[N:1]([CH:4]([CH3:6])[CH3:5])=[C:2]=[O:3].[C:7]([O:11][C:12]([NH:14][C@@H:15]([CH2:20][C:21]1[CH:26]=[CH:25][C:24]([OH:27])=[CH:23][CH:22]=1)[C:16]([O:18][CH3:19])=[O:17])=[O:13])([CH3:10])([CH3:9])[CH3:8].C(N(CC)CC)C>C(Cl)Cl>[C:7]([O:11][C:12]([NH:14][C@@H:15]([CH2:20][C:21]1[CH:26]=[CH:25][C:24]([O:27][C:2](=[O:3])[NH:1][CH:4]([CH3:6])[CH3:5])=[CH:23][CH:22]=1)[C:16]([O:18][CH3:19])=[O:17])=[O:13])([CH3:10])([CH3:8])[CH3:9]2-Isocyanatopropane (Aldrich, Cat. #141070) (22 mg, 0.25 mmol) was added to a solution of methyl (2S)-2-[(tert-butoxycarbonyl)amino]-3-(4-hydroxyphenyl)propanoate (50 mg, 0.2 mmol) and triethylamine (26 mg, 0.25 mmol) in methylene chloride (2 mL). The reaction was stirred for 1 h. The reaction was partitioned between water and ethyl acetate. The organic phase was washed with saturated brine, dried over magnesium sulfate, filtered and concentrated under reduced pressure to give crude product, (S)-methyl 2-(tert-butoxycarbonylamino)-3-(4-(isopropylcarbamoyloxy)phenyl)propanoate (50 mg, 80%) as a white solid. Analytical LCMS (M+H-Boc)+: m/z=281.0.
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