3-Arylpiperazinylethyl-1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-dione derivatives as novel, high-affinity and selective α1- adrenoceptor ligands

Article abstract of DOI:10.1016/j.bmcl.2005.09.027

The discovery of a new series of selective and high-affinity α₁-adrenoceptor (α₁-AR) ligands, characterized by a 1H-pyrrolo[2,3-d]-pyrimidine-2,4(3H,7H)-dione system, is described in this paper. Some synthesized compounds, including 20, 22, and 30, displayed affinity in the nanomolar range for α₁-ARs and substantial selectivity with respect to 5-HT_1A and dopaminergic D₁ and D ₂ receptors. Functional assays, performed on selected derivatives, showed antagonistic properties.

Full text of DOI:10.1016/j.bmcl.2005.09.027

Bioorganic & Medicinal Chemistry Letters 16 (2006) 150–153
3-Arylpiperazinylethyl-1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-
dione derivatives as novel, high-affinity and selective
a1-adrenoceptor ligands
a,
a
a
a
*
Valeria Pittala, Giuseppe Romeo, Loredana Salerno, Maria Angela Siracusa,
`
Maria Modica,a Luisa Materia,a Ilario Mereghetti,b Alfredo Cagnotto,b Tiziana Mennini,b
Gabriella Marucci,c Piero Angelic and Filippo Russoa
a
`
Dipartimento di Scienze Farmaceutiche, Universita di Catania, viale A. Doria 6, 95125 Catania, Italy
bIstituto di Ricerche Farmacologiche ÔMario NegriÕ, via Eritrea 62, 20157 Milan, Italy
`
Dipartimento di Scienze Chimiche, Universita di Camerino, via S. Agostino 1, 62032 Camerino, Italy
c
Received 28 July 2005; revised 6 September 2005; accepted 12 September 2005
Available online 10 October 2005
Abstract—The discovery of a new series of selective and high-affinity a1-adrenoceptor (a1-AR) ligands, characterized by a 1H-pyr-
rolo[2,3-d]pyrimidine-2,4(3H,7H)-dione system, is described in this paper. Some synthesized compounds, including 20, 22, and 30,
displayed affinity in the nanomolar range for a1-ARs and substantial selectivity with respect to 5-HT1A and dopaminergic D1 and D2
receptors. Functional assays, performed on selected derivatives, showed antagonistic properties.
ꢀ 2005 Elsevier Ltd. All rights reserved.
The adrenoceptors (ARs) play a key role in the modula-
tion of sympathetic nervous system activity and are a
site of action for many important therapeutic agents.
The ARs, members of the G-protein-coupled receptors
superfamily, are divided into three principal groups:
a1, a2, and b.1a–f
D1 and D2 receptors, and with respect to the different
a1-AR subtypes to finally clarify the pharmacological
profile of each receptor type and subtype.
As a part of our program addressed to the development
of selective a1-AR ligands, we have designed new
molecules based on an arylpiperazinyl moiety, a
well-known pharmacophoric portion present in several
classes of a1-AR ligands.2
The a1-ARs are further subdivided into a1A, a1B, and
a1D subtypes, and play a prominent role in regulating
vascular tone and hypertrophic growth of smooth
muscle and cardiac cells.1a–f
R2
H
O
O
N
N
R2
N
N
N
N
( )n
N
Particularly, a1A-AR seems to be primarily involved in
the development of benign prostatic hypertrophy
(BPH) and, in the last few years, it became an interesting
target for symptomatic treatment of BPH with reduced
side effects.1c,f
R1
N
H
N
N
H
O
O
H
R3
B
A
1: R1 = 4-CH3C6H4, R2 = Cl, n = 1
2: R1 = 4-ClC6H4, R2 = Cl, n = 1
Although a great number of a1-AR ligands are known,
there is still the need for high-affinity and selective
ligands with respect to 5-HT1A and dopaminergic
Recently, we reported the discovery of 3-aryl-
piperazinylalkylpyrrolo[3,2-d]pyrimidine-2,4-dione deriva-
tives (A) as ligands for the a1-AR subtypes endowed
with an excellent selectivity with respect to 5-HT1A
and dopaminergic D1 and D2 receptors.3 These mole-
cules, bearing between the pyrrolo[3,2-d]pyrimidine-
2,4-dione moiety and the phenylpiperazine (PPz) residue
Keywords: a1-Adrenoceptor antagonists; a1-Adrenoceptor subtypes;
Arylpiperazine; 1H-Pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-dione.
*
Corresponding author. Tel.: +39 095 7384273; fax: +39 095
222239; e-mail: vpittala@unict.it
0960-894X/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2005.09.027

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V. Pittala et al. / Bioorg. Med. Chem. Lett. 16 (2006) 150–153
151
a connecting alkyl chain with two or three carbon atoms
(n = 1 and 2), were structurally related to RN5, a known
a1-AR antagonist with affinity in the nanomolar range.2
Besides, some thienopyrimidine-2,4-dione derivatives,
related to compounds A, were described in a patent
and claimed as ligands selective for a1D- with respect
to a1A- and a1B-AR subtypes.4
worthy, all the compounds described herein presented
affinity for the a1-ARs in the low nanomolar range.
These results confirmed that the isomeric 1H-pyrrolo-
[2,3-d]pyrimidine-2,4(3H,7H)-dione system is a good
scaffold that, bearing appropriate substituents, can confer
to new ligands high affinity for a1-ARs.
In this paper, we report on the synthesis and pharmaco-
logical profile of novel a1-AR ligands (B) characterized
Regarding the selectivity, as general trend, derivatives in
which R2 is a chlorine residue showed an interesting
preference for a1-ARs with respect to 5-HT1A receptors
so that they were evaluated for their affinity toward D1
and D2 dopaminergic receptors, demonstrating often
good selectivity (20, 22, and 30). These results finally
confirmed that a chlorine residue at the 2-position of
the PPz moiety, instead of a methoxy group, is crucial
to address selectivity to a1-ARs (as previously reported
for compounds A).3
by
the
isomeric
1H-pyrrolo[2,3-d]pyrimidine-
2,4(3H,7H)-dione system, with the aim of getting a
new insight into the structural requirements that direct
selectivity toward a1-ARs.
Novel derivatives were synthesized as reported in
Scheme 1 and the synthetic route follows a strategy sim-
ilar to the one used for the preparation of compounds A.
Briefly, the appropriate 2-amino-5-substituted phenyl-
1H-pyrrole-3-carboxylic acid ethyl esters 3–9 were easily
accessible utilizing procedures described in the litera-
ture,5 starting from 3-amino-3-iminopropanoic acid eth-
yl ester and condensing it with the appropriate 2-bromo-
1-substituted phenylethanone. Aminoesters 3–9 were
then reacted with 2-chloroethyl isocyanate in refluxing
toluene. Obtained ureas 10–16 were reacted with the
suitable 1-(2-substituted phenyl)piperazine in THF in
the presence of NaI and NaHCO3, to afford intermedi-
ates that were converted into final products 17–30 in
refluxing methanolic potassium hydroxide solution.
The effects of the nature and the position of substituents
on the phenyl ring linked to heterobicyclic system were
investigated by introducing a methyl a methoxy or a
chlorine residue at the 2- or the 4-position. These substi-
tutions did not significantly affect affinity at a1-ARs,
while a modulation of the selectivity with respect to
the other tested receptors was observed. The introduc-
tion of a methyl or a methoxy residue at the 2-position
afforded interesting derivatives in terms of affinity and
selectivity (20 and 22). On the contrary, the same substi-
tution at the 4-position of the phenyl ring lead to a slight
loss of affinity for a1-ARs (26 and 28) and, as a conse-
quence, the selectivity ratios 5-HT1A/a1, D1/a1, and
D2/a1 were reduced (26 vs 20 and 28 vs 22).
A detailed synthetic procedure, along with analytical
characterization, is given for 3-[2-[4-(2-chlorophenyl)-
1-piperazinyl]ethyl]-6-(4-methylphenyl)-1H-pyrrolo[2,3-d]-
pyrimidine-2,4(3H,7H)-dione (26) and its precursors 7
and 14.6
The substitution at the 2-position of the phenyl ring
with a chlorine residue (23 and 24), while maintaining
affinity, was detrimental for selectivity.
All synthesized compounds were characterized by 1H
NMR spectra, IR, and elemental analysis and analytical
data were consistent with the proposed structures.
Compound 30 possesses one of the highest affinity val-
ues at a1-ARs and, although bearing chlorine residues
both at the 4-position of the phenyl ring linked to the
heterobicyclic nucleus and at the 2-position of the PPz
moiety, is the most selective derivative identified in this
study.
New compounds were evaluated in binding assays that
were performed as previously reported,3 and Ki values
of compounds 17–30 for a1-ARs and related receptors
are summarized in Table 1. In it, for sake of clarity,
affinity values of compounds 1 and 2, the most interest-
ing derivatives in the series A, are also reported. Note-
Derivatives 26, 28, and 30, along with compound 1, were
selected for further evaluation in functional assays for
O
O
O
O
O
O
O
N
a
b
c
N
R2
R2
N
NH
NH2
N
NH
Cl
N
N
H
N
N
H
H
N
N
N
O
N
H
O
R3
R3
R3
H
O
R3
H
H
3-9
17-30
10-16
3, 10: R3 = H
17: R3 = H, R2 = OCH3
18: R3 = H, R2 = Cl
24: R3 = 2-Cl, R2 = Cl
4, 11: R3 = 2-CH3
25: R3 = 4-CH3, R2 = OCH3
5, 12: R3 = 2-OCH3
19: R3 = 2-CH3, R2 = OCH3 26: R3 = 4-CH3, R2 = Cl
6, 13: R3 = 2-Cl
20: R3 = 2-CH3, R2 = Cl
21: R3 = 2-OCH3, R2 = OCH3 28: R3 = 4-OCH3, R2 = Cl
22: R3 = 2-OCH3, R2 = Cl
23: R3 = 2-Cl, R2 = OCH3
27: R3 = 4-OCH3, R2 = OCH3
7, 14: R3 = 4-CH3
8, 15: R3 = 4-OCH3
9, 16: R3 = 4-Cl
29: R3 = 4-Cl, R2 = OCH3
30: R3 = 4-Cl, R2 = Cl
Scheme 1. Reagents and conditions: (a) ClCH2CH2NCO, toluene, reflux, 6 h (75–85%); (b) 1-(2-substituted phenyl)piperazine, NaHCO3, NaI, THF,
reflux, 24 h (65–70%); (c) KOH/MeOH, reflux, 3 h (85–98%).

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V. Pittala et al. / Bioorg. Med. Chem. Lett. 16 (2006) 150–153
152
Table 1. Binding properties of 3-arylpiperazinylethyl-1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-dione derivatives 17–30
R2
R3
Ki (nM)a
5-HT1A/a1
D1/a1
D2/a1
b
b
b
Compound
a1
1.99 ( 0.45)
5-HT1A
D1
D2
17
18
19
20
21
22
23
24
25
26
27
28
29
30
RN5c
1d
OCH3
Cl
H
122.3 ( 15.2)
1540 ( 145.6)
295.1 ( 11.7)
1450 ( 79.9)
157.3 ( 8.83)
1030 ( 21.8)
19.10 ( 3.60)
64.90 ( 12.8)
333.34 ( 22.5)
1815 ( 39.3)
113.4 ( 8.46)
1070 ( 83.8)
401.3 ( 26.3)
1513 ( 172.8)
50.00 ( 4.70)
>10,000
ND
516 ( 45.2)
ND
ND
137.2 ( 10.8)
ND
61
327
248
518
97
H
4.71 ( 0.71)
1.19 ( 0.01)
2.80 ( 0.47)
1.63 ( 0.10)
3.32 ( 0.58)
2.18 ( 0.33)
6.28 ( 2.14)
4.97 ( 0.36)
11.71 ( 2.36)
5.47 ( 0.36)
16.40 ( 2.93)
4.08 ( 0.31)
2.64 ( 0.25)
0.21 ( 0.02)
17.26 ( 4.14)
11.70 ( 0.25)
110
371
>2500
90
29
224
109
57
OCH3
Cl
2-CH3
2-CH3
2-OCH3
2-OCH3
2-Cl
1040 ( 155.3)
ND
626.5 ( 99.6)
ND
OCH3
Cl
8550 ( 1480)
ND
363.2 ( 55.7)
ND
310
8.8
10
OCH3
Cl
2-Cl
566.8 ( 41.5)
ND
356.1 ( 53.1)
ND
OCH3
Cl
4-CH3
4-CH3
4-OCH3
4-OCH3
4-Cl
67
388.7 ( 45.4)
ND
1720 ( 506)
ND
155
21
33
147
10
OCH3
Cl
271.6 ( 41.1)
ND
544.4 ( 71.7)
164.6 ( 16.4)
ND
2370 ( 770)
65
98
17
OCH3
Cl
4-Cl
573
238
>500
>800
206
898
>10,000
>10,000
>10,000
>10,000
>500
>800
>500
>800
2d
>10,000
a Ki values were calculated as in Ref. 3 and are means ( SD) of three separate experiments.
b Ratio between Ki 5-HT1A/Ki a1; Ki D1/Ki a1; and Ki D2/Ki a1.
c Data taken from Ref. 2.
d Data taken from Ref. 3.
Table 2. Antagonist potency, expressed as pKb SEM values, and selectivities of compounds 26, 28, 30, and 1 at a1-ARs in isolated rat prostatic vas
deferens (a1A), spleen (a1B), and thoracic aorta (a1D
)
R2
R3
pKba1A or [pA2]b
pKba1B or [pA2]
pKba1D or [pA2]
a1D/a1A
a
1D/a1B
a
1B/a1A
a
c
Compound
26
28
Cl
Cl
Cl
4-CH3
4-OCH3
4-Cl
7.25 ( 0.07)
7.48 ( 0.06)
6.41 ( 0.09)
6.72 ( 0.11)
[9.51] ( 0.06)
7.50 ( 0.10)
8.03 ( 0.06)
6.64 ( 0.09)
6.37 ( 0.03)
[8.20] ( 0.10)
7.78 ( 0.07)
7.69 ( 0.13)
7.66 ( 0.05)
6.44 ( 0.05)
[8.80] ( 0.12)
3
2
2
0.5
2
4
2
30
1
WB4101
18
10
0.5
0.2
1
4
0.4
0.05
a pKb values were calculated according to van Rossum7 at a single concentration and each concentration was investigated at least four times.
b pA2 values were calculated according to Arunlakshana and Schild plots.8
c Antilog of the difference between the pKb values for a1A-, a1B-, and a1D-AR subtypes.
their activity at a1A-, a1B-, and a1D-AR subtypes. Tested
compounds, as shown in Table 2, behave as a1-AR
antagonists and their affinity was calculated at a single
concentration according to van Rossum.7 WB4101 was
included as a standard compound.
Further structural modifications of the 1H-pyrrolo[2,3-
d]pyrimidine-2,4(3H,7H)-dione system are currently
ongoing with the aim of ameliorating receptor subtype
selectivity.
Acknowledgment
The results in Table 2 show for derivatives 26 and 28
higher values of affinity at the three a1-AR subtypes
with respect to 30 and 1. However, compound 30 be-
haves as an a1D-AR selective antagonist displaying a
18 and 10 times higher affinity for this subtype when
compared with a1A- and a1B-AR ones.
This work was supported by grants from the University
of Catania and MIUR.
References and notes
In conclusion, we have discovered a new class of ligands
endowed with noteworthy affinity toward a1-ARs, char-
acterized by a 1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-
dione moiety. Furthermore, the 1H-pyrrolo[2,3-d]pyrim-
idine-2,4(3H,7H)-dione system showed to be able to ad-
dress properly a1-ARs selectivity over 5-HT1A, D1, and
D2 dopaminergic receptors (20, 22, and 30). In particu-
lar, compound 30, endowed with affinity in the nanomo-
lar range for the a1-ARs, showed the best profile in
terms of selectivity toward other tested receptors, and,
in functional assays, a preference for the a1D- with re-
spect to a1A- and a1B-AR of one order of magnitude.
1. (a) Ford, A. P. D. W.; Williams, T. J.; Blue, D. R.; Clarke,
D. E. Trends Pharmacol. Sci. 1994, 15, 167; (b) Hieble, J. P.;
Bondinell, W. E.; Ruffolo, R. R. J. Med. Chem. 1995, 38,
3415; (c) Ruffolo, R. R., Jr.; Bondinell, W.; Hieble, J. P. J.
Med. Chem. 1995, 38, 3681; (d) Hieble, J. P.; Bylund, D. B.;
Clarke, D. E.; Eikemburg, D. C.; Langer, S. Z.; Lefkowiz,
R. J.; Minneman, K. P.; Ruffolo, R. R. Pharmacol. Rev.
1995, 47, 267; (e) Zhong, H.; Minneman, K. P. Eur. J.
Pharmacol. 1999, 375, 261; (f) Romeo, G.; Materia, L.;
Salerno, L.; Russo, F.; Minneman, K. P. Expert Opin. Ther.
Pat. 2004, 14, 619.
2. Russo, F.; Romeo, G.; Guccione, S.; De Blasi, A. J. Med.
Chem. 1991, 34, 1850.

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V. Pittala et al. / Bioorg. Med. Chem. Lett. 16 (2006) 150–153
153
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`
3. Patane, E.; Pittala, V.; Guerrera, F.; Salerno, L.; Romeo,
G.; Siracusa, M.; Russo, F.; Manetti, F.; Botta, M.;
Mereghetti, I.; Cagnotto, A.; Mennini, T. J. Med. Chem.
2005, 48, 2420.
(3.44 mmol, 0.840 g) and 2-chloroethyl isocyanate
(5.16 mmol, 0.438 mL) in 8 mL of toluene was stirred for
6 h at reflux. The reaction mixture was then filtered, and the
obtained solid was washed several times with EtOH to give
the title compound as a pure solid (0.811 g, 67%): mp 112-
114 ꢁC; IR (KBr, selected lines) cmÀ1 3332, 2919, 2865,
1673, 1606, 1549, 1359, 1263, 1097, 778, 655. 1H NMR
(DMSO-d6) d 11.26 (br s, 1H, NH which exchanges with
D2O), 9.24 (br s, 1H, NHCONHCH2 which exchanges with
D2O), 7.99 (t, J = 5.8 Hz, 1H, NHCONHCH2 which
exchanges with D2O), 7.47–7.40 (m, 2H, aromatic), 7.20–
4. Meyer, M. D.; Carrol, W. A. U.S. 6,153,614; 2000.
5. (a) Toja, E.; DePaoli, A.; Tuan, G.; Kettenring, J. Synthesis
1987, 3, 272; (b) Cocco, M. T.; Congiu, C.; Maccioni, A.;
Plumitallo, A.; Schivo, M. L.; Palmieri, G. Farmaco 1988,
43, 103.
6. Experimental procedure for the synthesis of compounds 7,
14, and 26. 2-Amino-5-(4-methylphenyl)-1H-pyrrole-3-car-
boxylic acid ethyl ester (7). Ammonium chloride
(15.3 mmol, 0.820 g) was added to a suspension of 3-
ethoxy-3-iminopropanoic acid ethyl ester hydrochloride
(10.2 mmol, 2.00 g) and K2CO3 (17.9 mmol, 2.47 g) in
10 mL of anhydrous ethanol. The reaction mixture was
then stirred at 22 ꢁC for 24 h. The obtained suspension of 3-
amino-3-iminopropanoic acid ethyl ester was used for the
subsequent step without isolating it from the reaction
4
7.13 (m, 2H, aromatic), 6.61 (d, J = 2.8 Hz, 1H, pyrrole),
4.31 (q, J = 7.0 Hz, 2H, OCH2CH3), 3.69 (t, J = 6.0 Hz, 2H,
CH2Cl), 3.52–3.46 (m, 2H, NHCH2), 2.29 (s, 3 H, CH3),
1.34 (t, J = 7.0 Hz, 3H, OCH2CH3). Anal. (C17H20ClN3O3)
C, H, N. 3-[2-[4-(2-Chlorophenyl)-1-piperazinyl]ethyl]-6-(4-
methylphenyl)-1H-pyrrolo[2,3-d]pyrimidine-2,4(3H,7H)-dione
(26) A suspension of 2-[[[(2-chloroethyl)amino]carbon-
yl]amino]-5-(4-methyl)phenyl-1H-pyrrole-3-carboxylic acid
ethyl ester (14) (1.06 mmol, 0.370 g), 1-(2-chlorophenyl)pi-
perazine hydrochloride (1.27 mmol, 0.296 g), NaHCO3
(4.23 mmol, 0.356 g), and a catalytic amount of NaI in
2 mL of anhydrous THF was refluxed for 24 h. After this
period the obtained mixture was concentrated, and the
residue was diluted with water (50 mL) and extracted with
CH2Cl2 (3 · 50 mL). The combined organic layers were
dried over anhydrous Na2SO4, filtered, and evaporated.
The obtained crude material was dissolved in EtOAc,
filtered through a pad of silica gel, and concentrated. The
obtained urea was refluxed in methanolic KOH (1 N, 4 mL)
for 3 h. The reaction mixture was then allowed to cool at
22 ꢁC, acidified (pH 4) with glacial AcOH, and neutralized
with saturated NaHCO3. A white precipitate was isolated,
washed with water, and triturated in EtOAc to afford the
desired product 26 as a white solid (0.285 g, 58%): mp 254–
256 ꢁC; IR (KBr, selected lines) cmÀ1 2973, 2828, 1699,
1636, 1574, 1480, 1444, 1231, 1041, 932, 764, 708. 1H NMR
(DMSO-d6) d 7.60–7.52 (m, 2H, aromatic), 7.42–6.95 (m,
6H, aromatic), 6.58 (s, 1H, pyrrole), 4.06 (t, J = 6.6 Hz, 2H,
CONCH2CH2N), 2.99–2.94 (m, 4H, NCH2), 2.69–2.49 (m,
4H + 2H, NCH2 + CONCH2CH2N), 2.27 (s, 3H, CH3).
Anal. (C25H24ClN5O2) C, H, N.
environment.
2-Bromo-1-(4-methylphenyl)ethanone
(10.2 mmol, 2.18 g) and K2CO3 (5.11 mmol, 0.706 g) were
added to the previously obtained suspension of 3-amino-3-
iminopropanoic acid ethyl ester and the reaction mixture
was refluxed for 2 h. After this period the solvent was
evaporated at reduced pressure, and the residue was diluted
with NaOH 1 N (100 mL) and extracted with EtOAc (3·
100 mL). The combined organic layers were washed with
brine, dried over anhydrous Na2SO4, filtered, and evapo-
rated in vacuo. The obtained crude material was purified by
column chromatography (silica gel, 60,230–400 mesh,
Merck) using cyclohexane/EtOAc (6:4) as eluant, to afford
the title compound (7) as a white powder (1.03 g, 55%): mp
140–142 ꢁC; IR (KBr, selected lines) cmÀ1 3484, 3361, 1666,
1641, 1508, 1369, 1237, 1190, 1071, 799, 765. The 1H NMR
spectrum was recorded at 200 MHz on a Varian Inova
Unity 200 spectrometer in DMSO-d6. 1H NMR (DMSO-d6)
d 10.68 (br s, 1H, NH which exchanges with D2O), 7.41–
7.34 (m, 2H, aromatic), 7.14–7.07 (m, 2H, aromatic), 6.39
(d, 4J = 2.8 Hz, 1H, pyrrole), 5.63 (br s, 2H, NH2 which
exchanges with D2O), 4.13 (q, J = 7.0 Hz, 2H, OCH2CH3),
2.28 (s, 3H, CH3), 1.24 (t, J = 7.0 Hz, 3H, OCH2CH3).
Anal. (C14H16N2O2) C, H, N. 2-[[[(2-Chloroethyl)ami-
no]carbonyl]amino]-5-(4-methylphenyl)-1H-pyrrole-3-car-
boxylic acid ethyl ester (14). A suspension of 2-amino-5-(4-
methylphenyl)-1H-pyrrole-3-carboxylic acid ethyl ester (7)
7. van Rossum, J. M. Arch. Int. Pharmacodyn. 1963, 143, 299.
8. Arunlakshana, O.; Schild, H. O. Br. J. Pharmacol. Chemo-
ther. 1959, 14, 48.