General procedure for the synthesis of 3,5-diiodo-4-aminopyridine from 4-aminopyridine: 500 mL of methanol, 3000 mL of water, 94.11 g (1.0 mol) of 4-aminopyridine, 224.10 g (1.35 mol) of potassium iodide, and 149.80 g (0.70 mol) of potassium iodate were added sequentially to the reaction flask. 160 mL of hydrochloric acid was slowly added dropwise with controlled titration rate, and the titration process lasted for 3 hours. After the dropwise addition, the reaction was continued for 1 hour. Subsequently, 50 mL of an aqueous solution containing 17.73 g (0.20 mol) of sodium chlorite was added dropwise. After the dropwise addition was completed, the reaction process was monitored by HPLC and the reaction was continued for 0.5 hours. Upon completion of the reaction, the reaction solution was diluted with 4000 mL of water, and then 6000 mg of trichloromethane was added in three portions as an extractant. The organic phase was washed sequentially with an aqueous solution containing sodium thiosulfate and an aqueous solution of sodium bicarbonate, separated and dried with anhydrous magnesium sulfate. The solvent was removed by concentration under reduced pressure to give the crude product of 3,5-diiodo-4-aminopyridine in gray color. The crude product was recrystallized with a mixed solvent of ethanol and water to give 318.23 g of white crystalline solid in 92.0% yield.
