A facile one-pot domino reaction for the stereoselective synthesis of acryl derivatives promoted by Ca(OTf)2

Article abstract of DOI:10.1016/j.tetlet.2016.03.098

A facile one-pot domino reaction for the stereoselective synthesis of acryl derivatives has been reported using alkaline earth catalyst [Ca(OTf)₂]. Initially aryl amine reacts with ethyl propiolate to form β-enamino ester which further reacts w

Full text of DOI:10.1016/j.tetlet.2016.03.098

Tetrahedron Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
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A facile one-pot domino reaction for the stereoselective synthesis of
acryl derivatives promoted by Ca(OTf)2
a
a
b
Srinivasarao Yaragorla a, , Pyare Lal Saini , Abhishek Pareek , Abdulrahman I. Almansour ,
⇑
Natarajan Arumugam b
a Organic & Medicinal Chemistry Laboratory, Department of Chemistry, Central University of Rajasthan, Bandarsindri, NH-8, Ajmer, Rajasthan 305817, India
b Department of Chemistry, College of Science, King Saud University, PO Box 2455, Riyadh 11451, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile one-pot domino reaction for the stereoselective synthesis of acryl derivatives has been reported
using alkaline earth catalyst [Ca(OTf)2]. Initially aryl amine reacts with ethyl propiolate to form
b-enamino ester which further reacts with aryl aldehyde and indole in the presence of Ca(OTf)2 to give
indolyl acrylates. It is interesting to note that in the absence of indole the reaction leads to the formation
of benzylidene bisacrylates. Similarly b-enamino ester reacts with another electrophilic partner isatin in
the presence of Ca(OTf)2 to give oxindolyl acrylates. The products were isolated in good yields by simple
filtration.
Received 12 January 2016
Revised 23 March 2016
Accepted 27 March 2016
Available online xxxx
Keywords:
b-Enamino ester
Indolyl acrylates
Oxindolyl acrylates
Domino reactions
Heterocyclic compounds
Calcium catalysis
Ó 2016 Elsevier Ltd. All rights reserved.
Multicomponent reactions and domino reactions provide a sus-
tainable and diversity oriented synthesis of novel heterocyclic
molecules in one-pot.1 In addition they offer atom economy and
step economy synthesis, domino reactions are also treated as eco-
friendly reactions because they minimize the usage of large
amounts of organic solvents which are used to purify the interme-
diates in each step through column chromatography.1 On the other
hand, the active intermediate b-enamino ester which results from
the addition of primary amines to electron deficient alkynes is
known as a key building block for the synthesis of various hetero-
cycles and pharmaceutical compounds.2,3 Several domino reac-
tions were developed to trap the b-enamino ester with a wide
variety of electrophiles and nucleophiles in a sequence to generate
the novel nitrogen containing heterocyclic scaffolds.4–10 When we
looked into the literature there are very few reports available for
the synthesis of indolyl acrylates (5), oxindolyl acrylates (8), and
benzylidene (6) bisacrylates from the active b-enamino ester.11,12
Yan et al. reported an efficient FeCl3 catalyzed domino synthesis
of indolyl acrylates at rt with moderate yields and the reaction
took 30 h for completion.11 Later the same group reported the syn-
thesis of arylidene bisacrylates and oxindolyl acrylates using FeCl3
as the catalyst.12 Albeit the elegant methods available for the
synthesis of indolyl acrylates, oxindolyl acrylates, and arylidene
bisacrylates, it is highly desirable to develop an alternative cat-
alytic system (non-transition metal catalyst) to reduce the reaction
time and enhance the product yields. In continuation of our
research aimed toward the novel synthesis of heterocyclic mole-
cules using alkaline earth catalysts (as an alternative to transition
metal catalysts)13 herein we report a Ca(OTf)2 catalyzed domino
reaction for the synthesis of indolyl acrylates, oxindolyl acrylates,
and arylidene bisacrylates.
Initially we planned to study the synthesis of indolyl acrylates
which involves the four component domino reaction of aniline,
ethyl propiolate, aryl aldehyde, and indole. Literature studies illus-
trate that there are two stages in the reaction. First stage aniline
and ethyl propiolate react at rt in ethanol to give active b-enamino
ester, but the second stage requires activation (in the form of a cat-
alyst) to trap the b-enamino ester with suitable electrophilic
reagents.
We commenced our synthesis by reacting stoichiometric
amounts of aniline and ethyl propiolate in ethanol at rt and
observed the formation of b-enamino ester after 12 h; the stoichio-
metric amounts of benzaldehyde and indole were added to the
above reaction mixture along with 5 mol % of Ca(OTf)2 and 5 mol
% Bu4NPF6 (additive) and continuously stirred at rt for 12 more
⇑
Corresponding author. Tel.: +91 463 238535; fax: +91 463 238722.
E-mail address: Srinivasarao@curaj.ac.in (S. Yaragorla).
http://dx.doi.org/10.1016/j.tetlet.2016.03.098
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Yaragorla, S.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.03.098

2
S. Yaragorla et al. / Tetrahedron Letters xxx (2016) xxx–xxx
hours and isolated ethyl (Z)-2-((1H-indol-3-yl)(phenyl)methyl)-3-
(phenylamino)acrylate (5a) in 65% yield. The product was
confirmed by 1H & 13C NMR and mass data.15,16 The stereoselectiv-
ity of the product was established previously using single crystal
X-ray diffraction and proved that due to the hydrogen bonding
interactions between ester oxygen and amines proton the com-
pound exists only in the Z-form.11 In order to decrease the reac-
tion time the catalyst was added in the first stage and found
that there was no change in the reaction time. Nevertheless when
aniline and ethyl propiolate were refluxed in ethanol at 90 °C the
formation of b-enamino ester took place in 3.5 h only. Addition of
catalyst had no role in the first step as the reaction was thermo-
dynamically accelerated (reaction time decreased from 12 h to
3.5 h). Having improved the first stage of synthesis we were inter-
ested to optimize the second stage addition (Table 1). Among the
several catalyst loadings tried, we found that the second stage was
better with 10 mol % of Ca(OTf)2 and 10 mol % Bu4NPF6 at rt which
gave 79% yield after 8 h (Scheme 1). When the reflux was contin-
ued in the second stage we observed the formation of bis(indolyl
methane) as the major product (Table 1, entry 4) and hence reac-
tion was brought to rt and continued there. The reaction could not
yield better when refluxed in water (20%, 20 h) acetonitrile (35%,
20 h) and under solvent free conditions (50%, 24 h). Entry 12
(Table 1) describes that when KPF6 was used as the additive
instead of Bu4NPF6 only 30% of the product was isolated. Similarly
we replaced Ca(OTf)2 with another calcium salt CaCl2 which
yielded 48% of the product after 18 h (entry 13). After discovering
the suitable reaction conditions we were interested to study the
substrate scope of our methodology. Initially we chose to test
the participation of various aromatic aldehydes bearing electron
withdrawing substitutions, electron donating groups and halogen
atoms and found that all these aldehydes equally participated in
the reaction to give the desired products in moderate to good
yields (5a–5k, Table 2). Encouraged by the broad scope of aldehy-
des we further looked into the scope of 5-bromo indole and 5-
methoxy indole to furnish respective indolyl acrylates 5l–5p and
5q–5u in good yields (Table 2). 4-Chloro aniline was also used
along with indole and different aryl aldehydes to prepare the acry-
lates 5v–5y in good yields. 4-methyl aniline (electron donating
group) when treated with ethyl and methyl propiolates yielded
5z and 5aa in 73% and 70%, respectively (Table 2). When ortho-
substituted anilines (2-bromoaniline and 2-hydroxy aniline) were
used in the reaction with methyl propiolate we could notice only
traces of b-enamino ester and hence the second stage addition
was not performed. Probably the reaction is prohibited by the
steric factor.
Driven by this observation we went on for making the arylidene
bisacrylates (6, Table 3) taking the advantage that the above reac-
tion should be performed in the absence of external nucleophile
(indole). As proposed we treated aniline and ethyl propiolate in
ethanol at 90 °C for 3.5 then added 0.5 equiv of benzaldehyde along
with 10 mol % of catalyst and additive. The reaction was continued
at reflux for 6.5 h more to isolate diethyl (2Z,4Z)-3-phenyl-2,4-bis
((phenylamino)methylene)pentanedioate (6a) in 81% yield after
simple filtration (Table 3). Though the first step has a temperature
effect, the second one was not influenced by the raise in tempera-
ture (second stage took 7 h at rt) and hence we continued the sec-
ond step at rt. Benzylidene bisacrylates 6b and 6c were made using
same conditions in 82% and 80% yields. Methyl propiolate was
treated with aniline and 4-nitrobenzaldehyde to make compound
Table 1
Screening of reaction conditions for the second step of the reactiona
Entry
Catalyst (mol %)
Additive
(mol %)
Solvent
Temperature (°C)
Yieldc
(%)
& timeb
1
2
Ca(OTf)2,
5
Bu4NPF6, 5
Bu4NPF6, 5
Bu4NPF6,10
Bu4NPF6,10
Bu4NPF6,10
Bu4NPF6,10
Bu4NPF6,10
Bu4NPF6,10
—
EtOH
EtOH
EtOH
EtOH
Neat
Rt, 12 h
Rt, 12 h
Rt, 8 h
50
62
79
15e
50
30
20
35
30
35
5
Ca(OTf)2, 10
Ca(OTf)2, 10
Ca(OTf)2, 10
Ca(OTf)2, 10
Ca(OTf)2, 10
Ca(OTf)2, 10
Ca(OTf)2, 10
Ca(OTf)2,10
—
3d
4e
5
90, 12 h
Rt, 24 he
Rt, 24 h
Rt, 20 h
Rt, 20 h
Rt, 12 h
Rt, 12 h
Rt, 18 h
Rt, 12 h
Rt, 18 h
6
7
8
9
10
11
12
13
CH2Cl2
H2O
MeCN
EtOH
EtOH
EtOH
EtOH
EtOH
Bu4NPF6,10
—
KPF6, 10
—
Ca(OTf)2, 10
CaCl2, 10
30
48
Bu4NPF6,10
a
b
c
First step of the reaction proved to be efficient at reflux condition (3.5 h).
No progress was observed after the time mentioned.
Isolated yields.
Optimum conditions.
Bisindolyl methane was observed as the major product.
d
e
1+2, EtOH, 90 oC, 3.5 h
then bring to rt and
O
NH2
CHO
CO2Et
EtO
HN
N
H
N
H
add 3,4, along with 10 mol% Ca(OTf)2,
10 mol% Bu4NPF6, 8 h, 79%
1
2
3
4
5a
Scheme 1. Calcium catalyzed one-pot, 4-component domino approach for the synthesis of indolyl-3-acrylates.
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S. Yaragorla et al. / Tetrahedron Letters xxx (2016) xxx–xxx
3
Table 2
Ca(OTf)2 promoted one-pot, 4-component domino approach for the synthesis of indolyl-3-acrylatesa
X
O
Ar2
EtOH, Ca(OTf)2 10 mol%
Bu4NPF6 10 mol%
X
CO2R
Ar1NH2
Ar2CHO
EtO
N
H
N
H
HN
Ar1
1
2
3
4
5
R=Et,Me
Me
OMe
Br
F
O
O
O
O
O
EtO
EtO
EtO
EtO
EtO
N
H
N
N
H
N
H
N
H
HN
Ph
HN
Ph
HN
Ph
HN
Ph
HN
Ph
H
5a
5b
5c
5d
5e
73%, 12 h
79%, 11.5 h
77%, 13 h
75%, 13 h
74%, 14 h
Cl
NO2
Br
Cl
NO2
O
O
O
O
O
EtO
EtO
EtO
EtO
EtO
N
N
H
N
H
N
H
N
H
HN
HN
Ph
HN
Ph
HN
Ph
HN
Ph
H
Ph
75%, 15.5 h
5f
5g
5h
5i
5j
76%, 13.5 h
75%, 15.5 h
79%, 12 h
74%, 15.5 h
Me
OMe
Br
F
Br
Br
Br
Br
Cl
O
O
O
O
O
EtO
EtO
EtO
EtO
EtO
N
H
N
N
N
N
H
HN
Ph
HN
Ph
HN
Ph
HN
Ph
HN
Ph
H
H
H
5o
5l
5k
5m
5n
79%, 11.5 h
78%, 12.5 h
74%, 13.5 h
70%, 16.5 h
71%, 15.5 h
NO2
Me
OMe
Br
F
Br
OMe
OMe
OMe
OMe
O
O
O
O
O
EtO
EtO
EtO
EtO
EtO
N
H
N
H
N
H
N
H
N
H
HN
Ph
HN
Ph
HN
Ph
HN
Ph
HN
Ph
5t
5q
5s
5r
5p
76%, 13.5 h
80%, 11.5 h
74%, 14.5 h
76%, 14.5 h
76%, 15.5 h
OMe
NO2
Br
Br
O
O
O
O
OMe
O
EtO
EtO
HN
EtO
HN
EtO
HN
EtO
N
H
N
H
N
H
N
HN
H
N
H
HN
Ph
5u
5v
74%, 14.5 h
5w
75%, 12 h
5x
80%, 12 h
5y
Cl
71%, 15.5 h
Cl
Cl
Cl
75%, 12 h
O
O
O
Ph
Ph
Ph
EtO
MeO
HN
MeO
N
H
N
H
N
H
HN
HN
5aa
70%, 14 h
5z
73%, 13 h
5ab
73%, 14 h
Me
Me
a
Reaction conditions: 1.1 equiv of aniline and 1.1 equiv of ethyl propiolate were refluxed in ethanol at 90 °C for 3.5 h then reaction was
brought to rt and 10 mol % Ca(OTf)2, 10 mol % Bu4NPF6 were added along with 1 equiv of aldehyde and 1 equiv of indole. Yields are reported after
filtration.
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4
S. Yaragorla et al. / Tetrahedron Letters xxx (2016) xxx–xxx
Table 3
6d in 78% yield. Similarly benzylamine was also used to make the
arylidene bisacrylates 6e and 6f in 76% and 75%, respective yields
(Table 3).
Ca(II) catalyzed one pot three component domino reaction for the synthesis of
arylidene bisacrylatesa
1+2, EtOH, 90 oC, 3.5 h
CO2R2
Due to the potential applications of oxindole derivatives in
organic and medicinal chemistry14 we aimed to use isatin as the
electrophile in the second stage instead of aldehyde to make valu-
able oxindolyl derivatives. Aryl amine and ethyl propiolate were
refluxed in ethanol at 90 °C for 3.5 h then reaction was brought
to rt (as we did not notice any advantage in terms of time or yields
to continue the reaction further at reflux) and isatin was added
along with catalyst and additive (Table 4) to make the oxindolyl
acrylates 8a to 8f in good yields.
O
R3
O
R1 NH2
R3 CHO
R2O
HN
OR2
add 3, Ca(OTf)2 10 mol%,
Bu4NPF6 10 mol%,
3
1
2
NH
R1
6
R1
R1=Ar, Bn; R2=Et, Me; R3=Ar
NO2
Br
O
O
O
O
O
O
Plausible mechanism for the Ca(II) catalyzed domino reaction
for the synthesis of acryl derivatives has been described in the
Scheme 2. Initially conjugate addition of aniline to ethyl propiolate
will take to provide the b-enaminoester. Since the formed interme-
diate is nucleophilic in nature it reacts with isatin in the presence
of Ca(OTf)2 to yield oxindolyl acrylate 8 via nucleophilic substitu-
tion mechanism (S1N). Similarly b-enamino ester reacts with aryl
aldehyde 4 to yield alcohol 5A which will further react with
another mole of b-enamino ester in the presence of calcium triflate
to provide dimeric compound 6 via same mechanism. Intermediate
5A could also react with external nucleophile indole to give
indolylacrylate 5 via S1N mechanism.
In summary, we described a novel one pot domino reaction for
the stereoselective synthesis of acryl derivatives; indol-3-yl acry-
late, bisarylidene acrylates, and oxindolyl substituted acrylates
catalyzed by environmentally benign calcium catalyst. Substrate
scope has been thoroughly investigated. The products were iso-
lated in excellent yield by simple filtration. We believe that this
will be one of the best and economic ways for making the said
compounds which may be medicinally important due to the pres-
ence of indole and oxindole scaffolds.
EtO
HN
OEt
NH
EtO
HN
OEt
NH
EtO
HN
OEt
NH
6b
82%, 10 h
6c
6a
81%, 9 h
80%, 10 h
Cl
NO2
O
O
O
O
O
O
MeO
HN
OMe MeO
NH HN
OMe
NH
MeO
HN
OMe
NH
6e
76%, 10 h
6f
75%, 10 h
6d
78%, 8 h
a
Reaction conditions: aniline (1.1. equiv) and ethyl propiolate (1.1 equiv) were
refluxed in ethanol at 90 °C for 3.5 h then aldehyde (0.5 equiv) was added along
with catalyst and additive. Yields are reported after isolation by filtration.
Table 4
Ca(II) catalyzed one pot three component domino reaction for the synthesis of oxindolyl acrylatesa
O
NH2
O
1+2, EtOH, 90 oC, 3.5 h
EtO
HO
CO2Et
H
N
X
O
X
add 3, Ca(OTf)2 10 mol%,
Bu4NPF6 10 mol%, rt, 24 h
N
Y
O
H
N
Y
1
H
7
2
8
O
O
O
EtO
HO
EtO
HO
EtO
HO
H
N
H
N
H
N
Me
Cl
O
O
O
N
N
N
H
H
H
8a,
75%
8b, 76%
8c,
75%
O
O
O
EtO
HO
EtO
HO
EtO
HO
H
N
H
N
H
N
Me
F
Cl
Cl
O
O
O
N
N
N
H
H
H
8f, 76%b
8e, 74%
8d, 70%
bBenzyl amine was used instead of aniline.
a
Reaction conditions: aniline (1.1. equiv) and ethyl propiolate (1.1 equiv) were refluxed in ethanol at 90 °C for
3.5 h then reaction was brought to rt and isatin (1 equiv) was added along with catalyst and additive. Yields are
reported after isolation by filtration.
Please cite this article in press as: Yaragorla, S.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.03.098

S. Yaragorla et al. / Tetrahedron Letters xxx (2016) xxx–xxx
5
O
OEt
O
O
HO
H
N
O
OEt
O
Ar
N
H
CO2Et
EtO
O
H
ArNH2
+
Ca(OTf)2
EtOH
N
NH
Ar
N
Ar
H
1
2
8
-enamino ester
β
Ar1CHO
O
OEt Ar1 OEt
Ar1
Ar1
OEt
OEt
Ca(OTf)2
N
H
O
O
O
H
OH
NH
Ar
HN
Ar
NH
Ar
N
N
Ar
Ca(OTf)2
H
6
5
5A
Scheme 2. Plausible mechanism for the Ca(II) catalyzed domino reaction to synthesize acryl derivatives.
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Acknowledgments
P.L.S. and A.P. are thankful to UGC-India and Central University
of Rajasthan for the fellowships respectively. The authors acknowl-
edge the Deanship of Scientific Research at King Saud University
for the Research Grant No. RGP-VPP-026.
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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.2016.03.
098.
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15. General experimental experimental procedure for the synthesis of indolyl acrylates
(5): A mixture of aniline (1.1 equiv) and ethyl propiolate (1.1 equiv) was
refluxed in ethanol at 90 °C for 3.5 h then aromatic aldehyde (1.0 mmol), indole
(1.0 equiv) and Ca(OTf)2 (10 mol %) & Bu4NPF6 (10 mol %) were added to the
above reaction and solution was stirred at room temperature for 8–10 h. After
completion of the reaction (monitored by TLC) resulting precipitate was
collected by vacuum filtration and was washed with cold ethanol to give the
pure product.
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Please cite this article in press as: Yaragorla, S.; et al. Tetrahedron Lett. (2016), http://dx.doi.org/10.1016/j.tetlet.2016.03.098