Abstract: The present invention relates to novel compounds of the following formula, where the dotted line designates an optional bond: ##STR1## wherein "het" designates a five membered heterocyclic ring which may include 1 or 2 double bonds and 1-4 heteroatoms selected from nitrogen, oxygen or sulphur, provided that "het" may not designate a 1,2,4- or 1,3,4-oxadiazole;R.sup.1 is selected from hydrogen, lower alkyl, optionally substituted with phenyl which may be substituted with halogen, lower alkyl, or lower alkoxy, or a group R.sup.6 --CO--NH--CH.sub.2 -- or R.sup.6 --O--CO--, wherein R.sup.6 is lower alkyl, branched or unbranched, or phenyl optionally substituted with halogen, trifluoromethyl, lower alkyl, hydroxy, lower alkoxy, or lower acyloxy;R.sup.2 and R.sup.