The photo-Nazarov cyclization of 1-cyclohexenyl phenyl ketone revisited. Observation of intermediates

Abstract

The photochemical transformation of 1-cyclohexenyl phenyl ketone ( 1) to the hexahydrofluorenone 2 (“photo-Nazarov cyclization”) was reinvestigated on the level of conventional preparative chemistry backed up by nuclear magnetic resonance (NMR) spectroscopy and deuteration experiments. Hitherto unreported intermediates were detected. These include the enol 8 and, in non-protic media, the crystalline dimers 3 and 4. Dimer 3, which was characterized by single-crystal X-ray analysis, is a kinetically stable non-conjugated enol which readily isomerizes to 4 on acid-base catalysis. Dimer 4 is photolysed to give ultimately 2. The enol 8 is sufficiently long-lived to be seen in the 1H NMR spectra of freshly irradiated solutions of 1 in CD 3OD and of 4 in CD 3OD or CD 3CN before it isomerizes to 2. Enol 3 resuls from trapping of the oxyallyl 7, the presumed precursor of the enol 8, by 8 in a novel type of reaction. Irradiation of 1 in the presence of 1,3-cyclopentadiene (CPD) gives adducts 10–16. Adducts 13–16 appear to be derived from 7, whereas 10, 11 and 12 possess the structures of Diels-Alder adducts of CPD to the trans-cyclohexene isomer of 1 ( 6). The intermediacy of 6 as the primary photoproduct is also suggested by futher circumstantial evidence.