FIELD: medicine； pharmaceuticals.SUBSTANCE: invention relates to a compound represented by formula I, or a pharmaceutically acceptable salt thereof. In the IRa formula is hydrogen or -C1-C8 alkyl； Rb is selected from a group consisting of: 1) -C(O)NR'10R'11； 2) -C(O)NHSO2R1； and 3) -SO2R1； R1 is selected from a group consisting of: 1) -C1-C8 alkyl, unsubstituted or substituted with halogen, phenyl, carboxyl or hydroxyl； 2) -C3-C8 cycloalkyl, unsubstituted or substituted C1-C6 alkyl or halogen； bicyclo[2.2.1]heptane or 2-oxo-7,7-dimethylbicyclo[2.2.1]heptane； 3) phenyl unsubstituted or substituted with halogen, C1-C6 alkyl, halogen-C1-C6 alkyl, hydroxy-C1-C6 alkyl, C1-C6 alkoxy, halogen-C1-C6 alkoxy, piperidinyl, di(C1-C6 alkyl)amine, C3-C8 cycloalkyl, C3-C8 cycloalkyl substituted with halogen-C1-C6 alkyl, phenyl, carboxy, C1-C6 alkyloxycarbonyl, methoxycarbonyl-C1-C6 alkyl, 2-methylthiazol-4-yl, 4-methylthiazol-2-yl, isoxazol-5-yl, 2-methyloxazol-5-yl or 1,3,4-oxadiazol-2-yl； 1,3-benzodioxole； 1,4-benzodioxane； indane； naphthyl or 1,2,3,4-tetrahydronaphthalene； 4) 6-member heterocycloalkyl, in which two ring heteroatoms are selected from N, O and S, substituted with one or two groups selected from C1-C6 alkyl, oxo, C1-C6 alkylcarbonyl or 3-azabicyclo [3.1.0] hexane； 2-oxo-7-azaspiro[3.5]nonane or 6-azaspiro[2.5]octane； 5) five- or six-membered heteroaryl, in which one or two ring heteroatoms are selected from S and N, unsubstituted or substituted with C1-C6 alkyl, phenyl； 2,3-dihydroindole, substituted with acetyl, or quinoline, and 6) -NR10R11； R2 is hydrogen； m is selected from 0, 1, 2 and 3； R3 is hydrogen or hydroxyl； R4 is hydrogen； R5 and R6 are independently selected from hydrogen or a protecting group for a hydroxy group selected from tert-butyldimethylsilyl, acetyl and N-methylcarbamoyl； R7 is selected from -C1-C8 alkyl； R10 and R11 each is independently selected from hydrogen and -C1-C8 alkyl, or R10 and R11, taken together with the nitrogen atom to which
