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4-Bromo-3-nitro-1H-pyrrolo[2,3-b]pyridine (500 mg, 2.07 mmol) was used as starting material, which was dissolved in 48% aqueous hydrobromic acid (4 mL) and heated at 70 °C. Subsequently, tin(II) chloride dihydrate (2.26 g, 10 mmol) was added in batches. After the addition was completed, the reaction was continued at 70 °C for 1 hour. Upon completion of the reaction, the mixture was cooled and carefully poured into a stirred ice-water mixture (15 mL). The pH of the reaction mixture was adjusted to 12 with sodium hydroxide solution and the insoluble material was separated by filtration. The filtrate was extracted with dichloromethane (3 x 100 mL), the organic phases were combined, washed sequentially with water and saturated aqueous sodium chloride solution, dried over anhydrous magnesium sulfate and concentrated under reduced pressure. The target product 4-bromo-1H-pyrrolo[2,3-b]pyridin-3-amine (230 mg, 52.5% yield) was obtained, which could be used in the subsequent reaction without further purification.

[1] Patent: WO2013/114113, 2013, A1. Location in patent: Page/Page column 26
[2] Patent: US2015/11533, 2015, A1. Location in patent: Paragraph 0113
[3] Patent: WO2007/76423, 2007, A2. Location in patent: Page/Page column 154-155