Xylopidimers A-E, Five New Guaiane Dimers with Various Carbon Skeletons from the Roots of Xylopia vielana.

Five new guaiane dimers, xylopidimers A-E (1-5), were isolated and identified from the roots of Xylopia vielana. The structures of 1-5 were elucidated by spectroscopic analysis and further confirmed by single-crystal X-ray diffraction. On the basis of the results of single-crystal X-ray analysis, 1-5 showed different carbon skeletons. Among these compounds, the unique connecting patterns of 1 and 2 caused significant differences on their carbon skeletons, which have not been reported. Moreover, 3-5 were also three new guaiane dimers. Among these compounds, 4 exhibited potent inhibitory activity against the production of nitric oxide with an IC50 value of 4.59 μM in RAW264.7 cells stimulated by lipopolysaccharide.

Introduction

The genus Xylopia, which is one member of the family Annonaceae, contains about 160 species all over the world. Besides, only one species, Xylopia vielana, of this genus occurs in China.[1] On the basis of previous research studies, several dimeric guaianes have been continuously identified from this species.[2,6,7] However, all previous research studies were only focus on the aerial part of this plant.[8] In addition, some previous studies proved that some species from this genus showed potential bioactivities about hypotensive, diuretic, antibacterial, antifungal, and antioxidant effects.[3−5] These previous results highly motivated us to search for more novel compounds with promising bioactivities from the other parts of X. vielana, which resulted in five new guaiane dimers, xylopidimers A–E (1–5) and one known guaiane dimer, vielanin A (6) (Figure ), which is isolated and identified from the roots of X. vielana. Among these compounds, 1 and 2 exhibited two novel connecting patterns. The 4,5-linked exo-Diels–Alder adducts between two guaiane moieties of 2 were first reported by this research. In this paper, the structural identification and the inhibitory activities against the production of nitric oxide in RAW264.7 cells of these six guaiane dimers were described, as well.

Figure 1

Chemical structure of 1–6.

Results and Discussion

Xylopidimer A (1), [α]D20 −23.2 (c 0.109, MeOH), was obtained as colorless prismatic crystals. The molecular formula of 1 was found to be C30H36O4 (HRESIMS [M + Na+] at m/z 483.2519; calcd 483.2506), requiring 13 degrees of unsaturation. The structure of 1 was deduced by NMR analysis and further confirmed by single-crystal X-ray diffraction, which indicates that it was a guaiane dimer (Figure ).[9] The infrared (IR) spectrum indicated the presence of methyl groups (2967 cm–1) and carbonyl groups (1773 cm–1).

Figure 2

X-ray structure of 1.

From analyses of the 1H and 13C NMR spectra of 1, together with the examination of HSQC and HMBC experiments, 30 carbon signals of the 13C NMR spectrum were identified as eight CH3, four CH2, four CH, and 14 quaternary carbons (involving four carbonyl groups, two sp3, and eight sp2 carbons) (Table S1, Supporting Information). The planar structure of 1 was constructed by detailed analyses of 1H–1H COSY and HMBC spectra (Figure ).

First, the moiety 1b was confirmed by determining C-5′ (δc 172.4) because of its uncommonly high chemical shift caused by conjugation to a carbonyl group and the ring tension. The HMBC cross-peaks from H-2′ to C-1′/C-5′ and from H-15′ to C-1′/C-3′/C-4′/C-5′ suggested the appearance of a five-membered ring (Figure ). Moreover, a heptatomic ring was connected to this five-membered ring at C-1′ and C-5′ by analyses of the HMBC cross-peaks from H-6′ to C-5′/C-7′ and from H-9′ to C-1′/C-8′/C-10′ (Figure ). Additionally, two methyl groups were connected to C-11′, which was connected to C-7′, and were confirmed by observing the HMBC cross-peaks from H-12′ to C-7′/C-11′ and from H-13′ to C-7′/C-11′ (Figure ). What is more, two other methyl groups were connected to C-4′ and C-10′ by observing the HMBC cross-peaks from H-14′ to C-10′ and from H-15′ to C-4′, respectively (Figure ). On the basis of these evidences, the moiety 1b was confirmed as a guaiane moiety. Similar to moiety 1b, moiety 1a was also assigned as a guaiane moiety with the aid of key HMBC cross-peaks. Therefore, compound 1 was elucidated as a dimeric guaiane.

Figure 3

Chemical structure and selected NMR correlations of 1.

The connection between moiety 1a and moiety 1b through two direct C–C bonds (C-2 to C-2′ and C-4 to C-1′) was established by the key HMBC cross-peaks from H-15 to C-1′ combined with the 1H–1H COSY correlations of H-2/H-2′. This connecting pattern was probably caused by the Diels–Alder [4 + 2] cycloaddition of two guaiane moieties as the dienophile and diene themselves (Figure ) (Scheme ). This connecting pattern was also further confirmed by the Cu Kα X-ray crystallographic analyses (Figure ).[9] Besides, this connecting pattern is unique and has not been reported previously.

Scheme 1

Proposed Biogenetic Pathway for Xylopidimers A–E (1–5)

The relative configurations of 1 were identified by the key NOE correlations from the NMR spectra. The key NOE correlations of H3-14/H-2′, H3-15/H-2′, and H3-14′/H-2′ indicated that they are in the same face and were assigned as α-oriented, whereas the correlations of H-2/H-10 place them on the opposite side and assigned as β-oriented (Figure ). In order to further confirm this configuration, it was necessary to conduct a Cu Kα X-ray diffraction experiment. After trying several solvent systems, the single crystal of 1 was obtained from methanol/water (4:1). The single-crystal X-ray data unambiguously deduced the structure of 1, and its absolute configuration was also showed in the single-crystal X-ray data as 2S, 4R, 10R, 1′S, 2′S, and 10′R (Figure ).[9]

Xylopidimer B (2), [α]D20 −19.2 (c 0.102, MeOH), was obtained as colorless prismatic crystals. The molecular formula of 2 was found to be C30H36O4 (HRESIMS [M + Na+] at m/z 483.2515; calcd 483.2506), requiring 13 degrees of unsaturation. The structure of 2 was deduced by NMR analysis and further confirmed by single-crystal X-ray diffraction, which indicates that it was a guaiane dimer (Figure ).[10] The IR spectrum indicated the presence of methyl groups (2964 cm–1) and carbonyl groups (1777 cm–1).

Figure 4

X-ray structure of 2.

Extensive interpretation of the 1H, 13C NMR, 1H–1H COSY, and HMBC spectra showed that 2 possessed the guaiane moiety unit similar to those of 1 but possibly had a different linkage type for two moieties. The key HMBC cross-peaks from H-15 to C-4′ and from H-15′ to C-4, combined with the HMBC cross-peak from H-2 to C-5′, indicated that these two guaiane moieties were linked via two different C–C single bonds (C-2 to C-5′ and C-4 to C-4′) (Figure ). This connecting pattern was probably caused by the Diels–Alder [4 + 2] cycloaddition of two guaiane moieties as the dienophile and diene themselves (Scheme ). This connecting pattern was also further confirmed by the Cu Kα X-ray crystallographic analyses (Figure )[10] and proved to be first identified.

Figure 5

Chemical structure and selected NMR correlations of 2.

The relative configurations of 2 were identified from the NOE correlations from the NMR spectra. The key NOESY correlations of H3-14/H-2, H3-15′/H-2, and H3-14′/H-2 indicated that they are in the same face and were assigned as α-oriented, whereas the correlations of H3-15/H-10 place them on the opposite side and assigned as β-oriented (Figure ). In order to further confirm this configuration, the single crystal of 2 was obtained from dichloromethane/methanol (30:1). The single-crystal X-ray data unambiguously deduced the structure of 2, and its absolute configuration was also showed in the single-crystal X-ray data as 2R, 4S, 10R, 4′R, 5′R, and 10′R (Figure ).[10]

Xylopidimer C (3), [α]D20 −43.9 (c 0.121, MeOH), was obtained as colorless prismatic crystals. The molecular formula of 3 was found to be C30H38O5 (HRESIMS [M + Na+] at m/z 501.2617; calcd 501.2611), requiring 12 degrees of unsaturation. The structure of 3 was deduced by NMR analysis and further confirmed by single-crystal X-ray diffraction, which indicates that it was a guaiane dimer (Figure ).[11] The IR spectrum indicated the presence of methyl groups (2964 cm–1), carbonyl groups (1777 cm–1), and hydroxyl groups (3394 cm–1).

Figure 6

X-ray structure of 3.

Extensive interpretation of the 1H, 13C NMR, 1H–1H COSY, and HMBC spectra showed that 3 possessed the guaiane moiety units similar to those of 1 and 2. However, interestingly, another five-membered ring, which was connected to the heptatomic ring of moiety 3a, was observed based on the result of single-crystal X-ray diffraction (Figure ).[11] The chemical shift of C-8 (δc 102.8) and C-11 (δc 85.4) indicated that this five-membered ring involved a peroxide group between C-8 and C-11. In addition, hydroxyl was connected to C-8 simultaneously. What is more, the key HMBC cross-peak from H-15 to C-1′ combined with the 1H–1H COSY correlations of H-3/H-3′, indicated that the two guaiane moieties of 3 were linked via two C–C single bonds (C-3 to C-3′ and C-4 to C-1′), which was different from 1 and 2. This connecting pattern was probably caused by the Diels–Alder [4 + 2] cycloaddition of two guaiane moieties as the dienophile and diene themselves (Scheme ). This connecting pattern was also further confirmed by the Cu Kα X-ray crystallographic analyses (Figure ).[11]

The relative configurations of 3 were identified from the NOE correlations from the NMR spectra. The key NOESY correlations of H3-14/H-3, H-3/H-3′, H2-2′/H-3′, and H3-14′/H-3′ indicated that they are in the same face and were assigned as β-oriented, whereas the correlations of H3-15/H-10 place them on the opposite side and assigned as α-oriented (Figure ). However, the relative configuration at C-8 could not be determined by this NOE analysis. In order to fully confirm the structure of 3, the single crystal of 3 was obtained from dichloromethane/methanol (45:1). The single-crystal X-ray data unambiguously deduced the structure of 3, and its absolute configuration was also showed in the single-crystal X-ray data as 3R, 4R, 8S, 10R, 1′S, 3′R, and 10′R (Figure ).[11] Although these kinds of structures have been reported previously, the absolute configuration at C-8 was first determined in this research.

Figure 7

Chemical structure and selected NMR correlations of 3.

Xylopidimer D (4), [α]D20 −77.4 (c 0.113, MeOH), was obtained as colorless prismatic crystals. The molecular formula of 4 was found to be C30H40O4 (HRESIMS [M + Na+] at m/z 487.2811; calcd 487.2819). The structure of 4 was deduced by NMR analysis and further confirmed by single-crystal X-ray diffraction (Figure ).[12] The results also showed that it was a guaiane dimer. However, the 13C NMR showed only 15 carbon signals. Thus, 4 consisted of two equivalent C15H20O2 moieties. The IR spectrum indicated the presence of methyl groups (2964 cm–1) and carbonyl groups (1694 cm–1).

Figure 8

X-ray structure of 4.

These two moieties were also determined as a guaiane moiety through the same method as moiety 1a. The only difference was the reduction of the two olefinic carbon, C-7 and C-11, because of their chemical shifts, C-7 (δc57.5) and C-11 (δc 28.5) (Table S4, Supporting Information). Therefore, these two moieties were confirmed as two equivalent guaiane moieties. The Cu Kα X-ray crystallographic analyses indicated that these two guaiane moieties were linked via two C–C single bonds (C-3 to C-3′ and C-4 to C-4′) probably arisen from [2 + 2] cycloaddition of two guaiane moieties as the dienophile and diene themselves (Figure ) (Scheme ).

Figure 9

Chemical structure and selected NMR correlations of 4.

The relative configuration of 4 was also deduced from its NOE correlations. The key NOESY correlations of H3-14/H-11 and H3-14/H-3 indicated that they are in the same face and were assigned as α-oriented, whereas the correlations of H3-15/H-10 place them on the opposite side and assigned as β-oriented (Figure ). As 4 contains two equivalent moieties, the other part can also be deduced. Moreover, the single crystal of 4 was obtained from dichloromethane/methanol/water (5:2:1). The single-crystal X-ray data unambiguously deduced the structure of 4, and its absolute configuration was showed in the single-crystal X-ray data as 3R, 4R, 7S, 10R, 3′R, 4′R, 7′S, and 10′R (Figure ).[12]

Xylopidimer E (5), [α]D20 18.6 (c 0.107, MeOH), was obtained as a colorless prismatic crystal. The molecular formula of 5 was found to be C32H42O7 (HRESIMS [M + Na+] at m/z 561.2829; calcd 561.2823). The structure of 5 was deduced by NMR analysis and further confirmed by single-crystal X-ray diffraction (Figure ).[13] Compared to its chemical shifts, its planer construction was the same as one known compound, vielanin E (Table S5, Supporting Information).[6]

Figure 10

X-ray structure of 5.

However, as vielanin E not had the data of the single-crystal X-ray diffraction, only parts of its absolute construction were confirmed. In this research, on the basis of the Cu Kα X-ray crystallographic analyses of 5, it had different absolute construction compared with some of the known absolute constructing parts of the vielanin E and its absolute configuration was showed by the single-crystal X-ray data as 1S, 2S, 3R, 7S, 10R, 1′R, 2′R, 8′S, and 10′R (Figure ). Although these kinds of structures have been reported before, the absolute configuration at C-8 was first determined.

So far, according to previous studies, most of the sesquiterpene dimers were biogenetically synthesized through Diels–Alder cycloaddition reaction of some homo- or heterosesquiterpene moieties.[14] In this study, all of the five new guaiane dimers could be also synthesized through this method and the possible biogenetic pathway for 1–5 was proposed in Scheme , in which the Diels–Alder cycloaddition reaction is a key step.

Because nitric oxide plays a significant role in causing inflammation, 1–5 and vielanin A (6), which was also isolated and identified from X. vielana, this time were therefore tested for their inhibitory activities against the production of NO in RAW264.7 cells stimulated by lipopolysaccharide (LPS), as described previously.[15] Among these compounds, 4 exhibited a relatively high inhibitory effect against the production of NO, with an IC50 value of 4.59 ± 1.04 μM, and exhibited no cytotoxicity against RAW264.7 cells after MTT assay. In contrast, other compounds showed no activity against the production of NO even at the concentration of 10 μM.

Conclusions

In conclusion, six compounds, 1–5 and vielanin A, were isolated from the roots of X. vielana. Among these six compounds, five of them (1–5) were new compounds. Interestingly, 1 and 2 exhibited two unique connecting patterns that had not been reported. Although the moieties of these two compounds were two similar guaianes, based on the results of single-crystal X-ray analysis, these two connecting patterns caused huge differences on their carbon skeletons. In addition, the 4,5-linked exo-Diels–Alder adducts between two guaiane moieties was first reported by this research. Thus, 1 and 2 should be defined as two novel carbon skeletons and could lead to some interesting total synthesis. For 3 and 5, the absolute configuration at C-8 was also first confirmed on these two carbon skeletons. Additionally, these six compounds were tested for the activity against the production of NO in RAW264.7 stimulated by LPS and 4 exhibited a potent activity against the production of nitric oxide.

Experimental Section

General Experimental Procedures

All of the thin-layer chromatography analysis was conducted with HSGF254 silica gel plates (Yantai, China). Silica gel (Yantai, China) and silica gel H (Qingdao, China) were mainly used as packing materials in the silica gel column. Sephadex LH-20 (Pharmacia Co. Ltd.) was selected as a major packing material in the sephadex gel column. The purification step was also assisted by a series of preparative HPLC, The Shimadzu LC-6AD series together with a SPD-20 spectrophotometer. The Shimadzu PRC-ODS EV0233 C-18 column was selected as the separation column for this preparative HPLC. In addition, the Agilent 1200 series HPLC with DAD detector and a ZOBRAX Extend-C-18 column (250 mm × 4.6 mm, 5 μm) were used as the analytical HPLC. All of the 1H and 13C NMR spectra were recorded on the Bruker AVANCE-500. The TMS was utilized as an internal reference, chemical shifts are shown in δ (ppm), and coupling constants were shown in hertz. Crystal data were obtained on Bruker D8 VENTURE with a graphite monochromator with Cu Kα radiation (λ = 1.54178 Å) at 173 K. The high-resolution electrospray ionization mass spectra were recorded on Q-TOF micro YA019 (Waters, USA). A JASCO P-2000 polarimeter was selected as a major equipment for optical rotation analysis, and all of the IR spectra were tested on the Bruker FTIR Vector 22 with KBr pellets.

Plant Materials

The dry roots of X. vielana were acquired in Fangchenggang, Guangxi province, China in December 2016. Prof. Huang Baokang (School of Pharmacy, Second Military Medical University) identified the specimens, and one specimen (no. 201611MBS) was deposited in the herbarium of Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Pharmacy, Shanghai Jiao Tong University.

Extraction and Isolation

Compounds 1–5 were isolated from the extracts of X. vielana. 1–5 were obtained by using the following procedure: dried roots (22.3 kg) were extracted with 95% ethanol three times at room temperature. The extracts were then filtered and evaporated under normal pressure to get crude extraction (764.1 g). The crude extract was suspended in water and then further extracted with petroleum ether and ethyl acetate (EA) step by step. The EA fraction (70.2 g) was isolated by a silica gel column chromatography and eluted with a step gradient of CH2Cl2–MeOH (0:100–0:100, v/v). After that 11 fractions (EA-1–EA-11) were yielded. EA-2 (2.9 g) was further isolated by MPLC with the mobile phase MeOH–H2O (1:10–100:0, v/v), and three fractions were yielded (EA-2-1–EA-2-4). 2 (32.2 mg) was purified by preparative HPLC (MeOH–H2O, 60:40) from EA-2-4. EA-3 (2.3 g) was purified by the silica gel column chromatography with the gradient mobile phase of CH2Cl2–MeOH (100:1–0:100, v/v) and got five fractions (EA-3-1–EA-3-5). 3 (80.8 mg) was finally purified from EA-3-4 via preparative HPLC (MeOH–H2O, 65:35). EA-4 (5.2 g) was also isolated through the silica gel column chromatography with the gradient mobile phase of CH2Cl2–MeOH (100:1–0:100, v/v), and eight fractions (EA-4-1–EA-4-8) were yielded. EA-4-2 was then isolated and purified by preparative HPLC (MeOH–H2O, 60:40). 4 (7.3 mg), 5 (15.7 mg), and 6 (17.2 mg) were got from EA-4-2. 1 (70.9 mg) was purified form EA-4-8 by preparative HPLC (MeOH–H2O, 70:30), as well.

Xylopidimer A (1)

Colorless prismatic crystal; [α]D20 −23.2 (c 0.11 MeOH); IR (KBr): νmax 3444, 2967, 1773, 1696, 1449, 1382, 1337, 1204, 1044, 859, 597 cm–1; for 1H NMR and 13C NMR spectroscopic data, see Table S1; HRESIMS (positive): m/z 483.2519 [M + Na]+ (calcd for 483.2506).

Xylopidimer B (2)

Colorless prismatic crystal; [α]D20 −19.2 (c 0.10 MeOH); IR (KBr): νmax 3437, 2964, 1776, 1697, 1672, 1609, 1454, 1265, 1038, 864, 627 cm–1; for 1H NMR and 13C NMR spectroscopic data, see Table S2; HRESIMS (positive): m/z 483.2515 [M + Na]+ (calcd for 483.2506).

Xylopidimer C (3)

Colorless prismatic crystal; [α]D20 −43.9 (c 0.12 MeOH); IR (KBr): νmax 3429, 2863, 1693, 1400, 1262, 1011, 831, 702, 610 cm–1; for 1H NMR and 13C NMR spectroscopic data, see Table S3; HRESIMS (positive): m/z 501.2617 [M + Na]+ (calcd for 501.2611).

Xylopidimer D (4)

Colorless prismatic crystal; [α]D20 −77.3 (c 0.11 MeOH); IR (KBr): νmax 3993, 2853, 1663, 1591, 1378, 1291, 1238, 1153, 974, 898 cm–1; for 1H NMR and 13C NMR spectroscopic data, see Table S4; HRESIMS (positive): m/z 487.2811 [M + Na]+ (calcd for 487.2819).

Xylopidimer E (5)

Colorless prismatic crystal; [α]D20 18.6 (c 0.11 MeOH); IR (KBr): νmax 3377, 2828, 1744, 1669, 1386, 1237, 1165, 1131, 1037, 596 cm–1; for 1H NMR and 13C NMR spectroscopic data, see Table S5; HRESIMS (positive): m/z 561.2829 [M + Na]+ (calcd for 561.2823).

Computational Details

The theoretical calculations on 1–5 were performed using Gaussian 09. The geometries generated based on NMR were conducted by MestReNova 10.0, and the X-ray data were optimized using MM2.

Assay for Inhibition Ability against LPS-Induced NO Production

RAW 264.7 macrophages were incubated in 24-well plates (105 cells/well) together with test compounds and LPS (1 μg/mL) for 24 h. The concentration of the nitrite in the cultured RAW 264.7 macrophage was tested through the microplate reader. The concentration was therefore tested because it was equivalent to the nitrate concentration.