Studies on the Biosynthesis of Iridals and Cycloiridals

The incorporation of [2‐ 14 C]acetate, [2‐ 14 C]mevalonate, and [ 3 H]squalene proved the squalenoid nature of the iridals, novel triterpenoids from rhizomes of swordlilies. Methionine is readily incorporated into cycloiridalls of Iris pallida dalmatica , 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 3 H/ 14 C 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.