Free radical annulation of dichlorocyclobutanones with sequential ring expansion

A new synthetic sequence that adds the elements of ketene across the termini of a 1,5-diene is described. Dichlorocyclobutanone adducts from reaction of dichloroketene with 1,5-dienes undergo intramolecular free radical annulation. Lewis acid-promoted ring-opening of the cyclized cyclobutanones then yields novel ring expansion products. Reaction of (R)-carvone provides an unusual example leading to the formation of a tricyclic dione. The carbonyl group of carvone enhances the radical cyclization and influences the ring-opening pathway.