Beta-lactams

A beta-lactam (β-lactam) ring is a four-membered lactam. A ''lactam'' is a cyclic amide, and ''beta''-lactams are named so because the nitrogen atom is attached to the β-carbon atom relative to the carbonyl. The simplest β-lactam possible is 2-azetidinone. β-lactams are significant structural units of medicines as manifested in many β-lactam antibiotics Up to 1970, most β-lactam research was concerned with the penicillin and cephalosporin groups, but since then, a wide variety of structures have been described.

Clinical significance

The β-lactam ring is part of the core structure of several antibiotic families, the principal ones being the penicillins, cephalosporins, carbapenems, and monobactams, which are, therefore, also called β-lactam antibiotics. Nearly all of these antibiotics work by inhibiting bacterial cell wall biosynthesis. This has a lethal effect on bacteria, although any given bacteria population will typically contain a subgroup that is resistant to β-lactam antibiotics. Bacterial resistance occurs as a result of the expression of one of many genes for the production of β-lactamases, a class of enzymes that break open the β-lactam ring. More than 1,800 different β-lactamase enzymes have been documented in various species of bacteria. These enzymes vary widely in their chemical structure and catalytic efficiencies. When bacterial populations have these resistant subgroups, treatment with β-lactam can result in the resistant strain becoming more prevalent and therefore more virulent. β-lactam derived antibiotics can be considered one of the most important antibiotic classes but prone to clinical resistance. β-lactam exhibits its antibiotic properties by imitating the naturally occurring d-Ala-d-Ala substrate for the group of enzymes known as penicillin binding proteins (PBP), which have as function to cross-link the peptidoglycan part of the cell wall of the bacteria.

The β-lactam ring is also found in some other drugs such as the cholesterol absorption inhibitor drug Ezetimibe.

Synthesis

The first synthetic β-lactam was prepared by Hermann Staudinger in 1907 by reaction of the Schiff base of aniline and benzaldehyde with diphenylketene in a +2 cycloaddition (Ph indicates a phenyl functional group):
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Many methods have been developed for the synthesis of β-lactams.

The Breckpot β-lactam synthesis produces substituted β-lactams by the cyclization of beta amino acid esters by use of a Grignard reagent. Mukaiyama's reagent is also used in modified Breckpot synthesis.
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Reactions

Due to ring strain, β-lactams are more readily hydrolyzed than linear amides or larger lactams. This strain is further increased by fusion to a second ring, as found in most β-lactam antibiotics. This trend is due to the amide character of the β-lactam being reduced by the aplanarity of the system. The nitrogen atom of an ideal amide is sp²-hybridized due to resonance, and sp²-hybridized atoms have trigonal planar bond geometry. As a pyramidal bond geometry is forced upon the nitrogen atom by the ring strain, the resonance of the amide bond is reduced, and the carbonyl becomes more ketone-like. Nobel laureate Robert Burns Woodward described a parameter ''h'' as a measure of the height of the trigonal pyramid defined by the nitrogen (as the apex) and its three adjacent atoms. ''h'' corresponds to the strength of the β-lactam bond with lower numbers (more planar; more like ideal amides) being stronger and less reactive. Monobactams have ''h'' values between 0.05 and 0.10 angstroms (Å). Cephems have ''h'' values in of 0.20–0.25 Å. Penams have values in the range 0.40–0.50 Å, while carbapenems and clavams have values of 0.50–0.60 Å, being the most reactive of the β-lactams toward hydrolysis.

See also

* Azetidine
* Lactone
* Lectka enantioselective beta-lactam synthesis

References

External links

Synthesis of β-lactams

{{DEFAULTSORT:Lactam, beta-
Beta-lactams
Four-membered rings