Substituted α-Alkylidene Cyclopentenones via the Intramolecular Reaction of Vinyl Cations with Alkenes

Substituted alpha-alkylidene cyclopentenones are formed in up to 93% yield by the intramolecular capture of vinyl cations with pendent alkenes. An increased level of substitution at the beta-position of the beta-hydroxy-alpha-diazoketone starting material changed the course of the reaction to instead give a lactone product. A reaction path that involves bond reorganization via an acylium ion intermediate is proposed to explain these results. Substrate scope studies showed that more stable vinyl cations gave higher alpha-alkylidene cyclopentenone yields. This study provides a mild and efficient method to form alpha-alkylidene cyclopentenones that complements C-H insertion and Nazarov cyclization strategies.