SYNTHESIS AND PHOTOCHEMICAL REARRANGEMENTS OF BICYCLIC CROSS-CONJUGATED CYCLOHEXADIENONES CONTAINING DELTA-OXY SUBSTITUENTS

Several 6/5-fused cross-conjugated cyclohexadienones containing delta-oxy substituents on the B ring including the benzoyl, SEM, and TBDPS hydroxyl-protected systems 12b, 12f, and 12g were prepared from the corresponding enones by the phenylselenenylation-selenoxide elimination procedure. Attempted preparation of dienones 12a, 12c, and 12d containing tert-butyl, methyl, and trimethylsilyl hydroxyl-protecting groups failed. Instead, the phenolic aldehyde 13 was obtained after workup or column chromatography on silica gel. The THP-protected dienone 12e could be purified and characterized, but it gave aldehyde 13 on standing in acetic acid at room temperature. Irradiations of dienones 12b, 12f, and 12g in glacial acetic acid using a high-pressure mercury lamp housed in a Pyrex probe gave mixtures containing several photoproducts. The corresponding 5/6-fused acetoxy ketones 22a, 22c, and 22b were the major products in each case. The 5/6-fused hydroxy ketone 23 was also obtained from dienone 22a. Mixtures of tricyclic ketones of the type 27 and 28 resulting from rare examples of trapping of Zimmerman-Schuster cyclopropyl carbocation intermediates, i.e., 26, by the solvent were also obtained from all three dienones. Dienones 12g and 12f also gave tricyclic acetoxy ketones 31a and 31b, which presumably arose via a 1,4-sigmatropic rearrangement of an excited-state precursor of 26.