Anomerization of Furanose Sugars: Kinetics of Ring-Opening Reactions by 1H and 13C Saturation-Transfer NMR Spectroscopy

Article abstract of DOI:10.1021/ja00379a001

With the tetroses D-threose and D-erythrose, kinetic and thermodynamic parameters for the interconversion of α- and β-furanoses and the acyclic hydrate with the intermediate aldehyde form have been obtained from 1H and 13C NMR measurements.Unidirectional rate constants for the various equilibria involving the aldehyde have been determined, and from them the overall rate constants for interconversion of the abundant species.The approach can be applied to systems in which only solutions at tautomeric equilibrium are available or in which interconversions are too rapid to observe by other methods.Analyses by 13C NMR were facilitated by the use of <1-13C>tetroses.Aqueous solutions of the tetroses were examined at 17-81 deg C by 13C and 1H NMR spectroscopy to evaluate tautomeric composition.Solutions of D-threose contain ca. 51percent α-furanose and ca. 38percent β-furanose at all temperatures, while the amount of aldehyde (a) increases from 0.7 to 4.7percent and the amount of hydrate (h) decreases from 10.2 to 7.1percent. ΔG<*> (kcal mol^-1), ΔH<*> (kcal mol^-1), and ΔS<*> (cal mol^-1 K^-1) at 25 deg C were estimated for the reaction a -> α, -2.4, -6.0, and -12; a -> β, -2.2, -6.0, -13; a -> h, -1.4, -7.3, -20, respectively.Rate constants for ring opening of the tetroses in 2H2O were obtained by 1H and 13C saturation-transfer NMR spectroscopy by saturating H-1 or C-1 of the aldehyde, respectively, and observing the transfer of saturation to the α- and β-furanose and hydrate resonances.Rate constants (+/-10percent) for the ring-opening reactions of D-threose (unbuffered) are as follows: α-furanose, 0.034 s^-1 (51 deg C), 0.10 s^-1 (66 deg C); β-furanose, 0.083 s^-1 (38 deg C), 0.53 s^-1 (66 deg C).Ring-closing rate constants (+/-18percent), determined from equilibrium constants, and ring-opening rate constants are as follows: α-furanose, 0.74 s^-1 (51 deg C), 1.5 s^-1 (66 deg C); β-furanose, 2.0 s^-1 (38 deg C), 5.9 s^-1 (66 deg C).Activation energies were estimated for β-threofuranose ring-opening (14 +/- 3 kcal mol^-1) and ring-closing (8 +/- 2 kcal mol^-1) reactions.From the unidirectional rate constants, overall rate constants at 66 deg C for the conversion of α- and β-threofuranose were calculated as follows: k_αβ = 0.08 +/- 0.01 s^-1, k_βα = 0.11 +/- 0.01 s^-1.Rate constants for ring-opening of D-threose and D-erythrose in 50 mM sodium acetate (p2H 5.0) were compared.At 55 deg C, α-erythrose (0.40 s^-1) > β-threose (0.36 s^-1) > β-erythrose (0.19 s^-1) > α-threose (0.11 s^-1).Rate constants obtained by 13C and 1H ST NMR spectroscopy were in good agreement.