Privileged structure based ligands for melanocortin receptors - Substituted benzylic piperazine derivatives

Article abstract of DOI:10.1016/j.bmcl.2005.08.018

Replacement of the aryl piperazine moiety in compound 1 with a variety of substituted benzylic piperazines (6) yields compounds that afford melanocortin receptor 4 (MCR4) activity. Analogs with ortho substitution on the aromatic ring afforded the highest

Full text of DOI:10.1016/j.bmcl.2005.08.018

Bioorganic & Medicinal Chemistry Letters 15 (2005) 4973–4978
Privileged structure based ligands for melanocortin
receptors—Substituted benzylic piperazine derivatives
a
b
b
a
Matthew J. Fisher,a, Ryan T. Backer, Ivan Collado, Oscar de Frutos, Saba Husain,
´
*
´
Hansen M. Hsiung,a Steve L. Kuklish,a Ana I. Mateo,a Jeffrey T. Mullaney,a
a
b
a
a
´
Paul L. Ornstein, Cristina Garcıa Paredes, Thomas P. OÕBrian, Timothy I. Richardson,
Jikesh Shah,a John M. Zgombicka, and Karin Brinera
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 3 June 2005; revised 3 August 2005; accepted 3 August 2005
Available online 16 September 2005
Abstract—Replacement of the aryl piperazine moiety in compound 1 with a variety of substituted benzylic piperazines (6) yields
compounds that afford melanocortin receptor 4 (MCR4) activity. Analogs with ortho substitution on the aromatic ring afforded
the highest affinity. Resolution of the stereocenter of the benzylic piperazine based privileged structure revealed that the R-enantio-
mer was more active.
ꢀ 2005 Elsevier Ltd. All rights reserved.
The melanocortin receptor (MCR) family is composed
of 5 closely related receptors; all of which belong to
the Type I segment of the G-Protein Coupled Receptor
(GPCR) family.1 We were interested in identifying and
optimizing selective tools to facilitate pharmacological
characterization of MCRs with a primary focus on find-
ing ligands selective for MCR4.2,3 We recently reported
on our initial efforts, which used a privileged structure
based approach, for the identification of compounds
with the desired activity profile.4 We had found that a
privileged structure, containing both hydrophobic aro-
matic and polar functionalities (2), coupled with a
dipeptide address element (3), provided activity and
selectivity for the receptor of interest (Fig. 1). In order
to increase the dynamic range of our current series, we
sought to better understand what structural elements
were required and/or tolerated in privileged structures
that afford MCR activity.
coupled with the dipeptide address element 3.5 In both
cases described above, the piperazine moiety was direct-
ly attached to the phenyl ring, thus limiting overall flex-
ibility in this region. We were curious to see if MCR
activity would be retained when the piperazine moiety
was attached to a carbon chain that emanates from
the phenyl ring (Fig. 2). The resulting benzylic pipera-
zines would be interesting in that they offer an increased
level of flexibility both in regard to privileged structure
conformation and synthesis. Herein, we report the re-
sults of our continued progress toward the development
of privileged structures that can be used in the construc-
tion of ligands with MCR activity.
Synthesis of the desired benzylic piperazines begins as
outlined in Scheme 1. Condensation of a variety of ben-
zaldehydes (7) with cyanide ion and N-Boc piperazine,
using a modified Strecker protocol, affords the desired
amino nitriles 8 in good yield.6 These intermediates
could be carefully reduced with LAH at ꢀ50 ꢁC to af-
ford the 1ꢁ amines 9.7 Functionalization of amines 9
was easily accomplished by alkylation (12), acylation
(11, 13), or sulfonylation (10) using standard reaction
conditions as shown in Scheme 2. In some cases, reduc-
tion of the initial product of acylation (13) with BH3
afforded a 2ꢁ amine (14) that could be further deriva-
tized providing fully substituted amines 15–17.
We have previously shown that both substituted mono-
cyclic and substituted bicyclic aryl piperazines are
privileged structures that provide MCR activity when
Keywords: Melanocortin receptors; Priveleged structures.
*
Corresponding author. Tel.: +1 3172760632; fax: +1 3174333666;
e-mail: fisher_matthew_j@lilly.com
Deceased.
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.08.018

4974
M. J. Fisher et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4973–4978
Cl
Cl
O
Dipeptide Address Element
O
O
N
O
H
N
HN
N
N
H
OH
N
Boc
H
N
3
N
N
N
1 MC4R Ki = 14 nM
2 Privileged Structure
Figure 1. Disconnective analysis of lead series.
H
brominated with NBS and catalytic HBr, to provide the
desired a-halo acid chlorides in good yield. Allowing
this material to first react with methanol and then
N-Boc piperazine gave good yields of the desired homo-
logs 22. The ester moiety in these compounds was then
partially reduced with DIBAH to give aldehydes 23,
which were then subjected to reductive amination with
diethylamine giving amines 24.
H
N
N
N
b
N
a) cyclize
N
( )n N
a
5 (Bicyclic aryl piperazine)
4 (Aryl piperazine)
H
N
Resolution of the chiral benzylic piperazines described
in this paper was accomplished in two ways. Benzylic
piperazines devoid of substitution on the aromatic ring
were prepared via synthesis starting from optically
active phenylglycinols as shown in Scheme 4. Synthesis
of the R-enantiomer began with the alkylation of the
amino moiety contained in S-phenylglycinol 25 with ex-
cess ethyl iodide affording intermediate 26 in good yield.
This material was then subjected to the action of meth-
anesulfonyl chloride in ether and the resulting mesylate
was allowed to react with N-Boc-piperazine at room
temperature for 18 h. This afforded the desired interme-
diate benzylic piperazine 27 in good yield after workup
and chromatography. Analysis of the material produced
in this fashion by chiral chromatography (Chiralpak
AD, hexane–TFA 0.05%) indicated that product was
obtained in >98% ee. This transformation presumably
occurs via an intermediate aziridium ion, which is then
displaced at the benzylic center resulting in an overall
inversion of stereochemistry. Synthesis of the S-enantio-
mer was accomplished in the same way starting with
R-phenylglycinol. Alternatively, the desired privileged
structures could be resolved on a gram scale by chiral
chromatography (Chiralcel OD (4.6 · 250 mm) column,
eluting with 5% ethanol in heptane at 1 mL/min) of
the corresponding phthalimide derivatives (29). This
sequence is illustrated in Scheme 4 using the o-fluoro
derivative 28. The resolved phthalimides (29) were then
deprotected with excess hydrazine and the liberated
amine (30) was functionalized in an analogous fashion
to the racemic analogs.
b) change
connectivity
N
N
6 (Benzylic piperazine)
Figure 2. Evolution of priveleged structure connectivity.
Boc
N
Boc
N
O
b
a
N
N
H
R
NH
2
R
R
N
7
8
9
Scheme 1. Reagents and conditions: (a) Boc-piperazine, TMSCN, and
ZnI2: (b) LAH, ꢀ50 ꢁC.
An analog containing an a-amino amide moiety was
prepared as described in Scheme 3. The commercially
available ethyl ester of o-fluoro phenylacetic acid (18)
was brominated with N-bromosuccinimide forming the
a-bromo derivative, which was then allowed to react
with N-Boc-piperazine yielding 19. This material was
then hydrolyzed with sodium hydroxide and the result-
ing acid was transformed into the diethyl amide 20 using
standard conditions. Compounds derived from other
homologs of phenyl acetic acid were prepared using sim-
ilar chemistry. In these cases, the commercially available
phenyl carboxylic acids (21) were first transformed with
thionyl chloride into the acid chlorides, which were then
Final compounds described in this paper were con-
structed from the aforementioned intermediates as
outlined in Scheme 5. The piperazine-containing

M. J. Fisher et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4973–4978
4975
Boc
N
b
10 R2 = SO2Me, R3 = H
11 R2 = CO2Me, R3 = H
c
R3
N
9
N
12 R2 = Et, R3 = Et
d
R2
a
R
a
Boc
N
Boc
N
Boc
N
15 R2 = COCH3
16 R2 = SO2Me
17 R2 = CO2Me
e
b
c
N
N
N
H
N
NH
N
R2
R
R
R
O
13
14
Scheme 2. Reagents: (a) acetyl chloride; (b) methanesulfonyl chloride; (c) methylchlorofomate; (d) EtBr, K2CO3; and (e) BH3.
Boc
N
Boc
N
F
O
N
F
c, d
a, b
N
F
O
O
O
N
O
O
18
19
20
( )n
N
( )n
O
R
e, f
h
N
( )n
21
N
OH
N
Boc
N
Boc
22 R=OCH3
24
g
23 R=H
Scheme 3. Reagents: (a) NBS; (b) Boc-piperazine; (c) NaOH; (d) Et2NH, EDCI; (e) SOCl2; (f) NBS, HBr; (g) DIBAH; and (h) Et2NH, NaBH3CN.
H
N
NH2
N
b, c
OH
a
N
OH
N
25
26
27
Boc
N
Boc
N
Boc
N
Boc
N
O
d
e,f
N
F
N
F
N
F
N
F
+
NH2
N
NH2
NH2
O
28
30a
30b
29
Scheme 4. Reagents: (a) EtI, K2CO3, (b) MsCl, (c) Boc-piperazine, (d) phthalic anhydride, (e) chiral chromatography, and (f) hydrizine.
privileged structures were allowed to react with the
previously described dipeptide 3 in the presence of
HATU8 to afford the penultimate derivatives (31).
Treatment with TFA provided the desired deprotected
compounds 32 which could be used as the correspond-
ing TFA salt or subjected to ion exchange to obtain
the HCl salt. In the cases where the piperazines were
not prepared in chiral form, coupling with dipeptide 3
afforded diastereomeric pairs that were not separated
for biological evaluation.

4976
M. J. Fisher et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4973–4978
Table 2. Substituted benzyl piperazines
Cl
H
N
N
O
O
N
N
a
N
H
N
R1
N
N
N
R
R
R2 R3
31
32
b
R1
Compound
R
MC4R Ki (nM)12
R2 R3
39
40
41
42
43
44
45
46
47
o-CF3
m-CF3
p-CF3
o-Cl
40
150
200
70
Scheme 5. Reagents: (a) 3, HATU; (b) TFA, DCM.
o-CH3
o-F
o-OCH3
70
20
Compounds described for this report were evaluated for
binding affinity at human melanocortin 4 receptors
(hMCR4) by determining competitive inhibition of
125I-NDP MSH binding.9,10 Selected compounds were
evaluated for selectivity across human melanocortin
receptors 1, 3, and 5 in a similar fashion.11 We first
examined the effects that benzylic attachment would
have on this series of compounds. Compound 33 (Table
1) which lacks a polar group (thought to be key for
activity) afforded modest affinity for hMCR4 with a Ki
of 2 lM. Substitution on the benzylic carbon with a sim-
ple methyl group yielded compound 34, which showed a
3-fold increase in activity (Ki 0.7 lM) relative to the
unsubstituted analog 33. Conversion of this methyl
group into a diethylaminomethyl moiety, a functional
group that has been very productive in our previous ser-
ies, yielded compound 35, whose activity increased
8-fold (Ki of 0.09 lM) relative to the methyl analog
34. Stepwise elongation of the linker between the phenyl
group and the tertiary carbon provided analogs
36, 37, and 38 which had comparable affinities of
0.04 lM, 0.13, and 0.11 lM, respectively.
190
150
100
2,6-Difluoro
2,4-Difluoro
analog 35. Ortho substitution of the phenyl ring was also
beneficial for other substituents such as chloro, fluoro,
and methyl. Of the single substitutions examined in
the ortho position, the fluoro analog (44) appeared to
be optimal with a Ki of 0.02 lM. The only disubstituted
aromatic analogs examined (46 and 47) both utilized
fluorine as replacements of hydrogen and neither
showed an activity advantage relative to their mono-
substituted counterpart 44.
We next turned our attention to the polar functionality
found in this series of privileged structures through
examination of substitution on the amine moiety (Table
3). Relative to the dialkylamine moiety found in com-
pound 44 (Ki 0.02 lM), simple acylation (50) resulted
in a 5-fold erosion of activity (Ki 0.46 lM). Activity
was also diminished (Ki 0.47 lM) by sulfonylation
(49). Conversion of the dimethylamino moiety to a
carbamate (48) also decreased activity similar to that ob-
served from acylation. Interestingly, N-alkylation of the
acylated and sulfonylated compounds (49 and 50,
respectively) with an ethyl moiety provided tertiary
amides 52 and 53, that were more potent than their
nonalkylated counterparts and only 2-fold less active
As the binding affinities of the above-mentioned homo-
logs were similar, we chose to focus on optimizing the
more synthetically accessible benzylic analog 35 (Table
2). We next examined simple aromatic substitution
and a trifluoromethyl group was the first substituent
evaluated. The data reveal that ortho substitution (39)
was more active than meta (40) or para (41) substitution
by 4.7- and 3.6-fold, respectively. Ortho substitution
provided a 2-fold benefit relative to the unsubstituted
Table 3. Amine substitution
N
Table 1. Phenyl alkyl piperazines
N
F
R3
N
N
R2
Y'
Y
N
R
n
Compound
Y,Y0
R2
R3
MC4R Ki (nM)12
Compound
n
R
MC4R Ki (nM)12
48
49
50
51
52
53
54
H,H
H,H
H,H
H,H
H,H
H,H
Oxo
COOCH3
SO2CH3
COCH3
COOCH3
SO2CH3
COCH3
CH2CH3
H
483
472
467
261
157
106
125
H
33
34
35
36
37
38
0
0
0
1
2
3
H
CH3
2000
700
90
H
Et
Et
Et
Et
CH2N(CH2CH3)2
CH2N(CH2CH3)2
CH2N(CH2CH3)2
CH2N(CH2CH3)2
40
130
110

M. J. Fisher et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4973–4978
4977
than their tertiary amine counterpart 44. Changing the
point of acylation from an external position to an inter-
nal arrangement provided glycinamide derivative 54,
which was roughly 4-fold less potent (Ki 0.125 lM) than
the corresponding tertiary amine analog 44.
privileged structures as key components of GPCR li-
gands. While the use of privileged structures has prov-
en useful for the generation of active compounds, we
have found that efficient optimization requires an
accurate understanding of the pharmacophores and
topography displayed by the lead privileged structures.
This information can then be used in the design and
construction of a portfolio of physicochemically di-
verse, yet functionally equivalent, subunits. Identifica-
tion of these privileged structure families would in
principle allow the ready optimization of the address
elements of interest. Further development of this
hypothesis and its application to the further refine-
ment of ligands for the MCRs will be the subject of
future reports.
Finally, we examined the effect that the absolute config-
uration of the privileged structure had on the activity of
these compounds (Table 4). The data revealed a 4-fold
difference in the phenyl antipodes as seen by comparing
55 and 56. As this pair was from intermediates with
known absolute stereochemistry, we are able to assign
the stereochemistry of the benzylic carbon of most po-
tent isomer 56 as R while the same center of the less po-
tent isomer 55 was assigned the S configuration. Similar
differences in activity were observed for the fluoro ana-
logs, which were resolved chromatographically (see
text). In this case, the faster eluting isomer 57 afforded
more activity than the slower eluting isomer 58. Based
on the activity difference observed for both sets of com-
pounds, we have assigned the faster eluting isomer of
the fluoro analog 57 as having the R configuration at
the benzylic center and the slower isomer having the
S configuration.
Acknowledgment
The authors thank Mark Heiman for many helpful
discussions.
References and notes
The above data demonstrate that appropriately substi-
tuted benzylic piperazines act as privileged structures
when coupled to address element 3. Of interest is the fact
that the unsubstituted benzyl piperazine 33 affords mea-
surable activity, and functionalization of the benzylic
position with even a simple methyl group (34) affords
a compound with submicromolar activity. Consistent
with our earlier results, we found that the incorporation
of a polar group proximal to the aromatic group
enhances activity for this series. It appears that distance
between the phenyl moiety and the benzylic center to
which the polar group is attached is not very restrictive.
This may indicate that the putative binding site for this
pharmacophore is size tolerant. Substitution of the
phenyl moiety with halogens, especially fluorine, had a
measurable benefit, which suggests that this moiety
likely occupies a hydrophobic area in the receptor.
1. Cone, R. D.; Mounthoy, K. G.; Robbins, L. S.; Nadu, J.
H.; Johnson, K. R.; Roselli-Rehfuss, L.; Mortund, M. T.
Ann. N.Y. Acad. Sci. 1993, 680, 342.
2. For examples of efforts directed at the preparation of
selective ligands see: (a) Pan; Scott, M. K.; Lee, D. H. S.;
Fitzpatrick, L. J.; Crooke, J. J.; Rivero, R. A.; Rosenthal,
D. I.; Vaidya, A. H.; Zhao, B.; Reita, A. B. Bioorg. Med.
Chem. Lett. 2003, 11, 185; (b) Sebhat, I. K.; Martin, W. J.;
Ye, S.; Marakat, K.; Mosley, R. T.; Johnston, D. B. R.;
Bakshi, R.; Palucki, B.; Weinberg, D. H.; MacNeil, T.;
Kalyani, R. N.; Tang, R.; Sterns, R. A.; Tamvakopoulos,
C.; Strack, A. M.; McGowan, E.; Cashen, D. E.; Drisko,
J. E.; Hom, G. J.; Howard, A. D.; MacIntyre, D. E.;
vanderPloeg, L. H. T.; Patchett, A. A.; Nargund, R. P.
J. Med. Chem. 2002, 45, 4589; (c) Dyck, B.; Parfker, J.;
Phillips, T.; Carter, L.; Murphy, B.; Summers, R.;
Hermann, J.; Baker, T.; Cismowski, M.; Saunders, J.;
Goodfellow, V. Bioorg. Med. Chem. Lett. 2003, 13, 3793;
(d) Zhang, J.; Xiong, C.; Ying, J.; Wamg, W.; Hrubt, V. J.
Org. Lett. 2003, 5, 3115; (e) 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, 2337.
The activity afforded by both the phenyl and benzyl
piperazine series demonstrates the variety allowed in
privileged structures that yield MCR activity. These
observations underscore the practicality of employing
3. Richardson, T. I.; Ornstein, P. L.; Briner, K.; Fisher, M.
J.; Backer, R. T.; Biggers, C. K.; Clay, M. P.; Emmerson,
P. J.; Hertel, L. W.; Hsiung, H. M.; Husain, S.; Kahl, S.
D.; Lee, J. A.; Lindstrom, T. D.; Martinelli, M. J.; Mayer,
J. P.; Mullaney, J. T.; OÕBrien, T. P.; Pawlak, J. M.;
Revell, K. D.; Shah, J.; Zgombick, J. M. J. Med. Chem.
2004, 47, 744.
Table 4. Resolved examples
N
N
4. Fisher, M.J.; Backer, R.T.; Husain, S; Hsiung, H.M.;
Mullaney, J.T.; OÕBrian, T.P.; Ornstein, P.L; Rothhaar,
R.R.; Zgombick, J.M.; Briner, K. Bioorg. Med. Chem.
Lett. in press.
5. For a discussion of privileged structures see: Evans, B. E.;
Rittle, K. E.; Bock, M. G.; DiPardo, R. M.; Freidinger, R.
M.; Whitter, W. L.; Lundell, G. F.; Veber, D. F.;
Anderson, P. S.; Chang, R. S. L.; Lotti, V. J.; Cerino,
D. J.; Chen, T. B.; Kling, P. J.; Kunkel, K. A.; Springer, J.
P.; Hirshfield, J. J. Med. Chem. 1988, 31, 2235; Bondensg-
N
*
R
Compound
R
MC4R Ki (nM)12
*
55
56
57
58
H
H
F
S-isomer
R-isomer
Isomer 1
Isomer 2
190
50
14
111
F

4978
M. J. Fisher et al. / Bioorg. Med. Chem. Lett. 15 (2005) 4973–4978
aard, K.; Ankersen, M.; Thogersen, H.; Hansen, B. S.;
Wulff, B. S.; Bywater, R. P. J. Med. Chem. 2004, 47, 488.
6. Evans, D. S.; Carroll, G. L.; Truesdale, L. K. J. Org.
Chem. 1974, 38, 914.
7. Reduction at temperatures above ꢀ50 ꢁC resulted in
significant formation of the corresponding benzylic pip-
ereazines—presumably from loss of cyanide and reduction
of the resultant iminimum ion.
8. Speicher, A.; Klaus, T.; Eicher, T. J. Prakt. Chem./
Chemiker-Zeitung 1998, 340, 581.
293 cells stably transfected with hMC4R. Compounds
(both single antipodes and diastereomeric pairs) described
in this paper were agonists with relative efficacies 60–100%
of the maximum response obtained with NDP-a-MSH. Of
the diastereomeric pairs that were separated, we were
unable to detect any antagonist activity in the less active
diastereomer.
11. Compounds of this report were found to be generally
selective for MC4R relative to the related receptors MC1
and MC3. Moderate to good selectivity was observed for
MC4R relative to MC5R.
12. Each data point represents the average of at least two
determinations with an average error of the binding assay
being 15%.
9. The details of the binding assay have been recently
described. See Ref. 3.
10. Functional activity was determined by measuring cAMP
release with a standard luciferase assay employing HEK