A compound or pharmacologically acceptable salt thereof represented by theformula(I) has PDE9 inhibitory action, so that the intracerebral cGMP concentrationis anticipated tobe elevated. The PDE9 inhibitory action and the increase in cGMP lead to theimprovementof learning and memory behaviors, and the compound (I) has a potential use ofa therapeuticagent for cognitive dysfunctions in Alzheimer's disease.(See Formula I)wherein IV is a hydrogen atom； R2 is an aromatic ring group, etc.； R3 is ahydrogen atom, etc；R4 is a hydrogen atom； R5 is an oxepanyl group, etc.； R6 is a hydrogen atom.
