To a solution of dichloromethane (700 mL) containing traces of turbid 5-bromo-3-chloropyridine-2-carboxylic acid (60 g, 183.2 mmol), N,N-dimethylformamide (1 mL) and oxalyl chloride (24.9 mL, 286.9 mmol) were added slowly and dropwise. The reaction mixture was stirred at room temperature for 3 hours. Upon completion of the reaction, the resulting yellow solution was cooled to 10 °C and methanol (30.8 mL, 761.3 mmol) was slowly added dropwise at this temperature, keeping the reaction temperature between 15 °C and 20 °C. After the dropwise addition, the reaction mixture was continued to be stirred at room temperature overnight. At the end of the reaction, the reaction mixture was neutralized with saturated aqueous sodium bicarbonate solution, followed by washing the organic layer with brine. The organic layer was dried over anhydrous sodium sulfate, filtered and concentrated to give methyl 5-bromo-3-chloro-2-pyridinecarboxylate (55 g) as a yellow solid, which could be used in the next reaction without further purification.LCMS (Method B): m/z 250/252/254 (M + 1), retention time 1.12 min.
