This invention provides the compounds of the formula (II): ##STR1## and salts and esters thereof, wherein R.sub.1 is H and R.sub.3 is H or an aryl, aralkyl, lower alkyl or substituted lower alkyl group or R.sub.1 and R.sub.3 are joined so that the CHR.sub.1.NH.CO.R.sub.3 moiety forms a group of the sub-formula (a): ##STR2## wherein R.sub.4 is a hydrogen atom or a NH.CO.R.sub.5 group wherein R.sub.5 is a lower alkyl, lower alkoxy lower alkyl, aryl, aralkyl, aryloxyalkyl, lower alkoxy or aryloxy group.The compounds have .beta.-lactamase inhibitory and anti-bacterial properties.The invention also provides a process for their preparation, and pharmaceutical compositions containing them.