The general procedure for the synthesis of 4-amino-5-(ethylsulfonyl)-2-methoxybenzoic acid using amisulpride impurity 4 as a starting material was as follows: 30% hydrogen peroxide was slowly added to isopropanol (4.84 L) at ambient temperature containing a catalytic amount of sodium tungstate (0.0082 kg) in 4-amino-2-methoxy-5-ethylthiomethyl benzoate (X) (1.21 kg) solution. The reaction mixture was stirred at 40-45 °C for 3-4 hours and subsequently cooled to 5-10 °C. To the reaction mixture, 5% sodium thiosulfate solution (0.06 kg dissolved in 18.15 L of water) was added and stirring was continued for 60 min to produce methyl 2-methoxy-4-amino-5-ethylsulfonylbenzoate (XI) in situ. Subsequently, sodium hydroxide (1.00 kg dissolved in 10 L of water) was added to the reaction mixture, warmed to 60-65 °C and stirred for 2-3 hours. Upon completion of the reaction, the reaction material was cooled and the pH was adjusted to 4.0-4.5 with dilute hydrochloric acid (1:10). the product was isolated by diafiltration in 82% yield and 99% purity.
