[6-(2,4-Difluoro-phenoxy)-3-(2,5-difluoro-phenyl)-4-(2-methanesulfonyl-ethylamino)-pyrazolo[3,4-d]pyrimidin-1-yl]-methanol (170 mg, 0.33 mmol) was dissolved in methylene chloride, and Boc-(L)-Valine (216 mg, 0.99 mmol), dimethylamino pyridine (81 mg, 0.66 mmol), and triethylamine (0.23 mL, 1.6 mmol) were added. The reaction mixture was cooled to 0° C. and stirred under nitrogen, and isopropenyl chloroformate (54 uL, 0.49 mmol) was added. The reaction mixture was stirred for 10 minutes and then partitioned between water and methylene chloride. The organic layer was washed with brine, dried (MgSO4), filtered and concentrated under reduced pressure. The residue was purified by flash chromatography through silica (1:1 hexanes/EtOAc) to give 120 mg of 2-tert-butoxycarbonylamino-3-methyl-butyric acid 6-(2,4-difluoro-phenoxy)-3-(2,5-difluoro-phenyl)-4-(2-methanesulfonyl-ethylamino)-pyrazolo[3,4-d]pyrimidin-1-ylmethyl ester as an oil.
