2-(biphenyl-4-ylmethyl)-3-chloro-7-isobutyl-5-methyl-2H-pyrazolo[3,4-d]pyrimidine-4,6(5H,7H)-dione (25 mg, 0.056 mmol) is dissolved in anhydrous and degassed THF, and then (S)-tert-butyl 3-aminopyrrolidine-1-carboxylate (64 mg, 0.34 mmol), Pd2(dba)3 (25 mg, 0.026 mmol) and Xantphos (16 mg, 0.026 mmol) are added, followed by tBuOK (7.5 mg, 0.067 mmol). The reaction mixture is heated in microwave at 130° C. for 2.5 h. After cooling, solvent is removed. The residue is diluted with ethyl acetate (50 mL), and then washed with sodium carbonate aqueous solution three times. Solvent is removed under reduced pressure, the residue is treated with 50% TFA in CH2Cl2 (v/v) at room temperature for 6 hours. Evaporation to remove TFA and solvent, and the residue is purified by a semi-preparative HPLC to give pure product. MS (ESI) m/z 473.2 [M+H]+.
