DIPEA (1.53 mL, 8.78 mmol.) was added to a stirred solution of 4b (1.76 mmol), t-butyl carbazate (1.16 g, 8.78 mmol) and HATU (1.34 g, 3.52 mmol) in DMF (20 mL) at ambient temperature. After approximately 18 h, the reaction mixture was poured into saturated aqueous sodium bicarbonate and extracted three times with EtOAc. The combined organic extracts were washed with water, brine, dried (Na2SO4) and concentrated in vacuo. Purification of the crude residue by flash chromatography on silica gel (gradient elution; 0-35% EtOAc/hexanes as eluent) furnished the title compound 4c, m/z (ES) 564 (MH)+.
