Novel Natural Oximes and Oxime Esters with a Vibralactone Backbone from the Basidiomycete Boreostereum vibrans.

A variety of novel natural products with significant bioactivities are produced by the basidiomycete Boreostereum vibrans. In the present study, we describe 16 novel natural oximes and oxime esters with a vibralactone backbone, vibralactoximes, which were isolated from the scale-up fermentation broth of B. vibrans. Their structures were determined through extensive spectroscopic analyses. These compounds represent the first oxime esters from nature. The hypothetical biosynthetic pathway of these compounds was also proposed. Seven compounds exhibited significant pancreatic lipase inhibitory activity, while ten compounds exhibited cytotoxicities against five human cancer cell lines (HL-60, SMMC-7721, A-549, MCF-7, and SW480), with IC50 values comparable with those of cisplatin.

Introduction

Obesity and overweight, caused by multifaceted physical and environmental factors, have become global issues because of their association with a high risk of cardiovascular diseases,1 diabetes,2 musculoskeletal disorders,3 hypertension,4 and several types of cancers.5 According to the fact sheets of the World Health Organization, nearly 13 % of the world's adult population were obese and 39 % were overweight in 2014.6 Fighting obesity is one of the top priorities of health departments. Although pharmacotherapy is subordinate to individual lifestyle adjustments, it is an indispensable treatment in combating obesity. Many therapeutic agents used to treat obesity have been found to be associated with serious adverse reactions.7 Fenfluramine, an antiobesity medicine once approved by the US FDA in 1973, was withdrawn in 1997 due to the risk of valvular heart disease.8 Although orlistat is now available, it is currently involved in a debate regarding the likelihood of causing liver damage.9 Therefore, searching for new cost‐effective antiobesity medicines with fewer side effects has been one of the hot research topics over the past decades.10

Higher fungi belonging to the genus Stereum produce diverse bioactive secondary metabolites. Several sesquiterpenoid skeletons were first isolated from this genus, including hirsutane‐type,11 sterpurane‐type,12 and stereumane‐type13 sesquiterpenes. Moreover, some homo‐ and heterodimeric sesquiterpenes with various biological activities were recently obtained from this genus.14 We have investigated the basidiomycete Boreostereum vibrans (synonym Stereum vibrans), which is known to produce a significant pancreatic lipase inhibitor with an unusual fused β‐lactone, named vibralactone (IC50 of 0.4 μg mL−1).15 Some important progress has been made in both elucidating the biosynthetic pathway and modifying the structure of this molecule.16

To explore more interesting compounds from this productive strain, a scale‐up fermentation (1200 L) was conducted, which resulted in the isolation of sixteen oxime and oxime esters with a vibralactone backbone. Natural products possessing an oxime group are rare, and they have been found to originate from sponges, bacteria, fungi, and plants.17 These types of compounds exhibit various activities, including strong antibiotic activity,16a, 17j–17l cytotoxicity,[17  g] inhibition of insect juvenile hormone production,17d dexamethasone glucocorticoid receptor binding,[17  f] and phosphorylated cholinesterase regeneration.17d

Interestingly, the monomers vibralactoxime A (1) and vibralactoxime B (2) can play the role of building blocks to form polymers via the formation of an ester bond between the carbonyl and oxime hydroxy groups. All of the polymers can be divided into four categories depending on the starting and terminating scaffolds: a) starting with 1 and ending with the 2,3‐epoxy vibralactone derivative(3), b) starting with 1 and ending with the cyano group‐containing derivative of 1 (4, 5); c) starting with 1 and ending with vibralactone (6, 7), and d) starting with 1 or 2 and ending with 1 (8–16). Polyoxime ester bonds were found for the first time in natural products.

Herein, we describe the isolation, structural elucidation, biosynthetic supposition, cytotoxicity, and pancreatic lipase inhibitory activity of vibralactoximes 1–16 (Figure 1).

Figure 1

Structures of vibralactoximes A–P (1–16).

Results and Discussion

The filtrate of a scale‐up fermentation broth of B. vibrans (1200 L) was concentrated under reduced pressure and then partitioned between ethyl acetate and water four times to afford an ethyl acetate layer (856 g). Subsequently, the mycelia were soaked in choroform/methanol (1:1), and the extract was evaporated and then dissolved in water, which was extracted with ethyl acetate three times to afford an ethyl acetate layer (362 g). The total organic extract (1218 g) was purified using repeated silica gel, Sephadex LH‐20, medium‐pressure liquid chromatographic approaches and preparative highperformance liquid chrimatography (HPLC) to afford sixteen oximes and oxime esters (Figure 1).

Vibralactoxime A (1) was isolated as a pale yellow oil. Its molecular formula was established to be C12H15NO3 based on the high‐resolution electron ionization mass spectrometry (HR EI‐MS) analysis (m/z 221.1049, calcd 221.1052), requiring six degrees of unsaturation. The 1H and 13C NMR spectral data in combination with the heteronuclear single quantum coherence (HSQC) spectrum revealed the presence of a hydroxy (δ H=10.53 ppm), two methyls (δ H=1.70, s 3 H; 1.66, s, 3 H; δ C=25.9, 18.0 ppm), two methylenes, one oxygenated methine (δ C=78.9 ppm), one quaternary carbon (δ C=76.5 ppm), five olefinic carbons [δ C=118.5 (CH), 131.3 (CH), 146.5 (CH), 136.4, 141.1 ppm], and one carbonyl (δ C=172.4 ppm) (Table 1). All of these data, as well as the partial heteronuclear multiple bond correlations (HMBCs), were highly similar to those of vibralactone, except that the hydroxymethyl at δ C=61.3 ppm of vibralactone was changed to an sp2‐hybridized methane at δ C=146.5 ppm (C‐13), which was supported by the HMBC correlations from δ H=7.99 ppm (1 H, s, H‐13) to δ C=131.3 (d, C‐2), 141.1 (s, C‐3), and 36.7 ppm (t, C‐4).The above results indicated that the main difference between them was the substituent at C‐3. According to the molecular formula, the group substituted at C‐3 in 1 must have the composition CH2NO. In addition, the unique 3 J HMBC correlation from the hydroxyl proton at δ H=10.53 ppm (1 H, s) to C‐13 indicated that the carbon atom and hydroxyl group were separated by a nitrogen atom (Supporting Information, Figure 5 S). Therefore, considering aforementioned evidences and the chemical shift feature of C‐13 (δ C=146.5 ppm), the remaining unassigned atoms of a nitrogen, an oxygen, and two protons were determined to be an unusual oxime moiety. The key rotating‐frame Overhauser effect spectroscopy (ROESY) cross peaks between protons at δ H=6.04 (1 H, s, H‐2) and 7.99 ppm, and between δ H=7.99 and 10.53 ppm, suggested the conjugated diene (C2=C3−C13=N) was a trans configuration, while the oxime group was an E configuration. Other ROESY signals revealed that the relative configurations of remaining asymmetric centers in 1 to be 1 R′, 5 S’ (Figure 2).

Table 1

NMR data of 1 and 2 in [D6]acetone.

	Chemical shifts [ppm] and coupling constants (J) [Hz]
	1		2
No.	δ C, type	δ H, multi.	δ C, type	δ H, multi.
1	76.5, s		66.7, s
2	131.3, d	6.04, s	136.4, d	6.06, s
3	141.1, s		138.5, s
4	36.7, t	2.88, d (19.2) 2.94, dd (19.2, 5.4)	39.9, t	2.54, dd (17.0, 1.8) 2.84, overlapped
5	78.9, d	4.97, d (5.4)	78.5, d	4.27, ddd (6.0, 5.9, 1.8)
7	172.4, s		173.5, s
8	28.0, t	2.52, dd (15.0, 7.4) 2.67, dd (15.0, 7.4)	35.7, t	2.20, dd (13.9, 7.6) 2.62, dd (13.9, 7.6)
9	118.5, d	5.19, t (7.4)	120.4, d	5.06, t (7.6)
10	136.4, s		134.9, s
11	18.0, q	1.66, s, 3 H	17.9, q	1.57, s, 3 H
12	25.9, q	1.70, s, 3 H	26.0, q	1.66, s, 3 H
13	146.5, d	7.99, s	147.3, d	7.93, s
7‐OCH3			51.6, s	3.63, s, 3 H
N‐OH		10.53, s,		10.20, s
5‐OH				4.18, d (5.9)

Figure 2

a) Key 2 D NMR correlations of 1; b) key 1H−1H COSY and HMBC correlations of 4; c) key 2 D NMR correlations of 3.

In order to determine the absolute configurations of 1, a semisynthesis of this molecule was achieved from vibralactone by two steps (Scheme 1). The semisynthesis of 1 commenced with vibralactone, a major compound in this crude extract, which was oxidized with pyridinium chlorochromate (PCC) to give 17 as a colorless oil in 78 % yield.15b, 16a Aldehyde 17 was subjected to react with hydroxylamine hydrochloride to afford 1 a as a pale yellow oil in high yield (90 %).18 The 1H NMR data of 1 a, as well as circular dichroism (CD) and UV/Vis spectra matched those of the natural isolate 1 fabulously (Figure 3; Figures A, D, and E in the Supporting Information). Moreover, the presence of oxime group in the conjugated diene in 1 led to the red shift of its CD spectrum compared with that of vibralactone, as shown in Figure 3. All pieces of evidences above indicated that the absolute configurations of 1 were 1 R, 5 S, which were the same as vibralactone.

Scheme 1

Semisynthesis of 1 from vibralactone. Reagents and conditions: a) PCC, CH2Cl2, 0 °C, 30 min, 78 %; b) NH2OH⋅HCl, NaOAc, EtOH/H2O (1:1), 0 °C, 20 min, 90 %.

Figure 3

CD and UV/Vis spectra of compound 1, 1 a, and vibralactone (CH3CN).

Vibralactoxime B (2) was obtained as a light yellow oil and determined to have a molecular formula of C13H19NO4 based on the ion peak of HR‐EI‐MS at m/z 253.1314 (calcd 253.1312). Many of the structural characteristics of 2, including the oxime group and isopentenyl moiety, were found to be the same as those of vibralactoxime A (1) based on preliminary analyses of the 1 D and 2 D NMR spectra (Table 1). However, the protons at δ H=3.63 ppm (3 H, s) were correlated to the carbonyl at δ C=173.5 ppm (s, C‐7) in the HMBC spectrum, whereas the cross peak between the hydroxyl proton at δ H=4.18 ppm (1 H, d, J=5.9 Hz) and proton at δ H=4.27 ppm (1 H, ddd, J=6.0, 5.9, 1.8 Hz, H‐5) in the 1H‐1H COSY spectrum was also observed (Figure 12 S in the Supporting Information). All of the aforementioned spectral data suggested that 2 was a derivative of 1, in which the β‐lactone group was opened and the carboxyl was methyl esterified. The key ROESY correlation between H‐5 and H‐8 (2.62, dd, J=13.9, 7.6 Hz; 2.20, dd, J=13.9, 7.6 Hz) demonstrated the same configurations between 1 and 2.

The molecular formula of vibralactoxime C (3) was determined to be C26H33NO8 from the HR electrospray ionization (ESI)‐MS data, m/z 510.2102 [M+Na]+ (calcd for C26H33NO8Na, 510.2098). The 13C NMR spectroscopic data revealed the presence of 26 carbon signals, which were classified by distortionless enhancement by polarization transfer (DEPT) and HSQC spectra into the categories of five methyls, five methylenes (with one oxygenated), seven methines (with three oxygenated), and nine nonprotoned carbons (three carbonyls, three olefinic carbons, and one oxygen bearing carbon) (Table 2). Detailed analyses of the 1 D and 2 D NMR spectra indicated the existence of two isolated vibralactone skeletons. One of the moieties (fragment A, Figure 2) presented similar characteristic signals as vibralactoxime A (1). In fragment B, key HMBC correlations from δ H=4.49 (1 H, d, J=12.5 Hz, H‐13a), 4.20 (1 H, d, J=12.5 Hz, H‐13b), and 2.07 ppm (s, 3 H) to δ C=170.8 ppm (C=O), as well as the mutual 1H−1H COSY correlation between δ H=2.98 (5′‐OH) and 3.95 ppm (H‐5′) suggested that the hydroxyl of C‐13’ was acetylated, and that the β‐lactone group was opened. Moreover, obvious HMBC correlations were observed from H‐8’ (δ H=2.60, overlapped; 2.23 ppm, dd, J=14.3, 7.5 Hz) to three quaternary carbons at δ C=168.7 (C‐7′), 61.4 ppm (C‐1′), and 64.9 ppm (C‐2′), as well as H‐4’ (δ H=2.44, dd, J=15.2, 6.1 Hz; 2.10 ppm, d, J=15.2 Hz) to C‐1′, C‐2′, C‐3’ (δ C=65.3), and C‐13’ (δ C=64.0) (Figure 2). The above results in conjunction with the elemental composition revealed that the double bond of C‐2’ and C‐3’ was oxygenated into an epoxy ring. This enabled the completion of the planar structure of fragment B. Further analyses of the ROESY data enabled the relative configuration of 3, as described in Figure 2, simply as correlations between H‐2’ (δ H=3.77 ppm, s) and H‐8′, H‐2’ and H‐13′, as well as H‐2’ and H‐5’ (Figure 24 S in the Supporting Information). Finally, to satisfy the requirement of the molecular weight, fragments A and B were connected by an ester linkage between the hydroxyl group of the oxime substituent in fragment A and the carbonyl (C‐7′) in fragment B, which was also supported by the disappearance of the oxime hydroxyl of fragment A and the upfield shift of C‐7’ (δ C=168.7 ppm).The additional observed HMBC and 1H−1H COSY correlations were completely consistent with the assignments.

Table 2

NMR data of 3 and 4 in [D6]acetone.

	Chemical shifts [ppm] and coupling constants (J) [Hz]
	3		4
No.	δ C, type	δ H, multi.	δ C, type	δ H, multi.
1	77.1, s		77.0, s
2	138.9, d	6.48, s	138.9, d	6.48, s
3	139.3, s		139.2, s
4	36.6, t	3.08, dd (18.4, 5.8) 2.99, d (18.4)	36.5, t	3.08, dd (19.0, 5.7) 2.97, d (19.0)
5	79.1, d	5.08, d (5.8)	79.0, d	5.07, d (5.6)
7	171.7, s		171.6, s
8	28.0, t	2.74, dd (14.8, 7.5) 2.60, overlapped	27.9, t	2.73, overlapped 2.59, overlapped
9	118.4, d	5.20, br.t (7.5)	118.3, d	5.20, br. t (7.4)
10	136.8, s		136.7, s
11	18.1, q	1.67, s, 3 H	18.0, q	1.67, s, 3 H
12	26.1, q	1.71, s, 3 H	25.9, q	1.71, s, 3 H
13	154.2, d	8.44, s	154.3, d	8.45, s
1′	61.4, s		67.5, s
2′	64.9, d	3.77, s	149.4, d	6.82, s
3′	65.3, s		115.0, s
4′	37.2, t	2.44, dd (15.2, 6.1) 2.10, d (15.2)	43.3, t	3.11, dd (17.6, 5.8) 2.61, dd (17.6, 1.7)
5′	76.8, d	3.95, dd (7.5, 6.1)	77.8, d	4.44, ddd (5.8, 5.7, 1.7)
7′	168.7, s		168.2, s
8′	32.8, t	2.60, overlapped 2.23, dd (14.3, 7.5)	34.8, t	2.74, overlapped 2.33, dd (14.2, 7.8)
9′	118.8, d	5.16, br. t (7.5)	119.0, d	5.11, br. t (7.8)
10′	136.4, s		136.3, s
11′	18.1, q	1.63, s, 3 H	18.0, q	1.60, s, 3 H
12′	26.0, q	1.69, s, 3 H	26.0, q	1.68, s, 3 H
13′	64.0, t	4.49, d, (12.5) 4.20, d (12.5)	116.8, s
CH3CO‐	20.6, q	2.07, overlapped[a]
CH3CO‐	170.8, s
7‐OCH3
5′‐OH		2.98, d (7.5)		4.83, d (5.7)

[a] Signal overlapped in solvent residual peaks.

Vibralactoxime D (4) was obtained as a yellow oil with the elemental composition C24H28N2O5 (twelve degrees of unsaturation), which was determined by HR EI‐MS (ion peak at m/z: 424.1998, calcd 424.1987). In the 13C NMR and DEPT spectra, the signals were present in pairs, implying that 4 might be a dimer. Detailed analyses of the 1 D and 2 D NMR spectroscopic data showed that a moiety of 4 was the same as fragment A of 3, while the other part possessed similar signal characteristics to the β‐lactone‐opened vibralactone, except for the substituent at C‐3’ (Table 2). In this moiety, key HMBC correlations from the proton at δ H=6.82 ppm (s, H‐2′) to two quaternary carbons at δ C=115.0 (s, C‐3′) and 116.8 ppm (s, C‐13′) (Figure 32 S in the Supporting Information) suggested that the substituent at C‐3’ was a cyano group (Figure 2), which was biologically produced through the elimination of a water molecule of an oxime group. The infrared (IR) absorption bands at 2225 cm−1 confirmed the presence of the cyano group (Figure 35 S in the Supporting Information).19 Considering the overall formula of 4, an ester bond was determined to be between the oxime hydroxyl and C‐7′, as described in 3.

Vibralactoxime E (5) was obtained as a yellow oil. Its molecular formula was established to be C36H43N3O8 according to the pseudomolecular ion peak at m/z 668.2938 [M+Na]+ (calcd for C36H43N3O8Na, 668.2942) by HR ESI‐MS. The trimeric structural features of 5 were based on the comparison between the 1 D and 2 D NMR data of 4 and 5, as well as the requirement of molecular weight and IR spectroscopy (Figure 44 S in the Supporting Information), which revealed that two moieties of this trimer were the same as those of 4. Another moiety that was characterized by an opened β‐lactone group and invisible oxime hydroxyl signal was constructed from the remaining signals. These data led to the connectivity of the aforementioned three moieties of 5, as shown in Figure 1, for which we proposed the name vibralactoxime E.

Vibralactoxime F (6) presented a pseudomolecular ion peak at m/z 452.2046 [M+Na]+ in the HR ESI‐MS spectrum, indicating a molecular formula of C24H31NO6 (calcd for C24H31NO6Na, 452.2044). Preliminary analyses of its 1 D and HSQC data suggested the presence of four methyl singlets (δ H=1.58, 1.66, 1.67, 1.71 ppm; δ C=18.1, 18.0, 26.1, 25.9 ppm), five methylenes (one was oxygenated), seven methines, and eight quaternary carbons (two were sp3 hybridized and six were sp2 hybridized). Detailed analyses of the HMBC and 1H−1H COSY spectra revealed the presence of two isolated vibralactone moieties which corresponded to 1 and β‐lactone‐opened vibralactone, respectively. Finally, an ester linkage was assigned between the oxime hydroxyl of the first unit and the carbonyl of the second unit. The relative configuration in the second unit was elucidated by ROESY correlations from H‐5’ (δ H=4.29 ppm, br. t, J=6.5 Hz) to H‐8’ (δ H=2.65, overlapped; 2.21 ppm, dd, J=14.3, 8.0 Hz).

The molecular formula of vibralactoxime G (7) was established as C36H46N2O9 by HR ESI‐MS at m/z 673.3104 [M+Na]+, which was 221 mass units greater than that of vibralactoxime F (6), indicating that 7 was a trimer. This assumption was confirmed by 1 D and 2 D NMR spectral analyses. The 1H−1H COSY and HMBC spectra displayed three isolated moieties, which belonged to two vibralactoxime A (1) scaffolds and a vibralactone scaffold. The upfield shift of two carbonyls (δ C=170.6 and 169.1 ppm) and the disappearance of two oxime hydroxyls, as well as the active hydrogen at δ H=3.91 ppm (t, J=5.5 Hz), which correlated to a methylene at δ H=4.12 (dd, J=14.0, 5.5, H‐13′′a) and 4.13 ppm (dd, J=14.0, 5.5, H‐13′′b) in 1H−1H COSY spectrum, suggested that the β‐lactone groups of two units were opened and had been esterified, and that the vibralactone component is the tail unit. Therefore, the sequence of the aforementioned three moieties and the connected positions of 7 are described in Figure 1.

Vibralactoximes H–L (8–12) were isolated as yellow gums and identified as a group of polyether derivatives ranging from dimer to hexamer. The molecular formulas of compounds 8 to 12 were assigned as C24H30N2O6, C36H45N3O9, C48H60N4O12, C60H75N5O15, and C72H90N6O18, respectively (Figures 68 S, 75 S, 86 S, 89 S, and 92 S in the Supporting Information). These molecular weights generated a class of arithmetic sequence with a common difference of 221 mass units, which equaled C12H15NO3, just as a vibralactoxime A (1) moiety. It was possible to identify this group of compounds by 1H NMR spectra and HR MS analyses. The downfield of the 1H NMR spectra of these compounds could be classified into three characteristic signal areas: a) peaks at approximately 10.2 ppm, the oxime hydroxyl signal was roughly displayed as a singlet in this area; b) peaks distributed in 7.9–8.5 ppm, the methines of the oxime double bond (H‐13) exhibited in this area. The rules for this area were that H‐13 of the first part of these polymers was mainly present as singlets at δ H=8.42 ppm, while H‐13 of the last part was presented as singlets at δ H=7.95 ppm, and the others appeared as overlapped signals at approximately δ H=8.36 ppm; c) the singlets that appeared at 6.10–6.60 ppm were associated with H‐2. In vibralactoxime A (1), H‐2 was a singlet at δ H=6.04 ppm. When it polymerized, the H‐2 of the first moiety shifted downfield to 6.46 ppm, the last moiety shifted downfield to 6.11–6.13 ppm, and the other moieties shifted downfield to 6.54 ppm as a cluster of signals (Figure 4). The number of polymer units could be easily deduced from the number of the integration of hydrogen atoms in the (b) and (c) areas, as well as the mass spectra. Although there is a lack of 13C NMR spectra due to the scarcity of compounds 11 and 12, all these data could enable completion of the structures of 8 to 12, for which we proposed the name vibralactoximes H–L (Figure 1).

Figure 4

Chemical shifts (δ=10.7–4.8 ppm) of 1H NMR spectra of compounds 1, 8–12.

The chemical formula of vibralactoximes M–P (13–16), a group of polymers ranging from dimer to pentamer, were assigned by HR ESI‐MS/HR EI‐MS as C25H34N2O7, C37H49N3O10, C49H64N4O13, and C61H79N5O16, respectively (Figure 100 S, 108 S, 111 S, and 114 S in the Supporting Information). Compounds 15 and 16 were presented in limited quantities for their 13C NMR experiments. The differences between these polymers and vibralactoximes H–L (8–12) were that the β‐lactone group of the first scaffold was opened, and the carboxyl had been methyl esterified. The other parts of these polymers were the same as their counterparts, in which the β‐lactone group remained unopened.

The biogenetic pathway of vibralactone has previously been illustrated.16b Here, we present a proposed biogenetic pathway of all the compounds by referring to the biosynthetic pathway of vibralactone (Scheme 2). It is hypothesized that p‐hydroxybenzoyl methylamine was prenylated and oxidized by a series of oxidation reactions to produce vibralactamine (an assumed intermediate).20 According to the literature, the oxime group is oxidized from an amino group, which was catalyzed by an open reading frame, NcoL.21 Therefore, the amino group of vibralactamine was oxygenated followed by a dehydration process to produce vibralactoxime A (1). Vibralactoxime A could serve as a precursor to produce the other compounds. Because we have not yet isolated any C‐13 cyano vibralactone monomers, it was hypothesized that the dehydration procedure from oxime to cyano group was performed after the polymerization.

Scheme 2

Plausible biosynthetic pathway for compounds 1–16.

Because natural products containing an oxime group are rarely encountered, their bioactivities greatly aroused our curiosity. Vibralactone exerted pancreatic lipase inhibitory activity through nucleophilic attack on its β‐lactone group by the serine residue of pancreatic lipase to form a stoichiometric acyl‐enzyme complex.22 Some of the vibralactoximes still possessed the β‐lactone function, which is the reason why we tested their pancreatic lipase inhibitory activities. Interestingly, compounds 1, 4, 5, 7, 9, 10, and 11 exhibited stronger pancreatic lipase inhibitory activities than vibralactone, which was used as one of the positive controls (the other positive control was orlistat); compounds 6 and 12 exhibited low activities, and the other opened β‐lactone group compounds were inactives (Table 3 and Page 7 in the Supporting Information). From the results we could postulate that a) the cleavage of the β‐lactone function led to inactivity on pancreatic lipase, b) the existence of the oxime group could enhance the pancreatic lipase inhibitory activities, and c) it was more likely that the molecular lengths were related to the activities to a large extent because the pentamer vibralactoxime K (11) ranked first in terms of pancreatic inhibitory activity.

Table 3

Pancreatic lipase inhibitory activities (IC50) of compounds 1, 4, 5, 7, 9–11.

Compound	IC50 [μmol]	Compound	IC50 [μmol]
1	23.1	10	19.8
4	20.6	11	11.1
5	28.6	Vibralactone	48.7
7	16.8	Orlistat	0.0018
9	23.2

Moreover, all these compounds were evaluated for their cytotoxicities against five human cancer cell lines (promyelocytic leukemia HL‐60, hepatoma SMMC‐7721, lung adenocarcinoma A‐549, breast adenocarcinoma MCF‐7, and colon adenocarcinoma SW480 cells) using the MTT (3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyltetrazolium bromide) cell proliferation assay, as previously reported.23 Notably, compounds 7, 9, and 10 exhibited significant cytotoxicities against HL‐60, MCF‐7, and SW480 with IC50 values comparable with those of cisplatin. Compounds 4–6, 8, 11, 12, 14, and 15 exhibited moderate cytotoxic activities (Table 4).

Table 4

Cytotoxicities of compounds 4–10, 12, 14, 15.

	IC50 [μmol]
Compound	HL‐60	SMMC‐7721	A‐549	MCF‐7	SW480
4	14.5	29.5	>40	29.7	>40
5	7.6	14.6	15.7	15.6	16.1
6	12.1	18.5	24.6	21.3	17.4
7	2.9	12.9	12.2	15.3	7.5
8	10.9	13.0	18.6	17.7	17.4
9	3.1	13.2	14.2	9.4	8.1
10	3.4	14.3	15.0	14.4	8.0
11	14.4	21.6	27.8	20.5	16.0
12	34.2	>40	33.5	>40	>40
14	15.7	21.4	23.9	30.2	>40
15	16.3	20.1	33.7	24.5	>40
Cisplatin	1.2	4.5	6.2	15.2	12.0
Taxol	<0.008	<0.008	<0.008	<0.008	<0.008

Conclusion

A further chemical investigation of the culture broth of B. vibrans led to the discovery of sixteen structurally and biogenetically novel oxime‐group‐containing vibralactone derivatives. Notably, this work exemplified oxime‐group‐bearing natural products discovered from higher fungi for the first time. Vibralactoxime A (1) was a building block assembling various polymers with diverse activities, including pancreatic lipase inhibitory activity and cytotoxicity. A postulated biosynthetic pathway for these compounds was provided in Scheme 1. Their diverse bioactivities have broadened our horizons in structural modifications of vibralactone for developing stronger bioactive molecules. Moreover, further biosynthetic effort should shed light on the proposed biosynthetic pathway for these compounds.

Experimental Section

General

Optical rotations (OR) were recorded on a JASCO P‐1020 digital polarimeter (Horiba, Kyoto, Japan). UV/Vis spectra were obtained using a Shimadzu UV2401PC spectrometer (Shimadzu, Kyoto, Japan). CD spectra were tested on an Applied Photophysics Chirascan Circular Dichroism Spectrometer (Applied Photophysics Limited, Leatherhead, Surrey, UK). IR spectra were obtained using a Bruker Tensor 27 FT‐IR spectrometer (Bruker Optics, Inc., Billerica, MA) with KBr pellets. HR EI‐MS were recorded on a Waters AutoSpec Premier P776 instrument (Waters, Milford, MA, USA). HR ESI‐MS were recorded on an Agilent 6200 Q‐TOF MS system (Agilent Technologies, Santa Clara, CA, USA). NMR spectra were measured on a Bruker Avance III 600 MHz spectrometer (Bruker Biospin GmbH, Karlsruhe, Germany). Silica gel (200–300 mesh, Qingdao Haiyang Chemical Co., Ltd, P. R. China) and Sephadex LH‐20 (Amersham Biosciences, Sweden) were used for column chromatography (CC). Medium‐pressure liquid chromatography (MPLC) was performed on a Büchi Sepacore System equipped with pump manager C‐615, pump modules C‐605 and fraction collector C‐660 (Büchi Labortechnik AG, Switzerland), and columns packed with Chromatorex C‐18 (40–75 μm, Fuji Silysia Chemical Ltd., Japan). Preparative HPLC was performed on an Agilent 1260 liquid chromatography system equipped with two type of Zorbax SB‐C18 columns (9.4 mm×150 mm and 21.2 mm×150 mm, particle size 5 μm).

Five human cancer cell lines were used to evaluate the cytotoxicities of these isolated compounds: the HL‐60 (ATCC CCL‐240) human myeloid leukemia cell line, the SMMC‐7721 human hepatocellular carcinoma cell line, the A549 (ATCC CCL‐185) lung cancer cell line, the MCF‐7 (ATCC HTB‐22) breast cancer cell line, and the SW‐480 (ATCC CCL‐228) human colon cancer cell line. The cell line SMMC‐7721 was bought from China Infrastructure of Cell Line Resources (Beijing, China), and others were bought from American Type Culture Collection (ATCC, Manassas, VA, USA)

Fungus material and cultivation conditions

The fungus B. vibrans was collected in Kunming Botanical Garden. A voucher specimen was deposited at the Herbarium of Kunming Institute of Botany, Chinese Academy of Sciences (No. 20120920B). The culture medium consisted of potato (200.0 g), glucose (20.0 g), KH2PO4 (3.0 g), MgSO4 (1.5 g), vitamin B1 (10 mg), peptone from porcine meat (1.0 g), citric acid (0.1 g) in deionized water (1.0 L). The pH was adjusted to 6.0–6.5 before autoclaving 3400 bottles of 500 mL Erlenmeyer flasks, each containing 350 mL of above‐mentioned culture medium, which were then inoculated with B. vibrans strains. Fermentation was carried out on a shaker at 150 rpm for 25 d in darkness seven times.

Extraction and isolation

The culture broth of B. vibrans (1200 L) was filtered to separate the culture fluid and mycelia. The filtrate was concentrated in vacuo then extracted with EtOAc four times (total 200 L). Meanwhile, the mycelia were soaked with CHCl3/MeOH (1:1) (total 90 L, 3×30 L, 3 d), and the extraction was evaporated under reduced pressure then partitioned between EtOAc and water to give an EtOAc layer (90 L total, 5×18 L). The combination of total EtOAc solution (290 L) was concentrated under reduced pressure to give a crude extract of 1218 g. Then this residue was subjected to column chromatography over silica gel (200–300 mesh) eluting with a gradient of petroleum ether/acetone (10:1 → 0:1) to give five fractions (A–H). Fraction B (6.4 g) was separated by Sephadex LH‐20 CC (MeOH) followed by preparative HPLC (CH3CN/H2O, 40:60 → 70:30, 40 min, 20 mL min−1) to give four subfractions (B1–B4). Subfraction B1 (160 mg) was purified by preparative HPLC (CH3CN/H2O, 35:65 → 55:45, 35 min, 10 mL min−1) to yield 10 (12 mg, 9.85×10−6 % w/w),24 11 (26 mg, 2.13×10−5 % w/w), 14 (3 mg, 2.46×10−6 % w/w), 15 (3 mg, 2.46× 10−6 % w/w). Subfraction B2 (30 mg) was separated using preparative HPLC (CH3CN/H2O, 45:55 → 65:35, 30 min, 10 mL min−1) to give 12 (0.9 mg, 7.39×10−7 % w/w), 16 (0.6 mg, 4.93×10−7 % w/w). Fraction C (58.5 g) was separated by MPLC eluting with MeOH/H2O (20:80 → 100:0) to afford four subfractions (C1–C8). Subfraction C2 (16.8 g) was purified by Sephadex LH‐20 (acetone) to yield vibralactone (15.1 g). Subfraction C3 (4.1 g) was separated by preparative HPLC (CH3CN/H2O, 30:70 → 60:40, 30 min, 20 mL min−1) to obtain 1 (25 mg, 2.05×10−5 % w/w), 2 (13 mg, 1.07×10−5 % w/w), 3 (0.5 mg, 4.11×10−7 % w/w), and 6 (10 mg, 8.21× 10−6  w/w). Subfraction C6 (1.5 g) was separated using preparative HPLC (CH3CN/H2O, 36:64 to 56:44, 30 min, 20 mL min−1) to afford 4 (9 mg, 7.39×10−6 w/w), 5 (1 mg, 8.21×10−7 % w/w), 7 (4 mg, 3.28×10−6 % w/w), 8 (4 mg, 3.28×10−6 % w/w), 9 (6 mg, 4.93×10−6 % w/w), 13 (11 mg, 9.03×10−6 % w/w).

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Supplementary

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