Intramolecular 1,8-hydrogen-atom transfer reactions in (1 -> 4)-o-disaccharide systems: conformational and stereochemical requirements

The stereochemical and conformational factors controlling the intramolecular hydrogen-atom transfer (HAT) reaction between the two pyranose units in a (14)-O-disaccharide when promoted by a primary 6-O-yl radical are studied. Models with -D-Glcp-(14)--D-Glcp, -L-Rhamp-(14)--D-Galp or -D-Manp-(14)--L-Gulp skeletons led exclusively to the abstraction of the hydrogen from HC-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 -L-Rhamp-(14)--D-Glcp or -D-Manp-(14)--D-Galp exclusively abstract the hydrogen from HC-1 through a seven-membered transition state and, therefore, lead to an interglycosidic spiro ortho ester.