Compounds of formula ##STR1## wherein the radicals R.sub.1 and R.sub.2 are the same or different and represent hydrogen, halogen atoms, a trifluoromethyl group, a cyano, a nitro group, an amino group, a mono-C.sub.1 -C.sub.6 -alkylamino group, a di-C.sub.1 -C.sub.6 -alkylamino group, an amino group that is substituted by a phenyl C.sub.1 -C.sub.4 -alkyl radical, a C.sub.2 -C.sub.6 -alkanoyl-amino group, a C.sub.1 -C.sub.6 -alkoxycarbonyl amino group, a C.sub.1 -C.sub.6 -alkyl group optionally substituted by a phenyl radical, a hydroxy group, a C.sub.1 -C.sub.6 -alkoxy group, a C.sub.2 -C.sub.6 -alkanoyloxy group, a phenoxy group or represent a carbamoyl group optionally substituted by one or two C.sub.1 -C.sub.6 -alkyl groups and the radical A is an acyl group which is derived from an amino acid as well as processes for their preparation.