Hydrazine → 6-chloro-1H-pyrazolo[3,4-d]pyrimidine

A solution of 2,4-dichloropyrimidine-5-carbaldehyde (7.75 g, 43.8 mmol, Preparation #1) in THF (100 mL) was added to a 500 mL round-bottomed flask charged with anhydrous hydrazine (3.0 mL, 96 mmol) in THF (250 mL). The reaction mixture was allowed to stir at ambient temperature for about 1 h after which the reaction mixture was concentrated under reduced pressure. The crude product was suspended in DCM (100 mL) and filtered through a silica gel pad. The silica gel pad was washed with additional DCM (2×20 mL) and the combined organic layers were concentrated under reduced pressure. The crude material was purified by silica gel chromatography eluting with 30% EtOAc in heptane to provide 6-chloro-1H-pyrazolo[3,4-d]pyrimidine (2.3 g, 34%) as a white solid. LC/MS (Table 2, Method a) Rt=1.29 min; MS m/z: 155 (M+H)+.