First Total Synthesis and Cytotoxicity of Naturally Occurring Lespedezol E1

Full text of DOI:10.1007/s10600-016-1807-0

DOI 10.1007/s10600-016-1807-0
Chemistry of Natural Compounds, Vol. 52, No. 5, September, 2016
FIRST TOTAL SYNTHESIS AND CYTOTOXICITY
OF NATURALLY OCCURRING LESPEDEZOL E
1
1
1
1
1
Hua Sun, Ya Nan Lei, Wang Deng, Hao Meng Wang,
1
1
2
1*
Yu Ou Teng, Hong Ye Zhao, Qing Wei Yao, and Peng Yu
Lespedeza, a genus of the Fabaceae family, is widely distributed in Asia, Australia, and North America. In the last
several decades, many flavonoids and isoflavonoids isolated from Lespedeza species have been reported for a variety of
biological and pharmacological activities, such as hepatoprotective [1], antioxidant [2], antinematode [3], anticoccidial [4],
and antitumor activity [5–7]. Lespedeza E (1), a C -geranylated isoflavonoid, was first isolated from the stem of Lespedeza
1
8
homoloba in 1999 and was shown to exhibit antioxidant activity [2]. Recently, it was also isolated from Ficus benjamina and
evaluated for its inhibitory activity on BACE1 [8]. In 2007, the methylated homologues of 1,8-C-geranyl-7-O-methylbiochanin
A (3) and olibergin B were identified and isolated from the stems of Dalbergia species [9], but their biological activities have
not been further evaluated.
Recent studies have shown that some natural geranylated flavonoid/isoflavonoid possess potent antitumor activities
[10]. The development of convenient and efficient synthetic approaches for the preparation of large quantities of geranylated
flavonoid/isoflavonoid and their analogues would be highly desirable for their biological studies. Here, we report the first total
synthesis of the naturally occurring lespedezol E and its methyl derivatives 5,7,4ꢀ-trimethyl-8-C-geranylisoflavone (2) and 3.
1
In contemplating a general approach for the synthesis of C -geranylated isoflavanone natural products, we envisaged
8
a simple protocol involving the Pd-catalyzed Suzuki coupling of geranyl boronic esters [11] with a C -iodo derivative of
8
genistein. The latter is expected to be readily accessible based on our experience in the regioselective iodination of flavanoids
[12, 13]. Our initial synthesis commenced with the fully methylated genistein 5 (Scheme 1, A). The Suzuki coupling reaction
afforded 2 from regiospecific iodide 6. However, attempted removal of the methyl groups without affecting the geranyl side
chains proved unsuccessful. Thus, treatment of 2 in the presence of BCl at 0ꢁC for 30 min led to the formation of 3 in 85%
3
yield, with demethylation occurring at the C -position in accordance with the literature precedent that BCl selectively
5
3
demethylates the methoxyl group ortho to a carboxyl group [14]. Further demethylation of 3 using BCl or BBr was unsuccessful
3
3
because the geranyl group was unstable under an extended reaction time (> 30 min) and higher reaction temperature (> 0ꢁC).
The all MOM-protected isoflavone 7 was then prepared using MOMCl with the hope that the geranyl side chains would be
preserved under the reaction conditions for the deprotection of the MOM groups. Therefore, geranylated isoflavone 9 was
converted to the naturally occurring 1 in 81% yield (Scheme 1, B).
All these synthetic products and the unsubstituted genistein (4) itself were evaluated for their cytotoxicity in vitro
against tumor cell lines, including K562, HepG2, HT-29, and MGC-803 (Table 1). The preliminary SAR study showed that the
presence of a geranylated side chain on the isoflavonoid structure can enhance the cytotoxic activity of 1 by more than tenfold
for K562, HepG2, and HT-29, and sixfold for MGC-803 when compared to that of 4. A similar conclusion can be drawn when
comparing the trimethylated product 2 with 5. However, the 7,4ꢀ-dimethyl product 3 lost potency.
In summary, we have achieved the first and facile total synthesis of lespedezol E starting from genistein in four steps
1
with 47% overall yield. This approach allows the regioselective introduction of geranyl side chains in the natural isoflavonoid
in good yield. Furthermore, a comparison of the cytotoxic activity of the geranylated isoflavonoid with its mother core showed
that the geranyl is important to improve the antitumor activity in vitro. The synthesis of additional modified 1-analogues to
establish SARs and generate an optimized antitumor lead is in progress.
1) Key Laboratory of Industrial Fermentation Microbiology of Ministry of Education, Tianjin Key Laboratory of
Industry Microbiology, Tianjin University of Science and Technology, 300457, Tianjin, P. R. China, e-mail: yupeng@tust.edu.cn;
2) Sphinx Scientific Laboratory, 1250 East State Street, Sycamore, 60178, Illinois, USA. Published in Khimiya Prirodnykh
Soedinenii, No. 5, September–October, 2016, pp. 764–766. Original article submitted November 13, 2014.
896
0009-3130/16/5205-0896 02016 Springer Science+Business Media New York

TABLE 1. Cytotoxicity in vitro of Lespedezol E and Its Derivatives (IC , ꢃM)*
1
50
Compound
K562
HepG2
HT-29
MGC-803
1
2
3
4
5
13
3.14 ꢄ 0.52
7.78 ꢄ 2.16
47.25 ꢄ 6.78
59.64 ꢄ 6.14
32.65 ꢄ 2.24
52.68 ꢄ 6.85
0.13 ꢄ 0.04
19.58 ꢄ 3.59
25.19 ꢄ 3.39
>100
>100
87.04 ꢄ 0.01
>100
4.45 ꢄ 1.87
25.21 ꢄ 2.16
>100
>100
79.91 ꢄ 4.12
>100
8.30 ꢄ 4.50
16.72 ꢄ 4.27
>100
48.88 ꢄ 9.17
>100
>100
0.58 ꢄ 0.02
Camptothecin**
>10
0.38 ꢄ 0.03
______
*Results are the average of three independent experiments, each performed in duplicate. Standard deviations were below ꢄ10%.
**Reference compound.
R
1
R O
2
O
O
OR
3
OR
4
1 - 5, 13
1 - 3: R =
1
1: R = R = R = H; 2: R = R = R = CH
3
2
3
4
2
3
4
3: R = R = CH , R = H; 4: R = R = R = R = H
2
4
3
3
1
2
3
4
5: R = H, R = R = R = CH ; 13: R = R = H, R = R = CH
1
2
3
4
3
1
3
2
4
3
I
1
O
HO
7
RO
O
RO
O
c
O
O
3
a
b
5
OH
4'
O
OR
O
5, 7
OR
O
6, 8
R
B
OH
OR
OR
4
10''
8''
1''
3''
7''
5''
9''
8
RO
O
O
RO
O
O
d
3
4'
OH
OR
OR
OR
1, 3
1: R = H; 2, 3, 5, 6: R = CH ; 7 – 9: R = MOM
2, 9
3
A: a. CH I, K CO , DMF, 40°C, 4 h, 95%; b. NIS, DMF, 70°C, 4 h, 94%; c. Pd(dppf)4Cl , Cs CO , DMF, 110°C, microwave,
3
2
3
2
2
3
2 h, 66–76 %; d. BCl , DCM, 0°C, 30 min, 85 %. 4ꢅ5ꢅ6ꢅ2ꢅ3
3
B: a. MOMCl, 60% NaH, DMF, 0°C – r.t., 2 h, 87%; b. NIS, DCM, r.t., 2 h, 93%; c. Pd(dppf)4Cl , Cs CO , DMF, 110°C,
2
2
3
microwave, 2 h, 71–75 %; d. 2M4HCl, MeOH/THF (5:1), 0–60°C, 81%. 4ꢅ7ꢅ8ꢅ9ꢅ1
Scheme 1. Synthesis of compounds 2 and 3 (A) and compound 1 (B).
1
Lespedezol E (1). Yellow solid. H NMR (400 MHz, MeOD-d , ꢂ, ppm, J/Hz): 1.52 (3H, s), 1.57 (3H, s), 1.79 (3H, s),
1
4
1.97 (2H, t, J = 7.2), 2.05 (2H, m, J = 7.6), 3.41 (2H, d, J = 7.2), 5.01 (1H, t, J = 6.8), 5.20 (1H, t, J = 6.8), 6.27 (1H, s), 6.84
13
(2H, d, J = 8.8), 7.37 (2H, d, J = 8.4), 8.12 (1H, s). C NMR (100 MHz, MeOD-d , ꢂ, ppm): 14.8, 16.2, 20.7, 24.4, 26.1, 39.3,
4
98.1, 104.8, 106.5, 114.8, 122.0, 122.1, 122.9, 123.9, 130.0, 130.6, 134.5, 153.4, 155.4, 157.3, 160.0, 161.8, 181.2.
+
LR-MS(ESI) m/z 407.1 [M + H] .
1
5,7,4ꢀ-Trimethyl-8-C-geranylisoflavone (2). White solid. H NMR (400 MHz, CDCl , ꢂ, ppm, J/Hz): 1.56 (3H, s),
3
1.62 (3H, s), 1.79 (3H, s), 1.97 (2H, t, J = 8.8), 2.14 (2H, m), 3.45 (2H, d, J = 7.2), 3.82 (3H, s), 3.94 (3H, s), 3.96 (3H, s), 5.05
897

13
(1H, t, J = 6.8), 5.17 (1H, t, J = 6.8), 6.42 (1H, s), 6.93 (2H, d, J = 8.8), 7.49 (2H, d, J = 8.8), 7.82 (1H, s). C NMR (100 MHz,
CDCl , ꢂ, ppm): 16.1, 17.6, 21.6, 25.6, 26.6, 19.7, 55.3, 55.8, 56.4, 91.9, 109.6, 109.9, 113.6, 121.8, 124.2, 124.5, 125.1,
3
+
130.3, 131.3, 135.4, 150.2, 156.5, 159.3, 159.8, 160.8, 176.0. HR-MS(ESI) m/z calcd for C H O [M + H] 449.2323,
28 33
5
Found 449.2323.
1
5-Hydroxyl-7,4ꢀ-dimethyl-8-C-geranylisoflavone (3). Yellow solid. H NMR (400 MHz, CDCl , ꢂ, ppm, J/Hz):
3
1.57 (3H, s), 1.63 (3H, s), 1.79 (3H, s), 1.97 (2H, t, J = 7.2), 2.06 (2H, m), 3.42 (2H, d, J = 6.8), 3.84 (3H, s), 3.90 (3H, s), 5.05
(1H, t, J = 7.2), 5.16 (1H, t, J = 6.2), 6.42 (1H, s), 6.98 (2H, d, J = 8.0), 7.47 (2H, d, J = 8.0), 7.92 (1H, s), 12.94 (1H, s).
13
C NMR (100 MHz, CDCl , ꢂ, ppm): 16.0, 17.6, 21.3, 25.6, 26.6, 39.7, 55.3, 56.0, 95.2 105.7, 107.9, 114.1, 121.8, 123.0,
3
+
123.1, 124.2, 130.1, 131.3, 135.5, 152.8, 154.6, 159.7, 161.0, 162.9, 181.3. LR-MS(ESI) m/z 435.2 [M + H] .
ACKNOWLEDGMENT
We thank the Chinese National Natural Science Foundation (21502138), the International Science & Technology
Cooperation Program of China (No. 2013DFA31160) and the Scientific Research Foundation of Tianjin University of Science
& Technology (No. 20110115).
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