Hydroxypyruvic acid

The high stereoselectivity of the transketolase reaction also enables the resolution of racemic a-hydroxyaldehydes23,26. Treatment of racemic 2-hydroxyaldehydes and hydroxypyruvic acid with transketolase, gave the corresponding L-2-hydroxyaldehydes that are not substrates for the enzyme and, therefore, remained unreacted. The corresponding D-enantiomers were consumed and gave the condensation products. 

Dihydroxyacetone is not stable under the basic conditions preferred for oxidation of the primary function to give hydroxypyruvic acid (reaction e). Under acidic conditions the rate of oxidation of a 1 mol I l aqueous solution is veiy slow (5 mol h l mol i(Pt)). On platinum the initial rate of conversion for reduced concentrations of the starting material (0.3 mol whilst retaining the same amount of catalyst, was 42 mol h mol-i(Pt), as might be expected under non-favourable acidic conditions. Hydroxypyruvic acid is evolved with a selectivity of 82% at 40% conversion (see Figure 11). 

The Pt/C catalyst, compared with Pd/C, showed not only enhanced activity (vide supra) but also reduced selectivity for glyceric acid (only 55% at 90% conversion), favoring dihydroxyacetone formation up to 12%, compared with 8% for the Pd case [48]. The Pt/C catalyst promoted with Bi showed superior yields of dihydroxyacetone (up to 33%), at lower pHs. Glyceric and hydroxypyruvic acids, apparently, are formed as by-product and secondary product, respectively [48], The addition of Bi seems to switch the susceptibility of glycerol oxidation from the primary towards the secondary carbon atoms. 

Bi/Pt atomic ratios (changing from 0.1 to 0.4), as well as the precise preparation procedure of the BiPt/C catalysts, yielded differences in selectivity between glyceric acid, dihydroxyacetone and hydroxypyruvic acid that were not always easy to rationalize [72, 73]. The catalyst preparation procedures using acid-treated carbon (with enhanced number of surface carboxyl groups) are essentially as follows ... 

Furthermore, a base-catalyzed transformation by OH from the reaction medium between glycerate and hydroxypyruvate aldehyde (or hydroxypyruvic acid) could be excluded, while hydroxyacetone and glyceraldehyde interconversion was possible (Scheme 11.11). The existence of two major routes, of which hydroxyacetone and glyceric aldehyde are the primary oxidation products and glycolic and oxalic acid are the end-members, respectively, is now firmly established. Clearly, rapid oxidation of glyceraldehydes favors glyceric acid rather than hydroxyacetone formation. 

As shown in Scheme 3.4, there are several reaction pathways of glycerol oxidation to form dihydroxyacetone, glyceric acid, hydroxypyruvic acid, mesoxalic acid, and so on. Dihydroxyacetone is formed by the oxidation of a secondary hydroxy group under... 

In a critical study of the periodate oxidation of raffinose and related oligosaccharides, Courtois and Wickstrom showed78 that 1 mole of raffinose reduces 5 moles of periodate, with the formation of 2 moles of formic acid plus a hexaldehyde. Conversion of the aldehyde to the hexa-carboxylic acid, followed by hydrolysis, gave the expected amounts of glyoxylic acid, glyceric acid, hydroxypyruvic acid, and glycolaldehyde (by decarboxylation of hydroxypyruvic acid). 

Serine Hydroxypyruvic acid Ethanolamine Ethanolamine is a component of phosphatidyletha-nolamines... 

Figure 2.2.12 Reaction network of glycerol oxidation (GLY, glycerol DHA, dihydroxyace-tone GLA, glyceric aldehyde GLS, glyceric acid HBT, hydroxypyruvic acid MOS, mesoxalic acid TS, tartronic acid GOX, glyoxal GOS, glycolic acid GYS, glyoxylic acid OS, oxalic acid).

Scheme 5.55. Using hydroxypyruvic acid (HPA) as the ketol donor renders the transketolase reaction irreversible, making it of synthetic utility. Only D-hydroxyaldehyes are accepted by transketolase.

Sucrose was oxidized by Fleury and Courtois, who obtained a tetra-aldehyde (87) which was oxidized by bromine, in the presence of strontium or barium carbonate, to the corresponding tetracarboxylic acid salt the latter compound was hydrolyzed with acid to yield D-glyceronic acid, glyoxylic acid, and hydroxypyruvic acid, which spontaneously decomposed into carbon dioxide and glycolaldehyde. It was found that oxysucrose shows four aldehyde groups from hypoiodite estimation, but only two from mercurimetry the latter result was attributed to steric hindrance, but it would seem more likely that, under the conditions of the determination. 

Oxidation — Apart from C02 and H20, there are three series of products that result from the oxidation of saccharide. They are 2,3-dialdehydes (5.43 and 5.49) formed on the oxidative cleavage of saccharides (5.42 and 5.48) with periodates, the sole oxidants providing such course of oxidation. Such dialdehydes are considered toxic. Further oxidation of dialdehydes leads to glyceric acid (5.45), glyoxalic acid (5.47), hydroxypyruvic acid (5.46), and erythronic acid (5.51), as shown below for the oxidative cleavage of sucrose (5.42) and maltose (5.48). 

Selective oxidation with air of glyceric to hydroxypyruvic acid and tartronic to mesoxalic acid on PtBi/C catalysts... 

Figures 3(a-d) show the conversion and evolution of products as a function of time, and activity and selectivity data is presented in Table 1. Under acidic conditions, where the pH was determined by the acidity of the substrate and product acids (pH 2), an initial high rate of conversion was observed and very high selectivity in hydroxypyruvic acid was obtained. However, at about 50% conversion deactivation of the catalyst blocked reaction progress (maximum yield 53% at 58% conversion). At pH 4, the initial rate was reduced slightly but catalyst deactivation did not occur and conversion advanced to 93% after six hours. However, as the reaction progressed the selectivity fell as hydroxypyruvic acid was over-oxidised to oxalic and glycolic acids. At pH 5, conversion was total tdter just four hours, and a maximum yield in hydroxypyruvic acid was obtained after 1.6 hours (74% at 77% conversion). Unfortunately, the...

Figure 3 Product distribution for the oxidation of glyceric acid on Pt(4.3%)Bi(3.9%)/C at (a) pH 2, (b) pH 4, (c) pH 5 and (d) pH 6, as a function of time (A - glyceric acid, O -hydroxypyruvic acid, o- oxalic acid and - glycolic acid).

Activity and selectivity data for the oxidation of glyceric acid to hydroxypyruvic acid. 

Over-oxidation of the hydroxypyruvic acid to oxalic and glycolic acids reduces the selectivity in all cases and there appears to be a dependence on pH since increasing quantities of these by-products are evolved at higher pH. This may be because the rate of decarboxylation of hydroxypyruvic acid increases with pH and/or that the rate of the main reaction decreases with an increase in pH. At pH 2, levels of these by-products are negligible. 

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