Diterpenoids from the South China Sea soft coral Sarcophyton solidum

Diterpenoids from the South China Sea soft coral Sarcophyton solidum

Biochemical Systematics and Ecology 62 (2015) 6e10 Contents lists available at ScienceDirect Biochemical Systematics and Ecology journal homepage: w...

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Biochemical Systematics and Ecology 62 (2015) 6e10

Contents lists available at ScienceDirect

Biochemical Systematics and Ecology journal homepage: www.elsevier.com/locate/biochemsyseco

Diterpenoids from the South China Sea soft coral Sarcophyton solidum Jian-Yong Zhu, Wei Li, Jing-Mei Bao, Jun-Sheng Zhang, Sheng Yin, Gui-Hua Tang* School of Pharmaceutical Sciences, Sun Yat-sen University, Guangzhou 510006, Guangdong Province, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 July 2015 Received in revised form 20 July 2015 Accepted 26 July 2015 Available online xxx

Chemical investigation of the South China Sea soft coral Sarcophyton solidum has led to the isolation of one new (1) and seven known (2e8) diterpenoids, including three sarsolenanes (1e3), two capnosanes (4 and 5), and three cembranes (6e8). Sarsolilide B (4) was firstly confirmed by single-crystal X-ray diffraction. Compounds 1, 3, 4, and 6e8 were isolated from S. solidum for the first time, and 1, 2, and 4e7 were considered as the chemotaxonomic markers for the species S. solidum. © 2015 Elsevier Ltd. All rights reserved.

Keywords: Sarcophyton solidum Sarsolenane Capnosane Cembrane

1. Subject and source The corals belonging to the genus Sarcophyton are very prolific marine resources (Liang and Guo, 2013). Sarcophyton comprising approximately 1000 species, is widely distributed in the coral reefs of the world oceans (Li et al., 2013). Especially, it is represented by about 500 species along the shores of the China Seas (Li et al., 2013). Sarcophyton solidum samples were collected in Dongshan Island, Guangdong Province, China, in October 2014, at a depth of 1e4 m water. The biological materials were immediately frozen at 20  C until used. A voucher specimen has been deposited at School of Pharmaceutical Sciences, Sun Yat-sen University (Accession number YSSZ-201410-DS). 2. Previous work Previous chemical investigations of Sarcophyton species have resulted in the occurrence of sesquiterpenes, diterpenes, diterpene dimers, prostaglandins, steroids, and ceramides (Anjaneyulu and Rao, 1997; Liang and Guo, 2013). However, the chemical constituents of S. solidum have not been extensively researched previously. Up to now, only two diterpenes, sarsolilide A (Zhang et al., 1992) and sarsolinone (Zhang et al., 1995), and one batyl alcohol glycoside (Zhang and Long, 1995) were isolated from S. solidum collected from the South China Sea near the Xisha Islands in July 1987.

* Corresponding author. E-mail address: [email protected] (G.-H. Tang). http://dx.doi.org/10.1016/j.bse.2015.07.033 0305-1978/© 2015 Elsevier Ltd. All rights reserved.

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3. Present study 3.1. Extraction and isolation The frozen samples (400 g, wet weight) were homogenized with a pestle and extracted exhaustively with acetone (2 L, three times) by using ultrasound vibration. After filtration, the organic solvent was removed from the combined extracts under vacuum to afford a total crude extract (25 g), which was dissolved in water and then partitioned sequentially to give dried petroleum ether (PE) (2 g), EtOAc (6 g), and n-BuOH (7 g) extracts. The EtOAc extract was subjected to silica gel column chromatography (CC) eluted with a CH2Cl2/MeOH gradient (v/v 1:0 / 10:1) to afford Fr. IIV. Separation of the Fr. II (1 g) by Sephadex LH-20 eluted with MeOH led to Fr. IIaIId. Fr. IIa (300 mg) was subjected to silica gel CC using PE/acetone mixtures (v/v 1:0 / 0:1) to afford Fr. IIa1eIIa3. Fr. IIa2 was loaded onto a Sephadex LH-20 column and eluted with CH2Cl2/MeOH (1:1) to give 2 (10 mg). Fr. IIc (150 mg) was purified on a semi-preparative reversed-phase (RP) HPLC system equipped with a YMC column (MeOH/H2O, 8:2, 3 mL/min) to give 6 (32 mg, tR ¼ 15 min) and 7 (19 mg, tR ¼ 16 min). Fr. IId (200 mg) was purified by semi-preparative HPLC (MeOH/H2O, 7:3, 3 mL/min) to give 4 (30 mg, tR ¼ 13 min), 5 (12 mg, tR ¼ 14.5 min), and 8 (10 mg, tR ¼ 21 min). Fr. III (1.5 g) was subjected to Sephadex LH-20 eluted with MeOH to obtain Fr. IIIaIIId. Fr. IIIa (500 mg) was subjected to silica gel CC using PE/EtOAc mixtures (v/v 1:0 / 0:1) to afford Fr. IIIa1eIIIa3. Fr. IIIa2 was separated by semipreparative HPLC (MeCN/H2O, 45:55, v/v; 3 mL/min) to afford 1 (6 mg, tR ¼ 10 min). Fr. IIIb was purified by semipreparative HPLC (MeCN/H2O, 7:3, v/v; 3 mL/min) to give 3 (4.7 mg). The chemical structures of 18 were shown in Fig. 1. 3.2. Structure elucidation Compound 1, a colorless powder, had the molecular formula C20H28O4, as determined by the HRESIMS ion at m/z 331.1908 [MH] (calcd 331.1909), corresponding to seven degrees of unsaturation. The IR spectrum exhibited absorption bands for hydroxyl (3333 cm1), carbonyl (1642 cm1), and double bonds (1608 cm1) functionalities. The 1H NMR data (Table 1) of 1 showed three methyl singlets [dH 1.36 (3H, s, CH3-19) and 1.72 (6H, s, CH3-17 and CH3-18)], two terminal double bond protons [dH 4.93 (1H, s, H-16a) and 4.83 (1H, s, H-16b)], a trisubstituted olefinic proton [dH 4.84 (1H, s, H-3)], an oxygenated methine [dH 3.43 (1H, m, H-7)], and a series of aliphatic methylene multiplets. The 13C NMR data (Table 1) in combination with DEPT experiments resolved 20 carbon resonances attributable to a ketone group (dC 200.2), a tetrasubstituted double bond (dC 165.1 and 117.2), a trisubstituted double bond (dC 137.9 and 124.6), a terminal double bond (dC 150.0, 111.2), two sp3 oxygenated quaternary carbons, two sp3 methines (one oxygenated), six sp3 methylenes, and three methyls. These data implied that 1 possessed most of the structural features of sarsolenane diterpenes. Comparison of the NMR data of 1 with those of the coisolated sarsolinone (2) (Zhang et al., 1995; Liang et al., 2014), the major differences being the absence of one acetyl group and an upfield-shifted H-7 signal (dH 3.43 in 1; dH 4.69 in 2) indicated that 1 was the 7-deacetyl derivative of 2. The planar structure of 1 was further confirmed by analyses of its 2D NMR data (Fig. 2). The relative configuration of 1 was assigned to be

Fig. 1. Compounds 1e8 from Sarcophyton solidum.

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Table 1 1 H NMR (400 MHz) and No. 1 2 3 4 5a 5b 6a 6b 7 8 9a 9b 10a 10b

13

C NMR (100 MHz) data of compound 1 in acetone-d6 (dC in ppm, J in Hz).

dH 3.10, s 4.84, s 3.08, 1.86, 1.87, 1.78, 3.43,

m m m m m

2.14, 1.99, 2.61, 2.46,

m m dd (15.4, 12.0) dd (15.4, 7.8)

dC

No.

74.9, C 48.4, CH 124.6, CH 137.9, C 29.9, CH2

11 12 13a 13b 14a 14b 15 16a 16b 17 18 19 20

26.7, CH2 73.7, CH 87.3, C 35.0, CH2 38.9, CH2

dH

2.42, dd (17.4, 9.1) 2.22, m 2.00, m 1.86, m 2.33, m 4.93, s 4.83, s 1.72,s 1.72,s 1.36,s

dC 200.2, C 117.2, C 21.7, CH2 30.8, CH2 150.0, C 111.2, CH2 18.3, CH3 22.9, CH3 19.4, CH3 165.1, C

the same as that of 2 by comparing their 1D NMR and NOESY data. The absolute configuration of 1 was assigned to be the same as that of 2 by comparison of their ECD spectra, which showed similar cotton effects at 311 nm (Dε6.17), 272 nm (Dεþ15.16), 233 nm (Dε1.80), and 217 nm (Dεþ0.29) (Fig. 3). Thus, the structure of compound 1 was determined as depicted (Fig. 1) and named 7-deacetyl-sarsolenone. Compound 4, colorless crystals, mp 171e173  C, the 1D NMR spectra of 4 revealed the same structural features as sarsolilide B (Liang et al., 2014), whose absolute configuration was determined by TDDFT ECD calculations method. In the current study, the crystals of 4 were obtained and single-crystal X-ray crystallographic analysis was performed. The result established the absolute configuration of 4 as drawn in Fig. 4 using anomalous scattering of Cu Ka radiation, which confirmed the structure determined by TDDFT ECD calculations method. X-ray crystallographic study of sarsolilide B: C20H28O3, M ¼ 334.46, hexagonal, space group P65 (no.170), a ¼ 9.8128 (1) Å, b ¼ 9.8128(1) Å, c ¼ 32.8075(5) Å, a ¼b ¼ 90 , g ¼ 120 , V ¼ 2735.83 (6) Å3, Z ¼ 6, Dc ¼ 1.2179 g/cm3, F000 ¼ 1092.5751, Xcalibur, Onyx, Nova, Cu Ka radiation, l ¼ 1.54184 Å, T ¼ K, 2qmax ¼ 51.9 , 35421 reflections collected, 3557 unique (Rint ¼ 0.0887). Final GooF ¼ 1.001993, R1 ¼ 0.031858, wR2 ¼ 0.081205, R indices based on 3363 reflections with I  2 sigma (I) (refinement on F2), 228 parameters, 0 restraint. Lp and absorption corrections applied, m ¼ 0.083 mm1. Flack parameter ¼ 2.2 (7). Crystallographic data for the structure of 4 have been deposited in the Cambridge Crystallographic Data Centre (deposition number: CCDC 1409199). By comparison of their observed and reported NMR data, the known compounds (Fig. 1) were identified as sarsolinone (2) (Zhang et al., 1995; Liang et al., 2014), methyl dihydrosarsolenoneate (3) (Liang et al., 2014), sarsolilide B (4) (Liang et al., 2014), sarsolilide A (5) (Zhang et al., 1992; Liang et al., 2014), emblide (6) (Toth et al., 1980), (4Z,8S,9R,12E,14E)-9-hydroxy-1isopropyl-8,12-dimethyloxabicyclo[9.3.2]-hexadeca-4,12,14-trien-18-one (7) (Bowden et al., 1982; Gross et al., 2004), and ketoemblide (8) (Uchio et al., 1983). 3.3. 7-Deacetyl-sarsolenone (1) A colorless powder; [a]25D þ40 (c 0.10, MeOH); UV (MeOH) lmax (log ε) 224 (3.98) nm; CD (c 1.0  104 M, MeOH) lmax (Dε) 217 (þ0.29), 233 (1.80), 272 (þ15.16), 311 (6.17) nm; IR (KBr) nmax 3333, 2980, 1642, 1608, 1452, 1086, 1045, 666 cm1; 1H and 13C NMR data, see Table 1; HRESIMS m/z 331.1908 [MH] (calcd for C20H27O4, 331.1909). 4. Chemotaxonomic significance The genus Sarcophyton is characterized by sesquiterpenes, diterpenes, diterpene dimers, prostaglandins, steroids, and ceramides (Liang and Guo, 2013). Compounds 1e8 isolated in this study were classified as three categories of diterpenes:

Fig. 2. Selected 1He1H COSY (

) and HMBC (

) correlations of 1.

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Fig. 3. The ECD spectrum of 1 in MeCN.

Fig. 4. ORTEP diagram of compound 4.

sarsolenane-type (1e3), capnosane-type (4 and 5), and cembrane-type (6e8). The structural types of these isolates supported the taxonomic position of S. solidum under the genus Sarcophyton. Compounds 1, 3, 4, and 6e8 were isolated from S. solidum for the first time. Among them, compounds 2e8 were previously reported from different species of the same genus. In brief, compounds 2e5 were isolated from Sarcophyton trocheliophorum (Liang et al., 2014), while the known cembrane diterpenes 6e8 were previously described in Sarcophyton glaucum (Toth et al., 1980), Sarcophyton elegans (Uchio et al., 1983), and Sarcophyton crassocaule (Zhang et al., 2006), S. elegans (Uchio et al., 1983), and a new Sarcophyton species (Gross et al., 2004). Thus, these sarsolenane, capnosane, and cembrane diterpenes may signify a close phylogenetic relationship among these Sarcophyton species. As diterpenes were widely present in corals metabolites and isolated as main components in current study, compounds 1, 2, and 4e7 were considered as the chemotaxonomic markers for the species S. solidum. Acknowledgements The authors thank the Science and Technology Planning Project of Guangdong Province (No. 2013B021100009), the National Natural Science Foundation of China (No. 81402813), the Fundamental Research Funds for the Central Universities (No. 14ykpy10), and the Guangdong Natural Science Funds for Distinguished Young Scholar (No. 2014A030306047) for providing financial support to this work.

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Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.bse.2015.07.033. References Anjaneyulu, A.S.R., Rao, G.V., 1997. J. Indian Chem. Soc. 74, 272. Bowden, B.F., Coll, J.C., Willis, R.H., 1982. Aust. J. Chem. 35, 621. €enig, G.M., 2004. Org. Biomol. Chem. 2, 1133. Gross, H., Wright, A.D., Beil, W., Ko Li, Y., Liang, L., Xiao, W., Liang, J., Guo, Y., 2013. Chin. J. Org. Chem. 33, 1157. Liang, L.F., Guo, Y.W., 2013. Chem. Biodivers. 10, 2161. ndi, A., Gao, L.X., Li, J., Zhang, W., Guo, Y.W., 2014. Eur. J. Org. Chem. 2014, 1841. Liang, L.F., Kurt an, T., Ma Toth, J.A., Jay Burreson, B., Scheuer, P.J., Finer-Moore, J., Clardy, J., 1980. Tetrahedron 36, 1307. Uchio, Y., Nitta, M., Nakayama, M., Iwagawa, T., Hase, T., 1983. Chem. Lett. 12, 613. Zhang, C., Li, J., Su, J., Liang, Y., Yang, X., Zheng, K., Zeng, L., 2006. J. Nat. Prod. 69, 1476. Zhang, M., Long, K., 1995. Nat. Prod. Res. Dev. 7, 12. Zhang, M., Long, K., Huang, S., Shi, K., Mak, T.C.W., 1992. J. Nat. Prod. 55, 1672. Zhang, M., Long, K., Ma, K., Huang, X., Wu, H., 1995. J. Nat. Prod. 58, 414.