Novel heterocyclic thyromimetics

Article abstract of DOI:10.1016/j.bmcl.2005.02.028

Novel heterocycle-fused thyromimetics are presented carrying indoles or indazoles instead of the phenolic group in T3. Potent agonists were identified in both series. SAR trends are examined and found to be mostly consistent with previously published thyromimetics. Moderate THRβ selectivity (approx. 10-fold) was observed in the indole series using isoform-selective transient THR transfection assays

Full text of DOI:10.1016/j.bmcl.2005.02.028

Bioorganic & Medicinal Chemistry Letters 15 (2005) 1835–1840
Novel heterocyclic thyromimetics
Helmut Haning,* Michael Woltering, Ulrich Mueller, Gunter Schmidt, Carsten Schmeck,
Verena Voehringer, Axel Kretschmer and Josef Pernerstorfer
BAYER HealthCare AG, Business Group Pharma, D-42096 Wuppertal, Germany
Received 2 November 2004; revised 7 February 2005; accepted 8 February 2005
Abstract—Novel heterocycle-fused thyromimetics are presented carrying indoles or indazoles instead of the phenolic group in T3.
Potent agonists were identified in both series. SAR trends are examined and found to be mostly consistent with previously published
thyromimetics. Moderate THRb selectivity (approx. 10-fold) was observed in the indole series using isoform-selective transient THR
transfection assays
Ó 2005 Elsevier Ltd. All rights reserved.
Thyroid hormones are important endocrine signalling
hormones that are involved in a number of physiological
processes such as lipid metabolism, control of energy
expenditure and pre- and neonatal brain development.
Molecules mimicking only the beneficial effects of the
thyroid hormones and lacking their cardiac side effects
(tachycardia and arrhythmia) potentially could be used
to treat a number of conditions such as obesity and
dyslipidaemia. The tachycardic potential of natural thy-
roid hormones can at least in part be attributed to the
effect on mRNA expression of the pacemaker channel
HCN2.1
the binding pocket.3 Figure 1 depicts thyromimetics,
for which selectivity for metabolic activity versus cardiac
effects has been described.
GC1(3),4 CGS 26214 (4)5 and L-94901 (5)6 have been
described as potent ligands for THR and are void of car-
diovascular side effects.
For GC1, modest selectivity for THRb has been de-
scribed (6.6-fold), which originated from the oxyacetic
acid headgroup.7 For CGS 26214 data on binding to
THR subtypes have not been reported. For L-949018
however, it was suggested that liver-selective nucleic
transport rather than selective binding to THRb might
be responsible for its selective activity.
Thyroid hormones are ligands of the nuclear thyroid
hormone receptors (THR), which fall into several iso-
forms and splice variants. The natural hormone T3 (2)
does not show significant selectivity in binding to any
of the THR isoforms. However,2 tissue distribution
and knock-out animal studies as well as results with
selective ligands suggest that cardiac side effects can be
attributed to the THRa isoform. This provides a ratio-
nale for the search for THRb selective agonists as car-
diovascular treatment option.
As early as 19866,9 it had been noted, that modifications
of the THR ligands in the outer ring adjacent to the phe-
nol can lead to improved selectivity for metabolic versus
cardiovascular effects. Recently, several new chemotypes
have been described with selectivity for THRb.10–12 The
amino acid sequence difference between the inner ligand
binding domains of THRa and THRb is a single amino
acid exchange: Ser277 in THRa for Asn331 in THRb,
interacting with the polar head groups of THR ligands.
The binding pocket of THRb seems to be able to accom-
modate ligands with larger groups on the outer ring by
movement of amino acid residues (e.g., Met 44211). This
flexibility may in part be responsible for increased
selectivity.
All attempts to identify therapeutically useful thyromi-
metics have been structurally derived from the natural
hormone probably because the ligand–receptor interac-
tion is very tight and the ligand is completely buried in
Keywords: Thyromimetics; Indoles; Indazoles.
*
While indole replacement of the phenol in T3 has ap-
peared in one patent application, neither examples with
Corresponding author. Tel.: +49 202 364459; fax: +49 202 364160;
e-mail: helmut.haning@bayerhealthcare.com
0960-894X/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.028

1836
H. Haning et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1835–1840
"tail"
I
"head"
I
O
NH2
HO
I
COOH
HO
O
O
COOH
R
R=I: L-T4
R=H: L-T3
1
GC1 3
2
F
NH
N
Br
O
O
NH2
COOH
HO
HO
O
HO
Br
N
H
COOH
CGS 26214 4
L-94901 5
Figure 1. Thyroid hormones L-T4 (1) and L-T3 (2), GC1 (3), CGS 26214 (4) and L-94901 (5).
substituents in the indole 3-position nor biological activ-
ity were reported.13
The syntheses are depicted in Schemes 1–6. The Fischer
indole synthesis starting from para-benzyloxy phenyl-
hydrazine and the corresponding aldehydes, followed
by hydrogenolytic deprotection yields the hydroxy-
indoles as coupling partners. Nucleophilic substitution
is followed by elaboration of the nitro group into the
final head group.
Herein we show that incorporation of indoles and indaz-
oles into the tail group of the biphenylether framework
of T3 yields potent, single-digit nanomolar THR ago-
nists. For the indole-type thyromimetics moderate selec-
tivity for THRb is shown.
Cyclopropyl is introduced as a substituent on the inner
ring via cyclopropanation of the divinyl derivative using
diazomethane and subsequent nitrogen extrusion.
(Scheme 2) The necessary divinyl precursor is generated
from the dibromoderivative using tributylvinylstannan.
Since the hydrogen bond between the phenol and
His435 (in THRb, His381 in THRa) is essential for po-
tent THR interaction, our fused heterocyclic design mo-
tif offered the ability to attach substituents in a position
similar to the iodine atom in T3, whilst maintaining the
hydrogen bond donor capability of the heterocycle. A
range of literature-known headgroups was used.
For the synthesis of the methylene bridged analogues,
an approach was followed that had in part been estab-
lished during the course of the synthesis of GC1:
(Scheme 3).
Indoles, indazoles, benzimidazoles and carbazoles were
prepared with a typical THR ligand substitution pat-
tern. Benzimidazoles and carbazoles did not show
THR agonistic activity. On the other hand, indoles
and indazoles demonstrated comparable activity in the
single digit nanomolar range.
The methylene spacer is constructed from a tertiary
alcohol, which was generated from addition of a dili-
thium-organometallic reagent to an indole benzalde-
hyde. The dilithiated nucleophile is generated from
R3
R3
O
O
(a)
OH
(b)
H2N
N
H
N
H
N
H
R1
F
R1
R1
R3
R3
NO2
R2
(d)
O
O
O
N
N
NO2
R2
R2
N
H
R4
H
H
(c)
Scheme 1. Synthesis of indole type thyromimetics. Reagents and conditions: (a) cat. H2SO4, MeOH, RCH2CHO, 3 h, RF; (b) Pd/C, MeOH, rt,
1 atm; (c) K2CO3, DMSO, rt–125 °C; (d) R4COCl, Py, NEt3.

H. Haning et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1835–1840
1837
Br
O
O
(a)
N
H
N
H
NO2
Br
NO2
(b)
(c)
O
O
O
N
H
N
H
CO2Et
N
H
NO2
Scheme 2. Synthesis of a dicyclopropyl-thyromimetic. Reagents and conditions: (a) Pd2(PPh3)4, vinylSnBu3, Tol, RF, 20 h, 81%; (b) i. CH2N2, 0 °C–
rt, 48 h, ii. xylene, RF, 2.5 h 64%; (c) i. SnCl2, 69%, ii. ClCOCO2Et, NEt3, 96%.
OH
(a)
CHO
Br
+
N
NHBoc
N
NHBoc
tbdms
tbdms
(b)
O
N
H
CO2Et
N
H
Scheme 3. Synthesis of methylene bridged indole thyromimetics. Reagents and conditions: (a) i. MeLi, THF, ꢀ78 °C, ii. n-BuLi, THF, ꢀ78 °C 13%;
(b) i. Et3SiH, TMSOTf, ii. TFA, iii. ClCOCO2Et, DCM, NEt3, 27%.
Br
Br
B(OH)2
(b)
(a)
N
H
N
tbdms
N
tbdms
HO
O
CO2tBu
O
N
tbdms
O
CO2tBu
(c)
O
(d)
N
O
COOH
H
21
Scheme 4. Synthesis of indole oxyacetic acid thyromimetics. Reagents and conditions: (a) NaH, THF, rt, TBDMSCl, 89%; (b) nBuLi, THF, ꢀ78 °C,
BOiPr3, 58%; (c) CuOAc2, mol. sieves, DCM, Py, NEt3, 62%; (d) NaOH, 87%.
4-bromo-3,5-dimethyl-bocaniline first by deprotonation
with methyllithium and subsequent halogen–metal ex-
change with n-BuLi.
Consistent with earlier observations in thyromimetic
SAR trends,16 branched alkyl substituents at R3 yield li-
gands with higher potency, the order of activity is
H < Me < iPr (6, 7, 8). Benzyl (9) substituents are also
tolerated in this position, however phenyl (10) and ben-
zoyl (11) groups seem to lack the necessary structural
flexibility and result in ligands with considerably
The SAR trends for the indole series are shown in Table
1. Potency was determined in a HEP-G2 whole cell as-
say containing both receptor isoforms.14

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H. Haning et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1835–1840
CF3
CF3
O
(b)
O
CF3
N
H
N
H
CO2H
F3C
(a)
(c)
CO2H
F3C
O
22
23
N
H
F3C
CHO
CF3
CF3
O
O
(d)
CN
COOH
N
N
H
F3C
F3C
H
24
Scheme 5. Synthesis of indole thyromimetics with all-carbon headgroups. Reagents and conditions: (a) i. PPh3CHCO2Et, 88%, ii. dioxan, 1 N
NaOH, 79%; (b) Pd/C, H2, 1 atm, 57%; (c) i. NaBH4, MeOH, rt, 97%, ii. PPh3Br2, AcCN, Py, 55%, iii. NaCN, DMF, 50 °C, 74%; (d) HOAc, H2SO4,
105 °C, 4 h, 15%.
O
OH
OHC
O2N
OH
OBn
OH
(b)
(a)
(c)
O2N
H2N
OH
O
(e)
(d)
O
O
N
N
N
N
H
N
H
NO2
N
H
CO2Et
N
H
25
Scheme 6. Indazole thyromimetics. Reagents and conditions: (a) i. BnBr, K2CO3, 99%, ii. pentyne, EtMgBr, 45%; (b) MnO2, 83%, ii. H2, 91%; (c) i.
NaNO2, HCl, ii. SnCl2 27%; (d) K2CO3, DMSO, rt–100 °C, 5 h, 47%; (e) i. H2, EtOH, Pd/C, 56%, ii. ClCOCO2Et, NEt3, 77%, iii. cat. NaOEt, EtOH,
70%.
Table 1. EC50 data for compounds 6–20
R1
R3
X
O
N
H
R4
R2
N
H
14
EC50 (nM)
Compd
R1
R2
R3
R4
X
Sel15 (a/b)
6
Me
Me
Me
CF3
CF3
CF3
Me
Cl
Me
Me
Me
CF3
CF3
CF3
Me
Cl
H
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Et
CO2Me
CO2Et
CO2Et
O
O
O
O
O
O
CH2
O
O
O
O
O
O
O
O
72
15
4
—
—
10
—
—
—
—
—
—
10
—
—
—
—
10
0.5
7
Me
iPr
8
9
4F–Bn
4F–Ph
4F–Bz
iPr
11
115
495
18
15
23
3
10
11
12
13
14
15
16
17
18
19
20
2
H
CF3
CF3
Me
Cl
CF3
CF3
Cl
H
iPr
iPr
iPr
4
Cl
Br
4
Br
Vinyl
Me
iPr
iPr
1.2
7.8
8
Vinyl
Cl
iPr
CH2CO2Et
0.2

H. Haning et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1835–1840
1839
reduced activity. The position of the alkyl substituent on
the indole ring seems to be flexible since the 2-methyl and
the 3-methyl derivative demonstrate the same activity.
Indoles and indazoles show a slightly different SAR: for
indoles the classical isopropyl substituent showed the
highest potency, for the indazoles n-pentyl proved to
be the optimal substituent (25, EC50 = 8 nM). This find-
ing has some precedence since n-hexyl (equivalent to
n-pentyl substituents in the indazole series) as a substitu-
ent adjacent to the phenol had demonstrated the best
activity among linear alkyl chains in a series of 30substi-
tuted 3,5-diiodthyronines.16
Substituents in the inner ring also exert an influence
on the potency, more lipophilic groups result in higher
potency (vinyl < Me, CF3, Cl < Br). This had also been
demonstrated for KB141 and its dibromo analogue.10b,17
However, the relatively large difference (approx. 20-
fold) reported between dimethyl- and dichloro ana-
logues from an azauracil series12 was not reflected in
the indole series.
Our findings, when taken together with other recent re-
ports, show that structural modifications in the head
group, the tail group and on the inner ring lead to
THR b selective ligands even though the site of amino
acid difference interacts only with the head groups of
the agonists.
Replacement of the bridging oxygen atom with a meth-
ylene group (12) reduced the activity by a factor of 4.
This result is in agreement with findings for very early
T3 analogues.18 However, for GC1 and its ether ana-
logue and for T3 and its methylene analogue the oppo-
site effect was reported, that is, the methylene analogue
was more potent.6,9,19
References and notes
1. Pachuki, J.; Burmeister, L. A.; Larsen, P. R. Circ. Res.
1999, 85, 498.
Compounds 8, 15 and 20 demonstrate a 10-fold THRb
selectivity in a functional assay using isoform-selective
transient THR transfections.15 Under these assay condi-
tions, L-T3 (2) demonstrates a 2-fold selectivity for the
activation of THRa while GC1 displays a 10-fold selec-
tivity for THRb.
2. (a) Plateroti, M.; Angelin-Duclos, C.; Flamant, F.;
Samarut, J. Endocr. Updates 2004, 22, 13; (b) Trost, S.
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Bladh, L.-G.; Koehler, K.; Garg, N.; Collazo, A. M. G.;
Litten, C.; Husman, B.; Persson, K.; Ljunggren, J.;
Grover, G.; Sleph, P. G.; George, R.; Malm, J. J. Med.
Chem. 2003, 46, 1580.
3. Wagner, R. L.; Apriletti, J. W.; McGrath, M. E.; West, B.
L.; Baxter, J. D.; Fletterick, R. J. Nature 1995, 378, 690.
4. Chiellini, G.; Apriletti, J. W.; Yoshihara, H. A.; Baxter, J.
D.; Ribeiro, R. C. J.; Scanlan, T. S. Chem. Biol. 1998, 5,
299.
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In addition to amide head groups the oxyacetic acid
headgroup found in GC1 also was combined with the in-
dole motif. For this purpose, a copper mediated cou-
pling20 between a phenol and a boronic acid was used
as depicted in Scheme 4.
Compound 21 demonstrated somewhat reduced THR
agonistic activity (24 nM) compared to the oxamic acid
amide analogue (8) and 4-fold selectivity in activation of
THRb.
Scheme 5 shows our approach to indole thyromimetics
with all-carbon headgroups. Wittig elongation followed
by hydrogenation leads to propionic acid derivatives
with excellent potency (2.6 nM, 23). The cinnamic acid
precursor demonstrates comparable activity (3.2 nM,
22). Reduction, bromination and conversion to the cyano
headgroup, followed by hydrolysis gave the corre-
sponding acetic acid derivative and the most potent
indole thyromimetic in our series (EC50 = 0.5 nM, 24).
However, none of these carbon head group derivatives
showed THRb selectivity.
6. Underwood, A. H.; Emmett, J. C.; Ellis, D.; Flynn, S. B.;
Leeson, P. D.; Benson, G. M.; Novelli, R.; Pearce, N. J.;
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Kaneko, A.; Mori, J.; Hara, M.; Kumagai, M.; Takeda,
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9. Koerner, D.; Schwartz, H. L.; Surks, M. I.; Oppenheimer,
J. H. J. Biol. Chem. 1975, 250, 6417.
The syntheses of indazole containing thyromimetics and
the corresponding hydroxyindazoles are depicted in
Scheme 6. Protection of the phenol is followed by a
Grignard addition. Oxidation, simultaneous deprotec-
tion and reduction of the triple bond delivers the desired
amino acetophenon, which is diazotized. The resulting
diazoniumion is reduced to the hydrazine, which closes
in situ to the indazole ring system.21 In the case of the
indazoles acylation using oxalylethylchloride yields dou-
ble acylation products, that is, the indazole is also acyl-
ated. The indazole nitrogen is liberated using catalytic
NaOEt in EtOH.
10. (a) Hangeland, J. J.; Doweyko, A. M.; Dejneka, T.;
Friends, T. J.; Devasthale, P.; Mellstro¨m, K.; Sandberg, J.;
˚
Grynfarb, M.; Sack, J. S.; Einspahr, H.; Fa¨rnegardh, M.;
Husman, B.; Ljunggren, J.; Koehler, K.; Sheppard, C.;
Malm, J.; Ryono, D. E. Bioorg. Med. Chem. Lett. 2004,
14, 3549; (b) Grover, G.; Mellstro¨m, K.; Ye, L.; Malm, J.;
Li, Y.-L.; Bladh, L. G.; Sleph, P. G.; Smith, M. A.;
George, R.; Vennstro¨m, B.; Mookhtiar, K.; Horvath, R.;
Speelman, J.; Egan, D.; Baxter, J. D. Proc. Natl. Acad.
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1840
H. Haning et al. / Bioorg. Med. Chem. Lett. 15 (2005) 1835–1840
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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.;
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expression vector for standardization of transfection
efficacy. Twenty hour later, test compounds were added
and after 48 h incubation period the cells were lysed and
luciferase was quantified as above. As controls THR
expression vectors were omitted. EC50s were determined
by at least three independent transfection assays for either
THR. Each transfection assay comprised 96 single trans-
fections used for mean values of induction factor calcu-
lation for nine concentrations of the agonist and the
solvent control. Standard deviation did not exceed 30% of
the respective mean value. Selectivities are reported as
ratios EC50 (THRa)/EC50 (THRb).
13. Chiang, Y.-C. P.; Dow, R. L. PCT Int. Appl. 2000, WO
2000051971; Chem. Abstr. 2000, 133, 207681.
16. Leeson, P. D.; Ellis, D. E.; Emmett, J. C.; Shah, V. P.;
Showell, G. A.; Underwood, A. H. J. Med. Chem. 1988,
31, 37.
17. Ye, L.; Li, Y.-L.; Mellstro¨m, K.; Mellin, C.; Bladh, L. G.;
Koehler, K.; Garg, N.; Collazo, A. M. G.; Litten, C.;
Husman, B.; Persson, K.; Ljungren, J.; Grover, G.; Sleph,
P. G.; George, R.; Malm, J. J. Med. Chem. 2003, 46, 1580.
18. Tripp, S.; Block, F. B.; Barile, G. J. Med. Chem. 1973,
16(1), 60.
19. Jorgensen, E. C.; Murray, W. A. J. Med. Chem. 1974, 17,
434.
20. Evans, D. A.; Katz, J. L.; West, T. R. Tetrahedron Lett.
1998, 39, 2937.
14. HepG2 (containing both THRa and THRb) cells were
stably transfected with a firefly luciferase vector containing
a promoter with a THRE consisting of two inverted
palindromic THREs (AGGTCATGACCT) separated by
eight nucleotides. Cells were seeded into 96-well plates in
EagleÕs minimal essential medium supplemented with Glu,
non-essential amino acids, insulin, Se and transferrin and
were cultivated for 48 h. Test compounds and at-retinoic
acid were added and the cultures were incubated for 48 h.
After lysis, luciferin was added and luciferase activity was
measured by a camera system.
15. HUH-7 were transfected in 96-well microtitre plates by a
FuGENE-6 mix according to the manufacturerÕs protocol
(Roche Diagnostics) containing an expression vector for
either human THRa 1 or human THRb 1, a human RXR-
a expression vector and a firefly luciferase promoter
reporter with two THRE as above and a b-galactosidase
21. (a) Burnett, J. P.; Ainsworth, C. J. Am. Chem. Soc. 1958,
23, 1382; (b) Ainsworth, C. J. Am. Chem. Soc. 1957, 79,
5242; (c) Piozzi, F.; Ronchi, A. U. Gazz. Chim. Ital. 1963,
93, 3; (d) Simon, U.; Sues, O.; Horner, L. Justus Liebigs
Ann. Chem. 1966, 697, 17.