Abstract: The invention relates to compounds of the general formula (X) ##STR1## wherein R is as a removable protecting substituent of the amido group a phenyl group or a benzyl group substituted by one or more alkoxy group(s) having 1-4 carbon atoms andR.sup.1 stands for a hydrogen atom or an alkyl group having 1-4 carbon atoms and a process for the preparation thereof.The compounds of the general formula (X) can be prepared from the starting materials of the general formula (V), wherein R and R.sup.1 are as stated above and Z is an alkyl group having 1-4 carbon atoms.The compounds of the general formula (X) are useful pharmaceutical intermediates which can be used in the preparation of known antibiotics (e.g. Thienamycin and PS-5).