Stereoselective Photodimerization of 3-Arylindenones in Solution and in the Solid State.

Photodimerization of 3-arylindenones in solution and in the solid state was examined. Irradiation of 3-arylindenones in benzene exclusively gave C2-symmetric anti-HH cyclobutane dimers in good yields. In contrast, photolysis in the solid state afforded syn-HH cyclobutane dimers efficiently, which was considerably influenced by the molecular arrangement in the crystal lattice.