[CH:1](O)=[O:2].C(OC(=O)C)(=O)C.[NH2:11][CH:12]1[C:23](=[O:24])[N:14]2[CH:15]([C:20]([OH:22])=[O:21])[C:16](=[CH2:19])[CH2:17][S:18][C@H:13]12>CN(C)C=O>[CH:1]([NH:11][CH:12]1[C:23](=[O:24])[N:14]2[CH:15]([C:20]([OH:22])=[O:21])[C:16](=[CH2:19])[CH2:17][S:18][C@H:13]12)=[O:2]A mixture of formic acid (1 l) and acetic anhydride (2.5 l) was stirred at 50° C. for 1 hour and cooled at 40°C. To the mixture were added 7-amino-3-methylenecepham-4-carboxylic acid (1,070 g) and N,N-dimethylformamide (500 ml). The mixture was stirred for 1 hour at 55° C. and cooled in an ice-water bath. A resultant precipitate was collected by filtration and washed with a small amount of a mixture of ethyl acetate and diisopropyl ether (1:1) to give 7-formamido-3-methylenecepham-4-carboxylic acid (1.02 g).
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