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Advances in Carbohydrate Chemistry and Biochemistry

Volume 53

This Page Intentionally Left Blank

Advances in Carbohydrate Chemistry and Biochemistry Editor DEREK HORTON The American University Washington, DC

Board of Advisors LAURENS ANDERSON DAVIDR. BUNDLE STEPHEN J. ANGYAL STEPHEN HANESSIAN BENGT LINDBERG HANSH. BAER CLINTON E. BALLOU HANSPAULSEN NATHANSHARON JOHNS. BRIMACOMBE J. F. G. VLIEGENTHART J. GRANT BUCHANAN ROYL. WHISTLER

Volume 53

ACADEMIC PRESS San Diego London Boston New York Sydney Tokyo Toronto

This book is printed on acid-free paper. @ Copyright 0 1998 by ACADEMIC PRESS All Rights Reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information storage and retrieval system, without permission in writing from the Publisher. The appearance of the code at the bottom of the first page of a chapter in this book indicates the Publisher’s consent that copies of the chapter may be made for personal or internal use of specific clients. This consent is given on the condition, however, that the copier pay the stated per copy fee through the Copyright Clearance Center, Inc. (222 Rosewood Drive, Danvers, Massachusetts 01923), for copying beyond that permitted by Sections 107 or 108 of the US.Copyright Law. This consent does not extend to other kinds of copying, such as copying for general distribution, for advertising or promotional purposes, for creating new collective works, or for resale. Copy fees for pre-1998 chapters are as shown on the title pages. If no fee code appears on the title page, the copy fee is the same as for current chapters. 0065-23 18/98 $25.00

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CONTENTS PREFACE .............................................................

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John E Hodge. 1914-19% MILTONS. FEATHER Text

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Allene R Jeanes. 1906-1995

NINA M. ROSCHER AND PAULA . SANDFORD Text ................................................................

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Harriet L Frush. 1903-1996 HASSAN S. EL KHADEM Text ................................................................

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Applications of Tin-Containing Intermediates to Carbohydrate Chemistry T. BRUCEGRINDLEY I. I1. Ill . IV. V. VI . VI1. VIII .

Introduction .................................................... Nomenclature .................................................. Preparation of Tin-Containing Intermediates Physical Methods for the Study of Organotin Derivatives Structures ..................................................... Reaction Types and Conditions Factors That Influence Reaction Regioselectivity ....................... Trends in Regioselectivity ......................................... References

.......................... ................ .....................................

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17 18 18 19 25 32 33

44 133

Synthetic Applications of Selenium-ContainingSugars ZBIGNIEW J . WITCZAK AND STANISLAS CZERNECKI 1. Introduction

....................................................

I1. Preparations of Seleno Sugar Derivatives ............................. V

143 145

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CONTENTS

............................. ..................................................... References .....................................................

111. Application of Seleno Sugars in Synthesis IV . Conclusion

167 193 195

Anti-Carbohydrate Antibodies with Specificity for Monosaccharide and Oligosaccharide Units of Antigens JOHNH . PAZUR 1. Introduction .................................................... I1. Analytical Methods .............................................. 111. Preparation of Antigens Containing Carbohydrate Residues .............. IV . Immunization Procedure .......................................... V . Preparation and Properties of Anti-carbohydrate Antibodies .............. VI . Conclusions .................................................... References .....................................................

201 203 209 212 213 254 258

Complexes of Starch with Inorganic Guests PIOTRTOMASIK AND CHRISTOPHER H . SCHILLING

I . Introduction .................................................... I1. The Starch-Iodine Complex .......................................

......................................... References .....................................................

111. Starch-Water Complexes IV . Starch Complexes with Other Nonmetallic Guests ......................

263 264 298 312 328

Complexes of Starch with Organic Guests ROTR TOMASIK AND CHRISTOPHER H . SCHILLING V . Introduction Preparation Methods Complexes with Aromas and Flavoring Agents ........................ Complexes with Dyes Complexes with Lipids Starch Complexes with Mono- and Oligosaccharides Starch Complexes with Macromolecules References

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346 350 352 376 385 400 405 414

AUTHOR INDEX....................................................... SUBJECT INDEX ........................................................

427 461

VI . VII . VIII . IX . X. XI .

Synthetic work in the carbohydrate field has greatly benefited from new developments in protecting-group strategy and from the introduction of reagents based on heavier elements in the periodic table. Sugars are ideal models for the evaluation of reagents that can specifically bridge two or more oxygen sites. Developments during the past two decades have demonstrated the great value of organotin compounds, especially trialkylstannyl ethers and dialkylstannylene acetals, for effecting useful, high-yielding transformations with high regioselectivity. This volume of Advances features a comprehensive survey, by Grindley (Halifax, Nova Scotia), on the application of tin-containing intermediates to carbohydrate chemistry. In a comparable vein, the element selenium has found important applications in effecting organic transformations. The chapter by Witczak (Storrs, Connecticut) and Czernecki (Paris) on the synthetic applications of selenium-containing sugars surveys key aspects of this still rapidly evolving area. Much of the older work largely parallels the chemistry of the sulfur analogues, but newer developments show novel and versatile potential, particularly in the use of arylselenium reagents for stereochemically controlled addition to glycals and in transformations at the anomeric center. Molecular recognition between carbohydrates and proteins has become a vast area of broad significance in biochemistry and molecular biology. Anti-carbohydrate antibodies have importance in both medicine and technology. The preparation of protein antibodies having specificity for carbohydrate antigens requires application of a range of specialized techniques in molecular separation, the conjugation of carbohydrate ligands to suitable carriers for immunizing animals, and the isolation of pure antibodies from hyperimmune sera. In this volume Pazur (University Park, Pennsylvania), a seasoned Advances author, contributes an article on anti-carbohydrate antibodies with specificity for monosaccharide and oligosaccharide units of antigens. He provides a practical focus on experimental methodology in preparing antibodies having specificity for various sugars and polysaccharides, with many examples developed in his own laboratory. It has been almost two centuries since the first reports on the interaction between iodine and starch to produce a blue color, and more than fifty years since the discovery of the complexation between aliphatic alcohols and starch that revealed this polysaccharide to have a linear and a branched component. A large and diffuse body of published work exists on the interaction of starch and its components with other molecules and with ionic substances. Two closely related chapters in this volume, by Tomasik (Cracow, Poland) and Schilling (Ames, Iowa), provide comprehensive accounts of the extensive literature on complexes of starch with inorganic and with organic guests. vii

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PREFACE

Researchers working in industrial or government laboratories often have to rely on their individual experimental efforts in the laboratory and have frequently received less recognition for their accomplishments than have leading scientists in academia, whose work is usually based on extensive experimental work by teams of research students. The three individuals whose lives and scientific contributions are recalled in this volume all made notable discoveries in the carbohydrate field while working in government laboratories and conducting their own experimental work. John E. Hodge, whose career is reviewed by Feather (Columbia, Missouri), laid the major foundations of the chemical basis of the nonenzymatic browning (Maillard) reaction. In the same Peoria laboratory of the United States Department of Agriculture, Allene Jeanes, whose biography is presented by Roscher (Washington, DC) and Sandford (Santa Monica, California), conducted pioneering work on microbial polysaccharides that laid the basis for the commercial development of dextran and xanthan. The memoir contributed by El Khadem (Washington, DC) details the career of Harriet L. Frush, whose teamwork with her mentor, Horace S. Isbell, at the US. National Bureau of Standards and later at The American University in Washington, DC, spanned over sixty years and included seminal contributions in concepts of reaction mechanisms and on the synthesis of labeled sugars. With this volume we welcome David R. Bundle to the Board of Advisors and look forward to valuable input from the prestigious carbohydrate laboratory in Edmonton, Alberta. With regret we record the death, on April 16, 1997, of Elizabeth Percival, a noted authority on marine algal polysaccharides, and on October 14, 1997, of Iqbal R. Siddiqui, who authored the article “Sugars of Honey” in Volume 25 of this series. Washington, DC February 1998

DEREKHORTON


ADVANCES IN CARBOHYDRATE CHEMISTRY AND BIOCHEMISTRY. VOL. 53

JOHN E. HODGE 1914-1 996 John Edward Hodge, a distinguished carbohydrate chemist and prominent member of the African-American community, died of cancer on January 3, 1996. John was internationally known, primarily for his work in the area of nonenzymatic browning (the Maillard reaction), which was carried out during his years of service at the United States Department of Agriculture’s Northern Regional Research Laboratories in Peoria, Illinois, where he served on the staff for more than 40 years. John was born in 1914 in Kansas City, Kansas, and spent his early years in that area, before moving to Peoria in 1941. He and his younger sister Dorothy grew up in an atmosphere of learning in the home of their parents, John Alfred and Annabelle Hodge. John Alfred Hodge came to Kansas City from Indiana University in 1910 to teach science at Sumner High School, armed with a newly awarded Master of Science degree in physics. He taught science at Sumner from 1910 until 1916, and then was promoted to the position of principal, remaining in that position for the next 35 years. This was in the days of racial segregation, both in Kansas and the neighboring state of Missouri. Sumner High School, with John Alfred Hodge at the helm, developed a reputation as a serious school with high expectations, accepting only the very best students from the AfricanAmerican community. He was a powerful influence on his own children, and, academically, expected much from both John and his sister. Neither child would be a disappointment in that regard. During childhood, serious reading and science were topics of interest at home. Among some of young John’s interests were the building of model airplanes from balsa wood, and the construction of early radios with his father. John was also interested, as many scientists are, in solving puzzles and playing games of chance. He was well known by contemporaries as an efficient solver of crossword puzzles, and was an accomplished card and checker player, as well as a billiards player. In addition, John was an accomplished musician who played the piano and trumpet, which led to his lifelong interest, an appreciation and love of jazz music. John attended Sumner High School and then enrolled as an undergraduate student at the University of Kansas in nearby Lawrence, Kansas. H e Wfi5-231819R $25.00

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MILTON S. FEATHER

graduated with a degree in mathematics in 1936, having been elected to the prestigious Phi Beta Kappa academic honorary society, as well as the Pi Mu Epsilon honorary mathematics society. During the period from 1936 to 1940, he was employed as a teacher at Western University in Quindaro, Kansas, and as a chemist in the Kansas Department of Inspections, while pursuing advanced studies at the University of Kansas. In 1939, John married Beulah Payne, a fellow chemistry student from St. Louis, who was also enrolled at the University of Kansas. Beulah died tragically in 1942, leaving John with a son, John Laurent Hodge. In 1940, he was awarded an M.S. degree from the University of Kansas in the field of organic chemistry. In 1948 he remarried, to Justina Louise Williams, of Kewanee, Illinois. In the 1930s and 1940s in the USA, it was not an easy task even for a well-educated and intelligent black professional to find a way in life, but John was persistent, always having faith in his abilities and intellect. His opportunity came with the entry of the USA into World War 11, which stimulated considerable development of the national scientific infrastructure, and, with this development, jobs for scientists. The United States Department of Agriculture (USDA) opened four large laboratories in the United States, and John was successful in competing for a position in one of these, the USDA Northern Regional Research Laboratory, in Peoria, Illinois. This laboratory, when it initially opened, had the rather broad mandate to conduct research on agricultural crops that grew in the U.S. heartland, and to develop new products from these crops. John immediately threw himself into research on corn starch and related carbohydrates, and became a self-taught carbohydrate chemist. His initial studies were in the area of the production of D-glucose from corn starch and related technologies. This led to an interest in the interactions between glucose and amino groups, a degradative reaction that poses considerable problems (including loss of sugar) in the production of glucose based on the corn wet-millingprocess. These reactions (the Maillard reaction) became a major research interest from John for the remainder of his life. John was particularly interested in pyrolysis reactions and their role in the production of food flavors and aromas. With his typical thoroughness, he initiated a program that involved the synthesis of Amadori compounds (l-amino-ldeoxyfructose derivatives), since it was commonly considered that these are the first intermediates formed during the Maillard reaction and serve as precursors of many of the degradation products that are known to be formed. He was able to prepare a number of these compounds, in crystalline from, by reacting piperidine with glucose, as well as with maltose and lactose, and he used these products in many of his pyrolysis experiments. Out of this research came a number of new findings. He was able to demonstrate that isomaltol, a bakery aroma constituent, is directly produced

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from an Amadori compound and that it is, in fact, a furan derivative (2acetyl-3-hydroxyfuran).Isomaltol was first isolated in Germany in the late 1800s from the effluent from a bakery flue, and had been assigned a pyrone structure. John and his colleagues also were able to show that “piperidino lactulose,” an Amadori compound formed by reacting lactose with piperidine, undergoes pyrolysis to give galactosylisomaltol, a fact that provided confirming data on the mechanism of isomaltol formation from sugars via Amadori compounds. These Amadori compounds were also instrumental in his work on the mechanism for formation of maltol(3-hydroxy-2-methyl4H-pyran-4-one), a sugar-derived pyrone that is also a flavor and aroma constituent, often found in baked cereal products. John also investigated the reductones that are produced during the Maillard reaction, in collaboration with Professor Friedrich Weygand of the University of Munich. Together, these two scientists carried out numerous investigations on the mechanism of their formation. One of Hodge’s reviews, which appeared in the Brewing Chemists Society Proceedings in the 195Os,became a “citation classic,” and the scheme proposed in it for reductone and color formation during the Maillard reaction remains a widely cited publication today, more than 40 years after its publication. During his tenure at the Northern Regional Research Laboratory, John received numerous awards and recognitions. He served as the chairman of the American Chemical Society’s Division of Carbohydrate Chemistry, and he received a number of USDA awards, including a Superior Service Award in 1953.John served as an adjunct professor of chemistry at Bradley University in Peoria and taught a course in carbohydrate chemistry at that institution for a number of years. In 1972, he was designated a Visiting Professor at the University of Campinas in Silo Paulo, Brazil, where he spent a semester teaching carbohydrate chemistry and interacting with graduate students at that institution. Later, as one of the pioneers in his field of research, John was honored at the NIH Conference on the Maillard Reaction in Aging, Diabetes, and Nutrition, held in Bethesda, Maryland, in 1988. John was an engaging man of quiet temperament but firm determination. He was active in the African-American community and served as a role model for many aspiring African-American scientists. He had the respect of both black and white Americans. He was elected to Who’s Who among Black Americans in 1980, and served on a number of local committees in Peoria, including the Board of Directors of the Carver Community Center, the Citizens’ Committee for Peoria Public Schools, and the Central Illinois Agency on Aging. He was outspoken against the abolition of competency tests and the lowering of academic standards. John is survived by his wife, Justina Hodge, of Peoria; two sons, John Laurent of Brookline, Massachusetts, and Jay Mitchell of Peoria; two

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daughters, Judith Ann Dunmore of Milton, Massachusetts, and Justina Louise Mitchell of Washington, D.C.; and one sister, Dorothy Johnson of Kansas City, Missouri. This writer is deeply indebted to and acknowledges the following persons for their help in the preparation of this article: Mrs. Dorothy Johnson (sister) of Kansas City, Missouri, for a number of helpful discussions relative to John’s childhood and early life; Mrs. Justina Hodge (widow), of Peoria, Illinois, for access to archival materials; and Dr. William Doane (colleague) of Peoria, Illinois, for detailed information on John’s professional life while he was on the staff at the Northern Regional Research Laboratories in Peoria. MILTON S. FEATHER


ADVANCES IN CARBOHYDRATE CHEMISTRY AND BIOCHEMISTRY, VOL. 53

ALLENE R. JEANES 1906-1995 On December 11,1995, Allene Rosalind Jeanes passed away in Urbana, Illinois. No one who had met her will easily forget her. Dr. Jeanes, as she was usually called by her colleagues, will be remembered as an extremely serious and dedicated person who devoted most of her professional career to the characterization of dextran and xanthan, two microbial polysaccharides of exceptional practical importance. She was born in Waco, Texas, on July 19, 1906, to Largus Elonzo and Viola Herring Jeanes. She and her older brother, Perry, grew up in Waco. Their father was a switchman and later yardmaster for the Cotton Belt Route of the St. Louis Southwestern Railroad. Allene graduated “with honors” from Wac0 High School in 1924. In 1928, she graduated summa cum laude from Baylor University with the A.B. degree in chemistry. Next, she went to the University of California, Berkeley, where in 1929 she obtained the M.A. degree in organic chemistry. After a brief time as a high-school mathematics teacher in Laredo, Texas, she taught chemistry and biology from 1930 to 1935 at Athens College, Alabama, and was head of the Science Department. In 1936, Allene Jeanes’ yearning for research led her to graduate studies at the University of Illinois (Champaign-Urbana), where she studied in the laboratory of the famous organic chemist Roger Adams, majoring in chemistry with a minor in biochemistry. She was an instructor in the Chemistry Department (1936-1937) and later Chemical Foundation Fellow (19371938). In 1938 she was awarded the Ph.D. degree in organic chemistry. Fortunately for carbohydrate chemistry, and with Prof. Adams’ insistence, Dr. Jeanes decided to accept a position at the National Institutes of Health (NIH) in Washington D.C., in the laboratory of the celebrated carbohydrate pioneer Claude S. Hudson. Her support came from one of the first Fellowships of the Corn Industries Research Foundation. With Hudson she developed methods using periodate oxidation to determine the structure of starches, marking the start of her long fascination with carbohydrate polymers. Dr. Jeanes then continued her studies of carbohydrates with Dr. Horace Isbell at the National Bureau of Standards (now the National Institute of Standards and Technology). WS-2318198 $25.W

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In early 1941, Dr. Jeanes moved back to Illinois, this time to Peoria, just three months after the opening of the US. Department of Agriculture’s Northern Regional Research Laboratory (NRRL, later NRRC, and now the National Center for Agricultural Utilization). She continued to work there even after her retirement in 1976. Her more than 35 years of research at NRRL resulted in two major scientific and industrial developments, dextran and xanthan, and these are described in more than 60 publications, 24 presentations, and 10 patents. Her early endeavors to characterize the branch points in starch, and pursuit of the then-elusive a-(1 + 6) linkage in starch, drew Dr. Jeanes to dextrans, since these polysaccharides contain a-(1 + 6) linkages as their main structural feature. Dextrans are a family of D-glucans produced microbially from sucrose; they contain from 50 to 100% a-(1 + 6) linkages, depending on the microbial strain used. Dr. Jeanes became an authority on dextran sources, structures, and industrial applications. She published comprehensive bibliographies on dextran, the first in 1950 and another in 1978. These were a labor of love, produced with much effort before the days of automated data retrieval. A dextran-producing strain (NRRL B-512) of Leuconostoc mesenteroides was isolated from unpasteurized root beer in Peoria and brought to NRRL for characterization. This strain later became the source of a substrain, B512(F), that would be used widely to produce dextran for such pharmaceutical applications as “clinical dextran,” a blood-plasma extender. Crosslinked B-512 dextrans constitute the familiar Sephadex packing materials used for size-exclusion column-chromatographic separations, and dextrans are commonly used as molecular-weight standards in liquid chromatography. In 1950, during the time of the Korean conflict, Dr. Jeanes’ initial research with dextran formed the basis of a multidisciplinary crash program with 80 scientists that was started at NRRL to develop a blood-plasma substitute based on dextran. Dr. Jeanes and her immediate research group played an important role in the isolation and characterization of dextrans from more than 100 bacterial strains. Dr. Jeanes was a member of the Subcommittee on Plasma and Plasma Substitutes of the Medical Division, National Academy of Sciences National Research Council, from 1962 until 1968. In 1953, USDA’s highest award, the Distinguished Service Award, was given to Dr. Jeanes, and NRRL’s Dextran Team received the award in 1956. The American Chemical Society awarded her the Garvan Medal in 1956. Dr. Jeanes received the 1962 U.S. Civil Service Commission’s Federal Women’s Award, and in 1968, her alma mater, Baylor University, conferred on her the Outstanding Alumna Award. Based on the success with dextran, Dr. Jeanes and her colleagues started exploring other microbes as agents for producing useful polysaccharides.

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At NRRL the goals were first to find new outlets for cereal starches by using bacteria and fungi capable of converting them into new types of extracellular polysaccharides (gums), and second to develop domestic sources of thickening, suspending, and stabilizing agents to replace imported polysaccharide gums. At the start of this project about 30 microbial strains were identified as potential candidates. After small quantities of each were isolated for characterization, the 15 most interesting candidates were extensively characterized, both in terms of structure and rheological properties. The bacterial polysaccharide xanthan (Dr. Jeanes disliked the term “xanthan gum”), the second candidate on this screening list, produced by the plant pathogen Xunrhomonus campesrris NRRL B-1459 growing on a Dglucose medium, became the second blockbuster product to be commercialized based on the research of Dr. Jeanes’ team. Dr. Jeanes’ biopolymer team literally taught the fermentation giants of the world how to make xanthan, thus starting a new industry-microbial polysaccharides. Xanthan became the model of how to produce other microbial polysaccharides. Dr. Jeanes’ work demonstrated that the properties of each polysaccharide derive from its monosaccharide composition and linkage arrangements. Xanthan, used as a thickening agent in a myriad of food and nonfood applications, is currently manufactured in vast quantities by several companies worldwide. In 1968, the Biopolymer Research Team at NRRL was given the USDA Superior Service Award, based on their work that resulted in the commercialization of xanthan. Many people will remember Allene Jeanes as a rather thin, somewhat frail, gray- to white-haired woman who was soft-spoken but very intense and deliberate in her mannerisms. She never married. Woe to the person who thought she, being a woman, should be granted a lesser status as a researcher. She overcame with determination the many obstacles placed in the path of women researchers of her day. Her attitude was not one of boasting, but rather of sound reasoning and thorough preparation. She was extremely patient both with her colleagues and with the numerous visitors who came to her laboratory seeking detailed information about her polysaccharide research. She would spend as much time as desired with anyone willing to learn about her favorite polysaccharides. Some of her colleagues thought she spent too much time planning experiments and then finally writing and rewriting manuscripts. However, any co-experimenter or coauthor soon learned to pay attention to details. As for members of her team, she would fight for their rights and would try to advance their professional careers as much as possible, but her standards were high. Dr. Jeanes’ patience and dedication to details allowed her to make significant contributions to carbohydrate chemistry and biochemistry. She was a true pioneer in the use of microbial polysaccharides for industrial applications. In 1978

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she was recognized by research colleagues and friends worldwide who dedicated papers to her in a special issue of the journal Carbohydrate Research. Dr. Jeanes’ last published paper, “Immunological and Related Interactions with Dextrans Reviewed in Terms of Improved Structural Information,” was published in 1986 (Molecular Immunology, 23, 999-1028), 10 years after her “official retirement” and only 9 years before her death. It demonstrates the dedication this lovely woman had for her first lovedextran.

NINAM. ROSCHER PAULA. SANDFORD



HARRIET L. FRUSH* 1903- 1996

Harriet Louise Frush was one of the pioneering women scientists who entered the field of carbohydrate chemistry in the 1920s and with perseverance and hard work was able to succeed. At that time there were few women chemists in U.S. Government agencies, and hardly any on the faculty of most universities. Her career was not easy; to be accepted by her peers she had to prove herself in research, yet when she had done so, her position was terminated. Although later reinstated, she was overshadowed by her co-workers and never quite received the recognition she deserved. This memoir draws attention to her many accomplishments in the carbohydrate field. Harriet L. Frush, the daughter of Frank and Rose (Miller) Frush, was born on July 3, 1903, in Knoxville, Iowa, a little town southeast of Des Moines. When she was 3 years old, her parents moved to nearby Pella, Iowa, where she grew up and went to school. An excellent student, she was generally at the top of her class. In 1924 Harriet received a Bachelor of Science degree from Central College in Pella, Iowa, and enrolled in graduate school at the University of Iowa. There she received a Master of Science degree in chemistry in 1928, and was appointed in the following year as a chemist in the National Bureau of Standards in Washington, D.C. (presently the National Institute of Standards and Technology in Gaithersburg, Maryland). She was assigned to work in the Polarimetry Section under Dr. Horace S. Isbell, a rising star in the field of carbohydrate chemistry. She enjoyed working with him, and as time passed, the two developed a great admiration and profound respect for each other. The scientific collaboration that started between them at that time lasted for more than 60 years, making it one of the longest and most fruitful collaborations in the field of chemistry (probably the longest, if one discounts the work of such husband-and-wife teams as the Fiesers).

*

The information presented here was gathered during lengthy discussions with Dr. Harriet Frush and Dr. Horace Isbell. as well as from correspondence with Dr. Robert Schaffer, her superior at the National Bureau of Standards, and her niece Roselyn Lomax. The contributions of the last two are gratefully acknowledged.

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Harriet Frush’s first papers from the National Bureau of Standards dealt with the electrolytic oxidation of sugars to aldonic acids. The process she developed was so successful that it was adopted by industry for the production of calcium gluconate (extensively used pharmaceutically as a calcium supplement). The policy of the Bureau prevented employees from patenting their discoveries, but it was estimated that royalties from such a patent would have been worth millions in today’s dollars. It was therefore surprising that, when budget considerations necessitated a reduction in the staff of the National Bureau of Standards, Harriet was terminated in 1933. Fortunately the layoff was temporary, and a few months later she was rehired and assigned to the Cement Section, where she worked for some time as an analytical chemist. She enrolled in the Ph.D. program at the University of Maryland and selected as her thesis advisor Dr. Horace Isbell. A short time later, she succeeded in moving back to the Polarimetry Section, where she resumed work on her favorite subject, carbohydrate chemistry. She published several important papers on the reactions of glycosyl halides and received the Ph.D. degree in 1941. In her thesis she describes how the stereochemistry of glycosyl halides dictates the reactions that they undergo and the nature and configuration of the products formed. She found that orthoesters are produced when a leaving group is trans-oriented to an adjacent acetate group. Consequently, nucleophilic attack on a glycosyl halide can occur either on the carbonyl group of a properly oriented 2acetate to give a cyclic orthoester (which may undergo a second attack to afford a product having the same configuration as the starting material), or on the anomeric carbon to yield a product having an inverted configuration. The phenomenon was named the “neighboring-group effect.” Between 1941 and 1949, Harriet Frush worked on uronic acids and glyculosonic acids obtained from pectins, algins, and citrus products, and published her results in a number of pioneering papers. During the next 18 years she devoted her efforts to a project that generated her most important contribution to carbohydrate chemistry-namely, the synthesis of I4C- and 3H-labeled carbohydrates. The work was initiated in the 1950s by a grant from the then Atomic Energy Commission. Although most of the chemists in the Organic Chemistry Section in the National Bureau of Standards worked at one time or another on this major project, most of the work on 14C-labeledcarbohydrates was carried out by Horace Isbell, Harriet Frush, and Robert Schaffer, and the work on 3H compounds by Horace Isbell, Harriet Frush, and Lorna Sniegoski. First the carbon-labeled compounds were prepared by the action of 14C-labeledHCN on pentoses and tetroses, and later the tritium-labeled compounds were prepared by reduction of unlabeled aldonolactones with labeled borohydride or with sodium amalgam in tritiated water. From the resulting labeled products, saccharides

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could be produced having the radioactive isotopes of carbon and hydrogen in all the desired positions in their molecules. The samples prepared by the National Bureau of Standards were either used on campus in various research projects, or supplied to scientific institutions in the United States and elsewhere, where mechanistic studies helped solve important chemical and biological problems. Many of the methods Harriet developed were later adopted by industry for the manufacture of labeled sugars or modified in order to prepare the l3C-1abeled sugars needed for NMR work. Harriet Frush worked in the National Bureau of Standards from 1929 until 1968, during which period she published 60 papers, most of which appeared in the Journal of Research of the National Bureau of Standards. In 1968, Horace Isbell reached the mandatory retirement age in the Bureau of Standards, and it seemed that the much-admired work of the Isbell-Frush research team was about to come to an end. However, Isbell was not ready to stop his research work and decided to accept a position of Research Professor at American University in Washington, D.C. Harriet Frush, in turn, joined the Department of Chemistry at the American University, taking early retirement from the Bureau and being appointed Research Scientist at American University. This preserved the Isbell-Frush team and enabled the collaboration between the two chemists to continue for another 24 years, until 1992, when Isbell died. At American University, Harriet Frush and Horace Isbell worked on the mechanism of oxidation of carbohydrates by peroxides. They discovered that, in aqueous alkaline hydrogen peroxide, aldoses are quantitatively degraded to formic acid, so that hexoses produce six moles of this acid and pentoses produce five moles. A detailed study of the mechanism of the reaction revealed that degradation takes place by several pathways, the most rapid one involving the formation of peroxy radicals and hydroxy radicals. Thus, when a hydroperoxide-aldose adduct reacts with hydrogen peroxide, a peroxy radical is formed, which decomposes to a hydroxy radical, formic acid, and the next lower aldose. It was also found that, under basic conditions, hydroxy radicals oxidize alditols and aldonic acids to carbonyl compounds in much the same way they do with Fe2+in the Fenton reaction. During the years she spent at American University, Dr. Frush was able to publish 10 papers without help from any research assistant or laboratory technician. This brought her total to more than 70 papers. Harriet Frush was a very nice, devoted, and genteel person, reserved and unassuming, kind and always ready to help. She loved music and played the piano, organ, and flute quite well, often entertaining her friends with short pieces of baroque music. Her great hobby was travel, and she spent many of her vacations touring the United States and the world. Harriet was most of all a very modest person and hated being fussed about. One

16

HASSAN S. EL KHADEM

day, Dr. Horace Isbell wanted to nominate her for an important award and asked for her consent to put forward her name, but she refused, stating that she had done nothing to deserve such recognition. She was also a giving person who, in spite of her failing health, continued to deliver “Meals on Wheels” to the elderly until she could no longer drive. Harriet Frush never married; her closest relatives were the three children of her younger sister, Marion Vander Wert. The last major work of Harriet Frush at the American University was a collaborative effort with the present writer, the editing of the Complete Works ofHorace S. Isbell, a three-volume series published by the Carbohydrate Division of the American Chemical Society in 1988. Her health subsequently deteriorated, and she rarely came to the University, staying at home to take care of her ailing friend, Mary Jane Young. Later, the two went to a retirement home, in Knoxville, Illinois, to be near Harriet’s niece Roselyn Lomax. Harriet Frush died on November 16,1996, during surgery for a broken hip. HASSANS. EL KHADEM

ADVANCES IN CARBOHYDRATE CHEMISTRY AND BIOCHEMISTRY. VOL. 53

APPLICATIONS OF TIN-CONTAINING INTERMEDIATES TO CARBOHYDRATE CHEMISTRY

BYT. BRUCE GRINDLEY Department of Chemistry. Dalhousie University. Halifax. Nova Scotia. Canada B3H 4.73 I . Introduction ...................................................... 17 I1. Nomenclature 18 111. Preparation of Tin-Containing Intermediates ............................ 18 IV . Physical Methods for the Study of Organotin Derivatives ................... 19 19 1. Tin-119 NMR Spectroscopy ........................................ 2. Carbon-I3 and Proton NMR Spectroscopy., 25 V . Structures ........................................................ 25 1. Tributyltin Ethers 25 2. Dialkyltin Dialkoxides and Dialkylstannylene Acetals ................... 26 VI . Reaction Types and Conditions ....................................... 32 VII . Factors That Influence Reaction Regioselectivity ......................... 33 1. Tributylstannyl Ethers ............................................ 33 2. Dialkylstannylene Acetals ......................................... 36 VIII . Trends in Regioselectivity ........................................... 44 44 1. Symmetrical Diols and Polyols ..................................... 2 . trans-Diols and Polyols Containing Only trans-Diols .................... 44 3. cis-Diols and Polyols Containing cis-Diols ............................ 71 4 . Diols Adjacent to Deoxy Centers or Double Bonds ..................... 103 5. Terminal 1,2- and 1,3.Diols ........................................ 110 6. Glycoside Formation ............................................. 124 7. Miscellaneous .................................................. 129 133 References .......................................................

..................................................... .......................... ...............................................

1. INTRODUCTION Since the discovery’.’ of their utility in 1974. trialkylstannyl ethers and dialkylstannylene acetals have become widely used intermediates in the synthesis of carbohydrate derivatives . Although the use of these intermediates has been reviewed several times:-’ innovative developments continue to be described; marked advances have been made both in expanding the utility of these compounds and in understanding of the mechanisms of the reactions . The major reason that these intermediates have become widely employed is that they provide reliable. high-yielding methods for obtaining monosubsti0065-23111198 $25 IN1

17

Copyright 6 1998 by Academic Press . All rights of reproduction in any form reserved .

18

T. BRUCE GRINDLEY

tuted derivatives of diols or polyols, often with high regioselectivity. In addition, the reactions occur under much milder conditions or at rates that are much higher than those of the parent alcohols. As a result, these reactions have become part of the arsenal of techniques commonly used by organic chemists, and numerous publications in which they are used appear each year. This report is a comprehensive summary of this area. It is intended that all results using these intermediates should be included; however, these methods have become so common that routine results are often not highlighted, and some applications may have been inadvertently omitted. 11. NOMENCLATURE Dialkyltin dialkoxides, in which the dialkoxides are connected to form a ring are called dialkylstannylene acetals, or more properly, 2,2-dialkyl1,3,2-dioxastannolanes, if the ring is five-membered; 2,2-dialkyl-1,3,2dioxastannanes, if the ring is six-membered; or 2,2-dialkyl-1,3,2dioxestannepanes, if the ring is seven-membered. Trialkyltin alkoxides are normally termed trialkyltin ethers or trialkylstannyl ethers. The sugar derivatives are named by standard acetal (2-Carb-28) or ether (2-Carb-24.1) nomenclature.' 111. PREPARATION OF TIN-CONTAINING INTERMEDIATES The most widely employed tin-containing intermediates are dibutylstannylene acetals and tributylstannyl ethers. Dibutylstannylene acetals are usually prepared by reaction of diols with dibutyltin oxide in methanol with heating, or in benzene or toluene with azeotropic removal of water using a Dean-Stark apparatus (Fig. 1).The latter conditions probably result in complete conversion in 1-2 h at reflux, although they are normally left 4-24 h. Incorporation of a Soxhlet apparatus containing molecular sieves toward the end of the reaction ensures complete removal of water. Reaction in methanol, where dibutyldimethoxytin is an intermediate for the preparation, is more rapid, often being complete in 1 h; however, one group observed that yields were lower and that starting material remained after reaction workup if the dibutylstannylene acetal was formed by this method.' The technique used for preparation may also have been a factor in the

-(

0

+ Bu$nO

+

$0

FIG.1 .-Formation of a dibutylstannylene acetal.

APPLICATIONS OF TIN-CONTAINING INTERMEDIATES 2ROH + (Bu,Sn),O

*[ROH

+ Bu,SnOR + Bu,SnOH]-

FIG.2.-Formation

19

ZROSnBu, + H,O

of a tributyltin ether.

yields reported by KovhE and Edgar," who formed the stannylene acetal in toluene, which were higher than those reported for identical reactions by earlier workers, who formed it in methanol." Most reports in which dibutylstannylene acetals are formed in methanol give excellent yields; it is probably sufficient to use carefully dried methanol and to be very careful to remove the methanol completely at the end of formation of the dibutylstannylene acetal, to ensure complete conversion. The use of performed dibutyltin dimethoxide is advantage~us'~-'~; reactions of diols are complete in 5-15 min in benzene at reflux. This technique avoids the problems with working in methanol just mentioned, and gives better yields. The preparation of stannylene acetals is also accelerated by microwave irradiati~n,'~ and selective reactions can be performed, albeit in low yields, on some diols and polyols by microwave irradiation in the presence of a catalytic amount of dibutyltin oxide.16.17 Tributylstannyl ethers are prepared in the same manner by reaction of alcohols with hexabutyldistannoxane (more commonly known as bistributyltin oxide). Holzapfel et al. noted that the reaction in benzene requires only 0.5 molar equivalents of bistributyltin oxide to go to completion but takes 16 h at reflux (Fig. 2). This is probably because the tin-containing by-product of the first half of the reaction, tributyltin hydroxide, reacts much more slowly than the initial reagent.18 Dibutyltin oxide, dibutyltin dimethoxide, and hexabutyldistannoxane are commercially available. Di-t-butyltin dichloride is commercially available, but reactions of di-t-butylstannylene acetals are often very slow. In certain cases, there are advantages in using bulkier tin oxides or tin oxides that are sterically restricted, such as cyclic tin oxide^.^^.^^ Non-commercial dialkyltin oxides may be prepared conveniently by the reaction of dialkyldiphenyltin with chloroacetic acid followed by sodium hydroxide.21 This procedure works well for the preparation of stannacycloalkanes as well as bulky dialkyltin oxides. IV. PHYSICAL METHODS FOR THE STUDY OF ORGANOTIN DERIVATIVES

1. Tin-119 NMR Spectroscopy The most convenient technique available to study organotin compounds The Il9Sn nucleus has a spin of in solution is "'Sn-NMR %and a natural abundance of 8.7%;it is about 25.5 times more sensitive than

T. BRUCE GRINDLEY

20

13C,taking into account the isotopic abundances. The '"Sn isotope, also a spin ?hnucleus, with a natural abundance of 7.7%is slightly less sensitive and has been little used. It is about 19.7 times more sensitive than I3C.Since both of these nuclei have negative magnetogyric rations, nuclear Overhauser enhancements are negative. Thus, spectra are normally recorded with decoupling on only during acquisition. It should be noted that NOESonly occur if the tin atoms relax through dipole-dipole interactions with protons. A limited number of relaxation studies of organotin compounds have been performed. Tin atoms in simple compounds in which the tin atoms are tetracoordinate relax by the spin-rotation mechanism, but dipole-dipole relaxation becomes more important as the molecules get larger and the temperature becomes l o ~ e r ? ~Since - ~ l they are in still larger molecules, the tin atoms in tributyltin ethers of carbohydrates relax predominantly by the dipole mechanism, with TI values in the 2- to 7-sec range.32In contrast, tin atoms in the pentacoordinate and hexacoordinate environments present in stannylene acetals relax predominantly by chemical-shift anisotropy, with relaxation times in the 0.02- to 0.2-sec range at 8.5 T (360 MHz ~pectrometer).~~ This allows much shorter pulse intervals, and the effectiveness of this mechanism increases with the square of the applied magnetic field strength. These relaxation times are short enough that they cause the line widths to be broad; if TI is 20 msec, the contribution to the line width from relaxation is 16 Hz, offsetting the gain in intensity arising from short pulse intervals. Chemical shifts for tin atoms in typical carbohydrate-derived organotin compounds are listed in Tables I and 11. In general, the signals of Il9Sn

TABLE I Typical ll'Sn NMR Chemical Shifts of Tributvltin Ethersa Compound

Solvent

Chemical Shift

Reference

Tributyltin propoxide Tributyltin isopropoxide 1

Neat Neat Toluene Toluene Toluene Toluene Benzene Benzene Benzene Benzene Benzene Benzene

87 76 99.0 86.4 92.8, 98.6 77.5 (C-2), 91.2 (C-3) 78.5, 90.4 102.5 90.0 73.9 (C-3), 101.6 (C-4) 113.5 119.4

34 34 35 35 35 35 36 36 36 36 36 36

2 3 4 5 6 7 8 9 a

In ppm from tetramethyltin.

APPLICATIONS OF TIN-CONTAINING INTERMEDIATES

21

nuclei in tributyltin ethers appear between 75 and 105 ppm if no hydroxyl groups are on adjacent carbons, but are deshielded by up to 20 ppm more if hydroxyl groups are cis and vicinal to the tributyltin ether (Table I). The presence of a trans-hydroxyl group causes a lesser deshielding of about 10 ppm. Presumably, these effects are caused by equilibria in which there are small contributions from species in which hydroxyl oxygen atoms have added to tin atoms to form five-membered rings.

Me0

. 1

OMe

OSnBu,

OMe

2

3

Bu,SnO OMe

4

5

Bu,SnO

HO

OMe

6

OBn

7

Tin-carbon coupling constants can be useful in making assigments for tributyltin ethers. Values for 2JC,Sn lie between 22 and 32 Hz if the carbons are ~econdary,~'. 36 but are larger for primary carbons (46 Hz for one example3'). The 3JC,sn values are of similar magnitude, ranging35,36from 10

T. BRUCE GRINDLEY

22

SnBu,

FIG.3.-Three-bond tin-carbon coupling constants?6

to 29 Hz. These values probably follow a Karplus-type relationship since, in several examples, the magnitudes of coupling constants from tin atoms on axial oxygen atoms have values at the low end of the range to one carbon (presumably gauche) but at the high end of the range to the second carbon (presumably anti) (see Fig. 3). Tin atoms in dibutylstannylene acetals resonate between -115 and -150 pprn if they are pentacoordinate in five-membered rings. If they are hexacoordinate in five-membered rings, the tin nuclei are considerably shielded to between -220 and -330 ppm, with the more deshielded values being observed for polymers and higher oligomers (Table 11). Pentacoordinate tin atoms in six-membered rings are more shielded than those in fivemembered rings, by about 50 or 60 ppm to about -180 ppm (see results for compounds 16 and 23). Those in seven-membered rings appear at intermediate shifts (see 25 and 26).Replacement of butyl groups by t-butyl groups results in the shielding of the tin atom by about 100 ppm (compare data for compounds 10 and 13 and also for compounds 21b and 24), and intermediate changes in branching on the carbon attached to the tin atom bring intermediate shift change^.^' Bu\ Bu\

3"\

/Bu

Bu\ /Bu

8"\

/Bu

O W 0

10

/""\

H?3w w 11

12

t-Bu\

fBu

/""\

13

14

15

Bu,

,Bu

/""\ O U O

16


23

ph

k

Ph

ok

0

OBn

\

BuHSn\

No

Bu

Bu Bu

OBn

0

20

OBn

21a R = M e 21b R = B n

Bu

Nsnp+o& BnO

23

22

OR

OBn

OBn

24

H BnO

25

OBn

26

OMe

TABLE I1 Typical lI9Sn NMR Chemical Shifts for Dialkylstannylene Acetals

Solvent

Compound 10

Chloroform

11

Solid state Chloroform/dichlorofluoro-methane3 :1

14

Chloroform Chloroform Solid state Chloroform

15 16

Chloroform Chloroform

12

13

17

18 19

20

2lb

22

23 24 25 26

Solid state Chloroform Toluene Solid state Chloroform

Coordination Status 5 5, 6 5, 6 6 5

5, 6 5 5 5 5 4 4 5 5, 6 6 5 5 5 5

Toluene

5 5 5 5 5, 6 5, 6

Chloroform Chloroform Chloroform Chloroform Chloroform

5 5 5 5 5

Chloroform Solid state Chloroform Chloroform

Oliomer Name Dimer Trimer Tetrarner Polymer Dimer Trimer Dimer Dimer Dimers Dimer Monomer Monomer Dimer Trimer Polymer 3,3-Dimer 3,3-Dimer 3,3-Dimer 2,3-Dimer 2,2-Dimer 2,2-Dimer Dimer Dimer Trimer Trimer Tetramer 6.6-Dimer 6,6-Dimer Dimer Dimer Dimer

Chemical Shift -126.8 -126.8, 283.0 -131.4, -266.0 -231 - 141.5 -142.5, -291.8 -139.8 -225 -222, -224, -225, -226 - 127.9 80 -20.4 -187.8 -195.1, -346.1 -279.5, -281.8 -125.4, -124.9 -131.6 -126.8, -128.6 - 132.6 -132.8, 133.0 -145.1, -143.8 -141 -125.6 - 123.4 -117.4, -122.5, -283.3 -116.8, -124, -287 -124.1, -243.4 -119.6 -180 -217.7 - 150.8 - 160.2

Reference 38 39 38 38 40 39 41 41 42 43, 44 35 39 44 43, 44 39 43 41

37 43 39 43 45


25

2. Carbon-13 and Proton NMR Spectroscopy Carbon-13 and proton NMR specta are much less useful than “’Sn NMR spectra in providing information about the structures for these organotin intermediates, but can serve useful roles in mechanistic studies. HoleEek et al. have measured 13C NMR data for a wide range of dibutyltin( IV) corn pound^.^^ Chemical shifts for C-1 to C-4 of butyl groups in simple dibutyltin dialkoxides have values of about 19, 27, 26, and 13 ppm, re~pectively.~~ The chemical shifts of the C-1 carbons are somewhat dependent on the nature of the compound and its state of association. For 2,2-dibutyl-l,3,2-dioxastannane, the chemical shift of the butyl C-1 was about 20 pprn when the attached tin atom was pentacoordinate, but about 27 pprn when it was hexacoordinate?2 Probably a more useful parameter is the lJsn-lly,c coupling constant. The magnitude of this parameter has been shown to increase linearly with the size of the C-Sn-C b0nd-angle.4~9~~ These one-bond coupling constants are large and vary widely (307 to 1175 Hz), but have not been extensively employed for the study of carbohydrate derivatives. For butyl groups attached to pentacoodinate tin atoms, values of about 580 to 620 Hz and and 1,3650 Hz have been observed for simple 1,3-dio~astannolanes~~*~ dioxastannanes,42 respectively, at low temperature. Average roomare larger, 6534’ or temperature values for 2,2-dibutyl-l,3-dioxastannolane 643 Hz5’, indicating that the values for hexacoodinate tim must be still greater, consistent with the expected and observed larger C-Sn-C bond angles for hexacoordinate tin.42,S1*52 Chemical-shift changes for carbon nuclei in the diols on substitution of hydroxyl hydrogen atoms by a tin atom are small (39 -39% 2' 3' >37 >37 2' 70 3'0 2' 0 3' 70 2' 0 3' 70 2'24 3'72 2' 0 3' 87 2'0 3' 70 2'70 3'0 2' 24 3'some 2' 13 -

3'50

74

- 257 - 71 - 71 - 71 1 1 1 1 - 256

1 12 258

-

Miscellaneous Compounds

134 2'-O-Acetyltylosin (135)

Acetal Toluene Acetal Toluene

cis-3,4-Dihydroxy-2-piperidinone (136)

Acetal Acetal Acetal Acetal

Toluene Benzene Benzene

DMF

TBAI

TBAI

TBAI CsF

80 p-Methoxybenzyl bromide Amb. p-Methoxyphenylacetyl chloride Arnb. Phenylacetyl chloride Amb. Methoxymethyl chloride 80 p-Methoxybenzyl chloride Arnb. Methyl iodide

Added nucleophile; for abbreviations of nucleophile names, see Table IV. A > sign indicates that the yield reported was for more than the minimum number of steps. Partial ester exchange occurred the product contained 55% methyl ester plus 30% benzyl ester. * Not reported. 'Plus 21% of the 3.6-di-0-benzoyl derivative. 'Plus 8% of the 6-0-benzyl derivative. g With 15 equiv p-toluenesulfonyl chloride, 75% at 0-3' and 15% di-product (3' and 6'). With 2 equiv sulfur trioxide-trimethylamine complex, 76% at 0-3' and 10% di-product (3' and 6'). ' Based on starting material consumed. The yield was not improved by performing the reaction in toluene or acetonitrile or in DMF containing cesium fluoride. Ir Plus 10% dibenzoate. Plus 30% of the 3.6-di-0-benzoyl derivative. Only the di-0-benzyl derivative could be isolated. " Plus 20% of three mono-0-benzyl derivatives with the benzoates rearranged. " Obtained as a minor product. p Plus 35% 5-benzoate. 4 Plus 45% of an approximately 1:1:1 mixture of the three possible di-0-benzoyl derivatives. Also 18% of a mixture of 2.3- and 3,4-disubstituted products. Plus 23% of the 3.4-di-0-benzoyl derivative. ' As the lactone. J

'

-

84

0

259

4" 28

260

4" 18 4" 14 4 26 -

260 260

27

0

260 261

OBn

OH

HO HO

h:nOBn

55 R = B n 56 R = CH,CH,SiMe, 57 R = A l l

OAll \

OAc

54

HO

58 7

0

HO

&

OH

59 cAo*&:

OH

NH*

OBn

HO

OBn

O I

OAll

60

OAc

c q

61

HO

+

HO

&

OBn

0

AcO

4

62

OMe

63

OMe

HO

64

OH


89

66

OBn

H,C h

O

B

& p &

n

/OBn OpOMeBn NHAc

HO OH

67

hoBn

OBn

H3C

HO

,OBn

&hO OBn

OH

68

90

T. BRUCE GRINDLEY

HO NRR'

69 R = H , R'=Ac 70 R, R ' = Phth

HO OH

71 R = M e 72 R = B n 73 R = CH,CH,SiMe, 74 R =y-AllyloxyBn 75 R = 4-Pentenyl PI1

OpOMeBn

HO

NHAc

OH

76

OBn

OpOMeBii HO

77


h p $ $ /

91

0

.O

HO OH

78

HO OH

79

HO

80 R = O H 81 R = N H A c

HO H O3*c0Bn k o l I q

0

HO

% 83 HO OH&

OH

82

SPh

OH

OCH,CH,SiMe,

92

T. BRUCE GRINDLEY

NHCbz

Me0

HO HO OH

OH

85

84 7

OH

H

OH

H

OH

OH

88

4

HO

OH

&

OH

91

90

c

d

H , $OHH o M e

HO

OH OBn

OBn

OH

93

92

BnO

All0

RO

HO O

89

,

OH

87

86

RoH

OH

0

OBn

95 R = A l l 96 R = B n

OMe

OH

94

dOH &/ All0

OAl

p-MeOBnO

BnO

97

OBn

OBn

98


OH

Bn@oH

93

Bngn+&

BnO

BnO

BnF-

99

OBn BnO

OBn

BnO

RO

I

S

O

“ RO

100

?H

?H

I

RO

H AIIOAllO

OR

OAll

OH

AllO

’OR

OH

101 R = p-MeOBn

104 R = B n

103

102 R = B z

105 R = p-MeOBn OH

O B *n BnO

OH OAll

OH

OBn

106

OBn

107

108

OH

OH B BnO n

O

6OH

I

BnO

PrCOO PrCOO -0,

OBn

109 X = F 110

OBn

111

x=c1 OH

OBn

BnO

OH

I

AllO A AllO 1

’

o

S

O

OAll

112 OH

p”

I

OBn

OBii

114 Cr = -CH,CH=CHCH,

115

H

T. BRUCE GRINDLEY

94

OBEt,

OBEt, OBEt,

116

118

OH

117 Me

I

HO

OH

119

HO

121

120

HO

q,

C0,Me

OR

122 R = M e 123 R=All

HO

HO

SEt I

124

CO,El

125

OH

126 R = R = H 127 R - H R ' = M e 128 R = C N R'=Me 129 R = Me R' = Me

130

131

OAll

01


I

95

I

OH

HO

132

I H 136

The reactions performed on dibutylstannylene acetals formed from galactopyranose derivatives having only 0 - 3 and 0 - 4 unprotected are amazingly regioselective; as shown by numerous groups, treatment with every electrophile except bromine gives only substitution at the equatorial oxygen atom, 0-3. Bromine oxidizes this type of compound with high regioselectivity at 0-4. Figures 33 and 34 illustrate some of the wide range of conditions that give these two types of regioselectivity. Even when there are other unprotected hydroxyl groups in the substrate, 0-3 of dibutylstannylene acetals of the cis-diol of the gulucto configuration reacts with electrophiles with high regioselectivity. Figure 35 shows some of the most spectacular examples: those for P-lactosides, where the reactive oxygen atom is one of seven unprotected. The last example in Fig. 35, where the electrophile is t-butyldimethylsilyl chloride (e), illustrates a consistent pattern with this e l e c t r ~ p h i l e and ~ ~ -other ~ silylating agents88to yield the product of reaction with a primary oxygen atom, when available. One other reaction on a substrate with this configuration has yielded small amounts of primary product when the primary hydroxyl was free,'= but probably small amounts of second products are present in many cases when the yields reported are in the 50 to 80%range. Nevertheless, the regioselectivity obtained for these substrates is highly impressive and very useful synthetically. The regiochemistries of reactions on dibutylstannylene acetals of amannopyranosides having only the cis-1,Zdiol on 0 - 2 and 0-3 free are much more complicated than those just discussed. By far the greatest extent

T. BRUCE GRINDLEY

96

OBn

2b. AllBr, Bu,NI Toluene, 80 OC

55 R1 = Bn 56 R1 = CH,CH,SiMe,

2c. Bar-DMF, lO0OC

57 R1=All

a. R1 = Bn, R2 = S03NEt3 83% b. R' = CH,CH,SiMe,, R2 = All 91% c. R1 = All, R2= Bn 81%

FIG.33.-Reactions of the dibutylstannylene acetals of 2,3,6,2',6'-penta-O-benzyI-P-lactose glyc~sides.'~~~'~~~'~'

of substitution still occurs on the equatorial oxygen atom, 0-3, but, in contrast to the results for the galactose derivatives, significant amounts of substitution on the axial oxygen atom, 0-2, were often obtained. In fact, preferential substitution of this position was observed on reaction of the dibutylstannylene acetal of methyl 4,6-O-benzylidene-a-~mannopyranoside with benzoyl chloride in nonpolar solvents in the absence of added n u ~ l e o p h i l e s , ~and * ~ also ~ ~ ~with + ~ the ~ ~ triethylamine-sulfur trioxide complex in DMF (see Fig. 36).lo7However, in the presence of added

-

OH

1 . Bu,SnO

HO

OBn 2. Br.-CKCL Br2-C&C12

nBO- *HO ' OBn

OBn

60

76%

FIG. 34.-Oxidation of the dibutylstannylene acetal of benzyl 2-O-benzyI-P-~fucopyranoside with bromine.'62

97


1. Bu,SnO H HO "O&;

OH OR'

OH

71 R 1 = M e

2a. AlIBr, Bu,NBr-Benzene,

80 "C

2b. p-MeOBnCI, Bu,NI-Benzene,

80 OC

2c. MeI, Bu4NBr-CH3CN, 45 "C

72 Rl = Bn 75 Rl = 4-Pentenyl

2d. AlIBr, Bu,NBr-Benzene,

80 OC

2e. TBDMSCI- THF,RT

RzO

OR1

OH a. R1 = Me, R2 = All, R3 = H 70%

b. Rl = Me, R2 = p-MeOBn, R3 = H 70% c. R l = M e , R , = M e , R 3 = H

54%

d. R1 = Bn, RZ= All, R3 = H 61% e. RI = 4-Pentenyl, R2 = H, R3 = TBDMS 92% FIG.35.-Reactions of the dibutylstannylene acetals of p-lactose giyc~sides.'~~,'~.''~.'~.~~

nucleophiles, and under more vigorous conditions, as required for alkylation, the equatorial product becomes predominant. The X-ray crystal structure shows methyl 4,6-0-benzylidene-2,3-0dibutylstannylene-D-mannopyranosideto be a pentamer." In the pentamer, the two terminal monomer units have 0 - 2 dicoordinate. The

Ph

T*

1. Bu,SnO

AMe2. For reagents see below BzCI - Benzene BzCI, NMI - Benzene Et3N.S03- DMF

I

OMe

R' = Bz, R2 = H 85% R ] = H , R ~ = B 15% Z R' = H,R~= BZ 90% R1= Bz, R2 = H 0% R' = S03NEt,, R2= H 70% R' = H, R2= S03NEt3 14%

FIG.36.-Reactions of the dibutylstannylene acetal of methyl 4,6-O-benzylidene-a-~mannopyrano~ide.'~~'~~

98

T. BRUCE GRINDLEY

l19Sn NMR spectra of this compound indicate that it is less associated in solution, being present as a mixture of a symmetrical dimer and a trimer in chloroform-d and of the dimer, the trimer, and a tetramer in toluene-d8:' The populated dimer is probably the 3,3-dimer with 02 dicoordinate. Thus, these products could be the result of trapping of 'the more populated dimer in solution by reaction of reactive electrophiles with the more nucleophilic oxygen atom. Alternatively, the intermediate formed initially may rearrange during workup, as demonstrated for esterification of acyclic diols (see earlier).53 Alkylation reactions of dibutylstannylene acetals of compounds having this stereochemistry give variable degrees of regioselectivity for 0-3 in a rather unpredictable manner. However, none of the reactions on this type of substrate performed in DMF containing CsF gave anything other than complete regiospecificity for 0-3. Therefore, this method is the best choice to obtain this regiochemistry. Although not nearly as mainly results are available for other stereochemistries, regioselectivities seem to be in accord with those already noted. Thus, allopyranosides react at 0 - 2 when 0-4 is and talopyranosides have a reference for reaction at 0-3.9*205.233 Although not nearly as many reactions have been performed on tributylstannyl ethers of cis-diols, the effect of varying stereochemistry was examined in a systematic fashion for 1,6-anhydropyranoses by Martin-Lomas and his co-workers.68As shown in Fig. 37, the outcome is similar to that anticipated for similar reactions on dibutylstannylene acetals. Table VI also lists the numerous reactions that have been performed on dibutylstannylene acetals of inositol derivatives containing cis-diols, mostly in myo-inositol derivatives. Most reactions were alkylation reactions and, almost without exception, single products were obtained with the substituent on the equatorial oxygen atom. In most cases, yields were greater than 80%, and in only a few cases were yields lower than 70%. The fructopyranoside and arabinopyranoside reactions are listed in Table VI because the regiochemistry associated with the cis-diol is more important in controlling their regiochemistry of reaction than the lack of substitution of one side of the diol. Thus, the fructopyranoside derivatives react with electrophiles other than bromine at 0-4,8'J47the equatorial oxygen atom of the cis-diol, and with bromine at the axial oxygen, 0-5.81Although 0-3 is generally favored, the regioselectivity observed for arabinopyranosides is lower than for most of the other cisdiols, presumably because there are not great stability differences between the 'C, and 4C1conformations. Based on the foregoing observations, dibutylstannylene acetals of 8O-protected Kdo derivatives should display great selectivity for 0-4, an

99


DOH 1. Bu,SnOSnBu,

p

+

2. BnBr, NMI -toluene, 120 o c

I

OH

OH

86

7

OBn

25%

65%

0 1. Bu,SnOSnBu,

* 2. BnBr, NMI -toluene,

I

87

BnO

120 o c

OH

OH

93%

FIG. 37.-Regioselectivity obtained in benzylation of tributylstannyl ethers of two anhydropyranoses."R

equatorial oxygen oxygen atom that has a deoxy center at C-3 and a cis oxygen atom at C-5. This regioselectivity was obtained, but the reactions of a-glycosides are complicated by concomitant lactone formation, as shown Only when the large TIPS group is used to protect 0-7 and in Fig. 38.252.253 0-8 is lactone formation inhibited.262 The furanose derivatives listed in Table VI present an interesting contrast. The acylation reactions are fairly selective,' usually for 0-3', whereas the alkylation reactions are relatively n o n ~ e l e c t i v e . Presumably, ' ~ ~ ~ ~ ~ ~ ~the ~ regi-

1 . Bu,SnO / Benzene

HO

BnO

C0,Me

122

OMe

2. BnBr, Bu,NBr, NaI 80 O C 19h

72%

FIG.38.-Benzylation of a dibutylstannylene acetal of a cis-diol on a molecule that also contains a methyl ester, which results in concomitant lactone formatio11.2~~

OH

100

T. BRUCE GRINDLEY

oselectivity on acylation is the result of eq~ilibration?~ whereas the lack of regioselectivity in alkylation reactions indicates the lack of kinetic preference for 0 - 2 or 0-3. Nevertheless, the preferences of dibutylstannylene acetals to give monosubstitution, and reaction at a secondary site of a cisdiol over an unprotected primary oxygen atom, can be synthetically Figure 39 shows a spectacular example of the selectivity of reactions of dibutylstannylene acetals of vicinal cis-diols over isolated diols, for 2'-0acetyltylosin, where one secondary hydroxyl of the three available reacts because it is the only one vicinal to another hydroxyl group. In the sequence shown, reflux in methanol removes the 2'-O-acetyl group?60 Table VII lists the comparatively few di- or poly-substitution reactions that have been performed on dibutylstannylene acetals and tributylstannyl ethers of compounds containing cis-diols. Figure 40 (page 103) shows some of the

1. Bu,SnO

- toluene

2. p-Methoxyphenylacetyl chloride 3 . Methanol, reflux

11,111

OR

2 8% OMe FIG.39.-Synthesis of 4"-O-p-methoxyphenylacetyltylosin?m

TABLE VII Formation of Di-0-substituted Derivatives from cis-Diols

Compound Methyl 0-D-galactopyranoside

Reaction Conditions Acetal or Ether Solvent Nuc" Temp. ("C) Ether Ether Ether Acetal Ether Ether Ether Acetal

2-(Trimethylsilyl)ethyl 2,3,6-tri-O-benzyI-40(P-D-galactopyranosyl)-fl-D-glucopyranoside (136) 2-hidoethyl P-D-galactopyranoside Acetal Methyl a-D-galactopyranoside Ether

3,6-Di-O-benzyl-4-0-(2-0-benzyl-/3-~galactopyranosy1)-D-glucal

Acetal Acetal and ether

Toluene Toluene DMF BnBr BnBr AllBr Toluene Toluene

20 Amb. 25 85 85 80 60

4 Amb. 20

Pyridine Toluene

85

BnBr Benzene TBAB

80

Yield (%) Electrophile

23

0 Benzoyl chloride Pivaloyl chloride 0 ET3N . SO3 0 Benzyl bromide 38 Benzyl bromide 6 24 Ally1 bromide 6 11 Trityl chloride 2,6 72 Benzoyl chloride 0

Pyridine . SO3 Benzoyl chloride Benzyl bromide Benzyl bromide

0 6 22 2.6 10 0

2,4

3,4

Ref.

0 0 0 0 0 0 0

3.6 95 3,6 86 3,6 76 3,6 70 3,6 48 3,6 51 0

74 264 107 151 263 263 263

0 3',6' 89 120 0 3,6 71 115 3,6 21 2 3 74 40 0 3,6 63 151 3',6' 90 182 (continues)

TABLE VII (ConTinued)

Compound

Reaction Conditions Acetai or Ether Solvent Nuc’ Temp. (“C)

Methyl 6-chloro-6-deoxy-cr-~-mannopyranosideAcetal Methyl a-D-maMopyranoside Ether Ethyl 1-thio-a-D-mannopyranoside Acetal 1,6-Anhydro-/3-~-talopyranose (91) Ether l&O-isopropyhdene-/3-D-fructopyranose (119) Acetal Ether Methyl @-D-arabinopyranoside Ether Acetal (-)-chiro-Inositol (W7) Ether

Hexa-0-diethylboronyl-myo-inositol(116) Lactose

Etherb Ether

Benzene Toluene DMF CsF Toluene NMI Benzene Benzene Benzene Benzene CH3CN TBAI Toluene NMI Toluene

Added nucleophile; for abbreviations of nucleophile names, see Table IV Three equivalents.

80 20 Amb.

120 80 Amb. 50

80 82 Amb.

45

Yield (%) Eleetrophile

2,3

Benzoyl chloride 90 Benzoylchloride 0 Benzyl bromide 0 Benzyl bromide 0 Benzoylchloride Benzoylchloride Benzoyl chloride 0 Benzoyl chloride 17 Benzyl chloride 2,3,5

2,4

3,4

Ref.

0

0 0 0

18 74 218

3,6 90 3,659 95 50 0

2,3,5,6 32 6 Benzyl bromide 1,3 48 1,5 20 Benzoyl chloride 2,6,3’,6’ 72

0

4,5100 4 3 95 50

80

6

8

18 18 18 18 265 246 74


1. Bu,,SnO-toluene

HO

BnO

2. BnBr, 80 "C, 26 h

OH

HO

*

OMe

&

&

OMe

OH

R=Bn OH

1. (Bu3Sn),0

OMe

OH

103

R'O

70%

OR2

!&& 0

OMe ~

3

2a. BzCI- toluene

R1= R2= Bz R3 = H 95%

2b. E$N.SO,-DMF

R' = R2 = S03NEt, R3= H 76%

2c. BnBr, 80°C, 26 h 2d. TrCI-toluene, 60 oC, 35 h

Rl=RZ=Bn R3=H 48% Rl=R3=Tr R2=H 72%

FIG. 40.-Major products obtained in the di-0-substitution of methyl galactopyranoside.'4~'"7~"3~2h'

p-D-

reactions performed on methyl P-D-galactopyranoside.Both tributylstannyl ethers and dibutylstannylene acetals yield 3,6-di-O-substituted derivatives as the major products in most cases,74J07-151,263 although tritylation of the tributylstannyl ether afforded the 2,6-di-tritylether as the only di-0-substituted When no primary hydroxyl is available, di-0-benzoylation of either the dibutylstannylene acetal or the tributylstannyl ether was observed to occur at the site of the cis-diol, as observed for methyl 6-chlorod-deoxya-D-mannopyranoside or 1,2-O-isopropylidene-P-~-fructopyranoside.~~ However, di-0-benzylation for 1,6-anhydro-P-~-talopyranose (91),which has two equatorial hydroxyl groups adjacent to an axial hydroxyl, occurs in good yield on the equatorial oxygen atoms.68When primary hydroxyl groups are available, the major di-0-substituted product almost always has substitution at the primary oxygen atom.74~'07~'15~120~218

4. Diols Adjacent to Deoxy Centers or Double Bonds Numerous examples as listed in Table VIII demonstrate that lack of substitution adjacent to a 1,2-diol causes reactions of its dibutylstannylene acetal to occur on that sterically less hindered oxygen atom. Some examples that illustrate the trends are shown in Figs. 41 to 44 (page 107). The

TABLE VIII Reactions of Dibutylstannylene Acetals and Tributyltin Ethers of Diols Having Adjacent Deoxy Centers or Double Bonds

Yield (YO)

Reaction Conditions Compound

Acetal or Ether

1,5-Anhydro-l-amino-4,6-0- Acetal benzylidene-Nbenzyloxycarbonyl-1-deoxyD-glucitol (l39) Acetal Acetal Methyl 2,6-dideoxy-a-~arabino-hexopyranoside (140) Methyl 2.6-dideoxy-3-Cmethyl-a-D-arabinohexopyranoside (141) Butyl 2,6-dideoxy-P-~~arabino-hexopyranoside (142)

Methyl 2,6-dideoxy-a-~arabino-hexopyranoside (143)

p-Methoxybenzyl 2,6-dideoxydpL-arabinohexopyranoside (144) Methyl 2,6-dideoxy-a-~-lyxohexopyranoside (145) Methyl 2,6-dideoxy-P-~-lyxohexopyranoside (146)

Solvent

Nuc'

Temp. ("C)

Electrophie

0-3

0-4

Ref.

73

0

-

268

94

0-2

Benzene

Amb.

Benzoyl chloride

Benzene

Amb. 110

Tosyl chloride Benzyl bromide

0 85

-

-

0

268 273

DMF

Acetal

Toluene

Et3N

Amb.

Benzoyl chloride

-

80

12

267

Acetal

Toluene

TBAI

110

Benzyl bromide

-

86

0

266

Acetal Acetal

Toluene Benzene

TBAI Et3N

26 -45

Tosyl chloride Benzoyl chloride

-

98 70'

0 0

266 269


Benzene Benzene Benzene Toluene

TAB1 TBAI Et3N

Amb. 80 80 -40

Tosyl chloride Benzyl bromide Methyl iodide p-Nitrobenzoyl chloride

-

7s 70 72 72

0 0 18 0

269 269 269 274

Acetal

Toluene

TBAB

80

t-Butyldimethylsilyl chloride

-

85

0

178

Acetal Acetal

Benzene Benzene

TBAI

80 Amb.

Benzyl bromide Tosyl chloride

-

84 90

0 0

275 269

Acetal

Benzene

TBAI

80

Benzyl bromide

-

9s

0

79

-

Methyl 2,6-dideoxy-c~-~-lyxohexopyranoside (147)

Toys1 chloride

-

98

0

269

Benzyl bromide Benzyl bromide Benzyl bromide

-

48

-

95 60

0 0 40

270 269 271

Acetal

Benzene

Acetal Acetal Acetal

Benzene Benzene Benzene

Ether

Toluene

Amb.

Benzoyl chloride

Acetal

Toluene

111

Acetal

DMF

45


DMF DMF CHzC12

CsF CsF CsF

Amb. Amb. -45

Castanospermine (53)

Acetal Acetal Acetal Acetal Acetal Ether

MeOH MeOH MeOH Toluene Toluene Toluene

Et3N Et3N Et,N

Amb. Amb. Amb. - 10 Amb. -10

Methyl cY-D-xylopyranoside

Ether Acetal Acetal Acetal Acetal Acetal Acetal Acetal

CHCI, 1,4-Dioxane lP-Dioxane DMAP 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane 1,4-Dioxane Benzene TBAB

0 Amb. Amb. 100 100

Acetal Acetal Ether

Ip-Dioxane Benzene Toluene

Amb. Arnb. 0

Methyl 2,6-dideoxy-c~-~-ribohexopyranoside (148) Methyl 4,6-dideoxy-P-~-xylohexopyranoside (149) Methyl 2-deoxyl-P-~-eryrhropentopyranoside (150) Methyl 1,5-di-O-acetyl-(-)quinate (151) 152 153 154

Methyl 2-azido-2-deoxy-a-~xylopyranoside Methyl P-D-xylopyranoside

Amb. TBAl TBAI TBAl

80

80 80

50 100 80

-

94

0

Benzyl bromide

-

47

43

277

Trityl chloride

-

65

0

272

1' 99 2 ' 0 1' 63 2' 18 1 20 >66 6 40 0 0 6 39 0 0 0 0 644 0 0 6 94 6 76 0 0 0 0 1,6 75 0 0 92 52 0 30 52 32 0 6 3 4 57 6 23 47 28 50 12 0 28 37 70 13

278 278 279

p-Methoxybenzyl chloride p-Methoxybenzyl chloride 2-(Trimethylsilyl)ethoxymethyl chloride Acetyl chloride Decanoyl chloride Benzoyl chloride Benzoyl chloride Benzyl chloroformate Pivaloyl chloride (2 equiv) Bromine Benzoyl chloride Tosyl chloride Benzyl bromide Ally1 bromide Methoxymethyl chloride Methyl iodide Benzyl bromide Benzoyl chloride Benzoyl chloride Benzoyl chloride

0 0 0

0 0 0

276

-

loo 93 100

280 280 280 281 281 282 77 130 106 131 131 131 131 283 130 132 130

(continues)

TABLE VIII (Continued)

Yield (YO)


Ally P-D-xylopyranoside Benzyl P-D-xylopyranoside 2-Naphthyl &Dxylopyranoside 6-O-Trityl-~-gl~~al 6-O-t-Butyldimethyls~yl-Dglucal 6-Deoxy-~-glucal L-Rhamnal (155) D - F u (156) ~ D-Arabinal (157) D-Glucal

158 Methyl 5-O-acetyl-(-)shikamate (159)

A c e d or Ether

Solvent

Nuc." Temp. ("C)

Electrophile

Ether Acetal Acetal Acetal Acetal Acetal Acetal Acetal

CH2C12 1,4-Dioxane 1,4-Dioxane DMAP 1,4-Dioxane 1,4-Dioxane Benzene Benzene Benzene

Amb. Amb. Amb. 100 50 Amb. Amb. Amb.

Chloroacetyl chloride Phenoxythiocarbonyl chloride Tosyl chloride Benzyl bromide' Methoxymethyl chloride Benzoyl chloride Chloroacetyl chloride Chloroacetyl chloride

Acetal Acetal

CH2C12 Toluene

Et,N TBAB

Amb. 80

Benzoyl chloride Benzyl bromide

Acetal Acetal Acetal Acetal Acetal Acetal Ether Ether

Toluene DMF CH2C12 THF DMF Toluene Benzene Benzene

TBAB CsF EtSN CsF CsF TBAB

0 Amb. Amb. Amb. 80 70 Amb. Amb.

Methoxymethyl chloride Benzyl bromide Benzoyl chloride Methyl iodide p-Methoxybenzyl bromide Tosyl chloride NIS Benzyl bromide


DMF DMF DMF

CsF CsF

Amb. Amb. Amb.

t-Butyldimethylsilyl chloride Benzyl bromide Dimethoxytrityl chloride

TBAB

Added nucleophile; for abbreviations of nucleophile names, see Table IV. Also 9% of the di-0-benzoyl derivative. With 24 equiv: 6 equiv gave an 0-2, 0-3. 0-4 mixture of yields of 35 to trace to 356, respectively. Plus 10% 3.4-di-0-methoxymethyl derivative. 'Ketone product. With NIS in acetonitrile or with bromine or iodine in any solvent, 2-halo-1.6-anhvdro

derivativec are

0-3

0-4

Ref.

0 0 0 0 41 0 0 0

0 0 0 0 6 0 0 0

78 97 100 70 34 82 81 72

117 70 106 131 131 132 284 285

-

88

0

-

93

0

286 287

0-2

3,6 73 -

nhtPin-ri

70d 66 78 73

73 0 60' 0

28 38 68

288 219 286 289 0 210 60 290 0 291 0 182, 292 0 0 0 0

0 0 0

293 293 272


107

pllT:&

-

N-COOBn

HO

HO OH

OTs

2. TsCI,E&N

139

94%

FIG.41 .-p-Toluenesulfonation of the dibutylstannylene acetal of 4,6-O-benzylidene-Nbenzyloxycarbonyl-l-deoxy-norjirimycin.270 1. Bu,SnO

-

toluene

B

z

o

H

CH,

141

+

w OMe

CH3

12%

FIG.42.-Benzoylation

80%

of a secondary-tertiary dial?'

1. Bu,SnObenzene

H,C

2. BnBr,TBAI,

OH

80 "C

OH

95%

147 FIG.43.-Benzylation of a the dibutylstannylene acetal of a pyranose cis-diol having the equatorial oxygen atom next to a deoxy 1 Bu,SnO

benzene

B

n

2 BnBr,TBAI, 80 oc

o

M

OH

148

OMe

+

HoM OBn

40%

60%

1. Bu,SnO

C0,Me

2. TrCl / DMF

OAc

OTr OAc 65%

FIG.44.-Alkylation of dibutylstannylene acetals of a pyranose cis-diols having axial oxygen atoms next to a deoxy enter?^'.^^'

T. BRUCE GRINDLEY

108

dibutylstannylene acetal of compound 142, where the oxygen atoms are both equatorial, reacts only on the atom adjacent to the deoxy Even when the oxygen atom next to the deoxy center is tertiary, as in Fig. 42, reaction occurs mainly at this point.267When the equatorial oxygen atom of a cis-diol is next to the deoxy center, as in Fig. 43, only one product is formed. However, when the axial oxygen atom is next to the deoxy center, the two factors determining regioselectivity are in conflict and mixtures result, as shown in Fig. 44.

/ OBn

OH I

2

p h Y & J -.-

AH 138

HO

5

OH

139

How HO H

HO

H HO

O

W

OMe

CH,

OMe

140

W

I

,

e

,

142

I HO H

o

HO

141

OMe

HO H

CBZ

0

HO=OH HO'

?H

W

Ho&

W HO

OH

143

144

145

OMe


109

H3c-p0 H3cw0Me 146

OH

H

O

W

147

OH

O

M

e

148

C0,Me

OH OMe

149

OH

150

OAc

151

OH OH

155

154 R

= 3,4-dimethoxybenzyl

OH

156

OH

OH

157

159

T. BRUCE GRINDLEY

110

I . Bu2SnO-l ,4-dioxane

*

OMe

p,,ocso H

a OMe

2. PhOCSCl

OH

OH 97%

1. Bu2SnO-1,4-dioxane b B HO 2 -

q

+; H

,

2. PhCOCl

BzO

OMe

Ok

OMe 5 2%

3 0%

FIG.45.-Esterification of dibutylstannylene acetals of p- and a-~-xy~opyranosides.~"~~~"

Oxygen atoms at position 4 of dibutylstannylene acetals of pentopyranosides are also quite reactive, presumably because the lack of obstruction to the approach of electrophiles increases the rate constant for reaction at that position. Thus, dibutylstannylene acetals of P-xylopyranosides usually react exclusively at 0 - 4 (see Fig. 45), whereas a-xylopyranosides give mixtures of products at 0 - 2 and at 0 - 4 (see Fig. 46). Oxygen atoms in diols adjacent to double bonds exhibit a similar effect. Their dibutylstannylene acetals react preferentially on the allylic oxygen atom, as shown in Fig. 46. 5. Terminal

14-and 1,3-Diols

The reactions of tributylstannyl ethers and dibutylstannylene acetals of diols or polyols containing one hydroxyl group that is at the end of a chain (terminal diols) are listed in Table IX. Some reactions on compounds of this type, primarily unprotected glycosides, have been discussed earlier, but for those examples, the terminal hydroxyl group was not critical in

1 . Bu2SnO- toluene

HO

H

O

A

R,O

R = OTr

2. BzCI, Et,N-

CH2C12

R = OTBDMS 2. BnBr, TBAB - toluene, 80 "C R=H 2. BnBr, CsF - DMF, RT

R,=Bz

88%

R, = Bn 93%

R, = Bn 66%

FIG.46.-Reactions of dibutylstannylene acetals of glucal derivative^?'^.^,^^^

TABLE IX Reactions of Dibutylstannylene Acetals and Trihutyltin Ethers of Terminal 12- or l,3-Diols ~

~~

~~

Yield (%) Reaction Conditions Compound I,2 Diols 1,2-Propanediol

1-Phenyl-l,2-ethanediol

1,2-Hexanediol 12.4-Butanetriol

1,2,6-Hexanetriol 1-0-Methylglycerol 1-0-(p-Methoxyphenyl) glycerol

Acetal or Ether

Solvent

Nuc.” Temp. (“C)

Acetal Acetal

CHCI3 CHCI3


DMF CHC1, CHC13

CsF

Acetal Acetal Acetal Acetal Acetal Acetal Acetal Acetal Acetal Acetal

DMF CHC13 Benzene Benzene DMF CHCI3 Benzene Benzene DMF CHC13

CsF

Ether Acetal Acetal Acetal

Toluene CHC13 DMF DMF

Amb. Amb.

Et3N Et3M CsF Et3N EQN CsF

Amb. Amb. Amb. Amb. Amb. 0 0 Amb. Amb. 0 0 Amb. Amb.

0 CsF CsF

Amb. Amb. Amb.

Electrophile

Benzoyl chlorideb 1-Butyldimethylsilyl chloride Benzyl iodide Benzoyl chlorideh r-Butyldimethylsilyl chloride Benzyl bromide Benzoyl chloride” Acetyl chloride Tosyl chloride Benzyl bromide Benzoyl chloride‘ Acetyl chloride Tosyl chloride Benzyl bromide t-Butyldimethylsilyl chloride Benzoyl chloride Benzoyl chlorided Benzyl iodide 1-Bromohexadecane

Secondary Site

Terminal Site

Ref.

71 0

13 74

84 87

9 86

91

69

5

84

0

97

87

10

0 - 2 86 7 70 82

69

55

0 0 0 60 0 0 0 0

0 55

10 8

80 24 0 - 1 71 0-1 72 0-1 70 0-4 99

0-1 67 17 67 90

84

12 12 12 84 12 12 12 87 14 84

69 294

(continues)

TABLE IX (Continued) Yield (YO) Reaction Conditions Compound 1-0-Benzylglycerol

3 +

N

Methyl 2,3dihydroxypropanoate 12-O-Isopropylidene-Lthreitol (160) 2-O-Benzyl-l-O-rbutyldiphenylsilyl-L-threitol (161) 3,4-Di-O-benzyl-~-mannito1

12:3,4-Di-O-isopropylidene-

Acetal or Ether

Solvent

Nuc'

Temp. ("C)

Acetal Acetal Acetal Acetal Acetal

CH2ClZ-THF 4 : 1 DMF DMF DMF DMF CsF

Acetal

Toluene

Acetal

CHIC12

Acetal

Toluene

Acetal

CHC13

Acetalc Acetal

CHC13 CHC13

Acetalc Acetal

CHCL Dioxane

Acetal

Toluene

TBAI

20

Acetal Acetal

Toluene Toluene

TBAB TBAB

70 111

TBAI

TBAI

0 90 100

90 Amb.

Electrophile Acetyl chloride 1-Bromododecane 1-Bromohexadecane I-Bromooctadecane Benzyl iodide

Secondary Site

Terminal Site

Ref.

0 0 0 0 14

65 71 71 62

294 295 296 295 69

84

70

Benzyl bromide

0

84

297

40

Tosyl chloride

0

85

298

70

0

20

Benzyl bromide (4 equiv) Tosyl chloride

20 20

Tosyl chloride Tosyl chloride

1,6 74

299

2

96

20

58 12

38 80

20 20

60 1,6 46

17 1.5 20

20 300

1,6 67

Cyclized 15 301

1,6 60 1,6 54

302 300

D-mannitol 3,4-Di-O-benzylll,2-0-

isopropylidene-D-mannitol (2S,5S)-12,5,6-Hexanetetraol

20 Amb.

(162)

Tosyl chloride Benzoyl chloride (2.2 equiv) Tosyl chloride (2.1 equiv) Benzyl bromide Benzyl bromide (4 equiv)

(2S,3E,5S)-Hex-3-ene-l,2,5,6tetraol (163) (2S,4Z,6S )-7-)tButyldimethy1silyloxy)-hept4-ene-1,2,6-triol (164)

Acetal

Toluene

Acetal

MeOH

(2R,3S,4E)-3-Methyl-S-phenyl-Acetal 4-pentene-1,2-diol (165) 1,2-O-Isopropylidene-3-0Acetal methyl-a-D-glucofuranose

3-0-Benzyl-1,2-0isopropyhdene-a-Dglucofuranose c W

3-O-Benzoyl-1.2-0isopropyhdene-a-Dglucofuranose

1,2-O-Isopropylidene-3-O-ptoluenesulfonyl-a-Dglucofuranose 3-O-Benzyl-l,2-0isopropylidene-a-Dglucofuranose 3-Deoxy-3-fluoro-1,2-0isopropylidene-a-Dglucofuranose 3-Deoxy-3-iodo-l.2-0isopropylidene-a-Dallofuranose

Toluene

EtjN

O + 25

Tosyl chloride (2.1 equiv) Tosyl chloride

1,6 83

Tosyl chloride

0

20

301

303

1 68

82

14

0

76

Benzene

Amb.

Bromine

48

Acetal Acetal Acetalf Acetal

CHC13 Benzene CHC13 Toluene

Arnb. Amb. 20 20

NBS Benzoyl chloride Tosyl chloride Tosyl chloride

95 7.5 89 20

0 38' 5 78

82 76 20 20

Acetal Acetalf Ether

CHC13 CHC13 Benzene

20 20 Amb.

Tosyl chloride Tosyl chloride Bromine

39 95 65

59 4 0

20 20 304

Ether

Benzene

Amb.

Bromine

70

0

304

Acetal

CHC13

Amb.

NBS

90

0

82

Acetal

MeOH

0

75

305

Ether

Benzene

57

0

304

Et3N

0

Amb.

Benzoyl chloride

Bromine

(continues)

TABLE IX (Continued) ~

Yield (YO) Reaction Conditions Compound

Acetal or Ether

Solvent

Nuc" Temp. ("C)

Electrophile

Secondary Site

Tenninal Site

Ref.

3-O-Benzoyl-1,2-0isopropylidene-a-~allofuranose

Acetal

Pyridine

20

Benzoyl chloride

0

>65

80

1,2-O-Isopropylidene-3-0-

Acetal

CHC13

20

Tosyl chloride

0

98

20

Acetal

DMF

CsF

Amb.

8

85

306

3-Deoxy-l,2-O-isopropylidene- Acetal

DMF

CsF

Amb.

Benzyl bromide, TBAI Benzyl bromide

0

66

307

Amb.

Bromine

92

0

304

Tosyl chloride Tosyl chloride Bromine

5 51 5 92

94

0

20 20 77

Bromine NBS Benzoyl chloride Tosyl chloride Tosyl chloride

5 50

50

0 0 75 98 90

308 82 106 106 20

me thyl-a-o-allofuranose

3-C-methyl-a-~allofuranose (166) 1,2-0-Isopropylidene-3-deoxy- Ether a-D-ribo-hexofuranose (167) Acetal Acetalf 1,2-O-Isopropylidene-a-~Ether glucofuranose Acetal Acetal Acetal Acetal 1,2-O-Isopropylidene-a-~Acetal allofuranose Methyl 2.3-0-isopropylideneAcetal a-D-mannofuranose Acetalf

Benzene CHC13 CHCl3 CHC13 CHZCIz CHCl3 Dioxane Dioxane CHCl3

20 20 0

0 Amb. Arnb. Amb. 20

584 50 50

48

CHC13

20

Tosyl chloride

0

98

20

CHCl3

20

Tosyl chloride

69

28

20

+ L

Ethyl 2,3-di-O-benzyl-P-~galactofuranose (168) [2R(S),3S,5R(S)]-2-(1Benzyloxy-2-propyl)-5-(2,2dideuterio-l.2dihydroxyethyl) 3-methyltetrahydrofuran (169) Methyl (methyl 3-deoxy-4,50-isopropyfidene-a-Dmanno-2octu1opyranoside)onate (170) Methyl (methyl 4-0-benzoyl5-O-benzyl-3-deoxy-a-~manno-2octu1opyranosid)onate (17l)

1,3-Diols 1.3-Butanediol

Acetal

MeOH

Et3N

Acetal

MeOH

Et3N

Acetal

Benzene

TBAB

80

Acetal

Benzene

TBAB

100

Acetal Acetal

CHC13 CHCl3

(2S,3S,4R)-2,4-Dimethylhex-5-Acetal

0 Amb.

Amb. Amb.

Benzene

Etfi

Benzene Benzene CHzCl

EtjN TBAI

Benzene Toluene

Amb.

Tosyl chloride

0

81

309

Tosyl chloride

0

81

310

Benzyl bromide

0

8 81

252

Methyl iodide

0

873

311

63 0

17 99

87

0

77

12

88 98 0

12 12 78

Benzoyl chlorideb r-Butyldimethylsilyl chloride Benzoyl chloride

84

ene-1,3-diol (172) Acetal Acetal Ether

Methy 2,3-O-isopropylidene-aD-mannopyranoside Ethyl 2,3-dideoxy-a-~-eryfhro-Acetal hexopyranoside (173) 1,3-O-Ethylidene-~-erythritol Acetal (174)

0

0 0 0 - 4 768

0

Tosyl chloride Benzyl bromide Bromine

TBAB

80

Benzyl bromide

0

82

312

TBAI

70

Benzyl bromide

0

55

313

50

(continues)

TABLE IX (Continued) ~~~~

Yield (YO) Reaction Conditions

Compound

+

c)

QI

Benzyl 2-Acetamido-3-0benzyl-2-deoxy-a-~glucopyranoside t-Butyldimethylsily 2-azido-2deoxy-B-D-glucopyranoside Benzyl 2-acetamido-3-0benzyl-2-deoxy-P-~glucopyranoside Ally1 3-0-benzyl-2-deoxy-2phthalimido-P-Dglucopyranoside 2,2,2-Trichloroethyl 2-deoxy-2phthalimido-Paglucopyranoside f-Butyldimethylsilyl 2-azido-3O-benzyl-2-deoxy-P-~glucopyranoside Methyl-3-azido-3-deoxy-2-0methyl-P-Dglucopyranoside (175) Benzyl 2,3-di-O-benzyI-a-oglucopyranoside Tigogeninyl 2,3-di-O-benzyl-PD-galactopyranoside

A c e d or Ether

Solvent

Nuc" Temp. ("C)

Electrophile

Secondary Site

Terminal Site

Ref.

Ether

Toluene

TBAB

80

Benzyl bromide

All 0

86

314

Ether

Toluene

TBAI

95

Benzyl bromide

All 0

70

315

Ether

Toluene

TBAI

111

Benzyl bromide

All 0

78

316

Ether

BnBr

90

Benzyl bromide

All 0

76

317

Ether

BnBr

80

Benzyl bromide

AU 0

60

156

Acetal

Benzene

TBAI

50

Benzyl bromide

0

77

12

Acetal

CHiCN

TBAB

80

Benzyl bromide

0

81

318

Acetal

Benzene

Amb.

Benzoyl chloride

0

90

76

Acetal Acetal

Benzene Toluene

80 111

Benzyl bromide Benzyl bromide

0 0

80 91

63 319

TBAI NMI

Benzyl 3-O-benzyl-a-~galactopyranoside Benzyl 3-O-allyl-a-ogalactopyranoside Ally1 2-acetamido-3-0-benzyl2-deoxy-a-~galactopyranoside Ethyl 2,3-O-isopropylidene-lt hio-a-o-mannopyranoside Methyl 2,3,6,2’,3’-penta-Obenzyl-B-D-lactoside (176) a-Cyclodextrin

Acetal

Benzene

TBAI

80

Benzyl bromide

0

72

63

Acetal

Benzene

TBAI

80

Ally1 bromide

0

60

63

Acetal

Toluene

TBAI

70

Benzyl bromide

0

73

320

Acetal

DMF

CsF

Amb.

Benzyl bromide

0

75

218

Ether

CH,CN

NMI

45

Methyl iodide

0

Ether Ether

Toluene Toluene

50 50

Methyl B-D-glucopyranoside

Acetal Ether Acetal

1,CDioxane Toluene 1,4-Dioxane


1,4-Dioxane 1,4-Dioxane Toluene

Acetal

Toluene


DMF DMF DMF

Et3N

Amb. Amb. Amb.

Acetal

DMF

Et,N

Amb.

Acetal

Toluene

Tosyl chloride 2-NaphthalenesuLfonyI chloride Benzoyl chloride Benzoyl chloride Phenoxythiocarbonyl chloride Tosyl chloride Tosyl chloride f-Butyldimethylsilyl chloride Trimethylsilyl chloride Decanoic anhydride Decanoyl chloride Octadecanoyl anhydride Octadecanoyl chloride t-Butyldimethylsilyl chloride

Methyl a-D-glucopyranoside

Sucrose (48)

Methyl a-D-mannopyranoside

Amb. 0 Amb.

DMAP

Amb. Amb. 100 100

100

6’ 78

119

Penta 6 9 Hexa 6 32 Hexa 6 78

321 321

All 0 All 0 All0

86 19 85

130 130

All 0 All 0 AUO

92 92 90

70 70 88

All0

93

88

70

0 0 0

6 Mh 6 6Sh 6 Mh

322 322 322

0

6 47h

322

All 0

84

88

(continues)

TABLE IX (Continued) Yield (YO) Reaction Conditions Compound

A c e d or Ether

Secondary Solvent

Nuc" Temp. ("C)

Methyl a-D-galactopyranoside

Acetal

Toluene

100

Ethyl 1-thio-P-D-lactoside (79)

Ether Acetal

Toluene THF

0 Amb.

Phenyl 2,3-dideoxyl-l-thio-aAcetal ~-threo-hex-2enopyranoside (177) Ethyl 2,3-dideoxy-a-~-eryrhro- Acetal hex-2-enopyranoside (178) D-GIu~ Acetal Uridine (l26)

Acetal

Cytidine (130)

Acetal

Adenosine (131)

Acetal

CHCl3

TBAB

Amb.

Benzene

TBAB

80

Toluene DMF-1,C Dioxane' DMF-1,4DioxaneJ DMF-1,4Dioxanej

100 Amb. Amb. Amb.

Electrophile r-Butyldimethylsilyl chloride Benzoyl chloride r-Butyldimethylsilyl chloride Tosyl chloride

Benzyl bromide r-Butyldimethylsilyl chloride t-Butyldimethylsilyl chloride f-Butyldimethylsilyl chloride t-Butyldimethylsilyl chloride

Added nucleophile; for abbreviations of nucleophile names, see Table IV. Followed by reaction with chlorodimethylphenylsilane. Followed by reaction with oxalic acid. Followed by reaction with chlorotrimethylsilane. ' Plus 24% 5,6-di-O-benzoyl derivative. 'Hexamethylenestannylene acetal. Plus 20% of the dimeric ester. Longer reaction times gave slightly increased yields plus small (2-5%) amounts of the 3-0-substituted derivatives. ' Plus 32% of a mixture of the 2,6- and 3.6-di-0-benzoyl derivatives. Solvent ratio DMF to 1.4-dioxane. 1 to 4. a

Terminal Site

Ref.

92

88

59'

130 86

0

86

210

0

86

323

All 0

67

88

Site

All 0 All 0 All 0

6' 96

All 0

5 91

85

All 0

5 82

85

All 0

5 80

85


119

determining the regiochemistry of the reaction considered. The complicated factors that influence regiochemistry in reactions of these organotin intermediates are understood best for the derivatives of these terminal diols, as outlined in Section VI. From a synthetic point of view, the most important observation is that dibutylstannylene acetals and tributylstannyl ethers can be used to effect terminal substitution of diols in excellent yields, often better than can be obtained by direct reaction at low temperatures, even for benzoylation or p-toluenesulfonylation, where direct reaction is reasonably effective. Terminal 0-alkylation, which cannot be performed directly, is routine through choice of the appropriate conditions, as outlined in the sections following. These types of reactions are considered first, followed by reactions where the nonterminal oxygen atom is favored.

OBn

OH

162 160

161

163

164 H O l

166

Ho "7c 167

OMt?

170

T. BRUCE GRINDLEY

120

Bn&

BzO

H

C0,Me

171 OMe

O

173

172

W OMe

174 OH

OH

boMe N3

BnO h

175

o

OBn &

Brio &

OBii OM

176

Hoe -

I

177

SPh

178

OEt

Both tributylstannyl ethers and dibutylstannylene acetals of terminal triols yield in most cases the product of reaction with 1,2-diolsin preference to other hydroxyl groups, as shown in Figs. 47 and 48. Figure 47 also illustrates the tendency of t-butylchlorodimethylsilane to react with terminal 1,3-diols in preference to terminal 1,2-diols.” Figure 48 shows that the preference for reaction at terminal oxygen atoms ,is considerably stronger than the preference for reaction next to unsubstituted centers, discussed in the previous section. As the numerous examples in Table IX demonstrate, this selectivity is maintained over a wide range of structural features. The two sets of conditions most employed for benzylation of dibutylstannylene acetals are benzyl bromide with cesium fluoride in DMF at room temperature and benzyl bromide with tetrabutylammonium iodide or bromide in toluene or benzene at elevated temperatures. Although there are no examples with careful analysis of the product mixtures where the same substrate has been allowed to react under both sets of conditions with terminal 12-diols, examination of Table IX suggests that the latter condi-


121

OH H

0

L

O

H 2a. TsCI, Et,N-benzene,

0 OC

2b. BnBr, CsF-DMF 2c. TBDMSCI-CHCI,

Rl = TBDMS, R2 = H 99%

OH

1. (Bu,Sn),O

FIG. 47.-Reactions

R' = H, R2 = Ts 72% R1 = H, R2= Bn 70%

-

OH

of dibutylstannylene a c e t a l ~ ' ~and " ~ a tributylstannyle ether74 of

two triols.

tions provide more selective reactions for this type of substrate. Numerous examples have appeared in which the CsF conditions yielded mixtures containing small proportions of the secondary products, but no examples have been noted in which the latter conditions yielded mixtures. Figure 49 shows an example of methylation of a sensitive substrate under the latter condition^.^'' Dibutylstannylene acetals and tributylstannyl ethers of terminal 1,3-diols also react preferentially with most electrophiles on the terminal oxygen atom. The terminal 1,3-diol can be acyclic, or the secondary oxygen atom can be on a ring adjacent to an hydroxymethyl group, such as 0 - 4 and 06 of aldohexopyranoses or 0 - 3 and 0- 5 of aldopentofuranoses. Figures 50 to 53 show some examples. C&OBn HO 4 ' O H

1 . BySnO y

r

OH CyOH

162 FIG.4d.-Benzylation

*

2

2. BnBr, Bu,NBr

-

toluene, 70 "C, 5h

Hoi 7%

POH

CH,OBn

60%

of the dibutylstannylene acetal of 3,4-dideoxy-~-rhreo-hexitol.'"'~~

122

T. BRUCE GRINDLEY

pvoMe

Bn I

1. Bu,SnO

C0,Me

c

2. MeI, Bu,NBr benzene, 100 OC

BzO

I 171

FIG.49.-Methylation vessel?"

OMe

73%

OMe

of the dibutylstannylene acetal of a Kdo derivative in a sealed

I . BySnO

benzene, 0 "C

172

2b. BnBr, Bu,NI benzene, 50 "C

FIG.50.-Reactions

6 OMe

NHAc

OH

-

2. BnBr, Bu,NI-

Ho&/

NHAc OMe

BnO

toluene, 11 I "C

78%

of the tributylstannyl ether of a terminal 1,3-diol from a p-D-

OM.

2. BnBr, Bu,NBr CH,CN, 80 OC

175

R = B n 98%

1. (Bu,Sn),O

OH

N3

-

of the dibutylstannylene acetal of an acyclic terminal 1,3-diol.l2

HO BnO

FIG. 51.-Benzylation glucopyranoside

R = T s 88%

OH

OBn

k

O

M

e

N3

8 1%

FIG.52.-Benzylation of the dibutylstannylene acetal of a terminal 1.3-diol from a 0-0gulopyranoside derivative."'


123

OH CH,OTs

OH CH,OH I. B y S n O

b

-b

2. TsCI, Bu,NBr-CHCI,

SPh SPh 86% FIG.53.-p-Toluenesulfonylation of the dibutylstannylene acetal of the terminal 1,3-diol in phenyl 2,3-dideoxy- l-thi0-a-o-rhreo-2-enopyranoside.~"

As previously noted, dibutylstannylene acetals of all compounds that have a free primary hydroxyl group, including such compounds as methyl a-D-mannopyranoside or uridine, which have cis-1,2-diols unprotected, react with t-butylchlorodimethylsilane to give substitution on the primary oxygen atom in high ~ i e l d . There ~ ~ - ~is no evidence of any products of substitution on secondary oxygen atoms in these reactions. Oxidation of both dibutylstannylene acetals76~xz2.30x and tributylstannyl of terminal 1,2- and 1,3-diols with bromine or Nbromosuccinimide occur at the more substituted oxygen atom. The causes of this preference were discussed in Section VI. Two types of reaction conditions from stannylene acetals have been developed that effect preferential substitution on the oxygen at the more substituted carbon atom. The method of Roelens and his co-workers has been applied to a wide range of simple non-carbohydrate 1,2- and 13-diols. The dibutylstannylene acetals of these compounds are treated with benzoyl chloride followed by a chlorosilane, normally chlorodimethyphenylsilane, but chlorotrimethylsilane is nearly as effective (see Fig. 54). The two esterdibutylstannyl ether intermediates from the first stage can equilibrate. The bulky silylating agent has a strong preference for reaction at the primary oxygen atom, and equilibration takes place during the second step of the reaction sequence. Thus, products having the ester on the secondary or tertiary oxygen are obtained, with good r e g i o ~ p e c i f i c i t y It . ~has ~ ~ ~not ~~~~ yet been demonstrated that the dimethylphenylsilyl or trimethylsilyl groups

I . Bu,SnO

H O T P h OH

2 BzCI-CHCI,, RT, Ih

B z 0 y p h

OSiM5Ph 5

3 PhMe$CI-CHCI,, O°C, 1.5 h

FIG.54.-Benzoylation

+

Pm%SiO/YPh OBz 95 (59% isolated)

of 2-phenyl-I.2-enthanediolwith reversed regioselectivity?'R4

T. BRUCE GRINDLEY

124 HO

1. R,SnO D

2. TsCI-CHCI,

R=Bu 2R=(C&),

43% 5%

52% 91%

FIG. 55.-Tosylation of 3-O-benzyl-l,2-O-isopropylidene-cr-~-glucofuranose with reversed regioselectivity?"

can be removed without causing isomerization of the ester. For some substrates, oxalic acid in chloroform, which is presumably an aggregate, also gives preferential quenching at the primary center. A second method can be used for p-toluenesulfonylation reactions performed in the absence of added nucleophiles. As outlined in Section VI, dialkylstannylene acetals of terminal 12-diols are present mainly as dimers that have the secondary oxygen atom dicoordinate, and thus these oxygen atoms are more nucleophilic. In other less populated dimers, the primary oxygen atom is dicoordinate, and its inherently greater reactivity than the secondary oxygen atom makes this less populated dimer more reactive. If the population of the more populated dimer is sufficiently greater than that of the less populated (but more reactive) dimer, reaction at the secondary oxygen atom is preferred. These conditions are not met with dibutylstannylene acetals, but use of hexamethylenestannylene acetals leads to substitution on the secondary oxygen atoms with fair to excellent regioselectivity, depending on the substrate (see Fig. 55).'9.20337The starting materials for the latter type of stannylene acetal, hexamethylenetin oxide, is not available commercially but can be prepared conveniently.2' 6. Glycoside Formation

Some of the most exciting developments in the applications of organotin derivatives to carbohydrate chemistry have come in their use for the synthesis of glycosides. These intermediates have served as both glycosyl donors and glycosyl acceptors. Applications in the former area are discussed first and are listed in Table X. 1,2-cis-Methyl glycosides have been formed in variable yields from partially protected free sugars.224,325,326 For instance, the dibutylstannylene reacts with methyl iodide in acetal of 3,4,6-tri-0-benzyl-~-mannopyranose DMF at 45°C to give the 6-glycoside in 94% yield, or with ally1 bromide

TABLEX Glycoside Formation Using Dibntylstannylene Acetals as Glycosyl Donors Yield (YO)


Solvent

Nuc."

DMF 3,4,6-Tri-O-benzyl-~-mannopyranose 3,4.6-Tri-O-benzyl-o-mannopyranose DMF 3,4,6-Tri-O-benzyl-~-mannopyranose Benzene TBAI 3,4.6-Tri-O-benzyl-~-gIucopyranose DMF 3,5-Di-O-benzyl-~-ribofuranose DMF 3,5-Di-O-benzyl-o-ribofuranose KzC03 DMF Pyridine o-Mannose 4,rid in e D-Mannose Pyridine L-Rhamnose Pyridine L-Rhamnose Pyridine D-Lyxose 6-0-Trityl-D-talose Pyridine CH&N BbNF 3-O-Benzyk-~-mannose

Temp. ("C) 45 75 80 45 38 Amb. Amb. Amb. Amb. Amb. Amb. Amb. 25

3-O-Benzyl-o-mannose

DMF

CsF

25

D-Mannose

CH3CN

Bu~NF

25

o-Mannose

DMSO

L-Rhamnose

DMF

CsF

-5

L-Rhamnose

DMF

CsF

25

25

Electrophile

a

0 Methyl iodide 0 Ally1 bromide 0 Benzyl bromide 10 Methyl iodide 49 Methyl iodide 83 Benzyl bromide 0 Acetic anhydride 0 Benzoyl chloride 0 Acetic anhydride 0 Benzoyl chloride 0 Acetic anhydride 0 Acetic anhydride Methyl 2,3,4-tri-O-benzoyl-6-0-triflyl-a-~- 0 glucopyranoside Methyl 2,3,6-tri-O-benzoyl-4-0-triflyl-a-~0 galactopyranoside Methyl 2,3,4-tri-O-benzoyl-6-0-tnflyl-a-o0 glucopyranoside 0 Methyl 2.3,6-tri-0-benzoyl~-O-triflyl-a-~galactopyranoside 0 Methyl 2,3,4-tri-O-benzoyl-6-O-triflyl-a-~glucopyranoside Methyl 2,3,6-tn-O-benzoyl-4-0-triffyl-cY-~- 0 galactopyranoside

Added nucleophile; for abbreviations of nucleophile names, see Table IV. F'yranose form isolated as the pentaacetate. Pyranose form isolated as the pentabenzoate. Pyranose form isolated as the tetraacetate. ' 1.2.3,5-Tetra-O-acetyl-6-O-tntyl-~-~-talofuranose was accompanied by 13%of the a-furanose anomer.

p

Other

Ref.

94

0 0 0 2 70 243 2 13

224 224 325 224 326 326 327 327 327 327 327 327 230

Loo 70 0 0 0 85 72' 95 9w 85d 40d 75

8-f 13'

65

230

57

230

59

230

88

230

78

230

126

T. BRUCE GRINDLEY

at 75°C to give the P-glycoside in 100%yield.224However, the dibutylstannylene acetal of 3,4,6-tri-0-benzyl-~-glucopyranose, on treatment with methyl iodide in DMF at 45"C, mainly gives the 2-methyl ether (70% yield) and only 10% of the 1,2-cis-glycoside,the (Y a n ~ m e r . ~ ~ ~ cis-Glycosyl esters, either acetates or benzoates, can be formed with good stereochemical control from free sugars having axial hydroxyl groups at C2 by reaction of the dibutylstannylene acetals, formed in methanol, with acetic anhydride or benzoyl chloride in ~ y r i d i n e . ~ The '~ 1,2-0dibutylstannylene acetal is probably the major species present in solution,'27 but this useful result presumably arises because 0-1 in this species is the most nucleophilic oxygen atom in the mixture of stannylene acetals present (see Fig. 56). These reactions give mixtures if 0 - 2 is oriented e q ~ a t o r i a l l y . ~ ~ ~ A most promising development is the observation that dibutylstannylene acetals of L-rhamnose and D-mannose derivatives having 0-1 and 0 - 2 unprotected react with primary and secondary triflates of sugars under mild conditions to give in good yields cis-(1+2)-linked disaccharides with inversion in the glycosyl acceptor (see Fig. 57).230As noted in Table X, L-rhamnose derivatives give better yields than shown for the mannose derivatives in Fig. 57. Tributylstannyl and dibutylstannylene derivatives have also been employed as glycosyl acceptors. In particular, tributylstannyl groups have often been used to increase the nucleophilicity of oxygen and sulfur atoms. Simple alkyl and aryl glycosides and thioglycosides can.be prepared in useful yields by reaction of glycosyl acetates, halides, or sulfoxides with tributyltin ethers and thioethers in the presence of such Lewis acids as trimethylsilyl trifluoro m e t h a n e ~ u l f o n a t e or ~ ~tin '~~~~ The reaction has been applied to the synthesis of S-glycosylatedpep tide^.^^ Dibutyltin derivatives [ B u ~ S ~ ( S RR) = ~ ,Ph, Me, cyclohexyl,t-Bu; Bu2Sn(SePh),] have also been found to react in the same way, particularly when Bu2Sn(0Tf)* is used as the Lewis acid catalyst.336 Glycosyl acetates react with tributyltin methoxide to give glycosyl tributylstannyl ethers. As with glycosyl anions,337-339 the anomeric effect causes

PH c''lOri~ B

"

W

J

,

B ' O

U

z

~

W

Pyridine OBz

'Bu Bu

90%

Bu FIG.

O

56.-Formation of a cis-glycosyl ester from a dibutylstannylene acetal.'*'

B

z


H HO O

1. BySnO

a

O

BzO %

Bz 0

59%

Bz.0 BZO

127

OMe

I OMe

DMSO. 25 "C, 48 h

1 Bu,SnO

* HBnO O

a

O

%

BzO BzO

67%

Bz 0

DMF, CsF, 25 "C, 18 h FIG.57.-Formation of cb-1,2-linked disaccharides by reactions of glycosyl dibutylstannylene acetals with secondary triflate~.~~"

the axial orientation to be favored. Hydrolysis on workup is then a convenient method for freeing the anomeric center of a poly-0-acetyl derivative for conversion into a more reactive glycosyl d ~ n o r . ~Reaction ~ ' , ~ ~ with allyl acetate in the presence of catalytic palladium(0) compounds gives allyl glycosides with predominant retention of anomeric configuration, even in the presence of participating This type of reaction has also been used for the formation of disaccharides and oligosaccharides. Liu and Danishefsky have achieved very good regioselectivity for the primary oxygen atom in the reaction of the tributylstannyl ether of a terminal, 1,3-diol with a glycosidic oxirane in the presence of a Lewis acid catalyst, Zn(OTf)*, and also obtained good trunsstereoselectivity (see Fig. 58).343A number of other examples of this type of reaction have appeared, using this and other active glycosyl don o r ~ . ~ ~ ~Choice - ~ ~ of ' *a~different ~ " . ~ Lewis ~ ~ acid can lead to cis-(1+2) glycosidic linkages (Fig. 59).344In an interesting application, Danishefsky et ~ 1 . ~have ~ ' reacted tributylstannyl ethers derived from 6-0-tbutyldimethylsilyl-D-galactal and 6,6'-di-O-t-butyldimethylsilyl-lactalwith glyglycal-derived iodosulfonamides to give trans-2-deoxy-2-sulfonylamino

OMe

T. BRUCE GRINDLEY

128

TESo

a

A

1. 2,2-Dimethyldioxirane acetone, 0 "C

o

~

TESO

b

y3

OTES

2. 2eq. B

u

,

S

n

I OH

O\ w (CHZ),*CH,

2 eq. Zn(OTf), -THF, 0 "C to RT

oms TESO

TESO OTES

OH

OH 44% after desilylation and acetylation

FIG.58.-Formation of a fruns-(l-+ 2)-linked glycoside by a Lewis acid-catalyzed reaction of a glycosyl oxirane with a tributyltin either.'4'

cosides in yields of 52 and 42%,respectively (Fig. 60). It should be emphasized that, in most of these reactions, the role of the tributyltin ether is simply to enhance the nucleophilicity of the ethereal oxygen, and tributylstannyl ethers are used only when the reaction with the alcohol is slow.346 Peracetylated and perbenzylated glycosyl chlorides and acetates were investigated some years ago as glycosyl donors toward dibutylstannylene

&

N 3

2 eel

B Y s n o ~ ( c % ) , , c % OH

BnO BnO

0

2 eq. AgBF4- THF, 0 "C to RT b

BnO BnO

y3

o-i--(c%)'2cq OH

41% after acetylation

FIG.59.-Formation of a cis-( 1+2)-linked glycoside by a silver tetrafluoroborate-catalyzed reaction of glycosyl oxirane with a tributyltin either.j4'


129 OTBDMS

AcO AcO

10 eq.

I OBn

Bn 0

- %:& 5 eq. AgBF,

OTBDMS

AcO

AcO

THF

AcO

OAC

:&

I

I

NHR

52%

Bn 0

FIG.60.-Reaction of tributylstannyl ethers with iodos~lfonamides~~’

acetals, both in the absence and presence of Lewis acids, but the results were mixed.347In a few cases, disaccharides and trisaccharides were obtained, but, in the case of peracetates, orthoesters were often the major products and the products obtained were the same as those formed in the absence of organotin activation. However, it has since been demonstrated that (1+6)-glycosyl linkages can be formed efficiently in this manner. Martin-Lomas and his co-workers used the tributylstannyl ether of methyl P-lactoside as a glycosyl acceptor, with 2,3,4,6-tetra-O-benzyl-au-~-galactosyl bromide in toluene containing tetraethylammonium bromide, to give the a-(1+6)-linked trisaccharide in 58% yield plus a further 14% of the a-(1+6)-linked trisaccharide and also glycosylated 1,6-anhydro-P-~-galactopyranose.~~* Subsequently, Garegg et al. demonstrated that this procedure is general; dibutystannylene acetals of methyl a- and 0-D-galactopyranosides and methyl j3-D-glucopyranosides act as glycosyl acceptors toward glycosyl halides, with tetrabutylammonium iodide and glycosyl thioglycosides activated by DMTST, to form (1-6)linkages in yields of 4441% (see Figs. 61 and 62).349 7. Miscellaneous

Table XI lists results on acyclic diols whose structures cannot be characterized into one of the foregoing categories. The observations on the benzylation of the dibutylstannylene acetal of 2,3-dihydroxybutanoic acid derivatives in DMF in the presence of cesium fluoride, where more product is

130

T. BRUCE GRINDLEY

&

BnO BnO

OBn Bu,SnOOMe MeOH, reflux

HO

Bu,NI-CH,CI,, RT, 4 days

-

OH

Br

c

BnO BnO OHO O

78% "

N

k

O

M

e

OH

FIG.61.-Formation of a (1+6)-linked disaccharide by reaction of a dibutylstannylene acetals of unprotected glycosides with a glycosyl br0rnide.3~'

&

SEt

AcO AcO

-

B%snoOMe MeOH, reflux

HO

OH

DIVITST -CH,CI,, RT, 10 h

NPhth

C

, OAc

AcO NPhth HO 81%

OMe OH

FIG.62.-Formation of a (1+6)-linked disaccharide by reaction of a dibutylstannylene acetal of an unprotected glycoside with a thioglycoside and DMTST?5'


131

obtained on 0-3, is probably due the greater nucleophilicity of this oxygen atom in the monocoordinated dimer, although the original authors offered a different explanation>' The preference for reaction on 0 - 4 of 2-methyl2,4-pentanediol is simply due to steric hindrance. The results on benzoylation of the dibutylstannylene acetal of 5-0benzoyl-L-arabinose dialkyl dithioacetals and related compounds are interesting (see Fig. 63).350For the dialkyl dithioacetals and for the benzyloxime, there is a marked preference for reaction at 0-4, adjacent to the existing benzoyl group, followed by reaction at 0-2, adjacent to the C-1 group. The first preference is similar to that noted earlier for P-D-xylopyranoside derivatives, where replacement of a 4-0-benzyl group by a 4-0-benzoyl group caused the regioselectivity of the reaction to change drastically from close to a 1 : l mixture to reaction exclusively at 0-3.'32 However, the regioselectivity of these reactions on acyclic diols may be controlled by the rate of quenching.53Therefore, comparison with direct benzoylation may be more relevant, and it is interesting that preferences for the direct reaction are similar: For example, D-galactose diethyl dithioacetal forms the 5,6-dibenzoate, and then the 2,5,6-tri-benzoate under these ~onditions.3~~ In the dibutylstannylene acetal reaction, replacement of a dialkyl dithioacetal by a dimethyl acetal reverses the two regioselectivitiesjust mentioned, in line with the argument on electron-withdrawing effects. An interesting reaction that does not fit into any of these categories has been reported by Hodosi and K o v ~ i EThey . ~ ~ noted that, when aldoses and ketoses having 0 - 2 unprotected are heated at >60°C for an extended period of time (2 to 24 h) with dibutyltin oxide in methanol, ethanol, or benzene, epimerization occurs at C-2.73The products obtained contain much more of those compounds having 0-2 in an axial orientation than would be expected on thermodynamic grounds, and also more than that obtained in

WR),

HO{ HO

CHCR),

OH

1 . Bu,SnO 2. BzCI- benzene

HOToR1 R20

C%OH

CqOH

179

R-SEt

R = S E t R ' = H R 2 = B z 75%

180

R=OMe

R = O M e R 1 = B z R 2 = H 97%

FIG. 63.-Benzoylation arabinose?'*

of dibutylstannylene acetals of acyclic triols derived from L-

TABLE XI Reactions of Dibutylstannylene Acetals and Tributyltin Ethers of Acyclic Polyols

Compound

Solvent

Nuc

Temp. ("C)

Methyl (2S,3R)-2,3-dihydroxybutanoate N,N-Dimethyl (2S,3R)-2,3-dihydroxybutanamide 2-Methyl-2,4-pentanediol 5-O-Benzoyl-~-arabinosediethyl dithioacetal (179)


DMF DMF DMF Benzene

CsF CsF CsF

Amb. Amb. Amb. Amb.

Acetal

Benzene

Amb.

Acetal

Benzene

Amb.

Acetal

Benzene

Amb.

5-O-Pivaloyl-~-arabinosediethyl dithioacetal

Acetal

Benzene

Amb.

5-O-Benzoyl-~-arabinosebenzyloxime 5-O-Benzoyl-~-arabinosedimethvl acetal (180)

Acetal Acetal

Benzene Benzene

Amb. Amb.

5-0-Benzoyl-L-arabinose dibenzyl dithioacetal

Yield (YO)

Reaction Conditions

A c e d or Ether

Electrophile

Site 1

Benzyl iodide 0 - 2 30 Benzyl iodide 0 - 2 17 Benzyl iodide 0-2 0 Benzoyl chloride 0 - 4 75 (1 equiv) Benzoyl chloride (2 equiv) Benzoyl chloride 0-480 (1 equiv) Benzoyl chloride (2 equiv) Benzoyl chloride 0 - 2 18 (1 equiv) 0-4 23 Benzoyl chloride 0-4 91 Benzovl chloride 0 - 2 97

Site2

Ref.

0 - 3 55 0-3 75 0-450 2.4-Di-12

69 69 69 350

2,4-Di 98

350

2.4-Di 8

350

2.4-Di 98

350

2,4-Di 29

350

2.4-Di 4

350 350


133

epimerization using molybdenum salts.352-354 This observation most likely arises from equilibration of the dibutylstannylene acetals, which are most stable if 0-1 and 0 - 2 are cis. Thus, epimerization of 6-O-trityl-~-galactose gave 6-O-trityl-~-talosein yields of 60-70%?3

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(324) A. Ricci, S. Roelens, and A. Vannucchi, J. Chem. Soc., Chem. Commun., (1985) 14571458. (325) A. Dessinges, A. Olesker, G. Lukacs, and T. T. Thang, Carbohydr. Res., 126 (1984) C6-C8. (326) T.-L. Su, R. S. Klein, and J. J. Fox, J. Org. Chem., 47 (1982) 1506-1509. (327) G. Hodosi and P. KovfiE, Carbohydr. Rex 303 (1997) 239-243. (328) T. Ogawa, K. Beppu, and S. Nakabayashi, Carbohydr. Res., 93 (1981) C6-C9. (329) T. Ogawa and M. Matsui, Carbohydr. Rex, 51 (1976) C13-Cl8. (330) T. Ogawa, K. Katano, K. Sasajirna, and M. Matsui, Tetrahedron, 37 (1981) 2779-2786. (331) T. Ogawa and S. Nakabayashi, Curbohydr. Res., 97 (1981) 81-86. (332) T. Ogawa, S. Nakabayashi, and K. Sasajima, Carbohydr. Res., 95 (1981) 308-312. (333) T. Ogawa and M. Matsui, Curbohydr. Rex, 86 (1977) C17-C21. (334) M. Gerz, H. Matter, and H. Kessler, Angew. Chem. Int. Ed. Engl., 32 (1993) 269-271. (335) F. Clerici, M. L. Gelmi, and S. Mottadelli,J. Chem. Soc., Perkin Trans. I (1994) 985-988. (336) T. Sato, Y. Fujita, J. Otera, and H. Nozaki, Tetrahedron Lett., 33 (1992) 239-242. (337) R. R. Schmidt, Angew. Chem. Int. Ed. Engl., 25 (1986) 212-235. (338) R. R. Schmidt, Pure Appl. Chem., 61 (1989) 1251-1270. (339) K. Vogel, J. Serling, Y.Herzig, and A. Nudelman, Tetrahedron, 52 (1996) 3049-3056. (340) A. Nudelman, J. Herzig, H. E. Gottlieb, E . Kerinan, and J. Sterling, Curbohydr. Res., 162 (1987) 145-152. (341) M. K. Gurjar and U. K. Saha, Tetrahedron, 48 (1992) 4039-4044. (342) E. Keinan, M. Sahai, Z. Roth, A. Nudelman, and J. Herzig, J. Org. Chem., 50 (1985) 3558-3566. (343) K. K.-C. Liu and S. J. Danishefsky, J. Am. Chem. Soc., 115 (1993) 4933-4934. (344) K. K.-C. Liu and S. J. Danishefsky, J. Org. Chem., 59 (1994) 1895-1897. (345) S. J. Danishefsky, K. Koseki, D. A. Griffith, J. Gervay, J. M. Peterson, F. E. McDonald, and T. Oriyama, J. Am. Chem. Soc., 114 (1992) 8331-8333. (346) S. J. Danishefsky and M. T. Bilodeau, Angew. Chem., Int. Ed., 35 (1996) 1380-1419. (347) C. AugC and A. Veyrihes, J. Chem. Soc., Perkin Trans. I(1979) 1825-1832. (348) C. Cruzado, M. Bernabe, and M. Martin-Lomas, Curbohydr. Res., 203 (1990) 296-301. (349) P. J. Garegg, J.-L. Maloisel, and S. Oscarson, Synthesis (1995) 409-414. (350) M. W. Bredenkamp, C. W. Holzapfel, and A. D. Swanepoel, Tetrahedron Lett., 31 (1990) 2759-2762. (351) T. B. Grindley and R. Ponnampalam, Can. J. Chem., 58 (1980) 1365-1371. (352) V. Bflik, L. Petrus, and V. Farkas, Chem. Zvesti, 29 (1975) 690-696. (353) V. Bflik, Chem. Listy, 77 (1983) 496. (354) M. L. Hayes, N. J. Pennings, A. S. Serianni, and R. Barker, J. Am. Chem. Soc., 104 (1983) 6764-6769.


SYNTHETIC APPLICATIONS OF SELENIUM-CONTAINING SUGARS BY ZBIGNIEW J. WITCZAK AND STANISLAS CZERNECKI* Department of Pharmaceutical Sciences, School of Pharmacy, University of Connecticut, Storrs, Connecticut 62690, USA; and Laboratoire de Chimie des Glucides, Universiti Pierre el Marie Curie, F-7S00S, Paris, France 1. Introduction ....................................................... 11. Preparations of Seleno Sugar Derivatives.. ............................... 1. Early Work ..................................................... 2. Nucleophilic Displacement.. ........................................ 3. Opening of 1.2-Anhydro Sugar Derivatives.. ........................... 4. Addition to Protected Glycals ....................................... 111. Application of Seleno Sugars in Synthesis 1. Utilization of Selenoglycosides in Glycosylation Reaction. ................. 2. Organoselenium-Mediated Alkenylation: Synthesis of Unsaturated Sugars. 3. Cyclization-Mediated Reactions: Synthesis of Deoxy Sugars. IV. Conclusion ........................................................ References ........................................................

143 145 145 148 150 154 167 167 180 186 193 195

................................ .... ...............

I. INTRODUCTION Selenium and its compounds are by no means new in organic chemistry. For many years, organoselenium chemistry was regarded as quite specialized and was consequently largely ignored by organicchemists. Over the past decade, however, this situation has changed with the recognition that organoselenium reagents provide convenient tools for organic synthesis. The new wave of selenium-based methodology has provided useful routes for the introduction of alkenic bonds, as well as for performing many other transformations utilized in numerous syntheses of natural products, including alkaloids, antibiotics, steroids, terpenoids, and carbohydrates. In carbohydrate chemistry, the first article' reporting on the introduction of a selenium atom into a sugar derivative was published in this series in 1945. Since then, a number of reports dealing with the synthesis of deoxy and unsaturated sugars through organoselenium intermediates have been published.

*

Deceased October 16, 1997. 143


144

ZBIGNIEW J. WITCZAK AND STANISLAS CZERNECKI

Selenium is the chalcogen below sulfur in the periodic table. The general chemistry of seleno sugars resembles that of thio sugars,la and they are named by the same general principles.Ib However, the differences already evident between oxygen and sulfur progress further when sulfur and selenium are compared: Selenium is less basic but more nucleophilic than sulfur, the selenium atom is larger and more polarizable than sulfur, and the carbon-selenium bond is weaker and less polar than the carbon-sulfur bond. The acidity of the proton alpha to the chalcogen increases markedly from oxygen to sulfur to selenium, and selenoxides eliminate several orders of magnitude faster than sulfoxides.Formation of the very stable diselenides may be an important driving force in reactions, and the low polarity of the C-Se bond enhances the utility of free-radical methodology. Consideration of these fundamental factors offers the possibility of applying rational design rather than empirical experimentation in the optimization of synthetic targets. Accordingly, many useful reactions originally developed using sulfur chemistry may be effected more conveniently by using the selenium analogs; this is particularly the case for reactions introducing alkene functionality and for deoxygenation. In other situations, the use of selenium reagents offers totally new methodology for important synthetic targets, as in glycosidic coupling and the construction of carbon-linked sugars. Whereas the importance of sulfur in biomolecules has long been known, the first indication of the natural occurrence of seleno sugars, in the “selenium indicator plant” Astragalus racernosus, was not demonstrated until 1997. Generally, there was little early information on the specificity of selenium reagents3-$ however, the first one used in the synthesis of a seleno sugar (“selenoisotrehalose”),7 in 1917, was hydrogen selenide. Since then, a number of selenium reagents have been used in organic s y n t h e ~ i s . ~ - ~ Among them, the most common ones used in synthetic carbohydrate chemistry are benzyl selenide (PhCH2SeH), benzylselenol (PhCH2SeOH), phenyl selenide (PhSeH) and its salts, diphenyl selenide (PhSeSePh), phenylselenyl chloride (PhSeCl), phenyl selenocyanate (PhSeCN), o-nitrophenyl selenocyanate (SeCN-c6H4No2-o), potassium selenocyanate (KSeCN), selenium dioxide (Se02),N-phenylselenophthalimide(N-PSP), selenourea (MSBT), dimeth[H2NC(Se)NH2],3-methyl-2-selenoxo-1,3-benzothiazole ylaluminum methaneselenolate (CH3)2AlSeCH3,and phenylselenyl trifluoromethanesulfonate. Many of these reagents are now commercially available, and this circumstance should encourage their use. This article collates information on the reactivity of various organoselenium reagents in modern, synthetic carbohydrate chemistry. It illustrates some of the chemical properties that have contributed to the synthesis of

SELENIUM-CONTAINING SUGARS

145

seleno sugars and their application in the synthesis of unsaturated and deoxy sugars. 11. PREPARATIONS OF SELENO SUGARDERIVATIVES

1. Early Work The pioneering work of Wrede and co-w0rkers,7-~~ performed between 1917 and 1925, was rediscovered almost 30 years later by Bonner and RobinsonI4 during a synthetic approach to several selenoglycosides; they used benzeneselenol in a classic Koenigs-Knorr reaction with tetra-0acetyl-a-D-glucopyranosyl bromide (1). Oxidation of the selenoglycoside 3 with either hydrogen peroxide or potassium permanganate gave not the corresponding selenoxide, but exclusively the product (D-glucose) of C-Se bond cleavage, together with diphenyl selenide. Wagner and c~-worker'~-*~ applied the additional methodology of Wrede7-I3 and Bonner and Robinson14 in the preparation of a number of additional 1-seleno-P-D-glycosides.A similar approach, with a deacetylation step, was employed for the preparation of 0,Se bis(g1ycosides) such as 10 from 4-hydroxybenzeneselenol (7), and N-[(4a-D-g~ucopyranosy~se~eno)pheny~]-~-~-g~ucopyranosy~amine from 4arninobenzeneselenol" (8). Wagner and Nuhn" also detailed treatment of the 1-seleno-P-Dglucoside"*'2 with thiophosgene, a reaction which yielded a thiocyanato derivative after reaction with various classes of amine.

Rh R3

Q-se,

P

MeSO

AcO

Br

I R'=OA~.R~=H 4 R' = H I Rz= OAc

SCHEME1 .

ACO Sa-Sb 6a-6b

R' = Y me,4c1,ZAe

146


10

SCHEME 2.

Kocourek and co-workersZ4reported an alternative approach to selenoglycosides that starts from 1 and proceeds supposedly by SN2nucleophilic displacement of bromine with potassium selenobenzoate (PhCOSeK) and formation of 14 with inversion of configuration. Treatment of 14 with sodium methoxide produced the sodium salt of 6-O-acetyl-l-seleno-Dglucose (W), a good precursor in the preparation of selenoglycosides and other derivatives. Similarly, the potassium salt of 1-seleno-D-glucose has been prepared through a selenopseudoureido derivative, as reported by Wagner and Nuhn.” These approaches started with the per-0-acetyl-a-D-glucopyranosy1 bromide (1) and per-0-acetyl-a-D-xylopyranosyl bromide (16). Treatment of potassium salts 19 and 20 with sodium borohydride, followed by deacetylation, produced the diselenides 23 and 24 which, on treatment with metallic potassium in methanol, underwent cleavage of the Se-Se bond with formation of potassium salts 25 and 26. Products 19 and 20 are excellent precursors in the synthesis of symmetrical and unsymmetrical sugar selenides 27-32, as reported by Wagner and N ~ h n . ’ ~ It is noteworthy that all of the methods for the synthesis of selenoglycosides mentioned lead preferentially to the formation of the 1,2-trans prod-

Acb

AcO

11

12 R=N=C=S 13 R = NHCSNH~ R‘ = H. Me, Ph

SCHEME 3.

147


14

15

SCHEME4.

21 R’ = CHSAc 22 R‘ = H

d

2~ R‘ = CWH

25 = C W H 26R1=H

24d=H

SCHEME5.

R!

\

-m ACO

la

R1= CHSAc

20 R1 = H

Acb

27 R1= R z = C W A c , R3= H, R4= OAc IRI =w=c%oAc,rn=OAc,W=H 29 Rc = CHPAC, w = rn= y R4= OAC 30 RI = R = H, W=-, R4 = OAc S1 Rc = R4= H, W = , RS = OAc S2 Rc = rtZ = RS= H, R4 = OAc

SCHEME6.

148


CHzOAc

PhSeH

+:A c*

+

AcfllPyridine

Ac

ePh

33 34 SCHEME

7.

ucts, presumably under kinetic control. However, Wagner and Frenze126 found that formation of both anomers of selenoglycosides occurs during opening of the epoxide ring of the Brig1 anhydride 33 with benzeneselenol. The ‘H NMR spectra of 2 and 34 showed that the signals of protons cis to the aglycon are shifted to lower field compared with those of the glycosides and 1-thioglycosides.

2. Nucleophilic Displacement Potassium selenocyanate is the best-known selenium compound used in synthetic carbohydrate chemistry, and it has traditionally been employed for introduction of a selenocyanate group into the sugar moiety through s N 2 nucleophilic displacement of a sulfonyloxy group, as in the conversion 35 436. Depending on the reaction conditions (solvent and temperature), the major product may be, however, (as reported by Van Es and cow o r k e r ~ ) ? ~either - ~ ~ the 3,6-anhydro derivative, an unsaturated sugar,27or the diselenide. In a fully protected system37,s N 2 displacement of the p tolylsulfonyloxy group by benzyl selenolate ion in methanolic sodium methoxide produced methyl 5-(Se-benzyl)-2,3-O-isopropylidene-5-seleno-a-~ribofuranoside (40) in high ~ i e l d . ~ ’ - ~ ~ In a number of reports, Zingaro and c o - ~ o r k e r s ~ ~ described -~’ several transformations in seleno sugars that involved the application of such selenium reagents as selenourea and potassium selenocyanate in this synthesis of diselenides 43,46, and 49. The first approach involves the SN2 displace-

KSCN ____)

EtOH

35

36 SCHEME 8.

149


2 M e O k ,MeOH

PhCweH MeONa, MeOH

40

37

H ‘cM/a2

38

SCHEME 9.

ment of the p-tolylsulfonyloxy group in 41 by the selenocyanate ion:’ Subsequent comparable to the earlier reports of Van Es and ~o-workers.”~~’ reduction with sodium borohydride gave diselenide 43. Two alternative approaches to this class of compound (6-6’-diselenides) were also reported.36The first proceeded through selenoureido derivative 45, followed by reduction with sodium hydrogensulfite, and led to the formation of 46. The second approach proceeded by use of iodine oxidation and conversion of the product into diselenide 49. Analogously, Daniel and Zingar040-41 prepared diselenides 52 and 53 as convenient precursors for the synthesis of dimethylselenoarsine derivatives 54 and 55, investigated as potential carcinostatic agents against the P388 system (mouse lymphocyticleukemia). prepared by the method Another example of this class of previously employed in the synthesis of 1-seleno-D-galactose,is the synthesis of 2-acetamido-2-deoxy-1-(dimethylarsino)-l-seleno-c~-~-glucopyranoside (59) and a 6-substituted analog 62. Related examples of the application of

SCHEME 10.

150


48

49

SCHEME 11

similar ideas have been r e p ~ r t e d . ~ For ~ - ~example, ’ a-D-glycosyl bromides 1 and 4 have been shown43to react with the triethylammonium salt of 5,5-dimethyl-2-seleno-2-thio-1,4,3,2-dioxyphosphorinane (63) to yield two types of product: 64 and 65, and 66 and 67. These results on the glycosylation reaction of thio and selenothio organophosphorous acids led to two valuable transformations. The first is anomerization of the selenoate 68 by heating in boiling xylene. The second is a selenono-selenolo rearrangement of the seleno ester 70 to the selenoate 71. 3. Opening of l,2-Anhydro Sugar Derivatives

The reaction of 3,4,6-tri-0-acetyl-1,2-anhydro-a-~-glucopyranose (72) with the triethylammonium salt of 5,5-dimethyl-2-0x0-seleno-1,3,2dioxaphosphorinane leads to formation of the diselenide 74 and 3,4,6-

50 R1=OAc,RZ=H 51 Rl=H,RZ=OAc

52 R1= OH,RZ-H 53 R1= H,RZ=OH

SCHEME 12.

54 55


Cl

56

OAc

60

SCHEME13.

63

66 RI=OAc,RZ=H 67 R ~ = H , R Z = O A C SCHEME14.

151

152


68

69

SCHEME15.

tri-U-acetyl-D-glucal (75) in quantitative yield. The generalization of the reaction for other, model anhydro sugars constitutes a potential new entry into the unsaturated sugars, as exemplified by the conversion of 76 via 77 into diselenide 78, the 2,3-alkene 79, and the reagent by-product 80. Another class of seleno sugars derivatives is exemplified by the D-glucosyl selenophosphates. These are readily ~ynthesized~~ by the action of dicyclohexylammonium 0,O-di-tert-butylphosphoroselenoate(82) or its thio counterpart 81on glycosyl bromide 1 at low temperature (because of the thermal instability of the synthesized products 83 and 84). 0-dealkylation of derivatives 83 and 84 was effected with boiling toluene for 5 min or by the catalytic influence of trifluoroacetic acid (TFA) in benzene for 24 h at room temperature. It is noteworthy that both the thio compound 83 and the selenophosphate 84 exist only as /3 anomers under the reaction conditions, as well as during the dealkylation reactions.46Michalska’s continuing research on the sugar phosphoroselenoates, thio- and selenophosphates, and diselenides has been published in two review article^.^^,^' Various physicochemical properties of sugar diselenides, circular dichroism such as spectra, have been d i s c u ~ s e d .The ~ ~ *selenophosphorylation ~~ of 2-deoxy sugars and a seleno-selenolo isomerization which proceeds through an intermediate 2-deoxyglycosyl chloride have been p ~ b l i s h e d .Combination ~~ of the SN2 displacement of a p-tolylsulfonyloxy group by PhSe, with cleavage of the epoxide ring and simultaneous removal of the PhSe group, was used by

70

71

SCHEME 16.

SELENIUM-CONTAININGSUGARS

A

c Ac

0

v

.

-

L

72

33

153

+

B

75 74

SCHEME 17.

JoulliC and ~ o - w o r k e r sin~ the ~ * ~synthesis ~ of such muscarine analogs as Disoepiallomuscarine (90) from sulfonate 87 via intermediate 87a, 88, and 89, and the antibiotic turanomycin (94) via the route 89 + 91 + 92 + 93 + 94. The phenylselenyl group has also been found to be an excellent 95 and leaving group during the photolysis of 1-seleno-D-glycopyranosides

-Thp

OMe

+

72

79

I8

SCHEME 18.

80

154


CbOAc

85 X-S 86 X e S e

SCHEME19.

97 in the presence of trimethyltin radicals in toluene solutions4 to form Dglycosyl radicals 96 and 98. The radicals generated in these reactions exist

in conformations established by electron-spin In contrast, the 0-glycoside epoxide 99 under comparable reaction conditions gave simply the alkene 100 rather than radical products.

4. Addition to Protected Glycals a. Hydroxy- and Alkoxy-phenylseleny1ation.-The general procedure used for alkoxyphenylselenylation of both acyclic and cyclic vinyl was employed for alkoxyphenylselenylation of 3,4,6-tri-0-benzyl-~-glucal (99).60Since monosaccharide units were employed as the alkoxy groups (Scheme 22), this reaction afforded a new synthetic route to oligosaccharides. The glycal was first treated with an excess of phenylselenyl chloride in acetonitrile, and the protected monosaccharide (A, B, or C) was then added, together with 2,4,6-trimethylpyridine. A mixture of a-manno and p-glum disaccharides (100 and 101) was obtained, the product of antiaddition, tentatively explained as the result of regiospecific opening of an episelenonium ion. In the case of the selected glucal (99) and under the experimental conditions used, generation of the episelenonium ion that allows diaxial ring opening is favored and the a-manno derivative is predominant. The 2’-Se-phenyl-2’-selenodisaccharides (100a-100c) and (101a-101c) thus synthesize were reduced by triphenyltin hydride to give the correspond-

155


I

I

NaH ,THF

p)IseN.

90

OMe

/

PhSeNs

I

TsO&

OH

-

87

87.

.OM0

&""' /

a"" /

PhSeHS

AH

91

""1

OH

89

Prh

PhCNCHMe

I

94

93

SCHEME20.

ing 2'-deoxy disaccharides (102a-102c) and (103a-103c) in high yields (9095%). Several years later, hydroxyphenylselenylation of 3,4,6-tri-O-benzylmglucal(99) and also D-galactal(lO4) was realized under similar conditions (phenylselenyl chloride in THF, water, and triethylamine).61The observed

156


97 X=SePh

98

hv

-

OMe

100

99 SCHEME 21.

stereochemistry of the reaction differed from that reported by Sinay and co-workers!' The isolated product (-65%) resulted from phenylselenylation from the (Y face. The authors did not report whether another isomer was formed (Scheme 23). The resultant 2-deoxy-2-phenylselenoglycopyranoses105 and 106 were oxidized with m-chloroperoxybenzoic acid in methanol.6' A stereospecific ring-contraction (see 109) occurred, affording respectively the aldehydo derivatives 107 and 108 (no yields reported). Following the same line, studies were undertaken attempting to favor trans-diequatorial opening of the intermediate 12-episelenonium ion, a procedure which could afford, after reduction, a selective route to 2-deoxy-/3-glycosides. Preliminary attempts to produce P-linked glycosides from glycals in a one-step procedure by modifying the previously employed conditions (which afforded a-linked glycosides6'), gave uncontrollable a/P mixtures with low efficiency.60The


99

lOOa R1= SePh,R2 = A (80%) lOOb R1= SePh,R2 = B (69%)

lOOc R1= SePh,R2 = C (61%) 102a R ~ = H , R ~ = A 102b R1=H,R2=B 1 0 2 ~R ~ = H , R ~ = c

157

lOla R1= Seph, R2 = A (5%) lOlb R1= SePh,R2 = B (@A) lOlc R1= SeF'h, R2 = C (8%) 103a R l = H , R 2 = A 103b R 1 = H, R2 = B 103c R1=H,R2=C

SCHEME 22.

trans-diequatorial 2-Se-phenyl-2-seleno-~-glucopyranosides were, however, prepared with high selectivity by a two-step procedure:* In the first step, 2-Se-phenyl-2-seleno-/3-~-glucopyranosyl acetates were obtained by treatment of the protected D-glucal derivatives 99 and Wa with phenylselenyl chloride and silver acetate in a nonpolar solvent (toluene). Good yields (60-80%) and selectivity (P/a = 9/1) were obtained in the presence of benzyl ethers (Scheme 24). Although the results have not been rationalized, dramatic changes in the stereochemical course of this reaction were observed with different protecting groups. The ability of compounds 109 and 109a to function as glycosyl donors has been evaluated.62The choice of solvent was of prime importance to avoid formation of the (Y anomer. For example, reaction of 109 with monosaccharide derivatives B and D in diethyl ether, in the presence of a catalytic

158


=E

99 R ~ = o B ~ , R ~ = H

105 R1= OBn, R2 = H (67%)

107 ~

104 R1= H, R2 = OBn

106 R1= H,R2 = OBn (65%)

108 R1= H, R2 = OBr

1O= B ~~2 ,

amount of trimethylsilyl trifluoromethanesulfonate (Me3SiOTf) afforded the @-linkeddisaccharides 110 and 111 in good yield (92%) and selectivity (Pla = 16/1). Reductive removal of the phenylseleno group of 110 and 111by triphenyltin hydride led to the 2’-deoxy-@-~-disaccharides 112 and 113. Although these methodologies were reported several years ago, they have not often been employed to prepare a-to @-linked2-deoxy disaccharides.

b. Preparation of 2’-Deoxy Nuc1eosides.-A methodology similar to the foregoing was evaluated for the synthesis of 2’-Se-phenylselenonucleosides, which may be converted into 2’-deoxy nucleosides.h”@ Furanoid glycal derivatives 115 and 115 were successively treated with phenylselenyl chloride and silylated uracil or thymine in the presence of silver trifluoromethanesulfonate (AgOTf) as activator. The @-nucleoside derivatives 117a,b and 118a,b were obtained in good yield (90%), but they were contaminated by around 10% of the a anomers. Removal of the 2’-phenylseleno group was effected by reaction with tri-n-butyltin hydride, to afford 2’-deoxy

4 mFq


159

Bn BnJ &

99 R=OBn 99a R = H

110 R=OBn llOa R=H

111 Rl=B,R2=SePh 112 R1= D, R2 = SePh 113 R ~ = B , R ~ = H 11.1 R ~ = D , R ~ = H

SCHEME24.

nucleosides 1l9a,b and l20a,b. Excellent stereoselectivity was obtained in this reaction because the two diastereofaces of the starting “threo” glycals were strongly differentiated by the presence of the two bulky groups on the same side. It is not immediately apparent if such a high diastereoselectivity could be obtained with “eryrhro” furanoid glycals, precursors of natural 2‘-deoxy-ribo-nucleosides,since the bulky group at C-3 is on the a face. An extension of this approach for the synthesis of 2’-deoxyglycopyranosyluracil derivatives was reported by the same group.@ Peracetylated and perbenzylated D-glucal(99,99b), D-galactal(104,104b) and D-arabinal(120, 120b) were employed as starting materials. The stereoselectivity of the reaction was shown to be dependent on the protecting groups present in the glycal, the structure of the starting glycal, the phenylselenyl reagent used, and the solvent. The best results were obtained with perbenzylated glycals in diethyl ether, using phenylselenyl chloride (PhSeC1) as the activating agent in the presence of AgOTf at low temperature. Under these conditions, 2’4e-phenyl-2’selenoglycopyranosyluracil derivatives (121-127) were obtained in 70-85% yield. The stereoselectivity was high (80 :20) and nucleosides of the p-Dgluco (121), p-D-gafacto (123) and a-D-arabino (127) configurations were principally obtained. A rationalization of these results was presented following the one proposed by Horton et aL6’ for NIS-mediated addition of alcohols to glycals.

160


115R=Bn,R1=

117a R=Bn, R1=

116 R = Bn, R1= -CH2OBn 117bR=Bn,R1=

118a R = Bn, R1= -CH20Bn, R2 = H 118b R = Bn, R1= -CH20Bn, R2 = CH3

dy

% ,R~=H

119a R = B ~~, 1 =

o,yQ 119b R = Bn, R1=

,R2=CH3

l2Oa R = Bn, R1= -CH20Bn, R2 = H 120b R = Bn, R1= -CH20Bn, R2 = cH3 SCHEME25.


161

Q R

99 R = B n 75 R = A c

122 R = B n 122b R = A c

125 R = B n 125b R = A c

104 R = B n 104b R = A c

121 R = B n 121b R = A c

123 R=Bn 123b R-Ac

124 R = B n 124b R = Ac

J2 @ 126 R=Bn 126b R=Ac

127 R = B n 127b R = Ac

u = uracil- 1-yl SCHEME26.

The 2’-phenyl-2’-selenoglycopyranosyluracil derivatives obtained by this method were efficiently transformed into the corresponding 2’-deoxy nucleosides by tri-n-butyltin hydride. of double c. Azido-phenylselenylation.-Azido-phenylselenylation bonds is very powerful and versatile reaction because it allows the one-

162


step introduction of two functionalities in a m o l e c ~ l e .Moreover, ~ ~ * ~ ~ the regioselectivity can be controlled with unsymmetrical alkenes. When the reaction is initiated by electrophilic phenylselenium species (such as PhSeC1) in the presence of azide ion, Markovnikov adducts are prevalent.66 Anti-Markovnikov addition products can be obtained by treatment of an alkene with sodium azide and diphenyl diselenide in the presence of (diacetoxyiodo)benzeneF7These two methods of azido-phenylselenylation have been applied to diverse protected glycals, affording respectively 2-Sephenyl-2-selenoglycosyl azides or phenyl2-azido-2-deoxy selenoglycosides (Scheme 27). The overall stereochemical outcome is controlled by the conformation of the starting glycal derivative and the reaction conditions.

(i) Preparation of 24e-phenyl-%seknoglycosyl Azides.-Azido-phenylselenylation of tri-0-acetyl-D-glucal (75) with azide ion in the presence of phenylselenyl chloride in a polar solvent is very slow6' because of the withdrawing effect of the acetyl groups and the ionic mechanism proposed by Hassner.& With perbenzylated glycals, the reaction proceeds smoothly, and glycals 99,104, and 128 were efficiently transformed into 2-Se-phenyl-2-selenoglycosy1 a~ides.6'-~~ As solvent, N,N-dimethylformamide (DMF) was found to be superior to dimethyl sulfoxide (Me2SO), the solvent recommended by Hassner.@The addition was stereoselective and gave the trans-addition products, in agreement with a mechanism involving an episelenonium ion or a related bridged intermediate, which subsequently captures azide ion.

W k "Markovnikov"

seph

W k

"Anti-Markovnikov" SCHEME27.


163

From tri-0-benzyl-D-glucal (W), a mkture of p-D-gluco- (129) and a-Dmanno (130) pyranosyl azides was ~ b t a i n e d .The ~ . ~same ~ regioselectivity, affording 129 and 130 in similar proportions, was observed when N phenylselenophthalimide (N-PSP) was employed instead of PhSeCl in the presence of sodium azide.68The same outcome was observed with the rhamnal derivative 128,which afforded 131 and 132, whereas only isomer 133 was detected in the ‘H NMR spectrum of the product of addition to glycal 104.69

(ii) Preparation of Phenyl Zazido-2deoxy-se1enoglycosides.-Azidophenyl-selenylation of glycal derivatives by reaction with (diacetoxyiodo) benzene and sodium azide in the presence of diphenyl diselenide was r e p ~ r t e d ~by ” ~two ’ different groups in 1993. The results for glycals 75,99, 104, 104b, 128b, and 134 are listed in Table I. Good yields were obtained with peracetylated glycals 75,104b, and 128b, and the reaction was regiospecific,affording phenyl2-azido-2-deoxy-l-selenoglycopyranosides. From 75 and U?b, inseparable mixtures of gluco and manno derivatives (135-136 and 137,138) were obtained. Galactal triacetate 104b afforded exclusively the gulucto derivative 139 in high yield. Interestingly, in every case, only the a anomer was formed.

Blld

99 R1= OBn,R2 = H

129 R1= N3, R2 = R3 = H, R4 SePh

128

130 R1= R4=H, R2 =N3, R3 SePh

104 R1 =H, R2= OBn

Bn

131 R1 =R4=H, R2 =N3, R3 S e P h 132 R1= N3, R2 = R3 = H,R4 S e P h

133 SCHEME 28.


164

99 R=Bn 75 R = A c

104 R=Bn 104b R=Ac

134

128b

CHzOR

I

135 R = Ac, R1 =N3, R2 = H 136 R=Ac,Rl=H,R2=Ng

137 R1 =N3, R2 = H 138 R1= H, R2 =N3

139 R = Ac 144 R=Bn

142 R=Bn,R1 = N j , R 2 = H 143 R = Bn, R1= H, R2 = N3

%-bph fiPh CHflAc

ph

I

\

A

dAc

dAc

140

141 SCHEME 29.

These results are in good agreement with a rapid addition of an electrophilic radical7*(generated by oxidation of azide ion in situ) to C-2 of the electron-rich double bond, affording an anomeric radical stabilized in the (Y configuration by the anomeric effect. Further homolytic reaction with diphenyl diselenide affords the a-selenoglycoside. Extension of this methodology to disaccharidic glycals was reported later.'3 In contrast with the earlier results, only the manno configuration was obtained in the addition product and, although the (Y-D anomer was preponderant, some p-Danomer was formed. In all cases, reactions were

165

SELENIUM-CONTAINING SUGARS TABLE I Azido-phenylselenylation of Glycal Derivatives

Glycal 99 99 75 75 104 104b 104b l2Sb 134

Method"

Product(s)

Yield* (YO)

142 + 143' 142 + 143' 135 + 136c 135 + 136' 144 138 138 137 + 1388 140 + 141

4gd 82e 74d 88f

76 87d

92' 66d 26 + 47d

" Method A: PhI(OAc)z, (PhSe)2, CH2C12, rt. Method B: N-PSP. (CH&SiN,, (nBu)4NF, CH2C12,rt. " Calculated on isolated products. ' gluco/manno = 1/1. ,IRef. 71. ' Ref. 69. f Ref. 12. Rgluco/manno = 112.

slower and afforded lower yields (22-45%) as compared to monosaccharidic glycals. Because of the presence of a strong oxidant, these conditions of azidophenylselenylation did not give satisfactory yields when oxidatively cleav-

d R=R~ R=R~ R=R3

R=R~ R=R~ R=R3

SCHEME 30.

13 : 1 6 : 1 6 : 1

R = R1 (22%) R = R2 (45%) R = R3 (33%)

166


able protective groups (such as benzyl ethers or benzylidene acetals) were present in the glycal (see Table 1). With glycall34, cleavage of the benzylidene acetal occurred, as indicated by the resulting mixture of 140 (26%) and 141 (47%) after acetylation of the reaction mixture.” Interestingly, azido-phenylselenylation of perbenzylated glycals 99 and 104 was possible, with the same regioselectivity, when N-PSP was employed in the presence of trimethylsilyl azide and tetra-n-butylammonium fluoride in dichloromethane (see Table I).68771From 99, a mixture of phenyl 2azido-2-deoxy-l-seleno-a-~-gluco(142) and munno-pyranosides 143 was obtained in 82% yield.6*As before, from 104 the a-gulucto isomer 144 was obtained as the sole product.” Although the mechanism of this reaction is not yet fully understood, it should be pointed out that only the cy anomers were obtained, as was the case in reactions with (diacetoxyiodo)benzene. It was verified6*that the regio- and stereo-selectivity were the same for these different conditions of azido-phenylselenylation, by deacetylation of 139 followed by benzylation to furnish 144. The compatibilityof this methodology with many protecting groups was further demonstrated with diversely protected D-galactal derivatives (l&a-e).6R974 CH2OR

145a 145b 145c 145d 145e

qH20R

R=H R=Ac R=Bn R = Bu-t-Me 2% R=Allyl

146a 146b 146c 146d 146e

R = H (59%) R = Ac (91%) R = Bn (84%) R = Bu-t-Me $3(66%) R = Ally1 (74%)

148a R = H (68%) 148b R = A c (73%) 148c R=Bn (76%)

147a R = H 147b R=Ac 147c R=Bn SCHEME 31.


167

When a silyl ether was present (145d), tetra-n-butylammonium fluoride was omitted and the reaction time was longer than with other glycals, and the yield slightly lower. Furthermore, it was verified that the reaction proceeds faster with electron-rich double bonds and, therefore, can be successfully carried out in the presence of an ally1 ether (145e). Less than 3%of product resulting from double azido-phenylselenylation was isolated. Azido-phenylselenylation was also possible in the presence of one unprotected hydroxyl group (145a and 147a). The compatibility with the benzylidene acetal group, which is very sensitive to oxidation and radical reactions, was also d e m ~ n s t r a t e dwith ~ ~ glycal derivatives 147a-c. Azido-phenylselenylation of a furanoid glycal was also possible under these conditions and afforded exclusively the gluco isomer, which was transformed into a 2-azido-2-deoxy glucopyranosyl donor (see p. 170). In conclusion, these phenyl 2-azido-2-deoxy-l-seleno-~-glucopyranosides could serve as, or be transformed (see p. 170) into, glycosyl donors bearing a nonparticipating group at C-2. For complementarity, their transformation into glycosyl donors bearing a participating N-acetyl group at C-2 was realized by reduction of the azido followed by acetylation.68 OF SELENO SUGARS IN SYNTHESIS 111. APPLICATION

1. Utilization of Selenoglycosides in Glycosylation Reaction and Furana. Preparationof Protected 2-Azido-2-deoxy-glucopyranoses oses.-Protected 2-azido-2-deoxy derivatives of galactose and glucose are extensively used for the synthesis of biologically important 2-amino-2deoxy-D-galactose (and glucose)-containing oligosa~charides.~~*~~ Depending on the leaving group and the promoter employed, they can be directed toward either the 1,2-cis-ora 1,2-truns stereo~hemistry,7~ and regeneration of the amino function is possible under mild conditions. A new and very efficient access to this important class of compounds from diversely protected glycals has been disclosed in which azido-phenylselenylation afforded phenyl2-azido-2-deoxyl-selenoglycopyranosidesin which the anomeric hydroxyl group can be regenerated by hydrolysi~?~.~~ Owing to the soft nature of the selenium atom, sop catalysts such as heavy metal salts were employed for hydrolysis. When ethers or acetals were present as protecting groups, hydrolysis was rapid in the presence of mercury trifluoroacetate in wet tetrahydrofuran, and the protected 2-azido2-deoxyglycopyranoseswere obtained in good yield (Table 11). When mercuric acetate was employed, some 1-0-acetyl derivative was also formed. Hydrolysis of the glucolmunno mixture 142 and 143 resulting from azidophenylselenylation of tri-0-benzyl-D-glucal(99) afforded a mixture of 142a

168


W2OR

I

I

142 R=Bn, R1 =N3, R2 = H

142a R = Bn, R 1 = N3, R2 = H

143 R = B n , R l = H , R z = N - j

143a R = Bn, R 1 = H ,R2 = N 3

135 R = A c , R l = N 3 , R 2 = H

1351 R = Ac, R 1 = N3, Rz = H

136 R = Ac, R 1 = H,R2 = N3

136a R = Ac, R 1 = H, R 2 = N 3 CHZOR I

CHBR

I

139 R = A c 144 R = Bn

139a R = Ac 144b R = B n

146a R = Bn 146b R = Ac 146e R = Ally1

149a R = B n 149b R = A c 149e R = Ally1

4 148a R = Bn 148b R = A c

150a R = B n 150b R = Ac

SCHEME32.

169

SELENIUM-CONTAINING SUGARS TABLE I1 Preparation of Variously Protected 2-Azido-2-deoxy-glycopyranoses

Entry

Phenyl selenoglycoside

Method

Product

142 + 143 135 + 136 144 139

A'

142a + 143a 13% + 136s 144a 139a 149s 149b 149e l50a 150b

146C

146b 146e 148a 148b

BC A

B A

A A

A A

Yield 93h

90 87 87 87 79 88 82 86

"Method A: (CFKO&Hg (1.5 equiv), THF-H20, 30 min rt. " 142a was isolated in 66% yield by chromatography. 'Method B: NIS ( 5 equiv). THF-H20, 12 h. rt.

and 143a that could be separated by chromatography, and the gfuco derivative 142a was obtained crystalline (66%).Hydrolysis of the gafacfoderivative 144 proceeded smoothly, and 144a was obtained in 87% yield. Under these conditions, hydrolysis of peracetylated derivatives 135,136, and 139 was very slow, presumably because of the electron-withdrawing effect of the acetoxy groups. When N-iodosuccinimide was employed instead of mercury trifluoroacetate, the reaction was complete in 12 h at room temperature and the yield was good (Table 11). The complete stereochemical control obtained for the azido-phenylselenylation in the gafacfo series (139, 144) makes this procedure especially attractive. For convenience, the two steps can be carried out without purification of the intermediate phenyl 2-azido-2-deoxy-1-selenogalactopyranoside, and the resulting phenyl 2-azido-2-deoxy-1-selenogalactopyranoside is obtained in 72% yield from the galactal derivative. The mild conditions of hydrolysis are compatible with many protecting groups currently employed in oligosaccharide synthesis (see Table 11). To obtain this type of D-glucosamine donor in a stereocontrolled manner, another glycal derivative, namely 3-O-benzyl-2-deoxy-5,6-O-isopropylidene-D-arabino-l,4-anhydro-hex-l-enitol(151), was chosen as starting material because it is easily prepared from D-mannose and because the p-face of the double bond is very hindered.'9 As anticipated, azido-phenylselenylation of 151 afforded an alp mixture of phenyl 2-azido-2-deoxy-l-seleno-~glucofuranosides 152 in high yield. In order to obtain the glucosamine equivalent in pyranose form, compound 152 was treated under acidic conditions in the presence of mercury acetate to effect simultaneous cleavage of

170


the acetal group and the selenoglycoside. After acetylation, an culp mixture of 153 was obtained in high yield (83%). Selective deacetylation of the anomeric hydroxyl group afforded 154,which could be activated for glycosidic coupling as a trichloroacetimidate. This transformation of protected glycals into 2-azido-2-deoxyglycopyranoses constitutes a new and efficient method for the preparation of these compounds which are important intermediates in oligosaccharide synthesis. This methodology is compatible with a variety of protecting groups, including acetates, acetals, ally1 ethers, benzyl ethers, benzylidene acetals, and silyl ethers. The mild conditions employed would allow extension of this methodology to glycals derived from disaccharides.

b. Selenoglycosides as Glycosyl Donors.-(i) 0-Glycosidation with Selenog1ycosides.-Phenyl selenoglycosides have been introduced as glycosyl donors and acceptors in glycosidation rea~tions,8’-~~ and the most interesting application is the selective activation of one of the two partners. This may be realized in two different ways: by modulating the reactivities of the partners by suitable choice of protecting g r o ~ p s ~ or - ~by ’ using different types of glycosyl donors and acceptors, which can be activated by different promoters. The latter strategy was employed by Mehta and Pinto.80,82 Glycosylation of the methyl glycoside acceptor 157 with phenyl selenoglycosides 155 and 156 in the presence of AgOTf and potassium carbonate afforded the 1,2-truns-linked disaccharides 158 and 159 in good yield. This reaction was extended to other methyl glycosides acceptors, as depicted and as listed in Table 111. It was also established that, under the same conditions, neither the peracetylated nor the benzylated thioglycoside derivatives were activated. Consequently, the “armed” thioglycoside acceptors 160, 161, and 162 were efficiently glycosylated with “disarmed” selenoglycoside donors 155 and 156.

151

152 SCHEME 33.

153 R = h 154 R = H


CHZOAc

2R*‘

171

a2oR3

AcO A d

yo%

Ac

155 ~ 1 se=ph, ~2 = H

156

R1 R2 160 H SEt 161 SEt H 162 SEt H -0

A dn+Bgo+

+ r ; +g cA

R1 R2 R3 157 H OMe H 163 SePh H Bn

R3 R4

Bn H H Bn

Bn H

CXZOAc

BnO

AcO A

158 ~ 1 y= ~2 = OM^

159 Rl=H,R2=OMe

163 R1= H,R2 = SEt 164 R1= SEt, R2 = H

165 R1= SEt, R2 = H

Bnb

166 SCHEME 34.

As expected, such a selective activation of per ,enzylatec (“armed”) phenyl selenoglycoside 163 was also possible in the presence of the benzylated thioglycoside acceptor 162. In this case, an cr/p mixture (ratio 2.5 : 1) of disaccharide 166 was obtained in excellent yield (Table 111). The fact that phenyl selenoglycosides are rendered unreactive if an organic base such as collidine or 1,1,3,3-tetramethylurea is employed instead

R1

112


TABLE Ill Glycosylation With Selenoglycoside Donors ~

Entry

Donor

1

155 156 155 155 156 163

2 3 4 5 6

Acceptor Product Yield (%) 157 157 160 161 162 162

158 159 163 164 165 157

85 84 80

ia 84 90

of potassium carbonate was also exploited. Under these conditions, selective activation of the glycosyl bromide donor 167in the presence of selenoglycoside acceptors 168 and 169 was possible. The corresponding disaccharides 170 and 171 were obtained in -60% yield.

167 X = Br 172 X = O C o C C l 3

169

168

170

171

173

SCHEME35.


173

The selective activation of glycoside trichloroacetimidates over phenyl selenoglycosideswas also demonstrated by glycosidation of phenyl selenoglycoside acceptors 168 and 169 with glycosyl trichloroacetimidate 172. In the presence of catalytic amounts of triethylsilyl trifluoromethanesulfonate at -78”C, the disaccharide derivatives 170 and 171 were obtained in respectively 84 and 90% yields. The versatility of phenyl selenoglycosides in oligosaccharide synthesis was further illustrated by the synthesis of the trisaccharide 173. Compound 171 was directly employed as a glycosyl donor for the glycosidation of “armed” thioglycoside acceptor 162 in the presence of AgOTf and K2C03. The thioglycoside product 173 could be directly activated by thiophilic promoters for further glycosidation. The use of phenyl selenoglycosides as glycosyl donors in glycosidation reactions was also studied by Zuurmond et a1.81.83Activation was achieved by iodonium ions generated from Niodosuccinimide and a catalytic amount of trifluoromethanesulfonic acid (NIS-TfOH) or iodonium di-sym-collidine perchlorate (IDCP). Glycosidation of several glycoside acceptors 174,175, and 176 with perbenzoylated phenyl selenoglycoside 177 was possible in the presence of NIS-TfOH (Table IV). The expected 1,2-truns-disaccharides 178, 179, 180, and 182 were obtained in moderate to good yields. Selective activation of the selenoglycoside under these conditions was possible in the presence of benzoylated thioglycoside acceptor 181, and the corresponding disaccharide 182 was obtained in 79% yield (Table IV). Glycosylation with perbenzylated “armed” phenyl selenoglycoside 183 of the same glycoside acceptors (174,175,and 176) was efficient and an a/ /3 mixture of disaccharides 184, 185, and 186 was obtained (Table IV). Since a weak thiophilic promotor (IDCP) was employed in this case, glycosylation of “armed” ethyl thioglycoside acceptor 187 was also possible, affording an alp mixture of 188 (Table V). Furthermore, selective activation of differently protected selenoglycosides was also possible with IDCP as promoter. Perbenzylated “armed” phenyl selenoglycoside 183 was selectively activated in the presence of “disarmed” acceptor 189 and an alp mixture of disaccharide 190 was obtained. With the less-reactive secondary hydroxyl group of 191,the yield of 192 was lower. Some disappointing results were reported later by the same group in an attempt to use phenyl selenoglycoside donors for the synthesis of a tetrasaccharide.86No glycosylation was possible with phenyl selenoglycoside 193 in the presence of NIS-TfOH, whereas 40% yield was obtained with the corresponding trichloroacetimidate 194. The previously discussed selective activation methodologies80*81 were also evaluated with phenyl2-0-benzoyl-2,3-di-0-benzyl-l-seleno-~-~-ribofuranoside (195)as the ribofuranosyl donor.87

174

ZBIGNIEW J. WITCZAK A N D STANISLAS CZERNECKI

R20 -0Me \

OR1

L3

H

174R1 = R2=R3 = Bn, R4= H 175R1= H R2=R3= (CMe)z,R4=Tr

176

(OH R10

SePh

181 R-Bz 187 R-Bn

177 R1= R2 = Bz 183 R I = R ~=BII 180 R1 =Bz, R L H

191 SCHEME36.

Glycosylation of ribofuranoside 195 with pyranoside acceptors 197 and 1% in the presence of AgOTf-K2C03 afforded the &linked disaccharides 198 and 199 in 60% yields, but the condensation was accompanied by the formation of the dimer 200 in appreciable amount (Table VI). Attempted

193X = SePh 194R = O C O C C 1 3 SCHEME 37.

175

SELENI UM-CO NTAINING SUGARS

TABLE IV Glyeosidation with Perbenzoylated Phenyl Selenoglycoside 177 Entry

Donor

Acceptor

Disaccharide

Yield (YO)

0

91

BzO

1

177

174

Bzoa

n

O BnO

h

O BnO

M

e

178

2

177

67

175

179

3

177

50

176

180

4

177

181

BzO&T7P

0

19

BzO BzO 182

glycosylation of ribofuranoside derivatives 201-205 was also disappointing. With terminal ethyl P-D-ribofuranoside 201, the formation of dimer 200 was also apparent (Table VI). Selective activation of the phenyl selenoribofuranoside 195 in the presSurprisingly, ’ reacence of 1-thioribofuranoside acceptors was also s t ~ d i e d . ~ tion of 195 with benzylated ethyl 1-thio-P-D-ribofuranoside (202) afforded the product of trans glycosidation, 203 (Table VI). The same phenomenon


176

TABLEV Glycosidation with Perbenzylated Phenyl Selenoglycoside 183

Yield (YO) Entry Donor Acceptor

1

183

Disaccharide

(a//.ratio) ?

82 (2%)

175 BnO BnO 184

2

183

176

185

3

183

176

186

Bno BnO

4

183

187

79 (5/1)

BzO

tE&-+fozBonB

BzO 188

BnO BnO

S

5

183

B

z

O 87

189

BzO O 190

t

s BzO

L SePh

(4/1)


177

TABLE V (Continued) Yield (YO)

Entry Donor Acceptor

6

183

(a@ ratio)

Disaccharide

191

SePh

45 (911)

192

was previously reported with ethyl thioglycosides as acceptor^.^*-^^ When phenyl 1-thio-P-D-ribofuranoside(204) was employed as acceptor, the trunsglycosylation was suppressed, but the yield of disaccharide 208 was low because the dimer 200 was also formed (Table VI). Activation of 195 with iodonium species82did not significantly improve the results. Glycosidation with the weakly thiophilic IDCP gave exclusively (albeit in poor yield) the ortho-ether derivative 210. In the presence of a strong thiophilic promoter (NIS-TfOH), a mixture of 208 and 200 was obtained. Similar results were obtained with the 4-nitrophenyl 1-thioribofuranoside acceptor 205 (Table VI). On account of side reactions, glycosidation with selenoribofuranoside donors is not very efficient under the conditions presently employed87and further work is needed to find better conditions. (ii) Preparation of Seleno-disaccharides.-In recent years, much effort has been devoted to the synthesis of unnatural di- and oligosaccharides in which the bridging oxygen atom of the glycosidic linkage is replaced by a nitr0gen,9'-~' or carbon at0rn,9~.~~ because such compounds could act as glycosidase inhibit01-s~~ and might exhibit interesting biological properties including antidiabetic, anti-inflammatory, and antiviral activities. A versatile methodology for the preparation of a new class of such compounds in which the bridging atom is selenium has been reported.95 To achieve this goal two strategies were evaluated: Method A, condensation of a glycosyl selenoate of known anomeric configuration with a protected deoxyhalo sugar, by analogy with the method employed for the synthesis of thio-disaccharide~~'; and Method B, condensation of a selenosugar derivative with a glycosyl halide. A perbenzylated a-glucopyranosyl selenoate was generated by reduction of the protected diglycosyl diselenide 211. Further reaction with methyl


178

X = SePh; R = Bz X=OEt;R=H X=SEt;R=H X = SEt; R = Bz X=SPh;R=H X = SPhN%; R = H

195 201 202 203 204 205

BJ B~

A

196R=H 1 9 8 R = BnO

197R=H

BnO 206 x = O E t 207 X=SEt

-pJ ;;; U

BJ

+h 210

ABZ

SCHEME38.

2,3,4-tri-O-benzyl-6-deoxy-6-iodo-a-~-glucopyranoside~~ (212) afforded the a-seleno-disaccharide213 in 70%yield. The same sequence of reactions was not effective with the peracetylated diglycopyranosyl diselenide.


179

TABLE VI Glycosidations with Phenyl Selenoribofuranoside 195

Entry

Acceptor

Promoter

1 2 3

1% 197 201 202 204 204 204 205 205

AgOTF-K2CO3

4 5

6 7 8 9

Temp "C 20

0 20 20 20 20 -50

IDCP NIS-TfOH

Yield (YO) 60 56 52 (19) 66 25 (22) 34 20 (8) 39 (40) 55 (30)

Method B was evaluated with the diglucosyl diselenide 216,prepared in two steps by reaction of the 6-deoxy-6-iodo derivative 214 with selenourea and transformation of the pseudourea resulting 215 and 217.The selenoate was generated by reduction of 216 and allowed to react with 2,3,4,6-tetra0-acetyl-a-D-glucopyranosyl bromide in DMF, affording protected disaccharide 217. The free (1 + 6)-P-linked 6-selenodisaccharide 218 was obtained in 70% yield after deacetylation. This methodology could find

Method A:

bMe

Method B:

SCHEME 39.


I80

1 2

211

212 R = B n , Y = I 214 R = A c , Y = I 215 R = A c , Y C S e

E

0.18

0

5D

10.0

15.0

20.0 mL

ImM iodine solution FIG.2.-Potentiometric titration with 0.05%N aqueous Kls solution of butanol-precipitated corn amylose (100 mL of 0.01% solution) (upper line) and corn amylopectin (100 mL of 0.04% solution) (lower line). [Reprinted with permission from Bates er Copyright (1943) American Chemical Society.]

COMPLEXES OF STARCH WITH INORGANIC GUESTS

269

TABLE I Adsorption of Iodine from Aqueous II-KI Solutions by Starch of Various Origins69

Mass of Adsorbed Iodine per 100 g of Starch Starch Variety

Original

Gelatinized

Maize Potato Rice Sago Tapioca Wheat

18.61 18.18 19.01 17.32 18.73 18.52

20.18 19.74 20.28 18.97 20.40 20.12

more iodine than native starch, regardless of its origin (see Table I). The same authors observed that the smallest granules of starch exhibit the highest iodine uptake. The differences in iodine uptake vary from 4.5 up to 25.3 wt % of adsorbed i ~ d i n e . Amylose ~ ~ . ~ ~ from rice binds 18.9% of iodine, regardless of the molecular weight of the particular sample^.^' Table I1 and results by other a u t h o r P 6 show that the size of the granules is not the only factor controlling the dependence of the iodine uptake on the origin of starch. Figure 3 shows that differences in iodine uptake may be ascribed solely to the varying amylose and amylopectin content in particular starches.73The amount of iodine adsorbed by amylose is a function of the length of its helical chain; the amount of iodine adsorbed by amylopectin

TABLEI1 Adsorption of Iodine from Benzene Solutions by Different Varieties of Starch' Variety

Adsorbed Iodine, wlw%

Maize Potato Rice Sago Tapioca Wheat

4.85 2.97 15.99 1.28 1.29 3.08

a Reproduced by permission from Huebner and Venkataraman!'

270

PIOTR TOMASIK AND CHRISTOPHER H. SCHILLING

0.18 I

0

I

1

10.0

5.0

15.0

L

20.0 mL

1mM iodine solution

FIG.3.-Potentiometric titration with 0.001 N aqueo s K13 solution of 100 mL of 0.04% solution of the following starch varieties: x = wheat; = rice; 0 = corn; 0 = potato; A = tapioca. (From Bates et

4

depends on the degree of b r a n ~ h i n g . ~ ~ . "Table * ~ ~ -I11 ~ ~shows the iodinebinding power for amylopectin of various origins. The presence and content of phosphoric acid moieties in starch also does not affect the course of r e a c t i ~ n . For ~ ~ .several ~~ types of amylose, the iodine affinity varies between 18.5 and 20% in a 0.05 M solution of iodine.79 4. Solvent Effects

As already discussed on p. 265, iodine uptake by starch is facilitated by water (together with 13-ions), 1-butanol, or other alcohols. This is attributed to the conversion of A- and B-type starch into V-type starch. Huebner and Venkataramad9 demonstrated that starch can adsorb iodine from nonaqueous and not necessarily polar solvents, as shown in Table IV. Iodine adsorption by starch in aqueous ethanol increases as the ethanol content increases. Again, the solvent effect depends on the origin of the starch (Table 11). The varying effects observed using benzene, ethanol, and chloroform may also be ascribed to the use of starches of different origin TABLE 111 The Binding of Iodine by Different Varieties of Amylope~tin'~ Amylopectin Variety

Amount of Absorbed Iodine, O/O

Barley Hevea brasiliensis Oat Waxy maize Wrinkled pea

2.6 0.8 3.2 1.4 3.4


27 1

TABLE IV Absorption of Iodine by Starch from Various Organic Solvents@ Solvent

Amount Absorbed, %

Aqueous alcohol 99% 80% 60% 55% 50% 45% 40% 20%

4.05 8.39 12.44 13.42 15.13 17.07 21.18 53.42 61.87 6.71 12.34

0%

Benzene Chloroform

in every case. For instance, the enormously strong effect of benzene on starch was interpreted as the result of azeotropic evacuation of water from starch capillaries and he lice^.^ Such action could certainly change the original structure of the granules and the helix itself. However, frequently interpretations seem to be oversimplified. Ono et observed that the starchiodine reaction failed in dimethyl sulfoxide containing less than 28 moles of water per dm3. The amylose complex with iodine is slightly less sensitive to the addition of dimethyl sulfoxide than is the complex of amylopectin.81 W. T. and G. T. Smithx2estimated the minimum water contents with various solvents that would allow the formation of starch blue (Table V). The authors interpreted their results in terms of the stabilization of the amylose helix. Such stabilization is suggested to be a function of the polarity of the solvents investigated. As shown by Mazurkiewicz and Tomasik,83 different TABLE V Minimum Water Concentrations in Organic Solvents Providing Color as a Result of Starch-Iodine Reactions** Solvent

Minimum Water Concentrations, v/v%

N-Methylacetamide Methanol Ethanol Dimethyl sulfoxide N,N-Dimethylformamide 1.4-Dioxane Acetonc

39 47 52 53 58 58 68

212


organic solvents undergo hydration by various numbers of water molecules. In the case of high solvent-to-water ratios, bulky, stable solvates are formed which are characterized by, among other things, increased viscosity. Thus, under certain conditions of stoichiometry, water molecules are no longer available to the starch, and the solvent acts as a nonaqueous one with respect to starch. The conditions of solvation of the I2 molecule and the Ij- ion as well as the effect of the solvents on the I2 + I13- equilibrium may also be of significance (see, for instances, Tomasik et dM).

*

5. Electrolyte Effects The blue reaction is stronger with the addition of HI or KI.x5Electrolytes intensify the blue color by the coalescence of smaller particles into larger ionsx6;the size of the cation is an important However, at concentrations as low as 1 ppm, Fe(II1) (and also N203)” and H I 0 2 5 obstruct this reaction. Improved stability of the blue color is reported using NH41 and I2 instead of the KI-I2 mixture.g0More-detailed studies by Kuge and Ono91*y2 as well as by Miyazaki9jhave pointed to the importance of cations and anions in the observed effect. Thus, chlorides and bromides interact strongly with iodine ions to form iodine-bromide and iodine-chloride anions. This interaction affects the length of the chromophore of the complex that is responsible for the blue color. Figure 4 shows the effects of various salts on the formation of the amylose-iodine complex, as measured by the frequency of the longest-wavelength absorption band of the visible spectrum, and by the absorbance of that band. It may also be seen that the effect of both cations and anions depends on their concentration (Fig. 5). Cations of higher charge cause their effect at lower concentrations. The conclusions of the authors focus on the ionic strength of solutions in which the blue complex is intended to be formed. This problem points to the importance of the concentration of 13- ions themselves in achieving such a complex. Indeed, such an effect has been observed and is discussed later in connection with the mechanism of formation of the complex. On the other hand, nitrile anions inhibit the blue reaction. This inhibition can, however, be reversed by the addition of rhodanide anions.y4 Many electrolytes precipitate starch iodide from s o l ~ t i o n . ~ ~ ~ ~ ~ * ~ ~ 6. Effects of Proteins and Other Organic Compounds It was realized long ago that a number of organic compounds can obscure the blue reaction.y7 This effect is caused by the formation of inclusion complexes by such compounds. Hence, the iodine reaction is widely used as the test for complexation of starch, mainly amylose, with many organic


273

0.5

E

03 0.2.

0.1

'

[MI FIG.4.-Effects of various salts on the formation of the amylose-iodine complex. The effect is expressed as the shift of the 605-nm band (above) and the variation of the extinction of that band (below). 1, KCI; 2, NaN03; 3, (NH&SO,; 4, CaCI2; 5, ZnSO,; 6, KBr; 7, KI. (From Kuge and Ono.")

guest molecules, in both a qualitative and a quantitative sense. Since it is known that adrenaline98 and many other compounds99prevent the formation of the blue complex, and that proteins (such as those in blood, plasma, serum, or bacterial vaccines) also affect this reaction, the blue reaction has

0.4 ~

0.3

FIG.5.-Changes of absorbance of the amylose-iodine complex (at 610 nm with increasing concentrations of various salts): 1, A12(S04)3;2, Pb(NO&;3, Na,SO,; 4, NH4Cl;5 , CH3C02Na; 6, Sr(NO&: 7. KI; 8, KNO3. (From Kuge and Ono.")

274


significant value in toxicology studies and clinical analysis, and it can be a method of colorimetric estimation of proteins in immunity 7. Structure of the Complex

Although the stoichiometry of complexes can be defined, different compositions have been reported. The composition {[(C6HlnOS)4I]4KI},is given when the complex is prepared by blending approximately 0.5% of an aqueous suspension of starch with an excess of iodine in 0.1 N KI.lo2 Murrayln3 proposed the ionic structure of complexes obtained by shaking a starch solution containing KI with a solution of iodine in C C 4 . The author presented several possible formulae (C6Hl0O5),KISwith n = 15. A t high concentrations of KI, compositions of (ChHI0O5),,KI3and even (C6H100~)nK12 have been reported. Euler and MyrbacklW reported the formation of two defined compounds which are stable at a given dissociation pressure of iodine. The existence of these two compounds was later confirmed" by UV spectrophotometric studies. However, according to Meyer and Bernfeld,5x amylose forms several complexes, the composition of which ranges from (C6Hlo05)10-,,12,to (C6H1005)20n12n. A composition (C6H100s)8n12,1 was also reported.'"' In view of information presented in the next paragraph, it is not reasonable to anticipate a universally valid formula of the blue complex. Only general rules regarding the formation of the complex and general structural data are available. The amylose and amylopectin portions of the complex have obviously different structures (see, for instance, papers by Rundle et ul).65*106 The amylose moiety has a left-handed helical structure 18.65.70.106-109 with six glucose units per turn. In the complex, the diameter of the pitch is 7.91 L%,II~-I'I the outer diameter is 12.97 A,and that of the central cavity is 5 The helical structure is, however, deformed, which makes the amylose helix nonlinear (see Fig. 6).Il2 According to various sources, 10 is the minimum number of turns in the amylose helix which is necessary to provide the appearance of color,'ns-113-116 but a blue color is not necessarily produced in such circumstances.6' Bailey and WhelanMreported that saccharide chains become iodine-stained when they

A.

FIG.6.-The structure of the deformed helix in a solvated state. (Reprinted with permission from J. Hollo and J. Szejtli, Period. Polytech., 2 (1958) 25-37.)


275

are 18 units long in contrast to earlier'I7 and newer6' findings that the size limit is 30 units. Above 30 units there is a rapid uptake of iodine; however, the uptake is reduced at longer chains approaching 72 (ref. 64) or 100 (ref. 61) glucose units in length. It is suggested that when the degree of polymerization does not exceed 50, surface adsorption cannot be distinguished from the complex formation.l18 According to Bailey and Whelan,@ the interior of the helix has a hydrocarbon-like lining because of the inward-pointing hydrogen atoms of the glucose units. In contrast to this, Hollo and S ~ e j t l i " ~ . assumed "~ that iodine-amylose interactions take place via the hydroxylic groups of the six glucose units per turn (see Fig. 7). Stein and Rundles accepted the involvement of only one hydroxylic group per glucose unit. The first model explains the decreasing affinity of the complex toward enzymic degradation. It involves secondary hydroxylic groups, which are now engaged in holding iodine in the helix. The interiors of the helices are occupied either by iodine molecules or by accompanied iodide anions. There are two iodine atoms per turn when the cavity is saturated with iodine.12' Iodide anions are rather randomly distributed in the peripheral turns of helices (Fig. 8). Odd members on the chain-end bind neither iodine nor iodide According to many a ~ t h o r s , ~ ~ . ~the ' , ' ~13-' .ion ' ~ ~is the complexed unit, with the iodine atoms linearly arran~ed.~'.'1~123~124 The interatomic spacing is 3.10 (Cramer'2s reported 3.05 A). Such spacing provides about 3.9 glucose units per iodine atom. Inasmuch as amylose consists of 200-2000 glucose units, the amylose-13- complex thus incorporates a row of 50 to 500 iodine atoms. According to Cramer and Herbst,'I7 the shortest chain contains 14 iodine atoms, based on the assumption that the shortest blue-staining helix must have 30 to 35 glucose units. Based on the interpretationi26-'2Xof Raman and Mossbauer spectroscopy, the complex ( 13-* 12),, o r even ( Is-),, species are proposed to exist inside the helix, in close agreement with earlier suggestions.lZ9Also, 162-,Is2-, (312 + 21-), and Ilo2-species resulted from theoretical considerationsi3"and experiment^'^' with decreasing concentrations of KI. Among the three later species, 1;- is rather unlikely, as shown by

A

Fiti. 7.--Space pattern of one period of the amylose-iodine complex. (Reprinted with permission from J . Hollo and J. Szejtli, Period. folytech., 2 (1958) 25-37.)

216


FIG.S.-Structure of the arnylose-iodine complex. (Reprinted with permission from J. Hollo and J. Szejtli, Brauwissenschuff, 13 (1960) 380-386.)

X-ray s t ~ d i e s ' ~ on ~ - complexes '~~ of iodine with cyclodextrins. By analogy to iodine-cyclomaltoheptaose complexes, ( 12.1-.12)n, (15-)n, and even ( 12.13-.12)nor (17-),, species are also suggested. It is known"' that low concentrations of iodide ions (and low temperatures) favor an increase of the iodine chain length. The amylose-iodine interaction has a dipolar nature, as deduced from a distinct difference between the molecular coefficient of iodine in starch and in nonpolar solvents.'23 The additional stabilization may result from the formation of resonating polyiodine chains at high dipolar interactions. The composition of the total energy of stabilization is shown' in Table VI. This model is oversimplified because it applies solely to the absorption of iodine vapor by dry amylose. On the other hand, Schlamowitz'3s ascribed a predominantly nonpolar form to iodine inside the helix. Studies involving Raman spectroscopy of amylose-iodine complexes suggest a fully resonating polyiodine chain (electron gas model) as the most realistic description of the spectrum.'36Such a model has been proposed by Cramer.'37.'38Charge transfer between iodine atoms and amylose oxygen atoms (as suggested


211

TABLE VI Interaction Energies between Starch and Iodine" Interaction

Energy, kJ/mol of I2

Repulsion van der Waals attraction Electrostatic attraction Resonance attraction Total energy

-44.1 -119 - 13.0 -112

~

"

64.5

~~~

~~

Reproduced by permission from Stein and Rundle?

by Murakami,"" based on theoretical considerations, and by Ro~sott,'~' based on analysis of IR vibrational spectroscopy) is not possible, as shown by X-ray s t ~ d i e s . ' ~Hydrogen ~ - ' ~ ~ bonds are perhaps an essential part of the interactions."2*'19 Studies carried out in solution provide strong evidence that the helical amylose-iodine complex also exists in such circumstances.' The structure of the iodine-amylose complex has also received theoretical treatment,'42 and a three-dimensional free-electron model was developed for quantum mechanical calculations. Ultraviolet and visible spectra simulated in this manner reached close agreement with experiment observations. The color of iodine-polysaccharide complexes depends on the length of the iodine species within the helix. The large permanent dipole along the axis of the helix resulting from combination of dipoles of individual Dglucose units is consistent with the dipole of this iodine-iodide species entering the amylose cavity. The induced dipole of the latter increases with the length of both partners of the complex. In this manner, iodine binds prefer entially to long amylose chains because of the greater stability of the resultant complex. There is a relation between the length of the helix and the number of complexed iodine and iodide anions. Thus, there is also a relationship between the length of the helix and the color of the c ~ m p l e x . ' ~ "S. ' ~~ a~ n s o n established '~~ that chains having 4 to 6 glucose units do not produce any color, those from 8 to 12 units give a red color (520 nm), and those with 30-35 units display a blue color (600 nm). Table VII gives more detailed assignments of color to the chain length. A summary of all those arguments led to the conclusion that starch blue is a collection of left-handed amylose helices with six glucose units per turn, holding Isunits inside each fraction of the helix. It does not, however, explain the structure of the amylopectin-iodine complex because that complex is branched and apparently has no helical structure. The amylopectin-iodine complex has a purple-red or violet color with an absorption maximum at '03140.141

278

PIOTR TOMASIK AND CHRISTOPHER H. SCHILLING TABLE VII Relationship between the Chain Length of Synthetic Polysaccharides, the Color of Their Reaction with Iodine, and the Position of the Absorption Peak in the Visible Spectra of Relevant Complexes'45

Chain Length

Color with Iodine

u p to 5.7 7.4 10.2-12.9 13.3-18.3 20.2-21.2 25-5-29.3 33.1-146.5

None Faint red Red Lavender Purple Blue-purple Blue

Absorption Peak, Lax, nm 500

520 540-560 560-580 580 600

approximately 540 nm, regardless of both the origin of the polysaccharide and the degree of b r a n ~ h i n g . UV ' ~ ~ spectroscopy studies of dextrins and amylopectin suggest that the outer branches of amylopectin are composed of approximately 18 glucose units and that the inner branches consist of about 10 subunits.'45Mould'22 postulated that, in red-staining polysaccharides, the 13- species is responsible for the color because of the weak bonding of iodine to the polysaccharide chain. This low binding power (but not capacity) is attributed to disruption of helix formation, by the large number of branch points.I4' Higginb~tharn'~' found that the iodine-binding capacities of amylose and amylopectin are practically identical. Hence, another mechanism governs the bonding of iodine to amylopectin. He suggested that outer branches may coil into short helices under the influence of iodine. Such helices complex I2 and 13- species. Surface adsorption of those species is, however, also involved. According to Gilbert and Mari~tt,'~' red staining is due to the adsorption of eight atoms of iodine and one I- ion per molecule of the linear polysaccharide structure. Hydrogen bonds between amylopectin and iodine may be essential in such a c a ~ e . ~ ~ ' , ' ~ ' 8. The Mechanism of Complex Formation

The pathway for the formation of the amylose-iodine complex is well recognized, at least when aqueous solutions of KI and iodine are used. Relevant studies are lacking on the formation of the complexes between iodine vapor and both amylose and amylopectin. Thus, randomly coiled amylose binds 13- ions, and this interaction is responsible for the helical structure of amylose. Helical amylose arrests iodine and further 13- anions, a process which is cooperative. If the filling of one helix commences, then the filling of another helix proceeds once the former helix is full. The longest available helix has priority in the uptake


279

of iodine. After the uptake by the longest helix is completed, the completion of the next one begins. According to the results of Cesario and Brant,Il3 the limiting uptake expressed by the ratio of [Ij-]bound/{[12]bound+[13-]bound} reaches 0.3 ? 0.11. Additional polarization of the system can be caused by interactions between iodine molecules that are within the helix and/or adsorbed to the exterior surface of the helix and iodine molecules outside of this complex: (amylose . . . 1-1) + 1-1 F? (amylose . . . I . . . I . . . I)-I+. This process with iodine vapor is assumed to be responsible for the formation of the blue complex with starch without KI added.Is2 During the 1920s a suggestion was made'53-'55 that iodine is adsorbed by starch in two consecutive steps. The first step is rapid and is followed by slow sorption; equilibrium is not reached in most cases. The rate of reaction of iodine with starch was first rneasuredIs6 in 1939. The results depend on the variety of starch and on the amylose-to-amylopectin ratio, as illustrated by Huebner and Venkatararnad9 (Fig. 9). The rate of uptake of iodine differs in both consecutive steps, according to the origin of the starch. These differences can, moreover, be attributed to the different affinities of those starches to retrogradation. As shown by Hollo and S ~ e j t l i , ' ~ ~absorption -''~ of iodine decreases as a function of time and is also dependent on pH (Fig. 10). The binding capacity of iodine by starch was studied by spectrophotometric, potentiometric, amperometric, and rheological methods. A sharp increase in the viscosity of starch solutions occurred after the amylose helix

t

Days

FIG.9.-Effect of time (in days) on the amount of iodine adsorbed by 3 g of maize (M), rice (R), sago (S), tapioca (T), and wheat (W) starch. (Based on the data of Huebner and Venkataraman.")

280

PlOTR TOMASIK AND CHRISTOPHER H. SCHILLING

4

B

12

16 20 24 Time, h

h

FIG.lO.-Effect of time (in h) and pH on the iodine uptake by amylose at 28°C. The effcct is expressed as change of absorption of I2 (in %) with respect to the point of origin. (Reprinted with permission from J. Hollo and J. Szejtli, Przem. Spoz., (1957) 429-433.)

reached saturation. The process of helix saturation does not affect the viscosity. The second step, which appears to be adsorption of iodine on the surface, is responsible for the observed increase of viscosity due to bridging of individual helices by i ~ d i n e . ~ ~ ~ , ' ~ ~ , ' ~ " In spectroscopy studies it should be remembered that the LangmuirPerrin absorption law is obeyed, whereas the Beer-Lambert law is Increasing the proportion of iodine is accompanied by an increase in absorption intensity and a red shift of the absorption band, whereas increasing the proportion of KI decreases the intensity and shifts absorption toward the blue end. Another authory1observed the opposite effect of the KI concentration on the intensity of the absorption bands. When the concentrations of free iodine is farily high, amylopectin binds as much iodine as does amylose.16yThe effects of pH and temperature have been studied by S ~ e j t l i ' and ~ ~ ) Hatch.I7' The intensity of the 625-nm band (the longest wavelength band) decreased as the pH and temperature increased (see Fig. 11). Among the methods listed here, the amperometric determination has evoked the most i n t e r e ~ t . ' ~ ~ . ' ~ Potentiometric ,'~~-'~' mea~urements~~~.~~~* also confirm the findings of these methods, although the equilibrium concentrations of free iodine are lower by one order of magnitude than those determined by photometric titration. For the reaction Amylose

+ yI- + x12 = Amylose x12yI-,

the enthalpy, AH, is determined to be -46.8 kJ/m01.'~~ For the reaction Arnylose + nI-

+ n12 = Amylose nI2nI

,


281

w

39.0

10 1

2

3

4

5 PH

6

FIG. 1 I.-Effect of pH and temperature ( T ) on the formation of the amylose-iodine complex, expressed as the change (in %) of the relative extinction of the band measured at 625 nm. (Reprinted with permission from J. Hollo and J. Szejtli, Przem. Spoz., (1957) 429-433.)

A H = -64.9 kcJ/m01,""*~ and the equilibrium constant K = 1/[12] = -65.7 kJ/mol.IXs Am perometric methods indicated AH = -7.20 kcJ/ mol.' 1')~1s8-1h0~1Xh Spectrophotometric methods indicated values of AH ranging from -41 kJ/mol'*' through 53.6, (ref. 188) 16.6 (ref. 188) to 87.0 kJ/ mol. (refs. 91, 184). Calorimetric methods produced values of -70.3 (ref. 189) and 71.6 (ref. 113) kJ/mol. Approximately 10-20% of the overall value of AH (that is -6.7 to -12 kJ/mol) is attributed to nearest-neighbor interactions.I" More precise thermodynamic parameters are given by Yamamoto etaf.,"' who reported data for both steps of the complex formation. The temperature effect of the second cooperative step of the reaction is equal to zero, which suggests that this step is controlled by entropy and not by enthalpy. The free energy of formation of the iodine complex with amylose and amylopectin was estimated at 25 and 21 kJ/mol, respectively."* The stability constant at 25"CI9' is of the order of loh. The differences in AH presented here are attributable to two factors: (1) the character of the analytical methods employed, and (2) the KI concentration, which affects the position of the triiodide-iodine equilibrium because of the uptake of the constant ratio of the iodine and triiodide molecules in the ~ o m p l e x . " ~ The following diagrams show the effect of iodine concentration on the binding capacity of starch (Fig. 12), the effect of KI concentration (Fig. 13), the effect of temperature (Fig. 14), and the effects of the extent of

282


1

2

3

4

5

Pn 12 FIG.12.-The iodine-binding capacity (IBC) of starch (in %) as a function of the concentration of free molecular iodine (in mol). (Reprinted with permission from J. Hollo and J. Szejtli, Bruuwissenschafi, 13 (1960) 348-352.)

hydrolysis of amylose (the effect of the length of the amylose chain) (Fig. 15). According to Adkins and G r e e n ~ o o d ,the ' ~ ~effect of low temperature can only be observed in the case of short-chain amylomaize starch (see references 169, 196, and 197) for discussions of the temperature effects). Circular-dichroism (CD) analysis revealed three steps in the formation of the blue complex. The first step is shown by the shoulder at 480 nm in the UV-VIS absorption spectrum and is due to formation of the complex of the amylose fraction having a degree of polymerization (DP) from 10 to 30. The second step is the development of color from 13- ions with amylose of DP 30 to 100. The third step, which is the development of a deep-blue color, is associated with the shallowing of the dual CD band for high-DP

-s

N

20 15

10

0

1

2

3

4

n KI

FIG.13.--lodine-binding capacity (IBC) of amylose as the function of the concentration of 1- ions (in mol of KI). (Reprinted with permission from J. Hollo and J. Szejtli, Bruuwissenschufl, 13 (1960) 348-352.)


20

30

40

283

50 60 t em pemture.O C

FIG.14.-Variation of the absorbance at 600 nm of the amylose-iodine complex as the function of temperature. Crosses: measurements on heating; circles: measurements on cooling. (Reprinted with permission from Nature, 197, pp. 898-899, copyright 1963 Macmillan Magazines Limited.)

amyloses at high concentrations of iodine. Skewed ions located in the knot portions interact in the associated helices of the aggregate.61 The formation of the starch-iodine complex is affected by alcohols. This effect is interpreted as the result of the isolation of polyiodide chains inside the amylose helices as molecules of alcohol also occupy their i n t e r i o ~ - . " ~ * ' ~ ~

10

20

30

AE

FIG.15.-Variation of the iodine-binding capacity of amylose and the iodine concentration expressed in the weight of iodine as a function of the degree of hydrolysis ( A E ) . (Reprinted with permission from J . Hollo and J. Szejtli, Brauwissenschafr, 13 (1960) 380-386.)

284


When whole starch granules react with iodine or an iodine-iodide mixture, the structure of the granule changes. Iodine is more readily bound by the internal part of the granule than by the external part.'99 A similar effect is observed when granules react with zinc chloride iodide anions.2Mx' 9. Properties of the Iodine Complex

a. Physical Properties-(i) Ultraviolet and Visible SpectroscopyAbsorption spectroscopy in the region of 200-700 nm played a crucial role in understanding the structure of iodine complexes with starch and its components, as well as the mechanism of the complex formation. Investigations by Rundle et ~ l . , " S~ ~ a n s o n , ' ~Ono ' et ~ l . , ~Bailey ' and Whelan," and Kerr et d 2 0 1 revealed that the wavelength of maximum absorption of the amylose-iodine complex is related to the amylose chain length. This shift, which obviously has its effect on the color observed, is interpreted as the result of the state of aggregation of the complex.202Again, this property is governed by the average length of the uninterrupted amylose helices and the degree of crystalline order.203These can be primary factors affecting the spectra. Mokhnach and Rusakova" have shown that amylose-iodine and also starch-iodine complexes absorb at 226, 288-290, 344-360, and 585620 nm. The first band appears only when iodine is added to the solution simultaneously with KI, and it is absent when only iodine is added. The longest-wavelength absorption may be indicative of the molecular size of the carbohydrate portion of the complex, as demonstratedlMin Table VIII. Spectra of iodine-amylopectin complexes were measured by Archibald et The UV absorption spectrum of the starch-iodine complex is shown

TABLE VIII Relationship between the Molecular Size of Amylose and the Spectral Properties of Its Blue Complex with Iodine1& Molecular Size"

Amylose

A,,,, nm

~

Potato Tapioca Lily Corn Crystalline Synthetic Arnylodextrin ~~

&max ~~

500 450 310 250 175 85 44

628 625 622 618 605 590 580 ~~~

Number of glucose residues per molecule.

43,000 4 1,600 41,400 40.400 40,100 32,900 25,400


F

6.0 '

2

285

5.0.

E

OJ 4 . 0 .

3.0. 2.0.

i n

1 .o

in Fig. 16. The variation of the spectrum is attributed to the varying starchto-iodine ratio.204It had earlier been observed by Gilbert and M a r i ~ t t ' ~ ~ that the amylose complex with iodine is metastable and undergoes a physical change as a function of time. Pfannemuller and Z i e g a ~ t 'made ~ ~ similar observations by UV-VIS absorption spectroscopy (Fig. 17). 606

I X

0

E

u)

300

400

500

600

700

800

A ,nm FIG.17.-Absorption spectra of aqueous solutions of I2 (A) and amylose with 190 glucose units after the addition of iodine after 5,30,60, 150, and 270 min (B through F, respectively), and the complex of the same amylose with the 12-KI complex (G). (Reprinted with permission from B. Pfannemueller and G. Ziegast, Sfaerke, 35 (1983) 7-11.)

286


the IR spectra of (ii) Infrared Spectroscopy.-According to amylose and its blue complex are identical. Nevertheless, Greenwood and RossottiZo6found two small differences between these spectra. The spectrum of the complex exhibits two small peaks at 1101 and 1052 cm-' (Fig. 18).By analogy to the spectra of iodine in ethers (diethyl ether, 1,4-dioxane, tetrahydrofuran), where peaks appear in that region with assignment to the -I coordination band, these peaks in the spectrum of the amylose-iodine complex are ascribed to such interactions. These suggestions were presented subsequently by M~rakami,'~' and they were rejected by Noltenmeyer and Saenger.'32.'33Thus, this band assignment is doubtful. Assuming this type of interaction, the authorszMproposed a boatlike conformation of glucose units in the helix, but Murakami expressed a contradictory opinion in this re~pect.~" (iii) Raman Spectroscopy.-These are four characteristic maxima (162, 112,52,and 24 cm-I) in the Raman spectrum of the amylose-iodine complex (Fig. 19).'28~131~1"~142~208 The intensities and structures of individual peaks (shoulders) depend to a certain extent on the degree of polymerization of amylose, as well as on whether the complex was prepared from amylose and iodine or amylose and on 12-KI mixture. Despite thorough analysis of the spectra,I3' it was not possible to distinguish between stretching frequencies and bending frequencies in such units as 13- * I2 . 13- or 13- I2 * Iz . 13-, nor was it feasible to distinguish between possible iodine chain-units

1250

1000

750 Clli'

FIG.K-Infrared absorption spectrum of the amylose-iodine complex. [From Greenwood and Rosotti?" Copyright 0 1958 John Wiley & Sons. Reprinted by permission of John Wiley & Sons, Inc.]


287

164

0

300 cml

FIG.19.-Resonance Raman spectrum of the amylose (DP 25)-iodine complex. (Reprinted with permission from B. Pfannemueller and G. Ziegast, Stuerke, 35 (1983) 7-11.)

themselves. The existence of Is- units seems to be most likely. The opinion'36 was also expressed that there is a fully resonating polyiodine chain in the amylose helix instead of a polyiodine chain composed of discrete iodine and iodide species forming regularly dislocated subunits therein. (iv) Mossbauer Spectroscopy.-Analysis of the Mossbauer spectrum of the starch-iodine complex (Fig. 20 and Table IX) points to three unequivalent iodine sites in approximate relative populations of 2:2:1. In this manner the Is- is shown to be the predominant polyiodide unit inside the amylose helix."'

(v) Electron Resonance Spectroscopy.-In 1961, Bersohn and Isenberg"' observed a weak electron resonance in the amylose-iodine complex. Based on this observation, these authors deduced that if atoms of iodine in the chain are mutually covalently bonded, the band of the 5ps state is only partially filled. This would result in metallic properties, including weak paramagnetism and electronic conductivity. The fundamentals of this conclusion met with criticismz1"because the results of the earlier authorszw

-20.0 -10.0 a0

iao

20.0

mm/sec

FIG.20.-The iodine-129 Mossbauer spectrum of the starch-iodine complex. [Reprinted ' ~ ~ (1980) American Chemical Society.] with permission from Teitelbaum et ~ 1 . Copyright

288

PIOTR TOMASIK AND CHRISTOPHER H. SCHILLING TABLEIX '*'I Mossbauer Parameters for the Amylose-Iodine ComplexUS Site 1

a, mmlsec (versus ZnTe) e2qQ, MHz (for 12')1) r, mm/sec (line width) Relative population

1.22

- 1743 1.14 1.9

Site 2 8, mmlsec e2qQ, MHz r, mm/sec Relative population

0.53 -1187 2.13 1.8

d, mm/sec e2qQ, MHz r. mm/sec Relative population

-842 1.08

Site 3 0.14

1.o

could not be repeated when the samples were stored in the dark. Exposure to sunlight was evidently the only reason the ESR spectra could be recorded at all. Although Insberg and Bersohn211accepted the criticism, they did not reject their one-dimensional model for iodine inside the amylose activity. Their results suggest the possibility of a photochemical reaction of the complex, which can be either iodination or oxidation. Two years later, P e t i ~ o l a s demonstrated '~~ that the former observation could be done to moisture present in the sample. (vi) 'H and I3C Nuclear Magnetic Resonance Spectroscopy.-The I3C NMR spectrum depends on the type of starch (amylose-to-amylopectin ratio) and is associated with the numbers of carbon atoms in the branching points and thermal glucose units. Tables X and XI present I3Cspin-lattice relaxation times ( T I , $ )and nuclear Overhauser enhancement (n.0.e) for I3C nuclei of starches of various origins. Figure 21 shows IH NMR spectra of amylose and a high-amylopectin waxy sorghum starch. The amount of iodine complexed by starch of various origins depends on the same factors as the NMR spectra. There are satisfactory linear correlations between the iodine-binding capacity and the percentage of terminal glucose units of starch. The latter could be calculated from the intensities of the proton and carbon chemical shifts.81 (vii) Dichroism.-The flow of solutions of the starch-iodine complex exhibits dichroism.212This phenomenon is associated with the difference


289

TABLE X I3C NMR Spin-Lattice Relaxation Times for Starch of Various Origins" Relaxation Times, Sec Atom

Signals, ppm

Sorghum

Wheat

Potato

Corn

c-1

100.4" 100.2 100.1 99.9h 79.3 79.2 79.1 79.0 78.9" 70.4" 72.5" 72.0h 61.O"

0.29 0.22 0.19 0.19 0.19 0.23

0.30 0.19 0.18 0.18 0.18 0.18 0.20 0.20 0.20 0.23 0.27 0.20 0.12

0.28 0.20 0.18 0.18 0.19 0.22 0.21 0.17 0.20 0.30 0.25 0.19 0.12

0.25 0.22 0.15 0.19

c-4

c-2 C-6

0.19 0.20 0.28 0.27 0.21 0.12

c

0.15

0.17 0.20 0.23 0.26 0.20 0.12

"The signal is attributed to the glycosyl end group. *The signal is attributed to the glycosyl groups of the main chains. ' The signal is not adequately resolved.

in the absorption of light photons, with the electric vectors being parallel and normal to the flow lines. Dichroism of flow indicates the preference of absorption of photons with the electric vector parallel to the flow. Dichroism in this particular case requires parallelism of the long axes of the iodine molecules and the starch-iodine complex. Dichroism is readily observed

TABLE XI Values of Nuclear Overhauser Effect for 'jC Nuclei of Starch of Various Originss1 Nuclear Overhauser Effect Starch

C-1"

C-lb

C-4"

C-4b

C-6"

C-6'

Sorghum' Wheat Potato Corn"

2.15 2.22 2.58 1.99

1.69 1.76 1.66 1.65

1.99 1.91 2.11 2.35

1.71 I .74 1.71 1.71

2.27 1.76 2.52 2.37

1.84

The values for the glycosyl end groups.

'The values for the glycosyl groups of the main chain. ' Waxy starch. " High-amylose

starch.

1.92 1.82 1.87

290


P Pm FIG.21.-Partial 'H NMR spectra (300 MHz) of amylose (A) and waxy-sorghum starch (B) in MezSO-d6 at 23°C. (From Peng and Perlin.8')

in the case of iodine complexes of starch and amylose. A small amount of dichroism can be noted for red-staining starches, waxy maize starch, and glutinous rice starch. Starch stained with iodine, with subsequent removal of iodine from the sample, exhibits no dichroism. Hence, the dichroism of flow of iodine-stained solutions is a function of the size of the

(viii) Circular Dichroism and Optical Rotatory Dispersion.-Starch and complexes of iodine with starch, amylose, and amylopectin, are amendable to study by optical rotatory dispersion and circular d i ~ h r o i s r n , 6 ~ *be'~"~~~~ cause these polysaccharides exhibit a positive induced Cotton effect in the region of the absorption maximum (545 nm). Figures 22-25 show the UV-VIS absorption spectra, rotatory dispersion, and circular dichroism spectra of starch-, amylose-, and amylopectin-iodine complexes. The rotatory power and extinctions at 600 and 445 nm depend on the iodineto-carbohydrate ratio, as shown in Figs. 25 and 26. At constant extinction, the optical rotation of the complexes may increase with time, although it is not a general property. Both the absorption spectrum and the optical rotation are sensitive to heat and certain additives, because of changes in structure of the complexes.

(ix) X-Ray Diffraction.-Powder X-ray diffraction played a crucial role in clarifying the structure of starch-iodine complexes (Table XII)!8 The amylose-iodine complex forms a hexagonal unit cell with a, = 12.97, c, = 7.91, and dlM)= 11.23 A. (ref. 72)


291

AE

3 2 1

0

300

LOO

600

500

h ,nm FIG.22.-Absorption (-), ORD (- - -), CD (-..-) spectra of aqueous solutions of soluble starch ( a ) K13 ( b ) , and the starch-iodine complex (c). (Reprinted with permission from R. C. Schulz, R. Wolf, and H. Mayerhoefer, Kolloid-Z., Z. Polyrn., 227 (1968) 65-72.)

1500 1000

q:o 2000

II

500

1000

I

350 400

5qO"

600

700

'

0

FIG.23.-Absorption (-) and ORD spectra of the amylose-iodine spectrum.(Reprinted with permission from R. C. Schulz, R. Wolf, and H. Mayerhoefer, Kolloid-Z., Z. Polym., 227 (1968) 65-72.)

292


. 1500 -

2500

500

1

-500

hnm

--0.4

FIG.24.-Absorption (-), ORD (- - -), and CD (-.-) spectra of amylopectin (API) and the amylopectin-iodine complex. (Reprinted with permission from R. C. Schulz, R. Wolf, and H. Mayerhoefer, Kolloid-Z., Z. Polym., 227 (1968) 65-72.)

(x) Rheo1ogy.-Rheological studies complemented the amperometric analysis of the structure of starch-iodine complexes."2~"yThis method was refined by Liang et and served in conformational studies of those complexes. The authors sought relationships between intrinsic viscosity [ 771 and the concentration of iodine added to polysaccharides. Such a relationship displays a sigmoidal pattern, as shown in Fig. 27. The pronounced increase of 1771 on complexation with iodine is caused by a stiffening of the helix that is not accompanied by any conformational changes. Iodine does not affect the shape and/or extension of the chain in space. The complex-

4ooov 2000

01 0

I

0.05

01

I

0.15

0.2

0.25

12 . 9 / L

FIG.25.-The shift of the maximum of the specific rotation of the starch-iodine complex as the function of the concentration (c) of iodine (in 12-C6H1005, mollmol). (Reprinted with permission from R. C. Schulz, R. Wolf, and H. Mayerhoefer, Kolloid-Z., Z. Polym., 227 (1968) 65-72.)


293

*Oo0 05.1000

/--------

0,4

FIG.26.-Specitic rotation (- - -) and extinction coefficient (-) of the amylose-iodine complex as the function of the concentration (c) of iodine. (Reprinted with permission from R. C. Schulz, R. Wolf, and H. Mayerhoefer, Kolloid-Z., Z. Polym., 227 (1968) 65-72.)

ation also reduces the sensitivity of [v]to changes in temperature, as a rigid particle is less able to dissipate thermal kinetic energy than a flexible one. b. Chemical Properties.-Little is known regarding the effects of the iodine chain inside the amylose helix on the reactivity of that helix, and the effects of adsorbed iodine molecules on the reactivity of polysaccharides in general. Also, heating of the complex in aqueous solution causes its decomposition. This is due to the replacement of iodine inside of the helix by water .27.2 5-2 I7 Th e thermal decolorizing effect is due to the decrease of the length of the amylose helix2Is and not, as previously suggested, to the evaporation of iodine.21yIn contrast, the influence of iodine on chemical reactivity and its effect on the enzymic degradation of starch is well known.l5 Complexation of starch with iodine increases its resistance to enzymic degradation, as shown in Fig. 28. It is known that ultraviolet light affects the complex, both in the solid state and in aqueous solution. Complexes undergo gradual decolorization as a result of deterioration of the helix and reaction of the iodine-iodide complex anion with water to produce HI.220-222 It has also been shown209-21' that ordinary room lighting in the presence of atmospheric oxygen affects the complex by oxidation, but this process has not been studied in detail. Both X-rays and ultraviolet light cause similar disruption of the complex, but the process is faster with the former.223Radon has a similar effect.224 Boric acid is known to complex starch (see Section IV), and the same complexation takes place with the starch-iodine complexes. Figure 29 diagrams the assumed position of attached boric acid.lSyAs with starch, starch blue is capable of sorbing some dyes (Acridine Orange, Methylene Blue, and 1,9-dimethyl-rnethylene blue).22sThis phenomenon is discussed in more detail in the accompanying article (p. 345).

'

294


XI1 TABLE Results of X-Ray Powder Diffraction of the Amylose-Iodine Complex" Sin2 MA2 Hexagonal Indices

Observed

Calculated

100

0.001965 .005937"

0.001979 .005912 .005965 ,007916 .009925 .01192 .01386 .01782* .01797 .01786 .02189 .02193 .02375 .02391 .02573 .02976 .02985 .03162 .03767 .0415gh .04166 ,04174 .04558 ,04767 .04950 ,05344 .05542 ,06137 .07126 .07324

llOb

101 200 111 201 210 300b 211 102 301 112 220h 202 310 311 212 400 320 410" 321 312 411 402 500 330 420 510 600 430

.007922 .009955 .01158 .01384 .01781 .02182 .0237Sh .02573 .02974 ,03178 .03775

.04155 .04551 ,04763 .05360 .05549 ,06165 .07114 ,07310

Intensities

vs S S M VM

vw vs S

ww M MS

vw W M

M

vvw vvw nil

vw vw

vw vw

vw

Reproduced by permission from Rundle and Only the reflection (h,hzO) should be intense enough to influence the position of the line. I,

Iodine can be removed from the complex by means of thiosulfate ions; the activation energy of this fast reaction is 8 kcal/mo1.226 Several organic compounds either change the color of the complex or eliminate color altogether. Color disappears under treatment with egg albumen and several proteins, an effect caused by the formation of inclusion complexes with the proteins. Among a-amino acids, only tyrosine exhibited such behavior. Furfural also decomposes the blue complex and epinephrine


295

0.4-

40

0

8.0 12.0 16.0 20.0 rnV

FIG.27.-The relationship between the intrinsic viscosity (7)and concentration of 12 added to starch solution. (Reprinted with permission from Joan-Nan Liang, C. J. Knauss, and R. R. Myers, Rheol. Acra, 13 (1974) 740-744.)

5

10

15

20 X ,%

FIG. 28.-The variation of the iodine binding capacity of starch in the course of the enzymic hydrolysis of starch and the starch-iodine complex. (Reprinted with permission from J. Hollo and J. Szejtli, Brouwissenschufi, 13 (1960) 380-386.)

FIG.29.-The mode of iodine binding in the amylopectin-boric acid complex. (Reprinted with permission from J. Hollo and J. Szejtli, Ferre. Seifen. Anschtrichminel, 59 (1957) 94-98.)

296


instantly changes the color from blue to pink.” Certain corrections may be required in the quantitative determination of starch in the presence of such corn pound^.^^^^"^^^^^ 10. Applications of the Complexation and the Complex

a. Analytics.-Use of the reaction in iodometry is well known (see, for instance, reference~’’~-~~~, and in this section, only a few less-known facts and more sophisticated applications will be cited. Some authors have recommended the use of Cd12 instead of KI as the source of the 13- complex ion, and ZnC12.238 as well as cadmium Carboxylic acids separated by starch-impregnated paper chromatography can be visibly detected by using the 1-103- solution. The sensitivity depends on the dissocation constants of the acids. Addition of CaCI2 increases the ~ensitivity.’~’Microgram-level determination of thiosulfates by paper chromatography has also been reported. In this case, the chromatographic strip is impregnated with the starch-iodine complex.240 A method of determining airborne iodine has also been rep0rted.2~’ Here, iodine is absorbed into 5% aqueous KI and spectrophotometrically determined at 590 nm in the form of its complex with starch. This method is selective with respect to bromine and chlorine, and the sensitivity of this method is 0.25 mg of I2 per m3 of air. The concentration of the I3lI isotope in water can be determined by a method involving isotope exchange in the starch-iodine complex.242Flow-injection determination of ascorbic acid (0.1-40 mg/mL) has been proposed.243Iodine is generated in the flow system as 13-ions, which are in turn exposed to starch to produce a steady signal at 350 and 580 nm. Ascorbic acid provides inversed maxima which are measured. This method is recommended for analysis of ascorbic acid in fruit juice, jam, and vitamin-C preparations. Use of the blue complex has also been reported for determination of sodium dichloroisocyanurate in air.244Obviously the blue reaction is applicable in the determination of amylose, amylopectin, and s t a r ~ h ? ~ “as ’ ~well ~ as modified starchesa245,25%2SS Since the result of the starch reaction depends on the type of starch, the blue reaction is suitable for the characterization of different varieties of starchy materials,76.77.105.162,256-275 and a special coloristic scale has been developed for industrial purposes.276A negative correlation was observed257 between the color density of the complex and the stickness of cooked starchy material. The blue reaction is very useful in determining the amylose-amylopectin composition of ~ t a r c h , 6 ’ , ~ and ~ ~ +in~ ~ controlling ’ of the effect of mechanochemical and enzymic treatment and fractionation of s t a r ~ h ~ ~ - ~ ~ ~ ~


291

and staining enzymes.2yo-2y3 The usefulness of the starch-iodine reactions in the study of retrogradation, and was demonstrated by Hollo et u1.294*29s an investigation of the nature of various starch inclusion complexes has been reported by Rutschmann and S ~ l m sSzejtli . ~ ~ et~ul. published determination of the degree of polymerization of amylose, based on its reaction with iodine.297The use of a starch-iodine complex as a histochemical reagent has also been reported.298 b. Amylose Fractionation and Separation.-Fractionation of amylose achieved based on the sequence of iodine uptake by amylose, a process that is dependent on the length of its helix.263~2yy~30" Electrophoretic separation is described by Mould.'22

c. Concentration and Extraction of Iodine.-Attempts to utilize starch to extract iodine from dilute aqueous solutions have been made by Magidson,3"' and the applicability of this method for use on a large scale has been discussed. There is a French patent on iodine extraction from seaweed using starch.'02 d. BiologicallyActive Preparations.-Iodine is well known as a powerful disinfectingand antiseptic agent. It may be possible to attenuate this activity by iodine complexation with starch. It is reported that a mixture of soluble starch (25 g) and 1:lOO iodine-iodide combination (50 cm3)in boiling water (1 dm3)does not irritate tissues, does not harm clothing, and is stable. It also kills streptococcic, pyocyanic, and colic bacteria.3o3Another preparation for such purposes consists of starch, glycerol, water, and iodine.304Other modifications have also been p r o p o ~ e d .Among ~ ~ ~ , starch, ~ ~ ~ amylose, and amylopectin, the first two are superior carriers for iodine. The bacteriostatic properties against gram-positive and gram-negative bacteria are manifested at concentrations of 2.5-20 g/cm3 after 15 min of e x p o s ~ r e . ~ ~ ~ - ~ ' ~ Use of the starch-iodine complex has been reported for the disinfection of wounds and for the artificial insemination of animal^.^" Food containers may also be disinfected with this complex.311Uses of the starch-iodine complex in the iodization of food3'2,313 and ruminant fodde9l4 have also been described. This complex has been found effective in the storage preservation of cabbage and carrots,315potatoes?16 and apples?" The addition of 0.0005 to 0.005% of the complex to bee honey also improves its properties; it was reported that the application of a 1%aqueous solution of the complex for spraying bee houses increased the yield of honey.318 Intragastric injection of the complex (49 mg/dm3 of iodine content) to rats is reported to provide slight radioprotection from gamma rays (5 Gy at 1.8 Gy/min or 4.19 Gy at 1.235 G ~ / m i n ) . ~AI ~microbial fabric that

298


contains in its fibers a small quantity of starch-iodine complex has also been patented.’2” e. Miscellaneous Applications.-The starch-iodine complex was proposed as an additive for the production of special copying paper.”’ Iraqi scientists322demonstrated that this complex improves the thermal efficiency of liquid solar collectors and is more effective than potassium chromate, potassium, permanganate, and Indian ink additives.

111. STARCH-WATER COMPLEXES

1. Introduction There are several theoretical and technological problems associated with the presence of water in star~h.’~’ The amount of water in starch determines its nutritive value and functional properties. Knowledge of the amount of water is essential in selecting the mode of processing starch by drying, moisturizing (as in dough production), thermal treatment, extrusion, molding, and hydrolysis. Water also affects the stability of starch during storage, particularly with respect to mold formation (starch and cereals with 13-17% humidity are moldproof when stored324at 20°C),dusting, and dust explosivity. Water causes corrosion of the metal of silos and transporters. The possibility of molding with moist starch increases the risk of corrosion by mold slime.325As demonstrated by Muetgeert et uL,326 water in starch determines its chemical reactivity; anhydrous starch is unreactive. The reactivity of starch decreases with drying, and this is reversible when the starch is remoisturized. Reactivity is readily restored upon contact with acetic acid. Regarding the hydration of starch, two points must be distinguished. The first concerns saturation of starch with water (that is, the water-binding capacity of starch), a value that is usually related to water uptake and loss (sorption-desorption) from and to the atmosphere. This is a significant technological problem, because the water concentration varies according to the origin of starch and the temperature. Below the freezing point, it is reported that the starch matrix holds up to 46.5% of starch.327Another concern is that the estimated value of the water content depends to a certain extent on the method of its determination. Table XI11 presents estimates by many authors. It should be mentioned that the first studies of water in starch are dated” as early as 1834. The second point concerns water that is usually held by “natural” starch, that is, a state that can be considered as the natural starch-water inclusion complex. In this case also, the concentration of such water depends on the same factors as those just mentioned. Corresponding data are given in Table XIV.


299

TABLE XI11 Water Saturation Capacities (YO)for Starch of Different Origins at Room Temperature" Corn Potato Rice Sweet potato Tapioca Wheat Waxy corn

19.55-28.7 20.9-33.7 20.8-26.8

18.7-31.63

(39.9) (50.9) (40.5) (43.5) (42.4) (39.9) (51.6)

" The values recalculated on a dry starch basis are given in parentheses. Below the freezing point, the starch matrix of potato starch holds up to 46.2%of water. The estimation for the starch matrix at 20°C. given by the same author?20 is 35.5%.

Reviews on starch hydration were published by Schierbaum3*' in 1960 and by Nara et al.329in 1968 and (ref. 330) 1981. The first scientific approaches were based on an assumption that water in starch is held by sorption. R a k o ~ s k i ~observed ~I-~~~ hysteresis in measurements of the uptake and loss of humidity by solid starch; other authors334-336 subsequently confirmed the S-shape of the isotherms. At the time of these publications, more detailed knowledge of the structure of starch granules was not available, nor was the helical structure of amylose recognized. Nevertheless, Malfitano and M o s ~ h k o f described f~~~ part of the water residing in starch as the water of hydration and constitutional water. In this manner, they presented the idea of different fractions of water in starch. Several authors, using various techniques, confirmed these observations. Various methods have been proposed for determining the localization of water in starch materials. In the work of van der Haeve et ~ l . , ~a ~ ' portion of the water was identified as a constitutional water of micelles, and TABLE XIV Water Contents (YO)in Natural (Native) Starch of Various Origins Arrowroot Corn Potato Rice Sago Tapioca Wheat

11.37-16.50 11.61-16.00 13.34-22.42 13.71 (average) 12.80-18.83 11.37-16.50 10.52-16.10

300


same^^^' suggested capillaries as the location of water molecules. Further reported three water molecules bonded to one glucose unit of starch, although later studies have reported one and two bonded water molecules per glucose nit.^^^*^^^

2. The Status of Water in Starch The first essential observation in the middle of last century345distinguished two types of water absorbed from the atmosphere (the equilibrium water). However, R a k o ~ s k i ~performed ~ l - ~ ~ ~ kinetic studies of the drying of starch and observed that two-thirds of the total water in starch was desorbed during the first 24 hs, and it required another 2 weeks to remove the remaining water. In this 2-week period, 98%of this content desorbed in the first week, whereas the remaining water was desorbed during the second week. Infrared radiation studies on the dehydration of potato starch346led to the conclusion that at least four different types of water are held in starch. Further investigation^^^' validated this conclusion, but only in the case of potato starch. There are three well-observed transition points on a chart of weight loss as a function of time during the infrared drying of starch. In contrast, cereal starches exhibit only two transition points. The type of dehydration of gelatinized (swollen) starch is reflected by a curve of similar shape, but with the transition points that are not recognizable (see Fig. 30). Nevertheless, the authors distinguished only two types of water, namely, capillary water (between A and B transition points) and the water of adsorption (from the B transition point up to the end of

4p

At

HzO.’/O

FIG.30.-The course of starch dehydration (in dgldt, where g is weight and r is time) for potato (P), wheat (W), and rice (R) starch. (From Schierbaum and Taeufel.”’)


301

the diagram). The same conclusion can be reached based on the agreement of the sorption-desorption curves with the Freundlich isotherm348 a =

(y

pllrz

(4

where a = x/m is the total adsorbed water per unit weight of starch, p is the partial pressure of water vapor, a is an adsorption constant, and l / n is the adsorption exponent. Chapek et a1.34y*3‘0 pointed out that, among the two types of water of hydration, a significant major proportion of the water is weakly bound, weakly oriented, or osmotic. Its heat of adsorption is approximately 2 kcal/ mol. Later s t ~ d i e s ~with ~ l the , ~ ~NMR ~ spin-echo technique and dielectric methods illustrated that barely 3% or less of the water enters discrete cavities (micropores that are comparable in size to the size of water molecules) in the first stage of sorption. Subsequent sorption proceeds in these micropores with the formation of clusters of water molecules. Derivatographic studies of potato, wheat, rye, and maize ~ t a r c h ”have ~ shown that only the potato starch lost water at temperatures between 30 and 150°C (19%). The other starches retained their humidity over this temperature range. Hysteresis loops for sorption-desorption are nearly 100% reproducible, but a consistent trend is observed: a decrease of the amount of adsorbed water with increasing numbers of sorption-desorption cycles. In the seventh subsequent run of the sorption-desorption cycle at 35”C, the volume of adsorbed water is by a factor of 1/30. This decrease suggests the possibility of a small amount of damage to the structure of either the starch matrix or granules, and even the amylose helix. Such a conclusion may also be drawn by observation of the thermolysis of air-dried, azeotropically ” dried (with benzene), and oven-dried (2 h, 130 “C) potato s t a r ~ h . ~Drying affects the structure of starch, but this effect is rather insignificantat elevated temperatures (see Table XV). Again it is qualitatively in agreement with the observations of Ulmann and S ~ h i e r b a u m ~ who ~ ~ - reported ~’~ the alteration of starch as dependent on the mode of drying. The irreproducibility of sorption-desorption hysteresis may be due to irreversible changes in the swelling of starch granules. Swelling obviously increases the surface area of s t a r ~ h . ~ ~ ’ . ~ ~ ’ Later studies by Poliszko et ~ 1 . documented ~ ~ ’ that, in freeze-dried starch gels, several complex conformational transitions take place that increase the rigidity of starch chains nearly 105 times. The free energy of mechanical relaxations due to the reorientation of hydroxymethyl groups reaches 38 kJ/mol; the activation energy due to the local conformational mobility of polymeric chain is reported at 48.7 kJ/mol.


302

XV TABLE Differential Thermal Analysis (DTA), Thermogravimetric Analysis (TGA), and Differential Thermogravimetric Analysis (DTG) of Potato Starch Dried by Three Methods (All Temperatures in 0C)3ss -

Air-dried starch DTA”: lOO(730) en; 258 (89) en; 269 (30) en; 277s en; 283 ( tapioca > waxy rice > sweet potato > rice; S m 0 1 i n a ~reported ~~ the orders potato > maize > rice > wheat at low humidity and potato > rice > maize > wheat at high humidity. These orders do not agree with the order reported in Table XVII on the effect of desiccation on the water retention of starch. Based on these data. the

TABLE XVI Degree of Crystallinity as a Function of the Variety of Starch and the Partial Pressure of Water Vapor, as Determined by X-Ray Diffra~tion~'~ Fraction Crystallized Partial Pressure

Potato

Amylomaize

Amylose

Retrograded Potato

0 0.20 0.40 0.60 0.80

0.21 0.20 0.22 0.22 0.27

0.20 0.17 0.19 0.19 0.20

0.19 0.19 0.19 0.20 0.19

0.06 0.06 0.10 0.11 0.12


309

TABLE XVII Effect of Desiccation at Room Temperature on the Water Adsorption-Desorption Behavior of Starch420

Loss in Water Retention Capacity, mg HZWg of Dry Starch Starch

Desorption

Adsorption

Arrowroot

15.2 11.1 28.2 8.2 19.6 3.9

5.3 8.2 9.1

Corn Potato Rice Sago Wheat

4.0 8.6 4.4

Difference 9.9 2.9 19.1 9.9 11.0 +0.5

authors4’” classified starch into three groups: cereal starch (wheat, corn, and rice), pit or root starch (sago, arrowroot, and sweet potato), and tuber starch (potato). Furthermore, taking into account the theory of the involvement of hydroxyl groups in water binding and sorption, they deduced that potato starch has more hydroxyl groups available (that is, it is less intramolecularly associated). There is a suggestion423that, at low moisture concentrations, intragranular diffusion controls the sorption process, whereas at high moisture concentrations, the dissipation of heat due to the heat of sorption is the controlling factor. There is also a report that the values of the adsorption capacity of water vapor on various swelling starches are practically independent of the origin of the s t a r ~ h . 4Other ~ ~ attempts at interpreting the differences in sorption capacity by starch of different varieties failed; the authors correctly observed that surface condensation is not the only mode of ~orption.4’~ The hydration of starch evokes thermal effects which are obviously also dependent on the starch variety. This is true because the external temperature does not affect426the macrostructure and microstructure of starch below 140°C. An estimation of the heat of sorption is useful in determining the concentration of bound ~ a t e r . The ~ ~heat ~ -of~adsorption ~ ~ ranges from 0.255 to 0.100 kJ/mol and depends not only on the variety of starch, but also on the mode of drying of starch prior to measurement (Table XVIII).426 The effect of the mode of drying can be used to study the role of water in changing the chemistry and structure of the starch matrix. For example, a series of papers on dehydration of starch had particular emphasis on drying by infrared r a d i a t i ~ n .In~ this ~ ~ situation, . ~ ~ ~ drying can cause dehydration of terminal glucose groups and dextrinization. Bursting of starch granules always takes place (125°C for maize starch and 158°C for barley starch),

310

PIOTR TOMASIK AND CHRISTOPHER H. SCHILLING TABLEXVIII Effect of the Method of Drying of Starch on the Heat of Remoi~turization~~ (Wlmol)

Starch

Oven Drying

Vacuum Drying

Corn Potato Rice Wheat

0.225 0.255 0.150 0.113

0.025 0.100 0.234 0.134

and this fact helps to maximize the water uptake by exposing more surface area."3h The rate and extent of granule swelling is also an interesting factor. The swelling power depends on the variety of starch and is not well understood. Figure 34 presents the results of swelling studies of a few thick-boiled ~tarches.4~~

5. Analysis of the Water Content This problem, which is important from both theoretical and practical standpoints, has been solved in many ways. Some approaches have been limited to the estimation of the overall water content, a problem interesting mainly for practicians. Gravimetric methods seem to be the least sophisticated because the final result is available from the difference of the mass of sample before and after drying (usually for 2 h at 130°C). It is possible

50

60 70 80 90 Extraction tempemture,'C

FIG.34.-Swelling patterns of various thick-boiling starches (P, potato; T, tapioca; W, waxy corn; C, corn; S, sorghum). (From Elder and Schoch!")


311

to distinguish between the equilibrium water concentration and the total concentration of sorbed water by using a simple vacuum-drying procedure!0'.43x-"0 Eberius"' reported a convenient and precise method of estimating the water content by Karl Fischer titration. Dumanskii and Yakovkina442 proposed the measurement of starch humidity by selective adsorption of water from 15-25% aqueous solutions of sucrose. A more sophisticated technique involves measurements of the dieIectric properties of starch, which is compressed in an apparatus presentedM3in Fig. 35. Using this approach, dielectric properties are measured at low frequencies (316 Hz-10 kHz), and attenuation of electromagnetic waves is measured at a high frequency (9.3 GHz). A key result is that the electromagneticwave damping is linearly dependent on the starch humidity at a given applied pressure (Fig. 36). A group of Japanese workers369evaluated the dielectric properties of starch, and hence the water content, using audio frequency measurements. Infrared and Hertzian frequencies have also been used for this purpose.368*370 Freeman4" compared the results of bound water measurements by calorimetric, cryoscopic, and dilatometric methods. The results were comparable and slightly higher than those measured by the refractometric method of D u m a n ~ k i iFrom . ~ ~ ~heat-capacity data, the amount of starch in suspension may also be determined. Among various methods, the tensimetric method delivers the greatest amount of practical and theoretical information. This method has been reviewed in detail and refined by Schier(see also ref. 445 for a later modification). The infrared method

FIG.35.-Apparatus for measuring the effect of humidityon the damping of electromagnetic waves. 1, screw press; 2, measurementcell; 3, electrode; 4, bow dynanometer;5, starch sample. (Reprinted with permission from M. Boruch, S. Brzezinski, and A. Palka, Acta Aliment. Pol., 1 1 (1985) 115-124.)

312


736 kPa 521 kPa

265 kPa A,dB

15

17

19

21

23 25 27 Humidity, O/O

29

FIG.36.-Changes in electromagnetic wave damping, A, as a function of starch humidity at various pressures. Three potato-starchvarieties are denoted by three different point patterns. (Reprinted with permission from M. Boruch, S. Brzezinski, and A. Palka, Acra Aliment. Pol., 11 (1985) 115-124.)

was also presented by the same author jointly with Ulmann.446N e m i t ~ ~ ~ ~ presented a hygroscope for the determination of water content, and Nara et introduced a method involving centrifugal filtration. Among other methods, thermogravimetric analysis has also been d es ~ri b ed .~’ IV. STARCH COMPLEXES WITH OTHER NONMETALLIC GUESTS

1. Introduction BET (Braunnauer-Emmett-Taller) sorption isotherms of starch indiate^^^,‘"'^ that only very small volumes of nitrogen is achieved quickly (within 10 min). Sorption-desorption proceeded without hysteresis; however, complete degassing of starch was not possible by heating to 105°C at nitrogen partial pressures as low as torr. Air is also adsorbed in small amounts.48 Convincing evidence for the sorption of various gases on starch . ~observed ~ ~ similar amounts was presented as early as 1928 by K ~ d aHe of contraction of an isolated frog heart after exposure to starch samples that had been previously exposed to different gases, including COz, CO, coal gas, NO, N 2 0 , HzS, S02, C12, HZ,02,ethene, ethyne, and methyl chloride. The author concluded that inorganic gases are better sorbed than organic gases, and that a critical factor is the polarity and not the size of the gas molecule. As compared to charcoal and bolus, starch has rather poor sorptive affinity.


313

cr-cyclodextrin forms inclusion complexes with C 0 2ranging in their sorp1O:l to 1:l. Moreover, the inclusion of C 0 2inside the cavity tion from45".451 is the sole mechanism of adsorption involved in such cases. Therefore, it is not a good model system for the starch-C02 complex because other adsorption mechanisms are typically present in starch, for instance, capillary sorption and surface sorption. However, starch has a low (-W3 wt %) capacity to sorb C02, and the sorption is rever~ible.4~~ In addition, carbon dioxide cannot replace water from its starch c0rnplex.4~~ Sulfur dioxide and ammonia are adsorbed by starch in at an approximate amount of one molecule per glucose unitf' however, the structure of these complexes has not been investigated. As with iodine, starch forms an inclusion complex with bromine vapor.'"' Depending on the starch variety, different colors are developed by the complex. Maize and wheat produce an ochre color, rice produces a lightbuff color, potato and sago develop a pale-yellow color, and cassava forms a cream color.69 Iodine cyanide (and bromine)-amylose complexes are brown-black and dark brown, respectivelyFM The adsorption of chlorine and iodine proceeds according to the Freundlich isotherm. A discontinuity on the Freundlich isotherm plot is reported, which possibly results from the swelling of starch granules.454 Sorption of various gases (02, N2, C 0 2 ,and He) on wheat flour, soybean flour, potato starch, and wheat starch has been investigated in order to study the effect of those gases on the functional properties of processed food products. Results indicate that such properties improved only after flour and starch were treated with ~ h l o r i n e . ~The ~ ' *adsorptivity ~~~ of various gases by starch has been ree~amined.~" Liquid ammonia quickly forms a gelatinous paste with Tomasik et ~ 1attempted . ~ to~prepare ~ inclusion complexes of starch with colloidal sulfur. A key result was that inclusion inside the starch matrix was only possible in small amounts because of the large relative size of the sulfur micelles. It has been discovered that starch adsorbs onto graphite, a phenomenon that is employed in processing sulfide ores containing graphite.46" The adsorption of starch on quartz and hematite has been studied in more In this case, adsorption is related to the balance between electrostatic interactions and hydrogen bonding. Interparticle bridging is also observed as a result of the adsorption of starch molecules at the interface (flocculation). 2. Complexes with Arrhenius Acids and Bases

a. Complexes with Acids.-According to Rakowski,"62 starch adsorbs insignificant amounts of acid; however, other authors do not support this opinion. For example, Lloyd463reported that the sorption of hydrochloric

314


acid is weaker than the sorption of salts and hydroxides. He observed that the sorption of hydrochloric acid follows the sorption rule for acid concentrations below 0.4 M. Adsorption affinities vary among starches of different origins, as presented in Fig. 37. Hydrogen chloride gas is readily adsorbed by starch at concentrations of up to slightly more than one gas molecule per glucose unit. The resulting complex is black and water-soluble, indicating that hydrolysis accompanies adsorption.454Gaseous hydrogen chloride does not exhibit a strong ability to penetrate the inside of starch granules unless moisture is absent. Granules containing 3-4% moisture are readily penetrated by hydrogen chloride and undergo dextrinization."@The sorption of hydrogen chloride by starch sol causes its peptization, perhaps because of the discharge of the b. Complexes with Metal Hydroxides.-Interactions of alkali-metal hydroxides with starch have been studied in order to recognize unique aspects of the base-catalyzed degradation of starch. For example, R a k o ~ s k i ~ ~ ' . ~ ~ ~ recognized that the sorption of those hydroxides plays a key role in the degradation, which proceeds as a two-step process. The first step entails surface sorption and is followed by the hydroxide penetrating into the interior of grains as a second step. Since adsorption equations did not fit the experimental data very precisely, it is possible that something more

0.02

0.06

0.10

0.14

0.18

HCI absorbed,

FIG.37.-Adsorption of hydrochloride by starch of various origins (M, maize; R, rice; A, arrowroot; P, potato; C, cassava). [Reprinted with permission from Lloydjh3Copyright (191 1 ) American Chemical Society.]


315

than adsorption occurs in such cases. The equation which fits the experimental data best is c2 =

P + CUP,

(9)

where c2 is the concentration of a compound sorbed per gram of starch (in mequiv), c1 is the concentration of adsorbed compound at the point of equilibrium, and p and p are empirical constants. The P and p constants are respectively dependent and independent of several hydroxides: NH40H, LiOH, NaOH, KOH, Sr(OH)2, and Ba(OH)2. Among these hydroxides, ammonium hydroxide is only slightly sorbed. Later, R a k o w ~ k ide~~~,~~~ duced that the combination of alkali hydroxides with starch leads to the formation of “alkali starchates” according to a reversible reaction: St-H

+ Metal-OH

P St-Metal

+ H20.

Reychler independently presented a similar sugge~tion.~’ The following complexes with sodium hydroxide and starch have also been characterized in solution: (2C6Hl0OS* NaOH), (C6Hl0OS NaOH), and (ChHl0O5 2NaOH),,470.471 Lipatov reported studies of alkali diffusion in He discovered that the adsorption velocity from dilute solutions is higher than that from concentrated solutions and obeyed the empirical relation

-

k

=

(l/t) In [m/ (rn - y Q ) ] ,

-

(10)

where t is the time, m is the mass of adsorbate, y is a constant, and k is proportional to the diffusion constant and to the surface area of starch. In addition, k is inversely proportional to the thickness of the diffusion layer. As mentioned before, the mode of adsorption of calcium, stontium, and barium hydroxides differs from that of alkali-metal hydroxides. This difference is attributed to a lower adsorption velocity of metal hydroxides of the second nontransition group. Moreover, the quantitative results of adsorption, are different, as ~ h o ~in Table n ~ XIX. ~ ~ . ~ ~ ~ As discussed next, barium salts form insoluble complexes with starch. Similar behavior is exhibited by the complex of Ca(OH)2 with starch. This property is utilized for the precipitation of starch from aqueous solution, a process that is rather s10w.47s.476In his subsequent paper, Rako~ski~ studied ~ ’ the effect of salts on the adsorption of metal hydroxides; it was observed that increases in salt increased the adsorption of hydroxides. This effect is absent in the case of the adsorption of NH40H. More extensive demonstrated the following order of adsorption capacities: Ca(OH)2 = Ba(OH)2 > KOH > NaOH > LiOH > Me3(PhCH2) ~ ~ * ~ ~and ~ Leach et aL478accordingly NOH > Me4NOH. R a k o ~ s k i , 4Lloyd,”63

316


TABLE XIX Adsorption of Alkali-Metal Hydroxides by Different Varieties of Starch&’ Starch Arrowroot: Bermuda St. Vincent Potato

Wheat Rice

Alkali

Wt% Absorbed

NaOH NaOH NH40H LiOH NaOH WOW2 Sr(OH)2 Ba(W2 NaOH NaOH

43.3-36.1 44-35 4.2-1 41.1-35.5 45.8-38.6 19-76 17.2-75.8 78-73 47.6-36.9 42.8-43.9

stated that there are negligible differences in the hydroxide-adsorption behavior of different starch varieties. It was o b ~ e r v e d that ~ ~ ’starch ~ ~ ~retards ~ precipitation of AI(OH)3 formed from aluminates. This effect is caused by chemisorption and formation of a colloidal suspension, and is useful in organic synthesis, as in MeenveinPondorff-Verley redox reactions with aluminum alcoholates. Adsorption of cupric hydroxide from ammonium hydroxide by starch causes the sorption of CU(NH&(OH)~.The sorption of this copper species is more effective than the sorption of Ba(OH)2; however, the process is complex and cannot be expressed by a simple formula.48’On the other hand, fractionation of potato starch by means of A1(OH)3 has been reported.4E2The complex of CU(OH)~, ammonium hydroxide, and starch has been patented as a germicide and algicide composition.483

3. Complexes With Salts a. Introduction.-Native starch always contains a variety of metal cations which are held by phosphoric acid residues of the amylophosphoric portions of starch. These cations thus exist as cations of salts and not as a part of any inclusion complex. Several authors observed that the mechanical properties of starch gels (namely, their strength and elasticity) greatly depend on water hardness (see, for instance, refs. 484 and 485). Such changes can be attributed to the formation of complexes with salts. Intervention of the ion exchange in the phosphoric acid moieties is quite sufficient to cause this observed e f f e ~ t . ~ ~ ~ , ~ ~ ’ The formation of starch complexes with metal salts of metal oxides is of practical interest in, for instance, textile finishing (see discussion by Hal-


317

ler4"). As shown by Dykyj,"89metal cations influence the sorption of dyes on potato starch according to the acidity or basicity of a particular dye. In the case of basic dyes, cations sorbed on starch decrease the sorptive capacity in the order Ba2+> Sr2+>> Ca2+>> Cs+> Rb+ > NH4+> K+> Na+ > H+. Aluminum salts after hydrolysis produce a basic starch which is therefore capable of sorbing acidic dyes. Several authors observed the effects of salts (cations and anions) on the swelling and gelation of starch. It is accepted that peptization of starch by salts and metal hydroxides results from a disturbance of the hydration of The precipitation of starch is caused by the formation of sorption, capillary, and inclusion c o m p l e x e ~ . ~The ~ . ~formation ~ - ~ ~ ~ of complexes between starch and salts is supported by several experimental facts. For instance, the addition of CaC12to starch changes its optical rotation, which is further changed by the addition of stannic chloride. Such behavior is not uniform with respect to all salts tested and indicates the selectivity of starch in complexation with The addition of 0.3% KBr, 0.05% MnS04,or 0.05% C0S04 causes changes in the UV absorption spectra of starch. It should be emphasized that these changes become more pronounced after storage,"9xwhich in turn suggests that this process is not simple surface sorption. In her papers, Jane499*5"indicated that these effects are due to breaking of the water structure, that is, due to changing of the conditions of hydration and electrostatic interactions of ions with starch (complexation). Figure 38 illustrates the effects of selected cations on starch swelling and gelation. Rey~hler~'"-~''~ distributed selected salts into three groups according to their effect on starch at room temperature. It was observed that NaCl, CaC12,and MgS04 do not cause any swelling, whereas all of the

t

54'0 52.0

,

, \sr,

1

2

, 3

4

\

5

Ion Conc., N

FIG.38.-Effect of cations on the swelling and gelatinization of starch. (From S e i d e ~ n a n n . ~ ~ )

318


following cause typical swelling: KI, ZnC12,Na[HgC13], NH4N03,AgN03, Pb(C103)2, NaC104, KSCN, and NH4SCN. In contrast, NH4C1, SnCI2, HgCI2,Pb(N03)2,Ba(SCN)*,and sodium benzoate cause atypical swelling. The swelling rate is only slightly dependent on the chemistry of the salts.504 It was o b ~ e r v e by d ~'H~ NMR ~ ~ ~that ~ swelling of starch grains causes ordering of water molecules. In the case of wheat starch, the buildup of such a well-organized matrix is reached. It increases the electrolytic conduction of ions, perhaps because the ions are hydrated to a lesser extent.507 On the other hand, cations affect 'H NMR spectra mainly by the magnitude of splitting, and the effect of ions of the I1 nontransition groups is more pronounced than that of the ions of the I nontransition Figure 39 illustrates the effects of some anions on those processes. For example, anions that are poorly hydrated are adsorbed more strongly on the surfaces of starch micelles. The salts that are adsorbed in a polar manner bring water with them, since they are strongly hydrophilic and thereby 80 76 72 0

66 Frn 6L

60 56 52

40 44

40

36 32

20

24 20

FIG. 39.--Influence Seidemann."m)

06 0.8 1.2 16 2 0 24 28 3.2 36 4.0 4 4 48 Salt ,mole of some salts on swelling and gelatinization of starch. (From


319

increase the hydrophilic properties of the micelle. In turn, this influences swelling."'9 Starch readily adsorbs anions containing a polar sulfur atom, such as a thiocarbonyl group (also organic compounds bearing such groups). Thus, the adsorption becomes polar, and swelling takes place.509According to Jane,s"" the NCS- anion complexes with starch in a manner similar to that of the 13- anion. Perhaps because of complexation, starch inhibits the hemolytic action of cyanide anions on erythrocytes510and prevents the formation of humus from them.'l' The behavior of complexes of sodium salicylate is another trivial example of the role of anions in starch complexes. The phenolie group is acidic and can cause some hydrolysis of ~ t a r c h . ~ ' Usually, ~ * ' ~ ~ C1- and S042- retard swelling, but this effect also depends on the accompanying cation."' The type of anion also controls the possible mode of complex formation. The interior of amylose and some branches of amylopectin can assume a helical structure, the interior of which is hydrophobic. Thus, under favorable structural circumstances, inclusion complexes can be formed that hold salt anions inside the helix, so that the accompanying cation is excluded and becomes activated through lack of direct contact with the charge field of the anions. Such phenomena are observed in the case of salt-inclusion complexes of cycl~dextrins.~'~ However, studies on the effect of lithium chloride, bromide, and rhodanide, and also MgS04 on the specific rotation of starch suggest that there is an interaction of ions with C-2 and C'-3 OH groups of glucose units."6 Such complexes can readily be formed with long-chain carboxylates salts, sulfates, and so 0n.s17-520 Studies by Fischer and S ~ h w a i b o l d ~as~well l as by Seidemann491show that there are significant differences between starch varieties in response to their being treated with salts. A general method for synthesizing starch-metal complexes with Ba, Ca, wherein Sr, Be, Mg, Zn, Al, Fe, and Cu is given by a 1926 British alkali starches were treated with one or more metal salts. The cupric complex was reported to have disinfecting properties. Sorptive properties of starch in thin-layer chromatography with respect to such metal ions as Fe, Hg, Sn, As, Sb, and Cr have been presented in several paper^.^^^-"^ In the twentieth century, several papers were published on the degradation of starch by salts, especially those which cause hydrolysis (see, for instance, papers by B i e d e ~ - m a n n ~and ~ ' *Haehn529). ~~~ This observation was frequently criticized as an artifact and was finally abandoned.530Nevertheless, the changes of starch-sodium salicylate complexes previously menshow ' tioned,S12.513 as well as the known hydrolysis of starch by a l ~ r n , 5 ~ that the question cannot be generalized. Moreover, in 1964 a paper was

320


publisheds3’ on the hydrolysis of starch gels by chlorides (Table XX). The authors assumed that the salts undergo hydrolysis in aqueous solution, generating hydronium ions. The hydrolytic activity correlates with the radius of the salt cation, and the rate of hydrolysis varies significantly with the type of salt added. The water-binding capacity of starch is affected by salts in the following order for cations: K > Mg > A1 and K > Na > Li. Similarly, the waterbinding capacity is affected by anions in the following order: CNS > I > Br > NO3> Cl > SO4.The effect of cations is stronger than that of anions.s33 The addition of various salts to potato starch on its compression up to 1.2 X lo9 Pa had only a small effect on the thermal properties of starch, as measured by thermogravimetry/differential thermogravimetry/differential thermal analysis (TGIDTGIDTA). Only FeC13, CoC12, and I2 caused significant effects.62 The viscosity of starch gels is also affected by salts. For example, potassium salts decrease their viscosity, but manganese (MnS04) and cobaltous (CoCI2)salts increase the viscosity at concentrations as low as 0.1 M.s34As described by Merckel, the viscosity of gels (from starch sols) blended with salts increases as a function of the third power of the lyotropic number at a given concentration. The lyotropic series CNO > I > Br 2 1 0 3 > F > Br03> C1, which is also obeyed in the case of the sorption of sodium salts in liquid ammonia,4s8displays a concave relationship with respect to the viscosity, as shown in Fig. 40. Similar relations have also been found for potassium salts.s3sIn some cases, a linear relationship is observed between the gel viscosity and lyotropic numbers assigned to anions and ~ a t i o n s . ~ ~ ~ - ~ ~ The gelation characteristics of starch blended with salts also depend on the variety of starch; in addition, the time- and temperature-dependent rheological behavior (amylogram) varies in a more complex manner with the lyotropic numbers.s39 More extensive studiess4” have revealed that SO4’-, PO:-, S 2 0 3 ’ - , and NaF increase the temperature of gelation; NaCl.

TABLE XX Hvdrolvsis of Starch in the Presence of Some Metal Chloridess3*

Metal Ion A13+ Fe3+

cu2+ Ni2+ Mn2‘ Cd”

Goldschmidt Radius, A

pH

Period for Total Hydrolysis, Days

0.51 0.67 0.72 0.78 0.91 1.03

0.9 0.4 1.o 4.2 0.9 3.7

2 8 to 90 140

t


321

1.160 1

4

4

5

6

7

8

9

5

6

7

8

9 10 11

10 11 12 13 14 N

12 13 14

N FIG.40.-Relationships between the lyotropic number, N,and the viscosity of starch gels. (From MerckeLs")

NaBr, and NaI decrease that temperature; and CO2- and HC03- have no effect. Studies of the effects of electrolytes on the strength of starch gels were performed by S h i m i ~ u . It ~ ~should ' be mentioned that homogenous gels could not always be achieved. Neither wheat nor corn starch forms542 gels with ZnCI2,NH4CI, and (NH4)2S04. The effects of the electrolyte concentration on gel viscosity are typically insignificant. For example, it was reported543that a 100-fold increase of the electrolyte concentration changes the viscosity by only 23%. Such a result suggests that, again, only interactions of ions with a phosphoric acid moiety are involved. However, the effect of salts on the specific rotation of starch gels points to interactions between ions, and the amylose helix and amylopectin.516 Thermal effects of electrolyte adsorption on potato starch have also been observed.476For the alkali-metal salts, adsorption and accompanying swelling are reversible. Coacervation and flocculation of starch gels provide other evidence of starch-metal ion interactions. Coacervation can be achieved, for example, by blending starch gels with various salts, including NaC1, KC1, Na2C03, NaHC03, Na2S04, and CaC12,s""~s4s Flocculation of starch sols has been reported546with MgS04, sodium halides, acetates, sulfates, and isocyan a t e ~ . The * ~ ~state of hydration of starch seems to be the most critical factor. It was also observed that the rate of coagulation by three salts increases in the order KCl < CdC12< AlCl,; activation energies for these salts are, respectively, 1.9 X lo3, 25.1 X lo3, and 107.75 X lo3 kcal/m01~~~ and are related to the electrolyte concentration.

322


Rendelman published549a detailed review on starch-metal-ion complexes in 1978. b. Complexes with Nontransition Metal and Ammonium Salts.Relatively little attention has been paid to the combination of starch with ammonium salts. Starch has been reported to accelerate the decomposition of ammonium nitrate.ss0 S ~ h o c h showed ~~l that ammonium nitrate decreases the gelation temperature of starch to as low as room temperature. Ammonium sulfate increases the gelation temperature; with a 30% admixture of ammonium sulfate, starch does not gelatinize on boiling. The effect of ammonium carbonate on the viscosity of a starch gel is negligible.552 Ammonium chloride and sulfate do not form homogenous gels with some varieties of However, complexes of starch with ammonium chloride have been patented as hardening agents for amine resinsss3 and also to prevent the formation of brown spots during the ironing of clothes.”* Sorption of NaCl onto starch was recognized qualitatively by Dem o ~ s s yand ~ ~quantitatively ~ by Lloyd.463Maize starch exhibited better sorption than arrowroot starch, but the author noted that the adsorption rate is not a function of the surface area available for sorption. Because the process is fast ( 5 min) at room temperature, there is evidence of normal surface adsorption. The adsorption of NaCl has a so-called negative character in contrast to many other salts which exhibit positive adsorption?s5 The adsorption of NaCl increases the sorptivity of various organic dyes on This property is utilized in the textile printing of cellulose fibers that have been impregnated with starch. Because of negative sorption, acidic dyes are readily sorbed onto The use of calcium chloride and ammonium rhodanide also demonstrated good sorption behavior.5s8 It was reported that the electrolysis of 2-5% aqueous solution of starch containing sodium chloride or sodium sulfate yields a composition which is used for household cleanser applications.559Heating of an aqueous starch suspension in the presence of sodium sulfate at pH 6 increases the gelation temperature and vis~osity.”~~~’ Ironing starch can be prepared from starch and NaHF2.s62Gels of high viscosity are available from starch with sodium dihydrogenphosphate and disodium hydrogen-pho~phate.~~~ The presence of sodium phosphate in starch stiffens gels and stabilizes the v i s c o ~ i t y . ~ ~ Also, magnesium and calcium ions readily adsorb onto starch. In the case of magnesium, equilibrium is reached more slowly than in the case of calcium (see Fig. 41)s6sand the equilibrium saturation concentration is lower. Starch adsorbs approximately 86 mg of calcium per gram of starch independently of pH; equilibrium is reached within 20 min. The binding capacity of starch is not decreased by g e l a t i n i z a t i ~ n . ’ ~In~ .an ~ ~alkaline medium, CaC12forms a defined compound with starch that is free of chloride


323

OD060

- 0.0050 ‘0,

I *0.0040 0.0030

O.OOZSL, 0.0021

.

,

124 8

,

,

24

48 Tirne,h

FIG.41.-Adsorption of calcium and magnesium ions by starch as a function of time. The solid line represents adsorption of magnesium ions and the pointed line represents adsorption of calcium ions. P, potato starch; W, wheat starch; M, maize starch. (From Hollo et

anions.567A difference has been noted565in the salt-binding capacity of different starch varieties. For example, potato starch adsorbs almost twice as much as wheat and maize starch. The authors demonstrated that defatted maize starch exhibits the same binding capacity as potato starch. Another group of authors566demonstrated that the binding ability of cassava starch is higher than that of maize and waxy maize starch. Separation of amylose and amylopectin was attempted, based on the selective sorption of various cations.56xRussian workerP9 separated arnylose from amylopectin on calcium phosphate columns at pH 5.7. Amylose could be eluted with phosphate buffer, whereas amylopectin could not. The complexation of starch by calcium salts is readily observed polarimetrica11y570.571 and by conductivity meas~rements.~’~ The interaction of starch with CaC12 lowers the conductivity by a larger amount than with NaCI. Complexation of soluble starch with CaC12 progressively changes the specific rotation of starch from the initial [aylDZ0 = +196.68 to 196.80, 199.01, 200.80, and 201.60 at CaClz concentrations of 3.75,7.50,15.00, and 30.00%, respectively. The [aID2” value decreases in aqueous CaCI2 solutions, as shown in Table XXI. All of these results contradict former statements concerning the amount of calcium which is bound to starch5&if it is assumed that only one, uniform type of interaction exists between starch and calcium ions. Variations in the specific rotation coefficient of starch were observed at calcium concentrations in excess of the equilibrium calcium concentration. This effect is attributed to calcium ions clathrated in the gel net. The adsorption of starch on kaolinite does not exceed 4%, whereas on bentonite nearly 23% of maize starch is adsorbed. It has been proved by infrared spectroscopy that hydrogen bonds are involved without any ion e~change.~” The formation of a calcium complex also increases the digestibility of

324


TABLE XXI Temperature Effects on the Specific Rotation of Aqueous Starch Solutions571 Specific Rotation, [a]D, Degrees Temperature, 'C

30% CaCI2 Added

Hz0

Difference

80 70 65 60 55 50 45 40 35 30 25 20 15 10

194.53 196.00 196.51 196.54 197.06 197.54 197.55 197.59 198.06 199.65 201.oo 201.05 201.24 201.50

182.01 183.57 184.05 185.01 186.52 188.03 189.50 190.00 192.01 194.91 196.93 196.68 196.85 197.34

12.52 12.43 12.46 11.53 10.54 9.51 8.05 7.59 6.05 4.74 4.07 4.37 4.39 4.16

~tarch.~" Calcium chloride forms complexes with starch after it is pretreated with sodium silicate and alum; this product is used to produce pigment for rubber, paint, paper, and ~eramics.5~~ Complexes of starch with CaC12have been patented as drilling and described as dispersing media.576A complex of starch with calcium formate and sodium nitrite preserves silage and makes it difficult to ferment.577Blending glycerol and an aqueous solution of CaC12with potato starch gives a product used for printing rolls.57' The formation of a barium-starch complex is well k n ~ w n ~ ~ Stern"' ~-~"; determined its composition as (C6Hlo05)8Ba0.Complexation of starch with barium is also used to precipitate starch from its solution. The complex of starch with borax has interesting physical properties. The interaction between borax and starch is purely physi~al.~'~ The formation of this complex is described by a first-order equation with an activation energy of -16 kcal/moL5@' As shown by Leach et ul.,478starch sorbs the sodium metaborate (Na2B02)component of borax and rejects the H3B03component. It was reported that a colloidal solution of such a complex exhibits good ~ p i n n a b i l i t yThe . ~ ~ ~addition of this colloidal solution to paper was also reported to increase the wet strength, the degree of sizing, and its fire resistance.586The use of such a complex was also described as a container material for pesticide encapsulation587and for the sedimentation of alumi*~" preparation of a typical comnum ore residue^.^" M ~ r a k a m i ~ ' ~reported


325

plex: 3 mmol borax per gram of starch were heated at 90°C for 1 h. Calcium borate can be used instead5” in order to decrease the water solubility. A titanium oxide additive is also reported to improve the functional properties of the comp~sition.~’~ Several other modifications of starch-borax complexes are reported. For example, a starch-CaC12 (>35%) complex precipitated with borax gives a product that is dispersible in water and exhibits good adhesive proper tie^.^'^ A complex of starch with borax acidified with H2S04is also soluble in cold water.594Good adhesive properties may also be achieved by complexation of starch with borax in sodium hydroxide s o l ~ t i o n .A~ combination ~ ~ * ~ ~ ~ of starch with sodium tripolyphosphate and borax has been used as a treatment for textiles which is stable on ironing.597 A similar composition, without polyphosphate, is also patented598for use as a liquid laundry starch that is stable in freezing environments. Complexes of boric acid in borated starch have been d i s c ~ s s e d . 5 ~ ~ Starch is sorbed on alumina, more strongly on neutral alumina than on acidic alumina. The amylose and amylopectin components are sorbed with different degrees of effectiveness. Amylose can be eluted by an acidic buffer and amylopectin by a neutral buffer.600It is possible to fractionate starch components using a gel-separation column that is filleld with acidic and basic A1203 in the upper and lower parts of the column, respectively. Amylose adsorbs on basic alumina and amylopectin on acidic The complexation of sodium aluminate by starch accelerates decomposition of the ~ a l t , ’ ’ ~perhaps * ~ ~ ~ ~by hydrolysis of the salt and uptake of sodium hydroxide by starch. Starch complexed with sodium aluminate is used to agglomerate starch sludge when the complex has a low concentration of starch; it acts as a colloid when the complex contains a high concentration of starch.604Complexes simultaneously containing sodium aluminate (or A1203),borax, and trisodium phosphate are used in paper manufacturing to provide a high,filler percentage and mechanical strength.605Other alkalimetal aluminates complexed with starch have also been patented for this purpose.606Starch-aluminate complex solutions have a moderate effect on modulus stability (shape retention) in contact with solid phasesm7 An aluminum nitrate-starch complex has been patented for coating paper.608 A complex of starch (100 parts), aluminum formate (16.3 parts of 22% aqueous solution), and paraffin (16.3 parts) in water (62.2 parts) has been reported as a fabric-treating composition.609Complexes of cooking starch with alum are also used as thickeners for printing dyes, finishing compositions, and sizes.610An intramolecular complex of starch with AlCb was prepared by treating starch with AlC13 to yield a product containing AlC12 and AlCl (cross-linking) moieties. This complex is water-soluble but stable to hydrolysis (although not to hydration) and thermal treatment.611

326


A complex of starch with talc has been used for coating rubber granules."' Complexes of starch with SnC12 and Na2Sn03 are used as coatings, adhesives, and additives for drilling muds. They have exceptionally high viscosity and are stable on aging.613y6'4Starch complexes with lead are unknown, but complexation is utilized in dehydration and flocculation of lead slimes,h's protecting lead and lead alloys from corrosion in soft and also in chemical and environmental analysi~.~''A complex with As( 111) chloride (dichloroarsenous starchate) has been prepared.611Phosphorus( V) halides are reported618to form a 1:l complex with amylose having a stability constant of 1.5 X 10' at 25°C. c. Complexes with Salts of Transition Metals.-Starch forms two types of complex with CuS04: Positive and negative complexes are formed with diluted and concentrated solutions of CuS04, respectively.'" Sorption of silver(I) halides"' on starch (and on other hydrophilic polymers) influences the grain size of those halide^.^'' Complexation of starch with ZnC12results in insolubilization6" and resistance to microbial molding.564Complexation of starch with zinc fluorosilicate is patented for the ironing of clothes.562 Some varieties of starch form nonhomogenous gels with ZnC12.'40 Iodine zinc chloride causes layer swelling of starch granules, a phenomenon which may have some applications in the analysis of starch variety and chemistry.621Complexation with HgI preserves starch solutions.622An ammonium iodomercurate complex with starch is used to determine nitrogen concentrations in ~ t e e 1 . hComplexes ~~ which form between starch and organic titanium( IV) compounds of the general formula (Ti(OH),(OOCR)4.x (where R represents 1 to 4 carbon atom alkyl groups) have been reported and are perhaps responsible for stiffening of starch solutions.624Starch hydrated with titanium( IV) oxide or titanyl sulfate (5-10%) has been used in production of cardboard and p l y ~ o o dTitanium( . ~ ~ ~ IV) chloride reacts readily with starch, causing the evolution of hydrogen chloride. The resulting residue is readily hydrolyzed by water, causing precipitation of Ti02. The starch is also dextrinized, and its aqueous solutions have a low Coatings and adhesives result from the complexation of starch with titanates, zirconium( IV) chloride, or zirconium oxynitrate [ZrO(N03)2].6'4 Starch-iron complexes are reported as biologically active sources of iron.627,628 In one report, it is warned that such complexes may produce sarcomas as a result of long-term exposure.629investigation^^^' have shown that, independently of the ferric salt used, the maximum capacity of the salt in starch is 2-7 mg%. Because pH 5.8 is more conducive to stability than low pH values (3 or lower), higher concentrations of ferric salt are not advisable. Shi D e ~ h e n g recommended ~~l blending starch (100 g) with trisodium citrate (25g) and 2 M FeC13 (500 mL) in water (2500 mL) at


321

pH 7.5-8.5, adjusted with 20% aq. NaOH. In this case, the resulting iron was present in the form of FeO(0H). L e s z c z y n ~ ksaturated i~~~ potato starch with either FeC13 or ferric citrate, both at concentrations of 3.33 X lo-’, and 3.33 X 3.33 X M. The product is degradable by alpha amylase. These procedures are safe because FeC13 does not hydrolyze starch unless hydrogen peroxide is added; in this case the rate of hydrolysis increases with time.633Another procedure uses Fe(OH)3 (100 mg) in a colloidal state. It is stabilized by starch (10-50 parts) and made neutral by dilute ammonium hydroxide. It was not possible to precipitate a stable complex using CaCl’ or ethanolY4 it was noted that the presence of Ca’+ ions in the reaction mixture strongly inhibited the adsorption of ferric oxides from solution.635 S ~ n o w i e c kused i ~ ~ ~colloidal ferric oxide (136 mL), starch (6 g in 66 mL of water), and sodium hydroxide (0.7 g) in this procedure. After maintaining this reaction mixture for 1 h at 60°C, the complex was precipitated with alcohol. Studies by K r a ~ s e ~revealed ~ ’ , ~ ~that ~ only “meta ferric hydroxide” coagulates with starch, and amylose was not active in either case. It has also been reported that iron is released from starch-ferric complexes upon contact with ascorbic On working with mixtures of starch and ferric salts, redox reactions between components should be taken into account.531 The addition of potassium ferrocyanide to a starch solution increases the viscosity and the strength of the gel.@’ The Co(I1) ion is adsorbed on amylopectin in a manner similar to the Fe(II1) It is well known that starch-iron complexes are suitable for fortifying bread and flour with iron. The state of iron in flour, dough, and bread was investigated by Leichter and J ~ s l y n . @Iron ~ salts influence the whiteness of sweet-potato starch, but this effect is variable.614It was also reportedH5 that colloidal iron interacts with starch, a process which is used to fractionate starch into three portions: The first portion (80% of the total amount) is formed by colloidal iron itself, the second (9% of the total) is formed by iron and electrolytes, and the third portion (11%of the total) is not precipitated at all. Complexes of starch with salts of metals of the group VIII transition elements (particularly palladium complexes) are highly efficient regio- and stereoselective catalysts for the hydrogenation of plant 0ils.646*H7 Cerium( IV) salts are used to initiate graft polymerization of starch. In order to graft-copolymerize starch, a complex of Ce( IV) was prepared by blending cornstarch with Ce(NH4)z(N03)6-HN03reagent in the ratio of 75 glucose residues per Ce( IV) c ~ m p o n e n t . ~ ~ ~ @ ’ ACKNOWLEDGMENT The authors wish to thank the Office of Basic Energy Sciences at the U S . Department of Energy for supporting this research.

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(345) (346) (347) (348) (349) (350) (351) (352) (353) (354) (355) (356) (357) (358) (359) (360)

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ADVANCES IN CARBOHYDRATE CHEMISTRY AND BIOCHEMISTRY VOL . 53

COMPLEXES OF STARCH WITH ORGANIC GUESTS* BY PIOTRTOMASIK AND CHRISTOPHER H. SCHILLING Department of Chemistry and Physics. The Hugon Kollataj Academy of Agriculture. 30 059 Cracow. Poland; and Ames Laboratory** and Department of Materials Science and Engineering. Iowa State University. Ames. Iowa 50011

V. Introduction ...................................................... VI . Preparation Methods ............................................... VII . Complexes with Aromas and Flavoring Agents 1. Aliphatic and Aromatic Hydrocarbons .............................. 2. Haloalkanes and Halobenzenes .................................... 3. Nitrogen-Based Compounds 4 . Simple Sulfur-Containing Compounds ............................... 5 . Alcohols 6. Phenols ...................................................... 7. Ethers ....................................................... 8. Aldehydes .................................................... 9. Ketones ...................................................... 10. Carboxylic Acids ............................................... 11. Carboxylic Esters ............................................... 12. Nitriles ....................................................... 13. Amines 14. Carboxyamides ................................................ 15. Vitamins ..................................................... 16. Sulfates and Sulfonates (Surfactants) VIII . Complexes with Dyes .............................................. IX . Complexes with Lipids .............................................. 1. Introduction 2. Native Starch-Lipid Complexes ................................... 3. Preparation Methods ............................................ 4 . Structures and Properties of Complexes 5. Functional Properties of Starch-Lipid Complexes .....................

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346 350 352 352 357 357 359 360 364 365 365 366 367 372 373 373 374 374 375 376 385 385 386 387 392 396

* This is a companion

article to the immediately preceding Chapter “Complexes of Starch with Inorganic Guests. and the numbering of references. figures. tables. and the Table of Contents is consecutive from the prior article . ** Ames Laboratory is operated by Iowa State University under the contract number W7405-eng-82 with the U . S . Department of Energy . ”

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Copyright 0 19Y8 by Academic Press. All rights of reproduction in any form reserved .

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6. Digestibility of Starch-Lipid Complexes. ............................ 7. Analysis of Lipids in the Presence of Carbohydrates ................... X. Starch Complexes with Mono- and Oligosaccharides ...................... XI. Starch Complexes with Macromolecules 1. Starch-Protein Complexes. ....................................... 2. Complexes with Dextrins, Polysaccharide Gums, Alginates, Pectins, and Starch Derivatives .............................................. 3. Complexes with Cellulose and Modified Cellulose ..................... 4. Complexes with Synthetic Polymers ................................ References ......................................................

................................

399 400 400 405 405

41 1 412 413 414

V. INTRODUCTION Starch in plants is accompanied by water, metal ions, lipids, proteins, sterols (such as saponins), and alkaloids (as in such "exotic" plants as Dia~coracea).'~~ Several of these components can be washed out by the isolation of starch, some of them are extractable with organic solvents, and some are volatized by steam treatment. With the exception of metal ions (preceding article, p. 263), the foregoing components form physical mixtures with starch and do not chemically bond with either amylose or amylopectin. Therefore, one may assume that amylose and amylopectin form inclusion complexes with organic components that are similar to those mentioned in the preceding article. Bear4' and R ~ n d l e ~ observed '.~~ that starch of the A and B patterns transforms into V-type starch in contact with 1-butanol (namely, to a form capable of producing a blue color with iodine in KI). In this case, the randomly coiled amylose component is transformed into a helical structure. The hydrophobic carbon chain of 1-butanol, acted upon by its van der Waals forces as a complexing agent, causes a specific spatial orientation of the glucose residues in amylose. This orientation generates a helix having a hydrophobic cavity that includes a molecule of 1-butanol. A similar effect, forming either a helix or an extended coil, can be achieved by several other organic compounds and by alkali species.'85 Studies by Brant efu1.650-652indicate that only a small amount of complexing agent is required to form the helical structure of arnylose. On the other hand, attempts to form complexes with amorphous amylose have not been su~cessful."~~ Therefore, it may be readily deduced that complexes of organic compounds involve inclusion inside the helix rather than surface sorption. At first glance it may be assumed that the presence of an organic compound having a relatively long carbon chain is necessary for the coiling of amylose to trap organic molecules inside the coil. This could, for example, explain the observation of KodaU9 that ethane, ethyne, and chloromethane sorbed on starch less readily than did inorganic gases (CO, COz,NO, H2S, and SO?).

COMPLEXES OF STARCH WITH ORGANIC GUESTS

341

The carbon chains of these compounds were too short and, as a consequence, the sum of the van der Waals forces was too low to cause coiling. Another condition for the formation of the complex is the geometry of the guest molecule. For example, the size of the helix interior must be such that the guest molecule fits inside the helix. Several experiments have shown that the diameter of the helix cavity is variable and can expand from 4.5 to 6.0 This diameter also depends on the number of turns of the Thus, the helix has a variable inclusion capacity and it can hold even seemingly bulky guest molecules. The concept of a long enough hydrophobic portion of an organic component as a necessary condition for the complexation is only partly correct. Kuge and take^^^^ (Fig. 42) observed that a group of alcohols, ketones, and alkyl halides (among them such bulky compounds as cyclohexanol and cyclohexanone) are much better sorbed on amylose (area 1) than such compounds as benzene, toluene, branched alkyl halides, carbon tetrachloride, tert-butanol, isomeric xylenes, and P-pinene (area 2). Also, as shown in Fig. 42, linear, branched, and cyclic alkanes, such as n-hexane, isopentene, and cyclopentane, are sorbed in very small amounts (area 3). The polar group terminating each alkyl chain is also an important factor. The borderline between those three groups in Fig. 42 proceeds neither along the nature of the polar group nor along the steric properties of the hydrophobic moiety. In the case of monosubstituted benzenes, the inclusion capacity increases with increasing dipole moment and boiling point of the potential guest. Esters of benzoic acids form complexes less readily as the chain length of

A.

Amylose in column,o/o

FIG.42.-Relationship between retention volume and amylose content in colum11.6~~ (See text for explanation of areas 1 , 2, and 3.)

348


the esterifying alcohol exceeds 10 carbon atoms."' The curved shape of the relationship shown in Fig. 42 suggests that there are two types of sorption on amylose: (1) inclusion inside the helix, and (2) surface sorption."' The formation of surface-sorption complexes can explain why several dyes, which are usually rather large molecules, form complexes with starch. The branches of amylopectin can also undergo coiling, which in turn can be involved in complexation. French et ~ 2 1 . ~ confirmed ~' that amylopectin exhibits a weak tendency for complexation of certain organic compounds (see Table XXII) where the complexation ability is expressed by means of the iodine-binding capacity and p12 (namely, the negative logarithm of the apparent iodine concentration at half of its maximum potential binding capacity). Obviously, the complexing ability of starch depends on the variety of starch and hence the amylose-to-amylopectin ratio. Osman-Ismail and S01ms~~' investigated the formation of inclusion complexes of 1-hexanol, 1-octanol, 1-decanol, decanal, 1-menthol, 1-menthone, limonene, and 0pinene with rice, corn, and potato starch, and also potato amylose. The ability to form inclusion complexes with these materials is as follows: potato starch > rice starch > corn starch > potato amylose, a trend which has no relation to the content of amylose in these starch varieties. It should be emphasized that this order varies according to the nature of the guest molecule. For example, the relative percentages of acetone inclusion are as follows: potato (4.15) > cassava (3.40) > wheat (3.30) > maize (2.65); for methanol the corresponding order is potato (1.90) > cassava (1.30) > wheat (1.20) > maize (1.15).659 Inclusion complexes are commonly characterized either by the temperature of their formation"' or by the binding parameters of different ligands. These parameters originate from the Scatchard equation?'

r = n k cf/(l + k q ) ,

(11)

TABLE XXII Properties of Complexed Amylose and Amylope~tin~~'

Iodine-Binding Capacity Guest Molecule

Amylose

Amylopectin

Beta-Amylase Limit (Amylose)

Benzene Carbon tetrachloride Chloroform Cyclohexane 1,2-Dichloroethane 1-Pentanol 1.1,22-Tetrachloroethane

18.0 16.7 14.0 14.7 17.9 16.1 16.5

0.3 0.0 0.0 0.0

68.8 69.5 68.8 68.0 (-0.2) 67.5 67.0

-

0.3 0.0


349

where r is the average number of moles of bound ligand per mole of glucose unit of starch, n is the maximum number of moles of bound ligand per mole of glucose unit of starch, k is the intrinsic binding constant (an association constant), and cf is the molar concentration of unbound ligands at equilibrium. As just mentioned, starch and its components can adsorb by two mechanisms: inclusion or surface sorption. A two-parameter equation which accounts for these two mechanisms appears to be more appropriate: r

=

n1 kl cf/(l + kl cf) + n2 k2 cf/(l + n2 k2).

(14

In this expression, the subscripts 1 and 2 refer to inclusion and surface sorption, respectively. Other techniques for characterizing complexes include desorption methods, elemental analysis, X-ray diffraction, spectroscopic methods, thermogravimetry, and others. Since native complexes of starch, especially those with lipids and proteins, are known, it may be assumed that the evacuation of guest molecules can provide space for other guest molecules such as aromas, waxes, and various biologically active compounds (pesticides, pharmaceuticals, and so on). Such evacuation of natively residing guest molecules usually results in collapse of the helix. It is also known that the coiling and decoiling of amylose and certain branches of amylopectin is normally reversible. It may also be assumed that replacement of one guest molecule by another without the decoiling-coiling step should also be taken into account. Several observations suggest that wet starch (that is at least randomly coiled) accepts some organic molecules (such as alcohols) more readily than does dry starch (that is, more amorphous). In order to form a complex with methanol, a dry starch has to be used, whereas complexes with ethanol, 1-propanol, and benzyl alcohol are formed more readily with a native starch (that is, with a starch-water complex).661*662 Water in starch not only provides coiling favorable for the entry of guest molecules, but swelling also provides guest molecules access to the interior of granules. In addition, successive sorption and desorption of guest molecules from starch can affect the structure of granules.663Inclusion complexes may also form that involve inclusion of a second guest molecule into a preexisting inclusion complex. For example, starch containing 0.5%of included CaC12 adsorbs malt alpha-amylase,6@and starch-ZnCI2 complexes adsorb triethanolamine.665This is not always the case because, for instance, some bi- and trivalent cations inhibit inclusion of albumin and globulin into starch.666 There are several practical aspects to the formation of inclusion complexes. One of the earliest applications is the flocculation of starch from aqueous solutions. Among other compounds, the lower alcohols (Cl-C4) and acetone are superior in this respect.667The stabilization of flavoring and aromatizing compounds constitute the most common practical application of inclusion complexation. In this manner, oxidizable food components may be pro-

350


t e ~ t e d .Some ~ ~ ,biologically ~~~ active compounds can be introduced into plant and animal organisms in the form of starch complexes.670Stabilization of nitroglycerol in alkali starch has been patented.671Water-dispersible comhave also positions that are inclusion complexes of gossypol and been patented. The selectivity of various organic compounds in the complexation with components of starch can be utilized in fractionation of starch. Kuge and take^^^^ proposed (-)-menthone for this purpose. Other applications of complexation and complexes will be discussed next.

VI. PREPARATION METHODS There are three primary methods of complexation: (1) sorption from the gas phase, (2) sorption from a liquid phase, and (3) blending. Sorption from the gas phase is possible when the guest molecules can evaporate and form a saturated vapor. In this method, starch is placed in a chamber filled with vapor and left until sorption equilibrium is reached. Depending on the character of the guest molecule, equilibrium is reached after a time period which ranges from a few hours to several days. Figure 43 shows that equilibrium with n-hexane and ethyl acetate is reached within 1-2 days, whereas equilibrium with 1-butylamine and ethanol was still not reached even after 60 and 75 days, respectively.674The shapes of the curves in Fig. 43 suggest the possibility of a side effect, such as swelling. The extent to which guest molecules are complexed is related to their polarity; however, the kinetics of the process do not follow this trend (see also Table XXIII). Sorption of guest molecules from the liquid phase may proceed either when the guest molecules form a liquid phase by themselves, or when they are dissolved in a suitable solvent (such as water, alcohol, or Me2SO). In the latter case, the kinetics of complexation can be monitored by spontaneous precipitation of the complex. Acceptable results are achieved when the

0

25

50

75

T i m e , days

FIG.43.--Kinetic curves of sorption of volatile aromas on dry starch. (Reprinted with permission from H. G. Maier and A. Bauer, Sraerke, 24 (1972) 101-107.)


351

TABLE XXIII Content of Flavoring Agents in Air-Dried Starch Films674

Maximum of Sorption Flavoring Agent

After 3 Days, mg/g

Amount, mg/g

Acetic acid Acetone Acrolein I-Butylamine 2-Butylamine tert-Butylamine Die thylamine Ethanol Ethyl acetate Hexanal trans-2-Hexanal 3-Pentanone Propanal Propanoic acid 2-Propanol Pyrazine Pvridine

31 34 10

41 43 25 31

21 21 20 0.2

92 92 19 10 17 8 11 50 31 55 106 146

2 1 5 68 71 24 41 62 74 26 7 44

101

12 4 0 0 7 17 27 0 76 123

reaction mixture is autoclaved. Usually a 1-10% suspension of starch is used for c o m p l e x a t i ~ n . ~ Comparative ~ ~ ~ ~ ~ ' studies revealed that shaking at room temperature produces very similar results, regardless of the starch concentration.65' A conventional procedure for preparing the starchbenzaldehyde complex using alcohols as benzaldehyde-carrying solvents is presented in ref. 661 (see Scheme 1).There is also evidence that moisture in starch significantly improves the extent of inclusion from solutions of higher alcohols, although complex formation can be very slow. For instance, it was reported that a 1%solution of starch binds sorbic acid immediately, whereas a 0.1% solution binds only 1.76% of sorbic acid immediately and a further 44.38%binds after 4 weeks.675 In some instances the carrier of the guest plays the role of emulsifier. For example, alcohols and lower fatty acids or their esters are used in the formation of fat-starch complexes.676In this case, conditions for preparation of complexes resemble conditions for extraction, and unexpected results can accompany both processes. For example, it has been shown that extraction of lipids with 1-propanol from their surface complex with oat starch produced a helical lipid-starch complex that was absent prior to extraction.677 Preparation of complexes by blending entails mechanical grinding of starch and guest molecules (or a source containing the guest molecule) in

352

PIOTR TOMASIK AND CHRISTOPHER H. SCHILLING Starch ( 5 g dry basis)

.1 Drying at 100°C in vacuum (omitted in modified procedures) 1 + The solution of guest compound in a carrier-solvent (10 mL) .1 Standing overnight in a stoppered flask

.1 Drying at 40°C

1 Continuous extraction with In-hexane

1 Drying at 100°C in vacuum

.1 Product SCHEME1.-Conventional Process for Preparation of Starch Complexesbh'

a mortar. The resulting mixture is stored in a stoppered flask and then washed several times with a solvent such as ether or Me2S0. In every instance, repeated grinding increases the amount of inclusion c ~ m p l e x . " ~ VII. COMPLEXES WITH AROMAS AND FLAVORING AGENTS Starch complexes with aromas and flavoring agents are usually synthetic in origin. In nature, starch sometimes includes some aroma- and flavorgenic components that generate flavor and aroma on processing (see, for instance, ref. 678). Such agents include mainly aldehydes, ketones, and carboxylic esters; however, hydrocarbons, alcohols, carboxylic acids, and haloalkanes have also been used.

1. Aliphatic and Aromatic Hydrocarbons Published literature on starch complexes with aliphatic and aromatic guest molecules is scarce, and only limited generalizations can be made. For example, it may be observed from Fig. 42 that alkanes and cycloalkanes exhibit two distinct types of sorption behavior: (1) n-Hexane, isopentane, isoheptane, cyclopentane, cyclohexane, and methylcyclohexane are sparingly sorbed (they fall along line 3) whereas (2) isooctane and @-pineneare sorbed in larger amounts (in area2 of Fig. 42). Figure43 suggests that the equilibrium sorption occurs rapidly. For example, within the first 60 min, half of the equilibrium amount of n-hexane (6 mg/g of starch) is sorbed from the gas phase. This sorption is evidently strong, since n-hexane cannot be d e ~ o r b e d "at ~ temperatures as high as 80°C. In contrast, both @-pineneand isooctane, being more bulky compounds, are more readily sorbed. The curvature shown in Fig. 42 suggests that there are at least two mechanisms of sorption: (1) sorption


353

within the helix, and (2) sorption either in the capillaries or on the surface. The observation by KodaM9on the weak sorption of ethene and ethyne on starch, together with the conclusionsdrawn from Fig. 42, suggest that the size of hydrocarbon is a critical factor controlling sorption kinetics. Only P-pinene and isooctane have a size that fits the cavity of the helix. X-Ray diffraction analysis of an insoluble potato ?tarch-P-pinene complex revealed the following interplanar spacings (in A): 12.80 (strong), 7.70 (medium), and 4.83 (strong) (see Table XXIV). The Scatchard parameters are as follows: kl = 1.30 x lo1,k2 = 1.81, nl = 0.027, and n2 = 0.089 (Table XXV).656These data suggest, especially in comparison with data for other compounds mentioned in Table XXV, that sorption is weak and proceeds in two distinct regimes. The concentration of limonene in starch is linear679 against k. Reported temperatures of the formation of various complexes are shown in Table XXVI. The yields of complex formation (on the basis of dry weight of starch) are 17.2%for starch-cyclohexane, 20.4% for starchlimonene, and 9.2% for starch-P-pinene complexes.673Complexes of starch with petroleum ether (5.4-5.7%) are also known.681*682 Less information exists on the sorption behavior of aromatic hydrocarbons such as benzene, toluene, the three isomeric xylenes, mesitylene, and TABLEXXIV X-Ray Diffraction Patterns of Potato Starch and Some of Its Complexes6” Guest Molecule None

(-)-Menthol

(-)-Menthone

Decanoic acid Linoleic acid

Spacing, A

Intensity

15.8 8.90 7.94 6.14 5.16 4.54 4.00 3.70 3.38 2.60 12.75 7.60 4.90 12.75 7.60 4.90 11.70 6.75 12.70 6.80 4.80 4.40

m vw vw m s

Guest Molecule 1-Hexanol

1-Octanol

W

m m

1-Decanol

W W

s s s

Decanal

P-Pinene

S S

S

m s m m m m

Oleic acid

Octadecanoic acid

Spacing, A

Intensity

11.65 6.60 4.80 11.70 6.75 4.42 11.70 6.70 4.41 11.70 6.80 4.42 12.80 7.10 4.83 11.50 6.90 4.38 12.90 7.90 4.37

s s s S

s s s s s

m s

m s

m s m m m s S

s

354


TABLE XXV Scatchard Parameters of Different Ligands with Potato Starch at pH 7.06%

1-Hexanol 1-Octanol 1-Decanal Decanal Decanoic acid Octadecanoic acid Oleic acid Linoleic acid (-)-Menthone (-)-Menthol P-Pinene

54.5" 219 125 125 330 357" 666 959 184 143" 13.0

0.10 0.05 0.04 0.01 0.07 0.069 0.019

0.004 0.012 0.007 0.027

21.5 12.9

-

43.5

-

57.7 46.4 8.97 -

1.81

0.11 0.1 1 -

0.19 0.089 0.020 0.045 -

0.089

" k , is the total constant because the Scatchard plot is linear in its whole range.

p-cymene. Data for these hydrocarbons resides in area 2 of Fig. 42, along with P-pinene and i s ~ o c t a n e . ~The ' ~ temperatures of complex formation are reported in Table XXVI. Yields of complexation are 23.4, 22.9, and 18.2%,respectively.673It is apparent that they are higher than the complexation yields of alkanes and alkenes. The equilibrium sorption of benzene on both amylose and amylopectin is reached within 14 days. After this time period, 3 mg of benzene is sorbed per gram of amylose, and 4 mg of benzene per gram of a m y l ~ p e c t i nThe . ~ ~characteristics ~ of gelation of potato starch change dramatically after 3 h of refluxing in benzene; however, these changes are much smaller when starch is heated for 3 h in toluene at 100 "C. The dramatic changes observed with benzene are interpreted as resulting from the collapse of the helix on azeotropic evacuation of water. Observations of starch granules by optical microscopy over the temperature range of 60-245 "C confirm major changes in the appearance of granules after refluxing in benzene. Some differences in appearance are also evident in the case of air- and oven-dried potato starch. Analysis of the dry mass content shows that 12-13% of benzene remained in starch after such heat treatment; however, thermogravimetric analysis did not reveal any evidence of benzene being desorbed before decomposition of the s t a r ~ h . ~Ben~',~~~ zene can displace acetone, diethyl ether, or methanol from their inclusion complexes. Such replacement of one ligand by another is incomplete. The origin of starch plays a role in this repla~ement.6~~@~*~'*


355

TABLEXXVI Starch Inclusion C o m p I e ~ e s ~ ~ - ~ ~ ~ ~ Guest Molecule Hydrocarbons Benzene Bicyclo[l,2,2]heptene Cycloheptadiene Cycloheptane Cycloheptene Cyclohexane Cyclohexene Cyclooctane Cyclopentane p-Cymene 23-Dimethylbutane Limonene Methylcyclohexane Methylcyclopentane P-Pinene o-Xylene Halohydrocarbons Bromocyclopentane o-Bromotoluene Carbon tetrachloride Chloroform 2,3-Dibromobutane 1.2-Dichloroethane 1.2-Dichloropropane Fluorobenzene Hexachloroethane 1,1,2,2-Tetrabromoethane 1,1,2,2-TetrachIoroethane 1 ,l,l-Trichloroethane 1,1,2-Trichloroethane AIcohoIs Borneo1 I-Butanol fert-Butanol” Citronellol 1-Decanol Ethanolh Geraniol 1-Hexanol 4-Hydroxy-4-methyl-2-pentanoneh Linalool (-)-Menthol Methanol”

Temperature of Formation, “C

Yield’

56 70 57 81 61 67-69 65 53 41 49 26 25 41 40 23 30-32

23.4

53 32 66 52 27 50 53 52 78 69 83 57 50

17.2

22.9 20.4

9.2 18.2

18.2 20.0

26.1

67 46

25.7 26.6

62 37

25.5

45 42

25.5

65 66

22.8 26.4 (continues)

356

PIOTR TOMASIK AND CHRISTOPHER H. SCHILLING TABLE XXVI (Continued) Guest Molecule

1-Octanol 1-Pentanol Perillyl alcohol Pinacol Phenols p-Cresol 2,4-Dinitrophenol 1-Naphthol 2-Naphthol Phenol Aldehydes Decanal Octanal Perillaldehyde Ketones Acetoneh Camphor Carvone Cyclohexanone Dirnedone 2,6-Dimethyl-4-hexanone (-)-Menthone 4-Methyl-2-butanone Pinacolone Carboxylic acids 2-Bromopropanoic acid Butanoic acid Decanoic acid Dodecanoic acid Hexanoic acid Linoleic acid Octadecanoic acid Octanoic acid Oleic acid Tetradecanoic acid Miscellaneous Diethyl malonate Nitrobenzene Pyridine" Quinoline

Temperature of Formation, OC

Yield"

40 45 62 55

24.6 28.8

72 60 85 55 61

32.2 22.6 31.5 16.1 24.2

36 36 70

25.5

60 52 85 80 23 55 86 54 53

21.8 8.9 15.0 22.9 1Y.7 13.0 24.5 20.5 31.0

44 50 46 40 50-55 64 35 50 21

18.4

21.8 21.2 21.1

40

23 61

2.7

85

24.1

If available. The efficiency is dependent on the concentration of the guest molecule.


357

2. Haloalkanes and Halobenzenes Sorption data for haloalkanes are spread widely among all regions in Fig. 42. There seems to be no simple and logical relation between these positions and the dipole moments, boiling points, and size and shape of the guest molecules. Similarly, the temperatures of complex f o r m a t i ~ n ~ ~ ~ @ ~ are not amenable to simple interpretation. The sorption of alkyl halides is possible by replacing diethyl ether:59,681.682,pyridine,659*681,682 or dimethyl sulfoxide6" from their complexes with starch or amylose. In case of carbon tetrachloride, its sorption capacity is between 3.0 and 4.9%.The conversion of the pyridine complex is more complete, perhaps because of the known swelling effect of pyridine on starch (see later discussion). The low sorption capacity for C C 4 has been confirmed by Gupta and Bathia.3s4 These authors also observed good reproducibility of C C 4 adsorptiondesorption isotherms in successive experiments. The sorption-desorption isotherm of C C 4 on wheat flour proceeded without any hysteresis.6%The gelation characteristics of potato starch are only slightly affected by conditioning in chloroform (Table XXVII), but the appearance of the granules changes substantially (Table XXVIII).6x9 3. Nitrogen-Based Compounds

Whistler and Hilbert687observed that nitroalkanes form complexes with starch, a fact utilized from fractionating starch into amylose and amylopectin. The authors indicated that amylose forms complexes, whereas amyloTABLE XXVII Characteristics of Gelation of Air-Dried Potato Starch after Conditioning in Various Organic Solvents684

Solvent None Acetone Acetonitrile Benzene Chloroform N,N-Dimethylformamide Ethanol Nitrobenzene I-Propanol Pyridine Toluene "

Temperature Viscosities, mP-see Temperature of of Max. Gelation, "C Viscosity, "C Maximum At 96°C" At 50°C 63.5 62.5 67 62 62.5 74 63 63 62.5 83.5 63

73.5 73 79 71.5 72 95 75 76 76 96 72.5

4347 2724 369 3594 3764 263 2693 60 3152 397 2753

Values in parentheses represent viscosity maxima after 1200 seconds.

63 (322) 54 (178) 85 (73) 6 (76) 236 (263) 203 (138) 39 (393) 66 (130) 48 (337) 397 (163) 94 (241)

4194 2111 1999 6 1040 1184 2210 1279 4621 938 1939


358

TABLE XXVIII Microscopic Observations of Air-Dried Potato Starch aPter Conditioning in Organic SoIvents6&l Solvent None

Benzene

Conditions (Temp. "C)

Observation with Normal Light

Observation with Polarized Light

60

yellow points, dashes, crosses

yellow points, dashes, blue spots no change no change yellow crosses, edges of grains illuminated pale-blue spots (235°C) yellow crosses. pale blue spots yelow points, dashes and crosses no change no change yellow points and spots blue points and spots (180°C) yellow points and dashes, blue spots yellow dashes, pale blue spots no change no change yellow dashes and crosses shining ceases (230°C)

60 to 245 245 60 60 to 245 245

Toluene

Nitrobenzene

Pyridine

Chloroform

Acetonitrile

60 to 245 245 60 60 to 245 245

yellow points, dashes, and crosses no change no change yellow points and spots no change no change

60

yellow dashes, blue spots

60 to 245 245 60 60 to 245

no change no change yellow dashes and crosses grains melt, shining ceases dashes, crosses no change yellow dashes, crosses blue spots points turn into dashes

60

245 60 60 to 245 245

Ethanol

1-Propanol

DMF

no change no change yellow, dashes, blue points spots no change no change

60 60 to 245 245 60 60 to 245 245

60 60 to 245

245

no change yellow dashes, crosses blue spots yellow dashes and points blue spots yellow points and dashes yellow points and dashes blue spots blue spots yellow points, blue spots yellow dashes and crosses pale spots and dashes (185°C) points turn (130°C) yellow points and dashes yellow points and dashes yellow dashes, blue spots yellow dashes, blue spots points turn (130°C) points turn (130°C) yellow points, dashes edges of grains illuminated blue spots, yellow dashes yellow spots yellow dashes, pale spots yellow dashes yellow dashes (190'C) yellow dashes, blue spots (190°C) no change no change


359

TABLE XXVIII (Continued) Solvent Acetone

Conditions (Temp. "C) 60 60 to 245

245 Nitromethane

60 60 to 245 245

Observation with Normal Light yellow dashes no change no change yellow dashes, blue spots no change no change

Observation with Polarized Light illuminated yellow dashes, blue spots shining ceases (225°C) yellow dashes yellow dashes, blue spots no change yellow spots and dashes pale blue spots

pectin does not. The X-ray diffraction pattern of amylose complexes is essentially the same, regardless of which nitroalkane (nitromethane, nitroethane, l-nitropropane) is used. Such complexes readily retrograde. The effect of nitromethane on the gelation characteristics of potato starch, as well as on the appearance of the gran~les,6'~*~'~ was interpreted in terms of the low acidity of nitromethane (aci-form) as well as a possibility of the Henry6" reaction. Because of the sensitivity of the gelation characteristics to subtle structural changes, even traces of this reaction could affect the characteristics. There is little published information on starch complexes with nibrobenzene. Complexes have been reported, but their properties were not detailed.673@0Kuge and take^^^' reported that formation of the complex requires alkaline solutions. Ulmann and S ~ h i e r b a u m ~reported ' ~ . ~ ~ ~formation of the complex from a potato starch-acetone complex via a starchpetroleum ether complex boiled under reflux. The resulting complex contains 5.6-5.9% of the guest nitro molecule. Because of the possible oxidation of starch by nitrobenzene, the characteristics of gelation and the appearance of starch granules are strongly influenced by this n i t r ~ a l k a n e . ~ ~ ~ ~

4. Simple Sulfur-Containing Compounds The effects of sulfonic acids on starch are discussed later (p. 375). Dimethyl sulfoxide and carbon disulfide are the only other sulfur-containing compounds that have been examined with respect to their complex formation with starch. For example, a complex of potato starch with carbon disulfide was prepared via the starch-acetone complex on refluxing. It was reported that this complex contains 5.8-5.9% of CS2.682Dimethyl sulfoxide causes expanded coiling of amylose without the formation of a Banks and Greenw~od"~reviewed the Mark-Houwink exponent for Me2SO-starch solutions. Reported variations in this exponent are believed

360


to be caused by the reaction of Me2S0with starch.h8yErlander and Tobid'" observed that the helices of amylose and amylopectin are stabilized by either the dihydrate or anhydrous Me2S0. In acidic media, the stability increases, perhaps because of the formation of the protonated form of Me2S0 (Me2S+OH-).A complex of amylose with Me2S0 is also reported for the synthesis of amylose-alcohol and amylose-haloalkane c~mplexes."~ 5. Alcohols

Several reports discuss inclusion complexes of starch and its components with alcohols. In Fig. 42, all tested alcohols, with the exception of tertbutanol, follow line 1. This suggests that the size of the hydrophobic moiety of alcohols is a key factor.6s4The complexation yield of lower alcohols depends on the starch-to-alcohol ratio.h73The number of turns of the helix in the complex also depends on the complexed Previous reports on starch-alcohol interactions assumed that only physical sorption is involved, despite observations of the irreproducibility of successive adsorption-desorption isotherms for starch-methanol and starch-ethanol systems. This irreproducibility was assumed to be the result of swelling.354 Several years later Poliszko et aLh91studied the sorption of methanol on starch gels and confirmed that successive sorption-desorptions open new specific centers of sorption. Based on a comparison of the results of adsorption (entropying) of water and both of the aforementioned alcohols, a relationship between the volume of sorbate molecule and the entropying effect was suggested. Russian scientists studied the competitive sorption of water and alcohols by starch from water-alcohol solutions. For example, Chapek6y2observed that, in contrast to water, ethanol does not condense in capillaries. The amount of sorbed water was observed to decrease with increasing alcohol concentration in the solution. In contrast, the amount of sorbed alcohol first increases and then decreases with increasing water concentration.hh2It was also observed that water can be selectively adsorbed from water-ethanol solutions by minimizing the amylopectin-to-amylose ratio in starch. Water sorption by corn grits is usually decreased by grindingPY3Heats of sorption of starch with ethanol were determined by Dumanand Rebar et aL417The heat of sorption depend on the skii et al.6y4.6y5 concentration of water in starch as well as the concentration of the aqueous solution of alcohol (see Fig. 44). In contrast to water adsorption, the adsorption of ethanol is rather independent of the variety of ~ t a r c h . 4Like ~ ~ certain other molecules, ethanol has the effect of crystallizing starch and its component~,6~~ an effect that is not linearly proportional to concentration. The intrinsic viscosity of aqueous


361

FIG.44.-Relationship between heat, Q, of treating starch with water, ethanol, and mixtures thereof!y4

amylose solutions reaches a maximum at 12% ethanol and then displays a minimum at 16% ethanol. The authors6y7interpreted this phenomenon as a result of the aggregation of amylose, which increases its total length. On the other hand, Weige1698determined 30-35% of ethanol as the optimal concentration for starch crystallization. The effects of processing potato starch in ethanol and I-propanol on the gelation characteristics are similar, but the effect of this treatment on the appearance of starch granules in normal and polarized light is different (see Tables XXVII and XXVIII). The discovery of the V-type, helical amylose (see p. 265) that forms when amylose interacts with 1-butanol was crucial for the development of the chemistry of starch inclusion complexes. It soon appeared that 1-butanol complexes solely with the amylose component. This selectivity became the first convenient method of fractionating starch. This method was first described by S ~ h o c and h ~ ~later ~ developed by Kerr et uL700-7"2 and othe r ~ . ~ " , ~An " ~ improved procedure was subsequently ~atented.7"~ The amount of 1-butanol adsorbed in amylose is increased by the presence of moisture and is also dependent on two key factors: the time of contact with that alcohol and the origin of the amylose, as shown in Table XXIX.

TABLE XXIX Absorption of Butanol Vapor by Solid Amylose of Various Moisture content^'^ Absorption (YO)at Time (Days)

Amylose Origin Corn Tapioca Retrograded Amorphous

Moisture Content, 2 5-6 2 5-6 2 5-6 2 5-6

O/O

1

6

18

4.44 5.62 3.74 5.74 0.34 0.20 0.41 0.20

5.99 12.49 6.56 13.50 0 0.20 0 0.20

12.95 8.84 13.85 0 0.20 0 0.30

u)

25

27

32

39

18.2

9.02 21.4 10.39

7.86 21.4 9.44

9.23

7.90

16.0 9.23 18.1

18.2 0

0.2

0

0

0

0.2 0

0.25

0

0.35

41

62

18.1

17.4

0.2

0.4

0.30

0.25


363

Starch fractionation using l-pentan01,6~~~”~ cyclohexanol,7m2-butanol, and 2-propano16” instead of 1-butanol have also been proposed. Amylose complexes with all of the normal-chain alcohols have essentially the same X-ray diffraction pattern, which differs from the patterns of amylose complexes with branched a l ~ o h o l s . ~ ” ~ . ~ ~ ~ According to Hollo et ~ l . , ~the ” ’ low stability of starch-alcohol Complexes (lower than that of the starch-iodine complex) is caused by the relatively small amount of space inside the amylose helix that is available for the hydrophobic moiety of the alcohol. The data in Table XXX appear to confirm this assumption and indicate iodine sorption amounts by starch complexes with subsequent members of the homologous series of alcohols. With the exception of tert-butanol, the iodine uptake decreases as the alcohol inside of the helix becomes more bulky. Conditions for the formation of potato starch-alcohol complexes do not follow predictable trends. For example, all complexes are formed more readily from air-dried starch rather than from oven-dried starch. Only the starch-methanol complex favors room temperature for its formation; the other alcohols require elevated temperatures for effective complexations (see Table XXXI).65y~681~6x2 In contrast, moisture inhibits the formation of the starch-( -)-menthol complex, which is characterized as an interchain complex.710The Scatchard binding parameters show that (-)-menthol and 1-hexanol adsorb on starch by only one mode, whereas 1-octanol and 1decanol adsorb in two modes.656The results for the latter two alcohols indicate that the helices are not fully filled before the second mode of complexation starts. Temperatures of the formation of starch-alcohol complexes likewise do not follow any clear r e l a t i o n ~ h i p . 6 Bushuk ~ ~ * ~ ~ and reported that the amount of guest molecules (H20, MeOH,

TABLE XXX Effect of Various Alcohols on the Iodine Uptake, O/O, by S t a r ~ h ’ ~ Alcohol, V% Methanol Ethanol 1-Propano1 2-Propanol 1-Butanol 2-Butanol terr-Butyl alcohol 1-Pentanol 2-Pentanol Cyclohexanol

O.Oo0

0.415

0.830

1.679

3.340

8.350

4.22 4.22 4.22 4.22 4.22 4.22 4.22 4.22 4.22 4.22

-

4.17 4.09 3.97 4.10 3.92 4.08 4.12 3.80 3.95 3.51

4.07 3.98 3.76 3.99 3.65 3.76 3.91 3.46 3.69 2.85

3.95 3.72 3.50 3.80 3.38 3.57 3.74 3.14 3.46 2.07

3.87 3.50 3.08 3.57 2.85 3.24 3.49 2.51 3.04 0.85

-

4.14 4.10 4.08 4.08 3.99 -

3.82


364

TABLE XXXI Sorption of Alcohols by Starch Depending on the Reaction Conditionsm' Room Temperature

Reflux

Alcohol

Air-Dried

Vacuum-Dried

Air-Dried

Vacuum-Dried

Methanol Ethanol 1-Propanol 2-Propanol 1-Butanol 2-Butanol 2-Pentanol

1.1-1.6 2.4-3.3 0.5-1.1 0.1-0.9 0.1-0.5 0.1-0.5 0.2-0.7

0.3-0.9 0.1-0.7 0.1-0.2 0.1-0.4 0.4-1.6 0.4-1.6 0.6-1.1

0.6-2.0 3.3-3.6 4.5-4.8 3.6-4.3 3.9-4.6 4.0-5.0 1.6-2.2

0.9-2.3 1.4-2.4 0.3-0.8 0.3-0.9 0.7-1.4 0.4-0.9 0.5-1.2

EtOH, CClJ included in wheat flour decreases with the molar volume and increases with the dipole moment of these guests. The complex of 3,4-dinitrobenzyl alcohol with starch was prepared as a local radiation sensitizer in tumor radi~therapy.~~' Sorption of 2-phenoxyand 2-(ch1orophenoxy)-ethanol on starch was also studied by M ~ C a r t h y , ~ ' ~ who observed starch to be a suitable complexing agent. It was assurned7l3that other naturally occuring alcohols-sterols-form complexes with starch. Thus, complexes of cholest-5-ene-3P-01 (cholesterol), cholest-5-ene(24R)-24-methyl-3~-ol (campesterol), stigmast-5,Z,22diene-3/3-01 (stigmasterol), and stigmast-5-ene-3P-01 (sitosterol) were prepared by blending the sterols with a 0.2% solution of rice starch. Independent of the concentration of sterols, 33, 25, 22, and 20% of these sterols were complexed, respectively. Hydrolysis by cold acid led to recovery of approximately 20% of the sterols. Treatment with pyrogallol was completely ineffective, as less than 1% of the guest molecules could be liberated. Hydrolysis with beta amylase liberated 38-75% of complexed sterols. A starch-cholesterol complex was also studied by P01lak.~'~ 6. Phenols

Phenols readily form starch complexes, and they are also readily extractable. As indicated in Table XXXII, the extraction of phenols from their complexes with amylopectin and amylose using diethyl ether generally proceeds more readily with amylopectin. Relationships between the structure and stability of phenol-starch complexes are not clearly understood. Apart from the complexes of phenols listed in Table XXXII, complexes of chlorocresols,712three isomeric nitrophenols,715resorcinol, phloroglucinol, and rutin,'16 have been reported. The formation of complexes with nitrophenols has been monitored by ultraviolet


365

TABLE XXXII Diethyl Ether Extraction of Phenols fkom Amylose and Amyl~pectin~'~

Amount of Extracted Phenol, YO" Amylose

Amylopectin

Phenol

After 7 Weeks

After 1 Year

After 7 Weeks

After 1 Year

Phenol Guaiacol Vanillin Thymol

8 24 100 97

17 (42) 0 (11) 93 (7) 43 (35)

17 53 81 95

5 27 51 95

"The figures in parentheses relate to extraction with moist diethyl ether. The other extractions were performed with dry diethyl ether.

spectroscopy. In general, phenols require relatively high temperatures for the formation of c o r n p l e ~ e s . ~ ~ ~ ~ ~ ~ ~ Starch complexation with thymol was proposed for the fractionation of starch,704but the procedure is not as efficient as that with 1-butanol. Fractionation with thymol followed by the use of 1-butanol is used to produce high-purity components. Complexation with naphthols revealed that 1-naphthol forms complexes with starch more readily than does its 2isomer.6731-Naphthol complexes in two modes, as suggested by the Scatchard plot, and the binding is rather weak.717Complexes between a formaldehyde-hydroquinone polymer and starch have also been d e t e ~ t e d . ~ "

7. Ethers Ulmann and S c h i e r b a ~ m ~ published ~ ~ , ~ ~ ' ,some ~ ~ ~data on sorption of ethers and reported that moisturized starch takes up 1,4-dioxane when boiled under reflux. Diethyl ether and tetrahydrofuran complex more readily with air-dried starch than with oven-dried starch (see Table XXXIII ).

8. Aldehydes Earlier it was considered that formaldehyde adsorbs onto starch:l9 but later papers have shown that chemical reaction takes place with the formation of acetals and h e m i a ~ e t a l s . ~ ~ ~ ~ ' ~ ' Studies on complexes of higher aldehydes with starch were performed with a view to preserving natural food aromas. Among aldehydes, only trichloroacetaldehyde (chloral) has been studied extensively as a coacervant

366


TABLE XXXIII Complexation of Ethers by Potato S t a r ~ h ~ ~ ~ ~ @ * Preparation Method Ether Diethyl ether

Tetrahydrofuran 1.4-Dioxane

Ether Content, YO

Water Content in Starch, YO

Temp.

Source

3.3-3.8

20

Room temp.

5.5-6.1

40

Room temp.

3.2-3.6

20

Room temp.

5.0-7.0

20

Reflux

6.8-6.9 0.6-1.1 7.0-7.9 0.5-0.7 1.0-1.9

40 20 Vacuum-dried 20 Vacuum-dried

Starch-methanol complex Starch-methanol complex Starch-acetone complex Starch-acetone complex Native starch Native starch Native starch Native starch Native starch

Reflux Room temp. Reflux Room temp. Reflux

of and as an agent for starch f r a c t i o n a t i ~ n .Aromatic ~ ~ ~ . ~ ~alde~ hydes also readily undergo inclusion in Binding parameters and X-ray diffraction patterns of the complex of starch with 1-decanal do not differ significantly from corresponding analyses performed using l - d e c a n 0 1 . 6 ~The ~ ~ ~temperatures ~~ of formation of the aldehyde complexes are likewise close to those for relevant starch-alcohol complexes.673~hRn Analysis of the binding sites of the complex suggests that both amylose and amylopectin are involved in complexation, but these complexes are rather weak.717 The complexation of vanillin by starch is difficult and proceeds slowly over approximately 7 weeks. Among three varieties of starch (potato, waxy maize, and maize), potato starch is the most effective complexant. Gelatinized starch does not provide a suitable matrix to allow vanillin to be readily extracted.724Data from Maier and B a ~ eindicate r ~ ~ that ~ dry vanillin is better sorbed by amylopectin, and moist vanillin is equally well sorbed by both amylose and amylopectin. 9. Ketones

Acetone readily adsorbs on starch and is better sorbed on air-dried starch than on oven-dried starch. Complexation with the solvent boiling under reflux gives better yields than complexation at room temperature. The adsorption depends on the starch variety and decreases in the order po-


367

tat0 > cassava > wheat > maize. The overall amount of acetone sorbed ranges6SY.6Kl .682 from 4.2 to 2.6%. Higher temperatures favor the formation of ketone complexes.673~6Ko Comparative data are available for complexes of (-)-menthol and menthone; the X-ray diffraction patterns of these complexes are similar in both cases, but the binding parameter is slightly higher for menthone.6s6Menthone sorbs on starch solely by one Extensive X-ray diffraction studies were performed by Takeo and K ~ g on e ~ ~ ~ complexes of amylose with a series of n-aliphatic ketones of different chain length and different positions and numbers of the carbonyl groups. It was reported that the radius and chain length of the guest molecule are critical factors which determine the characteristics of the packing inside the helix. Butanone moderately alters the characteristics of gelation of potato starch and significantly changes the appearance of granules under normal and polarized

10. Carboxylic Acids Sorption of carboxylic acids on starch is useful in their thin-layer chromatograhic separation.727Amino acids are particularly well separated on thin layers of starch (see r e v i e ~ s ~ ~ ' Rice - ~ ~ starch ~ ) . is suitable for this purpose, whereas maize starch is not. Starch has been used for resolution of raceAgain, only a few studies are available on the adsorption of low molecular weight fatty acids on s t a r ~ h .Results ~ ~ ~ *indicate ~ ~ ~ that the adsorption follows the Freundlich isotherm (that is, chemisorption can be ruled out). The results also indicate that acetic acid is adsorbed onto potato starch more quickly and in greater quantities than propanoic acid. There is a rough reciprocal relationship between the number of carbon atoms in the fatty acid chain and the temperature of complex formaSuch complexation can be monitored by IR spectroscopy, as characteristic vibrations (mainly w , ~ ) undergo significant shifts.674Much more attention has been paid to complexes of fatty acids in the formation of starch-lipid complexes, and also in native starch, which usually contains higher fatty acids (in rice s t a r ~ h , 7 ~corn ~ v s~ t~a~r ~ h , ~ potato ~ ~ - ~starch73y) ~' and lipids.742M e r ~ k e observed l ~ ~ ~ that the adsorption of fatty acids is a function of the capillarity of starch. Further studies provide evidence that fatty acids increase the viscosity of gels of various starches, provided that they are nonwaxy, amylose varieties. Properties are obviously dependent on the amount of fatty material, the length of the fatty acid chain, the concentration of amylose, and the moisture content in the Starch hydrolyzed with alpha-amylase increases adsorption of fatty On the other hand, complexed fatty acids inhibit alpha-amylolytic degradation

368


30

60

90 120 Ti me,min

FIG.45.-Decomposition to glucose by alpha amylase of complexes of starch with various fatty acids given as the function of time. 1, behenic acid; 2, arachic acid; 3, octadecanoic acid; 4, hexadecanoic acid; tetradecanoic acid; decanoic acid; 7, hexoctanoic acid; 8, pure amylose. (By permission from Acker and Brauner-Glaesner.””’)

of starch in the manner presented746in Fig. 45. It may be seen from this figure that the inhibition increases with the length of the hydrophobic alkyl chain of the acids. Fatty acids included in starch cause swelling and solubilization of starch granules. Table XXXIV demonstrates this effect

TABLE XXXIV Effect of Fatty Acids on the Swelling and Solubility of Potato Starch747at 85°C Acid

Swelling Power

Solubility, YO

Arachidic Decanoic Dodeca noic Hexadecanoic LinoIeic Octadecanoic Octanoic Oleic Ricinoleic Tetradecanoic

29.9 19.3 14.9 13.5 16.4 20.8 23.3 13.1 14.7 11.4

3.2 13.6 2.8 1.3 3.9 1.7 13.6 0.9 3.1 1.2


369

Temp.’C 0

15 .$

3m 800

0

L

2 600

r

102

v)

5

3 400

UI

5 200 I I‘

0

. ! I

>

I

,

60

30

90

120

150

180

30

60

Time, min

90

120

150

180

Time,min

FIG.46.-Effect of various amounts of fatty acids on the Brabender viscosity (in B.U.) of potato starch. Starch concentration is 20 g/500 mL. Left side: effect of various concentrations of octadecanoic acid; right side: effect of various fatty acids at 1% concentration on dry starch basis (1, no adjunct; 2, caprylic; 3, lauric; 4, myristic; 5, arachidic; 6, stearic). (Reprinted with permission from V. M. Gray and T. J. Schoch, Staerke, 14 (1962) 239-246.)

for various acids in potato As shown in Fig. 46, the viscosity of gels depends on the acid and its concentration in starch. The preparation of starch-fatty acid complexes is based on the use of hydrophobic solvents, which in turn open the starch lattice for penetration by acids. Such solvent molecules can become guests of inclusion complexes and are subsequently displaced by fatty acids. Extrusion has been found useful for the synthesis of such c o m p l e x e ~ . ’ ~ Starch ~ , ~ ~should ~ be defatted prior to complexation by using m e t h a n ~ l ? ~ ~Cellosolve, .~~’ or 80% 1,4d i o ~ a n e . ~ ”Table ’ - ~ ~ XXXV ~ summarizes the results of a classical preparain Fig. 47, the amount of complex formation by extrusion t i ~ n . As ~ ~shown ’ is not linearly proportional to the concentration of fatty acids added.74yIn

TABLE XXXV Absorption of Fatty Acids, wt%, by Amylose at Different Moisture Levels”’ Amylose Corn Tapioca Retrograded Amorphous

Moisture Content

Oleic Acid

1.62 7.26 1.62 7.11 1.62 6.81 1.62 6.81

0.1 1.90 0.1 1.91 0 0.35 0 0.1

Hexadecanoic 3.40

0.06

370


U c

OA.* LA

60

E

n

II

2c

- . . 1.0

2.0

3.0

50

4.0

60

7.0

8.0

Lipids, '10

FIG. 47.-Effect of fatty acids on the formation of complexes with wheat (upper OA+LA and SA curves) and potato (lower OA+LA and SA curves) starch. OA, oleic acid; LA, linoleic acid; SA, octadecanoic a~id.7~'

every case, a saturation value of the fatty acid concentration is achieved (for instance, approximately 2 and 3% for potato and wheat starch, respectively). The practical value of wheat starch complexes (extrudates), given in terms of the characteristics of gelation, is given in Table XXXVI.74x

TABLE XXXVI Characteristics of Gelation of Wheat Starch-Lipid E ~ t r u d a t e ' ~ Characteristics of Gelation" Lipid Added Noneh None' Decanoic acid Hexadecanoic acid Octadecanoic acid Behenic Oleic/linoleic acid

Amount of Lipid Added YO

A

B

C

D

E

600 280 20 40 45 63 42

92 30

3.0 3.0 2.5 3.0 2.5

79 None None 30 30 30 30

90 84 None None None 92 None

1220 210 20 42 40 78 SO

92 92 92 92 ~~

A. Time at the first increase of the viscosity; B, viscosity (in Brabender units) at 96°C; C, time at the beginning of cooling; D. time at which the increase of viscosity is observed; E. viscosity after cooling to 50°C.

"The characteristics of gelation of the native starch. ' The characteristics of gelation of extruded starch without any lipid present.


371

Szejtli and Banky-Eloed optimized conditions for the formation of amylose complexes with unsaturated fatty The inhibiting effect of fatty acids on the blue reaction of amylose suggests that fatty acids are adsorbed by the amylose helix.751 X-ray diffraction patterns of amylose complexes with oleic, octadecanoic, hexadecanoic, and dodecanoic acids are very similar to one another. A key difference is that the amylose helix of the complex with dodecanoic acid has 17.6 glucose units per molecule of acid, and compounds with hexadecanoic acid and oleic acid have 22.6 and 25 glucose units per acid molecule, respectively. The diameters of the cavities in these three complexes are 19, 24, and 27 A, respectively. In amylose complexes the sorption capacities with respect to these acids are very similar. Amylopectin accepts a negligible amount of fatty acids, as shown in Table XXXVII for hexadecanoic (see also a paper by S c h o ~ h ~ ~The ' ) . X-ray diffraction patterns for pairs of hexanoic and oleic acids as well as octadecanoic and linoleic acids are also similar. Binding parameters for all those acids are of the same order of magnitude, but they increase with the chain length of the included acid.656 X-ray diffraction of a series of amylose complexes with lower and higher fatty acids revealed that the crystal structures depend on whether amylose was complexed in the dry or wet state. Both the 6, and 71 helical conformations of amylose were found in these complexes. The conformation appears to depend on the length of the hydrophobic moiety. Dry amylose forms crystalline complexes with a unit cell identical to that of the anhydrous 1-butanol-starch complex (lattice parameters a = b = 25.6 An orthorhombic unit cell was proposed for the 7, -helical structure of the wet complexes of monobasic acids (acetic, butanoic, pentanoic, hexanoic,

A).

TABLE XXXVIl Binding of Hexadecanoic Acid by Amylose and A m y I ~ p e c t i n ~ ~ * Hexadecanoic Acid Bound, wt% Amylose Variety Acid Added, g

V

B

Amylopectin

0.2 0.5 1.o 1.5 2.0 3.0 4.0

0.84-1.09 1.53-1.84 1.94-2.14 2.95-3.26 2.90-3.55 3.80-4.50 4.02-4.03

0.23 0.23 0.29 0.22 0.19 0.18 0.18

0.10 0.07-0.15 0.07 0.21 0.05-0.12 0.15 0.21

372


octanoic, dodecanoic, hexadecanoic, octadecanqic, undecylenic, elaidic, oleic, and linoleic; lattice parameters a = 13.7 A, b = 25.6 A, and c = 7.7 A). Propanoic acid gives rise to three kinds of X-ray diffraction patterns depending on the acid-to-amylose ratio. Dibasic acids give complexes with diffraction patterns that are characteristic of either the orthorhombic or hexagonal crystal structures. ’ ~ complexes ~ of corn starch with lower dioic Studies by Tomasik et ~ 1 . on acids [H02C-(CH2),-C02Hwith n = 0-41 revealed that oxalic acid forms a surface complex, with both of the carboxylic groups involved in interactions with the hydroxyl groups of the glucose residues. Malonic acid, which takes the form of an intramolecular, six-membered chelate, also reacts on the surface of starch, a process which involves interaction with only one carboxylic group. Succinic, glutaric, and adipic acids seem to enter the interior of the amylose helix. In all cases, complexes contain dioic acids at concentrations of approximately wt %. Inclusion protects unsaturated fatty acids from aging; however, this protection is not perfect, as amylose itself also complexes o ~ y g e n ~(see ~~.~’~ Fig. 48). Unsaturated acids have a specific pattern of the carbon chain because at least two sp2-hybridizedcarbon atoms are present, causing geometrical isomerism. The trans-isomer does not fit perfectly into the cavity of amylose. Another report indicates that starch complexation with tannic acid has no effect on its activity against experimental ulcers in laboratory rats.757

11. Carboxylic Esters Comparison of the sorption behavior of acetic acid and its ethyl ester (see Table XXIII)674reveals that ester sorption is slower and less efficient than that of the parent acid. The low yield of adsorption of diethyl malo-

f-#

100 l2OI

20

40 60 80 Time ,h

FIG.48.-Oxygen consumption of fatty acids complexed by amylose (I, 1 mg fatty acid; 2, 16.4 rng fatty acid; 3. 430 mg amylose). (Reprinted with permission from J. Szejtli and E. Banky-Eloed, Staerke, 27 (1975) 368-376.)


373

nate673suggests that this is a general property of esters. Thin-layer chromatography on rice and maize starch demonstrated the effect of complexation of esters of 3,5-dinitrobenzoic acid and alcohols, with the carbon chain from C1 to Cz0. A general trend is observed between the RF value and the length of the chain of the esterifying alcohol. Coumarin, which may be regarded as lactone, is sorbed on amylose and amylopectin. In the dry state this lactone is better sorbed on amylopectin, although moist coumarin is equally well sorbed on both of these polysaccharides.674It was also reported that breakdown of acetylcholine is reduced by inclusion in starch.759

12. Nitriles Conditioning in acetonitrile has significant effects on the characteristics of gelation of starch. It appears to act by ordering of the starch molecules. It increases the tendency of gels towards retrogradation. The appearance of starch granules is significantly affected on contact with acetonitrile, which suggests that the granules are easily penetrated.6x3@"

13. Amines

Sorption of amines on starch was recognized760as early as 1910 in studies of the interactions between starch and piperidine. Experiments with starch and 1-butylamine show extraordinarily high and fast adsorption. The equilibrium concentration of that amine reaches 1982 mg/g of starch, as compared to n-hexane (6 mg/g), ethyl acetate (27 mg/g), and ethanol (208 mg/ 8). Desorption is, in fact, also significant, but is much lower than in the case of the other complexes just mentioned. Because of their basic properties, amines decompose their host molecule.674Less-basic amines, such as aromatic t r y p ~ f l a v i n ~and ~ l aliphatic amine salts,'62 can be included into starch. Particular attention has been devoted to the interaction of starch with a specific tertiary amine, namely pyridine. Pyridine readily sorbs on starch, and the extent of complexation depends on the starch-to-pyridine ratio.h7',674,680 Despite the fact that pyridine has a higher basicity than do aliphatic amines, it does not compose starch to a significant large extent.669 Quinolineh7' and p y r a ~ i n are e ~ sorbed ~~ on starch in much smaller amounts. Pyridine, especially aqueous pyridine, also causes swelling of s t a r ~ h . ~ ~ ~ - ~ ~ Figure 49 shows that the swelling kinetics of potato starch depend on the concentration of aqueous pyridine. In addition, swelling kinetics depend on the variety of The affinity towards swelling decreases in the order potato > wheat > maize > rice starch. A r n y l ~ s e ~and ~ ~ amylopec,~~' tin76xcomplexes with pyridine can be prepared in crystalline forms. This process was utilized in starch fractionation, and amylopectin of 98%purity


314

60 E

50

0.

-2 430o -&

.-

f 3

m

20

lo o

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 Tirne,h

FIG.49.-Kinetics

of swelling starch as a function of the water content.77s

could be obtained. In addition, pyridine increases the gelation temperature of potato starch appreciably, and significantly affects the entire characteristics of g e l a t i ~ n . ~ ' ~ . ~ ~ 14. Carboxyamides

According to Banks and G r e n ~ o o d formamide ~~' increases the proportion of coiled amylose, but it is not able to completely transform amylose into a helical structure. As with pyridine, N,N'-dimethylformamide significantly affects the characteristics of gelation of potato starch. Based on thermal analysis, there is a suggestion that a reaction may occur between starch and that amide683~6'4 on 3 h of conditioning at 100 "C. Apart from various chemical modifications of starch with urea (see, for instance, Sroczynski et u L ~ ' ~ )a, true inclusion complex of urea with starch (17 :83) was prepared, and its usefulness in urea feeding was e~tablished.~~' It was reported that increases in the ammonia concentration in the blood of ruminants fed with this complex are generally much slower, and lower than in the case of the administration of urea by itself (Fig. 50). 15. Vitamins

It has been found that glucose, sucrose, casein, albumin, sodium chloride, flour groats, and starch stabilize ascorbic acid (vitamin C).771The addition of 5% of starch inhibits the decomposition of ascorbic Studies on the stability of ascorbic acid, sodium ascorbate, erythorbic acid, and sodium erythorbate showed that some stabilization is observed after the addition of 8% It was reported that the interaction is purely physical in nature.774


315

60 0

s 500

F- 400 0

I

z

; 300

0

m

200

100

I

I

1

2

L

I

3 4 Time,h

5

6

FIG.SO.--In vivo experiments with rumen-fistulated lambs with urea and starch-urea complex, measured as the concentration of ammonia in blood. (Reprinted with permission from J. Hollo, L. Fodor, and S . Gal, Stuerke, 31 (1979) 303-306.)

Potato starch adsorbs more than 2 mg of thiamine per gram. This vitamin is stable on storage at 38°C for 2 months and can be eluted by a 5% aqueous solution of sodium 16. Sulfates and Sulfonates (Surfactants)

Salts of long-chain alkyl sulfates and sulfonates are established surfactants. Thorough studies by S a i t and ~ ~ then ~ ~ by Yamamoto et ~ 1 with. sodium dodecylsulfate (SDS) on starch and amylose, respectively, revealed that a complex is formed in both cases. The long-chain anion occupies the interior of the starch helix, and the cations remain outside. Figure 51 illustrates that the response of a starch solution to an increase of SDS concentration is not linearly proportional to the reduced specific viscosity of the solution. The effect of SDS depends on the specific mode of its preparation and on the starch variety. The same trends are observed for such cationic detergents as cetyltrimethylammonium bromide. Such detergents cooperate with mineral salts, and their behavior becomes more like that of emulsifiers. Such behavior is specific for amylose. It is possible that amylopectin also interacts with detergents, but little is known about the nature of these interaction^.^'^ In general, anionic surfactants (monoglycerides and alkyl and aryl sulfates) restrict the hydration of starch and swelling. In contrast, cationic

~

~

~

376


-

0

0

02

04

06

Sodium Dodecyl Sulfate ,%

FIG.51.-Reduced specific viscosity of 0.93%starch solution as a function of concentration of sodium dodecylsulfate. (Reprinted with permission from S. Saito, Kolloid-Z., 154 (1957) 19-29.)

surfactants (quaternary ammonium salts) significantly increase hydration, and they reduce the swelling and solubilization of In this manner, water-soluble polysaccharides can be p r e ~ i p i t a t e d . ~ ' ~ . ~ ~ ~ The complexation of starch and thin-boiled starches with alkyl and aryl sulfonates gives a product that is readily soluble in warm water. This complexation also decreases the temperature of maximum swelling. Enzymic susceptibility is decreased and retrogradation is impeded.780 VIII. COMPLEXES WITH DYES Studies on the dyeing of starch are dated as early as the end of the nineteenth century, when the first observation was published on selective dyeing.781Because dyes belong to many chemical groups, the problem of dye complexation is rather involved.782However, this situation presents an exciting challenge for analysts. For example, it provided an excellent opportunity for differentiating starch varieties and for developing insight '~ also into the structure of starch dyes by selective d ~ e i n g . ~ ' ~It- ~should be noted that some dyes adsorb directly on starch and some of them merely impregnate the granules. For example, potato starch adsorbs Berlin Blue when impregnated with ferric chloride in solution with subsequent addition of potassium f e r r ~ c y a n i d e . ~ ~ ~ Several dye coloration scales are used to characterize starch varieties. Perhaps the oldest is the differentiation between starches based on the uptake of Saphranin and Gentiana Violet (see Table XXXVIII).787Cov e l l presented ~ ~ ~ ~ another coloration scale which is based on the use of six common acidic and basic dyes (see Table XXXIX). Like the Saphranin and Gentiana Violet dyes, these dyes adsorb directly on starch. Table XL presents a list of synthetic dyes tested in starch dyeing.78'.790Z ~ i k k e r ~ ~ ' observed the reactions of mechanically damaged starch granules and amylo-


311

TABLE XXXVIII Dyeability of Starches with Sapbranin and Gentiana Violet on the 15-Point Scalem7 Starch Source

Gentiana Violet

Saphranin

Aesculus hippocastanum Andropogon sorghum var. (W.K. Corn) Andropogon sorghum var. (Y.B. Sorgh.) Arum cornutum Arum italicum Avena sativaa var. Batatas edulis Canna edulis Canna roscoeana Canna var. (Jean Tissot) Castanea americana Castanea pumila Curcuma longa Curcunia petiolata Dolichos lahlab Hordeum sativum var. Iris alata Iris florentina Iris tingitana Jatropha curcas Lathyrus magellanicus var. albus Lathyrus odoratus var. Shahzada Lens esculenta Manihot utilissinia Maranta arundinacea Maranta leuconelira Musa cavendishii Musa cavendishii (Green fruit) Musa ensete Musa sapientum Oryza sativa var. Panicum crus-galli var. Phaseolus hinatus var. Phaseolus vulgaris var. Pisum sativum var. (Eugenie, yellow) Pisum sativum var. (Mam. G . Seeded) Polygonurn fagopyrum var. Quercus a h a Quercits rubra Secale cereale var. (Mammoth winter) Secalr cereale var. (Spring) Solanuni tiiberosurn Tacca pinnatifada

4 4 3.5 9.2 4 4 1 8.5 10 10.6 1 1.6 1 1.5 4

4 4 3.5 8.2 1 4 8.6 10 10 11.6 0.2 0.8 7 7.5 4 1 7 4 4.6 8.5 4.8 4 4 10 8.5 4 8.5 8.5 10.8 9.5 4 4 3.5 4 1 1.6 4 1 1.5 2.5 4 14 4

1 1 4 4.6 8.5 4.8 4 4 1 1 1 8.5 9.5 9.2 10.6 4 4 3.5 4 1 1.6 1

1 1 2.5 4 14 4

(continues)


378

TABLE XXXVIII (Continued)

Starch Source Triticum sativum var. vulgare Vicia faba Vicia sativa Vicia villosa Zea mays var. Everta (Golden Queen) Zea mays var. Everta (white rice) Zea mays var. Indurata Zea mays var. Saccharata Zingiber oncinale Zingiber offkinale var. (Jamaica)

Gentiana Violet

Saphranin

1 1.5 1 0.5 4 3.25 4 1 8.5 7.5

4 4.5

4 3.5 1 0.8 3.8 0.8 4 3.2

pectin bags with a variety of dyes (Table XLI). Rassow and Lobenstein7’* proposed characterizing native and processed starch based on their sorption behavior with respect to basic, acidic, and direct dyes. Gelation can also change the adsorptivity of starch towards dyes, depending on the starch variety and the type of dye. Schulz and S t e i n h ~ f presented f~~~ a coloration scale for the behavior of 5 varieties of starch as a function of 13 dyes (Table XLII). H ~ m b e r g e rand ~ ~ Seidemann795 ~ also proposed semiquantitative scales (Table XLIII). Huebner and V e n k a t a r a m a r ~studied ~ ~ ~ sorption of various dyes on starch of different varieties in air-dried and gelatinized forms (Table XLIV). Later studies by K ~ b a r n o t o ~ indicate ’~ that basic dyes adsorb to a greater extent on larger starch granules, whereas acidic dyes perform better on small granules. Neutral dyes adsorb independently of

TABLE XXXIX Dyeing of Starch with Acidic and Basic Dyesm

Fuchsin Starch Variety

Acidic

Basic

Methyl Violet

Methylene Blue

Potato Wheat Barley Rye Oat Rice Marantha Pea Bean

3+ 2+ 3+ 1+ I+ 1+ 1+ 2+ 1+

3+ 2+ 1+ 2+

3+ 2+ 2+ 2+

1+

1+

1+ I+ 2+ 1+

1+ 1+ 2+ I+

3+ 1+ 1+ 1+ 1+ 2+ I+ 0 0

Congo Red 0

I+ I+ I+ 0

0 I+ 0 0

Eosin 1+ I+ 1+ I+ I+ 0 I+ 1+ I+


TABLE XL Dyeing of Potato Starch with Coal-TarDyesm Little or No Dyeing Effect

Medium Dyeing Effect

Strong Dyeing Effect

Alizarin Yellow R Azo Bordeaux Azofuchsin B Cottonwool Yellow Benzopurpurin 10B Bordeaux Brilliant Crocein 3B Quinoline Yellow Chromic Brown R O Chrysoidine Diamino Bordeaux S Diamino Brown B Diamino Brown R Diamino Brown V Diamino Scarlett Diamino Deep Black 00 Direct Yellow G Direct Orange 2R Genuine Brown Genuine Acid Fuchsin B Eosin A Erythrosine Extra Fuchsin S Guinea Green B Hessian Purple NR Columbian Black Naphthol Yellow S Nyanza Black Orange I1 Patent Blue A Picric Acid Ponceau 4GB Primuline Resorcin Yellow Rose lndulin 2G Acid Violet 5BN Acid Violet 6B Salm Red Tartrazin

Alkaline Blue Alkaline Violet Hessian Brilliant Purple Hessian Yellow Towel Orange

Auramin 0 Bismarck Brown Extra Brilliant Green Chrysoidin A Diamin Blue BB Genuine Blue R Fuchsin Indazin M Janus Blue B Janus Blue R Janus Yellow G Cristal Violet Methylene Blue Neutral Blue Neutral Red Neutral Violet Parafuchsin Phosphine N Pyronin G Rhoduline Red Rhoduline Violet

319


380

TABLE XLI Behavior of Ground Starch and Amylopectin Granules with Respect to Organic Dyes"' Dye Nitro dyes Picric Acid Martius Yellow Naphthol Yellow S Azo Dyes Monoazo dyes Helianthine Orange G Chrysoidine Bismarck Brown Croceine Orange Ponceau 6R Bordeaux G Diazo dyes Brilliant Croceine Towel Red Biebrich Scarlett Chrysophenine Brilliant Yellow Direct Yellow Benzorit Benzazurine Benzo Blue 2B Benzo Orange Benzo Olive Benzopurpurine Benzo Blue-black Chrysamine Congo Red Congo Orange Brilliant Purpurine Benzo Brilliant Green Delta Purpurine Direct Black Direct Blue Toluine Orange " GS = Ground starch. A = Amylopectin.

+ = Strong dyeing effect. -t =

-

=

Weak dyeing effect. N o dyeing effect.

GS" -

-

Ab

Dye Acetone dyes (carbonyl dyes) Hematein Acid Carmin Ammonium Quinoline Yellow Acetonimino dyes Primulin

-

+ + -

-

+ -

rf.

+ + + + -

+

+ + + + + + + + + + + + +

Indigo dyes Indigo Carmin

Quinone Dyes Anthracene dyes Purpurine (Potassium Salt) Cyanine Quinimido dyes Cresil Brilliant Blue Magdala Red Methylene Blue Neo Blue Neutral Red Nigrosin e Saphranine Triphenylmethane dyes Erythrosine Eosine Koralline Fuchsin Acid Fuchsin Malachite Green Acid Green Iodine Green Gentiana Violet Acid Violet Water Blue Cottonwool Blue Toluidine Blue

GS"

Ab

381


TABLE XLII Dyeing of Starch Varietiesam3 Potato Dye Mixture of Black, White and Red Cresyl Violet Methyl Green Nigrosine-Sudan-Fuchsin Thionine Congo Red Gentiana Violet Neutral Red Bismarck Brown Methylene Blue Bromine water Metachrome Red Malachite Green Triacid mixture

A

B

Wheat

A

B

W

P

-

b

2 Ig

Ig

-

Ib Ib v

-

-

Y

-

bg Ib

Rye

P

-

dp Iv

-

-

-

-

_

b

gb

-

_

A W

Maize B

P

db

?lb

Ib

IP -

-

P

L

V

_

IY Ib

_

A

+ -

Rice

~

B

A

B

P

-

IP

-

Ib

-

-

-

Lv

-

IP Iv IP -

-

dP Ili IP 1Y Ib

Iv P IY b

-

b

_

~~

A. intact starch granules; B, swollen starch granules; b, blue; d, dark; g, green; I, light; li, lilac; p, pink; r. red: v. violet; w. white; y. yellow; 2.partly dyed; -no dye accepted. "

the granule size. Independent of the dye used, sorption is perturbed by chlorides of sodium, potassium, calcium, and barium. Lipatov"' recognized that the sorption of Methylene Blue on starch is a heterogenous neutralization reaction of neutralization in which starch acts as an acid. Adsorption isotherms of Methylene Blue on starch indicate that it is an equilibrium process and that swelling as well as other internal structural factors are responsible for the sigmoidal shape of the i ~ o t h e r m . ' ~ ~ *It' ~is' also known that the Langmuir isotherm is affected by equilibrium between the monomer and dimer forms of Methylene Blue, both of which are capable of adsorption on starch (Table XLV).'O' Anionic dyes readily adsorb on starch (Tables XXXIX-XLIII).802 However, there is evidence that this rule is violated in the case of potato s t a r ~ h . ~ "The ~ . ~simplest "~ intepretation of this fact has been presented in terms of the phosphate groups present in potato starch. A relationship exists between the phosphorus content in potato starch and the sorption capacity of that starch for Methylene Blue. The sorption follows the Langmuir isotherm. Extrapolation of the result to 0% phosphate leads to the conclusion that no Methylene Blue would be adsorbed on phosphate-free starch.'04 Methylene Blue is better adsorbed on potato starch, but is also

TABLE XLIII Dyeing of Starch Varietiesawm Potato

Wheat

Rye

Dye

A

B

A

B

A

B

A

B

A

Acid Fuchsin Bismarck Brown Brilliant Cresyl Blue Bromocresyl Green China Blue Chrysoidine Congo Red Crystal Violet Eosin, blueish Erythrosine Gentiana Violet Malachite Green Methyl Violet Neutral Red Nile Blue Saphranin Thionine Thiazole Yellow G Toluene Blue

3 0 0 2 0 4 0

4

3

3 1 3

1

0 0

2 0 2 1

4

4

3 0 1 2

4

2

2 4 0-2 4 4 1 2

0 2 3 3 2 3

0 2 2 0-1 2 0 3 1 1 3 3 1 3 0-1 2 3 2

4 1 3

3

3 3 1

1 1 2

0 3 3 0 0 1 1 0 1 2 1

3 0 0 1 0 3 0 0 0 1 3 2 1 4 2 3

1

4

1

1

2

1

2 1

1

1

4 2 4

1

4 1

4 2 0 0

1 1 1 1

3

0 2 2 0 2 0 3 1 2 3 3 1 3 0-1 3 3 2 2

1 4 1 4

2

0 1 1

1 1 1 3 1 1 2 1

2

0 2 0 1 0 0 3

2 3 2 1 2 2 1

2

Maranta

Maize

Rice

4

1

B

Pea

A

B

A

B

0

2

0

0

0

4

1

0

2 3

4

4

0

4

1 1

3

0 0 0

1

0 1 1

4 5

0 0 1

4

1

4

0 2 2

4

2

2

1

4 4

2 2

2 0-2

0

0 2 3 1 1 1 0

1

1

A. intact granules; B, swollen granules; data on a 5-point scale in which 0 = no dyeing, 1 = very weak dyeing, 2 = weak dyeing, 3 = good dyeing, 4 = strong dyeing.


383

TABLE XLIV Adsorption of Several Days on Starches of Various Origins7% Amount of Dyestuff Adsorbed, % Dye

Maize

Potato

Rice

Sago

Tapioca

Wheat

Acid Violet Benzopurpurine 48 Bismarck Brown Chrysoidine YRP Cutch Diamine Sky Blue Eosin A Indigo Carmine Magenta S Metanil Yellow Methylene Blue BB Methyl Violet Mikado Golden Yellow Naphthol Yellow S Orange 11 Ponceau R Water Blue

13-20 35 16 66 52 53 22-29 25 73-81 48 44-54 87 12 24 30

12 29

12 26

15 24

20 45

15 90

49 16 14 15 87 40 55

49 38 17 9 87

81

89

77 34 33 23 70 40 30

51 32 77 65 48

91 55 49 31 79 56 29

7 15

15 37

12 67

19 54

17 58

25 29

57

61 61

2

21 8 12 14 25

44

64

64

adsorbed on wheat and tapioca starch.'05 Starch inhibits Methylene Blue against fading.8MFluorescein, being an acidic dye, is sparingly sorbed on potato starch from very dilute aqueous solutions. The adsorbed dye retains its fluorescence, and starch granules assume an intense yellow-greenish CO~O~.~O~ The fluorescence of Acridine Orange adsorbed on potato starch depends on the concentration of that dye.'08

TABLE XLV Langmuir Constants for Absorption of Methylene Blue on Potato Starch from Aqueous Solutions at 26"c8O1 Dye Concentration

Langmuir Constants lo4 kl, L h o l

Low, predominantly monomer High. uredominantly dimer

0.466 1.54

lo-' *',

moVg

2.18 3.04

Linear Correlation Coefficient 0.996 0.998

384

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