Compounds of the formula I ##STR1## in which the carboxyl group on carbon atom 3 is orientated in the endo-position relative to the bicyclic ring system of cis-configuration, and in which R.sup.1 denotes hydrogen, allyl, vinyl or a side-chain of a naturally occurring .alpha.-aminoacid, which may be protected, R.sup.2 denotes hydrogen, alkyl, alkenyl or aralkyl, Y denotes hydrogen or hydroxyl and Z denotes hydrogen, or Y and Z together denote oxygen, and X denotes alkyl, alkenyl or cycloalkyl, or aryl which is optionally mono-, di- or tri-substituted by alkyl, alkoxy, hydroxyl, halogen, nitro, amino, alkylamino, dialkylamino or methylenedioxy, or denotes indol-3-yl, a process for their preparation, agents containing these compounds and their use.