The invention provides compounds of formula (I) ##STR00001## wherein R.sup.1 represents hydrogen or alkyl; R.sup.2 represents an unsubstituted or substituted heterocycle or R.sup.2 represents an unsubstituted or substituted aryl; R.sup.3 represents alkyl or halogen; X represents a single bond or an alkandiyl-group, optionally interrupted by one or more oxygen atoms or carbonyl groups or carbonyloxy groups in free base or acid addition salt form, processes for their preparation and their use as pharmaceuticals.