Synthesis and anti-HIV activity of isonucleosides

Synthesis of Triazole-Containing Furanosyl Nucleoside Analogues and Their Phosphate, Phosphoramidate or Phoshonate Derivatives as Potential Sugar Diphosphate or Nucleotide Mimetics

The synthesis of stable and potentially bioactive xylofuranosyl nucleoside analogues and potential sugar diphosphate or nucleotide mimetics comprising a 1,2,3-triazole moiety is reported. 3'-O-Methyl-branched N-benzyltriazole isonucleosides were accessed in 5-7 steps and 42-54 % overall yields using a Cu(I)-catalyzed cycloaddition of 3-O-propargyl-1,2-O-isopropylidene-α-D-xylofuranose with benzyl azide as key step. Related isonucleotides were obtained by 5-O-phosphorylation of acetonide-protected 3-O-propargyl xylofuranose and further "click" cycloaddition or by Staudinger-phosphite reaction of a 5-azido N-benzyltriazole isonucleoside. Hydroxy-, amino- or bromomethyl triazole 5'-isonucleosides were synthesized by thermal cycloaddition of 5-azido 3-O-benzyl/dodecyl xylofuranoses with propargyl alcohol, propargylamine or propargyl bromide. Better yields (82-85 %) were obtained when using propargyl alcohol and a high 1,4-regioselectivity was attained with propargyl bromide. Further O/N-phosphorylation or Arbuzov reaction led to (triazolyl)methyl phosphates, phosphoramidates or phosphonates. The latter were converted into uracil nucleoside 5'-(triazolyl)methyl phosphonates as prospective nucleoside diphosphate mimetics.

Synthesis and Biological Evaluation of Structurally Varied 5'-/6'-Isonucleosides and Theobromine-Containing N-Isonucleosidyl Derivatives

Isonucleosides are rather stable regioisomeric analogs of nucleosides with broad therapeutic potential. We have previously demonstrated the ability of 5′ and 6′-isonucleosides to inhibit the activity of acetylcholinesterase, a major target for Alzheimer’s disease therapy. Continuing with our research on this topic, we report herein on the synthesis and biological evaluation of a variety of novel terminal isonucleosides and theobromine isonucleotide analogs. Xylofuranose-based purine or uracil 5′-isonucleosides and xylofuranos-5′-yl or glucos-6′-yl theobromine derivatives were accessed via Mitsunobu coupling between partially protected xylofuranose or glucofuranose derivatives with a nucleobase using conventional or microwave-assisted heating conditions. Theobromine-containing N-isonucleosidyl sulfonamide and phosphoramidate derivatives were synthesized from isonucleosidyl acetate precursors. The most active compounds in the cholinesterase inhibition assays were a glucopyranose-based theobromine isonucleosidyl acetate, acting as a dual inhibitor of acetylcholinesterase (AChE, K_i = 3.1 µM) and butyrylcholinesterase (BChE, K_i = 5.4 µM), and a 2-O,4-O-bis-xylofuranos-5′-yl uracil derivative, which displayed moderate inhibition of AChE (K_i = 17.5 µM). Docking studies revealed that the active molecules are positioned at the gorge entrance and at the active site of AChE. None of the compounds revealed cytoxic activity to cancer cells as well as to non-malignant mouse fibroblasts.

Synthesis of glucopyranos-6′-yl purine and pyrimidine isonucleosides as potential cholinesterase inhibitors. Access to pyrimidine-linked pseudodisaccharides through Mitsunobu reaction

The synthesis of new isonucleosides comprising purine and pyrimidine-derived systems linked to methyl glucopyranosidyl units at C-6 and evaluation of their cholinesterase inhibitory profiles is reported. Their access was based on the Mitsunobu coupling of partially acetylated and benzylated methyl glucopyranosides with purine and pyrimidine derivatives. While the reactions with purines and theobromine proceeded with complete regioselectivity, affording exclusively N9- or N1-linked 6′-isonucleosides, respectively, the use of pyrimidine nucleobases led to N1 and/or N3-glucopyranosid-6′-yl pyrimidines and/or to N1,N3/2-O,4-O-pyrimidine-linked pseudodisaccharides through bis-coupling, depending on the substitution pattern of the sugar precursor and on the nature of the nucleobase. From this series of compounds, four were shown to be effective and selective inhibitors of acetylcholinesterase with inhibition constants in the micromolar concentration range. A tri-O-acetylated N1-glucopyranosid-6′-yl theobromine and a benzylated N1,N3-bis-glucopyranosid-6-yl thymine were the most active molecules with Ki values of 4 μM. A tri-O-benzylated glucopyranosid-6′-yl uracil displayed good and selective inhibition of butyrylcholinesterase (Ki=8.4±1.0 μM), similar to that exhibited by the standard galantamine. Molecular docking simulations, performed with the two most effective acetylcholinesterase inhibitors, showed interactions with key amino acid residues located at the enzyme’s active site gorge, which explain the competitive component of their inhibitory activities.

Nucleosides with Transposed Base or 4'-Hydroxymethyl Moieties and Their Corresponding Oligonucleotides

This review focuses on 4'-hydroxymethyl- or nucleobase-transposed nucleosides, nucleotides, and nucleoside phosphonates, their stereoisomers, and their close analogues. The biological activities of all known 4'-hydroxymethyl- or nucleobase-transposed nucleosides, nucleotides, and nucleoside phosphonates as potential antiviral or anticancer agents are compiled. The routes that have been taken for the chemical synthesis of such nucleoside derivatives are described, with special attention to the innovative strategies. The enzymatic synthesis, base-pairing properties, structure, and stability of oligonucleotides containing nucleobase- or 4'-hydroxymethyl-transposed nucleotides are discussed. The use of oligonucleotides containing nucleobase- or 4'-hydroxymethyl-transposed nucleotides as small oligonucleotide (e.g., human immunodeficiency virus integrase) inhibitors, in applications such as antisense therapy, silencing RNA (siRNA), or aptamer selections, is detailed.

9-Arylpurines as a novel class of enterovirus inhibitors

Here we report on a novel class of enterovirus inhibitors that can be structurally described as 9-arylpurines. These compounds elicit activity against a variety of enteroviruses in the low microM range including Coxsackie virus A16, A21, A24, Coxsackie virus B3, and echovirus 9. Structure-activity relationship (SAR) studies indicate that a chlorine or bromine atom is required at position 6 of the purine ring for antiviral activity. The most selective compounds in this series inhibited Coxsackie virus B3 replication in a dose-dependent manner with EC(50) values around 5-8 microM. No toxicity on different cell lines was observed at concentrations up to 250 microM. Moreover, no cross-resistance to TBZE-029 and TTP-8307 CVB3 resistant strains was detected.

New Synthesis of (±)-Isonucleosides

[Structure: see text] A novel method for synthesizing isonucleosides, a new class of anti-HIV nucleosides, is described. 2,2-Dimethyl-1,3-dioxan-5-one was converted into a dioxabicyclohexane derivative in six steps. After cleaving the epoxide group with thiophenol, the resulting product was subjected to the Mitsunobu reaction in the presence of a nucleobase to give the desired isonucleoside derivative via migration of the thiophenyl group. Removal of the thiophenyl group under radical conditions followed by deprotection led to the 4'-substituted 2',3'-dideoxyisonucleosides as a racemic mixture.

Convenient Synthesis of Nucleoside and Isonucleoside Analogues

[reaction: see text]. A very simple methodology to stereoselectively achieve tricyclic isonucleosides (nucleobase = thymine, uracil, and 5-fluoruracil) and 3'-C-branched nucleosides (nucleobase = theophylline) was performed by means of a DBU-mediated addition process using a readily available 2-bromo sugar. The mechanism for these transformations implies the loss of both substituents at C-2 and C-3 on the sugar moiety, and although it seems that DBU is probably involved, its involvement has not yet been ascertained. Cytosine did not react under these conditions.

Asymmetric synthesis of novel thioiso dideoxynucleosides with exocyclic methylene as potential antiviral agents

Novel thioiso pyrimidine and purine nucleosides substituted with exocyclic methylene have been synthesized, starting from D-xylose. The glycosyl donor 14 was synthesized from D-xylose, using cyclization of dimesylate 10 with sodium sulfide as a key step. Cyclization proceeded in pure S(N)2 reaction without going through S(N)1 reaction in the presence of an allylic functional group at low reaction temperature (0 degrees C) in polar solvent (DMF), affording compound 12 as a major product. At higher temperatures, S(N)2' product 11 was almost exclusively obtained as a major product. On the other hand, glycosylation of 14 with 6-chloropurine under Mitsunobu conditions afforded the desired S(N)2 product 26, while palladium-catalyzed glycosylation resulted in the sole formation of S(N)2' product 34.

Synthesis of d - and l -apio nucleoside analogues with 2′-hydroxyl group as potential anti-HIV agents

The present work describes the asymmetric synthesis of D- and L-apio-2',3'-dideoxynucleoside analogues, 4 and 5 with 2'-hydroxyl group via a common intermediate 9, starting from D-galactose. Stereoselective dihydroxylation and deoxygenation through radical inversion were successfully employed to synthesize the key intermediate 12 with D-apio structure, while stereoselecetive hydroboration-oxidation was used for the synthesis of another key intermediate 18 with L-apio structure.

Synthesis of novel (2R,4R)- and (2S,4S)-iso dideoxynucleosides with exocyclic methylene as potential antiviral agents

Novel (2R,4R)- and (2S,4S)-iso dideoxynucleosides with exocyclic methylene have been designed and synthesized, based on the lead BMS-200475 (3) which exhibited potent anti-HBV activity. For the synthesis of D types of (2R,4R)-nucleosides, L-xylose was converted to the key intermediate 14. The intermediate 14 was converted to the uracil derivative 4a and the cytosine derivative 4b. Compound 14 was also converted to the purine derivatives such as adenine derivative 4c, hypoxanthine derivative 4d, and guanine derivative 4e. The corresponding L types of (2S,4S)-enantiomers were more efficiently synthesized from the commercially available 1,2-isopropylidene-D-xylose (20) than the synthetic method used in the synthesis of (2R,4R)-nucleosides. The key intermediate 25 was converted to the pyrimidine analogues 5a and 5b and the purine derivatives 5c, 5d, and 5e using the similar method used in the preparation of 4c, 4d, and 4e. The synthesized final (2R,4R)- and (2S,4S)-nucleosides were tested against several viruses such as HIV-1, HSV-1, HSV-2, HCMV and HBV. (2R,4R)-Adenine analogue 4c exhibited potent anti-HBV activity (EC(50)=1.5 microM in 2.2.15 cells) among compounds tested, while (2R,4R)-uracil derivative 4a was the most active against HCMV among compounds tested and (2R,4R)-adenine derivative 4c was found to be moderately active against the same virus. However, the corresponding (2S,4S)-isomers were found to be totally inactive against all tested viruses. Both (2R,4R)-adenine derivative 4c and (2S,4S)-adenine analogue 5c were totally resistant to the adenosine deaminase like iso-ddA (1). From the molecular modeling study the hydroxymethyl side chains of BMS-200475 (3) and 4c were almost overlapped, indicating that 4c may be suitable for phosphorylation by cellular kinases like the lead 3, but some discrepancy between two bases was observed, indicating why 4c is less potent against HBV than 3. It is concluded that discovery of (2R,4R)-adenine analogue 4c as potent anti-HBV agent suggested that the sugar moiety of this series can be regarded as a novel template for the development of new anti-HBV agent and oxygen atom can be acted as a bioisostere of C-OH.

Syntheses and structure--activity relationships of novel apio and thioapio dideoxydidehydronucleosides as anti-HCMV agents

On the basis of the fact that apio dideoxynucleosides, in which the furanose oxygen and the C2 of the 2,3-dideoxyribose are transposed, exhibited potent anti-HIV activity and 2',3'-dideoxy-2',3'-didehydronucleosides also showed potent anti-HIV activity, we synthesized apio dideoxydidehydronucleosides in which the oxygen atom and the double bond of the 2,3-dideoxy-2,3-didehydroribose are exchanged. The thioapio dideoxydidehydronucleosides were also synthesized since sulfur serves as a bioisostere of oxygen. Apio dideoxydidehydronucleosides 13a--f were synthesized starting from 1,3-dihydroxyacetone, utilizing phenylselenenyl chemistry as a key step. The ratio of the anomeric mixture was variable from 1:1 to 5:1 during the condensation of nucleosidic bases with the phenylselenyl acetate 11 in the presence of a Lewis acid. This is in contrast with other glycosyl donors such as 5-O-(tert-butyldiphenylsilyl)-2-phenylselenenyl-2,3-dideoxyribosyl acetate which shows excellent neighboring group effect (alpha:beta = 1:99). Thioapio dideoxydidehydronucleosides 22a,b were synthesized from the lactone 9 via thiolactone 17 as a key intermediate which was synthesized from dicyclohexylcarbodiimide coupling of the mercapto acid produced from the basic hydrolysis of thioacetate 16. The majority of apio analogues synthesized in this study exhibited moderate to potent anti-HCMV activity, among which the 5-fluorouracil derivative 13c was found to be the most potent against HCMV, while thioapio analogues showed no activity against HCMV. However, all synthesized compounds did not exhibit any significant activities against HIV-1, HSV-1, and HSV-2. The fact that apio dideoxydidehydronucleosides were active against HCMV suggests that the apio dideoxydidehydro sugar moiety can serve as a novel template for the development of new antiviral agents.

Anti-human immunodeficiency virus activities of nucleosides and nucleotides: correlation with molecular electrostatic potential data

Examination of the anti-human immunodeficiency virus (HIV) data of some normal and isomeric dideoxynucleosides (ddNs and isoddNs), their three-dimensional (3-D) electron density patterns, their electrostatic potential surfaces (EPS), and their conformational maps reveals some interesting correlations. For example, the EPS of (S,S)-isoddA shows regions of high and low electrostatic potential remarkably similar to those of beta-D-3'-azido-3'-deoxythymidine (beta-D-AZT), (-)-oxetanocin A, and (-)-carbovir. Such correlations involving EPS data and anti-HIV activity were also found with many other active nucleosides. Conversely, inactive compounds had EPS different from those of compounds in the same series that were active. For example, apio-ddNs, which are inactive against HIV, exhibit clear differences in electrostatic potential and 3-D electron density shape from isoddNs that are active against HIV. Additionally, the inactivity of (S,S)-isoddC and (S,S)-isoddT can be correlated convincingly with a combination of their EPS data and their conformational energy maps. The electrostatic potential distributions of active nucleoside triphosphates show remarkable correlations. For example, (S,S)-isoddATP, AZT triphosphate (AZTTP), and oxetanocin A TP have similar 3-D electron density surface patterns and similar high and low regions of electrostatic potential, which may suggest that these compounds proceed through related mechanisms in their interactions with, and inhibition of, HIV reverse transcriptase (RT). Docking of AZTTP, (S,S)-isoddATP, and other active triphosphates into the active site of HIV RT and calculation of the EPS of both the nucleotide and the active site show that there is excellent matching between inhibitor and enzyme binding site EPS data. The structure-activity profile discovered has contributed to the development of a first predictive quantitative structure-activity relationship analysis in the area.

Development and optimization of anti-HIV nucleoside analogs and prodrugs: A review of their cellular pharmacology, structure-activity relationships and pharmacokinetics

Significant improvements in antiviral therapy have been realized over the past 10 years. Numerous nucleoside analogs, as well as prodrugs of active compounds, have been synthesized and tested for anti-HIV activity. In addition to the five nucleoside analogs currently used clinically for the treatment of HIV infection, a broad spectrum of anti-HIV nucleoside analogs (including 2',3'-dideoxynucleoside analogs, oxathiolanyl 2',3'-dideoxynucleoside analogs, dioxolanyl 2',3'-dideoxynucleoside analogs, carbocyclic 2',3'-dideoxynucleoside analogs and acyclic nucleoside analogs) and their prodrugs (including ester prodrugs, phospholipid prodrugs, dihydropyridine prodrugs, pronucleotides and dinucleotide analogs), targeted at HIV reverse transcriptase, are reviewed with focus on structure-activity relationships, cellular pharmacology and pharmacokinetics. Several of these anti-viral agents show promise in the treatment of AIDS.

Correlation of anti-HIV activity with structure: use of electrostatic potential and conformational analysis

Electrostatic potential distribution, 3-D electron density shapes, and conformational analysis were used to correlate anti-HIV activity or inactivity of AZT, (-) carbovir, oxetanocin A, and 16 isomeric dideoxynucleosides of different families. The structure-activity profile discovered is significant and is expected to contribute to the development of a much needed predictive QSAR analysis in this area.

Synthesis of Novel Iso-4'-thionucleosides Using the Mitsunobu Reaction

A novel class of isomeric 4'-thionucleosides with the base moiety at the 2'-position was synthesized from D-glucose. The coupling of 1,4-anhydro-4-thioarabitol (13) with various nucleobases using the Mitsunobu reaction was investigated. With both purines and N(3)-benzoyluracils, the reaction predominantly gave beta-isomers, suggesting that these were produced via an episulfonium intermediate. The beta-anomers produced by the reaction of N(3)-benzoyluracils included both N- and O-alkylated derivatives. Interestingly, only the reaction of N(3)-benzoyluracil gave a mixture of N-alkylated adduct (20d) and O-alkylated bipyrimidinyl adduct (22), the structure of which was unambiguously determined by NMR spectroscopic data including HMBC and NOE. Deprotection of the Mitsunobu reaction products gave the desired iso-4'-thionucleosides.

Synthesis and antiviral activity of apio dideoxy nucleosides with azido or amino substituent

Novel apio dideoxynucleosides with azido or amino or amino substituent were synthesized starting from 1,3-dihydroxyacetone utilizing an acid-catalyzed 1,4-conjugated addition as a key step and evaluated for antiviral activity. Unfortunately, they were found to be neither active against HIV-1, HSV-1,2 and poliovirus nor toxic.

Studies on the synthesis and biological activities of 4'-(R)-hydroxy-5'-(S)-hydroxymethyl-tetrahydrofuranyl purines and pyrimidines

A series of 4'-(R)-hydroxy-5'-(S)-hydroxymethyl-tetrahydrofuranyl purines and pyrimidines were synthesized by the reaction of 3,4-epoxy-5-(S,trans)-dimethoxymethyl-tetrahydrofuran and nucleobases under the catalysis of potassium tert-butoxide and 18-crown-6. Compounds 6a, 6c and 7b have shown significant inhibition on the growth of HL-60 cells. The phosphotriester and phosphodiester of isonucleoside 8a-d were synthesized and cytotoxic activities were reported. The conformation of isonucleosides in solution was studied by 1H NMR.

Antiviral, metabolic, and pharmacokinetic properties of the isomeric dideoxynucleoside 4(S)-(6-amino-9H-purin-9-yl)tetrahydro-2(S)-furanmethanol

4(S)-(6-Amino-9H-purin-9-yl)tetrahydro-2(S)-furanmethanol (IsoddA) is the most antivirally active member of a novel class of optically active isomeric dideoxynucleosides in which the base has been transposed from the natural 1' position to the 2' position and the absolute configuration is (S,S). IsoddA was active against human immunodeficiency virus type 1 (HIV-1) (strain IIIB), HIV-2 (strain ZY), and HIV-1 clinical isolates. Combinations of the compound with zidovudine (3'-azido-3'-deoxythymidine), 2',3'-dideoxyinosine, or 5-fluoro-2'-deoxy-3'-thiacytidine showed synergistic inhibition of HIV. A moderate reduction of activity was observed with clinical isolates resistant to zidovudine. An IsoddA-resistant virus (eightfold-increased 50% inhibitory concentration) was selected in vitro by repeated passage of HIV-1 (HXB2) in the presence of increasing concentrations of IsoddA. The reverse transcriptase-coding region of the mutant virus contained a single base change resulting in a change at codon 184 from Met to Val. IsoddA was also active against hepatitis B virus (HBV) in vitro; however, it lacked substantial selective activity in an in vivo HBV model. IsoddA was inefficiently phosphorylated in CEM cells; however, the half-life of the triphosphate was 9.4 h, and IsoddATP was a potent inhibitor of HIV-1 reverse transcriptase, with a Ki of 16 nM. The cytotoxicity 50% inhibitory concentrations of IsoddA were greater than 100 microM for CEM, MOLT-4, IM9, and the HepG2-derived HBV-infected 2.2.15 (subclone P5A) cell lines but were 12 and 11 microM for human granulocyte-macrophage (CFU-GM) and erythroid (BFU-E) progenitor cells, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Antiviral activities of isometric dideoxynucleosides of D- and L-related stereochemistry

In summary, many isomeric analogs of ddNs of both D-related and L-related absolute stereochemistries have been synthesized and evaluated in vitro for their antiviral activities. A few of these compounds exhibit potent antiviral activity and, interestingly, belong to both the D and L families. The synthetic methodologies developed will allow accessibility to many more novel modified nucleosides. While some structure-activity relationships are emerging from this work, it is clear that these chiral isomeric nucleosides have opened a new chapter in the field of antiviral nucleosides.