In N,N-dimethylformamide (20 ml) was dissolved 7β-(2-thienylacetamido)-3-hydroxymethyl-3-cephem-4-carboxylic acid sodium salt (3.86 g) and, at -5° C., triethylamine (1.40 ml) and diketene (1.50 ml) were added. The reaction was carried out at -5°-0° C. for 1 hour, after which time the reaction mixture was poured in ice-water (200 ml). The mixture was adjusted to pH 2.0 with 4 N-hydrochloric acid and extracted with ethyl acetate. The ethyl acetate layer was rinsed with water, diluted with water, brought to pH 7.0 with 5% aqueous sodium hydrogen carbonate solution and separated. The water layer was taken, concentrated and subjected to column-chromatography on Amberlite XAD-2, elution being carried out with water-methanol. The eluate was lyophilized. The above procedure provided 7β-(2-thienylacetamido)-3-(3-oxobutyryloxy)methyl-3-cephem-4-carboxylic acid sodium salt (4.48 g).
