Aldoses free radical oxidation

Scheme V. Free radical oxidation of aldoses (Pathway I)...

Two free-radical chain reactions, in addition to the ionic enolate mechanism, seem reasonable for the oxidation of the sugars by oxygen. With an aldose-2-f one of the free-radical mechanisms would yield non-labeled formic acid and the next lower aldonic acid the other would yield labeled formic acid and the same aldonic acid. 

Manganese(III) has been employed for the oxidation of aldoses, and a general mechanism for the oxidation has been proposed.167 The oxidation of hexoses, pentoses, hexitols, and pentitols by Mn(III), as well as by other cations, was proposed to proceed via a free-radical mechanism,168 as shown in Scheme 26. Oxidation of alditols produces the corresponding aldoses, which are further oxidized in the presence of an excess of oxidant to the lower monosaccharides and thence to formaldehyde, formic acid, and even carbon dioxide. The kinetics for the oxidation of aldoses and ketoses by Mn(III) in sulfuric acid medium have been reported.169... 

At The American University, Isbell s major interest in research turned to the study of the oxidation of saccharides with hydrogen peroxide. In collaboration with Dr. Frush, he published some forty papers on the subject. A number of major discoveries were made, including that of a stepwise degradative peroxidation, which is catalyzed by base or by such metals as iron(II). It starts at the anomeric carbon of an aldose, either in the acyclic or the cyclic form, and affords the lower aldose and formic acid (see Fig. 8). Two mechanisms were recognized an ionic one prevalent in strong alkali, and a free-radical process catalyzed by Fe(II) (see Fig. 9). 

A spectrophotometric study of the oxidation of aldoses by Ce(IV) in dilute sulphuric acid showed that the reactions are first order in both Ce(IV) and sugar. Similar kinetics for D-glucose, D-ribose and D-erythrose indicate a common oxidation mechanism involving cyclic (furanoid or pyranoid) forms in the rate determining radical formation step. Dissimilar kinetic and thermodynamic data for glyceraldehyde, on the other hand, point to a different oxidation pathway for the triose. Formation of an intermediate free radical species in the rate determining step has also been proposed for the Ce(IV) oxidation of L-rhanmose. ... 

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