α-D-Glucopyranosyl-D-fructoses: distribution of furanoid and pyranoid tautomers in water, dimethyl sulphoxide, and pyridine. Studies on ketoses. Part 4

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Abstract
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 (CD3)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 β-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-fand 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.

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