We developed a regio- and stereoselective method for introducing 1-hydroxyethyl, 2-hydroxyethyl, and vinyl groups at the position β to a hydroxyl group in halohydrins or α-phenylselenoalkanols using an intramolecular radical cyclization reaction with a dimethyl- or diphenylvinylsilyl group as a temporary connecting radical-acceptor tether (Scheme 14). Thus, when a vinylsilyl ether of halohydrins or α-phenylselenoalkanols (A) was subjected to the radical reaction with Bu
3SnH/AIBN, the selective introduction of both 1-hydroxyethyl and 2-hydroxyethyl groups can be achieved, depending on the concentration of Bu
3SnH in the reaction system, via a 5-
exo-cyclization intermediate E or a 6-
endo-cyclization intermediate F, respectively, after oxidative ring-cleavage by treating the cyclization products under Tamao oxidation conditions. A vinyl group can also be introduced by photo-irradiating the vinylsilyl ether A in the presence of (Bu
3Sn)
2, and then treating the resulting atom-transfer 5-
exo-cyclization product I with fluoride ion. The mechanistic studies showed that the kinetically favored 5-exo-cyclized radical C, formed from radical B, was trapped when the concentration of Bu
3SnH was high enough to give E. At lower concentrations of Bu3SnH and higher reaction temperatures, radical C rearranged into the more stable ring-enlarged 4-oxa-3-silacyclohexyl radical D, which was then trapped with Bu
3SnH to give F. The ring-enlarging rearrangement was experimentally proved to occur via a pentavalent-like silicon-bridging transition state X (Scheme 9). This radical reaction with a vinylsilyl tether has been successfully applied to the synthesis of biologically important 4'-branched-chain sugar nucleosides and
C-glycosides.
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