[NH:1]1[CH2:8][CH2:7][CH2:6][C@H:2]1[C:3]([OH:5])=[O:4].C(=O)([O-])[O-].[Na+].[Na+].[C:15]([S:18][CH2:19][CH:20]([CH2:24][S:25][CH3:26])[C:21](Cl)=[O:22])(=[O:17])[CH3:16].C(SCC(CSC)C(O)=O)(=O)C.S(Cl)(Cl)=O>O>[C:15]([S:18][CH2:19][CH:20]([CH2:24][S:25][CH3:26])[C:21]([N:1]1[CH2:8][CH2:7][CH2:6][C@H:2]1[C:3]([OH:5])=[O:4])=[O:22])(=[O:17])[CH3:16]To a solution of L-proline (1.44 g.) and sodium carbonate (2.7 g.) in water (25 ml.) in an ice bath, 3-(acetylthio)-2-(methylthiomethyl)propanoic acid chloride (prepared from the acid of Example 15 with thionyl chloride) (3.6 g.) is added, and the mixture is vigorously stirred at room temperature for two hours. After extraction with ethyl acetate, the aqueous layer is acidified and extracted with ethyl acetate. The organic layer is dried and concentrated to dryness in vacuo to give 1-[3-(acetylthio)-2-(methylthiomethyl)propanoyl]-L-proline.
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