IRIDALS AND CYCLOIRIDALS, PRODUCTS OF AN UNUSUAL SQUALENE METABOLISM IN SWORD LILIES (IRIDACEAE)

In search for the precursors of the irones, terpenoid C-14-ketones, wh ich 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 epoxy squalene to a bicyclic intermediate and subsequent opening of ring A b etween C3 and C4, accompanied by a methyl and hydride shift. Up to tod ay more than twenty different compounds of this family have been isola ted, which are derived from iridal by desaturations, oxidations and cy clizations. The introduction of an additional double bond by a dehydro genase 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 forma tion of a second cyclohexane ring. This way the cycloiridals are forme d, which represent the precursors of the irones and dihydroirones. The odoriferous ketones are released after oxidative degradation of the b icyclic C-31-triterpenoids. Even more remarkable is the formation of t he spiroiridals, which can be seen as products of an intramolecular Pr ins reaction. The compounds are not only present as free triterpenols but also esterified with fatty acids. There is experimental evidence t hat 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 com pounds, which are released by enzymatic hydrolysis, whenever needed.