A facile synthesis of 6,12-disubstituted 5,7-dihydroindolo[2,3-b] carbazoles from the reaction of 1H-indole and aldehydes catalyzed by molecular iodine

Article abstract of DOI:10.1055/s-2008-1000849

The reaction of 1H-indole and various aldehydes in the presence of molecular iodine as a catalyst in refluxing acetonitrile affords 6,12-disubstituted 5,7-dihydroindolo[2,3-b]carbazoles in good to excellent yields. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1000849

286
PAPER
A Facile Synthesis of 6,12-Disubstituted 5,7-Dihydroindolo[2,3-b]carbazoles
from the Reaction of 1H-Indole and Aldehydes Catalyzed by Molecular Iodine
F
acile Synthesis of 6, 1
o
2-Disubstitu
h
ted 5,7-Dihy
i
t
lo[2,3-
b
]carbLazoles . Deb, Biswajita Baruah, Pulak J. Bhuyan*
North East Institute of Science & Technology, Jorhat 785 006, India
Fax +91(376)2370011; E-mail: pulak_jyoti@yahoo.com
Received 24 September 2007
EtO2C
CO2Et
Abstract: The reaction of 1H-indole and various aldehydes in the
presence of molecular iodine as a catalyst in refluxing acetonitrile
affords 6,12-disubstituted 5,7-dihydroindolo[2,3-b]carbazoles in
good to excellent yields.
N
N
H
H
Key words: indolo[2,3-b]carbazole, indole, aldehyde, molecular
iodine, antitumor
OMe
Figure 1 Diethyl 6-methoxy-5,7-dihydroindolo[2,3-b]carbazole-
2,10-dicarboxylate
Since the first isolation1 of an indolocarbazole (ICZ) alka-
loid in 1977, the importance of this family of natural prod-
ucts has been recognized by chemists, biologists,
physicians, and pharmaceutical companies.2 Compounds
with this ring system possess significant biological activ-
ity. Therefore, considerable efforts have been made to
prepare and synthetically alter these molecules to find
useful compounds.3 As a result of these efforts a number
of ICZ analogues have been obtained with diverse biolog-
ical activity, some of which are currently potential drugs4
and others are being tested in clinical trials to evaluate
their future use against cancer and other diseases.2,5
itations in terms of their reaction conditions, yield, and
generality.
The use of molecular iodine in organic synthesis has been
known for a long time and, in recent years, it has received
considerable attention as an inexpensive, nontoxic, readi-
ly available, and mild Lewis acid catalyst for various or-
ganic reactions under convenient conditions affording the
desired products in excellent yields.11 Moreover, it has
high solubility in organic solvents and is easily removed
from the reaction mixture.
As a part of our continued12 work on indoles and the syn-
thesis of diverse heterocyclic compounds of biological
significance, we reporting here a very simple, highly effi-
cient, and cost-effective procedure for the synthesis of
Indolo[2,3-b]carbazole is one of the five possible isomeric
indolocarbazoles that possesses anti-tumor, anticancer ac-
tivity (Figure 1).6 Interestingly, a literature survey reveals
only a few reports on the synthesis of indolo[2,3-b]carba-
zoles. Von Dobeneck and Maas reported the first synthe-
sis of indolo[2,3-b]carbazole from an acid-catalyzed
reaction of 1H-indole and formaldehyde,7 which was later
contradicted by other reports.3c,8 Bearse et al. reported the
synthesis of indolo[2,3-b]carbazole from catalytic cyclo-
dehydrogenation of N,N-diphenyl-m-phenylenediamine
in 1961.8 The reactions were carried out in the vapor phase
at very high temperature (500 °C) and the products were
obtained in less than 5% yield. In the same year, as an ex-
tension of the work of Von Dobeneck, Venkiteswaran and
co-workers reported the synthesis of indolo[2,3-b]carba-
zoles using acetone with indoles in presence of maleic
acid or ethanolic hydrochloric acid.9 However, this reac-
tion has a number of disadvantages, e.g. long reaction
times, drastic conditions, a tedious workup procedure,
poor yields of the desired products, and, particularly, the
lack of generality. Since then only two reports10 have been
made in literature for the synthesis of indolo[2,3-b]carba-
zoles, but these also have a number of drawbacks and lim-
6,12-disubstituted
5,7-dihydroindolo[2,3-b]carbazoles
from the reaction of indoles with aldehydes using molec-
ular iodine (I2) as the catalyst.
The reaction of indoles and aldehydes is very complicated
and depends on the reaction conditions and the catalyst
used. In the reaction of indoles and aldehydes it was ob-
served that the use of protic acids like hydrochloric acid
or sulfuric acid gave indolo[3,2-b]carbazoles via isomer-
ization of an intermediate formed during the reaction.13
The mechanism proposed for such reaction shows the in-
volvement of acid proton (H+) in the reaction. On the other
hand, the use of a catalyst like phosphoryl chloride gives
a mixture of both the indolo[2,3-b]- and indolo[3,2-b]car-
bazoles via a different mechanism.10a In a recent study it
was reported that the reaction of two equivalents of 1H-in-
dole and one equivalent of an aldehyde in presence of io-
dine at room temperature in a very short time affords
bis(indol-3-yl)methanes.11a On the other hand when the
same reaction is allowed to continue for an extended peri-
od (14 h) at room temperature the bis(indol-3-yl)methane
isomerized to an (indol-2-yl)(indol-3-yl)methane.14 By
observing various mechanisms, we planned to study the
reaction using equimolar amounts of 1H-indole and an al-
dehyde under thermal conditions without giving time for
SYNTHESIS 2008, No. 2, pp 0286–0292
x
x
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x
x
.2
0
0
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Advanced online publication: 18.12.2008
DOI: 10.1055/s-2008-1000849; Art ID: P11307SS
© Georg Thieme Verlag Stuttgart · New York

PAPER
Facile Synthesis of 6,12-Disubstituted 5,7-Dihydroindolo[2,3-b]carbazoles
287
isomerization of the bis(indol-3-yl)methane formed dur- could not prepare the indolo[2,3-b]carbazoles of 4-ni-
ing the reaction and allowing a second aldehyde molecule trobenzaldehyde. 4-(Benzoyloxy)- (2j) and 4-chlorobenz-
to become involved in the reaction to afford indolo[2,3- aldehyde (2d) gave the desired compounds 3j and 3d,
b]carbazoles.
respectively, but in comparatively long reaction times. In
the case of aliphatic aldehydes, the yields were low. Het-
erocyclic aldehydes (e.g., 1H-pyrrole-2-carbaldehydes,
furfuraldehyde) were not found to react with 1H-indole
(1) in the presence of iodine to give the desired indo-
lo[2,3-b]carbazoles. The reaction stopped at the bis(indol-
3-yl)methane stage. Interestingly, the use of 1H-indole-3-
carbaldehyde as an aldehyde gave tris(indol-3-yl)methane
(TIM) in excellent yield (>98%). The reaction of 1H-in-
dole-3-carbaldehyde with 1H-indole (1) leading to the
formation of tris(indol-3-yl)methane has been reported by
several authors15 using some unfamiliar catalysts. There-
fore, iodine is potentially a much better alternative for this
reaction. Under identical conditions, bulky aldehydes
such as 2-chloroquinoline-3-carbaldehyde gave simply
the bis(indol-3-yl)methane products.
In our reaction strategy, utilizing equimolar amounts of
1H-indole (1) and benzaldehyde (2a) in the presence of a
catalytic amount of molecular iodine (2 mol%) in reflux-
ing acetonitrile afforded 6,12-diphenyl-5,7-dihydroindo-
lo[2,3-b]carbazole (3a) in excellent yield. The product
was isolated simply by filtration and was then recrystal-
lized from a mixture of N,N-dimethylformamide and chlo-
roform. The structure of the compound was determined
from spectroscopic data and elemental analysis. The gen-
erality of the reaction was established by utilizing 1H-in-
dole (1) with various aromatic or aliphatic aldehydes 2b–
l (Scheme 1) The products 3b–l were obtained in good to
excellent yields in short reaction times (4–45 min,
Table 1).
In the study it was observed that substituents in the aro-
matic ring of the aldehyde have a tremendous effect on the
reaction process. Electron-withdrawing groups in the aro-
matic ring of the aldehyde retarded the reaction while
electron-donating groups accelerated the reaction; we
1-Methyl-1H-indole did not react under these reaction
conditions even on refluxing for more than one hour. It
was also noticed that the yield of the products decreased
with increasing load of the catalyst.
A reasonable mechanism for the formation of compounds
3 is outlined in Scheme 2. The reaction occurs via initial
formation of bis(indol-3-yl)methane 4, which reacts with
the second aldehyde molecule in the presence of iodine
and eliminates a molecule of water to give the intermedi-
ate 5, which is not isolable and is subsequently oxidized
to the fully aromatized 6,12-disubstituted 5,7-dihydroin-
dolo[2,3-b]carbazole 3. The molecule of iodine first cata-
lyzed the reaction for the formation of bis(indol-3-
yl)methane 4 and then enhances the electrophilic charac-
R1
MeCN
R1CHO
+
N
I2, heat
N
N
H
H
H
R1
1
2
3
Scheme 1
Table 1 6,12-Disubstituted 5,7-Dihydroindolo[2,3-b]carbazoles 3
from 1H-Indole (1) and Aldehydes 2
R1
Entry Producta R1
Time Yieldb Mp
(min) (%) (°C)
R1CHO
+
N
I2
HN
H
N
1
2
3a
3b
3c
3d
3e
3f
Ph
5
4
75
80
79
50
46
70
77
75
85
72
25
20
395–396
H
1
2
4
4-MeC6H4
4-MeOC6H4
4-ClC6H4
2-ClC6H4
4-HOC6H4
2-HOC6H4
3-MeO-4-HOC6H3
3,4,5-(MeO)3C6H2
4-BzOC6H4
Me
>400
R1
R1
3
4
377
– H2O
4
25
25
5
355–357
348–351
398–399
384–386
381 (dec)
>400
R1
HN
N
N
N
R1CH
....
H
5
H
H
HO
6
O
I2
R1
7
3g
3h
3i
7
R1
8
7
N
N
N
N
H
H
9
4
H
R1
R1
5
10
11
12
3j
18
35
45
351 (dec)
282–283
311–312
R1
3k
3l
[O]
– H2
Et
N
N
H
H
a All the reactions were performed on a 2 mmol scale using I2 (2
3
R1
mol%).
b Isolated yield.
Scheme 2
Synthesis 2008, No. 2, 286–292 © Thieme Stuttgart · New York

288
M. L. Deb et al.
PAPER
ter of the second aldehyde molecule by a loose coordina- an electron-donating substituent in the aromatic ring, to
tion with oxygen, to react and give the product.
get the unsymmetrical indolo[2,3-b]carbazoles. We also
failed in the reverse process, i.e. by using an electron-do-
nating group in aromatic ring of the aldehyde for the syn-
thesis of the bis(indol-3-yl)methane and an electron-
withdrawing group in the aromatic ring of the aldehyde in
the next step.
As proposed in the mechanism the NH proton is required
for the formation of the desired product. It is supported by
the non-formation of indolo[2,3-b]carbazoles from the re-
action of 1-methyl-1H-indole and arylaldehydes.
The failure to synthesize indolo[2,3-b]carbazole by utiliz-
ing heterocyclic aldehydes might be due to conjugation of
the lone pair on the heteroatom with the exocyclic double
bond of the intermediate formed during the process,
which makes the exocyclic b-carbon less electrophilic for
further attack by the a-position of second indole moiety
(Figure 2).
R1
R2CHO
R1CHO
+
N
I2
H
HN
N
H
1
4
2
R1
R1
R1
[O]
N
N
HN
N
H
N
N
H
H
H
R2
R2
O
:
6
Scheme 3
Figure 2
In order to explore the synthetic utility of the process fur-
ther, we attempted the reaction of acetone with 1H-indole
(1) in the presence of iodine. Unfortunately we could not
determine the structure of the product, which has a molec-
ular weight of 386.2.
The mechanism and the formation of the product were es-
tablished by performing the reaction in two steps
(Scheme 3). First we synthesized bis(indol-3-yl)methanes
4 from 1 and 2 using iodine as a catalyst by following the
existing method.11a The bis(indol-3-yl)methanes 4 so ob-
tained were reacted with an equimolar amount of an alde-
hyde (R1 = R2) in refluxing acetonitrile, using iodine as
the catalyst. The products isolated from the reaction in
each case were characterized and found to be identical in
all respects with the compounds 3 obtained in the one-pot
process (Scheme 1). Then, we synthesized various un-
symmetrical 6,12-disubstituted 5,7-dihydroindolo[2,3-
b]carbazoles 6a–h following the same procedure but us-
ing a different aldehyde (R1 ≠ R2) in the second step
(Table 2, Scheme 3). The structures of the compounds
were confirmed by spectroscopic data and elemental anal-
In summary, we have reported a very simple, mild, and
highly efficient method for the synthesis of 6,12-disubsti-
tuted 5,7-dihydroindolo[2,3-b]carbazoles 3a–l from the
reaction of 1H-indole (1) and aldehyde in the presence of
molecular iodine as the catalyst. The formation of the
product is explained by a suitable mechanism. The mech-
anism was established by performing the reaction in two
steps. By exploring the two-step reaction sequence, a
number of unsymmetrical indolo[2,3-b]carbazoles 6a–h
were synthesized. This very simple, mild, and cost-effec-
tive procedure for the synthesis of various symmetrical
and unsymmetrical 6,12-disubstituted 5,7-dihydroindo-
lo[2,3-b]carbazoles is a valuable addition to the chemistry
of indolocarbazoles.
1
ysis. The appearance of a sharp singlet in the H NMR
spectra and in the IR for the two symmetrical NH protons
of the indolo[2,3-b]carbazoles ruled out the formation of
indolo[3,2-b]carbazoles and further evidenced their struc-
tures. It has already been mentioned that under identical
reaction conditions bulky aldehydes, e.g. 2-chloroquino-
line-3-carbaldehyde, give simply the bis(indol-3-yl)meth-
ane products. But when the bis(indol-3-yl)methanes so
obtained were reacted with simple aromatic aldehydes,
e.g. benzaldehyde, under similar conditions, the desired
6,12-disubstituted 5,7-dihydroindolo[2,3-b]carbazole, for
example 6g, were obtained in good yield (Table 2). In the
study we found that the use of aromatic aldehydes con-
taining strongly electron-withdrawing groups in the aro-
matic ring does not result in the formation of the desired
product; the reaction stops at the bis(indol-3-yl)methane
stage. We failed in our attempt to react such bis(indol-3-
yl)methane compounds with another aldehyde possessing
All reagents and solvents were of reagent grade. All chemicals, in-
cluding solvents, were used without drying. Indole, aldehydes, and
I2 were purchased from Aldrich Chemical Co. and other commer-
cially available reagents were used without further purification. IR
spectra were recorded on Perkin Elmer system-2000 FTIR spec-
trometer; 1H NMR and 13C NMR spectra were recorded on Bruker
Avance-DPX 300 MHz and 75 MHz FT NMR in DMSO-d6 using
TMS as an internal standard. MS spectra were recorded in Bruker
Daltonics ESQUIRE 3000 LC ESI ion trap mass spectrometer. Ele-
mental analyses were performed on Perkin Elmer-2400 spectrome-
ter. Analytical TLC was performed using E. Merck aluminum-
backed silica gel plates coated with a 0.2 mm thickness of silica gel;
petroleum ether = PE. Melting points (uncorrected) were deter-
mined on a Buchi B-540 apparatus. All products were characterized
by spectroscopic data and elemental analysis.
Synthesis 2008, No. 2, 286–292 © Thieme Stuttgart · New York

PAPER
Facile Synthesis of 6,12-Disubstituted 5,7-Dihydroindolo[2,3-b]carbazoles
289
Table 2 Unsymmetrical 6,12-Disubstituted 5,7-Dihydroindolo[2,3-b]carbazoles 6 from Bis(indol-3-yl)methanes 4 and Aldehydes 2
Producta
6a
R1
R2
Time (min)
Yieldb (%)
Mp (°C)
391–393
385–386
377–379
355–356
348–351
381–382
375–378
331–332
Ph
4-MeC6H4
Ph
5
4
75
80
79
72
74
69
55
40
6b
4-MeC6H4
4-MeC6H4
4-MeOC6H4
4-MeOC6H4
Ph
6c
4-MeOC6H4
4-MeC6H4
Ph
4
6d
5
6e
5
6f
4-MeOC6H4
Ph
5
6gc
6h
2-chloroquinolin-3-yl
Me
8
Ph
30
a All the reactions were performed on a 1-mmol scale using I2 (2 mol%).
b Isolated yield.
c The aldehyde has been prepared by a reported method.12h
6,12-Diaryl-5,7-dihydroindolo[2,3-b]carbazoles 3a–l; General
Procedure
MS: m/z = 435.3 [M – H]+.
Anal. Calcd for C32H24N2; C, 88.04; H, 5.54; N, 6.42. Found: C,
88.09; H, 5.55; N, 6.46.
In a simple experimental procedure, equimolar amounts of indole 1
(2 mmol) and aldehyde 2 (2 mmol) were added to a round-bottomed
flask containing MeCN (5 mL). I2 (2 mol%) was added to the mix-
ture and it was refluxed for 4–25 min (Table 1). The solid com-
pound obtained was filtered, dried, and recrystallized (DMF–
CHCl3). There was no need to remove I2 with Na2S2O3.
6,12-Bis(4-methoxyphenyl)-5,7-dihydroindolo[2,3-b]carbazole
(3c)
Whitish powder; yield: 370 mg (79%); mp 377 °C; Rf = 0.80 (7%
EtOAc–PE).
IR (KBr): 3392 (NH stretch), 3061 (w), 3031 (w), 2965 (w), 2837
(w), 1492 (w), 1456 (s), 1248 (m), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 3.40 (s, 3 H), 3.42 (s, 3 H),
6.75–6.79 (t, J = 7.41 Hz, 2 H), 6.91–6.93 (t, J = 7.34 Hz, 2 H), 7.1–
7.13 (d, J = 7.79 Hz, 4 H), 7.14–7.16 (d, J = 7.81 Hz, 2 H), 7.21–
7.24 (t, J = 7.56 Hz, 5 H), 7.81 (s, 1 H), 9.98 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 159.13, 158.68, 141.10,
139.32, 136.55, 136.16, 134.27, 134.18, 133.10, 132.26, 129.33,
128.05, 122.21, 122.0, 121.68, 120.22, 118.78, 109.11, 55.20,
55.06.
6,12-Diphenyl-5,7-dihydroindolo[2,3-b]carbazole (3a)
White cotton-like solid; yield: 306 mg (75%); mp 395–396 °C; Rf =
0.82 (7% EtOAc–PE).
IR (KBr): 3395 (NH stretch), 3062 (w), 3019 (w), 1492 (w), 1456
(s), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 6.72–6.77 (t, J = 7.41 Hz, 2
H), 6.87–6.92 (t, J = 7.32 Hz, 2 H), 6.98–7.0 (d, J = 7.71 Hz, 4 H),
7.05–7.08 (d, J = 7.83 Hz, 2 H), 7.14–7.19 (t, J = 7.14 Hz, 6 H),
7.54 (s, 2 H), 9.71 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 140.10, 139.42, 136.22, 134.0,
133.66, 133.37, 132.29, 132.18, 129.60, 129.20, 128.09, 126.5,
123.92, 123.06, 121.65, 120.0, 118.76, 112.42.
MS: m/z = 469.5 [M + H]+.
Anal. Calcd for C32H24N2O2; C, 82.03; H, 5.16; N, 5.98. Found: C,
82.07; H, 5.10; N, 5.93.
MS: m/z = 409.5 [M + H]+.
Anal. Calcd for C30H20N2; C, 88.21; H, 4.93; N, 6.86. Found: C,
88.26; H, 4.88; N, 6.89.
6,12-Bis(4-chlorophenyl)-5,7-dihydroindolo[2,3-b]carbazole
(3d)
Light yellowish powder; yield: 238 mg (50%); mp 355–357 °C;
Rf = 0.81 (7% EtOAc–PE).
6,12-Bis(4-methylphenyl)-5,7-dihydroindolo[2,3-b]carbazole
(3b)
Shiny white solid; yield: 348 mg (80%); mp >400 °C; Rf = 0.81 (7%
EtOAc–PE).
IR (KBr): 3385 (NH stretch), 3051 (w), 3032 (w), 1493 (w), 1456
(s), 741 (s), 701 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 6.76–6.80 (t, J = 7.47 Hz, 2
H), 6.86–6.89 (t, J = 7.43 Hz, 2 H), 7.0–7.03 (d, J = 7.75 Hz, 4 H),
7.07–7.10 (d, J = 7.76 Hz, 2 H), 7.19–7.22 (t, J = 7.55 Hz, 5 H),
7.71–7.73 (d, J = 3.07 Hz, 1 H), 9.98 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 139.29, 139.16, 137.20,
136.14, 135.21, 134.09, 133.23, 131.26, 131.01, 130.31, 129.07,
128.26, 128.14, 127.46, 123.44, 121.78, 118.77, 111.98.
IR (KBr): 3395 (NH stretch), 3061 (w), 3019 (w), 2930 (w), 2855
(w), 1492 (w), 1456 (s), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 2.29 (s, 3 H), 2.31 (s, 3 H),
6.85–6.90 (t, J = 7.61 Hz, 2 H), 6.99–7.04 (t, J = 7.25 Hz, 2 H),
7.07–7.10 (d, J = 7.68 Hz, 4 H), 7.16–7.18 (d, J = 7.79 Hz, 2 H),
7.24–7.26 (d, J = 7.56 Hz, 5 H), 7.44 (s, 1 H), 9.93 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 141.28, 139.02, 136.29,
134.07, 133.66, 133.23, 132.22, 132.05, 129.63, 129.06, 128.12,
126.15, 123.91, 123.06, 121.68, 120.0, 118.76, 112.06, 20.91,
20.74.
MS: m/z (%) = 477.2 ([M + H]+, 2 35Cl, 100) 479.2 ([M + H]+, 35Cl,
37Cl, 62), 481.2 ([M + H]+, 2 37Cl, 10).
Anal. Calcd for C30H18Cl2N2; C, 75.48; H, 3.80; N, 5.87. Found: C,
75.52; H, 3.86; N, 5.81.
Synthesis 2008, No. 2, 286–292 © Thieme Stuttgart · New York

290
M. L. Deb et al.
PAPER
6,12-Bis(2-chlorophenyl)-5,7-dihydroindolo[2,3-b]carbazole
(3e)
Yellowish fibrous solid; yield: 219.5 mg (46%); mp 348–351 °C;
Rf = 0.80 (7% EtOAc–PE).
6.92–6.94 (t, J = 7.38 Hz, 2 H), 7.14–7.16 (d, J = 7.14 Hz, 3 H),
7.14–7.16 (d, J = 7.20 Hz, 2 H), 7.21–7.24 (t, J = 7.56 Hz, 4 H),
7.83 (s, 1 H), 9.96 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 158.26, 158.09, 156.41,
156.10, 142.54, 140.41, 138.01, 135.14, 133.18, 132.41, 131.39,
131.06, 130.42, 128.42, 128.09, 122.21, 122.06, 121.49, 120.22,
118.76, 109.11, 55.36, 55.11.
IR (KBr): 3385 (NH stretch), 3051 (w), 3032 (w), 1493 (w), 1456
(s), 743 (s), 700 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 6.78–6.84 (t, J = 7.49 Hz, 2
H), 6.86–6.90 (t, J = 7.46 Hz, 2 H), 7.0–7.03 (d, J = 7.74 Hz, 4 H),
7.07–7.09 (d, J = 7.76 Hz, 2 H), 7.21–7.25 (t, J = 7.58 Hz, 5 H),
7.71–7.74 (d, J = 3.01 Hz, 1 H), 10.0 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 139.31, 138.98, 138.75,
137.54, 135.28, 136.24, 134.19, 134.07, 133.23, 131.22, 131.11,
130.31, 129.07, 128.66, 128.16, 127.43, 125.96, 123.44, 121.70,
119.70, 118.77, 111.99.
MS: m/z = 501.5 [M + H]+.
Anal. Calcd for C32H24N2O4; C, 76.79; H, 4.83; N, 5.60. Found: C,
76.74; H, 4.85; N, 5.65.
6,12-Bis(3,4,5-trimethoxyphenyl)-5,7-dihydroindolo[2,3-b]car-
bazole (3i)
Off white solid; yield: 500 mg (85%); mp >400 °C; Rf = 0.74 (7%
EtOAc–PE).
MS: m/z (%) = 477.5 ([M + H]+, 2 35Cl, 100), 479.5 ([M + H]+, 35Cl,
37Cl, 64), 481.5 ([M + H]+, 2 37Cl, 10).
IR (KBr): 3392 (NH stretch), 3061 (w), 2945 (w), 1492 (w), 1456
(s), 1248 (m), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 3.76–3.79 (m, 18 H), 6.85–
6.88 (t, J = 7.47 Hz, 2 H), 7.02–7.06 (t, J = 7.48 Hz, 2 H), 7.14–7.16
(d, J = 7.28 Hz, 3 H), 7.24–7.27 (t, J = 7.47 Hz, 4 H), 7.87 (s, 1 H),
10.06 (s, 2 H, NH).
Anal. Calcd for C30H18Cl2N2; C, 75.48; H, 3.80; N, 5.87. Found: C,
75.56; H, 3.86; N, 5.92.
6,12-Bis(4-hydroxyphenyl)-5,7-dihydroindolo[2,3-b]carbazole
(3f)
White shiny solid; yield: 308 mg (70%); mp 398–399 °C; Rf = 0.56
(7% EtOAc–PE). Due to the lower solubility of 3f, its 13C NMR
spectrum could not be recorded.
13C NMR (75 MHz, DMSO-d6): d = 158.80, 158.61, 158.33,
158.08, 139.13, 136.14, 134.23, 133.32, 132.39, 132.05, 130.88,
128.91, 128.50, 121.96, 121.49, 120.22, 118.70, 109.11, 55.61,
55.34, 55.21, 55.04.
IR (KBr): 3397 (br), 3051 (w), 2922 (w), 1491 (w), 1456 (s), 1216
(m), 741 (s), 698 cm–1 (m).
MS: m/z = 587.5 [M – H]+.
1H NMR (300 MHz, DMSO-d6): d = 5.68 (s, 1 H, OH), 5.70 (s, 1 H,
OH), 6.72–6.75 (t, J = 7.44 Hz, 2 H), 6.84–6.87 (t, J = 7.43 Hz, 2
H), 7.02–7.04 (d, J = 7.73 Hz, 4 H), 7.07–7.09 (d, J = 7.72 Hz, 2 H),
7.21–7.24 (t, J = 7.58 Hz, 5 H), 7.71–7.72 (d, J = 3.02 Hz, 1 H),
9.98 (s, 2 H, NH).
Anal. Calcd for C36H32N2O6; C, 73.45; H, 5.48; N, 4.76. Found: C,
73.51; H, 5.42; N, 4.79.
6,12-Bis[4-(benzoyloxy)phenyl]-5,7-dihydroindolo[2,3-b]carba-
zole (3j)
Orange crystals; yield: 466 mg (72%); mp 351 °C (dec); Rf = 0.72
MS: m/z = 441.4 [M + H]+.
(7% EtOAc–PE).
Anal. Calcd for C30H20N2O2; C, 81.80; H, 4.58; N, 6.36. Found: C,
81.84; H, 4.49; N, 6.31.
IR (KBr): 3394 (NH stretch), 3058 (w), 1737 (s), 1492 (w), 1456
(s), 1271 (m), 745 (s), 702 cm–1 (m).
6,12-Bis(2-hydroxyphenyl)-5,7-dihydroindolo[2,3-b]carbazole
(3g)
Light yellow solid; yield: 339 mg (77%); mp 384–386 °C; Rf = 0.59
(7% EtOAc–PE).
1H NMR (300 MHz, DMSO-d6): d = 6.81–6.84 (t, J = 7.43 Hz, 3
H), 7.06–7.09 (t, J = 7.43 Hz, 4 H), 7.18–7.23 (m, 9 H), 7.29–7.32
(t, J = 7.41 Hz, 3 H), 7.92–7.98 (m, 7 H), 10.09 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 166.04, 165.89, 152.91,
152.72, 141.21, 140.80, 138.31, 136.25, 136.12, 134.29, 133.56,
133.41, 131.23, 131.02, 129.73, 129.59, 128.52, 128.33, 124.29,
124.08, 122.33, 120.26, 118.85, 116.42, 111.13, 110.56, 109.83.
IR (KBr): 3397 (br), 2922 (w), 1592 (s), 1456 (m), 1216 (m), 771
(s), 668 cm–1 (w).
1H NMR (300 MHz, DMSO-d6): d = 6.03 (s, 1 H, OH), 6.06 (s, 1 H,
OH), 6.72–6.76 (t, J = 7.47 Hz, 2 H), 6.84–6.87 (t, J = 7.43 Hz, 2
H), 7.02–7.04 (d, J = 7.74 Hz, 4 H), 7.08–7.10 (d, J = 7.77 Hz, 2 H),
7.21–7.24 (t, J = 7.58 Hz, 5 H), 7.72 (s, 1 H), 10.02 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 158.0, 157.86, 141.11, 139.40,
138.36, 137.42, 136.15, 134.09, 133.28, 131.64, 131.10, 130.47,
128.66, 128.20, 127.44, 125.99, 123.44, 121.70, 119.73, 118.77,
111.98.
MS: m/z = 649.8 [M + H]+.
Anal. Calcd for C44H28N2O4; C, 81.47; H, 4.35; N, 4.32. Found: C,
81.52; H, 4.28; N, 4.37.
6,12-Dialkyl-5,7-dihydroindolo[2,3-b]carbazoles 3k,l; General
Procedure
In a simple experimental procedure, equimolar amounts of indole 1
(2 mmol) and aliphatic aldehyde 2 (2 mmol) were added to a round-
bottomed flask containing MeCN (5 mL). I2 (2 mol%) was added to
the mixture and it was refluxed for 35–45 min (Table 1). The sol-
vent was removed under reduced pressure and the desired com-
pound was isolated from the residue by column chromatography
[silica gel, 100–200 mesh, 10% EtOAc–PE (bp 60–80 °C)].
MS: m/z = 441.3 [M + H]+.
Anal. Calcd for C30H20N2O2; C, 81.80; H, 4.58; N, 6.36. Found: C,
81.87; H, 4.52; N, 6.41.
6,12-Bis(4-hydroxy-3-methoxyphenyl)-5,7-dihydroindolo[2,3-
b]carbazole (3h)
Bright yellow crystals; yield: 375 mg (75%); mp 381 °C (dec); Rf =
0.78 (7% EtOAc–PE).
6,12-Dimethyl-5,7-dihydroindolo[2,3-b]carbazole (3k)
Light yellow solid; yield: 71 mg (25%); mp 282–283 °C; Rf = 0.83
(7% EtOAc–PE).
IR (KBr): 3395 (br), 3061 (w), 3031 (w), 2965 (w), 1492 (w), 1456
(s), 1248 (m), 1234 (m), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 3.40 (s, 3 H), 3.42 (s, 3 H),
5.43 (s, 1 H, OH), 5.44 (s, 1 H, OH), 6.75–6.78 (t, J = 7.40 Hz, 2 H),
IR (KBr): 3396 (NH stretch), 3055 (w), 2933 (w), 1455 (s), 748
cm–1 (w).
Synthesis 2008, No. 2, 286–292 © Thieme Stuttgart · New York

PAPER
Facile Synthesis of 6,12-Disubstituted 5,7-Dihydroindolo[2,3-b]carbazoles
291
1H NMR (300 MHz, DMSO-d6): d = 2.29 (s, 3 H), 2.31 (s, 3 H),
6.94–6.99 (d, J = 7.68 Hz, 2 H), 7.12–7.17 (dd, J = 7.51, 7.55 Hz, 2
H), 7.29–7.34 (m, 2 H), 7.41–7.44 (d, J = 7.18 Hz, 2 H) 10.10 (s, 2
H, NH).
13C NMR (75 MHz, DMSO-d6): d = 138.18, 135.04, 133.29,
129.74, 129.23, 122.71, 118.0, 114.21, 112.20, 112.0, 20.38, 20.21.
J = 7.38 Hz, 6 H), 7.39–7.43 (d, J = 7.47 Hz, 2 H), 7.64 (s, 1 H),
9.98 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 141.30, 138.49, 136.17,
134.57, 133.63, 132.29, 132.05, 129.27, 129.14, 128.54, 128.11,
126.18, 123.98, 123.17, 121.68, 120.07, 118.79, 111.18, 21.31.
MS: m/z = 423.7 [M + H]+.
MS: m/z = 285.5 [M + H]+.
Anal. Calcd for C31H22N2; C, 88.12; H, 5.25; N, 6.63. Found: C,
88.18; H, 5.19; N, 6.58.
Anal. Calcd for C20H16N2; C, 84.48; H, 5.67; N, 9.85. Found: C,
84.42; H, 5.59; N, 9.93.
6-(4-Methoxyphenyl)-12-(4-methylphenyl)-5,7-dihydroindo-
lo[2,3-b]carbazole (6c)
Pale yellow solid; yield: 357 mg (79%); mp 377–379 °C; Rf = 0.80
(7% EtOAc–PE).
6,12-Diethyl-5,7-dihydroindolo[2,3-b]carbzole, (3l)
Orange solid; yield: 62 mg (20%); mp 311–312 °C; Rf = 0.81 (7%
EtOAc–PE).
IR (KBr): 3393 (NH stretch), 3061 (w), 2933 (w), 1455 (s), 748
cm–1 (w).
IR (KBr): 3392 (NH stretch), 3061 (w), 2965 (w), 2849 (w), 1492
(w), 1456 (s), 1248 (m), 745 (s), 702 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 1.63–1.67 (m, 6 H), 2.33 (q,
J = 6.61 Hz, 2 H), 2.34 (q, J = 6.64 Hz, 2 H), 6.98–7.03 (d, J = 7.40
Hz, 2 H), 7.16–7.19 (t, J = 6.98 Hz, 2 H), 7.32–7.36 (m, 2 H), 7.43–
7.49 (t, J = 7.73 Hz, 2 H), 9.99 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 144.47, 144.28, 136.25,
133.12, 130.11, 122.64, 118.10, 115.16, 111.46, 111.10, 29.19,
29.01, 15.47, 15.28.
1H NMR (300 MHz, DMSO-d6): d = 2.27 (s, 3 H), 3.41 (s, 3 H),
6.80–6.84 (t, J = 7.33 Hz, 2 H), 6.96–7.03 (t, J = 7.28 Hz, 2 H),
7.06–7.08 (d, J = 7.58 Hz, 2 H), 7.16–7.21 (t, J = 7.35 Hz, 2 H),
7.24–7.31 (m, J = 7.33 Hz, 5 H), 7.35–7.38 (d, J = 7.53 Hz, 2 H),
7.71 (s, 1 H), 10.10 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 159.18, 140.25, 136.79,
134.27, 133.93, 132.76, 132.36, 129.27, 129.18, 128.24, 128.12,
126.15, 123.87, 123.0 121.68, 120.12, 118.41, 111.09, 55.91, 20.82.
MS: m/z = 313.6 [M + H]+.
MS: m/z = 453.5 [M + H]+.
Anal. Calcd for C22H20N2; C, 84.58; H, 6.45; N, 8.09. Found: C,
84.63; H, 6.48; N, 8.03.
Anal. Calcd for C32H24N2O; C, 84.93; H, 5.35; N, 6.19. Found: C,
84.87; H, 5.40; N, 6.15.
Unsymmetrical Indolo[2,3-b]carbazoles 6a–h; General Proce-
dure
12-(4-Methoxyphenyl)-6-(4-methylphenyl)-5,7-dihydroindo-
lo[2,3-b]carbazole (6d)
Light yellow crystal; yield: 325 mg (72%); mp 355–356 °C; Rf = 0.82
(7% EtOAc–PE).
In a simple experimental procedure, equimolar amounts of bis(in-
dol-3-yl)methane 411a (1 mmol) and aldehyde (1 mmol) were added
to a round-bottomed flask containing MeCN (5 mL). I2 (2 mol%)
was added to the mixture and it was refluxed for 5–30 min
(Table 2). The solid compound obtained was filtered, dried and re-
crystallized (DMF–CHCl3).
IR (KBr): 3392 (NH stretch), 3061 (w), 2922 (w), 2856 (w), 1492
(w), 1456 (s), 1248 (m), 745 (s), 702 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 2.28 (s, 3 H), 3.40 (s, 3 H),
6.80–6.84 (t, J = 7.34 Hz, 2 H), 6.94–7.01 (t, J = 7.24 Hz, 2 H),
7.06–7.09 (d, J = 7.62 Hz, 2 H), 7.14–7.19 (t, J = 7.34 Hz, 2 H),
7.22–7.30 (m, J = 7.32 Hz, 5 H), 7.35–7.37 (d, J = 7.53 Hz, 2 H),
7.71 (s, 1 H), 9.98 (s, 2 H, NH).
6-(4-Methylphenyl)-12-phenyl-5,7-dihydroindolo[2,3-b]carba-
zole (6a)
White cotton-like solid; yield: 316 mg (75%); mp 391–393 °C; Rf =
0.82 (7% EtOAc–PE).
13C NMR (75 MHz, DMSO-d6): d = 159.14, 140.10, 136.74,
134.27, 133.95, 132.71, 132.38, 129.21, 129.11, 128.26, 128.11,
126.14, 123.88, 123.0 121.63, 120.10, 118.74, 111.09, 55.71, 21.0.
IR (KBr): 3395 (NH stretch), 3061 (w), 2930 (w), 2855 (w), 1492
(w), 1456 (s), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 2.28 (s, 3 H), 6.80–6.85 (t,
J = 7.38 Hz, 2 H), 6.94–7.0 (t, J = 7.23 Hz, 2 H), 7.06–7.08 (d,
J = 7.62 Hz, 2 H), 7.13–7.18 (t, J = 7.38 Hz, 2 H), 7.22–7.29 (m,
J = 7.35 Hz, 6 H), 7.35–7.37 (d, J = 7.53 Hz, 2 H), 7.68 (s, 1 H),
9.95 (s, 2 H, NH).
MS: m/z = 453.3 [M + H]+.
Anal. Calcd for C32H24N2O; C, 84.93; H, 5.35; N, 6.19. Found: C,
84.98; H, 5.39; N, 6.22.
13C NMR (75 MHz, DMSO-d6): d = 140.22, 138.88, 136.19,
134.57, 133.61, 132.23, 132.05, 129.27, 129.16, 128.52, 128.12,
126.15, 123.91, 123.09, 121.68, 120.0, 118.54, 111.06, 21.02.
12-(4-Methoxyphenyl)-6-phenyl-5,7-dihydroindolo[2,3-b]car-
bazole (6e)
Off-white solid; yield: 324 mg (74%); mp 348–351 °C; Rf = 0.82
(7% EtOAc–PE).
MS: m/z = 423.3 [M + H]+.
IR (KBr): 3392 (NH stretch), 3055 (w), 2933 (w), 2855 (w), 1492
(w), 1456 (s), 1247 (m), 745 (s), 698 cm–1 (m).
Anal. Calcd for C31H22N2; C, 88.12; H, 5.25; N, 6.63. Found: C,
88.07; H, 5.19; N, 6.69.
1H NMR (300 MHz, DMSO-d6): d = 3.41 (s, 3 H), 6.83–6.87 (t,
J = 7.36 Hz, 2 H), 6.98–7.03 (t, J = 7.29 Hz, 2 H), 7.10–7.12 (d,
J = 7.46 Hz, 2 H), 7.16–7.20 (t, J = 7.33 Hz, 2 H), 7.24–7.32 (m,
J = 7.43 Hz, 6 H), 7.36–7.38 (d, J = 7.50 Hz, 2 H), 7.70 (s, 1 H),
9.97 (s, 2 H, NH).
12-(4-Methylphenyl)-6-phenyl-5,7-dihydroindolo[2,3-b]carba-
zole (6b)
White crystalline solid; yield: 337 mg (80%); mp 385–386 °C; Rf =
0.81 (7% EtOAc–PE).
13C NMR (75 MHz, DMSO-d6): d = 158.98, 138.69, 136.98,
134.27, 133.63, 132.25, 132.05, 129.21, 129.12, 128.25, 128.09,
126.14, 123.91, 123.08 121.64, 120.06, 118.79, 111.06, 55.70.
IR (KBr): 3394 (NH stretch), 3061 (w), 2933 (w), 2855 (w), 1492
(w), 1456 (s), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 2.28 (s, 3 H), 6.82–6.86 (t,
J = 7.36 Hz, 2 H), 6.95–7.0 (t, J = 7.14 Hz, 2 H), 7.06–7.08 (d,
J = 7.61 Hz, 2 H), 7.13–7.17 (t, J = 7.37 Hz, 2 H), 7.22–7.29 (m,
MS: m/z = 439.6 [M + H]+.
Synthesis 2008, No. 2, 286–292 © Thieme Stuttgart · New York

292
M. L. Deb et al.
PAPER
Anal. Calcd for C31H22N2O; C, 84.91; H, 5.06; N, 6.39. Found: C,
84.86; H, 5.09; N, 6.33.
MS: m/z = 347.4 [M + H]+.
Anal. Calcd for C25H18N2; C, 86.68; H, 5.24; N, 8.09. Found: C,
86.76; H, 5.29; N, 8.15.
6-(4-Methoxyphenyl)-12-phenyl-5,7-dihydroindolo[2,3-b]car-
bazole (6f)
Off-white solid; yield: 302 mg (69%); mp 381–382 °C; Rf = 0.78
(7% EtOAc–PE).
Acknowledgement
M.L.D. thanks CSIR (India) for the award of a Senior Research Fel-
lowship.
IR (KBr): 3392 (NH stretch), 3055 (w), 2933 (w), 2855 (w), 1492
(w), 1456 (s), 1244 (m), 745 (s), 698 cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 3.40 (s, 3 H), 6.83–6.87 (t,
J = 7.36 Hz, 2 H), 6.95–7.0 (t, J = 7.32 Hz, 2 H), 7.11–7.14 (d,
J = 7.48 Hz, 2 H), 7.16–7.20 (t, J = 7.33 Hz, 2 H), 7.28–7.35 (m,
J = 7.45 Hz, 6 H), 7.39–7.41 (d, J = 7.48 Hz, 2 H), 7.71 (s, 1 H),
9.97 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 159.10, 138.71, 136.96,
134.29, 133.63, 132.28, 132.05, 129.20, 129.12, 128.29, 128.05,
126.12, 123.90, 123.10, 121.71, 120.09, 118.79, 111.0, 55.90.
References
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Tsuchya, H.; Takahashi, Y.; Masuma, R. J. Antibiot. 1977,
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MS: m/z = 439.4 [M + H]+.
Anal. Calcd for C31H22N2O; C, 84.91; H, 5.06; N, 6.39. Found: C,
84.90; H, 5.02; N, 6.44.
12-(2-Chloroquinolin-3-yl)-6-phenyl-5,7-dihydroindolo[2,3-
b]carbazole (6g)
Bright orange crystals; yield: 271 mg (55%); mp 375–378 °C; Rf =
0.65 (7% EtOAc–PE).
(5) Gribble, G. W.; Berthel, S. J. Stud. Nat. Prod. Chem. 1993,
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IR (KBr): 3417 (NH stretch), 2919 (w), 1631 (s), 1483 (w), 1455
(m), 772 (s), 741 cm–1 (m).
(7) Von Dobeneck, H.; Maas, I. Chem. Ber. 1954, 87, 455.
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1H NMR (300 MHz, DMSO-d6): d = 6.86–6.91 (t, J = 7.47 Hz, 2
H), 7.04–7.09 (t, J = 7.41 Hz, 2 H), 7.29–7.38 (m, J = 8.1 Hz, 4 H),
7.46–7.49 (d, J = 7.47 Hz, 4 H), 7.61–7.68 (m, J = 7.71 Hz, 3 H),
7.74 (s, 1 H), 7.91–7.94 (d, 2 H), 10.45 (s, 2 H, NH).
13C NMR (75 MHz, DMSO-d6): d = 149.54, 144.21, 139.41,
138.23, 138.02, 136.37, 136.13, 134.91, 134.65, 134.21, 133.45,
132.92, 131.49, 129.61, 128.42, 127.31, 127.08, 122.54, 120.87,
118.56, 118.12, 111.91, 111.63.
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26, 4263.
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MS: m/z (%) = 493.9 ([M + H]+, 35Cl, 100), 495.9 ([M + H]+, 37Cl,
31).
Anal. Calcd for C33H20ClN3; C, 80.24; H, 4.08; N, 8.51. Found: C,
80.32; H, 4.04; N, 8.56.
12-Methyl-6-phenyl-5,7-dihydroindolo[2,3-b]carbazole (6h)
White crystalline solid; yield: 138 mg (40%); mp 331–332 °C; Rf =
0.83 (7% EtOAc–PE).
IR (KBr): 3405 (NH stretch), 2931 (w), 1471 (w), 1454 (m), 741
cm–1 (m).
1H NMR (300 MHz, DMSO-d6): d = 2.29 (s, 3 H), 6.98–7.02 (d,
J = 7.17 Hz, 2 H), 7.07–7.11 (d, J = 7.71 Hz, 2 H), 7.19–7.26 (m, 4
H), 7.31–7.36 (m, 3 H), 7.51–7.54 (d, J = 7.15 Hz, 2 H), 10.12 (s, 2
H, NH).
13C NMR (75 MHz, DMSO-d6): d = 140.27, 138.07, 136.41,
133.24, 129.71, 128.23, 127.49, 126.54, 123.91, 118.11, 114.24,
114.10, 111.54, 109.21, 20.09.
Synthesis 2008, No. 2, 286–292 © Thieme Stuttgart · New York