The present invention relates to a compound represented by the formula (I): or a pharmacologically acceptable salt thereof, wherein R1, R2, R3, and R4 are the same or different and each represent a hydrogen atom or a C1-6 alkyl group; X1 represents a C1-6 alkylene group that may be substituted; Xa represents a methoxy group or a fluorine atom; Xb represents an oxygen atom or a methylene group, provided that Xb is only an oxygen atom when Xa is a methoxy group; and Ar1 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.