Methyl iodide → 5-Chloro-1-methyl-1H-pyrazolo[4,3-D]pyrimidine

Potassium t-butoxide (1.68 g, 14.67 mmol, 1.14 equiv) was added to a solution of 5-chloro-1H-pyrazolo[4,3-d]pyrimidine (2.1 g, 12.91 mmol, 1.00 equiv, 95%) in tetrahydrofuran (50 mL) at 0° C. After the reaction mixture was stirred at 0° C. for 30 min, iodomethane (3 g, 20.71 mmol, 1.60 equiv, 98%) was added dropwise at 0° C. The resulting solution was stirred for 4.5 h at room temperature and then concentrated under vacuum. The residue was loaded onto a silica gel column and eluted with dichloromethane/ethyl acetate (10/1) to give 300 mg (14%) of 5-chloro-1-methyl-1H-pyrazolo[4,3-d]pyrimidine as a white solid.