C([S:4][CH2:5][CH:6]([CH2:17][C:18]([O:20][CH3:21])=[O:19])[C:7]([N:9]1[CH2:16][CH2:15][CH2:14][C@H:10]1[C:11]([OH:13])=[O:12])=[O:8])(=O)C.Cl.[Cl-].[Na+]>O.N>[SH:4][CH2:5][CH:6]([CH2:17][C:18]([O:20][CH3:21])=[O:19])[C:7]([N:9]1[CH2:16][CH2:15][CH2:14][C@H:10]1[C:11]([OH:13])=[O:12])=[O:8]1-[3-(Acetylthio)-2-(methoxycarbonylmethyl)propanoyl]-L-proline (2.1 g.) is dissolved in a mixture of water (35 ml.) and concentrated ammonia (35 ml.) under a blanket of argon. After twenty minutes, the solution is chilled in an ice bath, made acidic with concentrated hydrochloric acid, saturated with sodium chloride and extracted with ethyl acetate. The organic layer is dried and concentrated to dryness in vacuo to yield 1.1 g. of 1-[3-mercapto-2-(methoxycarbonylmethyl)propanoyl]-L-proline that is purified by chromatography on silica gel (benzene:acetic acid 75:25). Rf :0.35 (silica gel, benzene:acetic acid, 75:25).
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