Pyruvic aldehyde hydrate

Step 6 is the final step in the cellulose-to-lactic acid cascade, involving the isomerization of the 2-keto-hemi-acetal (here pyruvic aldehyde hydrate) into a 2-hydroxy-carboxyhc acid. This reaction is known to proceed in basic media following a Cannizzaro reaction with 1,2-hydride shift [111], Under mild conditions, Lewis acids are able to catalyze this vital step, which can also be seen as an Meerwein-Ponndorf-Verley reduction reaction mechanism. The 1,2-hydride shift has been demonstrated with deuterium labeled solvents [110, 112], Attack of the solvent molecule (water or alcohol) on pymvic aldehyde (step 5) and the hydride shift (step 6) might occur in a concerted mechanism, but the presence of the hemiacetal in ethanol has been demonstrated for pyruvic aldehyde with chromatography by Li et al. [113] andfor4-methoxyethylglyoxal with in situ CNMRby Dusselier et al. (see Sect. 7) [114]. 

Step 5 converts the multifunctional and thus reactive pyruvic aldehyde into its hemiacetal or hydrate, when performed in water or alcohol. Brpnsted acidity typically catalyzes these reactions. Strong acidity even leads to di-acetalization, e.g., pyruvic aldehyde diethyl acetal in ethanol [65, 110]. 

With regard to carboxylic derivatives, there is generally little difference in chemical shift among the various trifluoroacetic acid derivatives, as exemplified by the examples in Scheme 5.30. Also, the effect of moving the CF3 farther from the carboxylic acid function is similar to that seen with the aldehydes and ketones. Trifluoromethyl ketones will often be in equilibrium with their hydrated form, in which case signals from both the hydrate and water-free ketone will be observed, as is the case for the following pyruvate example. 

Formaldehyde is an exception and is nearly completely hydrated in aqueous solution. Unhindered aliphatic aldehydes are approximately 50% hydrated in water. Aryl groups disfavor hydration by conjugative stabilization of the carbonyl group. Ketones are much less extensively hydrated than aldehydes. Aldehydes and ketones with highly electronegative substituents such as trichloroacetaldehyde and hexafluoroacetone are extensively hydrated. a-Dicarbonyl compounds, such as biacetyl and ethyl pyruvate, are also significantly hydrated. Table 7.4 gives the for a number of carbonyl compounds. Data on other compounds are available in Table 3.23. 

The enzyme Is capable of catalysing other reactions such as hydrolysis of some carboxylic, sulfonic and carbonic esters (2-4) and the reversible hydration of aldehydes (5-7) and pyruvic acid (8), although with much smaller rates as compared with the hydration of CO2. 

The subsequent metabolism of a-hydroxymuconic semialdehyde has been outlined by Nishizuka et al. (1962). The next step is oxidation of the aldehyde group by a dehydrogenase using DPN as electron acceptor, forming y-oxalocrotonate. Decarboxylation and hydration, catalyzed by enzymes not yet described, produce a-keto-a-hydroxyvalerate, which is oxidized in the presence of DPN, presumably to acetopyruvate, which is rapidly split to acetate and pyruvate. 

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