Highly enantioselective transformations on 3,4-dihydroxytetrahydrofurans catalyzed by lipases

Article abstract of DOI:10.1016/j.tetasy.2008.05.027

Enzymatic acylations and alkoxycarbonylations of cis- and trans-3,4-dihydroxytetrahydrofuran and hydrolysis of their diacetylated and dialkoxycarbonylated derivatives have been studied. High enantioselectivity is obtained using Pseudomonas cepacia lipase as a catalyst in the hydrolysis of the trans-diacetyl derivative, while for the desymmetrization of the cis-3,4-dihydroxytetrahydrofuran the best results are obtained in the acylation process catalyzed by Candida antarctica lipase B.

Full text of DOI:10.1016/j.tetasy.2008.05.027

Tetrahedron: Asymmetry 19 (2008) 1684–1688
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Highly enantioselective transformations on 3,4-dihydroxytetrahydrofurans
catalyzed by lipases
*
Verónica Recuero, Rosario Brieva, Vicente Gotor
Departamento de Química Orgánica e Inorgánica and Instituto Universitario de Biotecnología de Asturias, Universidad de Oviedo, 33006 Oviedo, Asturias, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Enzymatic acylations and alkoxycarbonylations of cis- and trans-3,4-dihydroxytetrahydrofuran and
hydrolysis of their diacetylated and dialkoxycarbonylated derivatives have been studied. High enantio-
selectivity is obtained using Pseudomonas cepacia lipase as a catalyst in the hydrolysis of the trans-diace-
tyl derivative, while for the desymmetrization of the cis-3,4-dihydroxytetrahydrofuran the best results
are obtained in the acylation process catalyzed by Candida antarctica lipase B.
Received 22 May 2008
Accepted 28 May 2008
Available online 2 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
2. Results and discussion
Optically active vicinal diols are versatile chemical intermedi-
ates for the production of pharmaceuticals, agrochemicals, ferro-
electric liquid crystals, flavors, and fragrances. In particular,
chiral oxygenated tetrahydrofuran derivatives are useful synthons
in enantioselective synthesis1 and are often found in physiologi-
cally active compounds.2 Typically, enantiomerically enriched
3,4-dihydroxytetrahydrofuran derivatives are synthesized by hy-
dride reduction of tartaric acid to give a tetralol, followed by sub-
sequent cyclization.3 Other procedures include the asymmetric
opening of a meso-epoxide with alcohols in the presence of acid
catalysts. The desymmetrization of a meso-diol by carbamoylation
catalyzed with a chiral Cu(II) has recently been described.4
Biocatalytic methods are also employed for the preparation of
enantiomerically pure trans-3,4-dihydroxytetrahydrofuran. Micro-
bial epoxide hydrolases have been successfully employed for the
asymmetric ring opening of the meso-epoxide.5 The enzymatic res-
olution of the trans-isomer has been carried out by hydrolysis of
the diacetyl derivative in the presence of lipase from Pseudomonas
sp., but the enantioselectivity of the process was very low
(E = 4.4).6 Better results were obtained when the mono alkyloxy
derivative was employed as a substrate, for example, in the resolu-
tion of the cis-3-benzyloxy-4-hydroxytetrahydrofuran6 or trans-3-
alkenyloxy-4-hydroxytetrahydrofuran.7
First, we studied the enzymatic hydrolysis of the trans-diacetyl
derivative ( )-3. This substrate has been prepared in almost quan-
titative yield by treatment of 2,5-dihydrofuran 1 with m-chloro-
8
perbenzoic acid (m-CPBA) in CH2Cl2 and subsequent opening of
the resulting epoxide
2
with acetic anhydride and BF3ꢀEt2O
(Scheme 1). According to the results reported by Seemayer and
Schneider,6 the enantioselectivity of the enzymatic hydrolysis cat-
alyzed by Pseudomonas cepacia lipase (PSL-C) in an aqueous media
showed a very low enantioselectivity. Next, we carried out the pro-
cess in a mixture 1:2 of phosphate buffer 1 M and 1,4-dioxane. We
tested several commercially available lipases [PSL-C, Candida ant-
arctica lipases A and B (CAL-A and CAL-B), Candida rugosa (CRL),
Aspegillus melleus, porcine pancreatic (PPL)]. Only the lipase PSL-C
catalyzed the process and, surprisingly, under these conditions
the enantioselectivity of the process was very high (E >200). The
reaction rate was moderated, after one day of reaction, 51% conver-
sion was achieved. Similar results were obtained when acetonitrile
or THF was used as reaction solvent (Table 1).
OAc
AcO
O
MCPBA
CH2Cl2
Ac2O
BF3.Et2O
O
O
O
As part of our research on the enzymatic preparation of opti-
cally active polyhydroxyheterocycles, we are interested in the
enzymatic resolution of trans-3,4-dihydroxytetrahydrofuran and
in the desymmetrization of the cis-isomer, which, to the best of
our knowledge, has not been studied.
2
1
( )-trans-
3
AcO
OH
AcO
OAc
AcO
OAc
Lipase
Buffer pH 7.0 /
Organic solvent
1:2
+
O
O
O
(3S,4S)-3
(3R,4R)-4
( )-trans-
3
* Corresponding author. Tel./fax: +34 985 103 448.
E-mail address: vgs@fq.uniovi.es (V. Gotor).
Scheme 1. Synthesis and enzymatic hydrolysis of ( )-trans-3.
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.05.027

V. Recuero et al. / Tetrahedron: Asymmetry 19 (2008) 1684–1688
1685
Table 1
reaction was carried out at 30 °C only a 7% of the monohydrolyzed
product cis-7a was isolated after 29 h of reaction. In order to im-
prove the yield of the reaction the same process was carried out
at 60 °C, but after a period of three days only 13% of the monocar-
bonate (ꢁ)-7a was isolated and its enantiomeric excess was low.
As in the case of the substrate cis-3, no product was detected when
CAL-B was used as catalyst.
In light of these results, we decide to test the meso-diol cis-5 as a
substrate for the enzymatic acylation. First, we carried out the pro-
cess at 30 °C, using 5 equiv of different acylating agents and
tBuOMe as solvent (Scheme 5).
PSL-C catalyzed hydrolysis of ( )-trans-3 at 30 °C in phosphate buffer 1 M pH 7.0/
organic solvent 1:2
ca (%)
ees (%)
eep (%)
Ec
b
b
Entry
Organic solvent
t (h)
1
2
3
1,4-Dioxane
Acetonitrile
THF
31
24
24
51
51
51
>99
99
>99
95
95
94
>200
>200
>200
a
Conversion, c = ees/(ees + eep).
b
c
Determined by chiral GC.
Enantiomeric ratio, E = ln[(1 ꢁ c)(1 ꢁ ees)]/ln[(1 ꢁ c)(1 + ees)].9
The absolute configurations of the product and the remaining
HO
O
R
substrate were assigned as follows. Hydrolysis of the remaining
substrate (+)-trans-3 using NaOMe in MeOH afforded the dihy-
droxy derivative (ꢁ)-trans-5, (Scheme 2) whose specific rotation
sign was in agreement with that reported for (3S,4S)-(ꢁ)-5.3a
CAL-B
organic solvent
O
O
HO
OH
O
(−)-8a-d
R
O
O
OH
HO
OAc
AcO
PSL-C
organic solvent
R
O
OH
cis-5
NaOMe
MeOH
a R = CH3
O
O
O
b R = CH2CH3
c R = (CH2)8CH3
d R = Ph
O
-(+)-3
(3S,4
S)
(−)-5
(
3S,4
S)-
(+)-8a-d
Scheme 2. Hydrolysis of (+)-3.
Scheme 5. Enzymatic acylation of cis-5.
In view of these successful results, similar biocatalytic condi-
tions were applied to the hydrolysis of the derivative cis-3, which
was prepared by acetylation of the commercially available cis-5
(Scheme 3).
Two of the enzymes tested catalyzed the reaction: PSL-C and
CAL-B. It is noteworthy that depending on the biocatalyst em-
ployed, an opposite stereochemical preference in the asymmetric
acylation was observed. PSL-C catalyzed the formation of deriva-
tives (3R,4S)-(+)-8, while CAL-B catalyzed the formation of deriva-
tives (3S,4R)-(ꢁ)-8. Unfortunately, the PSL-C showed low
enantioselectivity in these processes.
The absolute configuration of these compounds were assigned
as follows (Scheme 6): Compound (3S,4R)-(ꢁ)-8d was converted
by a Mitsunobu inversion in (3S,4S)-(ꢁ)-trans-5, whose specific
rotation was compared with the one established.3a
AcO
CH2Cl2/Py
OAc
HO
OH
Ac2O
O
O
cis-
3
cis-5
Scheme 3. Synthesis of cis-3.
HO
O
Ph
1. PNBA, PPh3, DEAD/THF
2. Na/MeOH
HO
OH
As a desymmetrization process, a maximum yield of 100% can
be attained in the enzymatic hydrolysis of the meso-derivative.
The PSL-C catalyzed hydrolysis of cis-3 in a 1:2 mixture of phos-
phate buffer 1 M and 1,4-dioxane but, unfortunately, showed a
very low selectivity. After 1 h of reaction, a mixture of the mono-
and dihydroxylated products was detected and, after 14 h of
reaction only the meso-diol cis-5 was detected. No reaction was
detected when CAL-B was used as catalyst of the reaction.
Taking into account that the low selectivity of the reaction
catalyzed by PSL-C could be due to intramolecular acyl migrations
under the hydrolytic conditions, we tested the dicarbonate cis-6a
as substrate for the enzymatic hydrolysis (Scheme 4). When the
O
O
O
(−)-(3
S
,4R
)-8d
(−)-(3S,4S)-5
Scheme 6. Assignation of the absolute configuration of (ꢁ)-8d.
Table 2 summarized the results obtained in the lipase-catalyzed
acylations of cis-5. In all cases, the reactions were finished when no
further progress of the process was observed by HPLC analysis.
Table 2
Lipase-catalyzed acylation of cis-5 in tBuOMe
O
Cl
O
O
O
O
Cl
c
OH
OH
Cl
CH2Cl2/Py
Entry
Lipase
Acylating agenta
T (°C)
t (h)
cb (%)
eep (%)
d
d
O
Cl
Cl
1
2
3
4
5
6
7
8
9
PSL-C
PSL-C
PSL-C
PSL-C
PSL-C
PSL-C
CAL-B
CAL-B
CAL-B
CAL-B
Vinyl acetate
30
30
30
30
60
30
30
30
30
30
4
24
48
48
48
30
4
24
24
24
—
—
O
O
Vinyl propionate
Vinyl decanoate
Vinyl benzoate
Vinyl benzoate
Ethyl acetate
—
—
O
O
22
65
69
—
24
25
30
—
cis-5
cis-6a
d
d
Vinyl acetate
—
—
PSL-C,60ºC, 72h
OH
O
O
∗∗
Vinyl propionate
Vinyl decanoate
Vinyl benzoate
—
29
68
—
32
74
Buffer pH 7.0 / Organic solvent
1:2
O
10
O
a
5 equiv.
(-)-7a
b
c
13% yield, 44% ee
Conversion determined by HPLC.
Determined by chiral HPLC.
A mixture of cis-3, ( )-8a, and the remaining cis-5 was obtained.
d
Scheme 4. Synthesis and enzymatic hydrolysis of cis-6a.

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V. Recuero et al. / Tetrahedron: Asymmetry 19 (2008) 1684–1688
Among the processes catalyzed by PSL-C only with the mono-
10 equiv of the acylating agent. The results obtained are summa-
rized in entries 7, 8, 13, 14, 17, and 18 of Table 3. Even though
an increment of the initial reaction rate was observed using
10 equiv of the acylating agent, in any case an improvement in
the final conversion of the process was observed, and the best
enantioselectivities were always observed in processes carried
out using 5 equiv of acylating agent.
In order to verify the possible racemization of the monoacylated
product in the reaction media, the product (ꢁ)-8d obtained in one
of the enzymatic processes was dissolved in 1,4-dioxane and sha-
ken at 30 °C for seven days. Effectively, after this time HPLC analy-
sis of the recovered product showed a racemization of 18%. This
slow racemization, probably due to intramolecular acyl migrations,
can be easily avoided by a convenient protection of the free hydro-
xyl group.
Another option to avoid the racemization of the product is the
formation of a monocarbonate through an enzymatic alkoxy-
carbonylation process. With this in mind, we tested several
carbonates in the lipase-catalyzed desymmetrization of substrate
cis-5. (Scheme 7). First, we tested the reactions in 1,4-dioxane, at
30 °C and using 5 equiv of the corresponding carbonate as an
alkoxycarbonylation agent. Entries 1–4 and 7–9 of Table 4 summa-
rize the most significant results obtained in the alkoxycarbonyla-
tion processes.
As in the case of the enzymatic acylation, PSL-C showed very
low selectivity: the reaction with dibenzyl carbonate afforded a
mixture of mono- and dialcoxycarbonylated products. Better
results were obtained using CAL-B as a catalyst. The best enantio-
acylated vinyl decanoate (+)-8c and vinyl benzoate (ꢁ)-8d were
obtained as products of respective reactions (Table 2, entries 3
and 4). However, the enantioselectivity and conversion of the
reaction were low in both cases. When we carried out the process
with vinyl benzoate at 60 °C (entry 5) the results were only slightly
improved. The same reaction carried out with vinyl acetate
afforded low conversion and a mixture of mono- and diacylated
products. Using other acylating agents such as vinyl propionate
or ethyl acetate, no product was detected after 24 h of reaction.
The preliminary results obtained in acylations catalyzed by
CAL-B are summarized in entries 7–10 of Table 2. The best results
were obtained when vinyl benzoate was used as an acylating agent
(entry 10). After 24 h of reaction, only the monoacylated product
(ꢁ)-8d was obtained with 68% of conversion and 74% enantiomeric
excess.
In view of these moderate but promising results, we decided to
study the influence of different reaction parameters in the asym-
metric acylation of cis-5 catalyzed by CAL-B, using vinyl benzoate
as an acylating agent (Table 3).
First, we studied the influence of the organic solvent at 30 °C,
using 5 equiv of vinyl acetate. The results obtained are summarized
in entries 1, 2, 9, 10, 15, and 19. The faster process was carried out
in diethyl ether (entry 15), after 17 h of reaction an 80% of the
monoacylated product was achieved with an 82% enantiomeric
excess. Similar conversions were achieved in 1,4-dioxane (81%,
entry 2) and THF (79%, entry 10) after longer reaction times. Higher
enantiomeric excess was obtained in THF (83% ee, entry 10). The
effect of the temperature was also analyzed in three of the sol-
vents. Apparently, an increase in the temperature enhanced the
initial reaction rate, but did not improve the final conversion of
the process. For example, when the reaction was carried out at
40 °C in 1,4-dioxane, 82% conversion was obtained after 24 h (en-
try 3), but after 48 h of reaction, the conversion was only slightly
higher (85%, entry 4). A small increase in the enantiomeric excess
of the product was observed at 40 °C, but this tendency was not
maintained at 50 °C (entry 5); both conversion and enantioselec-
tivity were lower than at 40 °C. We also studied the process at low-
er temperature using diethyl ether as solvent (entry 16). Under
these conditions the reaction was slower and less enantioselective
than at 30 °C.
O
HO
OH
HO
O
O
R
R
R
O
O
O
CAL-B
organic solvent
O
O
(+)-7b-g
cis-5
b R = CH2Ph
c R = CH2CH=CH2
d R = CH2CH3
e R = Ph
f R =
pNO2-Ph
g R = CH3
The influence of the concentration of the acylating agent was
also examined in 1,4-dioxane, THF, and diethyl ether, using 2 or
Scheme 7. Enzymatic alkoxycarbonylation of cis-5.
Table 3
CAL-B catalyzed acylation of cis-5 in organic solvents using vinyl benzoate as an acylating agent
ca (%)
eep (%)
b
Entry
Acylating agent (equiv)
Solvent
T (°C)
t (h)
1
2
3
4
5
7
8
9
10
11
12
13
14
15
16
17
18
19
5
5
5
5
5
2
10
5
5
5
5
2
10
5
5
2
10
5
tBuOMe
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
Acetonitrile
THF
THF
THF
THF
THF
30
30
40
40
50
30
30
30
30
40
50
30
30
30
15
30
30
30
24
64
24
48
24
48
19
72
96
48
48
72
24
17
27
24
24
72
68
81
82
85
78
68
70
75
79
75
62
74
72
80
78
70
75
41
74
78
80
80
77
73
75
73
83
70
46
72
70
82
80
71
73
13
Et2O
Et2O
Et2O
Et2O
Toluene
a
Conversion determined by HPLC.
Determined by chiral HPLC.
b

V. Recuero et al. / Tetrahedron: Asymmetry 19 (2008) 1684–1688
1687
Table 4
Lipase-catalyzed alkoxycarbonylation of cis-5 in 1,4-dioxane at 30 °C
c
Entry
Carbonatea
R
Enzyme
Solvent
T (°C)
t (h)
cb (%)
eep (%)
d
1
2
3
4
5
6
7
8
9
Benzyl
Benzyl
Allyl
Ethyl
Ethyl
Ethyl
Phenyl
p-Nitrophenyl
Methyl
PSL-C
CAL-B
CAL-B
CAL-B
CAL-B
CAL-B
CAL-B
CAL-B
CAL-B
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
Ether
1,4-Dioxane
1,4-Dioxane
1,4-Dioxane
30
30
30
30
40
30
30
30
30
24
24
24
15
24
24
72
48
72
—
—
—
—
d
52
30
55
22
—
38
78
31
16
—
—
—
—
—
a
5 equiv.
b
c
Conversion determined by HPLC.
Determined by chiral HPLC.
A mixture of cis-6b, ( )-7b, and the remaining cis-5 was obtained.
d
selectivity was observed when the diethyl carbonate was used as
the alkoxycarbonylation agent. In this case, the product of the
enzymatic process showed 78% enantiomeric excess but only 30%
conversion was observed after 24 h of reaction (entry 4). In order
to improve these results, we carried out the reaction at 40 °C (entry
5). Under these conditions, the conversion increased to 55% after
24 h of reaction, but the enantioselectivity was lower than in the
process carried out at 30 °C. We also tested this enzymatic process
in ether (entry 6), because in this media the acylation is faster than
in the other solvents tested; however, for the alkoxycarbonylation
process the reaction in ether is slower and less enantioselective
than in 1,4-dioxane.
used to record mass spectra on Hewlett–Packard 110 LC/MSD Ser-
ies spectrometers. The enantiomeric excesses were determined by
chiral HPLC analysis on a Hewlett–Packard 1100, LC liquid chro-
matograph, using a CHIRALPCK IA column (4.5 ꢂ 250 mm) or by
GC analyses on a Hewlett Packard 6890 Series II chromatograph
equipped with a Restek RtbDEXse (30 m ꢂ 0.25 mm ꢂ 0.25
lm,
1.0 bar N2) column. For all the analyses, the injector temperature
was 225 °C and the FID temperature was 250 °C.
4.2. Synthesis of ( )-trans-3,4-diacetoxytetrahydrofuran ( )-
trans-3
The absolute configuration of compound (3S,4R)-(ꢁ)-7d was as-
signed as in the case of (3S,4R)-(ꢁ)-8d by a Mitsunobu inversion
and hydrolysis of the resulting product to afford (3S,4S)-(ꢁ)-
trans-5, whose specific rotation was compared with the one
established.3a
To a solution of 2 (0.5 g, 5.1 mmol) in acetic acid (10 mL), acetic
anhydride was added (2 mL, 21 mmol). Then, boron trifluoride
etherate (635 lL, 5 mmol) was slowly added, and the mixture
was stirred at room temperature for 24 h. An aqueous saturated
solution of NaHCO3 (10 mL) was added, and the mixture extracted
with EtOAc (20 mL). The organic phase was washed with saturated
aqueous Na2CO3 solution (20 mL) followed by saturated aqueous
NaCl solution (20 mL), and then dried over Na2SO4. The solvent
was removed under reduced pressure, and the crude residue was
purified by flash chromatography on silica gel (hexane/EtOAc
1:1) to afford product ( )-trans-4 as a white solid (0.75 g, 78%).
Mp 34–36 °C; 1H NMR (CDCl3, 300 MHz): d 2.04 (s, 6H), 3.72–
3.76 (m, 2H), 4.06 (dd, 2H, J = 4.38 Hz, J = 10.53 Hz), 5.11–5.14
(m, 2H); 13C NMR (CDCl3, 100.5 MHz): d 21.38 (2CH3), 72.48
(2CH2), 77.28 (2CH), 170.55 (2CO); IR (KBr, cmꢁ1) 1021.1, 1072.8,
1228.7, 1373.2, 1436.0, 1467.3, 1740.0, 2877.2, 2990.8 cmꢁ1; MS
(ESI+ m/z): 189 [(M+H)+, 100].
3. Conclusions
In conclusion, we have reported an easy methodology for the
resolution of trans-3,4-dihydroxytetrahydrofuran via a PSL-C cata-
lyzed hydrolysis of its diacetyl derivative. The high enantioselectiv-
ity obtained in this process is particularly noteworthy. On the other
hand, the enzymatic acylation is preferable to alkoxycarbonylation
for the desymmetrization of the cis-3,4-dihydroxytetrahydrofuran;
the best results are obtained in the process catalyzed by C. antarc-
tica lipase B using vinyl benzoate as an acylating agent, a conver-
sion higher than 80% and a enantiomeric excess of 82% can be
obtained by the appropriate selection of the reaction parameters.
4.2.1. Analysis by chiral GC of ( )-trans 3
Determination of the ee by GC analysis: RtbDEXse, 70 °C
(5 min), 3 °C/min, 200 °C (10 min). tR (3R,4R) 29.8 min; tR (3S,4S)
30.3 min.
4. Experimental
4.1. General
Enzymatic reactions were carried out in a Gallenkamp incuba-
tory orbital shaker. Immobilized C. antarctica lipase B, CAL-B Nov-
ozym 435 (7300 PLU/g), was a gift from Novo Nordisk co. and
immobilized P. cepacia lipase (PSL-C, 783 U/g) is from by Amano
Pharmaceuticals.
4.3. General procedure for the enzymatic hydrolysis of ( )-
trans-3
The reaction mixture containing the substrate (100 g,
0.16 mmol), 1 mL of 1 M phosphate buffer (pH 7.0), the lipase
(100 mg) and the corresponding organic solvent (2 mL) was shaken
at 30 °C and 250 rpm in an orbital shaker. The progress of the reac-
tion was monitored by TLC (hexane/EtOAc 1:1) until the achieve-
ment of the required conversion. Then, the enzyme was removed
by filtration and washed with the same solvent that has been used
in the reaction, 3 ꢂ 5 mL. The solvent was removed under reduced
pressure, and the crude residue was purified by flash chromatogra-
phy on silica gel (hexane/EtOAc 1:1) to afford the monoacetylated
product (3R,4R)-4 and the remaining substrate (3S,4S)-3.
Chemical reagents were purchased by Aldrich, Fluka, Lancaster
or Prolabo. Solvents were distilled over an appropriate desiccant
under nitrogen. Flash chromatography was performed using Merck
Silica Gel 60 (230–400 mesh). Optical rotations were measured
using a Perkin–Elmer 241 polarimeter and are quoted in units of
10ꢁ1 deg cm2 gꢁ1 1H NMR, 13C NMR, and DEPT spectra were re-
.
corded in a Bruker AC-300, Bruker AC-300 DPX, or Bruker NAV-
400 spectrometer using CDCl3 as solvent. The chemical shift values
(d) are given in ppm. Positive electrospray ionization (ESI+) was

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V. Recuero et al. / Tetrahedron: Asymmetry 19 (2008) 1684–1688
4.3.1. (+)-(3S,4S)-3,4-Diacetoxytetrahydrofuran, (+)-(3S,4S)-3.
Determination of the ee by GC analysis
4.5. General procedure for the enzymatic alkoxycarbonylations
of cis-3,4-dihydroxytetrahydrofuran
RtbDEXse, 70 °C (5 min), 3 °C/min, 200 °C (10 min). tR (3S,4S)
30.0 min, ee >99%.
The lipase (100 mg) and the corresponding carbonate (5 equiv)
were added to a solution of cis-5 (100 mg, 1 equiv) in 1,4-dioxane
(4 mL). The mixture was shaken at the selected temperature and
250 r.p.m. in a rotatory shaker. The progress of the reaction was
monitored by TLC. Once the reaction was finished, the enzyme
was removed by filtration, washed with 1,4-dioxane, and the sol-
vent was evaporated under reduced pressure. The crude residue
was purified by flash chromatography on silica gel to afford the
corresponding monoalkoxycarbonyl derivatives cis-7b–g.
4.3.2. (ꢁ)-(3R,4R)-3-Acetoxy-4-hydroxytetrahydrofuran, (ꢁ)-
(3R,4R)-4
White solid. Mp 37–38 °C; 1H NMR (CDCl3, 300 MHz): d 2.22 (s,
3H), 3.56 (br s, OH), 3.79–3.90 (m, 2H), 4.01–4.09 (m, 2H), 4.24–
4.32 (m, 2H); 13C NMR (CDCl3, 100.5 MHz): d 21.31 (CH3), 72.73
(CH2), 72.98 (CH2), 77.35 (CH), 81.92 (CH), 170.28 (CO); IR (KBr,
cmꢁ1) 3122, 1743; MS (ESI+ m/z): 147 [(M+H)+, 100].
4.5.1. (ꢁ)-(3S,4R)-3-Ethoxycarbonyloxy-4-
4.3.3. Determination of the ee by GC analysis
hydroxytetrehydrofuran, (ꢁ)-(3S,4R)-7d
In order to determinate its enantiomeric excess, (ꢁ)-(3R,4R)-4
was converted into the diacetyl derivative (ꢁ)-(3R,4R)-3 and ana-
lyzed by GC. RtbDEXse, 70 °C (5 min), 3 °C/min, 200 °C (10 min).
tR (3R,4R) 29.8 min, ee >99%.
White solid. Mp 39–41 °C. 1H NMR (CDCl3, 300 MHz): d 1.42 (t,
3H), 2.29 (c, 2H), 3.57 (br s, OH), 3.77–3.87 (m, 2H), 4.01–4.13 (m,
2H), 4.19–4.31 (m, 2H). 13C NMR (CDCl3, 100.5 MHz): d 11.52
(CH3), 28.11 (CH2), 72. 91 (CH2), 73.06 (CH2), 77.15 (CH), 82.88
(CH), 172.98 (CO); IR (KBr, cmꢁ1) 1138.5, 1422.1, 1744.8; MS
(ESI+ m/z): 199 [(M+Na)+, 40].
4.4. General procedure for the enzymatic acylation
The reaction mixture containing the corresponding substrate
(0.15 mmol), the corresponding acylating agent (0.75 mmol),
the lipase (100 mg) and the corresponding organic solvent (4 mL)
was shaken at 30 °C and 250 rpm in an orbital shaker. The progress
of the reaction was monitored by TLC (hexane/EtOAc 7:3) until
the achievement of the required conversion. Then, the enzyme
was removed by filtration and washed with CHCl3 (3 ꢂ 5 mL).
The solvent was removed under reduced pressure and the crude
residue was purified by flash chromatography on silica gel
(hexane/EtOAc 7:3), to afford the corresponding monoacylated
derivative.
4.5.2. Determination of the ee by HPLC analysis
Chiralpack IA, 20 °C, hexane/2-propanol (80:20), UV 210 nm,
0.5 mL minꢁ1, tR 15,69 min (minor); tR 17.48 min (major). ½a D25
¼
ꢃ
ꢁ4:6 (c 0.25, HCCl3), ee = 78%.
Acknowledgments
This work was supported by the Principado de Asturias (project
PC06-019). We express our appreciation to Novo Nordisk Co. for
the generous gift of the lipase CAL-B.
References
4.4.1. (ꢁ)-(3S,4R)-3-Benzoyloxy-4-hydroxytetrahydrofuran,
(ꢁ)-(3S,4R)-8d
1. See for example: Aghmiz, M. A.; Aghmiz, A.; Masdeu-Bulto, Y.; Claver, C.;
Castillon, S. J. Org. Chem. 2004, 69, 7502–7510.
2. See for example: (a) Wang, W.; Jin, H.; Fuselli, N.; Mansour, T. S. Bioorg. Med.
Chem. Lett. 1997, 2567–2572; (b) Tatani, K.; Shuto, S.; Ueno, Y.; Matsuda, A.
Tetrahedron Lett. 1998, 39, 5065–5068; (c) Hwang, D. J.; Kim, S. N.; Choi, J. H.;
Lee, Y. S. Bioorg. Med. Chem. Lett. 2001, 1429–1437.
White solid. Mp 48–49 °C; 1H NMR (CDCl3, 300 MHz): d 2.42 (br
s, OH), 3.78–3.98 (m, 1H), 4.00–4.20, (m, 3H), 4.57–4.59 (m, 1H),
5.36–5.41 (m, 1H), 7.43–7.59 (m, 3H), 8.05–8.08 (d, 2H,
J
7.17 Hz), 13C NMR (CDCl3, 100.5 MHz): d 70.93 (CH2), 71.47
(CH2), 72.72 (CH), 74.57 (CH), 128.86 (2CH), 129.61 (2CH), 130.08
(2CH), 133.86 (C), 166.67 (CO); IR (KBr, cmꢁ1) 1120.9, 1274.5,
1451.5, 1710.0, 3428.4; MS (ESI+ m/z): 209 [(M+H)+, 100].
3. (a) Dulphy, H.; Gras, J.-L.; Lejon, T. Tetrahedron 1996, 52, 8517–8524; (b)
_
Skarzewski, J.; Gupta, A. Tetrahedron: Asymmetry 1997, 8, 1861–1867.
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4.4.2. Determination of the ee by HPLC analysis
Chiralpack IA, 20 °C, hexane/2-propanol (80:20), UV 210 nm,
0.5 mL minꢁ1, tR 13.43 min (minor); tR 15.23 min (major). ½a D25
¼
ꢃ
8. Xue, F.; Seto, C. T. Bioorg. Med. Chem. 2006, 14, 8467–8487.
9. Chen, C. S.; Fujimoto, Y.; Girdaukas, G.; Sih, C. J. J. Am. Chem. Soc. 1982, 104, 7294.
ꢁ12 (c 0.03, HCCl3), ee = 83%.