The synthesis and biological evaluation of a novel series of antimicrobials of the oxazolidinone class

Halogenated Antimicrobial Agents to Combat Drug-Resistant Pathogens

Antimicrobial resistance presents us with a potential global crisis as it undermines the abilities of conventional antibiotics to combat pathogenic microbes. The history of antimicrobial agents is replete with examples of scaffolds containing halogens. In this review, we discuss the impacts of halogen atoms in various antibiotic types and antimicrobial scaffolds and their modes of action, structure-activity relationships, and the contributions of halogen atoms in antimicrobial activity and drug resistance. Other halogenated molecules, including carbohydrates, peptides, lipids, and polymeric complexes, are also reviewed, and the effects of halogenated scaffolds on pharmacokinetics, pharmacodynamics, and factors affecting antimicrobial and antivirulence activities are presented. Furthermore, the potential of halogenation to circumvent antimicrobial resistance and rejuvenate impotent antibiotics is addressed. This review provides an overview of the significance of halogenation, the abilities of halogens to interact in biomolecular settings and enhance pharmacological properties, and their potential therapeutic usages in preventing a postantibiotic era. SIGNIFICANCE STATEMENT: Antimicrobial resistance and the increasing impotence of antibiotics are critical threats to global health. The roles and importance of halogen atoms in antimicrobial drug scaffolds have been established, but comparatively little is known of their pharmacological impacts on drug resistance and antivirulence activities. This review is the first to extensively evaluate the roles of halogen atoms in various antibiotic classes and pharmacological scaffolds and to provide an overview of their ability to overcome antimicrobial resistance.

Discovery of Novel Diamides Scaffold Containing Monofluoro-acrylamides Activating the Insect Ryanodine Receptor

The research and development of organofluorine chemistry has flourished; in particular, monofluoroalkene has aroused considerable interest from medicinal and organic chemists. It is a significant attempt to introduce monofluoroalkene into agrochemicals. In this study, monofluoroalkene was introduced into diamide molecules and inserted between the aliphatic amide and benzene ring, and 44 compounds have been successfully synthesized. The bioassay results showed that compounds with monofluoro-acrylamide moiety (Z-isomers) had excellent larvicidal activity against lepidopteran pests at 5 mg·L-1. The LC50 values of compounds B16, B18, and B21 against Mythimna separata were 1.02, 1.32, and 0.78 mg·L-1, respectively. 3D-QSAR analysis including the CoMFA model and the CoMSIA model was conducted to illustrate the contributions of steric, electrostatic, hydrophobic, and hydrogen bond fields on the bioactivity. Moreover, typical symptoms caused by chlorantraniliprole including dehydration, shrinkage, and blackening were also observed on the test larvae treated with monofluoro-acrylamide diamide compounds. M. separata central neurons calcium imaging experiment of compound B18 indicated that the monofluoro-acrylamide diamide compounds were potential insect ryanodine receptor activators. The molecular docking was performed in the CHL binding domain of Plutella xylostella RyR and revealed that the predicted binding mode of compound B21 was slightly different from that of CHL. The MM|GBSA dG Bind values of B21 and CHL with P. xylostella RyR were respectively -85.797 and -95.641 kcal·mol-1. The present work explored the insecticidal properties of a new diamide scaffold containing a monofluoro-acrylamide fragment and extended the application of monofluoroalkene in the agrochemical field.

Anthranilic Diamides Containing Monofluoroalkene Amide Linkers as Potential Insect RyR Activators: Design, Synthesis, Bio-evaluation, and Computational Study

In order to develop anthranilic diamides with novel chemotypes, a series of anthranilic diamides with acrylamide linkers were designed and synthesized. The results of preliminary bioassays indicated that compounds with a monofluoroalkene amide linker (Z-isomer) exhibited good larvicidal activity against lepidopteran pests. The LC₅₀ values of compound A23 against Mythimna separata and Plutella xylostella were 1.44 and 3.48 mg·L^-1, respectively, while those of chlorantraniliprole were 0.08 and 0.06 mg·L^-1, respectively. Compound A23 also exhibited the same level of lethal potency against resistant and susceptible strains of Spodoptera frugiperda at 50 mg·L^-1. Compound A23 exhibited similar symptoms as chlorantraniliprole in test larvae. Comparative molecular field analysis was conducted to demonstrate the structure-activity relationship. Central neuron calcium imaging experiments indicated that monofluoroalkene compounds were potential ryanodine receptor (RyR) activators and activated calcium channels in both the endoplasmic reticulum and the cell membrane. Molecular docking suggested that A23 had a better binding potency to P. xylostella RyR than chlorantraniliprole. The MM|GBSA dG bind value of A23 with P. xylostella RyR was 117.611 kcal·mol^-1. Monofluoroalkene was introduced into anthranilic diamide insecticides for the first time and brought a novel chemotype for insect RyR activators. The feasibility of fluoroalkenes as insecticide fragments was explored.

Platinum-Catalyzed Hydrofluorination of Alkynes: Hydrogen Bonding to Indolylphosphine Ligands to Provide Fluoride Reactivity

The reaction of the Pt complexes cis-[Pt(CH3 )2 {R2 P(Ind)}2 ] (Ind=2-(3-methyl)indolyl, R=Ph (1 a), 4-FC6 H4 (1 b), 4-CF3 C6 H4 (1 c)) with HF afforded the fluorido complexes trans-[Pt(F(HF)2 )(CH3 ){R2 P(Ind)}2 ] 2 a-c, which can be converted into trans-[Pt(F)(CH3 ){R2 P(Ind)}2 ] (3 a-c) by treatment with CsF. Addition of 3-hexyne to 2 a-c gave alkyne complexes trans-[Pt(C,C-η2 -C2 H5 C≡CC2 H5 )(CH3 ){R2 P(Ind)}2 {F(HF)2 }] (4 a-c) at which a fluoride is stabilised as polyfluoride in the coordination sphere by hydrogen bonding to the indolyl-substituted phosphine ligands. Subsequent heating of a solution of 4 a in the presence of PVPHF led to fluoroalkene formation. Selective catalytic hydrofluorination of alkynes to yield (Z)-fluoroalkenes were developed. The ability of hydrogen bonding to polyfluoride favours the fluorination step as demonstrated by studies with complexes bearing no indolyl groups at the phosphine ligands.

Iron-catalyzed decarboxylative and oxidative decarbonylative cross-coupling: a new strategy for the synthesis of monofluoroalkenes

Herein, an iron(ii)-catalyzed decarboxylative and oxidative decarbonylative cross-coupling of α-fluoro cinnamic acids with aliphatic aldehydes is presented.

Copper-catalyzed direct monofluoroalkenylation of C(sp3)–H bonds via decarboxylation of α-fluoroacrylic acids

Herein, a protocol for the copper-catalyzed direct monofluoroalkenylation of C(sp3)–H bonds is reported.

gem-Heteroatom-Substituted Fluoroalkenes as Mimics of Amide Derivatives or Phosphates: A Comprehensive Review

gem-Heteroatom-substituted fluoroalkenes have received little attention despite their great potential in medicinal chemistry or in fine chemistry. Indeed, due to the electronic and steric similarity between the fluoroalkene moiety and the amide bond as well as the high strength of the carbon-fluorine bond, these gem-heteroatom-substituted fluoroalkenes could be envisioned as stable mimics of various important organic functions, such as phosphates, carbamates, S-thiocarbamates and ureas. We present herein an overview describing the syntheses over the last decade of heteroatom-substituted fluoroalkenes in geminal position. This review will be divided into several sections covering each the common following heteroatom: oxygen-, nitrogen-, sulfur-, phosphorus-, boron- and silicon-substituted fluoroalkenes.

Cyclodimers and Cyclotrimers of 2,3 -Bisalkynylated Anthracenes, Phenazines and Diazatetracenes

The synthesis of novel (N-)acene-based cyclooligomers is reported. Glaser-Hay coupling of the bisethynylated monomers results in cyclodimers and cyclotrimers that are separable by column and gel-permeation chromatographies. For the diazatetracene, the use of sec-butyl-silylethynyl groups is necessary to achieve solubility. Diazatetracene-based cyclodimers and cyclotrimers were used as semiconductors in thin-film transistors. Although their optoelectronic properties are quite similar, their electron mobilities in proof-of-concept thin-film transistors differ by an order of magnitude.

Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents

The discovery of safe, general, and practical procedures to prepare vinyl fluorides from readily available precursors remains a synthetic challenge. The metal-free hydrofluorination of alkynes constitutes an attractive though elusive strategy for their preparation. Introduced here is an inexpensive and easily handled reagent that enables the development of simple and scalable protocols for the regioselective hydrofluorination of alkynes to access both the E and Z isomers of vinyl fluorides. These reaction conditions were suitable for a diverse collection of alkynes, including several highly functionalized pharmaceutical derivatives. Computational and experimental mechanistic studies support C-F bond formation through vinyl cation intermediates, with the E- and Z-hydrofluorination products forming under kinetic and thermodynamic control, respectively.

Ameliorative effect of silymarin against linezolid-induced hepatotoxicity in methicillin-resistant Staphylococcus aureus (MRSA) infected Wistar rats

Linezolid has a better choice for eradication of methicillin-resistant Staphylococcus aureus (MRSA) infections, but its use is limited because of linezolid-induced hepatotoxicity, myelosuppression, and lactic acidosis. This research elucidated the role of silymarin against hepatoxicity of linezolid therapy in MRSA infected Wistar rats. The rats were rendered neutropenic by an intraperitoneal injection of cyclophosphamide injection. The neutropenic rats were injected subcutaneously with 10⁶ CFU/ml of MRSA. The rats were divided into 6 groups. Normal control, Infected, Infected animals treated with linezolid 50 mg/kg/twice/day and Infected animals treated with linezolid and different dose of silymarin 25, 50, and 100 mg/kg/twice/day for 14 days. On the 15th day, the blood, liver, kidney, and bone marrow were collected for biochemical and histopathological examination. The MRSA was confirmed by PCR assay. The minimum inhibitory concentration of linezolid was 0.5-2 μg/ml. The linezolid induced liver damage was confirmed by elevation of marker enzymes alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), Lactate dehydrogenase (LDH) levels, serum bilirubin, lactate, and histopathological studies of the liver. The linezolid treated rats also showed myelosuppression, lactic acidosis, oxidative stress and decreased intestinal alkaline phosphatase (IAP). The silymarin administration exhibited marked hepatoprotective effect by significantly lowering the liver marker enzymes, serum parameters, and cytological findings reflect the hepatoprotection. Additionally, Silymarin showed protection against myelosuppression and lactic acidosis evidenced by bone marrow smear and serum lactate estimation. Antioxidant effect of silymarin was confirmed by decreased levels of lipid peroxidation, restored the enzymatic and non-enzymatic antioxidants of the liver nearer to normal. The present study indicates that the silymarin could be a better herbal therapeutic agent which protects against the linezolid induced hepatotoxicity in MRSA infected rats.

Selective Monodefluorination and Wittig Functionalization of gem-Difluoromethyl Groups to Generate Monofluoroalkenes

Monodefluorination of gem-difluoromethyl groups is achieved using a frustrated Lewis pair (FLP) approach. Triarylphosphines and group 13 Lewis acids were surveyed as FLP components, with the combination of P( o-Tol)₃ and B(C₆F₅)₃ found to provide the best results, although the reaction is feasible with more economical components (PPh₃ and BF₃·OEt₂). The α-fluoroalkylphosphonium products arising from the reaction were of lower activity, in regard to further fluoride abstraction, as compared to difluoride starting materials, leading to highly selective monodefluorination. The activated substrates were subject to Wittig reaction protocols to generate a variety of monofluoroalkenes in moderate to high yields.

Visible light photocatalytic decarboxylative monofluoroalkenylation of α-amino acids with gem-difluoroalkenes

α-Amino acids are among the most common biologically active molecules in nature, and their functionalization has attracted much attention. In this communication, a novel, efficient and general visible-light photocatalytic decarboxylative monofluoroalkenylation of N-protected α-amino acids with gem-difluoroalkenes is reported, affording the corresponding α-amino monofluoroalkenes which might find applications in medical chemistry and materials science. The reaction proceeded at room temperature with high efficiency and tolerance of various functional groups.

Mitochondrial Alterations (Inhibition of Mitochondrial Protein Expression, Oxidative Metabolism, and Ultrastructure) Induced by Linezolid and Tedizolid at Clinically Relevant Concentrations in Cultured Human HL-60 Promyelocytes and THP-1 Monocytes

Linezolid, the first clinically available oxazolidinone antibiotic, causes potentially severe toxicities (myelosuppression, lactic acidosis, and neuropathies) ascribed to impairment of mitochondrial protein synthesis and consecutive mitochondrial dysfunction. Tedizolid, a newly approved oxazolidinone, shows an enhanced activity compared to linezolid but is also a more potent inhibitor of mitochondrial protein synthesis. We compared linezolid and tedizolid for (i) inhibition of the expression of subunit I of cytochrome c-oxidase (CYTox I; Western blot analysis), (ii) cytochrome c-oxidase activity (biochemical assay), (iii) mitochondrial oxidative metabolism (Seahorse technology), and (iv) alteration of mitochondrial ultrastructure (electron microscopy) using HL-60 promyelocytes and THP-1 monocytes exposed to microbiologically (multiples of modal MIC against Staphylococcus aureus) and therapeutically (C_min - C_max) pertinent concentrations. Both drugs caused a rapid and complete (48 to 72 h) inhibition of CYTox I expression, cytochrome c-oxidase activity, and spare respiratory capacity, with conspicuous swelling of the mitochondrial matrix and loss of their cristae. Globally, tedizolid was a more potent inhibitor than linezolid. For both drugs, all effects were quickly (48 to 72 h) and fully reversible upon drug withdrawal. Using an alternation of exposure to and withdrawal from drug mimicking their approved schedule of administration (twice daily and once daily [qD] for linezolid and tedizolid, respectively), only partial inhibition of CYTox I expression was noted for up to 96 h. Thus, rapid reversal of toxic effects upon discontinuous administration may mitigate oxazolidinone toxicity. Since tedizolid is given qD, this may help to explain its reported lower preclinical and clinical toxicity.

(2-Fluoroallyl)boronates: new reagents for diastereoselective 2-fluoroallylboration of aldehydes

New reagents for anti-selective 2-fluoroallylboration of aromatic and aliphatic aldehydes—(2-fluoroallyl)pinacolboranes, were prepared by Pd-catalyzed borylation of 2-fluoroallyl chlorides.

Palladium-Catalyzed Defluorinative Coupling of 1-Aryl-2,2-Difluoroalkenes and Boronic Acids: Stereoselective Synthesis of Monofluorostilbenes

The palladium-catalyzed defluorinative coupling of 1-aryl-2,2-difluoroalkenes with boronic acids is described. Broad functional-group tolerance arises from a redox-neutral process by a palladium(II) active species which is proposed to undergo a β-fluoride elimination to afford the products. The monofluorostilbene products were formed with excellent diastereoselectivity (≥50:1) in all cases, and it is critical, as traditional chromatographic techniques often fail to separate monofluoroalkene isomers. As a demonstration of this method's unique combination of reactivity and functional-group tolerance, a Gleevec® analogue, using a monofluorostilbene as an amide isostere, was synthesized.

Diastereoselective Synthesis of Fluorine-Containing Pyrrolizidines via Triphenylcarbenium Tetrafluoroborate-Promoted Carbofluorination of N-3-Arylpropargylpyrrolidine-Tethered Tertiary Allylic Alcohols

Inexpensive and air stable triphenylcarbenium tetrafluoroborate efficiently promoted the carbofluorination of N-arylpropargylpyrrolidines bearing a tertiary allylic alcohol tether at the 2-position of the pyrrolidine ring to provide 1-isobutenyl-2-(fluoro(phenyl)methylenylhexahydro-1H-pyrrolizidines in a stereoselective fashion. When subjected to bis(trifluoromethane)sulfonamide, the same substrates underwent cycloisomerization reaction within minutes to generate 1-isobutenyl-2-benzoylhexahydro-1H-pyrrolizidines with excellent stereoselectivity.

Transition-metal-free carbofluorination of TBS-protected nitrogen-containing cyclic enynols: synthesis of fluorinated azabicycles

The synthesis of fluorinated azabicycles from tert-butyldimethylsilyl-protected N-containing cyclic enynols using inexpensive BF3·OEt2 is described. In this reaction, BF3 reacts as both the Lewis acid and the fluoride source for cyclization/fluorination of the TBS-protected cyclic N-containing enynols. The method provides an easy access to fluorinated azabicycles where a new C(sp(2))-F bond and a new bicyclic skeleton are generated at ambient temperature within 1-13 min under metal-free reaction conditions.

Antibacterial oxazolidinones: emerging structure-toxicity relationships

The oxazolidinones are an important class of synthetic bacterial protein synthesis inhibitors with activity against Gram-positive and some fastidious Gram-negative bacteria. Key toxicological issues with the class include reversible inhibition of monoamine oxidase enzymes and reversible myelosuppression that can occur in patients treated for longer than the recommended course of therapy. Recent studies have uncovered the likely molecular mechanism underlying oxazolidinone-related myelosuppression and other toxicities, and these will be discussed here. Also reviewed are recent reports of structural modifications that can attenuate one or more of the undesired effects of oxazolidinones, while retaining the desired antibacterial effect.

A distal methyl substituent attenuates mitochondrial protein synthesis inhibition in oxazolidinone antibacterials

Oxazolidinone analogs bearing substituted piperidine or azetidine C-rings are described. Analogs with a methyl group at the 3-position of the azetidine ring or the 4-position of the piperidine ring exhibited reduced mitochondrial protein synthesis inhibition while retaining good antibacterial potency.

The site of action of oxazolidinone antibiotics in living bacteria and in human mitochondria

The oxazolidinones are one of the newest classes of antibiotics. They inhibit bacterial growth by interfering with protein synthesis. The mechanism of oxazolidinone action and the precise location of the drug binding site in the ribosome are unknown. We used a panel of photoreactive derivatives to identify the site of action of oxazolidinones in the ribosomes of bacterial and human cells. The in vivo crosslinking data were used to model the position of the oxazolidinone molecule within its binding site in the peptidyl transferase center (PTC). Oxazolidinones interact with the A site of the bacterial ribosome where they should interfere with the placement of the aminoacyl-tRNA. In human cells, oxazolidinones were crosslinked to rRNA in the PTC of mitochondrial, but not cytoplasmic, ribosomes. Interaction of oxazolidinones with the mitochondrial ribosomes provides a structural basis for the inhibition of mitochondrial protein synthesis, which is linked to clinical side effects associated with oxazolidinone therapy.

Inhibition of mammalian mitochondrial protein synthesis by oxazolidinones

The effects of a variety of oxazolidinones, with different antibacterial potencies, including linezolid, on mitochondrial protein synthesis were determined in intact mitochondria isolated from rat heart and liver and rabbit heart and bone marrow. The results demonstrate that a general feature of the oxazolidinone class of antibiotics is the inhibition of mammalian mitochondrial protein synthesis. Inhibition was similar in mitochondria from all tissues studied. Further, oxazolidinones that were very potent as antibiotics were uniformly potent in inhibiting mitochondrial protein synthesis. These results were compared to the inhibitory profiles of other antibiotics that function by inhibiting bacterial protein synthesis. Of these, chloramphenicol and tetracycline were significant inhibitors of mammalian mitochondrial protein synthesis while the macrolides, lincosamides, and aminoglycosides were not. Development of future antibiotics from the oxazolidinone class will have to evaluate potential mitochondrial toxicity.

Recent developments in the identification of novel oxazolidinone antibacterial agents

The oxazolidinones are a promising new class of synthetic antibacterial agents. Here, we review recent efforts directed at the discovery of new antibacterial compounds of this class. New structures and structure-activity relationships (SAR) are discussed in the context of earlier work in the field. Key issues of potency, spectrum, selectivity, in vivo efficacy, and pharmacokinetic profile of the new analogs are addressed.

Oxazolidinones inhibit cellular proliferation via inhibition of mitochondrial protein synthesis

The oxazolidinones are a relatively new structural class of antibacterial agents that act by inhibiting bacterial protein synthesis. The oxazolidinones inhibit mitochondrial protein synthesis, as shown by [35S]methionine incorporation into intact rat heart mitochondria. Treatment of K562 human erythroleukemia cells with the oxazolidinone eperezolid resulted in a time- and concentration-dependent inhibition of cell proliferation. The cells remained viable, but an increase in doubling time was observed with eperezolid treatment. Inhibition was reversible, since washing and refeeding of cells in the absence of compound resulted in a resumption of growth. The growth-inhibitory effect of the oxazolidinones did not appear to be cell type specific, and inhibition of CHO and HEK cells also was demonstrated. Treatment of cells resulted in a decrease in mitochondrial cytochrome oxidase subunit I levels, consistent with an inhibition of mitochondrial protein synthesis. Eperezolid caused no growth inhibition of rho zero (rho0) cells, which contain no mitochondrial DNA; however, the growth of the parent 143B cells was inhibited. These results provide a direct demonstration that the inhibitory effect of eperezolid in mammalian cells is the result of mitochondrial protein synthesis inhibition.

Palladium-Catalyzed Cross-Coupling Reactions of Pyridylboronic Acids with Heteroaryl Halides Bearing a Primary Amine Group: Synthesis of Highly Substituted Bipyridines and Pyrazinopyridines

A range of halogenated aromatics and heteroaromatics bearing a primary amine group are shown to be suitable substrates for Suzuki cross-coupling reactions with arylboronic acids and pyridylboronic acids under standard conditions, without the need for protection/deprotection steps. New amino-substituted arylpyridines, bipyridines, and pyrazinopyridines have thereby been obtained. Conditions for the efficient syntheses of 2-methoxy-5-pyridylboronic acid 1 and 2-methoxy-3-pyridylboronic acid 2 in ca. 75 g batches have been defined. A 2-fold reaction of 2-amino-5-bromopyrazine with 2,5-dimethoxy-1,4-benzenediboronic acid affords 1,4-dimethoxy-2,5-bis[2-(5-aminopyrazyl)]benzene 31. The X-ray crystal structures of 1 and 31.DMF are reported.

Synthesis and biological evaluation of benzazepine oxazolidinone antibacterials

Novel benzazepine oxazolidinone antibacterials were synthesized and evaluated against clinically relevant susceptible and resistant organisms. The effect of ring nitrogen position and N-substitution on antibacterial activity is examined.