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