A solution, cooled to 0° C., of 2.0 g of 7β-phenylacetylamino-cepham-3-one-4ξ-carboxylic acid diphenylmethyl ester in 75 ml of methanol is treated with excess diazo-n-butane in diethyl ether. The mixture is stirred for 15 minutes at 0° C. and is then evaporated under reduced pressure. The oily residue is purified by means of preparative thin layer chromatography (silica gel plates, 6×100 cm; system: toluene/ethyl acetate, 3:1). The layer which is visible under ultraviolet light (λ=254 mμ) is removed and in each case eluted with acetone. This zone (Rf~0.33) yields 3-n-butoxy-7β-phenylacetylamino-3-cephem-4-carboxylic acid diphenylmethyl ester which after crystallisation from a mixture of methylene chloride and diethyl ether is in the form of colourless flakes which melt at 168°-170° C., [α]D20 =+55°±1° (c=0.38 in chloroform); ultraviolet absorption spectrum (in 95% strength aqueous ethanol): λmax =264 mμ (ε=7,300); infrared absorption spectrum (in methylene chloride): characteristic bands at 2.98μ, 5.62μ, 5.81μ, 5.92μ, 6.25μ and 6.62μ.
