A mixture of 4-(4-amino-1-cyclopentyl-1H-pyrazolo[3,4-d]pyrimidin-3-yl)phenol (0.107 g, 0.362 mmol, 1.0 equiv), DMSO (0.5 mL), sodium hydride (60%, 0.030 g, 0.72 mmol, 2.0 equiv), and methyl-5-nitro-2-furoate (0.062 g, 0.36 mmol, 1.0 equiv) was heated at 90° C. for 3 h. The reaction mixture was allowed to cool to room temperature, poured into ice water (10 mL), and extracted with three portions of CH2Cl2 (50 mL each). The combined organic extracts were washed with 5% aqueous KOH (50 mL) and the organic layer was dried over MgSO4, filtered, and concentrated to afford a red oil which was purified by column chromatography on silica gel (elution with 300 mL of 5% MeOH/CH2Cl2) to afford methyl 5-[4-(4-amino-1-cyclopentyl-1H-pyrazolo[3,4-d]pyrimidin-3-yl)phenoxy]-2-furoate as a red solid (0.070 g, 0.17 mmol):
