A compound having the structure of formula I: ** Formula ** or a pharmaceutically acceptable salt thereof, wherein: Y is a C2-4 alkylene or C2-4 alkenylene linker； R1 is selected from a group consisting of -C1-9 alkyl, -C2-9 alkenyl, -C2-9 alkynyl, -NR9R10, -C1-9R11 alkyl, -C2-9R11 alkenyl, -C2-9R11 alkynyl, -carbocyclyl -R11, -CH (OH) C1-9R9 alkyl, -CH (OH) C2-9R9 alkenyl, -CH (OH) C2-9R9 alkynyl, -CH (OH) carbocyclyl-R9, -C (>； = O) R9 , -C (>； = O) C1-9R9 alkyl, -C (>； = O) C2-9R9 alkenyl, -C (>； = O) C2-9R9 alkynyl, - C (>； = O) C2-9-R9 carbocyclyl , -C (>； = O) NR9R10, -N (R9) C (>； = O) R9, -N (R9) C (>； = O) NR9R10, -N (R9) C (>； = O) OR9, - N (R9) C (>； = O) C (>； = NR10) R9, -N (R9) C (>； = O) C (>； = CR9R10) R9, -N (R9) C (>； = O) C1 alkyl -4N (R9) C (>； = O) R9, -N (R9) C (>； = NR10) R9, - C (>； = NR10) NR9R10, -N>； = C (R9) NR9R10, -N (R9) SO2R9, -N (R9) SO2NR9R10, -N>； = CHR915, aryl, heteroaryl, carbocyclyl and heterocyclyl； R6 is H； R7 is H； R8 is H； each R9 is independently selected from a group consisting of H, -C1-9 alkyl, -C2-9 alkenyl, -C2-9 alkynyl, -C1-9R11 alkyl, -C2-9R11 alkenyl, -C2-9R11 alkynyl, - carbocyclyl-R11, aryl, heteroaryl, carbocyclyl and heterocyclyl； each R10 is independently selected from a group consisting of H, -C1-9 alkyl, -OR9, -CH (>； = NH), -C (>； = O) OR9, aryl, heteroaryl, carbocyclyl and heterocyclyl； each R11 is independently selected from a group consisting of aryl, heteroaryl, carbocyclyl and heterocyclyl； X is selected from the group consisting of -CO2R12, -P (O) (OR9) 2, -P (O) (R9) (OR9), -P (O) (OR12 ') 2, - P (O) ( R9) (OR12 '), -CON (R9) OH, -SO3H, -SO2N (R9) OH, -CONHNHSO2R9, -COHNSO2R9 and a carboxylic acid isoster selected from a 5-7 membered carbocycle or heterocycle, wherein the carbocycle or the 5-7 membered heterocycle is selected from the group consisting of: ** Formula ** and in which any atom of the ring structure of the carbocycle or of the 5-7 membered heter
