A New Cadinane Sesquiterpene from the Marine Brown Alga Dictyopteris divaricata

Abstract

A sample of the marine brown alga D. divaricata collected off the coast of Yantai (P.R. China) was dried, powdered, and extracted with the mixture of CHCl₃ and MeOH (1:1, v/v). By a combination of silica gel and Sephadex LH-20 column chromatography and preparative TLC, a new cadinane sesquiterpene 1,4-epoxymuurolan-5β-ol (1) was isolated from this species. Its structure was established by detailed MS and NMR spectroscopic analysis, as well as comparison with literature data.

Introduction

Marine brown algae of the genus Dictyopteris are prolific sources of sesquiterpenes, and cadinane is a main carbon skeleton type [1,2,3,4,5,6,7,8]. In our investigations on the structurally interesting and biologically active terpenes from Chinese marine algae, we examined the chemical constituents of D. divaricata collected off the coast of Yantai and as a result, a new cadinane sesquiterpene, 1,4-epoxymuurolan-5β-ol (1) has been isolated and characterized for the first time. This paper reports the isolation and structural elucidation of compound 1 (Figure 1).

Results and Discussion

The dried and powdered alga D. divaricata was extracted with a mixture of CHCl₃ and MeOH (1:1, v/v). The concentrated extracts were partitioned between H₂O and EtOAc. The EtOAc-soluble fraction was further purified by a combination of silica gel and Sephadex LH-20 column chromatography, as well as preparative TLC, to yield compound 1. Compound 1 was obtained as a colorless oil. The broad IR absorption at v_max 3,452 cm^-1 indicated the presence of a hydroxyl group in the molecule. The positive electrospray ionization mass spectrometry (ESIMS) exhibited a characteristic quasimolecular ion peak at m/z 261 [M+Na]⁺. The molecular formula was determined as C₁₅H₂₆O₂ on the basis of HRESIMS (m/z 261.1829 [M+Na]⁺, calcd. for C₁₅H₂₆O₂Na, 261.1830), suggesting three degrees of unsaturation. The ¹H-NMR spectrum of 1 (

Table 1. ¹H and ¹³C-NMR data and ¹H-¹H COSY correlations of compound 1 (in CDCl₃, δ in ppm, J in Hz).

No.	δC	δH	1H-1H COSY
1	87.1 s
2a	34.6 t	1.42 (ddd, 12.5, 9.6, 5.8)	H-2b, H-3a, H-3b
2b		1.94 (ddd, 12.5, 12.1, 4.0)	H-2a, H-3a, H-3b
3a	29.4 t	1.30 (ddd, 12.1, 11.9, 5.8)	H-3b, H-2a, H-2b
3b		2.21 (ddd, 11.9, 9.6, 4.0)	H-3a, H-2a, H-2b
4	85.7 s
5	85.5 d	3.46 (br s)	H-6
6	56.0 d	1.26 (d, 11.6)	H-5, H-7
7	47.3 d	1.14 (dddd, 12.1, 11.6, 2.1, 1.6)	H-6, H-8a, H-8b
8a	23.7 t	0.89 (dddd, 12.9, 12.6, 12.1, 2.1)	H-7, H-8b, H-9a, H-9b
8b		1.54 (br dd, 12.9, 3.1)	H-7, H-8a, H-9a, H-9b
9a	31.7 t	1.23 (m)	H-8a, H-8b, H-9b, H-10
9b		1.62 (m)	H-8a, H-8b, H-9a, H-10
10	34.9 d	1.59 (m)	H-9a, H-9b, H-14
11	27.3 d	1.80 (hept d, 6.9, 1.6)	H-7, H-12, H-13
12	16.0 q	0.80 (d, 6.9)	H-11
13	21.8 q	0.94 (d, 6.9)	H-11
14	15.3 q	1.01 (d, 6.5)	H-10
15	19.6 q	1.41 (s)

) displayed one methyl singlet, three methyl doublets, and one broad singlet, attributed to an oxygenated methine. The ¹³C-NMR spectrum (Table 1), along with the DEPT and HSQC experiments revealed the presence of four methyl, four methylene, five methine, and two quaternary carbon atoms. A detailed comparison of the above spectra data with those reported for 1,4-epoxymuurolan-5α-ol revealed that 1 differed from this last compound mainly at C-5 (δ_C 85.5 d) [9], suggesting that compound 1 may be a C-5 isomer of 1,4-epoxymuurolan-5α-ol. The ¹H-¹H COSY correlations as shown in Table 1 and the observed HMBC correlations from H-12 to C-7, C-11, and C-13, from H-13 to C-7, C-11, and C-12, from H-14 to C-1, C-9, and C-10, from H-15 to C-3, C-4, and C-5, and from H-5 to C-3, C-4, C-6, C-7, and C-15 confirmed the planar structure of 1. The relative configuration of 1 was determined by analysis of NOESY spectrum and coupling constants. The NOESY correlations between H-5 and H-7, H-2a indicated H-5, H-7 and C-2 to be located on the same face of the molecule. The same orientation of C-14 and C-2 was suggested on the basis of the NOESY correlation between H-14 and H-2b. H-6 and C-15 were assigned on the same face according to the observed NOESY correlation between H-6 and H-15. H-6 was located on the opposite face of H-7 based on the large coupling constant (11.6 Hz) between them. The above evidence established the structure of 1 to be 1,4-epoxymuurolan-5β-ol (Figure 1), the C-5 epimer of 1,4-epoxymuurolan-5α-ol [9]. Compound 1 was tested for the toxicity against brine shrimp (Artemia salina) [10]. However, it exhibited no toxicity against brine shrimp at 100 μg/mL.

Experimental

General

NMR spectra were recorded in CDCl₃ with TMS as internal standard on a Bruker Avance 500 MHz NMR spectrometer operating at 500 and 125 MHz for ¹H and ¹³C, respectively. Low and high resolution mass spectra were determined on a VG Autospec 3000 mass spectrometer. The IR spectrum was obtained on a JASCO FT/IR-4100 Fourier Transform infrared spectrometer. Optical rotation was measured on a JASCO P-1020 polarimeter. Column chromatography was performed with silica gel (200-300 mesh, Qingdao Haiyang Chemical Co., Qingdao, P.R. China), RP-18 reversed-phase silica gel (YMC), and Sephadex LH-20 (Pharmacia). TLC was carried out with precoated silica gel plates (GF-254, Qingdao Haiyang Chemical Co., Qingdao, P.R. China). All solvents were of analytical grade.

Algal Material

The brown alga Dictyopteris divaricata was collected off the coast of Yantai (lat. 37°31’15”N, long. 121°26’59”E), Shandong Province, P. R. China, in July 2008, and a voucher specimen (MBA0807) has been deposited at the Bio-Resource Laboratory of Yantai Institute of Coastal Zone Research for Sustainable Development, Chinese Academy of Sciences.

Extraction and Isolation

Dried and powdered alga D. divaricata (2 kg) was extracted with the mixture of CHCl₃ and MeOH (1:1, v/v). The concentrated extract was partitioned between H₂O and EtOAc. The EtOAc-soluble fraction (90 g) was fractioned by silica gel column chromatography [petroleum ether (PE)/EtOAc gradient] to give ten fractions, I-X. Fraction VII, eluted with PE/EtOAc 2:1, was further purified by Sephadex LH-20 (CHCl₃/CH₃OH) and RP-18 (CH₃OH/H₂O 3:1) column chromatography and preparative TLC (PE/EtOAc 3:1) to afford 1,4-epoxymuurolan-5β-ol (1, 9.1 mg) as a colorless oil; [α]²⁵_D –29.2° (c=0.33, CHCl₃); IR (KBr) cm^-1: 3,452, 2,962, 2,954, 2,870, 1,458, 1,377, 1,065; ¹H-NMR and ¹³C-NMR: see Table 1; ESIMS m/z: 261 [M+Na]⁺; HRESIMS m/z: 261.1829 [M+Na]⁺, calcd. for C₁₅H₂₆O₂Na, 261.1830.

Brine Shrimp Assays

Brine shrimp assays were performed as previously described [10].