Synthesis and characterization of novel unnatural di(8-daidzeinyl)methane

Article abstract of DOI:10.1007/s10600-014-0870-7

A novel unnatural biflavonoid analog, di(8-daidzeinyl)methane, was synthesized using the C-C coupling reaction of daidzein and formaldehyde catalyzed by aliphatic amine in DMF, and its structure was characterized by ESI-MS, NMR, and FT-IR. Meanwhile, the effect parameters (catalyst, catalyst loading, temperature, and molar ratio of the substrates) were investigated and the optimized reaction conditions obtained using 9 mol % triethylamine as the catalyst, with the reaction run at 80°C for 20 h.

Full text of DOI:10.1007/s10600-014-0870-7

Chemistry of Natural Compounds, Vol. 50, No. 1, March, 2014 [Russian original No. 1, January–February, 2014]
SYNTHESIS AND CHARACTERIZATION OF NOVEL
UNNATURAL DI(8-DAIDZEINYL)METHANE
Yong-Mei Xiao, Xiao-Liang Zhu, Jin-Wei Yuan,
Liang-Ru Yang, Ying Xue, Pu Mao, and Ling-Bo Qu*
A novel unnatural biflavonoid analog, di(8-daidzeinyl)methane, was synthesized using the C–C coupling
reaction of daidzein and formaldehyde catalyzed by aliphatic amine in DMF, and its structure was
characterized by ESI-MS, NMR, and FT-IR. Meanwhile, the effect parameters (catalyst, catalyst loading,
temperature, and molar ratio of the substrates) were investigated and the optimized reaction conditions
obtained using 9 mol % triethylamine as the catalyst, with the reaction run at 80ꢀC for 20 h.
Keywords: daidzein, flavonoid, coupling reaction, synthesis.
Flavonoids generally refer to a broad group of natural products possessing a C6–C3–C6 carbon framework, or more
specifically a phenylbenzopyran skeleton. Biflavonoids, one of the subclasses of flavonoids, are flavonoid dimers having a C–C
or C–O–C linkage between monomers. Several natural biflavonoids, such as ginkgetin and ochnaflavone isolated from natural
plants, were found to be inhibitors of group II secretory phospholipase A . Moreover, ginkgetin was found to exert inhibitory
2
activity against COX-2-mediated PGE production by inhibiting of COX-2 expression [1, 2]. Recently, some biflavonoids
2
were prepared and found to have cytotoxic, antibacterial, and anticancer activities [3, 4]. Bidaidzeins are very interesting
targets for synthesis since daidzein (4ꢁ,7-dihydroxyisoflavone), which is mainly present in leguminous plants, was previously
found to have antioxidant, antiallergic, anti-inflammatory, antimicrobial, and anticancer activities [5–8]. Therefore, efficient
synthetic pathways for obtaining biflavonoid derivatives of daidzein have potential application value.
The impressive biological activities of biflavonoids are well known, and many of their derivatives are found in
commercial products or in nature. The synthesis of novel biflavonoids has attracted considerable attention due to their increasing
applications in medicinal fields in recent years. Some methods for synthesizing biflavonoids have been developed. Al-Maharik's
group reported the synthesis of C–C linked biflavones by the reaction of 2-bromomethyl-7,4ꢁ-dimethoxyisoflavone with ethyl
cyanoacetate anion or tetraethylammonium cyanide by Pd-catalyzed ethoxycarbonylation [9]. Zheng's group synthesized
gem-diuoromethylenated biflavonoids via Suzuki coupling between 6-iodinated flavones and 3ꢁ-boronate flavone [10]. Sawyer
and Frlan et al. prepared C–O–C linked biflavones via intermolecular Ullmann ether syntheses using 6-bromo flavone analogs
and 7-hydroxyflavone analogs as key intermediates [11, 12]. However, these developments suffer from complicated starting
materials, multistep procedures, metal salts as the catalyst, etc. Zhang's group synthesized a kind of biflavone derivatives,
which included two flavones coupled with the CH group [13, 14]. However, the compounds were obtained by a two-step
2
reaction. The first step (hydroxymethylation) had no regioselectivity when strong base and acid are used as catalysts.
O
OH
OCH
3
OH
H CO
3
O
OH
O
O
OH
HO
O
O
O
OH
OH
O
O
OH
HO
Ginkgetin
Ochnaflavone
College of Chemistry & Chemical Engineering, Henan University of Technology, 450001, Zhengzhou, P. R. China,
e-mail: henangongda@yahoo.com. Published in Khimiya Prirodnykh Soedinenii, No. 1, January–February, 2014, pp. 71–73.
Original article submitted November 11, 2012.
76
0009-3130/14/5001-0076 02014 Springer Science+Business Media New York

1
13
TABLE 1. H and C NMR Data of 1 and 4 (DMSO-d , ꢂ, ppm)
6
C atom
ꢂC 4
ꢂC 1
ꢃꢂ
ꢂH 4
ꢂH 1
ꢃꢂ
2
3
4
5
6
7
8
9
10
1ꢁ
153.1
123.1
175.6
124.8
114.6
160.7
113.8
156.2
117.0
123.4
130.5
115.4
157.5
17.4
153.2
123.0
175.1
127.7
115.6
163.0
102.5
157.9
117.1
123.9
130.5
115.4
157.6
–
–0.1
0.1
0.5
8.31
8.29
0.02
–2.9
–1.0
–2.3
11.3
–1.6
–0.1
–0.5
0.0
7.83
6.88
7.98
6.95
–0.15
–0.07
–
6.87
–
7.37
6.78
7.39
6.83
–0.02
–0.05
2ꢁ, 6ꢁ
3ꢁ, 5ꢁ
0.0
–0.1
–
ꢁ
4
4.28
10.55
9.51
–
–
CH2
10.82
9.56
–0.27
–0.05
7-OH
ꢁ
4 -OH
OH
O
HO
7
9
O
A
C
O
a
1'
HCHO
3'
3
10
+
5
B
5'
HO
HO
O
OH
9
7
O
O
1
2
1'
b
OH
3'
3
10
5
HO
7
9
O
c
OH
5'
4
1'
3'
3
10
5
O
OH
3
5'
a. Triethylamine, 80ꢀC, DMF; b. HCHO, triethylamine; c. 1, triethylamine
Scheme 1
Daidzein is one of the most important flavonoids, while CH bridged bidaidzein has not been reported. Herein, we present a
2
one-pot method for direct synthesis of the novel unnatural biflavonoid analog, di(8-daidzeinyl)methane using daidzein and
formaldehyde catalyzed by aliphatic amine under very mild conditions. This method offers a new, convenient approach to the
synthesis of the unnatural biflavonoid analog in good yield, as shown in Scheme 1. It is especially worth mentioning that this
reaction has high regioselectivity and only di(8-daidzeinyl)methane was obtained.
The biflavonoid analog di(8-daidzeinyl)methane (4) was synthesized by the reaction of daidzein (1) and 37%
1
formaldehyde solution catalyzed by triethylamine in DMF (Scheme 1) and characterized by ESI-MS, FT-IR, H NMR, and
13
–
C NMR. In the mass spectrum, the ion peak at m/z 519 [M – H] indicated the formation of a daidzein dimer, the
–1
biflavonoid analog di(8-daidzeinyl)methane (4). The FT-IR spectrum of compound 4 showed a characteristik peak at 1420 cm ,
which could be attributed to the CH group. Table 1 shows the NMR data of compound 4 and daidzein 1. In the C NMR
13
2
spectrum of compound 4, the chemical shifts of C-8 on theA-ring appeared at 113.8 ppm, shifted 11.3 ppm downfield compared
with that of daidzein, and the signals of C-5, C-7, and C-9 were shifted 2.9, 2.3, 1.6 ppm upfield, respectively, indicating that
the dimer featured a C8-CH -C8 structure skeleton. The chemical shifts for carbons on the B-ring and C-ring do not visibly
2
1
change. In the H NMR spectrum of compound 4, the absence of a singlet at 6.87 ppm and the presence of a signal at 4.28 ppm
confirmed the formation of a C8–CH –C8 linked dimer.
2
77

Factors such as catalyst, temperature, molar ratio of substrates, and reaction time have important effects on the
coupling reactions. We initially conducted a brief catalyst screening by running the reaction of daidzein and formaldehyde
solution (molar ratio1:1) at 80ꢀC for 20 h in the presence of different aliphatic amines, including ethylamine, diethylamine,
propylamine, isopropamide, dipropylamine, n-butylamine, and di(n-butyl)amine. The result showed that the coupling reaction
performed smoothly in the presence of 9 mol% triethylamine, affording the coupling product in high yield with easy purification.
Decreasing the catalyst loading led to lower yields even with prolonged reaction time, and increasing the catalyst loading to
12 mol % or 20 mol% increased the reaction rate but did not improve the yield significantly. This is tentatively attributed to the
rapid formation of a large amount of intermediate 3 (8-hydroxymethyldaidzein), which consumed the starting material and
1
led to insufficient daidzein to form the dimer. The intermediate 3 was separated and characterized (white powder, H NMR
(DMSO-d + D O, ꢂ, ppm): 8.00 (1H, s, H-2), 7.58 (1H, d, J = 8.6 Hz, H-5), 7.33 (2H, d, J = 7.7 Hz, H-2ꢁ, 6ꢁ), 6.78 (2H, d,
6
2
–
J = 7.7 Hz, H-3ꢁ, 5ꢁ), 6.35 (1H, d, J = 8.6 Hz, H-6), 4.69 (2H, s, CH OH). ESI-MS m/z 283 [M – H] . The reaction process was
2
seen to go through the formation of intermediate 3, which was in agreement with the report of Zhang et al. [13] and He et al.
[14].
Running the coupling reaction at different temperatures (0ꢀC, 20, 50, 80, 110, 140, 165ꢀC) proved the reaction was
sensitive to reaction temperature, and 80ꢀC was the optimal one. Temperatures lower than 80ꢀC resulted in slow reaction rates
and low yields, while higher temperatures (ꢄ 80ꢀC) generated complicated side reactions and decomposition of the product.
The effect of the molar ratio of daidzein and 37% formaldehyde solution on the reaction was also investigated, and 1.5:1
proved to be the optimum ratio (yield 82.1%). Initially, increasing the amount of formaldehyde solution improved the yield of
di(8-daidzeinyl)methane, while a molar ratio greater than daidzein–formaldehyde 1.5:1 resulted in low yields of
di(8-daidzeinyl)methane and the major formation of 8-hydroxymethyldaidzein.
EXPERIMENTAL
General Procedures. Daidzein was purchased from the Shanxi Huike Botanical Development Company. All other
materials were of analytical reagent grade purity. FT-IR spectra were recorded using an IR-200 Fourier transform spectrometer
1
13
in KBr pellets. H and C NMR spectra were recorded on a Bruker Avance 400 MHz spectrometer operating at 400 MHz
using DMSO-d as solvent at room temperature (20 ꢅ 2ꢀC). The chemical shifts were expressed in ppm with reference to TMS,
6
and coupling constants (J) in Hz. Mass spectra were obtained on a Bruker Esquire 3000 instrument from ESI-MS measurements
at negative mode using methanol as solvent.
Di(8-daidzeinyl)methane (4). Daidzein (1) (0.5 g, 2.0 mmol) was dissolved in DMF (10 mL), and then triethylamine
(0.8 mL, 0.4 mmol) and 37% formaldehyde solution (2, 0.16 mL, 2.0 mmol) were added. The mixture was heated at 80ꢀC until
reaction completion after about 20 h. After cooling, the solvent was evaporated in vacuo, and a solid residue was obtained.
Recrystallization of the residue from methanol afforded the biflavonoid analog di(8-daidzeinyl)methane (4). White powder,
–1
1
13
FT-IR (KBr, , cm ): 3475 (OH), 1624 (C=O), 1420 (CH ). For H and C NMR spectral data, see Table 1. ESI-MS m/z 519
[M – H] .
2
–
In conclusion, we present here a simple protocol for the synthesis of a novel unnatural biflavonoid analog
di(8-daidzeinyl)methane using the triethylamine-catalyzed coupling reaction of daidzein and formaldehyde solution. Compound 4
was obtained with a yield of 82.1% when running the reaction of daidzein and formaldehyde (molar ratio 1.5:1) using 9 mol %
triethylamine at 80ꢀC for 20 h. Studies on the synthesis of other daidzein dimer derivatives will be reported in due course.
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China (No. 21172055), the Natural Science Foundation of the
Henan Educational Committee (No. 2011B150007, 12A150007), and the Plan for Scientific Innovation Talent (No. 11CXRC10)
and Doctoral Scientific Fund Project of the Henan University of Technology for financial support of this work.
78

REFERENCES
1.
M. Murakami, T. Kambe, S. Shimbara, K. Higashino, K. Hanasaki, H. Arita, M. Horiguchi, M. Arita, H. Hrai,
K. Lnoue, and I. Kudo, J. Biol. Chem., 274, 31435 (1999).
2.
3.
4.
5.
6.
H. Sawada, M. Murakami, A. Enomoto, S. Shimbara, and I. Kudo, Eur. J. Biochem., 263, 826 (1999).
E. V. Brandt, B. C. B. Bezuidenhoudt, and D. G. Roux, J. Chem. Soc., Chem. Commun., 24, 1409 (1982).
M. Deodhar, K. Wood, D. S. C. Black, and N. Kumar, Aust. J. Chem., 65, 1377 (2012).
E. Woo, J. Lee, I. J. Cho, and K. W. Kang, Pharmacol. Res., 51, 539 (2005).
S. S. Kang, J. Y. Lee, Y. K. Choi, S. S. Song, J. S. Kim, S. J. Jeon, Y. N. Han, K. H. Son, and B. H. Han,
Bioorg. Med. Chem. Lett., 15, 3588 (2005).
7.
8.
9.
10.
11.
12.
13.
14.
X. L. Pan, N. H. Tan, G. Z. Zeng, G. Z. Zeng, Y. M. Zhang, and R. R. Jia, Bioorg. Med. Chem., 13, 5819 (2005).
W. K. Liu, F. W. Cheung, B. P. Liu, C. Li, W. Ye, and C. T. Che, J. Nat. Prod., 71, 842 (2008).
N. I. Al-Maharik, S. A. Kaltia, I. Mutikainen, and K. Wahala, J. Org. Chem., 65, 2305 (2000).
X. Zheng, W. D. Meng, and F. L. Qing, Tetrahedron Lett., 45, 8083 (2004).
J. S. Sawyer, Tetrahedron, 56, 5045 (2000).
R. Frlan and D. Kikelj, Synthesis, 14, 2271 (2006).
Z. T. Zhang, R. L. Gao, and S. K. Zhuang, Acta Pharm. Sinica, 44, 873 (2009).
Y. He, L. L. Chen, and Z. T. Zhang, J. Chem. Crystallogr., 38, 27 (2008).
79