Four novel oxapenem compounds (i.e., AM-112, AM-113, AM-114, and AM-115) were investigated for their beta-lactamase inhibitory activity against a panel of isolated class A, C, and D enzymes, which included expanded-spectrum beta-lactamase enzymes (ESBLs). The oxapenems were potent beta-lactamase inhibitors. Activity varied within the group, with AM-113 and AM-114 proving to be the most active c...