Synthesis of 4-hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno(3,2-h) chromen-2-ones using cerium(III) chloride

Article abstract of DOI:10.1080/00397910802028481

A new series of 4-hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno(3, 2-h)chromen-2-ones were synthesized in good yields by reacting 4-hydroxy-7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones with various α,β-unsaturated ketones in presence of cerium(III) chloride in acetonitrile. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910802028481

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Synthesis of 4-Hydroxy-3-
(1-aryl-2-acetylethyl)-7,8-
diaryl-2H-thieno(3,2-
h)chromen-2-ones Using
Cerium(III) Chloride
P. Naveen Kumar a , M. Mohan Babu a , M.
Amaravathi a & G. V. P. Chandramouli a
a Department of Chemistry, National Institute of
Technology, Warangal, India
Published online: 14 Oct 2008.
To cite this article: P. Naveen Kumar , M. Mohan Babu , M. Amaravathi & G. V. P.
Chandramouli (2008): Synthesis of 4-Hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-
thieno(3,2-h)chromen-2-ones Using Cerium(III) Chloride, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry,
38:21, 3592-3600
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Synthetic Communications1, 38: 3592–3600, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802028481
Synthesis of 4-Hydroxy-3-(1-aryl-2-acetylethyl)-
7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones
Using Cerium(III) Chloride
P. Naveen Kumar, M. Mohan Babu, M. Amaravathi, and
G. V. P. Chandramouli
Department of Chemistry, National Institute of Technology, Warangal, India
Abstract: A new series of 4-hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-
2H-thieno(3,2-h)chromen-2-ones were synthesized in good yields by reacting
4-hydroxy-7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones with various a,b-unsatu-
rated ketones in presence of cerium(III) chloride in acetonitrile.
Keywords: Cerium(III) chloride hepta hydrate; electron-deficient olefins;
4-hydroxy-7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones; Michael condensation
INTRODUCTION
A large number of natural products contain the coumarin hetero-
cyclic nucleus, which has in biological and medicinal applications.[1,2]
Coumarin and its derivatives are widely used as additives in food, per-
fumes, cosmetics, pharmaceuticals, and agrochemicals.[3,4] Particularly,
3-substituted-4-hydroxy coumarin and its derivatives have powerful anti-
coagulant activity in vivo in many of animal species. The structure–
activity relationship of these compounds in rabbits has been thoroughly
studied.[5] 3-(2-Acetyl phenyl ethyl)-4-hydroxy coumarin (warfarin) is the
best-known oral anticoagulant rodenticide.[6] Warfarins were obtained in
very low yield by Michael condensation of 4-hydroxy coumarins with
benzal acetones (arylideneacetones) in the presence of piperidine.[7] So,
the preparation of new warfarin analogs are established using cerium(III)
Received November 2, 2007.
Address correspondence to G. V. P. Chandramouli, Department of Chemistry,
National Institute of Technology, Warangal, AP 506 004, India. E-mail:
gvpc_2007@yahoo.co.in; mohanorg@yahoo.com
3592

4-Hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno (3,2-h)chromen-2-ones 3593
chloride as catalyst in acetonitrile. This article the synthesis of 4-hydroxy-
3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones (3a–u)
by condensation of 4-hydroxy-7,8-diaryl-2H-thieno(3,2-h)chromen-2-
ones (1a–g) with various a, b-unsaturated ketones in the presence of cer-
ium(III) chloride in acetonitrile. We have observed that when cerium(III)
chloride heptahydrate was used instead of piperidine in Michael con-
densation, warfarin derivatives (3a–u) were obtained in good yields and
reaction times and temperature were also reduced. Cerium(III) chloride
heptahydrate had been used in organic synthesis as a substitute for a
Lewis acid in preparation of heterocyclic compounds and other conden-
sation products.[8] It is an important catalyst for condensation reactions
and it is also a commercially available lanthanide reagent, nontoxic, and
easy to handle without further purification.[9]
RESULTS AND DISCUSSION
In continuation of our work on the synthesis of novel fused heterocyclic
systems,[10] this article reveals an efficient synthesis of 4-hydroxy-3-
(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones (3a–u)
using CeCl3ꢀ7H20 as catalyst. Various a,b-unsaturated ketones (2a–c)
were condensed with 4-hydroxy-7,8-diaryl-2H-thieno(3,2-h)chromen-2-
ones (1a–g) at room temperature in acetonitrile in the presence of 10%
mol CeCl3ꢀ7H2O to give 4-hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-
2H-thieno(3,2-h)chromen-2-ones (3a–u) with more than 70% yields
(Scheme 1), (Table 1). These warfarins (3a–u) were also prepared by con-
densation of 4-hydroxy-7,8-diaryl-2H-thieno(3,2-h)chromen-2-ones (1a–
g) with electron-deficient arylidene acetones (2a–c) in piperidene and
dioxane. The yields were obtained in less than 50%, and 8–9 h is required
to complete the reaction at 70–80ꢁC. Cerium(III) chloride heptahydrate
with acetonitrile is highly suitable for the Michael condensation at room
temperature. It is interesting to note that by employing cerium(III) chlor-
ide heptahydrate for this reaction in acetonitrile medium, the reaction
time also decreased to 2–3 from 8–9 h.
All the compounds (3a–u) are confirmed by IR, 1H NMR, and
mass spectra. In IR, the absorption bands are observed around
3400–3500 cmꢂ1, due to enolic form of the coumarin. The lactone
Scheme 1. Synthesis of 4-hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno.

3594
P. N. Kumar et al.
Table 1. Synthesis of substituted 4-hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-
2H-thieno
Time Temp
(h) (ꢁC) Yield
Compound
R
R1
R2
3a
3b
3c
3d
3e
3f
3g
3h
3i
ꢂC6H5
ꢂC6H5ꢂ
ꢂC6H5
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
25–30 78
25–30 73
25–30 71
25–30 74
25–30 73
25–30 71
25–30 75
25–30 74
25–30 70
25–30 70
25–30 72
25–30 72
25–30 73
25–30 78
25–30 77
25–30 71
25–30 70
25–30 76
25–30 77
25–30 70
25–30 73
ꢂC6H3Cl2-(3,4) ꢂC6H5ꢂ
ꢂC6H5OH-(p) ꢂC6H5
ꢂC6H5
ꢂC6H5
ꢂC6H5
ꢂC6H5
ꢂC6H5
ꢂC6H5
ꢂC6H4Br(p)
ꢂC6H3Cl2-(3,4) ꢂC6H4Br(p)
ꢂC6H5OH-(p) ꢂC6H4Br(p)
ꢂC6H5
ꢂC6H4CH3-(p) ꢂC6H5
ꢂC6H3Cl2-(3,4) ꢂC6H4CH3-(p) ꢂC6H5
ꢂC6H5OH-(p) ꢂC6H4CH3-(p) ꢂC6H5
3j
3k
3l
ꢂC6H5
ꢂC6H3Cl2(3,4) ꢂC6H5
ꢂC6H3Cl2-(3,4) ꢂC6H3Cl2(3,4) ꢂC6H5
ꢂC6H5OH-(p) ꢂC6H3Cl2(3,4) ꢂC6H5
3m
3n
3o
3p
3q
3r
3s
3t
ꢂC6H5
ꢂC6H3Cl2(3,4) ꢂC6H4Br-(p)
ꢂC6H3Cl2-(3,4) ꢂC6H3Cl2(3,4) ꢂC6H4Br-(p)
ꢂC6H5OH-(p) ꢂC6H3Cl2(3,4) ꢂC6H4Br-(p)
ꢂC6H5
ꢂC6H3Cl2(3,4) ꢂC6H4CH3-(p)
ꢂC6H3Cl2-(3,4) ꢂC6H3Cl2(3,4) ꢂC6H4CH3-(p)
ꢂC6H5OH-(p) ꢂC6H3Cl2(3,4) ꢂC6H4CH3-(p)
ꢂC6H5
ꢂC6H5
ꢂC6H3F2(3,4)
ꢂC6H3F2(3,4)
ꢂC6H3F2(3,4)
ꢂC6H3Cl2-(3,4) ꢂC6H5
ꢂC6H5OH-(p) ꢂC6H5
3u
coumarin carbonyl stretching is observed around 1715–1720 cmꢂ1 and the
ketone carbonyl of the side chain is present at 1680 cmꢂ1. The IR band at
1
750 cmꢂ1 is due to the vibration of thiophene ring. The H NMR spectra
contains the characteristic signals both for the starting materials and
for the final products. The starting substituted 4-hydroxycoumarins
(1a–g) exhibit a signal around 6.2–6.4 d for the 3-H of the 4-hydroxy cou-
marin. This signal disappears in the final products (3a–u), indicating
the formation of the Michael product. The 1H NMR spectra also contain
the peaks for CH3-CO around 2.3 to 2.5 d and -CH2- at a little higher
field around 3 d. All the mass spectra indicate a fragmentation pattern
uniformly losing CH3CO of the side chain followed by the loss of the
lactone carbonyl.
EXPERIMENTAL
All materials were obtained from commercial suppliers and used without
further purification. Standard distilled water was used throughout the

4-Hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno (3,2-h)chromen-2-ones 3595
study. All reactions were carried out in air. NMR spectra were recorded
at 293 K on a 400-MHz instrument using CDCl3 and d6-DMSO as
solvent and TMS as internal reference. IR spectra were recorded as
KBr pellets using a Shimadzu 8010 FTIR spectrophotometer. Mass
spectra were recorded on a VG Micromass 7070H (F or CI) auto spec-
trometer. Products were purified by column chromatography (Aldrich)
using 100- to 200-mesh silica gel.
Synthesis of 4-Hydroxy-3-(1-aryl-2-acetyl-ethyl)-7,8-diaryl-2H-
thieno(3,2-h)chromen-2-ones
4-Hydroxy-7,8-diphenyl-2h-thieno(3,2-h)chromen-2-ones (3 mmol) were
added to a solution of benzylidene (3 mmol) and CeCl3ꢀ7H2O (10%) in
acetonitrile (6 ml) and stirred at 25–30ꢁC for 2–3 h. The reaction had been
monitored by thin-layer chromatography (TLC) (hexane–ethyl acetate 5: 1).
After completion of the reaction, demineralized (DM) water (6ml) was
added to the reaction mixture at 25–30ꢁC and extracted with ethyl acetate
(2 ꢃ 20ml). The combined organic layers were washed with 10% (w=v)
brine solution and then dried over Na2SO4, concentrated in vacuum, and
purified by column chromatography on silica gel to afford the pure product.
Data
4-Hydroxy-3-(1-phenyl-2-acetyl-ethyl)-7,8-diphenyl-2H-thieno(3,2-h)
chromen-2-one (3a)
1
IR (KBr, cmꢂ1): 1715, 1682, 756. H NMR (300 MHz, CDCl3): d: 2.38
(3H, s, CH3), 2.98–3.10 (2H, m, -CH2), 4.40–4.52 (1H, dd, -CH),
7.10–8.39 (17H, m, Ar-H); MS: m=z 516 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetyl-ethyl)-7,8-diphenyl-2H-
thieno(3,2-h)chromen-2-one (3b)
1
IR (KBr, cmꢂ1): 750, 1680, 1720 and 3446 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.41 (3H, s, CH3), 2.98–3.18 (2H, m, -CH2), 4.38–4.46
(1H, dd, -CH), 6.96–8.16 (15H, m, Ar-H); MS: m=z 584 (Mþ).
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7,8-diphenyl-2H-
thieno(3,2-h)chromen-2-one (3c)
1
IR (KBr, cmꢂ1): 750, 1680, 1720 and 3500 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 3.00–3.28 (2H, m, -CH2), 4.32–4.39

3596
P. N. Kumar et al.
(1H, dd, -CH), 7.06–8.28 (16H, m, Ar-H), 10.36 (1H, s, -OH), 11.86
(1H, s, -OH); MS: m=z 532 (Mþ).
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-(p-bromophenyl)-8-
phenyl-2H-thieno(3,2-h)chromen-2-one (3d)
1
IR (KBr, cmꢂ1): 755, 1680, 1715 and 3460 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.36 (3H, s, CH3), 3.04–3.26 (2H, m, -CH2), 4.35–4.43 (1H,
dd, -CH), 7.02–8.30 (16H, m, Ar-H); MS: m=z 595 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetyl-ethyl)-7-p-bromophenyl-8-
phenyl-2H-thieno(3,2-h)chromen-2-one (3e)
1
IR (KBr, cmꢂ1): 752, 1680, 1715 and 3440 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 2.80–3.15 (2H, m, -CH2), 4.40–4.48 (1H,
dd, -CH), 7.03–8.315 (14H, m, Ar-H); MS: m=z 663 (Mþ).
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-p-bromophenyl-8-
phenyl-2H-thieno(3,2-h)chromen-2-one (3f)
1
IR (KBr, cmꢂ1): 755, 1686, 1710 and 3480 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.42 (3H, s, CH3), 3.00–3.28 (2H, m, -CH2), 4.43–4.52(1H,
dd, -CH), 7.04–8.28 (15H, m, Ar-H); MS: m=z 611 (Mþ).
4-Hydroxy-3-(1-phenyl-2-acetyl-ethyl)-7-p-tolyl-8-phenyl-2H-thieno(3,2-
h)chromen-2-one (3 g)
1
IR (KBr, cmꢂ1): 756, 1682, 1720 and 3455 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 1.88 (3H, s, CH3), 2.33 (3H, s, CH3), 2.86–3.20 (2H, m,
-CH2), 4.36–4.45 (1H, dd, -CH), 7.00–8.34 (16H, m, Ar-H); MS: m=z
530 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetyl-ethyl)-7-p-tolyl-8-phenyl-
2H-thieno(3,2-h)chromen-2-one (3 h)
1
IR (KBr, cmꢂ1): 755, 1680, 1710 and 3440 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 1.72 (3H, s, CH3), 2.40 (3H, s, CH3), 2.98–3.10 (2H, m,
-CH2), 4.42–4.53 (1H, dd, -CH), 7.10–8.25 (14H, m, Ar-H); MS: m=z
598 (Mþ).

4-Hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno (3,2-h)chromen-2-ones 3597
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-p-tolyl-8-phenyl-2H-
thieno(3,2-h)chromen-2-one (3i)
1
IR (KBr, cmꢂ1): 755, 1680, 1715 and 3452 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 1.82 (3H, s, CH3), 2.36 (3H, s, CH3), 3.04–3.22 (2H, m,
-CH2), 4.45– 4.58 (1H, dd, -CH), 7.05–8.28 (15H, m, Ar-H); MS: m=z
546 (Mþ).
4-Hydroxy-3-(1-phenyl-2-acetyl-ethyl)-7-o,p-dichlorophenyl-8-phenyl-
2H-thieno(3,2-h)chromen-2-one (3j)
1
IR (KBr, cmꢂ1): 750, 1680, 1710 and 3486 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.35 (3H, s, CH3), 2.98–3.12 (2H, m, -CH2), 4.35–4.43 (1H,
dd, -CH), 7.15–8.35 (15H, m, Ar-H); MS: m=z 584 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetyl-ethyl)-7-o,
p-dichlorophenyl-8-phenyl-2H-thieno(3,2-h)chromen-2-one (3k)
1
IR (KBr, cmꢂ1): 750, 1680, 1710 and 3450 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 3.06–3.36 (2H, m, -CH2), 4.38–4.46 (1H,
dd, -CH), 7.16–8.25 (13H, m, Ar-H); MS: m=z 652 (Mþ).
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-o,
p-dichlorophenyl-8-phenyl-2H-thieno(3,2-h)chromen-2-one (3l)
1
IR (KBr, cmꢂ1): 750, 1680, 1715 and 3460 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 3.02–3.32 (2H, m, -CH2), 4.43–4.50 (1H,
dd, -CH), 6.96–8.15 (12H, m, Ar-H); MS: m=z 600 (Mþ).
4-Hydroxy-3-(1-phenyl-2-acetyl-ethyl)-7-o,p-dichlorophenyl-8-phenyl-
2H-thieno(3,2-h)chromen-2-one (3m)
1
IR (KBr, cmꢂ1): 756, 1680, 1710 and 3480 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.42 (3H, s, CH3), 3.00–3.38 (2H, m, -CH2), 4.40–4.53 (1H,
dd, -CH), 6.98–8.20 (14H, m, Ar-H); MS: m=z 663 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetylethyl)-7-o,
p-dichlorophenyl-8-p-bomophenyl-2H-thieno(3,2-h)chromen-2-one (3n)
1
IR (KBr, cmꢂ1): 755, 1675, 1710 and 3450 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 2.98–3.32 (2H, m, -CH2), 4.36–4.48 (1H,
dd, -CH), 7.02–8.22 (12H, m, Ar-H); MS: m=z 731 (Mþ).

3598
P. N. Kumar et al.
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-o,p-dichlorophenyl-
8-p-bomophenyl-2H-thieno(3,2-h)chromen-2-one (3o)
1
IR (KBr, cmꢂ1): 750, 1680, 1710 and 3468 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.38 (3H, s, CH3), 3.07–3.40 (2H, m, -CH2), 4.40–4.52 (1H,
dd, -CH), 7.12–8.26 (12H, m, Ar-H); MS: m=z 679 (Mþ).
4-Hydroxy-3-(1-phenyl-2-acetyl-ethyl)-7-o,p-dichlorophenyl-8-tosyl-2H-
thieno(3,2-h)chromen-2-one (3p)
1
IR (KBr, cmꢂ1): 750, 1675, 1720 and 3450 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 2.97–3.34 (2H, m, -CH2), 4.30– 4.40 (1H,
dd, -CH), 7.05–8.26 (14H, m, Ar-H); MS: m=z 598 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetyl-ethyl)-7-o,p-dichlorophe-
nyl-8-tosyl-2H-thieno(3,2-h)chromen-2-one (3q)
1
IR (KBr, cmꢂ1): 755, 1680, 1710 and 3436 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.36 (3H, s, CH3), 3.04–3.38 (2H, m, -CH2), 4.35–4.44 (1H,
dd, -CH), 7.12–8.18 (12H, m, Ar-H); MS m=z 666 (Mþ).
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-o,p-dichlorophenyl-
8-tosyl-2H-thieno(3,2-h)chromen-2-one (3r)
1
IR (KBr, cmꢂ1): 755, 1680, 1710 and 3490 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.42 (3H, s, CH3), 3.02–3.38 (2H, m, -CH2), 4.41–4.50 (1H,
dd, -CH), 7.08–8.15 (13H, m, Ar-H); MS: m=z 614 (Mþ).
4-Hydroxy-3-(1-phenyl -2-acetyl-ethyl)-7-phenyl-8-o,p-difluorophenyl-
2H-thieno(3,2-h)chromen-2-one (3s)
1
IR (KBr, cmꢂ1): 750, 1676, 1715 and 3450 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 3.00–3.39 (2H, m, -CH2), 4.38–4.49 (1H,
dd, -CH), 7.04–8.23 (13H, m, Ar-H); MS: m=z 552 (Mþ).
4-Hydroxy-3-(1-o,p-dichlorophenyl-2-acetylethyl)-7-phenyl-8-o,p-
difluorophenyl-2H-thieno(3,2-h)chromen-2-one (3t)
1
IR (KBr, cmꢂ1): 750, 1680, 1710 and 3480 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.40 (3H, s, CH3), 3.02–3.40 (2H, m, -CH2), 4.33–4.42 (1H,
dd, -CH), 7.02–8.16 (12H, m, Ar-H); MS: m=z 620 (Mþ).

4-Hydroxy-3-(1-aryl-2-acetylethyl)-7,8-diaryl-2H-thieno (3,2-h)chromen-2-ones 3599
4-Hydroxy-3-(1-p-hydroxyphenyl-2-acetyl-ethyl)-7-phenyl-8-o,p-difluor-
ophenyl-2H-thieno(3,2-h)chromen-2-one (3u)
1
IR (KBr, cmꢂ1): 755, 1680, 1720 and 3498 cmꢂ1. H NMR (300 MHz,
CDCl3): d: 2.42 (3H, s, CH3), 3.00–3.36 (2H, m, -CH2), 4.38–4.51 (1H,
dd, -CH), 7.06–8.12 (13H, m, Ar-H); MS: m=z 568 (Mþ).
CONCLUSIONS
When cerium(III) chloride heptahydrate was used instead of piperidine
in the Michael condensation, warfarin derivatives (3a–u) were obtained
in good yields, and reaction times and temperature were also reduced.
ACKNOWLEDGMENT
One of the authors (M. Amaravathi) gratefully acknowledges the Depart-
ment of Science and Technology, New Delhi, India, for sanctioning
Women Scientist Scheme A. We also thank to the National Institute of
Technology, Warangal, India, for providing laboratory facilities.
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