Lewis base activation of Lewis acids: Catalytic, enantioselective addition of glycolate-derived silyl ketene acetals to aldehydes

(Chemical Equation Presented) A catalytic system involving silicon tetrachloride and a chiral, Lewis basic bisphosphoramide catalyst is effective for the addition of glycolate-derived silyl ketene acetals to aldehydes. It was found that the sense of diastereoselectivity could be modulated by changing the size of the substituents on the silyl ketene acetals. In general, the trimethylsilyl ketene acetals derived from methyl glycolates with a large protecting group on the α-oxygen provide enantiomerically enriched α,β-dihydroxy esters with high syn-diastereoselectivity, whereas the tert-butyldimethylsilyl ketene acetals derived from bulky esters of α-methoxyacetic acid provide enantiomerically enriched α,β-dihydroxy esters with high anti-diastereoselecitvity.