Photochemical, Catalyst-Free [2+2] Cycloaddition of Coumarin-3-carboxylates with Alkenes

Herein, we report a visible light-promoted, photocatalyst-free [2+2] photocycloaddition of coumarin-3-carboxylates with alkenes, resulting in an expeditious and efficient synthesis of cyclobutane-fused coumarins. The reaction was demonstrated on a gram scale and can also be performed under solar irradiation. The wide substrate scope, short reaction times, and high yields are highlights of this method. Preliminary mechanistic studies indicate that the reaction occurs through the involvement of an excited triplet state intermediate.