One-pot synthesis of 3,4-dihydropyridin-2-one via michael addition of in situ-generated enaminones

Article abstract of DOI:10.1080/00397911.2012.761239

Herein we have reported a facile solvent-, catalyst-, and aldehyde-free, one-pot synthesis of 3,4-dihydropyridin-2-one from 1,3-diones using simple and mild reaction conditions. The substrate scope has been also extended to β-ketoesters.

Full text of DOI:10.1080/00397911.2012.761239

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One-Pot Synthesis of 3,4-
Dihydropyridin-2-one via Michael
Addition of in situ–Generated
Enaminones
J. B. Senthil Kumar a , Neeta Kumari b & Pratibha Mehta Luthra a
a Neuropharmaceutical Laboratory, Dr. B. R. Ambedkar Center for
Biomedical Research , University of Delhi , Delhi , India
b Chemical Synthesis and Process Technologies, Department of
Chemistry , University of Delhi , Delhi , India
Accepted author version posted online: 18 Jun 2013.Published
online: 08 Aug 2013.
To cite this article: J. B. Senthil Kumar , Neeta Kumari & Pratibha Mehta Luthra (2013) One-Pot
Synthesis of 3,4-Dihydropyridin-2-one via Michael Addition of in situ–Generated Enaminones, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
43:22, 3010-3019, DOI: 10.1080/00397911.2012.761239
To link to this article: http://dx.doi.org/10.1080/00397911.2012.761239
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Synthetic Communications1, 43: 3010–3019, 2013
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2012.761239
ONE-POT SYNTHESIS OF 3,4-DIHYDROPYRIDIN-2-ONE
VIA MICHAEL ADDITION OF IN SITU–GENERATED
ENAMINONES
J. B. Senthil Kumar,1 Neeta Kumari,2 and
Pratibha Mehta Luthra1
1Neuropharmaceutical Laboratory, Dr. B. R. Ambedkar Center for
Biomedical Research, University of Delhi, Delhi, India
2Chemical Synthesis and Process Technologies, Department of Chemistry,
University of Delhi, Delhi, India
GRAPHICAL ABSTRACT
Abstract Herein we have reported a facile solvent-, catalyst-, and aldehyde-free, one-pot
synthesis of 3,4-dihydropyridin-2-one from 1,3-diones using simple and mild reaction con-
ditions. The substrate scope has been also extended to b-ketoesters.
[Supplementary materials are available for this article. Go to the publisher’s online edi-
tion of Synthetic Communications1 for the following free supplemental resource: Full
experimental and spectral details.]
Keywords 3,4-DHP-2-ones; 1,3-dicarbonyls; enaminones; solvent free
Received October 25, 2012.
Address correspondence to Pratibha Mehta Luthra, Neuropharmaceutical Laboratory, Dr. B. R.
Ambedkar Center for Biomedical Research, University of Delhi, Delhi 110007, India. E-mail: pmlsci@
yahoo.com
3010

3,4-DIHYDROPYRIDIN SYNTHESIS
3011
INTRODUCTION
Development of new synthetic approaches and solvent-free, one-pot, multi
component reactions (MCRs) for the synthesis of N-heterocycles offer the products
cost-effectively.[1] Because of the wide distribution of biologically active N-heterocycles
in nature, the synthesis of novel bioactive nitrogen-containing heterocyclic compounds
has been an important research target.[2] Among different N-heterocycles, pyridine-2--
one derivatives and their partially reduced 3,4-dihyrdopyridin-2-ones (3,4-DHP-2-ones)
possess a variety of biological properties and can act in pharmaceuticals such as Rho
kinase inhibitor, P2X7 antagonists, a-adrenergic receptor antagonists, and nonselective
b-blockers[3] (Fig. 1).
Moreover 3,4-DHP-2-ones act as useful scaffolds for the synthesis of Clausena
alkaloids[4] and calcium channel modulators.[5] 3,4-DHP-2-ones have been further
elaborated by reduction to d-lactams,[6] and oxidation to 2-pyridones,[7] giving
further scope to the parent library. Wide-spectrum therapeutic effects associated
with 3,4-DHP-2-ones have generated interest in the synthesis of these scaffolds. Sub-
stantial synthetic methods are documented for pyridine-2-one and its derivatives;[8]
however, limited literature is available for the synthesis of 3,4-DHP-2-one and its
derivatives.[9] Reports involve the Michael addition of the 1,3-dione intermediate
to the arylidene Meldrum’s acid condensate followed by intramolecular nucleophilic
substitution,[9a] 1,4-dien-3-ones-mediated Nazarov electrocyclization and intermole-
cular trapping by various azides,[9b] reaction of 1-azabutadienes with enolates of sub-
stituted acetates,[9c] and solid-support aza anulation[9e] to form the cyclic amide.
3,4-DHP-2-one intermediate has been envisioned from the bicyclic vinylogous amide
Figure 1. Biologically important 3,4-dihydropyridin-2-one.

3012
J. B. S. KUMAR, N. KUMARI, AND P. M. LUTHRA
Scheme 1. (a) Reported method for the synthesis of bicyclic vinylogous amide. (b) One-pot synthesis of
bicyclic vinylogous amide.
(4a), which is used in the synthesis of aspidosperm-type alkaloids[10] and nonselective
a-blocker carteolol.[3d] The classical method for the synthesis of 4a involved the prep-
aration of enaminone intermediate (2) by passing ammonia gas in the solution of
cyclohexane-1,3-dione (1a) in benzene, followed by cyclization with acrylic acid to
give 4a in 95% yield[11] (Scheme 1a); however, less than 50% yield was obtained by
us and other research groups[12] using same method. The use of ammonia gas and
hazardous solvent benzene for the preparation of 2 involved special handling and
safety procedures. We have developed a simple, versatile, efficient, and economically
viable synthetic method, devoid of ammonia[13] gas and benzene,[14] for the prep-
aration of 3,4-DHP-2-one (4a) (Scheme 1b). Further, the substrate scope of this
methodology was investigated using various 1,3-dicarbonyls.
RESULTS AND DISCUSSION
Various parameters such as temperature, reaction duration, molar ratios, and
nitrogen source were examined to optimize reaction conditions (Table 1). The reac-
tion of 1a with ammonium chloride and 3 in equimolar ratios (entry 1, Table 1) for
3 h at 60 ꢀC did not result in product formation; however, increasing temperatures to
80 ꢀC and 110 ꢀC afforded traces of 4a (entries 2 and 3). Interestingly, the reaction of
ammonium acetate with 1a and 3 in equimolar ratio at 60 ꢀC resulted in the forma-
tion of 4a in 40% yield after 3 h (entry 4) and 55% yield after 5 h (entry 5), whereas
further increase in reaction time (8 h) resulted in reduced yield (entry 6). Ammonium
acetate is a salt of weak acid, and ammonia is easily released to form enaminone as
compared to ammonium chloride, which is a salt of strong acid. It was noticed that
reaction conditions with equimolar ratios of 1a and 3 to the formation of 4a always
proceeded with the contamination of 1a, which may be due to the escape of a small
amount of NH3 from the reaction vessel. The use of a slight excess of ammonium

3,4-DIHYDROPYRIDIN SYNTHESIS
3013
Table 1. Optimization of reaction conditions
1a–3–ammonium
acetate (mol ratio)
Entry
Nitogen source
T (ꢀC)
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
1:1:1
1:1:1
NH4Cl
NH4Cl
NH4Cl
60
80
3
3
3
3
5
8
5
5
No product
Traceb
Traceb
40b
1:1:1
1:1:1
110
60
NH4OAC
NH4OAC
NH4OAC
NH4OAC
NH4OAC
1:1:1
1:1:1
60
60
55b
50b,c
65
1:1.2:1.2
1:1.2:1.2
60
80
92
aIsolated yield.
bStarting materials were present.
cPolymerisation was observed.
acetate and 3 accomplished a complete transformation of the starting material to
afford 4a in quantitative yield (65%, entry 7). Subsequent optimization of the
reaction temperature offered 4a in 92% yield at 80 ꢀC (entry 8). Further, modulating
the reaction conditions did not result in an increase in yield. The product was iso-
lated by vacuum filtration of the reaction mass followed by recrystallization with
water. The sequence of reactions for the formation of 4a involved are 1) dispropor-
tionate amount of ammonium acetate to give acetic acid and ammonia, (2) 1a and
ammonia under acidic conditions formed enaminone intermediate, and (3) Michael
addition of the enaminone intermediate to acrylic acid to led to the formation of
3,4-DHP-2-one (4a).
This is modified E1cb mechanism. The initial reaction involves the protona-
tion of enol form, generated from acetic acid, which favors the nucleophillic attack
of ammonia, which results in in situ generation of enaminone intermediate, followed
by Michael addition, leading to the formation of 3,4-DHP-2-one. The economically
viable and significantly improved synthesis of 4a has been developed by precluding
the preparation of enaminone intermediate 2. Furthermore, the scope and limitation
of this method were evaluated using various 1,3-dicarbonyls to get 5,6-disubstituted
products 4a–m (Table 2). 1,3-Acyclic dione (entry 2, Table 2) afforded cyclized
product in 87% yield, whereas 1,3-diketo moiety bearing an aryl substituent next
to the carbonyl group did not form the cyclized product (entry 3, Table 2), which
may be due to the predominant existence of cis-enol enaminone.[15] The reaction
of benzoylacetone with ammonium acetate (entry 3) was investigated to envisage
1
the failure of 3,4-DHP-2-one formation. The H NMR showed (Fig. 2A) the acidic
proton at d 10.21 (OH) and an imino proton at d 5.27 (NH). In 13C NMR of
=
the enaminone, the C O carbon at d 200 is slightly upfield at d 190 due to the
C-OH tautomer[15b], indicating that the intermediate occurs mainly as stable imine
tautomer (Fig. 2B).
Various substituted acetoacetate esters (entries 4–12) afforded the cyclized
product in 68–89% yield. The chloroethylacetoacetate (entry 13) afforded a mix-
ture of unidentified products, which could be due to the negative inductive effect
of the chlorine atom that favors the enol-enaminone formation, leading to an

3014
J. B. S. KUMAR, N. KUMARI, AND P. M. LUTHRA
Table 2. Synthesis of dihydropyridin-2-ones via Michalis addition of in situ–generated enamines
1,3 Dicarbonyls
Entry
R1
R2
Product
Yield (%)a Melting point (ꢀC)
=
R1 and R2 -(CH2)3-
1
92
193–195
2
3
CH3
CH3
CH3
87
146–148b
-b
—
4
CH3
-O-CH3
89
150–153
5
6
7
CH3
CH3
CH3
-O-CH2CH3
86
81
78
152
115
125
8
9
CH3
68
128
120
-O-CH3
81
79
10
-O-CH2CH3
119
(Continued )

3,4-DIHYDROPYRIDIN SYNTHESIS
3015
Table 2. Continued
1,3 Dicarbonyls
R1
Entry
11
R2
Product
Yield (%)a Melting point (ꢀC)
-O-CH2CH3
-O-CH2CH3
-O-CH2CH3
72
115–117
12
13
70
90
b
—
aIsolated yield.
bInseparable mixture.
intractable mixture. It is evident from the table that by increasing the chain
length at the ester (entries 4–8) and carbonyl groups (entries 9–12), the yield tends
to be decreased. Compounds 4a, 4d, and 4e were reported earlier. Compound 4d
was synthesized by a two-step procedure involving condensation of 3-aza-4-
(dimethylamino)-2-(methylthio)-l,3-pentadiene with methylacrylate followed by
hydrolysis of the cyclic adduct,[16] whereas compound 4e was synthesized (76%)
by the reaction of ethyl a-acetoglutarate with ammonium acetate and glacial acetic
acid in benzene.[17]
Scheme 2. Mechanism of enaminone intermediate in the formation of 3,4-DHP-2-one.

3016
J. B. S. KUMAR, N. KUMARI, AND P. M. LUTHRA
Figure 2. (A) Proton NMR; (B) Carbon-13 NMR. (Figure is provided in color online.)
CONCLUSION
In summary, we have developed a facile solvent-, catalyst-, and aldehyde-free,
one-pot synthesis of 3,4-DHP-2-one from 1,3-diones using simple and mild reaction
conditions. The substrate scope has been also extended to b-ketoesters. The com-
pounds 4b, 4f, 4g, 4h, 4i, 4j, 4k, and 4l are novel and could be used to synthesize
novel therapeutic scaffolds. The products were formed with a yield range of 68%
to 92%. The substrate scope of other Michael acceptors such as methacrylate, methyl
acrylate, and methacrylic acid is under investigation. The methodology could be

3,4-DIHYDROPYRIDIN SYNTHESIS
3017
automated and applied to form the combinatorial library for the of synthesis of a
wide array of biologically important dihydropyrolidine-2-ones.
EXPERIMENTAL
1H=13C NMR (400=75 MHz; CDCl3) spectra were recorded using commer-
cially available deuterated solvents on a multinuclear spectrometer Bruker
400-MHz instrument using TopSpin software and Jeol 400-MHz instrument using
Delta software. The NMR is reported as follows: chemical shifts (multiplicity [singlet
(s), doublet (d), triplet (t), quartet (q), broad (br), and multiplet (m)], coupling con-
stant [Hz], integration). High-resolution mass spectra (HRMS) were recorded on
Bruker Daltonics MicroTof Focus spectrometer using Na-formate solution as cali-
brant. Infrared(IR) spectra were recorded on a Jasco FTIR 6300 spectrophotometer
and only significant peaks are presented. Thin-layer chromatography (TLC) was
performed on Merck (60 F254, 0.2 mm) using an appropriate solvent system. The
chromatograms were visualized under ultraviolet light. Separation of various pro-
ducts was carried out by column chromatography on silica gel (100–200 mesh).
All solvents and liquid reagents were dried with appropriate reagents before use.
Commercially available 1,3-dicarbonyls, acryllic acid, and ammonium acetate were
used without any further purification. All the reactions were performed in
Teflon-capped 4-ml glass vials under different temperatures with stirring.
General Procedure for Synthesis of
3,4,7,8-Tetrahydroquinoline-2,5(1H,6H)-dione (4a)
1,3-Dicarbonyls (8.9 mmol,
1 equiv.) followed by ammonium acetate
(10.6 mmol, 1.2 mmol, 1.2 equiv.) were added to the solution of acrylic acid
(10.6 mmol, 1.2 equiv.). The reactant mass was mixed thoroughly with a spatula
and heated at 80 ꢀC with stirring under a nitrogen atmosphere for 5 h. After cooling,
the reaction mixture was diluted with 10 mL distilled water and extracted twice with
ethyl acetate (2 ꢁ 10 mL). The organic extracts were dried over anhydrous Na2SO4.
After filtration, the solvent was evaporated under reduced pressure and subjected
to column chromatography (ethyl acetate=hexane) to obtain pure products. The
characterization data of the compounds are given.
3,4,7,8-Tetrahydroquinoline-2,5(1H,6H)-dione (4a)
1
Isolated yield 92%; white solid. H NMR (400 MHz, CDCl3): d 2.0 (m, 2H),
2.4–2.6 (m, 8H), 9.0 (brs 1H); 13C NMR (75 MHz, CDCl3): d 17.2, 21.3, 27.2,
30.2, 36.6, 112.5, 152.4, 173.0, 196.3; HRMS (ESI, Na-formate calibrant) calcd.
for C9H11NO2 [MH]þ: 166.0868; found [MH]þ 166.0864.
(Z)-3-Imino-1-phenylbut-1-en-1-ol (4c Intermediate)
Isolated yield 55%; solid; 1H NMR (400 MHz, CDCl3): d 2.1 (s, 3H), 5.25 (brs,
1H), 5.75 (s, 1H), 7.4 (m, 3H), 7.8 (m, 2H), 10.2 (br s, 1H); 13C NMR (75 MHz,
CDCl3): d 22.90, 92.32, 127.1, 128.2, 130.8, 140.2, 162.9, 189.5.

3018
J. B. S. KUMAR, N. KUMARI, AND P. M. LUTHRA
SUPPORTING INFORMATION
Full experimental detail, 1H and 13C NMR spectra of all synthesized
compounds can be found via the Supplementary Content section of this article’s
Web page.
ACKNOWLEDGMENTS
J. B. Senthil Kumar acknowledges the Department of Biotechnology (DBT),
New Delhi, and the Council for Scientific and Industrial Research (CSIR), New
Delhi, for providing financial support, and Neeta Kumari is thankful to Pratibha
Mehta Luthra, Neuropharmaceutical Laboratory, B. R. Ambedkar Centre for Bio-
medical Research, University of Delhi, for providing supervision and laboratory
facilities to carry out this work.
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