Stereoselective synthesis of alcohols. Part LIII. (E)-γ-Alkoxyallylboronates: generation and application in intramolecular allylboration reactions

Alkoxyalkynes 9 may be hydroborated with pinacol borane, preferentially under Cp2ZrHCl catalysis, to give the vinylboronates 10. The latter, when subjected to the Matteson–Brown homologation with LiCH2Cl, give rise to the (E)-γ-alkoxyallylboronates 3 in good yield. This reaction sequence has been used to generate the (E)-γ-alkoxyallylboronates 14, 21, 26 and 31, which were the starting point for intramolecular allylboration reactions leading to the trans-disubstituted tetrahydropyrans 8 and 22, as well as hydrooxepans 27 and 32.