To a solution of ethyl 3-bromopyrazole-4-carboxylate (5.6 g, 25.57 mmol) in DMF (200 ml) was added iodomethane (14.52 g, 102.28 mmol, 6.37 ml) and cesium carbonate (33.32 g, 102.28 mmol). The reaction mixture was stirred at 25°C for 12 hours. After completion of the reaction, the mixture was diluted with deionized water (1000 ml) and extracted with ethyl acetate (500 ml). The organic layer was subsequently washed with saturated brine (500 ml x 3), dried over anhydrous sodium sulfate and concentrated under reduced pressure. The residue was initially purified (4 g) by preparative HPLC (alkaline conditions). Further purification by silica gel column chromatography (eluent: petroleum ether/ethyl acetate = 10/1 to 5:1) afforded ethyl 5-bromo-1-methyl-1H-pyrazole-4-carboxylate (1 g, yield: 16.8%) and ethyl 3-bromo-1-methyl-1H-pyrazole-4-carboxylate (2 g, yield: 33.6%), both as white solid.
