General procedure for the synthesis of 2-hydroxy-5-iodopyridine-3-carboxylic acid from 2-hydroxynicotinic acid: a mixture of 2-hydroxynicotinic acid (24 g, 173 mmol) and N-iodosuccinimide (50.5 g, 224 mmol) in N,N-dimethylformamide (DMF, 300 mL) was reacted for 1 h at room temperature, protected from light, and then stirred for 20 h at 50 °C. . Upon completion of the reaction, about 250 mL of DMF was removed by distillation under reduced pressure and water (400 mL) was added to the residue. The mixture was stirred at room temperature for 1 h, followed by continued stirring at 0 °C for 1 h. The precipitated solid was collected by filtration. The resulting solid was suspended in methanol (MeOH, 200 mL) and stirred at room temperature for 2 h. The mixture was filtered again, washed with methanol (40 mL), and finally dried under vacuum to afford the target compound 2-hydroxy-5-iodopyridine-3-carboxylic acid (19, light yellow solid, 38.4 g, 76% yield). The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3) δ= 8.41 (d, J = 2.4 Hz, 1H), 8.23 (d, J = 2.4 Hz, 1H) and mass spectra (ESI+) m/z 265.9 ([M + H]+).
