Photochemistry of substituted cyclic enones. Part 5. 3-Aryl-5-(3-phenylprop-2-enyl)cyclopent-2-enones

The E- and Z- compounds (1) and (2; Ar = Ph, 4-MeOC6H4, and 4-NCC6H4), were synthesised by standard procedures. The E- and Z-isomers set up a rapid photochemical equilibrium prior to formation of tricyclic [2 + 2]-cycloaddition products, 6-aryl-4-exo- and endo-phenyltricyclo[3.2.1.03,6]octan-2-ones, (3) and (4) respectively. The exo-tricyclic ketones (3) undergo further photolysis to afford the cyclobutanecarbaldehydes (6), which rearrange thermally to the enol ethers (7). The 6-endo-phenyl isomers (4) do not photolyse further. Boron trifluoride-assisted rearrangement of compounds (4) affords the cyclohexanones (9) and, in the case of the 4-cyanophenyl compound, the cyclohexenone (10c). Similar rearrangement of compound (3b) affords the ketone (8b), but compound (3c) gives no corresponding ketone (8c) but rather, traces of the monocycle (1c).