Abstract
The rearrangement of trans-pinonic acid into homoterpenyl methyl ketone has been studied in order to elucidate its mechanism. Relatively strong acid-catalysis is necessary for the rearrangement to occur, and the rate of reaction depends upon the strength of the acidic solvent employed. In monochloroacetic acid at 100, the rearrangement is of the first order with regard to pinonic acid. Formation of homoterpenyl methyl ketone is accompanied by isomerisation of trans-pinonic acid into cis-pinonic acid, which also rearranges to the ketolactone. An intramolecular mechanism is proposed for the rearrangement. An attempt has been made to determine the mechanism, of the conversion of pinonic acid and homoterpenyl methyl ketone into 2:4-dimethylphenylacetic acid by the action of bromine. With pinonic acid, the initial reaction results in the formation of a-bromo-pinonic acid, which has been isolated and characterised. It is believed that under conditions of strong acid-catalysis this readily rearranges to a-bromo-homoterpenyl methyl ketone, which is probably also formed by the action of bromine upon homoterpenyl methyl ketone. The subsequent cyclisation of the bromo-ketolactone yields 2:4-dimethylphenylacetic acid. Mechanisms of bromination and of cyclisation are discussed. Pinonic acid is rapidly converted into homoterpenyl methyl ketone when heated with phosphoric acid at 100. Further heating causes decomposition, with loss of carbon dioxide. 2:4-Dimethylphenylacetic acid is found among the products, which represent the result of a complex process of hydrogen transfer associated with dehydration and decarboxylation. 1:2:4-Trimethyl benzene, 1:2:4-trimethyl cyclohexene, and the lactone of 2-hydroxy 2:4-dimethyl cyclohexylacetic acid have also been identified as products of the reaction. A study of the reactions of the products and of compounds similar to those likely to be present in the reaction-mixture, has afforded evidence relating to the probable mechanism of the reaction.