Compounds of the formula ##STR1## wherein R.sub.1 and R.sub.4 represent certain heterocyclic groups; R.sub.2 is hydrogen or lower alkyl; and R.sub.3 is hydrogen, lower alkyl, phenyl-lower alkyl, substituted phenyl-lower alkyl, diphenyl-lower alkyl, tri(lower alkyl)stannyl, tri(lower alkyl)silyl, a salt forming ion, or the group ##STR2## wherein R is lower alkyl, phenyl, phenyl-lower alkyl, or substituted phenyl and phenyl-lower alkyl are disclosed. These compounds are useful as antibacterial agents.