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The chemical graphs and scaffold of primary examples of major β-lactam antibiotic classes. Common atom numbering scheme of the chemotype is depicted and oriented above the example β-lactam ring. The X may be a carbon, an oxygen atom, or a sulfur atom. For example, penem structures with X=C are “carbapenems”; penem structures with X=O are “oxapenems”; and penem structures with X=S are “thiapenems,” or more commonly, simply “penems.”
