The development of effective new approaches to substituted cyclopentenones constitutes a worthwhile synthetic pursuit because of their ubiquitousness in nature and their usefulness as synthetic building blocks. The sequence developed several years ago in our laboratory, consisting of regio- and stereoselective [2 + 2] cycloaddition of dichloroketene with olefins, followed by diazomethane ring expansion and dehydrochlorination, leads efficiently to α-chlorocyclopentenones ( ...