General procedure for the synthesis of 4-bromo-8-hydroxyquinoline using 4-bromo-8-methoxyquinoline as starting material: 1.5 g (0.006 mol) of 4-bromo-8-methoxyquinoline (CAS No. 103028-31-5) was dissolved in 10 ml of 48% aqueous hydrobromic acid (CAS No. 10035-10-6). The reaction mixture was stirred under reflux conditions for 24 hours. Upon completion of the reaction, 50 ml of water was added for dilution, followed by neutralization of the reaction solution with 2 M aqueous sodium hydroxide solution. The neutralized solution was extracted with dichloromethane (2 x 50 mL). The organic phases were combined, dried with anhydrous magnesium sulfate and subsequently concentrated under reduced pressure. The crude product was purified by rapid chromatography on a silica gel column with the mobile phase being a solvent mixture of dichloromethane and methanol (initial ratio dichloromethane:methanol = 90:5, gradient elution with the proportion of methanol increased from 5% to 15%). Finally, 0.84 g of the target product 4-bromo-8-hydroxyquinoline was obtained in 59% yield. The product was analyzed by LC-MS (Method 2): retention time (Rt) = 0.94 min; mass-to-charge ratio (m/z) = 225.09 ([M+H]+).1H-NMR (300 MHz, DMSO-d6) data were as follows: δ [ppm] = 7.16-7.19 (m, 1H), 7.51-7.61 (m, 2H), 7.93 (d, 1H), 7.93 (d, 1H), 8.64 (d, 1H). 1H), 8.64 (d, 1H), 10.09 (s, 1H).
