Photochemistry of benzotriazole: an unprecedented tautomer-selective intermolecular [2+2] photocycloaddition.

[reaction: see text]. Irradiation of benzotriazole with a variety of maleimide derivatives leads to the stereo- and regioselective formation of aryl [2 + 2] photocycloaddition products. Further studies with 2-alkyl benzotriazole derivatives indicates that in the case of the parent benzotriazole this cycloaddition proceeds selectively via the 2H-tautomer.