Ethyl 1,3-benzothiazole-5-carboxylate (5.30 g, 25.6 mmol) was used as a raw material, which was dissolved in a solvent mixture of methanol (150 mL), tetrahydrofuran (40 mL) and water (5 mL). To this solution was added 50% aqueous sodium hydroxide (10 mL). The reaction mixture was stirred at room temperature for 18 hours and then concentrated. The concentrated residue was dispersed in a solvent mixture of water (300 mL) and ether (200 mL), and the organic layer was separated and discarded. Concentrated hydrochloric acid was added to the aqueous layer to adjust the pH to 4, followed by extraction with ethyl acetate (3 x 300 mL). The organic extracts were combined, washed with brine (200 mL), dried over anhydrous sodium sulfate and concentrated to give 4.30 g of benzothiazole-5-carboxylic acid in 94% yield.
