An improved synthesis of 6-o-methyl-scutellarein through selective benzylation

Article abstract of DOI:10.3184/174751915X14452625043383

An improved synthesis of 6-O-methyl-scutellarein is described. Benzyl bromide was selected to protect both the hydroxyl groups at C-4'and C-7 in scutellarein. The product was then methylated and deprotected to produce the target compound in high yield in four steps.

Full text of DOI:10.3184/174751915X14452625043383

674 RESEARCH PAPER
VOL. 39 NOVEMBER, 674–676
JOURNAL OF CHEMICAL RESEARCH 2015
An improved synthesis of 6-o-methyl-scutellarein through selective
benzylation
Wei Zhanga, Ze-Xi Donga, Ting Gua, Nian-Guang Lia*, Wen-Yu Wua, Peng-Xuan Zhanga, Yu-Ping Tanga,
Jian-Ping Yanga, Xin Xuea, Fang Fanga, He-Min Lia, Hai-Bo Chengb, Jin-Ao Duana and Zhi-Hao Shic
aNational and Local Collaborative Engineering Center of Chinese Medicinal Resources Industrialization and Formulae Innovative Medicine, Nanjing University
of Chinese Medicine, Nanjing, Jiangsu 210023, P.R. China
bKey Laboratory of SATCM for Empirical Formulae Evaluation and Achievements Transformation, Nanjing University of Chinese Medicine, Nanjing, Jiangsu
210023, P.R. China
cDepartment of Organic Chemistry, China Pharmaceutical University, Nanjing, Jiangsu 211198, P.R. China
An improved synthesis of 6-O-methyl-scutellarein is described. Benzyl bromide was selected to protect both the hydroxyl groups at C-4’
and C-7 in scutellarein. The product was then methylated and deprotected to produce the target compound in high yield in four steps.
Keywords: scutellarin, scutellarein, 6-O-methyl-scutellarein, metabolite, selective benzylation
Ischemic cerebrovascular disease is a leading cause of disability
and death worldwide.1 Increasing evidence suggests that thrombin
plays an important role in causing this disease.2 It catalyses the
proteolytic cleavage process in which the soluble plasma-protein
fibrinogen forms insoluble fibrin, thus leading to clot formation.3
Furthermore, oxidative stresses also play a critical role in ischaemic
cerebrovascular disease.4,5 The highly reactive oxygen species
(ROS) including the singlet oxygen (1O2), superoxide anion radical
(O2−•), hydroxyl radical (•OH), nitric oxide (NO•) and hydrogen
peroxide (H2O2) may increase the low-density lipoprotein (LDL)
level and impair the bioactions that are mediated by the endothelial
derived relaxing factor (nitric oxide, NO).6
Consequently, some drugs with anticoagulant activity and
antioxidant capacity have been proposed to treat cerebrovascular
disease. Traditional Chinese medicines (TCM) can be used to
reveal lead compounds because they have been used in clinics
for thousands of years. Breviscapine, which is a natural drug
containing total flavonoids in Erigeron breviscapus (Vant.) Hand-
Mazz. (Compositae), has been widely used in China to treat
angina pectoris, cerebral infarction and coronary heart disease.7
Pharmacological studies showed that scutellarin (1) (Fig. 1), which
is the main active ingredient (>85%) in breviscapine, exhibited
anticoagulant and antioxidant activities which reduced neuronal
damage, and could be used to treat brain injury induced by cerebral
ischemia/reperfusion.8-10
However, the results of pharmacokinetic studies on scutellarin (1)
indicated that its oral bioavailability was very poor11 in humans.12
The main cause was that scutellarin (1) is easily hydrolysed
into scutellarein (2) (Fig. 1) in the gastrointestinal tract before
absorption in vivo, and scutellarein (2) was then converted into its
methylated, sulfated or glucuronidated metabolites13 in the blood.
One circulating metabolite of scutellarin (1) in vivo, 6-O-methyl-
scutellarein (3) (Fig. 1) might have some interesting therapeutic
effects. Hence the synthesis of this metabolite (3) in large amount is
very important for the further study of its biological activities.
Recently we reported a synthesis of 6-O-methyl-scutellarein
(3) (Fig. 2),14 in which dichlorodiphenylmethane was used first to
protect the two adjacent hydroxyl groups at C-6 and C-7 positions
in scutellarein (2). Then benzyl bromide was used to selectively
and successively protect the hydroxyl group at C-4’ in 4 and C-7
positions in 5. Finally, 6-O-methyl-scutellarein (3) was obtained in
high yield after methylation of the hydroxyl group at C-6 followed by
removal of the two benzyl groups at C-7 and C-4’ by hydrogenation.
Unfortunately, using dichlorodiphenylmethane to selectively protect
OH
OH
OH
OH
O
O
HO
HO
O
O
O
HO
HO
O
O
HO
O
O
OH
HO
H3CO
OH
OH
OH
6-O-Methylscutellarein (3)
Scutellarin (1)
Scutellarein (2)
Fig. 1 The chemical structures of scutellarin (1), scutellarein (2) and 6-O-methyl-scutellarein (3).
OH
OH
HO
HO
O
O
O
7
6
O
O
Ph
Ph
Ph2CCl2, Ph2O
C
175°C, 30 min
OH
O
OH
2
4
OH
4' OCH2Ph
OCH2Ph
HO
O
O
HO
HO
O
O
PhH2CO
HO
O
PhCH2Br, K2CO3
DMF, 25°C, 12 h
H3CO
OH
OH
OH
O
3
5
6
Fig. 2 The previous synthetic route of 6-O-methyl-scutellarein (3).
* Correspondent. E-mail: zpx20130901@126.com

JOURNAL OF CHEMICAL RESEARCH 2015 675
the two adjacent hydroxyl groups at the C-6 and C-7 positions in
scutellarein (2) at 175 °C in the previous synthesis precluded the
preparation of 6-O-methyl-scutellarein (3) on the large-scale.
Therefore, we optimised the reaction conditions and used the benzyl
bromide to directly protect the two adjacent hydroxyl groups at C-4’
and C-7 positions in 2. 6-O-Methyl-scutellarein (3) was obtained
after methylation and deprotection in high yield. We now describe
this new synthetic route in detail.
this selective benzylation in acetone at room temperature, but
unfortunately, there was no indication of product formation
even when the reaction time was prolonged to 24 h (Table 1,
run 6). To make this reaction proceed easily, we increased the
temperature to accelerate this benzylation. The result showed
that a high yield of 53.8% was obtained within 6 h when this
reaction was carried out at reflux (Table 1, run 7). When the
molar ratio of benzyl bromide was decreased to 2.0 equiv.
(Table 1, run 8), the yield of the 4ʹ,7-dibenzyl scutellarein (6)
was reduced to 31.3% because some by-products of 4ʹ-benzyl
scutellarein and 7-benzyl scutellarein were found in the reaction
mixture. Furthermore, when the molar ratio of the benzyl
bromide was increased (Table 1, run 9), the yield of the 4ʹ,7-
dibenzyl scutellarein (6) was reduced to 27.5% and the by-
product of 4ʹ,6,7-tribenzyl scutellarein was detected by TLC in
the reaction mixture.
When the 4ʹ,7-dibenzyl scutellarein (6) was obtained, we
synthesised the 6-O-methyl-scutellarein (3) according to our
previous procedure. The methylation of 6 with iodomethane (1.2
equiv.) afforded 10 in 94% yield and the only methyl group was
at the C6-OH position. Removal of both the benzyl groups by
hydrogenation with 10% palladium on carbon catalyst in THF/
EtOH solution produced the target compound 3 in 96% yield.
Results and discussion
As shown in Scheme 1, scutellarein (2) was obtained from
scutellarin (1) by hydrolysis with 6N HCl in 90% ethanol
under reflux.15,16 The reactivity of the four hydroxyl groups in
scutellarein (2) were different and followed the order: 4ʹ > 7 > 6
>5. Direct benzylation of scutellarein (2) with benzyl bromide
led mainly to the formation of dibenzylated product 6. We
optimised the reaction conditions as shown in Table 1.
First, the DMF was used as a solvent in this benzylation.
When the reaction temperature was set at 25 ºC, the 4ʹ,7-
dibenzyl scutellarein (6) was obtained in 33.4% yield (Table
1, run 1) in the presence of 2.2 equiv. benzyl bromide and 2.4
equiv. K2CO3. However, the yield of the target compound was
reduced to 28.8% and 25.9% when the reaction temperature
was raised to 40 ºC and 60 ºC (Table 1, runs 2 and 3). The
amount of benzyl bromide played a crucial role in this selective
benzylation. We found that on decreasing (Table 1, run 4) or
increasing (Table 1, run 5) the ratio of benzyl bromide, the
yields of the 4ʹ,7-dibenzyl scutellarein (6) were both reduced.
Although this benzylation in DMF selectively afforded the
4ʹ,7-dibenzyl scutellarein (6) as the main product, the yield was
not high, and a great deal of ethyl acetate and water were used
to separate the main product from the DMF solution. In order
to avoid this complicated process of extraction and separation,
we chose acetone as a solvent for this reaction and the crude
product could be separated by direct filtration. At first, we tried
Conclusion
In conclusion, we have developed an improved process for the
synthesis of 6-O-methyl-scutellarein (3) in only four steps.
Since the reactivity of the four hydroxyl groups was in the order:
4ʹ > 7 > 6 > 5, benzyl bromide was selected to protect both
hydroxyl groups at C-4’ and C-7. This synthetic procedure was
very efficient which could be used for the selective synthesis
of other O-methyl-flavonoid derivatives as well as the other
metabolites of sulfate- and glucuronide-flavonoid.
Experimental
Reagents and solvents were used directly without further purification
unless otherwise stated. Air- and moisture-sensitive solvents were
transferred through a syringe or stainless steel cannula. The organic
solutions were concentrated by rotary evaporation at ~20 mm Hg and
below 45 °C. All the non-aqueous reactions were performed in dry
and freshly distilled solvents using flame-dried glassware under a
nitrogen atmosphere. Yields were calculated for chromatographically
homogeneous materials. The reaction process was monitored by TLC
which was conducted on 0.15–0.20 mm Yantai silica gel plates (RSGF
254) using UV light to visualise. Chromatographic separation was carried
out on Qingdao silica gel (160–200 mesh) using a mixture of petroleum
ether (60–90) and ethyl acetate as eluant. Melting points (m.p.) were
recorded on a WRS-1B apparatus. The 1H NMR and 13C NMR spectra
Table 1 Optimisation of reaction conditions in the synthesis of 6
Run Reaction conditions
Yield/%
33.4
1
2
3
4
5
6
7
8
9
BnBr (2.2 equiv.), K2CO3 (2.4 equiv.), DMF, 25 °C , N2, 8 h
BnBr (2.2 equiv.), K2CO3 (2.4 equiv.), DMF, 40 °C , N2, 8 h
BnBr (2.2 equiv.), K2CO3 (2.4 equiv.), DMF, 60 °C , N2, 8 h
BnBr (2.0 equiv.), K2CO3 (2.2 equiv.), DMF, 25 °C , N2, 8 h
BnBr (2.4 equiv.), K2CO3 (2.6 equiv.), DMF, 25 °C , N2, 8 h
28.8
25.9
27.5
31.3
BnBr (2.2 equiv.), K2CO3 (2.4 equiv.), acetone, r.t., N2, 24 h No product
BnBr (2.2 equiv.), K2CO3 (2.4 equiv.), acetone, reflux, N2, 6 h 53.8
BnBr (2.0 equiv.), K2CO3 (2.2 equiv.), acetone, reflux, N2, 8 h 31.3
BnBr (2.4 equiv.), K2CO3 (2.6 equiv.), acetone, reflux, N2, 8 h 27.5
OH
OH
OCH2Ph
OH
O
O
HO
HO
O
O
O
PhH2CO
HO
O
O
HO
HO
O
O
i
ii
OH
HO
OH
OH
OH
1
6
2
OH
OCH2Ph
iv
HO
O
O
PhH2CO
H3CO
O
iii
H3CO
OH
OH
O
3
7
Scheme 1 Reagents and conditions: (i) HCl, EtOH, N2, reflux, 48h, 17%; (ii) BnBr (2.2 equiv.), K2CO3 (2.4 equiv.), acetone, reflux, 6h, 53.8%; (iii)
CH3I (1.2 equiv.), K2CO3 (1.4 equiv.), DMF, 25 °C, 6h, 94%; (iv) Pd/C (10 wt%), H2 (1 atm), THF/EtOH, 8h, 96%.

676 JOURNAL OF CHEMICAL RESEARCH 2015
were obtained from Bruker AV-300. The chemical shifts downfield
from TMS were recorded in ppm. The J values were shown in Hz,
abbreviations used were s (singlet), d (doublet), t (triplet), q (quartet),
b (broad) and m (multiplet) respectively. ESI-MS spectra were obtained
from a Synapt HDMS spectrometer.
0.21 mmol) in ethanol (10 mL) and THF (10 mL), the atmosphere over
reaction mixture was then replaced by hydrogen. The mixture was
filtered after it was reacted for 8 h, the filtrate was concentrated and the
crude material was purified by chromatography on silica gel column
using 50% ethyl acetate in petroleum ether to afford 3 (60 mg, 96.1%)
1
as yellow solid; m.p. 279–282 °C (lit.18 281–283 °C). H NMR (300
5,6,7-Trihydroxy-2-(4-hydroxyphenyl)-4H-chromen-4-one
(2):
Concentrated hydrochloric acid (120 mL) and water (10 mL) were
added to a solution of 1 (10.0 g, 21.6 mmol) in ethanol (120 mL), and
the reaction mixture was refluxed for 36 h under a N2 atmosphere. The
reaction mixture was cooled to room temperature, and poured into cold
water. The solid which appeared was filtered, and was then purified
by chromatography on the silica gel column using 50% ethyl acetate in
petroleum ether afforded 2 (1.05 g, 17.0% yield) as yellow solid; m.p.
288–290 °C (lit.17 290–293 °C). 1H NMR (DMSO-d6, 300 MHz) δ 6.57
(s, 1H, C8H), 6.74 (s, 1H, C3H), 6.91 (d, J = 8.8 Hz, 2H, C3’,C5’H),
7.90 (d, J = 8.8 Hz, 2H, C2’,C6’H), 8.71 (s, 1H, C6OH), 10.29 (s, 1H,
C7OH), 10.44 (s, 1H, C4’OH), 12.78 (s, 1H, C5OH); 13C NMR (75
MHz, DMSO-d6) δ 98.53 (C8), 102.27 (C3), 103.22 (C10), 115.76
(C3’,C5’), 121.37 (C1’), 124.91 (C6), 128.47 (C2’,C6’),145.42 (C9),
152.98 (C5), 153.28 (C7), 160.99 (C4’), 163.47 (C2), 181.95 (C4); ESI-
MS: m/z 287 [M+H]+. Anal. calcd for C15H10O6: C, 62.94; H, 3.52;
found: C, 62.92; H, 3.53%.
MHz, DMSO-d6) δ 3.72 (s, 3H, –OCH3), 6.58 (s, 1H, C3H), 6.89 (d,
2H, J = 8.6 Hz, C3′,C5′H), 7.11 (s, 1H, C8H), 7.87 (d, 2H, J = 8.6 Hz,
C2′,C6′H), 10.19 (s, 1H, C4′OH), 10.72 (s, 1H, C7OH), 12.76 (s, 1H,
C5OH); 13C NMR (75 MHz, DMSO-d6) δ 55.48 (C(CH3)), 98.65 (C8),
102.99 (C3), 103.31 (C10), 114.44 (C3’,C5’), 127.74 (C2’,C6’), 128.55
(C1’), 153.05 (C6), 153.46 (C9), 162.24 (C5), 163.07 (C7), 173.10
(C4’), 173.14 (C2), 182.06 (C4); ESI-MS: m/z 299 [M-H]-. Anal. calcd
for C16H12O6: C, 64.00; H, 4.03; found: C, 64.02; H, 4.01%.
This work was supported by the National Natural Science
Foundation of China (No. 81274058, 21302225, 81373511,
81403079), Natural Science Foundation of Jiangsu Province
(BK20131416, BK20141467), the Program for New Century
Excellent Talents by the Ministry of Education (NCET-12-
0741), 333 High-level Talents Training Project Funded by
Jiangsu Province, Six Talents Project Funded by Jiangsu
Province (2013-YY-010), Science and Technology Program of
Jiangsu province (BE2012763), Technology Innovation Venture
Fund by Nanjing University of Chinese Medicine (CX201301),
Jiangsu Collaborative Innovation Center of Chinese Medicinal
Resources Industrialization (ZDXMHT-1-13) and Project
Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
7- (Benzyloxy)-2- (4- (benzyloxy)phenyl)-5,6-dihydroxy-4H-
chromen-4-one (6): Benzyl bromide (0.13 mL, 1.1 mmol, 2.2 equiv.)
and K2CO3 (166 mg, 1.20 mmol, 2.4 equiv.) were added to a stirred
solution of 2 (143 mg, 0.50 mmol) in dry acetone (20 mL), and this
mixture was refluxed for 6 h. The reaction mixture was cooled to
room temperature, and filtered and then concentrated under vacuum.
The crude material was purified by chromatography on silica gel
column using 25% ethyl acetate in petroleum ether afforded 6 (125
1
mg, 53.8% yield) as yellow solid; m.p. 139–141 °C. H NMR (300
MHz, DMSO-d6) δ 5.03 (s, 2H, –OCH2), 5.23 (s, 2H, –OCH2), 6.62
(s, 1H, C3H), 6.87 (s, 1H, C8H), 7.18 (d, 2H, J = 8.6 Hz, C3′,C5′H),
7.31–7.53 (m, 10H, -Ph), 8.04 (d, 2H, J = 8.6 Hz, C2′,C6′H), 10.82 (s,
1H, C6OH), 13.11 (s, 1H, C5OH); 13C NMR (75 MHz, DMSO-d6) δ
73.93 (C(PhCH2)), 75.16 (C(PhCH2)), 98.44 (C8), 102.52 (C3), 103.94
(C10), 116.02 (C3’,C5’), 121.03 (C1’), 127.50 (C6), 127.58 (PhC),
127.62 (PhC), 127.84 (2×PhC), 127.92 (PhC), 127.99 (2×PhC), 128.03
(PhC), 128.04 (2×PhC), 128.29 (2×PhC), 128.34 (C2’,C6’),148.89
(C9), 157.03 (C5), 157.29 (C7), 161.18 (C4’), 163.99 (C2), 181.72 (C4);
ESI-MS: m/z 465 [M-H]-. Anal. calcd for C29H22O6: C, 74.67; H, 4.75;
found: C, 74.68; H, 4.72%.
7-(Benzyloxy)-2-(4-(benzyloxy)phenyl)-5-hydroxy-6-methoxy-
4H-chromen-4-one (7): Iodomethane (0.014 mL, 0.22 mmol, 1.2
equiv.) and K2CO3 (48 mg, 0.35 mmol, 1.4 equiv.) were added to
a stirred solution of 6 (84 mg, 0.18 mmol) in dry DMF (20 mL) at
room temperature. This mixture was allowed to react for 12 h, and
then between 100 mL water and 100 mL ethyl acetate. Then the ethyl
acetate layer was washed with 100 mL brine, dried over Na2SO4,
filtered and concentrated under vacum. The crude material was
purified by chromatography on silica gel column using 25% ethyl
acetate in petroleum ether to afford 7 (81 mg, 94.0% yield) as a yellow
solid; m.p. 196–198 °C. 1H NMR (300 MHz, DMSO-d6) δ 4.00 (s, 3H,
–OCH3), 4.95 (s, 2H, –OCH2), 5.28 (s, 2H, –OCH2), 6.62 (s, 1H, C3H),
6.94 (d, 2H, J = 8.6 Hz, C3′,C5′H), 7.00 (s, 1H, C8H), 7.32–7.42 (m,
6H, -Ph), 7.44–7.54 (m, 4H, -Ph), 7.94 (d, 2H, J = 8.6 Hz, C2′,C6′H),
12.69 (s, 1H, C5OH); 13C NMR (75 MHz, DMSO-d6) δ 55.42 (C(CH3)),
70.41 (C(PhCH2)), 75.15 (C(PhCH2)), 97.02 (C8), 103.19 (C3), 103.97
(C10), 114.39 (C3’,C5’), 122.64 (C1’), 122.84 (C6), 127.57 (PhC),
127.84 (PhC), 127.94 (2×PhC), 128.06 (PhC), 128.14 (2×PhC), 128.30
(PhC), 128.33 (2×PhC), 128.36 (2×PhC), 128.36 (C2’,C6’), 148.91
(C9), 156.47 (C5), 157.36 (C7), 162.28 (C4’), 163.41 (C2), 181.93 (C4);
ESI-MS: m/z 481 [M+H]+. Anal. calcd for C30H24O6: C, 74.99; H, 5.03;
found: C, 75.01; H, 5.04%.
Received 22 September 2015; accepted 9 October 2015
Paper 1503612 doi: 10.3184/174751915X14452625043383
Published online: 1 November 2015
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