Privileged structure based ligands for melanocortin receptors-4,4-Disubstituted piperidine derivatives

Article abstract of DOI:10.1016/j.bmcl.2006.04.016

Homologation and cyclization back to the chiral methine of compound 3 yields achiral 4,4-disubstituted piperidine privileged structures (e.g., 8a) useful in the construction of melanocortin 4 receptor (MC4R) ligands. The piperidine nitrogen was replaced w

Full text of DOI:10.1016/j.bmcl.2006.04.016

Bioorganic & Medicinal Chemistry Letters 16 (2006) 3843–3846
Privileged structure based ligands for melanocortin receptors—4,4-
Disubstituted piperidine derivatives
Steven L. Kuklish,* Ryan T. Backer, Karin Briner, Christopher W. Doecke, Saba Husain,
Jeffrey T. Mullaney, Paul L. Ornstein, John M. Zgombick,
Thomas P. O’Brien and Matthew J. Fisher
Lilly Research Laboratories, A Division of Eli Lilly and Company, Lilly Corporate Center, Indianapolis, IN 46258, USA
Received 10 March 2006; revised 7 April 2006; accepted 7 April 2006
Available online 11 May 2006
Abstract—Homologation and cyclization back to the chiral methine of compound 3 yields achiral 4,4-disubstituted piperidine priv-
ileged structures (e.g., 8a) useful in the construction of melanocortin 4 receptor (MC4R) ligands. The piperidine nitrogen was
replaced with carbon, oxygen, sulfur, and sulfone with minor erosion of binding. The methyl cyclohexane substituent was the most
potent while significant affinity was still seen for smaller lipophilic groups such as ethyl.
Ó 2006 Elsevier Ltd. All rights reserved.
The melanocortin 4 receptor (MC4R) is one of five type
I G-protein-coupled receptors (GPCRs) which have the
2 Dipeptide Address Element
Cl
Cl
melanocortin peptides a-, b-, and c-melanocyte-stimu-
lating hormones (MSH) as their endogenous ligands.1
The MC4R is expressed in the hypothalamus and is
thought to regulate a variety of processes including feed-
ing, metabolism, and reproductive behaviors.2 We have
been interested in developing ligands for the melanocor-
tin receptors to facilitate their pharmacological charac-
terization, and our efforts to date have yielded
compounds primarily selective for MC4R. The com-
pound series developed thus far have all required a
dipeptide address element coupled to a C-terminal priv-
ileged structure cap (Fig. 1).3
O
O
*
*
O
O
N
N
N
H
N
H
H
H
N
OH
H
N
N
*
N
N
*
1 MC4 Ki = 5 nM
N
3 Priveleged Structure
Figure 1.
tertiary carbon atom to link the key lipophilic and polar
pharmacophores.
Pharmacophore requirements for the piperazine-con-
taining subunits have generally included both hydropho-
bic and polar moieties. We have found that a number of
structurally related piperazine-containing moieties
afford reasonable activity and a fair degree of inter-
changeability when attached to the dipeptide address
element. Recently, we have reported on several new
structurally related families, the latest of which is
exemplified by structure 3.4,5 This scaffold uses a chiral
While the activity achieved with these structures has
been adequate for pharmacological characterization,
the chiral center afforded an unnecessary level of com-
plexity in synthesis. We wondered if this chiral carbon
and attached polar moiety could be replaced with a sym-
metrical subunit that still allowed the desired privileged
structure pharmacophores to be presented in a similar
orientation. Specifically, we thought that cyclization of
the polar residue back to the methine position of the chi-
ral carbon of 3, affording an achiral 4,4-disubstituted
piperidine (8a), might provide topography similar to
our previously described privileged structures (Fig. 2).
Keywords: Melanocortin; MC4; Privileged structure; G-protein
coupled receptors; GPCR.
*
Corresponding author. Tel.: +1 317 433 6519; e-mail:
Kuklish_steven@lilly.com
0960-894X/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2006.04.016

3844
S. L. Kuklish et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3843–3846
Table 1.
H
N
H
N
P
N
N
N
b
R2MgBr
THF
N
6a Or 6b
7a-i
NR
N
N
3
Chiral
R2
8a Achiral
a
Figure 2. The design strategy to mitigate asymmetry in our previous
privileged structure was achieved by the cyclization and homologation,
a and b, respectively.
Compound
P
R
R2
Compound
6a
6a
6a
6a
6a
6a
6a
6b
6b
Boc
Boc
Boc
Boc
Boc
Boc
Boc
Bn
Me
Me
Me
Me
Me
Me
Me
Ph
Bn
7a
7b
7c
7d
7e
7f
c-C6H11ACH2
n-C6H13
Et
Simple modeling of the 4,4-disubstituted piperidine of
8a revealed that many of the conformations accessible
to 3 remain available to the more constrained 4,4-disub-
stituted piperidine 8a. Herein, we report on the reduc-
tion of this hypothesis to practice and the continued
progress on the preparation and use of novel 4,4-disub-
stituted piperidine structures in the construction of
ligands for the melanocortin receptors.
2-Propene
(CH3)2CHACH2
c-C6H11ACH2
7g
7h
7i
Boc
Boc
Bn
(CH3)2CHACH2
O
X
P
N
H
N
a, b, c
Preparations of structures 8a–l containing functional-
ized piperidines are outlined in Scheme 1. Intermediates
6a, b were conveniently prepared from the correspond-
ing protected piperidines 4a, b using a modified Strecker
synthesis.6 Treatment of amino nitrile 6a with alkyl
Grignards gave modest yields (10–50%) of the 4,4-disub-
stituted piperazines 7a–g.7,8 The reactions of amino ni-
trile 6b with alkyl Grignards were more efficient for
the preparation of intermediates 7h (86% yield) and 7i
(94% yield) (Table 1). Deprotection of the Boc-protected
piperazines 7a–g with TFA yielded the N-methyl piperi-
dines 8a–g. Piperidine intermediates 7h (R = Boc) and 7i
(R = Boc) were deprotected with TFA, then sulfonylat-
ed, acylated or reductively aminated to yield the
corresponding benzyl-protected, functionalized 4,4-di-
substituted piperidines. The functionalized piperidines
were then debenzylated under palladium-mediated
hydrogenolysis to yield structures 8h–l. Cyclic ketones
9a–c were transformed in a similar manner as outlined
for the above piperidine ketones (Scheme 2). Notably,
reaction of the amino nitrile with an alkyl Grignard
was again more efficient (91–98% in route to 10a–c) than
observed for compounds 7a–g. Compounds that contain
a basic piperidine nitrogen such as 7a–g were typically
N
H
N
9a X=CH2
9b X=O
9c X=S
5a P=Boc
X
10a X=CH2
10b X=O
10c X=S
d
10d X=SO2
Scheme 2. Reagents: (a) TMSCN, ZnI2; (b) iBu-MgBr, THF, 91–98%;
(c) TFA, DCM; (d) MsOH, mCPBA, DCM.
less efficient in Grignard substitution reactions when
compared to those without the basic amine. Oxidation
of the thiomorpholine 10c with mCPBA in the presence
of methanesulfonic acid yielded 10d. The piperazine
structures 8a–l and 10a–d were coupled to the Boc-pro-
tected dipeptide9,10 11 in the presence of HATU as
previously described.11 The penultimate compounds
were then treated with TFA to generate 12a–p as the
corresponding TFA salts (Scheme 3).
Final products 12a–p were evaluated for binding at the
MC4 receptor by determining competitive inhibition of
[
125I]NDP-a-MSH binding in HEK293 cells stably
transfected with human MC4 receptors as previously
described.12,13 Compound 12a, an achiral analog of
1, yielded high affinity with a Ki of 7 nM. This observa-
tion suggests that this new achiral structure can mimic
O
P
N
P
N
a
6a R=Me, P=Boc
6b R=Boc, P=Bn
85-98%
N
R
N
H
N
5a P=Boc
5b P=Bn
4a R=Me
4b R=Boc
N
NR
Cl
Cl
b
10-94%
O
O
e, f
P
N
H
N
*
*
8a-l
Or
10a-d
O
O
N
Boc
N
N
N
H
c
H
H
OH
N
N
7a-g
7h-i
8a-l
N
11
N
c, d, e
R2
R2
NR
NR
R2
NR, X
12a-p
Scheme 1. Reagents: (a) TMSCN, ZnI2; (b) R2MgBr, THF; (c) TFA,
DCM; (d) One of the following; MsCl, Alkyl-COCl, Alkyl-COH and
NaBH4; (e) Pd/C, H2, EtOH.
Scheme 3. Reagents: (e) HATU, DIEA, DCM; (f) TFA, DCM.

S. L. Kuklish et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3843–3846
3845
Table 3.
the key topography afforded by chiral analog 1. Using
12a as a lead structure, we next explored the nature of
the aliphatic group R2, while holding the N-methyl
piperidine constant (12a–g, Table 2). The aromatic
counterpart to 12a, where R2 = Bn (12b), displayed a
6-fold decrease in binding affinity (Ki = 44 nM). Remov-
al of the methylene spacer of 12b (R2 = Bn) yields com-
pound 12c, (R2 = Ph) whose Ki of 215 nM is 31-fold less
than analog 12a. The alkyl substituted compound 12d
(R2 = n-hexyl) was 10-fold less active than compound
12a. Contracting the aliphatic unit of 12d (R2 = n-hexyl)
to yield compound 12e (R2 = ethyl) decreased binding
affinity by 5-fold (Ki = 370 nM). The isopropyl analog
12f resulted in binding affinity 9-fold better (Ki = 42 nM)
than its non-branched counterpart 12e (R2 = ethyl).
Homologation of 12f afforded the isobutyl analog 12g
(R2 = isobutyl) which had a 3-fold improvement in bind-
ing relative to 12f.
N
N
X
12m-p
Compound
X
MC4 Ki (nM)
12m
12n
12o
12p
CH2
O
46.9
31.4
68.4
75.8
S
SO2
The Ki data are the average of at least two determinations with an
average error of 15%.
amine- and tertiary sulfonamide-containing counter-
parts 12g (R = methyl) and 12l (R = SO2CH3).
Next we turned our attention to functionalization of the
piperidine nitrogen, while holding R2 constant as iso-
butyl. We observed minimal effects on binding affinities
for the family of alkyl substitutions on nitrogen. Both
12h (R = ethyl) and 12i (R = isopropyl) failed to differ-
entiate from the parent compound 12g (R = methyl)
with binding affinities of 16, 13, and 13 nM, respectively.
Acylation of the nitrogen yielded 12j (R = acetyl) which
resulted in a 3-fold loss in binding affinity (Ki = 38 nM)
relative to 12g. The branched alkyl amide of 12k
(R = COCH(CH3)2) demonstrated lower binding
affinity (Ki = 101 nM) than its simple alkyl amine coun-
terpart 12g.
The above data demonstrate that replacement of the chi-
ral structure 3 with a 4,4-disubstituted piperidine yields
structures which can be employed in the construction of
a new and physicochemically diverse set of MC4R li-
gands. Consistent with our hypothesis, the presumed de-
crease in conformational mobility afforded by the ring
structure of 8a did not preclude this new scaffold from
attaining the required conformation for binding to the
MC4R. In addition to the elimination of a stereocenter,
the synthetic route was straightforward with many of
the piperazine fragments being prepared in three steps.
For the MC4R, the amine moiety of the disubstituted
piperidine can posses an alkyl group (12g, polar,
charged) or be converted to a sulfonamide (12l, polar,
non-charged) with minimal loss of affinity. Similarly,
this piperidine nitrogen can be replaced by carbon, oxy-
gen, sulfide, and sulfone while still displaying good
MC4R affinity.
The focus then turned to replacement of the piperidine
ring with carbocycles and non-basic heterocycles
(Table 3). Here we found that both all aliphatic carbo-
cycle (12m X = CH2) and heterocycles 12n–p (X = O,
X = S, and X= SO2, respectively) produced binding
affinities typically 2- to 6-fold less than their tertiary
In summary, we have described a convenient synthesis
of useful 4,4-disubstituted piperidines and heterocycles
as achiral privileged structures with widely varying
physical properties for the preparation of MC4R li-
gands. Further, the subunits described here may afford
an attractive path to probe numerous GPCRs given
the highly conserved nature of the binding domains
across GPCRs. Understanding of privileged structure
topographical requirements and developing a collection
of these subunits can present a significant opportunity to
rapidly prepare ligands for the pharmacological explora-
tion of a variety of GPCRs.
Table 2.
N
N
R2
NR
Compound
R
R2
MC4 Ki (nM)
12a
12b
12c
12d
12e
12f
12g
12h
12i
Me
Me
c-C6H11ACH2
Bn
Ph
6.8
43.9
214.6
69.6
369.9
41.8
12.7
15.6
12.9
37.5
Me
Me
n-C6H13
Et
Me
Me
References and notes
(CH3)2CH
(CH3)2CHACH2
(CH3)2CHACH2
(CH3)2CHACH2
(CH3)2CHACH2
Me
Et
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(CH3)2CH
COCH3
12j
12k
12l
COCH(CH3)2 (CH3)2CHACH2 100.7
SO2CH3 (CH3)2CHACH2 27.3
The Ki data are the average of at least two determinations with the
average error of 15%.
3. For a discussion of privileged structures, see: (a) Evans, B.
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3846
S. L. Kuklish et al. / Bioorg. Med. Chem. Lett. 16 (2006) 3843–3846
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8. General reaction of a-amino nitriles with alkyl Grignards:
Preparation of 12e. To a solution of 4-(4-cyano-1-methyl-
piperidin-4-yl)-piperazine-1-carboxylic acid tert-butyl
ester (0.5 g, 1.6 mmol) in THF (4 mL) was added ethyl
magnesium bromide 3 M in Et2O (5.4 mL, 1.6 mmol)
(additional alkyl Grignard may be added and heated to
40 °C to drive the reaction to completion if needed. This
extra effort was most commonly used for examples with
structures like 7a–g). The mixture was stirred for 18 h and
then poured into water (50 mL). The mixture was then