A New Flavanone Glucoside from Oroxylum indicum

Article abstract of DOI:10.1007/s10600-015-1431-4

A new flavanone glucoside, (2S)-dihydrobaicalein 7-O-(6-benzoylglucopyranoside) (1), along with the known compounds scutellarein 4′-methyl ether (2), pectolinarigenin (3), and β-sitosterol-3-O-β-Dglucopyranoside (4), were isolated from the MeOH extract of Oroxylum indicum stem bark. The structure of the new compound was established on the basis of detailed spectroscopic and chemical studies.

Full text of DOI:10.1007/s10600-015-1431-4

DOI 10.1007/s10600-015-1431-4
Chemistry of Natural Compounds, Vol. 51, No. 5, September, 2015
A NEW FLAVANONE GLUCOSIDE FROM Oroxylum indicum
I. Sil Sarma, P. S. Ghosh, R. Banik, and B. Dinda*
A new flavanone glucoside, (2S)-dihydrobaicalein 7-O-(6ꢀꢀ-benzoylglucopyranoside) (1), along with the
known compounds scutellarein 4ꢀ-methyl ether (2), pectolinarigenin (3), and ꢁ-sitosterol-3-O-ꢁ-D-
glucopyranoside (4), were isolated from the MeOH extract of Oroxylum indicum stem bark. The structure of
the new compound was established on the basis of detailed spectroscopic and chemical studies.
Keywords: Oroxylum indicum, Bignoniaceae, flavonoids.
Oroxylum indicum Vent. (Bignoniaceae) is a small tree growing in India, China, and Southeast Asia [1, 2]. The stem
bark, root, and root bark of this plant have been used as traditional medicine in India, Thailand, and other Asian countries for
the treatment of rheumatism, diarrhea, dysentery, and scorpion sting [3]. The seeds of this plant have been used as traditional
Chinese medicine for the treatment of cough, bronchitis, pharyngitis, pertussis, and other respiratory problems [4]. The plant
possesses antimicrobial [5], anti-inflammatory [6], anticancer [7, 8], antioxidant [6], gastroprotective [9], and immuno-stimulant
[10] activities. Three flavonoids, baicalein, oroxylinA, and chrysin, isolated from its stem bark exhibited significant inhibitory
activity against proprotein convertases, which play an important role in cancer and viral and bacterial infections [11]. Baicalein
also exhibited significant antileprotic activity against camptothecin-resistant Leishmania donovani strain [12] and antitumor
activity against colorectal carcinoma CT-26 cells [13]. Previous chemical studies on different parts of the plant reported the
isolation of flavonoids [9, 14–17], pterocarpans [18], naphthoquinones [19], anthraquinone [20], prenylated glycosides, and
cyclohexyl ethanoids [21].
Our further investigation on the stem bark of Oroxylum indicum led to the isolation of three more minor
flavonoids (1–3) and ꢁ-sitosterol-3-O-ꢁ-D-glucopyranoside (4). These minor flavonoids are (2S)-dihydrobaicalein
7-O-(6ꢀꢀ-benzoylglucopyranoside) (1), scutellarein 4ꢀ-methyl ether (2), and pectolinarigenin (= 5,7 dihydroxy-6,4ꢀ-
dimethoxyflavone) (3). Compound 1 is new and the known compounds 2–4 are reported for the first time from this plant.
4'''
O 7'''
1'''
O
OMe
6''
4'
O
1
HO
HO
O
7
O
GlcO
HO
O
O
HO
RO
O
O
1'
OH 1''
HO
3
10
5
OH
O
OH
OH
1a
2, 3
1
2: R = H; 3: R = Me
Department of Chemistry, Tripura University, 799022, Suryamaninagar, Tripura, India, fax: (+91381) 237 48 02,
e-mail: indrajitchemtu@gmail.com; parthasarathighosh123@gmail.com; rajarshi_ark@rediffmail.com; dindabtu@gmail.com.
Published in Khimiya Prirodnykh Soedinenii, No. 5, September–October, 2015, pp. 728–730. Original article submitted
February 20, 2014.
0009-3130/15/5105-0847 02015 Springer Science+Business Media New York
847

1
13
TABLE 1. H (400 MHz) and C (100 MHz) NMR Data of 1 (DMSO-d , ꢄ, ppm, J/Hz)
6
C atom
C atom
ꢄH
ꢄC (DEPT)
ꢄH
ꢄC (DEPT)
2
3
4
5
6
7
8
9
10
1ꢀ
5.64 (dd, J = 12.5, 2.5)
2.78 (dd, J = 16.8, 2.5), 3.16 m*
79.8 (CH)
42.6 (CH2)
196.8 (C)
150.1 (C)
128.2 (C)
154.8 (C)
96.2 (CH)
153.4 (C)
103.8 (C)
138.0 (C)
126.4 (CH)
128.1 (CH)
128.4 (CH)
4.86 (d, J = 7.5)
3.10–3.78
101.1 (CH)
72.8 (CH)
76.2 (CH)
69.8 (CH)
78.2 (CH)
64.8 (CH2)
131.3 (C)
130.8 (CH)
130.0 (CH)
134.8 (CH)
167.0 (C)
–
1ꢀꢀ
2ꢀꢀ
3ꢀꢀ
4ꢀꢀ
5ꢀꢀ
6ꢀꢀ
1ꢀꢀꢀ
2ꢀꢀꢀ, 6ꢀꢀꢀ
3ꢀꢀꢀ, 5ꢀꢀꢀ
4ꢀꢀꢀ
–
–
–
–
3.10–3.78
3.10–3.78
3.10–3.78
5.12 (br.d, J = 11.5)
4.98 (br.d, J = 11.5)
7.98 (dd, J = 8.5, 1.8)
7.39 m
6.22 s
–
–
–
7.58 (br.d, J = 8.5)
7.29 m
7.39 m
–
2ꢀ, 6ꢀ
3ꢀ, 5ꢀ
4ꢀ
7ꢀꢀꢀ
5-OH
6-OH
12.37 s
8.06 br.s
7.29 m
–
______
*Signal was overlapped with solvent signal.
+
The positive mode FAB-MS of compound 1 exhibited a molecular ion at m/z 539 [M + H] corresponding to its
molecular formula C H O . This was also supported by its negative mode FAB-MS, which showed a molecular ion at m/z
28 26 11
–
537 [M – H] . The UV spectrum in MeOH displayed absorption maxima, ꢂ
242 sh, 285 and 348 nm, characteristic of
max
flavanones [22]. In the presence of AlCl , the UV spectrum exhibited bathochromic shifts at ꢂ
248 sh, 312, 314, and 443 sh,
3
max
indicating the presence of a free hydroxyl group at the C-5 position [22]. The IR spectrum showed the bands for hydroxyl
–1
–1
–1
–1
(3427 cm ), ꢃ,ꢁ–unsaturated carbonyl (1653 cm ), and aromatic (1611 and 1578 cm ) and glycosidic (1169 and 1032 cm )
1
functions. The H NMR spectrum (Table 1) displayed signals for one aromatic proton [ꢄ 6.22(1H, s, H-8)], an unsubstituted B
ring of the flavone moiety [ꢄ 7.58 (2H, br.d, J = 8.5 Hz, H-2ꢀ, 6ꢀ) and 7.29 (3H, m, H-3ꢀ, 4ꢀ, 5ꢀ)], one methine proton
[ꢄ 5.64 (1H, dd, J = 12.5 and 2.5 Hz, H-2)], one methylene proton [ꢄ 2.78 (1H, dd, J = 16.8 and 2.5 Hz, H-3) and 3.16 (1H,
overlapped with solvent signal, H-3)], one anomeric sugar proton [ꢄ 4.86 (1H, d, J = 7.5 Hz, H-1ꢀꢀ)], and two phenolic
hydroxyl protons [ꢄ 12.37 (1H, s, 5-OH) and 8.06 (1H, br.s, 6-OH)], supporting its dihydrobaicalein glucoside structure.
1
In addition, the H NMR spectrum showed the signals for five aromatic protons of a benzoyl moiety [ꢄ 7.98 (2H, dd, J = 8.5
and 1.8 Hz, H-2ꢀꢀꢀ, 6ꢀꢀꢀ) and 7.39 (3H, m, H-3ꢀꢀꢀ, 4ꢀꢀꢀ, 5ꢀꢀꢀ)] indicating the presence of a benzoyl group as an acylating unit. The
13
C NMR spectrum (Table 1) with DEPTexperiments revealed the signals for two methylenes, 17 methines, and nine quaternary
carbons. Possibly some carbon signals represent more than one carbon as the molecule contains 28 carbons. The carbon
resonance values were very similar to those reported for dihydrobaicalein 7-O-glucoside (1a) [23]. The high chemical resonance
of the C-6ꢀꢀ sugar carbon (ꢄ 64.8 ppm) and the downfield chemical resonance of H-6ꢀꢀ protons [ꢄ 5.12 and 4.98] confirmed
C
H
the location of a benzoyl group at the C-6ꢀꢀ position of the glucose moiety. FAB-MS of the compound showed mass ions at
+
m/z 435 [C H O ] , indicating the presence of the dihydrobaicalein hexoside moiety in it. Its EI-MS showed mass ions at
21 23 10
+
+
+
m/z 272, [C H O ] , 168 [C H O ] , and 104 [C H ] , indicating its flavanone skeletal structure with three hydroxyl
15 12
5
7
4
5
8 8
+
groups in the A-ring and an unsubstituated B ring. The mass ion at m/z 105 [C H O] suggested the presence of a benzoyl
7
5
group as the acyl moiety. Saponification of the compound with NaOH solution afforded benzoic acid, confirming the attachment
of a benzoyl group as the acyl moiety. Acid hydrolysis with 2 M HCl afforded D-glucose, suggesting the presence of a glucose
moiety as hexose sugar. The configuration of the C-2 position was assigned as (S) from its CD spectrum in MeOH, which
displayed positive Cotton effects at 338 and 214 nm and a strong negative Cotton effect at 290 nm [24]. Therefore, the
structure of the compound was elucidated as (2S)-dihydrobaicalein 7-O-(6ꢀꢀ-benzoylglucopyranoside)[=(2S),5,6,7-
trihydroxyflavanone 7-O-(6ꢀꢀ-benzoylglucopyranoside)] (1).
EXPERIMENTAL
General. Melting points were determined with a Koffler apparatus and are uncorrected. UV-Vis spectra were recorded
1
with a PerkinElmer Lamda 25 spectrometer. IR spectra were obtained on a PerkinElmer FTIR-100 spectrometer. H and
13
C NMR spectra were measured on a Varian XL- 400 and Brucker 600 spectrometers with TMS as internal reference.
848

FAB and EI-MS were recorded on a JEOL JMS-AX505 HA spectrometer. CD spectrum was measured on a Jasco J805
spectropolarimeter.
Extraction and Isolation.Air-dried powdered stem bark (2.0 kg) of O. indicum collected fromAgartala was extracted
with MeOH (3 ꢅ 5 L). The MeOH extract was evaporated under reduced pressure to a thick condensate and dissolved in H O
2
(0.15 L). The aqueous solution was extracted three times each with hexane, EtOAc, and n-BuOH. The EtOAc extract on
repeated column chromatography (CC) over silica gel afforded 2 (15 mg) and 3 (9 mg). The BuOH extract on repeated CC
over Si gel afforded 1 (25 mg) and 4 (38 mg).
(2S)-Dihydrobaicalein 7-O-(6ꢀꢀ-Benzoyl-ꢁ-D-glucopyranoside) (1). Pale yellow amorphous powder, mp 176–178ꢆC.
+
–1
C H O . (M 538). CD (MeOH, c 0.02 mg mL ): [ꢇ] + 7862, [ꢇ] –32150, [ꢇ] +88724.
28 26 11
338
290
214
Alkaline Hydrolysis of 1. Compound 1 (3 mg) was refluxed with 0.1 M methanolic NaOH solution (5 mL) for 1 h.
The reaction mixture was concentrated, diluted with H O, and extracted with CHCl . The CHCl extract was evaporated to
2
3
3
dryness to a residue, which was identified as benzoic acid by comparing its mass spectrum and co-TLC with an authentic
sample.
Acid Hydrolysis of 1. Compound 1 (4 mg) was refluxed with 2 M methanolic HCl (5 mL) for 2 h. The reaction
mixture was cooled, concentrated, diluted with H O, neutralized with Ag CO , and filtered. The aqueous filtrate was
2
2
3
concentrated. The concentrated solution indicated the presence of D-glucose on co-TLC with a standard sample of D-glucose.
+
1
13
Scutellarein 4ꢀ-Methyl Ether (2). Yellow amorphous powder, mp 187–189ꢆC. C H O (M 300). H and C NMR
16 12
6
spectral data were similar to that of published data [23].
+
Pectolinarigenin (3). Yellow needles, mp 209–210ꢆC, C H O (M 314). Physical constants and spectral data
17 14
6
were identical with the literature data [25].
+
ꢁ-Sitosterol Glucoside (4). White amorphous powder, mp 284–285ꢆC, C H O (M 576). The spectral data were
35 60
6
identical with the literature data [26].
ACKNOWLEDGMENT
The authors thank Prof. Y. Harigaya, School of Pharmaceutical Sciences, Kitasato University, Tokyo, Japan;
Prof. S. Roy, Director, IICB, Kolkata for NMR, CD, and MS spectra; Prof. B. K. Datta, Dept. of Botany, TU for identification
of the plant material; and DST, New Delhi for financial support (Grant No. SR/ S1/OC-75/2009).
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