Dihalo-substituted cyclopentanones: Synthetic equivalents of cyclobutenes in Diels-Alder reactions

2,2-Dichlorocyclopentanone and 2,5-dibromocyclopentanone react with various dienes in the presence of base to afford cycloadducts derived from [4+3]cy cloaddition of the corresponding cyclopentenyl oxyallylic cations. These cy cloadducts are unstable but reduction with sodium borohydride affords alcoh ols which are stable and easily handled. Treatment of these alcohols with p otassium hydride results in the formation of cyclobutane containing product s. This transformation makes dihalogenated cyclopentanones synthetic equiva lents of a variety of substituted cyclobutenes and allows easy access to Di els-Alder adducts of various cyclobutanes, Further, the initial [4 + 3] cyc loadducts can be treated with carbon nucleophiles, and the resulting alkoxi des also undergo the quasi-Favorskii reaction.