Synthesis of substituted carbazoles via electrocyclization of in situ generated enamines from 1-phenylsulfonyl-2/(3)-methyl-3/(2)-vinylindoles and DMF·DMA/DMA·DMA

Article abstract of DOI:10.1016/j.tet.2009.03.010

Interaction of 2/(3)-methyl-3/(2)-vinylindoles and DMF·DMA/DMA·DMA at 110 °C led to the in situ generation of enamines, which on concurrent electrocyclization followed by subsequent aromatization afforded substituted carbazoles.

Full text of DOI:10.1016/j.tet.2009.03.010

Tetrahedron 65 (2009) 3582–3591
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Synthesis of substituted carbazoles via electrocyclization of in situ generated
enamines from 1-phenylsulfonyl-2/(3)-methyl-3/(2)-vinylindoles and
DMF$DMA/DMA$DMA
*
Radhakrishnan Sureshbabu, Ramalingam Balamurugan, Arasambattu K. Mohanakrishnan
Department of Organic Chemistry, University of Madras, Guindy Campus, Chennai 600 025, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Interaction of 2/(3)-methyl-3/(2)-vinylindoles and DMF$DMA/DMA$DMA at 110 ꢀC led to the in situ
generation of enamines, which on concurrent electrocyclization followed by subsequent aromatization
afforded substituted carbazoles.
Article history:
Received 24 December 2008
Received in revised form 10 February 2009
Accepted 4 March 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Available online 9 March 2009
Keywords:
Electrocyclization
Enamines
2,3-Divinylindole
DMF$DMA
Substituted carbazole
1. Introduction
dimethylformamide dimethyl acetal) at 100–120 ꢀC led to the for-
mation of corresponding enamines. These enamines on oxidative
Ever since the first isolation of the carbazole alkaloid, mur-
rayanine1 organic chemists have been interested in the synthesis of
carbazole alkaloids due to their promising biological activities. In
particular, biomimetic oxidation products of 3-methylcarbazole are
isolated in plenty and their syntheses have been widely reported.2
Recently Kno¨lker and Reddy extensively reviewed the synthesis of
biologically important carbazole alkaloids.3
hydrolysis using NaIO4 led to the formation of benzaldehydes,
pyridinealdehydes, naphthaldehydes and quinolinaldehydes.8–10
The enamine intermediate prepared from substituted 2-nitro-
toluenes on reductive cyclization furnished variety of inaccessible
4-, 5-, and 7-substituted indoles.11 Similarly, the enamines gener-
ated using DMF$DMA has also been utilized for the synthesis of
azaindoles,12 pyrroles13 and quinolones.14
Thermal electrocyclization methodology has been widely used
Kim and co-workers15 accomplished the formylation/acetyla-
tion of acetylenic systems via interaction with DMF$DMA/
DMA$DMA (N,N-dimethylformamide dimethyl acetal/N,N-dime-
thylacetamide dimethyl acetal) at 80 ꢀC. Very recently, Sasada and
co-workers reported16 the synthesis of tetra substituted pyridines
involving trimethylsilyl chloride-promoted three component cou-
pling reaction of functionalized enamine, DMF$DEA (N,N-dime-
thylformamide diethyl acetal) and DMAD. Thus, in all these cases
DMF$DMA/DMF$DEA has been used as potential one carbon syn-
thon. In further continuation of preliminary report on the synthesis
of N-protected carbazoles involving electrocyclization of in situ
generated enamines,17 we report herein our detailed study on the
synthesis of carbazoles using 1-phenylsulfonyl-2/(3)-methyl-3/(2)-
vinylindoles with DMF$DMA/DMA$DMA.
for the synthesis of carbazole based natural products.4 Moha-
nakrishnan and Srinivasan outlined
a versatile synthesis of
quino[4,3-b]carbazoles involving electrocyclization of stable N-
phenylsulfonyl-2,3-divinylindoles as key step.5 Very recently syn-
thesis of several multi functional carbazoles is reported6 involving
either cycloaddition or base mediated cyclization reactions.
2. Results and discussion
In continuation of our interest on annulated carbazole ana-
logues,7 we planned to utilize DMF$DMA (N,N-dimethylformamide
dimethyl acetal) as a one carbon electrophilic synthon. Accordingly,
a survey of literature revealed that the interaction of arene/hetero-
arene containing acidic methyl function with DMF$DMA (N,N-
As per Coe’s protocol,8–10 we presumed that the condensation of
vinyl ester 1a with DMF$DMA followed by oxidative hydrolysis
might lead to the aldehyde 2. However, when 1a was treated with
5 equiv of DMF$DMA at 110 ꢀC for 4 h followed by oxidative
* Corresponding author. Tel.: þ91 44 22202813; fax: þ91 44 22300488.
E-mail address: mohan_67@hotmail.com (A.K. Mohanakrishnan).
0040-4020/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2009.03.010

R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
3583
hydrolysis using NaIO4 in DMF–H2O, the expected aldehyde 2 was
not obtained (Scheme 1).
Me2N
N
DMF/POCl3
heat
-NMe2
9
11 (80%)
Me
CO2Et
CO2Et
H
10
i) 5 eq. DMF.DMA
110 °C, 4 h
X
Me
N
SO2Ph
1a
Scheme 4.
CHO
N
ii) NaIO4 / DMF-H2O
SO2Ph
2
Scheme 1.
The enamine-based electrocyclization methodology was then
tested with a variety of 2-methyl-3-vinylindoles and 3-methyl-2-
vinylindoles 1a–m via interaction with acetals DMF$DMA/
DMA$DMA to afford the respective carbazoles in 0–73% yields. The
exact conditions employed and the resulting carbazoles obtained
are presented in Table 1. Comparatively, the reaction of 2-methyl-3-
vinylesters 1a–c5 with DMF$DMA/DMA$DMA afforded the corre-
sponding carbazoles 3a–c/4a–c in better yields (entries 1–3). The
presence of 5-methoxy group in the case of 2-methyl-3-vinylindole
1c didn’t have much influence on the yield of the carbazole 3c.
Always, the interaction of 3-vinylindoles 1a–c with DMF$DMA
provided better yields carbazoles 3a–c than the corresponding
carbazoles 4a–c obtained using DMA$DMA (entries 1–3). As a rep-
resentative case, structure of carbazole 3a was confirmed19 by
a single crystal X-ray analysis.
When 1a was treated with DMF$DMA and the resulting crude
product was carefully analyzed using mass spectrometry, the
presence of Mþ ions at m/z 379, 365, 239 and 225, corresponding to
carbazoles 3a,b and 3a0,b0 were observed (Scheme 2).
R
R
N
N
H
SO2Ph
3a R = CO2Et
3b R = CO2Me
3a' R = CO2Et
3b' R = CO2Me
Scheme 2.
The expected enamine formation and its subsequent annula-
tion was failed with 3-methyl-2-vinylindole 1d5 even under forc-
ing conditions (140 ꢀC for 4 h), only the starting material was
recovered in quantitative yield (entry 4). Obviously, the observed
failure to form required enamine might be due to the relatively
enhanced nucleophilicity of the indole-3-position. Gratifyingly,
when the annulation of 3-carbomethoxymethyl-2-vinylindole
1e20 was performed with DMF$DMA, the respective carbazole 3e
could be isolated in 65% yield. Hence, it may be concluded that the
failure to form enamine in the case of 1d can be attributed to the
relatively less acidic nature of 3-methyl group. The annulation of
the isomeric 2-carbomethoxymethyl-3-vinylindole 1f20 with
DMF$DMA afforded the expected carbazole 3f in 65% yield (entry
6). Possibly, the formation of the enamine in the case of 1f may be
facilitated by the electron withdrawing ester group. However, the
isolation of carbazole 3f in 65% yield confirms that the subsequent
electrocyclization of the resulting enamine may be somewhat
unfavourable. Further, the interaction of 2-carbomethoxymethyl-
3-vinylindole 1f with DMA$DMA failed to produce expected car-
bazole 4f, which confirms that the carbanion stabilized by the
ester group has not reacted with iminium ion CH3C(OMe)¼þNMe2
derived from DMA$DMA to form the required enamine. Obviously,
for the same reason, the interaction of 3-vinylindole-2-methyl-
phosphonate ester 1g5 with DMF$DMA also failed to generate the
required enamine and hence carbazole 3g could not be prepared
(entry 7). Interaction of 3-vinylindole-2-sulfinylmethylindole 1h21
with DMF$DMA led to the formation of ether 3h via the cleavage of
the N-phenylsulfonyl group followed by nucleophilic displacement
of sulfoxide unit. The observed cleavage of N-phenylsulfonyl unit
can be visualized only through the nucleophilic attack of the
methoxy ion followed by subsequent elimination of methyl
benzenesulfonate.
The mechanism of formation of carbazole 3a can be understood
through the electrocyclization of an intermediate enamine 7 fol-
lowed by aromatization of dihydrocarbazole 8 via elimination of
dimethylamine (Scheme 3). The formation of the remaining car-
bazoles 3b, 3a0 and 3b0 must involve the secondary reaction of 3a
with methoxide ion. This was further confirmed by prolonging the
reaction period or by using excess of DMF$DMA, wherein carba-
zoles 3a0 and 3b were isolated as major products. When the re-
action was performed using 2 equiv of DMF$DMA for 3 h at 110 ꢀC,
the carbazole 3a was obtained as sole product in 73% yield.
CO2Et
DMF.DMA
H
Me
Me
1a
+
110 °C, 3 h
-MeOH
CH2
SO2Ph
5
N
-MeOH
N
OMe
6
CO2Et
NMe2
CO2Et
NMe2
3a
-NMe2
N
N
SO2Ph
7
SO2Ph
8
Scheme 3. Mechanistic rational for the formation of 3a.
The formation of carbazole 3a also supported our earlier ob-
servation5 that N-protection is essential for smooth electro-
cyclization of 2,3-divinylindole systems. Thus, the presence of the
electron withdrawing phenylsulphonyl unit on the indole nitrogen
provides typical triene character to the 2,3-divinylindole system
and there by electrocyclization is facilitated.
It should be noted that Bergman and Pelcman have reported the
formation of 2-methylcarbazole in excellent yield during Vilsmeier
formylation of 2-(2-methylprop-1-enyl)-1H-indole 9 at elevated
temperature. Indeed, the formation of 2-methylcarbazole 11 was
realized through enamine intermediate 10 (Scheme 4). However,
the preparations of 1-substituted-2-methylcarbazole analogues
under identical conditions were found to proceed only in very low
yields.18
The annulation of 3-vinylindole 1i with DMF$DMA furnished
expected carbazole 3i in 62% yield (entry 9). However, the reaction
of 1i with DMA$DMA led to the isolation of tri-ester 4i in 60% yield
(entry 10). The formation of 4i can be realized only through the
Michael addition of 1i with iminium-carbanion 12 to form the in-
termediate 13. The latter on subsequent hydrolysis may lead to the
tri-ester 4i (Scheme 5).
The structure of 4i was confirmed by a single crystal X-ray
analysis22 (Fig. 1).

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R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
Table 1
Synthesis of substituted carbazoles from 2-/(3)-methyl-3/(2)-vinylindoles and DMF$DMA/DMA$DMA
Entry
Vinylindoles
CO2Et
Acetal & condition
Carbazole
Yielda (%) mp
73 (180 ꢀC)
67 (174 ꢀC)
CO2Et
R1
DMF$DMA, 110 ꢀC, 3 h
DMA$DMA, 110 ꢀC, 3 h
Me
N
N
1
SO2Ph
SO2Ph
1a
3a R1 = H
4a R1 = Me
CO2Me
R1
CO2Me
DMF$DMA, 110 ꢀC, 3 h
DMA$DMA, 110 ꢀC, 3 h
70 (188 ꢀC)
68 (171 ꢀC)
N
2
Me
SO2Ph
N
SO2Ph
3b R1 = H
4b R1 = Me
1b
CO2Me
R1
MeO
CO2Me
DMF$DMA, 110 ꢀC, 3 h
DMA$DMA, 110 ꢀC, 3 h
68 (183 ꢀC)
55 (175 ꢀC)
MeO
N
3
Me
SO2Ph
N
SO2Ph
3c R1 = H
4c R1 = Me
1c
Me
CO2Me
CO2Me
DMF$DMA, 140 ꢀC, 5 h
0
N
4
N
SO2Ph
SO2Ph
3d
1d
MeO2C
CO2Me
CO2Me
DMF$DMA, 110 ꢀC, 3 h
65 (210 ꢀC)
CO2Me
CO2Me
5
N
N
SO2Ph
SO2Ph
1e
3e
CO2Me
CO2Me
65 (102 ꢀC)
0
R1
DMF$DMA, 110 ꢀC, 3 h
DMA$DMA, 110 ꢀC, 5 h
N
CO2Me
6
PhO2S
N
SO2Ph
3f R1 = H
4f R1 = Me
1f
CO2Me
CO2Me
PO(OEt)2
7
DMF$DMA, 110 ꢀC, 4 h
0
N
PhO2S
N
PO(OEt)2
SO2Ph
3g
1g
CO2Et
SOPh
CO2Et
OMe
8
DMF$DMA, 110 ꢀC, 3 h
56 (116 ꢀC)
N
H
N
SO2Ph
3h
1h

R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
3585
Table 1 (continued )
Entry
Vinylindoles
Acetal & condition
Carbazole
Yielda (%) mp
MeO2C
MeO2C
CO2Me
CO2Me
9
DMF$DMA, 110 ꢀC, 3 h
62 (128 ꢀC)
Me
N
N
SO2Ph
SO2Ph
3i
1i
MeO2C
MeO2C
CO2Me
CO2Me
CO2Me
10
11
12
DMA$DMA, 110 ꢀC, 4 h
DMF$DMA, 140 ꢀC, 5 h
DMF$DMA, 110 ꢀC, 3 h
60 (122 ꢀC)
Me
Me
N
N
SO2Ph
SO2Ph
4i
1i
Me
CO2Me
Me
CO2Me
Me
N
0
N
SO2Ph
SO2Ph
1j
3j
CO2Me
Ph
CO2Me
66 (142 ꢀC)
N
H
N
SO2Ph
3k
1k
O
O
Me
Me
13
DMF$DMA, 110 ꢀC, 1 h
56 (192 ꢀC)
Me
N
N
SO2Ph
SO2Ph
3l
1l
CO2Me
CO2Me
14
DMF$DMA, 110 ꢀC, 3 h
65 (130 ꢀC)
Me
Me
N
Me
N
Boc
3m
1m
a
Isolated yield after column chromatography.
Surprisingly, the reaction of 3-vinylindole 1j with DMF$DMA
at 140 ꢀC for 5 h didn’t give the expected carbazole 3j, only the
starting material was recovered unchanged (entry 11). Upon in-
teraction of 3-vinyl-2-benzylindole 1k with DMF$DMA, furnished
the corresponding 1-phenylcarbazole 3k in 66% yield with
cleavage of N-phenylsulfonyl group (entry 12). When, 2-methyl-
indole 1l5 tethered with sensitive ketone-methyl function was
reacted with DMF$DMA at 110 ꢀC for 1 h, the expected carbazole
3l could be obtained in 56% yield (entry 13). Finally, the
H2C
MeO2C
N
Me
Me
CO2Me
OMe
N
Me
Me
OMe
N
MeO
H3O+
12
Me
1i
4i
OMe
SO2Ph
13
Scheme 5. Mechanistic rational for the formation of 4i.

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R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
MeO2C
MeO2C
CO2Me
CO2Me
4 eq. DMF.DMA
Me
N
Me
110 °C, 4 h
N
Me
16
SO2Ph
15
MeO2C
CO2Me
4 eq. DMA.DMA
110 °C, 4 h
53%
15
Me
Me
N
Me
17
Scheme 7.
starting material was recovered and for 22, complex mixture was
obtained (Scheme 8).
EtO2C
EtO2C
CO2Et
CO2Et
4 eq. DMF.DMA
X
Me
Me
110 °C, 4 h
S
18
S
19
R1
CH2R1
MeO2C
MeO2C
CO2Me
CO2Me
6 eq. DMF.DMA
X
110 °C, 6 h
R1
Figure 1. ORTEP diagram of tri-ester 4.
R1H2C
CH2R1
R1
CO2Me
CO2Me
20 R1 = H
21 R1 = H
22 R2 = CO2Me
23 R2 = CO2Me
attempted annulation of 1-tert-butoxycarbonyl-3-vinyl-2-methyl-
indole 1m led to the isolation of 1,2-dimethyl-3-vinylindole 3m
in 65% yield (entry 14). The formation of 3m can be visualized
through thermal cleavage of the N-Boc unit followed by its
subsequent N-methylation of carbanion 14 with iminium ion 6
(Scheme 6).
Scheme 8.
Finally, the annulation of 1-phenylsulfonyl-2-methyl-3-acetyl-
indole 24 with DMF$DMA at 140 ꢀC for 4 h followed by column
chromatographic purification led to the isolation of N-phenyl-
sulfonyl-4-hydroxy-carbazole-1-carboxaldehyde 25 in 65% yield
(Scheme 9). The initial reaction of 24 with DMF$DMA may lead to
the formation of bis-enamine 26. Intramolecular Michael addition
of 26 followed by elimination of dimethylamine and subsequent
workup may furnish 23.
CO2Et
H
DMF.DMA
110 °C
Me
1m
+
3m
N
-DMF
Me
N
Me
O
Me
O
HO
14
Scheme 6. Mechanistic rational for the formation of 3m.
6
Me
Me
2 eq. DMF.DMA
140 °C, 4 h
A survey of literature revealed that interaction of sulfonamide
with DMF$DMA at 70–80 ꢀC led to the formation of the corre-
sponding methylated product.23
N
N
CHO
SO2Ph
SO2Ph
25
24
As expected, the bis-annulation of 3,4-divinyl-2,5-dimethyl-
pyrrole 15 with 4 equiv of DMF$DMA at 110 ꢀC for 4 h followed
by usual workup and crystallization from methanol furnished N-
methylcarbazole 16 in 55% yield. Presumably, dimethyl 1-phe-
nylsulfonylcarbazole-3,6-dicarboxylate underwent facile cleavage
of the phenylsulfonyl group followed by methylation to afford 16
(Scheme 7). However, the attempted bis-annulation of 15 with
DMA$DMA furnished only N-methyl pyrrole 17 in 53% yield.
A bis-annulation of thiophenyl methyl compound 18 was found
to be unsuccessful possibly due to the less acidic character of thio-
phenyl methyl protons. Similarly, attempted tris-annulation of
mesitylenes 20/22 with 6 equiv of DMF$DMA also failed to produce
the expected products 21/23. While in the case of compound 20,
O
O
i) -NMe2
ii) H3O+
NMe2
NMe2
NMe2
NMe2
N
N
SO2Ph
SO2Ph
26
27
Scheme 9.
It is worthwhile to mention that an attempted annulation of 2-
methylacetophenone with DMF$DMA at 120 ꢀC for 12 h failed to

R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
3587
produce the expected enamine, only the starting material was re-
covered unchanged.
a SHIMADZU FT-IR 8300 instrument. 1H and 13C NMR spectra were
recorded in CDCl3 using TMS as an internal standard on a JEOL 400
and Bruker-300 spectrometers. Elemental analyses were per-
formed on a Perkin–Elmer series II 2400 (IIT Madras) elemental
analyzer. The Mass spectra were recorded on a JEOL DX 303 HF
mass spectrometer.
During the above-mentioned annulation reactions, in some of
the cases N-methylation was observed as a prominent side re-
action. Hence, it was decided to explore methylation character of
DMF$DMA. Accordingly, when ethyl 2-methylindole-3-carboxylate
28 was treated with DMF$DMA at 110 ꢀC, N-methylindole 29 could
be isolated in 65% yield (Scheme 10).
4.1.1. (E)-Methyl 3-(3-(2-methoxy-2-oxoethyl)-1-(phenylsulfonyl)-
1H-indol-2-yl)acrylate (1e)
A two necked flask containing (E)-methyl 3-(3-(bromome-
thyl)-1-(phenylsulfonyl)-1H-indol-2-yl)acrylate5 (1 g, 2.31 mmol),
Pd(PPh3)2Cl2 (162 mg, 0.23 mmol) and K2CO3 (320 mg, 2.31 mmol)
was evacuated. To this, dry THF (35 mL) and MeOH (10 mL) were
added via syringe. The reaction mixture was then purged with dry
carbon monoxide gas for 5 min and stirred under a carbon mon-
oxide atmosphere at room temperature for 10 h. The reaction
mixture was then diluted with water (50 mL) and extracted with
ethyl acetate (2ꢁ20 mL). The combined extracts were washed with
H2O (10 mL) and dried (Na2SO4). Removal of solvent followed by
column chromatographic purification (20% EA/Hexane) afforded 1e
(0.20 g, 42%) as a yellow solid; mp 150 ꢀC. [Found: C, 61.28; H, 4.36;
N, 3.69; S, 7.50. C21H19NO6S requires: C, 61.01; H, 4.63; N, 3.39; S,
7.76%.] Rf (20% EA/Hexane) 0.42; nmax (KBr) 1726, 1716, 1362,
CO2Et
Me
CO2Et
Me
DMF.DMA
110 °C, 4 h
65%
N
N
Me
H
28
29
CO2Et
Me
CO2Et
Me
Me
Me
EtO2C
DMF.DMA
110 °C, 4 h
75%
N
EtO2C
N
Me
H
30
31
Scheme 10.
1158 cmꢂ1
; dH (300 MHz, CDCl3) 8.24 (2H, d, J 7.5 Hz, ArH), 7.70 (2H,
d, J 7.5 Hz, ArH), 7.52–7.28 (6H, m, ArH), 6.16 (1H, d, J 16.2 Hz, Vi-
nylic CH), 4.14 (2H, s, CH2), 3.72 (3H, s, CH3), 3.65 (3H, S, CH3); dC
(75.6 MHz, CDCl3) 169.3,167.9,138.3,136.3,134.8,134.5,129.7,127.1,
126.3, 125.7, 124.4, 121.1, 120.7, 118.7, 114.3, 52.7, 51.4, 32.2; m/z (EI)
413 (32, Mþ).
Under identical conditions, 2,3-dimethylindole failed to produce
the corresponding N-methylindole. However, pyrrole 30 on heating
with DMF$DMA at 110 ꢀC furnished expected product 31 in 75%
yield. Obviously, the observed N-methylation of indole/pyrrole is
taking place only when it possesses an electron withdrawing unit.
Attempted one-pot desulfonylation followed by N-methylation of
1-phenylsulfonylindole24 with excess of DMF$DMA even at 140 ꢀC
failed to produce the expected N-methylindole, only the starting
material was recovered unchanged. Now, it is clear that cleavage of
N-phenylsulfonyl unit is taking place only in the case compounds
containing electron withdrawing groups. However, the precise
mechanism for the selective cleavage of N-phenylsulfonyl group
and subsequent N-methylation using DMF$DMA is not clear.
4.1.2. (E)-Methyl 3-(2-(2-methoxy-2-oxoethyl)-1-(phenylsulfinyl)-
1H-indol-3-yl)acrylate (1f)
Following the above mentioned procedure, diester 1f (0.43 g,
45%) was obtained using (E)-methyl 3-(2-(bromomethyl)-1-(phe-
nylsulfonyl)-1H-indol-3-yl)acrylate5 (1 g, 2.31 mmol), Pd(PPh3)2Cl2
(162 mg, 0.23 mmol), K2CO3 (320 mg, 2.31 mmol) in dry THF
(35 mL) and methanol (10 mL) as a colourless solid; mp 124 ꢀC.
[Found: C, 61.29; H, 4.38; N, 3.70; S, 7.51. C21H19NO6S requires: C,
61.01; H, 4.63; N, 3.39; S, 7.76%.] Rf (20% EA/Hexane) 0.42; nmax (KBr)
3. Conclusions
1724, 1712, 1372, 1164 cmꢂ1
; dH (300 MHz, CDCl3) 7.98 (1H, d, J
8.4 Hz, ArH), 7.79 (2H, d, J 7.5 Hz, ArH), 7.73–7.68 (2H, m, ArH), 7.49
(1H, t, J 7.2 Hz, ArH), 7.38 (2H, t, J 7.8 Hz, ArH), 7.26 (2H, t, J 3.9 Hz,
ArH), 6.67 (1H, d, J 15.9 Hz, Vinylic CH), 4.19 (2H, s, CH2), 3.76 (3H, s,
CH3), 3.67 (3H,s, CH3); dC (75.6 MHz, CDCl3) 169.5, 167.4, 138.7,
136.4, 134.8, 134.2, 129.4, 127.1, 126.8, 125.5, 124.4, 120.3, 120.1,
118.5, 114.6, 52.5, 51.8, 32.4; m/z (EI) 413 (34, Mþ).
In conclusion, we have developed a convenient one-carbon
annulation protocol for the synthesis of functionalized carbazoles
involving electrocyclization of in situ generated enamine through
the interaction of 1-phenylsulfonyl-2/(3)-methyl-3/(2)-vinyl-
indoles with DMF$DMA or DMA$DMA at a moderate temperature.
While, the bis-annulation could be performed with N-phenyl-
sulfonyl-3,4-divinylpyrrole, the same could not be performed with
the corresponding thienyl system. Similarly, attempted tris-annu-
lation with mesitylenes was also found to be unsuccessful. Finally,
a facile N-methylation of indole/pyrrole containing electron with-
drawing ester function could be performed in good yields. Further
research work to utilize the N-methylation character of DMF$DMA
is in progress.
4.1.3. Dimethyl 2-(2-methyl-1-(phenylsulfonyl)-1H-indol-3-
yl)maleate (1i)
To a solution of 2-methylindole (2 g, 15.2 mmol) in dry diethyl-
ether (10 mL), oxalyl chloride (3. 9 g, 30.53 mmol) was slowly
added at 0 ꢀC and allowed to stir for 1 h. To the reaction mixture,
dry methanol (10 mL) was then added slowly and stirred for an-
other 1 h. The resulting reaction mixture was poured over crushed
ice (200 g) and solid formed was filtered and dried (CaCl2) to give
methyl 2-(2-methyl-1H-indol-3-yl)-2-oxoacetate (2.65 g, 80%),
which was used as such for N-protection; mp 156 ꢀC; m/z (EI) 217
(93, Mþ).
4. Experimental
4.1. General
To a solution of methyl 2-(2-methyl-1H-indol-3-yl)-2-oxoace-
tate (2 g, 9.21 mmol) in dry DCM (25 mL), Et3N (1.86 g, 18.4 mmol),
DMAP (100 mg, 0.92 mmol) were added and stirred at room tem-
perature for 15 min. To this, benzenesulfonylchloride (2.1 g,
11.9 mmol) in dry DCM (10 mL) was added and stirred for addi-
tional 6 h. It was then poured into 2% aq HCl (10 mL) and extracted
with DCM (2ꢁ20 mL). The combined extracts were washed with
water (10 mL), brine (10 mL) and dried (Na2SO4). Removal of
All melting points are uncorrected. Solvents were purified using
standard procedure. Reactions were done under atmosphere of dry
nitrogen with magnetic stirring. All chemicals were used as
received. Analytical thin layer chromatography (TLC) was per-
formed on silica and components were visualized by observation
under iodine or UV-light. Column chromatography was performed
using silica gel (60–120 mesh). IR spectra were recorded on

3588
R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
solvent followed by column chromatographic purification (10% EA/
hexane) afforded methyl 2-(2-methyl-1-(phenylsulfonyl)-1H-
indol-3-yl)-2-oxoacetate (2.13 g, 65%) as light brown solid; mp
86 ꢀC. [Found: C, 60.80; H, 3.95; N, 4.19; S, 8.71. C18H15NO5S re-
quires: C, 60.49; H, 4.23; N, 3.92; S, 8.97%.] Rf (20% EA/Hexane) 0.51;
CDCl3) 8.23 (1H, d, J 7.5 Hz, ArH), 7.89 (1H, d, J 16.5 Hz, Vinylic CH),
7.85 (1H, d, J 8.4 Hz, ArH), 7.43–7.36 (5H, m, ArH), 7.22–7.19 (5H, m,
ArH), 7.11–7.02 (2H, m, ArH), 6.59 (1H, d, J 16.2 Hz, Vinylic CH), 4.67
(2H, s, CH2), 3.78 (3H, s, CH3); dC (75.6 MHz, CDCl3) 167.6, 141.2,
138.3, 137.5, 136.7, 135.6, 133.7, 129.0, 128.6, 128.4, 127.2, 126.6,
126.5, 125.2, 124.3, 120.1, 119.2, 117.6, 115.1, 51.7, 29.7; m/z (EI) 431
(54, Mþ).
nmax (KBr) 1720, 1676, 1354, 1170 cmꢂ1
; dH (300 MHz, CDCl3) 8.25
(1H, d, J 8.0 Hz, ArH), 7.89–7.85 (3H, m, ArH), 7.62–7.59 (1H, t, J
7.0 Hz, ArH), 7.49 (2H, t, J 7.2 Hz, ArH), 7.38–7.31 (2H, m, ArH), 3.53
(3H, s, CH3), 2.64 (3H, s, CH3); dC (75.6 MHz, CDCl3) 183.5, 164.7,
146.4, 138.5, 136.0, 134.7, 129.8, 126.7, 126.5, 125.6, 125.0, 120.6,
115.8, 114.4, 62.6, 14.2, 14.0; m/z (EI) 357 (46, Mþ).
4.1.6. tert-Butyl 3-((E)-2-(methoxycarbonyl)vinyl)-2-methyl-1H-
indole-1-carboxylate (1m)
To a solution of 2-methyl-1H-indole-3-carboxaldehyde (2 g,
12.5 mmol) in dry DCM, Et3N (2.54 g, 25.1 mmol), DMAP (0.15 g,
1.2 mmol) and (Boc)2O (3.5 g, 16.3 mmol) were added at 0 ꢀC. After
4 h, the reaction mixture was poured in to 2% aq HCl (10 mL)
solution and extracted with DCM (2ꢁ20 mL). The combined
extracts were washed with water (10 mL) and dried (Na2SO4).
Evaporation of the solvent followed by crystallization from meth-
anol afforded 2-methyl-1tert-butoxycarbonylindole-3-carboxal-
dehyde as a colourless solid. It was then dissolved in dry toluene
(70 mL). To this (carbomethoxymethylene)triphenylphosphorane
(5.0 g, 15.1 mmol) was added and refluxed under N2 for 12 h.
Removal of solvent followed by column chromatographic separa-
tion (5% EA/hexane) afforded compound 1m [2.1 g, 53% (two steps)]
as a white solid; mp 58 ꢀC. [Found: C, 68.28; H, 7.02; N, 4.13.
C18H21NO4 requires: C, 68.55; H, 6.71; N, 4.44%.] Rf (20% EA/Hexane)
A solution of methyl 2-(2-methyl-1-(phenylsulfonyl)-1H-indol-
3-yl)-2-oxoacetate (2 g, 5.6 mmol) and (carbomethoxymethylene)
triphenylphosphorane (2.2 g, 6.7 mmol) in dry benzene (60 mL)
was refluxed under N2 for 12 h. Removal of solvent followed by
column chromatographic separation (5% EA/hexane) afforded
compound 1i (1.62 g, 70%) as a thick yellow liquid. [Found: C, 61.30;
H, 4.33; N, 3.68; S, 7.52. C21H19NO6S requires: C, 61.01; H, 4.63; N,
3.39; S, 7.76%.] Rf (20% EA/Hexane) 0.50; nmax (KBr) 1695, 1689,
1370, 1168 cmꢂ1
; dH (300 MHz, CDCl3) 8.05 (1H, d, J 7.8 Hz, ArH),
7.68–7.65 (2H, m, ArH), 7.42 (1H, t, J 7.3 Hz, ArH), 7.32 (2H, t, J 7.5 Hz,
ArH), 7.19–7.12 (2H, m, ArH), 7.10–7.02 (2H, m, ArH), 3.63 (3H, s,
CH3), 3.33 (3H, s, CH3), 2.36 (3H, s, CH3); dC (75.6 MHz, CDCl3) 166.1,
164.6, 139.2, 136.3, 136.1, 135.7, 133.6, 132.1, 129.3, 129.2, 126.3,
124.2, 123.7, 118.8, 115.2, 114.5, 52.9, 51.7, 14.0; m/z (EI) 413 (75, Mþ).
0.59; nmax (KBr) 1747, 1692 cmꢂ1
; dH (300 MHz, CDCl3) 8.14–8.11
4.1.4. (E)-Methyl 3-(2-methyl-1-(phenylsulfonyl)-1H-indol-3-
yl)but-2-enoate (1j)
(1H, m, ArH), 7.93 (1H, d, J 15.9 Hz, Vinylic CH), 7.80–7.77 (1H, m,
ArH), 7.29–7.26 (2H, m, ArH), 6.52 (1H, d, J 16.2 Hz, Vinylic CH), 3.82
(3H, s, CH3), 2.69 (3H, s, CH3), 1.69 (9H, s, Boc CH3); dC (75.6 MHz,
CDCl3) 168.2, 150.1, 140.9, 136.6, 136.2, 127.0, 124.2, 123.5, 119.4,
116.6, 115.5, 114.5, 84.7, 51.5, 28.2, 14.3; m/z (EI) 315 (48, Mþ).
To a solution of 2-methylindole (1 g, 7.6 mmol) in dry 1,2-di-
chloroethane (30 mL), methyl 3-oxobutanoate (1.06 g, 9.1 mmol)
and FeCl3 (0.25 g, 1.5 mmol) were added. The reaction mixture was
stirred at room temperature for 3 h. After the completion of the
reaction (monitored by TLC), DCE was evaporated under vacuo,
20 mL of H2O was added and extracted with ethyl acetate
(2ꢁ20 mL). The combined extracts were washed with water
(10 mL), brine (10 mL) and dried (Na2SO4). Evaporation of the sol-
vent gave crude indole, which was dissolved in benzene (30 mL). To
this solution, benzenesulfonylchloride (1.6 g, 9.1 mmol), tetra butyl
ammoniumhydrogensulfate (0.2 g) and 50% NaOH solution (10 mL)
were added and stirred for 3 h at room temperature. Benzene layer
was separated and dried (Na2SO4). Evaporation of the solvent gave
crude compound, which was recrystallized from methanol to give
pure 1j [2 g, 71% (two steps)] as dull white solid; mp 104 ꢀC.
[Found: C, 65.31; H, 4.91; N, 4.09; S, 8.44. C20H19NO4S requires: C,
65.02; H, 5.18; N, 3.79; S, 8.68%.] Rf (20% EA/Hexane) 0.60; nmax
4.1.7. Ethyl 9-(phenylsulfonyl)-9H-carbazole-3-carboxylate (3a)
To a stirred solution of vinyl ester 1a (0.5 g, 1.35 mmol) in dry
DMF (1.5 mL), DMF$DMA (322 mg, 2.71 mmol) was added. The
reaction mixture was heated at 110 ꢀC under N2 for 3 h. It was then
poured into 2% aq HCl (15 mL) and extracted with CHCl3 (2ꢁ20 mL).
The combined extracts were washed with water (10 mL), brine
(10 mL) and dried (Na2SO4). Removal of solvent followed by column
chromatographic purification (20% EA/hexane) afforded carbazole
3a [0.38 g, 73%] as a colourless solid; mp 180 ꢀC. [Found: C, 66.52; H,
4.59; N, 3.74; S, 8.31. C21H17NO4S requires: C, 66.47; H, 4.52; N, 3.69;
S, 8.45%.] Rf (20% EA/Hexane) 0.52; nmax (KBr) 1709, 1369,
1176 cmꢂ1
; dH (400 MHz, CDCl3) 8.61 (1H, s, ArH), 8.39–8.33 (1H, m,
ArH), 8.21–8.18 (2H, m, ArH), 7.98 (1H, d, J 7.32 Hz, ArH), 7.83 (2H, d,
J 7.32 Hz, ArH), 7.55–7.36 (5H, m, ArH), 4.43 (2H, q, J 7.32 Hz, CH2),
1.43 (3H, t, J 7.08 Hz, CH3); dC (100.6 MHz, CDCl3) 166.4, 141.0, 138.8,
137.7, 134.0, 129.1, 128.7, 128.0, 126.4, 126.3, 125.8, 124.3, 121.9,
120.4, 115.1, 114.6, 61.1, 14.4; m/z (EI) 379 (100, Mþ), 239 (68%), 194
(42%), 165 (72%).
(KBr) 1698, 1362, 1168 cmꢂ1
; dH (300 MHz, CDCl3) 8.22 (1H, d, J
8.1 Hz, ArH), 7.79 (2H, d, J 7.5 Hz, ArH), 7.55 (1H, d, J 7.2 Hz, ArH),
7.46–7.38 (3H, m, ArH), 7.35–7.23 (2H, m, ArH), 5.82 (1H, s, CH), 3.75
(3H, s, CH2), 2.57 (3H, s, CH3), 2.48 (3H, s, CH3); dC (75.6 MHz, CDCl3)
166.7, 149.7, 139.1, 136.2, 133.8, 132.8, 129.3, 128.5, 126.3, 124.7,
124.5, 123.7, 121.1, 119.2, 114.6, 51.1, 20.3, 13.6; m/z (EI) 369 (65, Mþ).
4.1.8. Ethyl 2-methyl-9-(phenylsulfonyl)-9H-carbazole-3-
carboxylate (4a)
4.1.5. (E)-Methyl 3-(2-benzyl-1-(phenylsulfonyl)-1H-indol-3-
yl)acrylate (1k)
Following the procedure similar to that of 3a, carbazole 4a
(0.35 g, 67%) was obtained using vinyl ester compound 1a (0.5 g,
1.35 mmol), DMA$DMA (360 mg, 2.7 mmol) and dry DMF (1.5 mL)
as a colourless solid; mp 174 ꢀC. [Found: C, 67.28; H, 5.02; N, 3.83; S,
8.42. C22H19NO4S requires: C, 67.16; H, 4.87; N, 3.56; S, 8.15%.] Rf
To a suspension of (E)-methyl 3-(2-(bromomethyl)-1-phenyl-
sulfonyl-1H-indol-3-yl)acrylate (1 g, 2.3 mmol) in dry benzene
(30 mL), anhydrous ZnBr2 (0.62 g, 2.8 mmol) was added and the
reaction mixture was refluxed under N2 atmosphere for 12 h. The
solvent was removed and then reaction mixture was quenched
with ice water (100 mL) containing 2 mL of concd HCl, extracted
with chloroform (3ꢁ10 mL) and dried (Na2SO4). Removal of solvent
followed by flash column chromatographic purification (4% EA/
Hexane) afforded 2-benzylindole 1k (0.4 g, 40%) as a colourless
solid; mp 152 ꢀC. [Found: C, 69.31; H, 5.21; N, 2.94; S, 7.61.
C25H21NO4S requires: C, 69.59; H, 4.91; N, 3.25; S, 7.43%.] Rf (20%
(20% EA/Hexane) 0.56; nmax (KBr) 1712, 1360, 1174 cmꢂ1
; dH
(400 MHz, CDCl3) 8.48 (1H, s, ArH), 8.29 (2H, d, J 8.32 Hz, ArH), 8.20
(1H, s, ArH), 7.92 (1H, d, J 7.84 Hz, ArH), 7.82 (2H, d, J 7.32 Hz, ArH),
7.49–7.45 (2H, m, ArH), 7.39–7.32 (2H, m, ArH), 4.40 (2H, q, J
6.84 Hz, CH2), 2.83 (3H, s, CH3), 1.43 (3H, t, J 7.08 Hz, CH3); dC
(100.6 MHz, CDCl3) 167.4, 140.4, 139.9, 138.6, 137.8, 134.0, 129.1,
127.5, 126.4, 126.1, 125.9, 124.2, 124.1, 122.8, 120.0, 117.2, 115.0, 60.9,
22.9, 14.4; m/z (EI) 393 (100, Mþ), 349 (17%), 209 (73%), 181 (37%).
EA/Hexane) 0.68; nmax (KBr) 1705, 1369, 1165 cmꢂ1
; dH (300 MHz,

R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
3589
4.1.9. Methyl 9-(phenylsulfonyl)-9H-carbazole-3-carboxylate (3b)
Yield: 0.36 g (70%); colourless solid; mp 188 ꢀC. [Found: C,
65.53; H, 4.37; N, 3.64; S, 9.03. C20H15NO4S requires: C, 65.74; H,
4.14; N, 3.83; S, 8.78%.] Rf (20% EA/Hexane) 0.53; nmax (KBr) 1710,
4.05; N, 3.31; S, 7.57%.] Rf (20% EA/Hexane) 0.49; nmax (KBr) 1716,
1710, 1382, 1192 cmꢂ1
dH (300 MHz, CDCl3) 8.43–8.38 (2H, m, ArH),
;
8.31 (1H, s, ArH), 7.78 (1H, d, J 8.0 Hz, ArH), 7.55–7.34 (5H, m, ArH),
7.26 (2H, t, J 3.9 Hz, ArH), 3.93 (3H, s, CH3), 3.80 (3H, s, CH3); dC
(75.6 MHz, CDCl3) 168.1, 167.2, 141.3, 139.1, 137.9, 133.3, 129.8, 128.9,
127.9, 126.9, 126.4, 126.2, 125.9, 124.7, 122.5, 120.7, 116.8, 116.0, 53.0,
52.7; m/z (EI) 423 (4, Mþ), 297 (100%), 283 (93%), 266 (53%).
1354, 1170 cmꢂ1
; dH (300 MHz, CDCl3) 8.61 (1H, s, ArH), 8.40–8.33
(2H, m, ArH), 8.19 (1H, d, J 8.6 Hz, ArH), 7.96 (1H, d, J 7.8 Hz, ArH),
7.84 (2H, d, J 7.8 Hz, ArH), 7.53 (1H, t, J 7.6 Hz, ArH), 7.45 (1H, t, J
7.0 Hz, ArH), 7.41–7.32 (3H, m, ArH), 3.98 (3H, s, CH3); dC (75.6 MHz,
CDCl3) 166.8, 141.1, 138.8, 137.7, 134.1, 129.2, 128.7, 128.0, 126.4,
126.3,125.9, 125.7,124.3,122.0, 120.3, 115.0,114.6, 52.2; m/z (EI) 365
(100, Mþ), 224 (98%), 193 (82%), 165 (66%).
4.1.15. (E)-Ethyl 3-(2-(methoxymethyl)-1H-indol-3-yl)acrylate (3h)
Yield: 0.13 g (56%); yellow solid; mp 116 ꢀC. [Found: C, 69.23;
H, 6.91; N, 5.13. C15H17NO3 requires: C, 69.48; H, 6.61; N, 5.40%.] Rf
(20% EA/Hexane) 0.59; nmax (KBr) 3314, 1708 cmꢂ1
; dH (300 MHz,
CDCl3) 10.35 (1H, s, NH), 8.30 (1H, d, J 15.9 Hz, Vinylic CH), 8.01
(1H, d, J 8.0 Hz, ArH), 7.95 (1H, d, J 7.2 Hz, ArH), 7.47 (2H, t,
J 7.5 Hz, ArH), 6.63 (1H, d, J 15.9 Hz, Vinylic CH), 4.33 (2H, q,
J 7.2 Hz, CH2), 4.11 (3H, s, OCH3), 3.05 (2H, s, CH2), 1.37 (3H, t, J
7.05 Hz, CH3); dC (75.6 MHz, CDCl3) 167.2, 138.3, 136.1, 134.2, 126.7,
125.7, 123.9, 120.2, 119.1, 114.8, 106.2, 63.1, 57.9, 51.7, 13.2; m/z (EI)
259 (70, Mþ).
4.1.10. Methyl 2-methyl-9-(phenylsulfonyl)-9H-carbazole-3-
carboxylate (4b)
Yield: 0.36 g (68%); colourless solid; mp 171 ꢀC. [Found: C,
66.38; H, 4.72; N, 3.72; S, 8.57. C21H17NO4S requires: C, 66.47; H,
4.52; N, 3.69; S, 8.45%.] Rf (20% EA/Hexane) 0.57; nmax (KBr) 1720,
1369, 1172 cmꢂ1
; dH (300 MHz, CDCl3) 8.51 (1H, s, ArH), 8.31 (1H, d,
J 8.17 Hz, ArH), 8.23 (1H, s, ArH), 7.94–7.84 (3H, m, ArH), 7.50–7.28
(5H, m, ArH), 3.96 (3H, s, CH3), 2.83 (3H, s, CH3); dC (75.6 MHz,
CDCl3) 167.7, 140.4, 140.1, 138.5, 137.7, 135.2, 134.0, 129.1, 127.5,
126.4, 124.2, 124.0, 122.9, 120.0, 117.2, 115.0, 51.9, 19.7; m/z (EI) 379
(92, Mþ), 238 (100%), 206 (34%).
4.1.16. Dimethyl 9-(phenylsulfonyl)-9H-carbazole-3,4-
dicarboxylate (3i)
Yield: 0.37 g (62%); brown solid; mp 128 ꢀC. [Found: C, 62.66; H,
3.80; N, 3.58; S, 7.34. C22H17NO6S requires: C, 62.40; H, 4.05; N,
3.31; S, 7.57%.] Rf (20% EA/Hexane) 0.49; nmax (KBr) 1720, 1708,1365,
4.1.11. Methyl 6-methoxy-9-(phenylsulfonyl)-9H-carbazole-3-
carboxylate (3c)
1176 cmꢂ1
; dH (300 MHz, CDCl3) 8.38–8.31 (2H, m, ArH), 8.17 (1H, d,
J 7.5 Hz, ArH), 7.93 (1H, d, J 7.5 Hz, ArH), 7.82 (2H, d, J 7.5 Hz, ArH),
7.54–7.29 (5H, m, ArH), 3.95 (3H, s, CH3), 3.79 (3H, s, CH3); dC
(75.6 MHz, CDCl3) 167.7, 166.8, 141.1, 138.8, 137.7, 134.1, 129.2, 128.7,
128.0, 126.4, 126.3, 125.9, 125.7,124.3,122.0,120.3,115.0,114.6, 52.2,
51.5; m/z (EI) 423 (15, Mþ), 297 (100%), 266 (66%), 251 (26%).
Yield: 0.34 g (68%); mp 183 ꢀC. [Found: C, 63.69; H, 4.39; N, 3.94.
C21H17NO5S requires: C, 63.79; H, 4.33; N, 3.54%.] Rf (20% EA/Hex-
ane) 0.52; nmax (KBr) 1698, 1354, 1168 cmꢂ1
; dH (300 MHz, CDCl3)
8.52 (1H, s, ArH), 8.32 (1H, d, J 7.5 Hz, ArH), 8.19 (2H, t, J 8.2 Hz,
ArH), 7.76 (2H, m, ArH), 7.44–7.30 (4H, m, ArH), 7.10 (1H, d, J 6.6 Hz,
ArH), 3.95 (3H, s, CH3), 3.88 (3H, s, CH3); dC (75.6 MHz, CDCl3) 166.8,
157.1, 141.7, 137.5, 134.0, 133.0,129.1, 128.6, 126.9, 126.5, 126.4, 125.8,
121.9, 116.3, 116.1, 114.9, 103.2, 55.7, 52.2; m/z (EI) 396 (80, Mþ), 254
(100%), 240 (46%).
4.1.17. (2-Methyl-(1-phenylsulfonyl)-1H-indol-3-yl)-1,2,3-tricarbo-
methoxypropane (4i)
Yield: 0.28 g (60%); colourless solid; mp 122 ꢀC. [Found: C,
59.44; H, 4.91; N, 3.16; S, 6.32. C24H25NO8S requires: C, 59.13; H,
5.17; N, 2.87; S, 6.58%.] Rf (20% EA/Hexane) 0.49; nmax (KBr) 1724,
4.1.12. 6-Methoxy-2-methyl-9-(methyl phenylsulfonyl)-9H-
carbazole-3-carboxylate (4c)
1712, 1695, 1354, 1160 cmꢂ1
; dH (300 MHz, CDCl3) 8.19 (21H, d, J
8.1 Hz, ArH), 7.72 (2H, d, J 8.1 Hz, ArH), 7.55 (2H, t, J 8.7 Hz, ArH),
7.43 (2H, t, J 7.2 Hz, ArH), 7.31–7.20 (2H, m, ArH), 4.35 (1H, d, J
11.4 Hz, CH), 3.82 (1H, quint, J 5.47 Hz, CH), 3.74 (3H, s, CH3), 3.59
(3H, s, CH3), 3.38 (3H, s, CH3), 2.56 (3H, s, CH3), 2.46 (1H, dd, J 4.5,
4.5 Hz, CH2), 2.05 (1H, dd, J 6, 6 Hz, CH2); dC (75.6 MHz, CDCl3)
173.5, 172.3, 171.0, 138.9, 136.5, 136.1, 133.7, 129.2, 128.2, 126.2,
124.5, 123.8, 119.4, 114.7, 114.3, 52.4, 52.3, 51.5, 42.5, 40.9, 33.4, 12.7;
m/z (EI) 487 (56, Mþ).
Yield: 0.29 g (55%); colourless solid; mp 175 ꢀC. [Found: C,
64.29; H, 4.95; N, 3.21; S, 8.09. C22H19NO5S requires: C, 64.53; H,
4.68; N, 3.42; S, 7.83%.] Rf (20% EA/Hexane) 0.50; nmax (KBr) 1702,
1363, 1166 cmꢂ1
; dH (300 MHz, CDCl3) 8.35 (1H, s, ArH), 8.11–8.09
(2H, m, ArH), 7.75–7.68 (3H, m, ArH), 7.40–7.22 (3H, m, ArH), 6.98
(1H, d, J 6.9 Hz, ArH), 3.86 (3H, s, CH3), 3.80 (3H, s, CH3), 2.71 (3H, s,
CH3); dC (75.6 MHz, CDCl3) 167.7, 157.1, 141.1, 137.6, 135.7, 134.0,
132.9, 129.1, 127.0, 126.3, 124.3, 122.9, 117.5, 116.1, 115.5, 112.9, 103.1,
55.7, 51.9, 13.2; m/z (EI) 409 (34, Mþ), 269 (53%), 268 (100%), 239
(15%).
4.1.18. Methyl 1-phenyl-9H-carbazole-3-carboxylate (3k)
Yield: 0.18 g (66%); colourless solid; mp 142 ꢀC. [Found: C, 79.41;
H, 5.29; N, 4.36. C20H15NO2 requires: C, 79.72; H, 5.02; N, 4.65%.] Rf
(20% EA/Hexane) 0.60; nmax (KBr) 3321, 1710 cmꢂ1
; dH (300 MHz,
4.1.13. Dimethyl 9-(phenylsulfonyl)-9H-carbazole-2,4-
dicarboxylate (3e)
CDCl3) 8.78 (1H, s, ArH), 8.55 (1H, s, NH), 8.12 (2H, d, J 6 Hz, ArH),
7.66 (2H, d, J 7.8 Hz, ArH), 7.53 (2H, t, J 7.35 Hz, ArH), 7.45–7.42 (3H,
m, ArH), 7.27 (1H, q, J 6.8 Hz, ArH), 3.96 (3H, s, CH3); dC (75.6 MHz,
CDCl3) 167.9, 140.1, 139.9, 138.0, 129.3, 128.3, 127.9, 127.1, 126.6,
124.7, 123.6, 123.5, 121.9, 120.7, 120.4, 111.0, 52.0; m/z (EI) 301
(66, Mþ).
Yield: 0.33 g (65%); pale yellow solid; mp 210 ꢀC. [Found: C,
62.22; H, 4.33; N, 3.09; S, 7.78. C22H17NO6S requires: C, 62.40; H,
4.05; N, 3.31; S, 7.57%.] Rf (20% EA/Hexane) 0.49; nmax (KBr) 1719,
1712, 1384, 1195 cmꢂ1
; dH (300 MHz, CDCl3) 8.25–8.21 (3H, m, ArH),
7.79 (1H, s, ArH), 7.70 (2H, d, J 7.5 Hz, ArH), 7.52–7.28 (5H, m, ArH),
3.73 (3H, s, CH3), 3.80 (3H, s, CH3); dC (75.6 MHz, CDCl3) 167.9, 167.0,
141.5, 138.8, 137.7, 133.1, 129.6, 128.7, 127.7, 126.7, 126.2, 125.9, 125.7,
124.5, 122.3, 120.5, 116.6, 115.8, 52.8, 52.5; m/z (EI) 423 (72, Mþ),
282 (100%), 251 (23%), 223 (11%).
4.1.19. 3-Acetyl 9-(phenylsulfonyl)-9H-carbazole (3l)
Yield: 0.29 g, (56%); orange solid; mp 192 ꢀC. [Found: C, 68.45;
H, 4.61; N, 3.72; S, 9.39. C20H15NO3S requires: C, 68.75; H, 4.33; N,
4.01; S, 9.18%.] Rf (20% EA/Hexane) 0.52; nmax (KBr) 1670, 1382,
4.1.14. Dimethyl 9-(phenylsulfonyl)-9H-carbazole-1,3-
dicarboxylate (3f)
Yield: 0.43 g (65%); pale yellow solid; mp 102 ꢀC. [Found: C,
62.13; H, 4.34; N, 3.08; S, 7.79. C22H17NO6S requires: C, 62.40; H,
1172 cmꢂ1
; dH (300 MHz, CDCl3) 8.59 (1H, s, ArH), 8.41–8.35 (2H, m,
ArH), 8.11 (1H, d, J 8.7 Hz, ArH), 7.99 (1H, d, J 8.1 Hz, ArH), 7.84 (1H,
d, J 7.2 Hz, ArH), 7.55 (2H, t, J 7.3 Hz, ArH), 7.49–7.42 (2H, m, ArH),
7.3 (2H, t, J 7.6 Hz, ArH), 2.70 (3H, s, CH3); dC (75.6 MHz, CDCl3)

3590
R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
198.9, 141.2, 138.9, 137.5, 134.1, 129.2, 128.2, 127.8, 126.4, 124.4,
3,4-dicarbaldehyde26 (1 g, 59.5 mmol) and (carbomethoxy-
methylene)triphenylphosphorane (4.77 g, 14.1 mmol) in dry tolu-
ene (30 mL) as a pale yellow solid; mp 102–104 ꢀC; [Found: C,
60.28; H, 5.47; S, 11.71. C16H20O4S requires: C, 59.98; H, 5.75; S,
120.6, 120.3, 117.1, 115.1, 114.7, 106.2, 26.7; m/z (EI) 349 (82, Mþ).
4.1.20. (E)-Methyl 3-(1,2-dimethyl-1H-indol-3-yl)acrylate (3m)
Yield: 0.235 g (65%); dark brown solid; mp 130 ꢀC. [Found: C,
73.63; H, 6.31; N, 6.39. C14H15NO2 requires: C, 73.34; H, 6.59; N,
11.44%.] Rf (20% EA/Hexane) 0.61; nmax (KBr) 1707 cmꢂ1
; dH
(300 MHz, CDCl3) 7.64 (1H, d, J 16.1 Hz, Vinylic CH), 6.03 (1H, d, J
16.1 Hz, Vinylic CH), 4.26 (2H, q, J 7.3 Hz, CH2), 2.46 (3H, s, CH3),
1.34 (3H, t, J 7.0 Hz, CH3); dC (100.6 MHz, CDCl3) 166.9, 137.5, 137.2,
132.2, 121.2, 60.5, 14.4, 14.3; m/z (EI) 308 (43, Mþ).
6.11%.] Rf (20% EA/Hexane) 0.64; nmax (KBr) 1705 cmꢂ1
; dH
(300 MHz, CDCl3) 7.92 (1H, d, J 15.9 Hz, Vinylic CH), 7.83 (1H, s,
ArH), 7.20 (3H, s, ArH), 6.38 (1H, d, J 15.6 Hz, Vinylic CH), 3.79 (3H, s,
CH3), 3.51 (3H, s, CH3), 2.38 (3H, s, CH3); dC (75.6 MHz, CDCl3) 169.1,
141.9, 137.9, 137.5, 125.6, 122.1, 121.3, 119.9, 110.8, 109.3, 108.8, 51.3,
29.7, 10.5; m/z (EI) 229 (100, Mþ).
4.1.25. (2E,4E,6E)-Trimethyl 3,30,300-(trimethylbenzene-1,3,5-
triyl)triacrylate (20)
Using the procedure similar to that of 15, benzene trivinylester
20 (3.31 g, 52%) was prepared using 2,4,6-trimethylbenzene-
1,3,5-tricarbaldehyde27 (3.5 g, 17.1 mmol) and (carbomethoxy-
methylene)triphenylphosphorane (20.6 g, 61.7 mmol) in dry THF
(75 mL) as dull white solid; mp 118 ꢀC. [Found: C, 67.99; H, 6.21.
C21H24O6 requires: C, 67.73; H, 6.50%.] Rf (20% EA/Hexane) 0.52;
4.1.21. (3E,4E)-Dimethyl 3,30-(2,5-dimethyl-1-(phenylsulfonyl)-1H-
pyrrole-3,4-diyl)diacrylate (15)
To a suspension of t-BuOK (5.9 g, 52.9 mmol) in dry THF
(25 mL) and 18-crown-6 (0.7 g, 2.6 mmol), 2,5-dimethyl-1H-pyr-
role-3,4-dicarbaldehyde25 (4 g, 26.5 mmol) in dry THF (25 mL) was
added slowly at room temperature and resulting reaction mixture
was stirred for 15 min. To this, benzenesulfonylchloride (5.6 g,
31.7 mmol) in dry THF (15 mL) was added and stirred for another
4 h. It was then poured over ice water (200 mL) and extracted with
CHCl3 (2ꢁ30 mL). The combined organic extracts were washed
with H2O (2ꢁ25 mL) and dried (Na2SO4). Removal of solvent gave
crude 2,5-dimethyl-1-(phenylsulfinyl)-1H-pyrrole-3,4-dicarbalde-
hyde, which was dissolved in dry toluene (50 mL). To this, (carbo-
methoxymethylene)triphenyl phosphorane (18.5 g, 55.6 mmol)
was added and refluxed under N2 for 12 h. Removal of solvent
followed by column chromatographic separation (10% EA/Hexane)
afforded 15 [3.6 g, 34% (two steps)] as a white solid; mp 136 ꢀC.
[Found: C, 59.82; H, 4.96; N, 3.77; S, 7.73. C20H21NO6S requires: C,
59.54; H, 5.25; N, 3.47; S, 7.95%.] Rf (20% EA/Hexane) 0.56; nmax
nmax (KBr) 1724 cmꢂ1
; dH (300 MHz, CDCl3) 7.79 (1H, d, J 16.2 Hz,
Vinylic CH), 5.96 (1H, d, J 16.2 Hz, Vinylic CH), 3.82 (3H, s, CH3), 2.23
(3H, s, CH3); dC (75.6 MHz, CDCl3) 166.6, 144.1, 133.8, 133.7, 125.1,
51.7, 18.8; m/z (EI) 372 (53, Mþ), 357 (60%), 340 (40%), 281 (66%).
4.1.26. (2E,4E,6E)-Trimethyl (1,3,5-tris(methoxycarbonylmethyl)-
benzene)triacrylate (22)
Following the procedure similar to that of 1e, compound 22
(0.56 g, 63%) was obtained using (2E,4E,6E)-trimethyl (1,3,5-tris
(bromomethyl)benzene)triacrylate (1 g, 1.6 mmol), Pd (PPh3)2Cl2
(350 mg, 0.49 mmol) and K2CO3 (1.36 g, 9.8 mmol) in dry THF–
MeOH (35–10 mL) under CO atmosphere as a yellow solid; mp
76 ꢀC. [Found: C, 59.65; H, 5.27. C27H30O12 requires: C, 59.34; H,
5.53%.] Rf (20% EA/Hexane) 0.44; nmax (KBr) 1726, 1715 cmꢂ1
; dH
(KBr) 1708, 1373, 1176 cmꢂ1
;
dH (300 MHz, CDCl3) 7.73 (2H, d, J
(300 MHz, CDCl3) 7.70 (1H, d, J 16.5 Hz, Vinylic CH), 5.99 (1H, d, J
16.5 Hz, Vinylic CH), 3.80 (3H, s, CH3), 3.66 (3H, s, CH3), 3.62 (2H, s,
CH2); dC (75.6 MHz, CDCl3) 171.1, 165.9, 142.3, 136.9, 130.4, 126.8,
52.2, 51.9, 37.3; m/z (EI) 546 (49, Mþ).
7.5 Hz, ArH), 7.65 (2H, m, ArH), 7.55–7.53 (3H, m, ArH & Vinylic
CH), 6.00 (2H, d, J 15.9 Hz, Vinylic CH), 3.78 (6H, s, CH3), 2.49 (6H,
s, CH3); dC (75.6 MHz, CDCl3) 167.1, 139.4, 136.4, 134.1, 132.9, 129.7,
126.4, 120.8, 119.6, 51.6, 13.2; m/z (EI) 403 (58, Mþ).
4.1.27. 9-Phenylsulfonyl-4-hydroxycarbazole-1-carboxal-
dehyde (25)
4.1.22. 3,6-Dimethyl 9-(phenylsulfonyl)-9H-carbazole-3,6-
dicarboxylate (16)
To a mixture of 2-methyl-3-acetylindole 24 (0.5 g, 1.6 mmol)
and DMF$DMA (380 mg, 3.2 mmol), dry DMF (1.5 mL) was added
and the reaction mixture was heated at 140 ꢀC for 4 h under N2
atmosphere. Usual workup followed by column chromatographic
purification afforded title compound 25 (0.36 g, 65%) as a light
brown solid; mp 190 ꢀC. [Found: C, 65.25; H, 3.46; N, 4.30; S,
8.89. C19H13NO4S requires: C, 64.95; H, 3.73; N, 3.99; S, 9.13%.] Rf
Following the procedure similar to that of 3a, carbazole 16
(0.20 g, 55%) was obtained using divinyl ester compound 15 (0.5 g,
1.2 mmol), DMF$DMA (591 mg, 4.9 mmol) and dry DMF (1.5 mL) as
a colourless solid; mp 192 ꢀC. [Found: C, 68.97; H, 4.83; N, 4.99.
C17H15NO4 requires: C, 68.68; H, 5.09; N, 4.71%.] Rf (20% EA/Hexane)
0.60; nmax (KBr) 1669 cmꢂ1
; dH (300 MHz, CDCl3) 8.78 (2H, s, ArH),
8.18 (2H, dd, J 1.2, 1.2 Hz, ArH), 7.36 (2H, d, J 8.7 Hz, ArH), 3.97 (6H, s,
CH3), 3.83 (3H, s, CH3); dC (75.6 MHz, CDCl3) 167.5, 144.1, 127.9,
122.8, 122.5, 121.8, 108.4, 52.0, 29.4; m/z (EI) 297 (100, Mþ).
(20% EA/Hexane) 0.61; nmax (KBr) 3340, 1648, 1366, 1158 cmꢂ1
; dH
(300 MHz, CDCl3) 12.01 (1H, s, OH), 9.94 (1H, s, CHO), 8.30 (2H, t,
J 7.3 Hz, ArH), 7.99 (1H, d, J 8.7 Hz, ArH), 7.86 (2H, d, J 7.8 Hz,
ArH), 7.62 (1H, d, J 8.7 Hz, ArH), 7.53–7.35 (5H, m, ArH); dC
(75.6 MHz, CDCl3) 195.8, 158.6, 143.9, 137.9, 137.8, 134.2, 132.3,
129.3, 127.2, 126.5, 124.8, 124.7, 123.4, 116.4, 114.6, 114.4, 106.9;
m/z (EI) 351 (100, Mþ).
4.1.23. (3E,4E)-Dimethyl 3,4-(2,5-dimethyl-1-(methyl)-1H-
pyrrole)diacrylate (17)
Following the procedure similar to that of 3a, pyrrole 17 (0.18 g,
53%) was obtained using divinyl ester compound 15 (0.5 g,
1.2 mmol), DMA$DMA (660 mg, 4.9 mmol) and dry DMF (1.5 mL) as
a colourless solid; mp 168 ꢀC. [Found: C, 65.27; H, 6.65; N, 5.36.
C15H19NO4 requires: C, 64.97; H, 6.91; N, 5.05%.] Rf (20% EA/Hexane)
4.1.28. Ethyl 1,2-dimethyl-1H-indole-3-carboxylate (29)
Following the procedure similar to that of 3a, N-methylated
product 29 (0.34 g, 65%) was obtained using ethyl 2-methyl-1H-
indole-3-carboxylate 28 (0.5 g, 2.5 mmol) and DMF$DMA
(588 mg, 4.9 mmol) as a colourless solid; mp 96 ꢀC. [Found: C,
72.18; H, 6.69; N, 6.74. C13H15NO2 requires: C, 71.87; H, 6.96; N,
0.42; nmax (KBr) 1705 cmꢂ1
; dH (300 MHz, CDCl3) 7.78 (2H, d, J
15.9 Hz, Vinylic CH), 5.96 (2H, d, J 15.9 Hz, Vinylic CH), 3.78 (6H, s,
CH3), 3.41 (3H, s, CH3), 2.30 (6H, s, CH3); dC (75.6 MHz, CDCl3) 168.2,
138.2, 131.5, 115.5, 115.1, 51.3, 30.7, 11.1; m/z (EI) 277 (100, Mþ).
6.45%.] Rf (20% EA/Hexane) 0.80; nmax (KBr) 1708 cmꢂ1
; dH
(300 MHz, CDCl3) 8.13–8.10 (1H, m, ArH), 7.28–7.20 (3H, m, ArH),
4.39 (2H, q, J 7 Hz, CH2), 3.65 (3H, s, CH3), 2.75 (3H, s, CH3), 1.44
(3H, t, J 7.05 Hz, CH3); dC (75.6 MHz, CDCl3) 166.2, 145.2, 136.5,
126.5, 121.9, 121.6, 121.4, 109.0, 103.9, 59.3, 29.5, 14.6, 11.8; m/z
(EI) 217 (87, Mþ).
4.1.24. (3E,4E)-Dimethyl 3,30-(2,5-dimethylthiophene-3,4-
diyl)diacrylate (18)
Using the procedure similar to that of 15, thiophene divinyl
ester 18 (1.04 g, 65%) was prepared using 2,5-dimethylthiophene-

R. Sureshbabu et al. / Tetrahedron 65 (2009) 3582–3591
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4.1.29. Diethyl 1,3,5-trimethyl-1H-pyrrole-2,4-dicarboxylate (31)
Following the procedure similar that of 3a, N-methyl pyrrole 31
(0.40 g, 75%) was obtained using diethyl 3,5-dimethyl-1H-pyrrole-
2,4-dicarboxylate 30 (0.5 g, 2.1 mmol) and DMF$DMA (498 mg,
4.2 mmol) as a colourless solid; mp 110 ꢀC. [Found: C, 61.92; H, 7.26;
N, 5.82. C13H19NO4 requires: C, 61.64; H, 7.56; N, 5.53%.] Rf (20% EA/
Hexane) 0.81; nmax (KBr) 1703,1710 cmꢂ1
; dH (300 MHz, CDCl3) 4.30
(4H, quint, J 7.12 Hz, CH2), 3.77 (3H, s, CH3), 2.53 (3H, s, CH3), 2.50
(3H, s, CH3), 1.36 (6H, q, J 6.3 Hz, CH3); dC (75.6 MHz, CDCl3) 165.6,
162.2, 141.0, 131.0, 120.3, 112.5, 59.9, 59.5, 32.9, 14.3, 12.7, 11.9; m/z
(EI) 253 (68, Mþ).
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We thank CSIR, New Delhi (01(2071)06/EMR-II) for the financial
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