The present invention provides compounds of the general formulae: ##STR1## wherein R.sub.1 and R.sub.2, which can be the same or different, are hydrogen atoms or C.sub.1 -C.sub.5 -alkyl radicals, C.sub.3 -C.sub.5 -alkenyl radicals or phenyl radicals or, together with the carbon atom to which they are attached, form a saturated or unsaturated C.sub.3 -C.sub.7 ring, R.sub.3 and R.sub.4, which can be the same or different, are hydrogen atoms, straight-chained or branched C.sub.1 -C.sub.10 -alkyl radicals, straight-chained or branched C.sub.3 -C.sub.7 -alkenyl radicals, C.sub.3 -C.sub.7 -cycloalkyl radicals, C.sub.3 -C.sub.7 -cycloalkenyl radicals, phenyl, arylalkyl or hetarylalkyl radicals, R.sub.5 is a hydrogen atom or a lower alkyl radical and X is an oxygen or sulphur atom or an imino group.The present invention also provides processes for the preparation of these compounds, as well as pharmaceutical compositions with immunosuppressive action containing them.