Diastereospecific Synthesis of 2′- or 3′-C-Branched Nucleosides Through Intramolecular Free-Radical Capture by Silicon-Tethered Acetylene.

Abstmct: The intramokcularfree-radical napping by a silicon tethered acerpknejimcdon in a ribonuckoside 20. 2). Zc, 8a,8b or 8c gave only the [3.3.0]-c&fused Z-vinylsiloxane 3a, 3b, 3c, 9a. 9b or 9c (>90%) in a diastereospecifk manner. The temporary silicon connection from the Zvinylsiloxane was removed by the oxidation to give the 2’- or 3’-C-branched a-keto-/3-D-ribonacl (45%). which ws diaslereospec$ically re&uzed IO give 1.3~syn diol(>90%. >97% ee) [2’- or 3*-C-branched-a-subsdbsritrued R or S-hydroxymethyl-ED-ribonucleosides Sa, Sb. SC. Ilo, Ilb or Ilc]. These are the first examples of highly diastereospecifc synthesis of 2’- or 3,-Cbranched ribonucleosides, through a 5-wig-exe radical cyclisadon, using the silicon-tethered appr&h.