Asymmetric Hosomi - Sakurai Reaction of Allylsilanes Containing Arabinose-Derived Alcohols as Chiral Auxiliaries

Five enantiopure allylsilanes la-e have been prepared, with arabinose-derived alcohols methyl and benzyl 3,4-O-isopropylidene- and 3,4-O-methylene-β-L-arabinopyranosides as chiral auxiliaries, and subjected to the asymmetric Hosomi - Sakurai reaction. The effect of Lewis acid on the stereochemical outcome of the reaction was investigated, and BF₃ was found to exhibit higher enantioselectivity than that of SnCl₄ or TiCl₄. The reaction of benzyl 2-O-(allyldimethylsilyl)-3,4-O-isopropylidene-β-L-arabinopyranoside (1b) with n-decanal gave the highest ee (45%) in the presence of BF₃. The steric effect of the chiral auxiliary on the asymmetric Hosomi-Sakurai reaction is demonstrated for the first time although the stereogenic center is remote from the reaction site.