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 (1→4)‐ O ‐disaccharide when promoted by a primary 6‐ O ‐yl radical are studied. Models with α‐ D ‐Glc p ‐(1→4)‐β‐ D ‐Glc p , α‐ L ‐Rham p ‐(1→4)‐α‐ D ‐Gal p or α‐ D ‐Man p ‐(1→4)‐β‐ L ‐Gul p 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 α‐ L ‐Rham p ‐(1→4)‐α‐ D ‐Glc p or α‐ D ‐Man p ‐(1→4)‐α‐ D ‐Gal p exclusively abstract the hydrogen from HC‐1′ through a seven‐membered transition state and, therefore, lead to an interglycosidic spiro ortho ester.