Recognition of insoluble tartaric acid in chloroform

Article abstract of DOI:10.1016/S0040-4020(01)00428-8

Simple receptors for the recognition and solubilisation of insoluble tartaric acid in chloroform were designed and synthesised for the first time. Receptors 2 and 3 were successful in solubilising tartaric acid into chloroform forming a 1:1 complex, and w

Full text of DOI:10.1016/S0040-4020(01)00428-8

TETRAHEDRON
Pergamon
Tetrahedron 57 52001) 4987±4993
Recognition of insoluble tartaric acid in chloroform
Shyamaprosad Goswami,a,p Kumaresh Ghoshb and Reshmi Mukherjeea
aDepartment of Chemistry, Bengal Engineering College ꢀDeemed University), Howrah 711103, West Bengal, India
bDepartment of Chemistry, J.K. College, Purulia, West Bengal, India
Received 28 November 2000; revised 28 March 2001; accepted 12 April 2001
AbstractÐSimple receptors for the recognition and solubilisation of insoluble tartaric acid in chloroform were designed and synthesised for
the ®rst time. Receptors 2 and 3 were successful in solubilising tartaric acid into chloroform forming a 1:1 complex, and were also found to be
useful as ¯uorescent probes for the detection of this substrate. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
a porphyrin which speci®cally recognises tartaric acid
derivatives with four-point hydrogen bonding.28 Lavigne
et al. corroborated the recognition of tartrate and malate
Intermolecular interactions form the basis of speci®c recog-
nition, reaction, transport and regulation which occur in
biological systems.1 Hydrogen bonding plays a key role in
molecular recognition.2±7 In general, the greater the number
of hydrogen bonds contributed by the receptor,8 the stronger
is the complex formed with the guest. The development of
synthetic receptors with de®ned architecture having mul-
tiple hydrogen bonding sites for complementary steric
match of the guest into the cavity of the receptor is thus
of considerable importance.
by their synthetic receptors.29 Recently Moran etal.
Â
reported the recognition of hydroxycarboxylates such as
lactic acid or mandelic acids with a macrocyclic receptor.30
But to our knowledge, no synthetic receptor has been
reported till now, for the recognition of tartaric acid itself,
which is an insoluble substrate in chloroform.
We report here the ef®cacy of the simple receptors 1, 2, 3
and 4 for tartaric acid binding and solubilisation into chloro-
form. Receptors 2 and 3, butnot 1 and 4, were effective in
tartaric acid binding. Receptors 2 and 3 have also been
found to be useful as ¯uorescent probes which can offer
the speci®c binding sites for the carboxylic acid function-
ality of the hydroxyacid at the core of the cavity. They
ef®ciently bind the highly insoluble naturally occurring
l-51)-tartaric acid and solubilise it into chloroform.
Synthetic receptors for peptides, amino acid derivatives and
hydroxyacids are of prime importance due to the inter-
molecular interactions involving these small molecules
with the peptide backbone in biological systems.9±18 In
continuation of our molecular recognition research,19±23
we are interested in the design and synthesis of simple
receptors as carriers for the transport of biologically impor-
tant organic substrates 5e.g. urea,21 aminoacids22,23 and
hydroxyacids which are soluble in water but insoluble in
organic solvents) into cells. In this regard, tartaric acid is
an important bio-active substrate. We report here our efforts
to design simple receptors for insoluble tartaric acid which
can solubilise itand can be developed as ist ¯uorescent
carriers. Lehmann et al. studied the role of hydrogen bond-
ing in the racemic as well as in the enantiomeric forms of
tartaric acid.24 Etter and Hitchcock et al. demonstrated the
hydrogen bonding arrangementof 5 S)-1-phenylethyl ammo-
nium hydrogen-l-tartarate.25,26 Hamilton et al. developed a
synthetic receptor that can successfully recognise the two
enantiomers27 of the diacetyltartaric acid derivatives.
Kuroda et al. devised a unique trench-type binding site on
Keywords: hydroxyacid; ¯uorescent receptors; molecular recognition;
tartaric acid.
p
Corresponding author. Tel.: 191-033-668-4561-63; fax: 191-033-668-
4564; e-mail: spgoswamical@yahoo.com
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020501)00428-8

4988
S. Goswami et al. / Tetrahedron 57 ꢀ2001) 4987±4993
Scheme 1.
The synthesis of all the receptors 51±4) is delineated in
Scheme 1. Receptor 1 was synthesised22 by reaction of 2,7-
dimethyl 1, 8-naphthyridine with butyllithium and 2-N-acetyl-
amino-6-bromomethylpyridine31 in tetrahydrofuran. Receptor
2 was synthesised by the high dilution reaction of 2-amino-6-
methylpyridine and 2-amino-7-methyl-1, 8-naphthyridine
with isophthaloyl chloride. Receptor 3 was synthesised by
the reaction of isophthaloyl chloride and 2-amino-7-methyl-
1, 8-naphthyridine. Receptor 4 was synthesised by the reac-
tion of butyryl chloride with bis-monoamide which was
obtained by the high dilution coupling of 2,6-diaminopyri-
dine with diacidchloride of 1,4-phenylenediacetic acid.
®xation of monocarboxylic acids with our designed recep-
tors8 tempted us to study the binding and solubilisation of
tartaric acid by our designed receptors 2 and 3. Receptor 1
does not bind tartaric acid although it has multiple hydrogen
bonding sites. This is probably because of its ¯exibility.
This ¯exibility was reduced by choosing an isophthaloyl
spacer between the heterocyclic units which effectively
participates in hydrogen bond formation with tartaric acid.
Also the 1,4-phenylenediacetyl spacer in receptor 4 is found
to be inactive in binding tartaric acid. The complexation and
solubilisation of l-51)-tartaric acid in chloroform were
1
studied by H NMR, UV and ¯uorescence experiments
with receptors 1, 2, 3 and 4. Receptors 2 and 3 in this regard
were only found to be effective to solubilise tartaric acid in
chloroform. Receptors 1 and 4 were totally non-interacting
showing an almost negligible shift of the amide protons,
perhaps due to steric mis-match. The addition of powdered
2. Results and discussion
Our previous report comparing two point and three point

S. Goswami et al. / Tetrahedron 57 ꢀ2001) 4987±4993
4989
1
Figure 1. H NMR spectrum of receptor 2.
l-51)-tartaric acid into CDCl3 solution of receptor 2
51.00£1022 M) led to its facile dissolution, as evidenced
by the appearance of the methine protons of tartaric acid
at d 5.00 ppm 5Figs. 1 and 2) and a signi®cantdown®eld
shiftof hte differentamide prootns of
2 5from d 8.86 to
10.81, Ddˆ1.95 ppm and d 9.33 to 10.96, Ddˆ1.63 ppm
of pyridine and naphthyridine amide protons, respectively)
in complex A.
The other receptor 321 is also effective in forming a 1:1
complex with l-51)-tartaric acid as suggested by the proton
integration ratio of the complex in H NMR. This is also
1
proved by the appearance of the methine protons of tartaric
acid at d 5.30 ppm in a CDCl3 solution of receptor 3 and a
signi®cant down®eld shift of the amide protons of the recep-
tor 3 5from d 9.44 to 10.70, Ddˆ1.26 ppm). This may pos-
sibly be due to the formation of a tight complex with tartaric
acid 5complex B) either by the formation of higher order
1
Figure 2. H NMR spectrum of complex A.

4990
S. Goswami et al. / Tetrahedron 57 ꢀ2001) 4987±4993
hydrogen bonds or long range electrostatic interactions with
all four hydrogens of carboxyl as well as of the hydroxyl
groups of tartaric acid. Effective association of tartaric acid
with both 2 and 3 was suggested from the fact that dilution
of each 1:1 complex in CDCl3 showed negligible change in
NMR spectrum.
The binding constants were evaluated from UV titration
experiments at lmax 352 nm.32 Fig. 3a and b show the effect
of dilution on the UV spectra of the 1:1 complexes with 2
and 3, respectively, at 352 nm. For the case of 1:1 complex
with 2 in Fig. 3a, on dilution a blue shift of the maximum at
l
max 249 nm occurs, while a slightred shiftof both maxima
was observed at lmax 310 and 352 nm. Similarly in the case
of 1:1 complex in Fig. 3b, on dilution a slight red shift of the
maxima of both the peaks at lmax 250 and 352 nm is
observed. This change in UV±vis spectrum could be used
to conveniently study the binding, since the lower concen-
trations used lead to a more accurate determination of the
value of the association constant for tartaric acid. The
absorbance effects at the corresponding peak positions are
shown graphically in Fig. 4a and b. Receptor 3 shows a
moderate binding constant with tartaric acid 5Table 1)
which is four times greater than 2. The UV titration experi-
ments using malic and succinic acids as guests with
receptors 2 and 3 were also carried out to get further indi-
cation of the mode of binding. The relatively lower values of
Figure 3. 5a) UV spectra of complex A and its change of absorbance on
dilution, 5b) UV spectra of complex B and its change of absorbance on
dilution.
Figure 4. 5a) Plot of absorbance vs concentration of the complex A, 5b) plot of absorbance vs concentration of the complex B.

S. Goswami et al. / Tetrahedron 57 ꢀ2001) 4987±4993
Table 1. Ka values of l-51)-tartaric acid, dl-malic acid and succinic acid
measured by UV titration
4991
Acid
Receptor 2 Ka 5M21
)
Receptor 3 Ka 5M21
)
l-51)-Tartaric acid
rac-Malic acid
Succinic acid
1.6£104
1.3£103
1.0£103
7.0£104
2.7£103
1.3£103
Ka of tartaric acid with 2 compared to 3 may be due to
the smaller number of hydrogen bonds formed with 2 com-
pared to 3 in the 1:1 complexes. The successively lower
values of Ka of the acids down the series with 3 may also
be due to their lower number of hydrogen bonds during
complexation.
This experiment suggests the probable mode of binding
could be as shown in complex B. Receptors 2 and 3 bind
weakly with l-51)-dibenzoyltartaric acid 5Kaˆ3.10£102
and 1.5£102 M21, respectively)33 compared to l-51)-tar-
taric acid which may be due to the greater steric hindrance
in the cavity of 2 and 3 to accommodate the bulky diben-
zoyltartaric acid. The binding and solubilisation of tartaric
acid in chloroform by receptors 2 and 3 are also corrobo-
rated from the enhancement of ¯uorescence intensity on
complexation 5Fig. 5). Receptors 2 and 3 were dissolved
individually in chloroform and then tartaric acid was
added to make the stoichiometry 1:1 in each case. Excitation
at 345 nm in both cases gave rise to an emission at 398 nm.
On complexation with tartaric acid, the ¯uorescence
emission of 2 and 3 were again observed; however, they
showed Dl of 15.5 nm 5397.50±382.00 nm) and 9.0 nm
5394.50±385.50 nm) with the increase in intensity, respec-
tively. This blue shift in ¯uorescence wavelength indicates
the strong hydrogen bonding interactions of 2 and 3 with
tartaric acid.
Figure 6. Ball and Stick representation of the complex B.
direct impact on the development of ¯uorescent receptors
for analytical use to detect insoluble substrates of biological
signi®cance.
3. Experimental
3.1. General
Melting points 5mp) were recorded on a Toshniwal hot-coil
stage melting point apparatus and are uncorrected. 1H NMR
spectra were recorded on a 200 MHz 5Bruker) spectrometer.
CDCl3 was used as solvent unless otherwise mentioned. 13
C
NMR spectra were recorded in 50 MHz Bruker machine as a
solution in CDCl3. IR spectra were recorded for receptors
1±3 on Perkin±Elmer 883 and that was recorded for recep-
tor 4 on FTIR-8300 SHIMADZU instrument. Fluorescence
studies were carried out on Perkin±Elmer LS 50B and
Perkin±Elmer MP F44B machines. UV spectra were
recorded using JASCO 7850.
The energy minimisation of receptor 3 with tartaric acid
5complex B) was performed to visualise the binding inter-
action between the host and guest by using pcmodel
5Serena software) with standard constant and Emin of the
1:1 complex was found to be 35.439 kcal/mol 5Fig. 6).
All solvents were dried prior to use. Dichloromethane and
triethylamine were distilled from calcium hydride. All
reactions were carried out under a nitrogen atmosphere.
For preparative thin layer chromatography 5PTLC) puri®-
cation, the layer was formed over a glass plate using water
gel-GF254 silica gel.
We have thus devised naphthyridine based ¯uorescent
probes 2 and 3 for the solubilisation and detection of insol-
uble tartaric acid, a substrate of biological signi®cance, in
non-competitive organic solvent. This approach can have a
3.2. Fluorescence study
Receptor 2 50.5 mg, 2.099£1024 M) was dissolved in 6 mL
dry chloroform and ¯uorescence spectrum was recorded
exciting at 345 nm wavelength. 3 mL of the solution
was taken and to this excess tartaric acid was added and
mixed thoroughly, and again the ¯uorescence spectrum
was recorded. Receptor 3 50.4 mg, 1.488£1024 M) was
dissolved in 6 mL dry chloroform and the experiment was
performed as for receptor 2.
3.3. UV binding study
Receptor 2 51.5 mg, 7.5567£1025 M) was dissolved in
50 mL dry chloroform. 25 mL of this stock solution was
taken and to this, excess dicarboxylic acid was added and
Figure 5. Fluorescence emission spectra of 5A) receptor 2, 5B) complex A,
5C) receptor 3, 5D) complex B.

4992
S. Goswami et al. / Tetrahedron 57 ꢀ2001) 4987±4993
sonicated for 10 min for complexation. The solution of the
complex was quickly ®ltered to remove uncomplexed dicar-
boxylic acid. The UV spectrum was recorded for uncom-
plexed receptor and then for the complex. Then the solution
of complex was taken in ®ve different volumetric ¯asks
510 mL) containing 7, 5, 3, 2 and 1 mL of the solution of
complex, respectively, and the volume was made up to
10 mL with dry chloroform in each case. The UV spectrum
was recorded for each solution and the binding constant was
calculated according to Colquhoun et al.32 Receptor 3
51.3 mg, 5.8036£1025 M) was dissolved in 50 mL dry
chloroform and the experiment was performed as in the
case of receptor 2.
5H), 7.32 5d, 2H, Jˆ8 Hz, naphthyridine-6H, pyr-3H),
6.96 5d, 1H, Jˆ8 Hz, pyr-5H), 2.78 5s, 3H, naphthyridine-
CH3), 2.50 5s, 3H, pyr-CH3). IR 5KBr) nmax: 3367, 2921,
1675, 1593, 1448, 1301, 1135 cm21. Anal. calcd for
C23H19N5O2: C, 69.51: H, 4.82: N, 17.62. Found C, 69.17:
H, 4.63: N, 17.39.
3.3.3.
N,N0-Bis-*7-methyl-[1,8]naphthyridin-2-yl)-iso-
phthalamide 3. The compound was made by coupling of
2-amino-7-methylnaphthyridine 5200 mg, 1.24 mmol) with
isophthaloyl chloride 5130 mg, 0.62 mmol) in presence of
triethylamine 50.8 mL) in dry dichloromethane 540 mL) by
the above procedure. The compound was isolated as a white
solid, yield 64%, mp 130±1328C. 1H NMR 5200 MHz,
CDCl3) d 9.44 5bs, 2H, NH), 8.78 5s, 1H, iso-2H), 8.54 5d,
2H, Jˆ8 Hz, iso-4H and 6H), 8.17 5d, 2H, Jˆ8 Hz,
naphthyridine-3H), 8.14 5d, 2H, Jˆ8 Hz, naphthyridine-
4H), 8.01 5d, 2H, Jˆ8 Hz, naphthyridine-5H), 7.67 5t, 1H,
Jˆ8 Hz, iso-5H), 7.28 5d, 2H, Jˆ6.5 Hz, naphthyridine-
6H), 2.7 5s, 6H, naphthyridine-CH3). 13C NMR 550 MHz,
CDCl3) d: 165.5, 162.3, 154.0, 153.7, 138.4, 135.9, 133.4,
131.7, 128.6, 126.2, 121.0, 117.9, 114.3, 25.0. IR 5KBr)
3.3.1. N-{6-[2-*7-Methyl-[1,8]naphthyridin-2-yl)-ethyl]-
pyridin-2-yl}-acetamide 1. To a stirred solution of 2,7-
dimethyl-1, 8-naphthyridine 5100 mg, 0.63 mmol) in dry
THF at 2788C, n-BuLi 50.2 mL, 1.5 mmol) was added
dropwise under a nitrogen atmosphere. Then the solution
was stirred at 2788C for 0.5 h until red colour persists.
After that a solution of 2-N-acetylamino-6-bromomethyl-
pyridine 5288 mg, 1.26 mmol) in THF was added dropwise.
The red colour gradually disappeared and ®nally reached to
yellowish colour. The solventwas evaporated in vacuo and
water was added to the mixture. The aqueous layer was
extracted with chloroform, dried over anhydrous sodium
sulphate. After evapoartion of the solvent, the crude product
was puri®ed by PTLC using 20% ethylacetate in chloroform
to afford the title compound as a white solid 50.08 mg, 42%),
mp 1458C. 1H NMR 5200 MHz, CDCl3) d: 8.30 5s, 1H, NH),
8.00 5d of d, 2H, Jˆ8, 2 Hz, naphthyridine-4H and -5H),
7.57 5t, 1H, Jˆ8 Hz, pyr-4H), 7.34±7.23 5m, 3H, pyr-3H,
naphthyridine-3H and -6H), 6.91 5d, 1H, Jˆ8 Hz, pyr-5H),
3.42±3.31 5m, 4H, ±CH2±CH2±), 2.79 5s, 3H, naphthyri-
dine-CH3), 2.20 5s, 3H, CH3±CO±). 13C NMR 550 MHz,
CDCl3) d: 168.6, 164.9, 162.8, 159.3, 155.5, 150.7, 149.8,
138.7, 136.6, 122.3, 121.7, 119.0, 116.3, 111.2, 38.4, 37.0,
25.4, 24.6. IR 5KBr) nmax: 3265, 3058, 2927, 2857, 2354,
1679, 1604, 1538, 1450 cm21. MS 5EI, m/e) 307 5M1), 117
5100%), 59. Anal. calcd for C18H18N4O: C, 70.59: H, 5.88:
N, 18.30. Found C, 70.44: H, 5.91: N, 18.19.
n
max: 3388, 1678, 1598, 1506, 1410, 1300 cm21. MS 5EI,
m/e) 448 5M1, 21%), 186 5100%), 160 5100%). Anal. calcd
for C26H20N6O2: C, 69.64: H, 4.46: N, 18.75. Found C,
69.68: H, 4.38: N, 18.65.
3.3.4. N-[6-*2-{4-[*6-Butyrylamino-pyridin-2-ylcarba-
moyl)-methyl]-phenyl}-acetylamino)-pyridin-2-yl]-butyr-
amide 4. To a stirred solution of 1,4-phenylenediacetic acid
5200 mg, 1.02 mmol) in dry dichloromethane 510 mL),
oxalyl chloride 50.89 mL) and one drop of dry DMF was
added and stirred under a nitrogen atmosphere for 3 h. The
volatiles were evaporated and dried. The acid chloride was
taken in a round-bottomed ¯ask and dissolved in 15 mL dry
dichloromethane and to this the solution of 2,6-diamino-
pyridine 5280 mg, 2.5 mmol) and triethylamine 50.27 mL,
2 equiv.) in dry dichloromethane 525 mL) were added drop-
wise under a nitrogen atmosphere and stirred overnight. The
volatiles were evaporated and the solid were dissolved in
dichloromethane and washed with 5% sodium bicarbonate
solution. The solvent was evaporated in a rotavapor to yield
a white solid 5280 mg, 72%). To a solution of the corre-
sponding amine 5110 mg, 0.29 mmol) and triethylamine
50.081 mL, 3 equiv.) in dry dichloromethane, butyryl
chloride 50.091 mL, 3 equiv.) was added dropwise and
stirred overnight under nitrogen. After evaporation of the
volatiles the solid was dissolved in dichloromethane and
washed with 5% sodium bicarbonate solution. The solvent
was evaporated to yield the desired product 4 as a white
solid 5130 mg, 86%), mp 2048C. 1H NMR 5200 MHz,
CDCl3) d 7.90 5d, 2H, Jˆ3.2 Hz, pyr-3H), 7.86 5d, 2H,
Jˆ2.8 Hz, pyr-5H), 7.68 5t, 2H, Jˆ8.1 Hz, pyr-4H), 7.60
5bs, 4H, NH), 7.35 5s, 4H, phenyl 2H, 3H, 6H and 5H),
3.73 5s, 4H, phenyl-CH2), 2.30 5t, 4H, Jˆ7.4 Hz,
±CH2CH2CH3), 1.77±1.63 5m, 4H, ±CH2CH3), 0.97 5t,
6H, Jˆ7.4 Hz, ±CH3). IR 5KBr) nmax: 3309.6, 2958.6,
2925.8, 1670.2, 1587.3, 1523.7, 1467.7, 1452.3, 1294.1,
1242.1, 1193.9, 802.3 cm21. Anal. calcd for C28H32N6O4:
C, 65.10: H, 6.24: N, 16.27. Found C, 64.76: H, 6.11: N,
16.10. 1H NMR of complex A 5200 MHz, CDCl3) d
10.96 51H, bs, NH), 10.81 51H, bs, NH), 8.71 51H, bs,
iso-2H), 8.67 51H, d, Jˆ8 Hz, iso-4H), 8.35 53H, m,
3.3.2. N-*7-Methyl-[1,8]naphthyridin-2-yl)-N0-*6-methyl-
pyridin-2-yl)-isophthalamide 2. A solution of 2-amino-7-
methylnaphthyridine 5100 mg, 0.62 mmol) and triethyl-
amine 50.08 mL) in dry dichloromethane 520 mL) and a
solution of 2-amino-6-methylpyridine 570 mg, 0.62 mmol)
and triethylamine 50.08 mL) in dry dichloromethane
520 mL) were simultaneously added dropwise separately
from two dropping funnels to isophthaloyl chloride
5130 mg, 0.62 mmol) in dry dichloromethane 515 mL)
under a nitrogen atmosphere during a period of 3 h. The
whole mixture was then washed with saturated sodium
bicarbonate solution followed by water. The separated
organic layer was dried over sodium sulfate. Volatiles
were removed under vacuum and the brown solid thus
obtained was puri®ed by column chromatography using
5% MeOH in CHCl3, giving the desired product as white
solid 5120 mg, 50%), mp 1268C. 1H NMR 5200 MHz,
CDCl3) d: 9.33 5s, 1H, NH), 8.86 5s, 1H, NH), 8.65 5s,
2H, iso-2H and -4H or -6H), 8.25±8.16 5m, 3H, naphthyri-
dine 3H and 4H, iso-4H or -6H), 8.06 5d, 1H, Jˆ8 Hz,
naphthyridine-5H), 7.68 5t, 2H, Jˆ8 Hz, pyr-4H and iso-

S. Goswami et al. / Tetrahedron 57 ꢀ2001) 4987±4993
4993
Â
12. Galan, A.; Andreu, D.; Echavarren, M. A.; Prados, P.;
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naphthyridine-3H and -4H, iso-6H), 8.07 51H, d, Jˆ8 Hz,
naphthyridine-5H), 7.79 51H, t, Jˆ8 Hz, pyr-4H), 7.67 51H,
t, Jˆ8 Hz, iso-5H), 7.32 52H, d, Jˆ8 Hz, naphthyridine-6H
and pyr-3H), 6.98 51H, d, Jˆ8 Hz, pyr-5H), 5.00 52H, s,
methine protons of tartaric acid), 2.87 5s, 3H, naphthyri-
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1
dine-CH3), 2.53 5s, 3H, pyr-CH3). H NMR of complex B
5200 MHz, CDCl3) d 10.70 52H, bs, NH), 8.93 51H, s, iso-
2H), 8.72 52H, d, Jˆ8 Hz, iso-4H and -6H), 8.28 54H, m,
naphthyridine-3H and -4H), 8.10 52H, d, Jˆ8 Hz,
naphthyridine-5H), 7.70 51H, t, Jˆ8 Hz, iso-5H), 7.64
52H, d, Jˆ8 Hz, naphthyridine-6H), 5.30 5s, 2H, methine
protons of tartaric acid), 2.83 5s, 6H, naphthyridine-CH3).
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Acknowledgements
19. Goswami, S.; Ghosh, K. Tetrahedron Lett. 1997, 38, 4503±
4506.
R. M. thanks CSIR for providing her with a senior research
fellowship and we thank Dr S. Dasgupta, IIT Kharagpur for
¯uorescence measurements. We are also thankful to IUPAC
for providing free AutoNom software for nomenclature.
20. Goswami, S.; Ghosh, K.; Dasgupta, S. J. Org. Chem. 2000, 65,
1907±1914.
21. Goswami, S.; Mukherjee, R. Tetrahedron Lett. 1997, 38,
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