3-Propenzl cephalosporin isomer separation process and derivative

The process for preparing and separating antibiotic sodium salt imidazolidinone derivative which comprises (1) contacting a composition containing the antibiotic 7-[D-2-amino-2-(4-hydroxyphenyl)acetamido]-3-[(Z)-1-propenyl]ceph-3-em-4-c arboxylic acid in admixture with the (E)-isomer thereof with a sodium salt forming reagent in the presence of an alkyl, alicyclic, aryl, or aralkyl ketone having a molecular weight of less than 200 under reaction conditions whereby antibiotic sodium salt imidazolidinone derivative and (E)-isomer sodium salt imidazolidinone derivative are formed (2) suspending the imidazolidinone derivatives formed in Step (1) in a solvent in which said (E)-isomer sodium salt imidazolidinone derivative is soluble and said antibiotic sodium salt imidazolidinone derivative is substantially insoluble thereby forming a suspension of said antibiotic derivative in a solution of said (E)-isomer derivative, and (3) recovering said insoluble antibiotic derivative from said suspension substantially free of said (E)-isomer derivative.