Pyruvate ketals

In addition to one or more sugars, exopolysaccharides from prokaryotes commonly contain pyruvate ketals and various ester-linked organic substituents. These are only rarely found in eukaryotic exopolysaccharides. 

Pyruvate ketals add to the anionic nature of the exopolysaccharide and are usually present in stoichiometric ratios with the carbohydrate component Pyruvate is normally attached to the neutral hexoses but may also be attached to uronic adds. In the absence of uronic acids, pyruvate alone contributes to the anionic nature of the exopolysaccharide. 

Pyruvate ketals contribute to the cationic nature of exopolysaccharides. 

Xanthan (Figure 11) is a commercially important polysaccharide produced by the bacterium Xanthomonas campestris.187 188 The xanthan backbone consists of a P(l-4)-linked D-glucopyranose chain with a trisaccharide side chain attached at C3 to alternate glucose residues. These side chains consist of an acetylated mannose residue, a glucuronic acid residue, and a pyruvate ketal linked to a terminal mannose residue. The acetate and pyruvate content depend on the fermentation and isolation conditions used by the supplier. 

Pyruvate ketals can be synthesized [161] by direct condensation of a pyruvate ester with a diol in the presence of a Lewis acid, but this is less preferred because of the electron-withdrawing effect of the adjacent carboxylate group [162,163]. Therefore, several indirect methods for the acetalization have been introduced including condensation with pyruvate derivatives [164,165] or generation of the carboxylate group by oxidation of a suitable precursor [166,167,168,169]. A more efficient route to pyruvic acid acetals starts from silylated diols [170] or by the reaction between diols and methyl pyruvate dialkyl dithioacetal [171,172] activated by methyl triflate, dimethyl(methylthio)sulfonium trifluoromethane sulfonate (DMTST), nitroso tetraflu-oroborate (NOBF4), S02Cl2-trifluoromethanesulfonic acid, or Al-Iodosuccinimide (NIS) and trifluoromethanesulfonic acid [173] (O Scheme 24). 

Although this question is off the subject of inhibitor design, what prospect do we see for a chemical synthesis of 8, the intermediate itself We are optimistic that chemistry recently demonstrated by Uday Maitra in a model system can be applied to the appropriate shikimate substrate. He showed that the pyruvate ketal phosphate of 2-phenylethanol (14) is formed on tin hydride reduction of bromide 13, prepared by the sequence in Figure 8.3 This reduction is carried out with UV irradiation at 0 °C (in place of the usual radical source and thermal initiation) and is facilitated by a cationic micellar catalyst (Figure 10). 

The relative stability of various pyruvate ketal substituents in polysaccharides to either permethylation-methanolysis or permethylation-reductive cleavage (EtjSiH - MejSiOTf) has been... 

In addition to the total carbohydrate assay for xanthan using the Dubois method, there are specific analytical methods that may be applied to determine how much of a specific group is present. For example, specific micromethods were applied by Nisbet et al (1982) to assay for 0-acetyl groups, pyruvate ketals and uronic acids. Further discussion of such methods is beyond the scope of this work. 

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