Expanding the Scope of Group Transfer Radical Reaction: Toward the Synthesis of Fluorinated Nucleoside Analogues Incorporating Difluorophosphonylated Allylic Ether Moieties.

Scope and limitations of the group transfer radical reaction of diisopropyl iododifluoromethylphosphonate onto carbohydrates and nucleosides are described. This key step allowed us to explore the synthesis of new fluorinated nucleoside analogues containing a difluorophosphonylated allylic ether moiety onto the 2'-position, in purine and pyrimidine series (B = A, C, G, T, U). Indeed, two unprecedented chemical approaches involving a late introduction of either the nucleobase or the fluorinated moiety are discussed.

The absence of surface D-alanylation, localized on lipoteichoic acid, impacts the Clostridioides difficile way of life and antibiotic resistance.

Introduction: The dlt operon encodes proteins responsible for the esterification of positively charged D-alanine on the wall teichoic acids and lipoteichoic acids of Gram-positive bacteria. This structural modification of the bacterial anionic surface in several species has been described to alter the physicochemical properties of the cell-wall. In addition, it has been linked to reduced sensibilities to cationic antimicrobial peptides and antibiotics. Methods: We studied the D-alanylation of Clostridioides difficile polysaccharides with a complete deletion of the dltDABCoperon in the 630 strain. To look for D-alanylation location, surface polysaccharides were purified and analyzed by NMR. Properties of the dltDABCmutant and the parental strains, were determined for bacterial surface's hydrophobicity, motility, adhesion, antibiotic resistance. Results: We first confirmed the role of the dltDABCoperon in D-alanylation. Then, we established the exclusive esterification of D-alanine on C. difficile lipoteichoic acid. Our data also suggest that D-alanylation modifies the cell-wall's properties, affecting the bacterial surface's hydrophobicity, motility, adhesion to biotic and abiotic surfaces,and biofilm formation. In addition, our mutant exhibitedincreased sensibilities to antibiotics linked to the membrane, especially bacitracin. A specific inhibitor DLT-1 of DltA reduces the D-alanylation rate in C. difficile but the inhibition was not sufficient to decrease the antibiotic resistance against bacitracin and vancomycin. Conclusion: Our results suggest the D-alanylation of C. difficile as an interesting target to tackle C. difficile infections.

Photocatalyzed Hydroaminodifluoroalkylation of Alkenes.

The synthesis of undescribed ß-aminodifluoroethylsulfinates and their uses in the hydroaminodifluoroalkylation of alkenes is reported. This reaction is performed in the presence of a photocatalyst (4CzIPN, Ru complexes) and enables the direct incorporation of a ß-difluoroamine moiety into vinylic aryls, unactivated alkenes, and electron-rich, or -deficient alkenes. The mechanism was studied, and the formation of a gem-difluoromethyl radical was observed after the selective oxidation of the sulfinate function.

Photocatalyzed (3+2) Cycloaddition for the Dearomatization of Electron-Poor Arenes under Flow Conditions.

The photocatalyzed dearomative reaction between various electron-deficient aromatic compounds and a non-stabilized azomethine ylide is successfully performed in a flow system. Whereas the use of supported eosin as organic photocatalyst exhibits limited efficiency, turning to the soluble Rose Bengal allows to transform a broad range of substrates from hetarenes (indole, benzofuran, quinoline, pyridine) to naphthalenes and benzenes. This photocatalyzed (3+2) dearomative cycloaddition under green light irradiation provides a simple and efficient access to tridimensional pyrrolidino scaffolds with a tetrasubstituted carbon center at ring junction and can be performed in the friendly ethyl acetate. Computational studies support the mechanism involving azomethine ylide as reactive species toward the electron-poor arene.

Convergent access to mono-fluoroalkene-based peptidomimetics.

The convergent and selective preparation of (Z)-monofluoroalkene-based dipeptide isosteres from functionalized fluorosulfones as a cornerstone is described. In this approach, the N-terminal amino group is introduced by a conjugate addition reaction of phthalimide onto fluorinated vinylsulfones containing α-amino-acid side chains while the C-terminal motif is linked to the fluorovinylic peptide bond mimic via the Julia-Kocienski reaction between fluorosulfones and substituted aldehydes bearing α-amino-acid side chains.

Metal-Free Aminomethylation of Aromatic Sulfones Promoted by Eosin Y.

A metal-free α-aminomethylation of heteroaryls promoted by eosin Y under green light irradiation is reported. A large variety of α-trimethylsilylamines as precursor of α-aminomethyl radical species were engaged to functionalize sulfonyl-heteroaryls following a Homolytic Aromatic Substitution (HAS) pathway. This method has provided a range of α-aminoheteroaryl compounds including a functionalized natural product. The mechanism of this late-stage functionalization of aryls was investigated and suggests the formation of a sulfonyl radical intermediate over a reductive quenching cycle.

Inhibition of d-alanylation of teichoic acids overcomes resistance of methicillin-resistant Staphylococcus aureus.

BACKGROUND: MRSA are high-priority multidrug-resistant pathogens. Although there are still some antibiotics active against MRSA, continuous efforts to discover new antibiotics and treatment strategies are needed because resistance to these new drugs has already been reported. OBJECTIVES: Here we explore if d-alanylation of teichoic acids (TAs) mediated by the dlt operon gene products might be a druggable target to overcome ß-lactam-resistance of MRSA. METHODS: MICs and bactericidal effects of several ß-lactam antibiotics were monitored in a panel of clinical MRSA strains with genetic or chemically induced deficiency in d-alanylation of TAs. Efficiency of the chemical inhibitor to rescue MRSA-infected larvae of Galleria mellonella as well as its ability to prevent or eradicate biofilms of S. aureus were analysed. RESULTS: Genetic inactivation of the Dlt system or its chemical inhibition re-sensitizes MRSA to ß-lactams. Among the 13 strains, the most pronounced effect was obtained using the inhibitor with imipenem, reducing the median MIC from 16 to 0.25 mg/L. This combination was also bactericidal in some strains and significantly protected G. mellonella larvae from lethal MRSA infections. Finally, inactivation of d-alanylation potentiated the effect of imipenem on inhibition and/or eradication of biofilm. CONCLUSIONS: Our combined results show that highly efficient inhibitors of d-alanylation of TAs targeting enzymes of the Dlt system should be promising therapeutic adjuvants, especially in combination with carbapenems, for restoring the therapeutic efficacy of this class of antibiotics against MRSA.

Selective preparation of tetrasubstituted fluoroalkenes by fluorine-directed oxetane ring-opening reactions.

The selective ring-opening reaction of fluoroalkylidene-oxetanes was directed by the presence of the fluorine atom, enabling a two-step access to tetrasubstituted fluoroalkenes with excellent geometry control. Despite its small van der Waals radii electronic, rather than steric influences of the fluorine atom governed the ring-opening reaction with bromide ions, even in the presence of bulky substituents.

Genetic and pharmacological inactivation of d-alanylation of teichoic acids sensitizes pathogenic enterococci to ß-lactams.

BACKGROUND: Enterococci intrinsically resistant to cephalosporins represent a major cause of healthcare-associated infections, and the emergence of MDR makes therapeutic approaches particularly challenging. OBJECTIVES: Teichoic acids are cell wall glycopolymers present in Gram-positive bacteria. Teichoic acids can be modified by d-alanylation, which requires four proteins encoded by the dltABCD operon. Our objective was to evaluate the Dlt system as a druggable target to treat enterococcal infections. METHODS: The susceptibility of a d-alanylation-deficient strain of Enterococcus faecalis to ß-lactam antibiotics individually and/or in combination was analysed. Moreover, a DltA inhibitor was synthesized to test pharmacological inhibition of d-alanylation in vivo and in host using the animal model Galleria mellonella with different clinical isolates of E. faecalis and Enterococcus faecium. RESULTS: Most cephalosporins used as mono treatment had no impact on survival of the parental strain, but were slightly lethal for the dltA mutant of E. faecalis. Addition of a very low concentration of amoxicillin significantly increased killing of the dltA mutant under these conditions. The most spectacular effect was obtained with a combination of cefotaxime (1 mg/L) and amoxicillin (0.03 mg/L). In the presence of the inhibitor, the WT strain was as susceptible to this combination treatment as the dltA mutant. This molecule associated with the antibiotics was also effective in killing other E. faecalis clinical isolates and successfully prevented death of Galleria infected with either E. faecalis or E. faecium. CONCLUSIONS: The combined results support the potential usefulness of the Dlt system as a target to potentiate antibiotic combination therapies for the treatment of drug-resistant enterococci.

Difluorophosphonylated Allylic Ether Moiety as a 2'-Modification of RNA-Type Molecules: Synthesis, Thermal, and Metabolic Studies.

The first synthesis of oligonucleotides incorporating URF, a uridine modified with a difluorophosphonylated allylic ether onto the 2'-position, is described. Fluorinated homouridylates and miR-342-3p analogues are efficiently prepared. UV-melting experiments and enzymatic degradation studies indicate this new series of fluorinated oligonucleotides exhibit good and thermal metabolic stability as well as an increased lipophilicity. Comparison with oligonucleotides containing 2'- O-allyluridine instead of URF reveals improvement of these chemical properties is related to the presence of the difluoromethylphosphonate group.

Photoinduced Nonstabilized Azomethine Ylide Formation for the Preparation of Fluorine Containing Pyrrolidines.

A mild and reproducible method for the formation of a nonstabilized azomethine ylide was developed by photoinduced reaction catalyzed with eosin Y under green light irradiation. Resulting 1,3-dipole was trapped with fluoroalkenes, fluoroalkylated alkenes, and representative dipolarophiles to access pyrrolidine scaffolds, including spirocyclic compounds. The mechanism involved in this transformation was investigated, showing clearly a catalytic redox cycle with eosin Y.

Access to Fluoropyrrolidines by Intramolecular Aza-Michael Addition Reaction.

Study of the intramolecular aza-Michael addition reaction from an aminofluorovinylsulfone opens a new route for the synthesis of pyrrolidine derivatives. An unexpected diastereoselective cyclization reaction was observed, leading preferentially to the anti-N-benzylpyrrolidine sulfone. The resulting sulfone was reacted with aldehydes to access ß-substituted α-fluoroalkenyl pyrrolidines in one step.

Fluorinated hydroxypiperidines as selective ß-glucosidase inhibitors.

A new series of fluoroallylamines derived from hydroxypiperidines was prepared and evaluated against various glycosidases. The short synthesis of target molecules involved the modified Julia reaction between aldehydes and functionalized fluoroaminosulfones. Biological studies revealed good and selective ß-glucosidase inhibition in the micromolar range for two compounds, while the non-fluorinated analogue of the most active compound was selective towards α-glucosidase.

Synthesis of fluorinated agonist of sphingosine-1-phosphate receptor 1.

The bioactive metabolite sphingosine-1-phosphate (S1P), a product of sphingosine kinases (SphKs), mediates diverse biological processes such as cell differentiation, proliferation, survival and angiogenesis. A fluorinated analogue of S1P receptor agonist has been synthesized by utilizing a ring opening reaction of oxacycles by a lithiated difluoromethylphosphonate anion as the key reaction. In vitro activity of this S1P analogue is also reported.

Synthesis of substituted exo-glucals via a modified Julia olefination and identification as selective ß-glucosidase inhibitors.

A series of fluorine and non-fluorine-substituted C-glucosylidenes (exo-glucals) has been synthesized via a modified Julia olefination. The deprotected exo-glucals were prepared in five steps from commercially available d-gluconolactone. The evaluation of this original family of compounds against a panel of glycosidases showed a highly specific in vitro activity towards mammalian ß-glucosidase depending on the double bond substituents.

Aza-Michael access to fluoroalkylidene analogues of biomolecules.

The synthesis of fluoroaminosulfones derived from piperidine and nucleic bases followed by the study of their chemical behavior in the modified Julia reaction are described. The resulting aminosulfones open a straightforward access to a series of new fluorinated biomolecules including a potent DPP-II inhibitor and acyclonucleoside analogues as potential enzyme inhibitors.

Ready synthetic access to enantiopure allylic α(F)-branched fluoroalkenes.

Convenient access to homochiral fluoroalkenes is described via a Julia-Kocienski olefination reaction. The required homochiral fluorosulfone is synthesized by a Mitsunobu reaction from readily available enantiopure secondary alcohols.

Fluorophosphonylated nucleoside derivatives as new series of thymidine phosphorylase multisubstrate inhibitors.

The synthesis of new class of potential TPase inhibitors containing a difluoromethylphosphonate function as phosphate mimic is reported. This new series was prepared from a readily available fluorinated building block in few steps. Two series were evaluated as potential inhibitors: a linear series and a conformational constrained series. The activity of these multisubstrate inhibitors depends on the size of the spacer introduced between the pyrimidine ring and the phosphonate function. Best results were observed from triazolyl derivatives, easily obtained from propargylthymine and corresponding azides.

Toward the synthesis of benzothiazolyl fluoroaminosulfones.

Due to the importance of allylamines in organic synthesis, the synthesis of reagents as potent precursors of aminofluoroolefins is reported from functionalized benzothiazolylsulfones. The key intermediate, a fluorovinyl sulfone, was prepared and functionalized by addition of aliphatic, aromatic amines and amino acid alkyl esters through an aza-Michael addition reaction.

Synthesis of fluorophosphonylated acyclic nucleotide analogues via copper(I)-catalyzed Huisgen 1-3 dipolar cycloaddition.

Preparation of several acyclonucleosides containing both a difluoromethylphosphonate group and a triazole moiety is described starting from a difluorophosphonosulfide. The key step of the synthesis involves a copper(I)-catalyzed Huisgen 1-3 dipolar cycloaddition between difluorophosphonylated azides and propargylated nucleobases derived from thymine and 2-amino-6-chloropurine.