Methyl 5-bromopyridine-2-carboxylate (CAS 29682-15-3, 50 g, 0.23 mol) was mixed with m-CPBA (CAS 937-14-4, 80 g, 0.46 mol) in 400 mL of dry dichloromethane, heated to 60 °C and maintained the reaction for 20 hours. Upon completion of the reaction, the reaction was quenched with saturated sodium sulfite solution followed by extraction with ethyl acetate (2 x 200 mL). The organic layer was washed with brine (2 x 200 mL) and evaporated to dryness. The residue was purified by column chromatography (silica gel, 300 g, with a petroleum ether solution of 15% ethyl acetate as eluent) to give 5-bromo-2-(methoxycarbonyl)pyridine 1-oxide (30 g, 0.13 mol) as a brown oil. The oily substance was slowly added to phosphorus trichloride (CAS 10025-87-3, 80 mL) at 0 °C. The addition process lasted for 1 h. The mixture was then heated to 95 °C and kept for 1 h. The reaction was completed by evaporation. At the end of the reaction, the solvent was removed by evaporation and the residue was dissolved in water (50 mL) and extracted with ethyl acetate (3 x 50 mL). The organic layers were combined and evaporated to dryness to give the white solid product methyl 5-bromo-6-chloropyridine-2-carboxylate (19 g, 59% yield); mass spectrum (EI): m/e = 249.9 [M + H]+.
