Piperazinyl benzamidines: Synthesis and affinity for the delta opioid receptor

Article abstract of DOI:10.1016/S0960-894X(01)00272-4

Piperazinyl benzamidines were prepared and found to bind to the rat delta (δ) opioid receptor. The most active compounds had a N,N-diethylcarboxamido group and a N-benzyl piperazine. The most potent among these was N,N-diethyl-4-[4-(phenylmethyl)-1-piperazinyl][2- (trifluoromethyl)phenyl]iminomethyl]benzamide (27) with a 1.22 nM K_i for the rat δ opioid receptor and ca. 1000× selectivity relative to the μ opioid subtype.

Full text of DOI:10.1016/S0960-894X(01)00272-4

Bioorganic& Medicinal Chemistry Letters 11 (2001) 1741–1743
Piperazinyl Benzamidines: Synthesis and Affinity
for the Delta Opioid Receptor
Samuel O. Nortey,* Ellen W. Baxter, Ellen E. Codd, Sui-Po Zhang and Allen B. Reitz
Drug Discovery Division, R. W. Johnson Pharmaceutical Research Institute, Welsh and McKean Roads, Spring House, PA 19477, USA
Received 13 March 2001; accepted 20 April 2001
Abstract—Piperazinyl benzamidines were prepared and found to bind to the rat delta (d) opioid receptor. The most active com-
pounds had a N,N-diethylcarboxamido group and a N-benzyl piperazine. The most potent among these was N,N-diethyl-4-[4-
(phenylmethyl)-1-piperazinyl][2-(trifluoromethyl)phenyl]iminomethyl]benzamide (27) with a 1.22 nM Ki for the rat d opioid recep-
tor and ca. 1000Â selectivity relative to the m opioid subtype. # 2001 Elsevier Science Ltd. All rights reserved.
There has been intensive research in the past 20 years
leading to the identification of three opioid receptor
subtypes, referred to as mu (m), delta (d), and kappa (k).
Compounds that are d opioid receptor agonists are
analgesicagents in animal models, and seletcive non-
peptide d opioid agonists have been reported in the lit-
erature.1À6 Selective d agonists are less likely to have the
liabilities associated with marketed opiate analgesics,
which bind at the m opioid site, such as constipation and
respiratory depression. Most small-molecule d-opioid
agonists are piperidines or piperazines, such as SNC 80
(1) and the racemic hydroxy analogue BW373U86 (2).7,8
4-Aminopiperidines of structure 3 have also been shown
to have activity at the d opioid receptor site (Fig. 1).9À11
The esters of 8 were then converted to dialkylamides 9
via the acid chloride.
Delta opioid receptor affinity was calculated from the
3
inhibition of H-DPDPE binding to d opioid receptors
from rat brain membranes. In a similar manner, m
One of the limitations associated with piperazines 1 and
2 is their complicated stereochemistry. In order to
obtain achiral d opioid agonists, we synthesized amidino
piperazines 4 (Table 1). Additionally, data from high-
throughput screening of our corporate compound col-
lection at the human d opioid receptor suggested that
amidine functionality incorporated as in 4 would be
tolerated with retention of biological activity.
Figure 1. Nonpeptide opioid agonists.
The syntheticroute to compounds of type 4 is shown in
Scheme 1.12 Substituted piperazines 5 reacted with ter-
ephthalic acid monomethyl ester chloride 6 under
Schotten–Baumann conditions to give benzamides 7.
These intermediates were then condensed with anilines
utilizing phosphorus oxychloride to provide amidines 8.
*Corresponding author. Fax: +1-215-628-4985; e-mail: snortey@
prius.jnj.com
Scheme 1. (a) 1 N NaOH, CH2Cl2; (b) POCl3, H2O; (c) ArNH2; (d)
3 N NaOH, 80 ꢀC; (e) 3 N HCl, SOCl2; (f) R2R3NH.
0960-894X/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(01)00272-4

1742
S. O. Nortey et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1741–1743
opioid receptor affinity was determined by competition
3
with compounds 23 (d, 20.5 nM Ki) and 24 (d, 157.4 nM
Ki). 3,5-Dichlorophenyl 15, fluoro compounds 16 and
17, and 3-bromophenyl 18 were active, but incorpora-
tion of the electron-releasing methoxy group into the 2-
position as in 19 decreased d affinity (1023 nM Ki), being
less active than directly-comparable 2-chloro analogue
12 (292 nM Ki).
experiments using H-DAMGO in the same rat brain
membrane preparations.13 The binding affinities of
morphine and SNC 80 are also included in Table 1.
The piperazinyl benzamidines bind to the d opioid
receptor, with lesser affinity for the m opioid subtype.
Variations on the core structure investigated included
N-substitution on the piperazine, replacement of the
carboxamide with an ester, and substitution on the N-
phenyl ring (X). Prototype structure 10, where R=Et
and bearing a 4-(diethyl)carboxamido group revealed a
16 nM Ki for the d opioid receptor, prompted us to
continue to investigate this series. Further, the ratio of d
to m opioid receptor affinity for 10 was 62.5, which we
reasoned could be improved by subsequent structural
changes. Two- and three-substitution on the N-phenyl
ring were examined to the greatest extent. 2-Chloro
compound 11 showed a relative decrease in activity at d
(91.1 nM Ki), but the d/m selectivity increased. Dipropyl
amide 12 was less active than 11. Surprisingly, methyl
ester 13 showed comparable activity to 12. When X
involved 3-substitution, some of the compounds had a
higher affinity for the d opioid receptor than for 2- or 4-
substituted derivatives. For example, compound 14 (d,
63 nM Ki) was more active than 11 (d, 91.0 nM Ki).
Additionally, 17 (d, 58.5 nM Ki) was more active than
16 (d, 118 nM Ki). However, the reverse was observed
When the N-ethyl group of 10–19 was replaced with
propyl (20 and 21) or allyl (22–24), less affinity for the d
opioid receptor was observed, except for allyl derivative
23 (d, 20.5 nM Ki). The most active members of this
series were those with N-benzyl substitution (25–28).
For example, the most potent compounds were where X
was 2-trifluoromethyl (27, 1.22 nM Ki) and 2-chloro (26,
11.8 nM Ki). 4-Chloro compound 28 was less active, as
was unsubstituted 25. Amidine 27 had a higher affinity
for the d-opioid receptor than SNC 80, and has the
advantages of being achiral and easier to prepare.
Importantly, 26 and 27 have ca. 1000Â selective affinity
for the d relative to the m opioid receptor. Compounds
26 and 27 were full d opioid agonists as determined in a
GTP-g-S functional test. Extending N-substitution on
the piperazine with phenethyl (29) or thienylethyl (30
and 31) groups lessened activity. For example, thienyl-
ethyl 31 (d, 95.3 nM Ki) had less d opioid receptor affi-
nity than N-benzyl derivative 26 (d, 11.8 nM Ki). N-(2-
Methoxyethyl) substitution was tolerated, with 32 and
Table 1. Binding affinity of piperazinyl benzamidines to d and m opioid receptors
#
R
X
Y
d Ki (nM)
mKi (nM)
d/m ratio
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Pr
Pr
Allyl
Allyl
Allyl
H
2-Cl
2-Cl
2-Cl
3-Cl
3,5-Cl2
2-F
NEt2
NEt2
NPr2
OMe
NEt2
NEt2
NEt2
NEt2
NEt2
NPr2
OMe
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
NEt2
—
16
91.1
292
222
63
1000
>1000
3930
>1000
>1000
>1000
>1000
>10,000
>10,000
3910
62.5
>111
13
>45
>159
>36
>85
>171
>174
3.8
24
54
1.2
370
>64
64
277
118
58.5
57.6
1023
87.3
177
859
20.5
157
212
11.8
1.22
142
545
764
95.3
22.9
53.5
90
3-F
3-Br
2-MeO
2-Cl
3-Cl
H
2-Cl
3-Cl
H
2-Cl
2-CF3
4-Cl
2-CF3
H
2-Cl
2-CF3
2-Cl
—
2070
9530
1000
7600
>10,000
>10,000
>10,000
1200
>10,000
1000
>10,000
>10,000
2960
>10,000
1.8
1300
PhCH2
PhCH2
PhCH2
PhCH2
Ph(CH2)2
2-Thienyl-(CH2)2
2-Thienyl-(CH2)2
MeO-(CH2)2
MeO-(CH2)2
>847
984
70
1.8
>13
>105
117
>437
0.02
765
Morphine
SNC 80 (1)
—
—
1.7

S. O. Nortey et al. / Bioorg. Med. Chem. Lett. 11 (2001) 1741–1743
1743
ring and N-benzyl substitution (d 11.8 and 1.22 nM Ki’s,
respectively). These two compounds also displayed very
little m opioid binding, and are among those selected for
extensive in vivo evaluation. Structural modifications
involving the piperazinyl benzamidines have provided
potent and selective ligands for the d opioid receptor,
and further structural changes in the future may result
in additional derivatives that provide further insight in
this area.
Figure 2. Additional piperazinyl benzamidines.
Acknowledgements
Table 2. Binding affinity of homopiperazine benzamidines to d and m
opioid receptors
The authors thank John Carson for advice and encour-
agement, and Scott Dax for a careful review of the
manuscript.
References and Notes
1. Calderon, S. N.; Rice, K. C.; Rothman, R. B.; Porreca, F.;
Flippen-Anderson, J. L.; Kayakiri, H.; Xu, H.; Becketts, K.;
Smith, L. E.; Bilsky, E. J.; Davis, P.; Horvath, R. J. Med.
Chem. 1997, 40, 695.
2. Williams, M.; Kowaluk, E. A.; Arneric, S. P. J. Med.
Chem. 1999, 42, 1481.
3. Boyd, R. E.; Carson, J. R.; Codd, E. E.; Gauthier, A. D.;
Neilson, L. A.; Zhang, S.-P. Bioorg. Med. Chem. Lett. 2000,
10, 1109.
#
X
Y
d Ki (nM)
m Ki (nM)
d/m ratio
36
37
38
39
H
Cl
H
NEt2
NEt2
OMe
OMe
543
29.6
820
101
396
618
232
197
0.73
21
0.28
2.0
Cl
4. Thomas, J. B.; Atkinson, R. N.; Rothman, R. B.; Burgess,
J. P.; Mascarella, S. W.; Dersch, C. M.; Xu, H.; Carroll, F. I.
Bioorg. Med. Chem. Lett. 2000, 10, 1281.
5. Plobeck, N.; Delorme, D.; Wei, Z.-Y.; Yang, H.; Zhou, F.;
Schwarz, P.; Gawell, L.; Gagnon, H.; Pelcman, B.; Schmidt,
R.; Yue, S. Y.; Walpole, C.; Brown, W.; Zhou, E.; Labarre,
M.; Payza, K.; St-Onge, S.; Kamassah, A.; Morin, P.-E.; Pro-
jean, D.; Ducharme, J.; Roberts, E. J. Med. Chem. 2000, 43,
3878.
6. Wei, Z.-Y.; Brown, W.; Takasaki, B.; Plobeck, N.;
Delorme, D.; Zhou, F.; Yang, H.; Jones, P.; Gawell, L.; Gag-
non, H.; Schmidt, R.; Yue, S.-Y.; Walpole, C.; Payza, K.; St-
Onge, S.; Labarre, M.; Godbout, C.; Jakob, A.; Butterworth,
J.; Kamassah, A.; Morin, P.-E.; Projean, D.; Ducharme, J.;
Roberts, E. J. Med. Chem. 2000, 43, 3895.
7. Calderon, S. N.; Rothman, R. B.; Porreca, F.; Flippen-
Anderson, J. L.; McNutt, R. W.; Xu, H.; Smith, L. E.; Bilsky,
E. J.; Davis, P.; Rice, K. C. J. Med. Chem. 1994, 37, 2125.
8. Negus, S. S.; Picker, M. J. CNS Drug Reviews 1996, 2, 52.
9. Pelcman, B.; Roberts, E. World Patent Application WO 98/
28270, 1998; Chem. Abstr. 1998, 129, 108996.
10. Carson, J. R.; Carmosin, R. J.; Fitzpatrick, L. J.; Reitz, A.
B.; Jetter, M. C. World Patent Application WO 99/33806,
1999; Chem. Abstr. 1999, 131, 87826.
33 being somewhat less active than their N-benzyl com-
parators 26 and 27, respectively. Moving the carbox-
amido substituent to the meta position resulted in a
significant loss of activity. Compounds 34 and 35 were
much less active than their para-substituted counter-
parts (d, 7030 and 487 nM Ki’s for 34 and 35, respec-
tively, vs 859 and 20.5 for 22 and 23) (Fig. 2).
Homopiperazine analogues were prepared by variation
of the chemistry shown in Scheme 1 and 36–39 are listed
in Table 2. 2-Chloro derivative 37 (d, 29.6 nM Ki) was
quite active, with a 21Â selectivity relative to the m sub-
type. The activity with the homopiperazines led us to
further vary this central ring and prepare dimethyl
piperazine analogues 40 and 41, which are direct analo-
gues of SNC 80. These compounds showed only modest
activity (d, 170 and 405 nM Ki’s, respectively), suggest-
ing that the most active piperazinyl benzamides (e.g.,
27) may bind to the d opioid receptor in a different or
additional orientation as does SNC 80.
11. Podlogar, B. L.; Poda, G. I.; Demeter, D. A.; Zhang, S.-
P.; Carson, J. R.; Neilson, L. A.; Reitz, A. B.; Ferguson, D. M.
Drug Des. Discov. 2000, 17, 34.
12. (a) For related synthesis methods, see: Schneider, J. US
Patent 3,962248, June 8, 1976; Chem. Abstr. 1974, 80, 14969.
(b) Shroff, J. R, Bandurco, A. US Patent 3,793,322, Feb. 19,
1974; Chem. Abstr. 1974, 80, 96018.
We have synthesized a new class of piperazinyl benza-
mides 4 that bind to the d opioid receptor. Both N,N-
diethylcarboxamides, N,N-dipropylcarboxamides, and
methyl esters have at least some affinity for the d opioid
receptor. The best compounds (26 and 27) had electron-
withdrawing groups on the 2-position of the N-phenyl
13. Codd, E. E.; Shank, R. P.; Schupsky, J. J.; Raffa, R. B. J.
Pharmacol. Exp. Ther. 1995, 274, 1263.