Sesquiterpene and acetogenin derivatives from the marine red alga Laurencia okamurai.

In addition to 13 known compounds, four new bisabolane sesquiterpenes, okamurenes A-D (1-4), a new chamigrane derivative, okamurene E (5), and a new C₁₂-acetogenin, okamuragenin (6), were isolated from the marine red alga Laurencia okamurai. The structures of these compounds were determined through detailed spectroscopic analyses. Of these, okamurenes A and B (1 and 2) are the first examples of bromobisabolane sesquiterpenes possessing a phenyl moiety among Laurencia-derived sesquiterpenes, while okamuragenin (6) was the first acetogenin aldehyde possessing a C₁₂-carbon skeleton. Each of the isolated compounds was evaluated for the brine shrimp (Artemia salina) lethal assay and 7-hydroxylaurene displayed potent lethality with LD₅₀ 1.8 μM.

1. Introduction

Marine red algae of the genus Laurencia are prolific sources of diversified secondary metabolites, predominantly sesquiterpenoids, diterpenoids, and nonterpenoid C15-acetogenins [1]. The red alga Laurencia okamurai, widely distributed along the coast of China, mainly yields sesquiterpenes and C15-acetogenins [2]. These compounds, with structurally diverse skeletons, have attracted much attention for total syntheses [3] as well as chemotaxonomic research [4,5,6]. In the past five years, we have systematically conducted chemical investigation towards eight Laurencia species, which have resulted in the isolation of more than 30 new compounds [2,7,8,9,10,11]. In the course of our phytochemical studies on Laurencia okamurai, a new, rearranged chamigrane sesquiterpene, laurenokamurin, was previously characterized [10]. Continuous effort on the chemical investigation of this algal species collected from Weihai coastline resulted in the isolation and identification of five new sesquiterpenes, okamurenes A–E (1–5), one new C12-acetogenin, okamuragenin (6) (Figure 1), as well as nine known sesquiterpenes and four known C15-acetogenins. We present herein the isolation, structure elucidation, and bioactivity of these compounds.

Figure 1

Structures of the isolated new compounds 1–6 from L. okamurai.

2. Results and Discussion

Structure Elucidation of the New Compounds

Okamurene A (1) was obtained as a colorless oil and its molecular formula was established by HRESIMS to be C15H21BrO, corresponding to five degrees of unsaturation. The 1H NMR spectrum of 1 (Table 1) exhibited resonances for a para-substituted phenyl unit, four methyl groups, and a brominated or oxygenated methine group. There were also four signals for two diastereotopic methylene protons. The 13C NMR and DEPT spectroscopic data (Table 1) revealed the presence of 15 carbon signals including six aromatic carbons (corresponding to a para-substituted phenyl unit) and nine aliphatic carbons (corresponding to four methyls, two methylenes, one brominated methine, and two oxygenated quaternary carbons). These units accounted for 4 degrees of unsaturation, requiring one additional ring to be present in 1.

Table 1

1H- and 13C-NMR data of compounds 1 and 2 in CDCl3 a.

No.	1 (CDCl3)		2
	δH (J in Hz)	δ C	δH (J in Hz)	δ C
1/5	7.34, d (8.0)	124.8, CH	7.34, d (8.1)	126.0, CH
2/4	7.11, d (8.0)	128.7, CH	7.12, d (8.1)	128.6, CH
3		136.1, C		136.4, C
6		146.0, C		143.2, C
7		74.6, C		74.4, C
8eq	2.16, m	34.1, CH2	2.56, m	36.0, CH2
8ax	2.10, m		2.18, m
9eq	2.28, m	28.2, CH2	2.27, m	29.4, CH2
9ax	2.25, m		1.82, m
10	4.05, dd (7.9, 4.4)	59.1, CH	4.04, dd (12.1, 4.1)	59.0, CH
11		75.2, C		76.4, C
12	1.47, s	27.8, CH3	1.35, s	22.5, CH3
13	1.14, s	29.4, CH3	0.78, s	30.8, CH3
14	1.50, s	31.8, CH3	1.36, s	35.8, CH3
15	2.23, s	20.9, CH3	2.34, s	21.0, CH3

a Measured at 500 MHz for 1H and 125 MHz for 13C.

The structure of the non-phenyl portion of 1 was determined by analysis of 2D NMR data (1H–1H COSY, HSQC, and HMBC). The 1H–1H COSY experiment established the connectivity for a –CH2–CH2–CH– unit (C-8 through C-10, Figure 2). The C-10 methine of this unit was connected to CH3-12 and CH3-13 via the oxygenated quaternary carbon C-11 (δC 75.2) as evidenced by the observed HMBC correlations from the methyl protons H3-12 and H3-13 to C-10 and C-11, while the C-8 methylene was linked to the CH3-14 via the oxygenated quaternary carbon C-7 (δC 74.6) as supported by the observed HMBC correlation from the methyl protons H3-14 to C-8 (Figure 2). Given the fact that only one oxygen atom existed in the structure, the linkage of C-7/O/C-11 could be constructed, leading to the formation of a tetrahydropyran moiety, which accounted for the remaining degree of unsaturation. Thus, the planar structure of 1 was assigned.

Figure 2

Key COSY (bold lines) and HMBC (arrows) correlations for compounds 1, 3/4, 5, and 6.

Analysis of the proton coupling constants and NOESY data enabled assignment of the relative configuration of 1. The appearance of the bromomethine proton H-10 as a double doublet, with coupling constants of 7.9 and 4.4 Hz, suggesting the equatorial orientation of H-10 for 1. In the NOESY spectrum, NOE correlations of H3-13 with both H-10 and H3-14 placed the methyl groups CH3-13 and CH3-14 on the same face (axial or pseudoaxial) of the tetrahydropyran ring (Figure 3). On the basis of the above evidence, the structure of 1 was determined, and the trivial name okamurene A was assigned.

Figure 3

Key NOESY correlations for compounds 1 and 2.

The 1H and 13C NMR spectroscopic data of okamurene B (2), an isomer of 1 as established by HRESIMS data, were very similar to those of 1 except for some chemical shift variations of signals corresponding to the C-8, C-9, and C-12 through C-14 (Table 1). Therefore, compound 2 was presumed to be a stereoisomer of 1. Detailed analysis of the 1H and 13C NMR data as well as 1H–1H COSY and HMBC correlations supported the conclusion that 2 possesses the same planar structure as 1. However, comparisons of the J-value and NOESY data of 2 with those of 1 revealed a difference in relative configuration at C-10. A trans-diaxial J-value for Hax-10 and Hax-9 (12.1 Hz) indicated an equatorial orientation for the Br-atom at C-10. The NOE correlation from H-10 to H3-12 in the NOESY spectrum indicated an equatorial face of CH3-12, while the NOE correlation from H3-13 to H3-14 placed these two methyl groups in axial orientation (Figure 3). Based on the above data, the structure of compound 2 was identified and it was named okamurene B.

Okamurenes C (3) and D (4) were obtained as a colorless oily mixture in a 2:1 ratio, as indicated by the 1H NMR spectrum. Attempts to separate the mixture by various CC steps using different solvent systems failed. On the other hand, there is no conjugated system in compounds 3 and 4, making these compounds unsuitable for HPLC separation using the available UV detector. A similar unseparable mixture containing (9S)- and (9R)-2-bromo-3-chloro-6,9-epoxybisabola-7(14),10-diene from L. saitoi was previously described [11]. Most of the NMR signals for compounds 3 and 4 were duplicated or overlapped. By detailed analysis of 1D and 2D NMR data, their structures were determined to be C-9 epimer of 6,9-epoxybisabola-2,7(14),10-triene.

The molecular formula of compounds 3 and 4 were determined to be C15H22O (five degrees of unsaturation) on the basis of HRESIMS data. Examination of the 1H and 13C NMR data (Table 2) revealed that they resembled 9S- and/or 9R-2-bromo-3-chloro-6,9-epoxybisabola-7(14),10-diene [11], except for the presence of signals for a trisubstituted vinyl group at C-2 and, accordingly, the lack of the resonances due to a brominated methine at C-2 and a chlorinated quaternary carbon at C-3 [11]. The chemical shifts for the vinyl carbons at δC 119.1/119.0 (C-2) and 133.4/133.7 (C-3) as well as for one of the neighboring methylene groups C-4 (δC 27.7/28.0) in the 13C NMR spectrum of 3 and 4 were very similar to those reported for 8-bromochamigra-1,11(12)-dien-9-ol (with C-2 at δC 119.4, C-3 at δC 132.9, and C-4 at δC 27.5) [12], and these data strongly supported the presence of the trisubstituted vinyl group at C-2 in 3/4. These data indicated that compounds 3 and 4 were the dehalogenated derivatives corresponding to 9S- and/or 9R-2-bromo-3-chloro-6,9-epoxybisabola-7(14),10-diene [11]. The 1H–1H COSY and HMBC correlations (Figure 2) further verified the planar structures of 3/4 to be 6,9-epoxybisabola-2,7(14),10-triene. Assignment of the relative configuration at C-6 by NOESY experiment is not applicable for compounds 3 and 4 since there is no proton around C-6 in the tetrahydrofuran ring. However, the C-6 relative configuration was tentatively assigned to be the same as that of 9S- and/or 9R-2-bromo-3-chloro-6,9-epoxybisabola-7(14),10-diene based on the similar NMR data around the chiral center, as well as on biogenetic consideration [11].

Table 2

1H- and 13C-NMR data of compounds 3–6 in CDCl3a.

No.	3		4		5		6
	δH (J in Hz)	δ C	δH (J in Hz)	δ C	δH (J in Hz)	δ C	δH (J in Hz)	δ C
1	2.25, m	38.1, CH2	2.18, m	36.8, CH2	5.54, d	131.2, CH	9.80, br s	199.3, CH
					(10.4)
2a	5.34, m	119.1, CH	5.34, m	119.0, CH	5.85, d	136.5, CH	2.67, dd	42.4, CH2
					(10.4)		(17.5, 6.2)
2b							3.06, dd
							(17.3, 7.9)
3		133.4, C		133.7, C		67.4, C	4.34, t (6.5)	72.7, CH
4a	1.93, m	27.7, CH2	1.93, m	28.0, CH2	1.56, m	28.5, CH2	4.65, dd	81.6, CH
							(8.7, 5.0)
4b	2.22, m		2.22, m		1.99, m
5a	1.58, m	31.5, CH2	1.66, m	34.5, CH2	1.78, m	36.3, CH2	2.75, m	21.7, CH2
5b	1.82, m		1.75, m				2.91, m
6		80.9, C		80.8, C		47.4, C	4.97, m	80.9, CH
7		156.3, C		156.5, C		139.5, C	4.21, m	50.4, CH
8a	2.38, m	40.1, CH2	2.38, m	40.3, CH2	5.23, m	120.8, CH	2.42, dd	41.7, CH2
							(14.1, 5.8)
8b	2.71, dd (15.7, 9.7)		2.61, dd (15.6, 9.5)				2.61, m
9	4.63, m	71.8, CH	4.63, m	72.8, CH	2.58, m	36.1, CH2	4.50, dd	74.4, CH
							(7.4, 3.5)
10	5.22, m	126.0, CH	5.22, m	126.2, CH	4.64, dd	61.4, CH	3.80, dt	64.1, CH
					(10.6, 6.4)		(11.5, 3.5)
11a		136.2, C		135.5, C		41.6, C	1.77, m	27.4, CH2
11b							1.88, m
12	1.69, s	18.2, CH3	1.70, s	18.3, CH3	1.02, s	18.1, CH3	1.07, t (7.7)	12.8, CH3
13	1.71, s	25.8, CH3	1.71, s	25.8, CH3	1.11, s	26.3, CH3
14a	4.78, br s	103.5, CH2	4.78, br s	103.8, CH2	1.57, s	21.9, CH3
14b	4.90, br s		4.91, br s
15	1.66, s	23.4, CH3	1.66, s	23.4, CH3	1.31, s	28.8, CH3

a Measured at 500 MHz for 1H and 125 MHz for 13C.

Okamurene E (5), a colorless oil, was shown to have the molecular formula of C15H23BrO by the interpretation of HRESIMS data. The IR absorption at 3401 cm−1 exhibited the presence of a hydroxyl group. The 1H NMR spectrum (Table 2) delineated four methyl singlets, one double doublet ascribable to an oxygenated/halogenated methine, and one multiplet and two doublets attributable to three olefinic protons. The 13C and DEPT NMR spectra (Table 2) displayed four methyls, three methylenes, four methines, and four quaternary carbons. Compared to the reported NMR data for 10-bromo-7α,8α-expoxychamigr-1-en-3-ol [12], compound 5 exhibited no resonances for the epoxy moiety in the NMR spectra. Instead, it showed additional signals at δH 5.23 (H-8) and δC 139.5 (C-7) and 120.8 (C-8) for a trisubstituted vinyl group, which was positioned at C-7 based on the observed HMBC correlations from H-14 to C-6, C-7, and C-8. Further analysis of the 1H–1H COSY and HMBC correlations (Figure 2) confirmed the structure of 5 as 10-bromo-1,7-chamigradien-3-ol. The relative configurations at C-3, C-6, and C-10 of 5 were deduced to be same as those of 10-bromo-7α,8α-expoxychamigr-1-en-3-ol [12] by the NOESY correlation between H-5 and H-10 as well as by their similar NMR data.

Okamuragenin (6), isolated as a colorless oil, was assigned the molecular formula C12H18Br2O3 on the basis of HRESIMS, consistent with three degrees of unsaturation. The IR spectrum exhibited strong absorptions at 2762 and 1728 cm−1, indicating the existence of an aldehyde group. In accordance with the IR signals, the 1H and 13C NMR data (Table 2) also indicated the presence of an aldehyde group at δH (9.80, H-1) and δC 199.3 (CH, C-1). The 1H–1H COSY spectrum revealed that the aldehyde group was extended to a straight spin system consisting of six methines, four methylenes, and terminated by a methyl group (Figure 2). Compound 6 was deduced to be bicyclic, since no other unsaturated functionalities were indicated by the NMR data (Table 2). The connectivity of C-3/O/C-9 was deduced by the correlation from H-3 to C-9 in the HMBC spectrum (Figure 2). Taking into account the downfield chemical shifts of C-4 (δC 81.6) and C-6 (δC 80.9) and the calculated 3 degrees of unsaturation, C-4 and C-6 had to be linked through an oxygen atom. Finally, the two remaining Br-atoms indicated by the molecular formula could only be located at C-7 and C-10 based on the chemical shifts [13]. The relative configuration was determined by NOESY experiment. The same orientation of CH2-2, H-4, and H-9 was evidenced by the NOE correlations of H-2 to H-4 and H-9, while H-9 was syn to H-7 based on the NOE correlation between them. The above data established the structure of 6, trivially named okamuragenin.

In addition to the six new compounds, the other nine sesquiterpenes including isobromocuparene [14], 7-hydroxylaurene [15], laurene [16], filiformin [17], debromofiliformin [18], 6-bromo-filiformin [19], deoxyprepacifenol [20], 2-bromo-3-chloro-2,7-epoxy-9-chamigren-8α-ol [11], and 2,10-dibromo-3-chloro-7-chamigren-9-ol [21], together with four C15-acetogenins including 3E, 12Z-laurediol [22], neolaurallene [23], E-stereoisomer of neoisoprelaurefucin [24], and 3Z-laurentin [25], were all identified by comparison of their spectral data with those previously reported.

The isolated compounds were evaluated for the brine shrimp (Artemia salina) lethal activity [26,27]. Among them, 7-hydroxylaurene was found to possess potent lethality with LD50 1.8 μM, which is more active than that of 7-hydroxylaurene acetate, allolaurinterol acetate, and laurene [12]. Analysis of structure-activity relationship showed that the 7-hydroxyl group in laurene sesquiterpenes may play a key role in the brine shrimp toxicity, and the activity reduced significantly after acetylation. The above data suggested that 7-hydroxylaurene may be a potent chemical defensive agent with cytotoxicity, although the hatchability test was not performed [27]. The other tested compounds only displayed moderate or weak activity (data not shown).

3. Experimental Section

3.1. General

IR spectra were measured on a Nicolet NEXUS 470 FT-IR spectrophotometer. Optical rotations were recorded on an Atago Polax-L polarimeter. UV spectra were determined on a Spectrumlab 54 UV-visible spectrophotometer. HRESIMS were run on a VG Autospec 3000 mass spectrometer. 1D and 2D NMR spectra were obtained at 500 and 125 MHz for 1H and 13C, respectively, on a Bruker Advance 500 MHz NMR spectrometer in CDCl3 with TMS as internal standard. Column chromatography (CC) was performed on Si gel (200–300 mesh, Qingdao Haiyang Chemical Co., Qingdao, China) and Sephadex LH-20 (Sigma). TLC was carried out with precoated Si gel plates (GF-254, Qingdao Haiyang Chemical Co., Qingdao, China).

3.2. Algal Material

The marine red alga Laurencia okamurai Yamada was collected along Weihai coastline in Shandong Province, China, in May, 2007, and was identified by B.-M. Xia, Institute of Oceanology, Chinese Academy of Sciences (IOCAS). A voucher specimen (HZ0705) has been deposited at the Key Laboratory of Experimental Marine Biology of IOCAS.

3.3. Extraction and Isolation

The dried and powdered alga L. okamurai (3.8 kg) 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 loaded to Si gel column, eluting with a step gradient of increasing EtOAc (0%–100%) in petroleum ether (PE) to give eight fractions I–VIII. Fraction II eluted with PE/EtOAc 100:1 and was further purified by preparative TLC to afford a mixture of 3 and 4 (5.6 mg). Fraction IV eluted with PE/acetone 100:1 and was further separated by preparative TLC to afford 1 (3.7 mg), 2 (4.7 mg), 6 (13.1 mg). Fraction VI eluted with PE/acetone 30:1 and was further separated by Sephadex LH-20 (MeOH) CC and preparative TLC to afford 5 (10.7 mg).

3.4. Computational Details

Okamurene A (1): Colorless oil; [α]18D +2.3 (c 0.11, MeOH); UV (MeOH) λmax (log ε) 221 (3.56) nm; IR (KBr) νmax 3065, 2964, 2857, 1514, 1479, and 1205 cm−1; 1H and 13C NMR data, see Table 1; HRESIMS m/z 297.0748 [M + H]+ (calcd for C15H2279BrO, 297.0854).

Okamurene B (2): Colorless oil; [α]18D +3.6 (c 0.06, MeOH); UV (MeOH) λmax (log ε) 221 (3.66) nm; IR (KBr) νmax 3068, 2964, 2857, 1514, 1477, and 1208 cm−1; 1H and 13C NMR data, see Table 1; HRESIMS m/z 319.0726 [M + Na]+ (calcd for C15H21BrONa, 319.0673).

Okamurenes C (3) and D (4): Colorless oil; IR (KBr) νmax 3096, 2924, 2854, 1637, 1457, and 1024 cm−1; 1H and 13C NMR data, see Table 2; HRESIMS m/z 219.1757 [M + H]+ (calcd for C15H23O, 219.1749).

Okamurene E (5): Colorless oil; [α]18D +7.6 (c 0.09, MeOH); IR (KBr) νmax 3401, 2971, 2928, 1549, 1447, 1367 and 1121 cm−1; 1H and 13C NMR data, see Table 2; HRESIMS m/z 281.0846 [M − H2O + H]+ (calcd for C15H2279Br, 281.0905), and 283.0860 [M − H2O + H]+ (calcd for C15H2281Br, 283.0884).

Okamuragenin (6): Colorless oil; [α]18D +11.2 (c 0.18, MeOH); IR (KBr) νmax 3060, 2926, 2854, 2762, 1728, 1421, and 1134 cm−1; 1H and 13C NMR data, see Table 2; HRESIMS m/z 385.9926 [M + NH4]+ (calcd for C12H22N79Br2O3, 385.9966), 387.9986 [M + NH4]+ (calcd for C12H22N79Br81BrO3, 387.9946).

3.5. Brine Shrimp Toxicity

Brine shrimp (Artemia salina) toxicity of crude extract and pure compounds was determined as detailed previously [26,27].

4. Conclusions

Four new bisabolane sesquiterpenes, okamurenes A–D (1–4), a new chamigrane derivative, okamurene E (5), and a new C12-acetogenin, okamuragenin (6), together with 13 known related metabolites, were isolated from the marine red alga L. okamurai. Among them, okamurenes A and B (1 and 2) are first examples of bromobisabolane sesquiterpenes possessing a phenyl moiety among Laurencia-derived sesquiterpenes, while okamuragenin (6) was the first acetogenin aldehyde possessing a C12-carbon skeleton. Each of the isolated compounds was evaluated for the brine shrimp (Artemia salina) lethal assay and 7-hydroxylaurene displayed potent lethality with LD50 1.8 μM.