The invention relates to compounds of the formula I, or pharmaceutically acceptable salts thereof: wherein: Z=O or S; n=1-3; R3=—CO2R8, —C(O)SR8, —C(O)NHR8, —C(S)OR8, —C(S)SR8, —C(S)NHR8, —C(NH)SR8 or —C(NH)NHR8, wherein R8 is —H or alkyl; R4=—H, —CH2R5 or —CH2CH2R5, wherein R5 independently has one of the meanings of R3; B=—NR2—, —CH2NR2—, —CH2CH2NR2—, —CH2CHR7— or —CH2O—, wherein R2 is H or a C1-3 alkyl, alkenyl or alkynyl group, and R7 is H or a C1-3 alkyl or alkoxy group; wherein R1=—NH2 or —OH, and C and D are each, independently, a 5- or 6-membered, substituted or unsubstituted, aromatic or non-aromatic ring which may also contain one or more heteroatoms, and C is connected to group B in any available position.