Melanocortin subtype 4 receptor agonists: Structure-activity relationships about the 4-alkyl piperidine core

Article abstract of DOI:10.1016/j.bmcl.2006.11.084

SAR about the piperidine core in a series of MC4R agonists is described. A number of alkyl substituents that furnish compounds with good affinity and functional potency are reported.

Full text of DOI:10.1016/j.bmcl.2006.11.084

Bioorganic & Medicinal Chemistry Letters 17 (2007) 5720–5723
Melanocortin subtype 4 receptor agonists: Structure–activity
relationships about the 4-alkyl piperidine core
Iyassu K. Sebhat,a,* Yingjie Lai,a Khaled Barakat,a Zhixiong Ye,a Rui Tang,b
Rubana N. Kalyani,b Aurawan Vongs,b Tanya MacNeil,b David H. Weinberg,b
M. Angeles Cabello,c Marta Maroto,c Ana Teran,c Tung M. Fong,b
Lex H. T. Van der Ploeg,b Arthur A. Patchetta and Ravi P. Nargunda
aDepartment of Medicinal Chemistry, Merck & Co., Inc, PO Box, 2000, Rahway, NJ 07065, USA
bDepartment of Metabolic Disorders, Merck & Co., Inc, PO Box, 2000, Rahway, NJ 07065, USA
cCentro de Investigacion Basica, Merck Sharp and Dohme de Espana, S.A., Madrid, Spain
Received 5 October 2006; revised 28 November 2006; accepted 30 November 2006
Available online 3 December 2006
Abstract—SAR about the piperidine core in a series of MC4R agonists is described. A number of alkyl substituents that furnish
compounds with good affinity and functional potency are reported.
Ó 2006 Elsevier Ltd. All rights reserved.
Through interactions with the endogenous corticotropin
and melanocortin ligands, the melanocortin family of
G-protein-coupled receptors mediate a wide array of
physiological functions. These range from the control
of feeding and sexual behavior to skin pigmentation
and neuroendocrine regulation. Five subtypes have been
cloned and sequenced since the early 1990s.1
firmed the food intake-lowering effects of MC4R agon-
ism and provided evidence that the sexual effects
exhibited by non-selective ligands may be mediated by
MC4R4a,b,c (see Fig. 1).
The primary endogenous ligands for the five receptor
subtypes are generated through cleavage of the 31–36
kDa protein pro-opio-melanocortin (POMC). This
mechanism generates an array of biologically active pep-
tides which share a His-Phe-Arg-Trp pharmacophoric
core unit.5 Recognizing the similarity of this pharmaco-
phore to the active core of the growth hormone secreta-
gogue peptide GHRP-66 and applying strategies that
had yielded a clinical candidate in that program, we
quickly identified compound 2 bearing a spirocyclic
‘privileged structure’ with a dipeptide cap.7
Melanocortin 4 receptor (MC4R) is expressed in the
hypothalamus, brain stem, and many other brain re-
gions. A significant amount of evidence points to the
role of the receptor in feeding behavior.2 The link be-
tween MC4R and feeding regulation is strengthened
by knock-out studies of mice in which targeted deletion
of the receptor results in obese mice with moderate levels
of obesity in heterozygous mice.3
Efforts have been mounted by various research groups
to identify suitable agonists of the receptor as possible
treatments for obesity. In recent years, small molecule
agonists have been reported in the literature.4 Many of
the more selective agonists have been based on the struc-
ture of compound 1—reported by our laboratories in
2004.4a Pharmacological testing of compound 1 con-
Lessons from the literature outlining the influence of the
phenylalanine residue8 allowed rapid movement into an
agonist series exemplified by compound 3b. Further
optimization led to the development of a series of 4-
substituted, 4-cyclohexylpiperidine-based structures. A
very potent series emerged where the piperidine was
substituted with a methylene bearing a heterocycle4a or
with a tert-butyl amide.4d Replacement of the imidazole
side chain of His with tetrahydroisoquinoline (TIC) or
piperazine further enhanced potency and selectivity
eventually leading to compounds 1 and 11a.
Keywords: Obesity; Erectile dysfunction; Privileged structure; G-pro-
tein-coupled receptor; Apetite; Metabolic disorder; Peptidomimetic.
*
Corresponding author. E-mail: iyassu_sebhat@merck.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.11.084

I. K. Sebhat et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5720–5723
5721
GHRP-6 Pharmacophore
MC-4 Pharmacophore
CO2Bn
N
Trp-Ala-DTrp-His
Trp-Arg-DPhe-His
O
N CO2Bn
a
NH
NH
4
R
HN
R
H2N
HN
EtO2C
CN
N
N
N
N
EtO2C
CN
b
HN
H
O
HN
O
H
HN
HN
N
N
O
S
O
O
S
O
N
H
O
N
H
O
NH
R
NH
HN
CO2Bn
N
CO2Bn
N
R
H
N
c
N
N
H2N
HN
H2N
HN
7
5
N
CN
N
N
R
R
d
N
O
N
O
O
O
O
N
N
O
O
O
O
Ar
d
2
3a: Ar = 2-napthyl
3b: Ar = Ph
HN
HN
HN
HN
N
R
R
HN
HN
HN
O
O
O
NC
N
N
N
N
O
HN
O
N
N
NH
N
N
N
N
O
O
N
O
O
1
11a
Cl
F
Cl
Cl
8
6
Figure 1. Lead discovery/optimization (a privileged structure
approach).
Me
Et
nBu
a =
f =
b =
g =
c =
d =
e =
j =
R
Our group and others working in the area have reported
some SAR about the core of these compounds.4 While
this work has described alterations to the dipeptide
cap as well as the more polar piperidine substituent,
there are no reports describing SAR of the lipophilic
piperidine substituent. This paper seeks to outline
SAR about this metabolically labile region and identifies
a number of groups that allow for structural variation
while maintaining good activity.
h =
Cl
i =
O
k =
l =
m =
n =
nHex
Bn
Figure 2. Synthesis of nitrile and tetrazole analogs. Reagents and
conditions: (a) NH4Cl/AcOH/C6H6, reflux (–H2O); (b) i—RMgX/
CuCN/THF; ii—LiCl/H2O/DMSO, 160 °C; (c) i—TMSN3/Bu2SnO/
toluene, reflux; ii—K2CO3/MeI/DMF; (d) i—Pd(OH)2/H2/HCl/MeOH;
ii—Boc-D-Phe(p-Cl)-OH/EDC/HOBt/NMM/CH2Cl2; iii—HCl/CH2
Cl2; iv—Boc-D-Tic-OH/EDC/HOBt/NMM/CH2Cl2; v—HCl/CH2Cl2.
The work was carried out in three distinct series bearing
nitrile, tetrazole, and amide functional groups. In all the
series, flexible methodologies were developed that al-
lowed for facile variation of the alkyl subunit.
The synthesis of the nitrile and tetrazole analogs pro-
ceeded initially with the Knoevenagel condensation of
N-Cbz-protected 4-piperidone with ethyl cyanoacetate.
The resultant unsaturated cyanoacetate 4 was treated
with a cuprate generated from an alkyl Grignard or lith-
ium species. The 4-alkylpiperidine was then thermally
decarboxylated to generate the nitrile 5. Deprotection
followed by sequential coupling to the dipeptide cap
and final deprotection afforded the nitrile analogs 6. Pri-
or cycloaddition of azidotrimethylsilane in the presence
of catalytic tin followed by methylation afforded the var-
ious tetrazole analogs 8 (see Fig. 2).
amide, deprotected, and coupled to the dipeptide cap.
Hydrogenation of the Cbz group and methylation prior
to a final deprotection allowed access to the various ago-
nists 11 (see Fig. 3).
Secondary alkyl substituents were generated in a
straightforward fashion via deprotonation and alkyl-
ation of N-Boc-4-carbethoxy piperidine 9. The ester
was then saponified to afford acid 10.
Tertiary alkyl-substituted structures were generated in
an analogous fashion to the nitrile/tetrazole-bearing
compounds utilizing a Henry reaction. The requisite ni-
tro olefin 12 was generated via addition of nitromethane
to N-Cbz-4-piperidone followed by dehydration. Addi-
tion of a Grignard reagent then furnished nitrile oxide
Synthesis of the amide analogs required three distinct
routes to access compounds bearing secondary, tertiary,
and quaternary or allylic alkyl centers. Each route led to
protected acid 10. This was converted to a tert-butyl

5722
I. K. Sebhat et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5720–5723
Boc
N
Boc
N
Boc
N
Finally quaternary and allylic alkyl substituents were ac-
cessed via Claisen rearrangement. N-Boc piperidine-4-
carboxylic acid was coupled with an allylic alcohol to
furnish the Claisen precursor allylic ester 14. Warming
the silyl enol ether generated on treatment with base
and TMSCl then effected the rearrangement to allylic-
substituted acid 15. This was either hydrogenated to
the saturated analog or maintained as the alkene.
f
e
R =3y and/or
allylic
15
R1 R3
R2
R3
OH
CO2H
O
O
R2
R1
O
14
g
N
O
HN
HN
Boc
N
H
N
R
O
h
a
R =2y
Given the ease of generating the nitrile compounds (6),
we used this series to rapidly determine the suitability
of a number of alkyl groups of varying size. The cyclo-
hexyl subunit (6a) emerged as the most potent
(IC50 = 7 nM; EC50 = 189 nM at hMC4R) with 4%
maximal activation of hMC3R and 3-fold selectivity
over hMC5R. Methyl, ethyl, isopropyl, and cyclopropyl
analogs (6b, 6c, 6e, and 6f) caused over 10-fold reduc-
tion in potency. Use of a cyclopentane or n-butyl substi-
tuent (6g and 6d) resulted in a smaller shift (2- and 3-
fold, respectively) suggesting our efforts should focus
on groups with larger steric bulk. Functional activity
of the series at the hMC4 receptor (maximal stimulation
of cAMP) did not exceed 52%, even in the case of com-
pounds that had good affinity for the receptor. We have
hypothesized that the nitrile group does not form opti-
mal interactions with the receptor. This is possibly due
to the lack of an additional lipophilic interaction that
the other functional groups are capable of forming. In
further investigating alkyl group SAR, we turned to
the more active tetrazole series (8). Again, the cyclohexyl
analog was potent and selective: 8a maintained an EC50
of 3 nM at hMC4R with 36- and 69-fold selectivity over
hMC3R and hMC5R, respectively. While n-hexyl and
benzyl analogs 8k and 8l exhibited large reductions in
potency, the neopentyl analog 8i shifted 7-fold and the
branched butyl- and chlorophenyl-substituted com-
pounds 8h, 8j, and 8m had <5-fold reductions in func-
tional potency. The THP analog 8n is the least potent
in the series, suggesting that heteroatoms are not well
tolerated in the region (Table 1).
NH
N
OH
R
CO2Et
O
O
10
9
F
11
d
CO2Bn
N
CO2Bn
N
Boc
N
b
c
13
R =3y
R
O
N+
NO2
O-
12
a =
b =
c =
d =
e =
R
O
f =
g =
h =
i =
Figure 3. Synthesis of amide analogs. Reagents and conditions: (a)
i—LDA/THF; ii—RCH2X; (b) i—MeNO2/NaOMe/MeOH;
ii—Ac2O/DMSO; (c) i—RMgX/THF; ii—H2SO4; (d) NaNO2/
AcOH/DMSO; (e) ROH/EDC/DMAP/HOBt/CH2Cl2; (f) LDA/
TMSCl/THF; (g) HCl/CH2Cl2; (h) i—Boc-D-Phe(p-Cl)-OH/EDC/
HOBt/NMM/CH2Cl2; ii—HCl/ CH2Cl2; iii—(2S)-1-Boc-4-methyl-
piperazine-2-carboxylic acid/EDC/HOBt/NMM/CH2Cl2; iv—HCl/
CH2Cl2.
13—a versatile intermediate that, for these purposes,
was oxidized to generate 10.
Table 1. Binding and functional activity of compounds 6a–6g and 8a–8m at human melanocortin receptors
Compound
MC4R (IC50, nM)a
EC50 (nM)a [% max]
MC4R
MC3R
MC5R
6a
6b
6c
6d
6e
6f
6.8 ( 2)
189 ( 38) [51]
na [7]
na [16]
na [4]
na [0]
640 ( 113) [42]
na [3]
na [8]
1490 ( 331)
759 ( 312)
22.2 ( 4)
63.0 ( 3)
173 ( 37)
16.4 ( 4)
0.85 ( 0)
6.0 ( 1)
na [1]
na [1]
281 ( 66) [40]
827 ( 107) [32]
na [12]
480 ( 57) [21]
1100 ( 265) [20]
na [10]
na [1]
na [0]
na [2]
6g
8a
8h
8i
196 ( 51) [52]
3.2 ( 1) [96]
12.5 ( 4) [98]
21.0 ( 7) [76]
3.9 ( 0) [107]
28.7 ( 7) [84]
111 ( 28) [70]
4.9 ( 2) [100]
157 ( 55) [90]
630 ( 78) [40]
222 ( 37) [57]
662 ( 242) [46]
380 ( 50) [27]
250 ( 55) [39]
684 ( 32) [15]
1760 ( 596) [20]
527 ( 167) [17]
2950 ( 132) [18]
115 ( 22) [50]
680 ( 115) [31]
na [9]
3.6 ( 0)
3.7 ( 1)
4.0 ( 1)
8j
263 ( 60) [33]
1001 ( 230) [34]
840 ( 100) [17]
434 ( 126) [92]
2950 ( 850) [18]
8k
8l
52.3 ( 14)
113 ( 55)
32 ( 19)
8m
8n
a Values are means of at least three experiments, standard error of the mean is given in parentheses (na, not active).

I. K. Sebhat et al. / Bioorg. Med. Chem. Lett. 17 (2007) 5720–5723
5723
Table 2. Binding and functional activity of compounds 11a–11z at human melanocortin receptors
Compound
MC4R (IC50, nM)a
EC50 (nM)a [% max]
MC4R
MC3R
MC5R
11a
11b
11c
11d
11e
11f
11g
11h
11i
13.6 ( 4)
840 ( 149)
177 ( 13)
6.8 ( 1)
6.5 ( 1) [98]
na [14]
na [2]
na [4]
977 ( 123) [31]
na [5]
na [8]
2174 ( 732) [54]
563 ( 146) [55]
10.0 ( 4) [81]
546 ( 312) [87]
151 ( 54) [55]
57.8 ( 26) [79]
84.6 ( 15) [76]
1769 ( 725) [59]
230 ( 58) [37]
241 ( 76) [42]
na [2]
888 ( 93) [27]
1117 ( 95) [19]
na [32]
162 ( 81)
66.2 ( 16)
36.6 ( 9)
64.0 ( 6)
343 ( 98)
na [5]
na [8]
na [3]
1980 ( 186) [56]
na [12]
na [2]
a Values are means of at least three experiments, standard error of the mean is given in parentheses (na, not active).
Fairhurst, A. M.; Van der Ploeg, L. H. T.; MacIntyre, D.
E.; Mellin, T. N. Neuropeptides 1998, 32, 491; (h) Giraudo,
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Schiotch, H. B. Biochem. Biophy. Res. Commun. 1998, 248,
245.
The most potent compounds in the first two series had
maintained either a cyclohexane, sec-butyl or iso-butyl
substituent. The amide series (11) allowed us to investi-
gate further substitutions about these core-structures.
While unsaturation (11h), substitution of the branch
point (11f and 11g), 4,4-di-substitution (11e) or cycliza-
tion (11b and 11c) all reduced potency compared to their
respective parents, 4-position mono-methylation (11d)
maintained very similar activity (see Table 2).
3. Huszar, D.; Lynch, C. A.; Fairchild-Huntress, V.; Dun-
more, J. H.; Fang, Q.; Berkemeier, L. R.; Gu, W.;
Kesterson, R. A.; Boston, B. A.; Cone, R. D.; Smith, F.
J.; Campfield, L. A.; Burn, P.; Lee, F. Cell 1997, 88,
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4. For a recent review, see: Nargund, R. P.; Strack, A. M.;
Fong, T. M. J. Med. Chem. 2006, 49, 4035; (a) Sebhat, I.
K.; Martin, W. J.; Ye, Z.; Barakat, K.; Mosley, R. T.;
Johnston, D. B. R.; Bakshi, R.; Palucki, B.; Weinberg, D.
H.; MacNeil, T.; Kalyani, R. N.; Tang, R.; Stearns, R. A.;
Miller, R. R.; Tamvakopoulos, C.; Strack, A. M.; McGo-
wan, E.; Cashen, D. E.; Drisko, J. E.; Hom, G. J.; Howard,
A. D.; MacIntyre, D. E.; Van der Ploeg, L. H. T.; Patchett,
A. A.; Nargund, R. P. J. Med. Chem. 2002, 45, 4589; Van
der (b) Ploeg, L. H. T.; Martin, W. J.; Howard, A. D.;
Nargund, R. P.; Austin, C. P.; Guan, X.; Drisko, J.;
Cashen, D.; Sebhat, I.; Patchett, A. A.; Figueroa, D. J.;
DiLella, A. G.; Connolly, D. H.; Weinberg, D. H.; Tan, C.
T.; Palyha, O. C.; Pong, S.; MacNeil, T.; Rosenblum, C.;
Vongs, A.; Tang, R.; Yu, H.; Sailer, A. W.; Fong, T. M.;
Huang, C.; Tota, M.; Chang, R. S.; Stearns, R.; Tamvak-
opoulos, C.; Christ, G.; Drazen, D. L.; Spar, B. D.; Nelson,
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In conclusion, the three sets of MC4 receptor agonists
(nitriles, tetrazoles, and amides) allowed us to identify
suitable alkyl substituents of the piperidine core in this
lead series. The low potency of compounds bearing
small alkyl groups suggests that this region of the mole-
cule interacts with a large lipophilic pocket in the hMC4
receptor. A number of larger groups, in particular cyclo-
pentyl, n-butyl, neo-pentyl, iso-butyl, sec-butyl, and
cyclohexyl derivatives, furnished compounds with good
potency at hMC4R. This variety of substituents may
prove useful in tempering the in vivo and physical char-
acteristics of the lead series.
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