2.8. Sesquiterpenoid polymers
A novel dimeric guaianolide xylopiana A (40 ), possessing an
unprecedented
pentacyclo[5.2.1.01,2.04,5′.05,4′]decane-3,2′-dione
core, has been found from the leaves of Xylopia vielana , which is
one of the main sources of guaiane-type sesquiterpenoid
dimers.[63] Furthermore, the case-shaped skeleton
of xylopiana A (40 ) is possibly biosynthesized through
Diels−Alder cycloaddition of two monomeric guaiane intermediates and
then underwent an intramolecular [2 + 2] cycloaddition of a
guaianoid dimer (Fig. 10). Some other dimeric guaianolides, hedyorienoid
A (41 ), vlasoulamine A (42 ), heliaquanoids A−E
(43 −47 ), and artematrovirenolides A−D
(48 −51 ), have been isolated from four plants includingHedyosmum orientale , Vladimiria souliei , Inula
helianthus-aquatica , and Artemisia atrovirens ,
respectively.[64-67] Structurally, 41possesses an unprecedented heterodimeric structure with two different
classes of sesquiterpenoids furnished by forming an unusual
1,3-dioxolane ring. 42 , an unprecedented sesquiterpene lactone
dimer featuring a fully hydrogenated pyrrolo[2,1,5-cd]indolizine
core, has exhibited neuroprotective activity. 43 represents the
first exo -2,4-linked Diels−Alder adduct between a
pseudoguaianolide dienophile and a guaianolide diene.48 −51 are four novel hetero-dimeric [4 + 2]
Diels–Alder adducts dimerized from a rotundane-type unit and a
guaiane-type monomer.
Fig. 10. Proposed biosynthetic pathway for 40 .
A novel lindenane sesquiterpene dimer spirolindemer A (52 ) and
a trimer spirolindemer B (53 ),[68]equipped with oxaspiro[4.5]decane unit, have been discovered from
the medicinal plant Chloranthus henryi . 52 showed
anti-inflammatory activity by inhibiting the expression of iNOS and
COX-2. Chromatography of extracts of plant Chloranthus fortuneiand Chloranthus holostegius afforded the two novel skeletal
lindenane sesquiterpene dimers fortunoid A (54 ) and
chlotrichene A (55 ), respectively.[69,
70] 54 , the rearranged lindenane dimer, showed moderate
antimalarial activities, and 55 , featuring a unique
3/5/6/6/6/6/5/3-fused octacyclic skeleton, possesses a new type of
spirocarboncyclic dimeric framework formed by endo -Diels−Alder
reaction. In addition, a possible biosynthetic pathway for 54would involve in an intermolecular Diels−Alder cycloaddition by an
unidentified [4 + 2] cyclase and oxidative cleavage of the
Δ4 double bond.
A novel cadinane sesquiterpene dimer commiphoratone A
(56 ),[71] containing a saddle shape
skeleton with a unique 6/6/5/5/6/6 heptacyclic architecture, has been
isolated from the plant Resina Commiphora , a Chinese medicine for
the treatment of blood stagnation. 56 showed significant
retardation of lipid metabolism in a concentration-dependent manner.
Another compound arteannoide A (57 ),[72]an unusual cadinane dimer featuring a rare fused
6,8-dioxabicyclo[3.2.l]octan-7-one ring system, has also been
isolated from the traditional Chinese medicine plant Artemisia
annua L. Moreover, a plausible biosynthetic pathway for 57 ,
involving in aldol and Michael addition reactions, has been proposed in
Fig. 11.
Fig. 11. Proposed biosynthetic pathway for 57 .
3. Diterpenoids
Diterpenoids (C20), consisted of four basic isoprene
units, are an important group of natural terpenoids with structural
diversity, ranking only second to sesquiterpenoids. Biosynthetically,
diterpenoids arise from GGPP, which is formed by addition of an IPP to
FPP under the catalysis of diterpene synthase. Structurally, they
have mainly cyclic carbon skeleton
especially tri- and tetracyclic ring systems.[73]The main carbon skeletons and their transformation between each other
have been shown in Fig. 12. Importantly, compounds of this family show
diverse biological activities, including anti-tumor, anti-renal
fibrosis, antimicrobial, and neurotrophic
effects.[74-77]In the following pages, diterpenoids
(abietane, kaurene, cembrane, etc.) with diverse skeletal types will be
introduce
Fig. 12. Classification of the diterpenoids.