Solid-state photodimerization of cholest-4-en-3-one

Crystalline cholest-4-en-3-one undergoes solid-state dimerization by UV radiation to give:two ring A - ring A connected dimers. No dimerization occurs in solution. The first dimer, characterized by a cyclobutane ring, is formed by connection of C-2 and C-3 of a moiety with C-5 ' and C-6 ' of another moiety, respectively. The latter dimer has a six-membered ketal ring formed by connection of C-2 with C-5 ' and of O, linked to C-3, with C-3 '. The structures have been determined by spectroscopic means. X-ray analysis of title compound evidences the proximity of the axial H-2 of a molecule to the C-4 ' of a molecule in the upper layer. The transfer of the hydrogen and the connection between C-2 and C-5 ' might be the driving force of dimerization.