Process for the preparation of a syn isomer or a mixture of syn and anti isomers of 7beta-acylamidoceph-3-em-4-carboxylic acids with antibiotic activity

Syn isomers or mixtures of syn and antiisomers (with a syn isomer content of at least 90%) of 7 beta -acylamidoceph-3-em-4-carboxylic acids with antibiotic activity and the formula I in which R is furyl, thienyl or phenyl; R is C1-C4-alkyl, C2-C4-alkenyl, C2-C4-alkynyl, C3 -C7-cycloalkyl or phenyl; R is C1-C4-alkyl, C2-C4-alkenyl, C2-C4-alkynyl, C3 -C7-cycloalkyl, benzyl or phenyl; and B is >S or >S->O are prepared by reacting a corresponding 7-amino compound, where appropriate with intermediate protection of the 4-carboxyl group, with an acid of the formula IV or with an amide-forming derivative thereof; the acid has the syn configuration indicated for formula I. Resulting 1-sulphoxides of the formula I can be reduced to the corresponding 1-sulphides; likewise, resulting ceph-2-em compounds can be isomerised to the corresponding ceph-3-em compounds. The compounds of the formula I can also be esterified. The products are distinguished by a broad spectrum of antibiotic activity in conjunction with a high stability to beta -lactamase and by good stability in vivo.