Solid-state photodimerization of steroid enones

Androst-4-ene-3,17-dione and 17a-methyltestosterone are dimerized in the solid-state by UV radiation. These substances were selected by a search in the Cambridge Structural Database among the steroid enones presenting in the crystalline state an intermolecular short contact between a hydrogen alpha to a carbonyl group and the oxygen of an enone system. Dimerization occurs by transfer of the hydrogen to the oxygen and connection between the two involved carbons. Androst-4-ene-3,17-dione affords dimer and trimer, both formed by connection of the C16 of a molecule with the C3 of a near one. Irradiation of 17a-methyltestosterone gives two isomeric trienones. These compounds are reasonably formed by dehydration of unisolated intermediate products obtained by coupling of two molecules through a linkage between the C2 and the C3' carbons. The formation mechanisms of the photoproducts are satisfactory explained on the basis of the molecular arrangement of the monomers in the crystal state. Modelling of the dimeric molecules was done using molecular mechanics calculations. Single crystal X-ray analysis of the dimer of androst-4-ene-3,17-dione confirms the structural interpretation of spectral data. The conformer found in the solid state well agrees with the results of molecular mechanics calculations.