Towards the synthesis of a 2-deoxy-2-fluoro-d-mannose building block and characterisation of an unusual 2-S-phenyl anomeric pyridinium triflate salt via 1 → 2 S-migration

Regio- and stereo-selective synthetic routes to 2-deoxy-2-fluoro-d-mannose building blocks are often experimentally challenging when using Selectfluor with the corresponding glycal. We targeted a late-stage method to introduce fluorine in a stereospecific manner using inversion via a triflate. Accordingly, synthesis of a conventionally protected 2-deoxy-2-fluoro-d-mannose β-thioglycoside donor, directly applicable to oligosaccharide synthesis, was attempted using C2-triflate inversion of the corresponding d-glucoside with TBAF. Unexpectedly, an anomeric pyridinium salt was isolated when attempting to form the C2-triflate using Tf2O in pyridine. Indicatively, this proceeds via a 1 → 2 S-migration delivering a 1,2-trans product with α-d-manno configuration and the anomeric pyridinium in a pseudo-equatorial position. The structure of this unexpected intermediate was confirmed in the solid-state using X-ray crystallography. Omission of the pyridine solvent led to dimer formation. Switching the aglycone to an O-para-methoxyphenyl enabled smooth C2 inversion to the desired 2-deoxy-2-fluoro d-mannose system, suitably equipped for further anomeric manipulation.

Towards New Delivery Agents for Boron Neutron Capture Therapy: Synthesis and In Vitro Evaluation of a Set of Fluorinated Carbohydrate Derivatives

Boron Neutron Capture Therapy (BNCT) is a cancer treatment which combines tumor-selective boron delivery agents with thermal neutrons in order to selectively eradicate cancer cells. In this work, we focus on the early-stage development of carbohydrate delivery agents for BNCT. In more detail, we expand upon our previous GLUT-targeting approach by synthesizing and evaluating the potential embedded in a representative set of fluorinated carbohydrates bearing a boron cluster. Our findings indicate that these species may have advantages over the boron delivery agents in current clinical use, e.g., significantly improved boron delivery capacity at the cellular level. Simultaneously, the carbohydrate delivery agents were found to bind strongly to plasma proteins, which may be a concern requiring further action before progression to in vivo studies. Altogether, this work brings new insights into factors which need to be accounted for if attempting to develop theranostic agents for BNCT based on carbohydrates in the future.

Photocatalyzed Perfluoroalkylation of Endoglycals

The visible light-induced perfluoroalkyl (RF) radical reactions on peracetylglycals derived from hexoses and pentoses (galactal, glucal, arabinal, and xylal derivatives) were investigated. Various photocatalysts and perfluoroalkyl iodides (RF-I) were employed as sources of RF radicals with LEDs as the irradiation source. Particularly noteworthy was the use of an Iridium photocatalyst, Ir[dF(CF3)ppy]2(dtbpy))PF6, which yielded two distinct product types when applied to glucal. On the one hand, the 2-RF-substituted glucal was formed, a trend observed even when utilizing organic dyes as photocatalysts. On the other hand, the unexpected addition product, namely the 1-RF-2-iodo-α-manno-configured C-glycosyl derivative, was also obtained, as a result of a highly regioselective addition reaction of the RF moiety into the anomeric carbon, followed by attachment of the iodine atom on C-2 in axial disposition. This result contrasted with other radical reactions carried out on 2-unsubstituted glycals, where the incipient radical adds to C-2, generating a stabilized 1-glycosyl radical. The photocatalyzed radical perfluoroalkylations of peracetyl glycals derived from galactose, arabinose, and xylose all afforded the 2-RF-substituted glycals in good yields as a result of the expected vinylic substitution reaction. Mechanistic studies revealed that the 1-RF-2-iodo-α-manno-configured C-glycosyl derivatives arise from a radical chain reaction, whereas the 2-RF-substituted glycals proceed from inefficient chain processes.

Drug Discovery Based on Fluorine-Containing Glycomimetics

Glycomimetics, which are synthetic molecules designed to mimic the structures and functions of natural carbohydrates, have been developed to overcome the limitations associated with natural carbohydrates. The fluorination of carbohydrates has emerged as a promising solution to dramatically enhance the metabolic stability, bioavailability, and protein-binding affinity of natural carbohydrates. In this review, the fluorination methods used to prepare the fluorinated carbohydrates, the effects of fluorination on the physical, chemical, and biological characteristics of natural sugars, and the biological activities of fluorinated sugars are presented.

The Synthesis of Fluorinated Carbohydrates Using Sulfuryl Fluoride as the Deoxyfluorination Reagent

Fluorination of carbohydrates has been one of the strategies to increase their enzymatic and chemical stabilities and reduce their hydrophilicities, making this modification attractive for drug discovery purposes. The synthesis of monofluorinated carbohydrates was achieved under mild conditions by using SO₂F₂ as the deoxyfluorination reagent in the presence of a base without extra fluoride additives. This method features low toxicity, easy availability, low cost, and high efficiency and can be subjected to diverse sugar units.

Fluoro-labelled sp2-iminoglycolipids with immunomodulatory properties

The unique electronic properties of the fluorine atom make its strategic incorporation into a bioactive compound a very useful tool in the design of drugs with optimized pharmacological properties. In the field of the carbohydrates, its selective installation at C2 position has proven particularly interesting, some 2-deoxy-2-fluorosugar derivatives being currently in the market. We have now transferred this feature into immunoregulatory glycolipid mimetics that contain a sp²-iminosugar moiety, namely sp²-iminoglycolipids (sp²-IGLs). The synthesis of two epimeric series of 2-deoxy-2-fluoro-sp²-IGLs, structurally related to nojirimycin and mannonojirimycin, has been accomplished by sequential Selectfluor-mediated fluorination and thioglycosidation of sp²-iminoglycals. Exclusively the α-anomer is obtained regardless of the configurational profile of the sp²-IGL (d-gluco or d-manno), highlighting the overwhelming anomeric effect in these prototypes. Notably, the combination of a fluorine atom at C2 and an α-oriented sulfonyl dodecyl lipid moiety in compound 11 led to remarkable anti-proliferative properties, featuring similar GI₅₀ values than the chemotherapy drug Cisplatin against several tumor cell lines and better selectivity. The biochemical data further evidence a strong reduction of the number of tumor cell colonies and apoptosis induction. Mechanistic investigations revealed that this fluoro-sp²-IGL induces the non-canonical activation mode of the mitogen-activated protein kinase signaling pathway, causing p38α autoactivation under an inflammatory context.

Synthetic Approaches to Break the Chemical Shift Degeneracy of Glycans

NMR spectroscopy is the leading technique for determining glycans' three-dimensional structure and dynamic in solution as well as a fundamental tool to study protein-glycan interactions. To overcome the severe chemical shift degeneracy of these compounds, synthetic probes carrying NMR-active nuclei (e. g., 13 C or 19 F) or lanthanide tags have been proposed. These elegant strategies permitted to simplify the complex NMR analysis of unlabeled analogues, shining light on glycans' conformational aspects and interaction with proteins. Here, we highlight some key achievements in the synthesis of specifically labeled glycan probes and their contribution towards the fundamental understanding of glycans.

An Improved Protocol for the Stereoselective Synthesis of β-d-Glycosyl Fluorides from 2-O-Acyl Thioglycosides

A safe and operationally simple protocol for the preparation of β-d-glycosyl fluorides is presented. We demonstrate that a precise combination of XtalFluor-M, N-bromosuccinimide, and Et₃N·3HF can mediate facile, high-yielding, and diastereoselective conversions of 2-O-acyl thioglycosides to β-d- and other 1,2-trans glycosyl fluorides. The key roles of these reagents are dissected in this work, as is the impact of their interplay on the fluorination stereoselectivity.

The Synthesis and Glycoside Formation of Polyfluorinated Carbohydrates

Fluorinated carbohydrates have found many applications in the glycosciences. Typically, these contain fluorination at a single position. There are not many applications involving polyfluorinated carbohydrates, here defined as monosaccharides in which more than one carbon has at least one fluorine substituent directly attached to it, with the notable exception of their use as mechanism-based inhibitors. The increasing attention to carbohydrate physical properties, especially around lipophilicity, has resulted in a surge of interest for this class of compounds. This review covers the considerable body of work toward the synthesis of polyfluorinated hexoses, pentoses, ketosugars, and aminosugars including sialic acids and nucleosides. An overview of the current state of the art of their glycosidation is also provided.

Design and Synthesis of New α-hydroxy β-fluoro/β-trifluoromethyl and Unsaturated Phosphonates from Carbohydrate-Derived Building Blocks via Pudovik and Horner–Wadsworth–Emmons Reactions

Herein, we present the application of fluorinated carbohydrate-derived building blocks for α-hydroxy β-fluoro/β-trifluoromethyl and unsaturated phosphonates synthesis. Pudovik and Horner–Wadsworth–Emmons reactions were applied to achieve this goal. The proposed pathway of the key reactions is supported by the experimental results, as well as quantum chemical calculations. The structure of the products was established by spectroscopic (1D, 2D NMR) and spectrometric (MS) techniques. Based on our data received, we claim that the progress of the Pudovik and HWE reactions is significantly influenced by the acidic protons present in the molecules as assessed by pKa values of the reagent.

Parameterization and Application of the General Amber Force Field to Model Fluoro Substituted Furanose Moieties and Nucleosides

Molecular mechanics force field calculations have historically shown significant limitations in modeling the energetic and conformational interconversions of highly substituted furanose rings. This is primarily due to the gauche effect that is not easily captured using pairwise energy potentials. In this study, we present a refinement to the set of torsional parameters in the General Amber Force Field (gaff) used to calculate the potential energy of mono, di-, and gem-fluorinated nucleosides. The parameters were optimized to reproduce the pseudorotation phase angle and relative energies of a diverse set of mono- and difluoro substituted furanose ring systems using quantum mechanics umbrella sampling techniques available in the IpolQ engine in the Amber suite of programs. The parameters were developed to be internally consistent with the gaff force field and the TIP3P water model. The new set of angle and dihedral parameters and partial charges were validated by comparing the calculated phase angle probability to those obtained from experimental nuclear magnetic resonance experiments.

The Effect of Deoxyfluorination on Intermolecular Interactions in the Crystal Structures of 1,6-Anhydro-2,3-epimino-hexopyranoses

The effect of substitution on intermolecular interactions was investigated in a series of 1,6-anhydro-2,3-epimino-hexopyranoses. The study focused on the qualitative evaluation of intermolecular interactions using DFT calculations and the comparison of molecular arrangements in the crystal lattice. Altogether, ten crystal structures were compared, including two structures of C4-deoxygenated, four C4-deoxyfluorinated and four parent epimino pyranoses. It was found that the substitution of the original hydroxy group by hydrogen or fluorine leads to a weakening of the intermolecular interaction by approximately 4 kcal/mol. The strength of the intermolecular interactions was found to be in the following descending order: hydrogen bonding of hydroxy groups, hydrogen bonding of the amino group, interactions with fluorine and weak electrostatic interactions. The intermolecular interactions that involved fluorine atom were rather weak; however, they were often supported by other weak interactions. The fluorine atom was not able to substitute the role of the hydroxy group in molecular packing and the fluorine atoms interacted only weakly with the hydrogen atoms located at electropositive regions of the carbohydrate molecules. However, the fluorine interaction was not restricted to a single molecule but was spread over at least three other molecules. This feature is a base for similar molecule arrangements in the structures of related compounds, as we found for the C4-F_ax and C4-F_eq epimines presented here.

Deoxyfluorination tunes the aggregation of cellulose and chitin oligosaccharides and highlights the role of specific hydroxyl groups in the crystallization process

Using synthetic oligosaccharides, we examined how deoxyfluorination (site and pattern) impact the solubility and aggregation of cellulose and chitin oligomers.

De Novo Synthesis of Orthogonally-Protected C2-Fluoro Digitoxoses and Cymaroses: Development and Application for the Synthesis of Fluorinated Digoxin

Inspired by Roush's pioneering work on rare sugars, we have developed a scalable, stereoselective, de novo synthesis of orthogonally protected C2-fluoro digitoxose and cymarose, utilizing Sharpless kinetic resolution and organocatalytic fluorination as key steps. The utility of this strategy is demonstrated by the synthesis of a fluorinated analogue of digoxin, which indicates the fluorine on the sugar ring may have a significant impact on biological activity.

Enzymatic glycosylation involving fluorinated carbohydrates

Fluorinated carbohydrates, where one (or more) fluorine atom(s) have been introduced into a carbohydrate structure, typically through deoxyfluorination chemistry, have a wide range of applications in the glycosciences. Fluorinated derivatives of galactose, glucose, N-acetylgalactosamine, N-acetylglucosamine, talose, fucose and sialic acid have been employed as either donor or acceptor substrates in glycosylation reactions. Fluorinated donors can be synthesised by synthetic methods or produced enzymatically from chemically fluorinated sugars. The latter process is mediated by enzymes such as kinases, phosphorylases and nucleotidyltransferases. Fluorinated donors produced by either method can subsequently be used in glycosylation reactions mediated by glycosyltransferases, or phosphorylases yielding fluorinated oligosaccharide or glycoconjugate products. Fluorinated acceptor substrates are typically synthesised chemically. Glycosyltransferases are most commonly used in conjunction with natural donors to further elaborate fluorinated acceptor substrates. Glycoside hydrolases are used with either fluorinated donors or acceptors. The activity of enzymes towards fluorinated sugars is often lower than towards the natural sugar substrates irrespective of donor or acceptor. This may be in part attributed to elimination of the contribution of the hydroxyl group to the binding of the substrate to enzymes. However, in many cases, enzymes still maintain a significant activity, and reactions may be optimised where necessary, enabling enzymes to be used more successfully in the production of fluorinated carbohydrates. This review describes the current state of the art regarding chemoenzymatic production of fluorinated carbohydrates, focusing specifically on examples of the enzymatic production of activated fluorinated donors and enzymatic glycosylation involving fluorinated sugars as either glycosyl donors or acceptors.

Simultaneous SPECT imaging with 123I and 125I - a practical approach to assessing a drug and its carrier at the same time with dual imaging

Radiolabeling of a drug with radioactive iodine is a good method to determine its pharmacokinetics and biodistribution in vivo that only minimally alters its physicochemical properties. With dual labeling, using the two radioactive iodine isotopes ¹²³I and ¹²⁵I, two different drugs can be evaluated at the same time, or one can follow both a drug and its drug delivery system using a single photon emission computed tomography (SPECT) imager. One difficulty is that the two radioisotopes have overlapping gamma spectra. Our aim was therefore to develop a technique that overcomes this problem and allows for quantitative analysis of the two radioisotopes present at varied isotope ratios. For this purpose, we developed a simple method that included scatter and attenuation corrections and fully compensated for ¹²³I/¹²⁵I crosstalk, and then tested it in phantom measurements. The method was applied to the study of an orally administered lipid formulation for the delivery of fenofibrate in rats. To directly compare a traditional study, where fenofibrate was determined in plasma samples to SPECT imaging with ¹²³I-labeled fenofibrate and ¹²⁵I-labeled triolein over 24 h, the drug concentrations were converted to standardized uptake values (SUVs), an unusual unit for pharmaceutical scientists, but the standard unit for radiologists. A generally good agreement between the traditional and the radioactive imaging method was found in the pharmacokinetics and biodistribution results. Small differences are discussed in detail. Overall, SPECT imaging is an excellent method to pilot a new formulation with just a few animals, replaces blood sampling, and can very quickly highlight potential administration problems, the excretion pathways and the kinetics. Furthermore, dual labeling with the two radioisotopes ¹²³I and ¹²⁵I clearly shows if a drug and its drug delivery system stay together when traveling through the body, if slow drug release takes place, and where degradation/excretion of the components occurs.

Approaches to the Synthesis of Perfluoroalkyl-Modified Carbohydrates and Derivatives: Thiosugars, Iminosugars, and Tetrahydro(thio)pyrans

Fluoroalkyl-substituted carbohydrates play relevant roles in diverse areas such as supramolecular chemistry, glycoconjugation, liquid crystals, and surfactants, with direct applications as wetting, antifreeze, and coating agents. In light of these promising applications, new methodologies for the late-stage incorporation of fluoroalkyl R_F groups into carbohydrates and derivatives are herein presented as they are relevant to the synthetic carbohydrate community. Previously reviewed protocols for the installation of R_F groups onto carbohydrates and derivatives will be succinctly summarized in the light of the new achievements. Fluoroalkyl-substituted iminosugars, on the other hand, are also interesting glycomimetic derivatives with prominent roles as glycosidases and glycosyltransferases inhibitors, as has recently been demonstrated. Also, they positively contribute to the study of sugar-protein interactions and enzyme mechanisms. New advances in the syntheses of fluoroalkyl-substituted iminosugars will also be presented here.

DFT Based Pharmacokinetic, Molecular Docking, and ADMET Studies of Some Glucopyranoside Esters

Monosaccharide esters (MEs) are getting more attention from bioorganic chemists due to their biodegradable and drug-likeness properties. As a consequence, carbohydrate derivatives (sugar-based esters, SEs) are an essential part of medicinal chemistry. In this context, density functional theory (DFT) with B3LYP/ 3-21G has been employed to optimize the methyl 4,6-O-benzylidene-α-D-glucopyranoside (3) of methyl α-D-glucopyranoside (2) and its protected acyl esters 4-6. The prediction of activity spectra for substances (PASS) of these compounds showed better antifungal functionalities than the antibacterial potentiality. Thermodynamic properties and molecular electrostatic potential (MEP) of these MEs indicated their stability and both the electrophilic and nucleophilic attack sites. Due to their better antifungal potentiality, molecular docking was conducted against fungal protein lanosterol 14α-demethylase (3JUS), and SARS-CoV-2 main protease (6LU7) along with absorption, distribution, metabolism, excretion, and toxicity (ADMET) studies. The study indicated a better binding affinity of some esters compared to the standard antifungal and COVID-19 related drug hydroxychloroquine (HCQ).

From Carbohydrates to Complex Organofluorines: Synthesis, Conformation, and Lipophilicity of Multivicinal-Fluorine-Containing Hexitol Analogues

There is growing interest in the preparation of fluorine-containing organic molecules. Multivicinal-fluorine analogues are among the most intriguing and promising compounds, but their physical and biological investigations are held back by challenging syntheses. Herein, we report on the synthesis of a large set of novel polyfluorohexitols. The dominant solution-state conformation of all trifluorohexitols was determined, and the solid-state conformations of some analogues were compared. Finally, the lipophilicity of a large set of polyfluorinated hexopyranose and hexitol analogues was attributed by using a log P determination method based on ¹⁹ F NMR spectroscopy.

Synthesis of Glycosyl Fluorides by Photochemical Fluorination with Sulfur(VI) Hexafluoride

This study describes a new convenient method for the photocatalytic generation of glycosyl fluorides using sulfur(VI) hexafluoride as an inexpensive and safe fluorinating agent and 4,4′-dimethoxybenzophenone as a readily available organic photocatalyst. This mild method was employed to generate 16 different glycosyl fluorides, including the substrates with acid and base labile functionalities, in yields of 43%–97%, and it was applied in continuous flow to accomplish fluorination on an 7.7 g scale and 93% yield.

Fluorinated Carbohydrates as Lectin Ligands: Simultaneous Screening of a Monosaccharide Library and Chemical Mapping by 19F NMR Spectroscopy

Molecular recognition of carbohydrates is a key step in essential biological processes. Carbohydrate receptors can distinguish monosaccharides even if they only differ in a single aspect of the orientation of the hydroxyl groups or harbor subtle chemical modifications. Hydroxyl-by-fluorine substitution has proven its merits for chemically mapping the importance of hydroxyl groups in carbohydrate-receptor interactions. 19F NMR spectroscopy could thus be adapted to allow contact mapping together with screening in compound mixtures. Using a library of fluorinated glucose (Glc), mannose (Man), and galactose (Gal) derived by systematically exchanging every hydroxyl group by a fluorine atom, we developed a strategy combining chemical mapping and 19F NMR T2 filtering-based screening. By testing this strategy on the proof-of-principle level with a library of 13 fluorinated monosaccharides to a set of three carbohydrate receptors of diverse origin, i.e. the human macrophage galactose-type lectin, a plant lectin, Pisum sativum agglutinin, and the bacterial Gal-/Glc-binding protein from Escherichia coli, it became possible to simultaneously define their monosaccharide selectivity and identify the essential hydroxyls for interaction.

Recent advances in 1,4-functional group migration-mediated radical fluoroalkylation of alkenes and alkynes

Recently, the combination of radical fluoroalkylation of alkenyl or alkynyl moieties and 1,4-functional group migration (1,4-FGM) has emerged as a powerful strategy for the synthesis of fluorine-containing compounds. In this article, some representative reactions of 1,4-FGM-mediated radical fluoroalkylation of N-(arylsulfonyl)acrylamides, tertiary alcohol-containing alkynes, tertiary alcohol-containing alkenes and intermolecular 1,4-FGM-type substrates have been discussed based on the types of substrates.

Fluorine effect in nucleophilic fluorination at C4 of 1,6-anhydro-2,3-dideoxy-2,3-difluoro-β-D-hexopyranose

In this work, we have developed a simple synthetic approach using Et3N·3HF as an alternative to the DAST reagent. We controlled the stereochemistry of the nucleophilic fluorination at C4 of 1,6-anhydro-2,3-dideoxy-2,3-difluoro-4-O-triflate-β-ᴅ-talopyranose using Et3N·3HF or in situ generated Et3N·1HF. The influence of the fluorine atom at C2 on reactivity at C4 could contribute to a new fluorine effect in nucleophilic substitution. Finally, with the continuous objective of synthesizing novel multi-vicinal fluorosugars, we prepared one difluorinated and one trifluorinated alditol analogue.

Photocatalyzed reductive fluoroalkylation of 2-acetoxyglycals towards the stereoselective synthesis of α-1-fluoroalkyl-C-glycosyl derivatives

A benign, efficient, regio- and stereoselective protocol for the syntheses of α-1-fluoroalkyl-C-glycosyl compounds bearing CF3, C4F9, and C6F13 substituents on the anomeric carbon has been developed by a new methodology starting from 2-acetoxyglycals for the first time. Remarkably, the reactions proceeded in only one step, through the visible light-photocatalyzed reductive fluoroalkylation of 2-acetoxyglycals by means of an Ir photocatalyst and employed commercially available fluoroalkyl iodides n-CnF2n+1-I (n = 1, 4, 6) as a source of fluoroalkyl radicals.

Synthesis, PASS Predication, Antimicrobial, DFT, and ADMET Studies of Some Novel Mannopyranoside Esters

Due to the biodegradability and drug-likeness properties sugar esters (SEs) are getting especial attention to the synthetic and bioorganic chemists. In this context, we have synthesized several 6-O-pentanoyl mannopyranoside esters (5-9) with alkanoyl and sylfonyl chains reasonably in good yields. Both the prediction of activity spectra for substances (PASS) and in vitro tests indicated that these mannopyranoside esters possess better potentiality against fungal pathogens than the bacterial organisms. These SEs were also optimized with quantum chemical density functional theory (DFT), and various thermodynamic properties like frontier molecular orbital, and molecular electrostatic potential (MEP) were calculated and discussed. Absorption, distribution, metabolism, excretion, and toxicity (ADMET) calculations indicated that these SEs can pass through blood brain barrier and less toxic. Drug-likeness results indicated good conditions for alkanoyl esters rather than sulfonyl esters despite their promising antifungal results. All the in vitro and in silico results indicated that the combination of pentanoyl (C5) and lauroyl (C12) chains in mannopyranoside framework, as in 9, might be a potential candidate for novel antifungal agent.

Molecular Tagging for the Molecular Characterization of Natural Organic Matter

Natural organic matter (NOM) is the product of microbial and abiotic decay of plant and animal remains in terrestrial and aquatic ecosystems. On a molecular level, NOM is a complex mixture of organic molecules, of which the vast majority of structures are unknown. By identifying these molecules, our understanding of the many functions of NOM could be greatly enhanced. However, given that they are chromatographically inseparable and number in the thousands, traditional analytical techniques have proven unable to achieve this goal. A promising approach to enumerate functional groups and elucidate molecular structures within NOM is based on a combination of molecular tagging and high resolution spectroscopic techniques, such as nuclear magnetic resonance spectroscopy and mass spectrometry. Molecular tagging involves the selective modification of particular functional groups, inserting nuclei to act as reporters on their surrounding chemical environment. This allows examination of only the tagged molecules within NOM, thereby reducing the complexity of the mixture. In this review, the effectiveness of molecular tagging methods incorporating carbon, silicon, nitrogen, phosphorus, and deuterium into NOM are discussed. Some potential tagging methods which have not yet been applied to NOM are also presented.

Site-Selective Photochemical Fluorination of Ketals: Unanticipated Outcomes in Selectivity and Stability

We report a method for the regioselective photochemical sp³ C-H fluorination of acetonide ketals that presents interesting problems in chemical reactivity. The question of why certain products of the reaction are stable while others are not is addressed, as is the question of why only select α-ethereal hydrogen atoms are targeted in the reaction. We demonstrate that the method can be employed to synthesize unprecedented fluorinated sugars and steroids, and it can also be applied toward the fluorination of carbamates. Though some substrates contain up to eight discrete α-ethereal C-H bonds, we observed site-selectivity in each case, prompting us to investigate potential transition states for the reaction. Finally, a remarkable regiochemical switch upon minor structural modification of a diketal is also analyzed.

Metabolic inhibitors of bacterial glycan biosynthesis

The bacterial cell wall is a quintessential drug target due to its critical role in colonization of the host, pathogen survival, and immune evasion. The dense cell wall glycocalyx contains distinctive monosaccharides that are absent from human cells, and proper assembly of monosaccharides into higher-order glycans is critical for bacterial fitness and pathogenesis. However, the systematic study and inhibition of bacterial glycosylation enzymes remains challenging. Bacteria produce glycans containing rare deoxy amino sugars refractory to traditional glycan analysis, complicating the study of bacterial glycans and the creation of glycosylation inhibitors. To ease the study of bacterial glycan function in the absence of detailed structural or enzyme information, we crafted metabolic inhibitors based on rare bacterial monosaccharide scaffolds. Metabolic inhibitors were assessed for their ability to interfere with glycan biosynthesis and fitness in pathogenic and symbiotic bacterial species. Three metabolic inhibitors led to dramatic structural and functional defects in Helicobacter pylori. Strikingly, these inhibitors acted in a bacteria-selective manner. These metabolic inhibitors will provide a platform for systematic study of bacterial glycosylation enzymes not currently possible with existing tools. Moreover, their selectivity will provide a pathway for the development of novel, narrow-spectrum antibiotics to treat infectious disease. Our inhibition approach is general and will expedite the identification of bacterial glycan biosynthesis inhibitors in a range of systems, expanding the glycochemistry toolkit.