Chemical Diversity and Biological Activity of Secondary Metabolites from Soft Coral Genus Sinularia since 2013

Sinularia is one of the conspicuous soft coral species widely distributed in the world’s oceans at a depth of about 12 m. Secondary metabolites from the genus Sinularia show great chemical diversity. More than 700 secondary metabolites have been reported to date, including terpenoids, norterpenoids, steroids/steroidal glycosides, and other types. They showed a broad range of potent biological activities. There were detailed reviews on the terpenoids from Sinularia in 2013, and now, it still plays a vital role in the innovation of lead compounds for drug development. The structures, names, and pharmacological activities of compounds isolated from the genus Sinularia from 2013 to March 2021 are summarized in this review.


Introduction
Secondary metabolites from marine organisms represented a plentiful source o structurally diverse and natural bioactive products. The soft corals of genus Sinular (phylum Cnidaria, class Anthozoa, subclass Octocorallia, order Alcyonacea, family Alcy oniidae) inhabiting the coral reefs or rocks in shallow water constitutes a dominant po tion of the biomass in the tropical coral reef systems in the world. There are more than 9 species of Sinularia, and more than 50 species have been chemically evaluated from 197 to 2013 [1]. As reported, Sinularia is well-known to produce a wealth of diverse and com plex secondary metabolites, such as sesquiterpenes (10%), diterpenes (46%), norsesqui erpenes (2%), norditerpenes (9%), steroids/steroidal glycosides (22%), and other type (11%) (Figure 1) [1][2][3]. These metabolites exhibit a wide range of biological activities, suc as cytotoxic, anti-inflammatory, antifouling, and anti-microbials [4][5][6][7][8][9][10][11][12][13][14]. There are some published literature reviews on the genus Sinularia. For example, Liang et al. [15] reviewed the secondary metabolites obtained from Sinularia and summarized the different types of chemical structures and/or biological investigations within a literature survey from 1975 to April 2013. Chen et al. [1] reviewed the terpenes from the soft coral of the genus Sinularia, highlighting their novel chemistry and pharmacological activities. Sheu et al. [16] reviewed the structures, names, and bioactivities of 75 marine diterpenoids from octocorals possessing a hydroperoxy group, of which 41 compounds exhibited potential biomedical activities. Lakshmi et al. [17] summarized the chemical constituents of ≈40 S. sp. covering a period of 1975-2004. Rodrigues et al. [18] reviewed the new isolated cembranoid derivatives from species of genera Sarcophyton, Sinularia, and Lobophytum as well as their biological properties during 2016-2018. Especially, the current review focuses on the chemical structures and the biological activities of secondary metabolites obtained from soft coral genus S. since 2013; the Scifinder and Web of Science databases were used for research (Table 1).

Chemistry and Bioactivity of Secondary Metabolites from Genus Sinularia
Genus Sinularia is one of the most widely distributed soft coral genera over the oceans. Many of them were chemically investigated. Especially Sinularia collected from the South China Sea, Taiwan, Vietnam, and Malaysia have provided diverse structures in recent years [4,36,38,59]. On the investigations of the chemical constituents from marine soft coral genus Sinularia, about 100 research papers have been published, and about 249 new compounds have been isolated and identified from more than 30 species in recent years, such as S. brassica, S. depressa, S. erecta, S. flexibilis, S. humilis, S. kavarattiensis, S. leptoclados, S. nanolobata, S. polydactyla, S. scabra, and other unidentified S. sp. (Table 1). Terpenoids and steroids/steroidal glycosides are the prominent metabolites in S. sp. Terpenoids from the genus Sinularia could be classified into sesquiterpenoids, diterpenes, norsesquiterpenes, norditerpenes, etc. (Scheme 1). Steroids/steroidal glycosides are mainly composed of normal steroids and seco-steroids. Marine-derived terpenoids have attracted the interest of natural product chemists around the world, leading to the discovery of structurally diverse metabolites, featuring many novel scaffolds equipped with a diverse array of functionality. Especially, some microbial origin terpenoids from sponges and tunicates were characterized by rare and unprecedented structures and biological activities [89,90].
Mar. Drugs 2021, 19, x FOR PEER REVIEW and tunicates were characterized by rare and unprecedented structures and biolo tivities [89,90]. ring structures are formed from one or more cyclizations of FPP by FPP cyclase. Various carbocation intermediates formed after cyclization may need further cyclization, hydrogen anion migration, methyl transfer, or Wagner-Meerwein rearrangement to remove protons or absorb protons to form the final products. In recent years, about 35 new sesquiterpenes ( Figure 2) have been isolated from the genus Sinularia. According to the carbon skeletons, it can be classified into 17 types, including four new carbon skeletons. (Figure 1). Various activities about these new sesquiterpenes, such as cytotoxicity [78], anti-inflammatory [76], antimicrobial [51], and antimalarial [10], were reported ( Table 2).  Four new sesquiterpenes, sinularianins C-F (1-4), were separated from a South China Sea soft coral S. sp. [78]. 1 and 2 were valerenane-type sesquiterpenes, and 3 and 4 were farnesane-type sesquiterpenes. Although compounds 3 and 4 displayed a quite different skeleton from that of 1 and 2, they are related to each other. From a biosynthetic aspect, sinularinins C-D (1-2), and F (4) could be generated from sinularinin E (3) via different reaction cascades [78], including dehydration, Diels Alder cyclization, Michael addition, epoxidation and dehydration, and aldol condensation.
Chemical examination of the soft coral S. capillosa from the Sanya Bay resulted in the isolation of eight new sesquiterpenoids named capillosananes S-Z (5-12) [76]. The CD effects and Mosher method were determined for the assignment of their absolute configurations. Capillosananes S-U (5-7) presented as the novel carbon skeletons with bicyclo[3 .6.0] and bicyclo[4.5.0] systems, while capillosanane V (8) was characteristic of an unprecedented tricyclic skeleton [76]. Capillosananes W-X (9, 10) were assigned to the unusual  Four new sesquiterpenes, sinularianins C-F (1-4), were separated from a South China Sea soft coral S. sp. [78]. 1 and 2 were valerenane-type sesquiterpenes, and 3 and 4 were farnesane-type sesquiterpenes. Although compounds 3 and 4 displayed a quite different skeleton from that of 1 and 2, they are related to each other. From a biosynthetic aspect, sinularinins C-D (1-2), and F (4) could be generated from sinularinin E (3) via different reaction cascades [78], including dehydration, Diels Alder cyclization, Michael addition, epoxidation and dehydration, and aldol condensation.
Chemical examination of the soft coral S. capillosa from the Sanya Bay resulted in the isolation of eight new sesquiterpenoids named capillosananes S-Z (5-12) [76]. The CD effects and Mosher method were determined for the assignment of their absolute configurations. Capillosananes S-U (5-7) presented as the novel carbon skeletons with bicyclo[3.6.0] and bicyclo [4.5.0] systems, while capillosanane V (8) was characteristic of an unprecedented tricyclic skeleton [76]. Capillosananes W-X (9, 10) were assigned to the unusual dumortane-type sesquiterpenes, and capillosananes Y-Z (11, 12) were eudesmane-type sesquiterpenes. It was the first report of dumortane analogues from marine organisms [76].
A new guaiane sesquiterpene (13) was isolated from the soft coral S. kavarattiensis, which was collected from the Mandapam coast of the Indian Ocean [70]. The relative stereochemistry of the methyl group and epoxy groups in compound 13 was confirmed in search of the most stable conformer of sesquiterpenoid by molecular mechanics (UFF) and density functional theory calculations, in addition to NOESY correlations [70].
An unprecedented highly fused benzosesquiterpenoid verrubenzospirolactone (19) and a new asteriscane-type sesquiterpenoid 10-deoxocapillosanane D (20) were isolated from a South China Sea collection of the soft coral S. verruca [49]. Compound 19 was a novel skeleton that could be derived from capillobenzopyranol via a series of oxidation and cyclization reactions [49].
A new cadinane-type sesquiterpene and a new oplopane-type sesquiterpene, nanolobatols A and B (21 and 22), were isolated from the Vietnamese soft coral S. nanolobata [44].
Three new sesquiterpenoids, sinuketal (28) and sinulins A and B (29 and 30), were isolated from the Xisha soft coral S. sp [10]. Compound 28 was the first example of marine-originated isopropyl branche undecane sesquiterpenoid, which possessed an unprecedented a bicyclo[6.3.0] undecane carbon skeleton with unique endoperoxide moiety, and compound 30 was a eudesmane sesquiterpenoid [10]. The relative configuration of 28 was established by a nuclear Overhauser effect spectroscopy (NOESY) experiment in combination with conformational analysis and computational approaches such as the density functional theory-NMR (DFT-NMR) method. The endoperoxide group should be on the same side of the cyclooctane ring, owing to unfavorable distortions on the basis of molecular model analysis [10].
A novel aza-spirocyclic valerenane sesquiterpenoid sinulaspirolactam A (31) was isolated from the soft coral S. sp. [31]. It was the first example of valerenane sesquiterpenoid bearing an aza-spiro [4.5] ring moiety [31]. The biogenetic pathway of 31 could be plausibly proposed. It was originated from the successive condensation of IPP and DMAPP. Valerendiene synthase (VoVDS) and then catalyzed by the cyclization of FPP into an intermediate, which underwent a series of subsequent conversions, including oxidation, double bond migration, addition, amination, and dehydration to generate 31 [31]. The cytotoxic activities were evaluated against SW480, MCF-7, HepG2, HeLa, and PANC-1 cell lines. However, 31 showed no obvious activity [31].
The bioactivities of new sesquiterpenes discovered from S. sp. were summarized in Table 2. It is revealed that sesquiterpene showed no significant cytotoxicity, but some of them have promising anti-inflammatory activity. These sesquiterpenes with the hydroperoxyl group showed attractive structures and a diversity of biological activities, such as anti-inflammatory activities (compounds 16-18 and 32) and antimalarial activities compound 28). As for compound 15, no activity screening was performed.

Diterpenes
Diterpenes are synthesized by four isoprene units, and their basic structure starts with geranylgeranyl-PP (GGPP, C20). GGPP turns into a highly active carbocation intermediate catalyzed by diterpene synthase, which induced a series of cyclization reactions to form diterpenoid skeletons. The diterpenoid intermediate modified by secondary enzymatic reactions, such as hydroxylation, peroxidation, methylation, acylation, or lytic rearrangement, to produce a final product with more structural diversity and biological activity. About 109 diterpenes with different structures belonging to different chemical classes were reported from the Sinularia species in the past 8 years ( Figure 3). In accordance with sesquiterpenes in this genus, the structures of diterpenoids are also variable, which could be categorized as many types [1], such as cembrane-type, casbane-type, lobane-type, and other types, including some new carbon skeletons. Among them, the cembrane-type diterpenoid has the most diverse structural variations with a multitude of functional groups (lactone, epoxide, furan, ester, aldehyde, hydroxyl, carboxyl moieties) and cyclizations [18], which could be classified into many subtypes, such as isopropyl cembranes, isopropenyl cembranes, cembranolides (5/6/7-membered lactone), furanocembranoids, and biscembranoids. Diterpenoids have been reported to display a variety of biological activities, including anti-inflammatory activity [85], cytotoxicity [72], antifouling activity against bryozoan and barnacle [14], antimicrobial activity [53], PTP1B inhibitory activity [34], anti-Aβ aggregation activity [32], α-glucosidase inhibitory activity [88], and so on.
A new cembranoidal diterpene flexibilin D (36) with a 6-membered lactone ring was isolated from the Taiwan soft coral S. flexibilis [85]. It was found to significantly inhibit the accumulation of the pro-inflammatory iNOS and COX-2 proteins of the lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage cells [85].
Twelve new cembranoids sinulariols T−Z 4 (37-47) and a new capnosane diterpene sinulariol Z 5 (48) were separated from S. rigida [14], which was collected in Sanya Bay, Hainan Island of China. Compound 43 displayed dose-dependency and showed significant inhibition against the larval settlement of the barnacle Ba. amphitrite and moderate inhibition against Bu. neritina [14]. In addition, compound 43 showed a high therapeutic ratio for the inhibition of Ba. amphitrite [14].
The casbanes are a small group of diterpenoids strictly related to the cembrane skeleton and characterized by the presence of a dimethyl-cyclopropyl moiety fused to the 14membered ring [77]. Six new casbane diterpenoids, sinularcasbanes A-F (50-55) were isolated from a South China Sea S. sp. soft coral [77]. These compounds were tested for cytotoxicity against a panel of tumor cell lines, but they showed no cytotoxic activity [77]. Compounds 51 and 54 showed moderate inhibition against lipopolysaccharide (LPS)induced nitric oxide (NO) production [77].
Two novel diterpenoids, sinularbols A (56) and B (57) with a novel sinularboranetype carbon skeleton (3,9-cyclized cembranoid), were isolated from the Taiwan soft coral S. arborea [75]. The relative configuration of 56 was elucidated from the NOESY experiment and was found to be compatible with the computer modeling results. Compound 57 displayed a moderate inhibitory effect on the generation of superoxide anion [75].
One novel nine-membered macrocyclic polysulfur cembranoid lactone, sinulariaoid A (59); and three new multioxygenated cembranoids, sinulariaoid B (60), sinulariaoid C (61), and sinulariaoid D (62), were isolated from the soft coral S. sp. [72], which was collected off of Sanya Bay in the South China Sea. The absolute stereochemical structure of sinulariaoid A (59) was elucidated by single crystal X-ray diffraction analysis, and it was the first reported nine-membered macrocyclic polysulfur cembranoid from soft coral [72]. The cembranoid carbon skeleton is a logical biosynthetic precursor, and it yields many types of cembrane diterpenoid lactones via lactonization, hydrogenation, ring-opening reaction, methyl esterification, oxidation, acid-catalyzed hydrolysis, and so on [72]. The GSH was speculated to be the sulfur donor, and a CYP450 monooxygenase and glutathione S-transferase (GST) play a key role in gliotoxin C-S bond formation [72]. 59 was shown to be cytotoxic toward some cancer cell lines. The bioactivities of new sesquiterpenes discovered from S. sp. were summarized in Table 2. It is revealed that sesquiterpene showed no significant cytotoxicity, but some of them have promising anti-inflammatory activity. These sesquiterpenes with the hydroperoxyl group showed attractive structures and a diversity of biological activities, such as anti-inflammatory activities (compounds 16-18 and 32) and antimalarial activities compound 28). As for compound 15, no activity screening was performed.

Diterpenes
Diterpenes are synthesized by four isoprene units, and their basic structure starts with geranylgeranyl-PP (GGPP, C20). GGPP turns into a highly active carbocation intermediate catalyzed by diterpene synthase, which induced a series of cyclization reactions to form diterpenoid skeletons. The diterpenoid intermediate modified by secondary enzymatic reactions, such as hydroxylation, peroxidation, methylation, acylation, or lytic rearrangement, to produce a final product with more structural diversity and biological activity. About 109 diterpenes with different structures belonging to different chemical classes were reported from the Sinularia species in the past 8 years (Figure 3). In accordance with sesquiterpenes in this genus, the structures of diterpenoids are also variable, which could be categorized as many types [1], such as cembrane-type, casbane-type, lobane-type, and other types, including some new carbon skeletons. Among them, the cembrane-type diterpenoid has the most diverse structural variations with a multitude of functional groups (lactone, epoxide, furan, ester, aldehyde, hydroxyl, carboxyl moieties) and cyclizations [18], which could be classified into many subtypes, such as isopropyl cembranes, isopropenyl cembranes, cembranolides (5/6/7-membered lactone), furanocembranoids, and biscembranoids. Diterpenoids have been reported to display a variety of biological activities, including anti-inflammatory activity [85], cytotoxicity [72], antifouling activity against bryozoan and barnacle [14], antimicrobial activity [53], PTP1B inhibitory activity [34], anti-Aβ aggregation activity [32], α-glucosidase inhibitory activity [88], and so on.    A new cembrane-type diterpenoid, arbolide C (63), was obtained from the Taiwan soft coral [69], and it was found to display in vitro anti-inflammatory activities with an inhibitory effect on the release of elastase by human neutrophils [69].
New cembranoids 4-carbomethoxyl-10-epigyrosanoldie E (74) 7-acetylsinumaximol B (75) diepoxycembrene B (76), dihydromanaarenolide I (77), and isosinulaflexiolide K (78) were isolated from cultured soft corals S. sandensis and S. flexibilis [63]. The absolute configurations of 74 and 78 were further confirmed by single-crystal X-ray diffraction analysis. It should be noticed that the hypothesis (that cembrane diterpenes possessing an absolute configuration of an isopropyl group at C1 obtained from Alcyonacean soft corals belong to the α series, whereas analogues isolated from Gorgonacean corals belong to the β series) is not applicable for a small number of cembranoids. Compound 78 was found to significantly reduce the levels of iNOS and COX-2 [63]. It was proposed that the presence of one seven-membered lactone functional group was critical for antiinflammatory activity [63].
Two new cubitane-type diterpenoids, nanoculones A and B (85 and 86) and three new cembranoids, nanolobols A-C (87-89), were isolated from Taiwan soft coral S. nanolobata [59]. Compound 86 could effectively reduce the levels of LPS-stimulated NO production in activated RAW264.7 cells. These compounds were not cytotoxic against P388, K562, and HT-29 cell lines [59]. Considering their prior work on the same species collected in different locations of Taiwan water, it is noteworthy that the metabolites from soft corals, even the skeletons, might vary with their geographical location [59].
Four new diterpenoids, pambanolides A-C (90-93), were isolated from the soft coral S. inelegans [58], which was collected in the Gulf of Mannar on the southern coast of India. A biosynthetic pathway has been proposed for the formation of pambanolides A (90). It could be obtained from the precursor metabolite mandapamate, via oxa-Michael addition and intramolecular rearrangement [58]. Compounds 91 and 92 showed weak activity against the DU145 and A549 cancer cell lines [58].
A solvent extract of S. polydactyla resulted in the isolation of two new casbane diterpenes: sinularcasbane M (94) and sinularcasbane N (95). Compounds were elucidated based on spectroscopic analyses; the absolute configuration was confirmed by X-ray analysis [57].
A cembranoid sinulerectol C (96) with a hydroperoxy group was separated from soft coral S. erecta [3], which was collected off the coast of Dongsha Atoll. Compound 96 showed cytotoxicity toward the K-562 cell line. Moreover, it exhibited potent inhibitory activity against elastase release [3].
Three new diterpenes with a new carbon skeleton, xishacorenes A−C (99-101), featuring an undescribed bicyclo[3.3.1]nonane nucleus bearing 1-vinyl and 13-[(E)-4-methylpenta-1,3-dien-1-yl] alkyl chains were isolated from the Xisha soft coral S. polydactyla [41]. The new skeleton was structurally related with lobane-type diterpenes by sharing some common moieties, such as the cyclohexane ring bearing a vinyl group and a conjugated diene side chain [41]. The research team proposed that an electron delivery from the ∆ 10/12 olefinic head to the activated hydroxyl tail accomplished the cyclization of a six-member ring [41]. These three compounds were found to promote the ConA-induced T lymphocytes proliferation with dose-dependency and had no activity on LPS induced B lymphocytes proliferation.
A new cembranoid sinularolide F (104) was isolated from the Bornean soft coral S. sp. [36]. It showed potential anti-inflammatory activities against lipopolysaccharidestimulated RAW 264.7 through inhibiting NO synthesis by reducing the expression of iNOS protein. In addition, compound 104 exhibited apoptosis activity against HL-60 cells. The expressions of apoptotic proteins suggested that 104 triggered the up-regulation of Bax, the downregulation of Bcl-xL, and the activation of caspase-3 in the apoptosis mechanism [36].
Five new cembranoid-related diterpenoids, flexibilisins D and E (106 and 107), secoflexibilisolides A and B (108 and 109), and flexibilisolide H (110) were obtained from Taiwan soft coral S. flexibilis [35]. Compound 108 possessed an unusual skeleton that could be biogenetically derived from cembranoid flexibilisolide D, which is a known compound that was also isolated from this coral. After oxidative cleavage, Michael addition, condensation, rearrangement, and reduction, flexibilisolide D could be converted to secoflexibilisolide A. Compound 109 could be derived from sinuflexolide via oxidative cleavage of the diol groups, epimerization of the vinyl alcohol, and subsequent esterification. These compounds were nontoxic toward cancer cell lines P-388, K-562, and HT-29 [35].
Three novel cembranoid esters xidaosinularide A-C (118-120) featuring an n-butyl alcohol moiety were obtained from Hainan soft coral S. flexibilis [27]. Compound 118 reduced the levels of TNF-α in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells [27]. A and B (124 and 125), and 8-epi-xiguscabrol B (126) were isolated from the South China Sea soft coral S. scabra [26]. Compounds 122 and 124-126 exhibited strong inhibitory activity on the proliferation of Con A-induced T lymphocyte cells [26]. It was suggested that the position and the number of hydroxyls might influence the toxicity of cembranoids, while it could not affect the immunological activity [26].
A novel diterpenoid, sinueretone A (128), featuring an unprecedented tricyclo pentadecane carbon framework, and two new casbane-type diterpenoids sinuereperoxide A (129) and 10-oxo-3,4,11,12-tetrahydrodepressin (130) were isolated from the South China Sea soft coral S. erecta [7]. Analysis of the X-ray data unambiguously confirmed the planar structure of 129 and the determination of its absolute configuration. Compounds 128 and 129 displayed anti-inflammatory activity of lipopolysaccharide (LPS)-induced tumor necrosis factor (TNF)-α protein release in RAW264.7 macrophages [7]. Compound 129 showed anti-inflammatory activity, indicating that the peroxide bridge might be helpful for the activity. Both of them exhibited no obvious cytotoxicity against RAW264.7 cells [7].
As shown in Table 3, diterpenoids from S. sp. were found to be of promising biofunctional diversities. The biological properties reported in several works were attributed to the variability of chemical structures. It could be concluded that nearly all the bioactive diterpenoids possess oxygenic rings or hydroxy groups, and the position of the oxygenic ring, as well as the position and number of hydroxyls, might influence the activities. For example, the presence of one seven-membered lactone functional group at position C1 is critical for the anti-inflammatory action of this class of compounds [63]. In particular, the multisulfide moiety of sinulariaoid A (59) played an important role in the cytotoxic bioactivities. The diterpenoids with a hydroperoxyl group also showed significant antiinflammatory activity or cytotoxicity.

Norterpenoid
The mechanisms leading to the occurrence of norterpenoids are not well understood, but they may include the production of anionic and radical intermediates along with competitive transannular carbon-to-carbon bond-forming reactions [18]. Norterpenoids could be commonly considered as one of the chemotaxonomic markers of soft corals of genus Sinularia. To date, several novel norsesquiterpenoids and norditerpenoids have been isolated and structurally elucidated from Sinularia species. The structures of norsesquiterpenoids could be variable as sesquiterpenoids, but there were not many reported. Up until now, about 10 norsesquiterpenoids were obtained from S. sp.; to the best of our knowledge, three of them were found since 2013 (Figure 4). Two were uncommon norhumulene-type norsesquiterpenoids sinuhirtins A (145) and B (146), which were isolated from the South China Sea soft coral S. hirta [30]. The norhumulene was suggested to be the precursor. Molestins D (147) was the first example of a norsesquiterpene with a de-isopropyl guaiane skeleton isolated from the genus Sinularia [40], and it showed significant inhibitory activities against protein tyrosine phosphatase 1B (PTP1B). now, about 10 norsesquiterpenoids were obtained from S. sp.; to the best of our knowledge, three of them were found since 2013 (Figure 4). Two were uncommon norhumulene-type norsesquiterpenoids sinuhirtins A (145) and B (146), which were isolated from the South China Sea soft coral S. hirta [30]. The norhumulene was suggested to be the precursor. Molestins D (147) was the first example of a norsesquiterpene with a deisopropyl guaiane skeleton isolated from the genus Sinularia [40], and it showed significant inhibitory activities against protein tyrosine phosphatase 1B (PTP1B). The norditerpenes from S. sp. are relatively abundant and novel in structures. Norcembranoids are the most structurally diverse norditerpenes produced by S. sp. Compared with cembranoids, norcembranoids are those that lack a C-18 carbon substituent on C-4 [1] and always have a furane heterocycle. They are a rich source of bioactive substances with intriguing and unique structural features (Figure 4). Other types of norditerpenes such as sinulariane-type, horane-type, normandapamatane-type, and sinulanorcembrane-type were also reported in recent years [13,28,71,85].
One new polycyclic furanobutenolide-derived norcembranoid was isolated from xiguscabrolide H (154), from the South China Sea soft corals S. scabra [24], and it exhibited potent inhibitory activities on the proliferation of Con A-induced T lymphocyte cells and LPS-induced B lymphocyte cells [24].
A seco-yonarane norditerpenoid, 4,5-secosinulochmodin C (158), was isolated from the soft coral S. inelegans [58]. Compound 158 was an unprecedented skeleton with an eightcarbon side chain bearing an isopropenyl substituent and terminated by an a-keto ester functionality, which appears to be a C4-C5 redox bond cleavage product of sinulochmodin C [58].
A new horane-type norditerpenoid, named kavaranolide (159), was isolated from the Indian soft coral S. kavarattiensis [71]. It possessed a tricyclic carbocycle with the trans-fused six and seven-membered rings. The replicon-inhibiting potential of compound 159 was evaluated, but it showed no obvious activity [71].
A new normandapamatane-type norditerpenoid 12-hydroxy-scabrolide A (160) and a new norcembranoid 13-epi-scabrolide C (161) were obtained from the soft coral S. maxima [13]. It was confirmed that 161 had little or no effect on cell viability and potently inhibited IL-12 and IL-6 production in LPS-stimulated bone marrow derived dendritic (BMDCs). Compound 161 did not exhibit inhibitory activity on TNF-α production [13].
A new sinulanorcembrane-type norditerpene, 1-epi-sinulanorcembranolide A (162) was isolated from the soft corals S. gaweli [85]. These in vitro anti-inflammatory effects were evaluated, and it did not attenuate the iNOS and COX-2 expression in LPS-stimulated macrophage cells.
As shown in Table 4, polycyclic furanobutenolide-derived norditerpenoids were reported to comprise a broad range of biological activities, such as cytotoxic, anti-inflammatory, and immunosuppression and antimalarial activities. Similar to cembranoids, oxygenic rings or hydroxy groups might be critical to the bioactivities. Other types of norditerpenoids showed intriguing structural features with novel carbon skeleton, but their biological activities still need to be explored.

Steroids/Steroidal Glycosides
Among the various classes of secondary metabolites produced by soft corals S. sp., steroids come next as a major group of metabolites after terpenoids ( Figure 5). The steroid is a kind of natural product with a perhydrocyclopentanophenanthrene skeleton by removing three methyl groups based on lanosterol triterpenoid skeleton. The variations of side chains, high degrees of oxygenation, and re-arrangement on the rings of the tetracyclic nucleus account for the fascinated diversity of such steroids [74]. Among them, withanolides are a group of C 28 steroidal lactones possessing mostly a C-22/C-26 δ-lactone or in some cases Mar. Drugs 2021, 19, 335 16 of 25 a C-23/C-26 γ-lactone in the side chain [48]. There is also secosteroid produced by S. sp. For example, the 9,11-secosteroids are structurally characterized by the C-9/11 oxidative cleavage of the C-ring [25]. Steroids isolated from soft corals also exhibit a broad range of biological activities, such as cytotoxic, anti-inflammatory, anti-microbial, etc.  A novel polyhydroxylated sterol, 3β-25-dihydroxy-4-methyl-5α,8α-epidioxy-2 ketoergost-9-ene (165), was isolated from the Red Sea soft coral S. candidula Safaga [86]. It showed antiviral activity against the highly pathogenic H5N1 avian influenza strain.
Seven novel withanolides, sinubrasolides A−G (166−172), have been isolated from the cultured soft coral S. brassica [84]. The cytotoxicities of these compounds against the proliferation of cancer cell lines were evaluated, including murine leukemia (P388) cells, lymphoid T carcinoma (MOLT 4) cells, human erythroleukemia (K562) cells, and human colon carcinoma (HT-29) cells. Compound 167 exhibited cytotoxicity toward P388, MOLT 4, and HT-29 cancer cell lines. Compound 170 was found to show cytotoxicity toward MOLT 4 and HT-29 cell lines. In addition, 166 showed cytotoxicity toward the K562 cell line.
A new steroid, dissesterol (175), was separated from a Vietnamese soft coral S. dissecta [82]. It was confirmed that 175 had no or little effect on the cell viability, and it showed strong suppression of LPS-stimulated IL-12 p40 production [82].
Four new steroids with methyl ester groups, sinubrasones A-D (197-200), were obtained from a reef soft coral S. brassica, which was cultured in a tank [12]. showed a significant inhibitory effect (53.6 ± 1.8%) against superoxide anion generation. Compounds 199 and 200 also exhibited inhibitory activities against elastase release [12].
A new ergostane-type steroid sinubrassione (208) and a new pregnene-type steroid glycoside sinubrassioside (209) were isolated from methanol extract of the Vietnamese soft coral S. brassica [43]. Cytotoxic activity was observed for compound 208 against PANC-1. No cytotoxicity was observed for compound 209, suggesting that the location of the sugar moiety may be important for the cytotoxicity [43].
Two new highly oxygenated ergostane-type sterols (212 and 213) were isolated from the soft coral S. sp. collected from the Xisha Islands, South China Sea [9]. Compounds 212 and 213 exhibited moderate anti-proliferation effects against five human cancer cell lines, including MDA-MB-436, A549, Hep3B, HT-29, and H157. Compound 212-treated H157 cells displayed apoptosis characteristics. Moreover, Western blot assays suggested that 212 could increase the expression of Bax and down-regulate the expression of Bcl-2 [9].
A pair of novel highly degraded steroid named Erectsterates A and B (216 and 217) were derived from the South China Sea soft coral S. erecta [2]. These two compounds are rare steroids with a high degradation in ring B and an ester linkage between the A and C/D rings. It was proposed that the ring C of 216 and 217 was formally oxidized by the Baeyer-Villiger reaction to provide an unprecedented seven-membered lactone moiety in ring C of steroid [2]. Compound 217 showed a weak inhibitory effect on the A549, HT29, SNU-398 and Capan-1 cell lines [2].
Six new steroids (218-223) were isolated from the organic extract of the soft coral S. polydactyla collected from the Hurghada reef in the Red Sea [6]. Compounds 218 and 219 displayed the rare 8,9-seco-cholestane steroidal nucleus. Compound 223 displayed increased inhibition of androgen receptors with decreasing concentrations [6].
A new 5α,8α-epidioxysterol yalongsterol A (224) was isolated from the South China Sea soft coral S. sp. [22]. Compound 224 exhibited moderate activities against the concanavalin A (ConA)-induced proliferation of T lymphocyte cells and LPS-induced proliferation of B lymphocyte cells [22].
A new steroid named 16,17-epoxy-23-methylergostane (225) was derived from S. variabilis, which is a soft coral from the Persian Gulf [19]. It showed cytotoxic activity in a dose-dependent reduction manner against MCF-7 and MDA-MB-231. Apoptosis was the underlying mechanism that was enrolled by 225 to induce cell death [19].
The biological activities of new steroids from S. sp. are summarized in Table 5. Many steroids showed interesting cytotoxicity to cancer cell lines, especially withanolides, with a C-22/C-26 δ-lactone or a C-23/C-26 γ-lactone, which showed significant bioactivities. Some of them displayed anti-inflammatory activity. It is worth mentioning that hydroxy or oxygenic rings on the side chain might be a critical point for the activities, in particular, the cytotoxicities.

Other Types
In addition to a rich harvest of terpenoids and steroids/steroidal glycosides, S. sp. also yields other kinds of metabolites, such as cyclopentenone, ceramides, alkaloid, quinones, and lipids ( Figure 6).
A new quinone derivative, flexibilisquinone (246), was isolated from the cultured soft coral S. flexibilis [92]. In the in vitro anti-inflammatory effects test, quinone 246 was found to significantly inhibit the accumulation of the pro-inflammatory iNOS and COX-2 proteins of the LPS-stimulated RAW264.7 macrophage cells. Both iNOS and COX-2 were significantly inhibited by compound 246 at 5-20 µM and 20 µM, respectively. Furthermore, compound 246 (1-20 µM) did not induce obvious cytotoxicity in macrophage cells. Eight new cyclopentenone and butenolide derivatives, sinulolides A-H (234-241), were isolated from the soft coral S. sp. [68]. At a concentration of 10 µg/mL, sinulolide E exhibited moderate effects with an inhibitory rate of 38.12% [68]. A new quinone derivative, flexibilisquinone (246), was isolated from the cultured soft coral S. flexibilis [92]. In the in vitro anti-inflammatory effects test, quinone 246 was found to significantly inhibit the accumulation of the pro-inflammatory iNOS and COX-2 proteins of the LPS-stimulated RAW264.7 macrophage cells. Both iNOS and COX-2 were significantly inhibited by compound 246 at 5-20 µM and 20 µM, respectively. Furthermore, compound 246 (1-20 µM) did not induce obvious cytotoxicity in macrophage cells.

Conclusions
Marine invertebrates have been regarded as a treasurable source of bioactive secondary metabolites for drug development. Coral reefs are among the most productive marine ecosystems, which produced a huge diversity of chemical structures with biological properties. The soft corals belonging to the family Sinularia are not an exception. An increasing number of compounds have been reported since the first report of this genus published in 1975 [93]. With the rapid development of analysis, separation, and structure identification technology, more and more new compounds have been isolated and identified each year from soft coral S. sp. Until now, more than 150 species of this genus have been found, 78 species among which have been chemically studied, and more than 700 metabolites have been reported. In this review, we have reported 249 compounds along with different structures and potential biological activity based on the data collected from the available literature. The chemical diversity of the structures can be attributed to the chemical groups and the arrangement of the core structure. In particular, sulfur or nitrogen-containing metabolites are a special, relatively rare, and important class of natural products from soft coral (such as compounds 59 and 249), which suggested that the symbiotic microorganisms are the real producers. According to the reported literature, soft coral also harbors a microbial community featuring sponges and ascidians [94], including fungus, bacteria, and actinobacteria [95][96][97][98], which also corresponds to the antimicrobial activity of coral-derived natural products. The extensive structure-activity relationship studies on the metabolites from Sinularia species are expected, which provide a direction for the discovery of lead compounds. There are also some compounds with novel structures including some new skeletons that tested negative for the limited activity test models and low yield. Therefore, the structure-based drug design methods [18] (e.g., molecular docking, structure-based virtual screening and molecular dynamics) can be utilized in accordance with activity screening.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The data presented in this study are available.