Bismuth(III) Triflate-Catalyzed Synthesis of Substituted 2?Alkenylfurans

摘要

A convergent synthesis of the title compounds is reported, which relies on a successive 2-fold S_N′-type substitution reaction at methoxy-substituted propargylic acetates. The furan C3?C4 bond is presumably established by silyl enol ether attack at a propargylic cation intermediate. The resulting α-methoxyallene is intramolecularly substituted, leading to cyclization by displacement of the methoxy group (O?C2 bond formation) and to simultaneous formation of the exocyclic alkene double bond. Despite the relatively mild conditions, the reactions were complete within 5 min.