[NH:1]([C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])[C@H:2]([C:4]([NH:6][C@H:7]([C:9]([OH:11])=O)[CH3:8])=[O:5])[CH3:3].CN1CCOCC1.Br.[NH2:27][CH2:28][CH2:29][CH2:30][Br:31].CN(C=O)C>C1COCC1>[NH:1]([C:12]([O:14][C:15]([CH3:18])([CH3:17])[CH3:16])=[O:13])[C@H:2]([C:4]([NH:6][C@H:7]([C:9]([NH:27][CH2:28][CH2:29][CH2:30][Br:31])=[O:11])[CH3:8])=[O:5])[CH3:3]To a cold (−15° C.) solution of Boc-Ala-Ala-OH (908 mg, 3.5 mmol) and NMM (570 μl, 5.23 mmols) in THF (15 ml) was added ECF (341 μl, 3.6 mmols) under N2 atmosphere and vigorously stirred. After 2 min of stirring, a solution of 1-amino-3-propylbromide hydrobromide (976 mg, 4.90 mmols) in a mixture of THF:DMF (4 mL: 3 mL) was added to the mixture followed by NMM (951 μl, 8.73 mmols) and stirred. After 10 min the mixture was warmed to 25° C. and stirred for further 8 h. THF was removed under reduced pressure and the resulting viscous solution was diluted with water (10 mL) and thoroughly extracted with ethyl acetate. The combined organic extracts were washed with 5 ml saturated citric acid, 5 ml saturated NaHCO3 and dried over by Na2SO4 and concentrated to give a residue, which was purified by silica gel flash column chromatograph (EtOAc:Hexane—3:7) to give the desired product as a solid (m.p. 146° C.) in good yields (1.022 g, 78%). (TLC:EtOAc—Rf—0.39).
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