The present invention relates to a compound represented by formula (I): (wherein R, R2, R3 and R4 each independently represent a hydrogen or halogen atom R5 and R6 each independently represent a hydrogen atom or lower alkyl or together represent oxo X represents C(O) or the like Y represents an oxygen atom or the like Z represents a hydrogen atom or the like R represents 5- or 6-membered heteroaryl having 1-3 hetero atoms selected from the group consisting of nitrogen, sulfur and oxygen atoms, contained within a ring, or the like) or to a pharmaceutically acceptable salt thereof.
