[NH2:1][C:2]1[CH:7]=[CH:6][C:5]([S:8]([OH:11])(=[O:10])=[O:9])=[C:4]([NH2:12])[CH:3]=1.[C:13](OC(=O)C)(=O)C.[OH-].[Na+].S(OC)(OC)(=O)=O.Cl.[Cl-].[Na+]>O.C(O)=O>[CH3:13][NH:12][C:4]1[CH:3]=[C:2]([CH:7]=[CH:6][C:5]=1[S:8]([OH:11])(=[O:9])=[O:10])[NH2:1]94 g of 1,3-diaminobenzene-4-sulfonic acid in 430 ml of water and 70 g of formic acid were boiled for 3 hours under reflux. The mixture was cooled to 20° C., and the resulting 3-(N-formylamino)aniline-4-sulfonic acid was filtered off in vacuo and stirred in 500 g of water with the addition of caustic soda solution to give a pH of 6.5. 70 g of acetic anhydride were then added and the pH was kept between 5.0 and 6.5 by the addition of caustic soda solution. After 30 minutes, the pH was adjusted to 12.5 with caustic soda solution, and 75 g of dimethyl sulfate were added dropwise. After 30 minutes, the pH was adjusted to -0.5 with hydrochloric acid, and the mixture was boiled under reflux for 3 hours. After cooling, the mixture was adjusted to pH 1.0 with caustic soda solution and salted out with 150 g of sodium chloride. The precipitated 3-(N-methylamino)aniline- 4-sulfonic acid was isolated by filtration in vacuo.
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