[N:1](OCCC(C)C)=[O:2].[CH3:9][N:10]1[C:14](=[O:15])[CH:13]=[C:12]([CH3:16])[NH:11]1.C[O-].[Na+]>C(O)C>[CH3:9][N:10]1[C:14](=[O:15])[C:13](=[N:1][OH:2])[CH:12]([CH3:16])[NH:11]1183 g (1.56 mol) of isopentyl nitrite are added dropwise to 175 g (1.56 mol) of 1,3-dimethyl-pyrazolin-5-one and 84.4 g (1.56 mol) of sodium methylate in 1 L of absolute ethanol, while stirring and cooling with ice so that the internal temperature does not rise above 25° C. to 30° C. When the addition has ended, stirring is continued at room temperature for 24 hours and the sodium salt of 1,3-dimethyl-4-hydroximino-pyrazolin-5-one which has precipitated is filtered off with suction. The crystalline product is dissolved in 1 L of water and the solution is acidified with glacial acetic acid. For complete precipitation, the mixture is cooled at 0° C. for several hours, and the product is then filtered off with suction. 162 g (74% of theory) of 1,3-dimethyl-4-hydroximino-pyrazolin-5-one of melting point 93° C. are obtained.
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