Streptenols F–I Isolated from the Marine-Derived ... - ACS Publications

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Streptenols F−I Isolated from the Marine-Derived Streptomyces misionensis BAT-10-03-023 Guillermo Tarazona, Carmen Schleissner, Pilar Rodríguez, Marta Pérez, Librada Ma Cañedo,* and Carmen Cuevas Research & Development Department, PharmaMar S. A., Pol. Ind. La Mina Norte, Avenida de los Reyes 1, 28770, Colmenar Viejo, Madrid, Spain S Supporting Information *

ABSTRACT: A marine-derived bacterium, Streptomyces misionensis BAT-10-03-123, has produced four new streptenol derivatives, F, G, H, and I (1−4), as well as the known streptenols A and C (5 and 6). Their planar structures were elucidated by detailed analysis of spectroscopic data. The absolute configurations of the new streptenol compounds were determined by chemical and spectroscopic methods, including Mosher’s ester method. All of the compounds were tested for cytotoxicity against four selected cancer cell lines.

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errestrial microorganisms have been distinguished as an important source of bioactive natural products with structural diversity. Many of them have been isolated from actinomycetes and have contributed to the discovery of pharmaceutically useful compounds.1 Marine-derived bacteria constitute a promising source of new secondary metabolites with diverse chemical structures and interesting biological activities for drug development,2,3 and particularly marine actinobacteria represent an attractive resource for new bioactive compound screening.4 During our effort to discover new cytotoxic compounds from marine microorganisms, Streptomyces misionensis BAT-10-03123 was selected for its cytotoxic activity against the human tumor cell lines A-549 (lung), HT-29 (colon), MDA-MB-231 (breast), and PSN1 (pancreas). Extensive chromatography of the active extract led to the isolation of four new compounds, streptenols F (1), G (2), H (3), and I (4), as well as the previously reported streptenols A (5) and C (6).5−7 Investigation of the biological activities of streptenols A, B, C, and D have shown that these compounds are potent cholesterol biosynthesis inhibitors,7 but only streptenol A has been reported to have antitumor and immunostimulating activity,6 with no activity data being available for streptenol E.8 Herein, details are reported regarding the structure elucidation of the new streptenols F−I (1−4), including their absolute configurations and their cytotoxic activities. The structures of 5 and 6 were confirmed by comparison of their spectroscopic data with previously reported values.5−7 Compound 1 was obtained as a colorless, amorphous oil that was shown to have the molecular formula C11H18O3 by (+)-HRESI-TOFMS in combination with 1H and 13C NMR © 2017 American Chemical Society and American Society of Pharmacognosy

spectroscopic data (Table 1). Comparison of the NMR data (CD3OD) for 1 with those for previously reported 6 (Table 1) revealed the two compounds to be almost identical, except that an additional methoxy group (δH 3.32, δC 57.4) was found in 1. The different chemical shifts of C-2, C-3, and C-4 suggested that this new methoxy group was located at C-3, and this was confirmed by HMBC correlations between the methoxy protons (δH 3.32) and C-3 (δC 76.6). Detailed analysis of the Received: November 16, 2016 Published: February 23, 2017 1034

DOI: 10.1021/acs.jnatprod.6b01057 J. Nat. Prod. 2017, 80, 1034−1038

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Table 1. NMR Spectroscopy Data for 1 and 2 in CD3OD 1

2

pos.

δC, type

δH, mult (J in Hz)

δC, type

1 2 3 4

59.4, 38.2, 76.6, 45.9,

CH2 CH2 CH CH2

3.64, 1.73, 3.87, 2.90, 2.69,

dd (6.2, 6.2) m m dd (15.6,7.4) dd (15.6,5.2)

67.2, 38.4, 66.3, 51.0,

CH2 CH2 CH CH2

3.57, 1.64, 4.15, 2.55,

m m m m

201.7, 129.1, 145.7, 142.4, 131.6, 18.8, 57.4,

C CH CH CH CH CH3 CH3

6.14, 7.25, 6.30, 6.30, 1.88, 3.32,

d (15.5) dd (15.5, 9.4) m (15.2a) m (15.2a) d (5.1) s

211.7, 44.2, 27.6, 131.0, 126.3, 18.0,

C CH2 CH2 CH CH CH3

2.52, 2.22, 5.44, 5.44, 1.63,

m m m m d (5.0)

59.4, 38.2, 75.2, 46.3,

CH2 CH2 CH CH2

3.66, 1.74, 3.96, 2.91, 2.70,

m m m ddd (15.4, 7.3, 1.7) ddd (15.4, 7.7, 5.1)

201.8, 129.2, 145.7, 142.4, 131.6, 18.9,

C CH CH CH CH CH3

6.15, 7.25, 6.31, 6.30, 1.88,

d (15.4) m m m d (5.0)

5 6 7 8 9 10 OMe 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10′ a

δH, mult (J in Hz)

Value measured by decoupling experiments.

Figure 1. ΔδS−R values of the MTPA esters of compounds 2 and 4.

eight methines, of which six were olefinic, and two ketone carbonyls. Analysis of the COSY spectrum revealed the presence of spin systems assignable to streptenol A and streptenol C units. The deshielded methine carbon signals with chemical shift values at δC 67.2 and 75.3, assigned to C-1 and C-3′, respectively, were consistent with an ether linkage. The connectivity of the two streptenol fragments was also determined by HMBC correlation from H-1 to C-3′ and H3′ to C-1. Thus, compound 2 was identified as an ether dimer of streptenol A and streptenol C. The absolute configuration of 2 at C-3 was elucidated by application of the modified Mosher’s ester method.12−14 Analysis of the 1H NMR data for the (S)MTPA and (R)-MTPA esters of 2 confirmed the absolute configuration of C-3 as S, as shown in Figure 1, identical to that of streptenols A and C with positive values for the specific rotation. Compound 2 practically lacks optical activity ([α]25D +0.80), which led us to consider an R absolute configuration for C-3′. However, without further data due to the small amount

2D NMR data established 1 as the 3-O-methyl analogue of 6, and its name was assigned as streptenol F. All of the C−C double-bond geometries were assigned as E, on the basis of the 3 JHH coupling constants (3JH6H7 = 15.5 Hz, 3JH8H9 = 15.2 Hz). Given the high degree of similarity of the NMR data between compounds 1 and 6, similar specific rotations (1 [α]25D +23.3, 6 [α]25D +23.4),5 and a likely biosynthetic relationship between the two metabolites, we tentatively propose the absolute configuration of the stereogenic center at C-3 in 1 to be S, the same as for (+)-streptenol A (5)7,10 and (+)-streptenol C (6),5 whose absolute configurations have been confirmed by stereoselective total syntheses. 9−11 This assumption is supported by literature examples.19−21 Compound 2 was obtained as a colorless oil. The molecular formula was established to be C20H32O5 on the basis of the (+)-HRESI-TOFMS data, indicating five degrees of unsaturation. The 13C NMR spectrum of 2 (Table 1) displayed 20 signals, which were assigned with the help of HSQC data to two methyls, eight methylenes, of which two were CH2-O, 1035

DOI: 10.1021/acs.jnatprod.6b01057 J. Nat. Prod. 2017, 80, 1034−1038

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Table 2. NMR Spectroscopy Data for 3 and 4 in CD3OD 3 pos. 1 2 3 4 5 6 7 8 9 10 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10′ a

δC, type 66.8, 38.2, 66.7, 48.0, 201.8, 129.2, 145.7, 142.4, 131.7, 18.9, 59.5, 38.4,

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

75.3, CH 46.3, CH2 202.0, 129.2, 145.7, 142.4, 131.7, 18.9,

C CH CH CH CH CH3

4 δH, mult (J in Hz) 3.59, 1.67, 4.16, 2.70,

m m m m

6.12, 7.24, 6.30, 6.30, 1.88, 3.66, 1.71,

d (15.7) m m m d (5.0) m m

δC, type 57.2, 38.1, 66.4, 51.2, 211.5, 44.3, 27.5, 130.9, 126.7, 18.0, 60.7, 35.9,

3.96, m 2.90, dd (15.6, 7.4) 2.69, dd (15.6, 5.2) 6.12, 7.24, 6.30, 6.30, 1.88,

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

65.1, CH 43.2, CH2 102.4, 37.8, 27.6, 125.9, 131.9, 18.0,

d (15.7) m m m d (5.0)

C CH2 CH2 CH CH CH3

δH, mult (J in Hz) 3.47, 1.68, 4.20, 2.60,

m m dddd (7.6, 7.6, 5.0, 5.0) m

2.53, 2.22, 5.48, 5.48, 1.62, 3.65, 1.84, 1.41, 3.96, 2.06, 1.22,

dd (7.3, 7.3) m m (15.2a) m (15.2a) m m m dddd (12.1, 12.1, 12.1, 5.6) dddd (12.1, 11.1, 4.6, 4.6) dd (12.5, 4.7) dd (12.5, 11.1)

1.78, 2.00, 5.48, 5.48, 1.62,

m m m (15.2a) m (15.2a) m

Value measured by decoupling experiments.

4′, and H-6′ with C-5′ at 102 ppm. Analysis of the ROESY NMR spectrum and 1H−1H coupling constants (J values) determined the relative configuration of the tethrahydropyran ring of 4. The coupling constants of H-3′ (δH 3.96, J = 12.1, 11.1, 4.6, 4.6) indicated the equatorial position of the hydroxy group, and the relative configuration at the ketal C-5′ in the ring was established by the ROEs found between axial protons H-1′ and H-3′ and both of them with H-1 (Figure 2).

isolated, we could not establish the absolute configuration of C3′ in compound 2. Compound 3 was also isolated as a colorless oil, with the molecular formula C20H30O5 being deduced from (+)-HRESITOFMS measurements, indicating six degrees of unsaturation. The 1H and 13C NMR data of 3 (Table 2) were very similar to those of 2, with the only difference in the NMR spectra between 3 and 2 being that two methylenes (δC 44.2, C-6 and δC 27.6, C-7) in 2 were replaced by a double bond (δC 129.2, C-6 and δC 145.7, C-7) in 3. These data suggested that compound 3 was composed of two sets of similar structural fragments, and detailed analysis of 2D NMR, COSY, HSQC, and HMBC experiments revealed that both components had a streptenol C framework. The connectivity of the two monomers of streptenol C by an ether linkage was clarified by the HMBC correlation of H-3′ to C-1. Compound 3 has two stereogenic centers (C-3 and C-3′) like 2, and we speculate that 3 has the same configuration (3S) as 2, as suggested by the very similar 1D NMR data, similar specific rotations (3 [α]25D +0.25; 2 [α]25D +0.80), and assuming the same streptenol biogenetic pathway. As in compound 2, the absolute configuration of C-3′ could not be assigned. Compound 4, isolated as an optically active, colorless oil ([α]25D +23.0 (c 0.05, MeOH), gave the molecular formula C20H34O5, as calculated from (+)-HRESI-TOFMS measurements. The molecular formula afforded four degrees of unsaturation. The 1H and 13C NMR data of 4 analyzed with the help of HSQC data showed the presence of two methyls, 10 methylenes, of which two were CH2-O, six methines, of which four were olefinic, and two nonprotonated carbons consisting of a ketone carbonyl and a ketal carbon. Detailed analysis of COSY and HMBC spectra allowed construction of a streptenol A moiety and a tetrahydropyran ring. The linkage from C-1 to C-10′, through a ketal carbon (C-5′, δC 102.4), was established by 2D NMR, including HMBC correlations from H-1, H-1′, H-

Figure 2. Selected HMBC and ROESY correlations for compound 4.

The absolute configurations at C-3 and C-3′ in 4 were also solved by synthesis of the MTPA-diesters, in which Δδ values obtained from the 1H NMR spectrum indicated that the absolute configuration was S for both stereocenters. The Δδ values of both MTPA-diesters of 4 are shown in Figure 1. On the basis of the relative configurations of the tetrahydropyran ring, the absolute configuration of C-5′ was shown to be R. All isolated compounds were evaluated for their potential cytotoxic activity against A-549 human lung carcinoma cells, MDA-MB-231 human breast adenocarcinoma cells, HT-29 human colorectal carcinoma cells, and PSN1 human pancreatic adenocarcinoma cells. None of them displayed any significant 1036

DOI: 10.1021/acs.jnatprod.6b01057 J. Nat. Prod. 2017, 80, 1034−1038

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cytotoxicity, with GI50 > 10 μM (Table S1, Supporting Information).

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system, and gradient elution using mixtures of n-hexane−EtOAc and EtOAc−MeOH gave seven fractions (A1−A7). The active fraction A3 that eluted with EtOAc (2.4 g) was sequentially extracted by liquid− liquid extraction, first with H2O−n-hexane and then with H2O− EtOAc. The EtOAc fraction (1.7 g) rich in the target metabolites (1− 6) was subjected to preparative reversed-phase HPLC using an XBridge column (19 × 150 mm, 5 μm) and a linear gradient of H2O− CH3CN from 20% to 50% of aqueous CH3CN in 31 min at a flow rate of 13 mL/min and afforded seven peak fractions (B1−B7). Fractions B1, B3, B5, B6, and B7 were further purified by semipreparative HPLC on an Ascentis Phenyl column (10 × 250 mm, 5 μm) using a linear gradient elution of H2O−MeOH from 60% to 80% aqueous MeOH in 20 min at a flow of 5 mL/min. Compound 6 (tR 4.0 min, 2.0 mg) was obtained from B1, compound 1 (tR 5.5 min, 1.0 mg) was obtained from B3, compound 5 (tR 4.5 min, 15.6 mg) and compound 2 (tR 6.5 min 1.3 mg) were isolated from B5, compound 3 (tR 24.0 min, 11.0 mg) was isolated from B6, and compound 4 (tR 28.0 min, 20.8 mg) was obtained from B7. Streptenol F (1): colorless oil; [α]25D +23 (c 0.04, MeOH); IR (CHCl3) νmax 3384, 2927, 1683, 1635, 1590, and 1375 cm−1; UV (MeOH) λmax (log ε) 193 (3.53), 227 (3.29), 275 (3.70) nm; 1H NMR (CD3OD, 500 MHz) and 13C NMR (CD3OD, 125 MHz), Table 1; ESIMS m/z 221 [M + Na]+; HRESIMS m/z 221.1152 [M + Na]+ (calcd for C11H18O3Na, 221.1148). Streptenol G (2): yellow oil; [α]25D +0.80 (c 0.08, MeOH); IR (CHCl3) νmax 3423, 2937, 2324, 1710, 1367, and 1367 cm−1; UV (MeOH) λmax (log ε) 194 (3.86), 222 (3.62), 275 (3.02) nm; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz), Table 1; ESIMS m/z 375 [M + Na]+; HRESIMS m/z 375.2138 [M + Na]+ (calcd for C20H32O5Na, 375.2142). Streptenol H (3): yellow oil; [α]25D +0.25 (c 0.11, MeOH); IR (CHCl3) νmax 3388, 2931, 1683, 1635, and 1375 cm−1; UV (MeOH) λmax (log ε) 194 (3.81), 225 (3.81), 272 (3.61) nm; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz), Table 2; ESIMS m/z 373 [M + Na]+; HRESIMS m/z 373.1994 [M + Na]+ (calcd for C20H30O5Na, 373.1985). Streptenol I (4): colorless oil; [α]25D +23 (c 0.05, MeOH); IR (CHCl3) νmax 3437, 2943, 2884, 2324, 1738, 1715, 1594, 1439, and 1367 cm−1; UV (MeOH) λmax (log ε) 194 (3.86) nm; 1H NMR (CD3OD, 400 MHz) and 13C NMR (CD3OD, 100 MHz), Table 2; ESIMS m/z 377 [M + Na]+; HRESIMS m/z 377.2303 [M + Na]+ (calcd for C20H34O5Na 377.2298). Spectroscopic data for the known compounds 5 and 6 are depicted in the Supporting Information. Preparation of the (S)- and (R)-MTPA Ester Derivatives of 2 and 4. Compounds 2 (0.50 mg, 1.42 μmol) and 4 (0.50 mg, 1.41 μmol) were dissolved in deuterated pyridine (500 μL), and each was transferred to a clean NMR tube. Use of deuterated pyridine as solvent allows the reaction mixture to be directly monitored by 1H NMR spectroscopy. R-(−)-MTPA-Cl (3 μL, 15.6 μmol) was added to each NMR tube, and the mixture shaken carefully. After the reaction was completed (approximately 1 h), a 1H NMR spectrum was recorded.14 The preparation of the R-MTPA ester was done repeating the process using S-(+)-MTPA-Cl instead of R-(−)-MTPA-Cl. (S)-MTPA ester of 2 (2a): 1H NMR (500 MHz, pyridine-d5) δH 7.25−6.15 (ovl (overlapped peaks), 3H), 5.97 (m, 1H), 5.44 (ovl, 2H), 3.96−3.66 (ovl, 3H), 3.47 (m, 1H), 3.04 (m, 1H), 2.95 (m, 1H), 2.91−2.22 (ovl, 6H), 1.99 (m, 1H), 1.63−1.74 (ovl, 5H). (R)-MTPA ester of 2 (2b): 1H NMR (500 MHz, pyridine-d5) δH 7.25−6.15 (ovl, 3H), 6.01 (m, 1H), 5.44 (ovl, 2H), 3.96−3.66 (ovl, 3H), 3.58 (m, 1H), 2.96 (m, 1H), 2.91−2.22 (ovl, 6H), 2.87 (m, 1H), 2.06 (m, 1H), 1.63−1.74 (ovl, 5H). (S)-MTPA ester of 4 (4a): 1H NMR (500 MHz, pyridine-d5) δH 6.03 (ddt, J = 10.2, 8.0, 5.1 Hz, 1H), 5.65 (tt, J = 11.2, 4.7 Hz, 1H), 5.40 (m, 4H), 3.64−3.58 (m, 2H), 3.47 (m, 2H), 3.15 (dd, J = 17.7, 8.2 Hz, 1H), 3.09 (dd, J = 17.6, 4.4 Hz, 1H), 2.56 (m, 2H), 2.38 (m, 1H), 2.31 (m, 1H), 2.06 (m, 4H), 2.03 (m, 1H), 1.93 (m, 1H), 1.81 (m, 1H), 1.69 (m, 1H), 1.62 (m, 1H), 1.55 (m, 1H), 1.53 (m, 6H). (R)-MTPA ester of 4 (4b): 1H NMR (500 MHz, pyridine-d5) δH 6.05 (dtd, J = 8.0, 6.0, 4.5 Hz, 1H), 5.66 (tt, J = 11.4, 4.8 Hz, 1H), 5.42 (m,

EXPERIMENTAL SECTION

General Experimental Procedures. Optical rotations were determined using a Jasco P-1020 polarimeter. UV spectra were performed using an Agilent 8453 UV−vis spectrometer. IR spectra were obtained with a PerkinElmer Spectrum 100 FT-IR spectrometer with ATR sampling. NMR spectra were obtained on a Varian “Unity 500” spectrometer at 500/125 MHz (1H/13C) and on a Varian “Unity 400” spectrometer at 400/100 MHz (1H/13C). Chemical shifts were reported in ppm using residual solvent signal of CD3OD (δ 3.31 ppm for 1H and 49.0 ppm for 13C) as an internal reference. COSY, HSQC, HMBC, and ROESY experiments were performed using standard pulse sequences. Data were processed using MestReNova software. (+)ESIMS were recorded using an Agilent 1100 Series LC/MSD spectrometer. High-resolution mass spectroscopy (HRMS) was performed on an Agilent 6230 TOF LC/MS system using the ESIMS technique. Bacterial Isolation and Taxonomy. Streptomyces misionensis BAT-10-03-123 was isolated from a marine sediment sample collected in March 2010 at a depth of 15 m in Ria de Vigo, Galicia, Spain. From a suspension of 2 g of sediment in 10 mL of ASW (artificial seawater, Tropic Marine PRO-REEF Meersalz), 200 μL was spread on nutrient agar plates of the following composition (g/L): L-asparagine, 2.5; glycerol, 20; KCl, 5.35; Na2SO4, 7.5; Mg2SO4·7H2O, 0.1; MgCl2· 6H2O, 2.4; humic acid, 1; FeSO4·7H2O, 0.1; CaCO3, 0.1; and agar, 20 (Isolation BEN medium) supplemented with B vitamins and nalidixic acid (0.79 mM). The Petri dish plates were incubated at 28 °C for one month at atmospheric pressure. After this period colonies were picked and isolated onto agar plates, for taxonomy and fermentation studies, in modified ATCC 172 medium (172 M) with the following composition (g/L): dextrose, 5; soluble starch, 10; yeast extract, 2.5; tryptone, 2.5; artificial marine salts, 10 (artificial seawater, Tropic Marine PRO-REEF Meersalz); CaCO3, 2; and agar, 15. A taxonomic evaluation of BAT-10-03-123 was conducted by partial sequencing of the 16S rRNA following standard procedures, using the actinobacteria primers F1/R5 described by Cook and Myers.15 The amplified 16S rRNA gene sequence of 827 bp was submitted to GenBank with accession number KY386295. The phylogenetic neighbors were identified, and pairwise 16S rDNA gene sequence similarities were calculated and compared with the SILVA LTPs123 database16 using Bio Numeric software (Applied Maths). These results showed that BAT-10-03-123 is closely related to Streptomyces misionensis (accession number EF178678) with a similarity value of 98.67% (816/827 nt). Fermentation and Extraction. A seed culture was developed in two scale-up steps, first in 100 mL Erlenmeyer flasks containing 20 mL of seed medium and then in 250 mL Erlenmeryer flasks with 50 mL of the same medium. The seed culture was grown on a medium containing dextrose (0.1%), soluble starch 2.4%, soy peptone 0.3%, yeast extract 0.5%, tryptone 0.5%, soya flour 0.5%, sodium chloride 0.54%, potassium chloride 0.02%, magnesium chloride 0.24%, sodium sulfate 0.75%, and calcium carbonate 0.4% in tap water, and the strain was cultivated for 3 days at 28 °C at 220 rpm. For production, 12.5 mL of the seed medium was transferred into 2 L Erlenmeyer flasks containing 250 mL of fermentation medium (MP12) containing soy peptone 0.1%, soya flour 1.2%, dextrose 0.25%, malt extract 0.1%, dextrin 4%, artificial seawater 2% (Tropic Marine PRO-REEF Meersalz), and calcium carbonate 0.8%, and the culture was grown at 28 °C and 220 rpm. After 7 days of cultivation, the entire culture volume (5 L) was centrifuged to separate the mycelial cake and other solids (371 g) from 4.8 L of clarified broth. The mycelial cake was homogenized using a mixture of 2-propanol (0.7 L) and EtOAc (1.1 L). After stirring vigorously the mixture was then filtered through a pad of Celite, and the clarified broth (4.8 L) extracted with 4.8 L of EtOAc. The organic phase was concentrated under reduced pressure to give 3.9 g of supernatant extract with cytotoxic activity. Isolation of Secondary Metabolites. The supernatant extract (3.9 g) was applied to a silica gel vacuum flash chromatography 1037

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4H), 3.71−3.68 (m, 2H), 3.58 (m, 2H), 3.09 (dd, J = 17.5, 8.0 Hz, 1H), 2.99 (dd, J = 17.5, 4.6 Hz, 1H), 2.46 (m, 2H), 2.32 (m, 1H), 2.24 (m, 2H), 2.13 (ddd, J = 6.6 Hz, 2H), 2.01 (m, 1H), 2.00 (m, 2H), 1.82 (m, 1H), 1.69 (m, 1H), 1.68 (m, 1H), 1.55 (d, J = 4.9 Hz, 3H), 1.53 (m, 1H), 1.52 (d, J = 4.6 Hz, 3H). Biological Assays. The cytotoxic activities of compounds 1−6 were tested against A-549 human lung carcinoma cells, MDA-MB-231 human breast adenocarcinoma cells, HT-29 human colorectal carcinoma cells, and PSN1 human pancreatic adenocarcinoma cells. The concentration giving 50% inhibition of cell growth (GI50) was calculated according to the procedure described in the literature.17 Cell survival was estimated using the National Cancer Institute algorithm.18 Three dose−response parameters were calculated for streptenols 1−6.

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(8) Puder, C.; Loya, S.; Hizi, A.; Zeeck, A. J. Nat. Prod. 2001, 64, 42− 45. (9) Li, D. R.; Murugan, A.; Falck, J. R. J. Am. Chem. Soc. 2008, 130, 46−48. (10) Enders, D.; Hundertmark, T. Eur. J. Org. Chem. 1999, 4, 751− 756. (11) Franck-Neumann, M.; Bissinger, P.; Geoffroy, P. Tetrahedron Lett. 1997, 38, 4469−4472. (12) Ohtani, I.; Kusumi, T.; Kashman, Y.; Kakisawa, H. J. Am. Chem. Soc. 1991, 113, 4092−4096. (13) Hoye, T. R.; Jeffrey, C. S.; Shao, F. Nat. Protoc. 2007, 2, 2451− 2458. (14) Su, B. N.; Park, E. J.; Mbwambo, Z. H.; Santarsiero, B. D.; Mesecar, A. D.; Fong, H. H. S.; Pezzuto, J. M.; Kinghorn, A. D. J. Nat. Prod. 2002, 65, 1278−1282. (15) Cook, A. E.; Myers, P. R. Int. J. Syst. Evol. Microbiol. 2003, 53, 1907−1915. (16) Yarza, P.; Richter, M.; Peplies, J.; Euzeby, J.; Amann, R.; Schleifer, K. H.; Ludwig, W.; Glöckner, F. O.; Rossello-Mora, R. Syst. Appl. Microbiol. 2008, 31, 241−250. (17) Skehan, P.; Storeng, R.; Scudiero, D.; Monks, A.; McMahon, J.; Vistica, D.; Warren, J. T.; Bokesch, H.; Kenney, S.; Boyd, M. R. J. Natl. Cancer Inst. 1990, 82, 1107−1112. (18) Shoemaker, R. H. Nat. Rev. Cancer 2006, 6, 813−82. (19) Trisuwan, K.; Rukachaisirikul, V.; Kaewpet, M.; Phongpaichit, S.; Hutadilok-Towatana, N.; Preedanon, S.; Sakayaroj, J. J. Nat. Prod. 2011, 74, 1663−1667. (20) Khalil, Z. G.; Raju, R.; Piggott, A. M.; Salim, A. A.; Blumenthal, A.; Capon, R. J. J. Nat. Prod. 2015, 78, 949−952. (21) Karwehl, S.; Jansen, R.; Huch, V.; Stadler, M. J. Nat. Prod. 2016, 79, 369−375.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.6b01057. 1 H, 13C, gHSQC, gCOSY, and gHMBC NMR spectra of streptenols F−I (1−4) along with the 1H NMR spectra of (R)- and (S)-MTPA ester derivatives of compound 2 and 4; spectroscopic data for the known compounds 5 and 6; table of cytotoxic activity of compounds 1−6 (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel (L. M. Cañedo): +34 91 823 4664. Fax: +34 91 846 6001. E-mail: [email protected]. ORCID

Librada Ma Cañedo: 0000-0002-1273-4138 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the help of our PharmaMar colleagues, C. de Eguilior for collecting the marine samples, P. Zúñ iga and A. Peñalver for isolation and molecular characterization of the microbial strain, X. Benitez for bacterial fermentation, and J. Garcia and B. Delgado for scale-up of the microbial cultures and the recovery of the fermentation broths. We also thank P. Martinez for the screening experiments, S. González for the HPLC/MS experiments, S. Munt for revision of the manuscript, and R. Fernández for all the support received. The present research was financed in part by a grant from the EU Horizon 2020 Project INMARE 634486.

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REFERENCES

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DOI: 10.1021/acs.jnatprod.6b01057 J. Nat. Prod. 2017, 80, 1034−1038