Production of rare sugars from common sugars in subcritical aqueous ethanol

A new isomerization reaction was developed to synthesize rare ketoses. d-Tagatose, d-xylulose, and d-ribulose were obtained in the maximum yields of 24%, 38%, and 40%, respectively, from the corresponding aldoses, d-galactose, d-xylose, and d-ribose, by treating the aldoses with 80% (v/v) subcritical aqueous ethanol at 180 °C. The maximum productivity of d-tagatose was ca. 80 g/(L h). Increasing the concentration of ethanol significantly increased the isomerization of d-galactose. Variation in the reaction temperature did not significantly affect the production of d-tagatose from d-galactose. Subcritical aqueous ethanol converted both 2,3-threo and 2,3-erythro aldoses to the corresponding C-2 ketoses in high yields. Thus, the treatment of common aldoses in subcritical aqueous ethanol can be regarded as a new method to synthesize the corresponding rare sugars.