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4-Chloro-1H-pyrazolo[4,3-c]pyridine (1.5 g, 9.77 mmol, 1.00 eq.) was used as a starting material and dissolved in 1,4-dioxane (25 mL). Subsequently, potassium hydroxide (2.0 g, 35.65 mmol, 3.60 eq.) was added and the reaction mixture was stirred at 75 °C. At this temperature, iodine (4.95 g, 19.50 mmol, 2.00 eq.) was slowly added and the reaction lasted for 4 hours. Upon completion of the reaction, the reaction was quenched with saturated sodium thiosulfate pentahydrate aqueous solution and the resulting solid was collected by filtration. 4-Chloro-3-iodo-1H-pyrazolo[4,3-c]pyridine (2.5 g, 92% yield) was finally obtained as a light yellow solid. The product was analyzed by LC-MS (ES, m/z) and showed a molecular ion peak of 280 [M + H].

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