Compounds of the general formula (I), optical isomers or prodrugs of the same, pharmaceutically acceptable salts of them, or solvates thereof: (I) wherein R<1> is hydroxyl or the like; R<2> is optionally substituted lower alkyl or the like; R<3> is hydrogen or the like; R<4> is optionally substituted arylene or the like; R<5> is -CC- or the like; and R<6> is optionally substituted naphthyl or the like.
