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Jan 3, 2019 - ABSTRACT: Six new (1−6) and two known (7 and 8) indole alkaloids were produced by the marine fish-derived fungus ..... NMR spectra for...
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Generation of Indoles with Agrochemical Significance through Biotransformation by Chaetomium globosum Wei Yan,† Shuang Shuang Zhao,† Yong Hao Ye,† Yang Yang Zhang,† Yue Zhang,† Jia Yun Xu,† Sheng Mei Yin,† and Ren Xiang Tan*,‡ †

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College of Plant Protection, State & Local Joint Engineering Research Center of Green Pesticide Invention and Application, Nanjing Agricultural University, Nanjing 210095, People’s Republic of China ‡ State Key Laboratory Cultivation Base for TCM Quality and Efficacy, Nanjing University of Chinese Medicine, Nanjing 210023, People’s Republic of China S Supporting Information *

ABSTRACT: Six new (1−6) and two known (7 and 8) indole alkaloids were produced by the marine fish-derived fungus Chaetomium globosum 1C51 through biotransformation. The structures of these alkaloids were elucidated by a combination of MS, NMR, and X-ray crystallography analyses. Chaetoindolone A (1) was shown to inhibit the growth of the rice-pathogenic bacteria Xanthomonas oryzae pv. oryzae (xoo) both in vitro and in vivo. Chaetogline A (7) was found to be fungicidal against Sclerotinia sclerotiorum, a pathogen causing rape sclerotinia rot. Collectively, this work provides access to new indole alkaloids with potential agrochemical significance.

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microbial biotransformation, have been exploited to access new natural products or natural product-like structures.7−9 One effective approach is the application of epigenetically active xenobiotics that can activate the expression of silent gene clusters.10,11 The validity of such strategies was showcased by the supplementation of 1-methyl-L-tryptophan (1-MT) into the Chaetomium globosum12−14 culture, where 1-MT activates the silent fungal Pictet−Spenglerase (FPS) gene to generate a collection of chaetogline alkaloids with unprecedented skeletons.15 As a follow-up to the work, we present herein that 1-MT not only could elicit the production of Pictet− Spengler reaction-based alkaloids (chaetoglines15 and 5−8) but can also be converted into novel indoles (1−4) through biotransformation (Figures S1 and S2). Biotransformation has advantages over conventional chemical synthesis due to its selective nature, environmental acceptability, mild conditions, and high reaction rates.16,17 Biological assay revealed that compounds 1, 3, 5, and 6 inhibit the growth of the crop pathogenic microbes Xanthomonas oryzae pv. oryzae (Xoo) and X. oryzae pv. oryzicola, and 7 inhibits Sclerotinia sclerotiorum.

lant diseases caused by phytopathogenic fungi and bacteria pose globally major problems associated with crop loss. Rapid and effective control of plant diseases is generally achieved by the use of synthetic pesticides. However, some if not all of these agrochemicals have been shown to have undesirable effects on the environment due to their slow biodegradation in nature and some toxic residues are harmful to mammals. Most phytopathogenic microbes have developed drug resistance due to the highly dosed and repeated use of synthetic pesticides.1 To help feed a growing global population, there must be an accelerated search for new leads for the development of the next generation of pesticides. Natural products with novel structure patterns and/or bioactivity attributes are among the most promising sources of lead molecules that may trigger the development of new agrochemicals.2−4 However, the traditional bioactivity-guided screening method often yields the undesired reisolation of known compounds, thus wasting resources. 5 Are we approaching the “final diversity status” of molecules from Nature? The answer is “No”, since microbial genomics has indicated the presence of many silent genes in nearly all genome-sequenced organisms.6 For this reason, new strategies, such as activation of cryptic biosynthetic gene clusters and © XXXX American Chemical Society and American Society of Pharmacognosy

Received: January 3, 2019

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DOI: 10.1021/acs.jnatprod.8b01101 J. Nat. Prod. XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION Chaetoindolone A (1) was obtained as a pale red powder with its molecular formula (C23H19N3O3) deduced from HRESIMS data. The 1H and 13C NMR spectra of 1 disclosed the presence of a 2-substituted 1-MT scaffold and a 2,3-disubstituted Nmethylindole moiety. In its HMBC spectrum, the olefinic proton H-1 (δH 7.77) showed correlations to C-11 (δC 164.7, −COOH), C-5a (δC 135.9), and C-10a (δC 125.8). A strong NOE correlation in its NOESY spectrum was observed between H-1 and H-10 (δH 7.89, d, J = 7.9 Hz), indicating that 1 contains an indole acrylic acid moiety. The H-5 signal (δH 5.82) had HMBC correlations with C-4 (δC 165.6), C-10b (δC 108.8), C-2′ (δC 127.7), C-3′ (δC 107.1), and C-3′a (δC 126.3), suggesting that the two indole rings could be connected by C-5, as shown in Figure 1. This assumption

the ECD curves calculated for the two optional stereoisomers (Figure 3).

Figure 1. Key COSY (bold) and HMBC (solid arrows, blue) correlations of 1−6a. Figure 3. Experimental and theoretical ECD spectra of 1 (a) and CD comparison between 2 and 3 (b) and 5 and 7 (c).

was supported by the NOE correlations of H-5 to H-4′ (δH 7.83, d, J = 8.0 Hz) and H-2′ (δH 6.50, br s) (Figure 2). In view of the 16 degrees of unsaturation of its molecular formula (see above), a remaining double-bond equivalent and a NH group could be accounted for by inserting the NH motif between C-2 and C-4 to form an ε-lactam. Such an assignment was supported by the chemical shift magnitude of C-4 at δ 165.6. The 5S-configuration of 1 was deduced by comparing its acquired electronic circular dichroism (ECD) spectrum with

Chaetoindolone B (2), a light yellow solid, was shown to have a molecular formula of C14H15NO3 by its HRESIMS. Its IR spectrum displayed absorption bands at 3365 and 1774 cm−1, characteristic of a hydroxy group and a carbonyl functionality, respectively. A close inspection of its 1H and 13C NMR spectral data assigned by HSQC and HMBC experiments revealed the presence of one lactone (C-1, δC 177.7), one N-methylindole moiety (C-6−C-14), two methylenes (C-

Figure 2. Key NOESY (dashed arrows) correlations of 1−3. B

DOI: 10.1021/acs.jnatprod.8b01101 J. Nat. Prod. XXXX, XXX, XXX−XXX

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3 and C-5), and two methines (C-2, δC 67.2 and C-4, δC 78.2). With this information in mind, the planar structure of 2 was formulated by the coupling sequence of its 1H NMR signals via 1 H−1H COSY and the HMBC correlations (H-3 with C-1 and C-2; H-5 with C-3, C-4, C-7, and C-14; Figure 1). The relative stereochemistry of 2 was established by comparing the observed coupling constants and NOE data with those of harzialactone A isolated from Trichoderma harzianum OUPS-N115.17 Namely, its coupling constant magnitude of H-3 with H-2 and H-4 (J2,3α = J2,3β = J3β,4 = 8.2 Hz and J3β,4 = 3.3 Hz; Table 2) was closely similar to that

Table 2. NMR Spectroscopic Data of 2 and 3 in CDCl3 2 position

Table 1. NMR Spectroscopic Data of 1 in Acetone-d6 position

δC

1 2 4 5 5a 6a 7 8 9 10 10a 10b 2′ 3′ 3′a 4′ 5′ 6′ 7′ 7′a 11 6-NMe 1′-NMe

113.2 128.7 165.6 44.2 135.9 138.4 110.4 122.9 120.9 118.3 125.8 108.8 127.7 107.1 126.3 119.1 119.1 121.7 109.4 137.6 164.7 29.2 31.8

δH (mult. J in Hz) 7.77 (s)

5.82 (br s)

7.60 7.36 7.28 7.89

(d, 7.7) (t, 7.7) (t, 7.7) (d, 7.7)

7.83 7.10 7.20 7.33

1 2 3

177.7 67.2 34.7

4 5

78.2 30.6

6 7 8 9 10 11 12 14 13-NMe 15

107.8 127.9 118.6 119.4 122.0 109.4 136.9 128.2 32.8

16 17 18 19 20 21 22 24 23-NMe

6.50 (br s)

(d, 7.7) (t, 7.7) (t, 7.7) (d, 7.7)

δC

δH (mult. J in Hz) 3.98 (t, 8.2) 2.24 (dt, 8.2, 13.2) 2.39 (ddd, 3.3, 8.2, 13.2) 4.96 (m) 3.11 (br d, 5.4)

7.56 7.14 7.24 7.30

(d, 7.9) (t, 7.9) (t, 7.9) (d, 7.9)

6.92 (br s) 3.76 (s)

3 δC 177.7 67.1 34.1

78.4 30.1 105.2 127.7 118.1 119.1 121.9 109.1 137.2 137.2 30.0 20.9 111.5 127.1 118.7 119.5 121.4 109.4 137.0 126.9 32.7

δH (mult. J in Hz) 4.01 (t, 8.3) 2.17 (dt, 8.3, 13.2) 2.38 (ddd, 3.0, 8.3, 13.2) 4.95 (m) 3.15 (dd, 5.2, 14.9) 3.20 (dd, 5.2, 14.9)

7.59 7.13 7.28 7.27

(d, 7.9) (t, 7.9) (t, 7.9) (d, 7.9)

3.57 (s) 4.19 (d, 17.0) 4.28 (d, 17.0)

7.57 7.13 7.21 7.19

(d, 7.9) (t, 7.9) (t, 7.9) (d, 7.9)

6.39 (br s) 3.64 (s)

Chaetoindolone D (4), a brown, amorphous solid, was demonstrated to have a molecular formula of C13H13NO according to HRESIMS. The 1H and 13C spectra of 4 showed the presence of an N-methylindole nucleus and an aldehyde group, which resonates at δH 9.69 as a one-proton singlet and at δC 194.4 as a CH signal. To account for the double-bond equivalents and elemental composition of its molecular formula, the side chain situated on the indole ring has to be a butenal group (C4H5O), which allowed the splitting pattern of H-11 as a quartet at δH 6.88 (J = 7.1 Hz) and of H-12 as a doublet at δH 2.06 (J = 7.1 Hz). The NOE cross-peak between H-11 and H-13 indicated the E-configuration of the double bond. Moreover, the HMBC correlations of H-13 with C-10 (δC 138.6) and C-3 (δC 105.7) and of H-11 with C-3 and C-12 (δC 17.1) confirmed the butenal group at C-3 (Figure 1) to finalize the structure determination of 4. 19-O-Demethylchaetogline A (5) was isolated as a red solid with a molecular formula of C19H16N2O5 as determined by its HRESIMS. The 1H and 13C NMR data of 5 (Table S1) were nearly identical to those of chaetogline A (7),15 ignoring the methoxy group in 7 (δH 3.87 (s)/δC 52.5), suggesting the presence of a 19-carboxylic group. This elucidation was substantiated by its 2D NMR spectra (Figure 1) to characterize 5 as 19-O-demethylchaetogline A, with its CD spectrum closely similar to that of 7 (Figure 3). 20-O-Demethylchaetogline F (6) was obtained as a dark green, amorphous powder. Its molecular formula was established to be C20H16N2O5 by HRESIMS. However, the poor resolution of its 1H NMR spectra recorded in CDCl3, acetone-d6, DMSO-d6, methanol-d4, and pyridine-d5 impeded

3.92 (s) 3.66 (s)

of harzialactone A (J2,3α = J2,3β = J3β,4 = 8.3 Hz and J3β,4 = 3.5 Hz).18 Furthermore, NOEs of H-4 with H-3β and of H-2 with H-3α and H-5 in the NOESY spectrum of 2 (Figure 2) agreed with those of harzialactone A.18 This observation reinforced the presence of a trans-γ-lactone motif in 2. The absolute configuration of 2 was determined through the CD spectral comparison. As ascertained,19 the trans-(2S,4S)and trans-(2R,4R)-chromophores of harzialactone A display positive and negative Cotton effects around 225 nm in the CD spectra, respectively. This enabled us to conclude that 2 most likely had a 2R,4R-configuration, responsible for the negative Cotton effect at 240 nm (Figure 3). Chaetoindolone C (3) was obtained as a light yellow solid and was determined to have the molecular formula C24H24N2O3 by HRESIMS. The 1D and 2D NMR spectra (Table 2) of 3 suggested that it resembles 2 structurally, but probably possesses an additional N-methylindole moiety. Such an extra indole nucleus was shown to be linked to C-6, as evidenced from the HMBC correlations of H-15 to C-6 (δC 105.2), C-24 (δC 126.9), and C-17 (δC 127.1) (Figure 1). The absolute configuration of 3 was clarified to be identical with that of 2 by its CD spectrum, which gives a negative Cotton effect at 240 nm (Figure 3).19 C

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the structural elucidation procedure through the NMR spectral interpretation. Thus, 6 was treated with MeI to give its methyl ester (6a) (Figure 4), which formed crystals from the

Table 3. NMR Spectroscopic Data of 4 in CDCl3 position

δC

2 3 4 5 6 7 8 9 10 11 12 13 1-NMe

129.6 105.7 127.1 120.4 119.7 121.9 109.7 136.9 138.6 151.1 17.1 194.4 33.2

δH (mult. J in Hz) 7.12 (s)

7.31 7.13 7.24 7.34

(d, 7.7) (t, 7.7) (t, 7.7) (d, 7.7)

6.88 2.06 9.69 3.82

(q, 7.1) (d, 7.1) (s) (s)

Table 4. In Vivo Efficacy of Chaetoindolone A (1) on Rice Bacterial Leaf Blight compound

treatment (μg/ mL)

lesion length (mm)

protection efficacy (%)

Figure 4. Preparation of 6a and its crystal structure.

chaetoindolone A (1)

200

11.5 ± 1.0

81.6

methanol−CH2Cl2 (1:1) mixture. Single-crystal X-ray diffraction analysis of 6a clarified its structure (Figure 4), indicating that 6 was a 20-demethylated product of chaetogline F (Table S2 and Figure 1).15 All isolated alkaloids were evaluated for antifungal activities against pathogenic fungi (S. sclerotiorum, Botrytis cinerea, Fusarium solani, and Rhizoctonia cerealis) with carbendazim as positive control and for antibacterial action against Xanthomonas oryzae pv. oryzae, Ralstonia solanacearum, Xanthomonas oryzae pv. oryzicola, and Pseudomonas syringae pv. lachrymans using streptomycin sulfate as a reference. Chaetogline A (7) was inhibitory against S. sclerotiorum with an EC50 value of 10.3 μg/mL (carbendazim: 0.17 μg/mL). Alkaloids 1, 3, 5, and 6 showed antibacterial activities with MICs ranging from 8 to 128 μg/mL (Table S3). Structure− activity relationship analysis suggested that the 19-ester group in 7 is essential for its antifungal action, and the 19-carboxylic residue in 5 is required for the antibacterial effect. Chaetoindolone A (1) was selected to evaluate the in vivo activity against rice bacterial leaf blight caused by X. oryzae pv. oryzae. As demonstrated in Figure 5 and Table 4, 1 exhibited protection efficacies of 82% and 61% at 200 and 100 μg/mL, respectively, comparable to those of streptomycin sulfate (92% and 78%) coevaluated at the same dosages.

streptomycin sulfatea

100 200

24.0 ± 1.4 5.0 ± 0.4

61.2 92.0

100

13.5 ± 1.3 62.5 ± 4.2

77.6

negative control a

Positive control.

The indole structure motif occurs in a variety of natural products as well as in synthetic agrochemicals. This motif is associated with a broad spectrum of biological profiles ranging from antifungal and antibacterial to nematicidal activities.20−23 Some indole-bearing pesticides such as amisulbrom24 and indole-3-acetic acid (a well-known plant hormone) have sparked numerous synthetic efforts to access new alkaloids with such indole-related substructure(s).25,26 Unlike the synthetic approaches, this work describes the application of an endophytic fungus for biotransformation of 1-MT to produce new indole alkaloids, some of which were shown to have promising antifungal and antibacterial activities. Collectively, this work provides an efficient and environmentally benign methodology for constructing structurally undescribed indoles, from which lead molecules can be recognized and optimized for new pesticide development.

Figure 5. Effects of 1 against rice bacterial leaf blight. D

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Chaetoindolone B (2): light yellow solid; [α]20D 8.0 (c 0.25, MeOH); UV (MeOH) λmax (log ε) 223 (2.74), 207 (2.82) nm; IR (KBr) νmax 3365, 2933, 1774, 1613, 1473, 1188, 743 cm−1; 1H and 13 C NMR data assigned and listed in Table 2; HRESIMS m/z 268.0948 [M + Na]+ (calcd for C14H15NO3Na, 268.0944). Chaetoindolone C (3): light yellow solid; [α]20D 12.2 (c 0.23, MeOH); UV (MeOH) λmax (log ε) 287 (2.26), 229 (2.92), 207 (2.71), 193 (2.32) nm; IR (KBr) νmax 3397, 3051, 2928, 1775, 1470, 1182, 742 cm−1.; 1H and 13C NMR data assigned and listed in Table 2; HRESIMS m/z 411.1674 [M + Na]+ (calcd for C24H24N2O3Na, 411.1679). Chaetoindolone D (4): gray, amorphous solid; UV (MeOH) λmax (log ε) 357 (3.15), 276 (3.81), 254 (3.64), 219 (4.34) nm; IR (KBr) νmax 2925, 2854, 1672, 1630, 1461 cm−1; 1H and 13C NMR data assigned and listed in Table 3; HRESIMS m/z 222.0890 [M + Na]+ (calcd for C13H13NONa, 222.0889). 19-O-Desmethylchaetogline A (5): red solid; [α]20D −70.2 (c 0.10, MeOH); UV (MeOH) λmax (log ε) 325 (2.39), 228 (2.83), 211 (2.74), 207 (2.76) nm; IR (KBr) νmax 3459, 3170, 3053, 2948, 2583, 1691, 1225, 741 cm−1; 1H and 13C NMR data assigned and listed in Table S1; HRESIMS m/z 375.0954 [M + Na]+ (calcd for C19H16N2O5Na, 375.0951). 20-O-Desmethylchaetogline F (6): dark green, amorphous powder; UV (MeOH) λmax (log ε) 363 (2.12), 350 (2.10), 292 (2.64), 262 (2.74), 219 (2.87), 194 (2.38) nm; IR (KBr) νmax 3389, 3104, 3067, 2958, 1677, 1627, 1206, 1120, 753 cm−1; 1H and 13C NMR data assigned and listed in Table S2; HRESIMS m/z 365.1130 [M + H]+ (calcd for C20H17N2O5, 365.1132). Preparation and X-ray Crystallography of 6a. Compound 6 (10 mg) was allowed to react with MeI (200 mg) and K2CO3 (400 mg) in dry acetone under reflux for 2 h. Upon completion of the reaction, the resulting mixture was dried through N2 stream, dissolved in CHCl3, and washed successively with water and brine, followed by the in vacuo evaporation of the organic solvent. The obtained solid was subjected to CC over Sephadex LH-20 eluted with methanol to give the anticipated methyl ether 6a, which formed the desired single crystals in a MeOH−CH2Cl2 mixture (v/v, 1:1). Diffraction data were collected on a Bruker Apex-II CCD single-crystal X-ray diffractometer using Cu Kα radiation (λ = 1.5418 Å). The structure was solved by direct methods using the SHELXS-2016/6 program of the SHELXTL package and refined by the full-matrix least-squares method using SHELXL-2016/6. Crystal data of 6a: C25H31IN2O7, triclinic, space group P1, a = 7.9480(4) Å, b = 11.1134(5) Å, c = 15.8240(7) Å, α = 105.623(2)°, β = 97.489(2)°, γ = 92.541(2)°, V = 1329.97(11) Å3, Z = 2, Dcalc = 1.494 g/cm3, R = 0.0487, wR2 = 0.1315, T = 153(2) K. Crystal size, 0.19 × 0.22 × 0.26 mm3. Crystallographic data for 6a have been deposited at the Cambridge Crystallographic Data Center with the number CCDC 1846487. Biological Assay. The antifungal27 and antibacterial assays were performed as described,28,29 with all the tests accomplished in triplicate.

EXPERIMENTAL SECTION

General Experimental Procedures. UV spectra were measured with a Hitachi U-3000 spectrophotometer (Hitachi, Tokyo, Japan). IR spectra (KBr) were recorded on a Nexus 870 FT-IR spectrometer (Thermo Nicolet, Minneapolis, MN, USA). CD spectra were acquired on a JASCO-810 spectropolarimeter (JASCO, Easton, MD, USA). Optical rotations were determined on a Rudolph Autopol III automatic polarimeter (Rudolph Research Analytical, Hackettstown, NJ, USA). NMR spectra were acquired on either a Bruker DRX-500 or a DRX-600 NMR spectrometer (Bruker, Fällanden, Switzerland) at room temperature. Tetramethylsilane and solvent signals were used for the chemical shift calibration. High-resolution electrospray ionization mass spectroscopies (HRESIMS) were recorded on an Agilent 6210 TOF LC-MS spectrometer (Agilent Technologies, Santa Clara, CA, USA). Silica gel (200−300 mesh) for column chromatography (CC) was purchased from Qingdao Marine Chemical Factory, Qingdao, China. Sephadex LH-20 was produced by Pharmacia Biotech, Uppsala, Sweden. ODS gel was purchased from YMC Group, Japan. Reversed-phase HPLC purification was carried out on an ODS-2 Hypersil column (5 μM, 250 × 10 mm, Thermo Scientific, Shanghai, China). All chemicals used in the study were of analytical or HPLC grade. Fungus. C. globosum 1C51 was isolated from the gut of the marine fish Epinephelus drummondhayi collected in the Yellow Sea, Yancheng City, China.15 The strain was identified according to the ITS sequence (GenBank accession no. MK955344), preserved on potato dextrose agar slants, and stored at 4 °C prior to experimentation. Fermentation and Biotransformation Experiments. The fungus was transferred onto the PDA medium and cultured at 28 °C for 5 days. The colonies that formed were transferred to 1 L Erlenmeyer flasks containing 400 mL of Czapek’s medium (30 g sucrose, 1 g yeast extract, 3 g NaNO3, 0.5 g MgSO4·7H2O, 10 mg FeSO4·7H2O, 1 g K2HPO4, 0.5 g KCl, and an appropriate amount of water until reaching a one-liter volume), periodically shaking at 130 rpm at 28 °C on a rotary shaker. Two days later, 1-MT was added to the fungal cultures at 4 × 12 h intervals until a concentration of 1 mM. After cultivating for 10 days, the culture was filtered through three layers of muslin cloth and extracted with ethyl acetate four times to afford a crude extract (ca. 40 g) after the in vacuo evaporation of solvent. Isolation and Purification. The crude extract was separated by CC over silica gel eluted with CH2Cl2−MeOH mixtures of a growing polarity (v/v; 100:0, 100:1, 100:2, 100:4, 100:8, and 0:100, ca. 6 L each) to afford six fractions (F1−F6) according to the TLC monitoring. F2 was fractionated by CC over silica gel eluted with an acetone−petroleum gradient (v/v, 100:0 → 0:100) to give six fractions (F2.1−F2.6). F2.2 was further separated into 10 fractions (F2.2.1−F2.2.10) through gel filtration over Sephadex LH-20 with MeOH. Purification of F2.2.5 by semipreparative HPLC with MeOH−H2O (65:35) afforded 3 (6 mg, tR = 20.5 min) and 2 (24 mg, tR = 33.4 min). F3 was separated over a Sephadex LH-20 column with MeOH, followed by RP-HPLC (MeOH−H2O, 70:30) to yield 1 (14 mg, tR = 17.5 min) and 4 (8 mg, tR = 30.5 min). Separation of F4 by CC over reversed-phase ODS using MeOH−H2O mixtures (v/v; 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, and 100:0, ca. 3 L each) afforded eight fractions (F4.1−F4.8). F4.2 was separated by CC over Sephadex LH-20 with MeOH and then purified by semipreparative HPLC (MeOH−H2O, 60:40) to give 5 (17 mg, tR = 22.8 min). The subfraction F5 was further separated by CC successively over silica gel (CH2Cl2−MeOH, 10:1) and a reversed-phase ODS using MeOH− H2O mixtures (v/v; 20:80, 40:60, 60:40, 80:20, and 100:0, each 4 L), followed by the semipreparative HPLC (MeOH−H2O, 55:45) to give 6 (19 mg, tR = 18.5 min). Chaetoindolone A (1): pale red powder; [α]20D −15.6 (c 0.18, MeOH); UV (MeOH) λmax (log ε) 291 (2.41), 218 (2.95) nm; IR (KBr) vmax 3324, 3052, 2922, 1695, 1615, 1430, 1231, 746 cm−1; 1H and 13C NMR data assigned and listed in Table 1; HRESIMS m/z 408.1317 [M + Na]+ (calcd for C23H19N3O3Na, 408.1319).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jnatprod.8b01101. NMR spectra for compounds 1−6 (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yong Hao Ye: 0000-0003-3538-6006 Ren Xiang Tan: 0000-0001-6532-6261 E

DOI: 10.1021/acs.jnatprod.8b01101 J. Nat. Prod. XXXX, XXX, XXX−XXX

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Notes

(25) Santner, A.; Estelle, M. Nature 2009, 459, 1071−1078. (26) Silva, S.; Ascenso, O. S.; Lourenco, E. C.; Archer, M.; Maycock, C. D.; Ventura, M. R. Org. Biomol. Chem. 2018, 16, 6860−6864. (27) Cao, L. L.; Zhang, Y. Y.; Liu, Y. J.; Yang, T. T.; Zhang, J. L.; Zhang, Z. G.; Shen, L.; Liu, J. Y.; Ye, Y. H. Pestic. Biochem. Physiol. 2016, 129, 7−13. (28) Yan, W.; Wuringege Li, S. J.; Guo, Z. K.; Zhang, W. J.; Wei, W.; Tan, R. X.; Jiao, R. H. Bioorg. Med. Chem. Lett. 2017, 27, 51−54. (29) Pan, X. Y.; Xu, S.; Wu, J.; Luo, J. Y.; Duan, Y. B.; Wang, J. X.; Zhang, F.; Zhou, M. G. Pestic. Biochem. Physiol. 2018, 145, 8−14.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was co-financed by National Key Research and Development Programs of China (2018ZX09711001-007-004 and 2017YFD0201100), the National Natural Science Foundation of China (21602109 and 31572043), the State Level College Students’ Innovative and Entrepreneurial Training Program of China (20181037017), Top-notch Academic Programs Project of Jiangsu Higher Education Institutions (PPZY2015B157), and Qing Lan Project of Jiangsu Province.

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DOI: 10.1021/acs.jnatprod.8b01101 J. Nat. Prod. XXXX, XXX, XXX−XXX