Highly stereoselective epoxidation of α-methyl-γ-hydroxy-α,β-unsaturated esters : Rationalization and synthetic applications

The diastereoselectivity of the nucleophilic epoxidation of γ-hydroxy-α,β-unsaturated esters having a methyl substituent at the α- or β-position was investigated. Epoxidation of the α-methyl-substituted enoate was highly stereoselective, giving rise to the syn isomer. This finding was used to perform an enantioselective synthesis of a natural product having a β-hydroxy-α-methylene-γ-butyrolactone motif. The nucleophilic epoxidation of enoates was found to be irreversible. Models to explain the observed stereoselectivities are proposed.