An intramolecular C-2 → C-1 hydrogen transfer during the acid-catalyzed conversion of d-xylose to d-threo-pentulose (d-xylulose)

Aldose-ketose isomerases are known to catalyze a partial and sometimes complete intramolecular hydrogen transfer between C-1 of the ketose and C-2 of the aldose. It was recently shown (Feather, M. S., and Harris, D. W. (1975) J. Amer. Chem. Soc.97, 178–181) that the same type of transfer occurs during the acid-catalyzed interconversion of d-fructose, d-glucose, and d-mannose. A similar transfer is demonstrated herein for the conversion of d-xylose to d-xylulose in acid solution. d-[2-³H]xylose was isomerized in aqueous sulfuric acid and the resulting d-[³H]xylulose was isolated in 6% yield. The ketose had 18.3% the activity of the starting aldose. Chemical degradation showed that all the carbon-bound tritium of the d-[³H]xylulose was located at C-1, thus indicating a C-2 → C-1 intramolecular hydrogen transfer. During the reaction, less than 1.2% of the total radiochemical activity was found in the solvent, and, the unreacted d-[2-³H]xylose was recovered, having an activity nearly the same as the starting material. The differences in activity, therefore, of the d-[2-³H]xylose and the d-[1-³H]xylulose are due to an isotope effect ($\text{K}{}_{\mathrm{<mtext>H</mtext>}}\text{K}{}_{\mathrm{<mtext>T</mtext>}}$) which is indicated to be 5.4. The data are discussed in terms of currently accepted models for isomerase mechanisms.