Structurally diverse stilbene dimers from Gnetum montanum Markgr.: studies on the ¹H chemical shift differences between dimeric stilbene epimers correlating to the relative configurations

Abstract

Twenty-seven structurally diverse stilbenoids, comprising nine new dimeric stilbenoids, gnemontanins A (1), B (2), C (4), D (5), E (7), F (8), and G (12), as well as (−)-gnetuhainin P (3) and (−)-gnetuhainin I (6), were isolated from the caulis of Gnetum montanum Markgr. The structures of those new compounds were elucidated by mean of extensive analysis of MS, 1D and 2D NMR spectroscopic data. Naturally occurring stilbene dimers polymerized through one bond of 8-O-4′ (1 and 2) as well as two bonds of 7-8′ and 6-7′ (5) are reported for the first time. The ¹H chemical shift differences between dimeric stilbene epimers on C-7 were analysed and summarized, which could be used as a diagnostic to determine the relative configuration of C-7 of several types' dimeric stilbenes. Two misassigned structures of gnetuhainins D and E were revised as gnetuhainin S and 12, respectively.

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Introduction

Stilbenoids are a class of plant polyphenols characteristic of monomeric or polymeric 1,2-diarylethylene, which have attracted intense interest for their intricate structures and diverse biological activities.^1–4 Gnetum montanum Markgr. (Gnetaceae) is an evergreen vine indigenous to southern China and Southeast Asia.⁵ The leaves, stems and rhizomes of this plant contain abundant stilbenes and stilbene-related alkaloids, and are used as folk medicine for the treatment of lumbago, rheumatic arthralgia and chronic bronchitis.^5–8 In our continuing search for bioactive and diverse ingredients from medicinal plants,^9–11 twenty-seven stilbenoids, including 24 stilbene dimers polymerized via diverse modes, such as one bond of 8-O-4′ (1 and 2) or 8-8′ (3), two bonds forming an indane (4–11, 22) or a benzofuran (15–21), and three bonds forming an indano[1,2-c]-chromene (12 and 13), a dibenzobicyclo[3.2.1]octane (14), an indano[2,1-a]indane (23), or a benzo[6,7]cyclohepta[1,2,3-cd]benzofuran (24), were isolated from the caulis of the title plant. Among them, gnemontanins A (1), B (2), C (4), D (5), E (7), F (8), and G (12), as well as (−)-gnetuhainin P (3) and (−)-gnetuhainin I (6) (Chart 1) were structurally elucidated as new compounds by extensive analysis of MS, 1D and 2D NMR spectroscopic data. The ¹H chemical shift differences between dimeric stilbene epimers on C-7 were analysed and summarized. Herein, the isolation, identification, and structural characterization of the new dimeric stilbenoids, as well as the key ¹H NMR shifts correlating to the configuration of C-7 of several types' dimeric stilbenes are described.

Chart 1 Structures of compounds 1–14.

Results and discussion

8-O-4′ Stilbene dimers

Gnemontanin A (1) was obtained as a pale amorphous powder. Its molecular formula of C₃₀H₂₈O₉ was determined by HRESIMS ion [M + Na]⁺ at m/z 555.1632 (calcd 555.1631). Its ¹H and ¹³C NMR spectra (Tables 1 and 2), with the aid of ¹H–¹H COSY and HSQC spectra, showed 30 carbon and 22 carbon-bearing hydrogen signals for two 1,2,4-trisubstituted benzene rings (ring A₁: δ_H 6.69, 6.66 and 6.66, each br s, correlated with δ_C 112.3, 115.5 and 121.2, respectively, in the HSQC spectrum; ring B₁: δ_H 6.73, 6.87 and 7.15, ABX model, J = 8.4, 2.0 Hz), two symmetric 1,3,5-trisubstituted benzene rings (ring A₂: δ_H 6.11 and 6.09, A₂X system, J = 2.1 Hz; ring B₂: δ_H 6.45 and 6.16, A₂X system, J = 2.1 Hz), an E-double bond (δ_H 6.92 and 6.83, each d, J = 16.2 Hz), two vicinal oxygenated methines (δ_H 4.92 and 4.84, each d, J = 7.6 Hz; δ_C 79.1 and 88.0), and two methoxyls (δ_H 3.97 and 3.72). The observed four aromatic rings and one double bond combined with 17 unsaturated degrees in the molecule demonstrated 1 to be a single bond connective stilbene dimer. The HMBC interactions established the jointed mode of 8-O-4′, and the two methoxyls on C-3 and C-3′, respectively, based on observed cross-peaks of H-10(14)/C-8, H-8/C-4′, as well as 3-OMe/C-3, and 3′-OMe/C-3′ (Fig. 1). The relative configuration of 7,8-threo for 1 was determined by the NOESY correlations of H-7/H-10(14), H-7/H-5′, and H-8/H-2(6) (Fig. 1). Accordingly, the structure of gnemontanin A (1) was elucidated as threo-7,8-dihydroxydihydroisorhapontigenin(8-O-4′)isorhapontigenin.

Table 1 ¹H NMR data of 1–7 (400 MHz, δ in ppm, mult, J in Hz)

No.	1^a	2^a	3^a	4^b	5^b	6^b	7^b
a Recorded in methanol-d4.b Recorded in acetone-d6.
2	6.69 (br s)	6.95 (d, 1.7)	6.85 (d, 1.7)	6.85 (br s)	7.00 (br s)	6.55 (d, 1.6)	6.93 (d, 1.7)
5	6.66 (br s)	6.68 (d, 8.1)	6.63 (d, 8.2)	6.66 (d, 8.2)	6.55 (br s)	6.67 (d, 8.0)	6.68 (d, 8.0)
6	6.66 (br s)	6.71 (dd, 8.1, 1.7)	6.74 (dd, 8.2, 1.7)	6.75 (dd, 8.2, 2.0)	—	6.51 (dd, 8.0, 1.6)	6.64 (dd, 8.0, 1.7)
7	4.84 (d, 7.6)	4.93 (covered)	5.06 (d, 9.7)	6.95 (br s)	—	4.50 (d, 8.0)	4.56 (d, 7.3)
8	4.92 (d, 7.6)	5.16 (d, 4.5)	3.71 (d, 9.7)	—	7.10 (br s)	3.38 (dd, 8.0, 3.5)	3.48 (dd, 7.3, 4.5)
10	6.11 (d, 2.1)	6.18 (d, 2.2)	6.08 (d, 2.1)	—	6.43 (d, 2.0)	—	—
12	6.09 (t, 2.1)	6.14 (t, 2.2)	6.04 (t, 2.1)	6.30 (d, 2.0)	6.18 (d, 2.0)	6.32 (d, 1.9)	6.22 (d, 1.9)
14	6.11 (d, 2.1)	6.18 (d, 2.2)	6.08 (d, 2.1)	6.76 (d, 2.0)	6.43 (d, 2.0)	6.62 (d, 1.9)	5.98 (br s)
2′	7.15 (d, 2.0)	7.11 (d, 2.0)	6.58 (d, 1.6)	—	6.93 (br s)	6.64 (d, 1.6)	6.61 (d, 1.8)
3′	—	—	—	6.45 (d, 2.0)	—	—	—
5′	6.73 (d, 8.4)	6.68 (d, 8.4)	6.61 (d, 8.2)	6.13 (d, 8.2, 2.2)	6.65 (d, 8.4)	6.72 (d, 8.1)	6.67 (d, 8.1)
6′	6.87 (dd, 8.4, 2.0)	6.85 (dd, 8.4, 2.0)	6.66 (dd, 8.2, 1.6)	6.25 (dd, 8.2)	6.62 (br d, 8.4)	6.46 (dd, 8.1, 1.6)	6.46 (dd, 8.1, 1.8)
7′	6.92 (d, 16.2)	6.91 (d, 16.2)	6.78 (s)	4.55 (br s)	4.06 (br s)	4.23 (d, 2.9)	4.22 (d, 4.2)
8′	6.83 (d, 16.2)	6.81 (d, 16.2)	—	4.07 (br s)	4.00 (br s)	2.96 (dd, 3.5, 2.9)	3.41 (dd, 4.5, 4.2)
10′(14′)	6.45 (d, 2.1)	6.44 (d, 2.2)	6.09 (d, 2.2)	6.44 (d, 2.0)	6.32 (d, 2.0)	5.91 (d, 2.1)	6.13 (d, 2.2)
12′	6.16 (t, 2.1)	6.16 (t, 2.2)	6.25 (t, 2.2)	6.18 (t, 2.0)	6.14 (t, 2.0)	6.12 (t, 2.1)	6.16 (t, 2.2)
3-OMe	3.72 (s)	3.79 (s)	3.72 (s)	3.57 (s)	3.84 (s)	3.64 (s)	3.72 (s)
3′-OMe	3.97 (s)	3.89 (s)	3.48 (s)	3.94 (s)	3.73 (s)	3.72 (s)	3.70 (s)

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Table 2 ¹³C NMR data of 1–7 (100 MHz)

No.	1^a	2^a	3^a	4^b	5^b	6^b	7^b
a Recorded in methanol-d4.b Recorded in acetone-d6.
1	132.6	133.1	136.4	130.3	130.1	136.7	138.6
2	112.3	112.8	111.9	112.2	111.4	111.4	111.9
3	148.3	148.3	147.4	148.0	147.2	148.0	147.8
4	147.0	146.9	146.1	146.4	147.2	146.2	146.4
5	115.5	115.1	114.9	115.5	117.1	115.0	114.7
6	121.2	121.6	120.7	123.4	127.1	120.4	121.4
7	79.1	78.2	75.9	122.9	136.2	77.2	77.2
8	88.0	86.0	64.8	143.2	125.7	61.8	62.1
9	141.7	141.4	143.9	148.0	144.2	148.9	147.3
10	107.3	107.5	108.5	123.2	105.1	122.3	123.1
11	159.3	159.1	158.5	155.9	159.2	155.0	154.9
12	103.0	102.8	101.4	103.4	102.4	102.3	102.2
13	159.3	159.1	158.5	159.6	159.2	158.8	158.6
14	107.3	107.5	108.5	98.4	105.1	106.1	105.5
1′	132.7	132.3	130.4	125.2	138.6	138.1	136.0
2′	110.9	111.2	112.7	158.5	111.9	112.2	111.9
3′	151.1	151.1	147.5	99.5	148.0	147.7	148.0
4′	148.6	148.7	146.1	157.9	145.9	145.5	145.4
5′	117.2	116.5	115.1	107.1	115.5	115.2	115.2
6′	120.9	120.9	123.9	128.1	120.6	120.8	120.9
7′	129.2	129.2	128.1	50.5	53.3	55.9	56.7
8′	128.4	128.1	141.8	59.4	52.3	59.4	59.0
9′	141.0	141.0	145.1	149.0	146.7	150.6	151.0
10′	105.9	105.9	108.8	106.9	107.0	106.0	106.3
11′	159.7	159.6	159.5	159.3	159.3	159.1	159.2
12′	102.9	102.9	101.9	101.2	101.7	101.0	101.0
13′	159.7	159.6	159.5	159.3	159.3	159.1	159.2
14′	105.9	105.9	108.8	106.9	107.0	106.0	106.3
3-OMe	56.3	56.2	56.2	55.5	56.2	56.0	56.2
3′-OMe	56.7	56.9	55.5	56.0	56.4	56.3	56.1

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Fig. 1 Selected HMBC and NOESY correlations of 1 and 3.

Gnemontanin B (2) shared the same molecular formula of C₃₀H₂₈O₉ with 1 based on the HRESIMS and NMR data. Its ¹H and ¹³C NMR spectra (Tables 1 and 2) displayed signals for the same fragments with those of 1, such as four aromatic rings (two 1,2,4-trisubstituted and two 1,3,5-trisubstituted), a trans-double bond, two vicinal methines (³J = 4.5 Hz), and two methoxyls. Analysis of 2D NMR spectra (¹H–¹H COSY, HSQC and HMBC, ESI Fig. S12–S14†) disclosed 2 to have the same gross structure with 1.

According to the possible staggered conformers with intramolecular hydrogen bonding of those vicinal diols,¹² the Newman projections along C₇–C₈ bond of 1 and 2 (Fig. 2) showed large/small J_7,8 for threo-/erythro-form, and the different shielded effect from the neighbour aryls, which interpreted the upfield shifts of H-7 and H-8 of 1 in comparison with those of 2. The above evidence illustrated 2 to be the 7,8-erythro epimer of 1, i.e., erythro-7,8-dihydroxydihydro-isorhapontigenin(8-O-4′)isorhapontigenin.

Fig. 2 Newman view along bond of C₇–C₈ of 1 and 2.

Gnemontanins A (1) and B (2) are the first example of 8-O-4′ jointed stilbene dimers obtained as natural forms. Shi et al. reported 7-O-methyl ethers of 1 and 2 that were synthesized from isorhapontigenin under treated with silver acetate in methanol,¹² which indicated isorhapontigenin to be the probable biogenetic precursor of 1 and 2.

8-8′ Stilbene dimer

Stilbene 3, [α]²⁵_D −8.3 (c 0.06, MeOH), showed the molecular formula of C₃₀H₂₈O₉ as deduced from the HRESIMS ([M − H]⁻ ion at m/z 531.1668, calcd 531.1655). The ¹H NMR spectrum of 3 (Table 1) displayed signals for two sets of ABX aromatic protons (ring A₁: δ_H 6.63, 6.74 and 6.85, J = 8.2, 1.7 Hz; ring B₁: δ_H 6.61, 6.66 and 6.58, J = 8.2, 1.6 Hz), two sets of A₂X aromatic protons (ring A₂: δ_H 6.08 and 6.04, J = 2.1 Hz; ring B₂: δ_H 6.25 and 6.09, J = 2.2 Hz), one olefinic proton (δ_H 6.78, s), two vicinal methines (δ_H 5.06 and 3.71, J = 9.7 Hz), and two methoxyls (δ_H 3.72 and 3.48). Compared the NMR data with those of 1, the important differences were the absence of an olefinic hydrogen and upfield ¹H and ¹³C shifts of one methine (δ_H 3.71 and δ_C 64.8, CH-8) in 3. The above evidence suggested 3 to be an isorhapontigenin dimer jointed with single C–C bond. The HMBC correlations of H-10(14)/C-8, H-8/C-8′, and H-10′(14′)/C-8′ indicated the 8-8′ dimerization (Fig. 1). The relative configuration of 7,8-threo and 7′Z of 3 were determined by the large coupling constant between H-7 and H-8 (J = 9.7 Hz) and NOESY correlations of H-7/H-7′ and H-7/H-10(14), as well as H-2′/H-10′(14′), respectively (Fig. 1). Stilbene 3 had the same relative structure and opposite optical rotation with gnetuhainin P ([α]²⁵_D +6.6), an 8-8′ isorhapontigenin dimer from G. hainanense.¹³ Stilbenoid 3 was hence determined to be (−)-gnetuhainin P.

The 7′Z-configuration of 3 was also in agreement with the upfield shifts of H-7′ and protons of rings B₁ and B₂, due to the broken conjugated system and enhancive shielded effects in contrast with those of E-form analogues such as 1 and 2.

1-Benzylidene-2,3-diaryl-indane-type stilbene dimers

Stilbene 4 was obtained as a brown amorphous powder with [α]²⁵_D +69.3 (c 0.04, MeOH). It had the molecular formula of C₃₀H₂₆O₈ as judged by the HRESIMS [M − H]⁻ ion at m/z 513.1548 (calcd 513.1549) in combination with the ¹³C NMR data (30 carbons and 20 carbon-bearing hydrogens). The ¹H and ¹³C NMR spectra (Tables 1 and 2) showed signals for four aromatic rings (ring A₁: δ_H 6.66, 6.75 and 6.85, ABX coupled; ring A₂: δ_H 6.76 and 6.30, meta-coupled; ring B₁: δ_H 6.25, 6.13 and 6.45, ABX model; ring B₂: δ_H 6.44 and 6.18, A₂X system), one olefinic proton (δ_H 6.95, br s; δ_C 122.9), two aryl-bearing methines (δ_H 4.55 and 4.07, each br s), and two methoxyls (δ_H 3.94 and 3.57). In the HMBC spectrum (Fig. 3), obvious long-range correlations of H-7′ with C-2′, C-11 and C-8, H-8′ with C-7, C-1′, and C-10′, and H-7 with C-2, C-9 and C-8′ were observed, indicating a dimeric stilbene linked through 8-8′ and 10-7′. Two methoxyls were assigned on C-3 (δ 148.0) and C-2′ (δ 158.5) based on the HMBC cross-peaks and ¹³C shifts. The 7E-double bond and trans-orientated H-7′ and H-8′ were established by NOESY cross-peaks of H-7/H-14, H-7′/H-10′(14′) and H-8′/H-6′, respectively (Fig. 3). The relative structure of gnemontanin C (4) was therefore elucidated to be 2′-O-methylgnetuhainin J.¹⁴

Fig. 3 Selected HMBC (left) and NOESY (right) correlations of 4.

Gnemontanin C (4) possesses the same carbon skeleton with that of (−)-ampelopsin D, whose absolute configuration was determined to be 7′S,8′S with negative optical rotation ([α]²²_D −5.0) and positive cotton effects (CEs) at 237 nm and 272 nm, as well as negative CE at 314 nm in the circular dichroism (CD) spectrum.^15,16 Based on the opposite optical rotation ([α]²⁵_D +69.3) and CD signs at 235 nm (−6.55 mdeg) and 314 nm (+6.29 mdeg), the absolute configuration of 4 was elucidated to be 7′R,8′R.

Gnemontanin D (5) isolated as a pink amorphous powder with [α]²⁵_D −3.9 (c 0.05, MeOH), which was an isomer of 4 as judged by HRESIMS ion [M − H]⁻ at m/z 513.1545. Analysis of the ¹H and ¹³C NMR spectra (Tables 1 and 2) discovered the existences of four phenyls (two symmetric 1,3,5-trisubstituted, one 1,2,4,5-tetrasubstituted, and one 1,2,4-trisubstituted), a trisubstituted conjugated double bond (δ_H 7.10, br s), two aryl-bearing methines (δ_H 4.06 and 4.00, each br s), and two methoxyls (δ_H 3.84 and 3.73). Further analysis of HMBC and NOESY spectra (Fig. 4) indicated 5 to be an 8-7′ and 7-6′ connected isorhapontigenin dimer with relative configuration of 7E and 7′,8′-trans. However, the CD spectrum of 5 didn't offer significant information correlating to the absolute configuration. Gnemontanin D (5) is the first instance of 7-8′ and 6-7′ connected stilbene dimer, representing a new polymerized mode.

Fig. 4 Selected HMBC (left) and NOESY (right) correlations of 5.

The 7E-configurations of 4 and 5 could be easily deduced from the distinct downfield shift of the olefinic proton (H-7 or H-8) in contrast with that of 7Z-isomer, e.g., parthenocissin A (δ_H 6.31, br s), due to the broken conjugated system in the latter.¹⁷

1-Hydroxybenzyl-2,3-diaryl-indane-type stilbene dimers

Stilbene 6 was obtained as a brown amorphous powder with [α]²⁵_D −8.5 (c 0.06, MeOH), which had the molecular formula of C₃₀H₂₈O₉ as deduced from its HRESIMS [M − H]⁻ ion at m/z at 531.1658 (calcd 531.1655). Its ¹H and ¹³C NMR spectra (Tables 1 and 2) showed 30 C-atom and 21 carbon-bearing H-atom signals for four aromatic rings (ring A₁: δ_H 6.67, 6.51, and 6.55, ABX, J = 8.0, 1.6 Hz; ring A₂: δ_H 6.32, and 6.62, meta-coupled, J = 1.9 Hz; ring B₁: δ_H 6.72, 6.46 and 6.64, ABX, J = 8.1, 1.6 Hz; ring B₂: δ_H 5.91 and 6.12, A₂X, J = 2.1 Hz), along with four methines whose protons coupled in the order (δ_H 4.50, 3.38, 2.96 and 4.23, J = 8.0, 3.5 and 2.9 Hz), and two methoxyls (δ_H 3.72 and 3.64). The above NMR data of 6 were identical with those of gnetuhainin I ([α]²⁵_D +18.5; C₃₀H₂₈O₉),¹⁸ indicating 6 to be (−)-enantiomer of gnetuhainin I.

Gnemontanin E (7), [α]²⁵_D −8.9 (c 0.05, MeOH), exhibited the same molecular formula of C₃₀H₂₈O₉ with 6 (HRESIMS [M − H]⁻ m/z 531.1658). The ¹H and ¹³C NMR spectra (Tables 1 and 2) displayed signals for the same moieties with those of 6, such as four aromatic rings (ring A₁: δ_H 6.68, 6.64 and 6.93, ABX, J = 8.0, 1.7 Hz; ring A₂: δ_H 6.22, and 5.98, meta-coupled, J = 1.9 Hz; ring B₁: δ_H 6.67, 6.46 and 6.61, ABX, J = 8.1, 1.8 Hz; ring B₂: δ_H 6.13 and 6.16, A₂X, J = 2.2 Hz), four methines arraying in sequence (δ_H 4.56, 3.48, 3.41 and 4.22, J = 7.3, 4.5 and 4.2 Hz), and two methoxyls (δ_H 3.72 and 3.70). The HMBC experiments established the same gross structure with that of 6 as judged by long-range correlations of H-7′/C-9, C-11, C-2′ and C-6′, H-8′/C-7, C-8, C-9 and C-10′(14′), as well as H-8/C-1, C-9 and C-8′ (Fig. 5). In contrast with the ¹H NMR data of 6, 7 had upshifted protons of ring A₂ and downfield shifts of rings A₁ and B₂, and H-8′. The finding indicated rings A₁ and A₂ being cis-oriented (H-14 and H-12 being shielded by ring A₁) in 7 but not trans-form as in 6 (H-2′, H-6′ and H-8′ being shielded by ring A₁, and H-2 and H-6 being shielded by ring B₂) (Fig. 6. More examples see ESI Tables S1 and S2†). This consequence was also confirmed by the NOESY correlations of H-2/H-14, H-7′/H-10′(14′), H-8′/H-2′ and H-6′, as well as H-8/H-10′(14′) (Fig. 5). Accordingly, gnemontanin E (7) was determined to be 7-epimer of (−)-gnetuhainin I.

Fig. 5 Selected HMBC (left) and NOESY (right) correlations of 7.

Fig. 6 Significant shielded effects of 6 and 7.

Gnemontanin F (8) possessed the molecular formula C₃₂H₃₂O₉ as determined by the HRESIMS [M + Na]⁺ ion at m/z 583.1945 (calcd 583.1944). The ¹H and ¹³C NMR data of 8 (Tables 3 and 4) were close similar to those of 7 except for an additional ethoxyl (δ_H 3.24 and 3.04, each 1H, dq, J = 9.3, 7.0 Hz; 0.99, 3H, t, J = 7.0 Hz) presence in the former, suggesting an ethoxylated derivative of 7. The ethoxyl was assigned on C-7 based on the HMBC cross-peaks of EtO (δ_H 3.24 and 3.04) with C-7 (δ_C 85.4), which was in consistence with the downfield shift of C-7 (Δδ +8.2) and upshifted C-8 (Δδ −1.2) in contrast with those of 7 due to the etherification effect and γ-gauche effect, respectively.

Table 3 ¹H NMR data of 8, 12 and 14 in acetone-d₆ (δ, mult, J in Hz)^a

No.	8^b	12	14
a Recorded in 500 MHz (8) or 400 MHz (12 and 14).b The remaining δH: 3.73 (3H, s, 3-OMe), 3.69 (3H, s, 3′-OMe), 3.24 and 3.04 (each 1H, dq, 9.3, 7.0, 7-OCH2CH3), 0.99 (3H, t, 7.0, 7-OCH2CH3).
2	6.85 (d, 1.7)	7.14 (d, 8.6)	7.09 (d, 8.5)
3	—	6.81 (d, 8.6)	6.70 (d, 8.5)
5	6.71 (d, 8.0)	6.81 (d, 8.6)	6.70 (d, 8.5)
6	6.60 (dd, 8.0, 1.7)	7.14 (d, 8.6)	7.09 (d, 8.5)
7	4.07 (d, 7.8)	4.67 (d, 8.7)	4.18 (d, 1.7)
8	3.43 (dd, 7.8, 4.5)	3.53 (dd, 8.7, 7.5)	3.40 (br s)
12	6.22 (d, 2.0)	6.12 (d, 2.1)	6.04 (d, 1.9)
14	5.93 (br s)	5.60 (d, 2.1)	6.59 (d, 1.9)
2′	6.54 (d, 1.8)	—	—
3′	—	6.30 (br s)	6.28 (d, 2.4)
5′	6.68 (d, 8.1)	6.29 (br d, 7.7)	6.03 (dd, 8.4, 2.4)
6′	6.44 (dd, 8.1, 1.8)	6.64 (d, 7.7)	6.57 (d, 8.4)
7′	4.23 (d, 4.0)	3.49 (covered)	3.91 (br s)
8′	3.36 (dd, 4.5, 4.0)	4.10 (d, 7.5)	4.14 (br s)
10′(14′)	6.16 (d, 2.0)	6.26 (br s)	6.44 (d, 2.4)
12′	6.18 (t, 2.0)	6.26 (br s)	6.14 (d, 2.4)

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Table 4 ¹³C NMR data of 8, 12 and 14 in acetone-d₆^a

No.	8^b		12		14
	Unit A	Unit B	Unit A	Unit B	Unit A	Unit B
a Recorded in 125 MHz (8) or 100 MHz (12 and 14).b The remaining δC 56.2 (q, 2C, 3 and 3′-OMe), 64.1 (t, 7-OCH2CH3), 15.5 (q, 7-OCH2CH3).
1	132.9	138.5	131.5	116.5	138.4	121.6
2	112.2	111.8	129.9	156.1	130.0	157.1
3	148.1	148.1	115.6	103.6	115.4	102.8
4	147.0	145.6	158.1	157.5	156.0	157.1
5	114.9	115.3	115.6	109.1	115.4	106.5
6	122.5	121.0	129.9	130.8	130.0	129.1
7	85.4	56.2	78.9	49.9	47.3	45.2
8	60.9	59.4	49.0	57.2	55.7	49.3
9	146.8	150.5	145.8	147.4	148.3	148.6
10	123.1	106.5	122.5	107.5	127.9	113.4
11	155.1	159.4	155.0	159.4	158.3	157.7
12	102.4	101.3	102.7	101.6	101.6	101.5
13	158.7	159.4	158.2	159.4	153.2	156.9
14	106.0	106.5	105.1	107.5	103.7	105.7

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Stilbenes 9–11 were identified to be 7-O-methylgnetuhainin I (9, [α]²⁵_D −8.3), 7-O-methylgnemontanin E (10, [α]²⁵_D −8.1), and 7-O-ethylgnetuhainin I (11, [α]²⁵_D −7.0), respectively, based on their MS and analysis of NMR data (ESI Table S1†), which were previous reported as lehmbachols A–C (9–11) with uncertain C-7 configuration.¹⁹ Notably, the ¹H assignment of δ_H ca. 4.8 ppm for H-10′(14′) and H-12′ of 10 therein was apparently incorrect, which was actually the water residue in methanol-d₄.¹⁹ Two pairs of C-7 epimers, 10/9 and 8/11, showed the similar ¹H chemical shift differences with that 7/6 (ESI Table S1†).

Indeno[1,2-c]chromene-type stilbene dimer

Gnemontanin G (12) was obtained as a yellow amorphous powder. The HRESIMS spectrum indicated the molecular formula of C₂₈H₂₂O₇ ([M + Na]⁺ m/z 493.1267, calcd 493.1263) with 18 unsaturated degrees. The ¹H NMR spectrum (Table 3) exhibited the presence of four methines which coupled in sequence in the ¹H–¹H COSY spectrum (δ_H 4.67, 3.53, 3.49 and 4.10, J = 8.7, 7.5 and 7.5 Hz), and four benzene rings composing of 12 protons (ring A₁: δ_H 7.14 and 6.81, A′ABB′ model, J = 8.6 Hz; ring A₂: δ_H 6.12 and 5.60, meta-coupled, J = 2.1 Hz; ring B₁: δ_H 6.64, 6.29 and 6.30, ABX model, J = 7.7 Hz and br s; ring B₂: δ_H 6.26, 3H), suggesting a three bonds connective stilbene dimer. In the HMBC spectrum (Fig. 7), obvious ³J_CH long range correlations were observed between H-2(6)/C-7, H-14/C-8, H-6′/C-7′, and H-10′(14′)/C-8′, indicating that rings A₁, A₂, B₁ and B₂ were attached on C-7, C-8, C-7′, and C-8′, respectively. The polymerized mode of 7-O-2′, 8-7′ and 10-8′ was established by HMBC correlations of H-7/C-2′ and C-7′, H-8/C-7′ and C-8′, as well as H-8′/C-10 and C-11. The relative configuration was established as rel-7R,8S,7′S,8′R by NOESY correlations of H-7/H-8′, H-7′/H-10′(14′), and H-8′/H-2′ (Fig. 7).

Fig. 7 Selected HMBC (left) and NOESY (right) correlations of 12.

Two stilbene dimers named as gnetuhainins E (12a) and D (13a) had almost superimposed NMR data with those of 12 and gnetuhainin S (13), respectively (Fig. 8).^20,21 It is evidently that the reported structures of 12a and 13a were not exact and should be revised as 12 and 13, respectively, based on following reasons: (i) as above analysis on the ¹H chemical shift differences between 6 and 7, H-14 at δ 5.59/5.47 in 12a/13a indicated the cis- but not trans-oriented rings A₁ and A₂; (ii) the ¹H NMR differences between 13a and macrostachyol C (authors described the structure as 7-O-methyl of 13a)²² are obviously mismatched with the alteration of their structures (ESI Table S3†); (iii) the coupling constant J_7′,8′ = 8.4/10.5 Hz of 12a/13a are far larger than the other analogues (J_7′,8′ < 4.6 Hz, ESI Tables S1–S4†); finally, the mixture of 12a and 13a showed positive HRFABMS ion at m/z 471.1465 as authors' description, which was actually the ion of [M + H]⁺ but not misassigned [M + H − H₂O]⁺.²¹

Fig. 8 The proposed structures of gnetuhainins E (12a) and D (13a).

Diaryl-dibenzobicyclo[3,2,1]octadiene-type stilbene dimer

Stilbene 14, [α]²⁵_D +22.4 (c 0.05, MeOH), was isolated as a pale white amorphous powder. All differences in the ¹H NMR spectra of 14 (C₂₈H₂₂O₇) and ampelopsin F (14b, C₂₈H₂₂O₆) could be ascribed to the additional presence of a hydroxyl at C-2′ of the former.²³ Upon comparison of the spectra of both, significant shifts were observed only for those signals due to H-3′, H-5′, H-6′ and H-7′ (Δδ_H −0.28, −0.53, −0.21 and +0.27, respectively, ESI Table S5†). This inference was firmly confirmed by analysis of 2D NMR spectra (¹H–¹H COSY, HSQC, HMBC and NOESY, ESI Fig. S74–S77†). It is puzzling that 14 possesses opposite optical rotation and similar CD curve with that of (−)-14b (+CEs at 240 nm and 250 nm, and −CEs at 220 nm and 280 nm),²⁴ the absolute configuration of 14 is still uncertain.

Tanaka et al. reported the same structure, named as 2b-hydroxy-ampelopsin F (14a, [α]²⁵_D +12), with 14.²⁵ The ¹H NMR spectrum of 14a showed the marked more upfield H-14 (δ 5.68) and H-3(5) (δ 6.17) than those of 14 and 14b (ESI Table S5†). The similar distinction was also observed between 14b and its C-7 epimer, isoampelopsin F.²³ The latter showed downshifted H-5, H-6 and H-7, as well as upfield shifts of H-2, H-3 and H-14, due to the cis-oriented rings A₁ and A₂ being severely shielded/deshielded from each other, and loss of the shielded effect of H-7 from ring A₂ in the case of 14 (ESI Table S5†). From this viewpoint, 14a is inclined to be an atropisomer of gnetuhainin C (=2′-hydroxyisoampelopsin F)²⁶ rather than 14.

The other known stilbenoids (ESI Chart S1†) were identified as stilbene monomers: resveratrol,⁷ gnetol,⁷ and isorhapontigenin;⁷ dimmers: (−)-ε-viniferin (15, [α]²⁵_D −33.9),⁷ rac-bisisorhapontigenin A (16, [α]²⁵_D 0),⁶ rac-gnetuhainin A (17, [α]²⁵_D 0),²¹ (−)-cis-ε-vineferin (18, [α]²⁵_D −38.6),²⁷ gnetin C (19, [α]²⁵_D −28.3),²⁸ gnetin D (20, [α]²⁵_D −2.0, probable inequality mixture of enantiomers),²⁸ shegansu B (21, [α]²⁵_D +4.6),⁷ (−)-gnetulin (22, [α]²⁵_D −16.7),⁷ gneafricanin F (23, [α]²⁵_D 0),²⁹ and (−)-ampelopsin A (24, [α]²⁵_D −137.3).³⁰

These isolates were evaluated the cytotoxicities against human breast cancer cell line MCF-7 and human pancreatic cell line PANC-1 in vitro by MTT method. However, all stilbenoids were inactive (IC₅₀ > 10 μM) for both cell lines.

Experimental

General experimental procedures

Optical rotations were measured on a Perkin-Elmer 341 polarimeter. UV spectra were recorded on a Varian Cary 50 spectrophotometer and IR on a Thermo Nicolet JS5 spectrophotometer. CD spectra were measured by a JASCO J-810 spectropolarimeter. ESIMS and HRESIMS spectra were obtained via a Bruker Esquire 3000 plus and a Waters Q-TOF-Ultima spectrometer, respectively. NMR data were carried out on a Bruker Advance III 500 or a Varian Mercury Plus, and chemical shifts were referenced to the residual solvent peaks (δ_H 3.31 and δ_C 49.0 for methanol-d₄, δ_H 2.05 and δ_C 29.8 for methyl of acetone-d₆). Silica gel (200–300 mesh, Qingdao Marine Chemical Factory, Qingdao, P. R. China), Sephadex LH-20 (Pharmacia Biotech AB, Uppsala, Sweden), and MCI Gel Chp20p (Mitsubishi Chemical, Tokyo, Japan) were used for column chromatography (CC). Semi-preparative HPLC was run on a Waters HPLC system (Waters) with Waters-2545-HPLC pump, Waters-2489 detector, and Xbridge-C₁₈ or YMC-C₁₈ column (5 μm, i.d. 10 mm × 250 mm).

Plant material

The chopped caulis of G. montanum were purchased from the Herbal Medicine Fair of Hechi in October, 2014 (collected in Hechi city, Guangxi Province, P. R. China), and were identified by Professor Da-Yuan Zhu of Shanghai Institute of Materia Medica. A voucher sample (no. 20141002) was deposited with the Herbarium of Shanghai Institute of Materia Medica, Chinese Academy of Sciences.

Extraction and isolation

The dried caulis of G. montanum (10 kg) were powdered and extracted with 95% EtOH (30 L × 3) at room temperature to afford a crude extract (800 g), which was partitioned successively between petroleum ether (3 × 2.5 L) and EtOAc (3 × 2.5 L) with water. The EtOAc layer (138 g) was separated into fr.1 (92 g) and fr.2 (40 g) through a MCI Gel CHP-20P CC eluted with EtOH–H₂O (40% and 60%, v/v). Fr. 1 was subjected to CC over silica gel using gradient CH₂Cl₂–MeOH (20 : 1–2 : 1, v/v) to afford frs 1A–1E. Fr. 1A (26.0 g) was chromatographed with a Sephadex LH-20 column eluted with MeOH to yield isorhapontigenin (5 g). Resveratrol (200 mg), gnetol (50 mg) and 16 (50 mg) were obtained from fr. 1C (5.5 g) after purification through CC of Sephadex LH-20 (MeOH) and silica gel (CH₂Cl₂–MeOH, 10 : 1). Fr. 2 was divided into frs 2A–2F through CC of MCI gel (EtOH–H₂O, 40–60%, v/v). Fr. 2B (4.0 g) was isolated by silica gel CC (CH₂Cl₂–MeOH, 20 : 1 → 10 : 1) then semi-preparative HPLC to afford 3 (24 mg), 7 (28 mg), 6 (54 mg) and 14 (15 mg). Fr. 2C (17.0 g) was purified via repeatedly CCs of silica gel (CH₂Cl₂–MeOH) and Sephadex LH-20 (MeOH) finally semi-preparative HPLC to furnish 9 (3 mg), 10 (4 mg), 11 (6 mg), 8 (5 mg), 23 (21 mg), 5 (80 mg), 12 (13 mg), 22 (6 mg), 4 (4 mg), 24 (10 mg), 17 (20 mg), 20 (13 mg), and 13 (5 mg). By the same procedures, 1 (10 mg), 2 (10 mg), 15 (7 mg), 21 (26 mg), 19 (13 mg), and 18 (3 mg) were isolated from fr. 2D (7.0 g).

Characteristic data of compounds

Gnemontanin A (1), pale amorphous powders, [α]²⁵_D −3.7 (c 0.04, MeOH); UV (MeOH) λ_max (log ε) 204 (4.21), 324 (3.56) nm; CD (MeOH) λ_max (Δε) 204 (−20.8), 207 (+9.2), 210 (−19.0), 215 (−2.6), 217 (−8.4), 222 (+2.5), 238 (−3.1), 299 (+2.9) nm. IR (KBr) ν_max 3351, 2918, 2850, 2360, 2342, 1598, 1512 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z 555 ([M + Na]⁺); HRESIMS: m/z 555.1632 ([M + Na]⁺, calcd for C₃₀H₂₈O₉Na, 555.1631).

Gnemontanin B (2), pale amorphous powders, [α]²⁵_D −5.4 (c 0.06, MeOH); UV (MeOH) λ_max (log ε) 204 (4.23), 324 (3.59) nm; CD (MeOH) λ_max (Δε) 203 (+17.8), 207 (−17.4), 210 (+12.6), 214 (−3.9), 217 (+1.3), 220 (−3.3), 242 (+2.3) nm. IR (KBr) ν_max 3374, 1601, 1512, 1263, 1155 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z 531 ([M − H]⁻); HRESIMS: m/z 555.1621 ([M + Na]⁺, calcd for C₃₀H₂₈O₉Na, 555.1631).

(−)-Gnetuhainin P (3), brown amorphous powders, [α]²⁵_D −8.3 (c 0.06, MeOH); UV (MeOH) λ_max (log ε) 275 (4.81) nm; IR (KBr) ν_max 3420, 1606, 1515, 1452, 1276, 1156 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z 531 ([M − H]⁻); HRESIMS: m/z 531.1668 ([M − H]⁻, calcd for C₃₀H₂₇O₉, 531.1655).

Gnemontanin C (4), brown amorphous powders, [α]²⁵_D +69.3 (c 0.04, MeOH); UV (MeOH) λ_max (log ε) 204 nm (4.82), 330 (4.19); CD (MeOH) λ_max (Δε) 205 (−17.8), 218 (+5.3), 235 (−6.6), 285 (+6.9), 314 (+6.3) nm. IR (KBr) ν_max 3405, 1600, 1514, 1284, 1158 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z 513 ([M − H]⁻); HRESIMS: m/z 513.1548 ([M − H]⁻, calcd for C₃₀H₂₅O₈, 513.1549).

Gnemontanin D (5), pink amorphous powder, [α]²⁵_D −12.0 (c 0.05, MeOH); UV (MeOH) λ_max (log ε) 205 nm (5.28), 220 nm (5.03), 330 (4.65). IR (KBr) ν_max 3448, 1618, 1512, 1157 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z 513 ([M − H]⁻); HRESIMS: m/z 513.1545 ([M − H]⁻, calcd for C₃₀H₂₅O₈, 513.1549).

(−)-Gnetuhainin I (6), brown amorphous powders, [α]²⁵_D −8.5 (c 0.06, MeOH); UV: (MeOH) λ_max (log ε) 204 (4.38), 282 (3.25) nm; CD (MeOH) λ_max (Δε) 209 (+2.7), 212 (−31.8), 215 (+22.1), 220 (−4.7) nm; IR (KBr) ν_max 3348, 1603, 1514, 1464, 1432, 1343, 1274, 1149, 1125, 1031 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z [M − H]⁻ 531 ([M − H]⁻), 1063 ([2M − H]⁻); HRESIMS: m/z 531.1668 ([M − H]⁻, calcd for C₃₀H₂₇O₉, 531.1655).

Gnemontanin E (7), white amorphous powders, [α]²⁵_D −8.9 (c 0.05, MeOH); UV (MeOH) λ_max (log ε) 204 (4.38), 283 (3.25) nm; CD (MeOH) λ_max (Δε) 209 (−25.3), 212 (+11.2), 215 (−11.6), 220 (+1.7) nm; IR (KBr) ν_max 3347, 2920, 2851, 1596, 1512, 1498, 1459, 1273 cm⁻¹; ¹H and ¹³C NMR (Tables 1 and 2); ESIMS: m/z 531 ([M − H]⁻); HRESIMS: m/z 531.1658 ([M − H]⁻, calcd for C₃₀H₂₇O₉, 531.1655).

Gnemontanin F (8), brown amorphous powders, [α]²⁵_D −8.1 (c 0.04, MeOH); UV (MeOH) λ_max (log ε) 204 (4.43), 282 (3.30) nm; CD (MeOH) λ_max (Δε) 209 (−3.9), 210 (+4.5), 212 (−5.7), 215 (−6.3), 220 (−3.6) nm. IR (KBr) ν_max 3364, 2920, 2850, 1956, 1498, 1461 cm⁻¹; ¹H and ¹³C NMR (Tables 3 and 4); ESIMS: m/z 559 ([M − H]⁻); HRESIMS: m/z 583.1945 ([M + Na]⁺, calcd for C₃₂H₃₂O₉Na, 583.1944).

Gnemontanin G (12), yellow amorphous powders, [α]²⁵_D −19.6 (c 0.05, MeOH); UV (MeOH) λ_max (log ε) 205 (4.46), 279 (3.32) nm; CD (MeOH) λ_max (Δε) 207 (−5.7), 210 (+1.5), 215 (−37.7), 217 (2.9), 220 (−19.4), 232 (−27.7), 281 (−2.7) nm. IR (KBr) ν_max 3352, 2920, 2850, 1598, 1461, 1158 cm⁻¹; ¹H and ¹³C NMR (Tables 3 and 4); ESIMS: m/z 469 ([M − H]⁻); HRESIMS: m/z 493.1267 ([M + Na]⁺, calcd for C₂₈H₂₂O₇Na, 493.1263).

2′-Hydroxyampelopsin F (14), pale white amorphous powders, [α]²⁵_D +22.4 (c 0.05, MeOH); UV (MeOH) λ_max (log ε) 205 (4.36), 281 (3.25) nm; CD (MeOH) λ_max (Δε) 208 (+10.5), 211 (+11.1), 220 (−43.1), 240 (+17.9), 250 (+17.5), 281 (−4.1) nm; IR (KBr) ν_max 3347, 2921, 2850, 2360, 1602, 1509, 1458 cm⁻¹; ¹H and ¹³C NMR (Tables 3 and 4); ESIMS: m/z 469 ([M − H]⁻); HRESIMS: m/z 493.1268 ([M + Na]⁺, calcd for C₂₈H₂₂O₇Na, 493.1263).

Conclusions

Gnetum montanum contains abundant stilbenes with diverse structure types. Natural occurring dimeric stilbenes polymerized through one bond of 8-O-4′ as well as two bonds of 7-8′ and 6-7′ are reported for the first time, which enriched the structural diversity of stilbenoids. The ¹H chemical shift differences between dimeric stilbene epimers on C-7 were analysed and summarized, which could be used as a diagnostic to determine the relative configuration of C-7. By mean of it, the misassigned NMR data (10), the uncertain configurations (9–11), and the erroneous structures (12a and 13a) of previous reported dimeric stilbenoids were resolved.

Acknowledgements

This study was financially supported by National Natural Science Foundation of China (21202182).

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Footnote

† Electronic supplementary information (ESI) available: Detailed analysis of shielded effect for cis-/trans-indane-based stilbene dimers, 1D and 2D NMR, IR and MS spectra of compounds 1–8, 12 and 14. See DOI: 10.1039/c6ra08238f

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