Beneficial Effect of Internal Hydrogen Bonding Interactions on the β-Fragmentation of Primary Alkoxyl Radicals. Two-Step Conversion ofd-Xylo- andd-Ribofuranoses intol-Threose andd-Erythrose, Respectively

Abstract

Primary alkoxyl free radicals were generated from their readily synthesized N-phthalimido derivatives under reductive conditions. Primary alkoxyl radicals derived from their corresponding xylo- and ribofuranose derivatives underwent, exclusively, an unusual beta-fragmentation affording L-threose and D-erythrose derivatives, respectively. This occurs because the alkoxyl radical is capable of achieving an internal hydrogen-bonding interaction leading to a stable six-membered ring intramolecular hydrogen-bonded structure. When the hydroxyl group is protected, the beta-fragmentation pathway is prevented and the hydrogen atom transfer (HAT) pathway occurs. Computational studies provided strong support for the experimental observations.