Selected compounds of formula (I) are antibacterial agents: formula (I) wherein R2 represents a substituted or unsubstituted C1-C6 alkyl, cycloalkyl (C1-C6 alkyl)- or aryl (C1-C6 alkyl)- group, and A represents a group of formula (IA), or (IB) wherein R4 represents the side chain of a natural or non-natural alpha amino acid, and R5 and R6 are each independently hydrogen or C1-C6 alkyl, heterocyclic or aryl (C1-C6 alkyl)-, R5 and R6 when taken together with the nitrogen atom to which they are attached from an optionally substituted saturated heterocyclic ring of 3 to 8 atoms which ring is optionally fused to a carbocyclic or second heterocyclic ring.