Phosphorus oxychloride → ethyl 4-chloro-1-ethyl-1H-pyrazolo[3,4-b]pyridine-5-carboxylate

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A solution of 2-[(2-ethyl-2H-pyrazol-3-ylamino)-methylene]-malonic acid diethyl ester (11.75 g, 0.041 mol) in POCl3 (70 mL) was heated at reflux (135° C.) for 8 hours. The excess of POCl3 were distilled off and the reaction mixture cooled to 0° C., before careful addition of water (100 mL) and then aqueous NaOH (3N) until pH reached 7. The product was extracted with CH2Cl2 several times, and the combined orgnic phase were dried over Na2SO4 and concentrated under vacuo. Column chromatography (SiO2, EtOAc/Heptane 1/5) afforded 7.22 g (69%) of the title product as a white solid. ES-MS m/e: 254.2 (M+H+).

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