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0 °C

To a solution of methyl 5-nitro-1H-pyrazole-3-carboxylate (1 g, 5.84 mmol) in acetone (30 mL) was added potassium carbonate (4.04 g, 29.2 mmol) and 1,2-dibromoethane (3.29 g, 1.51 mL, 17.5 mmol) and the resulting solution heated to reflux for 2 h. The resulting mixture was cooled to 0° C. filtered and concentrated and the residue purified by flash chromatography (silica gel, 60 g, 20% to 40% ethyl acetate in hexanes) to give methyl 1-(2-bromoethyl)-3-nitro-1H-pyrazole-5-carboxylate (1.03 g, 63%) as a white solid.

1. temperaturethe resulting solution heated
2. temperatureto reflux for 2 h
3. filtrationfiltered
4. concentrationconcentrated
5. customthe residue purified by flash chromatography (silica gel, 60 g, 20% to 40% ethyl acetate in hexanes)