Reactions with a vinylsilyl group as a radical-acceptor tether: The stereoselective introduction of C2-units and the ring-enlargement reaction of (3-oxa-2-silacyclopentyl) methyl radicals into 4-oxa-3-silacyclohexyl radicalsvia a pentavalent silicon-bridging radical transition state

We developed a regio- and stereoselective method for introducing l-hydroxye thyl, 2-hydroxyethyl, and vinyl groups at the position beta to a hydroxyl g roup in halohydrins or alpha -phenylselenoalkanols using an intramolecular radical cyclisation reaction with a dimethyl- or diphenylvinylsilyl group a s a temporary connecting radical-acceptor tether (Scheme 14). Thus, when a vinylsilyl ether of halohydrins or alpha -phenylselenoalkanols (A) was subj ected to the radical reaction with Bu3SnH/AIBN, the selective introduction of both l-hydroxyethyl and 2-hydroxyethyl groups can be achieved, depending on the concentration of Bu3SnH in the reaction system, via a 5-exo-cycliza tion intermediate E or a B-endo-cyclization intermediate F, respectively, a fter oxidative ring-cleavage by treating the cyclization products under Tam ao oxidation conditions. A vinyl group can also be introduced by photo-irra diating the vinylsilyl ether A in the presence of (Bu3Sn)(2), and then trea ting the resulting atom-transfer 5-exo-cyclization product I with fluoride ion. The mechanistic studies showed that the kinetically favored 5-exo-cycl ized radical C, formed from radical B, was trapped when the concentration o f Bu3SnH was high enough to give E. At lower concentrations of Bu3SnH and h igher reaction temperatures, radical C rearranged into the more stable ring -enlarged 4-oxa-3-silacyclohexyl radical D, which was then trapped with Bu3 SnH to give F. The ring-enlarging rearrangement was experimentally proved t o occur via a pentavalent-like silicon-bridging transition state X (Scheme 9). This radical reaction with a vinylsilyl tether has been successfully ap plied to the synthesis of biologically important 4'-branched-chain sugar nu cleosides and C-glycosides.