General procedure for the synthesis of 1H-pyrazolo[3,4-b]pyridine from 2-chloro-3-pyridinecarboxaldehyde: Hydrazine hydrate (10 mL) was added to a mixture of 2-chloro-3-formylpyridine (5.00 g, 35 mmol) and p-toluenesulfonic acid (3.50 g, 18 mmol). The reaction mixture was stirred at 130 °C for 3 hours. Upon completion of the reaction, the mixture was cooled with cold water and subsequently extracted with ethyl acetate. The organic phases were combined and dried with anhydrous magnesium sulfate. After filtration, the solvent was removed by distillation under reduced pressure to afford the target compound 1H-pyrazolo[3,4-b]pyridine (3.65 g, 87% yield) as a yellow solid. The melting point of the compound was 88-90 °C (literature value 97-98 °C). Its Fourier transform infrared spectrum (KBr) showed characteristic absorption peaks: νmax/cm-1 3450 (NH), 3089, 3026 (aromatic CH), 2958, 2915 (CH), 1606, 1588, 1509, 1470, 1429 (C=N, C=C). The NMR hydrogen spectrum (300 MHz, CDCl3) data are as follows: δ/ppm 1.69 (s, 1H, NH), 7.19 (dd, 1H, J=4.7 and 7.5 Hz, ArH, 5-H), 8.12 (s, 1H, ArH, 3-H), 8.15 (d, 1H, J=8.1 Hz, ArH, 4-H), 8.63 (d, 1H, J=4.5Hz, ArH, 6-H).
![1H-PYRAZOLO[3,4-B]PYRIDINE](/CAS/GIF/271-73-8.gif)