Substituted imidazo [1,5-a]pyrazines as PDE9 inhibitors

1. A compound having the structure (I):

and a tautomer or a pharmaceutically acceptable acid addition salt thereof,

wherein:

A is absent or —CH 2 —;

R1 is cyclized with R2 or R3 is cyclized with R2;

R1 is selected from the group consisting of —H and —CH 2 R7,

wherein:

R7 is selected from the group consisting of —H, —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 and —CH(CH 3 ) 2 ; or

R1, when cyclized with R2, is

wherein:

R7 is selected from the group consisting of —H, —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 and —CH(CH 3 ) 2 ; and

* is the cyclization point;

R2 is selected from the group consisting of:

wherein:

R8 is selected from the group consisting of —H, —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 and —CH(CH 3 ) 2 ;

R12 is selected from the group consisting of —H, —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 and —CH(CH 3 ) 2 ; and

* is the cyclization point;

R3 is —CH(R10)(R11),

wherein:

R10 is selected from the group consisting of —H, —CH 3 and —CH 2 CH 3 ; and

R11 is selected from the group consisting of optionally substituted C 6 -C 10 aryl and optionally substituted C 3 -C 9 heteroaryl;

wherein the substituents for C 6 -C 10 aryl and C 3 -C 9 heteroaryl are independently selected from the group consisting of —F, —Cl, —CN, —CF 3 , —OCF 3 , —OCH 3 , —CH 3 , —CH 2 CH 3 , —CH(CH 3 ) 2 , —N(CH 3 ) 2 and cyclopropyl; or

R3, when cyclized with R2, is

wherein:

R9 is selected from the group consisting of —H, —C 1 -C 6 alkyl, C 3 -C 6 cycloalkyl, C 6 -C 10 aryl, substituted phenyl, optionally substituted C 3 -C 9 heteroaryl, C 1 -C 6 alkoxy, C 3 -C 6 cycloalkoxy, optionally substituted C 6 -C 10 aryloxy and optionally substituted C 3 -C 9 heteroaryloxy; and

* is the cyclization point;

wherein the substituents for phenyl, C 3 -C 9 heteroaryl, C 6 -C 10 aryloxy and C 3 -C 9 heteroaryloxy are independently selected from the group consisting of —F, —Cl, —CN, —CF 3 , —OCF 3 , —OCH 3 , —CH 3 , —CH 2 CH 3 , —CH(CH 3 ) 2 , —N(CH 3 ) 2 and cyclopropyl;

R4 is selected from the group consisting of —H, —F, —Cl, —CN, —CF 3 , —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 and —CH(CH 3 ) 2 ;

R5 is selected from the group consisting of optionally substituted C 6 -C 10 aryl, optionally substituted C 3 -C 9 heteroaryl, optionally substituted C 3 -C 6 heterocyclyl and optionally substituted C 3 -C 6 cycloalkyl;

wherein the substituents for C 6 -C 10 aryl, C 3 -C 9 heteroaryl, C 3 -C 6 heterocyclyl and C 3 -C 6 cycloalkyl are independently selected from the group consisting of —F, —Cl, —CN, —CF 3 , —OCF 3 , —OCH 3 , —CH 3 , —CH 2 CH 3 , —CH(CH 3 ) 2 , —N(CH 3 ) 2 and cyclopropyl; and

R6 is selected from the group consisting of —H, —F, —Cl, —CN, —CF 3 , —CH 3 , —CH 2 CH 3 , —CH 2 CH 2 CH 3 and —CH(CH 3 ) 2 .

2. The compound of claim 1 , wherein R9 is selected from the group consisting of branched C 3 -C 6 alkyl and branched C 3 -C 6 alkoxy.

3. The compound of claim 1 , wherein the C 3 -C 9 heteroaryl of R5, the C 3 -C 9 heteroaryl of R9 and the C 3 -C 9 heteroaryl of R11 each independently comprise one or two nitrogen atoms.

4. The compound of claim 1 , wherein the compound is selected from the group consisting of:

(1) 3-(4-fluorophenyl)-6-[1-(3-methoxyazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(2) 6-[1-(cyclohexylmethylamino)ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(3) 6-[(3-methoxyazetidin-1-yl)methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(4) 3-phenyl-6-[(3-pyrazin-2-yloxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(5) 3-phenyl-6-[(3-pyrimidin-2-yloxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(6) 6-[[3-[(6-fluoro-2-pyridyl)oxy]azetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(7) 3-phenyl-6-[1-(3-pyrimidin-2-yloxyazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(8) 3-phenyl-6-[(3-phenylazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(9) 6-[[benzyl(methyl)amino]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(10) methyl 1-benzyl-4-(8-oxo-3-phenyl-7H-imidazo[1,5-a]pyrazin-6-yl)pyrrolidine-3-carboxylate,

(11) 6-[[3-(4-ethylphenyl)azetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(12) 6-[1-[3-(4-fluorophenyl)azetidin-1-yl]propyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(13) 3-phenyl-6-[(3-quinoxalin-2-yloxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(14) 6-[[3-(4,6-dimethylpyrimidin-2-yl)oxyazetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(15) 3-phenyl-6-[1-(3-pyrazin-2-yloxyazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(16) 6-[[3-(3-fluorophenoxy)azetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(17) 6-[(3-anilinopyrrolidin-1-yl)methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(18) 3-phenyl-6-[1-(3-phenylazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(19) 6-[[3-(4-fluorophenyl)azetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(20) 6-[1-(3-methoxyazetidin-1-yl)ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(21) 6-[(3-phenoxyazetidin-1-yl)methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(22) 6-[(3-methoxyazetidin-1-yl)-phenyl-methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(23) 6-[[3-(4-fluorophenoxy)azetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(24) 6-[1-[3-(3-fluorophenoxy)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(25) 6-[1-(3-phenoxyazetidin-1-yl)ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(26) 6-[1-[3-(4-fluorophenoxy)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(27) 6-[1-[3-[(6-fluoro-2-pyridyl)oxy]azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(28) 6-[1-[3-(4-fluorophenyl)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(29) 6-[1-[3-(4-methyl-2-pyridyl)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(30) 6-[1-[3-(4-ethylphenyl)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(31) 6-(1-benzyl-4-methyl-pyrrolidin-3-yl)-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(32) 6-[1-[3-(2-fluorophenoxy)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(33) 6-[[3-(2-fluorophenoxy)azetidin-1-yl]methyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(34) 6-[1-[3-(4-methoxyphenyl)azetidin-1-yl]ethyl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(35) 4-[1-[(8-oxo-3-phenyl-7H-imidazo[1,5-a]pyrazin-6-yl)methyl]azetidin-3-yl]benzonitrile,

(36) 3-phenyl-6-[1-(3-pyrimidin-2-ylazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(37) 3-phenyl-6-[[3-(2-pyridyl)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(38) 4-[1-[1-(8-oxo-3-phenyl-7h-imidazo[1,5-a]pyrazin-6-yl)ethyl]azetidin-3-yl]benzonitrile,

(39) 3-cyclopentyl-6-[(3-pyrimidin-2-yloxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(40) 3-cyclopentyl-6-[1-[3-(4-fluorophenyl)azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(41) 3-cyclopentyl-6-[[3-(4-methoxyphenyl)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(42) 3-cyclopentyl-6-[1-(3-pyrimidin-2-yloxyazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(43) 6-[4-methyl-1-(2-pyridylmethyl)pyrrolidin-3-yl]-3-phenyl-7H-imidazo[1,5-a]pyrazin-8-one,

(44) 6-[[3-(4-fluorophenyl) azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(45) 6-[(3-benzyloxyazetidin-1-yl)methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(46) 3-cyclopentyl-6-[[3-(3-methylquinoxalin-2-yloxyazetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(47) 3-cyclopentyl-6-[1-[3-(4-fluorophenoxy)azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(48) 3-cyclopentyl-6-[[3-(4-dimethylaminophenyl) azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(49) 6-[[3-(2-fluorophenoxy)azetidin-1-yl]methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(50) 6-[[3-(2-fluorophenoxy)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(51) 6-[[3-(4-dimethylaminophenyl)azetidin-1-yl]methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(52) 3-cyclopentyl-6-[1-[3-(2-fluorophenoxy)azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(53) 3-(4-fluorophenyl)-6-[(3-pyrimidin-2-ylazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(54) 6-[(3-pyrimidin-2-ylazetidin-1-yl)methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(55) 3-(4-fluorophenyl)-6-[(3-pyrazin-2-yloxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(56) 6-[(3-benzyloxyazetidin-1-yl)methyl]-3-cyclopentyl-7H-imidazo[1,5-a]pyrazin-8-one,

(57) 3-cyclopentyl-6-[(3-isopropoxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(58) 3-cyclopentyl-6-[(3-isobutoxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(59) 3-cyclopentyl-6-[1-[3-(4-dimethylaminophenyl)azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(60) 3-(4-fluorophenyl)-6-[[3-(6-fluoro-2-pyridyl)oxy]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(61) 3-(4-fluorophenyl)-6-[[3-(2-pyridyl)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(62) 6-[[3-(2-pyridyl)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(63) 6-[[3-(4-fluorophenoxy)azetidin-1-yl]methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(64) 3-tetrahydropyran-4-yl-6-[[3-[4-(trifluoromethoxy)phenoxy]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(65) 6-[[3-[(6-fluoro-2-pyridyl)oxy]azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(66) 6-[(3-anilinopyrrolidin-1-yl)methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(67) 6-[(3-benzyloxyazetidin-1-yl)methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(68) 6-[(3-phenylazetidin-1-yl)methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(69) 6-[[3-(4-methoxyphenyl)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(70) 3-tetrahydropyran-4-yl-6-[[3-[5-(trifluoromethyl)-2-pyridyl]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(71) 6-[[3-(3-fluorophenoxy)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(72) 6-[(3-quinoxalin-2-yloxyazetidin-1-yl)methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(73) 3-(4-fluorophenyl)-6-[(3-phenoxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(74) 3-(4-fluorophenyl)-6-[[3-[4-(trifluoromethoxy)phenoxy]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(75) 6-[[3-(4-fluorophenoxy)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(76) 3-(4-fluorophenyl)-6-[[3-(4-fluorophenyl) azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(77) 3-(4-fluorophenyl)-6-[[3-[5-(trifluoromethyl)-2-pyridyl]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(78) 6-[[3-(4-ethylphenyl) azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(79) 6-[4-methyl-1-[[6-(trifluoromethyl)-3-pyridyl]methyl]pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(80) 4-[1-[(8-oxo-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-6-yl)methyl]azetidin-3-yl]oxybenzonitrile,

(81) 6-[4-methyl-1-(2-pyridylmethyl)pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(82) 6-[[3-(4-pyridyloxy)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(83) 6-[[3-(3-pyridyloxy)azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(84) 6-[(3-pyrimidin-2-yloxypyrrolidin-1-yl)methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(85) 3-(4-fluorophenyl)-6-[(3-phenoxypyrrolidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(86) 3-(4-fluorophenyl)-6-[(3-pyrimidin-2-yloxypyrrolidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(87) 3-(4-fluorophenyl)-6-[[3-(4-pyridyloxy)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(88) 3-(4-fluorophenyl)-6-[[3-(3-pyridyloxy)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(89) 6-[1-[(4-fluorophenyl)methyl]-4-methyl-pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(90) 6-[4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(91) 6-[1-(3-pyrimidin-2-ylazetidin-1-yl)ethyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(92) 4-[1-[[3-(4-fluorophenyl)-8-oxo-7H-imidazo[1,5-a]pyrazin-6-yl]methyl]azetidin-3-yl]oxybenzonitrile,

(93) 3-[1-[[3-(4-fluorophenyl)-8-oxo-7H-imidazo[1,5-a]pyrazin-6-yl]methyl]azetidin-3-yl]oxybenzonitrile,

(94) 3-[1-[(8-oxo-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-6-yl)methyl]azetidin-3-yl]oxybenzonitrile,

(95) 3-(4-fluorophenyl)-6-[[3-[3-(trifluoromethyl)phenoxy]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(96) 4-[1-[[3-(4-fluorophenyl)-8-oxo-7H-imidazo[1,5-a]pyrazin-6-yl]methyl]azetidin-3-yl]benzonitrile,

(97) 6-[4-methyl-1-(quinoxalin-6-ylmethyl)pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(98) 4-[[3-methyl-4-(8-oxo-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-6-yl)pyrrolidin-1-yl]methyl]benzonitrile,

(99) 3-(4-fluorophenyl)-6-[1-[(4-fluorophenyl)methyl]-4-methyl-pyrrolidin-3-yl]-7H-imidazo[1,5-a]pyrazin-8-one,

(100) 1-bromo-3-(4-fluorophenyl)-6-[1-[(4-fluorophenyl)methyl]-4-methyl-pyrrolidin-3-yl]-7H-imidazo[1,5-a]pyrazin-8-one,

(101) 6-[1-[(6-methoxy-3-pyridyl)methyl]-4-methyl-pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(102) 6-[4-methyl-1-[(5-methylpyrazin-2-yl)methyl]pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(103) 6-[[3-[(4-fluorophenyl)methoxy]azetidin-1-yl]methyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(104) 3-tetrahydropyran-4-yl-6-[[3-[4-trifluoromethyl)phenoxy]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(105) 3-cyclopentyl-1,5-difluoro-6-[(3-phenylazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(106) 4-[1-[(8-oxo-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-6-yl)methyl]azetidin-3-yl]benzonitrile,

(107) 3-[1-[(8-oxo-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-6-yl)methyl]azetidin-3-yl]benzonitrile,

(108) 6-[[3-[(4-chlorophenyl)methoxy]azetidin-1-yl]methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(109) 3-(4-fluorophenyl)-6-[[3-[4-(trifluoromethyl)phenoxy]azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(110) 3-(4-chlorophenyl)-6-[1-[(4-fluorophenyl)methyl]-4-methyl-pyrrolidin-3-yl]-7H-imidazo[1,5-a]pyrazin-8-one,

(111) 1-bromo-3-(4-chlorophenyl)-6-[1-[(4-fluorophenyl)methyl]-4-methyl-pyrrolidin-3-yl]-7H-imidazo[1,5-a]pyrazin-8-one,

(112) 6-[1-(3-methoxyazetidin-1-yl)ethyl]-6-methyl-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(113) 3-(4-fluorophenyl)-6-[[3-(p-tolylmethoxy)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(114) 3-(4-fluorophenyl)-6-[4-methyl-1-(2-pyridylmethyl)pyrrolidin-3-yl]-7H-imidazo[1,5-a]pyrazin-8-one,

(115) 6-[1-(cyclohexylmethyl)-4-methyl-pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(116) 3-(4-fluorophenyl)-6-[1-(3-isopropoxyazetidin-1-yl)ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(117) 6-[1-(3-isopropoxyazetidin-1-yl)ethyl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one,

(118) 6-[[3-(4-chlorophenoxy)azetidin-1-yl]methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(119) 6-[1-[(4-fluorophenyl)methyl]-4-methyl-pyrrolidin-3-yl]-3-[4-(trifluoromethoxy)phenyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(120) 3-(4-fluorophenyl)-6-[1-[3-[4-(trifluoromethoxy)phenoxy]azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(121) 6-[[3-(2,6-difluorophenoxy)azetidin-1-yl]methyl]-3-(4-fluorophenyl)-7H-imidazo[1,5-a]pyrazin-8-one,

(122) 3-(3,5-difluorophenyl)-6-[[3-(4-methoxyphenyl)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(123) 3-(4-fluorophenyl)-6-[[3-(4-methoxyphenyl)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(124) 3-(4-fluorophenyl)-6-[1-[3-(4-fluorophenyl)azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(125) 3-tetrahydrofuran-3-yl-6-[[3-[3-(trifluoromethyl)phenyl]pyrrolidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(126) 3-(4-fluorophenyl)-6-[1-[3-(4-methoxyphenyl)azetidin-1-yl]ethyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(127) 3-(4,4-difluorocyclohexyl)-6-[[3-(4-methoxyphenyl)azetidin-1-yl]methyl]-7H-imidazo[1,5-a]pyrazin-8-one,

(128) 3-(4,4-difluorocyclohexyl)-6-[(3-methoxyazetidin-1-yl)methyl]-7H-imidazo[1,5-a]pyrazin-8-one, and

(129) 6-[[3-(4-fluorophenoxy)azetidin-1-yl]methyl]-3-tetrahydrofuran-3-yl-7H-imidazo[1,5-a]pyrazin-8-one

or a tautomer or a pharmaceutically acceptable acid addition salt thereof.

5. A pharmaceutical composition comprising a therapeutically effective amount of the compound of claim 1 , and one or more pharmaceutically acceptable carriers, diluents and excipients.

6. A method of inhibiting phosphodiesterase 9 activity in a subject, comprising administering a therapeutically effective amount of a compound of claim 1 .

7. The method of claim 6 , wherein the subject has a neurodegenerative disorder selected from the group consisting of Alzheimer's disease, mental retardation, cognitive impairment associated with schizophrenia, attention-deficit/hyperactivity disorder, age-related cognitive decline, and substance-induced psychotic disorder.

8. The method of claim 7 , wherein the substance-induced psychotic disorder is psychosis induced by alcohol, psychosis induced by amphetamine, psychosis induced by cannabis , psychosis induced by cocaine, psychosis induced by hallucinogens, psychosis induced by inhalants, psychosis induced by opioids or psychosis induced by phencyclidine.

9. A compound, wherein the compound is 6-[4-methyl-1-(pyrimidin-2-ylmethyl)pyrrolidin-3-yl]-3-tetrahydropyran-4-yl-7H-imidazo[1,5-a]pyrazin-8-one, and a tautomer or a pharmaceutically acceptable acid addition salt thereof.

10. A pharmaceutical composition comprising a therapeutically effective amount of the compound of claim 9 , and one or more pharmaceutically acceptable carriers, diluents and excipients.