For example, the nitrogen atom of all bicyclic -lactams fused to five-membered rings is labelled position 4, as it is in penams, while in cephems, the nitrogen is position 5.
For optimal activity and selectivity of the coxibs, a 4-methylsulfonylphenyl attached to an unsaturated (usually) five-membered ring with a vicinal lipophilic group is required (rofecoxib).
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.