Two‐dimensional NMR spectral study of the tautomeric equilibria of D‐fructose and related compounds

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Abstract
The use of two‐dimensional NMR techniques and spin simulation afforded a detailed set of chemical shift and spin–spin coupling data for the main tautomeric forms of D‐fructose in dimethyl sulfoxide and aqueous solutions. Differences in vicinal coupling parameters for β‐D‐fructofuranose in the two solvents are taken as evidence that this tautomer incorporates intramolecular hydrogen bonding in dimethyl sulfoxide, which helps to account for its exceptional prominence in this solvent. The enhanced proportion of α‐D‐fructofuranose in dimethyl sulfoxide over that in water also receives comment. Also described are corresponding data from NMR measurements on the tautomeric equilibria of disaccharides that contain a D‐fructose reducing‐end residue, and of the homomorphic ketose, L‐galacto‐2‐heptulose, i.e. structurally related compounds that may entail different kinds of hydrogen‐bonding possibilities. Evidence indicating the presence of extensive inter‐residue hydrogen bonding within the individual tautomers of the disaccharide turanose is provided by the pattern of hydroxyl proton chemical shifts in dimethyl sulfoxide.

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