ON THE PHOTOISOMERIZATION OF BICYCLODIENONES

Article abstract of DOI:10.1016/S0040-4020(01)92380-4

Two bicyclo<3.2.n>dienones, which differ from one another only by the presence or absence of an O atom as the "n" bridge, show very different photoisomerization behavior.Irradiation of hexamethyl-8-oxabicyclo<3.2.1>octadienone 12 gives endo-6-acetylpentamethylbicyclo<3.1.0>hexenone 16, probably via an initial <1,3> sigmatropic rearrangement (Scheme 3).In contrast, hexamethylbicyclo<3.2.0>heptadienone 14 photoisomerizes via an initial <3,3>-sigmatropic rearrangement, the isolated product being the relatively stable cyclopentadiene-ketene 26.The methylene analog of 14 (i.e.32) rearranges similarly, giving allene 33 and alkyne 34.The mechanisms of these photoisomerizations are discussed.