Synthesis of novel substituted pyrano annulated flavones

Article abstract of DOI:10.1134/S1070363216050248

A simple and efficient one pot method has been developed for the synthesis of some new functionalized pyrano fused flavone derivatives, alkyl 4,8-dioxo-2-phenyl-4,8-dihydropyrano[2,3-f]chromene-10-carboxylates and dialkyl 4-oxo-2-phenyl-4,8-dihydropyrano[2,3-f]chromene-8,9-dicarboxylates, from 7-hydroxy flavones and 7-hydroxy 8-formyl flavones using dialkylacetalynedicarboxylates in the presence of triphenyl phosphine. The structures of all synthesized compounds were elucidated by FT-IR, ¹H and ¹³C NMR and Mass spectral analysis.

Full text of DOI:10.1134/S1070363216050248

ISSN 1070-3632, Russian Journal of General Chemistry, 2016, Vol. 86, No. 5, pp. 1126–1131. © Pleiades Publishing, Ltd., 2016.
Synthesis of Novel Substituted Pyrano Annulated Flavones1
Y. Jayaprakash Raoa,b, G. Thirupathia, Ch. Prasad Raoa, and Y. Hemasric
a Department of Chemistry, Osmania University, Hyderabad, Telangana, 500007 India
e-mail: yjpr_19@yahoo.com
b Department of Chemistry, Telangana University, Nizamabad, 503322 India
c Department of Chemistry, Nizam College, Osmania University, 500001 India
Received November 8, 2015
Abstract—A simple and efficient one pot method has been developed for the synthesis of some new
functionalized pyrano fused flavone derivatives, alkyl 4,8-dioxo-2-phenyl-4,8-dihydropyrano[2,3-f]chromene-
10-carboxylates and dialkyl 4-oxo-2-phenyl-4,8-dihydropyrano[2,3-f]chromene-8,9-dicarboxylates, from 7-
hydroxy flavones and 7-hydroxy 8-formyl flavones using dialkylacetalynedicarboxylates in the presence of
triphenyl phosphine. The structures of all synthesized compounds were elucidated by FT-IR, 1H and 13C NMR
and Mass spectral analysis.
Keywords: 7-hydroxy flavones, 7-hydroxy 8-formyl flavones, dialkyl acetylenedicarboxylates, Wittig reaction,
pyrano flavones
DOI: 10.1134/S1070363216050248
INTRODUCTION
2H-1-Benzopyrans (2H-chromenes) are important
intermediates in the synthesis of many natural products
and medicinal entities [13–16]. The chromene units
have elicited thereptical interest as structural elements
in drug-like compounds. These biological activities of
flavones and coumarin derivatives initiated our interest
in developing alkyl 4,8-dioxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-10-carboxylate 4a–4f and
dialkyl 4-oxo-2-phenyl-4,8-dihydropyrano[2,3-f]chro-
mene-8,9-dicarboxylate 6a–6f from hydroxy flavones
3a–3c and dialkyl acetylenedicarboxylates with PPh3.
This methodology is the convenient approach to the
synthesis of ester functionalized coumarins from
phenols and vinyl triphenylphosphonium salts.
Flavonoids make a group of polyphenolic com-
pounds abundant in plants and mainly found in dietary
components, including vegetables, fruits, tea, olive oil,
and red wine [1]. Flavonoids are important in many
biological processes that are beneficial for humans due
to interacting with a large number of cellular targets
involved in critical cell signaling processes in the body
[2]. Various derivatives of flavones have been
synthesized and evaluated for different biological
activities such as antioestrogenic, anti-inflammatory,
antimicrobial [3], anti-allergic, antioxidant, cytotoxic,
and antitumor [4]. Thus, flavone moiety can be taken
as a lead compound for the synthesis of semi- and
purely synthetic flavone derivatives with different
functional groups at different positions of its scaffold.
RESULTS AND DISCUSSION
The reaction of substituted resacetophenones 1a–1c
with benzoyl chloride 2 under refluxing in acetone/
K2CO3 medium gave 7-hydroxy flavones 3a–3c [17].
Refluxing of compounds 3a–3c with dialkyl acetylene-
dicarboxylate (DMAD/DEAD) in the presence of
triphenyl phosphine in CH2Cl2 for 18h led to low yield
(18%) of the desired product. Reaction of the same
compounds carried out in the polar solvent, DMF, at
60°C for 12 h gave alkyl 4,8-dioxo-2-phenyl-4,8-
dihydropyrano[2,3-f]chromene-10-carboxylates 4a–4f
in moderate to high yields (68–82%) (Scheme 1). The
characteristic feature of DMAD/DEAD is in situ
Coumarins constitute an important class of benzo-
pyrones exhibiting a broad range of biological
activities like anticoagulants [5], antimicrobial [6],
antibacterial [7], anticancer [8, 9], and anti-HIV [10,
11]. They are also widely used in fragrances, agro-
chemicals, insecticides, and in food and cosmetics.
Coumarins are important dyes in laser technology,
optical brighteners, fluorescent indicators and photo
sensitizers [12].
1 The text was submitted by the authors in English.
1126

SYNTHESIS OF NOVEL SUBSTITUTED PYRANO ANNULATED FLAVONES
1127
Scheme 1. Synthesis of coumarin-flavone esters 4a–4f.
O
Cl
HO
O
HO
OH
O
K2CO3, Acetone, reflux, 8 h
+
Methanol : H2O (1 : 1), reflux, 2 h
R
R
O
R1
O
1a−1c
2
3a−3c
O
O
DMAD/DEAD
PPh3, DMF, 60oC, 12 h
O
O
R
O
4a−4f
1: R = H (a), Me (b), Et (c); 4: R = H, R1 = Me (a), R = H, R1 = Et (b), R = Me, R1 = Me (c), R = Me, R1 = Et (d), R = Et,
R1 = Me (e), R = Et, R1= Et (f).
Scheme 2. Proposed mechanism of the synthesis of 4a–4f.
RO2C
CO2R
PPh3
+
PPh3
CO2R
RO2C
H
O
O
O
O
Ph3P
CO2R
H
O
+
RO2C
R
R
O
−PPh3
R
O
H
O
O
CO2R
O
R
O
O
O
Intramolecular
cyclization
O
O
−OR
R
R
O
O
4a−4f
generation of electrophelic vinyl tryphenyl phospho-
nium cation in the reaction with PPh3. The products are
probably formed by regioselective aromatic elec-
trophilic attack of vinyl tryphenylphosphonium cation
at the 8 position of the aromatic ring rather than the 6
position involving intramolecular lactonization from
diesters (Scheme 2).
The structures of 4a–4f were deduced from spectral
analysis. In FT-IR spectrum of 4a carbonyl bands were
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1128
JAYAPRAKASH RAO et al.
Scheme 3. Synthesis of diesters of pyrano flavones 6a–6f.
CHO
Hexamine/AcOH
90oC, 12 h
HO
O
HO
O
HCl−H2O (1 : 1), 80oC, 1 h
R
R
O
O
3a−3c
5a−5c
R1
O
O
O
O
R1
O
O
DMAD/DEAD
PPh3, DMF, 60oC, 12 h
R
O
6a−6f
3: R = H (a), Me (b), Et (c); 6: R = H, R1 = Me (a), R = H, R1 = Et (b), R = Me, R1 = Me (c), R = Me, R1 = Et (d), R = Et,
R1 = Me (e), R = Et, R1= Et (f).
recorded at 1626 and 1734 cm–1. Its 1H NMR spectrum
exhibited characteristic signals at 3.71 ppm (s, –OCH3)
and 6.58 ppm (s, coumarin –CH). The structure of 4a
was supported by 13C NMR spectrum.
In 13C NMR spectrum of 6a signals of ester carbonyls
carbons were recorded at 168.7 and 164.9 ppm.
In conclusion, we have developed the simple and
efficient one-pot synthesis of alkyl 4,8-dioxo-2-phe-
nyl-4,8-dihydropyrano[2,3-f]chromene-10-carboxylate
4a–4f and dialkyl 4-oxo-2-phenyl-4,8-dihydropyrano-
[2,3-f]chromene-8,9-dicarboxylate 6a–6f, which
cannot be obtained easily or as the sole products by
other methods. 4-Ester and 2,3-diester derivatives may be
useful precursors in preparation of many other heterocycles.
The next step of the present study was preparation
of dialkyl 4-oxo-2-phenyl-4,8-dihydropyrano[2,3-f]chro-
mene-8,9-dicarboxylates 6a–6f from bifunctional 7-
hydroxy-8-formyl flavones [18]. 7-Hydroxy flavones
3a–3c introduced in the Duff reaction [19] with
hexamethylenetetramine (HMT) in AcOH gave 8-formyl-
7-hydroxyflavones 5a–5c, that reacted with dialkyl
acetylenedicarboxylate (DMAD/DEAD) in the presence
of triphenyl phosphine in DMF at 60°C for 12 h to
give 6a–6f in high yields (72–85%) (Scheme 3).
Mechanism of the reaction involved the initial forma-
tion of phosphorane ylide followed by the intra-
molecular Wittig cyclisation [20, 21] (Scheme 4).
EXPERIMENTAL
Melting points were determined in open capillaries.
Purity of all compounds was routinely tested by TLC
on silica gel. IR spectra were recorded in KBr pellets
1
on a Perkin Elmer 2000 spectrophotometer. H NMR
spectra were measured on a Bruker 400 MHz with
TMS as the internal standard. Mass spectra were
measured on a Hewelett Packard mass spectrometer
operating at 70 eV. Elemental analysis was performed
on a Thermo Finnigan Flash EA microanalyzer.
The structures of compounds 6a–6f were elucidated
1
by FT-IR, H and 13C NMR and mass spectra as well
as elemental analyses. FT-IR spectrum of 6a demon-
strated bands of the carbonyl bonds at 1610, 1632, and
1
1710 cm–1. H NMR spectrum of 6a exhibited two
single sharp lines at 3.72 and 3.93 ppm, characteristic
for ester methyl protons, 2H-chromene proton and H4
proton appeared at 5.99 ppm and 8.09 ppm as singlets.
General procedure for the synthesis of alkyl 4,8-
dioxo-2-phenyl-4,8-dihydropyrano[2,3-f]chromene-
10-carboxylate (4a–4f). A mixture of 7-hydroxy
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SYNTHESIS OF NOVEL SUBSTITUTED PYRANO ANNULATED FLAVONES
1129
Scheme 4. Proposed mechanism of the synthesis of 6a–6f.
PPh3
O
RO2C
CO2R
+
RO2C
PPh3
H
O
RO2C
O
O
Ph3P
CO2R
H
O
O
O
+
RO2C
R
R
O
−O=PPh3
Intramolecular
Wittig
R
O
O
O
O
O
R
O
R
O
6a−6f
flavones 3a–3c (1mmol) and PPh3 (1.5 mmol) in DMF
(8 mL) was stirred at room temperature for 10 min.
Dialkylacetalynedicarboxylate (1.5 mmol) in DMF
was added drop wise in 10 min. The brown solution
thus obtained was heated at 60°C for 12 h. Then the
reaction mixture was diluted with ice cold water and
extracted with CH2Cl2. The organic layer was washed
with brine and dried over Na2SO4. The solvent was
evaporated under reduced pressure to give a crude
mass. The product was isolated by silica gel (100–
200 mesh) column chromatography using EtOAc–
Petrolium ether (2 : 8) as an eluent, giving pure solid
products 4a–4f (65–82%).
ESI-MS: m/z 349 [M + H]+. Found, %: C 68.89, H
3.44. C20H12O6. Calculated, %: C 68.97, H 3.47.
Ethyl
4,8-dioxo-2-phenyl-4,8-dihydropyrano-
[2,3-f]chromene-10-carboxylate (4b). Yield 66%, mp
196–198°C. IR spectrum, ν, cm–1: 1620 (C=O), 1732
1
(C=O). H NMR spectrum, δ, ppm: 8.46 d (1H, J =
8.78 Hz), 7.78 m (2H), 7.58 m (3H), 7.44 d (1H, J =
8.78 Hz), 6.77 s (1H), 6.57 s (1H), 4.19 q (2H), 1.11 t
(3H). 13C NMR spectrum, δ, ppm: 176.2, 165.2, 164.7,
158.2, 157.9, 152.2, 143.6, 132.1, 131.6, 130.0, 129.2,
129.2, 127.0, 127.0, 120.0, 116.1, 115.2, 109.6, 106.9,
62.8, 13.7. ESI-MS: m/z 363 [M + H]+. Found, %: C
69.52, H 3.81. C21H14O6. Calculated, %: C 69.61, H
3.89.
Methyl 4,8-dioxo-2-phenyl-4,8-dihydropyrano-
[2,3-f]chromene-10-carboxylate (4a). Yield 69%, mp
209–211°C. IR spectrum, ν, cm–1: 1626 (C=O) and
Methyl 3-methyl-4,8-dioxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-10-carboxylate (4c). Yield
81%, mp 204–206°C. IR spectrum, ν, cm–1: 1618
1
1734 (C=O). H NMR spectrum, δ, ppm: 8.46 d (1H,
1
J = 8.78 Hz), 7.78 m (2H), 7.59 m (3H), 7.44 d (1H,
J = 8.78 Hz), 6.77 s (1H), 6.58 s (1H), 3.71 s (3H). 13C
NMR spectrum, δ, ppm: 176.2, 165.6, 164.7, 158.1,
157.8, 152.2, 143.2, 132.0, 131.5, 130.1, 129.1, 129.1,
127.1, 127.1, 120.5, 116.2, 115.3, 109.6, 106.2, 53.3.
(C=O), 1736 (C=O). H NMR spectrum, δ, ppm: 8.47
d (1H, J = 8.78 Hz), 7.57 m (5H), 7.40 d (1H, J =
8.78 Hz), 6.46 s (1H), 3.28 s (3H), 2.13 s (3H). 13C
NMR spectrum, δ, ppm: 176.9, 165.2, 161.1, 158.3,
157.6, 151.7, 143.5, 132.1, 130.7, 130.4, 129.2, 129.2,
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JAYAPRAKASH RAO et al.
128.6, 128.5, 119.0, 118.9, 115.2, 114.8, 105.4, 52.6,
11.4. ESI-MS: m/z 363 [M + H]+. Found, %: C 69.48,
H 3.83. C21H14O6. Calculated, %: C 69.61, H 3.89.
was washed with brine and dried over Na2SO4. The
solvent was evaporated under reduced pressure to give
a crude mass. The product was isolated by silica gel
(60–120 mesh) column chromatography using a
mixture of EtOAc-petrolium ether (1.5 : 8.5) as an eluent
giving pure solid products 6a–6f (72–88%).
Ethyl 3-methyl-4,8-dioxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-10-carboxylate (4d). Yield
76%, mp 186–188°C. IR spectrum, ν, cm–1: 1622
1
(C=O), 1740 (C=O). H NMR spectrum, δ, ppm: 8.47
Dimethyl
4-oxo-2-phenyl-4,8-dihydropyrano-
d (1H, J = 9.03 Hz), 7.56 m (5H), 7.40 d (1H, J =
9.03 Hz), 6.45 s (1H), 3.71 q (2H), 2.13 s (3H), 0.95 t
(3H). 13C NMR spectrum, δ, ppm: 176.8, 165.0, 161.2,
158.3, 157.6, 151.7, 143.5, 132.0, 130.4, 130.1, 129.1,
129.0, 128.8, 128.7, 119.0, 118.8, 115.4, 114.5, 105.8,
61.8, 13.9, 11.3. ESI-MS: m/z 377 [M + H]+. Found,
%: C 70.12, H 4.21. C22H16O6. Calculated, %: C 70.21,
H 4.29.
[2,3-f]chromene-8,9-dicarboxylate (6a). Yield 73%,
mp 144–146°C. IR spectrum, ν, cm–1: 1610 (C=O),
1632 (C=O), 1710 (C=O). 1H NMR spectrum, δ, ppm:
8.20 d (1H, J = 8.78 Hz), 8.09 s (1H), 7.90 m (2H),
7.55–7.69 m (3H), 7.12 d (1H, J = 8.78 Hz), 6.79 s
(1H), 5.99 s (1H), 3.93 s (3H), 3.72 s (3H). 13C NMR
spectrum, δ, ppm: 177.0, 168.7, 164.9, 163.0, 158.1,
153.4, 132.9, 132.1, 132.0, 131.9, 129.2, 128.5, 128.4,
126.6, 126.2, 120.5, 118.9, 114.8, 108.0, 71.9, 52.9,
52.5. ESI-MS: m/z 393 [M + H]+. Found, %: C 67.28,
H 4.06. C22H16O7. Calculated, %: C 67.35, H 4.11.
Methyl 3-ethyl-4,8-dioxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-10-carboxylate (4e). Yield
78%, mp 162–164°C. IR spectrum, ν, cm–1: 1606
1
(C=O), 1728 (C=O). H NMR spectrum, δ, ppm: 8.46
Diethyl 4-oxo-2-phenyl-4,8-dihydropyrano[2,3-f]-
chromene-8,9-dicarboxylate (6b). Yield 76%, mp
140–142°C. IR spectrum, ν, cm–1: 1601 (C=O), 1630
d (1H, J = 9.03 Hz), 7.56 m (5H), 7.39 d (1H, J =
9.03 Hz), 6.44 s (1H), 3.22 s (3H), 2.54 q (2H), 1.16 t
(3H). 13C NMR spectrum, δ, ppm: 181.1, 166.4, 162.3,
159.6, 158.2, 153.0, 145.2, 133.2, 131.0, 130.6, 129.8,
129.8, 129.0, 128.9, 120.1, 119.3, 115.9, 115.1, 104.8,
52.1, 14.9, 11.1. ESI-MS: m/z 377 [M + H]+. Found,
%: C 70.08, H 4.34. C22H16O6. Calculated, %: C 70.21,
H 4.29.
1
(C=O), 1705 (C=O). H NMR spectrum, δ, ppm: 8.18
d (1H, J = 8.78 Hz), 8.09 s (1H), 7.92 m (2H), 7.48–
7.69 m (3H), 7.02 d (1H, J = 8.78 Hz), 6.83 s (1H),
5.96 s (1H), 4.38 q (2H), 4.15 q (2H), 1.40 t (3H), 1.22
t (3H). 13C NMR spectrum, δ, ppm: 177.0, 168.2,
163.7, 162.9, 158.1, 153.3, 134.6, 132.8, 132.0, 131.9,
129.2, 128.5, 128.4, 126.1, 126.0, 121.2, 118.8, 114.7,
108.3, 72.1, 62.0, 61.5, 14.2, 13.9. ESI-MS: m/z 421
[M + H]+. Found, %: C 68.51, H 4.77. C24H20O7.
Calculated, %: C 68.57, H 4.80.
Ethyl
3-ethyl-4,8-dioxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-10-carboxylate (4f). Yield
71%, mp 153–155°C. IR spectrum, ν, cm–1: 1612
1
(C=O) and 1730 (C=O). H NMR spectrum, δ, ppm:
8.46 d (1H, J = 8.78 Hz), 7.56 m (5H), 7.39 d (1H, J =
8.78 Hz), 6.43 s (1H), 3.62 q (2H), 2.54 q (2H), 1.15 t
(3H), 0.95 t (3H). 13C NMR spectrum, δ, ppm: 179.9,
166.2, 162.0, 159.5, 158.2, 153.2, 145.1, 133.0, 131.1,
130.8, 129.9, 129.9, 129.1, 129.1, 120.6, 119.8, 115.6,
115.8, 104.9, 62.2, 14.8, 14.2, 11.3. ESI-MS: m/z 391
[M + H]+. Found, %: C 70.68, H 4.59. C23H18O6.
Calculated, %: C 70.76, H 4.65.
Dimethyl 3-methyl-4-oxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-8,9-dicarboxylate (6c). Yield
80%, mp 190-192°C. IR spectrum, ν, cm–1: 1596
1
(C=O), 1628 (C=O), 1722 (C=O). H NMR spectrum,
δ, ppm: 8.24 d (1H, J = 8.78 Hz), 7.95 s (1H), 7.57–
7.66 m (5H), 7.11 d (1H, J = 8.53 Hz), 5.97 s (1H),
3.89 s (3H), 3.73 s (3H), 2.16 s (3H). 13C NMR
spectrum, δ, ppm: 177.9, 168.8, 163.9, 160.5, 157.9,
153.6, 133.8, 131.0, 130.3, 130, 129.4, 129.0, 128.5,
126.9, 121.6, 118.0, 117.4, 114.8, 108.2, 72.0, 52.6,
52.1, 12.2. ESI-MS: m/z 407 [M + H]+. Found, %: C
67.89, H 4.34. C23H18O7. Calculated, %: C 67.98, H
4.46.
General procedure for the synthesis of dialkyl 4-
oxo-2-phenyl-4,8-dihydropyrano[2,3-f]chromene-8,9-
dicarboxylate (6a–6f). A solution of 7-hydroxy 8-
formyl flavones 5a–5c (1 mmol) and PPh3 (1.5 mmol)
in DMF (8 mL) was stirred at room temperature for
10 min. A solution of dialkylacetalynedicarboxylate
(1.5 mmol) in DMF was added drop wise in 10min,
and the brown solution was heated at 60°C for 12 h.
Then the reaction mixture was diluted with ice cold
water and extracted with CH2Cl2. The organic layer
Diethyl
3-methyl-4-oxo-2-phenyl-4,8-dihydro-
pyrano[2,3-f]chromene-8,9-dicarboxylate (6d). Yield
75%, mp 138–140°C. IR spectrum, ν, cm–1: 1608 (C=O),
1622 (C=O), 1690 (C=O). 1H NMR spectrum, δ, ppm:
8.20 d (1H, J = 8.78 Hz), 7.92 s (1H), 7.65 m (2H),
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 86 No. 5 2016

SYNTHESIS OF NOVEL SUBSTITUTED PYRANO ANNULATED FLAVONES
7.55 m (3H), 7.08 d (1H, J = 8.78 Hz), 5.92 s (1H),
REFERENCES
1131
4.34 q (2H), 4.15 q (2H), 2.15 s (3H), 1.33 t (3H), 1.21
t (3H). 13C NMR spectrum, δ, ppm: 177.7, 168.4,
163.8, 160.4, 157.9, 153.3, 133.0, 131.2, 130.3, 129.9,
129.2, 128.9, 128.6, 126.5, 120.7, 117.9, 117.5, 114.5,
108.7, 72.1, 61.9, 61.4, 14.2, 13.9, 11.6. ESI-MS: m/z
435 [M + H]+. Found, %: C 69.06, H 5.08. C25H22O7.
Calculated, %: C 69.12, H 5.10.
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1
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Diethyl 3-ethyl-4-oxo-2-phenyl-4,8-dihydropyrano-
[2,3-f]chromene-8,9-dicarboxylate (6f). Yield 72%,
mp 150–153°C. IR spectrum, ν, cm–1: 1603 (C=O),
1630 (C=O), 1708 (C=O). 1H NMR spectrum, δ, ppm:
8.20 d (1H, J = 8.78 Hz), 7.89 s (1H), 7.55–7.64 m
(5H), 7.08 d (1H, J = 8.78 Hz), 5.92 s (1H), 4.32 q
(2H), 4.15 q (2H),2.56 q (2H), 1.34 t (3H), 1.21 t (3H),
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13
1.17 t (3H). C NMR spectrum, δ, ppm: 179.5, 168.4,
14. Ashwood, V.A., Buckingham, R.E., Cassidy, F., Evans, J.M.,
Faruk, E.A., Hamilton, T.C., and Nash, D.J., J. Med.
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163.8, 160.1, 157.8, 153.8, 133.0, 131.4, 130.1, 129.6,
128.6, 128.5, 126.1, 120.8, 117.9, 116.6, 114.5, 107.9,
72.3, 62.3, 61.9, 14.8, 14.6 , 14.3, 11.4. ESI-MS: m/z
449 [M + H]+. Found, %: C 69.56, H, 5.32. C26H24O7.
Calculated, %: C 69.63, H 5.39.
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ACKNOWLEDGMENTS
17. Reddy, B.P. and Krupadanam, G.L.D., J. Heterocycl.
Chem., 1996, vol. 33, p. 1561.
The authors are grateful to the Head, Department of
Chemistry, Osmania University, Hyderabad for
providing Laboratory facilities and the Director,
CFRD, Osmania University, Hyderabad for providing
spectral analysis. One of the authors, G. Thirupathi, is
grateful to CSIR, New Delhi, India, for financial
support.
18. Anil Kumar, S., Krupadanam, G.L.D., and Sriman-
narayana, G., Synth. Commun., 2007, vol. 37, p. 795.
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20. Becker, K.B., Tetrahedron, 1980, vol. 36, p. 1717.
21. Himeda, Y., Hatanaka, M., and Ueda, I., J. Chem. Soc.,
Chem. Commun., 1995, p. 449.
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