Monocyclic Β-lactams Research Articles

Carbamoyl radical-mediated synthesis and semipinacol rearrangement of β-lactam diols.

In an approach to the biologically important 6-azabicyclo[3.2.1]octane ring system, the scope of the tandem 4-exo-trig carbamoyl radical cyclization—dithiocarbamate group transfer reaction to ring-fused β-lactams is evaluated. β-Lactams fused to five-, six-, and seven-membered rings are prepared in good to excellent yield, and with moderate to complete control at the newly formed dithiocarbamate stereocentre. No cyclization is observed with an additional methyl substituent on the terminus of the double bond. Elimination of the dithiocarbamate group gives α,β- or β,γ-unsaturated lactams depending on both the methodology employed (base-mediated or thermal) and the nature of the carbocycle fused to the β-lactam. Fused β-lactam diols, obtained from catalytic OsO4-mediated dihydroxylation of α,β-unsaturated β-lactams, undergo semipinacol rearrangement via the corresponding cyclic sulfite or phosphorane to give keto-bridged bicyclic amides by exclusive N-acyl group migration. A monocyclic β-lactam diol undergoes Appel reaction at a primary alcohol in preference to semipinacol rearrangement. Preliminary investigations into the chemo- and stereoselective manipulation of the two carbonyl groups present in a representative 7,8-dioxo-6-azabicyclo[3.2.1]octane rearrangement product are also reported.

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.

Epimerization and substrate gating by a TE domain in β-lactam antibiotic biosynthesis.

Nonribosomal peptide synthetases (NRPSs) are versatile engines of bioactive natural product biosynthesis that function according to the multiple carrier thiotemplate mechanism. C-terminal thioesterase (TE) domains of these giant modular proteins typically catalyze product release by hydrolysis or macrocylization. We now report an unprecedented, dual-function TE involved in nocardicin A biosynthesis, the paradigm monocyclic β-lactam antibiotic. Contrary to expectation, a stereodefined series of potential peptide substrates for the nocardicin TE domain failed to undergo hydrolysis. The stringent discrimination against peptide intermediates was dramatically overcome by prior monocyclic β-lactam formation at an L-seryl site. Kinetic data are interpreted such that the TE domain acts as a gatekeeper to hold the assembling peptide on an upstream domain until β-lactam formation takes place and then rapidly catalyzes epimerization, not previously observed as a TE catalytic function, and thioesterase cleavage to discharge a fully fledged pentapeptide β-lactam harboring nocardicin G, the universal precursor of the nocardicins.

A kinetic analysis of the inhibition of FOX-4 -lactamase, a plasmid-mediated AmpC cephalosporinase, by monocyclic -lactams and carbapenems

Class C β-lactamases are prevalent among Enterobacteriaceae; however, these enzymes are resistant to inactivation by commercially available β-lactamase inhibitors. In order to find novel scaffolds to inhibit class C β-lactamases, the comparative efficacy of monocyclic β-lactam antibiotics (aztreonam and the siderophore monosulfactam BAL30072), the bridged monobactam β-lactamase inhibitor BAL29880, and carbapenems (imipenem, meropenem, doripenem and ertapenem) were tested in kinetic assays against FOX-4, a plasmid-mediated class C β-lactamase (pmAmpC). The FOX-4 β-lactamase was purified. Steady-state kinetics, electrospray ionization mass spectrometry (ESI-MS) and ultraviolet difference (UVD) spectroscopy were conducted using the β-lactam scaffolds described. The K(i) values for the monocyclic β-lactams against FOX-4 β-lactamase were 0.04 ± 0.01 μM (aztreonam) and 0.66 ± 0.03 μM (BAL30072), and the Ki value for the bridged monobactam BAL29880 was 8.9 ± 0.5 μM. For carbapenems, the Ki values ranged from 0.27 ± 0.05 μM (ertapenem) to 2.3 ± 0.3 μM (imipenem). ESI-MS demonstrated the formation of stable covalent adducts when the monocyclic β-lactams and carbapenems were reacted with FOX-4 β-lactamase. UVD spectroscopy suggested the appearance of different chromophoric intermediates. Monocyclic β-lactam and carbapenem antibiotics are effective mechanism-based inhibitors of FOX-4 β-lactamase, a clinically important pmAmpC, and provide stimulus for the development of new inhibitors to inactivate plasmidic and chromosomal class C β-lactamases.

ChemInform Abstract: Design and Synthesis of New 4‐Alkylthio Monocyclic β‐Lactams.

AbstractThe designed products (VI and XIV) contain penicillin‐like structures.

Synthesis of new nanocopolymer containing β-lactams

Several new monocyclic β-lactam monomers bearing the NO2 group 2a–g were synthesized via a [2 + 2] ketene–imine cycloaddition reaction (Staudinger reaction). Calculation of coupling constant of H-3 and H-4, and the X-ray crystallography of β-lactam 2e confirmed the cis stereochemistry of these β-lactams. Then aminophenyl β-lactams were synthesized by the reduction of NO2 to NH2 group in the presence of Raney Ni and hydrazine hydrate. Treatment of these aminophenyl β-lactams with acryloyl chloride and sodium bicarbonate (NaHCO3), afforded the monomers bearing the NHCOCH=CH2 group. These acrylated β-lactam monomers were dissolved in a warm mixture of butyl acrylate and styrene and then were converted to the corresponding polyacrylate nano β-lactams by emulsion polymerization in water. A unique feature of this methodology is the ability to incorporate water-insoluble compounds directly into the nanoparticle framework. Structures of the synthesized compounds were confirmed by physical and spectral analyses. Dynamic light scattering analysis and transmission electron microscopy of the final emulsions show that the nanoparticles were about 50–70 nm in diameter.

Stereocontrolled syntheses of peptide thioesters containing modified seryl residues as probes of antibiotic biosynthesis.

Methods have been developed to synthesize tri- and pentapeptide thioesters containing one or more p-(hydroxyphenyl)glycine (pHPG) residues and L-serine, some where the latter is O-phosphorylated, O-acetylated, or exists as a β-lactam. Selection of orthogonal protection strategies and development of conditions to achieve seryl O-phosphorylation without β-elimination and to maintain stereochemical control, especially simultaneously at exceptionally base-labile pHPG α-carbons, are described. Intramolecular closure of a seryl peptide to a β-lactam-containing peptide and the syntheses of corresponding thioester analogues are also reported. Modification of classical Mitsunobu conditions is described in the synthesis of the β-lactam-containing products, and in a broadly useful observation, it was found that simple exclusion of light from the P(OEt)3-mediated Mitsunobu ring closure afforded yields of >95%, presumably owing to reduced photodegradation of the azodicarboxylate used. These sensitive potential substrates and products will be used in mechanistic studies of the two nonribosomal peptide synthetases NocA and NocB that lie at the heart of nocardicin biosynthesis, a family of monocyclic β-lactam antibiotics.

Persistence and degradation of new β-lactam antibiotics in the soil and water environment

The development of new antibiotics with low environmental persistence is of utmost importance in contrasting phenomena of antibiotic resistance. In this study, the persistence of two newly synthesized monocyclic β-lactam antibiotics: (2R)-1-(methylthio)-4-oxoazetidin-2-yl acetate, P1, and (2R,3R)-3-((1R)-1-(tert-butyldimethylsilanyloxy)ethyl)-1-(methylthio)-4-oxoazetidin-2-yl acetate, P2, has been investigated in water in the pH range 3–9 and in two (calcareous and forest) soils, then compared to amoxicillin, a β-lactam antibiotic used in human and veterinary medicine. P1 and P2 persistence in water was lower than that of amoxicillin with only a few exceptions. P1 hydrolysis was catalyzed at an acidic pH whereas P2 hydrolysis takes place at both acidic and alkaline pH values. P1 persistence in soils depended mainly on their water potential (t1/2: 35.0–70.7d at wilting point; <1d at field capacity) whereas for P2 it was shorter and unaffected by soil water content (t1/2 0.13–2.5d). Several degradation products were detected in soils at both water potentials, deriving partly from hydrolytic pathways and partly from microbial transformation. The higher LogKow value for P2 compared with P1 seemingly confers P2 with high permeability to microbial membranes regardless of soil water content. P1 and P2 persistence in soils at wilting point was shorter than that of amoxicillin, whereas it had the same extent at field capacity.

Origins of the β-lactam rings in natural products

Naturally occurring β-lactam compounds fall into four basic structural groups, the penicillins/cephalosporins, the clavams, the carbapenems and the monocyclic β-lactams. Biosynthetic studies have clarified the steps involved in the formation of the β-lactam ring for the first three of these groups, but the corresponding process or processes for the monocyclic β-lactams remains obscure. Isopenicillin N synthase is responsible for formation of the β-lactam ring in all penicillin/cephalosporin compounds, and the reaction catalyzed is completely separate from that of β-lactam synthetase, the enzyme responsible for ring formation in all clavam compounds. Conversely, carbapenam synthetase, the enzyme responsible for β-lactam ring formation for all carbapenem compounds, shows clear relatedness to β-lactam synthetase, despite differences in the substrates and the products for the two enzymes. The mechanism of ring formation has not yet been clarified for any of the monocyclic β-lactams, but a third distinct mechanism of β-lactam ring formation seems likely, and this group includes such a diverse collection of structures that even more new ring-forming reactions may be involved.

Design and Synthesis of New 4-Alkylthio Monocyclic β-Lactams

Sang Hyup LeeCollege of Pharmacy, Duksung Women’s University, Seoul 132-714, Korea. E-mail: sanghyup@duksung.ac.krReceived August 1, 2012, Accepted October 16, 2012New types of monocyclic β-lactams constitute an important class of compounds due to their unique structuresand natures. Here, the design and synthesis of new 4-alkylthio monocyclic β-lactams 2a and 3a are reported.Significantly, compounds 2a and 3a, while keeping a monocyclic system, were designed to contain all of thesubstructures provided by the cleavage of C(2)-C(3) bond in penicillins. Efficient synthetic pathways forcompounds 2a and 3a were established based on two different strategies. Compound 2a was synthesized fromraw materials, using 4-acetoxyazetidin-2-one as a key intermediate, through a ten-step synthetic sequence in3% overall yield. Compound 3a was synthesized from potassium salt of penicillin G (17), using the degradedproduct 20 as a key intermediate, through a six-step synthetic sequence in 11% overall yield. 4-Alkylthio-azetidin-2-one derivatives, introduced in this study, could serve as valuable intermediates for the developmentof new monocyclic β-lactams. Key Words : Monocyclic β-lactams, Penicillin, Isopropylthioazetidinone, t-Butylthioazetidinone, Degrada-tion of penicillin IntroductionThe four-membered ring appears to be the smallest cyclicsystem that accommodates the amide function as a con-stituent. Such four-membered cyclic amides, commonly re-ferred to β-lactams, possess physical and chemical pro-perties that diverge sharply from those of acyclic amides,partially due to ring strain. After numerous penicillins andcephalosporins, new types of nonclassical β-lactams haveappeared. Monocyclic β-lactam compounds

Non-ribosomal Propeptide Precursor in Nocardicin A Biosynthesis Predicted from Adenylation Domain Specificity Dependent on the MbtH Family Protein NocI

Nocardicin A is a monocyclic β-lactam isolated from the actinomycete Nocardia uniformis that shows moderate antibiotic activity against a broad spectrum of gram-negative bacteria. The monobactams are of renewed interest due to emerging gram-negative strains resistant to clinically available penicillins and cephalosporins. Like isopenicillin N, nocardicin A has a tripeptide core of non-ribosomal origin. Paradoxically, the nocardicin A gene cluster encodes two non-ribosomal peptide synthetases (NRPSs), NocA and NocB, predicted to encode five modules pointing to a pentapeptide precursor in nocardicin A biosynthesis, unless module skipping or other nonlinear reactions are occurring. Previous radiochemical incorporation experiments and bioinformatic analyses predict the incorporation of p-hydroxy-L-phenylglycine (L-pHPG) into positions 1, 3, and 5 and L-serine into position 4. No prediction could be made for position 2. Multidomain constructs of each module were heterologous expressed in Escherichia coli for determination of the adenylation domain (A-domain) substrate specificity using the ATP/PPi exchange assay. Three of the five A-domains, from modules 1, 2, and 4, required the addition of stoichiometric amounts of MbtH family protein NocI to detect exchange activity. On the basis of these analyses, the predicted product of the NocA and NocB NRPSs is L-pHPG-L-Arg-D-pHPG-L-Ser-L-pHPG, a pentapeptide. Despite being flanked by non-proteinogenic amino acids, proteolysis of this pentapeptide by trypsin yields two fragments from cleavage at the C terminus of the L-Arg residue. Thus, a proteolytic step is likely involved in the biosynthesis of nocardicin A, a rare but precedented editing event in the formation of non-ribosomal natural products that is supported by the identification of trypsin-encoding genes in N. uniformis.

Monocyclic β-lactams as antibacterial agents: Facing antioxidant activity of N-methylthio-azetidinones

A series of N-methylthio-β-lactams with antibacterial activity were thoroughly evaluated as antioxidants. We found that only the presence of a polyphenolic moiety anchored to the β-lactam ring ensured an adequate antioxidant potency. New compounds, efficiently combining in one structure antioxidant and antibacterial activity, may provide a promising basis for the development of new leads useful in adverse clinical conditions such as in cystic fibrosis patients, in whom colonization by MRSA and epithelial damage by chronic pulmonary oxidative stress take place.

Inhaled Aztreonam Lysine for Cystic Fibrosis Pulmonary Disease–Related Outcomes

To evaluate the pharmacology, clinical efficacy, and safety of aztreonam lysine for inhalation (AZLI) for cystic fibrosis (CF)-related signs and symptoms of pulmonary disease. Literature was searched in MEDLINE through PubMed and cross-referenced with EMBASE (1980-June 2012). The key search terms used were aztreonam lysine, nebulized, inhaled, and cystic fibrosis. Bibliographies of selected articles were used to identify additional references. Ongoing trials were identified through a review of Web site trial registries. Articles were limited to those written in English about studies conducted in humans. Studies included in this review examined both adult and pediatric patients with CF. Aztreonam lysine is an inhaled monocyclic β-lactam antibiotic approved for use in the CF population. Four completed clinical trials with peer-reviewed published data were reviewed to assess the efficacy and safety of single-course AZLI; a fifth trial assessed the safety and efficacy of repeat courses of AZLI. None of these trials compared AZLI in a head-to-head manner with tobramycin for inhalation. In patients with moderate to severe pulmonary disease, AZLI administration improved forced expiratory volume in 1 second measurements, decreased sputum bacterial Pseudomonas aeruginosa density, and improved symptoms. Adverse effects in clinical trials were generally mild and similar to those with placebo. AZLI is safe and effective for management of pulmonary-related symptoms in patients with CF who are colonized with P. aeruginosa and have moderate to severe pulmonary disease. Additional trial data comparing AZLI with tobramycin are warranted to further establish the place of AZLI in therapy.

Improving known classes of antibiotics: an optimistic approach for the future

New antibiotic agents are desperately needed to treat the multidrug-resistant pathogens that continue to emerge at alarming rates. Many of the agents that have entered full clinical development since 1995 have been members of previously accepted classes of antibiotics. Among these are a new aminoglycoside (plazomicin), anti-MRSA cephalosporins (ceftobiprole and ceftaroline), a monocyclic β-lactam (BAL30072), the β-lactamase inhibitor combination of tazobactam with the anti-pseudomonal cephalosporin ceftolozane, β-lactam combinations with new non-β-lactam inhibitors (MK-7655 with imipenem, and avibactam with ceftazidime and ceftaroline), new macrolides (cethromycin and solithromycin), oxazolidinones (tedizolid phosphate and radezolid), and quinolones (delafloxacin, nemonoxacin and JNJ-Q2). Resistance and safety issues have been circumvented by some of these new agents that have well-established mechanisms of action and defined pathways leading toward regulatory approval.

In Vivo Characterization of Nonribosomal Peptide Synthetases NocA and NocB in the Biosynthesis of Nocardicin A

Two nonribosomal peptide synthetases (NRPS), NocA and NocB, together comprising five modules, are essential for the biosynthesis of the D,L,D configured tripeptide backbone of the monocyclic β-lactam nocardicin A. We report a double replacement gene strategy in which point mutations were engineered in the two encoding NRPS genes without disruption of the nocABC operon by placing selective markers in adjacent genes. A series of mutants was constructed to inactivate the thiolation (T) domain of each module and to evaluate an HHxxxDR catalytic motif in NocA and an atypical extended histidine motif in NocB. The loss of nocardicin A production in each of the T domain mutants indicates that all five modules are essential for its biosynthesis. Conversely, production of nocardicin A was not affected by mutation of the NocB histidine motif or the R828G mutation in NocA.

Choline Promotes Growth and Tabtoxin Production in a &amp;lt;i&amp;gt;Pseudomonas syringae&amp;lt;/i&amp;gt; Strain

Some Pseudomonas syringae pathovars secrete tabtoxin, a monocyclic β-lactam antibiotic, responsible for chlorosis, the principal halo blight symptom in susceptible plants as oats, rye, barley, wheat and sorghum, among other. Here, we demonstrated that the production of tabtoxin in a P. syringae strain increased at least 150%, when choline, betaine or dimethylglycine were used as nitrogen source, or when choline was added as osmoprotectant in hyperosmolar culture media. Besides, we investigated the induction of phosphorylcholine phosphatase (PchP) activity when choline or its metabolites were used as nitrogen sources. PchP is an enzyme involved in Pseudomonas aeruginosa pathogenesis through its contribution to the breakdown of choline-containing compounds of the host cells. Considering these results and that the success of a pathogenic microorganism depends on its ability to survive and proliferate in its target tissue, we propose that choline is one of the plant signals that contribute to establishment of the infection by tabtoxin-producing strains of P. syringae.

Monocyclic &amp;#946;-Lactams: New Structures for New Biological Activities

The azetidinone core-structure offers a unique approach to the design and synthesis of new derivatives with unique biological properties. During the last two decades researches convincingly demonstrated that the prospect of structural modifications of monocyclic β-lactams with specific substituents is an effective procedure for the detection and improvement of important pharmacological effects different from antibacterial activity. As a matter of fact, new β-lactam compounds demonstrated biological activity as inhibitors of a wide range of enzymes. This review reports the latest developments on monocyclic β-lactam compounds activity as anticancer, antitubercular, HFAAH inhibitors, HDAC inhibitors, anti-inflammatory drugs (tryptase inhibitors), Cathepsin K inhibitors, and vasopressin inhibitors. We attempted to highlight the intertwined relationships between structural features and biological activities, by analysing groups anchored on the three positions of the azetidinone ring as sources of molecular diversity.

C4-Alkylthiols with activity against Moraxella catarrhalis and Mycobacterium tuberculosis

Antimicrobial resistance represents a global threat to healthcare. The ability to adequately treat infectious diseases is increasingly under siege due to the emergence of drug-resistant microorganisms. New approaches to drug development are especially needed to target organisms that exhibit broad antibiotic resistance due to expression of β-lactamases which is the most common mechanism by which bacteria become resistant to β-lactam antibiotics. We designed and synthesized 20 novel monocyclic β-lactams with alkyl- and aryl-thio moieties at C4, and subsequently tested these for antibacterial activity. These compounds demonstrated intrinsic activity against serine β-lactamase producing Mycobacterium tuberculosis wild type strain (Mtb) and multiple (n=6) β-lactamase producing Moraxella catarrhalis clinical isolates.

Synthesis of mono-, bis-spiro- and dispiro-β-lactams and evaluation of their antimalarial activities

Some new mono-, bis-spiro- and dispiro-β-lactams have been synthesized from imines derived from 9 H-fluoren-9-one and a ketene derived from 9 H-xanthene-9-carboxylic acid or phenoxyacetic acid by a [2+2] cycloaddition reaction in good to excellent yields varying from 45 to 83%. The biological activity of these monocyclic β-lactams was successfully investigated against Plasmodium falciparum K14 resistant strain with excellent EC 50 values up to 5 μM.

Synthesis and Antimicrobial Activity of Novel 3-Benzyloxy-4-Substituted-2- Azetidinones: Formation of a Hydrophobic Layer Via a Self-Organization Effect

We report the synthesis of new persulfide-spacer N-substituted-2-azetidinone-D-glucosamine in an attempt to potentially provide new antibiotics. The Schiff base ligands considered for this study were derived from D-glucosamine and 2-hydroxybenzaldehyde, 4-methoxy-benzaldehyde, cinnamaldehyde, 4-chlorobenzaldehyde, and 4-hydroxy-3-methoxy-benzaldehyde. Staudinger [2+2] cycloaddition of benzyloxyacetyl chloride to the newly reported per-O-allyl-N-substituted benzylidene-2-deoxy-β-D-glucosamine provided the sugar-based monocyclic β-lactams in moderate yields. Radical addition of 2-mercaptoethanol catalyzed by azobisisobutyronitrile to the per-O-allyl-N-substituted-2-azetidinone-D-glucosamine led to the corresponding persulfide-spacers in good yields. All new compounds were characterized by spectroscopic and spectrometric methods. The scanning electron microscopy image of 1,3,4,6-tetra-O-[3-(hydroxythioethyl)-propyl]-2-deoxy-2-N-[(3-benzyloxy-4-(4-chloropenyl)-2-azetidinone]-β-D-glucopyranoside, as a representative example, demonstrated a super hydrophobic layer formed via highly organized thioether spacers on gold as the adsorbate system through the formation of sulfur–gold bonds. The reported glucosides showed a moderate antifungal activity against Candida albicans while being slightly to moderately active against gram-positive and gram-negative bacteria used in this investigation at a concentration of 1 mg/mL dissolved in dimethyl sulfoxide.Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.GRAPHICAL ABSTRACT