Photocycloaddition reactions of 6,6′-dimethyl-4,4′-[bis(methylenoxy)phenylene]-di-2-pyrones (4a–c) with benzophenone (2a) by mixing in the solid state (solid solution) afforded the corresponding oxetane derivatives (5a–c; 1:2 adducts) with high site- and regioselectivity across the C5–C6 and C5′–C6′ double bonds in 4 via the triplet excited state of benzophenone. The oxetane formation proceeded more