Studies on the stereoselectivity of sulfur ylide mediated epoxidation and related reaction

Abstract

Epoxides are important functional groups in synthesis because they undergo stereospecific nucleophilic ring opening to yield bifunctional compounds. So, stereoselective syntheses of them are attracting much attention. We examined various solvent effects on the diastereoselectivity of the sulfur ylide mediated epoxidation reaction under salt free condition and discovered that diastereoselectivity is influenced by solvating power of solvents. It was found out that polar solvent produced higher trans/cis ratio of the epoxidation. However, protic solvents reduce diastereoselectivity. We also investigated diastereoselectivity related to the substituent on the epoxide. We found that vinyl aldehyde give trans epoxides exclusively under the condition using KHMDS as the base. But, in case of alkyl aldehyde, we got the mixture of epoxide under same condition. In catalytic cycle, in principle, the sulfide can be catalytic. However, with decreasing amounts of sulfide, it is observed that the yield of epoxide is reduced and yield of stilbene is increased. This indicated that sulfide was not turning over within the catalytic cycle and was being held intermediate stage. Also it is hard to synthesize the chiral sulfide which was used by Aggarwal. So, to overcome the drawbacks of this cycle, we operated epoxidation reaction.