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. 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 “C5” units,
called isoprene units. Thus, terpen-
Fig. 1. Classification
of the terpenoids.
oids can be classified based on the number of isoprene units, mainly
including monoterpenoids (C10), sesquiterpenoids
(C15), diterpenoids (C20),
sesterterpenoids
(C25), and triterpenoids (C30).
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).[1] Moreover, the rich
structural variation of terpenoids led to their diverse bioactivities as
well,[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
(C15−C30), 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.[8-15]
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).[1] Based on this
classification by biosynthetic pathways, a series of intriguing new
sesquiterpenoids will be introduced herein.
2.1. Cedrane
The genus 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.[16-20] Chromatography of extracts ofIllicium simonsii and Illicium henryi afforded two new
caged sesquiterpenoids illisimonin A
(1 ) and illihenin A (2 ),
respectively.[21, 22] 1 possesses a
unique 5/5/5/5/5 pentacyclic scaffold featuring a caged
2-oxatricyclo[3,3,0,14,7]nonane ring system fused
to a five-membered carbocyclic ring and a five-membered lactone ring.2 represents a class of novel 5/7/6 tricyclic sesquiterpenoids
featuring a caged tricyclo[6.2.2.01,5]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.
Fig. 3. Proposed biosynthetic pathways for 1 and2 .