The syn isomer of 2-(1-t-butoxycarbonylcyclobutoxyimino)-2-(2-tritylaminothiazol-4-yl)-acetic acid prepared according to Belgian Patent 876,541 (4.42 g) and 4-dimethylaminopyridine (0.25 g) are added to a solution of the E form of 7-amino-2-benzhydryloxycarbonyl-8-oxo-3-(2-tosyloxyvinyl)-5-thia-1-azabicyclo[4.2.0]oct-2-ene-5-oxide (product 7d) (4.34 g) in methylene chloride (50 cc). The mixture is cooled to 5° C. and a solution of N,N'-dicyclohexylcarbodiimide (1.69 g) in methylene chloride (30 cc) is added dropwise in the course of 10 minutes. The temperature is left to rise, the mixture is stirred for 4 hours at 20° C. and concentrated to dryness at 20° C. under 20 mm Hg (2.7 kPa) and the residue is taken up in ethyl acetate (200 cc). The solution is washed with water (2×100 cc), 0.1 N hydrochloric acid (100 cc), a saturated aqueous solution of sodium bicarbonate (100 cc) and water (100 cc), dried over sodium sulphate and filtered and the filtrate is concentrated to dryness at 25° C. under 20 mm Hg (2.7 kPa). The residue is chromatographed on a column of Merck silica gel (0.06-0.2) (180 g) (diameter of the column: 3 cm, height: 60 cm). Elution is carried out with a 70/30 (by volume) cyclohexane/ethyl acetate mixture (1 liter), 60 cc fractions being collected. Fractions 6 to 10 are concentrated to dryness at 20° C. under 20 mm Hg (2.7 kPa) and this yields a product (7a) (2.7 g) in the form of a cream-coloured powder.
