Step 1 4-((S)-2-((S)-2-(6-(2,5-Dioxo-2,5-dihydro-1H-pyrrol-1-yl)hexanamido)-3-methylbutanamido)-5-ureidopentanamido)benzyl 4-nitrophenyl carbonate 58 (30 mg, 0.041 mmol) was reacted with tert-butyl piperazine-1-carboxylate (5.3 mg, 0.0287 mmol) in anhydrous DMSO under argon atmosphere at room temperature (FIG. 7d). The reaction mixture was stirred for 1 h, until disappearance of the starting material (HPLC-MS analysis). Diethyl ether (80 ml) was then added to the reaction mixture and the precipitate thus obtained was collected by filtration to give N-[6-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)hexanoyl]-L-valyl-N-{4-[({[4-(tert-butoxycarbonyl)piperazin-1-yl]carbonyl}oxy)methyl]phenyl}-N5-carbamoyl-L-omithinamide 59 as a yellow solid, 22.0 mg) was isolated and used without further purification in the next step. MS (ESI): 785 [M+H]+. Retention time=4.87 min (method 1).
