Intramolecular allylboration of γ-(ω-formylalkoxy)allylboronates for syntheses of trans- or cis-2-(ethenyl)tetrahydropyran-3-ol and 2-(ethenyl)oxepan-3-ol

3-Alkoxy-1-alkynes were hydroborated with pinacolborane (HBpin) to give 3-alkoxy-1-alkenylboronates . The latter gave (E)-3-alkoxyallylboronates (: (E)-(MeO)2CHCH2(CH2)nCH2OCHCHCH2Bpin, n=1–3) when they were subjected to iridium-catalyzed isomerization of the double bond. The corresponding (Z)-isomers were synthesized by nickel-catalyzed isomerization of . Both allylboronates underwent intramolecular allylboration leading to the formation of trans-2-(ethenyl)tetrahydropyran-3-ol or 2-(ethenyl)oxepan-3-ol from and the corresponding cis-isomers from in the presence of Yb(OTf)3 (20 mol%) in aqueous acetonitrile at 90°C.