New Highlights in the Synthesis of 4-Aryl-1,4-dihydropyrazines

Article abstract of DOI:10.1002/jhet.989

The 4-aryl-1,4-dihydropyrazines were prepared via the cyclization of N,N-bisalkylated anilines with ammonium acetate. These reactions were aided by improvements in the synthesis of N,N-bisalkylated anilines which were alkylated with anilines using ethyl 2-diazo acetoacetate in a reaction catalyzed by rhodium acetate in the absence of oxygen. A possible mechanistic route is postulated on the basis of the isolation of the N-alkylation intermediates, which were determined to be N-aryloxamates by ¹H NMR data and X-ray diffraction.

Full text of DOI:10.1002/jhet.989

November 2012
New Highlights in the Synthesis of 4‐Aryl‐1,4‐dihydropyrazines
1357
*
Jing‐Yu He, Xiu‐Qing Song, Hong Yan, and Ru‐Gang Zhong
College of Life Science and Bio‐engineering, Beijing University of Technology, Pingleyuan Street
No. 100, Chaoyang District, Beijing 100124, People’s Republic of China
*
E-mail: hongyan@bjut.edu.cn
Received January 10, 2011
DOI 10.1002/jhet.989
View this article online at wileyonlinelibrary.com.
The 4‐aryl‐1,4‐dihydropyrazines were prepared via the cyclization of N,N‐bisalkylated anilines with
ammonium acetate. These reactions were aided by improvements in the synthesis of N,N‐bisalkylated
anilines which were alkylated with anilines using ethyl 2‐diazo acetoacetate in a reaction catalyzed by
rhodium acetate in the absence of oxygen. A possible mechanistic route is postulated on the basis of the
isolation of the N‐alkylation intermediates, which were determined to be N‐aryloxamates by 1H NMR data
and X‐ray diffraction.
J. Heterocyclic Chem., 49, 1357 (2012).
INTRODUCTION
and electron distribution. These altered electronic properties
should therefore provide us with additional information on
the synthesis and insight into the mechanism of this process.
The synthesis of 4‐aryl‐2,6‐dimethyl‐1,4‐dihydropyrazine‐
3,5‐dicarboxyl esters reported by Wehinger and Chorvat,
and improved by Sit was involved the cyclization of N,N‐
bisalkylated anilines with ammonium acetate [2, 3]. However,
two 4‐aryl‐2,6‐dimethyl‐1,4‐dihydropyrazines (Aryl = 3‐
nitrophenyl and 4‐fluoro‐3‐nitrophenyl) were prepared by Sit
with about 18% aggregate yield. The purpose of this article
is to synthesize a series of 4‐aryl‐1,4‐dihydropyrazines (3)
and elucidate the reason for the lower yields (Scheme 1).
1,4‐Dihydropyrazine is an important component of
flavin coenzymes and of several marine luciferins [1].
This compound is especially useful as a potent Neuropep-
tide Y antagonist and as a surrogate of dihydropyridine in
the study of biologically active compounds [2, 3]. The
1,4‐dihydropyrazine ring itself is unknown, various
substitutions of the ring system are required to produce
stable, isolable molecules. This has been achieved with
the placement of electron withdrawing or organometallic
groups on the nitrogen in the structure and with the
addition of bulky or conjugated substituents in other
locations [4]. In the course of our investigations on the
synthesis of nitrogen heterocyclic derivatives, we succeed
in the synthesis of 1,4‐dihydropyridines as precursors of
3,9‐diazatetraasteranes [5]. The 1,4‐dihydropyrazines are
useful structural modifications of the 1,4‐dihydropyridine
system, and these structures are substantially different from
the parent molecules in terms of molecular polarization
RESULTS AND DISCUSSION
The 4‐aryl‐1,4‐dihydropyrazine‐3,5‐dicarboxyl esters (3)
was synthesized by the cyclization of bis‐alkylated aniline
(2) with ammonium acetate; 2 was prepared by the alkyl-
ation of aniline (1) with diazo compound in the presence
of catalytic rhodium acetate dimmer (Table 1).
© 2012 HeteroCorporation

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J.-Y. He, X.-Q. Song, H. Yan, and R.-G. Zhong
Vol 49
Scheme 1
It is worth mentioning out that the yields of compounds 2
the rhodium acetate dimer was found to play an important
role in accelerating the oxidation from 4 to 5 besides
N‐alkylation of anilines with 2‐diazo acetoacetate, while 5
was not obtained from 4 in the absence of rhodium acetate
dimer after refluxing for 24 hours. Additional evidence is
the fact that oxygen takes part in the reaction leading to
the 5. When the conversion of 4 to 5 was repeated in an at-
mosphere of oxygen with the catalytic amount of rhodium
acetate dimer (0.2–0.4 mol %), the yield of the 5 increased
significantly. But there was no 5 observed in the absence of
oxygen under the same catalytic condition (Table 2). For
example, 5j was obtained in 24% yield under refluxing
conditions, 45% yield when refluxed in the presence of
oxygen, and none was obtained under an atmosphere of
nitrogen. From this data, it can be seen that it is very
important to avoid oxygen in the preparation of 2. Under
an atmosphere of nitrogen, 2 was obtained in acceptable
yields (40–69%).
seriously affected the synthesis of 3. Compounds 2a‐k were
synthesized by the N‐alkylation of anilines with 2‐diazo
acetoacetate catalyzed by rhodium acetate, and the end of
the reaction was identified by the conversion of mono‐
alkylated anilines (4) to N,N‐bisalkylated anilines (2) by
TLC. The resulting mixtures were filtered over silica gel,
eluting with ethyl acetate‐petroleum ether, and 2a‐k were
obtained in approximately 30% yields (Table 2). A significant
amount of by‐product (5) was always generated in these
reactions, sometimes equaling the amount of 2 (Scheme 2).
The structures of 5 were N‐aryl‐oxamates, which were
1
confirmed by H NMR spectroscopy and X‐ray diffraction
analysis (Fig. 1).
Based on the experimental results, 5 may be produced by
the oxidation of 4 in the presence of rhodium acetate dimer.
At the beginning of reaction, only a mono‐alkylated ani-
lines (4) was obtained after approximately 15 min, and then
5 began to emerge after approximately 40 min. To confirm
the transformation of 4 to N‐aryl‐oxamates (5), a catalytic
amount of cerium (IV) ammonium nitrate (CAN) was
added to a solution of 4. The results indicated that 4 was
completely transformed to 5. Without CAN as catalyst, no
product 5 was observed. The mechanism of the formation
of 5 could thus be similar to that of acetoacetamides
forming oxamates when exposed to cerium (IV) ammo-
nium nitrate under oxygenated conditions [7]. Moverover,
Furthermore, the electronic effects of substituents on the
anilines visibly affected the yields of 2 under an atmosphere
Table 2
Yields of 2 and 5 under different condition.a
Yieldb
(%)
Yieldc
(%)
Yieldd
(%)
Entry
X
2
5 [Lit.]e
2
5
2
5
a
b
c
d
e
f
g
h
i
H
4‐CH3
3‐CH3
4‐NO2
3‐NO2
4‐OCH3
4‐Cl
28
21
23
31
36
19
37
35
30
31
39
8 [6]
8 [6]
7 [6]
23 [6]
25 [6]
6 [6]
23 [6]
22 [6]
29 [6]
24
10
8
7
15
14
16
35
37
14
31
38
41
45
43
49
45
47
66
64
40
67
60
69
63
68
0
0
0
0
0
0
0
0
0
0
0
Table 1
The yield of 4‐aryl‐1,4‐dihydropyrazin‐3,5‐dicarboxyl esters (3).
14
12
5
15
11
13
11
18
Entry Products
X
Time (min)
Yield (%) [Lit.]
1
2
3
4
5
6
3a
3b
3c
3d
3e
3f
H
60
50
50
30
30
50
40
30
30
30
30
55 [3a]
48 [3a]
51 [3a]
69
67
33
63
60 [3a]
62
76
73
4‐CH3
3‐CH3
4‐NO2
3‐NO2
4‐OCH3
4‐Cl
3‐Cl
4‐COOC2H5
2,5‐diF
3,5‐diCF3
j
k
31 [6]
a2 not characterized; yields refer to partially purified material.
bYields under the reflux.
7
8
9
3g
3h
3i
3‐Cl
cYields in the presence of air.
4‐COOC2H5
2,5‐diF
3,5‐diCF3
dYields under an atmosphere of nitrogen.
10
11
3j
3k
eIsolated yields and their 1H NMR data were verified and compared with
those reported in the literature.
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New Highlights in the Synthesis of 4‐Aryl‐1,4‐dihydropyrazines
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Scheme 2
CONCLUSION
A series of 4‐aryl‐3,5‐bis(ethoxycarbonyl)‐2,6‐dimethyl‐
1,4‐dihydropyrazines (3) were synthesized from the reaction
of N,N‐bisalkylated aniline (2) with ammonium acetate (1),
producing the final products in fair to good yields by the im-
provement on the synthesis of 2. Ethyl N‐aryloxamates (5)
were by‐products of the N‐alkylation of anilines, which were
produced by the oxidation of mono‐alkylated anilines (4).
The yields of 2 were markedly improved in the absence of
oxygen, which is supported by the postulated mechanism.
Figure 1. ORTEP diagram of the crystal structure of compound 5i
(Drawn at the 50% thermal ellipsoids).
EXPERIMENTAL
of nitrogen. The anilines with electron‐donating substitu-
ents, the yields of 2 were low (40–47%). However, anilines
with electron‐withdrawing groups, 2 were obtained in
the higher yields (60–69%). For example, when X was
4‐CH3, 2b was obtained in 45% yield, but when X was
4‐NO2, 2d was obtained in 66% yield.
General. All chemicals and reagents were purchased
from commercial sources and used without further purification. M.
p.: X‐5 melting point apparatus (uncorrected). IR spectra: Vertex‐
70‐FTIR Spectrophotometer; KBr pellets. NMR spectra: Bruker‐
Avance‐II‐400 instrument (400 MHz) in CDCl3. MS spectra: ZAB‐
HS & ESQUIRE 6000 mass spectrometer. Elemental analyses:
CARLO ERBA 1106 elemental analysis instrument. X‐ray
diffractions were recorded on a Siemens P4 or Simart‐1000
diffractometer. Flash chromatography was carried out on silica gel
A speculated mechanism for the formation of 5 is
described in Scheme 3.
Scheme 3
Journal of Heterocyclic Chemistry
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J.-Y. He, X.-Q. Song, H. Yan, and R.-G. Zhong
Vol 49
(Merck 230‐400 mesh). All reactions were monitored using thin layer
chromatography (TLC) on silica gel plates (Merck 60F254).
33%. IR (KBr): 2975, 1672, 1634, 1505, 1473, 1370, 1330, 1303,
1
1243, 1181, 1105, 1058, 1039 cm−1. H NMR (CDCl3, 400 MHz)
δ: 1.24–1.28 (m, 6H, CH3), 2.38 (s, 6H, CH3), 3.73 (s, 3H,
OCH3), 4.20–4.26 (m, 4H, CH2), 5.74 (s, 1H, NH), 6.73 (s, 4H,
Ar-H). 13C NMR (CDCl3, 100 MHz) δ: 14.4, 17.8, 55.1, 60.3,
99.7, 104.6, 109.6, 129.2, 148.1, 152.1, 160.1, 166.0. MS (ESI):
m/z (%) = 361.1 [M−1]+. Anal. Calcd. for C19H24N2O5: C 63.32,
H 6.71, N 7.71; Found C 63.31, H 6.70, N 7.75.
General procedure for the synthesis of compounds (2). A
mixture of aniline (2.0 mmol), rhodium acetate dimmer (2.6 mg, 0.3
mol %) and 10 mL of dry benzene was warmed to reflux under an
atmosphere of nitrogen. A solution of ethyl 2‐diazo acetoacetate [8]
(5.0 mmol) in 10 mL benzene was added dropwise to the refluxing
solution over a period of 30 min. The resulting mixture was heated
to reflux until all of the mono‐alkylated anilines (4) was converted
into compound 2 within 60–100 min, as monitored by TLC. After
cooling the solution to room temperature, the volatile components
were removed in vacuo and the residue was purified by flash
chromatography using petroleum ether and EtOAc as eluents. 2 was
instable and used without further purification for the synthesis of 3.
3,5‐Bis(ethoxycarbonyl)‐4‐(4‐chlorophenyl)‐2,6‐dimethyl‐
1,4‐dihydropyrazine (3g). Pale yellow solid, mp. = 212.8–214.3°
C, yield 63%. IR (KBr): 3434, 3291, 1694, 1668, 1635, 1592,
1
1329, 1300, 1193, 1108, 814, 776 cm−1. H NMR (CDCl3, 400
MHz) δ: 1.24–1.27 (m, 6H, CH3), 2.41 (s, 6H, CH3), 4.21–4.26
(m, 4H, CH2), 5.79 (s, 1H, NH), 6.61 (d, 2H, J = 9.2 Hz,
Ar-H), 7.08 (d, 2H, J = 9.2 Hz, Ar-H). 13C NMR (CDCl3, 100
MHz) δ: 14.3, 17.8, 60.3, 109.4, 114.3, 124.3, 128.3, 148.4,
149.0, 165.8. MS (ESI): m/z (%) = 364.1 [M]+. Anal. Calcd. for
C18H21ClN2O4: C 59.26, H 5.80, N 7.68; Found C 59.21, H
5.81, N 7.70.
3,5‐Bis(ethoxycarbonyl)‐4‐(3‐chlorophenyl)‐2,6‐dimethyl‐1,4‐
dihydropyrazine (3h). Pale yellow solid, mp. = 181.2–183.0°C (lit
[3a], 214°C), yield (0.22g, 60%). 1H NMR (CDCl3, 400 MHz) δ:
1.26–1.30 (m, 6H, CH3), 2.43 (s, 6H, CH3), 4.25–4.26 (m, 4H,
CH2), 6.02 (s, 1H, NH), 6.56–7.08 (m, 4H, Ar-H).
General procedure for the synthesis of compounds (3).
Compound 2 (1.0 mmol) was heated to reflux with ammonium
acetate (1.1 mmol, 84.7 mg) in tert‐butanol (10 mL). The reaction
progress was monitored by TLC. The solvent was removed in
vacuo and the residue was recrystallized from ethyl acetate and
hexane to give pure compound 3.
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐phenyl‐1,4‐dihydropyrazine
(3a). Pale yellow solid, mp. = 226.5–227.7°C (lit [3a], 228°C), yield
1
(0.18 g, 55%). H NMR (CDCl3, 400 MHz) δ: 1.23–1.26 (m, 6H,
CH3), 2.40 (s, 6H, CH3), 4.21–4.26 (m, 4H, CH2), 5.80 (s, 1H, NH),
6.70 (d, 2H, J = 8.0 Hz, Ar-H), 6.70 (d, 2H, J = 9.2 Hz, Ar-H),
6.82–6.85 (m, 1H, Ar-H), 7.13–7.17 (m, 2H, Ar-H).
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(4‐methylphenyl)‐1,4‐
dihydropyrazine (3b). Yellow solid, mp. = 151.9–153.1°C (lit [3a],
211°C), yield (0.16 g, 48%). 1H NMR (CDCl3, 400 MHz) δ:
1.23–1.27 (m, 6H, CH3), 2.23 (s, 3H, CH3, Ar-CH3), 2.38 (s, 6H,
CH3), 4.20–4.25 (m, 4H, CH2), 5.76 (s, 1H, NH), 6.61 (d, 2H,
J = 8.4 Hz, Ar-H), 6.98 (d, 2H, J = 8.4 Hz, Ar-H).
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(4‐(ethoxycarbonyl)
phenyl)‐1,4‐dihydropyrazine (3i). Pale yellow solid, mp. =
198.6–200.3°C, yield 62%. IR (KBr): 3290, 2975, 1710, 1671,
1
1635, 1603, 1509, 1423, 1375, 1266, 1105, 1024, 768 cm−1. H
NMR (CDCl3, 400 MHz) δ: 1.23–1.27 (m, 6H, CH3), 1.34–1.38
(m, 3H, CH3), 2.44 (s, 6H, CH3), 4.23–4.26 (m, 4H, CH2),
4.29–4.35 (m, 2H, CH2), 6.11 (s, 1H, NH), 6.65 (d, 2H, J = 9.2
Hz, Ar-H), 7.84 (d, 2H, J = 9.2 Hz, Ar-H). 13C NMR (CDCl3,
100 MHz) δ: 14.3, 14.4, 17.5, 60.3, 60.4, 108.3, 111.9, 120.9,
130.4, 148.7, 153.8, 165.4, 167.0. MS (ESI): m/z (%) = 402.0
[M]+. Anal. Calcd. for C21H26N2O6: C 62.67, H 6.51, N 6.96;
Found C 62.69, H 6.52, N 6.98.
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(3‐methylphenyl)‐1,4‐
dihydropyrazine (3c). Pale yellow solid, mp. = 160.8–161.4°C
(lit [3a], 176°C), yield (0.17 g, 51%). 1H NMR (CDCl3, 400 MHz)
δ: 1.24–1.27 (m, 6H, CH3), 2.25 (s, 3H, CH3, Ar-CH3), 2.38
(s, 6H, CH3), 4.21–4.26 (m, 4H, CH2), 5.29 (s, 1H, NH), 6.49
(s, 1H, Ar-H), 6.64 (d, 1H, J = 7.2 Hz, Ar-H), 7.05 (m, 2H, Ar-H).
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(4‐nitrophenyl)‐1,4‐
dihydropyrazine (3d). Yellow solid, mp. = 212.8–214.3°C, yield
69%. IR (KBr): 2983, 1699, 1672, 1638, 1596, 1560, 1543, 1500,
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(2,5‐bis(fluorophenyl))‐
1,4‐dihydropyrazine (3j). Pale yellow solid, mp. = 143.2–145.4°C,
yield 76%. IR (KBr): 3316, 2982, 2932, 1696, 1624, 1503, 1470,
1
1370, 1309, 1188, 1108, 769 cm−1. H NMR (CDCl3, 400 MHz)
δ: 1.26‐1.29 (m, 6H, CH3), 2.39 (s, 6H, CH3), 4.20‐4.25 (m, 4H,
CH2), 5.99 (s, 1H, NH), 6.45‐6.91 (m, 3H, Ar-H). MS (ESI): m/z
(%) = 366.1 [M]+. Anal. Calcd. for C18H20F2N2O4 : C 59.01, H
5.50, N 7.65; Found C 59.15, H 5.51, N 7.67.
1373, 1317, 1280, 1194, 1107, 1105, 1063 cm−1 1H NMR
.
(CDCl3, 400 MHz) δ: 1.23–1.26 (m, 6H, CH3), 2.45 (s, 6H,
CH3), 4.21–4.27 (m, 4H, CH2), 6.52 (s, 1H, NH), 6.63 (d, 2H,
J = 9.2 Hz, Ar-H), 8.04 (d, 2H, J = 9.2 Hz, Ar-H). 13C NMR
(CDCl3, 100 MHz) δ: 14.3, 17.5, 60.6, 107.6, 111.7, 125.1,
139.8, 149.0, 154.8, 164.7. MS (ESI): m/z (%) = 375.0 [M]+.
Anal. Calcd. for C18H21N3O6: C 57.59, H 5.64, N 11.19; Found
C 57.65, H 5.65, N 11.17.
3,5‐Bis(ethoxycarbonyl)‐4‐(3,5‐bis(trifluoromethyl)phenyl)‐2,6‐
dimethyl‐1,4‐dihydropyrazine (3k). Pale yellow solid, mp. =
187.8–189.2°C, yield 73%. IR (KBr): 3287, 2988, 2918, 1704, 1669,
1615, 1378, 1278, 1182, 1124, 1108, 865, 778, 699 cm−1. 1H NMR
(CDCl3, 400 MHz) δ: 1.25–1.28 (m, 6H, CH3), 2.49 (s, 6H, CH3),
4.20–4.23 (m, 4H, CH2), 5.98 (s, 1H, NH), 7.02 (s, 2H, Ar-H), 7.31
(s, 1H, Ar-H). 13C NMR (CDCl3, 100 MHz) δ: 14.2, 17.9, 60.6,
108.2, 112.3, 112.5, 122.2, 124.9, 131.4, 131.7, 149.2, 150.6, 164.7.
MS (ESI): m/z (%) = 466.0 [M]+. Anal. Calcd. for C20H20F6N2O4 :
C 51.51, H 4.32, N 6.01; Found C 51.40, H 4.33, N 6.02.
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(3‐nitrophenyl)‐1,4‐
dihydropyrazine (3e). Yellow solid, mp. = 212–214°C, yield
67%. IR (KBr): 3293, 1696, 1668, 1531, 1332, 1193, 1109,
1
779, 734 cm−1. H NMR (CDCl3, 400 MHz) δ: 1.25–1.28 (m,
6H, CH3), 2.46 (s, 6H, CH3), 4.24–4.31 (m, 4H, CH2), 5.91 (s,
1H, NH), 6.95–7.68 (m, 4H, Ar-H). 13C NMR (CDCl3, 100
MHz) δ: 14.3, 17.8, 60.5, 107.8, 108.6, 114.0, 118.8, 129.0,
148.7, 149.1, 151.1, 165.1. MS (ESI): m/z (%) = 375.0 [M]+.
Anal. Calcd. for C18H21N3O6: C 57.59, H 5.64, N 11.19; Found
C 57.60, H 5.66, N 11.21.
General procedure for the synthesis of compound (5).
Compound 5 was synthesized by the alkylation of aniline with ethyl
ethyl 2‐diazo acetoacetate, as in the procedure used to synthesize
compound 2 in the presence of air, which was bubbled into the
reaction mixture at the speed of 2 L/h. The resulting residue was
purified by flash chromatography to give compound 5, using
petroleum ether and EtOAc as eluents.
3,5‐Bis(ethoxycarbonyl)‐2,6‐dimethyl‐4‐(4‐methoxyphenyl)‐1,4‐
dihydropyrazine (3f). Pale yellow solid, mp. = 171.4–171.8°C, yield
Journal of Heterocyclic Chemistry
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New Highlights in the Synthesis of 4‐Aryl‐1,4‐dihydropyrazines
1361
REFERENCES AND NOTES
Crystallographic data of 5i. Crystallographic data for the
structures of 5i reported in this article have been deposited with
the Cambridge Crystallographic Data Centre as supplementary
publication no. CCDC 753366. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [fax.: (internet) + 44 1223/
336‐033; e‐mail: deposit@ccdc.cam.ac.uk].
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet