Intramolecular (3 + 2) Cycloaddition of Olefins to Aromatic Aldehydes Mediated by Visible Light and a Lewis Acid Catalyst

Benzaldehydes carrying an alkenyloxy substituent in the 3-position undergo a photocycloaddition reaction, in which the olefin adds intramolecularly to the carbonyl carbon atom and to the carbon atom C2 of the benzene core forming a cyclopentane ring. The reaction was shown to proceed upon AlBr3 catalysis (10 mol %) by irradiation with visible light (λ = 435 nm). A total of 30 tricyclic products (44–94% yields) were obtained with high simple and facial diastereoselectivity. Transient absorption spectroscopy and mechanistic experiments suggested that both the uncatalyzed reaction at λ = 280–340 nm and the Lewis acid-catalyzed reaction occur from the triplet state.