Br[CH2:2][C:3]([O:5][CH2:6][CH3:7])=[O:4].[NH:8]([C:20]([O:22][CH2:23][C:24]1[CH:29]=[CH:28][CH:27]=[CH:26][CH:25]=1)=[O:21])[C@H:9]([C:11]([NH:13][C@H:14]([C:16]([NH:18][NH2:19])=[O:17])[CH3:15])=[O:12])[CH3:10].CN1CCOCC1.C(Cl)Cl>CN(C=O)C>[NH:8]([C:20]([O:22][CH2:23][C:24]1[CH:25]=[CH:26][CH:27]=[CH:28][CH:29]=1)=[O:21])[C@H:9]([C:11]([NH:13][C@H:14]([C:16]([NH:18][NH:19][CH2:2][C:3]([O:5][CH2:6][CH3:7])=[O:4])=[O:17])[CH3:15])=[O:12])[CH3:10]Ethyl bromoacetate (1.1 eq) was added dropwise to a stirred solution of Cbz-Ala-Ala-NHNH2 (1 eq) and NMM (1.1 eq) in DMF that was cooled to −10° C. The resulting solution was stirred for 30 min at −10° C., after which the mixture was allowed to react at room temperature for 36 hours. The DMF was evaporated, and the residue was purified on a silica gel column using 1:9 MeOH:CH2Cl2 as the eluting solvent system to afford 4a as a white solid (yield=36%). 1H NMR (DMSO-d6): 1.18 (t, 9H, CH3), 3.5 (d, 2H, NCH2COOEt), 4.0-4.15 (m, 3H, α-H and OCH2CH3), 4.2 (m, 1H), α-H), 5.03 (m, 2H, Cbz), 5.18 (m, 1H, NH), 7.22-7.40 (m, 5H, Ph), 7.4-7.5 (d, 1H, NH), 7.9 (m, 1H, NH), 9.35 (m, 1H, NH). MS (FAB) m/z 395 [(M+1)+].
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