Cherbonolides M and N from a Formosan Soft Coral Sarcophyton cherbonnieri

Two new isosarcophine derivatives, cherbonolides M (1) and N (2), were further isolated from a Formosan soft coral Sarcophyton cherbonnieri. The planar structure and relative configuration of both compounds were established by the detailed analysis of the IR, MS, and 1D and 2D NMR data. Further, the absolute configuration of both compounds was determined by the comparison of CD spectra with that of isosarcophine (3). Notably, cherbonolide N (2) possesses the unique cembranoidal scaffold of tetrahydrooxepane with the 12,17-ether linkage fusing with a γ-lactone. In addition, the assay for cytotoxicity of both new compounds revealed that they showed to be noncytotoxic toward the proliferation of A549, DLD-1, and HuCCT-1 cell lines. Moreover, the anti-inflammatory activities of both metabolites were carried out by measuring the N-formyl-methionyl-leucyl-phenylalanine/cytochalasin B (fMLF/CB)-induced generation of superoxide anion and elastase release in the primary human neutrophils. Cherbonolide N (2) was found to reduce the generation of superoxide anion (20.6 ± 6.8%) and the elastase release (30.1 ± 3.3%) in the fMLF/CB-induced human neutrophils at a concentration of 30 μM.


Introduction
The chemical constituents for soft corals of the genera Sarcophyton have been well studied. According to the statistics, more than 500 marine natural products have been isolated from this genus during the past two decades. Among these metabolites, over 300 diterpenoids with the 14 membered cembranoidal skeleton were discovered [1]. Therefore, the Sarcophyton genus has been frequently considered to be an important source of the 14 membered ring diterpenoidal skeleton [2][3][4]. These secondary metabolites help the organisms to defend themselves against the predators and adapt to the environment stress [5]. Furthermore, some of the isolates have been demonstrated to possess various biological activities, such as cytotoxic [6][7][8][9][10][11][12], anti-inflammatory [6,7,[13][14][15][16][17][18], neuroprotective [19], antibacterial [19], and antiviral activities [12,20]. The diversified structures and various biological applications of marine natural products thus prompt us to further explore the secondary metabolites from organisms of the Sarcophyton genus. and various biological applications of marine natural products thus prompt us to furthe explore the secondary metabolites from organisms of the Sarcophyton genus.
Sarcophine, the dihydrofuran-containing cembranoidal diterpene, is one of the majo metabolites in the soft corals of Sarcophyton genera [21]. Bernstein et al. discovered thi metabolite from the soft coral Sarcophyton glaucum in 1974 [22]. Subsequently, the absolute configuration of sarcophine was determined by Kashman et al. in 1977 [23]. Furthermore Frincke et al. indicated that sarcophine was converted from the other 14 membered diterpene, sarcophytoxide, by auto-oxidation in 1980 [24]. Isosarcophine, an isomer o sarcophine, was proven to be converted from isosarcophytoxide via auto-oxidation and isolated from the soft coral Sinularia mayi by Kusumi et al. in 1990 [25]. It was found tha isosarcophine showed significant cytotoxicities toward some cancer cell lines [25,26].
Our previous investigation of soft coral Sarcophyton cherbonnieri had contributed to the isolation of 13 new cembranoids derived from the isosarcophine [13,14]. In the presen study, the continuous chemical investigation of S. cherbonnieri resulted in the discovery o two new isosarcophine derivatives, cherbonolides M (1) and N (2) as shown in Figure 1 The planar structures and relative configurations of both compounds were elucidated by analyzing the infrared (IR), MS, and 1D and 2D NMR data. Furthermore, to determine the absolute configurations, the circular dichroism (CD) spectra of both new compounds were measured and compared with those of isosarcophine (3). Moreover, in order to discove bioactive natural products for the development of drug leads, the cytotoxicity against hu man lung adenocarcinoma (A549), human colorectal adenocarcinoma (DLD-1), and hu man intrahepatic cholangiocarcinoma (HuCCT-1) was examined. In addition, the anti-in flammatory activity of both isolates was investigated by measuring the N-formyl-methio nyl-leucyl-phenylalanine/cytochalasin B (fMLF/CB)-induced generation o superoxide anion and elastase release in the primary human neutrophils.

Results and Discussion
The chemical structures of metabolites 1 and 2 were elucidated by analyzing the MS IR, CD, and 1D and 2D NMR data (Supplementary Materials S1-S32). Additionally, the 13 C and 1 H chemical shifts of 1 and 2 are listed in Table 1

Results and Discussion
The chemical structures of metabolites 1 and 2 were elucidated by analyzing the MS, IR, CD, and 1D and 2D NMR data (Supplementary Materials Figures S1-S32). Additionally, the 13 C and 1 H chemical shifts of 1 and 2 are listed in Table 1.
Compound 1, cherbonolide M, was isolated as a colorless oil. The molecular formula of 1 was C 20 H 28 O 4 deduced from the pseudomolecular ion peak at m/z 355.1882 [M + Na] + (calculated 355.1880, C 20 H 28 O 4 Na) in the high-resolution electrospray ionization mass spectrometry (HR-ESI-MS). Its IR spectrum showed the absorptions at 3445 and 1749 cm -1 , indicating the presence of hydroxy and ester groups. The 13 C and distortionless enhancement by polarization transfer (DEPT) spectra displayed 20 carbon signals, including four methyls, five methylenes, five methines, and six quaternary carbons. 1 H and 13 C NMR spectra showed the signals of an α-methyl-α, β-unsaturated-γ-lactone  The planar structure of 1 was established according to the analysis of 2D NMR spectra as shown in Figure 2. The correlation spectroscopy (COSY) spectrum showed four partial moieties from the correlations of H-2 to H-3, H-5 via H 2 -6 to H-7, H 2 -9 via H 2 -10 to H-11, and H 2 -13 to H 2 -14. These partial structures were assembled by heteronuclear multiple bond correlation (HMBC) correlations from H 3 -17 to C-1, C-15, C-16; H 3 -18 to C-3, C-4, C-5; H 3 -19 to C-7, C-8, C-9; H 3 -20 to C-11, C-12, C-13; and H 2 -14 to C-1 and C-2. Furthermore, the NMR spectroscopic data of 1 were compared with those of the previous metabolite cherbonolide H [13] for structural elucidation. It was shown that compound 1 should possess a hydroxy group at C-5 and a 7,8-trisubstituted double bond from this NMR data comparison. According to the above evidence, the gross structure of 1 was elucidated.
The planar structure of 1 was established according to the analysis of 2D NMR spec tra as shown in Figure 2. The correlation spectroscopy (COSY) spectrum showed four par tial moieties from the correlations of H-2 to H-3, H-5 via H 2 -6 to H-7, H 2 -9 via H 2 -10 to H 11, and H 2 -13 to H 2 -14. These partial structures were assembled by heteronuclear multipl bond correlation (HMBC) correlations from H 3 -17 to C-1, C-15, C-16; H 3 -18 to C-3, C-4, C 5; H 3 -19 to C-7, C-8, C-9; H 3 -20 to C-11, C-12, C-13; and H 2 -14 to C-1 and C-2. Furthermore the NMR spectroscopic data of 1 were compared with those of the previous metabolit cherbonolide H [13] for structural elucidation. It was shown that compound 1 should pos sess a hydroxy group at C-5 and a 7,8-trisubstituted double bond from this NMR dat comparison. According to the above evidence, the gross structure of 1 was elucidated.  Position  The relative configuration of 1 was established by analyzing the nuclear Overhauser effect spectroscopy (NOESY) spectrum, as shown in  Figure 3. In order to compare the NMR data with those of 2 and other isosarcophine-derived metabolites measured in C 6 D 6 of our previous studies [13,14], the 1 H and 13 C NMR spectra of 1 were also measured, and the results (Table 1) can further confirm the structure of 1 was elucidated. The absolute configuration of 1 was determined by comparison the CD spectrum of 1 with that of isosarcophine (3), as shown in Figure 4. The CD spectrum of 1 showed the positive Cotton effect at 227.0 nm (∆ε = +34.8) for the π-π* transition, while the negative Cotton effect at 245.5 nm (∆ε = −15.6) for the n-π* transition. This evidence demonstrated the fact of 2S-configuration [27,28]. Due to the biogenesis of 1 from isosarcophine, the absolute configuration of 1 was defined as 2S,5R,11R,12R,3E,7E. field shifted methyl groups of C-18 (δ C 10.3) and C-19 (δ C 14.9). The 3 J va and H-3 (10.4 Hz), H-5 and H-6β (10.8 Hz), and H-6β and H-7 (10.8 Hz) consistent with the relative configuration shown in Figure 3. In order to c data with those of 2 and other isosarcophine-derived metabolites measu previous studies [13,14], the 1 H and 13 C NMR spectra of 1 were also m results (Table 1) can further confirm the structure of 1 was elucidated. T figuration of 1 was determined by comparison the CD spectrum of 1 w cophine (3), as shown in Figure 4. The CD spectrum of 1 showed the pos at 227.0 nm (Δε = +34.8) for the π-π* transition, while the negative Cot nm (Δε = −15.6) for the n-π* transition. This evidence demonstrated the uration [27,28]. Due to the biogenesis of 1 from isosarcophine, the absol of 1 was defined as 2S,5R,11R,12R,3E,7E.   and H 3 -20. Further, the E geometry of C-3/C-4 and C-7/C-8 was determine field shifted methyl groups of C-18 (δ C 10.3) and C-19 (δ C 14.9). The 3 J value and H-3 (10.4 Hz), H-5 and H-6β (10.8 Hz), and H-6β and H-7 (10.8 Hz) we consistent with the relative configuration shown in Figure 3. In order to com data with those of 2 and other isosarcophine-derived metabolites measured previous studies [13,14], the 1 H and 13 C NMR spectra of 1 were also mea results (Table 1) can further confirm the structure of 1 was elucidated. The figuration of 1 was determined by comparison the CD spectrum of 1 with cophine (3), as shown in Figure 4. The CD spectrum of 1 showed the positiv at 227.0 nm (Δε = +34.8) for the π-π* transition, while the negative Cotton nm (Δε = −15.6) for the n-π* transition. This evidence demonstrated the fa uration [27,28]. Due to the biogenesis of 1 from isosarcophine, the absolute of 1 was defined as 2S,5R,11R,12R,3E,7E.  showed the absorption peaks at 3481 and 1741 cm −1 , suggesting the prese droxy group and ester group. In the 13 C spectrum, 2 showed 20 carbon sig to three methyls, seven methylenes, four methines, and six quaternary ca DEPT spectrum assistance. 1 H and 13 C NMR spectra revealed the signals group (δC 172.3, C), a tetrasubstituted double bond (δC 166. The planar structure of 2 was also elucidated by the 2D NMR spec spectrum showed that compound 2 possesses four partial structures from H 5 via H2-6 to H-7, H2-9 via H2-10 to H-11, and H2-13 to H2-14. All of the pa were linked by HMBC correlations from H2-17 to C-1, C-15, C-16; H3-18 to H3-19 to C-7, C-8, C-9; H3-20 to C-11, C-12, C-13; and H2-13 and H2-14 to C-HMBC correlation from both H-2 (  (10.4 Hz) were also consistent with the re ration as shown in Figure 5. Based on the above evidence, the relative con was defined as 2S*,11R*,12S*,3E,7E. In the present study, we isolated two cembranoids, cherbonolide M an from the soft coral S. cherbonnieri. Structurally, both isolates belong to the In the present study, we isolated two cembranoids, cherbonolide M and N (1 and 2), from the soft coral S. cherbonnieri. Structurally, both isolates belong to the isosarcophine derivatives. By consideration of the chemical types of reported cembranoids, this is the first time to discover the cembranoidal scaffold possessing the unique tetrahydrooxepane with 12,17-ether linkage fusing with a γ-lactone. The plausible biosynthetic pathway was postulated, as shown in Scheme 1. Moreover, the CD spectrum of 2 ( Figure 6) also revealed the positive effect at 225.0 nm (∆ε = +25) and the negative effect at 255.4 nm (∆ε = −2.0) as that of isosarcophine (3). Similar to compound 1, metabolite 2 should also be biotransformed from isosarcophine and thus possessesed a (2S,11R,12S,3E,7E) absolute configuration. with 12,17-ether linkage fusing with a γ-lactone. The plausible biosynthetic pathway was postulated, as shown in Scheme 1. Moreover, the CD spectrum of 2 ( Figure 6) also revealed the positive effect at 225.0 nm (Δε = +25) and the negative effect at 255.4 nm (Δε = −2.0) as that of isosarcophine (3). Similar to compound 1, metabolite 2 should also be biotransformed from isosarcophine and thus possessesed a (2S,11R,12S,3E,7E) absolute configuration. In 1990, Kusumi et al. afforded isosarcophine from the Okinawan soft coral, Sinularia mayi, and demonstrated its moderate cytotoxicity against human colorectal carcinoma (HCT-116) cell line with the IC 50 value of 64 μg/mL [25]. Subsequently, in 1992, Wu et al. also isolated the same metabolite from a Formosan soft coral, Sarcophyton trocheliophorum, which was also found to exhibit significant cytotoxicities toward human lung epithelial cells (A549), human colon carcinoma cells (HT-29), human oral epidermoid carcinoma cells (KB), mouse lymphoid neoplasm cells (P388), and promyelocytic leukemia cells (HL-60) with the ED 50 values of 13.3, 16.9, 24.5, 0.7, and 6.7 μg/mL, respectively [26]. Many isosarcophine derivatives have been discovered from different marine soft corals and reported to display various biological activities [13,14,27].
For the discovery of bioactivities of new metabolites 1 and 2, both compounds were examined according to the cytotoxic and anti-inflammatory activities. In the evaluation of cytotoxicity, both isolates were shown to be inactive against the proliferation of A549, DLD-1, and HuCCT-1 cell lines at the concentration of 30 μM. On the other hand, in antiinflammatory assays at 30 μM, metabolite 2 showed inhibition of superoxide anion generation (20.6 ± 6.8%) and the elastase release (30.1 ± 3.3%), while 1 only displayed 12.9 ± 5.7% and 16.7 ± 5.9% inhibition of superoxide anion generation and elastase release, respectively, in the fMLF/CB-induced human neutrophils.   In 1990, Kusumi et al. afforded isosarcophine from the Okinawan soft coral, Sinularia mayi, and demonstrated its moderate cytotoxicity against human colorectal carcinoma (HCT-116) cell line with the IC 50 value of 64 µg/mL [25]. Subsequently, in 1992, Wu et al. also isolated the same metabolite from a Formosan soft coral, Sarcophyton trocheliophorum, which was also found to exhibit significant cytotoxicities toward human lung epithelial cells (A549), human colon carcinoma cells (HT-29), human oral epidermoid carcinoma cells (KB), mouse lymphoid neoplasm cells (P388), and promyelocytic leukemia cells (HL-60) with the ED 50 values of 13.3, 16.9, 24.5, 0.7, and 6.7 µg/mL, respectively [26]. Many isosarcophine derivatives have been discovered from different marine soft corals and reported to display various biological activities [13,14,27].
For the discovery of bioactivities of new metabolites 1 and 2, both compounds were examined according to the cytotoxic and anti-inflammatory activities. In the evaluation of cytotoxicity, both isolates were shown to be inactive against the proliferation of A549, DLD-1, and HuCCT-1 cell lines at the concentration of 30 µM. On the other hand, in anti-inflammatory assays at 30 µM, metabolite 2 showed inhibition of superoxide anion generation (20.6 ± 6.8%) and the elastase release (30.1 ± 3.3%), while 1 only displayed 12.9 ± 5.7% and 16.7 ± 5.9% inhibition of superoxide anion generation and elastase release, respectively, in the fMLF/CB-induced human neutrophils.

General Experimental Procedures
Specific optical rotations of both compounds 1 and 2 were measured on a JASCO P-1020 polarimeter (JASCO Corporation, Tokyo, Japan), and their IR spectra were recorded on an FT/IR-4100 infrared spectrophotometer (JASCO Corporation, Tokyo, Japan). The low-resolution electrospray ionization mass spectrometry (LR-ESI-MS) and HR-ESI-MS experiments were carried out on a Bruker APEX II (Bruker, Bremen, Germany) mass spectrometer. The CD spectra of 1-3 were recorded by Jasco J-815 spectropolarimeter (JASCO, Tokyo, Japan) in MeOH. NMR spectra of 1 were acquired on a Varian Unity Inova 500 FT-NMR (Varian Inc., Palo Alto, CA, USA) at 500 MHz for 1 H and 125 MHz for 13 C in C 6 D 6 at room temperature (25 • C). Further, NMR spectra of 1 and 2 were acquired on a Varian 400 MR FT-NMR (Varian Inc., Palo Alto, CA, USA) instrument at 400 MHz for 1 H and 100 MHz for 13 C in acetone-d 6 and C 6 D 6 at the same condition. Low-resolution electrospray ionization mass spectrometry (LRESIMS) and HRESIMS data were recorded on a Bruker APEX II (Bruker, Bremen, Germany) mass spectrometer. Normal-phase column chromatography was undertaken using silica gel (Merck, 230-400 mesh). Pre-coated silica gel plates (Merck, Kieselgel 60 F-254, 0.2 mm, Merck, Darmstadt, Germany) were used for analytical thin-layer chromatography (TLC). High-performance liquid chromatography was carried out for further purification of isolates, using a Hitachi L-2455 HPLC apparatus (Hitachi, Tokyo, Japan) with a Supelco C18 column (250 × 21.2 mm, 5 µm; Supelco, Bellefonte, PA, USA).

Animal Material
The collection and identification of the soft coral S. cherbonnieri were carried out as described previously [13,14].

Extraction and Isolation
The soft coral S. cherbonnieri (wet weight: 1.2 kg) was freeze-dried to remove the water. The dried sample was sliced into small pieces for EtOAc extraction. The combined EtOAc extract was concentrated under reduced pressure to afford an oily residue (10.2 g), which was chromatographed by a normal phase column with the gradient elution of acetone in n-hexane (0-100%) and methanol in acetone (0-100%) to separate 19 fractions. Fraction 13, eluting with n-hexane-acetone (1:1), was further purified over Sephadex LH-20 column using acetone to afford five subfractions (A-E). Subfraction 13-D was further purified by reversed-phase HPLC with the elution of acetonitrile-H 2 O (1.6:1, 5.0 mL/min) to afford 1 (7.1 mg, t R 61.9 min). Subfraction 13-E was also purified by reversed-phase HPLC with the elution of acetonitrile-H 2 O (1.7:1, 5.0 mL/min) to afford 2 (3.6 mg, t R 54.5 min).

Cytotoxicity Testing
To measure the cytotoxicities of 1 and 2, three different concentrations of both compounds were added to A549, DLD-1, and HuCCT-1 cell lines for 72 h. The results were detected using the Alamar Blue assay [29,30].

Anti-Inflammatory Assay
The generation of the superoxide anion and elastase release in the fMLF/CB-induced primary human neutrophils was screened according to the previous description [13].

Conclusions
Two new isosarcophine derivatives, cherbonolides M (1) and N (2), were isolated from the continuous investigation of the soft coral S. cherbonnieri. Structurally, metabolite 2 is the first cembranoid possessing the unique cembranoidal scaffold of tetrahydrooxepane with the 12,17-ether linkage fusing with a γ-lactone. A plausible biosynthetic pathway was postulated for compound 2. In the anti-inflammatory assay, compound 2 was found to show moderate activity in inhibiting the generation of superoxide anion and elastase release in the fMLF/CB-induced human neutrophils. Based on the above statements and our previous discoveries [13,14], the soft coral S. cherbonnieri was demonstrated to be an attractive source of bioactive diterpenoids. Further, it has been proven that the soft coral S. glacum existed in more than seven genetically distinct clades and led to the high structural diversification of natural products of this species [31]. Currently, over 300 cembranoids have been discovered from the Sarcophyton genera [1], which might also come from the same reason. According to our investigation and related studies of other research groups [13,14,[32][33][34][35][36][37][38], the soft corals of genus Sarcophyton could be considered ideal organisms for discovering natural products with diversified structures and biological activities.