[Na].[N+:2]([C:5]1[C:6]([O:17][C:18]2[CH:23]=[CH:22][CH:21]=[CH:20][CH:19]=2)=[C:7]([S:14](O)=O)[CH:8]=[C:9]([CH:13]=1)[C:10]([OH:12])=[O:11])([O-])=O.Cl>C(O)C.[Zn]>[NH2:2][C:5]1[C:6]([O:17][C:18]2[CH:19]=[CH:20][CH:21]=[CH:22][CH:23]=2)=[C:7]([SH:14])[CH:8]=[C:9]([CH:13]=1)[C:10]([OH:12])=[O:11]Zinc powder (38 g) is added to a warm solution (70° C) of the mono sodium salt of 5-nitro-4-phenoxy-3-sulfinobenzoic acid (10 g) in ethanol (190 ml), and while stirring 5 N hydrochloric acid (190 ml) is added dropwise. The reaction mixture is kept under nitrogen for an additional 2 hours at 70° C. After cooling and filtration, the ethanol is distilled off in vacuo. After standing in a refrigerator for 16 hours, the precipitated material is collected by filtration and washed with concentrated hydrochloric acid (15 ml). This material is then suspended in water (300 ml) and the pH adjusted to 2 by addition of 2 N sodium hydroxide. The obtained crude 5-amino-3-mercapto-4-phenoxybenzoic acid is collected and recrystallized from aqueous ethanol. The acid is obtained with a melting point of 199°-200° C.
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