The photochemistry of 1-cyclohexenyl-2,4,6-trimethylphenyl-methanone (1-mesitoyl-cyclohexene)

The photochemistry (direct irradiation in solution, lambda > 300 nm) of the title compound (8) has been investigated. Whereas, in contrast to the meth yl-free analog of 8, only undefined material of higher molecular weight was obtained upon irradiation without additives in acetonitrile and toluene, t his situation changed dramatically in the presence of co-reactants. Thus, i n ethyl vinyl ether, [4+2] adducts 11a and 11b were obtained in good combin ed yield. In acetonitrile, in the presence of acid which catalyses a skelet al rearrangement, the hexahydrofluorenone 14 was the main product. The resu lts point to the intermediacy of the trans-cyclohexene isomer of 8, viz. 9, and to the 2-oxyallyl 10 as consecutive intermediates, in analogy to the m ethyl-free system. The molecular structures of some unprecedented minor by- products such as 15a-c even more clearly reveal their origin from 10. Photo reactions of 8 in the presence of either water or phenol as co-reactants pr oceeded less cleanly but did give defined products such as 16-18 in moderat e yields. (C) 2001 Published by Elsevier Science B.V.