A compound of structural formula I: or a pharmaceutically acceptable salts thereof; wherein X 1 and X 2 are independently selected from the group consisting of -N- and -CR 5 -; R 1 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, aryl, heterocycle, aroyl, heteroaroyl and C 3 -C 10 cycloalkyl; wherein the C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, aryl, heterocycle, aroyl, heteroaroyl and C 1 -C 10 cycloalkyl are unsubstituted or substituted with 1-3 substituents from R 5; R 2 is selected from the group consisting of C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, benzyl, heterocycle and C 1 -C 6 alkylheterocycle; wherein the C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, benzyl, heterocycle and C 1 -C 6 alkylheterocycle are unsubstituted or substituted with 1-3 substituents each independently selected from R 3, R 4 and R 5; R 3 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl and aryl, wherein the C 1 -C 6 alkyl and aryl are unsubstituted or substituted with 1-3 substituents selected from R 5; R 4 is selected from the group consiting of hydrogen and C 1 -C 6 alkyl; R 5 is selected from the group consisting of hydrogen, halogen, C 1 -C 6 alkyl, halogen-substitutedC 1 -C 6 alkyl, -OH, C 1 -C 6 alkylOH, halogen-substitutedC 1 -C 6 alkylOH, -OC 1 -C 6 alkyl, -Ohalogen-substitutedC 1 -C 6 alkyl, -COOH, -COOC 1 -C 6 alkyl, -C 1 -C 6 alkylCOOC 1 -C 6 alkyl, -C 1 -C 6 alkylCOOH, -CN, C 1 -C 6 alkylCN, NHC 1 -C 6 alkyl, N(C 1 -C 6 alkyl) 2, C 1 -C 6 alkylCONH 2,-CONH 2, -CONHC 1 -C 6 alkyl, -NHCOC 1 -C 6 alkyl, -CON(C 1 -C 6 alkyl) 2, -NHSO 2 C 1 -C 6 alkyl, SO 2 aryl, -SO 2 C 1 -C 6 alkyl, C 1 -C 6 alkylSO 2 C 1 -C 6 alkyl, aryl, -NHCOaryl, and -NHCOheterocycle; and n is 0-4. or a pharmaceutically acceptable salts thereof; wherein X 1 and X 2 are independently selected from the group consisting of -N- and -CR 5 -; R 1 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, aryl, heterocycle, aroyl, heteroaroyl and C 3 -C 10 cycloalkyl; wherein the C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, aryl, heterocycle, aroyl, heteroaroyl and C 1 -C 10 cycloalkyl are unsubstituted or substituted with 1-3 substituents from R 5; R 2 is selected from the group consisting of C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, benzyl, heterocycle and C 1 -C 6 alkylheterocycle; wherein the C 1 -C 6 alkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, benzyl, heterocycle and C 1 -C 6 alkylheterocycle are unsubstituted or substituted with 1-3 substituents each independently selected from R 3, R 4 and R 5; R 3 is selected from the group consisting of hydrogen, C 1 -C 6 alkyl and aryl, wherein the C 1 -C 6 alkyl and aryl are unsubstituted or substituted with 1-3 substituents selected from R 5; R 4 is selected from the group consiting of hydrogen and C 1 -C 6 alkyl; R 5 is selected from the group consisting of hydrogen, halogen, C 1 -C 6 alkyl, halogen-substitutedC 1 -C 6 alkyl, -OH, C 1 -C 6 alkylOH, halogen-substitutedC 1 -C 6 alkylOH, -OC 1 -C 6 alkyl, -Ohalogen-substitutedC 1 -C 6 alkyl, -COOH, -COOC 1 -C 6 alkyl, -C 1 -C 6 alkylCOOC 1 -C 6 alkyl, -C 1 -C 6 alkylCOOH, -CN, C 1 -C 6 alkylCN, NHC 1 -C 6 alkyl, N(C 1 -C 6 alkyl) 2, C 1 -C 6 alkylCONH 2,-CONH 2, -CONHC 1 -C 6 alkyl, -NHCOC 1 -C 6 alkyl, -CON(C 1 -C 6 alkyl) 2, -NHSO 2 C 1 -C 6 alkyl, SO 2 aryl, -SO 2 C 1 -C 6 alkyl, C 1 -C 6 alkylSO 2 C 1 -C 6 alkyl, aryl, -NHCOaryl, and -NHCOheterocycle; and n is 0-4.