A compound represented by formula I: or a pharmaceutically acceptable salt, a stereoisomer, and/or an N-oxide thereof, wherein R b is selected from the group consisting of H, halogen, hydroxyl, cyano and 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 selected from the group consisting of 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, benzyloxycarbonyl, p-methoxybenzyloxycarbonyl, acetyl, trifluoroacetyl, benzoyl, benzyl, p-methoxybenzyl, p-methoxyphenyl, 3,4-dimethoxybenzyl, triphenylmethyl, 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 any aforementioned 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; and 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; R 4 is H or C 1 -C 6 alkyl; R 5 is -C 1 -C 6 alkylene-X, wherein X is selected from the group consisting of: (i) 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; and (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; or R 5 is X, wherein X is 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; wherein R 5 is optionally substituted with and R 6 is selected from the group consisting of H, halogen, hydroxyl, cyano, -O-C -C 1 -C 6 alkyl, 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 1 is H or C 1 -C 6 alkyl; R 2 is H or C 1 -C 6 alkyl; R 3 is selected from the group consisting of 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, benzyloxycarbonyl, p-methoxybenzyloxycarbonyl, acetyl, trifluoroacetyl, benzoyl, benzyl, p-methoxybenzyl, p-methoxyphenyl, 3,4-dimethoxybenzyl, triphenylmethyl, 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 any aforementioned 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; and 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; R 4 is H or C 1 -C 6 alkyl; R 5 is -C 1 -C 6 alkylene-X, wherein X is selected from the group consisting of: (i) 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; and (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; or (i) 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; and (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; or R 5 is X, wherein X is 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; wherein R 5 is optionally substituted with and R 6 is selected from the group consisting of H, halogen, hydroxyl, cyano, -O-C -C 1 -C 6 alkyl, C 1 -C 6 alkyl, and C 1 -C 6 alkoxy.