A 1.0 M boron tribromide solution in dichloromethane (38.2 ml, 38.2 mmol) is slowly added dropwise to a solution of 4-(8-benzyloxy-7-methoxy-3-methyl-3H-pyrazolo[3,4-c]quinolin-1-yl)benzonitrile (3.82 g, 9.09 mmol) in trifluoroacetic acid (38 ml) under a dry nitrogen atmosphere with ice-bath cooling. When the addition is complete, the mixture is subsequently stirred for a further 30 min. When the reaction is complete (HPLC-MS check), the reaction mixture is carefully added to water (800 ml) and extracted twice with ethyl acetate (200 ml each time). The combined organic phases are washed with water (150 ml) and semi-saturated NaHCO3 solution (200 ml), subsequently dried using Na2SO4 and filtered with suction. The filtrate is evaporated in vacuo, suspended in a little ethanol, filtered off with suction and dried in a high vacuum, giving the title compound 4-(8-hydroxy-7-methoxy-3-methyl-3H-pyrazolo[3,4-c]quinolin-1-yl)benzonitrile (2.86 g, 8.66 mmol) as a solid having a melting point of 288.9° C. MS: 331.1 (M+H+), TLC (HPTLC): Rf=0.34 (ethyl acetate/ethanol 8:1, parts by volume).
