Chemical constituents and absolute configuration of megastigmanes’ isolated from Sedum sarmentosum Bunge

Article abstract of DOI:10.1080/14786419.2020.1834549

Phytochemical investigation of a methanolic extract of Sedum sarmentosum collected from Vietnam resulted in the isolation of a new megastigmane glucoside, named sedumoside K (1), together with 17 previously reported compounds (2–18). Structural elucidation of the new compound was achieved by HRFABMS, NMR spectroscopic analysis, acid hydrolysis and quantum ECD calculations. The absolute configuration of compounds 2–6 has been revised. The major isolates were tested for cytotoxic activity against HeLa human cervical cancer cells, and all showed moderate activities.

Full text of DOI:10.1080/14786419.2020.1834549

Natural Product Research
Formerly Natural Product Letters
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gnpl20
Chemical constituents and absolute configuration
of megastigmanes’ isolated from Sedum
sarmentosum Bunge
Dinh Xuan Doan , Sijia Sun , Ashraf M. Omar , Dong Thuong Nguyen , Anh Le
Tuan Hoang , Hironori Fujiwara , Kinzo Matsumoto , Hang Thi Nguyet Pham
& Suresh Awale
To cite this article: Dinh Xuan Doan , Sijia Sun , Ashraf M. Omar , Dong Thuong Nguyen , Anh
Le Tuan Hoang , Hironori Fujiwara , Kinzo Matsumoto , Hang Thi Nguyet Pham & Suresh Awale
(2020): Chemical constituents and absolute configuration of megastigmanes’ isolated from Sedum
sarmentosum Bunge, Natural Product Research, DOI: 10.1080/14786419.2020.1834549
To link to this article: https://doi.org/10.1080/14786419.2020.1834549
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NATURAL PRODUCT RESEARCH
https://doi.org/10.1080/14786419.2020.1834549
Chemical constituents and absolute configuration of
megastigmanes’ isolated from Sedum sarmentosum Bunge
Dinh Xuan Doana, Sijia Sunb, Ashraf M. Omarb, Dong Thuong Nguyenc,
Anh Le Tuan Hoangd, Hironori Fujiwarab, Kinzo Matsumotob,
Hang Thi Nguyet Phamc and Suresh Awaleb
b
aMilitary Institute of Traditional Medicine, Hanoi, Vietnam; Institute of Natural Medicine, University
c
of Toyama, Sugitani, Toyama, Japan; Department of Pharmacology and Biochemistry, National
d
Institute of Medicinal Materials, Hanoi, Hanoi, Vietnam; Mien Trung Institute for Scientific Research,
Vietnam Academe of Science and Technology, Hue, Vietnam
ABSTRACT
ARTICLE HISTORY
Received 26 July 2020
Accepted 22 September 2020
Phytochemical investigation of a methanolic extract of Sedum sar-
mentosum collected from Vietnam resulted in the isolation of a
new megastigmane glucoside, named sedumoside K (1), together
with 17 previously reported compounds (2–18). Structural elucida-
tion of the new compound was achieved by HRFABMS, NMR
spectroscopic analysis, acid hydrolysis and quantum ECD calcula-
tions. The absolute configuration of compounds 2–6 has been
revised. The major isolates were tested for cytotoxic activity
against HeLa human cervical cancer cells, and all showed moder-
ate activities.
KEYWORDS
Megastigmane; Sedum
sarmentosum; ECD
calculation; absolute
configuration
1. Introduction
Sedum sarmentosum Bunge (Crassulaceae) is a herbaceous plant traditionally used to
treat burns, hepatitis, dysentery, snake bites, scabies, pimples and sore throat (Vv,
CONTACT S. Awale
Sugitani, Toyama 930-0194, Japan; H. T. N. Pham
suresh@inm.u-toyama.ac.jp
Institute of Natural Medicine, University of Toyama, 2630
nguyethangpt@nimm.org.vn Department of Pharmacology
and Biochemistry, National Institute of Medicinal Materials, Hanoi, Hanoi 100000, Vietnam
Supplemental data for this article can be accessed at https://doi.org/10.1080/14786419.2020.1834549.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
D. X. DOAN ET AL.
2012). Phytochemical investigation of S. sarmentosum resulted in the discovery of
many megastigmanes, flavonoids, alkaloids, and terpenoids with a wide range of bio-
logical activities (Bai et al., 2016; Cho et al., 2017; He et al., 1998; Ma et al., 2017;
Morikawa et al., 2012; Muraoka et al., 2009; Zhang et al., 2007). Chemical composition
of Vietnamese S. sarmentosum has been never investigated. In the present study,
phytochemical investigation S. sarmentosum, collected from Vietnam, was carried out
for the first time, which led the isolation of a new megastigmane-type sesquiterpene
(1), together with 17 known compounds. The isolated compounds can be classified as
six megastigmanes, five flavonoids, six phenolics and a lignan. The previously reported
compounds were identified as sedumoside C (2) (Yoshikawa et al., 2007), myrsiniono-
side A (3) (Otsuka et al., 2001), simplicifloranoside (4) (Yean et al., 2014), sedumoside I
(5) (Yoshikawa et al., 2007), sarmentol A (6) (Yoshikawa et al., 2007), luteolin (7) (Lin
et al., 2015), isorhamnetin-3,7-O-di-b-D-glucoside (8) (Mskhiladze et al., 2016), quecer-
tin-3-O-b-D-glucopyranose (9) (Zhang et al., 2014), 30-methoxyluteolin-7-O-b-D-gluco-
pyranoside (10) (Harput et al., 2006), 30-methoxy-3,5,40-trihydroxyflavone-7-
neohesperidoside (11) (Umehara et al., 1988), 2-phenylethyl-D-rutinoside (12)
(Umehara et al., 1988), ferulic acid (13) (Van et al., 2004), trans-p-coumaric acid (14)
(Ren et al., 2013), p-hydroxybenzoic acid (15) (Chen et al., 2010), 4-hydroxyphenylethyl
Figure 1. Structures of compounds 1–18 isolated from S. sarmentosum.

NATURAL PRODUCT RESEARCH
3
alcohol (16) (Capasso et al., 1992), 3,4-dimethoxybenzyl alcohol (17) (Kanho et al.,
2005) and lariciresinol-9-O-b-D-glucopyranoside (18) (Cho et al., 2014) (Figure 1).
Herein, we report the isolation and structural elucidation of the new compound 1 and
the absolute configuration revision of compounds 2–6.
2. Results and discussion
Sedumoside K (1) was isolated as a pale-yellow oil. Its molecular formula was
deduced by HRMS as C19H34O8 [m/z 425.1940 (M þ Cl)ꢀ]. The IR spectrum of 1
showed absorption bands of hydroxy (3449 cmꢀ1) and carbonyl (1652 cmꢀ1) groups.
1
The H NMR spectrum of 1 displayed the presence of signals corresponding to three
methyl groups [dH 0.81 (s, H-11), 1.12 (s, H-12) and 1.14 (d, J ¼ 6.5 Hz, H-13)], four
methylenes [dH 2.43 (d, J ¼ 13.0 Hz, H-2a), 2.25 (m, H-4a), 2.20 (m, H-4b), 2.01 (dd,
J ¼ 13.0, 2.0 Hz, H-2b), 1.80 (m, H-7a), 1.79 (m, H-8a), 1.69 (m, H-8b) and 1.25 (m, H-
1
7b),] in addition to two methines [dH 1.83 (m, H-5) and 1.20 (m, H-6)]. Moreover, H
NMR spectrum showed the presence of two oxymethylene groups [dH 3.59 (dd,
J ¼ 12.0, 6.0 Hz, 10b), 3.71 (m, H-10a and H-6’b) and 3.91 (dd, J ¼ 12.0, 2.0 Hz, H-6’a)]
and five oxymethines [dH 3.79 (m, H-9), 3.62 (m, , H-20), 3.42 (m, H-30) 3.35 (m, H-40)
1
and 3.33 (m, H-50)]. H NMR spectrum of 1 also showed the presence of an anomeric
proton [dH 4.48 (d, J ¼ 7.5 Hz, H-10’)] which is characteristic for one sugar unit. The
13C NMR spectrum of 1 exhibited signals of 19 carbons corresponding to three meth-
yls (dC 21.1, 21.5, 30.3), four methylenes (dC 26.0, 34.9, 50.9, 57.1), two methines (dC
37.6, 53.7), two oxymethylene groups (dC 62.9, 64.8), five oxymethines (dC 71.7, 75.5,
77.9, 78.0, 82.3), an anomeric carbon (dC 103.9), a quaternary carbon (dC 40.4) and a
carbonyl group (dC 214.6). Part of these data suggested the presence of sarmentol C
(dC 21.1, 21.5, 26.0, 30.3, 34.9, 37.6, 40.4, 50.9, 53.7, 57.1, 64.8, 82.3, 214.6) which was
confirmed from the COSY, HMQC and HMBC spectra of 1 (Figure S1, Supporting
1
Information) (Yoshikawa et al., 2007). Remaining H and 13C NMR signals suggested
the presence of a b-glucose unit (dC 103.9, 75.5, 78.0, 71.7, 77.9, 62.9). The HMBC cor-
relation from H-10 to C-9 confirmed the location of the b-glucose at C-9 of sarmentol
C unit (Figure S1, Supporting Information). Thus, the planar structure of 1 was
deduced to be 6-(3-b-D-glucopyranosyl-4-hydroxy-n-butyl)-3,3,5-trimethylcyclohexa-
none. The relative configuration of 1 was assigned by analyzing the coupling con-
stants and NOESY data (Figure S2, Supporting Information). The large coupling
constant (J1 ,2 ¼ 7.5 Hz) suggested the trans-disposition of H-10 (a) and H-20 (b). The
presence of NOESY correlations H-10/H-30 and H-10/H-50 suggested their a-axial orien-
tation, while correlations between H-20 and H-40 suggested their b-axial orientation
(Figure S2, Supporting Information). For the present study, conformational analysis at
MM2 (Cho et al., 1989), followed by geometry optimization using DFT at CAM-
0
0
ꢁ
B3LYP/6-31G level (Dewar and O’Connor 1987, Yanai et al., 2004), which gave the
most stable conformer as depicted in figure S2. Theoretical ECD calculation on the
optimized geometry was carried out at the same level of theory. The calculated ECD
data for 5R,6S,9R and 5R,6S,9S gave negative cotton effects at 280 nm. Since the
observed ECD is positive, the new compound should possess 5S,6R absolute config-
uration (Figure S3, Supporting Information). Furthermore, the rotatory strength of

4
D. X. DOAN ET AL.
the cotton effect was found to be in close agreement with the experimental data of
5S,6R,9R. In order to confirm absolute configuration of the sugar unit, we further car-
ried out acid hydrolysis of 1, which yielded a sugar with positive optical rotation
value of [a]22 þ13.2 (c 0.2, H2O), similar to the value reported for D-glucose [a]25
D
D
þ52 (c 0.02, H2O) (Kim et al., 2015). Therefore, the sugar unit in 1 was confirmed as
D-glucose. The aglycone 1a, obtained from acid hydrolysis of 1, showed specific
optical rotation, [a]22 ꢀ11.3 (c 0.22, MeOH), opposite to that reported for sarmentol
D
C, [a]23 þ11.9 (c 0.22, MeOH), having a 5R,6S configuration (Yoshikawa et al., 2007).
D
Furthermore, specific optical rotation calculation of the optimized geometry of an
aglycone (1a) was carried out and compared with the experimental data. The sign of
calculated specific optical rotation (ꢀ38.76) was found to be in close agreement with
experimental data (Figure S4, Supporting Information). This data further suggested
that the aglycone 1a should have a 5S,6R configuration. In addition, computational
ECD calculation for 1a was carried out. The experimental ECD spectrum of 1a was
found to match closely with the theoretical ECD spectrum calculated for the 5S,6R,9R
configuration, confirming the absolute configuration of 1a as 5S,6R,9R (Figure S4,
Supporting Information). All these evidences unequivocally conclude the 5S,6R,9R
absolute configuration of compound 1.
Since the new compound 1 possessed 5S,6R configuration, which was found to be
opposite to those previously reported for 2–6, we further studied their chiroptical
properties. Compounds 2–6 displayed negative optical rotations and were comparable
with those reported in the literature (Otsuka et al., 2001; Yean et al., 2014; Yoshikawa
et al., 2007). Furthermore, compounds 1–5 displayed positive cotton effects at
280 ꢀ 290 nm (Figure S5, Supporting Information), suggesting that the absolute config-
uration of C-5 and C-6 in 2–6 should be the same as 1. Therefore, reported absolute
configuration of 2–5 should be re-assigned as 5S,6R (Figure 1).
The major isolates (1–6, 8, 10, 13–16) were tested for their cytotoxic activity
against HeLa human cervical cancer cells. All the compounds showed moderate cyto-
toxic activity (Figure S14, Supporting Information).
3. Experimental
3.1. General experimental procedures
Optical rotations, UV, ECD, and IR spectra were recorded on A JASCO P-2100 digital
polarimeter, a Hitachi U-5100 UV ꢀ Visible ratio-beam spectrophotometer, a JASCO J-
805 spectropolarimeter and a JASCO FT/IR-460 Plus spectrophotometer, respectively.
NMR spectra were recorded using a Bruker 500 MHz spectrometer, using CDCl3 as solv-
ent and TMS as internal standard, and chemical shifts are expressed in d values. An
Agilent 6550 iFunnel Q-TOF LC/MS system was used for HRESIMS measurement.
Column chromatography was performed using adsorbents silica gel (70–230 mesh) or
reverse phase C-18 (RP-18) resins (150 mm). Thin layer chromatography (TLC) was car-
ried out using pre-coated silica gel 60 F254 and RP-18 F254S plates.

NATURAL PRODUCT RESEARCH
5
3.2. Plant material
Whole plant parts of S. sarmentosum Bunge were collected at Sa Pa, Lao Cai province,
Vietnam in June 2015, and voucher specimens (DL-300615) have been deposited in
Herbarium of Military Institute of Traditional Medicine and Herbarium of National
Institute of Medicinal Materials.
3.3. Extraction and isolation procedures
Air dried Sedum sarmentosum Bunge (9 kg) was ground to a fine powder then
extracted with methanol (3 ꢂ 15 L) to give a methanol extract (400 g). The extract
(400 g) was suspended in hot water and partitioned successively with n-hexane,
CH2Cl2 and EtOAc to obtain n-hexane (162 g), dichloromethane (41 g), EtOAc (14 g)
and H2O (155 g) soluble fractions. The EtOAc fraction (14 g) was subjected to silica gel
column chromatography using a CH2Cl2–MeOH (20:1) mixture to obtained 5 fractions
(fr. 1 to 5). Fraction 1 (0.4 g) was further chromatographed separated on a RP-silica gel
using an acetone–water (2:1) mixture to give 3,4-dimethoxybenzyl alcohol (17, 3.0 mg).
Fraction 2 (3.4 g) was chromatographed on RP-silica gel using MeOH–water (1:2) mix-
ture to afford 3 subfractions, (fr. 2-1 to 2-3). Subfraction 2-1 (500 mg) was separated
using silica gel column eluted with a CH2Cl2–acetone–water (5:1:0.1) mixture to give p-
hydroxybenzoic acid (15, 20 mg) and 4-hydroxyphenylethyl alcohol (16, 30 mg).
Subfraction 2-2 (200 mg) was further purified on a Shephadex LH-20 column, eluted
with MeOH–water (1:1) to yield ferulic acid (13, 60 mg). Subfraction 2-3 (250 mg) was
purified using silica gel column eluted with a CH2Cl2–MeOH (20:1) mixture to give
trans-p-coumaric acid (14, 20.0 mg). Fraction 3 (2.9 g) was subjected to RP-silic gel col-
umn chromatography column using MeOH–water (1:2) mixture to yield 4 fractions (fr.
3-1 to 3-4). Subfraction 3-1 (300 mg) was further purified on a Shephadex LH-20 col-
umn, eluted with MeOH–water (1:1) mixture to yield luteolin (7, 10 mg). Subfraction 3-
2 (900 mg) was separated using silica gel column eluted with CH2Cl2–acetone–water
(5:1:0.1) to give sarmentol A (6, 15 mg) and myrsinionoside A (3, 50 mg). Subfraction 3-
3 (150 mg) was further separated on a Shephadex LH-20 column, eluting with
MeOH–water (1:1) mixture to yield simplicifloranoside (4, 20 mg). Subfraction 3-4
(200 mg) was further purified on a Sephadex LH-20 column, eluting with MeOH–water
(1:1) mixture to yield sedumoside I (5, 15 mg). Fraction 4 (3.7 mg) was further purified
using RP-silica gel column chromatography using a MeOH–water (1:2) mixture to yield
2 fractions. Subfraction 4-1 (900 mg) was purified using silica gel column chromatog-
raphy eluted with a CH2Cl2–MeOH–water (6:1:0.05) mixture to give 30-methoxyluteolin-
7-O-b-D-glucopyranoside (10, 12 mg). The H2O soluble fraction (155 g) was further
purified on a Sephadex LH-20 column eluted with MeOH–water (25:75) mixture to
obtained 4 fractions, (fr. 1 to 4). Fraction 3 (13.7 g) was subjected to silica gel column
chromatography using a CH2Cl2–MeOH (20:1) mixture to afford 5 subfractions, (fr. 3-1
to 3-5). Subfraction 3-1 (2.1 g) was further purified using a RP-silica gel column chro-
matography eluted with a MeOH–water (1:2) mixture to afford quecertin-3-O-b-D-glu-
copyranose (9, 5 mg). Subfraction 3-4 (1.5 g) was separated on a RP-18 column eluted
with MeOH–water (1:2.5) to afford 5 subfractions, (fr.3-4-1 to 3-4-5). Subfractions 3-4-1
(200 mg) and 3-4-3 (250 mg) were subjected to a silica gel column chromatography

6
D. X. DOAN ET AL.
using a CH2Cl2–acetone-water (1:2:0.1) mixture to afford sedumoside K (1, 41 mg),
sedumoside C (2, 10 mg), isorhamnetin-3,7-O-di-b-D-glucoside (8, 12 mg), 30-methoxy-
3,5,40-trihydroxyflavone-7-neohesperidoside (11, 15 mg), 2-phenylethyl-D-rutinoside
(12, 15 mg) and lariciresinol-9-O-b-D-glucopyranoside (18, 4 mg).
Sedumoside K (1): pale-yellow oil, [a]23 þ26.6 (c 0.03, MeOH); UV (MeOH) kmax (log e)
D
208 (3.0) nm; IR (KBr) ꢀmax 3417, 2924, 2855, 2365, 1654, 1438, 1410, 1317, 1022, 954,
1
709,674 cmꢀ1; H NMR (400 MHz, CD3OD): dH 4.48 (1H, d, J¼ 7.5 Hz, H-10), 3.91 (1H, dd,
J¼ 12.0, 2.0 Hz, H-6’a), 3.79 (1H, m, H-9), 3.71 (2H, m, H-10a, H-6’b), 3.62, (1H, m, H-20), 3.59,
(dd, J ¼ 13.0, 6.0 Hz, H-10b), 3.42 (1H, m, H-30), 3.35 (1H, m, H-40), 3.33 (1H, m, H-50), 2.43
(1H, d, J¼ 13.0 Hz, H-2a), 2.25 (1H, m, H-4a), 2.20 (1H, m, H-4b), 2.01 (1H, m, H-2b), 1.83 (1H,
m, H-5), 1.80 (1H, m, H-7a), 1.79 (1H, m, H-8a), 1.69 (1H, m, H-8b), 1.25 (1H, m, H-7b), 1.20
(1H, m, H-6), 1.14 (1H, d, J¼ 6.5Hz, H-13), 1.12 (1H, s, H-12), 0.81 (1H, s, H-11). 13C NMR
(100 MHz, CD3OD): 214.6 (C-3), 103.9 (C-10), 82.3 (C-9), 78.0 (C-30), 77.9 (C-50), 75.5 (C-20), 71.7
(C-40), 64.8 (C-10), 62.9 (C-60), 57.1 (C-2), 53.7 (C-6), 50.9 (C-4), 40.4 (C-1), 37.6 (C-5), 34.9 (C-8),
30.3 (C-12), 26.0 (C-7), 21.5 (C-13), 21.1 (C-11). HRMS m/z 425.1940 [M þ Cl]ꢀ (calcd for
C19H34O8, 425.1942).
3.4. Cytotoxicity assay
The cytotoxicity assay against HeLa cancer cell line was carried out as described previ-
ously (Lombe et al., 2018).
3.5. Computational ECD calculations
The ECD calculations were performed as previously reported (Omar et al., 2019; Omar
et al. 2020). The conformational search was performed on Spartan’16 (Wave function,
Inc., Irvine, CA, U.S.A.) by using MM2 molecular force field (Cho et al., 1989). All pos-
sible conformers with Boltzmann distribution > 1% were reoptimized using DFT at the
ꢁ
CAM-B3LYP/6-31G level in the gas phase (Dewar and O’Connor 1987, Yanai et al.,
2004). The theoretical ECD calculations on these optimized geometries were carried
out at the same level of theory in PCM solvation model for EtOH Using Gaussian 09
(Frisch et al. 2009). The output files were summed to obtain Boltzmann weighed spec-
tra and compared with the experimental spectra by using SpecDis v. 1.71.2 (Bruhn
et al., 2017) with respect to the experimental data employing the default r (0.16 eV)
and without applying any UV correction.
Conclusions
In conclusion, phytochemical investigation of Sedum sarmentosum resulted in the isola-
tion of a new megastigmane glucoside (1) together with 17 previously reported com-
pounds. The absolute configuration of 1 was deduced through computational ECD
calculation and acid hydrolysis. The absolute configuration of compounds 2–6 was
revised. The major isolates were tested for cytotoxic activity against HeLa human cer-
vical cancer cells, and all showed moderate activities.

NATURAL PRODUCT RESEARCH
7
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the Kobayashi International Scholarship Foundation to S.A.
ORCID
Suresh Awale
http://orcid.org/0000-0002-5299-193X
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