Equipotent activity in both enantiomers of a series of ketopiperazine-based renin inhibitors

Article abstract of DOI:10.1016/j.bmcl.2005.02.085

We have found that both enantiomeric configurations of the 6-alkoxymethyl-1-aryl-2-piperazinone scaffold display equipotent renin inhibition activity and similar SAR patterns. This enantiomeric flexibility is in contrast to a previously reported 3-alkoxym

Full text of DOI:10.1016/j.bmcl.2005.02.085

Bioorganic & Medicinal Chemistry Letters 15 (2005) 2371–2374
Equipotent activity in both enantiomers of a series of
ketopiperazine-based renin inhibitors
Noel A. Powell,* Emma H. Clay, Daniel D. Holsworth, John W. Bryant, Michael J. Ryan,
Mehran Jalaie, Erli Zhang and Jeremy J. Edmunds
Pfizer Global Research and Development, Michigan Laboratories, Ann Arbor, MI 48105, USA
Received18 November 2004; revised25 February 2005; accepted28 February 2005
Available online 30 March 2005
Abstract—We have foundthat both enantiomeric configurations of the 6-alkoxymethyl-1-aryl-2-piperazinone scaffolddisplay equi-
potent renin inhibition activity andsimilar SAR patterns. This enantiomeric flexibility is in contrast to a previously reported3-alk-
oxymethyl-4-arylpiperidine scaffold.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
Several pharmaceutical companies have attemptedto
advance renin inhibitors to the clinic. Although potent
in vitro renin inhibitory activity was obtained, most
programs were based on peptidic or peptidomimetic
scaffolds. Poor pharmacokinetic properties, including
low oral bioavailability andhigh clearance, anda high
cost of goods for large scale synthesis greatly diminished
the clinical utility of these agents.4
Hypertension is a leading risk factor for cardiovascular
disease, such as congestive heart failure, stroke, myocar-
dial infarction, and is the leading cause of death in the
Western world.1 The renin angiotensin system (RAS)
is well establishedas an endrocrine system involvedin
bloodpressure regulation andfluidelectrolyte balance
(Fig. 1).2 Activation of the RAS is stimulatedby several
signals, including a drop in blood pressure, a decrease in
the circulating volume, or a reduction in plasma sodium
concentration. These signals stimulate the release of
renin, which cleaves angiotensinogen to angiotensin I
(AngI). Angiotensin converting enzyme (ACE) then
converts AngI into the vasopressor peptide angiotensin
II (AngII). The binding of AngII to the AT1 receptor
initiates a number of physiological effects, such as
sodium and water retention and vasoconstriction, lead-
ing to an increase in bloodpressure. Since renin is the
rate-limiting step in the RAS cascade, renin inhibition is
considered to be an attractive antihypertensive strategy.
Renin inhibitors have been predicted to be more effica-
cious with fewer side effects than ACE inhibitors and
AT1 receptor antagonists, which target downstream
events.3
In 1999, an initial series of non-peptidic renin inhibi-
tors were disclosed. These inhibitors were based on a
Angiotensinogen
Renin
Angiotensin I
Bradykinin
Chymase
ACE
Inactives
Angiotensin II
AT1 Receptor
Blood Volume
Peripheral Resistance
Vasoconstriction
Central pressor activation
Sympathetic tone
Aldosterone release
Sodium re-absorption
Thirst
Keywords: Renin inhibitor; Ketopiperazine.
*
Corresponding author. Tel.: +1 734 622 2151; fax: +1 734 622
3909; e-mail: noel.powell@pfizer.com
Figure 1. The renin angiotensin system (RAS).
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.02.085

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N. A. Powell et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2371–2374
H
N
H
N
(IC50 = 66 nM) as the R-enantiomer 4. This result con-
trastedwith the disparate inhibitory activities observed
between the enantiomers of the trans-3-alkoxymethyl-
4-aryl piperidine scaffold (i.e., 1 and 2).5a
O
O
OMe
OMe
To further examine the SAR of the S-enantiomer series,
several C ring analogs with the S-enantiomer configura-
tion were preparedandcomparedwith the correspond-
ing analogs of R-enantiomer configuration.7
Cl
Cl
(R,R)-1
IC50 = 26 µM
(S,S)-2
IC50 = 1200 µM
2. Chemical synthesis
H
H
N
N
A
O
N
The S-enantiomer 5 was synthesizedas previously de-
scribed.6 Quinoline andtetrahydroquinoline analogs 8,
10, 12, and 14 were preparedby the route shown in
Scheme 1. Mitsunobu coupling of alcohol 6 and7-
hydroxyquinoline, followed by deprotection of the
tert-Boc carbamate yielded analog 8. Reduction of the
quinoline ring and tert-Boc deprotection provided ana-
log 10. Alkylation of tetrahydroquinoline 9 with 2-
bromoethanol and3-bromo-1-propanol and tert-Boc
deprotection gave analogs 12 and 14, respectively.
O
O
C
B
D
O
O
O
O
OMe
OMe
3
4
IC50 = 2 nM
IC50 = 54 nM
Figure 2. Stereochemical dependence of the trans-3-alkoxymethyl-4-
arylpiperidine scaffold.
7,8
Analog 18, containing an optimizedC ring,
paredvia a similar Mitsunobu coupling of alcohol
was pre-
6
trans-3-alkoxymethyl-4-arylpiperidine scaffold 1 that
was discovered by a high throughput screen (Fig. 2).5
The absolute configuration of the piperidine ring was
shown to be quite important for renin activity, as the
(3R,4R)-enantiomer of 1 was >40· more potent than
the (3S,4S)-enantiomer 2.5a Further optimization of
the trans-3-alkoxymethyl-4-arylpiperidine scaffold re-
sultedin 3 (IC50 = 2 nM).
andphenol 16 (Scheme 2). Phenol 16 was preparedfrom
Boc
N
R
N
HO
N
OH
O
N
O
N
O
N
PS-PPh3, DIAD
DCM, 0 °C
Our previous work hadcenteredaroundthe discovery of
novel chemical matter relatedto the trans-3-alkoxy-
methyl-4-aryl piperidine scaffold by replacement of the
4-aryl piperidine chiral center in 3 with a nitrogen atom
to reduce the synthetic complexity. These efforts resulted
in the discovery of the 6-alkoxymethyl-1-aryl-2-ketopip-
erazine scaffold, exemplified by 4, as novel leadchemical
matter (Fig. 2).6
60 %
O
O
O
O
OMe
OMe
6
7 R = Boc
8 R = H
AcCl, MeOH
58%
R
N
H
N
O
n
NaBH4
NiCl2-6H2O
We focusedour early SAR efforts on the R-enantiomer
of 4.6 Since the aryl piperidine scaffold exhibited such a
dramatic stereospecificity for renin potency, we were
interestedin determining if the 6-alkoxymethyl-1-aryl-
2-ketopiperazine scaffolddemonstrateda similar stereo-
specific renin inhibition pattern. Accordingly, we pre-
paredthe corresponding S-enantiomer 5. Surprisingly,
5 exhibitedalmost iedntical renin inhibition activity
O
N
Br
HO
KI, NaHCO3
CH3CN, reflux
71-94%
MeOH, 0 °C
69%
O
O
OMe
9
R = Boc
AcCl, MeOH
53%
10 R = H
R
N
OH
n
H
N
O
N
AcCl
MeOH
54%
11 n = 1, R = Boc
12 n = 1, R = H
O
O
N
O
N
AcCl
MeOH
60%
13 n = 2, R = Boc
14 n = 2, R = H
O
O
O
O
OMe
OMe
5
IC50 = 66 nM
Scheme 1.

N. A. Powell et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2371–2374
2373
renin inhibition activity, unlike the previously reported
trans-3-alkoxymethyl-4-aryl piperidine scaffold.10 When
complexedwith inhibitors of the ketopiperazine scaf-
fold, the renin enzyme exists in the flap open conforma-
tion. The ketopiperazine NH of 10 and 25 formeda salt
bridge between the Asp32 and Asp215 residues, while
the 3-(2-methoxybenzyloxy)-propyl group occupiedthe
large hydrophobic pocket formed by a conformation
change of the flap region of the protein. The bicyclic
portion of both analogs binds in the large S1/S3 pocket.
The 3-hydroxypropyl chain of 25 extends into the S3
subpocket. Although the inversion of the chiral center
results in a different vector orientation for the C6
hydroxymethyl linker in analog 10, this linker was able
to position the tetrahydroquinoline ring in the S1/S3
pocket, closely overlapping the corresponding ring in
analog 25. The S1/S3 pocket is sufficiently large enough
to allow the change in ligandconformation without a
high penalty in binding energy.
O
1) BnBr, K2CO3, CH3CN,
reflux, 79%
2)
Br
OMe
H
N
HO
O
HO
N
O
NaH, DMF, 0 °C, 82%
3) H2, Pd/C, MeOH, 100%
15
16
O
R
N
6, DIAD
O
N
O
PS-PPh3
O
N
CH2Cl2
0 °C
38%
O
O
OMe
17 R = Boc
18 R = H
AcCl, MeOH
65%
Scheme 2.
In summary, we have demonstrated that both enantio-
meric configurations of the chiral 1-aryl-6-(hydroxy-
methyl)-2-ketopiperazine scaffoldwith bicyclic C rings
7-hydroxy-3,4-dihydro-1H-quinolin-2-one 15 in a three-
step sequence. Protection of the phenol as a benzyl
ether, alkylation of the amide with 3-bromo-1-methoxy-
propane, andhydrogenolysis of the benzyl ether all pro-
ceeded in good yield.
Table 1. SAR comparison of analogs with (S)- and (R)-configurations
H
N
H
N
3. Results and discussion
O
O
O
N
R
O
O
N
R
O
The SAR trends observed with bicyclic C rings in the
R-configuration series were generally paralleledby the
S-configuration series. (Table 1). Replacement of the
2-naphthalene with 7-quinoline resultedin a >10-fold
loss of activity in both enantiomers (8, IC50 = 860 nM
and 22, IC50 = 820 nM). Interestingly, reduction of the
quino- line ring to the tetrahydroquinoline provided a
3-foldincrease in activity of the S-enantiomer 10
(IC50 = 23 nM), while the R-enantiomer 23 showeda
>2-folddecrease in activity (IC 50 = 120 nM). In the S-
enantiomeric configuration series, introduction of a
hydrogen bond donating hydroxyl group linked by a
2- or 3-carbon chain to the tetrahydroquinoline nitrogen
O
O
(S)
(R)
OMe
OMe
R
(S)-analogs IC50
(nM)a
(R)-analogs IC50
(nM)a
5
8
66
4
54
N
860
23
22
820
120
atom ledto analogs
(IC50 = 35 nM) with activity comparable to the unsub-
stitutedtetrahyrdoquinoline 10 (IC50 = 23 nM), and
12 (IC50 = 33 nM) and 14
H
N
10
12
23
24
approximately 2-foldmore potent than the correspond-
ing naphthalene analog 5 (IC50 = 66 nM). This pattern
roughly paralleledthe activity trendobservedfor the
R-configuration series (analogs 24 and 25). Introduction
of a hydrogen bond accepting 3-methoxypropyl chain
ledto an increase in renin potency ( 18, IC50 = 17 nM).8
This result mirroredthe SAR of R-configuration ana-
logs with hydrogen bond acceptors in the S3 subpocket
side chain (26, IC50 = 7.0 nM). It is noteworthy that the
alkylatedtetrahydroquinoline analogs 10, 12, 14, and 18
retainedrenin inhibition activity while decreasing clogP
relative to the naphthalene analog 5.
OH
33
35
17
64
37
7
N
OH
14
18
25
26
N
N
O
O
X-ray crystal structures of analogs 10 (S-configuration)
and 25 (R-configuration) complexedwith renin were ob-
tained( Fig. 3) to help explain how both of the enantio-
mers of the ketopiperazine scaffoldexhibitedsimilar
9
a IC50 values obtainedin duplicate using a fluorescent tGFP assay.

2374
N. A. Powell et al. / Bioorg. Med. Chem. Lett. 15 (2005) 2371–2374
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Figure 3. Overlap of the X-ray co-crystallization structures of analogs
10 (white atom coloring) and 25 (yellow atom coloring) complexed
with renin.10 Residues in the flap region have been removed for clarity.
The blue regions of the Connelly protein surface represent hydrophilic
regions andthe brown regions represent hydrophobic regions.
display equipotent renin inhibition activity. This is in
contrast to the enantiomeric specificity exhibitedby a
previously reported trans-3-alkoxymethyl-4-aryl piperi-
dine scaffold, and is presumably due to the conforma-
tional flexibility of the C6 hydroxymethyl linker
between the ketopiperazine andthe C ring. The discov-
ery that both enantiomers of the 1-aryl-6-(hydroxy-
methyl)-2-ketopiperazine scaffoldpossess equipotent
renin inhibition activity is also significant because of
the possibility that the separate enantiomers may dis-
play disparate pharmacokinetic, pharmacodynamic, or
toxicology properties.
6. Holsworth, D. D.; Powell, N. A.; Downing, D. M.; Cai,
C.; Cody, W. L.; Ryan, M.; Ostroski, R.; Jalaie, M.;
Bryant, J. W.; Edmunds, J. J. Bioorg. Med. Chem. 2005,
13, 2657.
7. Powell, N. A.; Holsworth, D. D.; Cody, W. L.; Cheng,
X.-M.; Lee, C.; Erasga, N.; Downing, D. M.; Cai, C.;
Clay, E. H.; Jalaie, M.; Bryant, J. W.; Ryan, M.;
Edmunds, J. J.; Li, T.; Kasani, A.; Hall, E.; Subedi, R.;
Rahim, M.; Maiti, S. 228th National ACS Meeting,
Philadelphia, PA, MEDI271.
8. The C ring carbonyl in analogs 18 and 26 was introduced
to reduce the electron density of the tetrahydroquinoline
ring. See Ref. 7.
References and notes
9. Cody, W. L.; Holsworth, D. D.; Powell, N. A.; Jalaie, M.;
Zhang, E.; Wang, W.; Samas, B.; Bryant, J.; Ostroski, R.;
Ryan, M. J.; Edmunds, J. J. Bioorg. Med. Chem. 2005, 13,
59.
10. Coordinates for the renin/10 (2bkt) andrenin/ 25 (2bks)
complexes have been deposited at www.rcsb.org.
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