Synthesis and structure-activity relationships of a series of pyrrole cannabinoid receptor agonists

Article abstract of DOI:10.1016/S0968-0896(03)00413-9

We designed and synthesized a series of pyrrole derivatives with the aim of investigating the structure-activity relationship (SAR) for the binding of non-classical agonists to CB₁ and CB₂ cannabinoid receptors. Superposition of two pyrrole-containing cannabinoid agonists, JWH-007 and JWH-161, allowed us to identify positions 1, 3 and 4 of the pyrrole nucleus as amenable to additional investigation. We prepared the 1-alkyl-2,5-dimethyl-3,4-substituted pyrroles 10a-e, 11a-d, 17, 21, 25 and the tetrahydroindole 15, and evaluated their ability to bind to and activate cannabinoid receptors. Noteworthy in this set of compounds are the 4-bromopyrrole 11a, which has an affinity for CB₁ and CB₂ receptors comparable to that of well-characterized heterocyclic cannabimimetics such as Win-55,212-2; the amide 25, which, although possessing a moderate affinity for cannabinoid receptors, demonstrates that the 3-naphthoyl group, commonly present in indole and pyrrole cannabimimetics, can be substituted by alternative moieties; and compounds 10d, 11d, showing CB₁ partial agonist properties.

Full text of DOI:10.1016/S0968-0896(03)00413-9

Bioorganic & Medicinal Chemistry 11 (2003) 3965–3973
Synthesis and Structure–Activity Relationships of a Series of
Pyrrole Cannabinoid Receptor Agonists
Giorgio Tarzia,a,* Andrea Duranti,a Andrea Tontini,a Gilberto Spadoni,a Marco Mor,b
Silvia Rivara,b Pier Vincenzo Plazzi,b Satish Kathuriac and Daniele Piomellic
a
`
Istituto di Chimica Farmaceutica e Tossicologica, Universita degli Studi di Urbino ‘Carlo Bo’, Piazza del Rinascimento 6,
I-61029 Urbino, Italy
b
`
Dipartimento Farmaceutico, Universita degli Studi di Parma, Parco Area delle Scienze 27/A, I-43100 Parma, Italy
cDepartment of Pharmacology, 360 MSII, University of California, Irvine, CA 92697-4625, USA
Received 16 April 2003; accepted 13 June 2003
Abstract—We designed and synthesized a series of pyrrole derivatives with the aim of investigating the structure–activity relation-
ship (SAR) for the binding of non-classical agonists to CB1 and CB2 cannabinoid receptors. Superposition of two pyrrole-con-
taining cannabinoid agonists, JWH-007 and JWH-161, allowed us to identify positions 1, 3 and 4 of the pyrrole nucleus as
amenable to additional investigation. We prepared the 1-alkyl-2,5-dimethyl-3,4-substituted pyrroles 10a–e, 11a–d, 17, 21, 25 and the
tetrahydroindole 15, and evaluated their ability to bind to and activate cannabinoid receptors. Noteworthy in this set of compounds
are the 4-bromopyrrole 11a, which has an affinity for CB1 and CB2 receptors comparable to that of well-characterized heterocyclic
cannabimimetics such as Win-55,212-2; the amide 25, which, although possessing a moderate affinity for cannabinoid receptors,
demonstrates that the 3-naphthoyl group, commonly present in indole and pyrrole cannabimimetics, can be substituted by alter-
native moieties; and compounds 10d, 11d, showing CB1 partial agonist properties.
# 2003 Elsevier Ltd. All rights reserved.
Introduction
Various classes of compounds active at cannabinoid
receptors have been developed, and their structure–
activity relationship (SAR) properties have been exten-
sively investigated.8 Agonists reported in the literature
belong to two main classes: (i) dibenzo[b,d]pyrane deri-
vatives (generally referred to as ‘traditional’ cannabi-
Plant-derived and synthetic cannabimimetic agents such
as Á9-tetrahydrocannabinol1 (Á9-THC, 1, Fig. 1) bind
to specific G-protein coupled cannabinoid receptors,
which include the CB1 subtype,2 mainly present in the
central and peripheral nervous systems, and the CB2
subtype,3 localized on immune cells. The identification,
cloning, and biochemical characterization of cannabi-
noid receptors,4 along with the discovery of their endo-
genous lipid ligands,5 have fuelled a considerable
interest in the physiology and pharmacology of the
cannabinergic system.6 Agents that modulate the activ-
ity of this system may have a broad therapeutic poten-
tial: beside acute and persistent pain conditions,
additional therapeutic applications for CB1 and CB2
receptor agonists also may include stroke, glaucoma,
multiple sclerosis and spinal cord injury.7
9
noids), for example, 1, HU 21 0 (2, Fig. 1) and
structurally related molecules, for example CP 55,94010
(3, Fig. 1); (ii) N-aminoalkyl indoles (AAIs), for exam-
ple Win-55,212-211 (4, Fig. 2) and N-alkylindoles (non-
AAIs), for example JWH-00712 (5a, Fig. 2). A number
of compounds having an indene or pyrrole nucleus as their
basic feature has been also reported.8b These comprise
Huffman’s derivatives JWH-030 (7a) and 7b (Fig. 2).13
While the SAR of indole cannabimimetic agents have
been extensively studied, much remains to be done in
the area of pyrrole cannabinoids.
Molecular biology and theoretical studies have provided
important insights on the pharmacophoric interactions
occurring at cannabinoid receptors. The relatively high
CB1 affinity of the pentacyclic derivative JWH-161 (6,
Fig. 2), a hybrid structure in which the elements of
*Corresponding author. Tel.: +39-0722-328254; fax: +39-0722-2737;
e-mail: gat@uniurb.it
0968-0896/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0968-0896(03)00413-9

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G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
traditional and non-AAI cannabinoids are combined,14
supports the hypothesis that a common pharmacophore
exists for the two main classes of ligands.12 According
to this model, the 3-aroyl substituent of indole com-
pounds may mimic the cyclohexene ring of Á9-THC (1),
whereas the indole N-substituent and the 3-alkyl chain
of traditional cannabinoids may engage in lipophylic
interactions with the same region of the receptor.
abolishes the binding of Á9-THC (1) and 3, but not that
of Win-55,212-2. Interestingly, the affinity of the non-
AAI compound JWH-015 (5b, Fig. 2) is only partially
affected in the singly and doubly mutated receptor.16
Again, phenylalanine to valine mutation in the TM5
domain, F5.46, decreases the binding of Win-55,212-2.17
It is worth noting that the CB1 receptor affinity of Win-
55,212-2 is enhanced by substitution of a valine, corre-
sponding to CB2 F5.46, for phenylalanine. Such muta-
tions, however, did not affect the CB1 and CB2 binding
of traditional cannabinoids such as HU 210 (2) and CP
55,940 (3).17 Together with the results of docking
experiments,16,17 these observations suggest that hydro-
phobic aromatic interactions taking place in a region of
the receptor not occupied by traditional ligands may
play a crucial role in the binding of Win-55,212-2 and
related compounds to CB2 receptors, while polar inter-
actions through K192 and S112 may contribute to the
productive binding of ‘traditional’ ligands to CB1 and
CB2, respectively. The decisive role of aromatic stacking
interactions for cannabinoid binding has been
supported by two recent investigations.18
This hypothesis was questioned by experiments with
site-directed mutated receptors, which suggest that a
lysine in the third transmembrane (TM3) domain of
CB1, K192, is essential for the binding of CP 55,940 (3),
but not Win-55,212-2 (4).15 However, as Huffman and
co-workers pointed out, non-AAI were not included in
these tests, leaving open the possibility that the N-alkyl
group of these molecules may align to the side chain of
traditional cannabinoids, whereas the morpholine group
of Win-55,212-2 may bind to a different region of the
receptor.14 The CB2 subtype offers a somewhat different
scenario. In this case, K109, a lysine residue corre-
sponding to K192 in CB1, may not be essential for the
binding of either traditional or AAI ligands. On the
other hand, the double mutation K109A/S112G
In an attempt to extend Huffman’s observations
regarding the activity of pyrrole derivatives on CB
receptors, we have prepared and tested, on CB1 and CB2
receptors, a series of pyrroles with chemical modifi-
cations on positions 1, 3, and 4 of the heterocycle. Our
compounds were designed assuming that the pharmaco-
phoric interactions occurring at cannabinoid receptors
may be modelled by the superposition of the hybrid
ligand 6 with JWH-007 (5a) (see Fig. 3a), so that the
distal ring of the naphthoyl group of 5a corresponds to
the ‘ring A’ of traditional cannabimimetics.
In particular, the replacement, in compounds 10a,d, 11a
of the 3-naphthoyl, the standard C-3 substituent in
Huffman’s pyrrolic cannabimimetics (see compounds
7a,b,13 Fig. 2), with a benzoyl group, may give infor-
mation about the importance of the distal moiety of the
naphthoyl group itself. In principle, this group could be
replaced by fragments that fit the receptor in a similar
manner. Therefore, an N-(2-acetylphenyl)carboxamido
fragment, a planar pseudo-bicyclic substructure stabil-
ized by an intramolecular H bond, is attached in position
Figure 1. Representative ‘traditional’ cannabimimetic agents.
Figure 3. Superposition of 6 (yellow carbons) to 5a (a), 11a (b), 25 (c),
10d (d).
Figure 2. Representative ‘indole-based’ cannabimimetic agents.

G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
3967
3 of compound 21.19 Moreover, it should be possible to
place in position 3 substructures able to mimic the ‘ring
A’ of classical cannabinoids, for example the N-cyclo-
hexylcarboxamido group of compound 25 (see Fig. 3c).
Lastly, we attempted to substitute the naphthalene
moiety for a totally different structural element; there-
fore, in pyrrole 17 a linear alkyl chain is present bearing
an alcoholic function, possibly interacting with the
polar site of the receptor to which the 9- or 11-hydroxy
residue of traditional cannabinoids are assumed to bind.
congeners (5). It is commonly accepted that, compared
to indole compounds, pyrrole cannabimimetics are
endowed with a reduced affinity for CB1 receptor. This
conclusion, however, has been inferred from the analy-
sis of a limited number of structures, namely 4,5-
unsubstituted pyrroles.20
Another region of cannabinoid receptors, which is of
crucial importance for ligand binding, is the one corre-
sponding in our topographical model to the alkyl chain
attached to the pyrrole nitrogen. Compounds in which a
group bearing an aromatic ring replaces the typical
alkyl chain substituent may provide useful information
about the steric tolerance of this region. Therefore, we
prepared compounds 10d,e and 11d, whose p-chloro-
benzyl N-substitutent is a feature similar to that of dia-
rylpyrazolic CB2 antagonist SR144528.8c However,
most of our compounds retain an N-pentyl chain, a
group known to afford optimal cannabinoid binding to
non-AAI and pyrrole ligands;20,21 an N-propylic chain
was inserted in some cases (10c, 11c), since this shor-
tened alkyl fragment is reported to confer CB2 selectiv-
ity to AAI compounds.22
Concerning position 4, this could be substituted by a
group like bromo (as in 11a–d); these compounds, along
with the 4,5,6,7-tetrahydroindole derivative 15, in which
an alkyl chain connects positions 4 and 5 of the pyrrole
ring, are instrumental to further examine the concept
that pyrrole derivatives may be as active as their indole
Result and discussion
Compounds 10a–e and 11a–d were synthesized in a
straightforward manner according to Scheme 1. Thus,
aroylation of 2,5-dimethylpyrrole (8) in the presence of
aluminum chloride, followed by N-alkylation, afforded
compounds 10a–e; pyrroles 10a–d were then brominated
with N-bromosuccinimide to give 11a–d. The synthesis
of 4,5,6,7-tetrahydroindole derivative 15 (Scheme 2),
also proceeding smoothly, followed the same route
employed by Huffman and co-workers for the prepara-
tion of 1-pentyl-3-(1-naphtoyl)pyrrole (7a).13 Thus, 1223
was regioselectively acylated,24 deprotected by alkaline
treatment, and eventually alkylated in standard condi-
tions to give 15. Compound 17 was obtained from 2,5-
dimethyl-1-pentylpyrrole25 (16), prepared by the already
mentioned N-alkylation procedure, and g-butyrolactone
in polyphosphoric acid, according to the method of
Moussavi et al.26 (Scheme 3). Compounds 21 and 25
were prepared as outlined in Scheme 4. Thus, 2,5-dime-
thyl-3-pyrrole carboxylic acid ethyl ester27 (18), pre-
pared following a literature procedure for the synthesis
of the methyl ester analogue of 18,28 yielded, after
hydrolysis, the carboxylic acid 19,29 which was
transformed into the amide 20 via acid chloride. Finally,
Scheme 1. Reagents and conditions: (a) R1C(O)Cl, AlCl3, CH2Cl2,
25 ꢀC, 0.25–2 h; (b) NaH, DMF, n-alkylBr, 25 ꢀC, 3 h; (c) NBS, 1,4-
dioxane, AcOH, 25 ꢀC, 0.5 h for 11a,b,d and NBS, CH2Cl2, 25 ꢀC, 3 h
for 11c.
Scheme 2. Reagents and conditions: (a) R1C(O)Cl, AlCl3, CH3NO2,
CH2Cl2, 25 ꢀC, 4 h; (b) KOH, H2O–MeOH, reflux, 20 h; (c) NaH,
DMF, n-C5H11Br, 25 ꢀC, 5 h.
Scheme 3. Reagents and conditions: (a) NaH, DMF, n-C5H11Br,
25 ꢀC, 3 h; (b) PPA, g-butyrolactone, 135 ꢀC, 24 h.

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G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
human CB2 receptors. The former assay was conducted
using rat cerebellar membranes (27,000g), and the latter
using membranes of Chinese hamster ovary (CHO) cells
that overexpress CB2 receptors (Receptor Biology Inc.
Perkin Elmer, Wellesley, MA, USA) using [3H]Win-
55212-2 (NEN-Dupont, Boston, MA, USA, 40–60 Ci/
mmol, 10 nM) as a ligand.5a A summary of these results
is provided in Table 1. N-Pentyl-3-naphthoylpyrroles
10a, 11a, 15, which can be regarded as congeners of
Huffman’s pyrrole derivative 7a (Fig. 2), retain a mod-
erate to good affinity for CB receptors. In particular, the
concomitant presence of a C-2 and a C-5 methyl sub-
stituent, a distinctive feature of the present series of
pyrrole ligands influencing the conformational equili-
brium of the N-alkyl group, appears to be tolerated at
CB1, and to slightly increase the affinity at the CB2,
receptor subtypes. This trend, illustrated by compound
10a, which is 20 times more potent than 7a in CB2
binding affinity, persists over the entire group of our 1-
alkyl-3-naphthoyl pyrrole ligands which, therefore,
albeit having only a moderate preference for CB2
receptors, show a reversed CB1/CB2 selectivity, com-
pared with Huffman’s derivative 7a. Substituents in
position 4 cause different effects, depending on their
nature: in agreement with the topographic model
depicted in Fig. 3b, introduction of a bromo group
produces a slight increase in binding affinity for both
receptor subtypes (see 10a), whereas an unfavourable
effect is produced by the tetramethylene chain linking
positions 4 and 5 of compound 15. The substitution of
the 3-(1-naphthoyl) group for a benzoyl one is detri-
mental for affinity (10b,e, 11b), and this is consistent
with the previous observation that AAIs with mono-
cyclic aroyl nuclei in position 3 are far less active than
their 3-(1-naphthoyl) homologues.11,30 The replacement
of the C-3 naphthoyl substituent with groups of differ-
ent structural characteristics provides compounds with
reduced affinity for the cannabinoid receptors (21, 25),
or a complete loss of binding (17). In particular, the
N-(2-acetylphenyl)carboxamido group of 21 is only in
part, and only with CB1 receptor subtype, able to
reproduce the binding mode of the naphthoyl group. A
slightly better binding affinity, at least for the CB2 sub-
type, was obtained with compound 25. This supports
our hypothesis of a cycloalkyl fragment mimicking the
cyclohexene ring of classical cannabinoids, according to
the model of Figure 3c. Finally, the attempt to repro-
duce the interactions afforded by the hydroxy group of
classical cannabinoids by means of a hydroxyalkyl
chain (17) was unsuccessful. The N-propyl derivatives
10c and 11c proved to be less potent than the corre-
sponding N-pentyl analogues 10a and 11a. The decrease
of affinity is less marked for the CB2 receptor, resulting
in a certain degree of CB2 selectivity, which is consistent
with literature data,18,20 even though, in our pyrroles,
the CB2/CB1 affinity ratio enhancement caused by chain
shortening is not as prominent as that found in proto-
typic 3-aroyl indole cannabimimetics.31 Interestingly,
the replacement of the N-linear alkyl chain by a sub-
stituent of different steric and electronic nature, that is,
a p-chlorobenzyl group, yields compounds (10d, 11d)
that retain a certain affinity for cannabinoid receptors.
Indeed, such derivatives, especially relative to CB1
Scheme 4. Reagents and conditions: (a) see Ref. 29; (b) ClC(O)-
C(O)Cl, CH2Cl2, DMF, 25 ꢀC, 2 h; (c) 20-NH2C6H4C(O)CH3, 25 ꢀC, 6
h; (d) NaH, DMF, n-C5H11Br, 20 h for 21 or 3 h for 22; (e) LiOH–
H2O in H2O, EtOH, reflux, 24 h; (f) 20-NH2C6H4C(O)CH3, CH2Cl2,
DCC, 25 ꢀC, 24 h; (g) xylene, Et3N, reflux, 15 h.
N-alkylation of 20 gave the desired compound 21. We
originally conceived an alternative synthesis of this
compound (Scheme 4); thus, compound 24, a putative
immediate precursor of 21, was obtained from 18 by
N-alkylation, hydrolysis, and treatment with dicyclo-
hexylcarbodiimide. Even after prolonged heating, how-
ever, 24 did not react with 20-aminoacetophenone,
giving instead an intramolecular rearrangement to N-
cyclohexylcarboxamido pyrrole 25. We also attempted
to convert 23 to 21 via acid chloride. Unfortunately, by
treating 23 with oxalyl chloride/DMF in CH2Cl2, fol-
lowed by 20-aminoacetophenone, we obtained, after
workup, a mixture in which no trace of the desired
amide 21, and of unreacted starting material 23, was
present.
We used radioligand displacement assays to evaluate
the affinity of compounds 10a–e, 11a–d, 15, 17, 21, 25
for the native rat CB1 receptor and the recombinant

G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
3969
Table 1. Binding affinity values at rat native CB1 and human recombinant CB2 receptors
No.
R1
R2
R3
R4
R5
EC50 (nM) rCB1
EC50 (nM) hCB2
7a
n-C5H11
n-C5H11
n-C5H11
H
1-Naphthyl
1-Naphthyl
C6H5
1-Naphthyl
1-Naphthyl
C6H5
H
H
H
H
H
H
30.5ꢁ4.7
45.3ꢁ7.5
>1000
552ꢁ314
9.85ꢁ2.1
>1000
309.7ꢁ20.8
55.6ꢁ26.5
>1000
10a
10b
10c
10d
10e
11a
11b
11c
11d
15
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
n-C3H7
>1000
83.7ꢁ17.8
pClC6H4CH2
pClC6H4CH2
n-C5H11
n-C5H11
n-C3H7
pClC6H4CH2
n-C5H11
n-C5H11
n-C5H11
n-C5H11
H
>1000
13.3ꢁ0.5
>1000
1-Naphthyl
C6H5
Br
Br
Br
Br
6.8ꢁ1.0
>1000
1-Naphthyl
1-Naphthyl
1-Naphthyl
HO(CH2)3
o(CH3CO)C6H4NH
c-C6H11NH
780ꢁ326
691.3ꢁ101.3
194.5ꢁ27.5
139ꢁ55
38ꢁ7.2
(CH2)4
235.8ꢁ6.2
>3000
17
21
25
CH3
CH3
CH3
H
H
H
CH3
CH3
CH3
>10,000
>1000
367.3ꢁ31.2
415.5ꢁ79.5
483.5ꢁ211
Results are expressed as the meanꢁSEM of at least three independent experiments.
binding, are more potent than the short alkyl chain
analogues 10c, 11c. The binding mode of 10d and 11d
may be somewhat different from that of the remaining
N-alkylpyrroles, as suggested by the fact that introduc-
tion of a bromo group in position 4 does not increase
potency (cfr. 10a vs 10d, and 11a vs 11d).
In summary, our results extend previously established
SARs for indole and pyrrole cannabimimetics. In partic-
ular, we identified a 3-naphthoyl pyrrole 11a, which
displays a binding affinity and intrinsic activity com-
parable to that of 3-naphthoyl indoles. This suggests
that a suitable lipophylic group, attached to position 4
of the pyrrole nucleus, can compensate for the lack of
contribution of the benzo moiety present in indole
compounds. The steric and electronic characters of this
substituent seem to be strictly defined, as may be infer-
red from the reduced affinity of 4,5,6,7-tetrahydroindole
15. Compound 11a, the most active of our series, does
not display a significant CB1/CB2 selectivity; this paral-
lels, however, the behaviour of N-pentyl cannabimi-
metic indoles.31 Furthermore, the testing of several
compounds that have structural elements unusual in
We also investigated the intrinsic activity of selected
high-affinity ligands at CB1 receptors by testing their
ability to stimulate [35S]GTPgS binding in rat cerebellar
membranes. Figure 4 illustrates the effects of various
pyrrole-based compounds on the binding of [35S]GTPgS
to rat cerebellar membranes. The compound 11a stimu-
lates [35S]GTPgS binding with an EC50 value of
140.3ꢁ8.2 nM (meanꢁSEM, n=9) and a maximal
degree of stimulation (238ꢁ18%) identical to that of
Win-55,212-2 (Fig. 4). We obtained similar results with
compound 10a, which stimulates [35S]GTPgS binding
with an EC50 value of 324.0ꢁ20.8 nM. These findings
suggest that 11a and 10a are full agonist ligands at rat
CB1 with an efficacy comparable to that of known
cannabimimetic agents.32 By contrast, the compounds
11d (Fig. 4) and 10d (data not shown), which enhance
[35S]GTPgS binding with EC50 values of 186.3ꢁ42.2
and 179.3ꢁ69 nM, respectively, produce only a fraction
of the maximal [35S]GTPgS binding stimulation induced
by Win-55,212-2. Therefore, these compounds may be
considered as partial CB1 agonists. These results sup-
port the idea that it is possible to modulate the
pharmacological properties of cannabimimetic pyrroles
by suitable elaboration of their N-substituents. Finally,
the compounds 21 and 10c are very weak at stimulating
[35S]GTPgS binding (EC50 values 4052ꢁ4 and
4004ꢁ603 nM, respectively). This failure may be due
either to their modest affinity for CB1 receptors and/or
to a lack of efficacy.
Figure 4. Effects of various cannabinoid receptor ligands on [35S]-
GTPgS binding to rat cerebellar membranes. Shown are the effects
of Win-55,212-2 (closed circles), 11a (open squares) and 11d (open
circles).

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G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
heterocyclic cannabinoid ligands provided further
insights to the topography of cannabinoid receptor
binding sites. Lastly, the fact that the N-chlorobenzyl
compound 10d behaves as a partial agonist, indicates
that the standard linear alkyl chain of non-AAI can be
substituted with groups that interact with an area of the
receptor involved in modulating ligand efficacy.
CH2Cl2. The combined organic layers were dried
(Na2SO4) and concentrated. Purification of the residue
by column chromatography (cyclohexane/EtOAc 6:4)
gave 9a,b.
(2,5-Dimethyl-1H-pyrrol-3-yl)(naphthalen-1-yl)methanone
(9a).36 Light-yellow crystals. Yield: 39% (1.459 g). Mp
163–164 ꢀC (EtOAc) (lit.: 165–167 ꢀC).36
(2,5-Dimethyl-1H-pyrrol-3-yl)(phenyl)methanone (9b).37
Pale brown amorphous solid. Yield: 28% (0.839 g). MS
(EI) is in agreement with literature.37
Conclusions
Some of the pyrrole derivatives described in this study
exhibit interesting affinity and efficacy profiles for can-
nabinoid receptors. Functional tests indicate that these
compounds may behave as full or partial agonists at the
CB1 receptor subtype, depending on the presence of
specific substituents. Further investigations will be
necessary to optimize the affinity and efficacy of the
present class of compounds, and to explore in more
detail their SAR properties.
General procedure for the synthesis of 1-alkyl-2,5-dime-
thyl-3,4-(un)substituted pyrroles (10a–e, 15, 16, 21, 22).
To a stirred, cooled (0 ꢀC) solution of the convenient
pyrrole 8, 9a,b, 14, 18, 20 (5 mmol) in dry DMF (12.5
mL) under N2 atmosphere, NaH (0.173 g of an 80%
mineral oil dispersion, 5.75 mmol) was added. When H2
evolution had ceased, the opportune 1-bromoalkane
was added (5.75 mmol). After stirring the mixture for 2
h at room temperature a further amount of NaH (0.087
g, 2.88 mmol; 0.043 g, 1.44 mmol in the case of 16) and
1-bromoalkane (2.88 mmol; 1.44 mmol in the case of
16) were added and the mixture again allowed to react
Experimental
Chemistry
for 1h. CH Cl2 and H2O were then cautiously added
2
All chemicals were purchased from Aldrich in the high-
est quality commercially available. Solvents were RP
grade, unless otherwise indicated. Chromatographic
separations were performed on silica gel columns by
flash chromatography (Kieselgel 60, 0.040–0.063 mm,
Merck). TLC analyses were performed on precoated
and the organic layer washed with H2O, dried (Na2SO4)
and concentrated. Purification of the residue by column
chromatography (cyclohexane/EtOAc 9:1; 85:15 for
10a; 8:2 for 10c,e and 22; 95:5 for 16) gave 10a,b,d,e, 15,
and 21 as solids, and 10c, 16, 22 as oils. In the case of 21
a few modifications to the procedure were adopted;
thus, only 5 mmol of NaH was employed, and after the
addition of 1-bromopentane (5 mmol) the mixture was
allowed to react for 20 h, then worked up as above
(chromatography: cyclohexane/EtOAc 8:2, then 1:1).
silica gel on aluminium sheets (Kieselgel 60 F254
,
Merck). Melting points were determined on a Buchi
SMP-510 capillary melting point apparatus, unless
otherwise indicated, and are uncorrected. EI-MS ana-
lyses (70 eV) were recorded with a Fisons Trio 1000
spectrometer; only molecular ions (M+) and base peaks
(2,5-Dimethyl-1-pentyl-1H-pyrrol-3-yl)(naphthalen-1-yl)-
methanone (10a). White crystals. Yield: 95% (1.518 g).
Mp 48–50 ꢀC (cyclohexane). MS (EI): m/z 319 (M+),
1
are given. H NMR spectra were recorded on a Bruker
AC 200 spectrometer using CDCl3 as solvent; chemical
shifts (d scale) are reported in part per million (ppm)
relative to the central peak of the solvent; coupling
costants (J values) are given in hertz (Hz). IR spectra
were obtained on a Shimadzu FT-8300, or a Nicolet
Avatar, spectometer; absorbances are reported in n
(cmꢂ1). Elemental analyses were performed on a Carlo
Erba analyzers.
1
155 (100). H NMR (CDCl3): d 0.94 (t, 3H); 1.37 (m,
4H); 1.68 (m, 2H); 2.14 (s, 3H); 2.61 (s, 3H); 3.79 (t,
2H); 5.84 (d, 1H); 7.48 (m, 4H); 7.89 (m, 2H); 8.11 (m,
1H) ppm. IR (nujol): 1631, 1616 cmꢂ1. Anal. calcd for
C22H25NO (319.45): C, 82.72; H, 7.89; N, 4.38. Found:
C, 82.76; H, 7.93; N, 4.36.
(2,5-Dimethyl-1-pentyl-1H-pyrrol-3-yl)(phenyl)methanone
(10b). White crystals. Yield: 90% (1.211 g). Mp 55–
56 ꢀC (after trituration with petroleum ether/Et2O 9:1).
Molecular modeling
1
Three-dimensional models of the molecules were built
with Sybyl 6.7 software package33 and their geometry
was optimized using the standard Tripos force field,34
with the Powell method35 to an energy gradient of 0.01
Kcal/mol A, ignoring the electrostatic contribution.
MS (EI): m/z 269 (M+, 100). H NMR (CDCl3): d 0.94
(t, 3H); 1.36 (m, 4H); 1.67 (m, 2H); 2.22 (s, 3H); 2.59 (s,
3H); 3.80 (t, 2H); 6.07 (s, 1H); 7.42 (m, 3H); 7.79 (m,
2H) ppm. IR (nujol): 1625 cmꢂ1. Anal. calcd for
C18H23NO (269.39): C, 80.26; H, 8.61; N, 5.20. Found:
C, 79.81; H, 8.63; N, 5.13.
General procedure for the synthesis of 3-aroyl-2,5-dime-
thylpyrroles (9a,b). To a stirred, cooled (0 ꢀC) solution
of 2,5-dimethylpyrrole (8) (15 mmol) and the opportune
aroyl chloride (15 mmol) in CH2Cl2 (15 mL), AlCl3 (2 g;
15 mmol) was cautiously added. The mixture was stirred
at room temperature for 15 min, quenched with a
cooled saturated NaHCO3 solution, and extracted with
(2,5-Dimethyl-1-propyl-1H-pyrrol-3-yl)(naphthalen-1-yl)-
methanone (10c). Yellow oil. Yield: 82% (1.194 g). MS
1
(EI): m/z 291(M +, 100). H NMR (CDCl3): d 1.01 (t,
3H); 1.71 (m, 2H); 2.14 (s, 3H); 2.61 (s, 3H); 3.77 (t,
2H); 5.84 (s, 1H); 7.48 (m, 4H); 7.88 (m, 2H); 8.12 (m,
1H) ppm. IR (CHCl3): 1627, 1618 cmꢂ1. Anal. calcd for

G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
3971
C20H21NO (291.39): C, 82.44; H, 7.26; N, 4.81. Found:
C, 82.22; H, 7.40; N, 4.87.
was added portionwise and the reactants were stirred at
room temperature for 30 min. The mixture was then
poured onto a cooled (0 ꢀC), dilute solution of NaOH,
and extracted with CH2Cl2. The combined organic lay-
ers were washed with 2 N HCl and brine, dried
(Na2SO4), and concentrated. Purification of the residue
by column chromatography (cyclohexane/EtOAc 85:15)
and recrystallization gave 11a,b,d.
1-(4-Chlorobenzyl)-2,5-dimethyl-1H-pyrrol-3-yl naphtha-
len-1-yl methanone (10d). Pale yellow crystals. Yield:
83% (1.552 g). Mp 105–106 ꢀC (i-Pr2O). MS (EI): m/z
1
373 (M+), 125 (100). H NMR (CDCl3): d 2.06 (s, 3H);
2.51(s, 3H); 5.06 (s, 2H); 5.97 (s, 1H); 6.89 (d, 2H);
7.27–7.64 (m, 6H); 7.91 (m, 2H); 8.16 (m, 1H) ppm. IR
(CHCl3): 1628 cmꢂ1. Anal. calcd for C24H20ClNO
(373.88): C, 77.10; H, 5.39; N, 3.82. Found: C, 77.00; H,
5.38; N, 3.82.
(4-Bromo-2,5-dimethyl-1-pentyl-1H-pyrrol-3-yl)(naphtha-
len-1-yl)methanone (11a). White crystals. Yield: 15%
(0.060 g). Mp 74–77 ꢀC (Et2O/petroleum ether). MS
1
(EI): m/z 398 (M+), 318 (100). H NMR (CDCl3): d
[1-(4-Chlorobenzyl)-2,5-dimethyl-1H-pyrrol-3-yl](phenyl)-
methanone ꢀ(10e). White crystals. Yield: 92% (1.491 g).
Mp 93–95 C (cyclohexane). MS (EI): m/z 323 (M+),
125 (100). H NMR (CDCl3): d 2.13 (d, 3H, J=0.74
Hz); 2.50 (s, 3H); 5.07 (s, 2H); 6.18 (d, 1H, J=0.74 Hz);
0.93 (t, 3H); 1.35 (m, 4H); 1.65 (m, 2H); 2.22 (s, 3H);
2.31(s, 3H); 3.82 (t, 2H); 7.52 (m, 4H); 7.87 (m, 2H);
8.22 (m, 1H) ppm. IR (nujol): 1627, 1618 cmꢂ1. Anal.
calcd for C22H24BrNO (398.34): C, 66.34; H, 6.07; N,
3.52. Found: C, 66.32; H, 6.12; N, 3.48.
1
6.88 (d, 2H); 7.29–7.50 (m, 5H); 7.82 (m, 2H) ppm. IR
(Nujol): 1624 cmꢂ1. Anal. calcd for C20H18ClNO
.
0.25H2O (328.33): C, 73.17; H, 5.68; N, 4.27. Found: C,
72.99; H, 5.55; N, 4.22.
(4-Bromo-2,5-dimethyl-1-pentyl-1H-pyrrol-3-yl)(phenyl)-
methanone (11b). White crystals. Yield: 22% (0.077 g).
Mp 80–81 ꢀC (Et2O/petroleum ether). MS (EI): m/z 348
(M+), 105 (100). 1H NMR (CDCl3): d 0.94 (t, 3H); 1.37
(m, 4H); 1.65 (m, 2H); 2.24 (s, 3H); 2.28 (s, 3H); 3.82 (t,
2H); 7.42 (m, 3H); 7.77 (m, 2H) ppm. IR (nujol): 1617
cmꢂ1. Anal. calcd for C18H22BrNO (348.28): C, 62.08;
H, 6.37; N, 4.02. Found: C, 61.88; H, 6.36; N, 3.99.
(Naphthalen-1-yl)(1-pentyl-4,5,6,7-tetrahydro-1H-indol-
3-yl)methanone (15). Pale yellow amorphous solid.
Yield: 77% (1.329 g). MS (EI): m/z 345 (M+), 155
1
(100). H NMR (CDCl3): d 0.95 (t, 3H); 1.42 (m, 4H);
1.71 (m, 2H); 1.87 (m, 4H); 2.40 (t, 2H); 2.67 (t, 2H);
4.43 (t, 2H); 6.26 (s, 1H); 7.51 (m, 4H); 7.89 (m, 2H);
8.09 (m, 1H) ppm. IR (nujol): 1617 cmꢂ1. Anal. calcd
for C24H27NO (345.48): C, 83.44; H, 7.88; N, 4.05.
Found: C, 83.57; H, 8.03; N, 4.05.
[4-Bromo-1-(4-chlorobenzyl)-2,5-dimethyl-1H-pyrrol-3-
yl](naphthalen-1-yl)methanone (11d). White crystals.
Yield: 41% (0.186 g). Mp 119–121 ꢀC (subl.) (CH2Cl2/
petroleum ether). MS (EI): m/z 372, 125 (100). 1H NMR
(CDCl3): d 2.13 (s, 3H); 2.20 (s, 3H); 5.07 (s, 2H); 6.89
(d, 2H); 7.32–7.65 (m, 6H); 7.93 (m, 2H); 8.28 (m, 1H)
2,5-Dimethyl-1-pentyl-1H-pyrrole (16). Colorless oil.
Yield: 20% (0.165 g). MS (EI): m/z 165 (M+), 108 (100).
The product was not fully characterized, owing to the
impossibility to obtain a pure sample of it (apparently, 16
is not stable in our chromatographic conditions).
ppm. IR (KBr): 1628 cmꢂ1
.
Anal. calcd for
C24H19BrClNO.0.05CH2Cl2 (457.62): C, 63.25; H, 4.25;
N, 3.06. Found: C, 63.16; H, 4.19; N, 3.10.
Synthesis of (4-bromo-2,5-dimethyl-1-propyl-1H-pyrrol-
3-yl)(naphthalen-1-yl)methanone (11c). To a stirred
solution of 10c (0.293 g, 1mmol) in CH Cl2 (6 mL), N-
2
2,5-Dimethyl-1-pentyl-1H-pyrrol-3-carboxylic acid (2-
acetylphenyl)amide (21). White crystals. Yield: 50%
(0.816 g). Mp 78–79 ꢀC (petroleum ether). MS (EI): m/z
bromosuccinimmide (0.178 g, 1 mmol) was added por-
tionwise and the reactants were allowed to react at
room temperature for 3 h. The mixture was then poured
onto a cooled (0 ꢀC) 2N NaOH solution, and extracted
with CH2Cl2. The combined organic layers were washed
with 2 N HCl and brine, dried (Na2SO4), and con-
centrated. Purification of the residue by column
chromatography (cyclohexane/EtOAc 9:1) and recrys-
tallization gave 11c as a white solid. Yield: 94% (0.348
g). Mp 92 ꢀC (with decomposition, EtOH). MS (EI): m/z
1
326 (M+), 192 (100). H NMR (CDCl3): d 0.93 (t, 3H);
1.35 (m, 4H); 1.51 (m, 2H); 2.27 (s, 3H); 2.62 (s, 3H);
2.70 (s, 3H); 3.77 (t, 2H); 6.41(s, 1H); 7.05 (t, 1H); 7.55
(t, 1H); 7.91 (d, 1H); 8.95 (d, 1H), 12.10 (br s, 1H) ppm.
IR (KBr): 3274, 1668, 1645, 1606 cmꢂ1. Anal. calcd for
C20H26N2O2 (326.44): C, 73.59; H, 8.03; N, 8.58.
Found: C, 74.12; H, 7.78; N, 8.92.
2,5-Dimethyl-1-pentyl-1H-pyrrol-3-carboxylic acid ethyl
ester (22). Yellow oil. Yield: 88% (1.046 g). MS (EI):
1
370 (M+), 290 (100). H NMR (CDCl3): d 0.99 (t, 3H);
1
m/z 237 (M+), 108 (100). H NMR (CDCl3): d 0.92 (t,
1.70 (m, 2H); 2.22 (s, 3H); 2.31 (s, 3H); 3.80 (t, 2H); 7.51
(m, 4H); 7.91(m, 2H); 8.23 (m, 1H) ppm. IR (neat):
3H); 1.33 (m, 7H); 1.62 (m, 2H); 2.20 (s, 3H); 2.52 (s, 3H);
3.74 (t, 2H); 4.24 (q, 2H); 6.25 (s, 1H) ppm. IR (neat):
1694 cmꢂ1. The product tends to decompose on standing.
1634 cmꢂ1. Anal. calcd for C20H20BrNO 0.25c-C6H12
.
(391.33): C, 65.99; H, 5.92; N, 3.58. Found: C, 65.65; H,
5.70; N, 3.64.
General procedure for the synthesis of 1-alkyl-3-aroyl-4-
bromo-2,5-dimethylpyrroles (11a,b,d)
Synthesis of (1-benzenesulfonyl-4,5,6,7-tetrahydro-1H-in-
dol-3-yl)(naphthalen-1-yl)methanone (13). To a stirred
solution of AlCl3 (0.320 g, 2.4 mmol) in CH3NO2 (1.6
mL), a solution of 4,5,6,7-tetrahydro-1-phenylsulpho-
nylindole (12) (0.523 g, 2 mmol) in CH2Cl2 (13 mL),
To a stirred solution of the convenient pyrrole 10a,b,d
(1mmol) in a 2:1solution of dioxane and glacial acetic
acid (4.5 mL), N-bromosuccinimmide (0.178 g, 1 mmol)

3972
G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
then 1-naphthoyl chloride (0.458 g, 0.36 mL, 2.4 mmol)
were added and the reactants were allowed to react at
room temperature for 4 h. The mixture was then poured
onto ice and H2O and extracted with CH2Cl2. The
combined organic layers were washed with 5% aqueous
NaHCO3, dried (Na2SO4), and concentrated. Puri-
fication of the residue by column chromatography
(cyclohexane/EtOAc 85:15) gave 13 as a pale brown
foamy solid. Yield: 44% (0.366 g). MS (EI): m/z 415
(0.282 g). Mp 180–182 ꢀC (EtOH). MS (EI): m/z 256
(M+), 122 (100). 1H NMR (CDCl3): d 2.28 (s, 3H); 2.59
(s, 3H); 2.70 (s, 3H); 6.39 (s, 1H); 7.07 (t, 1H); 7.56 (t,
1H); 7.90 (br s, 1H); 7.92 (d, 1H); 8.95 (d, 1H); 12.13 (br
s, 1H) ppm. IR (KBr): 3308, 3276, 1653, 1641, 1608
cmꢂ1. Anal. calcd for C15H16N2O2 (256.30): C, 70.29;
H, 6.29; N, 10.93. Found: C, 70.59; H, 6.12; N, 11.10.
Synthesis of 2,5-dimethyl-1-pentyl-1H-pyrrole-3-car-
boxylic acid (23). To a stirred solution of 22 (0.237 g, 1
mmol) in EtOH (8 mL), LiOH.H2O (0.168 g, 4 mmol) in
H2O (2 mL) was added, and the reactants refluxed for
24 h. The mixture was then cooled to room tempera-
ture, poured onto H2O, washed with EtOAc to remove
side products, acidified with 2 N HCl, and extracted
with EtOAc. The organic layers were dried (Na2SO4),
and concentrated. Recrystallization of the residue gave
23 as a white solid. Yield: 72% (0.151 g). Mp 163–
165 ꢀC (EtOH) (with decarboxylation). MS (EI): m/z
1
(M+), 274 (100). H NMR (CDCl3): d 1.80 (m, 4H);
2.37 (t, 2H); 3.06 (t, 2H); 6.31(s, 1H); 7.39–7.76 (m,
8H); 7.93 (m, 2H); 8.30 (m, 2H) ppm. The product tends
to decompose on standing.
Synthesis of (naphthalen-1-yl)(4,5,6,7-tetrahydro-1H-in-
dol-3-yl)methanone (14). A stirred suspension of 13
(0.332 g, 0.8 mmol) and KOH (0.090 g, 1.6 mmol) in a
5:8 mixture of H2O and MeOH (3 mL) was refluxed for
20 h, and then concentrated. The residue was dissolved
in H2O/CH2Cl2, and the organic layer cautiously
washed with 2 N HCl, dried (Na2SO4), and con-
centrated. Purification of the residue by column
chromatography (cyclohexane/EtOAc 9:1) and recrys-
tallization gave 14 as a yellow solid. Yield: 71% (0.148
g). Mp 196–198 ꢀC (EtOAc). MS (EI): m/z 275 (M+,
1
209 (M+), 152 (100). H NMR (CDCl3): d 0.92 (t, 3H);
1.35 (m, 4H); 1.63 (m, 2H); 2.21 (s, 3H); 2.53 (s, 3H);
3.75 (t, 2H); 6.31(d, 1H) ppm. IR (KBr): 2994-2587,
1648 cmꢂ1
.
Synthesis of 2,5-dimethyl-1-pentyl-1H-pyrrole-3-car-
boxylic acid cyclohexyl amide (25). To a stirred, cooled
(0 ꢀC) solution of 23 (0.126 g, 0.6 mmol) and 1-(2-ami-
nophenyl) ethanone (0.081g, 0.07 mL, 0.6 mmol) in
CH2Cl2 (6 mL), DCC (0.124 g, 0.6 mmol) was added,
and the mixture allowed to react at room temperature
for 24 h. After addition of EtOAc, the mixture was fil-
tered, and the filtrate washed with NaHCO3 saturated
solution. The organic layer was dried (Na2SO4), and
concentrated. Purification of the residue by column
chromatography (cyclohexane/EtOAc 8:2) afforded
pure 1,3-dicyclohexyl-2-(2,5-dimethyl-1-pentyl-1H-pyr-
role-3-carbonyl)isourea (24) [MS (EI): m/z 415 (M+),
192 (100)], which was dissolved in xylene (1.2 mL),
added of Et3N (0.6 mL), refluxed overnight under stir-
ring, and concentrated. Purification of the residue by
column chromatography (cyclohexane/EtOAc 8:2) and
recrystallization gave 25 as pale yellow solid. Yield:
38% (0.066 g). Mp 88–89 ꢀC (petroleum ether). MS
1
100). H NMR (CDCl3): d 1.81 (m, 4H); 2.47 (t, 2H);
2.72 (t, 2H); 6.40 (d, 1H); 7.52 (m, 3H); 7.73 (m, 1H);
7.93 (m, 2H); 8.22 (m, 1H); 9.47 (br s, 1H) ppm. IR
(nujol): 3263, 1578 cmꢂ1. Anal. calcd for C19H17NO
(275.35): C, 82.88; H, 6.22; N, 5.09. Found: C, 82.67; H,
6.05; N, 5.16.
Synthesis of 1-(2,5-dimethyl-1-pentyl-1H-pyrrol-3-yl)-4-
hydroxybutan-1-one (17). To a stirred solution of 16
(0.495 g, 3 mmol) in polyphosphoric acid (6 mL),
g-butyrolactone (0.258 g, 0.24 mL, 3 mmol) was added,
and the reactants heated at 135 ꢀC for 24 h. The mixture
was then cooled at room temperature, poured onto H2O
and extracted with EtOAc. The combined organic layers
were dried (Na2SO4), and concentrated. Purification of
the crude material residue by column chromatography
(cyclohexane/EtOAc 9:1, then 1:1) gave 17 (0.036 g) as a
pale brown oil, together with a consistent amount of
unreacted 16 (0.294 g). Yield 12%, calculated on the
reacted 16. MS (EI): m/z 251(M +), 192 (100). 1H NMR
(CDCl3): d 0.92 (t, 3H); 1.34 (m, 4H); 1.60 (m, 2H); 1.94
(m, 2H); 2.21(s, 3H); 2.55 (s, 3H); 2.73 (br d, 1H); 2.89
(t, 2H); 3.75 (m, 4H); 6.23 (s, 1H) ppm. IR (CHCl3):
3393, 1636 cmꢂ1. The purity of the sample was ascer-
tained by NMR.
1
(EI): m/z 290 (M+), 192 (100). H NMR (CDCl3): d
0.91 (t, 3H); 1.07–1.68 (m, 14H); 1.98 (m, 2H); 2.20 (s,
3H); 2.55 (s, 3H); 3.73 (t, 2H); 3.88 (m, 1H); 5.52 (br d,
1H); 5.91 (s, 1H) ppm. IR (KBr): 3318, 3254, 1612
cmꢂ1. Anal. calcd for C18H30N2O (290.45): C, 74.44; H,
10.41; N, 9.64. Found: C, 74.43; H, 10.51; N, 9.64.
Synthesis of 2,5-dimethyl-1H-pyrrole-3-carboxylic acid
(2-acetylphenyl)amide (20). To a stirred, cooled (0 ꢀC),
suspension of 2,5-dimethyl-1H-pyrrole-3-carboxylic
acid (19) (0.696 g, 5 mmol) in CH2Cl2 (60 mL), oxalyl
chloride (0.762 g, 0.52 mL, 6 mmol) was added, fol-
lowed by a catalytic amount of DMF; the mixture was
warmed to room temperature, stirred for 2 h, and, after
addition of 1-(2-aminophenyl) ethanone (1.014 g, 0.90
mL, 7.5 mmol), stirred for another 6 h, and eventually
concentrated. Purification of the residue by column
chromatography (cyclohexane/EtOAc 8:2, then 7:3) and
recrystallization gave 20 as a white solid. Yield: 22%
Acknowledgements
This work was supported by grants from MIUR (Min-
istero dell’Istruzione, dell’Universita’e della Ricerca);
Universities of Parma and Urbino; and the National
Institute of Drug Abuse (S.K. and D.P.). The CCE
(Centro di Calcolo Elettronico) and CIM (Centro
Interfacolta Misure) of the University of Parma are
gratefully acknowledged for supplying the Sybyl soft-
ware. Thanks are due to Dr. Giuseppe Gatti for assis-
tance with NMR characterization of compound 21.

G. Tarzia et al. / Bioorg. Med. Chem. 11 (2003) 3965–3973
3973
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