ビニルシリル基をテザーに用いるラジカル反応 Ｃ２ユニットの立体選択的導入法の開発と５配位ケイ素ラジカル遷移状態を経由する新規ラジカル環拡大反応

Abstract

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 Bu3SnH/AIBN, the selective introduction of both 1-hydroxyethyl and 2-hydroxyethyl groups can be achieved, depending on the concentration of Bu3SnH 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 (Bu3Sn)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 Bu3SnH 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 Bu3SnH 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 andC-glycosides.