Studies on Ketoses, 1 Distribution of Furanoid and Pyranoid Tautomers of D‐Fructose in Water, Dimethyl Sulfoxide, and Pyridine via1H NMR Intensities of Anomeric Hydroxy Groups in [D6]DMSO

In [D6]DMSO the anomeric 2‐OH groups of furanoid and pyranoid tautomers of D‐fructose give distinct, clearly separated singlets, their intensities providing an exact measure of the tautomeric distribution and allowing its determination as a function of temperature and time. Equilibration of tautomers in DMSO being exceedingly slow, this OH‐NMR methodology can be applied to determine tautomeric ratios in other solvents, too, by freezing a sample (liquid N2), dissolution of the resulting ice‐matrix in DMSO and rapid recording of the intensities in the low‐field OH‐region. The data obtained in this way for aqueous solutions at equilibrium between 0 and 80°C correlate well with 13C NMR‐derived values yet are more consistent and considerably more easy to obtain; they exhibit a clearly linear relationship between equilibrium composition and temperature such that the two fructofuranoses and the α‐p‐tautomer increase with temperature on expense of the β‐p‐form, which at 80°C comprises 55%, at 110°C (i.e. in the molten state) only 45% of the tautomers. In pyridine, the equilibrium distribution shows a similar linear temperature dependence as does the rate of equilibration, being nearly nondetectable at 0°C versus essentially instantaneous adaption above 60°C, thereby explaining the different outcome of fructose derivatizations in pyridine.