Phosphorus oxychloride → 4-Chloro-1H-pyrazolo[3,4-b]pyridine

1,7-Dihydro-pyrazolo[3,4-b]pyridin4-one (1.2 g, 8.8 mmol) (Dorn, H. et al., Prakt. Chem., 324, 557-62 (1982)) in POCl3 (15 ml) at 0° C. was treated with PCd5 (2.5 mg, 0.01 mmol). The solution was allowed to warm to rt over 1 h, then heated to 90° C. and held 3h. The solution was concentrated under reduced pressure, then treated with ice and water (50 ml). The resulting mixture was extracted with ethyl acetate (100 ml), and the organic layer was washed with a saturated aqueous solution of sodium bicarbonate (30 ml). The organic layer was dried over MgSO4, then evaporated under reduced pressure to give 4-chloro-1H-pyrazolo[3,4-b]pyridine as a yellow solid (820 mg, 60%). 1H NMR (300 MHz, CDCl3) δ 8.57 (d, 1H, J=5.2 Hz), 8.25 (s, 1H), 7.28 (d, 1H, J=5.2 Hz).