Microwave-assisted synthesis of quinoxalines in peg-400

Article abstract of DOI:10.1080/00397911.2010.496134

A rapid and efficient procedure for the synthesis of quinoxaline derivatives has been achieved by condensation of aryl-1,2-diamines with 1,2-dicarbonyl compounds catalyzed by polyethylene glycol under CEM-focused microwave-irradiation conditions. Copyright

Full text of DOI:10.1080/00397911.2010.496134

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Microwave-Assisted Synthesis of
Quinoxalines in PEG-400
Xia-Zhong Zhang a , Jin-Xian Wang a & Lin Bai b
a Institute of Chemistry, Department of Chemistry, Northwest
Normal University, Lanzhou, China
b Institute of Green Chemistry Experiment and Teaching, Lanzhou
City University, Lanzhou, China
Published online: 20 May 2011.
To cite this article: Xia-Zhong Zhang , Jin-Xian Wang & Lin Bai (2011): Microwave-Assisted
Synthesis of Quinoxalines in PEG-400, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, 41:14, 2053-2063
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Synthetic Communications1, 41: 2053–2063, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.496134
MICROWAVE-ASSISTED SYNTHESIS OF
QUINOXALINES IN PEG-400
Xia-Zhong Zhang,1 Jin-Xian Wang,1 and Lin Bai2
1Institute of Chemistry, Department of Chemistry, Northwest Normal
University, Lanzhou, China
2Institute of Green Chemistry Experiment and Teaching, Lanzhou City
University, Lanzhou, China
GRAPHICAL ABSTRACT
Abstract A rapid and efficient procedure for the synthesis of quinoxaline derivatives has
been achieved by condensation of aryl-1,2-diamines with 1,2-dicarbonyl compounds cata-
lyzed by polyethylene glycol under CEM-focused microwave-irradiation conditions.
Keywords Microwave; polyethylene glycol; quinoxaline derivatives
INTRODUCTION
Quinoxaline derivatives are an important class of nitrogen-containing hetero-
cycles and constitute useful intermediates in organic synthesis. They display a broad
spectrum of biological activities, which has made them privileged structures in phar-
macologically active compounds.[1] Besides these, they are well known for their
application in rigid subunits in macrocyclic receptors or molecular recognition,[2]
efficient electroluminescent materials,[3] building blocks for the synthesis of anion
receptors,[4] cavitands,[5] and DNA cleaving agents.[6] Considering the significant
applications in the fields of medicinal, industrial, and synthetic organic chemistry,
Received August 3, 2009.
Address correspondence to Jin-Xian Wang, Institute of Chemistry, Department of Chemistry,
Northwest Normal University, 967 Anning Road (E), Lanzhou 730070, China. E-mail: wangjx@
nwnu.edu.cn
2053

2054
X.-Z. ZHANG, J.-X. WANG, AND L. BAI
there have been tremendous interest in developing efficient methods for the synthesis
of quinoxalines. Improved methods have been reported, including the Bi-catalyzed
oxidative coupling reaction[7] and condensation processes using cerium ammonium
nitrate (CAN),[8] CuSO4 ꢀ 5H2O,[9] SA=MeOH,[10] or manganese octahedral molecu-
lar sieves[11] as catalysts or promoters. Although great success has been obtained,
many of these processes suffer from drawbacks such as drastic reaction conditions,
poor product yields, tedious workup procedures, using toxic metal salts as catalysts,
or relatively expensive reagents. Hence, the search for the better method is still being
actively pursued.
Recently, microwave-assisted organic synthesis has had a profound impact on
the way that chemists approach organic and parallel synthesis. In many instances,
CEM focused microwave heating under sealed vessel conditions has had dramatical
influences on reducing reaction times, increasing product yields, improving selec-
tivity, using less solvent, and enhancing product purities by reducing unwanted side
reactions compared to conventional synthetic methods. At the same time, attention
has also focused on solvent-free conditions. Solvent-free methods are especially
adapted to organic synthesis under green chemistry conditions; when coupled to
microwave (MW) irradiation, they result in very efficient and clean procedures with
noticeable improvements over classical methods.[12]
Polyethylene glycol (PEG) is a nontoxic, inexpensive, thermally stable, recov-
erable, and biologically acceptable reagent, which represents a very attractive
medium for organic reactions and has been widely applied in biotechnology, medi-
cine, and organic synthesis.[13] In our continuous search for developing new pro-
cesses using PEG as catalyst or microwave irradiation conditions,[14] we report a
green and efficient method for the synthesis of quinoxaline derivatives in good to
excellent yields by the condensation of 1,2-diamines with 1,2-dicarbonyl compounds
catalyzed by PEG-400 under microwave irradiation conditions.
RESULTS AND DISCUSSION
As a starting point for the development of microwave-promoted synthesis of
quinoxaline derivatives, we initially studied the condensation of benzil and
4-methylbenzene-1,2-diamine using a CEM focused microwave for optimizing reac-
tion conditions. The results are summarized in Table 1. It can be gathered that poor
yield of the desired product was observed when CH3COOH, HCl, or H2SO4 was
used as catalyst (Table 1, entries 1–3). When NiCl2, ZnCl2, CoCl2, or FeCl3 were
chosen as the catalyst, the condensation reaction proceeded smoothly, and the
desired product was obtained in moderate yield (Table 1, entries 4–7). When
PEG-2000 was used, to our surprise, a satisfactory yield was obtained (Table 1, entry
9). In an effort to obtain improved yields, we screened kinds of PEG with different
molecular weights to catalyze the reaction, and PEG-400 was found to be the best
catalyst for the condensation reaction (Table 1, entries 8–11).
Next, we studied the effects of temperature and power on the yields of the reac-
tion, and the results are summarized in Table 2. It was found that there was no sig-
nificant increase in the yield of the desired product even after prolonged irradiation
time when the temperature was 100 ꢁC at low power (Table 2, entries 1 and 2). For-
tunately, elevating the power has noticeable effect, and 200 W is the best power for

MICROWAVE-ASSISTED SYNTHESIS OF QUINOXALINES
2055
Table 1. Effects of different catalysts on the condensation of benzil and
4-methylbenzene-1,2-diaminea
Entry
Catalyst (mol%)
Yieldb (%)
1
2
CH3COOH (15)
HCl (15)
H2SO4 (15)
35
20
32
52
41
43
55
71
75
85
97
70
78
97
3
4
NiCl2 (15)
FeCl3 (15)
5
6
ZnCl2 (15)
CoCl2 (15)
7
8
PEG-10000 (15)
PEG-2000 (15)
PEG-600 (15)
PEG-400 (15)
PEG-400 (5)
PEG-400 (10)
PEG-400 (20)
9
10
11
12
13
14
aReaction conditions: 4-methylbenzene-1,2-diamine (2.0 mmol), benzil (2.0 mmol),
and catalyst irradiating under CEM focused microwave for 3 min at 200 W and
120 ꢁC.
bIsolated yield.
the condensation (Table 2, entries 3 and 4). Further studies revealed that 120 ꢁC is
necessary to obtain a good yield of the condensation product. It is clear that the opti-
mized conditions for the condensation of benzil and 4-methylbenzene-1,2-diamine
are 120 ꢁC and 200 W.
Comparative reactions were also carried out in conventional heating con-
ditions and an improved reflux microwave oven[15] that cannot control the tempera-
ture. The results of the impact of the improved reflux microwave oven and
conventional heating for the yield are summarized in Table 2. It is clear that the yield
Table 2. Optimized conditions via the condensation of benzil and 4-methylbenzene-1,2-diaminea
Entry
Power (W)
Temperature (ꢁC)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
100
150
200
220
200
200
200
600
—
120
120
120
120
110
130
—
8
8
NR
40
3
97
96
3
3
87
3
70
3
NRc
84c
10d
—
120
3
4h
aReaction conditions: 4-methylbenzene-1,2-diamine (2.0 mmol), benzil (2.0 mmol), and PEG (0.3 mmol)
irradiating under focused microwaves.
bIsolated yield.
cReactions were performed with an improved reflux microwave oven.
dConventional heating was used, and the reaction was stirred in an oil bath at 120 ꢁC.

2056
X.-Z. ZHANG, J.-X. WANG, AND L. BAI
Scheme 1. Synthesis of quinoxalines using different amines and 1,2-diketones.
of the product under microwave irradiation is greater than with conventional heating
(Table 2, entry 9). In contrast to the improved reflux microwave oven, the CEM
focused microwave can increase the yield because it can efficiently stir the substrates
and accurately control the temperature and power (Table 2, entries 7 and 8).
Encouraged by the remarkable results, we examined the substrate scope of this
reaction by using various 1,2-diamines and 1,2-dicarbonyl compounds (Scheme 1),
and the results are summarized in Table 3. We first tried the reactions of benzil with
various 1,2-diamines under the optimized conditions (Table 3, entries 1–8).
From Table 3, it can be seen that either diamines with electron-withdrawing
groups or electron-donating groups give the corresponding products in excellent
yields, but substrates bearing electron-withdrawing groups give lower yields than
substrates bearing electron-donating groups. For example, 4-methylbenzene-1,2-
diamine gives
a greater yield than (3,4-diminophenyl)-(phenyl) methanone
and 4-nitrobenzene-1,2-diamine (Table 3, entries 2–5). Aliphatic and alicyclic
1,2-diamines also react smoothly under our conditions (Table 3, entries 6–8).
Table 3. Synthesis of quinoxalines using different amines and 1,2-diketonesa
Entry
Amines
R2
Products
Time (min)
Yieldb (%)
1
Ph
3
96
2
3
Ph
Ph
5
5
94
92
(Continued )

MICROWAVE-ASSISTED SYNTHESIS OF QUINOXALINES
2057
Table 3. Continued
Entry
Amines
R2
Products
Time (min)
Yieldb (%)
4
Ph
4
94
5
6
Ph
3
3
3
3
4
4
5
5
97
85
83
94
94
95
90
Ph
7
Ph
8
Ph
9
2-Py
2-Py
2-Py
2-Py
10
11
12
91
(Continued )

2058
X.-Z. ZHANG, J.-X. WANG, AND L. BAI
Table 3. Continued
Entry
13
Amines
R2
Products
Time (min)
Yieldb (%)
2-Py
3
90
14
CH3
3
95
15
16
17
CH3
CH3
CH3
3
3
3
92
93
90
18
CH3
3
93
aAll reactions were conducted in the same conditions: amines (2 mmol), ketones (2 mmol), PEG
(0.3 mmol), P ¼ 200 W if both the substrates are solid or P ¼ 10 W if one of the substrates is liquid, at
120 ꢁC for the appropriate time.
bIsolated yields.
Similarly, when 2,20-pyridil instead of benzil, the desired quinoxalines were also
obtained in good yields (Table 3, entries 9–13). Further studies indicated that biace-
tyl also provide the corresponding quinoxalines in high yields at 120 ꢁC and 10 W
(Table 3, entries 14–18). In fact, we found that when one of the substrates is liquid,
such as biacetyl, 10 W of microwave irradiation is sufficient. However, if both the
substrates are solid, 200 W is indispensable to obtain good yield.
In conclusion, we describe a simple, efficient, and ecofriendly method for the
synthesis of quinoxalines from various 1,2-diketones and 1,2-diamines in the pres-
ence of PEG-400 under CEM focused microwave-irradiation conditions. The present
method has the advantages of mild conditions, short reaction time, simple operation,
and excellent product yields, which makes it a useful and important addition to the
existing methods.
EXPERIMENTAL
Infrared (IR) spectra were obtained by using an Alpha Centauri Fourier trans-
form (FT)–IR spectrophotometer. Mass spectra were recorded on an HP 5988A and
GC=MS=DS instrument. Melting points were determined with an X-4 apparatus and
1
were not corrected. H (400 MHz) and 13C (100 MHz) NMR spectra were obtained

MICROWAVE-ASSISTED SYNTHESIS OF QUINOXALINES
2059
for CDCl3 solutions with tetramethylsilane (TMS) as an internal standard at room
temperature.
General Procedure for the Synthesis of Quinoxalines
A mixture of 1,2-diketone (2 mmol) and 1,2-diamine (2 mmol) and a magnetic
stir bar were placed in a 10-mL glass tube. The vessel was sealed with a septum and
placed into the microwave cavity. When microwave irradiation of 10 W or 200 W
was used, the temperature was ramped from room temperature to 120 ꢁC. Then
the reaction mixture was irradiated at this temperature for several minutes. After
completion of the reaction, the mixture was cooled to room temperature, and the
reaction vessel was opened. The product was recrystallized from acetone and col-
lected by filtration or purified by column chromatography on silica gel using pet-
roleum ether and acetone (10:1 v=v) as the eluent to give the analytically pure
1
quinoxalines. All products were well characterized using mp, IR, H NMR, 13C
NMR, and MS.
Characterization Data of All the Products
2,3-Diphenylquinoxaline 3a. White solid; mp (ꢁC): 129 (lit.[8] 126–127). IR
(t=cmꢂ1): 3057, 2928, 1633, 1439, 1340, 1055, 766, 692; 1H NMR (400 MHz, CDCl3):
d ¼ 8.18 (dd, J ¼ 6.4, 3.6 Hz, 2H), 7.77 (dd, J ¼ 6.0, 3.6 Hz, 2H), 7.53 (dd, J ¼ 4.0,
1.6 Hz, 4H), 7.39–7.30 (m, 6H); 13C NMR (100 MHz, CDCl3): d ¼ 153.5, 141.2,
139.0, 130.0, 129.8, 129.2, 128.8, 128.3. MS m=z (%): 282 (Mþ, 100), 179 (48), 76 (53).
6-Nitro-2,3-diphenylquinoxaline 3b. Brown solid; mp (ꢁC): 190–191 (lit.[9]
193–194). IR (t=cmꢂ1): 3055, 1614, 1520, 1340, 1053, 766, 698; 1H NMR
(400 MHz, CDCl3): d ¼ 9.07 (s, 1H), 8.53 (d, J ¼ 9.2 Hz, 1H), 8.30 (d, J ¼ 8.8 Hz,
1H), 7.58–7.45 (m, 4H), 7.43–7.26 (m, 6H); 13C NMR (100 MHz, CDCl3):
d ¼ 156.3, 155.7, 147.8, 143.6, 139.9, 138.1, 138.0, 130.7, 129.9, 129.8, 129.8, 129.7
129.6, 128.4, 125.6, 123.3. MS m=z (%): 327 (Mþ, 34), 281 (22), 178 (46), 75 (100).
Phenyl(2,3-diphenylquinoxalin-6-yl)methanone 3c. White solid; mp (ꢁC):
146. IR (t=cmꢂ1): 3053, 1658, 1605, 1435, 1394, 1266, 767, 693; 1H NMR (400 MHz,
CDCl3): d ¼ 8.54 (s, 1H), 8.29 (s, 2H), 7.91 (d, J ¼ 7.2 Hz, 2H), 7.64 (t, J ¼ 7.2 Hz,
1H), 7.57–7.51 (m, 6H), 7.42–7.32 (m, 6H); 13C NMR (100 MHz, CDCl3):
d ¼ 195.8, 155.1, 154.6, 143.0, 140.2, 138.6, 138.6, 138.3, 137.2, 132.8, 132.5, 130.1,
129.9, 129.8, 129.7, 129.7, 129.6, 129.3, 129.1, 128.5, 128.4. MS m=z (%): 386 (Mþ,
39), 105 (100), 77 (81).
7-Bromo-2,3-diphenylpyrido[2,3-b]pyrazine 3d. White solid; mp (ꢁC):
150–151 (lit.[16] 149–150). IR (t=cmꢂ1): 3052, 3031, 2943, 1627, 1439, 1338, 1029,
782, 658; 1H NMR (400 MHz, CDCl3): d ¼ 9.16 (d, J ¼ 2.4 Hz, 1H), 8.68 (d,
J ¼ 2.8 Hz, 1H), 7.63–7.53 (m, 2H), 7.44–7.40 (m, 2H), 7.39–7.27 (m, 6H); 13C
NMR (100 MHz, CDCl3): d ¼ 156.5, 155.5, 155.1, 148.2, 139.4, 138.1, 137.8, 136.4,
130.2, 129.8, 129.7, 129.6, 128.4, 128.2, 120.9. MS m=z (%): 361 (Mþ, 35), 179
(45), 127 (34), 76 (100).

2060
X.-Z. ZHANG, J.-X. WANG, AND L. BAI
6-Methyl-2,3-diphenyoquinoxaline 3e. White solid; mp (ꢁC): 112 (lit.[9]
116–117). IR (t=cmꢂ1): 3056, 3030, 2940, 1617, 1444, 1340, 1019, 772, 698; 1H
NMR (400 MHz, CDCl3): d ¼ 8.06 (d, J ¼ 8.8 Hz, 1H), 7.95 (s, 1H) 7.60 (d,
J ¼ 8.4 Hz, 1H), 7.52–7.49 (m, 4H), 7.35–7.30 (m, 6H), 2.61 (s, 3H); 13C NMR
(100 MHz, CDCl3): d ¼ 153.3, 152.5, 141.3, 140.5, 139.7, 139.2, 132.3, 129.8, 129.8,
129.7, 128.7, 128.7, 128.6, 128.2, 128.0, 109.7, 21.9. MS m=z (%): 296 (Mþ, 100),
192 (17), 89 (33).
2,3-Dihydro-5,6-diphenylpyrazine 3f. Pale yellow solid; mp (ꢁC): 165–166.
1
IR (t=cmꢂ1): 3077, 2943, 1550, 1437, 1259, 985, 764, 694; H NMR (400 MHz,
CDCl3): d ¼ 7.40 (d, J ¼ 8.0 Hz, 4H), 7.33–7.30 (m, 2H), 7.27–7.23 (m, 4H), 3.69
(m, 4H); 13C NMR (100 MHz, CDCl3): d ¼ 160.3, 137.7, 129.6, 128.1, 127.9, 45.8.
MS m=z (%): 234 (Mþ, 42), 103 (100), 77 (25).
2-Methyl-5,6-diphenyl-2,3-dihydropyrazine 3g. Pale yellow solid; mp
(ꢁC): 120 (lit.[17] 116–118). IR (t=cmꢂ1): 3027, 2935, 1555, 1439, 1290, 984, 765,
1
695; H NMR (400 MHz, CDCl3): d ¼ 7.39 (d, J ¼ 8.0 Hz, 4H), 7.35–7.22 (m, 6H),
4.02 (dd, J ¼ 16.0, 5.2 Hz, 1H), 3.52–3.46 (m, 1H), 3.15 (t, J ¼ 15.2 Hz, 1H), 1.47
(d, J ¼ 5.6 Hz, 3H); 13C NMR (100 MHz, CDCl3): d ¼ 160.3, 159.2, 137.8, 137.7,
129.5, 129.5, 128.1, 128.1, 128.0, 127.8, 52.1, 50.9, 19.0. MS m=z (%): 248 (Mþ,
16), 233 (8), 104 (100), 77 (24).
2,3-Diphenyl-4a,5,6,7,8,8a-hexzhydroquinoxaline 3h. Yellow solid; mp
(ꢁC): 170–171 (lit.[17] 167–169). IR (t=cmꢂ1): 3036, 2923, 1575, 1427, 1390, 1150,
1
995, 792, 698; H NMR (400 MHz, CDCl3): d ¼ 7.40–7.33 (m, 3H), 7.30–7.21 (m,
7H), 2.84 (t, J ¼ 4.4 Hz, 2H), 2.51 (d, J ¼ 14.0 Hz, 2H), 1.85 (d, J ¼ 3.2 Hz, 2H),
1.70–1.59 (m, 2H), 1.48–1.41 (m, 2H); 13C NMR (100 MHz, CDCl3): d ¼ 159.6,
137.8, 129.4, 128.1, 128.0, 59.5, 33.5, 25.4. MS m=z (%): 288 (Mþ, 44), 104 (100),
77 (23).
2,3-Di(pyridin-2-yl)quinoxaline 3i. Brown solid; mp (ꢁC): 185–186 (lit.[18]
183–184). IR (t=cmꢂ1): 3052, 2941, 1516, 1355, 1066, 995, 793; 1H NMR
(400 MHz, CDCl3): d ¼ 8.39–8.38 (m, 2H), 8.25–8.21 (m, 2H), 7.96 (d, J ¼ 8.0 Hz,
2H), 7.85–7.80 (m, 4H), 7.27–7.23 (m, 2H); 13C NMR (100 MHz, CDCl3):
d ¼ 157.4, 152.4, 148.6, 141.1, 136.6, 130.5, 129.4, 124.2, 123.0. MS m=z (%): 283
(Mþ ꢂ 1, 100), 179 (14), 78 (29).
6-Methyl-2,3-di(pyridin-2-yl)quinoxaline 3j. White solid; mp (ꢁC): 146. IR
1
(t=cmꢂ1): 3047, 3003, 2942, 1585, 1470, 1349, 1075, 994, 792; H NMR (400 MHz,
CDCl3): d ¼ 8.38–8.37 (m, 2H), 8.11 (d, J ¼ 4.8 Hz, 1H), 8.10 (s, 1H), 7.93 (t,
J ¼ 7.6 Hz, 2H), 7.83–7.78 (m, 2H), 7.66–7.21 (m, 1H), 7.26–7.21 (m, 2H), 2.64 (s,
3H); 13C NMR (100 MHz, CDCl3): d ¼ 157.5, 152.3, 151.6, 148.6, 148.6, 141.2,
141.2, 141.1, 139.6, 136.6, 132.8, 128.9, 128.2, 124.2, 124.2, 122.9, 122.8, 21.9. MS
m=z (%): 297 (Mþ ꢂ 1, 100), 89 (29), 78 (36).
6-Nitro-2,3-di(pyridin-2-yl)quinoxaline 3k. Yellow solid; mp (ꢁC): 191. IR
(t=cmꢂ1): 3068, 1581, 1519, 1348, 1061, 991, 814, 748; 1H NMR (400 MHz, CDCl3):
d ¼ 9.13 (s, 1H), 8.59 (d, J ¼ 9.2 Hz,1H), 8.38–8.35 (m, 3H), 8.09–8.05 (m, 2H),
7.91–7.86 (m, 2H), 7.31–7.27 (m, 2H); 13C NMR (100 MHz, CDCl3): d ¼ 156.5,

MICROWAVE-ASSISTED SYNTHESIS OF QUINOXALINES
2061
156.5, 155.3, 154.7, 148.7, 148.6, 148.2, 143.3, 139.9, 136.9, 136.8, 130.9, 125.7, 124.3,
124.2, 123.8, 123.6, 123.5. MS m=z (%): 329 (Mþ, 54), 282 (8), 78 (100).
Phenyl(2,3-di(pyridin-2-yl)quionxalin-6-yl)methanone 3l. Pale yellow
solid; mp (ꢁC): 164–166. IR (t=cmꢂ1): 3078, 2935, 1854, 1613, 1550, 1440, 1284,
1
1059, 977, 764, 692; H NMR (400 MHz, CDCl3): d ¼ 8.59 (s, 1H), 8.40–8.31 (m,
4H), 8.05–8.02 (m, 1H), 7.97–7.80 (m, 5H), 7.67–7.63 (m, 1H), 7.54 (t, J ¼ 6.8 Hz,
2H), 7.29–7.24 (m, 2H); 13C NMR (100 MHz, CDCl3): d ¼ 195.7, 157.0, 156.9,
154.1, 153.6, 148.7, 142.9, 140.1, 138.8, 137.0, 136.8, 136.7, 132.9, 132.5, 130.4,
130.1, 129.9, 128.5, 124.3, 124.2, 123.3, 123.2. MS m=z (%):387 (Mþ ꢂ 1, 84), 105
(54), 77 (100). Anal. calcd. for C25H1N4O: C, 77.30; H, 4.15; N, 14.42. Found: C,
77.72; H, 4.05; N, 14.71.
4a,5,6,7,8,8a-Hexahydro-2,3-di(pyridin-2-yl)quinoxline 3m. White solid;
mp (ꢁC): 188–189. IR (t=cmꢂ1): 3043, 2925, 1585, 1427, 1390, 1178, 1150, 995,
792; 1H NMR (400 MHz, CDCl3): d ¼ 8.46–8.45 (m, 2H), 7.69–7.65 (m, 2H),
7.60–7.58 (m, 2H), 7.27–7.17 (m, 2H), 3.11 (s, 4H), 2.17–1.99 (m, 4H); 13C NMR
(100 MHz, CDCl3): d ¼ 157.1, 151.6, 149.0, 136.2, 124.3, 122.6, 77.2, 31.9, 22.7.
MS m=z (%): 289 (Mþ, 100), 77 (26). Anal. calcd. for C18H18N4: C, 74.46; H,
6.25; N, 19.30. Found: C, 74.72; H, 6.05; N, 19.91.
2,3,6-Trimethylquinoxaline 3n. White solid; mp (ꢁC): 93. IR (t=cmꢂ1):
1
3015, 2925, 1623, 1157, 987, 831, 571, 430; H NMR (400 MHz, CDCl3): d ¼ 7.86
(d, J ¼ 8.0 Hz, 1H), 7.75 (s, 1H), 7.49 (d, J ¼ 8.8 Hz, 1H), 2.71 (s, 6H), 2.56 (s,
3H); 13C NMR (100 MHz, CDCl3): d ¼ 153.3, 152.4, 141.1, 139.5, 139.1, 131.0,
127.8, 127.2, 23.1, 23.0, 21.7. MS m=z (%): 172 (Mþ, 100), 131 (91), 89 (34), 63 (15).
2,3-Dimethyl-6-nitroquinoxaline 3o. Brown solid; mp (ꢁC): 136–137 (lit.[19]
134–135). IR (t=cmꢂ1): 3046, 2957, 2857, 1524, 1401, 1340, 1161, 846, 818, 743, 421;
1H NMR (400 MHz, CDCl3): d ¼ 8.87 (s, 1H), 8.43 (dd, J ¼ 2.8, 8.8 Hz, 1H), 8.10 (d,
J ¼ 9.2 Hz, 1H), 2.78 (s, 6H); 13C NMR (100 MHz, CDCl3): d ¼ 157.2, 156.3, 147.2,
143.7, 139.9, 129.9, 124.9, 122.3, 23.5, 23.3. MS m=z (%): 203 (Mþ, 100), 162 (93),
116 (77), 75 (94).
2,3-Dimethylquinoxaline 3p. White solid; mp (ꢁC): 108 (lit.[20] 104–106). IR
(t=cmꢂ1): 3000, 2914, 1485, 1392, 1319, 1161, 982, 759; 1H NMR (400 MHz, CDCl3):
d ¼ 7.99–7.96 (m, 2H), 7.67–7.64 (m, 2H), 2.72 (s, 6H); 13C NMR (100 MHz,
CDCl3): d ¼ 153.4, 141.1, 128.8, 128.3, 23.2. MS m=z (%): 158 (Mþ, 56), 117 (100),
76 (44), 50 (40).
(2,3-Dimethylquinoxalin-6-yl)(phenyl)methanone 3q. Brown solid; mp
(ꢁC): 124. IR (t=cmꢂ1): 3272, 2997, 2914, 1710, 1641, 1446, 1253, 851, 726, 759;
1H NMR (400 MHz, CDCl3): d ¼ 8.36 (s, 1H), 8.17 (d, J ¼ 8.4 Hz, 1H), 8.09 (d,
J ¼ 8.4 Hz, 1H), 7.87 (d, J ¼ 8.4 Hz, 2H), 7.64–7.60 (m, 1H), 7.53–7.49 (m, 2H),
2.78 (s, 3H), 2.75 (s, 3H); 13C NMR (100 MHz, CDCl3): d ¼ 195.9, 155.7, 154.8,
143.0, 140.0, 137.4, 137.3, 132.6, 131.7, 130.1, 128.8, 128.4, 23.4, 23.2. MS m=z
(%): 262 (Mþ, 44), 185 (85), 105 (100), 77 (82).
7-Bromo-2,3-dimethylpyrido[2,3-b]pyrazine 3r. White solid; mp (ꢁC): 149
(lit.[21] 150). IR (t=cmꢂ1): 3036, 2924, 1864, 1633, 1548, 1442, 1290, 1049, 972, 714;

2062
X.-Z. ZHANG, J.-X. WANG, AND L. BAI
1H NMR (400 MHz, CDCl3): d ¼ 9.05 (d, J ¼ 1.6 Hz, 1H), 8.50 (d, J ¼ 2.4 Hz, 1H),
2.80 (s, 3H), 2.78 (s, 3H); 13C NMR (100 MHz, CDCl3): d ¼ 157.6, 156.1, 153.6,
148.6, 138.8, 136.2, 119.8, 23.5, 23.1. MS m=z (%): 237 (Mþ, 71), 196 (80), 117
(92), 76 (100).
ACKNOWLEDGMENTS
The work was supported by the Natural Science Foundation of China (Grant
Nos. 20272047 and 20572086), the Gansu Natural Science Foundation of China
(No. 0308RJZA-100), and the Key Laboratory of Eco-environment-Related Poly-
mer Material (Northwest Normal University), Ministry of the Education of China.
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