Process for the preparation of a syn-isomer or a mixture of syn- and anti-isomers of novel antibiotically active 7-beta-acylamidoceph-3-em-4-carboxylic acids

Syn-isomers or mixtures of syn- and anti-isomers, the amount of the syn-isomer being at least 90%, of novel antibiotically active 7 beta -acylamidoceph-3-em-4-carboxylic acids of the formula I, in which R denotes furyl, thienyl or phenyl; R denotes C1-C4-alkyl, C2-C4-alkenyl, C2-C4-alkynyl, C3-C7-cycloalkyl or phenyl; R denotes C1-C4-alkyl, C2-C4-alkenyl, C2-C4-alkynyl, C3-C7-cycloalkyl, benzyl or phenyl; and B denotes &rdurule& S or &rdurule& S->O are prepared by reacting an appropriate 3-hydroxymethyl compound, optionally with intermediate protection of the 4-carboxyl group, with an isocyanate of the formula R-NCO. Resulting 1-sulphoxides of the formula I can be reduced to the corresponding 1-sulphides. Resulting ceph-2-em compounds are isomerised to the corresponding ceph-3-em compounds. The process products are distinguished by a broad spectrum of antibiotic action in combination with high beta -lactamase stability and by a good stability in vivo.