Generation of 2-substituted 2-metallo-1,3-dithiane derivatives and their coupling with 2,3-disubstituted oxiranes

2-Substituted 2-lithio-1,3-dithiane derivatives have been used as an excellent acyl anion equivalent for carbon-carbon bond formation. One of the representative examples is their coupling with monosubstituted oxiranes. We found that 2-substituted 1,3-dithiane derivatives were lithiated with n-BuLi at room temperature and the resulting anions reacted with 2,3-disubstituted oxiranes at room temperarure, giving the coupling products in satisfactory yield. This type of coupling reaction has not been used in natural product syntheses, because the temperature-dependent instability of dithianide anions and the poor electrophilicity of 2,3-disubstituted oxiranes should be discouraging factors. This article describes effective metallation conditions of 2-substituted 1,3-dithiane derivatives and the practical synthesis of four stereoisomers of bis-propionates via the dithiane coupling with oxiranes.