Efficient synthesis of biflavones having a ring-A ring of two flavone units using Suzuki cross-coupling reactions

Article abstract of DOI:10.3987/COM-10-S(E)36

Biflavones having a A ring-A ring of two flavone units were easily prepared by using Suzuki cross-coupling reaction of borylated flavones with bromoflavones or flavone-5-triflate in good to excellent yields. The Japan Institute of Heterocyclic Chemistry.

Full text of DOI:10.3987/COM-10-S(E)36

HETEROCYCLES, Vol. 81, No. 8, 2010
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HETEROCYCLES, Vol. 81, No. 8, 2010, pp. 1871 - 1879. © The Japan Institute of Heterocyclic Chemistry
Received, 20th May, 2010, Accepted, 21st June, 2010, Published online, 21st June, 2010
DOI: 10.3987/COM-10-S(E)36
EFFICIENT SYNTHESIS OF BIFLAVONES HAVING A RING-A RING
OF TWO FLAVONE UNITS USING SUZUKI CROSS-COUPLING
REACTIONS
Yoshihito Kohari, Yukio Hoshino, Haruo Matsuyama, and Hiroto Nakano*
Division of Applied Chemistry, College of Environmental Technology, Graduate
School of Muroran Institute of Technology, 27-1 Mizumoto, Muroran, Hokkaido
050-8585, Japan; E-mail: catanaka@mmm.muroran-it.ac.jp
Abstract – Biflavones having aꢀA ring-A ring of two flavone units were easily
prepared by using Suzuki cross-coupling reaction of borylated flavones with
bromoflavones or flavone-5-triflate in good to excellent yields.
INTRODUCTION
Flavonoids belong to a important class of natural compounds and occur naturally in fruits, vegetables,
nuts, seeds, flowers, and barks.1 Natural flavonoids are known to exhibit a wide range of biological
activity such as antioxidant, anti-inflammatory, antiviral etc.,2 and are increasingly being used as dietary
supplement. Many other related compounds have been classified in this group, and new ones continue to
be isolated and identified from various plants.
OH
O
OH
A
OH
B
HO
OH
O
C
B
HO
OH
O
C
A
C
O
HO
OH
HO
O
A
OH
O
C
B
B
A
OH O
C
O
A
HO
OH O
HO
O
A
C
OH
B
O
B
OH O
OH
cupressuflavone
succedaneaflavanone
agathisflavone
Figure 1. Biflavonoids having linkages at A ring-A ring
Biflavonoids form a subclass of flavonoids, of which they are dimers. Unlike the other flavonoids, the
biflavones are distributed in only a limited area in plants. Their major presence is in the gymnosperms.
Most biflavonoids are derived from carbon-carbon linking of two similar flavone units, but mixed dimers

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HETEROCYCLES, Vol. 81, No. 8, 2010
such as flavone-flavanone and flavanone-chalcone are also known. Furthermore, some biflavonoids such
as cupressflavone, succedaneaflavanone and agathisflavone, which are composed of two flavone units
linking at each A ring, have been identified in plants (Figure 1). Some attempts have been made at
constructing a biflavone framework.3 With regard to the biflavone units, however, there has been to our
knowledge no attempt to prepare unsymmetric biflavones with a flavone-flavone unit linked at each A
ring. Thus, our research has focused on the synthesis of biflavones having various patterns of linkage in
the A ring based on Suzuki cross-coupling using a borylated compound. We describe in this paper the
details of a new and efficient synthetic method utilizing Suzuki coupling for biflavones having an A
ring-A ring linkage.
RESULTS AND DISCUSSION
We first examined the reactions of 6-, 7-, and 8-bromoflavone (1a-c)4 with bis(pinacolato)diboron
(pin2B2) in the presence of palladium catalysts to afford borylated compound 2a using a precursor for the
the synthesis of biflavones (Scheme 1).
6
O
O
O
7
8
Pd catalyst
B
Br
solvent, base
80oC, 6h
(pinB)2
O
O
O
1a-c
2a-c
a: 6-Br, b: 7-Br, c: 8-Br
Scheme 1. Borylation reaction of 1
The borylation reaction using pin2B2 was carried out in the presence of Pd catalysts and KOAc in DMSO
at 80 °C under a nitrogen atmosphere.5 First, the effect of palladium catalyst were examined. The results
Table 1. Catalyst effects in the borylation of 1a
Yield of Yield of
2a [%]a) 3aa [%]b)
Run
Pd catalyst
27
98
98
98
Pd(OAc)2
PdCl2(dppf)
1
2
3
4
PdCl2(PPh3)2
PdCl2(PPh3)2+2PPh3
65
98
PdCl2(PPh3)2+2dppf
PdCl2(PPh3)2+2P(o-tol)3
5
6
62
7
8
9
Pd(PPh3)4
Pd(dba)2
Pd2(dba)3
85
83
13
15
a) Isolated yield.
b) Isolated yield.

HETEROCYCLES, Vol. 81, No. 8, 2010
1873
are summarized in Table 1. It can be seen that the Pd-catalysts used were effective in the borylation
reactions, except in the case of Pd(OAc)2 or Pd(PPh3)4 (runs 1 and 7). The reaction using Pd(OAc)2 as a
catalyst progressed slowly under the conditions employed to afford 2a in poor yield (27%, run 1), likely
due to the lack of a factor to reduce Pd(II) to Pd(0) (run 1). Pd(PPh3)4 was also relatively ineffective (run
7), and the yield was moderate (62%), probably due to the formation of phenyl-boronate derived from the
coupling with a phosphine-bounded phenyl group.6 In contrast, a small amount of 6,6’’-biflavone 3aa was
formed as a by-product (13ꢀ, run 9: 15%, run 8) in the reactions using Pd(dba)2 or Pd2(dba)3.
Table 2. Base effects in the borylation reaction of 1a
Yield [%]a)
2a 3aa
Run
Pd catalyst
Base
PdCl2(PPh3)2
Na2CO3
33
62
54
0
1
41
98
98
50
98
K2CO3
KOAc
2
3
4
5
6
"
"
"
NaOAc
Na2CO3
KOAc
0
PdCl2(PPh3)2-2PPh3
43
0
"
a) Isolated yield.
Next, the effects of a base in this borylation reaction were examined. In general, it has been well known
that Pd-catalyzed boron-containing cross-coupling reactions are strongly accelerated by suitable base.5
Thus, the reaction were carried out using several bases with the best catalysts PdCl2(PPh3)2 or
PdCl2(PPh3)2-2PPh3 (Table 2).
The bases such as KOAc or NaOAc afforded 2a in almost quantitative yields (runs 3, 4 and 6). In contrast,
a mixture of dimers 3aa and 2a was obtained by using stronger bases such as K2CO3 or Na2CO3 (runs 1, 2
and 5). Although the reason for this difference is not clear, a strong base might promote the further
reaction of the prepared 2a with 1a to afford biflavone 3aa. The borylation reactions of 7- and
8-bromoflavones 1b,c were also attempted with pin2B2 under the optimized conditions
(PdCl2(PPh3)2+2PPh3, KOAc system). As a result, the reactions using 1b or 1c afforded the desired
borylated compounds 2b,c in moderate to good yields (2b: 82%, 2c: 68%).
We next tried the Suzuki cross-coupling of bromoflavones 1a-c with borylated compounds 2a-c,
respectively, to obtain biflavones 3 (Table 3). The reactions of 2a-c with 1a-c carried out under the
employed conditions (2M-Na2CO3, in benzene, reflux, for 16 h)7 using Pd(PPh3)4 as a catalyst gave 3 in a
wide range of yields. Although the coupling reaction is usually carried out using a slightly excess of
boron compound, we utilized an excess of 1 to prevent the homocoupling of 2a.

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HETEROCYCLES, Vol. 81, No. 8, 2010
Table 3. Cross-coupling reaction of 2 with 1
Entry
Borylated flavone (2)
Bromoflavone (1)
Product (3)
Yield of 3 [%]a)
O
O
O
O
O
6
67
1
2
O
B
O
6"
6
3aa
3ab
O
O
O
Br
Br
2a
1a
O
O
O
7"
O
O
68
4
1b
O
O
Br
O
8"
6
3
4
5
6
3ac
O
O
1c
O
O
O
O
O
B
Br
O
O
O
7"
O
O
O
35
3
7
1b
1c
O
2b
3bb
3bc
8"
Br
Br
O
O
O
O
O
7
O
O
O
O
O
B
O
O
8
4
O
O
8"
O
O
1c
2c
3cc
a) Isolated yield.
The reactions of borylated flavone 2a with bromoflavones 1a-c gave the corresponding biflavones (3a,
3ab, and 3ac) in 68%, 67% and 4% yields, respectively (entries 1-3). In addition, the combinations of 2b
with 1b,c, respectively, afforded the corresponding biflavones 3bb or 3bc. It is clear from these results
that the formation of unsymmetric biflavone is difficult to achieve. This difficulty might be due to the
highly steric factors in the structures of unsymmetric flavones 3ac and 3bc. The cross-coupling reaction
of 2c with 1c was also carried out to prepare the unsymmetric biflavones 8,8”-biflavone 3cc. However,
the formation of this sterically bulkier 3cc was almost not observed (entry 2). We examined the
cross-coupling reactions of 2a-c with flavone-5-triflate 1d under the above-mentioned conditions (Table
4). Triflate 1d was easily prepared from the reaction of 5-hydroxyflavone with Tf2NPh 4 under
microwave conditions according to the procedure by Fitzmaurice et al (Scheme 2).8

HETEROCYCLES, Vol. 81, No. 8, 2010
1875
O
O
Tf2NPh
microwaves
OTf O
1d
OH O
4
Scheme 2. Triflation of 5-hydroxyflavone
As a result, the corresponding unsymmetric biflavones 3ad, 3bd and 3cd were obtained in fairly good
yields in every case (entries 1-3).
Table 4. Cross-coupling reaction of 2 with 1d
Entry
Borylated flavone (2)
Product (3)
Yield of 3 [%]a)
O
O
5
O
O
1
90
O
B
O
O
2a
6"
O
O
3ad
O
B
O
O
O
2
3
93
93
O
7"
O
O
2b
5
3bd
3cd
O
O
O
O
B
O
O
5 8"
O
O
2c
a) Isolated yield
In conclusion, we have developed an efficient synthesis for obtaining unsymmetric and symmetric
bisflavone by the Suzuki cross-coupling of borylated flavones 2a-c with bromoflavones 1a-c or
flavone-5-triflate 1d in good to excellent yields. Further studies to examine the scope and limitations of
our new synthetic methodology for the synthesis of flavonoids are now in progress.
EXPERIMENTAL
Unless otherwise stated, all chemicals and reagents were commercially available grades and were used
without further purification. All reactions were performed under a nitrogen atmosphere and monitored by

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HETEROCYCLES, Vol. 81, No. 8, 2010
thin-layer chromatography (TLC) using silica gel 60 F254 on aluminium pre-coated plates (0.25 mm).
1
13
Column chromatography was performed on silica gel (Wakogel C-200). H NMR and C NMR spectra
were recorded at 270 MHz and 67.8 MHz on a JEOL JNM-EX 270 FT NMR SYSTEM in CDCl3 using
tetramethylsilane as an internal standard.
General procedure for the preparation of borylated flavones (2a-c). The mixture of
bis(pinacolato)diboron (0.55 mmol), PdCl2(PPh3)2 (3 mol%), PPh3 (6 mol%), 1 (0.5 mmol), KOAc (1.5
mmol) which was dried by oven for 1 h, and DMSO (3 mL), were stirred for 6 h at 80 °C under a nitrogen
atmosphere. After the reaction mixture was cooled to room temperature, the products were extracted with
CHCl3. The organic layer was washed with water and brine, followed by dried over anhydrous MgSO4
and filtered. After the filtrate was concentrated, Kugelrohr distillation in vacuo gave flavone boronates
(2).
6-Pinacolatoborylflavone (2a). Colorless needles; mp 158-160 °C; 1H NMR (CDCl3, 270MHz, ppm) ꢀ
1.37 (12H, s), 6.84 (1H, s), 7.52-7.55 (3H, m), 7.56 (1H, d, J=8. 4 Hz), 7.91-7.95 (2H, m), 8.10 (1H, dd,
13
J=8.4, 1.6 Hz), 8.74 (1H, d, J=1.6 Hz); C NMR(CDCl3, 65MHz, ppm): ꢀ 24.9, 84.2, 107.9, 117.4,
123.2, 126.3, 129.1, 131.6, 131.7, 133.3, 139.6, 158.2, 163.2, 178.4.9
1
7-Pinacolatoborylflavone (2b). Colorless needles; mp 172-173 °C; H NMR (CDCl3, 270 MHz, ppm):
ꢀ 1.35 (12H, s), 6.81 (1H, s), 7.47-7.50 (3H, m), 7.78 (1H, d, J=7.6 Hz), 7.85-7.92 (2H, m), 8.01 (1H, s),
8.18 (1H, d, J=7.8 Hz); 13C NMR(CDCl3, 68 MHz, ppm): ꢀ 24.8, 84.5, 107.5, 124.5, 124.6, 125.5,
126.2, 129.0, 130.6, 131.6, 131.6, 155.6, 163.3, 178.5.
1
8-Pinacolatoborylflavone (2c). Colorless needles; mp 178-179 °C; H NMR (CDCl3, 270 MHz, ppm):
ꢀ 1.38 (12H, s), 6.82 (1H, s), 7.35(1H, dd, J=7.7, 7.3 Hz), 7.43-7.46 (3H, m), 8.07 (1H, dd, J=7.3, 1.9
13
Hz), 8.15-8.18 (2H, m), 8.27 (1H, dd, J=7.7, 1.9 Hz); C NMR(CDCl3, 68 MHz, ppm): ꢀ 25.1, 84.1,
106.5, 123.6, 124.7, 126.7, 128.7, 129.0, 131.5, 133.8, 141.6, 160.0, 163.3, 178.9.
General procedure for the preparation of biflavone (3). The mixture of borylated flavone (2) (0.55
mmol), tetrakis(triphenylphosphine)palladium as a catalyst (3 mol%), bromoflavone or flavone-5-triflate
(1) (0.5 mmol), 2M Na2CO3 (1 mL), and benzene (3 mL) were stirred for 16 h at reflux temperture under
a nitrogen atmosphere. After the reaction mixture was cooled to room temperature, the products were
extracted with CHCl3. The organic layer was washed with water and brine, dried over anhydrous MgSO4

HETEROCYCLES, Vol. 81, No. 8, 2010
1877
and filtered. After the filtrate was concentrated, biflavones (3) were isolated by silicagel column
chromatoglaphy.
6,6”-Biflavone (3aa). Colorless needles; mp 312-313 °C (lit.,10 312-313 °C); 1H NMR (CDCl3, 270 MHz,
ppm): ꢀ 6.89 (2H, s) 7.55-7.58 (6H, m), 7.72 (2H, d, J=8.9 Hz), 7.96-7.99 (4H, m), 8.09 (2H, dd, J=8.9,
2.4 Hz), 8.53 (2H, d, J=2.4 Hz); 13C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.7, 119.0, 123.7, 124.2, 126.4,
129.1, 132.0, 131.8, 132.7, 136.5, 156.0, 163.6, 178.3; HRMS (EI): calcd for C30H18O4: 442.1205; found:
442.1207.
1
6,7”-Biflavone (3ab). Colorless needles; mp 236-238 °C; H NMR (CDCl3, 270 MHz, ppm): ꢀ 6.88
(1H, s), 6.90 (1H, s), 7.52-7.59 (6H, m), 7.74(1H, d, J=8.8 Hz), 7.76 (1H, dd, J=8.1, 1.5 Hz), 7.93 (1H, d,
J=1.5 Hz), 7.96-8.00 (4H, m), 8.06 (1H, dd, J=8.8, 2.3 Hz), 8.33 (1H, d, J=8.1 Hz), 8.58 (1H, d, J=2.3
Hz); 13C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.8, 107.8, 116.4, 119.2, 123.1, 124.2, 124.4, 126.3, 126.4,
126.6, 129.2, 131.6, 131.7, 131.9, 132.5, 136.2, 144.9, 156.4, 156.7, 163.7, 178.2; HRMS (EI): calcd for
C30H18O4: 442.1205; found: 442.1207.
1
6,8”-Biflavone (3ac). Colorless needles; mp 292-293 °C; H NMR (CDCl3, 270 MHz, ppm): ꢀ 6.91
(1H, s), 6.93 (1H, s), 7.41-7.62 (6H, m), 7.54 (1H, dd, J =7.9, 7.4 Hz), 7.75 (1H, d, J=8.6 Hz), 7.75-7.81
(2H, m), 7.83 (1H, dd, J=7.4, 1.8 Hz), 7.96-8.03 (2H, m), 8.04 (1H, dd, J=5.8, 3.0 Hz), 8.31 (1H, dd,
J=7.9, 1.7 Hz), 8.59 (1H, d, J=2.0 Hz); 13C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.4, 107.8, 118.3, 124.1,
124.6, 125.4, 125.8, 126.4, 126.8, 129.2, 130.1, 131.6, 131.7, 131.8, 133.4, 134.9, 135.1, 153.1, 156.0,
163.5, 163.7, 178.1, 178.4; HRMS (EI): calcd for C30H18O4: 442.1205; found: 442.1199.
1
7,7”-Biflavone (3bb). Colorless needles; mp 356-358 °C (lit.,10 346 °C); H NMR (CDCl3, 270 MHz,
ppm): ꢀ 6.90 (2H, s), 7.52-7.61 (6H, m), 7.77 (2H, dd, J=8.3, 1.7 Hz), 7.92 (2H, d, J=1.5 Hz),
13
7.95-8.01 (4H, m), 8.37 (2H, d, J=8.3 Hz) ; C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.9, 116.9, 123.7,
124.4, 126.4, 126.4, 129.2, 131.6, 131.8, 144.7, 156.6, 163.8, 178.0; HRMS (EI): calcd for C30H18O4:
442.1205; found: 442.1207.
1
7,8”-Biflavone (3bc). Colorless needles; mp 293-294 °C; H NMR (CDCl3, 270 MHz, ppm): ꢀ 6.94
(2H, s), 7.44-7.60 (5H, m), 7.57 (1H, dd, J=7.5, 7.9 Hz), 7.76-7.80 (2H, m), 7.78 (1H, dd, J=8.3 1.5 Hz),
7.84 (1H, dd, J=7.9, 1.7 Hz), 7.91 (1H, d, J=1.5 Hz), 7.95-7.99 (2H, m), 8.36 (1H, dd, J=7.9, 1.7 Hz),
13
8.40 (1H, d, J=8.3 Hz); C NMR(CDCl3, 68 MHz, ppm:) ꢀ 107.4, 107.9, 119.1, 123.4, 124.6, 125.4,

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HETEROCYCLES, Vol. 81, No. 8, 2010
125.7, 126.2, 126.3, 126.5, 126.9, 129.2, 129.2, 129.9, 131.4, 131.6, 131.9, 134.8, 142.1, 153.0, 156.3,
163.4, 163.7, 178.2; HRMS (EI): calcd for C30H18O4: 442.1205; found: 442.1207.
8,8”-Biflavone (3cc). Colorless needles; mp 289-290 °C (lit.,10 290-291 °C); 1H NMR (CDCl3, 270 MHz,
ppm:) ꢀ 6.85 (2H, s), 7.24-7.30 (4H, m), 7.35-7.41 (6H, m), 7.62 (2H, dd, J=7.9, 7.3 Hz), 7.86 (2H, dd,
13
J=7.3, 1.8 Hz), 8.42 (2H, dd, J=7.9, 1.8 Hz); C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.1, 124.2, 125.1,
125.7, 126.3, 126.4, 129.0, 131.0, 131.6, 135.6, 135.4, 163.2, 178.3; HRMS (EI): calcd for C30H18O4:
442.1205; found: 442.1207.
1
5,6”-Biflavone (3ad). Colorless needles; mp 235-238 °C; H NMR (CDCl3, 270 MHz, ppm): ꢀ 6.69
(1H, s), 6.86 (1H, s), 7.28 (1H, dd, J=7.09, 1.49 Hz), 7.50-7.55 (6H, m), 7.58 (1H, d, J=8.6 Hz), 7.63 (1H,
dd, J=8.4, 1.5 Hz), 7.70 (1H, dd, J=8.4, 7.1 Hz), 7.72 (1H, dd, J=8.6, 2.1 Hz), 7.91-7.97 (4H, m), 8.19
(1H, d, J=2.1 Hz); 13C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.7, 108.6, 116.7, 118.4, 121.1, 123.3, 124.6,
126.1, 126.2, 128.6, 129.0, 131.4, 131.5, 131.5, 131.9, 132.6, 135.6, 138.5, 141.1, 155.6, 157.4, 162.0,
163.3, 178.1, 178.4; HRMS (EI): calcd for C30H18O4: 442.1205; found: 442.1205.
1
5,7”-Biflavone (3bd). Colorless needles; mp 271-273 °C; H NMR (CDCl3, 270 MHz, ppm): ꢀ 6.71
(1H, s), 6.85 (1H, s), 7.25 (1H, dd, J=6.9, 1.6 Hz), 7.38 (1H, dd, J=8.1, 1.5 Hz), 7.49-7.55 (6H, m), 7.56
(1H, d, J=1.5 Hz), 7.67 (1H, dd, J=8.4, 1.7 Hz), 7.73 (1H, dd, J=8.4, 6.9 Hz), 7.90-7.95 (4H, m), 8.24 (1H,
13
d, J=1.5 Hz); C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.8, 108.7, 117.8, 118.8, 121.3, 122.7, 124.6,
126.2, 126.3, 126.5, 128.0, 129.0, 129.1, 131.4, 131.4, 131.7, 132.0, 132.7, 141.0, 147.5, 155.7, 157.3,
162.3, 163.4, 177.9, 178.4; HRMS (EI): calcd for C30H18O4: 442.1205; found: 442.1207.
1
5,8”-Biflavone (3cd). Colorless needles; mp 208-209 °C; H NMR (CDCl3, 270 MHz, ppm): ꢀ 6.61
(1H, s), 6.80 (1H, s), 7.20-7.35 (3H, m), 7.33 (1H, dd, J=6.68, 1.90 Hz), 7.42-7.58 (5H, m), 7.48 (1H, dd,
J=7.9, 7.3 Hz), 7.61 (1H, dd, J=7.3, 1.7 Hz), 7.75 (1H, dd, J=8.5, 1.9 Hz), 7.80 (1H, dd, J=8.5, 6.7 Hz),
7.89-7.93 (2H, m); 13C NMR(CDCl3, 68 MHz, ppm): ꢀ 107.3, 108.5, 119.0, 122.3, 123.4, 124.6, 124.9,
125.9, 126.2, 128.5, 128.8, 129.1, 131.2, 131.3, 131.7, 131.8, 131.9, 133.0, 133.2, 136.3, 153.9, 157.0,
162.3, 162.7, 177.9, 178.7; HRMS (EI): calcd for C30H18O4: 442.1205; found: 442.1207.
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