Polyene Cyclization Promoted by the Cross-Conjugated α-Carbalkoxy Enone System. Observation on a Putative 1,5-Hydride/1,3-Alkyl Shift under Lewis Acid Catalysis

Abstract

Polyene cyclization of compounds 3 and 4 under catalysis with AlCl3 and/or SnCl4 gave rise to complex bicyclic products 8 and 9, structures of which were highly unexpected, and X-ray analyses were invoked for unambiguously structural identification. Mechanistically, a tandem sigma-bond rearrangement process, including an unusual through-space 1,5-hydride or 1,3-alkyl shift as a key operation, is proposed.