Trogopterins A–C: Three new neolignans from feces of <i>Trogopterus xanthipes</i>
Seven compounds, including three neolignans 1-3, a norlignan 4, and three diterpenoids 5-7, were isolated from the feces of Trogopterus xanthipes. Structures of these compounds were identified by 1D and 2D NMR as well as MS. The absolute configurations of compounds 1, 2, and 4 were determined by comparing CD spectra and optical rotations. Among the isolated compounds, 1-3 were novel and subsequently named trogopterins A, B, and C, respectively. Likewise, compound 4 was isolated from nature for the first time. Cytotoxic activities of compounds 1-4 were evaluated. Compounds 1-3 exhibited moderate cytotoxic activities against HL-60 cells with IC 50 values of 34.77-45.68 μM.
Introduction<!>Results and Discussion<!>Experimental General Methods<!>Material<!>Extraction and isolation<!>Assessment of cytotoxicity
<p>Chemical studies of natural products including ones derived from plants and microorganisms have led to the isolation of numerous novel metabolites with biological activities [1,2]. As a continuation of these investigations, feces from Trogopterus were selected since organic extracts from this material were found to exhibit potent cytotoxic activities in our preliminary study. Trogopterus feces, called "Wulingzhi", are dried excre-ments of Trogopterus (T.) xanthipes Milne-Edwars (Petauristidae) that are known as complex-toothed flying squirrels to eat branches, leaves, and fruits of pine trees [3]. Trogopterus feces have been reported to promote blood circulation and resolve stasis. Thus, this material has been used as traditional medicine for treating amenorrhea, dysmenorrheal, menstrual pain, and retained lochia due to stasis [4]. Recent studies have indicated that Trogopterus feces mainly consist of terpenoids [5][6][7][8][9], phenolic acids, sterols, aliphatics, fatty acids, and flavonoids [10,11]. They have been reported to possess various pharmacological properties such as controlling of antithrombin levels, inhibition of platelet aggregation, cytotoxic activity, immunity enhancement, and anti-inflammatory activities. Isolation of compounds from the methanol extract of Trogopterus feces was presented before by our group [12]. In the present investigation, chemical evaluation of fecal extracts led to the isolation of seven compounds including three new neolignans named trogopterins A-C (1-3). Here we describe the isolation, structure, and cytotoxic activities of compounds 1-3.</p><!><p>Seven compounds, including three neolignans (1-3), a phenolic compound 4, and three diterpenoids (5-7), were isolated from the methanolic extract of T. xanthipes feces. Their chemical names are methyl 3-((2S,3R)-7-hydroxy-2-(3-hydroxy-4methoxyphenyl)-3-(hydroxymethyl)-2,3-dihydrobenzofuran-5yl)propanoate ( 1), (S)-methyl 3-(3-hydroxy-5-(1-hydroxy-3-(3hydroxyphenyl)propan-2-yl)phenyl)propanoate ( 2), ((1RS,2RS,3RS)-6-hydroxy-2- [13], 8β-hydroxy-3oxopimara-15-ene (5) [14], 9β-hydroxy-9(11),13-abietadien-12one (6) [15], and ent-pimar-15-en-9α, 19-diol (7) [16] based on spectral data (Figure 1). Compounds 1-3 were identified to be new and compound 4 was isolated from nature for the first time. In addition, compounds 6 and 7 were reported from this species in this study for the first time.</p><p>Compound 1 was isolated as colorless oil. The molecular formula of compound 1 was determined to be C 20 H 22 O 7 (ten unsaturations) based on 1 H, 13 C, and HMQC data, and verified by HREIMS (Table 1 and Table 2). Nineteen protons were bound to carbons, so three exchangeable hydrogens were present. Detailed analysis of the 1 H, 13   13 C NMR findings (Table 1 and Table 2), compound 1 was hypothesized to have a lignan skeleton formed through oxidative coupling of two phenylpropanoids units.</p><p>This compound was also presumed to have three rings in its structure based on the unsaturation degree since it contained two aromatic moieties and an ester group. 1 H, 1 H COSY spectrum of compound 1 revealed the presence of two isolated with C-3 and C-4 suggested that C-8' was linked to the C-3 position of the first phenylpropanoid moiety. HMBC correlations of H-7' with C-4 allowed linkage of the oxygenated sp 3 methine carbon C-7' to the oxygenated sp 2 carbon C-4 through oxygen to form a dihydrofuran ring. The presence of a dihydrofuran moiety in compound 1 was also demonstrated by chemical shifts of C-4 (δ 146.9) and C-7' (δ 88.8) together with the unsaturation requirement. Thus, compound 1 was proved to be a neolignan containing a dihydrobenzofuran skeleton. HMBC correlations of the methoxy proton at δ H 3.80 with C-4' (δ 149.1) showed that the methoxy group was attached to C-4' of the 1,3,4-trisubstituted benzene ring, and a hydroxy group was presumed to be attached to C-3' based on the chemical shifts. Ultimately, the chemical structure of compound 1 was identified as shown in Figure 1 and named trogopterin A. Detailed 1 H and 13 C NMR data are presented in Table 1 and Table 2.</p><p>The relative stereochemistry of compound 1 was established by interpreting the NOESY data. A strong NOESY correlation  between H-7' and H-9' indicated that the 1,3,4-trisubstututed benzene ring and hydroxymethylene group were on opposite faces of the molecule. The absolute configurations of C-7' and C-8' in compound 1 were established by comparing the circular dichroism (CD) data to those of previously reported neoligans containing a dihydrobenzofuran skeleton [17]. The CD of compound 1 showed positive cotton effects at 255 and 327 nm along with a negative cotton effect at 234 nm. These features were very similar to those of neolignans [17], indicating that C-7' and C-8' in compound 1 have S and R configurations, respectively. Thus, compound 1 was determined to be methyl   In this way, the planar structure of compound 3 was elucidated as shown in Figure 1.</p><p>The relative stereochemistry of compound 3 was deduced based on NOESY data and comparison with interproton distances calculated by MM2 with ChemDraw. A strong NOESY correla-tion between H-7′ and H-8′ indicated that the phenyl group at C-7′ and the hydroxymethylene group at C-8′ are on the same face of the molecule (Figure 3). Additionally, a strong NOESY correlation between H-8 and H-8′ indicated that the hydroxymethylene group at C-8 and the hydroxymethylene group at C-8′ are on the same face of the molecule (Figure 3). A structure model of this compound was created using MM2 with ChemDraw 3.0 (Figure 3). The resulting calculated interproton distances were in close agreement with the NOESY data (Table 3). A careful modeling study considering all the possible conformers of compound 3 exhibited that interproton distances between H-7, H-8, and H-8′ are within 3 Å when two hydroxymethylene groups and a hydroxyphenyl group in compound 3 are on the same face of the molecule. Thus, the structure of compound 3 was determined to be ((1RS,2RS,3RS)-6-hydroxy-2-(3-hydroxyphenyl)-1,2,3,4-tetrahydronaphthalene-1,3diyl)dimethanol and named trogopterin C.</p><p>Compound 4 was isolated as light brown oil. The 1 H and 13 C NMR spectra contained signals similar to those of compound 2 except for those corresponding to the methyl propanoate moiety in compound 2. Based on the 1 H, 13 C, HMQC, and HMBC data, compound 4 had an aromatic proton instead of the methyl propanoate found in compound 2. Thus, compound 4 was determined to be ((R)-3,3'-(3-hydroxypropane-1,2-diyl)diphenol as previously reported [13]. Even though this compound has been described in the literature, the absolute stereochemistry was not elucidated. Similar to compound 2, the optical rotation of compound 4 was compared to those of (R)-2,3-diphenyl-1-propanol and (S)-2,3-diphenyl-1-propanol [18,19]. Compound 4 had a negative optical rotation of [a] D 25 −15.4 similar to (R)-2,3-diphenyl-1-propanol [19]. Thus, the structure of 4 was identified as (R)-3,3'-</p><p>The in vitro cytotoxic activities of compounds 1-4 against HL-60 (human leukemia), HeLa (human cervical carcinoma), and MCF-7 (human breast cancer) cells were evaluated using an MTT assay. As shown in Table 4, compounds 1-3 exerted moderate cytotoxic effects against HL-60 cells with IC 50 values of 34.77-45.68 μM using adriamycin as a positive control (IC 50 = 0.18 μM). Additionally, compound 1 showed very weak cytotoxic activity against MCF-7 cells with an IC 50 of 94.69 μM. None of the compounds affected the HeLa cells. The absolute configurations of compounds 1, 2, and 4 were determined by comparing CD spectra and optical rotations. The levels of cytotoxicity against three tumor cell lines (HL-60, HeLa, and MCF-7) were evaluated. Compounds 1-3 had moderate cytotoxic effects against HL-60 cells even though the activities were not significant. Thus, Trogopterus feces could be a potential source of lignans with cytotoxic activity.</p><!><p>Optical rotations were measured with a Jasco P1000 digital polarimeter. UV spectra were recorded by a Hewlett Packard 8453 UV-vis spectrometer. CD spectra were obtained with a Jasco J-715 circular dichroism spectrophotometer. 1D and 2D NMR were performed on a VNS 600 MHz spectrometer operating at 600 MHz for protons and 150 MHz for carbon. Chemical shifts are expressed in ppm and referenced relative to the residual solvent signals. Mass spectra were recorded with a Micromass LCT mass spectrometer, and the lock mass calibration was applied to accurately measure masses. Semi-preparative HPLC was performed with an Agilent system consisting of a vacuum degasser, quaternary pump, diode array detector (DAD), and Luna 5u C 18 (2) 100 Å column (250 × 10.00 mm; Phenomenex). Column chromatography (CC) was carried out on either silica gel (Merck KGaA, 70-230 mesh) or Sephadex TM LH-20 (GE Healthcare Sweden). TLC was performed with precoated silica gel 60 F254 plates (Merck KGaA) and spots were visualized under UV light (254 and 365 nm) or by spraying with 20% H 2 SO 4 and heating.</p><!><p>Dried T. xanthipes feces were provided by the oriental hospital of Dongguk University (Seoul, South Korea). A voucher specimen (No. YU-BT-2013-03) was deposited in the Natural Products Chemistry Laboratory of the School of Biotechnology, Yeungnam University (Gyeongsan, South Korea).</p><!><p>The air-dried Trogopterus feces (1 kg) were subjected to extraction three times (3 h per cycle) with refluxing methanol. The solvent was evaporated under reduced pressure to recover the methanolic extracts (98 g) that were partitioned successively between H 2 O and CHCl 3 and EtOAc. The EtOAc extract (6.5 g) was subjected to silica gel CC (5 × 60 cm column) using a gradient of methylene chloride (MC) and acetonitrile (ACN) as eluents to acquire 27 fractions (Fr.</p><!><p>Different types of cancer cells (HL-60, HeLa, and MCF-7) were maintained in RPMI 1640 medium supplemented with L-glutamine, 10% fetal bovine serum (FBS), and 2% penicillin-streptomycin. The cells were cultured at 37 °C in a 5% CO 2 incubator. Cytotoxic activity was measured using a modified MTT assay [20]. Viable cells were seeded with the growth medium (100 µL) in 96-well microtiter plates (1 × 10 4 cells per well) and incubated at 37 °C in a 5% CO 2 incubator. The test sample was dissolved in DMSO for the final sample concentrations to be adjusted from 5.0 to 150 µM by diluting with the growth medium. Each sample was prepared in triplicate. The final DMSO concentration was adjusted to be below 0.1%. After standing for 24 h, 10 µL of the test sample was added to each well. The same volume of DMSO alone was added to the control wells. The medium was removed after 48 h of incubation with the test samples, and 10 µL of MTT were then added to the each well (final concentration, 5 mg/mL). After an additional 4 h of incubation, the resulting formazan crystals were dissolved in 150 mL of DMSO and the optical density (O.D.) was measured at 570 nm. The IC 50 value was defined as the concentration of sample that reduced absorbance by 50% relative to the vehicle-treated control. Adriamycin was used as a positive control.</p>