A non-destructive reactivity index of carbohydrates: the hydroxy protonation rates in dimethyl sulphoxide

Abstract

Proton transfers to 26 hydroxylated monosaccharides have been measured in anhydrous dimethyl sulphoxide (DMSO) at 25 °C under acid-catalysed conditions, using dynamic ¹H n.m.r. techniques at 250 MHz. The results are summarized by the equation, rate of hydroxylic proton exchange =k[DMSO ⋯ H⁺], with k= 1–3 and 4–9 × 10⁶ l mol^–1 s^–1 for anomeric and non-anomeric hydroxy-groups, respectively. Anomeric hydroxy-groups are protonated slightly faster in ketopyranoses and furanoses than in aldopyranoses, a sequence roughly parallel to that observed for the acid-catalysed hydrolysis of the corresponding glycosides. The protonation rate of an anomeric hydroxy-group is sensitive to the equatorial or axial nature of neighbouring hydroxy-groups. The reverse is true for non-anomeric hydroxy-groups, except if the neighbouring substituent is itself an anomeric hydroxy-group. Steric and electronic effects of 1- and 6-substituents are discussed. The order of kinetic basicities of the hydroxy-groups being protonated is in fairly good agreement with the range of acidities reflected by the chemical shifts of the hydroxy-protons of pyranoses.