2,3-bis-((3R,5S)-3,5-bis-(tert-Butoxycarbonylamino)-piperidin-1-yl)-quinoxalin-6-ylamine (246) (0.067 g, 0.087 mmol) and p-nitro-benzoylchloride (0.0167 g, 0.09 mmol) were combined and dissolved in DMF (0.45 mL). iPr2EtN (0.016 mL, 0.09 mmol) was added and the mixture was shaken at room temperature for 16 h. TLC (Rf: 0.8 compared to Rf: 0.05 for compound 246 in 1:1 hexane/EtOAc) and LC-MS indicated complete coupling at this point. The mixture was diluted with EtOAc (30 mL), washed with saturated aqueous NaHCO3 (2×10 mL) and brine (2×10 mL) via extraction, dried over MgSO4 and concentrated to give the desired product (251) as an orange solid (0.063 g, 0.068 mmol, 78.6% yield). LC-MS (ESI): (exact mass: 920.48) m/e=921.8 [M+1]+.
