Lewis base activation of Lewis acids. Vinylogous aldol addition reactions of conjugated N,O-silyl ketene acetals to aldehydes

N,O-Silyl dienyl ketene acetals derived from unsaturated morpholine amides have been developed as highly useful reagents for vinylogous aldol addition reactions. In the presence of SiCl₄ and the catalytic action of chiral phosphoramide (R,R)-3, N,O-silyl dienyl ketene acetal 8 undergoes high-yielding and highly site-selective addition to a wide variety of aldehydes with excellent enantioselectivity. Of particular note is the high yields and selectivities obtained from aliphatic aldehydes. Low catalyst loadings (2-5 mol %) can be employed. The morpholine amide serves as a useful precursor for further synthetic manipulation.