Photochemistry of benzotriazoles: generation of 1,3-diradicals and intermolecular cycloaddition as a new route toward indoles and dihydropyrrolo[3,4-b]indoles.

Irradiation of benzotriazoles 1a-e at λ = 254 nm in acetonitrile solution generated the corresponding 1,3-diradicals which underwent intermolecular cycloaddition with maleimides to afford the corresponding dihydropyrrolo[3,4-b]indoles and with acetylene derivatives to afford indoles as the major products. This offers an interesting and simple access to such ring systems of potential synthetic and biological interest. The structures of the photoproducts were established spectroscopically and by single crystal X-ray crystallography.