The stereochemical and conformational factors controlling the intramolecular hydrogen atom transfer (HAT) reaction between the two pyranoseunits in a (1
4)-disaccharide when promoted by a primary 6-
O-yl radical are studied. Models with
-
D-Glc
p-(1
4)-
-
D-Glc
p or
-
L-Rham
p-(1
4)-
-
D-Gal
p skeletons lead exclusively to the abstraction of H-C-5' and the formation, through a nine-membered transition state, of a1,3,5-trioxocane ring system in a stable boat-chair conformation. Notwithstanding, derivatives of
-
L-Rham
p-(1
4)-
-
D-Glc
p abstract exclusivelyH-C-1' through a seven-membered transition state and therefore lead to an interglycosidic spiro ortho ester.