Convenient and efficient method for the synthesis of substituted quinolines via one-pot heteroannulation reaction of o-amino arylketones with α-methylene ketones under solvent-free conditions

Article abstract of DOI:10.1080/00397911.2016.1242746

A facile and practical approach to the synthesis of a wide range of functionalized quinolines was developed via a tandem heteroannulation reaction of o-aminoarylketones with diverse α-methylene ketones in high yields by using tetrabutylammonium peroxydisu

Full text of DOI:10.1080/00397911.2016.1242746

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
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Convenient and efficient method for the
synthesis of substituted quinolines via one-pot
heteroannulation reaction of o-amino arylketones
with α-methylene ketones under solvent-free
conditions
G. Vanajatha & V. Prabhakar Reddy
To cite this article: G. Vanajatha & V. Prabhakar Reddy (2016) Convenient and efficient
method for the synthesis of substituted quinolines via one-pot heteroannulation reaction
of o-amino arylketones with α-methylene ketones under solvent-free conditions, Synthetic
Communications, 46:23, 1953-1961, DOI: 10.1080/00397911.2016.1242746
To link to this article: http://dx.doi.org/10.1080/00397911.2016.1242746
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Date: 17 November 2016, At: 02:44

SYNTHETIC COMMUNICATIONS®
2016, VOL. 46, NO. 23, 1953–1961
http://dx.doi.org/10.1080/00397911.2016.1242746
Convenient and efficient method for the synthesis of
substituted quinolines via one-pot heteroannulation reaction
of o-amino arylketones with α-methylene ketones under
solvent-free conditions
G. Vanajatha and V. Prabhakar Reddy
Department of Chemistry, Osmania University, Tarnaka, Hyderabad, India
ABSTRACT
ARTICLE HISTORY
Received 20 June 2016
A facile and practical approach to the synthesis of a wide range of
functionalized quinolines was developed via a tandem heteroannula-
tion reaction of o-aminoarylketones with diverse α-methylene ketones
in high yields by using tetrabutylammonium peroxydisulfate as a
versatile reagent (25 mol%) at ambient temperature under solvent-free
conditions. This protocol was applied to the synthesis of drug-like
molecules, which are key intermediates of alkaloid camptothecin.
KEYWORDS
Friedländer annulation;
α-Methylene ketones;
o-aminoaryl ketones;
solvent-free conditions;
substituted quinolines;
tetrabutylammonium
peroxydisulfate
GRAPHICAL ABSTRACT
Introduction
In recent years, the importance of heterocycles in biological systems has stimulated interest
in designing and developing novel drug scaffolds via iterative manipulation of functional
groups around the basic skeletal systems. In particular, heterocyclic compounds embedded
with nitrogen are ubiquitous in nature and present in natural products and pharmaceutical
leads. Among the nitrogen heterocycles, the quinoline ring system in particular constitutes a
privileged structural motif widely found in many pharmacologically significant compounds
and in some bioactive natural products. Members of this family display remarkable biologi-
cal activities such as antibacterial, antimalarial, antihypersensitive, and antituberculosis
activity[1,2] and act as HIV-1 integrase inhibitors,[3] cytotoxic agents like benzo[5,6]
pyrrolizino[1,2-b]quinolines,[4] natural topoisomerase I inhibitors camptothecin[5] and
luotonin,[6] and cholinesterase inhibitor tacrine (Fig. 1).[7] In addition, quinoline derivatives
have also found utility in polymer chemistry, optoelectronics, and electronics because of
their enhanced electrical, mechanical, and optical properties.[8]
CONTACT V. Prabhakar Reddy
reddyvanajatha@gmail.com
Department of Chemistry, Osmania University, Tarnaka,
Hyderabad, 500007, T.S., India.
Supplemental data (full experimental details and characteristics for all new compounds and copies of 1H NMR, 13C NMR,
and mass spectral data) can be accessed on the publisher’s website.
© 2016 Taylor & Francis

1954
G. VANAJATHA AND V. PRABHAKAR REDDY
Figure 1. Structures of some bioactive compounds containing quinoline moiety.
Results and discussion
In view of the remarkable importance of this class of heterocyclic compounds, a great deal
of effort has been drawn to develop new and efficient synthetic routes to substituted
quinolines in both synthetic organic and medicinal chemistry. Several methods have been
developed for the construction of quinoline building blocks, which include the classical
Doebner–von Miller, Skraup, Combes, Friedländer,[9,10] Conrad–Limpach–Knorr,[11]
Niementowski,[12] and Pfitzinger[12] methods, as well as other recent methods. Among
these methods, the venerable Friedländer annulation is still one of the most simple and
straightforward approaches for the synthesis of polysubstituted quinolines. Classically,
the reaction consists of an acid- or base-catalyzed condensation of ortho-aminoaryl
ketones with carbonyl compounds containing a reactive α-methylene group followed by a
cyclodehydration. This reaction is generally carried out either by refluxing an aqueous or
an alcoholic solution of reactants in the presence of a base or by heating a mixture of the
reactants at high temperature, 150 to 220 °C, in the absence of a catalyst.[13] However, under
thermal (155 °C) or base catalysis (aqueous KOH) conditions, o-aminobenzophenone did
not react with simple ketones, such as cyclohexanone and β-keto esters.[14] Subsequent
literature reports established that acid catalysts are more effective than base catalysts for
the Friedländer annulation. Recently, modified protocols have been used in the reaction
including ZnCl2, phosphoric acid, AuCl3, sodium fluoride, Bi(OTf)2, Y(OTf)3, silver
phosphotungstate, microwaves, and ionic liquids.[15,16] Although some of the methods
provide useful strategies for the synthesis of these heterocyclic compounds, they suffer
drawbacks such as drastic reaction conditions, low yields, difficulties in workup, and the
use of stoichiometric quantities of reagents and/or relatively expensive reagents. Therefore,
a simple, convenient, and environmentally benign approach for the synthesis of quinolines
is still desirable.
The art of performing efficient chemical transformations, which involve the coupling of
two or more components in a single operation by a catalytic process avoiding stoichiometric
toxic reagents, large amounts of solvents, and extensive purification techniques, represents
a fundamental target of the modern organic synthesis. In this direction, recently,
n-tetrabutylammonium peroxydisulfate [(Bu4N)2S2O8][17] reagent has been used as a versa-
tile radical oxidant, and also involved acid-induced processes to accomplish synthetically
useful transformations in organic synthesis[18] such as oxidation of alcohols and amines,
synthesis of nitriles from primary alcohols and aldehydes, tetrahydrofuranylation of
=
alcohols, cleavage of C N bond, and α-iodination of functionalized ketones. In contrast
to the metal peroxydisulfates such as K2S2O8, Na2S2O8, or (NH4)2S2O8, this reagent is
readily soluble in various organic solvents such as dichloromethane, acetonitrile, acetone,

SYNTHETIC COMMUNICATIONS®
1955
and methanol. Herein, we report a practical and efficient method for Friedländer quinoline
synthesis by employing (Bu4N)2S2O8 as a suitable reagent under solvent-free conditions in
the absence of ligands or additives (Scheme 1).
Therefore, we first investigated the optimization of the reaction conditions using
o-amino benzophenone (1a) and ethyl acetoacetate (2a) as model substrates, and the role
of various conditions on the reaction system was studied with respect to potential reagents,
and appropriate solvents. The results are summarized in Table 1. The reaction was feasible
in all the reagents and solvents tested. The optimum results were obtained with a 1:1:0.25
ratio of o-aminoaryl ketone, α-methylene ketone or β-diketones, and freshly prepared
catalyst (Bu4N)2S2O8 at ambient temperature under solvent-free conditions, producing
the high yield of 3a in 45 min (Table 1, entry 12). Lower yields were obtained in the case
of solvent medium at 55 °C (Table 1, entry 10). A lower amounts of catalyst (Bu4N)2S2O8,
10 mol%, led to lower yield and a higher amount, 50 mol%, did not show any improvement
in the yield or reaction time (Table 1, entry 14). Moreover, this procedure avoids problems
associated with solvent use (cost, handling, safety, and pollution). The role of (Bu4N)2S2O8
catalyst was confirmed when the model reaction between o-aminobenzophenone 1a and
α-methylene ketone 2a was carried out in the absence of catalyst; the reaction did not yield
any product even after longer reaction times (>12 h, Table 1, entry 15). To our knowledge,
(Bu4N)2S2O8 catalyst has not yet been exploited for the synthesis of substituted quinolines.
With the optimized conditions in hand, we investigated the scope of this reaction to a
variety of α-methylene ketones including acetone (45% yield),[14] alky acetoacetates, acetyl
acetone, acyclic ketones, acetophenone (92% yield), and cyclic β-diketones reacted with
o-aminoaryl ketones to afford the corresponding functionalized quinolines 3a–3o without
any side products. Interestingly, cyclic ketones such as cyclopentanone, cyclohexanone
(84% yield), 4-tert-butylcyclohexanone, 5,5-dimethylcyclohexanedione (dimedone), and
cyclic β-diketones reacted with o-amino aryl ketones to afford the respective tricyclic
quinolines in good yields. After completion of the reaction, as indicated by thin-layer
chromatography (TLC), the product was isolated by simple filtration. All the products were
1
characterized by infrared (IR), H NMR, 13C NMR, and mass spectroscopy and also by
comparison with authentic samples. However, condensation of o-aminobenzophenone with
benzoylacetonitrile requires a higher molar ratio of catalyst (50 mol%) and longer reaction
time, and gives relatively low yield (Table 2, entry 6). Furthermore, the condensation of
o-aminobenzophenone with ethyl acetoacetate in the presence of concentrated H2SO4
afforded the quinoline product in only 65% yield (entry 1). In general, the reaction is very
clean, involves a simple workup procedure, and is free from side reactions such as self-
condensation of carbonyl compounds, which normally take place under basic conditions.
Scheme 1. Synthesis of substituted quinolines via Friedlander’s annulations of o-aminoaryl ketones
with α-methylene ketones.

1956
G. VANAJATHA AND V. PRABHAKAR REDDY
Table 1. Optimization of the reaction conditions.a
Entry
Reagent (mol %)
Solvent
Time (min)
Yield (%)b
1
K2S2O8 (10)
DCE
90
120
90
15
2
K2S2O8 (20)
CH3CN
25
3
K2S2O8 (20)
K2S2O8 (20)
THF
20
4
DMSO
90
25
5
K2S2O8/Et4NBr (10:10)
K2S2O8/Et4NBr(20)
(NH4)2S2O8 (20)
(Bu4N)2S2O8(10)
(Bu4N)2S2O8(10)
(Bu4N)2S2O8(25)
(Bu4N)2S2O8(20)
(Bu4N)2S2O8(25)
(Bu4N)2S2O8(25)
(Bu4N)2S2O8(50)
—
CH3CN
120
120
90
35
6
Solvent-free
CH3CN
40
7
25
8
DCE
120
60
30
9
CH3CN
45
10
11
12
13
14
15
CH3CN
45
52
THF
60
40
Solvent-free
Solvent-free
Solvent-free
Solvent-free
45
95c
90
82
93
45
>720
no reactiond
aReaction conditions: o-aminoarylketone (1a, 10 mmol), α-methyleneketone (2a, 10 mmol), (Bu4N)2S2O8 (25 mol %).
bIsolated and unoptimized yields.
cOptimum reaction conditions.
dRecovered starting materials.
Unlike previous reports, the current protocol does not require high temperatures, use of
strong acids or bases, or the use of toxic solvents to produce quinoline derivatives.
On the basis of our observations and the literature precedents, a proposed mechanistic
pathway for the Friedlander quinoline synthesis is delineated in Scheme 2. First, the aniline
reacted with a carbonyl species bearing a reactive methylene group, yielding an imine
intermediate, which underwent enamine tautomerism to form an enamine. Here, the rate-
limiting step is the formation of the Schiff base followed by an intramolecular aldol-type
condensation to the hydroxyimine and subsequent loss of water to obtain the quinoline.
Having established the novel catalytic route for the generation of quinoline scaffolds, we
next turned our attention further to expand the scope of this methodology. As depicted in
Scheme 1, compound 4 represents the key precursor of the whole synthetic strategy.
In this direction, initially we prepared ethyl 6-chloro-2-(chloromethyl)-4-phenylquino-
line-3-carboxylate (4) by the condensation of o-amino-5-chlorobenzophenone with
4-chloroethylacetoacetate using 10 mol% (Bu4N)2S2O8 under solvent-free conditions for a
period of 60 min in 90% yield (Scheme 3). As the starting point of our plan, we decided
to explore the nucleophilic substitution reaction at the 2-position, having reactive
chloromethyl group in 4, using natural amino acids as nucleophiles to produce the drug-like
molecules 5 and 6 respectively. Accordingly, compound 5 was prepared by reacting 4
with L-prolinol in the presence of a base N,N-diisopropylethylamine (DIPEA) in dry
tetrahydrofuran (THF) at rt for 8 h in 78% yield. Similarly, synthesis of compound 6, which
is structurally related to cytotoxic alkaloid camptothecin, was planned by reacting 4 with
L-phenylalaninol under identical conditions with the hope of obtaining alkylated product
5a exclusively. However, the major product isolated from the reaction was assigned
the structure 5a (85%) and minor product isolated as concomitant ring closure to give

SYNTHETIC COMMUNICATIONS®
1957
Table 2. (Bu4N)2S2O8-catalyzed synthesis of quinolines under solvent-free conditions.a
Entry o-Aminoarylketone (1)
Ketone (2)
Time (min)
Quinoline (3)b
Yield (%)c
1.
45
95,(65)
92
2.
3.
60
60
87
4.
5.
6.
120
45
75
90
90
81d
7.
80
78
8.
45
45
65
45
75
87
92
89
90
84
9.
10.
11.
12.
13.
60
89
(Continued)

1958
G. VANAJATHA AND V. PRABHAKAR REDDY
Table 2. Continued.
Entry o-Aminoarylketone (1)
Ketone (2)
Time (min)
80
Quinoline (3)b
Yield (%)c
84
14.
15.
120
9.2
aAll reactions were performed on 10 mmol scale of substrates with 25 mol % of reagent TBAPD.
bProducts were characterized by spectral analysis (IR, 1H-NMR, 13C NMR, Mass spectroscopy).
cYield of isolated product after column chromatography.
dHigher molar ratio of reagent used (50 mol%). Yield in parentheses refer to conc. H2SO4 used instead of TBAPD.
Scheme 2. A Plausible reaction mechanism for the synthesis of quinolines.
Scheme 3. New class of drug-like compounds.

SYNTHETIC COMMUNICATIONS®
1959
the quinolin-pyrrolone fused compound 6, which can be easily explained by the
thermodynamic stability factors for the formation of a five-membered cyclized product.
Complete conversion of 5a to compound 6 was achieved by heating the reaction mixture
at 45 °C for an additional 12 h.
Conclusion
In summary, we have demonstrated a simple and efficient procedure for the synthesis
ofquinolines, in high yield, by employing inexpensive catalyst (Bu4N)2S2O8 (25 mol%) at
ambient temperature under solvent-free conditions. This method offers a high degree of
flexibility with regard to the functional groups that can be placed on various positions
of the quinoline moiety that in turn generates scaffolds for medicinal chemistry. Further
applications of this reagent to other types of privileged heterocyclic ring systems are
currently under investigation in our research group, and will be reported in due course.
Experimental
Reagents and chemicals were obtained from commercial sources and used as received
without further purification. Melting points were determined in open capillaries with a
precision digital melting point Veego VMP-DS apparatus and are uncorrected. IR spectra
were recorded on a thermo Nicolet Nexus 670 FT-IR spectrophotometer. 1H and
1
13C NMR spectra were recorded on either a Bruker Avance 300 (300.132 MHz for H,
75.473 for 13C) or a Varian FT 200-MHz (Gemini) spectrometer in CDCl3. The chemicals
shifts (d) and coupling constants (J) quoted in hertz (Hz) are reported in parts per million
1
(ppm) relative to tetramethylsilane (TMS; d¼0.00 ppm) (for H) as an internal standard.
The resonances of residual proton and those of carbons in deuterated solvents CDCl3
(dH ¼ 7.26 ppm, dc ¼ 77.0 ppm), DMSO-d6 (dH ¼ 2.50 ppm, dc ¼ 39.52 ppm) were used as
internal standards. Elemental analyses were performed on a Elementar Vario EL microanalyzer.
Low-resolution mass spectra (ESI-MS) and HRMS were recorded on Quattro LC,
Micromass, and Q STAR XL, Applied Biosystems, respectively. Column chromatography
was performed using silica gel (Acme’s 60–120 mesh). Solvents for chromatography
(n-hexane, acetonitrile, cyclohexane, EtOAc) were distilled prior to use. For analytical
TLC, Merck precoated silica-gel 60 F-254 plates were used; the plates were visualized
using UV light (254 nm) or iodine vapor or by dipping the plates in phosphomolybdic acid
ceric(IV) sulfate–sulfuric acid (PMA) solution and heating the plates at 100 °C.
Ethyl-2-methyl-4-phenylquinoline-3-carboxylate (3a) (typical procedure)
A mixture of o-aminobenzophenone (1.97 g, 10 mmol), ethyl acetoacetate (1.30 g,
10 mmol), and (Bu4N)2S2O8, (0.108 g, 25 mol%) was stirred at ambient temperature under
solvent-free conditions for the appropriate time (Table 1) until starting material could
no longer be detected by TLC (petroleum ether/EtOAc, 9:1). After completion of the
reaction, the mixture was diluted with ethyl acetate (30 mL), washed with water (15 mL),
dried (Na2SO4), and concentrated. The residue was purified by silica-gel column chroma-
tography (10% ethyl acetate in hexane) to afford the pure product 3a as a pale yellow
solid (2.76 g, 95%). Rf (petroleum ether/EtOAc 9:1 v/v): 0.85. Mp 98–99 °C (lit.[14a] mp

1960
G. VANAJATHA AND V. PRABHAKAR REDDY
99–100 °C); IR (KBr): 3050, 2975, 2930, 1725, 1567, 1485, 1295, 1227, 905, 768 cmÀ 1
.
1H NMR (300 MHz, CDCl3), δ: 0.95 (t, J ¼ 7.35 Hz, 3H), 2.78 (s, 3H), 4.03 (q, J ¼ 7.35 Hz,
2H), 7.35–7.48 (m, 6H), 7.55 (d, J ¼ 8.0 Hz, 1H), 7.70 (t, J ¼ 8.0 Hz, 1H), 8.05 (d, J ¼ 8.0 Hz,
1H). 13C NMR (CDCl3) δ: 13.5, 23.3, 65.8, 96.0, 125.1, 126.0, 126.4, 127.8, 128.2, 129.0, 129.5,
129.7, 135.7, 145.5, 148.1, 154.0, 167.7. EI-MS: m/z (%): 291 (Mþ, 80), 246 (100), 218 (46),
177 (10), 176 (20), 75 (27), 43 (25). Anal. calcd. for C19H17NO2: C, 78.33; H, 5.88; N, 4.81.
Found: C, 78.31; H, 5.85; N, 4.82.
Acknowledgments
One of the authors (G. V.) thanks the commissioner, Collegiate Education, government of Telangana
for support and encouragement. We are also indebted to Prof. D. Ashok for valuable suggestions.
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[17] (Bu4N)2S2O8 is prepared by extraction of an aqueous solution of potassium peroxydisulfate and
2.0 equiv of tertabutylammonium hydrogensulfate in a suitable organic solvent.
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Peng, Z.-Z.; Fu, H.; Liu, J.-D.; Shao, L.-Y. J. Chem. Res., Synop. 1999, 726–727; (c) Sampath, V.;
Jadhav, V.; Lee, H. Y.; Kim, Y. H. Heterocycles 2005, 66, 51–56; (d) Chen, F.-E.; Li, Y.-Y.; Xu,
M.; Jia, H.-Q. Synthesis 2002, 1804–1806.