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

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1-Ethyl-4-hydroxy-1H-pyrazolo[3,4-b]pyridine-5-carboxylic acid, ethyl ester (15 g, 63.8 mmol) was dissolved in phosphorus oxychloride (100 mL), and the resulting solution was heated at reflux for 4 h. The remaining phosphorus oxychloride was removed via evaporation under reduced pressure. The residual light brown solid was recrystallized from EtOH-hexane to afford the title compound as a white solid (14 g, 55.3 mmol, 87%): HPLC (YMC S5 ODS 4.6×50 mm column, 4 minute gradient-0% B to 100% B, 4 mL/min flow, solvent A: 10% MeOH-90% H2O-0.2% H3PO4, solvent B: 90% MeOH-10% H2O-0.2% H3PO4) retention time 3.84 minutes showed a purity of 96%; LRMS (m/z) 254 (MH+).

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