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-0-isopropylidene-D-gulofuranose, 2,3;6,7-di-0-isopropylidene-D-glycero-D-gulo-heptose, 2,3-0-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.

The influence of boric acid on the acetylation of aldoses: ‘one-pot’ syntheses of penta-O-acetyl-β-D-glucofuranose and its crystalline propanoyl analogue

“…1. 16 When pure α--mannofuranose pentaacetate was subjected to the boric acid-acetic acid-acetic anhydridesulfuric acid acetolysing conditions, no epimerization was observed, although there was 22% conversion to the β-anomer. The latter observation is consistent with the initial formation of the furanoid borate intermediate 1 and its transesterification without sugar-ring Table 1 The effect of boric acid on the proportions of per-acetates formed on acetylation of aldoses a 2 61 3 80 9 25 13 60 66 34 3 34 3 17 2 53 24 15 16 9 9 2 33 2 37 5 15 7 4 26 33 3 60 1 12 16 39 18 8 30 54 0 1 0 3 Trace 3 Trace 6 1 1 Glucofuranose rearrangement.…”