A compound represented by formula I: ** Formula ** or a pharmaceutically acceptable salt, stereoisomer or N-oxide thereof, wherein Rb is selected from the group consisting of H, halogen, hydroxyl, cyano and C1- alkyl C6； R is H, C1-C6 alkyl or -C (O) -C1-C6 alkyl； R1 is H or C1-C6 alkyl； R2 is H or C1-C6 alkyl； R3 is C1-C6 alkyl； C1-C6 alkoxy； -O-C1-C6 alkylene-phenyl； C2-C6 alkenyl； C2-C6 alkynyl； C3-C6 cycloalkyl； phenyl； or heteroaryl which includes 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 (C1-C3 alkyl), O and S； wherein R3 is optionally substituted with one, two or three substituents independently selected from the group consisting of amino, protected amino, halogen, C1-C6 alkyl, C1-C6 alkoxy, hydroxyl, phenyl (optionally substituted with one, two or three substituents each independently selected from Ra), benzyl (optionally substituted with one, two or three substituents each independently selected from Ra), and -C1-C6-alkyl C3-C6 cycloalkyl (optionally substituted with one, two or three substituents independently selected from halogen and C1-C6 alkyl)； Ra is selected from the group consisting of halogen, C1-C6 alkyl (optionally substituted with one, two or three halogens), C3-C6 cycloalkyl (optionally substituted with one, two or three halogens), and C1-C6 alkoxy (optionally substituted with one, two or three halogens)； R4 is H or C1-C6 alkyl； R5 is H or C1-C6 alkyl； X is selected from the group consisting of: H； C1-C6 alkyl； -OH； C1-C6 alkoxy； -CO2H； -C (O) NRcRd； and heteroaryl that includes 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 (C1-C3 alkyl), O and S, wherein the heteroaryl ring may be optionally substituted with one, two or three substituents independently selected from the group consisting of halogen, C1-C6 alkyl, C1-C6 alkoxy, hydroxyl and phenyl； and Rc and Rd are each independently selected fro
