penem penem a penem is a type of β-lactam with an unsaturated five-member heterocycle containing a sulfur atom fused to the β-lactam ring. penems do not occur naturally; all are synthetic. related to penems are carbapenems, which have a carbon atom in place of the sulfur atom. an example is faropenem. structure faropenem, a penem. note sulfur and double bond in ring. imipenem, a carbapenem. although imipenem has a sulfur, the sulfur is not in the ring. benzylpenicillin, a penicllin. note the absence of the double bond in the ring. penem molecules do not occur naturally, and production of penems is an entirely synthetic process. five main penem subgroups — thiopenems, oxypenems, aminopenems, alkylpenems, and arylpenems — have been produced and are distinguished by the side chain (at position 2) of the unsaturated five-membered ring. one structurally distinct penem is brl 42715. this molecule has no substitution at the above position, but has a bulky group attached to the β-lactam ring, and it displays effective inhibition of class c β-lactamases, but no antimicrobial activity. one possible consequence of these structural differences of penems from other β-lactams may be reduced immunogenicity and immunogenic cross-reactivity.