Synthesis of bistetrahydroquinolines as potential anticholinesterasic agents by double diels-alder reactions

Article abstract of DOI:10.3390/molecules181012951

The tetrahydroquinoline ring system is a unit found in many biologically active natural products and pharmacologically relevant therapeutic agents. A new series of bistetrahydroquinolines (bis-THQs) was synthesized using imino Diels-Alder reactions betwee

Full text of DOI:10.3390/molecules181012951

Molecules 2013, 18, 12951-12965; doi:10.3390/molecules181012951
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis of Bistetrahydroquinolines as Potential
Anticholinesterasic Agents by Double Diels-Alder Reactions
Yorley Duarte 1, Margarita Gutiérrez 1,*, Luis Astudillo 1, Jans Alzate-Morales 2 and Natalia Valdés 2
1
Laboratorio Síntesis Orgánica, Instituto de Química de Recursos Naturales, Universidad de Talca,
Casilla 747, Talca 3460000, Chile; E-Mails: yduarte@utalca.cl (Y.D.); lastudi@utalca.cl (L.A.)
2
Centro de Bioinformática y Simulación Molecular, Universidad de Talca, 2 Norte 685, Casilla 721,
Talca 3460000, Chile; E-Mails: jalzate@utalca.cl (J.A.-M.); nvaldesp@alumnos.utalca.cl (N.V.)
* Author to whom correspondence should be addressed; E-Mail: mgutierrez@utalca.cl;
Tel.: +56-71-200-285; Fax: +56-71-200-448.
Received: 2 September 2013; in revised form: 27 September 2013 / Accepted: 8 October 2013 /
Published: 17 October 2013
Abstract: The tetrahydroquinoline ring system is a unit found in many biologically active
natural products and pharmacologically relevant therapeutic agents. A new series of
bistetrahydroquinolines (bis-THQs) was synthesized using imino Diels-Alder reactions
between dialdehydes, anilines and N-vinyl-2-pyrrolidone (NVP). The notable features
of this procedure are mild reaction conditions, greater selectivity and good yields of
products. In addition, the inhibitory activity against acetylcholinesterase (AChE) and
butyrylcholinesterase (BuChE) of some selected derivatives is reported. The feasible
binding modes of these active compounds, within AChE and BuChE binding sites, were
predicted by molecular docking experiments and their binding affinity was estimated by
means of free energy calculations through the MM-GBSA approximation.
Keywords: bistetrahydroquinolines; Diels-Alder reaction; AChE and BuChE inhibitors;
molecular docking; MM-GBSA
1. Introduction
Heterocyclic compounds, especially nitrogen heterocycles, are the most important class of
compounds in the pharmaceutical and agrochemical industries [1]. The tetrahydroquinoline (THQ)

Molecules 2013, 18
12952
system, constitutes a privileged substructure found in numerous biologically active natural products
and pharmacologically relevant therapeutic agents [2].
The THQ nucleus has been found to possess a wide range of biological activities, including
psychotropic activity [3], anti-allergenic [4], antitumoral [5], antimalarial [6], anti-bacterial [7],
antifungal [8,9], cardiovascular activity [10], antiviral activity [11,12], etc. Also, these kind of
molecules can act as γ-secretase inhibitors for the treatment of Alzheimer’s disease [13], as platelet
aggregation inhibitors [14] and modulators of HIV transcription [15].
Among those, Alzheimer’s disease (AD), characterized by progressive cognitive impairment, has
been raising much interest as the most common cause of dementia in elderly people. It is also a
multifactorial disorder involving the malfunction of different biochemical pathways in which certain
enzymes play a key role [16]. Among these enzymes are cholinesterases, which have become
important therapeutic targets for AD treatments. For example, AChE catalyzes the hydrolysis of
acetylcholine, decreasing its availability in the synaptic space [17], and inhibitors of AChE have been
used as palliative drugs for AD, including synthetic compounds as tacrine [18], donepezil [19],
galanthamine [20] and rivastigmine [21], which have all been proven to improve a little the situation of
AD patients. However, it is important to know that the use of different biological entities that are
involved in the same pathology (AChE and BuChE), it is widely accepted and can be a better strategy
to block the course of multifactorial diseases rather than just reducing their symptoms [22]. Therefore,
and taking into account the effectiveness of this methodology for the treatment of AD, it is necessary
to focus our research in obtaining better AChE and BuChE inhibitors.
Due to their broad biological activity, as mentioned in the background, THQ compounds have been
considered a good starting material in the search of novel inhibitors of the enzymes AChE and BuChE.
Many synthetic routes to THQs are known, but due to the importance associated with their
biological activity, the development of new synthetic approaches remains an active research area [23].
The imino Diels-Alder reaction between aldimines and electron-rich alkenes is probably the most
powerful synthetic tool for the construction of this kind of heterocyclic compound [24,25]. This
reaction has been extensively studied with protic acids; different Lewis [26] and Brönsted acids [27]
and also lanthanide triflates [28] have been used as efficient catalysts for the synthesis of THQs. This
method allows the generation of THQs derivatives with a high degree of structural diversification [29].
The main purposes of our work were: first, to develop a simple and efficient synthesis protocol for
bis-THQ derivatives with several degrees of structural diversity; second, to study their biological
activity as inhibitors of the enzymes AChE and BuChE and third, to explain their binding modes of
interaction with those molecular targets, aiming in particular at estimating the binding free energy of
the compounds.
2. Results and Discussion
2.1. Chemistry
We report here the synthesis of a new series of 1,4-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-
1',2',3',4'-tetrahydroquinolin-2-yl)benzenes 4a–c, 1,3-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-1',2',3',4'-
tetrahydroquinolin-2-yl)benzenes 5a–c, and 2,6-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-1',2',3',4'-

Molecules 2013, 18
12953
tetrahydroquinolin-2-yl)pyridine derivatives 6a–e. They were prepared by imino Diels-Alder
cycloaddition between different substituted anilines 1, dialdehydes (terephthalaldehyde (2a),
isophthalaldehyde (2b), 2,6-pyridinedicarboxaldehyde 2(c)), and NVP 3 as alkene, using acetonitrile as
solvent in the presence of 20 mol% of bismuth trichloride (III) as catalyst (Scheme 1). It must be taken
into consideration that bismuth compounds have attracted attention due to their low toxicity, low cost,
water tolerant catalyst [30] and good stability in several reactions such as the imino Diels-Alder
(Povarov) reaction.
Scheme 1. Synthesis of novel series of bis-THQs, through the r.t. Povarov reaction
promoted by BiCl3 and MeCN.
The reactions proceeded efficiently at room temperature under mild conditions to give the bis-THQ
products as stable solids in 22%–67% yields after purification using SiO2 column chromatography,
depending of the structural variations in aromatic anilines and dialdehydes (Table 1).
Table 1. Reaction times and yields of bis-THQs 4a c, 5a–c and 6a–e.
Compound
R1
H
H
H
H
H
H
H
H
H
H
H
R2
H
R3
H
R4
H
Mp (°C) Yield (%) Reaction Time (h)
4a
4b
4c
5a
5b
5c
6a
6b
6c
6d
6e
250–257
295–298
218–221
221–225
190–195
237–239
155–158
187–190
173–178
202–206
157–161
60
75
22
67
51
58
46
72
34
65
25
7
7
H
CH3
O-CH3
H
H
H
H
5
H
H
7
H
CH3
O-CH3
H
H
8
H
H
6
H
H
7
H
CH3
O-CH3
NO2
H
H
5
H
H
5
H
H
10
9
CH3
CH3

Molecules 2013, 18
12954
In some cases, the reaction provides the corresponding products depending on the substituents of
the aromatic ring of aniline. In anilines bearing electron-donating groups, it was found that the reaction
proceeded faster than the ones bearing electron-withdrawing groups (Table 1). This observation could
be attributed to the higher nucleophilic activity of aromatic amines with electron-donating groups.
The products were obtained predominantly as cis-diasteroisomers; this configuration was determined
1
by H-NMR spectroscopy and assigned on the basis of the corresponding coupling constants. The
1
stereochemistry was also confirmed by 1D selective NOE NMR experiments. All H-NMR spectra of
the synthesized bis-THQs were very similar and characterized by the presence of three groups of
signals at δ 8.0–6.5, indicating the presence of aromatic protons. Similarly, we observed the
tetrahydoquinolinic H-2', H-2'' and H-4', H-4'' proton signals at δ 6.0–4.5 and with large coupling
constant values (between 8.5–12.0 Hz for H-4' and H-2', respectively). Finally, the δ 2.9–1.5 zone
showed multiplets corresponding to the proton signals of the pyrrolidone group. NOE experiments
support the relative trans-orientation of H-2', H-3' and H-4'. Thus, in the case of 4b, 5b and 6c the
selective irradiation of H-2' produced an NOE effect over H-3' and H-4', for the three compounds.
Notably, the high value of the coupling constants found in the proton signals corresponding to H-2',
H-2'', H-4' and H-4'' confirmed the trans configuration. Therefore, and, based on these results, we can
conclude that the compounds obtained correspond to the endo cycloadducts of the imino Diels-Alder
reaction. Besides, by a comparison with previous reports [31–33] for analogous THQs, we assumed
that the major isomer has a cis-configuration of the C-2 and C-4 substituents.
From these results, we propose the following possible mechanism to explain the product formation
(Scheme 2). This procedure allows a cycloaddition reaction between substituted anilines 1 and
dialdehydes 2, generating a double Schiff base, which is activated by the presence of BiCl3 as Lewis acid,
and by double imino Diels-Alder (iDA) cycloaddition with VNP leads to the formation of the bis-THQs.
Scheme 2. Plausible mechanism reaction in the synthesis of the bis-THQs.

Molecules 2013, 18
12955
2.2. Biological Activities
The bis-THQs 4a–c, 5a–c and 6a–e were tested for inhibition of the enzymes AChE and BuChE,
and the inhibitory activity of the newly synthesized compounds was studied using the method
described by Ellman [34] to determine the rate of hydrolysis of acetylthiocholine/butyryltiocholine in
the presence of the inhibitor. The activity was assayed in comparison with galanthamine as a reference
compound. The main reason to use galanthamine was because it belongs to a group of natural products
corresponding to the alkaloid family [35], which structurally resembles the compounds that were
reported in this work. Galanthamine was also used taking into account previous theoretical studies [36],
where it is described as the compound with the major number of interactions and low free energy of
binding against AChE. The compound’s selectivity was also tested by determining their inhibitory
activity against BuChE, this with the aim to compare both enzymes with the same inhibitors.
BuChE, also called nonspecific cholinesterase or pseudocholinesterase, is able to act on hydrophilic
and hydrophobic choline esters. Even though BuChE is closely related to AChE, owing to a larger
active site gorge of BuChE, a broader variety of substrates and inhibitors are accepted in its binding
site, compared with AChE [37].
Among the 11 compounds synthesized, only two compounds, 5c and 4c, showed some biological
activity against AChE and BuChE, respectively, with IC50 values of 122 µM for 5c in AChE and 323 µM
for 4c in BuChE. However, these IC50 values were not comparable with the standard inhibitor
galanthamine (0.54 µM). The other synthesized compounds showed IC50 values higher than 500 µg/mL,
therefore, they were not interesting as feasible AChE/BuChE inhibitors.
The more active compounds contain a [6-methoxy-1,2,3,4-tetrahydroquinoline] substituent. The
presence of other substituents such as 6-methyl or hydrogen decreased the AChE and BuChE
inhibitory activity. Furthermore, the large size of the molecules hindered them to accommodate along
the gorge of AChE, which is smaller than the BuChE [38]. Interestingly, the compound 5c has a better
selectivity for AChE over BuChE (2.64), perhaps due to the fact this molecule allows greater flexibility
in the pocket of this enzyme, as opposed to the structure 4c that does appear to have a larger size.
2.3. Molecular Docking and Binding Affinity Calculations
Considering the presented in vitro results, and in order to obtain a theoretical model for potential
binding modes and affinity strength of the most active compounds within the AChE and BuChE binding
sites, molecular docking and Molecular Mechanics-Generalized Born Surface Area (MM-GBSA)
calculations were performed for compounds 5c and 4c against the studied molecular targets. A
graphical inspection of the molecular docking results was made in order to explain the possible
molecular interactions for the two most active AChE and BuChE inhibitors 5c and 4c (See Figure 1).
In Figure 1A is shown an overlay of the structures for compounds 5c, 4c and E2020 that is a
member of a large family of N-benzylpiperidine-based AChE inhibitors that were developed,
synthesized and evaluated by the Eisai Company in Japan [39], on the basis of QSAR studies [40,41].
The compound E2020 (green carbon atoms) establishes aromatic stacking interactions against the
indole ring of residues Trp84 and Trp279. On the other hand, the charged nitrogen of the piperidine
ring makes a cation–π interaction with the phenyl ring of Phe330 [42,43]. The nitrogen atom of the

Molecules 2013, 18
12956
piperidine ring makes a hydrogen bond (H-bond) interaction with residue Tyr121, which is mediated
by a water molecule. Finally E2020 does not interact with the catalytic triad [19].
Figure 1. (A) Main molecular interactions established by compounds E2020 (green carbon
atoms), 5c (gray carbon atoms and ball-stick representation) and 4c (gray carbon atoms
and stick representation) within AChE binding site. (B) Main molecular interactions
established by compounds 5c (green carbon atoms and thin stick representation) and 4c
(green carbon atoms and width stick representation) within BuChE binding site. Only main
amino acids within AChE and BuChE are sketched for the sake of clarity.
According to our molecular docking results, and comparing the interactions against model
compounds E2020, the compounds 5c and 4c could establish some of the above-mentioned
interactions within the AChE binding site. For instance, both compounds established CH-π interactions
with indole ring of Trp84 through the methoxy groups attached to one of the tetrahydroquinoline ring
of tested compounds. An aromatic stacking interaction is formed between tetrahydroquinoline ring of
compound 5c and the indole ring of residue Trp279. The 1,4 substitution pattern of tetrahydroquinoline
rings around the central phenylene group makes this interaction to be broken for compound 4c. On the
other hand both compounds established a hydrogen bond interaction between nitrogen atom in
tetrahydroquinoline ring and hydroxyl group at residue Tyr121. None of the compounds formed any
interactions with catalytic triad.

Molecules 2013, 18
12957
The molecular interactions, derived from docking experiments, for compounds 5c and 4c within the
BuChE active site are shown in Figure 1B. As can be seen, the binding site pocket in BuChE seems to
be less crowded that the corresponding one in AChE, which is in agreement with previous work that
reported that smaller residues like Leu286 and Val288, instead of Phe295 and Phe297 in AChE, are
responsible for this [35].
The only molecular interactions present in our models suggest that compound 5c makes multiple
H-bond contacts, through the oxygen atom at pyrrolidone ring, with two residues in the catalytic triad
in the BuChE active site, namely His438 and Ser198. On the other hand, compound 4c can only
establish H-bond interactions with residue His438. Both compounds seem to be in close contact with
residue Trp82 establishing some CH-π interactions through their methoxy groups on the
tetrahydroquinoline ring. The remaining molecular structure orientation of compounds within the
binding site are quite different due to the substitution patterns obtained for the tetrahydroquinoline
rings around the central phenylene scaffold.
Regarding the binding affinity energies obtained from molecular docking and MM-GBSA
approximations, and their relationship with molecular interactions discussed above; it is important to
clarify that our aim here was to get a rough estimation of the energetic differences observed in binding
affinity, and not to obtain a more detailed structure activity relationships for compounds studied. This
is supposed to provide us with fresh ideas, mainly from the structural point of view, to redirect the
synthesis of new potent and selective THQ derivatives active against AChE/BuChE.
In Table 2 the computational binding affinity energies obtained for compounds 5c and 4c in their
complexes with the proteins AChE and BuChE can be seen. The docking score for compound 5c
suggest a more favorable interaction within the AChE binding sites than in the BuChE one (−8.63 and
−7.98 kcal mol−1, respectively), when compared with compound 4c, with score energies of −7.37 and
−8.82 kcal mol−1, respectively. This roughly indicates that compound 5c could interact more strongly
with AChE than with BuChE, and the opposite is true for compound 4c. This is in agreement with the
experimental affinity data reported in this work. However a work of caution is important here because
the lack of more experimental affinity data to compare with computational findings (i.e., only two
compounds showed defined IC50 values), and due to the molecular size of the studied compounds that
should prove to be difficult to accommodate in the AChE binding site. Other important point from our
experimental findings is the solubility problems found with the majority of synthesized compounds. In
order to get a more precise binding affinity prediction for protein-ligand complexes studied in this
work, we performed MM-GBSA calculations.
Table 2. Protein-ligand binding affinities estimated through molecular docking and
MM-GBSA approaches.
Glide docking score (kcal mol−1) Binding Free Energy (kcal mol−1) a
Target/Ligand
AChE
−8.63
−7.37
BuChE
−7.98
−8.82
AChE
−30.64
−28.72
BuChE
−33.94
−29.27
5c
4c
a
Free binding energies were obtained through MM-GBSA approach implemented in Prime module of
Schrödinger suite.

Molecules 2013, 18
12958
This technique is supposed to describe in a more accurate way the solvation events occurring in
protein-ligand binding processes. According to those results, compound 5c binds more strongly to
AChE than compound 4c does (−30.64 and −28.72 kcal mol−1, respectively), and regarding the binding
of those compounds to BuChE, compound 5c also binds more strongly than compound 4c (−33.94 and
−29.27 kcal mol−1, respectively). The latter free binding energy results are not in agreement with either
previous docking results or with experimental binding affinity results, but could help us to understand
the molecular determinants for the interaction of these new compounds within the binding sites of the
enzymes AChE and BuChE.
Summarizing, molecular docking and MM-GBSA experiments could be interesting approaches to
try to predict the activity for this class of bis-THQs in AChE and BuChE receptors. Moreover, the
different binding patterns for the best inhibitors proposed in this work, could help us to better
understand the modes of interaction of new series of bis-THQs inhibitors synthesized in our laboratory.
3. Experimental
3.1. General
Melting points were determined on a Buchi apparatus and are uncorrected. The compounds’ purity
was checked by thin layer chromatography (TLC) on silica gel and they were purified by column
chromatography. Chemicals were used without further purification. FT-IR spectra were recorded in
potassium bromide pellets using a Thermo Nicolet NEXUS 670 FT-IR spectrophotometer, with
1
13
0.125 cm−1 spectral resolution. H-NMR (400 MHz) and C-NMR spectra (100 MHz) were recorded
in CDCl3 or DMSO-d6, using a Bruker AM-400 instrument. Chemical shifts are expressed as values
relative to TMS as internal standard. High-resolution mass spectrometry ESI-MS and ESI-MS/MS
analyses were conducted in a high-resolution hybrid quadrupole (Q) and orthogonal time-of-flight
(TOF) mass spectrometer (Waters/Micromass Q-TOF micro, Manchester, UK) with a constant
nebuliser temperature of 100 °C. The experiments were carried out in positive ion mode, and the cone
and extractor potentials were set at 10 and 3.0 V, respectively, with a scan range of m/z 150–600.
MS/MS experiments were carried out by mass selection of a specific ion in Q1, which was then
submitted to collision-induced dissociation (CID) with helium in the collision chamber. The product
ion MS analysis was accomplished with the high-resolution orthogonal TOF analyzer. The samples
were directly infused into the ESI source, via a syringe pump, at flow rates of 5 µL min−1, via the
instrument’s injection valve.
General procedure for the preparation of 1,4-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-1',2',3',4'-tetra-
hydroquinolin-2-yl)benzenes 4a–c, 1,3-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-1',2',3',4'-tetrahydro-
quinolin-2-yl)benzenes 5a–c, and 2,6-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-1',2',3',4'-tetrahydro-
quinolin-2-yl)pyridines 6a–e. A mixture of p-anisidine (3 mmol) and isophthalaldehyde (1.5 mmol) in
anhydrous CH3CN (5 mL) was stirred at room temperature under N2 for 1 h. BiCl3 (20 mol%) was
added. Over a period of 20 min, a solution the NVP (3.1 mmol) in CH3CN (5 mL) was added
dropwise. The resulting mixture was stirred for 8–10 h. After completion of the reaction as indicated
by TLC, the reaction mixture was diluted with (15 mL) and extracted with ethyl acetate (3 × 10 mL). The
organic layer was separated, and dried with Na2SO4. The organic solvent was removed in vacuo and the

Molecules 2013, 18
12959
resulting product was purified by column chromatography (silica gel, petroleum ether/ethyl-acetate) to
afford the pure bis-THQs.
1,4-bis(4'-(2-Oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzene (4a). White solid; Yield
60%; Mp 250–257 °C; IR (cm−1): 3315, 2920, 1659, 1486, 1282. 1H-NMR (CDCl3, DMSO-d6) δ ppm:
7.45 (4H, s, 2-H, 3-H, 5-H and 6-H), 7.08 (2H, t, J = 7.8 Hz, 7'-H and 7''-H), 6.88 (2H, d, J = 8.0 Hz,
5'-H and 5''-H), 6.73 (2H, t, J = 7.8 Hz, 6'-H and 6''-H), 6.60 (2H, d, J = 8.0 Hz, 8'-H and 8''-H),
5.74 (2H, t, J = 9.5 Hz 4'-H and 4''-H), 4.63 (2H, m, 2'-H and 2''-H), 3.99 (2H, s, N-H), 3.28–3.19 (4H,
m, 5-Hpyrr and 5'-Hpyrr), 2.59–2.43 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.13–2.10 (4H, m, 3'-H and 3''-H),
13
2.05–1.99 (4H, m, 4-Hpyrr and 4'-Hpyrr). C-NMR (CDCl3, DMSO-d6): 18.2 (2), 31.4 (2), 42.30 (2),
48.4 (4), 56.1 (2), 114.9 (2), 118.3 (2), 118.8 (2), 126.9 (4), 128.2 (4), 142.8 (2), 145.8 (2), 175.85 (2).
MS (ESI, m/z): 507.41 ([M+H])+, 529.34 ([M+Na])+.
1,4-bis(6'-Methyl-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzene (4b). White
1
solid; Yield 75%; Mp 295–298 °C; IR (cm−1): 3318, 2920, 1661, 1496, 1281. H-NMR (CDCl3)
δ ppm: 7.44 (4H, s, 2-H, 3-H, 5-H and 6-H), 6.88 (2H, d, J = 8.0 Hz, 7'-H and 7''-H), 6.69 (2H, s, 5'-H and
5''-H), 6.52 (2H, d, J = 8.0 Hz, 8’-H and 8''-H), 5.71 (2H, dd, J = 9.6, 8.4 Hz, 4'-H and 4''-H),
4.59–4.56 (2H, m, 2'-H and 2''-H), 3.86 (2H, s, N-H), 3.26−3.21 (4H, m, 5-Hpyrr and 5'-Hpyrr),
2.56–2.45 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.23 (6H, s, 6'-CH3), 2.11–2.09 (4H, m, 3'-H and 3''-H),
13
2.07–2.01 (4H, m, 4-Hpyrr and 4'-Hpyrr). C-NMR (CDCl3, DMSO-d6) δ: 18.6 (2), 20.9 (2), 31.4 (2),
42.30 (2), 48.4 (4), 56.1 (2), 114.9 (2), 118.3 (2), 118.8 (2), 126.9 (4), 128.2 (4), 142.8 (2), 145.8 (2),
175.85 (2). MS (ESI, m/z): 535.67 ([M+H])+, 557.68 ([M+Na])+.
1,4-bis(6'-Methoxy-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzene
(4c).
Crystalline solid; Yield 22%; Mp 218–221 °C; IR (cm−1): 3378, 2956, 1659, 1497, 1284. H-NMR
(CDCl3, DMSO-d6) δ ppm: 7.99 (4H, s, 2-H, 3-H, 5-H and 6-H), 6.71 (2H, d, J = 2.4 Hz, 5'-H and
5''-H), 6.60 (2H, d, J = 8.8 Hz, 7'-H and 7''-H), 6.50 (2H, d, J = 8.6 Hz, 8'-H and 8''-H), 5.74 (2H, dd,
J = 11.2, 10.8 Hz, 4'-H and 4''-H), 4.64–4.61 (2H, m, 2'-H and 2''-H), 3.90 (2H, s, N-H), 3.74 (6H, s,
O-CH3), 3.29–3.23 (4H, m, 5-Hpyrr and 5'-Hpyrr), 2.55–2.46 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.16–2.14
(4H, m, 3’-H and 3''-H), 2.13–2.01 (4H, m, 4-Hpyrr and 4'-Hpyrr). MS (ESI, m/z): 567.72 ([M+H])+,
589.71 ([M+Na])+.
1
1,3-bis(4'-(2-Oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzene (5a). Green solid; Yield
1
67%; Mp 221–225 °C; IR (cm−1): 3319, 2943, 1670, 1484, 1250 cm−1. H-NMR (CDCl3, DMSO-d6)
δ ppm: 7.52 (1H, s, 2-H), 7.37 (3H, m, 4-H and 6-H and 5-H), 7.07 (2H, t, J = 7.8 Hz, 7'-H and 7''-H),
6.88 (2H, m, 5'-H and 5''-H), 6.73 (2H, m, 6'-H and 6''-H), 6.61 (2H, m, 8'-H, 8''-H), 5.72 (2H, m, 4'-H
and 4''-H), 5.30 (2H, s, N-H), 4.62 (2H, 2'-H and 2''-H), 3.23–3.25 (4H, m, 5-Hpyrr and 5'-Hpyrr),
2.55–2.43 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.15–2.03 (8H, m, 3'-H and 3''-H, 4-Hpyrr and 4'-Hpyrr).
MS (ESI, m/z): 507.51 ([M+H])+, 529.45 ([M+Na])+.
1,3-bis(6'-Methyl-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzene (5b). Crystalline
solid; Yield 51%; Mp 190–195 °C; IR (cm−1): 3315, 2920, 1670, 1511, 1284 cm−1. 1H-NMR (CDCl3) δ
ppm: 7.55 (1H, s, 2-H), 7.37 (3H, s, 4-H and 6-H and 5-H), 6.88 (2H, d, J = 8.0 Hz, 7'-H and 7''-H),

Molecules 2013, 18
12960
6.69 (2H, s, 5'-H and 5''-H), 6.54 (2H, d, J = 8.0 Hz, 8'-H and 8''-H), 5.71 (2H, dd, J = 10.8, 10.8 Hz,
4'-H and 4''-H), 5.30 (2H, s, N-H), 4.59 (2H, d, J = 10.4, 10.0 Hz, 2'-H and 2''-H), 3.26–3.21 (4H, m,
5-Hpyrr and 5'-Hpyrr), 2.60–2.45 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.23 (6H, s, 6'-CH3), 2.14–2.10 (4H, m,
3'-H and 3''-H), 2.03–1.99 (4H, m, 4-Hpyrr and 4'-Hpyrr). MS (ESI, m/z): 535.71 ([M+H])+, 5557.71
([M+Na])+.
1,3-bis(6'-Methoxy-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzene (5c). White
1
solid; Yield 58%; Mp 237–239 °C; IR (cm−1): 3325, 2948, 1671, 1509, 1280 cm−1. H-NMR (CDCl3)
δ ppm: 7.54 (1H, s, 2-H), 7.38 (3H, m, 4-H and 6-H and 5-H), 6.72 (2H, d, J = 7.8 Hz, 5'-H and 5''-H),
6.58 (2H, d, J = 8.8 Hz, 7'-H and 7''-H), 6.50 (2H, s(br), 8'-H and 8''-H), 5.74 (2H, dd, J = 10.0,
10.0 Hz, 4'-H and 4''-H), 5.32 (2H, s, N-H), 4.55 (2H, dd, J = 10, 10 Hz, 2'-H and 2''-H), 3.75 (6H, s,
O-CH3), 3.31–3.22 (4H, m, 5-Hpyrr and 5'-Hpyrr), 2.57–2.45 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.14–2.11
(4H, m, 3'-H and 3''-H), 2.09–2.01 (4H, m, 4-Hpyrr and 4'-Hpyrr). MS (ESI, m/z): 567.66 ([M+H])+,
589.65 ([M+Na])+.
2,6-bis(4'-(2-Oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)pyridine (6a). White solid;
1
Yield 46%; Mp 155–158 °C; IR (cm−1): 3363, 2918, 1659, 1490, 1286. H-NMR (CDCl3, DMSO-d6)
δ ppm: 7.77 (1H, t, J = 7.8 Hz, 4-H), 7.39 (2H, d J = 7.8 Hz, 3-H and 5-H), 7.10 (2H, t, J = 7.8 Hz,
7'-H and 7''-H), 6.90 (2H, d, J = 8.0 Hz, 5'-H and 5''-H), 6.74 (4H, m, 6'-H, 6''-H and 8'-H and 8''-H),
5.80 (2H, m, 4'-H and 4''-H), 5.31 (2H, s, N-H), 4.75 (2H, m, 2'-H and 2''-H), 3.22 (4H, m, 5-Hpyrr and
5'-Hpyrr), 2.58–2.45 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.38–2.32 (4H, m, 3'-H and 3''-H), 2.06–2.00 (4H, m,
4-Hpyrr and 4'-Hpyrr). MS (ESI, m/z): 508.33 ([M+H])+, 530.32 ([M+Na])+.
2,6-bis(6'-Methyl-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)pyridine (6b). Black
1
solid; Yield 72%; Mp 187–190 °C; IR (cm−1): 3397, 2916, 1655, 1495, 1278. H-NMR (CDCl3)
δ ppm: 7.75 (1H, t, J = 8.0 Hz, 4-H), 7.37 (2H, d, J = 6.0 Hz, 3-H and 5-H), 6.91 (2H, d, J = 8.0 Hz,
7'-H and 7''-H), 6.70–6.67 (4H, m, 5'-H and 5''-H, 8'-H and 8''-H), 5.75 (2H, dd, J = 11.6, 12.4 Hz,
4'-H and 4''-H), 5.33 (2H, s, N-H), 4.70 (2H, m, 2'-H and 2''-H), 3.25–3.18 (4H, m, 5-Hpyrr and
5'-Hpyrr), 2.56–2.48 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.34–2.30 (4H, m, 3'-H and 3''-H), 2.24 (6H, s,
6'-CH3), 2.05–1.99 (4H, m, 4-Hpyrr and 4’-Hpyrr). 13C-NMR (CDCl3, DMSO-d6) δ: 17.9 (2),
20.31 (2), 31.1 (2), 42.1 (2), 47.9 (4), 55.9 (2), 115.4 (2), 119.0 (2), 126.7 (2), 128.7 (2), 138.1 (1),
145.0 (2), 159.1 (2), 175.5 (2). MS (ESI, m/z): 536.85 ([M+H])+, 558.85 ([M+Na])+.
2,6-bis(6'-Methoxy-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)pyridine
(6c).
crystalline solid; Yield 34%; Mp 173–178 °C; IR (cm−1): 3395, 2916, 1672, 1490, 1277. H-NMR
(CDCl3, DMSO-d6) δ ppm: 7.75 (1H, t, J = 7.7 Hz, 4-H), 7.36 (2H, d, J = 7.8 Hz, 3-H and 5-H),
6.74–6.68 (4H, m, 7'-H and 7''-H, 5'-H and 5''-H), 6.49 (2H, d, J = 2.1 Hz 8'-H and 8''-H), 5.76 (2H,
dd, J = 11.6, 11.4 Hz, 4'-H and 4''-H), 5.30 (2H, s, N-H), 4.69–4.65 (2H, dd, J = 11.5, 11.7 Hz, 2'-H
and 2''-H), 3.74 (6H, s, O-CH3), 3.25–3.20 (4H, m, 5-Hpyrr and 5'-Hpyrr), 2.57–2.48 (4H, m, 3-Hpyrr and
3'-Hpyrr), 2.36–2.32 (4H, m, 3'-H and 3''-H), 2.08–1.98 (4H, m, 4-Hpyrr and 4'-Hpyrr). 13C-NMR (CDCl3,
DMSO-d6) δ: 17.9 (2), 31.1 (2), 42.1 (2), 48.2 (4), 55.53 (2), 56.2 (2), 111.7 (2), 114.1 (2), 116.7 (2),
119.0 (2), 120.3 (2), 139.1 (2), 152.5 (1), 160.1 (2), 175.5 (2). MS (ESI, m/z): 568.57 ([M+H])+,
590.56 ([M+Na])+.
1

Molecules 2013, 18
12961
2,6-bis(6'-Nitro-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)pyridine (6d). Brown solid;
Yield 65%; Mp 202–206 °C; IR (cm−1): 3397, 2916, 1655, 1495, 1278. 1H-NMR (CDCl3) δ ppm: 8.09
(5H, d, J = 8,8 Hz, 4-H, 3-H and 5-H, 7'-H and 7''-H), 6.62–6.68 (4H, m, 5'-H and 5''-H, 8'-H and
8''-H), 5.81–5.74 (2H, m, 4'-H and 4''-H), 4.39 (2H, s, N-H), 4.70 (2H, m, 2'-H and 2''-H), 3.38–3.32
(2H, m, 5-Hpirr), 3.22–3.16(2H, m, 5'-Hpyrr), 2.49–2.40 (4H, m, 3-Hpyrr and 3'-Hpyrr), 2.05–1.89 (4H, m,
3'-H and 3''-H), 1.51–1.50 (4H, m, 4-Hpyrr and 4'-Hpyrr). MS (ESI, m/z): 598.35 ([M+H])+, 621.33
([M+Na])+.
2,6-bis(5',7'-Dimethyl-4'-(2-oxopyrrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)pyridine (6e). Brown
solid; Yield 25%; Mp 157–161 °C; IR (cm−1: 3397, 2916, 1655, 1495, 1282. 1H-NMR (CDCl3) δ ppm:
7.86 (1H, t, J = 7.7 Hz, 4-H), 7.47 (2H, d, J = 7.6 Hz, 3-H and 5-H), 6.47 (2H, s, 8'-H and 8''-H),
6.43 (2H, s, 6'-H and 6''-H), 5.62–5.58 (2H, m, 4'-H and 4''-H), 4.77 (2H, s, N-H), 4.62 (2H, d, 2'-H
and 2''-H), 2.93–2.89 (2H, m, 5-Hpirr), 2.76–2.73 (2H, m, 5'-Hpyrr), 2.69–2.65 (4H, m, 3-Hpyrr and
3'-Hpyrr), 2.54 (6H, s, 7'-CH3 and 7''-CH3), 2.50–2.45 (4H, m, 3'-H and 3''-H), 2.08 (6H, s, 5'-CH3 and
5''-CH3), 1.77–1.74 (4H, m, 4-Hpyrr and 4'-Hpyrr).
3.2. Biological Assays
These activities were evaluated in 96-well plates as described, de la Torre et al. [44].
All compounds were evaluated in the range of 500–32 µg/mL.
3.3. Computational Details
To obtain information about enzyme inhibitor interactions that might help us to explain the
structural requirements for AChE and BuChE activity and selectivity, a docking study was performed.
The crystallographic structures of AChE in complex with donepezil inhibitor (pdb code 1EVE) and
BuChE in complex with benzoic acid (pdb code 3O9M) were used to dock the derivatives under study.
The former crystallographic structure (1EVE) was selected because it represents the molecular
interactions of a well-studied inhibitor compound (E2020) within the AChE binding site. The site, the
reported data showed that compound E2020 has high selectivity (compared with BuChE) and high
affinity. All previously reported data helped use to rationalize the interactions of the new synthesized
compounds againt AChE/BuChE proteins.
The docking simulations were carried out using software Glide operating the standard-precision
(SP) mode. A grid box of 25 Å × 25 Å × 25 Å was centered on the center of mass of respective ligands
crystallized with AChE and BuChE protein structures. The remaining Glide docking parameters were
used as default. The docking poses for each ligand were analyzed by examining both their relative total
energy score and their interaction with residues at binding sites. The five more energetically favorable
conformations were selected as the best poses.
The Molecular Mechanics/Generalized Born Surface Area (MM/GBSA) approach was used as
implemented in Prime (Prime, version 3.1, Schrödinger, LLC, New York, NY, USA, 2012) module
from Schrödinger Suite and using default settings. Protein-ligand complexes were obtained from
docking experiments as described before.

Molecules 2013, 18
12962
4. Conclusions
We report here the synthesis of a new group of bis-THQ derivatives and the evaluation of their
biological activity as AChE and BuChE inhibitors. The iDA reaction was used as a key step of a good
methodology for the efficient and general synthesis of a selected series of 1,4- or 1,3- or
1,6-bis(heteroaryl-4'-(2-oxopirrolidinyl-1)-1',2',3',4'-tetrahydroquinolin-2-yl)benzenes (pyridines). Among
the obtained compounds, only derivatives 5c and 4c exhibited a low activity against both enzymes.
With the aim to understand the ligand–protein interactions at molecular level and to gain insight of
minimal structural requirements for biological activity, molecular docking and MM-GBSA studies
were performed for these two compounds.
Acknowledgments
The authors are grateful to the grant Universidad de Talca, project Fondecyt 1100481 for financial
support. Y.D. thanks Applied Sciences Ph.D. program and “Becas Universidad de Talca”.
Conflicts of Interest
The authors declare no conflict of interest.
References
1. Sankaran, M.; Kumarasamy, C.; Chokkalingam, U.; Mohan, P.S. Synthesis, antioxidant and
toxicological study of novel pyrimido quinoline derivatives from 4-hydroxy-3-acylquinolin-2-one.
Bioorg. Med. Chem. Lett. 2010, 20, 7147–7151.
2. Marella, A.; Prakash, T.O.; Saha, R.; Rahmat, M.A.; Srivastava, S.; Akhter, M.; Shaquiquzzaman, M.;
Mumtaz, A. Quinoline: A versatile heterocyclic. Saudi Pharm. J. 2013, 21, 1–12.
3. Nesterova, I.N.; Alekseeva, L.M.; Golovira, S.M.; Granik, V.G. Synthesis and study the
pharmacological activity of derivatives of 5-dimethylaminopyrano[3,2-c]quinolin-2-ones.
Pharm. Chem. J. 1995, 29, 111–114.
4. Yamada, N.; Kadowaki, S.; Takahashi, K.; Umezu, K. MY-1250, a major metabolite of the
anti-allergic drug repirinast, induces phosphorylation of a 78-kDa protein in rat mast cells.
Biochem. Pharmacol. 1992, 44, 1211–1213.
5. Wallace, O.B.; Lauwers, K.S.; Jones, S.A.; Dodge, J.A. Tetrahydroquinoline-based selective
estrogen receptor modulators (SERMs). Bioorg. Med. Chem. Lett. 2003, 13, 1907–1910.
6. Mahajan, A.; Yeh, S.; Nell, M.; Rensburg, C.E.; Chibale, K. Synthesis of new 7-chloroquinolinyl
thioureas and their biological investigation as potential antimalarial and anticancer agents.
Bioorg. Med. Chem. Lett. 2007, 17, 5683–5685.
7. Upadhayaya, R.S.; Vandavasi, J.K.; Vasireddy, N.R.; Sharma, V.; Dixit, S.S.; Chattopadhyaya, J.
Design, synthesis, biological evaluation and molecular modelling studies of novel quinoline
derivatives against Mycobacterium tuberculosis. Bioorg. Med. Chem. 2009, 17, 2830–2841.
8. Kumar, S.; Bawa, S.; Drabu, S.; Panda, B.P. Design and synthesis of 2-chloroquinoline
derivatives as non-azoles antimycotic agents. Med. Chem. Res. 2011, 20, 1340–1348.

Molecules 2013, 18
12963
9. Kouznetsov, V.; Meléndez, G.C.; Derita, M.G.; Vetaz, L.; del Olmo, E.; Zacchino, A.S. Synthesis
and antifungal activity of diverse C-2 pyridinyl and pyridinylvinyl substituted quinolines.
Bioorg. Med. Chem. 2012, 20, 6506–6512.
10. Bernotas, R.C.; Singhaus, R.R.; Kaufman, D.H.; Ullrich, J.; Fletcher, H.; Quinet, E.; Nambi, P.;
Unwalla, R.; Wilhelmsson, A.; Nilsson, A.G.; et al. Biarylether amide quinolines as liver
X receptor agonists. Bioorg. Med. Chem. 2009, 17, 1663–1670.
11. Chen, S.; Chen, R.; He, M.; Pang, R.; Tan, Z.; Yang, M. Design, synthesis, and biological
evaluation of novel quinoline derivatives as HIV-1 Tat-TAR interaction inhibitors. Bioorg. Med.
Chem. 2009, 17, 1948–1956.
12. Normand-Bayle, M.; Bénard, C.; Zouhiri, F.; Mouscadet, J.-F.; Leh, H.; Thomas, C.-M.; Mbemba, G.;
Desmaële, D.; d’Angelo, J. New HIV-1 replication inhibitors of the styryquinoline class bearing
aroyl/acyl groups at the C-7 position: Synthesis and biological activity. Bioorg. Med. Chem. Lett.
2005, 15, 4019–4022.
13. Asberom, T.; Bara, T.; Clader, J.W.; Greenlee, W.J.; Guzik, H.S.; Josien, H.B.; Li, W.; Parker, E.M.;
Pissarnitski, D.A.; Song, L.; et al. Tetrahydroquinoline sulfonamides as gamma-secretase
inhibitors. Bioorg. Med. Chem. Lett. 2007, 17, 205–207.
14. Chen, J.J.; Chang, Y.L.; Teng, C.M.; Su, C.C.; Chen, I.S. Quinoline alkaloids and anti-platelet
aggregation constituents from the leaves of Melicope semecarpifolia. Planta Med. 2002, 9, 790–793.
15. Bedoya, L.M.; Abad, M.J.; Calonge, E.; Astudillo, S.L.; Gutierrez, C.M.; Kouznetsov, V.V.;
Alcami, J.; Bermejo, P. Quinoline-based compounds as modulators of HIV transcription through
NF-kappaB and Sp1 inhibition. Antivir. Res. 2010, 87, 338–344.
16. Minati, L.; Edginton, T.; Bruzzone, M.G.; Giaccone, G. Current concepts in Alzheimer’s disease:
A multidisciplinary review. Am. J. Alzheimers Dis. Other Dement. 2009, 24, 95–121.
17. Munoz-Muriedas, J.; Lopez, J.M.; Orozco, M.; Luque, F.J. Molecular modeling approaches to the
design of acetylcholinesterase inhibitors: New challenges for the treatment of Alzheimer’s
disease. Curr. Pharm. Design 2004, 10, 3131–3140.
18. Kurz, A. The therapeutic potential of tacrine. J. Neural. Transm. Suppl. 1998, 54, 295–299.
19. Kryger, G.; Silman, I.; Sussman, J.L. Structure of acetylcholinesterase complexed with E2020
(Aricept): Implications for the design of new anti-Alzheimer drugs. Structure 1999, 7, 297–307.
20. Santos, M.D.; Alkondon, M.; Pereira, E.F.R.; Aracava, Y.; Eisenberg, H.M.; Maelicke, A.;
Albuquerque, E.X. The nicotinic allosteric potentiating ligand galantamine facilitates synaptic
transmission in the mammalian central nervous system. Mol. Pharmacol. 2002, 61, 1222–1234.
21. Bar-On, P.; Millard, C.B.; Harel, M.; Dvir, H.; Enz, A.; Sussman, J.L.; Silman, I. Kinetic and
structural studies on the interaction of cholinesterases with the anti-Alzheimer drug rivastigmine.
Biochemistry 2002, 41, 3555–3564.
22. Rizzo, S.; Bartolini, M.; Ceccarini, L.; Piazzi, L.; Gobbi, S.; Cavalli, A.; Recanatini, M.;
Andrisano, V.; Rampa, A. Targeting Alzheimer’s disease: Novel indanone hybrids bearing a
pharmacophoric fragment of AP2238. Bioorg. Med. Chem. 2010, 18, 1749–1760.
23. Kouznetsov, V.; Vargas, L.; Meléndez, C. Recent progress in the synthesis of quinolines.
Curr. Org. Chem. 2005, 9, 141–161.

Molecules 2013, 18
12964
24. Kouznetsov, V. Recent synthetic developments in a powerful imino Diels-Alder reaction
(Povarov reaction): Application to the synthesis of N-polyheterocycles and related alkaloids.
Tetrahedron 2009, 65, 2721–2750.
25. Glushkov, V.A.; Tolstikov, A.G. Synthesis of substituted 1,2,3,4-tetrahydroquinones by the
Povarov reaction. New potentials of the classical reaction. Russ. Chem. Rev. 2008, 77, 137–159.
26. Maiti, G.; Kundu, P. Imino Diels-Alder reactions: An efficient one-pot synthesis of pyrano and
furanoquinoline derivatives catalyzed by SbCl3. Tetrahedron Lett. 2006, 47, 5733–5736.
27. Grieco, P.A.; Bahsas, A. Role reversal in the cyclocondensation of cyclopentadiene with
heterodienophiles derived from aryl amines and aldehydes: Synthesis of novel
tetrahydroquinolines. Tetrahedron Lett. 1988, 29, 5855–5858.
28. Hadden, M.; Stevenson, P.J. Regioselective synthesis of pyrroloquinolines—Approaches to
Martinelline. Tetrahedron Lett. 1999, 40, 1215–1218.
29. Kouznetsov, V.; Romero, A.; Stashenko, E. Three-component imino Diels-Alder reaction with
essential oil and seeds of anise: Generation of new tetrahydroquinolines. Tetrahedron Lett. 2007,
48, 8855–8860.
30. Lenga R.E. BiCl3 is as Lethal as Sodium Chloride for the Rat. In Sigma-Aldrich Library of
Chemical Safety Data, 2nd ed.; Sigma-Aldrich Corporation: Milwaukee, WI, USA, 1988;
Volume 1, p. 442.
31. Mendez, L.Y.; Kouznetsov, V.; Poveda, J.C.; Yolacan, C.; Öcal, N.; Aydongan, F. Transformations of
4-N-arylamino-4-(8-quinolinyl)-i-butenes and 3-aryl-2-(8-quinolinyl)-4-thiazolidinones. Heterocicycl.
Commun. 2001, 7, 129–134.
32. Astudillo, L.; Vallejos, G.; Kouznetsov, V.; Gutiérrez C.M.; Meléndez, C.M.; Vargas, L;
Bermúdez, J. Synthesis of new Diversely linked biquinoline derivates by multicomponent
Imino-Diels-Alder Cycloaddition and Intramoleclar Friedel-Crafs Cyclization. Synthesis 2010, 4,
593–600.
33. Katritzky, A.; Rachwal, B.; Rachwal, S. Reactions of Alkyl-N-phenyl-1H-benzotriazole-1-
methanamines with N-Vinilamides and N-Vinylcarbazole. A Convenient Synthesis of
4-(Dialkylamino)tetrahydroquinolines. J. Org. Chem. 1995, 60, 3993–4001.
34. Ellman, G.L.; Courtney, K.D.; Andres, V., Jr. A new and rapid colorimetric determination of
acetylcholinesterase activity. Biochem. Pharmacol. 1961, 7, 88–95.
35. Rainer, M. Clinical studies with galanthamine. Drugs Today 1997, 33, 273–279.
36. Sharma, J.; Ramanathan, K.; Sethumadhavan, R. Identification of potencial inhibitors against
acetylcholinesterase associated with alzheimer diseases: A molecular docking aproach. J. Comput.
Methods Mol. Design 2011, 1, 44–51.
37. Radic, Z.A.; Pickering, D.C.; Vellom, S.; Camp, P. Three distinct domains in the cholinesterase
molecule confer selectivity for acetyl- and butyrylcholinesterase inhibitors. Biochemistry 1993,
32, 12074–12084.
38. Bajda, M.; Więckowska, A.; Hebda, M.; Guzior, N.; Sotriffer, C.A.; Malawska, B. Structure-based
search for new inhibitors of cholinesterases. Int. J. Mol. Sci. 2013, 14, 5608–5632.
39. Kawakami, Y.; Inoue, A.; Kawai, T.; Wakita, M.; Sugimoto, H.; Hopfinger, A.J. The rationale for
E2020 as a potent acetylcholinesterase inhibitor. Bioorg. Med. Chem. 1996, 4, 1429–1446.

Molecules 2013, 18
12965
40. Cardozo, M.G.; Imura, Y.; Sugimoto, H.; Yamanishi, Y.; Hopfinger, A.J. QSAR analyses of the
substituted indanone and benzylpiperidine rings of a series of indanone-benzylpiperidine
inhibitors of acetylcholinesterase. J. Med. Chem. 1992, 35, 584–589.
41. Cardozo, M.G.; Kawai, T.; Imura, Y.; Sugimoto, H.; Yamanishi, Y.; Hopfinger, A.J.
Conformational analyses and molecular-shape comparisons of a series of indanone-benzylpiperidine
inhibitors of acetylcholinesterase. J. Med. Chem. 1992, 35, 590–601.
42. Verdonk, M.L.; Boks, G.J.; Kooijman, H.; Kanters, J.A.; Kroon, J. Stereochemistry of charged
nitrogen-aromatic interactions and its involvement in ligand-receptor binding. J. Comput. Aided
Mol. Design 1993, 7, 173–182.
43. Dougherty, D. Cation-π interactions in chemistry and biology: A new view of benzene, Phe, Tyr,
and Trp. Science 1996, 271, 163–168.
44. De la Torre, P.; Astudillo, L.; Caballero, J.; Quiroga, J.; Alzate-Morales, J.; Gutiérrez, M.;
Trilleras, J. A novel class of selective acetylcholinesterase inhibitors: Synthesis and evaluation of
(E)-2-(Benzo[d]thiazol-2-yl)-3-heteroarylacrylonitriles. Molecules 2012, 17, 12072–12085.
Sample Availability: Samples of the compounds 4a–c, 5a–c and 6a–e are available from the authors.
© 2013 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/3.0/).