Deoxygenated and alkylated furanoses: Thorpe—Ingold effects on tautomeric equilibria and rates of anomerization

Abstract

2-Deoxy- d- glycero-tetrose, 3-deoxy- dl- glycero-tetrose, 3-deoxy-3,3-di- C-methyl- dl- glycero-tetrose, 3- C-methyl- dl-erythrose, 3- C-methyl- dl-threose, 2-deoxy-5- O-methyl- d- erythro-pentose and 3-deoxy-5- O-methyl- d- erythro-pentose have been prepared, in some cases with 13C-substitution at the anomeric carbon, and characterized by 1H- (300 and 620 MHz) and 13C-n.m.r. (75 MHz) spectroscopy. The proportions of cyclic (α and β furanoses) and acyclic (aldehyde and hydrate) forms were determined in aqueous ( 2H 2O) solution, and ring-opening ( k open) and ring-closing ( k close) rate constants were measured by 1H and 13C saturation-transfer n.m.r. spectroscopy at p 2H 5.0 (acetate buffer) and 60°. The degree of furanose ring substitution was found to significantly affect both the thermodynamics and kinetics of furanose anomerization. Increased substitution enhances the proportion of cyclic forms in solution by stimulating furanose k close. In contrast, furanose k open was less affected by the degree of substitution; however, kinetic studies of 2-deoxyfuranose anomerization implicate furanose ring conformation as a potential determinant of k open.