Transition-Metal-Free Quinoline Synthesis from Acetophenones and Anthranils via Sequential One-Carbon Homologation/Conjugate Addition/Annulation Cascade

Article abstract of DOI:10.1021/acs.orglett.7b02429

A transition-metal-free method for the construction of functionalized quinolines from readily available acetophenones and anthranils is reported. This one-pot reaction cascade involves in situ generation of α,β-unsaturated ketones from the acetophenone via one-carbon homologation by DMSO followed by the aza-Michael addition of anthranils and subsequent annulation. DMSO acted in this reaction not only as solvent but also as one carbon source, thus providing a highly atom-economical and environmentally benign approach for the synthesis of 3-substituted quinolines.

Full text of DOI:10.1021/acs.orglett.7b02429

Letter
pubs.acs.org/OrgLett
Transition-Metal-Free Quinoline Synthesis from Acetophenones and
Anthranils via Sequential One-Carbon Homologation/Conjugate
Addition/Annulation Cascade
Sandip Balasaheb Wakade,†,‡ Dipak Kumar Tiwari,†,‡ Pothapragada S. K. Prabhakar Ganesh,†
Mandalaparthi Phanindrudu,†,‡ Pravin R. Likhar,‡ and Dharmendra Kumar Tiwari*,†,‡
†Medicinal Chemistry and Biotechnology Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500007, India
‡Academy of Scientific & Innovative Research (AcSIR), New Delhi 110001, India
S
* Supporting Information
ABSTRACT: A transition-metal-free method for the con-
struction of functionalized quinolines from readily available
acetophenones and anthranils is reported. This one-pot reaction
cascade involves in situ generation of α,β-unsaturated ketones
from the acetophenone via one-carbon homologation by
DMSO followed by the aza-Michael addition of anthranils and
subsequent annulation. DMSO acted in this reaction not only as
solvent but also as one carbon source, thus providing a highly
atom-economical and environmentally benign approach for the
synthesis of 3-substituted quinolines.
he 3-substituted quinolines are ubiquitous in various
pharmacologically and medicinally relevant molecules
for the synthesis of α,β-unsaturated ketones.8 To eliminate the
need for the troublesome isolation of α,β-unsaturated ketones
they are in situ subjected to various organic transformations
such as arylation, amination, and conjugate addition to get β-
functionalized ketones.13 Very recently, we developed a copper-
catalyzed α,β functionalization of saturated ketones with
anthranils via sequential dehydrogenation/aza-Michael addi-
tion/annulations cascade reactions in one pot (Scheme 1).13f
Although this method allows a convenient access to 3-
ketoquinolines, the requirements of a transition metal and the
commercial unavailability of phenylethyl ketones are the key
limitations.
T
displaying a broad range of biological activities (Figure 1).1,2
Figure 1. Biologically active molecules containing 3-substituted
quinolines.
In the recent past, both the acetophenone and anthranils
have been recognized as good substrates for several organic
In addition, they have been exploited as synthetic intermediates
in the preparation of drugs and functional materials.3 Over the
past few years, numerous quinoline syntheses have been
developed, but most of the existing methods suffer from the
requirement of highly functionalized starting materials and
expensive transition metals.4−6 Therefore, development of new
synthetic methods for the quinoline synthesis in green, efficient,
and metal-free fashion is highly desirable.
Scheme 1. Synthesis of 3-Ketoquinolines
α,β-Unsaturated ketones are frequently encountered in
various bioactive compounds and generally regarded as versatile
synthetic intermediates in the syntheses of fine chemicals,
pharmaceuticals, and materials.7,8 Traditionally, the synthesis of
α,β-unsaturated ketones required multiple steps9,10 and
equimolar amounts of reagents such as 2-iodoxybenzoic acid
(IBX) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ).11,12 Over the past few years, transition-metal-catalyzed
(Pd, Ru, Ir, Ni, and Cu) oxidation of saturated ketones has
proven to be a concise, efficient, and atom-economical method
Received: August 7, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02429
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Organic Letters
Letter
transformations due to their distinct electronic properties.14 In
this context and in the frame of our current research interest,15
herein we report the transition-metal-free synthesis of 3-
ketoquinolines from readily available acetophenones and
anthranils by using DMSO as one carbon source. In this
newly developed protocol, the α,β-unsaturated ketones are
formed in situ through C(sp3)−H bonds coupling among two
methyl groups of ketones and dimethyl sulfoxide.
of K2S2O8 (1.5 equiv) furnished a low yield of 3aa (entry 14).
Using different solvents such as DMF, DMA, and NMP also
furnished the desired product 3aa, but in low yield (see the
Supporting Information for details).
With the optimal reaction conditions in hand (Table-1, entry
9), the scope of the reaction with respect to various aryl methyl
ketones and anthranils was evaluated (Scheme 2). A wide range
a
Our investigation started with the reaction of readily available
acetophenone (1a), anthranil (2a), and DMSO as model
substrates, and the details are summarized in Table 1. Our
Scheme 2. Substrate Scope
a
Table 1. Optimization of Reaction Conditions
b
yield (%)
entry
additive
K2S2O8
base
temp (°C)
3aa
4aa
1
DABCO
DABCO
DABCO
DBU
rt
60
n.o.
n.o.
26
n.o
n.o.
45
2
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
(NH4)2S2O8
KHSO4
TBHP
3
120
120
120
120
120
120
120
120
120
120
120
120
4
21
26
5
Et3N
11
15
6
K2CO3
Cs2CO3
NaOAc
54
trace
trace
trace
n.o.
n.o
7
51
8
43
9
76
10
11
12
13
14
37
n.o
n.o.
n.o.
52
n.o.
n.o.
n.o.
TEMPO
K2S2O8
c
n.o.
a
Reaction was performed using 1a (1.0 mmol), 2a (1.1 mmol),
DMSO (2.0 mL), and additive (2.5 mmol) under nitrogen 24 h.
b
c
Isolated yield. When (1.5 equiv) of K2S2O8 was used.
initial effort of reacting acetophenone (1a, 1.0 mmol), anthranil
(2a, 1.2 mmol), and K2S2O8 (2.5 mmol) in the presence of
DABCO 1.0 mmol) at room temperature was found to be
unsuccessful and did not furnish any desired product 3aa (entry
1). Unfortunately, a similar result was obtained even after the
reaction mixture was heated at 60 °C for 24 h (entry 2).
Gratifyingly, when the reaction mixture was heated at 120 °C
for 24 h, a mixture of the desired 3aa in 26% yield along with
undesired 4aa in 45% yield was obtained (entry 3). At this
stage, the structures of 3aa and 4aa were well characterized
using different spectroscopic techniques, and the spectral data
of 3aa exactly matched that of previously reported compounds.
In order to reduce the formation of 2-(methylthiomethyl)-1-
phenylprop-2-en-1-one (4aa), different organic bases, such as
DBU and Et3N, were screened, but they also furnished
undesired 4aa as a major product (entries 4 and 5). Compared
to the organic bases, the use of inorganic bases such as K2CO3,
Cs2CO3, and NaOAc showed better selectivity and produced
3aa as a major product in good yields (entries 6−8).
Unexpectedly, 3aa was obtained as a single product in 76%
yield when the reaction was run without any base under similar
reaction conditions (entry 9). Various oxidants such as
(NH4)2S2O8, KHSO4, TBHP, and TEMPO were also screened,
but most of them were found to be less efficient and produced
3aa in low yields (entries 10−13). The decrease in the amount
a
Reaction conditions: aryl methyl ketones (1a−v; 1.0 mmol),
anthranils (2a−e; 1.1 mmol), DMSO (2.0 mL), and K2S2O8 (2.5
mmol) under N2 at 120 °C for 24 h.
of aryl methyl ketones and anthranils could be used in this
tandem reaction. The aryl methyl ketones containing both
electron-withdrawing (EWGs) and electron-donating groups
(EDGs) at different positions of the aryl ring readily delivered
the corresponding 3-ketoquinolines in good to moderate yields
(3ba−ao). A variety of functional groups such as alkyl (1b, 1c),
tert-butyl (1d), chloro (1e), bromo (1f), iodo (1h), fluoro
(1j,k), trifluoromethyl (1k), cyano (1l), nitro (1m), biphenyl
(1n), and alkoxy (1o,p) were well tolerated in this reaction. To
our delight, a phenyl ring containing a free hydroxyl group (1v)
was also tolerated in this reaction and furnished the
corresponding 3-ketoquinoline 3va in 52% yield. This one-
pot tandem cascade reaction was further extended to a variety
of heteroaryl methyl ketones including 2-acetylthiophene (1q),
2-acetylfuran (1r), and 2-acetylthiazole (1s), and good yields of
B
DOI: 10.1021/acs.orglett.7b02429
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Letter
the corresponding 3-ketoquinolines (3q−s) were obtained.
Moreover, bicyclic and tricyclic aryl methyl ketones such as 6-
acetyltetralin (1t) and 9-acetylanthracene (1u) smoothly
underwent to this reaction and furnished the corresponding
3ta and 3ua in 74% and 72% yields, respectively. Furthermore,
various substituted anthranils (2b−e) were treated with
acetophenone (1a) under optimized reaction conditions to
provide the desired products (3ba−ea) in moderate yields
(64−66%).
light on reaction mechanism, we carried out deuterium-labeling
experiments by treating 1a with 2a in DMSO-d6 under standard
1
reaction conditions which produced 3aa-d2 in 69% yield. H
NMR analysis revealed 100% deuteration at the C-2 position,
and surprisingly, we also observed 43% of deuteration at the C-
4 position.16b
On the basis of the preliminary mechanistic experiments and
previous literature, a possible mechanism is proposed (Scheme
5). DMSO (3a) is activated by K2S2O8 to furnish intermediate
To assess the efficiency and potential for application of this
method, we have developed a one-pot sequential synthesis of
2,3-dihydro-5-(3-quinolinyl)-1H-1,4-benzodiazepine (4),13f and
nickel-catalyzed decarbonylation of 3aa to prepare 3-phenyl-
quinoline 516a (Scheme 3) (for details, see the SI).
Scheme 5. Plausible Reaction Mechanism
Scheme 3. Synthetic Utility
To understand the reaction mechanism, we carried out a
series of control experiments (Scheme 4). Initially, when the
3a′, which on reaction with enolate (1a′) generated from 1a
furnished intermediate C. The intermediate C undergoes
K2S2O8-assisted demethylthioation to form α,β-unsaturated
ketone (D).14a,17 The aza-Michael addition by anthranil on
intermediate D would give intermediate E, which eventually
cyclizes to form F, which upon auto oxidation leads to the
formation of 3-ketoquinoline 3aa.
Scheme 4. Mechanistic Insights
In conclusion, an efficient three-component reaction cascade
has been developed for the synthesis of 3-ketoquinolines from
the readily available acetophenones, anthranils, and DMSO in
the presence of K2S2O8. The present report provides a novel
method for the one-carbon homologation of acetophenones to
produce α,β-unsaturated ketones that was further utilized in
situ for the aza-Michael addition and annulation reactions with
anthranils. The reaction is robust enough, and a number of 3-
ketoquinolines were synthesized, reflecting the generality of the
method.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02429.
Complete experimental details and characterization data
for all products (PDF)
reaction was performed in the presence of radical scavenger
TEMPO, the desired product 3aa was obtained in 59% yield,
which indicates that the radical species is not involved in this
process. To verify the carbon source in this reaction, we carried
out reaction in the methyl phenyl sulfoxide solvent that led to
formation of 3aa in 41% yield. However, when the reaction was
performed in diphenyl sulfoxide solvent expectedly 3aa was not
formed. Furthermore, when the reaction was performed in the
absence of anthranil (2a), under the standard condition for 12
h, two products, C and D, were isolated in 46% and 20% yields,
respectively. By further heating the reaction for 24 h, only
product D was isolated in 72% yield, indicating that C and D
would be the key intermediates in this reaction. Next, treating
the intermediate C with 2a under the standard reaction
conditions gave the desired 3aa in 80% yield. To shed more
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: dkt80.org@gmail.com, dktiwari.iict@gov.in.
ORCID
Dharmendra Kumar Tiwari: 0000-0002-0978-4177
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Department of Science and Technology (DST),
New Delhi, India, for financial support in the form of a DST-
INSPIRE Faculty grant and a Startup Research Grant for young
researchers (GAP 0414 and GAP 0512). S.B.W. thanks the
C
DOI: 10.1021/acs.orglett.7b02429
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