Limaol: A Polyketide from the Benthic Marine Dinoflagellate

Note pubs.acs.org/jnp

Limaol: A Polyketide from the Benthic Marine Dinoflagellate Prorocentrum lima A Reum Yang,† Sangbum Lee,† Young Du Yoo,† Hyung Seop Kim,† Eun Ju Jeong,‡ and Jung-Rae Rho*,† †

Department of Marine Biotechnology, Kunsan National University, 558 Daehak-ro, Gunsan 54150, South Korea Department of Agronomy & Medicinal Plant Resources, Gyeongnam National University of Science and Technology, JinJu 660-758, South Korea

‡

S Supporting Information *

ABSTRACT: Limaol (1), along with a dinophysistoxin 1 derivative and an okadaic acid (OA) derivative, was isolated from the large-scale cultivation of the benthic marine dinoflagellate Prorocentrum lima. The structure of 1 was determined by a combination of NMR spectroscopy and mass spectrometry and contained tetrahydropyran, 1,3,5,7tetra(methylene)heptane, and octahydrospiro[pyran-2,2′-pyrano[3,2-b]pyran] moieties. The absolute configuration of 1 was completely elucidated on the basis of ROESY correlations, J-based configuration analysis, and modified Mosher’s ester analysis. Limaol showed moderate cytotoxicity when compared to OA against three cancer cell lines.

M

On the basis of its positive HRESI mass spectrum, 1 was found to have the molecular formula C47H74O12, which corresponds to a compound containing 11 degrees of unsaturation. The presence of hydroxy and olefinic groups was revealed by strong absorptions at 3388 and 1593 cm−1 in the IR spectrum. Following this, NMR experiments were performed to elucidate the fine structural details of 1. The 1H NMR spectrum of 1 in CD3OD displayed three methyl signals (two doublets and one singlet) and well-resolved resonances distributed over the 0.5−6.0 ppm range. The 13C and edited HSQC NMR spectra of 1 showed 47 carbon signals in the form of three methyl, 20 methylenic (including one oxymethylene and five exo-methylene groups), 17 methinic (one sp3hybridized methine, 13 oxymethines, and three olefinic methines), one ketal, and six quaternary carbons. The presence of five exo-methylene groups (δC 113.9, 114.5, 114.7, 115.1, 115.9, 145.3, 145.4, 146.0, 146.1, and 147.1) and one ketal (δC 97.8) in 1 was deduced by the distinctive chemical shift values of their corresponding carbons. Along with the data provided above, the five exo-methylene groups together with two other olefins accounted for seven unsaturations, and the remaining unsaturations indicated the presence of four rings in 1. Detailed analyses of the DQF-COSY and TOCSY spectra established four spin systems (I, II, III, and IV), as indicated by the bold lines in Figure 1. In particular, spin system III, containing highly overlapping proton resonances (H-22, H-23, and H-24), was clarified by HMBC cross-peaks between their corresponding carbons and nearby protons. All assignments in these spin systems were confirmed by an HSQC-TOCSY experiment. In addition to the four spin systems, a unique new

arine dinoflagellates are being paid increasing attention because they produce a variety of structurally complex and biologically toxic secondary metabolites.1,2 Prorocentrum is a representative genus that produces diarrhetic shellfish poisoning (DSP) toxins, such as okadaic acid (OA), dinophysistoxin-1 (DTX-1), and analogues.3 Apart from these compounds, skeletally diverse bioactive compounds have also been isolated from cultured Prorocentrum species, including prorocentrolide,4 spiro-prorocentrimine,5 hoffmanniolide,6 prorocentin,7 formosalides A and B,8 belizentrin,9 belizeanolide,10 and prorocentrol.11 In our search for bioactive compounds from cultured P. lima, a new polyhydroxy compound, limaol (1), was isolated by cytotoxicity-guided fractionation. In this paper, we describe the structural characterization and bioactivity of 1. The cells harvested from 400 L of culture were extracted with 100% MeOH and then partitioned between H2O and BuOH. The organic fraction was separated by reversed-phase silica gel and Sephadex LH20 open column chromatography to obtain the fractions showing cytotoxicity. Compound 1 (1.6 mg) was purified from the cytotoxic fraction by reversed-phase HPLC with isocratic elution.

Received: February 13, 2017 Published: April 6, 2017 © 2017 American Chemical Society and American Society of Pharmacognosy

1688

DOI: 10.1021/acs.jnatprod.7b00127 J. Nat. Prod. 2017, 80, 1688−1692

Journal of Natural Products

Note

Figure 1. COSY and selected HMBC correlations in 1.

moiety was present in 1. The two singlet methylene protons at δH 2.70 (H-7) and 2.73 (H-9) showed HMBC cross-peaks with their mutually opposite carbons, as well as with the nearby six carbons (C-6, C-8, C-10, C-42, C-43, and C-44). This unambiguous assignment was confirmed by HMBC crosspeaks with the exo-methylene protons. The contiguous longrange correlations of H3-44/C-11, H-11/C-45, and H-11/C-12 revealed another methylene-mediated exo-methylene group, indicating a 1,3,5,7-tetra(methylene)heptane substructure. Spin systems I and II were positioned on both sides of this subunit, which was confirmed by HMBC cross-peaks of H3-42/C-5 and H3-45/C-13. Spin system II extended to a trisubstituted double bond and is then connected to system III via a ketal carbon. This was further supported by the cross-peaks of H3-46/C-15, H3-46/C-16, H3-46/C-17, H-17/C-18, and H-19/C-18 in the HMBC spectrum. Finally, HMBC correlations of H3-47/C-28 and H3-47/C-30 allowed linkage of spin systems III and IV to both sides of the remaining exo-methylene group. Considering the remaining unsaturations from the molecular formula of 1, the connected linear chain should be reconstituted to contain four rings. Initially, the deuteriuminduced isotope effects on carbon chemical shifts were investigated to identify if the carbons possessed a free hydroxy group. Eight carbons (C-2, C-20, C-23, C-24, C-27, C-32, C-33, and C-40) showed chemical shift differences of more than 0.07 ppm between the two 13C NMR spectra measured in CD3OD and CD3OH, indicating hydroxy-bearing carbons. Seven remaining oxygen-bearing carbon atoms were responsible for the construction of rings through ether linkages. On the basis of this information, the HMBC correlations of H-22/C-19 and H25/C-21 constructed an octahydropyrano[3,2-b]pyran. Similarly, the HMBC correlation between H-35 and C-31 revealed the presence of a tetrahydropyran unit. It was deduced that the one remaining ring was formed by connecting C-14 with C-18 through an ether bond, although the HMBC cross-peak between H-14 and the ketal carbon was not observed. This linkage could be evident from one remaining degree of unsaturation and the molecular formula, as well as the ROE correlation of H-14 and H-22. Hence, compound 1 was characterized to be a linear molecule containing a tetrahydropyran, a 1,3,5,7-tetra(methylene)heptane subunit, and an octahydro-3′H-spiro[pyran-2,2′-pyrano[3,2-b]pyran]. The planar structure of 1 was supported by several fragments provided by MS/MS analyses (Figure S2). Following the determination of the planar structure of 1, the relative configuration was established on the basis of ROESY correlations (Figure 2) and J-based configuration analysis (JBCA)12 (Figure 3). The large coupling constant (J = 10.3 Hz) between H-21 and H-22 showed that the octahydropyrano[3,2-b]pyran unit is rigidly fused in a trans

Figure 2. Key ROE correlations from C-14 to C-36 in 1.

relationship. This configuration was corroborated with the ROESY cross-peaks of H-21/H-19a, H-21/H-23, and H-21/H26b. These correlations also allowed us to define the configurations of C-23 and C-25. Moreover, the orientations of H-20 and H-24 on this skeleton were determined to be equatorial and axial, respectively, from the coupling constants of the well-split neighboring proton signals: J = 2.7 Hz between H-20 and H-21 and J = 9.1 Hz between H-23 and H-24. Further ROESY correlations of H-17/H-19a, H-17/H-19b, and H-14/H-22 allowed determination of the relative orientation of the octahydropyrano[3,2-b]pyran and dihydropyran rings. On the other hand, the configurations of the stereogenic centers beyond C-25 were established by measurement of 3JHH values from the DQF-COSY and 2,3JCH values from the HECADE spectra and observation of ROESY correlations. As shown in Figure 3A, the coupling constants related with H-25, H-26a, and H-26b, in accordance with JBCA, determined the orientation of the two nonequivalent protons on C-26. On the basis of this configuration, the configuration of C-27 was clearly identified as S* by the large coupling constant of H-26a/H-27 and the small heteronuclear coupling constant of H-26b/C-27, implying an anti relationship between H-26b and the −OH group on C-27. Unlike the previous cases, the vicinal coupling constants of H-27/H-28a and H-27/H-28b were measured as intermediate values (3JHH ≈ 6.6 Hz), which could be rationalized by a pair of interconverting conformers. The exchangeable conformers between C-27 and C-28 were determined by measurable coupling constant values shown in Figure 3C. The configurations from C-26 to C-28 were confirmed by the ROEs of H-21/H-27, H-23/H-26b, H-26a/H28b, and H-26b/H-27. Furthermore, the ROESY correlations of H-27/H-30a, H-47a/H-28a, and H-47a/H-28b established the orientation of the next exo-methylene group. On the basis of this preferred conformation, the weak ROEs of H-28a/H-30a and H-28b/H-30b indicated the positions of the two nonequivalent protons on C-30, which in turn assigned the configuration of C-31 on the tetrahydropyran ring from a large coupling constant of H-30a/H-31 and a small coupling constant of H-30b/C-30 (Figure 3D). The stereochemically assigned H-31 then enabled the assignment of the relative configuration of the three stereogenic carbons in the tetrahydropyran ring by the observation of ROE correlations between H-31/H-33 and H-31/H2-36, as well as the large coupling with H-32. Therefore, the stereocenters in the C-13 to C-36 chain were assigned as 14S*, 18S*, 20R*, 21R*, 22S*, 1689



Note

Figure 3. Relative configurations based on JBCA (A) C-25−C-27, (B) C-26−C-28, (C) C-27−C-29, (D) C-30−C-32, and (E) C-2−C-5.

combination of the shielding/deshielding effects produced by the two Mosher’s esters at C-24 and C-27. Considering the stereochemical structure of 1, it is considered that H-25 is more affected by the phenyl ring in the Mosher’s ester at C-27, while H-26a and H-26b are more influenced by the Mosher’s ester at C-24. Accordingly, the absolute configurations of C-25 and C27 could be determined to be 25R, 27S, respectively. This was also supported by the agreement of the ΔδSR signs for H-23, H27, and H-28a/H-28b with those of the corresponding protons for 1,4-diol system by Riguera et al.13 with the exception of H24. On the basis of these two absolute configurations, the absolute configurations of all stereocenters between C-13 and C-36 were assigned. Moreover, the configurations of C-32 and C-33 were also confirmed by the ΔδSR signs of H-32 and H-33, consistent with the corresponding sign distribution of four possibilities of a 1,2-diol system by Riguera et al. Finally, the Mosher’s ester analysis was applied to determine the absolute configuration of C-2 to be R, which allowed C-4 to be assigned as R by the JBCA method, as shown in Figure 3E. The ROE correlation of H-2/H3-41 via a five-bond distance also confirmed the configuration of C-4. Limaol (1) isolated from P. lima was evaluated for cytotoxicity against HepG2 (hepatocellular carcinoma), HCT116 (colon adenocarcinoma), and Neuro2a (neuroblastoma) cell lines. Limaol induced cytotoxicity in these cells, with IC50 values of 3.7, 7.3, and 9.6 μM against HepG2, HCT-116, and Neuro2a, respectively. Its cytotoxicity was relatively moderate when compared to OA in these three cell lines; IC50 values for OA against these three cells lines are 0.54, 0.67, and 0.85 μM, respectively. Limaol (1) is a new linear polyketide isolated from the benthic marine dinoflagellate P. lima and showed moderate cytotoxicity against three cells lines: HepG2, HCT-116, and Neuro2a. The structure of 1 was determined by NMR

23R*, 24R*, 25R*, 27S*, 31R*, 32S*, 33R*, and 35R*. The Δ37 double bond on the chain was assigned to be E by the coupling constant (3JHH = 15.4 Hz) between the corresponding protons. The absolute configuration was established by a modified Mosher’s ester analysis in which 1 was reacted with R/SMTPA-Cl. From the proton chemical shifts and MS data for the two esterified products, the hydroxy groups on C-2, C-24, C-27, C-32, C-33, and C-40 in 1 were esterified; the differences in proton chemical shifts (ΔδSR) for the partial structure (C19−C-36) of the two ester derivatives are presented in Figure 4. Although the values and signs of ΔδSR for H-25, H-26a, and H26b are unusual compared with a well-established Mosher’s ester analysis, these could be reasonably explained by the

Figure 4. 1H NMR chemical shift differences (ΔδSR, ppm) for the S-/ R-MTPA esters of 1 in CDCl3: (A) C19−C36; (B) C-1−C4. 1690



Note

Table 1. 1H and 13C NMR Data for Limaol (1) in CD3OD (500 MHz for 1H, 125 MHz for 13C) no.

δC, type

1 2 3a 3b 4 5a 5b 6 7 8 9 10 11a 11b 12 13a 13b 14 15a 15b 16 17 18 19a 19b 20 21 22 23

24.4, CH3 66.1, CH 47.6, CH2 28.1, CH 45.0, CH2 147.1, 43.3, 146.1, 42.3, 146.0, 43.2,

C CH2 C CH2 C CH2

145.3, C 42.5, CH2 66.8, CH 36.2, CH2 138.4, 123.7, 97.8, 41.2,

C CH C CH2

68.1, 70.5, 69.9, 72.6,

CH CH CH CH

δH, mult (J in Hz) 1.14, 3.82, 1.07, 1.46, 1.87, 1.81, 1.97,

d (6.4) m ddd (13.2, 9.1, 3.9) ddd (13.2, 9.1, 4.1) m dd (13.0, 8.1) dd (13.0, 5.6)

2.73, s 2.70, s 2.88, d (14.7) 3.01, d (14.7) 2.22, 2.29, 4.23, 1.84, 1.93,

m dd (13.9, 4.2) m dd (13.5, 2.9) m

5.28, brs 1.86, 1.94, 3.95, 3.26, 3.65, 3.61,

m m dt (2.9,2.7) dd (10.3, 2.7) dd (10.3, 9.1) t (9.1)

δC, type 73.1, CH 74.8, CH 32.5, CH2 66.4, CH 46.5, CH2 145.4, C 39.0, CH2 73.7, 77.1, 70.0, 36.5,

CH CH CH CH2

73.3, CH 35.9, CH2 129.9, 130.3, 37.1, 62.8, 19.8, 113.9, 114.7, 115.1, 115.9, 22.8, 114.5,

CH CH CH2 CH2 CH3 CH2 CH2 CH2 CH2 CH3 CH2

δH, mult (J in Hz) 3.64, m 4.25, m 1.61,ddd (13.5, 7.5, 3.2) 1.87, m 3.91, m 2.23, dd (13.7, 6.6) 2.36, dd (13.7, 6.6) 2.12, 2.61, 3.59, 2.98, 3.73, 1.63, 1.92, 3.91, 2.24, 2.46, 5.47, 5.54, 2.22, 3.55, 0.87, 4.80, 4.88, 4.91, 4.91, 1.71, 4.86,

dd (15.2, 9.5) brd (15.2) ddd (9.5, 8.3, 1.7) t (8.3) ddd (10.8, 8.3, 4.7) ddd (13.2, 10.8, 5.9) m m m ddd (13.7, 7.3, 6.6) dt (15.4, 6.6) dt (15.4, 6.4) m t (6.9) d (6.4) brs; 4.81, brs s, 4.90, brs s; 5.01, brs brs; 4.96, brs s brs; 4.91, brs

to the thalli of the algae and then screened through a 202 μm Nitex mesh and placed in six-well tissue culture plates. A clonal culture of P. lima was established by two serial single-cell isolations. Bottles containing f/2 medium and P. lima were again filled to capacity with freshly filtered seawater, capped, and placed on a shelf at 20 °C under an illumination of 50 μE/m2/s. As the concentration of P. lima increased, P. lima was subsequently transferred to 50, 125, and 500 mL polycarbonate (PC) bottles containing fresh f/2 seawater medium. Once dense cultures of P. lima (ca. 5000 cells mL−1) were obtained, they were transferred approximately every 3 weeks to new 500 mL PC bottles containing fresh f/2 seawater medium. When the concentration of P. lima became high, we analyzed the DNA sequences of the cultured cells, and after genetic identification, the morphology was analyzed when culture volumes were large enough. The small subunit rDNA sequence of this species was identical to that of Prorocentrum lima. Following this, the culture was transferred to 2 L, cultivated, and then again transferred to 20 L polycarbonate bottles. The cells were cultivated in the stationary stage of the growth curve after 4 weeks. The same process was repeated to become a volume of 400 L 20 times. After this, P. lima cells were harvested by centrifugation (15 000 rpm, 10 h) when cell concentration reached around 70 000−90 000 cells mL−1. The collected cells were stored in a deep freezer (−75 °C) followed by removing moisture with a freeze-dryer (−55 °C, 5 mTorr, 24 h). Extraction and Isolation of 1. The harvested cells were extracted with 100% MeOH and partitioned into H2O and BuOH. The organic layer was repartitioned into n-hexane and 85% aqueous MeOH. The aqueous layer (900 mg) was subjected to a reversed-phase column, eluting stepwise with 10% MeOH increasing from 50% H2O to 100% MeOH to yield six fractions (I−VI) (Scheme S1). Among them, fraction V, showing cytotoxicity in the brine shrimp lethality test, was further divided into four fractions using a Sephadex LH20 column. An active fraction (V-2, 90 mg) was separated by reversed-phase HPLC (column: Phenomenex C8, 250 × 10 mm; gradient elution with

spectroscopy and mass spectrometry and contained a tetrahydropyran, a 1,3,5,7-tetra(methylene)heptane subunit, and an octahydro-3′H-spiro[pyran-2,2′-pyrano[3,2-b]pyran] unit. Moreover, the complete configuration of 1 was established by a combination of ROESY correlations and the JBCA method, along with a modified Mosher’s ester method through the esterification of six hydroxy groups in 1. Limaol is one of the diverse secondary metabolites isolated from the genus Prorocentrum cultured in a seawater medium and suggested that this organism is a promising source of structurally new metabolites.

■

no. 24 25 26a 26b 27 28a 28b 29 30a 30b 31 32 33 34a 34b 35 36a 36b 37 38 39 40 41 42 43 44 45 46 47

EXPERIMENTAL SECTION

General Experimental Procedures. The optical rotation was measured on a JASCO P-1010 polarimeter with a 5 cm cell. The IR spectrum was recorded on a JASCO FT/IR 4100 spectrometer. The NMR spectra were measured on a Varian VNMRS 500 MHz spectrometer with a 3 mm i.d. probe in solvents of MeOH-d4 (residual solvent peaks at δH 3.30 and δC 49.0) and a Bruker Avance II 900 MHz spectrometer in CDCl3 (residual solvent peaks at δH 7.26). The highresolution ESI mass spectrum was acquired using a Waters SYNAPT G2, courtesy of Korea Basic Science Institute, Ochang Center, Korea. The ESIMS/MS spectra were obtained on an ABSCIEX Qtrap 3200. The HPLC was performed using an Agilent 1200 system using Phenomenex C8 and polar columns. All solvents were distilled prior to use. Collection and Cultures of Prorocentrum lima. For the isolation and culture of P. lima, samples of the macroalga Sargassum f ulvellum were collected by divers at a depth of ∼3 m from Geomundo Island, Korea, in November 2012. The samples were placed in plastic bags, stored in ice boxes, and then transported to the laboratory. These samples were vigorously shaken to detach the dinoflagellates attached 1691



Note

solvent from 40% ACN + 60% H2O to 100% ACN for 40 min; flow rate: 1.8 mL/min) to yield a known OA-diol (MW = 928, 7.5 mg), a known DTX-1 derivative (MW = 956, 11.5 mg),3 and a mixture containing limaol (1, 3.2 mg). Following this separation, the mixture of 1 was purified by reversed-phase HPLC (column: Phenomenex Polar 250 × 4.6 mm; isocratic elution with 70% MeOH + 30% H2O + 0.1% formic acid; flow rate: 1 mL/min) to obtain limaol (1.6 mg). Limaol (1): colorless oil, [α]25D +63 (c 0.1, MeOH); IR νmax 3388, 2920, 1593, 1380, 1069 cm−1; 1H and 13C NMR data, see Table 1; HRESIMS m/z 853.5078 [M + Na]+ (calcd for C47H74O12Na, 853.5078). Preparation of (S)- and (R)-MTPA Ester Derivatives of 1. Limaol (1, 1.0 mg) was transferred into each of the two vials in a 0.5 mg amount. After placing 4-dimethylaminopyridine (20 μg) and anhydrous pyridine solvent (20 μL) into each vial, S(+)-MTPA-Cl (20 μL) was added to a stirred solution at 40 °C. After 2 days, the reaction mixture was quenched by the addition of H2O and dimethyl ether. The organic solvent was evaporated in vacuo. The mixture was purified by silica phase HPLC by eluting with 75% hexane and 25% EtOAc to give the R-MTPA-ester (tR ≈ 10 min): 1H NMR (500 MHz, CDCl3) 5.301 (H-33), 5.235 (H-2), 5.197 (H-27), 5.145 (H-24), 4.938 (H-32), 4.164 (H-25), 3.925 (H-20), 3.930 (H-31), 3.762 (H-22), 3.682 (H23), 3.545 (H-35), 3.057 (H-21), 2.447 (H-28b), 2.258 (H-30b), 2.262 (H-28a), 2.164 (H-36b), 2.086 (H-36a), 2.064 (H-30a), 1.976 (H-19b), 1.912 (H-26b), 1.885 (H-34b), 1.763 (H-3b), 1.701 (H-4), 1.680 (H-34a), 1.624 (H-26a), 1.588 (H-19a), 1.258 (H-1), 1.198 (H3a), 0.878 (H3-41); LRESIMS m/z 1086 [M + 2Na]2+. In an entirely analogous way, the S-MTPA-ester was obtained using R-(−)-MTPA-Cl after reaction for 4 days: 1H NMR (500 MHz, CDCl3) 5.259 (H-2), 5.217 (H-33), 5.127 (H-24), 5.099 (H-27), 4.849 (H-32), 4.367 (H-25), 4.102 (H-31), 3.983 (H-20), 3.832 (H23), 3.789 (H-22), 3.687 (H-35), 3.098 (H-21), 2.413 (H-30b), 2.156 (H-36b), 2.093 (H-30a), 2.080 (H-36a), 2.073 (H-28a), 2.048 (H19b), 1.826 (H-26b), 1.796 (H-34b), 1.740 (H-3b), 1.703 (H-19a), 1.687 (H-34a), 1.493 (H-4), 1.337 (H-1), 1.131 (H-3a), 1.218 (H26a), 1.131 (H-3a), 0.796 (H-41); LRESIMS m/z 1086 [M + 2Na]2+. In Vitro Cytotoxicity Assay. The cell lines HepG2 and HCT-116 were purchased from Korean Cell Line Bank, and Neuro2a cells were purchased from ATCC. All cells were maintained in Dulbecco’s modified Eagle medium, supplemented with 10% fetal bovine serum, 100 U/mL penicillin, and 100 μg/mL streptomycin in a 5% CO2 atmosphere and 95% relative humidity at 37 °C. Cells were seeded in a 96-well plate at a density of 1.0 × 104 per well. Cell number was determined on a hemocytometer by trypan blue exclusion. After 24 h of incubation, the cells were treated with compounds to be tested. All compounds were solubilized in DMSO and then diluted with corresponding culture media just before use. After 24 h of treatment of the compounds, cell viability was measured using the 3-(4,5dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT, Sigma Chemical Co.) assay, which is based on the conversion of MTT to MTT-formazan by mitochondrial enzyme. Reduction of MTT to MTT-formazan was assessed using an ELISA plate reader at 550 nm. IC50 values refer to the 50% inhibition concentration and were calculated from regression line with at least five different concentrations.

■

ORCID

Jung-Rae Rho: 0000-0001-6443-632X Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was supported by the National Research Foundation (NRF) grant by the Korean government (MSIP) (NRF-2015M1A5A1041808).

■

REFERENCES

(1) Satake, M. Top Heterocycl. Chem. 2006, 5, 21−51. (2) Gallardo-Rodríguez, J.; Sánchez-Miron, A.; García-Gamacho, F.; López-Rosales, L.; Chisti, Y.; Molina-Grima, E. Biotechnol. Adv. 2012, 30, 1673−1683. (3) Lee, S.; Hwang, B. S.; Kim, H. S.; Yih, W.; Jeong, E. J.; Rho, J. R. Bull. Korean Chem. Soc. 2015, 36, 395−398. (4) Torigoe, K.; Murata, M.; Yasumoto, T.; Iwashita, T. J. Am. Chem. Soc. 1988, 110, 7876−7877. (5) Lu, C.-K.; Lee, G.-H.; Huang, R.; Chou, H.-N. Tetrahedron Lett. 2001, 42, 1713−1716. (6) Hu, T.; Curtis, J. M.; Walter, J. A.; Wright, J. L. C. Tetrahedron Lett. 1999, 40, 3977−3980. (7) Lu, C.-K.; Chou, H.-N.; Lee, C.-K.; Lee, T.-H. Org. Lett. 2005, 7, 3893−3896. (8) Lu, C.-K.; Chen, Y.-M.; Wang, S.-H.; Wu, Y.-Y.; Chen, Y.-M. Tetrahedron Lett. 2009, 50, 1825−1827. (9) Domínguez, H. J.; Napolitano, J. G.; Fernández-Sánchez, M. T.; Cabrera-García, D.; Novelli, A.; Norte, M.; Fernández, J. J.; Daranas, A. H. Org. Lett. 2014, 16, 4546−4549. (10) Napolitano, J. G.; Norte, M.; Padrón, J. M.; Fernández, J. J.; Daranas, A. H. Angew. Chem., Int. Ed. 2009, 48, 796−799. (11) Sugahara, K.; Kitamura, Y.; Murata, M.; Satake, M.; Tachibana, K. J. Org. Chem. 2011, 76, 3131−3138. (12) Matsumori, N.; Kaneno, D.; Murata, M.; Nakamura, H.; Tachibana, K. J. Org. Chem. 1999, 64, 866−876. (13) Freire, F.; Seco, J. M.; Quiñoá, E.; Riguera, R. J. Org. Chem. 2005, 70, 3778−3790.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.7b00127. Isolation scheme and MS data of 1; 1D and 2D NMR spectra for 1 and 1D NMR spectra for the (S)/(R)MTPA ester of 1 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Tel (J.-R. Rho): +82 63 469 4606. E-mail: [email protected]. 1692


Limaol: A Polyketide from the Benthic Marine Dinoflagellate

Recommend Documents