General procedure for the synthesis of 4-amino-3-iodopyridine from 4-aminopyridine: A 2L three-necked round-bottomed flask was assembled with a mechanical stirrer, thermocouple, dosing funnel, nitrogen inlet, and reflux condenser fitted with a drying tube, and the flask was placed in a heating jacket. Glacial acetic acid (523 mL) was added to the flask and stirring was initiated. 4-Aminopyridine (20.95 g) was added to the reaction system in a single addition, and the temperature was raised to 30 °C exothermically during dissolution. Iodine monochloride (101 mL) was slowly added dropwise over a period of 2 hours, controlling the reaction temperature to no more than 45°C. The reaction temperature rose to 42°C at the end of the dropwise addition. At the end of the dropwise addition, the temperature was raised to 42℃. After the exothermic reaction subsided, the reaction mixture was heated, maintained at 45-50°C, and stirred continuously overnight and maintained for 10 days until the reaction was complete (the progress of the reaction was monitored by HPLC, MPP-LC1 (270) method, with retention times of 2.8 minutes and 8.1 minutes for the starting material and product, respectively). Upon completion of the reaction, the mixture was cooled to room temperature, diluted with 1.9 L of water, cooled to 0-5 °C in an ice-water bath, and the pH was adjusted to 10 with 50% sodium hydroxide solution (brown solid was produced during the adjustment, ice was added to control the temperature if necessary). Ethyl acetate (4L) was added for extraction, stirred and partitioned. The aqueous layer was extracted once more with fresh ethyl acetate (4L). The organic phases were combined and washed sequentially with 15% sodium thiosulfate solution (2 x 2L), water (2L) and saturated brine (2L) and dried over sodium sulfate. It was filtered and concentrated under reduced pressure to give a brown solid. The solid was dissolved in 1 L of dichloromethane containing 5% methanol, filtered through a silica plug (No. 2) and washed with 2 L of eluent. The brown waxy solid was evaporated under reduced pressure and dried under vacuum for at least 12 hours. The final product was 4-amino-3-iodopyridine (batch no. 1357-69-1) 173 g in 78% yield and 92.3% HPLC purity. The structure of the product was confirmed by mass spectrometry and 1H-NMR (300 MHz, CDCl3).
