Novel Terpenoids with Potent Cytotoxic Activities from Resina Commiphora

A novel sesquiterpene dimer, spirocommiphorfuran A (1); two new cadinane sesquiterpenoids, commiphorenes A (2) and B (3); along with three known terpenoids (4–6), were isolated from Resina Commiphora. The structures of these new compounds were characterized by NMR, HRESIMS, quantum chemical computation, and X-ray diffraction analysis. Compound 1 features a 7-oxabicyclo[2.2.1]heptane-2-ene core, representing the first example of germacrane-type sesquiterpene dimer fused via a spiro ring system. Compound 2 is a novel sesquiterpene with a completely new carbon skeleton, which is characteristic of an additional carbon attaching to the cadinane backbone via a carbon–carbon bond. Additionally, compounds 2 and 4 exert acceptable cytotoxicity toward normal cells and high selectivity in cancer cells, especially in HepG2 cells.


Introduction
The Commiphora genus in the family Burseraceae comprises over 150 species, spreading over the tropical and subtropical regions, especially occurring in Eastern Africa, Arabia, and India [1]. The resins from the bark of Commiphora plants, also known as myrrh, have been historically used as a principal folk in Ayurvedic medicine, traditional Chinese medicine, and other indigenous medical systems for the treatment of arthritis, hyperlipidemia, pain, wounds, fractures, and diseases caused by blood stagnation [2,3]. Chemical investigations on myrrh revealed the constituents to be terpenoids, flavonoids, lignans, steroids, and carbohydrates [4]; some of these constituents were reported to have biological activities, such as cytotoxic [5], anesthetic [6], anti-inflammatory [7], and antimicrobial effects [8,9]. In our previous research of the bioactive terpenoids from Resina Commiphora, novel compounds were isolated [10][11][12][13][14]. In the course of our continuing investigations on structurally diverse and biologically intriguing terpenoids from Resina Commiphora, three new terpenoids (1)(2)(3), along with three known analogues (4-6), were isolated. Interestingly, compound 1 is an unexpected sesquiterpene dimer bearing a rare 7-oxabicyclo[2.2.1]heptane-2-ene core and a spiro ring system [15], and compound 2 is an unusual cadinane sesquiterpene containing an additional carbon. Compounds 2 and 4 inhibit cancer cells growth with low cytotoxicity in normal cells.

Structure Elucidation of the Compounds
Spirocommiphorfuran A (1), obtained as colorless block crystals, has a molecular formula C 31 H 38 O 6 (13 degrees of unsaturation) on the basis of its HRESIMS (m/z 507.2736, calcd. 507.2741 [M + H] + ), 13 C-NMR, and DEPT spectra. The 1 H-NMR data ( Table 1) of 1 exhibit four methyl (δ H 2.07 (s), 2.05 (s), 1.13 (s), and 1.08 (d, J = 6.2 Hz)), one methoxy (δ H 3.18 (s)), and five olefinic methine protons (δ H 7.06 (s), 5.10 (overlap), 5.08 (overlap), 4.92 (overlap), and 4.88 (overlap)). The 13 C-NMR and DEPT spectra (Table 1) display 31 carbon resonances, including four methyl, one methoxy, seven methylene (one olefinic), nine methine (three olefinic with one oxygenated, and six aliphatic with four oxygenated), and ten quaternary carbons (two keto-carbonyls, six olefinic, including one oxygenated, and two aliphatic with one oxygenated). In consideration of the aforementioned data and the chemical profile of the genus commiphora, we speculated that 1 might be a sesquiterpene dimer. The structural architecture of 1 (Figure 1) was first elucidated by using 2D-NMR. The 1 H-1 H COSY spectrum of 1 shows the following correlations: H 2 -9/H-10/H 2 -1/H-2/H-3 and H-10/H 3 -15 ( Figure 2). Starting from this spin system, the observed HMBC correlations of H-2/C-4, H-3/C-5, H 2 -5/C-3, C-7, C-14, H-9/C-7, H-10/C-8, H 3 -13/C-7, C-11, C-12, H 2 -14/C-3, C-4, C-5, H 3 -15/C-1, C-9, and C-10 ( Figure 2) allowed us to conclude the presence of a germacrane-type sesquiterpene moiety; apart from these characteristic signals, an epoxy group is also observed in the 1D-NMR spectra, whose position is assigned to C-2 (δ C 62.6) and C-3 (δ C 61.1) due to the observation of the HMBC correlations of H-10/C-2 and H 2 -1, H 2 -5, and H 2 -14/C-3 ( Figure 2). Thus, part I of 1 was deduced as shown (red lines in Figure 1). Part II (blue lines in Figure 1) was established by analysis of the remaining carbon signals in the 13 C-NMR spectrum. Further studies suggest that part II is similar to those reported for the related furanogermacren derivatives [16]; the main difference is that the ∆ 4 (14 ) double bond is absent in part II. Parts I and II are connected via C-6-C-14 and C-12-C-4 supported by the HMBC correlations of H-12/C-3 , C-4 , C-5 , C-14 , H 2 -5/C-14 , H 2 -14 /C-5, C-6, C-7, H-3 , and H-5 /C-12. The presence of an oxygen bridge between C-6 and C-12 is supported by the chemical shifts of C-6 (δ C 91.2), C-12 (δ C 88.2), the HMBC correlation of H-12/C-6, the requirement of the molecular formula, and the remaining one degree of unsaturation. Therefore, the plane structure of 1 was deduced.   The ROESY correlations ( Figure 3) of H-2/H-10, Ha-1/H-3, and H 3 -15 are observed, indicating the relative configurations at C-2, C-3, and C-10. Meanwhile, H-2 interacting with H-12 in the ROESY spectrum, in consideration of the rigidity of the bridged ring, allows one to assign the stereochemistry of the bicyclo[2.2.1]heptane ring. Finally, the ROESY correlation of H-1 /H 3 -15 suggests that these protons are spatially vicinal. Due to the flexibility of two ten-membered rings, it is hard to detect the ROESY restrictions between these two middle rings. Fortunately, X-ray crystallographic analysis of 1 with the excellent Flack parameter [0.00 (11)] allows one to clarify the absolute configurations at each chiral center in the architecture ( Figure 3). In general, the geometry of ∆ 2 (3 ) double bond could be concluded from the coupling constant of H-2 or H-3 . However, this is impossible in the present structure due to the overlapped signals for these protons. The X-ray data also indicate an E configuration for the ∆ 2 (3 ) double bond. Collectively, the structure of 1 was deduced as 2R,3R,6R,10S,12S,1 S,4 S,10 R. Commiphorene A (2), obtained as yellow gums, was found to have the molecular formula C 18 H 20 O 4 by analysis of its HRESIMS, 13 C-NMR, and DEPT spectra, indicating nine degrees of unsaturation. The 1 H-NMR spectrum of 2 exhibits four methyl groups (δ H 2.71 (s), 2.47 (s), 2.10 (s), and 1.18 (d, J = 6.2 Hz)), and one olefinic/aromatic proton. The 13 C-NMR and DEPT spectra (Table S1) reveal 18 carbon signals classified into four methyl, three methylene, two methine (one olefinic), and nine quaternary carbons (one ketone-carbonyl, one ester carbonyl, and seven olefinic, including two oxygenated). The structure of 2 was mainly assembled by interpretation of its 2D-NMR data. The 1 H- 1
It was noted that there is only one chiral center in 2. The absolute configuration of 2 was verified by the quantum chemical electronic circular dichroism (ECD) calculation using the TDDFT method. The experimental ECD spectrum of 2 is similar to the calculated one of (4R)-2 ( Figure 4). Thus, the structure of 2 was finally deduced.
The OR values of (11-R)-3 and its enantiomer (11S)-3 were predicted to be +28.8 and −29.6, respectively. The OR value for compound (11S)-3 is closer to the experimental one (−21.6). Therefore, the absolute configuration of 3 was proposed to be 11S.
The known compounds were identified as myrrhone [19], commipholinone [20], and myrrhanolide C [21], respectively, by comparing their spectroscopic data with those reported in the literature.

Biological Evaluation
All of the compounds were evaluated for their cytotoxic activities against human cancer cells originating from the liver, gastric, and lung. All cells were exposed to various concentrations (0-160 µM) for 48 h, and cell viability was quantified by CCK-8 assay ( Table 3). As a result, only compounds 2, 4, and 5-FU (positive control) significantly inhibited the growth of four human cancer cell lines in a dose-dependent manner, and the other compounds were not active (data not shown). Further, we found that the IC 50 values of 2 against their cancer cells are from 48.67 µM to 152.50 µM, and the most sensitive cell line is HepG2, in which 2 and 4 are more potent than 5-FU. On the other hand, the IC 50 value could not be determined, even at a concentration of up to 160 µM in normal human umbilical vein endothelial cells, suggesting that 2 and 4 exert acceptable cytotoxicity in normal cells and high selectivity in cancer cells, especially in HepG2 cells.

Plant Material
The medicinal materials of Resina Commiphora (myrrh) were obtained from the Juhuacun Market of Material Medica, Kunming, Yunnan Province, China, in July 2013. The material was identified by Mr. Bin Qiu at Yunnan Institute of Materia Medica, and a voucher specimen (CHYX-0585-2) was deposited at School of Pharmaceutical Sciences, Shenzhen University, China in November 2017.

Extraction and Isolation
The dried myrrha (50 kg) was ground and soaked with 95% EtOH (180 L, 3 × 48 h) to give a crude extract, which was suspended in warm water, followed by extraction with EtOAc to afford an EtOAc soluble extract (8 kg). This extract was divided into six parts (Fr.A-Fr.F) by using a    (F2 > 4σ (F2)). The structure was solved by direct methods using the SHELXS-97 program and refined by the program SHELXL-97 and full-matrix least squares calculations. In the structure refinements, nonhydrogen atoms were placed on the geometrically ideal positions by the "ride on" method. Hydrogen atoms bonded to oxygen were located by the structure factors with isotropic temperature factors. The final refinement gave R = 0.0402(4692), Rw = 0.1061(4831), S = 1.045, and Flack = 0.00 (11). Crystallographic data for structure 1 has been deposited at the Cambridge Crystallographic Data Centre (CCDC 1878959), www.ccdc.cam.ac.uk/data_request/cif.

Quantum Chemical Computations of 2 and 3
The theoretical calculations of compounds 2 and 3 were performed using Gaussian 09 [18]. Conformation search using molecular mechanics calculations was performed in Discovery Studio 3.5 Client with MMFF force field with 17 kJ/mol (approximately 4 kcal/mol) upper energy limit. The optimized conformation geometries and thermodynamic parameters of all selected conformations were provided. The predominant conformers were optimized at B3LYP/6-31G(d) level. The optimized conformers of 2 were used for the ECD calculation, and the optimized conformers of 3 were used for OR calculation at the B3LYP/6-311+G(d,p) level. The solvent effects were taken into account by the polarizable-conductor calculation model (PCM, methanol as the solvent). Percentages for each conformation are shown in Figures S1-S4.

Cell Viability
Cell lines were obtained from the Cell Bank of China Science Academy (Shanghai, China), maintained in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal bovine serum and 100 U/mL penicillin-streptomycin and incubated at 37 • C in an atmosphere of 5% CO 2 . Cell viability was evaluated using the CCK8 assay kit (Dojindo Laboratories, Tokyo, Japan) according to the manufacturer's instructions. Exponentially growing cells were seeded at 3-8 × 10 3 cells per well in 96-well culture plates for 24 h. Cells were exposed to increasing concentrations (0-160 µM) of compounds 1-6, or 5-FU for 48 h. The equal volume of DMSO was used as the solvent control. CCK8 solution (10 µL) was added to each well and incubated for another 1-4 h. Light absorbance of the solution was measured at 450 nm (Epoch 2; BioTek Instruments, Inc., Winooski, VT, USA). The IC 50 values were calculated using the PrismPad program (version 5.0, GraphPad Software, San Diego, CA, USA).

Conclusions
Resina Commiphora is commonly used in the traditional Chinese medicine system, in which terpenoids are the main chemical constituents. Inspired by our previous results of structurally novel and biologically intriguing compounds from the material, we conducted an in-depth investigation that resulted in an unusual sesquiterpene dimer with a rare 7-oxabicyclo[2.2.1]heptane-2-ene core (1); a sesquiterpene with an additional carbon in the backbone (2); and a new cadinane-type sesquiterpene (3), adding the structural diversity of sesquiterpenes in nature. Due to the attractive construction features of the isolated compounds, their cytotoxic activities were tested. As a result, compounds 2 and 4 exert low cytotoxicity against normal cells and high selectivity in cancer cells, especially in HepG2 cells, which will be helpful for gaining deep insight into the biological role of such terpenoids in human diseases.