The condensation of 4-ethyl-2,3-dioxo-1-piperazinecarbonyl chloride (I) with threonine (II) in methylene chloride gives alpha-(4-ethyl-2,3-dioxo-1-piperazinecarboxamido)-beta-hydroxybutyric acid (III), which is then condensed with diphenylmethyl 7-beta-amino-3-[(1-methyltetrazol-5-yl)thiometnyl)-Delta3-cephem-4-carboxylate (IV) by means of ethyl chloroformate and N-methyl morpholine yielding diphenylmethyl 7-beta-[alpha-(4-ethyl-2,3-dioxo-1-piperazi-necarboxamido)-beta-hydroxybutyramido]-3-(11-methyltetrazol-5-yl)thiomethyl]-Delta3-cephem-4-carboxylate (V). The methoxylation of (V) with lithium methoxide-methylene chloride THF affords diphenyl methyl 7-beta-[alpha-(4-ethyl-2,3-dioxo-1-piperazinecarboxamido)-beta-hydroxybutyramido]-7-alpha-methoxy-3-[(1-methyltetrazol-5-yl)thiamethyl-Delta3-cephem-4-carboxylate (VI), which is finally hydrolyzed with trifluoroacetic acid, and treated with NaHCO3 to obtain the sodium salt.