Photochemical transformations of the [4 + 2] dimeric adduct of 2-benzoylnorborna-2,5-diene and its derivatives

Abstract

Irradiation of the [4 + 2] dimeric adduct (7) of 2-benzoylnorborna-2,5-diene yields the [2 + 2] dimeric adduct (9). The head–head combination in structure (9) has been established by its conversion into the known 3,3′-dibenzyl-2,2′-binorborn-5-ene (12). Irradiation of tetrahydro-derivative (8) of the adduct (7), however, gives the retrodiene product, 2-benzoylnorborn-2-ene (16), identified by comparison with synthetic material. The intermediacy of a cyclobutane (19) in the formation of compound (16) was ruled out by the isolation of the cyclobutanol (20) as the major product from irradiation of (19). Mechanistic pathways for these transformations are discussed.