Step A: Trimethylsilylated diazomethane (12.0 mL, 24 mmol, 2.0 M hexane solution) was slowly added dropwise to a solution of 1-methyl-4-pyrazolecarboxylic acid (0.59 g, 4.7 mmol) in methanol (20 mL) at 0 °C through an addition funnel. The reaction mixture was stirred at the same temperature for 20 min followed by concentration. The crude product was partitioned between ethyl acetate (100 mL) and water (50 mL). The organic phase was dried over anhydrous sodium sulfate, filtered and concentrated. The crude product was purified by fast column chromatography using hexane/ethyl acetate (4:1, v/v) as eluent to afford methyl 1-methyl-1H-pyrazole-4-carboxylate (0.50 g, 3.6 mmol, 76% yield) as a white solid. Mass spectrum (APCI-positive ion mode) m/z: [M+1]+ = 141.1.
