Intramolecular 1,8- versus 1,6-Hydrogen Atom Transfer between Pyranose Units in a (1→4)-Disaccharide Model Promoted by Alkoxyl Radicals. Conformational and Stereochemical Requirements

The stereochemical and conformational factors controlling the intramolecular hydrogen atom transfer (HAT) reaction between the two pyranose units in a (1→4)-disaccharide when promoted by a primary 6-<i>O</i>-yl radical are studied. Models with α-d-Glc<i>p</i>-(1→4)-β-d-Glc<i>p</i> or α-l-Rham<i>p</i>-(1→4)-α-d-Gal<i>p</i> skeletons lead exclusively to the abstraction of H−C-5‘ and the formation, through a nine-membered transition state, of a 1,3,5-trioxocane ring system in a stable boat-chair conformation. Notwithstanding, derivatives of α-l-Rham<i>p</i>-(1→4)-α-d-Glc<i>p</i> abstract exclusively H−C-1‘ through a seven-membered transition state and therefore lead to an interglycosidic spiro ortho ester.