Compound of formula I ': ** Formula ** or a pharmaceutically acceptable salt thereof, wherein A is -CHR5-； Ring C is a 5 or 6 membered phenyl or heteroaryl； R1 is C1-7 alkyl, haloalkyl, C3-7 cycloalkyl, C6-10 aryl, C6-10 aryl-C6-10 alkyl, C6-10 aryloxy-C1-7 alkyl, heteroaryl or heterocyclyl, in which alkyl, aryl, heteroaryl, heterocyclyl are optionally substituted with 1 to 5 R7； R2 is H, C1-7 alkyl, halo- C1-7 alkyl, C3-7 cycloalkyl, C3-7 cycloalkyl-C1-7 alkyl, hydroxy-C1-7 alkyl, C1-7-OC (O) -C1 alkyl alkyl -7, C1-7 alkoxy-C1-7 alkyl, heteroaryl, heterocyclyl or C6-10 aryl； wherein aryl and heteroaryl are optionally substituted with hydroxyl, C1-7 alkoxy, C1-7 alkyl, halo- C1-7 alkyl, halogen, CN or C3-7 cycloalkyl； R3 is H, halogen, C1-7 alkyl, halo- C1-7 alkyl, C3-7 cycloalkyl, cyano, C1-7 alkoxy, hydroxyl, nitro, -NH2, -NH (C1-7 alkyl) or -N (alkyl C1-7) 2； each R4 is independently selected from the group consisting of halogen, C1-7 alkyl, halo- C1-7 alkyl, C3-7 cycloalkyl, cyano, -NH2, -NH (C1-7 alkyl), -N (C1-7 alkyl ) 2, C1-7 alkoxy, halo- C1-7 alkoxy, hydroxyl, carboxyl, nitro, sulfonyl, sulfamoyl, sulfonamido, C6-10 aryl, heterocyclyl, C6-10 aryloxy, heterocyclyloxy, -SH, -S-C1-alkyl 7, - C (O) O-C6-10 aryl, -C (O) O-heterocyclyl, -C (O) O-heteroaryl, -C (O) NR2-C1-7 alkyl, -C (O) NR2 -C6-10 aryl, -C (O) NR2- heteroaryl, -C (O) NR2-heterocyclyl, -NR2C (O) -C1-7 alkyl, -NR2C (O) -C6-10 aryl, -NR2C (O ) -heteroaryl, -NR2C (O) - heterocyclyl, -OC (O) -C 1-7 alkyl, -OC (O) -C6-10 aryl, -OC (O) -heteroaryl and -OC (O) -heterocyclyl； wherein R4 is optionally substituted with 1 to 5 R7； or two adjacent R4 groups may form with the atoms to which a 5 or 6 membered phenyl or a ring heteroaryl is attached, wherein said phenyl or heteroaryl ring is optionally substituted with 1 to 4 R8； R6 is C1-7 alkyl, C3-7 cycloalkyl, halo- C1-7 alkyl, heterocyclyl, heteroaryl or C6-10 aryl； each R7 is independently selected from the group consisting of halogen, C1-7 alkyl, C3-7 cy
