Bismuth(III) triflate-catalyzed synthesis of substituted 2-alkenylfurans

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.