Photorearrangements of bridgehead-disubstituted dibenzobarrelene esters and lactones

Abstract

This study deals with the photorearrangements of a few bridgehead-disubstituted dibenzobarrelene, i.e. dimethyl-9-hydroxymethyl-10-methyl-9,10-dihydro-9,10-ethenoanthracene-11,12-dicarboxylate (1) and the dibenzobarrelene lactone derivatives (10a,b). Irradiation of 1 in benzene gives a 77% yield of a dibenzocyclooctatetraene (8) with C₂ symmetry with respect to its substituents. In contrast, the photolysis of 10a yields the dibenzosemibullvalene (15a) in nearly quantitative yield. Similarly, the dibenzobarrelene lactone (10b) undergoes facile photoisomerization to the corresponding dibenzosemibullvalene (15b) in 92% yield. The structures of the photoproducts 8 and 15a have been unambiguously established through X-ray crystallographic analysis.