The mechanism undoubtedly was a nucleophilic attack by ylide carbon (step-1) followed by ring closure with elimination of dimethyl sulfide (step-2). The cyclopropanation reaction is not stereoselective, either diastereomer 23 may be obtained from the diastereomeric mixture. The outcome of stereoselectivity totally depends on chiral substrate and solvent polarity. The carboethoxy methylene transfer to the unsaturated double bond is reminiscent of the methylene transfer with Dimethyl oxo-sulfonium methylide .