Substituted indole-5-carboxamides and -acetamides as potent nonpeptide GnRH receptor antagonists

Article abstract of DOI:10.1016/S0960-894X(01)00274-8

The 2-aryltryptamine class of GnRH receptor antagonists has been modified to incorporate carboxamide and acetamide substituents at the indole 5-position. With either a phenol or methanesulfonamide terminus on the N-aralkyl side chain, potent binding affin

Full text of DOI:10.1016/S0960-894X(01)00274-8

Bioorganic & Medicinal Chemistry Letters 11 (2001) 1723–1726
Substituted Indole-5-carboxamides and -acetamides as Potent
Nonpeptide GnRH Receptor Antagonists
Wallace T. Ashton,a,* Rosemary M. Sisco,a, Yi Tien Yang,b Jane-Ling Lo,b
a
Joel B. Yudkovitz,b Kang Chengb and MarkT. Goulet
aDepartment of Medicinal Chemistry, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
bDepartment of Biochemistry and Physiology, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
Received 16 March 2001; accepted 19 April 2001
Abstract—The 2-aryltryptamine class of GnRH receptor antagonists has been modified to incorporate carboxamide and acetamide
substituents at the indole 5-position. With either a phenol or methanesulfonamide terminus on the N-aralkyl side chain, potent
binding affinity to the GnRH receptor was achieved. A functional assay for GnRH antagonism was even more sensitive to struc-
tural modification and revealed a strong preference for branched tertiary amides. # 2001 Elsevier Science Ltd. All rights reserved.
The gonadotropin-releasing hormone (GnRH; also
known as luteinizing hormone-releasing hormone, or
LHRH) receptor is an attractive target for pharmacol-
ogical suppression of sex hormone levels.1 GnRH ago-
nists, which, upon chronic administration, down-
regulate the receptor and desensitize the pituitary to
GnRH stimulation, have been valuable agents in the
treatment of such conditions as hormone-dependent
cancers, endometriosis, and uterine fibroids, as well as
in assisted reproduction.1,2 More recently, peptide
GnRH antagonists, which avoid the initial ‘flare-up’
problem, have achieved clinical success.1,3 The first
nonpeptide GnRH antagonists, offering the possibility
of oral bioavailability, have recently been described in
three heterocyclic series: thieno[2,3-b]pyridin-4-ones,4
quinolones,5 and indoles.6 Indole GnRH antagonists
reported from these laboratories include 1a–c.6 In the
present investigation we have further explored sub-
stituents at the indole 5-position.
acid (2) with the 3-chloropropyl ketone 37 in ethanol
yielded, after complete esterification, the tryptamine
derivative 4. (See below for further discussion of this
reaction.) To prepare the ‘right-hand’ side chain, reduc-
tive amination of aldehyde 56a by 4 gave the secondary
amine 6. The use of anhydrous magnesium sulfate (5
equiv) promoted formation of the imine, which was
The first series of compounds (indole-5-carboxylates and-
carboxamides bearing phenol end groups) was prepared
according to Scheme 1. Heating 4-hydrazinobenzoic
Scheme 1. Conditions: (i) EtOH, Á; (ii) cat. H2SO4, EtOH, Á; (iii) (a)
MgSO4, CDCl3, ꢀ5 to ꢀ10 ꢁC; (b) NaBH4, MeOH, ꢀ5 ꢁC; (iv) H2,
Pd(OH)2/C, EtOH–EtOAc, AcOH; (v) Cbz-Cl, i-Pr2NEt, CH2Cl2–
THF, ꢀ78 ꢁC; (vi) (a) KOH, MeOH–H2O, Á; (b) H+; (vii) R1R2NH,
PyBOP, Et3N, CH2Cl2.
*Corresponding author. Fax: +1-732-594-5350; e-mail: wally_ashton@
merck.com
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00274-8

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W. T. Ashton et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1723–1726
reduced in situ by sodium borohydride. Debenzylation
of 6 by catalytic hydrogenation afforded 7. For further
elaboration, 6 was converted to its Cbz derivative and
then saponified to give the acid 8. Full deprotection by
hydrogenolysis yielded 9. Amide derivatives 10–14 were
obtained by coupling 8 with amines in the presence of
PyBOP reagent, followed by hydrogenation.
The preparation of tryptamines by reaction of arylhy-
drazines with 3 merits further comment. This variation
of the Fischer indole synthesis has been extensively
studied by Grandberg.9 Although the reaction was well
documented for g-chloro aldehydes and aliphatic
ketones, it was reported9,10 that reaction of phenylhy-
drazine with 3-chloropropyl phenyl ketone yielded only
a tetrahydropyridazine product, not the tryptamine. We
have confirmed a finding from these laboratories11 that
tryptamine products can indeed be isolated from the
reaction of arylhydrazones with 3. Although the tetra-
hydropyridazine byproduct always appears to pre-
dominate, it is readily separable. The initially formed
hydrazone 50 can cyclize by either of two pathways
(Scheme 4). Depending on which of the hydrazone
nitrogen atoms displaces the chloro group, ring closure
can proceed via path a to the six-membered tetra-
hydropyridazine 51 or via path b to a five-membered
ring intermediate 52. The latter, as proposed by Grand-
berg9 in related cases, can undergo a rearrangement
leading to the tryptamine 53. Our observations suggest
that the relative proportions of 51 and 53 depend, in
part, on the electronic character of the substituent X.
Electron-withdrawing substituents might be expected to
preferentially deactivate the nitrogen adjacent to the
aryl group, thus disfavoring path a relative to path b.
For example, tryptamine 4 (X=EtO2C) was obtained in
29% yield, whereas yields of 40, in which the ester is
removed by one carbon atom from the aromatic
ring, ranged from 11–16%. (For X=NO2, yields of
up to 40% for 53 have been obtained by this
route11).
The second series of compounds (Scheme 2) was similar,
except that the phenolic OH, a possible metabolic and
toxicological liability, was replaced by methanesulfon-
amide. Conversion of acid 15 to its Weinreb amide fol-
lowed by hydrogenation of the nitro group yielded 16.
Treatment with methanesulfonyl chloride in pyridine
afforded 17, which was reduced to aldehyde 18. Further
transformation to compounds 19–33 was conducted as
described for Scheme 1.
For the derivatives containing homologated ester or
amide substituents at the indole 5-position (Scheme 3),
(4-aminophenylacetate) esters 348 were converted to the
hydrazines 35 by diazotization followed by stannous
chloride reduction. Reaction of 35 with chloroketone 3
as in Scheme 1 afforded the tryptamines 36. Further
transformations according to the conditions of Schemes
1 and 2 yielded final products 37–49.
The compounds of this investigation were evaluated for
the ability to compete with [125I]-buserelin (a GnRH
receptor agonist) for binding to the rat GnRH recep-
tor.5b It eventually became apparent that, in the absence
of bovine serum albumin (BSA) (Protocol A), the test
compounds adhered significantly to the binding assay
tubes, thus reducing the available concentration in
solution. Because this effect was proportionally greater
at low compound concentration, the resulting artificial
elevation of IC50 levels became more of a concern as
potency increased. The addition of 0.1% BSA to the
assay mixture (Protocol B) mitigated this nonspecific
interaction, possibly by neutralizing charges on the
surface of the glass tubes. Consequently, we regard the
IC50 determinations using Protocol B as a more accu-
rate indication of receptor binding affinity. Results from
.
Scheme 2. Conditions: (i) MeONHMe HCl, BOP, Et3N, CH2Cl2; (ii)
H2, Pd(OH)2/C, EtOH; (iii) MeSO2Cl, pyridine; (iv) LiAlH4, THF,
0 ꢁC.
Scheme 3. Conditions: (i) (a) NaNO2, HCl, H2O; (b) SnCl2, concd
HCl; (c) Na2CO3; (ii) 3, EtOH, Á.
Scheme 4. Postulated mechanism of formation of tryptamines and
tetrahydropyridazines.

W. T. Ashton et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1723–1726
1725
both sets of assay conditions are provided in order to
facilitate comparisons within each series. In addition,
most compounds were tested in a functional assay that
measured inhibition of GnRH-stimulated LH release
from rat primary pituitary cells.5b
spacer between the indole 5-position and the ester or
amide carbonyl group. Although no clear trends were
evident in the binding assay, some structure–activity
relationships were apparent in the LH release assay.
Among the esters (37–39), increasing the substitution on
the spacer atom markedly improved potency, from
2000 nM in the parent acetate 37 to 150 nM in the gem-
dimethyl derivative 39. To a lesser extent, a similar trend
held true for the amides (44 vs 40, 45 vs 41, and 49 vs 42).
Several medium-sized tertiary amides in the gem-dime-
thyl series (45–49) were relatively potent inhibitors of
GnRH-induced LH release (IC50 values<100 nM). In
the case of the 5-carboxamides (Table 2), the 2,5-dime-
thylpyrrolidine amide 32 was 5-fold more potent in
blocking LH release than the corresponding unsub-
stituted pyrrolidine derivative 29. For the gem-dimethyl-
substituted acetamides, however, the unsubstituted
pyrrolidine derivative 48 was at least as active as the 2,5-
dimethylpyrrolidine 49.
In the first series of compounds bearing a phenolic ter-
minus (Table 1), an ethyl ester at the indole-5-position
(7) was well tolerated, but hydrolysis to the free acid (9)
led to a 20-fold loss of binding affinity. Carboxamides,
however, proved to be most propitious, particularly
those derived from secondary amines (10, 11, 13, and
14), which were approximately 10-fold more potent
than the parent analogue 1a. The dimethyl amide 10
also compared favorably with the similarly substituted
carbamate 1c. The only amide derived from a primary
amine (12) was less effective. In the LH release assay,
the amides showed significant inhibitory activity,
whereas the parent 1a was essentially inactive. Here the
most potent compound in the series was the diisobutyl
amide 14.
Table 2. Inhibition of GnRH receptor binding and LH release by
indole-5-carboxylates/carboxamides with methanesulfonamide end
groups
When the phenol terminus was replaced by methane-
sulfonamide (Table 2), similar trends were observed.
Again, tertiary amides were preferred. Although the
differences in binding affinity were not large, activity
trends were revealed in the functional assay. Most
effective in blocking LH release from pituitary cells were
the amides derived from certain branched, acyclic or
cyclic secondary amines (27, 28, and 32).
Compd
X
rGnRH IC50, nMa
Protocol Ac Protocol Bd
LH Release
IC50, nMb
The third set of compounds (Table 3) also contained the
methanesulfonamide end group but had a one-carbon
1be
19
20
21
22
23
24
25
26
27
28
[H]f
EtO
Me2N
Et2N
EtNH
n-BuN(Et)
i-PrN(Et)
t-BuN(Et)
n-Pr2N
i-Pr2N
7
14
2
2
6
>6200
690
270
2400
240
120
660
140
72
Table 1. Inhibition of GnRH receptor binding and LH release by
indole-5-carboxylates/carboxamides with phenol end groups
0.2
0.8
4
3
5
3
0.3
0.4
i-Bu2N
32
510
Compd
X
rGnRH IC50, nMa
LH Release
IC50, nMb
29
4
0.2
Protocol Ac Protocol Bd
30
31
2
2
220
400
1ae
1cg
7
[H]f
[Me2NC(O)O]f
EtO
HO
Me2N
Et2N
PhCH2NH
50
3
32
600
5
>6200
0.2
0.2
0.4
9
10
11
12
820
540
6
24
0.5
32
33
3
3
95
13
14
4
800
380
210
i-Bu2N
5.7h
aInhibition of binding of [125I]-buserelin to rat pituitary GnRH recep-
tor.
bInhibition of GnRH-stimulated LH release from rat pituitary cells.
aInhibition of binding of [125I]-buserelin to rat pituitary GnRH recep-
tor.
cAbsence of BSA.
dPresence of 0.1% BSA.
bInhibition of GnRH-stimulated LH release from rat pituitary cells.
eRef 6a.
cAbsence of BSA.
fSubstituent in place of XCO- at indole 5-position.
gRef 6b.
dPresence of 0.1% BSA.
eRef 6c.
hInhibition of binding to human GnRH receptor.
fSubstituent in place of XCO- at indole 5-position.

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W. T. Ashton et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1723–1726
Table 3. Inhibition of GnRH receptor binding and LH release by
indole-5-acetates/acetamides with methanesulfonamide end groups
Acknowledgements
The authors are grateful to Amy Bernickand Dr.
Lawrence Colwell for providing mass spectral data and to
MarkLevorse, Robert Frankshun, Glenn Reynolds, and
Peter Cicala for preparation of certain intermediates.
References and Notes
Compd
R
R0
X
rGnRH IC50, nMa
LH Release
IC50, nMb
Protocol Ac Protocol Bd
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H
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H
H
H
H
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0.2
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H
H
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Me
Me
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