Novel photorearrangement of a 4-chloromethylazulene

1-Chloro-4,6,8-trimethylazulene is formed almost exclusively upon UV irradiation of a deaereted benzene solution of 4-chloromethyl-6,8-dimethylazulene. This photoproduct was identified by 1H and 13C NMR spectroscopy of both the isolated photoproduct and the one prepared by a thermal route. The mechanism proposed involves the recombination of the intimate radical pair initially formed by the photohomolysis of the CCl bond, followed by a sigmatropic [1,9] hydrogen shift.