A tandem one-pot aqueous phase synthesis of thiazoles/selenazoles

Article abstract of DOI:10.1016/j.tetlet.2012.04.097

The first ever tandem one-pot synthetic protocol for the synthesis of thiazoles/selenazoles from alkynes via the formation of 2,2-dibromo-1- phenylethanone is reported. The reaction is catalyzed by β-cyclodextrin in aqueous medium and resulted in good yields.

Full text of DOI:10.1016/j.tetlet.2012.04.097

Tetrahedron Letters 53 (2012) 3835–3838
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A tandem one-pot aqueous phase synthesis of thiazoles/selenazoles
⇑
B. Madhav, S. Narayana Murthy, B.S.P. Anil Kumar, K. Ramesh, Y.V.D. Nageswar
Natural Product Chemistry, Indian Institute of Chemical Technology, Uppal Road, Tarnaka, Hyderabad 500 007, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The first ever tandem one-pot synthetic protocol for the synthesis of thiazoles/selenazoles from alkynes
via the formation of 2,2-dibromo-1-phenylethanone is reported. The reaction is catalyzed by b-cyclodex-
trin in aqueous medium and resulted in good yields.
Received 5 March 2012
Revised 19 April 2012
Accepted 21 April 2012
Available online 11 May 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Thiazole
Selenazole
Phenylacetylenes
N-Bromo succinimide
Water
Thiazoles, selenazoles are the principal core structures present
in a variety of natural products and have acquired significance
due to a wide variety of medicinal and biological properties
associated with them (Fig. 1).1 Selenazoles have been extensively
described as synthetic tools,2 as well as for their biological signifi-
cance.3 1,3-Selenazoles act as antibiotic and cancerostatic4 (Fig. 1).
2-Amino-1,3-selenazoles are known for their superoxide anion-
scavenging activity.5
Thiazole skeleton has been associated with numerous biologi-
cally active molecules,6 and its derivatives are useful in the treat-
ment of hypertension,7 schizophrenia,8 inflammation,9 allergies,10
bacterial,11 and HIV12 infections. In particular, aminothiazoles act
as ligands of estrogen receptors13 as well as a novel class of aden-
osine receptor antagonists14 whereas other analogs are used as
fungicides, inhibiting in vivo growth of Xanthomonas and as an
ingredient of herbicides and anthelmintic drugs.15
benign approaches for the synthesis of selenazoles/thiazoles, is
highly desirable.
In continuation of our efforts toward the development of novel
ecofriendly methodologies20 which include the synthesis of het-
erocyclic compounds, we report herein, a mild and efficient one-
pot protocol for the synthesis of substituted thiazole/selenazole
derivatives, for the first time by a three-component reaction,
involving phenyl acetylene, N-bromo succinimide and thiourea/
selenourea in aqueous medium (Schemes 1 and 2). To the best of
our knowledge there are no reports on the synthesis of thiazoles/
selenazoles from phenylacetylenes. Developing new synthetic ap-
proaches in aqueous phase has gained prominence in organic syn-
thesis, as it overcomes the harmful effects of organic solvents and
reduces the environmental pollution. Water is also economically
viable, nontoxic, and the most readily available reaction medium,
making it an environmentally acceptable solvent for the design
and development of green chemistry protocols.
A few reports have appeared on the synthesis of selenazoles and
thiazoles, which include both solid phase synthesis16 and solution
phase17 synthesis. Narender et al., reported the synthesis of
selenazoles/thiazoles by the condensation of phenacylbromides/
tosylates with selenourea/thiourea/thiobenzamide by employing
b-cyclodextrin as catalyst.18 Recently Varma and co-workers
Initialy, phenyl acetylene (1.0 mmol) was reacted with N-bromo
succinimide (1.0 mmol) and thiourea (1.0 mmol) in the presence of
O
synthesized diaryl thiazoles from various
a-tosyloxy ketones in
H2N
water.19 Several protocols are also described using promoters or
catalysts in different organic solvents for the synthesis of thiazoles
and selenazoles. However, development of novel environmentally
O
S
O
S
F3CO
S
N
N
N
Se
HO
N
H
NH2
O
H2N
HO
OH
Riluzole (anticonvulsant)
Sulfathiazole (antibacteria)l
Selenazofurin (antibacterial)
⇑
Corresponding author. Fax: +91 40 27160512.
E-mail address: dryvdnageswar@gmail.com (Y.V.D. Nageswar).
Figure 1. Biologically active thiazoles/selenazoles.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.04.097

3836
B. Madhav et al. / Tetrahedron Letters 53 (2012) 3835–3838
S
b-CD, resulting in lower yields of the desired product. It was ob-
served that the reaction proceeded efficiently when (2.0 equiv) of
N-bromo succinimide was used. In the absence of N-bromo succin-
imide, it was observed that the reaction did not proceed and start-
ing materials were recovered (Table 1).
NH2
S
NBS, β-CD
N
+
NH2-C-NH2
H2O , 70 oC
1
2
3
In the absence of b-CD, the product was obtained in 45% yield.
While evaluating the efficiency of the catalytic system, 10 mol %
of the catalyst loading was observed to be optimal for the reaction
to proceed. Furthermore when the same reaction was carried out
in the presence of NIS the corresponding product was produced
in lower yields.
Scheme 1. Synthesis of selenazoles catalyzed by b-CD in aqueous medium.
Se
NH2
NBS, β-CD
Se
N
+
NH2-C-NH2
H2O, 70 OC
It was observed that the reaction proceeded through the in situ
formation of 2,2-dibromo-1-phenylethanone, from N-bromo suc-
cinimide (2.0 equiv) and phenylacetylene (1.0 equiv) (Scheme 3),
which further reacted with thiourea/selenourea to afford the de-
sired product (Scheme 4).22
The scope of the reaction was expanded by reacting various
substituted selenourea/thiourea derivatives with phenylacetylene
substrates. In these reactions, substituents on the selenourea/thio-
urea did not have significant effect on the product yields. Similar
trends were also observed in the case of phenylacetylenes. Several
examples illustrating this simple and practical methodology are
summarized in Table 2. All the products were characterized by
1H, 13C NMR and mass spectra23 as well as by comparison with
the known compounds.21
Scheme 2. Synthesis of thiazoles catalyzed by b-CD in aqueous medium.
Table 1
Optimization of NBS equiv in the reactiona
Entry
NBS
Time (h)
T (°C)
Yieldb (%)
1
2
3
4
0
1
1.5
2
12
12
10
4
90
90
90
70
—
35
45
72
a
Reaction conditions: phenyl acetylene (1.0 mmol), NBS, thiourea (1.0 mmol),
water (10 mL).
b
Isolated yield.
S
S
O
O
H2N
NH2
NH2
Br
N
NBr
Br
O
2,2-dibromo-1-phenylethanone
Scheme 3. Domino synthesis of thiazole via the in situ formation of 2,2-dibromo-1-phenylethanone.
Br
S
S
O
O
HO
HN
NH2
Br
Br
N
Br
S
- HOBr,
H2N
NH
NH
S
H2N
NH2
Scheme 4. Possible reaction pathway for the synthesis of thiazole compounds.
Table 2
b-CD mediated synthesis of thiazolesa
Entry
1
Phenylacetylene
Thiourea
Product
Yieldb (%)
72
S
S
NH2
H2N
H2N
H2N
H2N
NH2
N
S
S
S
S
NH2
NH2
NH2
NH2
NH2
NH2
2
3
73
70
N
S
F
F
F
F
N
S
N
4
70

B. Madhav et al. / Tetrahedron Letters 53 (2012) 3835–3838
3837
Table 2 (continued)
Entry
Phenylacetylene
Thiourea
Product
Yieldb (%)
S
S
NH
F
H2N
H2N
NH
F
F
5
6
7
62
68
68
N
S
S
S
S
N
H
F
NH
N
F
F
S
N
H
Cl
H2N
H2N
NH
Cl
N
Cl
Cl
S
N
H
MeO
8
64
N
H
N
OMe
S
S
S
S
NH
CN
H2N
H2N
H2N
NH
NH
NH
CN
F
9
60
67
N
S
N
H
F
F
10
N
S
N
H
F
N
11
66
68
F
F
S
S
N
H
F
H2N
NH
F
N
12
F
F
F
F
a
Reaction conditions: phenylacetylene (1.0 mmol), NBS (2.0 mmol), thiourea (1.0 mmol) and catalyst (10 mol %) at 70 °C.
Isolated yields.
b
Table 3
Table 4
Recyclability of b-CD catalyst
b-CD mediated synthesis of Selenazolesa
Entry
1
Phenylacetylene
Selenourea
Product
Yieldb (%)
69
Cycles
Yield (%)
Catalyst recovered (%)
Se
Se
Native
72
69
66
64
90
86
84
82
NH2
1
2
3
H2N
H2N
NH2
N
Se
Se
NH2
The recyclability of the b-CD catalyst was examined by using phenylacetylene
(1.0 mmol), NBS (2.0 mmol) and thiourea (1.0 mmol) in H2O (10 mL), as a model
reaction.
NH2
N
2
68
F
F
Se
Se
Se
Se
N
N
N
N
3
4
H2N
H2N
70
72
N
stantial role of b-CD is illustrated by performing the reaction in
water without using b-CD at 70 °C, where low yields were ob-
served. NMR spectroscopy is one of the most important techniques
used for characterization of inclusion complexes. The formation of
inclusion complex results in the shift changes in the resonances of
the host cyclodextrin as well as the guest protons. These changes
were observed in the 1H NMR spectra (D2O) of b-CD, b-CD/phenyl-
acetylene complex and freeze dried reaction mixture of b-CD/
phenylacetylene complex with the thiourea at 1 h. Inclusion of
the phenyl acetylene into b-CD from the secondary side of cyclo-
dextrin was confirmed by an upfield shift of H3 (0.03 ppm) and
H5 (0.05 ppm) protons of cyclodextrin in the b-CD/phenyl acety-
lene complex as compared to b-CD. This clearly demonstrates that
the phenylacetylene is elegantly set for the addition reaction, in
the hydrophobic microenvironment of b-cyclodextrin cavity. The
catalyst b-CD can be easily recovered and reused.
Se
N
N
Se
N
H2N
H2N
N
5
6
61
64
N
N
Se
Se
N
N
N
NC
NC
a
Reaction conditions: phenylacetylene (1.0 mmol), NBS (2.0 mmol), selenourea
(1.0 mmol) and b-CD (10 mol %) at 70 °C.
b
Isolated yields.
In general, a model reaction was performed by the in situ for-
mation of 1:1 b-CD complex with phenylacetylene (1) in water fol-
lowed by the addition of NBS and thiourea (2) and stirred for 4 h at
70 °C to give the corresponding product (3) (Scheme 1). The sub-
In conclusion, we have developed a convenient one-pot aque-
ous phase synthesis of substituted thiazoles/selenazoles under
mild conditions in encouraging yields. This method has several
advantages over the existing methods such as in situ formation

3838
B. Madhav et al. / Tetrahedron Letters 53 (2012) 3835–3838
19. Dalip, K.; Kumar, N. M.; Patel, G.; Gupta, S.; Varma, R. S. Tetrahedron Lett. 2011,
52, 1983.
of 2,2-dibromo-1-phenylethanone, ability to proceed without
acids/bases or additives, under milder environmental friendly con-
ditions. This protocol will be an interesting addition to the green
chemistry (see Table 4).
20. (a) Murthy, S. N.; Madhav, B.; Kumar, A. V.; Rao, K. R.; Nageswar, Y. V. D. Helv.
Chim. Acta 2009, 92, 2118; (b) Murthy, S. N.; Madhav, B.; Kumar, A. V.; Rao, K.
R.; Nageswar, Y. V. D. Tetrahedron 2009, 65, 5251; (c) Madhav, B.; Murthy, S. N.;
Reddy, V. P.; Rao, K. R.; Nageswar, Y. V. D. Tetrahedron Lett. 2009, 50, 6025; (d)
Murthy, S. N.; Madhav, B.; Reddy, V. P.; Nageswar, Y. V. D. Tetrahedron Lett.
2010, 51, 3649; (e) Shankar, J.; Karnakar, K.; Srinivas, B.; Nageswar, Y. V. D.
Tetrahedron Lett. 2010, 51, 3938; (f) Murthy, S. N.; Madhav, B.; Nageswar, Y. V.
D. Tetrahedron Lett. 2010, 51, 5252; (g) Ramesh, K.; Murthy, S. N.; Nageswar, Y.
V. D. Tetrahedron Lett. 2011, 52, 2362; (h) Ramesh, K.; Murthy, S. N.; Karnakar,
K.; Nageswar, Y. V. D. Tetrahedron Lett. 2011, 52, 3937; (i) Ramesh, K.; Murthy,
S. N.; Karnakar, K.; Nageswar, Y. V. D. Tetrahedron Lett. 2011, 52, 4734; (j) Anil
Kumar, B. S. P.; Madhav, B.; Harsha Vardhan Reddy, K.; Nageswar, Y. V. D.
Tetrahedron Lett. 2011, 52, 2862.
Acknowledgments
We thank the CSIR, New Delhi, India, for fellowships to B.M.,
S.N.M., B.S.P. and the UGC for a fellowship to K.R.
Supplementary data
21. (a) Paul, S.; Gupta, V.; Gupta, R.; Loupy, A. Tetrahedron Lett. 2003, 44, 439; (b)
Valeria, C.; Pierluca, G.; Adriano, S.; Barbara, F. Adv. Synth. Catal. 2005, 347,
1341; (c) Ye, C.; Shreeve, J. M. J. Org. Chem. 2004, 69, 8561.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2012.04.
097.
22. General procedure: b-Cyclodextrin (10 mol %) was dissolved in water (15 ml) at
70 °C and phenylacetylene (1.0 mmol) was added followed by NBS. After
10 min, thiourea (1.0 mmol) was added to this reaction mixture and stirred at
70 °C until the reaction was complete (Table 1). The mixture was extracted
with ethyl acetate, and the extract was filtered. The organic layer was washed
with water and dried over anhydrous Na2SO4, the solvent was removed under
reduced pressure, and the resulting crude product was further purified by
column chromatography using hexane and EtOAc as eluent to give the title
compound. The aqueous layer was cooled to 5 °C to recover b-CD by filtration.
This b-CD was recycled without any loss of activity.
23. Data for representative examples: 4-(p-Tolyl)thiazol-2-amine (Table 2, entry 2):
Solid, mp 135–137 °C 1H NMR (CDCl3/DMSO-d6, 300 MHz): d 7.60–7.58 (m,
2H), 7.14-7.11 (m, 2H), 6.57 (s, 1H), 5.28 (br s, 2H), 2.35 (s, 3H); (CDCl3/DMSO-
d6, 200 MHz): d 131.2, 128.3, 126.6, 126.4, 126.0, 125.2, 123.6, 27.3; ESI-MS (m/
z): 191(M+H)+. Anal. Calcd for (C10H10N2S): C, 63.13; H, 5.30; N, 14.72; S, 16.85.
Found C, 63.01; H, 5.25; N, 14.65; S, 16.78.
References and notes
1. De Souza, M. V. N. J. Sulfur Chem. 2005, 26, 429.
2. (a) Casar, Z.; Majcen-Le Marechal, A.; Lorey, D. New J. Chem. 2003, 27, 1622; (b)
Koketsu, M.; Ishihara, H. Curr. Org. Chem. 2003, 7, 175; (c) Duddeck, H.;
Bradenahl, R.; Stefaniak, L.; Jazwinski, J.; Kamienski, B. Magn. Reson. Chem.
2001, 39, 709; (d) Archer, S.; McGarry, R. J. Heterocycl. Chem. 1982, 19, 1245.
3. Koketsu, M.; Choi, S. Y.; Ishihara, H.; Lim, B. O.; Kim, H.; Kim, S. Y. Chem. Pharm.
Bull. 2002, 50, 1594.
4. (a) Goldstein, B. M.; Kennedy, S. D.; Hennen, W. J. J. Am. Chem. Soc. 1990, 112,
8265. and references cited therein; (b) Shafiee, A.; Khashayarmanesh, Z.;
Kamal, F. J. Sci. Islam. Repub. Iran 1990, 1, 11; (c) Shafiee, A.; Shafaati, A.;
Khamench, B. H. J. Heterocycl. Chem. 1989, 26, 709.
5. Sekhiguchi, A.; Nishina, A.; Kimura, H.; Fukumoto, R.-H.; Koichi, K.; Ishihara, H.;
Koketsu, M. Chem. Pharm. Bull. 2005, 53, 1439.
6. Lewis, J. R. Nat. Prod. Rep. 1999, 16, 389.
7. Patt, W. C.; Hamilton, H. W.; Taylor, M. D.; Ryan, M. J.; Taylor, D. G., Jr.;
Connolly, C. J. C.; Doharty, A. M.; Klutchko, S. R.; Sircar, I.; Steinbaugh, B. A.;
Bately, B. L.; Painchand, C. A.; Rapundalo, S. T.; Michniewicz, B. M.; Olzon, S. C. J.
J. Med. Chem. 1992, 35, 2562.
8. Jaen, J. C.; Wise, L. D.; Caprathe, B. W.; Tecle, H.; Bergmeier, S.; Humblet, C. C.;
Heffner, T. G.; Meltzner, L. T.; Pugsley, T. A. J. Med. Chem. 1990, 33, 311.
9. (a) Haviv, F.; Ratajczyk, J. D.; DeNet, R. W.; Kerdesky, F. A.; Walters, R. L.;
Schmidt, S. P.; Holms, J. H.; Young, P. R.; Carter, G. W. J. Med. Chem. 1988, 31,
1719; (b) Clemence, F.; Marter, O. L.; Delevalle, F.; Benzoni, J.; Jouanen, A.;
Jouquey, S.; Mouren, M.; Deraedt, R. J. Med. Chem. 1988, 31, 1453.
10. Hargrave, K. D.; Hess, F. K.; Oliver, J. T. J. Med. Chem. 1983, 26, 1158.
11. Tsuji, K.; Ishikawa, H. Bioorg. Med. Chem. Lett. 1994, 4, 1601.
12. Bell, F. W.; Cantrell, A. S.; Hoberg, M.; Jaskunas, S. R.; Johansson, N. G.; Jordon,
C. L.; Kinnick, M. D.; Lind, P.; Morin, J. M., Jr.; Noreen, R.; Oberg, B.; Palkowitz, J.
A.; Parrish, C. A.; Pranc, J.; Zhang, H.; Zhou, X.-X. J. Med. Chem. 1995, 38, 4929.
13. Fink, B. A.; Mortensen, D. S.; Stauffer, S. R.; Aron, Z. D.; Katzenellenbogen, J. A.
Chem. Biol. 1999, 6, 205.
14. Van Muijlwijk-Koezen, J. E.; Timmerman, H.; Vollinga, R. C.; Von Drabbe
Kunzel, J. F.; De Groote, S.; Ijzerman, A. P. J. Med. Chem. 2001, 44, 749.
15. Comprehensive Heterocyclic Chemistry I; Metzger, J. V., Ed.; Pergamon Press,
1984; Vol. 6, p 328.
16. Kazzouli, S. E.; Raboin, S. B.; Mouadbib, A.; Guillaumet, G. Tetrahedron Lett.
2002, 43, 3193.
N-(3,4-Difluorophenyl)-4-phenylthiazol-2-amine (Table 2, entry 6): Solid, mp
103–105 °C. 1H NMR (DMSO-d6, 300 MHz): d 9.6 (s, 1H), 8.6 (m, 1H), 7.83 (m,
2H), 7.37 (m, 2H), 6.95–6.89 (m, 3H), 2.56 (s, 1H); 13C NMR (DMSO-d6,
75 MHz): d 162.0, 161.6, 158.8, 146.5, 134.8, 128.6, 123.8, 119.7, 118.9, 112.3,
112.0, 110.3,100.3; ESI-MS (m/z): 289 (M+H)+. Anal. Calcd for (C15H10F2N2S): C,
62.49; H, 3.50; N, 9.72; S, 11.12. Found 62.35; H, 3.43; N, 9.66; S, 11.06.
N-(2-Chloro-6-methoxyphenyl)-4-phenylthiazol-2-amine (Table 2, entry 8): Solid,
mp 130–132 °C. 1H NMR (CDCl3, 300 MHz): d 7.62–7.60 (m, 4H), 7.41–7.32 (m,
6H), 3.66 (s, 3H); 13C NMR (CDCl3, 75 MHz): d: 162.3, 145.3, 133.8, 128.7, 128.2,
127.8, 127.5, 125.6, 120.6, 116.0, 110.2, 101.8, 55.5; ESI-MS (m/z): 312(M+H)+.
Anal. Calcd for (C16H13ClN2OS): C, 60.66; H, 4.14; N, 8.84; S, 10.12. Found C,
60.52; H, 4.09; N, 8.78; S, 10.09.
N-(4-Fluorophenyl)-4-(m-tolyl)thiazol-2-amine (Table 2, entry 10): Solid, mp
105–107 °C. 1H NMR (DMSO-d6, 300 MHz): d 7.59–7.56 (m, 3H), 7.31–7.21 (m,
2H), 7.08–7.07 (m, 1H), 7.01–6.98 (m, 2H), 6.72 (s, 1H), 2.36 (s, 3H), 1.53 (br s,
1H); 13C NMR (DMSO-d6, 75 MHz): d: 160.6, 150.8, 137.9, 136.2, 134.1, 128.5,
126.9, 123.3, 121.0, 116.2, 115.9, 101.0, 21.5; ESI-MS (m/z): 285(M+H)+. Anal.
Calcd for (C16H13FN2S): C, 67.58; H, 4.61; N, 9.85; S, 11.28. Found C, 67.42; H,
4.49; N, 9.74; S, 11.19.
N-(3,4-Difluorophenyl)-4-(4-fluorophenyl)thiazol-2-amine (Table 2, entry 12):
Solid, mp 98–101 °C. 1H NMR (DMSO-d6, 300 MHz): d 9.57 (s, 1H), 8.57–8.51
(m, 1H), 7.68 (s, 1H), 7.60 (m, 1H), 7.52 (m, 1H), 7.36–7.31 (m, 1H), 6.96–6.86
(m, 3H); ESI-MS (m/z): 307(M+H)+. Anal. Calcd for (C15H9F3N2S): C, 58.82; H,
2.96; F, 18.61; N, 9.15; S, 10.47. Found C, 58.71; H, 2.88; N, 9.07; S, 10.38.
N,N-Dimethyl-4-phenyl-1,3-selenazol-2-amine(Table 3, entry 3): Solid, mp
112 °C 1H NMR (CDCl3/DMSO-d6, 300 MHz): d 7.83–7.80 (m, 2H), 7.34–7.19
(m, 4H), 3.16 (s, 6H). 13C NMR (CDCl3/DMSO-d6, 200 MHz): d 128.9, 128.7,
127.3, 127.1, 126.9, 126.8, 105.2, 41.1; ESI-MS (m/z): 253(M+H)+. Anal. Calcd
for (C11H12N2Se): C, 52.60; H, 4.82; N, 11.15. Found C, 52.50; H, 4.77; N, 11.12.
N,N-Dimethyl-4-(naphthalen-2-yl)-1,3-selenazol-2-amine (Table 3, entry 4):
Solid, mp 118–120 °C. 1H NMR (CDCl3/DMSO-d6, 300 MHz): d 7.03–7.00 (m,
2H), 6.72–6.67 (m, 4H), 6.55–6.38 (m, 2H), 2.32 (s, 6H). 13C NMR (CDCl3/DMSO-
d6, 200 MHz): d: 128.4, 127.9, 127.8, 127.3, 126.4, 105.1, 41.0; ESI-MS (m/z):30
(M+H)+. Anal. Calcd for (C15H14N2Se): C, 59.81; H, 4.68; N, 9.30. Found C, 59.70;
H, 4.59; N, 9. 23.
17. (a) Bailey, N.; Dean, A. W.; Judd, D. B.; Middlemiss, D.; Storer, R.; Stephen, P. W.
Bioorg. Med. Chem. Lett. 1996, 6, 1409; (b) Kearney, P. C.; Fernandez, M.;
Flygare, J. A. J. Org. Chem. 1998, 62, 196; (c) Goff, D.; Fernandez, J. Tetrahedron
Lett. 1999, 40, 423.
18. (a) Narender, M.; Somi Reddy, M.; Kumar, V. P.; Reddy, V. P.; Nageswar, Y. V. D.;
Rao, K. R. J. Org. Chem. 2007, 72, 1849; (b) Narender, M.; Somi Reddy, M.;
Sridhar, R.; Nageswar, Y. V. D.; Rao, K. R. Tetrahedron Lett. 2005, 46, 5953.