General procedure for the synthesis of 6-hydroxy-1,2,3,4-tetrahydroquinoline from 6-methoxy-1,2,3,4-tetrahydroquinoline: 6-methoxy-1,2,3,4-tetrahydroquinoline (3.00 g, 18.38 mmol) was dissolved in 50 mL of dichloromethane, and boron tribromide (13.81 g, 55.14 mmol) was slowly added at 0 °C. The reaction system was then warmed up to 15 °C with continuous stirring for 2 hours. After completion of the reaction, the reaction solution was concentrated and the reaction was quenched with 5 mL of methanol. To the reaction mixture, 20 mL of saturated sodium bicarbonate solution was added and stirred for 30 minutes. The reaction was extracted three times with 20 mL of ethyl acetate, the organic phases were combined and dried with anhydrous sodium sulfate. The organic phase was concentrated to give the crude product. The crude product was purified by silica gel column chromatography (eluent ratio: petroleum ether/ethyl acetate from 3:1 to 1:1 gradient elution) to finally obtain 6-hydroxy-1,2,3,4-tetrahydroquinoline (1.95 g, yield 71.11%). The structure of the product was confirmed by 1H NMR (400 MHz, DMSO-d6): δ 8.24 (s, 1H), 6.22-6.35 (m, 3H), 4.93 (br.s, 1H), 3.02-3.10 (m, 2H), 2.57 (t, J = 6.4 Hz, 2H), 1.67-1.80 (m, 2H).
