Stereoselective Cyclopropanation of 2‐[(S)‐(4‐Methylphenyl)sulfinyl]cyclopent‐2‐en‐1‐one with Sulfur Ylides and α‐Halo Carbanions. Preliminary Communication

Abstract

AbstractThe cyclopropanation of the title compound (S)‐2 with various sulfur ylides has been examined. The reaction with methylenesulfonium ylides gave the corresponding cyclopropanes 4 with low diastereoselectivity. The formation of the second product 5 arising from the subsequent methylenation of the CO group was also observed. A clean cyclopropanation of (S)‐2 took place with ethyl (dimethylsulfanylidene)acetate affording the cyclopropanes 6, with high π‐facial selectivity, but low endo/exo ratio. A high endo/exo selectivity, but low π‐facial selectivity was observed in the reaction of (S)‐2 with (2‐ethoxy‐2‐oxoethyl)(diphenyl)sulfonium tetrafluoroborate. The use of α‐bromoacetate carbanion as the cyclopropanation reagent resulted in the formation of 6 with very high facial and endo/exo‐selectivity. In a proposed explanation of the stereochemical outcome of the cyclopropanations investigated, the ground‐state conformation of the sulfoxide 2 and the transition‐state structure of the initial addition step were taken into account.