ARNIFOLIN AND RELATED NATURAL PRODUCTS. Yen-Hsiang Liu¹, Frank R. Fronczek¹ and Tom J. Schmidt². ¹Department of Chemistry, Louisiana State University, Baton Rouge, LA 70803, USA; ²Institut für Pharmazeutische Biologie der Heinrich-Heine-Universität Düsseldorf, Universitätsstraße 1, D-40225 Düsseldorf, Germany

Several sesquiterpene lactones of the 10[[alpha]]-methylpseudoguaianolide (=helenanolide) type compounds have been isolated from plants of the genus Arnica. This type of compound is thought to be responsible for the pharmacological activity of the Arnica species in phytomedicine. In this study, the crystal structures of arnifolin (I), 11[[alpha]]-13-dihydroarnifolin (II), 2-deacetyl-6-O-tigloylchamissonolide (III), and 2-deacetyl-4-O-tigloylchamissonolide (IV) have been determined. Conformational study of the ring system shows that compounds I and II possess twist-chair conformations while compounds III and IV possess twist-boat conformations. Hydrogen bonding will also be discussed.