An efficient synthesis of pyrrolocoumarins and pyrroloquinolones by acid-catalysed cyclisation of acetylenic amines in water

Article abstract of DOI:10.1055/s-0030-1259990

A simple H⁺-catalysed cyclisation of acetylenic amines for the synthesis of pyrrolocoumarins and pyrroloquinolones under conventional heating or microwave irradiation has been achieved. The reaction requires inexpensive catalyst and simple yet clean reaction conditions and offers potentially bioactive heterocycles in 82-95% yields. Georg Thieme Verlag Stuttgart New York.

Full text of DOI:10.1055/s-0030-1259990

PAPER
1489
An Efficient Synthesis of Pyrrolocoumarins and Pyrroloquinolones by
Acid-Catalysed Cyclisation of Acetylenic Amines in Water
S
ynthesis of Pyrro
.
locoumarins
C
and
P
yrroloquin
.
Department of Chemistry, University of Kalyani, Kalyani 741235, W. B, India
Fax +91(33)25828282; E-mail: kcm_ku@yahoo.co.in
Received 15 December 2010; revised 30 January 2011
als.14 Due to their potential application as conducting
materials15 coumarins and quinolones are subunits of
many natural products possessing broad-spectrum biolog-
ical activity and exhibit, for example, antifungal, antibac-
Abstract: A simple H+-catalysed cyclisation of acetylenic amines
for the synthesis of pyrrolocoumarins and pyrroloquinolones under
conventional heating or microwave irradiation has been achieved.
The reaction requires inexpensive catalyst and simple yet clean re-
action conditions and offers potentially bioactive heterocycles in terial, antiviral, and antimicrobial properties.16–20 In
82–95% yields.
particular, pyrano-indole tethered heterocycles have been
used for their antibacterial, monoamine oxidase (MAO)
inhibitory, and anthelmintic activities.21 We became inter-
ested to develop an efficient protocol for synthesising
substituted pyrano[3,2-e]indolone and pyrrolo[3,2-f]qui-
nolone derivatives. So far we have succeeded in synthe-
sising pyrano[3,2-e]indolone and pyrrolo[3,2-f]quinolone
derivatives through palladium(0)-mediated cross-cou-
pling followed by copper(I)-catalysed or gold(III)-cataly-
sed heteroannulations and palladium-catalysed Heck
reaction or IBX-induced intramolecular oxidative amina-
tion of alkenes.22 Herein we report the results of our inves-
tigation on the synthesis of biologically important
substituted pyranoindoles and pyranoquinolones in water
medium23 via Brønsted acid catalysed cyclisation of acet-
ylenic amines possessing an electron-donating group on
the nitrogen atom.
Key words: pyrrolocoumarin, pyrroloquinolone, Brønsted acid ca-
talysis, intramolecular cyclisation, microwave irradiation
Recently there has been a flurry of activity surrounding
the development of green organic syntheses, sustainable
and environmentally benign protocols. In this respect, use
of water as a versatile solvent in place of hazardous organ-
ic reaction media is particularly relevant. Water is nontox-
ic, inexpensive, harmless and environmentally benign.1,2
Water as solvent also eliminates some tedious protection
and deprotection processes for certain acidic hydrogen
containing functional groups, which contribute to the
overall synthetic efficiency. Due to hydrophobic effects
water not only accelerates the reaction rates but also en-
hances reaction selectivities. Water has been used for met-
al-mediated organic synthesis3 and reactions involving
water sensitive compounds.4 Solubility of the substances
and the metal catalyst are the main problems with the use
of water and this problem can be overcome by performing
the reaction at high temperature in a homogeneous medi-
um. At high temperature water behaves like a pseudo or-
ganic solvent.5,6 Because of its high dielectric constant
water is also potentially a very useful solvent for micro-
wave-mediated synthesis.7,8 When water is heated well
above its boiling point in a sealed vessel, or under micro-
wave-mediated condition organic substrate may be more
soluble.9 This has prompted the development of new syn-
thetic protocols10–12 involving both water and micro-
waves.
The starting materials for this study 3a–j were prepared
according to our earlier published procedure22a
(Scheme 1). We have conducted an optimisation study to
explore the influence of acids, temperature, and solvents
on the outcome of the cycloisomerisation of 5-(6-ami-
nocoumarin)ylacetylenic amines 3a–g and 5-(6-amino-
quinolone)ylacetylenic amines 3h–j to pyrrolocoumarins
4a–g and pyrroloquinolones 4h–j, both under convention-
al heating and microwave irradiation.
R2
Br
NHR1
NHR1
NHR1
Among the fundamental heterocycles, the pyrrole ring is
one, which is widely distributed as structural unit in a va-
riety of naturally and biologically important molecules
such as porphyrins, bile pigments, co-enzymes, and alka-
loids.13 In recent years, there has been a continuous quest
to synthesise pyrrole and its oligomer with different sub-
stituents efficiently from readily available starting materi-
O
X
O
X
O
X
3
1
2
a
X = O, R1 = Et, R2 = C5H11
X = O, R1 = Me, R2 = Ph
X = O, R1 = Et, R2 = Ph
X = O, R1 = H, R2 = Ph
X = O, R1 = Me, R2 = Bu
X = O, R1 = Et, R2 = Bu
X = O, R1 = H, R2 = Bu
X = NMe, R1 = H, R2 = Ph
X = NMe, R1 = Me, R2 = Ph
X = NMe, R1 = Et; R2 = Ph
b
c
d
e
f
g
h
i
SYNTHESIS 2011, No. 9, pp 1489–1493
x
x
.
x
x
.2
0
1
1
j
Advanced online publication: 04.04.2011
DOI: 10.1055/s-0030-1259990; Art ID: Z53510SS
© Georg Thieme Verlag Stuttgart · New York
Scheme 1 Preparation of the starting materials 3a–j

1490
K. C. Majumdar et al.
PAPER
No change was detected on TLC when the cycloisomeri- were synthesised in excellent yields both under conven-
sation reaction of the alkyne 3a was carried out in the ab- tional heating and microwave irradiation on an ordinary
sence of any acid as a catalyst in ethanol at 80 °C or water kitchen microwave oven using a power level of 150 W.
at 100 °C (Table 1, entries 1 and 2). In the presence of one Almost similar yields were also obtained under micro-
equivalent of acid (TsOH), the reaction proceeds smooth- wave heating (Table 2); the most important advantage be-
ly when refluxed in ethanol at 80 °C and gives 80% yield ing the significant reduction of the reaction time, from
of the product 4a (Table 1, entry 3). For optimisation of several hours to typically 20–30 minutes.
the reaction conditions we have carried out a series of ex-
Table 2 Acid-Catalysed Cycloisomerisation Reaction of 3a to 4a
periments, in which the solvent, temperature, and the
Brønsted acid were changed sequentially. It was found
that at 100 °C water is the best solvent and 1 equivalent of
C5H11
C5H11
the acid is sufficient to bring about the highest yield of the
product (entry 9).
NEt
NHEt
H+ (cat.), H2O
r.t., heat, or MW,
20–180 min
Table 1 Optimisation of the Cycloisomerisation Reaction of 3a to
4a
O
O
O
O
4a
3a
C5H11
Entry
H+ (equiv) Heating
r.t.
Time (min)
Yield (%)a
C5H11
1
2
3
4
2
180
180
20
–
NEt
NHEt
100 °Cb
MWc
95
80
94
H+ (cat.), solvent
1
2
1
r.t. to 110 °C, 1–10 h
O
O
O
O
4a
3a
MWc
20
Entry H+ (cat.)
Solvent
EtOH
H2O
Temp (°C) Time (h) Yield (%)a
a Isolated yields.
1
2
3
4
5
6
7
8
9b
–
80
100
80
10
10
3
n.r.
n.r.
80
85
83
81
n.r.
50
95
n.r.
95
94
92
93
b Conventional heating.
c Microwave heating at 150 W.
–
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
TsOH
H2SO4
HNO3
HCl
EtOH
toluene
MeCN
DMF
H2O
Having established the optimised reaction conditions, the
scope of this protocol was explored with different sub-
strates 3a–g and 3h–j for the synthesis of various hetero-
cycles (Scheme 2, Table 3). Few examples of the
cycloisomerisation of acetylenic amines have been report-
ed.24–26 Cycloisomerisation of acetylenic amines with
electron-donating substituents are very rare.22a,b Here we
have been able to demonstrate that acetylenic amines with
electron-donating as well as without electron-withdraw-
ing groups readily undergo cycloisomerisation to give the
pyrrolocoumarin and pyrroloquinoline derivatives.
110
80
3
3
110
r.t.
3
10
1
H2O
100
100
100
100
100
100
100
H2O
3
10
–
3
R2
11
12
13
14
H2O
3
R2
H2O
3
NHR1
R1
N
H2O
3
H+, H2O
AcOH
H2O
3
heat or MW
20–270 min
O
X
O
X
a Isolated yield; n.r. = no reaction.
b Optimised reaction conditions.
4a–g X = O; R1 = H, Me, Et;
R2 = Ph, C5H11, Bu
3a–g X = O
3h–j X = NMe
4h–j X = NMe; R1 = H, Me, Et;
R2 = Ph
The reaction was also carried out without any solvent and
the result was unsatisfactory (Table 1, entry 10). We sub-
sequently found that TsOH affords slightly higher or sim-
ilar yields in comparison to other acids (Table 1, entries
11–14). The reaction failed completely at room tempera-
ture (Table 1, entry 7). Therefore, the optimised reaction
conditions consist of TsOH (1 equiv), H2O, reflux at
100 °C for three hours. The optimisation experiments are
summarised in Table 1. Various 2-substituted indoles
Scheme 2 Acid-catalysed cycloisomerisation of 3 to 4
However, in the case of 6-amino-5-[(trimethylsilyl)ethy-
nyl]-2H-chromen-2-one (5), exclusively the hydration
product, 5-acetyl-6-aminochromen-2-one (6) was ob-
tained instead of the desired pyrrolocoumarin derivative
(Scheme 3).
Synthesis 2011, No. 9, 1489–1493 © Thieme Stuttgart · New York

PAPER
Synthesis of Pyrrolocoumarins and Pyrroloquinolones
1491
Table 3 Pyrrolocoumarins 4a–g and Pyrroloquinolines 4h–j
Entry
1
Substrate
3a
X
R1
Et
R2
Heatinga
100 °C
MW
Time (min)
180
20
Product
4a
4a
4b
4b
4c
Yield (%)b
O
C5H11
C5H11
Ph
95
94
94
93
95
89
89
90
89
93
89
87
85
87
86
82
2
3a
O
Et
3
3b
3b
3c
O
Me
Me
Et
100 °C
MW
180
20
4
O
Ph
5
O
Ph
100 °C
100 °C
MW
180
210
25
6
3d
3d
3e
O
H
Ph
4d
4d
4e
7
O
H
Ph
8
O
Me
Me
Et
Bu
Bu
Bu
Bu
Bu
Ph
100 °C
MW
180
20
9
3e
O
4e
10
11
12
13
14
15
16
3f
O
100 °C
100 °C
MW
180
240
25
4f
3g
O
H
4g
4g
4h
4i
3g
O
H
3h
3i
NMe
NMe
NMe
NMe
H
100 °C
100 °C
MW
270
240
30
Me
Me
Et
Ph
3i
Ph
4i
3j
Ph
100 °C
240
4j
a Conventional heating at 100 °C or microwave heating at 150 W.
b Isolated yields.
The mechanistic rationalisation for the formation of prod- Brønsted acid-catalysed formation of pyrrole by a 5-endo-
ucts 4a–j is depicted in Scheme 4. Initially the protonation dig cyclisation. More over the substrate they used were
of the amine 3 may give the intermediate cation 7, which activated ynamide derivatives.
may then add to the triple bond in a concerted manner
where a proton transfer and 5-endo-dig cyclisation may
In conclusion, to our knowledge the work reported here
represents the first example of H+-catalysed preparation of
occur to give the products 4 because 4-exo-dig is energet-
pyrrolocoumarins and pyrroloquinolones in excellent
yields. The notable feature of the proposed methodology
ically unfavourable. There is only one report27for the
includes the use of water as a reaction medium under con-
ventional heating. As expected considerable reduction of
TMS
reaction time under microwave irradiation was observed.
O
Moreover, the transformation proceeds well during the
NH2
NH2
course of the reaction without requiring an inert atmo-
sphere and the procedure offers several advantages in-
cluding inexpensive catalyst, improved yields and simple
yet clean reaction conditions, making it a useful and at-
tractive strategy for the synthesis of bioactive heterocy-
cles.
H+, H2O, Δ
O
O
O
O
5
6
Scheme 3 Hydration of compound 5
R2
R2
R2
NHR1
NH2R1
cyclisation
5-endo-dig
R1
N
O
X
O
X
O
X
7
3a–g X = O
3h–j X = NMe
4a–g X = O
4h–j X = NMe
Scheme 4 Probable mechanistic pathway of acid-catalysed cycloisomerisation
Synthesis 2011, No. 9, 1489–1493 © Thieme Stuttgart · New York

1492
K. C. Majumdar et al.
PAPER
The analytical and spectral data of the products 4b–d,g–j were in
agreement with the reported values.22b,e The new products are listed
below.
Melting points were determined in open capillaries and are uncor-
rected. IR spectra (cm–1) were recorded using samples as neat liq-
uids and solid samples were recorded in KBr disks. 1H NMR (400
MHz) spectra were recorded in CDCl3 (chemical shifts in d) with
TMS as internal standard. Microwave irradiations were done on an
ordinary kitchen microwave oven, Model No. BMO 800 TS. silica
gel (60–120, 230–400 mesh, Rankem, India) was used for chro-
matographic separation. Silica gel G (Spectrochem, India) was used
for TLC.
4a
Yield: 95%; colourless solid; mp 72–74 °C.
IR (KBr): 2932, 1716 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.94 (t, J = 6.8 Hz, 3 H), 1.37 (t,
J = 7.2 Hz, 3 H), 1.40–1.46 (m, 4 H), 1.75–1.83 (m, 2 H), 2.77 (t,
J = 8.0 Hz, 2 H), 4.19 (q, J = 7.2 Hz, 2 H), 6.44 (d, J = 9.6 Hz, 1 H),
6.49 (s, 1 H), 7.13 (d, J = 8.4 Hz, 1 H), 7.42 (d, J = 8.8 Hz, 1 H),
8.11 (d, J = 9.6 Hz, 1 H).
13C NMR (100 MHz): d = 14.0, 15.5, 22.5, 26.8, 28.8, 31.7, 38.0,
96.7, 109.5, 110.1, 112.9, 114.3, 124.4, 132.2, 141.1, 143.5, 149.8,
162.1.
The starting acetylenic amines were prepared according to our ear-
lier published procedure.22a Compounds 3b–d,g–j are all known.
The analytical and spectral data of the new acetylenic amines are
given below.
3a
Yield: 79%; yellow gum.
IR (KBr): 1716, 2217, 3402 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.94 (t, J = 7.2 Hz, 3 H), 1.31 (t,
J = 7.2 Hz, 3 H), 1.35–1.42 (m, 2 H), 1.44–1.53 (m, 2 H), 1.65–1.72
(m, 2 H), 2.58 (t, J = 7.2 Hz, 2 H), 3.24 (t, J = 7.2 Hz, 2 H), 4.99 (s,
1 H), 6.42 (d, J = 10.0 Hz, 1 H), 6.79 (d, J = 10.0 Hz, 1 H), 7.15 (d,
J = 8.8 Hz, 1 H), 8.05 (d, J = 10.0 Hz, 1 H).
HRMS: m/z calcd for C18H21NO2 + Na [M + Na]+: 306.1470; found;
306.1468.
4e
Yield: 90%; colourless solid; mp 86–88 °C.
IR (KBr): 2930, 1704 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.99 (t, J = 7.2 Hz, 3 H), 1.49 (q,
J = 7.2 Hz, 2 H), 1.74 (q, J = 7.6 Hz, 2 H), 2.79 (t, J = 7.2 Hz, 2 H),
3.72 (s, 3 H), 6.44 (d, J = 9.6 Hz, 1 H), 6.49 (s, 1 H), 7.13 (d, J = 8.8
Hz, 1 H), 7.41 (d, J = 8.8 Hz, 1 H), 8.10 (d, J = 9.6 Hz, 1 H).
MS: m/z = 283 [M+].
Anal. Calcd for C18H21NO2: C, 76.29; H, 7.47; N, 4.94. Found: C,
76.53; H, 7.54; N, 4.80.
13C NMR (100 MHz, CDCl3): d = 13.9, 22.5, 26.7, 19.8, 30.6, 96.6,
3e
109.5, 110.0, 112.8, 114.4, 124.1, 133.4, 141.1, 144.1, 149.8, 162.1.
Yield: 74%; yellow solid; mp 76–78 °C.
IR (KBr): 1716, 2216, 3379 cm–1.
MS: m/z = 255 [M+].
Anal. Calcd for C16H17NO2: C, 75.27; H, 6.71; N, 5.49. Found: C,
74.97; H, 6.68; N, 5.55.
1H NMR (400 MHz, CDCl3): d = 0.98 (t, J = 7.2 Hz, 3 H), 1.53 (m,
2 H), 1.67 (m, 2 H), 2.57 (t, J = 7.2 Hz, 2 H), 2.94 (s, 3 H), 4.62 (s,
1 H), 6.42 (d, J = 9.6 Hz, 1 H), 6.79 (d, J = 8.8 Hz, 1 H), 7.18 (d,
J = 9.2 Hz, 1 H), 8.05 (d, J = 9.6 Hz, 1 H).
4f
Yield: 93%; colourless solid; mp 78–80 °C.
MS: m/z = 255 [M+].
IR (KBr): 2930, 1705 cm–1.
Anal. Calcd for C16H17NO2: C, 75.27; H, 6.71; N, 5.49. Found: C,
75.31; H, 6.84; N, 5.41.
1H NMR (400 MHz, CDCl3): d = 1.00 (t, J = 7.2 Hz, 3 H), 1.37 (t,
J = 7.2 Hz, 3 H), 1.45–1.52 (m, 2 H), 1.73–1.81 (m, 2 H), 2.77 (t,
J = 7.6 Hz, 2 H), 4.19 (q, J = 7.2 Hz, 2 H), 6.44 (d, J = 9.2 Hz, 1 H),
6.49 (s, 1 H), 7.13 (d, J = 8.4 Hz, 1 H), 7.43 (d, J = 8.8 Hz, 1 H),
8.11 (d, J = 9.2 Hz, 1 H).
13C NMR (100 MHz, CDCl3): d = 13.9, 15.5, 22.6, 26.5, 30.6, 38.0,
96.6, 109.5, 110.1, 112.9, 114.4, 124.4, 132.2, 141.1, 143.4, 149.7,
162.2.
3f
Yield: 72%; yellow gum.
IR (KBr): 1732, 2212, 3400 cm–1.
1H NMR (400 MHz, CDCl3): d = 0.99 (t, J = 7.2 Hz, 3 H), 1.31 (t,
J = 7.2 Hz, 3 H), 1.52 (m, 2 H), 1.67 (m, 2 H), 2.59 (t, J = 7.2 Hz, 2
H), 3.23 (q, J = 7.2 Hz, 2 H), 4.48 (s, 1 H), 6.41 (d, J = 9.6 Hz, 1 H),
6.78 (d, J = 9.2 Hz, 1 H), 7.14 (d, J = 9.2 Hz, 1 H), 8.04 (d, J = 9.6
Hz, 1 H).
MS: m/z = 269 [M+].
Anal. Calcd for C17H19NO2: C, 75.81; H, 7.11; N, 5.20. Found: C,
75.64; H, 7.20; N, 5.16.
MS: m/z = 269 [M+].
Anal. Calcd for C17H19NO2: C, 75.81; H, 7.11; N, 5.20. Found: C,
75.68; H, 7.03; N, 5.11.
Acknowledgment
We thank CSIR (New Delhi) and DST (New Delhi) for financial
assistance. Two of us (S.P. and S.H.) are grateful to CSIR (New
Delhi) for their Research Fellowships. We also thank the DST (New
Delhi) for providing Bruker NMR spectrometer (400 MHz), Perkin-
Elmer CHN analyser, UV-VIS spectrometer, and Perkin-Elmer FT
IR under FIST programme.
Pyrrolocoumarins 4a–g and Pyrroloquinolines 4h–j; General
Procedure
A mixture of acetylenic amine 3 (1 equiv) and TsOH (1 equiv) was
refluxed in H2O (5 mL) for 3 h or charged in a 20 mL thick-walled
glass sealed tube and irradiated for 20–30 min. After completion of
the reaction as monitored by TLC, the reaction mixture was cooled,
diluted with sat. aq NaHCO3 (50 mL), and extracted with EtOAc
(3 × 25 mL). The combined organic extracts were washed with
brine (50 mL) and dried (Na2SO4). The solvent was distilled off and
the resulting crude product was purified by column chromatography
over silica gel (60–120 mesh) using hexane–EtOAc mixture (80:20)
as eluent to give the products 4a–g or 4h–j.
References
(1) Watanabe, M.; Sato, T.; Inomata, H.; Smith, R. L. Jr.; Arai,
K.; Kruse, A.; Dinjus, E. Chem. Rev. 2004, 104, 5803.
(2) Dallinger, D.; Kappe, C. O. Chem. Rev. 2007, 107, 2563.
Synthesis 2011, No. 9, 1489–1493 © Thieme Stuttgart · New York

PAPER
(3) For an introduction to the use of organometallic catalysts in
Synthesis of Pyrrolocoumarins and Pyrroloquinolones
1493
(14) The Chemistry of Heterocyclic Compounds, Parts 1 and 2;
Jones, R. A., Ed.; Wiley: Toronto, 1990.
aqueous phase catalysis, see: Aqueous-Phase
Organometallic Catalysis, Concepts and Applications;
Cornils, B.; Herrmann, W. A., Eds.; Wiley-VCH:
Weinheim, 1998.
(15) (a) Tietze, L. F.; Nordmann, G. Synlett 2001, 337.
(b) Groenendaal, L.; Meijer, E.-W.; Vekemans, J. A. J. M. In
Electronic Materials: The Oligomer Approach; Müllen, K.;
Wegner, G., Eds.; Wiley-VCH: Weinheim, 1997.
(16) Boyd, D. R.; Sharma, N. D.; Barr, S. A.; Carroll, J. G.;
Mackerracher, D.; Malone, J. F. J. Chem. Soc., Perkin Trans.
1 2000, 3397.
(17) Bar, G.; Parsons, A. F.; Thomas, C. B. Tetrahedron 2001,
57, 4719.
(18) Lee, Y. R.; Kim, B. S.; Kweon, H. I. Tetrahedron 2000, 56,
3867.
(4) (a) Chen, L.; Li, C. J. Org. Lett. 2004, 6, 3151. (b) Li, C. J.
Chem. Rev. 1993, 93, 2023.
(5) Siskin, K.; Katritzky, A. R. Science 1991, 254, 231.
(6) Strauss, C. R.; Trainor, R. W. Aust. J. Chem. 1995, 48, 1665.
(7) (a) Baudel, V.; Cazier, F.; Woisel, P.; Surpateanu, G. Eur.
Polymer. J. 2002, 38, 615. (b) An, J. Y.; Bagnell, L.;
Cablewski, T.; Strauss, C. R.; Trainor, R. W. J. Org. Chem.
1997, 62, 2505.
(8) For recent reviews on the use of microwaves in organic
synthesis, see: (a) Larhed, M.; Moberg, C.; Hallberg, A.
Acc. Chem. Res. 2002, 35, 712. (b) Lew, A.; Krutzik, P. O.;
Hart, M. E.; Chamberlin, A. R. J. Comb. Chem. 2002, 4, 95.
(c) Linström, P.; Tierney, J.; Wathey, B.; Westman, J.
Tetrahedron 2001, 57, 9225. (d) Perreux, L.; Loupy, A.
Tetrahedron 2001, 57, 9199. (e) Deshayes, S.; Liagre, M.;
Loupy, A.; Luche, J.-L.; Petit, A. Tetrahedron 1999, 55,
10851.
(19) Pirrung, M. C.; Blume, F. J. Org. Chem. 1999, 64, 3642.
(20) Dickinson, J. M. Nat. Prod. Rep. 1993, 10, 71.
(21) Hiremath, S. P.; Badiger, G. R.; Jivanagi, A. S.; Purohit, M.
G. Indian J. Chem., Sect. B: Org. Chem. Incl. Med. Chem.
1992, 31, 583.
(22) (a) Majumdar, K. C.; Mondal, S. Tetrahedron Lett. 2008, 49,
2418. (b) Majumdar, K. C.; Chattopadhyay, B.; Samanta, S.
Synthesis 2009, 311. (c) Majumdar, K. C.; Chakravorty, S.;
Shyam, P. K.; Taher, A. Synthesis 2009, 403.
(9) Katritzky, A. R.; Siskin, M. Chem. Rev. 2001, 101, 837.
(10) Polshettiwar, V.; Rajender, S. V. Chem. Soc. Rev. 2008, 37,
1546.
(11) Polshettiwar, V.; Rajender, S. V. Acc. Chem. Res. 2008, 41,
629.
(12) Wei, W. C.; Keh, C. K.; Li, C.-J.; Varma, E. S. Clean
Technol. Environ. Policy 2004, 7, 62.
(d) Majumdar, K. C.; Samanta, S.; Nandi, R. K.;
Chattopadhyay, B. Tetrahedron Lett. 2010, 51, 3807.
(e) Majumdar, K. C.; De, N.; Roy, B. Synthesis 2010, 4207.
(23) (a) Majumdar, K. C.; Taher, A.; Ponra, S. Tetrahedron Lett.
2010, 51, 2297. (b) Majumdar, K. C.; Taher, A.; Ponra, S.
Tetrahedron Lett. 2010, 51, 147. (c) Majumdar, K. C.;
Taher, A.; Ponra, S. Synthesis 2010, 4307.
(13) (a) Jones, R. A.; Bean, G. P. The Chemistry of Pyrroles;
Academic: London, 1977, 1–5. (b) Sundberg, R. J. In
Comprehensive Heterocyclic Chemistry, Vol. 4; Katritzky,
A. R.; Rees, C. W., Eds.; Pergamon: Oxford, 1984, 370–
376. (c) Sundberg, R. J. In Comprehensive Heterocyclic
Chemistry II, Vol. 2; Katritzky, A. R.; Rees, C. W., Eds.;
Pergamon: Oxford, 1996, 119–206. (d) Boger, D. L.;
Boyce, C. W.; Labrili, M. A.; Sehon, C. A.; Jin, Q. J. Am.
Chem. Soc. 1999, 121, 54; and references cited therein.
(24) Koradin, C.; Dohle, W.; Rodriguez, A. L.; Schmidt, B.;
Knochel, P. Tetrahedron 2003, 59, 1571.
(25) Zhang, Y.; Donahue, J. P.; Li, C. J. Org. Lett. 2007, 9, 627.
(26) Trost, B. M.; McClory, A. Angew. Chem. Int. Ed. 2007, 46,
2074.
(27) Knight, D. W.; Sharland, C. M. Synlett 2003, 2258.
Synthesis 2011, No. 9, 1489–1493 © Thieme Stuttgart · New York