Enzymatic resolution of (±)-2-endo-hydroxymethyl and acetoxymethyl substituted hexachloronorbornene derivatives

Article abstract of DOI:10.1016/j.tetasy.2005.06.014

(±)-2-endo-Hydroxymethyl-1,4,5,6,7,7-hexachlorobicyclo[2.2.1] hept-5-ene and (±)-2-endo-acetoxymethyl-1,4,5,6,7,7-hexachlorobicyclo[2. 2.1]hept-5-ene were resolved by using various hydrolases to afford enantiomerically enriched products with ees of 94-98%. The absolute configuration was determined by transforming 2-endo-acetoxymethyl-1,4,5,6,7,7- hexachlorobicyclo[2.2.1]hept-5-ene into 2-endo-hydroxymethyl-bicyclo[2.2.1]hept- 5-ene with known absolute configuration.

Full text of DOI:10.1016/j.tetasy.2005.06.014

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 16 (2005) 2315–2318
Enzymatic resolution of ( )-2-endo-hydroxymethyl and
acetoxymethyl substituted hexachloronorbornene derivatives
_
Yunus Emre Turkmen, Idris Mecidog˘lu Akhmedov* and Cihangir Tanyeli*
¨
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
Received 13 May 2005; accepted 13 June 2005
Abstract—( )-2-endo-Hydroxymethyl-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene and ( )-2-endo-acetoxymethyl-1,4,5,6,7,7-
hexachlorobicyclo[2.2.1]hept-5-ene were resolved by using various hydrolases to afford enantiomerically enriched products with
ees of 94–98%. The absolute configuration was determined by transforming 2-endo-acetoxymethyl-1,4,5,6,7,7-hexachlorobicy-
clo[2.2.1]hept-5-ene into 2-endo-hydroxymethyl-bicyclo[2.2.1]hept-5-ene with known absolute configuration.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
[2.2.1]hepta-2,5-diene and meso-bis(hydroxymethyl)-
1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hepta-2,5-diene.13 In
connection to these biotransformations, we investigated
the enzymatic resolution of ( )-2-endo-hydroxymethyl-
1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene ( )-1 and
( )-2-endo-acetoxymethyl-1,4,5,6,7,7-hexachlorobicyclo-
[2.2.1]hept-5-ene ( )-2. Both hydroxymethyl ( )-1 and
acetoxymethyl ( )-2 norbornene derivatives possess
more flexible structures than corresponding norbornadi-
ene derivatives. In particular, ( )-2-endo-hydroxymethyl
derivative ( )-1 has already been used as a precursor in
some intramolecular cyclization reactions to afford some
polycyclic natural products.2,14 These studies prompted
us towards the development of a practical method as
mentioned above to obtain enantiomerically enriched
products using the enzymatic resolution approach.
Screening reactions were first executed with various
hydrolases (i.e., PPL, CCL, PLE, HLE and PL) using
substrate:enzyme ratios from 1:1 to 1:0.5. Among the
hydrolases studied, CCL, PLE, PPL and HLE proved
to be suitable for the resolution of these substrates.
The observed promising preliminary results directed us
towards a thorough catalytic study. All of the enzymes
afforded (1S,2R,4R) configured acetoxy derivative (ꢀ)-
2 with high ee values. Herein, we report the highly effi-
cient resolution of the racemic substrates ( )-1 and
( )-2 with CCL, PLE, PPL and HLE (Scheme 1).
Syntheses of polychlorinated norbornene derivatives
have been of growing interest for some years.1 These
are used as starting materials leading to the synthesis
of a-diketones2 and c-lactone-fused cyclopentanoids,3
which serve as precursors for natural products.4 Asym-
metric synthesis of these compounds has further impor-
tance as they can easily be converted into biologically
active precursors5 and chiral ligands.6 A hexachloronor-
bornene carboxylic acid derivative was found to be a
suitable resolving agent of some optically active biologi-
cal molecules in high performance liquid chromatogra-
phy.7 Moreover, high endo selectivity can be obtained
during the Diels–Alder reactions of hexachlorocyclo-
pentadiene because of the steric repulsion of its chlorine
atoms at the C-5 position.8 In a stereoselective manner,
some asymmetric Diels–Alder reactions of hexachloro-
cyclopentadiene with various chiral dienophiles have
been investigated.9,10
In the literature, however, there have only been a few
examples of enzymatic resolution of hexa- and tetra-
chlorinated norbornene derivatives.5,6,11 Recently, we
reported the enzymatic resolution of ( )-2-hydroxy-
methyl-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hepta-2,5-diene,
( )-2-acetoxymethyl-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]-
hepta-2,5-diene12 and enzymatic desymmetrization of
meso-bis(acetoxymethyl)-1,4,5,6,7,7-hexachlorobicyclo-
2. Results and discussion
*
( )-1 was synthesized in 89% chemical yield in its pure
endo form, through a Diels–Alder reaction by heating
Corresponding authors. Tel.: +90 312 210 3222; fax: +90 312 210
1280; e-mail addresses: mecid@metu.edu.tr; tanyeli@metu.edu.tr
0957-4166/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2005.06.014

2316
Y. E. Tu¨rkmen et al. / Tetrahedron: Asymmetry 16 (2005) 2315–2318
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
enzyme
CCL
vinyl acetate
+
7
pH =
Cl
Cl
Cl
Cl
OH
Cl
( )-1
Cl
(-)-2
Cl
Cl
( )-2
OH
OAc
OAc
(+)-1
Scheme 1.
a mixture of hexachlorocyclopentadiene and allyl alco-
hol in a sealed tube. ( )-2 was subsequently obtained
by acetylation of ( )-1 with acetyl chloride in the pres-
ence of pyridine in 81% chemical yield.
Cl
Cl
Cl
Cl
Na/EtOH
liq. NH3
Cl
Cl
OH
OAc
(1S,2R,4R)-(-)-2
Scheme 2.
The first bioconversion was performed by the enzyme
catalyzed acetyl transfer to ( )-1 in the presence of vinyl
acetate as acetyl source and catalytic amount of CCL
(substrate:enzyme = 1:0.02). The conversion was moni-
tored by TLC and 50% conversion was obtained after
168 h. The products were separated by flash column
chromatography and (ꢀ)-2 was isolated with 97% ee
in 39% yield (Table 1, entry 1). All ee determinations
were done over acetylated derivative 2, since it was effec-
tively resolved by HPLC.
(1R,2S,4S)-(-)- 3
3. Conclusion
Hexachlorocyclopentadiene afforded only endo-adduct
1 in the Diels–Alder reaction with allyl alcohol. The
resultant hydroxymethyl and acetoxymethyl endo-
compounds 1 and 2, respectively, were subjected to
enzymatic resolution with various commercially avail-
able hydrolases in catalytic amounts. All enzymes gave
the same configured compound (1S,2R,4R)-(ꢀ)-2 with
high enantioselectivities between 94% and 98% ee.
Among the enzymes used, PLE and HLE revealed the
best enantioselectivity with a short reaction duration.
The absolute configuration of compound (ꢀ)-2 was
determined by transforming it into the corresponding
non-chlorinated norbornene derivative by reductive
dechlorination. PLE and HLE, used in catalytic levels,
renders the process very attractive for large scale
preparations.
Alternatively, acetoxymethyl derivative ( )-2 was
subjected to the enzyme catalyzed hydrolysis. The
hydrolysis reactions of ( )-2 were carried out in
pH = 7 buffer at 20 ꢁC using catalytic amounts of
PLE, HLE and PPL (Table 1, entries 2, 3 and 4, respec-
tively). The biotransformations were monitored by
TLC until 50% conversions were obtained. Among
the enzymes, PLE (entry 2) and HLE (entry 3)
appeared to be the most efficient in terms of enantio-
selectivity and the duration of reactions. In entry 4, PPL
showed the lowest enantioselectivity among all and
longer duration than entries 2and 3. All of the enzymes
afforded the same configured acetoxymethyl derivative
(ꢀ)-2.
For the absolute configuration determination of (ꢀ)-2, it
was transformed into the corresponding 2-endo-
hydroxymethyl-bicyclo[2.2.1]hept-5-ene 3 derivative via
dechlorination with Na in liquid NH3 (Scheme 2).15
During the reductive dechlorination, the acetyl group
was hydrolyzed to the corresponding alcohol. The abso-
lute configuration of compound (ꢀ)-2 was assigned as
(1S,2R,4R) by comparison of its specific rotation with
the previously determined value for (1R,2S,4S)-(ꢀ)-2-
endo-hydroxymethyl-bicyclo[2.2.1]hept-5-ene 3.16
4. Experimental
The H and 13C NMR spectra were recorded in CDCl3
1
on a Bruker Spectrospin Avance DPX 400 spectrometer.
Chemical shifts are given in parts per million downfield
from tetramethylsilane. Apparent splittings are given in
all cases. Infrared spectra were obtained from KBr pel-
lets on a Mattson 1000 FT-IR spectrophotometer and
are reported in cmꢀ1. Mass spectra were recorded on a
Varian MAT 212. Optical rotations were measured in
Table 1. Results of the enzyme catalyzed acetylation of ( )-1 and hydrolysis of ( )-2
20
½aꢁD
ꢀ1.5
ꢀ1.5
ꢀ1.5
ꢀ1.4
Entry
Substrate
Enzyme
Substrate:enzyme ratio
Time (h)
Ester
Yielda (%)
39
eeb (%)
1
2( )-
( )-1
2
CCL
PLE
HLE
PPL
1:0.02168
500 mg:100 lL
1:0.08
(
ꢀ)-2
ꢀ)3-2
97
98
98
94
2
20
49
40
47
(
3
4
( )-2
( )-2
(ꢀ)-2
(ꢀ)-2
1:0.08
100
a Yields (%) are given as the isolated esters.
b Enantiomeric excess values are determined by the Chiralcel OD-H chiral column HPLC-analysis.

Y. E. Tu¨rkmen et al. / Tetrahedron: Asymmetry 16 (2005) 2315–2318
2317
a 1 dm cell using a Bellingham and Stanley P20 polari-
meter at 20 ꢁC. HPLC measurements were performed
with ThermoFinnigan spectra system instrument. Sepa-
rations were carried out on Chiralcel OD-H analytical
column (250 · 4.60 mm) with hexane–2-propanol as
eluent. Column chromatography was performed on
silica gel (60-mesh, Merck). TLC was carried out on
Merck 0.2mm silica gel 60 F 254 analytical aluminium
plates. PLE (pig liver esterase) was purchased from Sig-
ma as a suspension in ammonium sulfate solution
(3.2mol/L). CCL (lipase, type VII, from Candida rug-
osa), HLE (horse liver acetone powder) and PPL (lipase,
type II, from porcine pancreas) were purchased from
Aldrich.
evaporated under reduced pressure. The products
(1R,2S,4S)-(+)-1 and (1S,2R,4R)-(ꢀ)-2 were purified by
flash column chromatography (EtOAc–hexane, 1:2).
(1R,2S,4S)-(+)-1: (0.15 g, 30% yield).9 (1S,2R,4R)-(ꢀ)-
2: (0.22 g, 39% yield). HPLC-analysis of (ꢀ)-2: Chiralcel
OD-H at room temperature, n-hexane–2-propanol =
98:2, 1.0 mL/min, 254 nm, t1 = 6.3 min (minor), t2 =
20
D
6.8 min (major), ½aꢁ = ꢀ1.5 (c 1.53, MeOH).
4.4. General procedure for enzymatic hydrolysis of
( )-2-endo-acetoxymethyl-1,4,5,6,7,7-hexachloro-
bicyclo[2.2.1]hept-5-ene, ( )-2
To a stirred solution of 500 mg ( )-2 in 50 mL pH 7.00
phosphate buffer, 40 mg of HLE (PPL or 100 lL PLE)
was added in one portion and the reaction mixture stir-
red at 20 ꢁC in a pH stat unit. The conversion was mon-
itored by TLC. The reaction mixture was extracted with
ethyl acetate, dried over MgSO4 and concentrated under
reduced pressure. The products (1R,2S,4S)-(+)-1 and
(1S,2R,4R)-(ꢀ)-2 were purified by flash column chroma-
tography (EtOAc–hexane, 1:2).
4.1. Synthesis of ( )-2-endo-hydroxymethyl-1,4,5,6,7,7-
hexachlorobicyclo[2.2.1]hept-5-ene, ( )-1
A mixture of allyl alcohol (1.74 g, 30 mmol) and hexa-
chlorocyclopentadiene (2.73 g, 10 mmol) containing a
few crystals of hydroquinone was sealed under vacuum
in a thick-walled Pyrex tube. The mixture was heated
at 145 ꢁC for 4 h. The crude product was purified by
flash column chromatography to afford ( )-1 (EtOAc–
PLE hydrolysis products:
(1R,2S,4S)-(+)-1 (98 mg, 22% yield). (1S,2R,4R)-(ꢀ)-
1
hexane, 1:2) (2.94 g, 89% yield). Mp: 159–160 ꢁC. H
20
2: (0.25 g, 49% yield). ½aꢁ = 1.5 (c 2.39, MeOH).
NMR: d 1.43 (t, 1H, OH, J = 5.0 Hz), 1.86 (dd, 1H, endo
CH2, J = 4.1 and 12.6 Hz), 2.59 (dd, 1H, exo CH2,
J = 8.8 Hz and 12.6 Hz), 2.96–3.03 (m, 1H, CH), 3.39–
3.45 (m, 1H, CH2O), 3.73–3.79 (m, 1H, CH2O). 13C
NMR: d 38.7, 49.4, 62.5, 79.1, 81.4, 103.0, 130.2, 132.4
IR (neat): 1599, 3345 cmꢀ1. HRMS: calcd for C8H6Cl6O
(M+H)+: 328.8628. Found: 328.8645.
D
HLE hydrolysis products:
(1R,2S,4S)-(+)-1 (110 mg, 25% yield). (1S,2R,4R)-
20
D
(ꢀ)-2: (0.20 g, 40% yield). ½aꢁ = ꢀ1.5 (c 0.97, MeOH).
PPL hydrolysis products:
(1R,2S,4S)-(+)-1 (44 mg, 10% yield). (1S,2R,4R)-(ꢀ)-
20
2: (0.24 g, 47% yield). ½aꢁ = ꢀ1.4 (c 1.44, MeOH).
D
4.2. Acetylation of ( )-2-endo-hydroxymethyl-1,4,5,6,7,7-
hexachlorobicyclo[2.2.1]-5-heptene, ( )-1
4.5. Dechlorination of (1S,2R,4R)-(ꢀ)-2-endo-acetoxy-
methyl-1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene
(1S,2R,4R)-(ꢀ)-2
To a stirred solution of ( )-1 (1.50 g, 4.5 mmol) in
CH2Cl2 (25 mL), dry pyridine (0.72 g, 9.1 mmol) was
added at 0 ꢁC and the mixture stirred for 30 min under
an inert atmosphere. Acetyl chloride (0.54 g, 6.8 mmol)
was added dropwise. The resultant mixture was stirred
for 17 h at rt. The organic phase was extracted with
To a stirred solution of metallic sodium (0.6 g, 26 mmol)
in liquid NH3 (30 mL), (1S,2R,4R)-(ꢀ)-2 (0.51 g,
1.37 mmol) in absolute EtOH–ether (12mL, 1:1 ratio)
was added dropwise under argon atmosphere over
20 min. The resultant mixture was stirred for an addi-
tional 20 min and then solid NH4Cl was added in small
portions until the solution became colourless. NH3 was
removed by passing N2 through the mixture and ice-
water was added. The resultant mixture was acidified
with 2M HCl and extracted with ether (3 · 50 mL).
Organic phase was washed with saturated NaHCO3
(3 · 50 mL), brine (2 · 50 mL), dried over MgSO4 and
evaporated under reduced pressure. The crude product
was purified by flash column chromatography to afford
0.1 M
HCl
(3 · 50 mL),
saturated
NaHCO3
(3 · 50 mL) and brine (2 · 50 mL), dried over MgSO4
and solvent evaporated under reduced pressure. The
crude product was purified by flash column chromato-
graphy to afford ( )-2 (EtOAc–hexane, 1:2) (1.38 g,
81%). 1H NMR: d 1.86 (dd, 1H, endo CH2, J = 4.2
and 12.6 Hz), 1.99 (s, 3H, CH3), 2.60 (dd, 1H, exo
CH2, J = 8.9 and 12.6 Hz), 3.00–3.13 (m, 1H, CH),
3.92(dd, 1H, CH 2O, J = 6.7 and 11.7 Hz), 4.08 (dd,
1H, CH2O, J = 5.8 and 11.7 Hz). 13C NMR: d 21.1,
38.6, 46.4, 62.7, 79.0, 81.3, 102.9, 130.5, 132.3, 170.8.
(1R,2S,4S)-(ꢀ)-3. (EtOAc–hexane, 1:2). (0.12 g, 70%).
20
½aꢁ = ꢀ72.0 (c 1.05, 95% EtOH). All spectroscopic data
D
IR(neat): 1745, 1585 cmꢀ1
.
HRMS: calcd for
and optical rotations are in accordance with the
literature.16
C10H8Cl6O2 (M+H)+: 370.8734. Found: 370.8717.
4.3. CCL Acetylation of ( )-2-endo-hydroxymethyl-
1,4,5,6,7,7-hexachlorobicyclo[2.2.1]hept-5-ene, ( )-1
Acknowledgements
To a stirred solution of 500 mg ( )-1 in 5 mL vinyl
acetate, CCL (10 mg) was added in one portion and the
reaction mixture stirred at 20 ꢁC (TLC monitoring).
The reaction mixture was filtered and vinyl acetate
We thank the Middle East Technical University for a
grant (BAP-2004) and the Turkish Scientific and Techni-
cal Research Council for grants [TBAG-2244(102T169)
and TBAG-2454(104T066)].

2318
Y. E. Tu¨rkmen et al. / Tetrahedron: Asymmetry 16 (2005) 2315–2318
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