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

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 β-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.

Synthesis of 4-Deoxy-4-Fluoro-d-Sedoheptulose: A Promising New Sugar to Apply the Principle of Metabolic Trapping

Fluorinated carbohydrates are important tools for understanding the deregulation of metabolic fluxes and pathways. Fluorinating specific positions within the sugar scaffold can lead to enhanced metabolic stability and subsequent metabolic trapping in cells. This principle has, however, never been applied to study the metabolism of the rare sugars of the pentose phosphate pathway (PPP). In this study, two fluorinated derivatives of d-sedoheptulose were designed and synthesized: 4-deoxy-4-fluoro-d-sedoheptulose (4DFS) and 3-deoxy-3-fluoro-d-sedoheptulose (3DFS). Both sugars are taken up by human fibroblasts but only 4DFS is phosphorylated. Fluorination of d-sedoheptulose at C-4 effectively halts the enzymatic degradation by transaldolase and transketolase. 4DFS thus has a high potential as a new PPP imaging probe based on the principle of metabolic trapping. Therefore, the synthesis of potential radiolabeling precursors for 4DFS for future radiofluorinations with fluorine-18 is presented.

Origin of High Diastereoselectivity in Reactions of Seven-Membered-Ring Enolates

Unlike many reactions of their six-membered-ring counterparts, the reactions of chiral seven-membered-ring enolates are highly diastereoselective. Diastereoselectivity was observed for a range of substrates, including lactam, lactone, and cyclic ketone derivatives. The stereoselectivity arises from torsional and steric interactions that develop when electrophiles approach the diastereotopic π-faces of the enolates, which are distinguished by subtle differences in the orientation of nearby atoms of the ring.

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.

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...

De Novo Synthesis of Orthogonally-Protected C2-Fluoro Digitoxoses and Cymaroses: Development and Application for the Synthesis of Fluorinated Digoxin

Inspired by Roush's pioneering work on rare sugars, we have developed a scalable, stereoselective, de novo synthesis of orthogonally protected C2-fluoro digitoxose and cymarose, utilizing Sharpless kinetic resolution and organocatalytic fluorination as key steps. The utility of this strategy is demonstrated by the synthesis of a fluorinated analogue of digoxin, which indicates the fluorine on the sugar ring may have a significant impact on biological activity.

Chemical synthesis of 4'-thio and 4'-sulfinyl pyrimidine nucleoside analogues

Analogues of the canonical nucleosides required for nucleic acid synthesis have a longstanding presence and proven capability within antiviral and anticancer research. 4'-Thionucleosides, that incorporate bioisosteric replacement of furanose oxygen with sulfur, represent an important chemotype within this field. Established herein is synthetic capability towards a common 4-thioribose building block that enables access to thio-ribo and thio-arabino pyrimidine nucleosides, alongside their 4'-sulfinyl derivatives. In addition, this building block methodology is templated to deliver 4'-thio and 4'-sulfinyl analogues of the established anticancer drug gemcitabine. Cytotoxic capability of these new analogues is evaluated against human pancreatic cancer and human primary glioblastoma cell lines, with observed activities ranging from low μM to >200 μM; explanation for this reduced activity, compared to established nucleoside analogues, is yet unclear. Access to these chemotypes, with thiohemiaminal linkages, will enable a wider exploration of purine and triphosphate analogues and the application of such materials for potential resistance towards relevant hydrolytic enzymes within nucleic acid biochemistries.

Diastereoselective and regioselective synthesis of adenosine thionucleoside analogues using an acyclic approach

An acyclic approach to synthesize thiofuranoside N-glycosides bearing an adenine nucleobase is presented herein. This approach provides a significant improvement in terms of regio- and diastereoselectivity compared with the current paradigms used for their formation. Activation of acyclic dithioacetal substrates bearing either a C2′-alkoxy or fluoro group and coupling with purine nucleobases selectively generates 1′,2′-syn thioaminals. The regiochemistry of the nucleobase coupling (N7 or N9) can also be controlled by using either silylated or unsilylated purines. A subsequent SN2-like cyclization of these 1′,2′-syn acyclic thioaminals results in the desired 1′,2′-cis thionucleoside analogues.

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.

A simple graphical method to pinpoint local pseudosymmetries in Z' > 1 cases

An intuitive method is presented for detecting pseudosymmetries in Z' > 1 cases as a complement to well-proven strategies already available in the literature. It is based in the simple idea that the mid-points between equivalent atoms in symmetrically related molecules are disposed according to simple well-known patterns, which are easily recognizable by optical inspection. A number of Z' = 4 cases in the literature are analyzed, which allows some of the potentialities of the method to be revealed.

Fluorinated nucleosides as an important class of anticancer and antiviral agents

Fluorine-containing nucleoside analogs (NAs) represent a significant class of the US FDA-approved chemotherapeutics widely used in the clinic. The incorporation of fluorine into drug-like agents modulates lipophilic, electronic and steric parameters, thus influencing pharmacodynamic and pharmacokinetic properties of drugs. Fluorine can block oxidative metabolism of drugs and the formation of undesired metabolites by changing H-bonding interactions. In this review, we focus our attention on chemical fluorination reagents and methods used in the NAs field, including positron emission tomography radiochemistry. We briefly discuss both the cellular biology and clinical properties of FDA-approved and fluorine-containing nucleoside/nucleotide analogs in development as well as common resistance mechanisms associated with their use. Finally, we emphasize pronucleotide strategies used to improve therapeutic outcome of NAs in the clinic.