[CH3:1][O:2][C:3](=[O:14])[C:4]1[CH:9]=[CH:8][C:7]([NH2:10])=[C:6]([C:11](=[O:13])[CH3:12])[CH:5]=1.S(=O)(=O)(O)O.[N:20]([O-])=O.[Na+]>C(O)(=O)C>[CH3:1][O:2][C:3]([C:4]1[CH:5]=[C:6]2[C:7](=[CH:8][CH:9]=1)[NH:10][N:20]=[CH:12][C:11]2=[O:13])=[O:14]To a solution of 3-acetyl-4-amino-benzoic acid methyl ester (2063 mg, 10.68 mmol) in acetic acid (39 mL) was added sulfuric acid (6.5 mL) on an ice bath, then, an aqueous solution (6.5 mL) of sodium nitrite (922 mg, 13.35 mmol) was added thereto, followed by stirring on the ice for 1 hour, and at room temperature for 2 days. The reaction mixture was concentrated until it reached half of the amount, water was added, then an aqueous solution of 2N sodium hydroxide was added on an ice bath to adjust the pH to 5. The insoluble matter was separated by filtration, then, the filtrate was extracted with ethyl acetate, the organic layer was dried over sodium sulfate and then evaporated. Diethyl ether was added to the resulting residue for solidification, the resulting solid was washed with diethyl ether, and 365 mg of the title compound (1.78 mmol, 16.6%) was obtained.
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