A compound of the formula: ##STR1## wherein R.sup.1 -A- is a group of the formula: ##STR2## in which R.sup.1 is aryl or a heterocyclic group, each of which may be substituted with substituent(s) selected from the group consisting of lower alkyl, lower alkoxy, lower alkylthio, halo(lower)alkyl, acyl, acylamino and halogen, or lower alkyl which may be substituted with a heterocyclic group optionally substituted with acyl, andX is O or S, andR.sup.2 is hydrogen or lower alkyl, provided that when R.sup.1 is lower alkyl, thenR.sup.1 -A- is a group of the formula: ##STR3## in which R.sup.1 and X are each as defined above, processes for the preparation thereof and pharmaceutical composition comprising the same.