasis is put on process design, engineering, and plant design.

Fig. 30.2. Development process for an agrochemical product [20].

30.2 Process Development

30.2.2

Combinatorial Chemistry in Process Development

In contrast to combinatorial chemistry efforts in drug discovery where hundreds of compounds (libraries of products) are desired, in process development only one molecule is of interest and synthetic ways of producing this molecule are the main focus (‘‘libraries’’ of reaction conditions). Thus, the evaluation of many possible combinations of reaction parameters has to be achieved by parallelization, automation, and miniaturization. Furthermore, split-and-mix approaches cannot be applied in this setting. Proper identification of reaction conditions requires very carefully controlled reaction parameters and the acquisition of many data sets including, for example, the control of nonuniform temperatures and stirring rates. Furthermore, standardized and repetitive runs have to be included to allow proper statistical validation of the reaction yields and conditions. Within the area of process development for basic chemicals, parallelization is mostly used for the development of heterogeneous catalysts and, here, predominately for gas-phase reactions. Catalyst screenings are classified by their degree of parallelization, the complexity, and type of information gained from each experiment. A typical primary screening classifies a large number of solvents or catalysts as ‘‘good’’ or ‘‘bad’’ according to a set of standard conditions, whereas a quaternary screening gives detailed information for a single catalyst and an optimized set of process parameters. These expressions do not correspond to the nomenclature used in the context of process development for fine chemicals. The discussion of combinatorial chemistry in heterogeneous catalysis would go beyond the scope of this article and the reader is referred to the corresponding literature. A current list of examples, where combinatorial methods have been used, can be found in an article by Senkan [22]. Recently, a special issue was devoted to topical developments in combinatorial heterogeneous catalysis [23]. 30.2.3

Demands on Process Development

For our discussion, it may be helpful to understand the special requirements that process development has to meet in the whole business workflow (Fig. 30.3). As already mentioned, one of the main goals of process development is to enable the manufacture of the desired product in the desired quantity to the required specification. But, additionally, often equally important goals include shortening the time to market, and achieving the least expensive manufacturing process. For the production department, robustness and simplicity of the process are the two guiding principles in this endeavor. The process, moreover, has to be environmentally benign and should be the best long-term route. Last, but not least, the synthetic route must be unimpeded by external patents and, in order to strengthen one’s intellectual property position, it should be patentable.

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Fig. 30.3. Business constraints for process development.

In this environment, time is the parameter where parallelization and combinatorial chemistry can help to achieve the desired information faster, and ‘‘valuable information per time’’ can serve as a measure. Another important parameter is the cost of the process development. Increasing the efficiency of the laboratories and thus reducing the overall laboratory costs can be achieved by a rather small investment in parallel process development equipment. However, compared with the benefit of cutting the development time, those laboratory cost savings are marginal. According to the priorities given to the topics presented in Fig. 30.3, at a first glance ‘‘least expensive’’ seems to be most important. This view is only true if all the prerequisites are given: there is no profit at all, if the process is not scalable, if it does not give the product in the demanded analytical specification, or if it is not safe. (This statement has significant impact on considerations in Section 30.3.1.) Additionally, if the solution cannot be worked out in the supposed time frame, mar-

30.2 Process Development

ket shares will be lost. This result can easily exceed the additional costs of a more expensive alternative process. All these associations have to be borne in mind when strategies for parallelization in process development are discussed. 30.2.4

Process Development for Different Applications

Process development is a complex and diverse field because the targets of process development, its strategies, and approaches differ depending on the kind of product and the production scale. These can range from millions of tons of basic chemicals, where an improvement in the reaction selectivity of less than 1% saves several million Euros, to sensitive pharmaceuticals, where world production varies between 100 kg and several tons per year under strict quality requirements (GMP). For any product in between these extremes, an adequate process needs to be developed, taking into account a number of specific basic conditions:

. Proportion of material costs

.

.

.

Any given product can be classified by the ratio of material costs to total production costs. For large-scale chemicals, material costs may exceed 70% of the total cost, whereas for fine chemicals the percentage may be well below 10%. The higher the percentage of the material costs in relation to total costs, and the higher the scale, the more will the process aim at ‘‘selectivity’’ rather than at ‘‘space–time yield.’’ Available budget The process development for the production of bulk chemicals requires a far larger budget than the process development for the production of fine chemicals. This factor influences the size and dedication of the combinatorial screening apparatus. For big projects, it is profitable to design dedicated, process-tailored workstations, whereas for small projects commercially available standard equipment is usually favored. Timescale/milestones The development of a new process for a bulk chemical can take more than 5 years, whereas the first 10 kg of a new pharmaceutical development candidate usually has to be delivered within a few months. If only a limited number of experiments can be performed before the next milestone, the effort to be put into the development of specific parallelization methods has to be considered precisely. Constraints given by existing equipment/plants Since new processes in the area of fine chemicals usually have to be developed in accordance with the existing production facilities, the chemist has to consider whether the reactor where the target process is to be realized is already fixed and known. In general, when developing a process for a dedicated plant, custom-made or specially adapted equipment is recommended, whereas for multipurpose plants a standard (commercial) parallel screening apparatus seems adequate.

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30 Concepts of Combinatorial Chemistry in Process Development Tab. 30.1. Factors which influence the design of parallelized equipment.

Variable/constraint

Basic chemicals

Fine chemicals

Scale of the process Plant

1000–100,000 tons Continuous Dedicated 1–10 years High High Dedicated, tailored Often primary catalyst screening

0.01–1000 tons Batch Multipurpose 10 days to 2 years Low Low Multifunctional, standard Late-stage process screening

Time line Ratio material costs per total costs Budget for process screening Screening apparatus Screening stage, where started

.

Given that in the fine chemical business dedicated plants will only be built for runaway successes, the production of most fine chemicals requires processes with a high degree of simplicity which fit a standard apparatus. Topical stage of development Improving an existing industrial process usually includes minimizing investments and changing as little as possible in the plant’s equipment. Often, process parameters are optimized within narrow margins determined by the already existing plant reactors, which can be changed only at extraordinary costs. In this case, it is not necessary to set up a primary screening for a totally new synthesis. If the goal is to find a new, revolutionary route to a product using new feedstocks or totally different chemistry (‘‘dream reactions’’), the situation is reversed.

All of these factors have to be taken into account when discussing parallelization in process development more precisely. Table 30.1 gives a short summary of the key issues.

30.3

Parallelization in Process Development 30.3.1

Number of Experiments Compared with Scale of Experiment

On the one hand, it is obvious that by carrying out more experiments a higher degree of optimization will be achieved and, thereby, a better process. Here, parallelization can help either by reducing the time for a given number of experiments or by performing more experiments in a given time. On the other hand, the process has to be scaled up: thermochemistry, speed of addition, and work-up (e.g. filtration, precipitation, extraction, or distillation) have to be adjusted to the available apparatus, or a suitable apparatus has to be constructed. Therefore, the reaction is run on different scales, and, if applicable, in dif-

30.3 Parallelization in Process Development

Fig. 30.4. Scaling up and screening intensity.

ferent modes (batch or continuously); typically, it is run in a miniplant and a pilot unit. These two goals – optimization and scaling up – are largely independent of one another (Fig. 30.4)! For example, even thousands of experiments carried out on a milligram scale cannot replace a pilot run. For an efficient process development, the ‘‘hits’’ from primary screening have to be followed up in another screen under more defined conditions and on a larger scale (‘‘secondary screen’’). Since a larger number of hits is expected from the primary screening, the secondary screening also needs a reasonable degree of parallelization (typically 1:5 to 1:100). The same holds for the tertiary screen. Therefore, the typical picture of a pyramid results (Fig. 30.5), with decreasing numbers of reactions in consecutive screens. At the top, one optimal set of parameters is created and verified in a single representative and fully scalable reactor (parallelization 1). For high efficiency in process development, the existing screens have to be combined and experiments have to be performed at the smallest relevant scale. It is crucial to check the results from earlier screens in the more precise later screens as soon as possible and to set up mass and mole balances to detect artifacts stemming from lack of scalability, errors in analysis, or a lack of reproducibility.

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Fig. 30.5. Different phases in process development.

30.3.2

Requirements and Equipment for Parallelization in Different Development Phases 30.3.2.1 Route Scouting

During route scouting, completely different routes to the target molecule are discussed and investigated. For this discussion, the knowledge and experience of process development chemists regarding scalability, safety, and specific plant capabilities are highly valuable. Therefore, the most fertile combinatorial approach lies in a parallelization of several ‘‘biocomputers,’’ called brainstorming. In this context, the Eli Lilly Internet venture InnoCentive recently suggested a new way of global brainstorming [24]. The venture publishes Lilly research problems in organic synthesis on the worldwide web and offers a reward of up to US$100,000 for the best verified route. It will be interesting to see whether this approach finds imitators in other pharmaceutical companies in the near future. For experimental investigation, the more promising routes are selected on the basis of first-shot experiments. Based on these results, the decision as to which route should be further developed has to be made. Sometimes, one could start with two or three routes in an orientating process screening to obtain a better basis for prioritization, and then cancel the worst ones. The equipment for first-shot experiments should be of a simple set-up adaptable to various different reactions. Since this approach overlaps with process-screening equipment, details will be discussed below. In the future, a shift of the ratio between the discussion and the experimental investigation of the route-scouting process is likely. The easier a large number of ‘‘first-shot’’ experiments can be carried out by parallel process development equipment covering more different routes in more detail at a time, the more the speculative discussion part will be replaced by early experimental facts. This consideration has an impact on first-shot experiments in steps where the inherent understanding of the process is low or the parameter space is large. If, for example, in the case of asymmetric hydrogenations a route is skipped on the basis of only a handful of experiments, this approach would be fatal.


In those cases, the first-shot experiments should involve several parallel runs over the chiral ligand database (e.g. 3 96 experiments) to obtain a true estimate of the potential of this route. 30.3.2.2 Process Screening

In process screening some basic knowledge of the reaction already exists. For example, if a certain Lewis acid has been selected in a Friedel–Crafts acylation, then different solvents at different temperatures should be screened. In general, process screening and route scouting proceed at the same scale and parameters are screened with similar equipment, therefore both areas are discussed in one section. What parameters are screened in process screening? A number of reaction conditions are screened in small-volume batch runs. The interesting target parameters are often only conversion and selectivity with an accuracy of G10%. Sometimes, a yes/mediocre/no answer or a relative order (X is better than Y . . .) are sufficient results from the process screening, especially in the early stage. The reaction variables screened are solvents, temperatures, additives, catalysts or other reagents, protecting groups, stoichiometry, reaction time, etc. for each synthetic step. The data acquisition is usually restricted to thin layer chromatography (TLC), gas chromatography (GC), or high-performance liquid chromatography (HPLC) analysis of the reaction mixture, either only at the end-point or in addition to one or two measurements at intermediate run times. What are the requirements for parallel process screening equipment? In this development phase, most chemists use standard apparatus such as 50- to 100 mL flasks with magnetic stirrers. If sufficient starting material is available, batches of 25–50 mmol are preferred. The more sophisticated the intermediates and the more valuable the 2–3 g of ‘‘step 8 material’’ are, the more opportune are microscale reactions: however, the handling of small amounts of substances becomes more difficult when changing from the few grams scale to the few milligrams scale. To transfer a standard flask set-up into a parallel screening set, it is desirable to keep the most common features constant, including working under an inert atmosphere (degas/nitrogen purge), mixing (preferably stirring), cooling/heating, reflux, adding liquids through a dropping funnel, adding solids altogether through an opening under counternitrogen flow, collecting small samples for analysis during the reaction, and no cross-contamination by neighboring cells. Some special features such as distilling a reaction component out of the reaction mixture for equilibration shifting are sometimes necessary. The preferred vessel size is around 20–50 mL, it should not be smaller than 5 mL and not larger than 100 mL. The temperature range should be 50 C to 200 C. Sometimes, systems operate at up to 120–140 C, which is often not high enough. An independent temperature control for each reaction vessel would be the best case, but often this increases apparatus costs exponentially. We regard 6–12 simultaneous reactions per set as a good throughput, especially when the temper-

841

842


ature for all vessels per set is the same. In the case of automated workstations in a dedicated laboratory, a higher throughput can be realized. Examples of commercially available automated workstations For standard chemistry, the full range of automated organic synthesizers can be considered. Most of them are based on an automated dispenser or x, y,z robotics. Within the widely known systems are the Benchmark station from Advanced ChemTech, the Tecan Genesis ChemSystem Workstation, the Chemspeed workstations ASW 1000 and 2000, Zinsser’s Sophas, Bohdan’s Neptune or the Anachem SK 233 (Table 30.2). For the different applications in primary screening and process screening, every user has to make his/her own decision: which reaction volume, temperature range, or mixing form is best suited to screened chemistry? The Anachem SK 233 station is one which has been used for process development purposes to a larger extent recently. In collaboration with Glaxo, a special reflux unit was developed, which is now commercially available as Reactarray SK 233. Preferably for late-stage process screening, the Bohdan process development workstation offers 12 magnetically stirred reaction vessels with independent temperature control per run. The Bohdan process development workstation and the Reactarray SK 233 are both good options for automated late-stage process screening. Recently, HEL has offered a system named Chem-Scan, consisting of a compact x, y,z robot with a maximum of three reaction blocks, each with ten stirred 10-mL reactor cells. The reaction blocks are available either with one temperature control for all cells or with individual temperature control for each cell. (See Fig. 30.6 for examples of apparatus for each process development phase, or visit the web sites of the companies; for internet addresses, see Tables 30.2 and 30.3.) Examples of commercially available low-automated multiple reaction stations Examples of commercially available low-automated multiple reaction stations are given in Table 30.2. The list in this table is not exhaustive, but it gives a diverse selection of appliances for parallel screening, which allow syntheses of 0.5–100 mL at temperatures from 80 C to 250 C. An important criterion is whether or not the system has individual temperature control for each vessel. Equipment without this feature is roughly an order of magnitude less expensive than the apparatus with it. The price categories listed in Table 30.2 refer to the basic set-up in mid2001, but this information is supplied without liability. Most simple reaction stations use test tubes as vessels, e.g. H þ P blocks, Stem blocks, Argonaut FirstMate, or Radleys Carousel. The stirring is mostly based on magnetically driven stirring bars. An interesting alternative is given by Argonaut: the stirring bar is moved vertically by a large pneumatically driven magnetic bar in the middle of the apparatus. To fill the gap between nonautomated and fully automated stations, Argonaut has produced a system called Quest. Originally designed for organic solid-phase synthesis, the Quest 210 (20 10 mL) and the Quest 205 (10 100 mL) are also suitable for process screening. Every user has to make his/her own decision as to whether there are additional requirements for the application under consideration, e.g. an independent inert gas supply for each vessel. The ease of adding liquids and solids during the reac-


Fig. 30.6. Examples of commercially available equipment.

843

24 6 10 6 or 12 48 6 12 20 10

12 24

HEL-Chem-Scan Bohdan, Process Development Workstation Advanced ChemTech, LabMate Calypso Reaction Block System STEM Reaction Block System H þ P, Variomag H þ P, 48 er Block Bohdan MiniBlock Radleys, Carousel Argonaut, Quest 210

Argonaut, Quest 205

Argonaut FirstMate Radleys, GreenHouse Parallel Synthesizer

magnetic, vertical 0.5–3 mL magnetic, vertical

1–60 mL 50 mL 25 mL, magnetic 100 mL, magnetic 10 mL, magnetic 40 mL, shaker 10–15 mL, magnetic 5–10 mL magnetic, vertical 100 mL magnetic, vertical

10 mL 25 mL magnetic

96-well reactors or reactor blocks 10 12

Reactarray Workstation, Anachem SK 233 Tecan, Genesis

ChemSpeed, MSW500, ASW 2000

4.5 mL, shaker 40 mL, shaker 13 mL, shaker 75 mL, shaker 25 mL, magnetic

8, 16, 40 and 96-well reactors 12 48 66 112 28 5 10

Advanced ChemTech, Benchmark Bohdan, Neptune TM , MiniBlock TM

E E

(70)–150 C, no 5–150 C, no

Argotech.com magnetic, vertical Argotech.com Radleys.co.uk

C

(78)–130 C, no RT–160 C, no

A A

peptide.com charybtech.com stemcorp.com hp-lab.de hp-lab.de Bohdan.com Radleys.co.uk Argotech.com

B A A A A A A C

(78)–150 C, no 5–150 C, no (80)–200 C, no (80)–200 C, no (40)–120 C, no RT–160 C, no (40)–130 C, no (2 temp. parallel) (40)–130 C, no (2 temp. parallel)

Helgroup.co.uk Bohdan.com

Tecan.com

Reactarray.com

chemspeed.com

peptide.com bohdan.com

(80)–300 C, no (optional) D E (20)–140 C, yes

D

E E

Price Internet; category a http://www

(70)–150 C, no (40)–120 C, no

Vessel size (mL), agitation Temperature control, individual (yes/no)

Number of simultaneous reactions

Workstations for route scouting and parallel process screening.

Workstationb

Tab. 30.2.

844


b

a

6

Price categories: A¼ 1 L). Between four and 16 independent reactors can be controlled simultaneously from a single computer interface, although the configuration of four reactors is most common. The system consists of a small container that constitutes an oil jacket with a central recess into which a close-fitting miniature reactor is placed. Reactors of different sizes and materials can be used interchangeably with the jacket. The reactor cover has an integral stirrer mounted on it and has ports for a thermocouple, pH probe, heater, reflux condenser, etc. The electrical heater that is placed within the reactor maintains the temperature (isothermal or nonisothermal), enabling the precise and rapid control of the reactor temperature and optionally allows calorimetric data to be acquired using the power compensation technique. Using this information, it is possible to determine reaction endpoints and to establish a first-pass screen for hazard assessment in scale-up. Reactions under elevated pressures, such as hydrogenations, can be carried out with the Auto-MATE using high-pressure reactors. Researchers at HEL demonstrated that the Auto-MATE was useful for process analysis and for refinement and scale-up in conjunction with calorimetric data [29]. To meet the increasing demands being created by the combinatorial revolution in drug discovery and the demanding timelines forced on chemical development departments, Pfizer development laboratories were interested in a flexible automated system that mimics a plant reactor with the capability of controlling reaction conditions very precisely [30]. Since no commercial instruments met their requirements, the custom system was built around a Zymark XP Track Robot especially to accelerate the late-stage process development, i.e. optimization coupled with DOE and validation. The key features of the system include:

. plant reactor mimics; . self-cleaning and draining for continuous operation; . solids dispensing; . accurate liquid handling; . online analysis; . flexible software.

879

880

31 High-Throughput Screening Applied to Process Development

Two plant reactormimics of the system consist of cone-bottomed glass reactors with 50–400 mL capacity equipped with a five-port flange lid incorporated in an automated addition port, overhead stirring, a condenser, a thermocouple, and a spare port for probe technology. The triple-jacketed vessel has a temperature range of 20 C to 150 C, controlled inert gas flow, and automated drain valve for postreaction collection and manipulation if necessary. Throughput limitations of two reactors were resolved by incorporating automated self-draining/self-cleaning modules into the system to allow continuous operation. The system is capable of dispensing solids at any time during the reaction, including seeding for crystallization studies. The software was developed internally in LabView, conforming to the evolving standard of Laboratory Equipment Control Interface Specification (LECIS). The resulting software was extremely flexible, functional, and modular. It was demonstrated that the system performs well when applied to process optimizations and validations that are time-consuming, tedious, and difficult to control when conducted manually. 31.2.3

Miscellaneous

Sumitomo Chemical in Japan has developed a robotic workstation for automated organic synthesis. The system consists of a computer, a robot, and several other devices (including a reaction device, a separation device, dispensing device, etc). The reaction device has four temperature regulation units, each of which can freely change the reaction temperature between 30 C and 160 C. Since each of the four units can handle four reaction containers, 16 reactions under different conditions can be performed simultaneously. Reactions beyond the boiling points of solvents are possible using reflux condensers. The separation-processing device automatically performs solution extraction operations, based on the detected liquid levels and interface positions. The dispensing device can automatically pump various reagents and/or solvents at various speeds into the reaction containers by switching valves through a digital syringe pump. This system can automate all synthesis and analysis processes and can perform at least 3000 experiments per year [31]. Lindsey and coworkers investigated the conditions for condensation of mesitaldehyde and pyrrole to provide tetramesitylporphyrin (27) (TMP) using an automated chemistry workstation (Scheme 31.9) [32]. The automated chemistry workstation consists of a 60-vessel reaction station, a 264-sample vial rack, reagent and work-up reagent racks, a solvent inlet line, a reagent and sample transfer syringe, a washing station for syringes, a robotic arm, and an ultraviolet (UV)-visible absorption spectrophotometer. Each reaction vessel consists of a 10-mL glass vial fitted with a septum cap. The entire station is controlled by thermostats, and a magnetic stirrer individually stirs each vessel. The workstation also has a space for additional analytical instruments. Using this workstation and the proprietary algorithm, catalyst–cocatalyst [BF3 O(Et)2 /alcohol] combinations and concentrations were exam-

31.2 Case Studies

ined in 284 reactions over a 10-week period, yielding 1704 data points. Three efficient cocatalysts were identified and the rate and reactor volume productivity were optimized. The authors concluded that the comprehensive set of data accumulated from the automated experiments establishes the scope of BF3/ethanol cocatalysis in the synthesis of TMP and should be useful for planning syntheses as well as for studying the mechanism(s) of catalysis.

Scheme 31.9. Condensation of mesitaldehyde and pyrrole to

provide tetramesitylporphyrin 27 (TMP) using an automated chemistry workstation.

Otera and coworkers have developed a new type of automated synthesizer with the ability to conduct a variety of synthetic reactions [33]. Although not intended for the process development, their results suggest that the system will be useful for automated process development with some modification. The system consists of a control unit [automatic reaction system (ARS) and a sequencer], a jacketed glass reactor (50 or 130 mL), reservoirs, volumetric ceramic valveless piston pumps, a syringe for quenching the reaction, and a cooling unit. Reactions can be run under inert atmosphere at reaction temperatures from 78 C to elevated temperatures. Using the synthesizer, air-sensitive organolithium and Grignard reagents as well as transition metal catalysts could be handled. Also, the dependence of chemical yields on the reaction temperature for Peterson alkene synthesis and on the addition rates of the aldehyde for aldol reactions were examined. Because the order of reagent addition is programmed and the reaction temperature is quickly tunable, sequential reactions can be conducted smoothly. An advanced control system was

881

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incorporated that allows a task to start immediately after the preceding one has finished, minimizing the time for completing the multistep process. In the current environment of intense market competition, batch process industries stand to benefit from faster process development. Two batch process areas, operating procedure synthesis (OPS) and process hazards analysis (PHA), are timeconsuming because they are often performed manually. Recently, a Purdue University Group developed two intelligent systems, iTOPS and Batch HAZOPExpert (BHE), to automate OPS and PHA. Two applications from the specialty chemical industry are presented to demonstrate the utility of the integrated system [34]. Resolution by forming diastereomeric salts is still an important method for obtaining enantiomerically pure chiral compounds. Generally, tedious trial-and-error experiments are required to identify the satisfactory combination of resolving agents. Researchers at Roche Discovery Welwyn described the use of differential scanning calorimetry (DSC) as a means to identify diastereomeric salts with a clear eutectic composition that is needed for effective resolution and for forming the basis of a resolving agent screening process. This work also included automated salt synthesis using the ACT robot in 96-well microtiter plate format. (ACT refers to the Advanced ChemTech synthesis robot, which is primarily used for solid-phase synthesis.) Automation showed good correlation with the nonautomated experiment, and is therefore suitable for future screening of resolving agents. Rapid data analysis was facilitated using the in-house software package Resolution Companion, which also enabled identification of the optimum crystallization conditions following a trial crystallization experiment. This software package enables (1) the construction of binary-phase diagrams using the Schroeder–van Laar equation, (2) the rapid analysis of data from DSC thermograms, and (3) the construction of ternary-phase diagrams for evaluation of optimal solution concentrations. The authors also point out that the method has some limitations: (1) a failure to crystallize under multiwell evaporation does not imply crystallization will not occur under other conditions, (2) polymorphism, degradation, and signal overlap can complicate DSC analysis, and (3) solvate formation can markedly alter the phase diagram. Nevertheless, the use of DSC to aid in the selection of a resolving agent has been demonstrated and forms the basis of an automated screening procedure [35].

31.3

Summary and Outlook

Implementation of medium- and high-throughput screening for chemical process development is developing rapidly. Several commercial systems have been introduced and publications are beginning to demonstrate the value and return on investment for such systems. The cases cited here demonstrate that medium throughput (ten-at-a-time approach) in conjunction with statistical DOE has taken hold. At the screening/optimization stages, process chemists generally have significant insights into the mechanism of the reaction, thus rendering this approach especially powerful and dramatically accelerating the process development.

References

In the case that an entirely novel process has to be developed, where only a minimal level of chemical knowledge exists, the true combinatorial high-throughput methodology (hundred-at-a-time or thousand-at-a-time approach) needs to be implemented. There will be, however, significant technical hurdles between the tenat-a-time technology and the hundred-at-a-time technology, and it will take considerable efforts to develop these systems [36]. Not only will the close collaboration between process chemists and combinatorial chemists be necessary, but the involvement of engineers and computer scientists will be likewise indispensable for full implementation. Nevertheless, the truly high-throughput systems will fundamentally change the way chemical process development is carried out in the future.

Acknowledgments

The authors wish to thank Dr Michael Cannarsa (Synthetech) for his contribution to the original ideas of this manuscript, Ms Cate Larsen (Symyx Technologies) for useful help, and Ms Silvia Lee (Symyx Technologies) and Ms Kathryn Boykin (XenoPort) for their invaluable support in reference and patent searches. The authors are grateful for Ron Krasnow (Symyx Technologies) for checking the manuscript.

References 1 C. Littlehales, Modern Drug Discovery

8 For design of experiments (DOE), see:

1999, 2, 21. M. J. Cannarsa, T. Uno, C. Larsen, Curr. Opin. Drug Discovery Dev. 2000, 3, 743–749. T. Laird, Org. Process Res. Dev. 2001, 5, 272. For recent reviews concerning combinatorial aspects of materials science and catalysis, see previous chapters or: B. Jandeleit, D. J. Schaefer, T. S. Powers, H. W. Turner, W. H. Weinberg, Angew. Chem. 1999, 111, 2648–2689; Angew. Chem. Int. Ed., 1999, 38, 2495–2532. M. Harre, U. Tilstam, H. Weinmann, Org. Process Res. Dev. 1999, 3, 304–318. M. R. Owen, S. H. Dewitt in: Process Chemistry in the Pharmaceutical Industry. Gadamasetti, K. G. (ed.), Marcel Dekker, New York 1999, pp. 429–455, and references cited therein. N. W. Hird, Drug Discovery Today 1999, 4, 265–274.

a) G. E. Box, W. G. Hunter, J. S. Hunter, Statistics for Experimenters. Wiley, New York 1978; b) D. R. Pilipauskas in: Process Chemistry in the Pharmaceutical Industry. Gadamasetti, K. G. (ed.), Marcel Dekker, New York 1999, pp. 411–428. M. R. Owen, C. Luscombe, L.-W. Lai, S. Godbert, D. L. Crookes, D. Emiabata-Smith, Org. Process Res. Dev. 2001, 5, 308–323. J. N. Cawse, Acc. Chem. Res. 2001, 34, 213–221. D. J. am Ende, P. J. Clifford, D. M. DeAntonis, C. SantaMaria, S. J. Brenek, Org. Process Res. Dev. 1999, 3, 319–329. D. Dorsett in: Proceedings of CombiCat2000. Lisbon, Portugal, October 2000. The Catalyst Group, Inc., 714 N. Bethlehem Pike Spring House, PA 19477, USA T. S. Powers, T. Uno in: Proceedings of the 10th Conference on

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Combinatorial Chemistry, Osaka, Japan, April 2000. Japan Combinatorial Chemistry Focus Group (JCCF), The Society of Synthetic Organic Chemistry, Japan, Kanda-Surugadai 1-5 Chiyoda-ku, Tokyo 101-0062, Japan. a) A. Guram, A. Hagemeyer, B. Jandeleit, D. M. Poojary, H. Turner, W. H. Weinberg in: International Congress on Catalysis 12th: 2000: Granada, Spain, 1st ed. (Amsterdam, Elsevier, 2000); b) P. Desrosiers, A. Guram, A. Hagemeyer, B. Jandeleit, D. M. Poojary, H. Turner, H. Weinberg, Catalysis Today 2001, 2448, 1–6. P. Desrosiers, B. Jandeleit, D. M. Lowe, T. Uno, A. Vimercati, A. Volpe Jr, T. Weskamp, J. Zhang in: Book of Abstracts by the Division of Catalysis and Fine Chemicals of the Catalysis Society of Japan, Tokyo, Japan. International Symposium on Catalysis and Fine Chemicals. Tokyo, Japan, March, 2001. D. F. Emiabata-Smith, D. L. Crookes, M. R. Owen, Org. Process Res. Dev. 1999, 3, 281–288. M. A. Armitage, G. E. Smith, K. T. Veal, Org. Process Res. Dev. 1999, 3, 189–195. M. J. Cannarsa, Chim. Oggi 2000, 18, 22–24. M. Owen, Automation in Process R&D. Boston, MA, 2000. Scientific Update, Maycraft Place Stone Cross Mayfield East-Sussex TN 20 6EW, UK. G. S. Proehl in: Proceedings of The 3rd International Conference on Organic Process Research and Development, Montreal, Canada, 2000. Scientific Update Maycraft Place Stone Cross Mayfield East-Sussex TN 20 6EW, UK. M. Harre, H. Neh, C. Schulz, U. Tilstam, T. Wessa, H. Weinmann,

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Combinatorial Methods in Catalysis Bill Archibald, Oliver Bru¨mmer, Martin Devenney, Sasha Gorer, Bernd Jandeleit, Tetsuo Uno, W. Henry Weinberg, and Thomas Weskamp 32.1

Introduction 32.1.1

Combinatorial Catalysis

The conventional process used to search for new materials in important areas such as biopharmaceuticals, materials science, and catalysis has traditionally been long and expensive, relying on a large number of iterative steps to produce new or optimized materials. Increasing competition in the chemical industry demands faster and more efficient identification and development of improved materials, including catalysts. This drive toward increased research productivity was first encountered in the pharmaceutical industry, where long development times and high research costs necessitated the introduction of new research and development (R&D) approaches designed to accelerate the drug discovery process. One approach intended to speed up the discovery process is to develop high-throughput methods for rapidly synthesizing and screening large numbers of diverse chemical compounds, searching for trends in desired parameters. Virtually every major drug manufacturer now utilizes this ‘‘new’’ research technology, called ‘‘combinatorial chemistry,’’ as an integral part of its research and development program [1–10]. The combinatorial process involves the design and synthesis of discovery libraries aimed at efficiently exploring large numbers of structurally or compositionally diverse compounds thought to be of interest as a result of a review of their chemical, biological, physical, and/or structural properties. Rapid, sensitive measurements of one or more relevant chemical or physical properties of each library member result in the identification of a family of lead compounds with a desired property. These compounds are then optimized by continuously varying the stoichiometries or structures of a more focused set of precursors in a focus library. Compounds with optimum compositions are then synthesized in quantities sufficient for detailed characterization and evaluation. Finally, with methods of combinatorial chemical Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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synthesis evolving together with rapid analysis and high-throughput screening, data management is becoming challenging and requires appropriate database technology. The biopharmaceutical industry has levered development of combinatorial tools from the broad availability of low-cost, high-performance computers, robotics, molecular modeling, database software tools, and tools created for the clinical diagnostics, optics, and semiconductor industries. Adoption of combinatorial methods has also created novel research areas such as bio- and chemoinformatics to manage and mine the large amounts of structural and functional information obtained from combinatorial investigations [11]. Application of combinatorial methods to nonbiological materials and catalyst development is still in its infancy. Work initiated by Schultz and coworkers in 1995 has initiated tremendous efforts in this field [12], although the concepts and their experimental realizations can be traced back to Hanak in the early 1970s [13]. The combinatorial paradigm, however, is beginning to be explored increasingly in industrial and academic laboratories around the world. Combinatorial methods have been applied to optical and electronic materials, magnets, polymers, and catalysts for commodity, specialty, and fine chemical applications. Traditional materials design and screening approaches, which are often based on an a priori mechanistic approach, typically only lead to structures that were known or expected by the chemist. This applies even more so to catalysis, where knowledge of the underlying mechanism of the catalytic reaction often drastically limits the number of probable metal/ligand combinations. Combinatorial catalysis methods allow for both the probable and improbable metal/ligand combinations to be investigated. Furthermore, combinatorial catalysis is rapidly taking advantage of the numerous solidand solution-phase synthetic methodologies that exist, including polymer-supported reagents [14–18]. In general, three different approaches exist for preparing and testing libraries of compounds (Fig. 32.1). Conventional research, performed in a ‘‘one-at-a-time’’ or serial fashion, provides thorough quality control over samples entering the screening process. Combinatorial methods that involve, for example, ‘‘split-and-pool’’ synthesis are much faster and make the preparation of relatively large numbers of compounds feasible; however, compounds entering the screening equipment often lack phase purity. Additional methods that are intermediate to the two extremes outlined above are based on the parallel synthesis of compounds in a spatially addressable format with usually one composition per site, which are then coupled to automated screens. This allows multidimensional problems in the discovery and optimization of catalysts to be efficiently addressed by coupling parallel synthesis with automated or high-throughput screens. A SciFinder= reference search performed in February 2001 using the key word ‘‘combinatorial’’ resulted in more than 9000 ‘‘hits.’’ An analysis of the literature search reveals a tremendous growth in scientific publications and patent applications in the combinatorial field. Most contributions deal with combinatorial applications to pharmaceutical, biological, and medical disciplines; however, significant advances in the development of combinatorial approaches to the discovery and

32.1 Introduction

Fig. 32.1. Conventional, parallel array, and combinatorial approaches to catalyst synthesis and screening.

optimization of new materials and catalysts has occurred in recent years. An increasing number of review articles have been written that address new methods in combinatorial chemistry and high-throughput screening for chemical process development [19] and, of course, ‘‘combinatorial catalysis’’ [20]. This chapter summarizes applications of combinatorial methodologies to the discovery and optimization of new catalysts published in the scientific literature between 1995 and August 2001; patents, patent applications, and conference proceedings have only been included if they are of the utmost importance. After this introductory section, the following section introduces combinatorial approaches to novel metal binders, combinatorially functionalized polymers, and combinatorially discovered metal complexes as enzyme mimetics. Section 32.3 discusses combinatorial catalysis in asymmetric synthesis. Section 32.4 describes examples of multidimensional screening in combinatorial catalysis. Section 32.5 summarizes recent efforts in one-pot multisubstrate screening approaches, while Section 32.6 summarizes some applications of combinatorial methods in the search for novel polymerization catalysts. Section 32.7 describes combinatorial inorganic homogeneous catalysis, and Section 32.8 deals with combinatorial approaches in heterogeneous catalysis. Section 32.9 emphasizes combinatorial electrocatalysis. Novel high-throughput screening tools are discussed in Section 32.10. Section 32.11 includes a summary and an outlook concerning the future of combinatorial catalysis science. 32.1.2

Combinatorial Organic and Organometallic Catalysis

In homogeneous catalysis [21], the catalytically active species is in most cases a metal ion stabilized by one or more ligating or chelating organic moieties. The relationship of ligand structure to the chemical and physical properties of derived metal complexes is a central theme in numerous fields such as selective catalysis, sensor discovery, and bioorganic chemistry. In an extremely challenging rational

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approach, the identification of metal–ligand complexes with new physical or chemical properties such as well-designed structural, electronic, and/or stereochemical features typically involves the synthesis of a small number of appropriate ligand derivatives, and, subsequently, the individual analysis of their metal-binding properties. This ‘‘empirical approach’’ often proves to be very labor intensive and not economical since the potential for optimization is limited by resources and time. In this context, a systematic methodology for the expedient generation of new classes of coordination complexes would clearly be of great value. The synthesis of a library of ligands for the discovery of new or improved catalysts may be carried out using combinatorial organic synthesis methodologies to overcome a limited ligand supply, and may subsequently be followed by metal complexation and screening for catalytic activity in a targeted chemical transformation.

32.2

Metal-binding Ligands, Polymeric Enzyme Mimetics, and Metal Complexes as Enzyme Mimetics

The application of combinatorial methodologies to the synthesis of metal-binding agents may be considered as one of the starting points in the development of organic and organometallic catalysts by combinatorial methods. Since combinatorial methods have their origin in the field of peptide library synthesis, it is not surprising that libraries of peptide-based compounds have been studied to identify new sequences with high affinity for transition metals. A traditional approach to new metal-binding agents is based on core receptors that have already established binding properties and the modification of their substitution patterns to induce changes in binding strength and selectivity. In their efforts to identify novel ligand structures, researchers may as well take advantage of constructing building blocks that incorporate metal ion coordinating functional groups and link them in a manner such that selective metal binding may result. 32.2.1

Combinatorial Approaches to Metal-binding Ligands 32.2.1.1 Combinatorial Functionalization of Metal-binding Core Structures

Cyclen, a cyclic tetramine, its derivatives, and several other related macrocylic ligands possess affinity to metal ions and have been widely investigated and used as synthetic transmembrane ionophores and fluorescent sensors [22, 23]. Still and coworkers synthesized libraries of up to 10 5 peptide-modified macrocyclic cyclens [24]. After attaching one of the ring nitrogen atoms of the cyclen core to a poly(ethylene)glycol-grafted polystyrene resin (PEG-PS-resin), the remaining three secondary nitrogen atoms of the cyclen scaffold were then functionalized using a ‘‘split-and-pool’’ protocol. Employing fluorenylmethoxycarbonyl (Fmoc) solid-phase peptide synthesis methodology, peptidic side arms derived from a pool of 19 sidechain-protected amino acids (AA)n , were linked via aminoethyl spacers. Four dif-

32.2 Metal-binding Ligands, Polymeric Enzyme Mimetics, and Metal Complexes as Enzyme Mimetics

Fig. 32.2. Libraries of peptidic cyclen-based metal Cu(II)binding agents synthesized on solid support. The numbers in parenthesis represent the appropriate library size. AAi ¼ amino acids.

ferent cyclen libraries were synthesized, differing in both length and nature of the peptidic side arms (Fig. 32.2). The side-chain-protected and -deprotected resins were agitated with diluted aqueous Cu(II) and Co(II) ion solutions. Selective metal binding was screened visually in a pooled assay identifying binding to Cu(II) and Co(II) by characteristic bright blue and red colors. Deconvolution of the chemically encoded polymer beads allowed the identification of ligand structures that were responsible for the selective binding of the metal ions [25]. The peptidic sequences generally differed for Cu(II) and Co(II) ions, and the length and chemical nature of the peptidic appendages in both protected and unprotected amino acid forms significantly influenced the affinity of the new peptidic functionalized tetraamines for Cu(II) and Co(II) ions relative to the corresponding unfunctionalized or alkylated cyclen core. In a related combinatorial procedure, the development of fluorescent sensors for nanomolar aqueous copper was reported by Sames and coworkers [26]. A library of ionophoric ligands was generated based on three different N-containing macrocyclic or tridentate scaffold using a split-and-pool strategy and an orthogonal tertbutyloxycarbonyl (Boc)/Fmoc protecting group protocol. The library was assayed visually by incubating the solid support-bound ligands with aqueous Cu(II) solution followed by reaction with a copper-selective staining reagent (blue color). All identified ionophores contained a carboxylic group as well as an aromatic nitrogen heterocycle. A pyrazine-containing ligand was selected as a lead structure for further development of a fluorescent sensor bearing a covalently linked dansyl fluorophore (Fig. 32.3).

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Fig. 32.3. Pyrazine-containing lead structure obtained from visual primary screening of the solid-phase-bound ionophore library. Structure of the identified sensitive and selective copper ion chemosensor bearing a covalently linked dansyl fluorophore.

After covalent attachment of the fluorescent sensor to polymeric microspheres (3.1 mm), the microsphere sensors were examined in the presence of 50 nM buffered CuCl2 solution using fluorescent optical microscopy equipped with a custom fiber optic [27]. Affinity (K d ¼ 10 nM) and binding selectivity (Cu > Co > Ni > Fe) were assessed by fluorescence quenching and competition experiments. The X-ray structure of metalloprotein recombinant isopenicillin N synthase (IPNS), which is involved in the biosynthesis of isopenicillin N, has served as a lead structure for library design. IPNS possesses at its active site two iron-binding peptides, IleaHisaArg and TrpaHisaGluaAspaVal, which are linked by a short peptidic sequence [28]. Hoping to discover potentially new catalysts, Baldwin, Wood, and coworkers combinatorially explored various versions of resin-bound peptide libraries of the general structure resinaArgaHisaIlea(AA i )n aValaAspaGluaHisa TrpaAc (AA i ¼ amino acid; n ¼ 1–3), and examined their metal-binding abilities (Fig. 32.4) [29]. Three different peptide libraries with a total of 7240 individual oligomers, each differing in length and amino acid content, were synthesized using Fmoc peptidecoupling chemistry in a split-and-pool protocol. Up to three variable amino acids were incorporated in between the conserved peptide sequences IleaHisaArg and ValaAspaGluaHisaAcTrp. Selective ion binding was assayed by agitating the resin beads with aqueous solutions of Cu(II), Fe(II), and Co(II) sources. Significant metal binding was accompanied by a color change of the beads. Co(II), for which the color changed to pink/purple, bound most selectively. No encoding/deconvolution strategy was implemented in order to characterize the ligand structure that gave


Fig. 32.4. A peptide based on isopenicillin N-synthase. Schematic diagram of the active site. The wavy lines represent the remainder of the protein. A peptidic library designed to mimic the metal-binding region. n ¼ 0–3; AA ¼ amino acid; Ac ¼ acetyl.

rise to colored metal complexes from the pooled assay, and catalytic activity of the new metal complexes in the ring closure of the Arnstein tripeptide (ACV) to isopenicilline N was not addressed. Metalloproteins, synthetically or biosynthetically appended to proteins or other biomolecules, are finding increasing utility in the biochemical analysis of noncovalent protein–nucleic acid and protein–protein interactions. The amino terminal Cu(II)- or Ni(II)-binding (ATCUN) motif is a structural feature of several naturally occurring proteins such as certain types of albumins (e.g. human serum albumin (HSA), bovine serum albumin (BSA), and rabbit serum albumin (RSA) and neuromedins C and K, among several others [30]. Generally, this domain is described as Ni(II)H2 NaAA1 aAA2 aHis. Long and coworkers employed a ‘‘positional scanning’’ combinatorial protocol to optimize the desoxyribose-based cleavage of B-form DNA by Ni(II)H2 NaAA1 aAA2 aHis metallopeptides [31]. Using a standard Boc protocol and ‘‘split-and-mix’’ technique on methylbenzhydrylamine (mBHA) resin, two libraries were generated from a selection of l-amino acids in which the first (AA1 ) and the second position (AA2 ) of the peptide ligand were varied within the H2 NaAA1 aAA2 aHis sequence. The libraries were assayed after cleavage from the solid support for increased direct DNA cleavage relative to Ni(II)H2 NaGlyaGlyaHis after incubation with a Ni(II) source and oxidative activation of the metal complex with KHSO5 or magnesium monoperoxophthalate (MMPP). Increased catalytic activity was found when the amino-terminal peptide position contained a hydrophobic amino acid and the second peptide position contained an ionic or polar amino acid. The optimized and resynthesized metallotripeptide domain Ni(II)HNaProaLysaHis was found to oxidatively cleave DNA

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an order of magnitude faster than the reference Ni(II)H2 NaGlyaGlyaHis. DNAbinding affinity was slightly increased relative to Ni(II)H2 NaGlyaGlyaHis, but metal complexation and the A/T-rich site DNA-binding selectivity were not altered. Another approach to generate large libraries of metal-binding proteins is to take advantage of the immune system. Janda and coworkers identified antibodies that bind unique metals with excellent affinity by combining a metallo-panning agent with the high-throughput screening of a combinatorial antibody library [32]. The single-chain antibody (scFv) library was constructed from the blood of 50 healthy volunteers, the resulting phage scFv antibody library was estimated to be 1 10 9 in diversity. This library was then screened against three metal pool mixtures and an immobilized phosphorodithioate metallo-panning reagent that binds metals, leaving them coordinatively unsaturated. Two single-chain antibodies were identified and isolated that bind lanthanum and yttrium in the mM range. Inductively coupled plasma mass spectroscopy analysis suggests that the selected antibodies contain a single metal ion binding site. 32.2.1.2 Combinatorial Synthesis of Metal-binding Ligands from Building Blocks

with Metal-coordinating Functionalities In this approach, selective metal-binding ligand libraries are synthesized de novo by incorporating building blocks possessing potential metal-coordinating functionalities. Jacobsen and his group reported the synthesis of a 12,000-member library using ‘‘split-and-pool’’ methodology and a chemical-encoding protocol [33]. The study aimed to discover highly selective metal-binding agents directly on a solid support. The general ligand structure comprised four variable modules (Fig. 32.5). It was hoped that the stereochemically defined building blocks were subject to conformational restrictions facilitating the formation of potential binding sites in which both amino acid side-chains interact with metal. The resin-bound ligands were screened in a ‘‘pooled’’ assay by agitation in meth-

Fig. 32.5. General structure of a turn-element-containing

metal-binding agent. Specific examples of Ni(II)-binding ligands synthesized and screened on solid support. AAi ¼ amino acid; Trt ¼ trityl.


anolic solutions of Ni(II) and Fe(III) salts. After staining with the appropriate indicators (dimethyglyoxime (DMG) in the case of Ni(II) or KSCN in the case of Fe(III)), beads containing high concentrations of Ni(II) or Fe(III) were visually identified by their red or orange–red color, respectively, using a light microscope. The chemical tags on the selected beads were cleaved from the solid support, allowing deconvolution of the chemical synthesis history of an individual selective metal-binding ligand. Several new ligand structures were disclosed that selectively bind Ni(II) or Fe(III), each of which consisted of histidine residues, and, in most cases, a set of two turning elements and/or terminating caps. Technetium-99m complexed to organic compounds, proteins, peptides, or antibodies plays an important role in the radioimmunodetection (RIAD) of organs and in tumor imaging because of its low cost, ideal physical properties, and broad availability. Schneider-Mergener and his group synthesized and screened a cellulosebound hexapeptide combinatorial library for the identification of technetium-99m ( 99m Tc)-binding peptides [34]. A combinatorial library of approximately 8000 hexapeptides of the general structure B1 aXaB2 aXaB3 aX was synthesized using automated synthesis on a cellulose platform (‘‘spot synthesis’’) yielding the hexapeptide library in a spatially addressable format. The amino acids Bi ði ¼ 1–3) were positioned on the defined positions 1, 3, and 5 within the hexapeptide sequence and each was one of the 20 naturally occurring l-amino acids. The amino acids Xi in positions 2, 4, and 6 were ‘‘systematically randomized.’’ After incubation with an aqueous pertechnate solution as a 99m Tc source, the library was screened for 99m Tc affinity by phosphorimaging. Besides a variety of cysteine-containing hexapeptides, known for their strong complexing capability toward 99m Tc, peptidic ligands containing the amino acids His, Lys, Arg, and Met showed significant affinity toward the radioisotope. In subsequent focused libraries, the three X-positions were defined by ‘‘positional scanning’’ and revealed the noncysteine-containing primary sequences LysaGlyaHisaSeraHisaVal and LysaAlaaMetaTyraHisaGly as superior 99m Tc ligands. Pirrung and Park disclosed the discovery of selective metal-binding peptoids using 19 F-encoded combinatorial libraries [35]. A method for encoding solid-phase split-and-mix combinatorial libraries was developed taking advantage of the large chemical shift dispersion of synthetic fluoroarenes (F-tags). Nine fluoroarenes ( 19 Ftags) bearing linkers for attachment to solid support through a photocleavable linker (Holmes photolinker) were prepared [36]. A 90-member library of N-alkylglycines bearing substituted succinamides was prepared on solid support from a set of nine amines, in which the amine was encoded by the fluorinated tag, and ten anhydrides (Fig. 32.6). Bead pools were visually assayed for metal binding by treatment with a Cu(II) ion source (blue color). Selection of ten blue beads was followed by irradiation to release the tag. Sensitive 19 F-nuclear magnetic resonance (NMR) was used for decoding. Two compounds, obtained from solution-phase synthesis, showed high affinity either for copper(II) (CuOTf2 ) (K d ¼ 44 mM), or iron(III) [Fe(2-ethylhexanoate)3 ] (K d ¼ 31 mM). Walt and coworkers reported the solution-phase synthesis and characterization of metal-binding indicators with diverse optical responses on exposure to various

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Fig. 32.6. Split-and-mix solid-phase synthesis of a N-

alkylglycine library for the discovery of selective metal binding. Specific examples of Cu(II)- and Fe(III)-binding ligands synthesized in solution.

heavy metal ions [37]. A combinatorial approach, based on azo coupling with subsets of either phenolic compounds or aromatic amines, generated a library of azo dyes. Each reaction mixture containing the product(s) of the azo coupling was incubated with a series of a heavy metal ion sources. Physical parameters such as absorbance and fluorescence spectra of the resulting complexes were measured. Of the azo dyes prepared, terdentate dyes were particularly useful, providing distinct spectral responses to three or more metal ions. Metal-binding agents that are immobilized on appropriate inorganic supports are important in environmental chemistry, in sensing chemistry, and in biochemistry. Bergbreiter and coworkers prepared a library of surface-bound metalcomplexing hydroxamic acids on silica-coated glass plates as selective and reversible metal sensors [38]. Aminopropylated silica was reacted with a set of 12 dicarboxylic acid derivatives as acylation agents to modify the aminopropylated surface into a carboxylic acid-containing surface. Subsequent attachment of three different hydroxylamines furnished the immobilized hydroxamic acids (Fig. 32.7). Binding of the hydroxamic acids to Fe(III) ions resulted in a reddish coloration which was assayed qualitatively or semiquantitatively by either visual inspection or using diffuse reflection spectrometry, respectively. The highest yields in the formation of an Fe(III) complex were obtained when glutaric anhydride or maleic anhydride was used as an acylation agent. The results were confirmed on bulk samples.


Fig. 32.7. Schematic representation of metal-binding hydroxamic acids immobilized on silica support.

32.2.1.3 Miscellaneous

Besides the potential utility of a particular class of ligands for a particular catalytic application, several features must be considered in the selection of ligand libraries. Reactions that use more than two different starting materials are called multicomponent reactions (MCRs) [39]. In the light of chemical productivity and generation of molecular diversity, an ideal MCR should comprise more than two components. High atomic economy (ideally the incorporation of all of the atoms that build the starting materials into the final product) is an asset [40], and, furthermore, ligand synthesis and purification procedures must be readily amenable to parallel synthesis techniques. With respect to their productivity, yield, conversion, and facile execution, MCRs occupy an outstanding position among other reactions – making them especially interesting for the concept of combinatorial chemistry. Attempts are now being made to discover new multicomponent reactions by means of combinatorial technology [39]. In this context, Lapointe reported the parallel synthesis of substituted aminomethylphosphines of the general structure R1 R 2 PCHR 3 NR 4 R 5 in 1999 [41]. By combining two secondary phosphines (R1 R 2 PH), a subset of six substituted aldehydes (R 3 CHO), and eight secondary amines (HNR 4 R 5 ) in a facile Mannich-type three-component condensation reaction, a 96-member library of substituted aminomethylphosphines was synthesized (Scheme 32.1). According to their backbonebuilding atoms, these ligands were termed PCN ligands. The relatively mild reaction conditions [tetrahydrofuran (THF), room temperature], the large number of substructures available, and the lack of protection/deprotection steps for the phosphine moiety made this protocol an attractive target for parallel synthesis. The delivery of the reagents and the mixing were performed by an automated liquid dispenser. The crude condensation products were of sufficient purity (@ 95% by 1 H- and 31 P-NMR spectroscopy) for their direct use as ligands. The parallel synthesis of libraries of metal complexes of these PCN ligands and their use as ethylene polymerization catalysts, for example, is currently being investigated [42]. This example is one of the first reports about parallel syntheses of organophosphines, and, owing to the broad availability of the appropriate building blocks, large numbers of diverse functionalized PCN ligands are accessible.

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Scheme 32.1. Parallel synthesis of aminomethylphosphine ligands.

Bidentate phosphorus-containing ligands play a major role in homogeneous catalysis [21]. Therefore, Li and coworkers at the DuPont Company have developed a facile and rapid polymer-supported synthesis of novel diphosphane, phosphane– phosphinite, phosphane–thiophosphane, and phosphane–chlorophosphane ligands to access a variety of different molecular architectures (Scheme 32.2) [43]. The synthesis starts by treating Merrifield’s resin with excess tert-butylamine to give the polymer-supported secondary amine A which was then reacted with 1,2-

Scheme 32.2. Polymer-supported synthesis of P/P ligands.


bis-(dichlorophosphanyl)ethane in the presence of triethylamine. The resulting precursor B was coupled with a variety of Grignards, organolithium reagents, and metal alkoxides, resulting in the complete substitution of all three PaCl bonds to give C. Subsequent cleavage of the PaP ligands from the resin proceeded selectively with PCl3 and with a variety of alcohols and thioalcohols to produce the bidentate phosphorus-containing ligands D and E in good yields (26–75%). In all 15 cases the crude cleaved products were obtained with a high degree of purity (b 95%). Functionalized 2,2 0 -bipyridines constitute an interesting class of ligands that form coordination complexes with a variety of transition metals. Gallop and coworkers developed a versatile solid-phase adaptation of the classical Hantzsch threecomponent condensation reaction for the synthesis of highly functionalized bipyridines (Scheme 32.3) [44]. In one of their approaches, Knoevenagel condensation of polymer-supported picolinoyl acetate with aromatic aldehydes yielded stable arylidene intermediates. Subsequent Hantzsch heterocyclization with enamino esters provided resin-bound 1,4-dihydro-2,2 0 -bipyridines that were oxidized with ceric ammonium nitrate and cleaved from the support with trifluoroacetic acid to give 2,2 0 bipyridine carboxylates. In an additional approach, different classes of b-ketoesters were immobilized and the pyridine moiety was introduced via the enamino ester building block. This methodology was applied to the generation of a library of 500 bipyridines by split-and-pool synthesis from five b-ketoesters, ten aldehydes, and ten electron-deficient enamines. These compounds were also exploited for their coordination capacity toward Pt(II).

Scheme 32.3. Solid-phase synthesis of 2,2 0 -bipyridines via Hantzsch condensation.

Wang and Schwabacher reported the synthesis of five pyridine-containing bidentate ligand building blocks with nucleophilic groups at different positions (Fig. 32.8) [45]. Solid-phase alkylation of these building blocks resulted in the construc-

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Fig. 32.8. Library of dipeptides each bearing two bidentate pyridine ligands.

tion of a small library of 25 dipeptides, each bearing two bidentate ligands. These ligands can serve as building blocks for the construction of disparate metal-binding molecules. Schmalz and coworkers reported a modular approach to a new class of structurally diverse bidentate P/N, P/P, P/S, and P/Se chelate ligands for transition metal catalysis [46]. Bis-protected bromohydroquinones that are accessible from hydrochinone were used as the central building blocks (Scheme 32.4). The first donor functionality (L 1 ) is either introduced by Pd-catalyzed amination, by Suzuki crosscoupling, or by lithiation and subsequent treatment with electrophiles (e.g. chlorophosphanes, disulfides, or carbamoyl chlorides). In the case of the Suzuki reaction, the bromohydroquinones were first transformed into the corresponding boronic acid via lithiation and reaction with triisopropylborate and then reacted with various heteroaryl bromides. After selective deprotection, the second ligand tooth is introduced via reaction of the phenolic OH moiety with a chlorophosphane, a chlorophosphite, or a related reagent. Some of the resulting ligands were converted into the corresponding PdCl2 complexes that were characterized by X-ray crystallography.


Scheme 32.4. Modular approach to structurally diverse

bidentate chelate ligands for transition metal catalysis.

The aforementioned contributions exemplify the successful implementation of combinatorial methods such as parallel synthesis on solid supports or in solution, ‘‘spot synthesis,’’ ‘‘split-and-mix’’ methodology, and encoding/deconvolution techniques for the identification of potential ligands and their metal organic complexes. Although the screening tools were in most cases relatively simple, the high grade of parallelism and the simplicity of these approaches allowed efficient and rapid screening of literally thousands of ligands. 32.2.2

Combinatorial Libraries of Polymeric Catalysts as Enzyme Mimetics

Enzymes have found widespread use in industry in a multitude of chemical reactions because of their often perfect selectivities and high activities under relatively mild reaction conditions [47, 48]. For a multitude of the most different reasons, chemists are obviously stimulated to rationally design and synthesize numerous classes of low- and high-molecular-weight synthetic enzyme catalysts (‘‘synzymes’’) that mimic, improve, substitute, or exceed a natural enzymatic function or structure. However, most of them have fallen short of the goal of efficiently reproducing enzymatic catalysis, but, nonetheless, these studies have brought more insights into enzymatic reactions [49]. Immobilized (polymer supported) catalysts are of increasing importance in synthetic organic chemistry and library synthesis. In this scenario, established (metal) organic catalysts are anchored to a solid support to facilitate product work-up, separation, isolation, and catalyst recycling. However, the scope and limitations of these catalysts are reviewed elsewhere [50]. A novel approach to identify enzyme-like new organic catalysts is by searching through a multitude of ‘‘randomly’’ generated polymeric systems [51]. Random

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Fig. 32.9. Examples of a combinatorially functionalized

polymeric catalysts based on poly(allylamine) (PAA) or poly(ethyleneimine)(PEI) facilitating the hydrolysis of phosphate esters or the reduction of a-keto carboxylates.

compounds are meant to be combinatorially functionalized polymers. Assuming a proper juxtaposition of multiple functional, catalytically active groups within the polymer, the groups of Menger and Ford developed synthetic methodology for combinatorially functionalized polymers hoping to effect catalysis of several target reactions. One of the first applications of this concept in combinatorial catalysis was the development of phosphatase-like catalysts [52], or a combinatorially developed polymeric reducing agent (Fig. 32.9) [53]. In these studies, commercially available poly(allylamine) (PAA) or poly(ethyleneimine) (PEI) was functionalized with various combinations and proportions of small diverse sets (three, four, and eight) of carboxylic acids to afford randomly functionalized polymers bearing variably substituted amide groups ranging in concentration from 5% to up to 60% and leaving


some amino functions unreacted. Automation facilitated the synthesis of up to 8198 combinatorial variations of the polymeric reducing agents. To introduce redoxactive capabilities into the reducing polymer catalysts, 1,4-dihydropyridine (DHP), known from NADH models to reduce activated ketones to alcohols, was incorporated. Depending on the kind of reaction to be catalyzed, metal ions such as Mg(II), Zn(II), and Fe(III) [52], or Cu(II), Mg(II), and Zn(II) ions were added [53]. In each case, the polymeric mixtures were then screened spectrophotometrically either in a one-at-a-time fashion [52], or in a parallel 96-at-a-time microtiter plate format [53]. Analysis of initial hydrolysis rates of the bis-( p-nitrophenyl)phosphate test substrate concluded some composition–activity relationships (degree and nature of carboxylic acid content with respect to catalytic activity). Significant rate accelerations (kcat =kuncat ) of up to 3 10 4 (exceeding that of a catalytic antibody for the same reaction) [54] were observed for a specific polymer composition in the presence of Fe(III) ions [52]. About 92% of the polymeric reducing agents were found to be catalytically inactive in the reduction of benzoylformate (a-keto carboxylate) to the mandelate (ahydroxy carboxylate) as the test reaction. Among others, two of the most active polymer compositions could be identified, and, for example, an active PEI-based polymer had a functional composition of 5% dihydropyridine, 2.5% 2-imidazolacetic acid, 15% 2-naphthyl carboxylic acid, 2.5% 3-mercaptopropionic acid, and 5% Zn(II) [53]. A library of 1344 polymeric variations with an amide bond functionalization ranging up to 20% was obtained by aminolysis of poly(acrylic anhydride) and sets of mixtures of three to four amines taken from a library of 11 amines and using an instrumentational set-up for library synthesis similar to that described above (Fig. 32.10) [55]. Assuming that these polymeric catalysts possessing both acid and basic functional groups in proper spatial orientation would likely catalyze the dehydration of

Fig. 32.10. Example of a combinatorially functionalized polymeric catalyst based on poly(acrylic anhydride) facilitating the dehydration of b-hydroxyketones.

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Fig. 32.11. Representative structural element of a hydrolytically

active trimethylammonium (TMA) functionalized polymer.

b-hydroxy ketones, the researchers spectrometrically monitored the appearance of the b-aryl-a,b-unsaturated carbonyl chromophore of the product. About 1% of the polymeric catalysts revealed a significant rate enhancement ðkcat =kuncat ¼ 920Þ. For comparison, an antibody-catalyzed dehydration of the test substrate afforded a catalytic rate enhancement of 1200 above background [56]. Several other interesting features could be assessed rapidly such as the observation of a ‘‘nonbiological induced fit,’’ i.e. a substrate-induced transformation into a catalytically active conformation upon variation of the reaction parameters such as temperature and pH. Ion exchange latexes, especially those bearing quaternary ammonium anion exchange sites, significantly increase the hydrolysis rate of activated esters such as the ever-popular p-nitrophenyl diphenyl phosphate [57]. In this context, Miller and Ford reported the parallel synthesis of a small library of 32 anion exchange alkyl methacrylate latexes via radical polymerization which contained @20% of quaternary ammonium units (Fig. 32.11) [58]. The latexes comprised monomers such as vinylbenzyl chloride, sets of aliphatic methacrylates, divinylbenzene (crosslinker), and (m-/ p-vinylbenzyl) trimethylammonium chloride (stabilizer). The number and combination of the aliphatic monomers were combinatorially varied and additional anion exchange sites were introduced by quaternization of the benzylchloride residues with either trimethylamine or tributylamine. Pseudo-first-order reaction rate coefficients were spectrophotometrically determined. Relative rate enhancements ranged from 2.3 for a polystyrene-trimethyl ammonium (TMA) latex to 16.5 for a 2ethylhexyl methacrylate-tributyl ammonium (TBA) latex. In addition, several other composition–activity relationships and performance trends could be rationalized from the molecular structure. The random nature of the synthesis of polymeric catalysts leads to a lack of structural information about catalytically active sites. However, a polymeric system that serves a useful function, even devoid of detailed structural information, may


be generally of great interest. The obvious ease and practicability of this approach may allow for a rapid evolution and optimization of polymeric catalysts and may be useful, especially if reproducibility issues are carefully addressed, to identify active polymeric catalysts for many other important chemical reactions. 32.2.3

Combinatorial Synthesis Enzyme Mimetics 32.2.3.1 Hydrolytically Active Metal Complexes

Enzyme mimics for activated ester or phosphate hydrolysis remain at the center of interest. Janda and his group synthesized and screened novel transition metal complexes as enzyme mimics that catalyze the hydrolysis of carboxylic esters [59]. A hydroxy-substituted azacrown ether was modified in a conventional solution-phase three-step sequence to give three additional macrocylic ligands (Scheme 32.5). The ligands were then complexed with ten transition metal ion sources. The 40-member transition metal complex library was screened in parallel, without prior purification or isolation of the intermediates, for hydrolytic activity towards p-nitrophenyl acetate and N-methoxycarbonyl-l-phenylalanine- p-nitrophenyl ester as model substrates. Initial reaction rates were determined by repetitive photometric scanning of the absorption of the released p-nitrophenolate. Zinc and cobalt complexes gave only moderate rate enhancements ðkcat =kuncat ¼ @3Þ. The kinetic data implied that the systems do not behave like hydrolytic enzymes because of their linear dependence on both substrate and metal complex concentration.

Scheme 32.5. General structure of transition metal complexes

of hydroxy or alkoxy azacrown ethers efficient for hydrolysis of activated carboxylic acid esters.

The researchers extended their methodology towards the synthesis of the corresponding azacrown ether–lanthanide complexes that were capable of hydrolyzing phosphate esters, a crucial chemical transformation in areas such as nucleic acid chemistry [60]. The lanthanide complexes synthesized catalyzed the cleavage of

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phospho di- and triesters and double-stranded DNA as well, which opens up the possibility of using this class of chemicals as therapeutics. Another important potential application is the hydrolytic decontamination of toxic organophosphates, phosphonates, and fluorophosphates which are widely used as insecticides and are stockpiled as chemical warfare agents. A 625-member library of undecapeptides has recently been assayed for catalytic activity in phosphate hydrolysis, using test substances that in the presence of oxygen form insoluble indigo dyes. Using a split-and-mix approach, Berkessel and He´rault synthesized on solid support sequences PheaXaGlyGlyaXaGlyGlyaXa GlyGlyaX, where X is one of Arg, His, Tyr, Trp, or Ser [61]. About 2500 beads were incubated with Cu(II), Zn(II), Fe(III), Co(III), Eu(III), Ce(IV), and Zr(IV) solutions; only in the presence of Zr(IV) did the observation of the blue indigo color indicate catalytic hydrolysis activity. The most promising candidates were isolated, identified, and synthesized on solid support and in solution to confirm the library screening results. 32.2.3.2 Evolutionary Solid-phase Synthesis of Oxygenase Mimics

The catalytic oxygen transfer activity of the enzyme cytochrome P450 oxygenase inspired Sames and his coworkers to develop polymer-supported diamide complexes of manganese for catalytic oxygenation reactions [62]. The significant modular character of the targeted ligand structures allowed for an evolutionary solidphase ligand synthesis strategy resulting in the synthesis of three subsequently improved generations of diamide-based ligands. Upon transformation into the corresponding manganese(III) complexes, the resulting catalysts were directly screened on-bead for the epoxidation of vinylbiphenyl (Scheme 32.6). The first-

Scheme 32.6. Evolution of diamide ligands for Mn(III)-catalyzed oxygenation on solid support.


generation library quickly showed that a number of structural variants led to respectable yields of the epoxide (74% yield for the first-generation ligand). The second round identified a ligand which led to nearly complete consumption of vinylbiphenyl in 16 h (96% yield). The third generation of ligands provided a cyclic system that exhibited the highest turnover rate of the entire assay: 5 mol% of this catalyst yielded 98% of epoxide in 2 min, which translates into two orders of magnitude rate enhancement in comparison with the second-generation catalyst. The same catalysts were also used for the oxidation of ethyl benzene derivatives. 32.2.3.3 Libraries of Organic Acylation Catalysts

Mimics for acyl transferases require a concerted interplay between tethered functional groups where the catalyst should possess a suitable base that is covalently tethered in close proximity and the correct orientation relative to a nucleophilic center. Taylor and Morken reported the preparation of a library of 3150 distinct nucleophilic acylation catalysts on approximately 7000 encoded polymer beads prepared by ‘‘split-and-pool’’ methods [63]. Their ‘‘trimeric’’ catalysts consisted of a primary amine, a protected amino acid including the capping agent N-4pyridylproline, and functionalized carboxylic acids (Scheme 32.7). Temperature

Scheme 32.7. Solid-phase synthesis of a nucleophilic acylation catalyst library. On the bottom are shown the structures of the catalytically most active compounds containing the N-4pyridylproline moiety. R1, R2, R3, functionalized organic residues; Boc, tert-butyloxycarbonyl.

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changes due to the exothermic catalytic acetylation of ethanol were monitored directly on the floating beads by infrared (IR) thermography. The ‘‘hottest’’ 23 beads were then manually selected, and decoding revealed three structures as effective catalysts (Scheme 32.7, bottom). Most of them have N-4-pyridylproline incorporated as the capping agent and contained either (R)- or (S)-3-aminoquinuclidine as the primary amine component.

32.3

Combinatorial Catalysis in Asymmetric Synthesis

The importance and practicality of asymmetric synthesis as a tool to obtain optically pure or enantiomerically enriched compounds have been fully acknowledged by chemists in the areas of synthetic organic, medicinal, agricultural, and natural products chemistry and their related industries [64]. Among the asymmetric transformations, the most desirable and also the most challenging is catalytic asymmetric synthesis, where one chiral catalyst molecule can create millions of targeted chiral product molecules in a process that has been termed ‘‘chirality multiplication.’’ Thus, asymmetric catalysis represents a significant economic advantage over stoichiometric asymmetric synthesis, and, consequently, the development of new methods for discovering asymmetric catalytic transformations stands as an important and emerging objective in chemistry. Among others, divergent ligand synthesis strategies, where the transformation of an advanced ligand intermediate into a series of different chiral ligands is performed, appear to be especially fruitful in this area. In most cases, the ligand bears the chiral information and, therefore, creates a discriminating chiral environment in close proximity to the active metal site of the catalyst. Asymmetric synthesis is an especially suitable area for combinatorial catalyst discovery since asymmetric catalysis is often mechanistically very complex and critical variables in the parameter space to be evaluated for catalyst development and optimization are mutually dependent. Thus, by rapid parallel synthesis and metal complexation of chiral ligand libraries, coupled to automated or high-throughput screening, multidimensional problems in the discovery and in the optimization of catalysts may be efficiently addressed. 32.3.1

Combinatorial Catalyst Libraries in Enantioselective Additions of Dialkyl Zinc Reagents

The enantioselective addition of dialkyl zinc reagents to aldehydes can be catalyzed by enantiopure amino alcohols [65]. This ligand class as well as other prolinebased ligands have also been used extensively to prepare asymmetric catalysts for enantioselective reductions of ketones and asymmetric Diels–Alder reactions [66]. Chiral 1,2-amino alcohols attached to polymers have been used to catalyze the enantioselective addition of organozinc derivatives to aldehydes [67]. In 1995, Liu and Ellman reported the parallel solid-phase synthesis of a small

32.3 Combinatorial Catalysis in Asymmetric Synthesis

library of substituted 2-pyrrolidine methanol ligands [68]. To anchor trans-4hydroxy-l-proline methyl ester as a ligand scaffold to Merrifield resin, a parallel solid-phase synthesis approach involving a cleavable tetrahydropyranol linker was used [69]. Subsequent nucleophilic addition of various Grignard reagents, followed by reductive N-alkylations, gave the desired amino alcohols (Scheme 32.8). Onbead screening of the asymmetric addition of diethyl zinc to a variety of aromatic (up to 94% ee) and aliphatic aldehydes (up to 85% ee) yielded slightly inferior enantioselectivity compared with that of the corresponding free ligand in solution (Scheme 32.8). Further studies revealed that the purification of the ligands was not necessary to yield catalysts of similar performance with both aromatic and aliphatic aldehydes.

Scheme 32.8. Parallel solid-phase synthesis of pyrrolidine-type

catalysts and their screening in the enantioselective addition of diethyl zinc to aldehydes.

In a more recent example, Gennari and coworkers disclosed a new family of chiral ligands for enantioselective catalysis of the Ti(OiPr)4 -mediated addition of diethyl zinc to various aliphatic and aromatic aldehydes [70]. The novel ligands were based on a modular building block strategy to incorporate disulfonamide moieties as metal chelating units. Related disulfonamide ligands have previously been shown to facilitate the title reaction as well as asymmetric cyclopropanation reactions [71]. The 30-compound ligand library was synthesized by coupling the commercially available vicinal diamine scaffolds 1,2-diphenylethylenediamine [(R,R)- and (S,S)isomers] and 1,2-diaminocyclohexane [trans-(R,R)-, trans-(S,S)-, cis-(R,S), and rac-trans-isomers] in the presence of the polymer-bound nucleophilic catalyst ‘‘dimethylamino pyridine’’ with five N-protected b-amino sulfonyl chlorides derived from the l-amino acids Ala, Val, Ile, Phe, and Pro (Scheme 32.9) [72]. To avoid the

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classical aqueous work-up and purification of the ligands, solid-phase extraction (SPE) was utilized [16a, 73]; excess reagents were scavenged by solid-phase-bound tris(2-aminoethyl)amine. This methodology also eliminated the need for additional handles on the scaffolds for attachment to a solid support and avoided any interplay of a solid matrix on the yields and enantiomeric ratios of the catalyzed reactions.

Scheme 32.9. Synthesis of bis-(sulfonamide) ligands with polymer-supported reagents and solid-phase extraction technology.

By using this methodology, the same group also synthesized a library of 60 chiral Schiff base ligands that contains a set of different metal-binding sites (a phenol, an imine, and a secondary sulfonamine) [74]. By means of a high-throughput screening approach, a ligand was identified that catalyzed the copper-mediated, enantioselective conjugate addition of diethyl zinc to cyclic enones with up to 90% ee (Fig. 32.12). In a highly related approach, Liskamp and coworkers developed a solid-phase synthesis of bis-(sulfonamide) ligands [75]. A library of these ligands was screened in a simultaneous substrate-screening procedure for the ability to enantioselectively catalyze the Ti(OiPr)4 -mediated addition of diethyl zinc to aldehydes. The best solidphase candidate was resynthesized in solution and gave enantioselectivities up to 66% (Fig. 32.12). 32.3.2

Ligands for the Lewis Acid-catalyzed Asymmetric Aza-Diels–Alder Reaction

The asymmetric aza-Diels–Alder reaction provides an effective route to optically active nitrogen-containing heterocycles, as well as to other compounds of fundamental importance [76]. Asymmetric catalysis of imine dienophiles has remained


Fig. 32.12. Ligands for enantioselective diethylzinc addition

generated from combinatorial solid-phase synthesis.

elusive, and the successful asymmetric induction in the aza-Diels–Alder reaction heavily relied on auxiliary-based methodology. However, Yamamoto and coworkers’ stoichiometric enantiopure triarylborate Lewis acids [77] and Kobayashi and coworkers’ ytterbium-based Lewis acid for azadienes represent recent exceptions [78]. Furthermore, aza-Diels–Alder reactions of a-imino ester dienophiles with activated dienes by Johannsen as well as Jørgensen and coworkers merit closer attention [79]. In 1999 Jnoff and Ghosez disclosed chiral copper(II) complex-catalyzed asymmetric Diels–Alder reactions of 2-azadienes to enantiomerically pure piperidones [80]. Kobayashi and coworkers reported a novel class of chiral zirconocene complexes that catalyze the aza-Diels–Alder reaction of aldimines with Danishefsky’s diene to afford the corresponding piperidine derivatives in high yields with high enantioselectivities [81]. In a solid-phase approach, polymer-supported (R)-1,1 0 -binaphthols (BINOLs) have been synthesized and rapid catalyst optimization could be achieved (Scheme 32.10). A tethered, methoxymethyl (MOM)-protected (R)-BINOL derivative was brominated and then attached to Merrifield resin. Suzuki coupling of a variety of boronic acids with the immobilized building block and subsequent MOM deprotection gave a library of polymer-bound BINOLs. The best catalyst was found by mixing the immobilized ligands with Zr(Ot-Bu)4 and screening the resulting complexes for their performance in a model aza-Diels–Alder reaction of an aldimine with Danishefsky’s diene. The authors then synthesized the most promising ligands in solution, further optimized the inorganic precursor, and finally identified the Zr complex A as the best catalyst, giving 68% yield and 94% ee. 32.3.3

Divergent Ligand Synthesis for Enantioselective Alkylations

In 1998, Burgess and coworkers highlighted in several examples the value of divergent ligand synthesis strategies for the preparation of focused ligand libraries

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Scheme 32.10. Solid-phase approach for the identification of a

catalyst for the asymmetric aza-Diels–Alder reaction.

and the use of automated ligand screening [82, 83]. By definition, divergent syntheses of chiral ligands are characterized by an optically pure material being produced and then subsequently being used to prepare many ligands of a structurally similar class of ligands. Based on l-serine, a chiral building block (‘‘chiral synthon ¼ chiron’’) was prepared as a key intermediate that was then converted into a family of 13 novel chiral phosphine oxazoline ligands using several different routes and employing conventional organic solution synthesis [82]. It was shown that the divergent ligand synthesis approach was very suitable for efficiently accessing this ligand class with good diversity in terms of steric bulk and/or electronic pertubation (Fig. 32.13). The ability of these ligands to asymmetrically catalyze the addition of dimethyl malonate to 1,3-diphenylprop-2-enyl acetate was investigated [84]. The best of these ligands gave a palladium complex that catalyzes the allylic alkylation with up to 94% ee. In a different modular approach to generate chiral catalysts for the same trans-


Fig. 32.13. Divergent approach to phosphine oxazoline ligands.

Boc, tert-butyloxycarbonyl; R, alkyl and aryl.

formation, Gilbertson and coworkers synthesized a phosphine oxazoline ligand library from amino acids and phosphinocarboxylate building blocks (Scheme 32.11) [85]. Following amide formation between phosphinocarboxylates and amino acids, the carboxy group of the amino acid was reduced to an alcohol. These molecules could then be cyclized with the Burgess reagent and reduced in the presence of Raney Ni to give a 13-member library of phosphine oxazoline ligands. The best of these ligands gave a palladium complex that catalyzes the addition of dimethylmalonate to 1,3-diphenylprop-2-enyl acetate in 99% yield and in 98% enantiomeric excess.

Scheme 32.11. Divergent synthesis of new phosphino-oxazoline ligands.

Gilbertson and coworkers also used solid-phase chemistry to synthesize a library of 77 chiral phosphines that contain the b-turn structural motif [86]. Coordination of palladium gave metal complexes that catalyzed the addition of dimethylmalonate to cyclopentenyl acetate in up to 80% ee. Kobayashi and coworkers developed a solid-phase strategy to synthesize novel pentamine ligands for the enantioselective a-alkylation of lithium amide enolates [87]. The synthesis started with polymer-supported piperazine which was coupled with three different amino acids using standard peptide synthesis protocols followed by the addition of N,N-dialkylglycine. At this stage, the resulting compounds were cleaved off the resin, the terminal secondary amine was methylated, and the

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amide moieties were reduced using BH3 THF to give a library of 20 chiral pentamines. From this set of ligands, only moderate selectivities were obtained. Through modification of the N-methylpiperazine moiety, one complex was identified that catalyzes the alkylation of piperidinepropionamide with good yields and up to 84% ee (Scheme 32.12).

Scheme 32.12. Chiral pentamine ligand for the enantioselective a-alkylation of lithium amide enolates.

32.3.4

Chiral Phosphine Ligands for Asymmetric Hydrogenation

In homogeneous catalysis, phosphines are particularly important ligands in latetransition metal-catalyzed processes such as hydrosilylation, hydroformylation, carbonylation, olefin dimerization, and, last but not least, hydrogenation reactions [88]. For example, industrially important hydrogenations are involved in the synthesis of l-dopa (Monsanto) and Merck’s novel HIV protease inhibitor, indinavir, marketed currently as Crixivan= [89]. Phosphine-containing amino acids were originally synthesized to effect conformational states of peptides and to stabilize and control peptide structures upon metal binding [90]. Their application in catalytic asymmetric hydrogenation was reported in 1996 by Gilbertson and Wang [91]. Geysen’s polyethylene pin technique was used to synthesize a 63-member library of peptidomimetic ligands for the Rh(I)catalyzed hydrogenation of methyl 2-acetamidoacrylate to the corresponding amino acid derivative N-acetyl alanine. The general primary structure of the ligands, best described as AcaAlaaAibaAlaa[P-containing, internal peptide]aAlaaAibaAlaaNH2 , shared terminal AlaaAibaAla sequences to force an overall a-helical secondary structure and to bring the phosphine groups of the internal peptide sequence into close proximity with each other (Scheme 32.13). For the internal sequence, three classes of structurally unique chiral tetra- or pentapeptides were synthesized by combining the novel amino acids dicyclohexyl- and diphenylphosphane serine (Cps and Pps respectively) with two or three hydrophobic amino acids. Each ligand was screened directly on-bead in a parallel 24-vial reactor that was coupled to a gas chromatograph (GC) equipped with a chiral stationary phase to determine enantioselectivity and overall conversion. Unfortunately, only modest enantiomeric excesses


up to 18% were observed, but some trends and correlation between peptide sequence and selectivity were established. In a more recent publication, the same group synthesized larger libraries, giving access to ligands that were used for the catalytic asymmetric hydrogenation of methyl-2-acetamidoacrylate in the presence of rhodium. Enantiomeric excesses up to 38% were obtained [92].

Scheme 32.13. Screening of a library of support-bound

a-helical peptide/phosphine-Rh(I) catalysts in enantioselective hydrogenation reactions of prochiral methyl-2acetamidoacrylate to N-acetyl alanine methyl ester.

32.3.5

Asymmetric Reactions Catalyzed by Schiff Base-type Ligands – the Positional Scanning Approach

In this section, the successful application of a modular approach based on wellestablished functional building blocks to the rapid discovery of novel catalysts is described [93, 94]. The philosophy of this ‘‘positional scanning approach’’ for identifying a chiral catalyst consisting of three modular subunits, for example, is to optimize each of the three modular subunits while keeping the other two subunits constant. Common to the positional scanning approach (‘‘iterative optimization approach’’) of the modules composing the ligand structure are the two assumptions of (1) an independent and additive influence of each of the building blocks, and (2) the absence of negative cooperative effects during their systematic variation. In addition, the ‘‘positional scanning’’ approach, especially when coupled with

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automated screening, not only offers unique opportunities for the identification of substrate-specific catalysts (fine tuning) but also permits the discovery of ligands that possess unusual properties and which might otherwise elude detection. In 1992, Hayashi and coworkers reported the addition of trimethylsilyl cyanide (TMSCN) to epoxides catalyzed by titanium Schiff base complexes to yield racemic b-cyanohydrins [95]. Based on this work, Snapper, Hoyveda, and coworkers disclosed a diastereo- and enantioselective version of this reaction in 1996 (Scheme 32.14) [93]. With combinatorially optimized chiral titanium Schiff base complexes, nonracemic b-cyanohydrins were obtained in good chemical and optical yields. Chiral dipeptidic hemisalen ligands of the general structure SBaAA2 aAA1 aGlyOMe (SB ¼ Schiff base or hemisalen; AA1 ¼ amino acid) were synthesized in a modular fashion from various amino acid and aldehyde building blocks using parallel solid-phase synthesis techniques. After cleavage from the solid support, complexation with a titanium alkoxide precursor, and parallel screening of the chiral Lewis acids for efficiency and enantioselectivity, a potent catalytic system for ring opening of cyclohexene oxide in 89% enantiomeric excess was discovered (Scheme 32.14).

Scheme 32.14. Discovery of a new Schiff base catalyst for the Ti-catalyzed enantioselective addition of trimethylsilyl cyanide (TMSCN) to meso epoxides by an iterative optimization approach using ‘‘positional scanning.’’

Subsequent work by the same group demonstrated that screening did not require cleaving the Schiff base ligand from the solid support, which significantly accelerated the optimization process [96]. Using a closely related approach for their ligand synthesis, Sigman and Jacobsen disclosed polymer-supported Schiff base catalysts of the general structure linker1 – amino acid–linker2 –diamine–phenylidene in 1998 [94]. In this example, the onbead iterative optimization of ligands for the asymmetric hydrocyanation of imines (Strecker reaction) is described. The Strecker reaction constitutes one of the most


direct and viable strategies for the asymmetric synthesis of a-amino acid derivatives [97]. Ligand synthesis was made amenable to solid-phase synthesis techniques by choosing readily accessible building blocks and reaction conditions (Scheme 32.15). Systematic but non-obvious permutation cycles of their building blocks (‘‘iterative optimization’’), such as sequential variation of the amino acid components, the nature of the linker, or the diamino bridging moiety, revealed a transition metal-free catalyst after the synthesis of only three ligand generations that mediated the reaction of both aromatic and aliphatic aldimines in more than 70% and 83% enantiomeric excess, respectively. Screening was performed sequentially by gas chromatography using a chiral stationary phase. The best catalyst was resynthesized in solution and showed further improved enantioselectivity of 91% ee for an aromatic aldimine. Meanwhile, independent investigations by the same group disclosed a very active chiral salen Al(III) complex for the same reaction, resulting in imine hydrocyanation products of enantiomeric excesses as high as 95% [98].

Scheme 32.15. Combinatorially optimized Schiff base ligand for the asymmetric Strecker reaction (hydrocyanation of imines). AA, amino acid; TFAA, trifluoroacetic anhydride.

Hoveyda, Snapper, and coworkers applied their modular approach to Schiff basetype ligands for identifying catalysts for a variety of asymmetric transformations. Scheme 32.16 summarizes the novel catalytic systems for enantioselective additions of cyanide to imines (A) [99], for Cu-catalyzed conjugate addition of alkylzinc reagents to cyclic enones (B) [100], for Zr-catalyzed addition of diethylzinc to imines (C) [101], and for Cu-catalyzed allylic substitutions (D) [102].

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Scheme 32.16. Novel chiral catalysts containing Schiff base ligands.

32.3.6

Identification of Novel Catalysts for the Asymmetric Epoxidations via the Positional Scanning Approach

In 1999 Francis and Jacobsen disclosed the discovery of a novel catalyst for alkene epoxidation [103]. Using a solid-phase synthesis protocol on an aminomethyl polystyrene resin, ligands bearing potential metal-binding moieties were prepared. The ligands comprised five amino acids with donor side arms (Asp, Cys, His, Met, and Ser), three different chiral linking elements each differing in rigidity (1-amino-2indanol, trans-1,2-diaminocyclohexane, and Ser), and 12 different capping agents

32.4 Multidimensional Combinatorial Screening

with functionalities such as heterocycles, phosphanes, and salicylimines, which in turn were attached through imine and amide bonds. Initially, the metal-binding ability of the resulting 192 ligands was determined in a pooled assay with 30 different transition metal ions to yield 5760 possible metal–ligand combinations. Metal binding was visually detected with selective inorganic staining reagents in a fashion similar to previous studies. The entire pooled catalyst library was screened for the epoxidation of trans-b-methylstyrene (TBMS) to determine suitable reaction conditions. Those catalysts prepared from VOSO4 and FeCl2 were most active. Since VOSO4 also displayed significant epoxidation activity in the absence of the ligand library, FeCl2 was chosen for further investigations. The screening of 12 FeCl2 -derived libraries each containing a mixture of 16 basic structures and a different end cap furnished ligands with pyridine-containing end caps as the most active epoxidation catalysts. Further deconvolution showed that the most active catalytic system comprised FeCl2 , ligand structures exhibiting a pyridine-containing end cap, a serine linker, and serine or cysteine as the amino acid bound to the solid support. These systems resulted in high levels of catalytic activity, but only low enantioselectivities for the epoxidation of TBMS was observed (ee ¼ 4–7%). A second-generation 96-member optimization library based on the identified epoxidation catalyst produced moderately enantioselective variants (ee ¼ 15–20%) for the target reaction. Hoshino and Yamamoto have described the vanadium-catalyzed asymmetric epoxidation of allylic alcohols using a-amino acid-based hydroxamic acid ligands [104]. In order to optimize the ligands, again the iterative positional optimization approach was used, which involves screening one compound of a ligand structure for selectivity, while holding the other units constant (Scheme 32.17). In the first step, the source of chirality for the ligand – the amino acid moiety – was optimized, and the best result in this case was achieved using the tert-leucine-derived hydroxamic acid. In the second step, the imido group was examined and optimized to the 1,8-naphthalenedicarbonyl-protected hydroxamic acid (87% ee). Finally, the aryl groups near the metal coordination site were changed, identifying N-bis-(1naphthyl)methyl-substituted hydroxamic acid as the most effective ligand for a range of disubstituted allylic alcohols. The asymmetric epoxidations were performed in the presence of 1 mol% of VO(OiPr)3 and 1.5 mol% of the best ligand giving rise to the corresponding chiral epoxy alcohols in 58–99% yield and 76– 96% ee.

32.4

Multidimensional Combinatorial Screening

Conventional research in a ‘‘one-at-a-time’’ or serial fashion provides thorough quality control of the samples entering the screening process to insure that accurate data are obtained. Combinatorial methods that involve, for example, ‘‘splitand-pool’’ synthesis are much faster and render the preparation of relatively large numbers of compounds feasible but often lack control over the purity of the com-

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The iterative positional optimization approach to identify ligands for chiral epoxidations.

Scheme 32.17.

pounds entering the assay screen. Methods intermediate between the two extremes outlined above are based on parallel or array syntheses or screening, in a spatially addressable format with usually one compound per well, coupled to automated screens. This approach facilitates a relatively high level of quality control and a significantly enhanced throughput compared with sequential methods. Thus, multidimensional problems in the discovery and in the optimization of catalysts may be efficiently addressed by coupling parallel ligand synthesis with automated or highthroughput multidimensional screens (e.g. metal precursors, ligands, solvents, and reactant ratios). An integrated instrumental set-up for parallel high-throughput catalyst screening should encourage researchers to investigate a broader variety of systems, some of which may be regarded as too unusual to warrant testing in a sequential fashion.


32.4.1

Catalyst Discovery and Optimization Using Catalyst Arrays

Intramolecular carbon–hydrogen insertion of metal carbenes generated by catalytic decomposition of diazocarbonyl compounds is a facile methodology for carbon–carbon bond formation. The reaction involves presumably the insertion of in situ-generated metallocarbenes, often catalyzed by rhodium complexes, into CaH bonds [105]. In 1995, Lim and Sulikowski disclosed active copper catalysts for this type of reaction and used them in the synthesis of chiral indolyl derivatives from diazoesters [106]. Subsequently, Burgess and colleagues used a highly parallel screening approach for the optimization of ligand/metal/solvent combinations for the same reaction (Scheme 32.18) [107].

Scheme 32.18. Discovery and optimization of high-throughput catalyst screening. l-Menth, a new catalyst for the intramolecular CaH l-menthyl; dr, diastereomeric ratio; DDQ, insertion reaction of Rh carbenes from a-diazo 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. esters for the synthesis of indolyl derivatives by

A library of 96 catalytic systems was created in a standard microtiter/filtration plate in order to accommodate incremental variation of the reaction parameters. Combinations of five different natural or synthetic chiral ligands, seven metal precursors, and four solvents were evaluated in a high-throughput fashion using an HPLC device equipped with an autosampler. Subsequent data analysis not only identified two superior catalyst systems based on a bis-(oxazoline) ligand but also led to the identification of a novel and unexpected Ag(I)-based catalytic system with an overall increased diastereoselectivity compared with the original system. Using the optimized set of reaction conditions (AgSbF6 , THF, 25 C, 24 h), the desired product was obtained in 75% yield and in a diastereomeric ratio of 3.5:1.

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Burgess and coworkers demonstrated in several examples the value of a multidimensional screening for catalyst optimization. Using a small library of a novel phosphine oxazoline ligand class obtained by a divergent ligand synthesis approach [82, 85], parallel assaying was employed to evaluate and optimize application of these ligands. The novel ligands were screened simultaneously in a customized parallel reaction block bearing a 27- or 34-well format. The asymmetric alkylation of 1,3-diphenylpropenyl acetate with malonate was chosen as a classical test reaction (Scheme 32.19) [84]. Product evaluation and analysis were performed in an automated serial fashion by means of a conventional HPLC equipped with a chiral stationary phase. The parallel screening format allowed for a rapid assessment of various influential reaction parameters, including solvent effects, ligand substitution pattern, presence or absence of ions, ligand-to-metal ratios, and solvent effects on the enantiomeric excess and absolute configuration.

Scheme 32.19. High-throughput screening of phosphine oxazoline ligands in Pd-catalyzed asymmetric allylic alkylation reactions.

Allylic alkylation products with an enantiomeric excess as high as 94% were observed. In a closely related experiment, using the same instrumental set-up, ligands, metal sources, and additives, but a more difficult to control methyl-substituted allylic substrate, the best enantiomeric excess was determined to be 74% [83]. The related divergent ligand synthesis strategy and an identical instrumental set-up for the high-throughput catalyst screening as described above revealed novel so-called ‘‘propeller-shaped,’’ C3 -symmetric triarylphosphines as chiral ligands [108, 109]. The ligands were screened in an allylic amination reaction with phthalimide as the nucleophilic component, resulting in N-Substituted phthalimide derivatives were obtained with up to 82% enantiomeric excess.


Taylor and Morken uncovered an effective catalyst for the diastereoselective reductive aldol reaction with the aid of an arrayed catalyst evaluation protocol [110]. An array of 192 experiments in glass 96-well microtiter plates was used to evaluate the interdependence of a variety of reaction variables such as the metal precursor, the ligand, and the hydride source in a parallel fashion. The parent test reaction was the reductive coupling of methyl acrylate and benzaldehyde (Scheme 32.20).

Scheme 32.20. Catalytic stereoselective reductive aldol reaction by parallel catalyst array evaluation.

Each reaction was analyzed for relative conversion and stereoisomer ratios by conventional gas chromatography employing chiral stationary phases (GCCSP ). Several relationships between reaction condition and yield could be rapidly concluded, including that catechol borane was a superior reducing agent with the largest number of catalysts under the reaction conditions. The novel catalyst derived from [(COD)RhCl]2 , Me-DuPhos, and Cl2 PhSiH provided generally moderate yields (15– 69%) for a variety of other substrates but with a high syn stereoselection of up to 23:1 syn/anti. The study demonstrated a rapid assessment of the interdependence of reaction parameters showing that traditional empirical catalyst development approaches, where reaction variables are independently optimized, may not have revealed all active catalyst formulations found within the array. The metal-catalyzed addition of amines to carbon double bonds, especially of acrylic acid derivatives yielding b-amino acid derivatives, which are useful in peptide analogs or as precursors of optically active amino alcohols, diamines, and lactams, is of utmost importance. Kawatsura and Hartwig uncovered several late transition metal complexes that catalyze the addition of amines to acrylic acid derivatives using a novel parallel colorimetric assay to analyze the addition of primary and secondary alkyl amines to substrates with CbC bonds [111]. All possible combinations of seven metal precursors and 11 phosphine ligands were evaluated in a glass 96-well microtiter plate format for catalytic activity in the addition of piperidine or octylamine to methacrylonitrile as representative experiments (Scheme 32.21).

Scheme 32.21.

Transition metal-catalyzed addition of amines to acrylic acid derivatives.

The colorimetric ‘‘spot’’ test allowed for qualitative evaluation of secondary amines present in the reaction mixture either by consumption of secondary amine or

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by formation of secondary amine products. Catalysts derived from [Rh(COD)2 ]BF4 , [Ir(COD)2 ]BF4 , and [Ru( p-cymeneCl2 )]2 and several phosphine ligands were catalytically active in the addition of piperidine to methacrylonitrile, while Pd(OAc)2 showed activity in the addition of octylamine to the substrate. The results were confirmed in scale-up experiments and the scope and limitations of the reaction of alkylamines with acrylic acid derivatives were assessed in more detail. In 1998, Whiting and colleagues reported a parallel array screening approach to chiral catalyst discovery in the Lewis acid-catalyzed aza-Diels–Alder reaction of an N-aryl imine with Danishefsky’s diene [112], a reaction known not to proceed under normal thermal cycloaddition conditions (Scheme 32.22) [113].

Scheme 32.22. Parallel screen for asymmetric induction in Lewis-acid catalyzed aza-Diels–Alder reaction of a N-aryliminodienophile with Danishefsky ’s diene.

Using multiple-well plates, discrete homochiral Lewis acid complexes were individually generated in solution from four different metal salts, three different common enantiopure ligands, three different solvents, and two different additives. Screening was performed sequentially in about 1 week by measuring the enantiomeric excesses and conversions by means of an automated HPLC equipped with a chiral stationary phase to result in 144 sets of approximate yields and enantiomeric excesses. Reproducibility was confirmed and the most efficient chiral Lewis acid was a combination of MgI2 and (R,R)-1,2-diphenylethylenediamine in acetonitrile in the presence of 2,6-lutidine to afford the N-arylpiperidinone in 97% enantiomeric excess. Hydrosilylation of substrates such as acetophenones with ruthenium catalysts requires mixed P/N ligands for activity and selectivity; neither pure P/P-chelating nor pure N/N-chelating ligands show activity and selectivity alone [114]. Based


on that knowledge and adapting Noyori and coworkers’ concept of mixed ligand– ruthenium complexes as precatalysts [115], Frost and coworkers developed an efficient ruthenium-catalyzed asymmetric hydrosilylation of ketones using highthroughput parallel screening to optimize ligand combinations for the target reaction [116]. A small 50-member mixed ligand library of ruthenium diamine/ diphosphine complexes (precatalysts) was prepared in situ by addition of enantiomerically pure tol-BINAP ligands to a ruthenium precursor [RuCl2 (C6 H6 )2 ], followed by addition of various monoimine or diamine ligands. Diphenylsilane and the precatalysts were added to a solution of acetophenone as the substrate to synthesize the corresponding 1-phenyl ethanols (Scheme 32.23).

Scheme 32.23. High-throughput screening of ruthenium hydrosilylation catalysts to determine the optimal P/P and N/N ligand combination.

Yield and enantiomeric excesses were assayed by serial high-performance liquid chromatography (HPLC) and gas chromatography (GC). For various ruthenium diamine/diphosphine complexes, catalytic performance trends (yield, enantioselectivity, and absolute configuration) could be observed. The results obtained from parallel screening were confirmed by traditional serial experiments. The most effective catalyst formulation could be further improved by the addition of catalytic amounts of AgOTf to yield a catalyst that is active and selective in the hydrosilylation of a variety of aromatic ketones. For example, 1-phenyl ethanol was obtained from acetophenone in 53% yield and in 82% enantiomeric excess. Huffman and Reider from Merck Research Laboratories reported the discovery of improved conditions for the diastereoselective reductive amination in the largescale synthesis of the angiotensin converting enzyme (ACE) inhibitor elanapril [117]. Using a multidimensional screening approach, the effects of various heterogeneous catalyst formulations and a variety of additives were assayed (Scheme 32.24).

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Scheme 32.24. High-throughput screening of the Raney nickelcatalyzed reductive amination between ketoester and dipeptide Ala–Pro for Elanapril synthesis.

Experiments were performed in a hydrogenation reactor in which up to 18 reactions in vials were stirred in a single vessel under 1 atm. of hydrogen pressure. Product yield and diastereomer ratio were determined by standard highperformance liquid chromatography (HPLC) equipped with an autosampler. Hundreds of reactions were run with four catalyst formulations, including Raney/Ni, Pt/Al2 O3 , Pd/Al2 O3 , and Pd/C, and one or two additives at 10 wt% against AlaaPro as the substrate. The additives were chosen from a number of classes, both chiral and achiral, including amino acid derivatives, carbohydrates, salts, organic acids, and Lewis acids. Several trends and effects of the reaction parameters (catalyst, additives) for the selectivity of the reductive amination were observed. It was found that acetic acid and potassium fluoride, which in isolation were detrimental to the stereochemical outcome of the reaction, worked together to improve the (S,S,S):(R,S,S) selectivity with Raney nickel in ethanol from 11:1 to 17:1. The value of simultaneously varying multiple reaction parameters was demonstrated in the rapidly increased diastereoselectivity, which in turn led to a significant isolated yield improvement [90%, (S,S,S) þ (R,S,S) isomers] of this high-volume, high-value drug just by the addition of two inexpensive compounds. The transition metal-catalyzed reaction of enamines derived from b-oxoesters and a-amino acid amides with Michael acceptors such as methylvinylketone (MVK) was studied by Christoffers and Mann [118]. In the initial reaction step, enaminoesters were reacted with MVK in the presence of 14 metal salts including transition metals and main group metals in dichloromethane. The auxiliaries based on a-amino acid derivatives derived from Val, Leu, Ile, and tert-Leu already gave rise to


enantioselectivities of up to 78% enantiomeric excess in the absence of a metal salt. This result could be improved by 20% (up to 98% ee) by addition of Cu(OAc)2 H2 O as catalyst. Variation of solvent and catalyst loading finally led to the discovery of the catalytic system depicted in Scheme 32.25. Although stoichiometric but readily available amounts of auxiliary had to be employed, Christoffers and Mann demonstrated the application of combinatorial techniques in auxiliary-controlled catalytic reaction.

Scheme 32.25. Synthesis of enaminoesters and high-

throughput screening copper(II)-catalyzed reaction with Michael acceptors.

32.4.2

Parallel Array Screening for Catalyst Optimization Using Discovery and Focused Ligand Libraries

In analogy to the pharmacological model of drug discovery, a large parent set of ligands can be screened for lead compounds. After screening of the ligand library and discovering successful hits, smaller, more focused ‘‘daughter’’ libraries can be created based on ligand structure–activity relationships, resulting potentially in a higher hit rate because of the closer diversity space. This screening approach is useful for the optimization of reaction parameters, especially when essential reaction parameters of the targeted reaction such as preferred ligand class, mechanism of the catalytic cycle, or natural metal sources are already known. A 96-member library of pyridine-containing ligands consisting of both rationally chosen and ‘‘randomly’’ selected members was used by Hauptman and her group from DuPont to screen copper(I)- or copper(II)-catalyzed Ullmann aryl etherforming reactions [119, 120]. Aryl ethers are a structural element common to many

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agrochemical and pharmaceutical lead compounds. The study aimed to screen commercially available pyridine and pyridine-like compounds to find the best ligand for copper to produce the highest yield with the least amount of arenereduction byproduct. The reaction of 2-bromo-4,6-dimethylaniline as test substrate with a variety of alkoxides, including potassium phenoxide and sodium methoxide, was investigated under different conditions, such as ligand-to-copper ratio or solvent variation with the aid of an automated liquid workstation for library preparation (Scheme 32.26).

Scheme 32.26. High-throughput screening of copper ‘‘pyridine’’ catalysts for the Ullmann aryl ether formation.

Product analysis was performed by serial gas chromatography (GC). The results of the initial 96-member library screening allowed for assessment of structure– activity profiles. 8-Hydroxyquinoline derivatives proved to be outstanding ligands for the copper-catalyzed phenoxylation of the test substrate, yielding the desired aryl ether in @80% yield. Among the best ligands for methoxylation in terms of yield and low arene-reduction byproduct were pyridines bearing amino groups in position 2 (60% to >90%). Subsequent smaller focused ligand libraries including 8quinolinols (30 member) and 3-hydroxypyridine (23 member) for the phenoxylation reaction and 2-aminopyridine-type ligands (41 member) for the methoxylation confirmed the results obtained. The most effective ligands were found to be generally useful for alkoxylation of a variety of substrates, including intramolecular etherforming reactions. Parallel solution-phase synthesis techniques and parallel array assaying facilitated the identification and optimization of new nonpeptidic chiral dirhodium(II) carboxylates useful in the enantioselective SiaH insertion reaction of diazoesters with silanes [121–123]. A systematic search through the Available Chemical Directory (ACD) resulted in a set of 2000 useful alternative chiral carboxylic acid ligands which were then clustered and the centroid of each of 80 clusters selected as a representative acid [124]. The representative chiral carboxylic acid ligand from each of the 80 clusters then underwent a 20-at-a-time parallel synthesis format ligand exchange with a dirhodium(II) carbonate precursor to yield the desired chiral dirhodium(II) carboxylate catalyst. A total of 69 chiral dirhodium(II) catalysts were assayed in the targeted enantioselective carbenoid SiaH insertion of methyl 2(diazo)phenylethanoate into silanes with varying degrees of steric bulk to yield insertion product (Scheme 32.27).

32.5 One-pot, Multisubstrate Screening

Scheme 32.27. High-throughput screening of dirhodium insertion catalysts for the carbenoid insertion of azo esters into Si–H bonds.

Enantioselectivities of the SiaH insertion products were assessed by automated HPLC after a short parallel purification step. The first-generation screen revealed a-hydroxy carboxylic acids and N-arene sulfonyl a-amino acids as promising ligand candidates. Based on these results, a focused second-generation array was constructed from the original 80 clusters to identify N-arene sulfonyl a-amino acids as superior ligands for the targeted reactions. Up to 76% enantiomeric excess for the product was obtained with N-(toluene-4-sulfonyl)-l-leucine as a ligand at 78 C. The use of parallel synthesis and screening techniques has resulted in the rapid identification of improved catalysts for enantioselective SiaH insertion reactions of diazoesters with silanes.

32.5

One-pot, Multisubstrate Screening

Hadamard transformation is a mathematical algorithm that can be used to identify the properties of a particular group of candidates by recording the sum or superpositions of signals from well-selected groups or pools of single wells, catalysts, etc. [125]. Subsequent deconvolution of a set of data obtained from an entire group coupled with computer simulations can provide analytical solutions in various branches of physical sciences, including homogeneous and heterogeneous catalysis. Many applications and scenarios of Hadamard transformations in combinatorial catalysis can be envisioned, including multisubstrate/one-catalyst screening, one-substrate/multicatalyst (multiple ligands, metals) screening, or simultaneous screening of a variety of catalysts against multiple substrates. Consequently, Gao and Kagan investigated a Hamadard-type approach to combinatorial catalysis [126]. The researchers checked the validity of the one-catalyst

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against the multiple substrate screening in the asymmetric reduction of mixtures (families) of prochiral aromatic ketones to their corresponding alcohols. Corey’s chiral oxazaborolidine catalyst was chosen as the chiral catalyst (Scheme 32.28) [127].

Scheme 32.28. One-pot, multisubstrate screening of a chiral oxazaborolidine catalyst in the enantioselective reduction of a family of aromatic ketones. DMS, dimethylsulfide.

Conversion and enantiomeric excesses of the resulting mixture of alcohols was monitored by conventional HPLC equipped with a chiral stationary phase (CSP). Key was the careful calibration of the screening conditions, including baseline separation of all of the ketones as starting materials from the corresponding alcohols, and, in addition, a sufficient resolution of the enantiomers of the enantiomeric alcohols. All ketone reductions were repeated individually in solution, with most of the discrepancies between the individual asymmetric reductions and the multisubstrate approach ranging from 1% to 7%. For example, asymmetric reduction of acetophenone resulted in the formation of (R)-1-phenylethanol with 97% enantiomeric excess under screening conditions (ketone mixture) and in the isolated ketone reduction. In another example of an application of a one-pot, multisubstrate screening approach, Gennari and coworkers disclosed a family of chiral ligands for enantioselective catalysis of the Ti(OiPr)4 -mediated addition of diethylzinc to aliphatic and aromatic aldehydes [70]. A 30-compound ligand library was synthesized based on a modular building block strategy. The disulfonamide ligands comprised six chiral and racemic vicinal diamine scaffolds which were reacted with a set of five l-amino acid-derived N-protected b-amino sulfonyl chlorides [72]. This ligand class has previously been shown also to facilitate asymmetric cyclopropanation reactions [71]. Modern combinatorial synthesis techniques such as employment of a polymerbound nucleophilic ‘‘acylation’’ catalyst [polymer-bound dimethylaminopyridine (DMAP)] and the consequent use of solid-phase extraction (SPE) facilitated the solution-phase ligand synthesis, and also eliminated the need for additional handles on the scaffolds for solid support attachment [73]. In analogy to Kagan’s protocol, the individual ligands were assayed in parallel and in spatially addressable format on a mixture of four aldehydes (multisubstrate). After optimization of the analytical conditions, product analysis was performed sequentially with capillary GC equipped with a chiral stationary phase. Excellent

32.5 One-pot, Multisubstrate Screening

enantiomeric ratios for aromatic and aliphatic aldehydes in favor of the (R)enantiomer [(R)/(S) 97–98:3–2] were obtained. A ligand derived from trans-(S,S)diaminocyclohexane and the N-protected b-amino sulfonyl chloride derived from l-Phe performed the title reaction best (Scheme 32.29). The purified ligand was subjected to characterization, and the screening results were confirmed by reaction with the four separate aldehydes.

Scheme 32.29. One-pot screening of bis-sulfonamide ligands in the enantioselective addition of diethylzinc to aldehydes. dr, diastereomeric ratio.

A one-catalyst/multiple substrate strategy for the pooled synthesis of libraries of internal olefins via metathesis was reported by Bra¨ndli and Ward [128]. It was hypothesized that cross-metathesis of only two internally disubstituted olefins should produce up to 20 statistically distributed olefins including (E )/(Z )-isomers [NðN þ 1Þ, N ¼ number of unequivalent olefinic termini]. In this study, biologically important oleic acid derivatives were subject to cross-metathesis using Grubb’s metathesis catalyst [Ru(bCHPh)Cl2 (Pcy3 )2 , Cy ¼ cyclohexyl]. Product analysis and distribution were conducted by coupled gas chromatography/mass spectrometry (GC/MS) analysis and further oxidative degradation experiments. The results demonstrated the applicability of the multisubstrate/one-catalyst approach in this reaction for the synthesis of small libraries of organic compounds. A conceptually related combinatorial, or family approach to the resolution of racemates, was reported by Vries and coworkers. The simultaneous addition of a family of resolving agents to a solution of a given racemate caused a very rapid precipitation of crystalline diastereomeric salts in good to high enantiomeric purity and yield [129]. The novel concept ‘‘one-pot, multisubstrate screening in asymmetric catalysis’’ may suffer from intrinsic drawbacks and limitations, such as undesired cooperative/ noncooperative interactions (between chiral catalyst and chiral reaction product, including autoinduction, or nonlinear behavior of catalytic activity, etc.) and neighboring element effects (cross-talk between library entities). However, the simplicity of this method makes it convenient for a preliminary evaluation of many different

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chiral reagents or catalysts in a targeted reaction once the substrate mixture is standardized. 32.6

Combinatorial Approaches to Olefin Polymerization Catalysts

An annual production of approximately 46 million metric tons exemplifies the industrial importance of polyolefins [130]. Combinatorial programs for catalysis and materials can be utilized for both discovery and optimization purposes. The discovery process involves, for example, the identification of catalyst systems for novel copolymerizations, whereas in the optimization process the influence of new ligands, metals, activators, alkylators, solvents, and temperature is tested. Recent reports have shown that certain late transition metal diimine-based catalyst systems exhibit olefin polymerization activities similar to those reported for commercially employed early-metal single-site metallocene-based systems [131– 133]. These new systems have sparked considerable interest in the polyolefins industry because of their high activity, ease of synthesis and handling, and tolerance toward functionalized olefins such as methacrylate and vinyl acetate. Over the past few years, an increasing effort has been expended, in both academic and industrial research laboratories, towards the discovery of new olefin polymerization catalysts differing dramatically from the forefront group 4 metallocenes and half-sandwich titanium amide catalysts [134]. Most commercial-scale polyolefin processes employ high-surface area supports for immobilizing olefin polymerization catalysts, and only few reports have appeared examining the use of polystyrene as a catalyst support [135]. The researchers Powers, Murphy and colleagues at Symyx Technologies have developed a parallel synthesis and screening protocol for a polymer-bound 96member library of 1,2-diimine–transition metal complexes (Scheme 32.30) [136, 137]. The key intermediate, a resin-bound diketone, was converted in a titaniummediated condensation with 48 commercially available anilines with varying steric and electronic substituents to furnish a 48-member 1,2-diimine library. In these catalytic systems both substituent topography and electronic perturbation have been reported to play a dramatic role in catalyst activity, molecular weight, and yield of the polymer [138]. Splitting of the ligand library followed by conversion into the corresponding 48 Ni(II) or Pd(II) complexes with (DME)NiBr2 or (COD)PdMeCl, respectively, afforded a polymer-bound 96-member library of 1,2-diimine–transition metal complexes. In order to compare performance of the resin-bound catalysts with that of the corresponding catalysts in solution, a corresponding 1,2-diimine library based on a related diketone framework was synthesized [136]. To efficiently complex the solution-phase 1,2-diimine library with Ni(II) and Pd(II), metal-delivery agents (MDAs), a novel class of polymeric reagents, were used as metal ion sources [139]. After activation of the Ni(II) or Pd(II) catalyst precursors (1,2-diimine complexes) with MAO (methylalumoxane) or sodium tetrakis-(3,5-bistrifluoromethyl) phenyl borate (Na[(3,5-(F3 C)2 C6 H3 )4 B]), respectively, a custom high-pressure par-

32.6 Combinatorial Approaches to Olefin Polymerization Catalysts

Scheme 32.30. Synthesis of Brookhart-type

polymer-bound Ni(II) or Pd(II) 1,2-diimine complexes and their use in the polymerization of ethylene. M ¼ Ni(II), X ¼ Y ¼ Br; M ¼ Pd(II), X ¼ Me, Y ¼ Cl; R, alkyl, aryl,

heteroaryl, halogen, functional groups; tag, chemical code; MAO, methylalumoxane; ArF, 3,5-(F3 C)2 C6 H3 ; dme, dimethoxyethane; cod, 1,5-cyclooctadiene.

allel polymerization reactor with a modular series of 48 reaction chambers was used to screen for ethylene polymerization. The device was equipped with individual ethylene pressure controls, and reactants were loaded using a three-axis liquidhandling robot (Fig. 32.14) [136]. Compared with the corresponding free complexes screened under identical conditions in solution, catalyst performance consistently proved to be decreased for the on-bead Ni(II) catalysts and increased for the solid support-bound Pd(II) complexes. For example, assaying the isolated discrete polyethylene granules by rapid high-temperature gel permeation chromatography, molecular weights (MWs) up to 59,000 g mol1 were found for resin-bound Ni catalysts and up to 213,000 g mol1 for the corresponding diimine–Ni catalysts in solution. Assuming that catalyst performance is proportional to the ‘‘growth’’ of the polystyrene support beads (2–10 times from the initial diameter of 70 mm), visual inspection of the beads allowed a distinction between Ni(II)- and Pd(II)-derived catalysts in mixed assays (Fig. 32.15). These results were confirmed by a chemical encoding/deconvolution strategy with cleavable tertiary amine tags, followed by HPLC analysis. The present work demonstrates the feasibility of applying a multitude of combinatorial techniques, including solid-phase synthesis, on-bead screening, and the encoding/deconvolution of pooled libraries of catalysts, for the discovery and optimization of new olefin polymerization catalysts. Moreover, the technology appears to be suitable for catalytic processes other than ethylene polymerization. In this

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Fig. 32.14. Symyx Technologies Parallel Polymerization Reactor-96.

Fig. 32.15. Representative samples of polymeric products

obtained from a pooled polymerization of ethylene with polymer-supported 1,2-diimine Ni(II) and Pd(II) catalysts.


respect, Symyx Technologies has developed general methodologies for the combinatorial synthesis, high-throughput screening, and characterization of libraries of supported and unsupported organometallic compounds and catalysts [139a, 140]. For the discovery of novel polymerization catalysts, libraries of ligands in combination with various metals are screened in the presence of different monomers. In order to optimize the yield and selectivity of a given organometallic complex in a polymerization reaction, a variety of factors are tested in a high-throughput fashion, including the form of the ancillary ligand precursor, the choice of the metal precursor, the reaction conditions (e.g. solvent, temperature, time), and the stability of the desired product. Mu¨llen and coworkers tagged silica- or polymer-supported heterogeneous catalysts for industrial olefin polymerization with fluorescent dyes [141]. Here, direct detection of the different product beads obtained by different catalysts is available through fluorescent dyes that exhibit different emission wavelengths. The approach starts with producing the tagged catalysts by supporting various metallocenes with silica, activating them with MAO, and labeling them with different rylene dyes (Fig. 32.16). The dyes were chosen because of their high chemical stability, their high tendency to physisorb on silica, their high fluorescence yield, and because of the fact that they do not influence the polymerization. Also, a great variety of rylene dyes with different emission wavelengths covering the entire visible spectrum is available. These labeled catalysts are then mixed and introduced in a single polymerization vessel. During the olefin polymerization each catalyst particle forms only one product granulate through a particular growth process and can be considered as a microreactor. To assign the different compounds of the product mixture to the employed catalyst, the polymer products are exposed to UV light, and can be directly identified, manually separated, and characterized because of the different emission wavelengths of the labels. The authors could demonstrate the feasibility of this concept for the polymerization of ethylene as well as for the copolymerization of ethylene/hexene.

Fig. 32.16. Catalysts and dyes for the synthesis of tagged, supported catalysts.

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Coates and coworkers used a pooled polymerization catalyst strategy to identify catalysts for the synthesis of syndiotactic polypropylene [142]. The basis of this concept is that the formed polymer itself serves as a stereochemical recording of the events of the polymerization catalyst. Assuming that the catalyst species do not react with one another, then a group of complexes for stereoselective polymerization can be screened simultaneously. When the desired polymer product has distinguishing chemical or physical properties, techniques such as solubility, spectroscopy, or chromatography can be used to quickly probe the crude product of a pooled polymerization reaction to see if a noteworthy catalyst is present. To probe this concept, a library of 12 salicylaldiminato ligands was synthesized separately by the condensation of three different salicylaldehydes and four amines (Scheme 32.31). Equimolar amounts of these ligands were combined, deprotonated with

Scheme 32.31. Synthesis of a library of salicylaldiminato titanium complexes.


BuLi, and reacted with 0.5 equiv. of TiCl 4 to give a library of 78 possible titanium species. This complex library was then activated with MAO ([Al]/[Ti] ¼ 100] in toluene and the resulting catalyst solution was exposed to propylene (2.7 atm.). Even though 90% of the formed polymer could be washed away in refluxing diethyl ether (atactic polypropylene), the remaining 10% of polymer was found to be syndiotactic polypropylene ( 13 C-NMR microstructural analysis). Deconvolution methods, i.e. the synthesis and testing of sublibraries, were used to identify successfully the most active catalyst. Hinderling and Chen reported the use of electrospray ionization tandem mass spectrometry (ESI-MS/MS) and gas-phase ion molecule reactions for the rapid screening of Brookhart-type Pd(II) olefin polymerization catalysts [143]. A test library of eight catalysts was prepared by reacting eight diimine ligands with [(cod)Pd(CH3 )(Cl)], washing and drying of the complex, and activation with AgOTf (Scheme 32.32). An electrospray mass spectrum of the mixture of complexes showed that all eight catalysts are present in similar concentrations. The mixture

Scheme 32.32. Rapid screening of Brookharttype Pd(II) olefin polymerization catalysts by ESI-MS/MS. (i) [(cod)Pd(CH3 )(Cl)], then AgSO3 CF3 ; (ii) excess ethylene, then quenching with DMSO; (iii) electrospray under mild

desolvation conditions to give polymeric ions (and loss of DMSO); (iv) reject all ions below a certain mass and subject the remaining highmass ions to collision with Xe to induce bhydride elimination.

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was then dissolved in dichloromethane, saturated with ethylene, reacted for 1 h, and quenched with dimethyl sulfoxide (DMSO). The electrospray mass spectrum of the crude reaction mixture was rather complex and showed multiple, overlapping series of oligomeric and polymeric ions corresponding to each catalyst species with between 0 and @100 ethylene units added. After selection of ions with m=z > 2200, these high-mass ions were collided with xenon to induce b-hydride elimination. From the corresponding daughter ion spectrum the structure of the best catalyst can be derived. Similarly, the daughter ion spectrum with a lower mass cut-off shows the next best catalysts. The advantages of this methodology are sensitivity, speed, direct assay, versatility, and possible automation.

32.7

Combinatorial Inorganic Catalysis 32.7.1

Combinatorial Libraries of Homogeneous Polyoxometalate-based Catalysts

Early transition metal oxygen anion clusters or polyoxometalates (POMs) have a significant impact in a variety of different research fields. Among catalysis by heteropolyacids, selective hetero- and homogeneous oxidation processes are of utmost importance [144]. Of extraordinary interest are POMs of the so-called Keggin type that possess the general formula fHð3þxÞ [PVx Mð12xÞ O40 ](aq)g (M ¼ Mo, W; x ¼ 0–5). Hill and Gall published the combinatorial synthesis of phosphorus-centered molybdenum and tungsten polyoxometalates of the Keggin structure [145]. The POM catalysts were evaluated in the selective aerobic oxidation of tetrahydrothiophene (THT) to its corresponding sulfoxide (THTO) (Scheme 32.33).

Aerobic oxidation of tetrahydrothiophene (THT) to tetrahydrothiophene sulfoxide (THTO) by combinatorially prepared POMs.

Scheme 32.33.

Catalyst library synthesis was performed by combinatorially mixing stock solutions of appropriate tungsten, molybdenum, and vanadium precursors under a constant phosphate concentration. Completion of catalyst formation was achieved by equilibration at room temperature, which was followed by 51 V-NMR spectroscopy. Conventional gas chromatography was used to quantify product (THTO) yields and selectivities. Promising catalysts were selected and subsequently identified with the aid of 51 V-NMR and IR spectroscopy, revealing the complex [a-1,4-

32.7 Combinatorial Inorganic Catalysis

PV2 W10 O40 ] 5 as the most effective among them. All the samples evaluated, including the starting materials, exhibited catalytic efficiency to a certain degree in the conversion of THT to THTO, suggesting that Keggin-type polymolybdato- or tungstophosphates have the ability to oxidize selectively THT to THTO regardless of their vanadium content. It was assumed that the catalytically most active species were primarily Keggin polyoxometalates rather than lower nuclearity polyoxometalates. Several other combinatorial libraries containing boron, silicon, and arsenic were evaluated under a variety of conditions, but the results were not quantified. Selective catalytic oxidations that proceed at satisfactory rates at ambient conditions and use oxygen or air as terminal oxidant are of utmost interest [146]. In a more recent contribution, Hill and his group reported a diversity-based discovery and mechanistic investigation of selective homogeneous thioether oxidation by the Au(III)Cl2 NO3 (thioether)/oxygen system [147]. A 150-member library of inorganic complexes was constructed by combinatorially combining polyoxometalate anions and redox-active cations. The library design comprised POMs with reversible redox chemistry and redox-active d-block ions or appropriate precursors including HAuCl 4 , as well as s-block and p-block cations as counterions for the POMs. The catalyst library included also POM-free control formulations such as the chloride, nitrate, or perchlorate salts of the appropriate redox-active d-block cations. Each library member was evaluated for its ability to catalyze the oxidation of the thioether mustard analog 2-chloroethyl ethyl sulfide (CEES) to the corresponding sulfoxide (CEESO), using only oxygen as the oxidant (Scheme 32.34).

Aerobic oxidation of 2-chloroethyl sulfide (CEES) to 2-chloroethyl sulfide sulfoxide (CEESO) by the POMfree combinatorially discovered catalysts system Au(III)Cl2 NO3 (thioether). Scheme 32.34.

Product analysis and distribution was assessed by conventional serial GC. Most catalyst preparations showed little or no activity in the target reaction but three catalyst compositions exhibited considerable catalytic activity. The catalytically active compositions included the POMs [CuPW11 O39 ] 5 and [MnPW11 O39 ] 5 , and the POM-free system AgNO3 , each of them with five equivalents of HAuCl 4 . The most optimized combination of HAuCl 4 and AgNO3 formed a catalyst that exhibited orders of magnitude higher reaction rates and higher turnover numbers (TONs) at ambient temperature and at 1 atm. of air or oxygen than previously reported catalysts. Extensive kinetic and mechanistic studies for the oxygen-based oxidation of thioethers catalyzed by the parent system Au/Ag/NO3 revealed two gold complexes as the catalytically active species (Fig. 32.17).

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Fig. 32.17. Proposed catalytically active gold complexes.

32.7.2

Combinatorial Libraries and High-throughput Screening of Heterogeneous Polyoxometalate Catalysts

Early efforts by Symyx Technologies led to the development of an fully integrated high-throughput technology-screening platform useful for pharmaceuticals and for fine chemicals process discovery, screening, and optimization. The platform includes a proprietary software package as an essential component in the design of combinatorial libraries. It also controls all the robotic functions, gathers data from each spatially addressable reaction entity, and automatically stores the process data in a central database. The hardware also comprises a variety of 96-well reactors equipped with a liquid- and slurry-handling robot. In one example, the integrated screening platform was employed for the high-throughput synthesis and screening of metal-doped polyoxometalate (POM) libraries in 96-well format [148]. Libraries of the heterogeneous POM catalysts were screened to discover selective aerobic oxidation catalyst systems. In this case, an imidazole alcohol was catalytically oxidized to the corresponding aldehyde, which represents an important key intermediate for the synthesis of DuPont’s antihypertensive compound Losartan (Scheme 32.35).

Application of high-throughput screening in the discovery of novel catalysts for the POM-catalyzed oxidation of imidazole alcohol to aldehyde, a key intermediate in the synthesis of the antihypertensive compound Losartan (DuPont Pharmaceuticals).

Scheme 32.35.

Within 4 weeks, 5000 experiments were performed to screen different catalyst compositions, effects of solvents, and additives under variable reaction conditions.

32.8 Combinatorial Heterogeneous Catalysis

Two classes of catalyst systems that gave more than 90% yield were identified. These results were confirmed and validated in subsequent scale-up experiments, but the proprietary catalyst composition was not disclosed.

32.8

Combinatorial Heterogeneous Catalysis 32.8.1

Introduction

The mechanism of catalytic activity in a heterogeneous system is complex [149]. The distribution of active sites in heterogeneous catalysts as well as the phase integrity is in general poorly defined. The phase integrity plays a major role because the extent of deviation from the equilibrium structure under reaction conditions controls the catalytic function. These types of inherent problems severely hamper or even prevent the rational design of a heterogeneous catalyst for a particular reaction. Conventional methods of catalyst discovery are primarily trial-and-error processes where one catalyst at a time is tested for activity followed by numerous modifications to achieve satisfactory activity. The process is therefore time-consuming and laborious, and in most cases only local minima in a predefined parameter space (composition, reaction conditions, etc.) have been identified rather than the absolute minimum. Combinatorial synthesis and screening offer a new dimension for the quick discovery and optimization of heterogeneous catalysts in terms of both composition and process conditions. The high-throughput approach allows rapid exploration of a much larger parameter space than the conventional approach and therefore leads more likely to the discovery of novel materials with significant performance improvements. In this section, we focus on recent developments in the area of high-throughput synthesis and screening of heterogeneous catalyst libraries. Special emphasis is given to some recently published literature on methods of sample preparation and parallel screening. A number of interesting patents and articles has appeared which deal mostly with the development of methods for high-throughput synthesis and screening of heterogeneous libraries. Integrated synthesis and screening of a plurality of catalysts in library format has been recognized as an essential factor [150]. In 2000, Gennari and coworkers summarized inorganic catalysis in a review on combinatorial technologies for catalyst design and development [20e]. Recently, Senkan published a more detailed review on combinatorial heterogeneous catalysis [20c]. Mirodatos and coworkers discussed the application of combinatorial chemistry to heterogeneous catalysis in terms of current strategies and perspectives on the industrial and academic levels [151]. There are basically two techniques to prepare libraries of heterogeneous catalytic materials: solution-based methods and thin-film deposition methods. The description of both techniques is omitted in this part since they are described in detail in Chapter 34. Impregnation of catalyst components onto a preformed solid support,

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however, is an important solution-based technique for the preparation of heterogeneous catalysts, but is seldom utilized for other material sciences applications. The solution-based methods are predominant in the field of combinatorial heterogeneous catalysis. The majority of commercial heterogeneous catalysts is manufactured by solution-based techniques such as coprecipitation, impregnation, and their variations [152], and, thus, results from library experiments can be translated more easily into a bulk catalyst using the solution-based techniques. In addition, the solution-based methods present fewer scale-up problems. Furthermore, many liquid-handling robots and inkjet-based liquid-dispensing systems can be employed to prepare the heterogeneous catalyst libraries of microgram scale to gram scale by the solution-based methods. On the other hand, the thin-film deposition methods are performed on custom-designed and assembled high-vacuum instruments that generally require high initial investment. Special care should be taken to ascertain that the phase integrity of each catalyst member is the desired one since the thinfilm deposition methods rely on the interlayer diffusion for mixing and the multicomponent layers occasionally result in phase separation. The thin-film-based method will perhaps be of choice for fused catalyst libraries, e.g. metal alloys. In most cases, the catalyst performance depends on the method of preparation. Therefore, it is very important to adopt proper methods to obtain catalysts with expected surface area, uniform metal distribution, and desirable particle size. Newsam and Schu¨th have described various routes for combinatorial catalyst synthesis that include hydrothermal synthesis, use of fluid precursors, carrier impregnation, precipitation [153]. A number of other research papers have outlined different methods of preparing combinatorial catalyst libraries. These include computer-controlled inkjet deposition of liquid reactants [154], methods based on solution precursors [155–157], sol-gel techniques [158–160], impregnation of solid supports [161], as well as standard methods such as precipitation and coprecipitation [162, 163]. Table 32.1 summarizes some examples of heterogeneous catalyst libraries. With the development of new and efficient technologies for the synthesis of large catalytic libraries, there is an ongoing effort for inventing fast and parallel screening tools to identify active and selective catalysts. Several new techniques have been proposed and demonstrated in the past few years, including the resonanceenhanced multiphoton ionization (REMPI), time resolved and differential IR thermography, scanning mass spectrometry, and colorimetry. While IR thermography and fluorescence or colored dye assays are established techniques, a few other new methods have emerged recently to identify active catalysts in an array. So far, no screening method offers a general solution to the problem of fast screening of libraries together with the complexity involved in parallel detection of the reactants, products, and side products. Therefore, it is very important to develop screening technologies according to the detection requirements of the reactants, products, and side products of the reaction. Accordingly, in some cases, it is essential to have a combination of tools for a better characterization of reaction products. It is also very important to characterize the time-dependent nature of the catalyst library for developing a practical catalyst. Some catalysts have a significant induction period

Propylene oxidation

Ethane and propane dehydrogenation

Solution-based sol-gel method Solution-based hydrothermal zeolite synthesis Solution-based hydrothermal zeolite synthesis Solution-based coprecipitation Thin film deposition: sputtering Solution-based impregnation Solution-based coprecipitation Solution-based sol-gel method

Solution-based coprecipitation

37, 1–10% Co, Cr, Cu, Fe, Ir, Mn, Ni, Pd, Pt, Rh, Ru, V, Zn on Si, Ti oxides

100 Na2 O/Al2 O3/SiO2 zeolites with Li and Cs

37 TiO2/Al2 O3/SiO2/TiO2/ZrO2 zeolites

645 clusters of Pt, Ru, Os, Ir on carbon

120 ternary thin film clusters of Pt, Pd, Rh, and Pd, Rh, Cu

66 ternary combinations of 1% Pt, Pd and In on Al2 O3

16 Au/Co3 O4 and Au/TiO2 powders

33 1–6% Ag, Au, Bi, Co, In, Cr, Cu, Fe, Mo, Ni, Re, Rh, Sb, Ta, Te, V, Y on Si, Ti, Zr oxides

50 ternary and quaternary oxides of Co, Cd, Fe, Ga, Ge, In, Mn, Mo, Ni, Nb, V, W, Zn

CO oxidation

Six-parallel gas chromatography

Spatially resolved analysis of an array of batch microreactors

Quadruple mass spectrometry

REMPI

Scanning quadruple mass spectrometry

CO oxidation, CO þ NO Cyclohexane dehydrogenation to benzene

Fluorescence acid–base indicator

a

a

Methanol direct fuel cell

a

a

Infrared thermography

Infrared thermography

H2 þ O2

Solution-based impregnation

0.5% Ag, Bi, Co, Cr, Cu, Er, Fe, Gd, Ir, Ni, Pd, Pt, Rh, Ti, V, Zn, on Al2 O3 pellets Hyrogenation of 1-hexyne

Screening method

Target reaction

Preparation method

Catalyst libraries

Tab. 32.1. Examples of combinatorial heterogeneous catalyst libraries.

170

169

162

161

168

157

167

166

165

164

Reference

32.8 Combinatorial Heterogeneous Catalysis 941

Ethane oxidative dehydrogenation Cyclohexane dehydrogenation to benzene C3 H8 total oxidation

NO reduction by C3 H6

Solution-based sol-gel deposition

Solution-based sol-gel deposition Solution-based impregnation


Solution-based impregnation Solution-based coprecipitation

66 ternary combinations of oxides of Mo, V and Nb

144 ternary combinations of oxides of V/Al/Nb, and Cr/Al/Nb

66 ternary combinations of 1% Pt, Pd, and In on Al2 O3

45 3- to 5-element combinations of Pt, Pd, Rh, Ru, Au, Cu, Ag, and Mn on TiO2 and Fe 2 O3

56 quaternary combinations of Pt, Pd, In, Na on Al2 O3

V2 O5/TiO2 mixtures

Oxidation on naphthalene to naphthoquinone

Ethane oxidative dehydrogenation

Oxidative coupling of methane and CO oxidation

Solution-based impregnation, deposition, and precipitation

30 binary combinations of Na2 WO4 and Mn on SiO2 ; Au and In on ZrO2 , TiO2 , SiO2 , MgO, ZnO, Nd2 O3 , Y2 O3 , CeO2 , Mn2 O3

Target reaction

Preparation method

Catalyst libraries


Laser-induced fluorescence imaging (LIFI)




Scanning quadruple mass spectrometry, photothermal deflection

Scanning quadruple mass spectrometry, photothermal deflection, gas chromatography


Screening method

177

176

175

174

173

172

171

Reference

942


Solution-based deposition Solution-based impregnation Solution-based sol-gel method

Solution-based sol-gel method

715 combinations of Pt, Ru, Os, Ir, Rh

280 V2 O5 , MoO3 , MnO2 , Fe 2 O3 , Ga2 O3 , La2 O3 , B2 O3 , MgO on Al2 O3

32 Ti-silsesquioxanes from 8 RSiCl3 /Ti (OiPr)4 ternary composition of cyclopentyl, cyclohexyl, and phenylsubstituted silanes

65 1 mol% main group, transition, and rare earth metal dopands on TiO2 , SnO2 , or WO3 These publications describe only the synthesis of heterogeneous catalyst libraries.

Solution-based impregnation onto Al2 O3 electrochemically formed on aluminum plate

52 Pt/Zr/V/Al2 O3

a


36 Pt/Zr/V/Al2 O3

Photooxidation of 4chlorophenol

Epoxidation of 1-octene with tert-butylhydroperoxide (TBHP)

Propane dehydrogenation

Standard HPLC equipped with sampling robot

Standard GC equipped with sampling robot


Fluorescence acid–base indicator

Scanning quadruple mass spectrometry coupled with 35 parallel microreactors

Oxidative dehydrogenation of isobutane

O2 reduction H2 O oxidation

Scanning quadruple mass spectrometry coupled with monolith multichannel reactor

Methane oxidation Co oxidation

182

181

180

179

178

32.8 Combinatorial Heterogeneous Catalysis 943

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before they become fully active, and most catalysts deactivate over time. These properties will also vary according to the reaction conditions. Some parallel screening schemes offer the capability of monitoring the reactions over time. For example, a multichannel fixed bed reactor coupled with parallel GC and an array microreactor equipped with REMPI allow library screening over a prolonged reaction time. Many techniques such as MS, IR, and other optical detection schemes can be applied in both homogeneous and heterogeneous catalysis. Each of these emerging techniques and tools for high-throughput catalyst screening will be described in detail in the screening section (see Chapter 32.10). Finally, the heterogeneous catalysis section will be concluded by case studies (see Section 32.8.2).

32.8.2

Case Studies 32.8.2.1 Oxidative Dehydrogenation of Ethane

The development of efficient heterogeneous catalysts for the gas-phase oxidative dehydrogenation of light paraffins is of particular interest because of the economic benefits of using paraffins directly as raw materials for the production of important base chemicals [174]. The low-temperature oxidative dehydrogenation of ethane to ethylene has been a research topic of consistent interest after the report of catalytic activity in the Mo/V/Nb/O system below 300 C in 1978 [183, 184]. The optimum compositional range for these catalysts was reported to be Mo0:61 – 0:77 V0:31– 0:19 Nb0:08 – 0:04 [183, 184]. Researchers at Symyx Technologies have reported efforts in reproducing and improving these catalyst systems using combinatorial methods. The strategy reported is based on a three-step approach: in the primary screen thousands of catalyst formulations are generated using automated solution-based methods as arrays of thick films on 7.5-cm wafer plates (dispensing volumes on a nanoliter to microliter scale). The films themselves were prepared from sol-gel precursors that were deposited by automated solution deposition methods. The typical layout of a ternary library is shown in Fig. 32.18. In this case the wafer holds a ternary AaBaC library consisting of two identical triangular matrices with 66 elements. The three corners in this library correspond to 100% A, 100% B, and 100% C. Thus, the compositional increments are 10% per matrix element for this kind of library layout. The libraries were then screened using simultaneous mass spectrometry and photothermal deflection spectroscopy. The best catalysts were consecutively synthesized on milligram scale (25–50 mg) for the secondary screen using parallel fixed bed reactors. The secondary screen provides conversion and selectivity data for the catalysts under realistic operating conditions. Catalysts that performed well in the secondary screen are further evaluated in tertiary screens using bench-scale reactors with gram quantities of the catalysts. To demonstrate the reliability of combinatorial methods compared with larger scale experimentation, Symyx Technologies scientists tried to reproduce Thorstein-

32.8 Combinatorial Heterogeneous Catalysis

Fig. 32.18. Library design for a 7.5-cm quartz wafer.

son’s catalyst formulations on a microgram scale and plotted their primary screening results versus Thorsteinson’s activity diagram [183]. Figure 32.19 shows that the primary screening data with conversions in the ppm range correlate well with bulk catalyst activity with percent level conversions. For greater compositional precision, researchers at Symyx Technologies investigated additional libraries focusing on the most active area. This fine-tuning of the catalyst resulted in an ethane conversion of 12.8% and an ethylene selectivity of 74.4% at an optimal composition of Mo73 V24 Nb3 . Including Sb, Ca, and Li as dopants improved the catalysts even further. Under identical synthesis and reaction conditions, the optimal composition reported by Thorsteinson, Mo73 V18 Nb9 , converts 6.1% at a selectivity of 83.2%. Considering new mixed metal oxide ternary libraries, for example Cr/Al/Nb/O systems, researchers at Symyx Technologies were able to identify catalysts that are significantly more active than the previously known MoaVaNbaO systems [173, 185, 186]. Oxidative Dehydrogenation of Propane Recently, Baerns, Senkan, and coworkers reported the impregnation method of catalyst preparation coupled with multitube packed bed reactors [171] and array microchannel reactors [187] within the framework of genetic algorithms [188] to find catalysts for the low-temperature oxidation of propane at low concentrations [175]. The catalyst libraries were prepared by an automatic liquid-dispensation system applying the incipient wetness method. A total of eight active components were selected as candidates for incorporation into TiO2 or Fe 2 O3 as carrier materials. 32.8.2.2

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Fig. 32.19. Comparison of the ethylene concentration reported

by Thorsteinson (left) in conventional experimentation (tens of grams of catalyst) and by Symyx Technologies (right) in their primary screen (micrograms of catalyst).

In the first step, a library containing 45 supported catalytic materials consisting of mixtures of active elements at different compositions was prepared in a stochastic manner in such a way that each catalytic material consisted of up to five of eight primary components. The libraries were then tested for their catalytic performance (conversion of C3 H8 to CO2 ) in both the multitube packed bed reactors [172] and in array channel microreactors [187]. The creation of the next library based on the results of the previous generation was accomplished using mutation and crossover operators of the genetic algorithm approach as applied to the previous generation catalysts [188], leading to the creation of another library also containing 45 supported catalytic materials. The authors were able to demonstrate that the application of genetic algorithm led to an improvement in activity. Significant propane conversions could be achieved at 50 C. Catalysts providing propane conversions on the order of 90% were also achievable at higher temperatures. The best catalytic materials were found to be rich in ruthenium; multimetal combinations were significantly superior [175]. More recently, the same group disclosed an evolutionary approach to optimize multimetal oxide (V2 O5 , MoO3 , MnO2 , Fe 2 O3 , Ga2 O3 , La2 O3 , B2 O3 , and MgO) catalysts supported on a-Al2 O3 for the oxidative dehydrogenation of propane to propene [180]. Thus, 56 catalytic materials were prepared for each generation, and this process was iterated five times according to the aforementioned genetic algorithm.

32.9 Combinatorial Electrocatalysis

Both manual and robotic synthesis were used for the library preparations, and these catalytic materials were tested in parallel. For the best materials, propene yields of 7% (first generation) to 9% (fifth generation) were achieved. Catalytic Oxidation of CO and the Reduction of NO These two processes were studied at Symyx using catalyst libraries of metal alloys of Rh, Pd, Pt, and Cu [168]. Triangular libraries ð15 15 15Þ containing approximately 2–4 mg of 120 independently prepared catalysts were synthesized by deposition methods using fully automated radiofrequency sputtering techniques. Libraries with similar catalysts were also prepared by sol-gel-based methods. The oxidation of CO was carried out in the presence of oxygen and the production of CO2 was determined by scanning mass spectroscopy. The amount of CO2 produced followed the trend Rh > Pd > Pt, which is in good agreement with results from experiments with single crystalline surfaces as well as supported catalysts. In subsequent libraries, one of the noble metals was replaced by transition metals. In the case of a library with Rh/Pd/Cu, the 1:1 composition of Cu/Rh showed activity very similar to that of pure Rh and the catalyst with 93% Cu and 7% Rh maintains 40% of the activity of pure Rh. In addition, Pt/Cu and Pd/Cu binaries show activity enhancements over the respective pure metals, with the enhancement being stronger at low temperatures than at high temperatures. The oxidation of CO by NO was investigated along the same lines using the abovementioned Rh/Pd/Pt library. The reaction was carried out at ambient pressure at various temperatures. As in the CO oxidation by oxygen, the activity for the oxidation of CO by NO follows the trend Rh > Pd > Pt similar to that reported in the literature for bulk catalysts. 32.8.2.3

32.9

Combinatorial Electrocatalysis

Electrochemical reactions play an important role – from energy production in fuel cells and batteries to industrially important processes such as the production of chlorine and the electrohydrodimerization of acrylonitrile to adiponitrile that is used in the production of nylon. Electrocatalysis deals with the modification of the overall rates of electrochemical reactions so that selectivity of these reactions, their yield, and efficiency are maximized. The main focus is on the electron-transfer reactions that take place at the electrode/electrolyte interface with electrochemical parameters such as potential and current being the major driving forces for the reaction. 32.9.1

Electrocatalysts for Fuel Cells

Fuel cell technology holds the promise of low to zero emission energy for power plants, backup generators, and in the transportation sector. Fuel cells will also offer

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fuel efficiency in the order of 50% or more. With the increasing interest in fuel cells as alternative energy sources for stationary power, portable, and automotive applications, there is a need to develop new anode and cathode electrocatalysts. The cost of the current precious metal-based electrocatalysts is a considerable hurdle to the successful commercialization of fuel cells. The oxidation of methanol in fuel cells has been extensively studied for several decades. At present, however, the commercialization of the direct methanol fuel cell (DMFC) is beset by several problems. Given the lower electrochemical activity of methanol in comparison with hydrogen, much higher loading levels of noble metal catalysts must be employed. The most efficient anode systems for the complete electro-oxidation of methanol consist of the platinum metals and mixtures or alloys thereof. Notwithstanding the high cost issues, presently employed anode systems suffer from complications due to catalyst poisoning by CO, which leads to both an unacceptable loss in cell voltage and degradation in long-term performance. Numerous studies have indicated that the most promising anode systems consist of either a supported or unsupported binary alloy of Pt/Ru [189, 190]. The traditional approach to evaluating anode materials has involved the relatively time-consuming and laborious practice of synthesizing and characterizing materials one at a time. Mallouk, Smotkin, and coworkers developed a rapid combinatorial synthesis and fluorescent screening method for the exploration of ternary and quaternary anode electrocatalysts for the DMFC (Fig. 32.20) [157]. The authors created libraries of alloys by using automated inkjet deposition of metal precursor aliquots onto a conducting fiber paper, followed by reduction of the metal salts with a borohydride reagent. The authors created 135 unique ternary compositions

Fig. 32.20. Photo images of the different

stages of discovery of a ternary alloy in array format using a quinine indicator. a) Image in white light. b) Fluorescence image at low

overpotential, identifying the most active region of composition space. c) Fluorescence image at high overpotential, where methanol oxidation occurs at every spot in the array.


on nine exploratory library arrays. These compositions were based on five transition metals: Pt, Ru, Os, Rh, and Pd. After treatment, the libraries were immersed in an aqueous methanolic sodium sulfate electrolyte containing a fluorescent pH indicator. Screening was performed with a typical three-electrode cell (electrode array as working electrode, Pt counter, and saturated calomel reference electrode) by conducting a single potential sweep from cathodic to anodic potentials. The electrocatalysts are all on a common working electrode, and, as a result, it is not possible to directly measure the electrochemical activity at each electrocatalyst. However, upon the oxidation of methanol, it is expected that the pH will drop close to the regions where methanol is being oxidized. The conducting Toray paper was illuminated with an ultraviolet lamp and observation of the highest fluorescent intensities due to local proton release (pH drop) upon electro-oxidation of methanol determined the catalytically most active spots on a given library. The composition performances were compared with Pt/Ru (50:50 atom%). The best composition was given as Pt/Os/Rh (60:25:15 atom%). Later, these authors prepared, screened, and analyzed a 645-member electrode array of Pt, Ru, Rh, Ir, and Os consisting of 80 binary, 280 ternary, and 280 quaternary combinations [191a–c]. The most active catalyst was found to be Pt/Ru/Os/Ir (47:29:20:4 atom%). Results of these ‘‘zoom screens’’ are shown in Fig. 32.21. Most recently, Mallouk and coworkers applied a combinatorial approach to the discovery

Anode polarization curves (V vs. RHE) of NaBH4 reduced catalysts from ‘‘zoom screens.’’ Fig. 32.21.

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Fig. 32.22. Symyx Technologies electrode array consisting of 64

individually addressable electrodes prepared on an insulating 7.5-cm quartz substrate using lithographic techniques.

of bifunctional oxygen reduction/water oxidation electrocatalysts for regenerative fuel cells [181]. Scientists at Symyx Technologies have developed another combinatorial approach for the rapid synthesis and screening of fuel cell electrocatalysts. Libraries of alloy materials are synthesized directly onto an 8 8 electrode array (Fig. 32.22) which consists of 64 individually addressable electrodes prepared on an insulating 7.5-cm quartz substrate using lithographic techniques. The electrode material can be made from gold or titanium. Each electrode is approximately 1.0 mm diameter and electrical contact is made with each electrode via a contact pad on the edge of the wafer. Electrocatalysts were synthesized by a combination of magnetron sputtering or by parallel electroplating. The electrode array was then sealed to a Teflon cylinder that was filled with aqueous electrolyte. All 64 electrodes were therefore exposed to the same electrolyte solution. A common reference electrode and working electrode was immersed in the cell. The array was interfaced to a 64-channel potentiostat via a PCB interface and the electrocatalytic performance of the resulting materials were tested by parallel monitoring of the current–voltage time behavior of each individual electrocatalyst. The authors tested a number of catalytic concepts for the anodic electrooxidation of methanol (DMFC) as well as the cathodic electroreduction of oxygen in aqueous acidic electrolytes [192]. The authors demonstrated that for known systems the electrochemical activity as measured directly on the thin-film samples on the addressable electrochemical array correlated with the activity of known powder samples.


32.9.2

Combinatorial Electrosynthesis

Ward and coworkers applied two different combinatorial approaches to study the electrochemical reduction of 1,4-benzoquinone to hydroquinone at organosulfurmodified gold electrodes [193]. The authors prepared an array of physically but not electrically isolated gold electrodes on a glass substrate. Monolayers of organosulfur reagents were prepared on selected electrodes on the 4 7 array by dispensing aliquots of either hexanethiol or hexadecanethiol dissolved in ethanol at the respective electrodes. The array was subsequently rinsed and housed in a thin-layer electrochemical cell. The electrolyte contained 1,4-benzoquinone dissolved in 0.5 M KOH acidified to pH 3.0 and also a fluorescent dye, fluorescein, which fluoresces green at pH > 6. A potential of 0.1 V (vs. Ag/AgCl) was applied to the array and the thin-layer cell illuminated with an ultraviolet lamp. Green fluorescence was observed in those electrode regions that had not been modified with an organosulfur reagent as well as on electrode regions that had been modified with a monolayer of hexanethiol. Benzoquinone is reduced by two electrons at this potential with the consumption of two protons. It is the decrease in protons at the electrodes where benzoquinone is reduced that causes the local pH to increase. No fluorescence was observed at the hexadecanethiol-modified electrodes. In a second approach, the authors created an electrochemically addressable electrode array from which it was possible to measure directly the electrochemical activity by monitoring the current–voltage behavior at each individual electrode. An 8 8 gold electrode array was created on an insulating thermally oxidized silicon substrate ensuring physical and electrical isolation of the individual electrodes using standard lithographic techniques. Electrical contact was made via standard 64-pin connectors with a single channel potentiostat and multiplexer. Selected electrodes were subsequently modified with hexanethiol, hexadecanethiol, or dodecanethiol monolayers through a series of adsorption and electrochemical desorption steps. An electrochemical cell was created by attaching the electrode array to a Delrin cylinder with a Delrin cap configured for a reference and counter electrode. Electrolyte containing benzoquinone and KOH acidified to pH 3 was added to the cell, exposing all 64 electrodes to the electrolyte. In a serial manner, the electrochemical reduction of benzoquinone was studied at each electrode by cyclic voltammetry in which the potential of each electrode was cycled and the current measured. The authors proposed that this direct measurement of the electrochemical activity was more sensitive than the fluorescent screen in quantifying the benzoquinone reduction at the modified electrodes with the results indicating that the activity in the library increases in the order of hexadecanethiol < dodecanethiol < hexanethiol < gold, illustrating suppression of current with increasing alkanethiol chain length. The authors propose that the fluorescent screen allows for measurement of active zones while electrochemical screens can be used to discriminate smaller differences in activity. Yudin and coworkers demonstrated how combinatorial electrochemistry could be used in the electrosynthesis of small organic molecules [194a]. The authors de-

951

952


veloped what they term a spatially addressable electrolysis platform (SAEP). Each electrochemical cell in the 4 4 array was equipped with a stainless-steel cathode and a graphite rod anode. The cathodes were welded directly onto a stainless-steel support that provided a common terminal for the current source. The authors explored the anodic oxidation of carbamates, amides, and sulfonamides leading to libraries of a-alkoxycarbamates, a-alkoxyamides, and a-alkoxysulfonamides, respectively. The authors have also demonstrated the intramolecular cyclization of hydroxyamides yielding heterobicyclic compounds as well as the generation of vicinal diamines by the reductive hydrocoupling of aldimines. In another application, Yudin and coworkers generated libraries of catalytic materials on electrode surfaces by the copolymerization of bithiophene and pyrrolecontaining TEMPO (2,2,6,6-tetramethilpiperidin-1-yloxy) catalysts [194b,c]. Diversity was created by electrochemical copolymerization and by creating surfaces with different ratios of bithiophene/pyrrole. These catalyst films were utilized in the electrochemical oxidation of primary alcohols to aldehydes, where cyclic voltammetry was used to screen the catalytic activities of the modified electrodes. Another example of a high-throughput screen for catalyst activity comes from Hillier and coworkers, who applied a scanning electrochemical microscope to characterize the hydrogen oxidation reaction on a polycrystalline platinum surface [195]. This technique utilizes tip-sample feedback and works reliably for determination of the kinetics of the reaction over a large range of substrate potentials from the hydrogen adsorption region to the platinum oxidation region. Hillier explored this technique further and directly demonstrated the measurement of the rate constant for hydrogen oxidation and performed reactivity mapping of heterogeneous electrodes consisting of catalytic and noncatalytic domains.

32.10

Novel High-throughput Screening Tools

The process of discovering new catalysts or materials from a large pool of potential candidates requires a reliable and robust screening process. Traditionally, this process presents itself as a bottleneck in the discovery effort, especially if large libraries of potential catalysts or materials have already been synthesized or prepared. The first step in combinatorial catalysis, like all materials design, involves the identification of a specific chemical transformation of interest. A collection of potential catalysts, prepared from a set of chemically diverse ligand sets and metal precursors, are combined in a parallel or combinatorial fashion and screened for activity in a high-throughput primary screen. Although the large volume of potential catalysts examined in a primary screen typically allows only relative activity to be established, the goal at this stage of discovery is to identify promising lead catalysts worthy of further investigation and follow-up. Lead materials are then further examined in a high-throughput secondary screen designed to screen for specific trends in the physical and chemical properties in greater detail and typically at a slightly lower throughput than the primary screen. The information obtained from the second-

32.10 Novel High-throughput Screening Tools

ary screen is then used to prepare additional generations of catalysts that can be optimized into a superior catalyst worthy of commercialization. Typically, the information necessary to classify new materials cannot be obtained from a single piece of characterization equipment. Therefore, a series of highthroughput screening tools is employed at various stages in the combinatorial process. While a large number of automated commercial systems exist for highthroughput analysis of microliter quantity samples for medical and pharmaceutical applications, the vast majority of the screening tools necessary for advanced materials research are custom-made instruments. Conventional analytical tools such as mass spectrometry, gas [196] and liquid chromatography, electrophoresis [197, 198], Raman [199] and nuclear magnetic resonance spectroscopy, X-ray fluorescence microprobe [182], and X-ray diffraction [200] have been automated and redesigned for rapid serial measurements of hundreds of samples per day [201]. A number of recent reviews of combinatorial catalysis discuss high-throughput screening techniques [20c,d,h, 151, 202–204]. Several examples of novel high-throughput screening systems are discussed below. 32.10.1

Infrared Screening Tools

Infrared detection has become an extremely popular technique for the analysis of combinatorial libraries because it is generally noncontact, nondestructive, and amenable to very high-throughput screening. Use of the infrared can be divided into two general categories: thermal imaging (thermography) and infrared spectroscopy. 32.10.1.1 Infrared Thermography

One of the first truly parallel high-throughput screens utilized infrared thermography to identify catalytic activity. Infrared radiation is emitted by all materials above 0 K according to the Stefan–Boltzmann Law, W ¼ esT 4 , where W is the emitted power density, e the emissivity (e ¼ 1 for a blackbody), s is the Stefan–Boltzmann constant, and T is the temperature. Modern infrared cameras utilize a photovoltaic focal plane array (FPA) detector made of InSb, HgCdTe, or PtSi to convert infrared radiation into digital images. It is important to note that the infrared cameras measure emitted or reflected intensity, not temperature, and a calibration must be performed to relate the intensity images to temperature. Pawlicki and Schmitz first reported using infrared thermography to monitor the dynamics of reactions on solid surfaces in 1987 [205], and Sermon and coworkers applied this technology to the analysis of temperature profiles of exothermic reactions on silicon oxide-supported platinum catalysts [206]. Time-resolved infrared thermographic detection and infrared emission analysis of temperature profiles enable virtually any reaction to be monitored in a truly parallel fashion [63, 207]. As the field of combinatorial materials science began to heat up in the late 1990s, several groups applied infrared thermography to the search for new catalysts. Willson and coworkers employed infrared thermographic imaging to identify possible

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Fig. 32.23. Schematic of a catalytic reactor for infrared thermography.

formulations of heterogeneous catalysts for the oxidation of hydrogen to water [164]. The catalyst entities on the array were prepared in a conventional manner by impregnating g-aluminum oxide pellets as a catalyst support with aqueous stock solutions of 16 metal salt precursors. After reduction to zero-valency metals by exposure to pure hydrogen, the pellets were manually placed in an aluminum reactor and assayed in a parallel manner in a spatially addressable format. The reactor, shown schematically in Fig. 32.23, was equipped with an infrared imaging camera and devices to control reaction conditions such as gas flow, heating rates, and data collection/analysis. After calibrating the radiation intensity with respect to temperature and equilibrating the reactor at 35 C with hydrogen, a gas stream containing 5 vol% of oxygen was introduced into the hydrogen feed stream. Hydrogen oxidation activity was found for pure Pd, Pt, and Ir within 10 s of oxygen introduction. Heating the reaction chamber up to 300 C ‘‘ignited’’ the Rh-loaded pellet at approximately 82 C and the activity was measured with the camera as an increase in catalyst temperature (relative to the background) while continuously heating the reaction chamber. Although basically a proof of concept, the experimental procedure proved useful for parallel screening of new catalyst formulations and evaluating operational issues such as catalyst lifetime, resistance to poisoning, and regenerability [208]. Direct thermal imaging of combinatorial libraries for activity is very useful as a qualitative or semiquantitative tool for determining whether catalytic activity is present in a particular material. True quantitative determination of the temperature profile for a given library of materials is complicated by the difference in emissivity between members of the library. The emissivity of an opaque material is related to the reflectivity (lower reflectivity means higher emissivity), which is dif-


ferent for most elements in a catalyst library. Furthermore, emissivity must be calibrated as a function of wavelength and temperature. The absolute reflectivity and emissivity characteristics of a new material are not known for most highthroughput experiments, which means that the proportionality factor used to calculate the temperature profile from the emitted infrared radiation is generally not available. Emissivity can be accounted for (to some degree) by the use of linear corrections to the detector response, and subtraction of a reference image typically taken at the beginning when the experiment is cold. These corrections minimize emissivity effects for systems where the emissivity varies slowly with temperature, such as metals and metal alloys. In 1998, Maier and coworkers applied emissivity-corrected infrared thermography to detect activity in heterogeneously catalyzed gas-phase reactions on a model library [165]. The researchers chose transition metal-impregnated, amorphous microporous mixed oxide (AMM) supports, a class of materials previously shown to possess unusual properties as bulk catalysts or as catalytic membranes in selective oxidation, hydrocracking, hydrogenation, etherification, and esterification reactions [209–211]. The catalytic hydrogenation of hexyne and the oxidation of isooctane and toluene were chosen as their test reactions. With automated sol-gel procedures, less then 200 mg of each catalyst was deposited on a low-reflection slate substrate followed by controlled drying, calcination, and reduction to afford a catalyst array. A high-sensitivity infrared camera with a platinum silicide (PtSi) FPA was used to monitor the heat evolution upon catalytic conversion. The same catalyst array was screened in a customized parallel reactor and on a catalyst microgram scale under appropriate conditions for three test reactions. Several effective catalysts for each of the targeted conversions were identified. Further extensions of the technology came in 1998 when Reetz and coworkers stressed the general usefulness of infrared thermography to the time-resolved screening of liquid-phase catalytic reactions, such as enantioselective hydrolytic ring opening of epoxides to nonracemic diols and a lipase-catalyzed acetylation of a secondary alcohol [207a]. The experiment utilized a modified microtiter plate consisting of a commercial Eppendorf Thermomixer with the top replaced by an aluminum plate in which holes were drilled then filled with 8 32 mm glass vials. As a model reaction, Reetz chose the enantioselective lipase-catalyzed acylation of (R)-, (S)-, and rac-1-phenylethanol with vinylacetate. The reaction was followed with the infrared camera, periodically acquiring 250 images of the library, which were then averaged and visually inspected. To demonstrate the screening capability of the infrared camera under homogeneous conditions, the activity and selectivity of three metal catalysts were tested thermographically in the hydrolysis of epichlorhydrin. Finally, relative substrate activity was screened studying the hydrolysis of three different chiral epoxides with the cobalt catalyst that was found to be most active in the previous screening of epichlorhydrin hydrolysis. In all cases, relative trends in the activity and selectivity of the catalytic reactions were reproduced from the available literature.

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The same group used IR thermographic screening for thermoneutral or endothermic transformations, in this case ring-closing olefin metathesis [207b]. Four different ruthenium-based olefin metathesis catalysts were screened for four different types of ring-closing metathesis reactions. Highest catalyst activity is identified by heat uptake from the surroundings, as monitored by the appearance of ‘‘cold spots.’’ The heat of vaporization of one of the reaction products (ethylene or propylene) plays a crucial role in this process. Infrared thermography holds promise for more extensive application of this technology in homogeneous, heterogeneous, organic, and inorganic catalysis research. However, thermal imaging does not resolve the product composition of the catalyst, an important limitation in chemical catalysis. 32.10.1.2 High-throughput Infrared Spectroscopy

Spectroscopic techniques are extremely popular for the analysis of combinatorial libraries because of their speed, nondestructive nature, and relative ease of use. In addition, infrared spectroscopy is very useful for identifying structural properties of organic solid-phase-supported combinatorial libraries. Indeed, many commercial Fourier transform infrared (FTIR) spectrometers have been equipped with an automated attachment to allow multiple compounds to be screened in rapid succession. Single-bead FTIR microspectroscopy has been utilized by chemists for quite some time to analyze spectral information of products on solid supports. Extension of this technique to combinatorial chemistry in the form of a scanned highthroughput screen has been reported by Jung and coworkers [212]. In Jung’s method, polymer-bound resin beads modified through combinatorial synthesis were withdrawn from the reaction vessel and embedded in a KBr window. The KBr window was then placed on the automated x/ y stage of an infrared microscope and the spectra mapped. Data were presented as a map of the infrared absorption as a function of position across the KBr plate. Direct identification of resin-bound molecules is possible by superposition of maps taken at different absorption wavenumbers. Jung and coworkers mapped approximately 300 different resin beads at a rate of approximately 5 h per wavenumber map. While extremely useful, scanning spectroscopic measurements have a number of limitations stemming from the relatively slow speed of the x/ y scanning methodology. As combinatorial libraries increase in size, the time necessary to screen libraries in a serial manner becomes cumbersome; the time being dependent on the number of scans averaged, the wavenumber resolution, and the number of elements in the library. Incorporation of the microscope and x/ y scanning stage into a chemical reaction chamber in which the pressure and temperature can be varied is also complicated and expensive. Finally, the time needed to scan from point to point virtually eliminates the ability to derive kinetic information from the measurement. True parallel spectroscopic measurements rely on the ability of the analytical technique to simultaneously collect information from multiple samples. Parallel measurements offer the throughput and temporal resolution necessary to measure the kinetics of the large number of catalytic reactions possible in combinatorial


Fig. 32.24. Optical layout for an imaging FTIR

spectrometer. The light source is composed of an infrared source (S), a KBr beam splitter (BS), a moving mirror (M1), and a stationary mirror (M2). The optical set-up is composed of

a bandpass filter (F), a KBr diffuser (D), CaF2 plano-convex lens (L1), reactor (R), and CaF2 biconvex lens (L2). The infrared camera with Hg/Cd/Te detector array (FPA) acts as the detector.

studies. Lauterbach and coworkers have constructed an imaging FTIR capable of collecting spatially resolved spectral data for 4096 samples in the 1360–2720 cm1 spectral range (8 cm1 resolution) in less than 20 s (Fig. 32.24) [213–215]. Depending on the optics used, the field of view can be varied from an area of a few hundred square microns to several square centimeters. The system, shown schematically in Fig. 32.24, consists of an FTIR spectrometer, infrared optics, and a 64 64 element HgCdTe (MCT) infrared camera. A detailed description of the instrument can be found in elsewhere [215]. Imaging FTIR spectroscopy was utilized by Lauterbach and coworkers to investigate the effects of adsorbed CO on both Cu-ZSM5 zeolite and silica-supported Pt/ SiO2 catalysts as a function of different process conditions [213a, 214]. CO was preadsorbed onto the catalysts and the temperature increased at a rate of 8 K min1 in flowing oxygen. Spectral images collected during the heating ramp identified distinct spectral bands assigned to CO adsorbed on the Cuþ oxidation state (2157 cm1 ) and to CO adsorbed on the Cuþ oxidation state with a water molecule in the coordination sphere of the Cuþ ion (2139 cm1 ). As the temperature increased and the water desorbed, a decrease in the band at 2139 cm1 was observed. The entire heating process took 11 min and 13 spectral images (4096 spectra per image) were acquired at a spectral resolution of 4 cm1 . Following a chemical process in a highthroughput manner demonstrates the ability of imaging techniques to monitor chemical reactions in situ. Imaging allows kinetic information and spectral data on reaction components to be gained from a combinatorial high-throughput screen. The same group also applied the parallel FTIR analysis to determine conversion during temperature-programmed complete oxidation of propene in the presence of platinum group metals, a reaction that is important for the automotive three-way catalyst [213b]. 32.10.2

Optical High-throughput Screening Techniques

The effort to create parallel assays for solution-phase catalysis has led to the development of a number of optical techniques. Many techniques include probe mole-

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cules that change color or fluoresce with catalytic behavior. Additional techniques incorporate ultraviolet or infrared lasers to ionize or heat catalytic products, followed by detection with electrodes or probe lasers. Finally, circular dichroism has been developed to study the effects of chirality on catalysis. 32.10.2.1 Colorimetric Assays

Visual detection remains one of the simplest and most practical methods for measuring catalyst activity. While direct assays such as thermal imaging are fast and generic to most chemical processes, they lack chemical selectivity and are unable to identify reaction products. One approach, used extensively in enzymatic assays, is to use indirect detection of a molecular probe that fluoresces upon detection of a desired reaction product. Crabtree and coworkers have developed reactive dyes that photobleach as a chemical reaction occurs [216, 217]. Designed for hydrosilation reactions, the probe must not have any interfering reactive groups, a criteria met by using a ferrocenyl group as an electron donor and a pyridinium group as an acceptor (Scheme 32.36); a benzylic tail was added to make the dye more soluble. When the reactive functionality (CbC or CbN bond) is saturated upon reaction with a catalytic species, the electronic overlap between the donor and acceptor groups is diminished, giving rise to a loss of the parent dye color. The dye color as a function of time was recorded, with a hit being indicated by the dye bleaching in a reaction well.

Scheme 32.36. Reactive dyes as a method for rapid screening of homogeneous catalysts.

Crabtree and coworkers assayed a 60-well discovery library of hydrosilation catalysts in parallel using a digital camera. Of the 12 catalysts examined by the authors, Wilkinson’s catalyst, which is a known hydrosilation catalyst, was among the most active of the catalysts screened. However, a palladacyclic Heck reaction catalyst was also quite active, a compound not previously reported as a hydrosilation catalyst. A limitation of the screen is that the dye substrate is a nonstandard alkene with a higher reactivity than conventional substrates and has a strong tendency to give competitive hydrogenation instead of hydrosilation. Despite this, relative trends in activity seem to be well represented. Hartwig and coworkers utilized resin beads tagged with fluorophors in the investigation of coupling reactions [218]. Their method allows for visual screening of


a large set of parallel chemical reactions in which two molecules are bound by covalent interactions. In their study, one substrate (A) is attached to a dye molecule and the other (B) to a solid support. After a successful coupling reaction, the solid supports and the dye molecule would be bound together by a covalent interaction between the two substrates (Scheme 32.37). Analysis under UV illumination following filtering of the reagents allowed the substrate combinations that are capable of covalent coupling to be identified. In their investigation, an acrylate containing a tethered coumarin was reacted with an aryl halide supported on a crosslinked polystyrene bead. Comparison of the results from the fluorescence assay with results from a standard GC analysis showed that the fluorescence assay accurately represented the trends for the Heck coupling of aryl bromides and chlorides. Two ligands identified in the assay, tri-(tert-butyl)phosphine and di-(tert-butylphosphino)ferrocene, were shown to be the most active systems for the olefination of unactivated aryl bromides, and di-(tert-butylphosphino)ferrocene the most efficient for olefination of unactivated aryl chlorides.

Scheme 32.37.

Visual assay for coupling reactions.

Hartwig and coworkers then focused on the development of solution-phase assays for homogeneous catalysis that are based on fluorescent resonance energy transfer (FRET) between a substrate with a tethered fluorophor and a second molecule that is attached to a solid support [219]. FRET occurs when two fluorophors within close proximity (20–80 A˚) interact such that the emission band of one molecule overlaps the excitation band of a second molecule. Excitation of the higher energy fluorophor (donor), followed by resonant energy transfer to the lower energy fluorophor (acceptor), leads to a quenching of the fluorescent intensity. With a constant total concentration of free and bound FRET pairs, the emission at the donor molecule wavelength is inversely proportional to the mole fraction of paired molecules. Standard plate readers can be used to track the covalent bond formation fluorometrically. In the investigation of Heck reactions, Hartwig and coworkers chose fluorophores that contained functionalities that are compatible with most cross-coupling chemistries. A dansyl fluorophore was tethered to a styrenyl group, and an azodye quencher tethered to an aryl bromide. Upon covalent linking, the danzyl group was quenched by the diazo compound (Scheme 32.38). The results of their investigation show that the FRET method is significantly faster than standard HPLC techniques, while only one in ten cases showed a yield by

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HPLC that was more than 10% different from the FRET result. It was suggested that the FRET pair developed for this study should have application to many different reactions, such as aryl halide amination, aryl halide etherification, carbonyl a-arylation, Suzuki coupling, and Hiyama coupling with silanes.

Assay for homogeneous catalysis based on fluorescent resonance energy transfer (FRET).

Scheme 32.38.

Copeland and Miller developed a similar method for the study of acetic acid evolution that employs aminomethylanthracenes as pH-sensitive fluorophors. Neutral aminomethylanthracenes undergo photoinduced electron transfer (PET) and fluoresce when protonated (Fig. 32.25a) [220]. Attachment of the aminomethylanthracenes to partially derivatized resin beads, followed by attachment of peptide catalysts, created a collection of resin beads related to the structure shown in Fig. 32.25b [221]. Beads functionalized with the most active catalysts appeared brightest as a given acylation reaction proceeded. Furthermore, the beads maintained their relative intensities when examined in separate vessels or pooled together as catalyst mixtures. Copeland and Miller have applied the same aminomethylanthracenes as pH sensors in solution-phase catalyst libraries as well [220]. In this investigation, seven unique catalysts (Fig. 32.26) were deposited into a standard 96-well plate at three


Fig. 32.25. a) Acetic acid sensor in acyl transfer reactions.

b) Representative polystyrene bead simultaneously functionalized with a sensor and a catalyst.

Fig. 32.26. Seven different acylation catalysts used for the

chemosensor-based approach to catalyst discovery.

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Fig. 32.27. Fluorescent gel system for the detection of bead-supported catalysts.

different catalyst loadings. The reaction progress was monitored as a function of time with a standard fluorescence plate reader, allowing individual reaction rates to be determined. The known superacylation catalysts 4-pyrollidimo-pyridine (PPY) and DMAP were the most active catalysts, producing rapidly growing fluorescent signals. N-methyl-imidazole (NMI) and pyridine gave very low reaction rates. Most significantly, the fluorescent intensities are consistent with the observation that the catalytic activity of A is greater than B and NMI. A sensor-functionalized polymeric gel for screening pooled catalyst libraries has been developed by Miller and coworkers [222]. The method involves deposition of resin-bound catalysts onto a polymeric matrix that is designed with sufficient permeability such that reagents can diffuse to the beads. The polymer also incorporates (by covalent attachment) a fluorescent probe that signals the presence of reaction products. The method (Fig. 32.27) relies on slow diffusion of reaction products out of the bead into the matrix, which triggers the probe and creates a fluorescent zone around the active catalyst. Miller and coworkers used the same aminomethylanthracenes as in the previous experiments to investigate acylation reactions of alcohols with acetic anhydride; poly(ethylene glycol)dimethylacrylamide (PEGA) was used for the polymer matrix. Smotkin, Mallouk, and coworkers have developed an optical screen for electrooxidation that utilizes a fluorescent pH indicator [157]. In their method, catalyst compositions were applied to Teflon-coated Toray carbon discs such that each 2mm-diameter catalyst spot contained the same molar concentration of metal at a


Fig. 32.28. Fluorescent pH indicators used in an optical screen for electro-oxidation.

loading of approximately 1 mg cm2 [179]. The catalyst array was analyzed using a three-electrode gas diffusion cell, with the Toray carbon substrate linking the catalyst elements as a working electrode, Pt gauze as the counter electrode, and a reversible hydrogen electrode as a reference. Electrochemical half-cell reactions either generate or consume ions, creating a change in the pH in the location of active catalysts. Utilizing an indicator that is fluorescent in the presence of an acid or conjugate base allows the determination of which elements within the library were most active for a particular anode or cathode reaction. Mallouk and coworkers have used quinine and Phloxine B as fluorescent indicators for neutral pH, and Ni 2þ complexed with 3-pyridine-2-yl-(4,5,6)triazolo-(1,5-a)pyridine (Ni-PTP) (Fig. 32.28) for low pH [157]. Using the fluorescence method, Mallouk and coworkers screened ternary libraries of metal alloys and identified novel electrocatalysts for methanol and bifunctional oxygen reduction/water oxidation regenerative fuel cells [157, 179]. Additional optical screening techniques have been developed for a number of different chemical processes. The methods include colorimetric assays where colorless 1-naphthol undergoes an electrophilic aromatic substitution with a diazonium salt to give a bright orange azo product [223], indigo [61], and Prussian blue staining of reaction products [224]. 32.10.2.2 Resonance-enhanced Multiphoton Ionization (REMPI)

In a proof of concept experiment, Senkan has described high-throughput screening technology for combinatorial catalyst libraries that may supply data on both activity and selectivity for a dehydrogenation reaction [187]. Senkan’s approach is

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Fig. 32.29. Illustration of high-throughput screening of

combinatorial catalyst libraries by resonance-enhanced multiphoton ionization (REMPI). See text for details.

based on in situ photoionization of the reaction products using tunable UV lasers, followed by the detection of the photoions or electrons by spatially addressable microelectrodes placed in the vicinity of the laser beam (Fig. 32.29). When the laser frequency matches an intermediate electronic state of a molecule, the cross-section for photoionization is enhanced, resulting in resonance-enhanced multiphotonionization (REMPI). Senkan and coworkers reported the application of the REMPI technology together with novel array microreactors to the discovery of an optimal composition in the ternary Pt/Pd/In metal catalyst system [161]. Preformed cylindrical g-Al2 O3 supports were treated with aqueous metal precursor stock solutions of H2 PtCl6 , PdCl2 , and InCl2 , dried, and calcined. The catalyst library consisted of 66 ternary combinations of Pt, Pd, and In in 0.1% weight increments of each, with 1% total metal loading on the alumina support. Catalyst screening was performed after conditioning the reactor unit and reduction of the metal precursors to metallic form with hydrogen. Contrary to a previous report [161], the researchers confirmed that no species other than benzene contributed to the signals measured by the microelectrodes. Indium proved to be catalytically inactive in the dehydrogenation and Pt and Pd individually exhibited catalytic activities, with Pt being more active than Pd. Most importantly, a ternary mixture (0.8% Pt, 0.1% Pd, 0.1% In) exhibited the best benzene productivity of all the 66 ternary metal combinations tested. The screen for the entire catalyst library (66 catalysts) required five cycles in a time window of approximately 23 h and the total time for discovery (library preparation


Fig. 32.30. Schematic of a photothermal deflection cell for the detection of ethylene.

and screening) took 2.5 days, including additional checks for reproducibility of the results. A similar method utilizing laser-induced fluorescence has been developed by Su and Yueng [177]; however, the technique is only applicable to fluorescent species, which limits its use. 32.10.2.3 Photothermal Deflection

A photothermal deflection method has been developed by Cong and coworkers at Symyx Technologies for the purpose of screening potential catalysts for the oxidative dehydrogenation of ethane to ethylene [172]. The experimental set-up (Fig. 32.30) utilizes a CO2 laser operating at a single line (10P14) as a pump in resonance with an ethylene mode at 949.48 cm1 . Absorption by the ethylene creates a local ‘‘hot’’ spot in the gas, which causes a change in the index of refraction (a mirage). A 10-mW HeNe laser is used to detect changes in the index of refraction through a slight deflection in the probe beam, which can be detected using a position-sensitive photodiode. In their studies, Cong and coworkers used a scanning probe (described with the scanning mass spectrometer later in this chapter) that sampled gaseous products into a small measurement chamber. The instrument’s sensitivity for eth-

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ylene is better than 0.1 ppm and has a very high discrimination factor (@ 10 6 ) against ethane and other species present in the reaction. Enantiomeric Excess by Circular Dichroism A number of reports have appeared utilizing combinatorial strategies to facilitate the discovery and optimization of chiral catalysts. These reports have successfully addressed the synthetic combinatorial aspects of chiral catalyst discovery. However, they still rely solely on conventional means determining the enantioselectivity using HPLC or GC with chiral columns, a time-consuming process that creates a potential bottleneck in any combinatorial discovery effort. Mikami and coworkers have developed a new screening strategy employing circular dichroism (CD) detection added to HPLC on nonchiral stationary phases for the rapid determination of enantiomeric excesses (ee values) of a reaction product without separation of the chiral products [225, 226]. Circular dichroism is observed when optically active matter absorbs left- and right-hand circular polarized light differently. The difference in left- and right-hand absorbance is very small, resulting in a small CD signal De. The absorption e provides a dissymmetry factor g ¼ De=e, which is independent of product concentration and is linearly related to the ee. In order to validate the approach, diethyl zinc was added to prochiral aldehydes in the presence of chiral diol ligands (L1*, L2*, . . .) and chiral nitrogen-based activators (A1*, A2*, . . .). Reactions were screened in 1-mL polypropylene microtubes and the crude reaction products were then autosampled with a HPLC instrument equipped with CD detection. Combining diethyl zinc with an assortment of chiral ligands and activators led the researchers to identify an extremely effective ligand/ activator combination that gave excellent ee values in high overall yield with a variety of aldehydes. Although the authors dubbed the above technique as ‘‘super highthroughput screening,’’ the technique may still be somewhat limiting in terms of its overall throughput (@ 3 min per sample) and it is probably best described as a rapid serial technique. 32.10.2.4

32.10.3

High-throughput Screening Using Mass Spectrometry

Mass spectrometry is an established technique used to analyze complex gaseous mixtures. The technique allows for the direct determination of many reaction products; however, it has been limited in application because of its serial nature and because it requires sample withdrawal. Several groups have created scanning probes to extract product gases from combinatorial experiments, and new techniques have been developed to extend the throughput of mass spectrometry. 32.10.3.1 Scanning Mass Spectrometry

Cong, Guan, and coworkers have developed an instrument for the rapid screening of heterogeneous catalysts using a scanning mass spectrometer (Fig. 32.31) [168, 227]. The instrument is divided into two parts: a reaction chamber where each library element is examined for catalytic activity, and an analysis chamber where


Fig. 32.31. Schematic of the scanning mass spectrometer for

screening catalyst libraries for catalytic activity. See text for details.

the products from the catalytic process are ‘‘sniffed’’ from the reaction chamber and analyzed by a mass spectrometer. In addition to the two chambers, ancillary equipment is used to control the positioning of the library and to maintain variables such as the flow rate of reactant gases, temperature, and reaction pressures. In a typical experiment, a catalyst library is placed onto a platform capable of translation in three orthogonal directions. Gas flows to and from the library through a cylindrical delivery tube containing a second concentric internal tube. A reactant gas flows down the annular region of the tube onto the quartz substrate. Product gas flows from an element in the library to the analysis chamber through the inner tube via a capillary connection between the two chambers. Product gas exits the capillary in the ionization zone of the quadruple mass spectrometer. The temperature of the individual library elements is controlled using a CO2 laser, an infrared sensor, and a feedback control loop. Analysis of the data collected from the entire library (approximately one library element per minute) allows the relevant parameters associated with the reaction, such as catalyst activity and selectivity, to be determined for each element in the library. Cong and coworkers reported a systematic and integrated approach for the synthesis and screening of libraries of mixed metal alloys containing rhodium, palladium, platinum, and copper [168]. Fully automated gas-phase thin-film deposition techniques (RF sputtering) were used to synthesize three unique 120-member libraries of Rh/Pt/Cu, Rh/Pd/Cu, and Rh/Pt/Pd alloys. The metal alloy combinato-

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rial libraries were screened in the scanning mass spectrometer as described previously for the gas-phase oxidation of carbon monoxide and the reduction of nitric oxide. In accordance with the literature and despite some discontinuities, rhodiumrich regions in the Rh/Pt/Pd ternary generally showed enhanced activity for carbon monoxide oxidation with oxygen and, interestingly, Rh/Cu binaries such as Rh/Cu (50:50 atom%) showed promising oxidation activity. A similar scanning mass spectrometer has been constructed by Maier and coworkers [169]. This spectrometer incorporated a similar concentric capillary gas feed and sensing design, but utilized simpler scanning robotics and an open-air environment. ‘‘Mass Tags’’ as Chirality Probes High-throughput screening of enantioselective catalysts using electrospray mass spectrometry has recently been addressed by Siuzdak, Finn, and coworkers and by Reetz and coworkers [228, 229]. Two different stereochemical processes were addressed including the kinetic resolution of racemates and asymmetric transformation of prochiral substrates due to the presence of enantiotropic groups. Both groups utilized a similar method based on the use of an equimolar mixture of pseudoenantiomeric ‘‘mass-tagged’’ compounds (Scheme 32.39), such that the mass of the molecule is correlated to its absolute configuration. The relative amounts of the products and/or reactants are detected by electrospray mass spectrometry. 32.10.3.2

Scheme 32.39. a) Asymmetric transformation of a mixture of pseudoenantiomers involving cleavage of the functional group FG and labeled functional group FG*. b) Asymmetric transformation of a mixture of

pseudoenantiomers involving either cleavage or bond formation at the functional group FG; isotopic labeling at R2 is indicated by the asterisk.

Kinetic resolution of racemates was investigated by preparing enantiomerically pure compounds A and B, which differ in absolute configuration and in labeling at the functional group FG*, and then mixing them in a 1:1 manner to simulate a racemate. True enantiomers C and D were formed following asymmetric functional group transformation, as well as labeled and nonlabeled achiral products E and F. The ratios of the total intensities of A/B and E/F in the mass spectra allow for the determination of enantioselectivity. Kinetic resolution of pseudoenantiomers A and G, in which mass labeling occurs at the residue R2, created a new pair of pseu-


doenantiomers C and H (Scheme 32.39b). The ratios of mass intensities A/G and C/H allow the conversion, enantioselectivity, and the selectivity factor to be obtained [229]. 32.10.4

Electronic High-throughput Methods

A new group of electronic high-throughput assays has been developed to screen for catalytic behavior that utilize multiplexed arrays of electrodes or solid-state electronic devices. A majority of the new screens are for electrochemical measurements, although a novel thermal sensor for activity and solid-state gas sensors have also been reported. 32.10.4.1 Electrochemical Analysis with Electrode Arrays

A number of high-throughput screens have been developed to screen for electrochemical activity of modified electrode surfaces. Smotkin, Mallouk, and coworkers, for example, have developed an indirect fluorescent screen for pH changes already described in this article [157]. Ward and coworkers have constructed a computer-controlled electrode array capable of rapid serial measurements of electrode kinetics for 64-element libraries [193]. Utilizing standard photolithographic techniques, the researchers created an 8 8 patterned array of 1 mm 2 Au contact pads and connections on a 100-mmdiameter thermally oxidized silicon wafer. The wafer was then coated with an insulating silicon nitride layer using plasma-enhanced chemical vapor deposition, followed by reactive ion etching to remove the silicon nitride from the contact pads. A Delrin cylinder was then glued to the wafer to provide an electrolyte reservoir with the electrode array in the center, and the electrode array was attached to a standard potentiostat through a computer-controlled multiplexer. A schematic of the electrode array and the set-up are shown in Fig. 32.32. The time required to cycle through all 64 electrodes depends on the cyclic voltammetry sweep rate (typically 100 mV s1 here), the voltage range of the sweep, and the number of sweeps. A typical dataset takes 2–3 h to complete. Unlike fluorescent screens, direct electrochemical measurements are capable of detecting small variations in current at differently modified electrodes. Matsumoto and coworkers have developed a high-throughput technique to screen libraries of potential photocatalysts that involves a silicon nitride-covered silicon sensor to image changes in pH [230]. The instrument, shown schematically in Fig. 32.33, sandwiches an electrolyte between the combinatorial library and the sensor array. A scanning laser provides a light source for photocatalysis. As an example, Matsumoto and coworkers investigated a library of doped TiO2 compounds as potential water cleavage catalysts. Reaction between the water and Fe 3þ ions on the TiO2 surface produced Fe 2þ ions, oxygen, and protons as the water decomposed. The resulting pH change was detected by the FET array, allowing for the rapid assay of large numbers of potential photocatalysts in library format. Yudin and coworkers have reviewed a number of electrochemical devices with

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The computer-controlled 64-element electrode array and control electronics used to record cyclic voltammograms.

Fig. 32.32.

application to combinatorial electrosynthesis and high-throughput screening. Of particular interest is the review of electrochemical biosensors that may have potential application to combinatorial catalysis research [194a–c]. 32.10.4.2 Solid-state Temperature Sensors

Connolly and Sutherland reported the use of a multiplexed array of 96 thermistors as temperature probes for catalytic reactions [231]. Thermistors, a term derived from ‘‘thermally sensitive resistor’’, are passive semiconductors that produce resistance values dependent on temperature. The researchers immersed an 8 12 array of thermistors in solutions such that the temperature change associated with catalytic activity could be detected as a change in the electrical resistance of the multiplexed thermistors. The method has the disadvantage, relative to thermal imaging, that the sensor must make contact with the catalytic reaction, offering


Fig. 32.33. Schematic of a library and electrochemical cell for

high-throughput investigation of photocatalysts.

the potential for chemical interference. Advantages of this method include the elimination of expensive thermal imaging cameras and infrared optics, and a potential increase in the temperature sensitivity. 32.10.4.3 Solid-state Gas Sensors

Yamada and coworkers have incorporated solid-state gas sensors into the effluent stream of a parallel eight-channel microreactor to measure the total oxidation, oxidative dehydrogenation, and selective oxidation of metal oxide catalysts [232]. Although gas sensors are not as selective as gas chromatographs or other research tools, they respond quickly and allow continuous observation of reaction products. In their investigation, Yamada and coworkers used several types of solid-state sensors that incorporate hot-wire resistance, semiconductor, potentiometric, and nondispersive infrared (NDIR) detection mechanisms. The first type of sensor, used to detect CO, was constructed from an alumina bead approximately 0.5 mm in diameter covered with platinum. A coiled wire of a Pt/Rh alloy was placed inside of the alumina bead as a heater/thermometer to maintain the element temperature at approximately 500 C. Catalytic combustion of CO increased the temperature of the sensing element, which was measured as a

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change in the resistance of the Pt/Rh wire. While there is no gas selectivity, i.e. any combustible gas will increase the sensor temperature, the detectable gas concentration range is between 0.01% and 10% in air. Semiconductor gas sensors were utilized to analyze catalytic effluents for olefins or oxygenates. Flammable gases change the temperature, and therefore the free carrier density, of an SnO2 semiconductor deposited on an alumina plate held at 600 C. To improve the selectivity, different semiconductor layers were deposited on seven SnO2 sensors. They included 13 wt% SiO2/Al2 O3 , 28 wt% SiO2/Al2 O3 , 28 wt% SiO2/ZnO, 28 wt% SiO2/TiO2 , 28 wt% SiO2/ZrO2 , 3 wt% Mo/SiO2 , and 3 wt% V/SiO2 . A preliminary investigation revealed that the SnO2 covered with 28 wt% SiO2/Al2 O3 was most selective for olefins, and SnO2 covered with 28 wt% SiO2/ TiO2 or 13 wt% SiO2/Al2 O3 the most selective for oxygenates. A NDIR sensor, which measures infrared absorption at specific wavelengths, and two SnO2 semiconductor sensors sensitized by TiO2 or by 13 wt% SiO2/Al2 O3 were used to evaluate catalysts for selective propane oxidation. The sensors were in a multichannel parallel sensor array for the CO oxidation and oxidative dehydrogenation studies, and in a serial configuration for the selective oxidation of propane (Fig. 32.34). Although the accuracy of the gas analysis by the sensor system was lower than a conventional gas chromatograph, the gas sensors have the advantage of rapid response, continuous measurements, small size, and lower cost. Furthermore, Yamada and coworkers are investigating new twodimensional miniaturized reactors where the solid-state sensor is integrated into the library.

Fig. 32.34. Schematic showing the gas sensor system for the

evaluation of selective propane oxidation.


32.10.5

Array Reactors

The activity and selectivity of catalysts can change significantly during extended exposure to product feedstock, creating the need for a system of high-throughput secondary screens capable of addressing reaction kinetics. Kinetic studies are indispensable to the catalytic rate as a function of the process variables, such as temperature, pressure, space–time, and composition of the reaction mixture. Stability studies that test the catalyst over an extended time period need to be conducted before a scale-up is initiated. Typically, these extended kinetic studies are performed in pilot plants with real feed and recycle streams and with the catalyst in its practical shape. The increased number of potential catalysts generated in a combinatorial program has led to the development of a new generation of array reactors that are capable of testing a larger number of catalysts in parallel for extended periods of time. Array reactors share common design features, differing primarily in their construction materials, and in their flow geometries. 32.10.5.1 Array Microreactors

Measuring the intrinsic properties of a group of catalysts requires a microreactor that maintains sufficient contact between the reactants and the catalyst, while avoiding limitations to the mass and heat transport around the catalyst. In general, these requirements are met in fixed bed reactors operating in a plug–flow format. In addition, the design is ideal for high-throughput screening because of the small sample requirements and because it supports both gas- and liquid-phase systems. The drawbacks are that a plug–flow reactor yields conversions rather than rates, low flow rates can lead to temperature and concentration gradients around the catalyst, and care must be taken to maintain the plug–flow behavior [233]. A reactor can be divided into three different sections: the gas or liquid mixing section, reactor section, and analysis section. Computer-controlled mass flow controllers for both liquid and gas streams provide stable molar flow rates necessary for kinetic studies. Products are typically mixed in a manifold and then introduced to a reaction block containing the reaction wells and cartridge heaters designed to keep the entire block at a constant temperature. Analysis typically involves multiplexing the product gases into a standard gas chromatograph, or mass spectrometer. Schu¨th and coworkers used a custom array microreactor to identify and optimize gold-based catalysts for the room temperature oxidation of CO [162]. Automated solution-phase synthesis, followed by calcination, was used to prepare 16 Au/Co3 O4 and Au/TiO2 catalyst compositions. The array reactor, shown schematically in Fig. 32.35, consisted of a brass block with 16 1-cm-diameter holes. The block was equipped with nine heating cartridges to ensure that the temperature would be uniform within 1 K across the block. The catalysts were placed on a plate with 16 small wells, which was then sealed against the brass body. The plate divided the gas introduction manifold from a series of capillaries that were equipped with three-way valves to switch an individual effluent stream to the analytical equip-

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Fig. 32.35. Cross-sectional drawing of a reactor for the parallel

testing of 16 catalysts under nearly conventional conditions.

ment. Capillaries placed in the effluent stream present the major flow resistance in the system, which means that the largest pressure drop is not over the catalyst bed but through the capillaries, meaning that differences in the catalyst packing do not significantly influence the flow through the individual wells. A nondispersive infrared sensor was used to determine CO and CO2 concentrations, and the authors report a factor of 15 increase in throughput relative to traditional methods. Senkan and coworkers have applied an array microreactor to a number of catalytic systems including a 66-element Pd/Pt/In library for the catalytic dehydrogenation of cyclohexane to benzene. The library consisted of 66 ternary combinations of Pt, Pd, and In prepared in 0.1 wt% increments, for a total metal loading of 1 wt% that was impregnated on 4-mm-diameter by 1-mm-long alumina supports [161, 176]. The catalyst library was prepared from solution-phase metal salt precursors using an automated liquid-handling robot. The compositions were slowly evaporated, dried, and finally calcined at 500 C for 2 h. The catalyst pellets were placed in a reactor array consisting of 20 rectangular channels that were micromachined on a flat nonporous silica slab. The 1-mm-wide, 1-mm-deep, and 20-mmlong channels had a 4 2 mm cylindrical well to hold the catalyst pellets. A similar silica slab was micromachined to fit onto the top, forming a silica block with cylindrical channels leading to and from the catalyst sample. Four microarray reactor blocks, each containing 20 samples, were stacked and placed into an aluminum heating block, making it possible to test up to 80 samples in parallel.


The entire reactor system was mounted on a stand that could be moved in three dimensions by a computer. The test samples were heated in Ar gas up to 350 C, then reduced under hydrogen gas, cooled to the desired reaction temperature, and finally exposed to a feed stream of 10% cyclohexane in Ar. The contact time between the sample and the feed gas was approximately 4 ms. The level of reactants, products, and inert carrier gas were determined by withdrawing a small sample from each microreactor channel using a capillary sampling probe (50 mm diameter) inserted 2 mm into the channels. The gas was analyzed by a quadruple mass spectrometer. The capillary was inserted into each channel for approximately 5 s via a computer-controlled positioning system. In this manner, the entire 80-element array could be screened in approximately 10 min. Building on an earlier microreactor design that used 15 quartz reactors [234], Claus and coworkers designed a ceramic monolith reactor with 2.2-mm-square channels, 150-mm long, that were arranged in a 16 16 array [178]. Each channel in the monolith represents a single fixed bed reactor, which allowed up to 256 catalysts to be tested in parallel up to 600 C. Product gases were analyzed using a capillary mounted on an x/ y/z scanning stage, and attached to a quadruple mass spectrometer (Fig. 32.36). A series of control experiments demonstrated that the axial and radial temperature profiles between channels (as measured without catalysts) was not in excess of 5 K, and the flow rates did not differ more than 10%. The performance of the monolithic reactor was evaluated using a 36-element library of Pt/Zr/V/Al2 O3 catalysts for methanol oxidation. The results showed that the reactor was valuable as a primary screen, distinguishing between poor and good catalysts at a rate of about 1 min per sample. The accuracy was lowered owing to difficulties in generating equal flows in all channels of the monolith, and, in certain cases, owing to diffusion of exhaust gases from adjacent catalysts into the end of a channel containing the catalyst currently under investigation.

Fig. 32.36. Monolithic reactor system for the parallel screening of heterogeneous catalysts.

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32.10.5.2 Micromachined Array Reactors

Advancements in microfabrication have led to a new generation of microchannel reactors, allowing very large heat and mass transfer rates, and enabling safe investigations into explosive and dangerous reactions. Claus and coworkers describe a microchannel reactor that consists of a very large number of parallel microchannels having a square cross-section of approximately 500 mm on a side [178]. The reactor module consists of a stack of metallic frames with micromachined inlays. Each microstructured inlay contains a catalyst as an active coating on top of its microchannels; stacking the frames creates a system of catalyst-coated microchannels (Fig. 32.37). The reactants flow through a diffuser, through the microchannels and over the catalysts, and finally to a product outlet where products are detected by a scanning mass spectrometer. The reactor module design allows for interchangeable catalyst inlays made of different materials such as metals, silicon, ceramics, and glass, allowing many different catalysts and channel geometries to be investigated rapidly. Channel-to-channel cross-talk was tested by placing an active catalyst next to an inactive one and then measuring both product streams with a mass spec-

Fig. 32.37. Reactor module consisting of stacked metallic

frames. The catalyst inlays are mounted and removed in the directions indicated by the arrow.


Fig. 32.38. Chip module for the catalysis-on-a-chip approach.

trometer. The detected amount of catalytic product in the noncatalytic zone was less than 2% of that found in the active position. A 52-catalyst methanol oxidation library used for further verification of the microreactor demonstrated excellent sensitivity and reproducibility. 32.10.5.3 Catalysis on a Chip

Claus and coworkers have created a set of microreactors for enabling catalysis on a chip [178]. The chip-based system allows for efficient thermal control and short response time (Fig. 32.38). Preliminary investigations of this new technology utilized parallel microchannels etched into silicon and glass with cross-sectional dimensions of 500 200 m. Similar to the micromachined metal reactors, each microchannel contains a different catalyst, which is micropipetted into the chips. Reactants are introduced through a single inlet in the chip, flowing through a prefabricated manifold to each of the catalyst-filled microchannels. The products are analyzed as they leave the chip at a series of outlets. The prototype chip modules contain eight or 16 parallel channels having different cross-sections and channel lengths. 32.10.6

Capillary Array Electrophoresis

Recently, capillary electrophoresis (CE) has taken a key role in the parallel screening of homogeneous catalysts, in addition to the ‘‘classical’’ methods such as GC or HPLC. Yeung and coworkers have used multiplexed capillary electrophoresis for the combinatorial screening of enzyme activity [197a] and homogeneous catalysis [197b]. In the latter case, this methodology was successfully implemented to opti-

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mize, in a multidimensional screening approach, the regiochemistry of a Pd-catalyzed annulation reaction. Reetz and coworkers have adapted a commercial 96-well capillary electrophoresis system for the determination of enantiomeric excess [198]. In their initial study, chiral amines, which are potentially accessible by catalytic reductive ammonation of ketones, Markovnikov addition of ammonia to olefins, or enzymatic hydrolysis of acetamides, were used as model substrates (Scheme 32.40). Various a- and bcyclodextrin derivatives were used as chiral selectors, which were then modified with fluorescent compounds to enable laser-induced fluorescence (LIF) to be employed as a detection mechanism. Known enantiomeric mixtures of the amines (with the fluorescent tag added) were then analyzed by a commercial instrument and by a capillary array electrophoresis (CAE) system. Results from the CAE system initially suffered from unstable runs, but results improved with the addition of a higher viscosity electrolyte composed of 40 mM 2-(N-cyclohexylamino)ethanesulfonic acid and 6.25 mM g-cyclodextrin diluted with a buffer containing polyacrylamide. The agreement between the ee values of the mixtures of the (R)and (S)-amines measured with the conventional capillary electrophoresis system, the CAE system, and a conventional GC was excellent. Enantiomeric separation with these systems requires approximately 19 min.

Test system for the high-throughput screening of enantioselective catalysts using capillary array electrophoresis (CAE).

Scheme 32.40.

32.11

Summary and Outlook

It is virtually certain that we find ourselves at the dawn of a new age of applying combinatorial methodologies to catalysts’ discovery and optimization. As this chapter demonstrates, significant first steps in that direction have been taken in the area of catalyst research, and a multitude of tools are now available using combinatorial technologies to appropriately accommodate the new tasks and requirements for combinatorially accelerated materials and catalyst research. A common underlying theme associated with these technologies is miniaturization, parallel-

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Acknowledgements

The authors are deeply indebted to Ms Silvia Lee (Symyx Technologies) and Ms Kathryn Boykin (XenoPort) for their invaluable support in reference and patent searches. The authors thank Mr Ron Krasnow (Symyx Technologies) for checking the manuscript.

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Diversity-Based Identification of Efficient Homochiral Organometallic Catalysts for Enantioselective Synthesis Amir H. Hoveyda 33.1

Introduction

The discovery and identification of an effective chiral catalyst that promotes a chemical reaction with desirable levels of efficiency and selectivity is a difficult business [1]. Because only small energy gaps separate an inactive or nonselective catalyst from one that is potent and selective (@ 1–2 kcal mol variance in transition-state energy), variations in reactivity and selectivity often arise unpredictably. Seemingly insignificant variations in the catalyst or substrate structure and reaction conditions (solvent polarity, temperature, etc.) can lead to entirely unexpected swings in yield, ee (enantiomeric excess), or both. To outline a new transformation and achieve maximum levels of reactivity and selectivity, myriad reaction parameters must therefore be explored and adjusted. In cases where an effective metalcatalyzed enantioselective process is the goal, the choice of an appropriate chiral ligand and metal salt is perhaps most crucial. In such instances, a blend of mechanistic knowledge and human intuition are typically used to identify a desirable metal–ligand combination. Mechanistic knowledge is useful in allowing chemists to appreciate the general contours of a reaction pathway. Such information is critical in catalyst discovery; it aids us in deciding what class or classes of chiral ligands and what type of metal salts should be included in a study. However, mechanistic data alone cannot provide us with sufficient information to fully ‘‘design’’ [2] a catalyst, without requiring any degree of trial and error. That is, a set of mechanistic data collected through examination of a single substrate under a particular set of conditions with a specific metal center and chiral ligand is often less general than one might like. Similar to mechanistic data, chiral catalysts are often not general. Once we do come across an attractive catalyst, it is seldom effective for a wide range of substrates. This result is not surprising; a selective catalyst that recognizes a certain structural type with great fidelity cannot – by nature – recognize and associate with a gamut of substrates and promote reactions selectively. To address the above general uncertainties and fundamental difficulties, chemists have recently turned their attention to advances in combinatorial and highHandbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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33 Diversity-Based Identification of Efficient Homochiral Organometallic Catalysts

throughput strategies [3]. This movement arises from the realization that if the study of a large number of potential catalysts leads to a discovery, then why not perform the investigation systematically and more efficiently by covering a broad range of catalyst candidates? Another factor that supports the adoption of diversitybased approaches is that screening of a large number of compounds may allow chemists to identify an optimal catalyst for each particular substrate and thus overcome the problem of generality that often looms large when a small number of catalyst candidates are available. High-throughput screening can give rise to new mechanistic scenarios that are by nature more comprehensive, derived from a more extensive collection of data points. It would be idle to assume that screening is carried out at random or that diversity approaches to catalyst discovery are devoid of any mechanistic basis. Mechanistic information gained through examination of a large set of data can serve as the driving force behind additional investigations; it can lead to the blossoming of a symbiotic relationship between mechanistic inquiry and diversity-based screening that results in the discovery of more powerful and efficient catalysts of all types, and a deeper and more mature appreciation of the inner workings of various classes of catalysts.

33.2

Factors Critical to the Success of Diversity-based Reaction Development

Three important issues need to be addressed for the successful implementation of diversity-based catalytic asymmetric reaction development: (1) sources of diversity, (2) high-throughput synthesis of catalysts, and (3) high-throughput catalyst screening. 1 Sources of diversity. Variation of the ligand structures can generate an exponential number of catalysts with different steric and electronic attributes. When organometallic complexes are intended for use as catalysts, metal centers can be modified as well. Reaction conditions represent another dimension of diversity. Cocatalysts, additives, solvent, concentration, temperature, and reaction times are potential parameters that can be altered. It must be noted that the above protocols toward enhancing diversity are not mutually exclusive. 2 Catalyst candidates that lend themselves to diversity-based screening. To employ diversity-based strategies in an efficient and productive manner, the basic structural features of the targeted class of catalysts must lend themselves to such an approach. Accordingly, the following fundamental catalyst attributes are critical: – Facile modularity. Depending on the nature of the metal salts involved and the type of transformation that is being developed, it should be possible to alter ligand structures readily so that reactivity and selectivity levels are rapidly improved. The modularity of the chiral ligands has been critical to acceleration of their preparation by making the fabrication of each catalyst identical, regardless of structural variations. It must be noted that, in principle, all chiral ligands can be modified and thus labeled modular. Some classes of chiral ligands are, however, more easily modified because their structural components

33.2 Factors Critical to the Success of Diversity-based Reaction Development

are joined by bonds that are readily formed. These ligands are therefore most suitable for parallel library synthesis. Schiff bases and peptide linkages are among disconnections that fall into the latter category. If CaC, CaP, or CaN (nonpeptide linkage) bonds are to be modified to alter a ligand (e.g. binol- or biphen-based systems), then these constructs should be viewed as less readily modular. – Symmetry. One of the more ‘‘established’’ dogma in the field of metal-catalyzed asymmetric catalysis is that symmetric, particularly C2 -symmetric, chiral ligands are preferred so that the number of energetically differentiable modes of catalyst–substrate association can be minimized and stereoselectivity can be more logically ‘‘designed.’’ Many useful, efficient, and selective transformations have indeed been developed based on C2 -symmetric chiral ligands [1]; it is likely that many more chiral ligands of this type will be developed in the future. However, recent studies from a number of laboratories indicate that non-C2 -symmetric chiral ligands can also give rise to outstanding levels of selectivity and efficiency in a variety of synthetically important transformations [4]. As far as diversity-based approaches are concerned, the less symmetric class of chiral ligands provides a more attractive option. With C2 -symmetric systems, any structural alteration must be mirrored at the complementary region of the ligand structure, thus reducing the degree of available diversity. – Multiple binding sites. Another common perception in the field of asymmetric catalysis is that the number of binding sites for the metal center within the chiral ligand should be minimized (often limited to a single point of binding). As such, chiral ligands have often served a single function in a particular transformation. The metal–ligand complex either serves as a Lewis acidic activator or as a nucleophilic agent. In a limited number of instances, both functions are delivered by two distinct molecules of the chiral catalyst (second order in catalyst) [5]. Incorporation of multiple binding sites raises the intriguing possibility of multimetal systems and multifunctional catalysts [6]. Care must be taken, however, that different sites possess sufficiently diverse coordination properties so that, through association with various metal salts, chiral ligands give rise to complementary rather than competing effects [7]. Multiple binding sites are attractive from the point of view of highthroughput screening studies: they incorporate additional elements of structural modification and open the possibility of multimetal systems, which, in itself, represents a critical dimension in diversity. 3 High-throughput catalyst screening. This aspect is often the greatest bottleneck in assaying each catalyst for asymmetric induction. Although in combinatorial approaches to biological activity, mixtures of compounds can be analyzed simultaneously, such a strategy is by nature problematic in studies that pertain to identification of effective homochiral catalysts. Because subtle structural variations can lead to unexpected changes, or even reversal of enantioselectivity, examination of mixtures of catalysts can lead to conclusions that are misleading (low net selectivity by two effective catalysts that afford high ee values but in the opposite sense). Accordingly, recent reports generally involve testing individual systems.

993

994


It merits mention that the parallel screening strategy has also been applied in therapeutic discovery efforts because of the difficulties involved in the accurate deconvolution of various mixtures and because of the synergism that may exist between several active compounds. As such, combinatorial chemistry does not necessarily involve the generation of mixtures of compounds; it may be better characterized by the modular nature of the constituent compounds that in different combinations provide large numbers of molecular ensembles. Analysis of mixtures of candidates for activity (vs. selectivity) can be feasible however and may be accomplished effectively [3f ].

33.3

Peptidic Schiff Bases as Chiral Ligands

In search of a class of chiral ligands that satisfies the above-mentioned attributes (facile modularity, nonsymmetry, and multiple binding sites) and can promote a variety of CaC bond-forming reactions enantioselectively, we took note of Schiff base peptide ligand 1. In 1992, Inoue and coworkers reported that in the presence of 10 mol% Ti(OEt)4 and 10 mol% 1, addition of trimethylsilyl cyanide (TMSCN) to various aldehydes occurs with appreciable enantioselectivity [8]. There are several characteristics that are represented by a ligand such as 1: (1) the ease of Schiff base and amide bond synthesis renders these entities as potential readily modular chiral ligands for a wide range of metal salts; (2) the requisite building blocks, aromatic aldehydes, and optically pure amino acids, are readily available; (3) peptidic structures can be prepared on solid support, allowing simultaneous synthesis and examination of a sizeable collection of catalyst candidates in an expeditious manner (see Figure 33.2).

Scheme 33.1. Ti-catalyzed enantioselective addition of cyanide to aldehydes in the presence of a dipeptide Schiff base.

33.3 Peptidic Schiff Bases as Chiral Ligands

33.3.1

Ti-Catalyzed Enantioselective Addition of Cyanide to Meso Epoxides

In 1995, we initiated a program wherein we utilized diversity-based protocols to introduce variations within a modular peptide-based ligand as the means to identify effective chiral ligands for enantioselective TMSCN addition to meso epoxides (Scheme 33.2) [9]. In principle, 8000 (20 3 ) different chiral catalysts could be made from the 20 natural amino acids and 20 different aldehydes. To circumvent such a daunting requirement and control the numbers of compounds synthesized and screened, a representational search strategy was employed (concept illustrated in Fig. 33.1).

Scheme 33.2. Peptidic Schiff bases may be screened for

identification of an effective chiral ligand for catalytic enantioselective addition of TMSCN to meso epoxides.

First, as shown in Scheme 33.3, we established that in the presence of 10 mol% ligand 4 and 10 mol% Ti(OiPr)4 the addition of TMSCN to cyclohexene oxide 5 proceeds smoothly to afford 6 in @25% ee ( 99% ee), assuming there will be no notable difference in efficiency, justify the additional effort that would be required to achieve it? In the approach described above, we have made certain assumptions about the additivity and absence of cooperativity between the three subunits of the ligand structure. At least for this small sampling, these assumptions seem to hold true, but without testing every combination we cannot definitively answer this question. Examination of every possibility would be taxing and detract from the efficiency of the general screening method. An important practical advantage of the above approach is that, in a relatively short amount of time, it allowed us to identify a selective catalyst for an entirely new asymmetric process. That is, the search strategy is not an open-ended odyssey but a well-structured

Fig. 33.2. Screening by positional optimization of various peptide Schiff base ligands for enantioselectivity in the addition of TMSCN to cyclohexane oxide (5).

33.3 Peptidic Schiff Bases as Chiral Ligands 997


Fig. 33.2. (continued)

998

33.3 Peptidic Schiff Bases as Chiral Ligands Tab. 33.1. Optimized ligands for catalytic enantioselective addition of TMSCN to meso epoxides.

Entry Substrate

Product

ee (%)

Yield (%)

1

83

72

2

89

65

3

84

68

4

78

69

Optimized ligand

Conditions: 20 mol% Ti(OiPr)4 , 20 mol% ligand, TMSCN, 4 C, toluene, 6–20 h.

program that allows a fairly comprehensive assessment of a ligand framework with respect to a specific asymmetric reaction in a finite and predictable period of time. When we applied the above search strategy to various other meso epoxide substrates, a number of crucial observations were made (see Table 33.1) [10]. One significant trend that emerged from these studies was that, for each epoxide substrate, a similar but unique chiral catalyst was identified. This type of catalyst/substrate selectivity is akin to that observed in Nature where many reactions have their own unique enzymes. The high levels of selectivity observed with enzymatic reactions is often accompanied by the lack of substrate generality. In this instance, however, because ligand modification is relatively straightforward, substrate specificity does not necessarily imply lack of generality. Another noteworthy issue raised by the data in Table 33.1 is that the search for a ‘‘truly general catalyst’’ is perhaps unrealistic: catalysts that afford exceptional selectivity do so because they associate with specific structures with great fidelity. To expect high specificity and broad range generality may be somewhat contradictory. Our studies indicate that the above method of catalyst identification increases the frequency with which unexpected observations are made. For example, as illustrated

999

1000


in Scheme 33.4, a subtle alteration in the structure – and not the stereochemical identity – of the peptide ligand leads to inversion of stereochemistry in the epoxideopening reaction (compare reaction with ligands 9 and 17). These observations validate our choice of individually synthesizing and testing each catalyst, as mixtures of catalysts can lead to racemic products.

Scheme 33.4. Subtle modifications in the structure of a peptide ligand may unexpectedly lead to significant variations in selectivity.

33.3.2

Ti-Catalyzed Enantioselective Addition of Cyanide to Imines

In the second phase of our program, we applied the above screening technology to identify specific Ti–peptide complexes that catalyze the addition of cyanide to imines [11, 12]. As the representative cases in Scheme 33.5 illustrate, the reactions proceed efficiently and with outstanding enantioselectivity. In addition to arylimines (e.g. formation of 19 and 22 in Scheme 33.5), acyclic a,b-unsaturated imines are effective substrates for these asymmetric CaC bond-forming reactions (cf. formation of 25 and 28 in Scheme 33.5). Initially, our catalyst-screening approach led us to identify catalysts that deliver amino nitriles with 90–97% ee, but in low conversion (< 25% conversion in 18 h) [13]. Based on certain mechanistic considerations, which ironically later proved to be incorrect, we argued that addition of protic additives may lead to an enhancement in reactivity. The latter hypothesis was based on the fact that when reactions are carried out in relatively large scale, where adventitious water is more easily avoided, conversions are lower. Thus, the effect of a variety of alcohols and amines on reaction efficiency and selectivity were systematically screened. These studies led us to establish that the Ti-catalyzed cyanide additions are significantly more efficient if one equivalent of iPrOH is added to the reaction mixture slowly (see below for mechanistic details). Accordingly, reactions of arylimines and unsaturated aliphatic imines were effected with high enantioselectivity and in good yield, as represented by the examples in Scheme 33.5. It is important to note that in the course of these studies we established that


an NnBu amide terminus provides the same levels of reactivity and efficiency as when Gly occupies the AA3 site.

Scheme 33.5. Ti-catalyzed enantioselective addition of cyanide to imines promoted by modular peptidic Schiff base ligands.

The majority of the optically enriched amino nitrile intermediates can be easily recrystallized to enantiopurity. The Ti-catalyzed asymmetric process is of notable utility in organic synthesis, since after a single hydrolysis/deprotection step optically pure amino acids can be obtained; the example shown in Scheme 33.6 is illustrative. It is worth noting that such amino acids cannot be accessed by the celebrated catalytic asymmetric hydrogenation protocols [14]. With effective and highly enantioselective catalytic Strecker reactions in hand, we set out to explore the mechanistic details of these important transformations [15]. It is our conviction that a better mechanistic appreciation, along with the ability to prepare and screen large collections of catalyst candidates, will allow us to extend the scope of this versatile class of chiral ligands to include a number of other critical catalytic enantioselective processes. Thus, kinetic, structural, and ster-

1001

1002


eochemical studies of Ti-catalyzed addition of cyanide to imines in the presence of Schiff base peptides were carried out. Noteworthy among our findings is that it is likely HCN (not TMSCN) which serves as the active nucleophile; the slow addition of iPrOH leads to the formation of the reactive HCN. Moreover, detailed kinetic studies suggest the reaction is first order only in the Tiligand complex. These studies also reveal a DSz ¼ 45:6 G 4:1 cal K1 mol1 , indicating a highly organized transition structure for the turnover-limiting step of the catalytic cycle.

Scheme 33.6. Synthesis of nonproteinogenic a-amino acids by Ti-catalyzed Strecker reaction.

Various structural features of the chiral peptide ligand were systematically altered and the corresponding relative rates and enantioselectivities were measured. These studies, summarized in Scheme 33.7, led to several important findings: (1) not only is the presence of the AA2 moiety critical to reactivity and enantioselectivity (compare reaction of 33 with that of 34), its stereochemical identity is of notable significance as well (compare reaction of 33 with those of 35 and 36); (2) the presence of a more Lewis basic amide carbonyl (vs. a carboxylic ester) has an influence on the rate of asymmetric CN addition, e.g. the initial rate of reaction (90 min) with ligand 7 is 2.3 times faster than that for the derived methyl ester 37. The importance of AA2 and the influence of local chirality suggest that the peptide segment of the ligand actively participates in the asymmetric CaC bondforming reaction. That is, the TiSchiff base coordinates with the substrate while an amide moiety within the neighboring peptide segment associates and delivers HCN to the activated imine. A mechanistic model consistent with the kinetic and stereochemical data is presented in Fig. 33.3. These findings thus underline the significance of the peptidic moiety of this class of chiral ligands – not as passive providers of a chiral environment, but as active participants in the asymmetric CaC bond formation. These data provide a solid mechanistic basis regarding the importance of available diversity at the two peptidic sites and the practical utility of the non-C2 -symmetric structure of these chiral ligands. The above findings are significant, since they demonstrate that, by virtue of their structural and stereochemical identity, peptidic Schiff bases may serve as bifunctional catalysts to deliver appreciable reactivity and high enantioselectivity. The above mechanistic par-


Scheme 33.7. Effect of the AA2 moiety on reactivity and

enantioselectivity in the Ti-catalyzed addition of cyanide to imines.

adigm thus suggests that, in addition to HCN, other reagents such as various alkyl metals may be delivered effectively and enantioselectively to the bound imine substrate. Such a hypothesis led us to the following aspects of our program.

Fig. 33.3. Delivery of HCN by the AA2 moiety of the peptide ligand.

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33.3.3

Zr-Catalyzed Enantioselective Addition of Dialkyl Zincs to Imines

To determine whether the above mechanistic paradigms can be employed to deliver alkyl metals to imines in an enantioselective fashion, we set out to examine the possibility of developing a catalytic asymmetric alkylation of imines. Such a process would deliver optically enriched or pure amines, a class of compounds that is of great significance to medicine and biology. Screening of parallel libraries was used to establish conditions for an effective set of protocols, including the optimal metal center, solvent, reaction temperature, and amine protecting group [16, 17]. Interestingly, as the representative examples in Scheme 33.8 illustrate, these screening studies indicated that Zr(OiPr)4 is the most appropriate metal center with dipeptide Schiff base 41 serving as the chiral ligand that delivers excellent reactivity and enantioselectivity; the previously employed Ti salt leads to significantly less reactive and enantioselective reactions. An efficient Zr-catalyzed alkylation of arylimines in the presence of 0.1–10 mol% chiral peptidic ligands and Et2 Zn was thus established (see 38 ! 39 in Scheme 33.8). These transformations afford the derived amines in >90% ee and >66% isolated yield. As illustrated in Scheme 33.8, oxidative removal of the o-anisyl protecting group delivers the derived optically enriched arylamines (e.g. 40).

Scheme 33.8. Parallel screening points to Zr as the optimal

transition metal for enantioselective addition of Et2 Zn to imines.

With aryl imines that contain electron-withdrawing or electron-releasing substituents, the Schiff base dipeptides such as 41 are ineffective (amine formation with electron-withdrawing groups and 98% conversion within 6 h. Pyridine dipeptide 56 delivers the highest level of enantioselectivity (34% ee), followed by phosphine 55 (26% ee). To ascertain the identity of the most efficient and selective ligand/Cu salt combination, formation of 52 was examined in two sets of experiments involving ligands 55 and 56 and a collection of Cu salts (see Scheme 33.10, second-phase screening). This study established that 56 and CuCN, overall, provide the most efficient regio- and enantioselective process. Next, we secured the following additional ligand attributes (third-phase screening, Scheme 33.10): (1) an amide terminus is critical to the enantioselectivity – replacement of the NnBu in 56 with an OMe group (57, Scheme 33.10) leads to significant reduction of ee; (ii) incorporation of a third amino acid (58, Scheme 33.10) or removal of one (59) is detrimental to enantioselectivity [25]. The stereochemical outcome from the reaction with 59 (14% ee) once again underlines the importance of the AA2 moiety and indicates that simple attachment of a chiral group to the pyridyl ligation site is not alone sufficient for high asymmetric induction. We then prepared chiral ligands 60–67 (Scheme 33.10) and examined their ability to initiate the enantioselective alkylation of 51 under the same conditions mentioned above. Thus, the catalytic ability of the derived amine (60), amide (61), and various a-substituted pyridyl systems (62–65) were investigated; in addition, the related indole-based 66 and C2 -symmetric 67 were probed. All reactions proceed to >98% conversion and exhibit high degrees of SN 2 0 /SN 2 selectivity, but it is the osubstituted ligand 64 that generates the highest ee. We thus selected o-OiPr pyridyl as the Schiff base moiety, and continued with the optimization of the AA1 and AA2 segments according to methods reported previously in the context of our work on the enantioselective addition of cyanide to meso epoxides and imines [3]. These studies uniformly suggest that l-Phe is the AA2 of choice, and that ligands which bear l-Val (56), l-t Leu (68, Scheme 33.11) and l-Chg (69, Scheme 33.11) at the AA1 position offer similarly superior enantioselection. When d-Val is used as AA1, the sense of enantioselection is reversed, indicating that the stereochemical identity of AA1 is critical to the sense of induction and that the d,l-ligand may deliver lower levels of enantioselectivity than the l,l isomer. The above chiral ligands were subsequently used in catalytic alkylation of a range of aryl olefins in the presence of Et2 Zn and 10 mol% CuCN in THF. The results shown in Scheme 33.11 are representative and point out which ligand provides the highest selectivity for a particular substrate (< 2% SN 2 product in all cases). The selectivity and reactivity levels for the disubstituted alkenes are competitive with the recently reported catalytic alkylations of allylic chlorides [26], in which sterically demanding dialkyl zinc reagents (e.g. dineopentyl zinc) are required for high enantioselectivity (a 50% ee with n-alkyl zinc reagents). The cata-


Scheme 33.11. Parallel screening of ligand libraries indicates that pyridyl dipeptides can serve as effective ligands for Cucatalyzed allylic substitution reactions that afford quaternary carbon centers enantioselectively.

lytic enantioselective synthesis of quaternary carbons (e.g. formation of 73 and 75 in Scheme 33.9), however, represents a new and effective method for the regio- and enantioselective preparation of this important class of homochiral compounds. The significance of the new technology was demonstrated in the context of a brief enantioselective total synthesis of fish deterrent sporochnol (Scheme 33.12) [27, 28]. 33.3.5

Cu-Catalyzed Enantioselective Conjugate Addition of Dialkyl Zincs to Unsaturated Ketones: Peptidic Phosphines as Chiral Ligands

Another important class of transformations is the catalytic asymmetric conjugate additions of unsaturated carbonyls [29]. We have examined the possibility of using the modular Schiff base polypeptides as ligands that promote this important class

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Scheme 33.12. Cu-catalyzed asymmetric allylic substitution with longer chain alkyl zinc reagents and total synthesis of sporochnol.

of transformations [30]. Screening of ligand, Cu salt, and solvent libraries, carried out in a similar manner as described above, indicated that phoshine-based dipeptides, such as 55 shown in Scheme 33.13, promote various conjugate addition reactions with excellent reactivity and efficiency in the presence of CuOTf. Use of a hydroxyl-containing ligand such as 53 (see Scheme 33.10) or even a pyridyl ligand such as 56 leads to the formation of racemic products. Thus, once again, the modular character of this class of chiral ligands allows facile examination of a different subset of potential ligand structures that provides access to high levels of efficiency and asymmetric induction. The Cu-catalyzed CaC bond-forming reactions require only 2 mol% (CuOTf )2 C6 H6 and 2.4 mol% of the peptidic phosphine and can be effected with a wide range of dialkyl zinc reagents. Most importantly, as depicted in Scheme 33.13 and in contrast to the previously reported protocols, the present method provides – for the first time – an efficient and highly enantioselective conjugate addition protocol for cyclopentenones. Unlike the alkyl zinc reagents shown in Scheme 33.13, use of iPr2 Zn leads to moderate levels of asymmetric induction (e.g. 62% ee with cycloheptenone 84 as the substrate). To address this selectivity problem, we set out to identify an improved chiral ligand through the positional optimization strategy, with cyclohexenone 82 serving as the substrate. As illustrated in Scheme 33.14, we established that reactions promoted by 86 (t-Leu at AA1, tBu-Tyr at AA2, and Gly at AA3) deliver 88 in 91% ee (vs. 72% ee with 55). Chiral phosphine ligand 86 also provides a better selectivity in reaction of cycloheptenone 84 with iPr2 Zn (81% ee vs. 62% ee with 55). In contrast, with cyclopentenone 78 as the substrate, 90 is obtained in 65% ee when 86 is used (vs. 79% ee with 55). When phosphine 87 (Gly replaced by nBu) is employed in the reaction of 78, however, cyclopentanone 90 is isolated in 85% ee (94%) [31]. The above observations once again demonstrate the utility of facile modularity and its attendant parallel screening. These data imply that if complete ligand screening is carried out specifically for each substrate, a different optimal chiral phosphine construct may emerge for each particular enone.


Scheme 33.13. Cu-catalyzed asymmetric conjugate additions of cyclic enones with peptidic phosphines as the chiral ligand.

Scheme 33.14. Cu-catalyzed asymmetric conjugate additions of cyclic enones with peptidic phosphines as the chiral ligand. Conditions: 1.0 mol% (CuOTf )2 C6 H6 , 2.4 mol% chiral ligand, 3 equiv. (iPr)2 Zn, 30 C.

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This last enantioselective transformation catalyzed by peptide-based ligands is also of notable potential synthetic utility. The sequential catalytic addition/alkylation of 84 provides 91 in 97% ee, 80% yield, and with >15:1 diastereoselectivity (Scheme 33.15). Pd-mediated regioselective oxidation (! 92), followed by Wacker oxidation, completes a four-step enantioselective synthesis of the anticancer agent clavularin B (42% overall from commercially available 84) [32].

Application of Cu-catalyzed asymmetric conjugate additions to the enantioselective total synthesis of anticancer agent clavularin B.

Scheme 33.15.

33.4

Conclusions and Outlook

The studies summarized in this article represent our research group’s attempts in the past 5 years to establish a reasonably general protocol for the identification and discovery of chiral catalysts that effect a range of important bond formations enantioselectively and efficiently. By selecting an appropriate class of chiral ligands that satisfies the important criteria of modularity, lack of high symmetry, and availability of multiple but differentiated binding sites, we have been able to develop catalytic asymmetric CaC bond-forming reactions that involve both early (Ti and Zr) and late (Cu and Zn) transition metals. Although optimal ligands, metal centers, protecting groups, and solvents have been typically determined through systematic screening of parallel libraries, mechanistic knowledge along with basic chemical intuition are also critical ingredients along the way. These research activities are the result of the appreciation of the principle that a priori ‘‘rational’’ design of a catalyst may be near impossible since highly detailed mechanistic principles are often not general; such principles can vary with subtle changes in reaction conditions or substrate structure so that, even within a single class of substrates, the identity of the ‘‘optimum catalyst’’ may change. These studies are based on the premise that although the design of a specific catalyst may be impossible, general mechanistic principles can be utilized to outline a systematic screening protocol for catalyst identification.

Endnotes and References

The research described above bears testimony to the fact that this line of research does not advocate that we abandon rational or rigorous investigations of detailed mechanisms of important processes. Elements of design and a priori decisions are still required in determining what collections of catalysts need to be prepared; the framework is simply broader and thus initial bias that may be based on a few initial observations has less of a chance to point us in the wrong direction. A diversity-based strategy allows us to base our mechanistic hypotheses on a much wider pool of data points – it discourages us from making naive generalities, which are more than often revised soon after a few additional experiments. As demonstrated in the above studies, mechanistic studies based on data that are collected from parallel libraries can provide additional logic and impetus for future efforts in reaction development.

Acknowledgments

First and foremost, I thank my friend, colleague, and collaborator Professor Marc Snapper. I am grateful to Dr Joseph Harrity, Dr Ken Shimizu, Dr Bridget Cole, Dr Clinton Krueger, Mr Kevin Kuntz, Dr Carolyn Dzierba, Mr James Porter, Mr John Traverse, Ms Courtney Luchaco-Cullis, Dr Hirotake Mizutani, Ms Sylvia Degrado, Dr Wolfgang Wirschun, Ms Kerry Murphy, Mr Nathan Josephsohn, and Mr John Gleason for making numerous invaluable intellectual and experimental contributions to the projects discussed in this article. Research in our laboratories is generously supported by the National Institutes of Health (GM-47480, GM-57212, and postdoctoral fellowships F32-GM-17821 and F32-GM-18209). Additional support has been provided by the National Science Foundation (CHE-9632278), Johnson and Johnson, Pfizer, DuPont, AstraZeneca, Albemarle, ArQule, Dreyfus Foundation, Sloan Foundation, and Deutsche Forschungsgemeinschaft (postdoctoral fellowship to W.W.).

Endnotes and References 1 Jacobsen, E. N., Pfaltz, A.,

Yamamoto, H. (eds), Comprehensive Asymmetric Catalysis. Springer, Berlin 1999. 2 The word ‘‘design’’ is described in the Unabridged Webster English Dictionary in the following manner: ‘‘To conceive and plan out in mind.’’ Based on such a definition, it is unlikely that a chemist can, in general, consider mechanistic data and directly design a catalyst that proves to be optimal in selectivity and reactivity. If by design it

is implied (as is often the case) that numerous catalysts are prepared, and then after a period of trial and error a catalyst is determined to be optimal, such an excercise should be referred to as a ‘‘linear screening.’’ In such a case, the plan for the direction of screening – and not the catalyst itself – is conceived based on the existing mechanistic data. 3 a) K. D. Shimizu, M. L. Snapper, A. H. Hoveyda, Chem., Eur. J. 1998, 4, 1885–1889; b) M. B. Francis, T. F.

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Jamison, E. N. Jacobsen, Curr. Opin. Chem. Biol. 1998, 2, 422–428; c) H. B. Kagan, J. Organomet. Chem. 1998, 567, 3–6; d) K. W. Kuntz, M. L. Snapper, A. H. Hoveyda, Curr. Opin. Chem. Biol. 1999, 3, 313–319; e) B. Jandeleit, D. J. Schaefer, T. S. Powers, H. W. Turner, W. H. Weinberg, Angew. Chem. 1999, 111, 2648–2689; Angew. Chem. Int. Ed. Engl. 1999, 38, 2494–2532; f ) M. L. Snapper, A. H. Hoveyda in: Combinatorial Chemistry. Fenniri, H. (ed.), Oxford University Press, Oxford 2000, pp. 433–455. For a recent review regarding the use of some non-C2 -symmetric chiral ligands in metal-catalyzed enantioselective reactions, see: a) A. Pfaltz in: Stimulating Topics in Organic Chemistry. Shibasaki, M., Stoddard, J. F., Vogtle, F. (eds), VCH-Wiley, Weinheim 2000, pp. 89–103; b) G. Helmchen, A. Pfaltz, Acc. Chem. Res. 2000, 33, 336–345. For example, see: K. B. Hansen, J. L. Leighton, E. N. Jacobsen, J. Am. Chem. Soc. 1996, 118, 10924– 10925. a) H. Steinhagen, G. Helmchen, Angew. Chem., Int. Ed. Engl. 1996, 35, 2339–2342; b) M. Shibasaki, H. Sasai, T. Arai, Angew. Chem., Int. Ed. Engl. 1997, 36, 1236–1256. For an example, see: M. Sawamura, H. Nagata, H. Sakamoto, Y. Ito, J. Am. Chem. Soc. 1992, 114, 2586– 2592. a) H. Nitta, D. Yu, M. Kudo, A. Mori, S. Inoue, J. Am. Chem. Soc. 1992, 114, 7969–7975; b) A. Mori, H. Abe, S. Inoue, App. Organomet. Chem. 1995, 9, 189–197. B. M. Cole, K. D. Shimizu, C. A. Krueger, J. P. A. Harrity, M. L. Snapper, A. H. Hoveyda, Angew. Chem. 1996, 108, 1776–1779; Angew. Chem., Int. Ed. Engl. 1996, 35, 1668– 1671. K. D. Shimizu, B. M. Cole, C. A. Krueger, K. W. Kuntz, M. L. Snapper, A. H. Hoveyda, Angew. Chem. 1997, 109, 1781–1785; Angew.

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13

14

15

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17

Chem., Int. Ed. Engl. 1997, 36, 1704– 1707. a) C. A. Krueger, K. W. Kuntz, C. D. Dzierba, W. G. Wirschun, J. D. Gleason, M. L. Snapper, A. H. Hoveyda, J. Am. Chem. Soc. 1999, 121, 4284–4285; b) J. R. Porter, W. G. Wirschun, K. W. Kuntz, M. L. Snapper, A. H. Hoveyda, J. Am. Chem. Soc. 2000, 122, 2657–2658. For a review on catalytic asymmetric additions to imines, see: S. Kobayashi, H. Ishitani, Chem. Rev. 1999, 99, 1069–1094. For related studies on catalytic asymmetric cyanide addition to imines, see: a) M. S. Iyer, K. M. Gigstad, N. D. Namdev, M. Lipton, J. Am. Chem. Soc. 1996, 118, 4910– 4911; b) M. S. Sigman, E. N. Jacobsen, J. Am. Chem. Soc. 1998, 120, 4901–4902; c) M. S. Sigman, E. N. Jacobsen, J. Am. Chem. Soc. 1998, 120, 5315–5316; d) H. Ishitani, S. Komiyama, S. Kobayashi, Angew. Chem., Int. Ed. Engl. 1998, 37, 3186– 3188; e) E. J. Corey, M. J. Grogan, Org. Lett. 1999, 1, 157–160; f ) H. Ishitani, S. Komiyama, Y. Hasegawa, S. Kobayashi, J. Am. Chem. Soc. 2000, 122, 762–766; g) M. Takamura, Y. Hamashima, H. Usuda, M. Kanai, M. Shibasaki, Angew. Chem., Int. Ed. Engl. 2000, 39, 1650–1652. a) See reference 12; b) M. C. Hansen, S. L. Buchwald, Org. Lett. 2000, 2, 713–715, and references cited therein. N. S. Josephsohn, K. W. Kuntz, M. L. Snapper, A. H. Hoveyda, J. Am. Chem. Soc. 2001, 123, 11594–11599. J. R. Porter, J. F. Traverse, A. H. Hoveyda, M. L. Snapper, J. Am. Chem. Soc. 2001, 123, 984–985. For other catalytic asymmetric alkylations of imines, see: a) S. E. Denmark, C. M. Stiff, J. Org. Chem. 2000, 65, 5875–5878; b) S. E. Denmark, N. Nakajima, O. J.-C. Nicaise, J. Am. Chem. Soc. 1994, 116, 8797–8798; c) H. Fujihara, K. Nagai, K. Tomioka, J. Am. Chem. Soc. 2000, 122, 12055–12056; for a related review,

Endnotes and References

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19

20

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22

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24

see: d) S. E. Denmark, O. J.-C. Nicaise, Chem. Commun. 1996, 999– 1004. J. R. Porter, J. F. Traverse, A. H. Hoveyda, M. L. Snapper, J. Am. Chem. Soc. 2001, 123, 10409–10410. For previous work from these laboratories in connection to catalytic asymmetric allylic substitutions with hard alkylmetals and other related studies, see: a) A. H. Hoveyda, N. M. Heron in: Comprehensive Asymmetric Catalysis. Jacobsen, E. N., Pfaltz, A., Yamamoto, H. (eds), Springer, Berlin 1999, pp. 431–454; for a review concerning catalytic asymmetric addition of soft nucleophiles to olefins, see: b) B. M. Trost, V. L. van Vranken, Chem. Rev. 1996, 96, 395–422. For a review of catalytic enantioselective methods for the synthesis of quaternary carbon stereogenic centers, see: E. J. Corey, A. Guzman-Perez, Angew. Chem. 1998, 110, 402–405; Angew. Chem. Int. Ed. Engl. 1998, 37, 388–401. C. A. Luchaco-Cullis, H. Mizutani, K. E. Murphy, A. H. Hoveyda, Angew. Chem., Int. Ed. Engl. 2001, 40, 1456– 1460. a) C. C. Tseng, S. D. Paisley, H. L. Goering, J. Org. Chem. 1986, 51, 2884–2891; b) J.-E. Backvall, M. Sellen, B. Grant, J. Am. Chem. Soc. 1990, 112, 6615–6621. For Cu-catalyzed (nonasymmetric) addition of organotitanium and organozirconium reagents to allylic phosphates, see: a) A. Masayuki, E. Nakamura, B. H. Lipschutz, J. Org. Chem. 1991, 56, 5489–5491; b) L. M. Venanzi, R. Lehmann, R. Keil, B. H. Lipschutz, Tetrahedron Lett. 1992, 33, 5857–5860. For Cu-catalyzed asymmetric alkylation of allylic phosphates with Grignard reagents, see: a) M. van Klaveren, E. S. M. Persson, A. del Villar, D. M. Grove, J-E. Backvall, G. van Koten, Tetrahedron Lett. 1995, 36, 3059–3062; for W-catalyzed asymmetric alkylation of allylic

25 26

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phosphates with soft nucleophiles, see: b) G. C. Lloyd-Jones, A. Pfaltz, Angew. Chem. 1995, 107, 534; Angew. Chem. Int. Ed. Engl. 1995, 34, 462– 464. Tripeptide 57 and its derived Me ester afford similar results. a) F. Dubner, P. Knochel, Angew. Chem. 1999, 111, 391–393; Angew. Chem. Int. Ed. Engl. 1999, 38, 379– 381; b) F. Dubner, P. Knochel, Tetrahedron Lett. 2000, 41, 9233–9237; moreover, in referene 24b, a catalytic alkylation of 1a with nBuMgCl to afford 2a is reported to proceed in 10% ee (92% SN 2 0 ). For previous enantioselective total syntheses of sporochnol, see: a) M. Takahashi, Y. Shioura, T. Murakami, K. Ogasawara, Tetrahedron: Asymmetry 1997, 8, 1235– 1242; b) T. Kambikubo, M. Shimizu, K. Ogasawara, Enantiomer, 1997, 2, 297–301; c) A. Fadel, L. Vandromme, Tetrahedron: Asymmetry 1999, 10, 1153–1162. For a review on asymmetric catalysis in target-oriented synthesis, see: A. H. Hoveyda in: Stimulating Topics in Organic Chemistry. Vogtle, F., Stoddart, J. F., Shibasaki, M. (eds), VCH-Wiley, Weinheim 2000, pp. 145– 162. For previous studies on the catalytic asymmetric conjugate additions, see: a) B. L. Feringa, M. Pineschi, L. A. Arnold, R. Imbos, A. H. M. de Vries, Angew. Chem., Int. Ed. Engl. 1997, 36, 2620–2623; b) E. L. Strangeland, T. Sammakia, Tetrahedron 1997, 53, 16503–16510; c) R. Naasz, L. A. Arnold, M. Pineschi, E. Keller, B. L. Feringa, J. Am. Chem. Soc. 1999, 121, 1104–1105; d) A. Alexakis, C. Benhaim, X. Fournioux, A. van den Heuvel, J.-M. Leveque, S. March, S. Rosset, Synlett 1999, 1811–1813; e) X. Hu, H. Chen, X. Zhang, Angew. Chem., Int. Ed. Engl. 1999, 38, 3518– 3521; f ) Y. Yamanoi, T. Imamoto, J. Am. Chem. Soc. 1999, 64, 2988–2989; g) I. H. Escher, A. Pfaltz, Tetrahedron 2000, 56, 2879–2888; h) I.

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33 Diversity-Based Identification of Efficient Homochiral Organometallic Catalysts Chataigner, C. Gennari, U. Piarulli, S. Ceccarelli, Angew. Chem., Int. Ed. Engl. 2000, 39, 916–918. 30 S. J. Degrado, H. Mizutani, A. H. Hoveyda, J. Am. Chem. Soc. 2001, 123, 755–756. 31 In all screenings, two sets of ligands involving Gly and nBu as AA3 were examined. Structural modifications

thus involved variations at AA1 and AA2. Further details will be disclosed in the full account of this work. 32 For previous total syntheses of clavularin B, see: a) R. Tamura, K. Watabe, N. Ono, Y. Yamamoto, J. Org. Chem. 1993, 58, 4471–4472; b) K. Hiroya, H. Zhang, K. Ogasawara, Synlett 1999, 592–532.

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Combinatorial Aspects of Materials Science Bill Archibald, Oliver Bru¨mmer, Martin Devenney, Daniel M. Giaquinta, Bernd Jandeleit, W. Henry Weinberg, and Thomas Weskamp Abstract

Combinatorial chemistry, coupled with high-throughput screening and integrated data management systems, has forever changed the drug discovery process, promising to bring to the marketplace more drug entities per unit time than ever before. With rising economic demands to increase efficiency in other areas of research and development, it is not surprising that a similar paradigm is taking hold in the chemical industry as a whole. In particular, combinatorial synthesis and sophisticated screening technologies are now being applied to the discovery of more efficient materials, and with these new technologies come the promise of faster commercialization rates and reduced research and development costs. The combinatorial process aims at efficiently exploring the large parameter space that controls the properties of a material through the application of rapid parallel or combinatorial synthesis and subsequent high-throughput characterization for a given application. Certain synthesis and screening protocols developed in the pharmaceutical industry can be adapted to the new areas of research, whereas, in other areas, a completely new set of techniques must be developed. Unlike in the pharmaceutical industry where aspects such as solvent, temperature, and additives are held constant to eliminate assay variability, the examination of processing conditions in the search for new materials is a critical component of the combinatorial search. The variation of process and reaction conditions, combined with parallel synthesis, results in an exponential increase in the total number of experiments, dramatically increasing the chances of identifying a new material. Over the past 6 years, since the initial application of combinatorial methods to materials science discovery research, tremendous advances in this rapidly growing field have been made in the academic, private, and public sectors. The goal of this review is to examine the contributions made during this time period and to provide insight as to what the future may hold for combinatorial materials research. Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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34 Combinatorial Aspects of Materials Science

34.1

Introduction

With increasing competition in the chemical industry, new products must be brought to the market place rapidly. Global market pressures are driving the chemicals and advanced materials industries to improve research and development (R & D) productivity as measured by return on investment. Principal drivers include reduced cycle times and higher quality at lower cost, with reduced environmental impact. This dilemma was first encountered in the pharmaceutical industry, where long development times and high research costs necessitated the introduction of new research approaches to accelerate the drug discovery process. Virtually every major drug manufacturer now applies this ‘‘new’’ research technology, ‘‘combinatorial chemistry,’’ as the cornerstone of its research and development program; routinely, libraries of up to 1,000,000 distinct compounds are synthesized and screened for biological activity. This approach is feasible both because of the fact that technology challenges have been addressed, as well as because of the broad availability of low-cost computers, reliable robotics, molecular modeling, experimental design strategies, and database software tools [1–10]. The advent of combinatorial chemistry thus created novel research areas such as chemoinformatics to manage the huge amount of structural and functional information obtained from library-based research [11]. This new paradigm is now taking hold in the R & D centers of American, European, and Japanese advanced materials industries where combinatorial methodologies are being increasingly applied. Among others, advanced materials comprise optical and electronic materials, polymers, and catalysts for commodity, specialty, and fine chemical application. Combinatorial techniques represent a powerful research strategy when applied to problems where complex interactions within an extensive parameter space dictate the properties of a material, and these techniques will unquestionably influence the way materials research is carried out in these fields in the future. For example, in the case of heterogeneous catalysis, where active sites exist on the exterior and/or interior surface of a porous solid-state inorganic material, library synthesis can be carried out by a variety of deposition methods. In the case of homogeneous catalysis, where the active site is most often a metal ion stabilized by organic ligands, library synthesis may be carried out using combinatorial ligand synthesis. This process allows combinatorial homogeneous catalysis to rapidly take advantage of the numerous solid- and solution-phase synthetic combinatorial methodologies, including polymer-supported reagents [12–17]. Three different approaches to the task of preparing and testing libraries of compounds exist (Fig. 34.1). Conventional research provides thorough quality control at the expense of throughput in a ‘‘one-at-a-time’’ or serial manner of synthesis and characterization. Truly combinatorial methods involving ‘‘split-and-pool’’ syntheses are much faster and allow for the preparation of large numbers of compounds, but often lack control over the purity of the compounds entering the assay

34.1 Introduction

Fig. 34.1. Conventional, parallel, and pooled approaches to synthesis and screening.

screen. Methods intermediate between these two extremes are based on parallel or array syntheses, in a spatially addressable format with usually one compound per well or region, coupled to automated screens. A SciFinder9 reference search performed in February 2001 using the key word ‘‘combinatorial’’ resulted in more than 9000 ‘‘hits.’’ An analysis of the literature search demonstrates the tremendous growth of scientific publications and patent applications in the combinatorial field. Although most contributions deal with combinatorial applications within pharmaceutical, biological, and medical disciplines, recent years have witnessed significant advances in the development of combinatorial approaches to the discovery and optimization of new materials and catalysts, as is clearly shown by the increasing number of scientific publications and patent applications during the last few years within these areas. Additionally, an increasing number of review articles on the application of combinatorial methods to catalyst discovery and optimization, a process dubbed ‘‘combinatorial catalysis,’’ has appeared in the recent literature [18]. Furthermore, recent advances of combinatorial chemistry and high-throughput screening for chemical process development have been reviewed [19, 20]. Consequently, several recent reviews address combinatorial high-throughput methodologies and experimental strategies in further detail [21–32]. This chapter, covering the years 1995 to 2001, summarizes the latest developments in the application of combinatorial methodologies to the discovery of new solid-state and organic polymeric materials. Patents, patent applications, and conference proceedings have generally not been included but are well appreciated and acknowledged. Combinatorial approaches to catalyst discovery and process optimization will be treated in a subsequent chapter. After this introductory section, the following section, Section 34.2, summarizes the efforts in combinatorial materials science beginning with a general overview of combinatorial materials synthesis techniques. Section 34.3 describes examples of novel high-throughput screening technologies, while Section 34.4 summarizes some applications of combinatorial methods in the search for novel or improved electronic, magnetic, and optical properties. Section 34.5 describes case studies showing the application of combinatorial techniques to the discovery of a new

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phosphor and an example of the application of combinatorial methodologies applied to the optimization of thin-film layers in an organic light-emitting device (LED) device. Combinatorial methods in polymers are described in Section 34.6. Section 34.7 includes a summary and an outlook concerning the future of combinatorial materials science.

34.2

Combinatorial Solid-state Materials Science

The properties of solid-state materials often arise from complex interactions involving the host structure, dopants, defects, and interfaces. Therefore, they depend sensitively on both composition and processing conditions. Few general principles have emerged that allow the prediction of structure beyond binary systems and the resulting properties of such solid-state compounds. Conventional ‘‘one-at-a-time’’ synthesis and characterization can be a long and expensive process, and combinatorial materials science holds great promise in facilitating the materials discovery and optimization enterprises. Conventional materials research typically begins with a decision on a generalphase space which targets a property of interest and which is based on a set of physical or chemical constraints, many of which may be empirically or intuitively grounded. This parameter space is then divided into discrete compositions that must be synthesized and screened for properties of interest. Chemical additions, substitutions, and modifications of synthesis and processing conditions allow the researcher to optimize the properties of a given system. This process is typically long and laborious, and may or may not lead to a promising material. The integrated application of rapid synthesis, high-throughput screening, and sophisticated data analysis allows for a promising alternative to the time-consuming classical methodology. However, a well thought-through experimental design of the experiments (DOE) is required to reduce the number of samples that will be necessary to define sample spaces within the experimental universe or to direct screening to other spaces (feedback loop). The combinatorial process relies on the implementation and coupling of highspeed synthesis and high-throughput screening techniques. These methods facilitate more efficient explorations of a given composition space and offer a valuable tool for the investigation of ternary and higher order systems. However, it is often impossible to rapidly synthesize materials the physical characteristics of which (e.g. composition, microstructure, grain size, and density) are exactly the same as materials made using the final production process. Similarly, screening of desired properties is often very slow. Thus, combinatorial studies are based on the predictive capabilities of synthesis and screening tools, and the challenge of the combinatorial process is to implement appropriate synthesis and screening techniques. Rather than comparing the properties of a few specific compositions within a phase space, entire phase spaces can now be examined in a single experiment. The first library of compounds is often a broad compositional search covering an entire phase space,

34.2 Combinatorial Solid-state Materials Science

Fig. 34.2. Schematic process of the combinatorial materials discovery and optimization paradigm. Large numbers of diverse compounds are rapidly synthesized and screened for desired properties.

e.g. an entire ternary composition diagram (Fig. 34.2). A primary screen specifically developed to evaluate the very large number of compositions within that library identifies a particular composition or range of compositions that is of further interest. Primary screens are typically designed to eliminate a large fraction of the compositions studied in the first library, while secondary and follow-up libraries examine a more narrow range of compositions as well as additional chemical substitutions and processing conditions. Optical and electronic properties such as capacitance or luminescence are examples of physical properties that may be efficiently examined in high-throughput primary screens. At the follow-up level, more detailed information can be obtained from highthroughput secondary screens because the number of compounds that must be screened has been greatly reduced. This process of synthesis, screening, and optimization continues until a manageable number of compositions has been reached.

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These selected compositions, all of which have passed previous screens, can now be studied using conventional methods to obtain the more precise chemical and physical data necessary to characterize a material completely. Combinatorial methods can thus act as an efficient filter for conventional methods by selecting only the best candidates for further, more detailed study. 34.2.1

Materials Library Synthesis

Several synthesis techniques have been developed for combinatorial materials library formation. Some materials can be made using solution deposition methods, while others are more suited for thin-film deposition [33–38]. By modifying technologies similar to those used to make integrated circuit (IC) chips, materials libraries or integrated materials (IM) chips were first developed and utilized by Schultz and coworkers [39]. The choice of synthesis technique is based on both the material being prepared as well as the primary screen employed after synthesis. Unlike the case of drug discovery, however, the synthesis of solid-state materials often relies on processing temperatures in excess of 400 C. High-temperature reaction conditions have been addressed through the creation of two-dimensional, spatially addressable arrays of samples deposited on thermally stable substrates. 34.2.2

Vapor Deposition Techniques

Vapor deposition is commonly used in the semiconductor industry to deposit thin films of material onto a substrate. Vapor deposition techniques that have been utilized in combinatorial library synthesis include sputtering [21, 22, 40, 41] and thermal evaporation [23, 42–46], electron-beam evaporation [29], pulsed laser ablation [21, 47, 48], ion-beam implantation [49–52], molecular-beam epitaxy [24, 53–56], and chemical vapor deposition [57]. One of the most straightforward thin-film approaches is the continuous composition spread (CCS) technique, which utilizes two or three off-axis sources to codeposit material on a substrate [41]. This technique relies on the nonuniform deposition of materials formed by the geometric arrangement between the sources and the substrate. The relative concentration of each component at a specific location on the substrate decreases with the distance from the source. As materials spread from the sources, they mix in the vapor and are deposited on the substrate creating atomic-level mixing that reduces or eliminates the need for high-temperature postprocessing of the library. This technique is also amenable to the isolation of metastable or low-temperature phases that are crystalline on deposition. The lack of precise stoichiometric control and limited compositional range have relegated this technique primarily to optimization and exploration of systems with only two independent variables. A typical vacuum deposition system for combinatorial materials science has several source materials and is used in conjunction with masking techniques (physical or shadow masks, movable shutters, or photolithography) to deposit different


Fig. 34.3. Binary and quaternary masking strategies for

combinatorial material library synthesis. See text for details.

materials, sequentially or simultaneously, in particular areas of the substrate. The design of the masks and the sequence in which they are employed determine which materials are deposited at any given location on the substrate. By altering the sequence, time, and rate of deposition, it is possible to control the exact chemical composition of each element in the library. The efficiency with which a particular compositional landscape may be examined is dictated by the masking strategy employed [58]. Simple binary and gradient masks are useful for optimizing the composition of a known material. In the binary masking strategy, half of the substrate is exposed to different masking patterns (1, 2, 4, . . . strips oriented in two different directions) in each step (Fig. 34.3). After N steps, the number of different compositions is 2 N , including all possible combinations of N elements. Gradient or x/ y shutter masking utilizes movable masks that either expose or block certain areas of the substrate, allowing for controlled concentration and/or thickness variations in the deposited films. Quaternary masking strategies have been developed to enable efficient generation of diverse libraries containing materials with very different compositions [30]. In the quaternary masking scheme, deposition is carried out using a series of N different masks that successively subdivide the substrate into a series of nested quadrant patterns (Fig. 34.3 and Fig. 34.4). Each mask is used for up to four depositions, but after each deposition the mask is rotated by 90 . With N different masks, this process will generate up to 4 N different compositions in just 4N deposition steps. The r th ð1 a r a NÞ mask contains 4 r1 windows, where each window exposes one-quarter of the area deposited using the preceding ðr 1Þ th mask. Within each window is an array of 4 Nr openings, which can be provided by means of an underlying contact mask or created directly on the substrate by photolithographic techniques [33, 34, 59–61]. Each section of the substrate is thus exposed to a different combination of precursors by depositing each layer through a different mask.

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Fig. 34.4. The quaternary masking system uses six fractal masks, each of which can be rotated 90 to allow four choices of materials at each level.

Thin-film deposition methods are synthetically quite versatile; they have progressed to enable atomic- and molecular-layer epitaxy and offer the ability to construct artificial lattices, epitaxial overlayers, and patterned films of a variety of materials. Dopants are usually sandwiched between layers of the host material to avoid evaporation and to assure proper interdiffusion. Subsequent thermal processing results in a library of materials or devices the physical properties of which can be screened with either contact or noncontact screening probes. The number of compounds that can be simultaneously synthesized by this technique is limited by the spatial resolution of the masks and by the degree to which synthesis can be carried out on a microscale. One can use either physical shadow masks [39] or photolithographic lift-offs [61] to carry out masking. Photolithography, a standard process in the semiconductor industry, has a high level of spatial resolution and accuracy and can generate chips with a high density of diverse compositions (up to 10 6 per square inch). The multilayer thin-film deposition may result in the synthesis of multiple binary phases rather than the desired multielement single-phase material, a result that may be avoided through the use of an effective two-step annealing process [33]. Early nucleation and thus binary-phase formation is avoided by deposition at relatively low temperatures. If the thickness of the deposited layer is less than a critical value (material dependent) of typically 1–10 nm, diffusion is dominant over nucleation for sequential precursor layers. Johnson and coworkers have advanced this elegant


technique based on the sequential controlled deposition of thin films [62]. Using Johnson’s method, an extended period of low-temperature (100–400 C) annealing is performed for proper interdiffusion of thin-film precursors. Subsequently, crystallization of the intermediate, amorphous material is induced by a high-temperature annealing process allowing growth of entire integrated materials chips [63]. Growing entire integrated materials chips in this fashion is crucial for many materials, where the material properties are closely tied to the crystalline quality of the films. Reflection high-energy electron diffraction (RHEED) has been proposed for in situ monitoring of molecular beam epitaxial (MBE) deposition [35]. Pessaud and coworkers utilized pulsed laser ablation of two compound targets made of superconducting YBa2 Cu3 O7 (YBCO) and the double-chain insulator MCuO2 (M ¼ Ca, Sr) in an attempt to create new metastable superconducting compounds [47]. The goal of this exploration was to deposit m consecutive layers of YBCO with n layers of MCuO2 in an attempt to use the two-dimensional structure of YBCO as a structural template so that MCuO2 would form additional conducting CuO2 planes upon deposition. The technique has led to the identification of a number of new phases with enhanced critical temperatures and extends the multilayer deposition concept from elemental targets to compound targets. Combinatorial thin-film methods are also being used to optimize the performance of multiwavelength emitting chips that show potential applications in wavelength-divisible multiplexing (WDM). Multiwavelength emission has been achieved using selective area epitaxy and postgrowth selective region intermixing. Layer intermixing techniques have achieved wavelength shifting through implantation-induced intermixing, which is difficult to control. Liu and coworkers used arsenic and proton implantation to create a multiwavelength-emitting library of doses and ‘‘species-dependent’’ intermixing, which they analyzed by microphotoluminescence at room temperature [50]. Al0:35 Ga 0:65 As/GaAs single quantum wells were grown on GaAs (001) substrates by MBE. Ion implantation was accomplished using four different pattern masks on an 8 8 hollow circle-patterned blank mask in sequence. Four rows of elements implanted with different As doses at an ion energy of 90 keV were obtained. The four masks were then rotated 90 for proton implantation at an ion energy of 40 keV. The fifth column and the fifth row were implanted with protons only and As only. The implanted chip was annealed in a rapid thermal annealer (RTA) at 950 C, and the microphotoluminescence measured at room temperature. Using this technique Liu and coworkers were able to generate more than 20 different wavelengths from a single chip. Amorphous materials have been investigated in a combinatorial optimization of hydrogenated amorphous silicon (a-Si:H)-based thin-film transistor (TFT) devices by Koinuma and coworkers using plasma-enhanced chemical vapor deposition [57b]. A contact mask placed over the indium–tin oxide substrate and sequential depositions were carried out through a moving slit mask. A 90 substrate rotation followed by deposition of hydrogenated amorphous silicon nitride (a-SiN:H) allows for systematic investigation of the effects of thickness and compositional variation across the library. Source and drain contacts were deposited on the top of the bi-

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layers by aluminum evaporation to allow the current–voltage characteristics of the devices to be characterized. 34.2.3

Alternative Library Synthesis Techniques

Synthesis for many materials, e.g. zeolites, polymers, polycrystalline phosphors, and many other oxides and sulfides is best accomplished using solution-phase methods. When applicable, solution-phase methods provide a large number of libraries at a significantly lower capital investment relative to automated vapor deposition equipment. Solution-phase methods may result in improved compositional control. One significant complication associated with materials discovery in thinfilm format is the occasional lack of correlation with bulk properties owing to differences caused by film microstructure, strain, and so on. Physical and chemical properties of compounds prepared using solution techniques generally show excellent correlation with bulk properties regardless of the quantity of material prepared. Thus, library-based data may be easily confirmed in bulk. Solution techniques allow mixing at the molecular level, reducing the need for high-temperature interdiffusion and also facilitating the isolation of metastable phases. Xiang, Schultz, and coworkers have demonstrated that a scanning multihead inkjet delivery system can be used to perform automated microsynthesis of solid-state libraries, enabling rapid delivery and accurate control of nanoliter deposition volumes [34, 64]. Droplets are delivered sequentially to single reaction wells; the droplet size is on the order of 500 pL with reproducibility better than 1% and a maximum delivery speed of 2000 droplets s1 . The system has been successfully used to generate libraries of 100 members per inch 2 (100 members per 6.49 cm 2 ), and a system to generate 1000 members per inch 2 (1000 members per 6.49 cm 2 ) is under development. Sol-gel processing of glasses and ceramics provides an effective method for fabricating inorganic nanostructured materials that is amenable to combinatorial methods. Molecular precursors, such as metal alkoxides, are typically used as starting materials [65, 66]. The process begins with the formation of arrays of homogeneous sols with desired compositions through the use of automated liquid-dispensing robots. The sol is then converted to a gel, while maintaining its homogeneity and purity. A solid network is obtained through hydrolysis and condensation reactions. The technique has been applied in the investigation of phosphors [65] and oxide semiconductors [66], and in the synthesis of polymer/ vanadium oxide nanocomposites [67]. Multicomponent zeolite synthesis is another area of advanced solid-state materials research in which combinatorial discovery and optimization can be successfully applied. A multiautoclave system capable of performing at least 100 crystallizations under hydrothermal conditions has been reported by Akporiaye and colleagues [68]. In its simplest form, the reactor consists of a Teflon block with cylindrical holes, designed to accommodate Teflon-coated septa, which is sandwiched between two steel plates. The capability to stack identical synthesis blocks allows


for the parallel synthesis of in the order of 1000 combinations in one experiment. The system was tested by reproducing the phase diagram of the intensively studied ternary phase system Na2 O/Al2 O3/SiO2/H2 O in a single experiment and under identical reaction conditions. With a total volume of each sample gel of not more than 0.5 mL, crystalline phases of zeolite A, faujasite, and gmelinite were obtained, with the region of zeolite A coinciding almost perfectly with earlier results for bulk materials. Good agreement was also obtained in the case of faujasite. The formation of sodalite was only observed in a more recent investigation. This partial agreement was attributed to better control of the water content in the parallel autoclave. The attractiveness of combinatorial methods for hydrothermal synthesis was also realized by Klein et al., who claim that the advantage of their reactor over the one described above is their much smaller reaction volume, in the order of 2 mL, as well as the direct preparation of a library of materials the components of which can be identified automatically on the library substrate by X-ray microdiffraction (Fig. 34.5) [69]. The central feature of Klein’s reactor is a silicon wafer that, after hydrothermal synthesis and calcination, contains the sintered reaction products and represents the library. The identification of the individual products may be directly performed with a GADDS (General Area Detector Diffraction System) microdiffractometer. Choi and coworkers designed a centrifuge apparatus that allows quantitative product recovery onto filter paper for X-ray microdiffraction without manipulation of the individual samples [70]. Furthermore, Lai and coworkers demonstrated parallel synthesis of zeolite films in a 21-well reactor using a vertical substrate configuration to provide uniform wetting that favored heterogeneous film growth in an organic-free clear solution [71]. The results of the synthesis of

Fig. 34.5. Schematic cross-section of the multireactor autoclave.

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template molecules and metal components, followed by a qualitative indication showing that the materials formed are ‘‘amorphous or crystalline,’’ confirm that zeolite synthesis on a microgram scale is possible and justified. A solution-based combinatorial strategy has been described by Baker and coworkers for synthesis of surfaces exhibiting nanometer-scale variation in mixedmetal compositions and architecture [72]. Continuous or stepped gradients in the size and number density of surface features can be generated simultaneously over different regions of a single substrate. Baker et al. and Kneipp et al. prepared libraries of Ag-clad colloidal Au arrays in which the coverage of colloidal Au and the extent of the reductively deposited Ag coating were varied in an attempt to optimize the enhancement factor for surface-enhanced Raman scattering (SERS) [72, 73]. A sulfide-functionalized glass slide was immersed at a constant rate into an aqueous solution of colloidal gold, and the plate was then rotated by 90 and lowered at a fixed rate into a solution of Agþ . As a result, a library was generated containing approximately 10 8 colloids that differed in their particle size and Ag coverage. Para-nitrosodimethylaniline was adsorbed onto this surface and the surface-enhancement factor of the Raman scattering of this compound was measured. The values spanned approximately three orders of magnitude across the library and allowed for identification of a region of the library that gave the greatest enhancement. Atomic force microscopy was then used to determine the nanometer scale morphologies of these regions of interest. The availability of dispersible metal-containing nanoparticles and the numerous routes to metal deposition from complexed metal cations should allow for extension of this method to a variety of other metals. A scheme for generating complex, spatially separated patterns of multiple types of semiconducting and/or metallic nanocrystals has been presented by Vossmeyer and colleagues [38]. Nanocrystals may play an important role in future technologies such as photovoltaics, switches, phosphors, light-emitting diodes, electronic data storage systems, and sensors. Most of the photonics and electronics applications will eventually require parallel schemes for the control of spatial positioning of the nanocrystals. Standard patterning techniques such as laser ablation of the material and deposition through a shadow mask do not work well for nanocrystals since most metal and semiconductor nanocrystals have covalently bound organic surfactants that tend to desorb at temperatures above 100 C. Consequently, any patterning approach must be carried out at low temperatures. The stepwise preparation of multiple particle arrays is based on lithographic patterning of amino-functionalized organic monolayers that contain a photolabile protecting group covalently bound to SiO2 surfaces. The photosensitive substrate is irradiated through a mask in the near UV, removing the protecting group in the areas exposed to the radiation. To prepare binary micropatterns consisting of Pt or Au and highly luminescent CdSe/ CdS core/shell nanocrystals, the substrate is treated with a solution of aminestabilized metal-containing nanoparticles, which assemble in the area of deprotected amino groups. Changing the orientation of the mask, the deprotection step is repeated and the amine-stabilized CdSe/CdS nanoparticles are assembled onto areas of freshly deprotected amino groups, yielding a binary nanoparticle array.


Fig. 34.6. Representation of protonated peptide ligand framework depicting both the variable spacer residues (X, Y, and Z ) and the individual regions used in informatic analysis. [X, Y, and, Z ¼ a-Glu, g-Glu, a-aminobutyric acid (GABA), SerGly, e-aminohexanoic acid (e-Ahx)].

Nanocrystal assembly occurs with high selectivity on just one type of the spatially and chemically distinct sites. The production of binary, ternary, and quaternary patterns of nanocrystals has been reported, and the procedure appears applicable for the preparation of multicolored nanocrystal-based color pixels. Moreover, the technique should be compatible with other lithographic patterning steps, such as those involved in the formation of electrical contacts, capacitors, and other device components of the photonics and microelectronics industries. Furthermore, the procedure has been demonstrated on a variety of substrates. A novel approach by Wright and coworkers has shown that combinatorially derived peptide and peptidomimetic ligands based on phytochelatins can stabilize CdS nanoclusters [74–76]. Using the 7-mer sequence XaCysaYaCysaZaCysaGly of n3 -phytochelatin as a lead structure, Wright and coworkers employed standard Fmoc protocols (Fmoc ¼ 9-fluorenylmethoxycarbonyl) for combinatorial peptide library synthesis. In their study, the cysteine residues are held constant while the spacer residues X, Y, and Z are varied over all combinations of the selected spacer amino acids a-Glu, g-Glu, g-amino butyric acid (GABA), SerGly, and e-Ahx (Fig. 34.6). This approach limits the number of peptides to 125, as opposed to the over one billion potential ligands obtained if all seven positions are varied over all 20 naturally occurring amino acids. The five selected spacer residues chosen have between three and seven atoms in the backbone and can span an idealized extended C a aC a distance between the cysteines. Using commercial software, quantitative structure–activity relationship (QSAR) methods were employed to calculate the molecular numerical properties (descriptors) of the combinatorial peptide library. The positional analysis of individual amino acids allows the correlation between the properties of the stabilizing (iso)peptidic ligand and the resultant peptideencapsulated CdS nanocluster to be determined. Many new advanced materials ranging from magnetic materials to bulk metallic glasses involve alloying multiple elements to control both intrinsic and extrinsic properties. Permanent magnets, for example, may have a dozen or more elements included to control grain boundaries and domain wall movement. Cohen-Adad

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and coworkers utilized a melting zone technique to investigate AlCo and AlCoFe alloys [77]. In their method, source materials are cut into wedges and then welded together into a bar. The bar is then placed in an alumina crucible and drawn through a horizontal zone furnace to facilitate mixing. The resulting bar is diced into pieces and the composition at each point along its length measured with microprobe analysis. The morphology and crystal structure of the materials were investigated with optical microscopy, differential thermal analysis, and X-ray diffraction. Inuoe has reviewed similar techniques used to search for new glasses, which typically have five or more elements present to inhibit crystallization [25].

34.3

High-throughput Screening

The large numbers of compounds and the speed with which they may be prepared using the combinatorial synthesis methods described above require automated high-throughput screening techniques that are capable of equally rapid characterization. Typically, the information necessary to classify new materials cannot be obtained from a single piece of characterization equipment. Therefore, a series of high-throughput screening tools is employed at various stages in the combinatorial process to create a screening hierarchy. While a large number of automated commercial systems exist for the high-throughput analysis of microliter samples for medical and pharmaceutical applications, the vast majority of the screening tools necessary for advanced materials research are custom-made instruments. Conventional analytical tools such as mass spectrometry, gas and liquid chromatography, optical and nuclear magnetic resonance spectroscopy, X-ray fluorescence microprobe [78], and X-ray diffraction have been automated and redesigned for rapid serial measurements of hundreds of samples per day [79, 80]. Several examples of custom-made high-throughput screening systems are discussed below. 34.3.1

Optical Screening

Optical screening methods, where applicable, offer high-throughput noncontact screening for combinatorial libraries. Optical screens typically involve either a laser or fiber optic probe that is serially scanned across a library or cameras capable of imaging library elements in parallel. In studies of luminescent materials, highthroughput screens have been developed using UV-excited photoluminescence [81, 82]. A quantitative measure of chromaticity was calculated from three images obtained using red (602 nm), green (553 nm), and blue (461 nm) bandpass tristimulus emission filters. The color was quantified by calculating the normalized CIE (Commission Internationale de l’Eclairage) chromaticity coordinates, and the relative extrinsic efficiencies were estimated from the ratio of the measured output of the compounds to commercial standards. Optical transmission and reflection measurements have also been utilized to

34.3 High-throughput Screening

determine the band gap of various oxide thin films. Jin and coworkers [56] and Makino and coworkers [83] calculated the optical band-gap from the measured optical absorbance of 81 elements in a library of ZnO thin films doped with nine independent 3d-metal ions. Similar transmission and reflection methods have been used in the infrared to estimate band-gaps for thermoelectric and photovoltaic materials. Furthermore, Xiang describes an apparatus for the measurement of electro-optic coefficients of thin-film samples that functions by measuring the polarization modulation of light passing through a sample [21]. The variation in the index of refraction upon application of an electric field is typically small, approximately 0.1%, making thin-film measurements difficult. The electro-optic measurements are conducted using the library, a compensator, and a quarter-wave plate which are located between two crossed polarizers and are illuminated by a HeNe laser. An electric field is applied to the sample at 45 to the incident polarization. Given the index of refraction and some geometric factors, the electro-optic coefficients can be determined. 34.3.2

X-ray Characterization

Structural characterization presents an important issue in high-throughput characterization of solid-state materials. Several groups have created machines capable of carrying out X-ray microdiffraction [78, 79] through the use of intense sources (rotating anode or synchrotron), and focusing mirrors resulting in an X-ray beam collimated onto a small spot capable of measuring diffraction of thin-film libraries (Fig. 34.7). The data collection times are still sufficiently long that the measurement of a significant number of library elements takes hours instead of minutes. Furthermore, exploratory libraries are typically mixed phase, resulting in a complicated analysis that has yet to be dealt with efficiently.

Fig. 34.7. Schematic representation of an X-ray microprobe synchrotron beamline capable of producing a 3 20 mm 2 X-ray spot on the sample on the library array.

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34.4

Applications 34.4.1

Superconductivity

One of the first demonstrations of combinatorial thin-film materials exploration by Schultz and coworkers involved the preparation of libraries of superconducting copper oxide thin films by sputtering several metal oxides in sequence onto an inert single-crystal support of either MgO or LaAlO3 [39, 84]. In their initial experiments, arrays containing different combinations, stoichiometries, and deposition sequences of BaCO3 , Bi 2 O3 , CaO, CuO, PbO, SrCO3 , and Y2 O3 were generated using a series of four and seven binary masks and primary masks having either 16 or 128 openings, respectively, yielding a spatially addressable library of 16 (2 4 ) 2 mm 2 mm and 128 (2 7 ) 1 mm 2 mm samples distributed over a chip with a surface area of 1 inch 2 . After annealing the entire library at 840 C in an oxidizing atmosphere, the superconductivity of each sample was evaluated by measuring its resistance as a function of temperature with an array of small four-point probes that analyze 64 samples at a time [34]. Superconducting films of BiSrCaCuOx , BiPbCaSrCuOx , and YBa2 Cu3 Ox were identified having critical temperatures in the range of 80–90 K. Xiang and coworkers have developed a novel scanning evanescent microwave microscope (SEMM) capable of nondestructive and quantitative mapping of electrical impedance with submicron resolution [21, 85]. Unlike conventional fourprobe methods, the SEMM is a noncontact method, which eliminates dependence on contact resistance and interfacial effects. The SEMM, shown schematically in Fig. 34.8, is based on a coaxial microwave resonator with a sharpened tip that extends through an aperture outside of the resonant cavity. When the probe tip is near a sample, the interaction of the evanescent field and the sample changes the resonant frequency and the ratio between the energy stored in the cavity and the energy dissipated in the sample, resulting in a measurable change in the electrical impedance. The microscope is routinely used for characterization of linear and nonlinear dielectric constants of ferroelectrics and dielectrics. A low-temperature version has also been developed to characterize superconducting materials. High-throughput methods are not new to high-TC superconductor research and several groups utilized pooled samples as a means to increase the throughput of a conventional magnetometer [86, 87]. Superconductivity is one of the few electronic properties that can be measured when samples are pooled (mixed together) in a single vial. In this method, several compositions are synthesized separately and then mixed together (without heating) to form a single sample. Measurements of the low-temperature magnetization are performed on the mixture with a Squid magnetometer searching for a measurable Meissner fraction indicative of superconductivity. The presence of a Meissner fraction indicates the presence of superconductivity in at least one of the compounds in the mix, the identity of which must be determined by individually measuring each of the components in that

34.4 Applications

Fig. 34.8. Schematic representation of a scanning evanescent microwave microscope.

particular mixed sample. In this manner, a slow, serial measurement technique is converted into a fast parallel tool capable of measuring large numbers of potential superconducting phases. Bozovic and coworkers at Oxxel utilized a sophisticated composition spread MBE system equipped with 16 metal sources, an ozone source, an electron microscope (LEEM/LEED/PEEM system), time-of-flight ion scattering and recoil spectroscopy, and scanning quartz microbalances in a search for new materials for oxide electronics [24]. 34.4.2

Ferromagnetic Semiconductors

Ferromagnetic semiconductors are important materials for spin-based electronics in which the correlation between charge and spin is used to generate spindependent electronic functionality such as giant magnetoresistance and spin field effect transistors. A majority of the materials currently being investigated are III–V and II–VI compounds. Among known materials, Mn-doped GaAs is the most promising for practical applications because it has the highest magnetic ordering (Curie) temperature TC @ 100 K. The search for new ferromagnetic semiconductors has led to the investigation of dilute magnetic oxides, where small concentrations of 3d ions are added to wide band-gap oxides such as ZnO. Kionuma et al. have applied a combinatorial approach to the search for new ferromagnetic semicon-

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ductors by adding dilute amounts of cobalt to the anatase form of TiO2 [88]. Anatase TiO2 was chosen as a candidate semiconductor because of its high electron carrier mobility at room temperature. TiO2 and Ti 0:5 Co0:5 O2 ceramic targets were ablated using a KrF exicmer laser in a combinatorial laser molecular beam epitaxy system [55]. Anatase films of Tix Co1x O2 were grown on LaAlO3 (001) and SrTiO3 (001) substrates and the composition mapped by microprobe analysis. X-ray diffraction studies show that the Codoped anatase compositions obey Vegard’s law up to a solubility limit of x ¼ 0:08. Magnetic images obtained by a scanning Squid microscope at 3 K showed domains of around 20 mm in size for the Co-doped compositions. The spontaneous magnetization systematically increases with increasing Co content to 0.32 mB (Bohr magnetron) observed for the x ¼ 0:07 sample. The ferromagnetic ordering temperature was estimated to be in excess of 400 K, a substantial increase over the Mn-doped GaAs compounds. 34.4.3

Magnetoresistant Materials

The preparative combination of thin-film deposition and physical masking techniques was subsequently used by Xiang and coworkers in the discovery of new cobalt oxide magnetoresistance materials [89, 90]. Magnetoresistance is the variation of electrical resistance with an applied magnetic field. Materials with particularly large magnetoresistance properties are useful in high-density magnetic storage technology such as ‘‘read heads’’ for computer hard drives [91]. Colossal magnetoresistance (CMR) was first discovered in Mn-based perovskites (La,R)1x Ax MnO3e , where R is a rare earth element, and A is Ca, Sr, or Ba. Magnetoresistance ratios DR=Rð0Þ ¼ R½ðH ¼ 0Þ RðHÞ=RðH ¼ 0Þ as large as 99.0%, 99.9%, and 99.99% have been reported for polycrystalline samples of La 0:6 Y0:07 Ca 0:33 MnO3 and epitaxial thin films of La 0:67 Ca 0:33 MnO3 and Nd 0:7 Sr0:3 MnO3e respectively. Two identical 128-member libraries of composition Lnx My CoOz (1 mm 2 mm) were synthesized on LaAlO3 single crystals to examine the magnetoresistivity of materials that contain cobalt rather than manganese in simple perovskite, LnCoO3 , and related Ln2 CoO4 or Lnnþ1 Con O3nþ1 structures, where Ln ¼ (La, Y) 3þ , partially substituted with different stoichiometries of M ¼ (Ca, Sr, Ba, Pb) 2þ . The two libraries were annealed and sintered under different conditions, and one showed an overall decrease in magnetoresistivity, highlighting that in combinatorial materials synthesis, postdeposition processing conditions represent important variables. Using a four-point probe method with a multichannel switching system in a liquid helium cryostat with a superconducting 12 T magnet, large magnetoresistance (DR=Rð0Þ > 5%) was measured in La x (Ba,Sr,Ca)y CoOz samples, while Y-based samples exhibited much smaller magnetoresistive effects. The magnetoresistance of the Co-containing compounds was found to increase as the size of the alkaline earth metal increases, in sharp contrast to Mn-containing compounds, in which the MR effect increases as the size of the alkaline earth metal decreases. The MR ratio DR=Rð0Þ of a subsequently synthesized bulk sample of the lead compound La 0:58 (Ba,Sr,Ca)0:41 CoOz

34.4 Applications

was measured to be significantly higher than that of the corresponding thin-film sample in the combinatorial library (DR=Rð0Þ ¼ 60% compared with DR=Rð0Þ ¼ 30%). This finding again illustrates that, in combinatorial materials discovery, libraries are screened to point the direction to a promising lead; synthesis and characterization of a bulk sample is then performed to determine the bulk properties which can differ considerably from the properties of the thin-film compound. 34.4.4

Dielectric and Ferroelectric Materials

Driven by the current trend toward the continued miniaturization of electronic devices, improved thin-film insulators are needed in, for example, dynamic random access memory (DRAM) computer chips. Using the composition spread technique (CCS), van Dover and coworkers from Lucent Technologies have reported new dielectric thin-film materials that have higher dielectric coefficients than amorphous silicon dioxide (a-SiOx ), the insulating material most commonly used today [41, 85, 92–94]. The CCS technique (see above) involves the co-deposition of three different reagents onto a substrate from a triangular arrangement of targets. van Dover and coworkers used off-axis RF magnetron sputtering guns positioned at 90 intervals around a rectangular TiN-coated Si wafer (66 mm 63 mm) to co-deposit a continuous ternary composition variation of Zr, Ti, and Sn. The three sputter guns operated simultaneously to create intimate reactant mixing. A continuous thin film was obtained and the position-related composition was inferred using Rutherford backscattering spectroscopy. To isolate the optimum thin-film transistor candidate (Zr0:15 Sn 0:3 Ti 0:55 O2e ), more than 30 combinatorial libraries were synthesized and evaluated at a rate of approximately one per day. The critical electrical properties, capacitance C (per unit area C/A) and breakdown voltage Vbr, were evaluated with a scanning Hg-probe instrument as a function of position at approximately 4000 points in the 66 mm 63 mm rectangle, and the figure of merit data (CVbr =A) were mapped onto a conventional ternaryphase diagram. It was found that the properties of the Zrx Sn y Tiz O2e thin films depend strongly on the deposition conditions, as does the region of composition that yields the highest values for the figure of merit. Full compositional scans for films prepared under various processing conditions thus proved essential. In their choice of elements, van Dover and coworkers have focused on deposition at substrate temperatures below 300 C to maintain compatibility with back-end Si IC fabrication technology, and did not apply any postdeposition annealing treatment. In contrast to the application of combinatorial materials synthesis for superconducting and magnetoresistant materials, in this application reproduction of the thin film results by bulk materials samples is not essential, since in the final production process the materials will be manufactured by thin-film deposition techniques. Combinatorial approaches have also recently been applied by Xiang and coworkers to ferroelectric materials in which the effects of transition metal dopants on the dielectric coefficient and tangent loss of a library of thin films of (Ba x Sr1x )TiO3

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a Fig. 34.9. a) Dopant map for the (Ba 0:8 Sr0:2 )TiO3 quadrant of a ferroelectric thin-film library; the same map applies to the other three quadrants containing different host

materials. A quaternary masking scheme with four masks was used to deposit 256 differently doped thin films of Ba x Sr1x TiO3 ð0:5 < x < 1Þ on a 2.54 cm 2.54 cm LaAlO3 substrate.

(BST) were determined [63, 95]. Ferroelectric materials are being widely used in the development of new microwave devices such as frequency agile filters, phase shifters, and tunable resonators, and (Ba x Sr1x )TiO3 compounds have been intensely studied for such applications owing to their low loss and high dielectric coefficients. Four libraries of 256 differently doped thin films of (Ba x Sr1x )TiO3 , where x ¼ 1, 0.8, 0.7, and 0.5, were generated on a 2.5 cm 2.5 cm LaAlO3 substrate using multistep thin-film RF sputtering together with a quaternary masking strategy with four physical shadow masks. These four hosts were then doped with different combinations of up to three out of nine different metallic elements with each dopant added in excess of 1 mol% with respect to the BST host; the resultant dopant map for the (Ba 0:8 Sr0:2 )TiO3 quadrant is shown in Fig. 34.9a. The same map applies to the other three quadrants of the library with the three other host materials, BaTiO3 , (Ba 0:7 Sr0:3 )TiO3 , and (Ba 0:5 Sr0:5 )TiO3 . Appropriate postannealing afforded high-quality epitaxial thin films, similar to those made by conventional in situ methods from stoichiometric targets on hot substrates. The rapid, quantitative and nondestructive characterization of the microwave dielectric properties, the dielectric coefficient, and loss tangent of samples in the library was achieved with a scanning-tip microwave near-field microscope, as shown in Fig. 34.8, at 1 GHz, and the results were found to be consistent with measurements made with interdigital electrodes [21, 33, 34, 59, 96]. In Fig. 34.9b the averaged dielectric coefficient and tangent loss data are displayed for the BaTiO3 (BTO) quadrant; each square corresponds to a different thin-film sample site, and darker shades represent lower dielectric coefficient. Specific dopants were found to

34.4 Applications

b Fig. 34.9. b) Dielectric coefficient and loss tangent images of doped BaTiO3 films, measured using a scanning-tip microwave near-field microscope (STMNM) with

submicron spatial resolution. Each square corresponds to a different thin-film sample site as given in the composition map above.

significantly affect the dielectric coefficient and the tangent loss, with La (sample 2) and Ce (sample 3) increasing the dielectric coefficient, whereas most dopants result in decreased values when compared with the undoped material. In the tangent loss data, a lighter shade implies a lower loss tangent. In the case of microwave applications, where the values of the dielectric coefficient are considered to be high, a minor reduction may be tolerable as long as there is a noticeable improvement in the tangent loss, which is especially important for the microwave application of tunable dielectric materials. Possible applications include radar, which steers the beam and quickly locates objects electrically rather than mechanically, and cellular phones, which instantaneously tune into the incoming signals. Subsequently, Xiang and coworkers evaluated the effects of dopants on device performance by incorporating a layer of the doped dielectric (Ba x Sr1x )TiO3 in a parallel-plate capacitor library between Pt and La 0:5 Sr0:5 CoO3 electrodes [97]. Like van Dover’s work on thin-film dielectrics, this investigation aims to aid in the iden-

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tification of optimal dielectric materials for integrated capacitors in dynamic random access memory (DRAM) chips. Multilayer device structure libraries of 240 different host/dopant combinations were epitaxially synthesized with sharp interfaces from amorphous layers on a LaAlO3 substrate. Addition of 1.5 mol% W was found to increase the figure of merit, the ratio of the effective dielectric current, and the leakage current density 220-fold and to reduce the high-frequency tangent loss fourfold. The epitaxial growth of heterostructures in thin-film device libraries is necessary in order to explore the intrinsic effects of compositional variations in the absence of polycrystalline microstructural effects which can provide sites for charge traps and other defects. Since the conventional techniques of high-temperature in situ epitaxial thin-film growth cannot be used for combinatorial library synthesis, Xiang and colleagues developed a synthesis method to form epitaxial multilayers from amorphous layers deposited at room temperature using pulsed laser deposition (PLD). The capacitor library consisted of a 100- to 200-nm-thick amorphous layer of La 0:5 Sr0:5 CoO3 (LSCO) on LaAlO3 ; the substrate is then annealed at 850 C for 1.5 h. Using two-dimensional shutters, the three different hosts BaTiO3 , Ba 0:7 Sr0:3 TiO3 , and Ba 0:5 Sr0:5 TiO3 were deposited from stoichiometric targets in the x-direction of the substrate, whereas in the y-direction typically up to four dopants were added to each host as a gradient from 0 to 3 mol%. Two-step annealing then ensured uniform interdiffusion of dopants and epitaxial crystal growth with sharp interfaces between the BST and the LSCO layer. To complete the device structure, individual Pt electrodes (50 nm) were deposited using a photolithographic mask. Approximately 5500 capacitors, each 50 mm 50 mm in size, were formed with 240 different host/dopant concentration combinations, affording about 23 capacitors for each host/dopant concentration and providing a test of the reproducibility of each device. 34.4.5

Luminescent Materials

The combinatorial methodology of thin-film deposition and masking techniques to create materials chip libraries has quickly found applications in the discovery and optimization of luminescent materials. Despite many years of intensive research, fewer than 100 useful commercial phosphor materials have been discovered through conventional one-at-a-time synthesis and testing. Phosphor materials with high quantum efficiencies find application in display technologies such as cathode ray tubes (CRTs) and are of great importance for developing improved plasma, field emission, and electroluminescent flat panel displays and lighting [98]. Phosphors are typically inorganic powders that consist of a polycrystalline host doped with ions of a rare earth and/or a transition metal. The dopants may act as a center for luminescence that generates visible light when excited by ultraviolet light, or as a sensitizer that absorbs and transfers the ultraviolet energy to the luminescent center. Although the photophysical processes leading to luminescence are relatively well understood, the specific spectral properties, luminescence efficiencies, and operational lifetimes depend on complex interactions between the excita-

34.4 Applications

tion source, host lattice, sensitizer, and luminescent center. Two groups reported application of combinatorial methods to the discovery and optimization of several luminescent metal oxide materials. Oxides are attractive host materials for the development of advanced phosphors owing to their ease of synthesis and stability. For example, the efficient refractive oxide phosphors Y2 O3 :Eu 3þ (red), Y3 Al5 O12 :Tb 3þ (green), and BaMgAl10 O17 :Eu 2þ (blue), have found applications in tricolor lamps, projection TVs, and plasma displays. Symyx Technologies’ red phosphor, Y0:845Al0:07 La 0:06 VO4 :Eu 0:025 , with a quantum efficiency approaching those of existing commercial red phosphors, was discovered in a library of 25,000 different compositions produced using electron-beam evaporation to deposit thin films onto a 7.5-cm silicon wafer [81]. In this discovery library, four constant thickness columns were first deposited consisting of SnO2 (480 nm), V2 O5 (160 nm), Al2 O3 þ V2 O5 (150 nm þ 80 nm), or Al2 O3 (300 nm). On top of these layers were deposited four rows of linearly varying thickness of La2 O3 , Y2 O3 , MgO, and SrCO3 to create 16 host lattice subregions. Finally, within each of the 16 subregions linearly varying thicknesses of the rare earths Eu2 O3 , Tb4 O7 , Tm2 O3 , and CeO2 were deposited, resulting in approximately 600 different chemical compositions per square centimeter, see Fig. 34.10. A stainless-steel primary mask consisting of 230-mm 2 elements spaced 420 mm apart was attached to the substrate to separate individual library elements. The spatial variation of materials deposited on the library was created using stationary and movable physical masks to control the thickness of specific evaporants in selected regions of the substrate. Because the constituents of each library element were deposited in layers, oxidative thermal processing at various temperatures was needed to mix the layers and create the desired chemical compounds. High-throughput screening for UV-excited photoluminescence was performed by photographing the visible emission of the library with a charge-coupled device (CCD) camera while exciting with a 254-nm broadband source. A quantitative measure of the chromaticity relative to calibration standards was calculated from three images obtained using, respectively, red, green, and blue tristimulus emission filters. To optimize the best host compositions of the Eu-doped red phosphors, Y1m Alm VO4 , identified in the initial high-density exploration library, a second library was deposited on a triangular grid, including La as a third host component. First, Eu2 O3 and V2 O5 were deposited uniformly over the entire substrate, followed by linearly varying the amount of Y2 O3 , Al2 O3 , and La2 O3 along three axes offset by a rotation of 120 , thus exploring all possible Y0:95mn Aln Lam VO4 :Eu 0:05 compounds. The maximum intensity with red chromaticity suitable for a commercial material corresponds to the phosphor composition Y0:82Al0:07 La 0:06 VO4 :Eu 0:05 . The activator concentration in this lead host was subsequently optimized with a third library in which the Eu 3þ concentration was varied between 0% and 20% using a single movable mask, probing the compositions Y0:87m Al0:07 La 0:06 VO4 :Eu m . Screening of this optimization library identified 2.5% Eu as the most efficient dopant, resulting in the final optimized composition Y0:845Al0:07 La 0:06 VO4 :Eu 0:025 with improved red chromaticity (x ¼ 0:67, y ¼ 0:32) compared with the more orange standard commercial red phosphor Y1:95 O3 :Eu 0:05 (x ¼ 0:64, y ¼ 0:35). Synthesis of bulk samples by conventional methods produced

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phosphor discovery library with thicknesses of the various materials deposited on a 7.62-cm silicon wafer shown. b) Photograph of the UVexcited photoluminescence of the discovery

Fig. 34.10. a) Deposition map of diverse

library containing approximately 25,000 different compositions. In this library, novel efficient red and blue phosphor compositions were identified and subsequently optimized in focus libraries.

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34.5 Case Studies

materials indistinguishable from the thin-film library samples and confirmed the performance of the newly identified phosphor composition. Xiang and coworkers prepared several 128-member libraries of aluminates of the formula Gd(La,Sr)AlOx doped with rare earth activators, and have evaluated the phosphorescence of all the samples simultaneously by photographing the entire library under broad wavelength UV light (254 nm) [99]. The resulting color photograph allowed a convenient qualitative evaluation of the library. The combinatorial libraries were used to survey a wide range of oxide compositions under a variety of processing conditions, facilitated by the simultaneous deposition of seven identical libraries on LaAlO3 , MgO, and Si single-crystalline substrates followed by different annealing and sintering conditions. Subsequently, Xiang and coworkers demonstrated that a scanning multi-inkjet delivery system (see above) can be used to achieve results identical to the vapor deposited libraries [64]. Multicomposition materials libraries of thin-film phosphors (Gd,La,Y)m (Ta,Zr,W,Mo,Zn)n (Al,Mg,Sr)k Ox :(Tm,Eu,Tb,Ce)y (with 0 a m; n; k a 1, 0:005 a y a 0:1) were generated by Sun and Xiang using sequential RF to create three identical libraries simultaneously on LaAlO3 [100]. Different annealing conditions and atmospheres completed the synthesis of the phosphor libraries, and photoluminescence images were acquired under 254-nm centered broad wavelength UV radiation of a Hg lamp using color photography. A new red phosphor with the composition (Gd1:54 Zn 0:46 )O3d :Eu 3þ 0:06 is claimed, and reported to have characteristics similar to the state-of-the-art orange–red phosphor Y2 O3 :Eu 3þ and also to possess excellent X-ray and cathodoluminesent properties. Schultz and coworkers used RF/DC sputtering to prepare three 1024-member libraries of silicate and gallate host materials on Si substrates, each library entity containing a number of different dopants [61]. After annealing, a parallel imaging system and a scanning spectrophotometer were used to identify and characterize compositions in the library. Few blue phosphor were identified by their emission and excitation spectra, but neither bulk nor thin film samples on LaAlO3 substrates exhibited blue luminescence. Subsequently, gradient libraries of Gd x Ga1x Oz :(SiO2 )y on LaAlO3 substrates were generated using pulsed laser deposition (PLD) and broadband blue emission was observed from these libraries.

34.5

Case Studies 34.5.1

Materials Discovery

The group from Symyx Technologies followed the red vanadate phosphor with a report on a novel luminescent oxide, Sr2 CeO4 , with a bluish white emission [82]. Whereas previous studies of luminescent materials had primarily focused on the synthesis of compositional variants within classes of materials with known struc-

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ture types, the discovery of this fundamentally new blue–white phosphor in an unexpected region of composition space exemplifies and validates the combinatorial methodology of materials science and discovery. The new luminescent phase Sr2 CeO4 was identified by automated high-throughput screening of the discovery library shown in Fig. 34.10, which suggested the possibility of a new phosphor with blue–white emission in the region of the library containing Sr, Sn, and Ce. A subsequent ternary focus library containing combinations of these three elements revealed that Sn was not needed for the observed emissive properties, and that maximum luminance was observed at Sr/Ce ratios greater than 1. A bulk sample with a 2:1 ratio of Sr/Ce was prepared from the reaction of CeO2 with SrCO3 in order to study the optical and other physical properties. The excitation and emission spectra of Sr2 CeO4 displayed broad maxima at 310 and 485 nm, with the emission appearing blue–white to the eye (x ¼ 0:198, y ¼ 0:292) having a quantum yield of 0.48. The phosphor is effectively excited by X-rays and exhibits efficient cathodoluminescence. The structure, determined by powder X-ray diffraction, is highly anistropic and reveals one-dimensional chains of edge-sharing distorted CeO6 octahedra that are linked together by Sr 2þ cations (Fig. 34.11) [101]. This unusual structure has not previously been found for rare earth-based oxide phosphors, and it is believed that the structure is critical in the observation of luminescence in Sr2 CeO4 . Strong evidence suggests the mechanism of luminescence to be based on ligand-to-metal charge transfer, from O 2 to Ce 4þ , and not from isolated valence transitions (daf ) from Ce 3þ defect centers as in all known cerium-based phosphors. To determine whether there were other luminescent phases within the M2 CeO4 (M ¼ Ba, Sr, Ca) composition space, a follow-up library was prepared with the compositions Ba2 CeO4 , Sr2 CeO4 , and Ca2 CeO4 at each corner of a triangular library. The library was prepared by automated liquid dispensing of sol-gel precursors and followed by heating to 900 C. Under 254-nm excitation the Sr-containing region was observed to have the brightest emission while no appreciable luminescence was observed from the pure Ba and Ca regions. Additionally, no evidence of solid– solution formation was seen by X-ray diffraction. This work shows that the combinatorial materials synthesis approach can identify fundamentally new and unexpected structures with properties that arise from unusual mechanisms. 34.5.2

Device Optimization

Thin-layer organic light-emitting devices (OLEDs) are an emerging technology that is beginning to be commercialized in flat panel displays. To develop this technology further, it is important to optimize different materials properties, layer thicknesses, deposition parameters, and layer compositions under similar and reproducible conditions. A combinatorial approach has been developed to create libraries of different OLEDs in order to screen and optimize device parameters with respect to power efficiency, photometric efficiency, and current–voltage characteristics [42–45, 102, 103]. Schmitz and coworkers used a combinatorial approach to optimize the Alq3 [8-hyroxyquinolinlato-Al(III)] layer thickness on top of a constant thickness hole

34.5 Case Studies

The crystal structure of the blue phosphor Sr2 CeO4 consists of linear chains of trans edge-sharing distorted CeO6 octahedra with four equatorial O atoms and two terminal CeaO bonds per octahedron, separated by interchain Sr 2þ cations. Fig. 34.11.

transport layer (HTL) to resolve whether an additional electron transport layer (ETL) can further enhance device performance [45]. Libraries of devices were prepared using a movable sledge developed to carry different types of masks or shutters in a vacuum deposition chamber. In addition, a substrate holder that can be rotated stepwise is positioned above the movable sledge. In this experiment (Fig. 34.12) a uniform layer of N,N 0 -bis-(4-dimethoxyphenyl)-N,N 0 -diphenylbenzidine (DMeOTPD) which is 40 nm in thickness is deposited on an unpatterned indium– tin oxide (ITO) glass substrate. Continuous movement of the shutter creates a gradient of Alq3 that varies from 0 to 130 nm. The substrate is then rotated 90 and a linear gradient of spiroquinoxaline ether (spiro-Qux) is deposited over two-thirds of the substrate. Finally, 400 nm of aluminum is deposited through a mask with 49 holes to create a cathode array. The current–voltage–luminance characteristics were measured with a computerized set-up consisting of a programmable power supply, multimeter, and luminometer. Results from this optimization experiment show that the additional spiro-Qux layer improves the photometric efficiency in certain

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The preparation of three-layer OLEDs with orthogonal linear gradients of Alq3 and a hole-blocking electron transport layer (ETL). Step 1, evaporation of HTL with constant thickness on an ITO substrate. Step 2, preparation of an Alq3 gradient by vapor deposition and simultaneous shutter movement. Step 3, rotation of substrate by 90 and Fig. 34.12.

evaporation of ETL by simultaneous shutter movement. Step 4, evaporation of aluminum through a mask with 49 holes. ITO, indium–tin oxide; HTL, N,N 0 -bis(4-dimethoxyphenyl)N,N 0 -diphenylbenzidine (DMeOTPD); tris(8hydroxyquinolinato)aluminum(III) (Alq3 ); spiroquinoxaline (spiro-Qux).

combinations of Alq3 /spiro-Qux layer thicknesses, which is attributed to better hole-blocking properties of spiro-Qux. The power efficiency, however, could not be improved by using an additional spiro-Qux layer because of the less efficient electron injection.

34.6 Organic Materials and Polymers

34.6

Organic Materials and Polymers

The application of combinatorial chemistry holds great promise in the discovery of novel functional organic materials. The current emphasis is on finding materials that either selectively bind small organic molecules or have improved physical and mechanical properties. 34.6.1

Schiff Bases for Nonlinear Optical (NLO) Materials

An investigation of the combinatorial synthesis procedures needed for the rapid formation of large libraries of polar compounds for the preparation of nonlinear optical (NLO) materials has been described by Nesterov and coworkers [104]. The structural characteristics of the Schiff base products (i.e. donor and acceptor groups connected to a p-conjugated chain) are believed to have potential as NLO or electrooptical materials. Nesterov and coworkers created a test library using four amines and two aldehydes as the building blocks for eight potential polar Schiff bases. Figure 34.13 illustrates the acid-catalyzed condensation of a set of electron-rich cinnamaldehydes with four electron-poor anilines intended to generate a small library of eight Schiff bases. The color of the product compared with that of the starting material was used as an indication that a chemical reaction had taken place. Crystalline precipitates were observed in every reaction, and crystals suitable for Xray diffraction were obtained from all but one of the reactions, where the crystals were too small. Their results demonstrate that a targeted library of Schiff base

Fig. 34.13. Structures of various aniline and cinnamaldehyde

building blocks and their corresponding Schiff base library as new materials for nonlinear optics.

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compounds can be synthesized and screened. Five crystal structures were determined, all with centrosymmetric space groups that preclude the compounds from being NLO candidates. 34.6.2

Artificial Receptors for Small Organic Molecules

Since the rational design of nanoporous materials and their inclusion cavities based on molecular structures is difficult, combinatorial methods were implemented to find superior lattice inclusion compounds [105]. When screened against a guest molecule (iPrOH), seventeen new ammonium carboxylate host compounds were discovered from a combinatorial library of 100 salts formed by mixing ten commercially available amines and ten carboxylic acids. One host compound was identified that forms inclusion compounds with 25 different organic solvents. Application to chiral recognition by the proper choice of amines and carboxylic acids to form a variety of host compounds is under investigation. Leblanc and coworkers applied combinatorial principles to surface chemistry by synthesizing and screening libraries of amphiphilic lipid molecules [106]. At the air–water interface, the hydrophilic polar moiety of these molecules is embedded into the water phase and the hydrophobic alkyl chain is oriented toward the air phase. When compressed at the interface, the amphiphilic molecule will move and assemble into organized supramolecular structures, known as Langmuir monolayers. The polar moiety was functionalized with a peptide library such that the selfassembly of these functional lipids formed protein-like supramolecular structures (Fig. 34.14). One library with 250 members and three sublibraries of 54 members each were assayed for molecular recognition of maltose as well as other sugar molecules. One sublibrary, containing both polar charged (Glu) and aromatic (Tyr) amino acid residues, was identified that selectively bound maltose but not d-glucose and sucrose. This selectivity can be explained by the three-dimensional combination of those two amino acids in the binding site with Tyr playing a more significant role for the binding activity. 34.6.3

New Materials for the Separation of Enantiomers

Given the enormous demand of enantiomerically pure compounds in the pharmaceutical field, novel chiral stationary phases (CSPs) for high-performance liquid

Fig. 34.14.

The structure of a combinatorial peptide lipid library (AA i ¼ amino acid).


chromatography (HPLC) were developed by combinatorial means [107]. Frećhet and coworkers prepared a model library of chiral stationary phases (selectors) 1 for HPLC by attaching a small model library of 36 chiral compounds that were obtained from three l-amino acids and 12 aromatic amines to synthetic polymer beads (Scheme 34.1) [108]. After the entire on-bead library 1 was packed into a HPLC column, selectivity was assayed by injecting various racemates of chiral targets. Despite the presence of 36 mixed selectors, racemic d,l-(3,5-dinitrobenzoyl)leucine diallylamide was separated. A deconvolution process was used to determine which of the 36 selectors in the HPLC column was the most powerful. In contrast to the ‘‘one column, one selector’’ approach, which would require the preparation and testing of 36 chiral stationary phases, this combinatorial scheme requires the preparation of only 11 columns for the discovery of the most selective material. Applying the same approach, a library of 240 selectors could be deconvoluted using only 17 columns.

Scheme 34.1. Preparation of a model library of chiral stationary

phases (selectors) 1 for HPLC by coupling a small model library of 36 chiral compounds to synthetic polymer beads.

The same group also applied the principle of reciprocity to the identification of new CSPs [109]. This approach is based on the following assumption: if a ligand of the stationary phase is selective toward an analyte, then the same analyte immobilized onto the proper support should result in a stationary phase that is very likely to be selective toward the original ligand [110]. Following this principle, the chiral substrate (S)-(3,5-dinitrobenzoyl)leucine acted as an ‘‘inverse’’ selector and was immobilized on synthetic polymer beads to give CSP 2 (Fig. 34.15). Three-component

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Fig. 34.15. ‘‘Inverse’’ selector CSP 2, library of racemic 4-aryl1,4-dihydropyrimidines (DHPM) 3, and the most efficient CSP 4.

Biginelli condensation was utilized to prepare a parallel library of 108 racemic 4aryl-1,4-dihydropyrimidines (DHPM) (3) which were then screened against the ‘‘inverse’’ selector CSP 2 by observing the enantioselectivity for resolution. The best candidate was then attached to monodisperse macroporous beads to prepare the new polymer-based chiral stationary phase 4, which proved efficient in the resolution of several analytes. Li and coworkers also utilized the principle of reciprocity to identify novel chiral selectors [111]. In order to study the chiral resolution of racemic (1-naphthyl)leucine ester 5, the l-enantiomer of 5 was immobilized onto silica gel (Scheme 34.2). A small (4 4) peptide library was synthesized in its d- and l-form, respectively, and the chromatograms of these two libraries on stationary phase 6 were compared. The visible difference between the two chromatograms indicates that immobilized 5 interacts differently with the enantiomers of at least one library component. Deconvolution methods were used to identify the most potent selector. The same group also turned the principle around in preparing two stationary phases (l-6, S-6), one immobilized with the R enantiomer of 5 and the other immobilized with the S enantiomer of 5 [112]. Potential chiral selectors were identified again by subsequent screening of the racemic (4 4) peptide library against the two stationary phases and comparison of the chromatograms. A different approach is to synthesize parallel combinatorial CSP libraries with up to 200 members and screen them for their ability to resolve a racemate of the analyte [113]. These libraries of potential selectors typically consist of immobilized dipeptides and a capping group. Racemic analyte is then added to each member of


Scheme 34.2. Immobilized analyte 5 utilized in the reciprocity approach.

the library and allowed to equilibrate. The enantiomeric ratio of the analyte in the supernatant is analyzed after the equilibration period using circular dichroism measurements or other methods. A selective absorption of one of the two enantiomers to the resin is indicative of a chiral selector. Welch and coworkers introduced an improved method for rapid liquid chromatography/mass spectrometry (LC/MS) screening of chiral stationary phases based on the use of isotopically labeled enantiomers [114]. ‘‘Pseudoracemates’’ made up of a pair of isotopically differentiated pseudoenantiomers were prepared and screened against different CSPs. The enantioenrichment of the supernatants can be rapidly estimated by comparing the MS abundance of the ions corresponding to each of the pseudoenantiomers. A visual screen for identifying chiral selectors was introduced by Still and coworkers [115]. The idea was to treat an equimolar mixture of a blue l-amino acid L-7 and a red d-amino acid D-7 with a library of chiral selectors on synthesis beads in which each bead carries a different selector (Fig. 34.16). Highly enantioselective binding results in red or blue beads, whereas unspecific binding yields brown

Fig. 34.16. Enantiomeric probe molecules 7 labeled with different dyes.

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beads. The reddest and the bluest beads found in the assay were picked and decoded to determine the structure of the bound chiral selector. 34.6.4

Molecular Imprinting

Molecularly imprinted polymers (MIPs) are another class of artificial receptors for small organic molecules. Their binding performances give these materials great potential in combinatorial approaches as recognition matrices for the screening and rapid selection of ligands from a combinatorial library [116]. Molecular imprinting is a technology where recognition sites can be generated within a macromolecular matrix by use of a molecular template in a casting procedure [117]. The selected ligand (print molecule) is first allowed to interact freely in solution via bond formation with one or more functional monomers. The resulting adducts are subsequently copolymerized with a large excess of crosslinker to give a rigid, insoluble polymer. Following extraction of the print molecule, specific recognition sites are left in the polymer. The technique has been applied to the screening of a combinatorial steroid library [116a], and a combinatorial library of molecularly imprinted polymers has been tested in chiral separations [116c]. Combinatorial methods proved to be advantageous since the preparation and evaluation of MIPs is usually very tedious and time-consuming. In 1999, Takeuchi and coworkers reported a rapid combinatorial approach to synthesize and evaluate artificial receptors [118]. The preparation of the MIPs was automatically performed using programmable liquid-handling equipment with a new in situ molecular imprinting protocol. The freshly synthesized MIP library (polymer films) was then subjected to a primary screening to estimate the affinity of the template to the resultant polymer. A secondary screening on purified MIPs then allows for a more precise evaluation of the affinities and selectivities of the new materials. MIPs that are artificial receptors for triazine herbicides ametryn 8 and atrazine 9 were prepared by the combinatorial molecular imprinting approach using variable amounts of two functional monomers, methacrylic acid (MAA) 10 and 2-(trifluoromethyl)acrylic acid (TFMAA) 11, as well as ethylene glycol dimethacrylate as a crosslinker and 2,2 0 -Azo-bis-isobutyryl-nitrile (AIBN) as the polymerization initiator (Fig. 34.17). On examination of the 49-member MIP library, it appears that, depending on the functional monomer used, the imprinting efficiency is different for each triazine herbicide. This result suggests that highthroughput synthesis and screening of large sets of polymers will provide the most efficient molecularly imprinted receptors for given target molecules. 34.6.5

Polymers with Novel Topologies and Functionalization

Combinatorial approaches have also been applied to polymer chemistry. For example, a small library of polyarylates (0.2 g per sample) (14) was prepared from 14 distinct tyrosine-derived diphenols (12) and eight aliphatic diacids (13), resulting in


Fig. 34.17. Structures of the template molecules ametryn 8

and atrazine 9 as well as the functional monomers MAA 10 and TFMAA 11.

112 structurally related strictly alternating copolymers (Scheme 34.3) [119]. These copolymers are biodegradable and potentially useful as medical implant materials. The structural variations provided incremental differences in polymer free volume, bulkiness, flexibility, and hydrophobicity. Up to 32 polymers were synthesized in parallel under identical reaction conditions and conventional methods were used to measure the molecular weight by GPC (gel permeation chromatography), Tg by DSC (differential scanning calorimetry), and the air–water contact angle (sessile drop) on a sample-by-sample basis, providing structure–property correlations. Diverse and focused combinatorial libraries of synthetic biodegradable polymers could also be designed by using molecular topology and genetic algorithm optimized quantitative structure–property relationships (QSPR) [120].

Scheme 34.3. Library of polyarylates 14 derived from diphenols 12 and diacids 13.

In a different approach, polymerization reactions between different combinations of two starting materials have been found to lead to many new, uniquely responding sensors with responses not simply related to the proportions of the starting materials [121]. Discrete polymer-sensing cone arrays or gradient sensors show

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diverse shifts in fluorescence emission wavelength in response to organic vapor pulses. Combinatorial polymer synthesis from a limited number of starting materials thus presents an attractive approach to generate large sets of unique and diverse polymer sensors for analyte detection rapidly. Radical polymerizations have been the focus of a variety of combinatorial approaches due mainly to the fact that a greater number of monomers are amenable to radical polymerizations than either anionic or cationic methods, and therefore a higher diversity of polymers may be obtained through free radical polymerizations [122]. Additional structural variation can be achieved through the synthesis of polymers based on either one monomer (homopolymer) or multiple monomers (copolymers). Block and graft polymerizations offer the possibility of a controlled alteration of physical properties by varying polymer block and graft length and composition [123]. Janda and coworkers applied a sequence of normal and ‘‘living’’ free radical polymerizations to generate a library of 20 block copolymers possessing either block or graft architecture with initiators 15 and 16 and a diverse set of vinyl monomers 17–21 (Fig. 34.18) [124]. The principle of the combinatorial approach is outlined in Scheme 34.4. Diazene and TEMPO (2,2,6,6-tetramethilpiperidin-1-yloxy) moieties are known to initiate/mediate free radical polymerizations at 70 C and 130 C, respectively. Therefore, bifunctional free radical ini-

Fig. 34.18. Free radical initiators 15, 16 and vinylic monomers

17–21 for the construction of a library of block polymers.


tiators of the type 15 were prepared that provide for two independent rounds of polymerization. Block copolymers 22 could be obtained in a temperature-controlled manner through sequential normal and ‘‘living’’ polymerizations. To add another dimension to the combinatorial diversity, initiator 16 participates as a monomer in the first polymerization with monomer A, resulting in statistic copolymers of type 23. The TEMPO-functionalized residues then mediate the ‘‘living’’ polymerization at 130 C, giving access to combinatorial polymers of type 24. The members of the block copolymer library were then screened for their ability to serve as a soluble polymer support for organic reactions. One block copolymer could be identified that has a solubility profile that is complementary to the current soluble polymer of choice in liquid-phase organic synthesis (LPOS) – polyethylene glycol (PEG). R1 and the nitrile groups in block copolymers of type 22 could be used to functionalize these molecules.

Scheme 34.4. Parallel polymer synthesis via sequential normal/living free radical process.

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Hawker and coworkers used combinatorial means to identify novel nitroxidebased mediators for ‘‘living’’ free radical polymerizations [125]. Nitroxide-mediated processes are potentially simpler systems then atom transfer radical processes (ATRP) since no metal complex needs to be added; thus, more functional groups are tolerated and purification is simplified. However, nitroxide-mediated processes are limited by their incompatibility with numerous vinyl monomer families. Since the structure of the nitroxide moiety plays an important role in the success of ‘‘living’’ free radical polymerizations, a structurally diverse, a-hydrogen-bearing nitroxide library was prepared and screened for the ability to control the polymerization of a variety of functionalized monomers (Scheme 34.5). The a-hydrido alkoxyamine 25 was identified as the most promising candidate to effect the ‘‘living’’ free radical polymerizations of a wide variety of monomers, e.g. styrenes, acrylates, acrylamides, and acrylonitrile-based monomers.

Scheme 34.5. Synthesis of a diverse alkoxyamine mediator library for living free radical polymerizations.

Very recently, Frećhet, Hawker, and researchers at Symyx Technologies presented a new technology to synthesize multiarm star polymers using nitroxidemediated ‘‘living’’ free radical polymerizations, giving access to well-defined macromolecules with three-dimensional architecture [125a]. Combinatorial techniques were used to rapidly evaluate the scope and limitations of this general methodology.


Safir, Petro, and colleagues at Symyx Technologies have focused on combining rapid synthesis and rapid characterization techniques with full automation to reap the maximum benefit of combinatorial chemistry in the area of polymer science [126]. To demonstrate this integration of fully automated synthesis and characterization, living free radical polymerizations, which allow for molecular weight control, were studied [127]. In particular, for atom transfer radical polymerizations (ATRP), which have been shown to polymerize a variety of monomers successfully, the effects of different initiators (organohalides) and catalyst/control agents (low oxidation state organometallic reagents) were studied. A 45-member library consisting of the combinations of five CuI initiators and nine organohalide control agents was screened for controlled styrene polymerization using 4,4 0 -di-(5-nonyl)2,2 0 -bipyridine (dNbpy) as the ligand [128]. To produce molecular weight diversity in an 80-member styrene ATRP library, the monomer-to-initiator ratio (M/I), the rate of propagation, the initiators, the solvents, and the concentrations were varied. Finally, in a 55-member library, structural and molecular weight diversity was introduced by using one initiator with five M/I ratios (100 to 500 in steps of 100) in the copolymerization of styrene and butylacrylate, creating a gradient in the polymer composition from 0% to 100% butylacrylate with 10% increments. Utilizing the standard microtiter plate format equipped with a Teflon-faced sealing mechanism, a 96-well, glass-lined, aluminum reactor was developed to conduct organic reactions above the boiling point of the reagents without loss of pressure. Two methods of rapid molecular weight determination, i.e. precipitation– redissolution chromatography (PRC) and rapid-fire light scattering (RFLS), were used to screen the resulting polystyrene samples [129]. Combinatorial approaches were also employed to produce p-conjugated polymers with novel electrical and/or photofunctional properties. The photoluminescence of p-conjugated polymer thin films makes them interesting materials for flat panel displays. Since film preparation parameters are numerous, combinatorial concepts have been applied for the synthesis of p-conjugated polymer thin films [130]. A variety of p-conjugated polymer building blocks was used for this study (Fig. 34.19). In addition, the diversity of the building blocks was increased by combinatorial copolymerization of PP (para-phenylene) and MP (meta-phenylene). This goal was achieved by copolymerizing different ratios of the two monomers in the presence of magnesium and nickel complexes to form poly-(PP-co-MP). Target disks of p-conjugated polymer building blocks were then evaporated by either vacuum evaporation (VE) or pulsed laser deposition (PLD) and the film was deposited on silica and/or Si substrates that were kept at room temperature. Combinatorial film preparation was achieved using a movable mask. Various mixed component and multilayer films can be fabricated by employing appropriate mask shapes and the substrate rotation synchronized with the target exchange. A variety of p-conjugated polymer films with interesting photoluminescent properties could be discovered by this method. Especially copolymers of PP and MP, poly-(PP-co-MP), showed a very high photoluminescence. The same group also identified novel p-conjugated polymers with high-quality pan junctions from a different thin-film library [131].

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Fig. 34.19. p-Conjugated polymers utilized for the combinatorial fabrication of thin films: PPP, poly-(para-phenylene); PMP, poly-(metaphenylene); Ppy, poly-(pyridine-2,5-diyl), PTh,

poly-(thiophene-2,5-diyl); PPh2BPV, poly(diphenylene biphenylene vinylene); Poly(PPco-PM), poly-(para-phenylene-co-metaphenylene).

34.7

Summary and Outlook

We find ourselves at the dawn of a new age of materials discovery and optimization. As this review demonstrates, significant first steps in that direction have been taken in various areas of materials science, and a multitude of tools are now available using combinatorial technologies to accommodate the new tasks and requirements for combinatorially accelerated materials research. A common underlying theme associated with these technologies is miniaturization, parallelization, and automation so that large numbers of samples can be synthesized and screened efficiently. Rapid serial and parallel adaptations of conventional analytical techniques will become increasingly important in the characterization of materials properties, as will the development and implementation of new and unconventional highthroughput screening tools. Software development and engineering support in the construction and design of synthesis and screening tools are as crucial as further advances in chemistry, even when appropriate tools or robots for synthesis and screening automation are commercially available. Finally, the combinatorial methodology generates data much faster than conventional research employing ‘‘empirical’’ and ‘‘rational’’ approaches to materials discovery, and, inevitably, data handling and storage advances must accompany the high-throughput synthesis and screening to maintain the integrity of research and development efforts. Full realization of the combinatorial methodology will require the integration of chemistry, physics, engineering, and informatics, enhancing the probability of finding materials with desired properties. In the future, the scientist’s efforts may be shifted, at least to some degree, toward programming and experimental design in addition to the already existing analysis of synthesis and data analysis.

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Reprogramming Combinatorial Biology for Combinatorial Chemistry* Sean V. Taylor 35.1

Introduction

As the other chapters in this book have nicely illustrated, combinatorial chemistry has made dramatic technical advances since its inception, with significant improvements in the design, synthesis, purification, and evaluation of combinatorial libraries. It is now commonplace for companies focusing on drug discovery to have 0.5–1.5 million compounds arrayed throughout their combinatorial libraries, which can be screened in a matter of weeks or months for activity against any number of biological targets. The potential success of this strategy is underscored by reports from numerous pharmaceutical companies of compounds that were identified or optimized using combinatorial chemistry and that have now entered clinical drug trials. It is quite likely that, over the next 100 years, a significant percentage of pharmaceuticals will be derived in some manner from combinatorial libraries. If we expand our definition of combinatorial libraries, however, the current importance of the combinatorial approach to drug discovery, as well as to most life processes, is more immediately clear. For instance, eight of the 20 top-selling pharmaceuticals are natural products or derived from natural products [1–3]. It is not a large leap in logic to consider these compounds as already being derived from combinatorial libraries – biological, or perhaps ‘‘evolutionary’’ combinatorial libraries. In contrast to combinatorial libraries produced by synthetic means, the combinatorial libraries that give rise to natural products are not produced at the ‘‘compound’’ level, but at the genetic level during the process of evolution, through mutation, gene duplication, and interspecies genetic transfer. The evaluation of these biological combinatorial libraries is simply the process of natural selection. Those compounds that provide some value toward the survival of the producing organism will be propagated and further improved upon through additional evolutionary rounds. *Portions of this article were previously published (Angew. Chem. 113 (2001) 3408–3436; Angew. Chem. Int. Ed. 40 (2001) 3310–3335). [Ref. 100] Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Vol. 2. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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Nature has utilized such ‘‘evolutionary’’ libraries over billions of years, but molecular biologists, biochemists, and chemists have only recently learned how to harness this combinatorial power, and for a number of different uses. In this chapter, the various efforts to engineer and apply biological combinatorial libraries for use in enzymology, drug discovery, bioprocessing, and also for addressing fundamental issues in chemistry and biology will be explored. Instead of an exhaustive review, specific examples will be used to highlight various combinatorial aspects of biology. In addition to asking the question ‘‘Where are we now?’’, I also hope to ask ‘‘Where are we (or where could we be) going?’’

35.2

Combinatorial Biosynthesis: Creation of Novel Small-molecule Natural Products

Many secondary metabolites are composed of polyketides, nonribosomally produced peptides, carbohydrates, or some combination of all three (Fig. 35.1) [4–6]. Polyketides are complex and highly stereogenic compounds that are constructed from acyl-coenzyme A building blocks, often by huge modular enzyme complexes called polyketide synthases (PKSs) [7]. Nonribosomal peptides (NRPs), as the name implies, are amino acid-derived molecules wherein the peptide bonds are not produced by the traditional translational machinery of the ribosome, but instead on large modular enzyme complexes called nonribosomal peptide synthases (NRPSs) [8]. Carbohydrate moieties, very commonly deoxysugars in secondary metabolites, are produced by more ‘‘traditional’’ nonmodular biosynthetic pathways [9–11]. Illustrations of the typical biosynthetic pathways are shown in Fig. 35.2. The relaxed specificity demonstrated by some of the enzymes involved in producing these compounds, as well as the modular nature of some of the systems, have allowed researchers to produce new ‘‘natural’’ products by cleverly reengineering the biosynthetic pathways. Such engineering is usually called combinatorial biosynthesis, and is often achieved by substituting (in combinatorial fashion) different non-natural building blocks and the enzymes that can utilize them into a polyketide, nonribosomal peptide, or deoxysugar biosynthetic pathway [12–18]. The reprogramming of biosynthetic pathways has the potential to be enormously useful for combinatorial chemistry, since it creates a method for the parallel synthesis of a large number of complex molecules whose conventional organic syntheses are generally too complex to be easily adapted for combinatorial production. Another advantage is that these combinatorial biosynthetic libraries are often closely related to natural products that have already shown some useful activity, so the chances of harvesting pharmaceutical lead compounds from them may be higher than those from a synthetic combinatorial library. Finally, combinatorial biosynthesis could provide a convenient method for creating libraries that could be further manipulated by synthesis, reducing some of the time and cost associated with synthetic combinatorial chemistry. For all of these reasons, many corporate and academic research groups are working to manipulate biosynthetic pathways for use in combinatorial biosynthesis [19].

35.2 Combinatorial Biosynthesis: Creation of Novel Small-molecule Natural Products

Fig. 35.1. Representative examples of polyketide-derived, nonribosomal peptide-derived, and deoxysugar-derived natural products.

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Fig. 35.2. Biosynthetic pathways. (A) The

modular erythromycin polyketide synthase produces 6-deoxyerythronolide B, which is then converted to erythromycin A by downstream nonmodular processing enzymes. The modular polyketide synthase has a loading module (L) and six chain elongation modules (1–6) expressed from three genes DEBS1–3. AT, acyl transferase domain; CP, acyl carrier protein; KS, ketosynthase domain; KR, ketoreductase domain; DH, dehydratase; ER, enoyl reductase; TE, thioesterase (cyclization) domain. (B) The modular peptide synthase-producing surfactin

A. Seven amino acid incorporation modules (1–7), coded by three genes srfA-A, srfA-B, and srfA-C, are shown with the growing chain. The b-hydroxy fatty acid side-chain is incorporated at the beginning of the biosynthesis by an unknown mechanism. P, incorporation of bhydroxy fatty acid (not part of module); A, amino acid adenylation domain; T, thiolation domain; C, condensation; E, epimerization domain; TE, thioesterase (cyclization) domain. (C) Biosynthesis pathway for d-desosamine, the deoxysugar of the secondary metabolites methamycin/neomethamycin.

Salient examples of the combinatorial biosynthetic engineering of polyketides, nonribosomal peptides, and carbohydrates are illustrated below. 35.2.1

Polyketide Combinatorial Biosynthesis

Polyketide biosynthesis has been the focus of intense study for several years. Several excellent reviews summarizing the current state of knowledge in this area have been published elsewhere [20–23]. The enzyme systems responsible for polyketide biosynthesis can be roughly assigned to four categories: the type I modular, type I iterative, type II, and higher plant PKSs, each with different characteristics. Type I modular PKSs are bacterial in origin and produce mostly macrolide-type natural products, such as erythromycin, and have a separate catalytic module for each chain elongation step (including modifications along the way, such as reductions). Type I iterative PKSs, generally found in fungi, have only one ‘‘module’’ which iteratively processes the polyketide chain through numerous steps [24]. In


Fig. 35.2. (continued)

these PKSs, some activities can be turned on and off at different points in the biosynthesis [25], and this process probably occurs through the intercession of ‘‘helper’’ proteins outside the module [26]. The complexity of the type I iterative systems has to this point precluded attempts to reengineer them for combinatorial biosynthesis. Type II PKSs are multisubunit enzymes that usually make aromatic polyketides, such as actinorhodin (see Fig. 35.3). Each subunit is responsible for catalyzing a specific type of reaction. Finally, the higher plant PKSs utilize paracoumaroyl-CoA as a starter unit and produce complex hydroxylated aromatics. Interestingly, they utilize a single enzyme active site for the chain elongation, decarboxylation, cyclization, and aromatization steps along the biosynthetic pathway,

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Fig. 35.3. Actinorhodin biosynthesis pathway. Reactions not occurring via the act minimal PKS are indicated with a reaction arrow. KR, ketoreductase; ARO, aromatization; CYC, cyclization.

an impressive feat of enzymatic control seemingly dictated by only a few highly conserved residues acting in concert [27, 28]. The modularity of type I PKSs perhaps makes them the most attractive targets for combinatorial biosynthesis efforts, and the majority of effort in combinatorial reengineering has been spent in mixing and matching modules through genetic engineering. However, a significant effort has also been placed on manipulating the type II PKSs to produce new natural products. A few examples are noted below. 35.2.1.1 Combinatorial Biosynthesis from Type II PKSs

The initial reports of the combinatorial biosynthesis of novel polyketides from type II PKSs were from Hopwood, Khosla, and coworkers [29–33], and were at least partially driven by efforts to unravel the mechanistic enzymology of the type II systems. These efforts were rewarded by the identification of the ‘‘minimal PKS,’’ which consists only of a ketosynthase, a chain length factor, and an acyl carrier protein, which is sufficient to produce a polyketide chain of specific size poised for cyclization at a specific position [29, 32, 34]. Every type II PKS system could be considered to be composed of a ‘‘minimal PKS,’’ along with other enzymes responsible for other functionalizations on the polyketide chain, such as additional cyclizations, aromatizations, or glycosylations (Fig. 35.3). Combinatorial biosynthetic libraries can thus be created by producing hybrid type II PKSs containing a minimal PKS with different modifying (or nonmodifying) subunits from various natural type II PKSs. The engineered actinorhodin PKS systems produced several novel


Fig. 35.4. Novel aromatic polyketides produced from

engineered variants of the actinorhodin PKS. Some of the products are derived from an actinorhodin/tetracenomycin (tem) or a frenolamycin/actinorhodin ( fre) hybrid.

aromatic polyketides, some of which are shown in Fig. 35.4. Subsequent reports of combinatorial biosynthesis using type II PKSs, while generally successful, showed that the hybrid enzyme systems did not always perform as expected [35, 36], which has reduced the ability to design combinatorial biosynthesis libraries from type II PKSs with predictable structures. Additionally, the assignments of the roles of different parts of the ‘‘minimal PKS’’ in specifying chain length and point of initial cyclization have been shown to be incorrect, at least for some type II PKSs [37–39]. Despite this limitation, or perhaps because of it, several previously unknown aromatic polyketides have been created, sometimes with quite intricate and unexpected structures (Fig. 35.5). Work on combinatorial biosynthesis of aromatic polyketides

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Fig. 35.5. Unpredicted products from the whiE minimal PKS.

from type II PKSs continues, but has perhaps been overshadowed by developments in engineering type I modular PKSs for production of combinatorial biosynthesis libraries [14, 18, 20, 40, 41]. 35.2.1.2 Combinatorial Biosynthesis from Type I Modular PKSs

To date, the best studied and most successful type I modular PKS combinatorial biosynthesis system is based around the erythromycin polyketide synthase complex (DEBS) [15, 20, 42]. DEBS is a three-unit PKS (DEBS1, DEBS2, and DEBS3) containing a loading module and six processing modules that ultimately produce


Fig. 35.6. Incorporation by feeding of alternative diketide precursors into the erythromycin pathway results in novel variants of erythromycin A.

6-deoxyerythronylide B (see Fig. 35.2A). This aglycon unit is processed by downstream enzymes that oxidize the backbone and append deoxysugars to give erythromycin A. To explore the potential of combinatorial biosynthesis with type I modular PKSs, the DEBS system has been manipulated in several ways to produce combinatorial biosynthetic libraries. At the entry of the pathway, new biosynthetic products can be made either by utilizing alternate diketides (Fig. 35.6), which has also been possible with type II PKSs [43], or by changing the loading module [44]. Propionyl CoA is the natural starter unit, but replacing the DEBS loading module with the avermectin PKS loading module, which demonstrates broader specificity, results in the production of new polyketides (Fig. 35.7) [44]. In the pathway, combinatorial libraries have been produced by (1) altering the extension of the chain by replacing the native acyl transferase units in modules with non-native acyl transferase domains [45], (2) altering the state of reduction of the ketone at each chain position through changes in the reduction domains in each module [46], (3) altering the stereochemistry at tertiary and quaternary carbons through alteration in the appropriate domains [47, 48], or (4) feeding substrates that can be utilized by downstream modules [49, 50]. Additionally, diverse products can be made by lengthening or shortening the total length of the chain through adding or removing modules [51– 53]. Most DEBS-derived libraries to date, while presenting exciting advances in combinatorial biosynthesis, have been small by combinatorial library standards [54], since genetic manipulation of the very large type I modular PKS systems can be cumbersome. A clever approach that should allow for easier manipulation and larger combinatorial libraries is to utilize multiple PKS gene-containing plasmids

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Fig. 35.7. Replacing the wild-type ery loading module with the avermectin loading module, which displays reduced specificity, results in iso-propyl- and sec-butyl-containing erythromycin variants.

(Fig. 35.8) [55–58]. For two or more plasmids (circular segments of DNA) to coexist in a single cell, they must be compatible (containing compatible origins of replication). In the multiplasmid PKS approach, compatible plasmids are introduced into an appropriate expression host, i.e. one that produces no polyketides, or has had the polyketide-producing pathways it normally contains completely eliminated. These plasmids contain different sets of genes coding for different PKS modules, and each plasmid may harbor PKS genes that have mutations providing further permutations. By introducing different combinations of plasmids through cotransformation, each host cell has the capability to produce a different polyketide. Such an approach has led to the simplified production of a library of novel polyketides based around the DEBS architecture (Fig. 35.9) [55]. As the collection of suitable PKS module-containing plasmids multiplies, the ability to produce more novel


Fig. 35.8. The multiplasmid approach to combinatorial biosynthesis. For each module a separate plasmid is used. Each plasmid also contains a different antibiotic resistance gene. Modifications are made to each module, then

possible combinations are transformed into a suitable host organism. Growth on all three antibiotics ensures that each host cell contains all three types of modules.

polyketides increases exponentially. Additionally, because the rational or predictive design of polyketides is possible due to the modular design of these type I PKSs, it should eventually be possible to produce almost any polyketide backbone with intelligently placed functionalities by transforming the right combination of plasmids. The production and manipulation should also be aided by expression of these systems in more easily handled organisms instead of the natural hosts [59]. Some progress toward this goal has been recently reported, with the high-level production of 6-deoxyerythronylide B in an engineered strain of Escherichia coli [60]. Finally, further understanding of how PKS modules interact and communicate throughout a polyketide construction will be very important to the proper mixing and matching of modules in combinatorial biosynthesis efforts [61–63].

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Fig. 35.9. A selection of different 6-deoxyerythronolide B variants that were produced using the multiplasmid approach with the erythromycin modules.

35.2.2

Combinatorial Biosynthesis of Nonribosomal Peptide Products

Nonribosomal peptides are produced over a wide range of species and probably function in their native organisms mainly as signaling, siderophoric, and defensive molecules [4]. In contrast to the PKSs, all known nonribosomal peptide synthases (NRPSs) appear to utilize a modular structure. Several excellent reviews summarize the state of knowledge concerning NRP biosynthesis from genetic, enzymologic, and structural viewpoints [22, 64–67]. NRP-containing products have a number of useful medical and biological functions, including biocidal, antibiotic, anticholesterolic, and immunosuppressive [8]. As with polyketide natural products, the ability to produce new NRPs would thus be a boon to drug discovery. While the


field of nonribosomal peptide combinatorial biosynthesis is still emerging, some significant progress has been made. The initial work on the reengineering of NRPSs for combinatorial biosynthesis was with the synthase that naturally produces surfactin A, a heptapeptide product from Bacillus subtilis [68, 69]. Three large gene products, srfA-A, srfA-B, and srfA-C, encode the seven modules involved in the biosynthesis after the incorporation of a fatty acid side-chain (Fig. 35.2B) [70]. Through genetic recombination, the second leucine incorporation module in srfA-A was replaced with modules from Bacillus brevis or from Penicillium chrysogenum that incorporate phenylalanine or ornithine, resulting in new surfactin analogs (Fig. 35.10A) [71, 72]. Truncated peptides were also created by moving the terminal thioesterase domain immediately downstream of the srfA-A gene (Fig. 35.10B) [73]. While these experiments are promising, a considerable amount of work remains to show whether true combinatorial libraries can be created by mixing and matching NRP modules. It also remains to be seen whether NRPs will be amenable to the multiplasmid approach (discussed above), which holds much promise for the engineering of PKS combinatorial biosynthesis. This approach depends on whether NRP modules that are expressed apart can properly reconstitute entire NRPs. Alternatively, direct recombinant engineering of NRPSs may be more feasible than for PKSs, provided workable fusion sites can be identified for mixing and matching NRP modules. Dimodular and trimodular hybrid NRPSs have been successfully created thus far through this approach [74, 75]. Another important issue in developing NRP combinatorial biosynthesis is the recognition of amino acids by adenylation domains. The potential to incorporate different amino acids and other starter units into NRPs exists, but we must first determine (and then subvert for our own uses) the strategy by which adenylation domains recognize the correct amino acid. Progress toward this goal has been made [76–78].

35.2.3

Combinatorial Biosynthesis from Hybrid PKS/NRPS Systems

A number of natural products are produced through hybrid PKS/NRPS biosynthetic pathways. A typical example is epothilone, the biosynthetic gene organization of which is shown in Fig. 35.11 [79]. These mixed systems are of course also suitable targets for combinatorial biosynthesis [80]. While no examples of such an approach yet exist, it is easy to imagine how novel epothilone analogs could be generated. For instance, the NRPS module could be replaced with another NRPS module, which would in effect provide alternate starter units for the PKS (Fig. 35.11). Alternatively, the PKS could be altered to create thiazole-containing combinatorial biosynthesis libraries. Much work remains to be done in elucidating the organization and structure of hybrid PKSs/NRPSs, however, before combinatorial biosynthesis with these systems can be undertaken. Some initial progress has been made in characterizing PKS/NRPS pathways [81].

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Fig. 35.10. (A) Surfactin A, which is produced

by the seven modules. Two new analogs were produced by replacing the d-Leu module with phenylalanine and ornithine incorporation modules. (B) Placing the thioesterase

domain immediately downstream of the srfA-A gene (which encodes the first three amino acid incorporation modules) results in the production of the expected truncated peptide.


Fig. 35.11. Biosynthesis of epothilone, a PKS/NRPS hybrid.

35.2.4

Combinatorial Biosynthesis of Carbohydrates

Deoxysugars are essential appendages for many secondary metabolites. Their biosynthesis has been reviewed extensively elsewhere [9–11, 82, 83]. In natural products they often act as guidance systems, determining the specificity and biological activity toward a target [84, 85]. The ability to create diverse deoxysugars and incorporate them into larger structures provides an opportunity to devise natural product analogs with altered targets, specificity, activity, and physical properties. As indicated in Fig. 35.2C, three types of enzymes are necessary for the incorporation of deoxysugar moieties into larger natural products: (1) nucleotidyltransferases are responsible for adding nucleotide diphosphate appendages to carbohydrates; (2) modifying enzymes, which, for example, alter the oxidation state of the molecule or replace hydroxyls with amines; and (3) glycosyltransferases, which couple the fully derivatized sugar with its intended aglycon partners. To produce novel deoxysugars, one can alter or relax the specificity of enzymes through the introduction of mutations, replace a ‘‘natural’’ enzyme in one pathway with those from other pathways, or take advantage of the relaxed specificity some of these enzymes already demonstrated to incorporate non-natural substrates. In one example, non-natural analogs of desosamine, the sugar moiety found in methymycin and neomethymycin, were produced (Fig. 35.12). The desosamine biosynthetic cluster was altered by deletion of the desV (reductive amination) and desVI (dimethylation) genes [86, 87]. Several new macrolides were produced from the deletion mutants, including some that were quite unexpected. In addition to

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Fig. 35.12. Deletion of DesV or DesVI results in the production

of two new methamycin analogs.

producing potentially useful new macrolides, this work also demonstrates that the glycosyltransferase involved has a relaxed substrate specificity, in that it could incorporate a variety of deoxysugars. The ability to swap genes between pathways is another important demonstration of the potential for deoxysugar combinatorial biosynthesis. In the methymycin pathway, the desVI gene can be replaced without difficulty by an analogous gene, tylM1, that is involved in the biosynthesis of mycaminose in the tylosin pathway (Fig. 35.13) [88]. Similarly, the tylB gene can replace desV [87]. The ease of such replacements might suggest that very few protein–protein interactions are essential in the deoxysugar biosynthetic pathway, thus reducing the chances that communication problems may be stumbling blocks when mixing and matching deoxysugar biosynthetic proteins in combinatorial biosynthesis. The appending of alternative sugar moieties to aglycons for combinatorial biosynthesis also rests upon our ability to reengineer or utilize the existing reduced specificity of the nucleotidyltransferases and glycosyltransferases involved. For instance, the wild-type a-d-glucopyranosyl phosphate thymidylyltransferase (Ep ) shows very broad acceptance of sugar substrates for coupling to thymidylate (Fig. 35.14) [89, 90]. Structural analysis suggested that introducing specific point mutations should further relax the specificity and better allow incorporation of deoxysugars modified at C2, C3 or C6 [91]. The pooled Ep mutants were capable of incorporating non-natural deoxysugars, including some that are unable to be in-


Fig. 35.13. DesV and DesVI can be replaced by analogous or

similar genes from the tylosin pathway.

corporated by the wild-type Ep . These results demonstrate the feasibility of a rational redesign approach. Another strategy for combinatorial biosynthesis using deoxysugars takes advantage of the broad specificity of some glycosyltransferases [92]. In one striking example, a cosmid (essentially, a very large piece of circular DNA) that contained 25 kb of the elloramycin biosynthetic pathway from Streptomyces olivaceus was transformed into mutants of Streptomyces fradiae and Streptomyces argillaceus (Fig. 35.15) [93]. The wild-type versions of these strains normally produce urdamycin A or mithramycin, respectively, but the PKS genes for these natural products were deleted in the mutant strains. The glycosyltransferase gene from the elloramycin pathway was capable of accepting the alternate deoxysugars produced by the mutants, resulting in the production of new glycosylated tetracenomycins. Further investigation revealed that this glycosyltransferase was capable of glycosylating tetracenomycins with several non-native deoxysugars, including a disaccharide. This glycosyltransferase element has been recently identified [94]. A related approach has also been used with the erythromycin-producing strain Saccharopolyspora erythraea [95]. The inverse approach – instead of employing different deoxysugars, using several different macrolides as substrates for a single glycosyltransferase – has also been successful with the glycosyltransferase from the picromycin biosynthetic pathway in Streptomyces lividans [96]. The ability to reengineer enzymes or to introduce wild-type enzymes into new deoxysugar biosynthetic pathways holds great promise for the combinatorial biosynthesis of a highly diverse array of deoxysugar-containing macrolides. While a substantial amount of work has already been devoted to understanding the genetics and enzymology of deoxysugar biosynthesis, further insight can only assist efforts to produce combinatorial deoxysugar-containing biosynthetic libraries. Recent identification of the elements in a glycosyltransferase responsible for substrate specificity and catalytic activity is an important advance along these lines [97].

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Fig. 35.14. (A) Reaction catalyzed by Ep , the glycosylation of thymine by glucose phosphate. (B) Ep shows broad specificity for a variety of glucose phosphate analogs. Analogs with numbers below them were evaluated by the

pooled mutants (described in the text). WT percentage, incorporation by the WT (wild type) Ep ; Pool percentage, the efficiency with which the pool was able to incorporate the sugar.

35.3 Other Combinatorial Aspects of Biology

Fig. 35.15. (A) Mithramycin and urdamycin A,

naturally produced by Streptomyces argillaceus and Streptomyces fradiae respectively. (B) Expressing the elloramycin PKS genes in PKS-

deficient mutants of Streptomyces argillaceus and Streptomyces fradiae results in the production of a variety of novel carbohydratecontaining elloramycins.

35.3

Other Combinatorial Aspects of Biology

Pools of random oligonucleotides (DNA or RNA), peptides, or proteins might also be considered combinatorial libraries, since they are also collections of diverse biomolecules (in this case, biopolymers) created in parallel. The fundamental difference between these libraries and those resulting from combinatorial biosynthesis is the method of production. In contrast to small-molecule natural products, larger biomolecules are synthesized in a template-directed fashion via the ribosome in vivo or in vitro. This method can provide numerous advantages compared with the production of small-molecule combinatorial libraries. First, it is quite easy to generate biopolymer libraries using recombinant DNA techniques, and the size, di-

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versity, and quality can be readily analyzed (most commonly via DNA sequencing) [98, 99]. This approach is in contrast to the wholly synthetic production of combinatorial biosynthesis libraries, in which complex product mixtures must be contended with, or sophisticated spatial addressing methods must be employed, if one is to productively screen compounds for activity. Second, several methods have been developed for the in vivo or in vitro selection or screening from DNA-encoded libraries, and so complex biopolymer mixtures can be screened or selected en masse and readily separated into active and inactive components [100]. Those members of biopolymer libraries with interesting properties can be easily identified by sequencing of the DNA template, and further optimization of a ‘‘hit’’ in a biopolymer library can be readily undertaken via simple mutagenesis and directed evolution methods. While combinatorial biosynthesis certainly contains potential for the drug discovery process through the generation of novel natural products, it has also been very useful as a tool for studying the enzymology and organization of the biosynthetic systems involved. Combinatorial libraries of DNA, RNA, small peptides, and proteins may also one day be very useful for pharmaceutical or medicinal purposes, but they are currently used mostly for the study of catalysis, folding, and molecular recognition. What follows are brief discussions of how these combinatorial libraries are currently being created and applied. 35.3.1

Combinatorial Libraries of Random DNA and RNA

Oligonucleotide molecules with a wide variety of functions have been selected from RNA or DNA combinatorial libraries. Such libraries are routinely generated with as many as 10 15 unique members. Because the structure, function, and genetic information is self-contained for each molecule, the in vitro recovery and amplification of rare genes with a desired phenotype is straightforward and very efficient. Consequently, combinatorial libraries of oligonucleotides can be easily subjected to multiple cycles of selection and enrichment. The selection of RNA aptamers with a broad range of receptor-like activities from large combinatorial RNA libraries has been particularly successful [101–104]. Using such an approach, and a number of clever in vitro selection methods, RNA aptamers have been identified with binding ability for small molecules [105–109], sometimes with very high specificity [110, 111]. Aptamers have also been selected from large oligonucleotide combinatorial libraries with affinity to a number of medically and biologically important protein targets, including the CD4 antigen [112], surface proteins specific to African trypanosomes [113], the cellular prion protein PrP c (but not the infectious scrapie prion form PrP Sc ) [114], the hepatitis C virus protein 3 protease [115], the human immunodeficiency virus (HIV) TAT-I protein [116], and human angiogenin [117]. RNA and DNA catalysts, termed ribozymes or DNAzymes, have also been selected from combinatorial ensembles for a wide array of reactions [118–122], including oligonucleotide modifications [123, 124], alkylation and acylation reactions


Fig. 35.16. Selection experiment for a self-ligating ribozyme.

[125, 126], peptide bond formation [127], and Diels–Alder cycloadditions [128]. While these ribozymes are often poorly active, in some cases very high levels of catalysis have been achieved through the further optimization of an initial ‘‘lead’’ [129]. The direct selection of highly efficient self-ligating ribozymes (i.e. phosphoryl transferases) from large RNA combinatorial libraries is one of the most impressive accomplishments thus far in the field of nucleotide-based catalysis (Fig. 35.16) [130]. A library of @10 15 different RNAs with 220 random nucleotides was constructed, and from this ensemble roughly 1 in 10 13 members had the desired ligase activity. Further cycles of diversification and selection were then used to evolve the initially selected group of ligase ribozymes, and one of these was subsequently

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reengineered with the ability to catalyze an intermolecular ligation with multiple turnovers [131]. This enzyme exhibited a rate constant of >1 s1 , which corresponds to a rate acceleration approaching 10 9 over background. Nucleic acid-based receptors and catalysts from combinatorial libraries are orthogonal analogs of their protein counterparts, and have potential value as drugs or diagnostic agents. The strategies used in their creation and selection, observations of how the initially identified ‘‘hits’’ further evolve to optimize function, and insight into the comparative advantages and disadvantages with respect to proteins should prove useful in our efforts to create and optimize novel protein catalysts (see Section 35.3.3). 35.3.2

Peptide Combinatorial Libraries

Short peptides are good inhibitors for a number of enzymes, and are thus suitable lead structures for developing potent nonpeptide inhibitors, which may have increased bioavailability and half-lives in vivo. Peptide ligands and inhibitors are also valuable for collecting a wealth of information about the structure, specificity, and size of receptor binding pockets and enzyme active sites. Since it is often difficult to predict which peptide sequence will have optimal inhibitory activity, selection from combinatorial peptide libraries is attractive. Additionally, the selection results may reveal new, previously unidentified components of the targeted pathway, which may, in turn, represent formidable targets for inhibition by small organic molecules. Because short peptides are often quickly degraded in the cell, peptide libraries for in vivo selection are usually displayed inside a larger protein scaffold. For example, combinatorial hexadecameric peptide libraries were displayed (by insertion at the genetic level) into a surface loop of a biologically inert carrier protein (an inactive mutant of staphylococcal nuclease) [132]. From a @10 6 -member yeasttransformed library, three peptides that inhibited the spindle checkpoint and 29 peptides that inhibited a mating pheromone signaling pathway (29 peptides) were identified by in vivo genetic selection in cleverly engineered yeast strains. The putative in vivo targets were subsequently identified using yeast two-hybrid analysis [133] and genetic dissection of the target pathways. A similar in vivo genetic selection approach was recently used to identify members from a combinatorial nonapeptide library that block intracellular dimerization of HIV-1 protease, albeit in E. coli [134]. Peptide combinatorial libraries can also be evaluated in vitro for binding or inhibitory activity. As opposed to in vivo peptide selections, in vitro selection has three particular advantages. First, no carrier protein is required to protect the short peptides from in vivo degradation. Of course, this freedom also increases the conformations available to the peptide, which may reduce the affinity of the peptide to a potential target [135, 136]. Secondly, much larger (@ 10 13 ) peptide libraries can be assayed in vitro than in vivo (maximally 10 9 , which is the practical upper limit because of plasmid transformation efficiencies [137, 138]). Finally, unnatural amino


Fig. 35.17.

The mRNA display method.

acids can be utilized in the peptide combinatorial libraries through the use of suppressor codons or other strategies [139–142]. Perhaps the most powerful method for in vitro selection of peptide combinatorial libraries is mRNA display, which works by utilizing direct covalent attachment of each member of the peptide combinatorial library to its encoding mRNA (Fig. 35.17) [143, 144]. Such covalent constructs are formed by in vitro translation of 3 0 puromycin mRNA templates. After mRNA translation, puromycin enters the peptidyl transferase site of the ribosome and is covalently attached to the C-terminus of the nascent peptide. The peptide moiety of the resulting mRNA–peptide conjugate can then be selected as described for the other formats. In a recent example, 20 different peptide aptamers to streptavidin were identified from a combinatorial library of 10 13 different 88 amino acid sequences [145]. These aptamers had binding affinities (KD 5–10 nM) that were three orders of magnitude better than a commercially available streptavidin-binding decapeptide (KD 13–72 mM). Truncation analysis showed that one of these peptides could be reduced to only 38 amino acids, and also indicated that the sequence motif of histidine–proline–glutamine (HPQ) was essential for binding activity (as in other peptides with affinity for streptavidin [146]). Interestingly, all 20 aptamers that were identified were not from the intended reading frame but from the third reading frame that results from a two-base insertion. This outcome is probably because the HPQ motif occurs more frequently in the third reading frame. 35.3.3

Protein Combinatorial Libraries

Protein design and engineering is an attempt to understand the factors involved in creating folded protein structures with specific behaviors, such as thermal stability or specific catalytic activity, and then utilize that understanding to create novel proteins with useful functions. Owing to our currently incomplete knowledge of the relationship between structure and function, however, the full de novo design of a

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Fig. 35.18. Experimental outline for the selection of ATPbinding proteins from a 10 12 -member combinatorial protein library.

functional protein from first principles is not yet an achievable goal. As an alternative, researchers have begun producing combinatorial libraries of proteins with varying levels of randomization, and then screening or selecting from these libraries for valued function. Given the size of typical protein domains, fully randomized protein combinatorial libraries are, for practical purposes, infinitely large. For a 100-amino-acid protein, for example, there are 20 100 (10 130 ) possible sequences, and it is thus impossible to examine a substantial portion of this diversity. To evaluate as large a fraction as possible, researchers have turned to in vitro selection methods (discussed in Section 35.3.2) such as mRNA–protein fusions. Using the same mRNA display system outlined in Section 35.3.2, but selecting for ‘‘protein’’ behavior instead of ‘‘peptide’’ behavior, Keefe and Szostak recently identified four ATP binders starting from a library of 6 10 12 proteins in which 80 contiguous amino acids were randomized (Fig. 35.18) [147]. One of these variants showed very high affinity (KD ¼ 5 nm) and specificity for ATP, and was capable of being further truncated to a protein of only 45 residues. Like most in vitro selection strategies, the ATP-binding proteins were identified based on their ability to elicit binding responses, but it is possible to select proteins on the basis of their catalytic activity [148–150], which may also allow the directed evolution of novel enzymatic activities. Fully randomized combinatorial protein libraries are of limited practical utility, since the frequency of foldable, much less functional, sequences in them is expected to be very low. As an alternative, partially randomized protein combinatorial libraries can provide the same benefits of randomization but in a sequence that is predicted to fold into a stable structure. For example, the utilization of basic structural information, such as the sequence preferences of helices and sheets, termed


Fig. 35.19. (A) Structure of the AroQ-scaffold

chorismate mutase. (B) Using a two-stage selection procedure, active chorismate mutase variants were selected that had a dramatically reduced amino acid palette

(X ¼ phenylalanine, isoleucine, leucine, or methionine; Z ¼ aspartate, glutamate, asparagine, or lysine) in the helical regions of the protein.

binary patterning [151–153], has been used to design ‘‘scaffolded’’ combinatorial libraries from which soluble proteins were selected [154–156]. To further probe the utility of such libraries, not only for folding structures but also for functional catalysts, we have randomized up to 80% of the total sequence of a six-helix bundle chorismate mutase, using a binary pattern and only eight different amino acids in the randomized regions (Fig. 35.19) [157]. Using in vivo genetic selection for chorismate mutase catalytic activity [158], we identified proteins that upon further analysis had physical properties characteristic of natural enzymes. Our results suggest that such patterned combinatorial protein libraries could be useful for protein design projects. Analogous to the way that some small-molecule chemotypes make attractive starting scaffolds for combinatorial chemistry libraries, some protein structures have been suggested as being very suitable scaffolds into which any number of possible activities could be built by randomizing or rationally mutagenizing specific regions of the scaffold. In particular, the (a/b)8 barrel, which is perhaps the most commonly occurring protein fold [159], has been suggested as the best starting point from which to produce combinatorial libraries to screen for new functions [160, 161]. One impressive example of the combinatorial redesign of the (a=b)8 barrel is the conversion of one enzyme on the tryptophan biosynthesis pathway, indole-3glycerol-phosphate synthase (IGPS), into another (phosphoribosylanthranilate isomerase, PRAI) with very high catalytic activity (Fig. 35.20) [162]. Rational redesign was used to remodel the IGPS structure by elimination of an a-helix and the re-

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Fig. 35.20. (A) Biosynthesis of tryptophan

from chorismate. AS, anthranilate synthase; PRPP, phosphoribosylpyrophosphate; APRT, anthranilate phosphoribosyl transferase; PRA, N-(5 0 -phosphoribosyl)anthranilate; PRAI, phosphoribosylanthranilate isomerase; CdRP, 10 -(2 0 -carboxyphenylamino)-10 -deoxyribulose 5 0 phosphate; IGPS, indole-3-glycerol phosphate

synthase; IGP, indole-3-glycerol phosphate; TS, tryptophan synthase; GAP, glyceraldehyde-3phosphate. (B) Directed evolution was used to create a novel PRAI, which catalyzes the conversion of PRA to CdRP, from IGPS, which catalyzes the cyclization of CdRP to IGP. (C) Both PRAI and IGPS are TIM barrel proteins but have somewhat different loop structures.


Fig. 35.21. Structure of triosephosphate isomerase. In separate

experiments, randomized libraries of the a/b-loops, b/a-loops, a-helices, and b-sheets were produced and variants selected in vivo for triosephosphate isomerase activity.

placement of two loop sequences, and then combinatorial protein libraries were created from the remodeled protein by randomizing positions in the loops. In vivo genetic selection was used to identify members from the combinatorial library capable of behaving as PRAIs. To further probe the mutability of the (a/b)8 scaffold, Harbury and coworkers created combinatorial protein libraries by randomizing portions of an (a/b)8 barrel – triose phosphate isomerase (TIM) (Fig. 35.21) [163]. They then selected for members of the libraries that were capable of complementing a TIM-deficient strain. Their results suggest that the (a/b)8 barrel is quite mutable in some of the loop regions, which would be the probable site for introducing combinatorial randomization for the creation of new catalytic activities. This study has set the platform for the rational design and creation of further combinatorial libraries from the (a/b)8 barrel from which new function can be selected. A third type of protein combinatorial library can be created through minor changes designed to optimize or alter activity or to improve physical properties, and then using selection or screening techniques to identify members of the library with the desired characteristic. Such a procedure is usually called directed evolution, and has been reviewed extensively elsewhere [164–170]. These directed evolution protein libraries could be considered analogous to lead optimization combinatorial libraries that are created to find an initial hit and optimize the scaffold for use as a drug. In directed evolution experiments, mutations are introduced by error-prone PCR [171] or DNA shuffling [172, 173] and related techniques [174, 175]. After a suitable selection step, selected library members can be recombined to blend additively positive mutations with the selection procedure carried out on the further evolved library (Fig. 35.22). Using such a strategy, a number of proteins have been further evolved to improve the activity [176], alter the selectivity [177–190], or increase the thermal stability [191, 192]. In some cases, even multistep processes utilizing sev-

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Fig. 35.22. DNA shuffling.

eral different proteins have been optimized by producing combinatorial libraries of entire genetic operons [193].

35.4

Perspectives

Perhaps the chief driving force in the development of combinatorial chemistry is the opportunity to streamline the discovery process of compounds for human benefit. Conversely, the combinatorial aspects of biology have been used mostly by Nature for evolutionary processes. As our understanding of biosynthetic systems, protein structure and function, and catalysis continues to improve, however, we are increasingly able to harness the combinatorial aspects of biology for our own

35.4 Perspectives

use. For instance, combinatorial biosynthesis holds great potential for the production of potential lead compounds for drug discovery. These leads could then be further optimized by more ‘‘traditional’’ synthetic and combinatorial chemistry. Additionally, combinatorial biosynthesis could be used to produce starting material intermediates (perhaps for further combinatorial chemistry) and final products that are not easily made using current synthetic methods. One example of this potential utility is the use of a thioesterase domain from the tyrocidine NRPS for catalyzing the cyclization of a variety of short, synthetic peptides [194, 195]. Combinatorial chemistry could play similar roles for combinatorial biology. For instance, an attractive scaffold that is identified through combinatorial chemistry could also act as the ‘‘lead’’ for which a more complexly functionalized (and synthetically challenging) combinatorial biosynthesis library could be prepared. Deoxysugar libraries prepared by combinatorial chemistry could be used as feeding precursors for more complex polyketide- or NRP-based combinatorial biosynthesis libraries utilizing low-specificity glycosyltransferases. Peptide combinatorial libraries, as mentioned above, are very attractive for producing initial leads for drug development, as well as for providing insight into the structure, activity, and specificity of biological receptors and enzymes. But their poor behavior in vivo often precludes their use as effective drugs. Combinatorial chemistry is thus essential for producing nonpeptide drugs based around the initial peptide leads. Clearly, combinatorial biology will never supplant combinatorial chemistry; rather, both fields will continually augment each other. Large-molecule combinatorial libraries of proteins and DNA or RNA may also find eventual use as drugs, but currently they are of interest to researchers mainly for their ability to probe issues of structure and catalysis. In addition to this, however, the production of such libraries has dramatically increased our knowledge and ability to engineer and develop suitable selection and screening systems, some of which have found their way to the combinatorial chemistry bench. Additionally, our ability to engineer and create proteins or oligonucleotides with desired function may one day provide combinatorial chemists with another catalytic repertoire, one that is based around environmentally friendly enzymatic reactions. Current work on structural redesign of nucleotidyltransferases [91], and on the utilization of glycosyltransferases with relaxed specificity [93], is a step in this direction. What does the future hold for our utilization of combinatorial biology for human benefit? Given the promise of the multiplasmid approach in polyketide combinatorial biosynthesis, and the predictive nature of modular biosynthetic systems, it is conceivable that we will someday have the ability to produce virtually any polyketideor NRP-derived product, appended with appropriate deoxysugars, in high yield, merely by transforming the right combination of plasmids into a suitable production strain. It is also likely, given our ever-increasing understanding of protein structure and function, that we will someday be able to design and engineer novel enzymes with catalytic activities that are useful for applications in chemistry (including combinatorial chemistry) or medicine. Additionally, our ability to rapidly select useful or interesting variants from combinatorial libraries steadily improves. Coupling binding or catalytic activity with genotypic information is currently pos-

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sible with DNA-encoded libraries, and the same linkage may someday also be possible with natural product combinatorial biosynthesis libraries. With the advent of the genomic, or postgenomic era, there will be a flood of potential new medical targets. The ability to utilize biological tools can only help our combinatorial efforts in designing drugs to control them.

Acknowledgments

The helpful suggestions and support of Professor Don Hilvert during the preparation of this chapter are gratefully acknowledged, as well as Kinya Hotta and Gavin Macbeath for the preparation of graphics.

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Index absorption 725, 744ff Accelab Arcosyn98 200 acetylation 906 N-acetyl-b-amino acid 520 acetylcholinesterase inhibitor 640 acid chlorides 347, 789 – reductions 418 acid fluorides 348 acrylate ester 280 acrylates of 1,3-oxazolin-2-ones 456 acrylic acids 232, 448 actinorhodin 1067 activated magnesium 488 active esters 348 acyl transferases 905 acylation 788, 790, 1082 acylcarbene tagging strategy 174 addition reaction 482, 915 addition electrophilic 313 addition (electrophilic) to isolated double bonds 306 1,2-addition 478 1,4-addition 503 1,4-addition to a,b-unsaturated carbonyl systems 317 addition of carbon nucleophiles 324 addition of dialkyl zincs 1004ff addition of N-nucleophiles to CN double bonds 331 addition of N-nucleophiles to nitriles 340 addition of phosphorus nucleophiles to CN double bonds 332 addition of sulfur nucleophiles 333, 342 addition of O-nucleophiles to CN double bonds 333 addition to CcC triple bonds 318 addition to CbS double bonds 338

addition to carbon-hetero multiple bonds 322 addition to CbC double bonds 306 addition to CN double bonds 322, 334 addition to CN triple bonds 339 addition to NbCbN 335 adenosine diphosphate 714 adenylation domains 1075 ADME properties 744f, 774 Advanced ChemTech 203 agricultural chemistry 3 AHB linker 65 AIBN 228, 231, 234 Ala-scan 592 albumins 751, 758, 891 alcoholysis 71, 134 aldehyde reductions 387, 417 aldol condensation 629, 673, 873 aldol diastereoselective reductive reaction 921 aldol reaction 148, 492, 580, 522 – Evans type 497 – using imines 324f alkanes 149 alkenes 149 alkene epoxidation 916 alkenylaziridines 483 alkenylstannanes 81 alkylation 508, 513, 1006, 1082 – enolates 506 – glycines 513 – phenolate 250 alkynes 150, 299, 481 alkynone 297 N-allylation 571 allyl boronic acid pinacolates 476 allyl ester 568 allyl glycosides 599 allyl indium 476

Handbook of Combinatorial Chemistry. Drugs, Catalysts, Materials. Edited by K. C. Nicolaou, R. Hanko, and W. Hartwig Copyright 8 2002 WILEY-VCH Verlag GmbH, Weinheim ISBN: 3-527-30509-2

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Index allyl transfer 625 allyl-based linkers 68 allylic amides 443 allylic phosphates 1006 allylic substitutions 94, 915 p-allylpalladium complex 568 allylstannane 232 aluminium hydride reagents 127, 390 amidines 142 amination of aromatic halides 561 amine tag 616 amines 234 a-amino acids 233, 234, 513, 785 b-amino acids 520, 649 b-amino alcohols 521 aminoimidazoles 701 b-aminoketone 520 aminolysis 489, 674 aminomethylanthracenes 960 aminomethylphosphines 895 aminoquinolinone 671 ammonation 978 ammonia process 833 amorphous microporous mixed oxide (AMM) supports 955 amphiphilic lipid molecules 1046 analysis of combinatorial libraries 217 anatase compositions 1034 anchoring of molecules 60 anemia 784 angiotensin converting enzyme (ACE) inhibitor enalapril 923 angiotensin I 769 anion exchange resins 211 anomeric configuration 707 anomeric stereochemistry 709 antibiotic resistance gene 1073 antibodies 892 antibody-catalyzed dehydration 902 anticancer agent clavularin B 1012 antitumor antibiotics 230 arene bromination 273 ArgoGel2 36 Argonaut Trident 208 Arnstein tripeptide (ACV) 891 Array microchannel reactors 945 Arrhenius equation 809 artificial lattices 1024 artificial receptors 1046 aryl halide 228 aryl iodides 150 aryl radical cyclizations 229 aryl sulfonates 79, 115 arylacetic acid 507

arylbromides 150 arylhydrazides 73 arylmagnesium bromides 295 arylspiropiperidines 736 aryltosylimines 461 arylzinc reagents 553 aspartic acid 233 asymmetric cyclopropanation reactions 907, 928 asymmetric Diels-Alder reactions 906 asymmetric dihydroxylation 311 asymmetric epoxidation 917 asymmetric induction 1010 asymmetric reductions 389 atactic polypropylene 935 atom transfer radical processes (ATRP) 1054 atom-transfer 242 ATP-binding proteins 1086 Auto-MATE 879 automated sample processing 219 automated synthesizer 802 Available Chemicals Directory (ACD) 729, 765, 926 aza-Diels-Alder reaction 325, 450, 655, 908f, 922 aza-Wittig reaction 335 azepanes 662 azepinone 662 2-azetidinones 649 azide reductions 319, 406 azides 129 aziridines 643 azo dyes 894 azomethine ylides 651ff baccatin III 620 Bacillus brevis 1075 Bacillus subtilis 1075 backbone amide linkers (BAL) 137 Bamford-Stevens reaction 110 Barton-McCombie deoxygenation 240 Barton esters 232 Baylis-Hillman reaction 317, 489, 524, 690 bead sizes 25 BEMP resin 505, 513 benzazepines 561, 662f benzimidazoles 104, 131, 260, 379 benzimidazolones 260 benzisoxazoles 131 benzodiazepines 4f, 75, 104, 259, 379, 735 benzodiazepinones 104 benzofurans 106, 109, 295, 676 benzofurans 66 551 benzofuroxanes 129

Index benzohydroquinones 479 benzophenones 295 benzopyrans 82, 287, 364, 636 benzopyrones 297 benzotriazole linker 483 benzotriazoles 86 benzoxazines 353 benzyl carbamates 71 benzyl-type linkers 63 biased libraries 739 Biginelli reaction 692 BINAP ligands 32 BINAP-Ru catalyst 422 binding affinity 772 binding proteins 595 bioavailability 745 biocatalyzed transformations 294 bioinformatics 886 bioisosteric pharmacophores 740 biological barriers 746 biological combinatorial libraries 1063 biological targets 4 biotage 214 biphenyl 237 biphenyltetrazole 736 Bischler-Napieralski cyclization 413 Bischler-Napieralski reaction 674, 277 bitstring 731 blood pressure 784 blood-brain barrier 747 Boba resin 65, 139 bond-scission 293 borane-amine adducts 430 borane-based reductions 399, 426 borohydride reagents 387, 392 boron enolates 495 boronates 76 boronic acids 96, 152 boronic acid ester 76 boronic acid Mannich reaction 326 brain-blood distribution 752 bromination 271, 471 bromoacetals 230 a-bromo esters 231 g-butyrolactones 230, 379, 676 (þ)-carvone 621 calcium channels 659 calorimetry 860 capacitance 1021 capacity factor 754 capillary array electrophoresis (CAE) 978 capillary electrophoresis (CE) 977 capital investments 19

carbamate reductions 400 carbene complexes 586 carbene complex-mediated cleavage reactions 597 carbenoid 300 carbodiimides 335 carbohydrates 706 carbohydrate orthosters 287 carbohydrate-binding proteins 595 carbohydrate-functionalized oligomers 596 carbon electrophiles 272 carbon nucleophiles 470 carbonyl insertion 352 carbonyl reactivity 401 carbopalladation 142 carbozincation 526 carpanone 634 cascade radical annulation 374 Castro sulfonamide betain 250 catalysts 3, 6, 885, 935 catalyst discovery 991 catalyst multifunctional 993 catalyst oxazaborolidine 428 catalyst pellets 974 catalyst research 831 catalytic acetylation 906 catalytic combustion 971 catalytic oxygenation 904 catch-and-release purification 430 cathepsin D 488, 773 cathode ray tubes (CRTs) 1038 cathodoluminesent 1041 cation exchanger resin 211 CBS catalyst 427 CaC bond-forming reactions 1012 CCK-A receptor 769 CD4 antigen 1082 cell phones 1037 cellular prion protein 1082 cellulose supports 4, 50, 183 cerium ammonium nitrate (CAN) 380 charge-coupled device (CCD) camera 1039 Charybdis Technologies Illiad PS 207 chelation-controlled reduction 389 chemical biology 3 chemical encoding methods 170, 171 chemical optimization 787 chemical space 795 chemical stability of resins 33 chemical vapor deposition 1022 chemoinformatics 886, 1018 Chemspeed ASW2000 system 202 chips 5 ‘‘chip’’ format 709

1101

1102

Index chiral auxiliary 234, 232, 120, 446, 928 chiral catalyst 991 chiral Lewis acids 450 chiral pentamines 912 chiral separation methods 7 chiral stationary phases (CSPs) 921, 1046 chiral synthon ¼ chiron 910 chirality multiplication 906 2-chlorotrityl resin 67 chorismate mutase 1087 Christ vacuum centrifuge 222 chromaticity 1030 chromatography 284 chromenones 75 chromium arene complexes 108 cinnolines 86 Ciprofloxacin2 262 circular dichroism (CD) 1049 – detection 966 Claisen 515 – condensations 515, 792 – reaction 791 – rearrangements 465 Claisen-Schmidt reaction 661 clearance 744, 752 clinical candidates 10 clinical research 12 C log P 726 clustering 732 cobalt enolates 521 b-C,O bond scission 292 cocatalysts 992 colorimetric ‘‘spot’’ test 921 colossal magnetoresistance 1034 CombiFlash TM from Isco 212 combinatorial antibody library 892 combinatorial biosynthesis 1064, 1075 combinatorial catalysis 886f, 927, 1019 combinatorial libraries 764 combinatorial semisynthesis 614 combinatorial total synthesis 626 composition conditions 1020 composition spread technique (CCS) 1035 computer-controlled multiplexer 969 condensation reactions 685 contamination 94 continuous composition spread (CCS) 1022 controlled pore glass (CPG) support 46 copper alkynes 329 Corey’s chiral oxazaborolidine catalyst 928 coumarin 519 para-coumaroyl-CoA 1067 coupling reagents 347f C-radicals 231

critical temperatures 1025 Crixivan2 912 cross metathesis 94, 587ff cross-coupling reactions 86, 531 crosslinked (insoluble) polymers 24 crosslinked acrylamides 41 crosslinked PEGs 44 crosslinked polymer beads 27 crosslinked-ethoxylate acrylate resins (CLEAR) 43 crosslinkers 25, 32 cross-metathesis 664, 929 crystallization 212 C2 -symmetric chiral ligands 993 C3 -symmetric triarylphosphines 920 cuprates 482 Curtis rearrangement 336 Curtius degradation 356 2-cyanoethylphosphate protecting group 291 4-cyanotrityl protection group 68 cyanuric chloride 255 cyclative cleavage 103, 526, 629, 631, 633, 654, 789 – to hydantoins 791 cyclic alkenes 588 cyclic amide 143 cyclic peptides 592, 594 cyclization reaction 314 5-exo cyclizations 228, 232 ½2+2 cycloaddition 463, 586 ½3+2 cycloadditions 453 ½4+2 cycloadditions 441 ½6+3 cycloaddition 464 cyclo cleavage 789 cycloaddition 649, 651, 698, 701 cycloaddition imine 459 cycloading 636 cyclobutanones 464 cyclofragmentation 295 cyclopentenones 1010 cyclorelease 631, 633 cycloreversions 299, 676 cyclotriphosphazene 49 cysteine 289 cytochrome P450 753 cytochrome P450 oxygenase 904 cytomegalovirus 628 Danishefsky’s diene 298, 325, 441, 480, 909, 922 DART (development automated reaction toolkit) 833, 870 data-handling system 854 daughter libraries 925

Index dealkylation 238 deconvolution methods 1048 degradation chemistry 614 dehydroalanine 280, 289, 446 Delrin cylinder 969 dendritic polyester 53 dendritic polyether 53 dendritic polymeric supports 51 dendritic TADDOL 32 6-deoxyerythronylide B 1071 deoxygenation 674 deoxysugar 1064 descriptors 731 design of libraries 11, 13 design of the experiments 1020 design process 11 desosamine 1077 Dess-Martin oxidation 307, 309, 372, 394 development phase 12 device optimization 1042 dialkylated amino acids 515 dialysis 47f diaminopyrimidine 772 diaryl ether synthesis 263 diaryl methane 79 diastereoselective ketone reductions 389 diastereoselective reductive aldol reaction 921 1,2-diazines 301 a-diazocarbonyl 300 diazo compounds 271 diazomethane 71 diazonium salts 83 diazotization of aromatic compounds 274 1,3-dicarbonyls compounds 508, 510, 513 – g-alkylation 510 1,5-dicarbonyls 506 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) 332 dicobaltoctacarbonyl 150 Dieckmann condensations 516 Diels-Alder reactions 13, 301, 441, 448, 450, 601, 634, 678, 1083 dienamines 441 1,3-dienes 97 diethylzinc 234 differential IR thermography 940 differential scanning calorimetry 882 differential thermal analysis 1030 diffusion coefficients 747 dihydrofolate reductase 772 dihydropyrans 678 dihydropyridazinones 785 dihydropyridine (DHP) 659, 689 – linker 653

dihydropyridones 482 2,3-dihydro-4-pyridones 298 dihydropyrimidine-2,4-diones 104 dihydroquinolinones 463, 818 dihydroquinone 399 dihydroxylation 309 diisoamylborane 388 diketomorpholine 104 b-diketones 516 diketopiperazines 64, 104 2,2-dimethylbenzopyran 737 dimethyldioxirane 80, 308, 309, 443 2,5-dioxopyrrolidines 653 diphenyl methane 736 1,3-cycloaddition 299 dipolarophiles 108, 453 dipoles 453 direct methanol fuel cell (DMFC) 948 directed evolution 1089 discovery phase 12 distribution coefficient 745, 747f, 749, 751, 754 dithianes 71 dithiocarbamate alkylation 251 diversity 730, 734, 776, 795, 802 diversity assessment 762 diversity-based reaction development 992 divinyl ketone 657 DNA shuffling 1089 DOCK 772 DOE 866, 872 Doebner-quinoline 674 dopants 1020 drug discovery 6, 802 drug optimization 727 drug-like character 744, 746, 763f, 770, 776 drug-like index 730 drug-like libraries 739 drug-like molecules 6, 778 drug-like properties 728 duocarmycin 230 dynamic random access memory (DRAM) chips 1037 Edman degradation 173 eight-channel microreactor 970 electrochemical cleaving step 79 electrochemical reduction 951 electrohydrodimerization 947 electroluminescent flat panel displays 1038 electron capture detection (ECGC) 174 electron transport layer (ETL) 1043 electron-beam evaporation 1022 electronic tracking 14

1103

1104

Index electro-optic coefficients 1031 electrophiles 88 electrophilic addition 313 electrophilic addition to isolated double bonds 306 electrophilic substitution 270 electrospray ionization mass spectroscopy 178 electrospray ionization tandem mass spectrometry (ESI-MS/MS) 935 eliminations – reactions 73, 279, 293, 745 b-elimination reaction 149, 228, 279, 285f, 288f b-H elimination 1005 1,4-elimination 295 1,6-elimination 293 elloramycin biosynthetic pathway 1079 enamines 442 enaminones 655 enantiopurity 1001 enantioselective Michael additions 505 enantioselective reducing agent 415 enantioselective reductions 906 enantioselective synthesis 991 encoding strategies 5 encoding technologies 17 Endeavor TM 873 enolates 492, 503, 793 – alkylation 506, 510 enones 494 environmental chemistry 894 environmental impact 1018 enzymatic synthesis of oligosaccharides 711 enzyme 98, 294 enzyme mimics 903 enzyme-sensitive linkers 710 epibatidine 452 (G)-epimaritidine 21, 417 epitaxial overlayer 1024 epothilones 97, 370, 494, 600, 628, 1075 epoxidation 287, 306, 707 epoxidation with dimethyldioxirane 443 epoxide 109, 296 epoxide opening 306 epoxide opening by amines 253 epoxide reductions 418 erythromycin 1066 erythromycin polyketide synthase complex 1070 erythropoietin 784 ester reductions 390 esterases 294 ether synthesis 263, 925, 249 ethylene polymerization 931

Evans oxazolidinone 446, 496f, 510 evaporation 221 everninomicin 287 evolutionary combinatorial libraries 1063 evolutionary selection 636 exo-glycosidase 714 extraction 284 faujasite 1027 feature trees 771 feedback loop 1020 ferroelectric materials 7, 1035 ferromagnetic semiconductors 1033 filtration 285 fine chemicals 838 fingerprints 755 – 2D finger prints 77 first-phase screening 1008 Fischer indole synthesis 666 fitness function 775 FlashMaster TM 213 flavilylium salts 147 flexible descriptors 731 FlexX 772 fluorene-based linkers 291 fluorescence detection 178 fluorescence dyes 933 fluorescent optical microscopy 890 fluorescent resonance energy transfer (FRET) 959 fluorescent sensors 888f fluorinated linkers 62 fluorous chemistry 5, 293, 241, 430 focal plane array (FPA) detector 953 Focus-2D 770 focused libraries 730, 743, 761, 765, 796 formation of amides 347 formation of carbamates 354 formation of carbon-carbon bonds 492 formation of esters 353 formation of guanidines 338 formation of ketones 355 formation of thioamides 357 Fourier transform infrared (FTIR) spectrometers 956 fraction dose absorbed 747 fragmentation reaction 107 free concentration 750 freeze-drying 221 Freidinger lactam 599 frequency agile filters 1036 Friedel-Crafts acylation 272, 877 FT-IR methods 217 FTIR spectrum 387

Index fumitremorgin 276 functionalized polypropylene pins 5 furans 300, 444, 460, 676 Gabriel-Cromwell reaction 253, 643 Gabriel synthesis 553 galactose 714 galanthamine 639 G-coupled protein receptor 282 gel 26 genes 1077 genetic algorithm 771, 775, 945, 1051 genetic operons 1090 Genevac vacuum centrifuge 222 germanium linker 75, 111 giant magnetoresistance 1033 Gilson ThermoQuest 216 Gilson-HPLC 215 glycals 708f glycobiology 720 glycolipids 706 glycopeptide 406 glycoproteins 706 a1 -glycoprotein 758 glycosidases 712 glycosidations 139, 238, 708 glycosides 83 glycosyl fluorides 708, 714 glycosyl halides 707f glycosyl phosphates 708, 710 glycosyl phosphites 708 glycosyl sulfoxides 707f glycosyltransferases 711, 716, 1077 glycosynthase 712, 714 gmelinite 1027 gold complexes 937 gold electrodes 951 G-protein-coupled receptors 735, 766 graft copolymers 26 graphical encoding 170 Grieco three-component reaction 691 Grignard reagents 146, 283, 295, 328, 441, 473, 478 group-transfer 242 Grubbs’ metathesis catalyst 929 guanidines 140 HAL linker 63, 65 halide reductions 418 a-haloacrylate 643 haloaromatic binary coding 173 Hantzsch condensation 661 Hantzsch dihydropyridine synthesis 125 Hantzsch reaction 689

Hantzsch synthesis 338 Hantzsch thiazole synthesis 251 Hantzsch three-component condensation reaction 897 HATU 616 Heck reaction 86, 95, 229, 555, 663, 666, 558 Heck reaction catalyst 958 a-helical peptide/phosphine-Rh(I) catalysts 913 hematocrit 784, 796 Henry reaction 522 hepatitis C virus protein 3 protease 1082 heterogenous catalysis 17, 586, 939, 1018 hexapeptide library 893 high internal phase emulsion polymers 40 high pressure reactions 846 high-energy electron diffraction (RHEED) 1025 high-loading immobilized catalysts 39 high-performance liquid chromatography (HPLC) 4, 15, 19, 198, 203, 215f, 788, 810, 1046f high-throughput catalyst screening 918, 993 high-throughput parallel synthesis 4 high-throughput screening 4, 190, 726, 761, 785, 806 history of Combinatorial Chemistry 4 HiTOPS system 182 HIV-1 protease 1084 (HIV) TAT-1 protein 1082 Hiyama coupling 960 Hofmann elimination 280 hole transport layer (HTL) 1042f Holmes photolinker 893 homochiral organometallic catalysts 996 homogeneous catalysis 887 homolytic processes 227 Horner-Emmons olefinations 357 Horner-Wadsworth-Emmons reaction 451, 632 Horner-Wittig condensation 633 host structure 1020 HPQ motif 1085 human angiogenin 1082 human genome project 634 Hunsdicker reaction 243 hybrid PKS/NRPS systems 1075 HYCRAM 69 HYCRON 69 hydantoins 5, 104, 789 hydrazines 128, 794 hydrazones 128 hydrobenzofuran 228 hydroboration 316

1105

1106

Index hydroboration/oxidation 316 hydrocarbons 148 hydrogen peroxide 310 hydrogenations 315, 422, 874 hydrolysis of esters 354 hydrometallation 316 hydrophobic collapse 736 hydrosilation catalysts 958 hydrostannylate 81 hydrostannylation 318 hydroxamic acids 142 b-hydroxy carbonyls 493 b-hydroxy ketones 494 hydroxylamine linkers 283 hyperbranched polymers 52 IBX oxidation 317 imidates 710 imidazoles 131, 300, 461, 695, 699, 857, 860 imidazolidinones 875 imidazoline 695 imines 479 imine cycloaddition 459 imine formation 401f imine reductions 411 imino aldol reaction 324f imino-Sakurai reaction 327 immobilized artificial membranes 754 immobilized dipeptides 1048 in silicio screening 761 indazoles 379 indigo 963 indinavir 912 indolactam 145 indoles 131, 565, 665, 812 indole-3-glycerol-phosphate synthase (IGPS) 1087 indolines 637 indolizines 379, 462 infrared (IR) thermography 906 infrared radiation 953 inkjet deposition 940 inorganic supports 46 insertion catalysts 927 insertion reaction 919 instrumentation 190 integrated circuit (IC) chips 1022 integrated data management 1017 interfaces 1020 intramolecular Diels-Alder reaction 448 intramolecular ether-forming reactions 925 intramolecular Heck reaction 558 iodination 274 iodine-magnesium exchange 475

iodoetherification 313 iodolactonization 313 iodo-lactonization 510 ion exchange resins 30 ion-beam implantation 1022 ionophores 889 ionophore library 890 Ireland-Claisen rearrangement 465, 525 Irori TM 209, 619, 637, 795 irradiation 237, 292, 233 isocyanides 693ff, 698f, 702 isopenicillin N synthase (IPNS) 890f isoquinolines 131, 379 isoxazoles 453, 794 isoxazolidines 453 isoxazolines 131, 379, 453 isra system 197 iterative optimization 915 iterative optimization approach 913 iTOPS 882 Jarvis-Patrick method 732 Kaiser’s oxime resin 341 Kenner linker 100 ketal/acetal-based linkers 70 ketene silyl acetals 498, 502, 520 a-ketoamides 297 b-ketoamides 689 a-ketoesters 297 b-ketoester 689 ketone reductions 388, 417 ketones 485 ketoreductase 1068 kinases 766 kinetic resolution 968 kinetic studies 1002 kinetics reaction 36, 61, 103 Knoevenagel condensations 450, 518, 661, 690, 897 Koenigs-Knorr method 707 Krohnke pyridine synthesis 661 laboratory automation 14 b-lactams 377, 463, 522, 671 b-lactam antibiotics 649 d-lactones 483 lactonization 314 laminar supports 183 Langmuir monolayers 1046 lanthanide complexes 903 Larock annulation 565 laser encoding 186 laser optical synthesis chip 186

Index laser-induced fluorescence (LIF) 978 Lawesson reagent 105 l-dopa (Monsanto) 912 lead compounds libraries 727 lead finding 778 lead optimization 778 lead structures 727, 786, 802 lead-finding libraries 762 ‘‘lead-like’’ 6, 764, 778 Lemieux-Johnson reagent 311 Lewis acid catalysis 452 Lewis acids for catalysis 451 Li enolates 495 LiBH4 388 libraries from libraries concept 400 libraries of mixtures 15 libraries of saccharides 707 library design 745 library optimization 774 ligands 888 light-directed synthesis 180 lipase-catalyzed acetylation 955 lipases 294 Lipinski’s rule of five 726, 764, 776, 778 lipophilicity 754 liquid chromatography/mass spectrometry 15 liquid-liquid extraction 203, 208, 212, 200, 211 liquid-liquid phase separation 48 lithiation 237, 471 lithium aluminium hydride 127 lithium iodide 283 lithium organyles 328 lithographic pattering 1029 living free radical polymerization 37, 1054f loop scan 592 Losartan 868, 938 luciferase reporter gene 785 LUDI 773 luminescence 1021 luminescent materials 1038 M log P 726 MACCS-II Drug Data Report 765 macrocyclic ligands 888 macrocyclization 254, 265 macrolactones 105 macrolides 406 macroporous resins 28 MADLRE 65 magnetoresistant materials 1034 MAMP linker 66 Mannich reactions 325, 481, 521, 685, 687f, 895

manual synthesis 802 mass encoding 178 mass spectrometry 818 Matassa’s sulfonamide betain 250 material costs 837 materials discovery 1041 materials industries 1018 materials science 1017 matrix-assisted laser desorption 178 mean tissue concentration 749 mechanical stability of resins 33 medical implant materials 1051 medicinal chemistry 6, 10, 190, 786, 802 medium-throughput screening 869 Meissner fraction 1031 Meldrums’s acid 654 melting temperature 756f melting zone technique 1030 membrane affinity 754 membrane filtration 47 mercaptoacyl prolines 653 Merrifield resin 65, 633 metabolism 745, 752 meta-chloroperbenzoic acid 306 metalated aromatics 471 metalation 471 metal-ligation 1006 metallocenes 930 metalloproteins 891 metaloproteases 769 metastable phases 1026 metathesis reaction 97, 290, 588, 443 – ring-closing 587, 600, 629, 659, 662, 956 – ring-opening 603, 587, 595 met-enkephalin 770 methanol oxidation 975ff 4-methoxytrityl resin 68 2-methyl butyryl-CoA 1072 N-methylmorpholine N-oxide (NMO) cooxident 310 4-methyltrityl resin 68 methymycin 1077 Mettler Toledo Myriad Core System 208 Michael addition 280, 317, 503f, 523 – nitroalkanes 523 Michael enantioselective additions 505 microarray reactor 974 microchannel reactors 976 microliter plate-based positional encoding 181 microphotoluminescence 1025 microporous resins 31 microreactor 185 microreactor eight-channel 970

1107

1108

Index microwave 447 miniaturization 1035 Mitsunobu reaction 238, 248ff, 618, 253 ‘‘mix-and-split’’ type library 709 mixed anhydride reductions 392 modular peptide-based ligand 995 molecular diversity 5 molecular fingerprints 731 molecular frameworks 738 molecular imprinting 1050 molecular volume 744, 747 molecular-beam epitaxy 1022 molecular-layer epitaxy 1024 molecularly imprinted polymers (MIPs) 1050 monitoring 61 monolithic reactor 975 morphine 770 morphology 27 mRNA display 1085 Mukaiyama aldol reaction 545, 498, 503 Mukaiyama’s reagent 335 multidimensional screening approach 920, 923 multifunctional catalysts 993 multipins 5, 182 multiplasmid approach 1073 Multiple Core Structure Libraries 473 multisubstrate/one-catalyst screening 927 MultiSyn Tech Syro II 204, 794 multitube packed bed reactors 945 (dl)-muscone 632 NaBH4 392 NaCNBH3 392 nanocrystals 1028 NanoKan TM 637 natural product-like libraries 634, 637 natural products 444, 613, 697 neighborhood behavior 730 neighboring group participation 709 neomethymycin 1077 neural networks 729 neuromedins C and K 891 nitrile reduction 413 nitro group reductions 409, 419 nitro-aldol reaction 522 NMR spectroscopy 61f, 217 NaN bond reductions 413 NaO bond reductions 413 nonbiological induced fit 902 nonchelation-controlled diastereoselective ketone reductions 389 non-chemical encoding methods 180 noncrosslinked (soluble) polymers 24

nonlinear optical (NLO) materials 1045 non-positional encoding 183 nonribosomal peptides (NRPs) 1064 normal phase chromatography 212 Norrish type I reaction 292 NPE linkers 291 nuclear hormone receptors 766 nucleic acids 706 nucleotidyltransferases 1077 nylon 947 octahydrobenzisoxazoles 636 olefin cross-metathesis 710 olefin metathesis 585 oligomerization 447 oligonucleotide modifications 1082 oligonucleotide-encoded libraries 171 oligosaccharides 97, 138, 706, 718, 721 oligosaccharides, enzymatic synthesis 711 on-bead analytics 36 one bead-one concept compound 5 one-pot synthesis 717 online analysis 850 Oppolzer’s camphorsultam 234 optical band-gap 1031 optical microscopy 1030 optical screening 1030 oral absorption 752 orally bioavailable 725 organic light-emitting device (LED) 1020 organic polymeric materials 1019 organoboron reagents 532 organolithium reagents 124, 475 organolithium species 475 organomagnesium compounds 149 organomercurial compounds as radical precursors 233 organometallic reagents 75 organometallics 93 organotin reagents 543 organozinc bromides 473 orthogonality-concept 88, 710, 721 osmium tetroxide 309f osmotic shock 33 overalkylation 488 oxabutadienes 678 oxazaborolidine catalyst 428 oxazole 695 oxazolidines 104 oxazolidinone synthesis 253 oxazolidinones 104 oxazolines 695 oxazolones 296 oxidation 369, 377, 457, 857

Index – – – – – – –

of alcohols 369 of aldehydes 375 of CO 947 of methanol 948 of oximes 454 of phenoles 380 of polymer-bound sulfides to sulfoxides and sulfones 376 – of selenium- and phosphorus-containing compounds 378 – oxidation-cyclization strategy 371 oxidative – aromatization 379 – cleavage 311, 379, 456 – coupling and cleavage 380 – coupling reactions 369 – cyclization or condensation 379 – dehydrogenation 944 – dehydrogenation 965 – oxidative formation of heterocycles 379 – thioester 937 oxygenation 904 oxime ethers 234ff oxindoles 560 (G)-oxomaritidine 21 Oxone2 308 oxyethylene chains 43 ozonolysis 144, 312 paclitaxel 619 PAL linker 63, 65 Palladium catalysts 69, 115, 131, 273, 1012 parallel flash chromatography 212 parallel optimization equipment 847 parallel processing 14 parallelization in process development 838 Parallex TM System 214 partition coefficients 754, 761 Passerini reaction 696 patterned films 1024 Pbs linker 75 Pd-mediated regioselective oxidation 1012 PEI-based polymer 901 Penicillium chrysogenum 1075 pentamine ligands 911 pentenyl glycosides 707f Pepsin-K resins 41 peptide 4, 18 – aldehydes 371 – bond formation 1083 – libraries 1084 – nucleic acids 69 – synthesis 190 – tags 173

– functionalized oligomers 596 – phosphines 1009 – schiff bases 994 peptidyl transferase site 1085 peptoid libraries 5, 180, 252 pericyclic reactions 465 Perkin-Elmer Solaris 530 permeability 725, 744, 746 Petasis reaction 326, 481 pH sensors 960 pharmacokinetic modeling 752 pharmacokinetic profile 796 pharmacophores 453, 732, 743, 770, 764 pharmacophoric fingerprints 770 phase I transformations 753 phase II transformations 753 phase shifters 1036 phenanthridines 379 phenolic oxidation 380 Phloxine B 963 Phoc linker 102 phosphine oxazoline ligand 911, 920 phosphites 707, 709 phosphonium salts 115 phosphorescent materials 7 phosphoribosylanthranilate isomerase (PRAI) 1087 photocatalysts 969 photochemistry 91, 180, 243 photolabile linkers 80, 91 photolabile protection group 5 photolithography 5, 180, 1024 photoluminescence 1055 photolysis 91 photometric efficiency 1043 photo-sensitive linkers 710 photovoltaic materials 1028, 1031 pH-sensitive fluorophors 960 phytochelatins 1029 Pictet-Spengler reaction 275, 294, 329, 489 pin technique 5, 617, 912 piperazine 281 piperazinones 105 piperidine 655 pipiridine alkaloids 298 piperidone 289 plasma mass spectroscopy 892 plasma protein binding 758 plasma-enhanced chemical vapor deposition 1025 plasmepsin II 773 plasmids 1071 platinum catalysts 953 plug-flow reactor 973

1109

1110

Index poly(ethylen glycol)-copoly(N,N 0 dimethylacrylamide) (PEGA) 42 poly(N-acrylylpyrrolidine) (PAP) 42 poly(styrene-b-1-vinyl-alcohol) 50 poly(vinyl alcohol-b-1-vinyl-2-pyrrolidone) 50 poly(vinyl) pyridinium dichromate 371 polyacrylamide 50 polyacrylic acid 50 polycyclic structures 602 polyethylene 16 polyethylene glycol (PEG) 50 polyethylene glycol (PEG) grafts 35 polyethylene glycol (PEG) resins 35 polyfunctional linear polymeric supports 49 PolyHIPE 40 polyketide synthases (PKSs) 1064 polyketides 389, 495, 1065 polymer carrier (bead) 16 polymer properties 27 polymer-capture concept 488 polymeric tin iodide 429 polymerization 576 polymer-supported acrylates 445 polymer-supported aldehyde 283 polymer-supported allyl stannane 242 polymer-supported amide coupling reagents 350 polymer-supported ammonium cyanoborohydride 323, 415, 901 polymer-supported ammonium perchromate 371 polymer-supported aryl diazonium salts 274 polymer-supported arylboron reagents 540 polymer-supported arylstannane 45 544 polymer-supported base 263 polymer-supported BINAP 580 polymer-supported carbodiimides 335 polymer-supported catalysts 574, 422 polymer-supported catalysts 901 polymer-supported dienes 441 polymer-supported dienophiles 445 polymer-supported dihydropyridine-mediated reductions 420 polymer-supported isocyanates 351 polymer-supported palladium catalysts 579, 574 polymer-supported perruthenata (PPS) 370, 371, 373, 457 polymer-supported phenylselenyl bromide 314 polymer-supported phenylselenyl phthalimide 314 polymer-supported reagents 20, 238, 1018 polymer-supported reagents 6

polymer-supported reducing agents 323 polymer-supported ruthenium carbene complexes 589 polymer-supported scandium catalyst 325 polymer-supported Schiff base catalysts 914 polymer-supported sulfide ozonide reductions 421 polymer-supported tinhydrides 239, 419 polymer-supported triphenylphosphine 359 polymer-supported triphenylphosphines for the reduction of azides 421 polymer-supported ylide 629 polymer-swelling/shrinking characteristics 16 polymerization 934 polynorbornene ROMF gel 50 polyolefins 930 polyoxometalate libraries 869, 935 polyoxyethylene-polyoxethane (SPOCC) 45 polyoxyethylene-polyoxypropylene (POEPOP) 44 polyoxyethylene-polystyrene (POEPS-3) 45 polypropylene 16 polypropylene mesh containers 5 polypropylene oxide (PPO) 50 polystyrene 16, 35, 38, 39, 50 polystyrene-trimethyl ammonium (TMA) latex 902 polyvinylalcohol 50 positional encoding 170, 180 positional scanning approach 913 position-sensitive photodiode 965 potassium ferricyanide 310 precipitation 47f preformed linker concept 61 primary screening libraries 730 privileged structures 636, 735, 774, 762 process characterization 834, 849 process development 831, 864f process discovery 867 process hazards analysis 882 process optimization 834, 847, 867 process screening 834, 841 process validation 834, 849, 869 processing conditions 1020 product-based design 734 product-based selection 774 propionyl-CoA 1072 prostaglandins 71, 482, 626, 543, 481 protection groups 5, 62, 290, 88, 710, 721, 20, 71, 68, 291, 706 protein catalysts 7, 1084 protein families 766 protein kinase inhibitors 618

Index Prussian blue 963 pseudo dilution 16 pulsed laser ablation 1022 purine 617 purine-based library 256 purity of combinatorial libraries 217, 806, 210, 15 puromycin 1085 pyran linker 627 pyrazoles 108, 462, 789, 791 pyrazolones 104, 521 pyridazines 379 pyridines 131, 379, 661 pyridones 657 pyrimidines 379, 258 pyrrole synthesis 299 pyrroles 462, 655, 695, 699 pyrrolidines 235, 458, 651 QSAR models 771 Quad3 TM from Biotage 213 quantitative structure-activity relationship (QSAR) 745, 1029 quartz wafer 945 quaternary masking 1023 quaternization 281, 283 quinazolinediones 104 quinazolinones 379 quinoline N-oxide 674 quinolines 672 quinolinones 104, 671 quinolones 261 quinone reductions 399 quinones 147 quinoxalinones 131 radical cyclization 678 radiofrequency (RF) encoding 618, 18, 802, 633 radiofrequency tags 183 radioimmunodetection (RIAD) 893 ramage linker 66 random libraries 743, 765, 1081 random selection 734 rasta silanes 37 rational design 4, 1089 – of a catalyst 1012 Rauwolfa alkaloids 615 reactant-based design 734 reactant-biased algorithm 734 REACTarray TM 871 reaction of CS in sp systems 339 reaction – kinetics 36, 61, 103 reactive intermediates 151

reagent-based selection 774 rearrangement ‘‘fingerprints’’ 822 receptor-assisted combinatorial synthesis 590 reduction 387, 789 reduction electrochemical 951 reduction of alkenes 316 reduction of alkynes 316 reduction of amides 399, 430 reduction of disulfide 396, 419 reduction of imines/enamines 361 reduction of sulfones 396 reduction of sulfurs 396 reduction of thioesters 393 reduction-chelation-controlled 389 reduction-enantioselective 415 reductions diastereoselective ketone 389 reductions of epoxides 418 reductions of esters 390 reductions of ketones 388, 417 reductions of nitriles 413 reductions of nitro groups 409, 429 reductions of NaN bonds 413 reductions of NaO bonds 413 reductions of quinones 399 reductive alkylation 280, 323, 448f, 792 reductive amination 401, 403, 416, 430 reductive cleavage 393 reductive cyclizations 419 reductive dehalogenations 428 reductive hydrocoupling 952 reductive hydrodeselenation 314 regenerative fuel cells 963 Reissert intermediates 328, 108 Reissert-type reaction 277 REM resins 126, 281 REMPI 940, 944, 964 repository 808 resin capture strategy 297 resin ylide 621 resin-supported MOP ligand 580 resolution of racemates 929 resonance-enhanced multiphoton ionization (REMPI) 940 retro-aldol reaction 493 retro-Diels-Alder reaction 444 retro-Michael reaction 80 retro-Michael addition reaction 131 return on investment 1018 reverse solid-phase peptide synthesis 64 reversed-phase high-performance liquid chromatography (HPLC) 176 rhodium(II) catalysts 300, 927 Richter reaction 86 rifgid descriptors 732

1111

1112

Index ring-closing metathesis 587, 600, 629, 659, 662 ring-closing reactions 254 ring-opening cross-metathesis 603 ring-opening metathesis polymerization (ROMP) 587, 595 Rink resin 136, 123, 67 RNA aptamers 1082 RNA combinatorial libraries 1083 robot-arm-based systems 190 ROMP spheres 36 ROMP-based polymers 51 route scouting 834, 840, 869, 869 RP-HPLC 214 Rutherford backscattering spectroscopy 1035 SAC linker 66, 75 Saccharopolyspora erythraea 1079 safety-catch linkers 73, 77, 80, 90, 99, 624 safety-catch procedure 444 safety-catch resin 282 Sakurai reaction 653, 327 SAL linker 66, 75 salicylaldiminato ligands 934 samarium iodide 229, 284 sample logistics 219 Sandmeyer reaction 129 sarcodictyns 375, 620 SASRIN resin 63, 650, 65 Savant vacuum centrifuge 222 Sc(OTf )3 502, 505, 521 SCAL (safety-catch) 67 scanning lectron microscope (SEM) 33 scanning electrochemical microscope 952 scanning evanescent microwave microscope (SEMM) 1031 scanning-tip microwave near-field microscope 1036 scavenger resins 20, 63, 212, 452 Schiff base ligands 908 screening intensity 839 screening multisubstrate/one catalyst 927 screening strategies 851 screening technologies 1017 secondary amine binary coding 175 selection algorithms 732 selenenyl bromide 623 selenides 237, 286 selenium 77 selenium bromide 636 selenium reductions 397 selenium-based linkers 81, 398 selenol 293 selenoxides 286

self metathesis 587, 589 self-ligating ribozymes 1083 self-similarity 763 SEM linker 75, 83, 73, 144 semiconductor gas sensors 972 sensing chemistry 894 serial processing 14 serine proteases 769 Sharpless asymmetric dihydroxylation 311 shelf life characterization 806 shikimic acid 455, 635 SiaH insertion reaction 926 sialyl trimetric Lewis X 713 sigmatropic rearrangements 465 silyl amide linkers (SAL) 289 silyl enol ethers 441, 498, 505f, 520f silyl ketene acetals 505, 525 silyl linkers 73 silylated resins 73 similarity 770 similarity radius 734 similarity searching 763 single compounds 3 size exclusion chromatography 47 sol-gel processing 1026, 940 solid-phase Bischler-Napieralski reaction 277 solid-phase extraction (SPE) 203, 208, 211, 908 solid-phase Friedel-Crafts acylation 272 solid-phase iodination 274 solid-phase Mitsunobu reaction 253 solid-phase phenolate alkylation 250 solid-phase Williamson ether synthesis 249 solid-state materials 1019 solubility 727, 744, 748f, 755 soluble polymeric supports 46 soluble polystyrene polymer 626 solution deposition methods 1022 Sonogashira reaction 329, 544, 688, 297 spacer group 61 spatial encoding 170 spatially addressable electrolysis platform (SAEP) 952 spin field effect transistors 1033 split-and-combine concept 4, 5, 17, 176, 886 sporochnol 1009 sputtering 1022 srfA-A 1075 stannane-based linkers 81, 77 Staphylococcus auerus 623 statistical design 852 Staudinger reaction 407, 463, 650 stereoselective alkylations 510 – enolates 510

Index steric hindrance 798 Stille reactions 81, 95, 104, 273, 543, 630 Strecker reaction 914, 1001 Streptomyces argillaceus 1079 Streptomyces fradiae 1079 Streptomyces lividans 1079 Streptomyces olivaceus 1079 structural fragment keys 731 Structure-Activity-Relationship (SAR) 795, 802, 374, 743, 785, 801 Structure-Activity-Relationship (SAR) by NMR 773 structure-property relationships 752 structure-stability relationships 806 Stuttgart Neural Network Simulator (SNNS) 766 substrate-controlled reduction 389 substructure analysis 738 sugar-based auxiliary 232 suicide inhibitor 307 sulfone elimination reaction 288 sulfur reductions 396 sulfur-based linkers 77 b-sultams 464 super high-throughput screening 966 superacylation catalysts PPY 962 superconducting 1025 superconducting materials 7 surface proteins 1082 surface-enhanced Raman scattering (SERS) 1028 surfactin A 1075 surrogate analytes 616 Surveyor TM 875 Suzuki 316 Suzuki reaction 95, 273, 868, 909, 960, 316, 76, 898, 79 Suzuki-Miyaura reaction 532 Swern oxidation 372, 370 syndiotactic polypropylene 934 syn-elimination 286 synthesis of b-lactams 143 synthesis of cyclopeptides 137 synthesis of six-member heterocycles 450 synthetic enzyme catalysts (‘‘synzyme’’) 899 synthon 289 T1 linker 84 T1 triazene linker 95, 522 T2 linker 84 Tamura tandem reaction 455 tandem Mannich-Michael mechanism 325 tandem Mannich-Michael reaction 325, 480 Tanimoto coefficient 732, 763

target family 765 targeted libraries 730 Taxol TM 629 Tayol TM 619 tea-bag strategy 5, 183 Tebbe olefination reaction 356, 443 tentagel resin 61, 16, 35 ternary composition diagram 1021 tert-butyl hydroperoxide 310 tetracenomycins 1079 tetrahydrocarbolines 104 tetrahydrofurans 675 tetrahydroisoquinolines 330, 674 tetrahydropyranol linker 907 tetrahydropyridines 131 tetrahydroquinolines 668 tetramic acids 517, 653 tetrazoles 130, 694 thermal decomposition 808 thermal evaporation 1022 thermally sensitive resistor 970 thiazoles 104, 338, 695 thiazole-containing combinatorial biosynthesis libraries 1075 thiazoline 695 thin-film deposition 1022, 1025, 939, 1020 thin-layer organic light-emitting devices (OLEDs) 1042 thioester reductions 393 thioether oxidation 937 thioglycosides 707ff, 718 thiohydantoins 104 thiohydroxamates 243 thioketene silyl acetals 502 thio-oligosaccharides 396 thiophenes 678 thiosilyl ketene acetal 521 THP linker 70, 130 THP-type linkers 144 three-component condensation 701 three-dimensional fingerprint keys 763 thrombin 773 Ti-catalyzed cyanide additions 1000 tin-mediated nitro reductions 409 Ti-peptide complexes 1000 Toray carbon 962 Toray paper 949 tosyl radicals 233 traceless cleavage 656 traceless linkers 19, 75, 90, 110, 237, 280, 659, 662, 676, 737, 81 Tramadol2 126 transesterification 71, 134, 282 transfer hydrogenations 424

1113

1114

Index transketalization 623 transmembrane ionophores 888 transmetalation reaction 483 transporters 726 b-trialkylsilyl 289 triangular libraries 947 triazene-based linkers 83, 115 triazine template 256 trichloroacetimidate 708f tridentate scaffold 889 triethylborane 234f 6-endo-trig cyclization 286 5-exo-trig cyclization 295 trimethylorthoformate 401, 788 (trimethylsilyl)acetylene 551 2-(trimethylsilyl)ethylester 290 triose phosphate isomerase (TIM) trityl resins 64, 67 tropene 566 tryptophan biosynthesis pathway turnable resonators 1036 b-turn mimetic library 396, 599 b-turn structural motif 911 two-component reactions 685 two-dimensional adressable arrays two-dimensional fingerprint keys two-resin system 284 tylosin pathway 1078

very-large-scale immobilized polymer synthesis 180 vicinal diols 309 vinyl cuprates 482 vinyl sulfone 282 vinyl triflates 666 vinylogous systems 296 vinylsulfones 456 vitamin D3 633

1089

1087

1022 762

UDP-galactose 714 Ugi reactions 409, 599, 601, 697, 699, 299, 461, 487 Ullmann aryl ether-forming reactions 925 ultrafiltration 48 ultrasound 85 g,d-unsaturated acids 231 a,b-unsaturated ketones 441, 494, 503 urazines 451 urdamycin A 1079 UV lasers 964 UV-excited photoluminescence 1030 vacuum centrifuges 221 vancomycin 69, 110, 590, 623f vapor deposition techniques 1022 Varian-HPLC 215

Wacker oxidation 1012 Wang linker 65 Wang resin 63 Waters Micromass 216 wavelength-divisible multiplexing (WDM) 1025 Weinreb amide linker 487 Weinreb amide reductions 394 Weinreb amides 144, 485 well reactors 938 wolf-lamb strategy 515 workstations 842 work-up 838 World Drug Index (WDI) 729, 777 xanthates 708 2-xanthates 709 X-ray microdiffraction 1027 Yb resin 298 Yb(OTf )3 503, 657, 670 yield determination 200 yohimbinic acid 615 ytterbium triflate 451f, 456 zearalenone 474 (S)-zearalenone 630 Zenyx Magellan synthesizer 205 zeolite synthesis 1026 zinc borohydride polymers 416 zinc organyles 328 Zinsser Sophas 206 zirconium borohydride polymers 416 Zn enolates 494, 526 zoom screens 949 Zr-catalyzed 1004, 1006 zymark 199

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