General procedure for the synthesis of 4-hydroxybenzothiazole from 4-methoxybenzothiazole: Boron bromide (3.09 mL, 1M DCM solution, 3.09 mmol) was slowly added dropwise to a stirring solution of 4-methoxybenzothiazole (510 mg, 3.09 mmol) in anhydrous dichloromethane (30 mL) at 0 °C. The reaction mixture was gradually warmed to 40 °C and stirred continuously overnight. Upon completion of the reaction, the solution was concentrated under vacuum and subsequently diluted with water and 2N hydrochloric acid. The aqueous phase was neutralized to pH 7 with sodium bicarbonate and then extracted with ethyl acetate (3 x 100 mL). The organic extracts were combined, dried with magnesium sulfate, and the solvent was removed under vacuum. The resulting oil was purified by fast chromatography on a silica gel column with the eluent ratio tapered from hexane:ethyl acetate [4:1] to hexane:ethyl acetate [7:3], resulting in 4-hydroxybenzothiazole as a brown solid (730 mg, 80% yield). The NMR hydrogen spectrum (300 MHz, CDCl3) data were as follows: δ 7.59 (1H, s, hydrogen on the thiazole ring), 7.46 (1H, dd, aromatic hydrogen), 7.36 (1H, t, aromatic hydrogen), 7.02 (1H, dd, aromatic hydrogen).
