Intermolecular and intramolecular reactions of resolved 2-alkoxytetrahydrofuran-3-yl and 2-alkoxytetrahydropyran-3-yl radicals

Diastereomeric trans-2-alkoxy-3-phenylselenenyltetrahydrofuranyl acetals and trans-2-alkoxy-3-phenyl-selenenyltetrahydropyranyl acetals were prepared from dihydrofuran, dihydropyran, phenylselenenyl bromide, and the alcohols (S)-methyl lactate, (S)-methyl hexahydromandelate, and (R)-pantolactone. The diastereomers were chromatographically separated and were subjected to intermolecular alkylation via generation and trapping of free radicals. Observed diastereoselectivity was higher for 2-alkoxytetrahydrofuran-3-yl radicals than for 2-alkoxytetrahydropyran-3-yl radicals, and highest for acetals involving (R)-pantolactone. Alcohol exchanges at the anomeric carbon were highly stereocontrolled, and permitted introduction of alkenes for intramolecular trapping of free radical intermediates. (C) 2000 Elsevier Science Ltd.