Synthesis of substituted quinolines from N-aryl-N-(2-alkynyl) toluenesulfonamides via FeCl3-mediated intramolecular cyclization and concomitant detosylation

Article abstract of DOI:10.1016/j.tetlet.2012.07.047

A series of substituted quinolines have been synthesized in moderate to good yields (55-81%) from easily available substrates N-aryl-N-(2-alkynyl) toluenesulfonamides via FeCl₃-mediated intramolecular cyclization and concomitant detosylation.

Full text of DOI:10.1016/j.tetlet.2012.07.047

Tetrahedron Letters 53 (2012) 5119–5122
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of substituted quinolines from N-aryl-N-(2-alkynyl)toluenesulfona-
mides via FeCl3-mediated intramolecular cyclization and concomitant
detosylation
⇑
⇑
B. Roy , Inul Ansary, Srikanta Samanta, K. C. Majumdar
Department of Chemistry, University of Kalyani, Kalyani 741 235, WB, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of substituted quinolines have been synthesized in moderate to good yields (55–81%) from easily
available substrates N-aryl-N-(2-alkynyl)toluenesulfonamides via FeCl3-mediated intramolecular cycliza-
tion and concomitant detosylation.
Received 3 April 2012
Revised 10 July 2012
Accepted 11 July 2012
Available online 20 July 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Sonogashira coupling
N-Aryl-N-(2-alkynyl)toluenesulfonamides
FeCl3
Substituted quinolines
The quinoline nucleus is found in many naturally occurring
compounds with remarkable biological activities.1 Luotonin A (1)
a pyrroloquinazolinoquinoline alkaloid extracted from the Chinese
medicinal plant Peganum nigellastrum2 acts as a cytotoxic agent to-
out either by refluxing an aqueous or alcoholic solution of the reac-
tants in the presence of a base or by heating at high temperatures
ranging from 150–220 °C in the absence of any catalyst.15 Subse-
quent work showed that acid catalysts are more effective than base
catalysts.16 Several acid catalysts have been used in the Friedländer
annulations viz. Bronsted acids like sulfamic acid, hydrochloric acid,
sulfuric acid, p-toluene sulfonic acid, phosphoric acid etc16 and Le-
ward murine leukemia P-388 cell line (IC50 1.8 lg/mL). Camptothe-
cin (2) is also an alkaloid, isolated from the bark and stem of
Camptotheca acuminata (Camptotheca, Happy tree), a tree native
to China used as an anticancer agent in traditional Chinese medi-
cine3 (Fig. 1).
wis acids such as FeCl3 or Mg(ClO4)2,17 SnCl2–ZnCl2,18 SnCl2Á2H2O,19
22
Bi(OTf)3,20 Yb(OTf)3,21 Ag3PW12O40
,
and NaAuCl4Á2H2O23 have
Moreover, members of this family possess antimalarial,4 antiin-
flammatory,5 antiasthmatic,6 antibacterial,7 antihypertensive,8 and
tyrosine kinase inhibiting9 activities. Besides, quinolines are valu-
able synthons for the preparation of nanostructures and polymers
that combine enhanced electronic, optoelectronic, or non-linear
optical properties with excellent mechanical properties.10
In view of the broad array of biological activity, several methods
for the construction of this heterocyclic nucleus have been re-
ported. Although methods such as the Skraup,11 Döebner-von Mill-
er,12 Friedländer,13 and Combes14 procedures have been reported in
the literature, the Friedländer annulations are the most straightfor-
ward method to produce polysubstituted quinolines. The reaction
involves an acid or a base catalyzed annulation reaction between
2-aminoaryl ketone and a carbonyl compound possessing a reactive
been reported to be effective for the synthesis of quinoline. Besides,
there are also several methods reported for the synthesis of quino-
lines using various types of catalysts, for example, molecular-io-
dine,24 In(OTf)3,25 CAN,26 RuCl2(PPh3)3,27 [RhCp⁄Cl2]2
etc.
28
Moreover, substituted quinolines have also been synthesized by
the Baylis–Hillman reaction29 as well as Photo-Fries rearrange-
ment.30 However, many of these procedures suffer from harsh reac-
tion conditions,27 longer reaction time,23 difficulties in work-up,19
and the use of relatively expensive reagents.21–23,25,27,28
O
O
N
N
N
N
a-methylene group. Classically, the Friedländer reaction is carried
N
2
O
1
Camptothecin
Luotonin A
⇑
OH
O
Corresponding authors. Tel.: +91 3325827521; fax: +91 3325828282.
E-mail addresses: broy@klyuniv.ac.in (B. Roy), kcm@klyuniv.ac.in, kcm_ku@
yahoo.co.in (K. C. Majumdar).
Figure 1. Naturally occurring biologically active quinoline alkaloids.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.047

5120
B. Roy et al. / Tetrahedron Letters 53 (2012) 5119–5122
Ts
N
Ts
N
NHTs
i
ii
R1
R1
R1
5a-i
(65-80%)
1
1
1
3a:
3b:
3c:
3d:
R
= Me
(72-85%)
R = OMe
1
1
4a:
R
= Me
R = Cl
4b:
R = OMe
1
4c: R1 = Cl
R = OEt
R2
4d: R1 = OEt
Reagents and conditions: (i) Propargyl bromide (1.5 equiv), anhydrous K2CO3 (4.0 equiv),
dry acetone, reflux, 6-8 h; (ii) substituted iodobenzene (1.2 equiv), Pd(PPh3)2Cl2 (3 mol%),
CuI (3 mol%), DMF-Et3N (4:1), r.t., 0.5-1.0 h.
Scheme 1. Synthesis of N-aryl-N-(2-alkylnyl)toluenesulfonamides.
Table 1
Table 2
Optimization of reaction conditions for synthesis of 6a
Synthesis of substituted quinolines
Ts
N
Ts
N
N
N
FeCl3 (1.0 equiv)
DCE, reflux.
R1
R1
Me
Me
5a-i
5a
6a-i
6a
R2
R2
Entry
Catalyst (equiv)
Solvent/T (°C)
Time (h)
Yielda (%)
1
2
FeCl3 (0.2)
FeCl3 (0.5)
FeCl3 (1.0)
FeCl3 (1.5)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3 (1.0)
FeCl3Á6H2O (1.0)
Fe(NO3)3 (1.0)
Fe2(SO4)3 (1.0)
DCE/reflux
DCE/reflux
DCE/reflux
DCE/reflux
DCE/reflux
DCE/25
CH3CN/reflux
EtOH/reflux
CHCl3/reflux
CH2Cl2/reflux
CH3NO2/100
Toluene/100
Dioxane/100
DCE/reflux
DCE/reflux
DCE/reflux
2
2
1
1
1
12
2
8
1.5
2.5
2
15
35
70
70
69
n.r.
50
45
68
52
35
17
22
10
Trace
Trace
Entry
Substrate
R1
R2
Product
Time (h)
Yielda (%)
3b
4
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
5f
5g
5h
5i
Me
Me
Me
OMe
OMe
Cl
Cl
Cl
OEt
H
6a
6b
6c
6d
6e
6f
6g
6h
6i
1.0
0.5
7.0
0.5
9.0
0.5
8.0
0.5
0.5
70
75
58
81
55
72
56
78
80
OMe
CO2Me
OMe
COMe
OMe
CO2Me
OEt
5c
6
7
8
9
10
11
12
13
14
15
16
OMe
3
3
2
3
a
Isolated yield of the product.
3
to react with FeCl3 (0.2 equiv) in 1,2–dichloroethane (DCE) under
reflux for 2 h, the quinoline derivative 6a33 was obtained in only
15% yield (Table 1, entry 1). By increasing the amount of FeCl3 up
to 1.0 equiv, the yield of the quinoline derivative 6a was increased
(Table 1, entries 2 and 3). Further increase in the amount of FeCl3
did not improve the yield of product 6a (Table 1, entry 4). The reac-
tion under nitrogen atmosphere also gave similar results (Table 1,
entry 5). But at 25 °C temperature any trace amount of the desired
quinoline derivative 6a was not obtained, instead the starting
material 5a was recovered unchanged (Table 1, entry 6). Next the
effect of solvent on the yield of quinoline derivative 6a was studied.
Use of CH3CN, EtOH, CHCl3, CH2Cl2, CH3NO2, toluene, and dioxane as
solvents led to lower yields of the desired product 6a (Table 1, en-
tries 7–13). Other iron-catalysts viz. FeCl3Á6H2O, Fe(NO3)3, and
Fe2(SO4)3 were also used for testing their effectiveness toward the
formation of quinoline 6a, but these were found to be less effective
for the reaction (Table 1, entries 14–16). It became apparent that
the optimized reaction condition is the use of 1.0 equiv of FeCl3 in
DCE under reflux which provides the best result (Table 1, entry 3).
To test the generality of the reaction, the other substrates 5b–i
were treated under optimized reaction condition and the substi-
tuted quinolines 6b–i were obtained in 55–81% yields (Table 2).
From Table 2 it is seen that the substrates having electron-donating
a
b
c
Isolated yield of the product.
Optimized reaction condition.
Reaction was carried out in nitrogen atmosphere, n.r. = no reaction.
In recent years, the development of sustainable, environmen-
tally friendly, and low cost C–C and C–X bond forming protocols
have attracted much attention in synthetic organic chemistry.
Many excellent works concerning direct C–C and C–X bond form-
ing reactions catalyzed or mediated by iron have been reported.31
In one report, Takaki et al.31j have used Fe(OTf)3 for the cyclization
of aryl-substituted alkynes to obtain 1,2-dihydroquinolines. But by
changing the catalyst Fe(OTf)3 to FeCl3 interestingly we have
achieved the synthesis of quinoline derivatives. Herein, we report
our results.
The starting precursors, N-aryl-N-(2-alkynyl)toluenesulfona-
mides 5a–i were prepared in 65–80% yields from 4a–d via Sonogash-
ira coupling reaction with substituted iodobenzene. Compounds
4a–d were in turn synthesized in 72–85% yields from 3a–d by the
reaction with propargyl bromide in the presence of anhydrous
K2CO3 in refluxing acetone for 6–8 h (Scheme 1).
We initiated our investigation with substrate 5a32 and the re-
sults are summarized in Table 1. When substrate 5a was subjected

B. Roy et al. / Tetrahedron Letters 53 (2012) 5119–5122
5121
Ts
N
Ts
N
Ts
N
6-endo-dig
FeCl3
R1
R1
FeCl3
R1
FeCl3
H
7
H
5
6
8
Ar
Ar
Ar
Me
FeCl3
H/
Ts
N
O
S
O FeCl2
N
FeCl3
N
H
Cl
R1
R1
H
9
R1
Ar
Ar
HCl
10
Ar
Scheme 2. A proposed mechanism for the FeCl3-mediated cyclization of N-aryl-N-(2-alkylnyl)toluenesulfonamides.
N-aryl-N-(2-alkynyl)toluenesulfonamides via FeCl3-mediatedintra-
molecular cyclization. This process can provide a diverse range of
quinoline derivatives from simple starting materials. The procedure
is simple and detosylation occurs under the reaction condition.
Ts
N
N
i
Me
Me
4a
11
Acknowledgements
We thank the CSIR (New Delhi) and DST (New Delhi) for finan-
cial assistance. We also thank Dr. G. V. Karunakar of IICT, Hydera-
bad for providing HRMS of three compounds reported in this letter.
One of us (K.C.M.) is thankful to UGC (New Delhi) for Emeritus Fel-
lowship and two of us (I.A. to UGC, New Delhi and S.S. to CSIR, New
Delhi) are grateful for their research fellowships. We also thank
DST (New Delhi) for providing Bruker DPX-400 NMR and Perkin-El-
mer L 120-000A IR spectrometers and a Perkin-Elmer 2400 series II
CHN-analyzer under the DST-FIST programme.
Reagents and conditions : (i) FeCl3 (1.0 equiv), DCE, reflux, 8 h.
Scheme 3. FeCl3-mediated reaction of N-propargyl aniline 4a.
groups like OMe and OEt present in the benzene ring of the terminal
acetylenic part gave higher yields of the products and in a relatively
shorter reaction time, whereas substrates with electron-withdraw-
ing groups like CO2Me and COMe gave lower yields and required
longer reaction time.
The structure of quinoline derivatives 6 was determined from
their analytical and spectral data. A proposed reaction mechanism
for the FeCl3-mediated cyclization of N-aryl-N-(2-alkynyl)toluene-
sulfonamides is depicted in Scheme 2. Initially the alkyne moiety of
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5 is activated by FeCl3 to generate p-complexes 7, which by subse-
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the charged species 8 which on simultaneous deprotonation-pro-
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In conclusion, we have successfully developed an inexpensive
and efficient method for the synthesis of substituted quinolines in
moderate to good yields from easily available starting materials

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white solid. Yield: 65%; mp 74–76 °C; IR (KBr):
mmax = 1163, 2246, 2919,
3055 cmÀ1 1H NMR (400 MHz, CDCl3): dH = 2.34 (s, 3H, CH3), 2.36 (s, 3H, CH3),
;
4.63 (s, 2H, NCH2), 7.12 (d, J = 8.0 Hz, 2H, ArH), 7.18–7.20 (m, 6H, ArH), 7.27–
7.31 (m, 3H, ArH), 7.60 (d, J = 8.0 Hz, 2H, ArH) ppm; MS (ESI): m/z = 398
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for 1 h and cooled to room temperature. CH2Cl2 (50 mL) was added and the
organic layer was successively washed with water (2 Â 20 mL), brine (20 mL)
and dried over anhydrous Na2SO4. The solvent was removed under reduced
pressure to give a crude mass which was flash chromatographed over silica gel
(230–400 mesh) using ethyl acetate-petroleum ether (3:17) as eluent to afford
the quinoline derivative 6a as a colorless gummy mass. Yield: 70%; IR (KBr):
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m
max = 1583, 2917, 3057 cmÀ1 1H NMR (400 MHz, CDCl3): dH = 2.45 (s, 3H,
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8.07 (d, J = 8.4 Hz, 1H, ArH), 8.86 (d, J = 4.4 Hz, 1H, ArH) ppm; 13C NMR
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136.5, 138.2, 147.3, 147.8, 149.0 ppm; HRMS (ESI): Calcd for
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[M+Na]+ 242.0946. Found: 242.0961.
34. 6-Methoxy-4-(4-methoxyphenyl)-1-tosyl-1,2-dihydroquinoline (9d): Yield: 15%;
white solid, mp 118–120 °C; IR (KBr):
max = 1158, 2932, 3037 cmÀ1 1H NMR
m
;
(400 MHz, CDCl3): dH = 2.27 (s, 3H, CH3), 3.69 (s, 3H, OCH3), 3.80 (s, 3H, OCH3),
4.47 (d, J = 4.0 Hz, 2H, NCH2), 5.52 (t, J = 4.4 Hz, 1H, @CH), 6.41 (d, J = 2.4 Hz,
1H, ArH), 6.64 (d, J = 8.4 Hz, 2H, ArH), 6.76 (d, J = 8.8 Hz, 2H, ArH), 6.86 (dd,
J = 8.8, 2.8 Hz, 1H, ArH), 7.02 (d, J = 8.0 Hz, 2H, ArH), 7.31 (d, J = 8.0 Hz, 2H,
ArH), 7.70 (d, J = 8.8 Hz, 1H, ArH) ppm; 13C NMR (100 MHz, CDCl3): dC = 21.3,
45.6, 55.3, 55.4, 111.7, 113.0, 113.4, 121.4, 127.7, 128.5, 128.8, 129.0, 129.6,
130.4, 132.3, 136.1, 138.2, 143.2, 158.0, 159.1 ppm; MS (ESI): m/z = 422
[M+H]+.