Enzymatic desymmetrization of pyrrolidine and pyrroline derivatives

Article abstract of DOI:10.1016/j.tetasy.2008.05.022

The enzymatic desymmetrization of various meso-N-Boc-2,5-cis-disubstituted pyrrolidines and pyrrolines compounds by ester hydrolysis or transesterification provided the corresponding monoesters in high enantiomeric excess.

Full text of DOI:10.1016/j.tetasy.2008.05.022

Tetrahedron: Asymmetry 19 (2008) 1333–1338
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Enzymatic desymmetrization of pyrrolidine and pyrroline derivatives
a
a
b
Robert Chênevert a, , Frédéric Jacques , Pascall Giguère , Mohammed Dasser
*
a Département de Chimie, CREFSIP, Faculté des Sciences et de Génie, Université Laval, Québec (Qc), Canada G1K 7P4
b OmegaChem, 8800 boul. de la Rive-Sud, Lévis (Qc), Canada G6V 9H1
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 13 March 2008
Accepted 19 May 2008
The enzymatic desymmetrization of various meso-N-Boc-2,5-cis-disubstituted pyrrolidines and pyrro-
lines compounds by ester hydrolysis or transesterification provided the corresponding monoesters in
high enantiomeric excess.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
CO2Me
N
Bn
MeO2C
MeO2C
Pyrrolidine and pyrroline rings are found in numerous natural
products, such as pyrrolizidine and indolizidine alkaloids.1–3
These five-membered heterocycles are common structural
features of synthetic pharmaceutical and agrochemical com-
pounds.2,4,5 Many polyhydroxylated pyrrolidines, also known as
imino or azasugars, are inhibitors of glycosidase enzymes, show-
ing therapeutic potential in the treatment of diabetes, AIDS, and
cancer.6–9 Chiral pyrrolidines have been used as organocata-
lysts10,11 and chiral auxiliaries or ligands in asymmetric synthe-
sis.12–15 The interest in pyrrolidine and pyrroline derivatives is
well displayed by the wealth of published material detailing
their synthesis.2,4,16–21 Enzymatic differentiation of enantiotopic
groups in meso-substrates (desymmetrization) is an efficient
method for the preparation of enantiomerically enriched prod-
ucts with multiple stereogenic centers.22 Herein, we report the
enzymatic desymmetrization of meso-pyrrolidines and meso-
pyrrolines.
1
CO2Me
N
MeO2C
Boc
a
d
5
CO2Me
N
Boc
2
N
OH
Boc
6
OH
b
e
N
c
OH
N
OAc
Boc
3
Boc
4
OH
OAc
Scheme 1. Reagents and conditions: (a) H2, Pd(OH)2/C, Boc2O, MeOH, 99%; (b)
LiAlH4, THF, 84%; (c) AcCl, Et3N, CH2Cl2, 88%; (d) H2, Pt/C, 96%; (e) H2, Pd/C, 95%.
2. Results and discussion
2.1. Substrate preparation
we sought a more suitable synthesis of 2 and 3. In an alternative
route, catalytic hydrogenation of known compound 528 provided
a nearly quantitative yield of a cis–trans mixture rich (93%) in the-
cis-compound 2. Similarly, hydrogenation of 629 gave 3 in high
yield.
The synthesis of substrates 9 and 10 is described in Scheme 2.
Diol 8 was prepared by the reaction of the known olefin 729 with
catalytic osmium tetroxide in the presence of N-methylmorpholine
N-oxide (NMO) as the co-oxidant. Reaction of diol 8 with dry ace-
tone in the presence of a catalytic amount of p-toluenesulfonic acid
(p-TsOH) and sodium sulfate as a dehydrating agent provided iso-
propylidene acetal 9. Saponification of the acetate groups with
K2CO3 in water–methanol gave diol 10.
N-Benzyl cis-diester 1, which was prepared in four steps start-
ing from adipic acid,23–26 was converted into the corresponding
N-tert-butyloxycarbonyl (N-Boc) cis-diester 2 by catalytic hydro-
genolysis in the presence of di-tert-butyl dicarbonate (Boc2O) in a
one-pot procedure (Scheme 1). Reduction of 2 with LiAlH4 in
THF27 gave meso diol 3. The corresponding meso diacetate 4 was
obtained by acetylation of diol 3 with acetyl chloride. The prepara-
tion of 1 from adipic acid is tedious and gives low yields; therefore
* Corresponding author.
E-mail address: robert.chenevert@chm.ulaval.ca (R. Chênevert).
0957-4166/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2008.05.022

1334
R. Chênevert et al. / Tetrahedron: Asymmetry 19 (2008) 1333–1338
phile to water and lowers the yield of monoester. We needed an
OH
HO
easily reproducible reaction on a preparative scale and a more con-
venient protecting group such as tert-butyloxycarbonyl (Boc).
Diester 2 was hydrolyzed by PLE in water at pH 7.5 to give opti-
cally active monoester 11 in good yield and high enantiomeric ex-
cess (ee) (Table 1). Candida antarctica lipase fraction B (CAL-B)
provided monoester 11 in high ee (P99%) but low yield (40%).
The reaction stopped after 24 h and the low yield was attributed
to product inhibition. The hydrolysis of the corresponding diethyl
ester gave identical results.
The results of the enzymatic desymmetrization of meso-sub-
strates 3–4, 6–7 and 9–10 via acylation or hydrolysis are summa-
rized in Tables 2–4. The enantiomeric excess of the monoesters
were determined by HPLC or GC on a chiral phase. The reactions
were monitored by this chromatographic analysis and terminated
when all of the starting material (diol or diacetate) was consumed
(conversion = 100%).
a
N
Boc
8
N
Boc
7
OAc
OAc
OAc
b
OAc
O
N
O
N
Boc
10
O
O
c
OAc
OH
Boc
OAc
OH
9
Scheme 2. Reagents and conditions: (a) OsO4, NMO, acetone, H2O, 91%; (b) acetone,
p-TsOH, 91%; (c) K2CO3, MeOH–H2O, 95%.
Acylation of diol 3 with vinyl acetate as the acylation reagent
and solvent in the presence of CAL-B provided monoester
(2S,5R)-12 in fair yield (70%) and high enantiomeric excess (97%)
(Table 2, entry 1). The opposite enantiomer (2R,5S)-12 was ob-
tained in better yield (83%) but in lower ee (85%) in the presence
of Candida rugosa lipase (CRL) (entry 2). Interestingly, acylation
and hydrolysis with CRL afforded the same enantiomer (entries 2
and 3). When both the meso-alcohol and the corresponding
meso-ester are substrates for a lipase, acylation and hydrolysis
are usually complementary and give the opposite enantiomers.
While acylation and hydrolysis represent reactions in opposite
directions, the enzyme displays the same selectivity (the same
enantiomer or the same prochiral group) in both cases. This empir-
ical rule applies to kinetic resolutions and desymmetrizations but
exceptions have been reported.30,31 However, with both CRL and
PLE, extensive overhydrolysis to achiral diol 3 occured and mono-
ester 12 was obtained in poor yield (entries 3 and 4).
Table 3 displays several biotransformations leading to pyrroline
monoester 13. CRL, Pseudomonas cepacia lipase (PCL), Pseudomonas
sp. lipase (PSL), and CAL-B were found to catalyze the enantioselec-
tive esterification of diol 6 (entries 1–4). Acylation of diol 6 by
treatment with PSL in vinyl acetate gave the best results
(yield = 85%, ee = 96%, entry 3). Hydrolysis of diester 7 in the pres-
ence of PSL and CRL gave monoester (2S,5R)-13 with high enantio-
selectivity, but yields were lowered by overhydrolysis. As the
desymmetrization of 6–7 was being developed, we became aware
of a report by Donohoe et al.29 describing similar and complemen-
tary results.
2.2. Enzymatic desymmetrization
We first completed some screening experiments in order to find
hydrolases with the ability to distinguish the enantiotopic groups
of meso-substrates 2, 3–4, 6–7, 9–10. The pig liver esterase (PLE)-
catalyzed hydrolysis of ester 1 has been reported and found to be
very dependent on the reaction conditions.25 High enantioselectiv-
ity is obtained only in the presence of a co-solvent (25% DMSO in
Tris buffer at pH 7.5). Reactions run in this buffer were faster and
gave higher enantioselectivity but Tris acts as a competitive nucleo-
Table 1
Enzymatic desymmetrization of diester 2
2
hydrolase
CO2Me
CO2H
5
N
N
MeO2C
MeO2C
H2O, pH 7.5
Boc
11
Boc
2
Enzyme
Time (h)
Yieldc (%)
eed (%)
Absolute configuration
PLEa
36
24
75
40
P99
P99
(+)-(2S,5R)
(+)-(2S,5R)
CAL-Bb
a
PLE: pig liver esterase.
CAL-B: Candida antarctica lipase B.
Isolated yield.
b
c
d
Determined by HPLC analysis using a Chiralcel OD-H column.
Table 2
Enzymatic desymmetrization of diol 3 and diacetate 4
5
2
2
5
Enzyme
Enzyme
N
N
N
OAc
OH
OR
Boc
Boc
Boc
OAc
OH
OR
Phosphate buffer
pH 7.5
vinyl acetate
3 R = H
(+)-(2R,5S)-12
(-)-(2S,5R)-12
4 R = Ac
Entry
Substrate
Reaction
Enzyme
Time (h)
Yieldd (%)
eee (%)
Absolute configuration
1
2
3
4
Diol 3
Diol 3
Diester 4
Diester 4
Acylation
Acylation
Hydrolysis
Hydrolysis
CAL-Ba
CRLb
CRLb
PLEc
36
22
26
26
70
83
30
15
97
85
94
75
(ꢀ)-(2S,5R)
(+)-(2R,5S)
(+)-(2R,5S)
(+)-(2R,5S)
a
CAL-B: Candida antarctica lipase B.
CRL: Candida rugosa lipase.
PLE: pig liver esterase.
Isolated yield.
b
c
d
e
Determined by GC analysis using a Chiraldex B-DM column.

R. Chênevert et al. / Tetrahedron: Asymmetry 19 (2008) 1333–1338
1335
Table 3
Enzymatic desymmetrization of diol 6 and diacetate 7
5
2
2
5
Enzyme
Enzyme
N
N
OAc
N
OH
OR
Boc
Boc
Boc
OH
OAc
OR
Phosphate buffer
pH 7.0
Vinyl acetate
(-)-(2S,5R)-13
(+)-(2R,5S)-13
6 R = H
7 R = Ac
Entry
Substrate
Reaction
Enzyme
Time (h)
Yielde (%)
eef (%)
Absolute configuration
1
2
3
4
5
6
Diol 6
Diol 6
Diol 6
Diol 6
Diester 7
Diester 7
Acylation
Acylation
Acylation
Acylation
Hydrolysis
Hydrolysis
CRLa
PCLb
PSLc
39
39
26
84
24
3.5
71
60
85
65
75
65
80
81
96
88
94
96
(+)-(2R,5S)
(+)-(2R,5S)
(+)-(2R,5S)
(ꢀ)-(2S,5R)
(ꢀ)-(2S,5R)
(ꢀ)-(2S,5R)
CAL-Bd
PSLc
CRLa
a
CRL: Candida rugosa lipase.
PCL: Pseudomonas cepacia lipase.
PSL: Pseudomonas sp. lipase.
CAL-B: Candida antarctica lipase B.
Isolated yield.
b
c
d
e
f
Determined by HPLC analysis using a Chiralcel OD-H column.
Table 4
Enzymatic desymmetrization of diacetate 9 and diol 10
O
O
O
O
O
O
Enzyme
4
6
Enzyme
6
Phosphate buffer
pH 7.0
4
N
N
N
OH
OAc
OR
Vinyl acetate
10 R = H
Boc
Boc
Boc
OAc
9 R = Ac
OH
OR
(-)-(3aR,4R,6S,6aS)-14
(+)-(3aS,4S,6R,6aR)-14
Entry
Substrate
Reaction
Enzyme
Time (h)
Yieldc (%)
eed (%)
Absolute configuration
1
2
3
4
Diol 10
Diol 10
Diester 9
Diester 9
Acylation
Acylation
Hydrolysis
Hydrolysis
CAL-Ba
PSLb
96
24
120
72
90
85
30
60
97
96
60
90
(ꢀ)-(3aR,4R,6S,6aS)
(+)-(3aS,4S,6R,6aR)
(+)-(3aS,4S,6R,6aR)
(ꢀ)-(3aR,4R,6S,6aS)
CAL-Ba
PSLb
a
CAL-B: Candida antarctica lipase B.
PSL: Pseudomonas sp. lipase.
Isolated yield.
b
c
d
Determined by HPLC analysis using a Chiralcel OJ-H column.
The results obtained for the desymmetrization compounds 9–
3. Experimental
3.1. General
10 are shown in Table 4. The enantioselective acylation of 10
was performed successfully with both PSL and CAL-B (entries 1
and 2). These enzymes have a complementary enantiopreference
and both enantiomers of monoester 14 were obtained in good yield
(85–90%) and high ee (96–97%). The hydrolysis of diester 9 by PSL
or CAL-B provided product 14 with opposite absolute stereochem-
istry with lower yield and ee (entries 3 and 4).
NMR spectra were recorded on a Varian Inova AS400 spectro-
meter (400 MHz). The majority of the 1H and 13C NMR spectra
exhibit doubling of some signals because of the presence of
N-Boc rotamers. Infrared spectra were recorded on a Bomem MB-
100 spectrometer. Optical rotations were measured using a JASCO
DIP-360 polarimeter (c as gram of compound per 100 mL). Flash
2.3. Determination of the absolute configurations
column chromatography was carried out using 40–63 lm (230–
The absolute configurations of compounds 12 and 14 (Scheme
3) were determined by chemical correlation with compound (ꢀ)-
13 of a known absolute configuration being (2S,5R).29 Hydrogena-
tion of 13 in methanol in the presence of 10% Pd/C gave monoester
(ꢀ)-(2S,5R)-12. Compound (ꢀ)-(3aR,4R,6S,6aS)-14 was prepared by
dihydroxylation of 13 followed by acetalization of intermediate 15
with acetone. Hydrogenolysis of the known (+)-(2S,5R)-1625 with
H2/Pd(OH)2/C in the presence of Boc2O provided (+)-(2S,5R)-11.
400 mesh) silica gel. The enantiomeric excesses (ee) were
determined by chiral HPLC analysis on Chiralcel OD-H or Chiralcel
OJ-H columns and by chiral GC analysis on a Chiraldex B-DM
column using racemic compounds as references. C. antarctica lipase
B (Chirazyme L-2) was obtained from Boehringer Mannheim.
Porcine liver esterase (PLE), Pseudomonas sp. lipase (PSL), P. cepacia
lipase (PCL), and C. rugosa lipase (CRL) were purchased from
Aldrich.

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R. Chênevert et al. / Tetrahedron: Asymmetry 19 (2008) 1333–1338
HO
OH
HO
HO
b
5
5
2
2
OAc
OAc
N
N
Boc
Boc
(-)-(2S,5R)-13
(-)-(2R,3R,4S,5S)-15
a
c
HO
O
O
5
2
HO
6
OAc
N
4
OAc
N
Boc
(-)-(2S,5R)-12
Boc
(-)-(3aR,4R,6S,6aS)-14
MeO
MeO
5
5
d
2
2
OH
OH
N
N
O
O
Boc
Bn
O
O
(+)-(2S,5R)-11
(+)-(2S,5R)-16
Scheme 3. Reagents and conditions: (a) Pd/C, MeOH, 95%; (b) OsO4, NMO, acetone–H2O, 90%; (c) acetone, p-TsOH, 90%; (d) H2, Pd(OH)2/C, Boc2O, MeOH, quantitative.
3.2. cis-1-tert-Butyl 2,5-dimethyl pyrrolidine-1,2,5-
tricarboxylate 2
MgSO4, and evaporated. The crude product was purified by flash
chromatography (hexanes–EtOAc, 3:7) to give cis-diol 327 (1.83 g,
84%) as
a
;
colorless oil: IR (NaCl) 3397, 2970, 1657, 1396,
Method 1: To a solution of diester 1 (3.16 g, 11.4 mmol) and
di-tert-butyl dicarbonate (2.73 g, 12.5 mmol) in freshly degassed
methanol (30 mL) was added 20% Pd(OH)2/C (54 mg). Hydrogenol-
ysis was performed at 54 psi for 24 h at room temperature. EtOAc
(75 mL) was added and the mixture was filtered through a pad of
Celite. The organic layer was washed with 1 N HCl (3 ꢁ 75 mL), sat-
urated aqueous NaHCO3 (3 ꢁ 75 mL) and brine (75 mL). The organ-
ic layer was dried over MgSO4 and evaporated. The crude product
was purified by flash chromatography (hexanes–EtOAc, 7:3) to
give 224,27 (3.25 g, 99%). IR (NaCl) 2983, 1748, 1687, 1365,
1043 cmꢀ1
1H NMR (400 MHz, CDCl3) d 1.45 (s, 9H), 1.72–2.00
(m, 4H), 3.48–3.52 (m, 2H), 3.86 (s, 2H), 3.70–4.02 (m, 4H); 13C
NMR (100 MHz, CDCl3) d 27.1, 28.7, 60.8, 63.6, 66.4, 80.9, 156.6.
Method 2: A solution of 6 (438 mg, 1.91 mmol) in MeOH (25 mL)
was stirred at room temperature with 10% Pd/C (100 mg) under
hydrogen (50 psi) for 4 h. The catalyst was then removed by filtra-
tion and the solvent evaporated. The crude product was purified by
flash chromatography (hexanes–EtOAc, 3:7) to give cis-diol 3
(421 mg, 95%) as a colorless oil. Spectral data as above.
1158 cmꢀ1
;
1H NMR (400 MHz, CDCl3) d 1.33 (s, 9H), 2.02–2.15
3.4. cis-1-tert-Butoxycarbonyl-(2,5)-bis(acetoxymethyl)
(m, 4H), 3.66 (s, 6H), 4.19–4.34 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 28.3, 28.9, 29.7, 52.2, 52.3, 59.7, 60.2, 80.9, 153.6, 172.3, 172.6.
Method 2: A solution of 5 (295 mg, 1.04 mmol) in MeOH (20 mL)
was vigourously stirred at room temperature and degassed with
argon for 10 min before Pt/C (90 mg) was added. Hydrogen
(1 atm) was added and the mixture stirred for 8 h. The mixture
was diluted with CH2Cl2 (20 mL) and the catalyst was removed
by filtration through a pad of Celite. The pad was washed with
CH2Cl2 (25 mL) and the solvent evaporated. The crude product
was purified by flash chromatography (hexanes–AcOEt, 7:3) to
give 2 (288 mg, 96%) as a colorless oil. Spectroscopic data as above.
pyrrolidine 4
To a solution of diol 3 (400 mg, 1.73 mmol) in anhydrous CH2Cl2
(15 mL) at 0 °C under dry atmosphere were added anhydrous Et3N
(532 lL, 3.82 mmol) and acetyl chloride (270 lL, 3.82 mmol). The
reaction mixture was stirred at room temperature for 2 h, washed
with water, dried over MgSO4, and evaporated. Flash chromatogra-
phy (hexanes–EtOAc, 1:1) provided diacetate 4 (480 mg, 88%) as a
colorless oil. IR (NaCl) 2916, 1742, 1657, 1359, 1036 cmꢀ1; 1H NMR
(400 MHz, CDCl3) d 1.45 (s, 9H), 1.74–2.03 (m, 4H), 2.06 (s, 6H),
3.96–4.19 (m, 6H); 13C NMR (100 MHz, CDCl3) d 21.1, 26.8, 27.7,
28.6, 56.9, 65.1, 80.4, 154.7, 171.1; HRMS (CI, NH3) calcd for
3.3. cis-1-tert-Butoxycarbonyl-(2,5)-bis-(hydroxymethyl)-
C
15H26NO6 (MH)+: 316.1760. Found 316.1765.
pyrrolidine 3
3.5. (2S,3S,4R,5R)-1-tert-Butoxycarbonyl-3,4-dihydroxy-2,5-
bis(acetoxymethyl) pyrrolidine meso-8
Method 1: To a suspension of LiAlH4 (0.718 g, 18.9 mmol) in
anhydrous THF (100 mL) at 0 °C under a dry nitrogen atmosphere
was added a solution of diester 2 (2.71 g, 9.43 mmol) in anhydrous
THF (100 mL). The reaction mixture was stirred at room tempera-
ture for 20 h and then quenched by the addition of saturated aque-
ous NH4Cl (75 mL) and EtOAc (100 mL). The organic layer was
washed with 1 M HCl (3 ꢁ 100 mL), brine (100 mL), dried over
To a solution of olefin 7 (344 mg, 1.1 mmol) in acetone–water
(4:1, 15 mL) was added N-methylmorpholine-N-oxide monohy-
drate (396 mg, 3.3 mmol) followed by a solution of osmium tetrox-
ide in water (200
lL, 4% w/w) and a catalytic amount of quinidine.
The mixture was stirred at room temperature for 24 h, then ex-

R. Chênevert et al. / Tetrahedron: Asymmetry 19 (2008) 1333–1338
1337
tracted with diethyl ether (2 ꢁ 25 mL). The organic phase was
washed with brine (1 ꢁ 25 mL), dried over MgSO4, and concen-
trated. The crude product was purified by flash chromatography
(Et2O) to yield 8 (348 mg, 91%) as a colorless oil. IR (NaCl) 3419,
The crude product was purified by flash chromatography (hex-
anes–EtOAc, 1:1) to provide (ꢀ)-(2S,5R)-12 (1.24 g, 70%) as a color-
less oil. ½a 2D0
¼ ꢀ10:9 (c 1.60, CHCl3); ee = 97%, chiral GC; IR (NaCl)
ꢂ
3397, 2922, 1657, 1395, 1042 cmꢀ1 1H NMR (400 MHz, CDCl3) d
;
1642, 1391, 1249, 1037 cmꢀ1
;
1H NMR (400 MHz, CDCl3) d 1.46
1.44 (s, 9H), 1.50–2.00 (m, 5H), 2.04 (s, 3H), 3.45–4.20 (m, 6H);
13C NMR (100 MHz, CDCl3) d 21.1, 27.0, 27.2, 28.6, 57.7, 61.7,
65.3, 67.5, 81.2, 154.6, 171.1; HRMS (CI, NH3) calcd for
(s, 9H), 2.09 (s, 6H), 3.80–4.40 (m, 10H); 13C NMR (100 MHz, CDCl3)
d 21.1, 28.5, 30.5, 62.2, 63.1, 72.9, 81.3, 155.1, 171.2; HRMS (CI,
NH3) calcd for C15H26NO8 (MH)+: 348.1658. Found: 348.1668.
C
13H24NO5 (MH)+: 274.1654. Found: 274.1649.
3.6. [(3aR,4R,6S,6aS)-5-(tert-Butoxycarbonyl)-2,2-dimethyl-
tetrahydro-3aH-[1,3]dioxolo[4,5-c]pyrrole-4,6-diyl]-
bis(methylene) diacetate meso-9
3.9.2. Enzymatic hydrolysis of diacetate 6
Diester 4 (100 mg, 0.32 mmol) was emulsified in a phosphate
buffer (10 mL, 0.5 M, pH 7.5) and C. rugosa lipase was added
(25 mg). After 26 h, the aqueous mixture was filtered and extracted
with CH2Cl2 (3 ꢁ 20 mL). The organic layer was dried over MgSO4
and concentrated. Flash chromatography (hexane–EtOAc, 1:1) gave
To a solution of diol 8 (298 mg, 0.86 mmol) in acetone (25 mL)
was added sodium sulfate (500 mg) followed by p-toluenesulfonic
acid (200 mg). The mixture was stirred at room temperature over-
night, then filtered and evaporated. Flash chromatography
(CH2Cl2–acetone, 3:1) provided 9 (310 mg, 91%) as a colorless oil.
(+)-(2R,5S)-12 (26 mg, 30%) as a colorless oil. ½a D20
¼ þ10:6 (c 1.60,
ꢂ
CHCl3); ee = 94%, chiral GC. Spectroscopic data as above.
IR (NaCl) 2981, 1748, 1391, 1245, 1037 cmꢀ1
;
1H NMR (400 MHz,
3.10. tert-Butyl 2-(acetoxymethyl)-5-(hydroxymethyl)-2,5-
dihydro-1H-pyrrole-1-carboxylate 13
CDCl3) d 1.31 (s, 6H), 1.44 (s, 9H), 2.11 (m, 6H), 3.35–4.61 (m,
8H); 13C NMR (100 MHz, CDCl3) d 15.4, 21.0, 25.4, 27.3, 28.4,
50.6, 63.3, 63.6, 63.8, 64.0, 66.0, 81.1, 81.9, 82.7, 112.4, 154.0,
170.9; HRMS (CI, NH3) calcd for C18H30NO8 (MH)+: 388.1971.
Found: 388.1974.
3.10.1. Enzymatic acylation of diol 6
To a solution of diol 6 (250 mg, 1.09 mmol) in vinyl acetate
(5 mL) was added Pseudomonas sp. lipase (90 mg). The mixture
was stirred at room temperature for 26 h. The enzyme was
removed by filtration, and the filtrate was evaporated to dryness.
The crude product was purified by flash chromatography
(hexane–EtOAc, 1:1) to provide (+)-(2R,5S)-1329 (251 mg, 85%) as
3.7. (3aR,4R,6S,6aS)-tert-Butyl 4,6-bis(hydroxymethyl)-2,2-
dimethyldihydro-3aH-[1,3]dioxolo[4,5-c]pyrrole-5(4H)-
carboxylate meso-10
a colorless oil. ½a D20
ꢂ
¼ þ79:6 (c 2.00, CHCl3); ee = 96%, chiral HPLC;
To a solution of 9 (310 mg, 0.80 mmol) in MeOH–H2O (20 mL,
1:1) was added K2CO3 (553 mg, 4.00 mmol). The solution was stir-
red vigourously at room temperature overnight, then extracted
with CH2Cl2 (5 ꢁ 20 mL), dried over MgSO4, and evaporated to dry-
ness. Flash chromatography (CH2Cl2–acetone, 1:1) gave 10
(231 mg, 95%) as a colorless oil. IR (NaCl) 3583, 2934, 2120,
IR (NaCl) 3442, 2922, 1745, 1395, 1039 cmꢀ1 1H NMR (400 MHz,
;
CDCl3) d 1.49 (s, 9H), 2.04 (s, 3H), 3.54–4.79 (m, 7H), 5.72 (s,
2H); 13C NMR (100 MHz, CDCl3) d 21.1, 28.6, 64.5, 64.6, 66.9,
68.4, 81.6, 128.1, 128.3, 155.3, 171.1.
3.10.2. Enzymatic hydrolysis of diacetate 7
1704, 1479 cmꢀ1
;
1H NMR (400 MHz, CDCl3) d 1.31 (s, 3H), 1.44
Diester 7 (120 mg, 0.38 mmol) was emulsified in a phosphate
buffer (10 mL, 0.25 M, pH 7.6) and C. rugosa lipase was added
(45 mg). After 3.5 h, the aqueous mixture was filtered and
extracted with CH2Cl2 (3 ꢁ 25 mL). The organic layer was dried
over MgSO4 and concentrated. Flash chromatography (hexanes–
EtOAc, 1:1) gave (ꢀ)-(2S,5R)-13 (67 mg, 65%) as a colorless oil.
(s, 9H), 3.67–4.12 (m, 9H), 4.69 (m, 2H); 13C NMR (100 MHz, CDCl3)
d 25.5, 27.6, 28.5, 63.1, 63.3, 66.5, 66.8, 81.0, 81.6, 82.2, 111.8,
155.2;HRMS (CI, NH3) calcd for C14H26NO6 (MH)+: 304.1760.
Found: 304.1755.
3.8. (2S,5R)-1-(tert-Butoxycarbonyl)-5-
(methoxycarbonyl)pyrrolidine-2-carboxylic acid 11
½
a 2D0
as above.
ꢂ
¼ ꢀ79:6 (c 1.30, CHCl3); ee = 96%, chiral HPLC. Spectral data
Diester 2 (292 mg, 1.30 mmol) was emulsified in a phosphate
buffer (15 mL, 0.5 M, pH 7.5). Porcine liver esterase was added
(50 mg). After 3 d, the aqueous mixture was filtered on a 0.22 lm
3.11. tert-Butyl 4-(acetoxymethyl)-6-(hydroxymethyl)-2,2-
dimethyldihydro-3aH-[1,3]dioxolo[4,5-c]pyrrole-5-(4H)-
carboxylate 14
nylon membrane filter. The aqueous solution was acidified to pH
4–5 with 1 M HCl and extracted with CH2Cl2 (5 ꢁ 50 mL). The
organic layer was dried over MgSO4 and concentrated. Flash
chromatography (EtOAc–MeOH, 9:1) gave (+)-11 (270 mg, 76%)
3.11.1. Enzymatic acylation of diol 10
To a solution of diol 10 (50 mg, 0.16 mmol) in vinyl acetate
(5 mL) was added C. antarctica lipase B (50 mg). The mixture was
stirred at room temperature for 96 h. The enzyme was removed
by filtration, and the filtrate was evaporated to dryness. The crude
product was purified by flash chromatography (CH2Cl2–acetone,
as a colorless oil. ½a D20
ꢂ
¼ þ12:9 (c 1.40, MeOH); ee P 99% chiral
HPLC; IR (NaCl) 3408, 2983, 1705, 1389, 1140 cmꢀ1
.
1H NMR
(400 MHz, CDCl3) d 1.40 (s, 9H), 1.98–2.30 (m, 4H), 3.72 (s, 3H),
4.23–4.37 (m, 2H), 4.87 (s, 1H); 13C NMR (100 MHz, CDCl3) d
28.3, 29.3, 30.0, 54.1, 60.0, 61.6, 82.9, 153.2, 172.9, 177.9; HRMS
(CI, NH3) calcd for C12H20NO6 (MH)+: 274.1291. Found: 274.1289.
3:1) to provide (ꢀ)-14 (51 mg, 90%) as
¼ ꢀ38:2 (c 1.20, CHCl3); ee = 97%, chiral HPLC. IR (NaCl)
3391, 2108, 1673, 1398, 869 cmꢀ1 1H NMR (400 MHz, CDCl3) d
a colorless oil.
½ ꢂ
a 2D0
;
0.86 (s, 1H), 1.31 (m, 6H), 1.45 (s, 9H), 2.07 (s, 3H), 3.65–4.53 (m,
8H); 13C NMR (100 MHz, CDCl3) d 21.2, 25.5, 27.5, 28.5, 63.6,
64.3, 64.5, 67.0, 81.6, 81.8, 82.1, 112.3, 153.9, 164.2; HRMS (CI,
NH3) calcd for C16H28NO7 (MH)+: 346.1866. Found: 346.1871.
3.9. 1-tert-Butoxycarbonyl-2-(acetoxymethyl)-5-(hydroxy-
methyl)pyrrolidine 12
3.9.1. Enzymatic acylation of diol 3
To a solution of diol 3 (1.50 g, 6.49 mmol) in vinyl acetate
(15 mL) was added C. antarctica lipase B (240 mg). The mixture
was stirred at room temperature for 36 h. The enzyme was re-
moved by filtration, and the filtrate was evaporated to dryness.
3.11.2. Enzymatic hydrolysis of diacetate 9
Diester 9 (120 mg, 0.31 mmol) was emulsified in a phos-
phate buffer (10 mL, 0.5 M, pH 7.6) and Pseudomonas sp. lipase
was added (50 mg). After 72 h, the aqueous mixture was

1338
R. Chênevert et al. / Tetrahedron: Asymmetry 19 (2008) 1333–1338
filtered and extracted with CH2Cl2 (3 ꢁ 25 mL). The organic layer
was dried over MgSO4 and concentrated. Flash chromatography
(CH2Cl2–acetone, 3:1) provided (ꢀ)-14 (64 mg, 60%) as a colorless
(c 1.20, MeOH); from PLE hydrolysis of 1 to (+)-16: ee = 82%, chiral
HPLC. Spectroscopic data as above.
oil. ½a 2D0
ꢂ
¼ ꢀ35:4 (c 0.80, CHCl3); ee = 90%, chiral HPLC. Spectral
Acknowledgment
data as above.
We acknowledge the financial support of this work by the Nat-
ural Sciences and Engineering Research Council of Canada (NSERC).
3.12. Determination of absolute configuration by chemical
correlation
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Dihydroxylation of (ꢀ)-(2S,5R)-13 following the procedure used
for the transformation of 7 into 8 provided (ꢀ)-(2S,5R)-15 as a col-
orless oil.
½
a 2D0
ꢂ
¼ ꢀ8:1 (c 1.50, CHCl3); from PSL hydrolysis
(ee = 94%); IR (NaCl) 3500, 2934, 2101, 1668, 1407, 1254, 1146,
1031 cmꢀ1 1H NMR (400 MHz, CDCl3) d 1.46 (s, 9H), 2.06 (s, 3H),
;
3.70–4.19 (m, 11H); 13C NMR (100 MHz, CDCl3) d 21.2, 28.5, 63.1,
63.2, 64.4, 65.1, 72.4, 72.5, 81.9, 156.7, 171.0; HRMS (CI, NH3) calcd
for C13H24NO7 (MH)+: 306.1553. Found: 306.1548.
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Acetalization of (ꢀ)-(2R,3R,4S,5S)-15 with acetone following the
procedure used for the transformation of 8 into 9 provided (ꢀ)-
(3aR,4R,6S,6aS)-14. Spectroscopic data as above.
3.12.4. Preparation of 11 from 16
In a hydrogenation cell, monoester (+)-(2S,5R)-16 (130 mg,
0.49 mmol) and tert-butoxycarbonyl anhydride (118 mg,
0.54 mmol) were dissolved in MeOH (20 mL). Under a nitrogen
atmosphere, Pd(OH)2/C (20% wt., 20 mg) was added. The cell was
purged twice (H2, then vacuum) and agitated mechanically
while hydrogen pressure was adjusted to 54 psi. After 24 h, the
mixture was diluted with EtOAc (50 mL), washed with 1 M HCl
(3 ꢁ 50 mL), NaHCO3 (3 ꢁ 50 mL), and brine (1 ꢁ 50 mL). The
organic phase was dried over MgSO4, concentrated, and the residue
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vide (+)-(2S,5R)-11 (134 mg, 100%) as a colorless oil. ½a D22
¼ þ10:7
ꢂ