Fusagerins A–F, New Alkaloids from the Fungus Fusarium sp.

Abstract Fusagerins A–F (1–6), six new alkaloids including a unique one with the rare a-(N-formyl)carboxamide moiety (1), a hydantoin (imidazolidin-2,4-dione) derivative (2), and four fungerin analogues (3–6), were isolated from the crude extract of the fungus Fusarium sp., together with the known compound fungerin (7). Compound 2 was isolated as a racemate and further separated into two enantiomers on a chiral HPLC column. The structures of 1–6 were determined mainly by NMR experiments, and the absolute configuration of 1 and 2 was assigned by electronic circular dichroism (ECD) calculations. Compound 7 showed antibacterial activity against Staphylococcus aureus and Streptococcus pneumoniae, and weak cytotoxicity against the T24 cells. Graphical Abstract Electronic supplementary material The online version of this article (doi:10.1007/s13659-015-0067-1) contains supplementary material, which is available to authorized users.


Introduction
Fungi are important sources of bioactive secondary metabolites [1], and those from special and competitive habitats are especially likely to produce structurally diverse and unique natural products due to their highly evolved secondary metabolism [2][3][4]. Based on this consideration, and our previous success in the discovery of new bioactive secondary metabolites from the of unique niches [5], a special group of fungi which were isolated from the fruiting body and larvae of Cordyceps sinensis were chemically investigated, leading to the isolation of a variety of cytotoxic natural products [6][7][8][9]. During the course, we also screened the fungal species isolated from the soil samples that were collected on the Qinghai-Tibetan plateau at altitudes above 3200 m, the environment in which Cordyceps sinensis typically reside [10][11][12][13]. A strain of Fusarium sp. was such a fungus, and an EtOAc extract prepared from solid-substrate fermentation products of the fungus was subjected to chemical investigation. Fractionation of the crude extract afforded fusagerins A-F ( Fig. 1; 1-6), six new alkaloids including a unique one with the rare a-(N-formyl)carboxamide moiety (1), a hydantoin (imidazolidin-2,4-dione) derivative (2), and four fungerin analogues (3)(4)(5)(6), along with the known compound fungerin ( Fig. 1; 7) [14]. Details of the isolation, structure elucidation, and biological activities of these compounds are reported herein.

Results and Discussion
The molecular formula of fusagerin A (1) was established as C 13  170.6, respectively). These data accounted for all the five unsaturations of 1. Interpretation of the 1 H-1 H COSY NMR data of 1 revealed the presence of three isolated spinsystems, which were C-2-C-3, C-9-C-10, and N-7-C-14, respectively. In the HMBC spectrum of 1, cross peaks from the H-2 and H-3 olefinic protons to the carboxylic carbon at 166.8 ppm (C-1) established an a,b-unsaturated ketone moiety (C-1-C-3). An HMBC correlation from the Omethyl proton signal H 3 -15 to the C-1 carboxylic carbon located the O-methyl group at C-1. Correlations of the H-10 olefinic proton with C-12 and C-13, and of H 3 -12 and H 3 -13 with C-10 and C-11 established of a prenyl unit. In turn, a correlation from H 3 -14 to the C-8 carboxylic carbon revealed the presence of the C-8/N-7 amide bond in the structure. In addition, a one-bond coupling was noted between the C-6 carbonyl resonance and the deshielded H-6 proton resonance in the HSQC spectrum, suggesting   [15,16]. Key HMBC correlations from H-2, H-3, H-6, and H-10 to C-4, and from H-3 and H 2 -9 to C-8 indicated that C-3, N-5, C-8, and C-9 are all connected to C-4. Although no HMBC correlations were observed for the two exchangeable protons, considering the relevant 1 H-1 H COSY correlation between H-7 and H-14, as well as the unsaturation requirement for 1, they were assigned as 5-and 7-NH, respectively. On the basis of these data, the gross structure of fusagerin A was established as shown in Fig. 1. The C-2/C-3 olefin was assigned the E-geometry based on a large coupling constant of 15.9 Hz observed for the corresponding olefinic protons. To determine the absolute configuration for the C-4 stereogenic center in fusagerin A (1), the theoretical CD spectra for the two enantiomers 4R-1 and 4S-1 (Fig. 2) were predicted using quantum chemical calculations. Due to the flexible nature of the structure, the conformational species of each enantiomer were defined using the Random Search approach in Sybyl8.0 software. Six conformations were selected from 380 results within a range of 6 kcal/mole above the global minimum. For each conformer, the respective CD spectrum was calculated by TD-B3LYP/6-311??g(2d, p), after optimization using DFT at the B3LYP/6-31G(d) level in Gaussian09 program [17]. The overall calculated CD spectra were then generated according to Boltzmann weighting of each conformer, and were further compared to the experimental CD curve (Fig. 2). The experimental CD spectrum of 1 matched the ECD spectrum for 4R-1, and was nearly opposite to that for 4S-1, leading to the deduction of the 4R absolute configuration for 1. Fig. 2 The experimental CD spectrum of 1 (solid) and the calculated ECD spectra (dash) of two enantiomers 4R-1 and 4S-1 , D ? 0.3 mmu), two mass units less than that of 1. Analysis of the 1 H and 13 C NMR spectroscopic data of 2 (Table 2) revealed the presence of structural features similar to those found in 1, except that the signals for the H-6 aldehyde proton and the H-7 exchangeable proton in 1 were not observed, and the N-methyl proton signal H 3 -14 was observed as a singlet instead of a doublet in the 1 H NMR spectrum of 2, implying that the C-6 and C-8 carboxylic carbons are connected to N-7 to form a 3-methylimidazolidine-2,4-dione moiety. This postulation was also supported by the HMBC correlations from H 3 -14 to C-6 and C-8. On the basis of these data, the planar structure of 2 was established as depicted ( Fig. 1).
Despite the presence of a stereogenic center at C-4, the measured optical rotation value of fusagerin B (2) was zero, and no Cotton effect was observed in its CD spectrum, suggesting that 2 is a racemate. A portion of 2 was separated using HPLC on a chiral column to afford two enantiomers, (-)-fusagerin B ( (Table 3) revealed structural similarity to the co-isolated known compound fungerin (7) [14], except that the C-11 olefinic carbon in 7 was reduced to an oxygenated sp 3 quaternary carbon (d C 70.8), and the C-9 methylene carbon in 7 was oxidized to an olefinic carbon (d H /d C 6.66/112.3), which were confirmed by the 1 H-1 H COSY NMR correlations of 9-H with 10-H, and HMBC cross peaks from the exchangeable proton OH-11 to C-10, C-12, and C-13. The C-2/C-3 and C-9/C-10 olefins were all assigned the E-geometry based on the large coupling constants observed for the relevant olefinic protons (15.3 and 16.2 Hz, respectively). Therefore, the structure of 3 was determined as shown in Fig. 1.
The molecular formula of fusagerin D (4) was determined to be C 13 (Table 3), which is the same as that of 3, implying isomeric relationship between the two compounds. The 1 H and 13 C NMR spectra of 4 showed nearly identical resonances to those of 3, except that the chemical shifts for the C-2/C-3 olefin (d H /d C 6.52/115.1 and 7.64/136.7 in 3; 5.74/118.2 and 6.68/129.5 in 4) were significantly different. Interpretation of the 2D NMR data of 4 established the same planar structure as 3, and a small coupling constant of 12.1 Hz observed between H-2 and H-3 in 4 compared to 15.3 Hz in 3, indicating that 4 is the 2Z isomer of 3.
Fusagerin E (5) was assigned a molecular formula of C 13 H 18 N 2 O 4 (six degrees of unsaturation) by HRESIMS (m/z 267.1339 [M ? H] ? , D 0 mmu), containing one more oxygen atom than that of 3. Analysis of its NMR data (Table 3) revealed structural characteristics similar to those of 3, but the 13 C NMR chemical shift of C-11 in 5 (d C 82.1) was significantly downfield compared to that in 3 (d C 70.9), suggesting that the C-11 hydroxy group was oxidized to a peroxy unit. A similar peroxy moiety was also found in the known compounds, peroxylippidulcine A [18] and bruguierin C [19]. The absolute configuration of 5 was deduced as shown by analogy to 3 and 4.
The molecular formula of fusagerin F (6) was established as C 12 H 16 N 2 O 2 (six degrees of unsaturation) on the basis of HRESIMS (m/z 221.1293 [M ? H] ? , D -0.8 mmu), which is 14 mass units less than that of the known compound fungerin (7) [14]. The NMR data of 6 ( Table 3) are nearly identical to those of 7, except that the C-15 methyl group (d H /d C 3.77/51.4) in 7 was replaced by a proton (d H 12.07), indicating that 6 bears a free carboxylic acid moiety at C-2.

General Experimental Procedures
Optical rotations were measured on a polAAr3005 polarimeter, and UV data were recorded on a Shimadzu Biospec-1601 spectrophotometer. The CD spectra were recorded on a JASCO J-815 spectropolarimeter. IR data were recorded using a Nicolet Magna-IR 750 spectrophotometer. 1 H and 13 C NMR data were acquired with Varian Mercury-400, -500, and -600 spectrometers using the solvent signals (acetone-d 6  were obtained using an Agilent Accurate-Mass-Q-TOF LC/ MS 6520 instrument equipped with an electrospray ionization (ESI) source. The fragmentor and capillary voltages were kept at 125 and 3500 V, respectively. Nitrogen was supplied as the nebulizing and drying gas. The temperature of the drying gas was set at 300°C. The flow rate of the drying gas and the pressure of the nebulizer were 10 L/min and 10 psi, respectively. All MS experiments were performed in positive ion mode. Full-scan spectra were acquired over a scan range of m/z 100-1000 at 1.03 spectra/ s. HPLC separations were performed on an Agilent 1260 instrument (Agilent, USA) equipped with a variable wavelength UV detector. Chiral HPLC analysis and separation were performed on a Kromasil 5-CelluCoat RP column (4.6 9 250 mm; 5 lm; AkzoNobel).

Fungal Material
The culture of Fusarium sp. was isolated from a soil sample collected on the Qinghai-Tibetan plateau at an altitude of 3800 m, in May, 2008. The isolate was identified based on morphology and sequence (Genbank Accession No. JQ284030) analysis of the ITS region of the rDNA. The fungal strain was cultured on slants of potato dextrose agar (PDA) at 25°C for 10 days. Agar plugs were cut into small pieces (about 0.5 9 0.5 9 0.5 cm 3 ) under aseptic conditions, 15 pieces were used to inoculate in three Erlenmeyer flasks (250 mL), each containing 50 mL of media (0.4 % glucose, 1 % malt extract, and 0.4 % yeast extract), and the final pH of the media was adjusted to 6.5. After sterilization, three flasks of the inoculated media were incubated at 25°C on a rotary shaker at 170 rpm for 5 days to prepare the seed culture. Spore inoculum was prepared by suspending the seed culture in sterile, distilled H 2 O to give a final spore/cell suspension of 1 9 10 6 /mL. Fermentation was carried out in eight Fernbach flasks (500 mL), each containing 80 g of rice. Distilled H 2 O (120 mL) was added to each flask, and the contents were soaked overnight before autoclaving at 15 psi for 30 min. After cooling to room temperature, each flask was inoculated with 5.0 mL of the spore inoculum and incubated at 25°C for 40 days.

Extraction and Isolation
The fermented material was extracted repeatedly with EtOAc (4 9 1.0 L), and the organic solvent was evaporated to dryness under vacuum to afford the crude extract (50.0 g), which was fractionated by silica gel VLC using petroleum ether-EtOAc-MeOH gradient elution. The fraction (150 mg) eluted with 10:90 petroleum ether-EtOAc was separated by Sephadex LH-20 column chromatography (CC) eluting with MeOH. The resulting subfractions were combined and further purified by semipreparative RP HPLC (Agilent Zorbax SB-C 18 column; 5 lm; 9.

Antimicrobial and Antifungal Bioassays
Antimicrobial and antifungal bioassays were conducted in triplicate by following the National Center for Clinical Laboratory Standards (NCCLS) recommendations [20,21]. The bacterial strains, Staphylococcus aureus (ATCC 6538), Streptococcus pneumoniae (CGMCC 1.1692), and Escherichia coli (CGMCC 1.2340) were grown on Mueller-Hinton agar, the yeasts, Candida albicans (ATCC 10231) and Geotrichum candidum (AS2.498), were grown on Sabouraud dextrose agar, and the fungus, Aspergillus fumigatus (ATCC 10894), was grown on potato dextrose agar. Targeted microbes (3-4 colonies) were prepared from broth culture (bacteria: 37°C for 24 h; fungus: 28°C for 48 h), and the final spore suspensions of bacteria (in MHB medium), yeasts (in SDB medium), and Aspergillus fumigatus (in PDB medium) were 10 6 , 10 5 cells/ mL, and 10 4 mycelial fragments/mL, respectively. Test samples (10 mg/mL as stock solution in DMSO and serial dilutions) were transferred to 96-well clear plate in triplicate, and the suspension of the test organisms was added to each well achieving a final volume of 200 lL (ampicillin and fluconazole were used as positive controls). After incubation, the absorbance at 595 nm was measured with a microplate reader (TECAN). The inhibition rate was calculated and plotted versus test concentrations to afford the IC 50 , whereas the MIC was defined as the lowest test concentration that completely inhibited the growth of the test organisms.

MTS Assay [52]
In a 96-well plate, each well was plated with (2-5) 9 10 3 cells (depending on the cell multiplication rate). After cell attachment overnight, the medium was removed, and each well was treated with 100 lL medium containing 0.1 % DMSO, or appropriate concentrations of the test compounds and the positive control cisplatin (100 mM as stock solution of a compound in DMSO and serial dilutions; the test compounds showed good solubility in DMSO and did not precipitate when added to the cells). The plate was incubated for 48 h at 37°C in a humidified, 5 % CO 2 atmosphere. Proliferation assessed by adding 20 lL of MTS (Promega) to each well in the dark, followed by a 90 min incubation at 37°C. The assay plate was read at 490 nm using a microplate reader. The assay was run in triplicate.