Chermesins A–D: Meroterpenoids with a Drimane ... - ACS Publications

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Chermesins A−D: Meroterpenoids with a Drimane-Type Spirosesquiterpene Skeleton from the Marine Algal-Derived Endophytic Fungus Penicillium chermesinum EN-480 Hui Liu,†,‡ Xiao-Ming Li,† Yang Liu,†,‡ Peng Zhang,†,‡ Jia-Ning Wang,†,‡ and Bin-Gui Wang*,† †

Laboratory of Marine Biology and Biotechnology, Qingdao National Laboratory for Marine Science and Technology, Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Nanhai Road 7, Qingdao 266071, People’s Republic of China ‡ University of Chinese Academy of Sciences, Yuquan Road 19A, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Chermesins A−D (1−4), four new spiromeroterpenoids containing a drimane-type sesquiterpene skeleton, were isolated and identified from the culture extract of Penicillium chermesinum EN-480, an endophytic fungus obtained from the inner tissue of the marine red alga Pterocladiella tenuis. The structures of these new spiromeroterpenoids were elucidated based on detailed spectroscopic analyses, and their absolute configurations were confirmed by single-crystal X-ray diffraction experiments and by ECD data. This is the first report of the crystal structures of spiromeroterpenoids that contain a drimane-type sesquiterpene skeleton with a rare cyclohexa-2,5-dienone unit, which resulted in the unambiguous assignment of their relative and absolute configurations. Compounds 1 and 2 exhibited antibacterial activity against the opportunistic pathogen Micrococcus luteus, with an MIC value of 8 μg/mL.

M

arine-derived fungi continue to be a prolific source for the discovery of diverse secondary metabolites with various biological activities.1,2 During our continuous search for structurally unique and biologically active compounds from marine algal-derived endophytic fungi,3−7Penicillium chermesinum EN-480, an endophytic fungus obtained from the marine red alga Pterocladiella tenuis, attracted our attention. The HPLC-UV profile of the culture extract showed several peaks, with a characteristic UV maximum absorption at approximately 295 nm, which showed no hits in our HPLC-UV database. Chemical investigation of the EtOAc extract of the fungus resulted in the isolation and identification of chermesins A−D (1−4), four new meroterpenoids that possess a drimane-type spirosesquiterpene skeleton. Drimane-type spirosesquiterpene derivatives are a rarely described class of compounds, generally produced by the fungal genera Stachbotrys,8−11Penicillium,12 and Memnoniella,13 as well as by the marine sponge Siphonodictyon coralliphagum.14 Majority compounds in this class having a phenyl unit in the D ring, and only three of them containing a cyclohexa-2,5-dienone moiety in the D ring have been described so far.12,14 Their absolute configurations have not yet been determined because of the mixed biogenetic origin of the unconventional spiro ring system. The single crystals of compounds 1−4 were obtained during our experiments and thus allowed the assignment of their relative and absolute configurations. Herein, we describe the isolation, structure elucidation, and biological evaluation of these compounds. © XXXX American Chemical Society and American Society of Pharmacognosy

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RESULTS AND DISCUSSION The EtOAc extract of the culture filtrate of P. chermesinum EN480 was fractionated by column chromatography (CC) on silica gel, Lobar LiChroprep RP-18, and Sephadex LH-20, as well as by preparative TLC, to yield compounds 1−4. Compound 1, originally obtained as a colorless, amorphous powder, had a molecular formula of C24H36O3 based on HRESIMS data. The UV spectrum showed absorption peaks at Received: October 7, 2015

A

DOI: 10.1021/acs.jnatprod.5b00893 J. Nat. Prod. XXXX, XXX, XXX−XXX

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λmax 202, 246, and 296 nm, which are similar to that of simplicissin, a pollen growth inhibitor identified from Penicillium cf. simplicissimum (Oudemans) Thom No. 410,12 suggesting that 1 might be an analogue of simplicissin. The 1H and 13C NMR and DEPT data of 1 indicated the presence of seven methyls, five methylenes, four methines (one oxygenated and one aromatic), and eight nonprotonated carbons (Tables 1

Table 2. 1H NMR Spectroscopic Data for Compounds 1−4 (δ in ppm, J in Hz)

Table 1. 13C NMR Spectroscopic Data for Compounds 1−4 (δ in ppm)

a

no.

1a

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10′ 11′

33.0, CH2 26.8, CH2 78.6, CH 39.0, C 49.9, CH 16.9, CH2 29.1, CH2 42.4, CH 98.4, C 41.3, C 39.8, CH2 16.9, CH3 28.2, CH3 15.1, CH3 17.4, CH3 48.2, C 177.6, C 110.8, C 188.6, C 125.6, CH 158.0, C 33.3, CH3 7.8, CH3 18.8,CH3

2a 34.3, 34.4, 215.4, 48.2, 51.4, 18.1, 29.0, 42.3, 97.9, 39.9, 41.1, 17.0, 25.5, 21.8, 16.8, 48.2, 176.9, 111.3, 188.4, 125.9, 158.0, 33.4, 8.0, 19.0,

CH2 CH2 C C CH CH2 CH2 CH C C CH2 CH3 CH3 CH3 CH3 C C C C CH C CH3 CH3 CH3

3b 32.3, 26.9, 76.6, 38.9, 45.9, 26.1, 68.5, 50.1, 98.5, 40.8, 45.0, 11.5, 28.6, 16.0, 17.6, 45.1, 181.4, 107.8, 192.9, 58.0, 42.9, 21.5, 8.3, 14.5, 171.5, 52.2,

CH CH2 CH C CH CH2 CH CH C C CH2 CH3 CH3 CH3 CH3 C C C C CH CH CH3 CH3 CH3 C CH3

4b 33.5, 34.3, 214.8, 47.6, 50.1, 17.8, 28.5, 42.4, 99.2, 40.9, 38.9, 17.1, 25.6, 21.8, 17.0, 48.3, 178.1, 109.2, 184.8, 131.6, 156.0, 33.3, 8.3, 16.7, 168.3,

1a

no.

CH2 CH2 C C CH CH2 CH2 CH C C CH2 CH3 CH3 CH3 CH3 C C C C C C CH3 CH3 CH3 C

a

1α

1.71, m

1β

1.48, m

2α 2β 3 5 6

1.49, m 1.72, m 3.22, dd (10.8, 4.7) 0.84, m 1.52, m

7 8 11a

1.58, m 1.56, m 1.70, d (12.8)

11b

2.16, d (12.8)

12 13α 14β 15 5′ 6′ 7′ 8′ 9′ 11′

1.10, 1.01, 0.84, 1.22, 5.90,

d (7.1) s s s s

1.48, s 1.81, s 1.98, s

2a 2.36, d (15.2) 2.03, d (15.2) 2.81, m 1.87, m

1.29, m 1.71, m 1.55, m 1.62, m 1.60, m 1.73, d (13.0) 2.12, d (13.0) 1.15, d (7.1) 1.10, s 1.10, s 1.45, s 5.90, s 1.46 s 1.83, s 1.97, s

3b 1.33, d (12.2) 1.44, d (12.2) 1.50, m (overlap) 3.06, dd (10.6, 4.5) 0.82, m 1.50, m (overlap) 3.68, m 1.83, d (6.2) 1.73, d (13.6) 2.28, d (13.6) 0.98, 0.92, 0.72, 1.08, 3.30, 2.23, 1.14, 1.60, 0.91, 3.64,

d (7.3) s s s d (3.5) dt (6.0, 3.5) s s d (6.0) s

4b 2.21, d (14.7) 1.94, d (14.7) 2.80, m 1.80, m

1.39, m 1.80, m 1.50, m 1.63, m 1.50, m 1.80, d (13.6) 2.44, d (13.6) 1.10, d (7.2) 1.00, s 0.99, s 1.44, s

1.34, s 1.70,s 1.98, s

Measured in CDCl3. bMeasured in DMSO-d6.

Measured in CDCl3. bMeasured in DMSO-d6.

and 2). Two methyl groups (C-13 and C-14) attached to the quaternary carbon C-4 were identified by their mutual HMBC correlations, along with correlations from H3-13 and H3-14 to C-3, C-4, and C-5 (Figure 1). The drimane-type sesquiterpene moiety was further confirmed by the HMBC correlations from H3-15 to C-1, C-5, C-9, and C-10 and from H3-12 to C-7 and C-8. The remaining part of 1 was established based on the HMBC correlations from H3-7′ to C-1′, C-2′, C-6′, and C-11; from H3-9′ to C-1′, C-5′, and C-6′; and from H3-8′ to C-2′, C3′, and C-4′. Careful comparison of the NMR data of 1 with that of simplicissin revealed that the structures of these two compounds are very similar, except that the signals for the methyl formate unit and the nonprotonated olefinic carbon (C5′) in simplicissin were absent in the NMR spectra of 1. Rather, resonances for an olefinic methine at δC 125.6 (C-5′) and δH 5.90 (H-5′) were observed in the NMR spectra of 1. The planar structure of 1 was thus determined. The relative configuration of 1 was assigned by analyzing its coupling constant (J-value) and NOESY data (Figure 2). A typical large coupling value for H-3 (J = 10.8 Hz) indicated the axial orientation of this proton, which is different from that of simplicissin.12 The NOE correlations from H-3 to H-5 and H313 and from H-5 to H-11b in the NOESY spectrum revealed

Figure 1. Key HMBC (arrows) and COSY (bold lines) correlations for compounds 1−3.

the cofacial relationship of these protons, while the correlations from H3-15 to H3-12 and H3-14 placed these groups on the opposite side of the molecule. However, the relative configuration of the stereogenic center C-1′ could not be determined by the NOESY experiment due to the vertical relationship from rings A and B to rings C and D; moreover, no diagnostic NOE correlation was observed that could correlate the relative configuration of C-1′ to rings A and B of the molecule. Upon slow evaporation of a solution of 1 in acetone, quality single crystals were obtained. The relative and absolute configurations of 1 were thus unambiguously established by a single-crystal X-ray diffraction experiment using Cu Kα radiation (Figure 3). The refined Flack parameter [0.0(3)] allowed for the establishment of the absolute configuration of 1 as 3S, 5S, 8S, 9S, 10S, and 1′R. On the basis of the above data, the structure of 1 was determined, and the trivial name chermesin A was assigned to this compound. B

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disappeared in the NMR spectra of 2 (Tables 1 and 2). Rather, a signal indicative of a keto group at δC 215.4 (C-3) was observed in the 13C NMR spectrum of 2. These data indicated the replacement of the −CHOH group in 1 by a keto group in 2, which was consistent with the difference in the molecular formula. The key HMBC correlations from H2-1, H2-2, H3-13, and H3-14 to C-3 verified the above deduction (Figure 1). The NOE correlation from H-5 to H-11b revealed that these protons are on the same side, while the correlations from H3-15 to H3-12 indicated that they are on the other side. However, the relative configuration of the chiral center C-1′ in compound 2 was not determined by NOESY. The relative and absolute configurations of 2 were also confirmed by a single-crystal X-ray diffraction experiment (Figure 3). The final refinement of the Cu Kα data resulted in a 0.0(5) Flack parameter, allowing for the unambiguous assignment of the absolute configuration as 5R, 8S, 9S, 10S, and 1′R. Chermesin C (3) was originally obtained as an amorphous powder and was found to have the molecular formula C26H40O6, with one oxygen and two proton atoms more than simplicissin (on the basis of HRESIMS data). The general features of the 1H and 13C NMR data of 3 (Tables 1 and 2) resembled those reported for simplicissin,12 and the major difference was observed for the signals of C-7, C-5′, and C-6′. Compared to simplicissin, deshielded resonances for the methine C-7 (δC 68.5/δH3.68, Tables 1 and 2) indicated the replacement of the methylene C-7 in simplicissin by an oxymethine in 3. Additionally, signals for the nonprotonated C5′ and C-6′ corresponding to Δ5 in simplicissin were replaced by two aliphatic methines at δC 58.0/δH 3.30 (CH-5′) and δC 42.9/δH 2.23 (CH-6′) in 3 (Tables 1 and 2). The observed COSY correlations from H2-6 to H-5 and H-7 and from H-6′ to H-5′ and H3-9′, as well as HMBC correlations from H-8 and H3-12 to C-7 and from H-5′ to C-4′, C-6′, C-9′, and C-10′ (Figure 1), supported the above conclusion. The NOESY spectrum of 3 revealed correlations from H-3 to H3-13 and from H-5 to H-3, H-7, and H-11b, demonstrating that these protons are on the same side, while NOE correlations from H315 to H3-12 and H3-14 revealed that these groups are on the other side. Although correlations from H-5′ to H3-7′ and H3-9′ were observed, which revealed these protons were on the same side of ring D, however, the relative configurations of C-1′, C5′, and C-6′ could not be unambiguously correlated to rings A and B due to the vertical relation of rings A and B with rings C and D. A single crystal suitable for an X-ray diffraction experiment of 3 was obtained after many attempts, and the Xray data enabled the assignment of the absolute configuration of 3 as 3S, 5S, 7S, 8S, 9S, 10S, 1′R, 5′S, and 6′S (Figure 3). Chermesin D (4), obtained as a colorless crystal, was determined to possess the molecular formula C25H34O5, with one CO2 unit more than 2 (on the basis of HRESIMS data). The 1D NMR data assignment of 4 matched well with those of the related signals for 2, except for the absence of the olefinic methine (C-5′) and the appearance of the carboxylic carbonyl carbon (C-10′) (Tables 1 and 2). Accordingly, the signals for the olefinic methine at δC 125.9 (C-5′) and δH 5.90 (H-5′) in the NMR spectra of 2 disappeared in those of 4. Rather, a nonprotonated olefinic carbon at δC 131.6 (C-5′) and a carboxylic carbonyl carbon at δC 168.3 (C-10′) were observed in the NMR spectra of 4 (Tables 1 and 2). The structure, including the relative and absolute configurations, of 4 was determined based on the X-ray diffraction analysis (Figure 3),

Figure 2. Key NOESY correlations for compounds 1−3.

Chermesin B (2) was initially obtained as an amorphous powder. Its molecular formula C24H34O3, with two proton atoms less than 1, was determined by HRESIMS. The 1H and 13 C NMR data of this compound are very similar to those of 1 (Tables 1 and 2), suggesting the same carbon skeleton for both compounds. However, resonances at δC 78.6 (C-3) and δH 3.22 (H-3) for the oxymethine group at the C-3 position on 1 C

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Figure 3. X-ray crystallographic structures of compounds 1−4 (note: different numbering systems are used for the structures in the text).

D, exhibited weak negative and positive CEs near 301 and 270 nm, respectively, which are quite different from those of compounds 1, 2, and 4 (Figure 4). Compounds 1−4 were evaluated for their antimicrobial activities against four human pathogens (Candida albicans, Escherichia coli, Micrococcus luteus, and Pseudomonas aeruginosa), five aquatic bacteria (Aeromonas hydrophila, Edwardsiella tarda, Vibrio alginolyticus, V. harveyi, and V. parahemolyticus), and five plant-pathogenic fungi (Alternaria brassicae, Colletotrichum gloeosporioides, Fusarium oxysporum, Gaeumannomyces graminis, and Physalospora piricola). Chermesins A (1) and B (2) exhibited activity against C. albicans, E. coli, M. luteus, and V. alginolyticus, with MIC values ranging from 8 to 64 μg/mL (Table 3), whereas chermesin D (4) showed only weak activity against E. coli (MIC = 64 μg/mL). Chermesin C (3) was not active against any of the tested strains. These results indicated that the double bond at C-5′ or the cyclohexa-2,5-dienone unit is essential for the antimicrobial activity (1 and 2 vs 3 and 4) and that the C-5′ substitution of the −COOH group significantly decreased the activity (2 vs 4). Compounds 1−4 were also examined for brine shrimp lethality against Artemia salina; none of the compounds exhibited activity at a concentration of 100 μg/mL (lethal rate 2σ(I)]. The Flack parameter was 0.0(3) in the final refinement for all 13 554 reflections, with 3857 Friedel pairs. Crystal data for compound 2: C24H34O3; fw = 370.51; orthorhombic space group P2(1)2(1)2(1); unit cell dimensions a = 8.4475(7) Å, b = 10.3954(9) Å, c = 24.175(2) Å, V = 2123.0(3) Å3, α = β = γ = 90°; Z = 4; dcalcd. = 1.159 mg/m3; crystal dimensions 0.43 × 0.12 × 0.08 mm; μ = 0.583 mm−1; and F(000) = 808. The 4566 measurements yielded 3072 independent reflections after equivalent data were averaged and Lorentz and polarization corrections were applied. The final refinement provided R1 = 0.0589 and wR2 = 0.1086 [I > 2σ(I)]. The Flack parameter was 0.0(5) in the final refinement for all 4566 reflections, with 3072 Friedel pairs. Crystal data for compound 3: C26H40O6·CH3OH; fw = 480.62; orthorhombic space group P2(1)2(1)2(1); unit cell dimensions a = 11.5247(6) Å, b = 12.6097(6) Å, c = 18.3600(10) Å, V = 2668.1(2) Å3, α = β = γ = 90°; Z = 4; dcalcd. = 1.196 mg/m3; crystal dimensions 0.35 × 0.30 × 0.27 mm; μ = 0.687 mm−1; and F(000) = 1048. The 5600 measurements yielded 3909 independent reflections after equivalent data were averaged and Lorentz and polarization corrections were applied. The final refinement provided R1 = 0.0534 and wR2 = 0.1171 [I > 2σ(I)]. The Flack parameter was 0.1(3) in the final refinement for all 5600 reflections, with 3909 Friedel pairs. Crystal data for compound 4: C25H34O5·CH3OH; fw = 446.56; orthorhombic space group P2(1)2(1)2(1); unit cell dimensions a = 8.0099(5) Å, b = 8.5188(4) Å, c = 35.9835(18) Å, V = 2455.3(2) Å3, α = β = γ = 90°; Z = 4; dcalcd. = 1.208 mg/m3; crystal dimensions 0.45 × 0.44 × 0.40 mm; μ = 0.682 mm−1; and F(000) = 968. The 5403 measurements yielded 3597 independent reflections after the equivalent data were averaged and Lorentz and polarization

Table 3. Antimicrobial Activity of Compounds 1−4 (MIC, μg/mL)a 1 2 3 4 positive control

CA

EC

ML

VA

32 64 n.a. n.a. 2b

32 64 n.a. 64 2c

8 8 n.a. n.a. 2c

32 64 n.a. n.a. 4c

a

CA: C. albicans. EC: E. coli. ML: M. luteus. VA: V. alginolyticus. n.a.: no activity. bPositive control: amphotericin B. cPositive control: chloramphenicol.

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EXPERIMENTAL SECTION

General Experimental Procedures. Melting points were measured using an SGW X-4 micro-melting-point apparatus. Optical rotations were determined on an Optical Activity AA-55 polarimeter. UV spectra were recorded on a PuXi TU-1810 UV−visible spectrophotometer. ECD spectra were acquired on a Chirascan spectropolarimeter. 1D and 2D NMR spectra were obtained at 500 and 125 MHz for 1H and 13C, respectively, on a Bruker Avance 500 MHz spectrometer with TMS as the internal standard. Mass spectra were acquired using a VG AutoSpec 3000 or an API QSTAR Pulsar 1 mass spectrometer. Analytical HPLC was performed on a Dionex HPLC system equipped with a P680 pump, ASI-100 automated sample injector, and UVD340U multiple wavelength detector controlled by Chromeleon software (version 6.80). Sodium glutamate was obtained from LianHua Weijing Co, Ltd. (Henan, China). Commercially available silica gel (200−300 mesh, Qingdao Haiyang Chemical Co.), Lobar LiChroprep RP-18 (40−63 μm, Merck), and Sephadex LH-20 (Pharmacia) were used for open-column chromatography. All solvents used were distilled prior to use. Fungal Material. The endophytic fungus Penicillium chermesinum EN-480 was isolated from the marine red alga Pterocladiella tenuis, which was collected at Rongcheng, Shandong Province of P. R. China, in July 2014, using a protocol described in our previous report.7 Its ITS region of the rDNA was used for taxonomic identification, as described in our previous report.7 The sequence of the fungus, which was the most similar (99%) to the sequence of Penicillium chermesinum (accession no. KJ767051), has been deposited in GenBank (accession no. KT119566). This strain is stored at the Key Laboratory of Experiment Marine Biology, Institute of Oceanology of the Chinese Academy of Sciences (IOCAS). Fermentation, Extraction, and Isolation. The fungal strain P. chermesinum EN-480 was grown on PDA medium at 28 °C for 4 days, which was inoculated statically for 30 days in 90 × 1 L Erlenmeyer flasks with rice solid medium at room temperature (each flask contained 70 g of rice, 0.1 g of corn flour, 0.3 g of peptone, 0.1 g of sodium glutamate, and 100 mL of natural, filtered seawater that was obtained from the Huiquan Gulf of the Yellow Sea near the campus of IOCAS, pH 6.5−7.0). The whole fermentation product was extracted three times with EtOAc, which was concentrated under reduced pressure to obtain an extract (23 g). The extract was fractionated by vacuum liquid chromatography (VLC) on silica gel, eluting with mixed solvent systems of increasing polarity from petroleum ether (PE)− acetone (from 50:1 to 1:1) to chloroform−methanol (from 20:1 to 1:1), to give 10 fractions (Fr. 1−10), as monitored by TLC analysis. Fr. 4 (1.2 g) was further separated by reversed-phase CC over Lobar LiChroprep RP-18 with a MeOH−H2O gradient (from 10:90 to 100:0) to yield 10 subfractions (Fr. 4.1−4.10). Fr. 4.4 was purified by CC on Sephadex LH-20 (MeOH) and then by preparative TLC (plate: 20 × 20 cm, developing eluent: CHCl3−acetone, 5:1) to yield compounds 1 (12.2 mg) and 2 (7.9 mg). Fr. 4.2 was recrystallized from MeOH to afford compound 4 (3.0 mg). Fr. 5 (1.0 g) was fractionated by CC over Lobar LiChroprep RP-18 (MeOH−H2O, from 10:90 to 100:0) and then over Sephadex LH-20 (MeOH) to obtain compound 3 (5.0 mg). E

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corrections were applied. The final refinement provided R1 = 0.0448 and wR2 = 0.1162 [I > 2σ(I)]. The Flack parameter was 0.0(3) in the final refinement for all 5403 reflections, with 3597 Friedel pairs. Antimicrobial Activity. The antibacterial assays against four human pathogens (C. albicans, E. coli, M. luteus, and P. aeruginosa) and five aquatic bacteria (A. hydrophila, E. tarda, V. alginolyticus, V. harveyi, and V. parahemolyticus), as well as five plant-pathogenic fungi (A. brassicae, C. gloeosporioides, F. oxysporum, G. graminis, and P. piricola), were performed using a microplate assay.19 Chloramphenicol and amphotericin B were used as positive controls against bacteria and fungi, respectively. Brine Shrimp Lethality Assay. The brine shrimp (Artemia salina) lethality was evaluated according to a previously reported method.20

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(13) Lam, Y. K. T.; Wichmann, C. F.; Meinz, M. S.; Guariglia, L.; Giacobbe, R. A.; Mochales, S.; Kong, L.; Honeycutt, S. S.; Zink, D.; Bills, G. F.; Huang, L.; Burg, R. W.; Monaghan, R. L.; Jackson, R.; Reid, G.; Maguire, J. J.; Mcknight, A. T.; Ragan, C. I. J. Antibiot. 1992, 45, 1397−1403. (14) Chan, J. A.; Freyer, A. J.; Carte, B. K.; Hemiing, M. E.; Hofmann, G. A.; Mattern, M. R.; Mentzer, M. A.; Westley, J. W. J. Nat. Prod. 1994, 57, 1543−1548. (15) Crystallographic data of compounds 1−4 have been deposited in the Cambridge Crystallographic Data Centre as CCDC 1412760, 1412761, 1412762, and 1412644, respectively. The data can be obtained free of charge via http://www.ccdc.cam.ac.uk/data_request/ cif (or from the CCDC, 12 Union Road, Cambridge CB21EZ, U.K.; fax: +44-1223-336-033; e-mail: [email protected]). (16) Sheldrick, G. M. SADABS, Software for Empirical Absorption Correction; University of Göttingen: Germany, 1996. (17) Sheldrick, G. M. SHELXTL, Structure Determination Software Programs; Bruker Analytical X-ray System Inc.: Madison, WI, 1997. (18) Sheldrick, G. M. SHELXL-97 and SHELXS-97, Program for X-ray Crystal Structure Solution and Refinement; University of Göttingen: Germany, 1997. (19) Pierce, C. G.; Uppuluri, P.; Tristan, A. R.; Wormley, F. L., Jr.; Mowat, E.; Ramage, G.; Lopez-Ribot, J. L. Nat. Protoc. 2008, 3, 1494− 1500. (20) Meyer, B. N.; Ferrigni, N. R.; Putnam, J. E.; Jacobsen, L. B.; Nichols, D. E.; McLaughlin, J. L. Planta Med. 1982, 45, 31−34.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.5b00893. Selected 1D and 2D NMR spectra of compounds 1−4 (PDF) X-ray crystallographic file of 1 (CIF) X-ray crystallographic file of 2 (CIF) X-ray crystallographic file of 3 (CIF) X-ray crystallographic file of 4 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-532-82898553. E-mail: [email protected]. cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Natural Science Foundation of China (NSFC Grant No. 31330009) and from the NSFCShandong Joint Fund for Marine Science Research Centers (U1406402) is gratefully acknowledged.

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REFERENCES

(1) Gerwick, W. H.; Fenner, A. M. Microb. Ecol. 2013, 65, 800−806. (2) Blunt, J. W.; Copp, B. R.; Keyzers, R. A.; Munro, M. H. G.; Prinsep, M. R. Nat. Prod. Rep. 2015, 32, 116−211. (3) Zhang, P.; Mandi, A.; Li, X. M.; Du, F. Y.; Wang, J. N.; Li, X.; Kurtan, T.; Wang, B. G. Org. Lett. 2014, 16, 4834−4837. (4) Zhang, P.; Li, X. M.; Li, X.; Wang, B. G. Phytochem. Lett. 2015, 12, 182−185. (5) Zhang, P.; Li, X. M.; Wang, J. N.; Li, X.; Wang, B. G. Chin. Chem. Lett. 2015, 26, 313−316. (6) Li, X.; Li, X. M.; Xu, G. M.; Li, C. S.; Wang, B. G. Phytochem. Lett. 2014, 7, 120−123. (7) Wang, S.; Li, X. M.; Teuscher, F.; Li, D. L.; Diesel, A.; Ebel, R.; Proksch, P.; Wang, B. G. J. Nat. Prod. 2006, 69, 1622−1625. (8) Kaise, H.; Shinohara, M.; Miyazaki, W.; Izawa, T.; Nakano, Y.; Sugawara, M.; Sugiura, K.; Sasaki, K. J. Chem. Soc., Chem. Commun. 1979, 726−727. (9) Ayer, W. A.; Miao, S. Can. J. Chem. 1993, 71, 487−493. (10) Sakai, K.; Watanabe, K.; Masuda, K.; Tsuji, M.; Hasumi, K.; Endo, A. J. Antibiot. 1994, 48, 447−456. (11) Kaneto, R.; Dobashi, K.; Kojima, I.; Sakai, K.; Shibamoto, N.; Yoshioka, T.; Nishida, H.; Okamoto, R.; Akagawa, H.; Mizuno, S. J. Antibiot. 1994, 48, 727−730. (12) Miyako, K.; Hirocultural, K.; Jun, U.; Shozo, F.; Tsuyoshi, K.; Yasuo, K. Biosci., Biotechnol., Biochem. 1997, 61, 2153−2155. F

DOI: 10.1021/acs.jnatprod.5b00893 J. Nat. Prod. XXXX, XXX, XXX−XXX