Design and syntheses of melanocortin subtype-4 receptor agonists: Evolution of the pyridazinone archetype

Article abstract of DOI:10.1016/j.bmcl.2003.09.026

The discovery and optimization of a new class of non-peptidyl, pyridazinone derived melanocortin subtype-4 receptor agonists is disclosed.

Full text of DOI:10.1016/j.bmcl.2003.09.026

Bioorganic & Medicinal Chemistry Letters 13 (2003) 4431–4435
Design and Syntheses of Melanocortin Subtype-4 Receptor
Agonists: Evolution of the Pyridazinone Archetype
Feroze Ujjainwalla,a,* Daniel Warner,a Thomas F. Walsh,a Matthew J. Wyvratt,a
Changyou Zhou,a Lihu Yang,a Rubana N. Kalyani,b Tanya MacNeil,b
Lex H. T. Van der Ploeg,b Charles I. Rosenblum,b Rui Tang,b Aurawan Vongs,b
David H. Weinbergb and Mark T. Gouleta
aDepartment of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065-0900, USA
bDepartment of Obesity and Metabolic Research, Merck Research Laboratories, Rahway, NJ 07065-0900, USA
Received 15 July 2003; revised 9 September 2003
Abstract—The discovery and optimization of a new class of non-peptidyl, pyridazinone derived melanocortin subtype-4 receptor
agonists is disclosed.
# 2003 Elsevier Ltd. All rights reserved.
The melanocortin receptors constitute a family of five
known seven-transmembrane Gas G-protein-coupled
receptor subtypes (MC1R–MC5R). These receptors and
their endogenous ligands, namely the melanocortins
and corticotropins, form peripheral and central signal-
ing networks which mediate a diverse number of
physiological functions, including skin pigmentation,
steroidogenesis, energy homeostasis, sexual function
and exocrine secretion.1 The association between mela-
nocortins and feeding behavior was suggested in 1995,
when it was determined that the agouti protein acted as
a melanocortin subtype-4 receptor (MC4R) antagonist.2
Taken in conjunction with an earlier observation that
agouti was overexpressed in the central nervous system
of the Ay agouti mice which were hyperphagic and
obese,3 these observations implied a link between cen-
tral MC4 receptors and feeding control. This link was
further fortified by the discovery of the Mc4rÀ/À mice
which developed morbid obesity.4 Moreover, the
Mc4r+/À heterozygous mice displayed a proportion-
ately milder obesity suggesting that the extent of MC4
receptor signaling was important in sustaining normal
body weight. The most compelling evidence indicating a
role for the MC4R in obesity was obtained in humans
in which both nonsense and frameshift mutations in
MC4R led to dominantly inherited obesity.5 These
observations in part, have fueled the recent surge of
efforts in drug discovery to identify novel MC4R ago-
nists for the potential treatment of obesity.
As part of our obesity research program, the design and
pharmacology of
a
novel tetrahydroisoquinoline
derived (1) MC4R agonist was recently described (Fig.
1).6 Herein, we disclose the discovery and optimization
of pyridazinone 2,7 which led to a new class of non-
peptidyl, functionally selective (versus hMC3R and
hMC5R) agonists of the MC4R.
Pyridazinones 9, 22, and 23 were prepared according to
the general method described below (Scheme 1).
Deprotonation of 38 and subsequent exposure to allyl
bromide, gave alkene 4 in excellent yield. Oxidative
cleavage of the terminal olefin via the Lemieux–Johnson
method,9 followed by reduction of the resultant alde-
hyde 5, afforded primary alcohol 6 in >85% overall
yield. Functional group interconversion to the azide
using the Mitsunobu-type protocol,10 followed by pal-
ladium catalyzed hydrogenation and subsequent acyl-
ation, furnished pyridazinone 9 as a 1:1 mixture of trans
diastereoisomers.
The general synthetic strategy to compounds 12 and
24–26 is presented in Scheme 2. Reaction of aldehyde 5
with methyl magnesium bromide provided the requisite
*Corresponding author. Tel.: +1-732-594-1719; fax: +1-732-594-
5966; e-mail: f_ujjainwalla@merck.com
0960-894X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2003.09.026

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F. Ujjainwalla et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4431–4435
secondary alcohol 10. Subsequent elaboration to the
target amides was conducted according to the synthetic
strategy described in Scheme 1.
To access compounds 9 and 12 in enantiomerically pure
form, asymmetric routes to carboxylic acids 16 and 17
and amines 20 and 21 were developed (Schemes 3 and
4). Enantiomeric resolution of N-BOC-4-phenyl-3-car-
boxypiperidine 13 was performed according to the
known method to afford the cis enantiomers 14 and 15
(Scheme 3).11 Esterification of either 14 or 15, followed
by epimerization and subsequent base catalyzed hydro-
lysis, furnished the trans enantiomers 16 and 17,
respectively.
Figure 1. MC4R agonists 1 and 2.
Asymmetric syntheses of amines 20 and 21 were
accomplished as shown in Scheme 4. Allylic deproton-
ation of 3 and subsequent treatment with either S- or R-
propylene oxide gave enantiomerically pure alcohols 18
and 19, respectively. Functional group interconversion
followed by hydrogenation afforded optically pure
amines 20 and 21, respectively.
Pyridazinone 2, which was discovered through an earlier
structure–activity relationship (SAR) effort,7 showed
good binding affinity and modest functional activity for
the MC4R (Table 1).
Deletion of the phenyl portion of the benzolactam resi-
due in 2, resulted in a dramatic loss of binding and
functional activity.13 Also, comparison of 2 with 1
and other compounds of that class, suggested that 2
lacked a key basic site found in other MC4R agonists.
We therefore reasoned that incorporation of an appro-
priately positioned basic residue could improve the
relatively modest functional activity observed for 2.
These observations guided us to a series of compounds
Scheme 1. Reagents and conditions: (i) LHMDS, allyl bromide, THF,
À78 ꢀC, 89%; (ii) (a) OsO4, NMO, acetone, H2O; (b) NaIO4, THF,
H2O; (iii) NaBH4, MeOH, 0 ꢀC to rt, 86% ꢀ(two steps); (iv)
.
Zn(N3)2 2Py, PPh3, imidazole, DEAD, CH2Cl2, 0 C to rt, 98%; (v)
H2 (50 psi), Pd/C, EtOH, rt, 85%; (vi) trans-1-(tert-butoxycarbonyl)-4-
phenylpiperidine-3-carboxylic acid, EDC, HOBT, CH2Cl2, N-methyl-
morpholine, rt; (vii) HCl, EtOAc, rt, 80% (two steps).
Scheme 2. Reagents and conditions: (i) MeMgBr, THF, À78 ꢀC, 64%;
Scheme 3. Reagents and conditions: (i) (a) (S)-(À)-a-methyl-benzyl-
amine, MeOH; (b) fractional crystallization; (c) 1 N HCl; (ii) diazo-
methane, Et2O, MeOH; (iii) (a) Na, MeOH, 65 ꢀC; (b) 5 N NaOH,
65 ꢀC; (iv) (R)-(+)-a-methylbenzylamine, MeOH; (b) fractional crys-
tallization; (c) 1 N HCl.
ꢀ
.
(ii) Zn(N3)2 2Py, PPh3, Imidazole, DEAD, CH2Cl2, 0 C to rt, 99%;
(iii) H2 (50 psi), Pd/C, EtOH, rt, 72%; (iv) trans-1-(tert-butoxy-
carbonyl)-4-phenylpiperidine-3-carboxylic acid, EDC, HOBT,
CH2Cl2, N-methylmorpholine, rt, 80%; (v) HCl, EtOAc, rt, 90%.

F. Ujjainwalla et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4431–4435
4433
Table 2. Binding affinity and functional activity of compounds at the
human MC4R12
Compda
X
H
Rb
Binding
IC50 (nM)
cAMP EC50
(nM) (%max)
9
1970
500 (20%)
22
23
12
24
25
26
H
2755
1245
41
>5000 (28%)
820 (22%)
227 (82%)
260 (79%)
250 (97%)
5800 (67%)
Scheme 4. Reagents and conditions: (i) LHMDS, (S)-(À)-propylene
CH2Cl2, 0 ꢀC to rt; (iii) H2 (50 psi), Pd/C, THF, EtOH, rt, 37% (three
steps); (iv) LHMDS, (R)-(+)-propylene oxide, THF, À78 ꢀC.
oxide, THF, À78 ꢀC; (ii) Zn(N3)2 2Py, PPh3, imidazole, DEAD,
.
H
R/S-Me
R/S-Me
R-Me
S-Me
Table 1. Binding affinity and functional activity of 1 and 2 at the
human MC4R12
Compd
Binding IC50 (nM)
cAMP EC50 (nM) (%max)
339
380
975
1
2
1.2
144
2.1 (97%)
3060 (34%)
typified by 9, 22, and 23, which contains both the
desired phenyl substituent and a basic contact (Table 2).
The in vitro data showed a modest improvement in
functional activity for 9 and 23 relative to 2, but this
was accompanied byꢁ10-fold erosion of binding affi-
nity for the MC4R. We envisioned that binding and
functional activity might be enhanced by restricting the
conformational freedom in the tether adjoining the pyr-
idazinone nucleus to the aryl-piperidine/pyrrolidine
portion. Accordingly, a methyl group was installed
proximal to the amide to limit the number of low energy
bond rotamers in this region. This resulted in an overall
improvement in binding and functional activity for
compounds 12, 24 and 25 relative to their corresponding
des-methyl counterparts. In the case of 24, the increased
functional activity was at least an order of magnitude
greater than 22. Unexpectedly, activation of the MC4R
was also increased in all cases.
aAll compounds are racemic, >95% pure (HPLC) and were tested as
either TFA or HCl salts.
bRelative stereochemistry.
Table 3. Binding affinity and functional activity of 27 and 28 at the
human MC4R12
The position of the methyl group was critical with
respect to potency, and attempts to relocate this moiety
b- or g- to the amide bond (27 and 28, respectively)
resulted in a profound loss of activity (Table 3).
Compda
Binding IC50 (nM)
cAMP EC50 (nM) (%max)b
The size of the a-amido substituent was investigated
briefly. While ethyl substitution could be tolerated with
impunity, the sterically larger isopropyl group resulted
in diminished binding and functional activity.14 Having
established the correct position and size for the a-amido
substituent, we focused our attention on determining
27
28
2170
3930
>5000 (35%)
>5000 (31%)
aAll compounds are racemic, >95% pure (HPLC) and were tested as
HCl salts.
bRelative stereochemistry.

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F. Ujjainwalla et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4431–4435
Table 5. MCR activity profile of 3612
Table 4. Binding affinity and functional activity of compounds at the
human MC4R12
Receptor
Binding IC50
(nM)
cAMP EC50
(nM)
Activation at
10 mM (%)
MC3R
MC4R
MC5R
2172
33
1410
—
177
—
8
77
5
the activities of the individual components in 12, since
this mixture of four compounds displayed the best
combination of binding and functional activity. Com-
pounds 31 and 32 which feature an S-methyl group and
a trans disposition of the carboxamide and aryl groups
(piperidine ring), displayed significantly reduced binding
and functional potency at the MC4R relative to 12
(Table 4). The corresponding cis substituted piperidine
variants 29 and 30, which were readily accessible from
14 and 15, resulted in an inactive design. Compounds
33–36 in which the methyl group exists in a R-configur-
ation experienced a modest to dramatic right-shift in
potency. Remarkably, the lack of activity observed for
the cis diastereoisomers 29 and 30 could be partially
rescued in the presence of a R-methyl substituent.
Compound 36 showed the most pronounced improve-
ment in binding and functional activity, and was ꢂ10-
fold more active than the methyl epimer 32. Therefore
the most active constituent of 12 was 36 and both were
judged equipotent in the context of assay reproduci-
bility. Relaying the preferred stereochemical pattern
found in 36 to compounds 24 and 25 gave 37 and 38,
respectively. The activity differences between 24 and 37
and between 25 and 38 were again inconsequential, and
these data suggested that the stereochemical relation-
ships found in 36, 37 and 38 were optimum in all three
cases, and moreover, independent of ring size or loca-
tion of the ring nitrogen atom.
Compda Stereo (Me)
Rb
Binding IC50
(nM)
cAMP EC50
(nM) (%max)
29
30
31
32
33
34
35
36
37
38
S
S
9020
13% @ 10 mM
82% @ 20 mM 12% @ 10 mM
S
2990
1197
2240
5% @ 10 mM
1733 (65%)
4050 (54%)
S
R
R
R
R
R
R
A more complete MCR activity profile for pyridazinone
36 is shown in Table 5. These data indicate that 36 is a
subtype selective (versus hMC3R and hMC5R) agonist
of the MC4R.
95% @ 20 mM >5000 (38%)
In conclusion, we have described the design and asym-
metric synthesis of a new class of non-peptidyl, MC4R
agonists derived from a pyridazinone architecture. This
preliminary SAR study has transformed the moderately
potent partial MC4R agonist 2 to the potent and func-
tionally selective (versus hMC3R and hMC5R) agonist 36.
2055
>5000 (32%)
177 (77%)
33
References and Notes
130
190
237 (85%)
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405 (103%)
aAll compounds are single enantiomers, >95% pure (HPLC) and
were tested as HCl salts.
bAbsolute stereochemistry.

F. Ujjainwalla et al. / Bioorg. Med. Chem. Lett. 13 (2003) 4431–4435
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