Compounds of general formula (I): ##STR1## in which: R.sub.1 and R.sub.2 each represent a hydrogen atom or, with the nitrogen and oxygen which bear them, form an --O--CO--N link, W represents a halogen atom or a lower alkyl or alkoxy group optionally substituted with one or more halogen atoms, such as trifluoromethyl, and m being between 0 and 3, A is a linear or branched alkyl radical comprising from 1 to 6 carbon atoms, R.sub.3 and R.sub.4 are defined in the description have medicinal properties.