Structure Revision of Xanthalongin and Further Sesquiterpene Lactones from Flowers of Arnica longifolia ¹

Abstract

In addition to the helenanolide and xanthanolide type sesquiterpene lactones previously isolated from A. longifolia D.C. Eaton, we report on 13 further sesquiterpene lactones (7 helenanolides, 1 xanthanolide, 3 1,5-trans-guaianolides, 1 1,5-cis-guaianolide and 1 eudesmanolide) occurring in the flowerheads of this species. The structure of xanthalongin is revised to a 10β-methyl configurated xanthanolide on the basis of new NMR spectral evidence. Furthermore, the 1,5-cis-guaianolide 2-O-acetylarnimollin, in contrast with our previous assignment, possesses a 2α-O-acyl configuration. A sesquiterpene lactone pattern as diverse as that of A. longifolia consisting of 1,5-trans-guaianolides and pseudoguaianolides (helenanolides), 1,5-cis-guaianolides and xanthanolides as well as eudesmanolides, which are postulated to be derived from three different biogenetic pathways, is particularly remarkable.

Key words

Arnica longifolia - Compositae - sesquiterpene lactones - pseudoguaianolides - guaianolides - xanthanolides - eudesmanolides