A New Epoxy-cadinane Sesquiterpene from the Marine Brown Alga Dictyopteris divaricata

Abstract

A new epoxy-cadinane sesquiterpene, 4β,5β-epoxycadinan-1β-ol (1), and six known cadinane sesquiterpenes: cadinan-1,4,5-triol (2), 4α,5β-dihydroxycubenol (3), cubenol (4), cadinan-3-ene-1,5-diol (5), cubenol-3-one (6), and torreyol (7), were isolated from a sample of marine brown alga Dictyopteris divaricata collected off the coast of Yantai (China). Their structures were established by detailed MS and NMR spectroscopic analysis, as well as comparison with literature data.

1. Introduction

Marine brown algae of the genus Dictyopteris (Dictyotales, Dictyotaceae) have been found to be a rich source of structurally unique sesquiterpenes, including cadinane, selinane, germacrane, and other rearranged skeleton types [1–9]. Continuing chemical investigation of the secondary metabolites of D. divaricata collected off the coast of Yantai has led to the isolation of one new cadinane sesquiterpene, 4β,5β-epoxycadinan-1β-ol (1), and six known cadinane sesquiterpenes: cadinan-1,4,5-triol (2) [5], 4α,5β-dihydroxycubenol (3) [6], cubenol (4) [6], cadinan-3-ene-1,5-diol (5) [5], cubenol-3-one (6) [6], and torreyol (7) [6]. The isolation of compounds 1–7 and structural determination of compound 1 are presented.

2. 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 purified by a combination of silica gel, reversed-phase silica gel, and Sephadex LH-20 column chromatography, as well as preparative TLC, to yield compounds 1–7 (Figure 1).

Compound 1 was obtained as a colorless oil. The IR absorption at v_max 3471 cm⁻¹ indicated the presence of a hydroxyl group in the molecule. The molecular formula was determined as C₁₅H₂₆O₂ on the basis of HRESIMS (m/z 261.1825 [M+Na]⁺, calcd. for C₁₅H₂₆O₂Na, 261.1830), suggesting three degrees of unsaturation. The ¹H-NMR spectrum displayed one methyl singlet, three methyl doublets, and two broad singlets attributed respectively to an oxygenated methine and a hydroxyl group. The ¹³C-NMR spectrum along with the DEPT and HSQC experiments revealed the presence of four methyls, four methylenes, five methines, and two quaternary carbon atoms. A detailed comparison of the above spectral data with those reported for cadinan-1,4,5-triol (2) revealed that 1 differed from 2 mainly at C-4 and C-5 [5], so 1 could be a dehydrated product of 2 between the hydroxyl groups at C-4 and C-5 according to the upfield-shifted C-4, C-5, and H-5 [10]. The ¹H-¹H COSY correlations as shown in

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

No.	δC	δH	1H-1H COSY
1	71.4 s
2a	31.7 t	1.16 (ddd, 13.4, 13.1, 7.4)	H-2b, H-3a, H-3b
2b		1.94 (ddd, 13.4, 7.0, 1.0)	H-2a, H-3a, H-3b
3a	25.3 t	1.87 (br dd, 13.0, 7.4)	H-3b, H-2a, H-2b
3b		2.20 (ddd, 13.0, 13.1, 7.0)	H-3a, H-2a, H-2b
4	59.5 s
5	62.6 d	3.22 (br s)	H-6
6	44.4 d	1.53 (br d, 11.6)	H-5, H-7
7	37.9 d	1.90 (m)	H-6, H-8a, H-8b, H-11
8a	24.3 t	1.08 (m)	H-7, H-8b, H-9a, H-9b
8b		1.72 (m)	H-7, H-8a, H-9a, H-9b
9a	30.1 t	1.47 (m)	H-8a, H-8b, H-9b, H-10
9b		1.58 (m)	H-8a, H-8b, H-9a, H-10
10	40.4 d	1.21 (m)	H-9a, H-9b, H-14
11	26.4 d	2.19 (m)	H-7, H-12, H-13
12	15.1 q	0.84 (d, 6.9)	H-11
13	21.3 q	1.00 (d, 6.9)	H-11
14	14.8 q	0.88 (d, 6.7)	H-10
15	24.6 q	1.38 (s)
OH		3.43 (br s)

and the observed HMBC (Figure 2) 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, from H-5 to C-1, C-4, C-6, and C-15, and from HO-1 to C-1, C-6, and C-10 confirmed the gross structure of 1.

The relative stereochemistry of 1 was determined by analysis of NOESY correlations (Figure 2) and coupling constants. The NOESY correlations between H-6 and H-2a, H-8a, H-10 indicated them to be axial and on the same face of the molecule. H-5 was located on the same face of H-6 and C-15 based on the NOESY correlation between H-5 and H-6, H-15 and the little coupling constant (only broad singlet) of H-5, while the large coupling constant (11.6 Hz) of H-6 suggested H-6 and H-7 to be opposite. The observed NOESY correlation between HO-1 and H-7 indicated them to be located on the same face of the molecule. The above evidence established the structure of 1 to be 4β,5β-epoxycadinan-1β-ol.

The structures of known compounds 2–7 were confirmed by detailed NMR data comparison with those in literature [5,6]. Compound 6 is firstly reported from D. divaricata, while 2–5, 7 have been isolated from this species before [5,9,11]. When we tried to purify compound 1 by preparative TLC, 2 was by-produced. So, compound 2 may be an artifact, though it has been isolated from a different fraction (fraction VIII). Compound 1 represents a new addition to the molecular diversity of cadinane sesquiterpenes, which may be a key intermediate in the biosynthesis from 4 to 2 and 3.

3. Experimental Section

3.1. General

NMR spectra were recorded in CDCl₃ at 500 and 125 MHz for ¹H and ¹³C, respectively, on a Bruker Avance 500 MHz NMR spectrometer with TMS as internal standard. Mass spectra were determined on a VG Autospec 3000 mass spectrometer. 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, 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, China). All solvents were of analytical grade.

3.2. 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, China, in July 2008. It was identified by one of the authors (Nai-Yun Ji) and a voucher specimen (MBA0807) has been deposited at the Bio-Resource Laboratory of Yantai Institute of Coastal Zone Research, Chinese Academy of Sciences.

3.3. Extraction and Isolation

Dried and powdered alga D. divaricata (2 kg) was extracted with a mixture of CHCl₃ and MeOH (2 L, 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 III, eluted with PE/EtOAc (50:1), was further purified by silica gel column chromatography (PE/EtOAc 10:1) to afford 4 (ca. 30 g). Fraction IV, eluted with PE/EtOAc (20:1), was further purified by silica gel and Sephadex LH-20 (CHCl₃/CH₃OH) column chromatography to afford 4β,5β-epoxycadinan-1β-ol (1, 0.8 mg). Colorless oil; [α]²¹_D–8.2° (c=0.13, CHCl₃); IR (KBr) cm⁻¹: 3471, 2954, 2923, 2877, 1450, 1392, 976; ¹H-NMR (CDCl₃, 500 MHz) and ¹³C-NMR (CDCl₃, 125 MHz): see Table 1; HRESIMS m/z: 261.1825 [M+Na]⁺, calcd. for C₁₅H₂₆O₂Na, 261.1830. 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 3 (11.0 mg), 5 (2.1 mg), 6 (14.0 mg), and 7 (8.0 mg). Fraction VIII, eluted also 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 1:4) to afford 2 (20.7 mg).