X-Ray crystallographic determination of the conformation of bicyclo[3.3.0]octane and bicyclo[3.2.0]heptane derivatives and evidence for an O ⋯ CO interaction

The crystal structures of (3a), (3b), and (6) have been determined and show that in both ring systems the cyclopentane ring takes up an endo conformation with the substituents pseudo-axial; this gives rise to trans-annular O ⋯ CO interactions, which are also seen in related epoxides.