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\begin{document}

\title{Natural Sesquiterpenoids, Diterpenoids, Sesterterpenoids, and
Triterpenoids with Intriguing Structures from 2017 to 2022}



\author[1]{Xiaotian Zhang}%
\author[1]{Meijia Zheng}%
\author[2]{Aimin Fu}%
\author[2]{Qin Li}%
\author[2]{Chunmei Chen}%
\author[2]{Hucheng Zhu}%
\author[1]{Yonghui Zhang}%
\affil[1]{Huazhong University of Science and Technology}%
\affil[2]{Tongji Medical College of Huazhong University of Science and Technology}%


\vspace{-1em}



  
  \date{May 23, 2024}


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\selectlanguage{english}
\begin{abstract}
This review covers the isolation, structural determination, plausible
biosynthetic pathways, and biological activities of 166 natural
terpenoids including 57 sesquiterpenoids, 65 diterpenoids, 15
sesterterpenoids, and 29 triterpenoids from January 2017 to December
2022.%
\end{abstract}%



\sloppy


\textbf{Cite this paper:}\emph{Chin. J. Chem.} \textbf{2021} , \emph{39}
, XXX---XXX. \textbf{DOI: 10.1002/cjoc.202100XXX}

Natural Sesquiterpenoids, Diterpenoids, Sesterterpenoids, and
Triterpenoids with Intriguing Structures from 2017 to 2022.

Xiaotian Zhang, \textsuperscript{\#} Meijia Zheng,
\textsuperscript{\#}Aimin Fu, \textsuperscript{\#} Qin Li, Chunmei Chen,
Hucheng Zhu, * Yonghui Zhang *

Hubei Key Laboratory of Natural Medicinal Chemistry and Resource
Evaluation, School of Pharmacy, Tongji Medical College, Huazhong
University of Science and Technology, Wuhan 430030, People's Republic of
China\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
Sesquiterpenoids \textbar{} Diterpenoids \textbar{} Sesterterpenoids
\textbar{} Triterpenoids \textbar{} Structural scaffolds \textbar{}
Biosynthetic pathways & Sesquiterpenoids \textbar{} Diterpenoids
\textbar{} Sesterterpenoids \textbar{} Triterpenoids \textbar{}
Structural scaffolds \textbar{} Biosynthetic pathways & Sesquiterpenoids
\textbar{} Diterpenoids \textbar{} Sesterterpenoids \textbar{}
Triterpenoids \textbar{} Structural scaffolds \textbar{} Biosynthetic
pathways & Sesquiterpenoids \textbar{} Diterpenoids \textbar{}
Sesterterpenoids \textbar{} Triterpenoids \textbar{} Structural scaffolds
\textbar{} Biosynthetic pathways\tabularnewline
This review covers the isolation, structural determination, plausible
biosynthetic pathways, and biological activities of 166 natural
terpenoids including 57 sesquiterpenoids, 65 diterpenoids, 15
sesterterpenoids, and 29 triterpenoids from January 2017 to December
2022. & This review covers the isolation, structural determination,
plausible biosynthetic pathways, and biological activities of 166
natural terpenoids including 57 sesquiterpenoids, 65 diterpenoids, 15
sesterterpenoids, and 29 triterpenoids from January 2017 to December
2022. & This review covers the isolation, structural determination,
plausible biosynthetic pathways, and biological activities of 166
natural terpenoids including 57 sesquiterpenoids, 65 diterpenoids, 15
sesterterpenoids, and 29 triterpenoids from January 2017 to December
2022. & This review covers the isolation, structural determination,
plausible biosynthetic pathways, and biological activities of 166
natural terpenoids including 57 sesquiterpenoids, 65 diterpenoids, 15
sesterterpenoids, and 29 triterpenoids from January 2017 to December
2022.\tabularnewline
& & &\tabularnewline
\textsuperscript{\#} The authors contributed equally to this work. *
E-mail: zhangyh@mails.tjmu.edu.cn (Y. Zhang), zhuhucheng@hust.edu.cn (H.
Zhu). & & &\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{longtable}[]{@{}l@{}}
\toprule
\textbf{Left to Right:} Xiaotian Zhang, Meijia Zheng, Aimin Fu, Qi Li,
Chunmei Chen, Huchen Zhu, Yonghui Zhang Xiaotian Zhang is currently a
PhD candidate at Tongji Medical College, Huazhong University of Science
and Technology (HUST). He joined the research group of Prof. Yong-Hui
Zhang in 2018. His research interests include isolation, structure
elucidation, and biological activity studies of natural products. Meijia
Zheng is currently a PhD candidate at Tongji Medical College, HUST. she
joined the research group of Prof. Yong-Hui Zhang in 2019. She focuses
on the isolation, structure elucidation, and biological activity studies
of natural products. Aimin Fu is currently a PhD candidate at Tongji
Medical College, HUST. He joined the research group of Prof. Yong-Hui
Zhang in 2020. He is carrying out research on the isolation, structure
elucidation, and biological activity studies of natural products. Qi Li
is currently a research associate at Tongji Medical College, HUST. She
was conferred a Ph.D. degree (2020) in Natural Product Chemistry after a
five-year doctoral career by the Tongji Medical College, HSUT, under the
supervision of Professor Yonghui Zhang. Her research interests include
isolation, structure elucidation, biological activity, and biosynthesis
studies of natural products. Chunmei Chen is an associate professor at
Tongji Medical College, HUST. She was conferred a Ph.D. degree (2015) in
Natural Product Chemistry after a five-year doctoral career by the
Tongji Medical College, HSUT, under the supervision of Professor Yonghui
Zhang. Her research interests include isolation, structure elucidation,
biological activity, and biosynthesis studies of natural products.
Huchen Zhu is a professor at Tongji Medical College, HUST. He was
conferred a Ph.D. degree (2014) in Natural Product Chemistry after a
five-year doctoral career by the Tongji Medical College, HSUT, under the
supervision of Professor Yonghui Zhang. His research interests include
isolation, structure elucidation, biological activity, and biosynthesis
studies of natural products. Yonghui Zhang received his Ph.D. degree in
2004 from HUST, where he studied natural products chemistry. He joined
the faculty of HUST as an associate professor (2005) and was then
promoted to professor (2008). He was elected as an associate dean (2010)
and then the dean (2014) of School of Pharmacy. He is the director of
``Key laboratory of natural pharmaceutical chemistry and resource
evaluation in Hubei Province'', and his research interest is the
discovery of bioactive metabolites from fungi and plants as well as the
innovative drugs. He has authored nine monographs, 16 patents, and 200+
scientific publications.\tabularnewline
\bottomrule
\end{longtable}

\textbf{Contents}

1. Introduction

2. Sesquiterpenoids

2.1. Cedrane 2.2. Cuparene 2.3. Humulane 2.4. Germacrane 2.5. Eudesmane
2.6. Guaiane 2.7. Miscellaneous sesquiterpenoids 2.8. Sesquiterpenoid
polymers

3. Diterpenoids

3.1. Abietane 3.2. \emph{Ent}-kaurane 3.3. Cembrane 3.4. Pimarane 3.5.
Fusicoccane 3.6. Jatrophane

4. Sesterterpenoids

5. Triterpenoids

6. Challenge and prospect

1. Introduction

Terpenoids, widely distributed in plants and fungi, are a large and
diverse class of secondary metabolites both in terms of their overall
number and the range of structural scaffolds. Structurally, terpenoids
are defined as a group of natural products (NPs) composed of the simple
``C\textsubscript{5}'' units, called isoprene units. Thus, terpen-

\textbf{Fig. 1.} Classification of the terpenoids.

oids can be classified based on the number of isoprene units, mainly
including monoterpenoids (C\textsubscript{10}), sesquiterpenoids
(C\textsubscript{15}), diterpenoids (C\textsubscript{20}),
sesterterpenoids (C\textsubscript{25}), and triterpenoids
(C\textsubscript{30}). Biosynthetically, it is demonstrated that the
origins of the isoprene units are isopentenyl pyrophosphate (IPP) and
dimethylallyl pyrophosphate (DMAPP), generated by the MVA (mevalonate)
or MEP

(methylerythritol phosphate) pathways. Both IPP and DMAPP can be
converted into hemiterpenes, and under the catalyzation of enzymes, they
can be condensed into geranyl diphosphate (GPP), farnesy diphosphate
(FPP), Geranylgeranyl diphosphate (GGPP), and Geranylfarnesyl
diphosphate (GFPP) and further derived into monoterpenes,
sesquiterpenoids, diterpenoids, sesterterpenoids, and triterpenoids
(Fig. 1).\textsuperscript{{[}1{]}} Moreover, the rich structural
variation of terpenoids led to their diverse bioactivities as
well,\textsuperscript{{[}2-7{]}} represented by the notable antimalarial
activity of artemisinin and antitumor activity of taxol.

This review will not comprehensively exhibit all terpenoids in recent
years, and it only focuses on representative natural terpenoids
(C\textsubscript{15}-C\textsubscript{30}), with intriguing skeletons and
bioactivities, from 2017 to 2022. We hope this review will provide a
wide horizon for all the scholars who are interested in terpenoids.

2. Sesquiterpenoids

As one of the largest classes and the most frequently reported
terpenoids, sesquiterpenoids, derived from FPP, have attracted
increasing attention from organic chemists and pharmacologists due to
their diverse carbon skeletons as exemplified by various ring systems
and polymer architectures and extensive biological activities including
neuroprotective, anti-inflammatory, and antimicrobial
effects.\textsuperscript{{[}8-15{]}}

\textbf{Fig. 2} . Classification of the sesquiterpenoids.

To date, many types of sesquiterpenoids with different carbon skeletons
have been reported, namely, germacrane, humulane, bisabolane, eudesmane,
cedrane, guaiane, etc (Fig. 2).\textsuperscript{{[}1{]}} Based on this
classification by biosynthetic pathways, a series of intriguing new
sesquiterpenoids will be introduced herein.

\textbf{2.1. Cedrane}

The genus \emph{Illicium} is a rich source of sesquiterpenoids, such as
seco-prezizaane, acorane, allo-cedrane, etc, and some of them have been
reported to exhibit diverse biological activities including
antimicrobial, antiviral, and neurotrophic
effects.\textsuperscript{{[}16-20{]}} Chromatography of extracts
of\emph{Illicium simonsii} and \emph{Illicium henryi} afforded two new
caged sesquiterpenoids illisimonin A (\textbf{1} ) and illihenin A
(\textbf{2} ), respectively.\textsuperscript{{[}21, 22{]}} \textbf{1}
possesses a unique 5/5/5/5/5 pentacyclic scaffold featuring a caged
2-oxatricyclo{[}3,3,0,1\textsuperscript{4,7}{]}nonane ring system fused
to a five-membered carbocyclic ring and a five-membered lactone
ring.\textbf{2} represents a class of novel 5/7/6 tricyclic
sesquiterpenoids featuring a caged
tricyclo{[}6.2.2.0\textsuperscript{1,5}{]}dodecane skeleton.
Furthermore, their structures have been determined by spectroscopic
analyses, ECD calculations, and single-crystal X-ray diffraction.
Additionally, the plausible biosynthetic of their skeletons are both via
a 5/6/6 tricarbocyclic allo-cedrane framework involved in the
Wagner-Meerwein (W-M) rearrangement as shown in Fig. 3.

\textbf{Fig. 3.} Proposed biosynthetic pathways for \textbf{1}
and\textbf{2} .

\textbf{2.2. Cuparene}

A chemical study on the extract of the fungus \emph{Coprinopsis
cinerea}has afforded two new skeletal sesquiterpenoids, which have been
named hitoyols A and B (\textbf{3} and \textbf{4}
).\textsuperscript{{[}23{]}}\emph{C. cinerea} from the genus
\emph{Coprinopsis} (Hitoyotake, in Japanese) plays an important role in
model basidiomycete, and\emph{Coprinopsis} sp. are rich sources of
cuparene-type sesquiterpenoids.\textsuperscript{{[}24-26{]}}
Structurally, hitoyol A (\textbf{3} ), as an unprecedented
norsesquiterpenoid, possesses an\emph{exo}
-tricyclo{[}5.2.1.0\textsuperscript{2,6}{]}decane core. The novel
skeletal sesquiterpenoid hitoyol B (\textbf{4} ), containing
4-cyclopentene-1,3-dione, has displayed antimalarial activity with an
IC\textsubscript{50} of 59 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M. Moreover, \textbf{3}
and\textbf{4} are possibly biosynthesized through
decarboxylation-induced cyclization of lagopodin B, a known
cuparene-type sesquiterpenoid (Fig. 4).

Trichothecrotocins A (\textbf{5} ) and B (\textbf{6} ) are two new
trichothecene derivatives, which have been obtained from the potato
endophytic fungus \emph{Trichothecium crotocinigenum}
.\textsuperscript{{[}27{]}} Biosynthetically, trichothecenes have been
demonstrated to be derived from an important precursor trichodiene, also
originated from cuparene-skeleton similar to\textbf{3} and \textbf{4} .
Moreover, one of the most critical steps of the proposed biosynthetic
pathway for\textbf{5} would be the formation of a new carbon-carbon bond
between C-9 and C-12 (Fig. 4).\textsuperscript{{[}28-30{]}} In
addition,\textbf{5} and \textbf{6} exhibit antiphytopathogenic
activities with MIC values of 16-32 \selectlanguage{greek}\emph{μ} \selectlanguage{english}g/mL.

\textbf{Fig. 4.} Proposed biosynthetic pathways for\textbf{3}
-\textbf{6} .

\textbf{2.3. Humulane}

Higher fungi (mushroom) are undoubtedly an important source of
sesquiterpenoids.\textsuperscript{{[}13{]}} In the past decade, many
interesting secondary metabolites of higher fungi have been reported by
Liu's group, such as trefolane A from culture of the
mushroom\emph{Tremella foliacea} and conosilane A from culture of the
mushroom\emph{Conocybe siliginea} .\textsuperscript{{[}31, 32{]}} In
this study, a novel skeletal sesquiterpenoid antroalbocin A (\textbf{7}
), possessing a bridged tricyclic system, has been obtained from the
higher fungus \emph{Antrodiella albocinnamomea} , a white-rot fungus
belonging to the Basidiomycota (widely distributed in northeast
China).\textsuperscript{{[}33{]}}\textbf{7} , an antibacterial
sesquiterpenoid, was found to inhibit\emph{Staphylococcus aureus} with
an MIC value of 169\selectlanguage{greek}\emph{μ} \selectlanguage{english}M,\textsuperscript{{[}34{]}} and its
structure has been confirmed by single crystal X-ray diffraction.
Furthermore, the key steps of the plausible biosynthetic pathway derived
from humulane would involve a cyclization of humulene building a
backbone of triquinane-type sesquiterpene and the formation of new C-C
bond between C-8 and C-9 (Fig. 5).

The endophytic Fungus \emph{Phomopsis} sp. TJ507A, isolated from the
medicinal plant \emph{Phyllanthus glaucus} , has provided a naturally
occurring 2,3-\emph{seco} -protoilludane-type sesquiterpenoid, named
phomophyllin A (\textbf{8} ).\textsuperscript{{[}35{]}} One new
cage-like cerapicane derivative, cerrenin A (\textbf{9} ), and two new
isohirsutane derivatives, cerrenins B and C (\textbf{10} and \textbf{11}
), have been obtained from the broth extract of \emph{Cerrena} sp.
A593.\textsuperscript{{[}36{]}} Three pairs of enantiomeric
sesquiterpenoids, ([?])-syringanoid A (\textbf{12a} and \textbf{12b} ) and
(\selectlanguage{ngerman}±)-pinnatanoids A (\textbf{13a} and\textbf{13b} ) and B (\textbf{14a}
and \textbf{14b} ) have been isolated from the peeled stems of
\emph{Syringa pinnatifolia} .\textsuperscript{{[}37{]}} \textbf{12}
\selectlanguage{english}-\textbf{14}represent unprecedented 5/4/6 tricyclic backbone and a rare
6/7 bicyclic backbone, respectively, and their cardiomyocyte-protective
and anti-in\selectlanguage{english}flammatory activities were evaluated. In addition, plausible
biosynthetic pathways for \textbf{7} and \textbf{12} \selectlanguage{english}-\textbf{14} have
been proposed (Fig. 5).

\textbf{Fig. 5.} Proposed biosynthetic pathways for \textbf{7}
and\textbf{12} \selectlanguage{english}-\textbf{14} .

\textbf{2.4. Germacrane}

The traditional Chinese medicine \emph{Anisodus tanguticus} , usually
used for promoting blood circulation, contain four novle
norsesquiterpenoids anisotanols A\selectlanguage{english}-D (\textbf{15} \selectlanguage{english}-\textbf{18} ) with a
congested tricyclic 6/3/5 ring system, which have been assumed to be
biosynthesized by W\selectlanguage{english}-M 1,4-hydride shift and cyclization of
germacane-type carbon skeleton (Fig.
6).\textsuperscript{{[}38{]}}Another unusual germacrane derivative
tomenphantadenine (\textbf{19} ), with an adenine-substituted unit, has
been isolated from the whole plant of \emph{Elephantopus tomentosus} L,
and \textbf{19} showed potent antibacterial activity against the
gram-positive \emph{Staphylococcusaureus} .\textsuperscript{{[}39{]}}

Four rearranged sesquiterpenoids eugenunilones C\selectlanguage{english}-F (\textbf{20}
\selectlanguage{english}-\textbf{23} ), possibly originated from the carbon skeleton of
germacrane, have been isolated from the fruits of \selectlanguage{english}\emph{Eugenia uniflora}
,\textsuperscript{{[}40{]}} an evergreen shrub native to South
America.\textsuperscript{{[}41-43{]}} Structurally, eugenunilones C and
D (\textbf{20} and \textbf{21} ) possess new skeletons assembled on
caged tricyclo{[}4.3.1.0\textsuperscript{3,7}{]}decane and
bicyclo{[}3.2.1{]}octane sca\selectlanguage{english}ffolds, respectively, while eugenunilones E
and F (\textbf{22} and \textbf{23} ) possess a rare
tricyclo{[}4.4.0.0\textsuperscript{2,10}{]}decane ring system.

\textbf{Fig. 6.} Proposed biosynthetic pathway for\textbf{15}
\selectlanguage{english}-\textbf{18.}

\textbf{2.5. Eudesmane}

Spiroalanfurantones A\selectlanguage{english}-D (\textbf{24} \selectlanguage{english}-\textbf{27} ) have been isolated
from the roots of \emph{Inula helenium} ,{[}44{]} which has been
reported to be one of the most important source of sesquiterpene
lactones, such as guaianolide, eudesmanolide, germacranolide,
etc.\textsuperscript{{[}45-49{]}} Structurally, spiroalanfurantones A\selectlanguage{english}-D
(\textbf{24} \selectlanguage{english}-\textbf{27} ), four eudesmanolide\selectlanguage{english}-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 di\selectlanguage{english}ffraction analysis. Biogenetically, {[}4 + 2{]}
cycloaddition between eudesmane and furan would be a key step to
construct the carbon skeleton in the proposed biogenetic pathway
of\textbf{24} \selectlanguage{english}-\textbf{27} (Fig. 7). Additional, \textbf{24}
\selectlanguage{english}-\textbf{25}showed inhibition activity of nitric oxide production in
lipopolysaccharide-induced RAW264.7 macrophages with
IC\textsubscript{50} values of 17.3 and 9.5 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M, respectively.

Artemilavanolides A (\textbf{28} ) and B (\textbf{29} ), two new
rearranged eudesmane-type sesquiterpenoids with an unprecedented
6-oxabicyclo{[}3.2.1{]}octane system, have been obtained from aerial
parts of \emph{Artemisia lavandulaefolia} .\textsuperscript{{[}50{]}}
Atralanols A and B (\textbf{30} and \textbf{31} ),{[}51{]} derived from
eudesmane derivativies, possess a rare spiro {[}4,4{]} skeleton, and a
plausible biosynthetic pathway for \textbf{30} and \textbf{31} has been
given in Fig. 7.

\textbf{Fig. 7.} Proposed biosynthetic pathways for\textbf{24}
-\textbf{27} and \textbf{30} -\textbf{31.}

\textbf{2.6. Guaiane}

The genus \emph{Artemisia} is famous because of artemisinin, an
amorphane sesquiterpenoid obtained from \emph{Artemisia annua}exhibiting
notable antimalarial activity.\textsuperscript{{[}52-55{]}}Two novel
cage-like sesquiterpenoids artatrovirenols A and B (\textbf{32} and
\textbf{33} ), have been found from the plant\emph{Artemisia atrovirens}
.\textsuperscript{{[}56{]}} Structurally, artatrovirenols A and B
(\textbf{32} and \textbf{33} ) possess a 5/5/6/5/5-pentacyclic and a
5/5/6/5-tetracyclic system with an unprecedented
tetracyclo{[}5.3.1.1\textsuperscript{4,11}.0\textsuperscript{1,5}{]}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 (\textbf{34} -\textbf{36} ) are three novel rearranged
guaiane-type sesquiterpenoids which have been isolated from the herb
of\emph{Daphne penicillata} by molecular networking
strategies.\textsuperscript{{[}57{]}} Daphnenoid A (\textbf{34} )
possesses a unique caged
tetracyclo{[}5.3.2.0\textsuperscript{1,6}.0\textsuperscript{4,11}{]}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
(\textbf{35} and \textbf{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.

\textbf{Fig. 8.} Proposed biosynthetic pathways for\textbf{32}
-\textbf{36} .

\textbf{2.7. Miscellaneous sesquiterpenoids}

Curcumanes A (\textbf{37} ) and B (\textbf{38} ) are two novel bicyclic
sesquiterpenoids with significant vasorelaxant activity, which have been
isolated from \emph{Curcuma longa} ,\textsuperscript{{[}58{]}} a
traditional Chinese medicine for promoting blood circulation.{[}59{]}
Structurally, \textbf{37} and \textbf{38} possess unprecedented
skeletons with a dicyclo{[}3.2.1{]}octane and a dicyclo{[}3.3.1{]}nonane
moiety, respectively. Biosynthetically, the cyclization from C-1'' to
C-1' and C-1'' to C-5 of a bisabolane derivative, respectively, are key
steps in the proposed biogenetic pathways of \textbf{37} and
\textbf{38}(Fig. 9), respectively.

A pair of caged norsesquiterpenoidal enantiomers (\selectlanguage{ngerman}±)-preuisolactone A
(±\textbf{39} ), featuring an unprecedented
tricyclo{[}4.4.0\textsuperscript{1,6}.0\textsuperscript{2,8}{]}decane
carbon sca\selectlanguage{english}ffold determined by X-ray crystallography, have been found from
the plant endophytic fungus \emph{preussia isomera}
.\textsuperscript{{[}60-62{]}} A plausible biosynthetic pathway for
(+-)-\textbf{39} has been proposed (Fig. 9). In addition, (+-)-\textbf{39}
exhibited remarkable antibacterial activity against\emph{Micrococcus
luteus} with an MIC value of 10.2 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M.

\textbf{Fig. 9.} Proposed biosynthetic pathways for\textbf{37}
-\textbf{39} .

\textbf{2.8. Sesquiterpenoid polymers}

A novel dimeric guaianolide xylopiana A (\textbf{40} ), possessing an
unprecedented
pentacyclo{[}5.2.1.0\textsuperscript{1,2}.0\textsuperscript{4,5'}.0\textsuperscript{5,4'}{]}decane-3,2'-dione
core, has been found from the leaves of \emph{Xylopia vielana} , which
is one of the main sources of guaiane-type sesquiterpenoid
dimers.\textsuperscript{{[}63{]}} Furthermore, the case-shaped skeleton
of xylopiana A (\textbf{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 (\textbf{41} ), vlasoulamine A (\textbf{42} ), heliaquanoids A-E
(\textbf{43} -\textbf{47} ), and artematrovirenolides A-D (\textbf{48}
-\textbf{51} ), have been isolated from four plants
including\emph{Hedyosmum orientale} , \emph{Vladimiria souliei} ,
\emph{Inula helianthus-aquatica} , and \emph{Artemisia atrovirens} ,
respectively.\textsuperscript{{[}64-67{]}} Structurally,
\textbf{41}possesses an unprecedented heterodimeric structure with two
different classes of sesquiterpenoids furnished by forming an unusual
1,3-dioxolane ring. \textbf{42} , an unprecedented sesquiterpene lactone
dimer featuring a fully hydrogenated pyrrolo{[}2,1,5-cd{]}indolizine
core, has exhibited neuroprotective activity. \textbf{43} represents the
first \emph{exo} -2,4-linked Diels-Alder adduct between a
pseudoguaianolide dienophile and a guaianolide diene.\textbf{48}
-\textbf{51} are four novel hetero-dimeric {[}4 + 2{]} Diels--Alder
adducts dimerized from a rotundane-type unit and a guaiane-type monomer.

\textbf{Fig. 10.} Proposed biosynthetic pathway for \textbf{40} .

A novel lindenane sesquiterpene dimer spirolindemer A (\textbf{52} ) and
a trimer spirolindemer B (\textbf{53}
),\textsuperscript{{[}68{]}}equipped with oxaspiro{[}4.5{]}decane unit,
have been discovered from the medicinal plant \emph{Chloranthus henryi}
. \textbf{52} showed anti-inflammatory activity by inhibiting the
expression of iNOS and COX-2. Chromatography of extracts of plant
\emph{Chloranthus fortunei}and \emph{Chloranthus holostegius} afforded
the two novel skeletal lindenane sesquiterpene dimers fortunoid A
(\textbf{54} ) and chlotrichene A (\textbf{55} ),
respectively.\textsuperscript{{[}69, 70{]}} \textbf{54} , the rearranged
lindenane dimer, showed moderate antimalarial activities, and
\textbf{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 \emph{endo} -Diels-Alder reaction. In addition, a possible
biosynthetic pathway for \textbf{54}would involve in an intermolecular
Diels-Alder cycloaddition by an unidentified {[}4 + 2{]} cyclase and
oxidative cleavage of the \selectlanguage{greek}Δ\selectlanguage{english}\textsuperscript{4} double bond.

A novel cadinane sesquiterpene dimer commiphoratone A (\textbf{56}
),\textsuperscript{{[}71{]}} containing a saddle shape skeleton with a
unique 6/6/5/5/6/6 heptacyclic architecture, has been isolated from the
plant \emph{Resina Commiphora} , a Chinese medicine for the treatment of
blood stagnation. \textbf{56} showed significant retardation of lipid
metabolism in a concentration-dependent manner. Another compound
arteannoide A (\textbf{57} ),\textsuperscript{{[}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 \emph{Artemisia
annua} L. Moreover, a plausible biosynthetic pathway for \textbf{57} ,
involving in aldol and Michael addition reactions, has been proposed in
Fig. 11.

\textbf{Fig. 11.} Proposed biosynthetic pathway for \textbf{57} .

3. Diterpenoids

Diterpenoids (C\textsubscript{20}), 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.\textsuperscript{{[}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.\textsuperscript{{[}74-77{]}}In the following
pages, diterpenoids (abietane, kaurene, cembrane, etc.) with diverse
skeletal types will be introduce

\textbf{Fig. 12.} Classification of the diterpenoids.

\textbf{3.1. Abietane}

The abietane diterpenoids are the main components of conifers
oleoresins, which play crucial roles in plant to resist pests and
pathogens. Abietane diterpenoids isolated in recent years can be divided
into five sub-classes, namely, classical abietanes, rearranged
abietanes,\textsuperscript{{[}78{]}} aromatic
abietanes,\textsuperscript{{[}79{]}} abietane
lactones,\textsuperscript{{[}80}{]} and abietane dimers.

In previous studies, more than 90 diterpenoids belonging to abietane
have been reported from the genus\emph{Isodon}
.\textsuperscript{{[}81{]}} During ongoing investigation on\emph{Isodon
rugosiformis} , rugosiformisin A, a skeleton-rearranged abietane-type
diterpenoid with potent NO inhibitory activity, has been obtained
(\textbf{58} ).\textsuperscript{{[}82{]}} The stereochemistry
of\textbf{58} has been determined using single-crystal X-ray diffraction
analysis.

Officinalins A (\textbf{59} ) and B (\textbf{60}
),\textsuperscript{{[}83}{]} which are a pair of unique
C\textsubscript{23} terpenoid epimers possessing an unprecedented
tetracyclic 6/7/5/5 carbon skeleton with a
tetracycline{[}9.6.0.0\textsuperscript{3,8}.0\textsuperscript{12,16}{]}heptadecane
core, have been isolated from the leaves of \emph{Salvia officinalis} .
Another rearranged abietane diterpenoid, salviyunnanone A (\textbf{61}
),\textsuperscript{{[}84}{]} has been isolated from\emph{Salvia
yunnanensis} . \textbf{61} possesses an unprecedented 7/5/6/3 fused ring
system comprising an unusual 9,11-epoxy moiety. Recently, a series of
highly oxidized abietane diterpenoids with potent inhibitory activity
against cancer cells were identified from\emph{Euphorbia fischeriana}
.\textsuperscript{{[}85{]}}

A unique abietane diterpenoid dimer, fischdiabietane A (\textbf{62}
),\textsuperscript{{[}86{]}} possessing 2-oxaspiro{[}4.5{]}decane-1-one
and 2-oxabicyclo{[}3.2.2{]}nonane framework, has been isolated from
\emph{Euphorbia fischeriana} . The absolute configuration of \textbf{62}
has been determined by single-crystal X-ray diffraction analysis.
\textbf{62} has induced apoptosis in T47D cells through the activation
of caspase-3 and the degradation of poly (ADP-ribose) polymerase.

\textbf{3.2. Ent-kaurane}

The simplest \emph{ent} -kaurane diterpenoid,\emph{ent} -kaurane
(\textbf{63} ),\textsuperscript{{[}87{]}} was identified from the
genus\emph{Agathis} in 1961. The plant is locally called \emph{kauri
pine} , therefore the diterpenoid with negative optical rotation was
called\emph{ent} -kaurane. Up to now, more than 1000 \emph{ent} -kaurane
diterpenoids have been isolated,\textsuperscript{{[}88{]}} and most of
them are derived from the genus \emph{Isodon}
.\textsuperscript{{[}89{]}}

Maoeriocalysin A (\textbf{64} ),\textsuperscript{{[}90{]}} a novel
rearranged \emph{ent} -kaurane diterpenoid with an unprecedented
4,5-\emph{seco} -3,5-cyclo-7,20-epoxy-\emph{ent} -kaurane skeleton, have
been isolated from \emph{Isodon eriocalyx} . Another highly rearranged
and dual-bridged spiro \emph{ent} -kaurane diterpenoid crokonoid A
(\textbf{65} ) has been isolated from \emph{Croton kongensis}
.\textsuperscript{{[}91{]}} \textbf{65} has displayed strong
cytotoxicity against HL-60 and A-549 cell lines. Pierisketolide A
(\textbf{66} ) and pierisketones B and C (\textbf{67} and \textbf{68} ),
three diterpenoids with an unusual A-\emph{homo} -B-\emph{nor}
-\emph{ent} -kaurane carbon skeleton, have been obtained from the roots
of \emph{Pieris formosa} .\textsuperscript{{[}92{]}} \textbf{66}
exhibited an analgesic effect with a 45\% writhe inhibition rate at a
dose of 10.0 mg/kg.

Grayannotoxane diterpenoids with a 5/7/6/5 ring system are derived from
the \emph{ent} -kaurane, and their tetracyclic skeleton is formed by the
rearrangement of the kaurane A/B ring system to a 5,7-ring
system.\textsuperscript{{[}93, 94{]}}

In recent years, Yao's group has reported a serious of grayanane
diterpenoids from the traditional Chinese medicine\emph{Rhododendron
mole} . Rhodomollacetals A-C (\textbf{69} -\textbf{71}
),\textsuperscript{{[}95}{]} three novel diterpenoids with an
unprecedented 2,3:5,6-di-\emph{seco} -grayanane carbon skeleton, have
been isolated in 2017. The absolute configurations of \textbf{69} and
\textbf{70} have been assigned by single-crystal X-ray crystallography,
and \textbf{69} -\textbf{71} show moderate PTP1B inhibitory activities.
Rhodomollanol A (\textbf{72} ) also has been reported in
2017,\textsuperscript{{[}96{]}} which possesses a unique 3/5/7/5/5/5
hexacyclic ring system featuring a rare 7-oxabicyclo{[}4.2.1{]}nonane
core decorated with three cyclopentane units, and \textbf{72} exhibited
moderate PTP1B inhibitory activity. Furthermore, two highly modified
grayanane diterpenoids, mollebenzylanols A (\textbf{73} ) and B
(\textbf{74} ),\textsuperscript{{[}97{]}} have been identified, and the
key steps of plausible biosynthetic pathway would involve in an
enzymatic W-M rearrangement and two retro-aldol reactions as shown in
Fig. 13. Three highly functionalized 5,6-\emph{seco} -grayanane
diterpenoids mollactones A--C (\textbf{75} --\textbf{77} ) have been
reported in 2020,\textsuperscript{{[}98{]}} and these compounds
exhibited significant PTP1B inhibitory activity. In 2021,
epoxymicranthol G (\textbf{78} )\textsuperscript{{[}99{]}} has been
obtained from \emph{Rhododendron micranthum} , which is a
5,9-epoxygrayanane diterpenoid expressed potent analgesic activity. The
structure-activity relationship for the analgesic effects of
5,9-epoxygrayanane diterpenoids has been discussed, and the 3\selectlanguage{greek}\emph{β}
\selectlanguage{english}-\selectlanguage{english}OH, 10\selectlanguage{greek}\emph{α} \selectlanguage{english}-\selectlanguage{english}OH, 14\selectlanguage{greek}\emph{α} \selectlanguage{english}-\selectlanguage{english}OH, and 16\selectlanguage{greek}\emph{α} \selectlanguage{english}-\selectlanguage{english}OH may be the
activating groups to the analgesic activity of 5,9-epoxygrayanane
diterpenoids. Bismollether A (\textbf{79} ), a dimeric grayanane
diterpenoid with a caged structrue has been isolated in
2022,\textsuperscript{{[}100{]}} and it exhibits significant analgesic
activities. An unprecedented 5/6/6/5 tetracyclic grayanane-derived
diterpenoid rhomollone A (\textbf{80} ) has been obtained from flowers
of\emph{Rhododendron molle.} \textsuperscript{{[}101{]}}

\textbf{Fig. 13.} Proposed biosynthetic pathway for\textbf{73}
-\textbf{74} .

\textbf{3.3. Cembrane}

Cembranolides, with a 14-membered macrocyclic skeleton, are the
precursors of casbane, tigliane, and related
diterpenoids.\textsuperscript{{[}102{]}} (\selectlanguage{ngerman}±)-Mangelonoids A and B
(\textbf{81} --\textbf{84} ),\textsuperscript{{[}103{]}} two pairs of
enantiomers featuring an unprecedented bicyclo{[}9.3.1{]}pentadecane
core and a bridgehead double bond, have been isolated from \emph{Croton
mangelong} . \textbf{81} exhibited NF-\selectlanguage{greek}\emph{κ} \selectlanguage{english}B inhibition with an
IC\textsubscript{50} value of 7.27 \selectlanguage{ngerman}± 1.30 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M. A novel
cembranoid, sarcomililate A (\textbf{85} ),\textsuperscript{{[}104{]}}
possessing a previously undescribed
tricyclo{[}11.3.0.0\textsuperscript{2,16}{]}hexadecane scaffold, has
been isolated from \emph{Sarcophyton mililatensis} . Populusene A
(\textbf{86} ) is a cembrane-type diterpenoid possessing an
unprecedented carbon skeleton,\textsuperscript{{[}105{]}}western
blotting analysis has confirmed that \textbf{86} significantly inhibits
LPS-induced activation of NF-\selectlanguage{greek}κ\selectlanguage{english}B in RAW264.7 cells. Sinudenoid A
(\textbf{87} ),\textsuperscript{{[}106{]}} a new
furanobutenolide-derived C\textsubscript{19}-norcembranoid diterpene,
has been isolated from the soft coral \emph{Sinularia densa} . Recently,
several casbane-type diterpenoids have been
reported,\textsuperscript{{[}107-112{]}} and sinunanolobatone A
(\textbf{88} ) from \emph{Sinularia nanolobata}
,\textsuperscript{{[}107{]}} featuring an unprecedented casbane related
carbon framework, showed significant inhibitory activity against
lipopolysaccharide (LPS) induced inflammation.

\textbf{3.4. Pimarane}

Pimarane, a type of tricyclic diterpenoids, generally obtained from
plants and fungi but seldom from other biological
resources.\textsuperscript{{[}113, 114{]}} Pimarane could be further
classified into pimarane, isopimrane, \emph{ent} -pimarane,
and\emph{ent} -isopimrane based on differed stereochemistry. Seven new
diterpenoids (\textbf{89} --\textbf{95} ) have been isolated from the
tuber of \emph{Icacina oliviformis}
.\textsuperscript{{[}115{]}}Oliviformislactones A (\textbf{89} ) and B
\textbf{(90} ) are the first examples of rearranged 3,4-\emph{seco}
-pimarane possessing a 6/6/5/5 tetracyclic ring system;
secopimaranlactone A (\textbf{91} ) and secocleistanthone A (\textbf{92}
) are the first examples of 3,4-\emph{seco} -pimarane and
3,4-\emph{seco} -cleistanthane type diterpenoids, respectively, which
have been obtained from the Icacinaceae family. Chromatographic
purification of \emph{Premna fulva}extracts led to one undescribed
isopimarane-type diterpenoid, named premnafulvol A (\textbf{93}
),\textsuperscript{{[}116{]}} and the plausible biotransformation
pathway of \textbf{93} has been proposed (Fig. 14). Glabresides A-C
(\textbf{94} -\textbf{96} ),\textsuperscript{{[}117{]}} three
undescribed\emph{ent} -pimarane diterpenoid dimers, have been isolated
from\emph{Sigesbeckia glabrescens} . \textbf{96} exhibited the most
potent inhibition activity on nitric oxide (NO) production induced by
LPS in BV2 microglia compared with the other two compounds. \textbf{96}
also increased the protein expression level of heme oxygenase-1 (HO-1),
and suppressed inducible nitric oxide synthase (iNOS) and cyclooxygenase
2 (COX-2) in LPS-stimulated BV2 cells.

\textbf{Fig. 14.} Proposed biosynthetic pathway for \textbf{93} .

\textbf{3.5. Fusicoccane}

In recent years, Zhang's group has reported a serious of fusicoccane
diterpenoids with unexpected carbon skeleton from the
genus\emph{Alternaria} and \emph{Talaromyces} . Alterbrassicene A
(\textbf{97} ),\textsuperscript{{[}118{]}} a fusicoccane diterpenoid
bearing an unprecedented 5/9/4-fused carbocyclic skeleton, has been
characterized from \emph{Alternaria brassicicola} . \textbf{97} is the
first fusicoccane derivative acting as a potent IKK\selectlanguage{greek}\emph{β} \selectlanguage{english}inhibitor in
the NF-\selectlanguage{greek}\emph{κ} \selectlanguage{english}B signaling pathway. Alterbrassicicene A (\textbf{98}
),\textsuperscript{{[}119{]}} a fusicoccane diterpenoid with a
degradatived framework, has been also obtained from \emph{A.
brassicicola} . \textbf{98} reprents the first fusicoccane-derived
diterpenoid functioning as a potent PPAR-\selectlanguage{greek}\emph{γ} \selectlanguage{english}agonist
(EC\textsubscript{50} = 744.1 nM). A plausible biosynthetic pathway
for\textbf{98} has been proposed and involved in a series of
cyclizations and rearrangement by the function of enzymes (Fig. 15).
Alterbrassinoids A-D (\textbf{99} -\textbf{102}
),\textsuperscript{{[}120{]}} the first examples of fusicoccane dimers
furnished by forming an undescribed C-12-C-18' bond, have been isolated
from the same fungus. Talaronoids A--D (\textbf{103} -\textbf{106} ),
four diterpenoids with an unexpected tricyclic 5/8/6 carbon skeleton,
have been isolated from\emph{Talaromyces stipitatus} . Talaromynoid A
(\textbf{107} ),\textsuperscript{{[}121{]}} the first fusicoccane
diterpenoid bearing an unexpected 5/7/5 tricyclic ring system, have been
obtained from the endophytic fungus \emph{Talaromyces} sp. DC-26.

\textbf{Fig. 15.} Proposed biosynthetic pathway for \textbf{98} .

\textbf{3.6. Jatrophane}

Jatrophane diterpenoids possess a bicyclo{[}10.3.0{]}pentadecane
skeleton, and occur exclusively in the
\emph{Euphorbiaceae}family.\textsuperscript{{[}3, 122{]}} Heliojatrones
A and B (\textbf{108} and \textbf{109} ), two jatrophane-derived
diterpenoids with an unprecedented \emph{trans}
-bicyclo{[}8.3.0{]}tridecane core, have been isolated from the whole
plants of \emph{Euphorbia helioscopia} . \textbf{109} showed significant
Pglycoprotein inhibitory activity. Euphopias A-C (\textbf{110}
-\textbf{112} ), three rearranged jatrophane-type diterpenoids with
unexpected carbon skeleton, have been isolated from \emph{Euphorbia
helioscopia} . \textbf{111} could ameliorate mitochondria damage,
thereby interrupting NLRP3 inflammasome activation. Euphohelioscoids A
(\textbf{113} ) and B (\textbf{114} ), diterpenoids with rearranged
9(10-11)-\emph{abeo} -10,12-cyclojatrophane skeleton and the first
(15\emph{S} )-jatrophane have been isolated from\emph{Euphorbia
helioscopia} .\textsuperscript{{[}123{]}} Yin's group has studied
\emph{Euphorbia} plants a lot, and they have identified a serious of
highly modified jatrophane diterpenoids, such as euphorhelipanes A
(\textbf{115} ) and B (\textbf{116} ) from the whole plants of
\emph{Euphorbia helioscopia} ,\textsuperscript{{[}124{]}}jatrofolianes A
(\textbf{117} ) and B (\textbf{118} ) from \emph{Jatropha gossypiifolia}
,\textsuperscript{{[}125{]}} euphohyrisnoids A (\textbf{119} ) and B
(\textbf{120} ) from the seeds of \emph{Euphorbia lathyris}
,\textsuperscript{{[}7{]}} and euphylonoids A (\textbf{121} ) and B
(\textbf{122} ) from \emph{Euphorbia hylonoma}
.\textsuperscript{{[}126{]}} \textbf{115} and \textbf{116}showed a
triglyceride-lowering effect in oleic-acid-stimulated HuH7 cells, and
\textbf{122} significantly inhibited early adipogenesis in 3T3-L1
adipocytes via activating AMP-activated protein kinase signaling.

4. Sesterterpenoids

Sesterterpenoids are a small group of terpenoids containing 25 carbons,
which are derived from the linear precursor GFPP. Sesterterpenoids have
been isolated from many kinds of organisms such as plants, fungi, and
marine sponge, with a great diversity of structures and skeletons and
broad biological functions. A review has introduced the structures and
bioactivities of marine sesterterpenoids since 2013 until
2017;\textsuperscript{{[}127{]}} another review has summarized the
distribution, chemistry, biological activities, biosynthesis and
evolution of plant sesterterpenoids since 1969 until
2021.\textsuperscript{{[}128}{]} In this contribution, some typical
sesterterpenoids with unprecedented carbon skeletons will be introduced,
including their chemical structures and biological activities.

Pre-leucosceptroid (\textbf{123} ) and isopre-leucosceptroid
(\textbf{124} ) are intriguing monocarbocyclic
sesterterpenoids.\textbf{123} has been identified from the leaves of
\emph{Leucosceptrum canum} , while its isomer \textbf{124} has been
isolated from the excrement of \emph{Nacna malachitis} larvae, a
specialist insect feeding on \emph{L. canum} . Extensive spectroscopic
analysis and quantum chemical calculations have confirmed their absolute
configurations.\textsuperscript{{[}129{]}}

A pair of enantiomeric bicyclic sesterterpenoids, (\selectlanguage{ngerman}±)-hippolide J
(\textbf{125} and \textbf{126} ), have been isolated from the marine
sponge \emph{Hippospongia lachne} collected from the South China
Sea.\textsuperscript{{[}130{]}} Both of them showed potent antifungal
activity against three strains of hospital-acquired pathogenic
fungi,\emph{Candida albicans} SC5314, \emph{Candida glabrata} 537,
and\emph{Trichophyton rubrum} Cmccftla, with MIC\textsubscript{50}
values of 0.125\selectlanguage{english}-0.25 \selectlanguage{greek}\emph{μ} \selectlanguage{english}g/mL.\textsuperscript{{[}130{]}}

Two unique tricyclic sesterterpenoids linderasesterterpenoids A
(\textbf{127} ) and B (\textbf{128} ), with an unprecedented
7-cyclohexyldecahy-droazulene carbon skeleton, have been separated from
the root of \emph{Lindera glauca}
.\textsuperscript{{[}131{]}}\textbf{127} and \textbf{128} showed good
inhibitory activities against LPS-induced NO production in RAW 264.7
cells with IC\textsubscript{50}values of 18.9 and 16.2 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M,
respectively, as compared to the positive control (indomethacin,
IC\textsubscript{50} = 30.9\selectlanguage{greek}\emph{μ} \selectlanguage{english}M).\textsuperscript{{[}131{]}}
Gentianelloids A (\textbf{129} ) and B (\textbf{130} ), a pair of
sesterterpenoid epimers, possessing an unusual 10,11-\emph{seco}
-gentianellane skeleton, have been obtained from the traditional Uighur
medicine \emph{Gentianella turkestanorum} . Their structures including
absolute configurations have been determined by extensive spectroscopic
and single-crystal X-ray diffraction analyses. Moreover, the plausible
biosynthetic origins of their skeletons are both via the tetracyclic
precursor nitidasin, which must be derived from GFPP via cyclization and
oxidation reactions (Fig. 16).\textsuperscript{{[}132{]}}

Eurysoloids A (\textbf{131} ) and B (\textbf{132} ), two novel
diastereomeric sesterterpenoids possessing a pentacyclic 5/6/5/10/5
framework with an unusual macrocyclic ether system, have been reported
from \emph{Eurysolen gracilis} Prain, and their absolute configurations
have been assigned by single-crystal X-ray diffraction and
DP4\textsuperscript{+}analyses.\textsuperscript{{[}133{]}}Peniroquesines
A-C (\textbf{133} -\textbf{135} ), possessing an 5/6/5/6/5 fused
pentacyclic ring system, identified based on NMR spectroscopy, MS,
Mosher's method, and single-crystal X-ray crystallography, have been
isolated from the fungus \emph{Penicillium
roqueforti}YJ-14.\textsuperscript{{[}134{]}} Additionally, compounds
\textbf{133}and \textbf{134} showed a potent inhibitory effect on nitric
oxide production in LPS-activated RAW264.7 macrophages with
IC\textsubscript{50} values of 16.13 \selectlanguage{ngerman}± 1.61 and 11.74 ± 0.26 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M,
respectively, making them more potent than the positive control (L-NMMA,
IC\textsubscript{50} = 42.24 \selectlanguage{ngerman}± 0.68 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M). Biosynthetically, a
plausible cyclization mechanism to yield the scaffold of\textbf{133}
-\textbf{135} via a series of W-M rearrangement and alkyl shift has been
proposed (Fig. 16).\textsuperscript{{[}134{]}}

\textbf{Fig. 16.} Proposed biosynthetic pathways for\textbf{129}
-\textbf{130} and \textbf{133} -\textbf{137} .

Niduterpenoids A (\textbf{136} ) and B (\textbf{137} ), isolated
from\emph{Aspergillus nidulans} , represent the first examples of
sesterterpenoids with a highly congested hexacyclic 5/5/5/5/3/5 carbon
skeleton. Their structures have been determined via a combination of
spectroscopic data and single-crystal X-ray diffraction
analyses.\textsuperscript{{[}135{]}} Compounds \textbf{136}
and\textbf{137} are estrogen receptor\selectlanguage{greek}\emph{α} \selectlanguage{english}inhibitors, and
\textbf{136} abolished 17-estradiol induced human breast tumor cell
proliferation in a dose-dependent manner (IC\textsubscript{50} = 11.42 \selectlanguage{ngerman}±
0.85 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M). Furthermore, a plausible biosynthetic pathway for
\textbf{136} and \textbf{137} has been proposed and involved in a series
of cyclization and W-M hydride and alkyl shift reactions (Fig.
16).\textsuperscript{{[}135{]}}

5. Triterpenoids

Triterpenes are a class of terpenoids with 30 carbons in the basic
parent nucleus. They exist in plants in free form or as glycosides, and
have various biochemical activities. Among them, three systematic
reviews have covered the published data on triterpenoids isolated from
all organisms from 2013 to 2015.\textsuperscript{{[}136-138{]}} In
recent years, many triterpenoids have been reported with unusual
tetracyclic,\textsuperscript{{[}139-144{]}}pentacyclic,\textsuperscript{{[}140,
145-149{]}} and a few tricyclic ring
systems.\textsuperscript{{[}150-153{]}} Among them, a series of dimers
bearing different scaffold have been designed and synthesized, and their
anti-HCV entry activities have also been
evaluated.\textsuperscript{{[}154{]}}

Belamchinenin A (\textbf{138} ), an unprecedented 6/5/6 fused tricyclic
triterpenoid with a novel carbon skeleton, has been isolated from the
rhizomes of \emph{Belamcanda chinensis} .\textsuperscript{{[}151{]}} The
absolute configuration of tricyclic nucleus system has been
unequivocally assigned by ECD calculation. In addition,
\textbf{138}exhibits cytotoxicity against NCI-H1650, HepG2, BGC 823,
HCT-116, and MCF-7 cells with IC\textsubscript{50} values of 2.48, 2.55,
4.47, 2.29, and 2.85 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M,
respectively.\textsuperscript{{[}151{]}}Nototronesides A-C (\textbf{139}
-\textbf{141} ), three triterpenoid saponins with a 6/6/9 fused
tricyclic tetranordammarane carbon skeleton, have been isolated from the
leaves of \emph{Panax notoginseng} .\textsuperscript{{[}152{]}}Moreover,
\textbf{140} showed a moderate neuroprotective effect on serum
deficiency-induced cellular damage.\textsuperscript{{[}152{]}}

\textbf{Fig. 17.} Proposed biosynthetic pathway for \textbf{142} .

Ganolearic acid A (\textbf{142} ), a hexanorlanostane triterpenoid with
a 3/5/6/5 fused tetracyclic skeleton from \emph{Ganoderma cochlear} ,
has been found using a LC-UV/MS screening
approach.\textsuperscript{{[}155{]}} The stereochemical structure has
been established by spectroscopic data, \textsuperscript{13}C NMR and
ECD calculations, and then a plausible biosynthetic pathway
for\textbf{142} has been given (Fig.
17).\textsuperscript{{[}155{]}}Alismanin A (\textbf{143} ), a novel
protostane-type triterpenoid with a C\textsubscript{34} skeleton, has
been isolated from the rhizomes of\emph{Alisma orientale}
.\textsuperscript{{[}156{]}} Its structure has been determined by a
combination of HRESIMS, 2D NMR spectra, ECD calculation. \textbf{143}
displayed significant agonistic effects on pregnane X receptor at a
concentration of 10 nM.\textsuperscript{{[}156{]}} Colqueleganoids A
(\textbf{144} ) and B (\textbf{145} ), with the first methyl-30
incorporated 6/6/6/6 tetracyclic carbon skeleton, have been isolated
from the root of \emph{Colquhounia elegans}
.\textsuperscript{{[}157{]}}Interestingly, \textbf{144} and \textbf{145}
showed significant immunostimulatory effects and enhanced the production
of cytokines TNF-\selectlanguage{greek}\emph{α} \selectlanguage{english}and IL-6 in the RAW 264.7 cells stimulated
with LPS.\textsuperscript{{[}157{]}}

Applanoids A-E (\textbf{146} -\textbf{150} ) represent the first example
of\emph{Ganoderma} triterpenoids with 6/6/5/6/5 pentacyclic system and
the formation of the ether ring between C-15 and C-20 involves Michael
addition reaction.\textsuperscript{{[}158{]}} Among of them,\textbf{146}
, \textbf{147} , and \textbf{148} could significantly activate human
pregnane X receptor (hPXR), and molecular docking revealed that they
were docked well into the ligand binding domain by the establishment of
hydrogen bonds and van der Waals effects.\textsuperscript{{[}158{]}}
Irpexolidal (\textbf{151} ), a triterpenoid with an unprecedented
6/5/6/5/6/5 fused polycyclic skeletal system carbon skeleton, has been
isolated from the fruiting bodies of the medicinal fungus \emph{Irpex
lacteus} .\textsuperscript{{[}159{]}} The structure of \textbf{151} has
been established by ECD calculation, and
DP4\textsuperscript{+}probability based on GIAO NMR chemical shift
calculations.\textsuperscript{{[}159{]}}

Belamchinanes A-D (\textbf{152} -\textbf{155} ), four triterpenoids with
an unprecedented 4/6/6/6/5 pentacyclic system, have been isolated from
the seeds of \emph{Belamcanda chinensis} .\textsuperscript{{[}160{]}}
Biological studies reveal that they have dose-dependently protect
age-related renal fibrosis in\emph{vitro} . In addition, a key step of
plausible biosynthetic pathway would involve in Michael addition
reaction of the 3,4-\emph{seco}fernane (Fig.
18).\textsuperscript{{[}160{]}} Alstoscholarinoids A (\textbf{156} ) and
B (\textbf{157} ), representing new subtypes of pentacyclic
triterpenoids, with unique 6/6/6/7/5 and 6/6/5/6/6/6 ring systems have
been isolated from \emph{Alstonia scholaris}
.\textsuperscript{{[}161{]}} Their absolute structures have been
established by single-crystal X-ray diffraction, and ECD calculations.
Surprisingly, both compounds exhibited potent antihyperuricemic
bioactivity in \emph{vitro} and in\emph{vivo}
.\textsuperscript{{[}161{]}}

\textbf{Fig. 18.} Proposed biosynthetic pathway for\textbf{152}
-\textbf{155}

Schincalactones A (\textbf{158} ) and B (\textbf{159} ), featuring a
unique 5/5/6/11/3 fused schinortriterpenoids with a 13-membered carbon
ring system, have been obtained from \emph{Schisandra incarnata}
.\textsuperscript{{[}162{]}} The absolute configuration of\textbf{158}
has been confirmed by single-crystal X-ray diffraction, and then a
plausible biosynthetic pathway for \textbf{158} and \textbf{159}via
8,9-oxidative cleavage has been given (Fig. 19). Moreover,
schilancidilactone C (\textbf{160} ), representing the first
3-norlancischiartane with unusual configurational inversions occurring
at C-1 and C-10 compared to the reported schindilactones A-C, has been
isolated from \emph{Schisandra lancifolia} .\textsuperscript{{[}163,
164{]}}

Dichagelinoids A-E (\textbf{161} -\textbf{165} ) have been isolated and
characterized by solid data from \emph{Dichapetalum gelonioides}
.\textsuperscript{{[}165{]}} Particularly,\textbf{161} -\textbf{163}
showed significant cytotoxic activities against A549 and HL-60 cells
with IC\textsubscript{50} values ranging from 1.00 to 1.35 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M
and 0.38 to 4.23\selectlanguage{greek}\emph{μ} \selectlanguage{english}M, respectively.\textsuperscript{{[}165{]}}

Phainanolide A (\textbf{166} ), a highly modified triterpenoid
incorporating an unprecedented 6/9/6 heterotricyclic system in the
down-left and a highly oxygenated 5,5-spirocyclic ketal lactone motif in
the up-right, has been isolated from \emph{Phyllanthus hainanensis}
.\textsuperscript{{[}166{]}} Particularly, \textbf{166}exhibited potent
activity against HL-60 cell lines with IC\textsubscript{50} values of
0.079 \selectlanguage{ngerman}± 0.037 \selectlanguage{greek}\emph{μ} \selectlanguage{english}M, compared to that of the positive control
(Adriamycin, IC\textsubscript{50} = 0.073 \selectlanguage{ngerman}± 0.015 \selectlanguage{greek}\emph{μ}
\selectlanguage{english}M).\textsuperscript{{[}166{]}}

\textbf{Fig. 19.} Proposed biosynthetic pathway for \textbf{158} and

\textbf{159}

6. Challenge and prospect

Natural products (NPs) have been increasingly attracting attention from
various fields due to their structural diversity and biological
activities.\textsuperscript{{[}13{]}} The skeletal types of NPs are
closely related to their biological evolution. As the evolution
progresses, organisms must continuously change their biosynthetic
pathways to adapt the change in the various environment or resist
external threats, which result in changes in the skeletal types of NPs.
This relationship between NPs structure and evolution makes NPs an
important area in the fields of chemistry and
biology.\textsuperscript{{[}167{]}}

The discovery of new carbon skeletons in NPs is often accompanied by the
discovery of new biological activities. For example, the famous
sesquiterpenoid artemisinin and diterpenoid taxol show notable
antimalarial and antitumor activities respectively, which highlights the
significance of the discovery of novel skeletal NPs and their potential
impact on the development of new drugs.

However, the quantity of novel skeletal NPs is generally limited,
makeing it difficult to evaluate the biological activities of novel
skeleton compounds,\textsuperscript{{[}168{]}} which might limit the
potential of NPs or hinder the development of new drugs based on these
compounds. Therefore, these challenges may inspire strong interests
among organic chemists, biologists, and pharmacologists. As terpenoids
are one of the largest classes of NPs, this review focuses on 166
natural terpenoids with intriging structures from 2017-2022, including
the isolation, structural determination, plausible biosynthetic
pathways, and biological activities. Hopefully, this review would
provide a new perspective for the scholars who are interested in NPs.

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