ALPHA-D-GLUCOPYRANOSYL-D-FRUCTOSES - DISTRIBUTION OF FURANOID AND PYRANOID TAUTOMERS IN WATER, DIMETHYL-SULFOXIDE, AND PYRIDINE - STUDIES ON KETOSES .4.

The tautomeric distributions of trehalulose (2), turanose (3), maltulose (4), leucrose (5), and palatinose (6) have been established using an 1H NMR methodology based on integration of the low-field anomeric OH singlets in dimethyl sulphoxide. Equilibration in dimethyl sulphoxide being exceedingly slow the method can be applied to determine tautomeric ratios in essentially any solvent, by freezing a small probe of the respective solution (liquid N2), dissolving the resulting ice-matrix in (CD 3)2SO and rapid recording of the intensities in the low-field OH-region. In dimethyl sulphoxide, fructose, trehalulose (2), turanose (3), and maltulose (4) adopt equilibria with a high proportion of the furanoid tautomers (∼ 60% at 20°C) in an approximate 2:1 β-f/α-f-ratio versus 10:1 for the β-p/α-p-forms. For leucrose (5), the β-p ⇌ α-p equilibrium lies at the α-p side with a 9:1 preference, whereas palatinose (6) establishes an 11:4 β-f ⇌ α-f equilibrium with a comparably high proportion of the acyclic keto-form (6%), that, in all other cases, is ≤ 1%. There is little temperature dependence of the anomeric compositions in the 20-70°C range. In water, the β-p form invariably is the most abundant at equilibrium, decreasing substantially though with rising temperature in favour of the β-f and α-f-tautomers. The equilibrium compositions of fructose, trehalulose, and maltulose being almost identical, that of turanose is markedly shifted towards higher proportions of furanoid forms, amounting to two-thirds of the mixture at 70°C. Aqueous solutions of leucrose contain the β-p form almost exclusively, palatinose establishes a 5:1 to 3:1 β-f ⇌ α-f equilibrium over the 1-70°C range. In pyridine, equilibrium tautomeric compositions are similar to those found for water, except for distinctly slower tautomerization rates. The preparative implications for derivatizations of these glycosyl-fructoses in pyridine are discussed, their now predictable outcome being strongly dependent on the mode of dissolution of the substrate, and the temperature of the reaction.