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

Alkoxyalkynes 9 may be hydroborated with pinacol borane, preferentially und er Cp2ZrHCl catalysis, to give the vinylboronates 10. The latter, when subj ected to the Matteson-Brown homologation with LiCH2Cl, give rise to the (E) -gamma -alkoxyallylboronates 3 in good yield. This reaction sequence has be en used to generate the (E)-gamma -alkoxyallylboronates 14, 21, 26 and 31, which were the starting point for intramolecular allylboration reactions le ading to the trans-disubstituted tetrahydropyrans 8 and 22, as well as hydr ooxepans 27 and 32.