Calix[4]quinone. Part 2: Intramolecular Michael-addition of calix[4]diquinone

Article abstract of DOI:10.1016/S0040-4020(01)00788-8

The oxidation of calix[4]arene dibenzoate 1 with chlorine dioxide yielded the corresponding calix[4]diquinone 2 and an intramolecular Michael-addition product 3. Reaction of diquinone 2 with ethylene glycol under acidic conditions produced the half-protected ketal derivative 4. The removal of benzoate moieties from compound 4 in basic conditions produced a phenoxide anion, which underwent intramolecular Michael-addition and yielded product 5. In acidic ketal cleavage conditions, the ketal moieties of product 5 were removed, but the intramolecular Michael-addition structure was maintained in the product 6.

Full text of DOI:10.1016/S0040-4020(01)00788-8

TETRAHEDRON
Pergamon
Tetrahedron 57 G2001) 8101±8105
Calix[4]quinone. Part 2: Intramolecular Michael-addition
of calix[4]diquinone
Ker-Ming Yang, Ming-Dar Lee, Rong-Fua Chen, Yi-Lin Chen and Lee-Gin Linp
Institute of Applied Chemistry, Chinese Culture University, Taipei 11114, Taiwan, ROC
Received 2 May 2001; revised 5 July 2001; accepted 26 July 2001
AbstractÐThe oxidation of calix[4]arene dibenzoate 1 with chlorine dioxide yielded the corresponding calix[4]diquinone 2 and an
intramolecular Michael-addition product 3. Reaction of diquinone 2 with ethylene glycol under acidic conditions produced the half-protected
ketalderivative 4. The removalof benzoate moieties from compound 4 in basic conditions produced a phenoxide anion, which underwent
intramolecular Michael-addition and yielded product 5. In acidic ketalcleavage conditions, the ketalmoieties of product 5 were removed, but
the intramolecular Michael-addition structure was maintained in the product 6. q 2001 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
2.1. Calix[4]arene dibenzoate 1
Calix[4]quinone, which is a sort of calix[4]arene comprising
cyclic arrays of p-quinone and methylene residues, was ®rst
synthesized by a direct oxidation of calix[4]arene with
chlorine dioxide in 1986.1 However, the exact structure of
calix[4]quinone which was con®rmed by X-ray crystallo-
graphy was reported later by Morita and co-workers from
a very different multi-step synthetic approach.2b Since then,
severalpapers on this new cailxarene system had been
published,2±4 and among these reports, the p-hydroxy-
calix[4]arene and calix[4]quinone pair have been proposed
Although, the calix[4]arene 1,3-dibenzoate 1 could be
prepared according to the literature procedure,6 undesired
side products were frequently observed during the prepara-
tion. It was later shown that the majority of the side product
was calix[4]arene tetrabenzoate with a `1,3-alternate'
conformation,7 and this tetrabenzoate became the only
product when a large excess of NaH was used in the
synthesis. Further investigations indicated that the
reaction conditions for the calix[4]arene 1,3-dialkyl ethers8
were also suitable for the preparation of the calix[4]arene
dibenzoate 1, in which the calix[4]arene was re¯uxed with
large excess of benzoyl chloride in CH3CN in the presence
of K2CO3 to afford calix[4]arene dibenzoate 1 in over 80%
yield.
4
as the enzyme redox modeland/or electron transfer system.
In our previous work,5 we have established the use of
chlorine dioxide to introduce an oxygen atom at the
calixarenes para-position, and the synthesis of p-mono-
hydroxycalix[4]arene was achieved in a subsequent route.
It was anticipated that the calix[4]diquinone molecule
would also behave similar to its calix[4]monoquinone
counterpart, and that the corresponding p-dihydroxy-
calix[4]arene could be achieved accordingly. However, we
were surprised to ®nd that the calix[4]diquinone derivatives
behaved quite differently from calix[4]monoquinone, and a
totally different set of calix[4]arene derivatives was
produced. In this paper, we report on the intramolecular
Michael-addition behavior of the calix[4]diquinone
molecular system.
2.2. Calix[4]diquinone dibenzoate 2
When calix[4]arene dibenzoate 1 was oxidized with
chlorine dioxide in phosphate-buffered solution at room
temperature, a procedure described by L. Rosik,1 a pale
orange minor product 3 was obtained along with the bright
yellow calix[4]diquinone 2 GFig. 1). Calix[4]diquinone 2
was puri®ed from the reaction mixture by a simple
recrystallization, but the isolation of product 3 required
column chromatography. The structure of calix[4]diquinone
2 was accurately assigned based on all available spectral
data, whereas, the structuralassignment of compound
presented some uncertainty.
3
The FAB-mass spectraldata indicated that compound 3 has
a molecular weight of 678 a.m.u, which was 18 a.m.u more
Keywords: calix[4]quinones; chlorine dioxide oxidation; Michael-addition.
Corresponding author; e-mail: lglin@faculty.pccu.edu.tw
1
then the expected calix[4]diquinone's mass; and the H
p
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020G01)00788-8

8102
K.-M. Yang et al. / Tetrahedron 57 &2001) 8101±8105
Figure 1.
NMR spectra indicated that 30 hydrogens were present in
compound 3, which was 2 hydrogens more than the
expected calix[4]diquinone. These data indicated that the
structure of compound 3 was equivalent to adding one
water molecule onto calix[4]diquinone 2 in some fashion.
Severalpossibiilties existed for the structure of product 3,
but most of the proposed structures were shown to be
incorrect when D2O treated NMR sample indicated that
only one single hydrogen was exchanged with deuterium.
arene.9 They also suggested, based on Dreiding models
studies, that the distance is too long to allow the formation
of a transannular bond between phenolic oxygen atom and
9a
the meta-position of the distalrings. Therefore, the distal
transannular ether linkage in product 3 was assigned to link
between the `upper rim' oxygen atom and the meta-position
of the distalring. However, the exact structure for product 3
still required further evidence, e.g. a single crystal X-ray
crystallography, to reveal the exact location of the hemiketal
moieties.
1
With the availability of the two-dimension H NMR spec-
trum, we were able to deduce every hydrogen signal and
proposed a hemiketalstructure with distalGi.e. 1,3-realted)
transannular ether linkage for product 3. Biali et al. reported
a similar structure with proximal G1,2-related) transannular
ether linkage in the oxidative studied of p-tert-butylcalix[4]-
Even though, the structure of compound 3 seemed to be the
result of addition of one water molecule onto diquinone 2,
the hydration studies indicated that diquinone 2 did not
convert to compound 3 in acidic conditions. It was also
Figure 2.

K.-M. Yang et al. / Tetrahedron 57 &2001) 8101±8105
8103
noticed that the hemiketalstructure of compound 3 was
exceptionally stable, as an attempt to cleave the hemiketal
moieties by re¯uxing compound 3 with 6N HClfor 96 h
produced no trace of diquinone 2 and/or any degraded
compounds. These results implied that instead of adding
one water molecule onto diquinone 2 in an acidic oxidation
reaction, compounds 2 and 3 were both produced simul-
taneously in chlorine dioxide oxidation via a different
mechanism.
moieties, added to the nearby conjugate enone system in
1,4-addition fashion forming a Michael-addition product
5. The intramolecular 1,4-addition that occurred would
hinder the free rotation, and the product 5 was expected to
possess chirality similar to the spiro products reported by
Biali.9 This unique structure of intramolecular Michael-
addition product 5 was established with two-dimension H
NMR spectrum and all the hydrogen signals were assigned
accurately.
1
During the preparation of diquinone 2, the oxidation was
attempted in one instance without the buffer solution.
Although the oxidation reaction proceeded as with the
buffer solution conditions, the amount of compound 3 was
increased and the diquinone 2 was no longer able to be
isolated directly from the reaction mixture by a simple
recrystallization. The separation of this mixture by column
chromatography resulted in 32% of diquinone 2 and 19% of
compound 3. The amount of compound 3 was largely
increased from less than 1% to 19% in this reaction. This
result turned our attention to search for better reaction
conditions for the chlorine dioxide oxidation, and prelimin-
ary results indicated that a pH 9 buffer system gave a better
overall yield of both oxidative products 2 and 3. A detailed
study on the effect of pH on the chlorine dioxide oxidation is
already underway in our laboratory.
The ketalmoieties in monoketal 5 were very stable and can
only be cleaved by re¯uxing with strong acid, but the intra-
molecular phenyl±cyclohexyl ether linkage was retained in
hydrolysis conditions. We were surprised to notice that the
hydrolysis product 6, which possessed an 1,4-diketone
structure did not undergo double keto-enol tautomerization
to give the stable aromatic hydroquinone structure in acidic
conditions. We believe that the rigid structure of compound
6 is one of the reasons for its failure to undergo keto-enol
tautomerization, and a simple molecular structure model
does indicate the in¯exible structure of compound 6.
3. Experimental
3.1. General
2.3. Calix[4]diquinone dibenzoate monoketal 4
All reagents were obtained from Commercial Chemical
Companies and used without further puri®cation. Melting
points were taken in capillary tubes on a Mel-Temp
apparatus GLaboratory Devices, Cambridge, MA) and were
As in case of calix[4]monoquinone tribenzoate,5 the ketali-
zation of calix[4]diquinone dibenzoate 2 also produced only
the corresponding half-protected ketal derivative 4 GFig. 2).
One may speculate that steric hindrance of the diquinone 2
is much smaller as compared with the previous mono-
quinone case, and hence, the failure of the formation of
the fully protected quinones may not arise strictly from
the benzoate moieties. We suggest that the molecular struc-
ture of calix[4]arene contains a smaller opening on the
`lower rim', and steric hindrance from the calix[4]arene
itself may prevent the formation of the `lower rim' cyclic
ketalstructure. The position of the masked carbonyl, on this
assumption, was assigned to the less hindered `upper rim'.
1
uncorrected. H NMR spectra were recorded on Burker
DMX-300 WB and/or Burker DMX-500 SB spectrometer
and chemicalshifts were reported as d values in ppm
relative to TMS Gdˆ0.00) as an internalstandard. IR spectra
were recorded on Perkin±Elmer Paragon 1000 FT-IR
Spectrometer, FAB-MS spectra were taken on a JOEL
JMS-HX 102 spectrometer, and elemental analyses were
taken on a Perkin±Elmer 248C analyzer. Chromatographic
separations were performed with Merck silica gel G230±400
mesh ASTM) on columns of 25 mm diameter ®lled to height
of 150 mm. TLC analyses were carried out on Merck
aluminum back silica gel 60 F254 plates Gabsorbant thickness
0.2 mm).
2.4. Intramolecular Micheal-addition products 5 and 6
It was anticipated, as in previous cases,5 that the benzoate
moieties of dibenzoate 4 should be easily removed and
produce the corresponding calix[4]diquinone diketal
product. It was also expected that the resultant product,
which possessed a C2v symmetry would display a simple
1H NMR spectrum with one exchangeable hydrogen signal.
To our surprise, the basic hydrolysis product 5 displayed a
very complex but well-de®ned 1H NMR spectrum with a set
of three doublets and one unusually up®eld double doublet
for the calix[4]arene's methylene hydrogens. We were more
surprised to notice that no hydrogen signaldisappeared
when the NMR sample was treated with D2O. This informa-
tion not only indicated that the phenolic hydroxy hydrogen
was non-existant but also implied that a proton was adjacent
to calixarene's methylene bridge in the structure of hydro-
lysis product 5. The only rational explanation for such
phenomenon was that the phenoxide anion, which was
produced in basic hydrolysis of the phenyl benzoate
3.1.1. 25,27-Dibenzoyloxy-26,28-dihydroxycalix[4]arene
01). A solution of 2.12 g G5.00 mmol) of calix[4]arene,
0.69 g G5.00 mmol) of K2CO3, and 1.15 mL G1.40 g,
10.0 mmol) of benzoyl chloride in 100 mL of CH3CN was
re¯uxed for 10 h. The solvent was removed, and the residue
was treated with water to give a white solid. The solid
material was collected and recrystallized from CHCl3 and
CH3OH to give 2.47 g G78%) of colorless crystals: mp 262±
2648C Glit.6 mp 262±2648C). The spectralproperties of this
6
product was identicalto the ilterature reported.
3.1.2. 25,27-Dibenzoyloxy-26,28-calix[4]diquinone 02)
and intramolecular Michael-addition product 3. Method
A: with pH 7 buffer solution. A slurry of 3.35 g G5.30 mmol)
of 1 was dissolved in 150 mL of acetone, and 50 mL of
concentrated buffer solution10 was added. A portion of
115 mL of yellowish ClO2 aqueous solution11 was then
added, and the reaction mixture was stirred at room

8104
K.-M. Yang et al. / Tetrahedron 57 &2001) 8101±8105
temperature for 48 h. The pale yellow solid material was
collected and recrystallized three times from CHCl3 and
CH3OH to give 1.52 g G47%) of 2 as an yellow crystals:
mp 298±2998C; 1H NMR GCDCl3) d 8.24 Gbs, 4H,
o-Ar0H), 7.74±7.77 Gm, 2H, p-Ar0H), 7.57±7.60 Gm, 4H,
m-Ar0H), 6.92±7.07 Gm, 6H, ArH), 6.34 Gbs, 4H, quinone-
H), 3.47 Gbs, 8H, ArCH2Ar); IR GKBr) cm21: 1732 Gs,
CvO), 1655 Gs, CvO); FAB-MS m/z: 661 GM111), 662
GM112). Anal.12 Calcd for C42H28O8: C, 76.36; H, 4.27.
Calcd for C42H28O8´1/2CHCl3: C, 70.86; H, 3.99. Found:
C, 70.88; H, 3.90.
from CHCl3 and CH3OH, weighing 0.27 g G8%) was the
yellow compound 2. The second eluted product, after
recrystallization from CHCl3 and CH3OH, weighing
0.11 g G3%) was the yellow product 3.
3.1.3. 25,27-Dibenzoyloxy-26,28-calix[4]diquinone-5,17-
bis0ethylene ketal) 04). A slurry of 0.99 g G1.50 mmol) of
2, 5.6 mL G6.20 g, 100 mmol) of ethylene glycol, and 0.10 g
of p-TsOH in 70 mL of benzene was equipped with a Dean±
Stark water trap and re¯uxed for 48 h. The solvent was
removed, and the residue was treated with water to leave
a pale yellow solid. The solid material was collected and
recrystallized from CHCl3 and CH3OH to afford 0.81 g
The mother liquor was concentrated and subjected to
column chromatography separation with CHCl3 as an
eluent. The ®rst eluted component, after recrystallization
from CHCl3 and CH3OH, weighing 0.15 g G4%) was the
yellow compound 2. The second eluted product, after
recrystallization from CHCl3 and CH3OH, weighing
0.02 g G0.5%) was the yellow product 3; mp 296±2978C;
1H NMR GCDCl3) d 8.36 Gd, Jˆ7.5 Hz, 2H, o-Ar0H), 8.32
Gd, Jˆ7.5 Hz, 2H, o-Ar0H), 7.75 Gt, Jˆ7.5 Hz, 1H, p-Ar0H),
7.72 Gt, Jˆ7.5 Hz, 1H, p-Ar0H), 7.61 Gt, Jˆ7.5 Hz, 2H,
m-Ar0H), 7.58 Gt, Jˆ7.5 Hz, 2H, m-Ar0H), 7.19 Gbd,
Jˆ7.0 Hz, 1H, m-ArH), 7.08 Gbd, Jˆ7.5 Hz, 1H, m-ArH),
6.92 Gt, Jˆ7.5 Hz, 1H, p-ArH), 6.87±6.91 Gm, 2H, m-ArH),
6.78 Gt, Jˆ7.7 Hz, 1H, p-ArH), 6.41±6.43 Gm, 2H, quinone-
H and COCH), 6.25 Gs, 2H, quinone-H), 3.88±3.89 Gm, 1H,
COCH), 3.58±3.68 G3d, 3H, ArCH2Ar), 3.42 Gd, Jˆ13.7 Hz,
1H, ArCH2Ar), 3.34 Gd, Jˆ13.6 Hz, 1H, ArCH2Ar), 3.21 Gd,
Jˆ14.0 Hz, 1H, ArCH2Ar), 2.53 Gs, 1H, ArOH2), 2.51 Gd,
Jˆ14.0 Hz, 1H, ArCH2Ar), 1.99 Gd, Jˆ14.0 Hz, 1H,
ArCH2Ar); IR GKBr) cm21: 3503 Gs, OH), 1722 Gs, CvO),
1659 Gs, CvO); FAB-MS m/z: 678 GM1), 680 GM112).
Anal. Calcd for C42H30O9: C, 74.34; H, 4.42. Calcd for
C42H30O9´H2O: C, 72.41; H, 4.60. Found: C, 72.56; H, 4.38.
1
G72%) of light yellow crystals: mp 315±3178C; H NMR
GCDCl3) d 8.46 Gd, Jˆ7.6 Hz, 4H, o-Ar0H), 7.70 Gt,
Jˆ7.6 Hz, 2H, p-Ar0H), 7.57 Gt, Jˆ7.7 Hz, 4H, m-Ar0H),
7.09 Gbs, 4H, m-ArH), 6.86 Gm, 2H, p-ArH), 6.11 Gbs, 4H,
quinone-H), 3.88 Gbs, 8H, ketal-H), 3.44 Gbs, 4H, ArCH2Ar),
3.13 Gbs, 4H, ArCH2Ar); IR GKBr) cm21: 1732 Gs, CvO),
1655 Gs, CvO); FAB-MS m/z: 749 GM111). Anal. Calcd
for C46H36O10: C, 73.80; H, 4.81. Calcd for C46H36O10´1/
2H2O: C, 72.92; H, 4.89. Found: C, 72.63; H, 4.90.
3.1.4. Intramolecular Michael-addition product 5Ð
product of basic hydrolysis of compound 4. A slurry of
0.75 g G1.00 mmol) of 4 was dissolved in 40 mL of THF,
and a portion of 25 mL of ethanolic alkali solution G1.54 g of
NaOH dissolved in 7 mL of water and 18 mL of ethanol)
was added. The reaction mixture was re¯uxed for 20 h, and
was neutralized with diluted HCl. The solvent was removed
to leave an oily residue, and a large amount of water was
then added to induce an off white solid. The solid material
was collected and recrystallized from CHCl3 and CH3OH to
afford 0.33 g G61%) of pale yellow crystals: mp 325±3278C;
1H NMR GCDCl3) d 7.01 Gs, 2H, CvCH), 7.00 Gd,
Jˆ7.5 Hz, 2H, m-ArH), 6.69 Gd, Jˆ7.4 Hz, 2H, m-ArH),
6.52 Gt, Jˆ7.5 Hz, 2H, p-ArH), 4.36±4.37 Gm, 2H, OCH),
4.31 Gdd, Jˆ11.2, 6.1 Hz, 2H, ketal-H), 4.25 Gdd, Jˆ14.8,
7.7 Hz, 2H, ketal-H), 4.15 Gdd, Jˆ11.4, 6.4 Hz, 2H, ketal-
H), 3.99 Gdd, Jˆ14.0, 7.0 Hz, 2H, ketal-H), 3.38 Gd,
Jˆ12.8 Hz, 2H, ArCH2Ar), 3.31 Gd, Jˆ15.8 Hz, 2H,
ArCH2Ar), 3.25 Gd, Jˆ12.8 Hz, 2H, ArCH2Ar), 3.08±3.10
Gm, 2H, COCH), 3.22 Gdd, Jˆ15.9, 5.8 Hz, 2H, ArCH2Ar);
IR GKBr) cm21: 1673 Gs, CvO); FAB-MS m/z: 541
GM111). Anal. Calcd for C32H28O8: C, 71.11; H, 5.19.
Calcd for C32H28O8´1/2H2O: C, 69.94; H, 5.28. Found: C,
69.82; H, 5.25.
Method B: without buffer solution. A slurry of 3.34 g
G5.28 mmol) of 1 was dissolved in 150 mL of acetone, and
50 mL of water was added. A portion of 115 mL of yellow-
ish ClO2 aqueous solution was then added, and the reaction
mixture was stirred at room temperature for 48 h. The
organic solvent was removed, and a large amount of water
was added to generate a pale yellow solid. The solid
material was collected, dissolved in CHCl3, and subjected
to column chromatography separation with CHCl3 as an
eluent. The ®rst eluted component, after recrystallization
from CHCl3 and CH3OH, weighing 1.17 g G32.5%) was
the yellow compound 2. The second eluted product, after
recrystallization from CHCl3 and CH3OH, weighing 0.66 g
G19%) was the yellow product 3.
3.1.5. Intramolecular Michael-addition product 6Ð
product of acidic hydrolysis of compound 5. Method A:
p-TsOH as catalyst. A slurry of 0.43 g G0.80 mmol) of 5 and
0.04 g of p-TsOH in 200 mL of acetone was re¯uxed for 6 h.
The color of the reaction mixture was gradually change
from colorless to greenish yellow, and ®nally to brown.
The organic solvent was removed, and the residue was
treated with water to leave an yellowish solid. The solid
material was collected and recrystallized from CHCl3 and
n-hexane to afford 0.27 g G75%) of yellow crystals: mp over
4558C; 1H NMR GCDCl3) d 7.06 Gd, Jˆ7.4 Hz, 2H, m-ArH),
6.82 Gd, Jˆ7.5 Hz, 2H, m-ArH), 6.71 Gs, 2H, CvCH), 6.67
Gt, Jˆ7.5 Hz, 2H, p-ArH), 4.71±4.72 Gm, 2H, OCH), 3.52 Gd,
Jˆ12.8 Hz, 2H, ArCH2Ar), 3.36±3.40 Gm, 4H, ArCH2Ar),
3.18±3.20 Gm, 2H, COCH), 2.79 Gdd, Jˆ15.9, 5.6 Hz, 2H,
Method C: with pH 9 buffer solution. A slurry of 3.35 g
G5.30 mmol) of 1 was dissolved in 150 mL of acetone, and
50 mL of concentrated buffer solution13 was added. A
portion of 115 mL of yellowish ClO2 aqueous solution
was then added, and the reaction mixture was stirred at
room temperature for 60 h. The pale yellow solid material
was collected and recrystallized three times from CHCl3 and
CH3OH to give 1.88 g G54%) of 2 as an yellow crystals.
The mother liquor was concentrated and subjected to
column chromatography separation with CHCl3 as an
eluent. The ®rst eluted component, after recrystallization

K.-M. Yang et al. / Tetrahedron 57 &2001) 8101±8105
8105
ArCH2Ar); IR GKBr) cm21: 1686 Gs, CvO); FAB-MS m/z:
453 GM111). Anal. Calcd for C28H20O6: C, 74.34; H, 4.42.
Calcd for C28H20O6´CH3OH: C, 71.88; H, 5.00. Found: C,
71.85; H, 4.83.
4. Ga) Kim, N. Y.; Chang, S.-K. J. Org. Chem. 1998, 63, 2362.
Gb) Toth, K.; Lan, B. T. T.; Jeney, J.; Horvath, M.; Bitter, I.;
Grun, A.; Agai, B.; Toke, L. Talanta 1994, 41, 1041.
5. The ®rst part of calix[4]quinone series, Lee, M. D.; Yang,
K.-M.; Tsoo, C.-Y.; Shu, C.-M.; Lin, L.-G. Tetrahedron
2001, 57, 8095±8099.
Method B: H2SO4 as catalyst. A slurry of 0.20 g
G0.37 mmol) of 5 dissolved in 150 mL of CHCl3 and
50 mL of 60% of ethanol, and a portion of 15 mL of conc.
H2SO4 was then slowly added as a catalyst. The reaction
mixture was re¯uxed for 18 h., and the color of the reaction
mixture changed from colorless to a greenish yellow. The
reaction mixture was washed, sequentially with, water three
times, saturated NaHCO3 solution, and ®nally water. The
organic portion was separated and dried with MgSO4. The
®ltrate was concentrated and recrystallized from CHCl3 and
n-hexane to afford 0.14 g G84%) of yellow crystals.
6. Shu, C.-M.; Liu, W.-C.; Ku, M.-C.; Tang, F.-S.; Yeh, M.-L.;
Lin, L.-G. J. Org. Chem. 1994, 59, 3730.
7. Gutsche, C. D.; Lin, L.-G. Tetrahedron 1986, 42, 1633.
8. Van Loon, J.-D.; Arduini, A.; Coppi, L.; Verboom, W.;
Pochini, A.; Ungaro, R.; Harkema, S.; Reinhoudt, D. N.
J. Org. Chem. 1990, 55, 5639.
9. Ga) Litwak, A. M.; Grynszpan, F.; Aleksiuk, O.; Cohen, S.;
Biali, S. E. J. Org. Chem. 1993, 58, 393. Gb) Grynszpan, F.;
Aleksiuk, O.; Biali, S. E. J. Org. Chem. 1994, 59, 2070.
Gc) Aleksiuk, O.; Grynszpan, F.; Litwak, A. M.; Biali, S. E.
New. J. Chem. 1996, 20, 473. Gd) Van Gelder, J. M.; Brenn, J.;
Thondorf, I.; Biali, S. E. J. Org. Chem. 1997, 62, 3511.
10. Concentrated aqueous buffer solution was obtained by
dissolving 4.36 g of Na2HPO4´2H2O and 2.42 g of
NaH2PO4´2H2O in 100 mL of deionized water. A pH 7 buffer
solution was afforded, when this concentrated solution was
diluted ®ve times in volume.
Acknowledgements
We thank Ms S.-L. Huang of NSC InstrumentalCenter in
Taipei for taking all the NMR measurements. Financial
support of this work from the NationalScience Councilof
the Republic of China GGrant NSC-88-2113-M034-001) is
gratefully acknowledged.
11. Aqueous chlorine dioxide solution was prepared by mixing
equal volume of sodium chlorite solution GNaClO2´2H2O,
31.60 g, 0.25 molin 500 mL of deionized water) and sodium
persulfate solution GNa2S2O8, 29.70 g, 0.25 molin 500 mL of
deionized water). The solution was then stored in a brown
bottle at 08C prior being used.
References
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1986.
12. All the new compounds, which were submitted to Elemental
Analysis GEA), were dried at 1208C under vacuum for 48 h
prior to the analysis. If the analysis value was different from
the calculated value, the sample was dried at 1408C under
vacuum for 48 h prior to another analysis. The drying period
will be increased further, if the sample still received a different
EA value from the theoretical value. The procedure was
continued untila constant EA vaul e was attained.
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13. Concentrated aqueous buffer solution was obtained by
dissolving 2.86 g of Na2B4O7´10H2O and 2.76 mL of 1N
HClin 100 mL of deionized water. A pH 9 buffer soultion
was afforded, when this concentrated solution was diluted 5
times in volume.