General procedure for the synthesis of 2-amino-6-hydroxybenzothiazole from 1-(4-hydroxyphenyl)thiourea: 1-(4-hydroxyphenyl)thiourea (5 g, 29.7 mmol) was dissolved in chloroform (30 mL) in a 100 mL two-necked round-bottomed flask fitted with a stirrer. The chloroform (15 mL) solution of bromine (5 g, 29.7 mmol) was transferred to a dropping funnel and slowly added dropwise to the reaction mixture under stirring while controlling the reaction temperature to be lower than 5 °C. After the dropwise addition, the reaction was continued with stirring for 4-6 hours. Subsequently, the reaction mixture was refluxed on a water bath until no more hydrogen bromide gas escaped. After completion of the reaction, the resulting solid was treated with ice water and filtered. The filtrate was neutralized with ammonia solution and the precipitated white precipitate was filtered and washed with water. Finally, the product 2-amino-6-hydroxybenzothiazole was recrystallized from ethanol to give a white solid powder (4 g, 80% yield). The structure of the product was confirmed by 1H NMR (DMSO-d6, 400 MHz): δ 9.10 (s, 1H, -OH), 7.13 (d, J = 8.4 Hz, 1H, ArH, C5-H), 7.06 (s, 2H, -NH2), 7.01 (d, J = 2.4 Hz, 1H, ArH, C4-H), 6.65 (dd, J = 2.6 , 8.6 Hz, 1H, ArH, C7-H).
