C([NH:4][N:5]1[CH2:10][C:9]([CH3:11])=[N:8][NH:7][C:6]1=[O:12])(=O)C.Cl.O.[OH-].[Na+]>CO>[NH2:4][N:5]1[CH2:10][C:9]([CH3:11])=[N:8][NH:7][C:6]1=[O:12]85 g of 4-acetylamino-6-methyl-3-oxo-2,3,4,5-tetrahydro-1,2,4-triazine in 250 ml of methanol are stirred together with 63 ml of hydrochloric acid (37%) for 4 hours at 50° C. After cooling to 5° C., 250 ml of icecold water and 125 ml of sodium hydroxide solution (50%) are added. The solvent is removed by evaporation in vacuo. The residue is taken up in 100 ml of ethanol, and the solvent is removed again in vacuo. The residue is taken up in 1200 ml of acetonitrile, undissolved salt precipitates are removed by filtration, the filtrate is concentrated to a small volume, the residue is caused to crystallize by adding 100 ml of acetic acid ethyl ester at 0° C., and the colorless crystals are filtered off and dried yielding 60.4 g (92% of the theoretical amount) of the title compound having a melting point of 118°-120° C.
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