Upon irradiation a regio- and stereoselectivecycloaddition occurred intramolecularly between two acyl-1,4-benzoquinone moieties connected by a polymethylene bridge. The regio- and stereoselectivities are different from those of the corresponding intermolecularcycloaddition; this was interpreted in terms of the Diels-Alder reaction between an acyl-1,4-benzoquinone moiety and a photochemically produced