[CH:1]1[C:2]2[C:9](=O)[NH:8][CH:7]=[N:6][C:3]=2[NH:4][N:5]=1.P(Cl)(Cl)([Cl:13])=O.C(N(C(C)C)CC)(C)C>C1(C)C=CC=CC=1>[Cl:13][C:9]1[N:8]=[CH:7][N:6]=[C:3]2[NH:4][N:5]=[CH:1][C:2]=12To a solution of allopurinol (20 g, 146.94 mmoles) in toluene (205.71 mL), add phosphoryl chloride (68.27 mL, 734.68 mmoles) and diisopropylethylamine (56.38 mL, 323.26 mmoles) and heat the mixture at 80° C. for 2 hours. Remove the solvent in vacuo to half and pour the mixture into 2 M potassium phosphate, dibasic (734.68 mL, 1.47 moles) in water at 4° C. Stir the mixture overnight at room temperature. Filter off the precipitate through a pad of celite and wash it subsequently with EtOAc. Separate the filtrate, wash the aqueous layer with more EtOAc, join the organic layers, dry it over MgSO4, filter and concentrate in vacuo to afford 4-Chloro-1H-pyrazolo[3,4-d]pyrimidine (16 g, 70.45% yield) as a yellow solid.
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