Synthesis and antifungal activities of novel 5,6-dihydro-indolo[1,2-a] quinoxaline derivatives

Article abstract of DOI:10.1016/j.ejmech.2011.02.035

A series of new 5,6-dihydro-indolo[1,2-a]quinoxaline derivatives has been prepared in moderate to excellent yields from 2-(indol-1-yl)phenylamines with aromatic aldehydes by an efficient and economical iron-catalyzed Pictet-Spengler reaction. Meanwhile, a

Full text of DOI:10.1016/j.ejmech.2011.02.035

European Journal of Medicinal Chemistry 46 (2011) 1919e1925
Contents lists available at ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Short communication
Synthesis and antifungal activities of novel 5,6-dihydro-indolo[1,2-a]quinoxaline
derivatives
,
b
,
a
Hui Xu a , Ling-ling Fan
*
a Laboratory of Pharmaceutical Design and Synthesis, College of Sciences, Xinong Road 22#, Northwest A&F University, Yangling 712100, China
b State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new 5,6-dihydro-indolo[1,2-a]quinoxaline derivatives has been prepared in moderate to
excellent yields from 2-(indol-1-yl)phenylamines with aromatic aldehydes by an efficient and economical
iron-catalyzed PicteteSpengler reaction. Meanwhile, as compared with hymexazol, a commercially
Received 2 August 2010
Received in revised form
14 February 2011
Accepted 15 February 2011
Available online 23 February 2011
available agricultural fungicide at the concentration of 50
mg/mL, some 5,6-dihydro-indolo[1,2-a]qui-
noxalines exhibited promising antifungal activities in vitro against the phytopathogenic fungi, and might be
considered as novel promising lead candidates for further design and synthesis of agricultural fungicides.
Ó 2011 Elsevier Masson SAS. All rights reserved.
Keywords:
5,6-Dihydro-indolo[1,2-a]quinoxaline
Iron catalysis
Cyclization
PicteteSpengler reaction
Antifungal activity
1. Introduction
and investigated their effect on phytopathogenic fungi. Addition-
ally, the structureeantifungal activity relationships (SAR) of
The indole moiety (I, Fig. 1) represents an important structural
component associated with a variety of alkaloids [1,2] and wide-
ranging biological activities, such as antiviral activities [3e7],
antitumor activities [8e10], antimicrobial activities [11], and anti-
inflammatory activities [12e14]. Meanwhile, the quinoxaline-con-
taining polycyclic ring derivatives, such as pyrroloquinoxalines,
imidazoquinoxalines and triazoloquinoxalines, exhibited the
interesting activities [15]. In continuation of our program aimed at
the discovery and development of bioactive molecules [16,17], we
wanted to design other quinoxaline-containing polycyclic ring
derivatives, 5,6-dihydro-indolo[1,2-a]quinoxalines (3a-q, Fig.1), via
combining the indole unit with tetrahydroquinoxaline (II, Fig. 1)
moiety together. On the other hand, since phytopathogenic fungi
are hard to control and easily infect many crops, the development
of new compounds that effectively inhibit those agricultural
diseases is still highly necessary. In this paper, therefore, we
prepared a series of novel 5,6-dihydro-indolo[1,2-a]quinoxalines
(3a-q) via a minor modification of the PicteteSpengler reaction,
compounds 3a-q were also preliminarily described.
2. Chemistry
As compared with other methods [18,19], PicteteSpengler
reaction, which has been extensively used for the construction of
a variety of natural products and novel heterocycles [20e22], is still
a powerful methodology for the synthesis of 5,6-dihydro-indolo
[1,2-a]quinoxalines. Although Kundu and coauthors have reported
the synthesis of indolo[1,2-a]quinoxalines in situ via the interme-
diates, 5,6-dihydro-indolo[1,2-a]quinoxalines, excess amounts
(1.5 equiv.) of trifluoroacetic acid (TFA), a strong Brønsted acid,
were required to prepare 5,6-dihydro-indolo[1,2-a]quinoxalines by
the PicteteSpengler reaction [23]. Consequently, the development
of environmentally friendly and less expensive catalysts for the
construction of a library of 5,6-dihydro-indolo[1,2-a]quinoxalines is
very important.
It is well-known that iron as one of the most abundant, inex-
pensive and environmentally friendly metals, recently, has been
widely used as a catalyst in many reactions [24e26]. To the best of
our knowledge, the iron-catalyzed synthesis of 5,6-dihydro-indolo
* Corresponding author. Laboratory of Pharmaceutical Design and Synthesis,
College of Sciences, Xinong Road 22#, Northwest A&F University, Yangling 712100,
China. Tel./fax: þ86 29 87091952.
[1,2-a]quinoxalines
from
2-(indol-1-yl)phenylamines
with
E-mail address: orgxuhui@nwsuaf.edu.cn (H. Xu).
aromatic aldehydes has not yet been investigated. As described in
0223-5234/$ e see front matter Ó 2011 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.ejmech.2011.02.035

1920
H. Xu, L.-l. Fan / European Journal of Medicinal Chemistry 46 (2011) 1919e1925
Table 1
2
R
Combinatorial optimization
Optimization studiesa.
1
R
1
R
N
N
H
NH
Entry
Catalyst (mol%)b
Time (h)
Yield (%)c
1
2
3
4
5
6
7
None
24
1
1
24
5
9
0
83
85
0
23
16
89
I
FeCl3$6H2O (2.5)
FeCl3$6H2O (5)
I2 (2)
5% TFA (110)
p-TsOH (10)
SnCl2$2H2O (2.5)
H
N
3a-q
2
N
H
R
1
II
Fig. 1. Design strategy of the target compounds 3a-q.
a
Reaction conditions: The mixture of 1a (0.25 mmol) and 2a (0.25 mmol) in DCM
(2 mL) in the presence of catalyst was reacted at room temperature.
b
The amount of catalyst was referred to mol% of 1a and given in parentheses.
c
Isolated yields.
Scheme 1, therefore, the reaction of 2-(indol-1-yl)phenylamine
(1a), prepared from 2-(indol-1-yl)nitrobenzene [27], with p-nitro-
benzaldehyde (2a) in dichloromethane (DCM) at room temperature
was screened as a model reaction. The results were summarized in
Table 1. Firstly, we carried out the reaction without any catalyst, and
6-(p-nitrophenyl)-5,6-dihydro-indolo[1,2-a]quinoxaline (3a) was
not obtained at all even if the reaction time was prolonged to 24 h
(Table 1, entry 1). The reaction was then carried out by using
FeCl3$6H2O (2.5 mol%) as the catalyst for 1 h, compound 3a was
obtained in a 83% yield (Table 1, entry 2). Increasing the amount of
FeCl3$6H2O to 5 mol% resulted in the formation of 3a in a 85% yield
(Table 1, entry 3). Finally, the effect of other catalysts on this reac-
tion was also evaluated. The catalysts such as I2, TFA or p-TsOH gave
no product or lower yields of 3a (Table 1, entries 4e6). Although 3a
was obtained in a 89% yield by using SnCl2$2H2O (2.5 mol%) as the
catalyst (Table 1, entry 7), it was not an environmentally friendly
catalyst. Consequently, the optimum reaction conditions for the
synthesis of 3a were to use 2.5 mol% of FeCl3$6H2O as the catalyst in
DCM at room temperature (Table 1, entry 2).
Encouraged by the above-mentioned results, a variety of 2-
(indol-1-yl)phenylamines (1, R1 ¼ H, Me, and CN) reacting with
aromatic aldehydes (2, R2 ¼ H, OH, NMe2, OMe, and NO2) were
investigated to explore the scope for the synthesis of 5,6-dihydro-
indolo[1,2-a]quinoxalines. As outlined in Table 2, all reactions
generally proceeded via a PicteteSpengler intermolecular cycliza-
tion to give 5,6-dihydro-indolo[1,2-a]quinoxalines in moderate to
excellent yields. The electronic effects of the 2-(indol-1-yl)phenyl-
amines and aromatic aldehydes on this reaction were observed. In
general, introduction of the electron-donating functional group on
the indole rings of 2-(indol-1-yl)phenylamines (Table 2, entries 4, 6
and 13 vs. 15), and introduction of the electron-withdrawing func-
tional group on the phenyl rings of aromatic aldehydes (Table 2,
entries 1 vs. 2 and 3; 4 vs.5; 6 vs.7e12; 13 vs.14) would easily and
rapidly result in PicteteSpengler cyclization to give the products in
excellent yields, and vice versa. For example, as compared with 1a-d,
when the electron-deficiency 2-(5-cyanoindol-1-yl)phenylamine
(1e) reactedwith2a, the reaction time shouldbe prolongedto24h at
room temperature to obtain the good yield of 6-(4-nitrophenyl)-5,6-
dihydro-9-cyanoindolo[1,2-a]quinoxaline (3o) (Table 2, entries 1, 4,
6 and 13 vs. 15). Additionally, 1a reacted with 2a for 1 h at room
temperature to give 3a in a 83% yield, while 1a reacted with 3-
methoxy-4-hydroxybenzaldehyde (2c) to produce 6-(3-methoxy-4-
hydroxyphenyl)-5,6-dihydro-indolo[1,2-a] quinoxaline (3c) only in
a 23% yield, even if the reaction time was prolonged to 15 h at room
temperature (Table 2, entries 1 vs. 3). Especially 4-(N,N-dimethyla-
mino)benzaldehyde (2e) did not react with 2-(4-methylindol-1-yl)
phenylamine (1c) at room temperature, and when the temperature
was raised to reflux for 31 h, 6-(4-N,N-dimethylaminophenyl)-5,6-
dihydro-8-methylindolo[1,2-a]quinoxaline (3i) was obtained in
a 27% yield (Table 2, entry 9).
However, the steric effects of 2-(indol-1-yl)phenylamines and
aromatic aldehydes on this reaction were not obvious. For example,
when 2a reacted with 2-(indol-1-yl)phenylamine (1a) or 2-(3-
methylindol-1-yl)phenylamine (1b), the corresponding yields of 3a
and 6-(p-nitrophenyl)-5,6-dihydro-7-methylindolo[1,2-a]quinoxa-
line (3d) were 83% and 90%, respectively (Table 2, entries 1 and 4).
Meanwhile, when 1c reacted with 4-hydroxybenzaldehyde (2b) or
2-hydroxybenzaldehyde (2g), the corresponding 6-(p-hydrox-
yphenyl)-5,6-dihydro-8-methylindolo[1,2-a]quinoxaline (3g) and
6-(o-hydroxyphenyl)-5,6-dihydro-8-methylindolo[1,2-a]quinoxa-
line (3l) were obtained in 75% and 71% yields, respectively (Table 2,
entries 7 and 12).
We also attempted to extend the scope to other carbonyl
compounds such as acetophenon (2h) and 2-furaldehyde (2i). To
our delight, the corresponding 5,6-dihydro-indolo[1,2-a]quinoxa-
lines were obtained in moderate to good yields (Scheme 2). For
example, when 1c reacted with 2h at reflux for 48 h, 6-methyl-6-
phenyl-5,6-dihydro-8-methylindolo[1,2-a]quinoxaline (3p) was
obtained in a 48% yield, while 1b reacted with 2i at room
temperature only for 0.5 h to give 6-(furan-2-yl)-5,6-dihydro-7-
methylindolo[1,2-a]quinoxaline (3q) in a 95% yield. The structures
of all new target compounds 3a-q were well characterized by 1H
NMR, 13C NMR, HRMS, IR, and m.p.
3. Biological evaluation
The antifungal activities of 17 5,6-dihydro-indolo[1,2-a]qui-
noxaline derivatives (3a-q) against five phytopathogenic fungi (i.e.,
Fusarium graminearum, Pyricularia oryzae, Fusarium oxysporum f.
sp. vasinfectum, Alternaria alternata, and Alternaria brassicae) were
investigated at the concentration of 50
food technique [28]. Hymexazol, a commercially available agricul-
tural fungicide, was used as a positive control at 50 g/mL. As
mg/mL in vitro by poisoned
m
NO
2
depicted in Table 3, among all the derivatives, compounds 3b, 3e,
3g, 3n, and 3o exhibited the good and broad spectrum of antifungal
activities against the five phytopathogenic fungi. Meanwhile, some
interesting results through a comparative study on the SAR of 3a-q
were found as follows: (1) Generally, when the hydroxy group was
introduced at the 4-position on the E-ring, the corresponding
derivatives showed the potent activities. Interestingly, if the
methoxy group was simultaneously introduced at the 3-position on
the E-ring of 3b or 3g to give 3c or 3k, the corresponding antifungal
CHO
catalyst
DCM, rt
N
N
NH
+
2
NH
NO
2
1a
2a
3a
Scheme 1. Model reaction.

H. Xu, L.-l. Fan / European Journal of Medicinal Chemistry 46 (2011) 1919e1925
1921
Table 2
Synthesis of 5,6-dihydro-indolo[1,2-a]quinoxalines via PicteteSpengler reaction promoted by FeCl3$6H2O.a
E
1
.
R2
R1
R1
FeCl3 6H2O
DCM
A
B
2 3N
CHO
6
N
C
5
NH2
NH
+
rt or reflux
0.5-48 h
4
R2
D
2
3
1
Entry
1
2
Product (3)
Condition
Time (h)
Yield (%)b
NO2
CHO
N
1
rt
1
83
73
23
NH
2
2a
N
N
N
NH
NH
NH
3a
1a
NO2
OH
CHO
2
3
1a
1a
rt
rt
24
15
2b
3b
OH
OH
OMe
CHO
OH
2c
MeO
3c
NO
2
Me
Me
4
5
6
2a
2b
2a
rt
rt
rt
1
90
79
98
N
N
NH2
1b
NH
NH
NH
3d
OH
Me
1b
48
N
3e
NO2
Me
Me
Me
0.5
N
N
NH2
1c
3f
OH
7
1c
2b
rt
24
75
N
NH
3g
(continued on next page)

1922
H. Xu, L.-l. Fan / European Journal of Medicinal Chemistry 46 (2011) 1919e1925
Table 2 (continued ).
Entry
1
2
Product (3)
Condition
Time (h)
Yield (%)b
Me
CHO
CHO
8
1c
1c
1c
1c
rt
6
95
N
2d
NH
3h
NMe2
Me
Me
2e
9
reflux
31
24
25
27
78
44
N
N
NH
NH
3i
NMe
2
OMe
CHO
OMe
2f
10
rt
3j
OH
OMe
Me
Me
11
2c
rt
N
N
NH
NH
3k
HO
CHO
OH
12
13
14
1c
rt
rt
rt
24
71
94
89
3l
2g
NO
2
N
Me
NH2
2a
2b
1
N
Me
NH
NH
1d
3m
OH
1d
24
N
Me
NC
3n
NO
2
NC
N
15
2a
rt
24
85
NH2
N
NH
1e
3o
a
Reaction conditions: The mixture of 1 (0.25 mmol), 2 (0.25 mmol), and FeCl3.6H2O (2.5 mol%) in DCM (2 mL) was reacted at room temperature or reflux under N2, and the
reaction process was checked by TLC.
b
Isolated yields.

H. Xu, L.-l. Fan / European Journal of Medicinal Chemistry 46 (2011) 1919e1925
1923
clearly demonstrated that the substituent R2 as hydroxy group at
the 4-position on the E-ring, and introduction of cyano group on
the A-ring of 5,6-dihydro-indolo[1,2-a]quinoxalines were very vital
for their antifungal activities. Further structural modification on
5,6-dihydro-indolo[1,2-a]quinoxalines and in-depth study on the
antifungal activities are currently in progress in our research group.
Me
Me
Me
FeCl3.6H2O
DCM
Me
O
N
N
NH
NH2
+
reflux, 48 h
48%
3p
2h
1c
Me
5. Experimental protocols
Me
O
FeCl3.6H2O
DCM
5.1. Chemistry
N
+
N
NH2
NH
CHO
O
2i
rt, 0.5 h
95%
5.1.1. General remarks
All reagents and solvents were of reagent grade or purified
according to standard methods before use. Thin-layer chromatog-
raphy (TLC) and preparative thin-layer chromatography (PTLC)
were used with silica gel 60 GF254 (Qingdao Haiyang Chemical Co.,
Ltd., China). Melting points were determined on a digital melting-
point apparatus and were uncorrected. Infrared spectra (IR) were
recorded on a Bruker TENSOR 27 spectrometer. Proton nuclear
magnetic resonance spectra (1H NMR) and carbon-13 nuclear
magnetic resonance spectra (13C NMR) were recorded on Bruker
Avance DMX 300, 400 or 500 MHz and 125 MHz instruments,
respectively, using TMS as the internal standard and CDCl3 as the
solvent. High-resolution mass spectra (HR-MS) were carried out
with APEX II Bruker 4.7T AS instrument.
1b
3q
Scheme 2. Synthesis of 3p and 3q.
activities were decreased sharply (3b vs. 3c; 3g vs. 3k). Moreover,
introduction of hydroxyl group at the 2-position on the E-ring of 3h
gave 3l, the antifungal activities of which were decreased as
compared with 3g bearing hydroxy group at the 4-position on the
E-ring. (2) Introduction of the electron-withdrawing group (e.g.,
cyano) on the A-ring of 3a usually led to the more potent compound
than those possessing the electron-donating group (e.g., methyl) on
the A-ring (3o vs. 3d, 3f and 3m). (3) When the methyl group was
introduced at the 6-position on the C-ring of 3h to give 3p, or the E-
ring of 3e was substituted by the furanyl ring to afford 3q, the
corresponding activities of 3p and 3q were not increased at all.
5.1.2. General procedures for the synthesis of 5,6-dihydro-indolo
[1,2-a]quinoxalines (3a-q)
A mixture of 1 (0.25 mmol) and 2 (0.25 mmol) in DCM (2 mL) in
the presence of FeCl3$6H2O (2.5 mol%) was reacted at room
temperature or reflux under N2, and the reaction process was
checked by TLC analysis. When the reaction was complete, the
organic solvent was removed, and the residue was directly purified
by preparative thin-layer chromatography (PTLC) to give the
desired products 3a-q, which were characterized by 1H NMR, 13C
NMR, HRMS, IR and m.p.
4. Conclusion
In summary, we have reported a convenient, economic and
efficient cyclization reaction using FeCl3$6H2O as the catalyst for
the synthesis of 5,6-dihydro-indolo[1,2-a]quinoxalines. The reac-
tions proceed via a PicteteSpengler intermolecular cyclization to
give 5,6-dihydro-indolo[1,2-a]quinoxalines in moderate to excel-
lent yields from 2-(indol-1-yl)phenylamines with aromatic alde-
hydes. It is noteworthy that this protocol uses inexpensive and
environmentally friendly FeCl3$6H2O (2.5 mol%) as the catalyst.
Especially compounds 3b, 3e, 3g, 3n and 3o exhibited promising
and pronounced antifungal activities in vitro against the phyto-
pathogenic fungi, and might be considered as new lead structures
for further design of agricultural fungicides. Generally, the SAR
5.1.2.1. 6-(p-Nitrophenyl)-5,6-dihydro-indolo[1,2-a]quinoxaline
(3a). Yellow solid, m.p. 168e170 ꢀC; IR cmꢁ1: 735, 745, 1350, 1455, 1510,
1598, 3329; 1H NMR (400 MHz, CDCl3)
d: 4.22 (1H, s, NH), 5.63 (1H, s),
5.85 (1H, s), 6.90e6.91 (1H, m), 7.04e7.06 (2H, m), 7.15 (1H, t, J ¼ 7.6 Hz),
7.27 (1H, t, J ¼ 7.2 Hz), 7.52 (1H, d, J ¼ 7.6 Hz), 7.65e7.67 (2H, m),
7.93e8.02 (2H, m), 8.23 (2H, d, J ¼ 8.4 Hz); 13C NMR (125 MHz, CDCl3)
d:
Table 3
Antifungal activities of compounds 3a-q at 50
m
g/mL.
Compd
Antifungal activities (inhibition %)
F. graminearum
P. oryzae
F. oxysporium f. sp. vasinfectum
A. alternata
A. brassicae
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
Hym
17.2 (ꢂ0.7)
52.1 (ꢂ0.7)
15.7 (ꢂ0.6)
26.6 (ꢂ0.7)
48.0 (ꢂ0.7)
24.6 (ꢂ0)
5.8 (ꢂ0.7)
50.8 (ꢂ1.2)
4.4 (ꢂ1.2)
11.8 (ꢂ0.7)
46.5 (ꢂ07)
10.1 (ꢂ1.2)
50.8 (ꢂ1.2)
22.1 (ꢂ1.2)
7.7 (ꢂ1.2)
15.1 (ꢂ1.3)
16.0 (ꢂ1.3)
7.8 (ꢂ0.7)
1.6 (ꢂ1.2)
43.7 (ꢂ0)
9.2 (ꢂ1.1)
39.2 (ꢂ1.0)
11.0 (ꢂ1.5)
11.4 (ꢂ0.5)
44.0 (ꢂ1.2)
5.3 (ꢂ0.5)
41.2 (ꢂ0.6)
8.8 (ꢂ1.0)
6.9 (ꢂ1.0)
9.7 (ꢂ1.2)
20.2 (ꢂ1.2)
11.7 (ꢂ1.2)
11.7 (ꢂ1.2)
37.3 (ꢂ0.6)
60.8 (ꢂ0.6)
23.6 (ꢂ0.6)
17.8 (ꢂ1.2)
62.7 (ꢂ1.0)
19.4 (ꢂ0.6)
58.3 (ꢂ0)
11.2 (ꢂ0.6)
31.4 (ꢂ0)
9.2 (ꢂ0.6)
18.2 (ꢂ1.2)
55.6 (ꢂ1.2)
17.7 (ꢂ1.0)
72.9 (ꢂ0)
15.7 (ꢂ0.5)
13.3 (ꢂ0.6)
53.0 (ꢂ0.5)
10.4 (ꢂ0.6)
55.9 (ꢂ1.0)
30.4 (ꢂ1.0)
10.4 (ꢂ1.1)
7.2 (ꢂ1.6)
51.3 (ꢂ0.8)
32.0 (ꢂ0.7)
17.6 (ꢂ0.4)
12.2 (ꢂ0.7)
12.3 (ꢂ1.0)
8.8 (ꢂ1.2)
29.2 (ꢂ1.0)
19.8 (ꢂ1.0)
14.9 (ꢂ0.6)
17.9 (ꢂ0.6)
12.0 (ꢂ0.6)
7.8 (ꢂ1.5)
56.3 (ꢂ0.6)
64.6 (ꢂ0.4)
32.7 (ꢂ0.6)
37.3 (ꢂ0)
21.0 (ꢂ1.2)
55.8 (ꢂ0.5)
13.0 (ꢂ0.5)
49.0 (ꢂ1.5)
52.4 (ꢂ0.6)
29.9 (ꢂ0.5)
30.1 (ꢂ0.5)
47.1 (ꢂ0.5)
12.2 (ꢂ0.7)
41.2 (ꢂ0.4)
56.3 (ꢂ0.7)
43.0 (ꢂ0.4)
31.5 (ꢂ1.0)
57.0 (ꢂ0.4)
62.8 (ꢂ1.2)
34.6 (ꢂ1.3)
33.4 (ꢂ0.7)
53.5 (ꢂ0.7)
68.8 (ꢂ0)

1924
H. Xu, L.-l. Fan / European Journal of Medicinal Chemistry 46 (2011) 1919e1925
56.7, 100.8, 112.0, 116.5, 117.3, 121.0, 121.4, 121.5, 123.3, 124.3, 124.6, 127.5,
129.4, 129.7, 134.4, 136.9, 137.5, 147.6, 148.3; HRMS-ESI: Calcd. for
C21H15N3O2 [M þ H]þ: 342.1237. Found: 342.1239.
m), 7.15 (1H, t, J ¼ 8.0 Hz), 7.37 (2H, d, J ¼ 8.4 Hz), 7.83 (1H, d,
J ¼ 8.4 Hz), 7.90e7.93 (1H, m); 13C NMR (125 MHz, CDCl3)
d: 18.7,
56.7, 98.4, 109.3, 115.5, 115.9, 116.9, 120.1, 121.1, 122.5, 123.9, 127.4,
129.3, 129.8, 130.4, 132.4, 133.8, 137.7, 139.0, 155.7; HRMS-ESI:
Calcd. for C22H18N2O [M þ H]þ: 327.1492. Found: 327.1488.
5.1.2.2. 6-(p-Hydroxyphenyl)-5,6-dihydro-indolo[1,2-a]quinoxaline
(3b). Pale yellow solid, m.p. 146e148 ꢀC; IR cmꢁ1: 738, 747, 1229,
1452, 1508, 1598, 3307; 1H NMR (300 MHz, CDCl3)
d: 4.11 (1H, s,
5.1.2.8. 6-Phenyl-5,6-dihydro-8-methylindolo[1,2-a]quinoxaline
(3h). Pale yellow solid, m.p. 158e160 ꢀC; IR cmꢁ1: 628, 699, 739,
769, 1281, 1433, 1505, 1556, 1598, 3336; 1H NMR (400 MHz, CDCl3)
NH), 4.92 (1H, s, OH), 5.42 (1H, s), 5.87 (1H, s), 6.80e6.88 (3H, m),
6.99e7.04 (2H, m), 7.11 (1H, t, J ¼ 7.5 Hz), 7.22e7.28 (1H, m), 7.35
(2H, d, J ¼ 8.7 Hz), 7.51 (1H, d, J ¼ 7.5 Hz), 7.90e7.93 (1H, m), 7.98
d: 2.43 (3H, s, CH3), 4.16 (1H, s, NH), 5.51 (1H, s), 5.88 (1H, s),
(1H, d, J ¼ 8.1 Hz); 13C NMR (125 MHz, CDCl3)
d
: 56.6, 100.1, 111.6,
6.84e6.86 (1H, m), 6.94 (1H, d, J ¼ 7.2 Hz), 7.00e7.02 (2H, m), 7.15
115.5,116.0,116.9,120.1,120.8,121.0,122.5,124.0,127.3,129.6,129.8,
132.3, 134.2, 137.7, 139.6, 155.77; HRMS-ESI: Calcd. for C21H16N2O
[M þ H]þ: 313.1335. Found: 313.1339.
(1H, t, J ¼ 8.0 Hz), 7.41e7.45 (3H, m), 7.52e7.53 (2H, m), 7.84 (1H, d,
J ¼ 8.4 Hz), 7.92e7.94 (1H, m); 13C NMR (125 MHz, CDCl3)
d: 18.7,
57.3, 98.5, 109.3, 116.0, 116.9, 120.1, 121.2, 122.6, 123.9, 127.4, 128.4,
128.6, 128.8, 129.3, 130.4, 133.8, 137.6, 138.6, 140.1; HRMS-ESI:
Calcd. for C22H18N2 [M þ H]þ: 311.1543. Found: 311.1549.
5.1.2.3. 6-(3-Methoxy-4-hydroxyphenyl)-5,6-dihydro-indolo[1,2-a]
quinoxaline (3c). Pale yellow solid, m.p. 212e214 ꢀC; IR cmꢁ1: 745,
779, 1031, 1150, 1227, 1266, 1454, 1505, 1595, 3317, 3485; 1H NMR
5.1.2.9. 6-(4-N,N-Dimethylaminophenyl)-5,6-dihydro-8-methyl-
indolo[1,2-a]quinoxaline (3i). Pale yellow solid, m.p. 186e188 ꢀC; IR
cmꢁ1: 613, 668, 731, 807, 1276, 1348, 1506, 1560, 1609, 3366; 1H
(400 MHz, CDCl3) d: 3.84 (3H, s, OCH3), 4.14 (1H, s, NH), 5.43 (1H, s),
5.70 (1H, s), 5.90 (1H, s), 6.87e6.89 (1H, m), 6.94e6.96 (1H, m),
7.00e7.08 (4H, m), 7.13 (1H, t, J ¼ 7.6 Hz), 7.25e7.29 (1H, m), 7.53
(1H, d, J ¼ 7.6 Hz), 7.93e7.95 (1H, m), 8.00 (1H, d, J ¼ 8.4 Hz); 13C
NMR (500 MHz, CDCl3) d: 2.44 (3H, s, CH3), 3.00 (6H, s, N(CH3)2),
4.09 (1H, s, NH), 5.41 (1H, s), 5.93 (1H, s), 6.76 (2H, d, J ¼ 9.0 Hz),
6.83e6.85 (1H, m), 6.94 (1H, d, J ¼ 7.0 Hz), 6.99e7.01 (2H, m), 7.14
(1H, t, J ¼ 7.5 Hz), 7.38 (2H, d, J ¼ 8.5 Hz), 7.83 (1H, d, J ¼ 8.5 Hz),
NMR (125 MHz, CDCl3) d: 56.0, 57.1, 100.1, 110.4, 111.6, 114.0, 116.0,
116.9, 120.1, 120.8, 121.0, 121.6, 122.4, 124.0, 127.3, 129.6, 131.8, 134.1,
137.7, 139.7, 145.9, 146.9; HRMS-ESI: Calcd. for C22H18N2O2
[M þ H]þ: 343.1441. Found: 343.1448.
7.91e7.93 (1H, m); 13C NMR (125 MHz, CDCl3)
d: 18.7, 40.5, 56.8,
98.3, 109.3, 112.4, 115.9, 116.9, 119.8, 121.0, 122.3, 123.8, 127.4, 127.5,
129.3, 129.4, 130.3, 133.9, 138.1, 139.5, 150.7; HRMS-ESI: Calcd. for
C24H23N3 [M þ H]þ: 354.1965. Found: 354.1963.
5.1.2.4. 6-(p-Nitrophenyl)-5,6-dihydro-7-methylindolo[1,2-a]qui-
noxaline (3d). Yellow solid, m.p. 160e162 ꢀC; IR cmꢁ1: 704, 737,
1342, 1454, 1507, 1596, 3367; 1H NMR (400 MHz, CDCl3)
d
: 2.12 (3H,
5.1.2.10. 6-(p-Methoxyphenyl)-5,6-dihydro-8-methylindolo[1,2-a]qui-
noxaline (3j). Orange solid, m.p. 194e196 ꢀC; IR cmꢁ1: 742, 763, 825,
1027, 1234, 1262, 1561, 1597, 3318, 3343; 1H NMR (400 MHz, CDCl3)
s, CH3), 4.39 (1H, s, NH), 5.78 (1H, s), 6.76 (1H, d, J ¼ 6.8 Hz),
6.94e7.01 (2H, m), 7.21e7.25 (1H, m), 7.30e7.36 (3H, m), 7.59 (1H,
d, J ¼ 8.0 Hz), 7.89 (1H, d, J ¼ 6.8 Hz), 8.01 (1H, d, J ¼ 8.0 Hz),
d: 2.44 (3H, s, CH3), 3.85 (3H, s, OCH3), 4.12 (1H, s, NH), 5.45 (1H, s),
8.07e8.09 (2H, m); 13C NMR (125 MHz, CDCl3)
d: 8.4, 53.9, 107.9,
5.87 (1H, s), 6.84e6.86 (1H, m), 6.94 (3H, d, J ¼ 8.8 Hz), 7.00e7.02
111.9, 116.50, 116.58, 119.2, 120.5, 120.7, 123.0, 124.0, 124.1, 127.3,
127.8, 130.4, 131.0, 133.5, 134.4, 147.4, 148.8; HRMS-ESI: Calcd. for
C22H17N3O2 [M þ H]þ: 356.1394. Found: 356.1399.
(2H, m), 7.15 (1H, t, J ¼ 8.0 Hz), 7.44 (2H, d, J ¼ 8.4 Hz), 7.83 (1H, d,
J ¼ 8.0 Hz), 7.91e7.94 (1H, m); 13C NMR (125 MHz, CDCl3)
d: 18.7,
55.3, 56.7, 98.4, 109.3, 114.0, 115.9, 116.9, 120.0, 121.1, 122.5, 123.9,
127.4, 129.3, 129.6, 130.4, 132.2, 133.9, 137.8, 139.0, 159.8; HRMS-ESI:
Calcd. for C23H20N2O [M þ H]þ: 341.1648. Found: 341.1651.
5.1.2.5. 6-(p-Hydroxyphenyl)-5,6-dihydro-7-methylindolo[1,2-a]qui-
noxaline (3e). Pale yellow solid, m.p. 204e206 ꢀC; IR cmꢁ1: 740,
754, 1174, 1222, 1454, 1509, 1597, 3300; 1H NMR (400 MHz, CDCl3)
5.1.2.11. 6-(3-Methoxy-4-hydroxyphenyl)-5,6-dihydro-8-methyl-
indolo[1,2-a]quinoxaline (3k). White solid, m.p. 192e194 ꢀC; IR
cmꢁ1: 621, 750, 1029, 1230, 1263, 1321, 1430, 1508, 1560, 1598, 1610,
d
: 1.99 (3H, s, CH3), 5.59 (1H, s), 6.65 (2H, d, J ¼ 8.0 Hz), 6.73e6.75
(1H, m), 6.93e6.95 (2H, m), 7.06 (2H, J ¼ 8.4 Hz), 7.16e7.29 (2H, m),
7.54 (1H, d, J ¼ 7.6 Hz), 7.86e7.89 (1H, m), 7.98 (1H, d, J ¼ 8.8 Hz);
3350, 3506; 1H NMR (400 MHz, CDCl3)
d: 2.44 (3H, s, CH3), 3.84 (3H,
13C NMR (125 MHz, CDCl3)
d: 8.3, 54.6, 107.2, 111.7, 115.5, 116.2,
s, OCH3), 4.12 (1H, s, NH), 5.42 (1H, s), 5.72 (1H, s), 5.89 (1H, s),
6.85e6. 88 (1H, m), 6.95e6.97 (2H, m), 7.01e7.03 (3H, m), 7.08 (1H,
s), 7.16 (1H, t, J ¼ 8.0 Hz), 7.84 (1H, d, J ¼ 8.4 Hz), 7.91e7.94 (1H, m);
116.4, 118.9, 119.8, 120.3, 122.5, 123.6, 127.4, 128.7, 130.7, 132.9,
133.3,134.1,135.7, 155.2; HRMS-ESI: Calcd. for C22H18N2O [M þ H]þ:
327.1492. Found: 327.1499.
13C NMR (125 MHz, CDCl3)
d: 18.7, 56.0, 57.2, 98.5, 109.3, 110.4,
114.0, 115.9, 116.9, 120.1, 121.1, 121.7, 122.5, 123.9, 127.4, 129.3, 130.4,
132.0, 133.8, 137.7, 139.1, 145.9, 146.9; HRMS: Calcd. for C23H20N2O2
[M þ H]þ: 357.1598. Found: 357.1603.
5.1.2.6. 6-(p-Nitrophenyl)-5,6-dihydro-8-methylindolo[1,2-a]quinoxa-
line (3f). Yellow solid, m.p. 218e220 ꢀC; IR cmꢁ1: 730, 758, 1277,
1342, 1504, 1515, 1561, 1595, 3362; 1H NMR (400 MHz, CDCl3)
d: 2.44
(3H, s, CH3), 4.23 (1H, s, NH), 5.67 (1H, s), 5.86 (1H, s), 6.90e6.92 (1H,
m), 6.97 (1H, d, J ¼ 7.6 Hz), 7.04e7.08 (2H, m), 7.19 (1H, t, J ¼ 7.6 Hz),
7.70 (2H, d, J ¼ 8.4 Hz), 7.85 (1H, d, J ¼ 8.8 Hz), 7.94e7.96 (1H, m),
5.1.2.12. 6-(o-Hydroxyphenyl)-5,6-dihydro-8-methylindolo[1,2-a]
quinoxaline (3l). White solid, m.p. 192e194 ꢀC; IR cmꢁ1: 730, 758,
824, 1243, 1278, 1491, 1509, 1560, 1590, 3299; 1H NMR (400 MHz,
8.27 (2H, d, J ¼ 8.4 Hz); 13C NMR (125 MHz, CDCl3)
d: 18.7, 56.5, 98.9,
CDCl3) d: 2.42 (3H, s, CH3), 5.58 (1H, s), 5.93 (1H, s), 6.95e6.99 (4H,
109.4, 116.2, 117.0, 120.7, 121.5, 123.1, 124.0, 124.2, 127.3, 129.1, 129.2,
130.6, 133.8, 136.6, 136.7, 147.4, 148.1; HRMS-ESI: Calcd. for
C22H17N3O2 [M þ H]þ: 356.1394. Found: 356.1387.
m), 7.05 (1H, t, J ¼ 7.6 Hz), 7.13e7.21 (2H, m), 7.23e7.25 (1H, m),
7.34e7.38 (1H, m), 7.81 (1H, d, J ¼ 8.4 Hz), 7.94 (1H, d, J ¼ 8.0 Hz);
13C NMR (125 MHz, CDCl3)
d: 18.7, 57.5, 99.3, 109.2, 117.1, 117.4, 117.8,
119.8, 121.5, 122.1, 122.4, 123.0, 123.9, 128.4, 129.3, 129.9, 130.4,
130.8, 134.2, 135.7, 136.0, 156.7; HRMS-ESI: Calcd. for C22H18N2O
[M þ H]þ: 327.1492. Found: 327.1490.
5.1.2.7. 6-(p-Hydroxyphenyl)-5,6-dihydro-8-methylindolo[1,2-a]qui-
noxaline (3g). Pale yellow solid, m.p. 178e180 ꢀC; IR cmꢁ1: 735,
766, 1220, 1275, 1427, 1508, 1567, 1596, 1611, 3309; 1H NMR
(400 MHz, CDCl3)
OH), 5.43 (1H, s), 5.88 (1H, s), 6.84 (3H, d, J ¼ 8.8 Hz), 6.94e7.02 (3H,
d
: 2.44 (3H, s, CH3), 4.10 (1H, s, NH), 4.98 (1H, s,
5.1.2.13. 6-(p-Methoxyphenyl)-5,6-dihydro-10-methylindolo[1,2-a]
quinoxaline (3m). Yellow solid, m.p. 216e218 ꢀC; IR cmꢁ1: 712, 742,

H. Xu, L.-l. Fan / European Journal of Medicinal Chemistry 46 (2011) 1919e1925
1925
796, 807, 830, 851, 1343, 1434, 1491, 1515, 1598, 3318; 1H NMR
(300 MHz, CDCl3) : 2.52 (3H, s, CH3), 4.20 (1H, s, NH), 5.63 (1H, s),
control. For each treatment, three replicates were conducted. The
radial growths of the fungal colonies were measured and the data
were statistically analyzed. The inhibitory effects of the test
compounds on these fungi in vitro were calculated by the formula:
d
5.80 (1H, s), 6.88e6.91 (1H, m), 6.99e7.07 (3H, m), 7.39 (1H, d,
J ¼ 7.8 Hz), 7.65 (2H, d, J ¼ 9.0 Hz), 7.81 (1H, s), 7.92e7.95 (1H, m),
8.23 (2H, d, J ¼ 8.7 Hz); 13C NMR (125 MHz, CDCl3)
d: 22.1, 56.4,
Inhibition rate ð%Þ ¼ ðC ꢁ TÞ ꢃ 100=C
where C represents the diameter of fungi growth on untreated PDA,
and T represents the diameter of fungi on treated PDA.
100.3, 111.8, 116.2, 117.0, 120.70, 120.77, 122.8, 123.9, 124.1, 127.1,
127.3, 129.1, 132.9, 134.5, 136.6, 136.7, 147.4, 148.0; HRMS-ESI: Calcd.
for C22H17N3O2 [M þ H]þ: 356.1394. Found: 356. 1401.
5.1.2.14. 6-(p-Hydroxyphenyl)-5,6-dihydro-10-methylindolo[1,2-a]qui-
noxaline (3n). White solid, m.p. 154e156 ꢀC; IR cmꢁ1: 740, 751, 811,
833, 877, 941, 1235, 1251, 1346, 1432, 1508, 1602, 3294; 1H NMR
Acknowledgments
This work was financially in part supported by the Program for
New Century Excellent University Talents, State Education Ministry
of China (NCET-06-0868), the National Natural Science Foundation
of China (no. 31071737), the Fok Ying Tong Education Foundation
for Young Talents (no.121032), the Special Funds of Central Colleges
Basic Scientific Research Operating Expenses (QN2009045), and the
State Key Laboratory of Applied Organic Chemistry, Lanzhou
University. We also would like to thank the editor and the referees
for very constructive suggestions and comments.
(300 MHz, CDCl3) d: 2.51 (3H, s, CH3), 4.09 (1H, s, NH), 4.91 (1H, s, OH),
5.39 (1H, s), 5.81 (1H, s), 6.80e6.85 (3H, m), 6.96e7.04 (3H, m),
7.33e7.41 (3H, m), 7.80 (1H, s), 7.90e7.93 (1H, m); 13C NMR (125 MHz,
CDCl3) d: 22.1, 56.6, 99.9, 111.7, 115.4, 115.9, 116.9, 120.0, 120.6, 122.4,
123.8, 127.41, 127.48, 129.8, 132.2, 132.4, 134.6, 137.7, 139.0, 155.7;
HRMS-ESI: Calcd. for C22H18N2O [M þ H]þ: 327.1492. Found: 327.1493.
5.1.2.15. 6-(4-Nitrophenyl)-5,6-dihydro-9-cyanoindolo[1,2-a]qui-
noxaline (3o). Yellow solid, m.p. 180e181 ꢀC; IR cmꢁ1: 684, 718,
753, 850, 888, 1332, 1349, 1513, 1589, 2213, 3118; 1H NMR
Appendix. Supplementary data
(400 MHz, CDCl3
)
d
: 6.72 (1H, d, J ¼ 2.8 Hz), 7.30e7.34 (2H, m),
7.38e7.42 (2H, m), 7.47e7.49 (1H, m), 7.53e7.57 (2H, m), 7.74 (2H, d,
J ¼ 8.8 Hz), 8.02 (1H, s), 8.20 (2H, d, J ¼ 8.8 Hz), 8.50 (1H, s); HRMS-
ESI: Calcd. for C22H14N4O2 [M þ H]þ: 367.1190. Found: 367.1193.
Supplementary data related to this article can be found online
version at doi:10.1016/j.ejmech.2011.02.035.
References
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noxaline (3p). White solid, m.p. 174e176 ꢀC; IR cmꢁ1: 699, 737, 765,
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d: 1.98 (3H,
s, CH3), 2.57 (3H, s, CH3), 6.45 (1H, s), 6.87e7.01 (4H, m), 7.16e7.22
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line (3q). Pale yellow solid, m.p. 44e46 ꢀC; IR cmꢁ1: 743, 751, 801,
1012, 1090, 1456, 1507, 1598, 2916, 3364; 1H NMR (300 MHz, CDCl3)
d: 2.25 (3H, s, CH3), 4.52(1H, s, NH), 5.79e5.82 (2H, m), 6.15 (1H, s),
6.81e6.97 (3H, m), 7.21e7.30 (3H, m), 7.60 (1H, d, J ¼ 7.5 Hz),
7.83e7.86 (1H, m), 7.97 (1H, d, J ¼ 8.1 Hz); 13C NMR (125 MHz,
CDCl3) d: 8.0, 48.2, 107.2, 107.6, 110.3, 111.7, 116.4, 119.1, 120.2, 120.4,
122.6,123.6, 127.3,130.1, 130.3, 133.5,135.2,142.2, 153.8; HRMS-ESI:
Calcd. for C20H16N2O [M þ H]þ: 301.1335. Found: 301.1337.
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tion of test compounds in the medium was fixed at 50 mg/mL. The
medium was then poured into sterilized Petri dishes. All types of
fungi were incubated in PDA at 28 ꢂ 1 ꢀC for 5 d to get new
mycelium for the antifungal assays, and a mycelia disk of approx-
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with a sterilized inoculation needle and inoculated in the center of
the PDA Petri dishes. The inoculated Petri dishes were incubated at
28 ꢂ 1 ꢀC for 4 d. Acetone without any compounds mixed with PDA
was served as the control, while hymexazol, a commercially avail-
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able agricultural fungicide at 50
mg/mL, was used as a positive