To a stirring suspension at room temperature under N2 of (S)-3-(1-(tert-butoxycarbonyl)-1H-imidazol-5-yl)-2-(3-(tert-butoxycarbonylamino)propanamido)propanoic acid (45c, 35.74 g, 83.80 mmol) and (R)-2-acetamido-3-mercaptopropanamide (45b, 13.59 g, 83.78 mmol) in 500 mL of THF:EtOAc (1:1) was added dropwise triethylamine (26 mL, 186 mmol). After addition, the suspension was stirred for 15 minutes then propylphosphonic anhydride, 50 wt. % solution in EtOAc (55 mL, 92 mmol) was added dropwise. The resultant cloudy solution was stirred at room temperature for 2 h then saturated NaHCO3 solution (˜500 mL) was added. The layers were separated and the organic extract was washed with saturated KH2PO4 and then saturated NaCl solution. The aqueous phases were back extracted (in the same order) with fresh EtOAc (2×500 mL). The combined organic extracts were dried using MgSO4, filtered, concentrated and chromatographed twice (silica gel, 50% THF in EtOAc) to give 35.53 g (74%) of tert-butyl 5-((S)-3-((R)-2-acetamido-3-amino-3-oxopropylthio)-2-(3-(tert-butoxycarbonylamino)propanamido)-3-oxopropyl)-1H-imidazole-1-carboxylate (45d) as white foamy solid. MS (APCI) m/z 571 [M+H]+.
