A compound of formula II: wherein L 1 is a linker group selected from the group consisting of a single bond, -C(O)-O-, - C(O)- and -C(O)NR 2 ； R 1 is selected from the group consisting of H, OH, C 1-20 alkyl, C 3-20 cycloalkyl and aryl, said latter three groups may be optionally substituted and in which one or more of the carbons of the alkyl, cycloalkyl or aryl groups may optionally be replaced with O, S, N, NR 2 or N(R 2 ) 2 ； R 2 is H or C 1-6 alkyl； and v is an integer from 1 to 4； wherein aryl is mono- or bi-cyclic aromatic radical containing from 6 to 14 carbon atoms having a single ring or multiple condensed rings； and wherein the optional substituents are selected from the group consisting of halo, OH, OC 1-6 alkyl, C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkenyloxy, NH 2 , NH(C 1-6 alkyl), N(C 1-6 alkyl)(C 1-6 alkyl), CN, NO 2 , C(O)C 1-6 alkyl, C(O)OC 1-6 alkyl, SO 2 C 1-6 alkyl, SO 2 NH 2 , SO 2 NHC 1-6 alkyl, phenyl and C 1-6 alkylenephenyl.
