Enol silanes reaction with chiral acetals

Table 20 Ratio of Diastereoisomers in the TiCU-mediated reactions of Enol Silanes with Chiral Acetals...

Some of the most impressive advances in the area of catalytic, enantioselective aldol addition reactions have taken place in the development of catalytic methods for enantioselective acetate aldol additions, a reaction type that has long been recalcitrant. Thus, although prior to 1992 a number of chiral-auxiliary based and catalytic methods were available for diastereo- and enantiocontrol in propionate aldol addition reactions, there was a paucity of analogous methods for effective stereocontrol in the addition of the simpler acetate-derived enol silanes. However, recent developments in this area have led to the availability of several useful catalytic processes. Thus, in contrast to the state of the art in 1992, it is possible to prepare acetate-derived aldol fragments utilizing asymmetric catalysis with a variety of transition-metal based complexes of Ti(IV), Cu(II), Sn(II), and Ag(I). 

Titanium-based Lewis acids are widely used in the Mukaiyama reaction, i. e. the condensation of an enol silane and an aldehyde, giving 3-hydroxycarbonyl derivatives. In this context, BINOL-derived chiral titanium complexes were used in the condensation of thioester-derived ketene silyl acetals with a large range of aldehydes, in high yields and enantioselectivities (Scheme 7.39). ... 

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