Example 5 Synthesis of 4-amino-3-hydroxypyridine: 3-methoxy-4-aminopyridine (200 mg, 1.6 mmol) was dissolved in dichloromethane (15 mL) under argon protection and stirred at -70°C. Boron tribromide (10 mL, 1 M solution in dichloromethane, 6 eq.) was added slowly over 10 min. After stirring under cooling conditions for 15 minutes, the reaction mixture was gradually warmed to room temperature and continued to stir overnight. After completion of the reaction, the solvent was removed by distillation under reduced pressure. The residue was mixed with methanol and saturated sodium bicarbonate solution and stirred for 25 minutes. The mixture was filtered and the filtrate was evaporated. The residue was stirred with a solvent mixture of dichloromethane/methanol/ammonia (75/25/2, v/v/v), filtered, and purified by silica gel column chromatography using the same solvent mixture as eluent to give 145 mg (81% yield) of the target product 4-amino-3-hydroxypyridine with a melting point of 205°C (literature value: 188°-191°C). The structure of the product was confirmed by proton NMR spectroscopy. Elemental analysis results: Calculated values (C5H6N2O-0.2H2O): C, 52.81; H, 5.67; N, 24.63. Measured values: C, 53.02; H, 5.63; N, 24.40.
