General procedure for the synthesis of 5-amino-1H-pyrazol-3-one from 5-oxo-2,5-dihydro-1H-pyrazole-3-carboxylic acid: to a reaction flask was added toluene (500 mL), triethylamine (138.6 mL, 1.0 mol, 2.0 eq), and 5-hydroxy-4-carboxylic acid ester (64 g, 0.5 mol, 1.0 eq) made in step (1). After raising the temperature of the reaction system to 80°C, diphenylphosphoryl azide (DPPA, 162.5 mL, 0.75 mol, 1.5 eq) was slowly added and stirred continuously for 5 hours at this temperature. After the reaction was completed, the system was cooled to room temperature. The reaction solution was concentrated, 500 mL of saturated aqueous ammonium chloride solution was added, the precipitated white precipitate was collected and dried to give 5-amino-1,2-dihydro-pyrazol-3-one 44.7 g in 90% yield.
