Photocyclization of a conjugated triaryl ‘Y-enyne’

Upon irradiation at 350 nm, in the presence/absence of oxygen, ‘Y-enyne’ 1 undergoes electrocyclic ring closure to photoproduct 4. A mechanism involving an allene intermediate is proposed. In nonpolar solvents a [1,5] H shift affords the photoproduct, while in methanol protonation of the central allenic carbon occurs. Quantum yields of the photoreaction in the different solvents were measured.