[Br:1][C:2]1[CH:3]=[C:4]2[C:10]([CH3:11])=[N:9][N:8]([C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])[C:5]2=[N:6][CH:7]=1.[Br:19]N1C(=O)CCC1=O.N(C(C)(C)C#N)=NC(C)(C)C#N>C(Cl)(Cl)(Cl)Cl>[Br:1][C:2]1[CH:3]=[C:4]2[C:10]([CH2:11][Br:19])=[N:9][N:8]([C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])[C:5]2=[N:6][CH:7]=1A mixture of tert-butyl 5-bromo-3-methyl-1H-pyrazolo[3,4-b]pyridine-1-carboxylate x154 (10.8 mmol, 1 eq, 3.37 g), N-bromosuccinimide (1.92 g, 1 eq, 10.8 mmol) and azo-bisiso-butyronitrile (AIBN) (10.8 mmol, 1 eq, 1.77 g) in CCl4 (50 ml) is heated for 2 hours at 80° C. The solvent is removed under reduce pressure, water is added and the resulting mixture is extracted with dichloromethane. The combined organic phases are dried over MgSO4, filtered and volatiles removed under reduced pressure. The product is purified by chromatography on silicagel (hexane/AcOEt 8/2) and recristallized in AcOEt to afford 0.28 g of pure tert-butyl 5-bromo-3-(bromomethyl)-1H-pyrazolo[3,4-b]pyridine-1-carboxylate x155.
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