Photoinduced Thiol−Ene Coupling as a Click Ligation Tool for Thiodisaccharide Synthesis

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.

Photoclick Chemistry: A Bright Idea

At its basic conceptualization, photoclick chemistry embodies a collection of click reactions that are performed via the application of light. The emergence of this concept has had diverse impact over a broad range of chemical and biological research due to the spatiotemporal control, high selectivity, and excellent product yields afforded by the combination of light and click chemistry. While the reactions designated as "photoclick" have many important features in common, each has its own particular combination of advantages and shortcomings. A more extensive realization of the potential of this chemistry requires a broader understanding of the physical and chemical characteristics of the specific reactions. This review discusses the features of the most frequently employed photoclick reactions reported in the literature: photomediated azide-alkyne cycloadditions, other 1,3-dipolarcycloadditions, Diels-Alder and inverse electron demand Diels-Alder additions, radical alternating addition chain transfer additions, and nucleophilic additions. Applications of these reactions in a variety of chemical syntheses, materials chemistry, and biological contexts are surveyed, with particular attention paid to the respective strengths and limitations of each reaction and how that reaction benefits from its combination with light. Finally, challenges to broader employment of these reactions are discussed, along with strategies and opportunities to mitigate such obstacles.

Photoactive Metal-Organic Frameworks for the Selective Synthesis of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical Generation

Metal-organic framework (MOF) materials are intriguing photocatalysts to trigger radical-mediated chemical transformations. We report herein the synthesis and characterization of a series of isomorphic MOFs which show a novel structure, wide visible-light absorption, high chemical stability, and specific redox potential. The prepared MOFs were explored for the photoinduced single-electron oxidation of thiol compounds, generating reactive thiyl radicals to afford thioethers via a convenient thiol-olefin reaction. Importantly, we provide a widely applicable strategy by combing a photoactive MOF with phosphine to modulate the generation of thiyl radical in the reaction, thereby producing a single product of the thioether without the formation of a disulfide byproduct due to the dimerization of thiyl radicals. The photocatalytic reaction takes advantage of this strategy, showing great generality where tens of thiols and olefins have been examined as coupling partners. In addition, the strategy has also been demonstrated to be effective for the reactions catalyzed by other MOFs. Mechanism studies reveal that the selective synthesis of C-S products relies on a synergy between the photoinduced generation of a thiyl radical over the MOF and the in situ cleavage of S-S bond into a S-H bond by phosphine. It is notable that the synthesized MOFs show advanced performance in comparison with classical MOFs. The work not only provides a series of novel MOF photocatalysts that are capable of photoinduced thiol-olefin coupling but also indicates the great potential of MOFs for photochemical transformations mediated by reactive radicals.

Bioinspired zwitterionic polyphosphoester modified porous silicon nanoparticles for efficient oral insulin delivery

The intestinal epithelial and mucus barriers on the gastrointestinal tract limit the bioavailability of oral protein or peptide drugs. Therefore, efficient mucus permeability and cellular internalization are required properties for oral delivery systems. To overcome these two obstacles, porous silicon nanoparticles were modified with poly (pyridyl disulfide ethylene phosphate/sulfobetaine) polymers to make P(PyEP-g-SBm)n-AmPSiNPs (m = 0.1, 0.2, 0.3 and n = 10, 20, 30) nanoparticles (NPs). The insulin-loaded P(PyEP-g-SB)-AmPSiNPs showed favorable stability and good biocompatibility in vitro. The zwitterionic dodecyl sulfobetaine (SB) coated nanoparticles improved the mucus permeability. P(PyEP-g-SBm)20 with the optimal conjugated ratio (m = 0.3) of SB units was determined by evaluating the mucus diffusion rate of NPs. The cellular uptake of P(PyEP-g-SB0.3)n-AmPSiNPs (n = 10, 20, 30) was much higher than AmPSiNPs in the presence of inhibitors (N-acetylcysteine solution and sodium chlorate) (p < 0.01) due to the enhanced charge shielding effect of P(PyEP-g-SB) modification. The P(PyEP-g-SB0.3)20-AmPSiNPs showed about 1.4-1.7 fold increase in the apparent permeability of insulin across Caco-2/HT-29-MTX cell monolayers, compared to AmPSiNPs (p < 0.01). Finally, the in vivo study showed that insulin-loaded P(PyEP-g-SB0.3)20-AmPSiNPs generated 20% reduction of the blood glucose level with an 2-fold increase in oral bioavailability. These suggested that zwitterionic polyphosphoester modified porous silicon nanoparticles, which were of enhanced mucus permeability and cellular internalization, represent a promising carrier for oral delivery of peptide and protein.

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.

Selected Research Topics of the Dondoni Group over the Last Two Decades (2000–2020)

From a selection of research topics carried out in our laboratory during the last twenty years it becomes apparent that our main target was the discovery of new or improved synthetic methods together with new properties. Our efforts were made with the aim of being of some utility to other fields of research, with particular emphasis to glycobiology and heterocyle-based bioorganic chemistry. We performed new chemistry mainly in the field of carbohydrate manipulations taking as a primary rule the simplicity and efficiency manners. Toward this end, modern synthetic tools and approaches were employed such as heterocyle-based transformations, multicomponent reactions, organocatalysis, click azide–alkyne cycloadditions, reactions in ionic liquids, click photoinduced thiol-ene coupling, and click sulfur–fluoride exchange chemistry. With these potent methodologies in hand, the syntheses of carbohydrate containing amino acids up to proteins glycosylation were performed.

1 Heterocyclic Glycoconjugates and Amino Acids

2 Triazole-Linked Oligonucleotides: Application of Click CuAAC

3 Non-Natural Glycosyl Amino Acids

4 Non-Natural Oligosaccharides

5 Calixarene-Based Glycoclusters

6 Carbohydrate-Based Building Blocks

7 Homoazasugars and Aza-C-disaccharides

8 Synthesis of Glycodendrimers

9 Peptide and Protein Glycoconjugates

10 Conclusions

Photoinitiated Thiol-Ene Reactions of Various 2,3-Unsaturated O-, C- S- and N-Glycosides - Scope and Limitations Study

The photoinitiated thiol-ene addition reaction is a highly stereo- and regioselective, and environmentally friendly reaction proceeding under mild conditions, hence it is ideally suited for the synthesis of carbohydrate mimetics. A comprehensive study on UV-light-induced reactions of 2,3-unsaturated O-, C-, S- and N-glycosides with various thiols was performed. The effect of experimental parameters and structural variations of the alkenes and thiols on the efficacy and regio- and stereoselectivity of the reactions was systematically studied and optimized. The type of anomeric heteroatom was found to profoundly affect the reactivity of 2,3-unsaturated sugars in the thiol-ene couplings. Hydrothiolation of 2,3-dideoxy O-glycosyl enosides efficiently produced the axially C2-S-substituted addition products with high to complete regioselectivity. Moderate efficacy and varying regio- and stereoselectivity were observed with 2,3-unsaturated N-glycosides and no addition occurred onto the endocyclic double bond of C-glycosides. Upon hydrothiolation of 2,3-unsaturated S-glycosides, the addition of thiyl radicals was followed by elimination of the thiyl aglycone resulting in 3-S-substituted glycals.

Addition of Heteroatom Radicals to endo-Glycals †

Radical reactions have found many applications in carbohydrate chemistry, especially in the construction of carbon–carbon bonds. The formation of carbon–heteroatom bonds has been less intensively studied. This mini-review will summarize the efforts to add heteroatom radicals to unsaturated carbohydrates like endo-glycals. Starting from early examples, developed more than 50 years ago, the importance of such reactions for carbohydrate chemistry and recent applications will be discussed. After a short introduction, the mini-review is divided in sub-chapters according to the heteroatoms halogen, nitrogen, phosphorus, and sulfur. The mechanisms of radical generation by chemical or photochemical processes and the subsequent reactions of the radicals at the 1-position will be discussed. This mini-review cannot cover all aspects of heteroatom-centered radicals in carbohydrate chemistry, but should provide an overview of the various strategies and future perspectives.

Stereoselective Synthesis of Carbon-Sulfur-Bridged Glycomimetics by Photoinitiated Thiol-Ene Coupling Reactions

Oligosaccharides and glycoconjugates are abundant in all living organisms, taking part in a multitude of biological processes. The application of natural O-glycosides in biological studies and drug development is limited by their sensitivity to enzymatic hydrolysis. This issue made it necessary to design hydrolytically stable carbohydrate mimetics, where sulfur, carbon, or longer interglycosidic connections comprising two or three atoms replace the glycosidic oxygen. However, the formation of the interglycosidic linkages between the sugar residues in high diastereoslectivity poses a major challenge. Here, we report on stereoselective synthesis of carbon-sulfur-bridged disaccharide mimetics by the free radical addition of carbohydrate thiols onto the exo-cyclic double bond of unsaturated sugars. A systematic study on UV-light initiated radical mediated hydrothiolation reactions of enoses bearing an exocyclic double bond at C1, C2, C3, C4, C5, and C6 positions of the pyranosyl ring with various sugar thiols was performed. The effect of temperature and structural variations of the alkenes and thiols on the efficacy and stereoselectivity of the reactions was systematically studied and optimized. The reactions proceeded with high efficacy and, in most cases, with complete diastereoselectivity producing a broad array of disaccharide mimetics coupling through an equatorially oriented methylensulfide bridge.

Recent development in the synthesis of C-glycosides involving glycosyl radicals

C-Glycosylation involving glycosyl radical intermediates is a particularly effective approach to access C-glycosides, which are core units of a great number of natural products, bioactive compounds and marketed drugs.

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.

Metal-free synthesis of imino-disaccharides and calix-iminosugars by photoinduced radical thiol–ene coupling (TEC)

Deprotected iminosugar alkenes were subjected to thiol–ene coupling with deprotected sugar thiols to afford new imino-disaccharides. Two thiol–ene couplings converted these alkenes into iminosugar thiols and then multivalent iminosugars.

Stereoselective Thioconjugation by Photoinduced Thiol-ene Coupling Reactions of Hexo- and Pentopyranosyl d- and l-Glycals at Low-Temperature-Reactivity and Stereoselectivity Study

A comprehensive optimization and mechanistic study on the photoinduced hydrothiolation of different d- and l- hexo- and pentoglycals with various thiols was performed, at the temperature range of RT to -120 °C. Addition of thiols onto 2-substituted hexoglycals proceeded with complete 1,2-cis-α-stereoselectivity in all cases. Hydrothiolation of 2-substituted pentoglycals resulted in mixtures of 1,2-cis-α- and -β-thioglycosides of varying ratio depending on the configuration of the reactants. Hydrothiolation of unsubstituted glycals at -80 °C proceeded with excellent yields and, except for galactal, provided the axially C2-S-linked isomers with high selectivity. Cooling was always beneficial to the efficacy, increased the yields and in most cases significantly raised the stereoselectivity. The suggested mechanism explains the different conformational preferences of the intermediate carbon-centered radicals, which is a crucial factor in the stereoselectivity of the reactions.

Towards the preparation of synthetic outer membrane vesicle models with micromolar affinity to wheat germ agglutinin using a dialkyl thioglycoside

A series of alkyl thioglycosides and alkyl thiodiglycosides bearing glucose and N-acetylglucosamine residues were prepared by thiol-ene coupling in moderate to good yields (40-85%). Their binding ability towards wheat germ agglutinin was measured by competitive enzyme-linked lectin assays. One of the synthetic compounds presenting two GlcNAc units showed the highest inhibitory effect of this study with an IC₅₀ of 11 µM corresponding to a 3182-fold improvement compared to GlcNAc. These synthetic molecules were used to produce giant vesicles, alone or in mixture with phospholipids, mimicking bacterial outer membrane vesicles (OMV) with potential antiadhesive properties.

Theoretical modeling of pKa's of thiol compounds in aqueous solution

The pK_a's of different kinds of thiols (R-SH) were investigated by using the M06-2X method with a SMD_sSAS model.

Metal-free photocatalytic thiol–ene/thiol–yne reactions

We report a metal-free photocatalytic approach to the synthesis of glycoconjugates, highlighting the mild nature of the reaction conditions.

Design and synthesis of structurally defined heparan sulfate (HS)-FK506 conjugates as an exogenous approach to investigate biological functions of nucleus HS

Although heparan sulfate (HS) is widely implicated in numerous physiological and pathological processes, the biological function of nucleus HS remains underexplored, largely due to its complex structure and high hydrophilic property. To supplement these efforts, ideal vehicles are drawing attention as they combine attractive features including lipid solubility for penetrating cell membrane, high affinity binding to its target receptor, metabolic stability, and no cellular actions resulting in toxicity. Herein, we develop a convenient and promising strategy to prepare HS-FK506 conjugates for membrane transport and entry into nucleus, where click chemistry takes easily place between the exocyclic allyl group of a clinic drug FK506 and thiol as a handle incorporated into HS analogues. HS derivatives for constructing the conjugates were synthesized using a cutting-edge chemoenzymatic method. Meantime, [³⁵S] labeled 3'-phosphoadenosine 5'-phosphosulfate (PAP³⁵S) and [¹⁴C] glucuronic acid (Glc A) were adopted to label HS-FK506 conjugates, respectively, to evaluate their efficiency of nucleus entry, as a result, ¹⁴C Glc A was sensitive, effective and reliable whereas PAP³⁵S gave rise to a mixture of labeled compounds, hampering the understanding of structure-function relationship of nucleus HS. Compared with the corresponding HS, the amount of HS-FK506 conjugates to translocate into nucleus from radioactive assay experiments sharply increased, e.g. tridecasaccharide-FK506 1d increased by approximate 10 folds, offering a simple and robust platform for enabling hydrophilic compounds including carbohydrates to translocate into nucleus and shedding light on their biological functions.

A low-temperature, photoinduced thiol–ene click reaction: a mild and efficient method for the synthesis of sugar-modified nucleosides

While studying the radical mediated hydrothiolation of nucleoside enofuranosides, an unusual temperature effect was observed by the exploitation of which various thio-substituted nucleoside analogues were produced.

Unprecedented β-manno type thiodisaccharides with a C-glycosylic function by photoinitiated hydrothiolation of 1-C-substituted glycals

Unique thiodisaccharides of β-manno type featuring C-glycosylic structure with an anomeric functional group for further elaboration were obtained by radical-mediated hydrothiolation of 1-C-acceptor-substituted glycals.

Recent advances in photoinduced glycosylation: oligosaccharides, glycoconjugates and their synthetic applications

Carbohydrates have been demonstrated to perform imperative act in biological processes. This review highlights recent uses of photoinduced glycosylation in carbohydrate chemistry for the synthesis of oligosaccharides, thiosugars, glycoconjugates and glycoprotein.

The photochemical thiol-ene reaction as a versatile method for the synthesis of glutathione S-conjugates targeting the bacterial potassium efflux system Kef

The thiol-ene coupling reaction is emerging as an important conjugation reaction that is suitable for use in a biological setting. Here, we explore the utility of this reaction for the synthesis of glutathione-S-conjugates (GSX) and present a general, operationally simple, protocol with a wide substrate scope. The GSX afforded are an important class of compounds and provide invaluable molecular tools to study glutathione-binding proteins. In this study we apply the diverse library of GSX synthesised to further our understanding of the structural requirements for binding to the glutathione-binding protein, Kef, a bacterial K⁺ efflux system, found in many bacterial pathogens. This system is vital to the survival of bacteria upon exposure to electrophiles, and plays an essential role in the maintenance of intracellular pH and K⁺ homeostasis. Consequently, Kef is an appealing target for the development of novel antibacterial drugs.

Cyclopropenone-caged Sondheimer diyne (dibenzo[a,e]cyclooctadiyne): a photoactivatable linchpin for efficient SPAAC crosslinking

The first fully conjugated bis-cyclopropenone (photo-DIBOD), a derivative of dibenzo[a,e][8]annulene, has been synthesized. 350-420 nm irradiation of this robust compound results in the efficient formation of dibenzo [a,e] cyclooctadiyne, an unstable, but useful SPAAC cross-linking reagent. Since photo-DIBO doesn't react with organic azides, this method allows for the spatiotemporal control of the ligation of two azide-tagged substrates.

Photochemical radical thiol–ene click-based methodologies for silica and transition metal oxides materials chemical modification: a mini-review

The photochemical radical thiol–ene addition reaction (PRTEA) is a highly powerful synthetic technique for surface modification.

Photoinitiated hydrothiolation of pyranoid exo-glycals: the d-galacto and d-xylo cases

Radical-mediated addition reactions of thiols to O-peracetylated exo-galactal and exo-xylal with 2,2-dimethoxy-2-phenylacetophenone as the photoinitiator resulted in high yielding formation of the corresponding β-d-glycopyranosylmethyl-sulfide derivatives (2,6-anhydro-1-deoxy-1-S-substituted-1-thio-alditols) with exclusive regio- and very high stereoselectivity, including disaccharide mimicks with Gly-CH2-S-Gly scaffolds.

Facile synthesis of zwitterionic polymer-coated core-shell magnetic nanoparticles for highly specific capture of N-linked glycopeptides

Highly selective and efficient capture of glycosylated proteins and peptides from complex biological samples is of profound significance for the discovery of disease biomarkers in biological systems. Recently, hydrophilic interaction liquid chromatography (HILIC)-based functional materials have been extensively utilized for glycopeptide enrichment. However, the low amount of immobilized hydrophilic groups on the affinity material has limited its specificity, detection sensitivity and binding capacity in the capture of glycopeptides. Herein, a novel affinity material was synthesized to improve the binding capacity and detection sensitivity for glycopeptides by coating a poly(2-(methacryloyloxy)ethyl)-dimethyl-(3-sulfopropyl) ammonium hydroxide (PMSA) shell onto Fe3O4@SiO2 nanoparticles, taking advantage of reflux-precipitation polymerization for the first time (denoted as Fe3O4@SiO2@PMSA). The thick polymer shell endows the nanoparticles with excellent hydrophilic property and several functional groups on the polymer chains. The resulting Fe3O4@SiO2@PMSA demonstrated an outstanding ability for glycopeptide enrichment with high selectivity, extremely high detection sensitivity (0.1 fmol), large binding capacity (100 mg g(-1)), high enrichment recovery (above 73.6%) and rapid magnetic separation. Furthermore, in the analysis of real complicated biological samples, 905 unique N-glycosylation sites from 458 N-glycosylated proteins were reliably identified in three replicate analyses of a 65 μg protein sample extracted from mouse liver, showing the great potential of Fe3O4@SiO2@PMSA in the detection and identification of low-abundance N-linked glycopeptides in biological samples.

Multivalent thioglycopeptoids via photoclick chemistry: potent affinities towards LecA and BC2L-A lectins

The straightforward access to peptoid-based multivalent thioglycoclusters displaying 1-thio-β-d-galactose or 1-thio-α/β-d-mannose and their evaluation towards two bacterial lectins are described.

Combining ring-opening metathesis polymerization and thiol-ene coupling chemistries: facile access to novel functional linear and nonlinear macromolecules

The aim of this article is to highlight recent examples in which two powerful synthetic tools, namely ring-opening metathesis polymerization (ROMP) and thiol-ene (including the thiol-Michael variant) click chemistry have been combined to facilitate the preparation of novel functional materials of varying topology.

Expedient synthesis of an α-S-(1→6)-linked pentaglucosyl thiol

An α-S-(1→6)-linked pentaglucosyl thiol (1) is synthesized via the longest linear sequence of eleven steps from an α-glucosyl thiol (7).

New glycopolymers as multivalent systems for lectin recognition

New types of glycopolymers have been designed and evaluated as multivalent systems for lectin recognition.