D-Talose Anomerization: NMR Methods To Evaluate the Reaction Kinetics

Article abstract of DOI:10.1021/ja00189a050

The kinetics of anomerization of the aldohexose, D-talose, have been studied by several NMR methods in order to evaluate their limitations, complementarity, and internal consistency and to futher explore the effect of monosaccharide structure on reactivity.By use of D<1-13C>talose and 13C NMR spectroscopy, six tautomeric forms were detected and quantitated in aqueous solution: α- and β-talofuranoses, α- and β-talopyranoses, hydrate (1,1-gem-diol), and aldehyde.The 13C (75-MHz) and 1H (620-MHz) NMR spectra of D-talose have been interpreted, yielding chemical shiftsand coupling constants (J_HH, J_CC, J_CH) that have been evaluated in terms of ring configuration and conformation.By use of 13C saturation-transfer NMR (ST-NMR), ring-opening rate constants (k_open) of the four cyclic forms were measured, and ring-closing rate constants (k_close) were calculated from k_open and equilibrium constants.NMR-derived rates of tautomer equilibration obtained after dissolving α-D-<1-13C>talopyranose in aqueous solution were predicted accurately from a computer treatment of the unidirectional rate constants determined by ST-NMR under similar solution conditions.Two-dimensional 13C exchange spectroscopy was applied to obtain overall rate constants of tautomer interconversion; rate constants obtained in this fashion compared favorably with those calculated from the ST-derived unidirectional rate constants using the steady-state approximation.Kinetic results show that anomeric configuration and ring size significantly affect ring-opening and ring-closing rates of monosaccharides.