Synthesis of 4'-Thionucleoside Analogues Bearing a C2' Stereogenic All-Carbon Quaternary Center.

The design of novel 4'-thionucleoside analogues bearing a C2' stereogenic all-carbon quaternary center is described. The synthesis involves a highly diastereoselective Mukaiyama aldol reaction, and a diastereoselective radical-based vinyl group transfer to generate the all-carbon stereogenic C2' center, along with different approaches to control the selectivity of the N-glycosidic bond. Intramolecular SN2-like cyclization of a mixture of acyclic thioaminals provided analogues with a pyrimidine nucleobase. A kinetic bias favoring cyclization of the 1',2'-anti thioaminal furnished the desired ß-D-4'-thionucleoside analogue in a 7:1 ratio. DFT calculations suggest that this kinetic resolution originates from additional steric clash in the SN2-like transition state for 1',4'-trans isomers, causing a significant decrease in their reaction rate relative to 1',4'-cis counterparts. N-glycosylation of cyclic glycosyl donors with a purine nucleobase enabled the formation of novel 2-chloroadenine 4'-thionucleoside analogues. These proprietary molecules and other derivatives are currently being evaluated both in vitro and in vivo to establish their biological profiles.

Sialyl LewisX glycomimetics bearing an extended anionic chain targeting E- and P- selectin binding sites.

Neutrophil binding to vascular P- and E-selectin is the rate-limiting step in the recruitment of immune cells to sites of inflammation. Many diseases, including sickle cell anemia, post-myocardial infarction reperfusion injury, and acute respiratory distress syndrome are characterized by dysregulated inflammation. We have recently reported sialyl Lewisx analogues as potent antagonists of P- and E-selectin and demonstrated their in vivo immunosuppressive activity. A key component of these molecules is a tartrate diester that serves as an acyclic tether to orient the fucoside and the galactoside moiety in the required gauche conformation for optimal binding. The next stage of our study involved attaching an extended carbon chain onto one of the esters. This chain could be utilized to tether other pharmacophores, lipids, and contrast agents in the context of enhancing pharmacological applications through the sialyl Lewisx / receptor-mediated mechanism. Herein, we report our preliminary studies to generate a small library of tartrate based sialyl Lewisx analogues bearing extended carbon chains. Anionic charged chemical entities are attached to take advantage of proximal charged amino acids in the carbohydrate recognition domain of the selectin receptors. Starting with a common azido intermediate, synthesized using copper-catalyzed Huisgen 1,3-dipolar cycloadditions, these molecules demonstrate E- and P-selectin binding properties.

Synthesis of Sialyl LewisX Mimetics with E- and P-Selectin Binding Properties and Immunosuppressive Activity.

E- and P-selectins are adhesion proteins implicated in immune cell recruitment at sites of infection, making them important drug targets for diseases involving excessive and uncontrolled inflammation. In this study, we developed an efficient strategy to synthesize bicyclic galactopyranosides through a key stereoselective equatorial C4-propiolate addition and TMSCN axial C-glycosidation. The nitrile group can then be converted to the carboxyl and different bioisosteres at a late stage in the synthesis, allowing for various derivatizations to potentially enhance biological activity. The sialyl LewisX glycomimetic featuring this rigidified bicyclic galactopyranoside moiety prevents neutrophil adhesion to endothelial cells in vitro by binding to both E- and P-selectins. We show here that the axial carboxyl analogue blocks immune cell recruitment in vivo, demonstrating its potential as an immunomodulator.

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.

Identification of a C3'-nitrile nucleoside analogue inhibitor of pancreatic cancer cell line growth.

A synthetic strategy to access a novel family of nucleoside analogues bearing a C3'-nitrile substituted all-carbon quaternary center is presented herein. These purine bearing scaffolds were tested in two pancreatic cancer cell lines harboring either wild-type (BxPC3) or G12V KRAS (Capan2) mutations. A promising compound was shown to have significantly greater efficacy in the Capan2 cell line as compared to Gemcitabine, the clinical gold standard used to treat pancreatic cancer.

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.

Photoredox-Catalyzed Stereoselective Radical Reactions to Synthesize Nucleoside Analogues with a C2'-Stereogenic All-Carbon Quaternary Center.

The design of novel nucleoside analogues bearing a C2' all-carbon quaternary center is described. The construction of this all-carbon stereogenic center involves the use of photoredox catalysis to initiate an intramolecular attack of a silyl-tethered vinyl functionality on a tertiary radical. Density functional theory calculations were performed to explore the origin of the high syn diastereoselectivity obtained through the preferred 5-exo-trig cyclization mode. The intramolecular vinyl addition also enables the preparation of the complementary configuration of the C2' all-carbon stereocenter when performed after lactonization.

Synthesis of Sialyl LewisX Glycomimetics Bearing a Bicyclic 3- O,4- C-Fused Galactopyranoside Scaffold.

Reported herein is the synthesis of sialyl LewisX analogues bearing a trans-bicyclo[4.4.0] dioxadecane-modified 3- O,4- C-fused galactopyranoside scaffold that locks the carboxylate pharmacophore in either the axial or equatorial position. This novel series of bicyclic galactopyranosides are prepared through a stereocontrolled intramolecular cyclization reaction that has been evaluated both experimentally and by density functional theory calculations. The cyclization precursors are obtained from ß-d-galactose pentaacetate in a nine-step sequence featuring a highly diastereoselective equatorial alkynylation and Cu(I) catalyzed formation of the acetylenic α-ketoester moiety. Preliminary biological evaluations indicate improved activity as P-selectin antagonists for the axially configured analogues as compared to their equatorial counterparts.

Diastereoselective Radical Hydrogen Transfer Reactions using N-Heterocyclic Carbene Boranes.

Reported herein are the first diastereoselective and Lewis acid-mediated radical reactions of N-heterocyclic carbene (NHC) boranes. We applied these reactions to the synthesis of four propionate diastereoisomers combining an aldol reaction, followed by a stereoselective radical-based reduction in which the NHC borane serves as the hydrogen donor, thus obviating the use of tin-based reagents. The 2,3-syn isomer is obtained by combining an NHC-borane and a Lewis acid (MgBr2·OEt2), while using a reverse polarity strategy provides the 2,3-anti isomer.

Diastereoselective Synthesis of C2'-Fluorinated Nucleoside Analogues Using an Acyclic Approach.

Nucleoside analogues bearing a fluorine in the C2'-position have been synthesized by SN2-like cyclizations of acyclic thioaminal precursors. This strategy provides access to two scaffolds, d-1',2'-cis-thiofuranosides and d-1',2'-trans-furanosides, which are difficult to generate using the standard approach for nucleoside synthesis. The addition of silylated nucleobases onto model C2-fluorinated dithioacetal substrates resulted in 1,2-syn diastereoselectivity, which is consistent with the C2-F and S-alkyl moiety being in close proximity. A new series of analogues bearing a C3' all-carbon quaternary center along with a C2'-F atom have also been synthesized using this approach and are being investigated as potential antimetabolites.

Dual-face nucleoside scaffold featuring a stereogenic all-carbon quaternary center. Intramolecular silicon tethered group-transfer reaction.

The design of a novel nucleoside scaffold that exhibits an all-carbon quaternary center is reported. This allows for both α- and ß-anomers of a given 2'-deoxy-2',2'-difluoro nucleoside analog (NA) to have potential biological activity. Using an intramolecular atom-transfer reaction, an all-carbon quaternary center was obtained without the use of heavy metals and/or harsh conditions. The chemistry developed is efficient, easily scalable and leads to novel libraries of molecules.

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,ß-bis-alkoxydithioacetals.

Acyclic tethers mimicking subunits of polysaccharide ligands: selectin antagonists.

We report on the design and synthesis of molecules having E- and P-selectins blocking activity both in vitro and in vivo. The GlcNAc component of the selectin ligand sialyl Lewis(X) was replaced by an acyclic tether that links two saccharide units. The minimization of intramolecular dipole-dipole interactions and the gauche effect would be at the origin of the conformational bias imposed by this acyclic tether. The stereoselective synthesis of these molecules, their biochemical and biological evaluations using surface plasmon resonance spectroscopy (SPR), and in vivo assays are described. Because the structure of our analogues differs from the most potent E-selectin antagonists reported, our acyclic analogues offer new opportunities for chemical diversity.

Diastereoselective hydrogen-transfer reactions: an experimental and DFT study.

Radical reductions of halogenated precursors bearing a heterocycle exo (α) to the carbon-centered radical proceed with enhanced anti-selectivity, a phenomenon that we termed "exocyclic effect". New experimental data and DFT calculations at the BHandHLYP/TZVP level demonstrate that the origin of the exocyclic effect is linked to the strain energy required for a radical intermediate to reach its reactive conformation at the transition state (ΔE(≠)(strain)). Furthermore, radical reductions of constrained THP systems indicate that high 2,3-anti inductions are reached only when the radical chain occupies an equatorial orientation. Hydride deliveries to different acyclic substrates and calculations also suggest that the higher anti-selectivities obtained with borinate intermediates are not related to the formation of a complex mimicking an exocycle. From a broader standpoint, this study reveals important conformational factors for reactions taking place at a center vicinal to a heterocycle or an α-alkoxy group.

A study of exocyclic radical reductions of polysubstituted tetrahydropyrans.

Exocyclic radical reductions were thoroughly investigated in the context of the synthesis of polysubstituted tetrahydropyrans, which are found in numerous macrolides. The radical precursors studied herein were generated by tandem cycloetherification and iodoetherification reactions or, alternatively, by semicyclic acetals substitutions. DFT calculations (BHandHLYP/TZVP) performed at the transition-state level for the hydrogen radical delivery are in good accordance with the experimental data and enabled the identification of important conformational factors that govern the selectivities obtained. This study demonstrates that both the preferred reactive conformation of the radical and steric interactions with the incoming hydride have to be considered in order to fully rationalize the levels of diastereoselection generated in acyclic free-radical processes.

Study of the endocyclic versus exocyclic C-O bond cleavage pathways of α- and ß-methyl furanosides.

The activation and ring-opening of methyl furanosides in the four natural sugar scaffolds (ribo, lyxo, arabino, and xylo) efficiently afforded acyclic thioacetals with high S(N)2-like selectivity at the acetal center in the presence of Me2BBr and thiophenol. The stereochemical outcome of these reactions provides important mechanistic insights into the activation pathway of five-membered semicyclic acetals. The thioacetal products should find applications in oligosaccharides synthesis and allow further development of acyclic strategies for the synthesis of novel nucleoside analogues.

A stereoselective approach to ß-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 ß-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.

A new approach to explore the binding space of polysaccharide-based ligands: selectin antagonists.

The discovery of molecules that interfere with the binding of a ligand to a receptor remains a topic of great interest in medicinal chemistry. Herein, we report that a monosaccharide unit of a polysaccharide ligand can be replaced advantageously by a conformationally locked acyclic molecular entity. A cyclic component of the selectin ligand Sialyl Lewis(x), GlcNAc, is replaced by an acyclic tether, tartaric esters, which link two saccharide units. The conformational bias of this acyclic tether originates from the minimization of intramolecular dipole-dipole interaction and the gauche effect. The evaluation of the binding of these derivatives to P-selectin was measured by surface plasmon resonance spectroscopy. The results obtained in our pilot study suggest that the discovery of tunable tethers could facilitate the exploration of the carbohydrate recognition domain of various receptors.

A bidirectional approach to the synthesis of polypropionates: synthesis of C1-C13 fragment of zincophorin and related isomers.

The structure-activity study of a bioactive natural product containing polypropionate subunits requires that its stereoisomers also be evaluated. Therefore, a general approach to synthesize these motifs is necessary. We describe herein the synthesis of the C1-C13 polypropionate subunit of zincophorin and isomers thereof using a two-reaction sequence: an aldol reaction using a mixture of tetrasubstituted enoxysilanes and a hydrogen-transfer reaction, both under Lewis acid control. Selection of the appropriate Lewis acid dictates the stereochemical outcome of these reactions. From a tactical standpoint, this study shows how a polypropionate sequence can be read and constructed in two directions, either the east-west or the west-east approaches. The choice of the optimal route is influenced by the number of complexation sites that can interfere in the aldol step under bidentate Lewis acid control.

Synthesis of 1',2'-cis-nucleoside analogues: evidence of stereoelectronic control for SN2 reactions at the anomeric center of furanosides.

We are reporting a highly diastereoselective route to 1',2'-cis-nucleoside analogues in the D-ribo, D-lyxo, D-xylo, and D-arabinoside series. Five-membered ring lactols undergo highly selective N-glycosidation reactions in the presence of dimethylboron bromide with different silylated nucleobases. Stereoelectronic control plays a crucial role for the observed induction, and the products are proposed to be formed through S(N)2 "exploded" transition states. This approach shows great potential considering its simplicity and selectivity for the synthesis of nucleoside analogues, an important class of molecules in medicinal chemistry.