Simpterpenoid A, a Meroterpenoid with a Highly Functionalized

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Letter Cite This: Org. Lett. 2018, 20, 1465−1468

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Simpterpenoid A, a Meroterpenoid with a Highly Functionalized Cyclohexadiene Moiety Featuring gem-Propane-1,2-dione and Methylformate Groups, from the Mangrove-Derived Penicillium simplicissimum MA-332 Hong-Lei Li,†,§ Rui Xu,†,‡,§ Xiao-Ming Li,*,† Sui-Qun Yang,† Ling-Hong Meng,† and Bin-Gui Wang*,† †

Key Laboratory of Experimental Marine Biology, Institute of Oceanology, Chinese Academy of Sciences, Laboratory of Marine Biology and Biotechnology, Qingdao National Laboratory for Marine Science and Technology, Nanhai Road 7, Qingdao 266071, China ‡ Department of Life Sciences, Heze University, Daxue Road 2269, Heze 274015, China S Supporting Information *

ABSTRACT: Simpterpenoid A (1), an unprecedented meroterpenoid possessing a highly functionalized cyclohexadiene moiety (ring C) featuring gem-propane-1,2-dione and methylformate groups, was characterized from mangrove-derived Penicillium simplicissimum MA-332. Due to the highly oxygenated and functionalized ring C and lack of some key correlations, the unambiguous assignment of the planar structure and steric configuration was solved by X-ray crystallographic analysis. Compound 1 exhibited inhibitory activity against influenza neuraminidase in nanomolar quantities.

M

dione and methylformate groups, which has not been reported in natural products. Compound 1 exhibited potent inhibitory activity against influenza neuraminidase but no obvious antimicrobial activity. This paper describes the isolation, structure determination, stereochemical assignment, and influenza neuraminidase inhibitory and antimicrobial assays of 1. The plausible biogenetic pathway is also discussed. The fungus P. simplicissimum MA-332, which was identified on the basis of its ITS (internal transcript spacer) sequence (deposited in Genbank, with accession no. KU612221) analysis, was cultivated in solid rice medium, and the culture was exhaustively extracted with EtOAc to afford an organic extract, which was further purified by repeated column chromatography on silica gel, Sephadex LH-20, and Lobar LiChroprep RP-18, to afford simpterpenoid A (1) (Figure 1). Simpterpenoid A (1),6 originally obtained as yellowish amorphous powder, has the molecular formula C27H36O7 as determined on the basis of HRESIMS data. Inspection of its 1H

arine microbial natural products have received great attention in past two decades due to their unique structures and significant biological activities.1,2 A number of marine microbial metabolites such as alkaloids, peptides/cyclic peptides, polyketides, and terpenoids/meroterpenoids have been isolated and identified from marine-derived microbes, and these compounds display various biological activities such as antimicrobial, antitumor, antiviral, and enzymatic inhibitory properties.1,3 Fungal-derived meroterpenoids are a class of natural products that originated from polyketide and terpenoid precursors and usually possess a wide range of bioactvities.4 In the course of our continuous research on marine-derived fungi,5 we recently performed chemical investigation on the fungal strain Penicillium simplicissimum MA-332 that was obtained from the rhizospheric soil of the marine mangrove plant Bruguiera sexangula var. rhynchopetala, which was collected from Hainan island in the South China sea. As a result, a new meroterpenoid, namely, simpterpenoid A (1), was isolated and identified from the culture extract of the fungal strain. This compound is an unconventional meroterpenoid containing a highly functionalized cyclohexadiene moiety (ring C) with gem-propane-1,2© 2018 American Chemical Society

Received: January 29, 2018 Published: February 16, 2018 1465

DOI: 10.1021/acs.orglett.8b00327 Org. Lett. 2018, 20, 1465−1468

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Organic Letters

Figure 1. Chemical structure of compound 1.

Figure 2. Selected 2D NMR correlations of compound 1.

NMR and HSQC data (Table 1 and Supporting Information) revealed the presence of eight methyls (with one oxygenated),

presence of unsaturated ring C was also verified by the HMBC correlations, and the linkage of rings B and C was confirmed by correlations from H3-17 to C-8, C-9, and C-14. Other HMBC correlations revealed the presence of the gem-propane-1,2-dione and methylformate moieties as speculated by the cross-peaks from H3-24 to C-22 and C-23 and from H3-27 to C-21 (Figure 2). However, the attachment of these two moieties to C-14 could not be unambiguously assigned due to lack of supportive HMBC evidence. NOESY experiment of 1 only provided one diagnostic NOE correlation from H-18 to H3-17, which suggested the same orientation of the −CHO and CH3-17 groups (Figure 3). No

Table 1. NMR Data for Compound 1 in CDCl3 (δ in ppm)a no.

δC

δH (mult., J in Hz)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

27.0, CH2 24.1, CH2 77.3, CH 37.3, C 44.1, CH 17.5, CH2 34.9, CH2 40.9, C 147.5, C 55.8, C 125.1, CH 133.8, C 122.2, C 66.1, C 26.9, CH3 21.2, CH3 20.1, CH3 200.3, CH 18.6, CH3 16.7, CH3 168.9, C 182.8, C 199.4, C 25.3, CH3 170.6, C 21.2, CH3 51.6, CH3

α 2.38 (1 H, m), β 1.54 (1 H, m) α 1.74 (1 H, m, overlap), β 1.77 (1 H, m, overlap) 4.65 (1 H, br s) 2.43 (1 H, br d, 11.6) α 2.00 (1 H, m), β 1.76 (1 H, m, overlap) α 1.25 (1 H, m), β 2.24 (1 H, td, 4.2, 13.1)

5.53, s

0.97 (3 H, s) 0.85 (3 H, s) 1.03 (3 H, s) 9.87 (1 H, s) 1.67 (3 H, s) 1.81 (3 H, s)

Figure 3. Key NOESY correlations observed for compound 1. 2.18 (3 H, s)

other obvious NOE correlations could be observed, thus making the relative configurations at the chiral centers C-3, C-5, and C14 unable to be determined. These problems encouraged us to make an effort toward a single-crystal X-ray experiment. With many attempts, single crystals suitable for an X-ray diffraction experiment were finally obtained by slow evaporation and slow diffusion of DMSO into a saturated mixture of CHCl3−MeOH (1:1) after 3 months. The structure and configurations of 1 were thus unambiguously established by the X-ray diffraction experiment using Cu Kα radiation (Figure 4).7 The resulting Flack parameter, 0.0(4), allowed for the establishment of its absolute configuration as 3S, 5R, 8S, 10S, and 14S. On the basis of the above data, the structure of 1 was determined and was named simpterpenoid A. Compound 1 is a tricyclic meroterpenoid possessing a highly functionalized ring C with gem-propane-1,2-dione and methylformate groups at C-14, and this skeleton has not appeared in natural products. From a biogenetic point of view, simpterpenoid A (1) might be derived from 3,5-dimethylorsellinic acid (DMOA).8 Briefly, the tetracyclic intermediate (III) could be biosynthesized by cyclization of the prenylated intermediate (II), which is derived from DMOA (I) by prenylation (Scheme 1), and this prenylation was also known in the biosynthesis of several

2.10 (3 H, s) 3.65 (3 H, s)

a Recorded at 500 and 125 MHz for 1H and 13C, respectively. Data were assigned by DEPT and HSQC as well as by COSY and HMBC experiments.

four aliphatic methylenes, and four methines (with one aldehydic, one olefinic, and one oxygenated), and carbon signals in 1. The 13C NMR and DEPT data (Table 1 and Supporting Information) showed signals corresponding to the 1H NMR data and exhibited the presence of 27 resonances comprising eight methyls (with one oxygenated), four aliphatic methylenes, four methines, and 11 nonprotonated (with three olefinic and four carbonyl) carbons. Comprehensive analysis of the COSY and HSQC spectra of 1 interpreted two discrete proton−proton spin-coupling systems, corresponding to a −CH2−CH2−CH(O)− unit (C-1 to C-3) and a −CH−CH2−CH2− unit (C-5 to C-7), and these two units were connected to each other via C-4 and C-8−C-10 to form rings A and B, as evidenced by the observed HMBC correlations (Figure 2), and the groups attached at C-3 and C-4 were confirmed by corresponding HMBC correlations, as well. The 1466

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evaluated against human pathogens (Aeromonas hydrophila, Escherichia coli, and Staphylococcus aureus), aquatic bacteria (Edwardsiella tarda, Micrococcus luteus, Pseudomonas aeruginosa, Vibrio alginolyticus, V. harveyi, and V. parahemolyticus), and plantpathogenic fungi (Alternaria brassicae, Colletotrichum gloeosporioides, Fusarium oxysporum, Gaeumannomyces graminis, and Physalospora piricola).11 Compound 1 showed weak activity against P. piricola, with an MIC value of 64 μg/mL, whereas amphotericin B, the positive control, has a MIC value of 4.0 μg/ mL. Compound 1 exhibited no potent inhibitory activity against other tested strains (MIC > 64 μg/mL). In conclusion, a new meroterpenoid, simpterpenoid A (1), was isolated and identified from the culture extract of the fungus P. simplicissimum MA-332, which was obtained from the rhizospheric soil of the marine mangrove plant B. sexangula var. rhynchopetala. This compound is a tricyclic meroterpenoid possessing a highly substituted and unsaturated ring C with a rare gem-propane-1,2-dione and methylformate groups. To our knowledge, this skeleton has never appeared in natural products. Simpterpenoid A (1) might attract attention from synthetic chemists due to its unique structure, which is a challenge for organic synthesis. Compound 1 exhibited potent inhibitory activity against influenza neuraminidase, which suggests that it could be an alternative therapeutic agent against the influenza virus or may serve as a drug lead for further study toward medicinal research and development to magnify the activity against the influenza virus.

Figure 4. X-ray crystallographic structure of compound 1.

Scheme 1. Proposed Biosynthetic Pathway for Compound 1

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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.8b00327. Experimental procedures, fungal material, extraction, and isolation, influenza neuraminidase inhibitory and antimicrobial assays, and full spectroscopic data of compound 1 (PDF) Accession Codes

CCDC 1498045 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

natural products such as andrastin in Penicillium chrysogenum, austinol in Aspergillus nidulans, and terretonin in Aspergillus terreus.9 The key intermediate (V) was speculated to be the product of the intermediate IV, the keto−enol tautomer of III, by multistep oxidization. Simpterpenoid A (1) could be produced from V by a series of reactions including decarboxylation, oxidization, and acetylation. Compound 1 was assayed for in vitro influenza neuraminidase inhibitory activity.10 The result (Figure 5) showed that 1 has potent inhibitory activity with an IC50 value of 8.1 nM. Oseltamivir was used as positive control, which has an IC50 value of 3.2 nM. The antimicrobial activities of compound 1 were

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Bin-Gui Wang: 0000-0003-0116-6195 Author Contributions §

H.-L.L. and R.X. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Natural Science Foundation of China (Grant No. 31570356) and by the NSFCShandong Joint Fund for Marine Science Research Centers

Figure 5. Influenza neuraminidase inhibitory curves of simpterpenoid A (1) (IC50 = 8.1 nM) and oseltamivir (positive control, IC50 = 3.2 nM). 1467

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Organic Letters (U1606403). B.-G.W. appreciates the support of Taishan Scholar Project from Shandong Province of China.

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REFERENCES

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