Asymmetric Organic Synthesis Via New Enantioenriched Allylsilanes

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Highly enantioenriched allylsilanes bearing an ω-hydroxy group were synthesized stereoselectively via a sequence involving palladiumcatalyzed intramolecular bis-silylation of allylic alcohols. The new functionalized allylsilanes reacted with aldehydes in the presence of Lewis acids, giving cyclic ethers via acetalization followed by intramolecular Hosomi-Sakurai reaction. The allylsilanes with an ω-hydroxy group on their γ-alkyl group afforded 2-substituted cyclic ethers with 3-(1-alkenyl) group stereoselectively. For instance, reactions of (S)-(E)-6-(dimethylphenylsilyl) dec-4-en-1-ol (93% ee) and (R)-(E)-7-(dimethylphenylsilyl) undec-5-en-1-ol (96% ee) with heptanal afforded trans-2-hexyl-3-((E)- 1-hexenyl) tetrahydropyran (92% ee) and trans-2-hexyl-3-((E)-1-hexenyl) oxepane (94% ee), respectively, in good yields with >97% stereoconservation. On the other hand, allylsilanes with an ω-hydroxy group on their α-alkyl chain afforded 2, 3-disubstituted oxacycloalk-4-enes stereoselectively. In these reactions, not only high diastereoselectivity but also highly stereoselective transfer of chirality from the allylsilanes to the cyclic ethers was accomplished.