Asymmetric Synthesis of (-)-6'-β-Fluoro-aristeromycin via Stereoselective Electrophilic Fluorination

Gilman reagent toward the synthesis of natural products

With the ever-increasing scope of organocuprates, a well-established Gilman reagent has been considered as an unprecedented synthetic tool in modern organic chemistry. The broad research profile of the Gilman reagent (R2CuLi in THF or Et2O) is owing to its propensity to carry out three kinds of reactions, i.e., epoxide ring opening reactions, 1,4-conjugate addition reactions, and SN2 reactions in a regioselective manner. This review examines the applications of Gilman reagent in the total synthesis of both abundant and scarce natural products of remarkable synthetic pharmaceutical profile reported since 2011. The presented insights will be of a vital roadmap to general organic synthesis and it will contribute to the development of new natural products and their analogues in future drug discovery.

Stereoselective Approach for the Synthesis of Diverse 1'-Modified Carbanucleosides

We describe an efficient and stereoselective synthesis of 1'-substituted-β-carbocylic nucleosides 5 via gem-dichlorooxirane intermediate 7, which directly condensed with weak nucleophiles such as pyrimidines or purines. The formation of gem-dichlorooxirane 7 and direct nucleobase condensation exclusively proceeded in protic polar solvents like MeOH. This method provides a general and modular route for the late-stage diversification of 1'-modified nucleosides.

Synthesis of Enantiomerically Pure Pyrimidine Ribonucleosides Locked in the South Conformation

The conformation of the central five-membered ring of a nucleoside plays an important role in enzyme recognition. Bicyclo[3.1.0]hexane, also known as the methanocarba (MC), serves as a template that can mimic the locked forms of the two distinctive conformations, namely, the north and south conformations. While modified nucleosides locked in the north conformation have been actively investigated, the south counterpart remains largely unexplored because it is difficult to synthesize. Herein, we report a concise synthetic route that can provide the key amino sugar intermediate essential for the synthesis of (S)-MC ribonucleosides in a 100% stereoselective manner. Also, through the proposed synthetic approach, we report the first synthesis of enantiomerically pure (S)-MC cytidine 1. We believe our findings would greatly contribute to the field of nucleoside chemistry and provide opportunities for novel nucleoside discovery.

Synthesis of fluorinated carbocyclic pyrimidine nucleoside analogues

Analogues of the canonical nucleosides have a longstanding presence and proven capability within medicinal chemistry and drug discovery research. The synthesis reported herein successfully replaces furanose oxygen with CF₂ and CHF in pyrimidine nucleosides, granting access to an alternative pharmacophore space. Key diastereoselective conjugate addition and fluorination methodologies are developed from chiral pool materials, establishing a robust gram-scale synthesis of 6'-(R)-monofluoro- and 6'-gem-difluorouridines. Vital intermediate stereochemistries are confirmed using X-ray crystallography and NMR analysis, providing an indicative conformational preference for these fluorinated carbanucleosides. Utilising these 6'-fluorocarbauridine scaffolds enables synthesis of related cytidine, ProTide and 2'-deoxy analogues alongside a preliminary exploration of their biological capabilities in cancer cell viability assays. This synthetic blueprint offers potential to explore fluorocarbanucleoside scaffolds, indicatively towards triphosphate analogues and as building blocks for oligonucleotide synthesis.

Fluorinated Nucleosides: Synthesis, Modulation in Conformation and Therapeutic Application

Over the last twenty years, fluorination on nucleoside has established itself as the most promising tool to use to get biologically active compounds that could sustain the clinical trial by affecting the pharmacodynamics and pharmacokinetic properties. Due to fluorine's inherent unique properties and its judicious introduction into the molecule, makes the corresponding nucleoside metabolically very stable, lipophilic, and opens a new site of intermolecular binding. Fluorination on various nucleosides has been extensively studied as a result, a series of fluorinated nucleosides come up for different therapeutic uses which are either approved by the FDA or under the advanced stage of the clinical trial. Here in this review, we are summarizing the latest development in the chemistry of fluorination on nucleoside that led to varieties of new analogs like carbocyclic, acyclic, and conformationally biased nucleoside and their biological properties, the influence of fluorine on conformation, oligonucleotide stability, and their use in therapeutics.

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.

Identification of 6'-β-fluoro-homoaristeromycin as a potent inhibitor of chikungunya virus replication

We have reported on aristeromycin (1) and 6′-fluorinated-aristeromycin analogues (2), which are active against RNA viruses such as Middle East respiratory syndrome coronavirus (MERS-CoV), severe acute respiratory syndrome coronavirus (SARS-CoV), Zika virus (ZIKV), and Chikungunya virus (CHIKV). However, these exhibit substantial cytotoxicity. As this cytotoxicity may be attributed to 5′-phosphorylation, we designed and synthesized one-carbon homologated 6′-fluorinated-aristeromycin analogues. This modification prevents 5′-phosphorlyation by cellular kinases, whereas the inhibitory activity towards S-adenosyl-l-homocysteine (SAH) hydrolase will be retained. The enantiomerically pure 6′-fluorinated-5′-homoaristeromycin analogues 3a-e were synthesized via the electrophilic fluorination of the silyl enol ether with Selectfluor, using a base-build up approach as the key steps. All synthesized compounds exhibited potent inhibitory activity towards SAH hydrolase, among which 6′-β-fluoroadenosine analogue 3a was the most potent (IC₅₀ = 0.36 μM). Among the compounds tested, 6′-β-fluoro-homoaristeromycin 3a showed potent antiviral activity (EC₅₀ = 0.12 μM) against the CHIKV, without noticeable cytotoxicity up to 250 μM. Only 3a displayed anti-CHIKV activity, whereas both3a and 3b inhibited SAH hydrolase with similar IC₅₀ values (0.36 and 0.37 μM, respectively), which suggested that 3a’s antiviral activity did not merely depend on the inhibition of SAH hydrolase. This is further supported by the fact that the antiviral effect was specific for CHIKV and some other alphaviruses and none of the homologated analogues inhibited other RNA viruses, such as SARS-CoV, MERS-CoV, and ZIKV. The potent inhibition and high selectivity index make 6′-β-fluoro-homoaristeromycin (3a) a promising new template for the development of antivirals against CHIKV, a serious re-emerging pathogen that has infected millions of people over the past 15 years.

Design, Synthesis, and Anti-RNA Virus Activity of 6'-Fluorinated-Aristeromycin Analogues

The 6'-fluorinated aristeromycins were designed as dual-target antiviral compounds aimed at inhibiting both the viral RNA-dependent RNA polymerase (RdRp) and the host cell S-adenosyl-l-homocysteine (SAH) hydrolase, which would indirectly target capping of viral RNA. The introduction of a fluorine at the 6'-position enhanced the inhibition of SAH hydrolase and the activity against RNA viruses. The adenosine and N⁶-methyladenosine analogues 2a-e showed potent inhibition against SAH hydrolase, while only the adenosine derivatives 2a-c exhibited potent antiviral activity against all tested RNA viruses such as Middle East respiratory syndrome-coronavirus (MERS-CoV), severe acute respiratory syndrome-coronavirus, chikungunya virus, and/or Zika virus. 6',6'-Difluoroaristeromycin (2c) showed the strongest antiviral effect for MERS-CoV, with a ∼2.5 log reduction in infectious progeny titer in viral load reduction assay. The phosphoramidate prodrug 3a also demonstrated potent broad-spectrum antiviral activity, possibly by inhibiting the viral RdRp. This study shows that 6'-fluorinated aristeromycins can serve as starting points for the development of broad-spectrum antiviral agents that target RNA viruses.

An efficient synthesis of fluoro-neplanocin A analogs using electrophilic fluorination and palladium-catalyzed dehydrosilylation

An alternative and efficient approach to neplanocin A analogs 1b and 1d has been developed using electrophilic fluorination and Pd-catalyzed dehydrosilylation.