INFRARED STUDY OF INTRAMOLECULAR HYDROGEN-BONDS IN ALPHA-L-RHAMNOSE DERIVATIVES IN SOLUTION

Three benzyl derivatives of alpha-L-rhamnopyranoside in dilute solutions of CCl4, CS2, CHCl3 and CH2Cl2 were studied in the fundamental OH-stretching region. Two types of H bonds were found to be present, viz., a 5-R OH ... O and an OH ... pi-bond, absorbing in distinct frequency regions. The latter are present only because the fixed pyranose skeleton is favoured with respect to the former. Three OH groups mutually positioned gauche at adjacent carbon atoms form an H-bond "chain". Inversion of the pyranose ring occurs in compounds with only O(1)H or O(1)H and O(4)H protected, although one chair conformation is less favourable owing to steric interactions.