2.5. Eudesmane
Spiroalanfurantones A−D (24 −27 ) 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
(24 −27 ), 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 of24 −27 (Fig. 7). Additional, 24 −25showed 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 for24 −27 and 30 −31.
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 (34 −36 ) 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 for32 −36 .