Light induced synthesis of symmetrical and unsymmetrical dihydropyridines in ethyl lactate-water under tunable conditions

Article abstract of DOI:10.1016/j.tetlet.2012.10.106

A highly efficient environment-friendly one-pot green methodology has been developed for the synthesis of symmetrical and unsymmetrical 1,4-dihydropyridines and polyhydroquinolines following the multicomponent Hantzsch synthesis under visible light irradiation in ethyl-l-lactate-water solution at room temperature. The present methodology offers several advantages such as simple procedure, greener condition, excellent yields and short reaction time sans any catalyst, support or promoter. The developed protocol has been materialized with the involvement of a household compact tungsten lamp as the visible light source, and the manifested high selectivity of the reaction performed in ethyl lactate-water solvent mixture under tunable conditions. The Ca²⁺ channel blocker nitrendipine and nemadipine B were also successfully synthesized applying the developed methodology in high yields.

Full text of DOI:10.1016/j.tetlet.2012.10.106

Tetrahedron Letters 54 (2013) 138–142
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Light induced synthesis of symmetrical and unsymmetrical dihydropyridines
in ethyl lactate–water under tunable conditions
⇑
Partha Pratim Ghosh, Sanjay Paul, Asish R. Das
Department of Chemistry, University of Calcutta, Kolkata 700009, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A highly efficient environment-friendly one-pot green methodology has been developed for the synthesis
of symmetrical and unsymmetrical 1,4-dihydropyridines and polyhydroquinolines following the multi-
component Hantzsch synthesis under visible light irradiation in ethyl-L-lactate–water solution at room
temperature. The present methodology offers several advantages such as simple procedure, greener con-
dition, excellent yields and short reaction time sans any catalyst, support or promoter. The developed pro-
tocol has been materialized with the involvement of a household compact tungsten lamp as the visible
light source, and the manifested high selectivity of the reaction performed in ethyl lactate–water solvent
mixture under tunable conditions. The Ca2+ channel blocker nitrendipine and nemadipine B were also
successfully synthesized applying the developed methodology in high yields.
Received 17 August 2012
Revised 18 October 2012
Accepted 21 October 2012
Available online 1 November 2012
Keywords:
Visible light
Ethyl lactate–water
1,4-Dihydropyridine
Hantzsch condensation
Ó 2012 Elsevier Ltd. All rights reserved.
In recent years, multicomponent reactions (MCRs)1 have at-
tracted much attention and frequently used to synthesize highly
functionalized molecules in a single flask operation. In addition,
MCRs in water have been shown to be a powerful tool for develop-
ing libraries of medicinal scaffolds as well as for the requirements
of green chemistry because of its cheapness, easy availability and
benign character.2
In continuation of our ongoing research work employing water
as a solvent in the multicomponent organic transformation,3 we
have found that the Hantzsch reaction can be designed very
smoothly in the presence of water, tuning the polarity by ethyl lac-
tate in the presence of visible light.
1,4-Dihydropyridines (DHPs) are an important class of N-
heterocyclic scaffolds of low molecular weight in medicinal field,
providing important ligands for biological receptors.4 These com-
pounds, although described for the first time by Arthur Hantzsch
in 18825 by successive structural modifications involving addi-
tions, reductions and condensations mainly in the 1,2 and 6-posi-
tions of the dihydropyridine ring, its unique structural features
have recently been recognized as vital drugs in the treatment of
angina pectoris,6 blood pressure and hypertension.7 Many DHPs
are already commercial products such as: amlodipine, felodipine,
isradipine, lacidipine, nicardipine, nitrendipine, nifedipine and
nemadipine B, of which nitrendipine and nemadipine B exhibit po-
tent calcium channel blocking activities (Fig. 1).
These molecules have gained therapeutic success due to the
efficient binding to the active site of the receptors of calcium chan-
nels.8 Their pharmacological properties include neuro and radio
protective effects,9,10 anti-inflammatory11, and HIV protease inhi-
bition and in the treatment of Alzheimer’s disease.12 Due to the un-
ique molecular structure, certain Hantzsch dihydropyridines are
considered as imitates of the biological redox system NAD/NADH+
and have emerged as hydrogen transfer agents in biomimetic
reductions13 and also as photoactive materials.14
A variety of methods have been emerged to achieve the synthesis
of this dihydropyridine nucleus. Most of the methods15–27 reported
previously have focused on the modification and the optimization of
the process parameters of the Hantzsch reaction to minimize reac-
tion time and maximize reaction conversion to achieve the desired
1,4-DHP in high purity. These methodologies are generally based on
the usage of ionic liquids, metal/metal-free catalysts, microwave
and ultrasound irradiation.28 Ananthakrishnan et al.29 have recently
reported visible light irradiated Hantzsch reaction in the presence of
[Ru(bpy)3]2+ complex under a molecular oxygen atmosphere, where
substituted 2-arylpyridine was formed (Scheme 1, case I). But all the
literature reports revealed that there still remains the scope to work
on the Hantzsch reaction in materializing the synthesis of 1,4-dihy-
dropyridine structural core if visible light can trigger the reaction.
Our thinking and untiring effort gave us the success and hence,
we report for the first time, a clean, safe, high yielding green meth-
odology for the visible light induced synthesis of symmetrical and
unsymmetrical 1,4-dihydropyridines (Scheme 1, case II) in ethyl
lactate–water medium.
⇑
Corresponding author. Tel.: +91 3323501014/9433120265; fax: +91
3323519754.
In the preliminary stage of investigation, we focused on the ef-
fect of the heat and visible light on the isolated yield of the model
E-mail addresses: ardas66@rediffmail.com, ardchem@caluniv.ac.in (A.R. Das).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.106

P. P. Ghosh et al. / Tetrahedron Letters 54 (2013) 138–142
139
N
O
Cl
t-BuO2C
EtO2C
Cl
MeO2C
N
Cl
MeO2C
CO2Et
CO2Et
i-PrO2C
CO2Et
CO2Et
N
H
H3C
N
H
CH2O(CH2)2NH2
N
H
N
H
Lacidipine
Isradipine
Felodipine
Amlodipine
Cl
O2N
O2N
Cl
EtO2C
O2N
MeO2C
CO2Et
MeO2C
CO2(CH2)2NHBn
CO2Me
CO2Me
MeO2C
N
H
N
H
N
H
N
H
Nicardipine
Nitrendipine
Nifedipine
Nemadipine B
Figure 1. Some bioactive dihydropyridines.
Previous work(case I) :
O
OEt
O
CHO
[Ru(bpy)3]2+,visible light
MeCN/O2-Balloon
EtO
OEt
NH4OAc
N
O
O
NO2
NO2
NO2
Present work (case II):
CHO
O
O
OEt
visible light
50% Ethyl lactate in water
EtO
OEt
HCOONH4
O
O
N
H
NO2
Scheme 1. Comparative study for the formation of 1,4-dihydropyridine.
Table 1
Synthesis of 1,4-dihydropyridines under thermal and light mediated conditions
NO2
CHO
O
O
+
HCOONH4
O
O
+
O
50% Ethyl-L-lactate in water
O
O
NO2
N
H
Reaction condition
Temperature (°C)
Time
72 h
150 min
90 min
120 min
135 min
Yielda (%)
b
Thermal
27
27
27
27
27
—
Visible light (150 W tungsten lamp)c
Visible light (150 W tungsten lamp)d
Visible light (150 W tungsten lamp)e
Visible light (150 W tungsten lamp)f
92
95
88
85
a
Isolated yield of the pure compound.
b
A mixture of 4-nitrobenzaldehyde (1.0 mmol), ethyl acetoacetate (2.0 mmol) and ammonium formate (1.5 mmol) in 3 mL solvent (1:1 mixture of
-lactate and water) was stirred at room temperature (27 °C).
A mixture of 4-nitrobenzaldehyde (1.0 mmol), ethyl acetoacetate (2.0 mmol) and ammonium formate (1.5 mmol) in 3 mL solvent (1:1 mixture of
ethyl- -lactate and water) was irradiated with a 150 W tungsten lamp at room temperature (27 °C).
A mixture of 4-nitrobenzaldehyde (1.0 mmol), ethyl acetoacetate (2.0 mmol), ammonium formate (1.5 mmol) and 10 mol % benzoyl peroxide in 3 mL
solvent (1:1 mixture of ethyl- -lactate and water) was irradiated with a 150 W tungsten lamp at room temperature (27 °C).
A mixture of 4-nitrobenzaldehyde (1.0 mmol), ethyl acetoacetate (2.0 mmol), ammonium formate (1.5 mmol) and 5 mol % benzoyl peroxide in 3 mL
solvent (1:1 mixture of ethyl- -lactate and water) was irradiated with a 150 W tungsten lamp at room temperature (27 °C).
A mixture of 4-nitrobenzaldehyde (1.0 mmol), ethyl acetoacetate (2.0 mmol), ammonium formate (1.5 mmol) and 1 mol % benzoyl peroxide in 3 mL
solvent (1:1 mixture of ethyl- -lactate and water) was irradiated with a 150 W tungsten lamp at room temperature (27 °C).
ethyl-
L
c
L
d
L
e
L
f
L

140
P. P. Ghosh et al. / Tetrahedron Letters 54 (2013) 138–142
Table 2
In addition, to obtain a high yielding neat protocol, the polarity
of the solvent was tuned30 by employing water as a cosolvent with
Screening of solvents for the synthesis of 1,4-dihydropyridines under light mediated
conditions
ethyl-
case II) was performed in pure ethyl-
was tuned with water for better results. Table 3 clearly indicates
that 50% ethyl- -lactate in water produced the optimum polarity
L
-lactate in different proportions. The reaction (Scheme 1,
Entry
Solvent
Yielda (%)
L-lactate and then the polarity
1
2
3
4
5
6
Toluene
Acetonitrile
DMSO
Ethanol
Water
49
52
67
70
60
75
L
of the solvent in which the speedy reaction took place with maxi-
mum isolated yield.
To examine the versatility of this methodology, the preparation
of symmetrical 1,4-DHPs (Table 4) including the synthesis of the cal-
cium channel blockers nitrendipine and nemadipine B (Table 4, en-
tries 19 and 20), some new unsymmetrical 1,4-DHPs (Table 5) and
polyhydroquinolines (Table 6) were carried out using a modified
Hantzsch procedure (Scheme 1, case II), developed by us (electronic
supplementary information). Investigations of the reaction scope
revealed that various aromatic (bearing electron-withdrawing and
electron-donating groups), heteroaromatic, aliphatic aldehydes as
well can be utilized in this protocol (Tables 4 and 5). The choice of
ammonium formate in water as an in situ ammonia and formic acid
donor in the presence of visible light drives the reaction very effi-
ciently and eventually satisfied our demand in obtaining optimum
yield than other conventional ammonia donor for the specific
synthesis of symmetrical and unsymmetrical 1,4 DHPs.19,25
It has been observed that better yields are obtained with sub-
strates having electron-withdrawing groups. In case of ortho
substituted aldehydes, the yields are slightly lower (probably due
to steric hindrance) than para-substituted ones. Moreover, the
enhancement of the reaction rate was observed on addition of cat-
alytic amount (10 mol %) of benzoyl peroxide to the reaction mix-
ture at room temperature and the yield of the product was
decreased when the amount of benzoyl peroxide reduced to
1 mol %, whereas the reaction failed to give any product in the ab-
sence of visible light source even after 72 h at room temperature.
This observation may help us to propose that presumably the reac-
tion takes the course of one-pot photochemical pathway31 and
probably the first step is the hitherto unknown visible light in-
duced Knoevenagel condensation in the presence of ammonium
formate to generate the intermediate (I). After that, intermediate
Ethyl-
L-lactate
a
Isolated yield of the pure compound.
Table 3
Effect of solvent polarity on the yield for the one pot synthesis of
1,4-dihydropyridines
Entry
Solvent
Yielda (%)
1
2
3
4
5
6
7
100% Ethyl-
L
-lactate
75
80
86
92
82
73
66
85% Ethyl-
75% Ethyl-
50% Ethyl-
25% Ethyl-
10% Ethyl-
L-lactate in water
L-lactate in water
L-lactate in water
L-lactate in water
L-lactate in water
5% Ethyl-L-lactate in water
a
Isolated yield of the pure compound.
reaction of 4-nitrobenzaldehyde (1.0 mmol), ethyl acetoacetate
(2.0 mmol) and ammonium formate (1.5 mmol) using ethyl-
L
-
lactate as a solvent. Table 1 clearly exposed the greater efficiency
of the visible light mediated protocol than the thermal one.
In order to optimize the reaction condition we applied some po-
lar and nonpolar solvents in the three-component reaction for pre-
paring dihydropyridine derivatives (Scheme 1, case II) and also to
understand the effect of the solvents. In each case, the substrates
were mixed together with 3 mL solvent and were irradiated with
a 150 W tungsten lamp for 150 min. at room temperature. The re-
sults are shown in Table 2. It is noteworthy to mention that the po-
lar solvents afforded a better yield than the nonpolar ones.
Table 4
Synthesis of symmetrically substituted 1,4-dihydropyridines under modified Hantzsch conditions
O
R
O
O
O
visible light
50% Ethyl-L-lactate in water
R1
R1
+
RCHO
+
HCOONH4
R1
N
H
4a-r
Entry
R
R1
Time (mins)
Product
Yielda (%)
1
2
3
4
5
6
7
8
C6H5
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
OEt
Me
Me
Me
Me
Me
Me
Me
OMe
OEt
150
150
160
155
150
160
150
150
150
155
150
160
155
150
160
165
160
160
145
155
4a15
4b17
4c19
4d19
4e19
4f20
4g19
4h19
4i18
90
92
81
86
91
92
90
92
82
81
80
90
92
85
87
87
89
85
92
90
4-O2N C6H4
4-MeO C6H4
4-HO C6H4
4-Cl C6H4
4-F C6H4
3-O2NC6H4
2-Furyl
n-Propyl
2-Pyridyl
4-Me2NC6H4
C6H5
4-O2NC6H4
4-MeOC6H4
4-HOC6H4
4-ClC6H4
2-Furyl
9
10
11
12
13
14
15
16
17
18
19
20
4j16
4k17
4l17
4m22
4n23
4o23
4p23
4q22
4r21
4s25
4t25
n-Propyl
3-NO2
2,3-diCl
a
Isolated yield of the pure compound.

P. P. Ghosh et al. / Tetrahedron Letters 54 (2013) 138–142
141
Table 5
Synthesis of unsymmetrically substituted 1,4-dihydropyridines under modified Hantzsch conditions
O
R
O
O
O
O
O
O
visible light
50% Ethyl-L-lactate in water
+
+
HCOONH4
+
R1
R2
R1
R2
R
H
N
H
5a-c
Product
Entry
R
R1
R2
Time (mins)
Yielda (%)
1
2
3
4-O2NC6H4
4-F C6H4
4-OMe C6H4
OEt
OEt
OEt
Me
Me
Me
155
150
160
5a
5b
5c
81
77
74
a
Isolated yield of the pure compound.
Table 6
Synthesis of polyhydroquinoline derivatives under modified Hantzsch conditions
O
R
O
O
O
O
visible light
50% Ethyl-L-lactaten in water
R1
+
R1
+
RCHO
+
HCOONH4
N
H
O
6a-d
Entry
R
R1
Time (mins)
Product
Yielda (%)
1
2
3
4
4-O2NC6H4
OEt
OEt
Me
Me
155
160
160
155
6a27
6b27
6c26
6d24
91
88
83
80
4-MeOC6H4
4-O2NC6H4
4-MeOC6H4
a
Isolated yield of the pure compound.
NO2
O
O
visible light
EtO
NO2
OHC
O
OEt
O
O
NO2
(I)
NO2
O
O
O
O
O
H2O
EtO
EtO
O
HCOO NH4
NH3
HCOOH
visible light
EtO
OEt
EtO
OEt
(I)
OH
O
O
NH
O
H
(II)
(III)
HCOOH
NO2
NO2
NO2
NO2
H
H
H
O
H
O
H
O
O
O
O
O
O
O
O
H
H
EtO
OEt
EtO
OEt
EtO
OEt
EtO
OEt
H
N
H
N
H
N
H
N
H
OH
OH
O
H
Scheme 2. Plausible mechanism for the formation of 1,4-dihydropyridine.
(I) suffers nucleophilic attack by the enol form of dicarbonyl com-
pound (already formed a complex with water and ethyl lactate
possibly) and to follow the Michael addition sequence to generate
1,5-dioxo compound (II) photochemically. Ammonia and formic
acid generated from ammonium formate in the presence of water
at room temperature convert intermediate (II) into intermediate

142
P. P. Ghosh et al. / Tetrahedron Letters 54 (2013) 138–142
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ates 1,4-dihydropyridine by following conventional acid catalysis
(formic acid from ammonium formate) (Scheme 2) pathway. We
have tried to isolate intermediate (I) by the two component reac-
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in the presence of UV-visible light but the reaction offered no prod-
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catalytic amount of ammonium formate (10 mol %), 1,4-DHP (trace
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presence of visible light and hence could not be detected and
evaluated.
In summary, we have developed a new, potentially efficient,
absolutely clean, versatile and environment-friendly light induced
green procedure devoid of any catalyst support or promoter for the
synthesis of 1,4-dihydropyridine derivatives. Possibly, this is the
first report for the successful synthesis of 1,4-DHP and polyhydro-
quinoline derivatives without using any photo-catalyst in the pres-
ence of visible light at room temperature. Additionally, the calcium
channel blockers nitrendipine and nemadipine B were also suc-
cessfully synthesized in this developed methodology with excel-
lent yields. Moreover, milder conditions, shorter reaction times,
low costs, easy work-up and high yields will make this process
attractive over the other available methods and may be considered
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We gratefully acknowledge the financial support from U.G.C,
New Delhi, India and also from the University of Calcutta. P.P.G.
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Supplementary data
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.10.
106.
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