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 .