Enantioselective Construction of Quaternary Stereogenic Carbons by the Lewis Base Catalyzed Additions of Silyl Ketene Imines to Aldehydes

The development of catalytic, enantioselective methods for the construction of quaternary stereogenic centers represents a continu-ing challenge in organic chemistry.1 Creating these centers rapidly and selectively is difficult because of the steric repulsion that is encountered in the C-C bond-forming event. Moreover, achieving high levels of enantiotopic face selectivity is difficult because of the relatively similar steric environments presented by the non-hydrogen substituents. The need for more general methods is underscored by a growing number of biologically active natural products and pharmaceutical agents that possess quaternary ste-reogenic carbon atoms. Quaternary stereogenic centers can be generated through the aldol addition of R,R-disubstituted enolates to aldehydes, and given the edifice of work on catalytic, enantioselective aldol-type reactions, this strategy seems logical.2 However, this approach is limited b