Lewis Acid Induced Conjugate Addition of Alkenes to α,β-Unsaturated Ketones or Aldehydes

Abstract

α,β-Unsaturated ketones or aldehydes form a 1:2 complex with ethylaluminum dichloride which reacts with alkenes either intermolecularly or intramolecularly to give a zwitterion. The zwitterion collapses reversibly to a cyclobutane in geometrically favorable cases and undergoes hydride and alkyl shifts to generate a,d-unsaturated carbonyl compounds. The intramolecular reactions proceed with high regio- and stereospecificity (e.g, 18 → 20). The reaction is quite general for a variety of enone and alkene substitution patterns. Other Lewis acids do not give similar reactions. Use of less than 1 equiv of EtAlCl2 in intramolecular reactions gives concerted ene reactions in geometrically favorable cases and complex mixtures of products where an ene reaction cannot occur. In one case, transfer of the β hydrogen of the ethyl group to the carbenium ion of the zwitterion results in reduction to give a saturated ketone. © 1980, American Chemical Society. All rights reserved.