A new Bisabolane Derivative of Leontopodium andersonii

The genus Leontopodium (Asteraceae) comprises about 58 species in Asia and Europe with a center of diversity in China bearing 36 species including 18 endemic species [1a]. The current taxonomic revision of the genus prompted us to investigate one of the endemic Chinese species, L. andersonii C. B. Clarke (syn.: L. bonatii Beauverd; L. subulatum (Franchet) Beauverd; L. subulatum var. bonatii (Beauverd) Handel-Mazzetti) in comparison to other already investigated species [1b-1d]. Since previous investigations of the aerial plant parts of different Leontopodium species [1e] showed a highly comparable metabolite profile, we decided to investigate the root material which afforded, beside several known compounds (isocomene, β-isocomene, leoligin (=[(2S,3R,4R)-4-(3,4-dimethoxybenzyl)-2-(3,4,-dimethoxy-phenyl)tetra-hydro-furan-3-yl]methyl-(2Z)-2-methylbut-2-enoate) and its 5-methoxy-derivative as well as the cumarine obliquin), one new compound 1. The molecular formula of 1 was determined as C24H34O7 by high-resolution (HR) EI-MS (M+ m/z 434.2315; calc. 434.2305). 1H NMR and phase sensitive HSQC spectra exhibited signals of eight methyl groups (δH 0.98 - 2.18), one methylene group (δH 1.66), and eight methine groups (δH 1.80-2.60 and 4.98 to 6.50). Cross peaks in the HMBC-spectrum identified seven quaternary carbons. Comparison of this experimental data with those of already known bisabolane derivatives of the genus Leontopodium [1b,1d,2a] indicated a close resemblance. Three downfield shifted quaternary carbon signals (δC 170.7, 169.8 and 167.2) indicated the presence of two acetoxy groups at C5 and C9, respectively, and one angeloyl moiety positioned at C1. Localization of the ester groups could be established by HMBC correlations. A further quaternary signal at δC 193.3 could be assigned to a ketonic function and HMBC contacts indicated substitution at C4. Thus, structure of 1 was deduced as in Figure 1. The substitution pattern is identical with that of a pair of stereoisomeric bisabolane derivatives of the roots of L. alpinum [2a]. However, 1 shows a higher oxidation level at position C4. Assignments of the relative configurations of carbons 1, 5 and 6 of the cyclohex-2-en-4-on ring are based on multiplicities and coupling constants of the corresponding protons in comparison with NMR data of carvotaceton derivatives [2b], as well as Phorbasin B [2c], a diterpene of marine origin. The relative configuration of Phorbasin B was established by comparison of the theoretical coupling constants for J1,6 and J5,6 and experimental values (calculated: J1,6 2.8 Hz; J5,6 12.2 Hz, Hz; experimental: J1,6 3.6 Hz, J5,6 12.2 Hz). Figure 1 Structure of 1. For the carvotacetone-derivatives coupling constants of J1,6 3.0 Hz and J5,6 12.5 Hz are reported. Coupling constants of J1,6 (2.0 Hz and 10.0 Hz) and J5,6 (13.2 Hz) determined for 1 support the 1R*,5S* and 6S* configuration. The relative configuration of C7 and C9 in compound 1 remains to be elucidated. Since some of the bisabolane derivatives showed a significant inhibition of in vitro leukotriene biosynthesis, 1 was tested by the same test model as described in [1b]. Additionally the substance was tested for a direct inhibition of COX-1 and COX-2. Interestingly the compound showed at a concentration of 50 μM no activity in all test systems. This observation might be related to the substitution at position C4, since derivatives without this substitution showed a pronounced effect.