Epimerization of Monosaccharides Under Acetolysis Conditions

Abstract

Monosaccharides with a cis-configuration at C-2 and -3 readily undergo inversion at C-2 in an acetolysis medium composed of acetic acid, acetic anhydride, and sulfuric acid. The acetolysis-inversion reaction has been carried out with 2,3;5,6-di-O-isopropylidene-D-gulofuranose, 2,3;6,7-di-O-isopropylidene-D-glycero-D-gulo-heptose, 2,3-O-isopropylidene-D-ribose, D-ribose, and D-mannose using chromatography for isolation of the deacetylated epimeric products. The reaction requires a medium with a high concentration of acetic acid (ca. 90%) and appears to take place only with furanose structures. Inversions observed with the unsubstituted pyranoses D-ribose and D-mannose probably occur because of tautomerization to furanose forms. The epimerization proceeds in the direction which yields a smaller number of cis-oriented groups at C-2, -3, and -4 of the furanose forms of monosaccharides.