Furanose ring anomerization: a kinetic study of the 5-deoxypentoses and 5-O-methylpentoses

The anomerization of 5-deoxy-L-pentoses (1-4) and 5-O-methyl-D-pentoses (5-8) in aqueous solution has been studied by 13C saturation-transfer n.m.r. (s.t.-n.m.r.) spectroscopy, using compounds substituted with 13C at the anomeric carbon atom. Unidirectional rate-constants of ring-opening (k open) and ring-closing (k close) have been obtained for these compounds under identical solution conditions (50mM acetate buffer, pH 4.0 at 60 degrees), and have been compared to those measured for the D-tetroses (9 and 10) and the four D-pentose 5-phosphates (11-14). Based on these comparisons, several correlations between furanose structure and reactivity have been revealed, and models have been proposed to explain the observed kinetic behavior of compounds (1-10). The effect of exocyclic structure on acid-catalyzed rate-constants was also examined by comparing the behavior of 5-deoxy-L-(1-13C)-lyxose and 5-O-methyl-D-(1-13C)lyxose. Some consideration has been given to identifying the factors (enthalpic and entropic) that may play roles in determining the effect of structure on anomerization reactivity.