LITMIR.BIZ

PREFACE TO THE FIRST EDITION

      Biological systems, in most cases, recognize a pair of enantiomers as different substances, and the two enantiomers will elicit different responses. Thus, one enantiomer may act as a very effective therapeutic drug, whereas the other enantiomer is highly toxic. The sad example of thalidomide is well‐known. It is the responsibility of synthetic chemists to provide highly efficient and reliable methods for the synthesis of desired compounds in an enantiomerically pure state, that is, with 100% enantiomeric excess (% ee), so that we shall not repeat the thalidomide tragedy. It has been shown for many pharmaceuticals that only one enantiomer contains all of the desired activity, and the other is either totally inactive or toxic. Recent movements of the Food & Drug Administration (FDA) in the United States clearly reflect the current situation in “Chiral Drugs,” that is, pharmaceutical industries will have to provide rigorous justification to obtain the FDA’s approval of racemates. Several methods are used to obtain enantiomerically pure materials, which include classical optical resolution via diastereomers, chromatographic separation of enantiomers, enzymic resolution, chemical kinetic resolution, and asymmetric synthesis.

      The importance and practicality of asymmetric synthesis as a tool to obtain enantiomerically pure or enriched compounds have been fully acknowledged to date by chemists in synthetic organic chemistry, medicinal chemistry, agricultural chemistry, natural products chemistry, pharmaceutical industries, and agricultural industries. This prominence is due to the explosive development of newer and more efficient methods during the last decade.

      This book describes recent advances in catalytic asymmetric synthesis with brief summaries of the previous achievements as well as general discussions of the reactions. A previous book reviewing this topic, Asymmetric Synthesis, Vol. 5—Chiral Catalysis, edited by J. D. Morrison (Academic Press, Inc., 1985), compiles important contributions through 1982. Another book, Asymmetric Catalysis, edited by B. Bosnich (Martinus Nijhoff, 1986) also concisely covers contributions up to early 1984. In 1971, an excellent book, Asymmetric Organic Reactions, by J. D. Morrison and H. S. Mosher, reviewed all earlier important work on the subject and compiled nearly 850 relevant publications through 1968, including some papers published in 1969. In the early 1980s, a survey of publications dealing with asymmetric synthesis (in a broad sense) indicated that the total number of papers in this area of research published in the 10 years after the Morrison/ Mosher book, that is, 1971–1980, was almost the same as that of all the papers published before 1971. This doubling of output clearly indicates the attention paid to this important topic in 1970s. Since the 1980s, research on asymmetric synthesis has become even more important and popular when enantiomerically pure compounds are required for the total synthesis of natural products, pharmaceuticals, and agricultural agents. It would not be an exaggeration to say that the number of publications on asymmetric synthesis has been increasing exponentially every year.

Among the types of asymmetric reactions, the most desirable and the most challenging is catalytic asymmetric synthesis because one chiral catalyst molecule can create millions of chiral product molecules, just as enzymes do in biological systems. Among the significant achievements in basic research: (i) asymmetric hydrogenation of dehydroamino acids, a groundbreaking work by W.S. Knowles et al.; (ii) the Sharpless epoxidation by K.B. Sharpless et al.; and (iii) the second‐generation asymmetric hydrogenation processes developed by R. Noyori et al. deserve particular attention because of the tremendous impact that these processes have made in synthetic organic chemistry. Catalytic asymmetric synthesis often has significant economic advantages over stoichiometric asymmetric synthesis for industrial‐scale production of enantiomerically pure compounds. In fact, a number of catalytic asymmetric reactions, including the “Takasago Process” (asymmetric isomerization), the “Sumitomo Process” (asymmetric cyclopropanation), and the “Arco Process” (asymmetric Sharpless epoxidation), have been commercialized in the 1980s. These processes supplement the epoch‐making “Monsanto Process” (asymmetric hydrogenation), established in the early 1970s. This book uncovers other catalytic asymmetric reactions that have high potential as commercial processes. Extensive research on new and effective catalytic asymmetric reactions will surely continue beyond the year 2000, and catalytic asymmetric processes promoted by man‐made chiral catalysts will become mainstream chemical technology in the twenty‐first century.

      This book covers the following catalytic asymmetric reactions: asymmetric hydrogenation (Chapter 1), isomerization (Chapter 2), cyclopropanation (Chapter 3), oxidations (epoxidation of allylic alcohols as well as unfunctionalized olefins, oxidation of sulfides, and dihydroxylation of olefins) (Chapter 4), hydrocarbonylations (Chapter 5), hydrosilylation (Chapter 6), carbon–carbon bond‐forming reactions (allylic alkylation, Grignard cross‐coupling, and aldol reaction) (Chapter 7), phase‐transfer reactions (Chapter 8), and Lewis acid‐catalyzed reactions (Chapter 9). The authors of the chapters are all world leaders in this field, who outline and discuss the essence of each catalytic asymmetric reaction. In addition, a convenient list of the chiral ligands appearing in this book, with citation of relevant references, is provided as an Appendix.

      This book serves as an excellent reference for graduate students as well as chemists at all levels in both academic and industrial laboratories.

      Iwao Ojima

      March, 1993