2.5. Eudesmane
Spiroalanfurantones A−D (2427 ) have been isolated from the roots of Inula helenium ,[44] which has been reported to be one of the most important source of sesquiterpene lactones, such as guaianolide, eudesmanolide, germacranolide, etc.[45-49] Structurally, spiroalanfurantones A−D (2427 ), four eudesmanolide−furan sesquiterpene adducts, have an unprecedented pentacyclic 6/6/5/5/5 skeleton. Moreover, their structures have been confirmed by spectroscopic data analysis and single-crystal X-ray diffraction analysis. Biogenetically, [4 + 2] cycloaddition between eudesmane and furan would be a key step to construct the carbon skeleton in the proposed biogenetic pathway of2427 (Fig. 7). Additional, 2425showed inhibition activity of nitric oxide production in lipopolysaccharide-induced RAW264.7 macrophages with IC50 values of 17.3 and 9.5 μ M, respectively.
Artemilavanolides A (28 ) and B (29 ), two new rearranged eudesmane-type sesquiterpenoids with an unprecedented 6-oxabicyclo[3.2.1]octane system, have been obtained from aerial parts of Artemisia lavandulaefolia .[50] Atralanols A and B (30 and 31 ),[51] derived from eudesmane derivativies, possess a rare spiro [4,4] skeleton, and a plausible biosynthetic pathway for 30 and 31 has been given in Fig. 7.
Fig. 7. Proposed biosynthetic pathways for2427 and 3031.
2.6. Guaiane
The genus Artemisia is famous because of artemisinin, an amorphane sesquiterpenoid obtained from Artemisia annuaexhibiting notable antimalarial activity.[52-55]Two novel cage-like sesquiterpenoids artatrovirenols A and B (32 and 33 ), have been found from the plantArtemisia atrovirens .[56] Structurally, artatrovirenols A and B (32 and 33 ) possess a 5/5/6/5/5-pentacyclic and a 5/5/6/5-tetracyclic system with an unprecedented tetracyclo[5.3.1.14,11.01,5]dodecane scaffold, respectively, as demonstrated by X-ray crystallography definitely. Additionally, a plausible biosynthetic pathway has been proposed from a guaiane derivative via the key intramolecular Diels−Alder cycloaddition (Fig. 8).
Daphnenoids A−C (3436 ) are three novel rearranged guaiane-type sesquiterpenoids which have been isolated from the herb ofDaphne penicillata by molecular networking strategies.[57] Daphnenoid A (34 ) possesses a unique caged tetracyclo[5.3.2.01,6.04,11]dodecane scaffold by unexpected cyclizations of C-1/C-11 and C-2/C-14 in hypothetical biogenetic pathway (Fig. 8). Daphnenoids B and C (35 and 36 ) represent the first natural sesquiterpenoids with unique 5/5 spirocyclic system. Moreover, they showed potential inhibitory activities on the production of NO against LPS-induced BV2 microglial cells.
Fig. 8. Proposed biosynthetic pathways for3236 .