Diastereospecific synthesis of 2′- or 3′-c-branched nucleosides through intramolecular free-radical capture by silicon-tethered acetylene

Abstract The intramolecular free-radical trapping by a silicon tethered acetylene function in a ribonucleoside 2a , 2b , 2c , 8a , 8b or 8c gave only the [3.3.0]-cis-fused Z-vinylsiloxane 3a , 3b , 3c , 9a , 9b or 9c (&>90%) in a diastereospecific manner. The temporary silicon connection from the Z-vinylsiloxane was removed by the oxidation to give the 2′- or 3′-C-branched α-keto-,β-D-ribonucleoside (∼85%), which was diastereospecifically reduced to give 1,3-syn diol (&>90%, &>97% ee) [2′-or 3′-C-branched-α-substituted R or S-hydroxymethyl-β-D-ribonucleosides 5a , 5b , 5c , 11a , 11b or 11c ]. These are the first examples of highly diastereospecific synthesis of 2′-or 3′-C-branched ribonucleosides, through a 5-trig-exo radical cyclization, using the silicon-tethered approach.