L-Tyrosine loaded nanoparticles: An efficient catalyst for the synthesis of dicoumarols and Hantzsch 1,4-dihydropyridines

Article abstract of DOI:10.1039/c4ra16627b

Environmentally benign l-tyrosine loaded nanoparticles are fabricated and characterized by PCS, TEM, FT-IR and AFM studies. A novel straightforward green approach was applied for the synthesis of dicoumarols and Hantzsch 1,4-dihydropyridines using this catalyst. The structures and purity of these compounds were confirmed by FT-IR and NMR (¹H, ¹³C and DEPT). The flexible and swelling properties of the polymer coating increase l-tyrosine dispersion and its high catalytic activity in organic reactions. This journal is

Full text of DOI:10.1039/c4ra16627b

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L-Tyrosine loaded nanoparticles: an efficient
catalyst for the synthesis of dicoumarols and
Cite this: RSC Adv., 2015, 5, 13366
Hantzsch 1,4-dihydropyridines†
a
a
Anamika Khaskel, Pranjit Barman and Utpal Janab
*
Environmentally benign L-tyrosine loaded nanoparticles are fabricated and characterized by PCS, TEM,
FT-IR and AFM studies. A novel straightforward green approach was applied for the synthesis of
dicoumarols and Hantzsch 1,4-dihydropyridines using this catalyst. The structures and purity of
these compounds were confirmed by FT-IR and NMR (1H, 13C and DEPT). The flexible and swelling
properties of the polymer coating increase L-tyrosine dispersion and its high catalytic activity in
organic reactions.
Received 18th December 2014
Accepted 19th January 2015
DOI: 10.1039/c4ra16627b
www.rsc.org/advances
Several modied methods have been reported nowadays
for example such as free nano Fe2O3,16 Zn(L-proline)2,17
PPh3,18 L-proline under ultrasound condition19 etc. Further-
Introduction
Implementation of several transformations through a multi-
component reaction strategy is highly compatible with the
goal of green chemistry.1 Use of environmentally benign
solvents like water2 and solvent-free reactions3 offers a powerful
and green protocol from both synthetic and economical points
of view. It has several advantages like reduced pollution,
simplicity of processing and low cost, which is useful to the
industry as well as for the environment.
more, ionic liquid such as glycine nitrate,20 Ni2+ containing
ionic liquid,21a Silica functionalized sulphonic acid coated
with ionic liquid,21b enzymes such as Baker's yeast,22 lipase
B23 demonstrated catalytic activity in 1,4-DHPs synthesis.
Some heterogeneous catalysts have also been reported such
as sulfonic acid supported g-Fe2O3,24 cellulose,25 alginic
acid.26
Two recent studies have established L-tyrosine as an effi-
cient organocatalyst in multi-component reactions for green
synthesis.27 Thitherto, there was no report of any catalytic
activity of this amino acid. Researchers in the past few years
have investigated the pharmacological importance L-tyrosine
loaded nanoparticles (LTNPs). It has been found to increase
the antitumoural activity of direct electric current in a meta-
static melanoma cell model.28 Beside, L-tyrosine poly-
phosphate nanoparticles are also used in gene therapy.29a So
these studies reveal that encapsulating L-tyrosine inside nano-
sized polymeric coating possibly increases its pharmacolog-
ical activity. Consequently, the next question that arises is
whether this encapsulation of L-tyrosine affects its ability to
catalyze organic reactions and if yes, in which direction. The
fact that L-proline functionalized polymeric nanoreactors
have been used as catalyst in Aldol reaction,29b makes the
above question more pertinent. With this aim, LTNPs have
been used as catalyst for preparation of dicoumarols and
Hantzsch 1,4-DHPs following a fully green synthetic method.
To the best of our knowledge, this is the rst report of LTNPs
acting as catalyst in any organic reaction. The products
are fully characterized by FT-IR, 1H NMR, 13C NMR,
DEPT NMR and also by comparison with authentic samples
(Scheme 1).
Dicoumarols and their derivatives are of interest because
of their anticoagulant (Antivitamin K activity), spasmolytic
and rodenticidal activities.4 Chemically, it is designated as
3,30-methylenebis[4-hydroxycoumarin]. Dicoumarol and its
synthetic derivative warfarin sodium (Coumadin) help to
decrease metastases in animal model.5 Recently, Xian-Xing
Luo and his co-workers reported the anti-bacterial activity
of pyridine substituted dicoumarols.6 Similarly, Hantzsch
1,4-dihydropyridines (1,4-DHPs) have remarkable pharma-
cological efficiency.7 The ferrocene moiety is known to play
an important role in organometallic drugs because of its high
affinity towards amino acids, proteins, DNA and carbohy-
drates.8 Therefore, the attempts to modify the conditions of
Hantzsch reaction are still of growing importance. Dicou-
marols have been synthesized using various catalysts such as
Zn(proline)2,9 molecular iodine,10 sulphated titania,11 SDS,12
MnCl2,13 Biodegradable Task-specic ionic liquid,14 nano
silica chloride15 etc. For Hantzsch's 1,4-DHPs synthesis
aDepartment of Chemistry, National Institute of Technology, Silchar 788010, Assam,
India. E-mail: barmanpranjit@yahoo.co.in; Fax: +91 3842 224797
bSchool of Pharmacy, Chouksey Engineering College, Lal Khadan, NH-49, Bilaspur
495004, India
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra16627b
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Scheme 1 Synthesis of dicoumarols using LTNPs as catalyst.
C-4 and electron withdrawing carbonyl group at C-2. As the
carbon–carbon double bond and lone pair of electrons
present on the oxygen atom of the OH group is in conjugation,
this make coumarin ring very convenient at position 3 to
react with carbonyl carbon of the aldehydes. In Scheme 3
we have mentioned the plausible mechanism for the
synthesis of various dicoumarols using LTNPs as catalyst. At
the outset, we tried the condensation of 4-methoxy benzal-
dehyde and 4-hydroxy coumarin as a model reaction to
synthesize the dicoumarol (3a) using LTNPs catalyst in water
at 70 ꢀC in a water bath. A broad range of dicoumarols (3a–3j)
have been synthesized. In a comparative study a large number
of catalysts were used to show the effectiveness of LTNPs over
other catalysts (Table 1). Increment in the amount of
LTNPs catalyst to 0.05 g did not show any improvement in the
yield (Table 1, Entry-7) whereas, the yield was found to be
Results and discussions
LTNPs were prepared with polymer Eudragit® RS100 using
the solvent evaporation (single emulsion) technique with
slight modication as previously reported by Jana et al.30
Eudragit is the co-polymer of poly(ethylacrylate, methyl-
methacrylate and chlorotrimethyl-ammonioethyl methacry-
late) containing quaternary ammonium group.30 The ammo-
nium groups are present as salts and make the polymers
permeable. This polymer has high permeability and pH
independent swelling properties. The prepared nanoparticles
(ESI-1,† for details) are characterized by particle size analysis,
TEM (average size 50 nm), FT-IR and AFM studies (ESI-2†)
(Scheme 2).
The C-3 position of 4-hydroxycoumarin (1) ring is highly
reactive as it is anked by electron donating hydroxyl group
Scheme 2 Synthesis of 1,4-DHPs using LTNPs as catalyst.
Scheme 3 Plausible mechanism for the synthesis of dicoumarols using LNTPs (LTNPs dispersed through Eudragit coating).
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Table 1 Screening of various catalysts on the synthesis of 3a using
effective (Table 2) both in terms of product yield and reaction
time.
water as solventa
Among the three protic solvents tried, an interesting
trend was observed which made our choice of reaction
medium very easy and straightforward. With increment in
the dielectric constant of solvents the catalyst was found to
give higher yield in lesser time, and when the yield to time
ratio was plotted against the dielectric constants of the
solvents, it showed a monotonic relationship between the
two (Fig. 1).
The highest yield (90%) in shortest period (5 minutes) was
obtained for the solvent with highest dielectric constant among
the three solvents tried, water. Considering the undeniable
importance of water as solvent in organic reactions when the
primary goal is search for a green synthetic protocol, it was
promptly chosen as the solvent for dicoumarol synthesis. A
range of dicoumarols were synthesized using the various alde-
hydes and 4-hydroxycoumarin. The results are summarized in
Table 3.
Entry
Catalyst
Catalyst loading (g)
Time
Yieldb (%)
1
2
3
4
5
6
7
8
PTS
0.02
0.02
0.02
0.02
0.02
0.02
0.05
0.01
No reaction
1.5 h
2 h
20 min
15 min
5 min
NA
65
50
72
75
90
90
78
Glycine
L-Proline
L-Serine
L-Tyrosine
LTNPs
LTNPs
LTNPs
5 min
15 min
a
Reaction temperature: 70 ꢀC; molar ratio of aldehydes : 4-
b
hydroxycoumarin: 1 : 2. Isolated yields.
Table 2 Effect of various solvents on the synthesis of 3a using LTNPsa
Entry
Solvent
3r
Time
Yieldb (%)
Regardless of substitution (electron withdrawing and
electron donating) of the aromatic aldehydes, the products
were obtained in good yields. Similar results were also
obtained with the heterocyclic aldehydes. The excellent
catalytic performance of this catalyst prompted us to explore
its further applications toward the synthesis of various
substituted 1,4-DHPs under solvent-free condition at room
temperature stirring.
1
2
3
4
5
H2O
MeOH
CH3COOH
CH2Cl2
CH3CN
80
32
6
9
37
5 min
15 min
20 min
20 h
90
86
65
30
NA
NA
a
Reaction conditions: molar ratios of aldehydes : 4-hydroxycoumarin ¼
b
1 : 2; 3r is the dielectric constant; catalyst loading: 0.02 g. Isolated
yields.
Here too, a number of different catalysts were tried and
LTNPs was easily the best of the lot in terms of both yields and
time (Table 4).
Once the reaction condition was established, the scope of
this reaction was investigated with various aldehydes and b-
ketoesters yielding a variety of highly functionalized 1,4-DHPs
(Table 5, 5a–5o). In Scheme 4 we have mentioned the plausible
mechanism for the synthesis of various 1,4-DHPs using LTNPs
as catalyst.
When reaction was carried out with ferrocenyl aldehydes to
synthesize their corresponding 1,4-DHPs using free L-tyrosine
as catalyst, it resulted in moderate yields (72%) in relatively
longer time (25 min). Replacing it with LTNPs showed
striking improvements in both yield and time (5d and 5e).
Owing to the potential of ferrocene moiety to be used in
drugs, a fast, easy and green method for synthesizing 1,4-
DHPs possessing ferrocenyl side chains can lead up to much
easier synthetic protocols for large scale production of
important drug molecules. The reduced particle size and
improved surface area of the L-tyrosine loaded polymeric
nanoparticulate system increase the penetrability of L-tyro-
sine in the reaction mixture which in turns gives the high
yields. Moreover the polymer Eudragit® RS100 has the ability
of swelling, which represents the good material for L-tyrosine
dispersion.
Fig. 1 Relative efficiency of three protic solvents as reaction medium
for dicoumarol (3a) synthesis using LTNPs.
lesser when the catalyst loading reduced to 0.01 g (Table 1,
Entry-8).
The next aim was to look for a suitable medium in which
the chosen catalyst, i.e. LTNPs provides high yield in short
reaction time. Both protic as well as aprotic solvents
were tried and the former type proved to be way more
Experimental
All commercially available chemicals were obtained from
Merck and Aldrich, and used without further purications.
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Table
3
Reaction of 4-hydroxycoumarin with aromatic/hetero- Table 4 Effect of various catalysts on the synthesis of 5a using LTNPs
aromatic aldehydes catalyzed by LTNPs in water (3a–3j)
as catalyst
Entry Aldehydes
Product Timea (min) Yieldb (%) Entry
Catalyst
Catalyst loading (g)
Timea
Yieldb (%)
1
2
3
4
5
6
7
8
PTS
0.02
0.02
0.02
0.02
0.02
0.02
0.05
0.01
10 h
1.5 h
2 h
30 min
20 min
10 min
10 min
25 min
70
63
68
72
75
91
91
80
Glycine
L-Proline
l-Serine
l-Tyrosine
LTNPs
LTNPs
LTNPs
1
3a
5
90
a
All reactions were monitored by TLC. b Isolated yields.
2
3
3b
3c
10
10
88
91
Melting points were uncorrected. IR spectra were recorded as
KBr pellets on a Perkin Elmer 782 spectrophotometer. The 1H
NMR and 13C NMR spectra in DMSO were run on a Bruker
AM-300L instrument operating at 300 MHz and 75 MHz
respectively.
4
3d
10
87
a. Materials
L-Tyrosine and Lutrol® F-68 (Poloxamer 188) were obtained
from Sigma, Mumbai. Eudragit® RS100 (Evonik Industries AG,
Germany) was obtained from Sandoz Ltd. Mumbai. Distilled-
deionized water was prepared with Milli-Q plus System (Elix
10, Millipore corp. India). All other chemicals used were of the
highest available grade.
5
6
3e
3f
10
10
93
89
b. Catalyst preparation
The L-tyrosine loaded-polymeric nanoparticles were
prepared with polymer Eudragit® RS100 using the solvent
evaporation (single emulsion) technique with slight modi-
cation.30 In brief, the polymeric solution was prepared by
adding 100 mg of Eudragit® RS100 in the mixture of meth-
anol and acetone (20 : 80 v/v) at room temperature. Weighed
quantity of L-tyrosine (equivalent to 10% w/w dry weight of
polymer) was dissolved in 1(N) HCl and added to the poly-
meric solution. The resultant solution was poured into 25 ml
of aqueous phase containing 1% (w/v) of poloxamer-188 with
a constant ow rate of 1 ml minꢁ1. The mixture was then
homogenized using a probe homogenizer (VIRTIS, Cyclone
IQ, USA), at constant agitation speeds of 10 000 rpm in an ice
bath. The formed emulsion was kept at room temperature
under gentle stirring for 24 h to evaporate the organic
solvents. The prepared polymeric nanoparticles were
centrifuged at 18 000 rpm, for 15 min (Sorvall Ultracentri-
fuge, USA). The nanoparticle was recovered and freeze dried
7
8
9
3g
3h
3i
15
10
15
91
85
90
ꢀ
for 2 days (ꢁ80 C and <10 mm mercury pressure, Freezone
6lt, Labconco Corp., MO) to get powdered nanoparticles and
stored in freeze.
10
3j
10
92
c. Characterization of nanoparticles
Determination of particle size. Particle size analysis was
performed by Photon Correlation Spectroscopy (PCS) with
a
All reactions are monitored by TLC. b Isolated yields.
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Table 5 LTNPs catalyzed synthesis of Hantzsch 1,4-DHPs scaffolds (5a–5o)a
Entry
Aldehyde
1,3-Diketone
Product
Timeb (min)
Yieldc (%)
1
5a
10
91
2
3
5b
5c
15
85
89
5
4
5
Ferrocene-3-carboxaldehdye
Ferrocene-3-carboxaldehyde
5d
5e
15
15
91
90
6
7
5f
10
12
91
94
5g
8
5h
15
10
5
85
87
93
9
5i
10
5j
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Table 5 (Contd.)
Entry
11
Aldehyde
1,3-Diketone
Product
Timeb (min)
Yieldc (%)
5k
10
89
12
5l
7
92
13
14
5m
5n
10
15
90
86
15
5o
15
85
a
Reaction conditions – aldehyde : 1,3-diketone : ammonium acetate ¼ 1 : 2 : 1; catalyst loading ¼ 0.02 g. b Reaction progress monitored by TLC.
c
Isolated yields.
Zetasizer 3000 (Malvern Instruments). The freeze dried potassium hydroxide corresponding to a 1 : 1 ratio before
powdered samples were suspended in Milli-Q water (1 mg examination. One drop of sample was placed for one minute on
mlꢁ1) at 25 ꢀC and sonicated for 30 s in an ice bath before a copper grid coated with a formvar carbon lm. The excess of
measurement to prevent clumping. The mean particle diam- sample was wicked away with the aid of lter paper. The sample
eter and size distribution of the suspension were assessed. was then ready for analysis by TEM.
Analysis was carried out for three times for each batch of
sample under identical conditions and mean values were chemical integrity and possible chemical interaction between L-
reported. tyrosine and polymer can be determined by FTIR analysis
Fourier transforms infrared spectroscopy (FT-IR). The
Atomic force microscopy (AFM). The surface morphology of (Perkin Elmer, FT-IR Spectrometer). Samples were mixed sepa-
prepared nanoparticles was carried out using atomic force rately with potassium bromide (200–400 mg) and compressed
microscopy (AFM). The nanoparticles suspension was prepared by applying pressure of 200 kg cmꢁ2 for 2 min in hydraulic press
with milliQ water and dried overnight in air on a clean glass to prepare the pellets.
surface and observation was performed with AFM consisting of
silicon probes with pyramidal cantilever having force constant
of 0.2 N mꢁ1. To avoid damage of the sample surface, the tip to
sample distance was kept constant. The scan speed of 2 Hz and
312 kHz resonant frequency was used for displaying amplitude,
signal of the cantilever in the trace direction and to obtained
images.
Transmission electron microscopy (TEM). Morphology of
the particles was also examined using transmission electron
microscope. A sample of particle suspension was diluted with
3% w/v phosphotungstic acid adjusted to pH 7.5 with
Conclusions
In summary, we have demonstrated that LTNPs as an organo-
catalyst in dicoumarols and 1,4-DHPs synthesis expecting
advantages in (i) L-tyrosine can easily disperse through the
exible thin polymer coating (ii) increment in surface area (iii)
easy separation of the catalyst (iv) fully green methodology and
(v) very small catalyst loading is sufficient for reactions.
Henceforth, this new catalyst works well in various organic
transformations under green methodology.
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Scheme 4 Plausible mechanism for the synthesis of 1,4-DHPs using LNTPs (LTNPs dispersed through Eudragit coating).
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Nov 1968; 11.
Acknowledgements
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