Photochemistry of benzotriazoles

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Abstract
We have studied the photolysis of benzotriazoles with the following 1-substituents: 2-pyridyl, 3-methyl-2-pyridyl, isoquinolin-1-yl, thiazol-2-yl, and benzothiazol-2-yl. Condensed heterocycles are obtained by elimination of nitrogen from the benzotriazolyl group and selective ring closure on the nitrogen of the heterocyclic substituent. Benzimidazo[2,1-a] isoquinoline and pyrido[1,2-a]- and thiazolo[3,2-a]-benzimidazoles are formed. The reaction is different from the classical acid-catalysed decomposition of benzotriazoles in which the cyclization does take place on the nitrogen atom of the heterocycle except when the other position is blocked by a substituent: this steric requirement is necessary to obtain the same product by photolysis as by thermoiysis but the yields are lower.

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