2-Arylindoles as gonadotropin releasing hormone (GnRH) antagonists: Optimization of the tryptamine side chain

Article abstract of DOI:10.1016/S0960-894X(02)00026-4

A series of 2-arylindoles containing novel heteroaromatic substituents on the tryptamine tether, based on compound 1, was prepared and evaluated for their ability to act as gonadotropin releasing hormone (GnRH) antagonists. Successful modifications of 1 included chain length variation (reduction) and replacement of the pyridine with heteroaromatic groups. These alterations culminated in the discovery of compound 27kk which had excellent in vitro potency and oral efficacy in rodents.

Full text of DOI:10.1016/S0960-894X(02)00026-4

Bioorganic & Medicinal Chemistry Letters 12 (2002) 827–832
2-Arylindoles as Gonadotropin Releasing Hormone (GnRH)
Antagonists: Optimization of the Tryptamine Side Chain
Jonathan R. Young,a,* Song X. Huang,a Thomas F. Walsh,a Matthew J. Wyvratt, Jr.,a
Yi Tien Yang,b Joel B. Yudkovitz,b Jisong Cui,b George R. Mount,b Rena Ning Ren,b
Tsuei-Ju Wu,b Xiaolan Shen,c Kathryn A. Lyons,d An-Hua Mao,d Josephine R. Carlin,d
a
Bindhu V. Karanam,d Stella H. Vincent,d Kang Chengb andMark T. Goulet
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
bDepartment of Biochemistry & Physiology, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
cDepartment of Comparative Medicine, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
dDepartment of Drug Metabolism, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
Received22 October 2001; accepted10 January 2002
Abstract—A series of 2-arylindoles containing novel heteroaromatic substituents on the tryptamine tether, based on compound 1,
was prepared and evaluated for their ability to act as gonadotropin releasing hormone (GnRH) antagonists. Successful modifica-
tions of 1 included chain length variation (reduction) and replacement of the pyridine with heteroaromatic groups. These alterations
culminatedin the discovery of compound 27kk which hadexcellent in vitro potency andoral efficacy in rodents. # 2002 Elsevier
Science Ltd. All rights reserved.
Previous communications from these laboratories
described 3-arylquinolones1 and2-arylindoles 2 as potent
non-peptidyl GnRH antagonists (Fig. 1). Notable dis-
coveries within the 2-arylindole class included the
potency enhancing 3,5-dimethylphenyl2a andthe C(5)-
dimethylacetamide substituents,2d,e introduction of a
pyridine side chain terminus as a replacement for the
earlier metabolically labile phenol group,2f andincor-
poration of the (S)-b-methyl group which improvedthe
selectivity profile.2g Indole 1 was characterizedby
excellent in vitro potency in both our binding (hGnRH)
tether andendterminus. In this letter, we will present a
series of heteroaromatic replacements for the pyridine
moiety of GnRH antagonist 1.
The tryptamine core of 1 (25, Scheme 1) was assembled
by either Fischer4 or Larock5 methods as previously
described. Attachment of the heteroaromatic side chain
was accomplishedby either reudctive amination of
aldehydes 23 with tryptamines 25, or by the Fukuyama6
amine synthesis which requiredalcohols 24 andsulfo-
namides 26. The requisite aldehydes and alcohols were
preparedby stanadrdmethosd that will be outlined
below. Installation of the side chains found in com-
pounds 23 and 24 was envisionedto be derivedfrom a
halogenatedprecursor such as 22.
3
andfunctional assays (hPI) andhadgoodoral bio-
availability in dogs.
Despite these favorable characteristics, the in vivo
duration of action for indole 1 was low. We attributed
this in part to low systemic drug levels and fast clear-
ance. At this juncture, we were confident that the 2-aryl
and5-acetamide positions were optimizedfor potency
and pharmacokinetics. To improve this lead, we studied
the structure–activity relationships of the tryptamine
A number of heterocyclic syntheses are outlinedin
Schemes 2–6. 6-Bromoisoquinoline 4 was preparedfrom
*Corresponding author. Fax:+1-732-594-2210; e-mail: jonathan_
young@merck.com
Figure 1. Biological properties of potent GnRH antagonist 1.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00026-4

828
J. R. Young et al. / Bioorg. Med. Chem. Lett. 12 (2002) 827–832
.
Scheme 4. Reagents andconidtions: (a) Cl 3Fe 6H2O, H2NNH2,
MeOH (97%); (b) (i) MeOC(NH)CCl3, 5% AcOH (aqueous) (62%);
(ii) MeOH, 67 ꢀC (90%); (c) NaH, DMF; SEMCl (77%); (d) (i)
DIBAL-H, CH2Cl2, ꢁ78 ꢀC (74%); (ii) NaBH4, MeOH (100%); (e)
TBSCl, imidazole, DMF (98%); (f) HC(OEt)3, 125 ꢀC (79%); (g)
NaNO2, 5% AcOH (75%).
Scheme 1. Retrosynthetic preparation of indole analogues.
Scheme 2. Reagents andconditions: (a) NaBH 4, MeOH (100%); (b)
H2SO4, C6H6, 80 ꢀC (92%); (c) (i) O3, CH2Cl2–MeOH, ꢁ78 ꢀC; (ii)
Me2S; (d) NH4OH (37%).
Scheme 5. Reagents andconditions: (a) (i) Ac 2O, DMAP, pyridine,
CH2Cl2 (97%); (ii) H4NO2CH, 10% Pd/C, MeOH, 60 ꢀC (78%); (b) (i)
KNO3, TFAA (89%); (ii) K2CO3, MeOH; (c) ClCO2Me, 1,2-DCE,
pyridine (60%); (d) K2CO3, MeOH (88%); (e) (i) Ph3P, DEAD, 26,
benzene; (ii) n-PrNH2, CH2Cl2 (64% two steps); (f) H4NO2CH, 10%
Pd/C, MeOH, 60 ꢀC (100%); (g) AcOH, 110 ꢀC (60%).
Scheme 3. Reagents andconditions: (a) (i) ethoxymethylene malonate,
xylenes, 135 ꢀC (91%); (ii) Ph2O, 230 ꢀC (48%); (b) KOH, EtOH;
K2CO3, BnBr, DMF; (c) (i) NaOH, EtOH; (ii) KCN, DMSO, 100 ꢀC
(78%); (d) H4NO2CH, Pd(OH)2, MeOH (76%); (e) POCl3, toluene,
100 ꢀC (76%).
Scheme 6. Reagents andconditions: (a) NaH, DMF; SEMCl (60%);
.
(b) Cl3Fe 6H2O, H2NNH2, MeOH (97%); (c) NaNO2, HBF4, H2O;
(d) tributyl(vinyl)tin, Pd2(dba)3, MeCN (40%).
5-bromoindanone 2 by ketone reduction and sub-
sequent acidcatalyzedelimination of the alcohol to
indene 3 (Scheme 2). Ozonolysis of indene 3 andsub-
sequent treatment with ammonium hydroxide gave iso-
quinoline 4.7
9 with methyl 2,2,2-trichloroacetimidate10a followedby
methanolysis11 and N-SEM protection12 gave 10.
Reduction of 10 followedby TBS protection of the
alcohol provided 11. Alternatively, exposure of 9 to
triethyl-orthoformate10b andsubsequent protection
furnished 12. Benzotriazole 13 was readily prepared by
treatment of 9 with sodium nitrite13 in acetic acidand
subsequent protection of the heterocyclic nitrogen.
Condensation of 2-amino-6-methylpyridine
5 and
ethoxymethylene malonate followedby thermal cycliza-
tion, ester hydrolysis, and double benzylation afforded
the vinylogous-amide 6 (Scheme 3).8 The C(6)-methyl
group provedcritical for the Friedel–Crafts cyclization.
Omission of this substituent resultedin intramolecular
acylation on the pyridine nitrogen. Removal of the ester
was accomplishedin two steps by first, hydrolysis of the
ester followedby decarboxylation of the acidwith KCN
in hot DMSO.9 Napthyridine 7 was completedby
hydrogenation of the N-benzyl substituent andchlor-
ination (POCl3).
For benzimidazalinone 18, we foundit expeident to
begin with the tether pre-installed. After acetylation of
14 andnitro group reudction, nitration (potassium
nitrate and trifluoroacetic anhydride) occurred ortho to
the aniline concomitant with trifluoroacetamide protec-
tion of the aniline (Scheme 5). Unfortunately, selective
hydrolysis proved to be capricious. Thus, treatment
under basic methanol conditions provided amino-
alcohol 15. Global protection of 15 using methyl chlor-
oformate andselective hydrolysis of the carbonate gave
intermediate 16. Using the protocol of Fukuyama, 16
and 26 were subjectedto Mitsunobu conditions and
subsequent desulfonylation under basic conditions
Benzimidazoles 10, 11 and 12 along with benzotriazole
13 were preparedfrom diamine 9, which was available
from hydrazine reduction of 8 (Scheme 4). Treatment of

J. R. Young et al. / Bioorg. Med. Chem. Lett. 12 (2002) 827–832
829
C, H4NO2CH).21 Finally, alcohols 24 were combined
with sulfonamides 26 under Mitsunobu conditions fol-
lowed by base catalyzed desulfonylation which provided
indoles 27 (Scheme 8).
As a continuation of the pyridylbutyl end group SAR,
we have recently prepareda series of fusedpyridine
mimetics (i.e., quinolines andisoquinolines) with varied
tether lengths in order to improve the in vivo properties
Scheme 7. Reagents and conditions: (a) 2-propenal 1,1-diethyl acetal
(or 3-butenal 1,1-diethyl acetal), 9-BBN, THF; Cl2Pd(dppf), K3PO4,
70 ꢀC; (b) TFA, H2O; (c) Cl2Pd(Ph3P)2, NaO2CH, DMF, 95 ꢀC (n=0);
(d) tributyl(vinyl)tin, Cl2Pd(Ph3P)2, DMF, 95 ꢀC; (e) 9-BBN, THF;
H2O2, NaOH; (f) NBS, DMSO-H2O; (g) t-BuOK, THF-t-BuOH; (h)
10% Pd/C, H4NO2CH, MeOH, 65 ꢀC.
Table 1. SAR of 27 with quinoline andisoquinoline end-groups
27
a
R1
R2
hGnRH
IC50 (nM)a
hPI
IC50 (nM)b
Scheme 8. Reagents and conditions: (a) 2,4-dinitrobenzenesulfonyl
chloride, NaHCO3, H2O–CH2Cl22; (b) 23, 25, NaCNBH3, HOAc-
THF-MeOH; (c) 24, 26, Ph3P, DEAD, benzene; (d) n-PrNH2, CH2Cl2.
27.5
420
furnished 17. After reduction of the nitro group, the o-
14
b
c
d
e
f
15.3
2.4
1.9
3.0
1.1
2.1
2.1
0.7
3.6
0.6
0.9
1.5
315
131
46.4
17.8
11.5
7.5
aminocarbamate was refluxedin acetic acid
afforded benzimidazalinone 18.
which
Diazonium salt 20 was utilizedthe as the nucleophillic
component in the Stille coupling reaction (Scheme 6).
Protection of heterocycle 19 with SEM-Cl was followed
by nitro group reduction and oxidation which afforded
diazo compound 20. The ensuing Stille reaction with
tributyl(vinyl)tin was very sensitive to the counter ion.
In fact, the tetrafluoroborate ion was requiredfor the
successful coupling. In contrast to couplings with aryl
bromides or triflates, diazonium intermediate 20 parti-
cipatedin the Stille reaction at temperatures as low as
ꢁ20 ꢀC.15
g
h
i
Specifically for the 3- and4-atom tethers ( n=3 and4,
respectively), the appropriate alkenal 1,1-diethyl acetal
was transformedto the trialkyl borane andreactedwith
the heteroaromatic intermediates 22 under the Suzuki
coupling protocol (Scheme 7).16 Subsequent hydrolysis
of the acetal with aqueous TFA liberated aldehydes 23.
The benzaldehydes (n=0) were in turn preparedby a
palladium catalyzed formylation.17 Aldehydes 23 were
then coupledto amines 25 under reductive-amination
conditions (Scheme 8).
10.2
4.6
j
5.8
Since phenylacetaldehydes failed to couple cleanly
under reductive amination protocols, we opted for
Fukuyama’s methodfor amine synthesis which required
alcohols 24. Stille18 coupling of intermediates 22 with
tributyl(vinyl)tin furnishedthe styrene derivatives which
were transformedto alcohols 24 via hydroboration–
oxidation sequence for many substrates. In some cases
however, certain heterocycles were susceptible to reduc-
tion with 9-BBN. In those instances, the styrene was
k
l
34.7
5.0
m
6.8
convertedto the bromohyrdin (NBS-H
2O),19 base
aInhibition of [125I]buserelin binding to human pituitary GnRH
receptor.
bInhibition of GnRH-stimulated[ 3H]inositol phosphate hydrolysis.
induced closure to the epoxide (t-BuOK),20 followedby
benzylic reduction to give the desired alcohol (10% Pd/

830
J. R. Young et al. / Bioorg. Med. Chem. Lett. 12 (2002) 827–832
Table 2. SAR of 27 with novel heteroaromatic end-groups
andoff-target activities of the indole lead 1 (Table 1). In
our initial series, 5-alkylquinolines 27a, 27c, and 27d
were synthesizedwithout the chiral methyl substituent.
Comparison of 27a and 27b revealedthat the ( S)-b-
methyl group afforded a two fold improvement in binding
affinity to the GnRH receptor. In addition, this group
also impartedgreater selectivity versus off target activ-
ities. Compounds 27c and 27d were noticeably more
potent than 27a, indicating a preference for the longer
tether designs (3 or 4 atoms). The remaining quinolines
andisoquinolines ( 27e–m) in Table 1 displayed similar
in vitro activities to one another (hPI IC50’s=5–35 nM).
Compound 27k, the only two-atom tether analogue in
Table 1, demonstrated the best binding affinity to the
GnRH receptor but the functional activity was less than
predicted.
27
n
R1
R2
hGnRH
IC50 (nM)a
hPI
IC50 (nM)b
1.1
0.6
16.2
8.4
o
p
q
To distinguish the two-atom and four-atom tether
designs, we obtained pharmacokinetic data (dog) on
compounds 27k and 27m. The shorter tether revealed
several advantages during this in vivo comparison. For
example, the terminal plasma half-life of 27k was dou-
bled(1.9 vs 1.0 h), the plasma clearance rate was nearly
10 times less (5.8 vs 49.2 mL/min/kg) andthe oral
bioavailability was over 2-foldhigher than 27m (10 vs
4%). These findings were consistent with other studies
in our group andwas later foundto be consistent across
heteroaromatic termini.
0.8
0.4
10.9
7.9
r
0.4
0.4
0.3
0.4
0.3
16.5
0.5
0.4
0.6
0.3
1.1
4.6
2.5
4.5
2.6
4.5
43.9
11.4
2.0
3.9
9.0
1.5
Encouragedby the initial results from the quinoline
class, we surveyeda variety of more sophisticatedhet-
erocycles. Napthyridines 27n–o, pyrazinylpyridines 27p–q
along with imidazopyridine 27r displayed very pro-
mising in vitro results, although no improvement in
potency over the quinolines andisoquinolines was
observed(Table 2). A modest preference for the four-
atom tether was observedin this series. Interestingly,
imidazopyridine 27s, attachedthrough a two-carbon
tether, demonstrated functional antagonism equal to
27r. This was in contrast to that observedin the iso-
quinoline series (cf. 27k and 27m). The closely related
benzimidazole analogue 27u was equipotent to 27s.
Additionally, the isoquinuclidine amides (27t and 27v)
were indistinguishable in terms of in vitro potency from
the azanorbornane amides (27s and 27u).2i Heteroatom
variations of the five-memberedring, incluidng ben-
zothiadiazole 27x, benzoxazoles 27y, 27z and 27aa and
indazole 27bb, provided additional examples of func-
tionally potent GnRH antagonists.
s
t
u
v
w
x
y
Basedon the attractive properties of 27s–v, we obtained
pharmacokinetic data (dog) on these analogues. In gen-
eral, the benzimidazoles were cleared from plasma more
slowly andhadlonger terminal half-lives as compared
to the imidazopyridines. This data in part accounts for
the large discrepancy in oral bioavailability between 27v
and 27t (63% vs 16%, respectively). During these stud-
ies, however, we discovered that 27t and 27v were
z
aa
bb
potent inhibitors of the cytochrome P450 3A4 enzyme
22
0
with IC50 s of 4 mM and100 nM, respectively. In fact,
Cyp3A4 was a common liability in this class of analo-
gues; for example, 27k, 27x and 27y were foundto have
activities between (IC50=1–2 mM).
aInhibition of [125I]buserelin binding to human pituitary GnRH
receptor.
bInhibition of GnRH-stimulated[ 3H]inositol phosphate hydrolysis.

J. R. Young et al. / Bioorg. Med. Chem. Lett. 12 (2002) 827–832
Table 3. SAR of 27 with novel heteroaromatic end-groups
831
Basedon the superior pharmacokinetic profile of the
benzimidazole class, we formulated a strategy to
attenuate the P450 inhibitory properties while maintain-
ing GnRH potency in this series. Incorporation of a
C(2)-alkyl substituent on the benzimidazole (27cc and
27dd) successfully diminished the P450 inhibition (5 and
>10 mM, respectively, vs 100 nM), yet with con-
comitant reduction of functional GnRH potency (Table
3). The functional potency was restoredwith ester
(27ee), acid( 27ff) andhydroxymethyl ( 27gg) substitu-
tion. Acid 27ff hada low plasma clearance rate in dog
(24.6 mL/min/kg) compared to all other acids tested,
but hadno oral bioavailability.
27
cc
R1
R2
hGnRH
IC50 (nM)a
hPI
IC50 (nM)b
0.4
13
The potency of benzimidazoles 27ee–gg reinforcedthe
notion that basic substituents were not requiredfor
2c
functional antagonism (cf., phenols andsulfonamides)
dd
ee
ff
0.9
0.3
0.4
0.7
0.6
0.7
0.6
0.3
0.6
101
7.6
6.5
5.0
2.0
27.9
2.8
2.5
2.0
andthat polar groups capable of participating in
hydrogen bonding interactions were well tolerated.
Benzimidazolinone 27 hh was a promising compoundin
terms of GnRH potency (PI: IC50=2.0 nM) but suf-
feredfrom low oral bioavailability (5%) in odgs.
Fluorine incorporation onto the benzimidazolinone
(27ii) hada negative impact on GnRH functional
activity. Finally, benzotriazole derivatives 27jj, 27kk,
and 27ll demonstrated an acceptable balance between
functional activity andP 450 inhibition (IC50=7.5 mM for
27kk). Moreover, 27kk and 27ll hadmoedrate oral
bioavailability (20 and37% in dogs, respectively) with
low plasma clearance (5.0 and9.7 mL/min/kg, respec-
tively) andreasonable terminal half-lives (7.2 and2.7 h,
respectively).
gg
hh
ii
jj
Benzotriazole 27kk provedto be efficacious in the
castratedrat assay which measures the ability of a
GnRH antagonist to reduce the circulating levels of LH.
In an intact rat, the levels of LH are pulsatile andvar-
ied, whereas in a castrated rat the levels are more con-
sistent andelevated. The first six h of the experiment
servedto establish the basal level of circulating LH for
the specific rat (Fig. 2). Compound 27kk effectively
inhibitedLH release over a range of oral doses (1, 5, 10,
and20 mpk). In a representative experiment, a single
oral dose at 10 mpk substantially reduced plasma LH
levels for approximately 14 h.
kk
ll
aInhibition of [125I]buserelin binding to human pituitary GnRH
receptor.
bInhibition of GnRH-stimulated[ 3H]inositol phosphate hydrolysis.
In conclusion, we have demonstrated that the pyridine
ring can be replacedwith a variety of heteroaromatic
rings while maintaining GnRH potency. In addition, a
basic substituent on the tryptamine tether was not cri-
tical for GnRH antagonist activity as hadbeen pre-
viously shown with the phenol andsulfonameid
analogues. In this study, benzotriazole 27kk was a
potent, orally active GnRH antagonist that sub-
stantially decreased the cytochrome P450 3A4 inhibi-
tory activity over the closely relatedpyriidne and
benzimidazole series.
Acknowledgements
Figure 2. Effect of oral 27kk (administration at time=6 h indicated by
arrow) on LH release in a castratedmale rat, as describedin ref 2i.
The data shown are for a single male rat (n=1).
We wouldlike to thank Amy Bernick andDr. Lawrence
Colwell for providing mass spectral data and to Gerard

832
J. R. Young et al. / Bioorg. Med. Chem. Lett. 12 (2002) 827–832
Kieczykowski, Robert Frankshun, Amanda Makar-
ewicz andJoseph Simeone for preparation of several
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3. See ref 1c for details concerning the in vitro assays. The
0
IC50 s reportedherein are minimally an average of two titra-
tions.
4. For synthesis of the tryptamine core via a Fischer-indole
method, see ref 2g.