Germacranolide, Guaianolide und Xanthanolide aus Blüten vonArnica mollisund Röntgenstrukturanalyse von Baileyinacetat1

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Abstract
The flowers of Arnica mollis afforded, in addition to baileyin (1a), baileyin acetate (1b), xanthalongin (3), and 2-deacetoxy-11α,13-dihydroxanthuminol (6), the new 1,5-cis-guaianolides arnimollin acetate (2a) and arnimollin tiglinate (2a) and the new xanthanolides 11β3,13-dihydroxanthalongin (4) and 11α,13-dihydroxanthalongin (5). No traces of helenanolides were discernible. The structures were elucidated mainly by high-field NMR spectroscopy. The stereochemistry of 1b has been revised on the basis of an X-ray crystallographic study to be a 4α,5β-epoxygermacrolide instead of a 4α,5β-epoxymelampolide. The implications for the biosynthesis of cis- and trans-fused guaianolides are discussed. The co-occurrence of 1a with cis-fused guaianolides and xanthanolides (no helenanolides) in A. mollis and with trans-fused guaianolides and helenanolides (no xanthanolides) in A. acaulis supports the hypothesis that germacrolides represent the biogenetic precursors for cis- and trans- fused guaianolides.