Chemical constituents of the Vietnamese plants Dalbergia tonkinensis Prain and Cratoxylum formosum (Jack) Dyer in Hook and their DPPH radical scavenging activities

Article abstract of DOI:10.1007/s00044-019-02383-9

Phytochemical investigations of the leaves and roots of Dalbergia tonkinensis led to the isolation of a new isoflavone glycoside derivative, isocaviunin 7-O-β-D-apiofuranosyl-(1 → 6)-β-D-glucopyranoside (1), and a new scalemic sesquiterpene lactone, 3,7-d

Full text of DOI:10.1007/s00044-019-02383-9

MEDICINAL
CHEMISTRY
RESEARCH
Medicinal Chemistry Research
https://doi.org/10.1007/s00044-019-02383-9
ORIGINAL RESEARCH
Chemical constituents of the Vietnamese plants Dalbergia
tonkinensis Prain and Cratoxylum formosum (Jack) Dyer in Hook and
their DPPH radical scavenging activities
Ninh The Son1,2 Mari Kamiji1 Tran Thu Huong2 Miwa Kubo1 Nguyen Manh Cuong2 Yoshiyasu Fukuyama1
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Received: 29 April 2019 / Accepted: 7 June 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
Phytochemical investigations of the leaves and roots of Dalbergia tonkinensis led to the isolation of a new isoflavone
glycoside derivative, isocaviunin 7-O-β-D-apiofuranosyl-(1 → 6)-β-D-glucopyranoside (1), and a new scalemic sesqui-
terpene lactone, 3,7-dimethyl-3-vinylhexahydro-6,7-bifuran-3(2 H)-one (2), along with the previously known compounds 3-
16, and nine other known compounds 17-25 were isolated from the leaves of Cratoxylum formosum. The chemical structures
of the isolated compounds were elucidated by 1D- and 2D-NMR analyses as well as MS spectroscopic data. The results
suggest that flavonoids are characteristic of both plants. In the DPPH radical scavenging assay, (3 R)-vestitol (5) and
isoquercetin (24) possessed the strongest antioxidative IC50 values of 42.20 µg/mL and 45.63 µg/mL, respectively, and their
values were comparable to that of the positive control catechin (IC50 42.98 µg/mL).
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Keywords Dalbergia tonkinensis Cratoxylum formosum leaves roots DPPH radical scavenging activity
Introduction
inhibition (Nguyen et al. 2018), but to date, the phyto-
chemical studies on this plant have been quite limited. On
the other hand, Cratoxylum formosum (Jack) Dyer in Hook
(other names include Mampat or Cratoxylum formosum
subsp. formosum species) belongs to the family Clusiaceae
(Guttiferae) and is widely distributed in countries in
southeastern Asia; the plant C. formosum subsp. pruni-
florum (Kurz) Gogelein is a subspecies of Cratoxylum for-
mosum (Jack) Dyer in Hook (Gogelein 1967). To the best of
our knowledge, there has been only one report on its phy-
tochemical constituents, as well as on its intracellular anti-
oxidant and anti-inflammatory activities until now (Choi
et al. 2012).
The rare plant D. tonkinensis, locally called “Sua do”, is an
endemic and perennial species native to Vietnam. In terms
of its conservation status, this plant is classified as vulner-
able on the Red List of Threatened Species of the Interna-
tional Union for Conservation of Nature and Natural
Resources (IUCN); thus, its exploitation, shipping and
storage is prohibited (Cuong et al. 2017; Son et al.
2017, 2018a, 2018b). The preliminary screenings suggested
that this species possessed a wide spectrum of biological
activities such as anti-bacterial activity and alpha-glucoside
In this paper, we report the isolation and structural elu-
cidation of new compounds 1-2 as well as the DPPH radical
scavenging activities of the isolated compounds.
Supplementary information The online version of this article (https://
doi.org/10.1007/s00044-019-02383-9) contains supplementary
material, which is available to authorized users.
* Yoshiyasu Fukuyama
Experimental
fukuyama@ph.bunri-u.ac.jp
General experimental procedures
1
Faculty of Pharmaceutical Sciences, Tokushima Bunri University,
Tokushima 770-8514, Japan
1H-NMR (500 MHz) and 13C-NMR (125 MHz) were mea-
sured using a Bruker 500 MHz. HR-FAB-MS/FAB-MS
were obtained from a JEOL MStation JMS-700. IR, UV,
2
Institute of Natural Products Chemistry, Vietnam Academy of
Science and Technology (VAST), 18 Hoang Quoc Viet,
Caugiay, Hanoi, Vietnam

Medicinal Chemistry Research
and optical rotation spectra were recorded by a JASCO FT-
IR 410 infrared spectrophotometer, JASCO V-650 spec-
trophotometer, and JASCO P-2300 polarimeter, respec-
tively. Column chromatography was carried out on silica
gel (Si 60 F254, 40–63 mesh, Merck). Precoated TLC plates
(Si60 F254) were used for analytical purposes. The com-
pounds were visualized under UV radiation (254 and
365 nm) and by spraying plates with 10% H2SO4 followed
by heating with a heat gun.
The root CH2Cl2 fr. (45.5 g) underwent chromatography
on a normal silica gel column [CH2Cl2–CH3COCH3 (3:1, v/
v)] to afford 6 frs. D1-D6. Compounds 3 (15.0 mg) and 4
(5.5 mg) were separated from fr. D1 (2.1 g) by silica gel
column chromatography [n-hexane–CH3COCH3 (3:1, v/v)].
The fr. D2 (3.2 g) then underwent chromatography on a
Sephadex LH-20 column [MeOH–H2O (1:1, v/v)] to yield
compounds 5 (3.1 mg), 9 (4.2 mg), 10 (2.1 mg) and 11
(3.99 mg). Compounds 2 (1.1 mg), 15 (2.0 mg) and 16
(2.5 mg) were purified from fr. D3 (2.7 g) by silica gel
column chromatography [CHCl3–MeOH (3:1, v/v)] and
HPLC [Cosmosil 5C18-AR-II column (10 × 250 mm),
Plant material
The whole plant of D. tonkinensis was collected in
Quangbinh Province, Vietnam in 2016. The leaves of C.
formosum were collected in Ninhbinh Province, Vietnam in
2010. These plants were identified by Dr. Nguyen Quoc
Binh, Vietnam National Museum of Nature, Hanoi, Viet-
nam. The voucher specimens (C-575 for D. tonkinensis and
C-437 for C. formosum) have been deposited at the
Department of Bioactive Products, Institute of Natural
Products Chemistry, VAST, Hanoi, Vietnam.
MeOH–H2O–HCOOH (3:1:0.1%,
2
mLmin−1), UV
254 nm]. The EtOAc root fr. (30.5 g) was purified on a
normal silica gel column [CHCl3–MeOH–H2O (3:1:0.1, v/
v/v)] to afford frs. E1-E7. Then, fr. E2 (0.85 g) was purified
by HPLC [Cosmosil 5C18-AR-II column (10 × 250 mm),
MeOH–H2O (7:3, 2 mLmin−1), UV 210 nm] to give com-
pounds 6 (0.7 mg) and 7 (1.3 mg).
Dried leaves (2.0 kg) of C. formosum were extracted with
MeOH (10.0 L, 3 times) at room temperature to produce the
MeOH crude extract (220.0 g). This extract was then sub-
jected to silica gel column chromatography and eluted with
Extraction and isolation
n-hexane,
CH2Cl2,
CH2Cl2–EtOAc
(8:2,
v/v),
The leaves (2.0 kg) of D. tonkinensis were extracted with
MeOH (10.0 L) for 4 h using a Soxhlet extractor to produce
the MeOH crude extract (300.0 g). This extract was then
suspended in hot MeOH–H2O (1:1, v/v) and successively
partitioned with CHCl3 and EtOAc to yield the corre-
sponding fractions (frs.). The H2O-soluble residue (fr. W,
95.5 g) underwent chromatography on a Diaion HP-20
column [H2O–MeOH (1:0, 1:3, 0:1, v/v)], to afford 3 frs.
W1-W3. The fr. W2 (4.57 g) was separated by silica gel
column chromatography [CHCl3–MeOH–H2O (3:1:0.1, v/
v/v)] to yield compound 14 (20.6 mg) and nine frs. W21-
W29. Then, fr. W27 (0.9 g) was purified by HPLC [Cos-
mosil 5C18-AR-II column (10 × 250 mm) with MeOH–H2O
(7:3, v/v, 1 mLmin-1), UV 210 nm] to give compound 1
(1.2 mg), and compound 12 (4.6 mg) was obtained from fr.
W28 (1.9 g) in the same manner. Utilizing HPLC [Cosmosil
5C18-AR-II column (10 × 250 mm), MeOH–H2O–HCOOH
(3:1:0.1%, 2 mLmin−1), UV 254 nm], compound 13
(1.6 mg) was obtained from fr. W29 (0.6 g). Using column
chromatography over silica gel [CHCl3–MeOH (99:1, v/v)],
the CHCl3 fr. (60.7 g) was separated to give eight frs.
(C1–C8). Compound 8 (15.6 mg) was isolated from fr. C6
CH2Cl2–EtOAc (1:1, v/v), EtOAc, EtOAc–MeOH (8:2, v/v)
and EtOAc–MeOH (1:1, v/v), yielding the corresponding
frs. Fr1-Fr7. Compounds 17 (45 mg) and 18 (15 mg) were
obtained from fr. Fr1 (30.5 g) as white needles by silica gel
column chromatography with n-hexane–CH3COCH3 (3:1,
v/v). Using silica gel column chromatography
[CHCl3–MeOH (8:1, v/v)], fr. Fr4 (16.7 g) was separated to
give four frs. Fr41-Fr44. Utilizing Sephadex LH-20 column
chromatography with MeOH–H2O (1:1, v/v), compound 19
(37.5 mg) was obtained from fr. Fr42 (2.1 g). Meanwhile,
compound 20 (200 mg) was precipitated out of fr. Fr44
(2.3 g)
by
silica
gel
column
chromatography
[CH2Cl2–MeOH–H2O (3:1:0.1, v/v/v)]. Similarly, the fr.
Fr6 (20.5 g) was subjected to Sephadex LH-20 column
chromatography with MeOH–H2O (2:1, v/v) to afford
compound 21 (15.0 mg) and fr. Fr61. Then, fr. Fr61
(150 mg) was further subjected to HPLC [Cosmosil 5C18-
AR-II column (10 × 250 mm), MeOH–H2O (7:3, v/v,
1.0 mL/min), UV 210 nm], to yield compounds 22 (5.4 mg),
23 (4.2 mg) and 24 (3.2 mg). Compound 25 (240 mg) was
crystallized from the fr. Fr7 (30.1 g) in MeOH.
Isocaviunin-7-O-β-D-apiofuranosyl-(1 → 6)-β-D-gluco-
25
(5.7 g)
by
silica
gel
column
chromatography
pyranoside (1): White amorphous powder; [α]D = –266.9
[CHCl3–MeOH (3:1, v/v)].
(c 0.24, EtOH); IR (film): 3510, 3378, 2903, 1654, 1617,
1581, 1525, 1506, 1467, 1371, 1282, 1211, 1148, 1065,
980, 905, 853, 783, 678 cm-1; UV (EtOH) λmax nm (log ε):
295 (3.61), 263 (3.87), 205 (3.96); HR-FAB-MS: m/z
707.1577 [M + K]+ (100%) (calcd. for C30H36O17K
707.1576), FAB-MS: m/z 707 [M + K]+, 669 [M + H]+,
The dried powdered roots (2.0 kg) were mixed with EtOH
(10.0 L) for 4 h in a Soxhlet extractor, affording the crude
EtOH extract (200.0 g). This crude extract was resuspended
in hot MeOH–H2O (1:1, v/v) and partitioned with CH2Cl2
and EtOAc to yield the corresponding fractions.

Medicinal Chemistry Research
Table 1 1H and 13C-NMR data
of compound 1
Isocaviunin -7-O-β-D-apiofuranosyl-(1 → 6)-β-D-glucopyranoside (1)
Position
δH (J in Hz)
δC
Position
δH (J in Hz)
δC
74.5
2
8.24 (s)
155.7
120.6
181.2
106.9
157.8
100.1
157.2
130.0
150.9b
111.3
152.6
98.9
2″
3″
4″
5″
6″
1″′
2″′
3″′
4.34 (m)
3
4.78, (m)
77.7
71.2
77.3
68.6
111.0
78.4
80.2
75.0
65.6
4
4.11 (m)
4a
5
4.27 (m)
4.71 (d, 11.0 Hz) 4.14 (m)
5.70a (d, 2.5 Hz)
4.32 (m)
6
7.30 (s)
7
8
8a
1′
2′
3′
4′
5′
6′
1″
4″′
5″′
4.63 (d, 9.0 Hz) 4.36 (m)
4.20 (t, 11.5 Hz)
13.20 (s)
5-OH
8-OCH3
2′-OCH3
4′-OCH3
5′-OCH3
6.78 (s)
3.99 (s)
61.5
55.9
56.3
56.7
150.9b
143.6
117.0
102.2
3.73 (s)
3.80 (s)
7.21 (s)
5.68a (d, 7.5 Hz)
3.83 (s)
b
athe assignments by HSQC and HMBC correlations, May be interchangeable.
Table 2 1H and 13C-NMR data of compound 2
Acid hydrolysis of compound 1
Position
δH (J in Hz)
δC
88.27
Acid hydrolysis of 1 was performed according to the pro-
cedure of Cuong and partners (Cuong et al. 2015). Briefly,
compound 1 (0.85 mg) was added into 1 N HCl (2 mL) and
then heated to 80 °C for 3.0 h. The acidic solution was
extracted with chloroform (2 mL × 2 times). The organic
layer was washed with distilled water (2 mL × 3 times),
dried over Na2SO4, and evaporated to give an aglycone
(0.45 mg) as a white amorphous powder. The aglycone was
identified as isocaviunin by NMR spectroscopy techniques
and HR-ESI-MS: m/z 397.0872 (100%) [M + Na]+ (calcd.
2
2-CH3
1.35 (s)
22.39
28.12
29.05
178.42
83.57
26.70
36.87
83.16
25.71
143.67
110.62
3
1.91 (m) 2.31 (m)
2.62 (m)
4
5
2'
4.10 (dd, 3.0, 10.0)
1.70 (m) 1.99 (m)
1.76 (m) 1.95 (m)
-
1.28a (s) 1.30a (s)
5.97 (dd, 11.0, 17.5)
5.18 (d, 17.5) 4.99 (d, 10.5)
3'
4'
5'
5’-CH3
for
C18H19O8Na
397.0899);
Rf
(TLC):
0.4
(CH2Cl2–MeOH, 6:1).
6'
7'
aMay be interchangeable
Results and discussion
1
667 [M–H]–; H-NMR (500 MHz, C5D5N, δH ppm) and
13C-NMR (125 MHz, C5D5N, δC ppm); the corresponding
data are given in Table 1.
Compound 1 was obtained as white amorphous powder. Its
molecular formula C30H36O17 was deduced from its quasi-
molecular ion peak at m/z 707.1577 [M + K]+ in its positive
HR-FAB-MS data and from the m/z 707 [M + K]+, 669 [M
+ H]+, and 667 [M–H]– peaks in its positive (negative)
FAB-MS data. The IR spectrum of 1 showed absorption
bands at 3510, 3378, 1654, and 1581 cm−1, which could be
attributed to hydroxyl, chelated hydroxyl, α,β-unsaturated
carbonyl, and carbon-carbon double bond conjugated car-
bonyl groups, respectively. The UV spectrum of 1 showed
absorption maxima at λmax values of 205, 263 and 295 nm,
which are characteristic of an isoflavone derivative (Dixit
et al. 2012). The 1H-NMR spectrum of 1 displayed a
2,5′-Dimethyl-5′-vinylhexahydro-2,2′-bifuran-5(2 H)-
one (2): Yellowish amorphous powder; [α]D 25= –1.2 (c 1.1,
MeOH), IR (film): 2973.7, 1771.3, 1607.4, 1452.1, 1271.8,
1163.8, 1084.8, 944.9, 914.1, 829.2, 715.5, 617.1, 598.8,
564.1 cm-1; UV λmax (MeOH) nm (log ε): 206.5 (2.462),
257.5 (1.339); HR-EI-MS: m/z [M]+ 210.1254 (calcd. for
C12H18O3; found: 210.1253), EI-MS: m/z [M]+ 210; H-
1
NMR (500 MHz, CD3OD, δH ppm) and 13C-NMR
(125 MHz, CD3OD, δC ppm); the corresponding data are
given in Table 2.

Medicinal Chemistry Research
spectral pattern similar to that of isocaviunin 7-gentibioside
(compound isolated from D. sissoo) with resonances from
two components (Sharma et al. 1980), which include an
isocaviunin nucleus [5-OH (δH 13.20, 1 H, s), H-2 (δH 8.24,
1 H, s), H-6 (δH 7.30, 1 H, s), H-6′ (δH 7.21, 1 H, s), H-3′
(δH 6.78, 1 H, s), 8-OCH3 (δH 3.99, 3 H, s), 5′-OCH3 (δH
3.83, 3 H, s), 4′-OCH3 (δH 3.80, 3 H, s), and 2′-OCH3 (δH
3.73, 3 H, s)] and an β-D-apiofuranosyl-(1 → 6)-β-D-glu-
copyranosyl moiety [H-1″′ (δH 5.70, d, 2.5 Hz), H-1″ (δH
5.68, d, 7.5 Hz), and 11 H (δH 3.99-4.78)] (Table 1 and
Fig. 1). The 13C-NMR and 1H-13C HSQC spectra of 1
(C5D5N) contained 30 carbon signals (12 × C, 11 × CH, 3 ×
CH2, 4 × CH3), which were assignable to an isoflavone
aglycone, a β-D-apiofuranosyl residue, and a β-D-gluco-
pyranosyl bridge (Table 1 and Fig. 1). Regarding the 2D-
NMR analysis, the 1H-13C HMBC spectrum established the
isocaviunin ring by the J2 and J3 correlations H-2/C-3, C-4
and C-8a; H-6/C-4a, C-5, C-7, and C-8; 5-OH/C-4a, and C-
5; H-3′/C-1′, and C-5′; H-6′/C-5′, and C-3; 8-OCH3/C-8;
2′-OCH3/C-2; 4′-OCH3/C-4; and 5′-OCH3/C-5, and a gly-
cone unit was confirmed by the cross peaks H-2″/C-3″;
H-3″/C-4″; H-4″′/C-1″′, and C-2″′; and H-5″′/C-4″′ in the
1H-13C HMBC spectrum, as well as H-1″/H-2″; H-4″/H-5″;
H-4′/C-5′. Furthermore, the COSY correlation H-6′/H-7′
and the HMBC interactions H-6′/C-5′; 5′-CH3/C-5′ and C-
4′; and 2-CH3/C-2 and C-3 indicated that the vinyl group
occurred at C-5′, and two methyl groups were attached at
the C-2 and C-5′ positions. The connection between the
THP ring and the γ lactone ring can be explained by the key
correlation 2-CH3/C-2′. In contrast to rel-(2 R,2′R,5′S)-2,5′-
dimethyl-5′-vinylhexahydro-2,2′-bifuran-5(2 H)-one, H-2′
had no NOE on 5′-CH3; therefore, H-2′ and the 5′-methyl
group were trans-orientated in compound 2 (Marshall and
Hann 2008; Wang et al. 2014).
Compound 2 was found to have a small optical rotation
of –1.2 (c 1.1, MeOH) (Deng et al. 2017; Song et al. 2018).
As a consequence, we concluded that compound 2 was an
enantiomeric mixture. Thus, compound 2 was further ana-
lyzed by a chiral HPLC column (OD-RH, 150 × 4.6 mm)
with a mobile phase of n-hexane-2-propanol (80:20, v/v),
thereby showing two peaks at tR = 11.2 (35%) and tR = 15.81
(65%) (Fig. S14). Therefore, compound 2 was identified as
a scalemic mixture of the two enantiomers rel-(2 S,2′S,5′S)-
2,5′-dimethyl-5′-vinylhexahydro-2,2′-bifuran-5(2 H)-one
(2a) and rel-(2 R,2′R,5′R)-2,5′-dimethyl-5′-vinylhexahy-
dro-2,2′-bifuran-5(2 H)-one (2b).
Apart from the new isoflavone glycoside 1 and new
scalemic mixture 2, based on comparing their NMR spec-
troscopic data with the previous literature, thirteen known
compounds 3-16 were also identified, including two mono-
acylglycerides 1-monolaurin (3) and 1-monomyristin (4);
one isoflavan (3 R)-vestitol (5); two pterocarpans medi-
carpin (6) and melilotolcarpan D (7); eight isoflavones and
isoflavone glycosides orobol (8), tectorigenin (9), daidzein
(10), 3’-hydroxyldaidzein (11), ambocin (12), biochanin A-
7-β-D-apiofuranosyl-(1 → 6)-β-D-glucopyranosside (13),
dalsissooside (14), and formononetin-7-O-β-D-glucopyr-
anoside (15); and one mono-phenol killitol (16) (Cho et al.
2014; Dixit et al. 2012; Lotti et al. 2010; Parthasarathy
et al. 1974; Romo et al. 2018; Saha et al. 2013; Shimada
et al. 1997; Zhao et al. 2011). All of the known compounds
3-16 were isolated from D. tonkinensis species for the first
time. 1-Monolaurin (3), 1-monomyristin (4) and killitol
(16) were never isolated from the family Fabaceae before.
Ambocin (12) had been reported in the family Fabaceae,
but this is the first time it was found in the genus
Dalbergia.
In current paper, our phytochemical work also resulted
in the appearance of two phytosterols β-sitosterol (17)
and stigmasterol (18); one flavan (+)-catechin-3-O-(3,4-
dihydroxybenzoyl) (19); four flavonol and flavonol
glycosides quercetin (20), quercitrin (21), hyperin
(quercetin-3-O-β-galactopyranoside) (22), afzelin (23),
and isoquercetin (quercetin-3-O-β-glucopyranoside) (24);
and one xanthone glycoside mangiferin (25) from the
leaves of C. formosum (Choi et al. 2012; Khanam,
1
H-5″/H-6″; and H-1″′/H-2″′ in the H-1H COSY spectrum
(Fig. 2). Furthermore, the key 1H-13C HMBC correlation H-
1″′/C-6″ evidently generated a 1 → 6 linkage between two
sugar units, and the cross peak H-1″/C-7 indicated the
connection between glycone and aglycone at C-7. In addi-
tion, the acid hydrolysis of 1 yielded the aglycone, the
NMR and MS data of which were identical to those of
isocaviunin (Sharma et al. 1980). Based on these data,
compound 1 was elucidated to be isocaviunin-7-O-β-D-
apiofuranosyl-(1 → 6)-β-D-glucopyranoside.
Compound 2 was obtained as yellowish amorphous
powder. The molecular formula of 2 corresponded to
C12H18O3, as deduced from its molecular ion peak at m/z
210.1253 [M]+ in its positive HR-EI-MS data, as well as the
peak m/z 210 [M]+ in its positive EI-MS data. The
absorptions at 1771 and 1607 cm−1 in the IR spectrum
showed the presence of a γ-lactone and a double bond,
respectively. The 1H and 13C-NMR data of 2 were included
two methine groups, five methylene groups, two methyl
groups, two oxygenated quaternary carbons, and one lac-
tone carbon (Table 2 and Fig. 1). Based on the HSQC,
HMBC, COSY, and NOESY extensive analysis and com-
parison with the literature (Wang et al. 2014), compound 2
was assigned as a diastereomeric isomer of rel-(2 R,2′R,5′
S)-2,5′-dimethyl-5′-vinylhexahydro-2,2′-bifuran-5(2 H)-
one (a component was isolated from D. odorifera T. Chen)
(Wang et al. 2014). As shown in Fig. 2, the appearance of
tetrahydrofuran (THP) and γ-lactone rings was confirmed
by the COSY correlations H-3/H-4, H-2′/H-3′, and H-3′/H-
4′, as well as the HMBC cross peaks H-3 and H-4/C-5 and

Medicinal Chemistry Research
Fig. 1 Structures of isolated compounds
Sultana (2012); Kitanov et al. 1988; Mai 2017; Tung
et al. 2008). It should be noted that flavan 19 was found
in genus Cratoxylum for first time.
The DPPH radical scavenging assay was carried out on
several isolated compounds according to the protocol
described by Tram and partners (Tram et al. 2017), and the

Medicinal Chemistry Research
Fig. 2 Structures and HMBC
and COSY key correlations of
compounds 1-2
glycose moiety and hydroxyl groups attached at C-3 and C-
3’ caused an increase in the antioxidant capacity. Similarly,
3-glucopyranosylated flavonol (compound 24) showed a
better antioxidant potency than 3-galactopyranosylated fla-
vonol (compound 22).
Table 3 DPPH radical scavenging activity
Compounds
IC50 (µg/mL)
(3 R)-Vestitol (5)
Medicarpin (6)
42.20
69.45
54.14
91.18
75.37
66.54
66.22
> 100
45.63
64.03
42.98
Melitocarpan D (7)
Orobol (8)
Acknowledgements This work was supported by the JSPS RON-
PAKU Ph.D program for FY 2017 and NAFOSTED under grant
number 104.01-2015.49.
Catechin-3-O-(3,4-dihydroxylbenzoyl) (19)
Quercetin (20)
Hyperin (22)
Compliance with ethical standards
Afzelin (23)
Conflict of interest The authors declare that they have no conflict of
interest.
Isoquercetin (24)
Magniferin (25)
Catechin
Publisher’s note: Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations.
results are given in Table 3. With regards to several tested
compounds from D. tonkinensis species, (3 R)-vestitol (5)
possessed the strongest antioxidative IC50 value of 42.20 µg/
mL, which was comparable to that of the positive control
catechin (IC50 42.98 µg/mL). Pterocarpan 7 revealed a strong
IC50 value of 54.14 µg/mL when compared with the mod-
erate IC50 value of 69.45 µg/mL of pterocarpan 6 and the
weak one of 91.18 µg/mL of isoflavone 8. It can therefore be
concluded that isoflavans from D. tonkinensis species should
be the best choice for an antioxidant model compared with
pterocarpans or isoflavones. In addition, comparing the two
pterocarpans medicarpin (6) and melilotolcarpan D (7),
hydroxylation occurred at carbons C-1 and C-8, and meth-
oxylation happened at carbon C-2, which can be claimed to
be responsible for the antioxidant increase.
Regarding the isolated compounds from C. formosum
leaves, the IC50 values established a consistent arrangement
as follows: catechin (IC50 42.98 µg/mL) > isoquercetin 24
(IC50 45.63 µg/mL) > magniferin 25 (IC50 64.03 µg/mL) >
hyperin 22 (IC50 64.22 µg/mL) > quercetin (20) (IC50
64.54 µg/mL) > catechin-3-O-(3,4-dihydroxylbenzoyl) 19
(IC50 75.37 µg/mL) > afzelin 23 (IC50 > 100 µg/mL, inac-
tive). As a consequence, flavonols, flavonol glycosides, and
xanthone glycosides may be thought of as promising anti-
oxidant agents, but the flavans cannot be considered as
such. Taking flavonol derivatives into consideration, a
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