The myxocoumarins A and B from Stigmatella aurantiaca strain MYX-030

Summary The myxobacterial strain Stigmatella aurantiaca MYX-030 was selected as promising source for the discovery of new biologically active natural products by our screening methodology. The isolation, structure elucidation and initial biological evaluation of the myxocoumarins derived from this strain are described in this work. These compounds comprise an unusual structural framework and exhibit remarkable antifungal properties.

But also from an agrochemical point of view, myxobacteria have already furnished a large set of promising lead structures, in particular in the context of antifungal compounds with around 50% of myxobacteria-derived natural products exhibiting such properties [10]. This activity of myxobacterial metabolites results from diverse modes of action, such as inhibition of DNA replication, transcription and translation, as well as interference with cell membrane stability [21]. Inhibition of acetyl- CoA carboxylase is exerted by soraphen A 1α (2), which has been shown to be a potent and broad spectrum fungicide [22,23]. The most dominant target of myxobacterial antifungal agents, however, is the mitochondrial respiratory chain [21]. The compounds thereby either target complex I (NADH dehydrogenase) of the electron transport chain, as for example pyrrolnitrin (3) [24,25], which is in use for the treatment of fungal skin infections [26] and served as a lead structure for the development of the agrochemical agents fenpiclonil (4) and fludioxonil [27], or complex III (cytochrome bc 1 ), as for example myxothiazol A (5) [28,29]. Compounds of the same structural family can also target different target complexes simultaneously, as for example in the case of aurachins (e.g. aurachin A (6), Figure 1) [30][31][32]. Due to the remarkably high percentage of antifungal myxobacterial natural products, we investigated the potential of new myxobacterial isolates towards the production of novel, agrochemically relevant antifungal lead structures. The results obtained with strain Stigmatella aurantiaca MYX-030 are described herein.

Results and Discussion
The myxobacterial strain Stigmatella aurantiaca MYX-030 has been selected as interesting producer of bioactive natural products by our standardized hit follow-up process. Initial dereplication of known compounds was performed by using HPLC-UV-MS [33] combined with Peak-Activity-Correlation (PAC) data. Taken together these results indicated the presence of the known antifungal myxobacterial compound myxothiazole A (5), as well as the aurachins A (6) and C in the extract. In addition to these metabolites, a series of non-polar compounds exhibiting strong antifungal activity was detected. Comparison of UV and molecular mass data of these secondary metabolites with an in-house compound library resulted in no hits, thus prompting us to their isolation and structural elucidation.
The UV spectrum of 7 revealed maxima at 218 and 286 nm, strongly suggesting the presence of an aromatic system within the structure. Negative mode ESIMS analysis of the most abundant congener 7 of this set of natural products showed strong signals at 378.  (Table 1). These signals were assigned to three methyl groups (14.0, 16.5, and 21.2 ppm), eight methylene units (22.6, 23.4, 29.2, 29.3, 29.4, 29.8, 31.8, and 38.3 ppm), a quaternary carbon at 46.5 ppm, a heteroatom-substituted quaternary carbon -most likely a tertiary alcohol -at 79.1 ppm, an ester-type functionality at 172.5 ppm, and six sp 2 -hybridized carbon atoms as part of an highly heteroatom-substituted aromatic system (103.2, 108.0, 118.6, 148.3, 149.2, and 157.8 ppm). The presence of the latter moiety was further corroborated by the 1 H NMR spectral data: two doublets with chemical shifts at 7.43 and 7.35 ppm and a coupling constant of 2 Hz clearly evidenced a highly substituted aromatic ring system bearing two protons in metaposition to each other.
The proton signals at 7.43 and 7.35 ppm correlated in an HSQC experiment with the 13 C NMR signals at 108.0 and 103.2 ppm, respectively. The structural organization of the aromatic system was further investigated by HMBC spectroscopy. The proton at  7.43 ppm showed strong correlations with two of the heteroatom-substituted carbons at 157.8 and 148.3 ppm, the putatively alkyl substituted position at 118.6 ppm, as well as the C atom at 103.2 ppm. The proton at 7.35 ppm, in turn, correlated with the C atoms at 149.2, 148.3, and 118.6 ppm, leading to overall structural fragment I (F-I) shown in Figure 2. In addi-tion to the aromatic protons, three methyl signals were observed in the 1 H NMR spectra: a triplet at 0.86 ppm ( 3 J = 7 Hz), suggesting a methyl group directly attached to a CH 2 -unit, as well as two singlets at 1.18 and 1.33 ppm. Furthermore, a pair of diastereomeric protons at 1.56 and 1.99 (higher order multiplets with a triplet of doublets structure) and a set of diastereomeric protons at 1.08 and 1.49 ppm (m) were observed. The signals of the other six methylene units formed a multiplet at 1.14-1.30 ppm, indicating the presence of a linear alkyl chain in the molecule. This is corroborated by the HSQC correlations to the C atoms at 29 ppm, as well as 22.6 and 31.8 ppm. In the HMBC data, strong correlations of the isolated methyl groups to each other, to the putative ester functionality, the quaternary carbon at 46.5 ppm, and the tertiary alcohol carbon at 79.1 ppm led to the assembly of structural fragment II (F-II, Figure 2). Further substitution of the latter was obvious from HMBC cross peaks of the diastereotopic CH 2 group (1.56 and 1.99 ppm) to the latter position (79.1 ppm) and to the carbon bearing the two methyl groups (46.5 ppm), as well as to the aromatic carbon at 118.6 ppm. In addition, this methylene unit exhibited correlations with two further CH 2 groups of the linear alkyl chain. This side chain turned out to consist of 8 neighboring CH 2 units with the methyl functionality at 0.86 ppm as the end group, as evident from HMBC correlations of the latter to the final two methylene units of this chain (F-III, Figure 2). Taken together, this information allowed the assembly of all aforementioned structural fragments to give a draft structure 8. Given the chemical shifts of the carbonyl C-2 (172.5 ppm) and of carbon C-8' (149.2 ppm), the presence of a lactone moiety can be deduced. The resulting overall structure still lacks three oxygens, a hydrogen, and a nitrogen atom combined with one degree of unsaturation when compared to the calculated molecular formula of 7. Consequently, the substituents at C-5 and C-7 can be identified as an OH and a nitro function, respectively, as also indicated by the chemical shifts of C-5 (157.8 ppm) and C-7 (148.3 ppm). This results in the final overall structure 7 of this natural product. To unambiguously prove the substitution pattern at the aromatic ring system, the phenolic OH-group was selectively methylated using diazomethane. The resulting methyl ether was irradiated in a 1D NOE experiment, which resulted in the expected strong increase of the proton bound to C-6. This observation thus firmly validated that the methyl ether had to be situated at C-5, as its alternative location at C-7 would have led to a strong increase in signal intensity of the protons bound to both, C-6 and C-7.
Owing to its coumarin core structure and its biological origin, compound 7 was named myxocoumarin A (7). Besides 7, a second metabolite 9 with a highly similar NMR spectrum was isolated. ESIMS analysis of 9 showed a molecular ion peak at 346.  7). In addition, no loss of water was observable in the MS data, suggesting the hydroxy function at C-4 in 7 not to be present in 9. Comparison of the 1 H NMR spectra of 7 and 9 furthermore revealed the missing signals of the methyl groups 18 and 19 of 7 in the spectrum of 9, along with a new singlet at 2.20 ppm integrating for three protons. In conclusion, this lead to the proposed structure of myxocoumarin B (9) as shown in Figure 3, which was unambiguously verified by in-depth analysis of chemical shifts in comparison with literature data (see Supporting Information File 1). Unfortunately, the amount of 7 isolated from the initial fermentation broth was not sufficient for stereochemical investigations, nor did the obtained amount of 9 suffice to obtain high quality 13 C NMR data. Attempts to produce more crude material were not successful, as cryo cultures of the producing strain failed to be transformed into actively growing cultures again.
Coumarins in general constitute a large group of bioactive natural products, mostly isolated from plants. Most of these plant-derived compounds are polyphenols that are often further modified by most diverse functionalization reactions, such as O-methylation, glycosylation, prenylation, oxygenation and subsequent cyclization or dimerization events [34]. Nitrogenbearing congeners are rather scarce, with the bacterial aminocoumarins, such as the gyrase inhibitor novobiocin, being the most well-known examples [35]. Despite the larger number of existing coumarin structures in literature, the 5-hydroxy-7nitro substitution pattern combined with the unusual long-chain, fully saturated alkyl substituent at C-4 are unique to the myxcoumarins 7 and 9.
With the production of myxothiazol A (5) and aurachin A (6), the investigated strain Stigmatella aurantiaca MYX-030 resembles the secondary metabolite profile of previously chemically investigted S. aurantiaca and S. erecta strains. Interestingly, both, S. aurantiaca Sg a15 and S. erecta Pd e21, were reported to produce 5-nitroresorcinol (13) [36]. The biosynthesis of 13 was investigated in S. erecta Pd e21 using isotope-labeling experiments that revealed its precursor molecules to be glucosederived erythrose-4-phosphate (10)  To verify the initially measured antifungal activity from the PAC, myxocoumarin A (7) was subjected to leaf disk assays against a series of agriculturally relevant pathogenic fungi. The compound revealed interesting antifungal activities with complete inhibition of agronomically important pathogens such as Botrytis cinerea at 200 mg/L as well as Magnaporthe grisea and Phaeosphaeria nodorum at 67 mg/L. Partial effects were found against Blumeria graminis (67 mg/L). The compound was, however, inactive against Drechslera teres and Phytophthora infestans. These findings indicate a broad range antifungal activity of 7, comparable to commercial standard fungicides, although on a somewhat lower activity level. To further substantiate these results, liquid culture assays against a set of fungal pathogens were performed, showing strong intrinsic antifungal activities against M. grisea (100% inhibition at 0.7 mg/L) as well as B. cinerea, Fusarium culmorum, and P. nodorum (all at 2 mg/L), with no activity against Pythium Scheme 1: Postulated biosynthetic pathway to myxocoumarins A (7) and B (9).
ultimum and Rhizoctonia solani. So far, no insecticidal and herbicidal activities have been observed. Due to the problems arising from the lacking culturability of the producing strain S. aurantiaca, further tests with 7 were so far not possible. Structure-activity relationship (SAR) studies on synthetic myxocoumarin derivatives as well as in-depth investigation of their antibacterial potential are currently performed in our laboratory and will be reported in due course.

Experimental
Cultivation and extraction. The preculture of S. aurantiaca MYX-030, inoculated directly from a single culture of an agar plate, was incubated by shaking (150 rpm) at 30 °C for 7 days in a 500 mL shake flask without baffle containing 80 mL of medium MIX-5 (composition: 2 g glucose, 5 g potato starch, 2 g peptone, 0.5 g MgSO 4 , 0.

Supporting Information
Supporting Information File 1 NMR and MS spectra of myxocoumarin A (7) and B (9). In-depth discussion and analysis of chemical shifts for the verification of the structure of 9.