A novel photodimerization of 4-aryl-4H-pyrans for cage compounds

Irradiation of 4-aryl-4H-pyrans in either the solid state or in solution gave rise to the formation of novel oxa-cage compounds, 3,9-dioxatetraasteranes, derived from a double [2 + 2] cycloaddition reaction. Meanwhile, an unexpected oxetane formed by the Paternò–Büchi reaction of benzophenone with the pyrans, was established by 1H NMR data as well as by X-ray crystallographic analysis. Experimental observations and theoretical calculations confirmed that the photodimerization was effectively catalyzed by the triplet excited state of benzophenone, while the Paternò–Büchi reaction was competitive with the process.