Deuterium isotope effects in a - pyranose and in pyranose-furanose interconversions

: Rates of mutarotation, catalytic coefficients, and isotope effects are reported for the mutarotations of α-d-xylose, α-d-glucose, and β-d-fructose in H2O and in D2O at 20 °C. The isotope effects (kH/kD ) for the mutarotation of β-d-fructose (a pyranose-furanose interconversion) parallel in striking manner the isotope effects for the mutarotation of α-d-glucose (an α-β pyranose anomerization). For each sugar, the isotope effect is lowest for the acid-catalyzed reaction, and highest for the water-catalyzed reaction. The parallelism of the values obtained for the isotope effects under various conditions shows that the rate-determining steps in the two reactions are similar. Presumably, in both instances, the overall mutarotation arises from concurrent reactions operating on different species of the sugar and showing substantially different isotope effects. The gradual change in the isotope effect indicates that, under the conditions studied, three reactions take place concurrently. The following isotope effects were found for the mutarotations at 20 °C: For α-d-glueose, kH3O+/kD3O+ = 1.39 : kH2O+/kD2O = 3.87 ; and kB/kB*=1.83 . For β-d-fructose, kH3O+/kD2O+ = 1.39 ; kH2O/kD2O = 3.87 ; and kB/kB*=1.92 . Mechanisms are presented for the several concurrent acid- and base-catalyzed mutarotation reactions.