Acid-Controlled Chemodivergent Synthesis of Three Differently Substituted Quinolines via Site Selective Coupling of ortho- Aminoaryl Ketones with α-Enolic Dithioesters

Article abstract of DOI:10.1002/adsc.201500962

A straightforward approach for the chemodivergent synthesis of quinolines is described through site-selective coupling of ortho-aminoaryl ketones with α-enolic dithioesters (DTEs) under solvent-free conditions. The operationally and user-simple one-pot me

Full text of DOI:10.1002/adsc.201500962

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DOI: 10.1002/adsc.201500962
Acid-Controlled Chemodivergent Synthesis of Three Differently
Substituted Quinolines via Site Selective Coupling of ortho-
Aminoaryl Ketones with a-Enolic Dithioesters
Suvajit Koley,a Tanmoy Chanda,a B. Janaki Ramulu,a Sushobhan Chowdhury,a
and Maya Shankar Singha,*
a
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi – 221005, India
Fax: (+91)-542-268-127; e-mail: mayashankarbhu@gmail.com (homepage: www.drmssinghchembhu.com)
Received: October 15, 2015; Revised: February 10, 2016; Published online: March 31, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500962.
Abstract: A straightforward approach for the che-
modivergent synthesis of quinolines is described
through site-selective coupling of ortho-aminoaryl
ketones with a-enolic dithioesters (DTEs) under
solvent-free conditions. The operationally and user-
simple one-pot methodology is based on the trifunc-
tional nature of DTEs. Both the carbonyl and the
thiocarbonyl moiety in a-enolic dithioesters were
employed for the efficient construction of three dif-
ferently substituted quinolines in a chemoselective
manner simply by variation of an easy to handle
acid catalyst.
more economical and eco-efficient strategies for their
synthesis. In the course of our research programme
on the development of simple and efficient methods
to access quinolines through a modified Friedländer
approach, we explored the synthesis of functionalized/
annulated quinolines by the reaction of ortho-amino-
aryl ketones with alkynes/active methylenes/a-oxoke-
tene dithioacetals under varying conditions.[12a] Re-
cently, we have developed an efficient one-pot
method for the synthesis of diverse quinolines directly
from ortho-haloacylbenzenes using an eco-efficient
CuSO4-d-glucose catalyst system.[12b]
During the past ten years, cascade reactions have
attracted great attention for their applications in the
construction of simple and complex molecules. Our
group has focused on the research of cascade reac-
tions based on a-enolic dithioesters to construct di-
verse useful heterocycles.[13] Recently, a-enolic di-
thioesters have been widely utilized as valuable inter-
mediates and building blocks because of their multi-
Keywords: acid catalyst; diverse quinolones; a-
enolic dithioesters; solvent-free conditions; trifunc-
tionalization
Quinoline derivatives are of particular interest due to ple reactive sites and versatile reactivities.[14] As part
their presence in a wide range of natural products[1] of our research in this field, this time we envisioned
with a broad spectrum of biological activities.[2] In ad- to utilize the a-enolic dithioester as an alternative to
dition, quinoline frameworks have also been exten- the active methylene component in the Friedländer
sively exploited as potential building blocks in organic reaction. In this context, we herein disclose a straight-
synthesis.[3] Furthermore, they are valuable synthons forward and practical one-pot, two-component Fried-
for the preparation of materials with unique electron- länder approach for the synthesis of three differently
ic and photo-physical properties.[4] Therefore, they substituted quinolines employing a-enolic dithioesters
have drawn significant attention of synthetic, medici- as one component under minor conditional modifica-
nal and industrial chemists. Besides the prevailing tions (Scheme 1).
conventional methods such as Friedländer,[5] Skraup–
To optimize the reaction conditions for the synthe-
Dçebner–von Miller,[6] Conrad–Limpach–Knorr[7] and sis of 2,3,4-trisubstituted quinolines, 5-chloro-2-amino-
Combes[8] syntheses, several other synthetic strategies benzophenone (1a) and methyl 3-hydroxy-3-phenyl-
for quinoline derivatives have been developed.[9,10] prop-2-enedithioate (2a) were used as model sub-
However, a large portion of the available literature is strates. Substrates 1a and 2a were subjected to an
based on modifications of the Friedländer approach[11] array of reaction conditions, and the results are listed
due to its efficiency and operational simplicity. The in Table 1. Initially, the mixture of 1a and 2a (1 mmol
highly privileged status of quinolines still demands each) in the presence of 10 mol% of InCl3 was re-
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Scheme 1. Synthesis of differently substituted quinolines.
fluxed in acetonitrile. Unfortunately, no reaction took entry 20) with a trace of 3a. To improve the yield of
place even after 48 h of reflux and the starting materi- 5a, the loading of SiO2-H2SO4 was optimized, and it
als remained completely unconsumed (Table 1, was found that 30 mol% of SiO2-H2SO4 furnished the
entry 1). Next, different aprotic solvents such as tolu- best result (Table 1, entries 21–23). Next, the effec-
ene, THF and DMF and protic solvents like EtOH tiveness of SiO2-InCl3 was checked but it produced all
and MeOH were investigated, and were found to be three products without any specific selectivity
ineffective (Table 1, entries 2–6). Keeping in mind the (Table 1, entry 26). The use of H2SO4 as catalyst gave
benefits of solvent-free conditions, the above model only a small amount 2,8-dichloro-6,12-diphenyldiben-
reaction was performed under solvent-free conditions zo[b,f][1,5]diazocine[12a] and most of the starting mate-
at 808C. Notably, the reaction was completed within rial remain unconsumed even after 24 h of reaction.
2 h, and work-up of the reaction afforded a mixture Screening of P2O5 was less effective in the current
of two different quinolines 3a and 4a in 15% and reaction providing 5a in 55% yield along with the
70% yield, respectively (Table 1, entry 7).
2,8-dichloro-6,12-diphenyldibenzo[b,f][1,5]diazocine
The success of this protocol prompted us to opti- formed by dimerization of 5-chloro-2-aminobenzo-
mize the temperature of the reaction. Decreasing or phenone 1a (Table 1, entry 26). Without any catalyst
increasing the temperature could not improve the no reaction occurred (Table 1, entry 27).
result (Table 1, entries 8 and 9). The yield of quino-
Systematic optimization of the reaction parameters
lines 3a and 4a was not further improved by increas- allowed us to identify two-component coupling condi-
ing or decreasing the amount of InCl3 (Table 1, en- tions that were tolerant of a wide range of functional
tries 10 and 11). Thus, it was observed that the use of groups, thereby providing three different sets of
10 mol% of InCl3 provided the best result for the for- densely functionalized quinolines 3, 4 and 5 that
mation of 4a (Table 1, entry 7). Further screening of could not easily be prepared by other methods.
InBr3, FeCl3, Sc(OTf)3, Yb(OTf)3, SnCl4, Cu(OTf)2, Under the optimized reaction conditions, the scope of
BF3·Et2O, etc. separately could not furnish a better various ortho-aminoaryl ketones 1 and a-enolic di-
result (Table 1, entries 12–18). In an attempt to in- thioesters 2 was investigated, and the results are sum-
crease the efficiency of the reaction, the model reac- marized in Table 2 and Table 3. Various a-enolic di-
tion was carried out in the presence of 10 mol% of thioesters 2 with different substitution patterns at R1
SiO2 under solvent-free conditions at 808C. Interest- such as aromatic (containing both electron-donating
ingly, TLC monitoring of the reaction after 16 h and electron-withdrawing substituents) and heteroar-
showed an entirely new spot, which was isolated and omatic (such as 2-furyl, 2-thienyl) groups were toler-
characterized as 5a in 57% yield with a trace amount ated well and afforded the desired products in moder-
of 3a (Table 1, entry 19). No trace of the quinoline 4a ate to good yields. Electronic and steric effects of the
was observed.
substituents at R1 did not substantially influence the
Inspired by this result, we used 10 mol% of SiO2- outcome of the reaction. a-Enolic dithioesters with R1
H2SO4 under solvent-free conditions at 808C. The as aliphatic substituents are low boiling liquids, and
yield of quinoline 5a was increased to 70% (Table 1, are not stable at higher temperature (808C). Al-
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Table 1. Optimization studies for the synthesis of quinolines 3a, 4a, and 5a.[a]
Entry
Catalyst (mol%)
Solvent
Temperature [8C]
Time [h]
Yield [%][b]
3a
4a
5a
1
2
3
4
5
6
7
8
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (10)
InCl3 (20)
InCl3 (5)
InBr3 (10)
FeCl3 (10)
Sc(OTf)3 (10)
Yb(OTf)3 (10)
SnCl4 (10)
BF3.Et2O (10)
Cu(OTf)2 (10)
SiO2 (10)
SSA (10)
CH3CN
toluene
THF
DMF
EtOH
MeOH
reflux
reflux
reflux
80
reflux
reflux
80
60
100
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
48
48
48
48
48
48
2
24
2
2
15
2.5
6
4
4.5
7
24
4
16
5
3.5
3
3
15
24
5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
trace
trace
–
-
–
15
5
12
15
trace
trace
–
-
–
70
40
65
68
54
62
30
45
41
36
–
45
–
–
–
–
–
45
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
10
10
trace
trace
trace
trace
–
trace
trace
trace
trace
8
[c]
–
–
[d]
57
70
81
90
88
30
SSA[e] (20)
SSA (30)
SSA (50)
SiO2/InCl3 (10)
H2SO4
P2O5 (30)
–
10
8
–
–
[c]
–
55[c]
[f]
80
24
–
–
[a]
All reactions were carried out using 1.0 mmol each of 1 and 2.
Isolated yield.
[b]
[c]
A small amount of 2,8-dichloro-6,12-diphenyldibenzo[b,f][1,5]diazocine (dimerized product of the corresponding 5-
chloro-2-aminobenzophenone 1a) was observed along with 5a.[12a]
The compound 2-(4-benzoyl-5-(methylthio)-3H-1,2-dithiol-3-ylidene)-1-phenylethanone was observed.[13b]
SSA=SiO2-H2SO4.
[d]
[e]
[f]
Starting materials remain unconsumed.
though we tried to involve them in the reaction under
The structural characterization of the prepared
13C NMR, IR and mass) studies and unequivocally es-
the optimized conditions, the reaction remained un- compounds was accomplished by spectroscopic (1H,
successful.
When R2 was ethyl, propyl or n-pentyl, the prod- tablished by the X-ray single crystal diffraction analy-
ucts 3 were obtained in traces and could not be isolat- sis of one representative compound of each series,
ed. In case of products 4, the SR2 group was eliminat- i.e., 3a, 4a and 5e (see Scheme 2 and the Supporting
ed during the course of the reaction. Therefore, the Information).[15]
a-enolic dithioesters bearing R2 as ethyl, propyl or n-
Based on the entire experimental outcomes and lit-
pentyl afforded 4 in almost comparable yields to that erature reports, a possible mechanistic proposal for
of a-enolic dithioester containing R2 as methyl. Fur- the formation of three differently substituted quino-
thermore, the R2 substituents at the DTE 2 as methyl, lines 3, 4 and 5 is presented in Scheme 3. Initially, the
ethyl, propyl and n-pentyl were also tolerated well nucleophilic attack of the enolic carbon of a-enolic
and the reactions proceeded smoothly affording the dithioester to the carbonyl carbon of ortho-aminoaryl
quinolines 5 in excellent yields (Table 3).
ketones forms an open-chain intermediate A. The in-
termediate A undergoes dehydration to form D,
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Table 2. Scope of substrates 1 and 2 for the synthesis of qui-
nolines 3 and 4.[a]
Table 3. Scope of substrates 1 and 2 for the synthesis of qui-
nolines 5.[a]
[a]
All reactions were carried out using 1.0 mmol each of
1 and 2.
Isolated yield.
[b]
Scheme 2. ORTEP diagrams of 3a, 4a and 5e.
which immediately undergoes intramolecular N-cycli-
zation to give cyclic intermediate E. Next, intermedi-
ate E aromatizes via two ways either by the elimina-
tion of SH or SR2 group. Removal of the SH group
[a]
All the reactions were carried out using 1.0 mmol each of
1 and 2.
Isolated yield.
[b]
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Scheme 3. Plausible mechanism for the formation of quinolines 3, 4 and 5.
promotes the formation of 3 while the extrusion of thioesters. The salient features of this domino proto-
the SR2 group yielded 4. Intermediate A exists in its col are its methodical simplicity, structural diversity,
rotameric form B. The intermediate B in the presence carbon-economy, viable substrates, absence of toxic
of SiO2-H2SO4 undergoes dehydration to form C, reagents/solvents, environmental friendliness, and the
À
À
which is followed by intramolecular N-cyclization to formation of two new bonds (one C C and one C N)
form the desired quinoline 5. and one ring in a single operation. The scope and di-
The reason behind the regioselective formation of 5 versity of the tolerated substrates in this work is
may be attributed to the greater electrophilicity of rather broad in comparison to the reported ones.
the carbonyl carbon compared to the thiocarbonyl Thus, this solvent-free methodology should prove
carbon because of the preferable protonation of the useful for the facile synthesis of such scaffolds on an
carbonyl oxygen[16] in SiO2-H2SO4 medium. In the industrial scale. The presence of diverse functional
case of InCl3, the d-orbital interaction (between metal groups at quinoline rings makes these compounds ex-
and sulfur)[17] favors attack of the lone-pair of nitro- cellent precursors for further synthetic renovations.
gen to the thiocarbonyl carbon leading to the forma-
tion of 3 and 4. The preferential formation of 4 may
be due to its intermolecular hydrogen bonding stabili-
zation (see the Supporting Information).
Experimental Section
To support the experimental observations and the
mechanistic course, we performed DFT studies using
B3LYP (see the Supporting Information). From these
DFT studies, it was found that both the intermediates
E and F have comparable energies, however, the opti-
General Procedure for the Synthesis of 2-Alkylthio-
quinolines (3a–j) and 3-Aroyl-2-mercaptoquinolines
(4a–j)
To a mixture of ortho-aminoaryl ketone 1 (1.0 mmol) and a-
enolic dithioester 2 (1.0 mmol), InCl3 (0.1 mmol) was added
and the reaction mixture was heated at 808C until comple-
mized geometry of intermediate E is quite different
2
À
from its expected geometry. In E the C SR bond
length (1.90 Š) is quite a bit longer than the normal tion of the reaction (Table 2). After completion of the reac-
2
À
tion (monitored by TLC), water (20 mL) was added to the
reaction mixture, followed by extraction with ethyl acetate
(2”10 mL). The combined organic layer was dried over an-
hydrous Na2SO4 and then evaporated under vacuum. The
crude residue was purified by column chromatography over
silica gel using ethyl acetate/hexane as eluent to afford pure
3-aroyl-2-mercaptoquinolines 4 as the major product along
with 2-alkylthioquinolines 3 as the side product. All the syn-
thesized quinolines 3 were previously reported by our group
and the recorded spectral data match with the reported val-
bond length (1.78 Š). The elongation of the C SR
bond length favored SR2 elimination to form the
major product 4 supporting our proposed mechanism.
Energy calculations of products 3 and 4 also support-
ed the product 4 as being more stable.
In summary, we have successfully developed
a simple and efficient one-pot practical solvent-free
reaction methodology for the synthesis of diverse
highly functionalized quinolines directly by the reac-
tion of ortho-aminoaryl ketones with a-enolic di- ues.[12a]
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Angew. Chem. 2012, 124, 2744; Angew. Chem. Int. Ed.
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General Procedure for the Synthesis of the 3-Thio-
carbothioalkoxy-2,4-diarylquinolines (5a–k)
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W. Qiu, T. Liu, F. Yang, X. Liu, J. Tang, Tetrahedron
2011, 67, 7090; d) R. Martínez, D. J. Ramón, M. Yus,
Eur. J. Org. Chem. 2007, 1599; e) S. Atechian, N. Nock,
R. D. Norcross, H. Ratni, A. W. Thomas, J. Verron, R.
Masciadri, Tetrahedron 2007, 63, 2811; f) J. Wu, H.-G.
Xia, K. Gao, Org. Biomol. Chem. 2006, 4, 126; g) J.
Wu, L. Zhang, T.-N. Diao, Synlett 2005, 2653; h) S. S.
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Miller-type reactions; see: a) N. Sakai, D. Aoki, T. Ha-
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reactions; see: a) A. C. W. Curran, J. Chem. Soc. Perkin
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To a mixture of ortho-aminoaryl ketone 1 (1.0 mmol) and a-
enolic dithioester 2 (1.0 mmol), SiO2-H2SO4 (0.3 mmol) was
added and the reaction mixture was heated at 808C until
completion of the reaction (Table 3). After completion of
the reaction (monitored by TLC), water (20 mL) was added
to the reaction mixture followed by extraction with ethyl
acetate (2”10 mL). The combined organic layer was dried
over anhydrous Na2SO4 and then evaporated under vacuum.
The crude residue was purified by column chromatography
over silica gel using ethyl acetate/hexane as eluent to afford
pure desired quinolines 5. The catalyst SiO2-H2SO4 was pre-
pared following the literature procedure.[18b]
Acknowledgements
We gratefully acknowledge the generous financial support
from the Science and Engineering Research Board (Grant
No. SB/S1/OC-30/2013), and the Council of Scientific and In-
dustrial Research, New Delhi, India (Grant No. 02(0072)/12/
EMR-II).
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Adv. Synth. Catal. 2016, 358, 1195 – 1201

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[15] The crystallographic coordinates have been deposited
with the Cambridge Crystallographic Data Centre;
deposition nos. CCDC 1051090 (3a), CCDC 1051091
Adv. Synth. Catal. 2016, 358, 1195 – 1201
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1201