Asymmetric reduction of acetophenone and propiophenone by NaAl(IPTOLate)H2 combined with enantiomeric enrichment of the reaction product as an inclusion complex with IPTOL

Article abstract of DOI:10.1023/A:1026117026241

An improved procedure was developed for asymmetric reduction of acetophenone and propiophenone by the chiral reagent NaAl(IPTOLate)H ₂. This procedure is based on isolation of the chiral alcohol that formed as a crystalline host - guest complex with the IPTOL ligand. The enantiomeric enrichment of the product was as high as 97% ee. The ability of IPTOL and its analogs to form host - guest complexes with a number of ether-type solvents, 1-phenylethanol, and 1-phenylpropan-1-ol as well as thermal stabilities of IPTOL-containing complexes with these alcohols were studied.

Full text of DOI:10.1023/A:1026117026241

Russian Chemical Bulletin, International Edition, Vol. 52, No. 8, pp. 1841—1846, August, 2003
1841
Asymmetric reduction of acetophenone and propiophenone
by NaAl(IPTOLate)H combined with enantiomeric enrichment
2
of the reaction product as an inclusion complex with IPTOL
M. G. Vinogradov,aꢀ L. S. Gorshkova, G. V. Chel´tsova, D. V. Kurilov, V. A. Ferapontov,
a
a
a
a
b
c
O. V. Shishkin, and G. L. Heise
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. Eꢀmail: ving@cacr.ioc.ac.ru
b
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
2
8 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5085
Cambrex Corporation, One Meadowlands Plaza,
c
East Rutherford, NJ 07073, USA
An improved procedure was developed for asymmetric reduction of acetophenone and
propiophenone by the chiral reagent NaAl(IPTOLate)H . This procedure is based on isolation
2
of the chiral alcohol that formed as a crystalline host—guest complex with the IPTOL ligand.
The enantiomeric enrichment of the product was as high as 97% ee. The ability of IPTOL and
its analogs to form host—guest complexes with a number of etherꢀtype solvents, 1ꢀphenylethanol,
and 1ꢀphenylpropanꢀ1ꢀol as well as thermal stabilities of IPTOLꢀcontaining complexes with
these alcohols were studied.
Key words: asymmetric reduction of alkyl phenyl ketones, enantiomeric enrichment of
alcohols, chiral host—guest complex, sodium aluminum hydride, α,α,α´,α´ꢀtetraarylꢀ1,3ꢀ
dioxolaneꢀ4,5ꢀdimethanol.
In recent years, a number of efficient catalytic proꢀ
cesses have been developed for asymmetric reduction of
prochiral C=C, C=O, and C=N bonds.1 At the same
time, it is often convenient to perform organic syntheses
—8
with the use of reagents containing a stoichiometric
amount of a chiral inducing agent,9
—11
particularly, if the
latter can be immobilized on the surface of a solid carrier
or recovered after completion of the reaction and thus
used repeatedly.1
1—13
Chiral aluminum hydride reagents find wide applicaꢀ
tion in asymmetric reduction of the carbonyl group. In
these reactions, BINOLꢀ and TADDOLꢀcontaining* reꢀ
1
4—16 16—18
19,20
agents 1,
2,
and 3
and their close structural
analogs 1
4—20
exhibit the highest enantioselectivity.
However, reduction of ketones by reagents 1 and 2 did
not give reproducible results due to low stability of comꢀ
pounds 1 and 2.1
4,17
In addition, high enantioselectivity
of reduction of ketones by reagent 2 was achieved only
*
BINOL is 1,1´ꢀbiꢀ2ꢀnaphthol; TADDOL is α,α,α´,α´ꢀtetraꢀ
arylꢀ1,3ꢀdioxolaneꢀ4,5ꢀdimethanol; IPTOL is 2,3ꢀOꢀisopropylꢀ
ideneꢀ1,1,4,4ꢀtetraphenylthreitol as their (R) and (S) or (R,R)
and (S,S) enantiomers.
with the use of freshly purified LiAlH (with the storage
time of no longer than a week).
4
1
7
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1746—1750, August, 2003.
066ꢀ5285/03/5208ꢀ1841 $25.00 © 2003 Plenum Publishing Corporation
1

1842
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
Vinogradov et al.
Unlike monohydride reagents 1 and 2, dihydride reꢀ
Scheme 1
agent 3a and its analogs prepared with the use of NaAlH4
give stable solutions in THF, DME, or diglyme and retain
their enantioselectivity even upon storage of solutions at
room temperature for one month. Reduction of ketones
by reagent 3a and its analogs 3b—h based on other
1
9,20
TADDOL ligands (4b—h) gave reproducible results.
From the aforesaid, it can be concluded that among presꢀ
ently known chiral aluminum hydrides, compounds 3 are
reducing reagents of choice for asymmetric reduction of
ketones.
It is known that the TADDOL ligands not only can
act as chiral inducing agents in the asymmetric syntheꢀ
1
8
sis but also can be used for the resolution of racemic
compounds into enantiomers through the formation of
2
1—23
crystalline molecular host—guest complexes (HGC).
Compound
2 R
Ar
R´
For example, cocrystallization with TADDOL can allow
a
b
c
d
2 Me
2 Et
(CH2)4
(CH2)5
Ph
Ph
Ph
Ph
Me
Et
—
one to resolve various alcohols, including 1ꢀarylalkanꢀ
ꢀols, into enantiomers.2
4—26
1
The fact that the latter are
produced upon asymmetric reduction of the correspondꢀ
ing ketones by TADDOLꢀcontaining aluminum hydride
reagents, makes it possible to increase the optical yields of
alcohols owing to their isolation as inclusion complexes
with TADDOL. A combination of asymmetric reduction
of ketone and enantiomeric enrichment (AREE) of the
alcohol that formed in one tandem process was carried
out for the first time1 with the use of hydride complex 2
—
e
f
Ph
—
—
2 Me
7
g
h
2 Me
2 Me
—
—
in the step of reduction of ketone. However, this apꢀ
proach has not gained further acceptance18 apparently
2ꢀNaphthyl
1
7
because of insufficient stability of solutions of 2.
The aim of the present study was to perform AREE
using reagents 3 and develop a convenient preparative
procedure for the synthesis of chiral alcohols from alkyl
phenyl ketones 5 based on this process (Scheme 1). To
accomplish this aim, the TADDOL ligands should satisfy
particular requirements. In addition to their ability to
stereoselectively form host—guest complexes with one of
the enantiomers of alcohols 6, the TADDOL ligands
should not form stable host—guest complexes with the
solvent used for performing AREE. Since reduction by
the complex are similar in geometry. The spiroꢀfused fiveꢀ
membered rings adopt an envelope conformation. The
C(5) and O(2) atoms deviate from the plane through the
remaining atoms of the ring by 0.60 Å (A), 0.62 Å (B) and
—
0.49 Å (A), –0.50 Å (B), respectively. The substituents
at C(6) and C(7) have an equatorial orientation (torsion
angles: O(1)—C(6)—C(7)—C(9), 144.2(5)° (A) and
1
43.6(4)° (B); C(1)—O(1)—C(6)—C(8), –128.2(5)° (A)
and –127.0(5)° (B)). In the complex, the diglyme molꢀ
1
9,20
reagents 3 is carried out in etherꢀtype solvents,
we
studied the ability of TADDOL to give host—guest comꢀ
plexes with a number of ethers.
Table 1. Ability of compounds 4a—h to form crystalline
host—guest complexes with etherꢀtype solvents
As can be seen from Table 1, the TADDOL ligands
most readily formed host—guest complexes with diglyme.
Solvent
na
By contrast, no host—guest complexes with Et O were
isolated. Tetrahydrofuran and DME are intermediate beꢀ
tween the two aboveꢀmentioned solvents.
According to the data from Xꢀray diffraction analysis
of the molecular complex of diol 4c with diglyme (DGM)
2
4
a
4b
4c
4d
4e
4f
4g 4h
b
THF
DME
Diglyme
0
0
0.5
0
0
0
1
0.5
0.5
1
1
0.5
1
0
0.5
0
0
0
0
—
c
—c
1
—
1.5
(
Fig. 1), the ligand and diglyme molecules are linked to
a
n is the number of moles of the solvent per mole of TADDOL
each other by the O(3)H...O(5) (H...O, 2.03 Å; O—H...O,
1
1
in the crystalline product; for Et O, n = 0.
b
2
53.9°) and O(8)H...O(7) (H...O, 1.89 Å; O—H...O,
68.0°) hydrogen bonds. The diol molecules involved in
Crystallization did not occur.
Crystallization was not carried out.
c

Asymmetric reduction of ketones
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
1843
C(15)
C(13)
O(6)
O(7)
C(12)
O(4)
C(14)
O(8)
C(11)
C(9)
O(5)
C(7)
O(2)
O(3)
C(10)
C(1)
C(4)
C(6)
C(8)
C(2)
C(3)
O(1)
C(5)
A
B
Fig. 1. Association of the diglyme molecule with two molecules of diol 4c (A and B) in the crystalline complex 2•(4c)•DGM.
ecule assumes a gꢀgttꢀgt conformation. The torsion angles
are given below.
The crystals of the complex of (R,R)ꢀ4a with diglyme
are characterized by a high degree of order (Fig. 2). In the
crystals, the diol and ether molecules form linear helical
chains, and the unit cell corresponding to one turn of the
helix contains six diol molecules and three ether molꢀ
ecules.
Torsion angle
ϕ/deg
C(15)—O(7)—C(14)—C(13)
O(7)—C(14)—C(13)—O(6)
C(14)—C(13)—O(6)—C(12)
C(13)—O(6)—C(12)—C(11)
O(6)—C(12)—C(11)—O(5)
C(12)—C(11)—O(5)—C(10)
–61.3(7)
–57.1(7)
148.9(6)
–159.6(5)
–73.5(6)
170.9(5)
To examine the possibility of performing AREE, we
chose the model asymmetric reduction of acetophenone
(5a) by reagent 3a in THF, because it proceeds with an
1
9
enantioselectivity up to 90% ee, and diol 4a forms a
a
b
0
c
Fig. 2. Crystal structure of the complex 2•(4c)•DGM projected along the axis c.

1844
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
Vinogradov et al.
Table 2. Ability of compounds 4a—h to form crystalline
host—guest complexes with alcohols 6a,b
Table 4. Thermal stabilities of the complexes
(R)ꢀ6a•2[(S,S)ꢀIPTOL] (I) and (S)ꢀ6b•2[(R,R)ꢀIPTOL] (II)
Alcohol
na
HGCa
I
T/°C
τ/h
Loss of 6a
ee (%)
(
mol.%)
(GLC)b
4
a
4b
4e
4f
4g
25
7
7
0
0
3
33
80
100
0
90
88
91
89
90
88
85
86
6
6
a
b
0.5 b
0.5
0.5
0.5
0
—
0
1
0
0
7
0
1
00
5 min
0.5
1
5
7
a
b
100
100
100
25
See the note in Table 1.
The composition of the complex corresponds to the data pubꢀ
2
5
lished in the literature.
II
1
00
0.5
100
complex with alcohol 6a (Table 2) and does not give a
host—guest complex with THF (see Table 1).
a
The host—guest complexes were heated at 1—2 Torr. The deꢀ
gree of thermal dissociation of the host—guest complexes was
With the aim of modeling the second step of AREE
1
determined by H NMR spectroscopy.
b
(
enantiomeric enrichment of product 6), we carried out
The optical purity of 6a involved in the hostꢀguest complex.
experiments on resolution of racemic alcohols 6a and 6b
into enantiomers by inclusion crystallization with
(
S,S)ꢀIPTOL in a suspension of the latter in hexane, beꢀ
literature, we estimated their thermal stabilities. As can be
seen from Table 4, the complex 6b•2 IPTOL was more
readily subjected to thermal decomposition, viz., it was
completely decomposed at 100 °C after 0.5 h. It should be
emphasized that thermal decomposition of the host—guest
complex was not accompanied by racemization of alcohols
6a and 6b as well as of diol 4a involved in the complex.
Based on the aboveꢀmentioned results, we develꢀ
oped a preparative AREE process with the use of
NaAl(IPTOLate)H2 (3a) as the reducing reagent, which
made it possible to prepare chiral alcohols 6a,b from keꢀ
tones 5a,b with higher enantioselectivity (up to 97% ee)
compared to that achieved under standard conditions of
2
5
cause this procedure allows one to minimize losses of
the ligand and product in the course of crystallization.
The results of our study (Table 3) demonstrated that the
yield of the host—guest complex containing (R)ꢀ6a
(
90—95% ee) depended substantially on the concentraꢀ
tion of racꢀ6a in hexane (runs 1—4) (IPTOL is virtuꢀ
ally insoluble in hexane), the maximum yield of the
host—guest complex (95%) being achieved when the conꢀ
–
1
centration of racꢀ6a was higher than 0.06 mol L . After
twofold crystallization of the host—guest complex in a
suspension of (S,S)ꢀIPTOL in hexane, racꢀ6a involved in
the host—guest complex was virtually completely resolved;
the yield of the latter was 90% (run 5). It is remarkable
that the efficient resolution of alcohol 6b in a suspension
of IPTOL in hexane was achieved at its substantially higher
concentration in solution (run 6).
1
9,20
reduction.
It should also be noted that the optical
purity of the final product is virtually independent of the
enantioselectivity of reduction of ketone 5 by reagent 3a
(the first step of the AREE process) if it is in the range of
75—90% ee. As a result, reduction need not be carried out
with deep cooling (from —70 to –100 °C) because coolꢀ
ing to –20 °C is often sufficient to achieve the aboveꢀ
Since data on the properties of the host—guest comꢀ
plexes of diols 4 with 1ꢀarylalkanꢀ1ꢀols are lacking in the
1
9,20
mentioned values of ee.
The procedure developed in
Table 3. Dependence of the degree of enantiomeric enrichment
of racꢀ6a,b on the conditions of inclusion crystallization in a
suspension of (S,S)ꢀIPTOL in hexane
the present study opens up possibilities for improving the
enantioselectivity of reduction of carbonyl compounds,
which react with TADDOLꢀcontaining aluminum hydride
complexes with lower enantioselectivity compared to alkyl
Run
Alcohol
Ca
mol L–1
Yield of HGC ee (R) (%)b
2
0
aryl ketones.
/
(%)
1
2
3
4
5
6
6a
6a
6a
6a
6a
6b
0.0125
0.025
0.0625
0.125
0.17
<20
40
95
95
90
93
95
90
92
98
88
Experimental
The NMR spectra were recorded on a Bruker ACꢀ200 specꢀ
trometer. The ligands were synthesized and the reaction prodꢀ
ucts were analyzed as described earlier.
The Xꢀray diffraction data were collected on an automated
Siemens P3/PC diffractometer (λMoKα, graphite monochroꢀ
mator, θ/2θ scanning technique, 2θmax = 50°).
c
c
1
9,20
0.5
78
a
The concentration of racꢀ6 in the initial mixture.
The degree of enantiomeric enrichment of 6a involved in the
b
host—guest complex.
The result was obtained after twofold inclusion crystallization.
Study of the ability of etherꢀtype solvents to form host—guest
complexes with TADDOL (4). A solution of TADDOL in a choꢀ
c

Asymmetric reduction of ketones
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
1845
sen solvent was concentrated almost to dryness at atmospheric
pressure and room temperature for several hours. The solid preꢀ
cipitate was filtered off, washed with cold light petroleum, and
dried without heating at 1—2 Torr.
In the case of the use of diglyme as the solvent, crystallizaꢀ
tion was induced by adding hexane. The composition of the
crystalline product was analyzed by NMR spectroscopy. The
results of the study are given in Table 1.
Study of the ability of alcohols 6a,b to form host—guest comꢀ
plexes with various TADDOL. Alcohol 6 and TADDOL in a
molar ratio of 4 : 1 were dissolved in a 1 : 1 ether—hexane
mixture and crystallization was performed by slow evaporation
of the solvent. The crystalline product was dried at ∼ 20 °C and
then at 70 °C and 1—2 Torr for 3 h. The results of the study are
given in Table 2.
97% ee, and (S,S)ꢀIPTOL was recovered in a yield of 7.9 g
(85% of the amount used in the reaction). The latter can be
again used in the same process.
Under similar conditions, alcohol 6b was prepared from
ketone 5b in 53% yield with an optical purity of 96% ee.
Xꢀray diffraction study of the host—guest complex
2 (4c)•DGM. The crystals of the complex belong to the trigonal
system. At 20 °C, a = 9.357(2) Å, c = 58.21(2) Å, V = 4414(2) Å,
dcalc = 1.263 g cm–3, space group P3 , Z = 3.
1
The structure was solved by direct methods using the
SHELXTL PLUS program package. The positions of the hydroꢀ
gen atoms were calculated geometrically (except for the H atꢀ
oms of the hydroxy groups, which were revealed from a differꢀ
ence electron density synthesis) and refined using the riding
model with fixed Uiso = nUeq of the nonhydrogen atoms to which
the corresponding hydrogen atoms are bound (n = 1.5 for the
methyl groups and 1.2 for the remaining hydrogen atoms). The
refinement based on F 2 by the fullꢀmatrix leastꢀsquares method
with anisotropic thermal parameters for nonhydrogen atoms usꢀ
ing 5162 reflections converged to wR = 0.154 (R = 0.058 using
Resolution of racꢀ6a,b by the formation of the host—guest
complex in a suspension of IPTOL in hexane. Alcohol 6
(
0.125 mmol) was added to a suspension of IPTOL (0.25 mmol)
in hexane (the amount of the solvent varied from 0.5 to 10 mL)
and the suspension was stirred at ∼ 20 °C for 72 h. The crystalline
2
1
1
product was analyzed by H NMR spectroscopy. The yield of
3405 reflections with F > 4σ(F ), S = 1.02).
the host—guest complex (IPTOL : 6 = 2 : 1) was calculated from
the integral intensities of the signals corresponding to IPTOL
and alcohol 6. The degree of enantiomeric enrichment of 6
involved in the host—guest complex was determined by GLC.
The results of the study are given in Table 3.
This study was financially supported by the
Cambrex Co. (USA).
When modeling the second step of AREE, heptane was used
along with hexane, and the identical results were obtained. Unꢀ
der the conditions of the real AREE process, heptane (having
the higher boiling point) is the solvent of choice in the step of
formation of the inclusion complex. Upon partial concentration
of the ether—heptane mixture (see the procedure described beꢀ
low), the remaining solution was to a larger degree enriched
with the hydrocarbon solvent, which is favorable for an increase
in the yield of the crystalline complex 6•2 IPTOL.
References
1. R. Noyori and S. Hashiguchi, Acc. Chem. Res., 1997, 30, 97.
2. R. Noyori and T. Ohkuma, Angew. Chem., 2001, 40, 41.
3. M. J. Burk, Acc. Chem. Res., 2000, 33, 363.
4. G. Helmchen and A. Pfaltz, Acc. Chem. Res., 2000, 33, 336.
5. F. Fache, E. Schulz, M. L. Tommasino, and M. Lemaire,
Chem. Rev., 2000, 100, 2159.
Synthesis of alcohol (R)ꢀ6a from ketone 5a under conditions
6. V. A. Pavlov, E. V. Starodubtseva, M. G. Vinogradov, V. A.
Ferapontov, O. R. Malyshev, and G. L. Heise, Izv. Akad.
Nauk, Ser. Khim., 2000, 725 [Russ. Chem. Bull., Int. Ed.,
2000, 49, 728].
7. S. Wallbaum and J. Martens, Tetrahedron: Asymmetry, 1992,
3, 1475.
of the AREE process. A solution of NaAlH (1.08 g, 20 mmol) in
4
freshly distilled THF (40 mL) was added dropwise with stirring
to a solution of (S,S)ꢀIPTOL (9.32 g, 20 mmol) in THF (70 mL)
for 1 h. Then the reaction mixture was stirred for 1 h. Acetopheꢀ
none (1.17 mL, 10 mmol) was added with stirring to the resultꢀ
ing solution cooled to –20 °C through a rubber septum using a
syringe. The reaction mixture was kept at a temperature from
8. K. D. Sugi, T. Nagata, T. Yamada, and T. Mukaiyama,
Chem. Lett., 1997, 493.
–
18 to –20 °C for 2 days and then allowed to slowly warm to
9. E. R. Grandbois, S. I. Howard, and J. D. Morrison, in
Asymmetric Synthesis, Academic Press, 1983, 2, 71.
10. H. Haubenstock, in Topics in Stereochemistry, Eds. N. L.
Allinger, E. L. Eliel, and S. H. Wilen, 1983, 14, 231.
11. N. E. Leadbeater and M. Marco, Chem. Rev., 2002,
102, 3217.
∼
20 °C. An excess of the hydride was quenched by adding 90%
methanol. The solvent was distilled off in vacuo and the residue
was extracted with ether (3×40 mL). The extract was succesꢀ
sively washed with 1 М HCl and water and then dried with
Na SO . The solution was concentrated to 20—25 mL, a threeꢀ
2
4
fold excess (by volume) of heptane was added, and the resulting
suspension was stirred for 5 h. The solvent was distilled off to
12. McNamara, M. J. Dixon, and M. Bradley, Chem. Rev., 2002,
102, 3275.
∼
1/2 of the initial volume. The suspension was stirred for 5 h and
13. Q.ꢀH. Fan, Y.ꢀM. Li, and A. S. C. Chan, Chem. Rev., 2002,
102, 3385.
14. R. Noyori, I. Tomino, Y. Tanimoto, and M. Nishizawa,
J. Am. Chem. Soc., 1984, 106, 6709.
15. R. Noyori, I. Tomino, M. Yamada, and M. Nishizawa, J. Am.
Chem. Soc., 1984, 106, 6717.
kept at ∼ 20 °C for one day. The crystalline host—guest complex
was filtered off, washed with a small amount of cold heptane,
and dried at 1—2 Torr and ∼ 20 °C for 1 h and then at 70 °C
for 3 h. To isolate alcohol 6a from the host—guest complex, the
latter was heated in an apparatus for vacuum distillation at
1
00—150 °C and 1—2 Torr, the alcohol that eliminated being
16. A. K. Beck, R. Dahinden, and F. N. M. Kühnle; Reductions
in Organic Synthesis. Recent Advances and Practical Applicaꢀ
tions; ACS Symposium Series, 1996, 641, 53.
collected in a trap cooled with dry ice. Compound (R)ꢀ6a was
obtained in a yield of 0.74 g (62%) with an optical purity of

1
846
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 8, August, 2003
Vinogradov et al.
1
1
1
7. D. Seebach, A. K. Beck, R. Dahinden, M. Hoffmann, and
F. N. M. Kühnle, Croatica Chim. Acta, 1996, 69, 459.
8. D. Seebach, A. K. Beck, and A. Heckel, Angew. Chem., Int.
Ed., 2001, 40, 93.
9. M. G. Vinogradov, L. S. Gorshkova, V. A. Pavlov, O. V.
Mikhalev, G. V. Chel´tsova, I. V. Razmanov, V. A.
Ferapontov, O. R. Malyshev, and G. L. Heise, Izv. Akad.
Nauk, Ser. Khim., 2000, 459 [Russ. Chem. Bull., Int. Ed.,
22. Molecular Inclusion and Molecular Recognition — Clathrates I,
Ed. E. Weber, Topics in Current Chemistry, Springer—Verlag,
Berlin, 1987, 140, 165 pp.
23. F. Toda, in Molecular Inclusion and Molecular Recognition —
Clathrates II, Ed. E. Weber, Topics in Current Chemistry,
Springer—Verlag, Berlin, 1988, 149, 212.
24. F. Toda, S. Matsuda, and K. Tanaka, Tetrahedron: Asymꢀ
metry, 1991, 2, 983.
2
000, 49, 460].
25. F. Toda and Y. Tohi, J. Chem. Soc., Chem. Commun.,
1993, 1238.
26. F. Toda and H. Takumi, Enantiomer, 1996, 1, 29.
2
2
0. M. G. Vinogradov, L. S. Gorshkova, G. V. Chel´tsova,
V. A. Pavlov, I. V. Razmanov, V. A. Ferapontov,
O. R. Malyshev, and G. L. Heise, Izv. Akad. Nauk,
Ser. Khim., 2003, 450 [Russ. Chem. Bull., Int. Ed., 2003,
5
2, 471].
Received April 4, 2003;
in revised form May 27, 2003
1. F. Toda, Supramolecular Science, 1996, 3, 139.