4.10 Enantioselective Aldol Reactions Catalyzed by Chiral Lewis Bases

Abstract

Recent, significant developments in aldol reactions are mostly based on the conventional Mukaiyama-type catalysis (activation of aldol acceptor) using various chiral Lewis acids as catalysts. In this chapter, base-catalyzed aldol reactions (activation of aldol donor) via hypervalent silicates as reaction intermediates are discussed. The trichlorosilyl enol ethers were invented as a new type of aldol donors for the enantioselective aldol reactions using chiral phosphoramides, phosphine oxides, or N-oxides as Lewis base catalysts, affording the aldol adducts in high diastereo- and enantioselectivities. The reductive in situ preparation of trichlorosilyl enol ethers from enones with trichlorosilane and the successive asymmetric aldol reactions (enantioselective reductive aldol reactions) afforded the aldol adducts without the isolation of the trichlorosilyl enol ethers. The enantioselective direct-type aldol reactions were accomplished by the in situ generation of silyl enol ethers from ketones with tetrachlorosilane and the following asymmetric aldol reactions. Base-catalyzed aldol reactions of other silyl enol ethers such as trimethoxylsilyl enol ethers and trimethylsilyl enol ethers are also discussed.