Reassignment of the methine resonances of D-fructose based on the carbon-13 n.m.r. spectrum of 3-O-methyl-D-fructose

Several disagreements in the ¹³C n.m.r. assignments of the methine carbons of D-fructose exist in the literature. In order to settle these inconsistencies, we examined the ¹³C n.m.r. spectrum of 3-O-methyl-D-fructose. By following the methyl induced shift in this spectrum, as compared to the parent sugar, we identified the alkylated C-3 resonance of all four tautomeric forms of D-fructose. This information, together with our previous identification of the C-5 resonances of the α- and β-forms of D-fructofuranose 6-phosphate, allow the unambiguous identification of all methine carbons of D-fructose in its ¹³C n.m.r. spectrum. The tautomeric composition of 3-O-methyl-D-fructose at 16.5°, in aqueous solution, was found to be as follows: α-pyranose 18%, β-pyranose 37%, α-furanose 11% and β-furanose 34%.