Letter Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Trematosphones A and B, Two Unique Dimeric Structures from the Desert Plant Endophytic Fungus Trematosphaeria terricola Bo Song,†,⊥ Ling-Yu Li,† Hai Shang,† Yang Liu,† Meng Yu,† Gang Ding,*,† and Zhong-Mei Zou*,† †
Institute of Medicinal Plant Development, Peking Union Medical College and Chinese Academy of Medical Sciences, Beijing 100193, People’s Republic of China ⊥ College of Pharmacy, Qiqihar Medical University, Qiqihar 161006, Heilongjiang, People’s Republic of China
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S Supporting Information *
ABSTRACT: Trematosphones A (1) and B (2), two unique dimers with novel structural features, were isolated from the special bioenvironmental desert plant endophytic fungus Trematosphaeria terricola. Their structures were assigned on the basis of spectroscopic approaches including NMR, calculated electronic circular dichroism data, and X-ray diffraction analysis. Possible biosynthetic pathways of these two compounds were proposed. Trematosphone A (1) exhibited protective activities against the corticosterone-induced damages in PC12.
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pecial bioenvironmental microbes from deep sea, Arctic zones, and symbiotic environments are vital sources of natural products for the discovery of drugs and agricultural chemicals as they produce many secondary metabolites with novel structural features and diverse biological effects.1 For instance, endophytic fungi, which inhabit in desert plants suffering from strong light intensity, high salt-alkali conditions, and water and nutrition shortage, are a unique group of special bioenvironmental microbes. They are ecologically important, but their value in chemistry is rarely investigated. Our group recently initiated the chemical investigation of this special fungal resources collected from desert plants inhabiting in Northwest of China. From these fungi we isolated a series of secondary metabolites originated from diverse biosynthetic pathways with a wide range of biological effects.2 In our ongoing search for bioactive and novel metabolites from this special group of plant endophytic fungi, two novel dimers trematosphones A (1) and B (2) with unusual structural features were isolated from the desert plant endophytic fungus Trematosphaeria terricola. Trematosphone A (1) features a unique 6/7/6/5-fused tetracyclic carbon skeleton, whereas trematosphone B (2) possesses an axial symmetrical tricyclic system (Figure 1). The isolation, structural elucidation, possible biogenetic pathway, and biological activities of 1 and 2 are provided in this report. The molecular formula of trematosphone A (1) was determined to be C18H22O8 (8 degrees of unsaturation) on the basis of HRESIMS (m/z 389.1200, [M + Na]+, calcd 389.1212). Analysis of the 1H, 13C and HSQC NMR data of 1 (Table 1 and Figures S1−S4) revealed the presence of 18 carbons, consisting of three carbonyl carbons, two olefinic carbons, a methine unit, six quaternary carbons, four methyl © XXXX American Chemical Society
Figure 1. Structures of trematosphones A (1) and B (2).
groups, two methoxy groups, and two exchangeable protons. These data accounted for all 1H and 13C NMR resonances and required compound 1 to be a tetracyclic ring system. Since there are many quaternary carbons in 1, the structural elucidation of 1 was mainly characterized by the HMBC spectrum (Figure S5−S6). The HMBC correlations from H-2 to C-1, C-3, C-4, and C-6, from 13-Me to C-1, C-5, and C-6, from 5-OH to C-4, C-5, and C-6, from 14-Me to C-3, C-4, and C-5 together with the correlation from −OMe to C-3 established a cyclohex-2-en-1-one ring system; the HMBC correlations of 15-Me with C-9, C-10, and C-11, of 10-OH with C-9, C-10, and C-11, of 16-Me with C-7, C-10, and C-12, Received: February 3, 2019
A
DOI: 10.1021/acs.orglett.9b00454 Org. Lett. XXXX, XXX, XXX−XXX
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Organic Letters Table 1. 1H (500 MHz) and 13C NMR (125 MHz) Data (DMSO-d6) of 1
with the experimental spectrum (Figures 3 and S8), which indicated the absolute configuration to be 4R,5R,6S,7R,8S,10S,12S.
1 no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 3-OCH3 7-OCH3 5-OH 10-OH
δH, mult 5.56, s
3.82, s
1.43, 0.82, 1.33, 1.03, 3.69, 3.27, 6.68, 6.27,
s s s s s s s s
δC 192.0 100.1 174.4 49.9 88.0 80.6 107.6 55.8 202.5 74.1 206.1 63.2 17.5 19.2 18.0 9.5 56.3 52.4
Figure 3. Experimental and calculated ECD spectra of 1.
Trematosphone B (2) was obtained as a colorless crystal, and its molecular formula was determined to be C18H22O9 by HRESIMS on the [M + Na]+ (m/z 405.1168, calcd 405.1162 for C18H22O9Na) (Figure S9) with 8 degrees of unsaturation. Analysis of the 1H and 13C data of 2 (Table 2 and Figures S10
of H-8 with C-7, C-9, C-10, and C-12, together with HMBC of a −OMe with C-7 constructed a cyclohexane-1,3-dione ring system with a −OMe group anchored at C-7; and the key HMBC correlations from 14-Me to C-12, from 16-Me to C-4, and from 5-OH to C-8 established the connectivity of C-4 with C-12 and C-5 with C-8, respectively. Considering the degrees of unsaturation, molecular formula, and 13C chemical shift values of C-6 (δC 80.6)/C-7 (δC 107.6), an ether bond must exist between C-6 and C-7, which formed a tetracyclic system with a ketal carbon at C-7. Thus, the planar structure of 1 was elucidated (Figure 2.).
Table 2. 1H (500 MHz) and (DMSO-d6) of 2
13
C NMR (125 MHz) Data 2
no. 1, 2, 3, 4, 5, 6, 7, 8, 3, 5, 6,
1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 3′-OCH3 5′-OH 6′-OH
δH, mult
1.78, 1.22, 3.61, 6.57, 5.36,
s s s s s
δC 200.6 120.8 159.7 128.6 112.7 80.2 9.0 22.0 59.1
and S11) revealed that only 11 protons and nine carbons were observed in the NMR spectra of 2, implying that this compound might be a symmetrical dimer. The HSQC NMR (Figure S12) revealed the presence of one carbonyl carbon, three olefinic carbons, two oxygenated quaternary carbons, two methyl groups, a methoxy group, and two exchangeable protons. The gross structure of 2 was elucidated by the HMBC analysis (Figures 4 and S13). HMBC correlations from 7-Me to C-1, C-2, and C-3 indicated that C-1, C-3, and C-7 are all connected to C-2; correlations from 6-OH (δH 5.36) to C-1, C-5, C-6, and C-8 and from 8-Me to C-1, C-5, and C-6 led to the connection of C-6 with the hydroxyl group (δH 5.36), C-1, C-5, and C-8; cross-peaks in the HMBC spectrum from 5-OH (δH 6.57) to C-4, C-5, and C-6 revealed the connectivity of C5 to the hydroxyl group (δH 6.57), C-4, and C-6; a methoxy group (δH 3.61) was placed at C-3 based on the HMBC correlations between these two groups; and the key long-range HMBC correlation from 7-Me to C-4 linked C-3 with C-4,
Figure 2. Key HMBC and NOESY correlations of 1.
The relative stereochemistry of 1 was determined by NOESY correlations (Figure 2 and Figure S7). The NOESY correlations from 5-OH to 13-Me and 14-Me assigned these groups on the same side of the cyclohex-2-en-1-one ring; correlations from H-8 to 5-OH and 13-Me suggested the same orientation of these groups on the tetrahydrofuran, whereas the correlations from 15-Me to 16-Me and 7-OMe together with the one from 16-Me to 7-OMe placed these groups on the opposite position of the corresponding cyclohexane-1,3-dione ring. Thus, the relative configuration of 1 was determined (Figure 2). The absolute configuration of 1 was determined by the comparison of the ECD spectrum recorded in MeOH and the DT-DFT-calculated spectrum of 1 at the B3LYP16-311+G(d,p) level.3,4 The calculated ECD spectrum of 1 matched well B
DOI: 10.1021/acs.orglett.9b00454 Org. Lett. XXXX, XXX, XXX−XXX
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addition, the core skeleton of 1 and 2 is essentially the same as that of dihydrodibenzofuran through different postmodifications upon biosynthesis via the polyketide pathway.9 In this regard, trematosphones A (1) and B (2) were tentatively suggested to be the origin of biosynthetic polyketides (Scheme 1). The monomer 2-hydroxy-2,4-dimethylcyclohexane-1,3,5trione was first formed in the PKSs pathway, through different postmodifications, including oxidation, cyclization, and methylation; hence, the complex end products 1 and 2 were shaped. The corticosterone-induced damage in PC12 cells model is a classic antidepressant example in vitro.10 The protective effects of compounds 1 and 2 on corticosterone-induced PC 12 cell injuries were examined via the MTT method.11 Compound 1 exhibited a protective effect against induced injury in PC 12 cells at 6.25 μM concentration (P < 0.05, vs corticosterone treated group), while compound 2 was inactive. PC 12 cells were incubated with various concentrations of trematosphone A (0−100 μM) for 24 h; there was no significant difference in cell survival rate compared with the control group (P > 0.05). The result indicated that compound 1 may not be toxic to PC12 cells. Trematosphone A (1) possesses a unique 6/7/6/5-fused tetracyclic system with seven stereocenters (six carbons C-4-C5(C-6)-C-8-C-7-C-12 are continued), and eight oxygen atoms forming different groups including carbonyl, ether bond, −OH, and −OMe are contained in this complex secondary metabolite, which will be a potential star molecule to organic synthesis. Trematosphone B (2) possesses a highly axial symmetrical structure as a rigid conjugated system with nine oxygen atoms contained in this small molecule, which is unique in natural products. Our result further implied that special bioenvironmental microbes such as endophytic fungi from desert plants with little chemical investigation might be a new resource for novel and bioactive secondary metabolites.
Figure 4. Key HMBC correlations of 2.
which established a cyclohex-2-en-1-one ring system (Figure 4). Accounting for the chemical shift value of C-4, C-4′ (δC 128. 6) and C-5, C-5′ (δC 112.7), molecular formula, and degrees of unsaturation, the other identical parts must be present in the structure of 2 to form a completely symmetrical structural system. Fortunately, a single-crystal of 2 was grown for X-ray diffraction analysis, which determined the absolute configuration of 2 as 5S, 6S, 5′S, 6’S (Figure 5).
Figure 5. X-ray crystal structure of 2.
Elbandy et al. isolated polyketide pyranones and cyclohexenones from a sponge-derived fungus Paecilomyces lilacinus.5 Actually, we also isolated polyketide pyranones (5 and 6),6 polyketide cyclohexenone wasabidienone B0 (3),7 and the possible monomeric precursor (4) (Table S1 and Figures S16−S18) of 1 and 2 from the this fungus. Compound 4 possesses the same key skeleton of cyclohexenones as that of wasabidienone B1, which rearranged to wasabidienone B0 (3). These two compounds were recently totally synthesized.8 In
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ASSOCIATED CONTENT
S Supporting Information *
TThe Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b00454.
Scheme 1. Plausible Biosynthetic Pathways of 1 and 2
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DOI: 10.1021/acs.orglett.9b00454 Org. Lett. XXXX, XXX, XXX−XXX
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Experimental procedures, calculated ECD spectrum for compound 1, X-ray crystal data for compound 2, HRESIMS, 1D NMR, and 2D NMR spectra of 1 and 2 (PDF) Accession Codes
CCDC 1895035 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
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Gang Ding: 0000-0002-7297-116X Zhong-Mei Zou: 0000-0002-2466-3146 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We acknowledge financial support from the CAMS Innovation Fund for Medical Sciences (CIFMS, 2016-I2M-3-015, for Z.Z.M.), the Chinese National S&T Special Project on Major New Drug Innovation (2018ZX09711001-008, for Z.Z.M.), and the National Key Research and Development Program of China “Research and Development of Comprehensive Technologies on Chemical Fertilizer and Pesticide Reduction and Synergism” (2017YFD0201402, for D.G.). We acknowledge Dr. Bing-Da Sun (Institute of Microbiology, Chinese Academy of Sciences, Beijing 100101, People’s Republic of China) for providing the fungus used in this work.
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DOI: 10.1021/acs.orglett.9b00454 Org. Lett. XXXX, XXX, XXX−XXX
