L-Proline-catalyzed diastereoselective synthesis of cis-isoquinolonic acids and evaluation of their neuroprotective effects

Article abstract of DOI:10.1016/j.tetlet.2012.04.089

l-Proline efficiently catalyzes the one-pot, three-component reaction of homophthalic anhydride, amines, and aldehydes in CH₃CN at room temperature, to afford the corresponding cis-isoquinolonic acids in high yields and with excellent diastereo

Full text of DOI:10.1016/j.tetlet.2012.04.089

Tetrahedron Letters 53 (2012) 3440–3443
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
L-Proline-catalyzed diastereoselective synthesis of cis-isoquinolonic acids
and evaluation of their neuroprotective effects
Ali Reza Karimi a, , Hamid Reza Momeni b, Rahim Pashazadeh a
⇑
a Department of Chemistry, Faculty of Science, Arak University, Arak 38156-8-8349, Iran
b Department of Biology, Faculty of Science, Arak University, Arak 38156-8-8349, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
L
-Proline efficiently catalyzes the one-pot, three-component reaction of homophthalic anhydride, amines,
Received 2 January 2012
Revised 2 April 2012
Accepted 19 April 2012
Available online 25 April 2012
and aldehydes in CH3CN at room temperature, to afford the corresponding cis-isoquinolonic acids in high
yields and with excellent diastereoselectivities. The synthesized compounds are screened for neuropro-
tective activities.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
L-Proline
Isoquinolonic acids
Multi-component reaction
Diastereoselective synthesis
Neuroprotective effects
Multicomponent reactions (MCR) have emerged as powerful
tools in modern synthetic organic chemistry, which allows the intro-
duction of molecular complexity and diversity as required in combi-
natorial chemistry for the preparation of bioactive compounds.1
Several strategies are available for diastereoselective catalysis,
including Brønsted or Lewis acid and base catalysis, heterogeneous
catalysis, homogeneous transition-metal catalysis, and organocatal-
ysis. Among organocatalysts,2 proline is effective in several asym-
metric transformations, including the aldol,3 Mannich,4 Michael,5
disease20 and amyotrophic lateral sclerosis21 in which apoptosis
or programed cell death might play an important role in neuronal
degeneration. A variety of mechanisms have been proposed as hav-
ing a role in motor neuron cell death in the spinal cord including
glutamate excitotoxicity,22 caspase activation23 and calpain activa-
tion.24 Accordingly, glutamate receptor antagonists,25 caspase,26
and calpain27 inhibitors have been shown to protect neurons in
the nervous system.
Isoquinoline-containing compounds have shown a wide range of
biological and medicinal properties.28 Some of these compounds
have been reported as neuroprotective substances in a variety of
neurons such as mesencephalic,29 cerebral,30 and substantia nigra31
neurons.
Hence, the synthesis of isoquinolonic acids is an important and
useful task in organic chemistry. The cyclocondensation reaction is
a straightforward method for the synthesis of these compounds.
Recently, the three-component condensation of homophthalic
anhydride, amines, and aldehydes has gained popularity. Lewis
acids such as KAl(SO4)2Á12H2O,32 BF3ÁEt2O,33 ZnCl2, FeCl3, AlCl3,
CH3COOH, HCl, TEA, DIEA,34 sulfonic acid functionalized silica,34
ionic liquids,35 trimethyl orthoformate,36 Yb(OTf)3,37 and TiCl4/DI-
PEA38 have been utilized for the two- or three-component synthesis
of isoquinolonic acids.
However, the use of metal catalysts in the synthesis of biolog-
ically active compounds can be disadvantageous as the presence of
even minute amounts of residual metal can lead to serious conse-
quences. In view of the above, we were interested in developing
a nonmetallic, organocatalytic, atom economic method for the
a
-amination,6 Diels–Alder,7 Knoevenagel,8 Robinson annulation,9
Baylis–Hillman,10
a
-selenenylation,11 oxidation,12 and chlorination
reactions.13
Proline-catalysis has been extended to direct asymmetric multi-
component reactions including: three- or four-component synthe-
ses of imidazoles,14 three-component diastereoselective syntheses
of hexahydropyrano pyrimidinones,15 five-component syntheses of
antimalarial tetrahydropyridines,16 three-component synthesis of
3,4-dihydropyrano[c]chromene17 and multicomponent syntheses
of diarylmethanes4 and highly substituted pyridines.18
The rigid ring structure, easy availability, lack of toxicity, and its
low cost, make L-Proline a desirable organocatalyst for synthesiz-
ing small molecules of biological interest.
The degeneration of motor neurons in the central nervous sys-
tem (CNS) is an important phenomenon following spinal cord inju-
ries,19 and in neurodegenerative disorders such as Parkinson’s
⇑
Corresponding author. Tel.: +98 861 4173400; fax: +98 861 4173406.
E-mail address: a-karimi@araku.ac.ir (A.R. Karimi).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.089

A. R. Karimi et al. / Tetrahedron Letters 53 (2012) 3440–3443
3441
diastereoselective synthesis of isoquinolonic acids and then to
evaluate them for neuroprotective activity (Scheme 1).
BF3ÁEt2O and TiCl4/DIPEA were used for the preparation of trans-
isoquinolonic acids, while trimethyl orthoformate, ionic liquids,
KAl(SO4)2Á12H2O, sulfonic acid functionalized silica, and Yb(OTf)3
were employed for the synthesis of the cis isomers. Using the other
catalysts mentioned above led to mixtures of cis and trans products
or complicated mixtures.
organocatalyst was based on the facts that it acts as an excellent
Lewis acid/base catalyst in a variety of organic reactions.
In order to optimize the conditions, reactions were carried out
at different temperatures and molar ratios of L-Proline and in var-
ious solvents. The condensation of homophthalic anhydride with
3-chlorobenzaldehyde and 4-methylaniline was chosen as a model.
Initially, the effect of catalyst quantity on the yield was evalu-
ated by varying the amount of catalyst from 0 to 20 mol % at room
temperature (Table 1, entries 1–5). The results indicated that in the
Our method consists of the reaction of aromatic aldehydes, ani-
lines, and homophthalic anhydride under the influence of 10 mol %
absence of
L-Proline, product 4h was obtained in low yield. In addi-
L-Proline as the organocatalyst. The choice of
L-Proline as the
tion, the yield decreased on using 20 mol % of L-Proline. The best
result was achieved with 10 mol % of the catalyst (Table 1).
An investigation of the solvent effect (Table 1, entries 3 and 6–
10) on the reaction at room temperature using catalyst (10 mol %)
indicated that the best yield was obtained using CH3CN. Side prod-
ucts were formed when EtOH/H2O (3:2), H2O, and THF were used
as solvents. Hence, the optimized conditions were as follows:
O
CHO
O
R1
L-Proline
2 h, r.t.
N
O
R2
+ R1-NH2
+
O
R2
H
HOOC
10 mol % of L-Proline, CH3CN, room temperature. A high tempera-
1
2
3
4
H
ture improved the reaction rate, but favored the occurrence of side
reactions. We next examined a wide variety of aldehydes and ani-
lines to establish the scope of this catalytic transformation (Table
2). The synthesis was carried out in CH3CN at room temperature
to give products 4a–h in good to high yields.
Scheme 1. Synthesis of cis-isoquinolonic acids.
Table 1
Optimization of the reaction conditions for the synthesis of 4ha
It is noteworthy that the use of L-Proline resulted in a complete
diastereoselective process. The stereochemistry of the products
was determined from the coupling constants between the vicinal
methine protons. In compounds 4a–h the coupling constant
(3JHH ꢀ 4–6 Hz) is typical for the cis configuration. In the trans com-
Entry
Solvent
Yieldb
%
L
-Proline (mol %)
1
2
3
4
5
6
7
8
9
—
5
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
H2O
EtOH
THF
EtOH/H2O (3:2)
CH2Cl2
15
59
85
83
77
30
45
49
42
46
10
15
20
10
10
10
10
10
3
pounds the coupling constant JHH is significantly smaller (typical
value = 0–2 Hz).37 While the enantioselectivity was low, the diaste-
reoselectivity of this reaction was found to be excellent.
We hypothesized that the derivatives of isoquinolonic acids
might demonstrate neuroprotection and delay apoptosis in motor
neurons. To test this hypothesis we used adult mouse spinal cord
slices to examine the effect of isoquinoline-containing compounds
on the apoptosis of motor neurons in cultured slices.
10
a
Reaction time: 2 h at rt.
b
Yields refer to isolated products.
Fluorescent staining was used to determine apoptotic cell death
based on morphological changes (Fig. 1).
Table 2
In freshly prepared slices (0 h) the motor neurons showed large
cell bodies, large nucleus, and the expected distribution of nuclei
material; no apoptotic signs were observed within the motor neu-
rons (Fig. 1A). After 6 h, many of the motor neurons displayed char-
acteristic morphological futures of apoptosis such as cell shrinkage
as well as nuclear and chromatin condensation (Fig. 1B). The appli-
Synthesis of cis-isoquinolonic acids 4a–h
Product
R1
R2
Yielda (%)
Mp/mpLit (°C)
4a
4b
4c
4d
4e
4f
C6H6–
C6H6–
4-CH3-C6H6–
4-CH3-C6H6–
4-CH3-C6H6–
PhCH2–
4-Cl
H
H
4-Cl
4-CH3
3-OCH3
H
85
86
88
89
91
87
90
86
195–196/—
201–202/20132
183–185/18532
231–233/23034
212–214/22534
222–224/22234
234–236/23334
200–202/21634
cation of 4f (300 lM as an effective concentration), effectively pre-
vented the appearance of nuclear apoptotic changes in the motor
neurons after 6 h (Fig. 1C) compared to the control (Fig. 1B). Fur-
thermore, cis-isoquinolonic acid 4f increased significantly
(P <0.01) the percentage of motor neuron viability in the ventral
4g
4h
PhCH2–
4-CH3-C6H6–
3-Cl
a
Yield refer to isolated products.
Figure 1. The inhibition of apoptosis in motor neurons. Motor neurons were stained with a combination of propidium iodide (red) and Hoechst (blue). (A) Normal motor
neuron at 0 h. (B) Motor neurons from slices cultured for 6 h (control) showed nuclear and chromatin condensation. (C) 4f (300 M) inhibited nuclear apoptotic changes after
6 h compared to the control. Arrows show motor neurons. Scale bar = 25 m.
l
l

3442
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horns after 6 h compared with the control (Fig. 2). Compounds 4a–
e,g,h had no effect on apoptosis inhibition nor on the viability of
the motor neurons (data not shown).
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fects through different mechanisms such as poly (ADP-ribose)
polymerase (PARP) inhibition,30 glutamate receptor antagonist,39
and free radical scavenging.40 How 4f prevents apoptosis in the
motor neurons remains to be investigated. One possibility might
be related to the substituent on this component. A methoxy group
on the benzene-fused ring is reported to be crucial for producing
pharmacological effects.41 In this context, isoquinoline compounds
with this substituent have been demonstrated as 2-amino-3-hy-
droxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antag-
onists.39 Over activation of this type of glutamate receptor in the
CNS is documented to induce neural cell death in neurological dis-
orders.42 Therefore, it is reasonable to assume that 4f with its
methoxy group was able to exert its neuroprotective effects by
reducing apoptosis of the motor neurons as an AMPA receptor
antagonist.43
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In conclusion, we have reported a simple and convenient proto-
col for the multicomponent synthesis of cis-isoquinolonic acids
using L-Proline as the organocatalyst. The proline can catalyze effi-
ciently this reaction with high yields and very good diastereoselec-
tivity. The products may show potential in the development of new
chemotherapeutics.
43. General procedure for the preparation of products 4a–h:
homophthalic anhydride (1 mmol), amine (1 mmol), aldehyde
(1 mmol) and -Proline (10 mol %) in CH3CN (15 mL) was stirred at room
A mixture of
1
2
3
L
Acknowledgment
temperature for 2 h (in.Table 2). Upon completion of the reaction as monitored
by TLC [n-hexane/EtOAc (2/1)], the solvent was evaporated under reduced
pressure and hot EtOH (5 mL) was added to the residue which was then
filtered. A few drops of H2O were added which was cooled to 0 °C. The crystals
resulting were separated by filtration and washed with Et2O and H2O.
2-benzyl-3-(3-methoxyphenyl)-1-oxo-1,2,3,4-tetrahydroisoquinoline-4-carboxylic
We gratefully acknowledge financial support from the Research
Council of Arak University.
acid (4f): White powder: IR (KBr) (m
max, cmÀ1): 3400–2700 (COOH), 3026, 2955,
Supplementary data
1712, 1643. 1H NMR (DMSO-d6, 300 MHz) dH: 3.63 (s, 3H, CH3), 3.88 (d, 1H,
J = 14.1 Hz, CH2), 4.12 (d, 1H, J = 4.6 Hz, CH), 5.23 (br s, 2H, Hmethylene and methine),
6.57 (m, 2H, Harom), 6.76 (s, 1H, Harom), 7.26 (m, 7H, Harom), 7.42 (m, 2H, Harom),
7.98 (s, 1H, Harom), 12.99 (br s, 1H, COOH). Biology: Animals and the culture of
spinal cord slices: Adult female Balb/c mice (23–25 g) were purchased from the
Pasteur Institute of Iran, Tehran. The animals were housed in plastic cages at
20 °C, over 12 h light/dark cycles, and fed with standard commercial laboratory
chew and water. The experiments were approved by the local ethical
committee at Arak University. The animals were deeply anesthetized via an
intraperitoneal injection of sodium pentobarbital (60 mg/kg) and subsequently
killed by heart puncture. The spinal cord was dissected and placed in ice-cold
phosphate-buffered saline (PBS, pH 7.4). The thoracic region of the spinal
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.04.089.
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cord27 was then sliced transversally into 400 lm thick sections using a
McIlwain tissue chopper (Stoelting, USA). The slices were cultured in a medium
(50% minimum essential medium, 25% Hanks balanced salt solution, 25% horse
serum, 25 mM HEPES, 6 g/L glucose and penicillin–streptomycin, pH 7.3–7.4) in
the absence (control) or presence of the isoquinolins 4a–h. The cultured slices
were then incubated at 37 °C in a humidified atmosphere of 5% CO2 in air for
6 h. Fixation and sectioning: Freshly prepared (0 h) and cultured slices were
fixed in Stefanini’s fixative (2% paraformaldehyde, 0.2% picric acid in 0.1 M
phosphate buffer, pH 7.2) for at least 2 h. The fixed slices were washed in PBS
(3 Â 5 min) and incubated overnight in 20% sucrose in PBS at 4 °C. The slices
were cut into 10 lm thick sections in a cryostat. The sections were collected
and mounted on Poly-l-lysine coated glass slides. Assessment of apoptosis: To

A. R. Karimi et al. / Tetrahedron Letters 53 (2012) 3440–3443
3443
study the morphological features of apoptosis, a combination of propidium
iodide (Sigma, USA, 10 g/ml in PBS, 15 min at room temperature) and Hoechst
33342 (Sigma, USA, 5 g/ml in PBS, 1 min at room temperature) was used. The
the synthesized materials were dissolved in dimethylsulfoxide (DMSO) as
stock solutions. Each solution was added directly to the medium and tested at
100, 200, 300, 400, and 500 lM to find an effective concentration. The controls
l
l
cryostat sections were washed in PBS (3 Â 5 min) and mounted in glycerol/PBS
(1:1) and a cover slip applied. Photographs were taken with an Olympus
camera attached to an Olympus fluorescence microscope (Olympus Optical Co
Ltd, Japan) using the appropriate excitation and emission filters. Chemicals: All
received a corresponding amount of DMSO. Statistical analysis: Results were
expressed as mean SD. The statistical significance was examined by analysis
of variance (ANOVA). In all cases, tests were considered significant at a level of
P <0.05.