Stereoselective reactions. I. A highly efficient asymmetric synthesis of .BETA.-substituted aldehydes via 1,4-addition of Grignard reagents to optically active .ALPHA.,.BETA.-unsaturated aldimines.

The 1, 4-addition of Grignard reagents to the chiral α, β-unsaturated aldimines (3d, e), prepared from α, β-unsaturated aldehydes (1) and optically active tert-leucine tert-butyl ester (2d), afforded, after hydrolysis, optically active β-substituted aldehydes (4) in 91-98% enantiomeric excess. The present method has advantages in giving aldehydes (4) in high enantiomeric purities, allowing easy preparation of the aldimines as well as easy recovery of optically active tert-leucine tert-butyl ester (2d) without any racemization for reuse, and exhibiting general utility. The possible mechanism of the reaction, by which the absolute configuration of the aldehydes (4) is unequivocally predictable, is proposed.