Total synthesis of plagiochin G and derivatives as potential cancer chemopreventive agents

Article abstract of DOI:10.1016/j.tetlet.2014.10.038

A new and efficient total synthesis has been developed to obtain plagiochin G (22), a macrocyclic bisbibenzyl, and four derivatives. The key 16-membered ring containing biphenyl ether and biaryl units was closed via an intramolecular S_NAr reaction. All synthesized macrocyclic bisbibenzyls inhibited Epstein-Barr virus early antigen (EBV-EA) activation induced by the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) in Raji cells and, thus, are potential cancer chemopreventive agents.

Full text of DOI:10.1016/j.tetlet.2014.10.038

Tetrahedron Letters 55 (2014) 6500–6503
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Total synthesis of plagiochin G and derivatives as potential cancer
chemopreventive agents
Rui-Juan Li a,b, Yu Zhao , Harukuni Tokuda , Xiao-Ming Yang , Yue-Hu Wang , Qian Shi ,
b
c
b
b
b
b
a,
⇑
b,d,
⇑
Susan L. Morris-Natschke , Hong-Xiang Lou , Kuo-Hsiung Lee
a
Department of Natural Products Chemistry, Key Lab of Chemical Biology of Ministry of Education, School of Pharmaceutical Science, Shandong University, Jinan 250012, PR China
Natural Products Research Laboratories, UNC Eshelman School of Pharmacy, University of North Carolina, Chapel Hill, NC 27599, United States
Department of Biochemistry, Kyoto Prefectural University of Medicine, Kamigyo-ku, Kyoto 602-0841, Japan
b
c
d
Chinese Medicine Research and Development Center, China Medical University and Hospital, Taichung, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
A new and efficient total synthesis has been developed to obtain plagiochin G (22), a macrocyclic
Received 11 June 2014
Revised 6 October 2014
Accepted 7 October 2014
Available online 13 October 2014
bisbibenzyl, and four derivatives. The key 16-membered ring containing biphenyl ether and biaryl units
was closed via an intramolecular S Ar reaction. All synthesized macrocyclic bisbibenzyls inhibited
N
Epstein–Barr virus early antigen (EBV-EA) activation induced by the tumor promoter 12-O-tetradecanoyl-
phorbol-13-acetate (TPA) in Raji cells and, thus, are potential cancer chemopreventive agents.
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Bisbibenzyls
Plagiochin G
Intramolecular S Ar reaction
N
Cancer chemopreventive agents
Epstein–Barr virus early antigen (EBV-EA)
Cancer, the second leading cause of death in humans, is a group
of illnesses resulting from abnormal growth of cells in the body.
Many cancer therapies, especially various anticancer agents, have
been developed since the beginning of the last century. However,
several problems, such as adverse side effects and drug resistance,
have also encouraged scientists to explore strategies to prevent
premalignant cells from completing the process of carcinogenesis.
This concept known as ‘cancer chemoprevention’ has been devel-
includes natural plagiochins A–D isolated from the liverwort
1
0
Plagiochila acanthophylla by Hashimoto et al. and plagiochins
1
1
E–H synthesized by Speicher et al.
The unusual structures and intriguing biological activities of
macrocyclic bisbibenzyls have made them attractive synthetic
1
2
targets. In 1992, Keseru et al. synthesized plagiochins C and D
by using a Wurtz-type coupling at position a to close the
16-membered ring (Scheme 1). In 1999, Fukuyama et al.1
3,14
used
1
oped over the past few decades, and the Epstein–Barr virus early
an intramolecular Still-Kelly reaction at position c to accomplish
the macrocyclization in the syntheses of plagiochins A and D.
‘Plagiochin E’, initially reported as a natural product from the liver-
antigen (EBV-EA) activation assay has been established to quickly
evaluate chemopreventive activity in vitro.2
–4
1
5
Macrocyclic bisbibenzyls are phenolic natural products that
occur mainly in liverworts and exhibit remarkable biological activ-
ities, such as 5-lipoxygenase, cyclooxygenase, and calmodulin
inhibitory effects, as well as antifungal, anti-HIV, antimicrobial,
wort Marchantia polymorpha, was totally synthesized in 2009 by
Speicher et al., who revised the structure of the isolated ‘plagiochin
1
6,17
E’ to that of riccardin D.
Subsequently, in 2010, Speicher et al.
reported the syntheses of plagiochins E–H by employing an intra-
5
–7
and cytotoxic activities.
These compounds are divided into four
molecular McMurry reaction at position a as the key macrocycliza-
distinct structural types (I–IV, Fig. 1),8 each containing four aro-
matic rings (labeled A–D) and two ethylene bridges, and originate
biosynthetically from bibenzyl lunularin or its precursor lunularin
tion step (Scheme 1). In 2011, Cortes Morales et al. used an
intramolecular S Ar reaction at position d to form the 16-mem-
11
18
N
bered macrocyclic ring of plagiochin D (Scheme 1). Recently, Jiang
et al. prepared plagiochin E using an intramolecular McMurry
reaction at position a for the macrocyclization (Scheme 1).19
In the course of our ongoing efforts to find bioactive macrocy-
clic bisbibenzyls, we developed a new route to synthesize plagio-
chin G and several ester derivatives. We initially synthesized two
7
,9
acid. The plagiochin family of type II macrocyclic bisbibenzyls
⇑
Corresponding authors. Tel.: +86 531 88382012; fax: +86 531 88382019
(
H.-X. L.); tel.: +1 (919) 962 0066; fax: +1 (919) 966 3893 (K.-H.L.).
E-mail addresses: louhongxiang@sdu.edu.cn (H.-X. Lou), khlee@unc.edu
K.-H. Lee).
(
http://dx.doi.org/10.1016/j.tetlet.2014.10.038
040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
0

R.-J. Li et al. / Tetrahedron Letters 55 (2014) 6500–6503
6501
OH
B Suzuki-Miyaura
R
4
HO
3
5
5'
4'
A Wittig,
D
2
3'
hydrogenation
6
B
OH
A Wittig,
hydrogenation
6
'
1
1
'
2'
7
8
9
7
'
14'
8
'
1
4
10 13'
HO
Lunularin
9'
C
A
1
3
11 12'
10'
R = H
12
O
11'
Lunularin acid R = COOH
OH
C etherification (SNAr)
Plagiochin G (22)
Figure 2. Construction unit system for the synthesis of plagiochin G.
HO
HO
B
D
HO
B
OH
D
A
C
A
reduced to a benzyl alcohol with NaBH
pound (3) was treated with PPh
ÁHBr to give the phosphonium salt
, and the resulting com-
4
C
O
OH
OH
3
OH
4.
I
II
Ring C was developed from commercially available 3-hydroxy-
4-methoxybenzaldehyde (5). Protection of the phenol moiety with
O
O
B
2
1
D
chloromethyl methyl ether yielded compound 6. Units 4 and 6
were coupled with a Wittig reaction in the presence of K CO
and 18-crown-6 to give the D–C segment 7 (obtained as an E/Z
B
OH
HO
D
2
3
2
2
mixture in a ratio 1:1), which was hydrogenated over Pd/C to
give bibenzyl 8. The deprotected hydroxy group in 8 was then con-
verted to the corresponding triflate in 9, which underwent a
A
C
A
C
O
OH
OH
OH
PdCl
2
(dppf) mediated conversion to the pinacolboronate ester 10,
III
IV
2
3
produced in 62% overall yield from 5.
Figure 1. Four distinct structural types of macrocyclic bisbibenzyls.
The synthesis of the second bibenzyl sub-unit (16) began with
commercially available 4-fluorobenzaldehyde (11), corresponding
to ring A. Compound 11 was nitrated to yield compound 12, which
was linked with phosphonium salt 4 by using a Wittig reaction to
give A–B segment 13 (obtained as an E/Z mixture in a ratio 7:1).22
The double bond of each isomer was reduced with Wilkinson’s
catalyst to afford a single compound 14; the benzyl ether was
MeO
OMe
HO
c
D
B
D
O
B
TfO
B
OH
a
O
b
+
24
retained under the mild hydrogenation conditions. Subsequently,
C
A
O-debenzylation was accomplished using concentrated HCl in
C
A
OMOM
MeO
NO2
18
O
HOAc to yield compound 15, and the free hydroxyl group was
d
OMe
F
10
23
OH
then converted to a triflate in 16.
16
Plagiochin G (22)
The triflate 16 and the pinacolboronate ester 10 were combined
via an aryl-aryl bond between rings B and D by using the
Suzuki–Miyaura coupling reaction to afford 17, followed by
deprotection of the phenol methoxymethyl ether using p-toluene-
sulfonic acid. Macrocyclization to give 19 was achieved in 89%
CHO
CHO
D/B
C
A
OBn
OMOM
NO2
N 2 3
yield through an intramolecular S Ar reaction using K CO in
OMe
PPh3Br
F
6
4
12
DMF at room temperature. Next, the nitro group was removed in
a two-step sequence of reduction and deamination18 to give
plagiochin trimethyl ether (21). Plagiochin G (22) was finally
Scheme 1. Retrosynthetic analysis of plagiochin G.
1
1,26
obtained after cleavage of the methyl ethers.
The acetyl ester derivative 22a was prepared by reaction of 22
with acetyl chloride and Et N in CH Cl with DMAP as a catalyst.
lunularin precursors (10 and 16), then formed the arylAaryl bond
between rings B and D by using a Pd-catalyzed Suzuki–Miyaura
3
2
2
coupling reaction, and finally applied an intramolecular S
N
Ar reac-
tion at position d to achieve the key 16-membered ring closure
Scheme 1 and Figure 2). Furthermore, we found that all macrocy-
Derivatives 22b–22d were synthesized from 22 by esterification
with 3-(1H-imidazol-1-yl)propanoic acid, 3-(1H-1,2,4-triazol-1-
yl)propanoic acid, and 3-(1H-tetrazol-1-yl)propanoic acid, respec-
tively (Scheme 3).
2
0
(
clic bisbibenzyls produced by this new route exhibited potential
cancer chemopreventive activity. Herein, we report the synthetic
details for producing plagiochin G (22) and its derivatives (19–21
and 22a–22d) as well as the evaluation of their cancer chemopre-
ventive activity.
Biological evaluation
To evaluate the cancer chemoprevention effects of compounds
(
19–22 and 22a–22d) in vitro, we assayed the eight compounds
2
7
Chemistry
for inhibition of EBV-EA activation. Glycyrrhetic acid was used
as a positive control. In this assay, all tested compounds showed
inhibitory effects toward EBV-EA activation without cytotoxicity
to Raji cells. As shown in Table 1, plagiochin G (22) exhibited the
The total synthesis of plagiochin G was achieved in 12.5%
overall yield in 14 steps as shown in Scheme 2. Both rings B and
D were produced from the commercially available 2-hydroxy-5-
methoxybenzaldehyde (1). The phenol moiety of 1 was protected
by reaction with benzyl bromide. Then, the aldehyde moiety was
3
highest potency with 88%, 45%, and 19% inhibition at 1 Â 10 ,
2
2
5 Â 10 , 1 Â 10 mol ratio/TPA, respectively, and IC50 value of
481 lM, with highly preserved viability of Raji cells. The four ester

6
502
R.-J. Li et al. / Tetrahedron Letters 55 (2014) 6500–6503
MeO
MeO
MeO
MeO
a
b
D/ B
c
D/B
D/B
D/ B
OBn
OBn
OH
OBn
CHO
CHO
PPh Br
OH
3
1
2
3
4
MeO
MeO
MeO
MeO
D
D
D
D
O
CHO
CHO
+ 4
e
OBn
OH
OTf
h
B
O
d
f
g
C
C
OH
OMOM
C
C
C
C
OMe
5
OMe
6
OMOM
OMOM
OMOM
OMOM
OMe
OMe
OMe
OMe
7
8
9
10
OMe
OMe
k
OMe
OMe
B
B
B
B
CHO
CHO
BnO
BnO
HO
TfO
+
4
e
i
j
g
A
A
NO2
A
A
A
A
F
F
12
1
1
NO2
NO2
NO2
NO2
F
F
F
F
1
3
14
15
16
MeO
MeO
MeO
D
D
D
B
OMe
B
OMe
B
OMe
l
m
n
10
+
16
C
A
C
A
C
A
NO2
NO2
OMOM
OH
O
OMe
F
OMe
F
OMe
NO2
1
7
18
19
MeO
MeO
HO
D
D
D
B
OMe
p
B
OMe
q
B
OH
o
C
A
C
A
C
A
O
O
O
OMe
NH2
OMe
OH
2
0
21
22
Scheme 2. Synthesis of plagiochin G. Reagents and conditions: (a) BnBr, K
d) chloromethyl methyl ether, diisopropylethylamine, CH Cl
CH Cl , 0 °C, trifluoromethanesulfonic anhydride, then rt, 0.5 h; (h) PdCl
catalyst, H , THF–t-BuOH (1:1), rt, 24 h; (k) conc. HCl, HOAc, 70 °C, 6 h; (l) EtOH, Na
n) K CO , DMF, rt, 24 h; (o) Pd/C (10%), 1 bar H
À78 °C, then rt, 1 h.
2
CO
3
, acetone, reflux; (b) NaBH
CO , 18-crown-6, CH
N, pinacolborane, dioxane, reflux; (i) H
CO (2 M), Pd(PPh , toluene, reflux, 18 h; (m) p-toluenesulfonic acid, MeOH, 40 °C, 4 h;
(1.2 M in H O), NaHSO (1.0 M in H
4
, MeOH, 0 °C, 20 min, then rt, 1 h; (c) PPh
Cl , reflux, 8 h; (f) Pd/C (10%), 1 bar H , MeOH, rt, 24 h; (g) Et
SO , HNO
, À5 °C, then rt 1 h; (j) Wilkinson’s
3
ÁHBr, MeCN, reflux, 1 h;
(
2
2
, 0 °C, then rt, 6 h; (e) K
(dppf), Et
2
3
2
2
2
3
N,
2
2
2
3
2
4
3
2
2
3
3 4
)
(
2
3
2
, THF, rt, 4 h; (p) NaNO
3
2
3
2
O), EtOH–HOAc (5:4), rt, 3 h; (q) BBr
3
(1 M in CH
2
Cl
2
), CH
2
Cl
2
,
O
2
2
2
2
2a R1 = R2 = R3
=
HO
R O
1
N
N
D
D
O
O
O
N
N
N
B
OH
B
OR2
2b R1 = R2 = R3 =
2c R1 = R2 = R3 =
2d R1 = R2 = R3 =
C
A
C
A
N
N
O
O
N
N
OH
OR3
22
Scheme 3. Syntheses of ester derivatives of plagiochin G. Reagents and conditions: (22a) acetyl chloride, Et
yl)propanoic acid/3-(1H-1,2,4-triazol-1-yl)propanoic acid/3-(1H-tetrazol-1-yl)propanoic acid, EDCI, DMAP, CH
3
N, DMAP, CH
Cl , rt 4 h.
2
2
Cl
2
, rt, 1 h; (22b/22c/22d) 3-(1H-imidazol-1-
2

R.-J. Li et al. / Tetrahedron Letters 55 (2014) 6500–6503
6503
M)
Table 1
Relative ratioa of EBV-EA activation with respect to positive control (100%) in the presence of 22 and related compounds
Compound
Compound concentration (mol ratio/TPAb)
IC50c
(l
1
000
500
100
10
1
2
2
2
2
2
2
2
9
0
1
2
2a
2b
2c
2d
14.9 ± 0.5 (70)d
15.3 ± 0.4 (70)
13.0 ± 0.5 (70)
11.5 ± 0.6 (70)
13.9 ± 0.5 (70)
13.0 ± 0.4 (60)
13.8 ± 0.5 (60)
14.0 ± 0.5 (60)
7.4 ± 0.5 (60)
58.2 ± 0.7
59.6 ± 0.6
56.8 ± 0.5
54.3 ± 0.6
57.9 ± 0.5
55.4 ± 1.5
56.0 ± 1.6
57.6 ± 1.4
35.7 ± 0.8
83.1 ± 2.4
84.6 ± 2.3
81.1 ± 2.5
80.1 ± 2.3
82.4 ± 2.5
79.1 ± 2.3
80.0 ± 2.5
81.6 ± 2.4
83.2 ± 2.0
100 ± 0.5
100 ± 0.4
100 ± 0.5
100 ± 0.5
100 ± 0.6
100 ± 0.5
100 ± 0.3
100 ± 0.5
100 ± 0.3
491
500
490
481
495
479
482
488
413
e
Glycyrrhetic acid
a
b
c
Values represent percentages relative to the positive control value (100%).
TPA concentration is 20 ng/mL (32 pmol/mL).
The molar ratio of compound, relative to TPA, required to inhibit 50% of the positive control activated with 32 pmol TPA.
Values in parentheses are viability percentages of Raji cells. In all other experiments, viability was >80%.
Positive control.
d
e
derivatives (22a–22d) showed similar inhibitory effects, while the
three synthetic precursors (19–21) of 22 were comparably or
slightly less potent.
7. Asakawa, Y. Chemical Constituents of the Bryophytes; Springer, 1995.
8
9
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Harrowven, D. C.; Kostiuk, S. L. Nat. Prod. Rep. 2012, 29, 223.
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1
1
1
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Tetrahedron 1992, 48, 913.
6
Conclusions
6
In this study, a new and efficient total synthesis of 22 and four
ester derivatives (22a–22d) was successfully accomplished in
3. Fukuyama, Y.; Yaso, H.; Mori, T.; Takahashi, H.; Minami, H.; Kodama, M.
Heterocycles 2001, 54, 259.
1
2–16 steps. An intramolecular S
N
Ar reaction was used for the
14. Fukuyama, Y.; Yaso, H.; Nakamura, K.; Kodama, M. Tetrahedron Lett. 1999, 40,
formation of the 16-membered ring. All tested synthetic macrocy-
clic bisbibenzyls exhibited potential cancer chemopreventive
activity as evaluated by an EBV-EA activation assay. To the best
of our knowledge, this is the first report of macrocyclic bisbibenz-
yls with cancer chemopreventive activity. The new synthetic route
reported herein is an additional effective strategy to construct
variously substituted macrocyclic bisbibenzyls and should greatly
facilitate our further synthesis and SAR study of cancer-preventa-
tive derivatives of 22.
105.
1
1
5. Niu, C.; Qu, J.-B.; Lou, H.-X. Chem. Biodivers. 2006, 3, 34.
6. Speicher, A.; Groh, M.; Zapp, J.; Schaumlöffel, A.; Knauer, M.; Bringmann, G.
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011, 3165.
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Acknowledgments
2
2
4. Jourdant, A.; González-Zamora, E.; Zhu, J. J. Org. Chem. 2002, 67, 3163.
5. Konoshima, T.; Konishi, T.; Takasaki, M.; Yamazoe, K.; Tokuda, H. Biol. Pharm.
Bull. 2001, 24, 1440.
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Chem. 2012, 2012, 6878.
This work was financially supported by the National Natural
Science Foundation of China (No. 81172956) and by NIH grant
CA177584 from the National Cancer Institute awarded to K.H. Lee.
2
2
7. Inhibition of EBV-EA activation assay. Inhibition of EBV-EA activation was
assayed using Raji cells (virus nonproducer type), an EBV genome-carrying
human lymphoblastoid cell, which were cultivated in 10% fetal bovine serum
Supplementary data
6
(
FBS) RPMI 1640 medium. The indicator cells (Raji, 1 Â 10 /mL) were
incubated at 37 °C for 48 h in 1 mL of medium containing n-butyric acid
(4 mM as trigger), TPA (32 pM = 20 ng in 2 L of DMSO as inducer), and various
amounts of the test compounds dissolved in 5 L of DMSO (ca. 0.7% DMSO).
Supplementary data (experimental section and spectroscopic
data) associated with this article can be found, in the online ver-
sion, at http://dx.doi.org/10.1016/j.tetlet.2014.10.038.
l
l
Smears were made from the cell suspension. The EBV-EA inducing cells were
stained with high titer EBV-EA positive serum from NPC patients and detected
by an indirect immunofluorescence technique.25 In each assay, at least 500
cells were counted, and the number of stained cells (positive cells) was
recorded. Triplicate assays were performed for each data point. The average
EBV-EA induction of the test compound was expressed as IC50 value and as a
relative ratio to the positive control experiment (100%), which was carried out
with n-butyric acid (4 mM) plus TPA (32 pM). In the experiments, the EBV-EA
induction was normally around 35%, and this value was taken as the positive
control (100%). n-Butyric acid (4 mM) alone induced 0.1% EA-positive cells. The
viability of treated Raji cells was assayed by the trypan blue staining method.
The cell viability of the TPA positive control was greater than 80%. Therefore,
only the compounds that induced less than 80% (% of control) of the EBV-
activated cells (those with a cell viability of more than 60%) were considered
able to inhibit the activation caused by promoter substances. Student’s t-test
was used for all statistical analyses.
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