Synthesis of two LewisX trisaccharides using regiospecific glycosylation reactions

Article abstract of DOI:10.1081/CAR-200058529

Synthesis of two Lewis^X trisaccharides, namely, 2-(trimethylsilyl)ethyl 2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl-(1 → 4)-6-O-benzyl-2-deoxy-3-O-(2,3,4-tri-O-benzyl-α-L-fucopyranosyl)-2- phthalimido-β-D-glucopyranoside and 2-(trimethylsilyl)ethyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenylsilyl-β-D-galactopyranosyl-(1 → 4)-6-O-benzyl-2-deoxy-3-O-(2,3,4-tri-O-benzyl-α-L-fucopyranosyl) -2-phthalimido-β-D-glucopyranoside, has been achieved using a regiospecific glycosylation strategy under NIS-TfOH activation. Two trisaccharides were prepared from monosaccharides without any protecting group manipulation. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1081/CAR-200058529

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Synthesis of Two LewisX Trisaccharides
Using Regiospecific Glycosylation
Reactions
Debaraj Mukherjee a , Sujit Kumar Sarkar b , Partha Chattopadhyay a
& Uday Shankar Chowdhury a
a Synthetic, Biophysical, and Natural Product Chemistry Division ,
Indian Institute of Chemical Biology , Jadavpur, Kolkata, India
b Department of Biological Chemistry , Indian Association for the
Cultivation of Science , Jadavpur, Kolkata, India
Published online: 16 Aug 2006.
To cite this article: Debaraj Mukherjee , Sujit Kumar Sarkar , Partha Chattopadhyay & Uday Shankar
Chowdhury (2005) Synthesis of Two LewisX Trisaccharides Using Regiospecific Glycosylation Reactions,
Journal of Carbohydrate Chemistry, 24:3, 251-259, DOI: 10.1081/CAR-200058529
To link to this article: http://dx.doi.org/10.1081/CAR-200058529
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Journal of Carbohydrate Chemistry, 24:251–259, 2005
Copyright # Taylor & Francis, Inc.
ISSN: 0732-8303 print 1532-2327 online
DOI: 10.1081/CAR-200058529
Synthesis of Two LewisX
Trisaccharides Using
Regiospecific Glycosylation
Reactions
Debaraj Mukherjee
Synthetic, Biophysical, and Natural Product Chemistry Division, Indian Institute of
Chemical Biology, Jadavpur, Kolkata, India
Sujit Kumar Sarkar
Department of Biological Chemistry, Indian Association for the Cultivation of Science,
Jadavpur, Kolkata, India
Partha Chattopadhyay and Uday Shankar Chowdhury
Synthetic, Biophysical, and Natural Product Chemistry Division, Indian Institute of
Chemical Biology, Jadavpur, Kolkata, India
Synthesis of two LewisX trisaccharides, namely, 2-(trimethylsilyl)ethyl 2,3,4,6-tetra-
O-acetyl-b-D-galactopyranosyl-(1 ! 4)-6-O-benzyl-2-deoxy-3-O-(2,3,4-tri-O-benzyl-a-L-
fucopyranosyl)-2-phthalimido-b-D-glucopyranoside and 2-(trimethylsilyl)ethyl 2,3,4-tri-
O-acetyl-6-O-t-butyldiphenylsilyl-b-D-galactopyranosyl-(1 ! 4)-6-O-benzyl-2-deoxy-3-O-
(2,3,4-tri-O-benzyl-a-L-fucopyranosyl)-2-phthalimido-b-D-glucopyranoside, has been
achieved using a regiospecific glycosylation strategy under NIS-TfOH activation. Two
trisaccharides were prepared from monosaccharides without any protecting group
manipulation.
Keywords Synthesis, Trisaccharide, LewisX, N-Iodosuccinimide, Trifluoromethane-
sulfonic acid
Received November 29, 2004; accepted February 25, 2005.
Address correspondence to Uday Shankar Chowdhury, Synthetic, Biophysical, and
Natural Product Chemistry Division, Indian Institute of Chemical Biology, 4 Raja
S.C. Mullick Road, Jadavpur, Kolkata 700032, India. Fax: þ91-33-24735197/4730284;
E-mail: uschowdhury@iicb.res.in
251

D. Mukherjee et al.
252
INTRODUCTION
Lewisx and sulfated Lewisx oligosaccharides have been attractive targets for
synthesis owing to the important biological role of these compounds as poten-
tial ligands for selectins.[1] In all of these oligosaccharides sialic acid is
present as NAcNeu a-(2 ! 3)-Galp and is important for the trisaccharide to
maintain a high affinity for E-Selectins.[2] However, replacement of this sialic
acid residue with a sulfate group retains its affinity.[3] Therefore, synthesis of
this suitably protected Lewisx trisaccharide is very important so that the
sialic acid or sulfate group can be introduced in proper position. Various
groups[4–17] synthesized this trisaccharide backbone using different glycosyla-
tion strategies. Most of the synthetic strategy involves mainly two procedures:
(1) galactosylation or fucosylation of the suitably protected glucosamine
derivatives, having 3-OH or 4-OH unprotected, and (2) regioselective galacto-
sylation or fucosylation of a glucosamine acceptor, having both 3-OH and
4-OH unprotected. The first synthetic strategy requires an elaborate protection
of the acceptor while the second requires high regioselectivity. Different groups
have synthesized the N-acetyllactosamine derivative by this regioselective
glycosylation strategy[13,18,19] but either the regioselectivity of the glycosyla-
tion[18] was low or the yield of the reaction was not very good.[19]
In this paper, we describe the regioselective synthesis of N-acetyllactosamine
derivatives under NIS-TfOH activation and their conversion to Lewisx trisac-
charides, whichare suitablefor the synthesisofsilylLewisx orsulfatedanologues.
RESULTS AND DISCUSSION
The known glucosamine acceptor 2-(trimethylsilyl)ethyl 6-O-benzyl-2-deoxy-2-
phthalimido-b-D-glucopyranoside (3)[20] has two unprotected hydroxyl groups
(3-OH and 4-OH) with different reactivities. We observed that disarmed galacto-
pyranosyl donors 1 and 2 react regioselectively with the acceptor 3 under NIS-
TfOH activation and the yield of the glycosidation reaction was high. The
donor 1 reacts with the acceptor 3 to afford the disaccharide 2-(trimethylsilyl)
ethyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-(1 ! 4)-6-O-benzyl-2-deoxy-
2-phthalimido-b-D-glucopyranoside (4) in 80% yield, whereas the donor,
phenyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenylsilyl-1-thio-b-D-galactopyranoide
(2), under similar condition of glycosidation reaction afforded the disaccharide

Synthesis of Two LewisX Trisaccharides
253
2-(trimethylsilyl)ethyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenylsilyl-b-D-galacto-
pyranosyl-(1 ! 4)-3-O-acetyl-6-O-benzyl-2-deoxy-2-phthalimido-b-D-glucopyra-
noside (5) in 88% yield. No undesired (1 ! 3)-linked disaccharide was detected
in the reaction mixture. Acetylation of the disaccharide (4) afforded the corre-
sponding 3-O-acetate derivative, which shows a downfield shift of the H-3A
signal at d 5.62 (dd, J2,3 ¼ 8.9 Hz, J3,4 ¼ 10.6 Hz) in its 1H NMR spectrum.
The downfield shift of the H-3A signal also confirms that the disaccharide (4)
is (1 ! 4)-linked. The yield of these reactions clearly indicates that the 6-O-
TBDPS group in the galactose residue has influence on the regioselectivity of
glycosylation. For a-fucosylation, we have used phenyl 2,3,4-tri-O-benzyl-1-
thio-b-L-fucopyranoside (6). The disaccharide acceptor 4 was reacted with the
donor (6) under NIS-TfOH activation to afford the trisaccharide (2-trimethyl-
silyl)ethyl
2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-(1 ! 4)-6-O-benzyl-2-
deoxy-3-O-(2,3,4-tri-O-benzyl-a-L-fucopyranosyl)-2-phthalimido-b-D-glucopyra-
noside (8) in good yield (65%). In spite of the steric hindrance of the phthalimido
group, we were successful in introducing fucosyl group at 3-OH of the disaccha-
ride acceptor 4 in good yield. Ellervik and Magnusson[13] reported that fucosyla-
tion at this hindered 3-OH was difficult under various reaction conditions. The
a-anomeric configuration of this newly formed glycosidic linkage of trisaccharide
8 was confirmed from the signal at d 4.75 (J1,2 ¼ 3.3 Hz) in the 1H NMR
spectrum and the signal at d 97.5 in the 13C NMR spectrum. Mass spectroscopy
of 8 also confirmed the formation of this trisaccharide. However, under similar
glycosylation conditions, the donor 7 was reacted with the acceptor 5 to afford
the trisaccharide (2-trimethylsilyl)ethyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenyl-
silyl-b-D-galactopyranosyl-(1 ! 4)-6-O-benzyl-2-deoxy-3-O-(2,3,4-tri-O-benzyl-
a-L-fucopyranosyl)-2-phthalimido-b-D-glucopyranoside (9) in poor yield (35%).
Under CuBr2-TBAB activation the yield of compound 9 slightly improved
(44%). From this glycosylation reaction we can conclude that the 6-O-TBDPS
group at the galactose moiety has influence in the glycosylation at 3-OH of
1
the glucosamine acceptor. The compound was characterized from H, 13C, and
DEPT-135 NMR and mass spectroscopic data.
In conclusion, we have synthesized two Lewisx trisaccharides using a
minimal number of synthetic steps. These oligosaccharides are suitable
for introducing sialic acid residues for conversion to the corresponding 3/6-
O-sulphate analogues (Sch. 1).
EXPERIMENTAL
General Methods
Reactions were monitored by TLC on Silica Gel 60F254 (E. Merck). Column
chromatography was performed on 100–200 mesh Silica Gel (SRL, India). The

D. Mukherjee et al.
254
Scheme 1: (a) NaOMe, MeOH, 308C, 30 min, then IR-120 Hþ resin; (b) TBDPS-Cl, pyridine, 308C,
2 hr, then Ac2O, 0–308C, 16 hr; (c) NIS,TfOH,CH2Cl2,–208C,1 hr; (d) NIS, TfOH, CH2Cl2,–208C,
2 hr; (e) CuBr2, TBAB, ClCH2CH2Cl-DMF(5:1), 308C, 16 hr.
organic extracts were dried over anhydrous Na2SO4. All solvents were distilled
and/or dried before use and all evaporations were conducted at or below 508C
under reduced pressure unless stated otherwise. Optical rotations were
1
measured at 258C with a JASCO DIP 360 polarimeter. The H and 13C NMR
spectra were recorded with a Bruker DPX 300 spectrometer at 300 MHz
using CDCl3 as solvent and tetramethylsilane as the internal standard
unless stated otherwise. Mass spectra were recorded on a Q-Tof of micromass
spectrometer using electron spray ionization (ESI). Compound 1,[23] 3,[20]
6,[22] and 7[21] were synthesized as described in the literature.
Phenyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenylsilyl-1-thio-b-D-galacto-
pyranoside (2). Compound 1 (5 g) was dissolved in MeOH (30 mL) and
50 mg NaOMe (in 5 mL MeOH) was added. The mixture was stirred for
30 min, neutralized by IR-120 Hþ resin, and filtered. The filtrate was coconcen-
trated twice with toluene to give phenyl 1-thio-b-D-galactopyranoside. The
solid residue (3.6 g) was dissolved in pyridine (60 mL), and TBDPSCl (5 mL,
19.5 mmol) was added at rt. The mixture was stirred for 2 hr at 308C and
then Ac2O (6.2 mL, 65 mmol) was added at 08C. The mixture was kept over-
night at rt and then coconcentrated twice with toluene. The residue was chro-
matographed (4:1 petroleum ether-EtOAc) to afford 2 (6 g, 82%) as an
1
amorphous solid; [a]2D8 þ 5.51 (c 2.5, CHCl3). H NMR d 7.65–7.25 (m, 15H,
aromatic protons), 5.56 (d, 1H, J3,4 ¼ 3.2 Hz, H-4), 5.22 (t, 1H,
J1,2 ¼ J2,3 ¼ 9.9 Hz, H-2), 5.07 (dd, 1H, J2,3 ¼ 9.9 Hz, J3,4 ¼ 3.2 Hz, H-3), 4.71

Synthesis of Two LewisX Trisaccharides
255
(d, 1H, J1,2 ¼ 9.9 Hz, H-1), 3.84–3.76 (m, 2H, H-6), 3.68–3.62 (m, 1H, H-5),
2.08, 1.99, 1.97 (s, 9H, 3 COCH3), 1.03 [s, 9H, (CH3)3CSiPh2]; 13C NMR d
170.0, 169.9, 169.4 (3 COCH3), 135.5–127.7 (aromatic carbons), 86.7 (C-1),
77.2, 72.3, 67.4, 67.4, 61.4 (C-6), 26.6 [(CH3)3CSiPh2], 20.8, 20.6, 20.5 (3
COCH3), 18.9 [(CH3)3CSiPh2]. TOF MS (ESI þ ) Found: 659[M þ Na]þ.
Anal. Calcd for C34H40SSiO8 (636.22): C, 64.12; H, 6.33. Found: C, 64.01;
H, 6.23.
2-(Trimethylsilyl)ethyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-
(1 ! 4)-6-O-benzyl-2-deoxy-2-phthalimido-b-D-glucopyranoside (4). A
mixture of donor 1 (109 mg, 0.24 mmol), acceptor 3 (100 mg, 0.2 mmol), and
˚
4 A MS (250 mg) in CH2Cl2 (2 mL) was allowed to stir at rt for 2 hr. The
mixture was then cooled to –208C and NIS (64.8 mg, 0.288 mmol) and TfOH
(2 mL, 0.024 mmol) were added. After stirring for 45 min, when TLC (7:1
toluene-EtOAc) showed optimum conversion, reaction was quenched with the
addition of Et3N, and then diluted with CH2Cl2 (25 mL) and filtered through
a Celite bed. The filtrate was washed successively with aq. Na2S2O3, saturated
aqueous NaHCO3, and water. The organic layer was dried (Na2SO4) and evap-
orated to a syrup. Column chromatography with 6:1 toluene-EtOAc yielded 4
(132 mg, 80%) as a glassy syrup; [a]2D8 þ 13.71 (c 1.05, CHCl3). 1H NMR d
7.24–7.82 (m, 9H, aromatic protons), 5.30 (d, 1H, J3,4 ¼ 3.1 Hz, H-4B), 5.19
(d, 1H, J1,2 ¼ 8.5, H-1A), 5.17 (t, J ¼ 8.2 Hz, H-2B), 4.91 (dd, 1H, J2,3
¼
10.5 Hz, J3,4 ¼ 3.39 Hz, H-4B), 4.73, 4.52 (2 d, 2H, J ¼ 12.1 Hz, CH2Ph), 4.47
(d, 1H, J1,2 ¼ 8.0 Hz, H-1B), 4.39 (m, 1H, H-3A), 4.13 (m, 1H, H-2A),
3.89 (m, 1H, H-5A), 3.59–3.74 (m, 5H, H-5B, H-6A, H-6B), 3.49 (m, 2H,
OCH2CH2SiMe3), 2.13, 2.00, 1.95, 1.89 (s, 12H, 4 COCH3), 0.82 (m, 2H,
OCH2CH2SiMe3), –1.5 (s, 9H, SiMe3); 13C NMR d 170.4, 170.0, 169.9, 169.1
(4 COCH3), 138.1–127.5 (aromatic carbons), 101.5 (C-1B), 97.7 (C-1A), 74.1,
73.6, 73.6 (OCH2CH2Si), 71.1, 69.7, 68.7, 67.9 (C-6A), 66.9, 66.8, 65.9, 61.4
(C-6B), 56.0 (C-2B), 20.6, 20.6, 20.5, 20.5 (4COCH3), 17.7 (OCH2CH2Si), –1.5
(SiMe3); TOF MS (ESI þ ) Found: 852[M þ Na]þ.
Anal. Calcd for C40H51SiNO16 (829.30): C, 57.89; H, 6.19; N, 1.69. Found: C,
57.67; H, 6.32; N, 1.61.
2-(Trimethylsilyl)ethyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenylsilyl-b-D-
galactopyranosyl-(1 ! 4)-3-O-acetyl-6-O-benzyl-2-deoxy-2-phthalimido-
b-D-glucopyranoside (5). The donor 2 (152.4 mg, 0.24 mmol) was reacted
with the acceptor 3 (100 mg, 0.2 mmol) in the same manner as in the case of
4 and the resulting product was purified by column chromatography with
7:1 toluene-EtOAc to afford the disaccharide 5 (184 mg, 88%) as amorphous
1
solid. [a]2D8–11.21 (c 2.14, CHCl3). H NMR d 7.65–7.32 (m, 19H, aromatic
protons), 5.35 (d, 1H, J3,4 ¼ 3.2 Hz, H-4B), 5.17 (d, 1H, J1,2 ¼ 8.6 Hz, H-1B),
5.14 (t, 1H, J ¼ 8.1 Hz, H-2B), 4.91 (dd, 1H, J2,3 ¼ 10.5 Hz, J3,4 ¼ 3.3 Hz,

D. Mukherjee et al.
256
H-3B), 4.73, 4.53 (2 d, 2H, J ¼ 12.1 Hz, CH2Ph), 4.52 (d, 1H, J1,2 ¼ 7.9 Hz,
H-1A), 4.44 (t, 1H, J ¼ 8.8 Hz, H-3A), 4.13 (dd, 1H, J1,2 ¼ 8.6 Hz,
J2,3 ¼ 10.7 Hz, H-2A), 4.02 (d, 1H, J ¼ 1.3 Hz, 3-OH), 3.92 (m, 1H, H-5A),
3.76–3.61 (m, 5H, H-5B, H-6A, H-6B), 3.47 (m, 2H, OCH2CH2Si), 1.98, 1.94,
1.94 (3 s, 9H, 3 COCH3), 0.83 [s, 9H, (CH3)3CPh2Si], 0.75 [M, 2H,
OCH2CH2Si], –0.15 [s, 9H, Si(CH3)3]; 13C NMR d 170.0, 169.9, 169.3 (3
COCH3), 138.3–127.7 (aromatic carbons), 101.5 (C-1B), 97.8 (C-1A), 81.9 (C-
4A), 74.3, 73.8, 73.7 (OCH2CH2Si), 71.0, 69.9, 69.1, 68.1 (C-6A), 66.9, 66.9,
61.4 (C-6B), 55.8 (C-2B), 26.5 [(CH3)3CPh2Si], 20.7, 20.6, 20.5 (3 COCH3), 18.7
[(CH3)3CPh2Si], 17.7 [OCH2CH2Si], –1.5 [Si(CH3)3]. TOF MS (ESI þ ) Found:
1048[M þ Na]þ.
Anal. Calcd for C54H67Si2NO15(1025.40): C, 63.20; H, 6.58; N, 1.36. Found:
C, 63.02; H, 6.41; N 1.45.
2-(Trimethylsilyl)ethyl 2,3,4,6-tetra-O-acetyl-b-D-galactopyranosyl-
(1 ! 4)-6-O-benzyl-2-deoxy-3-O-(2,3,4-tri-O-benzyl-a-L-fucopyranosyl)-
2-phthalimido-b-D-glucopyranoside (8). The donor 6 (102 mg, 0.19 mmol)
was reacted with the acceptor 4 (134 mg, 0.16 mmol) in the same manner as
in the case of 4 and the product was purified on column chromatography
with 3:1 toluene-EtOAc to yield trisaccharide 8 (130 mg, 65%). [a]2D8 þ 3.35 (c
1
1.8, CHCl3); H NMR d 7.41–7.03 (m, 24 H, aromatic protons), 5.23 (d, 1H,
J ¼ 3.4 Hz, H-4B), 5.08 (d, 1H, J ¼ 8.4 Hz, H-1A), 5.01 (t, J ¼ 8.4, H-2B), 4.75
(d, 1H, J ¼ 3.2, H-1C), 4.67 (d, 1H, J ¼ 7.9, H-1B), 2.01, 2.00, 1.94, 1.85 (4 s, 12
H, 4 COCH3), 1.20 (d, 3H, J ¼ 6.3 Hz, H-6C), 0.72 (m, 2H, OCH2CH2Si), –0.15
[s, 9H, SiMe3]; 13C NMR d 170.2, 170.1, 170.0, 168.8 (4 COCH3), 140.9–127.0
(aromatic carbons), 99.5 (C-1B), 97.8 (C-1A), 97.5 (C-1C), 79.8, 77.2, 75.4, 75.3,
74.6 (5 CH), 74.2, 73.6, 72.9 (3 CH2Ph), 72.6 (CH), 72.4 (PhCH2), 71.0, 70.4,
69.1 (3 CH), 67.8 [OCH2CH2Si], 66.8 (C-6A), 66.4 (CH), 60.3 (C-6B), 56.4
(C-2A), 20.7, 20.6, 20.5 (3 COCH3), 17.8 (OCH2CH2Si), 16.7 (C-CH3), –1.5
[Si(CH3)3]. TOF MS (ESI þ ) Found: 1268[M þ Na]þ.
Anal. Calcd for C67H79NSiO20(1245.50): C, 64.56; H, 6.69; N, 1.12. Found:
C, 64.82; H, 6.74; N, 1.18.
2-(Trimethylsilyl)ethyl 2,3,4-tri-O-acetyl-6-O-t-butyldiphenylsilyl-b-D-
galactopyranosyl-(1 ! 4)-6-O-benzyl-2-deoxy-3-O-(2,3,4-tri-O-benzyl-
a-L-fucopyranosyl)-2-phthalimido-b-D-glucopyranoside (9). A solution
of 5 (100 mg, 0.095 mmol) and 7 (91 mg, 0.19 mmol) in 2 mL ClCH2CH2Cl-
DMF (5:1) was added to a stirred mixture of CuBr2 (52 mg, 0.23 mmole), tetra-
butylammonium bromide (74 mg, 23 mmole), and freshly activated powdered
MS 4A (200 mg) under nitrogen. The reaction was stirred for 50 hr at rt. The
reaction mixture was filtered through Celite and washed with CH2Cl2. The
combined filtrate and washings were washed with saturated NaHCO3
solution and then with water, dried (Na2SO4), and concentrated.

Synthesis of Two LewisX Trisaccharides
257
Column chromatography of the residue with ethyl acetate-pet ether (3:7)
yielded the solid product 9 (61 mg, 44%). [a]2D8–19.9 (c 1.6, CHCl3); H NMR
1
d 7.49–6.86 (m, 34 H, aromatic protons), 5.50 (d, 1H, J ¼ 2.93 Hz, H-4B), 5.03
(d, 1H, J ¼ 8.51 Hz, H-1A), 4.82 (d, 1H, J ¼ 4.17 Hz, H-1C), 4.62 (d,1H, J ¼
5.66 Hz, H-1B), 2.02, 1.96, 1.74 (s, 12H, 4 COCH3), 1.11 (d, 1H, J ¼ 6.5 Hz,
H-6C), 0.98 (s, 9H, Me3CPh2Si), 0.79 (m, 2H, OCH2CH2Si), –0.15 (s, 9H,
SiMe3); 13C NMR d 170.1, 169.7, 168.8 (3 COCH3), 138.4–123.5 (aromatic
carbons), 99.7 (C-1B), 97.8 (C-1A), 97.3 (C-1C), 79.5, 77.2, 75.4, 75.3, 74.4, 74.3
(6 ring CH), 73.5, 72.8, 72.7, 72.6, 72.4, 71.4, 69.4, 67.7, 66.8, 66.8
[OCH2CH2Si], 66.2 (C-6A), 59.9 (C-6B), 56.4 (C-2A), 26.7, 21.0, 20.8, 20.7 (3
COCH3), 18.9 (OCH2CH2Si), 17.8, 16.5 (C-CH3), –1.5 [Si(CH3)3]. TOF MS
(ESI þ) Found: 1464[M þ Na]þ.
Anal. Calcd for C81H95Si2NO19 (1441.60): C, 67.43; H, 6.64; N, 0.97. Found:
C, 67.65; H 6.81; N, 1.11.
ACKNOWLEDGMENTS
We are grateful to DST (Govt. of India) for financial assistance and CSIR for a
J.R.F to D.M. The authors thank Dr. R. Mukhopadhyay for spectral analysis.
We are indebted to Pradip Kr Roy of our department for his interest in this
work.
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