2,4-Disubstituted pyrroles: Synthesis, traceless linking and pharmacological investigations leading to the dopamine D4 receptor partial agonist FAUC 356

Article abstract of DOI:10.1016/S0960-894X(02)00316-5

Solution-phase synthesis and a solid-phase supported approach to piperazinylmethyl substituted pyrroles are described. Receptor binding studies and the measurement of D4 ligand efficacy led to the ethynylpyrrole 1d (FAUC 356) exerting selective D4 binding and substantial ligand efficacy (66%, EC₅₀=1.9 nM). This activity profile might be of interest for the treatment of ADHD.

Full text of DOI:10.1016/S0960-894X(02)00316-5

Bioorganic & Medicinal ChemistryLetters 12 (2002) 1937–1940
2,4-Disubstituted Pyrroles: Synthesis, Traceless Linking and
Pharmacological Investigations Leading to the Dopamine D4
Receptor Partial Agonist FAUC 356
Markus Bergauer, Harald Hubner and Peter Gmeiner*
Department of Medicinal Chemistry, Emil Fischer Center, Friedrich-Alexander University, Schuhstraße 19, D-91052 Erlangen, Germany
Received 5 March 2002; revised 3 May2002; accepted 6 May2002
Abstract—Solution-phase synthesis and a solid-phase supported approach to piperazinylmethyl substituted pyrroles are described.
Receptor binding studies and the measurement of D4 ligand efficacy led to the ethynylpyrrole 1d (FAUC 356) exerting selective D4
binding and substantial ligand efficacy(66%, EC 50=1.9 nM). This activityprofile might be of interest for the treatment of ADHD.
# 2002 Elsevier Science Ltd. All rights reserved.
Recent investigations using molecular cloning technol-
ogyhave led to the characterization of a number of
dopamine receptor subtypes that can be divided into
two classes, D1-like (D1 and D5) and D2-like (D2, D3,
D4).1 D4 receptors have been attributed to involvement
in the pathogenesis of neuropathological diseases,2
when a polymorphism of the dopamine D4 receptor
gene has been implicated with ADHD (attention deficit
hyperactivity disorders).3 Due to these findings, selective
agonists or partial agonists might offer a chance for an
effective treatment of ADHD.
involving an associated non-aromatic p-system being
a-positioned (adjacent to the pyrrole nitrogen).10 Pro-
jecting analogous binding modes of the structural
families of type A and B, modification of the substituted
pyrrole moiety should influence not only receptor bind-
ing but also ligand efficacy. As a consequence, we plan-
ned to prepare and pharmacologicallyinvestigate the
pyrroles 1 containing different aromatic and non-aro-
matic p-systems in the pyrrole b-position. Besides
exploiting solution-phase synthesis involving DEM-pro-
tection of the pyrrole nitrogen,11 we envisioned to
demonstrate our recentlydescribed solid-phase supported
approach to substituted pyrroles.12
SAR studies in the field of selective D4 ligands led to a
varietyof piperazine derivatives of tpye A containing
bicyclic heteroaromatic moieties, when 1H-pyrrolo[2,3-
b]pyridines,4 pyrazolo[1,5-a]pyridines5 or 5-cyanoindoles6
turned out to be superior. Interestingly, mitogenesis
assays with compounds of type A showed that the
intrinsic activityof the ligands stronglydepends on the
nature and the spatial orientation of the heterocyclic
system.7 Piperazines of type B, which are characterized
byimidazole, imidazoline, thiazoline or oxazoline sub-
structures8,9 as monocyclic heteroaromatic systems, also
proved to have preferential binding to the D4 recep-
tor when substituted bya phenly residue extending
the p-system of the heteroarene unit. Previously, we
developed D4 active pyrrole derivatives of type B
As a common starting material for the selected target
compounds, we chose the DEM-protected 4-iodo-
pyrrole-2-carbaldehyde 2 which was obtained byour pre-
viouslydescribed methodology. 11 Applying Pd-catalyzed
*Corresponding author. Tel.: +49-9131-852-9383; fax: +49-9131-
852-2585; e-mail: gmeiner@pharmazie.uni-erlangen.de
0960-894X/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved.
PII: S0960-894X(02)00316-5

1938
M. Bergauer et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1937–1940
coupling reactions, the introduction of pharmacophoric
elements into position 4 could be accomplished (Scheme
1). Thus the phenyl derivative 3a and the alkynes 3b,c
were synthesized according to Suzuki13 and Sonoga-
shira14 using the catalysts Pd(PPh3)4 and Pd(PPh3)2Cl2/
CuI, respectively(Scheme 1). Reductive amination of
the pyrrolecarbaldehydes 3a–c with phenylpiperazine
and sodium triacetoxyborohydride furnished the piper-
azines 4a–c in 63–88% yield. The trimethylsilyl group of
the alkyne 4c was removed using tetrabutylammonium
fluoride to give 4d (82%). After deprotection of the
DEM-pyrroles 4a, 4b, and 4d with TFA and subsequent
treatment with NaOH, we obtained the final products
1a,b,d15 in 56–94% yield.
syntheses of pyrrole derived compound libraries. In
detail, traceless linking was facilitated bytransacetali-
zation of the DEM-protected iodopyrrolecarbaldehyde
2 and the 1,3-diol linker of Leznoff resin16 to give
the polymer-bound dioxolane 5. Subsequent Suzuki
coupling gave access to the arylation product 6 when a
catalyst based on Pd2(dba)3 and P(t-Bu)3 was utilized.
Finally, reductive amination gave the resin-bound
piperazine 7 that was hydrolytically cleaved to afford
the final product 1a.
Stimulated bythe high D4 affinityof our previously
described indolecarbonitrile derivatives of type A, we
envisioned to introduce a cyano function enlarging the
p-system and inducing a negative electrostatic region
around the sp-nitrogen. Thus, we approached the test
compound 1f as well as its reversed substituted regio-
isomer 13 (Scheme 2).
Choosing the phenylpyrrole 1a as a representative target
compound, we elaborated a reaction protocol on solid
phase being suitable for two-dimensional parallel
In detail, the aldehyde 2 was reacted with phenylpiper-
azine and sodium triacetoxyborohydride to give the
amination product 8 in 83% yield. Sakamoto et al.17
described an effective protocol for the palladium cata-
lyzed cyanation of bromo- and iodo-substituted ben-
zenes, indoles and pyrroles. Employing these conditions
(CuCN/Pd2(dba)3/dppf in 1,4-dioxane) gave the desired
cyano pyrrole 9 in low yield (15%). Alternatively, we
tried a halogen–metal exchange reaction using n-BuLi
and subsequent trapping with TsCN. This reaction
sequence resulted in formation of the carbonitrile 9 in
28% yield. N-Deprotection with TFA/NaOH gave the
piperazine 1f (86%). The reversed functionalized isomer
13 was synthesized starting from the cyano pyrrole 1011
when iodine–magnesium exchange and reaction with
DMF gave the aldehyde 11 that was subjected to
reductive amination with phenylpiperazine and hydro-
lytic deprotection to furnish 81% of the final product
13.
Receptor binding profiles of the test compounds 1a,b,
1d–f, and 13 were determined in vitro bymeasuring
3
their abilityto displace [ H]spiperone from the cloned
18
human dopamine receptor subtypes D2long, D2short
,
Scheme 1. (a) 3a, 3c: see ref 11; PhCCH, Pd(PPh3)2Cl2, CuI, 1,4-
dioxane/NEt3 (2:1), rt, 15 min (3b: 85%); (b) phenylpiperazine, NaB-
H(OAc)3, 1,2-dichloroethane, rt (4a: 19 h, 88%; 4b: 22 h, 63%; 4c:
19 h, 77%; 7: 22 h); (c) tetrabutylammonium fluoride, THF, rt, 1 h
(82%); (d) 4a: TFA (3 equiv), CH3CN, rt, 2 days; 2 N NaOH (6 equiv),
rt, 16 h (1a: 69%); 4b: TFA (4 equiv), CH3CN, rt, 0.5 h; 2 N NaOH
(8 equiv), rt, 0.5 h (1b: 94%); 4d: TFA (3 equiv), CH3CN, rt, 24 h; 2 N
NaOH (6 equiv), rt, 1 h (1d: 56%); (e) 1,4-dioxane, p-TsOH, rt;12 (f)
phenylboronic acid, 15% Pd2(dba)3, P(t-Bu)3, Cs2CO3, 1,4-dioxane,
80 ꢀC, 24 h; (g) (1) 1,4-dioxane/CH3CN/TFA (2:6:1), 60 ꢀC, 48 h; (2)
2 N NaOH, rt, 3 h (53%; purity70%, indicated by 1H NMR spectro-
scopy).
Scheme 2. (a) Phenylpiperazine, NaBH(OAc)3, 1,2-dichloroethane, rt,
4 h (83%); (b) (1) n-BuLi (1.1 equiv), THF, À78 ꢀC, 1 h; (2) TsCN
(1.5 equiv), À78 ꢀC, 30 min (28%); (c) TFA (4 equiv), CH3CN, 60 ꢀC,
22 h; 2 N NaOH (8 equiv), rt, 1 h (86%); (d) ref 11; (e) phenylpiper-
azine, NaBH(OAc)3, rt, 14 h (83%); (f) (1) TFA (4 equiv), CH3CN,
60 ꢀC, 22 h; (2) 2 N NaOH (8 equiv), rt, 2 h (81%).

M. Bergauer et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1937–1940
1939
Table 1. Binding data of the test compounds for human dopamine D2, D3 and D4 receptors, bovine D1 and porcine 5-HT1A and 5-HT2A
receptors [Ki values (nM) based on the means of 2–5 experiments each performed in triplicate]
Ki values (nM)
[3H]SCH23390
[3H]Spiperone
hD3
[3H]8-OH-DPAT
p5-HT1A
[3H]Ketanserin
p5-HT2A
bD1
hD2short
hD2short
hD4
1a
1b
1d
1e
1f
13
2100
2700
2000
5400
4600
1200
12,000
420
230
760
770
3100
3700
2400
3200
41
170
550
580
3000
8000
3400
4300
28
540
670
850
570
1800
720
5000
960
12
83
5.9
100
21
18
3.1
16
790
nd
270
nd
450
510
nd
nd
nd
1300
nd
380
nd
610
450
nd
nd
nd
FAUC 113
Clozapine
Quinpirolea
nd
64/3100
52/4000
24/420
1.6/49
nd, values not determined.
aHigh/low affinitybinding sites.
Table 2. Intrinsic activityof the piperazines 1a, 1d, 1f and 13 in relation to the full agonist quinpirole, the partial agonist FAUC 113 and the
antagonist clozapine at the human D4.2 receptor established bymeasuring the stimulation of mitogenesis
Test compounds
1a
1d
1f
13
FAUC 113
Quinpirole
Clozapine
Intrinsic effecta (%)
EC50 (nM)
43
6.8
66
1.9
38
5.6
36
7.8
27
16
100
2.2
0
—
aRate of incorporation of [3H]thymidine as evidence for mitogenetic activity relative to the maximal effect of the full agonist quinpirole (100%) as
the means of quadruplicates from 6 to 12 experiments. EC50 values in nM are derived from the mean curves of the experiments.
D319 and D4.420 stablyexpressed in CHO (Chinese
hamster ovary) cells.21 D1 receptor affinities were investi-
gated employing bovine striatal membranes and the D1
selective radioligand [3H]SCH 23390.21 The resulting Ki
values of the test compounds are listed in Table 1 in
comparison to quinpirole, the D4 partial agonist FAUC
113 and the atypical neuroleptic clozapine.
To investigate the intrinsic effects of the most interesting
test compounds 1a,d,f and 13, a mitogenesis assaywas
performed employing CHO10001 cells stably expressing
the human D4.2 receptor.6,22 Agonist activation of
dopamine receptors can be determined bymeasuring the
rate of [3H]thymidine incorporation into growing het-
23
erologouslytransfected cell lines.
Comparative
experiments were done with the full agonist quinpirole,
the partial agonist FAUC 113 and the full antagonist
clozapine. Intinsic activities and EC50 values are depic-
ted in Table 2, clearlyindicating ligand efficacyfor the
phenylpyrrole 1a (43%) and the cyano substituted
regioisomers 1f and 13 (36–38%) that is comparable to
the D4 partial agonist FAUC 113 (27%). Both these
The test compounds displayed substantial D4 selectiv-
ity. The highest affinities for the D4 binding site were
determined for the phenylpyrrole 1a and its alkynyl
substituted bioisostere 1d with Ki values of 12 and
5.9 nM, respectively. Within the D2 receptor family, the
alkyne 1d showed a significantlystronger D4 preference
than the phenyl derivative 1a when the selectivityover
D2long, D2short and D3 was 7 and 3 times higher,
respectively. Increase of the p-system by joining toge-
ther the acetylene unit and the phenyl moiety resulted in
significant reduction of D4 affinityfor 1b (Ki=83 nM).
Substitution of the ethynyl group of 1d with the more
polarized cyano function led to a slight reduction of D4
affinitywhen the position of the NH-function exerted
onlya minor influence onto the binding profiles of the
regioisomers 1f and 13. The binding data of the unsub-
stituted aminomethylpyrrole 1e10 show that an exten-
sion of the pyrrole p-system is, in fact, necessary to
provide substantial D4 affinity. In order to further
characterize the binding properties of the potent D4
ligands 1a,d,f and 13, 5-HT1A and 5-HT2 receptor
recognition was evaluated. Employing porcine brain
homogenates and the radioligands [3H]8-OH-DPAT
and [3H]ketanserin, respectively, the resulting Ki values
indicated onlymoderate 5-HT1A (270–790 nM) and 5-
HT2 affinities (380–1300 nM).
Figure 1. Stimulation of mitogenesis as a functional assayto estimate
the effect of 1d at the human dopamine D4.2 receptor in relation to the
full agonist quinpirole, the antagonist clozapine and the D4 ligand
FAUC 113.

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M. Bergauer et al. / Bioorg. Med. Chem. Lett. 12 (2002) 1937–1940
parameters, however, are substantiallyimproved for the
alkyne 1d (66%, EC50=1.9 nM), which might be due to
the abilityof the acetlyene substructure to act as a
hydrogen bond donating functionality, possibly stabi-
lizing active receptor conformations.24 Figure 1 shows
dose–response curves for 1d (FAUC 356) in comparison
to the full agonist quinpirole, the full antagonist cloza-
pine and the previouslydescribed D4 ligand FAUC 113.
Marwood, R.; Patel, S.; Patel, S.; Ragan, C. I.; Leeson, P. D.
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In conclusion, solution-phase synthesis and a solid-
phase supported approach to piperazinylmethyl sub-
stituted pyrroles are described. Receptor binding studies
and the measurement of D4 ligand efficacyindicated
analogous binding modes for the general compound
families of type A and B. The most interesting activity
profile was discovered for the ethynylpyrrole 1d (FAUC
356) exerting selective D4 recognition and substantial
ligand efficacy(66%), which might be of interest for the
treatment of ADHD.
10. Haubmann, C.; Hubner, H.; Gmeiner, P. Bioorg. Med.
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Acknowledgements
15. 1d: 1H NMR (CHCl3, 360 MHz): d 2.58 (t, J=5.0 Hz, 4H,
CH2NCH2CH2), 2.92 (s, 1H, HCC), 3.17 (t, J=5.0 Hz, 4H,
CH2NPh), 3.49 (s, 2H, pyrrole–CH2), 6.17 (m, 1H, H-3), 6.99
(dd, J=2.4, 1.7 Hz, 1H, H-5), 6.8–7.0, 7.2–7.3 (m, 5H, Ph),
8.58 (s, 1H, NH); 13C NMR (CHCl3, 90 MHz): 49.11
(CH2NCH2CH2), 53.02 (CH2NPh), 54.97 (pyrrole–CH2),
75.31 (HCC), 79.48 (HCC), 102.98 (C-4), 111.23 (C-3), 116.05
(Ph–C-2/6), 119.80 (Ph–C-4), 122.82 (C-5), 128.58 (C-2),
129.18 (Ph–C-3/5), 151.17 (Ph–C-1); EIMS: 265 (M+). Anal.
calcd for C17H19N3 (265.36): C, 76.95; H, 7.22; N, 15.84.
Found: C, 76.74; H, 7.07; N, 15.53.
16. Leznoff, C. C.; Wong, J. Y. Can. J. Chem. 1973, 51, 3756.
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The authors wish to thank Dr. H. H. M. Van Tol
(Clarke Institute of Psychiatry, Toronto, Canada), Dr.
J.-C. Schwartz and Dr. P. Sokoloff (INSERM, Paris,
France) as well as Dr. J. Shine (The Garvan Institute of
Medical Research, Sydney, Australia) for providing
dopamine D4, D3 and D2 receptor expressing cell lines,
respectively. Dr. R. Huff (Pharmacia & Upjohn, Inc.,
Kalamazoo, MI, USA) is acknowledged for providing a
D4 expressing cell line employed for mitogenesis.
Thanks are also due to Mrs. H. Szczepanek, Mrs. P.
Schmitt and Mrs. P. Hubner for skillful technical assis-
tance. This work was supported bythe Deutsche For-
schungsgemeinschaft, the BMBF, and the Fonds der
Chemischen Industrie.
18. Hayes, G.; Biden, T. J.; Selbie, L. A.; Shine, J. Mol.
Endocrinol. 1992, 6, 920.
19. Sokoloff, P.; Andrieux, M.; Besanc¸ on, R.; Pilon, C.;
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