A compound represented by formula I: or a pharmaceutically acceptable salt thereof, wherein R b is selected from the group consisting of H, halogen, hydroxyl, cyano and C 1 -C 6 alkyl; R is H or C 1 -C 6 alkyl; R 1 is H or C 1 -C 6 alkyl; R 2 is H or C 1 -C 6 alkyl; R 3 is -CONR'R', wherein R' for each occurrence is independently selected from H, C 1 -C 6 alkyl and a nitrogen protecting group; wherein the nitrogen protecting group is selected from the group consisting of 9-fluorenylmethyloxycarbonyl, tert-butoxycarbonyl, carbobenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, acetyl, trifluoroacetyl, benzoyl, phthalimido, benzyl, p-methoxybenzyl, p-methoxyphenyl, 3,4-dimethoxybenzyl, triphenylmethyl, benzylidene, p-toluenesulfonyl, -C OR 31 and -C R 32; R 31 is selected from the group consisting of: C 1 -C 6 alkyl, C 1 -C 6 haloalkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 10 cycloalkyl, -CH 2 -C 3 -C 10 cycloalkyl, -CH 2 -phenyl, and -CH 2 -pyridyl, wherein the cycloalkyl of the C 3 -C 10 cycloalkyl or -CH 2 -C 3 -C 10 cycloalkyl is optionally substituted with from 1-3 independently selected C 1 -C 3 alkyl, and wherein the phenyl is optionally substituted with from 1-2 substituents independently selected from C 1 -C 3 alkyl, C 1 -C 3 haloalkyl, C 1 -C 3 alkoxy, C 1 -C 3 haloalkoxy, nitro, halo, SO 2 Me, cyano, and -OC CH 3; R 32 is selected from the group consisting of: H, C 1 -C 6 alkyl, C 1 -C 6 haloalkyl, phenyl, and pyridyl, wherein the phenyl is optionally substituted with from 1-2 substituents independently selected from C 1 -C 3 alkyl, C 1 -C 3 haloalkyl, C 1 -C 3 alkoxy, C 1 -C 3 haloalkoxy, nitro, halo, SO 2 Me, cyano, and -OC CH 3; and X is H, X', -C 1 -C 6 alkylene-X', or -C 1 -C 6 alkylene-C -X', wherein X' is selected from the group consisting of: (i) C 3 -C 6 cycloalkyl; (ii) heterocyclyl including from 3 to 6 ring atoms wherein 1, 2, or 3 of the ring atoms are independently selected from the group consisting of N, NH, N(C 1 -C 3 alkyl), O, and S; (iii) phenyl; and (iv) heteroaryl including from 5 to 6 ring atoms wherein 1, 2, or 3 of the ring atoms are independently selected from the group consisting of N, NH, N(C 1 -C 3 alkyl), O, and S; wherein X' is optionally substituted by one or more substituents independently selected from the group consisting of halogen, hydroxyl, C 1 -C 6 alkyl and C 1 -C 6 alkoxy. R b is selected from the group consisting of H, halogen, hydroxyl, cyano and C 1 -C 6 alkyl; R is H or C 1 -C 6 alkyl; R 1 is H or C 1 -C 6 alkyl; R 2 is H or C 1 -C 6 alkyl; R 3 is -CONR'R', wherein R' for each occurrence is independently selected from H, C 1 -C 6 alkyl and a nitrogen protecting group; wherein the nitrogen protecting group is selected from the group consisting of 9-fluorenylmethyloxycarbonyl, tert-butoxycarbonyl, carbobenzyloxycarbonyl, p-methoxybenzyloxycarbonyl, acetyl, trifluoroacetyl, benzoyl, phthalimido, benzyl, p-methoxybenzyl, p-methoxyphenyl, 3,4-dimethoxybenzyl, triphenylmethyl, benzylidene, p-toluenesulfonyl, -C OR 31 and -C R 32; R 31 is selected from the group consisting of: C 1 -C 6 alkyl, C 1 -C 6 haloalkyl, C 2 -C 6 alkenyl, C 2 -C 6 alkynyl, C 3 -C 10 cycloalkyl, -CH 2 -C 3 -C 10 cycloalkyl, -CH 2 -phenyl, and -CH 2 -pyridyl, wherein the cycloalkyl of the C 3 -C 10 cycloalkyl or -CH 2 -C 3 -C 10 cycloalkyl is optionally substituted with from 1-3 independently selected C 1 -C 3 alkyl, and wherein the phenyl is optionally substituted with from 1-2 substituents independently selected from C 1 -C 3 alkyl, C 1 -C 3 haloalkyl, C 1 -C 3 alkoxy, C 1 -C 3 haloalkoxy, nitro, halo, SO 2 Me, cyano, and -OC CH 3; R 32 is selected from the group consisting of: H, C 1 -C 6 alkyl, C 1 -C 6 haloalkyl, phenyl, and pyridyl, wherein the phenyl is optionally substituted with from 1-2 substituents independently selected from C 1 -C 3 alkyl, C 1 -C 3 haloalkyl, C 1 -C 3 alkoxy, C 1 -C 3 haloalkoxy, nitro, halo, SO 2 Me, cyano, and -OC CH 3; and X is H, X', -C 1 -C 6 alkylene-X', or -C 1 -C 6 alkylene-C -X', wherein X' is selected from the group consisting of: (i) C 3 -C 6 cycloalkyl; (ii) heterocyclyl including from 3 to 6 ring atoms wherein 1, 2, or 3 of the ring atoms are independently selected from the group consisting of N, NH, N(C 1 -C 3 alkyl), O, and S; (iii) phenyl; and (iv) heteroaryl including from 5 to 6 ring atoms wherein 1, 2, or 3 of the ring atoms are independently selected from the group consisting of N, NH, N(C 1 -C 3 alkyl), O, and S; (i) C 3 -C 6 cycloalkyl; (ii) heterocyclyl including from 3 to 6 ring atoms wherein 1, 2, or 3 of the ring atoms are independently selected from the group consisting of N, NH, N(C 1 -C 3 alkyl), O, and S; (iii) phenyl; and (iv) heteroaryl including from 5 to 6 ring atoms wherein 1, 2, or 3 of the ring atoms are independently selected from the group consisting of N, NH, N(C 1 -C 3 alkyl), O, and S; wherein X' is optionally substituted by one or more substituents independently selected from the group consisting of halogen, hydroxyl, C 1 -C 6 alkyl and C 1 -C 6 alkoxy.