Chemoenzymatic synthesis of enantiomerically enriched 2-oxobicyclo[m.1.0] alkan-3-yl acetate derivatives

Article abstract of DOI:10.1016/j.tetasy.2006.01.004

Racemic α′-acetoxy α,β-unsaturated cyclopentanone and cyclohexanone have been resolved into the corresponding enantiomerically enriched α′-hydroxylated and acetoxylated compounds with 96-97% ee via PLE hydrolysis. Stereoselectivity in the palladium(II)-ca

Full text of DOI:10.1016/j.tetasy.2006.01.004

Tetrahedron: Asymmetry 17 (2006) 287–291
Chemoenzymatic synthesis of enantiomerically enriched
2-oxobicyclo[m.1.0]alkan-3-yl acetate derivatives
b
c
a
a,
*
¨
Fazilet Devrim Ozdemirhan, Murat C¸ elik, Selin Atlı and Cihangir Tanyeli
aDepartment of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
b
_
Department of Chemistry, Abant Izzet Baysal University, 14280 Bolu, Turkey
cDepartment of Chemistry, Atatu¨rk University, 25240 Erzurum, Turkey
Received 21 November 2005; accepted 9 January 2006
Abstract—Racemic a0-acetoxy a,b-unsaturated cyclopentanone and cyclohexanone have been resolved into the corresponding enan-
tiomerically enriched a0-hydroxylated and acetoxylated compounds with 96–97% ee via PLE hydrolysis. Stereoselectivity in the
palladium(II)-catalyzed reaction between the enantiomerically enriched a0-acetoxylated compounds and diazomethane has been
investigated. In the a0-acetoxylated cyclopentenone, preferential cyclopropanation occurs in the anti-form, whereas a0-acetoxylated
cyclohexenone affords both syn- and anti-products (syn:anti, 61:36%). The relative configuration of the products was determined by
NOE experiments.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
In connection with our work on the development of
novel procedures for the direct oxidation of a,b-unsatu-
rated cyclic ketones with Mn(OAc)3 in the synthesis of
9
Cyclopropanes are attractive and versatile building
blocks for synthetic targets, since the specific reactivity
of this carbocyclic ring system1 renders them useful as
synthetic intermediates. Cyclopropane rings are also
found in a variety of natural products and biologically
active compounds.2 Consequently, a great deal of effort
is devoted to the development of methods, which are
aimed at the synthesis of cyclopropanes with high
enantioselectivities.2,3 The transition metal-catalyzed
cyclopropanation of olefinic bonds using diazo com-
pounds as a carbene source is among the best developed
and most useful methods available to the synthetic
organic chemist.3,4 Alternatively, cyclopropanes can be
prepared by addition of trimethyl sulfoxonium iodides
to enones.5
( )-5-acetoxy-2-cyclopentenone 1a and ( )-6-acetoxy-
2-cyclohexenone 1b and subsequent PLE catalyzed enzy-
matic resolution of them into enantiomerically enriched
forms,10 we herein report the results of Pd(OAc)2 catal-
yzed cyclopropanation with diazomethane.
2. Results and discussion
2.1. Enzymatic hydrolyses of racemic substrates 1a and 1b
The bioconversions of racemic 1a9 and 1b10 were per-
formed using PLE according to the following proce-
dure, which has already been developed in our group
(Scheme 1).10 To a stirred solution of 1a or 1b
(500 mg) in a phosphate buffer (pH 7.00, 50 mL), PLE
(100 lL) was added in one portion and the reaction
mixture stirred at 20 ꢁC in a pH stat unit. The conver-
sions were monitored by TLC. After 5 h, (S)-(+)-5-acet-
oxy-2-cyclopentenone 1a was obtained with 96% ee in
45% chemical yield (Table 1, entry 1). The bioconver-
sion of substrate 1b using PLE under the same condi-
tions as above was completed in 3 h. (S)-(ꢀ)-6-
Acetoxy-2-cyclohexenone 1b was obtained with 97% ee
in 46% isolated yield (entry 2). In our previous work,
In particular, cyclopropyl ketones have proven useful as
synthetic intermediates.6 Angle strain and polarization
by the carbonyl, impart special reactivity onto these mole-
cules. The most widely applied methods for the synthe-
sis of cyclopropyl ketones include the direct cyclo-
propanation of a,b-unsaturated ketones7 and cyclo-
propanation of allylic alcohols followed by oxidation.8
*
Corresponding author. Tel.: +90 312 210 3222; fax: +90 312 210
1280; e-mail: tanyeli@metu.edu.tr
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.01.004

¨
F. D. Ozdemirhan et al. / Tetrahedron: Asymmetry 17 (2006) 287–291
288
O
O
O
O
O
O
OAc
OAc
OH
OAc
OAc
OAc
OAc
OAc
PLE
Pd(OAc)2
CH2N2
+
+
+
n
n
n
n
n
n
rac-1a-b
(S)-1a-b
(R)-2a-b
(
S
)-1a-b
anti-3a-b
syn-3a-b
AcCl
Pyridine
a: n=1
b: n=2
O
O
O
Pd(OAc)2
CH2N2
OAc
O
OAc
n
n
n
syn-3a-b
anti-3a-b
(
R
)-1a-b
n
(R)-1a-b
a: n=1
b: n=2
Scheme 1.
Scheme 2.
we showed that 5-hydroxy-2-cyclopentenone 2a and 6-
hydroxy-2-cyclohexenone 2b could not be isolated in
enantiomerically enriched forms due to the racemization
and/or rearrangement of a0-hydroxylated cyclic enones
as observed in our previous work.11 In this work, after
purification of substrates 2a and 2b, they were immedi-
ately subjected to acetylation to afford corresponding
(R)-(ꢀ)-5-acetoxy-2-cyclopentenone 1a and (R)-(+)-6-
acetoxy-2-cyclohexenone 1b with 94% ee in 42% chemi-
cal yield and 92% ee in 44% chemical yield, respectively
(entries 3 and 4).
by NMR (Table 2, entry 1). The relative configuration
of (ꢀ)-3a was determined by differential NOE experi-
ments. The characteristic methine proton of the acetoxy
attached carbon at 5.13 ppm and one of the methylene
proton of cyclopropane ring at 1.16–1.25 ppm show
significant NOE enhancements, which indicate the
anti-relationship between the acetoxy group and the
cyclopropane moiety. This is presumably the thermo-
dynamically most stable of the possible diastereomers.
Thus, the absolute configuration of anti-(ꢀ)-3a was
determined as anti-(1S,3S,5S)-(ꢀ)-3a.
2.2. Cyclopropanation of substrates (S)-(+)-1a, (R)-(ꢀ)-
1a, (S)-(ꢀ)-1b and (R)-(+)-1b
The next attempt involved substrate (S)-(ꢀ)-1b using
Pd(OAc)2 and diazomethane under the same conditions
as above. The chemical yield of the reaction (entry 2)
was as high as in the five-membered ring case (97%). In
contrast to entry 1, 1H NMR spectra showed the mixture
of two diastereomers. The mixture was separated by flash
column chromatography to afford (ꢀ)-3b and (+)-3b in
61% and 36% chemical yields, respectively. The configu-
rations of (ꢀ)-3b and (+)-3b were determined by differen-
tial NOE experiments. In the differential NOE
experiment of (+)-3b (as the minor product), we observed
significant NOE enhancement between the methine pro-
ton of the acetoxy attached carbon at 5.05 ppm and one
of the methylene proton of cyclopropane ring at
1.12 ppm and decided that the acetoxy group and the
cyclopropane moiety are in an anti-relationship. Hence,
the configuration of the minor diastereomer anti-(+)-3b
was determined as anti-(1S,3S,6R)-(+)-3b. In contrast
to the minor diastereomer, the major diastereomer (ꢀ)-
3b did not show any NOE enhancement between the
characteristic methine proton of acetoxy attached carbon
at 4.83 ppm and methylene protons of cyclopropane moi-
ety at 1.05–1.11 and 1.15–1.19 ppm. The configuration of
syn-(ꢀ)-3b was determined as syn-(1R,3S,6S)-(ꢀ)-3b.
We envisaged the stereoselective synthesis of various
2-oxobicyclo[m.1.0]alkan-3-yl acetates using the enantio-
merically enriched a0-acetoxy a,b-unsaturated cyclo-
pentenone and cyclohexenone derivatives. Since the
palladium-catalyzed cyclopropanation using diazo-
methane and a,b-unsaturated carbonyl compounds,
including cyclopentenones and cyclohexenones, has been
used as an efficient method12, we applied this method to
all enantiomerically enriched substrates (S)-(+)-1a, (R)-
(ꢀ)-1a, (S)-(ꢀ)-1b and (R)-(+)-1b to afford the possible
stereoisomers (Scheme 2).
The first cyclopropanation was performed with substrate
(S)-(+)-1a using Pd(OAc)2 and diazomethane prepared
in situ according to the following general procedure.
To a stirred solution of (S)-(+)-1a (300 mg, 2.14 mmol)
and Pd(OAc)2 (13.32 mg, 0.06 mmol) in ether (30 mL),
an ether solution of diazomethane (10 mmol) prepared
from N-methyl-N-nitrosourea was added by distillation
and the mixture stirred at 0 ꢁC. The conversion was moni-
tored by TLC. After 4 h, (ꢀ)-3a was obtained in 98%
chemical yield as the single diastereomer, as established
Table 1.
20
Entry
Substrate
Time (h)
Acetoxy product
Yield (%)a
½aꢁD
ee (%)b
1
2
3
4
(
(
)-1a
)-1b
5
3
—
—
(S)-(+)-1a
(S)-(ꢀ)-1b
(R)-(ꢀ)-1a
(R)-(+)-1b
45
46
42
44
+60.3
ꢀ88.7
ꢀ59.1
+84.2
96
97
94
92
(ꢀ)-2a
(+)-2b
a Yields (%) are given as the isolated yields. In entries 3 and 4, yields (%) are calculated with respect to corresponding racemic substrates.
b Enantiomeric excess values are determined using Chiralcel ODH chiral column HPLC analysis.

¨
F. D. Ozdemirhan et al. / Tetrahedron: Asymmetry 17 (2006) 287–291
289
Table 2.
20
20
Entry
Substrate
Time (h)
syn-Product
Yield (%)
½aꢁD
anti-Product
Yield (%)
½aꢁD
1
2
3
4
(S)-(+)-1a
(S)-(ꢀ)-1b
(R)-(ꢀ)-1a
(R)-(+)-1b
4
3
4
3
—
—
61
—
63
—
(1S,3S,5S)-(ꢀ)-3a
(1S,3S,6R)-(+)-3b
(1R,3R,5R)-(+)-3a
(1R,3R,6S)-(ꢀ)-3b
98
36
97
34
ꢀ22.0
(1R,3S,6S)-(ꢀ)-3b
ꢀ22.05
+5.0
—
—
+21.3
ꢀ4.2
(1S,3R,6R)-(+)-3b
+20.52
In entry 3, (R)-(ꢀ)-5-acetoxy-2-cyclopentenone 1a was
also subjected to cyclopropanation under the same con-
ditions as above to confirm the yield and absolute con-
figuration assignment of the resultant oppositely
configured product. Comparison of the specific rotation
value of the product with (ꢀ)-3a showed an opposite
sign, which proves the enantiomeric relationship. In entry
4, a similar comparison was carried out by the cyclo-
propanation products of (R)-(+)-1b with the results
obtained in entry 2. We observed the enantiomeric rela-
tionships between the pairs of the products in entries 2
and 4.
4.1. General procedure for enzymatic hydrolyses of ( )-1a
and 1b
To a stirred solution of 500 mg rac-1a,1b in 50 mL of
pH 7.00 phosphate buffer, 100 lL PLE was added in
one portion and the reaction mixture stirred at 20 ꢁC
in a pH stat unit. The conversion was monitored by
TLC. The reaction mixture was extracted with ethyl ace-
tate, dried over MgSO4 and concentrated under reduced
pressure. The product was purified by flash column
chromatography (EtOAc–hexane, 1:3). Compounds
(S)-(+)-1a and (S)-(ꢀ)-1b were isolated and are in accor-
dance with the literature data.10 The isolated products
(R)-2a and (R)-2b were directly subjected to an acetyl-
ation reaction to prevent their complete racemization.
To a stirred solution of (R)-2a,2b (250 mg, 2.55 and
2.23 mmol, respectively) in CH2Cl2 (10 mL), dry pyri-
dine (300 mg, 3.75 mmol) was added at 0 ꢁC and stirred
for 30 min. Acetylchloride (300 mg, 3.75 mmol) was
then added dropwise. The resultant mixture was stirred
for 12 h at room temperature. The organic phase was
extracted with 0.1 M HCl (3 · 10 mL), saturated NaHCO3
(3 · 10 mL) and brine (2 · 10 mL), dried over MgSO4
and solvent was evaporated under reduced pressure to
afford quantitatively (R)-(ꢀ)-1a and (R)-(+)-1b. All are
in accordance with the literature data.10
3. Conclusion
Herein, we have improved the direct stereoselective
cyclopropanations of enantiomerically enriched a0-acet-
oxy a,b-unsaturated cyclopentane (S)-(+)-1a and cyclo-
hexane (S)-(ꢀ)-1b, respectively, using Pd(OAc)2 and
diazomethane in good yields, and have shown that five-
membered ring enone (S)-(+)-1a afforded only anti-dia-
stereomers (1S,3S,5S)-(ꢀ)-3a whereas the six-membered
enone (S)-(ꢀ)-1b afforded the both the syn- and anti-dia-
stereomer (1R,3S,6S)-(ꢀ)-3b and (1S,3S,6R)-(+)-3b as
the major and minor products, respectively. The config-
urations of anti-(ꢀ)-3a, syn-(ꢀ)-3b and anti-(+)-3b were
determined by differential NOE experiments. The enantio-
mers (R)-(ꢀ)-1a and (R)-(+)-1b were also subjected to
the palladium-catalyzed cyclopropanation reactions
and the findings confirmed the results obtained in the
first set of experiments.
4.1.1.
(S)-(+)-5-Acetoxy-2-cyclopentenone
(S)-(+)-
20
1a. (0.23 g, 45%) as a colourless oil; 96% ee ½aꢁD
¼
þ60:3 (c 0.2, CHCl3).
4.1.2. (S)-(ꢀ)-6-Acetoxy-2-cyclohexenone (S)-(ꢀ)-1b.
20
(0.23 g, 46%) as a colourless oil; 97% ee ½aꢁD ¼ ꢀ88:7
(c 0.5, MeOH).
4. Experimental
4.1.3. (R)-(ꢀ)-5-Acetoxy-2-cyclopentenone (R)-(ꢀ)-1a.
20
1
The H and 13C NMR spectra were recorded in CDCl3
(0.25 g, 49%) as a colourless oil; 94% ee ½aꢁD ¼ ꢀ59:1
on a Brucker Spectrospin Avance DPX 400 spectro-
meter. Chemical shifts are given in parts per million
downfield from tetramethylsilane. Apparent splittings
are given in all cases. Infrared spectra were obtained
from KBr pellets on a Mattson 1000 FT-IR spectro-
photometer. Mass spectra were recorded on a Varian
MAT 212. Optical rotations were measured in a 1 dm
cell using a Rudolph Research Analytical Autopol III
polarimeter at 20 ꢁC. HPLC measurements were per-
formed with ThermoFinnigan Spectra System instru-
ment. Separations were carried out on Chiralcel OD-H
analytical column (250 · 4.60 mm) with hexane/2-pro-
pyl alcohol as eluent. Column chromatography was
performed on silica gel (60-mesh, Merck). TLC was car-
ried out on Merck 0.2-mm silica gel 60 F254 analytical
aluminium plates. PLE (Pig Liver Esterase) was
purchased from Sigma as a suspension in ammonium
sulfate solution (3.2 mol/L).
(c 0.2, CHCl3).
4.1.4. (R)-(+)-6-Acetoxy-2-cyclohexenone (R)-(+)-1b.
20
(0.22 g, 44%) as a colourless oil; 92% ee ½aꢁD ¼þ84:2
(c 0.5, MeOH).
4.2. General procedure for cyclopropanation of (S)-(+)-
1a, (S)-(ꢀ)-1b, (R)-(ꢀ)-1a and (R)-(+)-1b
Pd(OAc)2 (13.32 mg, 0.059 mmol) was added to an ice-
cooled solution of substrate (S)-1a,1b or (R)-1a,1b
(300 mg, 2.14 mmol) in ether (30 mL). To this solution,
an etheral solution of diazomethane prepared from
N-methyl-N-nitrosourea (720 mg, 10.0 mmol) and KOH
(2.0 g, 35.6 mmol) was distilled. The resultant mixture
was stirred at 0 ꢁC for 4 h. The conversion was moni-
tored by TLC. The reaction mixture was allowed to
warm to room temperature and then filtered through a

¨
F. D. Ozdemirhan et al. / Tetrahedron: Asymmetry 17 (2006) 287–291
290
pad of Celite and concentrated. The product was purified
by flash column chromatography (EtOAc–hexane 1:3).
Research Council for
2244(102T169)].
a
grant [no. TBAG-
4.2.1. (1S,3S,5S)-(ꢀ)-2-Oxobicyclo[3.1.0]hexan-3-yl ace-
tate anti-(1S,3S,5S)-(ꢀ)-3a. (294 mg, 98%) as a colour-
20
References
less oil; 96% ee ½aꢁD ¼ ꢀ22:0 (c 0.2, CHCl3); mmax (neat)
2989, 2956, 2925, 1732, 1639 cmꢀ1; dH (400 MHz,
CDCl3) 5.12 (1H, t, J = 8.4 Hz, CHOAc), 2.58 (1H,
ddd, J = 12.5, 8.1, 4.4 Hz, CHaHbCHOAc), 2.13 (3H,
s, MeCO2), 2.05–2.16 (2H, m, CHaHbCHOAc and
cyclopropylCHCO), 1.89–1.92 (1H, m, cycloprop-
ylCHCHCO), 1.26–1.29 (1H, m, cyclopropylCHexo-Hen-
do), 1.16–1.19 (1H, m, cyclopropylCHexoHendo); dC
(100.6 MHz, CDCl3) 207.6, 170.5, 70.9, 29.2, 24.9,
21.0, 20.3, 15.2; HRMS (EI): M+, found 154.0629,
C8H10O3 requires 154.0630.
1. (a) Wong, H. C. N.; Hon, M. Y.; Tse, C. W.; Yip, Y. C.;
Tanko, J.; Hudlicky, T. Chem. Rev. 1989, 89, 165; (b)
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Rappoport, Z., Ed.; Wiley: Chichester, UK, 1987; Chapter
8, p 375.
2. Liu, H. W.; Walsh, C. T. In The Chemistry of the
Cyclopropyl Group; Rappoport, Z., Ed.; Wiley: Chiches-
ter, UK, 1987; Chapter 16, p 959.
3. (a) Pfaltz, A. In Comprehensive Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.;
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(b) Lydon, K. M.; McKervey, M. A. In Comprehensive
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Academic Press: New York, 1974; Vol. 12, Chapter 3.
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sekhar, S.; Narasihmulu, Ch.; Jagadeshwar, V.; Venkatram,
K. R. Tetrahedron Lett. 2003, 44, 3629.
4.2.2. (1R,3R,5R)-(+)-2-Oxobicyclo[3.1.0]hexan-3-yl ace-
tate anti-(1R,3R,5R)-(+)-3a. (320 mg, 97%) as a col-
20
ourless oil; 94% ee ½aꢁD ¼ þ21:3 (c 0.2, CHCl3).
4.2.3.
acetate syn-(1R,3S,6S)-(ꢀ)-3b. (199 mg, 61%) as a col-
(1R,3S,6S)-(ꢀ)-2-Oxobicyclo[4.1.0]heptan-3-yl
20
ourless oil; 97% ee ½aꢁD ¼ ꢀ22:05 (c 0.2, CHCl3); mmax
(neat) 2980, 2952, 2870, 1746, 1712 cmꢀ1
;
dH
(400 MHz, CDCl3) 4.83 (1H, dd, J = 13.0, 6.4 Hz,
CHOAc), 2.13–2.20 (2H, m, CH2CH2CHOAc), 2.11
(3H, s, MeCO2), 1.82–1.88 (1H, m, CHaHbCHOAc),
1.70–1.79 (2H, m, CHaHbCHOAc and cycloprop-
ylCHCO), 1.59–1.69 (1H, m, cyclopropylCHCHCO),
1.16 (1H, q, J = 5.5 Hz, cyclopropylCHaHb), 1.05–1.11
(1H, m, cyclopropylCHaHb); dC (100.6 MHz, CDCl3)
202.3, 170.3, 74.2, 24.4, 21.6, 21.3, 21.0, 15.0, 8.9; HRMS
(EI): M+, found 168.0783, C9H12O3 requires 168.0786.
6. (a) Julia, M.; Julia, S.; Tchen, S. Y. Bull. Soc. Chim. Fr.
1961, 1849; (b) Mongrain, M.; Lafontaine, J.; Belanger,
A.; Deslongchamps, P. Can. J. Chem. 1970, 48, 3273; (c)
McCurry, P. M. Tetrahedron Lett. 1971, 1845; (d) Ireland,
R. E.; Dawson, M. I.; Welch, S. C.; Hagenbach, A.;
Bordner, J.; Trus, B. J. Am. Chem. Soc. 1973, 95, 7829; (e)
Ireland, R. E.; Walba, D. M. Tetrahedron Lett. 1976,
1971; (f) Corey, E. J.; Balanson, R. D. Tetrahedron Lett.
1973, 3153; (g) Bhaleras, U. T.; Plattner, J. J.; Rappoport,
H. J. Am. Chem. Soc. 1970, 92, 3429; (h) Marshall, J. A.;
Johnson, P. C. J. Org. Chem. 1970, 35, 192; (i) Trost, B.
M.; Taber, D. F.; Alper, J. B. Tetrahedron Lett. 1976,
3857; (j) Piers, E.; Banville, J. J. Chem. Soc., Chem.
Commun. 1979, 1138; (k) Hudlicky, T.; Kutchan, T. M.;
Wilson, S. R.; Mao, D. T. J. Am. Chem. Soc. 1980,
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Lahiri, S.; Go¨rls, H.; Langer, J. J. Org. Chem. 2004, 69,
9128.
7. (a) Marchand, A. P. In The Chemistry of Double Bonded
Functional Groups: Supplement A; Patai, S., Ed.; Wiley:
New York, 1977; Vol. 1, p 533; (b) Bethell, D. In Organic
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8. (a) Miller, R. D.; McKean, D. R. J. Org. Chem. 1981, 46,
2412; (b) Mohamadi, F.; Still, W. C. Tetrahedron Lett.
1986, 893.
4.2.4.
(1S,3R,6R)-(+)-2-Oxobicyclo[4.1.0]heptan-3-yl
acetate syn-(1S,3R,6R)-(+)-3b. (206 mg, 63%) as a col-
ourless oil; 92% ee ½aꢁD ¼ þ20:5 (c 0.2, CHCl3).
20
4.2.5.
(1S,3S,6R)-(+)-2-Oxobicyclo[4.1.0]heptan-3-yl
acetate anti-(1S,3S,6R)-(+)-3b. (118 mg, 36%) as a col-
20
ourless oil; 97% ee ½aꢁD ¼ þ5:0 (c 0.2, CHCl3); mmax
(neat) 2985, 2946, 2886, 2853, 1754, 1720 cmꢀ1; dH
(400 MHz, CDCl3) 5.05 (1H, dd, J = 11.0, 5.5 Hz,
CHOAc), 2.15–2.29 (2H, m, CH2CH2CHOAc), 2.14
(3H, s, MeCO2), 1.98–2.06 (1H, m, CHaHbCHOAc),
1.88–1.96 (2H, m, CHaHbCHOAc and cyclopro-
pylCHCO), 1.78–1.94 (1H, m, cyclopropylCHCHCO),
1.32–1.38 (1H, m, cyclopropylCHexoHendo), 1.11 (1H,
q, J = 5.5 Hz, cyclopropylCHexoHendo); dC (100.6 MHz,
CDCl3) 203.2, 170.3, 72.1, 32.2, 26.4, 22.0, 21.2, 21.0,
18.4; HRMS (EI): M+, found 168.0784, C9H12O3
requires 168.0786.
4.2.6.
acetate anti-(1R,3R,6S)-(ꢀ)-3b. (110 mg, 34%) as a
(1R,3R,6S)-(ꢀ)-2-Oxobicyclo[4.1.0]heptan-3-yl
20
colourless oil; 92% ee ½aꢁD ¼ ꢀ4:2 (c 0.2, CHCl3).
Acknowledgements
9. Tanyeli, C.; Tosun, A.; Turkut, E.; Sezen, B. Tetrahedron
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We thank Dr. Cavit Kazaz for NMR measurements. We
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