Add time:09/25/2019         Source:sciencedirect.com

Because, in the presence of a large excess of alkaline hydroperoxides, aldoses are oxidized stepwise to formic acid, it was expected that alduronic acids would be degraded to formic acid and carbon dioxide by the mechanism previously proposed. However, in addition to the products expected, substantial yields of oxalic acid were found, indicating a second mechanism, proposed here. With alkali-metal hexuronates, one mechanism yields five moles of formic acid and one mole of carbon dioxide per mole of substrate, whereas the second mechanism yields four moles of formic acid and one mole of oxalic acid. The results show that, under highly alkaline conditions, the two mechanisms are of approximately equal importance. Oxidation of a D-xylo-5-hexulosonate begins with the cleavage of glycolic acid, and this is followed by degradation of the resulting tetruronic acid by the two mechanisms described for the degradation of hexuronic acids. With 2-hexulosonates, also, two reaction-mechanisms appear to be necessary to account for the products formed under highly alkaline conditions; the main reaction (80%) yields one mole each of carbon dioxide and pentonic acid per mole, whereas the other yields one mole of oxalic acid and four moles of formic acid.Under moderately alkaline conditions, both the alduronic acids and the 2-keto acids react almost entirely by the mechanism that yields carbon dioxide; no detectable amount of oxalic acid was found. In all cases, small amounts of unknown products, not further investigated, are formed.The following compounds were studied: sodium D-galacturonate, sodium D-glucuronate, potassium D-mannuronate, sodium D-lyxo-2-hexulosonate, sodium D-arabino-2-hexulosonate, calcium D-xylo-5-hexulosonate, and glyoxylic acid.

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