Cyclic allenes or diradicals in the cyclization reactions of dienynes generated by photolysis of alkynylcyclohexadienones

Reported here are some rearrangements involving the electrocyclic ring closure of dieneynes 7. Such ring closures are envisaged to possibly give strained substituted cyclic allenes 8 which could also behave as diradicals 8a. The results show that compounds such as 5 rearrange to cyclohexadienones 9a, 9b, or 11 through these kind of intermediates. Theoretical calculations performed on simple models similar to the intermediates suggest that the nature of these intermediates correspond to that of cyclic allenes.