An improved synthesis of scutellarin-7-O-glucuronid

Article abstract of DOI:10.3184/174751913X13813183192507

An eight-step synthesis of scutellarin-7-O-glucuronide via a novel preparation of the intermediate 5,6,7,4′-trimethoxyflavone from a chalcone derivative is described. The conditions of certain steps were studied and optimised to give an overall yield of 20%.

Full text of DOI:10.3184/174751913X13813183192507

JOURNAL OF CHEMICAL RESEARCH 2013
NOVEMBER, 671–673
RESEARCH PAPER 671
An improved synthesis of scutellarin-7-O-glucuronid
Duo-Zhi Chen, Ting Wu, Zhao Zhao, Xi-Quan Lin, Tao Yang and Jian Yang*
Faculty of Life Science and Technology, Kunming University of Science and Technology, (Chenggong Campus), Kunming 650500, P.R. China
An eight-step synthesis of scutellarin-7-O-glucuronide via a novel preparation of the intermediate 5,6,7,4′-trimethoxyflavone from
a chalcone derivative is described. The conditions of certain steps were studied and optimised to give an overall yield of 20%.
Keywords: scutellarein-7-O-glucuronide, eight-step synthesis, chalcone derivative, 5,6,7,4′-trimethoxyflavone, herbal medicine
Erigeron breviscapus (Vant.) Hand-Mazz is an important
herbal medicine which has been widely used in China.1–3 There
90
are many commercially available preparations of it, such as
dengzhanhua injection (DI), and breviscapine tablet.4 It has
80
been widely used to promote blood circulation, analgesic and
anti-pyretic activity.5 Modern pharmacological studies and
clinical practice have demonstrated that it also possesses
70
various biological activities including neuroprotective action,
60
antibacterial activity, antifungal activity, anti-HIV-1 effects,
and regulatory and preventative effects for early liver injury.6–8
50
Therefore, as the main active component of E. breviscapus.,
scutellarein-7-O-glucuronide (Fig. 1) has attracted much
attention.
40
Compound 1 have several other activities including
0.0
0.1
0.2
0.3
0.4
0.5
protection from acute cerebral ischemia–reperfusion injury,
inhibition of thrombus formation, acceleration the rate of
blood flow of microcirculation and the myocardium protecting
function.9–12 Until now, only two total syntheses of 1 have
been reported. One was reported by Farkas et al. in 1974, used
2,5-dihydroxyl-4,6-dimethoxyacetophenone as the starting
material in eight steps gave 0.6% total yield.13–16 The second
synthesis was by Cui et al. in 2005, starting with 2-hydroxy-
4,5,6-trimethoxyacetophenone, afforded 1 in eight steps with a
total yield of 13%.17
potassium hydroxide (mol)
Fig. 2 Correlations between yields and the dosage of KOH.
obtained from 8 by glycosylation with methyl (tri-O-acetyl-α-
D-glucopyranurosyl-bromide)-uronate which had previously
been prepared by us (Scheme 2).18 Finally, compound 1 was
obtained by the hydrolysis of 9. It is notable that the preparation
of the key intermediate (6) through a synthesis of 4. was the
significant improvement of the synthesis procedure of 1.
The preparations of 4 and 5 were the key steps of the synthesis
and so the reaction conditions were studied and optimised.
Theoretically, the correct amount of KOH was important for
a Claisen–Schmidt condensation. Therefore, we studied the
correlations between yields and the amount of KOH. The
results showed that the reaction gave the highest yield when the
substrate ratio (compound 3: KOH) was 1:2 (Fig. 2).
We now report an effective procedure leading to scutellarein-
7-O-glucuronide (1) which featured a different preparation of
the key intermediate 5,6,7,4′-trimethoxyflavone (6) through the
synthesis of a chalcone derivative.
Results and discussion
As shown in the Scheme 1, 1 has been synthesised from
3,4,5-trimethoxyphenol (2) in eight steps. Firstly, compound
2 was acetylated to give 2-acetyl-3,4,5-trimethoxyphenol
Next, different amounts of I2 were used to investigate the
effect of I2 on the preparation of 5. The results indicated that
when the substrate ratio (compound 4: I2) was 50:1, the yield
reached the highest of n80%. More or less I2 led to a decrease
in the yield (Fig. 3).
(3) by Fries rearrangement. Then,
3 was condensed
with p-methoxybenzaldehyde to afford 4. Thirdly,
5,6,7-trimethoxyflavone (5) was obtained by cyclisation.
Compound 5 was then treated with pyridine hydrochloride for
demethylation and then acetylated to obtain 6 in good yield.
Protection of the C-7 hydroxyl group of compound 6 with
benzyl chloride afforded 7.15–17 Subsequent hydrogenolysis
of the benzyl group furnished compound 8. Compound 9
80
70
60
50
40
30
20
OH
HOOC
OH
OH
OH
O
O
O
O
HO
OH
1
0.000
0.001
0.002
0.003
0.004
0.005
Fig. 1 The structure of scutellarein-7-O-glucuronide (1).
iodine (mol)
* Correspondent. E-mail: yangjiankmust@hotmail.com
Fig. 3 Correlations between yields and the amount of I2.
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672 JOURNAL OF CHEMICAL RESEARCH 2013
OH
O
OH
OCH3
O
H3CO
a
b
H3CO
H3CO
OCH3
OCH3
H
3CO
OH
OCH3
OCH3
OCH3
c
2
3
4
OAc
OCH3
AcO
O
H3CO
H3CO
O
e
d
,
AcO
OAc
O
OCH3 O
OAc
6
5
f
H3COOC
OAc
OAc
Ph
OAc
OAc
OAc
O
O
O
HO
O
O
O
O
g
h
AcO
AcO
AcO
OAc
O
OAc
OAc
O
8
9
7
i
OH
HOOC
OH
OH
OH
O
O
O
HO
OH
O
1
Scheme 1 The synthesis of scutellarein-7-O-glucuronide (1). Reagents and conditions: (a) acetic anhydride and BF3–Et2O, 75 °C, 2 h, 86%;
(b) p-methoxybenzaldehyde, KOH, 50 h, 90%; (c) I2, DMSO, 110 °C, 4 h, 80%; (d) pyridine hydrochloride, 190 °C, 8 h; (e) NaOAc, acetic anhydride,80 °C, 4 h,
77%; (f) K CO3 and benzyl chloride, 60 °C, 8 h, 65%; (g) 10% Pd/C, H2, 50 h, 76%; (h) methyl (tri-O-acetyl-α-D-glucopyranurosyl-bromide)-uronate, CaSO4,
AgO and q2uinoline, 10 h, 94%; (i) NaOH, 1 h, 82%.
HO
HO
HO
O
COOMe
OH
AcO
AcO
O
Br
MeOOC
AcO
O
COOMe
OAc
a
b
c
O
O
OH
OAc
O
OH
OAc
OAc
H
O
methyl (tri-O-acetyl- -D-
α
glucuronolactone
glucopyranurosyl -bromide)-uronate
Scheme 2 The synthesis of methyl (tri-O-acetyl-α-D-glucopyranurosyl -bromide)-uronate. Reagents and conditions: (a) NaOH and CH OH, r.t., 1 h, 80%;
(b) acetic anhydride and pyridine, 10 h, 90%; (c) bromide reagent (prepared from Br2, red phosphorus and acetic acid), CHCl2, 4 h, 90%. 3
temperature. Ethyl acetate (300 mL) was then added and stirred for
a minute. The mixture was kept in the refrigerator for 2 h. Then, it was
filtered to give the filter cake which was poured into a solution of H2O
(400 mL) and ethanolamine (40 mL). The mixture was stirred for 1 h and
then extracted with ethyl acetate (300 mL). The extract was dried with
anhydrous magnesium sulfate overnight. The solvent was evaporated to
obtain compound 3 (97 g, 86%). 1H NMR (CDCl3, 500 MHz): δ 13.43 (s,
1H, –OH), 6.22 (s, 1H, H-3), 3.97 (s, 3H, –OCH3), 3.87 (s, 3H, –OCH3),
3.76 (s, 3H, –OCH3), 2.63 (s, 3H, –COCH3). ESI+MS m/z: 227 [M+H]+.
Conclusion
In conclusion, an efficient and convenient synthesis of
scutellarein-7-O-glucuronide has been reported. This
route featured a different preparation of a key intermediate
5,6,7,4′-trimethoxyflavone (6) through a synthesis of a chalcone
derivative. The conditions of certain reactions were studied and
optimised to raise the yield to 20%.
Experimental
2-Hydroxy-4,5,6,4′-tetramethoxychalcone (4):
A
mixture of
Melting points were measured on a YRT-3 temp apparatus and were
uncorrected. NMR spectra and MS data were recorded on a Bruker
DRX-500 NMR spectrometer and a ZAB-2F mass spectrometer,
respectively. TLC was carried out on silica gel layers (Haiyang
Chemical Co., Ltd, Qingdao, China).
2-Acetyl-4,5,6-trimethoxyphenol (3): A dry round-bottomed flask
containing compound 2 (92 g, 0.5 mol), acetic anhydride (150 mL) and
BF3–Et2O (5 mL) was heated at 75 °C for 2 h, and then cooled to room
compound (22.4 g, 0.1 mol), p-methoxylbenzaldehyde (16.4g,
3
0.12 mol), and methanol (300 mL) were placed into a dry round-
bottomed flask. Potassium hydroxide (11.2 g, 0.2 mol) was slowly
added and the solution was stirred for 50 h at room temperature. Then
dilute hydrochloric acid was added to bring the pH to 6–7 to precipitate
yellow crystals. The reaction mixture was filtered and the cake was
washed by water (200 mL). The solid was recrystallised from ethanol
14
1
to give compound 4 (31 g, 90%); m.p. 134–136 °C (lit. 137 °C)；H
JCR1302055_FINAL.indd 672
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JOURNAL OF CHEMICAL RESEARCH 2013 673
NMR(CDCl3, 400 MHz): δ 13.80 (s, 1H, –OH), 7.86 (d, J=12 Hz, 1H),
7.62 (d, J=8.5 Hz, 2H, H-2′, H-6′), 6.96 (d, J=8.5 Hz, 2H, H-3′, H-5′),
6.89 (d, J=12 Hz, 1H),6.31 (s, 1H, H-3), 3.94 (s, 3H, –OCH3), 3.92 (s,
3H, –OCH3), 3.88 (s, 3H, –OCH3), 3.86 (s, 3H, –OCH3); HREIMS m/z
344.1272 [M]+ (calcd for C19H20O6, 344.1260).
Recrystallisation from C2H5OH gave compound 9 (13.7 g, 94%). m.p.
271–274 °C (lit. 17 270–272 °C); 1H NMR(CDCl3, 400 MHz): δ 7.89 (d,
2H, J=8.6 Hz, H-2′, H-6′), 7.27 (d, 2H, J=8.6 Hz, H-2′, H-6′), 7.04 (s,
1H), 6.59 (s, 1H), 5.29–5.39 (m, 4H), 4.34 (d, 2H, J=8.6 Hz), 3.77 (s, 3H,
–COOCH3), 2.43 (s, 3H, –COCH3), 2.35 (s, 3H, –COCH3), 2.31 (s, 3H,
–COCH3), 2.09～2.06 (m, 9H, 3×–COCH3); ESI+MS m/z: 729 [M+H]+.
Scutellarin-7-O-glucuronide (1): A three-necked round-bottomed
flask equipped with a dropping funnel and thermometer containing
9 (14.5 g, 0.02 mmol) and acetone (200 mL) was cooled to 0 °C with ice
under a N2 atmosphere. A solution of sodium hydroxide (2.5 M, 100 mL)
was added dropwise and the temperature was maintained below 0 °C
for 1 h. Dilute hydrochloric acid was then added dropwise at the same
temperature. The resultant mixture was stirred under 0 °C for 0.5 h, and
then the solvent was evaporated. The yellow precipitate was separated
and dried to give scutellarin-7-O-glucuronide (7.6 g, 82%). m.p. ﹥300 °C
(lit. 16 300 °C); 1H NMR (DMSO, 500 MHz): δ 10.38 (s, 1H, –OH), 8.61
(s, 1H, –OH), 7.92 (d, 2H, J=8.5 Hz, H-2′, H-6′), 6.98 (s, 1H), 6.93 (d,
2H, J=8.5 Hz, H-3′, H-5′), 6.82 (s, 1H), 5.20–5.44 (br, 3H), 5.21 (d, 1H,
J=7.5 Hz), 4.04 (d, 1H, J=9.6 Hz), 3.42–3.37 (m, 4H); HREIMS m/z
462.0790 [M]+ (calcd for C21H18O12, 462.0798).
5,6,7,4′-Trimethoxyflavone (5): A dry round-bottomed flask equipped
with a reflux condenser containing compound 4 (17.2 g, 0.05 mol),
DMSO (50 mL) and iodine (0.5 g, 0.05 mol) was heated at 110 °C for 4 h.
The mixture was then poured into sodium hydrogen sulfite solution
(300 mL, 10%) and left to stand overnight. Then the mixture was filtered
and the cake was recrystallised from ethanol to give compound 5 as light
14
1
yellow crystals (13 g, 80%); m.p. 160–162 °C (lit. 158–160 °C); H
NMR(CDCl3): δ 7.81 (d, J=8.5 Hz, 2H, H-2′, H-6′), 7.01 (d, J=8.5 Hz,
2H, H-3′, H-5′), 6.81 (s, 1H, H-3), 6.59 (s, 1H, H-8), 4.00 (s, 3H,–OCH3),
3.99 (s, 3H,–OCH3), 3.93 (s, 3H, –OCH3), 3.89 (s, 3H, –OCH3); HREIMS
m/z 342.1114 [M]+ (calcd for C19H18O6, 342.1103).
5,6,7,4′-Tetra-O-acetoxyflavone (6): A three-necked round-bottomed
flask equipped with a reflux condenser containing excess pyridine
hydrochloride (57.5 g, 0.5 mol) and compound 5 (17.1 g, 0.05 mol)
under N2 atmosphere was heated at 190 °C and refluxed for 8 h. Then,
the residue was cooled to room temperature and treated with acetic
anhydride (260 mL) and pyridine (180 mL). The mixture was stirred for
4 h at 110–120 °C and ethanol (50 mL) was added and cooled to 5 °C. The
residue was filtered and the cake was recrystallised with EtOAc to give 6
(17.5 g, 77%); 1H NMR(CDCl3, 400 MHz): δ 7.88 (d, 2H, J=8.6 Hz, H-2′,
H-6′), 7.49 (s, 1H), 7.26 (d, 2H, J=8.6 Hz, H-3′, H-5′), 6.62 (s, 1H), 2.44 (s,
3H, –COCH3), 2.38 (s, 3H, –COCH3), 2.35 (s, 3H, –COCH3), 2.34 (s, 3H,
–COCH3); ESI+MS m/z: 455 [M+H]+.
7-Benzyloxy-5,6,4′-triacetoxyflavone (7): A round-bottomed flask
equipped with a reflux condenser containing 6 (22.7 g, 0.05 mol), K2CO3
(20 g, 0.12 mol), KI (2.1 g, 12.6 mmol) and PhCH2Cl (10 mL, 0.08 mol)
in dry acetone (300 mL) was heated at 60 °C for 8 h. It was diluted with
water and extracted with CH2Cl2. The extract was dried and evaporated,
Received 9 July 2013; accepted 23 August 2013
Paper 1302055 doi: 10.3184/174751913X13813183192507
Published online: 11 November 2013
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J=8.5 Hz, H-3′, H-5′), 6.96 (s, 1H), 6.70 (s, 1H), 2.35–2.37 (m, 9H, 3×–
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0.06 mol), AgO (11.2 g, 0.04 mol) and quinoline (160 mL) was stirred at
room temperature for 10 h. It was then diluted with CH2Cl2, and filtered.
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