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General procedure for the synthesis of 5-bromo-1-methyl-1H-pyrazole-3-carboxylic acid methyl ester from 1-methyl-5-hydroxypyrazole-3-carboxylic acid: firstly, 1-methyl-5-hydroxypyrazole-3-carboxylic acid methyl ester (64 mmol) was dissolved in acetonitrile (200 mL) and then phosphorus tribromide (POBr3, 322 mmol) was added. The reaction mixture was transferred to a sealed flask and heated at 80 °C for 15 hours. Upon completion of the reaction, the mixture was cooled to room temperature and subsequently slowly poured into a saturated aqueous sodium bicarbonate solution pre-cooled to 0 °C. The reaction solution was extracted three times with ethyl acetate and the organic phases were combined. The organic phase was decolorized by activated carbon, dried with anhydrous sodium sulfate, filtered, and the filtrate was concentrated under reduced pressure to afford a brown oily product, which solidified to the target compound 5-bromo-1-methyl-1H-pyrazole-3-carboxylic acid methyl ester in a yield of 11.5 g and 72% yield after standing. The product was analyzed by LC/MS with ESI m/z (M+H)+ = 220.2.

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