ANALOGIFREMGANGSMAADE TIL FREMSTILLING AF CEPHALOSPORINFORBINDELSER ELLER UGIFTIGE SALTE, BIOLOGISK ACCEPTABLE ESTERE, 1-OXYDER ELLER SOLVATER DERAF

Preparation of the syn isomer or of a mixture of syn isomer and anti isomer, where the content of the syn isomer is at least 90%, of 7-acylaminoceph-3-em-4-carboxylic acids with antibiotic activity and the formula I in which B is > S or > S->O; R is furyl, thienyl or phenyl; and R is C1-C4-alkyl, C3-C7-cycloalkyl or phenyl, and of their non-toxic salts, including their solvates, and esters by acylation of the corresponding 7-amino compound, where appropriate with intermediate protection of the 4-carboxyl group and/or of the 3-carbamoyloxymethyl group, and subsequent isomerisation of a ceph-2-em compound which is obtained where appropriate to the ceph-3-em compound. The compounds are distinguished by a broad spectrum of action on pathogenic microorganisms, the ability to resist beta -lactamases and good stability in vivo.