Privileged structure based ligands for melanocortin-4 receptors-Aliphatic piperazine derivatives

Article abstract of DOI:10.1016/j.bmcl.2006.04.002

Aliphatic carbocyclic replacement of the benzyl group of compound 1 yielded compounds with high affinity for the melanocortin-4 receptor (MC4R). Compounds with a cyclohexyl group showed a consistent high affinity, while different polar groups with less basicity were good replacements for the original diethyl amines. Substitution of the polar group found in these privileged structures with an aliphatic moiety produced compounds with high affinity for MC4R.

Full text of DOI:10.1016/j.bmcl.2006.04.002

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3449–3453
Privileged structure based ligands for melanocortin-4
receptors—Aliphatic piperazine derivatives
a
b
a
b
a
´
´
Karin Briner, Ivan Collado, Matthew J. Fisher, Cristina Garcıa-Paredes, Saba Husain,
Steven L. Kuklish,a Ana I. Mateo,b Thomas P. O’Brien,a Paul L. Ornstein,a
John Zgombicka,ꢀ and Oscar de Frutos
´
b,
*
aLilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46258, USA
bEli Lilly and Company, Lilly S.A., Avenida de la Industria, 30, 28108 Alcobendas, Madrid, Spain
Received 21 February 2006; revised 3 April 2006; accepted 3 April 2006
Available online 2 May 2006
Abstract—Aliphatic carbocyclic replacement of the benzyl group of compound 1 yielded compounds with high affinity for the
melanocortin-4 receptor (MC4R). Compounds with a cyclohexyl group showed a consistent high affinity, while different polar
groups with less basicity were good replacements for the original diethyl amines. Substitution of the polar group found in these priv-
ileged structures with an aliphatic moiety produced compounds with high affinity for MC4R.
Ó 2006 Elsevier Ltd. All rights reserved.
The five melanocortin receptors form a family of type I
G-protein-coupled receptors (GPCRs). Each member of
the melanocortin receptor family has been cloned and
characterized.1 The MC4R is expressed in the hypothal-
amus and is thought to play a critical role in regulating
metabolism, feeding, and reproductive behavior.2 We
have been interested in developing tools for the pharma-
cological characterization of the melanocortin receptors
(MCRs) with a primary focus on finding ligands selec-
tive for MC4R.3 Our approach has been to employ a
privileged structure-based paradigm for the generation
of the initial active leads.4,5 In these studies, we have
found that compounds presenting an aromatic hydro-
phobic and a polar group can be coupled with a dipep-
tide address element to afford products with activity and
selectivity for the MC4R (Fig. 1).3 We have shown that
there is a fair amount of flexibility in the privileged
structure with monocyclic and bicyclic aryl piperazines,
and substituted benzylic piperazines affording interest-
ing activity.6
previous investigations we operated under the assump-
tion that an aromatic group and a polar group were
required elements for privileged structure design and
function. In this paper, we examined if aliphatic privi-
leged structures (with different steric and hydrophobic
requirements) would influence the melanocortin activity.
Simultaneously we explored whether a polar group was
required in the privileged structure. Herein we present
our findings in the further optimization of this family
of melanocortin ligands.
The preparation of privileged structures with aliphatic
hydrophobic groups is outlined in Scheme 1. Treatment
of the carboxylic acids (4, X = X0 = H) with thionyl
chloride formed the acid chloride followed by bromina-
tion with NBS and HBr to provide the a-bromo acid
chloride, which was then reacted with MeOH to afford
the a-bromo methyl esters (4, X = Br, X0 = Me).7 For
the compound derived from ethyl propionate, bromina-
tion was accomplished by forming the enolate with
LHMDS followed by reaction with NBS. The final
tertiary amines 5 were then obtained by nucleophilic
displacement of the bromine with N-Boc-piperazine,
transformation to the aldehyde (LiAlH4 reduction then
Swern oxidation), and subsequent reductive amination
with the desired amine using NaBH(OAc)3.8
This structural flexibility initially encountered in the
privileged structure motif suggested that additional
opportunity might exist for finding active constructs
with varied physicochemical properties. In all of our
Keywords: Melanocortin receptors; Privileged structures.
*
The a,a-disubstituted cyclopentyl derivative was synthe-
Corresponding author. E-mail: de_frutos_oscar@lilly.com
Deceased.
sized from cyclopentanone using a Strecker synthesis9 as
ꢀ
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.04.002

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K. Briner et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3449–3453
Figure 1. Disconnective analysis of lead series.
Different polar groups were prepared as replacement for
the tertiary amine. Using the cyclohexyl ester intermedi-
ate 10 as a precursor (Scheme 3), the amide 11 was pre-
pared by transformation of the ester moiety to the acid
followed by amide formation. The succinimide 12a and
phthalimide 12b were obtained by reduction of 10 to the
alcohol and subsequent Mitsunobu reaction with succin-
imide and phthalimide, respectively.
Scheme 1. Reagents: (a) 1—SOCl2 CH2Cl2; 2—NBS, HBr, CH2Cl2;
3—MeOH, 45–90%; (b) Boc-piperazine, K2CO3, n-Bu4NI, MeCN,
35–45%; (c) 1—LiAlH4, THF, 90–95%; 2—(COCl)2 DMSO, NEt3,
CH2Cl2, 80%; (d) R02N, NaBH(OAc)3; 1,2-DCE, 50–85%.
The N-Boc-piperazine-phthalimide derivative 12b was a
key intermediate for the preparation of most of the
remaining compounds as shown in Scheme 4. Treatment
of this derivative with hydrazine afforded the primary
amine 13 (R1 = R2 = H), which was used to prepare dif-
ferent sulfonamides, amides, and carbamates. Reaction
of 13 with acetyl chloride or methanesulfonyl chloride
yielded the secondary amide 14 (R1 = COMe, R2 = H)
and the secondary sulfonamide 15 (R1 = SO2Me,
shown in Scheme 2. Hydrolysis of the intermediate ami-
no nitrile 6 provided the ethyl ester 7 which was allowed
to condense with N,N-bis-(2-chloroethyl)-p-toluenesul-
fonamide giving rise to the piperazine intermediate 8.
Further transformation to the diethylamine and depro-
tection of the tosyl-protecting group afforded 9.
Scheme 2. Reagents: (a) KCN, NH4Cl H2O (b) HCl, MeOH; (c) TsN(CH2CH2Cl)2, i-Pr2NEt, DMF, 10% over three steps; (d) 1—LiAlH4, THF,
99%; 2—(COCl)2, DMSO, NEt3, CH2Cl2, 99%; 3—Et2NH, NaBH(OAc)3, 1,2-DCE; (e) HBr, HOAc, 38% over 2 steps.
Scheme 3. Reagents: (a) 1—LiAlH4, THF, 96%; 2—CrO3, H2SO4, acetone, 70%; (b) Et2NH, HOBt, EDCl, CH2Cl2, DMF, 58%; (c) succinimide or
phthlalimide, DEAD, PPh3, THF, 75–80%.

K. Briner et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3449–3453
3451
Scheme 4. Reagents: (a) MeCOCl, NEt3 CH2Cl2, 63%; (b) MeSO2Cl, NEt3, CH2Cl2; 70%; (c) BH3ÆTHF, THF, 86%; (d) Cl(CH2)3COOH HOAt,
HATU, i-Pr2NEt, DMF, 69%; (e)tBuOK, THF, 54%.
R2 = H), respectively. Furthermore, the primary amine
13 (R1 = R2 = H) was transformed to the ethylamine
16 (R1 = Et, R2 = H) (acetylation with acetyl chloride
and reduction with BH3ÆTHF) and converted to the
tertiary amide 17 (R1 = Et, R2 = COMe) and tertiary
sulfonamide 18(R1 = Et, R2 = SO2Me), respectively.
Scheme 4 also shows the preparation of the cyclic amide
19 from the same intermediate 13 (R1 = R2 = H) by ami-
dation with 3-chlorobutaric acid and intramolecular
cyclization promoted by base.
of the Boc group with TFA) and the dipeptide 21 in
the presence of HATU as previously described.3 The
dipeptide 21 incorporated 2 different moieties (Y in
Scheme 6): tetrahydroisoquinoline (TIC) and isoindol
(IIN).12 Treatment of the penultimate intermediates
with TFA provided the desired compounds as the corre-
sponding salt. These materials were either used as isolat-
ed or subjected to salts exchange with HCl (Scheme 6).
In the cases where the piperazines were prepared as race-
mates, the resultant diasteroisomeric pairs were not
separated.
The enantiomers of phthalimide 12b were separable on a
gram scale with chromatography on a chiral AD
column, allowing for the preparation of specific antipo-
des of different compounds.10 The absolute stereochem-
istry of the enantiomers was not determined; rather it
was tracked by the order of elution and designated as
isomer a (first eluting) and isomer b (second eluting).
The primary assays were conducted as follows: affinities
were determined by competitive inhibition of 125I-NDP-
a-MSH binding in HEK293 cells stably transfected with
human MC4 receptors.13,14
We initially focused on analogs in which the o-fluoro
benzylic group of 1 was replaced with an aliphatic
substituent. We observed a significant loss of affinity
when the aryl group was replaced by a methyl group,
as in 22 (Table 1).15 However, the introduction of carbo-
cyclic moieties, as in 23–27, afforded compounds with
equal or higher affinity than that of the parent benzylic
derivative 1 (Ki 0.2 lM). Both the cyclopentyl 23 and
cyclohexyl 24 compounds showed low nanomolar affin-
ities: 4 nM. Other groups like cycloheptyl (25) and
cyclohexylmethyl (26) had comparable affinities (13
Finally, a set of compounds was prepared where the polar
group was replaced by an all-carbon moiety as shown in
Scheme 5. The route began with a Strecker synthesis using
N-Boc-piperazine as the amine counterpart, followed by
displacement of the cyano group with various Grignard
reagents,11 providing the all-aliphatic A-domains 20.
The final compounds for this study were constructed
from the privileged structures (previous deprotection
Scheme 5. Reagents: (a) TMSCN, Boc-piperazine, MeOH, 70–80%; (b) R4MgBr, THF, 50–60%.

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K. Briner et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3449–3453
Scheme 6. Reagents: (a)HOAt, HATU, i-Pr2NEt, CH2Cl2, DMF, 55–85%; (b) TFA, DCM or HCl, EtOAC, 99%.
Previously we had shown that address elements incorpo-
Table 1.
rating an isoindole (IIN) moiety in place of the tetrahy-
droisoquinoline (TIC) provide active molecules with
melanocortin activity.12 We therefore opted to try this
address element with these new privileged structures. It
was interesting to find that the substitution afforded
comparable (although slightly higher) activity (28 and
29) in comparison with the analogous TIC derivatives
(24 and 26). At this point, we chose to continue our
investigation with the more potent isoindole containing
address element.
Cl
O
O
A
N
Y
H
N
N
B
Compound
A
B
Y
Ki (nM)15
We next turned our attention to the nature of the po-
lar group. It was observed that other amine polar
groups such as pyrrolidine and morpholine (com-
pounds 30–31) did not produce a significant variation
in the affinity. The introduction of a diethylcarboxa-
mide group (32) was accompanied by a decrease in
affinity (148 nM). Also, N-methylsulfonamide 33
showed a decrease in affinity with a Ki of 26 nM com-
pared with 28. The N-ethylation of 33 afforded the
tertiary sulfonamide 34 which provided an 8-fold
improvement (3 nM) relative to 33. This trend was
continued with the tertiary amide 35, showing a simi-
lar affinity. Finally, the succinimide, 36, afforded a Ki
of 10 nM. As with our related series,6 we wanted to
learn the effect that the absolute configuration of the
privileged structure had on melanocortin affinity. We
prepared the two diasteroisomers of the final com-
pounds (only the diasteroisiomeric pairs for four com-
pounds 28, 34, 35, and 36 are shown).16 Both isomers
had comparable MC4R affinity.17 Unlike their aro-
matic counterparts, these aliphatic privileged struc-
tures did not show a strong enantiopreference.
1
o-FPh
Me
CH2NEt2
CH2NEt2
CH2NEt2
CH2NEt2
CH2NEt2
CH2NEt2
TIC 20
TIC 3427
22
23
24
25
26
C5H9
C6H11
C7H13
TIC
TIC
TIC
TIC
4
4
13
13
C6H11CH2
NEt2
27
TIC
70
B
A
28
C6H11
C6H11
C6H11
CH2NEt2
CH2NEt2
CH2NEt2
CH2NEt2
IIN
IIN
IIN
IIN
IIN
3
3
28aa
28ba
29
5
C6H11CH2
C6H11CH2
C6H11CH2
C6H11
5
30
31
CH2NC4H8
12
10
148
26
3
CH2morpholine IIN
CONEt2 IIN
32
33
C6H11
C6H11
CH2NHSO2Me IIN
CH2NEtSO2Me IIN
CH2NEtSO2Me IIN
CH2HEtSO2Me IIN
CH2NEtCOMe IIN
CH2NEtCOMe IIN
CH2NEtCOMe IIN
CH2succinimide IIN
CH2succinimide IIN
CH2succinimide IIN
34
34aa
34ba
35
C6H11
C6H11
6
7
C6H11
C6H11
8
35aa
35ba
36
36aa
36ba
37
2
C6H11
C6H11
3
10
10
18
18
35
54
326
C6H11
C6H11
Finally we turned our attention to the compounds in
which an aliphatic group replaced the polar group.
Unlike our previous benzylic piperazine series where
these modifications decreased affinity,6 we found that
the all-aliphatic compounds (37, 38, and 39) only suf-
fered a 5- to 10-fold decrease in activity when compared
with the related congeners, that is, 28 and 29. Notably,
these hydrophobic privileged structures still provided a
good potency range (18–54 nM). The compound 40 with
larger aliphatic groups showed a more significant
decrease in affinity (326 nM).
CH2CH(CH3)2 CH2CH(CH3)2
C6H11
IIN
IIN
IIN
IIN
38
39
C6H11
C6H11
C6H11CH2
CH2CH(CH3)2
C6H11CH2
40
a Resolved diasteroisomers.
nM) compared to those of 23 and 24. In contrast, the
achiral a,a-disubstituted cyclopentyl 27 presented a
roughly 10-fold decrease in affinity (70 nM).

K. Briner et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3449–3453
3453
15, 5237; (e) Bakshi, R. K.; Hong, Q.; Tang, R.; Kalyani,
R. N.; MacNeil, T.; Weinberg, D. H.; Van der Ploeg, L.
H. T.; Patchett, A. A.; Nargund, R. P. Bioorg. Med. Chem.
Lett. 2006, 16, 1130.
Data herein illustrate that the use of an aliphatic carbo-
cyclic piperazine-based privileged structure can provide
molecules with potent MC4R affinity.6 As we have seen
with previous privileged structures, the polar groups are
an important component for activity, but in contrast to
the aromatic series, less basic or nonbasic polar groups
showed excellent affinity. That privileged structures de-
void of polar functionality provide active constructs
suggests that our initial hypothesis regarding require-
ments for privileged structure design may have been
oversimplified. In fact, these results suggest that consid-
erable dynamic range with regard to required privileged
structure functionality exists and that careful systematic
evaluation of all new privileged structures is needed.
These results demonstrate the usefulness of the privi-
leged structure approach to find ligands with melano-
cortin activity. Further refinements of these structures
will be presented in due course.
5. For a discussion of priviledge structures (a) Evans, B. E.;
Rittle, K. E.; Bock, M. G.; DiPardo, R. M.; Freidinger, R.
M.; Whitter, W. L.; Lundell, G. F.; Veber, D. F.;
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6. Fisher, M. J.; Backer, R. T.; Collado, I.; de Frutos, O.;
Husain, S.; Kuklish, S. L.; Mateo, A.; Mullaney, J. T.;
Ornstein, P. L.; Garcia-Paredes, C.; Richardson, T. I.;
Zgombick, J. M.; Briner, K. Bioorg. Med. Chem. Lett.
2005, 15, 4973.
7. Cooke, M. P. J. Org. Chem. 1992, 57, 1495.
8. Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.;
Maryanoff, C. A.; Shah, R. D. J. Org. Chem. 1996, 61,
3849.
9. O’Brien, P. M.; Sliskovic, D. R.; Blankley, C. J.; Roth, B.
D.; WilsonMichael, W.; Hamelehle, K. L.; Brian, R.;
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10. Chiral chromatography conditions: 8 cm Novasep Col-
umn packed with 1Rq DAICEL Chiralpak AD. Mobile
phase = MeOH + 0.2% DMEA.
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4. For some related work in melanocortin receptor agonist,
see (a) Fotsch, C.; Smith, D. M.; Adams, J. A.; Cheetham,
J.; Croghan, M.; Dorhety, E. M.; Hale, C.; Jarosinski, M.
A.; Kelly, M. G.; Norman, M. H.; Tamayo, N. A.; Xi, N.;
Baumgartner, J. W. Bioorg. Med. Chem. Lett. 2003, 13,
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Z.; Sebhat, I. K.; Pollard, P. G.; Kalyani, R. N.; Tang, R.;
MacNeil, T.; Weinberg, D. H.; Vongs, A.; Rosenblum, C.
I.; Doss, G. A.; Miller, R. R.; Stearns, R. A.; Peng, Q.;
Tamvakopoulos, C.; McGowan, E.; Martin, W. J.; Metz-
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377; (d) Pontillo, J.; Tran, J. A.; White, N. S.; Arellano,
M.; Fleck, B. A.; Marinkovic, D.; Tucci, F. C.; Saunders,
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13. Functional activity was determined by measuring cAMP
release with a standard luciferase assay employing HEK
293 cells stably transfected with hMC4 (data not shown).
Compounds of this paper were agonists with relative
efficacies 60–100% of the maximum response obtained
with NDP-a-MSH.
14. Compounds of this report were found to be generally
selective for MC4 relative to the related receptors MC1
and MC3. Moderate to good selectivity was observed for
MC4 relative to MC5 (data not shown).
15. Each data point represents the average of at least two
determinations with an average error of the binding assay
being 15%.
16. The numeration of the 2 epimers comes from the order of
elution in the chiral separation of intermediate 9b.
17. This same trend was also observed in other less-active
derivatives.