Determination of the absolute configuration of hydroxyiridals by chiroptical and NMR spectroscopic methods

Hydroxyiridals - monocyclic triterpenoids - are found abundantly in sword lilies. They feature a homofarnesyl side chain that is hydroxylated at one of the allylic methylene groups, which gives rise to a chiral carbon atom. The absolute configuration of six hydroxyiridals from different sources was determined by the exciton chirality method and by NMR spectroscopy of the diastereoisomeric MTPA and MPA esters. All three measurements gave consistent results, establishing the (R) configuration for all of the hydroxyiridals. The reliability of these methods for the stereochemical analysis of acyclic secondary allylic alcohols was confirmed by applying them to a synthetic model compound of known configuration. In the course of this study, two new iridals - 16,26-dihydroxyiridal 7 and 18,19-epoxyiridal 8 - were isolated from extracts of I. versicolor and the iridal fraction of I. missouriensis was analyzed thoroughly.