Remote steric effect on the regioselectivity of Sharpless asymmetric dihydroxylation

Studies on the regioselectivities for the Sharpless asymmetric dihydroxylation (AD) of conjugated dienoates, trienoates, dienones and dienamides are described. Excellent regioselectivities were obtained in straight chain dienoates, all trienoates, ketones and amides. The remote branched iso-propyl and tert-butyl groups of dienoates greatly lowered the normally excellent regiocontrol. This observation is rationalized in terms of substrate conformational changes, and the steric interaction between the branched methyl group of iso-propyl or tert-butyl groups and the ethyl group on the (DHQD)₂PHAL ligand.