Photorearrangements of Epoxy Ketones. Epoxides of Hexamethylbicyclohepta-2,5-dienone, a Valence Tautomer of Hexamethyltropone

Article abstract of DOI:10.1021/jo01299a012

The double bond in the four-membered ring of hexamethylbicyclo<3.2.0>hepta-3,6-dien-2-one (1) is selectively oxidized by m-chloroperbenzoic acid, whereas the enone double bond is selectively epoxidized by alkaline hydrogen peroxide to give 2 and 3, respectively.In each case, a single stereoisomer is formed.The exo stereochemistry of the epoxides was deduced from X-ray structures on diepoxides 4 and 5 obtained by further epoxidation of 2 and 3.Irradiation of 2 gives pentamethylphenol (6) and ketene; if 2 is labeled at the C₄ methyl with deuterium, the resulting 6 is labeled para to the hydroxyl group.A novel rearrangement of the four-membered ring to give a 7-norbornenyl-type intermediate is suggested to rationalize the labeling result.Irradiation of 3 gives, in addition to minor amounts of pentamethylphenol and its acetate, a tetracyclic product formulated as 10, which rearranges thermally to 1-acetylpentamethylcyclopentadiene (12).A mechanism involving photochemical C-C bond cleavage of the epoxide ring in 3 is suggested to account for the results.