An efficient microwave accelerated three component reaction of phenacyl azides and pyridinium phenacyl salts: A facile greener approach to 2-amino-2-ene-1,4-diones/pyrrolidin-2-ones

Article abstract of DOI:10.1080/00397911.2020.1787447

A mild and efficient base promoted, microwave assisted, green synthesis of 2-amino-2-ene-1,4-diones has been described by the decomposition of phenacyl azides followed by treatment with pyridinium salts of phenacyl bromides in aqueous media. A diverse range of substrates bearing electron-releasing and electron-withdrawing substituents were well tolerated and delivered corresponding 2-amino-2-ene-1,4-diones in good yields. Synthesized 2-amino-2-ene-1,4-diones have been further explored in the synthesis of various substituted 4-hydroxypyrrolidin-2-ones.

Full text of DOI:10.1080/00397911.2020.1787447

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
An efficient microwave accelerated three
component reaction of phenacyl azides and
pyridinium phenacyl salts: A facile greener
approach to 2-amino-2-ene-1,4-diones/
pyrrolidin-2-ones
Venkateswarlu Chimaladenne , Ramesh Manda , Ashok Reddy Gudipally ,
Krishna Reddy Valluru , Pradeep Kumar Brahman & Vijaya Laxmi Somarapu
To cite this article: Venkateswarlu Chimaladenne , Ramesh Manda , Ashok Reddy Gudipally ,
Krishna Reddy Valluru , Pradeep Kumar Brahman & Vijaya Laxmi Somarapu (2020): An efficient
microwave accelerated three component reaction of phenacyl azides and pyridinium phenacyl
salts: A facile greener approach to 2-amino-2-ene-1,4-diones/pyrrolidin-2-ones, Synthetic
Communications, DOI: 10.1080/00397911.2020.1787447
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https://doi.org/10.1080/00397911.2020.1787447
An efficient microwave accelerated three component
reaction of phenacyl azides and pyridinium phenacyl salts:
A facile greener approach to 2-amino-2-ene-1,4-diones/
pyrrolidin-2-ones
a,b
b
b
Venkateswarlu Chimaladenne , Ramesh Manda , Ashok Reddy Gudipally ,
b
a
a,c
Krishna Reddy Valluru , Pradeep Kumar Brahman , and Vijaya Laxmi Somarapu
a
Koneru Laksmaiah Education Foundation, Green Fields, Vaddeswaram, Guntur, Andhra Pradesh, India;
Chemistry Services, GVK Biosciences Pvt. Ltd, IDA Mallapur, Hyderabad, India; Department of
b
c
Chemistry, Palamuru University, Mahabubnagar, India
ABSTRACT
ARTICLE HISTORY
A mild and efficient base promoted, microwave assisted, green syn-
thesis of 2-amino-2-ene-1,4-diones has been described by the
decomposition of phenacyl azides followed by treatment with pyridi-
nium salts of phenacyl bromides in aqueous media. A diverse range
of substrates bearing electron-releasing and electron-withdrawing
substituents were well tolerated and delivered corresponding 2-
amino-2-ene-1,4-diones in good yields. Synthesized 2-amino-2-ene-
Received 5 March 2020
KEYWORDS
b-Enamino carbonyls; 4-
hydroxypyrrolidin-2-ones;
microwave; phenacyl azide;
pyridinium salts
1,4-diones have been further explored in the synthesis of various
substituted 4-hydroxypyrrolidin-2-ones.
GRAPHICAL ABSTRACT
Introduction
In the recent past, environmental awareness drove the synthetic community toward
greener technologies in synthetic chemistry. Consequently, the usage of nontoxic sol-
vents to minimize the environmental impact has been evolved as a trend since several
[
1]
years, complying with one of the 12 principles of green chemistry. Viewed in this
light, water termed as a “green solvent” is immensely advantageous over the conven-
tional organic solvents. It is an indisputable and unattainable solvent of choice and is
CONTACT Vijaya Laxmi Somarapu
vijaya1214@gmail.com
Department of Chemistry, Palamuru University,
Bandameedipally, Mahabubnagar, Telangana, 509001, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2020 Taylor & Francis Group, LLC

2
V. CHIMALADENNE ET AL.
inherently safer, non-flammable, non-explosive, non-carcinogenic and possesses high
[
2]
heat capacity. Consequently, the development of organic reactions in aqueous-phase
is inevitable in the biochemical reactions as well as the syntheses of pharmaceutical
[
3]
products and is a challenge for the synthetic community to be tackled. Lately, phe-
nacyl azides have attracted significant attention as valuable building blocks in the fields
of organic chemistry, particularly in the synthesis of aza-heterocyclic systems and bio-
[
4]
medical sciences due to their high reactivity and excellent photophysical properties.
By nature they are 1,3-dipoles, and are generally believed as masked phenacyl amines.
Phenacyl azides generate reactive nitrene species after degradation and are also key sub-
strates for iminophosphorane synthesis which are considered to be key starting materi-
[
4,5]
als for the aza-Wittig reactions.
On the other hand, enaminone motifs represent
potential intermediates in organic synthesis, as they possess two nucleophilic and three
[
6]
electrophilic centers. They also have the ability to involve in the radical and pericyclic
reactions. Hence, the enaminone functional group is believed to contribute immensely
[
7]
to the synthesis of the natural and synthetic bioactive molecules. Consequently, few
common protocols have been evolved for the synthesis of enaminone functionality.
[
8]
Some of them are condensation of dicarbonyl compounds with amines, conjugate
[
9]
addition of amines to a,b-unsaturated carbonyl compounds, the reaction of lactims or
[
10]
lactams containing active methylene group with other nucleophiles,
nucleophilic add-
[
11]
[12]
ition of nitrile compounds,
reactions,
Eschenmoser coupling reaction,
hydroamination of alkynes,
intramolecular aza-Wittig
[
13]
[14]
Friedel ꢀ Crafts and Vilsmeier reactions of enecarbamates,
the
[
15]
and the metal-catalyzed coupling of thioamides with
[
16]
a-diazocarbonyl compounds.
Despite numerous reports available for the synthesis of
enaminones, only few were reported for the synthesis of 2-amino-2-ene-1,4-diones,
which are very important intermediates for the construction of N-heterocycles such as
pyrrolidin-2-ones. Initially, Oae et al. have synthesized 2-amino-2-ene-1,4-diones by
[
17]
employing S,S-Diphenylsulfilimine (Figure 1a).
Later, H. S. P. Rao and S. P. Senthil
Kumar et al. have reported the Ba(OH) -mediated synthesis of 2-amino-2-ene-1,4-dio-
2
[
18]
nes using sodium azide (Figure 1b).
Due to insufficient literature reports and the
importance of 2-amino-2-ene-1,4-diones in the synthesis of N-heterocycles, there is
necessity for the development of new methods to synthesize 2-amino-2-ene-1,4-diones.
Recently, microwave assisted organic synthesis has emerged as an alternative approach
for the conventional heating because of its increased reaction rate, expanded range of
reactions, less decomposition of reactants and/or formation of less by-products. It has
also additional advantage of rapidly increasing the temperature beyond the conventional
[
19]
boiling point in a pressurized system.
Herewith, we describe our efforts toward base
promoted, water mediated microwave accelerated reaction of phenacyl azides with pyri-
dinium salts of phenacyl bromide for the preparation of 2-amino-2-ene-1,4-diones
(Figure 1c).
Results and discussion
Our journey toward the optimization of the reaction conditions for the three-compo-
nent construction of 2-amino-2-ene-1,4-diones derivatives was begun by reacting the
pyridinium salt 2a with phenacyl azide 1a as model substrates (Table 1). Initially, the

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3
Figure 1. Comparison of the present work with the previous reports for the synthesis of 2-amino-2-
ene-1,4-diones.
a
Table 1. Optimization of the reaction conditions.
ꢁ
b
Entry
Base (equiv.)
Solvent
Time (min)
10 h
Temp. ( C)
Yield (%)
c
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
a
K
K
K
K
K
K
K
K
2
2
2
2
2
2
2
2
CO
3
CO
3
CO
3
CO
3
CO
3
CO
3
CO
3
CO
3
(2.0)
(2.0)
(2.5)
(3.0)
(3.5)
(3.0)
(3.0)
(3.0)
H
H
H
H
H
H
H
H
H
H
H
H
H
2
2
2
2
2
2
2
2
2
2
2
2
2
O
80
45
45
45
45
80
45
45
45
45
45
45
45
45
45
45
45
45
ꢁ
48
O
O
O
O
O
O
O
O
O
O
O
O
45 min
45 min
45 min
45 min
45 min
1 h
30 min
45 min
45 min
45 min
45 min
45 min
45 min
45 min
45 min
45 min
45 min
74
80
84
82
72
84
78
33
NF
39
53
55
46
54
75
68
NF
NaOH (3.0)
BuOK (3.0)
t
0
1
2
3
4
5
6
7
8
Na
Et
DBU (3.0)
2 3
CO (3.0)
3
N (3.0)
K
K
K
K
–
2
2
2
2
CO
3
CO
3
CO
3
CO
3
(3.0)
(3.0)
(3.0)
(3.0)
Toluene
CH CN
3
DMF
THF
H O
2
b
c
Reaction conditions: 1a (1.1 mmol), 2a (1 mmol), Base (3 mmol), solvent (5 mL), MW, 45 C, 45 min. Isolated yields. 1a
ꢁ
(
1.1 mmol), 2a (1 mmol), Base (3 mmol), solvent (5 mL), 80 C, 10 h. NF: Not formed; THF: tetrahydrofuran; DMF: N,N-
dimethyl formamide.

4
V. CHIMALADENNE ET AL.
pyridinium salt 2a was prepared by treating an equimolar mixture of pyridine and phe-
nacyl bromides in ethylacetate at ambient temperature for 12 h. The crude pyridinium
salt 2a was sufficiently pure for further transformation, hence after the formation of 2a,
the solvent was removed under reduced pressure and the same reaction vessel was
charged with phenacyl azide 1a and base (2.0 equiv.).
In order to study the comparative effectiveness of microwave irradiation over conven-
tional heating, initially, a representative reaction was performed under conventional
conditions, where the reaction mixture of pyridinium salt 2a and phenacyl azide 1a was
ꢁ
loaded with 2 equiv., of aqueous K CO and heated at 80 C for 10 h, which resulted in
2
3
the formation of 48% of the required product (entry 1, Table 1). On the other hand,
the same reaction proceeded for completion within 45 min with a yield of 74% under
ꢁ
microwave irradiation at 45 C (entry 2, Table 1). Inspired by these results, next we
turned to focus our attention on other reaction parameters such as amount of base,
reaction time, temperature and the type of solvent. It is observed that increase in the
amount of base from 2 to 3 equiv., led to improvement in the yield of the product
(entries 3 and 4, Table 1). However further increase in the amount of base beyond 3.0
equiv., did not lead to any improvement in the yield of the product (entry 5, Table 1).
ꢁ
Increase in the reaction temperature to 80 C led to a detrimental effect on the yield of
product (entry 6, Table 1). Increase in the reaction time has no substantial increase in
the yield of the reaction, moreover, decrease in the reaction time led to decrease in the
yield of the product (entries 7 and 8, Table 1). It is evident that K CO is the best base
2
3
of choice for this transformation over other organic and inorganic bases screened such
t
as NaOH, BuOK, Na CO , Et N and DBU (entries 9–13, Table 1). In addition, in an
2
3
3
attempt to improve the yields of the product, various solvents such as toluene, CH CN,
3
DMF and THF were also screened, water was found to be the superior solvent for this
transformation (entries 14–17, Table 1), with good absorbing capacity for microwave
energy because of which has been successfully utilized as a solvent for various organic
syntheses and it turned out to be one of the best choices considering its environmen-
tally-friendly characteristics. A control experiment in the absence of base led to the for-
mation of no desired product even in trace amounts, which signifies the necessity of
base for this transformation (entry 18, Table 1). Therefore, systematic screening of the
reaction conditions revealed that the formation of enaminone is efficient while using
ꢁ
K CO (3 equiv.) in H O at 45 C for 45 min under microwave irradiation.
2
3
2
In order to validate the generality of this methodology, a series of pyridinium salts of
phenacyl bromides and phenacyl azides were screened for the construction of 2-amino-
2-ene-1,4-diones (3a–m) by employing the optimized reaction conditions (Figure 2).
The reaction was found to be very robust and a number of electron-releasing and elec-
tron-withdrawing substituents on both the aryl moieties of phenacyl bromide pyridi-
nium salts and phenacyl azides were well tolerated and delivered corresponding
products in good yields. It is worth mention that phenacyl bromides and phenacyl
azides bearing halogen group such as chlorine are also amenable for this transformation
and yielded corresponding enaminone products in decent yields (82–93%, Figure 2).
The tolerance of halogens toward the present protocol makes it worthier as these halides
are needed in wide range of transition metal-catalyzed reactions for further synthetic
modifications.

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5
Figure 2. Substrate scope of a-azido ketones and pyridinium ylides for the synthesis of b-enamino
carbonyls 3a–m; Reaction conditions: 1 (1.1 mmol), 2 (1 mmol), Base (3 mmol), solvent (5 mL), MW,
ꢁ
4
5 C, 45 min; Yields of isolated products were mentioned.
In order to demonstrate the synthetic utility of these b-enamino carbonyls 3, acetyl-
ation of the amino group of 3 followed by base mediated condensation of the acetylated
amine was planned (Figure 3). Acetylation of 3 was performed using acetyl chloride
and condensation was carried out employing t-BuOK as a base. Even though there is a
feasibility to form both six and five membered rings, the five membered heterocycle
was obtained exclusively. This selectivity for the formation of the five membered rings
can be explained by considering the higher electrophilicity of the unconjugated carbonyl
over the conjugated one. Consequently, several b-enamino carbonyls 4 were subjected

6
V. CHIMALADENNE ET AL.
Scheme 1. Control experiments.
to acetylation/intramolecular condensation to yield pyrrolidin-2-ones 5a-e in good yields
(70–75%, Figure 3).
In order to investigate the reaction mechanism for the formation of b-enamino
carbonyls several control experiments were performed and the results are depicted in
Scheme 1. In this context, we performed the reaction under oxygen and nitrogen
atmosphere by employing standard optimized reaction conditions. However, reaction
yield does not alter either in the presence of oxygen (Scheme 1a) or nitrogen atmos-
phere (Scheme 1b). Moreover, required product formation was not observed by
replacing the pyridinium salt (2a) with phenacyl bromide (I) as a starting material
and the reaction was performed under standard reaction optimized conditions
(Scheme 1c).
A plausible reaction mechanism for the formation of substituted (Z)-2-amino-1,4-
diphenylbut-2-ene-1,4-dione derivatives (3) was shown in Figure 4. Initially, it is
assumed that the phenacyl azide 1 decomposes into corresponding imine D via the
elimination of N in the presence of K CO (Figure 4b). The imine D then reacts with
2
2
3
the ylide A which is generated in situ from the corresponding pyridinium salt of the
phenacyl bromide 2 to form intermediate E which after subsequent elimination of pyri-
dine in the presence of K CO affords desired product, 2-amino-2-ene-1,4-diones 3
2
3
(Figure 4b). The 2-amino-2-ene-1,4-dione 3 after N-acetylation leads to formation of N-
acetylated enamino-1,4-diones 4 which finally undergoes intramolecular cyclization to
afford the hydroxypyrrolidin-2-ones 5 (Figure 3).
Conclusion
In conclusion, an efficient microwave assisted, base promoted one pot protocol has
been developed for the construction of various 2-amino-2-ene-1,4-diones by the reac-
tion of phenacyl azides with pyridinium phenacyl bromide salts in the presence of

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7
Figure 3. Synthesis of pyrrolidin-2-ones 5 via acetylation of b-enamino carbonyls followed by base
mediated condensation; Yields of isolated products were mentioned.
K CO
2
3
(a)
N
N
Br
R1
R1
O
O
2
A
O
O
O
K CO
2
3
(b)
N
N
N
R2
N
R2
N
R2
N
N
1
B
C
NH2
O
O
N
O
K CO
2
3
H
R2
A
R1
R1
NH
R2
R2
O
D
NH2
O
3
E
Figure 4. Plausible reaction mechanism.

8
V. CHIMALADENNE ET AL.
water. The obtained 2-amino-2-ene-1,4-diones were further acetylated and condensed
with t-BuOK to yield 4-hydroxypyrrolidin-2-ones in good yields. Metal free milder con-
ditions, operational simplicity, high functional group compatibility and aqueous reaction
medium are some of the salient features that render this protocol to be a greener alter-
native to the conventional organic synthesis.
Acknowledgments
The authors to GVK Biosciences for providing basic research and analytical facility. Special
thanks to Dr. Sudhir Kumar Singh and Dr. Somesh Sharma for his valuable scientific inputs.
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[
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