Penam

Penam

Names
IUPAC name (5R)-4-thia-1-azabicyclo[3.2.0]heptan-7-one
Other names 1-Aza-7-oxo-4-thiabicyclo[3.2.0]heptane
Identifiers
CAS Number	53908-04-6 N[ChemSpider]
3D model (JSmol)	Interactive image
Beilstein Reference	4374479
ChEBI	CHEBI:35991 Y
ChemSpider	7827765
PubChem CID	9548842
InChI InChI=1S/C5H7NOS/c7-4-3-5-6(4)1-2-8-5/h5H,1-3H2/t5-/m1/s1 Key: WSHJJCPTKWSMRR-RXMQYKEDSA-N InChI=1S/C5H7NOS/c7-4-3-5-6(4)1-2-8-5/h5H,1-3H2/t5-/m1/s1 Key: WSHJJCPTKWSMRR-RXMQYKEDSA-N
SMILES C1CSC2N1C(=O)C2
Properties
Chemical formula	C5H7NOS
Molar mass	129.18 g·mol−1
Related compounds
Related compounds	clavam
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Benzylpenicillin, an example of a penam

Not to be confused with penem.

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

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かい = 54°) and twist of the C-N bond (τたう = 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.

Penams are strained due to the angle strain on the four-member βべーた-lactam ring, whose internal bond angles are 90º. Consequently, penams are susceptible to acid- and base-catalyzed hydrolysis.