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New Resveratrol Oligomer Derivatives from the Roots of Rheum lhasaense

Authors:
  • Yunnan Nationality Institute

Abstract and Figures

Two new resveratrol trimer derivatives, named rheumlhasol A (1) and rheumlhasol B (2) were isolated from the methanolic extract of roots of Rheum lhasaense A. J. Li et P. K. Hsiao together with four known resveratrol dimer derivatives, including maximol A (3), gnetin C (4), e-viniferin (5), and pallidol (6). The structures were determined by combined spectroscopic methods and by comparison of their spectral data with those reported in the literature. All the compounds isolated from R. lhasaense were tested for their ability to scavenge1,1-diphenyl-2-picrylhydrazyl (DPPH) radical.
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Molecules 2013, 18, 7093-7102; doi:10.3390/molecules18067093
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
New Resveratrol Oligomer Derivatives from the Roots of
Rheum lhasaense
Wen-Bo Liu
1
, Lin Hu
1,2,
*, Qun Hu
2
, Na-Na Chen
1
, Qing-Song Yang
1
and Fang-Fang Wang
1
1
Key Laboratory of Chemistry in Ethnic Medicinal Resources, State Ethnic Affairs Commission &
Ministry of Education, Yunnan University of Nationalities, Kunming 650031, Yunnan, China;
E-Mails: liuwenbokm@126.com (W.-B.L.); chennana1111@126.com (N.-N.C.);
smkms@126.com (Q.-S.Y.); foreverfang98@126.com (F.-F.W.)
2
Kunming Xianghao Technology Co., Ltd., Kunming 650204, Yunnan, China; E-Mail:
huqun871@163.com
* Author to whom correspondence should be addressed; E-Mail: hulin66@163.com or
hulin@ynni.edu.cn; Tel./Fax: +86-871-6521-2813.
Received: 27 May 2013; in revised form: 10 June 2013 / Accepted: 13 June 2013 /
Published: 18 June 2013
Abstract: Two new resveratrol trimer derivatives, named rheumlhasol A (1) and
rheumlhasol B (2) were isolated from the methanolic extract of roots of Rheum lhasaense
A. J. Li et P. K. Hsiao together with four known resveratrol dimer derivatives, including
maximol A (3), gnetin C (4), ε-viniferin (5), and pallidol (6). The structures were
determined by combined spectroscopic methods and by comparison of their spectral data
with those reported in the literature. All the compounds isolated from R. lhasaense were
tested for their ability to scavenge1,1-diphenyl-2-picrylhydrazyl (DPPH) radical.
Keywords: Rheum lhasaense A. J. Li et P. K. Hsiao; resveratrol oligomers; rheumlhasol A;
rheumlhasol B; DPPH radical
1. Introduction
Natural resveratrol oligomers, commonly consisting of two to eight resveratrol units, have
drawn increasing attention across the world due to their intriguing structures and pharmacological
potential [1–3]. Resveratrol oligomers provide dazzling chemical diversities with regard to the degree
and pattern of polymerization, as well as their stereochemistry [4]. Most of them possess antioxidant
OPEN ACCESS
Molecules 2013, 18 7094
activities because they have polyphenol functions in the molecules and are considered to be promising
new sources of natural antioxidants [5,6]. However, resveratrol oligomers have been isolated from a
relatively small assemblage of plant families. Vitaceae, Diterocarpaceae, Gnetaceae and Fabaceae
provide a significant number of oligostilbenes [7–9].
The genus Rheum Linn consists of approximate 60 species and is mainly distributed in sub-alpine
and alpine zones of Asia [10]. The underground part of Rheum spp. is commonly known as Da-Huang
(rhubarb), and is used in traditional medicine for the treatment of constipation, inflammation, cancer,
renal failure, and infectious diseases [11,12].
Rheum lhasaense A. J. Li et P. K. Hsiao is a stout herb primarily confined to the mountainous areas
of eastern Tibet and adjacent regions [13]. The rhizomes and roots of this plant are locally known as
“Qu Zha” and are traditionally used to help soothe the stomach (stomachic).
Previous phytochemical investigation on R. lhasaense mainly focused on the analysis of
anthraquinones, one of the most common and abundant substances in the roots of Rheum plants [14].
Surprisingly, R. lhasaense is very different from other species because of the absence of anthraquinones.
No biological study on this special rhubarb has been conducted. A preliminary 1,1-diphenyl-2-
picrylhydrazyl (DPPH) radical scavenging assay conducted by us demonstrated strong antioxidant
activities in the methanolic extract of R. lhasaense. Therefore, the present study was carried out to
investigate the bioactive constituents present in the medicinally important part of R. lhasaense plant.
Herein, we report the isolation and identification of two new resveratrol trimer derivatives named as
rheumlhasol A (1) and rheumlhasol B (2) from the roots of R. lhasaense, together with four known
resveratrol dimer derivatives, including maximol A (3), gnetin C (4),
ε
-viniferin (5), and pallidol (6).
The antioxidant activities of all the isolated compounds were evaluated by the DPPH free
radical-scavenging assay.
2. Results and Discussion
2.1. Structural Elucidation of the New Compounds
The isolated compounds were identified by different spectroscopic analyses, including the extensive
use of 1D (
1
H and
13
C) and 2D NMR techniques (HH COSY, HMBC, HMQC, and NOESY), and by
comparing the experimental NMR data to the values reported in literature. The structures of isolated
compounds are shown in Figure 1.
Rheumlhasol A (1) was isolated as a yellow amorphous powder. The molecular formula, deduced to
be C
42
H
32
O
9
by negative HR-ESI-MS ([MH]
at m/z 679.1964, calcd for C
42
H
31
O
9
), fitted well for a
resveratrol trimer. The
1
H-NMR spectra of 1 (Table 1) showed two sets of A
2
B
2
-type signals [δ
H
ppm
7.12 (d, J = 8.5 Hz, 2H)
,
and 6.77 (d, J = 8.5 Hz, 2H); 7.16 (d, J = 8.5 Hz, 2H), and 6.75 (d, J = 8.5 Hz,
2H)] and two sets of AX
2
-type signals [δ
H
ppm 6.10 (d, J = 2.0 Hz, 2H), and 6.14 (t, J = 2.0 Hz, 1H);
6.10 (d, J = 2.0 Hz, 2H), and 6.17 (t, J = 2.0 Hz, 1H)] that were assigned to two p-substituted phenyl
moieties (A1 and C1 rings) and two 1,3,5-trisubstituted aromatic rings (A2 and C2 rings), respectively,
characteristic of the two resveratrol structural units. The presence of two sets of mutually coupled
methine hydrogen signals [each set containing two deshielded oxymethine signals: δ
H
ppm 5.28 (d,
J = 5.4 Hz, 1H), and 4.31 (d, J = 5.4 Hz, 1H); 5.36 (d, J = 8.3 Hz, 1H), and 4.36 (d, J = 8.3 Hz, 1H)]
Molecules 2013, 18 7095
instead of the olefinic proton signals suggesting that the olefinic bond got reduced thereby resulting in
trimerisation of these carbons in the two resveratrol structural units. Furthermore, the
1
H-NMR spectra
of 1 displayed two signals [δ
H
ppm 6.40 (d, J = 12.2 Hz, 1H), and 6.46 (d, J = 12.2 Hz, 1H)] that were
assigned to the cis-coupled olefinic protons in the third resveratrol structural unit. Two aromatic rings
(e.g., B1 and B2) of resveratrol structural units took part in the trimerisation process, as was evident
from the
1
H-NMR signals as follows: two singlets for m-hydrogens resonating at δ
H
ppm 6.34 (s, 1H)
and 6.32 (s, 1H) were assigned to a 1, 3, 4, 5-tetrasubstituted ring (B1), and two doublets for protons
resonating at δ
H
ppm 6.74 (d, J = 8.0 Hz, 1H), and 7.21 (d, J = 8.0 Hz, 1H) together with a singlet at δ
H
ppm 6.94 (br s, 1H) were assigned to a 1, 3, 4-trisubstituted ring (B2).
Figure 1. Chemical structures of compounds 16.
O
O
HO
HO
OH
OH
HO
OH
OH
A1
A2
B1
B2
C2
C1
2a
3a
5a
6a
7a
8a
10a
12a
14a
2b
6b
7b
8b
10b
13b
14b
2c
3c
5c
6c
7c
8c 10c
12c
14c
O
HO
O
HO
HO
HO
HO OH
OH
A1
A2
B1
B2
C1
C2
2a
3a
5a
6a
10a
12a
14a
7a
8a
2b
6b
7b
8b
10b
13b
14b
2c
3c
5c
6c
7c
8c
10c
12c
14c
1
2
O
HO
OH
HO
HO
OH
3
O
OH
HO
HO
OH
OH
HO
OH
HO
OH
OH
HO
O
OH
OH
OH
HO
HO
4
5
6
7a and 8a are trans
7a and 8a are trans
Further, with the aid of HH COSY, HMBC, and HMQC NMR techniques, the chemical shift
values were assigned and the structural connections between the three resveratrol units were established.
In the HMBC spectrum of 1 (Figure 2): the correlations of H-8a with C-3b, C-4b, and C-5b; H-8c with
C-10b, C-11b and C-12b; and H-10b with C-8c clearly revealed that unit A is connected to B1 ring
through C-8a/C-4b, and unit C is connected to B2 ring through C-8c/C-11b. Furthermore, the presence
of two dihydrofuran rings (e.g., 7a-8a-4b-5b-O and 7c-8c-11b-12b-O) was deduced by calculating the
degrees of unsaturation and confirmed by the correlation of the cross-peaks: H-7a/C-5b and H-7c/C-12b
in the HMBC spectrum.
Molecules 2013, 18 7096
Table 1.
1
H,
13
C-NMR and ROSEY (500M Hz) data of 1 and 2 (CD
3
OD, δ in ppm).
Positions
1
2
δ
H
(mult, J in Hz, I) δ
C
ROSEY δ
H
(mult, J in Hz, I) δ
C
ROSEY
1a 134.2 134.3
2a(6a) 7.12 (d, 8.5, 2H) 128.1 7a, 8a 7.13 (d, 8.5, 2H) 128.1 7a, 8a
3a(5a) 6.77 (d, 8.5, 2H) 116.3 6.76 (d, 8.5, 2H) 116.3
4a 158.5 158.4
7a 5.28 (d, 5.4, 1H) 94.5 5.30 (d, 5.4, 1H) 94.5
8a 4.31 (d, 5.4, 1H) 56.7 4.33 (d, 5.4, 1H) 56.6
9a 146.5 146.6
10a(14a) 6.10 (d, 2.0 Hz, 2H) 107.1 7a, 8a 6.13 (d, 1.7 ,2H) 107.0 7a, 8a
11a(13a) 159.6 159.6
12a 6.14 (t, 2.0 Hz, 1H) 102.0 6.16 (t, 1.7, 1H) 102.0
1b 141.2 141.5
2b 6.34 (s, 1H) 110.1 6.50 (s, 1H) 108.0
3b 155.5 155.6
4b 115.1 115.4
5b 163.0 163.3
6b 6.32 (s, 1H) 102.5 6.60 (s, 1H) 99.6
7b 6.40 (d, 12.2, 1H) 129.8 8b 6.85 (d, 16.2, 1H) 127.4
8b 6.46 (d, 12.2, 1H) 131.0 7b 7.02 (d, 16.2, 1H) 129.3
9b 131.7 132.3
10b 6.94 (br s, 1H) 127.4 7.20 (br s, 1H) 124.1
11b 131.7 132.4
12b 160.4 160.9
13b 6.74 (d, 8.0, 1H) 109.8 6.84 (d, 8.3, 1H) 110.3
14b 7.21 (d, 8.0, 1H) 130.6 7.37 (d, 8.3, 1H) 128.7
1c 132.9 132.8
2c(6c) 7.16 (d, 8.5, 2H) 128.7 7c, 8c 7.17 (d, 8.5, 2H) 128.7 7c, 8c
3c(5c) 6.75 (d, 8.5, 2H) 116.3 6.79 (d, 8.5, 2H) 116.3
4c 158.7 158.6
7c 5.36 (d, 8.3, 1H) 94.7 8c 5.39 (d, 8.4, 1H) 94.9 8c
8c 4.36 (d, 8.3, 1H) 58.7 7c, 10b 4.40 (d, 8.4, 1H) 58.7 7c, 10b
9c 145.4 145.3
10c(14c) 6.10 (d, 2.0, 2H) 107.7 7c, 8c 6.15 (d, 1.7, 2H) 107.9 7c, 8c
11c(13c) 159.8 159.8
12c 6.17 (t, 2.0, 1H) 102.5 6.22 (t, 1.7, 1H) 102.5
Figure 2. Main HMBC (indicated by blue arrows from
1
H to
13
C) and H-H COSY
correlations (indicated by bold lines) of compounds 1 and 2.
O
O
HO
HO
OH
OH
HO
OH
OH
A1
A2
B1
B2
C2
C1
1
O
HO
O
HO
HO
HO
HO OH
OH
A1
A2
B1
B2
C1
C2
2
Molecules 2013, 18 7097
The relative stereochemistry of the two dihydrofuran rings was assigned by the ROESY correlations
(Table 1, Figure 3). Significant NOE interactions between H-7a/H-10a(14a) protons on A2 benzene
ring and H-8a/H-2a(6a) protons on A1 benzene ring suggested that H-7a/H-8a protons are situated in a
trans-orientation, which was confirmed by comparing the value of coupling constant (e.g., J = 5.4 Hz)
to that of related resveratrol oligomers reported in literature [15–17]. Significant NOE interactions
between H-7c/H-8c protons suggested that H-7c/H-8c protons are situated in a cis-orientation.
However, no NOE interactions between either H-7c/H-7a(8a) or H-8c/H-7a(8a) protons were observed
in the ROESY experiment due to their remote distance, and therefore the complete relative
stereochemistry of 1 could not be assigned.
Figure 3. Main ROSEY (indicated by red arrows) of compound 1.
O
O
OH
OH
OH
OH
OH
OH
OH
A
1
A
2
B
1
B
2
C
2
C
1
2a
6a
7a
8a
10a
14a
6b
10b
13b
2c
6c
7c
8c
10c
14c
7a and 8a are trans
The molecular formula C
42
H
32
O
9
for rheumlhasol B (2) was deduced from negative HR-ESI-MS
([MH]
at m/z 679.1943, calcd for C
42
H
31
O
9
). The
1
H-NMR and
13
C-NMR data (Table 1) of 2 are
very similar to those of 1, except for the appearance of two new signals [δ
H
ppm 6.85 (d, J = 16.2 Hz,
1H), and 7.02 (d, J = 16.2 Hz, 1H)] with relatively low-field chemical shifts and large coupling
constants owing to trans-olefinic coupling (instead of cis-olefinic coupling). Thus, rheumlhasol B (2)
was characterized as the (E)-geometrical isomer of rheumlhasol A (1) and from these results, the
structure of 2 was determined as shown in Figure 1. The remaining known compounds 36 were
identified by comparison of their spectroscopic data with literature data.
2.2. Antioxidant Activities by DPPH Scavenging Capacities
The resveratrol oligomers 16 isolated from R. lhasaense were screened for their antioxidant
activities by DPPH free radical-scavenging assay that has been widely used for the evaluation of
antioxidant activities of natural products. The results obtained in this study are summarized in Table 2.
Among these compounds, 2 and 3 exhibited relatively high antioxidant activities with IC
50
values of
28.7 and 31.3 µM, respectively, which was comparable to that of α-tocopherol; while 1, 4, and 5
showed moderate activities with IC
50
values in the range of 49.7 to 69.8 µM. Compound 6 showed
lowest antioxidant activity with IC
50
values of 190.2 µM.
Molecules 2013, 18 7098
Table 2. Antioxidant Activities of the Compounds 16.
Compds. DPPH radical IC
50
(μM)
a
1 49.7 ± 2.3
2 31.3 ± 1.5
3 28.7 ± 1.0
4 69.8 ± 2.3
5 52.6 ± 1.1
6 190.2 ± 3.8
Vitamin E 27.9 ± 0.9
a
IC
50
values were expressed as means ±standard deviation.
3. Experimental
3.1. General
The
1
H-,
13
C-, and 2D NMR spectra were recorded on Bruker DRX-500 (500 MHz) spectrometer
with TMS as internal standard. The ESI-MS (negative ion mode) and HR-ESI-MS (negative ion mode)
spectra were recorded on VG AutoSpe 3000 and API Qstar P ulsar LC/TOF spectrometers,
respectively. The UV spectra were measured by using a Shimadzu double-beam 210A spectrophotometer.
The IR spectra were recorded on a Bio-Rad FTS-135 spectrometer, in KBr pellets. The optical
rotations were measured by using a SEPA-3000 automatic digital polarimeter. The column
chromatographic separations were performed on silica gel (200–300 mesh size; Qingdao Marine
Chemical Inc., Qingdao, China), or Lichroprep RP-18 gel (40–63 µm mesh size; Merck, Darmstadt,
Germany). The column fractions obtained were monitored by TLC, and spots were visualized by
heating the silica gel plates after spraying with 15% H
2
SO
4
in water. The TLC and PTLC separations
were performed on silica gel Gf 254 pre-coated plates (Qingdao Marine Chemical Inc.).
3.2. Plant Materials
R. lhasaense A. J. Li et P. K. Hsiao plant materials were collected in August 2010 from LhaSa,
Tibet Autonomous Region, China, and authenticated by Professor Anjen Li of Institute of Botany,
Chinese Academy of Sciences. A voucher of the specimen (No. 2004080203) collected was deposited
at School of Chemistry & Biotechnology, Yunnan University of Nationalities.
3.3. Extraction and Isolation of the Compounds
The air-dried powder roots (1 kg) of R. lhasaense A. J. Li et P. K. Hsiao were extracted
exhaustively with 95% aqueous EtOH (5 × 10 L) at room temperature . The EtOH extract was
concentrated in vacuo to yield a brown residue (200 g), which was suspended in water (200 mL), and
extracted with EtOAc (3 × 200 mL). The combined organic phase was concentrated to yield a residue
(89 g), which was loaded on a silica gel (SiO
2
) column (2 kg) and eluted with petroleum ether
(PE)/acetone gradient to give five fractions (1–5). Fraction 3 eluted with PE/acetone (2:1) was
subjected to repeated column chromatography (CC) (SiO
2
; CHCl
3
/MeOH, 10:1) to afford 3 (25 mg).
Fraction 4 eluted with PE/acetone (1:2) was subjected to repeated CC (SiO
2
; CHCl
3
/MeOH, 10:1–8:2),
Molecules 2013, 18 7099
followed by PTLC (CHCl
3
/MeOH, 9:1) to afford 4 (27 mg) and 5 (15 mg). Fraction 5 eluted with
acetone was subjected to repeated CC (SiO
2
; CHCl
3
/MeOH, 10:1–5:1) to afford 6 (30 mg) and a
sub-fraction containing 1 and 2. This sub-fraction was subjected to repeated CC on RP
18
gel eluted by
MeOH/water (58:42) to afford 1 (10 mg) and 2 (12 mg).
3.4. Spectroscopic Data
Rheumlhasol A (1): white powder; [α]
D
= +10.2262
ο
(c = 0.0056, MeOH); IR (KBr)
ν
max
3419, 1603,
1515, 1486, 1449, 1339, 1301, 1233, 1155, 998, and 831 cm
1
; UV (MeOH)
λ
max
(log
ε
) 202 (4.9), 227
(3.3), 285 (2.4), and 300 (2.2) nm; negative ESI-MS [MH]
at m/z 679; negative HR-ESI-MS
[MH]
at m/z 679.1964 (calcd for C
42
H
31
O
9
679.1968);
1
H and
13
C-NMR data (Table 1).
Rheumlhasol B (2): white powder; [α]
D
= +5.4321
ο
(c = 0.0054, MeOH); IR (KBr)
ν
max
3396, 1600,
1516, 1486, 1450, 1341, 1303, 1235, 1155, 999, 960, and 832; UV (MeOH)
λ
max
(log
ε
) 202 (4.9), 225
(3.3), 310 (2.3), and 335 (2.4) nm; negative ion ESI-MS [MH]
at m/z 679; negative ion HR-ESI-MS
m/z 679.1943 (calcd for C
42
H
31
O
9
679.1968);
1
H and
13
C-NMR spectra (Table 1).
Maximol A (3): brown amorphous powder, positive ESI-MS [M]
+
at m/z 454;
1
H-NMR (500 MHz,
acetone-d
6
) δ ppm 7.43 (d, J = 8.3 Hz, 1H, H-6), 7.24 (overlapping signals, 3H, H-2, H-6, and H-2),
7.06 (d, J = 16.3 Hz, 1H, H-7), 6.90 (d, J = 16.3 Hz, 1H, H-8), 6.85 (d, J = 8.3 Hz, 1H, H-5), 6.78
(overlapping signals, 3H, H-3, H-5, and H-8), 6.53 (d, J = 1.7 Hz, 2H, H-10 and H-14), 6.28 (t,
J = 1.7 Hz, 1H, H-12), 6.25 (t, J = 1.7 Hz, 1H, H-12), 6.19 (d, J = 1.7 Hz, 2H, H-10 and H-14), 5.45
(d, J = 8.0 Hz, 1H, H-7), and 4.46 (d, J = 8.0 Hz, 1H, H-8);
13
C-NMR (125 MHz, acetone-d
6
) δ ppm
160.54 (C-4), 159.72 (C-11 and C-13), 159.52 (C-11 and C-13), 158.40 (C-4), 145.15 (C-9), 140.70
(C-9), 132.44 (C-1), 132.14 (C-3), 131.68 (C-1), 129.04 (C-7), 128.56 (C-2 and C-6), 128.56 (C-5),
127.15 (C-8), 123.86 (C-2), 116.12 (C-3 and C-5), 110.11 (C-5), 107.37 (C-10 and C-14), 105.64
(C-10 and C-14), 102.68 (C-12), 102.34 (C-12), 94.00 (C-7), and 57.77 (C-8). These data are consistent
with those reported in literature [18].
Gnetin C (4): yellow powder; negative ESI [MH]
at m/z 453;
1
H-NMR (500 MHz, CD
3
OD) δ ppm
7.35 (d, J = 8.4 Hz, 2H, H-2 and H-6), 7.16 (d, J = 8.4 Hz, 2H, H-2 and H-6), 7.01 (d, J = 16.2 Hz,
1H, H-7), 6.89 (d, J = 16.2 Hz, 1H, H-8), 6.80 (overlapped signals, 4H, H-3, H-5, H-3, and H-5), 6.67
(br s, 1H, H-14), 6.58 (br s, 1H, H-10), 6.26 (t, J = 1.7 Hz, 1H, H-12), 6.21 (d, J = 1.7 Hz, 2H, H-10
and H-14), 5.35 (d, J = 5.4 Hz, 1H, H-7), 4.41 (d, J = 5.4 Hz, 1H, H-8);
13
C-NMR (125 MHz,
CD
3
OD) δ ppm 163.36 (C-5), 159.54 (C-11 and C-13), 158.27 (C-4), 158.09 (C-14), 155.57 (C-3),
146.71 (C-9), 141.85 (C-1), 134.31 (C-9), 130.63 (C-1), 129.56 (C-7), 129.11 (C-10 and C-14),
128.4 (C-2 and C-6), 127.15 (C-8), 116.6 (C-3 and C-5), 116.51 (C-11 and C-13), 115.42 (C-4),
108.15 (C-2), 107.40 (C-10 and C-14), 102.26 (C-12), 99.94 (C-6), 94.64 (C-7), and 56.67 (C-8).
These data are in accordance with those reported in literature [19].
ε
-Viniferin (5): pale white powder; positive ESI [M+H]
+
at m/z 455;
1
H-NMR (400 MHz, CD
3
OD) δ
ppm 7.14 (d, J = 8.4 Hz, 2H, H-2 and H-6), 7.04 (d, J = 8.4 Hz, 2H, H-2 and H-6), 6.82 (d, J = 16.3 Hz,
1H, H-7), 6.77 (d, J = 8.4 Hz, 2H, H-3 and H-5), 6.65 (d, J = 8.4 Hz, 2H, H-3 and H-5), 6.63 (d,
Molecules 2013, 18 7100
J = 1.6 Hz, 1H, H-14), 6.57 (d, J = 16.3 Hz, 1H, H-8), 6.25 (d, J = 1.6 Hz, 1H, H-12), 6.18 (t, J = 1.7 Hz,
1H, H-12), 6.16 (d, J = 1.7 Hz, 2H, H-10 and H-14), 5.36 (d, J = 6.6 Hz, 1H), and 4.34 (d, J = 6.7 Hz,
1H);
13
C-NMR (125 MHz, CD
3
OD) δ ppm 162.73 (C-3), 160.05 (C-11 and C-13), 159.75 (C-5),
158.54 (C-12), 158.40 (C-4), 147.35 (C-9), 136.90 (C-1), 133.87 (C-1), 130.36 (C-9), 130.31 (C-7),
128.77 (C-2 and C-6), 128.21 (C-10 and C-13), 123.66 (C-8), 120.04 (C-2), 116.36 (C-3 and C-5),
116.29 (C-11 and C-13), 107.44 (C-10 and C-14), 104.30 (C-6), 102.18 (C-12), 96.83 (C-4), 94.83
(C-7), and 58.30 (C-8). These data are in accordance with those reported in literature [20,21].
Pallidol (6): white powder; positive ESI-MS [M+H]
+
at m/z 455;
1
H-NMR (500 MHz, CD
3
COCD
3
) δ
ppm 6.97 (d, J = 8.5 Hz, 4H, H-2, H-6, H-2, and H-6), 6.69 (d, J = 8.5 Hz, 4H, H-3, H-5, H-3 and
H-5), 6.61 (d, J = 2.0 Hz, 2H, H-10 and H-10), 6.18 (d, J = 2.0 Hz, 2H, H-12 and H-12), 4.55 (s, 2H,
H-7 and H-7), and 3.80 (s, 2H, H-8 and H-8);
13
C-NMR (125 MHz, CD
3
COCD
3
) δ ppm 159.30 (C-11
and C-11), 156.30 (C-4 and C-4), 155.28 (C-13 and C-13), 150.24 (C-9 and C-9), 137.69 (C-1 and
C-1), 128.98 (C-2, C-6, C-2, and C-6), 123.18 (C-14 and C-14), 115.75 (C-3, C-5, C-3, and C-5),
103.26 (C-10 and C-10), 102.43 (C-12 and C-12), 60.45 (C-8 and C-8), and 53.89 (C-7 and C-7).
These data are in accordance with those reported in literature [22].
3.5. DPPH Assays
Sample stock solutions (1 mM) were diluted to concentrations of 25, 50, 100, 150, 200, and 250 µM
in methanol. One milliliter of DPPH methanol solution (500 µM, final concentration = 125 µM)
was added to 3.0 mL of a MeOH solution of various sample concentrations. The mixtures were shaken
vigorously and then kept in the dark at room temperature. After 30 min, the absorbance values
were measured at 518 nm and converted into the percentage inhibition of DPPH (Ip) by using the
following formula:
Ip = [ (Abs
sample
Abs
control
)/Abs
control
] × 100
A mixture of DPPH solution (1.0 mL, 160 µM) and methanol (3.0 mL) was used as negative control
while dl-α-tocopherol solution was used as positive control. The IC
50
values obtained represent the
concentrations of the tested samples and standards that caused 50% inhibition of DPPH, and were
calculated by linear regression of plots where the abscissa represent the concentration of tested
compounds and the ordinate represent the average percentage of inhibition from three separate tests.
The experiments were done in triplicate, and the results are given as mean ± standard deviation (SD).
4. Conclusions
Two new isomeric resveratrol trimers named rheumlhasol A (1) and rheumlhasol B (2) were
isolated from the roots of Rheum lhasaense A. J. Li et P. K. Hsiao, together with four known dimers
36. Apparently, compound 2 is derived from the coupling of gnetin C (4) with another resveratrol
unit. The benzofuran ring connects B unit and C unit may formed from condensation of oxygen radical
(C12-O
) and carbon C-11 of B2 ring with C-7c and carbon radical (C-8c) of C unit. This is the first
time that Rheum plants have been reported to contain resveratrol trimers. In addition, the free radical
scavenging activities of all the isolated compounds against DPPH radical have been evaluated in this
Molecules 2013, 18 7101
study. Compounds 15 showed moderate antioxidant activities, with IC
50
values in the range of 28.7 to
69.8 μM, while compound 6 showed low antioxidant activity with an IC
50
value of 190.2 μM.
Acknowledgments
The financial support from Yunnan Natural Science Foundation (2012FB172) and Kunming
Science and Technology Project (11H010401) is gratefully acknowledged.
Conflict of Interest
The authors declare no conflict of interest.
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distributed under the terms and conditions of the Creative Commons Attribution license
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Five new stilbene glucosides, gnemonosides F, G, H, I, and J were isolated from the stem lianas of Gnetum gnemonoides BRONGN and Gnetum africanum WELW along with nine known stilbenoids. The structures of the new compds. were elucidated as gnetin E 4a,4b,4c-O-β-triglucopyranoside (2), gnetin E 4a,4c-O-β-diglucopyranoside (3), gnetin C 4a,4b,11a-O-β-triglucopyranoside (4), gnetin D 4a,4b-O-β-diglucopyranoside (5), and gnetuhainin A 4a,4b-O-β-diglucopyranoside (6) on the basis of spectroscopic evidence. [on SciFinder(R)]
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A review of the distribution of resveratrol oligomers in plants is presented and their biosyntheses are collated. A revised structure for gnetin I on biogenetic grounds is proposed.
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Gnetum leyboldii and G. schwackeanum (Gnetaceae) are lianas of Amazonia. The former contains in its wood the gnetins A,B,C,D and E for which the structures of rel-(1R,5R,6R,7R)-6-(4-hydroxyphenyl)-7-(3,5-dihydroxyphenyl)-2-(E) - (4-hydroxystyryl) - 4,8-dioxobicyclo | 3.2.1 | oct-2-ene; 1-hydroxy-5-(4-hydroxyphenyl)-6-(3,5-dihydroxyphenyl)-3-(E)-(4-hydroxystyryl)-7- oxobicyclo |2.2.2| oct-2-ene; rel-(2S, 3S)-4-hydroxy-2-(4-hydroxyphenyl)-3- (3,5-dihydroxyphenyl)-6-(E)-(4-hydroxystyryl)-2,3-dihydrobenzofuran; rel-(2S,3S)-4-hydroxy-2-(2,4-dihydroxyphenyl)-3-(3,5-dihydroxyphenyl)-6-(E)-(4- hydroxystyryl)-2,3-dihydrobenzofuran; and rel-(2S,3S)-4-hydroxy-2-(4-hydroxyphenyl)-3-[6-rel-(2S,3S)-4-hydroxy-2-(4- hydroxyphenyl)-3-(3,5-dihydroxyphenyl)-2,3-dihydrobenzofuranyl]-6-(E)-(4- hydroxystyryl)-2,3-dihydrobenzofuran were proposed, respectively. The fruits of G. schwackeanum contain the gnetins C and E.
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Stilbenoids are formed by a particular branch of the flavonoid biosynthetic pathway. They are of special interest to natural product researchers for their roles in plant resistance to fungal pathogens and their biological effects. This review in the volume of Bioactive Natural Products provides a comprehensive account of the occurrence, chemistry, biological roles and activities of the stilbenoids. Nearly 800 stilbenoids, isolated from natural sources in the recent 12 years, are grouped into structural types and discussed in terms of their reported pharmacological activity. The major groups of stilbenoids which are discussed in detail include stilbenes, bibenzyls, bisbibenzyls, phenanthrenoids, stilbene oligomers etc. Detailed tables and figures list the occurrence of stilbenoids in major plant species, and methods used for extracting and analyzing stilbenoids are discussed. Biosynthetic pathways and chemical synthesis are reviewed and the biological activities of stilbenoids are also addressed. The coverage of the new structures is from 1994 to 2006.