C(=O)([S:3][CH2:4][CH2:5][C@H:6]([NH:16][C:17]([O:19][CH2:20][C:21]1[CH:26]=[CH:25][CH:24]=[CH:23][CH:22]=1)=[O:18])[C:7](=[O:15])[NH:8][CH2:9][CH2:10][CH2:11][CH2:12][CH:13]=[O:14])C.C[O-].[Na+]>CO>[SH:3][CH2:4][CH2:5][C@H:6]([NH:16][C:17](=[O:18])[O:19][CH2:20][C:21]1[CH:22]=[CH:23][CH:24]=[CH:25][CH:26]=1)[C:7](=[O:15])[NH:8][CH2:9][CH2:10][CH2:11][CH2:12][CH:13]=[O:14]A solution of (S)—S-3-(benzyloxycarbonylamino)-4-oxo-4-(5-oxopentylamino)butyl ethanethioate (195 mg, 0.494 mmol) in methanol (4 mL) was purged with argon for 10 min and then cooled to 0° C. Sodium methoxide in MeOH (25%, 0.12 mL) was added to the reaction. The mixture was stirred at 0° C. for 10 min. The reaction was quenched with saturated aqueous NH4Cl solution. The reaction was extracted with EtOAc (2×). The combined EtOAc extracts were washed with saturated aqueous NH4Cl solution, water, saturated aqueous NaCl, dried over Na2SO4, filtered and concentrated. The white solid residue was dried under vacuum to give (S)-benzyl 4-mercapto-1-oxo-1-(5-oxopentylamino)butan-2-ylcarbamate (150 mg, 0.426 mmol, 86% yield) as a white solid. Anal. Calcd. for C12H24N2O4S m/z 352.4. found: 353.23 (M+H)+.
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