Step I: Synthesis of methyl 3-hydroxy-1-methyl-1H-pyrazole-5-carboxylate (3_41_2)
Triethylamine (66 mL, 474.4 mmol) was added to a mixed solution of methylhydrazine sulfate (30.4 g, 211 mmol) in water (150 mL) and methanol (300 mL). The reaction mixture was stirred at room temperature for 0.5 h. Dimethyl butynedioate (30 g, 211 mmol) was subsequently added. The reaction system was warmed up to 70 °C with continuous stirring for 18 hours. After completion of the reaction, the mixture was allowed to stand at room temperature for two days, the precipitated solid was collected by filtration and dried to give 12 g of the target product. The filtrate was concentrated, cooled in an ice bath and filtered again to collect the solids and dried to give an additional 5 g of the target product 3_41_2 (total 17 g, 52% yield) as a yellow solid.
NMR (400 MHz, DMSO-d6): δ = 3.79 (s, 3H), 3.88 (s, 3H), 6.01 (s, 1H), 10.05 (s, 1H).
