Montmorillonite K-10 catalyzed, microwave-assisted cyclization of acetylenic amines: An efficient synthesis of pyrrolocoumarins and pyrroloquinolones

Article abstract of DOI:10.1055/s-0031-1291161

A microwave-assisted montmorillonite K-10 catalyzed synthesis of pyrrolocoumarins and pyrroloquinolones from acetylenic amine is described. The reaction utilizes montmorillonite K-10 as an inexpensive, recyclable, and environmentally benign acid catalyst. Reactions were completed in few minutes under microwave irradiation and provided excellent yields. The efficient and eco-friendly catalyst and the convenience of the product isolation make the process an attractive alternative for the synthesis of these important heterocycles. Georg Thieme Verlag Stuttgart ? New York.

Full text of DOI:10.1055/s-0031-1291161

PAPER
▌2079
paper
Montmorillonite K-10 Catalyzed, Microwave-Assisted Cyclization of
Acetylenic Amines: An Efficient Synthesis of Pyrrolocoumarins and
Pyrroloquinolones
Synthesis of Pyrrolocoumarins and Pyrroloquinolones
K. C. Majumdar,* Sudipta Ponra, Tapas Ghosh
Department of Chemistry, University of Kalyani, Kalyani 741235, W. B., India
Fax +91(33)25828282; E-mail: kcm@klyuniv.ac.in
Received: 22.02.2012; Accepted after revision: 24.04.2012
inexpensive, strong solid acid, active under microwave
conditions, and can be used without any pretreatment.16
The combination of montmorillonite K-10 and micro-
wave irradiation provided a number of successful process-
Abstract: A microwave-assisted montmorillonite K-10 catalyzed
synthesis of pyrrolocoumarins and pyrroloquinolones from acetyle-
nic amine is described. The reaction utilizes montmorillonite K-10
as an inexpensive, recyclable, and environmentally benign acid cat-
alyst. Reactions were completed in few minutes under microwave es.17,18 As a part of our continuing effort towards the
irradiation and provided excellent yields. The efficient and eco-
friendly catalyst and the convenience of the product isolation make
development of new protocols for the expeditious synthe-
sis of biologically relevant heterocyclic compounds,19 we
the process an attractive alternative for the synthesis of these impor-
became interested in developing an efficient protocol for
tant heterocycles.
synthesizing substituted pyrano[3,2-e]indolone and pyr-
Key words: microwave irradiation, montmorillonite K-10, acety-
lenic amine, pyrrolocoumarin, pyrroloquinolone
rolo[3,2-f]quinolone derivatives. Herein, we describe an
environmentally benign one-pot domino approach for the
synthesis of pyrrolocoumarins and pyrroloquinolones
(Scheme 1).
Among the fundamental nitrogen containing heterocy-
cles, the pyrrole ring has attracted considerable attention
and is widely distributed as structural unit in a variety of
naturally and biologically important molecules, for exam-
ple, porphyrins, bile pigment, alkaloids, and co-enzymes.1
In recent years, there has been a continuous quest to syn-
thesize pyrrole and its oligomer with different substituents
efficiently from readily available starting materials,2 due
to their potential application as conducting materials.3
Coumarins and quinolones are subunits of many natural
products possessing broad-spectrum biological activity
and exhibit, for example, antifungal, antibacterial, antivi-
ral, and antimicrobial properties.4–8 In particular, the
pyrrolocoumarin derivatives exhibit photobiological
activity9a and antiproliferative effect,9b DNA binding
properties,10 photophysical properties,11 anti-inflammato-
ry, and antioxidant activities.12 These compounds also act
as ideal dyes for various modern fluorescent imaging
technologies such as FRET.13
R1
R1
N
NHR2
R2
K-10, MW
120 °C
O
X
O
X
4
3
X = O, NMe
R
R
1 = Ph, C6H13, C5H11, Bu, Pr
2 = H, Me, Et
Scheme 1
The starting materials for this study were prepared accord-
ing to our published procedure (Scheme 2).19a We have
conducted an optimization study to explore the influence
of acids, temperature, and solvents on the outcome of the
cycloisomerization of coumarin and quinolone to pyrrolo-
coumarin and pyrroloquinolone both under conventional
heating and microwave irradiation.
Solid acidic materials are safer alternatives to traditional
mineral acids. Hence, the present choice is to use clays,
zeolites, metal oxides, and acidic ion-exchange resins.14
These catalysts are stable, easy to store and handle, and do
not produce hazard wastes. Microwave-assisted organic
synthesis (MAOS)14 is also now widely used. The appli-
cation of heterogeneous catalysts15 under microwave con-
ditions includes an additional advantage; reactions can be
carried out with minimum solvent use or without any sol-
vent. Montmorillonite K-10 with a Hammett acidity con-
stant (H0) of about –8, is a commercially available, stable,
R1
Br
NHR2
NHR2
NHR2
O
X
O
X
O
X
3
1
2
X = O, NMe
R1 = C6H13, C5H11
,
Bu, Pr, Ph
R2 = H, Me, Et
SYNTHESIS 2012, 44, 2079–2083
Advanced online publication: 31.05.2012
Scheme 2
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0031-1291161; Art ID: SS-2012-Z0188-OP
© Georg Thieme Verlag Stuttgart · New York

2080
K. C. Majumdar et al.
PAPER
perature and reaction time. The same reaction was then
carried out at 120 °C for 10, 15, and 20 minutes. Ninety-
six per cent yield was obtained when the reaction was car-
ried out at 120 °C for 15 minutes (entry 9). Longer irradi-
ation did not increase the yield of the final product (entry
10). For comparison, the same reaction was carried out in
a pressure vessel using conventional heating in the ab-
sence of any solvent at different temperatures (entries 12,
13). As expected the formation of the desired product was
very slow under conventional heating. The optimized con-
dition for the reaction has been established to be: MWI
250 W power, montmorillonite K-10 as catalyst, 15 min-
utes at 120 °C (entry 9).
Table 1 Effects of Activation Method, Temperature, and Time on
the K-10 Catalyzed Synthesis
C6H13
C6H13
NMe
NHMe
K-10
Δ
O
O
O
O
3a
4a
Entry
Methoda
Temp (°C) Time (min) Yield (%)b
1
2
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
MW
CH
80
80
5
10
15
5
30
45
62
42
53
68
77
79
96
96
95
70
84
Next, our attention was turned towards studying the scope
of the methodology. Initially a broad variety of substrates
with variation of substituents in the terminal alkyne were
used. It is found from the examples in Table 2 that both
aromatic and aliphatic substituents are equally effective.
The scope of this method was then tested by using various
substituted coumarin- and quinoline-containing acetyle-
nic amines and in both the cases desired pyrrolocoumarins
and pyrroloquinolones were obtained in excellent yields.
Cycloisomerization of acetylenic amines with electron-
donating groups as well as unprotected, unactivated
amines successfully gave pyrrolocoumarins and pyrrolo-
quinolones in excellent yields. The catalyst montmoril-
lonite K-10 is reusable.
3
80
4
100
100
100
100
120
120
120
140
90
5
10
15
20
10
15
20
15
90
90
6
7
8
9
10
11
12
13
The mechanistic rationalization for the formation of prod-
ucts 4 is depicted in Scheme 3. Initially the montmorillon-
ite K-10 may activate the triple bond of 3 to give the
intermediate 5. The resulting electron-deficient triple
bond of 3 undergoes intramolecular nucleophilic attack by
the less basic nitrogen of the amine group, by a 5-endo-dig
cyclization to give the products 4 because 4-exo-dig is
energetically unfavorable.
CH
120
a MW: Microwave irradiation, CH: conventional heating.
b Isolated yield.
Initially, we have used 3a as a model substrate to optimize
the condition for our reaction. The reactions were carried
out in a CEM Discover Benchmate microwave reactor. In
order to optimize the reaction parameters, several test re-
actions were carried out and the effect of temperature,
heating method, and time was studied. The results are pre-
sented in Table 1. No change was detected on TLC when
the cycloisomerization reaction of the alkyne 3a was car-
ried out without any catalyst under microwave for a long
time. In the presence of K-10 as a solid recyclable Lewis
acid catalyst, the reaction proceeds rapidly in a matter of
few minutes when irradiated in a microwave and forms
the expected product 4a. For optimization of the reaction
condition a series of experiments were carried out, in
which temperature, activation method, and the reaction
time were changed sequentially. First the reaction was
carried out at 80 °C for 5 minutes but only 30% yield of
the final product was obtained (Table 1, entry 1). The re-
action time at 80 °C was then gradually increased, and af-
ter 15 minutes the yield of the product was increased to
62% (entries 2, 3). The reaction temperature was then
raised to 100 °C and the reaction was continued for 5, 10,
15, 20 minutes, respectively (entry 4–7). It was found that
the yield of the product increases with the increase of tem-
R1
R1
K-10
NHR2
NHR2
O
X
O
X
3 X = O
X = NMe
5
R1
R2
N
cyclization
5-endo-dig
O
X
4 X = O
X = NMe
Scheme 3 Probable mechanistic pathway of K-10 catalyzed cyclo-
isomerization
According to literature there are many reports on the syn-
thesis of pyrrolocoumarin and pyrroloquinolone deriva-
tives using longer reaction routes or expensive
reagents.12,13,20 We have earlier reported the synthesis of
pyrano[3,2-e]indolone and pyrrolo[3,2-f]quinolone deriv-
atives by Pd(0)-mediated cross coupling followed by
Synthesis 2012, 44, 2079–2083
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis of Pyrrolocoumarins and Pyrroloquinolones
2081
Table 2 Heteroannulated Products 4 Prepared
R1
O
O
R1
K-10
MW
N
X
N
Me
R2
NHR2
4
3
Entry
Heteroannulated precursor
Time (min)
15
Heteroannulated product
Yield (%)
1
X = O, R1 = C6H13, R2 = Me
X = O, R1 = Pr, R2 = Et
X = O, R1 = C5H11, R2 = Et
X = O, R1 = Bu, R2 = Me
X = O, R1 = Bu, R2 = Et
X = O, R1 = Bu, R2 = H
X = O, R1 = Ph, R2 = Me
X = O, R1 = Ph, R2 = Et
X = O, R1 = Ph, R2 = H
X = NMe, R1 = Ph, R2 = H
X = NMe, R1 = Ph, R2 = Me
X = NMe, R1 = Ph, R2 = Et
X = O, R1 = C6H13, R2 = Me
X = O, R1 = Pr, R2 = Et
X = O, R1 = C5H11, R2 = Et
X = O, R1 = Bu, R2 = Me
X = O, R1 = Bu, R2 = Et
X = O, R1 = Bu, R2 = H
X = O, R1 = Ph, R2 = Me
X = O, R1 = Ph, R2 = Et
X = O, R1 = Ph, R2 = H
X = NMe, R1 = Ph, R2 = H
X = NMe, R1 = Ph, R2 = Me
X = NMe, R1 = Ph, R2 = Et
96
95
95
91
93
89
94
95
92
89
87
86
2
20
20
18
20
20
15
20
20
20
20
20
3
4
5
6
7
8
9
10
11
12
Cu(I)-catalyzed, or Au(III)-catalyzed heteroannulations Melting points were determined in open capillaries and are uncor-
rected. IR spectra (cm–1) were recorded using samples as neat liq-
and palladium-catalyzed Heck reaction or IBX-induced
intramolecular oxidative amination of alkene,
iron/palladium-catalyzed intramolecular hydroamination,
uids and solid samples were recorded in KBr disks. 1H NMR (400
MHz) spectra were recorded in CDCl3 (chemical shift in δ) with
TMS as internal standard. HRMS and mass spectra were recorded
on a Qtof Micro instrument. Microwave irradiation was done on a
CEM Discover instrument. Silica gel (60–120, 230–400 mesh,
or Brønsted acid catalyzed cyclization of acetylenic
amines.21 The aforesaid methodologies suffer from disad-
Rankem, India) was used for chromatographic separation. Silica gel
G (Spectrochem, India) was used for TLC.
vantages, such as, the use of expensive metal catalyst,
long reaction time, hazardous organic solvents and re-
agents, hazardous workup, separation problem, poor The starting materials 3 were prepared according to our published
procedure (Scheme 2).19a The analytical and spectral data for 3a,b
are given below.
yield, etc. Herein we have reported a cleaner and environ-
ment-friendly protocol for the synthesis of biologically
important substituted pyranoindoles and pyranoquino-
lones via montmorillonite K-10 mediated cyclization of Yield: 78%; yellow gum.
3a
IR (KBr): 3385, 2930, 1713, 1574 cm–1.
acetylenic amines possessing an electron-donating group
on the nitrogen atom, under microwave avoiding any or-
ganic solvent. To the best of our knowledge the work re-
ported here represent the first example of montmorillonite
K-10 catalyzed preparation of pyrrolocoumarins and pyr-
roloquinolones in excellent yields.
1H NMR (400 MHz, CDCl3): δ = 0.92–0.94 (m, 3 H), 1.36 (t,
J = 3.6 Hz, 4 H), 1.45–1.53 (m, 2 H), 1.64–1.72 (m, 2 H), 2.57 (t,
J = 7.2 Hz, 2 H), 2.94 (s, 3 H), 4.63 (s, 1 H), 6.41 (d, J = 9.6 Hz, 1
H), 6.78 (d, J = 9.2 Hz, 1 H), 7.17 (d, J = 8.8 Hz, 1 H), 8.04 (d,
J = 9.6 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ = 14.0, 19.9, 22.6, 28.7, 28.8, 30.7,
31.3, 73.4, 102.9, 104.3, 112.9, 116.8, 116.9, 119.7, 142.1, 145.9,
147.1, 161.2.
In conclusion, a novel, montmorillonite K-10 catalyzed
synthesis of substituted pyrrolocoumarins and pyrroloqui-
nolones derivatives is described. The notable feature of
the proposed methodology includes rapid reaction, excel-
lent yield of the products from easily available and inex-
pensive starting materials and catalyst. In addition to
efficiency, the solvent-free reaction condition, limited en-
ergy consumption, and its waste-free nature make the pro-
cess very attractive for environmentally benign synthesis
of bioactive heterocycles.
MS: m/z = 283 [M+].
Anal. Calcd for C18H21NO2: C, 76.29; H, 7.47; N, 4.94. Found: C,
76.48; H, 7.42; N, 5.01.
3b
Yield: 73%; yellow gum.
IR (KBr): 3387, 2961, 1726, 1571 cm–1.
1H NMR (400 MHz, CDCl3): δ = 0.92 (t, J = 7.2 Hz, 3 H), 1.27 (t,
J = 7.2 Hz, 3 H), 1.67–1.75 (m, 2 H), 2.80 (t, J = 7.6 Hz, 2 H), 3.19
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2079–2083

2082
K. C. Majumdar et al.
PAPER
(q, J = 7.2 Hz, 2 H), 4.60 (s, 1 H), 6.44 (d, J = 9.6 Hz, 1 H), 6.95 (d,
J = 9.2 Hz, 1 H), 7.30 (d, J = 9.2 Hz, 1 H), 7.71 (d, J = 9.2 Hz, 1 H).
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
m
tgioSrantnugIifoop
r
itmnatr
13C NMR (100 MHz, CDCl3): δ = 13.6, 14.7, 21.8, 22.2, 38.3, 73.5,
102.6, 104.1, 113.2, 116.5, 116.6, 119.6, 142.0, 145.6, 146.1, 161.0.
MS: m/z = 255 [M+].
References
(1) (a) Jones, R. A.; Bean, G. P. The Chemistry of Pyrroles;
Academic: London, 1977, 1. (b) Sundberg, R. J. In
Comprehensive Heterocyclic Chemistry; Vol. 4; Katritzky,
A. R.; Rees, C. W., Eds.; Pergamon: Oxford, 1984, 370.
(c) Sundberg, R. J. In Comprehensive Heterocyclic
Chemistry II; Vol. 2; Katritzky, A. R.; Rees, C. W.; Scriven,
E. F. V., Eds.; Pergamon: Oxford, 1996, 149. (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.
(2) The Chemistry of Heterocyclic Compounds; Vol. 48, Parts 1
and 2; Jones, R. A., Ed.; Wiley: New York, 1990.
(3) (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.
(4) 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.
(5) Bar, G.; Parsons, A. F.; Thomas, C. B. Tetrahedron 2001,
57, 4719.
(6) Lee, Y. R.; Kim, B. S.; Kweon, H. I. Tetrahedron 2000, 56,
3867.
(7) Pirrung, M. C.; Blume, F. J. Org. Chem. 1999, 64, 3642.
(8) Dickinson, J. M. Nat. Prod. Rep. 1993, 10, 71.
(9) (a) Rodighiero, P.; Chilin, A.; Pastorini, G.; Guiotto, A.
J. Heterocycl. Chem. 1987, 24, 1041. (b) Guiotto, A.; Chilin,
A.; Manzini, P.; Dall’Acqua, F.; Bordin, F.; Rodighiero, P.
Farmaco 1995, 50, 479.
Anal. Calcd for C16H17NO2: C, 75.27; H, 6.71; N, 5.49. Found: C,
74.99; H, 6.64; N, 5.56.
Compounds 4; General Procedure
Acetylenic amine 3 (1 mmol) was dissolved in CH2Cl2 (3 mL) in a
round-bottomed flask, followed by addition of K-10 (200 mg). Af-
ter stirring for 5 min, the solvent was evaporated to obtain a dry
mixture, which was transferred into a glass reaction vessel, and ir-
radiated in the microwave reactor (CEM Discover, 120 °C). During
the optimization process, the progress of the reaction was monitored
by TLC (eluent: hexane–EtOAc, 80:20). After completion of the re-
action as monitored by TLC, the reaction mixture was cooled, and
extracted with EtOAc (3 × 25 mL). The product was separated from
the catalyst by filtration and the solvent was distilled off. The result-
ing crude product was purified by column chromatography over sil-
ica gel (60–120 mesh) using hexane–EtOAc (80:20) as eluent to
give 4.
All products showed satisfactory spectral data (1H, 13C NMR, and
MS). Full spectral characterization is given for only the previously
unknown products. Such data for the known compounds synthe-
sized in this study are available from the authors.
4a
Yield: 272 mg (96%); yellow solid; mp 102–104 °C.
IR (KBr): 2932, 1711, 1701, 1592 cm–1.
1H NMR (400 MHz, CDCl3): δ = 0.92 (t, J = 7.2 Hz, 3 H), 1.34–
1.37 (m, 4 H), 1.44–1.48 (m, 2 H), 1.75 (t, J = 7.6 Hz, 2 H), 2.78 (t,
J = 8.0 Hz, 2 H), 3.73 (s, 3 H), 6.43 (d, J = 9.2 Hz, 1 H), 6.48 (s, 1
H), 7.12 (d, J = 8.8 Hz, 1 H), 7.40 (d, J = 9.2 Hz, 1 H), 8.09 (d,
J = 9.2 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ = 14.1, 22.6, 27.0, 28.5, 29.1, 29.8,
31.6, 96.6, 109.5, 109.8, 112.8, 114.3, 124.2, 133.4, 141.1, 144.2,
149.8, 162.1.
(10) Gia, O.; Mobilio, S.; Chilin, A.; Rodghiero, P.; Palumbo, M.
J. Photochem. Photobiol., B. 1988, 2, 435.
(11) Quanten, E.; Adriaens, P.; Schryver, F. C.; Roelandts, R.;
Degreef, H. Photochem. Photobiol. 1986, 43, 485.
(12) Kontogiorgis, C.; Litinas, K. E.; Makri, A.; Nicolaides, D.
N.; Vronteli, A.; Litina, D. J. H.; Pontiki, E.; Siohou, A.
J. Enzyme Inhib. Med. Chem. 2008, 23, 43.
HRMS: m/z [M + Na]+ calcd for C18H21NO2 + Na: 306.1470; found:
(13) Chen, L.; Hu, T.-S.; Yao, z.-J. Eur. J. Org. Chem. 2008,
6175.
306.1468.
4b
(14) (a) Gates, B. C. Catalysis by Solid Acids, In Encyclopedia of
Catalysis; Vol. 2; Horváth, I., Ed.; Wiley: New York, 2003,
104. (b) Molnár, Á. Curr. Org. Chem. 2008, 12, 159.
(c) Polshettiwar, V.; Varma, R. S. Acc. Chem. Res. 2008, 41,
629. (d) Dasgupta, S.; Török, B. Curr. Org. Synth. 2008, 5,
321. (e) Loupy, A. Microwaves in Organic Synthesis;
Wiley-VCH: Weinheim, 2005. (f) Kappe, C. O.; Stadler, A.
Microwaves in Organic and Medicinal Chemistry; Wiley-
VCH: Weinheim, 2005. (g) Kappe, C. O.; Dallinger, D. Nat.
Rev. 2006, 5, 51.
(15) (a) Augustine, R. L. Heterogeneous Catalysis for the
Synthetic Chemists; Marcel Dekker: New York, 1996.
(b) Smith, G. V.; Notheisz, F. Heterogeneous Catalysis in
Organic Chemistry; Academic Press: San Diego, 1999.
(16) (a) Benesi, H. A.; Winquest, B. H. C. Adv. Catal. 1978, 27,
97. (b) Vaccari, A. Appl. Clay Sci. 1999, 14, 161.
(c) Nikalje, M. D.; Phukan, P.; Sudalai, A. Org. Prep.
Proced. Int. 2000, 32, 1. (d) Varma, R. S. Tetrahedron 2002,
58, 1235. (e) Dasgupta, S.; Török, B. Org. Prep. Proced. Int.
2008, 40, 1.
Yield: 242 g (95%); yellow solid; mp 94–96 °C.
IR (KBr): 1712, 1703, 1593 cm–1.
1H NMR (400 MHz, CDCl3): δ = 1.10 (t, J = 7.32 Hz, 3 H), 1.38 (t,
J = 6.84 Hz, 3 H), 1.83 (q, J = 7.32 Hz, 2 H), 2.77 (t, J = 7.8 Hz, 2
H), 4.20 (t, J = 6.88 Hz, 2 H), 6.44 (d, J = 9.16 Hz, 1 H), 6.50 (s, 1
H), 7.13 (d, J = 8.72 Hz, 1 H), 7.43 (d, J = 8.72 Hz, 1 H), 8.11 (d,
J = 9.16 Hz, 1 H).
13C NMR (100 MHz, CDCl3): δ = 14.0, 15.5, 21.9, 28.8, 38.0, 96.8,
109.5, 110.1, 112.9, 114.4, 124.4, 132.2, 141.0, 143.2, 149.8, 162.1.
MS: m/z = 255 [M+].
Anal. Calcd for C16H17NO2: C, 75.27; H, 6.71; N, 5.49. Found: C,
75.11; H, 6.76; N, 5.53.
Acknowledgment
We thank CSIR (New Delhi) and DST (New Delhi) for financial
assistance. Two of us (S.P. and T.G.) are grateful to CSIR (New
Delhi) for their research fellowships. We also thank the DST (New
Delhi) for providing Bruker NMR spectrometer (400 MHz) and
Perkin-Elmer CHN analyzer, UV-VIS spectrometer, and Perkin-
Elmer FT-IR under its FIST program.
(17) (a) Varma, R. S.; Saini, R. K. Synlett 1997, 857. (b) Varma,
R. S.; Kumar, D. Tetrahedron Lett. 1999, 40, 7665.
(c) Vass, A.; Dudas, J.; Varma, R. S. Tetrahedron Lett. 1999,
40, 4951.
Synthesis 2012, 44, 2079–2083
© Georg Thieme Verlag Stuttgart · New York

PAPER
Synthesis of Pyrrolocoumarins and Pyrroloquinolones
2083
(18) (a) Abid, M.; De Paolis, O.; Török, B. Synlett 2008, 410.
(b) Landge, S. M.; Berryman, M.; Török, B. Tetrahedron
Lett. 2008, 49, 4505. (c) Landge, S. M.; Schmidt, A.;
Outerbridge, V.; Török, B. Synlett 2007, 1600.
(d) Suslov, V. V.; Gordeev, E. N.; Traven, V. F. Chem.
Heterocycl. Compd. 2004, 40, 1315. (e) Jiang, W.; Alford,
V. C.; Qiu, Y.; Bhattacharjee, S.; John, T. M.; Haynes-
Johnson, D.; Kraft, P. J.; Lundeen, S. G.; Sui, Z. Bioorg.
Med. Chem. 2004, 12, 1505. (f) Gonzalez, J. C.; Lobo-
Antunes, J.; Perez-Lourido, P.; Santana, L.; Uriarte, E.
Synthesis 2002, 475. (g) Traven, V. F.; Suslov, V. V.;
Gordeev, E. N. ARKIVOC 2000, (iv), 603. (h) Dumas, C.;
Kan-Fan, C.; Carniaux, J-F.; Royer, J.; Husson, H-P.
Tetrahedron Lett. 1999, 40, 8211. (i) Fujihara, M.;
Kawazu, M. Chem. Pharm. Bull. 1972, 20, 88.
(19) (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. Synlett 2010, 735. (d) Majumdar, K. C.;
Ray, K.; Ponra, S. Tetrahedron Lett. 2010, 51, 5437.
(e) Majumdar, K. C.; Taher, A.; Ponra, S. Synthesis 2010,
4043. (f) Majumdar, K. C.; Ponra, S.; Ganai, S. Synlett 2010,
2575. (g) Majumdar, K. C.; Ponra, S.; Ghosh, D.; Taher, A.
Synlett 2011, 104. (h) Majumdar, K. C.; Ponra, S.; Ghosh, D.
Synthesis 2011, 1132. (i) Majumdar, K. C.; Ponra, S.; Nandi,
R. K. Eur. J. Org. Chem. 2011, 6909.
(21) (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, K. P.; Taher, A. Synthesis 2009, 403. (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. (f) Majumdar, K. C.;
Ponra, S.; Hazra, S.; Roy, B. Synthesis 2011, 1489.
(20) (a) Mei, De-S.; Qu, Y.; He, J-X.; Chen, L.; Yao, Z-J.
Tetrahedron 2011, 67, 2251. (b) Mentel, M.; Peters, M.;
Albering, J.; Breinbauer, R. Tetrahedron 2011, 67, 965.
(c) Chen, L.; Xu, M-H. Adv. Synth. Catal. 2009, 351, 2005.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2079–2083