Modification of the pyridine moiety of non-peptidyl indole GnRH receptor antagonists.

Article abstract of DOI:10.1016/S0960-894X(02)00751-5

The synthesis of a number of indole GnRH antagonists is described. Oxidation of the pyridine ring nitrogen, combined with alkylation at the two position, led to a compound with an excellent in vitro activity profile as well as oral bioavailability in both rats and dogs.

Full text of DOI:10.1016/S0960-894X(02)00751-5

Bioorganic & Medicinal Chemistry Letters 12 (2002) 3329–3332
Modification of the Pyridine Moiety of Non-peptidyl Indole
GnRH Receptor Antagonists
Joseph P. Simeone,a,* Robert L. Bugianesi,a Mitree M. Ponpipom,a Yi Tien Yang,b
Jane-Ling Lo,b Joel B. Yudkovitz,b Jisong Cui,b George R. Mount,b Rena Ning Ren,b
Mellissa Creighton,c An-Hua Mao,c Stella H. Vincent,c Kang Chengb
and Mark T. Gouleta
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
cDepartment of Drug Metabolism, Merck Research Laboratories, PO Box 2000, Rahway, NJ 07065-0900, USA
Received 10 July 2002; accepted 12 August 2002
Abstract—The synthesis of a number of indole GnRH antagonists is described. Oxidation of the pyridine ring nitrogen, combined
with alkylation at the two position, led to a compound with an excellent in vitro activity profile as well as oral bioavailability in
both rats and dogs.
# 2002 Elsevier Science Ltd. All rights reserved.
With the recent discovery of non-peptidyl Gonado-
tropin Releasing Hormone (GnRH) receptor antago-
nists the possibility arises for new and better treatment
for hormone dependent diseases such as endometriosis,
breast cancer, and prostate cancer. Many of the current
therapeutic agents act as agonists to the receptor.1
However, one of the major drawbacks of these agonists
are that they cause an initial rise in hormone levels
before the desensitization of the receptor occurs. This
troublesome phenomenon, known as the ‘flare effect’,
actually makes the symptoms of the disease worse for a
short time. Due to the inherent nature of antagonist
therapy, this problem does not arise. In addition, non-
peptidyl receptor antagonists offer the possibility for
oral administration.
well as their pharmacokinetic profile. Recently, our
efforts have concentrated on modification of the pyri-
dine portion of the molecule. Results of this study are
described in this letter.
Figure 1. A Merck GnRH receptor antagonist.
Assembly of the compounds of interest was achieved via
either coupling of tryptamine 16 with pyridine acetic acid
derivative 2 (Scheme 1) or dinitrobenzene sulfonamide
Previously reported efforts in these laboratories have led
to the discovery of two novel classes of non-peptide
GnRH antagonists.2,3 Recent modifications of the
2-aryl indole class (Fig. 1), such as incorporation of a
chiral b-methyl substituent4 and the truncation of the
distal side chain to a two carbon tether5 have produced
improvements in these compounds in vitro potency as
*Corresponding author. Tel.: +1-732-594-4307; fax: +1-732-594-
4307; e-mail: joe_simeone@merck.com
Scheme 1. (a) Et3N, pyBOP, CH2Cl2, rt, 16 h; (b) 2 M (CH3)2SBH3,
rt, 16 h.
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00751-5

3330
J. P. Simeone et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3329–3332
tryptamine 36 with pyridine ethanol derivatives 4
(Scheme 2). For the acids, standard peptide coupling
conditions resulted in a diamide, of which, the second-
ary amide was selectively reduced using borane dime-
thylsulfide. In the case of the alcohols, coupling was
achieved under Fukuyama Mitsunobu conditions7 fol-
lowed by deprotection with n-propylamine.
Compound 11 (Scheme 5) was prepared via the Comins
method and converted to 12 by standard ozonolysis
conditions followed by in situ sodium borohydride
reduction. The resulting alcohol was protected as the
acetate and allylated via Stille coupling in one pot to
yield 13. A second ozonolysis and reduction gave the
acetoxy derivative 14 which was coupled to tryptamine
3. Further elaboration of 14 was achieved via protection
of the alcohol as the TBS ether 15 and saponification of
the acetate. This was followed by alkylation of the
alcohol 16 to the methyl ether 17 with iodomethane and
removal of the TBS protecting group to give 18.
Scheme 2. (a) PPh3, DEAD, CH2Cl2, rt, 3 h; (b) n-PrNH2, CH2Cl2, rt,
20 min.
The acid and alcohol derivatives themselves were pre-
pared through one of four different routes outlined
below.
Methoxycarbonylation
Methyl substituents were introduced around the ring via
methoxycarbonylation of the appropriate lutidine or
collidine with dimethyl carbonate (Scheme 3). After
hydrolysis of the resulting methyl ester, pyridine acetic
acids 7a–c were obtained.
Scheme 5. (a) (1) Ozone, 2 N HCl, H2O, rt, 90 min; (2) NaBH4,
CH3OH, NaOH, 0 ꢀC to rt 2 h; (b) (1) acetic anhydride, triethylamine,
rt, 3 h; (2) Allyl tributyltin, Pd(PPh3)2Cl2, 90 ꢀC, 90 min; (c) (1) ozone,
2N HCl, H2O, rt 1 h; (2) NaBH4, CH3OH, NaOH, 0 ꢀC to rt 20 min;
(d) TBSOTf, 2,6-lutidine, CH2Cl2, 0 ꢀC, 10 min; (e) 0.5 N KOH/
CH3OH, H2O, rt, 10 min; (f) NaH, CH3I, DMF, 0 ꢀC, 1 h; (g) TBAF,
THF, rt, 48 h.
Reissert Reaction
The third synthetic route (Scheme 6) involved commer-
cially available 4-(2-hydroxyethyl)pyridine 19. This was
protected as the TBS ether and oxidized with hydrogen
peroxide under methyl trioxorhenium catalysis10 to give
the N-oxide 20. Treatment with trimethylsilylcyanide
and dimethyl carbamoyl chloride under Reissert reac-
tion conditions11 resulted in the nitrile 21 which was
deprotected with TBAF to give 22. Compound 21 was
further converted to the carboxymethyl analogue 24 via
hydrolysis of the nitrile and esterification of the car-
Scheme 3. (a) Diisopropylamine, n-BuLi, dimethyl carbonate, 0 ꢀC, 2
h; (b) concd HCl, reflux, 1 h or NaOH, H2O, THF, 4 h.
Comins Alkylation
Alternatively, the reaction conditions first reported by
Comins8 were used to prepare 2-ethyl and propyl deri-
vatives (Scheme 4). The alkylation proceeds via activa-
tion of the pyridine ring with phenyl chloroformate.
This is followed by addition of a Grignard reagent and
treatment with o-chloranil. Stille coupling9 of the alky-
lated bromides followed by ozonolysis and sodium boro-
hydride reduction resulted in pyridine ethanols 10a and
10b.
Scheme 6. (a) TBSOTf, 2,6-lutidine, CH2Cl2, 0 ꢀC, 10 min; (b)
MeReO3, H2O2, CH2Cl2, rt, 1 h; (c) TMSCN, dimethylcarbamoyl
chloride, CH2Cl2, rt, 23 h; (d) TBAF, THF, rt, 48 h; (e) 6 N HCl,
reflux, 24 h; (f) methanol, cat. H2SO4, reflux, 16 h; (g) CH3MgBr,
THF, À20 ꢀC, 30 min; (h) acetic anhydride, pyridine, rt, 16 h; (i)
CH3MgBr, THF, 0 ꢀC to rt.
Scheme 4. (a) (1) RMgX, phenylchloroformate À78 ꢀC, 30 min; (2)
o-chloranil, acetic acid, toluene, rt, 16 h; (b) allyl tributyltin,
Pd(PPh3)2Cl2, 90 ꢀC, 90 min; (c) (1) ozone, 2 N HCl, H2O, rt 1 h; (2)
NaBH4, CH3OH, NaOH, 0 ꢀC to rt 20 min.

J. P. Simeone et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3329–3332
3331
boxylic acid 23. In addition, ester 24 was treated with
two equivalents of methyl Grignard thereby yielding the
tertiary alcohol 25.
Table 1. Substituent effects along various positions of the pyridine
ring
Conversion of the nitrile 22 to the ketone 26 was
achieved in two steps, via treatment with acetic anhydride
and pyridine followed by methyl Grignard addition.
Compd
R1
R2
R3
hGnRH
IC50 (nM)
PI turnover
IC50 (nM)
Sharpless Oxidation
The N-oxide derivatives (Scheme 7) were prepared
starting from the appropriate pyridine ethanol and
using hydrogen peroxide under methyl trioxorhenium
catalysis to oxidize the pyridine nitrogen.
29
30
31
32
33
34a
35
36
37
38
39
40
41b
42c
H
H
Me
H
H
H
H
H
H
H
H
H
H
H
Me
Me
H
Et
H
Me
H
H
H
H
H
H
H
H
H
H
H
H
0.9
0.4
0.8
3.3
4.6
8.2
0.4
0.7
0.9
0.7
0.7
0.6
0.6
0.4
8.7Æ4.6
24
48
19
62
371
22
55
25
106
11
12
20
7.5Æ1.1
R-Pr
COOH
CH2OAc
CH2OMe
COOMe
C(CH3)2OH
CN
C(O)Me
C(OH)Me
CH2OH
aObtained from 37 under the following conditions: 0.5 N KOH in
CH3OH, H2O, rt, 1 h.
Scheme 7. (a) MeReO3, H2O2, CH2Cl2, rt, 1–48 h.
bObtained from 40 under the following conditions: NaBH4, CH3OH,
rt, 20 min.
Tables 1 and 2 list the activities of compounds12 pre-
pared using the protocols described above. Data from
the hGnRH assay are expressed in terms of IC50s for the
inhibition of [I125] labeled buserelin binding to the
human GnRH receptor.13 For the PI Turnover assay,
activities are expressed as IC50 values representing the
inhibition of phosphatidyl inositol hydrolysis in Chinese
hamster ovary cells stably expressing human GnRH
receptors.13 All in vitro data represents an average of
two experiments (n=2); except in cases where the stan-
dard error is given (nꢁ4). Pharmacokinetic studies were
conducted via IV and oral dosing of at least two subject
animals with blood collected at various time points for
analysis .
cObtained from 35 under the following conditions: 0.5 N KOH/
CH3OH, H2O, rt, 10 min.
Of particular interest was the hydroxymethyl pyridine
analogue 42, which exhibited an excellent in vitro pro-
file. However, its pharmacokinetic properties proved to
be less than desirable, exhibiting only moderate bio-
availability in dogs (15%) with a short terminal plasma
half-life (2 h). Further exploration into substituents
containing nitrogen and sulfur proved to be fruitless,14
thus we turned our attention to modification of the
pyridine nitrogen.
Oxidation of the pyridine nitrogen was thought to be a
metabolic event upon oral dosing in certain members of
this class of compounds. Therefore, we set out to pre-
pare N-oxide derivatives with the hope of retaining
GnRH antagonist activity, while at the same time
improving the pharmacokinetic profile. We were pleased
to find that in almost every case, the presence of the
We began by assessing the effect of methyl group sub-
stitution at different locations on the ring. As previously
disclosed, having the pyridine in a para linked con-
figuration was important for maintaining oral activity.5
As seen in Table 1, entries 29–31 are all potent binders
of the human GnRH receptor. However, analogue 29,
which has a methyl group ortho to the pyridine nitro-
gen, was a much more effective antagonist in the PI
turnover assay. In addition, 29 had excellent oral bioa-
vailability in dogs (36%). Increasing the size of the alkyl
chain by one or two carbons had a deleterious effect on
activity in both assays, as seen from entries 32 and 33.
Table 2. The effect of N-oxidation on GnRH antagonist activity
The introduction of heteroatom functionality at the
position ortho to the pyridine nitrogen, produced a
number of subnanomolar binders to the human recep-
tor, with the exception of the carboxylic acid 34. In the
PI turnover assay, those analogues containing an ether
or ester functionality, such as 35–37, performed poorly,
as did the tertiary alcohol 38. Substituents with less
steric bulk around the heteroatom produced the most
effective inhibitors, as can be seen from entries 39–42.
Compd
R1
R2
R3
hGnRH
IC50 (nM)
PI turnover
IC50 (nM)
43
44
45
46
H
H
H
Me
H
Me
Me
H
H
H
Me
H
0.3
0.3
0.5
0.8
6.4
5.9Æ0.4
10Æ3.4
26

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J. P. Simeone et al. / Bioorg. Med. Chem. Lett. 12 (2002) 3329–3332
N-oxide boosted the activity in both assays (with the
exception of 435). Table 2 shows the results of this
study. While compounds 45 and 46 showed improve-
ment over their reduced counterparts, they were weaker
antagonists than both the unsubstituted pyridine N-
oxide 43 and the ortho methyl pyridine N-oxide 44.
However, compound 45 did exhibit favorable pharma-
cokinetics in a single species (bioavailability 29%, half-
life 7 h in dogs). In contrast, the bioavailability of 43
was rather low (14%). However, further evaluation of
44 resulted in calculated oral bioavailabilities of 25% in
dogs (half-life 5 h) and 23% in rats (half-life 2 h).
2. DeVita, R. J.; Walsh, T. F.; Young, J. R.; Jiang, J.;
Ujjainwalla, F.; Toupence, R. B.; Parikh, M.; Huang, S. X.;
Fair, J. A.; Goulet, M. T.; Wyvratt, M. J.; Lo, J.-L.; Ren, N.;
Yudkovitz, J. B.; Yang, Y. T.; Cheng, K.; Cui, J.; Mount, G.;
Rohrer, S. P.; Schaeffer, J. M.; Rhodes, L.; Drisko, J. E.;
McGowan, E.; MacIntyre, D. E.; Vincent, S.; Carlin, J.;
Cameron, J.; Smith, R. G. J. Med. Chem. 2001, 44, 917.
3. Chu, L.; Hutchins, J. E.; Weber, A. E.; Lo, J.-L.; Yang,
Y. T.; Cheng, K.; Smith, R. G.; Fisher, M. H.; Wyvratt, M. J.;
Goulet, M. T. Bioorg. Med. Chem. Lett. 2001, 11, 509.
4. Simeone, J. P.; Bugianesi, R. L.; Ponpipom, M. M.; Gou-
let, M. T.; Levorse, M. S.; Desai, R. C. Tetrahedron. Lett.
2001, 42, 6459.
5. Simeone, J. P.; Bugianesi, R. L. Unpublished work from
Merck Research Laboratories.
In conclusion, modifications to the pyridine portion of
the 2-aryl indole class of Gonadotropin Releasing Hor-
mone receptor antagonists resulted in several potent
new inhibitors. It was quickly determined that the posi-
tion ortho to the nitrogen atom in the ring tolerated
methyl substitution as well as a variety of oxygen con-
taining functional groups. In addition, the ortho methyl
analogue 29 exhibited excellent oral bioavailability in
dogs. Oxidation of the pyridine nitrogen coupled with
methyl substitution at various positions on the ring
resulted in three compounds with excellent in vitro pro-
files. Among these, compound 45 had significant oral
levels in dogs, while the ortho methyl pyridine N-oxide 44,
showed greater than 20% bioavalibility in two species.
6. Ashton, W. T.; Sisco, R. M.; Kieczykowski, G. R.; Yang,
Y. T.; Yudkovitz, J. B.; Cui, J.; Mount, G. R.; Ren, R. N.;
Wu, T. J.; Shen, X.; Lyons, K. A.; Mao, A.; Carlin, J.; Kar-
anam, B. V.; Vincent, S. H.; Cheng, K.; Goulet, M. T. Bioorg.
Med. Chem. Lett. 2001, 11, 2597.
7. Fukuyama, T.; Cheung, M.; Jow, C.; Hidai, Y.; Kan, T.
Tetrahedron. Lett. 1997, 38, 5831.
8. Comins, D. L.; Mantlo, N. B. J. Org. Chem. 1985, 50, 4410.
9. Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
10. Coperet, C.; Adolfsson, H.; Khuong, T. V.; Yudin, A. K.;
Sharpless, K. B. J. Org. Chem. 1998, 63, 1740.
11. Shuman, R. T.; Ornstein, P. L.; Paschal, J. N.; Gesellchen,
P. D. J. Org. Chem. 1990, 55, 738.
12. All compounds exhibited >98% purity (@210 and 254
nm) by analytical HPLC and were further characterized by
1
NMR (500 MHzH field strength) and mass spectrometry
(ESI-MS).
13. Walsh, T. F.; Toupence, R. B.; Young, J. R.; Huang, S. X.;
Ujjainwalla, F.; Devita, R. J.; Goulet, M. T.; Wyvratt, M. J.;
Fisher, M. H.; Lo, J. L.; Ren, R. N.; Yudkovitz, J. B.; Yang,
Y. T.; Cheng, K.; Smith, R. G. Bioorg. Med. Chem. Lett. 2000,
10, 443.
References and Notes
1. For a recent review on this subject: Kutscher, B.; Bernd,
M.; Beckers, T.; Polymeropoulos, E.; Engel, J. Angew. Chem.,
Int. Ed. Engl. 1997, 36, 2149.
14. Simeone, J. P.; Bugianesi, R. L., Ponpipom, M. M. Unpub-
lished work from Merck Research Laboratories.