The formula (I): ##STR00001## wherein each of R.sup.2 and R.sup.3 is, same or different, C2-C4 alkyl or the like; or R.sup.2 and R.sup.3 are taken together with the adjacent carbon atom to form a 5 to 8 membered non-aromatic carbocyclic ring; R.sup.4 is C1-C6 alkyl or the like; X is an oxygen atom or a sulfur atom; A is the group of the formula: ##STR00002## wherein R.sup.1 is, same or different, alkyl or the like; W is C2-C6 alkylene which may contain an optionally substituted heteroatom(s) or the like; n is an integer of 0 to 7, a pharmaceutically acceptable salt, or a solvate thereof.