[OH-].[K+].NC1[S:5][C:6]2[C:12]([C:13]([O:15]C)=[O:14])=[CH:11][CH:10]=[CH:9][C:7]=2[N:8]=1>O.O1CCOCC1>[NH2:8][C:7]1[C:6]([SH:5])=[C:12]([CH:11]=[CH:10][CH:9]=1)[C:13]([OH:15])=[O:14]28.9 g of potassium hydroxide (85%), dissolved in 38 ml of water, are added dropwise to 3.5 g of methyl 2-amino-benzothiazole-7-carboxylate in 38 ml of dioxane under a nitrogen atmosphere in the course of 20 minutes, while stirring and cooling with ice, such that the internal temperature of 25° C. is not exceeded. Thereafter, the mixture is refluxed at a bath temperature of 140° C., and towards the end of the reaction the internal temperature is increased to 170° C. and dioxane is distilled off by means of a descending condenser. The mixture is then cooled to 0° C. and filtered under a nitrogen atmosphere and the residue is rinsed with 30 ml of ice-water. The title compound can be isolated from the filtrate by acidification to pH 5.5, with severe cooling and stirring at a maximum of 0° C., and extraction with ethyl acetate/tetrahydrofuran (8:2) and washing with concentrated sodium chloride solution. Because of the risk of the formation of the disulfide, the potassium salt in the filtrate (potassium 3-amino-2-mercapto-benzoate) is preferably further reacted directly.
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