The flowers of ARNICA MOLLIS afforded, in addition to baileyin ( 1A), baileyin acetate ( 1B), xanthalongin ( 3), and 2-deacetoxy-11alpha,13-dihydroxanthuminol ( 6), the new 1,5- CIS-guaianolides arnimollin acetate ( 2A) and arnimollin tiglinate ( 2A) and the new xanthanolides 11beta3,13-dihydroxanthalongin ( 4) and 11alpha,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 4alpha,5beta-epoxygermacrolide instead of a 4alpha,5beta-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.
The synthesis of five-and six-membered rings 3% 4, Sb, 5% and 17% containing the structural moiety 3-pentene-l,2,5-trione, is described The chromophoric properties of these vinylogous tricarbonyls together with related compoutlds rue investigated. The conformation in solution, particularly the torsion angle rindsidechain, can be derived from measurements of coupling constants, NOE difference spectra, and force-field calculations. X-ray structure analyses of It., 17r, 184 and 35 afford geometries in the solid state. Ionization energies are determined by means of photoelectron spectroscopy. The theoretical calculations and interpretation o f absorption spectra and colour are carried out using the models CNDO/S-CI and HAM/3.As a result of both synthetic and spectroscopic studies the nature of vicinal triketones l a or 1 b has been determined').On the other hand, little information has been presented for vinylogous tricarbonyl compounds such as 2, although the spectral properties of this chromophore are important in the interpretation of the colour of violerythrine"), analogous norcarotinoid-dione~~~), and of compounds containing the
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