Design and synthesis of 3-arylpyrrolidine-2-carboxamide derivatives as melanocortin-4 receptor ligands

Article abstract of DOI:10.1016/j.bmcl.2008.01.125

Based on 3-phenylpropionamides, a series of 3-arylpyrrolidine-2-carboxamide derivatives was designed and synthesized to study the effect of cyclizations as melanocortin-4 receptor ligands. It was found that the 2R,3R-pyrrolidine isomer possessed the most

Full text of DOI:10.1016/j.bmcl.2008.01.125

Available online at www.sciencedirect.com
Bioorganic & Medicinal Chemistry Letters 18 (2008) 1931–1938
Design and synthesis of 3-arylpyrrolidine-2-carboxamide
derivatives as melanocortin-4 receptor ligands
Joe A. Tran,a Fabio C. Tucci,a, Melissa Arellano,a Wanlong Jiang,a Caroline W. Chen,a
Dragan Marinkovic,a Beth A. Fleck,b Jenny Wen,c Alan C. Fosterd and Chen Chena,*
aDepartment of Medicinal Chemistry, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
bDepartment of Pharmacology, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
cDepartment of Preclinical Development, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
dDepartment of Neuroscience, Neurocrine Biosciences, Inc., 12790 El Camino Real, San Diego, CA 92130, USA
Received 11 December 2007; revised 30 January 2008; accepted 31 January 2008
Available online 7 February 2008
Abstract—Based on 3-phenylpropionamides, a series of 3-arylpyrrolidine-2-carboxamide derivatives was designed and synthesized
to study the effect of cyclizations as melanocortin-4 receptor ligands. It was found that the 2R,3R-pyrrolidine isomer possessed the
most potent affinity among the four stereoisomers.
Ó 2008 Elsevier Ltd. All rights reserved.
The melanocortin-4 receptor (MC4R) is a member of
the G-protein-coupled receptor (GPCR) superfamily
and plays an important role in regulating feeding behav-
ior.1 While MC4R agonists are pursued for reducing
body weight,2 MC4R antagonists have been shown to
reverse both lean body mass loss and food intake reduc-
tion in animal models, suggesting their potential to be
used for the treatment of cancer cachexia.3,4 In addition,
recent studies have shown that selective MC4R antago-
nists may also be useful in the treatment of anxiety and
depression.5
log 2c (Ki = 31 nM), indicating a strong steric effect
caused by the additional a-methyl moiety on the 3-phe-
nylpropionyl group. Therefore, we can conclude that the
position and orientation of the substituted phenyl ring
in a low-energy conformation is critical for high
potency. When a nitrogen-containing moiety is used to
replace the a-methyl group of 2a, binding affinity is
further improved.8 For example, the acetamido 4
(Ki = 1.9 nM) is significantly more potent than 2a. How-
ever, this improvement could result from the direct
interaction of the acetyl group with the receptor.
Since a small change in this region of the molecule has
a large impact on its biological activity, we decided to
synthesize constrained derivatives of 4 by cyclizing the
acetamide moiety to lock the location of the important
4-chlorophenyl group as shown in Figure 2. Here we
report the design, synthesis and structure–activity rela-
tionship study of these compounds.
We have previously identified a series of a-benzylpropi-
onylpiperazines, such as 2a (Ki = 26 nM, Fig. 1), as
MC4R antagonists.6 Compound 2a is more potent than
the 3-phenylpropionyl analog 1 (Ki = 74 nM)7 but simi-
lar to the phenylalanine 2b, suggesting a small role of the
methyl group in 2a or the amino moiety in 2b. In con-
trast, the a,a-dimethyl analog 3 (Ki = 810 nM) displayed
a much lower binding affinity than its monomethyl ana-
2-Oxo-4-(4-chlorophenyl)methyloxazolidine-4-carbox-
ylic acid 13 was synthesized using a procedure similar to
that described by Qi et al. from 4-chlorophenylalanine
8.9 This was converted to the corresponding acid chlo-
ride 14, which was coupled with the phenylpiperazine
15a10 to afford the desired product 6 after deprotection
with HCl in methanol (Scheme 1).
Keywords: Synthesis; Pyrrolidine; Melanocortin-4 receptor; Stereoiso-
mer; Structure–activity relationship.
*
Corresponding author. Tel.: +1 858 617 7600; fax: +1 858 617
7602; e-mail: cchen@neurocrine.com
Present address: Department of Medicinal Chemistry, Tanabe
Research Laboratories, U.S.A., Inc., 4540 Towne Centre Ct., San
Diego, CA 92121, USA.
Ethyl 5-oxo-3-(4-chlorophenyl)pyrrolidine-2-carboxyl-
ate 18 was synthesized using a procedure similar to that
0960-894X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2008.01.125

1932
J. A. Tran et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1931–1938
Cl
Cl
F3C
Y
N
Ra
N
N
Rb
NH
N
N
NH2
N
O
O
O
1: Ra = Rb = H, Y = Cl;
HN
2a: Ra = H, Rb = Me, Y = Cl
2b: Ra = H, Rb = NH2, Y = Cl
2c: Ra = H, Rb = Me, Y = H
3: Ra = Rb = Me, Y = H
THIQ
Figure 1. Chemical structures of previously reported MC4R antagonists 1–3 and agonist THIQ.
Cl
Cl
Cl
F3C
F3C
F3C
Y
N
Y
Y
c
a
N
N
N
b
d
N
N
N
NH2
NH2
N
H
O
NH2
NH
a
O
O
b
O
O
O
5a: Y = Cl
5b: Y = H
4: Y = Cl
c
d
Cl
Cl
Cl
F3C
F3C
N
N
N
NH2
7
O
O
N
N
N
N
N
H
NH2
NH2
HN
O
O
O
O
O
cis-30a-II
(2 ,3
6
R
R)
Figure 2. Strategy for constrained analogs of MC4R antagonists 4.
Cl
Cl
Cl
b
c
a
O
N
N
H
O
O
HOOC
NH2
O
O
O
8
10
9
F3C
Cl
Cl
g
d,e,f
N
6
+
NH
COX
HN
NH
S
CO2Me
H2N
HO
O
O
12: X = OMe
13: X = OH
14: X = Cl
O
11
15a
Scheme 1. Reagents and conditions: (a) i—PhCOCl/NaOH/acetone/H2O/rt, 2 h; ii—Ac2O/80 °C, 20 min, 63%; (b) CH2O/Py/H2O/rt, 16 h, 63%; (c)
i—5 N HCl/reflux, 3 h; ii—H2SO4/MeOH/rt, 8 h; (d) COCl2/DMAP/DCM/0 °C to rt, 1.5 h, 60%, three steps; (e) LiOH/dioxane/65 °C, 8 h, 46%; (f)
(COCl)2/DMF(cat.)/DMC/rt, 1 h; (g) i—Et3N/DCM/rt, 8 h; ii—HCl/MeOH/rt, 1 h, 4% for 3 steps.
described by Soloshonok.11 This was then converted to
compound 7 as shown in Scheme 2.
with acetamidomelonate gave the intermediate 21,
which was hydrolyzed, followed by decarboxylation
and Boc-protection to afford 1-tert-butoxycarbony-3-
(4-chlorophenyl)pyrrolidine-2-carboxylic acid 22 and
23a.13 Coupling reactions of 22 and 23a with phenyl-
2E-3-Arylpropenals 20 were prepared from 4-iodobenz-
enes as described by Haemers et al.12 Cyclization of 20

J. A. Tran et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1931–1938
1933
Cl
Cl
Cl
a,b
c,d
e
7
CO2Et
CO2Et
O
O
O
COOR
N
N
H
OMe
O
OBu-t
17
16
18: R = Et (cis
19: R = H
/trans: 2/1)
Scheme 2. Reagents and conditions: (a) AcNHCH(COOEt)2/NaOEt/EtOH/reflux, 24 h, 53%; (b) (Boc)2O/Et3N/DMAP/THF/rt, 16 h, 91%; (c)
NaCl/DMSO/150 °C, 40 h, ꢀ100%; (d) NaOH/EtOH/H2O/rt, 3 h, 78%; (e) i—15a/EDC/HOBt/DMF/DCM/rt, 16 h; ii—HCl/MeOH/rt, 1 h, 53%.
piperaines 15a–d gave the desired products 24a–c after a
double deprotection with TFA followed by HCl in
methanol (Scheme 3).
was also resolved to establish the stereochemistry
(Fig. 3, right).14
Coupling reactions of the acids cis-23a with phenylpip-
erazines 15d–g provided the amides cis-28 after TFA
treatment. Deprotection of cis-28a with HCl/MeOH
gave the diamine cis-24f. Reductive alkylations of
cis-28a afforded the tertiary amines cis-29 after HCl/
MeOH treatment. Similarly, coupling reactions of
cis-28a with several carboxylic acids gave the amides
cis-30a–d; reactions of cis-28a–d with chloro alkylfor-
mates afforded the carbamates cis-31a–f. The urea
cis-32 was obtained from cis-28a and ethyl isocyanate,
and the sulfonamide cis-33 was obtained from methane-
sulfonyl chloride (Scheme 5).
Alternatively, 1-acetylpyrrolidine-2-carboxylic acids 26a
were synthesized from 21b using a procedure described
in Scheme 4. When the ester 25, as a mixture of about
1:1, was subjected to basic conditions in aqueous
solution (1 N NaOH/EtOH/rt, 20 h), the trans-isomer
(trans-25) was hydrolyzed much faster than the cis-iso-
mer, resulting in a mixture of cis-ester (cis-25) and a
trans-acid (trans-26a), which were easily separated by
chromatography. Coupling reaction of the acid trans-
26a with (R)-4-benzyl-2-oxazolidinone resulted in a pair
of diastereoisomers, which were separated by chroma-
tography to give the amides trans-27-I and trans-27-II.
The crystal structure of trans-27-I was resolved (Fig. 3,
left), and its stereochemistry was therefore established.
Compounds trans-30 and trans-31 were synthesized
from trans-26a using a procedure similar to that for
the corresponding cis-isomers.
The ester cis-25 was then subjected to an acidic hydroly-
sis (6 N HCl/AcOH/reflux, 8 h). Under these conditions,
the acetyl group was also removed, therefore, treatment
with acetic anhydride was required before purification
which provided the desired acid cis-26a. Alternatively,
treatment of the intermediate with (Boc)2O provided
cis-23a. The single diastereoisomers cis-26a-I and
cis-26a-II were obtained through the oxazolidinones
cis-27-I and cis-27-II. The crystal structure of cis-27-II
Compounds cis-34–38 with various 4-substituted phenyl
group were synthesized as described in Scheme 6. Cycli-
zation of the di-ester 40 under basic conditions gave
3-oxo-1,2-pyrrolidinedicarboxylic acid 1-(tert-butyl)-2-
ethyl ester 41,15 which was converted to the correspond-
ing triflate 42 in a moderate yield. Coupling reactions of
42 with various 4-substituted phenylboronic acids
provided the unsaturated intermediates 43a–f,16 which
Cl
Y
Cl
Y
I
Cl
Y
b,c
a
O
EtOOC
N
EtOOC
X
O
20a: Y = Cl
20b: Y = H
21a: Y = Cl
21b: Y = H
N
d,e
Y
NH
R'
NH
Cl
Cl
S
O
X
Y
f,g
15a-d
HO
N
Boc
N
15b: X = 4-Cl, R' = iBu
15c: X = 6-F, R' = iPr
15d: X = 4-Me, R' = iPr
N
R1
NH2
O
N
H
22: Y = Cl
23a: Y = H
O
24a-f
Scheme 3. Reagents and conditions: (a) CH2 = CHCH(OEt)2/Pd(OAc)2/nBu4NOAc/K2CO3/KCl/rt, 87% for 20a (Y = Cl); (b) AcNHCH(COOEt)2/
Na/EtOH/rt, 8 h, 70% from 20a; (c) Et3SiH/TFA/CHCl3/rt, 4 h, 90% for 21a; (d) i—NaOH/H2O/rt, 20 h; ii—toluene/75 °C, 1 h; (e) (Boc)2O/
NaHCO3/THF/H2O, rt, 16 h; (f) 15a–d/EDC/HOBt/Et3N/DCM/rt, 18 h; (g) i—TFA/DCM/rt, 1 h; ii—HCl/MeOH/rt, 1 h.

1934
J. A. Tran et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1931–1938
Cl
Cl
Cl
Cl
a
b
+
CO2Et
CO2Et
CO2Et
N
Ac
CO2Et
N
CO2H
N
N
Ac
Ac
Ac
cis-25
trans-26a
21b
25 (cis
/
trans: ~1)
d
c
Cl
Cl
Cl
Cl
Cl
c
+
+
O
N
O
S
O
N
O
N
R
R
S
O
O
N
O
O
S
R
S
R
CO2H
N
N
N
N
N
O
R"
O
O
O
Ac
Ac
Ac
Ac
Bn
Bn
Bn
Bn
cis-23a: R' = Boc
cis-26a: R' = Ac
trans-27-I
(2 ,3
cis-27-I
(2 ,3
cis-27-II
(2 ,3
trans-27-II
(2R,3S)
S
R)
S
S
)
R
R
)
Scheme 4. Reagents and conditions: (a) i—NaOH/H2O/rt, 20 h; ii—toluene/75 °C, 1 h, 97%; (b) i—NaOEt/EtOH/rt, 8 h; ii—1 N NaOH/EtOH/rt,
20 h, then separation, 41% for cis-25 and 38% for trans-26a; (c) i—Me3CCOCl/Et3N/THF/ꢁ20 °C, 20 h; ii—(R)-4-benzyl-2-oxazolidinone/LiCl/THF/
rt, 8 h, iii—chromatography separation, 20–30%; (d) i—6 N HCl/AcOH/reflux, 8 h; ii—Ac2O/Et3N/DCM/rt, 4 h; or (Boc)2O/NaHCO3/THF/H2O, rt,
16 h, ꢀ100%.
Figure 3. X-ray crystal structures of (R)-3-[(2S,3R)-1-acetyl-3-(4-chlorophenyl)-pyrrolidine-2-carbonyl]-4-benzyl-oxazolidin-2-one (trans-27-I, left)
and (R)-3-[(2R,3R)-1-acetyl-3-(4-chlorophenyl)-pyrrolidine-2-carbonyl]-4-benzyl-oxazolidin-2-one (cis-27-II, right).
were hydrogenated to give the pyrrolidines cis-44.17
Hydrolysis of cis-44 using LiOH gave the corresponding
acids cis-23, which were deprotected, followed by
acetylation to afford cis-26. Coupling reactions of cis-
26 with the phenylpiperazine 15d provided the cis-34–
38 after deprotection.
onyl group displayed a Ki of 11 nM, indicative of a
minor role of the 2-chloro group of 5a. The oxazolinone
6 as a 1:1 mixture of two diastereoisomers, which con-
tains a CONH functionality, only exhibited weak bind-
ing affinity (Ki = 550 nM) which is quite similar to the
a,a-dimethyl compound 3 (Ki = 810 nM). These results
strongly suggest that the substituent at the a-position
of the phenylpropionyl group of these compounds con-
tributes to the structural conformation instead of direct
interactions with the receptor.
These compounds were then tested for their binding
affinity at the human MC4 receptor stably expressed in
HEK293 cells using [125I]-NDP-MSH as previously
reported.18
The 3-phenyl-5-oxopyrrolidine-2-carboxamide 7, which
also contains the CONH moiety, displayed only moder-
ate binding affinity (Ki = 340 nM) as a mixture of cis-
and trans-isomers (ꢀ2:1). Removing the 5-oxo moiety
of 7 resulted in a derivative with improved potency
(24a, Ki = 98 nM). This structural change might slightly
alter the conformation of the five-membered ring. As a
While the acetamido 4 (Ki = 1.9 nM) was about 40-fold
more potent than the early lead compound
1
(Ki = 74 nM), the cyclic pyrrolidinone 5a (Ki = 4.5 nM)8
was only slightly less active than 4, demonstrating the
amide proton is not critical for ligand-receptor interac-
tion. The pyrrolidinone 5b with a 4-chlorophenylpropi-

J. A. Tran et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1931–1938
1935
Cl
Cl
X
X
N
N
c
a,b
N
NH
N
N
H
N
NH
NH
N
H
NH2
S
S
O
O
O
O
cis-24f
cis-28a-d
15d-g
cis-28a: X = 4-Me
15e: X = H;
15f: X = 4-F;
15g: X = 4-Cl
e, f, or g,
h, c
d
then c
Cl
Cl
Cl
X
N
N
N
N
N
N
N
NH2
N
NH2
N
NH2
S
R2
R1
O
O
O
O
O
O
cis-33
cis-29
cis-30-32
Scheme 5. Reagents and conditions: (a) cis-23a/EDC/HOBt/Et3N/DMF/rt, 16 h; (b) TFA/DCM/rt, 1 h; (c) HCl/MeOH/rt, 1 h; (d) aldehyde/
NaBH(OAc)3/DCM/rt, 8 h; (e) R2COOH/EDC/DCM/rt, 16 h; (TFA treatment for cis-30c); (f) ROCOCl/Et3N/DCM/rt, 2 h; (g) EtNCO/DCM/rt,
0.5 h; (h) MeSO2Cl/Et3N/DCM/rt, 0.5 h.
OTf
O
O
O
O
Y'
O
c
b
O
O
N
R4
O
N
N
O
O
O
O
39: R4 = H
40: R4 = Boc
O
a
42
41
Y'
Y'
f
d
e
OH
O
O
N
N
N
O
O
O
O
O
O
O
O
O
43a-f
cis-44
cis-23
43a: Y' = Cl; 43b: Y' = MeO;
43c: Y' = Me; 43d: Y' = F;
43e: Y' = CF3; 43f: Y' = H
Y'
Y'
g,h
i,j
N
OH
N
N
N
NH2
O
O
O
O
cis-30a, cis-34-38
cis-26
Scheme 6. Reagents and conditions: (a) (Boc)2O/NaOH/H2O/DCM/rt, 100%; (b) tBuOK/toluene/0 °C, 2 h, 45%; (c) KHMDS/THF/ꢁ78 °C, 0.5 h,
then (Tf)2O/rt, 2 h, 52%; (d) 4-Y0C6H4B(OH)2/PdCl2(dppf)/K2CO3/toluene/MeOH/reflux, 17 h, ꢀ80%; (e) NiCl2/NaBH4/MeOH/0 °C, 1 h, 80% for
43a (Y0 = Cl); or H2 (1 atm)/PtO2/EtOH/rt, 6 h; (f) LiOH/THF/H2O/reflux, 20 h, ꢀ85%; (g) TFA/DCM/rt, 1 h; (h) Ac2O/Et3N/DCM/0 °C, 1 h,
ꢀ70% for two steps; (i) 15 d/HBTU/DIEA/DCM/rt, 8 h; (j) HCl/Et2O/MeOH/rt, 1 h.
mixture of cis-/trans-isomers (ꢀ2:1), 24a was only mod-
erately less potent than the R-configured amine 2b
(Ki = 39 nM), indicating that using cyclization to pro-
vide a much more constrained molecule is worth for fur-
ther exploration.
and 24f possessed similar or better potency compared
to the a-isobutyl 24a and 24b. Interestingly, the 2,4-
dichlorophenyl 24c as a 1:1 cis/trans mixture was less
potent than the monochloro 24b, implying that the
ortho-chlorine at the phenyl group of 24c hinders the
rotation of this aromatic ring from adopting an optimal
orientation for receptor interactions. Similar results
were also obtained from 24d/24e. Finally, for these dia-
mines, the cis-isomer (cis-24f, Ki = 52 nM) displayed
A quick survey on the left-side phenyl ring showed that
the chloro analog 24b had similar affinity to 24a (Table
1), while the a-isopropylbenzylamine derivatives 24d

1936
J. A. Tran et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1931–1938
Table 1. Summary of SAR of oxazolinone 6, pyrrolidinone 7 and
pyrrolidines 24a–f at MC4Ra
Table 3. Binding affinity of trans-pyrrolidines trans-30–31 at MC4Ra
Cl
Cl
X
Y
N
N
NH2
24a-f
N
N
R
NH2
N
N
H
R'
O
trans-30-31
R
O
Compound
Ki (nM)
Compound
X
R0
Y
Ki (nM)
trans-30a-I (2S,3R)
trans-30a-II (2R,3S)
trans-30b
Ac
Ac
78
180
79
6
550
340
98
7b
t-BuCO
Ticb
EtCO
24ac
24bc
24cc
24dc
24ec
24fc
cis-24f
4-CF3
4-Cl
4-Cl
6-F
i-Bu
i-Bu
i-Bu
i-Pr
i-Pr
i-Pr
i-Pr
H
H
Cl
H
Cl
H
H
trans-30c
trans-30d
92c
62
96
190
110
560
56
trans-30e
trans-30f
n-PrCO
c-BuCO
MeOCO
EtOCO
52
55
6-F
4-Me
4-Me
trans-31a
trans-31b
trans-31c
59
42
52
i-PrOCO
68
a Data are average of two or more independent measurements.
b Ratio of cis-/trans-isomers was about 2:1.
c Ratio of cis-/trans-isomers was about 1.
a Data are average of two or more independent measurements.
b Tic: 3R-tetrahydroisoquinolinyl-3R-carbonyl.
c Very weak functional agonist with an EC50 of 15 lM and Emax of
21%.
Table 2. SAR of cis-pyrrolidines cis-29–38 at MC4Ra
Y'
similar binding affinity to the mixture (24f, Ki = 56 nM),
suggesting that the cis-isomer might be a preferred
stereoisomer.
X
N
To examine the effect of a substituent at the pyrrolidine
nitrogen, compounds 29–32 were studied for their bind-
ing affinity at MC4R (Tables 2 and 3). For cis-isomers,
the tertiary amines cis-29a–b were slightly less potent
than the secondary amine cis-24f. Acylation of cis-24f
resulted in about 2-fold improvement (cis-30a,
Ki = 24 nM). Between the two individual diastereoiso-
mers of cis-30a, the 2R-configured cis-30a-II
(Ki = 11 nM) was much more potent than the 2S-config-
ured cis-30a-I (Ki = 120 nM), demonstrating the 2R-ste-
reo is preferred. These results match the stereo
preference for acyclic analogs such as 4. The bulky
isobutylcarbonyl compound cis-30b (Ki = 7.2 nM)
showed similar binding affinity to the acetyl analog
cis-30a, indicating an open space in this area. This is fur-
ther confirmed by the Tic (tetrahydroisoquinoline-3R-
carbonyl) compound cis-30c, which displayed a Ki value
of 1.1 nM. While combining with 4-chlorophenylala-
nine, the Tic group has been used for many potent
MC4R agonists such as THIQ (Fig. 1). However,
cis-30c only exhibited weak agonist activity in a cAMP
assay (EC50 = 3500 nM, Emax = 44%), suggesting the
Tic group in this compound might not be at a preferred
position for receptor activation.
N
N
R
NH2
O
cis-29-38
Compound
X
Y0
R
Ki (nM)
cis-29a
cis-29b
cis-30a
4-Me
4-Me
4-Me
Cl
Cl
Cl
Me
iPr
Ac
160
110
24
cis-30a-I (2S,3S)
cis-30a-II (2R,3R)
cis-30b
120
11
4-Me
4-Me
4-Me
Cl
Cl
Cl
tBuCO
Ticb
MeOCO
7.2
1.1c
17
cis-30c
cis-31a
cis-31a-I (2S,3S)
cis-31a-II (2R,3R)
cis-31b
53
18
4-Me
Cl
EtOCO
16
cis-31b-I (2S,3S)
cis-31b-II (2R,3R)
cis-31c
130
11
4-Me
H
4-F
Cl
Cl
Cl
Cl
Cl
iPrOCO
EtOCO
EtOCO
EtOCO
Me2NCO
12
94
cis-31d
cis-31e
74
18
cis-31f
cis-32
4-Cl
4-Me
7.4
190
6.5
40
cis-32-I (2S,3S)
cis-32-II (2R,3R)
cis-33
4-Me
4-Me
4-Me
4-Me
4-Me
4-Me
Cl
MeSO2
Ac
Ac
cis-34
cis-35
MeO
Me
F
45
97
Several carbamates cis-31a–f were also studied. The
three close derivatives cis-30a–c were essentially equipo-
tent, and for both cis-31a and cis-31b, the 2R-stereoiso-
mers (cis-31a-II and cis-31b-II) possessed higher binding
affinity than their 2S-counterparts (cis-31a-I and
cis-31b-I). The urea cis-32 had a Ki of 7.4 nM, which
was substantially higher than the methylsulfonamide
cis-33 (Ki = 40 nM).
cis-36
cis-37
cis-38
Ac
Ac
330
54
CF3
H
Ac
1800
a Data are average of two or more independent measurements.
b Tic: tetrahydroisoquinoline-3R-carbonyl.
c Functional partial agonist with an EC50 of 3.5 lM and Emax of 44%.

J. A. Tran et al. / Bioorg. Med. Chem. Lett. 18 (2008) 1931–1938
1937
The effect of a substituent at the left-side phenyl group
was examined using compounds cis-31b and cis-31d–f.
The 4-fluoro compound cis-31e had a binding affinity
similar to the phenyl analog cis-30d, while the more lipo-
philic chloro compound cis-31f increased affinity by
5-fold. For the substituent at the right-side phenyl
group, the 4-chloro-compound cis-30a showed higher
binding affinity than other compounds examined
(cis-34–38). The 4-methoxy cis-34 (Ki = 45 nM) dis-
played 40-fold improvement from the unsubstituted
cis-38 (Ki = 1800 nM). These data agree with our early
findings from an acyclic series at this site,19 indicating
the significant contribution of the 4-chlorophenyl group
to receptor-binding.
Instead, all compounds showed dose-dependent inhibi-
tion of a-MSH-stimulated cAMP production. For
example, cis-30a-II and cis-32a-II had IC50 values of
520 and 93 nM, respectively, in this functional assay.
Compound cis-32a-II was profiled for its pharmacoki-
netic properties in rats. After an intravenous injection
at a 5 mg/kg dose, cis-32a-II exhibited a moderate plas-
ma clearance of 20 ml/min kg and a high volume of dis-
tribution (Vd = 17.2 L/kg), resulting in a half-life of
4.3 h. After an oral dose of 10 mg/kg, cis-32a-II reached
a maximal concentration of 35 ng/ml to give an area un-
der the curve of 533 ng/ml h, resulting in an absolute
bioavailability of 7.2%. The whole brain concentration
reflected by area under the curve was 80% of the plasma.
The trans-pyrrolidine derivatives trans-30–31 were
also studied in the binding assay, and the results are
summarized in Table 3. Unlike their cis-isomers, these
compounds showed flat SAR. For example, the 2S-con-
figured trans-30a-I (Ki = 78 nM) was only slightly more
potent than the 2R-isomer trans-30a-II (Ki = 180 nM),
and both were less potent than the cis-30a. The Tic-ana-
log trans-30c had a Ki of 92 nM, which was almost
90-fold less potent than cis-30c. These results indicate
that the acyl group of trans-pyrrolidines is not at a place
to interact with the receptor. In contrast, for a series of
4-arylpyrrolidine-3-carboxamide derivatives as MC4R
agonists,20 the trans-isomers have higher binding affinity
than the cis-analogs. The ‘Y’ shape conformation for the
MC4R agonist Tic-D(4-Cl)Phe piperazine THIQ has
been observed in a solid structure21 in which the 4-chlo-
rophenyl group is almost parallel to the piperidine ring.
Recently, we have also shown that a close analog of 2c
displays a similar relationship in a crystal structure.22
The MC4R agonists and antagonists might have a sim-
ilar conformation in binding to the receptor,23,24 but the
Tic or its replacement is required for receptor activation.
Apparently in the current pyrrolidine series, the Tic
group was not in the right position.
In conclusion, a series of 3-phenylpyrrolidine-2-carbox-
amide derivatives were designed and synthesized to com-
pare with their acyclic analogs as MC4R ligands.
Optimization led to several potent compounds. It was
determined that the 2R,3R-pyrrolidine was the preferred
stereoisomer for receptor-binding. These results provide
further insights into the structure–activity relationship
of the 3-phenylpropionyl derivatives and related com-
pounds as MC4R ligands.
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cis-30a-II
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1938
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