New Benzofuranoids and Phenylpropanoids from the Mangrove Endophytic Fungus, Aspergillus sp. ZJ-68

Three new benzofuranoids, asperfuranoids A–C (1–3), two new phenylpropanoid derivatives (6 and 7), and nine known analogues (4, 5, and 8–14) were isolated from the liquid substrate fermentation cultures of the mangrove endopytic fungus Aspergillus sp. ZJ-68. The structures of the new compounds were determined by extensive spectroscopic data interpretation. The absolute configurations of 1–3 were assigned via the combination of Mosher’s method, and experimental and calculated electronic circular dichroism (ECD) data. Compounds 4 and 5 were a pair of enantiomers and their absolute configurations were established for the first time on the basis of their ECD spectra aided with ECD calculations. All isolated compounds (1–14) were evaluated for their enzyme inhibitory activity against α-glucosidase and antibacterial activities against four pathogenic bacteria (Staphylococcus aureus, Escherichia coli, Bacillus subtilis, and Pseudomonas aeruginosa). Among them, compound 6 exhibited potent inhibitory activity against α-glucosidase in a standard in vitro assay, with an IC50 value of 12.4 μM, while compounds 8 and 11 showed activities against S. aureus, E. coli, and B. subtilis, with MIC values in the range of 4.15 to 12.5 μg/mL.


Introduction
The mangrove forest, growing in tropical and subtropical intertidal estuarine zones, is a diverse group of salt-tolerant plants and represents a rich resource of fungal endophytes [1,2]. Endophytic fungi are distributed in the tissues of every plant to establish a remarkable mutualistic association with their host plants [3]. As a consequence of this unusual lifestyle, endophytic fungi are widely considered to be a pivotal and prolific reservoir of structurally unique and biologically active secondary metabolites with promising medicinal, agricultural, or industrial applications [4]. The genus Aspergillus was first described by Micheli in 1729 and consists of over 300 species distributed worldwide that are able to grow and sporulate under very different environmental conditions including significant changes of temperature, osmolarity, or pH [5,6]. Moreover, Aspergillus species are well known for their prolific production of chemically versatile bioactive secondary metabolites [7], such as the potent cholesterol-lowering agent lovastatin obtained from the culture extracts of Aspergillus terreus [8].
In the past decade, our research group has focused on mangrove endophytic fungi from the South China Sea to discover new and bioactive metabolites [9][10][11][12]. As part of our ongoing research, a fungal strain Aspergillus sp. ZJ-68 was collected from fresh leaves of the mangrove plant Kandelia candel. Further chemical investigations of its crude extract led to isolation of five new compounds, including three new benzofuranoids, namely asperfuranoids A-C (1-3) and two new phenylpropanoid
Asperfuranoid A (1) was obtained as a pale yellow oil with a molecular formula C 15 Figure S6) as five nonprotonated carbons (four olefinic carbons and one sp 3 oxygen-bearing carbon), four methine carbons (two olefininc carbons and two sp 3 oxygen-bearing carbons), three sp 3 methylene carbons, and three methyl carbons. The molecular formula of 1 required five degrees of unsaturation, but only six olefinic carbons resonating at δ C 158.6 (C, C-9b), 126.6 (C, C-3a), 124.7 (CH, C-4), 118.5 (CH, C-5), 142.5 (C, C-5a), and 119.8 (C, C-9a) were detected, indicating the bicyclic nature of 1. The planar structure of 1 was further established by the 2D NMR data ( Figure 2). A hydrogenated benzofuran ring was deduced by the COSY correlations (H-2/H 2 -3 and H-4/H-5) and the HMBC correlations from H-2 to C-9b (δ C 158.6) and C-3a (δ C 126.6), from H-3 to C-3a, C-4 (δ C 124.7), and C-9b (δ C 158.6), and from H-4 to C-5a (δ C 142.5) and C-9b. The locations of two hydroxy groups at C-6 and C-11 were suggested by the chemical shifts of C-6 (δ C 72.6, δ H 4.74) and C-11 (δ C 71.7). The additional COSY correlations of H-6/H 2 -7/H 3 -10 and the HMBC correlations from H-6 (δ H 4.74) to C-5 (δ C 118.5), C-5a, and C-9a, and from H 2 -7 to C-5a allowed the establishment of a propan-1-ol group, which was linked to C-5a of the benzofuran moiety. The presence of a hydroxyisopropyl group which was assigned at C-2 was deduced from the HMBC correlations from H 3 -12 and H 3 -13 to C-11 (δ C 71.7) and C-2 (δ C 89.6). Finally, the planar structure of 1 was established by connecting the hydroxymethyl group to C-9a as evidenced by the HMBC correlations from H 2 -9 to C-5a, C-9a, and C-9b ( Figure 2). further established by the 2D NMR data ( Figure 2). A hydrogenated benzofuran ring was deduced by the COSY correlations (H-2/H2-3 and H-4/H-5) and the HMBC correlations from H-2 to C-9b (δC 158.6) and C-3a (δC 126.6), from H-3 to C-3a, C-4 (δC 124.7), and C-9b (δC 158.6), and from H-4 to C-5a (δC 142.5) and C-9b. The locations of two hydroxy groups at C-6 and C-11 were suggested by the chemical shifts of C-6 (δC 72.6, δH 4.74) and C-11 (δC 71.7). The additional COSY correlations of H-6/H2-7/H3-10 and the HMBC correlations from H-6 (δH 4.74) to C-5 (δC 118.5), C-5a, and C-9a, and from H2-7 to C-5a allowed the establishment of a propan-1-ol group, which was linked to C-5a of the benzofuran moiety. The presence of a hydroxyisopropyl group which was assigned at C-2 was deduced from the HMBC correlations from H3-12 and H3-13 to C-11 (δC 71.7) and C-2 (δC 89.6). Finally, the planar structure of 1 was established by connecting the hydroxymethyl group to C-9a as evidenced by the HMBC correlations from H2-9 to C-5a, C-9a, and C-9b ( Figure 2).  The absolute configuration of the C-6 was determined by the modified Mosher's method [21]. The △δ SR values between 1a and 1b (S-and R-MTPA esters of 1 on 6-OH) were negative for H-4/H-5 and positive for H2-7/H3-10, which indicated the 6R configuration ( Figure 3). Then the other stand-alone stereogenic center C-2 of 1 was deduced and confirmed by comparison of its experimental and theoretical electronic circular dichroism (ECD) spectra, the latter calculated at the b3lyp/6-311+G (d,p) level ( Figure 4). On the basis of the matching of the experimental and computed ECD spectra, the absolute configuration of 1 was defined as 2R, 6R ( Figure 1). The absolute configuration of the C-6 was determined by the modified Mosher's method [21]. The δ SR values between 1a and 1b (S-and R-MTPA esters of 1 on 6-OH) were negative for H-4/H-5 and positive for H 2 -7/H 3 -10, which indicated the 6R configuration ( Figure 3). Then the other stand-alone stereogenic center C-2 of 1 was deduced and confirmed by comparison of its experimental and theoretical electronic circular dichroism (ECD) spectra, the latter calculated at the b3lyp/6-311+G (d,p) level ( Figure 4). On the basis of the matching of the experimental and computed ECD spectra, the absolute configuration of 1 was defined as 2R, 6R (  , and the absence of two aliphatic signals at δH 1.87 and 1.78 (H2-7), and δH 0.94 (H3-10), suggesting a double bond at △ 7 (10) . This deduction was further corroborated by the cross-peaks of H2-10/H-7/H-6 in the COSY spectrum and the HMBC correlations ( Figure 2) from H-6 to C-7 (δC 139.3) and C-10 (δC 115.3), and from H-7 to C-5a (δC 141.0). In addition, the CD curve and specific rotation of 2 was similar to the CD spectrum and specific rotation of 1 in MeOH (Figure 4), respectively. Therefore, the absolute configuration of compound 2 was also determined as 2R, 6R.
Asperfuranoid C (3) has a molecular formula of C15H16O4, as determined by the HRESIMS and NMR data, requiring eight degrees of unsaturation. The 2D NMR data ( Figure 2) provided the structure of 3 to be partially related to 1, possessing a benzofuran ring with the substitution of hydroxyisopropyl at C-2. These functionalities accounted for six sites of unsaturation. The remaining NMR resonances (Table 1) were attributed to two olefinic carbons (δC 101.7 and 160.7) for a double bond, a ketone group (δC 199.9), and a methyl group (δC 20.1), providing two additional sites of unsaturation. The HMBC correlations from the hydroxymethyl protons (H2-9) at δH 5.28 and 5.22 to C-5a (δC 142.6), C-7 (δC 160.7), C-9a (δC 108.9), and C-9b (δC 167.9), from the olefinic proton H-6 (δH 5.69) to C-5 (δC 117.6), C-5a, and C-9a, and from the methyl protons at δH 1.98 (s, H3-10) to C-6 (δC 101.7), and C-7 ( Figure 2) allowed the establishment of a 7-methyl-9H-pyran ring between C-5a and C-9a, accounting for the remaining one site of unsaturation. Additionally, the HMBC correlations from the H-2 and H-4 to C-3 (δC 199.9) (Figure 2) confirmed the presence of the ketone group, which was located at C-3a. Thus, the planar structure of 3 was confirmed and its absolute configuration was designated as 2S by experimental and theoretical ECD data ( Figure 4).   7(10) . This deduction was further corroborated by the cross-peaks of H 2 -10/H-7/H-6 in the COSY spectrum and the HMBC correlations ( Figure 2) from H-6 to C-7 (δ C 139.3) and C-10 (δ C 115.3), and from H-7 to C-5a (δ C 141.0). In addition, the CD curve and specific rotation of 2 was similar to the CD spectrum and specific rotation of 1 in MeOH (Figure 4), respectively. Therefore, the absolute configuration of compound 2 was also determined as 2R, 6R.
Asperfuranoid C (3) has a molecular formula of C 15 H 16 O 4 , as determined by the HRESIMS and NMR data, requiring eight degrees of unsaturation. The 2D NMR data ( Figure 2) provided the structure of 3 to be partially related to 1, possessing a benzofuran ring with the substitution of hydroxyisopropyl at C-2. These functionalities accounted for six sites of unsaturation. The remaining NMR resonances (Table 1) were attributed to two olefinic carbons (δ C 101.7 and 160.7) for a double bond, a ketone group (δ C 199.9), and a methyl group (δ C 20.1), providing two additional sites of unsaturation. The HMBC correlations from the hydroxymethyl protons (H 2 -9) at δ H 5.28 and 5.22 to C-5a (δ C 142.6), C-7 (δ C 160.7), C-9a (δ C 108.9), and C-9b (δ C 167.9), from the olefinic proton H-6 (δ H 5.69) to C-5 (δ C 117.6), C-5a, and C-9a, and from the methyl protons at δ H 1.98 (s, H 3 -10) to C-6 (δ C 101.7), and C-7 ( Figure 2) allowed the establishment of a 7-methyl-9H-pyran ring between C-5a and C-9a, accounting for the remaining one site of unsaturation. Additionally, the HMBC correlations from the H-2 and H-4 to C-3 (δ C 199.9) (Figure 2) confirmed the presence of the ketone group, which was located at C-3a. Thus, the planar structure of 3 was confirmed and its absolute configuration was designated as 2S by experimental and theoretical ECD data ( Figure 4).  , and from H-7 to C-5a (δC 141.0). In addition, the CD curve and specific rotation of 2 was similar to the CD spectrum and specific rotation of 1 in MeOH (Figure 4), respectively. Therefore, the absolute configuration of compound 2 was also determined as 2R, 6R.
Asperfuranoid C (3) has a molecular formula of C15H16O4, as determined by the HRESIMS and NMR data, requiring eight degrees of unsaturation. The 2D NMR data (Figure 2) provided the structure of 3 to be partially related to 1, possessing a benzofuran ring with the substitution of hydroxyisopropyl at C-2. These functionalities accounted for six sites of unsaturation. The remaining NMR resonances (Table 1) were attributed to two olefinic carbons (δC 101.7 and 160.7) for a double bond, a ketone group (δC 199.9), and a methyl group (δC 20.1), providing two additional sites of unsaturation. The HMBC correlations from the hydroxymethyl protons (H2-9) at δH 5.28 and 5.22 to C-5a (δC 142.6), C-7 (δC 160.7), C-9a (δC 108.9), and C-9b (δC 167.9), from the olefinic proton H-6 (δH 5.69) to C-5 (δC 117.6), C-5a, and C-9a, and from the methyl protons at δH 1.98 (s, H3-10) to C-6 (δC 101.7), and C-7 ( Figure 2) allowed the establishment of a 7-methyl-9H-pyran ring between C-5a and C-9a, accounting for the remaining one site of unsaturation. Additionally, the HMBC correlations from the H-2 and H-4 to C-3 (δC 199.9) (Figure 2) confirmed the presence of the ketone group, which was located at C-3a. Thus, the planar structure of 3 was confirmed and its absolute configuration was designated as 2S by experimental and theoretical ECD data (Figure 4).   (Table 2) led to the identification of 4 and 5 as penicisochroman A, a previously reported and structurally characterized compound isolated from Penicillium sp. [13]. The near zero optical rotation, congruent with the crystal data (centrosymmetric space group P _ 1)) which have been reported from our research [15], indicated a racemic mixture. The chiral HPLC separation of (±)-penicisochroman A was further performed on an Acchrom S-Chiral A column to yield a pair of enantiomers, (−)-penicisochroman A (4) and (+)-penicisochroman A (5), with the opposite Cotton effects and the opposite optical rotations. To determine the absolute configurations of (−)-4 and (+)-5, the ECD spectra of (−)-4 and (+)-5 were measured in MeOH and compared with the calculated ECD spectra of the enatiomers ( Figure 5). Thus, the absolute configurations of (−)-4 and (+)-5 were determined as 7R and 7S (Figure 1), respectively. C16H18O4 based on HRESIMS data. Analysis of the NMR data ( Table 2) led to the identification of 4 and 5 as penicisochroman A, a previously reported and structurally characterized compound isolated from Penicillium sp. [13]. The near zero optical rotation, congruent with the crystal data (centrosymmetric space group P1 __ ) which have been reported from our research [15], indicated a racemic mixture. The chiral HPLC separation of (±)-penicisochroman A was further performed on an Acchrom S-Chiral A column to yield a pair of enantiomers, (−)-penicisochroman A (4) and (+)-penicisochroman A (5), with the opposite Cotton effects and the opposite optical rotations. To determine the absolute configurations of (−)-4 and (+)-5, the ECD spectra of (−)-4 and (+)-5 were measured in MeOH and compared with the calculated ECD spectra of the enatiomers ( Figure 5). Thus, the absolute configurations of (−)-4 and (+)-5 were determined as 7R and 7S (Figure 1), respectively.   Asperpanoid A (6) had a molecular formula of C 10 H 14 O 3 , as determined by the HRESIMS and NMR data, requiring four degrees of unsaturation. The 1 H NMR data (Table 3) and HSQC spectrum (Supplementary material, Figure S49) (Figure 2) established the gross structure of 6 to be similar as a known phenylpropanoid analogue, 2-(hydroxymethyl)-3-propylphenol (8) [16]. The signals at δ H 6.66 (d, J = 8.1 Hz) and 6.51 (d, J = 8.1 Hz) indicated the presence of a 1,2,3,4-tetrasubstituted benzene system in 6, instead of the trisubstituted benzene in 8. The 13 C NMR spectrum (Table 3) revealed that a deshielded aromatic nonprotonated carbon (δ C 144.1, C-1) in 6 replaced an aromatic methine carbon (δ C 114.4, C-1) in 8. These observations coupled with the MS data suggested that the position of the additional hydroxy group was located at C-1. The COSY correlation between H-2 and H-3, and the HMBC correlations from H-3 to C-1 and C-5 confirmed the above deductions. Thus, compound 6 was the 1-hydroxylated analogue of 8 (Figure 1). Asperpanoid B (7) was assigned the molecular formula C 11 H 12 O 3 on the basis of HRESIMS data. The 1 H and 13 C NMR data (Table 3) of 7 matched well with those for the known compound peniciphenol (9) and indicated the same structural features present in 9 except for the presence of methoxy group at C-6, which is consistent with the difference in the molecular formula [13]. Accordingly, the signal for the additional methoxy group at δ H 3.85 and δ C 56.1 were observed in the NMR spectra of 7. These observations coupled with the MS data indicate that the hydroxyl group at C-6 in 9 was replaced by a methoxy group in 7. The location of the methoxy group at C-6 was further confirmed by the HMBC correlation from the methoxy proton to C-6. The similar NOESY coupled patterns ( Figure 2) and coupling constant between H-7 and H-8 (J 7,8 = 11.5 Hz) to 9 assigned the same Z-∆ 7 -double bond in 7. Therefore, compound 7 was the methoxy derivative of 9 ( Figure 1).

Biological Activity
The isolated compounds 1-14 were evaluated for their inhibitory activity against α-glucosidase in vitro, and acarbose was selected as the positive control (IC 50 = 453.3 µM) ( Table 4). Compounds exhibiting inhibitory activity against α-glucosidase with values greater than 40% at 100 µM were further tested and IC 50 values calculated. Compound 6 was more potent than acarbose, with an IC 50 value of 12.4 µM. Additionally, the other compounds revealed weak or no inhibitory effects at a concentration of 100 µM.  All isolated compounds (1−14) were also tested for their antibacterial effects against four pathogenic bacteria (S. aureus, E. coli, B. subtilis, and P. aeruginosa) ( Table 5). Compounds 8 and 11 showed activities against S. aureus, E. coli, and B. subtilis, with MIC values in the range of 4.15 to 12.5 µg/mL, while the other compounds exhibited weak or no antibacterial activities (MIC values > 100 µg/mL). None of the compounds were active against P. aeruginosa (MIC values > 100 µg/mL).  1-7, 9, 10, and 12-14 showed no activity (MIC > 100 µg/mL); b ciprofloxacin was tested as positive control.

Fungal Material
The fungus Aspergillus sp. ZJ-68 (Supplementary material, Figure S67) was isolated from fresh leaves of the mangrove plant Kandelia candel, which were collected in July 2016 from the Zhanjiang Mangrove Nature Reserve in Guangdong Province, China. The fungal strain was identified according to a molecular biology protocol by rDNA amplification and sequencing of the internal transcribed spacer (ITS) region [22]. A BLAST search result showed that it was most similar (99%) to the sequence of Aspergillus sp. (compared to JF312217.1). The sequence data have been submitted to GenBank with accession number MK629267. The isolate was stored on PDA medium (potato 200 g, dextrose 20 g, sea salt 3 g, and agar 15 g in 1.0 L of H 2 O, pH 7.4-7.8) slants at 4 • C.

Fermentation
The fungus Aspergillus sp. ZJ-68 was cultured on PDA agar plate at 28 • C for 7 days. The fungal colony was further inoculated into the PDB medium (potato 200 g, dextrose 10 g, and sea salt 3 g in 1.0 L of H 2 O, pH 7.4-7.8) at 28 • C for 3 days on a rotating shaker (140 rpm). Then, a large-scale fermentation of the strain was performed. The fungal seed broth (10 mL) was added to one hundred flasks (1000 mL), each containing 300 mL of liquid medium that was composed of potato 200 g, dextrose

Extraction and Isolation
The whole fermentation broth (30 L) was filtered by cheesecloth to separate the mycelia from the filtrate. The mycelia were extracted three times by CH 2 Cl 2 , while the filtrate was extracted three times by the equivalent volume of EtOAc. The CH 2 Cl 2 and EtOAc solutions were concentrated under reduced pressure to give an organic extract. This extract was fractionated by silica gel (200-300 mesh) column chromatography using a petroleum ether (PE, 60-90 • C) and EtOAc gradient system (from 1:0 to 0:1) to give 10 fractions (F1 to F10). Fraction F2 was applied to Sephadex LH-20 eluting with  Figure S68). Fraction F3 was separated into two subfractions (F3a and F3b) by CC on silica gel eluting with a step gradient of PE/EtOAc (80:20 to 70:30, v/v). Compounds 3 (3.4 mg, t R : 20.1 min), 7 (6.8 mg, t R : 19.6 min), and 10 (8.9 mg, t R : 18.0 min) were obtained from the F3a which was subjected on RP-HPLC column (70% MeOH-H 2 O). F3b was submitted to silica gel CC (CH 2 Cl 2 -MeOH, 2:100, v/v) to yield 9 (4.5 mg), 11 (9.8 mg), and 14 (7.4 mg). Fraction F4 was fractionated by CC on silica gel into two subfractions (F4a and F4b) eluting with gradient CH 2 Cl 2 and MeOH (2:100 to 3:100, v/v). Compounds 1 (10.8 mg, t R : 19.7 min) and 2 (5.3 mg, t R : 20.8 min) were purified by RP-HPLC column (70% MeOH-H 2 O) from the F4b. F4a was submitted to silica gel CC eluting with CH 2 Cl 2 -MeOH (3:100, v/v) and further purified by Sephadex LH-20 eluting with CH 2 Cl 2 -MeOH (1:1, v/v) to yield compounds 6 (6.8 mg) and 8 (5.9 mg). 3.5. Inhibitory Activity Against α-Glucosidase α-Glucosidase inhibitory activity was assessed in 96-well plates using 0.01 M KH 2 PO 4 −K 2 HPO 4 (pH 7.0) buffer solution, and the enzyme solutions were prepared to give 2.0 units/mL in buffers. The assay was conducted in the 200 µL reaction system which contains 168 µL of buffers, 10 µL of diluted enzyme solution, and 2 µL of dimethyl sulfoxide (DMSO) or sample (dissolved in DMSO). The detailed methodology for biological testing has already been described in a previous report [12]. Acarbose was used as the positive control and all assays were performed in three replicates.

Antibacterial Assay
Two Gram-positive bacteria B. subtilis (ATCC 6633) and S. aureus (ATCC 25923), and two Gram-negative bacteria E. coli (ATCC 25922) and P. aeruginosa (ATCC 27853) were used. The antibacterial assay and the determination of the MIC were assessed according to continuous dilution method in 96-well plates which has been described in our published paper [23]. The tested substances were dissolved in DMSO and ciprofloxacin was the positive control.

Conclusions
In summary, three new benzofuranoids, asperfuranoids A-C (1-3), and two new phenylpropanoid derivatives (6 and 7) were isolated from the mangrove endopytic fungus Aspergillus sp. ZJ-68. The absolute configurations of 1-5 were unambiguously established by a combination of Mosher's method, and experimental and calculated ECD data. Compounds 4 and 5 were a pair of enantiomers and their absolute configurations were established as 7R and 7S for the first time. This study further expanded the structural diversity of naturally occurring benzofuranoid and phenylpropanoid derivatives. In the bioactivity assays, compound 6 exhibited potent inhibitory activity against α-glucosidase with an IC 50 value of 12.4 µM, and compounds 8 and 11 showed significant activities against S. aureus, E. coli, and B. subtilis with MIC values in the range of 4.15 to 12.5 µg/mL. Compounds 6, 8, and 11 may be considered as potential new drug leads.