Multicomponent approach towards the synthesis of substituted pyrroles under supramolecular catalysis using β-cyclodextrin as a catalyst in water under neutral conditions

Article abstract of DOI:10.1002/hlca.200900098

Synthesis of substituted pyrroles in H₂O by using β-cyclodextrin as a supramolecular catalyst is described. This reaction has several advantages over existing methods and provides substituted pyrroles in good-to-excellent yields (79-89%). The s

Full text of DOI:10.1002/hlca.200900098

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Helvetica Chimica Acta – Vol. 92 (2009)
Multicomponent Approach Towards the Synthesis of Substituted Pyrroles
under Supramolecular Catalysis Using b-Cyclodextrin as a Catalyst in Water
Under Neutral Conditions
by Sabbavarapu Narayana Murthy, Bandaru Madhav, Akkiligunta Vijay Kumar, Kakulapati Rama Rao,
and Yadavalli Venkata Durga Nageswar*
Organic Chemistry Division-I, Indian Institute of Chemical Technology, Uppal Road,
Hyderabad 500-607, India (phone: þ 914027160512; e-mail: dryvdnageswar@gmail.com)
Synthesis of substituted pyrroles in H2O by using b-cyclodextrin as a supramolecular catalyst is
described. This reaction has several advantages over existing methods and provides substituted pyrroles
in good-to-excellent yields (79 – 89%). The supramolecular catalysis of the reaction was studied using
1H-NMR spectroscopy. b-Cyclodextrin can be recovered and reused several times without loss of activity.
Introduction. – Multicomponent reactions (MCRs) are excellent strategies, being
employed in the synthesis of many natural products. These MCRs are generally defined
as reactions where more than two starting materials react to form a product,
incorporating more or less all the atoms of the starting materials. Unlike the usual
stepwise formation of individual bonds in a target molecule, in MCRs the formation of
several bonds occurs in one single step. MCRs are convergent, in analogy to the
convergent synthesis and in contrast to a divergent multistep synthesis. These reactions
are classified into various ways based on the number of components involved in the
reaction or their intrinsic variability [1]. The MCRs offer a wide range of advantages
such as a single-step procedure, avoiding complicated purification processes and saving
both solvents and reagents. The MCRs have attracted considerable interest owing to
their exceptional synthetic efficiency. Recently, there has been tremendous develop-
ment in three- and four-component reactions [2].
The pyrrole ring is widely distributed in many natural and biologically important
molecules such as porphyrins, coenzymes, and alkaloids [3]. There has been an
enhanced interest in the synthesis of pyrrole and its oligomers due to their potential
application as conducting materials [4]. Although there are numerous routes reported
for the synthesis of pyrroles [3d][5], most of them involve multistep synthetic
operations, which lower the overall yields and reduce the use of expensive transition
metal catalysts [6] in combination with organic solvents. Recently, a few one-step
procedures [7] have been reported for the synthesis of pyrroles. However, these reports
are far from satisfactory with regard to the reaction parameters such as costly transition
metal catalysts, longer reaction times, and halogenated solvents, which are a threat to
the environment. In view of these shortcomings, there is a need to develop a mild and
ecofriendly synthetic methodology for the synthesis of pyrroles by replacing organic
solvents, most of which are toxic, flammable, or carcinogenic by H2O, and by using a
ꢀ 2009 Verlag Helvetica Chimica Acta AG, Zꢁrich

Helvetica Chimica Acta – Vol. 92 (2009)
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recyclable catalyst as a part of green chemistry approach. H2O is a naturally occurring,
cheap, and non-toxic ecofriendly solvent. In continuation of our efforts towards b-
cyclodextrin-mediated biomimetic approaches [8], we report herein for the first time
the versatile synthesis of substituted pyrroles from readily available building blocks in
aqueous medium under supramolecular catalysis.
Cyclodextrins (CDs) and substituted cyclodextrins are cyclic oligosaccharides
possessing hydrophobic cavities, which bind substrates selectively and catalyze
chemical reactions with high selectivity. They catalyze reactions by supramolecular
catalysis involving reversible formation of hostꢀguest complexes by non-covalent
bonding as seen in enzymes. These attractive features of CDs prompted us to carry out
the synthesis of pyrrole derivatives (Table 1) from phenacyl bromide (¼2-bromo-1-
phenylethanone; 1), pentane-2,4-dione (2), and amines in H2O in presence of b-CD, as
this is the most useful synthetic methodology involving MCRs in H2O medium.
Results and Discussions. – In general, the reactions were carried out by in situ
formation of the b-CD complex of phenacyl bromide (1) in H2O at 508, followed by the
addition of pentane-2,4-dione (2) and amine 3, stirring at 60 – 708, giving the substituted
pyrroles 4 as products in excellent yields (Table 1), as confirmed by 1H- and 13C-NMR,
ESI-MS, and elemental analysis. However, the generality of the three-component
reaction was checked by a model reaction done in PEG-400 under catalyst-free
conditions without using any base at 1208, and the reaction was sluggish. When
0.1 equiv. of bases like Cs2CO3, K2CO3, and Na2CO3 were used, product formation was
observed in low yields.
It was found that aromatic amines with electron-donating groups in p-position gave
excellent yields as shown in Table 1 (Entries 2 and 4), whereas electron-withdrawing
groups in p-position gave relatively lower yields. In case of aliphatic amines, the yields
were lower, as shown in Table 1 (Entries 11 and 12). This may be due to the electronic
factors of substituents.
In this synthesis, the role of b-cyclodextrin appears to be to activate phenacyl
bromide through H-bonding interactions, thereby promoting the reaction with
pentane-2,4-dione and completing the reaction sequence with an amine. In the absence
of b-CD, a reaction did not take place. b-CD can be recovered and reused. A plausible
b-CD/phenacyl bromide complex was suggested by us as shown in Fig. 1, which was
further supported by the isolation and characterization studies on b-CD/phenacyl
1
bromide inclusion complex. H-NMR of the inclusion complex indicated the upfield
shift of HꢀC(3) and HꢀC(5) of b-CD, as shown in Fig. 2 and Table 2. The inclusion
complex was prepared by taking b-cyclodextrin and phenacyl bromide in equal
amounts.
Conclusions. – We have demonstrated for the first time that substituted pyrroles can
be synthesized with readily available starting materials by using b-cyclodextrin as a
catalyst, in H2O as the reaction medium. This novel environmentally friendly
methodology following the green-chemistry approach may find a wide range of
applications.

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Table 1. Synthesis of Substituted Pyrroles by b-Cyclodextrin as a Catalyst
Entry Amine 3
Product 4
Reaction time [h] Yield [%]a)
1
8
86
88
84
89
87
83
85
2
3
4
5
6
7
7.5
8.5
7
8.5
7.5
8

Helvetica Chimica Acta – Vol. 92 (2009)
2121
Table 1 (cont.)
Entry
8
Amine 3
Product 4
Reaction time [h]
Yield [%]a)
85
8
9
NH4OAcb)
7.5
8.5
8.5
80
79
78
10
11
12
8.5
80
a) Yields after isolation after CC. b) 5 equiv. of NH4OAc.
Fig. 1. A plausible b-CDꢀphenacyl bromide complex for the mechanistic path-
way of the reaction in the b-cyclodextrin cavity

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Fig. 2. 1H-NMR Spectra of a) b-CD and b) b-CD/phenacyl bromide inclusion complex
Table 2. 1H-NMR Chemical Shifts of HꢀC(3) and HꢀC(5) in Native b-CD and b-CDꢀPhenacyl
Bromide Inclusion Complexa)
H-atoms
b-CD
b-CDꢀphenacyl bromide complex
Dd
HꢀC(3)
HꢀC(5)
514.14
499.31
509.05
489.78
5.09
9.53
a) Chemical shifts are expressed in Hz.
We thank CSIR, New Delhi, India, for fellowships to S. N. M. and B. M., and UGC for a fellowship to
V. K. A.
Experimental Part
General. All reactions were carried out without any special precautions in an atmosphere of air.
Chemicals were purchased from Fluka and S. D. Fine Chemicals and used for the syntheses. TLC:
precoated silica gel plates (60 F254, 0.2 mm layer; E. Merck). M.p.: Fischer– Johns melting-point
apparatus; uncorrected. 1H-NMR Spectra: Varian 200 or Bruker 300 spectrometers; in CDCl3; d in ppm,
J in Hz. MS: VG Autospec; in m/z.
General Procedure for the Synthesis of Substituted Pyrroles Using b-Cyclodextrin as a Catalyst. b-
Cyclodextrin (1 mmol) was dissolved in H2O (20 ml). To this clear soln., phenacyl bromide (1; 1.0 mmol)
was added and stirred for 10 min, and then pentane-2,4-dione (2; 3.0 mmol) followed by amine 3
(1.0 mmol) were added, after which the mixture was heated at 60 – 708 until completion of the reaction, as
indicated by TLC. The mixture was extracted with AcOEt (3 ꢁ 10 ml). The org. layers were washed with
H2O and sat. brine soln., and dried (Na2SO4). The combined org. layers were evaporated under reduced
pressure, and the resulting crude product was purified by column chromatography (CC) with AcOEt/
hexane 3 :7 as eluent. The aq. layer was cooled to 58 to recover b-CD by filtration.

Helvetica Chimica Acta – Vol. 92 (2009)
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Preparation of the b-CDꢀPhenacyl Bromide Inclusion Complex. b-CD (1 mmol) was dissolved in
H2O (15 ml) by warming at 608 to get clear soln., and then phenacyl bromide (1; 1 mmol) was added, and
the mixture was allowed to stirr for 30 min at 608 and then cooled to r.t. Then it was further cooled over
night in a refrigerator. The obtained solid was filtered, dried, and analyzed by 1H-NMR spectroscopy. The
characterization data for all the isolated compounds are given below.
1-(2-Methyl-1,5-diphenyl-1H-pyrrol-3-yl)ethanone (4a; Table 1, Entry 1). Light yellow oil. IR: 2923,
1
1654, 1501, 1405, 1223. H-NMR (300 MHz): 7.51 – 7.31 (m, 10 H); 6.67 (s, 1 H); 2.41 (s, 3 H); 2.08 (s,
3 H).13C-NMR (75 MHz): 12.5; 31.1; 120.5; 126.2; 126.8; 128.0; 128.2; 129.3; 135.2; 135.9; 138.7; 197.6.
ESI-MS: 298 ([M þ Na]þ). Anal. calc. for C19H17NO (275.34): C 82.88, H 6.22, N 5.09; found: C 82.90, H
6.15, N 5.11.
1-[2-Methyl-1-(4-methylphenyl)-5-phenyl-1H-pyrrol-3-yl]ethanone (4b; Table 1, Entry 2). Light
1
yellow oil. IR: 2922, 1654, 1517, 1407, 1223. H-NMR (300 MHz): 7.29 – 7.24 (m, 5 H); 7.16 (q, J ¼ 9.82,
4 H); 6.53 (s, 1 H); 2.37 (s, 3 H); 2.32 (s, 3 H); 1.97 (s, 3 H). 13C-NMR (300 MHz): 12.7; 21.0; 30.8; 95.9;
120.3; 125.8; 126.5; 128.0; 129.1; 129.6; 135.1; 136.1; 137.7; 196.6. ESI-MS: 290 ([M þ 1]þ). Anal. calc. for
C20H19NO (289.37): C 83.01, H 6.62, N 4.84; found: C 83.07, H 6.65, N 4.87.
1-[1-(4-Fluorophenyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4c; Table 1, Entry 3). Yellow oil.
IR: 2923, 1653, 1508, 1407, 1217, 759. 1H-NMR (300 MHz): 7.38 – 7.34 (m, 5 H); 7.33 (d, J ¼ 4.9, 2 H); 7.30
(d, J ¼ 4.9, 2 H); 6.63 (s, 1 H); 2.38 (s, 3 H); 2.07 (s, 3 H). 13C-NMR (300 MHz): 12.7; 31.0; 116.1; 116.4;
122.5; 126.3; 127.9; 128.0; 128.3; 129.2; 134.7; 135.3; 135.8; 160.4; 197.6. ESI-MS: 316 ([M þ Na]þ). Anal.
calc. for C19H16FNO (293.33): C 77.80, H 5.50, N 4.77; found: C 77.82, H 5.48, N 4.79.
1-[1-(4-Methoxyphenyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4d; Table 1, Entry 4). Oil. IR:
1
2924, 1652, 1511, 1249. H-NMR (300 MHz): 7.37 (d, J ¼ 4.7, 2 H); 7.33 – 7.22 (m, 3 H); 7.23 (d, J ¼ 8.8,
2 H); 6.97 (d, J ¼ 8.8, 2 H); 6.62 (s, 1 H); 3.85 (s, 3 H); 2.38 (s, 3 H); 2.07 (s, 3 H). 13C-NMR (300 MHz):
12.7; 31.0; 55.4; 96.0; 114.3; 120.7; 122.1; 126.0; 127.3; 128.1; 129.2; 131.5; 135.5; 136.0; 159.2; 197.3. EI-
MS: 328 ([M þ Na]þ). Anal. calc. for C20H19NO2 (305.37): C 78.66, H 6.27, N 4.59; found: C 78.65, H 6.32,
N 4.60.
1-[1-(3,4-Dimethoxyphenyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4e; Table 1, Entry 5). Yel-
low oil. IR: 2928, 1658, 1511, 1415, 1225. 1H-NMR (300 MHz): 7.62 – 7.21 (m, 8 H); 6.63 (s, 1 H); 3.78 (s,
6 H); 2.37 (s, 3 H); 2.02 (s, 3 H). 13C-NMR (300 MHz): 12.5; 31.2; 55.8; 101.0; 114.3; 120.7; 126.6; 127.6;
128.5; 129.0; 131.5; 133.8; 135.5; 139.7; 159.1; 197.5. ESI-MS: 336 ([M þ H]þ). Anal. calc. for C21H21NO3
(335.40): C 75.20, H 6.31, N 4.18; found: C 75.24, H 6.33, N 4.17.
1-[1-(4-Chlorophenyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4f; Table 1, Entry 6). Light
1
brown oil. IR: 2924, 1651, 1510, 1401, 1219. H-NMR (300 MHz): 7.36 – 7.32 (m, 5 H); 7.31 (d, J ¼ 4.7,
2 H); 7.32 (d, J ¼ 4.8, 2 H); 6.65 (s, 1 H); 2.36 (s, 3 H); 2.04 (s, 3 H). 13C-NMR (300 MHz): 12.6; 31.2;
116.0; 116.5; 122.1; 126.3; 127.8; 128.3; 128.6; 129.2; 133.7; 135.3; 135.6; 161.4; 197.9. ESI-MS: 311 ([M þ
H]þ). Anal. calc. for C19H16ClNO (309.79): C 73.66, H 5.21, N 4.52; found: C 73.68, H 5.26, N 4.49.
1-[1-(4-Butylphenyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4g; Table 1, Entry 7). Brown oil.
IR: 2926, 1655, 1514, 1246. 1H-NMR (300 MHz): 7.96 (dd, J ¼ 6.7, 2 H); 7.64 – 7.40 (m, 7 H); 6.69 (s, 1 H);
2.48 (s, t, J ¼ 6.0, 5 h); 2.24 (s, 3 H); 1.55 – 1.41 (m, 2 H); 1.33 – 1.23 (m, 2 H); 0.91 (t, J ¼ 7.5, 3 H).
13C-NMR (300 MHz): 12.5; 17.8; 25.4; 30.9; 33.8; 36.2; 108.2; 120.9; 126.8; 127.3; 128.5; 129.1; 132.5;
133.5; 136.6; 198.3. ESI-MS: 332 ([M þ H]þ). Anal. calc. for C23H25NO (331.45): C 83.34, H 7.60, N 4.23;
found: C 83.36, H 7.63, N 4.21.
1-[1-(2,6-Diethylphenyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4h; Table 1, Entry 8). Oil. IR:
2925, 1651, 1520, 1407, 1223. 1H-NMR (300 MHz): 7.35 – 6.87 (m, 8 H); 6.67 (s, 1 H); 2.57 – 2.31 (m, 7 H);
2.13 (s, 3 H); 1.20 – 1.09 (m, 3 H); 0.99 (t, J ¼ 7.5, 3 H).13C-NMR (300 MHz): 12.8; 19.8; 27.6; 31.3; 114.3;
120.7; 123.1; 126.0; 126.6; 127.7; 128.1; 129.2; 131.5; 135.2; 136.2; 158.6; 197.8. ESI-MS: 332 ([M þ H]þ).
Anal. calc. for C23H25NO (331.45): C 83.34, H 7.60, N 4.23; found: C 83.36, H 7.63, N 4.21.
1-(2-Methyl-5-phenyl-1H-pyrrol-3-yl)ethanone (4i; Table 1, Entry 9). Light yellow oil. IR: 3291,
2926, 1655, 1514, 1246. 1H-NMR (300 MHz): 7.50 – 7.26 (m, 5 H); 6.65 (s, 1 H); 5.23 (br. s, 1 H); 2.44 (s,
3 H); 2.05 (s, 3 H). 13C-NMR (300 MHz): 12.2; 31.3; 118.4; 120.5; 127.2; 129.3; 133.2; 135.9; 138.7; 197.8.
ESI-MS: 222 ([M þ Na]þ). Anal. calc. for C13H13NO (199.25): C 78.36, H 6.58, N 7.03; found: C 78.37, H
6.61, N 7.06.

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1-(1-Benzyl-2-methyl-5-phenyl-1H-pyrrol-3-yl)ethanone (4j; Table 1, Entry 10). Yellow oil. IR: 2923,
1654, 1511, 1244. 1H-NMR (300 MHz): 7.43 – 7.12 (m, 10 H); 6.69 (s, 1 H); 5.23 (s, 2 H); 2.38 (s, 3 H); 2.14
(s, 3 H).13C-NMR (300 MHz): 12.9; 31.2; 61.3; 118.2; 126.4; 128.7; 132.4; 136.3; 138.7; 149.3; 198.9. ESI-
MS: 290 ([M þ H]þ). Anal. calc. for C20H19NO (289.37): C 83.01, H 6.62, N 4.84; found: C 83.04, H 6.66,
N 4.80.
1-[1-(3-Methoxybenzyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4k; Table 1, Entry 11). Brown
1
oil. IR: 2923, 1651, 1514, 1241. H-NMR (300 MHz): 7.48 (d, J ¼ 6.8, 2 H); 7.34 – 7.09 (m, 7 H); 6.67 (s,
1 H); 5.21 (s, 2 H); 3.78 (s, 3 H); 2.36 (s, 3 H); 2.12 (s, 3 H). 13C-NMR (300 MHz): 12.7; 31.1; 56.2; 61.1;
118.2; 120.2; 124.6; 126.4; 128.7; 134.3; 136.4; 145.3; 163.5; 198.6. ESI-MS: 320 ([M þ H]þ). Anal. calc.
for C21H21NO2 (319.40): C 78.97, H 6.63, N 4.39; found: C 78.94, H 6.62, N 4.40.
1-[1-(3-Bromobenzyl)-2-methyl-5-phenyl-1H-pyrrol-3-yl]ethanone (4l; Table 1, Entry 12). Yellow
oil. IR: 2922, 1658, 1516, 1238. 1H-NMR (300 MHz): 7.46 – 7.11 (m, 9 H); 6.67 (s, 1 H); 5.24 (s, 2 H); 2.37
(s, 3 H); 2.11 (s, 3 H). 13C-NMR (300 MHz): 12.7; 31.0; 61.2; 118.4; 124.3; 127.4; 128.7; 133.6; 136.3; 138.7;
149.4; 198.7. ESI-MS: 369 ([M þ H]þ). Anal. calc. for C20H18BrNO (368.27): C 65.23, H 4.93, N 3.80;
found: C 65.26, H 4.96, N 3.79.
REFERENCES
´
[1] ꢂMulticomponent Reactionsꢃ, Eds. J. Zhu, H. Bienayme, Wiley-VCH, Weinheim, 2005.
[2] V. Nair, A. U. Vinod, C. Rajesh, J. Org. Chem. 2001, 66, 4427; B. List, C. Castello, Synlett 2001, 1687;
A. M. Shestopalov, Y. M. Emeliyanova, A. A. Shestiopolov, L. A. Rodinovskaya, Z. I. Niazimbe-
tova, D. H. Evans, Org. Lett. 2002, 4, 423; F. Bertozzi, M. Gustafsson, R. Olsson, Org. Lett. 2002, 4,
3147; Y. Yuan, X. Li, K. Ding, Org. Lett. 2002, 4, 3309; J.-F. Cheng, M. Chen, T. Arrhenius, A.
Nadzan, Tetrahedron Lett. 2002, 43, 6293; H. Z. S. Huma, R. Halder, S. S. Kalra, J. Das, J. Iqbal,
Tetrahedron Lett. 2002, 43, 6485; U. Bora, A. Saikia, R. C. Boruah, Org. Lett. 2003, 5, 435; D.
Dallinger, N. Y. Gorobets, C. O. Kappe, Org. Lett. 2003, 5, 1205.
[3] a) R. A. Jones, G. P. Bean, ꢂThe Chemistry of Pyrrolesꢃ, Academic Press, London, 1977, pp. 1 – 5;
b) R. J. Sundberg, in ꢂComprehensive Heterocyclic Chemistryꢃ, Eds. A. R. Katritzky, C. W. Rees,
Pergamon Press, Oxford, 1984, Vol. 4, p. 370; c) R. J. Sundberg, in ꢂComprehensive Heterocyclic
Chemistryꢃ, Eds. A. R. Katritzky, C. W. Rees, Pergamon Press, Oxford, 1996, Vol. 2, p. 149; D. L.
Boger, C. W. Boyce, M. A. Labroli, C. A. Sehon, Q. Jin, J. Am. Chem. Soc. 1999, 121, 54.
[4] L. F. Tietze, G. Nordmann, Synlett 2001, 337; L. Groenendaal, E. W. Meijer, J. A. J. M. Vekemans, in
ꢂElectronic Materials: The Oligomer Approachꢃ, Eds. K. Mꢁllen, G. Wegner, Wiley-VCH,
Weinheim, 1997.
[5] S. Su, J. A. Porco Jr., J. Am. Chem. Soc. 2007, 129, 7744; M. Shindo, Y. Yoshimura, M. Hayashi, H.
Soejima, T. Yoshikawa, K. Matsumoto, K. Shishido, Org. Lett. 2007, 9, 1963; M. Rodrꢄguez Rivero,
S. L. Buchwald, Org. Lett. 2007, 9, 973; D. J. S. Cyr, N. Martin, B. A. Arndtsen, Org. Lett. 2007, 9, 449;
J. T. Binder, S. F. Kirsch, Org. Lett. 2006, 8, 2151; D. J. Gorin, N. R. Davis, F. D. Toste, J. Am. Chem.
Soc. 2005, 127, 11260; A. R. Bharadwaj, K. A. Scheidt, Org. Lett. 2004, 6, 2465; R. Dhawan, B. A.
Arndtsen, J. Am. Chem. Soc. 2004, 126, 468; A. V. Kelꢃin, A. W. Sromek, V. Gevorgyan, J. Am.
Chem. Soc. 2001, 123, 2074.
[6] M. Periasamy, G. Srinivas, P. Bharathi, J. Org. Chem. 1999, 64, 4202.
[7] H. Shiraishi, T. Nishitani, S. Sakaguchi, Y. Ishii, J. Org. Chem. 1998, 63, 6234; H. Shiraishi, T.
Nishitani, T. Nishihara, S. Sakaguchi, Y. Ishii, Tetrahedron 1999, 55, 13957.
[8] K. Surendra, N. Srilakshmi Krishnaveni, M. Arjun Reddy, Y. V. D. Nageswar, K. R. Rao, J. Org.
Chem. 2003, 68, 9119; K. Surendra, N. Srilakshmi Krishnaveni, Y. V. D. Nageswar, K. Rama Rao, J.
Org. Chem. 2003, 68, 4994; K. Surendra, N. Srilakshmi Krishnaveni, M. Arjun Reddy, Y. V. D.
Nageswar, K. Rama Rao, J. Org. Chem. 2003, 68, 2058; R. Sridhar, B. Srivivas, V. Pavan Kumar, V.
Prakash Reddy, A. Vijay Kumar, K. Rama Rao, Adv. Synth. Catal. 2008, 350, 1489.
Received March 18, 2009