General procedure for the synthesis of 5-nitropyridin-3-ol from 3-amino-5-hydroxypyridine: A solution was prepared by grinding and dissolving potassium persulfate (56.8 g, 210 mmol) in 31.5 mL of concentrated sulfuric acid. The solution was slowly added to a concentrated sulfuric acid solution (27 mL) of 3-amino-5-hydroxypyridine (2.75 g, 25 mmol). The reaction mixture was allowed to stand at room temperature for 72 hours. Upon completion of the reaction, the mixture was slowly poured into ice water and the pH was adjusted with concentrated ammonia to 6. Subsequently, the aqueous phase was extracted with ethyl acetate (4 x 100 mL). The organic phases were combined, dried with anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The crude product was purified by silica gel column chromatography (eluent: chloroform/methanol, 99:1 to 9:1 gradient elution) to afford 1.65 g (47% yield) of the target compound 5-nitropyridin-3-ol. The product was characterized by 1H NMR (CDCl3, 300 MHz) and mass spectrometry (DCI/NH3) with the following data: 1H NMR δ 8.81 (d, J = 3 Hz, 1H), 8.51 (d, J = 3 Hz, 1H), 7.82 (dd, J = 2.5 Hz, 1H); MS (DCI/NH3) m/z 141 (M + H)+, 158 ( M + NH4)+.
