The Chemical Reactivity of β-Lactams, β-Sultams and β-Phospholactams

Studies at the ionizable position of cephalosporins and penicillins: hydroxamates as substitutes for the traditional carboxylate group

Classically, β-lactams need an ionizable group to potentiate antibacterial activity. Sets of cephalosporins and penicillins featuring different substituted hydroxamates in place of the traditional carboxylate group have been synthesized and tested for antibiotic activity. Many of the compounds exhibited anti-bacterial activities with notable MIC values in the range of 6-0.2 μM.

Kinetic and Theoretical Studies of Beta-Lactone Reactivity-A Quantitative Scale for Biological Application

Natural products comprise a rich source for bioactive molecules with medicinal relevance. Many of these contain electrophilic scaffolds that bind conserved enzyme active sites covalently. Prominent examples include beta-lactams and beta-lactones, which specifically acylate serine residues in diverse peptidases. Although these scaffolds appear similar, their bioactivities and corresponding protein targets vary. To quantify and dissect these differences in bioactivities, the kinetics of the reactions of beta-butyrolactone with a set of reference amines in buffered aqueous solution at 37 °C have been analyzed. Different product ratios of C1 versus C3 attack on the beta-butyrolactone have been observed, depending on the aliphatic or aromatic nature of the standard amine used. Quantum mechanics/molecular mechanics (QM/MM) calculations reveal that a H₃ O⁺ molecule has a crucial role in stabilizing C3 attack by aniline, through coordination of the lactone ring oxygen. In agreement with their weak proteome reactivity, monocyclic beta-lactams did not react with the set of standard nucleophiles studied herein. Bicyclic beta-lactams, however, exhibited a lower activation barrier, and thus, reacted with standard nucleophiles. This study represents a starting point for semiquantitative reactivity scales for natural products, which, in analogy to chemical reactivity scales, will provide predictions for electrophilic modifications in biological systems.

Syntheses and Biological Evaluations of Highly Functionalized Hydroxamate Containing and N-Methylthio Monobactams as Anti-Tuberculosis and β-Lactamase Inhibitory Agents

Both the resurgence of tuberculosis (TB) and antibiotic resistance continue to threaten modern healthcare and new means of combating pathogenic bacterial infections are needed. The syntheses of monobactams possessing hydroxamate and N-methylthio functionality are described, as well as their anti-TB, in vitro β-lactamase inhibitory, and general antimicrobial evaluations. A number of compounds exhibited significant anti-TB and β-lactamase inhibitory activity, with MIC values in the range of 25 to < 0.19 μM against Mycobacteria tuberculosis (M.tb), and K_i values in the range of 25-0.03 μM against purified NDM-1 and VIM-1 lystate metallo β-lactamases. This work suggests that these scaffolds may serve as promising leads in developing new antibiotics and/or β-lactamase inhibitors.

Molecular Targets of β-Lactam-Based Antimicrobials: Beyond the Usual Suspects

The common practice in antibacterial drug development has been to rapidly make an attempt to find ever-more stable and broad-spectrum variants for a particular antibiotic, once a drug resistance for that antibiotic is detected. We are now facing bacterial resistance toward our clinically relevant antibiotics of such a magnitude that the conversation for antimicrobial drug development ought to include effective new antibiotics with alternative mechanisms of action. The electrophilic β-lactam ring is amenable for the inhibition of different enzyme classes by a suitable decoration of the core scaffold. Monocyclic β-lactams lacking an ionizable group at the lactam nitrogen exhibit target preferences toward bacterial enzymes important for resistance and virulence. The present review intends to draw attention to the versatility of the β-lactams as antimicrobials with "unusual" molecular targets.

New β-phospholactam as a carbapenem transition state analog: Synthesis of a broad-spectrum inhibitor of metallo-β-lactamases

In an effort to test whether a transition state analog is an inhibitor of the metallo-β-lactamases, a phospholactam analog of carbapenem has been synthesized and characterized. The phospholactam 1 proved to be a weak, time-dependent inhibitor of IMP-1 (70%), CcrA (70%), L1 (70%), NDM-1 (53%), and Bla2 (94%) at an inhibitor concentration of 100μM. The phospholactam 1 activated ImiS and BcII at the same concentration. Docking studies were used to explain binding and to offer suggestions for modifications to the phospholactam scaffold to improve binding affinities.

Suppressed beta-effect of silicon in 3-silylated monocyclic beta-lactams: the role of antiaromaticity

The beta-stabilizing effect of silicon substituent at C-3 on a C-4 cation and a radical in the 2-azetidinone systems is studied using NMR kinetics. While the beta-effect is virtually nonexistent in the case of a cation, a foiled beta-effect (only a 3-fold rate enhancement) is observed for a radical intermediate. From both the experimental and theoretical studies, it is demonstrated that antiaromaticity is playing the prime role in suppressing the beta-stabilizing effect of silicon.

3-Alkenyl-2-azetidinones as fatty acid amide hydrolase inhibitors

A series of novel 2-azetidinones (beta-lactams) bearing short alkenyl chains at C3 and N1 have been prepared and evaluated in vitro as inhibitors of human FAAH. Compound 9c (1-(4'-pentenoyl-3-(4'-pentenyl)-2-azetidinone)) featured an IC(50) value of 4.5 microM and a good selectivity for FAAH versus MGL.

Synthesis of Strained Tricyclic β-Lactams by Intramolecular [2+2] Cycloaddition Reactions of 2-Azetidinone-Tethered Enallenols: Control of Regioselectivity by Selective Alkene Substitution

A convenient metal-free methodology for the preparation of structurally novel, strained tricyclic beta-lactams containing a cyclobutane ring has been developed. The first examples accounting for the intramolecular [2+2] cycloaddition reactions in beta-lactams have been achieved by the thermolysis of 2-azetidinone-tethered enallenols, which have been prepared in aqueous media by regio- and diastereoselective indium-mediated carbonyl allenylation of 4-oxoazetidine-2-carbaldehydes. Notably, the regioselectivity of the cycloaddition can be tuned in the allene component just by a subtle variation in the substitution pattern of the alkene component.

THEORETICAL STUDY ON HYDROLYSIS MECHANISM OF β-PHOSPHOLACTAMS

The neutral hydrolysis mechanisms of a simple β-phospholactam with and without water-assisted reaction have been studied by using quantum chemical method at HF/6-31G**, MP2/6-31G** and B3LYP/6-31G** levels, respectively. The reaction can proceed by two different mechanisms: concerted and stepwise. There are two pathways in stepwise, i.e. pathway a and b, and the energy barriers of them are close. The energy barriers of water-assisted hydrolysis of β-phospholactam are obviously lower than those of no-water-assisted hydrolysis system. The energy barriers of stepwise mechanism are much lower than those of the concerted pathway in both cases. The solvent effects have been considered by means of a polarizable continuum model. The hydrolysis mechanism of β-phospholactam with that of the β-lactam and β-sultam was compared.

Kinetics and Mechanism of Hydrolysis of N-Acyloxymethyl Derivatives of Azetidin-2-one

The pH-independent, acid-catalyzed and base-catalyzed hydrolyses of N-acyloxymethylazetidin-2-ones all occur at the ester function. The pH-independent hydrolysis involves rate-limiting alkyl C-O fission and formation of an exocyclic beta-lactam iminum ion. This iminium ion is then trapped by water at the exocyclic iminium carbon atom, rather than at the beta-lactam carbonyl carbon atom, to form the corresponding N-hydroxymethylazetidin-2-ones. Calculations carried out at the B3LYP/6-31+G(d) level of theory also support that nucleophilic attack by water takes place at the exocyclic carbon rather than at the beta-lactam carbonyl carbon of the iminium ion. The mechanism for the acid-catalyzed pathway involves a preequilibrium protonation, probably at the beta-lactam nitrogen, followed by rate-limiting alkyl C-O fission with formation of an exocyclic iminum ion. The base-catalyzed hydrolysis involves rate-limiting hydroxide attack at the ester carbonyl carbon. These results imply formation of a beta-lactam system containing a positively charged amide nitrogen atom that hydrolyzes via a pathway that preserves the beta-lactam structure in the product and provide further evidence that cleavage of the beta-lactam C-N bond is not as facile as is commonly imagined.

Structurally Novel Bi- and Tricyclic β-Lactams via [2 + 2] Cycloaddition or Radical Reactions in 2-Azetidinone-Tethered Enallenes and Allenynes

[reaction: see text] Thermolysis of beta-lactam-tethered enallenyl alcohols gave tricyclic ring structures via a formal [2 + 2] cycloaddition of the alkene with the distal bond of the allene, while the tin-promoted radical cyclization in 2-azetidinone-tethered allenynes proceeded to provide bicyclic beta-lactams containing a medium-sized ring. The access to cyclization precursors was achieved by regio- and stereoselective metal-mediated carbonyl allenylation of 4-oxoazetidine-2-carbaldehydes in an aqueous environment.

Asymmetric synthesis of unusual fused tricyclic beta-lactam structures via aza-cycloadditions/ring closing metathesis

Conveniently substituted bis-beta-lactams, pyrrolidinyl-beta-lactams, and piperidinyl-beta-lactams undergo ring-closing methatesis using Grubbs' carbene, Cl(2)(Cy(3)P)(2)Ru=CHPh, to give medium-sized rings fused to bis-2-azetidinone, pyrrolidinyl-2-azetidinone, or piperidinyl-2-azetidinone systems. The diolefinic cyclization precursors can be obtained from optically pure 4-oxoazetidine-2-carbaldehydes bearing an extra alkene tether at position 1 or 3 of the beta-lactam ring via [2 + 2] cycloaddition of imino 2-azetidinones, N-metalated azometine ylide [3 + 2] cycloaddition, and subsequent N-acylation of the pyrrolidinyl nitrogen atom, or through aza-Diels-Alder cycloaddition of 2-azetidinone-tethered imines. Under standard reaction conditions, the combination of cycloaddition reactions of 2-azetidinone-tethered imines with ring-closing methatesis offers an asymmetric entry to a variety of unusual fused tricyclic 2-azetidinones bearing two bridgehead nitrogen atoms.