4-Amino-2-(aryl)-butylbenzamides and their conformationally constrained analogues. Potent antagonists of the human neurokinin-2 (NK2) receptor

Article abstract of DOI:10.1016/S0960-894X(03)00343-3

A library, evaluating a range of piperazines, piperidines and acyclic amines, as replacements for the 4-hydroxy-4-phenylpiperidine moiety in lead (1b) was prepared. These efforts identified the 4-((N)-benzimidazolone)piperidine analogue (2a) which was further optimised using classical single-compound synthesis to yield the 3-((N)-morpholino)azetidine (2j). Conformationally constrained analogues of (2j), generally offered no potency advantage in this particular series.

Full text of DOI:10.1016/S0960-894X(03)00343-3

Bioorganic & Medicinal Chemistry Letters 13 (2003) 2211–2215
4-Amino-2-(aryl)-butylbenzamides and Their Conformationally
Constrained Analogues. Potent Antagonists of the Human
Neurokinin-2 (NK2) Receptor
A. Roderick MacKenzie,a,y Allan P. Marchington,a,{ Donald S. Middleton,a,*
Sandra D. Newman,a Christopher N. Selwayb and Nicholas K. Terrettb
aDepartment of Discovery Chemistry, Pfizer Global Research and Development, Sandwich, Kent, CT139NJ, UK
bDepartment of Medicinal Technologies, Pfizer Global Research and Development, Sandwich, Kent, CT139NJ, UK
Received 2 December 2002; revised 28 February 2003; accepted 19 March 2003
Abstract—A library, evaluating a range of piperazines, piperidines and acyclic amines, as replacements for the 4-hydroxy-4-phe-
nylpiperidine moiety in lead (1b) was prepared. These efforts identified the 4-((N)-benzimidazolone)piperidine analogue (2a) which
was further optimised using classical single-compound synthesis to yield the 3-((N)-morpholino)azetidine (2j). Conformationally
constrained analogues of (2j), generally offered no potency advantage in this particular series.
# 2003 Elsevier Science Ltd. All rights reserved.
Introduction
(1a) with a 4-hydroxy group (1b) also resulted in a
compound with excellent functional potency in the RPA
assay (pA2 9.5).
Tachykinins and their receptors have been postulated to
play a significant role in the pathophysiology of a wide
variety of diseases including gastrointestinal disorders,1
emesis,2 chronic pain,3 depression4 and asthma.5 Our
own long standing efforts in this field have focused on
investigating the role of neurokinin-2 antagonists in
urological disorders.6 Our initial studies in this area,
however, were hampered by the lack of attractive che-
mical leads both as mechanistic tools and as starting
points for a medicinal chemistry programme.
In this communication we wish to report our own pre-
liminary SAR from within 4-amino-butylbenzamide
series (2), using both parallel chemistry and single com-
pound synthesis to explore both the role of the cyclic
amine and the ring substitution in conferring potency in
this series.
In addition, we also wish to report our preliminary
findings from within a series of conformationally
constrained analogues (3) containing a 3-((N)-mor-
pholino)azetidine substituent, as a replacement for the
4,4-disubstituted piperidine moiety present in series (1).
In 1991, however, the Sanofi group7 reported the first
examples of potent, non-peptidic NK2 antagonists,
exemplified by (1), Figure 1, precipitating significant
activity in this structural series.8 Benzamide, (1a), shows
excellent potency against the human NK2 receptor in
binding studies (pIC50 8.9) and functionally in the Rab-
bit Pulmonary Artery (RPA) assay (pA2 9.1). Similarly,
replacing the 4-N-acetyl group on the piperidine ring in
*Corresponding author. Fax:+44-1304-651987; e-mail: don_s_
middleton@sandwich.pfizer.com
yCurrent address; Discovery Technology Center, Pfizer Inc.,
620 Memorial Drive, Cambridge, MA 02139, USA.
{Current address; Millennium Pharmaceuticals, Memorial Drive,
Cambridge, MA, USA.
Figure 1.
0960-894X/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0960-894X(03)00343-3

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A. R. MacKenzie et al. / Bioorg. Med. Chem. Lett. 13(2003) 2211–2215
Chemistry
aldehyde reductively aminated using 3-((N)-morpholi-
no)azetidine hydrochloride11 in the presence of NaB-
H(OAc)3. Targets (3c) and (3e) were accessed via
conversion of the appropriate alcohols (9) to their
mesylates using methanesulphonyl chloride in CH2Cl2
and subsequent displacement using 3-((N)-morpho-
lino)azetidine hydrochloride in MeCN at reflux using
NEt3 and K2CO3 as base.
4-Amino-2-(3,4-dichlorophenyl)butylbenzamides (1a, 1b,
1d–1f and 2a–2j) were prepared as shown in Scheme 1.
Alkylation of commercially available 3,4-dichloro-
phenylacetonitrile with 2-bromoethyl tetrahydro-(2H)-
pyran-2-yl ether in the presence of NaH as base and
subsequent reduction of the nitrile using hydrogen in
the presence of Raney nickel as catalyst under acid
conditions, yielded (4). Formylation of the primary
amine using paraformaldeheyde and borane reduction
in THF as solvent, followed by treatment of the result-
ing secondary amine with benzoylchloride gave the
benzamide (5). Alcohol (5) was then converted to the
target compounds by one of two methods; oxidation to
the aldehyde (6) under Swern conditions9 and reductive
amination of the resulting aldehyde with the appro-
priate amine, or alternatively, conversion to the mesy-
late, using methanesulphonyl chloride and displacement
of the resulting mesylate using the appropriate amine in
the presence of K2CO3 as base, in DMF at elevated
temperature.
Results and Discussion
Library compounds
NK2 receptor binding experiments on library com-
pounds were carried out according to the method of
Bergstrom12 on rat duodenum membranes. Data were
analysed using programmes to calculate percentage
specific binding for each sample.
Singleton compounds
Competition binding experiments were performed in a
NK2 receptor assay using labeled 125I NKA binding to
human NK2 receptors expressed in Chinese Hamster
Ovary (CHO) cells. Functional activity was determined
by testing their ability to antagonise the contractile
effects of the selective NK2 receptor agonist [B-
ala8]NKA(4ꢀ10) in the rabbit pulmonary artery (RPA),
using the method of Pataccini and Maggi.13
Pyrrolidines and piperidines (3a–-3e)10 were prepared as
shown in Scheme 2. Alkylation of (7), prepared as
described in Scheme 1, with either ethyl bromoacetate
or ethyl bromopropionate, using LDA as base in THF,
gave (8a) and (8b), respectively. These homologues were
then independently reduced to the amino esters, using
hydrogen and Raney nickel as catalyst at room tem-
perature, undergoing in situ lactamisation under the
reduction conditions. Reduction of the g- and d-lactams
to their respective cyclic amines, was then achieved
using either LiAlH4 or BH3.SMe2. Acylation with ben-
zoyl chloride (for 3a and 3b), ortho-methoxybenzoyl
chloride (for 3c), 2-(cyclopropyl)acetyl chloride (3d) or
phenylsulphonyl chloride (3e) and alcohol deprotection,
using HCl in THF, then furnished the intermediates (9a)
and (9b). The target compounds were then accessed
using two separate routes. For targets (3a), (3b), and
(3d), the appropriate alcohols (9) were oxidised to the
aldehyde under Swern conditions and the resulting
The key objective of this work was to rapidly identify
novel SAR for NK2 activity in the piperidine region of
lead (1). It is well precedented from the literature that
the biological activity resides in the (S)-enantiomer of
(1)7c and with this fact in mind, we chose to focus our
efforts, in this initial phase of the programme, in the
synthetically more accessible racemic series.
Initial exploration of the effects on potency of modify-
ing piperidine substitution in (1b) produced some
intriguing SAR, Table 1. For example, removal of the
Scheme 2. Reagents and conditions: (a) Ethyl bromoacetate (n=0) or
ethyl bromopropionate (n=1), LDA, THF, ꢀ78 ꢁC to rt; (b) H2,
Raney nickel, MeOH; (c) LiAlH4, Et2O or BH3.SMe2, THF, reflux;
(d) acylation or sulphonylation, NEt3, CH2Cl2; (e) HCl, THF, rt; (f)
(COCl)2, NEt3, DMSO, CH2Cl2; (g) 3-((N)-morpholino)azetidine
hydrochloride, NaBH(OAc)3, AcOH, NEt3, THF, rt; (h) MeSO2Cl,
NEt3, CH2Cl2, rt; (i) 3-((N)-morpholino)azetidine hydrochloride,
NEt3, K2CO3, MeCN, reflux.
Scheme 1. Reagents and conditions: (a) Br-(CH2)2-OTHP, NaH,
DMF; (b) H2, Raney nickel; (c) (CHO)n, BH3, THF; (d) PhCOCl,
NEt3 CH2Cl2, rt; (e) (COCl)2, DMSO, NEt3, CH2Cl2; (f) cyclic amine,
NaBH(OAc)3, AcOH, NEt3, THF; (g), MeSO2Cl, NEt3, CH2Cl2; (h)
amine, K2CO3, DMF.

A. R. MacKenzie et al. / Bioorg. Med. Chem. Lett. 13(2003) 2211–2215
2213
Table 1. Human NK2 binding and RPA functional data for 4-piper-
idinyl-2-(3,4-dichlorophenyl)-butylbenzamides (1b–f)
Table 2. NK2 binding inhibition data for 3-(diethylcarbox-
amido)piperidine (9) and benzimidazolone analogue (2a)
Compda
R
NK2 binding (CHO)
Compda
X
Y
NK2 binding
(CHO, IC50, nM)b
NK2 Functional
(RPA, pA2)
9
61% inhibition @ 100nM
1b
1c
1d
1e
–Ph
–Ph
–H
–OH
–H
–OH
–H
8.9
<7c
6.4
9.5
NT
NT
NT
2a
81% inhibition @ 100nM
–H
6.3
1f
–OH
6.6
NT
aAll compounds are racemic.
NT=not tested.
aAll compounds are racemic.
bAll determinations nꢂ2 (each experiment performed in triplicate).
cLiterature data (ref 7c).
both the substitution on the piperidine ring and the
cyclic amine ring size. For example, 3-piperidine sub-
stituted isomer (2b) (pIC50 7.1) was found to be sig-
nificantly weaker in NK2 binding studies than the
4-isomer (2a). Changing the piperidine into a pyrroli-
dine (2c) was also found to reduce potency (pIC50 7.4).
Increasing the amine ring size to give the homo-
piperidine analogue (2d) retained functional potency
(pA2 8.4) relative to the piperidine. Conversion of the
benzimidazolone (2a) to the N-linked benzimidazole
(2e) resulted in a small reduction in binding activity
(pIC50 8.1), although functional activity in the RPA
assay was retained (pA2 8.6), indicating that the imida-
zolone carbonyl in (2a) was not needed for high NK2
activity. To explore this observation further, the (C)-2
linked benzimidazole (2f) isomer was also prepared.
Disappointingly, this compound was found to be 20-
fold weaker than the (N)-1 isomer (2e) in the RPA
assay. Interestingly, removal of the fused phenyl ring in
(2e) to give the simple 4-((N)-imidazolyl)piperidine (2g),
as a strategy to reduce MWt, effectively retained func-
tional potency (pA2 8.3), although the triazole analogue
(2h) was >10-fold weaker than the imidazole. Repla-
cing the piperidine ring present in (2g) with the azetidine
(2i) resulted in only a small reduction in functional
potency (pA2 7.6), suggesting that the 3-substituted
azetidine effectively mimicked the directionality of the
4-substituted piperidine, placing the imidazole sub-
stituent in a similar region of space to its larger ring
homologue. Building on this azetidine SAR further, we
investigated non-aromatic substituents at the 3-posi-
tion.16 From these studies, the 3-(N)-morpholino ana-
logue (2j) emerged. This compound was found to
possess excellent functional potency (pA2 8.9).
4-hydroxyl group (1c)7c resulted in a precipitous drop in
potency (pIC50 <7).7c Similarly, removal of the 4-
phenyl ring, while retaining the 4-hydroxy substituent,
(1d) (pIC50 6.4) also resulted in a large reduction in
activity relative to the 4-phenyl-4-hydroxy analogue (1b)
(pIC50 8.9).
Removal of both substituents to give the unsubstituted
piperidine derivative (1e) gave a compound of roughly
equivalent potency to either of the two mono-sub-
stituted analogues (1c) or (1d). Based on these initial
data we believed that the key feature imparting activity
observed in (1b) was the presence of an axial hydroxyl
group. To explore this hypothesis further, we prepared
the 4-imidazo-4-hydroxy analogue (1f).14 Unfortu-
nately, this compound was found to possess only weak
activity against the human NK2 receptor (pIC50 6.6),
suggesting that the two piperidine 4-substituents in (1b)
were working synergistically to give the high NK2
activity observed.
As a strategy to follow-up this intriguing SAR, we pre-
pared a library15 of compounds to rapidly explore the
spatial region of the receptor occupied by the 4,4-dis-
ubstituted piperidine in (1). From these studies, several
monosubstituted compounds emerged showing excellent
inhibition profiles in the NK2 binding assay. For exam-
ple, Table 2, the 3-diethylcarboxamido analogue (9) and
the 4-((N)-benzimidazolone)piperidine (2a) were found
to possess high NK2 inhibitory activity in the rat duo-
denum screen (61 and 81% inhibition, respectively, at
100 nM).
Resynthesis and rescreening of (2a) revealed the com-
pound to possess excellent NK2 binding activity (pIC50
8.9) and high functional potency in the RPA assay (pA2
8.4). Following up this exciting new lead, a series of
analogues were prepared, Table 3.
Attempts to further increase the potency of lead (2j) b y
introducing conformational constraint, Figure 2, were
also explored.
Cyclisation of the amide methyl group in (2j) into a
ring,17 (3), Figure 2, gave a series of acylpyrrolidines
(n=0) and piperidines (n=1), Table 4.
Initial analogues of library-derived lead (2a), focused on
retaining the N-linked benzimidazolone and modifying

2214
A. R. MacKenzie et al. / Bioorg. Med. Chem. Lett. 13(2003) 2211–2215
Table 3. Human NK2 binding and RPA functional data for 4-piper-
idinyl-2-(3,4-dichlorophenyl)-butylbenzamides (2a–2j)
Compda
NK2 binding
(CHO, IC50, nM)b
NK2 Functional
(RPA, pA2)
Figure 2. Cyclisation as a strategy to improve potency in lead (2j).
Table 4. Human NK2 binding and RPA functional data for cyclic N-
acylated pyrrolidines and piperidines (3a-3e)
2a
2b
2c
2d
8.9
7.1
7.4
8.4
8.4
NT
NT
8.4
Compda
R
n
NK2 (CHO,
IC50, nM)b
NK2 Functional
(RPA, pA2)
2j
—
—
0
7.7
7.2
8.9
8.1
3a
3b
3c
3d
3e
1
1
1
1
7.5
6.5
6.3
6.7
8.4
7.7
NT
7.2
2e
2f
2g
2h
2i
8.1
7.3
7.7
6.8
6.8
7.7
8.6
7.3
8.3
7.1
7.6
8.9
PhSO2ꢀ
NT=not tested.
aAll compounds are racemic except (3d) (S-enantiomer).
bAll determinations nꢂ2 (each experiment performed in triplicate).
In summary, initial analogues of literature lead (1b)
revealed that the two piperidine 4,4-disubstituents were
working in a synergistic manner, with analogues posses-
sing either substituent alone found to be only weakly
active in the NK2 binding assay. To more fully explore
this intriguing SAR as rapidly as possible, we employed a
library-based strategy, evaluating a range of piperazine,
piperidines and acyclic amines, as replacements for the 4-
hydroxy-4-phenylpiperidine moiety. These efforts identi-
fied the 4((N)-benzimidazolone)piperidine analogue (2a)
as a potent 4-mono-substituted piperidine replacement for
the 4-hydroxy-4-phenylpiperidine group in (1a), which
demonstrated similar functional activity in the RPA
assay. Further optimisation of this library-derived lead,
using classical single-compound synthesis identified the
3-((N)-morpholinoazetidine (2j), which possessed simi-
lar functional potency to both (1b) and (2a). Con-
2j
NT=not tested.
aAll compounds are racemic.
bAll determinations nꢂ2 (each experiment performed in triplicate).
For example, N-benzoylpyrrolidine (3a) was found to
be slightly weaker than the acyclic analogue (2j) in both
binding and functional studies, while the N-benzoylpi-
peridine was found to be equipotent with (2j). The
ortho-methoxy analogue of (3b) was found to be 10-fold
weaker in NK2 binding studies. Similarly, the 2-(cyclo-
propyl)acetyl analogue (3d) (pIC50 6.3) was also found
to be weaker than the N-benzoyl analogue (3b). The
phenylsulphonamide analogue (3e), was also found to
be functionally 10-fold weaker in the RPA assay than
(3b).
formationally
constrained
analogues
of
(2j),
acylpyrrolidine (3a) and acylpiperidine (3b), designed to
further increase NK2 activity, whilst active, generally
offered no potency advantage in this particular series.

A. R. MacKenzie et al. / Bioorg. Med. Chem. Lett. 13(2003) 2211–2215
2215
Further studies to optimise the profiles of NK2 antago-
nists from these and related, stereochemically defined,
series will be the subject of future communications from
these laboratories.
P.; Proietto, V.; Van Broeck, D.; Naline, E.; Neliat, G.; Le
Fur, G.; Breliere, J. C. Life Sci. 1992, 50, PL-101–PL-106.
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A. P.; Middleton, D. S.; Meadows, S. D. US Patent 2001, 6,
242 438.
11. The synthesis and spectral data of 3-((N)-morpholino)-
azetidine has been reported previously. See: MacKenzie, A. R.;
Marchington, A. P.; Meadows, S. D.; Middleton, D. S. US
Patent 1996, 5, 963 923.
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Acknowledgements
The authors wish to thank Dr. Mark Bushfield for
advice and acknowledge the able assistance of Mrs. J.
Banks, Mrs. D. Davey and Miss S. Broadbent for
ligand binding assays. We also wish to thank Mr. M.
Corless, Dr. D. Hardstone, Miss C. Howard and Mr. A.
Warren for compound preparation. We also thank the
staff of the Structural and Separation Sciences Depart-
ment for analytical and spectroscopic data.
13. Patacchini, R.; Maggi, C. A. Eur. J. Pharmacol. 1993, 236,
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References and Notes
14. 4-(2-imidazo)-4-hydroxy-piperidine used in the prepara-
tion of (1f) was prepared in two steps from commercially
available N-benzylimidazole and N-benzylpiperidine-4-one,
via 2-lithiation of the imidazole at ꢀ60 ꢁC in THF, followed
by quenching with N-benzylpiperidine-4-one. Following work
up and chromatography of the tertiary alcohol product,
hydrogenolysis of the two N-benzyl groups was affected using
10% Pd/C under an atmosphere of hydrogen at 30 psi, 50 ꢁC
for 4 h.
15. (a) The library was prepared by the reaction of the mesy-
late formed in Scheme 1 with substituted piperazines, piper-
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using acetonitrile as solvent and heating the reaction mixtures
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