Synthesis and antiviral activities of 8-alkynyl-, 8-alkenyl-, and 8-alkyl-2'-deoxyadenosine analogues

Synthesis of Fluorescent C-C Bonded Triazole-Purine Conjugates

A design toward C-C bonded 2,6-bis(1H-1,2,3-triazol-4-yl)-9H-purine and 2-piperidinyl-6-(1H-1,2,3-triazol-4-yl)-9H-purine derivatives was established using the combination of Mitsunobu, Sonogashira, copper (I) catalyzed azide-alkyne cycloaddition, and SNAr reactions. 11 examples of 2,6-bistriazolylpurine and 14 examples of 2-piperidinyl-6-triazolylpurine intermediates were obtained, in 38-86% and 41-89% yields, respectively. Obtained triazole-purine conjugates expressed good fluorescent properties which were studied in the solution and in the thin layer film for the first time. Quantum yields reached up to 49% in DMSO for bistriazolylpurines and up to 81% in DCM and up to 95% in DMSO for monotriazolylpurines. Performed biological studies in mouse embryo fibroblast, human keratinocyte, and transgenic adenocarcinoma of the mouse prostate cell lines showed that most of obtained triazole-purine conjugates are not cytotoxic. The 50% cytotoxic concentration of the tested derivatives was in the range from 59.6 to 1528.7 µM.

Bond Formation at C8 in the Nucleoside and Nucleotide Purine Scaffold: An Informative Selection

This paper presents methods for the introduction and exchange of substituents in a nucleobase and its nucleosides and nucleotides with emphasis on the C8-position in the purine skeleton. The nucleobase is open for electrophilic and nucleophilic chemistry. The nucleophilic chemistry consists mainly of displacement reactions when the C8-substituent is a good leaving group such as a halogen atom. The heteroatom in amines, sulfides, or oxides is a good nucleophile. Halides are good reaction partners. Metal-promoted cross-coupling reactions are important for carbylations. Direct oxidative metalation reactions using sterically hindered metal amides offer chemo- and regio-selectivity besides functional tolerance and simplicity. The carbon site is highly nucleophilic after metalation and adds electrophiles resulting in chemical bond formation. Conditions for metal-assisted reactions are described for nucleobases and their glycosides.

Steroid-triazole conjugates: A brief overview of synthesis and their application as anticancer agents

Steroids are biomolecules that play pivotal roles in various physiological and drug discovery processes. Abundant research has been fuelled towards steroid-heterocycles conjugates over the last few decades as potential therapeutic agents against various diseases especially as anticancer agents. In this context various steroid-triazole conjugates have been synthesized and studied for their anticancer potential against various cancer cell lines. A thorough search of the literatures revealed that a concise review pertaining the present topic is not compiled. Therefore, in thus review we summarize the synthesis, anticancer activity against various cancer cell lines and structure activity relationship (SAR) of various steroid-triazole conjugates. This review can lay down the path towards the development of various steroid-heterocycles conjugates with lesser side effects and profound efficacy.

Diversely C8-functionalized adenine nucleosides via their underexplored carboxaldehydes

The potentially versatile N-unprotected 8-formyl derivatives of adenosine and 2'-deoxyadenosine are highly underexploited for C8 modifications of these nucleosides. Only in situ formation of 8-formyladenosine is known and a single application of an N-benzoyl derivative has been reported. On the other hand, 8-formyl-2'-deoxyadenosine and its applications remain unknown. Herein, we report straightforward, scalable syntheses of both N-unprotected 8-formyladenine nucleoside derivatives, and demonstrate broad diversification at the C8 position by hydroxymethylation, azidation, CuAAC ligation, reductive amination, as well as olefination and fluoroolefination with modified Julia and a Horner-Wadsworth-Emmons reagents.

Interaction of Adenosine, Modified Using Carborane Clusters, with Ovarian Cancer Cells: A New Anticancer Approach against Chemoresistance

Platinum compounds remain the first-line drugs for the treatment of most lethal gynecological malignancies and ovarian cancers. Acquired platinum resistance remains a major challenge in gynecological oncology. Considering the unique physicochemical properties of the metallacarboranes modifier and the significant role of nucleoside derivatives as anticancer antimetabolites, we designed and synthesized a set of adenosine conjugates with metallacarboranes containing iron, cobalt, or chromium as semi-abiotic compounds that influence the cisplatin sensitivity of ovarian cancer cells. Adherent cultures of ovarian carcinoma cell lines and multicellular spheroids, ranging from sensitive to highly resistant including experimental cell lines "not responding" to platinum drugs were used. Iron-containing metallacarborane conjugates showed the best anticancer activity, especially against resistant cells. Compound modified at the C2' nucleoside position showed the best activity in resistant cancer cells and highly resistant cancer spheroids exposed to cisplatin, increasing cell cycle arrest, apoptosis or necrosis, and reactive oxygen species production. Moreover, it showed high cellular accumulation and did not induce cross-resistance to cisplatin, carboplatin, doxorubicin, paclitaxel, or gemcitabine in long-term cultures. The reference nido-carborane derivative (no metal ions) and unmodified nucleosides were not as effective. These findings indicate that metallacarborane modification of adenosine may sensitize ovarian cancer cells to cisplatin in combination treatment.

Rational Design of Nucleoside-Bile Acid Conjugates Incorporating a Triazole Moiety for Anticancer Evaluation and SAR Exploration

Herein we report a study on the synthesis and biological evaluation of a library of nucleoside-bile acid conjugates prepared by combining 2′-deoxyadenosine, 2′-deoxyguanosine, 2′-deoxyuridine as well as adenosine and guanosine derivatives with cheno-, urso-, nor-cheno-, nor-urso- and taurourso-desoxycholic acid derivatives by means of the click reaction. The new nucleoside-bile acid conjugates incorporating a triazole moiety were tested in vitro against leukemic K562 and HCT116 colon carcinoma, as well as on normal fibroblast cells. Six compounds displayed interesting anti-proliferative activity against the selected cancer lines and no cytotoxic effects against normal fibroblasts. A possible structure activity relationship was also investigated.

Basic chemokine-derived glycosaminoglycan binding peptides exert antiviral properties against dengue virus serotype 2, herpes simplex virus-1 and respiratory syncytial virus

Chemokines attract leukocytes to sites of infection in a G protein-coupled receptor (GPCR) and glycosaminoglycan (GAG) dependent manner. Therefore, chemokines are crucial molecules for proper functioning of our antimicrobial defense mechanisms. In addition, some chemokines have GPCR-independent defensin-like antimicrobial activities against bacteria and fungi. Recently, high affinity for GAGs has been reported for the positively charged COOH-terminal region of the chemokine CXCL9. In addition to CXCL9, also CXCL12γ has such a positively charged COOH-terminal region with about 50% positively charged amino acids. In this report, we compared the affinity of COOH-terminal peptides of CXCL9 and CXCL12γ for GAGs and KD values in the low nM range were detected. Several enveloped viruses such as herpesviruses, hepatitis viruses, human immunodeficiency virus (HIV), dengue virus (DENV), etc. are known to bind to GAGs such as the negatively charged heparan sulfate (HS). In this way GAGs are important for the initial contacts between viruses and host cells and for the infection of the cell. Thus, inhibiting the virus-cell interactions, by blocking GAG-binding sites on the host cell, might be a way to target multiple virus families and resistant strains. This article reports that the COOH-terminal peptides of CXCL9 and CXCL12γ have antiviral activity against DENV serotype 2, clinical and laboratory strains of herpes simplex virus (HSV)-1 and respiratory syncytial virus (RSV). Moreover, we show that CXCL9(74-103) competes with DENV envelope protein domain III for binding to heparin. These short chemokine-derived peptides may be lead molecules for the development of novel antiviral agents.

Palladium-catalyzed modification of unprotected nucleosides, nucleotides, and oligonucleotides

Synthetic modification of nucleoside structures provides access to molecules of interest as pharmaceuticals, biochemical probes, and models to study diseases. Covalent modification of the purine and pyrimidine bases is an important strategy for the synthesis of these adducts. Palladium-catalyzed cross-coupling is a powerful method to attach groups to the base heterocycles through the formation of new carbon-carbon and carbon-heteroatom bonds. In this review, approaches to palladium-catalyzed modification of unprotected nucleosides, nucleotides, and oligonucleotides are reviewed. Polar reaction media, such as water or polar aprotic solvents, allow reactions to be performed directly on the hydrophilic nucleosides and nucleotides without the need to use protecting groups. Homogeneous aqueous-phase coupling reactions catalyzed by palladium complexes of water-soluble ligands provide a general approach to the synthesis of modified nucleosides, nucleotides, and oligonucleotides.

Labeling deoxyadenosine for the preparation of functional conjugated oligonucleotides

Herein we present a versatile synthetic method for the 8-thioalkylation of (deoxy)adenosine with a short carbon linker having on the other side a variety of molecules (psoralen, acridine) and functional groups (alkyne). After conventional protections, the modified adenosine can be phosphytylated and inserted into an oligonucleotide without affecting the standard protocols for supported oligonucleotide synthesis. The hybridization properties of a generic oligonucleotide containing the above conjugated moieties toward both DNA and RNA are evaluated both in the case of a perfectly complementary strand and in the case of a single mismatch. This methodology is suitable for the preparation of several types of derivatives and—through the alkynyl moiety—provides fast access to click-chemistry transformations.

8-Modified-2'-deoxyadenosine analogues induce delayed polymerization arrest during HIV-1 reverse transcription

The occurrence of resistant viruses to any of the anti-HIV-1 compounds used in the current therapies against AIDS underlies the urge for the development of new drug targets and/or new drugs acting through novel mechanisms. While all anti-HIV-1 nucleoside analogues in clinical use and in clinical trials rely on ribose modifications for activity, we designed nucleosides with a natural deoxyribose moiety and modifications of position 8 of the adenine base. Such modifications might induce a steric clash with helix αH in the thumb domain of the p66 subunit of HIV-1 RT at a distance from the catalytic site, causing delayed chain termination. Eleven new 2′-deoxyadenosine analogues modified on position 8 of the purine base were synthesized and tested in vitro and in cell-based assays. In this paper we demonstrate for the first time that chemical modifications on position 8 of 2′-deoxyadenosine induce delayed chain termination in vitro, and also inhibit DNA synthesis when incorporated in a DNA template strand. Furthermore, one of them had moderate anti-HIV-1 activity in cell-culture. Our results constitute a proof of concept indicating that modification on the base moiety of nucleosides can induce delayed polymerization arrest and inhibit HIV-1 replication.

Highly efficient Suzuki-Miyaura coupling of heterocyclic substrates through rational reaction design

A dicyclohexyl(2-sulfo-9-(3-(4-sulfophenyl)propyl)-9H-fluoren-9-yl)phosphonium salt was synthesized in 64% overall yield in three steps from simple commercially available starting materials. The highly water-soluble catalyst obtained from the corresponding phosphine and [Na(2)PdCl(4)] enabled the Suzuki coupling of a broad variety of N- and S-heterocyclic substrates. Chloropyridines (-quinolines) and aryl chlorides were coupled with aryl-, pyridine- or indoleboronic acids in quantitative yields in water/n-butanol solvent mixtures in the presence of 0.005-0.05 mol % of Pd catalyst at 100 degrees C, chloropurines were quantitatively Suzuki coupled in the presence of 0.5 mol % of catalyst, and S-heterocyclic aryl chlorides and aryl- or 3-pyridylboronic acids required 0.01-0.05 mol % Pd catalyst for full conversion. The key to the high activity of the Pd-phosphine catalyst is the rational design of the reaction parameters (i.e., the presence of water in the reaction mixture, good solubility of reactants and catalyst in n-butanol/water (3:1), and the electron-rich and sterically demanding nature of the phosphine ligand).

Solution structure of DNA/RNA hybrid duplex with C8-propynyl 2'-deoxyadenosine modifications: Implication of RNase H and DNA/RNA duplex interaction

Solution structures of DNA/RNA hybrid duplexes, d(GCGCA*AA*ACGCG): r(cgcguuuugcg)d(C) (designated PP57), containing two C8-propynyl 2'-deoxyadenosines (A*) and unmodified hybrid (designated U4A4) are solved. The C8-propynyl groups on 2'-deoxyadenosine perturb the local structure of the hybrid duplex, but overall the structure is similar to that of canonical DNA/RNA hybrid duplex except that Hoogsteen hydrogen bondings between A* and U result in lower thermal stability. RNase H is known to cleave RNA only in DNA/RNA hybrid duplexes. Minor groove widths of hybrid duplexes, sugar puckerings of DNA are reported to be responsible for RNase H mediated cleavage, but structural requirements for RNase H mediated cleavage still remain elusive. Despite the presence of bulky propynyl groups of PP57 in the minor groove and greater flexibility, the PP57 is an RNase H substrate. To provide an insight on the interactions between RNase H and substrates we have modeled Bacillus halodurans RNase H-PP57 complex, our NMR structure and modeling study suggest that the residue Gly(15) and Asn(16) of the loop residues between first beta sheet and second beta sheet of RNase HI of Escherichia coli might participate in substrate binding.

An unusual occurrence on attempted purine C-8 electrophilic fluorination of 5'-noraristeromycin

Reaction of the in situ generated purine C-8 carbanion of a protected 5'-noraristeromycin derivative with N-fluorobenzenesulfonimide gave 8-phenylsulfonyl-5'-noraristeromycin rather than the expected 8-fluoro derivative. A single electron transfer (SET) mechanism is proposed for this occurrence. The phenylsulfonyl product offers a structural feature common to some anti-HIV agents. [reaction: see text]

John Montgomery's legacy: carbocyclic adenosine analogues as SAH hydrolase inhibitors with broad-spectrum antiviral activity

Ever since the S-adenosylhomocysteine (AdoHcy, SAH) hydrolase was recognized as a pharmacological target for antiviral agents (J. A. Montgomery et al., J. Med. Chem. 25:626-629, 1982), an increasing number of adenosine, acyclic adenosine, and carbocyclic adenosine analogues have been described as potent SAH hydrolase inhibitors endowed with broad-spectrum antiviral activity. The antiviral activity spectrum of the SAH hydrolase inhibitors include pox-, rhabdo-, filo-, arena-, paramyxo-, reo-, and retroviruses. Among the most potent SAH hydrolase inhibitors and antiviral agents rank carbocyclic 3-deazaadenosine (C-c3 Ado), neplanocin A, 3-deazaneplanocin A, the 5'-nor derivatives of carbocyclic adenosine (C-Ado, aristeromycin), and the 2-halo (i.e., 2-fluoro) and 6'-R-alkyl (i.e., 6'-R-methyl) derivatives of neplanocin A. These compounds are particularly active against poxviruses (i.e., vaccinia virus), and rhabdoviruses (i.e., vesicular stomatitis virus). The in vivo efficacy of C-c3 Ado and 3-deazaneplanocin A has been established in mouse models for vaccinia virus, vesicular stomatitis virus, and Ebola virus. SAH hydrolase inhibitors such as C-c3Ado and 3-deazaneplanocin A should in thefirst place be considered for therapeutic (or prophylactic) use against poxvirus infections, including smallpox, and hemorrhagic fever virus infections such as Ebola.

Synthesis and biological activities of nucleoside-estradiol conjugates

Nucleosides were coupled to estradiol via a 17alpha-ethynyl spacer group using Pd(II) as a catalyst. The conjugates were evaluated in vitro for estrogen receptor (ER) binding affinity and cytotoxicity against cell lines with and without ER. The highest receptor binding affinities (RBA approximately 3) were observed with conjugates coupled via a relative long spacer group, while none of the conjugates exhibited cytotoxicity against either cell lines.

Inhibitory effects of the guanine moiety on Suzuki couplings of unprotected halonucleosides in aqueous media

In the Suzuki arylations of unprotected halonucleosides in aqueous media, 8-bromo-2'-deoxyguanosine (8BrdG) couplings were slower to reach completion than the corresponding 8-bromo-2'-deoxyadenosine (8BrdA) couplings. The guanine moiety has an acidic proton, which under our Suzuki conditions (pH congruent with 10) may be deprotonated to give an anion that can coordinate to palladium. The possibility that guanine coordination was responsible for the observed slower rates was explored using additive experiments in which nonhalogenated nucleosides were added to the Suzuki coupling reaction of 8BrdA or 4-bromotoluene and PhB(OH)2 and the reaction progress monitored by HPLC or GC. Adding dG slowed these reactions, and an induction period was observed. The addition of dA or 1-methyl-2'-deoxyguanosine (1MedG) to these couplings did not affect the rate of conversion to product. Guanine coordination was further explored using 13C and 31P NMR spectroscopy, which implies that guanine is coordinating to palladium through N-1 or O-6, or both. Furthermore, the presence of dG inhibited the formation of the active palladium(0) catalytic species, which may account for both the observed induction period and the sluggishness of reactions where guanine is involved.

Modified Tricyclic Analogues of Acyclovir. A Direct Alkynylation in the Fused Ring

Two types of new 7-alkynylated tricyclic analogues (3,9-dihydro-5H-imidazo[1,2-a]purin-9-ones) of acyclovir differing by the presence of N-5 substituent, a temporary 2-(4-nitrophenyl)ethyl or a permanent 3-hydroxypropyl were obtained by a Sonogashira coupling. 7-Alk-1-ynyl-5-(3-acetoxypropyl) compounds (19a-19d, 21a-21c) were efficiently prepared from 7-iodo, 7-iodo-6-methyl precursors 12 and 11, respectively, and deprotected while the products with unsubstituted N-5 were unstable (e.g. 17). Iodide 12 was generally less reactive than 11 and underwent a preferable reduction (48%) to deiodinated 8 when coupled with ethynyltrimethylsilane.

Efficient one-step Suzuki arylation of unprotected halonucleosides, using water-soluble palladium catalysts

Modification of nucleosides to give pharmaceutically active compounds, mutagenesis models, and oligonucleotide structural probes continues to be of great interest. The aqueous-phase modification of unprotected halonucleosides is reported herein. Using a catalyst derived from tris(3-sulfonatophenyl)phosphine (TPPTS) and palladium acetate, 8-bromo-2'-deoxyguanosine (8-BrdG) is coupled with arylboronic acids to give 8-aryl-2'-deoxyguanosine adducts (8-ArdG) in excellent yield in a 2:1 water:acetonitrile solvent mixture. The TPPTS ligand was found to be superior to water-soluble alkylphosphines for this coupling reaction. The coupling chemistry has been extended to 8-bromo-2'-deoxyadenosine (8-BrdA) and 5-iodo-2'-deoxyuridine (5-IdU), as well as the ribonucleosides 8-bromoguanosine and 8-bromoadenosine. Good to excellent yields of arylated adducts are obtained in all cases. With use of tri(4,6-dimethyl-3-sulfonatophenyl)phosphine (TXPTS), the Suzuki coupling of 8-BrdA and 5-IdU can be accomplished in less than 1 h at room temperature. This methodology represents an efficient and general method for halonucleoside arylation that does not require prior protection of the nucleoside.

Vaccinia virus inhibitors as a paradigm for the chemotherapy of poxvirus infections

Poxviruses continue to pose a major threat to human health. Monkeypox is endemic in central Africa, and the discontinuation of the vaccination (with vaccinia virus) has rendered most humans vulnerable to variola virus, the etiologic agent of smallpox, should this virus be used in biological warfare or terrorism. However, a large variety of compounds have been described that are potent inhibitors of vaccinia virus replication and could be expected to be active against other poxviruses as well. These compounds could be grouped in different classes: (i) IMP dehydrogenase inhibitors (e.g., EICAR); (ii) SAH hydrolase inhibitors (e.g., 5'-noraristeromycin, 3-deazaneplanocin A, and various neplanocin A derivatives); (iii) OMP decarboxylase inhibitors (e.g., pyrazofurin) and CTP synthetase inhibitors (e.g., cyclopentenyl cytosine); (iv) thymidylate synthase inhibitors (e.g., 5-substituted 2'-deoxyuridines); (v) nucleoside analogues that are targeted at viral DNA synthesis (e.g., Ara-A); (vi) acyclic nucleoside phosphonates [e.g., (S)-HPMPA and (S)-HPMPC (cidofovir)]; and (vii) polyanionic substances (e.g., polyacrylic acid). All these compounds could be considered potential candidate drugs for the therapy and prophylaxis of poxvirus infections at large. Some of these compounds, in particular polyacrylic acid and cidofovir, were found to generate, on single-dose administration, a long-lasting protective efficacy against vaccinia virus infection in vivo. Cidofovir, which has been approved for the treatment of cytomegalovirus retinitis in immunocompromised patients, was also found to protect mice, again when given as a single dose, against a lethal aerosolized or intranasal cowpox virus challenge. In a biological warfare scenario, it would be advantageous to be able to use a single treatment for an individual exposed to an aerosolized poxvirus. Cidofovir thus holds great promise for treating human smallpox, monkeypox, and other poxvirus infections. Anecdotal experience points to the efficacy of cidofovir in the treatment of the poxvirus infections molluscum contagiosum and orf (ecthyma contagiosum) in immunosuppressed patients.

Synthesis of 8-(omega-Hydroxyalkyl)-, 8-(omega-hydroxyalk-1-enyl)-, and 8-(omega-hydroxyalk-1-ynyl)adenines using the tert-butyldimethylsilyloxymethyl group, a new and versatile protecting group of adenine

The synthesis of 12 analogues of adenine substituted at C-8 by an omega-hydroxyalkyl, omega-hydroxyalk-1-enyl, or omega-hydroxyalk-1-ynyl chain of various length has been carried out in five or six steps starting from adenine. The analogues were obtained using a new protecting group of adenine, the tert-butyldimethylsilyloxymethyl group. 9-tert-Butyldimethylsilyloxymethyl-adenine is more soluble than adenine in organic solvents. It was prepared regiospecificaly in two steps from adenine and was amenable to C-8 iodination under basic conditions and to subsequent introduction of the various carbon chains at C-8 by palladium-catalyzed cross-coupling reactions (Stille or Sonogashira). The protecting group was removed under acidic conditions, thus demonstrating its versatility.

Synthesis and stability of oligodeoxynucleotides containing C8-labeled 2'-deoxyadenosine: novel redox nucleobase probes for DNA-mediated charge-transfer studies

[reaction: see text] An efficient and convenient synthetic strategy to redox-labeled C8-derivatives of 2'-deoxyadenosine is described. The Pd(0) cross-coupling chemistry is amenable to both oxidative and reductive redox probes. The corresponding phosphoramidites of phenothiazine and anthraquinone nucleosides are amenable to automated DNA synthesis. The resulting labeled oligodeoxynucleotide strands form stable B-form duplexes with melting temperatures and CD spectra similar to those of the unlabeled analogues.

2'-Deoxy-8-(propyn-1-yl)adenosine-containing oligonucleotides: effects on stability of duplex and quadruplex structures

2'-Deoxy-8-(propyn-1-yl)adenosine has been incorporated in synthetic oligodeoxyribonucleotides and its influence on thermal stability of duplex and quadruplex structures investigated by UV, CD and 1H NMR. The obtained results seem to indicate that the presence of the modified base negatively affects the stability of double stranded DNA whereas remarkably increases the stability of parallel quadruplex structures.

Pyrrolo[2,3-d]pyrimidine nucleosides: synthesis and antitumor activity of 7-substituted 7-deaza-2'-deoxyadenosines

A one step synthesis, using the nucleoside 7-iodo-2'-deoxytubercidin (2b) in a Pd(0)/Cu(I)-catalyzed cross coupling reaction furnished a series of 7-alkynyl-2'-deoxytubercidin derivatives. The 7-iodo-, 7-chloro- or 7-bromo 2'-deoxytubercidins 2b-d as well as certain 7-alkynyl derivatives show significant activity against several tumor cell lines, with 7-iodo-2'-deoxytubercidin (2b) as the most effective compound.

Synthesis and antiviral effects of 2-heteroaryl substituted adenosine and 8-heteroaryl substituted guanosine derivatives

2-(2"- and 3"-Thienyl)adenosine and the corresponding furyl derivatives were prepared though Pd(0)-catalyzed coupling of 2',3',5'-tri-O-(t-butyldimethylsilyl)-2-iodoadenosine with the appropriate tributyltin derivatives followed by deprotection. Preparation of the 8-(2"- and 3"-thienyl)guanosines and 8-(2"- and 3"-furyl)guanosines followed a similar route. Antiviral properties of these compounds and the related 2,6-diaminopurine ribofuranosides were of no pharmacological interest.