1,3-Dibromo-5,5-dimethylhydantoin → 2-(1-Bromoethyl)benzo[b]thiophene

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A solution of 9.78 g (60 mmol) of 2-ethylbenzo[b]thiophene in 10 ml of heptane is added, over 10 minutes, to a mixture of 9.43 g (33 mmol) of 1,3-dibromo-5,5-dimethylhydantoin and 0.49 g (3 mmol) of azobisisobutyronitrile in 117.5 ml of heptane preheated to 60° C. The mixture is maintained for 1 additional hour at this temperature. After cooling to 20° C., stirring is continued for 1 hour and then the 5,5-dimethylhydantoin which has precipitated is filtered off and washed with 10 ml of heptane. The filtrate is washed with an aqueous sodium thiosulphate solution (3 g/100 ml) and then with 3 times 100 ml of water. After drying over sodium sulphate, the heptane phase is concentrated to approximately 40 ml and then cooled to -10° C. for one hour and a half. The precipitate formed is filtered off and dried under partial vacuum at 20° C. to provide 9.05 g of the desired compound in the form of a beige powder, which represents a yield of 57% (M.p.: 53° C., purity by liquid phase chromatography: 92%). A second, less pure sample is obtained by concentrating the mother liquors (16%).

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