AChE inhibitor: A regio- and stereo-selective 1,3-dipolar cycloaddition for the synthesis of novel substituted 5,6-dimethoxy spiro[5.3′]-oxindole- spiro-[6.3″]-2,3-dihydro-1H-inden-1″-one-7-(substituted aryl)-tetrahydro-1H-pyrrolo[1,2-c][1,3]thiazole

Article abstract of DOI:10.1016/j.bmcl.2011.10.087

Pyrrolothiazolyloxindole analogues share vital pharmacological properties, considered useful in Alzheimer's disease (AD). The aim of this study was synthesis and evaluate pyralothiazolyloxindole analogues if possess acetyl cholinesterase (AChE) inhibitory

Full text of DOI:10.1016/j.bmcl.2011.10.087

Bioorganic & Medicinal Chemistry Letters 22 (2012) 508–511
Contents lists available at SciVerse ScienceDirect
Bioorganic & Medicinal Chemistry Letters
journal homepage: www.elsevier.com/locate/bmcl
AChE inhibitor : A regio- and stereo-selective 1,3-dipolar cycloaddition
for the synthesis of novel substituted 5,6-dimethoxy spiro[5.30]-oxindole-
spiro-[6.300]-2,3-dihydro-1H-inden-100-one-7-(substituted aryl)-tetrahydro-
1H-pyrrolo[1,2-c][1,3]thiazole
Mohamed Ashraf Ali a, , Rusli Ismail a, Tan Soo Choon a, Raju Suresh Kumar b, Hasnah Osman b,
⇑
Natarajan Arumugam c, Abdulrahman I. Almansour c, Karthikeyan Elumalai e, Abhimanyu Singh d
a Institute for Research in Molecular Medicine, Universiti Sains Malaysia, Minden 11800, Penang, Malaysia
b School of Chemical Science, Universiti Sains Malaysia, Minden 11800, Penang, Malaysia
c Department of Chemistry, College of Sciences, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia
d New Drug Discovery Research, Department of Medicinal Chemistry, Alwar Pharmacy College, Alwar, Rajasthan 301 030, India
e Faculty of Pharmacy, Department of Drug Discovery Research, Sunrise University, Alwar, Rajasthan 301 030, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyrrolothiazolyloxindole analogues share vital pharmacological properties, considered useful in Alzhei-
mer’s disease (AD). The aim of this study was synthesis and evaluate pyralothiazolyloxindole analogues
if possess acetyl cholinesterase (AChE) inhibitory activity. The easily accessible one-pot synthesis of these
compounds resulted to be significantly less difficult and expensive than that of donepezil. Several com-
pounds possess anti-cholinesterase activity in the order of micro and sub-micromolar. Particularly, com-
pound 6a was the most potent inhibitors of the series against acetyl cholinesterase enzyme with IC50
Received 3 August 2011
Revised 13 October 2011
Accepted 27 October 2011
Available online 20 November 2011
Keywords:
0.11 lmol/L.
Alzheimer diseases
Acetyl cholinesterase
AchE inhibitor
Ó 2011 Elsevier Ltd. All rights reserved.
1,3-Dipolar cycloaddition
Azomethine ylide
Principal role of acetylcholinesterase (AChE) is the termination
of nerve impulse transmission at the cholinergic synapses by rapid
hydrolysis of acetylcholine (ACh). Inhibition of AChE serves as a
strategy for the treatment of Alzheimer’s disease (AD), senile
dementia, ataxia, myasthenia gravis and Parkinson’s disease.1 Alz-
heimer’s disease is one form of senile dementia which occurs due
to various neuropathological conditions such as senile plaques
and neurofibrillary tangles. It is the most widely spread dementias
that affects 50% of the population aged 85 years.2,3 It is the seventh
leading cause of death in the world affecting 5.3 million people. In
AD, increasing numbers of nerve cells deteriorate and die along
with loss in synapse through which information flows from and
to the brain, this leads to cognitive impairment and dementia.4
The neuropathology of AD is generally characterized by the
presence of numerous plaques of amyloid b-peptide (Ab) plaques,
neurofibrillary tangles (NFT), and degeneration or atrophy of the
basal forebrain cholinergic neurons. The loss of basal forebrain
cholinergic cells results in an important reduction in acetylcholine
(ACh), which plays an important role in the cognitive impairment
associated with AD. Acetylcholine esterase plays an important role
in accelerating Ab plaque deposition. The levels of MAO-B are in-
creased in temporal, parietal and frontal cortex of AD patients.
The high levels of MAO-B increase hydroxyl radicals which in turn
lead to the formation of Ab plaques.5
The increasing level of ACh has been regarded as one of the
most promising approaches for the symptomatic treatment of
AD. Acetyl choline esterase is an enzyme that inhibits the hydroly-
sis of acetylcholine there by it increases the level of acetyl choline.6
Several drugs are under development targeting acetylcholine
esterase (AChE). The commercially available AChE inhibitors in-
clude galanthamine, donepezil, rivastigmine and tacrine (Fig. 1).6
Based on the prevalence of disease and the availability of
limited options of drugs for the treatment of AD, in the present
investigation potent pyrrolothiazolyloxindole analogues were
developed targeting AChEI involved in decreasing the acetyl cho-
line levels.
In continuation of our earlier work7 in the construction of novel
heterocycles via domino and cycloaddition reactions and/or their
screening for biological activities prompted us to investigate the
[3+2]-cycloaddition of azomethine ylides generated in situ from
⇑
Corresponding author.
E-mail address: asraf80med@rediffmail.com (M. Ashraf Ali).
0960-894X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2011.10.087

M. Ashraf Ali et al. / Bioorg. Med. Chem. Lett. 22 (2012) 508–511
509
5a–e and 6a–e, respectively) were purified by crystallization. The
results obtained for both methods are summarized in Table 1,
and clearly show that microwave irradiation led to an enhance-
ment in the yield of the product (90–96%) over the conventional
thermal method (72–88%). As shown in Table 1, the reaction works
well regardless of the position and electronic or steric properties of
the substituents at the aromatic rings of compounds 1.
The purity of the compounds ( 4a–e, 5a–e and 6a–e) was
checked by TLC and elemental analyses, the results were within
0.4% of the theoretical values. Both analytical and spectral data
(IR and 1H-NMR) of all the synthesized compounds were in full
agreement with the proposed structures. In general, Infra red spec-
tra (IR) of compound 6a shows characteristic bands at 1525, 1690,
1724 and 3290 cmꢀ1. In the 1H NMR spectra, the signals of the
respective protons of the synthesized title compounds were veri-
fied on the basis of their chemical shifts, multiplicities and cou-
pling constants.
N
NH2
N
N
O
HO
O
O
O
N
H
O
Rivastigmine
Galantamine
Tacrine
O
CH3
O
O
H
N
N
H3C
NH2
Huperzine A
Donepezil
Figure 1. Structure of some acetylcholine esterase inhibitors.
In the 1H NMR spectrum of 6a, the two doublets at d 2.78 and d
4.02 ppm (J = 17.0 Hz) corresponding to CH2 group; doublet of dou-
blets at d 2.98 and d 3.18 ppm (J = 10.94 and 4.4 Hz) corresponding
to CH2 group; doublets at d 3.58 and d 3.92 ppm (J = 9.1 Hz) corre-
sponding to CH2 group; multiplet at d 4.72–4.76 ppm correspond-
ing to CH group; doublet at d 4.2–4.14 ppm corresponding to CH
group; singlet at d 3.40, 4.13 ppm corresponding to OCH3 group,
multiplet at d 6.80–7.40 ppm corresponding to aromatic protons,
multiplet at d 7.70–8.70 ppm corresponding to pyridine ring pro-
ton, singlet at d 10.48 ppm corresponding to NH proton (Fig. 3).
It was observed that the non-stabilized azomethine ylide added
regioselectively to the exocyclic double bond of the dipolarophiles
(1) to afford the novel spiroheterocycles.
Among the novel substituted spiro-pyrrolothiazolyloxindoles
derivatives for treating AD, their anticholinesterase activities
(4a–e, 5a–e and 6a–e) was assayed according to Ellmann’s meth-
od9 on AChE from electric eel using commercial donepezil HCl as
the reference standard. Inhibition of AChE activities of the synthe-
sized compounds is shown in Table 1. The results clearly showed
that most of the designed compounds exhibited good to moderate
inhibitory activities toward the AChE inhibition are summarized in
Table 1. All spiro-pyrrolothiazolyloxindoles derivatives were po-
tent inhibitors of AChE, with IC50 values ranging from micromolar
isatin and L(ꢀ)thiazolidine-4-carboxylic acid to 5,6-dimethoxy-2-
[(E)-1-arylmethylidene]-1-indanone. Interestingly, this three-com-
ponent reaction furnished novel pyrrolothiazolyloxindole with the
assembly of new heterocyclic rings and these confine compounds
were assayed for their acetylcholinesterase inhibitory activity
and the results are reported here.
1,3-Dipolar cycloaddition of azomethine ylide generated by the
decarboxylative condensation of appropriate isatin (2) and thiazol-
idine-4-carboxylic acid (3) with dipolarophiles 1 were initially per-
formed by heating an equimolar mixture of the starting materials
under reflux in methanol to afford the title compounds in
72–88% yield after recrystalization (Scheme 1). The reactions were
subsequently investigated also under microwave irradiation in a
focused microwave synthesizer, as this technique has evolved as
a valuable alternative to conventional heating for the introduction
of energy into reactions and has clear benefits in many chemical
transformations, including cycloadditions (Fig. 2),8 in terms of rate
accelerations and yield enhancements. As in the case of the ther-
mal reactions, an equimolar mixture of the starting materials 1–3
in methanol was subjected to microwave irradiation for 4–7 min
until completion of the reaction (TLC) and the products ( 4a–e,
NH
R
O
O
S
O
O
R
O
MeOH
Reflux 4-6 h
+
+
N
O
HOOC
N
H
S
Ar
N
H
O
O
O
Ar
1
2
3
Ar = Phenyl, Pyridyl,
4-fluorophenyl,
4 R=H
5 R=Cl
6 R=NO2
3,4-dimethoxyphenyl,
2-chlorophenyl
Scheme 1. Protocol for the synthesis of spiro-pyrrolothiazolyloxindoles.
S
O
R
O
CO2H
S
N
H
NH
N
Ar
O
O
3
N
O
1
O
+
-CO2
S
O
O
R
R
NH
O
Ar
O
N
H
2
Figure 2. Plausible mechanism for the regioselective formation of spiro-pyrrolothiazolyloxindoles.

510
M. Ashraf Ali et al. / Bioorg. Med. Chem. Lett. 22 (2012) 508–511
Table 1
Comparison of the reaction times, yields for the conventional and microwave-assisted three-component cycloadditions and Inhibition of AchE activities of the synthesized
compounds
NH
R
O
O
O
N
S
O
Ar
Entry
Compd
Ar
R
Rf value Conventional
Microwave method
mp (°C) AchE inhibition (IC50 SD)a
(lmol/L)
method (MeOH,
reflux)
(100 °C, 100 W)
Yield
Time (h)
Yield
Time(min)
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
5a
5b
5c
5d
5e
6a
6b
6c
6d
6e
Pyridyl
H
H
H
H
H
Cl
Cl
0.67
0.64
0.69
0.62
0.78
0.73
0.76
0.74
0.66
0.70
74
65
72
80
82
85
92
85
77
82
90
56
81
76
72
—
4
5
5
5
5
6
5
6
6
5
5
6
5
5
5
—
94
92
93
90
92
92
95
92
96
95
94
94
92
93
90
—
4
5
5
5
3
5
5
5
7
5
5
6
5
5
5
—
204
183
194
161
138
113
126
105
154
162
126
178
194
172
182
—
0.48 0.02
1.12 0.02
6.20 0.02
8.32 0.1
5.80 0.01
0.34 0.01
1.56 0.1
2.24 0.1
3.28 0.1
2.6 0.1
0.11 0.1
1.16 0.1
2.10 0.1
2.12 0.1
3.16 0.1
0.12 0.01
4-Fluorophenyl
3,4-dimethoxyphenyl
2-Chlorophenyl
Phenyl
Pyridyl
4-Fluorophenyl
3,4-Dimethoxyphenyl Cl
2-Chlorophenyl
Phenyl
Pyridyl
4-Fluorophenyl
9
Cl
Cl
10
11
12
13
14
15
NO2 0.78
NO2 0.82
3,4-Dimethoxyphenyl NO2 0.85
2-Chlorophenyl
Phenyl
—
NO2 0.59
NO2 0.83
—
Standard Donepezil HCl
—
a
Data are means standard deviation of duplicate independent experiments
2.78,d,1H,J,17.0.1Hz
4.02,d,1H,J,17.0Hz
can be further modified to exhibit better potency than the standard
drugs. Further studies to acquire more information about quantita-
tive structure–activity relationships (QSAR) and in vivo studies are
in progress in our laboratory. The spiro-pyrrolothiazolyloxindoles
derivatives discovered in this study may provide valuable thera-
peutic intervention for the treatment of AD.
56.3
O
H
N
145.7
N
172.2
O
O
148.0
130.5
153.6
28.4
56.1
O2N
62.0
83.8
66.7
H
S
Acknowledgments
3.58, d, 1H, J=9.1 Hz
3.90, d, 1H, J=9.1 Hz
207.2
H
33.2
H
56.3
O
Author’s gratefully acknowledge Deanship of Scientific Re-
search, College of Science, King Saud University and this work
was funded through Research Grant No. RGP-VPP-026 and thanks
to Universiti Sains Malaysia for the providing research facilities.
4.14,d,1H,J=10.2 Hz
H
38.3
H
2.98, dd, 1H, J=10.94, 4.4 Hz
3.18, dd, 1H, J=10.94, 4.4 Hz
H
N
4.72-4.76,m,1H
Supplementary data
Figure 3. Selected 1H and 13C NMR chemical shifts of 6a.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.bmcl.2011.10.087.
to sub-micromolar. A simple structure–activity relationship analy-
sis showed that the AChE inhibitory potency closely related to the
substitution of the spiro-pyrrolothiazolyloxindoles ring. Com-
pounds 6a, 5a, and 4a with pyridine ring, were the best inhibitors
in their series. Especially, compound 6a showed the best AChE
inhibitory activity of all the spiro-pyrrolothiazolyloxindoles deriv-
References and notes
1. Rahman, A. U.; Choudhary, M. I. Pure Appl. Chem. 2001, 73, 555.
2. Olson, R. E.; Thompson, L. A. Annu. Rep. Med. Chem. 2000, 35, 31.
3. Mudher, A.; Lovestone, S. Trends Neurosci. 2002, 25, 22.
4. Corinne, B. H. J. Med. Chem. 2000, 44, 3195.
5. Castro, A.; Martinez, A. Mini-Rev Med. Chem. 2001, 1, 267.
6. Angela, R. J. Med. Chem. 2001, 44, 3810.
atives, with an IC50 value of 0.10 lM. Other compounds in this ser-
ies showed a remarkably moderate inhibitory activity compared to
the nitro group substitute with pyridine ring containing spiro-pyr-
rolothiazolyloxindoles derivatives. Nitro group with pyridine ring
substituted derivatives fashioned most potent inhibitory activity
than other substitution containing analogues. The relative rigidity
of the nitro with pyridine substituted scaffold could encumber
penetration into the AChE ravine to reach the binding site.
From the IC50 values of the compounds, it appeared that the
spiro-pyrrolothiazolyloxindoles derivatives 4b, 5b, 6b showing a
better inhibitory activity of AChE. Among the newer derivatives,
it is conceivable that derivatives showing AChE inhibitory activity
7. (a) Ashraf Ali, M.; Govindasamy, J.; Manogaran, E.; Sellappan, V.; Hasan, M. Z.;
Ahsan, M. J.; Pandian, S.; Yar, M. S. Bioorg. Med. Chem. Lett. 2009, 19, 7000.(b)
Ashraf Ali, M.; Ismail, R.; Choon, T. S.; Yoon, Y. K.; Wei, A. C.; Pandian, S.; Suresh
Kumar, R.; Osman, H.; Manogaran,E. Bioorg. Med. Chem. 2010, 20, 7064. (c) 5,6-
Dimethoxy spiro[5.30]-50nitrooxindole-spiro-[6.300]-2,3-dihydro-1H-inden-100-
one-7-(pyridyl)-tetrahydro-1H-pyrrolo[1,2-c][1,3]thiazole (6a). IR (cmꢀ1):
1525, 1690, 1724 and 3290 cmꢀ1 1H NMR (300 MHz, CDCl3) dH: 2.78 (d, 1H,
.
J = 17.0 Hz, H-3’’), 2.98 (dd, 1H, J = 10.94 and 4.4 Hz, H-1), 3.18 (dd, 1H, J = 10.94,
4.4 Hz, H-1), 3.40 (s, 3H, OCH3), 3.58 (d, 1H, J = 9.1 Hz, H-3), 3.90 (d, 1H,
J = 9.1 Hz, H-3), 4.02 (d, 1H, J = 17.0 Hz, H-3’’), 4.13 (s, 3H, OCH3), 4.14 (d, 1H,
J = 10.2 Hz, H-7), 4.72–4.76 (m, 1H, H-7a), 6.80–7.40 (m, 5H, Ar-H), 7.70–8.70
(m, 4H, pyridine), 10.48 (s, 1H, NH); 13CNMR (75 MHz, CDCl3) dC: 28.4, 33.2,
38.3, 56.3, 62.0, 66.7, 83.8, 108.6, 109.8, 111.8, 121.3,121.6, 121.7, 124.7, 130.5,

M. Ashraf Ali et al. / Bioorg. Med. Chem. Lett. 22 (2012) 508–511
511
143.3, 145.7, 146.9, 147.8, 148.0, 149.7, 149.7, 153.6, 154.0, 154.6, 154.7, 172.2,
207.2. Anal. Calcd for C28H24N4O6S: C, 61.75; H, 4.44; N, 10.29%. Found C, 61.68;
H, 4.40; N, 10.20%. EI-MS m/z 545 (M+1).
E.; Caliendo, G. Curr. Med. Chem. 2002, 9, 1251; (f)Microwaves in Organic
Synthesis; Loupy, A., Ed.; Wiley-VCH: New York, 2002; (g) Varma, R. S. Advances
in Green Chemistry: Chemical Synthesis Using Microwave Irradiation; AstraZeneca
Research Foundation: India, 2002; (h) Zhu, J.; Bienayme, H. Multicomponent
Reactions; Wiley-VCH: Weinheim, 2005; (i) Ramon, D. J. Int. Ed. 2005, 44, 1602.
9. Ellman, G. L.; Courtney, K. D.; Andres, B.; Featherstone, R. M. Biochem. Pharmacol.
1961, 7, 88.
8. For representative reviews and books on the use of microwave irradiation in
synthetic organic chemistry, see: (a) Caddick, S. Tetrahedron 1995, 38, 10403; (b)
de la Hoz, A.; Díaz-Ortiz, A.; Moreno, A.; Langa, F. Eur. J. Org. Chem. 2000, 22,
3659; (c) Perreux, L.; Loupy, A. Tetrahedron 2001, 57, 9199; (d) Tierney, J.;
Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225; (e) Santagada, V.; Perissutti,