1-Hydroxy-naphthalene-2-carboxylic acid (142) (0.016 g, 0.085 mmol) was dissolved in DMF (0.8 mL) and HBTU (0.032 g, 0.085 mmol) followed by iPr2EtN (0.022 mL, 0.128 mmol) were added. The mixture was shaken at room temperature for 25 minutes. 4-Amino-N-(2,3-bis-((3R,5S)-3,5-bis-(tert-Butoxycarbonylamino)-piperidin-1-yl)-quinoxalin-6-yl)-benzamide (252) (0.06 g, 0.068 mmol) was added and the mixture was shaken at room temperature for 16 h. The mixture was diluted with EtOAc, washed with sodium bicarbonate and brine via extraction, dried over MgSO4 and concentrated under reduced pressure. Purification by flash chromatography on silica gel using a gradient of hexane and EtOAc gave the desired product (253) (0.009 g, 0.0085 mmol, 12.5% yield). LC-MS (ESI): (exact mass: 1060.54, mol. wt: 1061.23) m/e=1062.0 [M+1]+.
