Steric influence of silyl groups in titanium- and lithium-mediated aldol condensations

Reactions of lithium enolates of α-silyloxyketones 1a, 1b, 2a, and 2b with benzaldehyde yield opposite diastereoselection depending upon the size of the silyl group, but all four titanium enolates lead to high diastereofacial selectivities. These results implicate a nonchelated transition structure which is insensitive to the steric size of the silyl group for the titanium-mediated aldol reactions. Ketone 1a is shown to give high selectivities for the two opposite syn adducts (99:1 with Ti; 10:90 with Li). Because it is readily prepared in monochiral form, 1a may offer advantages as a chiral precursor for preparation of either product stereochemistry, as desired.