Asymmetric Organocatalysis in Natural Product Syntheses
This book provides the reader with an illustrative overview concerning successful and widely used applications of organocatalysis in the field of natural product synthesis. The main focus will be on organocatalytic key-steps for each (multi-step) synthesi
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Over the ages, organisms have developed the capacity to elaborate a fascinating variety of natural products with an almost infinite diversity in structure and biological activity. It is impressive (and sometimes may also be a bit frustrating) to synthesis-oriented organic chemists to recognize the ease with which Nature biosynthesizes such important compounds like nucleic acids, saccharides, amino acids (proteins), or various highly complex secondary metabolites (1). Due to the high chemical diversity of compounds available from natural sources, the identification and isolation of novel biologically active natural products represents a major goal in contemporary biomedical and agrochemical science and a large percentage of today’s major drugs have their origins in Nature. However, not all of the potentially useful natural compounds can be isolated readily or in such large quantities as can amino acids or saccharides. Accordingly, the paucity of such natural products may require a (total) synthesis approach to obtain sufficient quantities for initial biological investigations, and, if promising, for further development. The field of natural product synthesis is definitely one of the most challenging and attractive areas of organic chemistry and numerous contributions focusing on the development of synthesis routes for natural products are reported constantly (2–4). Among the different types of transformations that are necessary to successfully achieve a complex total synthesis, those enabling the stereoselective introduction of a stereogenic center have attracted special interest. The field of asymmetric synthesis has made spectacular progress over the last few decades. Among the various ways of creating enantiomerically enriched products, catalytic methods are considered to be the most appealing as the use of stoichiometric amounts of valuable chiral reagents can be avoided, thus resulting in highly efficient approaches. New methods have been emerging recently, enabling more selective, environmentally friendly, and economically more cost-effective transformations. Besides enzymatic and metal-catalyzed asymmetric transformations, the use of substoichiometric amounts of organic molecules (with so-called “organocatalysts”) has proven to possess an enormous potential for the catalysis of stereoselective reactions (5, 6). M. Waser, Asymmetric Organocatalysis in Natural Product Syntheses, Progress in the Chemistry of Organic Natural Products 96, DOI 10.1007/978-3-7091-1163-5_1, # Springer-Verlag Wien 2012
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1 Introduction
Fig. 1 Annual number of publications covering the topic organocatalysis (SciFinder®, Chemical Abstracts Service, Columbus, OH, U.S.A.) O
O H
N
N
OH
HO
H
N
N 2
3
H OH CHO + HCN
2 or 3
1
CN 4
Scheme 1 Cinchona alkaloid-mediated addition of HCN to 1
Although the use of small molecules for the catalysis of a variety of organic reactions has been known for decades (7–26), it was only just over 10 years ago, when the seminal publications of MacMillan et al. (27) and List, Barbas, and Ler
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