CH625526

Cephalosporin antibiotics in which the 7 beta -acylamido group has the structure (WHERE R is phenyl, thienyl or furyl; Ra and Rb are each selected from hydrogen, C1-4 alkyl, C2-4 alkenyl, C3-7 cycloalkyl, phenyl, naphthyl, thienyl, furyl, carboxy, C2-5 alkoxycarbonyl, aminocarbonyl, N-substituted aminocarbonyl and cyano, or Ra and Rb together with the carbon atom to which they are attached form a C3-7 cycloalkylidene or cycloalkenylidene group; Rc is hydrogen or C1-4 alkyl; and m and n are each 0 or 1 such that the sum of m and n is 0 or 1) exhibit broad spectrum antibiotic activity characterized by particularly high activity against gram negative microorganisms, including those which produce beta -lactamases. The compounds, which are syn isomers or exist as mixtures of syn and anti isomers containing at least 90% of the syn isomer, have particularly good activity against Proteus organisms including indole positive strains and, especially when both Ra and Rb are other than hydrogen, against Pseudomonas organisms.