Furans with basic side chains: Synthesis and biological evaluation of a novel series of antagonists with selectivity for the estrogen receptor alpha

Article abstract of DOI:10.1016/S0960-894X(01)00488-7

3-Alkyl-2,4,5-triarylfurans with basic side-chain substituents were prepared as ligands for the estrogen receptor. Those analogues having the basic side chain on the C(4) phenol were high-affinity, ERα-selective antagonists.

Full text of DOI:10.1016/S0960-894X(01)00488-7

Bioorganic & Medicinal ChemistryLetters 11 (2001) 2521–2524
Furans with Basic Side Chains: Synthesis and Biological
Evaluation of a Novel Series of Antagonists with Selectivity
for the Estrogen Receptor Alpha
Deborah S. Mortensen,a Alice L. Rodriguez,a Jun Sun,b
Benita S. Katzenellenbogenb and John A. Katzenellenbogena,*
aDepartment of Chemistry, University of Illinois, Urbana, IL 61801, USA
bDepartment of Molecular andIntegrative Physiology, University of Illinois, Urbana, IL 61801, USA
Received 23 May2001; accepted 9 July2001
Abstract—3-Alkyl-2,4,5-triarylfurans with basic side-chain substituents were prepared as ligands for the estrogen receptor. Those
analogues having the basic side chain on the C(4) phenol were high-affinity, ERa-selective antagonists. # 2001 Published by
Elsevier Science Ltd.
We have recentlydescribed the synthesis and biological
evaluation of a series of 3-alkyl-2,4,5-triaryl-substituted
furans, several of which proved to be ligands for the
estrogen receptor (ER) with veryhigh selectivityfor
ERa over ERb, both in terms of binding affinityand
nor that the product BSC furans would retain the ERa
selectivityof the parent furans, because neither ralox-
ifene nor tamoxifen demonstrates pronounced ER sub-
type selective antagonism.7 We were able to adapt the
synthetic route originally used to prepare the tetra-sub-
stituted furan systems for the preparation of the BSC
analogues,1 bydifferential protection of the phenol
1
potencyof transcriptional activation. Because there is
great interest in the development of antagonists as well
as agonists that are ER subtype-selective,2,3 we have
investigated a wayto convert these ER a-selective furan
agonists into ERa-selective antagonists.
The antagonist character of the antiestrogens raloxifene
and hydroxy-tamoxifen (now more properly designated
‘selective estrogen receptor modifiers’ or SERMs; cf.
Fig. 2)4 relies on a tertiaryamino ethoxysubstituent,
termed a basic side chain (BSC). When these ligands are
bound to ERa, this bulkyBSC projects outward from
the ligand binding pocket and effects a structural reor-
ganization of the receptor surface, repositioning helix 12
in a manner that blocks the binding of coactivator pro-
teins important in mediating transcriptional activity.5,6
Thus, we surmised that we might be able to impart
antiestrogenic character to the furan series of agonist
ligands, byappending a BSC through one of the three
phenols of our furan ligands (Fig. 1). It is not immedi-
atelyevident which attachment site would be preferred,
*Corresponding author. Tel.: +1-217-333-6310; fax: +1-217-333-
7325; e-mail: jkatzene@uiuc.edu
Figure 1. Three potential antiestrogenic furan basic side-chain (BSC)
derivatives.
0960-894X/01/$ - see front matter # 2001 Published byElsevier Science Ltd.
PII: S0960-894X(01)00488-7

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D. S. Mortensen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2521–2524
functions in the starting materials. We recentlyused a
similar approach to attach BSC groups to pyrazoles.2
Here, we report the results of our efforts to identifythe
preferred furan position for attachment of the BSC, to
obtain ER antagonists that have some selectivityfor
ERa over ERb, and to elucidate the orientation with
which these furans bind to the receptor.
The affinityof the BSC furans 9a–f and 13 for the
estrogen receptor was assayed by a competitive radio-
metric binding assay, using purified, full-length human
ERa and ERb from recombinant sources.8,9 The results
of these assays are expressed as relative binding affinity
(RBA) values, where the affinityof estradiol is con-
sidered to be 100%, and theyare summarized in Table 1.
Treatment of the differentiallyprotected 1,2-diarly-
ethanones (1–2), ketone (3), or desoxyanisoin (4) with
potassium hexamethyl-disilyl-amide (KHMDS), fol-
lowed bythe addition of an a-bromoketone (5a–e),
provided the desired, differentiallyprotected 1,3,4-
triaryl-2-alkyl-butane-1,4-diones (6a–g) in 66–98%
yields (Scheme 1). Although these diones were mixtures
of diastereomers, no separation was required, because
these stereocenters become trigonal in the furan pro-
ducts.
Bycomparison of the ER a affinities for the isomeric
series of 3-ethyl-substituted BSC furans 9b, 9f, and 13, it
appears that the preferred position for the attachment
of the BSC is on the C(4) phenol (9b); much lower affi-
nities were observed when it was attached to the C(2) or
Diones 6a–f were refluxed in toluene with a catalytic
amount of p-toluenesulfonic acid (TsOH) to effect furan
formation and desilylation, giving mono-deprotected
furans 7a–f. The piperidinyl-ethoxy side chain was
added using Mitsunobu chemistry,1 and the inter-
mediates 8a–f were demethylated using AlBr3/EtSH to
afford BSC furans 9a–f.4
In contrast to the behavior of diones 6a–f, when
refluxed with a catalytic amount of TsOH in toluene,
dione 6g cyclized to the furan but did not undergo TIPS
deprotection (Scheme 2). However, treatment of furan
10 with tetrabutylammonium fluoride at 0 ꢀC did result
in desilylation, giving the mono-deprotected furan 11
that was converted to BSC furan 13, as described above.
Scheme 1. Synthesis of basic side-chain furans 9a–f (BSC=CH2CH2–
piperidine).
Table 1.
Ligand
ER-a
ER-b
a/b
9a
9b
9c
9d
9e
9f
32Æ9
75Æ20
34Æ2
0.47Æ0.1
3.1Æ0.1
67.2-fold
24.3-fold
8.5-fold
3.0-fold
8.8-fold
2.7-fold
6.1-fold
4.0Æ1.2
60Æ1
20Æ3.5
3.8Æ0.9
2.5Æ0.3
0.36Æ0.11
0.43Æ0.04
0.91Æ0.02
0.06Æ0.02
13
Determined bya competitive radiometric binding assaywith
[3H]estradiol.
Figure 2. Two possible A-ring mimics of estradiol for furan 9b.

D. S. Mortensen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2521–2524
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C(5) phenols (13 and 9f, respectively). The BSC-mod-
ified furans did retain much of the selective affinityfor
ERa shown bytheir parent furans (which for the tri-
phenol series, with R3=Me, Et, and Pr, ranged from 48-
to 65-fold),1 the most selective BSC furan 9a having a
67-fold ERa/ERb binding selectivity. Even the analo-
gues with low affinities for ERa maintained some of this
selectivity.
playing this important role in binding by deleting each
of them in turn, and then evaluating the effect that this
had on their ERa binding affinity.10
The X-raycrystal structures for ER a complexed with
either raloxifene6 or 4-hydroxy-tamoxifen5 show that the
BSC on these ligands has a common orientation that
places it roughlyin the 11 b direction with respect of a
standard steroid core, where a salt bridge can then form
between the tertiaryamine and aspartate 351 in helix-3.
Thus, as indicated in Figure 2, there are likelyto be only
two binding modes for a C(4)-BSC furan, such as 9b, mode
A, with the C(2) phenol in the A-ring binding pocket, and
mode B, with C(5) phenol mimicking the A-ring.
The initial series of BSC furans we prepared have two
free phenols, either of which could be serving as the
analogues of the A-ring of estradiol, a group that func-
tions as a crucial hydrogen-bonding partner in the
ligand binding pocket and is essential for the high affi-
10
nityof the natural in estrogen. We sought to deter-
mine which of these phenols in the C(4)-BSC furans was
Comparison of the ERa binding affinityof BSC furans
9d and 9e, mono-phenol analogues of the high affinity
BSC furan diol 9b (Table 1), clearlyshows that the
phenol attached at position C(5) serves as the A-ring
mimic: there was a 20-fold loss of affinitywhen this
phenol was deleted (9e), whereas deletion of the C(2)
phenol caused less than a 1.3-fold drop (9d). Thus, we
conclude that the highest affinityfuran with the basic
side chain, ligand 9b, binds to ER in orientation B (Fig.
2), and that in this orientation, having a second phe-
nolic hydroxyl on the C(2) phenyl ring does not seem to
be veryimportant for high affinitybinding ( 9b vs 9d);
this second phenol does, however, playa significant role
in the selective affinityof these BSC furans for ER a.
We selected the BSC furans that combined high affinity
with good ERa binding selectivity(namely 9a–c) to be
assayed for their capacity to activate transcription
through either ERa or ERb, or to antagonize the tran-
scriptional activityof estradiol through these ERs.
These assays were done by cotransfection in human
endometrial carcinoma (HEC-1) cells using a luciferase
reporter gene system;11 dose–response studies with the
BSC furans alone were conducted to measure agonist
activity, and in the presence of 1 nM estradiol to deter-
mine antagonist activity. The results are summarized in
Figures 3 and 4.
As expected, none of the three BSC furans showed
appreciable agonist activityon either ER a or ERb. The
Scheme 2. Synthesis of basic side-chain furan 13.
Figure 3. Transcriptional activityof the BSC furans 9a–c and their antagonism of estradiol activitythrough ER a and ERb.

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D. S. Mortensen et al. / Bioorg. Med. Chem. Lett. 11 (2001) 2521–2524
is of note in both the furan and pyrazole series that the
parent ligands appear to bind in a different orientation
than do the BSC derivatives, placing the C(2) phenol of
the furan (and the corresponding phenol of the pyra-
zole) in the A-ring binding pocket. The BSC derivatives
of ERa-selective ligands should prove to be useful
agents to investigate the biological functions of the two
ER subtypes.
Acknowledgements
We are grateful for support of this research through
grants from the National Institutes of Health, the US
Army, and the NSF and Keck Foundation. We thank
Kathryn Carlson for excellent assistance.
Figure 4. Full dose–response, showing the ERa-selective antagonism
of BSC furan 9b.
methyl analogue (9a) has low potencyon both ERs, and
shows essentially no selectivity; the propyl analogue (9c)
was somewhat more potent and showed a small degree
of selectivityin antagonizing ER a in preference to ERb.
The most potent and selective antagonist was the ethyl
BSC furan, 9b, and a more complete dose–response
curve for this compound (Fig. 4) shows that, at 0.1 mM,
it almost fullysuppresses the transcriptional activityof
estradiol through ERa, yet it has no effect on ERb at
this concentration. The IC50 values for 9b on ERa and
ERb are approximately6.5 Â10À8 and 4.8Â10À7 M,
respectively, which corresponds to a nearly 10-fold
antagonist potencyselectivityfor ER a over ERb.
References and Notes
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