A full conformational characterization of natural ionones and irones, as well as 13-alkyl-substituted alpha-ionones

A modeling study at the B3LYP/6–31G(d) level was per- formed on a group of natural odorants. These included α-, β-, and γ-ionones, β-irone, cis- and trans-α- and -γ-irones, and three synthetic α-ionone analogues, all containing an iden- tical E-enone moiety and differing in the endo or exo posi- tions of another double bond and an additional alkyl group at C(2) or at C(13). Data showed a shift of the conformational preference of the butenone chain from an axial or pseudoax- ial orientation, as favored in α-ionone and in trans-α- and -γ- irones, to an equatorial or pseudoequatorial orientation, as favored in γ-ionone and in cis-α- and -γ-irones. These changes have been correlated with the enhanced olfactory potencies of the latter set of compounds. In the synthetic α- ionone analogues, bearing an ethyl, propyl, or isobutyl group at C(5) instead of the methyl group present in α-ionone, the hindrance due to this alkyl group does not affect the overall conformational behavior of the molecules. The odor proper- ties seem to be modulated by specific hydrophobic interac- tions of each carbon of this C(5) alkyl chain with some olfac- tory receptors rather than by different distributions of the conformational populations.