Photochemical reactivities of cyclic α‐phenyl‐β,γ‐enones. Singlet 1,3‐acyl shift, decarbonylation and unquenchable oxa‐di‐π‐methane reactions upon direct irradiation

The photochemistry of the α‐phenyl‐β,γ‐enones 3–6 has been studied under conditions of direct (λ 300 nm) and triplet‐sensitized irradiation. Upon direct irradiation, 3 exhibits decarbonylation, a 1,3‐acyl shift (1,3‐AS) and an oxa‐di‐π‐methane (ODPM) rearrangement. Under the same conditions, 4 , on the other hand, affords only the ODPM product whereas 5 gives the decarbonylation, 1 ,3‐AS and ODPM products. The formation of the ODPM product of 5 is efficiently quenched by triplet quenchers in contrast to the ODPM products of 3 and 4 , the quantum yields of which are not reduced upon the addition of typical triplet quenchers. Triplet‐sensitized irradiation of 5 leads to the ODPM rearrangement, whereas surprisingly 3 and 4 are unreactive. It is suggested that the ODPM rearrangement, upon direct irradiation of 3 and 4 , proceeds from the S 2 (ππ*) state. The ODPM product formed upon direct irradiation of 5 is thought to be formed from the T 1 (ππ*) state populated by ISC from the S 1 ( n π*) state. Upon direct irradiation, 6 exhibits a reversible 1,3‐AS and in addition ketene formation via an intramolecular 1,5‐H shift. Under conditions of triplet photosensitization, compound 6 appears to be stable. Given the apparently different photochemical behaviour of these constrained compounds, a general reactivity pattern for the direct and sensitized irradiation cannot be defined.