Synthesis of brominated quercetin derivatives using distinct brominating systems

Article abstract of DOI:10.14233/ajchem.2014.16175

6,8-Dibromoquercetin derivatives were obtained through reaction of quercetin with HBr-H₂O₂. On the other hand, quercetin reacted with bromine to synthesize poly brominated derivatives. The reaction procedures were monitored by either

Full text of DOI:10.14233/ajchem.2014.16175

Asian Journal of Chemistry; Vol. 26, No. 15 (2014), 4701-4703
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16175
Synthesis of Brominated Quercetin Derivatives Using Distinct Brominating Systems
1
1
1
2
1,*
MEI PENG , FENGXIAN LIU , XIN FENG , FAN YANG and XIAOPING YANG
1
2
Department of Pharmacy, School of Medicine, Hunan Normal University, Changsha 410013, P.R. China
State Key Laboratory of the Discovery and Development of Novel Pesticide, Shenyang Research Institute of Chemical Industry Co. Ltd.,
Shenyang 110021, P.R. China
Corresponding author: Fax: +86 731 88912417; Tel: +86 731 88912400; E-mail: simonxyoung@yahoo.com
Received: 10 August 2013; Accepted: 17 November 2013; Published online: 16 July 2014;
*
AJC-15554
2 2
6,8-Dibromoquercetin derivatives were obtained through reaction of quercetin with HBr-H O . On the other hand, quercetin reacted with
bromine to synthesize poly brominated derivatives. The reaction procedures were monitored by either TLC or HPLC or both in some
cases. The reaction products were further purified via either recrystallization or silica gel column chromatography. Then the chemical
1
structures were further confirmed by H NMR and LC-MS. Two new quercetin brominated derivatives, namely 6,8-dibromoquercetin and
6,8,2',5',6'-pentabrominated quercetin were obtained. Two distinct brominated approaches were studied to obtain two different quercetin
brominated derivatives, 6,8-dibromoquercetin and 6,8,2',5',6'-pentabrominated quercetin with the yields of 80 and 70 %, respectively.
Keywords: Quercetin, Selectivity, Bromination.
Considering these factors, we have designed two different
brominated systems: one of which had mild reaction conditions
while the other was violent. We expected that the mild condition
will result in low level of hydrogen replacement with mono
brominated derivative while violent condition will produce
high level of hydrogen replacement with poly brominated
product. We tested the role of solvents, the temperature and
time of reaction and developed two effective bromination
methodologies. The structures of the compounds obtained have
been confirmed by LC-MS and NMR spectra.
INTRODUCTION
Quercetin is widely distributed in varying concentrations
1
in vegetables and fruits . It has extensive pharmacological
effects, mainly concentrated on antitumor, antioxidation and
2,3
antiinflammatory activity . In the United States, quercetin
has been approved as a non-prescription drug widely used to
treat prostate cancer (http://baike.baidu.com/view/140548.htm).
So far, people have done much researches to modify the
chemical structure of quercetin for the purpose of obtaining
4
5
more active compounds via etherification , esterification ,
6 7,8
Sydney Mannich reaction and metal complexes of quercetin .
EXPERIMENTAL
However,the synthesis of quercetin brominated derivatives is
9
,10
.
rare to report
6,8-Dibromoquercetin: To a suspension of quercetin
In general, chemical reagents commonly used for the
bromination are hydrobromic acid, bromine and N-bromo
succinimide. For flavonols, the reaction mechanism is that
hydrogens on the benzene rings are replaced by bromine atom
through electrophilic substitution reaction, forming brominate
derivatives. However, the similarity of chemical activity of
various hydrogens on the benzene rings may lead to a number
of different derivatives, which are difficult to isolate and
purified. Therefore, it is challenging to regional selectively
control brominated levels with different number of hydrogen
replacement on benzene ring. On the other hand, the hydroxyl
of flavonols is also prone to cause side effects, resulting in
decrease in yields.
2 2
(1 g) in MeOH (50 mL), 30 % H O (3.5 mL) was added
dropwise at 0-5 °C and then 47 % HBr (1 mL) was added
dropwise for 10 min. After being stirred at 0-5 °C for 3 h, 1.5
mL H
and further stirred for 2 h. The reaction mixture obtained was
poured into H O (300 mL) to quench. The precipitate was
collected by suction, washed with H O and dried. The preci-
2 2
O was dropped and 0.5 mL 47 % HBr was slowly added
2
2
pitate was dissolved in ethyl acetate solvent and then extracted
with ethyl acetate and water (1:2), dried over anhydrous
magnesium sulfate, removed the solvents under rotor vacuum
and then recrystallized from acetone-DMF. This gave 1.2 g of
yellow acicular crystals red-brown solids with mp 250-252 °C.
The yield of 6,8-dibromoquercetin was 80 %.

4
702 Peng et al.
Asian J. Chem.
OH
6
,8,2',5',6'-Pentabrominated quercetin: A solution of
OH
Br
3
g (0.01 mol) of quercetin in 100 mL of glacial acetic acid
OH
OH
O
O
was treated dropwise with 4.3 mL (0.08 mol) of bromine in
0 mL of glacial acetic acid. The reaction mixture was stirred
HO
O
HBr, H O
2 2
OH
OH
3
MeOH, T = 0-5 °C
T = 5 h
Br
OH
at room temperature for 3 h and was then increased to 85 °C
in an oil bath and further stirred for 2 h and evaporated under
vacuum. The residue was washed with distilled water to
eliminate traces of acid and bromine and then extracted with
ethyl acetate, evaporated under reduced pressure. This directly
gave 5 g of red-brown solids with mp 260-262 °C. The yield
of 6,8,2',5',6'-pentabrominated quercetin was 70 %.
OH O
OH
Scheme-I
critical to obtain targeted product with high yield and good
purity. Interestingly, we could not obtain high content of mono
brominated quercetin -even under milder conditions including
decreasing the temperature, lower levels of hydrobromic acid
and shortening the reaction time. This indicates that two
hydrogens at 6- and 8-positions are equivalently active and
not discriminative under these conditions.
HPLC analysis was measured on a Waters e2695-2998.
Melting points were determined on a Buchi (M-565) melting
point apparatus and were uncorrected. NMR spectra were
acquired on a Gemini 300 MHz NMR spectrometer and were
reported relative to tetramethylsilane as an internal standard,
dimethylsulfoxide as solvent. LC-MS of compounds were
recorded by Agilent 1100 Series LC/MSD Trap SL.
The NMR spectra of quercetin were characterized by two
narrow doublets at approximately δ6.20 and δ6.45 for the
6- and 8-hydrogens, respectively. The NMR spectra for this
product at mild conditions displayed neither hydrogen peaks
near δ6.20 nor δ6.45, indicating that the product is 6.8-
dibromoflavonol. Its mass spectra showed that its molecular
mass was 460.7 and had the A, A + 2 pattern in the correct
ratio which is characteristic of fragments containing two
bromine atoms.
The samples dissolved in ethyl acetate was loaded into
the TLC plate and then developed in ethyl acetate :methanol
3
0:1 solvent system. After the solvents reached the top of the
TLC plate, the plate was taken out and dried. The TLC plate
was coloured and then image was taken by Iphone camera.
Lane 1: starting material quercetin, Lane 2: mixture of material
and product, Lane 3: reaction product, lane 4:mixture of
reaction product and crystalline product, lane 5:crystalline
product (Fig. 1).
We have investigated the effects of reaction temperature
and molar ratio of quercetin and HBr on the components of
product (Scheme-I). It is found that the reaction temperature
played a major role in controlling the products. If the reaction
temperature was controlled at 0-5 °C, the product would be
pure 6,8-dibromoquercetin with little by-product showed by
TLC. However, if the reaction temperature was gradually
increased, reaching at 20 °C, the products would be a variety
of different components, including a series of polybrominated
quercetin, which were analyzed by HPLC and were found that
it was difficult to separate the products. The ratio of starting
materials is another critical factor to affect the products. Using
the similar TLC and HPLC monitoring approaches, we studied
the effect of the ratio of quercetin and HBr on distribution of
products. We found that 1:1.5 ratio of quercetin: HBr gave best
final target product 6,8-dibromoquercetin (Table-1). Therefore,
to obtain high quality target product 6,8-dibromoquercetin,
the optimized reaction conditions are reaction temperature:
0
1
-5 °C, ratio of starting materials: quercetin: HBr: H
:1.5:1.5(g/mL/mL)(Table-1). To obtain a better quality of
2 2
O =
products, we investigated the purification processes under
various solvent systems. It turned out that recrystallization
using acetone with a few drops of DMF as a solvent system is
the best.
TABLE-1
REACTION OF QUERCETIN WITH HBR IN THE FOLLOWING
DIFFERENT CONDITIONS AND THE EXPLORATION RESULT
1
2
3
4
5
Entry
Ratio of material
quercetin: 47 % HBr)
Reaction
temperature
2 2
Fig. 1. TLC image of the reaction of quercetin with HBr-H O system
(
1
2
3
4
1 g : 0.7 mL
1 g : 0.7 mL
1 g :1.5 mL
1 g:1.5 mL
0-5 °C
0-5 °C, 20 °C
0-5 °C
RESULTS AND DISCUSSION
0-5 °C, 20 °C
Scheme-I: Hydrobromic acid in an oxidation system had
a marked tendency to lead to 6,8-dibromoflavonols. Contro-
lling the ratio of starting materials and reaction temperature is
Scheme-II: In this design,vigorous reaction conditions
including reaction media, reaction temperature, different ratio

Vol. 26, No. 15 (2014)
Synthesis of Brominated Quercetin Derivatives Using Distinct Brominating Systems 4703
OH
OH
Br
Conclusion
In summary, two efficient bromination methodologies
have been developed for modifying quercetin using HBr and
Br under different chemical conditions. In general, the product
Br
Br
Br2
OH
O
O
O
HO
OH
AcOH, Tmax = 85 °C
T = 5 h
2
OH
Br
OH
OHBr
OH O
OH
yields are high and work-up is straightforward.
Scheme-II
ACKNOWLEDGEMENTS
of starting materials and reaction time were studied. At the
end, we found that glacial acetic acid as solvent, ratio of starting
This work is supported by Initial Xiaoxiang Endowed
Professorship grant provided by Hunan Normal University.
The authors thank Dr. Changling Liu, CTO from Shenyang
Research Institute of Chemical Industry for his constructive
designs and creative discussions.
2
materials: quercetin: Br = 1:8 (molar ratio), Tmax = 85 °C and
5
h reaction time are optimized reaction conditions to obtain
final product 6,8,2′,5′,6′-pentabrominated quercetin. Interes-
tingly, under these conditions, single component, 6,8,2′,5′,6′-
pentabrominated quercetin was obtained with simple purifi-
cation and high yield (Table-2).
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