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.
