Unstable 1,1,2-enetriols as (probable) intermediates in the decarboxylation of .alpha.,.beta.-diketo acids

Abstract

During the acid hydrolyis of (hydrated) 4-aryl-2,3-diketobutyramide 2 (aryl = phenyl, o-chlorophenyl, pmethoxyphenyl), 3-aryllactic acid ( 5) is formed by rapid decarboxylation of the intermediate diketo acid (3). In the decarboxylation step, a further unstable intermediate is formed. The latter manifests itself by reducing 1 mol of added iodine during the hydrolysis-decarboxylation reaction, thereby forming 3-arylpyruvic acid (6), isolated instead of 5. Thus, the oxidation of the unstable intermediate by iodine is more rapid than its ketonization.

The Chemistry of Vicinal Polycarbonyl Compounds

“…The intermediacy of protonated 108 was supported by iodine trapping experiments, although it has not been observed directly. 145 We note, however, that it is not necessary to postulate two different oxyanions to explain the observed results. Both types of reaction can be explained via the single oxyanion 102 formed by hydroxide attack at the central carbonyl group of the tricarbonyl system, as shown in reactions 4 and 5 in the general scheme.…”

“…Protonation of 108 was suggested to give the neutral enetriol when ketonized to α-hydroxy acid (and not α-ketoaldehyde). The intermediacy of protonated 108 was supported by iodine trapping experiments, although it has not been observed directly …”

“…Dahn and co-workers have investigated the reaction of a considerable variety of open chain triketones 144a (alkyl and aryl substituted) and β-arylsubstituted R,β-diketo ethyl esters 145 and one amide 145 in 1-2 N aqueous or aqueous methanolic sodium or potassium hydroxide. Competing benzilic acid rearrangement and cleavage to carboxylic acid plus R-hydroxyacid were observed.…”

“…The base-catalyzed cleavage reactions reported by Dahn , could proceed via diacyl carbinol rearrangement followed by hydrolysis of the product esters in the strong (aqueous) base used. This will be discussed in detail in the following section.…”

Stable acyclic aliphatic solid enols: synthesis, characterization, X-ray structure analysis and calculations

“…The concentrations of the enols are usually too low that they have to be determined indirectly. Moreover, most of these enols are stabilized by aryl272829 or existed as cyclic structures373839, they rapidly tautomerized to their carbonyl isomers as a result of their kinetic instability under inappropriate conditions232440. Synthesis of stable acyclic aliphatic enols is still a challenge for chemists.…”