The Vilsmeier cyclization of 2′-azido and 2′-aminochalcones - A mild one pot synthesis of 2-aryl-4-chloroquinoline and its N-formyl-1,2-dihydro derivatives

Article abstract of DOI:10.1016/S0040-4020(01)00220-4

The Vilsmeier cyclization of 2′-aminochalcones provides a mild one pot synthesis of 2-aryl-4-chloro-N-formyl-1,2-dihydroquinolines. The scope of the reaction has been extended for the synthesis of quinolines themselves, by replacing 2′-aminochalcones with 2′-azidochalcones as the starting material. The yields are reasonably good and a plausible mechanism for the formation of the products in each case has been discussed.

Full text of DOI:10.1016/S0040-4020(01)00220-4

TETRAHEDRON
Pergamon
Tetrahedron 57 <2001) 3465±3469
The Vilsmeier cyclization of 20-azido and
20-aminochalconesÐa mild one pot synthesis of
2-aryl-4-chloroquinoline and its N-formyl-1,2-dihydro derivatives
Shanmugam Akila, Srinivasan Selvi and Krishna Balasubramanianp
Organic Chemistry Division, Central Leather Research Institute, Adyar, Chennai 600020, India
Received 7 November 2000; revised 29 January 2001; accepted 15 February 2001
AbstractÐThe Vilsmeier cyclization of 20-aminochalcones provides a mild one pot synthesis of 2-aryl-4-chloro-N-formyl-1,2-dihydro-
quinolines. The scope of the reaction has been extended for the synthesis of quinolines themselves, by replacing 20-aminochalcones with 20-
azidochalcones as the starting material. The yields are reasonably good and a plausible mechanism for the formation of the products in each
case has been discussed. q 2001Elsevier Science Ltd. All rights reserved.
Quinolines and their derivatives occur in numerous natural
products.1 Many quinolines display interesting physio-
logical activities and have found attractive applications as
pharmaceuticals and as general synthetic blocks.2 Many
syntheses have been developed for quinolines,3 but due to
their great importance, the development of novel synthetic
methods remains an active research area. Convenient
synthesis of 20-aminochalcone and its amide derivatives
and the ready cyclization of these compounds to 20-aryl-
1,2,3,4-tetrahydro-4-quinolone has been explored widely.4
However, until now, the cyclization of 20-aminochalcones
under Vilsmeier conditions has remained an uncharted
territory.
1. Results and discussion
The prerequisite 20-aminochalcones were prepared from
o-aminoacetophenone and the corresponding benzalde-
hydes following the reported procedure.4b Initially, the
reactions were carried out at room temperature in antici-
pation of the formation of 2-aryl-N-formyl-1,2-dihydro-
quinolones. We envisaged the amino group to undergo
formylation, followed by ring closure to yield an N-formyl
Vilsmeier±Haack±Arnold reactions that were initially used
for the formylation of activated aromatic substrates5 and
carbonyl compounds6 have now evolved into a powerful
synthetic tool for the construction of many heterocyclic
compounds such as quinolines, indoles, quinazolines, pyri-
dines,7 etc. Although various substituted quinolines have
been synthesized by both normal8 and reverse Vilsmeier
approaches,9 the synthesis of quinoline derivatives from
20-aminochalcones using the Vilsmeier reagent has not
been reported so far. This led us to conduct a systematic
investigation of the feasibility of cyclization of 20-amino-
chalcones under Vilsmeier conditions.
Scheme 1.
Table 1. Reaction products of 20-aminochalcones with the Vilsmeier
reagent
Entry
Producta
Ar
Yield <%)b
1
2
3
4
5
6
7
2ac
2b
2c
2d
2e
2f
Ph
p-ClC6H4
p-CH3C6H4
p-OCH3C6H4
o-ClC6H4
76
80
78
68
75
82
85
In this paper, we report a novel and mild method towards
the synthesis of 2-aryl-4-chloro-N-formyl-1,2-dihydro-
quinolines and 2-aryl-4-chloroquinolines from 20-amino-
chalcones and 20-azidochalcones respectively.
o-CH3C6H4
m-ClC6H4
2g
a
All products were characterized using 1H and 13C NMR, IR, mass spectra
and CHN analysis.
Isolated yield.
Dense oil.
b
c
Keywords: quinolines; aminochalcones; Vilsmeier reaction; azides.
Corresponding author; e-mail: kbalasubramanian@hotmail.com
p
0040±4020/01/$ - see front matter q 2001Elsevier Science Ltd. All rights reserved.
PII: S0040-4020<01)00220-4

3466
S. Akila et al. / Tetrahedron 57 (2001) 3465±3469
Scheme 2.
quinoline derivative. However, these reactions resulted only
in the formation of the corresponding uncyclized products,
N-formyl-20-aminochalcones. Prolonged reaction time or
variation in the DMF/POCl3 ratio could not impart any
change in the formation of products.
chalcones 4a±f necessary for the study, were prepared
from the corresponding 20-aminochalcones, by diazotization
followed by treatment with NaN3.
When various substituted 20-azidochalcones were treated
with the Vilsmeier reagent, a smooth conversion of the
starting material to 2-aryl-4-chloroquinolines 5a±f was
observed in moderate yields at 908C <Scheme 3, Table 2).
Attempts to accomplish the cyclization at lower tempera-
tures, using excess of reagents or by prolonging the reaction
time were not rewarded with favorable results.
Gratifyingly, when the reaction temperature was raised to
908C, the desired cyclization proceeded smoothly to give
2-aryl-4-chloro-N-formyl-1,2-dihydroquinolines 2a±g in
good yields <Scheme 1, Table 1).
1,2-Dihydroquinolines are important target molecules,
mainly due to their synthetic utility as precursors for a
variety of biologically active compounds.10 Literature also
reveals a number of patent applications concerning their
industrial uses.11 To the best of our knowledge this is the
®rst report on the one-pot synthesis of N-formyl-1,2-di-
hydroquinolines.12 Also, it is worth emphasizing the
dif®culty in preparing 1,2-dihydroquinolines by traditional
methods, which mainly depend upon the addition of nucleo-
philes to N-alkylquinolinium salts,13 and suffer from the
simultaneous production of 1,4-dihydro derivatives.
The reaction may be proceeding through initial cyclization
followed by reductive elimination of nitrogen as proposed in
Scheme 4.
In conclusion, we have unraveled a new, ef®cient and one
pot synthesis of N-formyl-4-aryl-1,2-dihydroquinolines and
2-aryl-4-chloroquinolines from the corresponding 20-
aminochalcones and 20-azidochalcones, respectively.
The reaction seems to proceed through N-formylation
followed by ring closure to give the intermediate 3, which
on hydrolysis furnishes the corresponding dihydroquinoline
as represented in Scheme 2.
The mild reaction conditions employed and the ready avail-
ability of the starting materials enticed us to adapt this
useful methodology for the synthesis of quinolines them-
selves.
Scheme 3.
Table 2. Reaction products of 20-azidochalcones with Vilsmeier reagent
We selected 20-azidochalcone as a suitable starting material
since the in situ elimination of nitrogen in the course of the
reaction could provide the desired 2-aryl-4-chloroquino-
lines. We were also encouraged by recent reports on the
cyclization of the azido moiety with iminium salts generated
in-situ under Vilsmeier conditions.14 Application of azides
in organic synthesis has attracted considerable attention in
recent years, owing to their versatility for synthetic
manipulations.15 In addition, a wide array of synthetic
methods are available for the introduction of the azido
moiety into organic substrates. The prerequisite 20-azido-
Entry
Producta
Ar
Yield <%)b
1
2
3
4
5
6
5a
5b
5c
5d
5e
5f
p-ClC6H4
72
71
62
51
63
68
p-CH3C6H4
p-CH3OC6H4
o-ClC6H4
o-CH3C6H4
m-ClC6H4
a
All products were characterized using 1H and 13C NMR, IR, mass spectra
and CHN analysis.
Isolated yield.
b

S. Akila et al. / Tetrahedron 57 (2001) 3465±3469
3467
Scheme 4.
2. Experimental
51.7; MS m/z 304 <M1), 306 <M12), 308 <M14); Anal.
Calcd for C16H11Cl2NO; C, 63.17; H, 3.64; N, 4.60; Found
C, 63.13; H, 3.62; N, 4.52.
Mass spectra were recorded on Varian VG 70-70H mass
spectrometer. Melting points were measured in capillary
tubes and are uncorrected. Analytical thin layer chromato-
graphy was performed on precoated sheets of silica gel G of
0.25 mm thickness containing PF 254 indicator <Merck,
Darmstadt). Column chromatography was performed with
silica gel <60±120 mesh; SD Fine, Boisar). IR spectra were
recorded as solids in KBr pellets on Nicolet Impact-400
spectrometer. NMR spectra were obtained on a Bruker
2.1.3. 4-Chloro-1-formyl-2-(4-methyl)phenyl-2H-quino-
line (2c). 1.1 g of colorless crystals <78%); mp 118±
;
1198C; IR <KBr) 1668, 910, 750 cm21 1H NMR
<300 MHz, CDCl3) d 8.65 <s, 1H), 7.71±7.68 <m, 1H),
7.31±7.22 <m, 3H), 7.15 <d, 2H, Jˆ8.1Hz), 7.04 <d, 2H,
Jˆ8.1Hz,), 6.38 <d, 1H, Jˆ6.3 Hz), 6.32 <d, 1H, Jˆ6.3 Hz),
2.25 <s, 3H); 13C NMR <75 MHz, CDCl3) d 161.4, 138.2,
134.9, 134.7, 130.0, 129.6, 129.4, 128.4, 127.4, 125.7,
125.2, 124.6, 118.0, 52.5, 21.1; MS m/z 283 <M1), 285
<M12); Anal. Calcd for C17H14ClNO; C, 71.96; H, 4.97;
N, 4.94; Found C, 72.18; H, 4.82; N, 4.94.
1
spectrometer. H NMR spectra were recorded at 300 MHz
in CDCl3 and the chemical shifts are given in d relative to
the internal standard TMS. 13C NMR spectra were
recorded at 75 MHz in CDCl3 and the chemical shift was
given in d relative to the solvent <77.0). DMF was vacuum
distilled from calcium hydride and dried over 4 A molecular
sieves.
Ê
2.1.4. 4-Chloro-1-formyl-2-(4-methoxy)phenyl-2H-quino-
line (2d). 1.02 g of oil which solidi®ed on storage <68%);
;
mp 68±698C IR <KBr) 1660, 831 cm21 1H NMR
<300 MHz, CDCl3) d 7.69 <d, 1H, Jˆ6.6 Hz), 7.25±7.22
<m, 3H), 7.17 <d, 2H, Jˆ8.7 Hz), 6.99 <d, 1H, Jˆ7.5 Hz),
6.73 <d, 2H, Jˆ8.7 Hz), 6.34 <d, 1H, Jˆ6.3 Hz), 6.27 <d, 1H,
Jˆ6.3 Hz), 3.68 <s, 3H); 13C NMR <75 MHz, CDCl3) d
161.3, 159.5, 134.5, 129.8, 129.2, 128.5, 125.9, 125.5,
125.4, 125.2, 124.4, 117.9, 114.2, 55.1, 52.1; MS m/z 300
<M1). Anal. Calcd for C17H14ClNO2; C, 68.12; H, 4.7; N,
4.67; Found C, 68.20; H, 4.79; N, 4.71.
2.1. General procedure for the preparation of 2-aryl-4-
chloro-N-formyl-1,2-dihydro quinolines (2a±g)
To an ice-cold magnetically stirred solution of 1a±g
<5 mmol) in DMF <10 mL), POCl3 <2.8 mL, 30 mmol) was
added dropwise. The reaction mixture was allowed to attain
room temperature and was heated over a water bath for 3 h,
after which it was poured into crushed ice, neutralized with
10% NaOH solution and extracted with ethyl acetate. The
organic layer was washed with water, dried over anhydrous
Na2SO4, ®ltered and the solvent evaporated. The crude reac-
tion product was puri®ed by column chromatography using
5% ethyl acetate in petroleum ether as eluent.
2.1.5. 4-Chloro-1-formyl-2-(2-chloro)phenyl-2H-quino-
line (2e). 1.14 g of colorless crystals <75%); mp 125±
;
1268C; IR <KBr) 1666, 831, 756 cm21 1H NMR
<300 MHz, CDCl3)
d 8.77 <s, 1H), 7.67 <d, 1H,
Jˆ7.5 Hz), 7.43±7.24 <m, 4H), 7.18±7.08 <m, 1H), 7.04
<s, 1H), 7.02 <s, 1H), 6.69 <d, 1H, Jˆ6.0 Hz), 6.47 <d, 1H,
Jˆ6.0 Hz); 13C NMR <75 MHz, CDCl3) d 161.5, 137.2,
135.8, 130.9, 130.4, 129.9, 129.3, 127.9, 126.8, 126.1,
125.8, 125.7, 123.9, 123.8, 117.2, 51.5; MS m/z 304 <M1),
306 <M12), 308 <M14); Anal. Calcd for C16H11Cl2NO;
C, 63.18; H, 3.64; N, 4.60; Found C, 63.10; H, 3.67;
N, 4.42.
2.1.1. 4-Chloro-1-formyl-2-phenyl-2H-quinoline (2a).
1.02 g of dense oil <76%); IR <KBr) 1923, 1665,
1
769 cm21; H NMR <300 MHz, CDCl3) d 8.65 <s, 1H),
7.69 <dd, 1H, Jˆ1.5, 2.1 Hz), 7.30±7.21 <m, 7H), 7.03
<dd, 1H, Jˆ1.5, 2.1 Hz), 6.41 <d, 1H, Jˆ6.3 Hz), 6.33 <d,
1H, Jˆ6.3 Hz); 13C NMR <75 MHz, CDCl3) d 161.4, 137.8,
134.7, 130.0, 128.9, 128.7, 128.5, 128.3, 127.2, 125.6,
124.9, 124.4, 117.9, 52.6; MS m/z 270 <M1), 272 <M12);
Anal. Calcd for C16H12ClNO; C, 71.24; H, 4.48; N, 5.19;
Found C, 71.29; H, 4.52; N, 5.21.
2.1.6. 4-Chloro-1-formyl-2-(2-methyl)phenyl-2H-quino-
line (2f). 1.16 g of colorless crystals <82%); mp 130±
1
1318C; IR <KBr) 1678, 759 cm21; H NMR <300 MHz,
CDCl3) d 8.66 <s, 1H), 7.69 <m, 1H,), 7.37±7.24 <m, 5H),
7.14±7.09 <m, 3H), 7.00±6.75 <m, 2H), 6.52 <d, 1H,
Jˆ6.3 Hz), 6.26 <d, 1H, Jˆ6.3 Hz); 13C NMR <75 MHz,
CDCl3) d 161.72, 136.9, 135.6, 134.8, 130.9, 130.2,
128.3, 127.5, 126.7, 126.4 <2), 125.7, 125.6, 124.5, 117.9,
50.9, 19.6; MS m/z 283 <M1). 285 <M12); Anal. Calcd for
C17H14ClNO; C, 71.95; H, 4.97; N, 4.93; Found C, 71.89; H,
4.91; N, 4.87.
2.1.2. 4-Chloro-1-formyl-2-(4-chloro)phenyl-2H-quino-
line (2b). 1.2 g of colorless crystals <80%); mp 119±
1
1108C; IR <KBr) 1666, 1598, 823, 760 cm21; H NMR
<300 MHz, CDCl3)
d 8.64 <s, 1H), 7.70 <d, 1H,
Jˆ7.0 Hz), 7.32±7.19 <m, 6H), 7.03 <d, 1H, Jˆ7.5 Hz),
6.37 <d, 1H, Jˆ6.3 Hz), 6.29 <d, 1H, Jˆ6.3 Hz); 13C NMR
<75 MHz, CDCl3) d 161.1, 136.1, 134.2, 134.1, 130.1,
128.9, 128.7, 128.6, 128.5, 128.4, 125.6, 124.1, 117.8,

3468
S. Akila et al. / Tetrahedron 57 (2001) 3465±3469
2.1.7. 4-Chloro-1-formyl-2-(3-chloro)phenyl-2H-quino-
line (2g). 1.29 g of colorless crystals <85%); mp 77±788C;
IR <KBr) 1676, 823, 769 cm21; 1H NMR <300 MHz, CDCl3)
d 8.50 <s, 1H), 7.65±7.62 <m, 1H), 7.27±7.17 <m, 3H),
7.16±7.07 <m, 3H), 7.00 <d, 1H, Jˆ9 Hz), 6.32 <d, 1H,
Jˆ6.6 Hz), 6.24 <d, 1H, Jˆ6.6 Hz); 13C NMR <75 MHz,
CDCl3) d 162.0, 140.4, 135.1, 134.9, 130.8, 130.5, 129.7,
129.0, 127.9, 126.4, 126.3, 125.9, 124.7, 124.4, 118.5, 52.3;
MS m/z 304 <M1) 306 <M12); Anal. Calcd for
C16H11Cl2NO; C, 63.18; H, 3.64; N, 4.60; Found C, 63.22;
H, 3.65; N, 4.58.
C15H9Cl2N; C, 65.72; H, 3.31; N, 5.11; Found C, 65.63;
H, 3.28; N, 5.26.
2.2.5. 4-Chloro-2-(2-methyl)phenyl quinoline (5e). 0.79 g
of crystalline solid <63%); mp 67±688C; <67±688C).16
2.2.6. 4-Chloro-2-(3-chloro)phenyl quinoline (5f). 0.92 g
of crystalline solid <68%); mp 58±598C; IR <KBr) 2957,
1110, 843, 735 cm21 1H NMR <300 MHz, CDCl3) d
;
8.20±8.13 <m, 3H), 7.97 <t, 1H, Jˆ1.5 Hz), 7.89 <s, 1H),
7.76 <t, 1H, Jˆ8.2 Hz), 7.62 <t, 1H, Jˆ7.5 Hz), 7.42 <d, 2H,
Jˆ5.1Hz); 13C NMR <75 MHz, CDCl3) d 155.5, 149.0,
143.4, 140.3, 135.1, 130.7, 130.0, 129.9, 129.8, 129.7,
127.6, 127.5, 125.5, 124.0, 118.7; MS m/z 273 <M1), 275
<M12), 277 <M14); Anal. Calcd for C15H9Cl2N; C, 65.72;
H, 3.31; N, 5.11; Found C, 65.68; H, 3.28; N, 5.22.
2.2. General procedure for the preparation of 2-aryl-4-
chloroquinolines (5a±g)
To an ice-cold magnetically stirred solution of 20-azido-
chalcones 4a±g <5 mmol) in DMF <10 mL), POCl3
<2.8 mL, 30 mmol) was added dropwise. The reaction
mixture was allowed to attain room temperature and was
heated over a water bath for 3 h, after which it was poured
into crushed ice, neutralized with 10% NaOH solution and
extracted with ethyl acetate. The organic layer was washed
with water, dried over anhydrous Na2SO4, ®ltered and the
solvent evaporated. The crude reaction product was puri®ed
by column chromatography using 5% ethly acetate in
petroleum ether as eluent.
Acknowledgements
We thank the Council of Scienti®c and Industrial Research,
New Delhi, India for ®nancial support.
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2.2.1. 4-Chloro-2-(4-chloro)phenyl quinoline (5a). 0.98 g
of crystalline solid <72%); mp 99±1008C; IR <KBr) 2927,
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1
1584, 831, 753 cm21; H NMR <300 MHz, CDCl3) d 8.20
<d, 1H, Jˆ8.4 Hz), 8.13 <d, 1H, Jˆ8.1Hz), 8.10 <d, 2H,
Jˆ8.1 Hz), 7.90 <s, 1H), 7.76 <t, 1H, Jˆ7.5 Hz), 7.60 <t,
1H, Jˆ7.5 Hz), 7.47 <d, 2H, Jˆ8.1Hz); 13C NMR
<75 MHz, CDCl3) d 155.9, 149.0, 143.4, 136.9, 136.1,
130.8, 130.1, 129.1, 128.7, 127.5, 125.4, 124.0, 118.6.;
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0.83 g of crystalline solid <62%); mp 77±788C; IR <KBr)
2897, 1154, 831, 753 cm21; 1H NMR <300 MHz, CDCl3) d
8.15±8.05 <m, 4H), 7.86 <s, 1H) 7.70 <t, 1H, Jˆ7.5 Hz), 7.52
<t, 1H, Jˆ7.5 Hz), 6.99 <d, 2H, Jˆ8.7 Hz), 3.83 <s, 3H); 13C
NMR <75 MHz, CDCl3) d 161.1, 156.6, 148.9, 142.8, 131.0,
130.3, 129.7, 128.7, 126.7, 124.9, 123.8, 118.4, 114.2, 55.3;
MS m/z 269 <M1), 271<M 12); Anal. Calcd for
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