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General procedure for the synthesis of 2-chloro-3-methyl-3H-imidazo[4,5-b]pyridine using 3-methyl-1H-imidazo[4,5-b]pyridin-2(3H)-one as starting material: 3-methyl-1H-imidazo[4,5-b]pyridin-2(3H)-one (60 mg, 0.40 mmol) was dissolved in phosphorochloridic acid (4.2 mL) in a microwave reaction vessel and heated at 125 °C for 30 min. Upon completion of the reaction, the crude reaction mixture was slowly poured into pre-cooled saturated sodium bicarbonate (NaHCO3) solution and the pH was adjusted to neutral with sodium bicarbonate, followed by extraction with ethyl acetate (EtOAc). The organic phase was separated, washed sequentially with water and saturated saline, dried over anhydrous sodium sulfate (Na2SO4), and concentrated under reduced pressure to remove the solvent. All organic phase extracts were combined and the residue was purified by column chromatography with a gradient solvent system of hexane to ethyl acetate as eluent, resulting in the target product 2-chloro-3-methyl-3H-imidazo[4,5-b]pyridine as a white solid in 43% yield.

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[3] Patent: US2011/319394, 2011, A1. Location in patent: Page/Page column 79