lridals and Cycloiridals, Products of An Unusual Squalene Metabolism in Sword Lilies (lridaceae)

In search for the precursors of the irones, terpenoid C14-ketones, which are responsible for the violet like scent of the orris oil, a new family of hitherto unknown triterpenoids, the iridals, was found. The monocyclic parent compound iridal is derived from cyclization of epoxysqualene to a bicyclic intermediate and subsequent opening of ring A between C3 and C4, accompanied by a methyl and hydride shift. Up to today more than twenty different compounds of this family have been isolated, which are derived from iridal by desaturations, oxidations and cyclizations. The introduction of an additional double bond by a dehydrogenase into the terpenoid side chain leads to iridals with conjugated triene moiety. Addition of a methyl group from S-adenosylmethionine to the terminal double bond of the iridal side chain initiates the formation of a second cyclohexane ring. This way the cycloiridals are formed, which represent the precursors of the irones and dihydroirones. The odoriferous ketones are released after oxidative degradation of the bicyclic C31-triterpenoids. Even more remarkable is the formation of the spiroiridals, which can be seen as products of an intramolecular Prins reaction. The compounds are not only present as free triterpenols but also esterified with fatty acids. There is experimental evidence that the iridals serve the plants as membrane constituents and protect the tissue against oxidative damage and other noxious influences. The iridal esters may be used by the plant as a pool of these valuable compounds, which are released by enzymatic hydrolysis, whenever needed.