C(=O)(O)[O-].[Na+].[S:6]=[C:7]1[NH:11][CH2:10][N:9]=[N:8]1.C(O[CH2:16][C:17]1[CH2:24][S:23][CH:22]2[N:19]([C:20](=[O:35])[CH:21]2[NH:25][C:26](=[O:34])[CH2:27][C:28]2[S:29][C:30](=[O:33])[S:31][CH:32]=2)[C:18]=1[C:36]([OH:38])=[O:37])(=O)C>>[C:36]([C:18]1[N:19]2[CH:22]([S:23][CH2:24][C:17]=1[CH2:16][S:6][C:7]1[N:11]=[CH:10][NH:9][N:8]=1)[CH:21]([NH:25][C:26](=[O:34])[CH2:27][C:28]1[S:29][C:30](=[O:33])[S:31][CH:32]=1)[C:20]2=[O:35])([OH:38])=[O:37]Sodium bicarbonate (5.55 g.) and 3-thioxo-1,2,4-triazoline (3.63 g.) are added to 3-acetoxymethyl-2-carboxy-8-oxo-7-[(1,3-dithiol-2-on-4-yl)-acetamido]-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene (12.9 g.) in a buffer (250 cc.) of pH 6.4. The mixture is heated to 60° C. for 7 hours. It is allowed to cool, sodium bicarbonate (1 g.) is added and the mixture is washed with ethyl acetate (250 cc.), which is discarded. The aqueous solution is treated with decolourising charcoal and is acidified to pH 2 by adding 4 N hydrochloric acid in the presence of ethyl acetate (200 cc.). The organic phase is separated off and the aqueous phase is then extracted with ethyl acetate (750 cc.). The combined organic extracts are dried over sodium sulphate, treated with decolourising charcoal and concentrated to dryness under reduced pressure (20 mm Hg). A residue (5.5 g.) is obtained, which is triturated with acetonitrile (25 cc.). The solid is filtered off and washed with acetonitrile (25 cc.) and isopropyl ether (20 cc.) to give 2-carboxy-8-oxo-7-[(1,3-dithiol-2-on-4-yl)-acetamido]-3-[(1,2,4-triazol-3-yl)-thiomethyl]-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene (2.9 g.), m.p. 227° C. (decomposition).
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