Note pubs.acs.org/jnp
Pyristriatins A and B: Pyridino-Cyathane Antibiotics from the Basidiomycete Cyathus cf. striatus Christian Richter,† Soleiman E. Helaly,†,‡ Benjarong Thongbai,§ Kevin D. Hyde,§ and Marc Stadler*,† †
Department of Microbial Drugs, Helmholtz Centre for Infection Research; and German Centre for Infection Research (DZIF), partner site Hannover/Braunschweig, Inhoffenstrasse 7, 38124 Braunschweig, Germany ‡ Department of Chemistry, Faculty of Science, Aswan University, Aswan 81528, Egypt § Institute of Excellence in Fungal Research and School of Science, Mae Fah Luang University, Chiang Rai 57100, Thailand S Supporting Information *
ABSTRACT: Two novel pyridino-cyathane diterpenoids, pyristriatins A and B (1 and 2), together with striatin C (3) were isolated from cultures of Cyathus cf. striatus, a basidiomycete that was found during a field trip in northern Thailand. The pyristriatins showed antimicrobial effects against Gram-positive bacteria and fungi. The isolation, structure elucidation, relative configuration, and biological and cytotoxic activity are described. Their structures were assigned by HRMS and NMR spectroscopy. We also describe the first 2D NMR assignment of striatin C. Pyristriatins A and B are the first cyathane natural products featuring a pyridine ring.
A
ntibiotic-resistant bacteria and emerging infectious diseases are on the rise, new therapeutic agents are needed more than ever.1 The Basidiomycota represent a relatively neglected source for promising natural products.2 Cyathane diterpenoids are a structurally diverse class of secondary metabolites that are primarily produced by basidiomycetes. The first compounds of this class of metabolites were isolated from Cyathus helenae in the 1960s.3 Later, several similar compounds were isolated from related species of the Nidulariaceae and in species of other fungal orders.4,5 In particular, the genus Cyathus is still a promising source for novel bioactive metabolites.6 Depending on their sources, the cyathane compounds were named cyathins, striatins, sarcodonins, scabronines, and erinacines. All of these compounds share a characteristic 5−6−7 tricyclic carbon skeleton.7 Many of these diterpenoids show biological activity, such as antimicrobial, anti-inflammatory, and antiproliferative properties, as well as osteoclast-forming suppressing, nerve growth factor activating, and agonistic effects toward the kappa-opioid receptor.8 Herein, we report on the isolation, structure determination, and biological evaluation of two novel cyathane compounds, pyristriatins A and B (1 and 2), and the known striatin C (3). The new metabolites are members of the cyathane diterpenoid family; they are related to the striatins but differ by the presence of a pyridine ring. The antimicrobial and cytotoxic properties of these metabolites are described. Furthermore, the morphological description and the phylogenetic position of the producing organism is illustrated and discussed. Pyristriatin A (1) was obtained as a white, amorphous powder. Molecular ion peaks at m/z 442.13 [M + H]+ and 440.17 [M − H]− revealed the molecular mass of 441.13. It © XXXX American Chemical Society and American Society of Pharmacognosy
Figure 1. Pyristriatin A (1), pyristriatin B (2), and striatin C (3).
displayed a molecular ion peak at m/z 442.2623 [M + H]+ (calcd 442.2588) in the HRESIMS spectrum, which was consistent with the molecular formula of C26H35NO5 with 10 degrees of unsaturation. The 1H NMR data (Table 1) showed a Received: March 4, 2016
A
DOI: 10.1021/acs.jnatprod.6b00194 J. Nat. Prod. XXXX, XXX, XXX−XXX
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Table 1. NMR Spectroscopic Data (700 MHz, CD3OD) for Pyristriatin A (1), Pyristriatin B (2), and Striatin C (3) 1 pos.
δC, type
1a 1b 2 3 4 5 6 7a 7b 8a 8b 9 10 11a 11b 12 13 14 15 16 17 18 19 20 1′ 2′ 3′ 4′ 5a′ 5b′ 6′
39.9, CH2 29.3, CH2 140.4, qC 139.1, qC 47.9, CH 47.5, qC 33.1, CH2 38.5, CH2 50.5, qC 68.8, CH 43.4, CH2 135.6, qC 150.8, qC 90.4, CH 151.9, CH 21.1, CH3 24.9, CH3 27.2, CH 22.5, CH3 22.0, CH3 107.9, CH 134.5, qC 145.9, qC 200.5, qC 67.2, CH2 57.4, OCH3
2 δH (J in Hz) 1.56, m 1.64, m 2.35, m
2.24, brs 1.18, 1.48, 1.40, 1.54,
m m m m
4.73, brd (4.3) 3.14, d (19.2) 3.34, d (16.7)
5.85, 8.45, 1.31, 0.81, 3.31, 1.04, 1.05, 6.47,
s s s s m d (6.7) d (6.7) s
4.91, d (19.3) 5.21, d (19.5) 3.59, s
δC, type
3 δH (J in Hz)
39.9, CH2
1.56, m 1.64, m 2.35, m
29.4, CH2 140.4, qC 139.0, qC 48.5, CH 47.8, qC 32.1, CH2
2.19, brs 0.89, 1.55, 1.38, 1.52,
38.4, CH2 50.4, qC 68.7, CH 43.5, CH2
m m m m
4.72, brd (4.3) 3.14, d, (18.9) 3.34, d (16.5)
135.9, qC 150.9, qC 89.3, CH 151.9, CH 21.0, CH3 24.9, CH3 27.2, CH 22.4, CH3 22.0, CH3 107.7, CH 135.1, qC 146.1, qC 200.5, qC 68.7
5.92, 8.46, 1.32, 0.79, 3.31, 1.04, 1.05, 6.58,
s s s s m d (7.0) d (6.7) d (2.1)
4.91, d (19.5) 5.21, d (19.5) 3.50, s
56.6, OCH3
characteristic aromatic singlet (δH 8.45) and together with the nitrogen atom implied by the molecular formula suggested the presence of a heterocyclic ring in compound 1. HSQC data established all 1J1H−13C connectivities. The planar structure of pyristriatin A was established by comprehensive analysis of the 2D NMR data, particularly COSY and HMBC data (Figure 2A), in which the HMBC correlations network from H-1 to C3/C-4/C-9, H-2 to C-4/C-9, H-5 to C-3/C-4/C-7/C-9/C-11/ C-16, H-10 to C-4/C12/C-6, H-11 to C-5/C-12/C-13, H-14 to C-13/C-6/C-7, H-7 to C-5/C-8/C-14, and H-8 to C-4/C-6/C7/C-9 enabled the construction of the characteristic 5−6−7 tricyclic carbon skeleton (rings A, B, and C) in 1. In addition, HMBC correlations from the methyls H3-16 to C-14/C-5/C-6/ C-7, H3-17 to C-8/C-9/C-1, and H-18 to C-2/C-3/C-4 and from the two germinal methyls H3-19/H3-20 to C-18/C-3 indicated the presence of 10-hydroxycyathane in compound 1. The previously mentioned data and biosynthetic reasoning suggested that 1 was likely a new cyathane derivative containing a heterocyclic ring. Furthermore, HMBC correlations from H15 (δH 8.45) to C-11/C-12/C-13/C-3′ enabled the construction of a pyridine ring attached at C-12/C-13 of the cyathane skeleton (ring E). The five-membered 2-methoxytetrahydrofuran D-ring was established by the HMBC correlation from H-1′ (δH 6.47) to C-14 and C-13 together with an HMBC correlation from H-14 (δH 5.85) to C-2′ and a correlation from the methoxyl group H-6′ to C-1′. Finally, the presence of a 2hydroxyethanone moiety was deduced by an HMBC
δC, type 40.7, CH2 29.5, CH2 140.7, qC 138.1, qC 48.2, CH 41.4, qC 30.3, CH2
δH (J in Hz) 1.52, m 1.72, m 2.33, m
2.23, brs
38.4, CH2
1.44, m 1.72, m 1.56, m
50.6, qC 70.2, CH 133.4, CH
4.63, d (7.0) 6.09, m
136.4, qC 48.6, CH 91.6, CH 99.4, CH 21.8, CH3 24.5, CH3 26.9, CH 22.2, CH3 22.4, CH3 107.3, CH 81.3, qC 97.9, qC 70.2, CH 65.2, OCH2 56.7, OCH3
2.99, 4.65, 5.24, 1.22, 1.02, 3.39, 0.97, 1.02, 4.94,
d (9.5) d (9.1) brs s s m d (7.1) d (6.4) s
3.88, 3.66, 3.75, 3.56,
dd (4.9, 9.2) dd (4.9, 10.1) dd (9.1, 10.7) s
Figure 2. Key HMBC (arrows) and COSY (green bonds) correlations of 1 and 2 (A) and 3 (B).
B
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C-5, C-6, and C-9, and consequently the tentative absolute configuration of 1 is assumed to be 5R,6R,9R,10S,14R,1′R. Pyristriatin B (2) was obtained as a white, amorphous powder. The EIMS data of 2 were identical with compound 1, which exhibited a molecular mass of 441.13, and similarly the HRESIMS data (m/z 442.2625, [M + H]+) gave the molecular formula of C26H35NO5 (calcd 442.2588). Nevertheless, compound 2 showed a difference of 0.35 min in the HPLC chromatogram. The 2D NMR data analysis, particularly the HMBC (Figure 2A) revealed that compound 2 has the same planar structure as 1. Its 1H and 13C NMR data revealed a high similarity with those of 1, and the slight differences observed were closely associated with C-1′, with the H-7 (δH 1.18/1.48, 1; 0.89/1.55, 2), H-14 (δH 5.85, 1; 5.92, 2), H-1′ (δH 6.47, 1; 6.58, 2), and the methoxy group (δH 3.59, 1; δH 3.50, 2) suggesting that they were C-1′ epimers. The relative configuration of pyristriatin B was assigned on the basis of its NOESY spectrum, which showed that the stereocenters from rings A to C were identical with those of 1. Nevertheless, the key NOE correlation between H-1′ and H-7a and between H7a and H-5 indicated that the plane of rings D and E is vertical on the cyathane rings and H-1′ is β-oriented (Figure 3B). Furthermore, on comparing its NMR data with those of 1 (Table 1), H-7a in 2 was significantly more shielded due to the effect of the methoxy group (H-6′), while H-14 was more deshielded owing to the absence of the methoxy group (Figure 4), which confirmed the different configurations at C-1′. Thus,
correlation from the oxygenated methylene to the carbonyl carbon (C-4′); its position attached to C-3′ of the pyridine ring was determined by a weak HMBC correlation from H-5′ to C3′. Consequently, compound 1 was established as a novel pyridino-cyathane diterpenoid, which was named pyristriatin A, representing the first member of the cyathane diterpenoid family possessing a pyridine ring. The relative configuration of 1 was established by examination of its NOESY data (Figure 3A). A strong NOE
Figure 3. NOESY correlations of 1 (A) and 2 (B). Figure 4. Newman projections along C-1′/C-14/C-7 bonds for 1 and 2, describing the deshielding effect of the methoxy group on H-7a and H-14.
correlation between H-14 and H3-16 showed that they were on the same side of the cyathane skeleton and were assigned to be α-oriented. In addition, NOEs between H-5 and H-10/H3-17/ H2-7a and between H-10 and H2-11b/H3-18/H3-20 showed that H-5, H-10, and H3-17 were cofacial and β-oriented. Furthermore, no correlations were observed between H-1′ and H2-7, while a weak correlation between the methoxy group H36′ and H-7b was observed, indicating that rings D and E are vertical to the cyathane unit and H-1′ is α-directed. Thus, the relative configuration of pyristriatin A was assigned as shown in (Figure 3A), which was in good accordance with those of the striatal/striatin group.6 Several attempts including crystallization and Mosher esterification, as well as recording the 1H NMR of 1 in the chiral solvent BMBA-p-Me reported by Kobayashi et al.,9 were unsuccessful in determining the absolute configuration of 1. The NMR data revealed that the esterification using (R)- and (S)-MTPA chlorides occurs on the less sterically hindered hydroxy 5′-OH rather than the targeted hydroxy 10-OH. Nevertheless, the diterpenoid part of striatals/striatins is formed via the mevalonate pathway,10 and to the best of our knowledge all 105 cyathane diterpenoids hitherto reported proved to have the same absolute configuration at C-5, C-6, and C-9 of the cyathane scaffold.7 We assume that the cythane scaffold of pyristriatin A (1) is formed by the same pathway, as it has been coisolated with the known striatin C (3) from the same strain. Therefore, compound 1 should have the same absolute configuration at
compound 2 was established as the 1′-epimer of 1 and named pyristriatin B. The absolute stereochemistry of pyristriatin B was determined, on the same basis previously argued for compound 1, as 5R,6R,9R,10S,14R,1′S. Compound 3 was isolated from the same fermentation as a white powder possessing a molecular formula of C26H38O8 as determined by HRESIMS, which showed a peak at m/z 461.2550 [M + H − H2O]+ (calcd 461.2534). A database search revealed that the molecular formula of 3 is identical with that of striatin C, an antibiotic previously isolated from Cyathus striatus.6 To the best of our knowledge there were no 2D NMR data published for striatin C. Thus, herein we described the first 2D NMR assignment of striatin C (3). The 1H NMR spectrum exhibited signals for four methyls (one methoxy), five methylenes (one oxygenated), and nine methines (five oxygenated and one olefinic). The 13C NMR data with a DEPT spectrum showed 26 carbon resonances. Furthermore, comprehensive analysis of the 2D NMR data, particularly COSY and HMBC correlation networks (Figure 2B), confirmed the planar structure of 3 to be identical with striatin C. In addition, further analysis of the NOESY data in which the key correlation between H-5 and H-10/H-13/H3-17 and between H-13 and H-5/H2-7a showed that H-5, H-10, H-13, C
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Table 2. Antimicrobial and Cytotoxic Activities of Compounds 1 and 2
a
test strains
1
2
reference
Bacteria Bacillus subtilis DSM 10 Chromobacterium violaceum DSM 30191 Escherichia coli DSM 498 Micrococcus luteus DSM 1790 Pseudomonas aeruginosa PA 14 Staphylococcus aureus DSM 346 Fungi Candida albicans DSM 1665 Mucor plumbeus MUCL 49355 Pichia anomala DSM 6766 Rhodotorula glutinis DSM 10134 Schizosaccharomyces pombe DSM 70572 Cell Lines Huvec KB 3.1 HeLa mouse fibroblasts L929
MIC (μg/mL) 9.4 n.a.a n.a. 8.3 n.a. 8.3
9.4 n.a. n.a. 16.7 n.a. 16.7
≤2.3 2.1 ≤2.3 8.3 0.5 1
ciprofloxacin oxytetracycline ciprofloxacin oxytetracycline gentamicin oxytetracycline
n.a. 150 n.a. 16.7 16.7
16.7 9.4 16.7 66.7 66.7
nystatin nystatin nystatin nystatin nystatin
5.9 14.7 16.3
0.00055 0.00022 0.0038
n.a. 150 n.a. 8.3 8.3 IC50 (μM) 6.3 12.7 15.4
epothilone B epothilone B epothilone A
n.a. no activity.
and H3-17 are cofacial and were assigned to be in a βconfiguration. In addition, H-14, H3-16, and H-4′ were assigned to the α-configuration due to strong correlations between H-14 and H3-16/H-4′. Finally correlations between H-10 and H-11, and H-11 and H-15, revealed that H-15 is α-oriented. Consequently, our 2D NMR assignment including NOESY data confirmed that compound 3 is striatin C, and this is the first report of the 2D NMR assignment. The producing organism is named Cyathus cf. striatus based on morphological and molecular data of the specimen and culture (Supporting Information). To the best of our knowledge, pyristriatins A and B are the first cyathane diterpenoids featuring a pyridine ring and therefore also constitute the first members of a novel heterocyclic diterpene carbon skeleton. We assume that the pyristriatins are descend from the striatals/striatins biosynthetic pathway.10 In addition, the incorporation of ammonia may lead to the formation of the pyridine ring. Pyristriatins A and B were tested for antimicrobial and cytotoxic properties against various bacteria, fungi, and mammalian cell lines (Table 2). They showed antibacterial effects exclusively against Gram-positive bacteria such as the pathogenic Staphylococcus aureus, antifungal effects against filamentous fungi as well as yeasts, and moderate cytotoxic activities. The striatins are known to possess broad-spectrum antimicrobial activities, but compared to the pyristriatins also act against some Gram-negative bacteria.11
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Collection and Characteristics of the Strain. The fungal material was collected in August 2014 in the tropical rainforest near the MRC (http://www.mushroomresearchcentre.com/), Chiang Mai Province, Thailand. A voucher specimen and the corresponding culture have been deposited at the mycological herbarium of Mae Fah Luang University, Chiang Rai (MFLU15-1416 and MFLUCC14-0770, respectively). Sequences of the rDNA (5.8S gene region, the internal transcribed spacer 1 and 2 (ITS) and part of the large subunit (LSU)) are deposited in GenBank with acc. nos. KU865513 (ITS) and KU865514 (LSU). DNA extraction was performed with the EZ-10 Spin Column Genomic DNA Miniprep kit (Bio Basic Canada Inc., Markham, Ontario, Canada). A Precellys 24 homogenizer (Bertin Technologies, France) was used for cell disruption at a speed of 6000 rpm for 2 × 40 s. The gene regions were amplified with primers ITS 1f and ITS4 (ITS) and LR0R and LR7 (LSU). According to the BLAST search results of the submitted sequences, the fungal culture belongs to the striatum group within the genus Cyathus and is closely related to Cyathus striatus and C. stercoreus.12 A morphological investigation of the specimen confirmed a close relationship of the first-mentioned species, but also revealed differences. For a detailed description of the producer strain and the morphological characterization and comparison of the specimen see the Supporting Information. Fermentation, Extraction, and Isolation. Small pieces from well-grown YMG agar were used to inoculate 200 mL of YM medium contained in eight 500 mL Erlenmeyer flasks, which were shaken on a rotary shaker at 23 °C and 140 rpm. After 20 days the free glucose was used up and the cultures were harvested. The mycelium was separated from the culture broth by using gauze and filtration. The mycelium was extracted two times with acetone in an ultrasonic bath at 40 °C for 30 min, and the solvent was evaporated in vacuo (40 °C). The remaining aqueous residue was diluted with the same amounts of ethyl acetate and water and extracted three times. The extracts were combined, dried over sodium sulfate, and again evaporated in vacuo (40 °C) to dryness. After filtration using an RP solid-phase cartridge (Strata-X 33 μm, polymeric reversed phase; Phenomenex, Aschaffenburg, Germany) 1 g of crude product was obtained. Preparative HPLC. The crude mycelial extract was fractionated by preparative RP-MPLC [column 480 × 30 mm (Kronlab), ODS-AQ C18, 15 μm; solvent A: deionized water (Milli-Q), solvent B: methanol; gradient system: 30% B for 5 min increasing to 100% B in 170 min, holding at 100% B for 20 min; flow rate 15 mL/min, UV detection at 254 nm]. RP-HPLC [VP 250/21 Nucleodur100-5 C18 ec column (Macherey−Nagel) equipped with a Kromasil 100 precolumn (50 × 20 mm, 7 μm; AkzoNobel); acetonitrile−water gradient with 0.05% trifluoroacetic acid, 5 min at 80% to 100% solvent B in 20 and 5
EXPERIMENTAL SECTION
General Experimental Procedures. Optical rotations were determined with a PerkinElmer 241 polarimeter. NMR data were recorded on a Bruker 500 MHz Avance III spectrometer with a BBFO(plus) SmartProbe (1H 500 MHz, 13C 125 MHz) and a Bruker 700 MHz Avance III spectrometer with a 5 mm TCI cryoprobe (1H 700 MHz, 13C 175 MHz). HRESIMS mass spectra were obtained with an Agilent 1200 series HPLC-UV system combined with an ESI-TOFMS (Maxis, Bruker) [column 2.1 × 50 mm, 1.7 μm, C18 Acquity UPLC BEH (Waters), solvent A: H2O + 0.1% formic acid; solvent B: AcCN + 0.1% formic acid, gradient: 5% B for 0.5 min, increasing to 100% B in 19.5 min, maintaining 100% B for 5 min, RF = 0.6 mL min−1, UV detection 200−600 nm]. D
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min at 100%, flow 15 mL/min] provided 4.5 mg of 1 and 5.3 mg of 2. The compounds eluted at 14−15 min (1) and 16−17 min (2), respectively. Pyristriatin A (1): white powder; [α]25D −185 (c 0.1, MeOH); 1H NMR and 13C NMR see Table 1; LCMS m/z 442.17 [M + H]+ (100), 440.12 [M − H]− (93), 486.09 [M − H + HCOOH]− (100), 422 (23), 390 (16); HRESIMS m/z 442.2623 [M + H]+ (calcd for C26H35NO5, 442.2588). Pyristriatin B (2): white powder; [α]25D −100 (c 0.07, MeOH); 1H NMR and 13C NMR see Table 1; LCMS m/z 442.13 [M + H]+ (100), 440.17 [M − H]− (87), 486.11 [M − H + HCOOH]− (100), 422 (20), 390 (17); HRESIMS m/z 442.2625 [M + H]+ (calcd for C26H35NO5, 442.2588). Striatin C (3): white powder; [α]25D −15 (c 1, MeOH); 1H NMR and 13C NMR see Table 1; LCMS m/z 447.20 [M + H − CH3OH]+ (69), 461.22 [M + H − H2O]+ (19), 429 (100), 411 (15), 445.05 [M − H − CH3OH]− (100), 491.03 [M − H + HCOOH]−, 385 (10), 373 (13); 357 (22), 339 (19); HRESIMS m/z 461.2550 [M + H − H2O]+ (calcd for C26H38O8, 461.2534). Serial Dilution Assay and Cytotoxicity Assay. The MIC and the in vitro cytotoxicity (IC50) were determined according to our previously reported procedure.13
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(6) Hecht, H.-J.; Höfle, G.; Steglich, W.; Anke, T.; Oberwinkler, F. J. Chem. Soc., Chem. Commun. 1978, 15, 665−666. (7) Tang, H.-Y.; Yin, X.; Zhang, C.-C.; Jia, Q.; Gao, J.-M. Curr. Med. Chem. 2015, 22, 2375−2391. (8) De Silva, D. D.; Rapior, S.; Sudarman, E.; Stadler, M.; Xu, J.; Aisyah Alias, S.; Hyde, K. D. Fungal Divers. 2013, 62, 1−40. (9) Kobayashi, Y.; Hayashi, N.; Kishi, Y. Org. Lett. 2002, 4, 411−414. (10) Anke, T.; Rabe, U.; Schu, P.; Eizenhöfer, T.; Schrage, M.; Steglich, W. Z. Naturforsch., C: J. Biosci. 2002, 57, 263−271. (11) Anke, T.; Oberwinkler, F.; Stegich, W.; Höfle, G. J. Antibiot. 1977, 30, 221−225. (12) Zhao, R.-L.; Desjardin, D. E.; Soytong, K.; Hyde, K. D. Persoonia - Mol. Phylogeny Evol. Fungi 2008, 21, 71−76. (13) Surup, F.; Thongbai, B.; Kuhnert, E.; Sudarman, E.; Hyde, K. D.; Stadler, M. J. Nat. Prod. 2015, 78, 934−938.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.6b00194. Experimental procedures, 1D and 2D NMR data, LCMS data, morphological and phylogenetic details of the producing organism (PDF)
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AUTHOR INFORMATION
Corresponding Author
*Tel: +49 531 6181-4240. Fax: +49 531 6181 9499. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We are grateful to W. Collisi for conducting the bioassays and C. Kakoschke, A. Gollasch, C. Schwager, and H. Steinmetz for recording NMR and HPLC-MS data. Financial support by the German Academic Exchange Service (DAAD) and the Thai Royal Golden Ph.D. Jubilee-Industry program (RGJ) for a joint TRF-DAAD PPP (2012−2014) academic exchange grant to K.D.H. and M.S. and the RGJ for a personal grant to B.T. (No. Ph.D/0138/2553 in 24.S.MF/53/A.3) is gratefully acknowledged. K.D.H. would like to thank the Thailand Research Fund for a grant (BRG5580009). S.E.H. would like to thank the Alexander von Humboldt Foundation for funding through a Georg Forster research fellowship (HERMES).
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REFERENCES
(1) Spellberg, B.; Shlaes, D. Clin. Pharmacol. Ther. 2014, 96, 151− 153. (2) Stadler, M.; Hoffmeister, D. Front. Microbiol. 2015, 6, doi: 10.3389/fmicb.2015.00127. (3) Allbutt, A. D.; Ayer, W. A.; Brodie, H. J.; Johri, B. N.; Taube, H. Can. J. Microbiol. 1971, 17, 1401−1407. (4) Mudalungu, C. M.; Richter, C.; Wittstein, K.; Abdalla, M. A.; Matasyoh, J. C.; Stadler, M.; Süssmuth, R. D. J. Nat. Prod. 2016, 79, 894. (5) Thongbai, B.; Rapior, S.; Hyde, K. D.; Wittstein, K.; Stadler, M. Mycol. Prog. 2015, 14, 1−23. E
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