General procedure for the synthesis of 1,2-dihydro-3H-pyrazol-3-one from methyl propargylate: Hydrazine hydrate (3.62 mL, 48.53 mmol) was slowly added dropwise to a solution of methyl propargylate (4.23 mL, 47.58 mmol) dissolved in methanol (40 mL) under cooling conditions in an ice bath. The reaction mixture was stirred at room temperature for 30 min. Subsequently, brine (10 mL) was added and methanol was removed by distillation under reduced pressure. The remaining aqueous layer was extracted with ethyl acetate (4 × 75 mL), the organic layers were combined, dried with anhydrous magnesium sulfate, filtered and concentrated under reduced pressure to give 1,2-dihydro-3H-pyrazol-3-one (3.82 g, 95% yield) as a milky white solid. Nuclear magnetic resonance hydrogen spectrum (400 MHz, DMSO-d6): δ 5.44 (1H, d, J = 2.27 Hz), 7.35 (1H, d, J = 2.23 Hz), 9.50 (1H, br s, NH), 11.45 (1H, br s, NH); nuclear magnetic resonance carbon spectrum (101 MHz, DMSO-d6): δ 89.25, 129.71, 160.73; MS (electrospray positive ionization mode): m/z 85.04 (100, [M+H]+); high-resolution MS (electrospray ionization): measured value of [M+H]+ 85.03951, calculated value of C3H5N2O 85.03964.
