Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nesteretal A, A Novel Class of Cage-Like Polyketide from MarineDerived Actinomycete Nesterenkonia halobia Chun-Lan Xie,†,§ Renzhi Chen,‡ Sihan Yang,‡ Jin-Mei Xia,† Gai-Yun Zhang,† Chao-Hong Chen,† Yandong Zhang,‡ and Xian-Wen Yang*,†
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†
Key Laboratory of Marine Biogenetic Resources, Third Institute of Oceanography, Ministry of Natural Resources, 184 Daxue Road, Xiamen, Fujian 361005, China ‡ Department of Chemistry and Key Laboratory of Chemical Biology of Fujian Province, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, Fujian 361005, China § School of Pharmaceutical Sciences, Xiamen University, South Xiangan Road, Xiamen, Fujian 361102, China S Supporting Information *
ABSTRACT: An intriguing cage-like polyhemiketal, nesteretal A (1), was isolated from the coral-derived actinomycete Nesterenkonia halobia. Its structure was established by extensive spectroscopic and computational methods. Nesteretal A is a highly oxygenated compound featuring an unprecedented 5/5/5/5 tetracyclic scaffold. A possible biosynthetic pathway of 1 from naturally occurring diacetyl was proposed. Compound 1 showed a weak retinoid X receptor-α (RXRα) transcriptional activation effect.
C
oral reefs are among the most biologically productive ecosystems of the world,1 from which a plethora of natural products with novel structures and diverse bioactivities have been discovered.2 The mucus layer, skeleton, and tissues of healthy corals usually contain large populations of eukaryotic algae, bacteria, and archaea.3 The coral host benefits from the wide range of bioactive secondary metabolite produced by its associated microorganisms. Chemically driven interactions protect the coral host from predation and fierce competition.4 Recently, we identified some secondary metabolites from the deep-sea-derived Nesterenkonia f lava including two macrolides and a novel polyether.5 Moreover, we conducted a comparative metabolomics investigation on four strains of N. f lava respectively isolated from terrestrial and marine environments. The preliminary results demonstrated that marine actinomycetes could produce secondary metabolites with a novel scaffold, which is different from their terrestrial relatives. This phenomena can be attributed to their special metabolic patterns in the unique living environment.6 As part of our continuing interest in exploring novel secondary metabolites from marine-derived actinomycetes, we herein report the isolation, identification, and bioactivity of a novel polyhemiketal named nesteretal A (Figure 1) from Nesterenkonia halobia E5.1, a marine actinomycete isolated from a scleractinian coral Platygyra. Nesteretal A (1)7 (10.5 mg) was obtained, by large-scale fermentation (ca. 40 L) on N. halobia followed by a systematic isolation and purification, as an amorphous powder. The © XXXX American Chemical Society
Figure 1. Chemical structure of compound 1.
molecular formula C12H18O6 was established on the basis of its negative HRESIMS at m/z 257.1010 [M − H]−, suggesting four degrees of unsaturation. The 1H NMR spectrum exhibited four methyl singlets (δH 1.04 s, 1.12 s, 1.15 s, 1.21 s), one methylene (δH 1.51, d, J = 14.1 Hz, H-3α; 2.11, d, J = 14.1 Hz, H-3β), one methine (δH 2.55 s), and three exchangeable protons (δH 4.56 s, 5.17 s, 7.69 s). These signals were documented in the 13C NMR spectrum as four methyls (δC 13.3 q, 18.1 q, 19.4 q, 21.0 q for Me-6, 7, 12, and 1, respectively), one methylene (δC 47.4 t, C-3), and one methine (δC 64.4 d, C-9). In addition, the 13C NMR of 1 also exhibited six oxygenated nonprotonated carbons (δC 84.4 s, 90.2 s, 90.3 s, 104.1 s, 107.0 s, 111.6 s). In the HMBC, correlations originated from four methyls established three fragments of C1/C-2/C-3/C-7/C-8/C-9, C-4/C-5/C-6, and C-10/C-11/C12 (Figure 2). Further HMBC correlations of C4-OH to C-3/ C-4/C-9 and C10-OH to C-9/C-10 connected these three Received: July 26, 2019
A
DOI: 10.1021/acs.orglett.9b02634 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Table 1. Experimental 1H, 13C, and the Calculated 13C NMR Spectroscopic Data for 1 in DMSO-d6 (J in Hz) δH
position 1 2 3α 3β 4 5 6 7 8 9 10 11 12 OH (C2) OH (C4) OH (C10)
Figure 2. Key HMBC (solid arrow) and NOESY (dashed arrow) correlations of 1.
fragments to be a cyclopentane moiety, leaving the other threering systems unresolved. Taking its molecular formula and the chemical shifts of C-10 (δC 111.6 s), C-11 (δC 107.0 s), and C5 (δC 104.1 s) into consideration, it is easy to conclude that 1 could have three hemiketal moieties on C-5, C-10, and C-11. Obviously, C11−Oα could not form a ketone group with C11−Oβ (a) nor an oxirane moiety (a three-membered hemiketal) with C10−O (b). Moreover, it could not be connected with C5−O to generate highly unstable c, which bears a four-membered hemiketal and a 1,3-dioxetane moiety. Therefore, there is only one possible connection of C11−Oα with C8−O, C11−Oβ with C5−Oα, and C5−Oβ with C10− O, which established the planar structure as d, possessing an unprecedented 5/5/5/5 ring system (Figure 3).
a
1.04, s 2.11, d (14.1) 1.51, d (14.1)
1.21, s 1.12, s 2.55, s
1.15, 4.56, 5.17, 7.69,
s s s s
δCa
δCb
Δδ
21.0, CH3 84.4, C 47.4, CH2
19.0 86.5 45.7
−2.0 2.1 −1.7
90.3, C 104.1, C 13.3, CH3 18.1, CH3 90.2, C 64.4, CH 111.6, C 107.0, C 19.4, CH3
92.6 102.5 14.4 18.1 92.1 66.5 109.4 105.5 19.0
2.3 −1.6 1.1 0 1.9 2.1 −2.2 −1.5 −0.4
Experimental data. bCalculated data.
Figure 3. Possible structures of 1 (a−d).
In the NOESY spectrum, correlations were found of H-9 (δH 2.55 s) to C2-OH/C4-OH/C10-OH/7-Me, C10-OH to 12Me, C4-OH to 6-Me/H-3β, C2-OH to H-3β, and H-3α to 1Me (Figure 2). This suggested the coface of C2-OH, C4-OH, 6-Me, 7-Me, H-9, C10-OH, and 12-Me. Accordingly, the relative configurations of 1 were established as 2R*, 4S*, 5R*, 8R*, 9R*, 10R*, and 11R*, respectively. For a verification, the quantum chemical calculation on the 13 C NMR spectroscopic data was performed using the densityfunctional theory (DFT) method. In general, conformational analyses were carried out via random searching in the Sybyl-X 2.0 using the MMFF94S force field with an energy cutoff of 10.0 kcal/mol. Due to the rigid skeleton, only one lowest energy conformer was obtained. Subsequently, this conformer was reoptimized at the B3LYP/6-31G* level in gas phase by the GAUSSIAN 09 program.8 The 13C NMR shielding constants of 1 were calculated with the GIAO method at the MPW1PW91/6-31G(d,p) level in gas phase. The computational 13C NMR data were finally obtained by the linear regression analysis method,9 which agreed well with the experimental ones (Table 1), with the correlation coefficient (R 2 ) of 0.99779 (Figure 4), confirming the relative configuration of 1. To determine the absolute configuration of 1, the theoretical optical rotation (OR) was further calculated in the B3LYP/ AUG-CC-pVDZ level with the polarizable continuum models (PCM) in MeOH for (2R,4S,5R,8R,9R,10R,11R)-1 (1a) and
Figure 4. Regression analysis of experimental versus calculated 13C NMR chemical shifts of 1 at the mPW1PW91/6-311+G(d,p) level in gas phase.
its enantiomer (2S,4R,5S,8S,9S,10S,11S-1, 1b) after the conformer was reoptimized using DFT at the B3LYP/631G* level in MeOH. The OR value of 1a was predicted to be −27.1, which was close to the experimental data (−35.7), while opposite to 1b (+27.1). This allowed an obvious assignment of the absolute configurations to be 2R, 4S, 5R, 8R, 9R, 10R, and 11R. Structurally, polyhemiketal nesteretal A (1) could be conceived as a cyclotrimer of diacetyl (2,3-butanedione) (Figure 5). Diacetyl was widely found in nature, especially in some fermentative bacteria,10 which is usually formed through thiamine pyrophosphate (TPP)-mediated condensation of pyruvate with acetyl-CoA11 (Figure 5). It is noteworthy that its reduced derivative 2,3-butanediol has recently been discovered by us from the deep-sea-derived actinomycete N. f lava.5 Thus, we believe that diacetyl also occurs naturally in N. halobia, and it may serve as a key building block in the biosynthesis of 1. On the basis of these considerations, the biogenetic origin of nesteretal A (1) in N. halobia is proposed to involve a linear trimerization process to produce the advanced intermediate 5 either through a double aldol reaction B
DOI: 10.1021/acs.orglett.9b02634 Org. Lett. XXXX, XXX, XXX−XXX
Letter
Organic Letters
Figure 5. Biosynthetic proposal for nesteretal A (1).
(pathway A) or through two sequential condensations with malonyl-CoA and the acetaldehyde−TPP complex 2 (pathway B). Finally, the cyclotrimer nesteretal A (1) can be formed by a thermodynamically favorable cascade cyclization process (ΔG = −17.94 kcal/mol, calculated at M06-2X/def2-SVP//M062X/def2-TZVP/SMD level, solvent = H2O, 298 K) involving an intramolecular aldol reaction followed by three wellorganized hemiketalizations. Retinoid X receptor-α (RXRα) represents a unique intracellular target for pharmacologic interventions. It plays a key role in various biological processes, especially for cancer.12 Compound 1 was investigated for the RXRα transcriptional activity by the reporter gene assay. It only showed a weak activation effect under the concentrations ranging from 10 to 40 μM but showed an interesting competitive effect with 9-cisretinoic acid, a natural RXRα ligand, at a high dose level (Figure S1). Not surprisingly, it did not show cytotoxicity against five tumor cell lines of BIU-87, ECA-109, Hela-S3, PANC-1, and BEL-7402 at a concentration of 40 μM (Figure S2). In conclusion, nesteretal A, a polyhemiketal with an unprecedented scaffold, was discovered and characterized from the coral-derived actinomycete Nesterenkonia halobia. A possible biosynthetic pathway was proposed, which will inspire further chemical and biological synthesis interests.
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chemical calculations, bioassays, and NMR, HR-ESIMS, and IR spectra for 1 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] ORCID
Yandong Zhang: 0000-0002-5558-9436 Xian-Wen Yang: 0000-0002-4967-0844 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by grants from the National Natural Science Foundation of China (41676130 and 21772164) and the Xiamen Southern Oceanographic Center (17GYY026NF05).
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b02634. General experimental procedures, fungal material, fermentation, extraction and isolation, the quantum C
DOI: 10.1021/acs.orglett.9b02634 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters (4) (a) Penesyan, A.; Kjelleberg, S.; Egan, S. Mar. Drugs 2010, 8, 438. (b) Egan, S.; Thomas, T.; Kjelleberg, S. Curr. Opin. Microbiol. 2008, 11, 219. (5) Xie, C. L.; Liu, Q.; Xia, J. M.; Gao, Y.; Yang, Q.; Shao, Z. Z.; Liu, G.; Yang, X. W. Mar. Drugs 2017, 15, 71. (6) Xie, C. L.; Xia, J. M.; Wang, J. S.; Lin, D. H.; Yang, X. W. Mar. Drugs 2018, 16, 356. (7) Nesteretal A (1): Amorphous powder; [α]20 D −35.7 (c 0.03, MeOH); IR (KBr) νmax (cm−1) 3414, 2992, 2941, 1644, 1448, 1383, 1345, 1256, 1149, 1103, 1048, 955, 904, 868, 836, 744; for 1H and 13 C NMR data, see Table 1; HRESIMS m/z 257.1010 [M − H]− (calcd for C12H17O6, 257.1025). (8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. GAUSSIAN 09W, (Revision A.02); Gaussian, Inc.: Wallingford, CT, 2009. (9) Lodewyk, M. W.; Siebert, M. R.; Tantillo, D. J. Chem. Rev. 2012, 112, 1839−1862. (10) Wang, Y.; Sun, W.; Zheng, S.; Zhang, Y.; Bao, Y. J. Biotechnol. 2019, 301, 2. (11) Speckman, R. A.; Collins, E. B. J. Bacteriol. 1968, 95, 174. (12) (a) Ye, X.; Wu, H.; Sheng, L.; Liu, Y. X.; Ye, F.; Wang, M.; Zhou, H.; Su, Y.; Zhang, X. K. Nat. Commun. 2019, 10, 1463. (b) Chen, L.; Aleshin, A. E.; Alitongbieke, G.; Zhou, Y.; Zhang, X.; Ye, X.; Hu, M.; Ren, G.; Chen, Z.; Ma, Y.; Zhang, D.; Liu, S.; Gao, W.; Cai, L.; Wu, L.; Zeng, Z.; Jiang, F.; Liu, J.; Zhou, H.; Cadwell, G.; Liddington, R. C.; Su, Y.; Zhang, X. K. Nat. Commun. 2017, 8, 16066. (c) Chen, F.; Liu, J.; Huang, M.; Hu, M.; Su, Y.; Zhang, X. K. ACS Med. Chem. Lett. 2014, 5, 736.
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DOI: 10.1021/acs.orglett.9b02634 Org. Lett. XXXX, XXX, XXX−XXX
