[Cl:1][C:2]([Cl:7])([Cl:6])[C:3](Cl)=[O:4].[CH3:8][C:9]1[CH2:16][S:15][CH:14]2[N:11]([C:12](=[O:27])[CH:13]2[NH:17]C(=O)CC2C=CC=CC=2)[C:10]=1[C:28]([O:30][CH2:31][C:32]([Cl:35])([Cl:34])[Cl:33])=[O:29].O>N1C=CC=CC=1.C1C=CC=CC=1>[CH3:8][C:9]1[CH2:16][S:15][CH:14]2[N:11]([C:12](=[O:27])[CH:13]2[NH:17][C:3](=[O:4])[C:2]([Cl:7])([Cl:6])[Cl:1])[C:10]=1[C:28]([O:30][CH2:31][C:32]([Cl:35])([Cl:33])[Cl:34])=[O:29]Trichloroacetyl chloride (1.04 cc.) is added, over the course of 30 minutes and with vigorous stirring, to a solution, cooled to -10°C, of 3-methyl-8-oxo-7-phenylacetamido-2-trichloroethoxycarbonyl-5-thia-1-aza-bicyclo[4,2,0]oct-2-ene (1.38 g) in pyridine (15 cc.). Thereafter, the mixture is stirred for 2 hours 30 minutes at a temperature of between -3° and -1°C. The mixture is then poured into water and crushed ice (100 cc.) and a maroon paste-like product settles out. After removing the liquid phase by decanting, this product is triturated in water (50 cc.) and is then taken up in ethyl acetate (200 cc.). The organic phase is washed with water (3 times 200 cc.), dried over sodium sulphate, filtered and concentrated to dryness under reduced pressure (12 mm Hg) at 30°C. The solid obtained is dissolved in benzene (20 cc.) and the solution obtained is chromatographed on a column of silica gel (20 g) (0.05-0.20 mm, pH neutral; diameter of the column: 1.5 cm, height: 20 cm). Elution is carried out successively with benzene (300 cc.), a mixture of benzene and ethyl acetate (99.5/0.5 by volume) (300 cc.) and a mixture of benzene and ethyl acetate (99/1 by volume) (600 cc.), collecting 25 cc. fractions. Fractions 6 to 36 are combined and concentrated to dryness under reduced pressure (12 mm Hg) at 30°C. 3-methyl-8-oxo-7-trichloroacetamido-2-trichloroethoxycarbonyl-5-thia-1-aza-bicyclo[4,2,0]oct-2-ene (450 mg) is obtained in the form of a light maroon solid, the characteristics of which are as Follows:RF = 0.63 [silica gel; chloroform/ethyl acetate (80/20 by volume)]
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