Boron tribromide → 6-Bromo-5-hydroxy-1H-indole-2-carboxylic acid ethyl ester

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The solution of 8.30 g (27.8 mmol) 6-bromo-5-methoxy-1H-indole-2-carboxylic acid ethyl ester (prepared according to J. Org. Chem. 1974, 39, 3580) in 160 mL dichloromethane was cooled to −78° C. At this temperature, 55.7 mL boron tribromide (55.7 mmol; 1M solution in dichloromethane) were added. The solution was allowed to warm to room temperature and after 30 min. the solution was poured on 10% aqueous sodium bicarbonate solution, the phases were separated and the aqueous phase was extracted three times with ethyl acetate. The combined organic layers were washed with water followed by brine, dried over magnesium sulfate, filtered and evaporated. The residue was flash-chromatographed on silica gel with n-hexane:ethyl:acetate (2:1 v/v) as eluant to give 5.7 g (72%) of the product as a light yellow solid.

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