Cinchona‐Derived Chiral Phase‐Transfer Catalysts for Other Asymmetric Synthesis

Abstract

Phase-transfer catalysis has been recognized as one of the most powerful synthetic tools for establishing practical protocols because it offers several advantages, such as operational simplicity, mild conditions with aqueous media, suitability for large-scale reaction, and an environmentally benign nature. Since the pioneering studies on asymmetric alkylation promoted by chiral phase-transfer catalysts (PTCs) [1,2], this research area has served as an attractive area for the pursuit of green chemistry, and many types of chiral quaternary ammonium salts for various asymmetric phasetransfer catalyses have been developed over the past few decades [3,4]. For example, two-center organocatalysis using bis-ammonium salts derived from tartrate represents a new concept in asymmetric PTC chemistry [5].In particular, it is not only the cinchona alkaloids that are suitable chiral sources for asymmetric organocatalysis [6], but also the corresponding ammonium salts. Indeed, the latter are particularly useful for chiral PTCs because: (1) both pseudo enantiomers of the starting amines are inexpensive and available commercially; (2) various quaternary ammonium salts can be easily prepared by the use of alkyl halides in a single step; and (3) the olefin and hydroxyl functions are beneficial for further modification of the catalyst. In this chapter, the details of recent progress on asymmetric phase-transfer catalysis are described, with special focus on cinchonaderived ammonium salts, except for asymmetric alkylation in a-amino acid synthesis. Asymmetric Darzens ReactionThe Darzens reaction is a fundamental reaction, the products of which are extremely useful for the synthesis of other molecules. Despite these advantages, the reaction Asymmetric Phase Transfer Catalysis. Edited by Keiji Maruoka