Design and synthesis of a novel series of [1-(4-hydroxy-benzyl)-1H-indol-5-yloxy]-acetic acid compounds as potent, selective, thyroid hormone receptor β agonists

Article abstract of DOI:10.1016/j.bmcl.2015.02.062

The design, synthesis, and structure activity relationships for a novel series of indoles as potent, selective, thyroid hormone receptor β (TRβ) agonists is described. Compounds with >50× binding selectivity for TRβ over TRα were generated and evaluation of compound 1c from this series in a model of dyslipidemia demonstrated positive effects on plasma lipid endpoints in vivo.

Full text of DOI:10.1016/j.bmcl.2015.02.062

Bioorganic & Medicinal Chemistry Letters 25 (2015) 1377–1380
Contents lists available at ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
Design and synthesis of a novel series of
[1-(4-hydroxy-benzyl)-1H-indol-5-yloxy]-acetic acid compounds
as potent, selective, thyroid hormone receptor b agonists
Timothy P. Burkholder a, Brian E. Cunningham a, Joshua R. Clayton a, Peter A. Lander a, Matthew L. Brown a,
Robert A. Doti a, Gregory L. Durst a, Chahrzad Montrose-Rafizadeh b, Constance King b, Harold E. Osborne b,
Robert M. Amos b, Richard W. Zink b, Lawrence E. Stramm b, Thomas P. Burris c, Guemalli Cardona c,
Debra L. Konkol d, Charles Reidy d, Michael E. Christe d, Michael J. Genin a,
⇑
a Discovery Chemistry Research and Technology, Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA
b Lead Optimization Biology, Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA
c Bone and Inflammation Therapeutic Area, Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA
d Diabetes Therapeutic Area, Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46285, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The design, synthesis, and structure activity relationships for a novel series of indoles as potent, selective,
Received 3 November 2014
Revised 18 February 2015
Accepted 24 February 2015
Available online 3 March 2015
thyroid hormone receptor b (TRb) agonists is described. Compounds with >50ꢀ binding selectivity for
TRb over TRa were generated and evaluation of compound 1c from this series in a model of dyslipidemia
demonstrated positive effects on plasma lipid endpoints in vivo.
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Thyroid hormone
Agonist
TR-b
Indole
Thyroid hormone is an important mediator of a variety of biolo-
gical processes including basal metabolic rate and lipid metabo-
lism. Hyperthyroid individuals have an increased basal metabolic
rate leading to weight loss and increased cholesterol metabolism
and excretion leading to low serum cholesterol levels.1
have been reported (SKF-94901, CGS-23425, CGS-26214, and
GC-1,4 GC24,5 Pfizer azauracil,6 KB-1417).
Here we report the design and synthesis of
a series of
[1-(4-hydroxy-benzyl)-1H-indol-5-yloxy]-acetic acid compounds
as potent, selective, thyroid hormone receptor b (TRb) agonists.
Computational studies were performed by superimposing pro-
posed compounds from this series with T3 using the published
PDB X-ray crystal structure 1BSX of the TRb receptor.8
3,5,30-Triiodo-
is a potent agonist of TR
L-thyronine (T3), the major active thyroid hormone,
a
and TRb receptors. The utility of T3 as
a treatment for obesity and hypercholesterolemia has been limited
by unacceptable cardiac side effects. The major isoform of thyroid
Modeling low energy conformations where the carboxylic acid
was in a position to interact with key Arg residues and the phenol
was in a position to interact with His-435 resulted in the indole
core overlapping with the diiodo-phenyl ring of T3. In these
conformations the 2- and 3-positions of the indole overlapped with
one of the iodine atoms of T3 and the 7-position of the indole
overlapped with the other iodine atom of T3, (see Fig. 1). A series
of 2-, 3-, and 7-substituted indole analogs were designed and syn-
thesized to investigate the effect of the indole ring substituents on
hormone receptor in human cardiac tissue is TRa 2
. This led to the
speculation that identification of a TRb selective agonist would
increase metabolic rate and beneficially affect cholesterol metabo-
lism, be devoid of cardiac liabilities, and would therefore be a use-
ful therapeutic agent.
Investigation of the structure activity relationships for thyroid
hormone receptor modulators has been studied for several
decades.3 Recently selective thyroid hormone receptor agonists that
have higher affinity for TRb receptors compared to TR
a
receptors
affinity for TRb receptors and selectivity relative to TRa (see
Table 1).
[1-(4-Hydroxy-benzyl)-1H-indol-5-yloxy]-acetic acid compounds
(1a–o) were synthesized by alkylating the corresponding indoline
⇑
Corresponding author. Tel.: +1 317 651 3849; fax: +1 317 277 2035.
E-mail address: geninmi@lilly.com (M.J. Genin).
http://dx.doi.org/10.1016/j.bmcl.2015.02.062
0960-894X/Ó 2015 Elsevier Ltd. All rights reserved.

1378
T. P. Burkholder et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1377–1380
(Scheme 2a). Protection of the phenol (6) as the triisopropylsilyl
ether was accomplished under standard conditions.10 Reduction
of the indole ring with sodium cyanoborohydride in acetic acid
gave the 3-methyl indoline intermediate (8a). The unsubstituted
indoline intermediate (8b) was prepared in an analogous manner
from 5-hydroxyindole (6b) (Scheme 2b). The 2-methyl indoline
intermediate (11) was synthesized from 5-methoxy-2-methyl-
indole (9) by reduction with sodium cyanoborohydride in acetic
acid, removal of the methyl ether with boron tribromide, and pro-
tection of the phenol as the triisopropylsilyl ether (Scheme 2c).
The substituent at the 7 position was introduced by a directed
metalation strategy on the t-butylcarbamate of the indoline
(12).11 The t-butyl carbamate was synthesized with ditert-
butyldicarbonate and the carbamate was deprotonated with
sec-butyl lithium in the presence of TMEDA. The resulting anion
was allowed to react with either methyl iodide to give the
7-methyl derivative or dichloroethane to give the 7-chloro deriva-
tive (2) (Scheme 2d).
The requisite benzaldehyde (3a) and benzyl chloride (3b)
intermediates were synthesized as described in Scheme 3.
2-Bromo-anisole (13) was coupled with an alkyl Grignard reagent
via a Kumada reaction to give alkylated anisole intermediate
(14).12 This anisole was subsequently deprotected with boron tri-
bromide to afford the phenol (15). Several commercially available
phenols (cyclopentyl, cyclohexyl, t-butyl, isopropyl, benzyl) were
also used. Bromination with HBr in acetic acid and DMSO gave
the desired para bromo intermediate (16). Silylation of the phenol
with triisopropylsilyl chloride or t-butyldimethylsilyl chloride gave
the desired silyl ether (17).10 Lithium–halogen exchange (t-butyl
lithium or n-butyl lithium) followed by quenching with DMF pro-
vided the benzaldehyde intermediate (3a). The benzyaldehyde (3a)
was used in the reductive aminations of the 7-H indolines.
Alternatively, the benzyaldehyde was reduced with sodium boro-
hydride to give the benzyl alcohols (19) which were converted to
the appropriate benzyl chlorides (3b) by treatment with methane-
sulfonyl chloride. These benzyl chlorides (3b) were used for the
alkylation of the 7-methyl and 7-chloro indolines.
Figure 1. Superimposition of T3 (green) and the indole system 1c (orange) in TRb
(1BSX).
intermediate (2) through a reductive amination with the requisite
benzaldehyde derivative (3a). This reaction worked well with the
7-H indoline intermediates. The reductive amination strategy with
the 7-methyl and 7-chloro indoline intermediate did not give any
of the desired product, therefore, these indoline intermediates
were alkylated with the benzyl chloride (3b). The resulting alkylat-
ed indoline (4) was oxidized with DDQ to give the indole (1).9 The
yield for this oxidation was higher for the 2-methyl or 3-methyl
indoline oxidations compared to the oxidation of the unsubstituted
indoline. The triisopropylsilyl ether was removed by treatment
with TBAF and the ethyl ester was saponified with sodium hydrox-
ide to give the targeted compounds (Scheme 1).
The 2-methyl-7-chloro analog (1g) was the most potent TRb
agonist in this series and it was a potent full agonist in the Gal 4
CTF cell-based assay (TRb EC50 = 29 nM, TR
3-methyl analog (1c) was the most selective analog in this series
(Ki TR /TRb = 20) and it was selected for evaluation in vivo (see
a EC50 = 181 nM). The
a
The intermediate indolines were prepared as described in
Scheme 2. 5-Hydroxy-3-methyl-indole (6) was synthesized from
5-benzyloxy-gramine (5) by hydrogenation with Pd/C in ethanol
below). The 2,7-methyl (1f) and 3,7-dimethyl (1h) analogs were
more selective than the 2-methyl-7-chloro (1g) or the 3-methyl-
7-chloro (1i) analogs (Ki TRa/TRb = 15 and 10 for 6 and 7, respec-
tively, and Ki TR /TRb = 15 and 6 for 8 and 9, respectively).
a
We designed and synthesized analogs with different sub-
stituents ortho to the phenol in an attempt to improve the receptor
selectivity for TRb receptors in the 3-methyl indole series. The
results are summarized in Table 2.
Table 1
3
O
R
O
HO
2
The benzyl analog (1k) was found to be the most selective
R
(Ki TR
a/TRb = 68) and it was a full agonist in the Gal 4 CTF
N
7
cell-based assay (TRb EC50 = 406 nM, TR
a
EC50 = 2466 nM). The
R
OH
improved receptor selectivity for the benzyl analog is similar to
that recently reported for GC-24.5 The phenyl analog (1n) was
found to be a weak partial agonist/partial antagonist. This result
highlights the importance of the interactions between this region
of the ligand for efficacy in the TRb receptor.
In an effort to assess in vivo properties compound 1c was
administered orally to fructose fed rats once a day for 7 days. A diet
consisting of 60% fructose (Teklad #89247) was administered to six
week old male Sprague-Dawley rats for three weeks. This diet and
time duration significantly raised plasma cholesterol and triglyc-
erides. Vehicle (10% Acacia) or Compound 1c was then adminis-
tered by oral gavage, q.d., in fructose-treated animals that were
randomized into groups based on non-fasted serum cholesterol
Compound
R2
R3
R7
TRb Ki (nM)
TR
a
Ki (nM)
Ki TRb/TRa
1a
1b
1c
1d
1e
1f
1g
1h
1i
H
Me
H
H
H
Me
Me
H
H
H
Me
H
H
H
H
Me
Me
H
H
H
Me
Cl
Me
Cl
Me
Cl
1.04
1.12
0.38
0.26
0.07
0.14
0.04
0.18
0.09
13.45
16.33
7.74
1.09
0.35
2.05
0.38
2.77
0.51
13
15
20
4
5
15
10
15
6
H

T. P. Burkholder et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1377–1380
1379
AcOH, NaCNBH3
O
OSi(R)
3
o
3
O
R
R
3
O
3a
R
O
EtO
2
O
or
R
EtO
2
R
N
N
7
K2CO3
Cl
H
R
7
R
OSi(R)
3
o
OSi(R)
4
2
3
R
o
R
3b
3
O
R
O
1. DDQ
HO
2
R
N
2. TBAF
3. NaOH
7
R
OH
1
o
R
Scheme 1. Synthesis of [1-(4-hydroxy-benzyl)-1H-indol-5-yloxy]-acetic acid compounds.
N
RoMgX
BBr3
Br
HO
o
O
o
Br
R
H2, Pd/C
R
(a)
Ni coupling
O
O
OH
15
N
H
N
H
13
14
R3SiCl,
Imidazole
6
5
Br
HBr, DMSO,
R3
AcOH
R3
R3
Tips
BuLi
DMF
Tips
O
HO
O
NaBH3CN
HOAc
o
o
imidazole
Tips-Cl
R
R
(b)
N
OH
16
O-Si(R)
17
N
N
H
3
H
H
8a, R3 = Me
8b, R3 = H
6a, R3 = Me
7a, R3 = Me
7b, R3 = H
Cl
OH
6b, R3 = H
CHO
Tips
NaBH4
MsCl, Et3N
HO
1. NaBH3CN
HOAc
O
O
o
imidazole
o
o
R
R
R
(c)
N
H
N
Tips-Cl
N
H
2. BBr3
O-Si(R)
H
O-Si(R)
3
O-Si(R)
3
3
10
9
11
3a
19
3b
R3
R3
Tips
Tips
O
O
Scheme 3. Synthesis of benzaldehyde and benzylic chloride intermediates.
1. TBAF
1. Boc2O
R2
R2
(d)
2. K2CO3,
BrCH2CO2Et
3. TFA
2. sec-BuLi
TMEDA
X-R7
N
N
H
R7
8 or 11
Boc
12
Table 2
R3
O
O
O
O
EtO
R2
HO
N
Boc
N
R7
2
OH
Scheme 2. Synthesis of indoline intermediates.
Ro
TR
Compound
Ro
TRb Ki (nM)
a
Ki (nM)
Ki TRb/TRa
and triglyceride concentrations. Animals were maintained on fruc-
tose diet for the course of the study. After eight days, non-fasted
animals were sacrificed and serum values of cholesterol and
triglycerides were generated. A pooled sample of serum from each
group was submitted to size exclusion chromatography to deter-
mine HDL content. Animals exhibited dose-dependent effects on
lipoprotein–cholesterol and serum triglycerides (Table 3). HDL
was elevated by up to 55% and triglycerides were reduced up to
64% while food intake was not affected.
1c
1j
1k
1l
1m
1n
1o
i-Propyl
t-Butyl
Benzyl
Cyclopentyl
CH2-cyclohexyl
Phenyl
0.38
0.30
0.54
0.64
1.11
3.14
8.04
7.74
9.98
20
33
68
51
42
29
25
36.53
32.33
46.55
89.89
199.97
Cyclohexyl
In summary, we designed and synthesized
a series of
ortho to the phenol. Evaluation of compound 1c from this series in
a model of dyslipidemia demonstrated positive effects on plasma
lipid endpoints.
[1-(4-hydroxy-benzyl)-1H-indol-5-yloxy]-acetic acid compounds
that were potent selective TRb agonists. The selectivity for TRb
receptors was further improved by modification of the substituent

1380
T. P. Burkholder et al. / Bioorg. Med. Chem. Lett. 25 (2015) 1377–1380
Blange, I.; Drvota, V.; Yen, P. M.; Sylven, C. Biol. Pharm. Bull. 1997, 20, 1123;
Table 3
Gomberg-Maitland, M.; Frishman, W. H. Am. Heart J. 1998, 135, 187; Klein, I.;
Ojamaa, K. N. Engl. J. Med. 2001, 344, 501.
3. Dietrich, S. W.; Bolger, M. B.; Kollman, P. A.; Jorgensen, E. C. J. Med. Chem. 1977,
20, 863; Jorgensen, E. C. In Thyroid Hormones and Analogs. II. Structure-Activity
Relationships; Choh Hao Li, Ed.; Academic Press: New York, 1978; Vol. VI, pp
107–204. Chapter 3.
4. Scanlan, T. S.; Yoshihara, H. A. I.; Nguyen, N. H.; Chiellini, G. Curr. Opin. Drug
Discovery Dev. 2001, 4, 614.
5. Borngraeber, S.; Budny, M. J.; Chiellini, G.; Cunha-Lima, S. T.; Togashi, M.;
Webb, P.; Baxter, J. D.; Scanlan, T. S.; Fletterick, R. J. Proc. Natl. Acad. Sci. U.S.A.
2003, 100, 15358.
6. Dow, R. L.; Schneider, S. R.; Paight, E. S.; Hank, R. F.; Chiang, P.; Cornelius, P.;
Lee, E.; Newsome, W. P.; Swick, A. G.; Spitzer, J.; Hargrove, D. M.; Patterson, T.
A.; Pandit, J.; Chrunyk, B. A.; LeMotte, P. K.; Danley, D. E.; Rosner, M. H.;
Ammirati, M. J.; Simons, S. P.; Schulte, G. K.; Tate, B. F.; DaSilva-Jardine, P.
Bioorg. Med. Chem. Lett. 2003, 13, 379.
Dose (mg/kg)
HDL
Serum triglycerides
0.1
0.3
1.0
3.0
10.0
Vehicle
22
39
55
42
35
—
204 10*
214 21*
177 20*
149 22*
106 19*
296 20
Compound 1c, dosed once a day. HDL values are HDL-cholesterol (mg/dL) percent
change from vehicle group from pooled samples for each group. Serum triglycerides
are mg/dL. N = 6 per group.
Significantly different from vehicle group (ANOVA followed by Fisher HSD post-
hoc test, p <0.05).
*
7. Grover, G. J.; Mellstrom, K.; Ye, L.; Malm, J.; Li, Y. -L. ; Bladh, L. -G. ; Sleph, P. G.;
Smith, M. A.; George, R.; Vennstrom, B.; Mookhtiar, K.; Horvath, R.; Speelman,
J.; Egan, D. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 10067; Hangeland, J. J.;
Doweyko, A. M.; Dejneka, T.; Friends, T. J.; Devasthale, P.; Mellstrom, K.;
Sandberg, J.; Grynfarb, M.; Sack, J. S.; Einspahr, H.; Farnegardh, M.; Husman, B.;
Ljunggren, J.; Koehler, K.; Sheppard, C.; Malm, J.; Ryono, D. E. Bioorg. Med. Chem.
Lett. 2004, 14, 3549.
8. Darimont, B. D.; Wagner, R. L.; Apriletti, J. W.; Stallcup, M. R.; Kushner, P. J.;
Baxter, J. D.; Fletterick, R. J.; Yamamoto, K. Genes Dev. 1998, 12, 3343.
9. Borror, A. L.; Chinoporos, E.; Filossa, M. P.; Herchen, S. R.; Petersen, C. P.; Stern,
C. A. J. Org. Chem. 1988, 53, 2047.
Supplementary data
Supplementary data (Experimentals for the synthesis of com-
pound 1c and procedures for TRa and TRb binding and functional
agonist assays.) associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.bmcl.2015.02.062.
10. Landi, J. J., Jr.; Ramig, K. Synth. Commun. 1991, 21, 167; Corey, E. J.;
Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190.
References and notes
11. Dillard, R. D.; Bach, N. J.; Draheim, S. E.; Berry, D. R.; Carlson, D. G.; Chirgadze,
N. Y.; Clawson, D. K.; Hartley, L. W.; Johnson, L. M.; Jones, N. D.; McKinney, E.
R.; Mihelich, E. D.; Olkowski, J. L.; Schevitz, R. W.; Smith, A. C.; Snyder, D. W.;
Sommers, C. D.; Wery, J. J. Med. Chem. 1996, 39, 5137.
12. Tamao, K.; Sumitani, K.; Kumada, M. J. Am. Chem. Soc. 1972, 94, 4374; Corriu, R.
J. P.; Masse, J. P. Chem. Commun. 1972, 144.
1. Yoshihara, H. A. I.; Scanlan, T. S. Curr. Top. Med. Chem. 2003, 3, 1601.
2. Lazar, M. A. Endocr. Rev. 1993, 14, 184; Forrest, D. Semin. Cancer Biol. 1994, 5,
167; Wilkstrom, L.; Johansson, C.; Salto, C.; Barlow, C.; Barrow, A. G.; Baas, F.;
Forrest, D.; Thoren, P.; Vennstrom, B. EMBO J. 1998, 17, 455; Johansoson, C.;
Gothe, S.; Forrest, D.; Vennstrom, B.; Thoren, P. Am. J. Physiol. 1999, 276, H2006;