Novel chiral amide hosts derived from mandelic acid: A marked difference in inclusion abilities between host stereoisomers

Article abstract of DOI:10.1002/ejoc.200300281

Novel chiral amide host compounds (2-8) have been synthesized from mandelic acid (1). A marked difference between the inclusion properties of analogous isomers such as 5 and 6 was observed. Host compounds 4 and 5 showed a relatively high chiral recognition in the optical resolution of guest compounds by inclusion crystallization. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003.

Full text of DOI:10.1002/ejoc.200300281

SHORT COMMUNICATION
Novel Chiral Amide Hosts Derived from Mandelic Acid: A Marked Difference
in Inclusion Abilities between Host Stereoisomers
[a]
[a]
[b]
Koichi Tanaka,* Takaichi Hiratsuka, and Zofia Urbanczyk-Lipkowska
Keywords: Host-guest chemistry / Chiral resolution / Supramolecular chemistry
Novel chiral amide host compounds (2−8) have been synthe-
sized from mandelic acid (1). A marked difference between pounds by inclusion crystallization.
the inclusion properties of analogous isomers such as 5 and ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
was observed. Host compounds 4 and 5 showed a relatively
high chiral recognition in the optical resolution of guest com-
6
Germany, 2003)
Introduction
In recent years, there has been increasing interest in host-
guest inclusion systems because of their potential appli-
[
1]
cations in analytical, synthetic and material sciences. Pre-
viously, we have designed chiral alcohol host compounds
derived from tartaric acid, and found them to be very useful
hosts for the optical resolution of racemic guests and for
the enantioselective reaction of prochiral guests.[ We have
now designed and synthesized novel chiral amide host com-
pounds (2Ϫ8) derived from mandelic acid (1), which have a
rigid amide group connected to the benzene or cyclohexane
framework, bulky phenyl substituents, and an OH group
for possible hydrogen bonding to guest molecules. Of the
phenylene-substituted host compounds, the o- and p-phen-
ylene derivatives (2, 4) showed a broad inclusion ability al-
though the m-isomer 3 showed poor inclusion ability. The
host compound (R,R,R,R)-(ϩ)-5 includes the greatest num-
ber of guest molecules, whereas the host (R,R,S,S)-(ϩ)-6
2]
(the stereoisomer of 5) showed little inclusion abilities.
reaction of (R,R)-(Ϫ)-trans-1,2-cyclohexanediamine with
Results and Discussion
(R)-(Ϫ)-mandelic acid and (S)-(ϩ)-mandelic acid, respec-
tively, in the presence of N-hydroxysuccinic anhydride and
The amide host compounds 2Ϫ4 were synthesized by the
condensation reaction of (R)-(Ϫ)-mandelic acid and o-, m-
and p-diaminobenzene, respectively, in the presence of 4-
dicyclohexylcarbodiimide (DCC) as a condensing agent ac-
[
4]
cording to the reported method. The amide host com-
pounds 7 and 8 were synthesized by the condensation reac-
tion of (R)-(Ϫ)-mandelic acid with cis-1,2-cyclohexane-
diamine and trans-1,4-cyclohexanediamine, respectively, in
the presence of N-hydroxysuccinic anhydride and dicyclo-
hexylcarbodiimide (DCC) as a condensing agent. The
(
4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium
[
3]
chloride hydrate as a condensing agent. The amide host
compounds 5 and 6 were prepared by the condensation
[
[
a]
b]
Department of Applied Chemistry, Faculty of Engineering,
Ehime University,
yields, melting points and [α] value of these new host com-
D
Matsuyama, Ehime 790Ϫ8577, Japan,
Fax: (internat.) ϩ81-89/927-9920
pounds are listed in Table 1.
The inclusion behavior of the host compounds 2Ϫ8 was
studied. Of the o-, m- and p-phenylene-substituted host
compounds (2Ϫ4), the o- and p-substituted hosts (2 and 4,
E-mail: ktanaka@eng.ehime-u.ac.jp
Polish Academy of Sciences,
Kasprzaka 44/52, 01Ϫ224 Warsaw, Poland
Eur. J. Org. Chem. 2003, 3043Ϫ3046
DOI: 10.1002/ejoc.200300281
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3043

K. Tanaka, T. Hiratsuka, Z. Urbanczyk-Lipkowska
SHORT COMMUNICATION
Table 1. Yield, melting point and [α]
pounds
D
value of new host com- was resolved most efficiently with host 4 in 44% ee after
one complexation. The efficiency of the optical resolution
of 2-methylcyclohexanone is much higher than that pre-
viously reported by using (R,R)-(Ϫ)-1,6-bis(2-chloro-
Host
Yield [%]
Mp [°C]
D
[α] (c, solv.)
phenyl)-1,6-diphenylhexa-2,4-diyne-1,6-diol as the optically
active host compound. Piperidine derivatives were re-
Ϫ27.0 (0.30, MeOH) solved most efficiently by complexation with host 5. For
2
3
4
5
6
7
8
44
65
66
63
57
78
71
170Ϫ173
180Ϫ183
215Ϫ219
152Ϫ155
190Ϫ195
200Ϫ205
223Ϫ227
Ϫ72.8 (0.35, MeOH)
Ϫ33.7 (0.32, MeOH)
[
5]
ϩ3.7 (0.21, CHCl
ϩ108 (0.20, CHCl
3
)
example, when a solution of a 1:4 mixture of 5 and (Ϯ)-2-
ethylpiperidine in EtOAc was kept at room temperature for
3
)
Ϫ84.6 (0.32, MeOH)
Ϫ82.6 (0.31, MeOH)
12 h, 1:2 inclusion crystals of 5 and (ϩ)-2-ethylpiperidine
were formed as colorless needles. Four recrystallizations of
the crystals from EtOAc gave pure inclusion crystals, which,
upon heating in vacuo, gave (ϩ)-2-ethylpiperidine of 100%
ee in 21% yield by distillation. γ-Butyrolactone derivatives
were also resolved by complexation with host 5.
respectively) showed a higher inclusion ability for some
typical guest compounds than the m-substituted host 3
(Table 2). A marked difference between the inclusion ability
of analogous isomers (5 and 6) was observed. Host 5, which
Table 3. Optical resolution of guest compounds by inclusion com-
was prepared from the condensation reaction between (R)- plexation
Ϫ)-1 and (R,R)-(Ϫ)-trans-1,2-cyclohexanediamine, took up
all guest compounds tested, whereas host 6, prepared from
S)-(ϩ)-1 and (R,R)-(Ϫ)-trans-1,2-cyclohexanediamine, in-
(
Guest
2
4
5
6
8
(
2
3
2
-Methylcyclohexanone 17% ee Ϫ[a]
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
cluded only two guest compounds (Table 2). The chirality
of the stereogenic center of mandelic acid (1) is very import-
ant for inclusion complexation. The stereochemistry of 3-Methylpiperidine
trans-1,2-cyclohexanediamine is also important for stable
inclusion, since the cis derivative 7 showed no inclusion
properties. On the other hand, the 1,4-trans derivative 8
showed moderate inclusion properties, as shown in Table 2.
-Methylcyclohexanone rac
-Methylpiperidine
44% ee 36% ee Ϫ
16% ee 29% ee Ϫ
rac
36% ee 70% ee Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
21% ee 18% ee 21% ee
2
3
3
-Ethylpiperidine
Ϫ
Ϫ
Ϫ
-Methyl-γ-butyrolactone Ϫ
-Ethyl-γ-butyrolactone
17% ee Ϫ
32% ee Ϫ
Ϫ
[a]
No inclusion complexation occurred.
Table 2. Host-guest ratio of inclusion crystals[a]
The chiral-discrimination properties of the new hosts are
to a large extent due to their solid-state structures. A crystal
of the 1:1 inclusion complex of (R,R)-(Ϫ)-4 and (R)-2-ethyl-
piperidine suitable for X-ray analysis was obtained. The
host molecule is characterized by an extended conformation
of the central part with one intramolecular hydrogen bond:
Guest
MeOH
Acetone
Cyclopentanone
Cyclohexanone
γ-Butyrolactone
THF
Dioxane
Acetonitrile
DMSO
2 [
Ϫ
Ϫ
1:1
1:1
Ϫ
Ϫ
1:1
Ϫ
3
4
Ϫ
5
6
7
8
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
1:1
1:1
Ϫ
1:1
Ϫ
b]
Ϫ
Ϫ
Ϫ
1:2
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
2:1
3:1
1:1
1:1
1:2
2:1
1:1
2:1
1:1
1:1
1:1
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
1:2
Ϫ
1:1
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
Ϫ
3:1
1:1
1:1
Ϫ
2:1
1:1
Ϫ
2:1
2:1
1:1
˚
N15···O4 2.633(5), H15···O4 2.20 A, angle N15ϪH15···O4
110.9° (Figure 1).
1:1
1:1
Ϫ
DMF
Pyridine
1:1
[
a]
The host-guest ratios were determined by TG analysis.[b] No in-
clusion complexation occurred.
Figure 1. ORTEP diagram of the 1:1 complex of (R,R)-(Ϫ)-4 and
The results of optical resolution by one complexation (R)-2-ethylpiperidine showing thermal ellipsoids at 30% probability
level (significantly higher for the guest)
with the host compounds 2, 4, 5, 6 and 8 are summarized
in Table 3. Racemic 2-methylcyclohexanone was partially
resolved by complexation with host compound 2. 3-Methyl-
The host molecules form channels where the molecules
cyclohexanone was included in host compounds 2Ϫ4, and of (R)-2-ethylpiperidine are included partially due to inter-
Table 4. Hydrogen bonding pattern
DϪH
d(DϪH)
d(H···A)
Angle
d(D···A)
A
O4ϪH4
0.82
0.82
0.86
1.01
2.14
2.23
2.07
2.37
118.44
146.64
152.11
167.4
2.633(5)
2.951(4)
2.860(4)
2.645 (5)
N15 intramolecular
O23ϪH23
N15ϪH15
N30ϪH30A
O4 [Ϫx ϩ 1, y Ϫ 1/2, Ϫz ϩ 1]
O2 [Ϫx ϩ 1, y ϩ 1/2, Ϫz ϩ 1]
O4 [Ϫx, y ϩ 1/2, Ϫz]
3044
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 3043Ϫ3046

Novel Chiral Amide Hosts Derived from Mandelic Acid
SHORT COMMUNICATION
actions between the hydrophobic groups of the host and (Nujol): ν˜ ϭ 3432 cmϪ1 (OH), 1661 (CϭO). H NMR (300 MHz,
1
CDCl
OH), 7.2Ϫ7.4 (m, 14 H, Ar), 8.51 (s, 2 H, NH) ppm. C22
376.14): calcd. C 70.20, H 5.36, N 7.44; found C 70.23, H 5.54,
N 7.52.
3
), δ ϭ 3.38 (d, J ϭ 3 Hz, 2 H, CH), 4.95 (d, J ϭ 3 Hz, 2 H,
guest. Additionally, a relatively strong intermolecular hy-
drogen bond can be postulated between N30 and O4 on the
basis of a short donor···acceptor distance and the calculated
position of the N30ϪH atom (for geometry see Table 4 and
Figure 2). Such a structure promotes fast release of the gu-
est from the hosting structure.
20 2 4
H N O
(
(
R,R)-(؊)-2-Hydroxy-N-[3-(2-hydroxy-1-phenylvinyloxyamino)-
phenyl]-2-phenylacetamide (3): Colorless prisms; m.p. 182Ϫ185 °C.
Ϫ1
[α]
D
ϭ Ϫ33.7 (c ϭ 0.72, MeOH). IR (Nujol): ν˜ ϭ 3367 cm (OH),
1
1
2
8
5
665 (CϭO). H NMR (300 MHz, CDCl
H, CH), 5.75 (d, J ϭ 4 Hz, 2 H, OH), 7.2Ϫ7.5 (m, 14 H, Ar),
.95 (s, 2 H, NH) ppm. C22 (376.14): calcd. C 70.20, H
.36, N 7.44; found C 70.18, H 5.66, N 7.55.
3
), δ ϭ 5.16 (d, J ϭ 4 Hz,
20 2 4
H N O
(
R,R)-(؊)-2-Hydroxy-N-[4-(2-hydroxy-1-phenylvinyloxyamino)-
phenyl]-2-phenylacetamide (4): Colorless prisms; m.p. 216Ϫ219 °C.
Ϫ1
[α]
D
ϭ Ϫ27.0 (c ϭ 0.30, MeOH). IR (Nujol): ν˜ ϭ 3249 cm (OH),
1
1
2
9
5
613 (CϭO). H NMR (300 MHz, CDCl
H, CH), 6.00 (d, J ϭ 4 Hz, 2 H, OH), 7.2Ϫ7.5 (m, 14 H, Ar),
.31 (s, 2 H, NH) ppm. C22 (376.14): calcd. C 70.20, H
.36, N 7.44; found C 70.10, H 5.61, N 7.65.
3
), δ ϭ 5.17 (d, J ϭ 4 Hz,
20 2 4
H N O
Preparation of (R,R)-(؊)-2-Hydroxy-N-[2-(2-hydroxy-2-phenylace-
tylamino)cyclohexyl]-2-phenylacetamide (7). General Procedure: Di-
cyclohexylcarbodiimide (9.0 g, 43.8 mmol) was added to a stirred
solution of (R)-(Ϫ)-mandelic acid (1; 5.3 g, 35.0 mmol), cis-1,2-
cyclohexanediamine (2.0 g, 17.5 mmol) and N-hydroxysuccinic an-
hydride (5.0 g, 43.8 mmol) in anhydrous tetrahydrofuran (150 mL)
at room temperature under an argon atmosphere. The reaction
mixture was stirred overnight. The solvent was removed under re-
duced pressure, and the residue, dissolved in ethyl acetate, was
washed successively with saturated sodium carbonate, water, 1 
HCl, water and brine and dried over MgSO
4
. The crude product
was purified by recrystallization from MeOH to give 5.2 g of 7
(
78% yield) as colorless prisms [mp, 203Ϫ205 °C, [α]
D
ϭ Ϫ84.6
Figure 2. Crystal structure of the 1:1 complex between (R,R)-(Ϫ)-
Ϫ1
(
c ϭ 0.32, MeOH)]. IR (Nujol): ν˜ ϭ 3414 cm (OH), 1685 (Cϭ
4
and (R)-2-ethylpiperidine; the host molecules form channels con-
1
taining hydrogen-bonded guest molecules
O). H NMR (300 MHz, CDCl
J ϭ 3 Hz, 1 H), 3.49 (d, J ϭ 3 Hz, 1 H), 3.66 (s, 2 H), 4.89 (d, J ϭ
Hz, 1 H), 5.00 (d, J ϭ 3 Hz, 1 H), 6.49 (d, J ϭ 7.5 Hz, 1 H), 6.79
d, J ϭ 7.5 Hz, 1 H), 7.2Ϫ7.5 (m, 10 H) ppm. C22
382.19): calcd. C 69.09, H 6.85, N 7.32; found C 69.33, H 7.01,
3
), δ ϭ 1.2Ϫ1.75 (m, 8 H), 3.37 (d,
3
(
(
26 2 4
H N O
Experimental Section
N 7.59.
1
(R,R)-(؊)-2-Hydroxy-N-[4-(2-hydroxy-1-phenylvinyloxyamino)-
cyclohexyl]-2-phenylacetamide (8): Colorless prisms, m.p. 224Ϫ227
General Remarks: H NMR spectra were recorded in CDCl
3
on a
JEOL Lambda 300 spectrometer. IR spectra were recorded with a
JASCO FT-IR 200 spectrometer. All melting points were deter-
mined using a Yanaco micro melting-point apparatus and are un-
corrected. Optical rotations were determined with Jasco DIP 1000
polarimeter. The host-guest inclusion crystals were prepared by
recrystallization of host compounds from the neat guest (solvent)
solution; the host-guest ratios were determined by thermogravi-
metric analysis (TG).
Ϫ1
°
(
0
C. [α]
OH), 1640, 1623 (CϭO). H NMR (300 MHz, CDCl
.9Ϫ2.0 (m, 8 H), 3.37 (d, J ϭ 3 Hz, 2 H), 3.74 (s, 2 H), 5.00 (d,
J ϭ 3 Hz, 2 H), 5.98 (br. s, 2 H), 7.2Ϫ7.4 (m, 10 H) ppm.
(382.19): calcd. C 69.09, H 6.85, N 7.32; found C
9.19, H 6.99, N 7.54.
D
ϭ Ϫ82.6 (c ϭ 0.31, MeOH)). IR (Nujol): ν˜ ϭ 3503 cm
1
3
), δ ϭ
22 26 2 4
C H N O
6
Optical Resolution of 3-Methylcyclohexanone by Complexation with
(R,R)-(؊)-4: When a solution of (R,R)-(Ϫ)-4 (1.0 g, 2.66 mmol) in
(Ϯ)-3-methylcyclohexanone (4.0 g, 35.5 mmol) was kept at room
Preparation of (R,R)-(؊)-2-Hydroxy-N-[2-(2-hydroxy-2-phenylace-
tylamino)phenyl]-2-phenylacetamide (2). General Procedure: 4-[4,6-
Dimethoxy-1,3,5-triazin-2-yl]-4-methylmorpholinium chloride hy- temperature for 12 h, a 1:1 inclusion complex of (R,R)-(Ϫ)-4 and
drate (11.0 g, 39.9 mmol) was added to a mixture of (R)-(Ϫ)-man-
delic acid (1; 25.0 g, 32.9 mmol) and o-phenylenediamine (1.8 g,
(Ϫ)-3-methylcyclohexanone was formed as colorless prisms (1.05 g,
81% yield, no sharp m.p.). Heating the inclusion crystals at 180
16.6 mmol) in MeOH (100 mL) at room temperature. After stirring °C/30 Torr gave (Ϫ)-3-methylcyclohexanone with 44% ee {0.11 g,
for 6 h at room temperature, the MeOH was evaporated. The re-
sulting residue was dissolved in EtOAc and washed successively
with saturated sodium carbonate, water, 1  HCl, water and brine
[α]
D 3
ϭ Ϫ6.35 (c ϭ 1.1, CHCl )} as a distillate. The optical purity
of (Ϫ)-3-methylcyclohexanone was determined by comparison of
the measured [α]
value with the literature value.[ An X-ray
6]
D
and dried over MgSO
4
. The crude product was purified by recrys-
sample of the pure 1:1 complex of (R,R)-(Ϫ)-4 and (Ϫ)-3-methyl-
tallization from MeOH to give 2.72 g of 2 (44% yield) as colorless cyclohexanone was prepared by complexation of (R,R)-(Ϫ)-4 with
prisms [mp, 173Ϫ175 °C, [α] ϭ Ϫ72.8° (c ϭ 0.35, MeOH)]. IR (Ϫ)-3-methylcyclohexanone of 100% ee.
D
Eur. J. Org. Chem. 2003, 3043Ϫ3046
www.eurjoc.org
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3045

K. Tanaka, T. Hiratsuka, Z. Urbanczyk-Lipkowska
Optical Resolution of 2-Ethylpiperidine by Complexation with Crystallographic Data Centre, 12, Union Road, Cambridge
SHORT COMMUNICATION
(R,R,R,R)-(؉)-5: A solution of a 1:4 mixture of (R,R,R,R)-(ϩ)-5 CB2 1EZ, UK; Fax: (internat.) ϩ44-1223/336-033; E-mail:
(1.5 g, 3.92 mmol) and (Ϯ)-2-ethylpiperidine (1.77 g, 15.7 mmol) in
deposit@ccdc.cam.ac.uk].
EtOAc (15 mL) was kept at room temperature for 12 h, forming
:2 inclusion crystals of (R,R,R,R)-(ϩ)-5 and (ϩ)-2-ethylpiperidine
of 70% ee as colorless needles (1.99 g). Four recrystallizations of
1
Acknowledgments
This work is supported by Grants-in Aid for Scientific Research
the inclusion crystals from EtOAc gave pure inclusion crystals
(0.52 g). Heating the inclusion crystals at 180 °C/30 Torr gave (ϩ)-
(
C), No. 13640538 to K.T. from the Ministry of Education, Cul-
2-ethylpiperidine of 100% ee {0.18 g, 21% yield, [α]
D
ϭ ϩ11.3 (c ϭ
ture, Sports, Science and Technology, Japan, and the Asahi Glass
Foundation. The authors thank Tokuyama Corporation, Japan for
kindly providing us with a sample of the condensing reagent, 4-
0
.06, CHCl )} as a distillate. The optical purity of (Ϫ)-3-methyl-
3
1
cyclohexanone was determined by H NMR spectroscopy in the
presence of (Ϫ)-bis(2-naphthol).[
7]
[4,6-dimethoxy-1,3,5-triazin-2-yl]-4-methylmorpholinium chloride
hydrate.
X-ray Crystallographic Study of the 1:1 complex of (R,R)-(؊)-4 and
(
R)-2-ethylpiperidine: A suitable crystal of dimensions 0.35 ϫ 0.35
3
[1]
ϫ 0.21 mm , covered by epoxy glue, was used for data collection
Solid-State Supramolecular Chemistry: Crystal Engineering,
on an Nonius BV Kappa CCD system at 283 K. Crystal data: a ϭ
Vol. 6, in Comprehensive Supramolecular Chemistry (Eds.: J. L.
Atwood, J. E. D. Davies, D. D. Macnicol, F. Vögtle), Perga-
mon, 1996.
2]
˚
9.6110(4), b ϭ 10.3610(4), c ϭ 14.1490(6) A, β ϭ 103.1710(10)°;
1 α
Z ϭ 2, monoclinic space group P2 , Mo-K radiation, λ ϭ 0.7107
[
Ϫ3
K. Tanaka, F. Toda, Chem. Rev. 2000, 100, 1025Ϫ1074; K.
Tanaka, S. Honke, Z. Urbanczyk-Lipkowska, F. Toda, Eur. J.
Org. Chem. 2000, 3171Ϫ3176.
˚
A, Dcalcd. ϭ 1.185 Mg·m . A total of 3331 independent reflections
were measured in the θ-range 2.18Ϫ21.97°. These were used for
structure solution using SHELXS-97[ and refinement with
8]
[3]
M. Kunishima, C. Kawachi, K. Hioki, K. Terao, S. Tani, Tetra-
hedron 2001, 57, 1551Ϫ1558.
[
9]
SHELXL-97. . All non-H atoms were refined in the anisotropic
mode. All non-hydroxyl hydrogens were placed geometrically and
refined with a riding mode with Uiso constrained to 1.2 times that
of the carrier atom. The positions of the N,O-protons were found
from ∆ρ maps and refined without constraints. The position of the
H30 proton, essential for host-guest binding, could not be found
from difference maps. It was therefore placed geometrically and not
[
[
[
[
4]
5]
6]
7]
A. J. A. Cobb, C. M. Marson, Tetrahedron: Asymmetry 2001,
12, 1547Ϫ1550.
F. Toda, K. Tanaka, T. Omata, K. Nakamura, T. Oshima, J.
Am. Chem. Soc. 1983, 105, 5151Ϫ5152.
G. Adolphen, E. J. Eisenbraun, G. W. Keen, P. W. K. Flanagan,
Org. Prep. Proc. 1970, 69, 165Ϫ167.
K. Tanaka, M. Ootani, F. Toda, Tetrahedron: Asymmetry 1992,
1 2
refined. Final R and wR parameters are 0.0693 and 0.1789,
respectively.
CCDC-209871 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge at
www.ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge
3, 709Ϫ712.
[8]
[9]
G. M. Sheldrick, Acta Crystallogr., Sect. A 1990, 46, 467Ϫ473.
G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement, University of Göttingen, Germany, 1997.
Received May 11, 2003
3046
 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
Eur. J. Org. Chem. 2003, 3043Ϫ3046