Photochemical and thermal reactions of heterocycles. Part 2. Photolysis of 2,3-diphenylnaphthoquinone 2,3-epoxide: trapping of a cyclic carbonyl ylide and photoisomerisation

Abstract

Photolysis of 2,3-diphenylnaphthoquinone epoxide (1) caused epoxy-carbon–carbon bond cleavage to form a cyclic carbonyl ylide (4), which was trapped by dipolarophiles. The norbornadiene primary adduct (3) underwent further photochemical isomerisation, via methylenephthalide derivatives (5), to spiro[oxetan-phthalide] derivatives (7), which reverted thermally to methylenephthalides (5). Thermal fragmentation of an oxabicycloheptadiene adduct (18a) gave a formal acetylene adduct (11) and a furan (21). Carbon-13 n.m.r. spectroscopy was useful in establishing the structures of the products. Photolysis of the epoxide (1) in the absence of a dipolarophile gave 3-(α-benzoylbenzylidene)phthalides (24).