PHOTOTRANSFORMATIONS OF 9-ETHYL-SUBSTITUTED DIBENZOBARRELENE

Photolysis of benzoyl-9,10-dihydro-9-ethyl-9,10-ethenoanthracene (1) i n benzene or acetone in the complete absence of oxygen gives the 4b-et hyl-substituted dibenzosemibullvalene (5) (45-48%) whereas, in the pre sence of oxygen, 1 in acetone gives a mixture of 5 (32%), together wit h two oxygenated products, namely the peroxy compound 12 and the lacto ne 20. The structures of 5,12 and 20 have been confirmed through singl e-crystal X-ray diffraction studies. Compound 5 arises through a di-pi -methane rearrangement of 1, whereas 12 is formed through the oxygen q uenching of a diradical intermediate, derived from an isomeric 8b-ethy l-substituted dibenzosemibullvalene (6). Compound 20 arises through th e oxygen quenching of a diradical intermediate, implicated in the phot orearrangements of 1,2-dibenzoylalkenes.