Heteroarylisopropylamines as MAO inhibitors

Article abstract of DOI:10.1016/j.bmc.2005.04.045

The in vitro monoamine oxidase inhibitory (MAOI) activities of 11 heteroarylisopropylamines vis-a-vis MAO-A and MAO-B were described and interpreted in terms of possible interactions with the enzyme active site. Molecular dynamics simulations allowed a comparison between the most active MAO-A inhibitor of the series, the 1-(2-benzofuryl)-2-aminopropane, and the specific, analogous MAO-A substrate serotonin.

Full text of DOI:10.1016/j.bmc.2005.04.045

Bioorganic & Medicinal Chemistry 13 (2005) 4450–4457
Heteroarylisopropylamines as MAO inhibitors
a
a
a,
*
´
Gabriel Vallejos, Angelica Fierro, Marcos Caroli Rezende,
Silvia Sepu´lveda-Bozab and Miguel Reyes-Paradab
a
´
´
Facultad de Quımica y Biologıa, Universidad de Santiago, Casilla 40, Correo 33, Santiago, Chile
Facultad de Ciencias Medicas, Universidad de Santiago, Casilla 442, Correo 2, Santiago, Chile
b
´
Received 6 January 2005; revised 18 April 2005; accepted 18 April 2005
Available online 23 May 2005
`
Abstract—The in vitro monoamine oxidase inhibitory (MAOI) activities of 11 heteroarylisopropylamines vis-a-vis MAO-A and
MAO-B were described and interpreted in terms of possible interactions with the enzyme active site. Molecular dynamics simula-
tions allowed a comparison between the most active MAO-A inhibitor of the series, the 1-(2-benzofuryl)-2-aminopropane, and the
specific, analogous MAO-A substrate serotonin.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
introduction of electron-withdrawing substituents like
nitro, trifluoromethyl, alkylsulfoxyl and sulfonyl,
markedly reduced their MAOI activity was taken as
an indication of possible HOMO–LUMO interactions
between these compounds and aromatic fragments at
the active site.6
Considerable progress has been made in the past years
in deciphering the mechanisms of action and inhibition
of MAO. The human MAO-B isoform has been
crystallized with a variety of inhibitors.1–3 The structure
of a rat MAO-A bound with clorgyline has been
recently elucidated from X-ray diffraction analysis.4
Replacement of an aryl by a heteroaryl group has led in
some cases to interesting, novel MAO inhibitors. This,
for instance, was the case of a 5-hydroxyindole deriva-
tive related to pargyline, which was found to be more
potent and selective than the MAO-A inhibitor clorgy-
line.10 For a series of aryloxazolidinones, the introduc-
tion of a benzothiazolyl ring led to a potent and
selective MAO-A inhibitor.11,12
These developments have made possible
a more
detailed understanding of the binding between mono-
amine oxidase inhibitors and the protein. Computa-
tional simulations coupled with structure–activity
studies may now be used to mimic interactions in the
active site of the enzyme and to understand the subtle
factors, that govern MAO inhibitory activity and
selectivity.5
As a natural extension of our studies on arylalkylam-
ines, we decided to evaluate some heteroaromatic sys-
tems and compare their activity with that of their
homoaromatic analogues. A few thienyl derivatives
had been assessed in vitro previously as MAO inhibi-
tors.13 Some chlorinated derivatives showed a moderate
to good (20–70%) MAO inhibition at 0.2 lM concentra-
tion. Since then, no other studies have appeared in the
literature dealing with the MAOI activity of this family
of compounds.
SAR studies on the activity and selectivity of arylalkyl-
amines as MAO inhibitors have been a subject of
recent interest.6–9 Our previous QSAR studies of
substituted phenylisopropylamines have identified some
features responsible for an increased monoamine oxi-
dase inhibitory (MAOI) activity.6,7 Soft, lipophilic thio-
alkyl substituents at position 4 of the aromatic ring
tended to increase their activity. The fact that the
Our previous work with substituted arylalkylamines
emphasized the importance of interactions between the
aromatic rings of the pharmacophore and the hydro-
phobic fragments in the active site of the enzyme.6,7
Keywords: MAO inhibitors; Heteroarylisopropylamines; Molecular
dynamics simulation.
*
Corresponding author. Tel.: +56 2 681 2575; fax: +56 2 681 2108;
e-mail: mcaroli@lauca.usach.cl
0968-0896/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmc.2005.04.045

G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
4451
The present report, dealing with heteroaromatic
CHO
Br
CHO
AlCl3
py
Br2
systems, aims at contributing to a better understanding
of these interactions by taking advantage of the recently
elucidated structure of the MAO isoforms.
MeO
MeO
OH
OH
1
2. Results
Br
CHO
Br
CHO
BrCH2CH2Br
The racemic heteroarylaminopropanes, obtained in the
form of the hydrochloride salts 15–25, were prepared
from the corresponding heteroarylcarboxaldehydes by
conversion into the intermediate nitropropenes 4–14
and reduction with lithium aluminium hydride.
Na2CO3 / CuO
O
HO
O
OH
2
3
obtained MAOI activities, expressed as IC50 values,
are given in Table 1. For the sake of comparison, the
percentage of MAO inhibition at 100 lM (the highest
concentration of the inhibitors tested) is also given for
all compounds with an IC50 >100.
NO2
EtNO2
BuNH2
NH2.HCl
i) LiAlH4
ii) HCl
Het
Het CHO
Het
Me
Me
Het
4–14
15–25
O
4
15
The possible time-dependence of MAO inhibitory acti-
vity was assessed by preincubating the reaction mixture
with different compounds at appropriate concentrations
for 30 min, and then measuring the enzymatic activity.
Table 2 shows the results obtained for some representa-
tive compounds after preincubation (0 and 30 min) with
MAO-A.
O
Br
5
16
O
O
6
7
17
18
3. Discussion
S
Inspection of Table 1 shows that most of the hetero-
arylaminopropanes are selective MAO-A inhibitors.
Among the best inhibitors (IC50 values <100), the only
exception was compound 22, which exhibited, in
addition to its MAO-A inhibitory activity, an interesting
MAO-B inhibitory activity. Such selectivity is shared
with the majority of other previously evaluated 1-aryl-
2-amino- propanes.6,7,14,15
8
19
20
21
22
23
24
25
9
O
Br
10
11
12
13
14
S
Thienyl derivatives were in general better MAO-A
inhibitors than their furyl analogues. This emerges from
a comparison of pairs 17/19 and 22/23 in Table 1. Also,
among the less active compounds (IC50 >100), the 3-thi-
enyl derivative 18 is a better inhibitor (30% inhibition)
than the furyl analogue 20 (18% inhibition) at a
100 lM concentration. This trend parallels the greater
MAO-A inhibitory activity in the series of 1-aryl-2-
Br
Br
S
O
aminopropanes of compounds substituted with
a
S
thioalkyl group, when compared with their oxygen
analogues.6,14 Substitution by sulfur renders an aro-
matic ring softer and more polarizable than analogous
substitution by oxygen. Hydrogen-bond formation with
the hard oxygen atom is also greatly diminished in a sul-
fur analogue.
O
Most aldehydes were commercial. An exception was
3-bromo-4,5-ethylenedioxybenzaldehyde, which was
Substitution of the heteroaromatic (thienyl or furyl) ring
with a bromine atom led to an improvement of the
inhibitory activity of the parent aminopropanes, as can
be seen in a comparison of compounds 21 and 22 with
17 or 23 with 19 in Table 1. A similar trend had been re-
ported for ring-chlorinated thienylisopropylamine deriva-
prepared
(vanillin) by the route below.
from
4-hydroxy-3-methoxybenzaldehyde
The prepared heteroarylaminopropanes were assayed in
vitro as inhibitors of the two MAO isoforms. The

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G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
Table 1. In vitro MAO-A and MAO-B inhibitory activities of aminopropane hydrochlorides 15–25
Compd
Heteroaryl group
IC50 (lM)a
MAO-A
MAO-B
15
16
17
18
19
20
21
22
23
24
25
3,4-Ethylenedioxyphenyl
3,4-Ethylenedioxy-5-bromophenyl
2-Thienyl
11.0 2.4
18.3 3.0
64.4 4.0
>100 [30.1]b
>100 [20.6]b
>100 [18.3]b
17.8 0.4
15.8 1.6
44.8 0.8
16.2 0.4
0.8 0.1
>100 [16.4]b
>100 [26.2]b
>100 [31.5]b
>100 [32.4]b
Inactive [0]b
>100 [18.0]b
>100 [36.0]b
12.9 0.5
3-Thienyl
2-Furyl
3-Furyl
4-Bromo-2-thienyl
5-Bromo-2-thienyl
5-Bromo-2-furyl
3-Benzothienyl
2-Benzofuryl
>100 [12.5]b
>100 [36.5]b
>100 [34.0]b
a Compounds described as inactive were completely devoid of activity at a 100 lM concentration; compounds showing less than 50% inhibition at
this concentration are reported with IC50 >100.
b Values between square brackets correspond to the percentage of MAO inhibition at a 100 lM concentration of the inhibitor.
Table 2. MAO-A inhibition after preincubation with compounds 22,
23 and 25
charge-transfer interactions with other aromatic frag-
ments in the active site. This possibility gains support
% Inhibition of MAO-Aa
from the present study, where a soft, polarizable het-
eroaromatic ring showing an extended conjugation, like
compound 25, was found to be the best MAO-A inhib-
itor of the series. In order to gain some insight into these
hypothetical interactions, we carried out simulations
with this compound in the active site of the enzyme.
Compd [concn, M]
22 [10ꢀ5
23 [5 · 10ꢀ5
25 [10ꢀ6
]
49.3 4.9b
56.0 2.8c
61.1 0.2c
59.5 3.5c
]
58.5 7.2b
60.5 0.7b
]
a Average values of three measurements.
b After 0 min of preincubation.
c After 30 min of preincubation.
Due to the presence of a chiral centre on the arylamino-
propane structures, both enantiomers of the benzo-
furylalkylamine 25 were docked into the active site of
MAO-A by using the crystallographic data available
for the rat enzyme isoform (1O5W).16
tives.13 This effect might be explained on the basis of an
increased polarizability induced by the introduction of a
halogen atom on the ring. An increased polarizability
could also explain the greater MAO-A inhibitory activ-
ity of the benzo derivatives 24 and 25, when compared
with 18 and 19, respectively.
The benzofuran ring in both isomers inserted itself into
a hydrophobic pocket formed by residues, which in-
cluded Tyr407, Tyr444, Gln215 and Phe208. The mole-
cules were further stabilized by hydrogen bonds between
the side-chain amino group and neighbouring amino
acids, like Thr336 for the S-enantiomer and Phe208
and Asn181 for the R-isomer.
Negligible changes in MAO-A inhibition were observed
after different preincubation times (0 and 30 min, Table
2), indicating that blockade of the enzyme was not time-
dependent, and that the inhibition was not due to
substrate competition. These experiments showed that
compounds 22, 23 and 25 were not metabolized by the
enzyme and that the observed inhibition resulted from
a real blockade of the enzymatic activity. The present
results agree with similar observations reported
previously by us for several other arylisopropylamine
derivatives.14,15
The heterocyclic ring system in 25 is related with the
indolyl group present in 5-HT, a specific substrate of
MAO-A. This structural similarity might suggest similar
interactions between the two heterocyclic systems and
the hydrophobic cavity of MAO-A. In Figure 1 we
superimpose the structures of the S-benzofuryl deriva-
tive 25 and 5-HT, after docking both compounds into
the enzyme active site.
The structural requirements leading to an enhanced
MAOI activity in the series of prepared heteroarylamino-
propanes parallel similar observations with other aryl-
aminopropanes. We had previously investigated the
hypothesis of charge-transfer interactions between the
aromatic ring of these inhibitors and the flavin cofactor
in determining the MAOI activity of these compounds.6
The crystallographic elucidation of the interactions
between some inhibitors and the FAD cofactor in the
active site of MAO-B1–3 and MAO-A4 made this
hypothesis no longer tenable.5 However, the obtained
correlations of MAO-A inhibitory activity with the
frontier orbital energies of the inhibitors6 may reflect
As can be seen, both heterocyclic rings occupy the same
region of the cavity, with one important difference.
While the side-chain amino group of serotonin ap-
proaches the FAD ring system, hydrogen bonding with
ring N5, the corresponding amino group of 25 points
away from the flavin ring, forming a hydrogen bond
with Thr336. A similar difference in side-chain orienta-
tion occurs for 5-HT and the R-isomer of 25 (not
shown). Thus, different alignments are obtained for the
two heterocyclic compounds, with their ring systems
sharing the same region and similar interactions with
the aromatic fragments of the hydrophobic cavity.

G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
4453
Figure 1. Superimposed structures of 5-HT (blue) and the S-enantio-
mer of 25 (white), docked into the active site of rat MAO-A (grey
fragments), showing the different orientations of the side chains of
both compounds. The 5-hydroxy group of the 5-HT indole ring
hydrogen bonds with the phenolic OH substituent of Tyr 444, and its
side-chain amino group forms bonds with the flavin N5 and Tyr407.
By contrast, the side-chain amino group of compound 25 interacts with
Thr336.
Figure 2. Superimposed binding sites of rat MAO-A (grey) and human
MAO-B (pink fragments). The S-enantiomer of 25 is shown in white.
Steric repulsion with Tyr326 in MAO-B would prevent accommoda-
tion of the inhibitor in the enzyme active site. Such interactions are not
present with the corresponding MAO-A fragment Ile335.
of a high similarity between the active sites of rat and
human MAO-B. Mutation of specific residues in MAO
from both species has not always resulted in identical
functional changes.19,20 Likewise, the conclusions from
docking studies regarding the inhibitory mechanism
are limited because they were obtained using the enantio-
mers of 25 and not its racemate. Further experiments
assessing the inhibitory properties of both optical iso-
mers of aminopropane derivatives are necessary to con-
firm the computational predictions. Despite these
limitations, our results and conclusions highlight the
usefulness of coupling computational simulations with
structure–activity studies in order to understand the
subtle factors governing MAO inhibitory activity.
Therefore, in spite of occupying the active site and con-
sequently blocking access of any substrate to the flavin
ring, the benzofuryl derivative 25 would avoid deamina-
tion by preferring a different side-chain orientation with
respect to the FAD cofactor. This result provides a
rationale for the observed inhibitory activity. This pro-
posal is supported by the lack of substrate behaviour
shown by 22, 23 and 25 (Table 2) as evaluated in prein-
cubation studies.
An 85–88% identity is observed between the same iso-
forms (MAO-A or MAO-B) from human and rat,17
and a high homology at the active site of MAO-B from
both species could be inferred from the analysis of their
primary sequences. This allows, in principle, an analysis
of the structural mechanisms underlying the selectivity
shown by our compounds (data obtained with the rat
isoform), using the available crystallographic data (rat
MAO-A and human MAO-B). With this aim, the active
site of MAO-B was superimposed onto the correspond-
ing site of MAO-A already docked with 25 (Fig. 2). As
can be seen, the presence of Tyr326 in MAO-B (Ile335
being the corresponding residue in MAO-A), would pre-
vent the accommodation of 25 into the active site of
MAO-B. This suggests a possible explanation for the
selectivity exhibited by this compound. It is worth point-
ing out that fragments Ile335 and Tyr326 in MAO-A
and MAO-B, respectively, have been regarded as major
determinants of selectivity for both substrates and
inhibitors.4,18,19
4. Experimental
1H and 13C NMR spectra were recorded on a Bruker
Avance 400 MHz instrument, employing tetramethylsi-
lane as internal standard. IR spectra were obtained with
a Perkin Elmer 750 spectrometer. Melting points were
measured with a Microthermal apparatus and were
not corrected.
The following aldehydes were purchased from Aldrich: 4-
hydroxy-3-methoxybenzaldehyde,
3,4-ethylenedioxy-
benzaldehyde, 2- and 3-thiophenecarboxaldehydes, 2- and
3-furancarboxaldehydes, 4- and 5-bromo-2-thiophene-
carboxaldehydes, 5-bromo-2-furancarboxaldehyde, 3-
benzothiophenecarboxaldehyde and 2-benzofurancar-
boxaldehyde. 3-Bromo-4-hydroxy-5-methoxybenzalde-
hyde (1) was prepared in 54% yield by bromination of
4-hydroxy-3-methoxybenzaldehyde (vanillin) in acetic
acid,21 mp 162–163 ꢁC, lit.21 mp 158 ꢁC.
The results from our simulations should be regarded
with caution, firstly because biochemical data obtained
using rat enzymes were analysed with the assumption

4454
G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
4.1. 3-Bromo-4,5-dihydroxybenzaldehyde (2)
(CDCl3) d 8.00 (1H, s, CH@C(Me)NO2); 6.90 (3H, m,
Ar–H); 4.30 (4H, s, OCH2CH2O); 2.35 (3H, s,
To a stirred solution of 3-bromo-4-hydroxy-5-methoxy-
benzaldehyde (15 g, 65 mmol) in chloroform (150 mL),
cooled in a water bath (5–10 ꢁC), was added AlCl3
(12.4 g, 93 mmol) followed by careful, dropwise addition
of pyridine (23 mL). The resulting green solution was re-
fluxed with stirring for 24 h. The deep violet solution
was then concentrated by distillation of the solvent.
To the cooled dark residue was then added HCl 3 M un-
til the reaction mixture was acidic, and the solid that
separated was triturated, washed with acid and filtered
to give, after recrystallization in aqueous ethanol,
11.7 g (83% yield) of the 3-bromo-4,5-dihydroxybenzal-
dehyde, mp 230–232 ꢁC, lit.22 mp 230 ꢁC. IR (KBr) mmax
CH@C(CH3)NO2). 13C NMR (CDCl3)
d 146.09,
145.39, 143.62, 133.40, 125.54, 124.27, 119.01, 117.78,
64.53, 64.16 and 14.05.
4.3.2. 1-(3,4-Ethylenedioxy-5-bromophenyl)-2-nitropro-
pene (5). Yield 82%, yellow crystals, mp 140–142 ꢁC. IR
(KBr) mmax 2950, 1680, 1600, 1580, 1500, 1300, 1160 and
850 cmꢀ1. 1H NMR (CDCl3) d 7.94 (1H, s, CH@C(Me)-
NO2); 7.29 (1H, d, J = 1.7 Hz, H-6); 7.05 (1H, d, J =
1.7 Hz, H-2); 4.45 (2H, m, OCH2CH2O); 4.35 (2H, m,
OCH2CH2O); 2.45 (3H, s, CH@C(CH3)NO2). 13C
NMR (CDCl3) d 146.80, 144.24, 142.32, 131.91, 126.86,
125.71, 118.26, 110.70, 65.07, 63.92 and 13.86.
3250, 3020, 1650, 1590, 1310 and 1000 cmꢀ1. H NMR
1
(CDCl3) d 10.30 (2H, br s, OH), 9.70 (1H, s, CHO);
7.10 (1H, d, J = 2.0 Hz, H-2); 6.92 (1H, d, J = 2.0 Hz,
H-5).
4.3.3. 1-(2-Thienyl)-2-nitropropene (6). Yield 79%, yel-
low crystals, mp 65–67 ꢁC, lit.13 69 ꢁC. 1H NMR
(CDCl3) d 8.23 (1H, s, CH@C(Me)NO2); 7.58 (1H, d,
J = 5.1 Hz, H-5); 7.36 (1H, d, J = 3.6 Hz, H-3); 7.12
(1H, dd, J = 5.1 Hz, J0 = 3.6 Hz, H-4); 2.48 (3H, s,
CH@C(CH3)NO2).
4.2. 3-Bromo-4,5-ethylenedioxybenzaldehyde (3)
The preparation employed CuO as catalyst, which was
prepared as follows: To a solution of CuSO4Æ5H2O
(3.13 g, 12.5 mmol) in water (50 mL), heated at 80–
90 ꢁC, was added with stirring a solution of NaOH
(1.1 g, 27.5 mmol) in water (25 mL). The precipitate
was filtered and washed thoroughly with water, and then
dried in an oven at 200 ꢁC for 3 h.
4.3.4. 1-(3-Thienyl)-2-nitropropene (7). Yield 89%, yel-
low crystals, mp 73–75 ꢁC. IR (KBr) mmax 3100, 1650,
1560, 1500, 1300, 1280 and 800 cmꢀ1. 1H NMR (CDCl3)
d
8.08 (1H, s, CH@C(Me)NO2); 7.60 (1H, dd,
J = 2.9 Hz, J0 = 1.3 Hz, H-2); 7.44 (1H, dd, J = 5.0 Hz,
J0 = 2.9 Hz, H-5); 7.28 (1H, dd, J = 5.0 Hz, J0 = 1.3 Hz,
H-4); 2.49 (3H, s, CH@C(CH3)NO2).
A mixture of 3-bromo-4,5-dihydroxybenzaldehyde
(1.1 g, 4.8 mmol), 1,2-dibromoethane (0.64 mL,
4.3.5. 1-(2-Furyl)-2-nitropropene (8). Yield 81%, yellow
crystals, mp 46–47 ꢁC. IR (KBr) mmax 3100, 1640,
7.4 mmol), anhydrous Na2CO3 (2.3 g, 21.8 mmol) and
CuO (0.15 g, 1.8 mmol) in DMF (6 mL) was heated at
120–130 ꢁC for 4 h. The cooled mixture was then acidi-
fied with dilute HCl (5%, 20 mL) and extracted with
dichloromethane (3 · 20 mL). The organic extracts were
rotary evaporated and the residue purified by column
chromatography, employing CH2Cl2 as eluent. The 3-
bromo-4,5-ethylenedioxybenzaldehyde was obtained as
a white solid, 35% yield, mp 77–79 ꢁC. 1H NMR
(CDCl3) d 9.71 (1H, s, CHO); 7.60 (1H, d, J = 2.3 Hz,
H-2); 7.30 (1H, d, J = 2.3 Hz, H-6); 4.38 (2H, m, OCH2-
CH2O); 4.25 (2H, m, OCH2CH2O). 13C NMR (CDCl3)
d 189.64, 146.29, 144.60, 130.46, 127.45, 117.65, 111.63,
65.46 and 63.94.
1
1480, 1500, 1280 and 750 cmꢀ1. H NMR (CDCl3) d
7.87 (1H, s, CH@C(Me)NO2); 7.66 (1H, d, J = 1.7 Hz,
H-5); 6.84 (1H, d, J = 3.5 Hz, H-3); 6.60 (1H, dd,
J = 3.5 Hz,
CH@C(CH3)NO2).
J0 = 1.7 Hz,
H-4);
2.60
(3H,
s,
4.3.6. 1-(3-Furyl)-2-nitropropene (9). Yield 73%, yellow
crystals, mp 84–85 ꢁC. IR (KBr) mmax 3100, 1640,
1500, 1160 and 800 cmꢀ1 1H NMR (CDCl3) d 7.93
.
(1H, s, CH@C(Me)NO2); 7.76 (1H, br s, H-5); 7.53
(1H, br s, H-2); 6.64 (1H, br s, H-4); 2.44 (3H, s,
CH@C(CH3)NO2). 13C NMR (CDCl3)
145.18, 124.63, 119:23, 110.46, 110.05 and 14.19.
d 146.21,
4.3. Preparation of 1-heteroaryl-2-nitropropenes 4–14.
General procedure
4.3.7. 1-(4-Bromo-2-thienyl)-2-nitropropene (10). Yield
86%, yellow crystals, mp 87–88 ꢁC. IR (KBr) mmax
1
3100, 1640, 1500, 1300, 1160 and 800 cmꢀ1. H NMR
A solution of nitroethane (1.6 mL, 22.4 mmol), n-butyl-
amine (0.9 mL, 9.1 mmol) and the corresponding het-
eroarylcarboxaldehyde (7.9 mmol) in acetic acid
(4 mL) was heated at 80 ꢁC for 2 h. The crude product
that separated on cooling was filtered and recrystallized
from methanol, and had its structure confirmed by its
1H and 13C NMR spectra.
(CDCl3) d 8.14 (1H, s, CH@C(Me)NO2); 7.51 (1H, s,
H-5); 7.31 (1H, s, H-3); 2.55 (3H, s, CH@C(CH3)NO2).
13C NMR (CDCl3) d 145.98, 136.34, 135.93, 128.52,
125.69, 111.99 and 14.45.
4.3.8. 1-(5-Bromo-2-thienyl)-2-nitropropene (11). Yield
78%, yellow crystals, mp 138–140 ꢁC. IR (KBr) mmax
1
In this way the following nitropropenes were prepared.
3100, 1620, 1480, 1300, 1400, 1280 and 800 cmꢀ1. H
NMR (CDCl3) d 8.16 (1H, s, CH@C(Me)NO2); 7.17
(2H, s, H-3 and H-4); 2.49 (3H, s, CH@C(CH3)NO2).
13C NMR (CDCl3) d 144.80, 136.90, 135.14, 131.27,
126.76, 119.92 and 14.47.
4.3.1. 1-(3,4-Ethylenedioxyphenyl)-2-nitropropene (4).
Yield 65%, yellow crystals, mp 97–98 ꢁC. IR (KBr) mmax
1
1620, 1580, 1520, 1300, 1260 and 900 cmꢀ1; H NMR

G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
4455
4.3.9. 1-(5-Bromo-2-furyl)-2-nitropropene (12). Yield
82%, yellow crystals, mp 91–93 ꢁC. IR (KBr) mmax 2959,
ice water, the excess hydride was decomposed by careful
addition of water, the mixture was extracted with diethyl
ether (3 · 25 mL), the organic extracts were dried over
MgSO4 and rotary evaporated. The residue was then
dissolved in dry diethyl ether and the hydrochloride
(16) precipitated by passing gaseous HCl through the
solution. Yield 30%, mp 148–150 ꢁC. Anal. Calcd for
C11H14BrNO2ÆHClÆ1/2H2O: C, 41.59; H, 5.08; N,
4.41. Found: C, 41.84; H, 4.88; N, 4.26. IR (KBr):
mmax 3450, 2950, 1600, 1480, 1300, 1200 and 1080
1729, 1651, 1496, 1301, 1022, 940 and 797 cmꢀ1 1H
.
NMR (CDCl3) d 8.16 (1H, s, CH@C(Me)NO2); 6.76
(1H, d, J = 3.5 Hz, H-4); 6.52 (1H, d, J = 3.5 Hz, H-3);
2.56 (3H, s, CH@C(CH3)NO2). 13C NMR (CDCl3) d
149.93, 131.06, 127.48, 121.20, 119.62, 115.23 and 14.26.
4.3.10. 1-(3-Benzothienyl)-2-nitropropene (13). Yield
84%, yellow crystals, mp 110–112 ꢁC. IR (KBr) mmax
1
1
1620, 1580, 1500, 1300, 1200 and 800 cmꢀ1. H NMR
cmꢀ1. H NMR (DMSO-d6) d 8.02 (3H, s, NH3+); 6.94
(CDCl3) d 8.33 (1H, s, CH@C(Me)NO2); 7.89 (2H, m,
H-4 and H-7); 7.67 (1H, s, H-2); 7.47 (2H, m, H-5 and
H-6); 2.53 (3H, s, CH@C(CH3)NO2).
(1H, d, J = 2.3 Hz, H-6); 6.72 (1H, d, J = 2.3 Hz, H-2);
4.20 (4H, s, CH2O); 3.42 (1H, m, CH2CH(CH3)-
NH3+); 3.00 (1H, m, CH2CH(CH3)NH3+); 2.78 (1H,
m, CH2CH(CH3)NH3+); 1.03 (3H, d, J = 6.3 Hz,
CH2CH(CH3)NH3+). 13C NMR (DMSO-d6) d 145.82,
141.06, 132.15, 126.67, 119.11, 114.10, 66.37, 65.57,
49.43, 36.52 and 15.16.
4.3.11. 1-(2-Benzofuryl)-2-nitropropene (14). Yield 75%,
yellow crystals, mp 104–105 ꢁC. IR (KBr) mmax 3072,
1656, 1516, 1312, 1139, 985 and 748 cmꢀ1
.
1H
NMR (CDCl3) d 7.94 (1H, s, CH@C(Me)NO2); 7.64
(1H, d, J = 7.7 Hz, H-4); 7.53 (1H, d, J = 8.4 Hz, H-7);
7.42 (1H, dd, J = 8.4 Hz, J0 = 7.4 Hz, H-6); 7.30 (1H,
dd, J = 7.7 Hz, J0 = 7.4 Hz, H-5); 7.13 (1H, s, H-3);
2.73 (3H, s, CH@C(CH3)NO2). 13C NMR (CDCl3) d
156.32, 149.65, 147.09, 127.94, 127.40, 124.02, 122.19,
121.03, 115.45, 111.82 and 14.63.
Following the same procedure described above for com-
pound 16, the aminopropane hydrochlorides 17–25 were
obtained from the corresponding nitropropenes 6–14. In
this way the following compounds were prepared.
4.5.1. 1-(2-Thienyl)-2-aminopropane hydrochloride (17).
Yield 63%, mp 143–145 ꢁC (lit.13 146 ꢁC). 1H NMR
(DMSO-d6 and D2O) d 7.23 (1H, m, H-5); 6.90 (2H,
m, H-3 and H-4); 3.55 (1H, m, CH2CH(CH3)NH3+);
3.08 (2H, m, CH2CH(CH3)NH3+); 1.21 (3H, d,
J = 6.5 Hz, CH2CH(CH3)NH3+).
4.4. 1-(3,4-Ethylenedioxyphenyl)-2-aminopropane hydro-
chloride (15)
To a stirred suspension of lithium aluminium hydride
(1.20 g, 31.6 mmol) in dry THF (10 mL) was added a
solution of 1-(3,4-ethylenedioxyphenyl)-2-nitropropene
(4) (0.72 g, 3.26 mmol) in dry THF (15 mL). The result-
ing mixture was refluxed for 3 h. The excess hydride was
then decomposed by careful addition of water, the mix-
ture was extracted with diethyl ether (3 · 15 mL), the or-
ganic extracts were dried over MgSO4 and rotary
evaporated. The residue was then dissolved in dry
diethyl ether and the hydrochloride (15) precipitated
by passing gaseous HCl through the solution. Yield
50%, mp 167–170 ꢁC. HRMS m/z 193.0822, calcd for
C11H15NO2 193.1103. Anal. Calcd for C11H15NO2ÆHClÆ
1/2H2O: C, 55.34; H, 6.77; N, 5.87. Found: C, 55.39;
H, 6.50; N 6.31. IR (KBr): mmax 2900, 1600, 1520,
4.5.2. 1-(3-Thienyl)-2-aminopropane hydrochloride (18).
Yield 57%, mp 135–137 ꢁC HRMS m/z 141.0619, calcd
for C7H11NS, 141.0612. IR (KBr): mmax 2950, 1600,
1
1500, 1380, 1000, 720 and 680 cmꢀ1. H NMR (D2O) d
7.35 (1H, dd, J = 4.9 Hz, J0 = 2.9 Hz, H-5); 7.15 (1H,
dd, J = 2.9 Hz, J0 = 1.1 Hz, H-2); 6.94 (1H, dd,
J = 4.9 Hz, J0 = 1.1 Hz, H-4); 3.41 (1H, m,
CH2CH(CH3)NH3+); 2.80 (2H, m, CH2CH(CH3)NH3+);
1.11 (3H, d, J = 6.6 Hz, CH2CH(CH3)NH3+).
4.5.3. 1-(2-Furyl)-2-aminopropane hydrochloride (19).
Yield 32%, mp 120–121 ꢁC. HRMS m/z 125.0838, calcd
for C7H11NO, 125.0841. IR (KBr): mmax 3422, 3000,
1300, 1200 and 1080 cmꢀ1
.
1H NMR (DMSO-d6) d
1593, 1493, 1392, 1014 and 765 cmꢀ1 1H NMR
.
8.22 (3H, s, NH3+); 6.77 (1H, d, J = 8.1 Hz, H-5); 6.73
(1H, d, J = 2.1 Hz, H-2); 6.65 (1H, dd, J = 8.1 Hz,
J0 = 2.1 Hz, H-6); 4.19 (4H, s, CH2O); 3.29 (1H, m,
CH2CH(CH3)NH3+); 2.92 (1H, m, CH2CH(CH3)-
(DMSO-d6) d 8.25 (3H, s, NH3+); 7.36 (1H, dd,
J = 2.1 Hz, J0 = 0.7 Hz, H-3); 6.30 (1H, dd, J = 3.3 Hz,
J0 = 2.1 Hz, H-4); 6.18 (1H, dd, J = 3.3 Hz, J0 = 0.7 Hz,
H-5); 3.55 (1H, m, CH2CH(CH3)NH3+); 2.87 (2H, m,
CH2CH(CH3)NH3+); 1.19 (3H, d, J = 6.5 Hz,
CH2CH(CH3)NH3+).13C NMR (DMSO-d6) d 152.10,
143.92, 112.33, 109.28, 47.40, 33.88 and 19.03.
+
+
NH3 ); 2.52 (1H, m, CH2CH(CH3)NH3 ); 1.10 (3H, d,
J = 6.5 Hz, CH2CH(CH3)NH3+). 13C NMR (DMSO-
d6) d 143.68, 142.65, 130.21, 122.43, 118.16, 117.49,
64.49, 64.43, 48.53, 40.52 and 17.94.
4.5.4. 1-(3-Furyl)-2-aminopropane hydrochloride (20).
Yield 38%, mp 125–127 ꢁC. HRMS m/z 125.0830, calcd
for C7H11NO, 125.0841. IR (KBr): mmax 3400, 2950,
4.5. 1-(3,4-Ethylenedioxy-5-bromophenyl)-2-aminopro-
pane hydrochloride (16)
1600, 1500, 1400, 1020 and 800 cmꢀ1
.
1H NMR
To a suspension of LiAlH4 (0.85 g, 22.4 mmol) in dry
diethyl ether (6 mL), cooled in a dry ice/acetone bath
at ꢀ10 ꢁC, was added dropwise a solution of 1-(3,4-ethyl-
enedioxy-5-bromophenyl)-2-nitropropene (5) (1.23 g,
4.3 mmol) in dry diethyl ether (12 mL) and benzene
(8 mL). The suspension was stirred for 5 h at 0 ꢁC in
(DMSO-d6) d 8.28 (3H, s, NH3+); 7.60 (1H, br s, H-5);
7.54 (1H, br s, H-2); 6.24 (1H, br s, H-4); 3.30 (1H, m,
CH2CH(CH3)NH3+); 2.80 (2H, dd, J = 4.8 Hz,
J0 = 14.1 Hz, CH2CH(CH3)NH3+); 2.57 (2H, dd,
J = 9.1 Hz, J0 = 14.1 Hz, CH2CH(CH3)NH3+); 1.13
(3H, d, J = 6.4 Hz, CH2CH(CH3)NH3+). 13C NMR

4456
G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
(DMSO-d6) d 143.44, 140.56, 119.67, 111.31, 46.92,
29.39 and 17.61.
(DMSO-d6) d 154.28, 153.91, 128.27, 123.77, 122.74,
120.68, 110.83, 104.82, 45.60, 32.87 and 17.99.
4.5.5. 1-(4-Bromo-2-thienyl)-2-aminopropane hydrochlo-
ride (21). Yield 31%, mp 165–167 ꢁC. Anal. Calcd for
C7H10BrNSÆHCl: C, 32.77; H, 4.32; N, 5.46; S, 12.49.
Found: C, 33.32; H, 3.94; N, 5.58; S, 11.76. IR (KBr):
4.6. Evaluation of the inhibitory activity of the
heteroarylaminopropanes
`
The inhibitory activity of all compounds vis-a-vis MAO-
mmax 3400, 2900, 1600, 1520, 1380, 1100 and 800 cmꢀ1
.
A and MAO-B was assayed in vitro following a protocol
described previously.14 Briefly, mitochondrial suspen-
sions from rat brain (male Sprague Dawley rats weigh-
ing 180–230 g, sacrificed by decapitation) were
employed as a source of crude MAO. The activities of
both isoforms in the presence of variable concentrations
of the inhibitors were measured in triplicate by HPLC
with electrochemical detection (Merck-Hitachi L-7110
pump with a Lichrospher C18 5 lm column and a
Labchrom L-3500A amperometric detector). Serotonin
(5-HT, 100 lM) or 4-dimethylaminophenethylamine
(4-DMAPEA, 5 lM)23 were used as selective substrates
for MAO-A and MAO-B, respectively. Control experi-
ments were carried out without inhibitor and blanks
were run without mitochondrial suspension. The heights
of the chromatographic peaks of 5-HT, DMAPEA and
their main MAO metabolites (5-hydroxyindolacetic acid
and 4-dimethylaminophenylacetic acid) were used to
calculate MAO activity. IC50 values were determined
using the Prism Graph Pad software, from plots of inhi-
bition percentages (calculated in relation to a sample of
the enzyme treated under the same conditions without
inhibitors) versus the logarithm of the inhibitor
concentration.
1H NMR (DMSO-d6) d 8.26 (3H, s, NH3+); 7.54 (1H,
s, H-5); 7.01 (1H, s, H-3); 3.37 (1H, m,
CH2CH(CH3)NH3+); 3.19 (1H, dd, J = 14.5 Hz,
J0 = 5.5 Hz, CH2CH(CH3)NH3+); 2.97 (1H, dd,
J = 14.5 Hz, J0 = 8.4 Hz, CH2CH(CH3)NH3+); 1.17
(3H, d, J = 6.5 Hz, CH2CH(CH3)NH3+). 13C NMR
(DMSO-d6) d 141.13, 129.77, 123.47, 109.11, 48.40,
34.62 and 18.27.
4.5.6. 1-(5-Bromo-2-thienyl)-2-aminopropane hydrochlo-
ride (22). Yield 43%, mp 122–124 ꢁC. Anal. Calcd for
C7H10BrNSÆHCl: C, 32.77; H, 4.32; N, 5.46; S, 12.49.
Found: C, 33.13; H, 4.40; N, 5.60; S, 12.09. IR (KBr):
mmax 3400, 2950, 1600, 1450, 1400 and 800 cmꢀ1 1H
.
NMR (DMSO-d6) d 8.19 (3H, s, NH3+); 7.01 (1H, d,
J = 3.4 Hz, H-3); 6.76 (1H, d, J = 3.4 Hz, H-4); 3.42
(1H, m, CH2CH(CH3)NH3+); 3.08 (1H, dd,
J = 14.9 Hz, J0 = 5.2 Hz, CH2CH(CH3)NH3+); 2.85
(1H, dd, J = 14.9 Hz, J0 = 8.6 Hz, CH2CH(CH3)NH3+);
1.09 (3H, d, J = 6.9 Hz, CH2CH(CH3)NH3+).
4.5.7. 1-(5-Bromo-2-furyl)-2-aminopropane hydrochloride
(23). Yield 37%, mp 124–126 ꢁC. HRMS m/z 202.9953,
calcd for C7H11BrNO, 202.9925. IR (KBr): mmax 3400,
1
2950, 1600, 1500, 1400, 1200 and 800 cmꢀ1. H NMR
The time course of MAO inhibition by the drugs was as-
sessed by preincubating the reaction mixture with differ-
ent inhibitors at 37 ꢁC for 30 min. In all cases the
employed inhibitor concentrations, without preincuba-
tion, produced only partial enzyme inhibition. Percent
inhibition of deamination of 5-HT (100 lM) was deter-
mined by HPLC with electrochemical detection. After
preincubation, MAO activity was measured as
described.14
(DMSO-d6) d 8.26 (3H, s, NH3+); 6.49 (1H, d,
J = 3.3 Hz, H-4); 6.35 (1H, d, J = 3.3 Hz, H-3); 3.40
(1H, m, CH2CH(CH3)NH3+); 3.04 (1H, dd;
J = 15.0 Hz, J0 = 5.3 Hz, CH2CH(CH3)NH3+); 2.83
(1H, dd; J = 15.0 Hz, J0 = 8.4 Hz, CH2CH(CH3)NH3+);
1.18 (3H, d, J = 6.6 Hz, CH2CH(CH3)NH3+). 13C NMR
(DMSO-d6) d 153.35, 120.64, 113.11, 111.70, 46.57,
33.16 and 18.44.
4.5.8. 1-(3-Benzothienyl)-2-aminopropane hydrochloride
(24). Yield 52%, mp 189–191 ꢁC. HRMS m/z 191.0796,
calcd for C11H13NS, 191.0769. IR (KBr): mmax 3412,
4.7. Molecular simulation
The crystallographic data of MAO-A (PDB: 1O5W)
were used for all calculations. The hydrogen atoms of
the protein and the FAD molecule were built using In-
sight II24 and then were relaxed following a minimiza-
tion protocol using Discover_324 and the force field
ESFF. The calculations were performed using a dis-
tance-dependent dielectric constant of 80. All docking
simulations were performed with Autodock 3.02.25 The
protein and substrate were assigned partial charges
using the force field ESFF. Atomic solvation parameters
and fragmental volumes were assigned to the protein
atoms using the addsol option. The grid maps were cal-
culated using autogrid3 and were centred on the puta-
tive ligand-binding site. The volume chosen for the
grid maps were made up of 40 · 40 · 40 points, with a
3025, 1598, 1570, 1458, 1022, 758 and 738 cmꢀ1 1H
.
NMR (DMSO-d6) d 7.83 (1H, m, H-5); 7.75 (1H, m,
H-6); 7.33 (3H, m, H-2, H-4 and H-7); 3.54 (1H, m,
+
+
CH2CH(CH3)NH3 ); 3.11 (1H, m, CH2CH(CH3)NH3 );
2.99 (1H, m, CH2CH(CH3)NH3+); 1.13 (3H, d,
J = 6.5 Hz, CH2CH(CH3)NH3+).
4.5.9. 1-(2-Benzofuryl)-2-aminopropane hydrochloride
(25). Yield 26%, mp 151–153 ꢁC. Anal. Calcd for
C11H13NOÆHCl: C, 62.41; H, 6.67; N, 6.62. Found: C,
62.48; H, 6.71; N, 6.37. IR (KBr): mmax 3400, 2900,
1
1600, 1450, 1050, 800 and 750 cm1. H NMR (DMSO-
d6) d 8.33 (3H, s, NH3+); 7.53 (2H, m, H-4 and H-7);
7.22 (2H, m, H-5 and H-6); 6.76 (1H, s, H-3); 3.55
(1H, m, CH2CH(CH3)NH3+); 3.22 (1H, dd,
J = 14.9 Hz, J0 = 5.4 Hz, CH2CH(CH3)NH3+); 3.01 (H,
dd, J = 14.9 Hz, J0 = 8.5 Hz, CH2CH(CH3)NH3+); 1.24
(3H, d, J = 6.4 Hz, CH2CH(CH3)NH3+). 13C NMR
˚
grid-point spacing of 0.375 A. The autotors option was
used to define the rotating bonds in the ligand. In the
Lamarckian genetic algorithm (LGA) dockings, an ini-
tial population of random individuals with a population

G. Vallejos et al. / Bioorg. Med. Chem. 13 (2005) 4450–4457
4457
size of 50 individuals, a maximum number of 1.5 · 106
energy evaluations, a maximum number of generations
of 27,000, a mutation rate of 0.02, and a cross-over
rate of 0.80 were employed. Proportional selection was
used, where the average worst energy was calculated
over a window of the previous 10 generations. In the
LGA dockings, the pseudo-Solis and Wets local search
method was used, with a maximum of 300 iterations
per local search. The probability of performing local
search on an individual in the population was 0.06.
The docked compound complexes were built using the
lowest free-energy binding positions.
8. Yoshida, S.; Meyer, O. G.; Rosen, T. C.; Haufe, G.; Ye,
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This work was supported by DICYT-USACH and by
Fondecyt project 1000776.
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