Facile synthesis and estrogenic activity of arylpyrrole-based bisphenol derivatives

Article abstract of DOI:10.3987/COM-12-S(N)56

Novel estrogen candidates 6a and 6b incorporating arylpyrrole bisphenol structure were synthesized in only three steps from commercially available materials by means of McMurry coupling and an unexpected BBr₃- mediated aromatization. These comp

Full text of DOI:10.3987/COM-12-S(N)56

HETEROCYCLES, Vol. 86, No. 1, 2012
165
HETEROCYCLES, Vol. 86, No. 1, 2012, pp. 165 - 170. © 2012 The Japan Institute of Heterocyclic Chemistry
Received, 26th June, 2012, Accepted, 6th August, 2012, Published online, 14th August, 2012
DOI: 10.3987/COM-12-S(N)56
FACILE
SYNTHESIS
AND
ESTROGENIC
ACTIVITY
OF
ARYLPYRROLE-BASED BISPHENOL DERIVATIVES
Kiminori Ohta, Fumi Taguchi, and Yasuyuki Endo*
Faculty of Pharmaceutical Sciences, Tohoku Pharmaceutical University, 4-4-1
Komatsushima, Aoba-ku, Sendai 981-8558, Japan; e-mail:
yendo@tohoku-pharm.ac.jp
Abstract – Novel estrogen candidates 6a and 6b incorporating arylpyrrole
bisphenol structure were synthesized in only three steps from commercially
available materials by means of McMurry coupling and an unexpected
BBr3-mediated aromatization. These compounds showed ER-binding affinity in
competitive binding assay and estrogenic activity in MCF-7 cell proliferation
assay.
17-Estradiol (E2, 1) is an endogenous estrogen that plays important roles in the female and male
reproductive systems, as well as in bone maintenance, in the central nervous system, and in the
cardiovascular system (Figure 1a).1 Most estrogenic signaling is mediated through estrogen receptors
(ERs)  and , which are members of the nuclear receptor superfamily of transcription factors.2
Compounds that either induce or inhibit cellular estrogen responses have potential value as biochemical
tools and candidates for drug development.3 Since the discovery of non-steroidal estrogens, many
estrogen agonists, antagonists, and selective estrogen receptor modulators (SERMs) have been developed
as agents for regulating fertility, preventing and controlling hormone-responsive breast cancer,
post-menopausal hormone replacement, and treating osteoporosis.4 Binding of the ligands to the ER
ligand binding domain (LBD) generally requires a phenolic ring, which forms hydrogen bonds with
amino acid residues Glu353 and Arg394 of the hER LBD (Figure 1a).5 It is also well known that the
secondary alcohol group of E2 interacts with His524 of hER.5 The hydrophobic group should closely
match the hydrophobic surface of the ER. Raloxifen (2), which is a well-known SERM, has a rigid and
flat skeleton with two phenolic hydroxyl groups for hydrogen bond formation, and a basic side chain that
affords partial agonistic activity (Figure 1b).
Dedicated to Professor Dr Ei-ichi Negishi on the occasion of his 77th birthday.

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HETEROCYCLES, Vol. 86, No. 1, 2012
Figure 1. (a) Structure of E2 and binding mode of E2 with ER. (b) Structure of raloxifene, a widely
used SERM.
We have found that a simple 1,2-bis(4-hydroxyphenyl)-o-carborane, BE360 (3), exhibited high binding
affinity for ER and ER.6 Furthermore, in vivo evaluation indicated that 3 exhibited estrogenic action in
bone, ameliorating bone loss without showing estrogenic action in uterus of ovariectomized (OVX) and
orchiectomy (ORX) mice, suggesting its possible application as a new type of SERM to treat
osteoporosis.7 Therefore, we next designed BE1060 (4) and BE1054 (5), which have three-dimensional
hydrocarbon units as new hydrophobic pharmacophores.8 Interestingly, bicyclo[2,2,2]octane-based
bisphenol 4 showed potent estrogenic activity but tetramethylcyclohexene-based bisphenol 5 acted as a
partial agonist, like 2. A vicinal bisphenol structure and a hydrophobic moiety should provide a basis for
development of various ER ligands with agonistic, partial agonistic or antagonistic activities.
Therefore, we have focused on the development of novel bisphenol derivatives. Compound 2 has a phenol
ring on the opposite side of benzothiophene ring, which plays an important role for ER binding. In
consideration of raloxifene-like geometry between two phenol rings and synthetic advantages such as
Figure 2. Structures of ER modulators and newly designed ER modulator candidates based on a
bisphenol structure.

HETEROCYCLES, Vol. 86, No. 1, 2012
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symmetric structure and simple synthesis, we chose a pyrrole ring as a central core structure and designed
compounds (6) as novel ER modulators. In this paper, we describe the facile synthesis and biological
activities of the novel pyrrole-based vicinal bisphenol compounds 6a and 6b.
Aromatic rings can be introduced into a pyrrole ring by means of Cu-mediated9 or Pd-catalyzed10
coupling reaction, but we chose pyrrole ring synthesis using McMurry coupling11,12 because of the easily
oxidizable character of pyrrole derivatives. The synthesis of the target compounds 6a and 6b is
summarized in Scheme 1. Commercially available 2-bromo-4’-methoxyacetophenone (7) was reacted
with aniline or p-anisidine to afford tertiary amines (8). Intramolecular McMurry coupling of 8 using
TiCl4 and Zn powder afforded dihydropyrrole derivatives (9) in low yields. We suggested that the low
yields of coupling products 9 are due to a property of a dihydropyrrole ring, which is susceptible to
oxidation. When the methoxy groups were deprotected with BBr3, an unexpected aromatization
proceeded simultaneously to afford the target compounds 6a and 6b in high yields. In general, the
oxidation of dihydropyrroles to pyrroles is achieved under oxidative conditions using metal or organic
oxidants.13 During this reaction, dihydropyrrole derivatives could not be observed on TLC and were not
isolated. Although the mechanisms involved are not clear, we suspected that the driving forces may be
extension of the -conjugated system and activation of -hydrogen of the dihydropyrrole ring by
coordination of the amine with BBr3.
R
MeO
HO
NH2
O
a
b
c
N
R
N
R
N
R
MeO
O
7
9
MeO
HO
R = H (6a)
OH (6b)
MeO
8
Scheme 1. Synthesis of bisphenol derivatives 6a and 6b; Reagents and conditions: (a) aniline or
p-anisidine, Na2CO3·10H2O, EtOH, reflux, 70% (for R = H) and quant (for R = OMe); (b) TiCl4, Zn, THF,
reflux, 21% (for R = H) and 28% (for R = OMe); (c) BBr3, CH2Cl2, rt, 88% (for 6a) and 61% (for 6b).
Figure 3 shows dose-response curves for competitive binding of compounds 6a and 6b with
[2,4,6,7-3H]E2 using human recombinant ER and ER.14 Both bisphenols 6a and 6b bound to ER, but
not to ER, and acted as ER-selective ligands. Unfortunately, their potencies were around 100 times
weaker than that of E2. Since there was no significant difference between compounds 6a and 6b in
ER-binding properties, the phenolic hydroxyl group in the N-substituent of 6b appears not participate in
secondary hydrogen bond formation with His524 or hydrogen-bonding amino acid residues around
His524. On the other hand, compound 6b has three phenolic hydroxyl groups and its binding mode to

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HETEROCYCLES, Vol. 86, No. 1, 2012
ER might be quite different from that of 6a. In that case, these compounds might show different
biological activities.
Figure 3. Competitive binding assay of compounds 6a and 6b with [3H]E2 (4 nM), using ER and ER.
The cell proliferative activity of compounds 6a and 6b was evaluated using human breast cancer MCF-7
cells, which show ER-dependent growth (Figure 4).15 Neither of the bisphenols showed anti-proliferative
activity. Compounds 6a and 6b showed dose-dependent enhancement of cell proliferation, although their
activity was weaker than that of E2. These compounds acted as ER agonists and the EC50 values of
compounds 6a and 6b were estimated to be 72 nM and 19 nM, respectively, from the dose-response
curves.
Figure 4. MCF-7 cell proliferation in the presence of compounds 6a and 6b.
Although the bisphenol compounds acted as moderately potent pure ER agonists, not partial agonists, in
cell proliferation assay, the synthetic method allows convenient modification of the benzene and pyrrole
rings, which might be effective to alter the activities. For example, it might be possible to produce a

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partial agonist by the introduction of a basic side chain at a phenolic hydroxyl group of the bisphenol
structure, as in the case of raloxifen. It seems that an agonistic activity is induced by a small and planar
hydrophobic structure in the bisphenol derivatives. Therefore, an introduction of bulky hydrophobic
substituents at the 2 and 5 positions on the pyrrole ring might conduct partial agonistic activity. These
easily available compounds, 6a and 6b, are expected to be useful as lead compounds. Further
derivatization and structure-activity relationship studies are in progress.
In conclusion, bisphenol derivatives 6a and 6b were prepared in short step synthesis from commercially
available materials by using McMurry coupling as a key reaction, together with a unique BBr3-mediated
aromatization. Compounds 6a and 6b bound weakly to ER and showed estrogenic activity in MCF-7
cell proliferation assay. Although their activities were not strong, there is scope for improvement by
means of structural modifications on the nitrogen atom, benzene rings, and pyrrole ring. A triarylpyrrole
structure might be a promising scaffold for drug discovery, because of the fixed structure in three
different directions, availability of various substituted derivatives, and ease of synthesis.
ACKNOWLEDGEMENTS
This research was supported by a Grant-in-Aid for High Technology Research Program, a Grant-in-Aid
for Scientific Research (B) (No. 20390035) and a Grant-in-Aid for Young Scientists (B) (No. 21790116)
from the Ministry of Education, Culture, Sports, Science and Technology, Japan.
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