[CH3:1][O:2][C:3]1[CH:11]=[C:10]([NH2:12])[C:9](S)=[CH:8][C:4]=1[C:5]([OH:7])=O.[Na].C(O[S:18]([O-])(=O)=O)C.Cl.CCO[CH2:26][CH3:27]>O>[CH3:1][O:2][C:3]1[CH:11]=[C:10]([NH2:12])[C:9]([CH2:26][CH3:27])=[CH:8][C:4]=1[C:5]([OH:7])=[S:18]159 g of 2-methoxy-4-amino-5-mercaptobenzoic acid, 355 cm3 of water and 160 cm3 of 30% sodium hydroxyde solution are placed in a flask fitted with a condenser. The mixture is heated until the solid dissolves, then 123 g of ethylsulfate is added. The mixture is heated to reflux, treated with 10 cm3 of 30% sodium hydroxyde solution, then heated to reflux for 1 hour. After cooling, 800 cm3 of water is added and the solution is filtered. The precipitate obtained by adding 100 cm3 of concentrated hydrochloric acid in the presence of ether is drained, washed with water and dried. 162 g of 2-methoxy-4-amino-5-ethylthio benzoic acid is obtained (yield=88%).
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