Reconstructive Methodology in the Synthesis of 2-Aminopurine

A fundamentally new synthetic approach to the synthesis of 2-aminopurine has been developed. It consists in the combination of the creation of a condensed polyazotic heterocyclic tetrazolopyrimidine structure, its transformation into triaminopyrimidine, and its subsequent cyclization into 2-aminopurine. The structure of the obtained compounds was established based on spectral characteristics, and the structure of the intermediate compound 5 was established directly by X-ray diffraction analysis.

The electrochemical behavior’s character of a potential antiviral drug 3-nitro-4-hydroxy-7-methylthio-4H-[1,2,4]triazolo[5,1-c][1,2,4]triazinide monohydrate

The results of this study of the electrochemical transformation of 3-R-4-hydroxy-1,4-dihydro-7-X-1,2,4-triazolo[5,1-c][1,2,4] obtained by voltammetry are presented. It was found that 3-R-4-hydroxy-1,4-dihydro-7-X-1,2,4-triazolo[5,1-c][1,2,4] derivatives are capable of electrochemical reduction in the potential range of –0.28 to –0.33 V (relative to Ag/AgCl) in Britton–Robinson buffer at pH = 2. The electrochemical behavior of the sodium salt of 3-nitro-4-hydroxy-7-methylthio-4H-[1,2,4]triazolo[5,1-c][1,2,4]triazinide monohydrate (compound 1), which in silico modeling predicted possible biological activity against various tick-borne encephalitis and Coxsackie B3 viruses. At the potentials of the first stage of electroreduction at pH = 2, the main transformation process is the three-electron reduction scheme of the nitro group of compound 1. It was established that compound 1 in an aprotic medium is reduced in ionic form, most likely in the form of an ion pair with the Na+ cation, and in an aqueous medium in the form of a protonated particle. Based on this, a scheme was proposed for the probable electrochemical transformation of the studied compound.

Triazavirin-A Novel Effective Antiviral Drug

This review outlines the data of numerous studies relating to the broad-spectrum antiviral drug Triazavirin that was launched on the Russian pharmaceutical market in 2014 as an anti-influenza drug (the international non-patented name is Riamilovir). The range of antiviral activity of Triazavirin has been significantly expanded during recent years; in particular, it has been shown that Triazavirin exhibits activity against tick-borne encephalitis, Rift Valley fever, West Nile fever, and other infections of viral etiology. This drug has been approved for treatment of influenza and acute respiratory infections by the Russian Ministry of Health on the basis of comprehensive clinical trials involving over 450 patients. Triazavirin was found to be a highly effective and well-tolerated drug, allowing its over-the-counter sale. The recently published data on the use of Triazavirin in clinical practice for the treatment of patients with COVID-19 are discussed, with special attention paid to potential biological targets for this drug.

4-(Aryl)-Benzo[4,5]imidazo[1,2-a]pyrimidine-3-Carbonitrile-Based Fluorophores: Povarov Reaction-Based Synthesis, Photophysical Studies, and DFT Calculations

A series of novel 4-(aryl)-benzo[4,5]imidazo[1,2-a]pyrimidine-3-carbonitriles were obtained through the Povarov (aza-Diels-Alder) and oxidation reactions, starting from benzimidazole-2-arylimines. Based on the literature data and X-ray diffraction analysis, it was discovered that during the Povarov reaction, [1,3] sigmatropic rearrangement leading to dihydrobenzimidazo[1,2-a]pyrimidines took place. The structures of all the obtained compounds were confirmed based on the data from ¹H- and ¹³C-NMR spectroscopy, IR spectroscopy, and elemental analysis. For all the obtained compounds, their photophysical properties were studied. In all the cases, a positive emission solvatochromism with Stokes shifts from 120 to 180 nm was recorded. Aggregation-Induced Emission (AIE) has been illustrated for compound 6c using different water fractions (fw) in THF. The compounds 6c and 6f demonstrated changes in emission maxima or/and intensities after mechanical stimulation.

Mechanochemical Approach towards Multi-Functionalized 1,2,3-Triazoles and Anti-Seizure Drug Rufinamide Analogs Using Copper Beads

Highly regiospecific, copper-salt-free and neat conditions have been demonstrated for the 1,3-dipolar azide-alkyne cycloaddition (AAC) reactions under mechanochemical conditions. A group of structurally challenging alkynes and heterocyclic derivatives was efficiently implemented to achieve highly functionalized 1,4-disubstituted-1,2,3-triazoles in good to excellent yield by using the Cu beads without generation of unwanted byproducts. Furthermore, the high-speed ball milling (HSBM) strategy has also been extended to the synthesis of the commercially available pharmaceutical agent, Rufinamide, an antiepileptic drug (AED) and its analogues. The same strategy was also applied for the synthesis of the Cl-derivative of Rufinamide. Analysis of the single crystal XRD data of the triazole was also performed for the final structural confirmation. The Cu beads are easily recoverable from the reaction mixture and used for the further reactions without any special treatment.

Searching for novel antagonists of adenosine A1 receptors among azolo[1,5-a]pyrimidine nitro derivatives

Introduction: Ligands of adenosine A1Rs are potential candidates for the development of drugs for the treatment of paroxysmal supraventricular tachycardia, angina pectoris, hypertriglyceridemia, type 2 diabetes mellitus, neuropathic pain, and heart failure. At the same time, there is a deficiency of drugs that can regulate the functions of A1 receptors. A number of A1-antagonists are at the various stages of clinical trials; other drugs are not very selective or are characterized by an insufficient breadth of their therapeutic action. Therefore, the search for new medicinal compounds for the prevention and treatment of A1-depended diseases among nitro derivatives of tetrazolo[1,5-a]pyrimidine and 1,2,4-triazolo[1,5-a]pyrimidine is of scientific interest.

Materials and methods: The search for active compounds was carried out by in silico and in vitro methods. At the first stage, a computer forecast of A1-antagonistic activity was carried out using the Microcosm BioS software. At the second stage, the prediction results were verified in vitro in a model of isolated mouse atria.

Results and discussion: Based on the results of the prediction by the method of maximum similarity to standards, the most active compounds III, VIII, and XVII were selected. After testing the prediction results by the isolated atria method, the compound VIII was characterized by A1-blocking effect in vitro at a concentration of 10 μmol/L.

Conclusion: The most promising compound with A1-blocking effect in vitro was identified; it is a derivative of tetrazolo[1,5-a]pyrimidine under the code of VIII. It is of interest for us for further in-depth study of its pharmacological properties.

Synthetic approaches to 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-ones as basic heterocyclic structures of the antiviral drug Riamilovir ("Triazavirin®") active against SARS-CoV-2 (COVID-19)

Fragments of 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-one are found in many compounds with various types of biological activities, including the antiviral drug Riamilovir (Triazavirin®), which shows activity against SARS-CoV-2 (COVID-19). Therefore, the development of convenient methods for the synthesis of new derivatives of 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-one is always in demand. This review systematizes the information on the most common synthetic methods for constructing the 1,2,4-triazolo[5,1-c][1,2,4]triazin-7-one heterocyclic system.

Azolo[1,5-a]pyrimidines and Their Condensed Analogs with Anticoagulant Activity

Hypercytokinemia, or cytokine storm, is one of the severe complications of viral and bacterial infections, involving the release of abnormal amounts of cytokines, resulting in a massive inflammatory response. Cytokine storm is associated with COVID-19 and sepsis high mortality rate by developing epithelial dysfunction and coagulopathy, leading to thromboembolism and multiple organ dysfunction syndrome. Anticoagulant therapy is an important tactic to prevent thrombosis in sepsis and COVID-19, but recent data show the incompatibility of modern direct oral anticoagulants and antiviral agents. It seems relevant to develop dual-action drugs with antiviral and anticoagulant properties. At the same time, it was shown that azolo[1,5-a]pyrimidines are heterocycles with a broad spectrum of antiviral activity. We have synthesized a new family of azolo[1,5-a]pyrimidines and their condensed polycyclic analogs by cyclocondensation reactions and direct CH-functionalization and studied their anticoagulant properties. Five compounds among 1,2,4-triazolo[1,5-a]pyrimidin-7-ones and 5-alkyl-1,3,4-thiadiazolo[3,2-a]purin-8-ones demonstrated higher anticoagulant activity than the reference drug, dabigatran etexilate. Antithrombin activity of most active compounds was confirmed using lipopolysaccharide (LPS)-treated blood to mimic the conditions of cytokine release syndrome. The studied compounds affected only the thrombin time value, reliably increasing it 6.5–15.2 times as compared to LPS-treated blood.

Regiospecific way to N9-alkylated thioxanthines

A regiospecific way to N9-alkylated thioxanthines as novel acyclic nucleoside analogues has been developed. This approach is based on a cleavage methodology involving the construction of a target heterocyclic scaffold...

Redox Conversions of 5-Methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4triazolo[1,5-a]pyrimidinide L-Arginine Monohydrate as a Promising Antiviral Drug

This article presents the results of a study of electrochemical transformations in aqueous and aprotic media of 5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a]pyrimidinide l-arginine monohydrate (1a, Triazid) obtained by electrochemical methods and ESR spectroscopy. The effect of pH on the current and the reduction potential of 1a in an aqueous Britton-Robinson buffer solution was studied. It was found that 1a is irreversibly reduced in aqueous acidic media on a glassy carbon electrode in one stage with the participation of six electrons and the formation of 5-methyl-6-amino-7-oxo-1,2,4-triazolo[1,5-a]pyrimidin. The electroreduction of 1a in DMF on a background of tetrabutylammonium salts proceeds in two stages, controlled by the kinetics of second-order reactions. In the first stage, the reduction of 1a is accompanied by protonation by the initial compound of the basic intermediate products formed in the electrode reaction (self-protonation mechanism). The second quasi-reversible stage of the electroreduction 1a corresponds to the formation of a dianion radical upon the reduction of the heterocyclic anion 5-methyl-6-nitro-7-oxo-4,7-dihydro-1,2,4-triazolo[1,5-a]pyrimidin, which is formed upon the potentials of the first peak. The ESR spectrum of the radical dianion was recorded upon electroreduction of Triazid in the presence of Bu4NOH. The effect of the formation of ion pairs on the reversibility of the second peak of the 1a transformation is shown. A change in the rate and regioselectivity of the protonation of the dianion radical in the presence of Na+ and Li+ ions is assumed. The results of studying the electroreduction of 1a by ESR spectroscopy with a TEMPO trap make it possible to assume the simultaneous formation of both a nitroxyl radical and a radical with the spin density localized on the nitrogen at the 4 position of the six-membered ring.

Oxidative Aromatization of 4,7-Dihydro-6-nitroazolo[1,5-a]pyrimidines: Synthetic Possibilities and Limitations, Mechanism of Destruction, and the Theoretical and Experimental Substantiation

The reaction tolerance of the multicomponent process between 3-aminoazoles, 1-morpholino-2-nitroalkenes, and aldehydes was studied. The main patterns of this reaction have been established. Conditions for the oxidation of 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines were selected. Previous claims that the 4,7-dihydro-6-nitroazolo[1,5-a]pyrimidines could not be aromatised have now been refuted. Compounds with an electron-donor substituent at position seven undergo decomposition during oxidation. The phenomenon was explained based on experimental data, electro-chemical experiment, and quantum-chemical calculation. The mechanism of oxidative degradation has been proposed.

Benzimidazoazapurines: Design, Synthesis, and Photophysical Study

A highly efficient approach to a new class of polycyclic 8-azapurines, benzo[4,5]imidazo[1,2-a][1,2,3]triazolo[4,5-e]pyrimidines (BITPs), with good photophysical characteristics is proposed. The approach comprises condensation of aminobenzimidazoles with 3-oxo-2-phenylazopropionitrile to form 3-(arylazo)benzo[4,5]imidazo[1,2-a]pyrimidine-4-amines, which undergo oxidative cyclization by the catalytic action of copper(II) acetate, resulting in BITPs with 73-84% yield. Spectral investigations demonstrated the fluorescent properties of BITPs, exhibiting good quantum yields (up to 60%) with maxima absorption at 379-399 and emission at 471-505 nm.

Antiviral drug Triazavirin, selectively labeled with 2H, 13C, and 15N stable isotopes. Synthesis and properties

Isotope-labeled antiviral drug Triazavirin containing 2H, 13C, and 15N atoms in its structure has been synthesized. 13C2H3I and KS13CN served as donors of 13C isotopes. The use of 13С-MeI containing 2H atoms made it possible to additionally incorporate deuterium labels into the structure of the compound. The 15N atoms were incorporated using 15N-enriched sodium nitrite, aminoguanidine carbonate, and ethyl nitroacetate. The resulting 2H3,13C2,15N3-Triazavirin was characterized by NMR spectroscopy.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s10593-021-02927-1.

Methods of Synthesis and Antiviral Activity of New 4-Alkyl-3-Nitro-1,4-Dihydroazolo[5,1-c][1,2,4]Triazin-4-ols

An azo coupling reaction of α-nitro ketones with 5-diazoazoles was used to obtain 4-alkyl-3-nitro-1,4-dihydroazolo[5,1-с][1,2,4]triazines, which were characterized with respect to their antiviral activity against influenza and Coxsackie B3 viruses.

Aerosol Inhalation Delivery of Triazavirin in Mice: Outlooks for Advanced Therapy Against Novel Viral Infections

Under pandemic-caused emergency, evaluation of the potential of existing antiviral drugs for the treatment of COVID-19 is relevant. Triazavirin, an antiviral drug developed in Russia for per-oral administration, is involved in clinical trials against SARS-CoV-2 coronavirus. This virus has affinity to epithelial cells in respiratory tract, so drug delivery directly in lungs may enhance therapeutic effect and reduce side effects for stomach, liver, kidneys. We elaborated ultrasonic method of triazavirin aerosol generation and investigated the inhalation delivery of this drug in mice. Mean particle size and number concentration of aerosol used in inhalation experiments are 560 nm and 4 × 105 cm-3, respectively. Aerosol mass concentration is 1.6 × 10-4 mg/cm3. Inhalation for 20 min in a nose-only chamber resulted in 2 mg/kg body delivered dose and 2.6 μg/mL triazavirin concentration in blood plasma. Elimination rate constant determined in aerosol administration experiments was ke = 0.077 min-1, which agrees with the value measured after intravenous delivery, but per-oral administration resulted in considerably lower apparent elimination rate constant of pseudo-first order, probably due to non-linear dependence of absorption rate on triazavirin concentration in gastrointestinal tract. The bioavailability of triazavirin aerosol is found to be 85%, which is about four times higher than for per-oral administration.

A PASE Approach to the Synthesis of Benzimidazopurines as Polycondensed Purine Derivatives

A highly efficient PASE approach to a new class of polycyclic purine derivatives has been proposed. The strategy includes a consecutive reduction, auto-aromatization, and heterocyclization of the initial nitrobenzimidazopyrimidines obtained by a three-component condensation. It was shown that reduction of nitrobenzimidazopyrimidines by metals in acidic media was more efficient than heterogeneous hydro­genation. Novel derivatives of benz[4,5]imidazo[1,2-a]purines were obtained­ in good yields and the proposed structure was confirmed by X-ray crystal structure analysis. The obtained convergent benzimidazopurines combine two relevant medicinal chemistry scaffolds – benz­imidazole and purine.

Synthesis and biological evaluation of 6-nitro-1,2,4-triazoloazines containing polyphenol fragments possessing antioxidant and antiviral activity

Stable σ-adducts of azolo[5,1-c]triazines and azolo[1,5-a]pyrimidines with different polyphenols were synthesized and their antioxidant and antiviral activity were investigated. Their affinity to viral hemagglutinin was assessed using molecular modelling. The phloroglucinol-modified azolo-azines possessed the highest virus-inhibiting activity. According to the results of the study of antioxidant properties of compounds, the most promising ones exhibiting highest antioxidant capacity were adducts containing in their structure pyrogallol and catechol residues and 6-nitro-triazolotriazin-7-ol scaffold. No correlation between antioxidant and virus-inhibiting activity of compounds studied was detected. The most active compounds demonstrated the ability to prevent binding of viral hemagglutinin with cellular receptor as shown in hemagglutination inhibition assay. Our results demonstrate that polyphenol-modified azolo-azines are prospective for further optimization as potential antivirals and that their action is directed against viral hemagglutinin.

Tripod-type 2,2′-bipyridine ligand for lanthanide cations: synthesis and photophysical studies on coordination to transition metal cations

New tripod-type 2,2′-bipyridine ligand consisting of a central polyaminocarboxylic moiety for the coordination to lanthanide cations and three appended 5-phenyl-2,2′-bipyridine fragments for the coordination to various transition metal cations have been prepared. A europium complex of this ligand was prepared, and its photophysical properties and a luminescent response towards transition metal salts (particularly, CdI2, Cd(OAc)2, Zn(ClO4)2, Cu(OAc)2, and HgCl2) have been studied. Europium cation luminescence quenching in the presence of transition metal salts in solution was observed in all cases. In addition, it was observed that the fluorescent response of the europium complex was quite individual depending on the type of the metal salt. The obtained data were compared with the earlier published data for some lanthanide complexes bearing additional sites for the chelation of transition metal cations.

15N-Labelling and structure determination of adamantylated azolo-azines in solution

Determining the accurate chemical structures of synthesized compounds is essential for biomedical studies and computer-assisted drug design. The unequivocal determination of N-adamantylation or N-arylation site(s) in nitrogen-rich heterocycles, characterized by a low density of hydrogen atoms, using NMR methods at natural isotopic abundance is difficult. In these compounds, the heterocyclic moiety is covalently attached to the carbon atom of the substituent group that has no bound hydrogen atoms, and the connection between the two moieties of the compound cannot always be established via conventional 1H-1H and 1H-13C NMR correlation experiments (COSY and HMBC, respectively) or nuclear Overhauser effect spectroscopy (NOESY or ROESY). The selective incorporation of 15N-labelled atoms in different positions of the heterocyclic core allowed for the use of 1H-15N (JHN) and 13C-15N (JCN) coupling constants for the structure determinations of N-alkylated nitrogen-containing heterocycles in solution. This method was tested on the N-adamantylated products in a series of azolo-1,2,4-triazines and 1,2,4-triazolo[1,5-a]pyrimidine. The syntheses of adamantylated azolo-azines were based on the interactions of azolo-azines and 1-adamatanol in TFA solution. For azolo-1,2,4-triazinones, the formation of mixtures of N-adamantyl derivatives was observed. The JHN and JCN values were measured using amplitude-modulated 1D 1H spin-echo experiments with the selective inversion of the 15N nuclei and line-shape analysis in the 1D 13С spectra acquired with selective 15N decoupling, respectively. Additional spin-spin interactions were detected in the 15N-HMBC spectra. NMR data and DFT (density functional theory) calculations permitted to suggest a possible mechanism of isomerization for the adamantylated products of the azolo-1,2,4-triazines. The combined analysis of the JHN and JCN couplings in 15N-labelled compounds provides an efficient method for the structure determination of N-alkylated azolo-azines even in the case of isomer formation. The isomerization of adamantylated tetrazolo[1,5-b][1,2,4]triazin-7-ones in acidic conditions occurs through the formation of the adamantyl cation.

Unsymmetrically functionalized 5,5″-diaryl- and 5,6,5″-triaryl-2,2′:6′,2″-terpyridines: an efficient synthetic route and photophysical properties

An efficient approach for the synthesis of 5,5″- or 5,6,5″-aryl substituted 2,2′:6′,2″-terpyridines, bearing an anneleted cyclopentene unit in one of the side-chain pyridine rings for the improved solubility in organic solvents, via their 1,2,4-triazine analogues has been developed. By using this approach, various aromatic substituents were introduced in the 2,2′:6′,2″-terpyridine core. Depending on the nature of the aromatic substituents, the obtained terpyridines exhibited an intense emission in a range of ca. 344–394 nm in acetonitrile solutions. For the most representative compounds, pronounced bathochromic shifts in both absorption and emission spectra were observed compared with previously reported substituted terpyridines.