CAN-catalyzed regioselective synthesis of pyrido[2,3-c ]carbazoles by the Povarov reaction

Article abstract of DOI:10.1055/s-0031-1289967

A simple route for the synthesis of 3-methyl-3H-pyrido[2,3-c]carbazoles via Povarov reaction of 3-aminocarbazoles, and ethyl vinyl ether is described. This transformation, catalyzed by cerium(IV) ammonium nitrate (CAN) under mild reaction conditions, prov

Full text of DOI:10.1055/s-0031-1289967

PAPER
253
CAN-Catalyzed Regioselective Synthesis of Pyrido[2,3-c]carbazoles by the
Povarov Reaction
S
ynthesisof Pyrido[2,3
a
-
c
]carbazo
y
les avan Viji, Rajagopal Nagarajan*
School of Chemistry, University of Hyderabad, Hyderabad 500 046, India
E-mail: rnsc@uohyd.ernet.in
Received 20 October 2011; revised 16 November 2011
thesis of functionalized carbocyclic and heterocyclic
compounds15 and various Lewis acid catalysts have been
used to study this reaction.16 Cerium(IV) ammonium ni-
trate (CAN) is a lanthanide reagent that has gained special
Abstract: A simple route for the synthesis of 3-methyl-3H-pyri-
do[2,3-c]carbazoles via Povarov reaction of 3-aminocarbazoles,
and ethyl vinyl ether is described. This transformation, catalyzed by
cerium(IV) ammonium nitrate (CAN) under mild reaction condi-
tions, provides the pyrido[2,3-c]carbazoles in good yields with high attention as a catalyst and it has been employed as a mul-
tipurpose promoter for useful organic transformations17
regioselectivity.
due to its high reactivity, stability in various solvents, low
toxicity, ease of handling, inexpensiveness, and eco-
friendly nature. In this paper we disclose the synthesis of
pyrido[2,3-c]-, pyrido[2,3-b]-, and pyrido[2,3-a]carba-
zoles from various 3-amino- and 1-aminocarbazoles via
Povarov reaction with ethyl vinyl ether (2) as the dieno-
phile, CAN as the catalyst, and acetonitrile as the solvent.
This reaction proceeds under very mild conditions with-
out a co-catalyst or ligand.
Key words: Povarov reaction, annulations, heterocycles, Lewis
acids, regioselectivity
Over the last few decades numerous efforts have been in-
vested in developing aryl- and heteroaryl-annulated car-
bazoles, which have been reported and comprehensively
reviewed.1 In particular, pyridocarbazoles are important
constituents of heteroannulated carbazoles, for example,
ellipticine, olivacine, strellidimine, and janetine, belong
to a group of pyrido[4,3-b]carbazole alkaloids that show
potential biological activity2 (Figure 1). Ellipticine and 9-
methoxyellipticine were isolated from the stems of
Ochrosia elliptica Labill. which belongs to the Apocyn-
aceae family.3 Among them, ellipticine is one of the sim-
plest naturally occurring alkaloids, it occupies a pivotal
position in the field of medicinal chemistry because of its
promising antitumor activity.4 Some of these pyridocarba-
zole derivatives are associated with a wide spectrum of
biological and medicinal activity such as treatment of
breast cancer,5 intercalation into the DNA double helix,6
inhibition of topoisomerase II,7 and anti-HIV agent.8
The synthetic strategy adopted for the synthesis of the py-
rido[2,3-c]carbazole system is outlined in Scheme 1. At
the outset of our studies, 9-ethyl-3-amino-9H-carbazole
(1a) was used as a model substrate to optimize the reac-
tion conditions together with ethyl vinyl ether (2). We
probed the effect of catalysts, catalyst concentration, and
solvents on this Povarov reaction and the results are sum-
marized in Table 1. Our preliminary investigations re-
vealed that zinc triflate was able to catalyze this
transformation but generated the product pyridocarbazole
3a in low yield (entry 23), as a result we searched for al-
ternative Lewis acid catalysts that proved to work more
efficiently in this Povarov reaction.
Numerous methods have been reported for the synthesis
of ellipticine and its structurally modified derivatives.9
However, much less attention has been paid to the synthe-
sis of its isomeric pyridocarbazoles.10 Recently our group
reported the preparation of isomeric pyridocarbazoles in
good yields.11 Several methods for the synthesis of pyri-
docarbazoles are known, however there have been contin-
uous efforts to develop pyridocarbazoles using mild
reaction conditions.
CAN was found to be more effective than other catalysts
examined for the reaction (Table 1). It catalyzes this
Povarov reaction smoothly and furnishes aromatized py-
N
H
N
N
N
O
NH
N
H
The hetero-Diels–Alder reaction has been recognized as
one of the most fascinating areas of current research.12
This method has been used as an efficient approach in the
synthesis of biologically active natural and unnatural
compounds.13 The Povarov (imino-Diels–Alder)
reaction14 has been proven to be a powerful tool for syn-
ellipticine
sterllidimine
N
NH
N
H
N
H
SYNTHESIS 2012, 44, 253–258
Advanced online publication: 20.12.2011
x
x
.x
x
.2
0
1
1
janetine
olivacine
DOI: 10.1055/s-0031-1289967; Art ID: Z99011SS
© Georg Thieme Verlag Stuttgart · New York
Figure 1 Some pyrido[2,3-b]carbazole alkaloids

254
M. Viji, R. Nagarajan
PAPER
Table 1 Optimization Conditions for the Preparation of 3aa
NH2
N
Entry
1
Catalyst
CAN
Solvent
CHCl3
CH2Cl2
dioxane
toluene
MeCN
MeCN
MeCN
THF
Time (h) Yieldb (%)
catalyst
8
6
53
56
28
61
51
78
78
32
25
61
32
72
68
68
42
30
33
46
71
22
16
22
15
32
–
OEt
+
solvent
time, r.t.
2
N
N
2
CANc
Et
1a
Et
3a
3
CAN
12
7
Scheme 1 Preparation of pyrido[2,3-c]carbazole 3a
4
CAN
5
CANd
6
ridocarbazoles in good yields. Pyrido[2,3-c]carbazole 3a
was formed in low yields when copper(II) triflate and sil-
ver(I) triflate were employed as catalysts in this reaction
(entries 20 and 22). On the other hand, the reaction pro-
ceeded smoothly and gave the desired product 3a in good
yields when indium(III), ytterbium(III), lanthanum(III)
and scandium(III) triflates were used as the catalysts (en-
tries 12–14 and 19). The reaction did not proceed when
ethanol, benzene, carbon tetrachloride, or dimethyl sulf-
oxide were used as solvents with CAN as the catalyst (en-
try 25). Further screening revealed that the best yield of 3a
(78%) was obtained with acetonitrile (entry 6). Other sol-
vents such as chloroform, dichloromethane, and toluene
provided reasonable yields in this Povarov reaction (en-
tries 1, 2, and 4). The expected product 3a was not ob-
tained, even using a prolonged reaction time, when no
catalyst was used (entry 26), which indicated the essential
role of cerium(IV) in this transformation.
6
CAN
6
7
CANc
8
8
CAN
12
12
12
8
9
InCl3
toluene
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
toluene
MeCN
MeCN
DMF
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
InCl3
I2
In(OTf)3
Yb(OTf)3
La(OTf)3
Montmorillonite
AlCl3
8
8
8
8
12
12
12
8
When the amount of ethyl vinyl ether (2) was decreased to
two equivalents, a slightly lower yield of 3a was obtained;
no improvement was observed when the amount of ethyl
vinyl ether (2) was increased to four equivalents. The use
of triphenylphosphine or 1,2-bis(diphenylphosphi-
no)ethane as a ligand in the reaction did not improve the
yield (data not shown). A lower concentration of catalyst
could also be used with moderate success. For instance, 5
mol% of CAN resulted in 3a in 51% yield in six hours (en-
try 5). However, no improvement in the yield of 3a was
observed when the catalyst loading was increased to 15
mol% (entry 7). After careful screening, the optimal reac-
tion conditions were found to be aminocarbazole 1a (1.0
equiv), ethyl vinyl ether (2, 2.5 equiv), and CAN (10
mol%) in acetonitrile (10 mL) at room temperature.
ZnCl2
PTSA
Sc(OTf)3
Cu(OTf)2
La(OTf)3
Ag(OTf)
Zn(OTf)2
CAN
8
8
8
12
12
24
24
CAN
solventse
solventse,f
–
–
a Conditions: 1a (1 equiv), ethyl vinyl ether (2, 2.5 equiv), catalyst (10
mol%), solvent (10 mL), r.t.
To demonstrate the generality of this transformation, the
optimized conditions were applied to a variety of ami-
nocarbazoles. Table 2 and Scheme 2 summarize the re-
b Isolated yield.
c 15 mol% of catalyst was used.
sults for the Povarov reaction of
a series of
d 5 mol% of catalyst was used.
aminocarbazoles 1a–j with ethyl vinyl ether (2) to pro-
duce the corresponding pyridocarbazoles 3a–j in good
yields. When the 3-aminocarbazole was substituted by a
methyl group at C4, a 3-amino-4-methylcarbazole, the
products have the pyrido[2,3-b]carbazole skeleton 3h,i
(Scheme 2). The optimized conditions were examined us-
ing unprotected carbazoles 1b,h,j and they afforded the
pyridocarbazoles 3b,h,j in good yields. For example, py-
rido[2,3-a]carbazole 3j was obtained as the product when
1-aminocarbazole 1j was subjected to the Povarov reac-
tion. Product 3a–j were confirmed by NMR analysis. Ad-
ditionally, the structure of 3g is confirmed by X-ray
e EtOH, benzene, CCl4, DMSO.
f Toluene, MeCN, dioxane, CH2Cl2, CHCl3, THF.
crystallographic analysis. The ORTEP diagram of 3g is
shown in Figure 2.18
Extremely simple reagents and conditions were used in
this Povarov reaction. Mechanistically, shown in
Scheme 3 for 1a and 2, their reaction in acetonitrile con-
taining a catalytic amount of CAN (10 mol%) affords the
imine intermediate 4, which subsequently undergoes an
intermolecular Povarov with electron-rich ethyl vinyl
Synthesis 2012, 44, 253–258
© Thieme Stuttgart · New York

PAPER
Synthesis of Pyrido[2,3-c]carbazoles
255
Table 2 Synthesis of Pyrido[2,3-c]carbazoles from Various 3-Ami-
NH2
+
N
nocarbazoles by the Povarov Reactiona
CAN
(10 mol%)
OEt
MeCN
r.t., 3–6 h
N
R
N
R
R1
NH2
+
R1
2
N
CAN
(10 mol%)
3h,i
1h,i
OEt
MeCN
r.t., 3–7 h
N
R3
1a–g
N
R3
2
R2
R2
if R = H, yield = 82% (3 h)
if R = Et, yield = 80% (6 h)
3a–g
CAN
(10 mol%)
Entry Substrate R1
R2
R3
Time Product Yieldb
(h)
(%)
78
79
80
75
79
76
76
OEt
+
N
MeCN
r.t., 5 h
H
N
N
H
2
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
H
H
H
H
Cl
H
H
H
Et
6
3a
3b
3c
3d
3e
3f
NH2
3j, 83%
1j
H
H
6
Scheme 2 Preparation of pyrido[2,3-b]- and pyrido[2,3-a]carba-
Br
Cl
Cl
H
Bu
Bu
Me
Me
Bn
4
zoles
6
2
7
1g
H
7
3g
a Conditions: 1a–g (1.0 equiv), ethyl vinyl ether (2, 2.5 equiv), CAN
(10 mol%), MeCN (10 mL), r.t., stirring.
b Isolated yields.
ether (2) as a dienophile to afford compounds 5 and 5¢ that
undergo aromatization to furnish 3-methyl-7H-pyri-
do[2,3-c]carbazole 3a in good yield. This mechanism was
proposed based on the literature.17l
Figure 2 ORTEP diagram of 3g
OEt
NH
NH2
N
δ–
Ce(IV)
H2C
+
N
– EtOH
– Ce (IV)
N
N
EtO
δ+
Et
Et
Et
1a
2
4
CH2
EtO
EtO
OEt
NH
+N
– Ce(IV)
NH
+
– Ce (IV)
N
N
N
Et
Et
Et
EtO
EtO
N
N
NH
NH
[O]
+
– EtOH
N
N
Et
5
Et
5'
Et
Scheme 3 Possible mechanism for synthesis of pyridocarbazoles
© Thieme Stuttgart · New York
Synthesis 2012, 44, 253–258

256
M. Viji, R. Nagarajan
PAPER
1H NMR (400 MHz, CDCl3): d = 11.01 (s, 1 H), 8.18 (d, J = 8.3 Hz,
1 H), 8.13–8.09 (m, 2 H), 7.52 (d, J = 8.4 Hz, 1 H), 7.34–7.32 (m, 2
H), 7.25–7.23 (m, 2 H), 2.76 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 157.4, 139.0, 136.99, 136.95,
135.1, 125.3, 125.1, 123.8, 121.3, 120.1, 119.6, 119.4, 118.4, 111.3,
24.9.
In conclusion, we have described an efficient route for the
construction of potentially active pyridocarbazole frame-
work via CAN-mediated Povarov reaction. This transfor-
mation proceeds with high regioselectively through the
Povarov reaction and subsequent aromatization. It is note-
worthy that this is the first example, to the best of our
knowledge, of the construction of pyrido[2,3-c]-, pyri-
do[2,3-b]-, and pyrido[2,3-a]carbazoles under such mild
conditions.
LC-MS (EI+): m/z = 233 (M + H)+.
Anal. Calcd for C16H12N2: C, 82.73; H, 5.21; N, 12.06. Found: C,
82.59; H, 5.21; N, 12.11.
10-Bromo-7-butyl-3-methyl-7H-pyrido[2,3-c]carbazole (3c)
Yellow-colored solid; yield: 0.046 g (80%); mp 62 °C; Rf = 0.37
(hexane–EtOAc, 7:3).
1
All H and 13C NMR spectra were recorded on a Bruker AV-400
spectrometer operating at 400 and 100 MHz, respectively; 1H NMR
referenced to TMS, 13C NMR referenced to CDCl3 (d = 77.0).
IR (KBr): 2964, 2924, 2858, 1612, 1579, 1518, 1467, 1375, 1317,
1153, 810, 734, 688, 434 cm–1.
1
Chemical shifts of common trace H NMR impurities (CDCl3):
H2O, d = 1.56 ppm; EtOAc, d = 1.26, 2.05, 4.12 ppm; CH2Cl2, d =
5.30 ppm; CDCl3, d = 7.26 ppm. IR spectra were recorded on Jasco
FT/IR-5300 spectrophotometer. Mass spectra were recorded on ei-
ther VG7070H mass spectrometer using EI technique or Shimadzu-
LCMS-2010A mass spectrometer. Elemental analyses (CHN) were
recorded on a Thermo Finnigan Flash EA 1112 analyzer in School
of Chemistry, University of Hyderabad. Routine monitoring of the
reactions was performed by TLC silica gel plates Merck 60 F254
were used. Compounds were visualized with a UV light at 254 nm.
Further visualization was achieved by staining with iodine. Column
chromatography was carried out employing neutral alumina; all
products 3a–j were purified by column chromatography. Commer-
cially available reagents and solvents were used without further pu-
rification and were purchased. Melting points were measured in
open capillary tubes and are uncorrected.
1H NMR (400 MHz, CDCl3): d = 8.94 (d, J = 8.4 Hz, 1 H), 8.46 (d,
J = 7.9 Hz, 1 H), 8.08 (d, J = 9.1 Hz, 1 H), 7.83–7.80 (m, 1 H),
7.55–7.53 (m, 1 H), 7.44 (d, J = 8.4 Hz, 1 H), 7.37–7.35 (m, 1 H),
4.40 (t, J = 7.1 Hz, 2 H), 2.79 (s, 3 H), 1.91–1.84 (m, 2 H), 1.44–
1.37 (m, 2 H), 0.94 (t, J = 7.3 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 154.9, 144.1, 139.6, 137.2, 131.3,
127.3, 124.4, 123.2, 123.0, 122.0, 121.7, 199.7, 114.3, 113.9, 109.6,
42.9, 31.6, 25.0, 20.5, 13.8.
LC-MS (ESI+): m/z = 368 (M + H)+, 369 (M + 1).
Anal. Calcd for C20H19BrN2: C, 65.40; H, 5.21; N, 7.63. Found: C,
65.26; H, 5.21; N, 7.56.
7-Butyl-8,10-dichloro-3-methyl-7H-pyrido[2,3-c]carbazole (3d)
Yellow-colored solid; yield: 0.043 g (75%); mp 132 °C; Rf = 0.25
(hexane–EtOAc, 7:3).
7-Ethyl-3-methyl-7H-pyrido[2,3-c]carbazole (3a); Typical Pro-
cedure
IR (KBr): 3447, 2959, 2928, 2868, 1739, 1647, 1583, 1545, 1520,
1456, 1367, 1300, 1211, 1080, 1014, 841, 810 cm–1.
An oven-dried 50-mL round-bottom flask equipped with a mechan-
ical stirrer and charged with CAN (0.013 g, 10 mol%), and 9-ethyl-
3-aminocarbzole (1a, 0.50 g, 1.0 equiv), MeCN (10 mL) along with
ethyl vinyl ether (2, 0.056 mL, 2.5 equiv), and it was stirred at r.t.
for 6 h. After completion of the reaction (TLC monitoring), the sol-
vent was evaporated and dissolved in EtOAc. The combined organ-
ic layers were washed with H2O, dried (anhyd Na2SO4), and then
concentrated under reduced pressure to give a crude product. The
crude product was purified by column chromatography (neutral alu-
mina, hexane–EtOAc). The solvent was evaporated to dryness to
give pure 3a as a yellow-colored solid; yield: 0.048 g (78%); mp 48
°C; Rf = 0.29 (hexane–EtOAc, 7:3).
1H NMR (400 MHz, CDCl3): d = 8.65 (d, J = 8.5 Hz, 1 H), 8.18 (d,
J = 1.8 Hz, 1 H), 8.07–8.05 (m, 1 H), 7.73–7.70 (m, 1 H), 7.40 (d,
J = 8.4 Hz, 1 H), 7.38 (d, J = 1.8 Hz, 1 H), 4.67 (t, J = 7.7 Hz, 2 H),
2.79 (s, 3 H), 1.88–1.80 (m, 2 H), 1.46–1.37 (m, 2 H), 0.96 (t,
J = 7.3 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 155.6, 144.4, 138.8, 133.2, 130.6,
129.0, 126.6, 125.9, 124.9, 122.4, 122.3, 119.7, 116.9, 113.8, 113.2,
44.5, 33.2, 25.0, 20.0, 13.8.
LC-MS (ESI+): m/z = 357 (M + H)+, 359 (M + 2).
IR (KBr): 2920, 1649, 1537, 1466, 1020, 752 cm–1.
Anal. Calcd for C20H18Cl2N2: C, 67.23; H, 5.08; N, 7.84. Found: C,
67.32; H, 5.08; N, 7.79.
1H NMR (400 MHz, CDCl3): d = 8.97 (d, J = 8.0 Hz, 1 H), 8.49 (d,
J = 8.0 Hz, 1 H), 8.10 (d, J = 9.2 Hz, 1 H), 7.87–7.84 (m, 1 H),
7.59–7.57 (m, 1 H), 7.54–7.50 (m, 1 H), 7.47 (d, J = 8.0 Hz, 1 H),
7.38 (t, J = 7.0 Hz, 1 H), 4.52 (q, J = 7.0 Hz, 2 H), 2.81 (s, 3 H), 1.50
(t, J = 7.0 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 154.9, 144.2, 139.1, 136.7, 131.3,
127.4, 124.5, 123.3, 123.0, 122.0, 121.8, 119.7, 114.4, 113.6, 109.4,
37.7, 25.1, 14.4.
10-Chloro-3,7-dimethyl-7H-pyrido[2,3-c]carbazole (3e)
Yellow-colored solid; yield: 0.048 g (79%); mp 52 °C; Rf = 0.33
(hexane–EtOAc, 7:3).
IR (KBr): 3360, 2961, 1633, 1606, 1483, 1323 cm–1.
1H NMR (400 MHz, CDCl3): d = 8.11 (d, J = 8.4 Hz, 1 H), 7.79 (d,
J = 9.0 Hz, 1 H), 7.74 (s, 1 H), 7.29 (d, J = 9.0 Hz, 1 H), 7.20–7.18
(m, 1 H), 7.12 (d, J = 8.4 Hz, 1 H), 7.01–6.99 (m, 1 H), 3.45 (s, 3
H), 2.70 (s, 3 H).
LC-MS (ESI+): m/z = 261 (M + H)+.
Anal. Calcd for C18H16N2: C, 83.04; H, 6.19; N, 10.76. Found: C,
83.15; H, 6.08; N, 10.65.
13C NMR (100 MHz, CDCl3): d = 154.8, 143.7, 137.8, 137.5, 130.4,
127.6, 124.7, 124.0, 123.3, 122.0, 121.8, 120.5, 112.9, 112.8, 109.7,
28.8, 20.0.
3-Methyl-7H-pyrido[2,3-c]carbazole (3b)
Yellow-colored solid; yield: 0.049 g (79%); mp 78 °C; Rf = 0.50
(hexane–EtOAc, 7:3).
LC-MS (ESI+): m/z = 281 (M + H)+, 282 (M + 1).
Anal. Calcd for C17H13ClN2: C, 72.73; H, 4.67; N, 9.98. Found: C,
72.65; H, 4.58; N, 9.88.
IR (KBr): 3404, 2924, 2858, 1722, 1655, 1601, 1529, 1431, 1379,
1329, 1240, 1124, 837, 746 cm–1.
Synthesis 2012, 44, 253–258
© Thieme Stuttgart · New York

PAPER
Synthesis of Pyrido[2,3-c]carbazoles
2-Methyl-11H-pyrido[2,3-a]carbazole (3j)
White-colored solid; yield: 0.052 g (83%); mp 136 °C; Rf = 0.45
(hexane–EtOAc, 7:3).
257
3,7-Dimethyl-7H-pyrido[2,3-c]carbazole (3f)
Yellow-colored solid; yield: 0.047 g (76%); mp 118 °C; Rf = 0.78
(hexane–EtOAc, 7:3).
IR (KBr): 2918, 2856, 1645, 1527, 1113, 1020 cm–1.
IR (KBr): 3186, 2916, 1601, 1527, 1431, 1379, 1327, 1246, 839,
742, 445 cm–1.
1H NMR (400 MHz, CDCl3): d = 11.22 (s, 1 H), 8.19 (d, J = 8.3 Hz,
1 H), 8.11 (t, J = 8.6 Hz, 2 H), 7.52 (d, J = 8.4 Hz, 1 H), 7.34–7.31
(m, 2 H), 7.25–7.19 (m, 2 H), 2.76 (s, 3 H).
1H NMR (400 MHz, CDCl3): d = 8.99 (d, J = 8.3 Hz, 1 H), 8.47 (d,
J = 7.9 Hz, 1 H), 8.15 (d, J = 8.7 Hz, 1 H), 7.87–7.84 (m, 1 H),
7.58–7.54 (m, 2 H), 7.49 (d, J = 8.0 Hz, 1 H), 7.41–7.37 (m, 1 H),
4.01 (s, 3 H), 2.83 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 154.7, 140.2, 137.7, 131.9,
126.98, 126.90, 124.7, 123.0, 122.9, 122.1, 121.6, 119.9, 114.3,
114.0, 109.5, 29.3, 24.7.
13C NMR (100 MHz, CDCl3): d = 157.5, 139.0, 137.0, 136.9, 135.2,
125.3, 125.1, 123.8, 121.3, 120.1, 119.6, 119.4, 118.4, 111.3, 24.9.
LC-MS (ESI+): m/z = 233 (M + H)+.
LC-MS (ESI+): m/z = 247 (M + H)+.
Anal. Calcd for C16H12N2: C, 82.73; H, 5.21; N, 12.06. Found: C,
82.58; H, 5.21; N, 12.15.
Anal. Calcd for C17H14N2: C, 82.90; H, 5.73; N, 11.37. Found: C,
82.79; H, 5.61; N, 11.25.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
7-Benzyl-3-methyl-7H-pyrido[2,3-c]carbazole (3g)
Yellow-colored solid; yield: 0.049 g (76%); mp 156 °C; Rf = 0.37
(hexane–EtOAc, 7:3).
IR (KBr): 2922, 2852, 1647, 1464, 1311, 1126, 1018, 744 cm–1.
Acknowledgment
1H NMR (400 MHz, CDCl3): d = 8.93 (d, J = 8.4 Hz, 1 H), 8.46 (d,
J = 8.0 Hz, 1 H), 8.03 (d, J = 9.0 Hz, 1 H), 7.74–7.72 (m, 1 H),
7.46–7.41 (m, 3 H), 7.37–7.31 (m, 1 H), 7.22–7.21 (m, 3 H), 7.08–
7.06 (m, 2 H), 5.57 (s, 2 H), 2.78 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 155.1, 144.4, 139.9, 137.3, 136.9,
131.3, 128.8, 127.7, 127.6, 126.2, 124.7, 123.4, 122.9, 122.1, 121.8,
120.2, 114.7, 113.9, 109.8, 46.5, 25.1.
We gratefully acknowledge DST (Project number: SR/S1/OC-70/
2008) for financial support and for the single-crystal X-ray diffrac-
tometer facility in our school. M.V. thanks CSIR for a senior
research fellowship. M.V. also thanks Nagarjuna Reddy, Nagaraju
for helping in crystal studies.
References
LC-MS (ESI+): m/z = 323 (M + H)+.
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Anal. Calcd for C23H18N2: C, 85.68; H, 5.63; N, 8.69. Found: C,
85.66; H, 5.51; N, 8.59.
2,5,11-Trimethyl-6H-pyrido[3,2-b]carbazole (3h)
Yellow-colored solid, yield: 0.050 g (82%); mp 84 °C; Rf = 0.45
(hexane–EtOAc, 7:3).
IR (KBr): 3443, 2970, 2926, 2856, 1741, 1635, 1456, 1201, 1113
cm–1.
1H NMR (400 MHz, CDCl3): d = 8.40 (d, J = 7.0 Hz, 1 H), 8.33 (d,
J = 8.8 Hz, 1 H), 7.93 (s, 1 H), 7.52–7.46 (m, 2 H), 7.32–7.27 (m, 2
H), 3.39 (s, 3 H), 2.81 (s, 3 H), 2.78 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 154.1, 142.1, 141.5, 137.1, 131.4,
128.4, 126.8, 125.5, 124.8, 124.3, 124.0, 120.2, 119.4, 110.1, 108.2,
25.5, 13.9, 12.1.
LC-MS (ESI+): m/z = 261 (M + H)+.
Anal. Calcd for C18H16N2: C, 83.04; H, 6.19; N, 10.76. Found: C,
83.15; H, 6.23; N, 10.68.
Jaureguiberry, C.; Barsi, M. C.; Le Pecq, J. B.; Roques, B. P.
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Bisagni, E. Heterocycles 1993, 36, 2753.
6-Ethyl-2,5,11-trimethyl-6H-pyrido[3,2-b]carbazole (3i)
Yellow-colored solid; yield: 0.048 g (80%); mp 72 °C; Rf = 0.78
(hexane–EtOAc, 7:3).
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IR (KBr): 3466, 2970, 2926, 1599, 1479, 1224, 744 cm–1.
1H NMR (400 MHz, CDCl3): d = 8.37 (d, J = 7.7 Hz, 1 H), 8.28 (d,
J = 8.8 Hz, 1 H), 7.51–7.47 (m, 1 H), 7.32–7.30 (m, 1 H), 7.26–7.22
(m, 1 H), 7.20 (d, J = 8.8 Hz, 1 H), 4.43 (q, J = 7.16 Hz, 2 H), 3.35
(s, 3 H), 2.88 (s, 3 H), 2.76 (s, 3 H), 1.40 (t, J = 7.12 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 154.0, 144.3, 141.2, 137.7, 131.5,
128.5, 126.8, 126.5, 124.9, 124.4, 124.3, 120.2, 119.1, 108.9, 108.2,
40.4, 25.4, 15.2, 14.1, 13.5.
LC-MS (ESI+): m/z = 289 (M + H)+.
Anal. Calcd for C20H20N2: C, 83.30; H, 6.99; N, 9.71. Found: C,
83.21; H, 7.05; N, 9.65.
(5) (a) Paoletti, C.; Le Pecq, J. B.; Dat Xuong, N.; Lesca, P.;
© Thieme Stuttgart · New York
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M. Viji, R. Nagarajan
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(18) The CCDC deposition number for compound 3g is 845183;
molecular formula: C23H18N2. Chemical formula weight is
322.39; orthorhombic; unit cell parameters: a = 6.6003(8)
Å, b = 18.947(3) Å, c = 27.539(4) Å, a = b = g = 90°. Space
group Pbca.
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