Chemistry and Pharmacological Activity of Sesquiterpenoids from the Chrysanthemum Genus

Plants from the Chrysanthemum genus are rich sources of chemical diversity and, in recent years, have been the focus of research on natural products chemistry. Sesquiterpenoids are one of the major classes of chemical constituents reported from this genus. To date, more than 135 sesquiterpenoids have been isolated and identified from the whole genus. These include 26 germacrane-type, 26 eudesmane-type, 64 guaianolide-type, 4 bisabolane-type, and 15 other-type sesquiterpenoids. Pharmacological studies have proven the biological potential of sesquiterpenoids isolated from Chrysanthemum species, reporting anti-inflammatory, antibacterial, antitumor, insecticidal, and antiviral activities for these interesting molecules. In this paper, we provide information on the chemistry and bioactivity of sesquiterpenoids obtained from the Chrysanthemum genus which could be used as the scientific basis for their future development and utilization.


Introduction
The Chrysanthemum genus belongs to the family Compostae and is comprised of more than 40 accepted species of flowering plants (http://www.theplantlist.org/ accessed on 22 March 2021), which are perennial herbs mainly distributed in East Asia; more than 20 species are found in China [1,2]. The flowers of several Chrysanthemum species, including ornamental cultivars and hybrids are economically the second most important floricultural crop after rose, and one of the most important sources of flower arrangement and potted plants around the world [3,4]. In addition, the flower is regularly consumed in many countries as both food and medicine. Traditionally, in China and Japan, some Chrysanthemum species, such as Chrysanthemum morifolium Ramat. (Juhua in Chinese) and Chrysanthemum indicum L. (Yejuhua in Chinese), are used as sedative, anti-inflammatory, antitussive, and a general tonic. C. morifolium and C. indicum are slightly cold in effect with a sweet and bitter taste. Chinese Pharmacopoeia (2015 edition) records their good effects on liver and heart. They are effective in reducing heat and detoxification, treating swollen furuncle carbuncle, swelling and eye pain, headache, and dizziness [5,6]. Modern pharmacological studies have shown that the Chrysanthemum genus has antitumor, antioxidant, anti-inflammatory and antibacterial properties. The species have been found effective against cardiovascular diseases and for reducing fat and cholesterol contents in the blood [7][8][9][10]. With the traditional Chinese Medicine Fufang, C. morifolium granule, C. indicum granule, and C. indicum injections are used to treat prostatitis, chronic pelvic

Sesquiterpenoids
Sesquiterpenoids are compounds made up of three isoprene units, containing 15 carbons [20]. Some sesquiterpenoids with different skeleton types have been isolated and identified from the Chrysanthemum genus, including germacrane-type, eudesmane-type and guaianolide-type sesquiterpenoids. Interestingly, most of these compounds have been reported from just one species, except angeloylcumambrin B (53), cumambrin A (54), and handelin (104) which were reported from more than one species. The following is a classification of the sesquiterpenoids found in the Chrysanthemum genus. A complete profile of these compounds is given in Table 1. Note: NMR, nuclear magnetic resonance; HRESIMS, high resolution electrospray ionization mass spectroscopy; ESIMS, electron ionization mass spectrometry; X-ray, X-ray crystallography; ECD, electronic circular dichroism.

Germacrane-Type Sesquiterpenoids
Germacrane-type sesquiterpenoids are monocyclic sesquiterpenoids, which are composed of a 10-membered carbon ring, a methyl group at C-4 and C-10, and an isopropyl group at C-7. Twenty-six compounds of this type have so far been reported from the genus.

Eudesmane-Type Sesquiterpenoids
Eudesmane-type sesquiterpenoids are bicyclic compounds, composed of two sixmembered carbon rings with methyl groups at C-4 and C-10 positions, and an isopropyl group at C-7. Twenty-six compounds with this skeleton have been isolated from this genus, however, no eudesmanolides have been reported so far. Compounds 35 and 38 are rare due to α-CH3 at C-10, others having a β-CH3 at the same position. Examples of structures are shown in Figure 2.

Guaianolide-Type Sesquiterpenoids
Guaianolide-type sesquiterpenoids have ternary rings consisting of a five-membered ring, a seven-membered ring, a five-membered γ-lactone ring, and methyl groups at C-4 and C-10 positions. The structures are quite complicated with up to nine stereocenters, mostly highly oxidized. Moreover, they are also the most abundant compounds in the Chrysanthemum genus. Compound 90 contains chlorine atoms. So far, 60 chlorinecontaining guaianolide-type sesquiterpenoids have been isolated from Compositae [39,46,47]. Examples of structures are given in Figure 3.

Eudesmane-Type Sesquiterpenoids
Eudesmane-type sesquiterpenoids are bicyclic compounds, composed of two sixmembered carbon rings with methyl groups at C-4 and C-10 positions, and an isopropyl group at C-7. Twenty-six compounds with this skeleton have been isolated from this genus, however, no eudesmanolides have been reported so far. Compounds 35 and 38 are rare due to α-CH 3 at C-10, others having a β-CH 3 at the same position. Examples of structures are shown in Figure 2.

Eudesmane-Type Sesquiterpenoids
Eudesmane-type sesquiterpenoids are bicyclic compounds, composed of two sixmembered carbon rings with methyl groups at C-4 and C-10 positions, and an isopropyl group at C-7. Twenty-six compounds with this skeleton have been isolated from this genus, however, no eudesmanolides have been reported so far. Compounds 35 and 38 are rare due to α-CH3 at C-10, others having a β-CH3 at the same position. Examples of structures are shown in Figure 2.

Guaianolide-Type Sesquiterpenoids
Guaianolide-type sesquiterpenoids have ternary rings consisting of a five-membered ring, a seven-membered ring, a five-membered γ-lactone ring, and methyl groups at C-4 and C-10 positions. The structures are quite complicated with up to nine stereocenters, mostly highly oxidized. Moreover, they are also the most abundant compounds in the Chrysanthemum genus. Compound 90 contains chlorine atoms. So far, 60 chlorinecontaining guaianolide-type sesquiterpenoids have been isolated from Compositae [39,46,47]. Examples of structures are given in Figure 3.

Guaianolide-Type Sesquiterpenoids
Guaianolide-type sesquiterpenoids have ternary rings consisting of a five-membered ring, a seven-membered ring, a five-membered γ-lactone ring, and methyl groups at C-4 and C-10 positions. The structures are quite complicated with up to nine stereocenters, mostly highly oxidized. Moreover, they are also the most abundant compounds in the Chrysanthemum genus. Compound 90 contains chlorine atoms. So far, 60 chlorine-containing guaianolide-type sesquiterpenoids have been isolated from Compositae [39,46,47]. Examples of structures are given in Figure 3.

1,10-Seco Guaianolide Sesquiterpenoids
1,10-Seco guaianolide sesquiterpenoids have a binary ring system. The sevenmembered ring in the middle is disconnected at C-1 and C-10 of guaianolide-type sesquiterpenoids. Six compounds of this subclass have been reported from the genus under focus (Figure 4).

Disesquiterpenoids and a Trisesquiterpenoid
Disesquiterpenoids and a trisesquiterpenoid are composed of an electron donor conjugated diene and an electrophilic double bond fragment formed through [4 + 2] Diels-Alder reaction. An electron donor diene is a C5/C7/C5 guaianolide-type structure, while the electron deficient double bond is provided with the associated α-methylene-γ-lactone fragment [48]. Guaianolide-type compounds in the Chrysanthemum genus form dimers or trimers by cyclization to form five-membered ring. Examples of structures are given in Figure 5.

1,10-Seco Guaianolide Sesquiterpenoids
1,10-Seco guaianolide sesquiterpenoids have a binary ring system. The seven-membered ring in the middle is disconnected at C-1 and C-10 of guaianolide-type sesquiterpenoids. Six compounds of this subclass have been reported from the genus under focus (Figure 4).

1,10-Seco Guaianolide Sesquiterpenoids
1,10-Seco guaianolide sesquiterpenoids have a binary ring system. The sevenmembered ring in the middle is disconnected at C-1 and C-10 of guaianolide-type sesquiterpenoids. Six compounds of this subclass have been reported from the genus under focus (Figure 4).

Disesquiterpenoids and a Trisesquiterpenoid
Disesquiterpenoids and a trisesquiterpenoid are composed of an electron donor conjugated diene and an electrophilic double bond fragment formed through [4 + 2] Diels-Alder reaction. An electron donor diene is a C5/C7/C5 guaianolide-type structure, while the electron deficient double bond is provided with the associated α-methylene-γ-lactone fragment [48]. Guaianolide-type compounds in the Chrysanthemum genus form dimers or trimers by cyclization to form five-membered ring. Examples of structures are given in Figure 5.

Disesquiterpenoids and a Trisesquiterpenoid
Disesquiterpenoids and a trisesquiterpenoid are composed of an electron donor conjugated diene and an electrophilic double bond fragment formed through [4 + 2] Diels-Alder reaction. An electron donor diene is a C5/C7/C5 guaianolide-type structure, while the electron deficient double bond is provided with the associated α-methylene-γ-lactone fragment [48]. Guaianolide-type compounds in the Chrysanthemum genus form dimers or trimers by cyclization to form five-membered ring. Examples of structures are given in Figure 5.

Other Types of Sesquiterpenoids
Nineteen other types of sesquiterpenoids have also been reported from this genus including, four bisabolane-type, one eremophilane-type, one cadinane-type, one oplopanane-type, one dodecane-type, and a carabranolide-type sesquiterpenoids ( Figure  6).

Other Types of Sesquiterpenoids
Nineteen other types of sesquiterpenoids have also been reported from this genus including, four bisabolane-type, one eremophilane-type, one cadinane-type, one oplopananetype, one dodecane-type, and a carabranolide-type sesquiterpenoids ( Figure 6).

Other Types of Sesquiterpenoids
Nineteen other types of sesquiterpenoids have also been reported from this genus including, four bisabolane-type, one eremophilane-type, one cadinane-type, one oplopanane-type, one dodecane-type, and a carabranolide-type sesquiterpenoids ( Figure  6).

Pharmacological Activities
The main pharmacological activities of sesquiterpenoids are antitumor, antiinflammatory, antibacterial and antiviral. Some of the monomers are very good antitumor and anti-inflammatory agents [58].

Antitumor Activity
Current studies have shown that sesquiterpenoids have low cytotoxic effects on normal cells while playing an anticancer role, and therefore more and more experts and scholars are focusing on the antitumor properties of sesquiterpenoids [59]. Among them, sesquiterpenoid lactone have strong antitumor activity. It has been found that αmethylene-γ-butyrolactone ring is the effective group with antitumor activity [60].

Pharmacological Activities
The main pharmacological activities of sesquiterpenoids are antitumor, antiinflammatory, antibacterial and antiviral. Some of the monomers are very good antitumor and anti-inflammatory agents [58].

Antitumor Activity
Current studies have shown that sesquiterpenoids have low cytotoxic effects on normal cells while playing an anticancer role, and therefore more and more experts and scholars are focusing on the antitumor properties of sesquiterpenoids [59]. Among them, sesquiterpenoid lactone have strong antitumor activity. It has been found that αmethylene-γ-butyrolactone ring is the effective group with antitumor activity [60].

Pharmacological Activities
The main pharmacological activities of sesquiterpenoids are antitumor, antiinflammatory, antibacterial and antiviral. Some of the monomers are very good antitumor and anti-inflammatory agents [58].

Antitumor Activity
Current studies have shown that sesquiterpenoids have low cytotoxic effects on normal cells while playing an anticancer role, and therefore more and more experts and scholars are focusing on the antitumor properties of sesquiterpenoids [59]. Among them, sesquiterpenoid lactone have strong antitumor activity. It has been found that α-methyleneγ-butyrolactone ring is the effective group with antitumor activity [60].
Six compounds, angeloylcumambrin B (53), chrysanolide G (56), tigloylcumambrin B (57), chrysanolide E (106), 8 -tigloylchrysanolide D (107), and 8,8 -ditigloylchrysanolide D (109) were isolated and identified from C. indicum by the Xu Jun research group. These six compounds showed multiple cytotoxic activities against four human nasopharyngeal carcinoma (NPC) cell lines (CNE1, CNE2, HONE-1, and SUNE-1) and one human intestinal epithelial cell line . The IC 50 value of some compounds was lower than that of the positive control drug, among which compound 109 had the strongest activity against cancer. It was found that compound 109 concentration-dependently induces G2/M cell arrest. Thus, investigation of the mechanism of action and structure-activity relation (SAR) for compound 109 is worth exploring further [31]. In addition, the Kong Lingyi research group found that the new compound Chrysanthemulide A (CA) (92) demonstrated significant anti-osteosarcoma potential; therefore, its mechanism was studied. The c-Jun N-terminal kinase (JNK) signaling pathway was activated by CA, and treatment with JNK siRNAs or inhibitor SP600125 significantly reduced CA-mediated autophagosome accumulation and DR5-mediated cell apoptosis. CA can induce apoptosis according to upregulated DR5 via JNK-mediated autophagosome accumulation and the combined treatment of CA and TRAIL might be a promising therapy for osteosarcoma [61].

Antibacterial Activity
Two sesquiterpenoids were isolated and purified from the flower of C zawadskii and identified as angeloycumambrin B (53) and cumambrin A (54). Compared to the positive control benzoic acid and sorbic acid which showed antibacterial activity against Bacillus subtilis, Staphylococcus aureus, and Vibrio parahemolyticus with 10-13 mm diameter of clear zone (500 µg/disk), compound 53 (100 µg/disk) showed antibacterial activity against B. subtilis, S. aureus, and V. parahemolyticus with diameter of clear zone 12, 10, and 11 mm, respectively, and compound 54 (100 µg/disk) exhibited the activity against B. subtilis and V. parahemolyticus with 13 and 12 mm diameters of clear zone, respectively. Compounds 53 and 54 showed about five-fold stronger antibacterial activity against B. subtilis and V. parahemolyticus [30].

Conclusions
Sesquiterpenoids are one of the main chemical constituents of the Chrysanthemum genus which have various chiral centers and configurations. At present, 135 sesquiterpenes have been reported from the Chrysanthemum genus (most of them were isolated from C. morifolium and C. indicum), mainly including germacrane-type sesquiterpenoids, eudesmane-type sesquiterpenoids, and guaianolide-type sesquiterpenoids. The compounds have been found to exhibit wide spectrum pharmacological activities such as anti-inflammatory, antitumor, antibacterial, antiviral, antidiabetic, and anti-obesity activities. The activity of some compounds has been found to be better than that of positive control drugs in vitro. Some compounds, especially guaianolide-type sesquiterpenoids, showed better anticancer and anti-inflammatory activity, and their mechanism of action was explained from the aspects of signal pathway and target, which was consistent with the traditional function of clearing heat and removing toxicity exhibited by the Chrysanthemum genus, which is widely distributed in China and has unique advantages in number and species. Therefore, further research and development of the Chrysanthemum genus based on sesquiterpenoids need to be conducted.
At present, some progress has been made in the study of the sesquiterpenoids of the Chrysanthemum genus, but there are still some species to be explored. Most of the sesquiterpenoids have complicated structures, in which there are many chiral centers, conformers, and configurational isomers, and therefore it is difficult to separate and identify them. In terms of pharmacological activities, studies have mainly focused on the in vitro activity screening of pure compounds, while studies on pharmacodynamic evaluation and structure-activity relationship in vivo are almost nonexistent. However, it may be related to the insufficient yield and configuration of isolated compounds.
Therefore, for further research and development of the Chrysanthemum genus the following suggestions should be considered: Firstly, focus on other species of the Chrysanthemum genus that have not been fully studied, especially those whose chemical composition has not been studied. Secondly, try to obtain more sesquiterpenoids from Chrysanthemum genus plants by using more advanced methods. Thirdly, screen the sesquiterpenoids with strong activity and carry out synthesis and structural modification to study structureactivity relationships, efficacy evaluation, and mechanism targets in vivo, and to develop drugs for clinical use.
Author Contributions: Writing-original draft preparation, S.J. and M.W.; writing-review and editing, Z.J., S.Z., Q.X., Y.Y., Y.L. and Y.J.; funding acquisition, H.Y. and W.W. All authors have read and agreed to the published version of the manuscript.