Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Transformations of carbohydrate derivatives enabled by photocatalysis and visible light photochemistry

This review article highlights the diverse ways in which recent developments in the areas of photocatalysis and visible light photochemistry are impacting synthetic carbohydrate chemistry. The major topics covered are photocatalytic glycosylations, generation of radicals at the anomeric position, transformations involving radical formation at non-anomeric positions, additions to glycals, processes initiated by photocatalytic hydrogen atom transfer from sugars, and functional group interconversions at OH and SH groups. Factors influencing stereo- and site-selectivity in these processes, along with mechanistic aspects, are discussed.

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.

New syntheses of thiosaccharides utilizing substitution reactions

Novel synthetic methods published since 2005 affording carbohydrates containing sulfur atom(s) are reviewed. The review is divided to subchapters based on the position of sulfur atom(s) in the sugar molecule. Only those methods that take advantage of substitution are discussed.

Strong Binding of C-Glycosylic1,2-Thiodisaccharides to Galectin-3─Enthalpy-Driven Affinity Enhancement by Water-Mediated Hydrogen Bonds

Galectin-3 is involved in multiple pathways of many diseases, including cancer, fibrosis, and diabetes, and it is a validated pharmaceutical target for the development of novel therapeutic agents to address unmet medical needs. Novel 1,2-thiodisaccharides with a C-glycosylic functionality were synthesized by the photoinitiated thiol-ene click reaction of O-peracylated 1-C-substituted glycals and 1-thio-glycopyranoses. Subsequent global deprotection yielded test compounds, which were studied for their binding to human galectin-3 by fluorescence polarization and isothermal titration calorimetry to show low micromolar Kd values. The best inhibitor displayed a Kd value of 8.0 μM. An analysis of the thermodynamic binding parameters revealed that the binding Gibbs free energy (ΔG) of the new inhibitors was dominated by enthalpy (ΔH). The binding mode of the four most efficient 1,2-thiodisaccharides was also studied by X-ray crystallography that uncovered the unique role of water-mediated hydrogen bonds in conferring enthalpy-driven affinity enhancement for the new inhibitors. This 1,2-thiodisaccharide-type scaffold represents a new lead for galectin-3 inhibitor discovery and offers several possibilities for further development.

In vitro and in vivo antiplasmodial evaluation of sugar-modified nucleoside analogues

Drug-resistant Plasmodium falciparum (Pf) infections are a major burden on the population and the healthcare system. The establishment of Pf resistance to most existing antimalarial therapies has complicated the problem, and the emergence of resistance to artemisinin derivatives is even more concerning. It is increasingly difficult to cure malaria patients due to the limited availability of effective antimalarial drugs, resulting in an urgent need for more efficacious and affordable treatments to eradicate this disease. Herein, new nucleoside analogues including morpholino-nucleoside hybrids and thio-substituted nucleoside derivatives were prepared and evaluated for in vitro and in vivo antiparasitic activity that led a few hits especially nucleoside-thiopyranoside conjugates, which are highly effective against Pf3D7 and PfRKL-9 strains in submicromolar concentration. One adenosine derivative and four pyrimidine nucleoside analogues significantly reduced the parasite burden in mouse models infected with Plasmodium berghei ANKA. Importantly, no significant hemolysis and cytotoxicity towards human cell line (RAW) was observed for the hits, suggesting their safety profile. Preliminary research suggested that these thiosugar-nucleoside conjugates could be used to accelerate the antimalarial drug development pipeline and thus deserve further investigation.

Synthesis of thiodisaccharides related to 4-thiolactose. Specific structural modifications increase the inhibitory activity against E. coli β-galactosidase

In the search for new glycosidase inhibitors, a set of benzyl β-D-Gal-S-(1→4)-3-deoxy-4-thio-α-D-hexopyranosides was synthesized. Diverse configurations were installed at C-2 and C-4 of the glucose residue. The benzyl glycosidic group was kept intact or substituted by an electron-donating or electron-withdrawing group that could also participate in hydrogen bonding. All thiodisaccharides were found to be inhibitors of E. coli β-galactosidase. In general, benzyl thiodisaccharides were better inhibitors than those substituted (NO₂ or NH₂) on the benzyl ring. Thiodisaccharides containing a hexopyranoside, instead of a pentopyranoside, showed a weaker inhibitory activity, except for those having the α-D-xylo configuration, which exhibited inhibition constants of the same order of magnitude. These and previous results indicated that the inhibition process by thiodisaccharides is strongly dependent on the configuration of the 3-deoxy-4-thiopyranoside, as well as its substitution pattern (such as the presence of a benzyl glycoside). The enzyme-inhibitor interaction during the hydrolysis process involves a conformational selection resulting from rotation around the thioglycosidic bond and the flexibility of the terminal six-membered ring. Thus, the mentioned structural features of the inhibitor could give rise to favorable ground state conformations for the interaction with the enzyme, similar to those found for selected thiodisaccharides in the bound state. These studies demonstrated that the performance of thiodisaccharides as enzyme inhibitors could be increased by selecting the appropriate configuration and substitution of the hexopyranoside replacing the glucose moiety of 4-thiolactose.

Synthesis of a Heparinoid Pentasaccharide Containing l-Guluronic Acid Instead of l-Iduronic Acid with Preserved Anticoagulant Activity

l-Iduronic acid is a key constituent of heparin and heparan sulfate polysaccharides due to its unique conformational plasticity, which facilitates the binding of polysaccharides to proteins. At the same time, this is the synthetically most challenging unit of heparinoid oligosaccharides; therefore, there is a high demand for its replacement with a more easily accessible sugar unit. In the case of idraparinux, an excellent anticoagulant heparinoid pentasaccharide, we demonstrated that l-iduronic acid can be replaced by an easier-to-produce l-sugar while maintaining its essential biological activity. From the inexpensive d-mannose, through a highly functionalized phenylthio mannoside, the l-gulose donor was prepared by C-5 epimerization in 10 steps with excellent yield. This unit was incorporated into the pentasaccharide by α-selective glycosylation and oxidized to l-guluronic acid. The complete synthesis required only 36 steps, with 21 steps for the longest linear route. The guluronate containing pentasaccharide inhibited coagulation factor Xa by 50% relative to the parent compound, representing an excellent anticoagulant activity. To the best of our knowledge, this is the first biologically active heparinoid anticoagulant which contains a different sugar unit instead of l-iduronic acid.

Synthesis and Anticancer and Antiviral Activities of C-2′-Branched Arabinonucleosides

d-Arabinofuranosyl-pyrimidine and -purine nucleoside analogues containing alkylthio-, acetylthio- or 1-thiosugar substituents at the C2’ position were prepared from the corresponding 3’,5’-O-silylene acetal-protected nucleoside 2’-exomethylenes by photoinitiated, radical-mediated hydrothiolation reactions. Although the stereochemical outcome of the hydrothiolation depended on the structure of both the thiol and the furanoside aglycone, in general, high d-arabino selectivity was obtained. The cytotoxic effect of the arabinonucleosides was studied on tumorous SCC (mouse squamous cell) and immortalized control HaCaT (human keratinocyte) cell lines by MTT assay. Three pyrimidine nucleosides containing C2’-butylsulfanylmethyl or -acetylthiomethyl groups showed promising cytotoxicity at low micromolar concentrations with good selectivity towards tumor cells. SAR analysis using a methyl β-d-arabinofuranoside reference compound showed that the silyl-protecting group, the nucleobase and the corresponding C2’ substituent are crucial for the cell growth inhibitory activity. The effects of the three most active nucleoside analogues on parameters indicative of cytotoxicity, such as cell size, division time and cell generation time, were investigated by near-infrared live cell imaging, which showed that the 2’-acetylthiomethyluridine derivative induced the most significant functional and morphological changes. Some nucleoside analogues also exerted anti-SARS-CoV-2 and/or anti-HCoV-229E activity with low micromolar EC₅₀ values; however, the antiviral activity was always accompanied by significant cytotoxicity.

Synthesis of Oligosaccharides Containing the S-Galp(α1 → 3)Galp Unit, Glycomimetic of the Epitope Recognized by Lytic Antibodies

Two important activities take place in the surface of Trypanosoma cruzi, the agent of Chagas disease: the trans-sialidase (TcTS) catalyzes the transfer of sialic acid from the host glycoconjugates to the mucin-like glycoproteins from the parasite and the presence of lytic antibodies recognize the epitope α-Galp(1 → 3)-β-Galp(1 → 4)-α-GlcNAcp. This antigenic structure is known to be present in the parasite mucins; however, in order to be substrates of trans-sialidase, some of the galactose residues should be in the β-Galp configuration. To study the interaction between both activities, it is important to count the synthetic structures as well as the structural-related glycomimetics. With this purpose, we addressed the synthesis of a trisaccharide and two isomeric tetrasaccharides containing the 1-S-α-Galp(1 → 3)-β-Galp motif, the thio analog of the epitope recognized by lytic antibodies. Starting with a common lactose precursor, the sulfur function was incorporated by double inversion of the configuration of the galactose residue that was further glycosylated using different activated donors. Both tetrasaccharides were good acceptors of sialic acid in the reaction catalyzed by TcTS, as determined by high-performance anion exchange chromatography.

The Very First Modification of Pleuromutilin and Lefamulin by Photoinitiated Radical Addition Reactions-Synthesis and Antibacterial Studies

Pleuromutilin is a fungal diterpene natural product with antimicrobial properties, semisynthetic derivatives of which are used in veterinary and human medicine. The development of bacterial resistance to pleuromutilins is known to be very slow, which makes the tricyclic diterpene skeleton of pleuromutilin a very attractive starting structure for the development of new antibiotic derivatives that are unlikely to induce resistance. Here, we report the very first synthetic modifications of pleuromutilin and lefamulin at alkene position C19–C20, by two different photoinduced addition reactions, the radical thiol-ene coupling reaction, and the atom transfer radical additions (ATRAs) of perfluoroalkyl iodides. Pleuromutilin were modified with the addition of several alkyl- and aryl-thiols, thiol-containing amino acids and nucleoside and carbohydrate thiols, as well as perfluoroalkylated side chains. The antibacterial properties of the novel semisynthetic pleuromutilin derivatives were investigated on a panel of bacterial strains, including susceptible and multiresistant pathogens and normal flora members. We have identified some novel semisynthetic pleuromutilin and lefamulin derivatives with promising antimicrobial properties.

Transition-metal-free synthesis of aryl 1-thioglycosides with arynes at room temperature

A mild, convenient and transition-metal-free protocol for the synthesis of aryl 1-thioglycosides is presented via arynes generated in situ combined with glycosyl thiols in the presence of TBAF(tBuOH)₄. The methodology provides a general and efficient way to prepare a series of functionalized thioglycosides in good to excellent yields with a perfect control of the anomeric configuration at room temperature. In addition, the reaction conditions tolerate a variety of the pentoses and hexoses, and the reaction also performs smoothly on protected monosaccharides and disaccharides.

Synthesis of Thiodisaccharides Bearing N-Acetylhexosamine Residues: Challenges, Achievements and Perspectives

Carbohydrate-protein interactions are involved in a myriad of biological processes. Thus, glycomimetics have arisen as one of the most promising synthetic targets to that end. Within the broad variety of glycomimetics, thiodisaccharides have proven to be excellent tools to study these processes, and even more, some of them unveiled interesting biological activities. This review brings together research made on the introduction of N-acetylhexosamine residues into thiodisaccharides to date, passing through classic substitution (as S_N 2, thioglycosylation and ring-opening reactions) and addition (as thiol-ene coupling and Michael-type additions) reactions. Recent and interesting developments regarding addition reactions to vinyl azides, cross-coupling reactions and novel chemoenzymatic methods are also discussed.

The synthesis of thermoresponsive POSS-based eight-arm star poly(N-isopropylacrylamide): A comparison between Z-RAFT and R-RAFT strategies

Z-Type POSS-based eight-arm star poly(N-isopropylacrylamide), POSS-(PNIPAM)₈-Z, is synthesized and demonstrated to be a thermoresponsive switchable emulsifier.

A thiol–ene click reaction with preservation of the Si–H bond: a new approach for the synthesis of functional organosilicon compounds

This work presents an approach for the preparation of functional hydrosilanes.

Synthesis and oligomerization of cysteinyl nucleosides

Nucleoside and nucleic acid analogues are known to possess a considerable therapeutic potential. In this work, by coupling cysteine to nucleosides, we successfully synthesized compounds that may not only have interesting biological properties in their monomeric form, but can be used beyond that, for oligomerization, in order to produce new types of synthetic nucleic acids. We elaborated different strategies for the synthesis of cysteinyl nucleosides as monomers of cysteinyl nucleic acids using nucleophilic substitution or thiol-ene coupling as a synthetic tool, and utilised on two complementary nucleosides, uridine and adenosine. Dipeptidyl dinucleosides and pentameric cysteinyl uridine were prepared from the monomeric building blocks, which are the first members of a new class of peptide nucleic acids containing the entire ribofuranosyl nucleoside units bound to the peptide backbone.

Photoinitiated Thiol-ene Reactions of Enoses: A Powerful Tool for Stereoselective Synthesis of Glycomimetics with Challenging Glycosidic Linkages

Thioglycosides and C-glycosides represent pharmacologically useful classes of glycomimetics that possess a high degree of biological stability. One emerging tool for the stereoselective synthesis of thioglycosides is the photoinitiated addition of thiols to unsaturated sugars. Moreover, thiyl radical-mediated reactions of exo-glycals and 1-substituted endo-glycals offer facile routes to β-C-glycosidic structures. This Concept article summarizes the thiol-ene coupling strategies developed recently by our group and Somsák's group for the synthesis of several kinds of glycomimetics which are difficult to synthesize by conventional methods. One unusual characteristic of the thiol-ene reactions of endo-glycals is that heating inhibits, whereas cooling promotes the reaction. This unique temperature dependence as well as the effects of the enose structures and thiol configurations on the efficacy and stereoselectivity of the reactions are also discussed.

Synthesis and photoinitiated thiol–ene reactions of exo-mannals – a new route to C-β-d-mannosyl derivatives

Syntheses of acyl protected exo-mannal derivatives were developed starting from O-peracylated mannopyranoses via the corresponding anhydro-aldose tosylhydrazones under modified Bamford–Stevens conditions. The synthesis of analogous O-peralkylated (benzylated and isopropylenated) derivatives was carried out from pyranoid and furanoid mannonolactones using methylene transfer reagents. Photoinitiated thiol–ene additions of these exo-mannals resulted in the corresponding C-(mannopyranosyl/mannofuranosyl)methyl sulfides in medium to good yields with exclusive regio- and β(d) stereoselectivities.

A synthetic procedure was elaborated for O-peracylated exo-mannals. Thiol-ene additions to pyranoid and furanoid exo-mannals gave mannosylmethyl sulfide type adducts with exclusive regio- and β(d)-stereoselectivities including disaccharide mimetics.