General procedure for the synthesis of 2-amino-5-fluoro-3-iodopyridine from 2-amino-5-fluoropyridine: Concentrated sulfuric acid (H2SO4, 120 mL) was added slowly and dropwise to a mixed solution of acetic acid (AcOH, 4 L) and water (H2O, 1 L) of 2-amino-5-fluoropyridine (1 kg, 8.9 mol) over 5 min. Periodic acid (H5IO6, 450 g, 1.97 mol, 0.22 eq.) and iodine (I2, 1 kg, 3.94 mol, 0.44 eq.) were subsequently added, and the reaction mixture was stirred at 82 °C (internal temperature) overnight. The reaction progress was monitored by TLC (sample treatment: dilution with water, alkalinization with 30% sodium hydroxide (NaOH), extraction with ethyl acetate (EtOAc), concentration), showing that 13-15% of the starting material remained unreacted. Periodic acid (H5IO6, 80 g) and iodine (I2, 180 g) were added additionally and stirring was continued at 80 °C overnight. External heating was removed and the reaction mixture was allowed to stir at room temperature overnight. The reaction mixture was slowly poured into ice water (8 L), alkalized with 33% aqueous sodium hydroxide (NaOH) solution (~6.5 L) and stirred for 2 hours. The precipitated product was collected by filtration and washed with hot water (8 x 3 L). The washings were filtered overnight and then the product was washed with heptane (3×). The product was dried in an oven at 45 °C over the weekend to give the compound 2-amino-5-fluoro-3-iodopyridine (2b, 1390 g, 65% yield) as a black solid. Water (H2O) was added to the heptane layer and allowed to stand over the weekend. The dark aqueous and light yellow organic layers were separated and the organic layer was concentrated to dryness to give more compound 2b (95 g, 70% overall yield) as a yellow solid. Nuclear magnetic resonance hydrogen spectrum (1H NMR, DMSO-d6, 300 MHz): δ 7.95-7.88 (m, 2H) ppm. nuclear magnetic resonance hydrogen spectrum (1H NMR, CDCl3, 300 MHz): δ 7.95-7.90 (m, 1H); 7.68-7.62 (m, 1H); 4.85 (s, NH2) ppm.
