A compound selected from the group consisting of a syn isomer of a compound of the formula ##STR1## wherein R is selected from the group consisting of hydrogen, alkyl of 1 to 6 carbon atoms, alkenyl and alkynyl of 2 to 6 carbon atoms and cycloalkyl of 3 to 6 carbon atoms, all optionally substituted with at least one member of the group consisting of optionally esterified or salified carboxy, alkoxy carbonyl, carbamoyl, dimethylcarbamoyl, amino, alkylamino, dialkylamino, halogen, alkoxy and alkylthio of 1 to 4 carbon atoms, aryl, heterocyclic aryl, arylthio and heterocyclic arylthio optionally substituted by alkyl of 1 to 4 carbon atoms, R.sub.1 is selected from the group consisting of ##STR2## and A is selected from the group consisting of hydrogen, alkali metal, alkaline earth metal, magnesium, --NH.sub.4 and an organic amine or A is selected from the group consisting of the residue of an easily cleavable ester group or --COOA is --COO-- and the wavy line indicates --CH.sub.2 --R.sub.1 is in the E or Z position and their non-toxic, pharmaceutically acceptable acid addition salts having good antibacterial activity.