Studies on the Biosynthesis of Iridals and Cycloiridals

Abstract

The incorporation of [2-I4C]acetate, [2-14C]mevalonate, and [3H]squalene proved the squalenoid nature of the iridals, novel triterpenoids from rhizomes of swordlilies. Methionine is readily incorporated into cycloiridals of Iris pullida dalrnatica, thus indicating that the methylation of iridals via S-adenosyl-L-methionine leads to the formation of the irone moiety of the bicyclic compounds. The 3H/'4C ratio of the transferred labelled methyl group remained unchanged. Therefore, the methylation/cyclization of the terpenoid side chain of iridals must proceed by a concerted mechanism without formation of a cyclopropanoid intermediate.Introduction. -The precursors of the irones 1 and 2, the odoriferous principle of the 'essence d'iris', belong to a new class of triterpenoids [l-31 which we named iridals [4]. There are several such compounds in extracts of the plants, mainly distinguished by their degree of unsaturation or hydroxylation. The iridals usually have in common 30 Catoms, one highly substituted cyclohexane ring (ring B), and a terpenoid non-cyclic (EE)-homofarnesyl side chain [4]. Some typical structures are depicted in Scheme 1. In the irone precursors, the cycloiridals (see 5 and 6), however, as a new structural feature, the unsaturated side chain is cyclized and the resulting ring system ('E' ring) is substituted by an additional CH, group [2]. There are no cycloiridals with an ionone ring and only 30 C-atoms. This makes it plausible that the formation of the cycloiridals is initiated by the addition of an active CH, group to the terminal double bond of iridals. To validate this assumption, experiments on the biosynthesis of iridals and cycloiridals were undertaken.We describe here our investigations on the squalenoid origin of the iridals and of their methylation by feeding labelled precursors to young Iris plantlets. By these experiments, the question was answered if the cyclization to ring E proceeds by an epimethylene intermediate, analogous to the epoxide start of lanosterol biosynthesis, or as a concerted process initiated by a CH: ion, in parallel to the proton-triggered cyclization of squalene in the hopane series.

A metabolomic approach to quality determination and authentication of raw plant material in the fragrance field. Iris rhizomes: A case study

“…Ten discriminant components for I. pallida from Morocco were found, in particular two iridals, spiroiridal-3 and spiroiridal-3 isomer (98, 101) and two trihydroxy-trimethoxy isoflavone 7-O-(6 -glucosylglucoside) isomers (21,25), iso-swertiajaponin (26), three iridin isomers (28,33,46), a homotectorigenin-7-glucoside isomer (29), and an irigenin isomer (47).…”

“…32,42,51,121,122) in the unknown samples and, on the other, by comparing those identified as characteristic of I. pallida from Morocco (i.e. 21,25,26,28,29,33,46,47,98,101,105). The profiles of the marker trend views of the unknown samples were found to be very similar to those of reference samples of I. pallida from Italy, and completely different from those from Morocco, thus unequivocally classifying origin and species of the unknown samples.…”

“…1). Subsequently, Marner's group [15,16] isolated and described several mono-cyclic [17][18][19] and bicyclic iridal structures [20], for a total of more than 40 triterpenic compounds, and also clarified their biosynthetic pathway [21][22][23][24]. In 1993, Bicchi and Rubiolo [25] showed that iridals were also present as esters of myristic acid.…”

“…Several studies on species belonging to the Iris genus have dealt with their biological activity [10][11][12][13], with the formation of irones and identification of their precursors [14][15][16][17][18][19][20][21][22][23][24][25], and have led to the isolation of different classes of secondary metabolites, namely triterpenoids, flavonoids and phenols [25][26][27]. In 1983, Krick [14] showed that iripallidal and iriflorental are the irone precursors ( Fig.…”

ChemInform Abstract: Structure Determination of New Iridals from Iris pallida and Iris foetidissima.

“…1-7) [ 11 [2]. All compounds have a seco-ring-A moiety in common and are I 25 2 R = O H I 4 R = C H j 5 R = CH20H 6 R = C H~O A C 7 2036 HELVETICA CHIMICA ACTA ~ Vol .74 (1991) derived from squalene [3], most probably via iridal (1) which gave this family of triterpenoids its name. Thus, iridal is oxygenated in various positions to give, e.g., the 16-hydroxyiridal2.…”