A compound of structural formula I: or a pharmaceutically acceptable salt thereof; wherein "a" is a single bond or a double bond, provided that if "a" is a double bond, then R 5 and R 6b are absent; T is CH; U is CR 1; V is CR 2; W is CH; X is selected from the group consisting of: Oxygen, sulfur, S 2, -CR g R g, and C=O; Y is selected from the group consisting of: oxygen, and -CR 4a R 4b, provided that if X is oxygen or sulfur, then Y is not oxygen, and further provided that if X is S 2, then Y is not oxygen; A is selected from the group consisting of: aryl, and C 3-5 cycloheteroalkyl, wherein each aryl and cycloheteroalkyl is unsubstituted or substituted with one to five substituents independently selected from R a; B is selected from the group consisting of: aryl, aryl-C 1-10 alkyl-, and heteroaryl, wherein each alkyl, aryl and heteroaryl is unsubstituted or substituted with one to five substituents independently selected from Rb; R 1 and R 2 are each independently selected from: hydrogen, and -C 1-6 alkyl, wherein one of R 1 and R 2 is -C 1-6 alkyl substituted with R 7, and wherein alkyl is unsubstituted or substituted with one to three substituents independently selected from R L; R 3 is absent or hydrogen; R 4a is hydrogen; R 4b is hydrogen; R 5 is absent or hydrogen; R 6a is hydrogen; and R 6b is absent or hydrogen; R 7 is -CO 2 R 8; R 8 is hydrogen; R a is halogen; each R b is independently selected from the group consisting of: -C 1-10 alkyl, -CF 3, halogen, -OC 1-10 alkyl, and -OC 3-6 cycloalkyl; R g is hydrogen; and each R L is independently selected from the group consisting of: -C 1-10 alkyl, and -C 3-6 cycloalkyl; or a pharmaceutically acceptable salt thereof. "a" is a single bond or a double bond, provided that if "a" is a double bond, then R 5 and R 6b are absent; T is CH; U is CR 1; V is CR 2; W is CH; X is selected from the group consisting of: Oxygen, sulfur, S 2, -CR g R g, and C=O; Oxygen, sulfur, S 2, -CR g R g, and C=O; Y is selected from the group consisting of: oxygen, and -CR 4a R 4b, provided that if X is oxygen or sulfur, then Y is not oxygen, and further provided that if X is S 2, then Y is not oxygen; oxygen, and -CR 4a R 4b, A is selected from the group consisting of: aryl, and C 3-5 cycloheteroalkyl, wherein each aryl and cycloheteroalkyl is unsubstituted or substituted with one to five substituents independently selected from R a; aryl, and C 3-5 cycloheteroalkyl, B is selected from the group consisting of: aryl, aryl-C 1-10 alkyl-, and heteroaryl, wherein each alkyl, aryl and heteroaryl is unsubstituted or substituted with one to five substituents independently selected from Rb; aryl, aryl-C 1-10 alkyl-, and heteroaryl, R 1 and R 2 are each independently selected from: hydrogen, and -C 1-6 alkyl, wherein one of R 1 and R 2 is -C 1-6 alkyl substituted with R 7, and wherein alkyl is unsubstituted or substituted with one to three substituents independently selected from R L; hydrogen, and -C 1-6 alkyl, R 3 is absent or hydrogen; R 4a is hydrogen; R 4b is hydrogen; R 5 is absent or hydrogen; R 6a is hydrogen; and R 6b is absent or hydrogen; R 7 is -CO 2 R 8; R 8 is hydrogen; R a is halogen; each R b is independently selected from the group consisting of: -C 1-10 alkyl, -CF 3, halogen, -OC 1-10 alkyl, and -OC 3-6 cycloalkyl; R g is hydrogen; and each R L is independently selected from the group consisting of: -C 1-10 alkyl, and -C 3-6 cycloalkyl;