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A mixture of 104 g (1 mol) of ethyl hydrazinoformate, 200 g of phenol, 50 ml of dimethyl formamide and 106 g (1 mol) of sodium carbonate is initially introduced under a water-jet vacuum and a solution of 171 g (1 mol) of tetramethylchloroformamidinium chloride in 200 g of phenol is added in such a manner that a temperature of 40° C. is not exceeded, water being gradually distilled off. Then the mixture is heated slowly, 55 g (1.1 mol) of hydrazine hydrate are added at 80° C. and the mixture is heated under atomspheric pressure in the course of 3 hours until phenol is refluxed (at about 190° C.), dimethylamine being eliminated. The phenol is then distilled off under a water-jet vacuum, the residue is treated with 750 ml of (hot) ethoxyethanol, the salt is separated off by filtration under heat and the filtrate is cooled. The crystalline product obtained in this manner is isolated by suction filtration. 49g (34% of theory) of 4-amino-5-dimethylamino-2,4-dihydro-3H-1,2,4-triazol-3-one of melting point 205° C. are obtained. ##STR350##

1. additionis initially introduced under a water-jet vacuum
2. distillationwater being gradually distilled off
3. temperatureThen the mixture is heated slowly
4. distillationThe phenol is then distilled off under a water-jet vacuum
5. additionthe residue is treated with 750 ml of (hot) ethoxyethanol
6. customthe salt is separated off by filtration
7. temperatureunder heat
8. temperaturethe filtrate is cooled
9. customThe crystalline product obtained in this manner
10. customis isolated by suction filtration