Synthesis and chiral recognition abilities of new calix[6]arenes bearing amino alcohol moieties

Article abstract of DOI:10.1016/j.tetasy.2006.04.013

Herein the synthesis and recognition abilities towards amino acids and amino alcohols of new d-/l-phenylalaninol substituted p-tert-butylcalix[6]arenas are reported. These compounds, 6 and 7 have been synthesized via nucleophilic substitution reactions in

Full text of DOI:10.1016/j.tetasy.2006.04.013

Tetrahedron: Asymmetry 17 (2006) 1258–1263
Synthesis and chiral recognition abilities of new calix[6]arenes
bearing amino alcohol moieties
Serkan Erdemir, Mustafa Tabakci and Mustafa Yilmaz*
Selc¸uk University, Department of Chemistry, 42031 Konya, Turkey
Received 14 March 2006; accepted 11 April 2006
Abstract—Herein the synthesis and recognition abilities towards amino acids and amino alcohols of new D-/L-phenylalaninol substituted
p-tert-butylcalix[6]arenas are reported. These compounds, 6 and 7 have been synthesized via nucleophilic substitution reactions involving
5,11,17,23,29,35-tert-butyl-37,38-dimethoxy-39,40,41,42-(p-tosylethoxy)calix[6]arene 5 with D-/L-phenylalaninol in dry THF. The extrac-
tion properties of 6 and 7 towards some selected amino acid methylesters and amino alcohols have been studied by liquid–liquid extrac-
tion. These results show that chiral calix[6]arene derivatives exhibit a good affinity towards all amino acids and amino alcohols.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
ity and efficiency of binding.8 The introduction of an amino
acid or an amino alcohol could lead to chirality in the artifi-
cial receptors. Due to the important role played by amino
acid and amino alcohol units in several recognition processes
of natural and artificial systems,9 chiral discrimination10 and
stereoselective synthesis, synthetic calix[4]arenes containing
amino acid have been extensively studied.11 Readily avail-
able calix[6]arenes could constitute as ideal platforms for
the design of such enantioselective endo-receptors.1,12 In
fact, their cavity is large enough for the deep inclusion of
organicmolecules. Surprisingly,examplesofenantiomerically
pure calix[6]arenes are rare13 and their host–guest properties
towards chiral guests have not been investigated much. This
is mainly due to the fact that calix[6]arenes suffer from their
reputation as highly flexible molecules, which is not compat-
ible with efficient syntheses and good host properties.
Recently we reported that the synthesis of two chiral
calix[4]arenes bearing an (RS)-phenyl ethylamine moiety
andevaluationoftheirextractionabilitytowardssomeamino
acid methylesters.14 Herein, we report on the synthesis of
some new chiral calix[6]arene amine derivatives bearing a
D-/L-phenylalaninol group and on their use as ligands in
amino acid and amino alcohol recognition studies.
A macrocyclic compound forms a stable complex with a
guest molecule by entrapping it into its cavity. Since the
1980s, calixarene, which is formed by the condensation be-
tween p-alkyl phenol and formaldehyde under basic condi-
tions, has attracted much attention as a macrocyclic
platform. The scaffold of calix[n]arenes can be modified by
introducing many kinds of functional groups and/or struc-
tural groups, to create a specific interaction to a target guest
molecule. A number of books have been published concern-
ing the synthesis, structural features and host–guest interac-
tions.1 More specifically, the subject of chemical recognition
and separation of ions was addressed in several publica-
tions.2 On the other hand, only a few reviews concerning
calixarenes for biochemical recognition are available, for
example, on peptido- and glycoconjugates and the role of
hydrogen-bonding interactions,3 on neoglycoconjugates
with large rigidified cavities4 and on synthetic receptors.5
Molecular recognition, and in particular chiral recognition,
is one of the most fundamental and significant processes in
living systems.6 Chiral recognition can contribute to the
understanding of biochemical systems and offer new per-
spectives for the development of pharmaceuticals, enantio-
selective sensors, catalysts and other molecular devices.7
Among the several kinds of host molecule for recognition,
calixarenes offer a number of advantages in terms of selectiv-
2. Results and discussion
2.1. Synthesis
*
We were interested in the synthesis of calix[6]arene-based
ionophores having chiral binding sites in order to under-
Corresponding author. Tel.: +90 332 2232774; e-mail addresses:
myilmaz@selcuk.edu.tr; myilmaz42@yahoo.com
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.04.013

S. Erdemir et al. / Tetrahedron: Asymmetry 17 (2006) 1258–1263
1259
stand their recognition ability towards amino acid methyl-
2.2. Two-phase solvent extraction studies
esters and amino alcohols via the two phase solvent extrac-
tion systems. The synthesis of 1–7 depicted in Figure 4
have been carried out as follows: After compounds 1 and 2
have been synthesized according to previous literature meth-
ods,15,16 compound 2 was reacted with ethylbromoacetate in
dry acetone in the presence of potassium carbonate at reflux
for 2.5 days in order to obtain 3 in 63% yield. This compound
The present work is focused in order to elaborate the stra-
tegic requirements for the two-phase extraction measure-
ments, therefore, the binding abilities of parent p-tert-
butylcalix[4]arene 1, its ester derivative 3, its alcohol deriv-
ative 4 and the synthesized chiral calix[6]arenes 6 and 7
towards some selected amino acids and amino alcohols, have
been evaluated by means of solvent extraction of their
ammonium picrates and the results have been summarized
in Tables 1 and 2. These data have been obtained by using
dichloromethane solution of the ligands to extract ammo-
nium picrates from aqueous solution. The equilibrium con-
centration of ammonium picrate in an aqueous phase was
then determined spectrophotometrically. From the extrac-
tion data given in Tables 1 and 2, it has been observed that
parent p-tert-butylcalix[6]arene 1 and ester derivative 3
transfer both amino acids and amino alcohols from aque-
ous phase into the organic phase in trace amounts, alcohol
derivative 4 displays a noteworthy extraction ability
towards these species, and chiral calix[6]arenes 6 and 7 recog-
nize both amino acids and amino alcohols in high yields.
According to our experience and knowledge from our pre-
vious study and the other studies,14,17 increasing the extrac-
tion properties of chiral calix[6]arene amines 6 and 7 can be
explained by multiple hydrogen bonding between chiral ca-
lix[6]arene amines and amino acids or amino alcohols. In
addition, the guests are stabilized by CH–p interactions
with the aromatic walls of the hydrophobic cavity of the
chiral calix[6]arenes 6 and 7. Also although they are small,
similar interactions have been observed for alcohol deriva-
tive 4 due to its hydroxyl groups. An expectation of this
study is also to observe chiral discrimination between
amino acids or amino alcohols by using these new chiral
calix[6]arene amines as a ligand. From the results, this goal
is relatively true, especially for phenylglycinols because of
their extraction abilities being different according to each
other, when compared to other species.
1
has been characterized by a combination of H NMR, IR
and elemental analysis. The IR spectra shows an ester band
1
at 1766 cmꢀ1, and H NMR exhibits three singlets at 3.09,
3.20 and 3.35 ppm corresponding to the four ester groups.
Subsequent reduction of these ester groups of 3 by lithium
aluminium hydride yielded alcohol derivative 4 in 65% yield.
Completion of the reaction was followed by FTIR-spectro-
scopy, which showed the disappearance of the band due to
ester groups at 1766 cmꢀ1 and appearance of the band due
to alcohol hydroxyl groups at 3490 cmꢀ1. 1H NMR spectra
confirmed the reduction due to observing new peaks at
3.13–3.36 and 3.40 ppm, which correspond to the alcohol
1
groups. After compound 4 had been characterized by H
NMR, it was treated with p-toluenesulfonyl chloride at
ꢀ4 ꢁC and tosylate derivative 5 has been obtained in 74%
yield. Conversion of the alcohol groups into tosylate groups
has been confirmed by FTIR, which showed the disappear-
ance of the alcohol band at 3490 cmꢀ1 and appearance of
the 921 cmꢀ1 (S@O), 741 and 823 cmꢀ1 (S–O) bands. Conse-
quently compound 5 has been reacted with D-/L-phenylalan-
inol in dry THF at room temperature for 5 h to obtain chiral
calix[6]arene derivatives 6 and 7 in 51% and 45% yields,
respectively. Compounds 6 and 7 have been characterized
1
by a combination of H NMR, IR and elemental analysis.
IR spectra of 6 and 7 show the amine bands at 3102 cmꢀ1
and 3105 cmꢀ1, respectively. Compounds 6 and 7 are asym-
metric due to the formation of chiral sub-unitsontothe lower
rim of calix[6]arene. The splitting patterns of protons (see
Experimental) reflect the presence of the chiral moieties in
the molecules.12
Table 1. Extraction percentage of selected amino acid methylesters with 3, 4, 6 and 7a
Ligand
L-AlaOMe
D-AlaOMe
L-PheOMe
D-PheOMe
D-TrpOMe
L-TrpOMe
1b
3
<1.0
<1.0
17.4
91.4
84.3
<1.0
<1.0
16.9
89.1
89.6
<1.0
<1.0
16.1
90.3
87.2
<1.0
<1.0
16.5
90.7
82.5
<1.0
<1.0
15.8
87.5
85.4
<1.0
<1.0
16.3
93.2
89.8
4
6
7
a Aqueous phase, [ammonium picrate] = 2.0 · 10ꢀ5 M; organic phase, dichloromethane, [ligand] = 1.0 · 10ꢀ3; at 25 ꢁC, for 1 h.
b Chloroform was used as organic phase.
Table 2. Extraction percentage of selected amino alcohols with 3, 4, 6 and 7a
Ligand
D-Phegly
L-Phegly
(R)-Hyd-Me-Pyr
(S)-Hyd-Me-Pyr
1b
3
<1.0
<1.0
16.8
92.3
83.5
<1.0
<1.0
14.1
72.5
87.6
<1.0
<1.0
15.9
89.1
84.4
<1.0
<1.0
16.3
90.4
91.2
4
6
7
a Aqueous phase, [ammonium picrate] = 2.0 · 10ꢀ5 M; organic phase, dichloromethane, [ligand] = 1.0 · 10ꢀ3; at 25 ꢁC, for 1 h.
b Chloroform was used as organic phase.

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S. Erdemir et al. / Tetrahedron: Asymmetry 17 (2006) 1258–1263
The extraction data for 6 has been analyzed by a classical
slope analysis method. Assuming the extraction of an
ammonium cation ðR–NHþ3 Þ by the receptor 6 according
to the following equilibrium:
derivatives 6 and 7 were excellent extractants for all the
amino acids and amino alcohols used, the chiral discrimi-
nation between the amino acids and amino alcohols could
be obtained. It should be noted that the hydrophobic cavity
of chiral calix[6]arenes and hydrogen bonding led us to rec-
ognize these amino acids and amino alcohols. This work
should be useful with regards to the synthesis of chiral
and enantioselective receptors.
½R–NHþ3 ꢁaq þ ½Picꢀꢁaq þ x½Lꢁorg ꢀ ½R–NH3PicðLÞ ꢁ
x org
The extraction constant Kex is defined by
½R–NH3PicðLÞxꢁ
Kex
¼
ð1Þ
ð2Þ
x
½R–NHþ3 ꢁ½Picꢀꢁ½Lꢁ
4. Experimental
Eq. (1) can be rewritten as
log DA ¼ log KexPic þ x log½Lꢁ
4.1. Materials and general methods
where the distribution ratio DA is defined as ratio of the
concentrations of the ammonium cation ðR–NHþ3 Þ in the
two phases:
Melting points were determined on a Gallenkamp appa-
ratus in a sealed capillary and are uncorrected. H NMR
1
spectra were recorded on a Bruker 250 MHz spectrometer
in CDCl3 with TMS as the internal standard. IR spectra
were recorded on a Perkin Elmer 1605 FTIR spectrometer
as KBr pellets. UV–vis spectra were obtained on a
Shimadzu 160A UV–visible recording spectrophotometer.
FAB-MS spectra were taken on a Varian MAT 312
spectrometer. Elemental analyses were performed on
a Leco CHNS-932 analyzer. Specific rotations were
DA ¼ ½R–NH3PicðLÞxꢁorg=½R–NHþ3 ꢁaq
ð3Þ
Consequently a plot of logDA versus log[L] leads to a
straight line, whose slope allows the stoichiometry of the
extracted species to be determined.
Figure 1 shows the extraction into dichloromethane at dif-
ferent concentrations of 6 for the ammonium ion. A linear
relationship between logDA versus log[L] is observed with
a slope for ammonium ion by 6, which equals 2.14, suggest-
ing that 6 forms a 2:1 complex with an ammonium cation.
The analytical data of 6 shows that the complexation reac-
tion takes place according to the following equilibrium:
measured on A Kruss Optronic polarimeter.
¨
Analytical TLC was performed on precoated silica gel
plates (SiO2, Merck PF254), while silica gel 60 (Merck, par-
ticle size 0.040–0.063 mm, 230–240 mesh) was used for pre-
parative column chromatography. Generally, solvents were
˚
Kext
dried by storing them over molecular sieves (Aldrich; 4 A,
2ðLÞorg þ ðR–NHþ3 PicꢀÞaq ꢀ ðL2; R–NHþ3 PicꢀÞorg
8–12 mesh). Acetone and CH2Cl2 were distilled from
CaSO4 and CaCl2, respectively. Dry THF was distilled
from sodium and benzophenone. All aqueous solutions
were prepared with deionized water that had been passed
through a Millipore Milli-Q Plus water purification system.
According to the experimental data, if the Eq. 2 rearrange-
ment for 6, logKex has the value 7.24 0.2.
1
0.7
The following amino acid methylester hydrochlorides and
amino alcohols obtained from Aldrich or Merck at the
highest commercially available purity were used in this
study: L-phenylalanine methylester hydrochloride (L-Phe-
OMe), D-phenylalanine methylester hydrochloride (D-Phe-
OMe), L-alanine methylester hydrochloride (L-AlaOMe),
D-alanine methylester hydrochloride (D-AlaOMe), L-tryp-
tophan methylester hydrochloride (L-TrpOMe), D-tryptop-
han methylester hydrochloride (D-TrpOMe), L-phenyl-
glycinol (L-Phegly), D-phenylglycinol (L-Phegly), (R)-(5)-
(hydroxymethyl)-2-pyrolidinone [(R)-Hyd-Me-Pyr], (S)-(5)-
(hydroxymethyl)-2-pyrolidinone [(S)-Hyd-Me-Pyr] (Figs.
2 and 3).
0.4
0.1
-0.2
-0.5
-0.8
-3.7
-3.5
-3.3
-3.1
-2.9
Log [L]
Figure 4 illustrates the successive synthetic steps of the
extractants (1–7) used. Compounds 1 and 2 were prepared
according to the literature methods.15,16 Other compounds
(3–7) were synthesized by adapting known synthetic
procedures.
Figure 1. LogD versus log[L] for the extraction of D-PEA by 6 from an
aqueous phase into dichloromethane phase at 25 ꢁC.
3. Conclusions
4.2. Syntheses
A p-tert-butylcalix[6]arene was modified with chiral amino
alcohol groups in order to examine the extraction and chi-
ral discrimination ability towards some selected amino
acids and amino alcohols. Although the chiral calix[6]arene
4.2.1. 5,11,17,23,29,35-tert-Butyl-37,38-dimethoxy-39,40,
a
mixture of 2 (7 g, 7.00 mmol) and K2CO3 (3.86 g,
41,42-(methoxycarbonylmethoxy)calix[6]arene 3. To

S. Erdemir et al. / Tetrahedron: Asymmetry 17 (2006) 1258–1263
1261
H2N
OH
28.00 mmol) in acetone (400 mL), ethylbromoacetate
H
H
OH
(2.6 mL, 28.0 mmol) was added and the reaction mixture
was stirred at reflux for 2.5 days. After cooling, the solvent
was removed under reduced pressure. The remaining
solid was taken up CH2Cl2 (250 mL) and washed with
1 M HCl (2 · 250 mL) and water (250 mL). The organic
layer was dried over MgSO4 and evaporated to give a white
powder. The product was crystallized from methylene
chloride and hexane to obtain pure 3 (63%). IR (KBr):
NH2
L
-Phenylglycinol
H
D
-Phenylglycinol
OH
OH
O
O
N
N
1
1766 cmꢀ1 (C@O). H NMR (CDCl3): d 1.14, 1.18, 1.23
H
H
H
(R)-5-(Hydroxymethyl)-2-pyrolidinone
(S)-5-(Hydroxymethyl)-2-pyrolidinone
(s, 18H), 3.09 (s, 6H), 3.20 (br s, 4H), 3.35 (s, 2H), 3.69
(s, 2H), 3.74 (s, 4H), 3.60 (br s, 6H), 3.92 (br s, 8H), 3.98
(br s, 6H), 7.02 (br s, 6H), 7.14 (br s, 2H), 7.21 (br s,
4H). mp: 138–140 ꢁC. FAB-MS m/z: (1312.7) [M+Na]+
(calcd 1312.7). Calculated for C80H104O14 (1289.67): C,
74.50; H, 8.13. Found: C, 74.54; H, 8.24.
Figure 3. The chemical structures of some selected amino alcohols used in
experiments.
lated for C76H104O10 (1177.63): C, 77.51; H, 8.90. Found:
C, 77.73; H, 9.10.
4.2.2. 5,11,17,23,29,35-tert-Butyl-37,38-dimethoxy-39,40,
41,42-(hydroxyethoxy)calix[6]arene 4. LiAlH4 (0.5 g)
was added to a solution of 3 (2 g, 1.55 mmol) in anhydrous
diethyl ether at ꢀ4 ꢁC over a 5 min period. The solution
began refluxing. The solution was allowed to remain un-
disturbed until it returned to ambient temperature. Heat
was applied, and the solution refluxed for 16 h. Any excess
LiAlH4 was destroyed by the addition of cold HCl (8 mL,
2 M), and the organic layer separated. The ether layer was
further washed with HCl (2 · 50 mL), brine and then dried
over MgSO4. A white solid residue was obtained after
evaporation of the ether under reduced pressure. Tritur-
ation of this solid residue with boiling hexane gave the
product as white feathery crystals. Yield, 65% (1.2 g);
mp: 175–180 ꢁC. IR (KBr): 3490 cmꢀ1 (OH). 1H NMR
(CDCl3): d 1.10, 1.28 (s, 27H), 3.13–3.36 (m, 14H), 3.40
(s, 4H), 3.53 (br s, 6H), 3.85 (br s, 8H), 3.91 (br s, 6H),
6.74–6.99 (m, 6H), 7.02–7.21 (m, 2H), 7.21 (br s, 4H).
FAB-MS m/z: (1200.6) [M+Na]+ (calcd 1200.6). Calcu-
4.2.3. 5,11,17,23,29,35-tert-Butyl-37,38-dimethoxy-39,40,
41,42-(p-tosylethoxy)calix[6]arene 5.
A solution of 4
(1 g, 0.85 mmol) in pyridine (25 mL) was treated with
para-toluenesulfonyl chloride (1.96 g) at ꢀ4 ꢁC. The mix-
ture was stirred for 5 min to ensure a homogeneous solu-
tion. The solution was then kept at ꢀ4 ꢁC for 5 days. To
this solution was added HCl (25 mL, 2 M) and a precipitate
formed. This precipitate was filtered, and then dissolved in
methylene chloride. This solution was washed with HCl
(2 · 10 mL, 2 M), brine (2 · 50 mL) and then dried over
MgSO4. Evaporation of the solution gave a white solid res-
idue, which was crystallized from a mixture of methylene
chloride and hexane to give product 5 in 74% yield. mp:
145–150 ꢁC. IR (KBr): 921 cmꢀ1 (S@O), 741, 823 cmꢀ1
(S–O).
4.2.4. Preparation of chiral p-tert-butyl-calix[6]arene amines
with D/L-phenylalaninol. The preparation of compounds 6
and 7 is carried out following the general procedure: D/L-
phenylalaninol (0.92 mmol) was added to a solution of 5
(0.22 mmol) dissolved in dry THF (50 mL). After the mix-
ture was stirred at room temperature for 5 h, distilled water
(15 mL) was added to the solution. The mixture was ex-
tracted with dichloromethane, washed with distilled water
(2 · 50 mL), brine (2 · 50 mL) and then dried with aqueous
magnesium sulfate. The solvent was evaporated under
vacuo, and recrystallized with dichloromethane to give
H2N
CH3
H2N
CH3
H
H
O
O
.
.
HCl
HCl
O
O
CH3
CH3
D
-Alanine methylester hydrochloride
L-Alanine methylester hydrochloride
6/7 as white crystals. For compound 6, yield: 51%, mp:
H
H2N
H2N
H
22
209 ꢁC. ½aꢁD ¼ ꢀ3:9 (c 3.3, CHCl3); IR (KBr): 3102 cmꢀ1
.
.
O
O
HCl
HCl
1
(NH), 3414 cmꢀ1 (OH); H NMR (CDCI3): d 1.15 (br s,
CH3
CH3
O
O
54H), 2.67 (dd, J = 9 Hz, 4H), 2.87 (dd, J = 4.5 Hz, 4H),
3.30–3.51 (m, 16H), 3.57 (d, J = 8.5 Hz, 8H), 3.67 (t,
J = 6.5 Hz, 8H), 4.37 (br s, 8H), 5.00 (br s, 6H), 6.77 (s,
4H), 6.90–7.11 (m, 32H), 7.66 (d, J = 8 Hz, 4H). FAB-
MS m/z: (1733.4) [M+Na]+ (calcd 1733.4). Calculated for
C112H148N4O10 (1710.39): C, 78.65; H, 8.72; N, 3.27.
Found: C, 78.81; H, 9.01, N, 3.25.
D
-Phenylalanine methylester hydrochloride
L-Phenylalanine methylester hydrochloride
H2N
H
H
H2N
O
O
CH3
HCl
CH3
O
O
.
.
HCl
22
For compound 7, yield: 45%, mp: 213 ꢁC. ½aꢁD ¼ þ4:2 (c
N
H
N
H
3.3, CHCl3); IR (KBr): 3105 cmꢀ1 (NH), 3420 cmꢀ1
(OH); 1H NMR (CDCI3): d 1.11 (br s, 54H), 2.78 (m,
8H), 3.28–3.52 (m, 16H), 3.53 (d, J = 8.5 Hz), 3.64 (t,
J = 6.5 Hz), 4.39 (br s, 8H), 5.05 (br s, 6H), 6.76 (s, 4H),
6.90–7.17 (m, 32H), 7.65 (d, J = 8 Hz, 4H). FAB-MS
D
-Tryptophan methylester hydrochloride
L
-Tryptophan methylester hydrochloride
Figure 2. The chemical structures of some selected amino acid methylest-
ers used in experiments.

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S. Erdemir et al. / Tetrahedron: Asymmetry 17 (2006) 1258–1263
OH
OH
OMe
CH3 I
K2CO3/Acetone
2
6
4
Ethylbromoacetate
OR
K2CO3/Acetone
OR
OMe
OMe
LiAlH4
Diethylether
2
2
4
4
R=CH2CH2OH
R=CH2COOCH3
4
3
Pyridine
Tos-CI
HO
H
HN
O
OMe
THF
OR
OMe
D
-phenylalaninol
2
4
2
4
6
L
-phenylalaninol
R=CH2CH2OTs
5
HO
THF
H
HN
O
OMe
2
4
7
Figure 4. Schematic representation of synthesis of chiral calix[6]arene derivatives 6 and 7.
m/z: (1733.4) [M+Na]+ (calcd 1733.4). Calculated for
C112H148N4O10 (1710.39): C, 78.65; H, 8.72; N, 3.27.
Found: C, 78.84; H, 8.91, N, 3.26.
remained in the aqueous phase was then determined
spectrophotometrically. Blank experiments showed that
no picrate extraction occurred in the absence of calixarene.
The percent extraction (E%) has been calculated as:
4.3. Analytical procedure
ðE%Þ ¼ ðA0 ꢀ A=A0Þ ꢂ 100
ð4Þ
Picrate extraction experiments were performed following
Pedersen’s procedure.18 A 10 mL of a 2.0 · 10ꢀ5 M aque-
ous picrate (the picrate solutions were prepared as our
previous study14) and 10 mL of 1 · 10ꢀ3 M solution of
calixarene 6 or 7 in CH2Cl2 were vigorously agitated in a
stoppered glass tube with a mechanical shaker for 2 min,
then magnetically stirred in a thermostated water-bath at
25 ꢁC for 1 h, and finally left standing for an additional
30 min. The concentration of the picrate ion, which
where A0 and A are the initial and final concentrations of
the metal picrate before and after the extraction,
respectively.
Acknowledgement
We thank the Research Foundation of The Selc¸uk Univer-
sity (BAP) for financial support of this work.

S. Erdemir et al. / Tetrahedron: Asymmetry 17 (2006) 1258–1263
1263
11. (a) Samsone, E.; Barboso, S.; Casnati, A.; Fabbi, M.;
Pochine, A.; Ugozzdi, F.; Ungaro, R. Eur. J. Org. Chem.
1998, 897; (b) Xu, X.; He, J.; Chan, A. S. C.; Han, X.; Cheng,
J.-P. Tetrahedron: Asymmetry 1999, 10, 2685; (c) Xu, H.;
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