Utilization Of Tetrabutylammonium Triphenyldifluorosilicate (TBAT) In The Synthesis of 6-Fluoropurine Nucleosides

Abstract

Tetrabutylammonium triphenydifluorosilicate (TBAT) has been found to be a useful reagent for the conversion of 6-chloropurine nucleosides to 6-fluoropurine derivatives. The 6-chloropurine nucleosides were reacted with trimethylamine to form quaternary trimethylammonium salts which were treated in situ with TBAT in DMF to effect conversion to the 6-fluoro derivatives in yields of 59–72%.     

Synthesis and biological evaluation of novel thioapio dideoxynucleosides

On the basis of the bioisosteric rationale to apio dideoxynucleosides, novel thioapio dideoxynucleosides have been synthesized, starting from 1,3-dihydroxyacetone via thioapio sugar acetate as a key intermediate. The intermediate was condensed with silylated pyrimidine bases such as N(4)-benzoylcytosine, uracil or thymine in the presence of TMSOTf to give the beta-anomers and alpha-anomers, respectively. The intermediate was also condensed with silylated 6-chloropurine to give the 6-chloropurine derivatives and which were converted to adenine derivatives and, N(6)-methyladenine derivatives and, and hypoxanthine derivatives and, respectively. The guanine analogues and were also synthesized from the condensation of sugar acetate with 2-acetamido-6-chloropurine. All synthesized final compounds were tested against HIV-1. Most of the synthesized compounds exhibited toxicity-dependent anti-HIV-1 activity, among which 6-chloropurine derivative was found to be the most cytotoxic and showed good cytotoxicity against colon cancer cell lines. Although we could not find good anti-HIV agents in this study, findings of some anticancer activity in this series will allow this class of nucleosides to be the new template for the development of new anticancer agents (Fig. 1).     

Facile synthesis of 1',2'-cis-beta-pyranosyladenine nucleosides

1',2'-cis-beta-Glycosyladenine nucleosides, such as beta-altroside, beta-mannoside, and beta-idoside, were efficiently synthesized from the corresponding 1',2'-trans-beta-6-chloropurine derivatives, beta-glucoside, and beta-galactoside. Nucleophilic substitution of the O-trifluoromethanesulfonyl groups at the C-2' and/or 3' was carried out using tetrabutylammonium acetate or cesium acetate under mild conditions. Subsequent deprotection and amidation afforded the desired compounds, 1',2'-cis-beta-pyranosyladenine nucleosides.     

Lithiation-Stannylation Chemistry of Nucleosides

Abstract

Two examples of anionic stannyl migration practically useful for nucleoside synthesis are presented. One involves the migration from the 8- to 2-position of 6-chloropurine derivatives, which provided a new entry to 2-substituted purine nucleosides. The other is that from the 6- to 2′-position of 1′,2′-unsaturated uridine. The latter enabled the preparation of a hitheroto unknown class of nucleoside analogues, 2′-substituted 1′,2′-unsaturated uridines.     

Bicyclic nucleoside synthesis: a photochemical approach

The synthesis of a series of bicyclic nucleosides using photolytic ring-expansion of cyclobutanones is reported. The cyclobutanone precursors were prepared by [2+2] cycloaddition of a series of cyclic alkenes with chlorinated ketenes, derived from dichloro- and trichloroacetyl chloride. The synthesis of the nucleosides was achieved through photolysis of cyclobutanone precursors with 6-chloropurine by UV irradiation. The generality of this method was investigated and the absolute stereochemistry was assigned by NMR spectroscopy. The photoproducts demonstrated a marked preference for the 2'-exo conformation.     

Bicyclic Nucleoside Synthesis—A Photochemical Approach

The synthesis of a series of bicyclic nucleosides using photolytic ring-expansion of cyclobutanones is reported. The cyclobutanone precursors were prepared by [2+2] cycloaddition of a series of cyclic alkenes with chlorinated ketenes, derived from dichloro- and trichloroacetyl chloride. The synthesis of the nucleosides was achieved through photolysis of cyclobutanone precursors with 6-chloropurine by UV irradiation. The generality of this method was investigated and the absolute stereochemistry was assigned by NMR spectroscopy. The photoproducts demonstrated a marked preference for the 2′-exo conformation.     

2′-Azido-2′-deoxy- and 2′-amino-2′-deoxy-β-d-arabino-furanosyl purine nucleosides

A general method for the preparation of 2′-azido-2′-deoxy- and 2′-amino-2′-deoxyarabinofuranosyl-adenine and -guanine nucleosides is described. Selective benzoylation of 3-azido-3-deoxy-1,2-O-isopropylidene-α-d-glucofuranose afforded 3-azido-6-O-benzoyl-3-deoxy-1,2-O-isopropylidene-α-d-glucofuranose (1). Acid hydrolysis of 1, followed by oxidation with sodium metaperiodate and hydrolysis by sodium hydrogencarbonate gave 2-azido-2-deoxy-5-O-benzoyl-d-arabinofuranose (3), which was acetylated to give 1,3-di-O-acetyl-2-azido-5-O-benzoyl-2-deoxy-d-arabinofuranose (4). Compound 4 was converted into the 1-chlorides 5 and 6, which were condensed with silylated derivatives of 6-chloropurine and 2-acetamido-hypoxanthine. The condensation reaction gave α and β anomers of both 7- and 9-substituted purine nucleosides. The structures of the nucleosides were determined by n.m.r. and u.v. spectroscopy, and by correlation of the c.d. spectra of the newly prepared nucleosides with those published for known purine nucleosides.     

2′-Azido-2′-deoxy- and 2′-amino-2′-deoxy-β-d-arabino-furanosyl purine nucleosides

A general method for the preparation of 2′-azido-2′-deoxy- and 2′-amino-2′-deoxyarabinofuranosyl-adenine and -guanine nucleosides is described. Selective benzoylation of 3-azido-3-deoxy-1,2-O-isopropylidene-α-d-glucofuranose afforded 3-azido-6-O-benzoyl-3-deoxy-1,2-O-isopropylidene-α-d-glucofuranose (1). Acid hydrolysis of 1, followed by oxidation with sodium metaperiodate and hydrolysis by sodium hydrogencarbonate gave 2-azido-2-deoxy-5-O-benzoyl-d-arabinofuranose (3), which was acetylated to give 1,3-di-O-acetyl-2-azido-5-O-benzoyl-2-deoxy-d-arabinofuranose (4). Compound 4 was converted into the 1-chlorides 5 and 6, which were condensed with silylated derivatives of 6-chloropurine and 2-acetamido-hypoxanthine. The condensation reaction gave α and β anomers of both 7- and 9-substituted purine nucleosides. The structures of the nucleosides were determined by n.m.r. and u.v. spectroscopy, and by correlation of the c.d. spectra of the newly prepared nucleosides with those published for known purine nucleosides.     

Synthesis of novel purine nucleosides towards a selective inhibition of human butyrylcholinesterase

The search for new and potent cholinesterase inhibitors is an ongoing quest mobilizing many organic chemistry groups around the world as these molecules have been shown to treat the late symptoms of Alzheimer’s disease as well as to act as neuroprotecting agents. In this work, we disclose the synthesis of novel 2-acetamidopurine nucleosides and, for the first time, regioselective N⁷-glycosylation with 2-acetamido-6-chloropurine, promoted by trimethylsilyl triflate, was accomplished by tuning the reaction conditions (acetonitrile as solvent, 65 °C, 5 h) starting from 1-acetoxy bicyclic glycosyl donors, or by direct coupling of a methyl glucopyranoside with the nucleobase to obtain only N⁷ nucleosides in reasonable yield (55–60%). The nucleosides as well as their sugar precursors were screened for acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) inhibition. While none of the compounds tested inhibited AChE, remarkably, some of the N⁷ nucleosides and sugar bicyclic derivatives showed potent inhibition towards BChE. Nanomolar inhibition was obtained for one compound competing well with rivastigmine, a drug currently in use for the treatment of Alzheimer’s disease. Experimental results showed that the presence of benzyl groups on the carbohydrate scaffold and the N⁷-linked purine nucleobase were necessary for strong BChE inactivation. A preliminary evaluation of the acute cytotoxicity of the elongated bicyclic sugar precursors and nucleosides was performed indicating low values, in the same order of magnitude as those of rivastigmine.     

Modification of bovine pancreatic ribonuclease A with 6-chloropurine riboside

The chemical modification of bovine pancreatic ribonuclease A by 6-chloropurine riboside was studied to obtain information about the role of the purine nucleoside moiety of the ribonucleic acid in the enzyme-substrate interaction. The residues involved in the reaction were identified, after performic acid oxidation and trypsin digestion, by reverse-phase HPLC peptide mapping. The labeled peptides were detected by following the absorbance at 254 nm, and amino acid analyses of these peptides showed that the reaction had taken place with the amino groups of Lys-1, -37, -41, and -91. The specificity of the reaction was unaffected by changing the ligand:protein molar ratio. Partial separation of the reaction products was accomplished by means of chromatography on CM-Sepharose: four labeled fractions corresponding to mono- and bisubstituted derivatives were found. One of the monosubstituted fractions (fraction E) contained a homogeneous protein with the nucleoside bound to the alpha-amino group of Lys-1 whereas the other (fraction D) was a mixture of derivatives labeled in the epsilon-amino group of Lys-1, -37, -41, and -91. Kinetic studies of these two monosubstituted fractions were performed with cytidine 2',3'-phosphate and ribonucleic acid as substrates. These derivatives showed a noncompetitive inhibition-like behavior with respect to RNase A. Results support the existence of several RNase A regions with affinity for purine nucleosides.     

The search of novel inhibitors of HIV-1 integrase among 5-(4-halogenophenyl)-5-oxopentyl derivatives of nucleic bases

New nucleic base derivatives were obtained by alkylation of uracil, thymine, cytosine, adenine, 6-chloropurine, and 2-amino-6-chloropurine with 5-chloro-1-(4-halogenophenyl)-1-pentanones, and their physical and chemical properties were studied. The influence of the compounds synthesized on the HIV-1 integrase activity was studied.     

Study of Adenine Aminohydrolase in the Yeast, Schizosaccharomyces pombe

Observation of the growth of some adenineless mutants of Schizosaccharomyces pombe on six substituted purine analogs leads to the hypothesis that an enzyme is present which catalyzes the conversion of these analogs into hypoxanthine. The enzyme adenase (adenine aminohydrolase, EC 3.5.4.2) has been found to be active in cell-free extracts of S. pombe. Results are reported which are in agreement with the hypothesis that this enzyme is responsible for the in vivo utilization of 6-chloropurine. This evidence comes mainly from a study of adenine aminohydrolase in two mutants selected for partial inability to grow on 6-chloropurine.     

The synthesis of oligoribonucleotides containing N6-alkyladenosines and 2-methylthio-N6-alkyladenosines via post-synthetic modification of precursor oligomers

The N⁶-alkyladenosines and 2-methylthio-N⁶-alkyladenosines are the most common modified adenosine nucleosides and transfer ribonucleic acids (tRNA) are particularly rich in these modified nucleosides. They are present at position 37 of the anticodon arm and the contribution of these hypermodified nucleosides to codon–anticodon interactions, as well as translation, are significant, although not fully understood. Herein we described a new chemical synthesis method of the oligoribonucleotides containing N⁶-alkyladenosines and 2-methylthio-N⁶-alkyladenosines via post-synthetic modifications of precursor oligoribonucleotides. To obtain oligoribonucleotides containing N⁶-alkyladenosines, the precursor oligoribonucleotide carrying 6-methylthiopurine riboside residue was used, whereas for the synthesis of oligoribonucleotides containing 2-methylthio-N⁶-alkyladenosines the precursor oligoribonucleotide carrying the 2-methylthio-6-chloropurine riboside was applied. Among the modified oligoribonucleotides of different length and secondary structures, there were several containing naturally occurring modified nucleosides such as: N⁶-isopentenyladenosine (i⁶A), N⁶-methyladenosine (m⁶A), 2-methylthio-N⁶-isopentenyladenosine (ms²i⁶A), and 2-methylthio-N⁶-methyladenosine (ms²m⁶A), as well as several unnaturally modified adenosine derivatives.     

Preparation of 8-Chloropurine Nucleosides Through the Reaction Between their C-8 Lithiated Species and p-Toluenesulfonyl Chloride

Abstract

Chlorination of purine nucleosides protected with tert-butyldimethylsilyl (TBDMS) group was examined by the reaction of the C-8 lithiated species, generated by LDA, with p-toluenesulfonyl chloride as an electrophile. This provides a new method for the preparation of 8-chloropurine nucleosides.     

Synthesis of 2-Alkynylcordycepins and Evaluation of Their Vasodilating Activity

Abstract

Based on the recently developed lithiation-mediated stannyl migration of 6-chloropurine derivatives, 2-iodocordycepin was prepared from cordycepin. The reaction of this compound with terminal alkynes was carried out to synthesize a series of 2-alkynyl derivatives. The vasodilating effect of these compounds was evaluated.     

Synthesis of new nucleoside analogues comprising a geminal difluorocyclopropane moiety as potential antiviral/antitumor agents

Geminal difluorocyclopropane analogues of nucleosides 7a-7e were synthesized. Compounds 7a and 7c-7e were obtained by alkylation of nucleic acid bases or their appropriate precursors with (cis)-1-benzyloxymethyl-2-bromomethyl-3,3-difluorocyclopropane+ ++ (8). Analogue 7b was prepared by hydrolysis of 2-amino-6-chloropurine derivative 7e. Compounds 7a-7d did not exhibit any antiviral activity against HCMV, HSV-1, HSV-2, EBV, VZV, HBV and HIV-1 or antitumor effects against murine leukemia L1210, mouse tumors PO3 or C38 and human tumor H15.     

Synthesis of 4-formyl-4-imidazolin-2-one nucleosides, isomers of uridine and 2'-deoxyuridine

The syntheses of the ribo- and deoxyribonucleoside derivatives of 4-formyl-4-imidazolin-2-one, isosteric isomers of uridine and 2'-deoxyuridine, respectively, were carried out by ring contraction of the corresponding 5-bromouracil nucleosides, followed by conversion of the carboxyl side-chain of the products to the respective carboxaldehyde derivatives.     

Novel substituted 9-norbornylpurines and their activities against RNA viruses

We report on the synthesis and the study of the structure-activity relationship of novel 9-norbornyl-6-chloropurine derivatives, which exert selective antiviral activity on the replication of Coxsackievirus B3. In particular, the synthetic approaches towards norbornyl derivatives bearing diverse side chains were studied. The main goal of the study was to determine the influence of the norbornane moiety substitution at positions 5' and 6' on selective antiviral activity with special regard to the liphophilicity profile of the substituent.     

Synthesis of 9-alkyl and 9-heteroalkyl substituted 2-amino-6-guanidinopurines and their influence on the NO-production in macrophages

9-Alkyl and 9-heteroalkyl substituted derivatives of the 2-amino-6-guanidinopurine were synthesized by alkylation of 2-amino-6-chloropurine and subsequent guanidinolysis. The activity of the thus prepared compounds on murine macrophages was examined. Compounds 4a, 4b, and 4d inhibit the LPS+IFN-gamma-induced NO production in murine macrophages while compound 4h stimulates this production.     

Synthesis of base substituted 2-hydroxy-3-(purin-9-yl)-propanoic acids and 4-(purin-9-yl)-3-butenoic acids

Alkylation of 6-chloropurine and 2-amino-6-chloropurine with bromoacetaldehyde diethyl acetal afforded 6-chloro-9-(2,2-diethoxyethyl)purine (3a) and its 2-amino congener (3b). Treatment of compounds 3 with primary and secondary amines gave the N6-substituted adenines (5a-5c) and 2,6-diaminopurines (5d-5f). Hydrolysis of 3 resulted in hypoxanthine (6a) and guanine (6b) derivatives, while their reaction with thiourea led to 6-sulfanylpurine (7a) and 2-amino-6-sulfanylpurine (7b) compounds. Treatment with diluted acid followed by potassium cyanide treatment and acid hydrolysis afforded 6-substituted 3-(purin-9-yl)- and 3-(2-aminopurin-9-yl)-2-hydroxypropanoic acids (8-10). Reaction of compounds 3 with malonic acid in aqueous solution gave exclusively the product of isomerisation, 6-substituted 4-(purin-9-yl)-3-butenoic acids (15).