One-pot synthesis of indolo[2,3-c]quinolin-6-ones by sequential photocyclizations of 3-(2-azidophenyl)-N-phenylacrylamides

Article abstract of DOI:10.1055/s-0032-1317703

A one-pot synthesis of indolo[2,3-c]quinolin-6(7H)-ones was achieved by sequential photocyclizations of 3-(2-azidophenyl)-N-phenylacrylamides in moderate to high yields. The reactions proceeded via photochemical cyclization of aryl azides to form N-phenylindol-2-carbamides and subsequent 6π-electrocyclic reaction and oxidative aromatization to afford the corresponding indolo[2,3-c]quinolin-6(7H)-ones. Georg Thieme Verlag Stuttgart · New York.

Full text of DOI:10.1055/s-0032-1317703

LETTER
▌73
letter
One-Pot Synthesis of Indolo[2,3-c]quinolin-6-ones by Sequential Photocycliza-
tions of 3-(2-Azidophenyl)-N-phenylacrylamides
One-Pot Synthesis of Indolo[2,3-c]quinolin-6-ones
Zhanshan Li, Weixia Wang, Xiaotian Zhang, Congcong Hu, Wei Zhang*
State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, P. R. of China
Fax +86(931)8625657; E-mail: zhangwei6275@lzu.edu.cn
Received: 19.09.2012; Accepted after revision: 05.11.2012
(2-iodophenyl)-1H-indole-2-carboxamides6a or of 3-(2-
Abstract: A one-pot synthesis of indolo[2,3-c]quinolin-6(7H)-ones
bromophenylamino)quinolines;6b photostimulated intra-
was achieved by sequential photocyclizations of 3-(2-azidophenyl)-
molecular SRN1 reactions of N-(2-chlorophenyl)-1H-in-
N-phenylacrylamides in moderate to high yields. The reactions pro-
ceeded via photochemical cyclization of aryl azides to form N-phe-
dole-2-carboxamides;6c and cyclocondensation of 3-
nylindol-2-carbamides and subsequent 6π-electrocyclic reaction formaloxindoles with phenylhydrazines,6d and oxidative
and oxidative aromatization to afford the corresponding indolo[2,3-
c]quinolin-6(7H)-ones.
photochemical cyclization of N-phenylindole-2-carbox-
amides.6e However, all these methods generally required
Key words: indolo[2,3-c]quinolin-6(7H)-ones, photocyclization,
3-(2-azidophenyl)-N-phenylacrylamides, aryl azides, 6π-electrocy-
clic reaction
either an indole or a quinoline as the precursor and then in
order to reach to indolo[2,3-c]quinolin-6(7H)-ones struc-
ture, construction of another ring.
We have been long interested in the synthesis of polycy-
clic heterocycles by one-pot multistep reactions. Recent-
ly, we reported a new synthesis of indolo[3,2-c]quinolin-
6-ones by the photoreaction of 3-(2-azidobenzylidene)in-
dol-2-ones.7 In this route, photocyclization of 3-(2-azido-
benzylidene)indol-2-ones first lead to formation of indole
ring and subsequent ring expansion of indol-2-one afford
the quinoline ring. Here, we report a new method for the
synthesis of indolo[2,3-c]quinolin-6(7H)-ones (iso-
neocryptolepinone) by a one-pot two-step photocycliza-
tion of 3-(2-azidophenyl)-N-phenylacrylamides (Scheme
1).
Indoloquinolines are present in many natural alkaloids1
and pharmaceuticals2 possessing important biological ac-
tivities, such as alkaloid cryptolepine (A; Figure 1) (5-
methyl-5H-indolo[3,2-b]quinoline), cryptotackieine (B)
(5-methyl-5H-indolo[2,3-b]quinoline) and cryptosan-
guinolentine (C) (5-methyl-5H-indolo[3,2-c]quinoline),
all having potent antiplasmodial and antimalarial activi-
ties.1–3 Isoneocryptolepine (indolo[2,3-c]quinoline) (D), a
synthetic analogue of the above natural products, also
shows low resistant activity to chloroquine and pyrimeth-
amine, but has much better selectivity index (cytotoxici-
ty/antiplasmodial activity ratio) than cryptolepine, which
makes it a better compound for further validation of the
indoloquinolines as a potential antiplasmodial drugs. The
indoloquinoline alkaloids have been used extensively as
lead compounds for the discovery of new antiplasmodial
substances.
The reactions proceeded via photochemical cyclization of
aryl azides 1 to form the corresponding indole ring 2 first
and then photochemical 6π-electrocyclic reaction to form
the cyclic dihydroquinolin-2-one intermediate which is
aromatized by oxidative dehydrogenation to give the
product indolo[2,3-c]quinolin-6(7H)-ones 3.
We first optimized the photoreaction conditions using 3-
(2-azidophenyl)-N-phenylacrylamide (1a) as a model
substrate which was prepared by the Wittig reaction of 2-
azidobenzaldehyde with triphenyl phenylcarbamoylmeth-
yl phosphonium salt according the reported procedure
(Scheme 2).8a,b
Many reports have been found for the synthesis of indolo-
quinoline alkaloids,1–4 but for the unnatural isoneocrypto-
lepine (indolo[2,3-c]quinoline) derivatives, especially
indolo[2,3-c]quinolin-6(7H)-ones which was found to
possess antitumor activities,5 only a few of synthetic
methods were found in literature,6 for example,
Pd(OAc)2-catalyzed intramolecular Heck reaction of N-
CH3
N
H3C
N
N
CH3
N
N
CH3
N
N
N
cryptosanguinolentine (C)
isoneocryptolepine (D)
cryptolepine (A)
cryptotackieine (B)
Figure 1
SYNLETT 2013, 24, 0073–0078
Advanced online publication: 05.12.2012
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9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0032-1317703; Art ID: ST-2012-W0794-L
© Georg Thieme Verlag Stuttgart · New York

74
Z. Li et al.
LETTER
R3
R3
R3
R2
R1
hν
R2
R1
hν, O2
R2
R4
R5
R4
R5
N
O
R4
R5
C6H6
CH2Cl2
C6H6
CH2Cl2
N
N
O
R1
N
O
N
N3
H
H
1
2
3
Scheme 1 One-pot two-step photocyclization of 3-(2-azidophenyl)-N-phenylacrylamides
The reaction of 1a was examined in different oxygen-sat-
urated solvents (Scheme 3) in a Pyrex flask (λ ≥280 nm)
because it was suggested that the irradiation of aromatic
amide systems with the 254 nm light largely induced pho-
to-Fries rearrangement,9 but the irradiation with λ ≥300
hν, O2
solvent, r.t.
NH
O
NH
O
N
N3
nm light rather promoted cyclization.6e
H
3a
1a
NHCOCH2P+(Ph)3 Cl–
NHCOCH2Cl
NH2
Scheme 3 Photoreaction of 1a in different solvents
ClCH2COCl
Ph3P
toluene
reflux
K2CO3
acetone
Table 1 Optimization of Reaction Conditions for the Synthesis of
3aa
5a
6a
Entry Solvent
Time
(h)
Conv.
(%)b
Yield
(%)c
O
O
NHCOCH2P+(Ph)3 Cl–
+
Ph
DAP
N
H
H
1
2
3
4
5
6
7
8
9
MeCN
20
20
20
16
15
14
12
11
13
93
83
70
82
85
89
90
96
91
0d
75e
30
45
68
66
70
76
70
CHCl3
N3
N3
4a
benzene
6a
1a
benzene–acetone (5:1)
benzene–EtOH (5:1)
benzene (5:1)
Scheme 2 The procedure for the preparation of 1a
As shown in Table 1, the reaction efficiency was not sat-
isfactory in nonpolar solvent benzene or polar solvent ace-
tonitrile because of the low solubility of substrate 1a in
benzene and no 3a was formed in acetonitrile except for
the indole-2-carbamide 2a. Thus, the photoreaction was
then examined in a mixture of solvents. It was observed
that the photoreaction efficiency of 1a increased in sol-
vent mixture and reached its highest value in a 9:1 mixture
of benzene and dichloromethane. This result was probably
ascribed to the solvent effects on the two photocycliza-
tions because the weak polar solvents such as benzene, cy-
clohexane and dioxane favored the oxidative
photocyclization of amide systems,6e,10 whereas polar sol-
vents such as dichloromethane and acetonitrile were fa-
vorable to the photochemical cyclization of aryl azides7
and photo-fries rearrangement.9 The addition of small
quantity of dichloromethane to benzene not only en-
hanced the reaction of aryl azides, but also increased the
solubility of substrates. Therefore, irradiation of sub-
strates 1 in a mixture of solvents benzene and dichloro-
methane (9:1) in Pyrex flask was selected as the optimal
conditions for the synthesis of indolo[2,3-c]quinolin-
6(7H)-ones.
benzene–CH2Cl2 (2:1)
benzene–CH2Cl2 (5:1)
benzene–CH2Cl2 (9:1)
benzene–CH2Cl2 (15:1)
a Reaction conditions: compound 1a (128 mg, 0.5 mmol) was dis-
solved in anhyd benzene (54 mL) and anhyd CH2Cl2 (6 mL). The so-
lution was bubbled with oxygen for 10 min and irradiated at λ ≥280
nm with a medium-pressure mercury lamp (500 W) at ambient tem-
perature.
b Conversions calculated on the basis of substrate.
c Yields of isolated product based on consumed substrate.
d No 3a was separated except for the N-phenyl-2-indolecarbamide
(2a).
e The substrate 1a was not dissolved completely because of its low
solubility in benzene.
We then examined the photocyclization of a group of 3-
(2-azidophenyl)-N-phenylacrylamides with different sub-
stituents on both azidobenzene ring and aniline ring under
the optimized conditions.11The photoreactions of all these
substrates (1b–p) afforded the corresponding cyclization
products (3b–p) in moderate to high yields after irradia-
tions for 5–25 hours at room temperature (Table 2). All
products were fully identified by H NMR, 13C NMR,
1
HRMS and IR.11 It can be observed from Table 1 that the
Synlett 2013, 24, 73–78
© Georg Thieme Verlag Stuttgart · New York

LETTER
One-Pot Synthesis of Indolo[2,3-c]quinolin-6-ones
75
hν
+
N
N
N
H
H
H
– N2
–
N
••
N
O
O
N3
O
Ia
IIa
1a
+
N
H
H
N
hν
hν
N
H
O–
O
N
N
O
H
N
H
H
IIIa
2a
H
+
O2
– H2O2
N
O
H
N
H
H
O–
N
N
H
H
3a
IVa
Scheme 4 A plausible photoreaction mechanism of 1a
photoreaction efficiency was different for the substrates decomposition product methyl 2-aminobenazoate and
with different substituents. The electron-donating groups, photo-Fries rearrangement products could be separated
such as the methoxy and methyl groups (1f and 1g) on the from the reaction mixture besides 2k and 3k after irradia-
aniline ring, retarded the photoreaction of 1f and 1g and tion of 1k for 25 hours. This kind of result was also report-
more time was needed to reach the high conversion (en- ed in the photocyclization of benzanilides under the
tries 6 and 7; 18 h and 16 h, respectively). In contrast, irradiation at λ ≥280 nm.9b On the other hand, the substit-
electron-attracting groups, such as chlorine atom (1h, 1i) uents on the azidophenyl ring also had some effects on the
and carboxylate group (1j) had no significant effects on photoreactions of 1m–p. For example, meta-chlorine
the photoreaction rate of 1h–j (entries 8–10) in compari- atom in 1l,m (entries 13 and 14) accelerated the photore-
son with 1a, but deceased the yields of 3h–j greatly (entry action in comparison to that of 1b,c (entries 2 and 3). Dif-
10). Moreover, the ortho-ethoxycarbonyl group in 1k led ferently, para-ethoxycarbonyl group led to the decrease of
to a significant decrease in both the reaction rate and the both the reaction rate of 1l,m and the yield of products
yield of product 3k (entry 11). These results were proba- 3l,m (entries 13 and 14) as compared with the reactions of
bly derived from the decrease of pericyclic reaction prob- 1d,e (entries 4 and 5); the alkoxy groups in 1l lowered the
ability of the intermediate product 2k and the increase of yield of 3l (entry 12) as compared with that of 3d (entry
photo-Fries rearrangement products of 2k because one or- 4) although the reaction times needed were similar.
tho-position was blocked in 2k. In fact, small quantities of
Table 2 Photocyclization of (E)-3-(2-Azidophenyl)-N-phenylacrylamides in Benzene–Dichloromethanea
Entry
Substrate
Time (h)
Conv. (%)b Product
Yield (%)c
1
1a
11
96
92
3a
3b
76
NH
O
NH
N
H
N3
O
N
O
Me
2
1b
15
68
N
Me
N
H
N3
O
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 73–78

76
Z. Li et al.
LETTER
Table 2 Photocyclization of (E)-3-(2-Azidophenyl)-N-phenylacrylamides in Benzene–Dichloromethanea (continued)
Entry
3
Substrate
Time (h)
10
Conv. (%)b Product
Yield (%)c
1c
94
3c
60
N
O
N
N
H
N3
N3
N3
O
N
O
4
5
1d
1e
6
5
98
99
3d
3e
78
85
N
N
H
O
N
O
N
N
H
O
OMe
MeO
6
7
1f
1g
1h
1i
18
16
11
12
9
92
90
96
96
95
3f
3g
3h
3i
75
65
80
83
52
NH
O
NH
N
H
N3
O
Me
Me
NH
O
NH
N
H
N3
O
Cl
Cl
8
NH
O
NH
N
H
N3
O
Cl
Cl
9
NH
O
NH
N3
O
N
H
COOMe
MeOOC
10
1j
3j
NH
O
NH
N
N3
H
O
Synlett 2013, 24, 73–78
© Georg Thieme Verlag Stuttgart · New York

LETTER
One-Pot Synthesis of Indolo[2,3-c]quinolin-6-ones
77
Yield (%)c
40
Table 2 Photocyclization of (E)-3-(2-Azidophenyl)-N-phenylacrylamides in Benzene–Dichloromethanea (continued)
Entry
11
Substrate
Time (h)
25
Conv. (%)b Product
COOMe
COOMe
NH
O
1k
93
96
95
95
95
94
3k
3l
NH
N
N3
O
H
N
O
12
13
14
15
16
1l
5
8
7
9
9
60
57
56
50
52
O
O
O
O
N
N
N3
O
H
N
O
Me
1m
1n
1o
1p
3m
3n
3o
3p
N
Me
N
Cl
N3
O
Cl
H
N
O
N
N
H
N3
O
Cl
Cl
MeOOC
N
O
MeOOC
N
N
N3
O
H
N
O
MeOOC
MeOOC
N
N
N3
O
H
a Reaction conditions: compound 1 (0.5 mmol) was dissolved in anhyd benzene (54 mL) and anhyd CH2Cl2 (6 mL). The solution was bubbled
with oxygen for 10 min and irradiated at λ ≥280 nm with a medium-pressure mercury lamp (500 W) at ambient temperature.
b Conversions calculated on the basis of substrate.
c Yields of isolated product based on consumed substrate.
The intermediate products N-phenylindol-2-carbamides tack of nitrene Ia to the β-position of the adjacent double
2, produced in the first photocyclization of 1a–p (Scheme bond to form a zwitterion IIa10 and the subsequent shift of
1), were monitored in the progress of photoreactions. It hydrogen atom from β-carbon to nitrogen atom to give the
was found that no intermediate products 2 were detected intermediate aryl anilide 2a. Photochemical 6π-electrocy-
in most cases except for the photoreaction of 1c, 1d, 1e clic reaction of 2a forms the cyclic intermediate (IVa)
and 1k in which a small quantity of intermediate 2c, 2d, which then is dehydrogenated by oxygen to give the final
2e and 2k , respectively, could be separated. It meant that product 3a.10a,12
the oxidative photocyclization of intermediates 2 (the sec-
ond photocyclization) proceeded more quickly than the
photocyclization of 1 in most cases; only for 1c,1d, 1e and
In summary, we have developed an efficient method for
the synthesis of indolo[2,3-c]quinolin-6(7H)-ones, indo-
lo[2,3-c]pyrrolo[3,2,1-i,j]quinolin-7-ones and indolo[2,3-
1k, the photocyclization of intermediates 2c, 2d, 2e and
c]pyrido[3,2,1-i,j]quinolin-8-ones by a one-pot photocy-
2k was slower than the photocyclization of 1c, 1d, 1e and
clization of 3-(2-azidophenyl)-N-phenylacrylamides. The
1k.
reactions proceed by two sequential cyclization reactions,
A reasonable mechanism to account for the formation of namely photocyclization of 2-azidocinnamides and 6π-
indolo[2,3-c]quinolin-6(7H)-ones is presented in Scheme electrocyclic reaction of the formed intermediate aryl an-
4. Photocyclization of azide 1a is believed to occur via at- ilides.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 73–78

78
Z. Li et al.
LETTER
T.; Vanelle, P. Synlett 2005, 3047. (f) Fresneda, P. M.;
Acknowledgment
Molina, P. Synlett 2004, 1.
We thank the National Natural Science Foundation of China (Grant
Nos. 20872056 and J1103307) for financial support.
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(11) Typical Experimental Procedure: A solution 1a (128 mg,
0.5 mmol) in a mixture of anhyd benzene (54 mL) and anhyd
CH2Cl2 (6 mL) was bubbled with oxygen for 10 min and
irradiated at λ ≥280 nm with a medium-pressure mercury
lamp (500 W) in three 25-mL Pyrex flasks at ambient
temperature. The progress of reaction was monitored by
TLC at regular intervals. After the solvent was removed
under reduced pressure, the residue was separated by silica
gel column chromatography eluted with hexane–EtOAc
(5:1) to yield the product 3a.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett.SnoIufproig
m
iotSrat
ungIifoop
r
t
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Synlett 2013, 24, 73–78
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