The present invention relates to a compound represented by the formula (I): ##STR00001## or a pharmacologically acceptable salt thereof, wherein R.sup.1, R.sup.2, R.sup.3, and R.sup.4 are the same or different and each represent a hydrogen atom or a C1-6 alkyl group; X.sub.1 represents a C1-6 alkylene group that may be substituted; X.sub.a represents a methoxy group or a fluorine atom; X.sub.b represents an oxygen atom or a methylene group, provided that X.sub.b is only an oxygen atom when X.sub.a is a methoxy group; and Ar.sub.1 represents an aryl group, pyridinyl group, aryloxy group, or pyridinyloxy group that may have a substituent such as a halogen atom; and to use of the compound or salt as a pharmaceutical agent.