Lactam

(CC) Image: David E. Volk
Lactam structures.

In chemistry, a lactam is a cyclic amide. The name is derived from two chemical terms, lactone, referring to a cyclic ketone, and amide, a compound containing a nitrogen atom next to a carbonyl group. Lactams are named according to the size of the cyclic ring in the lactam: $\alpha$-lactams, $\beta$-lactams, $\gamma$-lactams and $\delta$-lactams contain rings made of three, four, five or six atoms, respectively. $\alpha$-lactams are also called aziridinones. Many widely used antibiotic drugs, including the penicillins and cephalosporins, owe their activity to the presence of a $\beta$-lactam structure. The lactams may have substitutions added to the nitrogen atom or any of the non-carbonyl carbon atoms in the base structure.

Synthesis of lactams

Synthesis of alpha-lactams

(CC) Image: David E. Volk
A general purpose alpha-lactam synthesis

The cyclization reaction of an $\alpha$-haloamide precuror in the presence of sodium hydride and 15-crown-5 ether at room temperature in dichloromethane (CH₂Cl₂) is a high-yielding, general route to $\alpha$-lactam (aziridinone) products. The hydrogen gas and sodium halide by-products are readily removed.

Antibiotics

(CC) Image: David E. Volk
Base structure of all cephalosporins.

The $\beta$-lactam forms the center structure of many antibiotic drugs, such as the cephalosporins and the penicillins, as shown above. In the penicillins, the non-lactam ring is one atom smaller compared to the cephalosporins. The activity of cephalosporins, penicillins, and some other antibiotics are due to the presence of the beta-lactam, which binds irreversibly, via acylation, to penicillin-binding proteins, thereby inhibiting the peptidogycan layer of bacterial cell wall synthesis. Cephalosporins and penicillins are often made semi-synthetically, using a core structure obtained from a natural organism, such as a fungus, due to the difficulty and expense of synthesizing these lactams.

Beta-lactams are four-membered cyclic amides, best known for the penicillins based on a bicyclo-thiazolidine, as well as the cephalosporins based on a bicyclo-thiazine, and including monocyclic monobactams. The beta-lactamases hydrolyze the beta lactam ring, accounting for beta-lactam resistance of infective bacteria.