A suspension of 0.250 g of 7β-amino-3 -methoxy-3-cephem-4-carboxylic acid diphenylmethyl ester in 25 ml of methylene chloride is treated with 1 ml of pyridine and 0.5 ml of phenylacetic acid chloride at 0° C. under a nitrogen atmosphere and stirred for 30 minutes at this temperature. The reaction mixture is evaporated under reduced pressure; the residue is stirred for 10 minutes with 20 ml of a 1:1 mixture of dioxane and water and the mixture is diluted with methylene chloride. The aqueous phase is separated off and extracted with methylene chloride. The combined organic phases are washed with a saturated aqueous sodium bicarbonate solution and a saturated aqueous sodium chloride solution, dried over sodium sulphate and evaporated under reduced pressure. The crude product is purified by means of preparative thin layer chromatography, using a 1:1 mixture of toluene and ethyl acetate as the solvent. The zone visible under ultraviolet light of λ=254 mμ (Rf~0.35) is eluted with a 4:1 mixture of acetone and methanol and yields 3-methoxy-7β-phenylacetylamino-3-cephem-4-carboxylic acid diphenylmethyl ester, which is identical with the product obtainable according to the process of Example 1.
