| Chapter one provides the background and significance of the thesis. Anitviral drugs derived from nucleoside have played an essential role in therapy against epidemical diseases caused by virus infection. Most of nucleoside drugs on the market or drug candidates in clinical phases structurally belong to a category of 2′,3′-modified nucleosides. Several synthetic methods have been developed for the preparation of these nucleoside analogues, including: 1) nucleophilic addition or nucleophilic substitution reaction; 2) radical reaction; 3) palladium catalyzed coupling reaction; 4) 2′,3′-double bonds formation. Sugar modified nucleosides not only exert biological activities but also generate substantial impact on antisense therapy by virtue of conformational restriction. The so called"antisense"strategy is to design and synthesize an oligonucleotide fragment with defined sequence complementary with the target mRNA by Watson-Crick base paring and thus selectively form duplex with the target mRNA blocking its translation process in formation of biologically functional proteins. Construction of a new ring system on sugar moiety of nucleoside and incorporation of such modified nucleoside into oligonucleotide constitutes the so called"locked nucleic acid"(LNA). It has been reported that LNA/DNA polymers demonstrated distinct"antisense"activity. Therefore, it is an effective strategy to search for potent and low toxic antiviral drug via fused ring nucleosides which are with novel structure feature, capability for fine turning the conformation of sugar and practicability inserted into oligonucleotide.The research work in this thesis is to synthesize 2′,3′-bicyclic nucleosides and to study their molecular structure and biological activity. The significant interest of these molecules rely on :1) retain the 5′-hydroxyl for phosphorylation and nucleobase for recognition; 2) restrict the conformation of nucleoside by the ring fusion to the 2′,3′-positions of the furanose ring especially with the six member ring; 3) adjust the conformation of furanose by introduction of hetero atoms on different positions with different valence; 4) subsititution of 2′or 3′position with NH allows the formation of LNA; 5) molecular structure and biological activities is of interest for the unique nucleosides.The Second Chapter discusses the synthesis of 2′,3′-fused nucleoside via 2′,3′-eneselenone nucleosides as the key intermediates. Selenonyl is a strong electron withdrawing group when connected with a unsaturated carbon, while turns to be a good leaving group when connected with a saturated carbon (sp3 carbon). Combining these characters, the carbon-carbon double bond in eneselenone possesses chemical properties equivalent to dication +CH2=CH2+.In this chapter, bicyclic nucleosides were designed to be synthesized via reaction of 2′or 3′-selenonyl-2′,3′-double nucleosides with reagents containing two nucleophilic groups. The sulfur containing bicyclic nucleosides could be oxidized to corresponding sulfoxide and sulfone nucleosides. The synthesis of dinucleotides was designed when the substituents were NH group. Starting from uridine 2′,3′-anhydrouridine was synthesized, the epoxide was opened up by an attack of PhSe- ion and two region isomers were isolated in 28% and 57% respectively. Mesylation 2′-hydroxyl of 3′-phenylselenyl uridine and followed elimination by the treatment with base produced 3′-phenyl-2′,3′- unsaturated uridine, oxidation of this product with m-CPBA resulted 3′-eneselenone uridine. But under the same condition elimination could not be achieved with the regioisomer of 3′-hydroxyl- 2′-phenylselenyl uridine. Thus Mitsunobu reagent was first introduced to produce the 2′-phenylselenyl-2′,3′- double bond which was oxidized with m-CPBA to offer 2′-eneselenone uridine. Synthesis of eneselenone adenosine was also studied, 3′-phenyl selenyl adenosine was synthesized via 2′,3′-O-anhydro-andenosine, but only elimination of phenyl selenyl was obtained when mesylation of 2′-hydroxy, and possible mechanism of the elimination was given.Reactions of 3′-eneselenone uridine with 1,2-Ethylenediamine and 1,2-Ethanedithiol were intensively examined under different conditions (such as changing solvent, temperature, concentration, bases, ratio of nucleophiles and protecting groups) to find suitable condition for the synthesis of six-membered fused ring. Reaction of 3′-eneselenone uridine with acetamide produced oxazole fused bicylic uridine, but with thioacetamide only gave complicated mixtures.Chapter three presents the results of synthesis, structure elucidation and conformational analysis of 2′3′-bicyclic nucleosides. Intermolecular Michael addition was unprecedented employed in synthesis the target molecules. Mitsunobu reaction was intended for cyclization to achieve 2′3′-oxathiine and the further derivatizations.2′,3′-anhydrouridine was reacted with 2-(tert-butyldimethylsilyloxy) ethanethiol in the presence of TMG to generate a mixture of 2′-S-CH2CH2 -OTBS-3′-hydroxyl and its regioisomer, which were separated by silica gel in 26% and 57% isolated yield respectively. Then 3′-isomer was oxidized and 2′-hydroxyl eliminated to double bond. Silyl group was deprotected by TBAF, in same time cyclization was furnished. Depreotection of trityl of 5′-hydroxyl to produce the final 2′,3′-oxathiine fused uridine. Similarly, tert-butyl-2- mercaptoethylcarbamate attacked 2′,3′-epoxide uridine, the isolated 3′-S -CH2CH2NHBoc isomer via oxidation and elimination gave vinyl sulfonyl uridine, After depretecting both the Boc and trityl group in acidic condition, cyclization was carried out in the presence of potassium carbonate, and desired 2′,3′-thiazine fused uridine was obtained.2,2′-anhydro-uridine was used to synthesis 2′-sulfonyl bicylic uridine by the same procedure, but the 3′-hydroxyl cannot be eliminated in the presence of base. Ph3P/DIAD was also used to eliminate 3′-hydroxyl group and 2′- SO2 -CH2CH2NHBoc-2′,3′-double bond uridine was obtained in 65% yield. Deprotection and cylization of the 2′-vinyl sulfonyl uridine however led to depyrimidination under many tested conditions.Reaction condition of oxidation sulfide to sulfoxide was optimized dur- ing the synthsis of 3′-sulfinyl bicylic uridine. 2′-Hydroxyl was also success- fully eliminated by Mitsunobu reagent in high yield. Both of the Michael precursors were obtained and cylization was attempted. It was disclosed that sulfoxide was not as efficient as sulfone to be an electon-withdrawing group in this cyclization.The synthesized 2′,3′-oxathiine and 2′,3′-thiazine bicyclic nucleosides were characterized by 1HNMR, 13CNMR, NOSY, COSY, HRMS and X-ray Single Crystal Diffraction. The conformation of the products was analysed by crystal data, two novel bicyclic nucleosides adopt 4′-exo, 3′-endo conforma- tion with pesudoroation phase angle of 36.8(2)°, 36.9(2)°and sugar puckering amplitude of 47.5(3)°, 49.9(3)°respectively. The glycosydic torsion angleγshows the orientation of the pyrimidine ring in both nucleosi- des to be anti with respect to the sugar group, and the conformations of 5′-hydroxyl group are ap and +sc respectively.The biological tests of the bicyclic nucleosides and their incorporation into oligonucleotide are to proceed in near future.
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