A compound or pharmacologically acceptable salt thereof represented by the formula (I) has PDE9 inhibitory action, so that the intracerebral cGMP concentration is anticipated to be elevated. The PDE9 inhibitory action and the increase in cGMP lead to the improvement of learning and memory behaviors, and the compound (I) has a potential use of a therapeutic agent for cognitive dysfunctions in Alzheimer's disease. wherein R 1 is a hydrogen atom； R 2 is an aromatic ring group, etc.； R 3 is a hydrogen atom, etc； R 4 is a hydrogen atom； R 5 is an oxepanyl group, etc.； R 6 is a hydrogen atom.式(I)所表示之化合物或其藥理學上容許之鹽因具有PDE9抑制作用，可期待使腦內之cGMP濃度上升。PDE9抑制作用或cGMP之上升可改善學習記憶行為，具有用作阿茲海默氏病之認知功能障礙之治療劑的可能性。[式中，R1為氫原子，R2為芳香環基等，R3為氫原子等，R4為氫原子，R5為氧雜環庚基等，R6為氫原子]。
