Synthesis of 1′,2′-cis-Nucleoside Analogues: Evidence of Stereoelectronic Control for SN2 Reactions at the Anomeric Center of Furanosides

Catalytic Activation of Thioglycosides with Copper-Carbenes for Stereoselective 1,2-Cis-Furanosylations

Thioglycoside activation, crucial for oligosaccharide synthesis, faces challenges with the need for stoichiometric promoters, additives, and cryogenic conditions, particularly in stereoselective 1,2-cis-linkage formation. This study introduces a carbene-based catalytic method using Cu(OTf)2 for thioglycoside activation, enabling efficient 1,2-cis-furanosylation in ribose and arabinose. The method is orthogonal to conventional thioglycoside and alkyne donors, accommodates sterically demanding acceptors, and achieves stereoselectivity independent of the donor anomeric configuration and C2-protecting groups, with copper chelation playing a key role.

B(C6F5)3-Catalyzed Stereoselective 1,2-cis Arabinofuranosylation with a Conformationally Constrained Donor

Compared with stereoselective glycosylation methods mainly addressed on the preparation of pyranose glycosides, the furanosylation has been more limited, especially for the 1,2-cis arabinofuranosylation. Herein, we report a novel stereoselective 1,2-cis-arabinofuranosylation strategy using a conformationally restricted 3,5-O-xylylene-protected arabinofuranosyl donor on activation with B(C6F5)3 for desired targets in moderate to excellent yields and β-stereoselectivity. The effectiveness of the 1,2-cis-arabinofuranosylation strategy was demonstrated successfully with various acceptors, including carbohydrate alcohols.

Multistep Synthesis of Analogues of Remdesivir: Incorporating Heterocycles at the C-1' Position

Studies suggest that the 1'β-CN moiety in remdesivir sterically clashes with the Ser861 residue of the RNA-dependent-RNA polymerase (RdRp), causing a delayed chain termination in the RNA replication process. Replacing C1'β-CN with 5-membered heterocycles such as tetrazoles, oxadiazoles, and triazoles can augment the inhibitory activity and pharmacokinetic profile of C-nucleotides. Synthesis of tetrazole-, triazole-, and oxadiazole-integrated C1' analogues of remdesivir was attempted using general synthetic routes. The final compounds 26, 28, and 29 did not inhibit viral replication; however, the synthetic intermediates, i.e., 27 and 50, exhibited an IC90 = 14.1 μM each. The trifluoromethyl-substituted 1,2,4-oxadiazole 59 showed an IC90 of 33.5 μM. This work adds to the growing evidence of the beneficial medicinal impact of C1,1'-disubstituted C-nucleotides.

Nucleotide Analogues Bearing a C2' or C3'-Stereogenic All-Carbon Quaternary Center as SARS-CoV-2 RdRp Inhibitors

The design of novel nucleoside triphosphate (NTP) analogues bearing an all-carbon quaternary center at C2′ or C3′ is described. The construction of this all-carbon stereogenic center involves the use of an intramoleculer photoredox-catalyzed reaction. The nucleoside analogues (NA) hydroxyl functional group at C2′ was generated by diastereoselective epoxidation. In addition, highly enantioselective and diastereoselective Mukaiyama aldol reactions, diastereoselective N-glycosylations and regioselective triphosphorylation reactions were employed to synthesize the novel NTPs. Two of these compounds are inhibitors of the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2, the causal virus of COVID-19.

Side Chain Conformation and Its Influence on Glycosylation Selectivity in Hexo- and Higher Carbon Furanosides

We describe the synthesis and side chain conformational analysis of a series of four 6-deoxy-2,3,5-tri-O-benzyl hexofuranosyl donors with the d-gluco, l-ido, d-altro, and l-galacto configurations. The conformation of the exocyclic bond of these compounds depends on the relative configuration of the point of attachment of the side chain to the ring and of the two flanking centers and can be predicted on that basis analogously to the heptopyranose analogs. Variable-temperature nuclear magnetic resonance (VT NMR) spectroscopy of the activated donors reveals complex, configuration-dependent mixtures of intermediates that we interpret in terms of fused and bridged oxonium ions arising from participation by the various benzyl ethers. The increased importance of ether participation in the furanoside series compared to the pyranosides is discussed in terms of the reduced stabilization afforded to furanosyl oxocarbenium ions by covalent triflate formation. The stereoselectivities of the four donors are discussed on the basis of the benzyl ether participation model.

Chemical and chemoenzymatic stereoselective synthesis of β-nucleosides and their analogues

β-Nucleosides are fundamental building blocks of biological systems that are widely used as therapeutic agents for treating cancer and viral infections among others. In the last two years, nucleoside analogues...

Surfactant-mediated thioglycosylation of 1-hydroxy sugars in water

Thioglycosides are an important class of sugars, since they can be used as non-ionic biosurfactants, biomimetic glycosides, and building blocks for carbohydrate synthesis. Previously, Brønsted- or Lewis-acid-catalyzed dehydrative glycosylations between a 1-hydroxy sugar and a thiol have been reported to yield open-chain dithioacetal sugars as the major products instead of the desired thioglycosides. These dithioacetal sugars are by-products derived from the endocyclic bond cleavage of the thioglycosides. Herein, we report dehydrative glycosylation in water mediated by a Brønsted acid-surfactant combined catalyst (BASC). Glycosylations between 1-hydroxy furanosyl/pyranosyl sugars and primary, secondary, and tertiary aliphatic/aromatic thiols in the presence of dodecyl benzenesulfonic acid (DBSA) provided the thioglycoside products in moderate to good yields. Microwave irradiation led to improvements in the yields and a shortening of the reaction time. Remarkably, open-chain dithioacetal sugars were not detected in the DBSA-mediated glycosylations in water. This method is a simple, convenient, and rapid approach to produce a library of thioglycosides without the requirement of anhydrous conditions. Moreover, this work also provides an excellent example of complementary reactivity profiles of glycosylation in organic solvents and water.

Stereospecific Furanosylations Catalyzed by Bis-thiourea Hydrogen-Bond Donors

We report a new method for stereoselective O-furanosylation reactions promoted by a precisely tailored bis-thiourea hydrogen-bond-donor catalyst. Furanosyl donors outfitted with an anomeric dialkylphosphate leaving group undergo substitution with high anomeric selectivity, providing access to the challenging 1,2-cis substitution pattern with a range of alcohol acceptors. A variety of stereochemically distinct, benzyl-protected glycosyl donors were engaged successfully as substrates. Mechanistic studies support a stereospecific mechanism in which rate-determining substitution occurs from a catalyst-donor resting-state complex.

Applying the Bent Bond/Antiperiplanar Hypothesis to the Stereoselective Glycosylation of Bicyclic Furanosides

The glycosylation stereoselectivities for a series of bicyclic furanoside models have been carried out in the presence of weak nucleophiles. These results were analyzed through the bent bond/antiperiplanar hypothesis (BBAH) orbital model to test its validity. According to the BBAH, incoming nucleophiles displace one of the two bent bonds of bicyclic oxocarbenium ion intermediates in an antiperiplanar fashion. The glycosylation stereoselectivity is then governed by the displacement of the weaker bent bond as determined by the presence of electron-withdrawing or -donating substituents at C₂. Overall, the BBAH analysis expands Woerpel's "inside/outside attack" glycosylation model by considering the stereoelectronic influence of neighboring electron-withdrawing and -donating groups on the nucleophilic addition to oxocarbenium ion intermediates.

Site-selective redox isomerizations of furanosides using a combined arylboronic acid/photoredox catalyst system

In the presence of an arylboronic acid and a hydrogen atom transfer mediator under photoredox conditions, furanoside derivatives undergo site-selective redox isomerizations to 2-keto-3-deoxyfuranosides. Experimental evidence and computational modeling suggest that the transformation takes place by abstraction of the hydrogen atom from the 2-position of the furanoside-derived arylboronic ester, followed by C3-O bond cleavage via spin-center shift. This mechanism is reminiscent of the currently accepted pathway for the formation of 3'-ketodeoxynucleotides by ribonucleotide reductase enzymes.

Diastereoselective Synthesis of Arabino- and Ribo-like Nucleoside Analogues Bearing a Stereogenic C3' All-Carbon Quaternary Center

The synthesis of novel nucleoside analogues bearing a C3' all-carbon quaternary center and a C2'-hydroxy substituent is described. The all-carbon stereogenic center was generated through an intramolecular 7-endo attack of a silyl-tethered allyl moiety on a tertiary radical using photoredox catalysis. Subsequent allylic oxidation and diastereoselective hydride reductions provided the hydroxy substituent at C2', which then controls the stereoselective introduction of pyrimidine nucleobases on the corresponding furanose scaffold. Density functional theory (DFT) calculations provided insights into the origin of the high syn diastereoselectivity resulting from the radical cyclization. This original methodology grants access to a wide range of 1',2'-cis and 1',2'-trans arabino- and ribo-like analogues bearing an all-carbon quaternary center at C3'. These molecules are currently being tested for their antiviral and anticancer properties.

The Experimental Evidence in Support of Glycosylation Mechanisms at the SN1-SN2 Interface

A critical review of the state-of-the-art evidence in support of the mechanisms of glycosylation reactions is provided. Factors affecting the stability of putative oxocarbenium ions as intermediates at the SN1 end of the mechanistic continuum are first surveyed before the evidence, spectroscopic and indirect, for the existence of such species on the time scale of glycosylation reactions is presented. Current models for diastereoselectivity in nucleophilic attack on oxocarbenium ions are then described. Evidence in support of the intermediacy of activated covalent glycosyl donors is reviewed, before the influences of the structure of the nucleophile, of the solvent, of temperature, and of donor-acceptor hydrogen bonding on the mechanism of glycosylation reactions are surveyed. Studies on the kinetics of glycosylation reactions and the use of kinetic isotope effects for the determination of transition-state structure are presented, before computational models are finally surveyed. The review concludes with a critical appraisal of the state of the art.

Investigation of diastereoselective acyclic α-alkoxydithioacetal substitutions involving thiacarbenium intermediates

Reported herein is an experimental and theoretical study that elucidates why silylated nucleobase additions to acyclic α-alkoxythiacarbenium intermediates proceed with high 1,2-syn stereocontrol (anti-Felkin-Anh), which is opposite to what would be expected with corresponding activated aldehydes. The acyclic thioaminals formed undergo intramolecular cyclizations to provide nucleoside analogues with anticancer and antiviral properties. The factors influencing the selectivity of the substitution reaction have been examined thoroughly. Halothioether species initially form, ionize in the presence (low dielectric media) or absence (higher dielectric media) of the nucleophile, and react through SN2-like transition structures (TS A and D), where the α-alkoxy group is gauche to the thioether moiety. An important, and perhaps counterintuitive, observation in this work was that calculations done in the gas phase or low dielectric media (toluene) are essential to locate the product- and rate-determining transition structures (C-N bond formation) that allow the most reasonable prediction of selectivity and isotope effects for more polar solvents (THF, MeCN). The ΔΔG(⧧) (G(TSA-TSD)) obtained in silico are consistent with the preferential formation of 1,2-syn product and with the trends of stereocontrol displayed by 2,3-anti and 2,3-syn α,β-bis-alkoxydithioacetals.

Solvent effects in the nucleophilic substitutions of tetrahydropyran acetals promoted by trimethylsilyl trifluoromethanesulfonate: trichloroethylene as solvent for stereoselective C- and O-glycosylations

The selectivities of nucleophilic substitution reactions of tetrahydropyran acetals promoted by trimethylsilyl trifluoromethanesulfonate depend upon the reaction solvent. Polar solvents favor the formation of S(N)1 products, while nonpolar solvents favor S(N)2 products. Trichloroethylene was identified as the solvent most likely to give S(N)2 products in both C- and O-glycosylation reactions.

A rapid and divergent access to chiral azacyclic nucleoside analogues via highly enantioselective 1,3-dipolar cycloaddition of β-nucleobase substituted acrylates

A rapid and divergent access to chiral azacyclic nucleoside analogues was developed via enantioselective 1,3-dipolar cycloaddition of β-nucleobase substituted acrylates.

A stereoselective approach to β-L-arabino nucleoside analogues: synthesis and cyclization of acyclic 1',2'-syn N,O-acetals

Reported herein is a novel and versatile strategy for the stereoselective synthesis of unnatural β-L-arabinofuranosyl nucleoside analogues from acyclic N,OTMS-acetals bearing pyrimidine and purine bases. These unusual acetals undergo a C1' to C4' cyclization where the OTMS of the acetal serves as the nucleophile to generate 2'-oxynucleosides with complete retention of configuration at the C1' acetal center. N,OTMS-acetals are obtained diastereoselectively from additions of silylated nucleobases onto acyclic polyalkoxyaldehydes in the presence of MgBr(2)·OEt(2). The strategy reported is addressing important synthetic challenges by providing stereoselective access to unnatural L-nucleosides starting from easily accessible pools of D-sugars and, as importantly, by allowing the formation of the sterically challenging 1',2'-cis nucleosides. A wide variety of nucleoside analogues were synthesized in 7-8 steps from easily accessible D-xylose.

Ribosylation of adenosine: an orthogonally protected building block for the synthesis of ADP-ribosyl oligomers

A method to ribosylate adenosine on the 2' hydroxyl function in an α-selective fashion and in good yield is presented. Protective groups chosen for the acceptor and donor used in this glycosylation not only direct α-selectivity but also allow the construction of a fully orthogonally protected building block for the future assembly of oligo-ADP-ribosylated peptides and proteins.

Stereoelectronic effects determine oxacarbenium vs β-sulfonium ion mediated glycosylations

Activation of a glycosyl donor protected with a 2-O-(S)-(phenylthiomethyl)benzyl ether chiral auxiliary results in the formation of an anomeric β-sulfonium ion, which can be displaced with sugar alcohols to give corresponding α-glycosides. Sufficient deactivation of such glycosyl donors by electron-withdrawing protecting groups is, however, critical to avoid glycosylation of an oxacarbenium ion intermediate. The latter type of glycosylation pathway can also be suppressed by installing additional substituents in the chiral auxiliary.