[CH3:1][O:2][C:3]1[CH:11]=[C:10]([O:12][CH3:13])[CH:9]=[C:8]2[C:4]=1[C:5](=[O:15])C(=O)[NH:7]2.OO.Cl.C(O)(=[O:21])C>[OH-].[Na+]>[NH2:7][C:8]1[CH:9]=[C:10]([O:12][CH3:13])[CH:11]=[C:3]([O:2][CH3:1])[C:4]=1[C:5]([OH:15])=[O:21]
To a mixture of 4,6-dimethoxyindoline-2,3-dione (20.28 g, 97.9 mmol) in 30% NaOH solution (100 mL) at 100° C. was carefully dropped a 50% H2O2 solution. It was heated at 100° C. for 20 minutes. It was cooled down and neutralized by concentrated HCl to pH 8, followed by acetic acid to pH 5 to form a solid. It was filtered, washed with water, and dried under vacuum with P2O5 to afford 2-amino-4,6-dimethoxybenzoic acid as a yellow solid (15.034 g, 78%). 1H NMR (300 MHz, DMSO-d6) δ 6.00 (d, 1H), 5.85 (d, 1H), 3.83 (s, 3H), 3.77 (s, 3H), 3.41 (br, 2H); LC-MS (ESI) m/z 198 (M+H)+.
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