Radical cyclisation based approach to lignans. Synthesis of 4-arylmethyldihydrofuran-2-ones

Article abstract of DOI:10.1081/SCC-100104837

Bromoacetalisation of the cinnamyl alcohols 7a-d, obtained from the corresponding benzaldehydes, generated the bromoacetals 10a-d. The 5-exo trig radical cyclisation of the bromoacetals 10a-d followed by one step hydrolysis-oxidation of the resulting cyclic acetals 11a-d furnished the title compounds 6a-d, respectively, well-established intermediates of a variety of lignans.

Full text of DOI:10.1081/SCC-100104837

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
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RADICAL CYCLISATION BASED APPROACH
TO LIGNANS. SYNTHESIS OF 4-
ARYLMETHYLDIHYDROFURAN-2-ONES
A. Srikrishna & S. Danieldoss
To cite this article: A. Srikrishna & S. Danieldoss (2001) RADICAL CYCLISATION BASED
APPROACH TO LIGNANS. SYNTHESIS OF 4-ARYLMETHYLDIHYDROFURAN-2-ONES, Synthetic
Communications, 31:15, 2357-2364, DOI: 10.1081/SCC-100104837
To link to this article: http://dx.doi.org/10.1081/SCC-100104837
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Date: 18 January 2016, At: 01:31

SYNTHETIC COMMUNICATIONS, 31(15), 2357–2364 (2001)
RADICAL CYCLISATION BASED
APPROACH TO LIGNANS.
SYNTHESIS OF
4-ARYLMETHYLDIHYDROFURAN-2-ONES
A. Srikrishna* and S. Danieldoss
Department of Organic Chemistry, Indian Institute of Science,
Bangalore 560 012, India
ABSTRACT
Bromoacetalisation of the cinnamyl alcohols 7a–d, obtained
from the corresponding benzaldehydes, generated the bro-
moacetals 10a–d. The 5-exo trig radical cyclisation of the bro-
moacetals 10a–d followed by one step hydrolysis-oxidation of
the resulting cyclic acetals 11a–d furnished the title com-
pounds 6a–d, respectively, well-established intermediates of
a variety of lignans.
Lignans of many structural types, widely distributed in plants, have
been found to exhibit a variety of biological activities such as cytotoxic,
fungitoxic, growth inhibitory activity, etc.1 Consequently, lignans have
received a great deal of attention. Among the numerous strategies devel-
oped, diasteroselective alkylation of 3-arylmethylbutyrolactones followed
by cyclisations remain the most general and versatile method for the
*Corresponding author. Fax: 91-80-3600683; E-mail: ask@orgchem.iisc.ernet.in
2357
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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SRIKRISHNA AND DANIELDOSS
synthesis of lignans. Based on the bromoacetal cyclisation originally devel-
oped by Stork et al., and Ueno et al.,2 a radical cyclisation reaction based
methodology was initiated for the synthesis of 3-(arylmethyl)butyrolac-
tones. It was conceived that the 5-exo trig radical cyclisation reaction of
the bromoacetal 1 results in the formation of 2-alkoxytetrahydrofuran 2,
which could be converted into the 3-substituted butyrolactone 3 via hydro-
lysis and oxidation sequence. The bromoacetal 1 could be generated by
bromination of an alkyl vinyl ether 4 in the presence of an appropriate
alcohol 5.3 Substituted cinnamyl alcohols 7a–d were readily identified as
the requisite starting materials.
The synthetic sequence for the generation of the lactones 6a–d is
depicted in Scheme 1. The cinnamyl alcohols 7a–d were obtained from the
benzaldehydes 8a–d, via Horner-Wadsworth-Emmons reaction with triethyl
phosphonoacetate and sodium hydride in THF followed by reduction of the
resulting E-cinnamates 9a–d with lithium aluminium hydride (LAH) in dry
ether at low temperature in good yields. Slow addition of a methylene
chloride solution of N-bromosuccinimide (NBS) to a solution of the cinna-
myl alcohols 7a–d and an excess of ethyl vinyl ether in methylene chloride
at low temperature furnished the bromoacetals 10a–d, respectively, whose
structures was established from their spectral data. The key 5-exo trig radi-
cal cyclisation reaction was carried out employing an in situ generated cat-
alytic tri-n-butyltin hydride using a combination of a catalytic amount of
tri-n-butyltin chloride and stochiometric amount of sodium cyanoboro-
hydride.4 Thus, refluxing a 0.2 M solution of the bromoacetals 10a–d in
tertiary butyl alcohol with 0.15 equivalents of tri-n-butyltin chloride and
2–3 equivalents of sodium cyanoborohydride in the presence of a catalytic
amount of azobisisobutyronitrile (AIBN) furnished the 2-ethoxytetrahydro-
furans 11a–d, respectively, as a mixture of ethoxy epimers. Sonochemically
accelerated reaction5 of the acetals 11a–d with a freshly prepared 1.6 M
Jones reagent in acetone furnished the butyrolactones 6a–d, respectively,

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2359
which were identified by comparison of the spectral data with that reported
in the literature. The butyrolactones 6a–d have been employed in the
synthesis of a wide variety of naturally occurring lignans of various
types,6 such as diarylbutanes, butyrolactones and tetrahydrofurans; arylter-
lin based lignans; dibenzocyclooctadiene based lignans, etc.
Scheme 1.
EXPERIMENTAL SECTION
4-[(3,4-Dimethoxyphenyl)methyl]dihydrofuran-2-one (6a)
To a cold (ꢀ70ꢁC) magnetically stirred solution of the cinnamyl alcohol
7a (2 g, 10.3 mmol) and ethyl vinyl ether (5.05 ml, 3.8 g, 53 mmol) in methylene
chloride (10 ml) was added a methylene chloride (20 ml) solution of NBS
(3.67 g, 20 mmol) over a period of 15 min. The reaction mixture was stirred
at the same temperature for 2 h. It was then diluted with methylene chloride
(30 ml), washed with 0.5 N aqueous NaOH followed by water and brine, and
dried (Na2SO4). Evaporation of the solvent and purification of the product
over a silica gel column using ethyl acetate-hexane (1:9) as eluent furnished
the bromoacetal 10a (2.8 g, 78%) as an oil, which was found to decompose on
standing. IR (neat): vmax 1595, 1580, 1500, 1120, 1010 cmꢀ1 1H NMR
.
(270 MHz, CDCl3): d 6.95 (1 H, s, H-20), 6.93 and 6.81 (2 H, AB q, J ¼ 8 Hz,

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SRIKRISHNA AND DANIELDOSS
H-60 and 50) [aromatic H], 6.58 (1 H, d, J ¼ 15.8 Hz, Ar-CH ¼ CH), 6,16
(1 H, t of d, J ¼ 15.8 and 6 Hz, Ar-HC ¼ CH), 4.78 (1 H, t, J ¼ 5.4 Hz,
OCHOEt), 4.33 and 4.23 (2 H, d of AB q, J ¼ 12.4 and 6 Hz, OCH2), 3.90
(3 H, s) and 3.88 (3 H, s) [2 Â Ar-OCH3], 3.64 (2 H, m, OCH2CH3), 3.42 (2 H,
d, J ¼ 5.4 Hz, CH2Br), 1.26 (3 H, t, J ¼ 7 Hz, OCH2CH3).
A magnetically stirred suspension of the bromoacetal 10a (295 mg,
0.85 mmol), tri-n-butyltin chloride (0.16 ml, 192 mg, 0.59 mmol), sodium
cyanoborohydride (238 mg, 3.7 mmol) and AIBN (catalytic) in tertiary
butyl alcohol (10 ml) was refluxed for 90 min. After the reaction, the solvent
was evaporated under reduced pressure, the residue was taken in water and
extracted with ether (3 Â 7 ml). The ether extract was washed with 1% aqu-
eous ammonia followed by brine and dried (Na2SO4). Evaporation of the
solvent and purification of the product on a silica gel column using ethyl
acetate-hexane (1:4) as eluent gave an epimeric mixture of the cyclised product
11a (171 mg, 75%) as an oil. IR (neat): vmax 1510, 1160, 1140, 1030 cmꢀ1
.
1H NMR (90 MHz, CDCl3, 1:1.5 mixture of diastereomers): d 6.78 (1 H, d,
J ¼ 8.1 Hz, H-50), 6.7 (1 H, dd, J ¼ 8.1 and 1.8 Hz, H-60) and 6.70 (1 H, d,
J ¼ 1.8 Hz, H-20) (aromatic H), 5.13 and 5.12 (1 H, t, J ¼ 2.7 Hz, O-CH-OEt),
4.1–3.4 (4 H, m), 3.87 (3 H, s) and 3.82 (3 H, s) [2 Â Ar-OCH3], 2.71 (2 H,
d, J ¼ 8 Hz, Ar-CH2), 2.49 (1 H, m), 2.1 (1 H, ddd, J ¼ 10.8, 8.1 and 5.4 Hz)
and 1.65 (1 H, m), 1.23 and 1.18 (3 H, t, J ¼ 6.8 Hz, OCH2CH3). 13C NMR
(50 MHz, CDCl3, DEPT, 1:1.5 mixture of diastereomers): d 148.9 (C) and
147.5 (C), 133.5 and 133.3 (C), 120.6 and 120.5 (CH), 112.1 (CH), 111.4 (CH),
104.4 and 104.0 (CH, O-CH-O), 71.7 (CH2), 63.1 and 62.7 (CH2), 55.9 (CH3),
55.8 (CH3), 40.3 and 39.0 (CH), 39.6 and 39.2 (CH2), 38.9 and 38.8 (CH2),
15.4 and 15.3 (CH3). Mass: m/z 266 (Mþ, 100%), 221 (100), 203 (50), 177 (50),
151 (100). HRMS: m/z for C15H22O4, Calcd: 266.1518. Found: 266.1505.
To a solution of the acetal 11a (150 mg, 0.56 mmol) was added a freshly
prepared 1.6 M Jones reagent (1 ml, 1.6 mmol) and the reaction mixture was
irradiated in a ultrasonic cleaning bath for 5 min. A few drops of isopropanol
was added to consume the excess reagent. It was then diluted with water and
extracted with ether (2 Â 7 ml). The ether extract was washed with aqueous
NaHCO3 and brine and dried (Na2SO4). Evaporation of the solvent and
purification of the product on a silica gel column using ethyl acetate-
hexane (1:4) as eluent furnished the lactone 6a (90 mg, 68%) as an oil.6
b.p.: 130–132ꢁC/0.2 torr. IR (neat): vmax 1758, 1590, 1503, 1140, 1008 cmꢀ1
.
1H NMR (270 MHz, CDCl3): d 6.79 (1 H, d, J ¼ 8.1 Hz, H-50), 6.68 (1 H, dd,
J ¼ 8.1 and 1.8 Hz, H-60) and 6.65 (1 H, d, J ¼ 1.8 Hz, H-20) [aromatic H], 4.34
(1 H, dd, J ¼ 9.1 and 6.75 Hz) and 4.04 (1 H, dd, J ¼ 9.1 and 5.7 Hz) [CH2O],
3.87 (3 H, s) and 3.86 (3 H, s) [2 Â ArOCH3], 2.83 (1 H, m), 2.70 (2 H, m), 2.60
(1 H, dd, J ¼ 17.5 and 7.8 Hz) and 2.29 (1 H, dd, J ¼ 17.5 and 7.3 Hz)
[CH2C ¼ O]. 13C NMR (100 MHz, CDCl3): d 176.8 (OC ¼ O), 149.0 and

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4-ARYLMETHYLDIHYDROFURAN-2-ONES
2361
147.8 [C-30 and C-40], 130.7 (C-10), 120.5 (C-60), 111.7 and 111.4 (C-20 and
C-50), 72.5 (CH2-O), 55.8 (2 C, 2 Â Ar-OCH3), 38.4, 37.1, 34.0 (CH2C ¼ O).
4-[(3,4-Methylenedioxy)phenylmethyl]dihydrofuran-2-one (6b)
Bromoacetalisation reaction of the alcohol 7b furnished the bromoa-
1
cetal 10b in 81% yield. IR (neat): vmax 1650, 1605, 1026 cmꢀ1. H NMR
(200 MHz, CDCl3): d 6.94 and 6.80 (2 H, 2 Â d, J ¼ 8 Hz, H-50 and 60), 6.77
(1 H, s, H-20) [aromatic H], 6.55 (1 H, d, J ¼ 15.8 Hz, ArHC ¼ CH ), 6.11
(1 H, t of d, J ¼ 15.8 and 6 Hz, ArHC ¼ CH), 5.9 (2 H, s, OCH2O), 4.76 (1H,
t, J ¼ 5.6 Hz, O-CH-O), 4.26 (2 H, m, CH2-O), 3.66 (2 H, m, OCH2CH3),
3.42 (2 H, d, J ¼ 5.6 Hz, CH2Br), 1.25 (3 H, t, J ¼ 7 Hz, OCH2CH3).
The radical cyclisation reaction of the bromoacetal 10b gave a 1:1
epimeric mixture of cyclised product 11b in 80% yield. IR (neat): vmax
1
1600, 1500, 1150, 1040 cmꢀ1. H NMR (270 MHz, CDCl3, 3:2 mixture of
diastereomers): d 6.72 (1 H, d, J ¼ 7.8 Hz), 6.65 (1 H, s), 6.60 (1 H, dd,
J ¼ 7.8 and 3.4 Hz), 5.92 (2 H, s, OCH2O), 5.12 (1 H, t, J ¼ 3.2 Hz, O-CH-
O), 3.30–4.10 (4 H, series of multiplets), 2.55–2.8 (2 H, m), 2.42 (1 H, m),
2.17 (ddd, J ¼ 13.5, 8.91 and 5.4 Hz), 1.98 (dd, J ¼ 13.5 and 6.9 Hz) and
1.5–1.8 (m), 1.22 and 1.17 (3 H, t, J ¼ 7.1 Hz). 13C NMR (50 MHz,
CDCl3, 3:2 mixture of diastereomers): d 147.7, 145.9, 134.7, 134.4, 121.4,
121.5, 109.0, 108.2, 104.4, 103.9, 100.8, 71.6, 63.0, 62.6, 40.3, 39.6, 39.7, 39.1,
15.4, 15.3. Mass: m/z 250 (Mþ, 50%), 204 (50), 187 (30), 162 (25), 135 (100).
HRMS: m/z for C14H18O4, Calcd: 250.1205. Found: 250.1213.
Reaction of the acetal 11b with Jones reagent furnished the lactone 6b
in 71% yield.6 m.p.: 32–34ꢁC (Lit. 31–33.5ꢁC). IR (neat): vmax 1770, 1500,
1
1040, 1010 cmꢀ1. H NMR (270 MHz, CDCl3): d 6.74 (1 H, d, J ¼ 7.8 Hz,
H-50), 6.63 (1 H, d, J ¼ 1.4 Hz, H-20), 6.6 (1 H, dd, J ¼ 7.8 and 1.4 Hz, H-50),
5.94 (2 H, s, OCH2O), 4.33 (1 H, dd, J ¼ 9.2 and 6.8 Hz) and 4.02 (1 H, dd,
J ¼ 9.2 and 5.9 Hz) [OCH2], 2.77 (1 H, m, ArCH2CH), 2.6–2.8 (2 H, m,
ArCH2), 2.60 (1 H, dd, J ¼ 17.4 and 8 Hz) and 2.27 (1 H, dd, J ¼ 17.4 and
6.8 Hz) [CH2C ¼ O]. 13C NMR (100 MHz, CDCl3): d 176.7 (O-C ¼ O), 147.9
and 146.3 (C-30 and C-40), 131.9 (C-10), 121.6 (C-60), 108.8 (C-50), 108.4
(C-20) [aromatic C], 100.9 (O-CH2-O), 72.5 (CH2O), 38.6 (ArCH2CH),
37.2 (ArCH2), 34.1 (CH2C ¼ O).
4-[(3,4,5-Trimethoxyphenyl)methyl]dihydrofuran-2-one (6c)
Bromoacetalisation reaction of the alcohol 7c furnished the bromo-
1
acetal 10c in 84% yield. IR (neat): vmax 1581, 1110, 1032 cmꢀ1. H NMR

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2362
SRIKRISHNA AND DANIELDOSS
(200 MHz, CDCl3): d 6.62 (2 H, s, aromatic H), 6.57 (1 H, d, J ¼ 16.1 Hz),
6.21 (1 H, t of d, J ¼ 16.1 and 6.1 Hz), 4.76 (1 H, t, J ¼ 5.4 Hz, OCHO), 4.28
(2 H, d of AB q, J ¼ 14.5 and 6.1 Hz, OCH2), 3.67 (6 H, s) and 3.64 (3 H, s)
[3 Â ArOCH3], 3.80–3.50 (2 H, m, OCH2CH3), 3.43 (2 H, d, J ¼ 5.4 Hz,
CH2Br), 1.26 (3 H, t, J ¼ 6.6 Hz, OCH2CH3).
Radical cyclisation reaction of the bromoacetal 10c furnished a 3:2
epimeric mixture of the cyclised product 11c in 80% yield. IR (neat): vmax
1
1590, 1510, 1180, 1120 cmꢀ1. H NMR (270 MHz, CDCl3, 3:2 mixture of
diastereomers): d 6.37 and 6.36 (2 H, s), 5.14 (1 H, t, J ¼ 4 Hz), 4.0–3.35 (4 H,
m), 3.84 (6 H, s), 3.81 (3 H, s), 2.70 (2 H, d, J ¼ 7.6 Hz), 2.45 (1 H, m),
1.5–2.3 (2 H, series of multiplets), 1.20 and 1.18 (3 H, t, J ¼ 7 Hz). 13C NMR
(50 MHz, CDCl3, DEPT, 3 : 2 mixture of diastereomers): d 153.1 (2 C), 136.5
(C), 136.3 (C), 105.6 (2 C, CH), 104.3 and 103.8 (CH, O-CH-O), 71.6 (CH2),
62.9 and 62.5 (CH2), 60.6 (CH3), 56.0 (2 C, CH3), 40.1 and 38.8 (CH), 40.2
and 39.5 (CH2), 39.1 and 38.6 (CH2), 15.3 and 15.2 (CH3). Mass: m/z 296
(Mþ, 85%), 250 (50), 207 (25), 182 (40), 181 (50), HRMS: m/z for C16H24O5,
Calcd: 296.1624. Found: 296.1633.
Reaction of the acetal 11c with Jones reagent furnished the lactone 6c
in 70% yield.6 m.p.: 80–82ꢁC (Lit. 79–83ꢁC). IR (neat): vmax 1760,
1
1580 cmꢀ1. H NMR (270 MHz, CDCl3): d 6.35 (2 H, s, aromatic H), 4.34
(1 H, dd, J ¼ 9.2 and 6.9 Hz) and 4.04 (1 H, dd, J ¼ 9.2 and 5.8 Hz) [OCH2],
3.84 (6 H, s, 2 Â m-ArOCH3), 3.82 (3 H, s, p-ArOCH3), 2.87–2.74 (1 H, m,
Ar-CH2CH), 2.7 (2 H, m, Ar-CH2), 2.63 (1 H, dd, J ¼ 17.4 and 8 Hz) and
2.29 (2 H, dd, J ¼ 17.4 and 6.6 Hz) [CH2C ¼ O]. 13C NMR (50 MHz,
CDCl3): d 176.8 (OC ¼ O), 153.3 (2 C, C-30 and 50), 136.7 (C-40), 133.9
(C-10), 105.6 (2 C, C-20 and 60), 72.5 (OCH2), 60.7 (p-ArOCH3), 56.0 (2 C,
2 Â m-Ar-OCH3), 39.2 (ArCH2CH), 37.0 (ArCH2), 34.1 (CH2C ¼ O).
4-[3-Methoxy-(4-phenylmethoxy)phenylmethyl]dihydrofuran-
2-one(6d)
Bromoacetalisation reaction of the alcohol 7d furnished the bromoa-
cetal 10d in 81% yield. IR (neat): vmax 1584, 1503, 1110, 1023 cmꢀ1
.
1H NMR (90 MHz, CDCl3): d 7.45–7.25 (5 H, m), 6.95 (1 H, s), 6.8 (2 H,
s), 6.6 (1 H, d, J ¼ 17 Hz), 6.16 (1 H, t of d, J ¼ 17 and 6 Hz), 5.15 (2 H, s),
4.75 (1 H, t, J ¼ 5.4 Hz), 4.28 (2H, d, J ¼ 6 Hz), 3.9 (3 H, s), 3.65 (2 H, m),
3.41 (2 H, d, J ¼ 5.4 Hz), 1.25 (3 H, t, J ¼ 7.5 Hz). 13C NMR (22.5 MHz,
CHCl3): d 149.3, 147.8, 136.7, 132.3, 129.8, 128.2 (2 C), 127.5, 126.9 (2 C),
123.0, 119.4, 113.5, 109.2, 100.5, 70.6, 67.1, 62.1, 55.6, 31.5, 14.9. Mass: m/z
421 (Mþ þ 1, 25%), 419 (25), 269 (25), 253 (25), 179 (40), 153 (25), 151 (25),
125 (20), 123 (20), 91 (100).

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4-ARYLMETHYLDIHYDROFURAN-2-ONES
2363
Radical cyclisation reaction of the bromoacetal 10d furnished a 2:1
epimeric mixture of the acetal 11d in 85% yield. IR (neat): vmax 1590, 1500,
1116, 1020 cmꢀ1. 1H NMR (90 MHz, CDCl3, 2:1 mixture of diastereomers):
d 7.45–7.20 (5 H, m), 6.79 (1 H, d, J ¼ 8.1 Hz), 6.70 (1 H, d, J ¼ 1.8 Hz) 6.67
(1 H, dd, J ¼ 8.1 and 1.8 Hz), 5.13 (2 H, s), 4.0–3.3 (4 H, m), 3.87 (3 H, s),
2.70 (2 H, d, J ¼ 7.5 Hz), 2.44 (1 H, m), 2.19 (ddd, J ¼ 18.5, 8.1 and 5.4 Hz),
1.99 (dd, J ¼ 13.1 and 7.2 Hz), 1.5–1.8 (m), 1.22 and 1.18 (3 H, t, J ¼ 7.1 Hz).
13C NMR (50 MHz, CDCl3, 1:1.8 mixture of diastereomers): d 149.8 (C) and
146.7 (C), 137.5 (C), 134.3 and 134.0 (C), 128.6 (2 C, CH), 127.9 (CH), 127.4
(2 C, CH), 120.7 and 120.6 (CH), 114.5 (CH), 112.8 (CH), 104.5 and 104.0
(CH), 71.8 (CH2), 71.4 (CH2), 63.1 and 62.8 (CH2), 56.1 (CH3), 40.4 and
39.0 (CH2), 39.3 and 38.9 (CH2), 15.5 and 15.4 (CH3). Mass: m/z 342 (Mþ,
25%), 296 (25), 205 (20), 91 (100). HRMS: m/z for C21H26O4, Calcd:
342.1831. Found: 342.1816.
Reaction of the acetal 11d with Jones reagent furnished the lactone 6d
in 79% yield.6 m.p.: 73–74ꢁC (Lit. 73–74.5ꢁC). IR (neat): vmax 1764, 1590,
1503 cmꢀ1. 1H NMR (270 MHz, CDCl3): d 7.45–7.26 (5 H, m, OCH2C6H5),
6.81 (1 H, d, J ¼ 8.1 Hz, H-60), 6.67 (1 H, s, H-20), 6.62 (1 H, d, J ¼ 8.1 Hz,
H-50), 5.13 (2 H, s, OCH2C6H5), 4.33 (1 H, dd, J ¼ 9.1 and 6.9 Hz) and 4.03
(1 H, dd, J ¼ 9.1 and 6 Hz) [OCH2], 3.88 (3 H, s, Ar-OCH3), 2.75 (1 H, m,
ArCH2CH ), 2.70 (2 H, m, ArCH2), 2.60 (1 H, dd, J ¼ 17.4 and 8 Hz) and
2.28 (1 H, dd, J ¼ 17.4 and 6.7 Hz) [CH2C ¼ O]. 13C NMR (100 MHz,
CDCl3): d 176.9 (OC ¼ O), 149.9 and 147.2 (C-30 and C-40), 137.2 (C-10),
131.5 (C-100), 128.6 (2 C, C-300 and C-500), 127.9 (C-400), 127.3 (2 C, C-200 and
C-600), 120.7 (C-60), 114.5 (C-50) and 112.6 (C-20), 72.7 (CH2O), 71.2
(PhCH2O), 56.1 (ArOCH3), 38.6 (ArCH2CH), 37.3 (ArCH2), 34.3
(CH2C ¼ O). Anal. for C19H23O4, Calcd: C, 73.06; H, 6.4. Found: C,
73.11; H, 6.5%.
ACKNOWLEDGMENT
We thank the Council of Scientific and Industrial Research, New
Delhi for the award of a research fellowship to SD.
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Received in the Netherlands November 1, 2000

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