STEREOCONTROLLED SYNTHESIS OF TETRAHYDROFURANS AND TETRAHYDROPYRANS BY CYCLIZATION OF HYDROXYSELENIDES

An efficient stereocontrolled synthesis of 2,3,5-trisubstituted tetrah ydrofuran and 2,4,6-trisubstituted tetrahydropyran rings from homoally lic alcohols (16) and (18) was achieved by: i) epoxidation; ii) ring o pening of the epoxide with sodium phenyl selenide; iii) cleavage of th e TBDMS group with a stereoconvergent elimination of water followed by intramolecular oxygen nucleophile capture. The presence of a branchin g near to the selenonium ring made the attack in the exo mode faster t han the attack in the endo/exo mode.