Synthesis
General procedure for the synthesis of 2-aminobenzothiazole-7-carboxylic acid from 1-(3-carboxyphenyl)-2-thiourea: 3-isothiocyanatobenzoic acid (1.0 g, 5.6 mmol) was dissolved in 40 mL of ethanol and stirred for 1 h at room temperature while ammonia was passed into the solution. The resulting white precipitate was separated by filtration. The filtrate was concentrated to induce more white solid precipitate, which was again separated by filtration.The total yield of 3-thiodibenzoic acid was 1.06 g (97%). 3-Thiobisbenzoic acid (1.06 g, 5.41 mmol) was suspended in 25 mL of chloroform at room temperature and a solution of bromine (0.4 mL, 8.0 mmol) was added slowly and dropwise. The reaction mixture was heated to reflux for 3 hours followed by stirring at room temperature for 60 hours. The solvent was removed by evaporation and after addition of water, the resulting solid was separated by filtration. The crude product was washed with water and dried to give 1.03 g (98%) of 2-aminobenzothiazole-7-carboxylic acid as a gray solid.
