A compound of structural formula I: or a pharmaceutically acceptable salts thereof; wherein X is O or S; V is selected from the group consisting of: Ar is phenyl optionally substituted with one to five R 1 substituents; each R 1 is independently selected from the group consisting of halogen, cyano, hydroxy, C 1-6 alkyl, optionally substituted with one to five fluorines, and C 1-6 alkoxy, optionally substituted with one to five fluorines; R2 is selected from the group consisting of: hydrogen; cyano; CONH2; C 1-3 alkyl, wherein alkyl is optionally substituted with one to five fluorines; and C 3-5 cycloalkyl, wherein cycloalkyl is optionally substituted with one to three substituents independently selected from halogen, hydroxy, and C 1-4 alkyl, wherein alkyl is optionally substituted with one to five fluorines; and R 8 is selected from the group consisting of: -phenyl; -heteroaryl -C3-6 cycloalkyl; -CH 2 SO 2 C 1-3 alkyl; wherein each alkyl residue is optionally substituted with one to five substituents independently selected from fluorine and hydroxy and wherein phenyl, heteroaryl, and each cycloalkyl residue are optionally substituted with one to five substituents independently selected from halogen, hydroxy, C 1-6 alkyl, and C 1-6 alkoxy, wherein alkyl and alkoxy are optionally substituted with one to five fluorines. -SO 2 C 3-6 cycloalkyl and -C 1-5 alkyl wherein each alkyl residue is substituted with one to five substituents independently selected from fluorine and hydroxyl and each cycloalkyl residue is substituted with one to five substituents independently selected from halogen, hydroxy, C 1-6 alkyl, and C 1-6 alkoxy, wherein alkyl and alkoxy are optionally substituted with one to five fluorines. X is O or S; V is selected from the group consisting of: Ar is phenyl optionally substituted with one to five R 1 substituents; each R 1 is independently selected from the group consisting of halogen, cyano, hydroxy, C 1-6 alkyl, optionally substituted with one to five fluorines, and C 1-6 alkoxy, optionally substituted with one to five fluorines; halogen, cyano, hydroxy, C 1-6 alkyl, optionally substituted with one to five fluorines, and C 1-6 alkoxy, optionally substituted with one to five fluorines; R2 is selected from the group consisting of: hydrogen; cyano; CONH2; C 1-3 alkyl, wherein alkyl is optionally substituted with one to five fluorines; and C 3-5 cycloalkyl, wherein cycloalkyl is optionally substituted with one to three substituents independently selected from halogen, hydroxy, and C 1-4 alkyl, wherein alkyl is optionally substituted with one to five fluorines; and hydrogen; cyano; CONH2; C 1-3 alkyl, wherein alkyl is optionally substituted with one to five fluorines; and C 3-5 cycloalkyl, wherein cycloalkyl is optionally substituted with one to three substituents independently selected from halogen, hydroxy, and C 1-4 alkyl, wherein alkyl is optionally substituted with one to five fluorines; and R 8 is selected from the group consisting of: -phenyl; -heteroaryl -C3-6 cycloalkyl; -CH 2 SO 2 C 1-3 alkyl; wherein each alkyl residue is optionally substituted with one to five substituents independently selected from fluorine and hydroxy and wherein phenyl, heteroaryl, and each cycloalkyl residue are optionally substituted with one to five substituents independently selected from halogen, hydroxy, C 1-6 alkyl, and C 1-6 alkoxy, wherein alkyl and alkoxy are optionally substituted with one to five fluorines. -phenyl; -heteroaryl -C3-6 cycloalkyl; -CH 2 SO 2 C 1-3 alkyl; -SO 2 C 3-6 cycloalkyl and -C 1-5 alkyl