1. Introduction
Soft corals have attracted a great deal of attention in light of the structural diversity and wide range of biological activities of their metabolites [1]. Recently, in the investigation of the bioactive metabolites from the Formosan soft corals, many bioactive cembranoids have been isolated from soft corals (Alcyonaceae) belonging to the genera Sinularia[2–11], Sarcophyton[12–16] and Lobophytum[17–19]. Some of these metabolites have been found to possess several kinds of biological activities, such as cytotoxic [4,9–18] and anti-inflammatory activity [2–8,12–14,18,19]. During the course of our investigation on new natural substances from wild and cultured soft coral Sinularia flexibilis, a number of cembrane-based diterpenoids were discovered, and some were found to be bioactive [20]. In continuation of our search for biologically active secondary metabolites from a soft coral Sinularia flexibilis (Figure 1), we have isolated two new cembrane-based diterpenoids, 11-acetylsinuflexolide (1) and 11-acetyldihydrosinuflexolide (2), along with three known cembranoids, sinuflexolide (3) [21], sinularin (4) [22] and dihydrosinularin (5) [22] (Figure 2). The structures of 1 and 2 were established by extensive spectroscopic analysis, including careful examination of 2D-NMR (1H–1H COSY, HMQC, HMBC and NOESY) (Figures S1–S10) correlations and by comparison of their NMR data with those of related compounds. The cytotoxicity of compounds 1–5 against human cervical epitheloid carcinoma (HeLa), laryngeal carcinoma (HEp-2) and breast carcinoma (MCF-7 and MDA-MB-231) cell lines was also investigated.
2. Results and Discussion
Frozen samples of Sinularia flexibilis were extracted with EtOAc. The dry EtOAc extracts were fractionated by silica gel gravity column chromatography, and the eluted fractions were further purified by HPLC to yield cembranoids 1–5.
The HR-ESI-MS (m/z 417.2250 [M + Na]+) of 11-acetylsinuflexolide (1) established the molecular formula C22H34O6, appropriate for six degrees of unsaturation. Inspection of the 13C-NMR and DEPT spectroscopic data (Table 1) (Figures S1 and S2) showed signals of four methyls (including one acetate methyl), seven sp3 methylenes, one sp2 methylene, three sp3 methines (including two oxymethines), one sp2 methine, two sp3 and four sp2 quaternary carbons (including two ester carbonyls). The 13C NMR signals appearing at δC 166.6 (C), 140.4 (C), 125.5 (CH2), 84.5 (CH), 36.7 (CH), and 29.3 (CH2) were assigned to an α-exomethylenic–δ-lactone ring functionality by comparing the very similar NMR data of the cembranoids with the same six-membered lactone ring [23,24]. Resonances in the 13C NMR spectrum of 1 at δC 170.6 (C) supported the presence of one additional ester group (Table 1). The ester was identified as acetate by the presence of one methyl resonance in the 1H NMR spectrum at δH 2.11 (3H, s) (Table 1). Furthermore, carbon signals of three methyls (δC 16.1, 25.4 and 25.5), one trisubstituted double bond (δC 135.1, C; 127.2, CH), two oxygen-bearing methines (δC 84.5 and 77.5), and two oxygenated quaternary carbons (δC 74.8 and 73.7) were also determined. The 1H-NMR spectral data revealed the presence of two olefinic methylene protons (δ 6.43, d, J = 2.0 Hz and 5.63, d, J = 2.0 Hz) and one olefinic methine proton (δ 5.26, dd, J = 7.5, 7.5 Hz). Furthermore, two oxygenated methines (δ 4.79, dd, J = 6.5, 2.5 Hz and 4.05, d, J = 11.5 Hz) were also designated from the 1H NMR signals. By interpretation of 1H-1H COSY correlations (Figure S5), it was possible to establish three partial structures from H-1 to H-3, from H2-5 to H-7, from H2-9 to H-11, and from H2-13 to H-1 through H2-14 (Figure 3). These data, together with the HMBC correlations (Figure 3) (Figure S4) from H2-5 to C-3 and C-4, H2-9 to C-7 and C-8, and H2-13 to C-11 and C-12 established the connectivity within the 14-membered ring. Three methyl groups attached at C-4, C-8 and C-12 were confirmed by the HMBC correlations from H3-18 to C-3, C-4 and C-5, H3-19 to C-7, C-8 and C-9, H3-20 to C-11, C-12 and C-13. A 1,1-disubstituted double bond attached at C-15 was confirmed by the HMBC correlations from H2-17 to C-1, C-15 and C-16. Moreover, one acetoxy group positioned at C-11 was confirmed from the HMBC correlations of H-11 (δ 4.79) and protons of an acetate methyl (δ 2.11) to the ester carbonyl carbon at δ 170.6 (C). The E–configuration of one double bond at C-7/C-8 was assigned based on the 13C NMR chemical shifts at C-19 (δC 16.1). Thus, 1 was revealed as a cembranoid possessing an α-exomethylenic–δ-lactone ring, based on the above analysis. Furthermore, the relative stereochemistry of 1 was mostly confirmed to be the same as that of the known metabolite sinuflexolide (3) by comparison of the proton chemical shifts and coupling constants [24]. Further comparison of the 1H and 13C NMR data of 1 with those of 3, showed that 1 contains an extra acetyl group relative to 3. The chemical shift of H-11 in 3 (δH 3.47, dd, J = 6.4, 2.4 Hz) was shifted downfield (δH 4.79, dd, J = 6.5, 2.5 Hz) in 1, suggesting that 1 is the 11-acetyl derivative of 3. This was further supported by acetylation of 3 with acetic anhydride in pyridine to yield 1. Thus, compound 1 was established as the 11-acetyl derivative of 3.
11-acetyldihydrosinuflexolide (2) obtained as a white powder. The HRESIMS (m/z 419.2411, [M + Na]+) and NMR data of 2 indicated the molecular formula, C22H36O6. Both the 1H and 13C NMR signals of 2 were found to be very closely related to those of compound 1, suggesting the same skeleton. Further comparison of NMR data of 2 with those of 1 (Table 1) (Figures S1–S10), revealed that the two exomethylene proton signals (δH 6.43 and 5.63) in 1 was replaced by a methyl proton signal (δH 1.35 d, J = 7.0 Hz) in 2. This was confirmed by the HMBC correlations (Figure 3) from H3-17 to C-1, C-15 and C-16. The relative stereochemistry of all stereocenters except C-15 of 2 was determined to be the same as that of 1 by comparison of the proton shifts and coupling constants. The methyl group at C-15 was assigned the β-configuration primarily due to the NOE correlation between H3-17 and H-1. Furthermore, comparison of the NMR data between 2 and 5 confirmed both compounds have the same relative stereochemistry at C-15 [22]. Thus, the structure of 2 was established.
Finally, a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay was used to examine the cytotoxic activities of compounds 1–5 against four cancer cell lines, including human cervical epitheloid carcinoma (HeLa), laryngeal carcinoma (HEp-2) and breast carcinoma (MCF-7 and MDA-MB-231) cancer cells. Cells were treated with different concentrations of 1–5 for 72 h. The results show that compound 3, the most potent of compounds 1–5, exhibited cytotoxicity against the HeLa, HEp-2, MCF-7 and MDA-MB-231 cancer cell lines with IC50 values of 8.6, 8.2, 16.0 and 11.3 μg/mL, respectively. Furthermore, compounds 1 and 4 were found to exhibit weak cytotoxicity towards some of the cell lines (Table 2).
3. Experimental Section
3.1. General Procedures
Optical rotation values were measured using a Jasco P-1010 digital polarimeter. IR spectra were recorded on a Varian Digilab FTS 1000 Fourier transform infrared spectrophotometer. NMR spectra were recorded on a Varian Mercury Plus 400 FT-NMR (or Varian Unity INOVA 500 FT-NMR) instrument at 400 MHz (or 500 MHz) for 1H-NMR and 100 MHz (or 125 MHz) for 13C-NMR, respectively, in CDCl3. ESIMS and HRESIMS data were recorded with a Bruker APEX II mass spectrometer. Gravity column chomatography was performed on silica gel (230–400 mesh, Merck, Darmstadt, Germany). Thin layer chomatography (TLC) was carried out on precoated Kieselgel 60 F254 (0.2 mm, Merck) and spots were visualized by spraying with 10% H2SO4 solution followed by heating. HPLC was performed using a system comprised of a Hitachi L-7100 pump (Tokyo, Japan) and a Rheodyne 7725 injection (Cotati, CA, USA) port. A preparative normal phase column (Hibar 250 × 21.2 mm, Supelco, silica gel 60, 5 μm, Bellefonte, PA, USA) was used for HPLC.
3.2. Animal Material
The marine soft coral Sinularia flexibilis (Quoy and Gaimard, 1833) was collected by scuba divers at a depth of around 10 m off the coast of Pingtung County, Taiwan, in July 2012, and the sample was frozen immediately after collection. A voucher sample was deposited at the National Museum of Marine Biology and Aquarium, Taiwan (specimen No. 2012-0709-10).
3.3. Extraction and Separation
The soft coral (2.0 kg, wet wt.) was stored frozen and then freeze dried. The freeze-dried material (450 g) was minced and extracted five times with EtOAc (2 L) for 24 h each time at room temperature. The organic extract was evaporated to yield a residue (60.5 g), which was subjected to open column chomatography on silica gel eluting with gradients of n-hexane (H)–EtOAc (E), to give 14 fractions: Fr-1 (eluted by n-hexane), Fr-2 (eluted by H–E 100:1), Fr-3 (eluted by H–E 50:1), Fr-4 (eluted by H–E 30:1), Fr-5 (eluted by H–E 20:1), Fr-6 (eluted by H–E 15:1), Fr-7 (eluted by H–E 10:1), Fr-8 (eluted by H–E 8:1), Fr-9 (eluted by H–E 5:1), Fr-10 (eluted by H–E 3:1), Fr-11 (eluted by H–E 2:1), Fr-12 (eluted by H–E 1:1), Fr-13 (eluted by H–E 1:2), and Fr-14 (eluted by EtOAc). Fraction 10 was further separated by silica gel column chomatography with gradient elution (n-hexane–EtOAc, 5:1 to 1:1) to yield five subfractions (10A–E). Subfraction 10C was subjected to normal phase HPLC with n-hexane–EtOAc (4:1) as the eluent (flow rate 2 mL/min) to obtain compounds 4 (250 mg, 0.41% dry wt. of extract) and 5 (330 mg, 0.55% dry wt. of extract). Fraction 12 was further separated by silica gel column chomatography with gradient elution (n-hexane–EtOAc, 1:1 to 1:3) to yield seven subfractions (12A–G). Subfraction 12C was subjected to normal phase HPLC with n-hexane–EtOAc (1:1) as the eluent (flow rate 2 mL/min) to obtain compounds 1 (8.0 mg, 0.013% dry wt. of extract) and 2 (6.5 mg, 0.011% dry wt. of extract). Subfraction 12F was subjected to normal phase HPLC with n-hexane–acetone (1:1) as the eluent (flow rate 2 mL/min) to obtain compound 3 (6.5 mg, 0.011% dry wt. of extract).
11-Acetylsinuflexolide (1): white solid; mp 82.0–85.0 °C; [α]D25 −12 (c 0.7, CHCl3); IR (neat) νmax 3434, 2974, 2937, 1712, 1622, 1452, 1376 and 1256 cm−1; UV (MeOH) λmax (log ɛ) 212 (3.9) nm; 13C and 1H NMR data, see Table 1; ESIMS m/z 417 [M + Na]+; HRESIMS m/z 417.2250 [M + Na]+ (calcd for C22H34O6Na, 417.2253).
11-Acetyldihydrosinuflexolide (2): white solid; mp 75.0–78.0 °C; [α]D25−15 (c 0.6, CHCl3); IR (neat) νmax 3434, 2975, 2938, 1714, 1639, 1458, 1377 and 1245 cm−1; 13C and 1H NMR data, see Table 1; ESIMS m/z 419 [M + Na]+; HRESIMS m/z 419.2411 [M + Na]+ (calcd for C22H36O6Na, 419.2409).
Sinuflexolide (3): white solid; [α]D24 −7.0 (c 0.2, CHCl3); IR (neat) νmax 3400, 2972, 1714, 1458, 1381, and 1235 cm−1; UV (MeOH) −max 215 (log ɛ =3.8); [lit. [α]D25 −8.6 (c 0.17, CHCl3)] [21].
Sinularin (4): white solid; [α]D25 –105 (c 1.0, CHCl3); IR (neat) νmax 3404, 2965, 1710, 1455, 1381, and 1237 cm−1; UV (MeOH) λmax 212 (log ɛ = 3.8); [lit. [α]D20 −127] [22].
Dihydrosinularin (5): white solid; [α]D25 –30 (c 0.3, CHCl3); IR (neat) νmax 3400, 2960, 1714, 1459, 1385, and 1231 cm−1; [lit. [α]D20 −45] [22].
Acetylation of 3: A solution of 3 (5.0 mg) in pyridine (0.5 mL) was mixed with Ac2O (0.1 mL), and stirred at room temperature for 24 h. After evaporation of excess reagent, the residue was subjected to column chromatograph over silica gel using n-hexane–EtOAc (1:2) to give the acetyl derivative 1 (4.8 mg, 81%).
3.4. Cytotoxicity Testing
Cell lines were purchased from the American Type Culture Collection (ATCC). Cytotoxicity assays of compounds 1–5 were performed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) colorimetric method [25,26].