A facile ionic liquid promoted synthesis, cholinesterase inhibitory activity and molecular modeling study of novel highly functionalized spiropyrrolidines

Article abstract of DOI:10.3390/molecules20022296

A series of novel dimethoxyindanone embedded spiropyrrolidines were synthesized in ionic liquid, [bmim]Br and were evaluated for their inhibitory activities towards cholinesterases. Among the spiropyrrolidines, compound 4f exhibited the most potent activi

Full text of DOI:10.3390/molecules20022296

Molecules 2015, 20, 2296-2309; doi:10.3390/molecules20022296
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
A Facile Ionic Liquid Promoted Synthesis, Cholinesterase
Inhibitory Activity and Molecular Modeling Study of Novel
Highly Functionalized Spiropyrrolidines
Abdulrahman I. Almansour 1, Raju Suresh Kumar 1,*, Natarajan Arumugam 1, Alireza Basiri 2,
Yalda Kia 3, Mohamed Ashraf Ali 4, Mehvish Farooq 5 and Vikneswaran Murugaiyah 2
1
Department of Chemistry, College of Science, King Saud University, P.O. Box 2455,
Riyadh 11451, Saudi Arabia; E-Mails: almansor@ksu.edu.sa (A.I.A.); aruorgchem@gmail.com (N.A.)
2
School of Pharmaceutical Sciences, Universiti Sains Malaysia, Minden 11800, Malaysia;
E-Mails: diba115920@gmail.com (A.B.); vicky@usm.my (V.M.)
3
School of Chemical Sciences, Universiti Sains Malaysia, Minden 11800, Malaysia;
E-Mail: kia.yalda@yahoo.com
4
Pharmacogenetic and Novel Therapeutic Institute for Research in Molecular Medicine,
Universiti Sains Malaysia, Minden 11800, Malaysia; E-Mail: asraf80med@yahoo.com
5
Institute of Pharmaceutical Science, King’s College London, London SE1 9NH, UK;
E-Mail: Mehvish.farooq@kcl.ac.uk
* Author to whom correspondence should be addressed; E-Mail: sraju@ksu.edu.sa or
drrajusureshkumar@gmail.com; Tel.: +966-4675907; Fax: +966-4675992.
Academic Editor: Jean Jacques Vanden Eynde
Received: 17 November 2014 / Accepted: 23 January 2015 / Published: 29 January 2015
Abstract: A series of novel dimethoxyindanone embedded spiropyrrolidines were
synthesized in ionic liquid, [bmim]Br and were evaluated for their inhibitory activities
towards cholinesterases. Among the spiropyrrolidines, compound 4f exhibited the most
potent activity with an IC50 value of 1.57 µM against acethylcholinesterase (AChE).
Molecular docking simulation for the most active compound was employed with the aim of
disclosing its binding mechanism to the active site of AChE receptor.
Keywords: acetylcholinesterase (AChE), butyrylcholinesterase (BChE), Alzheimer’s
disease (AD), spiropyrrolidines, ionic liquid and molecular docking study

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1. Introduction
Alzheimer’s disease (AD), the most common cause of dementia, is a neurodegenerative disorder
characterized by progressive decline of memory and cognition [1]. One of the major therapeutic
strategies adopted for primarily symptomatic AD is based on the cholinergic hypothesis targeting
cholinesterase enzymes [2]. Inhibition of the hydrolysis of acetylcholine (ACh) by blocking its
metabolic enzyme acetylcholinesterase (AChE) increases the ACh concentration and provides a
possible symptomatic treatment option for AD. On the other hand, butyrylcholinesterase (BChE) has
recently been considered as a potential target because it also plays an important role in regulating ACh
level [3]. Although there are many ongoing research activities in the search of drugs for treating AD,
only four drugs are currently approved for treatment and these drugs do not show potential cure rates.
However, from academic and practical standpoints, it is still desirable to develop more effective agents
for the treatment of AD. The increasing mortality rate and the reduced therapeutic potential of the
currently available options have led us to focus our research activities on the development of potential
cholinesterase inhibitors for the treatment of AD.
Among the five-membered heterocyclic systems, spiropyrrolidines occupy an important place as
this structural motif appears as an integral part of many natural products besides exhibiting
anticonvulsant [4–6], potential antileukemic [7], local anaesthetic [8] and antiviral activities [9]. Ionic
liquids are widely recognized as “green” solvents in organic synthesis because of their unique
properties, such as low vapor pressure, high chemical and thermal stability, solvating ability,
non-flammability, behavior as acidic or basic catalysts and recyclability [10–14]. In this context, ionic
liquids have emerged as new green solvents to replace the volatile organic compounds and they are
found suitable for executing many diverse organic reactions [15–17]. The development of
multi-component reactions in ionic liquids, although relatively unexplored [18–22] is of great interest
and to the best of our knowledge, this is the first report on 1,3-dipolar cycloaddition reaction of
2-arylmethylidene-5,6-dimethoxy-2,3-dihydro-1H-inden-1-one with an unexplored and new class of
azomethine ylide generated in situ from L-phenylalanine and isatin in ionic liquid medium, namely
1-butyl-3-methylimidazolium bromide ([bmim]Br).
As a part of our ongoing research project aimed at identifying potential cholinesterase inhibitors for
AD therapy [23–33], we report here our efforts in search of novel potent cholinesterase inhibitors
using dipolar cycloaddition strategy. In addition, molecular modeling studies were also performed to
disclose the binding modes of the most active inhibitors to the amino acid residues that compose the
active site of the AChE enzyme.
2. Results and Discussion
2.1. Chemistry
The target spiropyrrolidines 4a–j were synthesized as outlined in Scheme 1. The required starting
precursors 2-arylmethylidene-5,6-dimethoxy-2,3-dihydro-1H-inden-1-ones 1a–j were synthesized
following the synthetic protocol reported earlier by us [34] by the reaction of 5,6-dimethoxy-1-
indanone with appropriate aromatic aldehydes in dilute methanolic sodium hydroxide. The three
component 1,3-dipolar cycloaddition reaction of a series of 1a–j with a new class of relatively

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unexplored azomethine ylide generated in situ from isatin (2) and phenylalanine (3) in [bmim]Br
afforded the functionalized spiropyrrolidines 4a–j in good yields.
Scheme 1. Synthesis of spiropyrrolidines 4a–j.
Solvent optimization for this cycloaddition reaction was investigated by choosing the model
reaction between azomethine ylide and 2-(2-methylphenyl)methylidene-5,6-dimethoxy-2,3-dihydro-
1H-inden-1-one (1b). The reaction of an equimolar mixture of 1b, isatin and phenylalanine in
refluxing methanol, methanol:dioxane and dioxane for 5 h afforded the spiropyrrolidine (4b) in 44%,
52% and 58% yield whilst the reaction in [bmim]Br furnished 4b in an excellent yield of 87% in a
short reaction time of 1 h, suggesting [bmim]Br as the appropriate solvent in terms of enhanced yield
and reaction time for the cycloaddition reactions of this kind. Having determined the optimal
conditions, all the subsequent reactions were effected by heating an equimolar mixture of the reactants
in [bmim]Br (200 mg) in an oil-bath at 100 °C for 1 h. After completion of the reaction as evidenced
by TLC, the product was isolated and purified through column chromatography, whilst the ionic liquid
is recovered and reused, its efficacy being significantly undiminished in subsequent runs. As shown in
Table 1, the cycloaddition reaction works well regardless of the position and electronic or steric properties
of the substituents at the aromatic ring of compound 1 affording the cycloadduct 4 in good yields.
The structure of spiropyrrolidines 4 was elucidated unambiguously with the help of one and
two-dimensional NMR spectroscopic studies. As a representative case the chemical shift assignments
1
of 4b are discussed. In the H-NMR spectra of 4b, the doublet at 4.10 ppm (J = 8.8 Hz) can be
assigned to H-4'. H,H-COSY correlation of H-4' assigns the multiplet at 4.95–5.01 ppm to H-5'. The
C,H-COSY correlation of H-4' and H-5' assigns the carbon signals at 55.44 and 64.85 ppm to C-4' and
C-5', respectively. Further, the H,H-COSY correlation of H-5' assigns the doublet of doublets at 2.93
(J = 14.0, 8.8 Hz) and 3.02 (J = 14.0, 4.4 Hz) to the benzylic protons (6'-CH2). The C,H-COSY
correlation of 6'-CH2 assigns the carbon signal at 40.45 ppm to C-6'. The remaining doublets at 2.62

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and 2.73 ppm (J = 18.3 Hz) are due to 3-CH2. Further, H-4' shows HMBCs with the C=O of indanone
ring, C-5', C-6', C-3 besides showing correlation with the aromatic ring carbons. The carbon signals at
66.69 and 75.08 ppm are due to the spiro carbons C-3' and C-2', respectively. The carbon chemical shifts of
C-3, C-4', C-5' and C-6' were also confirmed from DEPT 135 NMR spectrum (vide supplementary
Materials).The singlets at 1.92, 3.70, 3.80 and 8.01 ppm can be attributed to the methyl, two methoxy
of aromatic rings and the NH group of oxindole ring, respectively. The aromatic protons appear as
singlets, doublets and multiplets around 6.19–8.05 ppm. The signals at 179.63 and 207.10 ppm are due
to the carbonyl groups of oxindole and indanone ring, respectively. The structure of other
spiropyrrolidines was also assigned by similar straightforward considerations. The stereochemistry of
compound 4b has been arrived from its NOESY spectra.
Table 1. Physical data, AChE and BChE inhibitory activities of 4a–j.
AChE Inhibition
IC50 µM (±SD)
11.29 ± 0.21
10.15 ± 0.17
12.23 ± 0.19
15.17 ± 0.22
2.13 ± 0.11
BChE Inhibition
IC50 µM (±SD)
15.73 ± 0.22
21.32 ± 0.19
11.39 ± 0.12
9.63 ± 0.15
Entry
Comp 4
R
Yield (%) a
mp °C
1
2
a
H
85
87
82
85
84
89
92
80
88
90
-
134–135
141–142
126–127
136–137
127–128
133–134
138–139
129–130
132–133
135–136
-
b
2-CH3
2-OCH3
2-Cl
3
c
4
d
5
e
3-OCH3
3-O2N
4-CH3
4-OCH3
4-Cl
17.44 ± 0.23
15.32 ± 0.18
10.46 ± 0.24
14.69 ± 0.17
16.11 ± 0.15
12.79 ± 0.25
19.21 ± 0.12
3.6 ± 0.11
6
f
1.57 ± 0.09
7
g
7.11 ± 0.13
8
h
3.67 ± 0.15
9
i
5.83 ± 0.20
10
11
12
j
2,4-Cl2
-
6.72 ± 0.18
Galantamine
Donepezil
2.09 ± 0.07
-
-
-
0.21 ± 0.02
a Isolated yield.
A mechanism proposed to rationalize the formation of 4 is summarized in Scheme 2. Hydrogen
atom of [bmim]+ being electron-deficient could form hydrogen bond between the imidazole ring
hydrogen atom of [bmim]+ and the carbonyl group of isatin facilitating the attack of lone pair of
phenylalanine on isatin and subsequent dehydration to furnish azomethine ylide 5. In the same way,
hydrogen atom of [bmim]+ forms hydrogen bond with the carbonyl group of 2-arylmethylidene-5,6-
dimethoxy-2,3-dihydro-1H-inden-1-one providing the easy addition of azomethine ylide to the more
electron deficient β-carbon to afford spiropyrrolidine 4, regio- and stereoselectively.
The presence of NH and benzyl groups in adjacent positions of the pyrrolidine ring of 4 prompted
us to explore their further reaction with paraformaldehyde (Pictet–Spengler cyclisation) in the presence
of trifluoroacetic acid to synthesize novel isoquinoline heterocyclic hybrids. Thus, the spiropyrrolidine
(4b) (1 mmol) was treated overnight with paraformaldehyde (1 mmol) in the presence of a catalytic
amount of trifluoroacetic acid (10 mol %) in CH2Cl2 at room temperature (Scheme 3). This reaction
afforded the mixture of products with same Rf value, which could not be separated through
column chromatography.

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Scheme 2. Plausible mechanism for the formation of spiropyrrolidine 4.
Scheme 3. Pictet-Spengler cyclisation of spiropyrrolidine 4b.
2.2. Cholinesterase Inhibitory Activity Study
All newly synthesized compounds were evaluated in vitro for their cholinesterase inhibitory activity
against AChE enzyme from electric eel and BChE enzyme from equine serum. As summarized in
Table 1, compound 4f, bearing nitro moiety at meta position of aromatic ring displayed highest
AChE inhibitory activity with calculated IC50 value of 1.57 µM, followed by 4e (Ar=m-OCH3),
4h (Ar=p-OCH3), 4i (Ar=p-Cl), 4j (Ar=o,p-Cl2) and 4g (Ar=p-CH3) all displaying remarkable IC50
values of lower than 10 µM. The rest of the compounds 4a–d displayed moderate AChE inhibitory
activities ranging from 10.15–15.17 µM. From Table 1, it is clear that the presence of electron
withdrawing nitro moiety, had greater impact on the AChE inhibitory potencies compared to electron
donating moieties such as methoxy (strong electron donor), methyl and chloro (mild electron donor).
The active site of AChE enzyme is located at bottom of a narrow channel composed of amino acid
residues with aromatic side chains such as tryptophan and tyrosine. The potency of inhibitors is closely
related to the ease of insertion into this channel and their ability to interact with its composing
residues. In fact, inhibitors having more facilitated insertion and better interactions with this gorge
should possess higher enzyme inhibitory potentials. Thus, the inhibitor 4f, owing to the unique
electronic withdrawing nature of its nitro group, strongly binds to the AChE active site and effectively
inhibited choline substrate hydrolysis. Interestingly, compounds bearing a strong electron donor
methoxy moiety, also showed good inhibitory activities but at much lower degree when compared to
those containing a nitro group.

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It is also worth to mention that these newly compounds are benefited from their specific chemical
structure of including three aromatic cores, by showing moderate to weak BChE inhibitory potencies
with calculated IC50 values ranging from 9.63–21.32 µM and good selectivity toward AChE enzyme.
The aromatic core multiplicity effect explains how compounds bearing three aromatic cores have
higher AChE inhibitory potencies than their analogs with only two aromatic cores and introduces the
aromatic content of inhibitor as an important factor to design potent AChE inhibitors [28].
Compared to standard drugs, galantamine and donepezil, compound 4f displayed significantly
higher and compound 4e showed comparable AChE inhibitory activity to galantamine. None of the
inhibitors had comparable activity to donepezil, due to its different planar chemical structure. For
BChE, except for compound 4b, all the inhibitors showed higher activities than galantamine. As
expected, none of the compounds showed comparable BChE activity to donepezil.
2.3. Molecular Docking Studies
The most active AChE inhibitor, compound 4f was docked into the active site of AChE enzyme
derived from the crystal structure of Torpedo californica in complex with donepezil (Figure 1).
Molecular docking analysis clearly demonstrated the influence of aromatic core multiplicity in the
remarkable activity observed for this compound. Compound 4f strongly bounded to the residues
having aromatic side chains such as Trp279, Tyr70, Tyr334 and Phe331, which compose peripheral
anionic site (PAS) of the AChE enzyme through hydrophobic interactions took place dominantly
between their aromatic entities. It was proposed that deposition of neurotoxin amyloid plaque in AD
could be accelerated or even triggered by interaction of β-amyloids with the peripheral anionic site of
the enzyme. Therefore, PAS inhibitors not only symptomatically improve the AD by blocking active
site gorge entrance and prevention of acetylcholine hydrolysis, but also can have effective disease
modifying properties [35].
Figure 1. Schematic representation of compound 4f bounded to active site of AChE enzyme.

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3. Experimental Section
3.1. General Methods
1
13
Melting points were measured using open capillary tubes and are uncorrected. H- and C-NMR
spectra were recorded on Jeol 500 and 400 MHz instruments (Tokyo, Japan) in CDCl3 using TMS as
internal standard. Chemical shifts are given in parts per million (δ scale) and coupling constants are
given in Hertz. IR spectra were recorded on a PerkinElmer spectrum 100 FT-IR instrument (KBr
pellet, Shelton, AL, USA). Elemental analyses were performed 10 on a PerkinElmer 2400 Series II
Elemental CHNS analyzer (Waltham, MA, USA). Column chromatography was performed on silica gel
(230–400 mesh) using petroleum spirit/ethyl acetate as eluent.
3.2. General Procedure for the Synthesis of 4
An equimolar mixture of 2-arylmethylidene-5,6-dimethoxy-2,3-dihydro-1H-inden-1-one (1), isatin
(2) and phenylalanine (3) was heated with stirring at 100 °C in 200 mg of [bmim]Br for 1 h. After
completion of the reaction (TLC), ethyl acetate (10 mL) was added and the reaction mixture stirred for
15 min. The ethyl acetate layer was separated, washed with water and dried. The product obtained in
good yield was purified through column chromatography. [bmim]Br after extraction of the product
was completely dried under vacuum and reused for subsequent reactions.
3.3. Characterization Data for Compounds 4a–j
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-phenyldispiro[1H-indene-2,3'-pyrrolidine-2',3''-indoline]-
1
1,2''-dione (4a). IR (KBr) νmax 3405, 1721, 1685, 1620, 1594 cm−1; H-NMR (400 MHz, CDCl3): δH
2.65 (d, J = 17.6 Hz, 1H, 3-CH2), 2.86–2.97 (m, 2H, 3-CH2 and 6'-CH2), 3.04–3.08 (m, 1H, 6'-CH2),
3.65 (s, 3H, OCH3), 3.78 (s, 3H, OCH3), 3.94 (d, J = 8.8 Hz, 1H, H-4'), 4.70–4.81 (m, 1H, H-5'), 6.19
(s, 1H, Ar-H), 6.58 (d, J = 7.2 Hz, 1H, Ar-H), 6.86–7.40 (m, 14H, Ar-H), 8.37 (s, 1H, NH). 13C-NMR
(100 MHz, CDCl3): δC 34.27 (C-3), 40.10 (C-6'), 56.09 (OCH3), 56.15 (OCH3), 59.47 (C-4'), 64.00
(C-5'), 67.16 (C-3'), 74.96 (C-2'), 104.08, 106.49, 109.63, 122.52, 126.37, 128.30, 128.45, 129.19,
129.27, 129.40, 129.62, 129.74, 130.21, 135.48, 136.57, 138.85, 148.17, 149.42, 155.44, 180.13
(C-2''), 206.35 (C-1). Anal. calcd for C34H30N2O4: C, 76.96; H, 5.70; N, 5.28. Found: C, 76.82; H,
5.47; N, 5.19%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(2-methylphenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4b). IR (KBr) νmax 3427, 1720, 1687, 1619, 1591 cm−1; H-NMR (400 MHz,
CDCl3): δH 1.92 (s, 3H, CH3), 2.62 (d, J = 18.32 Hz, 1H, 3-CH2), 2.73 (d, J = 18.32 Hz, 1H, 3-CH2),
2.93 (dd, J = 13.96, 8.0 Hz, 1H, 6'-CH2), 3.02 (dd, J = 13.92, 4.4 Hz, 1H, 6'-CH2), 3.70 (s, 3H, OCH3),
3.80 (s, 3H, OCH3), 4.10 (d, J = 8.8 Hz, 1H, H-4'), 4.95–5.01 (m, 1H, H-5'), 6.19 (s, 1H, Ar-H), 6.57
(d, J = 7.32 Hz, 1H, Ar-H), 6.86 (t, J = 7.32 Hz, 1H, Ar-H), 6.90 (s, 1H, ArH), 6.99–7.31 (m, 9H,
13
Ar-H), 7.36 (d, J = 7.36 Hz, 1H, Ar-H), 8.01 (s, 1H, NH), 8.04 (d, J = 8.0 Hz, 1H, Ar-H). C-NMR
(100 MHz, CDCl3): δC 20.33 (CH3), 35.16 (C-3), 40.45 (C-6'), 55.44 (C-4'), 55.95 (OCH3), 56.04
(OCH3), 64.85 (C-5'), 66.69 (C-3'), 75.08 (C-2'), 103.96, 106.41, 109.35, 122.51, 126.13, 126.23,
126.51, 126.69, 127.05, 127.71, 128.23, 129.00, 129.14, 129.66, 130.08, 137.46, 137.91, 138.92,

Molecules 2015, 20
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140.92, 148.09, 149.27, 155.36, 179.63 (C-2''), 207.10 (C-1). Anal. calcd for C35H32N2O4: C, 77.18; H,
5.92; N, 5.14. Found: C, 77.39; H, 5.78; N, 5.02%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(2-methoxyphenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4c). IR (KBr) νmax 3429, 1722, 1688, 1621, 1595 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.40–2.54 (m, 2H, 3-CH2), 2.91–3.09 (m, 2H, 6′-CH2), 3.66 (s, 3H, OCH3), 3.69 (s, 3H,
OCH3) 3.87 (s, 3H, OCH3), 4.14 (d, J = 9.2 Hz, 1H, H-4'), 5.02–5.10 (m, 1H, H-5'), 6.10 (s, 1H, Ar-H),
6.53 (d, J = 7.2 Hz, 1H, Ar-H), 6.61 (d, J = 8.0 Hz, 1H, Ar-H), 6.82–7.40 (m, 11H, Ar-H), 7.80 (d,
13
J = 7.2 Hz, 1H, Ar-H), 8.59 (s, 1H, NH). C-NMR (100 MHz, CDCl3): δC 34.72 (C-3), 41.02 (C-6'),
53.48 (C-4'), 54.41 (OCH3), 56.13 (OCH3), 61.68 (OCH3), 65.53 (C-5'), 67.15 (C-3'), 75.01 (C-2'),
104.11, 106.40, 109.52, 109.79, 120.65, 122.59, 126.16, 126.67, 127.71, 128.15, 128.30, 128.98,
129.15, 129.25, 135.42, 139.34, 148.72, 149.09, 155.06, 158.17, 179.63 (C-2''), 208.06 (C-1). Anal.
calcd for C35H32N2O5: C, 74.98; H, 5.75; N, 5.00. Found: C, 75.21; H, 5.61; N, 5.12%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(2-chlorophenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4d). IR (KBr) νmax 3422, 1716, 1684, 1620, 1595 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.51 (d, J = 18.0 Hz, 1H, 3-CH2), 2.64 (d, J = 18.0 Hz, 1H, 3-CH2), 2.96–3.10 (m, 2H,
6'-CH2), 3.69 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 4.39 (d, J = 8.8 Hz, 1H, H-4'), 4.92–4.99 (m, 1H,
H-5′), 6.30 (s, 1H, Ar-H), 6.60 (d, J = 7.6 Hz, 1H, Ar-H), 6.92–7.47 (m, 10H, Ar-H), 7.95 (s, 1H,
ArH), 8.07 (d, J = 7.6 Hz, 1H, Ar-H), 8.36 (s, 1H, NH). 13C-NMR (100 MHz, CDCl3): δC 34.77 (C-3),
40.78 (C-6'), 56.28 (OCH3), 56.39 (OCH3), 61.10 (C-4'), 66.28 (C-5'), 67.16 (C-3'), 75.37 (C-2'),
105.27, 106.54, 107.18, 109.66, 122.72, 126.81, 127.95, 128.26, 128.56, 129.08, 129.18, 129.84,
131.11, 133.87, 136.13, 137.39, 138.53, 149.39, 149.78, 155.45, 179.60 (C-2''), 207.11 (C-1). Anal.
calcd for C34H29ClN2O4: C, 72.27; H, 5.17; N, 4.96. Found: C, 72.06; H, 5.41; N, 4.87%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(3-methoxyphenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4e). IR (KBr) νmax 3438, 1716, 1692, 1618, 1587 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.69 (d, J = 17.6 Hz, 1H, 3-CH2), 2.88–3.10 (m, 3H, 3-CH2 and 6'-CH2), 3.67 (s, 3H,
OCH3), 3.72 (s, 3H, OCH3) 3.77 (s, 3H, OCH3), 4.08 (d, J = 8.8 Hz, 1H, H-4'), 4.75–4.82 (m, 1H,
H-5'), 6.22 (s, 1H, Ar-H), 6.60 (d, J = 8.0 Hz, 1H, Ar-H), 6.71–7.28 (m, 13H, Ar-H), 8.72 (s, 1H, NH).
13C-NMR (100 MHz, CDCl3): δC 34.42 (C-3), 40.28 (C-6'), 55.22 (OCH3), 56.02 (OCH3), 56.14
(OCH3), 60.00 (C-4'), 64.26 (C-5'), 67.14 (C-3'), 74.94 (C-2'), 104.10, 106.65, 109.71, 112.54, 115.79,
122.53, 122.74, 123.17, 126.31, 128.10, 128.40, 129.21, 129.36, 129.40, 138.79, 140.45, 148.29,
149.32, 155.52, 159.68, 180.27 (C-2''), 206.44 (C-1). Anal. calcd for C35H32N2O5: C, 74.98; H, 5.75;
N, 5.00. Found: C, 75.10; H, 5.86; N, 5.08%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(3-nitrophenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4f). IR (KBr) νmax 3425, 1721, 1684, 1623, 1590 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.55 (d, J = 17.6 Hz, 1H, 3-CH2), 2.90 (d, J = 17.6 Hz, 1H, 3-CH2), 2.94–2.98 (m, 1H, 6'-CH2),
3.10–3.19 (m, 1H, 6'-CH2), 3.73 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 3.88 (d, J = 8.8 Hz, 1H, H-4′),
4.78–4.84 (m, 1H, H-5'), 6.39 (s, 1H, Ar-H), 6.68 (d, J = 7.2 Hz, 1H, Ar-H), 6.85–7.86 (m, 10H,
Ar-H), 7.67 (d, J = 7.2 Hz, 1H, Ar-H), 7.97 (d, J = 8.0 Hz, 1H, Ar-H), 8.28 (s, 1H, ArH), 8.47 (s, 1H,
NH). 13C-NMR (100 MHz, CDCl3): δC 34.52 (C-3), 40.76 (C-6'), 56.12 (OCH3), 56.18 (OCH3), 60.09

Molecules 2015, 20
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(C-4'), 65.47 (C-5'), 67.16 (C-3'), 75.15 (C-2'), 104.32, 106.69, 109.70, 122.84, 122.98, 126.41,
126.53, 126.78, 127.82, 128.36, 129.18, 129.27, 136.75, 137.93, 141.00, 141.72, 147.74, 148.29,
149.66, 155.90, 179.76 (C-2''), 206.80 (C-1). Anal. calcd for C34H29N3O6: C, 70.94; H, 5.08; N, 7.30.
Found: C, 70.68; H, 5.25; N, 7.41%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(4-methylphenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4g). IR (KBr) νmax 3420, 1723, 1691, 1620, 1589 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.30 (s, 3H, CH3), 2.67 (d, J = 17.6 Hz, 1H, 3-CH2), 2.85–2.94 (m, 2H, 3-CH2 and 6'-CH2),
3.07 (dd, J = 14.0, 2.8 Hz, 1H, 6'-CH2), 3.66 (s, 3H, OCH3), 3.77 (s, 3H, OCH3), 3.81 (d, J = 10.4 Hz,
1H, H-4'), 4.75–4.82 (m, 1H, H-5'), 6.14 (s, 1H, Ar-H), 6.56 (d, J = 8.0 Hz, 1H, Ar-H), 6.83–7.29 (m,
13
13H, Ar-H), 8.48 (s, 1H, NH). C-NMR (100 MHz, CDCl3): δC 21.15 (CH3), 34.24 (C-3), 40.13
(C-6'), 56.01 (OCH3), 56.14 (OCH3), 59.44 (C-4'), 63.92 (C-5'), 67.21 (C-3'), 74.84 (C-2'), 104.07,
106.61, 109.65, 122.54, 126.31, 128.22, 128.43, 129.14, 129.26, 129.37, 129.60, 129.75, 130.15,
135.45, 136.62, 138.87, 148.16, 149.30, 155.46, 180.26 (C-2''), 206.25 (C-1). Anal. calcd for
C35H32N2O4: C, 77.18; H, 5.92; N, 5.14. Found: C, 77.43; H, 5.74; N, 5.01%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(4-methoxyphenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4h). IR (KBr) νmax 3426, 1722, 1691, 1622, 1596 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.69 (d, J = 17.6 Hz, 1H, 3-CH2), 2.85–2.96 (m, 2H, 3-CH2 and 6'-CH2), 3.03–3.08 (m, 1H,
6'-CH2), 3.71 (s, 3H, OCH3), 3.76 (s, 3H, OCH3), 3.78 (s, 3H, OCH3), 3.85–3.98 (m, 1H, H-4'),
3.67–3.75 (m, 1H, H-5'), 6.29 (s, 1H, Ar-H), 6.57 (d, J = 7.2 Hz, 1H, Ar-H), 6.79–7.62 (m, 13H,
13
Ar-H), 8.09 (s, 1H, NH). C-NMR (100 MHz, CDCl3): δC 34.15 (C-3), 40.22 (C-6'), 55.27 (OCH3),
56.04 (OCH3), 56.15 (OCH3), 59.16 (C-4'), 64.12 (C-5'), 67.26 (C-3'), 74.73 (C-2'), 104.13, 106.70,
109.51, 113.88, 122.58, 126.31, 128.30, 128.42, 129.15, 129.35, 130.58, 131.31, 132.42, 138.89,
148.18, 149.31, 155.49, 158.55, 180.01 (C-2''), 206.30 (C-1). Anal. calcd for C35H32N2O5: C, 74.98; H,
5.75; N, 5.00. Found: C, 74.70; H, 5.96; N, 5.13%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(4-chlorophenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4i). IR (KBr) νmax 3389, 1718, 1685, 1615, 1594 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.68 (d, J = 17.6 Hz, 1H, 3-CH2), 2.89–2.99 (m, 2H, 3-CH2 and 6'-CH2), 3.06–3.10 (m, 1H,
6'-CH2), 3.68 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 3.95 (d, J = 8.8 Hz, 1H, H-4'), 4.72–4.84 (m, 1H,
H-5'), 6.21 (s, 1H, Ar-H), 6.57 (d, J = 7.6 Hz, 1H, Ar-H), 6.83–7.45 (m, 13H, Ar-H), 8.40 (s, 1H, NH).
13C-NMR (100 MHz, CDCl3): δC 34.30 (C-3), 40.17 (C-6'), 56.10 (OCH3), 56.22 (OCH3), 59.45 (C-4'),
64.04 (C-5'), 67.13 (C-3'), 75.05 (C-2'), 104.12, 106.45, 109.62, 122.47, 126.46, 128.35, 128.43,
129.24, 129.31, 129.47, 129.61, 129.80, 130.29, 135.52, 136.61, 138.87, 148.20, 149.46, 155.49,
180.21 (C-2''), 206.63 (C-1). Anal. calcd for C34H29ClN2O4: C, 72.27; H, 5.17; N, 4.96. Found: C,
72.50; H, 5.02; N, 4.87%.
5'-Benzyl-5,6-dimethoxy-2,3-dihydro-4'-(2,4-dichlorophenyl)dispiro[1H-indene-2,3'-pyrrolidine-2',3''-
1
indoline]-1,2''-dione (4j). IR (KBr) νmax 3394, 1721, 1682, 1614, 1583 cm−1; H-NMR (400 MHz,
CDCl3): δH 2.47 (d, J = 18.0 Hz, 1H, 3-CH2), 2.62 (d, J = 18.0 Hz, 1H, 3-CH2), 2.93 (dd, J = 14.0,
5.8 Hz, 1H, 6'-CH2), 2.99 (dd, J = 14.0, 5.8 Hz, 1H, 6'-CH2), 3.72 (s, 3H, OCH3), 3.81 (s, 3H, OCH3),
4.31 (d, J = 8.8 Hz, 1H, H-4'), 4.84–4.90 (m, 1H, H-5'), 6.30 (s, 1H, Ar-H), 6.59 (d, J = 7.2, 1H, Ar-H),

Molecules 2015, 20
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6.84–7.35 (m, 9H, Ar-H), 7.47 (s, 1H, ArH), 7.60 (d, J = 8.8 Hz, 1H, Ar-H), 8.00 (d, J = 8.8 Hz, 1H,
13
Ar-H), 8.40 (s, 1H, NH). C-NMR (100 MHz, CDCl3): δC 34.66 (C-3), 40.88 (C-6'), 55.67 (OCH3),
56.08 (OCH3), 64.73 (C-4'), 66.06 (C-5'), 67.15 (C-3'), 75.37 (C-2'), 104.27, 106.55, 109.70, 122.81,
126.32, 126.51, 127.17, 127.83, 128.30, 128.73, 129.11, 129.53, 132.19, 132.79, 136.16, 136.60,
138.27, 141.23, 144.96, 148.02, 149.51, 155.59, 179.31 (C-2''), 206.98 (C-1). Anal. calcd for
C34H28Cl2N2O4: C, 68.12; H, 4.71; N, 4.67. Found: C, 68.30; H, 4.94; N, 4.56%.
3.4. General Procedure for the Synthesis of 6
To a stirred solution of 4b (1 mmol) in 5 mL of dry dichloromethane was added paraformaldehyde
(1 mmol), followed by catalytic amount of trifluoroacetic acid at 0 °C. The reaction mixture was
allowed to stir overnight and after completion of the reaction, the mixture was washed with water and
dried over Na2SO4. The solvent was removed under reduced pressure to afford 6b as mixture of
products with same Rf value, which could not be separated through column chromatography.
3.5. Cholinesterase Inhibition Assays
Cholinesterase inhibitory activity of the synthesized compounds was evaluated using the Ellman’s
microplate assay. For acetylcholinesterase (AChE) inhibitory assay, 140 μL of 0.1 M sodium
phosphate buffer (pH 8) was first added to a 96-wells microplate followed by 20 μL of test samples
and 20 μL of 0.09 units/mL acetylcholinesterase enzyme from Electrophoruselectricus (Sigma; St. Louis,
MO, USA). After 15 min of incubation at 25 °C, 10 μL of 10 mM 5,5'-Dithiobis-2-nitrobenzoic acid
(DTNB) was added into each well, followed by 10 μL of acetylthiocholine iodide (14 mM). At 30 min
after the initiation of enzymatic reaction, absorbance of the colored end product was measured using
BioTek Power Wave X 340 Microplate Spectrophotometer (BioTek; Winooski, VT, USA) at 412 nm.
For butyrylcholinesterase (BChE) inhibitory assay, the same procedure described above was followed,
except for the use of enzyme and substrate, instead of which, butyrylcholine esterase from equine
serum and S-butyrylthiocholine chloride were used. Galanthamine was used as positive control. Test
samples and galanthamine were prepared in DMSO at an initial concentration of 1 mg/mL (1000 ppm).
The concentration of DMSO in final reaction mixture was 1%. At this concentration, DMSO has no
inhibitory effect on acetylcholinesterase enzyme.
The initial screening was carried out at 10 µg/mL of test samples in 1% DMSO and each test was
conducted in triplicates. Absorbance of the test samples was corrected by subtracting the absorbance of
their respective blank. Percentage enzyme inhibition is calculated using the following formula:
(1)
Subsequently, the determination of IC50 was carried out using a set of five concentrations.
3.6. Molecular Modeling
Using GlideTM, (version 5.7, Schrödinger, LLC, New York, NY, USA, 2011), most active
compound was docked onto the active site of TcAChE derived from three-dimensional structure of the
enzyme complex with anti-Alzheimer’s drug, galanthamine (PDB ID: 4EVE).

Molecules 2015, 20
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Water molecules and hetero groups were deleted from enzyme beyond the radius of 5 Å of
reference ligand (galanthamine), resulting protein structure refined and minimized by Protein
Preparation WizardTM (Version 2014-1, Schrödinger, LLC, New York, NY, USA) using OPLS-2005
force field. Receptor Grid Generation program were used to prepare TcAChE grid and the ligand was
optimized by LigPrepTM (Version 2014-1, Schrödinger LLC, New York, NY, USA) program by using
OPLS-2005 force field to generate lowest energy state. Docking stimulations were carried out on
bioactive compound, handed in 5 poses per ligand, in which the best pose with highest score was
displayed for each ligand.
4. Conclusions
In conclusion, the synthesis of a series of novel spiropyrrolidines was accomplished via [bmim]Br
mediated one-pot three component 1,3-dipolar cycloaddition strategy employing a new class of
azomethine ylide. Among the spiropyrrolidines 4a–j, compound 4f displayed the highest AChE
enzyme inhibitory activity, Molecular-docking analysis, disclosed binding interactions of this
compound to the active site residues of the AChE enzyme.
Supplementary Materials
Copy of FT-IR, 1H and 13C spectra of a representative compound is available on supplementary file.
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/20/02/2296/s1.
Acknowledgments
The authors acknowledge the Deanship of Scientific Research at King Saud University for the
Research Grant RGP-VPP-026.
Author Contributions
The contributions of the respective authors are as follows: AIM, RSK, NA—design and synthesis of
compounds; AB, YK, VM, MAA, MF—Cholinesterase inhibitory activity and Molecular modeling
study. The manuscript was written by RSK and AB.
Conflicts of Interest
The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
© 2015 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
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