Steroid photochemistry : The photocycloaddition of an enone and dienone to norbornene and norbornadiene

The photocycloaddition of a steroidal 4-en-3-one and a 4,6-dien-3-one to bicyclo2.2.1]heptene and bicyclo2.2.1]heptadiene has been studied. The photocycloaddition to norbornene furnished only the trans-4α,5β-cyclobutane, while the enone gave a mixture of cis and trans-2+2] adducts as well as the first example of hydrogen transfer in this series to form a 4-(2-norbornanyl 1)-conjugated enone. The photocycloaddition of the dienone to norbornadiene formed the 4-(7-norbornenyl)-dienone by a 1,4-hydrogen shift as the major product, together with lesser amounts of the unrearranged trans-4α,5β-2+2]-cyclobutane. On the other hand, the cycloaddition of the enone to norbornadiene yielded the 4α-(7-norbornene)-β,γ-enone as the major product; accompanied again by the unrearranged trans-2+2]-adduct. In addition, the 4α,5α-cis-3+2] adduct was obtained and was formed by rearrangement of the norbornenyl radical portion of the intermediate diradical.