Some aspects of the chemistry of the dried fruit aroma

Abstract

Many compounds of alleged dried frult-llke aroma have been prepared with a view to a structure-activity relationship (S.A.R.) analysis of dried fruit odorants. Of the oxaspiro[2,5]octane type compounds, all the diastereoisomers of ethyl 4,4,7-trimethyl-2- (1-oxaspiro[2,5]octyl)carboxylate were prepared. The glycidate esters, derived from the Darzens condensation of α-ionone with methyl and ethyl chloroacetate, were also prepared. Megastigma-5,7-(E),9-trien-4-one was synthesised from β-ionone. All of the compounds were claimed to have dried fruit-like or related aromas, but these claims were not confirmed by us. Some novel chemistry and compounds were prepared during the syntheses of these compounds. In a serendipitous discovery by workers at Gallahers Ltd., cyclohexylacetic acid was found to possess a dried fruit note. In order to Investigate the stereoelectronic requirements of the presumed dried fruit aroma receptor, several commercial compounds were assessed and several analogues were prepared and assessed. The 2-, 3- and 4- methyl substituted acids were synthesised by two novel routes from a suitable methyl substituted cyclohexanone. The ring methyl substituted α,β-unsaturated acids were also prepared and assessed, as were the α,β-, β,γ-and γ,δ-unsaturated analogues of cyclohexylacetic acid itself. In addition, several bicyclic acetic acids and their unsaturated analogues were prepared and assessed. Thus, 9-bicyclo- [3,3,1]nonylacetlc acid and 9-bicyclo[3,3,1]nonylideneacetic acid were made. Likewise, the isomers of 2-blcyclo[2,2,1]heptyl acetic acid and 2-bicyclo[2,2,1]heptylideneacetic acid were assessed. Furthermore, a variety of the intermediates were prepared and assessed. The results of the S.A.R. study of these substituted acetic acids were inconclusive. The most realistic dried fruit-llke odorant was cyclohexylacetic acid and the odorant of lowest threshold was exo-2-bicyclo[2,2,1]heptyl acetic acid. Of interest, some methyl esters of these types of compound had dried fruit-like notes in their aroma profile on smoking in a ciarette.