Oxalyl chloride (3.43 mL, 39.9 mmol) was slowly added dropwise to a mixed solution containing DMF (30 mL) and dichloromethane (80 mL) at 0 °C while maintaining stirring. After the dropwise addition was completed, stirring was continued at 0 °C for 20 min. Subsequently, a solution of ethyl 5-bromoindole-2-carboxylate (10 g, 37.3 mmol) dissolved in DMF (80 mL) was added. The reaction mixture was transferred to room temperature and stirred continuously for 24 hours. Upon completion of the reaction, the mixture was poured into saturated aqueous sodium bicarbonate solution and extracted with dichloromethane. The organic phases were combined, washed sequentially with water and saturated saline, dried over anhydrous sodium sulfate and concentrated under reduced pressure. Finally, the target product ethyl 5-bromo-3-formyl-1H-indole-2-carboxylate was obtained by ethanol recrystallization in a yield of 8.9 g and 81%.
