The effect of methyl groups on the photolysis of dienones: pentamethylcyclohexa-2,4-dienones

Abstract

Pentamethylbenzene has been oxidised with peroxytrifluoroacetic acid–boron fluoride in methylene chloride to give mainly a mixture of pentamethylcyclohexa-2,4-dienones. The 2,4,5,6,6,- and 2,3,4,6,6-pentamethyl isomers were formed in yields of 33 and 37%, respectively, and the 3,4,5,6,6-pentamethyl isomer was formed in 7% yield. The 2,3,5,6,6-pentamethyl isomer was not isolated. The oxidation mechanism suggested is consistent with these results. The three dienones all gave Diels–Alder adducts with maleic anhydride, and these assist in the structure elucidation. Duroquinone, 2,3,4,4,6-pentamethylcyclohexa-2,5-dienone, and p-hydroxytetramethylbenzaldehyde have all been isolated from the oxidation in yields of about 7%. Irradiation of 2,3,4,6,6-pentamethylcyclohexa-2,4-dienone in ether or methanol afforded a bicyclo[3,1,0]hexane derivative. This reaction is thermally reversible. The 2,4,5,6,6- and 3,4,5,6,6-pentamethyl isomers were photochemically inert in ether, but in methanol the latter underwent ring opening to give a methyl dienoate while the former compound gave two major products—a bicyclo[3,1,0]hexane derivative and a second methyl dienoate.