Practical synthesis of 4′-selenopurine nucleosides by combining chlorinated purines and ‘armed’ 4-selenosugar

Synthesis of 6-Selanyl-2-triazolylpurine Derivatives Using 2,6-Bistriazolylpurines as Starting Materials

Two pathways toward 6-selanyl-2-triazolylpurine derivatives were designed. The first method involved the synthesis of 2-chloro-6-selanylpurine derivatives, further SNAr reaction with NaN3, and following CuAAC using different alkynes. The second method was based on the synthesis of 2,6-bistriazolylpurine derivatives as starting materials followed by SNAr reaction with commercial or in situ generated selenols as nucleophiles. A series of 2-chloro-6-selanylpurine derivatives were obtained in yields up to 84%. It was found that in the latter compounds, 6-selanyl moiety was the better leaving group compared to 2-chlorosubstituent in SNAr reactions. On the other hand, the SNAr reaction between 2,6-bistriazolylpurines and selenols or diselenides was successful, and 13 examples of 6-selanyl-2-triazolylpurine derivatives were obtained in yields up to 87%. This direct approach for the Se-C bond formation proved the ability of the 1,2,3-triazolyl ring at the C6 position of purine to act as a good leaving group.

Organoselenium Compounds: Chemistry and Applications in Organic Synthesis

Selenium, originally described as a toxin, turns out to be a crucial trace element for life that appears as selenocysteine and its dimer, selenocystine. From the point of view of drug developments, selenium-containing drugs are isosteres of sulfur and oxygen with the advantage that the presence of the selenium atom confers antioxidant properties and high lipophilicity, which would increase cell membrane permeation leading to better oral bioavailability. In this article, we have focused on the relevant features of the selenium atom, above all, the corresponding synthetic approaches to access a variety of organoselenium molecules along with the proposed reaction mechanisms. The preparation and biological properties of selenosugars, including selenoglycosides, selenonucleosides, selenopeptides, and other selenium-containing compounds will be treated. We have attempted to condense the most important aspects and interesting examples of the chemistry of selenium into a single article.

Synthesis and properties of fully-modified 4'-selenoRNA, an endonuclease-resistant RNA analog

A large number of chemically modified oligonucleotides (ONs) have been developed for RNA-based technologies. In most modified RNAs, the characteristic 2'-hydroxyl (2'-OH) groups are removed to enhance both nuclease resistance and hybridization ability. However, the importance of the 2'-OH group in RNA structure and function is well known. Here, we report the synthesis and properties of 4'-selenoRNA in which all four nucleoside units retain the 2'-OH groups but contain a selenium atom instead of an oxygen atom at the 4'-position of the furanose ring. 4'-SelenoRNA has enhanced ability to form duplexes with RNA, and high endonuclease resistance despite the presence of the 2'-OH groups. X-ray crystallography analysis showed that the 4'-selenoRNA duplex adopts an A-conformation, similar to natural RNA, although one 4'-selenocytidine residue has unusual South-type sugar puckering. Furthermore, preliminary studies using 4'-seleno-modified siRNAs suggest that 4'-selenoRNA may be applicable to RNA interference technology. Collectively, our results raise the possibility of a new class of modified RNA in which 2'-OH groups do not need to be masked.

Synthesis of 4-Selenothreofuranose Derivatives via Pummerer-Type Reactions of trans-3,4-Dioxygenated Tetrahydroselenophenes Mediated by a Selenonium Intermediate

Selenosugars are interesting targets of organic synthesis as they would possess potential biological activities. However, 4-selenotherofuranose derivatives, which have trans configuration for the two dihydroxy substituents at the 2,3-positions and a glycoside bond at the anomeric position, are not available in the current selenosugar library. In this study, racemic 4-selenothreofuranose derivatives were synthesized from trans-3,4-dioxygenated tetrahydroselenophenes in 77-99% yields with the α/β selectivity about 7:3 via oxidation and subsequent seleno-Pummerer rearrangement. The acetoxy group introduced at the anomeric position was then substituted with various nucleophiles, including activated 6-chloropurine, which afforded 4'-selenothreonucleoside derivatives, in the presence of BF₃·OEt₂ or TMSOTf. The stereochemistry of the selenosugar products was established by ¹H NMR spectroscopy as well as X-ray analysis. The similar α/β selectivity observed in the latter glycosylation reaction to that in the former seleno-Pummerer rearrangement suggested the mediation of a common selenonium intermediate (-Se⁺=C<). It was also suggested that an unexpected interaction between the ester protecting group at the 3-position of the selenofuranose ring and the anomeric carbon atom decreases the α/β selectivity.

Solid-phase synthesis and structural characterisation of phosphoroselenolate-modified DNA: a backbone analogue which does not impose conformational bias and facilitates SAD X-ray crystallography

Oligodeoxynucleotides incorporating internucleotide phosphoroselenolate linkages have been prepared under solid-phase synthesis conditions using dimer phosphoramidites. These dimers were constructed following the high yielding Michaelis-Arbuzov (M-A) reaction of nucleoside H-phosphonate derivatives with 5'-deoxythymidine-5'-selenocyanate and subsequent phosphitylation. Efficient coupling of the dimer phosphoramidites to solid-supported substrates was observed under both manual and automated conditions and required only minor modifications to the standard DNA synthesis cycle. In a further demonstration of the utility of M-A chemistry, the support-bound selenonucleoside was reacted with an H-phosphonate and then chain extended using phosphoramidite chemistry. Following initial unmasking of methyl-protected phosphoroselenolate diesters, pure oligodeoxynucleotides were isolated using standard deprotection and purification procedures and subsequently characterised by mass spectrometry and circular dichroism. The CD spectra of both modified and native duplexes derived from self-complementary sequences with A-form, B-form or mixed conformational preferences were essentially superimposable. These sequences were also used to study the effect of the modification upon duplex stability which showed context-dependent destabilisation (-0.4 to -3.1 °C per phosphoroselenolate) when introduced at the 5'-termini of A-form or mixed duplexes or at juxtaposed central loci within a B-form duplex (-1.0 °C per modification). As found with other nucleic acids incorporating selenium, expeditious crystallisation of a modified decanucleotide A-form duplex was observed and the structure solved to a resolution of 1.45 Å. The DNA structure adjacent to the modification was not significantly perturbed. The phosphoroselenolate linkage was found to impart resistance to nuclease activity.

Synthesis and biological evaluation of novel imidazole nucleosides as potential anti-dengue virus agents

In this work, we developed imidazole nucleoside derivatives with anti-dengue virus (DENV) activity was examined. First, compounds in a nucleosides library were screened to find lead compounds which inhibit replication of DENV. As a result, 5-ethynyl-(1-β-d-ribofuranosyl)imidazole-4-carboxamide (1; EICAR) and its 4-carbonitrile derivative EICNR (2) were selected as promising antiviral compounds. However, both of them also exhibited cytotoxicity. In order to develop an effective and less toxic compound, 4'-thio and 4'-seleno derivatives of EICAR and EICNR 3-6 were prepared. The resulting 4'-thioEICAR and 4'-thioEICNR showed inhibitory effect on DENV replication without cytotoxicity as potent as ribavirin, a positive control.

Glycosylation reactions mediated by hypervalent iodine: application to the synthesis of nucleosides and carbohydrates

To synthesize nucleoside and oligosaccharide derivatives, we often use a glycosylation reaction to form a glycoside bond. Coupling reactions between a nucleobase and a sugar donor in the former case, and the reaction between an acceptor and a sugar donor of in the latter are carried out in the presence of an appropriate activator. As an activator of the glycosylation, a combination of a Lewis acid catalyst and a hypervalent iodine was developed for synthesizing 4'-thionucleosides, which could be applied for the synthesis of 4'-selenonucleosides as well. The extension of hypervalent iodine-mediated glycosylation allowed us to couple a nucleobase with cyclic allylsilanes and glycal derivatives to yield carbocyclic nucleosides and 2',3'-unsaturated nucleosides, respectively. In addition, the combination of hypervalent iodine and Lewis acid could be used for the glycosylation of glycals and thioglycosides to produce disaccharides. In this paper, we review the use of hypervalent iodine-mediated glycosylation reactions for the synthesis of nucleosides and oligosaccharide derivatives.