A compound of the general formula I or a salt thereof: in which: R represents a group selected from -Y-OH, -Y-SH, -Y-S 2 NH-R x, -Y-NHS 2 -R x, -Y-CO 2 H, -Y-C NH-R x, -Y-NHC -R x, -Y-NHR y or -Y-S 2 NH 2, in which either Y is not present or Y represents a C 1-6 alkylene; R x represents a C 1-6 alkyl which is substituted by -OH, -SH, -NH 2 or -CO 2 H; R y represents a hydrogen atom or a C 1-4 alkyl group; X represents OH; Ra represents a C 1-4 alkyl group; Rb represents hydrogen; Rc represents hydrogen or methoxy; Rd represents a C 1-4 alkyl group; each Re independently represents a halogen atom, a CF 3 group, a C 1-4 alkyl group, a C 1-4 alkoxy group, a -CN group, or a -NO 2 group; n is 0, 1, or 2; Rf represents a C 1-4 alkyl group; Rg represents a hydrogen atom or an optionally substituted alkyl, cycloalkyl, alkenyl, alkynyl, aryl, or heteroaryl group, and wherein when Rg is an optionally substituted alkyl group, said alkyl group may be optionally substituted by: i) one or more halogen atoms and/or hydroxyl, alkoxy, amino, alkylamino or dialkylamino groups, in which any alkyl moiety has from 1 to 4 carbon atoms; ii) one or more amino acids; iii) a group N(R i)(R ii), in which R i represents hydrogen or a C 1-4 alkyl group, and in which R ii represents a C 1-4 alkyl group, a -C -C 1-6 alkyl group, a -C -C 2-6 alkenyl group, or a -C -C 3-6 cycloalkyl group, wherein said C 3-6 cycloalkyl group may be unsubstituted or substituted by up to 2 methyl groups. R represents a group selected from -Y-OH, -Y-SH, -Y-S 2 NH-R x, -Y-NHS 2 -R x, -Y-CO 2 H, -Y-C NH-R x, -Y-NHC -R x, -Y-NHR y or -Y-S 2 NH 2, in which either Y is not present or Y represents a C 1-6 alkylene; R x represents a C 1-6 alkyl which is substituted by -OH, -SH, -NH 2 or -CO 2 H; R y represents a hydrogen atom or a C 1-4 alkyl group; X represents OH; Ra represents a C 1-4 alkyl group; Rb represents hydrogen; Rc represents hydrogen or methoxy; Rd represents a C 1-4 alkyl group; each Re independently represents a halogen atom, a CF 3 group, a C 1-4 alkyl group, a C 1-4 alkoxy group, a -CN group, or a -NO 2 group; n is 0, 1, or 2; Rf represents a C 1-4 alkyl group; Rg represents a hydrogen atom or an optionally substituted alkyl, cycloalkyl, alkenyl, alkynyl, aryl, or heteroaryl group, and wherein when Rg is an optionally substituted alkyl group, said alkyl group may be optionally substituted by: i) one or more halogen atoms and/or hydroxyl, alkoxy, amino, alkylamino or dialkylamino groups, in which any alkyl moiety has from 1 to 4 carbon atoms; ii) one or more amino acids; iii) a group N(R i)(R ii), in which R i represents hydrogen or a C 1-4 alkyl group, and in which R ii represents a C 1-4 alkyl group, a -C -C 1-6 alkyl group, a -C -C 2-6 alkenyl group, or a -C -C 3-6 cycloalkyl group, wherein said C 3-6 cycloalkyl group may be unsubstituted or substituted by up to 2 methyl groups. i) one or more halogen atoms and/or hydroxyl, alkoxy, amino, alkylamino or dialkylamino groups, in which any alkyl moiety has from 1 to 4 carbon atoms; ii) one or more amino acids; iii) a group N(R i)(R ii), in which R i represents hydrogen or a C 1-4 alkyl group, and in which R ii represents a C 1-4 alkyl group, a -C -C 1-6 alkyl group, a -C -C 2-6 alkenyl group, or a -C -C 3-6 cycloalkyl group, wherein said C 3-6 cycloalkyl group may be unsubstituted or substituted by up to 2 methyl groups.