Penams are the primary skeleton structures that define the penicillin subclass of the broader β-lactam family of antibiotics and related compounds. They are bicyclic ring systems containing a β-lactam moiety fused with a five-member thiazolidine ring. ^[1] Due to ring strain and limitations on amide resonance, the structure is unstable and highly susceptible to catalytic cleavage at the amide bond.^[2] Benzylpenicillin (penicillin G) is the natural product parent that contains the penam structure.

Penams have inflexible structures. The structure is locked in a puckered (i.e. bent) shape due to the pyramidal geometry of the bridgehead nitrogen. The pyramidalization (χ = 54°) and twist of the C-N bond (τ = 18°) is caused by the strain from the lone pair's exclusion from planarity with the cyclic rings and electrostatic repulsion effects. As a result, the distorted C-N bond causes misalignment the orbitals of the carbonyl carbon and the nitrogen lone pair that allow for resonance overlap. The amide C-N bond length is 1.406 Å and displays greater single bond character than in noncyclic tertiary amides. The C-O bond length is 1.205 Å which is shorter than C-O bonds in noncyclic tertiary amides.^[3]

Penams are strained due to the angle strain on the four-member β-lactam ring, whose internal bond angles are 90º.^[4][3] Consequently, penams are susceptible to acid- and base-catalyzed hydrolysis.^[1][4]