¹³C Nuclear Magnetic Resonance Spectra and the Tautomeric Equilibriums of Ketohexoses in Solution

The proportions of pyranose and furanose forms at equilibrium in aqueous solutions of L-sorbose, D-fructose, D-tagatose, and D-psicose have been determined by ¹³C nuclear magnetic resonance spectroscopy. The assignments of the ¹³C resonances were based on a study of configurationally related 1,5-anhydrohexitols and 1,4- and 2,5-anhydropolyols. With the exception of D-psicose, pyranose forms predominate, and the observed conformations and α,β ratios of these forms are in good agreement with values calculated from the interaction energies of nonbonded substituents. The α,β ratios of the furanose forms are determined principally by the geometry of hydroxymethyl and hydroxyl groups at C-2 and C-3. In every case, the C-2 hydroxymethyl group and the C-3 hydroxyl group are trans in the predominant furanose anomer.