Synthesis of 2,4-diketoacids and their aqueous solution structures

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Abstract
The synthesis of several 2,4-diketo carboxylic acids by standard methods was undertaken to study the substrate specificity of the carbon-carbon bond hydrolases. It was shown by 1H- and 13C-NMR experiments that compounds with 4-alkyl, 4-alkenyl and 4-alicyclic substituents exist in three main forms: 2,4-diketo, 2-enol-4-keto and 2-hydrate-4-keto. The equilibrium ratios of these aqueous solution structures were similar, but were markedly affected by the pH values (1.5–10.5). At pH 7.5 the ratio of these structures was approximately 4:5:1, but at low pH values the 2-hydrate predominated (≈50%) and at high pH values the 2-enolate carboxylate was dominant (≈80%) while the 2-hydrate was not detected. 4-Aryl substituents gave one pH-independent isomer formulated with C-2, C-3 and C-4 electrons delocalized in conjugation with the arene at C-4. This interpretation of a very rapid equilibrium between 2- and 4-enolate isomers to give a π-delocalized six-membered ring in conjugation with arene substituents is supported by the effect of divalent metal ions on the structural forms. Rate and equilibrium constants for several of these solution interconversions are influenced by pH. Mg2+ coordinates to the 2-enolate dianion of alkyl analogues, whereas Cu2+ forms a six-membered π-delocalized ring with the 2- and 4-oxo atoms in conjugation with the arenes. Exchange of 2H from 2H2O-enriched solvent occurs with the protons at C-3. The dimers of the 4-alkyl analogues inaqua were characterized as a product of self-aldol condensations. These data have facilitated enzyme mechanism studies of C–C bond hydrolysases (β-ketolases).

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