A Full Conformational Characterization of Natural Ionones and Irones, as well as 13‐Alkyl‐Substituted α‐Ionones

A modeling study at the B3LYP/6–31G(d) level was performed 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 identical E ‐enone moiety and differing in the endo or exo positions 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 pseudoaxial 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 properties seem to be modulated by specific hydrophobic interactions of each carbon of this C(5) alkyl chain with some olfactory receptors rather than by different distributions of the conformational populations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)