A compound represented by structural formula I: or a pharmaceutically acceptable salt thereof, wherein: X represents NR, or O; R is selected from the group consisting of H and C 1-6 alkyl, A 2' , A 3' , A 4' , A 5' , and A 6' are represented respectively as follows: 1) A 2' =N, A 3' =CR 4 , A 4' =CR 4 , A 5' =N, A 6' =CH; 2) A 2' =CH, A 3' =CH, A 4' =N, A 5' =N, A 6' =CH; 3) A 2' =CH, A 3' =CH, A 4' =N, A 5' =CH, A 6' =N; 4) A 2' =CH, A 3' =N, A 4' =N, A 5' =N, A 6' =CH; One of R 1 and R 2 is hydrogen and the other is selected from the group consisting of CH 3 , CH(CH 3 ) 2 , C(CH 3 ) 2 OH, C(CH 3 ) 2 CH 2 OH, C(CH 3 ) 3 , CH 2 CH 3 , CH(CH 3 )OH, (CH 2 ) n OH, C(CH 3 ) 2 NH 2 , C(CH 2 CH 3 ) 2 OH, OCH 2 CH 3 , and (CH 2 ) n OCH 3 ; or R 1 and R 2 can combine with the carbon to which they are attached to form a C 3-10 cycloalkyl or C 3-10 heterocycloalkyl, said cycloalkyl and heterocycloalkyl optionally substituted with 1 to 3 groups of R a ; R 3 represents halo, C 1-6 alkyl, C 1-4 haloalkyl, (CH 2 ) n OR, C 3-10 cycloalkyl, NR 2 , said alkyl and cycloalkyl optionally substituted with 1 to 3 groups of R a ; R 4 is selected from the group consisting of hydrogen, (CH 2 ) n OR, C 1-6 alkyl, (CH 2 ) n C 1-4 haloalkyl, CN, and NR 2 , said alkyl optionally substituted with one to three groups of R a ; R a is selected from the group consisting of halo, CN, C 1-6 alkyl, (CH 2 ) n OR, (O)pC 1-4 haloalkyl, NR 2 , SCF 3 , SF 5 , C 3-10 cycloalkyl, C 6-10 aryl, and C 4-10 heterocyclyl; n represents 0, 1, 2, 3, or 4; p represents 0 or 1. X represents NR, or O; R is selected from the group consisting of H and C 1-6 alkyl, A 2' , A 3' , A 4' , A 5' , and A 6' are represented respectively as follows: 1) A 2' =N, A 3' =CR 4 , A 4' =CR 4 , A 5' =N, A 6' =CH; 2) A 2' =CH, A 3' =CH, A 4' =N, A 5' =N, A 6' =CH; 3) A 2' =CH, A 3' =CH, A 4' =N, A 5' =CH, A 6' =N; 4) A 2' =CH, A 3' =N, A 4' =N, A 5' =N, A 6' =CH; 1) A 2' =N, A 3' =CR 4 , A 4' =CR 4 , A 5' =N, A 6' =CH; 2) A 2' =CH, A 3' =CH, A 4' =N, A 5' =N, A 6' =CH; 3) A 2' =CH, A 3' =CH, A 4' =N, A 5' =CH, A 6' =N; 4) A 2' =CH, A 3' =N, A 4' =N, A 5' =N, A 6' =CH; One of R 1 and R 2 is hydrogen and the other is selected from the group consisting of CH 3 , CH(CH 3 ) 2 , C(CH 3 ) 2 OH, C(CH 3 ) 2 CH 2 OH, C(CH 3 ) 3 , CH 2 CH 3 , CH(CH 3 )OH, (CH 2 ) n OH, C(CH 3 ) 2 NH 2 , C(CH 2 CH 3 ) 2 OH, OCH 2 CH 3 , and (CH 2 ) n OCH 3 ; or R 1 and R 2 can combine with the carbon to which they are attached to form a C 3-10 cycloalkyl or C 3-10 heterocycloalkyl, said cycloalkyl and heterocycloalkyl optionally substituted with 1 to 3 groups of R a ; R 3 represents halo, C 1-6 alkyl, C 1-4 haloalkyl, (CH 2 ) n OR, C 3-10 cycloalkyl, NR 2 , said alkyl and cycloalkyl optionally substituted with 1 to 3 groups of R a ; R 4 is selected from the group consisting of hydrogen, (CH 2 ) n OR, C 1-6 alkyl, (CH 2 ) n C 1-4 haloalkyl, CN, and NR 2 , said alkyl optionally substituted with one to three groups of R a ; R a is selected from the group consisting of halo, CN, C 1-6 alkyl, (CH 2 ) n OR, (O)pC 1-4 haloalkyl, NR 2 , SCF 3 , SF 5 , C 3-10 cycloalkyl, C 6-10 aryl, and C 4-10 heterocyclyl; n represents 0, 1, 2, 3, or 4; p represents 0 or 1. <EOS>