[NH:1]([C:23]([O:25][C:26]([CH3:29])([CH3:28])[CH3:27])=[O:24])[C@H:2]([C:4]([NH:6][C@H:7]([C:20]([OH:22])=[O:21])[CH2:8][CH2:9][C:10](=[O:19])[O:11][CH2:12][C:13]1[CH:18]=[CH:17][CH:16]=[CH:15][CH:14]=1)=[O:5])[CH3:3].[CH:30]1[C:35]([N+:36]([O-:38])=[O:37])=[CH:34][CH:33]=[C:32](O)[CH:31]=1.C1CCC(N=C=NC2CCCCC2)CC1>CCOC(C)=O>[NH:1]([C:23]([O:25][C:26]([CH3:28])([CH3:27])[CH3:29])=[O:24])[C@H:2]([C:4]([NH:6][C@H:7]([C:20]([O:22][C:32]1[CH:31]=[CH:30][C:35]([N+:36]([O-:38])=[O:37])=[CH:34][CH:33]=1)=[O:21])[CH2:8][CH2:9][C:10](=[O:19])[O:11][CH2:12][C:13]1[CH:14]=[CH:15][CH:16]=[CH:17][CH:18]=1)=[O:5])[CH3:3]Boc-L-Ala-L-Glu(OBzl)-OH (40.5 g, 0.1 mol) was dissolved in 300 ml of EtOAc and combined with 17 g (0.12 mol) of p-nitrophenol. The reaction was kept at 0° C. for 1 hour. DCC (24.7 g, 0.12 mol) was then added. The reaction was stirred for 1 hour at 0° C. and for 4 hours at room temperature. The precipitate of DCU was filtered off and the solvent was evaporated in vacuum. The residual oil was then dissolved in ether. The precipitate was filtered off and washed with ether and hexane. The yield was 35 g (64%). Rf=0.7 (CHCl3:Et-Ac:MeOH=6:3:1).
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