Synthetic Approaches to α-Methylene γ-Lactones via Cycloadditions of Ketenes

Methylchloroketene was cycloadded to several cycloalkenes and cycloalkadienes to produce fused substituted cyclobutanones which can be transformed by Baeyer-Villiger oxidation into lactones. Exocyclic elimination of HC1 from the latter produces ring-fused a-methylene 7-lactones 4. This route, adaptable to a larger scale, serves as a three-step fair-yield synthesis of 4 from cyclic olefins.