Structure Determination of New Iridals from Iris pallida and Iris foetidissima

The four novel iridals 8 – 10 and 18 were isolated from rhizome extracts of Iris pallida and Iris foetidissima, and their structures were established by spectroscopic methods and oxidative degradation. The compounds 8 – 10 bearing a conjugated triene moiety are extremely labile and decompose rapidly. Thus, the monocyclic triene 8 was only isolated after its conversion to a Diels-Alder adduct with 4-phenyl-3H-1,2,4-triazole-3,5(4H)-dione ( 14 ), The tricyclic iridal 18 is a hitherto unknown precursor of α-irone. The possible biogenesis of these unusual triterpenoids is discussed.