Photoinduced Cyclization of (o-Alkylbenzoyl)phosphonates to Benzocyclobutenols.

(o-Alkylbenzoyl)phosphonates readily cyclize to highly strained benzocyclobutenols simply upon irradiation with UV light. The remarkable efficiency is ascribed to the electron-accepting character of the phosphonate substituent, which facilitates thermal ring closing of the o-quinodimethane intermediate and suppresses reversion to the starting carbonyl compound.