Stereoselective Synthesis of Functionalized <i>trans</i>-2,5-Disubstituted Tetrahydrofurans

The addition of the titanium enolates of <i>N</i>-acetyl, <i>N</i>-propionyl, and <i>N</i>-bromoacetyl (<i>R</i>)-oxazolidin-2-ones to γ-lactol <b>2</b>, derived from (<i>S</i>)-glutamic acid, afforded <i>trans</i>- and <i>cis</i>-2,5-disubstituted tetrahydrofurans (<i>trans</i>/<i>cis</i> ratio:  R = H, 2:1; R = Me, 8:1; R = Br, 10:1) after desilylation with aqueous HF/CH<sub>3</sub>CN. Chromatographic separation and LiBH<sub>4</sub> reduction allowed the efficient preparation of the corresponding <i>trans</i>-2,5-disubstituted tetrahydrofuran diols and the recovery of the chiral auxiliary.