Identification of iridals as piscicidal components of Iridaceous plants and their conformations associated with CD spectra

Upon toxicity-guided fractionation of the extractives of the Iris germanica rhizomes and I. japonica roots using killiefish (Oryzias latipes), 12 iridal-type triterpenoids including three new iridals were isolated as piscicidal constituents. Of these, iriflorental, iripallidal, and gamma-irigermanal exhibited a potent activity at a concentration of less than 1 mu g/mL. The structures of new iridals, irisgermanicals A (7), B (10), and C (11), from I. germanica were elucidated based on the spectral analysis. The absolute configuration at C-13 of the known spiroiridal (15) from I. japonica, which remained to be solved, was defined as R on the basis of NOE measurement. The hitherto unreported correlation between the Cotton effects in the CD spectra of some iridals and their conformations was also discussed on the basis of molecular modeling.