Unexpected formation of 9,10-dibenzoylphenanthrene

When heated in the presence of atmospheric oxygen, either in diffuse room light or in the dark, phencyclone (1) undergoes autoxidation to afford 9,10-dibenzoylphenanthrene (3). The structure of 3 was elucidated via application of single crystal X-ray crystallographic techniques. A mechanism is suggested to account for the formation of 3 via ( i) stepwise 1,4-addition of O-3(2) to the 1,3-diene system in the cyclopentadiene moiety of 1 to form endoperoxide 5 followed by (ii) cheletropic extrusion of carbon monoxide from 5. The structure of 3 is described: P2(1)/C, a = 12.116(2), b = 9.6850(2), c = 16.839(3) Angstrom, beta= 100.331(3) degrees.