ERβ ligands. Part 6: 6H-Chromeno[4,3-b]quinolines as a new series of estrogen receptor β-selective ligands

Article abstract of DOI:10.1016/j.bmcl.2007.04.068

A new class of estrogen receptor beta (ERβ) ligands based on the 6H-chromeno[4,3-b]quinoline scaffold has been prepared. Several C7-substituted analogues displayed high affinity and modest selectivity for ERβ.

Full text of DOI:10.1016/j.bmcl.2007.04.068

Bioorganic & Medicinal Chemistry Letters 17 (2007) 4053–4056
ERb ligands. Part 6: 6H-Chromeno[4,3-b]quinolines
as a new series of estrogen receptor b-selective ligands
An T. Vu,a,* Alison N. Campbell,a Heather A. Harris,b Rayomand J. Unwalla,c
Eric S. Manasc and Richard E. Mewshawa
aChemical and Screening Sciences Medicinal Chemistry, Wyeth Research, 500 Arcola Road, Collegeville, PA 19426, USA
bWomen’s Health and Musculoskeletal Biology, Wyeth Research, 500 Arcola Road, Collegeville, PA 19426, USA
cChemical and Screening Sciences Structural Biology, Wyeth Research, 500 Arcola Road, Collegeville, PA 19426, USA
Received 19 March 2007; accepted 23 April 2007
Available online 4 May 2007
Abstract—A new class of estrogen receptor beta (ERb) ligands based on the 6H-chromeno[4,3-b]quinoline scaffold has been pre-
pared. Several C7-substituted analogues displayed high affinity and modest selectivity for ERb.
Ó 2007 Elsevier Ltd. All rights reserved.
Estrogens are known to play a crucial role in mamma-
lian reproductive system and also in many other non-
reproductive organs such as skeletal, cardiovascular,
and central nervous systems.1 Estrogens exert their
effects mainly by interactions with the estrogen receptor
(ER), which is ligand-activated transcription factor and
belongs to a superfamily of nuclear hormone receptors.
The unexpected discovery of a second subtype of estro-
gen receptor, estrogen receptor beta (ERb)2, in 1996
necessitated renaming of the first estrogen receptor
ERa. Because ERb possesses unique tissue distribution
patterns and transcriptional properties from those of
ERa,3 its discovery has prompted intense research to
investigate ERb as a potential new drug target4 as well
as to develop novel, tissue and cell-selective estrogens.5
Recently, studies from Wyeth laboratories have demon-
strated certain therapeutic potentials of ERb-selective
ligands.6
type-selective ligands, medicinal chemistry efforts from
our laboratories and others in recent years have yielded
a number of structural motifs with impressive ERb
selectivity.8
We recently reported a series of 2-phenylquinoline ERb-
selective ligands,9 which was developed based on the
6-phenylnapthalene scaffold.10 The best compound in
this report was 4 (Scheme 1), which exhibited ERb bind-
ing IC50 value of 3.4 nM and 83-fold selectivity for ERb.
Docking of 4 into the ligand binding domain of the ERb
LBD/6-phenylnapthalene 3 cocrystal structure10 placed
the bromo group in close proximity to the ERb
Ile373 ! ERa Met421 residue substitution (Fig. 1A), sug-
gesting that the observed ERb selectivity of 4 may be
due to a differential interaction of the bromo group with
these amino acid residues.9,12 The docked structure of 4
also revealed a 16° dihedral angle between the pendant
phenyl ring and the quinoline core. We anticipated that
locking this dihedral angle by introducing a ring would
restrict the rotational freedom about the bond connect-
ing the two aryl planes, and generate a rigid quinoline
framework that would mimic the docked conformation
of 4. This anticipation led us to investigate the 6H-chro-
meno[4,3-b]quinoline scaffold (Scheme 1). To retain
similar geometrical arrangement as that of the 2-phenyl-
quinoline, a hydroxyl group will be placed at positions 3
and 9, which are known to be essential for binding to
ER. Docking of 6H-chromeno[4,3-b]quinoline-3,9-diol
7 (R = Br) into the ERb LBD/3 binding pocket reveals
a 13° torsion angle between the phenyl ring and the
Although the ligand binding domains (LBD) of ERa
and ERb share only modest homology (58% identity),
their ligand binding cavities are highly conserved, differ-
ing by only two amino acid residues (ERa Leu384 is re-
placed by ERb Met336, and ERa Met421 is replaced by
ERb Ile373).7 While this slight difference in the binding
cavities poses a great challenge in developing ER sub-
Keywords: Estrogen receptor b ligands; Chromenoquinoline.
*
Corresponding author. Tel.: +1 484 865 8432; fax: +1 484 865
9399; e-mail: vua@wyeth.com
0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2007.04.068

4054
A. T. Vu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4053–4056
OH
carried out using the Niementowski modification of
the Freidla¨nder synthesis.16 Thus, condensation of
7-methoxychroman-4-one and 5-methoxyanthranilic
acid gave 5. Subsequent reaction with phosphorus oxy-
chloride, followed by removal of the methyl protecting
group, afforded the 7-chlorochromenoquinoline 6.
Treatment of 5 with phosphorus oxybromide, followed
by demethylation furnished the bromo analogue 7.
HO
O
OH
HO
O
Genistein
2
OH
17β-Estradiol
1
4'
HO
F
Compound 7 was also the common intermediate from
which a number of 7-substituted 6H-chromeno[4,3-
b]quinolines could be prepared using various transition
metal-mediated cross-coupling reactions (Scheme 3).
Thus, heating 7 with copper cyanide gave the cyano ana-
logue 8, whereas Stille coupling of 7 with tribu-
tyl(vinyl)tin afforded the vinyl derivative 9. Similarly,
reaction of 7 with (trimethylsilylethynyl)tributyltin17 fol-
lowed by desilylation yielded the alkynyl compound 10,
which upon reduction furnished the ethyl analogue 11.
Suzuki reaction of 7 with substituted phenylboronic
acids provided targets 12–18.
2
OH
8
CN
6-Phenylnaphthalene
3
4'
HO
F
3
HO
N
N
6
9
O
OH
OH
4
7
Br
R
2-Phenylquinoline
6H-Chromeno[4,3-b]quinoline
4
The 7-substituted 6H-chromeno[4,3-b]quinolines were
evaluated in a competitive radioligand binding assay
measuring the relative binding affinity (IC50) of the com-
pounds for the human ERa and ERb LBD.18 Results
are presented in Table 1. As expected, endogenous
ligand 17b-estradiol bound equally well to both ER
isoforms in this assay.
Scheme 1. Compounds of interest.
quinoline core (Fig. 1B), which is similar to the dihedral
angle of the docked 4. Overlaying docked structures of 4
and 7 shows a near perfect alignment (Fig. 1B). In
addition, a variety of functional groups including
electronegative, aliphatic, and aromatic substituents
will also be introduced at position 7, which corresponds
to position 4 of the 2-phenylquinoline scaffold and
has been shown to be the preferred position to gain
ERb selectivity and/or binding affinity (Scheme 1 and
Fig. 1B).9 Herein, we describe the synthesis and struc-
ture–activity relationships of this novel series of 6H-
chromeno[4,3-b]quinolines.
The chloro analogue 6 exhibited high ERb binding affin-
ity (3.3 nM IC50) comparable to that of 17b-estradiol,
and a respectable ERb selectivity (27-fold). Larger elec-
tronegative bromo substituent of 7 and electron-with-
drawing cyano group of 8 also showed strong affinity
for ERb (nanomolar ERb IC50 values), but with slightly
lowered selectivity. Small aliphatic groups such as vinyl
(9), alkynyl (10), and ethyl (11) all showed some ERb
selectivity, although with considerably weaker binding
affinities. This affinity reduction for both ER isoforms
may be attributed to a greater desolvation penalty
caused by the increased basicity of the quinoline core
in the absence of an electron-withdrawing substituent
of analogues 9–11 (relative to derivatives 6–8). Phenyl
groups of analogues 12–18 bearing either electron-rich
All 6H-chromeno[4,3-b]quinolines in Table 1 were
synthesized as shown in Schemes 2 and 3. The synthesis
began with alkylation13 of 3-methoxyphenol and subse-
quent intramolecular Friedel–Crafts acylation14 of the
resulting 3-(3-methoxyphenyl)-propionic acid to give
7-methoxychroman-4-one (Scheme 2).15 The construc-
tion of the 6H-chromeno[4,3-b]quinoline core was
Met336
(ERα Leu384
Met336
(ERα Leu384
A
B
)
)
His475
Glu305
Glu305
His475
Arg346
Arg346
Ω = 16 ˚
Ω = 13 ˚
Ile373
(ERα Met421
Ile373
(ERα Met421)
)
Figure 1. (A) Compound 4 docked to ERb LBD/3 complex; (B) Overlaying of docked 4 (cyan) with docked 6H-chromeno[4,3-b]quinoline 7 (white).
Calculations were performed as described in Ref. 11. Only key residues and a Connolly surface of the ERb binding site are shown for simplicity.

A. T. Vu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4053–4056
4055
Table 1. Binding affinity for human ERa and ERb ligand binding domain
HO
HO
N
N
O
OH
OH
R
R'
ERb IC50 (nM) ERa IC50 (nM) a/bb Compoundc R0
ERb IC50 (nM) ERa IC50 (nM) a/bb
a
a
a
a
Compound
R
1
2
17b-Estradiol 3.6 1.6
3.2 1.0
395 181
88 19
1
41
27
16
16
24
12
14
15
16
16
6
Genistein
Cl
10
4
6
3.3 3.2
3.6 3.5
6.1 0.7
22 8.6
52 18
19
20
21
22
23
24
Cl
Br
4.6 2.1
4.3 2.3
213 38
212 87
455 107
504
46
50
16
8
7
Br
CN
56 20
98 82
8
CN
28
60
7
9
CH2@CH
HC„C
Et
526 31
624 510
641 27
2345 1054
1017 570
5235 1209
549 272
504 117
1130
CH2@CH
HC„C
Et
10
11
12
13
14
15
16
17
18
75 15
52 13
1500 113
634 287
20
12
46
160
5
0
4-Cl–Ph
4-CN–Ph
4-CF3–Ph
4-MeO–Ph
3-Cl–Ph
3-CN–Ph
3-MeO–Ph
65 24
319 257
94 47
144 115
150
4
8
10
86 18
820
a IC50 values are the means of at least two experiments STD, determined from eight concentrations (performed in triplicate). Values without STDs
are for a single determination only.
b ERb selectivity is expressed as ERa IC50/ERb IC50 ratio.
c See Ref. 9 for the 2-phenylquinolines 19–24.
O
a
b
COOH
O
O
OH
O
O
O
O
(31%)
(67%)
O
O
H
N
H2N
c
+
HO
O
O
O
O
O
O
f, e
5 (68%)
d, e
N
HO
HO
N
O
O
OH
OH
Br
Cl
7 (74%)
6 (86%)
Scheme 2. Reagents and conditions: (a) BrCH2CH2COOH, NaHCO3, NaOH, H2O, 100 °C, 3 h; (b) TfOH, 0 °C–rt, 3 h; (c) Ph2O, 170 °C, 1 h, then
200 °C, 7 h; (d) POCl3, reflux, 1 h; (e) BBr3, cyclohexene, ClCH2CH2Cl, 35 °C, 12 h; (f) POBr3, DMF, 70 °C, 30 min.
or electron-deficient substituents did not offer any
further improvement in either binding affinity or
selectivity.
between the methylene moiety of the chromane ring
and surrounding hydrophobic residues.
In summary, we have identified the 6H-chromeno[4,3-b]
quinolines as a new series of ERb selective ligands,
which was developed by rigidifying the 2-phenylquino-
line framework. Analogues with halogen or cyano sub-
stitution at the C7 position displayed high binding
affinities (comparable to that of 17b-estradiol) and were
moderately selective toward the ERb receptor. Our
Compared to the corresponding 2-phenylquinolines (19–
24, Table 1),9 the chromenoquinolines (6–11) generally
showed a similar SAR trend. However, they appeared
to exhibit slightly stronger affinity for both ER isoforms.
This affinity enhancement may be attributed to a favor-
able hydrophobic effect and van der Waals interactions

4056
A. T. Vu et al. / Bioorg. Med. Chem. Lett. 17 (2007) 4053–4056
3. (a) Kuiper, G. G. J. M.; Carlsson, B.; Grandien, K.;
HO
HO
Enmark, E.; Ha¨ggblad, J.; Nilsson, S.; Gustafsson, J.-A.
Endocrinology 1997, 138, 863; (b) Weihua, Z.; Andersson,
S.; Cheng, G.; Simpson Evan, R.; Warner, M.; Gustafs-
son, J.-A. FEBS Lett. 2003, 546, 17; (c) Kian, T. M.;
Rogatsky, I.; Tzagarakis-Foster, C.; Cvoro, A.; An, J.;
Christy, R. J.; Yamamoto, K. R.; Leitman, D. C. Mol.
Biol. Cell 2004, 15, 1262.
N
N
a
O
O
OH
OH
Br
C
N
b
7
8 (23%)
c, d
4. (a) Nilsson, S.; Kuiper, G.; Gustafsson, J. A. Trends
Endocrinol. Metab. 1998, 9, 387; (b) Koehler, K. F.;
HO
HO
f
N
´
Helguero, L. A.; Haldosen, L.-A.; Warner, M.; Gustafs-
son, J.-A. Endocr. Rev. 2005, 26, 465.
N
O
OH
O
5. For reviews, see: (a) Miller, C. P. Curr. Pharm. Des. 2002,
8, 2089; (b) Wallace, O. B.; Richardson, T. I.; Dodge, J. A.
Curr. Top. Med. Chem. 2003, 3, 1663.
OH
9 (83%)
6. (a) Harris, H. A.; Albert, L. M.; Leathurby, Y.; Malamas,
M. S.; Mewshaw, R. E.; Miller, C. P.; Kharode, Y. P.;
Marzolf, J.; Komm, B. S.; Winneker, R. C.; Frail, D. E.;
Hederson, R. A.; Zhu, Y.; Keith, J. C., Jr. Endocrinology
2003, 144, 4241; (b) Malamas, M.; Keith, J. C., Jr.; Harris,
H. A. Drugs Future 2005, 30, 333; (c) Harris, H. A. Mol.
Endocrinol. 2007, 21, 1.
7. Pike, A. C. W.; Brzozowski, A. M.; Hubbard, R. E.;
Bonn, T.; Thorsell, A. G.; Engstrom, O.; L-junggren, J.;
Gustafsson, J. A.; Carlquist, M. EMBO J. 1999, 18, 4608.
8. For recent reviews, see: (a) Veeneman, G. H. Curr.
Med. Chem. 2005, 12, 1077; (b) Henke, B. R.; Heyer,
D. Curr. Opin. Drug Discovery Dev. 2005, 8, 437; (c)
Ullrich, J. W.; Miller, C. P. Expert Opin. Ther. Pat.
2006, 16, 559.
10 (63%)
HO
N
e
O
HO
OH
R
N
12, R = 4-Cl-Ph (94%)
13, R = 4-NC-Ph (96%)
14, R = 4-F3C-Ph (95%)
15, R = 4-MeO-Ph (84%)
16, R = 3-Cl-Ph (94%)
17, R = 3-NC-Ph (74%)
18, R = 3-MeO-Ph (87%)
O
OH
11 (96%)
Scheme 3. Reagents and conditions: (a) CuCN, DMF, 200 °C, sealed
tube, 5 h; (b) Bu3SnCH@CH2, Pd(PPh3)4, tol., reflux, 1 h; (c)
Bu3SnC„C–TMS, Pd(PPh3)4, tol., reflux, 1 h; (d) K2CO3, MeOH,
30 min; (e) H2, 1 atm, Pd/C, EtOAc, 1 h; (f) substituted Ph–B(OH)2,
Pd(PPh3)4, DME, aq Na2CO3, reflux, 1 h.
9. Vu, A. T.; Cohn, S. T.; Manas, E. S.; Harris, H. A.;
Mewshaw, R. E. Bioorg. Med. Chem. Lett. 2005, 15,
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Zhang, B. X.; Henderson, R. A.; Keith, J. C., Jr.; Harris,
H. A. J. Med. Chem. 2005, 48, 3953.
11. Edsall, R. J.; Harris, H. A.; Manas, E. S.; Mewshaw, R. E.
Bioorg. Med. Chem. 2003, 11, 3457.
current efforts focus on multiple substitution strategy to
maximize ERb binding affinity and/or selectivity.
12. Manas, E. S.; Unwalla, R. J.; Xu, Z. B.; Malamas, M. S.;
Miller, C. P.; Harris, H. A.; Hsiao, C.; Akopian, T.; Hum,
W.-T.; Malakian, K.; Wolfrom, S.; Bapat, A.; Bhat, R. A.;
Stahl, M. L.; Somers, W. S.; Alvarez, J. C. J. Am. Chem.
Soc. 2004, 126, 15106.
13. Srikanth, N.; Kon, O.-L.; Ng, S.-C.; Sim, K.-Y. J. Chem.
Res. Miniprint 1997, 6, 1412.
14. Orita, A.; Yaruva, J.; Otera, J. Angew. Chem., Int. Ed.
1999, 38, 2267.
Acknowledgments
The authors thank the Wyeth Discovery Analytical
Chemistry department for the spectral data. Special
thanks to Dr. Chris Miller for serving as chemistry team
leader, Dr. Puwen Zhang for critical review of this man-
uscript, and to Drs. Gene Trybulski and John Yardley
for providing helpful advices and suggestions in the
ERb program.
15. Siddaiah, V.; Rao, C. V.; Venkateswarlu, S.; Krishnaraju,
A. V.; Subbaraju, G. V. Bioorg. Med. Chem. 2006, 14,
2545.
16. (a) Yamato, M.; Takeuchi, Y.; Hashigaki, K.; Ikeda, Y.;
Ming-rong, C.; Takeuchi, K.; Matsushima, M.; Tsuruo,
T.; Tashiro, T.; Tsukagoshi, S.; Yamashita, Y.; Nakano,
H. J. Med. Chem. 1989, 32, 1295; (b) Khajavi, M.;
Mohammadi, A. A.; Hosseini, S. S. S. Synth. Commun.
2001, 31, 3647.
17. Logue, M. W.; Teng, K. J. Org. Chem. 1982, 47, 2549.
18. Harris, H. A.; Bapat, A. R.; Gonder, D. S.; Frail, D. E.
Steroids 2002, 67, 379.
References and notes
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