[CH3:1][C:2]1[CH:11]=[CH:10][C:9]2[NH:8][C:7](=[O:12])[C:6](=[O:13])[NH:5][C:4]=2[C:3]=1[C:14]([O:16][CH3:17])=[O:15].[N+:18]([O-])([O-:20])=[O:19].[K+]>OS(O)(=O)=O>[CH3:17][O:16][C:14]([C:3]1[C:4]2[NH:5][C:6](=[O:13])[C:7](=[O:12])[NH:8][C:9]=2[CH:10]=[C:11]([N+:18]([O-:20])=[O:19])[C:2]=1[CH3:1])=[O:15]To a solution of 6-methyl-2,3-dioxo- 1,2,3,4-tetrahydro-quinoxaline-5-carboxylic acid, methyl ester (1.11 g, 4.74 mmol) in conc. H2SO4 (15 mL) at room temperature was added in one portion with vigorous stirring potassium nitrate (0.529 g, 5.23 mmol). The reaction mixture was stirred for 23 hours and poured over ice. The precipitated product was thoroughly washed with water upon collection to give 1.28 g (97%).
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