Novel Phototransformations of Bridgehead-Dimethyl-Substituted Dibenzobarrelene. Structure of the Photoproducts

Article abstract of DOI:10.1021/jo00020a034

Single-crystal X-ray diffraction studies have revealed that the phototransformation products of 11,12-dibenzoyl-9,10-dihydro-9,10-dimethyl-9,10-ethenoanthracene (1) are correctly represented as 1,5-dibenzoyl-4,8-dimethyl-2,3:6,7-dibenzocyclooctatetraene (9), the dibenzopentalene derivative 13, and the hexacyclic peroxycarbinol 17 and not the earlier reported structures 1,4-dibenzoyl-5,8-dimethyl-2,3:6,7-dibenzocyclooctatetraene (3), the benzonaphthalene derivative 6, and the carbinol 5, respectively.Both singlet-state-mediated "tri-?-methane" rearrangement and the triplet-state-mediated "di-?-methane" rearrangement of 1 could give rise to a 1,4-diradical intermediate 8, which could then lead to the cyclooctatetraene 9 by a Grob-type fragmentation or undergo a 1,2-rearrangement to the dibenzopentalene 13.The hexacyclic peroxycarbinol 17 could arise through the oxygen quenching of the diradical intermediate 14, derived from 12, followed by extensive rearrangements.