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

The synthesis of several 2,4-diketo carboxylic acids by standard methods wa s undertaken to study the substrate specificity of the carbon-carbon bond h ydrolases. It was shown by H-1- and C-13-WMR experiments that compounds wit h 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 a pproximately 4 : 5 : 1, but at low pH values the 2-hydrate predominated(app roximate to 50%) and at high pH values the 2-enolate carboxylate was domina nt (approximate to 80%) while the 2-hydrate was not detected. 4-Aryl substi tuents gave one pi-I-independent isomer formulated with C-2, C-3 and C-4 el ectrons delocalized in conjugation with the arene at C-4. This interpretati on of a very rapid equilibrium between 2- and 4-enolate isomers to give a p i-delocalized six-membered ring in conjugation with arene substituents is s upported by the effect of divalent metal ions on the structural forms. Rate and equilibrium constants for several of these solution interconversions a re influenced by pH. Mg2+ coordinates to the 2-enolate dianion of alkyl ana logues, whereas Cu2+ forms a six-membered pi-delocalized ring with the 2- a nd Core atoms in conjugation with the arenes. Exchange of H-2 from (H2O)-H- 2-enriched solvent occurs with the protons at C-3. The dimers of the 4-alky l analogues in aqua were characterized as a product of self-aldol condensat ions. These data have facilitated enzyme mechanism studies of C-C bond hydr olysases (beta-ketolases).