lactam

A lactam is a cyclic amide in which the carbonyl group is part of a ring that includes a nitrogen atom. They form when the carboxyl group of an amino acid or related molecule reacts with an amino group within the same molecule, closing a ring and releasing water. Lactams are classified by ring size, with alpha-lactams (three-membered), beta-lactams (four-membered), gamma-lactams (five-membered), and delta-lactams (six-membered) being common. Larger lactams also occur in natural products and synthetic compounds.

Lactam formation is typically achieved by intramolecular amidation or cyclization of amino acid derivatives or peptide precursors. The amide bond in lactams is stabilized by resonance, which reduces the basicity of the nitrogen and influences reactivity. The ring size largely determines stability and reactivity: very small rings, such as beta-lactams, are highly strained and prone to ring-opening, while larger lactams are comparatively less reactive.

Industrial and medicinal significance is broad. Beta-lactams are a major class of antibiotics, including penicillins and cephalosporins, whose antibacterial activity arises from interference with bacterial cell wall synthesis. Caprolactam, a six-membered delta-lactam, is a key precursor in the production of nylon-6. Gamma-lactams such as 2-pyrrolidinone are common building blocks in pharmaceuticals and agrochemicals. Lactam chemistry also plays a central role in natural product synthesis, drug development, and materials science.