Parallel synthesis and evaluation of N-(1-phenylethyl)-5-phenyl-imidazole-2-amines as Na+/K+ ATPase inhibitors

Article abstract of DOI:10.1016/S0960-894X(00)00296-1

A series of N-(1-phenylethyl)-5-phenyl-imidazole-2-amines was prepared using solution-phase, parallel synthesis and evaluated for Na⁺/K⁺ ATPase inhibition. (C) 2000 Elsevier Science Ltd. All rights reserved.

Full text of DOI:10.1016/S0960-894X(00)00296-1

Bioorganic & Medicinal Chemistry Letters 10 (2000) 1543±1545
Parallel Synthesis and Evaluation of N-(1-Phenylethyl)-5-phenyl-
imidazole-2-amines as Na+/K+ ATPase Inhibitors
Benjamin E. Blass,* C. T. Huang, Richard M. Kawamoto, Min Li, Song Liu,*
David E. Portlock, William M. Rennells and Melanie Simmons
Procter & Gamble Pharmaceuticals, Health Care Research Center, 8700 Mason Montgomery Road, Mason, OH 45040, USA
Received 11 April 2000; accepted 28 April 2000
AbstractÐA series of N-(1-phenylethyl)-5-phenyl-imidazole-2-amines was prepared using solution-phase, parallel synthesis and
evaluated for Na+/K+ ATPase inhibition. # 2000 Elsevier Science Ltd. All rights reserved.
Congestive heart failure (CHF), a disease characterized
by the failure of the heart to pump sucient blood ¯ow
to meet the metabolic needs of the body,1 a€ects 4.6
million Americans with an additional 400,000 new cases
every year. The ®ve-year mortality rate for CHF is nearly
50%,2 and symptoms of the disease include peripheral
edema, dyspnea, tachypnea, tachycardia, and decreased
exercise capacity. Current treatment regimens for the
disease include several di€erent classes of drugs. Diure-
tics have been available since the 1950s and decrease the
¯uid load on the heart. Angiotensin converting enzyme
(ACE) inhibitors were introduced in the 1980s, and have
since been shown to increase life expectancy sub-
stantially. The oldest treatment for CHF, however, is the
cardiac glycoside digoxin (1). Treatment of CHF with
digoxin can be traced back to at least the 18th century, and
has been shown to increase the exercise capacity of CHF
patients.3 This e€ect has been linked to the inhibition
of myocardial Na+/K+ ATPase, which strengthens the
heart's pumping action (positive inotropy). Digoxin
plasma concentrations must be carefully maintained
between 0.5 ng/mL to 2.0 ng/mL, as potentially life threat-
ening e€ects begin to occur when plasma concentrations
exceed 2.0 ng/mL. Treatment with digoxin is further
complicated by high plasma protein binding and lipid
solubility, which can greatly extend the time and dosing
required to reach steady-state plasma concentrations
(up to 2 weeks in some patients).4 In an e€ort to develop
positive inotropes with a more favorable therapeutic
index, we examined a series of aminoimidazoles based
on one of the few known nonsteriodal inotropes, BIIA
(2). This imidazoisoquinoline is inotropic (EC50=1.0 mM,
guinea pig papillary muscle), and we believe that unlike
digoxin, BIIA does not bind to the ouabain binding site.
Instead, BIIA binds at a Na+ site (the guanidine of BIIA
acts as a sodium mimetic), blocking ion transport.5 In an
e€ort to develop more potent nonsteroidal inotropes,
we prepared and examined a series of BIIA analogues
using solution phase parallel synthesis methods.
Our initial e€orts to produce more ecacious analogues
of BIIA focused on the preparation of des-B-ring com-
pounds (e.g,. 3). We believed that removal of the B-ring
would permit greater conformational mobility, possibly
allowing these analogues to adopt unique conformations
that would enhance their interaction with the binding site
of Na+/K+ ATPase, thereby increasing their potency. A
retrosynthetic analysis suggested imidazole 4 as a central
building block that could be elaborated with a combi-
nation of organometallic cross coupling chemistry and
nucleophilic substitution reactions to establish the A-ring
and the guanidine/d-ring, respectively. Imidazole 4 could
in turn be prepared from an N-alkylated imidazole (5)
by taking advantage of the di€erence in acidity between
the 2- and 5-positions (Scheme 1).6
*Corresponding authors. Tel.: +1-513-622-3797; fax: +1-513-622-
2675; e-mail: blassbe@pg.com; Tel.: +1-513-622-0975; fax: +1-513-
622-2675; e-mail: lius1@pg.com
Installation of the desired functional groups was readily
accomplished by selectively deprotonating N-methyl
0960-894X/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(00)00296-1

1544
B. E. Blass et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1543±1545
Scheme 1.
imidazole at the 2-position with n-BuLi and adding
diphenyl disul®de. Further deprotonation of the 5-position
with n-BuLi, followed by addition of iodine and oxida-
tion with mCPBA in methylene chloride produced 6.7
The desired des-B-ring analogues of BIIA could then
be approached by Suzuki coupling of 6 with phenyl
boronic acid in aqueous dimethoxyethane in the pre-
sence of Pd(PPh3)4 and NaOH to produce 7. Treatment
of 7 with the lithium amide of a-methyl benzyl amine
produced the desired product (8) in excellent yield.
Table 1. Representative examples of des-B-ring analogues of BIIA
(11) and isolated enzyme activity
Entry
Ar
R1
R2
Yield
IC50 (mM)c
a
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
CH3
(R)-CH3
(S)-CH3
CH3
H
H
H
4-Cl
4-F
H
4-Cl
H
H
H
H
H
38
10
61
1.3
10
77
0.8
2.1
14
33
52
88
99
63%
72%
62%
70%
67%
82%
29%b
57%b
41%b
29%b
40%b
45%b
CH3
sec-Butyl
CH2CH3
CH3
CH3
CH3
CH3
CH3
CH3
2-Napthyl
3-Biphenyl
4-CF3O-Ph
3,5-di-F-Ph
3,5-di-CF3-Ph
3,4-di-CH3O-Ph
9
10
11
12
13
H
aSee Scheme 2 for yields.
bYield over two steps.
cCompounds were evaluated once over a 2 log range. Digoxin
IC50=30 nM.
Biological screening of 8 in an isolated enzyme assay
revealed that the desired activity had been maintained
(ATPase activity was measured by monitoring the release
of inorganic phosphate by colorimetric determination;
myocardial Na+/K+ ATPase was isolated from canine
cardiac sarcolema).8,9 Therefore, we turned our attention
to the preparation of directed libraries of analogous
compounds. Libraries exploring the rami®cations of
modifying the d-ring were produced by condensing 7 with
a series of amines as their lithium amides. The desired
products were isolated by column chromatography and
tested in the isolated enzyme assay. Some representative
examples are shown in Table 1, entries 2±7. Modi®cation
of the A-ring was also accomplished by preparation of
solution-phase libraries, but by a slightly di€erent
method. The number of commercially available aryl
boronic acids is relatively small, and we felt that we
could produce a much larger library of compounds by
inverting the positions of the metal and halide by
employing Stille rather than Suzuki chemistry. Starting
with N-methyl imidazole, selective deprotonation in the
2-position, followed by addition of diphenyl disul®de
and mCPBA oxidation in methylene chloride provided
the necessary electrophile in the 2-position. Further
deprotonation with n-BuLi and addition of tributyltin
chloride produced 9 in good yield (Scheme 3). Instal-
lation of the tin functionality on the imidazole ring
allowed us to explore a wide range of commercially
available aryl bromides as potential coupling partners.
The des-B-ring analogues of BIIA were then prepared
by a two-step, one-pot procedure. Stille coupling of 9
with an aryl bromide was performed in re¯uxing 1,4
dioxane in the presence of BHT and Pd(PPh3)4. When
Scheme 2. Reagents: (a) (i) n-BuLi, THF, 78 ꢀC; (ii) Ph2S2, THF, 73%; (b) (i) n-BuLi, THF 78 ꢀC; (ii) I2, THF, 96%; (c) mCPBA, CH2Cl2, 83%;
(d) PhB(OH)2, Pd(PPh3)4, NaOH, DME, H2O, re¯ux, 73%; (e) PhSnMe3, Pd(PPh3)2Cl2, DMF, 80 ꢀC, 62%; (f) a-methyl benzyl amine, n-BuLi,
0 ꢀC, 76%.
Scheme 3. Reagents: (a) (i) n-BuLi, THF, 78 ꢀC; (ii) Ph2S2, THF, 73%; (b) mCPBA, CH2Cl2, 97%; (c) (i) n-BuLi, THF 78 ꢀC; (ii) SnBu3Cl, THF,
89%; (d) ArBr, Pd(PPh3)4, 1,4-dioxane, re¯ux, 24 h; (e) RNH2, n-BuLi, 0 ꢀC, 76%.

B. E. Blass et al. / Bioorg. Med. Chem. Lett. 10 (2000) 1543±1545
1545
the reaction was complete, a solution of the lithium salt of
the desired benzyl amine in THF was added. The desired
products were isolated by column chromatography and
tested in the isolated enzyme assay. Representative
examples from this library are also shown in Table 1
(entries 8±13).9
Rackley, C. E., Sonnenblick, E. H., Wenger, N. K., Eds.;
McGraw-Hill: New York, 1990.
4. Martindale: The Complete Drug Reference; Par®tt, K., Ed.;
Pharmaceuticals: Taunton, 1999; 849.
5. Borchard, U.; Boesken, R.; Gree€, K. Arch. Int. Pharma-
codyn. Ther. 1982, 256, 253. Fox, A. A. L.; Gree€, K. Bio-
chem. Pharmacol. 1981, 30, 611. Peer, D.: Mayan, H.; Cohen,
H.; Tal, D.M.; Karlish, S. J. D. J. Biol. Chem. 1992, 267, 1141.
6. Iddon, B. Heterocycles 1985, 23, 417.
7. For related chemistry see Bierer, D. E.; O'Connell, J. F.;
Parquette, J. R.; Thompson, C. M.; Rapoport, H. J. Org.
Chem. 1992, 57, 1390.
The results shown in Table 1 lead to several conclusions
about the nature of the binding of BIIA to myocardial
Na+/K+ ATPase. First, the B-ring is clearly not a
required aspect of the pharmacophore, as its removal
does not appear to dramatically decrease the range of
observed activity. Second, binding is sensitive to changes
in the chiral center of the benzyl amine/D-ring portion
of the compound, as indicated by the di€erence in activ-
ity observed in entries 1±3 of Table 1. Third, the A-ring is
probably associated with a hydrophobic pocket of the
binding site of Na+/K+ ATPase. This is suggested by
the nearly 50-fold increase in activity between entries 8
and 13, as well as the overall trend of entries 8±13.
8. MacGregor, S. E.; Walker, J. M. Appl. Biochem. Biotech.
1994, 49, 135. Jones, L. R.; Maddock, S. W.; Besch, H. R. Jr.
J. Biol. Chem. 1980, 255, 9971.
9. Typical library preparation: Bromobenzene (76 mg, 50.6
uL, 0.48 mmol), imidazole 9 (225 mg, 0.44 mmol), and 0.5 mg
of BHT were dissolved in 1.5 mL of 1.4 dioxane, and 9 mg of
Pd(PPh3)4 was added. The reaction was heated to re¯ux for 24
h and is then cooled to 12 ꢀC. Separately, 0.55 mL of n-BuLi
(2.4 M in hexane) was added to a 12 ꢀC solution of a-methyl
benzylamine (180 mg, 0.17 mL, 1.49 mmol) in 2.0 mL of 1.4
dioxane. After 30 min, the lithium amide solution was added
to the imidazole solution in a dropwise fashion, and the reac-
tion was allowed to warm to room temperature. After 15 h,
the reaction was quenched with 10 mL of NaHCO3, and the
solution was extracted with EtOAc (3Â10 mL). The combined
EtOAc was washed with aqueous KF (10 mL), dried over
MgSO4, ®ltered and stripped of solvents to yield a yellow
solid. Chromatography with 1:1 hexane:EtOAc provided 92.6
In summary, we have developed a series of des-B-ring
analogues of BIIA (2), which are potent inhibitors of
myocardial Na+/K+ ATPase using solution-phase,
parallel-synthesis methods.
1
References and Notes
mg (76 %) of the desired product as a pale-yellow solid. H
NMR (300 MHz, CDCl3) d 7.4 (10H, m), 6.81 (1H, s), 5.08
(1H, m, J=9.6 Hz), 3.87 (1H, bs), 3.38 (3H, t), 1.64 (3H, t,
J=9.6 Hz), 13C NMR (75.4 MHz, CDCl3) d 150.3, 144.9,
1. The Heart Arteries and Veins; Hurst, J. W., Schlant, R. C.,
Rackley, C. E., Sonnenblick, E. H., Wenger, N. K., Eds.;
McGraw Hill: New York, 1990.
2. American Heart Association statistics from 1996.
3. The Heart Arteries and Viens; Hurst, J. W., Schlant, R. C.,
130.9, 129.9, 128.9, 128.8, 127.9, 127.4, 127.1, 126.5, 123.4,
1
53.6, 30.4, 23.4. IR 3010, 2980, 1550, 1225, 1175, 925 cm
Mass spectrum m/z (%): 278 (M+1, 100%).
.