A compound of formula (I), salts and metabolically labile esters thereof, wherein R represents optionally substituted aryl or heteroaryl group; A represents a propylene chain or A is a chain of 3 members one of which is selected from an oxygen or sulphur atom or the group NH or a substituted derivative thereof and the other two members are methylene groups, having antibacterial activity, processes for their preparation and to their use in medicine.