[CH3:1][O:2][C:3]([C@H:5]1[NH:8][C:7](=[O:9])[C@H:6]1[NH2:10])=[O:4].C(=O)([O-])O.[Na+].[Cl:16][CH2:17][C:18]([NH:20][C:21]1[S:22][CH:23]=[C:24]([C:26](=[N:30][O:31][CH3:32])[C:27](Cl)=[O:28])[N:25]=1)=[O:19].O>O1CCCC1>[CH3:1][O:2][C:3]([C@H:5]1[NH:8][C:7](=[O:9])[C@H:6]1[NH:10][C:27](=[O:28])[C:26]([C:24]1[N:25]=[C:21]([NH:20][C:18](=[O:19])[CH2:17][Cl:16])[S:22][CH:23]=1)=[N:30][O:31][CH3:32])=[O:4]Under ice-cooling and stirring, 6 ml of an aqueous solution containing 288 mg of cis-3-amino-2-oxoazetidine-4-carboxylic acid methyl ester and 561 mg of sodium hydrogen carbonate is added to a suspension of 1.18 g of 2-(2-chloroacetamido-4-thiazolyl)-2-methoxyiminoacetyl chloride (anti isomer) in 10 ml of tetrahydrofuran. The mixture is stirred for 2 hours at room temperature and the solvent is then distilled off in vacuo to leave a residue, which solidifies. Water is added to the solidified residue, followed by filtration. The filtrate is neutralized with an aqueous sodium hydrogen carbonate solution and then extracted with chloroform. The chloroform layer is dried over anhydrous sodium sulfate and the solvent is distilled off in vacuo to give cis-3-[2-(2-chloroacetamido-4-thiazolyl)-2 -methoxyiminoacetamido]-2-oxoazetidine-4-carboxylic acid methyl ester (anti-isomer). m.p. 119°-129° C.
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