Asymmetric Phase-Transfer Catalysis in Organic Synthesis

Asymmetric Phase-Transfer Catalysis in Organic Synthesis 

Over the past few decades, the increasing demand for optically pure compounds has stimulated the development of asymmetric PTC. The examination of asymmetric PTC by the use of structurally well-defined chiral, nonracemic catalysts began in the mid-1970s [4], and has continued at a fast pace to become one of the most active fields in asymmetric catalysis. Today, a large number of chiral phase-transfer catalysts with diverse structures have been developed, and their applications in asymmetric PTC have resulted in notable achievements. 

Bridging Heterogsneous and Homogeneous Catalysis Concepts, Strategies, and Applications, First Edition. 

The aim of this chapter is to provide a general overview of this continuously growing field, focusing not only on the design of various types of chiral phase-transfer catalysts but also on their representative applications. In addition, the aim is to encourage chemists to direct their efforts toward further continuous development in asymmetric PTC. It is inevitable that some chiral phase-transfer catalysts and their appHcations are still missed in this chapter since there exists a vast amount of hterature on asymmetric PTC covering them. Fortunately, many previous books [5] and reviews [6] on this field may alleviate this problem. 

The development of efficient chiral phase-transfer catalysts is at the center of asymmetric PTC. Over the past 30 years, numerous chiral phase-transfer catalysts have been reported, and most of them can be divided into several different categories based on their activation modes and structures. 

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I 72 AsYmmetric Phase-Transfer Catalysis in Organic Synthesis... 

In contrast to the maturity of asymmetric synthesis utilizing chiral transition metal catalysts, asymmetric phase transfer catalysis is still behind it and covers organic reactions to lesser extent. Thus, it is further necessary in wide range to explore efficient asymmetric phase transfer catalysis keeping its superiority of easy operation, mild reaction conditions, and environmental binignancy. 

Enantioselective oxidation is one of the most important and yet useful transformations in organic synthesis, and the asymmetric phase-transfer catalysis has made notable contributions to this field. The stereoselective epoxidation of electron-deficient olefins with peroxides is a representative example, and Taylor demonstrated the synthetic utility of this system by accomplishing the total syntheses of three natural products of manumycin family, (-l-)-MT 35214 131, (-l-)-manumycin A 132, " and (—)-alisamycin 133 (Scheme 4.31). The syntheses were undertaken by the... 

Phase-transfer catalysis is a vast area of organic chemistry, and there are many different examples of the applications of phase-transfer catalysts to asymmetric synthesis, particularly in the area of enantioselective a-ami-noacid synthesis. This area has been extensively reviewed by Maruoka, Lygo and O Donnell. However, there are limited examples where asymmetric phase-transfer catalysis has been applied to drug discovery. One notable example is the asymmetric allylation of glycine imine 337, catalysed by quaternary ammonium bromide 340, which has been used by Kumar et al, as the key step in the synthesis of local anaesthetic levobupivacaine " (339) (Scheme 14.102). 

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