Design, chemical synthesis and antiviral evaluation of 2'-deoxy-2'-fluoro-2'-C-methyl-4'-thionucleosides.

Nucleoside analogues represent an historically accomplished class of antiviral drug. Notwithstanding this, new molecular scaffolds are required to overcome their limitations and evolve pharmacophore space within this established field. Herein, we develop concise synthetic access to a new 2'-deoxy-2'-fluoro-2'-C-methyl-4'-thionucleoside chemotype, including the ProTide form of the uridine analogue. Biological evaluation of these materials in the Hepatitis C replicon assay shows little activity for the canonical pyrimidine forms, but the phosphoramidate of 2'-deoxy-2'-fluoro-2'-C-methyl-ß-d-4'-thiouridine has an EC50 of 2.99 µM. Direct comparison to the established Hepatitis C drug Sofosbuvir shows a 100-fold drop in activity upon substituting the furanose chalcogen; the reasons for this are as yet unclear.

8-Mercaptoguanine-based inhibitors of Mycobacterium tuberculosis dihydroneopterin aldolase: synthesis, in vitro inhibition and docking studies.

The dihydroneopterin aldolase (DHNA, EC 4.1.2.25) activity of FolB protein is required for the conversion of 7,8-dihydroneopterin (DHNP) to 6-hydroxymethyl-7,8-dihydropterin (HP) and glycolaldehyde (GA) in the folate pathway. FolB protein from Mycobacterium tuberculosis (MtFolB) is essential for bacilli survival and represents an important molecular target for drug development. S8-functionalized 8-mercaptoguanine derivatives were synthesised and evaluated for inhibitory activity against MtFolB. The compounds showed IC50 values in the submicromolar range. The inhibition mode and inhibition constants were determined for compounds that exhibited the strongest inhibition. Additionally, molecular docking analyses were performed to suggest enzyme-inhibitor interactions and ligand conformations. To the best of our knowledge, this study describes the first class of MtFolB inhibitors.

Recent Advances in the Chemical Synthesis and Evaluation of Anticancer Nucleoside Analogues.

Nucleoside analogues have proven to be highly successful chemotherapeutic agents in the treatment of a wide variety of cancers. Several such compounds, including gemcitabine and cytarabine, are the go-to option in first-line treatments. However, these materials do have limitations and the development of next generation compounds remains a topic of significant interest and necessity. Herein, we discuss recent advances in the chemical synthesis and biological evaluation of nucleoside analogues as potential anticancer agents. Focus is paid to 4'-heteroatom substitution of the furanose oxygen, 2'-, 3'-, 4'- and 5'-position ring modifications and the development of new prodrug strategies for these materials.

Synthesis of 4'-Thionucleosides as Antitumor and Antiviral Agents.

Many attempts have been made to synthesize structurally novel nucleoside derivatives in order to identify effective compounds for the treatment of tumors and virus-caused disease. At our laboratories, as part of our efforts to synthesize 4'-thionucleosides, we have identified and characterized biologically active nucleosides. During the course of our synthetic study, we developed the Pummerer-type thioglycosylation reaction. As a result, we synthesized a potent antineoplastic nucleoside, 1-(2-deoxy-2-fluoro-ß-D-4-thio-arabino-furanosyl)cytosine (4'-thioFAC), and several novel 4'-thionucleosides that possess antiherpes virus activities.

Absorption Characteristics and Quantum Yields of Singlet Oxygen Generation of Thioguanosine Derivatives.

6-Thioguanine (1a) is considered to be photochemotherapeutic due to its specific characteristics of photosensitivity to UVA light and singlet molecular oxygen generation. To extend its phototherapeutic ability, two related thioguanines, 8-thioguanine (2a) and 6,8-dithioguanine (3a), have been designed and explored. Since the solubility of these thioguanines in dehydrated organic solvents is too poor to study, their triacetyl-protected ribonucleosides, that is, 2',3',5'-tri-O-acetyl-6-thioguanosine (1c), 2',3',5'-tri-O-acetyl-8-thioguanosine (2c) and 2',3',5'-tri-O-acetyl-6,8-dithioguanosine (3c) were prepared and investigated. The absorption maxima of 1c, 2c and 3c in acetonitrile were found at longer wavelengths than that of unthiolated guanosine (4c). Especially, 3c has the longest wavelength for absorption maximum and the highest value in terms of molar absorption coefficient among all thionucleobases and thionucleosides reported. These absorption properties were also well reproduced by quantum chemical calculations. Quantum yields of singlet oxygen generation of 2c and 3c were determined by near-infrared emission measurements to be as large as that of 1c. These results suggest that the newly synthesized thioguanosines, in particular 3c, can be further developed as a potential photosensitive agent for light-induced therapies.

Practical Synthesis of 4'-Thioribonucleosides from L-Arabinose via Novel Reductive Ring-Contraction Reaction and Pummerer-Type Thioglycosylation.

The detailed practical synthesis of 4'-thionucleosides starting from L-arabinose is described here. 1,4-Anhydro-2,3-O-isopropylidene-4-thioribitol, which is the key intermediate for the synthesis of 4'-thionucleosides, is obtained from L-arabinose in several steps, including a novel reductive ring-contraction reaction. After oxidation of the key intermediate, the sulfoxide is subjected to Pummerer-type thioglycosylation in the presence of persilylated nucleobases to obtain the 4'-thioribonucleosides in good yield and ß-selectively. © 2017 by John Wiley & Sons, Inc.

Vibration Ball Milling for the Synthesis of 5'-Thioadenosine 5'-Pyrophosphate (P'→5') Adenosine (dASppA).

Using vibration ball milling, 5'-chloro-5'-deoxyadenosine (CldA) reacts cleanly with 4-methoxybenzyl mercaptan (MobSH), under basic conditions, to the corresponding thioether (MobSdA), which is isolated following precipitation and trituration. Under acidic conditions, in a one-pot, two-step process, MobSdA is transformed into 5'-deoxy-5'-(5-nitropyridyl-2-disulfanyl)-adenosine (NPySSdA). Michaelis-Arbuzov (M-A) reaction of NPySSdA with tris(trimethylsilyl) phosphite proceeds to completion within 30 min as determined by 31 P NMR, and the persilylated M-A product thus formed can be stored in solution under anhydrous conditions at room temperature for several days (in contrast, the anionic phosphorothiolate monoester is labile to hydrolysis). Following evaporation, mechanochemical mixing of the crude M-A product with the nucleotide donor adenosine 5'-monophosphomorpholidate under acidic activation in the presence of additional water gives rapid hydrolytic desilylation and phosphate coupling, so that essentially complete reaction is observed after 90 min and dASppA isolated following C-18 reversed phase HPLC and desalting (>99% pure as determined by monitoring at 260 nm). © 2017 by John Wiley & Sons, Inc.

Design, synthesis, and anticancer activity of C8-substituted-4'-thionucleosides as potential HSP90 inhibitors.

A series of C8-substituted-4'-thioadenosine analogs 3a-3g, 15, and 17 and their truncated derivatives 4a-4j, 23-25, and 27 have been successfully synthesized from d-ribose and d-mannose, respectively, employing Pummerer type or Vorbrüggen condensation reactions and the functionalization at the C8-position of nucleobase via Stille coupling or nucleophilic aromatic substitution reactions as key steps. All the synthesized compounds were assayed for their HSP90 inhibitory activity, but they were found to be inactive up to 100µM. However, the 8-iodo derivatives 15, 17, and 27 exhibited potent anticancer activity, indicating that different mechanism of action might be involved in their biological activity.

Novel Nucleoside Analogues: First Synthesis of Pyridine-4-Thioglycosides and Their Cytotoxic Evaluation.

The reaction of sodium 2,2-dicyanoethene-1,1-bis(thiolate) with 2-cyano-N-arylacetamides afforded sodium pyridine-4-thiolates, coupling of the latters with 2,3,4,6-tetra-O-acetyl-D-gluco- and D-galactopyranosyl bromides, respectively, afforded new pyridine-4-thioglycosides. Ammonolysis of the latter compounds afforded the free thioglycosides. The antitumor activities of the synthesized compounds were tested against human tumor cell lines; lung (A549), colon (HCT116), liver (HEPG2), and prostate (PC3).

The 5'-nor aristeromycin analogues of 5'-deoxy-5'-methylthioadenosine and 5'-deoxy-5'-thiophenyladenosine.

To extend the potential of 5'-noraristeromycin (and its enantiomer) as potential antiviral candidates, the enantiomers of the carbocyclic 5'-nor derivatives of 5'-methylthio-5'-deoxyadenosine and 5'-phenylthio-5'-deoxyadenosine have been synthesized and evaluated. None of the compounds showed meaningful antiviral activity.

Antiherpesvirus activities of two novel 4'-thiothymidine derivatives, KAY-2-41 and KAH-39-149, are dependent on viral and cellular thymidine kinases.

The emergence of drug-resistant herpesviruses represents a significant problem in clinical practice, primarily in immunocompromised patients. Furthermore, effective antiviral therapies against gammaherpesvirus-associated diseases are lacking. Here, we present two thiothymidine derivatives, KAY-2-41 and KAH-39-149, with different spectra of antiviral activity from those of the reference antiherpetic drugs, showing inhibitory activities against herpes simplex virus, varicella-zoster virus (VZV), and particularly against Epstein-Barr virus, with high selectivity in vitro. While KAY-2-41- and KAH-39-149-resistant herpesviruses were found to harbor mutations in the viral thymidine kinase (TK), these mutations conferred only low levels of resistance to these drugs but high levels to other TK-dependent drugs. Also, antiviral assays in HeLa TK-deficient cells showed a lack of KAY-2-41 and KAH-39-149 activities against herpes simplex virus 1 (HSV-1) and HSV-2 TK-deficient mutants. Furthermore, enzymatic TK assays showed the ability of HSV-1 TK, VZV TK, and cellular TK1 and TK2 to recognize and phosphorylate KAY-2-41 and KAH-39-149. These results demonstrate that the compounds depend on both viral and host TKs to exert antiviral activity. Additionally, the antiviral efficacy of KAH-39-149 proved to be superior to that of KAY-2-41 in a mouse model of gammaherpesvirus infection, highlighting the potential of this class of antiviral agents for further development as selective therapeutics against Epstein-Barr virus.

Synthesis and cytotoxic evaluation of novel pyrimidine deoxyapiothionucleosides.

The synthesis of novel pyrimidine deoxyapiothionucleosides of D- and L-series was realized following application of a versatile and high-yielding scheme, which utilized inexpensive L- and D-arabinose as starting materials, respectively, and which makes use of a regio- and stereo-selective Pummerer rearrangement reaction for the coupling of the nucleobase with the thiosugar moiety. Some of the targeted compounds have shown selective cytotoxic effects (with IC50<10 µM) against specific cancer cell lines. All of the tested compounds had no cytotoxic effect on the normal cell line tested.

An access to the ß-anomer of 4'-thio-C-ribonucleosides: hydroboration of 1-C-aryl- or 1-C-heteroaryl-4-thiofuranoid glycals and its regiochemical outcome.

We have developed a novel method for the synthesis of the ß-anomer of 4'-thio-C-ribonucleosides from 3,5-O-(di-tert-butylsilylene)-4-thiofuranoid glycal. Palladium-catalyzed coupling of 1-tributylstannyl-4-thiofuranoid glycal with iodobenzene or a heteroaryl halide gave 1-C-phenyl- or 1-C-heteroaryl-glycals. Hydroboration of these glycals proceeded at the α-face, and subsequent alkaline hydrogen peroxide treatment of the resulting 2'-α-borane furnished the respective ß-anomer of 4'-thio-C-ribonucleosides. These results demonstrate that this synthetic method has a wider scope in terms of heterocyclic base structure. During this study, unexpected Markovnikov-oriented hydroboration has been observed to lead to the respective 1'-α-boranes. These 1'-boranes were converted into either the ring-opened structure or the 2'-deoxy derivatives depending upon their stability.

Design, synthesis, and binding of homologated truncated 4'-thioadenosine derivatives at the human A3 adenosine receptors.

We synthesized homologated truncated 4'-thioadenosine analogues 3 in which a methylene (CH(2)) group was inserted in place of the glycosidic bond of a potent and selective A(3) adenosine receptor antagonist 2. The analogues were designed to induce maximum binding interaction in the binding site of the A(3) adenosine receptor. However, all homologated nucleosides were devoid of binding affinity at all subtypes of adenosine receptors, indicating that free rotation through the single bond allowed the compound to adopt an indefinite number of conformations, disrupting the favorable binding interaction essential for receptor recognition.

Improved syntheses of 5'-S-(2-aminoethyl)-6-N-(4-nitrobenzyl)-5'-thioadenosine (SAENTA), analogues, and fluorescent probe conjugates: analysis of cell-surface human equilibrative nucleoside transporter 1 (hENT1) levels for prediction of the antitumor efficacy of gemcitabine.

5'-S-(2-aminoethyl)-6-N-(4-nitrobenzyl)-5'-thioadenosine (SAENTA), 5'-S-(2-acetamidoethyl)-6-N-[(4-substituted)benzyl]-5'-thioadenosine analogues, 5'-S-[2-(6-aminohexanamido)]ethyl-6-N-(4-nitrobenzyl)-5'-thioadenosine (SAHENTA), and related compounds were synthesized by S(N)Ar displacement of fluoride from 6-fluoropurine intermediates with 4-(substituted)benzylamines. Conjugation of the pendant amino groups of SAENTA and SAHENTA with fluorescein-5-yl isothiocyanate (FITC) gave fluorescent probes that bound at nanomolar concentrations specifically to human equilibrative nucleoside transporter 1 (hENT1) produced in recombinant form in model expression systems and in native form in cancer cell lines. Transporter binding effects were studied and the ability of the probes to predict the potential antitumor efficacy of 2'-deoxy-2',2'-difluorocytidine (gemcitabine) was demonstrated.

Beyond the Pummerer reaction: recent developments in thionium ion chemistry.

Since the early 1960s the Pummerer reaction has evolved to become an indispensable tool for synthesis, and continues to serve as a source of inspiration for organic chemists. In recent years, many exciting advances have demonstrated the broad scope and synthetic utility of Pummerer methodology and the versatility of thionium ion intermediates.

Synthesis and antitumor activity of 5'-deoxy-4'-thio-L-nucleosides.

A series of novel 5'-deoxy-4'-thio-L-nucleosides was designed and synthesized. The absolute configuration of the target compound 23alpha was confirmed by X-ray crystallography. The antitumor activities of the target compounds were tested against the growth of human carcinoma of colon (LOVO), human leukemia cell line (CEM) and human breast cancer cell line (MDA-MB-435) cells in vitro. 6-cyclopentylamino and 6-cyclohexylamino purine compounds 26 and 27, both in alpha-configuration and in beta-form, exhibited strong inhibition to CEM.

Synthesis and antitumor activity of novel 2',3'-dideoxy-2',3'-diethanethionucleosides bearing 1,2,3-triazole residues.

A series of novel 2',3'-dideoxy-2',3'-diethanethioribonucleosides and those modified with a triazole ring were prepared in excellent yields and their antitumor activity was evaluated. Nucleosides with a triazole ring, 16a-16c, showed significantly improved activity towards a broad range of tumor cell lines.

Synthesis and conformational analysis of d-2'-deoxy-2',2'-difluoro-4'-dihydro-4'-thionucleosides.

An efficient synthesis of d-2'-deoxy-2',2'-difluoro-4'-dihydro-4'-thionucleosides is described. The conformations of d-2'-deoxy-2',2'-difluoro-4'-dihydro-4'-thiouridine were studied by X-ray crystallography, NMR spectroscopy and molecular modeling in an attempt to explore the roles of the two gem-difluorine atoms in the puckering preferences of the thiosugar ring. No matter which conformation (south or north) the thiosugar adopts, there is always one fluorine in a pseudoaxial position, with the other in a pseudoequitorial position and thus the strong antiperiplanar (ap) effects from C-H and C-C sigma-bonds to sigma*C-F are equal to each other in these two conformers. Therefore, the other weak effects, such as dipole-dipole interactions and electrostatic attractions, become more important for determining the overall conformation of the sugar ring. Based on the results of NMR spectroscopy, high-level quantum computations and molecular dynamic simulations were performed to study the preferred pucker of the thiosugar ring in solution. Our results showed that the strong antiperiplanar preference of C-H and C-C sigma-bonds to sigma*C-F and sigma*C-O seemed to be responsible for the favored S-conformation in solution, and the weak electrostatic attractions between (delta+)C2-Fbeta(delta-) and (delta+)H6-C6(delta-) may stabilize the preferred structure further, and keep the base moiety in a high anti-rotamer population in solution. In contrast, the packing forces (hydrogen bond OH...O=C, dipole-dipole interaction C-F..C=O) in the solid state compensated the energetic disadvantage of the relatively less stable N-conformation, and drove the thiouridine to crystallize in the N-conformation. These results, along with the earlier empirical rules regarding proton chemical shifts in carbohydrates and nucleosides, were used to propose a method based on proton chemical shifts for the analysis of the N<==>S equilibrium of the fluorinated sugar ring.

Design and synthesis of N(6)-substituted-4'-thioadenosine-5'-uronamides as potent and selective human A(3) adenosine receptor agonists.

On the basis of a bioisosteric rationale, 4'-thionucleoside analogues of IB-MECA (N(6)-(3-Iodo-benzyl)-9-(5'-methylaminocarbonyl-beta-d-ribofuranosyl)adenine), which is a potent and selective A(3) adenosine receptor (AR) agonist, were synthesized from d-gulonic acid gamma-lactone. The 4'-thio analogue (5h) of IB-MECA showed extremely high binding affinity (K(i)=0.25 nM) at the human A(3)AR and was more potent than IB-MECA (K(i)=1.4 nM). Bulky substituents at the 5'-uronamide position, such as cyclohexyl and 2-methylbenzyl, in this series of 2-H nucleoside derivatives were tolerated in A(3)AR binding, although small alkyl analogues were more potent.