Photochemical rearrangement of bicyclo[3,3,1]nona-3,7-diene-2,6-diones

Abstract

Bicyclo[3,3,1]nona-3,7-diene-2,6-dione (1c) rearranges photochemically to the triasteranedione (2)(tetracyclo-[3,3,1,0^2,8,0^4,6]nonane-3,7-dione) by successive 1,2-acyl shifts and to a dihydrocoumarin by a 1,5-acyl shift. The 4,8-dimethyl analogue (1a) similarly gives a substituted triasteranedione (7) and a dimethyldihydrocoumarin, but the 4,8-dimethyl-9-phenyl compound (1b) gives, in addition to a triasteranedione (11), the epimeric diones (14) and (15)(4,8-dimethyl-6-phenylbicyclo[3,3,1]nona-3,7-diene-2,9-diones), products of a 1,3-acyl shift; no product of a 1,5-acyl shift is observed. Mechanistic aspects of these and related rearrangements are discussed.