Stereoselective Synthesis of 2‘,3‘-Dideoxy-3‘-fluoro-2‘-phenylselenenyl- β-nucleosides from Phenyl 1-Seleno-α-arabino-furanosides through Consecutive 1,2-Migration and Glycosylation under Mitsunobu Conditions. A New Entry to 2‘,3‘-Dideoxy-3‘-fluoronucleosides

Nucleophilic Substitution Reactions of 2‐Phenylthio‐Substituted Carbohydrate Acetals and Related Systems: Episulfonium Ions vs. Oxocarbenium Ions as Reactive Intermediates

“…[13,16] With optimization, the sulfur system could be rendered diastereoselective, but only low selectivity was observed for the selenium series. From these results, the authors concluded that these reactions do not involve episulfonium or episelenonium ions.…”

“…From these results, the authors concluded that these reactions do not involve episulfonium or episelenonium ions. [13,16] Scheme 10. Unselective 1,2-sulfur migration/glycosylation reaction.…”

“…[12] Although mechanisms involving bridged intermediates such as the episulfonium ions 4 and 8 provide concise and satisfying explanations for observations such as those shown in Schemes 1 and 2, accumulating evidence suggests that such intermediates may not be involved in stereoselective substitution processes. The fact that reactions that would be expected to proceed through episulfonium, [6,7,[13][14][15] episelenonium, [13,16] or iodonium ions [9,10] (9, Figure 1) are not completely selective [17] indicates that these reactions may occur by a different pathway or through multiple pathways. The formation of the product that would not correspond to stereospecific opening of the onium ion 9 (i.e., the 1,2-cis product) is typically explained by invoking oxocarbenium ions 10 as reactive intermediates.…”

Stereoselective Synthesis of 2-Deoxy-furanosides from 2,3-Anhydro-furanosyl Thioglycosides

“…Although the mechanism by which 8 is produced from 1 has not been studied, a plausible explanation (Figure ) is that the triflic acid generated in the course of the glycosylation protonates the epoxide, leading to migration of the thioaryl group from C1 to C2 with concomitant glycosylation. This reaction is thus related to other tandem chalcogenoglycoside migration glycosylation processes, − which have been postulated to proceed through episulfonium ion intermediates (e.g., 9 ).

3 Proposed mechanism for the formation of 8 from 1 .

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Biotechnology of Nucleic Acid Constituents - State of the Art and Perspectives

“…Pyrimidine nucleosides are produced by chemical methods employing the natural nucleosides as starting compounds [76,77], or by a convergent approach via the condensation of the carbohydrate precursor and heterocyclic base [78][79][80][81][82][83][84][85]. A chemical transglycosylation of purine bases using the corresponding thymine or uracil nucleosides as the deoxyfluoro sugar donor [86] or by a convergent synthesis [85,87] suffers from many drawbacks, most serious of which are the low stereospecificity of the glycosydic bond formation and ambiguous regiospecificity.…”