Prevalence of oxetanose forms in the tautomeric equilibrium of beta-hydroxy-1,5-dicarbonyl monosaccharides

The synthesis of 4-deoxy- and 4-deoxy-4-C-methylhexos-5-uloses, starting from 4-deoxyhex-4- enopyranosides, and the NMR study of their isomeric composition, are reported. The NMR spectra show that the two δ-dicarbonyl sugars exist as two anomeric α- and β-oxetanosyl forms, deriving from the hemiacetalization of the C-3 hydroxyl group with the aldehydic carbon. The observed tautomeric equilibria have been rationalized with computational calculations. Interestingly, this is the first time that dicarbonyl derivatives are mostly present in their oxetanose forms, offering a new entry into this very interesting type of scaffold.