Repairing the Thiol-Ene Coupling Reaction

Recent Developments on Photochemical Synthesis of 1,n-Dicarbonyls

1,n-dicarbonyls are one of the most fascinating chemical feedstocks finding abundant usage in the field of pharmaceuticals. Besides, they are utilized in a plethora of synthesis in general synthetic organic chemistry. A number of 'conventional' methods are available for their synthesis, such as the Stetter reaction, Baker-Venkatraman rearrangement, oxidation of vicinal diols, and oxidation of deoxybenzoins, synonymous with unfriendly reagents and conditions. In the last 15 years or so, photocatalysis has taken the world of synthetic organic chemistry by a remarkable renaissance. It is fair to say now that everybody loves the light and photoredox chemistry has opened a new gateway to organic chemists towards milder, more simpler alternatives to the previously available methods, allowing access to many sensitive reactions and products. In this review, we present the readers with the photochemical synthesis of a variety of 1,n-dicarbonyls. Diverse photocatalytic pathways to these fascinating molecules have been discussed, placing special emphasis on the mechanisms, giving the reader an opportunity to find all these significant developments in one place.

Structure of the human heparan sulfate polymerase complex EXT1-EXT2

Heparan sulfates are complex polysaccharides that mediate the interaction with a broad range of protein ligands at the cell surface. A key step in heparan sulfate biosynthesis is catalyzed by the bi-functional glycosyltransferases EXT1 and EXT2, which generate the glycan backbone consisting of repeating N-acetylglucosamine and glucuronic acid units. The molecular mechanism of heparan sulfate chain polymerization remains, however, unknown. Here, we present the cryo-electron microscopy structure of human EXT1-EXT2, which reveals the formation of a tightly packed hetero-dimeric complex harboring four glycosyltransferase domains. A combination of in vitro and in cellulo mutational studies is used to dissect the functional role of the four catalytic sites. While EXT1 can catalyze both glycosyltransferase reactions, our results indicate that EXT2 might only have N-acetylglucosamine transferase activity. Our findings provide mechanistic insight into heparan sulfate chain elongation as a nonprocessive process and lay the foundation for future studies on EXT1-EXT2 function in health and disease.

Metal-free bioorthogonal click chemistry in cancer theranostics

Bioorthogonal chemistry is a powerful tool to site-specifically activate drugs in living systems. Bioorthogonal reactions between a pair of biologically reactive groups can rapidly and specifically take place in a mild physiological milieu without perturbing inherent biochemical processes. Attributed to their high selectivity and efficiency, bioorthogonal reactions can significantly decrease background signals in bioimaging. Compared with metal-catalyzed bioorthogonal click reactions, metal-free click reactions are more biocompatible without the metal catalyst-induced cytotoxicity. Although a great number of bioorthogonal chemistry-based strategies have been reported for cancer theranostics, a comprehensive review is scarce to highlight the advantages of these strategies. In this review, recent progress in cancer theranostics guided by metal-free bioorthogonal click chemistry will be depicted in detail. The elaborate design as well as the advantages of bioorthogonal chemistry in tumor theranostics are summarized and future prospects in this emerging field are emphasized.

Solvent-mediated switching between oxidative addition and addition–oxidation: access to β-hydroxysulfides and β-arylsulfones by the addition of thiols to olefins in the presence of Oxone

Solvent-switching allows formation of either β-hydroxy-2-arylethyl aryl sulfides or 2-arylethyl aryl sulfones exclusively in thiol–ene ‘click’ reactions conducted with Oxone.

Forskolin Editing via Radical Iodo- and Hydroalkylation

The modification of highly oxygenated forskolin as well as manoyl and epi-manoyl oxide, two less functionalized model substrates sharing the same polycyclic skeleton, via intermolecular carbon-centered radical addition to the vinyl moiety has been investigated. Highly regio- and reasonably stereoselective iodine atom transfer radical addition (ATRA) reactions were developed. Unprotected forskolin afforded an unexpected cyclic ether derivative. Protection of the 1,3-diol as an acetonide led the formation of the iodine ATRA product. Interestingly, by changing the mode of initiation of the radical process, in situ protection of the forskolin 1,3-diol moiety as a cyclic boronic ester took place during the iodine ATRA process without disruption of the radical chain process. This very mild radical-mediated in situ protection of 1,3-diol is expected to be of interest for a broad range of radical and non-radical transformations. Finally, by using our recently developed tert-butyl­catechol-mediated hydroalkylation procedure, highly efficient preparation of forskolin derivatives bearing an extra ester or sulfone group was achieved.

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.

Synthesis of Fucose Derivatives with Thiol Motifs towards Suicide Inhibition of Helicobacter pylori

The syntheses of six thiol-exhibiting monosaccharides towards suicide inhibition of Helicobacter pylori are reported. Blood group Antigen Binding Adhesin (BabA), a bacterial membrane-bound lectin, binds to human ABO and Lewis b blood group structures displayed on the surface of host epithelial cells. Crystal structures of the carbohydrate-recognition domain revealed a conserved disulfide bonded loop that anchors a critical fucose residue in these blood group structures. Disruption of this loop by N-acetylcysteine results in reduced BabA-mediated adherence to human gastric tissue sections and attenuated virulence in Lewis b-expressing transgenic mice. With a view of creating specific inhibitors of the lectin, we designed and successfully synthesised six fucose-derived compounds with thiol motifs to engage in a thiol-disulfide exchange with this disulfide bond of BabA and form a glycan-lectin disulfide linkage. Branching and extending the fucose backbone with 2- and 3-carbon thiol motifs delivered a range of candidates to be tested for biological activity against BabA.

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.

Photo-induced thiol-ene reactions for late-stage functionalization of unsaturated polyether macrocycles: regio and diastereoselective access to macrocyclic dithiol derivatives

Double hydrothiolation of bis enol ether macrocycles was achieved under photo-mediated conditions. The thiol-ene reactions afford a fully regioselective anti-Markovnikov post-functionalization. Thanks to the use of ethanedithiol as reagent, moderate to excellent diastereoselectivity was accomplished leading to macrocycles containing four defined stereocenters in only three steps from 1,4-dioxane, tetrahydrofuran (THF) or tetrahydropyran (THP).

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.

Visible-Light-Mediated Organocatalyzed Thiol-Ene Reaction Initiated by a Proton-Coupled Electron Transfer

A convenient method for performing a thiol-ene reaction is described. The reaction is performed under blue-light irradiation and catalyzed by photoactive Lewis basic molecules such as acridine orange or naphthalene-fused N-acylbenzimidazole. It is believed that the process is initiated by a proton-coupled electron transfer process within the complex between the thiol and the Lewis basic catalyst.

Iron-Catalyzed Oxyalkylation of Terminal Alkynes with Alkyl Iodides

A general oxyalkylation of terminal alkynes enabled by iron catalysis has been developed. Primary and secondary alkyl iodides acted as the alkylating reagents and afforded a range of α-alkylated ketones under mild reaction conditions. Acetyl tert-butyl peroxide (TBPA) was used as the radical relay precursor, providing the initiated methyl radical to start the radical relay process. Preliminary mechanistic studies were conducted, and late-stage functionalizations of natural product derivatives were performed.

Highlights from the 53rd EUCHEM conference on stereochemistry, Bürgenstock, Switzerland, May 2018

When we first heard of the Bürgenstock conference it was described as a guarded meeting in a remote location, undisturbed by modern diversions, with mysterious customs and a secret handshake. All of these rumors turned out to be completely true. We arrived at an undisclosed location and three young men, who knew our names upon sight, greeted us and gave us an agenda in which we discovered the identity of the speakers, moderators and other participants. We then had a few moments to bask in their glory before being treated to a delicious dose of chemistry. During the banquet, before the first lecture, this year's president, Prof. Ilan Marek, gave the opening address from a balcony reserved only for these meetings, rumors hold that it stands vacant all year in anticipation. He mentioned several of the traditions of the meeting, which we are not allowed to share of course, and described the joy of putting together this year's exciting program with the help of an exceptional organizing committee that included Prof. Cristina Nevado, Prof. Christian Bochet, Dr Fabrice Gallou and Dr Alain De Mesmaeker. He also introduced this year's guest of honor, Prof. Yitzhak Apeloig, and wished the best luck to the current Vice President and future President, Prof. Véronique Gouverneur, not only for preparing the 54th Bürgenstock Conference, but also with her duty this year, maintaining the good weather for the whole week. Although the official title of the conference is the EUCHEM conference on stereochemistry, the topics covered span a broad range of cutting edge chemical transformations and insights, which can appeal to anyone working in chemistry and its interfaces with other disciplines. We will briefly describe each of the talks following one of our favorite citable quotes from these inspiring speakers.

Merging Art and Science-The 53rd Bürgenstock Conference

For the 53rd time, the Bürgenstock Conference gathered some of the most gifted scientists and rising stars in organic, physical, and bioorganic chemistry. Orchestrated by Ilan Marek (President) and his successor, Véronique Gouverneur, the synergy between art and science took place in Brunnen, Switzerland, with a beatiful view over Lake Lucerne.

Radical chain repair: The hydroalkylation of polysubstituted unactivated alkenes

The concept of repair is widely used by nature to heal molecules such as proteins, lipids, sugars, and DNA that are damaged by hydrogen atom abstraction resulting from oxidative stress. We show that this strategy, rather undocumented in the field of synthetic organic chemistry, can be used in a radical chain reaction to enable notoriously intractable transformations. By overcoming the radical chain inhibitor properties of substituted alkenes, the radical-mediated hydroalkylation of mono-, di-, tri-, and even tetrasubstituted unactivated olefins could be performed under mild conditions. With a remarkable functional group tolerance, this reaction provides a general coupling method for the derivatization of olefin-containing natural products.

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.

Design of α-S-Neoglycopeptides Derived from MUC1 with a Flexible and Solvent-Exposed Sugar Moiety

The use of vaccines based on MUC1 glycopeptides is a promising approach to treat cancer. We present herein several sulfa-Tn antigens incorporated in MUC1 sequences that possess a variable linker between the carbohydrate (GalNAc) and the peptide backbone. The main conformations of these molecules in solution have been evaluated by combining NMR experiments and molecular dynamics simulations. The linker plays a key role in the modulation of the conformation of these compounds at different levels, blocking a direct contact between the sugar moiety and the backbone, promoting a helix-like conformation for the glycosylated residue and favoring the proper presentation of the sugar unit for molecular recognition events. The feasibility of these novel compounds as mimics of MUC1 antigens has been validated by the X-ray diffraction structure of one of these unnatural derivatives complexed to an anti-MUC1 monoclonal antibody. These features, together with potential lack of immune suppression, render these unnatural glycopeptides promising candidates for designing alternative therapeutic vaccines against cancer.

A convenient access to β-iodo sulfone by the iodine-mediated iodosulfonylation of alkenes

A novel iodine-mediated intermolecular iodosulfonylation reaction of alkenes for dual C–S and C–I bond formation was achieved.

“Onion peel” dendrimers: a straightforward synthetic approach towards highly diversified architectures

We report herein a novel “onion peel strategy” for the divergent construction of glycodendrimers using different building blocks at each layer of the dendritic growth.