Brain function improving agent

A method for use in treatment of cognitive dysfunction selected from the group comprising Alzheimer's disease, Parkinson disease, Pick's disease, and Huntington's disease, schizophrenia, bipolar disorder, depression, phobia, sleep disorder, drug dependence, autism, Asperger's syndrome, mental deficiency, polyergic disorder, or tic disorder, comprising administering to a subject in need thereof an effective amount of a compound represented by formula (I):

• wherein, R1 is a hydrogen atom, C1-6 alkyl, cyano, —C(═O)NR11R12, or —C(═O)OR13;
• R2 is a hydrogen atom, C1-6 alkyl, hydroxy, —X1—R14, or —NR15R16;
• R3, R4, R5, and R6 are each independently selected from the group consisting of a hydrogen atom, a halogen atom, C1-6 alkyl, hydroxy, C1-6 alkoxy, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, cyano, nitro, —C(═O)NR17R18, and —C(═O)OR19;
• R11 and R12 are each independently selected from the group consisting of a hydrogen atom, C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl;
• or
• R11 and R12, together with a nitrogen atom to which they are attached, form a nitrogen-containing heterocycle, and the nitrogen-containing heterocycle may be substituted with one or more substituents selected from the group consisting of C1-6 alkyl, hydroxy, hydroxyC1-6 alkyl, C1-6 alkoxy, C1-6 alkoxyC1-6 alkyl, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, amino C1-6 alkyl, (C1-6 alkylamino)C1-6 alkyl, [di(C1-6 alkyl)amino]C1-6 alkyl, —C(═O)NR22R23, —C(═O)OR24, —(C1-6 alkyl)C(═O)NR22R23, and —(C1-6 alkyl)C(═O)OR24;
• R13 is selected from the group consisting of a hydrogen atom, C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl, wherein the alkyl group may be substituted with one or more substituents selected from the group consisting of C6-10 aryl, hydroxy, C1-6 alkoxy, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, —C(═O)NR22R23, and —C(═O)OR24;
• X1 is —O—, —S—, —SO—, or —SO2—;
• R14 is selected from the group consisting of C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl;
• R15 is hydrogen atom, C1-6 alkyl, C6-10 aryl, 5- to 10-membered heteroaryl, or —C(═O)—R21, wherein the alkyl group may be substituted with one or more substituents selected from the group consisting of C6-10 aryl, hydroxy, C1-6 alkoxy, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, —C(═O)NR22R23, and —C(═O)OR24;
• R16 is a hydrogen atom, C1-6 alkyl, C6-10 aryl, or 5- to 10-membered heteroaryl, wherein the alkyl group may be substituted with one or more substituents selected from the group consisting of C6-10 aryl, hydroxy, C1-6 alkoxy, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, —C(═O)NR22R23, and —C(═O)OR24;
• or
• R15 and R16, together with a nitrogen atom to which they are attached, form a nitrogen-containing heterocycle, and the nitrogen-containing heterocycle may be substituted with one or more substituents selected from the group consisting of C1-6 alkyl, hydroxy, hydroxyl C1-6 alkyl, C1-6 alkoxy, C1-6 alkoxyC1-6 alkyl, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, aminoC1-6 alkyl, (C1-6 alkylamino)C1-6 alkyl, [di(C1-6 alkyl)amino]C1-6 alkyl, —C(═O)NR22R23, —C(═O)OR24, —(C1-6 alkyl)C(═O)NR22R23, and —(C1-6 alkyl)C(═O)OR24;
• R17 and R18 are each independently selected from the group consisting of a hydrogen atom, C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl;
• or R17 and R18, together with a nitrogen atom to which they are attached, form a nitrogen-containing heterocycle;
• R19 is selected from the group consisting of a hydrogen atom, C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl;
• R21 is selected from the group consisting of a hydrogen atom, C1-6 alkyl, C1-6 alkoxy, C6-10 aryl, and 5- to 10-membered heteroaryl, wherein the alkyl group and the alkoxy group may be substituted with one or more substituents selected from the group consisting of C6-10 aryl, hydroxy, C1-6 alkoxy, C1-6 alkylthio, C1-6 alkylsulfinyl, C1-6 alkylsulfonyl, amino, C1-6 alkylamino, di(C1-6 alkyl)amino, —C(═O)NR22R23, and —C(═O)OR24;
• R22 and R23 are each independently selected from the group consisting of a hydrogen atom, C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl, or R22 and R23, together with a nitrogen atom to which they are attached, form a nitrogen-containing heterocycle group;
• R24 is each independently selected from the group consisting of a hydrogen atom, C1-6 alkyl, C6-10 aryl, and 5- to 10-membered heteroaryl, or a pharmaceutically acceptable salt thereof.