General procedure for the synthesis of methyl 5-oxo-2,5-dihydro-1H-pyrazole-3-carboxylate from dimethyl acetylenedicarboxylate: to a solution of hydrazine monohydrate (44.8 g, 894 mmol) in toluene (300 mL) was added acetic acid (180 mL), followed by the slow addition of dimethyl acetylenedicarboxylate (100 mL, 813 mmol). The reaction mixture was stirred at room temperature for 2 hours. Upon completion of the reaction, the mixture was slowly poured into ice-cooled water. The resulting precipitate was collected by filtration, washed sequentially with water and hexane, and dried to afford methyl 5-oxo-2,5-dihydro-1H-pyrazole-3-carboxylate (92.2 g, 80% yield) as a white solid. The product was characterized by 1H NMR (DMSO-d6): δ 3.77 (s, 3H), 5.96 (br s, 1H), 10.04 (br s, 1H), 12.79 (br s, 1H).
