(E)-1-ethoxy-3-trifluoromethyl-1,3-butadiene (ETFBO, 270 g, 1.61 mol, 1.00 eq.) was dissolved in methanol (750 mL). The resulting solution was cooled in an ice-water bath, followed by the slow dropwise addition of methylhydrazine (88.3 mL, 77.7 g, 1.69 mol, 1.05 eq.) over 20 min. The reaction mixture was heated to 55 °C and stirred in dry air (via a drying tube) at this temperature for 20 hours. Upon completion of the reaction, the solvent was removed by rotary evaporation and the residue was subjected to decompression distillation: the first fraction was collected at 150 mbar, bath temperature of 130 °C, and collection temperature of 73-85 °C to give 129 g of the mixture of isomers, and the second fraction was collected at 70 mbar, bath temperature of 150 °C, and collection temperature of 75-85 °C to give 24 g of 1 -methyl-3-trifluoromethyl-1H-pyrazole. the total yield was 153 g (1.02 mol), 63% yield.
