Synthesis of highly substituted 1,6-naphthyridines: A reinvestigation

Article abstract of DOI:10.1081/SCC-120006476

A reinvestigation of the recently described method of synthesis of highly substituted 1,6-naphthyridines was carried out. It was found out that the reaction of chalcones with malononitrile catalysed by pyrrolidine afforded the mixture of four products both when thermal as well as microwave heating was used. The claimed^[1] 1,6-naphthyridines formation were exceptions.

Full text of DOI:10.1081/SCC-120006476

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SYNTHESIS OF HIGHLY SUBSTITUTED 1,6-
NAPHTHYRIDINES: A REINVESTIGATION
Eva Veverková a , Marika Nosková a & Štefan Toma b
a Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University,
Bratislava, SK-842 15, Slovakia
b Department of Organic Chemistry, Faculty of Natural Sciences, Comenius University,
Bratislava, SK-842 15, Slovakia
Version of record first published: 18 Oct 2011.
To cite this article: Eva Veverková , Marika Nosková & Štefan Toma (2002): SYNTHESIS OF HIGHLY SUBSTITUTED 1,6-
NAPHTHYRIDINES: A REINVESTIGATION, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 32:18, 2903-2910
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SYNTHETIC COMMUNICATIONS
Vol. 32, No. 18, pp. 2903–2910, 2002
SYNTHESIS OF HIGHLY SUBSTITUTED
1,6-NAPHTHYRIDINES:
A REINVESTIGATION
Eva Veverkova, Marika Noskova, and Stefan Tomay
*
Department of Organic Chemistry,
Faculty of Natural Sciences, Comenius University,
SK-84215 Bratislava, Slovakia
ABSTRACT
A reinvestigation of the recently described method of
synthesis of highly substituted 1,6-naphthyridines was carried
out. It was found out that the reaction of chalcones with
malononitrile catalysed by pyrrolidine afforded the mixture
of four products both when thermal as well as microwave
heating was used. The claimed[1] 1,6-naphthyridines forma-
tion were exceptions.
Key Words: Microwaves; 1,6-Naphthyridines; Reinvestiga-
tion
INTRODUCTION
The 1,6-naphthyridines are interesting substance from the view of their
biological properties. Murugan et al. have disclosed very recently in this
*E-mail: veverkova@fns.uniba.sk
yCorresponding author. Fax: 421-2-60296-690; E-mail: toma@fns.uniba.sk
2903
DOI: 10.1081/SCC-120006476
Copyright & 2002 by Marcel Dekker, Inc.
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2904
VEVEROKA, NOSKOVA, AND TOMA
Journal[1] an interesting one-step method of their synthesis, by just refluxing
of ethanolic solution of chalcones with malononitrile using pyrrolidine as
the catalyst. They observed formation of the desired 1,6-naphthyridines, as
the single products, in 60–76% yield after 25 h refluxing of the reaction
mixture. It is well known that the reaction time of many heterogeneous as
well as homogeneous reactions can be considerably shortened when micro-
wave heating is applied to the reaction mixture.[2–7] We decided therefore to
check if microwave effects would be observed in the synthesis of 1,6-
naphthyridines.
RESULTS AND DISCUSSION
We started our experiments using unsubstituted chalcone as the reagent
and ethanol as the solvent. After 2min of the microwave irradiation only one
highly substituted benzene derivative (IVa) in 68% yield, but no 1,6-
naphthyridine irradiation was isolated from the reaction mixture. (Scheme 1,
R ¼ H). It is well known that ethanol is not a good solvent for microwave
reactions, therefore we decided to perform the reaction in chlorobenzene as
the solvent as well as the solvent free reaction. Two products were isolated
from the reaction carried out in chlorobenzene (reaction time 5 min,
Tfin ¼ 125ꢀC) one being 3-cyano-2-(pyrrolidine-1-yl)-4,6-diphenylpyrridine
Scheme 1.

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1,6-NAPHTHYRIDINES
2905
(IIIa, 56%) and the other one 1-amino-2,6-dicyano-4,5-diphenylbenzene
(IVa, 25%). From the solvent free reaction were isolated 33% of 2-(pyrroli-
dine-1-yl)-4,6-diphenylpyrridine (IIa), 18% of IIIa and 18% IVa.
In order to see if these unusual results are not caused by microwave
irradiation we decided to repeat exactly the procedure described by
Murugan et al., that is using thermal heating of the reaction mixture in
ethanol for 25 h. To our surprise the result was similar as in the solvent-
free reaction under microwave irradiation. Compounds IIa (7%), IIIa
(38%) and IVa (27%) were isolated but no 1,6-naphthyridine was isolated.
The experiments with substituted chalcones were carried out to check if these
results can be influenced by substituents on the benzene ring of chalcone.
From the data given in the Table 1 similar results were observed with
different chalcones,and observed product/substituent dependence is difficult
Table 1. Microwave Assisted Reaction of Chalcones Ia–e with
Malonodinitrile and Pyrrolidine, P ¼ 120 W
Isolated Yields
[%]
React.
Comp. Condition*
React. Time Tfin
[ꢀC]
[min : sec]
II
III
IV
V
a
A
B
C
D
2: 00
5 : 00
2: 00
25
72
125
132
78
–
–
33
–
56
18
68
25
18
–
–
–
–
7
38
27
b
c
A
B
C
2: 30
5 : 00
2: 30
70
115
164
–
–
18
17
54
40
43
23
18
–
–
–
A
B
C
D
2: 00
5 : 00
2: 40
25
72
116
186
78
18
–
42
12
33
30
–
1522 –
11
17
18
43
–
–
d
e
A
B
C
D
2
: 00
121
178
85
71
–
17
–
–
5 : 00
2: 00
2
–
19
–
23
11
72
6
25
–
–
8
–
–
A
B
C
D
3 : 00
5 : 00
2 : 20
25
64
105
122
78
–
–
14
–
–
74
7
50
47
–
65
23
42
–
10
9
*A-ethanol, B-chlorbenzene, C-free solvent, D-thermal heating (time in
hour).

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2906
VEVEROKA, NOSKOVA, AND TOMA
to rationalize. On the other hand we can conclude that microwave heating in
chlorobenzene or at solvent-free conditions prefer the pyridine ring forma-
tion over the benzene ring formation (IV). Small amount of 1,6-naphthyr-
idine derivative (Ve) was isolated only in the reaction with p-methyl
derivative of chalcone. We made also an unsuccessful attempt on the 1,6-
naphthyridine synthesis in which a 10 molar excess of malononitrile was used.
It is of interest to note that Victory et al.[8,9] made a thorough inves-
tigation of the reaction of chalcone with malononitrile in anhydrous ethanol
using piperidine as the catalyst. They never isolated 1,6-naphthyridine deri-
vative, only benzene derivative III and 3-benzoyl-4-hydroxy-2,4,5-triphenyl-
1,1-cyclohexanecarbonitrile. They isolated pyridine derivatives[8] only in
experiments when sodium alkoxide was used as the catalyst. We can
assume therefore that the results we achieved in ethanole (conditions
A,D) could be modified by traces of sodium ethoxide which could be
found in ethanol distilled from sodium.
The fact that we could repeat results described by Murugan et al.[1] and
the described procedure for 1,6-naphthyridine synthesis simply prompted us
to write to Professor Ramakrishnan and ask him if there was not something
omitted from the experimental procedure. His answer was negative and he
sent us a detailed procedure in which was stated that they have used lime
distilled ethanol. We have used both 96% ethanol as well as very dry ethanol
pre-distiled from lime and then dried by natrium, but the results were the same:
negative. The possible explanation for the very different results of Professor
Ramakrishnan and ours could be that some special, not specified salt (cata-
lyst) could be extracted into ethanol from the Indian lime.
EXPERIMENTAL
1H NMR and 13C NMR spectra were measured on Varian Gemini
2000 instrument, working frequency 300 MHz and 75 MHz respectively.
CDCl3 was used as the solvent and tetramethylsilane as an internal stan-
dard. Microanalysis were carried out on a Carlo Erba Strumenstacione
Milano CHN analyser, model 1106. Melting point were determined on a
Kofler apparatus and are not corrected. All microwave experiments were
carried out in the SYNTHEWAVE 402, PROLABO reactor.
General Procedure for the Microwave Experiments
A mixture of 3-aryl-1-fenyl propenone Ia–e (0.005 mol), malononitrile
(0.6 g, 0.01 mol) and pyrrolidine (0.84 mL, 0.01 mol) in solvent (40 mL) was

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1,6-NAPHTHYRIDINES
2907
irradiated in the microwave reactor (120 W input power). After removing
the solvent under reduced pressure, the residue was chromatographed over a
silica gel column. Using hexane–ethyl acetate mixture (5 : 1) as eluent was
obtained the products II and III, with hexane–ethyl acetate (3 : 1) was
obtained the product IV and with ethyl acetate was obtained the product
V. The reaction conditions (solvent or solvent-free), the reaction time and
the yields are given in Table 1 (reaction condition A,B,C).
General Procedure for Reactions Under Thermal Heating
A mixture of 3-aryl-1-fenyl propenone I (0.005 mol), malononitrile
(0.6 g, 0.01 mol) and pyrrolidine (0.84 mL, 0.01 mol) in ethanole (40 mL)
was refluxed for 25 h. After removing the solvent under reduced pressure,
the residue was chromatographed over a silica gel column. Using hexane–
ethyl acetate mixture (5 : 1) as eluent was obtained the products II and III,
with hexane–ethyl acetate (3 : 1) was obtained the product IV and with ethyl
acetate was obtained the product V. The yields are given in Table (reaction
conditions D).
2,4-Diphenyl-6-pyrrolidin-1-yl-pyridine (IIa): m.p. 158–159ꢀC (Ref. [10]
161–162ꢀC); spectroscopic and analytical data are in agreement with the
reported ones.[10]
4-(2-Chlorphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine (IIb): m.p.
143–144ꢀC; H NMR (CDCl3) d 2.03 and 3.60 (2t, 8H), 6.36 (d, 1H), 7.08
1
(d, 1H), 7.29–7.49 (m, 7H), 8.06–8.09 (m, 2H); 13C NMR (CDCl3) ꢀ 25.84,
49.24, 104.95, 110.49, 124.15, 124.66, 127.04, 129.00, 129.15, 129.25, 131.54,
134.72, 137.04, 141.51, 150.53, 156.17, 158.14. Anal. Calcd. for C21H19ClN2:
C, 75.33; H, 5.72; N, 8.37; Cl, 10.58. Found: C, 74.83; H, 5.73; N, 8.38;
Cl 10.23.
4-(4-Chlorphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine
(IIc):
m.p.
159–161ꢀC; H NMR (CDCl3) d 2.05 and 3.62 (2t, 8H), 6.44 (d, 1H), 7.17
(d, 1H), 7.43–7.47 (m, 7H), 7.61 (d, 2H), 8.06–8.09 (m, 2H); 13C NMR
(CDCl3) d 25.83, 47.06, 102.97, 106.77, 127.12, 128.63, 128.69, 128.79,
129.21, 134.67, 139.00, 140.41, 149.32, 156.39, 157.93. Anal. Calcd. for
C21H19ClN2: C, 75.33; H, 5.72; N, 8.37; Cl, 10.58. Found: C, 74.81;
H, 5.76; N, 8.38; Cl 10.21.
1
4-(4-Methoxyphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine (IId): m.p.
143–144ꢀC; H NMR (CDCl3) d 2.04 and 3.63 (2t, 8H), 3.87 (s, 3H), 6.46
1
(d, 1H), 7.01 (d, 2H), 7.20 (d, 1H), 7.37–7.47 (m, 3H), 7.63 (d, 2H),
8.08–8.11 (m, 2H); 13C NMR (CDCl3) d 25.52, 49.43, 55.15, 105.78,
110.48, 111.45, 124.05, 126.27, 128.98, 131.23, 136.85, 137.95, 146.75,

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2908
VEVEROKA, NOSKOVA, AND TOMA
153.42, 158.79, 159.85. Anal. Calcd. for C22H22N2O: C, 79.97; H, 6.71;
N, 8.48. Found: C, 80.06; H, 6.73; N, 8.50.
4-(4-Methylphenyl)-2-phenyl-6-(pyrrolidin-1-yl) pyridine (IIe): m.p.
122–124ꢀC; H NMR (CDCl3) d 2.04 and 3.62 (2t, 8H), 2.42 (s, 3H), 6.49
1
(d, 1H), 7.13 (d, 2H), 7.22 (d, 1H), 7.38–7.48 (m, 3H), 7.58 (d, 2H),
8.08–8.11 (m, 2H); 13C NMR (CDCl3) d 21.21, 25.62, 46.84, 102.86,
106.88, 116.92, 126.99, 128.39, 128.84, 129.75, 137.46, 138.35, 140.49,
150.27, 155.92, 157.81. Anal. Calcd. for C22H22N2: C, 84.02; H, 7.05;
N, 8.91. Found: C, 83.98; H, 6.97; N, 8.93.
2,4-Diphenyl-6-pyrrolidin-1-yl-pyridine-5-carbonitrile
(IIIa):
m.p.
133ꢀC; H NMR (CDCl3) d 2.04 and 3.94 (2t, 8H), 7.11 (s, 1H), 7.45–7.50
(m, 7H), 7.51–7.58 (m, 2H), 8.05–8.08 (m, 2H); 13C NMR (CDCl3) d 25.84,
47.09, 102.85, 105.72, 126.05, 126.33, 126.55, 127.16, 129.01, 135.73, 139.82,
147.42, 154.43, 157.90. Anal. Calcd. for C22H19N3: C, 81.20; H, 5.89; N,
12.91. Found: C, 80.54; H, 5.09; N, 12.54.
1
4-(2-Chlorphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine-5-carbonitrile
(IIIb): m.p. 175–176ꢀC; H NMR (CDCl3) d 2.04 and 3.94 (2t, 8H), 7.03
1
(s, 1H), 7.36–7.54 (m, 7H), 8.05–8.08 (m, 2H); 13C NMR (CDCl3) d 25.65,
49.25, 88.69, 109.57, 118.35, 126.92, 128.65, 129.99, 130.05, 130.25, 130.39,
132.50, 137.08, 138.19, 157.25, 158.14. Anal. Calcd. for C22H18ClN3:
C, 73.43; H, 5.04; N, 11.68; Cl, 9.85. Found: C, 72.91; H, 5.04; N, 11.49;
Cl, 9.74.
4-(4-Chlorphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine-5-carbonitrile
(IIIc): m.p. 168–169ꢀC; H NMR (CDCl3) d 2.04 and 3.93 (2t, 8H), 7.06
1
(s, 1H), 7.45–7.54 (m, 7H), 8.06–8.09 (m, 2H); 13C NMR (CDCl3) d 25.69,
49.48, 86.95, 109.06, 118.99, 127.12, 130.11, 135.57, 136.47, 138.18, 156.29,
158.12, 158.37. Anal. Calcd. for C22H18ClN3: C, 73.43; H, 5.04; N, 11.68;
Cl, 9.85. Found: C, 72.91; H, 5.03; N, 11.47; Cl 9.70.
4-(4-Methoxyphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine-5-carbonitrile
(IIId): m.p. 179–181ꢀC; H NMR (CDCl3) d 2.03 and 3.92 (2t, 8H), 3.04
1
(s, 3H), 7.02 (d, 2H), 7.09 (s, 1H), 7.44–7.46 (m, 3H), 7.56 (d, 2H), 8.06–8.09
(m, 2H); 13C NMR (CDCl3) d 25.96, 49.74, 55.61, 87.23, 109.48, 114.33,
115.12, 127.51, 128.86, 130.12, 130.31, 130.50, 138.71, 157.41, 158.27,
158.72, 160.85. Anal. Calcd. for C23H21N3O: C, 77.72; H, 5.96; N, 11.82.
Found: C, 78.06; H, 6.21; N, 11.99.
4-(4-Methylphenyl)-2-phenyl-6-pyrrolidin-1-yl-pyridine-5-carbonitrile
(IIIe): m.p. 156ꢀC; 1H NMR (CDCl3) d 2.03 and 3.93 (2t, 8H), 2.43 (s, 3H),
7.09 (s, 1H), 7.31 (d, 2H), 7.44–7.47 (m, 3H), 7.49 (d, 2H), 8.06–8.09 (m,
2H); 13C NMR (CDCl3) d 21.37, 24.71, 87.24, 109.29, 119.35, 127.26,
128.54, 128.63, 128.84, 129.34, 129.92, 135.13, 138.39, 139.44, 157.58,
158.04, 158.29. Anal. Calcd. for C23H21N3: C, 81.39; H, 6.24; N, 12.38.
Found: C, 81.09; H, 6.21; N, 12.65.

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1,6-NAPHTHYRIDINES
2909
2-Amino-4,6-diphenylbenzene-1,3-carbodinitrile (IVa): m.p. 223–224ꢀC
(Ref. [8], 223–224ꢀC), spectroscopic and analytical data are in agreement
with the reported ones.[9]
2-Amino-4-(2-chlorphenyl)-6-phenylbenzene-1,3-carbodinitrile (IVb): m.p.
208–211ꢀC (Ref. [11], 213–215ꢀC), analytical data are in agreement with the
reported ones.[11] 1H NMR (CDCl3) d 5.39 (s, 2H), 6.84 (s, 1H), 7.35–7.61 (m,
9H); 13C NMR (CDCl3) d 94.92, 95.42, 115.79, 120.91, 122.74, 127.33, 128.75,
129.53, 129.77, 134.72, 135.05, 137.79, 140.01, 140.14, 151.73, 152.13, 153.92.
2-Amino-4-(4-chlorphenyl)-6-phenylbenzene-1,3-carbodinitrile
(IVc):
m.p. 249–251ꢀC (Ref. [11], 250–251ꢀC), analytical data are in agreement
with the reported ones.[11] 1H NMR (CDCl3) d 5.40 (s, 2H), 6.86 (s, 1H),
7.47–7.59 (m, 9H); 13C NMR (CDCl3) d 94.72, 95.28, 115.75, 119.88,
128.38, 128.98, 129.24, 129.75, 129.85, 135.76, 136.11, 137.24, 148.75,
150.32, 153.16.
2-Amino-4-(4-methoxyphenyl)-6-phenylbenzene-1,3-carbodinitrile (IVd):
m.p. 176–179ꢀC (Ref. [11], 178–179ꢀC), analytical data are in agreement
with the reported ones.[11] 1H NMR (CDCl3) d 3.87 (s, 3H), 5.36 (s, 2H),
6.88 (s, 1H), 7.02(d, 2H), 7.49–7.59 (m, 7H), 13C NMR (CDCl3) d 55.42,
94.95, 96.16, 111.32, 114.79, 122.45, 123.75, 126.33, 133.15, 133.96, 134.02,
136.92, 152.08, 154.25, 155.78, 159.63.
2-Amino-4-(4-methylphenyl)-6-phenylbenzene-1,3-carbodinitrile (IVe):
m.p. 202–203ꢀC (Ref. [11], 207ꢀC), analytical data are in agreement with
the reported ones.[11] 1H NMR (CDCl3) d 2.43 (s, 3H), 5.36 (s, 2H), 6.89 (s,
1H), 7.31 (d, 2H), 7.48–7.59 (m, 7H); 13C NMR (CDCl3) d 21.55, 95.01,
95.19, 116.22, 116.31, 120.29, 128.57, 128.66, 129.15, 129.88, 129.90,
134.83, 137.79, 140.23, 150.26, 150.42, 153.44.
5-Amino-4-(4-methylphenyl)-2-phenyl-7-(pyrrolidin-1-yl)-1,6-naphthyri-
dine-8-carbonitrile (Ve): m.p. 249–250ꢀC, H NMR (CDCl3) d 1.37 (t, 4H),
1
2.47 (s, 3H), 3.87 (t, 4H), 4.96 (s, 2H), 7.31–7.49 (m, 8H), 8.27–8.30 (m, 2H);
13C NMR (CDCl3) d 21.36, 25.52, 49.04, 103.71, 116.87, 120.24, 127.72,
128.32, 128.76, 129.79, 130.17, 136.53, 138.05, 139.27, 148.93, 156.88,
157.72, 157.91, 159.44. Anal. Calcd. for C26H23N5: C, 77.01; H, 5.72; N,
17.27. Found: C, 76.83; H, 5.58; N, 17.48.
ACKNOWLEDGMENTS
Our thanks are due to Dr. Eva Solcaniova for NMR measurements,
Dr. Katerina Gaplovska (both of Institute of Chemistry, Faculty of Natural
Sciences, Comenius University) for microanalysis and to the Slovak Grant
Agency VEGA for financial support (Grant No. 1/7348/20).

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2910
VEVEROKA, NOSKOVA, AND TOMA
NOTE ADDED IN PROOF
Authors of the paper in reference 1 made an unfair command. After an
exchange of letters, they have published in this journal a paper ‘‘A Facile
One Pot Synthesis of m-Terphenyl and Biaryl Derivatives’’ (Synth.
Commun. 2001, 31, 3497). They are using the same procedure as described
in reference 1, receiving different products, but they did not mention that
their results described in 1 are not correct. They did not even mention this
previous paper in the references.
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Received in the UK August 6, 2001