A flask was charged with 4-hydroxy-N-((6-(5-methoxy-1-methyl-1H-indol-3-yl)-5-((2-(trimethylsilyl)ethoxy)methyl)-5H-pyrrolo[2,3-b]pyrazin-2-yl)methyl)butanamide (0.130 g, 0.248 mmol) and DCM (5 mL). The mixture was cooled to about −5° C. To the mixture was added 2,6-lutidine (0.064 mL, 0.54 mmol). After about 2 min, TBDMSOTf (0.063 mL, 0.273 mmol) was added. The mixture was stirred at about −5 to 0° C. for about 2 h. The mixture was concentrated under reduced pressure and the residue was purified using silica gel chromatography with an elution gradient of 0 to 80% EtOAc/heptane to give 4-(tert-butyldimethylsilyloxy)-N-((6-(5-methoxy-1-methyl-1H-indol-3-yl)-5-((2-(trimethylsilyl)ethoxy)methyl)-5H-pyrrolo[2,3-b]pyrazin-2-yl)methyl)butanamide (0.117 g, 74%): LC/MS (Table 2, Method a) Rt=3.40 min; MS m/z 638 (M+H)+.
