Thionyl chloride (0.72 mL, 9.80 mmol) was slowly added dropwise to a solution of 3,4-diaminobenzoic acid (1.00 g, 6.58 mmol) in methanol (15.0 mL) for a controlled time of 10 minutes. The reaction mixture was stirred at room temperature for 4 hours before the solvent was removed by evaporation under reduced pressure. The residue was partitioned between water and ethyl acetate (EtOAc) to separate the organic layer. The organic layer was diluted with ethyl acetate and washed sequentially with saturated aqueous sodium bicarbonate, saturated brine and water and then dried with anhydrous magnesium sulfate. The desiccant was removed by filtration and the solvent was evaporated under reduced pressure to afford methyl 3,4-diaminobenzoate (993 mg, 95.0% yield) as a brown solid. The product characterization data were as follows: 1H NMR (CDCl3, 500 MHz) δ 7.49 (d, 1H, J = 7.01 Hz, Ar-H), 7.47 (s, 1H, Ar-H), 6.68 (d, 1H, J = 7.01 Hz, Ar-H), 3.87 (s, 3H, -COOCH3), 3.80 (br s, 2H, -NH2 ), 3.35 (br s, 2H, -NH2); 13C NMR (CDCl3, 125 MHz) δ 166.56, 140.89, 138.16, 120.87, 118.23, 118.16, 52.11; LCMS m/z calculated value (MNa)+ 189.06, measured value 189.00.
