Intramolecular [2 + 2] cycloaddition and sequential ring expansion

Free radical ring expansion of fused-cyclobutanones gives bicyclic ketones expanded by three and four carbons, with the carbonyl group β to the ring junction. Four-carbon ring expansion shows stereoselectivity favoring the trans-fused product 6-trans. The radical precursor cyclobutanones are readily prepared by intramolecular [2 + 2] cycloaddition of ketenes or keteniminium salts to olefins.