Preparation of isoxazolidinyl nucleoside enantiomers by lipase-catalysed kinetic resolution

Article abstract of DOI:10.1016/j.tetasy.2009.02.026

An efficient biocatalytic procedure to obtain chiral N,O-nucleoside derivatives consisting of a lipase-catalysed resolution of the corresponding racemates in organic solvent has been developed. Enantioselective esterification of thymine and cytosine deriv

Full text of DOI:10.1016/j.tetasy.2009.02.026

Tetrahedron: Asymmetry 20 (2009) 425–429
Contents lists available at ScienceDirect
Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Preparation of isoxazolidinyl nucleoside enantiomers by lipase-catalysed
kinetic resolution
b,
Caterina Carnovale a, Daniela Iannazzo a, Giovanni Nicolosi b, Anna Piperno a, , Claudia Sanfilippo
*
*
a Dipartimento Farmaco-Chimico, Università di Messina, Viale SS. Annunziata, 98168 Messina, Italy
b Istituto di Chimica Biomolecolare del C.N.R, Via P. Gaifami, 18, 95126 Catania, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
An efficient biocatalytic procedure to obtain chiral N,O-nucleoside derivatives consisting of a lipase-cat-
alysed resolution of the corresponding racemates in organic solvent has been developed. Enantioselective
esterification of thymine and cytosine derivatives, ( )-9a and ( )-9b, has been investigated by comparing
the efficiency of different lipases and acyl donors. Since esterification of ( )-9a and ( )-9b, occurred with
low enantioselectivity (E 6 14) in the presence of the lipases considered, for preparative purposes we
resorted, with success, to a double sequential kinetic resolution, using Lipozyme IM as the best catalyst.
This approach enables us to obtain all the enantiomers with ee P 95%. The absolute configuration of the
new chiral N-acetyl cytosine derivative 9b was established by circular dichroism spectroscopy.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 2 December 2008
Accepted 19 February 2009
Available online 25 March 2009
1. Introduction
O
O
P
OEt
F
N,O-Nucleosides constitute a particular class of modified nucleo-
side bearing a heterocyclic system (isoxazolidine) instead of the
ribose ring. These compounds have been shown to be endowed with
importantbiologicalproperties:1,2 ADFU1isa goodinductorofapop-
tosis on lymphoid and monocytoid cells, acting as a strong potentia-
tor of Fas-induced cell death3 and phosphonated N,O-nucleosides 2
are potent reverse transcriptase inhibitors on different retroviruses4
(Fig. 1). Recently, compounds 2, synthesised as a racemic mixture
from the corresponding tosylated N,O-nucleoside by Arbuzov reac-
tion,5 have been proposed to ensure long lasting control of HTLV-1,
an oncogen retrovirus associated with adult Leukemia/Limphoma
(ATLL) and with myelopathy, tropical spastic Paraparesis.6
NH
EtO
B
N
O
N
CH3
O
HN
O
2
1
ADFU
HO
B
HO
B
X
N
R
O
OH
Several asymmetric routes have been reported for the synthesis
of enantiopure N,O-nucleosides, such as; (i) the 1,3-dipolar cycload-
dition of chiral nitrones;7,8 (ii) nucleophilic addition of enolates to
N,O-Nucleoside
X = O
Deoxyribonucleoside
X = CH2 Carbonucleoside
B = Nucleobase
a
-alkoxy- and a
-aminonitrones;9 and (iii) the Michael addition of
N-methyl hydroxylamine to unsaturated esters and lactones.10
Figure 1. Modified nucleosides.
However, the reported strategies do not constitute as a general
way for the synthesis of b-D and b-L enantiopure N,O-nucleosides
since the stereochemical result is strictly correlated to the structural
features of the reagents, the chosen route and the experimental con-
enzymes as chiral catalysts has useful applications in preparative
organic chemistry, so that the chemo-enzymatic approach for
asymmetric synthesis is largely accepted in synthetic design.12 To
the best of our knowledge, nothing has been reported for the
enzymatic resolution of racemic N,O-nucleosides, whereas the
lipase resolution has been applied for a few classes of modified
nucleosides, such as carbanucleosides and 20-deoxynucleosides.13
Herein, we report an efficient method for the preparation of
enantiopure chiral isoxazolidine nucleosides 9a and 9b via
lipase-catalysed transesterification reactions in an organic solvent.
ditions, and in many instances the undesired
a-anomer
predominates.11,7c
Lipases are able to catalyse asymmetric transesterification reac-
tions in a wide range of hydrophobic substrates. The use of these
* Corresponding authors. Tel.: +39 090 356230; fax: +39 090 6766562 (A.P.); tel.:
+39 095 7212136; fax: +39 095 7212141 (C.S.).
E-mail addresses: apiperno@pharma.unime.it (A. Piperno), claudia.sanfilippo@
icb.cnr.it (C. Sanfilippo).
0957-4166/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2009.02.026

426
C. Carnovale et al. / Tetrahedron: Asymmetry 20 (2009) 425–429
lipases used catalysed the esterification of substrates but lacking
2. Results and discussion
enantioselectivity, probably due to the distance between the
hydroxyl function and the stereogenic centre, as reported for the
chiral recognition of primary alcohols.14 The best one was Lipo-
zyme IM allowing E = 7 for ( )-9a and E = 8 for ( )-9b.15
2.1. Synthesis of racemic N,O-nucleosides ( )-9a,b
Racemic compounds ( )-9a and ( )-9b were synthesised by a
three-step procedure starting from the nitrone 3 as previously
reported.5 Thus, the 1,3-dipolar cycloaddition of nitrone 3 with vi-
nyl acetate, performed in anhydrous ether at room temperature in
3 h afforded a mixture of isoxazolidines ( )-4 and ( )-5 with a 90%
yield. The crude mixture of isoxazolidines was subjected to nucle-
osidation with silylated thymine or N-acetyl cytosine in dry aceto-
nitrile in the presence of 0.4 equiv of TMSOTf as a catalyst. The
Generally, it proved to be difficult to attain highly enantioselec-
tive resolution by lipase-catalysed reaction when E <20. To
improve the enantiomeric discrimination, several strategies con-
cerning physical and/or chemical modifications of the enzyme or
of the substrate have been proposed.16 One of these consists of a
change of acyl donor in order to induce a favourable conforma-
tional variation of substrate-binding site, so resulting in an
improvement in enzymatic enantioselectivity. With this intention,
the lipozyme-assisted esterification in 1,4-dioxane of ( )-9a and
( )-9b was repeated using vinyl propionate and vinyl benzoate as
acyl donors, and the results concerning the influence of the acyl
donor are summarised in Table 1.
a- and b-anomers have been separated by flash chromatography
and the cis-adducts ( )-7a,b were treated with TBAF in THF to give
the corresponding nucleosides ( )-9a and ( )-9b (Scheme 1).
2.2. Lipase-catalysed resolution of ( )-9a and ( )-9b
The use of vinyl propionate instead of vinyl acetate in the ester-
ification of ( )-9a has a positive influence on reaction rate but not
on the enantioselectivity. Conversely, the use of the bulkier benzo-
ate did not affect the reaction rate but gave a significant enhance-
ment in enantioselectivity (Table 1, entry 3). The change of acyl
donor in the esterification of ( )-9b leads to a consequent decline
in enantioselectivity although a very fast reaction is observed using
vinyl propionate (Table 1, entries 5 and 6).
A preliminary esterification screening for the resolution of thy-
mine, ( )-9a, and acetyl cytosine, ( )-9b, derivatives was carried
out using three native lipases from Pseudomonas cepacia, Pseudo-
monas fluorescens (lipase AK) and Candida rugosa and five as immo-
bilised form P. cepacia (PS-C, PSC-I and PS-D), Mucor miehei
(Lipozyme IM) and Candida antarctica (Novozyme 435). Due to
the scarce solubility of ( )-9a and ( )-9b in the organic solvents
of ordinary use in lipase catalysis we chose 1,4-dioxane as the reac-
tion medium, and vinyl acetate was exploited as acyl donor. All the
Considering these poor results, in order to access our enantiopure
compounds we resorted to trying a double kinetic resolution
TBDPSO
TBDPSO
OAC
H
OTBDPS
+
N
N
N
+
Me
OAc
Me
O
4
OAc
–
O
Me
O
3
( )-
5
( )-
TBDPSO
Me
TBDPSO
Me
N
N
+
a
B
b
O
B
O
6a,b
7a,b
( )-
( )-
O
NHAc
N
6/ 7 = 3 : 7
NH
B
HO
Me
HO
Me
N
O
N
O
N
N
+
B
O
B
O
( )-8a,b
( )-9a,b
Scheme 1. Synthesis of racemic N,O-nucleosides ( )-9a and ( )-9b.
Table 1
Acyl donor influence on the enantioselectivity of biocatalysed resolutiona of ( )-9a and ( )-9b
Entry
Substrate
Acyl donor
Time (h)
Conv.b (%)
ee % alcohol
ee % ester
Ec
1
2
3
4
5
6
( )-9a
( )-9a
( )-9a
( )-9b
( )-9b
( )-9b
Vinyl acetate
3
1
3
1.5
0.5
3
38
47
32
25
46
37
40
56
39
24
45
25
65
63
81
72
53
43
7
Vinyl propionate
Vinyl benzoate
Vinyl acetate
Vinyl propionate
Vinyl benzoate
8
14
8
5
3
a
The reactions were performed in the presence of Lipozyme IM in 1,4-dioxane at 45 °C and shaken at 300 rpm.
Substrate conversions and enantiomeric excesses were determined by chiral HPLC analysis using a Chiralcel OJ column.
See Ref. 15.
b
c

C. Carnovale et al. / Tetrahedron: Asymmetry 20 (2009) 425–429
427
HO
Me
BzO
HO
Lipozyme
1,4-dioxane
N
+
N
N
vinyl benzoate
Me
Ty
Ty
O
O
Me
Ty
O
Conv. 60%
E=14
( )-9a
(-)-9a, ee>98%
(+)-10, ee 64%
yield 40%
yield 58%
1.preparative
chromatography
2.chemical hydrolysis
chemical
hydrolysis
Lipozyme
HO
OPr
OH
1,4-dioxane
N
N
vinyl propionate
Conv. 70%
N
Me
Ty
Me
Me
O
Ty
Ty
O
O
9a
(+)- , ee>98%
11
(+)- , ee>98%
( )-9a, ee 64%
+
yield 41%
Scheme 2. Double enzymatic kinetic resolution of ( )-9a.
procedure. This, in theory, is realisable by a lipase-assisted sequen-
tial acylation followed by deacylation. Alternatively, if the biocatal-
ysed alcoholysis step is not efficient, the enantioenriched ester can
be transformed by chemical hydrolysis into the corresponding
alcohol, and subjected to a second lipase-assisted esterification.
To this end, ( )-9a was subjected to an esterification reaction,
catalysed by Lipozyme IM in 1,4-doxane using vinyl benzoate as
the acyl donor (Scheme 2). The reaction was monitored by HPLC
chiral analysis and the residue (ꢀ)-9a was obtained with ee >98%
(yield 40%) by protracting the reaction to 60% conversion. The
benzoate derivative (+)-10 after chromatographic purification (ee
64%, yield 58%) was dissolved in 1,4-dioxane and an alcoholysis
using n-butanol in the presence of Lipozyme IM was attempted.
Unfortunately, this reaction occurred very slowly giving a negligi-
ble amount of the desired product after 3 days. As a consequence of
this result, in order to have the desired (+)-9a in satisfactory enan-
tiomeric excess, the ester (+)-10 was transformed into alcohol (+)-
9a with 64% ee, by a chemical hydrolysis with aqueous NH4OH/
methanol mixture, so in the presence of Lipozyme IM and vinyl
propionate, (+)-9a was subjected to a new resolution cycle. Since
this substrate was enantioenriched, the use of vinyl propionate
instead of vinyl benzoate does not compromise the efficiency of
the resolution, while it allows us to have the ester (+)-11 in high
enantiomeric excess (ee >98%) and 41% overall yield in a shorter
time. Alkaline hydrolysis of (+)-11, carried out in aqueous NH4OH
/methanol mixture, gave the desired product (+)-9a quantitatively.
In the resolution of the N-acetyl-cytosine derivative ( )-9b, the
first step of a biocatalysed esterification was realised by choosing vi-
nyl acetate as an acyl donor, but due to the low enantioselectivity
(E = 8, Table 1, entry 4) the reaction required to reach 69% conversion
to give the unreacted alcohol (ꢀ)-9b with a satisfactory 95% ee. Pre-
parative column chromatography furnished (ꢀ)-9b in 27% yield and
acetyl derivative (+)-12 with 42% ee. In the second step of the reac-
tion, ester (+)-12 was subjected to alcoholysis in presence of Lipo-
zyme IM using n-butanol as nucleophile, (Scheme 3). After 42 h, a
46% substrate conversion was reached and chiral HPLC analysis of
the reaction evidenced the alcohol (+)-9b possessing 87% ee. After
chromatographic purification and crystallisation, alcohol (+)-9b
possessing ee >98% was recovered from the mother liquor.
2.3. Determination of absolute configurations
An absolute configuration of (3R,5S) was assigned to compound
(ꢀ)-9a by comparing its specific rotation to the literature value9 so
HO
Me
AcO
HO
Lipozyme
1,4-dioxane
N
+
N
N
vinyl acetate
Me
Cyt
Cyt
O
O
Me
Cyt
O
Conv. 69%
E=8
9b
yield 27%
12
(-)- , ee 95%
(+)- , ee 42%
( )-9b
preparative
chromatography
Lipozyme
HO
HO
AcO
1,4-diossano
crystallization
n-BuOH
N
N
N
Me
Cyt
Me
Me
Cyt
Cyt
O
O
O
Conv. 46%
9b
(+)- , ee>98%
9b
12
(+)- , ee 87%
(+)- , ee 42%
yield 30%
Scheme 3. Double enzymatic kinetic resolution of ( )-9b.

428
C. Carnovale et al. / Tetrahedron: Asymmetry 20 (2009) 425–429
that a stereopreference (S) of Lipozyme IM was determined. The
absolute configuration at C3 and C5 of (ꢀ)-9b was unknown, there-
fore it was determined by comparison of a CD spectrum with that
of (ꢀ)-9a whose configuration was known. The CD spectra of these
compounds are shown in Figure 2. Since the spectrum obtained for
(ꢀ)-9b was found to be a good approximation matching to that
registered for (3R,5S)-isomer (ꢀ)-9a, the same absolute configura-
tion was assigned to the stereogenic centres of (ꢀ)-9b. Moreover
these data obtained from CD analysis were in agreement with
the stereopreference of the lipase showing an (S)-selectivity in
the esterification of a –CH2OH group at C3 of ( )-9a.
enantiomer), for the benzoate ( )-10 (tR 35.2 and 45.2 min for
(3R,5S)- and (3S,5R)-enantiomer) and for the propionate ( )-11
(tR 27.1 and 47.5 min for (3R,5S)- and (3S,5R)-enantiomer). A 7:3
n-hexane/EtOH mixture and
a
0.7 ml/minꢀ1 flow rate were
employed for the alcohol ( )-9b (tR 11.5 and 13.6 min for (3R,5S)-
and (3S,5R)-enantiomer) and for the acetyl derivative ( )-12 (tR
25.1 and 28.6 min for (3R,5S)- and (3S,5R)-enantiomer).
CD spectra were registered at 20 °C in MeOH (0.1 cm cell
length) on a JASCO J-810 spectropolarimeter.
4.2. General procedure for analytical enzyme-catalysed
esterification of ( )-9a and ( )-9b
10
To a solution of substrates 9a and 9b (0.04) in 1,4-dioxane
(6 ml) were added vinyl ester (0.25 mmol) and lipase (100 mg).
The mixture incubated at 45 °C was shaken at 300 rpm. At regular
times, aliquots of the mixtures were analysed by chiral HPLC, and
the reactions stopped at desired substrate conversion (in range of
30–50%).
(-)-9a
8
(-)-9b
6
4
2
0
200
220
240
260
280
300
320
340
4.3. Preparative resolution procedure of ( )-9a
-2
-4
Substrate ( )-9a (250 mg, 1 mmol) was dissolved in 150 ml of
1,4-dioxane containing 2.5 g of Lipozyme IM and vinyl benzoate
(0.138 ml, 1 mmol) was added. The obtained mixture was incu-
bated at 45 °C and shaken (300 rpm) until 60% of substrate conver-
sion detected by HPLC analysis. Then the reaction was stopped by
filtering off the lipase and the mixture chromatographed on silica
gel eluting with CH2Cl2/MeOH 98:2, to furnish (+)-10 yield 58%
and ee 64% (194 mg, 0.58 mmol) and (ꢀ)-9a yield 40% (97 mg,
0.40 mmol) ee >98%.
-6
(+)-9a
-8
-10
(+)-9b
λ
Figure 2. CD spectra recorded in methanol of 9a and 9b enantiomers.
Specific rotation data of (ꢀ)-9a: ½a D25
ꢂ
¼ ꢀ113:5 (c 0.8, EtOH) and
3. Conclusions
½
a 2D5
ꢂ
¼ ꢀ8:1 (c 0.8, CHCl3) {lit.9
½ ꢂ
a 2D0
D
¼ ꢀ11 (c 0.8 CHCl3)}; CD (c
In this work we have applied, for the first time, an enzymatic
kinetic resolution procedure to obtain chiral non-racemic N,O-
nucleosides. Despite the low enantioselectivity shown by the
lipases investigated due to large distance between the –OH groups
and the stereogenic centres, single enantiomers of ( )-9a and ( )-
9b have been anyway obtained in high enantiomer purity by devel-
oping a lipase-catalysed double sequential kinetic resolution route.
The absolute configuration of new (3R,5S)-(ꢀ)-9b was assigned by
CD analysis by comparison of its spectrum with that of known (ꢀ)-
9a. Since (+)-9a and (+)-9b displayed the same C-3 stereochemis-
try, it could be deduced that the lipase recognition was mainly
affected by the isoxazolidine moiety with less influence for the
chemical nature of the pyrimidine base.
6.3 ꢁ 10ꢀ4 M, CH3OH): kext 202 (
e
e
+1.59), 219 (
D
e
ꢀ1.31), 238
(
(
D
e
e
+2.59), 254 (
D
e
+2.67), 274 (
D
+4.87), 300 (
D
e
+0.23), 346
D
+0.21).
The NMR spectra and elemental analysis were identical with
the literature data.17
Ester (+)-10 was dissolved in NH4OH/MeOH and hydrolysed by
heating at 50 °C for 8 h. After removal of the solvents under
reduced pressure, the residue was dissolved in CH2Cl2 and then
extracted with water containing NaHCO3. The organic layer was
dried over anhydrous sodium sulfate and concentrated to obtain
(+)-9a (ee >98%) quantitatively.
To a solution of (+)-9a (140 mg, 0.58 mmol) in 1,4-dioxane
(100 ml) containing Lipozyme IM (1.9 g) was added vinyl propio-
nate (0.170 ml, 1.56 mmol) and shaken at 45 °C and 300 rpm until
70% of substrate conversion detected by HPLC analysis. The reac-
tion was stopped and the mixture purified by chromatography
using CH2Cl2/MeOH 95:5 mixture as eluent to give ester (+)-11
ee >98%. Chemical hydrolysis, carried out in NH4OH/MeOH mix-
ture, gave (99 mg, 0.41 mmol) of the homochiral alcohol (+)-9a in
41% overall yield.
4. Experimental
4.1. Materials and methods
Lipases from P. cepacia, P. fluorescens (lipase AK), C. rugosa,
immobilised P. cepacia (PS-C, PSC-I and PS-D), M. miehei (Lipozyme
IM) and C. antarctica (Novozyme 435) were purchased from Aldrich
and were used as received. Optical rotations were recorded on a
DIP 135 JASCO instrument using a / 3.5 ꢁ 100 mm cell. 1H and
13C NMR spectra were recorded in CDCl3, on a Varian 300 instru-
ment. Chemical shifts (d) are reported in ppm relative to TMS
and coupling constants (J) in hertz. The enantiomeric excesses
and substrate conversions were determined by chiral HPLC analy-
sis using a ChiralcelÒ OJ (Daicel Chemical Industries) column
eluting with n-hexane/ethanol mixtures with simultaneous detec-
tion at k 225, 250, 275 and 300 nm. Chiral HPLC conditions: Chiral-
celÒ OJ column eluting with a 7:3 n-hexane/EtOH mixture at
0.7 ml/minꢀ1 for 20 min and then 1.3 ml/min flow rate was used
for alcohol ( )-9a (tR 12.9 and 14.4 min for (3R,5S)- and (3S,5R)-
4.4. Preparative resolution procedure for ( )-9b
Alcohol ( )-9b (300 mg, 1.3 mmol) was dissolved in 45 ml of
1,4-dioxane containing vinyl acetate (0.245 ml, 2.6 mmol) then
the reaction was started by the addition of lipase Lipozyme IM
(1.5 g). After 7 h at 300 rpm and 45 °C the reaction was stopped,
at 69% substrate conversion detected by chiral HPLC analysis. The
enzyme was filtered off and the mixture was chromatographed
on silica gel column eluting with CH2Cl2/2-propanol 6:1 to obtain
the alcohol (ꢀ)-9b with ee 95% and yield 27% (80 mg, 0.35 mmol).
After crystallisation from CH2Cl2/2-propanol (ꢀ)-9b was obtained
with ee >98%: ½a 2D5
ꢂ
¼ ꢀ19:8 (c 0.2 EtOH). 1H NMR (500 MHz,

C. Carnovale et al. / Tetrahedron: Asymmetry 20 (2009) 425–429
429
4. Chiacchio, U.; Balestrieri, E.; Macchi, B.; Iannazzo, D.; Piperno, A.; Rescifina, A.;
Romeo, A.; Saglimbeni, M.; Sciortimo, M. T.; Valveri, V.; Mastino, A.; Romeo, G.
J. Med. Chem. 2005, 48, 1389–1394.
5. Chiacchio, U.; Rescifina, A.; Iannazzo, D.; Piperno, A.; Romeo, R.; Borrello, L.;
Sciortimo, M. T.; Balestrieri, E.; Macchi, B.; Mastino, A.; Romeo, G. J. Med. Chem.
2007, 50, 3747–3750.
6. Balestrieri, E.; Matteucci, C.; Ascolani, A.; Piperno, A.; Romeo, R.; Romeo, G.;
Chiacchio,U.;Mastino,A.;Macchi,B.Antimicrob.AgentsChemother.2007,52,54–64.
7. (a) Rios, R.; Ibrahem, I.; Vesely, J.; Zhao, G.-L.; Córdova, A. Tetrahedron:
CDCl3): d 2.31 (s, 3H), 2.48 (ddd, 1H, J = 4.3, 9.5, and 13.5 Hz), 2.91
(s, 3H), 2.92 (m, 1H), 3.12 (ddd, 1H, J = 6.5, 7.2 and 13.5 Hz), 3.62
(dd, 1H, J = 4.0 and 12.0 Hz), 3.73 (dd, 1H, J = 4.0 and 12.0 Hz),
5.30 (br s, 1H), 6.06 (dd, 1H, J = 4.3 and 6.5 Hz), 7.45 (d, 1H,
J = 8.0 Hz), 8.40 (d, 1H, J = 8.0 Hz). 13C NMR (500 MHz, CDCl3): d
23.6, 39.2, 43.5, 60.1, 68.4, 88.9, 95.5, 141.65, 155.2, 162.8, 171.5.
HRMS (EI) for (M+): 268.1164. Anal. Calcd for C11H16N4O4: C,
49.25; H, 6.01; N, 20.88. Found: C, 49.31; H, 6.15; N, 21.04.
Asymmetry 2007, 48, 5701–5705; (b) Fisera, L.. In Top Heterocycl. Chem.;
¯
Springer: Berlin, Heidelberg, 2007; Vol. 7. pp 287–323; (c) Chiacchio, U.;
Corsaro, A.; Gumina, G.; Rescifina, A.; Iannazzo, D.; Piperno, A.; Romeo, G.;
Romeo, R. J. Org. Chem. 1999, 64, 9321–9327; (d) Chiacchio, U.; Corsaro, A.;
Iannazzo, D.; Piperno, A.; Pistarà, V.; Rescifina, A.; Romeo, R.; Sindona, G.;
Romeo, G. Tetrahedron: Asymmetry 2003, 14, 2717–2723.
CD: (c 1.7 ꢁ 10ꢀ4 M) kext 202 (
D
e
+2.53), 207 (
ꢀ4.26), 250 (
+0.10).
D
e
+4.56), 214
ꢀ0.24), 296
(
D
e
e
ꢀ0.26), 221 (
D
e
ꢀ5.70), 235 (
D
e
D
e
(D
+2.86), 318 (
D
e
+0.66), 370 (
D
e
The isolated ester (+)-12 (256 mg, 0.90 mmol) was dissolved in
8. (a) Merino, P.; Tejero, T.; Mates, J. A.; Chiacchio, U.; Corsaro, A.; Romeo, G.
Tetrahedron: Asy mmetry 2007, 11, 1517–1520; (b) Merino, P.; Del Alamo, E. M.;
Franco, S.; Merchan, F. L.; Simon, A.; Tejero, T. Tetrahedron: Asymmetry 2000, 11,
1,4-dioxane (40 ml) and then Lipozyme IM (1.2 g) was added. The
reaction was started by addition of n-butanol (0.084 ml,
0.95 mmol) and shaken at 45 °C and 300 rpm. The reaction was
monitored by chiral HPLC and stopped at 46% substrate conversion.
The alcohol (+)-9b was isolated with ee 87% and 30% overall yield
(90 mg, 0.40 mmol) by preparative chromatography eluting with
CH2Cl2/2-propanol 6:1. After crystallisation from CH2Cl2/n-hexane
(+)-9b was obtained with ee >98%.
´
´
1543–1554; (c) Hyrošová, E.; Medvecky, M.; Fisera, L.; Hametner, C.; Fr?hlich,
¯
J.; Marchetti, M.; Allmaier, G. Tetrahedron 2008, 64, 3111–3118.
9. Merino, P.; Del Alamo, E. M.; Bona, M.; Franco, S.; Merchan, F. L.; Tejero, T.;
Vieceli, O. Tetrahedron Lett. 2000, 41, 9239–9243.
10. (a) Xiang, Y.; Gi, H. J.; Niu, D.; Schinazi, R. F.; Zhao, K. J. Org. Chem. 2007, 72,
7430–7434; (b) Xiang, Y.; Chen, J.; Schinazi, R. F.; Zhao, K. Tetrahedron Lett.
1995, 36, 7193–7196; (c) Xiang, Y.; Gong, Y.; Zhao, K. Tetrahedron Lett. 1996, 37,
4877–4880.
11. Chiacchio, U.; Corsaro, A.; Iannazzo, D.; Piperno, A.; Procopio, A.; Rescifina, A.;
Romeo, G.; Romeo, R. Eur. J. Org. Chem. 2001, 1893–1898.
12. (a) Koeller, K. M.; Wong, C.-H. Nature 2001, 409, 232–240; (b) Carrea, G.; Riva,
S. Angew Chem., Int. Ed. 2000, 39, 2226–2254.
References
1. (a) Merino, P. Curr. Med. Chem. 2006, 13, 539–545; (b) Ichiwawa, E.; Kato, K.
Curr. Med. Chem. 2001, 8, 385; (c) Romeo, G.; Iannazzo, D.; Piperno, A.; Romeo,
R.; Corsaro, A.; Rescifina, A.; Chiacchio, U. Mini-Revi. Org. Chem. 2005, 2, 59–77.
2. Piperno, A.; Chiacchio, M. A.; Iannazzo, D.; Romeo, R. Curr. Med. Chem. 2006, 13,
3675–3695.
3. Chiacchio, U.; Corsaro, A.; Iannazzo, D.; Piperno, A.; Pistarà, V.; Rescifina, A.;
Romeo, A.; Valveri, V.; Mastino, A.; Romeo, G. J. Med. Chem. 2003, 46, 3696–
3702.
13. (a) Siddiqui, M. A.; Hughes, S. H.; Boyer, P. L.; Mitsuya, H.; Van, Q. N.; George,
C.; Sarafinanos, S. G.; Marquez, V. E. J. Med. Chem. 2004, 47, 5041–5048; (b)
Albert, M.; De Souza, D.; Feiertag, P.; H?nig, H. Org. Lett. 2002, 4, 3251–3254; (c)
Ferrero, M.; Gotor, V. Chem. Rev. 2000, 100, 4319–4347.
14. Bazcko, K.; Larpent, C. J. Chem. Soc., Perkin Trans. 2000, 521–526.
15. Chen, C-S.; Sih, C. J. Angew. Chem., Int. Ed. Engl. 1989, 28, 695–707.
16. Berglund, P. Biomol. Eng. 2001, 18, 13–22.
17. Xiang, Y.;Gi,H-J.;Niu,D.;Schinazi, R.F.;Zhao,K. J.Org. Chem.1997, 62, 7430–7434.