Compounds of the formula ##STR1## wherein R.sup.1 is hydrogen or a conventional amino-protecting group, R.sup.2 is a straight or branched chain alkyl group containing from 1 to 4 carbon atoms, allyl, 2-butenyl, 3-butenyl, propargyl, 2-butynyl, 3-butynyl, cycloalkyl containing from 3 to 6 carbon atoms or a group of the formula ##STR2## in which R.sup.3 and R.sup.4 each are independently hydrogen, methyl or ethyl, or R.sup.3 and R.sup.4, taken together with the carbon atom to which they are attached, may be a cycloalkylidene ring containing from 3 to 6 carbon atoms or a substituted cyclobutylidene ring of the formula ##STR3## wherein X is halogen, hydroxy or (lower)alkoxy, and R.sup.5 and R.sup.6 each are independently hydrogen, halogen, amino, carbamoyl, acylamino, acyloxy, (lower)alkyl, (lower)alkenyl, (lower)alkoxy, (lower)alkylamino or (lower)alkylthio, in which each (lower)alkyl, (lower)alkoxy, (lower)alkylamino or (lower)alkylthio group may contain 1 or 2 substituents selected from amino, carboxy, halogen and nontoxic pharmaceutically acceptable acid addition salts, physiologically hydrolyzable esters and solvates thereof, as well as processes for their preparation, are disclosed. The compounds in which R.sup.1 is hydrogen are potent antibacterial agents.