A selective transformation of flavanones to 3-bromoflavones and flavones under microwave irradiation

Article abstract of DOI:10.1002/adsc.200505223

This paper presents the first report of a highly selective transformation of flavanones to 3-bromoflavones or flavones by microwave irradiation of the corresponding flavanone reactants and N-bromosuccinimide (NBS) in the presence of a catalytic amount of 2,2′-azobis(isobutyronitrile) (AIBN). The combination of good to excellent yields, shorter reaction time (10 min), and high levels of functional group compatibility make this an attractive synthetic approach to 3-bromoflavones and flavones.

Full text of DOI:10.1002/adsc.200505223

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DOI: 10.1002/adsc.200505223
A Selective Transformation of Flavanones to 3-Bromoflavones
and Flavones Under Microwave Irradiation
Zhongzhen Zhou, Peiliang Zhao, Wei Huang, Guangfu Yang*
Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal
University, Wuhan 430079, Peopleꢀs Republic of China
Fax: (þ86)-27-6786-7141, e-mail: gfyang@mail.ccnu.edu.cn
Received: May 31, 2005; Accepted: October 10, 2005
Supporting Information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: This paper presents the first report of a
highly selective transformation of flavanones to 3-
bromoflavones or flavones by microwave irradiation
of the corresponding flavanone reactants and N-bro-
mosuccinimide (NBS) in the presence of a catalytic
amount of 2,2’-azobis(isobutyronitrile) (AIBN).
The combination of good to excellent yields, shorter
reaction time (10 min), and high levels of functional
group compatibility make this an attractive synthetic
approach to 3-bromoflavones and flavones.
a,b-unsaturated carbonyl compounds into the corre-
[
5]
sponding a- or a’-bromo derivatives. Although the de-
hydrogenation of flavanones to form the corresponding
flavones using NBS as a brominating reagent has been
[
4j, k]
reported,
we could find no report of the direct trans-
formation of flavanones into 3-bromoflavone by NBS-
bromination.
Microwave irradiation has emerged as a powerful
technique for promoting a variety of chemical reac-
[
6]
tions. The main benefits of performing reactions under
microwave irradiation conditions are significant rate-
enhancements and higher product yields. As part of an
ongoing program in our laboratory to synthesize a vari-
ety of 3-substituted flavones under mild conditions, we
herein report the first example of a highly selective
method for synthesizing 3-bromoflavones and flavones
via microwave-assisted bromination of the correspond-
Keywords: 3-bromoflavones; N-bromosuccinimide;
flavanones; flavones; microwave irradiation
3-Substituted flavones are of considerable interest be- ing flavanones using NBS.
cause of their widespread occurrence in nature as well As a starting point for the development of our meth-
[
1]
as their biological activities. Many methods have odology, we first examined the dehydrogenation of fla-
been developed for the synthesis of 3-substituted fla- vanones to form the corresponding flavones using
[2]
vones, most of which involve C-3 modification of fla- NBS as a brominating reagent under microwave irradi-
vones by simple chemical transformations via 3-halofla- ation. The reaction of 4’-chloroflavanone with an equi-
[
2c–e]
vones, especially 3-bromoflavones.
Among various molar NBS in CCl was chosen as a model to optimize
4
[
3]
methods reported to date, 3-bromoflavones have the reaction condition. Microwave reactions were per-
been prepared by brominating flavones using Br / formed in sealed heavy-walled Pyrex tubes under con-
2
[3b]
[3c]
AcOH,
2
Bu NBr/PhI(OAc) ,
2,4,4,6-tetrabromo- trolled conditions in a safe and reproducible manner.
,5-cyclohexadienone or pyridinium bromide perbro- Single mode microwave irradiation was used at a fixed
mide/pyridine system. However, most of these meth- temperature, pressure, and irradiation power during
ods are of limited use because they have low yields and the reaction time by automatic power control. The re-
give mixtures of products containing different A-ring sults of optimization experiments are summarized in Ta-
bromination patterns. Furthermore, these procedures ble 1.
4
2
[
3d]
3e]
[
require prolonged reaction times, toxic metal ions, or re-
As shown in Table 1, raising the reaction temperature
agents that are not commercially available. We noticed from 80 to 1008C for the same irradiation time of 10 min
that flavones have been prepared by the dehydrogena- improved the yield of 4’-chloroflavone remarkably from
[
4]
tion of flavanones for many years, but could find no re- 4% to 96%. On further increases of the temperature
port of the synthesis of 3-bromoflavones directly using above 1008C, the high yield was maintained even
flavanones as starting materials. As one of the most use- when the reaction temperature was raised to 1308C.
ful and convenient brominating reagents, N-bromosuc- Moreover, for the reaction at 1008C, prolonging the re-
cinimide (NBS) has been commonly used to transform action time from 10 min to 20 min did not affect the yield
Adv. Synth. Catal. 2006, 348, 63 – 67
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
63

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Zhongzhen Zhou et al.
Table 1. Optimization of the bromination of 4’-chloroflava- the solvent (e.g., CH Cl , CHCl and CCl ) indicated
2
2
3
4
none.
that CCl gave the best results. Thus, the optimal condi-
4
[a]
tions for the synthesis of flavones are 1008C for 10 min
No
Temp. [8C]
Time [min]
Yield [%]
using CCl as solvent. These conditions were used for
4
1
2
3
4
5
6
7
8
9
80
90
10
10
10
10
10
10
5
4
34
96
95
95
95
70
95
96
the transformation of other flavanones bearing various
substituents. The results obtained are shown in Table 2,
which also shows the results of control experiments in
which the reactions were carried out under conventional
heating.
The results in Table 2 indicate that the proposed meth-
odology gives good to excellent yields for a range of sub-
strates. The exceptions to this are 4’-methylflavone and
100
110
120
130
100
100
100
15
20
6
-methylflavone (Table 2, entries 4 and 8), which were
[
a]
The yield was measured by HPLC.
obtained in moderate yield and showed a tendency to
undergo bromination of the methyl group on the phe-
nyl-ring. This latter tendency has been observed previ-
[3]
whereas reducing the reaction time to 5 min caused the ously using other methods for brominating flavanones.
yield to decrease considerably. The increasing demand Furthermore, the reaction under microwave irradiation
for clean and efficient “eco-friendly” chemical synthe- required only 10 min, compared to 12 h for the conven-
ses has increased our interest in solvent-free reactions tional heating technique.
which, when combined with microwave irradiation,
have advantages from both economic and environ- nate flavones to 3-bromoflavones. In the presence of cat-
We were keen to see if it was possible to further bromi-
[
7]
mental standpoints. Hence we attempted the solvent- alytic amounts of 2,2’-azo-bis(isobutyronitrile) (AIBN),
free synthesis of flavones by NBS bromination of flava- refluxing of a mixture of flavone and NBS in a 1:2 molar
nones under microwave irradiation. Unfortunately, the ratio in CCl for 24 hours gave 3-bromoflavone in 50%
4
mixture of NBS and flavanones in a molar ratio of 1:1 yield. This finding prompted us to develop a strategy
did not react at 1008C under microwave irradiation for directly synthesizing 3-bromoflavones from flava-
and, when the temperature was increased to 1408C, nones under microwave irradiation.
the products were so complex that it is very difficult to
Initial optimization studies were carried out using a
obtain the desired flavones. Further optimization of variety of brominating agents (e.g., CuBr , B r/pyridine,
2
2
Table 2. Microwave-assisted dehydrogenation of flavanones with NBS.
1
2
No
R
R
Microwave irradiation
Time [min]
Traditional heating
Time [h]
[a]
[a]
Yield [%]
Yield [%]
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
7-MeO
6-Me
6-Cl
7-MeO
H
10
10
10
10
10
10
10
10
10
10
90 (88)
98 (95)
96 (92)
73 (69)
97 (94)
89 (85)
89 (87)
73 (69)
97 (95)
97 (94)
16
12
12
12
12
12
12
12
12
12
65 (61)
74 (70)
79 (74)
60 (55)
75 (70)
62 (58)
65 (62)
62 (59)
64 (60)
66 (63)
3’-Cl
4’-Cl
4’-Me
3’-Br
4’-MeO
H
H
H
4’-Cl
1
0
[
a]
The yield was measured by HPLC; the value in parentheses is the isolated yield.
64
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Adv. Synth. Catal. 2006, 348, 63 – 67

Transformation of Flavanones to 3-Bromoflavones and Flavones
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Table 3. Microwave-assisted preparation of 3-bromoflavones 3.
1
2
[a]
No.
R
R
Solvent-free Microwave
Conventional Refluxing
[
b]
[b]
Temp. [ 8C]
Time [min]
Yield [%]
Time [h]
Yield [%]
2
3
2
3
1
1
1
1
1
1
1
1
a
b
c
d
e
f
H
H
H
H
H
140
140
140
140
140
140
140
140
10
10
10
10
10
10
10
10
0
7
6
2
6
9
5
9
100 (98)
93 (90)
94 (91)
98 (95)
94 (93)
91 (87)
95 (94)
91 (89)
24
24
24
24
24
24
24
24
10
28
8
30
26
15
18
16
70 (65)
54 (51)
80 (77)
52 (50)
55 (51)
61 (58)
66 (64)
65 (63)
3’-Cl
4’-Cl
3’-Br
4’-MeO
H
H
7-MeO
6-Cl
7-MeO
g
h
H
4’-Cl
[
[
a]
The irradiation power is 150–300 W.
The yield was measured by HPLC; the value in parentheses is the isolated yield.
b]
2,4,4,6-tetroabromo-2,5-cyclohexadienone, pyridinium minated products, 3-bromoflavones, were isolated in ex-
bromide perbromide, NBS/AIBN), the 1:3 (molar ra- cellent to quantitative yields. It was previously reported
tio) flavanones/NBS system was found to be most effec- that bromination of partial acetate esters of flavanones
tive for the transformation of flavanones to 3-bromofla- with NBS or Br /pyridine system in CCl gave two A-
2
4
[
8]
vones. Further optimizations were carried out by vary- ring brominated by-products, namely 6,8-diflavanone
ing the reaction temperature from 1008C to 1608C, the esters and 6,8-dibromoflavone esters. However, we ob-
irradiation time from 10 min to 25 min, and by using var- served no such A-ring brominated by-products in spite
ious solvents (e.g., CH Cl , CHCl , CCl , and solvent- of the fact that we detected dehydrogenated product fla-
2
2
3
4
free). These experiments indicated that the optimized vones (0–9%).
reaction conditions for the clean, rapid and efficient
Thus, by using microwave irradiation, we have devel-
transformation of flavanones were the solvent-free reac- oped an efficient synthesis of flavones under mild condi-
tion of flavanones 1 (1 equiv.) with NBS (3 equivs.) at tions by the bromination of flavanones. In addition, we
the presence of 1 mol % of AIBN. Reaction mixtures explored the synthesis of 3-bromoflavones also starting
were irradiated with microwaves for 10 min at 1408C from flavanones, by adjusting the molar ratio of NBS to
followed by work-up to afford the corresponding 3-bro- flavanones. Microwave irradiation of 1 equiv. of flava-
moflavones 3 in excellent to quantitative yields. This nones 1 and 1 equiv. of NBS for 10 min at 1008C pro-
methodology was found to work with a wide range of fla- duced the flavones 2 in good to excellent yields, and mi-
vanones bearing electron-donating and electron-with- crowave irradiation of a solvent-free mixture of 1 equiv.
drawing substituents. As a control experiment, reactions of flavanones 1 and 3 equivs. of NBS at 1408C for 10 min
were also performed under conditions of conventional afforded the 3-bromoflavones 3 in excellent to quantita-
heating in CCl ; the results are listed in Table 3.
tive yields.
4
As shown in Table 3, it takes 24 hours to obtain 3-bro-
In conclusion, we have presented the first report of a
moflavones in moderate yields under conventional con- selective and fast transformation of flavanones with var-
ditions. Moreover, prolonging the reaction time or add- ious substituents to the corresponding 3-bromoflavones
ing excess brominating reagents under conventional and flavones by using NBS as a brominating reagent un-
conditions did not improve the yield because the fla- der microwave irradiation. This highly selective and ef-
vones 2 formed during the reaction were very difficult ficient protocol involves the use of a cheap, stable, and
to convert completely to the 3-bromoflavones 3. Under safe reagent, and a simple work-up. Moreover, this pro-
microwave irradiation, by contrast, the solvent-free re- tocol outperforms other currently available methods for
action was completed in 10 min and highly selective bro- the synthesis of 3-bromoflavones.
Adv. Synth. Catal. 2006, 348, 63 – 67
ꢁ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
65

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Experimental Section
Zhongzhen Zhou et al.
Preparation of 3-Bromoflavones 3 under Solvent-Free
Microwave Irradiation
Traditional Preparation of Flavones
NBS powder (3 mmol), a catalytic amount of AIBN and flava-
none 1 powder (1 mmol) were mixed and added into a micro-
wave tube. The sealed tube was placed in a Smithsynthesi-
zerTM and irradiated at 1408C for 10 min. The resulting mix-
ture was chromatographed or recrystallized to give 3-bromo-
NBS (1 mmol) and acatalytic amountof AIBN were added to a
solution of flavanone 1 (1 mmol) in anhydrous CCl (5 mL).
The resulting solution was refluxed and monitored by TLC. Af-
ter refluxing for 12 h, the reaction mixture was filtered. The fil-
trate was condensed under reduced pressure and the residue
was chromatographed to give flavone 2 and a small amount
of 3-bromoflavone 3.
4
flavone 3.
1
3
-Bromo-3’-chloroflavone (3b): mp 173–1758C; H NMR
(
400 MHz, CDCl ): d¼7.47–7.55 (m, 4H), 7.72–7.78 (m,
3
2
H), 7.83 (t, J¼1.6 Hz, 1H), 8.29 (dd, J¼8 Hz, 1.6 Hz, 1H);
1
3
C NMR (100 MHz, CDCl ): d¼109.6 (C-3), 117.9 (C-8),
3
1
21.7(C-10), 125.9 and 126.6 (C-5 or C-6’), 127.6 (C-6), 129.3
and 129.7 (C-1’ or C-2’), 131.2 (C-5’), 132.3 (C-4’), 133.8 and
1
34.4 (C-7 or C-3’), 155.5 (C-9), 160.4 (C-2), 172.9 (C¼O); EI-
Preparation of Flavones 2 under Microwave
Irradiation
þ þ
MS: m/z¼336 (M ), 334 [(Mꢀ2) ]; anal. calcd. for
C H BrClO : C 53.69, H, 2.40; found: C 53.86, H 2.44.
1
5
8
2
1
NBS (1 mmol) and acatalytic amountof AIBN were added to a
3,3’-Dibromoflavone (3d): mp 178–1798C; H NMR
solution of flavanone 1 (1 mmol) in anhydrous CCl (5 mL) in a
(400 MHz, CDCl ): d¼7.40–7.53 (m, 4H), 7.68–7.83 (m,
4
3
microwave tube. The sealed tube was placed in a Smithsynthe-
sizerTM and irradiated at 1008C for 10 min. The resulting mix-
ture was filtered. The filtrate was condensed under reduced
pressure and the residue was chromatographed or recrystal-
2H), 7.99 (t, J¼1.8 Hz, 1H), 8.31 (dd, J¼8.2 Hz, 1.8 Hz, 1H);
1
3
C NMR (100 MHz, CDCl ): d¼109. 7 (C-3), 117.9 (C-8),
3
121.7 and 122.3 (C-5 or C-10), 125.8 and 126.4 (C-6’ or C-6),
128.0 (C-2’), 129.9 (C-1’), 131.9 and 132.1 (C-4’ or C-5’), 134.1
and 134.4 (C-3’ or C-7), 155.6 (C-9), 160.3 (C-10), 172.9
lized to give flavone 2.
1
þ
þ
3
’-Chloroflavone (2b): mp 109–1108; H NMR (300 MHz,
(C¼O); EI-MS: m/z¼382 [(Mþ2) ], 380 (M ), 378 [(Mꢀ
þ
CDCl ): d¼6.81 (s, 1H), 7.43–7.48 (m, 2H), 7.51(d, J¼
2) ]; anal. calcd. for C H Br O : C 47.41, H 2.12; found: C
3
15
8
2
2
8
.1 Hz, 1H), 7.60 (d, J¼8.4 Hz, 1H), 7.71–7.74 (m, 1H), 7.79
47.61, H 2.10.
1
3
(
d, J¼7.8 Hz, 1H), 7.94 (s, 1H), 8.25 (d, J¼8.4 Hz, 1H);
C
3-Bromo-4’-chloro-7-methoxyflavone (3h): mp 188–1898C;
1
NMR (100 MHz, CDCl ): d¼108.0 (C-3), 118.0 (C-8), 123.7
H NMR (400 MHz, CDCl ): d¼3.91 (s, 3H), 6.85 (d, J¼
3
3
(
C-10), 124.3 (C-6’), 125.4 and 125.6 (C-6 or C-5), 126.2 (C-
2.0 Hz, 1H), 7.03 (dd, J¼9.0 Hz, 2.6 Hz, 1H), 7.50 (dd, J¼
6.8 Hz, 2 Hz, 2H), 7.80 (dd, J¼6.6 Hz, 1.8 Hz, 2H), 8.19 (d,
2
1
’), 130.3 (C-5’), 131.4 (C-4’), 133.4 and 133.9 (C-1’ or C-7’),
35.1 (C-Cl), 156.0 (C-9), 161.5 (C-2), 178.2 (C¼O); anal. calcd.
1
3
J¼8.8 Hz, 1H); C NMR (100 MHz, CDCl ): d¼55.9(CH -
3
3
for C H ClO : C 70.19, H 3.53; found: C 69.99, H 3.42.
O), 99.8 (C-8), 109.5 (C-3), 115.3 and 115.5 (C-6 or C-10),
127.9 (C-5), 128.6 (C-2’ and C-6’), 130.6 (C-3’ and C-5’), 131.2
(C-1’), 137.2 (C-4’), 157.3 (C-9), 160.2 (C-2), 164.5 (C-7),
1
5
9
2
1
3
’-Bromoflavone (2d): mp 114–1158C; H NMR (300 MHz,
CDCl ): d¼6.80 (s, 1H), 7.40–7.45 (m, 2H), 7.58 (d, J¼8.4 Hz,
3
þ
þ
1
1
H), 7.68 (d, J¼7.2 Hz, 1H), 7.72 (m, 1H), 7.85 (d, J¼7.8 Hz,
172.2 (C¼O); EI-MS: m/z¼366 (M ), 364 [(Mꢀ2) ]; anal.
1
3
H), 8.09 (s, 1H), 8.24 (d, J¼6.6 Hz, 1H); C NMR
calcd. for C H BrClO : C 52.56, H 2.76; found: C 52.36, H
1
6
10
3
(
100 MHz, CDCl ): d¼108.0 (C-3), 118.1 (C-8), 123.1 and
2.59.
3
1
5
7
23.7 (C-10 or C-6’), 124.80 (C-6), 125.4 and 125.6 (C-2’ or C-
), 129.1 (C-5’), 130.5 (C-4’), 133.6 (C-1’), 134.0 and 134.4 (C-
or C-3’), 156.0 (C-9), 161.6 (C-2), 178.2 (C¼O); anal. calcd.
Acknowledgements
for C H BrO : C 60.11, H 2.90; found: C 59.83, H 3.01.
15
9
2
1
The present work was supported by the National Key Project for
Basic Research (2002CCA00500, 2003CB114400), National
Natural Science Foundation of China (No. 20432010,
7
-Methoxy-4’-chloroflavone (2h): mp 159–1618C; H NMR
(
2
(
3
6
1
300 MHz, CDCl ): d¼3.99 (s, 3H), 6.78 (s, 1H), 7.00–7.10 (m,
3
1
3
H), 7.45–7.90 (m, 4H), 8.18 (d, J¼8.1 Hz, 1H); C NMR
2
0476036, and 20172017), Program for New Century Excellent
100 MHz, CDCl ): d¼55.8 (CH -O), 99.8 (C-8), 107.7 ( C-
3
3
Talents in University of China and Program for Excellent Re-
search Group of Hubei Province (No. 2004ABC002).
), 114.4 (C-6), 117.5 (C-10), 126.8 (C-5), 129.0 (C-2’ and C-
’), 131.2 and 131.6 [(C-3’ and C-5’) or C-1’), 134.7 (C-4’),
57.9 (C-9), 162.8 (C-2), 164.4 (C-7), 178.8 (C¼O); anal. calcd.
for C H ClO : C 67.03, H 3.87; found: C 67.34, H 3.56.
16
11
3
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NBS (3 mmol) and acatalytic amountof AIBN were added to a
solution of flavanone 1 (1 mmol) in anhydrous CCl (5 mL).
4
The resulting solution was refluxed and monitored by TLC. Af-
ter refluxing for 24 h, the reaction mixture was filtered. The fil-
trate was condensed under reduced pressure and the residue
was chromatographed to give 3-bromoflavone 3 and a small
amount of flavone 2.
3
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Adv. Synth. Catal. 2006, 348, 63 – 67

Transformation of Flavanones to 3-Bromoflavones and Flavones
COMMUNICATIONS
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