[C:1]([CH2:4][O:5][N:6]=[C:7]([C:11]1[N:15]=[C:14]([NH2:16])[S:13][N:12]=1)[C:8]([OH:10])=[O:9])([OH:3])=[O:2].[CH3:17][S:18](Cl)(=[O:20])=[O:19].C(=O)([O-])O.[K+].Cl>CN(C)C(=O)C.O.C(OCC)(=O)C>[CH3:17][S:18]([O:9][C:8](=[O:10])[C:7](=[N:6][O:5][CH2:4][C:1]([OH:3])=[O:2])[C:11]1[N:15]=[C:14]([NH2:16])[S:13][N:12]=1)(=[O:20])=[O:19]
To a solution of 2-carboxymethoxyimino-2-(5-amino-1,2,4-thiadiazol-3-yl)acetic acid (syn isomer) (45.0 g) in N,N-dimethylacetamide (432 ml) was added methanesulfonyl chloride (29.8 g) under stirring and cooling in an ice bath. The mixture was stirred for 25 minutes at 4° to 6° C. and finely powdered potassium hydrogen carbonate (48.4 g) was added thereto. The reaction mixture was stirred for 2.5 hours at 4° to 6° C. and then poured into a cold mixture of ethyl acetate (1.2 l), 1N hydrochloric acid (484 ml) and water (500 ml) below 10° C. under stirring. The organic layer was separated out and the aqueous phase was extracted with ethyl acetate (300 ml). The combined extracts were washed with cold water (800 ml) and a saturated aqueous solution of sodium chloride (800 ml), dried over magnesium sulfate and evaporated. The residue was triturated in methylene chloride (200 ml) to give 2 -carboxymethoxyimino-2-(5-amino-1,2,4-thiadiazol-3-yl)acetic methanesulfonic anhydride (syn isomer) (28.0 g), mp. 143° to 146° C. (dec.).
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