Asymmetric syntheses, superbases

Surprisingly, there have been only few synthetic studies on polymer-supported asymmetric superbase reagents. Recently, Wannaporn and Ishikawa prepared a new chiral guanidine based polymer catalyst and applied it to the asymmetric Michael addition reaction of iminoacetate with methyl vinyl ketone [39] (Scheme 6.7). Although the catalyst shows only moderate levels of reactivity and enantioselectivity, the result demonstrates the possibility of expanding an exciting field of asymmetric synthesis using polymer-supported chiral superbase catalysts. 

In this chapter, the synthetic utility of Proton Sponge (1) was reviewed. This superbase, although not a main player, is indispensable for various mild and selective transformations in organic synthesis. Despite the unique characteristics of superbases, their exploitation is still limited. Recently, various types of proton sponges, including chiral ones, have been developed, and are likely to have a wide range of applications in organic and asymmetric synthesis. 

Preparation and use of supported TMG (1) as a novel base catalyst is discussed in a review elsewhere [6]. Heterogeneous guanidines are provided as environmentally friendly base catalysts and, thus, precise discussion on supported superbases is given in Chapter 6. Guanidine chemistry has been excellently surveyed in books [7]. This chapter focuses on the synthetic utility of TMG (1) and its analogues in organic synthesis and application of modified guanidine to asymmetric reactions. 

In this chapter, applications of amidine, guanidine and phosphazene superbases to the synthesis of natural products have been discussed. Many structurally complex natural products have been synthesized efficiently and elegantly by making use of the reactions described. Currently, much attention is focussed on the development of chiral superbases and their application to asymmetric reactions. Such catalytic asymmetric reactions are expected to offer exciting and efficient new approaches to the synthesis of natural products and biologically active compounds. 

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