RING CONTRACTION DURING THE LEAD TETRAACETATE OXIDATION OF REDUCING SUGARS

Abstract

The rate of oxidation of D-glucose by lead tetraacetate in acetic acid is affected little by a large change in concentration of reactants, showing that the glycol cleavage step is not rate-controlling. Oxidation of 3-O-methyl-6-O-trityl-D-glucose yields 3-O-formyl-2-O-methyl-5-O-trityl-D-arabinose; the latter is not derived by production initially of a 4-O-formyl derivative that undergoes acyl migration. It is probable that the oxidation of D-glucose proceeds with conversion of the pyranose sugar into a furanose intermediate. By contrast, D-mannose appears to be oxidized mainly in the pyranose form, as shown by the oxidation patterns of D-mannose-1- and -2-14C and 5-O-methyl-D-mannofuranose, and the effects of concentration change on the reaction of D-mannose and derivatives. The proximity of the ring oxygen atom to the 1,2-diol group may contribute to the unusually high reactivity of D-mannopyranose as compared with other types of monocyclic six-membered ring vic-diols.Intermediate proportions of furanose and pyranose pathways characterize the lead tetraacetate oxidation of the other six aldohexoses. Similarities in the behavior of glucose and idose, gulose and mannose, and allose and talose may originate in steric similarities between these pairs of sugars.