Chemoenzymatic synthesis of (3R,4S)- and (3S,4R)-3-methoxy-4-methylaminopyrrolidine

Article abstract of DOI:10.1016/j.tetasy.2006.10.031

An efficient and a convenient enantioselective synthesis of (3R,4S)-3-methoxy-4-methylaminopyrrolidine has been carried out by a lipase-mediated resolution protocol. This method describes the preparation of (±)-1-Cbz-cis-3-azido-4-hydroxypyrrolidine start

Full text of DOI:10.1016/j.tetasy.2006.10.031

Tetrahedron: Asymmetry 17 (2006) 2876–2883
Chemoenzymatic synthesis of (3R,4S)- and
(3S,4R)-3-methoxy-4-methylaminopyrrolidine
Ahmed Kamal,* Ahmad Ali Shaik, Mahendra Sandbhor, M. Shaheer Malik and Shaik Azeeza
Biotransformation Laboratory, Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 24 August 2006; accepted 23 October 2006
Abstract—An efficient and a convenient enantioselective synthesis of (3R,4S)-3-methoxy-4-methylaminopyrrolidine has been carried out
by a lipase-mediated resolution protocol. This method describes the preparation of ( )-1-Cbz-cis-3-azido-4-hydroxypyrrolidine starting
from commercially available diallylamine followed by ring-closing metathesis (RCM) via SN2 displacement reactions. Pseudomonas cepa-
cia lipase immobilized on diatomaceous earth (Amano PS-D) provides (3R,4S)-11 and (3S,4R)-12 in an excellent enantiomeric excess.
ꢀ 2006 Elsevier Ltd. All rights reserved.
1. Introduction
3a subunit at the C-7 position of naphthyridine ring 2 to
obtain a high cytotoxicity against murine P388 leukaemia
cells. Furthermore, detailed structure–activity relationship
(SAR) studies in this area reveal that a cis-configuration
and stereochemistry at the C-3,4 positions of the C-7
substituted pyrrolidine ring is responsible for the antibacte-
rial activities in vitro as well as in vivo of these compounds.
Literature studies indicate that (3R,4S)-3-methoxy-4-methyl-
aminopyrrolidine 3a linked to a quinolone carboxylic acid
exhibits a superior in vitro antibacterial activity than did
the corresponding racemate. Thus, it is important to devel-
op a practical synthetic strategy for the preparation of this
pyrrolidine intermediate in an enantiomerically pure form.
There are very few methods that have been reported for the
stereospecific synthesis of this biologically active pyrrol-
idine intermediate. These methods involve either a chiron
approach6,7 or a resolution of the racemates.3
Many synthetic and naturally occurring biologically active
compounds contain chiral pyrrolidines as subunits in their
structures.1 It was first demonstrated by Okada et al.2,3
that (3R,4S)-3-methoxy-4-methylaminopyrrolidine 3a is
an important subunit linked at the C-7 position of quino-
lone carboxylic acid derivative 1 and has shown a higher
in vitro and in vivo antibacterial activity for both Gram
positive and Gram negative bacteria.
O
NH2
O
CO2H
F
CO2H
·HCl
N
N
N
N
N
H3CHN
H3CO
H3CHN
H3CO
F
N
S
2
1
H3CO
NCH3R
Recently, we have reported an highly efficient lipase-medi-
ated resolution protocol for the synthesis of (3S,4S)- and
(3R,4R)-3-methoxy-4-methylaminopyrrolidines.8
These
N
results encouraged us to develop a convenient chemoenzy-
matic method for the synthesis of (3R,4S)- and (3S,4R)-3-
methoxy-4-methylaminopyrrolidines involving ring-closing
metathesis and lipase-mediated kinetic resolution strategies.
R'
(R,S)
3a: R = R' = H
3b: R = R' = Boc
Recently, Tomita et al.4 as well as Tsuzuki et al.5 have
reported a (3R,4S)-3-methoxy-4-methylaminopyrrolidine
2. Results and discussion
*
In the present synthetic strategy, inexpensive commercially
Corresponding author. Tel.: +91 40 27193157; fax: +91 40 27193189;
e-mail: ahmedkamal@iict.res.in
available diallylamine 4 has been utilized as the starting
0957-4166/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.10.031

A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
2877
HO
N3
material. The Cbz protection of diallylamine 4 in the pres-
ence of a base gives Cbz protected diallylamine 5 followed
by ring-closing metathesis employing Grubbs’ catalyst to
yield 1-Cbz-3-pyrroline 6. The epoxidation of 6 with m-
chloroperbenzoic acid gives lower yields9 (maximum
46%). In order to improve the yields of the epoxidation
procedure, a two-step procedure has been employed. In
the first step, compound 6 has been converted into its
trans-bromohydrin 7 employing N-bromosuccinimide in
DMSO–H2O. In the second step, alkaline treatment of
compound 7 with 1 M NaOH gave pyrroline epoxide 8 in
high yields. Epoxide 8, upon treatment with sodium azide,
yielded 1-Cbz-trans-3-azido-4-hydroxypyrrolidine 9. The
required cis configuration of the azido alcohol has been
attained by mesylation of compound 9 to give mesylated
trans-azido alcohol 10. The SN2 displacement reaction of
mesylated product 10 with potassium acetate in DMF
under reflux conditions gave cis-azido acetate 11 with a
complete inversion of configuration. The deacetylation
was carried out under mild conditions with anhydrous
K2CO3 to give ( )-1-Cbz-cis-3-azido-4-hydroxypyrrolidine
12, which is required for the lipase-mediated kinetic resolu-
tion (Scheme 1).
HO
Br
N3
HO
i
+
N
N
N
Cbz
12
Cbz
Cbz
( )-9
( )-
cis, 20%
( )-7
trans, 80%
Scheme 2. Reagents and conditions: (i) NaN3, DMF, reflux, 10 h.
Br
Br
N3
HO
AcO
AcO
i
ii
N
N
N
Cbz
Cbz
13
Cbz
( )-11
7
Scheme 3. Reagents and conditions: (i) CH3COCl, Et3N, 0 ꢁC, 3 h; (ii)
NaN3, DMSO, 100 ꢁC, 6 h.
of compound 7 is protected as its acyl derivative 13 under
mild conditions. The next step is the SN2 displacement
reaction with sodium azide of compound 13 in dry DMSO
at 100 ꢁC, which gives racemic cis-azido acetate 11 as
shown in Scheme 3. The resulting cis-azido acetate, which
upon treatment with anhydrous K2CO3 gave racemic cis-
azido alcohol 12. In view of our interest in enzyme-medi-
ated kinetic resolution of chiral building blocks,10 we
performed the resolution of ( )-11 and ( )-12 by employ-
ing different lipases.
Attempts have been made to reduce the number of
synthetic steps. With this in mind, an SN2 displacement
reaction of the bromo functionality of compound 7 was
carried out by an azide nucleophile and it was observed
that a mixtures of cis- and trans-azido alcohols (20:80%)
was obtained during this reaction. A plausible mechanism
for the formation of trans-azido alcohol ( )-9 from com-
pound 7 may be by the formation of an epoxide under
refluxing conditions, followed by the ring opening by azide
nucleophile. The products are confirmed by 1H NMR spec-
troscopy and their percentage ratio is calculated by HPLC
analysis (Scheme 2).
2.1. Lipase screening and enzyme concentration
The selection of suitable lipases is an important aspect for
developing an efficient resolution protocol.11 Herein we
carried out the transesterification as well as alcoholysis.
Ten lipases from different sources were screened for this
lipase-mediated transesterification of racemic vicinal azido
alcohol ( )-12 with isopropenyl acetate in diisopropyl
ether. The results obtained are summarized in Table 1. It
was observed that lipases from Pseudomonas cepacia
To eliminate the formation of trans-azido alcohol and to
obtain cis-azido alcohol exclusively, a two-step procedure
was employed. In the first step, the hydroxyl functionality
Br
HO
i
iii
ii
N
N
N
H
Cbz
N
Cbz
6
Cbz
7
5
4
N3
O
N
N3
HO
N3
AcO
MsO
vi
vii
iv
v
N
N
N
Cbz
Cbz
Cbz
Cbz
( )-11
9
10
8
N3
HO
viii
N
Cbz
-12
( )
Scheme 1. Reagents and conditions: (i) CbzCl, Et3N, CH2Cl2, rt, 10 h; (ii) (Pcy2)2Cl2Ru@CHPh, CH2Cl2, rt, 6 h; (iii) NBS, DMSO–H2O, rt, 2 h; (iv) 1 M
NaOH, rt, 1 h; (v) NaN3, 1,4-dioxane–H2O, reflux, 12 h; (vi) MsCl, Et3N, CH2Cl2, rt, 4 h; (vii) AcOK, DMF, reflux, 3 h; (viii) anhydrous K2CO3, MeOH,
rt, 1 h.

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A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
Table 1. Transesterification of ( )-1-Cbz-cis-3-azido-4-hydroxypyrrolidine 12a
Lipase
Equivalents (w/w)
Timeb (h)
eec (%)
Conversion (%)
Alcohol (3S,4R)-12
Acetate (3R,4S)-11
PS-C
PS-D
PS-C
PS-D
PS
CAL
CRL
CCL
Lipozyme
PFL
1
1
0.5
0.5
1
1
1
1
1
8
9
12
14
68
97
94
94
92
64
47
52
38
14
—
—
—
59
86
72
97
88
65
89
74
86
—
—
—
62.1
52.2
56.6
48.6
42.1
41.9
36.8
33.9
14
72
60
120
120
168
168
168
1
1
1
—
—
—
AK-20
PPL
a Conditions: 12 (1 mmol), diisopropyl ether (10 mL), lipase, isopropenyl acetate (6 mmol).
b Time taken for transesterification.
c Determined by chiral HPLC analysis of the reaction mixture employing a Daicel Chiralcel OD column (0.46 · 25 cm); eluent: hexane/isopropa-
nol = 80:20; flow rate: 0.5 mL/min; detector: 254 nm.
(PS), Candida antarctica (CAL), Candida rugosa (CRL),
Candida cylindracea (CCL) and Mucor miehei (Lipozyme)
showed comparatively low enantioselectivities, whereas
Pseudomonas fluorescens (PFL), AK-20 and porcine pancre-
atic lipase (PPL) did not show any conversions, even after
prolonged reactions. The lipases from P. cepacia immobi-
lized on ceramic particles (PS-C) and lipase P. cepacia
immobilized on diatomaceous earth (PS-D) provided
encouraging results with respect to conversion and enantio-
selectivity. Immobilized P. cepacia on ceramic particles
(PS-C) gives a lower enantioselectivity of the azido acetate
(3R,4S)-11 59% ee, when 1 equiv (w/w) of lipase is em-
ployed and gives a 72% ee, with 0.5 equiv (w/w) of lipase.
However, the corresponding azido alcohol (3S,4R)-12
was obtained in a 97% ee and a 94% ee. Immobilized P.
cepacia on diatomaceous earth (PS-D) has also been exam-
ined under similar reaction conditions. When 1 equiv (w/w)
of lipase was used, azido alcohol (3S,4R)-12 was obtained
in a 94% ee, whereas azido acetate (3R,4S)-11 in a 86% ee.
However, when 0.5 equiv (w/w) lipase was used, azido
alcohol (3S,4R)-12 was obtained in a 92% ee and the corre-
sponding azido acetate (3R,4S)-11 in a 97% ee. Therefore,
based on these results, lipase PS-D was chosen for further
studies. However, in earlier studies the transesterification
of trans-azido alcohol ( )-9 using lipase PS-C gave better
results compared to lipase PS-D.8
2.2. Effect of solvent
Solvent variation in lipase-catalyzed kinetic resolution is
known to influence the enantiomeric and enantiotopic
selectivity as well as the reaction rate.12 In the present
study, five different solvents were examined using PS-D
lipase and isopropenyl acetate. It has been observed that
THF, tert-butyl methyl ether (TBME) and hexane are
not suitable solvents for obtaining good enantioselectivities
and conversions. Toluene takes a longer reaction time to
reach 50% conversion and shows comparatively lower
enantiomeric purities as shown in Table 2. Diisopropyl
ether gives good results with respect to conversions and
enantioselectivities. Moreover, it takes a shorter reaction
time. Therefore, diisopropyl ether has been selected as a
suitable solvent for these resolution processes.
2.3. Effect of acyl donor
Acyl donors also play a crucial role in achieving better
enantioselectivities.13 In the present protocol two different
acyl donors, that is, isopropenyl acetate and vinyl acetate,
have been employed using lipase PS-D in diisopropyl ether
at 40 ꢁC. It was observed that vinyl acetate takes 9 h to
reach about 50% conversion and the enantiomeric excess
of azido alcohol (3S,4R)-12 and azido acetate (3R,4S)-11
Table 2. Effect of different solvents on transesterification of ( )-1-Cbz-cis-3-azido-4-hydroxypyrrolidine 12a
Solvent
Lipase equivalents (w/w)
Timeb (h)
eec (%)
Conversion (%)
Alcohol (3S,4R)-12
Acetate (3R,4S)-11
DIPE
0.5
0.5
0.5
0.5
0.5
14
66
47
60
48
92
81
61
38
41
97
67
88
72
89
48.6
54.7
40.9
34.5
31.5
Toluene
TBME
Hexane
THF
a Conditions: 12 (1 mmol), solvent (10 mL), lipase PS-D, isopropenyl acetate (6 mmol).
b Time taken for transesterification.
c Determined by chiral HPLC analysis of the reaction mixture employing Daicel Chiralcel OD column (0.46 · 25 cm); eluent: hexane/isopropanol = 80:20;
flow rate: 0.5 mL/min; detector: 254 nm.

A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
2879
N3
are 82% and 68% ee, respectively. However, isopropenyl
AcO
HO
N3
N3
HO
acetate takes 14 h to reach about 50% conversion and azido
alcohol (3S,4R)-12 is obtained in a 92% ee and azido ace-
tate (3R,4S)-11 in a 97% ee.
i
+
N
N
N
Cbz
Cbz
Cbz
(3S,4R)-12
(3R,4S)-12
(3R,4S)-11
ii
( )-12
2.4. Enzymatic alcoholysis
Scheme 5. Reagents and conditions: (i) lipase PS-D, IPA, DIPE, 40 ꢁC,
14 h; (ii) anhydrous K2CO3, CH3OH, rt, 1 h.
Lipase-mediated alcoholysis of ( )-1-Cbz-cis-3-acetyloxy-
4-azidopyrrolidine 11 has been attempted by employing
two different alcohols, namely, n-butanol and 2-propanol.
The reaction rates of lipase-mediated alcoholysis are quite
slow when 1 equiv of lipase (w/w) was used, as shown in
Scheme 4. The lipase PS-D in n-butanol gives moderately
better results. This reaction required nearly 54 h to reach
about 50% conversion and azido alcohol (3R,4S)-12 was
obtained in a 76% ee and azido acetate (3S,4R)-11 in a
84% ee. The enantiopurities of the corresponding products
are lower in comparison to the transesterification process.
Therefore, lipase-mediated alcoholysis was not considered
practical for achieving good enantiopurities although some
good results obtained in this investigation have been
summarized in Table 3.
kinetic resolution of ( )-12 was converted to its corre-
sponding methyl ether (3R,4S)-14 using CH3I with NaH
as a base. The azide functionality of (3R,4S)-14 was selec-
tively reduced under mild reaction conditions using TPP in
THF and H2O at room temperature to give the amine,
which has been protected in the same pot with (Boc)2O
to give Boc protected amine (3R,4S)-15. N-Methylation
of compound 15 was performed with CH3I and NaH in
dry DMF under an N2 atmosphere to give (3R,4S)-16.
Finally, Cbz deprotection of (3R,4S)-16 was carried out
under extremely mild conditions employing poly(methyl-
hydrosiloxane) (PMHS) over 10% Pd–C in ethanol fol-
lowed by treatment with (Boc)2O to give the desired
product (3R,4S)-3b with a good enantiomeric excess
(97%). The overall yields of these reactions are also good.
The other enantiomer (3S,4R)-12 was obtained by employ-
ing the same reaction sequence and was converted to
(3S,4R)-3b in a 92% ee (Scheme 6). The absolute configura-
tion of these enantiomers (3R,4S)- and (3S,4R)-3b were
Thus, the lipase-catalyzed transesterification of ( )-1-Cbz-
cis-3-azido-4-hydroxypyrrolidine 12 has been carried out
by employing 0.5 equiv of PS-D lipase (w/w) and isoprop-
enyl acetate in diisopropyl ether at 40 ꢁC. The reaction
progress was monitored by chiral HPLC employing a ‘Chi-
ralcel OD’ column. In order to obtain good enantioselectiv-
ities and yields, the reaction was stopped when it reached
about 50% conversion. The pure products were isolated
by silica gel column chromatography to obtain azido alco-
hol (3S,4R)-12 in a 92% ee and a 48% yield, while the cor-
responding azido acetate (3R,4S)-11 in a 97% ee and a
52% yield. The deacetylation of (3R,4S)-11 using anhydrous
K2CO3 in methanol gave (3R,4S)-12 in a 97% ee (Scheme 5).
N3
NHBoc
iv
HO
N3
MeO
MeO
NMeBoc
MeO
i
ii
N
N
N
iii
N
Cbz
Cbz
(3R,4S)-14
Cbz
(3R,4S)-15
Cbz
(3R,4S)-16
(3R,4S)-12
The enantiomerically enriched (3R,4S)-1-Cbz-3-azido-4-
hydroxypyrrolidine 12 obtained by the lipase-mediated
v
NMeBoc
NMeBoc
MeO
HO
N3
MeO
i, ii, iii, iv, v
N
N3
N
N3
HO
N3
N
AcO
AcO
Boc
Boc
(3S,4R)-3b
Cbz
i
+
(3R,4S)-3b
(3S,4R)-12
N
N
N
Cbz
Cbz
Cbz
Scheme 6. Reagents and conditions: (i) NaH, MeI, dry THF, 14 h; (ii)
PPh3, THF:H2O, rt, 2 h; (iii) (Boc)2O, Et3N, rt, 6 h; (iv) NaH, MeI, DMF,
rt, 6 h; (v) 10% Pd–C, PMHS, (Boc)2O, Et3N, EtOH, rt, 6 h.
( )-11
(3S,4R)-11
(3R,4S)-12
Scheme 4. Reagents and conditions: (i) lipase PS-D, n-butanol, DIPE.
Table 3. Enzymatic alcoholysis of ( )-1-Cbz-cis-3-acetyloxy-4-azidopyrrolidine 11a
Lipase
Timeb (h)
Alcohol
Solvent
eec (%)
Conversion (%)
Alcohol (3R,4S)-12
Acetate (3S,4R)-11
PS-D
PS-D
PS-D
PS-C
54
68
84
72
n-Butanol
2-Propanol
n-Butanol
n-Butanol
DIPE
DIPE
Toluene
DIPE
76
72
63
72
84
86
80
75
52.5
54.4
55.9
51.0
a Conditions: 11 (0.5 mmol), diisopropyl ether (5 mL), lipase (1 equiv w/w), alcohol (3 mmol).
b Time taken for alcoholysis.
c Determined by chiral HPLC analysis of the reaction mixture employing a Daicel Chiralcel OD column (0.46 · 25 cm); eluent: hexane/isopropa-
nol = 80:20; flow rate: 0.5 mL/min; detector: 254 nm.

2880
A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
confirmed by comparison of their specific rotations with
those previously reported in the literature.3
4.2.2. 1-Cbz-3-Pyrroline 6. The 1-Cbz-N,N-diallylamine 5
(6.5 g, 28.1 mmol) was dissolved in dry CH2Cl2 (950 mL)
under nitrogen. Grubbs’ catalyst (0.464 g, 0.588 mmol,
2 mol %) was added and the solution stirred at room
temperature under nitrogen. After 6 h, the solvent was
evaporated and the residue purified on silica gel column
chromatography to afford 5.14 g pure 1-Cbz-3-pyrroline
3. Conclusion
In conclusion we have developed a simple and highly
efficient method for the preparation of optically active
(3R,4S)-3b and (3S,4R)-3b. The lipase-mediated kinetic
resolution has been carried out by using P. cepacia lipase
immobilized on diatomaceous earth PS-D to give excellent
results for the cis-azido alcohol. We have demonstrated an
alternative chemoenzymatic approach for the synthesis of
(3R,4S)- and (3S,4R)-3-methoxy-4-methylaminopyrrol-
idine. Moreover, this protocol provides good conversions
with a high enantiomeric excess.
6. Yield: 90%; IR (neat): 3061, 1705, 1624 cmꢀ1 1H
;
NMR (200 MHz, CDCl3): d 4.19 (4H, s), 5.12 (2H, s),
5.67–5.85 (2H, q, J = 13.91, 6.59 Hz), 7.32 (5H, m); EIMS
(m/z): 203 (M+). Anal. Calcd for C12H13NO2: C, 70.92; H,
6.45; N, 6.89. Found: C, 70.86; H, 6.35; N, 6.82.
4.2.3. 1-Cbz-trans-3-Bromo-4-hydroxypyrrolidine 7. To a
stirred mixture of 6 (5.1 g, 24.8 mmol), DMSO (70.7 mL)
and H2O (3.4 mL), NBS (5.36 g, 29.8 mmol) was gradually
added over 15 min at 0 ꢁC. After stirring at room temper-
ature for 2 h, water was added and the reaction mixture
extracted with ethyl acetate. The organic layer was washed
with a saturated solution of NaCl, dried over anhydrous
Na2SO4 and then concentrated. The residue obtained was
chromatographed on silica gel to give 6.48 g of pure
bromohydrin 7. Yield: 86%; IR (neat): 3380, 3010,
4. Experimental
4.1. Material and methods
Enzymatic reactions were carried out on a ‘Lab-line envi-
ron-shaker’ at 150 rpm. Infrared spectra of neat samples
are reported in wave numbers (cmꢀ1). 1H NMR was
recorded as solutions in CDCl3 and chemical shifts are
reported in parts per million (ppm, d) on a 200 MHz instru-
ment. Coupling constants are reported in hertz (Hz).
LSIMS mass spectra were recorded on Autospec M. with
a 7 kV acceleration voltage and 25 kV gun voltage. HPLC
analysis was performed on an instrument that consisted of
a Shimadzu LC-10AT system controller, SPD-10A fixed
wavelength UV monitor as the detector. Specific rotations
were recorded on SEPA-300 Horiba high sensitive polari-
meter, fixed with a sodium lamp of wavelength 589 nm.
1
1710 cmꢀ1; H NMR (200 MHz, CDCl3): d 3.44 (1H, d,
J = 12.02 Hz), 3.68–4.16 (5H, m), 4.36 (1H, s), 5.09
(2H, s), 7.30 (5H, m); FABMS (m/z): 302 (M++2). Anal.
Calcd for C12H14BrNO3: C, 48.02; H, 4.70; N, 4.67. Found:
C, 47.96; H, 4.64; N, 4.62.
4.2.4. 1-Cbz-trans-3-Acetyloxy-4-bromopyrrolidine 13. To
a stirred solution of 7 (0.3 g, 1 mmol) in dry CH2Cl2
(10 mL) was added Et3N (0.16 mL, 1.5 mmol) and then
acetyl chloride (0.078 mL, 1.1 mmol) gradually at 0 ꢁC.
After stirring at room temperature for 3 h, water was
added and the reaction mixture extracted with CH2Cl2.
The organic layer was washed with the saturated solution
of NaCl, dried over anhydrous Na2SO4 and then concen-
trated. The residue obtained was chromatographed on
silica gel to give 0.29 g of pure 13. Yield: 86%; IR (neat):
4.2. Chemicals and enzymes
3021, 1705, 1739 cmꢀ1 1H NMR (200 MHz, CDCl3): d
;
Diallylamine, sodium azide, NBS, Grubb’s catalyst, meth-
ane sulfonylchloride, PMHS, sodium hydroxide and
solvents were obtained commercially and used without
purification. P. cepacia lipase immobilized on ceramic
particles (PS-C) and P. cepacia lipase immobilized on
diatomaceous earth (PS-D) were purchased from Amano
(Nagoya, Japan).
2.07 (3H, s), 3.82–4.07 (4H, m), 4.23 (1H, d, J = 3.3 Hz),
5.13 (2H, s), 5.27 (1H, d, J = 4.2 Hz), 7.34 (5H, m);
FABMS (m/z): 344 (M++2). Anal. Calcd for
C14H16NBrO4: C, 49.14; H, 4.71; N, 4.09. Found: C,
49.08; H, 4.64; N, 4.01.
4.2.5. 1-Cbz-3,4-Epoxypyrrolidine 8. Bromohydrin
7
4.2.1. 1-Cbz-N,N-Diallylamine 5. To a solution of diallyl-
amine 4 (4.85 g, 50 mmol) and Et3N (9 mL, 65 mmol) in
dry CH2Cl2 (75 mL) was added benzyl chloroformate
(18.4 mL, 55 mmol of 50% solution in toluene) dropwise
over a period of 30 min at 0 ꢁC. The reaction mixture
was stirred at room temperature for 10 h. The reaction
mixture was diluted with water and extracted with CH2Cl2.
The combined organic layers were washed with brine and
dried over anhydrous Na2SO4. The solvent was evaporated
and the residue was purified by silica gel column chroma-
tography to give pure 10.62 g of 5. Yield: 92%; IR (neat):
(6.2 g, 20.6 mmol) was dissolved in MeOH (72.9 mL) and
a solution of aqueous NaOH (30.9 mL solution of 1 M,
30.9 mmol) was added at 0 ꢁC. The reaction mixture was
stirred at room temperature for 1 h. Methanol was evapo-
rated under reduced pressure, and water added and
extracted with ethyl acetate. The organic layer was washed
with a saturated solution of NaCl, dried over anhydrous
Na2SO4 and then concentrated, the residue obtained was
chromatographed on silica gel to give of 3.68 g pure
epoxide 8. Yield: 82%; IR (neat): 3023, 1698 cmꢀ1 1H
;
NMR (300 MHz, CDCl3): d 3.35 (2H, d, J = 12.08 Hz),
3.63 (2H, d, J = 3.3 Hz), 3.78–3.90 (2H, t, J = 12.84 Hz),
5.08 (2H, m), 7.30 (5H, m); EIMS (m/z): 219 (M+). Anal.
Calcd for C12H13NO3: C, 65.74; H, 5.98; N, 6.39. Found:
C, 65.69; H, 5.91; N, 6.35.
3072, 1703, 1629 cmꢀ1 1H NMR (300 MHz, CDCl3): d
;
3.86 (4H, s), 5.11 (6H, m), 5.74 (2H, s), 7.30 (5H, m); EIMS
(m/z): 231 (M+). Anal. Calcd for C14H17NO2: C, 72.70; H,
7.41; N, 6.06. Found: C, 72.61; H, 7.32; N, 5.99.

A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
2881
4.2.6. 1-Cbz-trans-3-Azido-4-hydroxypyrrolidine 9. Epox-
ide 8 (3.65 g, 16.5 mmol) was dissolved in 1,4-dioxane
(50 mL) and water (10 mL), after which sodium azide
(1.40 g, 20 mmol) was added at room temperature. The
reaction mixture was stirred at 100 ꢁC for 12 h. After com-
pletion of the reaction, the reaction mixture was brought to
room temperature and water was added and extracted with
ethyl acetate. The organic layer was washed with brine,
dried over anhydrous Na2SO4. The solvent was evaporated
and the residue subjected for silica gel column chromato-
graphy to give 3.78 g of pure racemic trans-azido alcohol
4.2.9. 1-Cbz-cis-3-Azido-4-hydroxypyrrolidine 12. cis-Azi-
do acetate 11 (3 g, 10.1 mmol) was dissolved in methanol
(20 mL) and then anhydrous K2CO3 (3.49 g, 25.2 mmol)
was added and the reaction stirred at room temperature
for 1 h. The reaction mass was filtered on a Celite pad,
the methanol evaporated and subjected to silica gel column
chromatography to give 2.53 g pure cis-azido alcohol 12.
1
Yield: 98%; IR (neat): 3384, 3030, 2097, 1665 cmꢀ1; H
NMR (300 MHz, CDCl3): d 3.31–3.78 (4H, m), 3.90–4.05
(1H, m), 4.32 (1H, br s), 5.10 (2H, m), 7.32 (5H, m);
FABMS (m/z): 263 (M++1). Anal. Calcd for
C12H14N4O3: C, 54.96; H, 5.38; N, 21.36. Found: C,
54.88; H, 5.34; N, 21.28.
1
9. Yield: 86%; IR (neat): 3401, 3033, 2107, 1689 cmꢀ1; H
NMR (200 MHz, CDCl3): d 3.31–3.73 (5H, m), 3.89 (1H,
s), 4.13 (1H, s), 5.07 (2H, s), 7.31 (5H, m); FABMS
(m/z): 263 (M++1). Anal. Calcd for C12H14N4O3: C,
54.96; H, 5.38; N, 21.36. Found: C, 54.91; H, 5.31; N,
21.29.
4.2.10. General procedure for lipase-mediated alcoholy-
sis. To a solution of racemic cis-azido acetate 11
(100 mg, 0.32 mmol) in diisopropyl ether (5 mL) lipase
(100 mg, 1 equiv w/w) and alcohol (1.97 mmol) were
added. The suspension was shaken at 220 rpm at 40 ꢁC.
The reaction was monitored by chiral HPLC and at 50%
conversion the reaction was stopped, filtered and solvent
was evaporated. The residue was chromatographed on sil-
ica gel to give enantiopure azido alcohol (3R,4S)-12 and
the corresponding azido acetate (3S,4R)-11. The enantio-
purity of these products was analyzed by chiral HPLC
and compared with the corresponding racemic products.
4.2.7. 1-Cbz-trans-3-Azido-4-[(methylsulfonyl)oxy]pyrrol-
idine 10.
A
solution of methanesulfonyl chloride
(1.2 mL, 15 mmol) in dry CH2Cl2 (10 mL) was added to
a mixture of 9 (3.75 g, 14 mmol) and Et3N (2.49 mL,
17 mmol) in dry CH2Cl2 over a period of 30 min at 0 ꢁC.
The reaction mixture was stirred at room temperature for
4 h. The reaction mixture was diluted with water and
extracted with CH2Cl2. The organic layer was washed with
brine, dried over anhydrous Na2SO4 and solvent removed
under reduced pressure. The residue was chromatographed
on silica gel to give 4.37 g of pure mesylated product 10.
Yield: 90%; IR (neat): 3026, 2110, 1705, 1357, 1176, 936,
4.3. General procedure for lipase-mediated
transesterification
1
906 cmꢀ1; H NMR (200 MHz, CDCl3): d 3.04 (3H, s),
To a solution of racemic cis-azido alcohol 12 (2.4 g,
9.1 mmol) in diisopropyl ether (50 mL) lipase PS-D
(1.2 g, 0.5 equiv w/w) and isopropenyl acetate (6 mL,
54 mmol) were added. The suspension was shaken at
220 rpm at 40 ꢁC. The reaction was monitored by chiral
HPLC analysis, and 50% conversion occurred after 14 h.
The reaction mixture was filtered and the solvent was evap-
orated. The residue was chromatographed on silica gel.
The enantiopure products were analyzed by chiral HPLC
and compared with the corresponding racemic products.
Azido alcohol (3S,4R)-12 was obtained (1.15 g). Yield:
48%; 92% ee; determined by HPLC analysis using Chiralcel
3.52–3.81 (4H, m), 4.29 (1H, m), 4.95 (1H, m), 5.12 (2H,
s), 7.33 (5H, m); FABMS (m/z): 341 (M++1). Anal. Calcd
for C13H16N4O5S: C, 44.88; H, 4.74; N, 16.46. Found: C,
44.79; H, 4.70; N, 16.39.
4.2.8. 1-Cbz-cis-3-Acetyloxy-4-azidopyrrolidine 11. Method
A: Bromo acetate 13 (0.25 g, 0.728 mmol) was dissolved in
dry DMSO (15 mL) and sodium azide (0.7 g, 1.09 mmol)
was added at room temperature. The reaction mixture stir-
red at 100 ꢁC for 6 h. After completion of the reaction, the
reaction mixture was brought to room temperature and ice
cold water was added and extracted with ethyl acetate. The
organic layer was washed with brine, dried over anhydrous
Na2SO4. The solvent was evaporated and the residue sub-
jected for silica gel column chromatography to give
0.170 g of pure racemic cis-azido acetate 11. Yield: 78%.
OD column (hexane/isopropanol, 80:20) with 0.5 mL/min
25
flow rate (tmajor = 19.59, tminor = 22.28 min); ½aꢁD ¼ ꢀ9:5
(c 1.01, CHCl3) and the azido acetate (3R,4S)-11 in
1.44 g. Yield: 52%, 97% ee; determined by the HPLC anal-
ysis using a Chiralcel OD column (hexane/isopropanol,
80:20) with 0.5 mL/min flow rate (tmajor = 33.65, tminor
28.32 min); ½aꢁD ¼ ꢀ29:3 (c 1.04, CHCl3).
=
25
Method B: A mixture of 10 (4 g, 11 mmol) and potassium
acetate (2.3 g, 23.5 mmol) in dry DMF (40 mL) was heated
at 100 ꢁC for 3 h under nitrogen atmosphere. The reaction
mixture was poured into an cold ice water and extracted
with ether. The ether layer was washed with brine, dried
over anhydrous Na2SO4. The solvent was removed under
reduced pressure. The residue was chromatographed on
silica gel to give 3 g of pure cis-azido acetate 11. Yield:
4.3.1. (3R,4S)-1-Cbz-3-Azido-4-hydroxypyrrolidine 12. Pre-
pared from (3R,4S)-11 by a deacetylation procedure using
anhydrous K2CO3 in methanol at room temperature to
give 1.23 g of pure (3R,4S)-12 cis-azido alcohol. Yield:
98%; 97% ee; determined by the HPLC analysis using a
Chiralcel OD column (hexane/isopropanol, 80:20) with
1
86%; IR (neat): 3032, 2107, 1740, 1706 cmꢀ1; H NMR
0.5 mL/min flow rate (tmajor = 22.77, tminor = 21.18 min);
25
(200 MHz, CDCl3): d 2.15 (3H, s), 3.37–3.58 (2H, m),
3.64–3.81 (2H, m), 4.08 (1H, m), 5.10 (2H, m), 5.22–5.36
(1H, m), 7.32 (5H, m); FABMS (m/z): 305 (M++1). Anal.
Calcd for C14H16N4O4: C, 55.26; H, 5.30; N, 18.41. Found:
C, 55.19; H, 5.24; N, 18.33.
½aꢁD ¼ þ11:9 (c 1.03, CHCl3).
4.3.2. (3R,4S)-1-Cbz-3-Azido-4-methoxypyrrolidine 14.
A
solution of azido alcohol (3R,4S)-12 (1.1 g, 4.1 mmol) in
dry THF (10 mL) was added to a suspension of NaH

2882
A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
(60% dispersion in oil, 0.2 g, 5 mmol, washed with dry hex-
ane before use) in dry THF (20 mL). After stirring at room
temperature for 1 h, the mixture was gradually warmed to
50 ꢁC over 2 h. The reaction mass was cooled to 0 ꢁC and
CH3I (0.31 mL, 5.03 mmol) added and left at room temper-
ature for 11 h. The reaction mass was quenched with satu-
rated NH4Cl and extracted with ethyl acetate. The organic
layer was washed with brine and dried over anhydrous
Na2SO4. The solvent was evaporated under vacuum. The
resulting mass was subjected for silica gel column chroma-
tography to give 0.98 g of the methylated product (3R,4S)-
14. Yield: 86%; 97% ee; determined by HPLC analysis
using Chiralcel OD-H column (hexane/isopropanol,
4.3.6.
(3R,4S)-1-Cbz-3-[(tert-Butoxycarbonyl)methyl-
amino]-4-methoxypyrrolidine 16. Compound (3R,4S)-15
(1 g, 2.8 mmol) was dissolved in dry DMF (10 mL) and
added into a suspension of NaH (0.13 g, 3.4 mmol, washed
with dry hexane before use) in dry DMF at 0 ꢁC over a per-
iod of 15 min CH3I (0.21 mL, 3.6 mmol) was added to the
reaction mass at 0 ꢁC and stirred at room temperature for
6 h. The reaction mass was quenched with saturated
NH4Cl and extracted with ether. The ether layer was
washed with brine, dried over anhydrous Na2SO4 and the
solvent removed in vacuo. The residue obtained was sub-
jected to silica gel column chromatography and gave
0.925 g of pure (3R,4S)-16. Yield: 89%; 97% ee; determined
by the HPLC analysis using a Chiralcel OD-H column
90:10) with 0.5 mL/min flow rate (tmajor = 33.47, tminor
=
25
32.00 min); ½aꢁD ¼ ꢀ48:4 (c 1, CHCl3); IR (neat): 2943,
(hexane/isopropanol, 80:20) with 0.5 mL/min flow rate
25
2104, 1693 cmꢀ1 1H NMR (400 MHz, CDCl3): d 3.45
;
(tmajor = 11.12, tminor = 15.79 min); ½aꢁD ¼ ꢀ42:6 (c 1.05,
1
(3H, s), 3.46–3.66 (4H, m), 3.97 (2H, m), 5.10 (2H, q,
J = 12.44 Hz), 7.33 (5H, m); FABMS (m/z): 277 (M++1).
Anal. Calcd for C13H16N4O3: C, 56.51; H, 5.84; N, 20.28.
Found: C, 56.46; H, 5.78; N, 20.17.
CHCl3); IR (neat): 2980, 1726, 1406, 1367, 1166 cmꢀ1; H
NMR (200 MHz, CDCl3): d 1.46 (9H, s), 2.89 (3H, s),
3.32 (3H, d, J = 3.03 Hz), 3.42–3.68 (4H, m), 3.92 (1H,
s), 4.57 (1H, s), 5.12 (2H, m), 7.32 (5H, m); FABMS (m/
z): 365 (M++1). Anal. Calcd for C19H28N2O5: C, 62.62;
H, 7.74; N, 7.69. Found: C, 62.57; H, 7.69; N, 7.58.
4.3.3. (3S,4R)-1-Cbz-3-Azido-4-methoxypyrrolidine 14. Pre-
pared from (3S,4R)-12 under the above mentioned condi-
tions to give 0.96 g of (3S,4R)-14. Yield: 85%; 92% ee;
determined by HPLC analysis using a Chiralcel OD-H
4.3.7.
(3S,4R)-1-Cbz-3-[(tert-Butoxycarbonyl)methyl-
column (hexane/isopropanol, 90:10) with 0.5 mL/min
amino]-4-methoxypyrrolidine 16. Prepared from (3S,4R)-
15 under the above mentioned conditions to give 0.92 g
of (3S,4R)-16. Yield: 88%; 92% ee; determined by HPLC
analysis using a Chiralcel OD-H column (hexane/isopropa-
25
flow rate (tmajor = 31.12, tminor = 34.18 min); ½aꢁD ¼ þ46:0
(c 1.05, CHCl3).
nol, 80:20) with 0.5 mL/min flow rate (tmajor = 15.22,
tminor = 11.20 min); ½aꢁD ¼ þ40:6 (c 1.02, CHCl3).
4.3.4.
(3R,4S)-1-Cbz-3-[(tert-Butoxycarbonyl)amino]-4-
25
methoxypyrrolidine 15. The methylated azide (3R,4S)-14
(0.9 g, 3.3 mmol) was dissolved in THF/H2O (15:15 mL)
and triphenylphosphine (1.73 g, 6.6 mmol) added at room
temperature. The reaction was monitored by TLC until
the starting material disappeared (approximately 2 h).
The reaction mass was cooled to 0 ꢁC after which Et3N
(0.73 mL, 5.2 mmol) and di-tert-butyldicarbonate (1.07 g,
4.9 mmol) were added dropwise. The reaction mixture
was stirred at room temperature for 6 h and THF then
removed under reduced pressure. The reaction mixture
was diluted with water and extracted with ether. The
organic layer was washed with brine, dried over anhydrous
Na2SO4 and concentrated. The residue was chromato-
graphed on silica gel to give 1.06 g of pure (3R,4S)-15.
Yield: 92%; 97% ee; determined by HPLC analysis using
a Chiralcel OD-H column (hexane/isopropanol, 80:20)
4.3.8.
(3R,4S)-1-tert-Butoxycarbonyl-3-methoxy-4{[tert-
butoxycarbonyl]methylamino}pyrrolidine 3b. To a stirred
solution of (3R,4S)-16 (0.8 g, 2.1 mmol) in ethanol
(10 mL) were added 3 mL of PMHS and 10% Pd–C
(80 mg) under an N2 atmosphere. Di-tert-butyldicarbonate
(0.71 g, 3.2 mmol) and Et3N (0.44 mL, 3.2 mmol) were
added to the reaction mass and stirred at room temperature
for 6 h. The reaction mixture was filtered on a Celite pad
and the solvent evaporated in vacuo. The residue was puri-
fied on silica gel column chromatography to give 0.6 g of
25
pure (3R,4S)-3b. Yield: 84%; 97% ee; ½aꢁD ¼ ꢀ53:2 (c
20
1.05, MeOH) [lit.3 ½aꢁD ¼ ꢀ54:4 (c 1.07, MeOH), >99%
ee]; IR (neat): 2939, 2358, 1689, 1376, 1156 cmꢀ1 1H
;
with 0.5 mL/min flow rate (tmajor = 12.90, tminor = 14.42 -
NMR (200 MHz, CDCl3): d 1.46 (9H, s), 1.47 (9H, s),
2.91 (3H, s), 3.34 (3H, s), 3.36–3.60 (4H, m), 3.90 (1H, br
s), 4.57 (1H, br s); 13C NMR (50 MHz, CDCl3): d 28.14,
28.18, 31.22, 44.99, 48.58, 49.33, 57.19, 57.46, 79.33,
79.63, 79.89, 80.51, 154.25, 155.64; FABMS (m/z): 331
(M++1). Anal. Calcd for C16H30N2O5: C, 58.16; H, 9.15;
N, 8.48. Found: C, 58.09; H, 9.12; N, 8.41.
25
min); ½aꢁD ¼ ꢀ7:3 (c 1.07, CHCl3); IR (neat): 3336, 2973,
1
1702 cmꢀ1; H NMR (200 MHz, CDCl3): d 1.44 (9H, s),
3.37 (3H, s), 3.39–3.85 (5H, m), 4.03–4.32 (1H, m), 5.09
(2H, m), 7.32 (5H, m); FABMS (m/z): 351 (M++1). Anal.
Calcd for C18H26N2O5: C, 61.70; H, 7.48; N, 7.99. Found:
C, 61.67; H, 7.40; N, 7.87.
4.3.5.
(3S,4R)-1-Cbz-3-[(tert-Butoxycarbonyl)amino]-4-
methoxypyrrolidine 15. Prepared from (3S,4R)-14 under
the above mentioned conditions to give 1.06 g of (3S,4R)-
15. Yield: 92%; 92% ee; determined by HPLC analysis
using a Chiralcel OD-H column (hexane/isopropanol,
4.3.9.
butoxycarbonyl]methylamino}pyrrolidine
(3S,4R)-1-tert-Butoxycarbonyl-3-methoxy-4{[tert-
3b. Prepared
from (3S,4R)-16 under the above mentioned conditions
to give 0.56 g of (3S,4R)-3b. Yield: 80%; 92% ee;
25
20
80:20) with 0.5 mL/min flow rate (tmajor = 14.30, tminor
=
½aꢁD ¼ þ50:9 (c 1.01, MeOH), {lit.3 ½aꢁD ¼ þ53:7 (c 1.00,
25
12.82 min); ½aꢁD ¼ þ5:4 (c 1.07, CHCl3).
MeOH), >99% ee}.

A. Kamal et al. / Tetrahedron: Asymmetry 17 (2006) 2876–2883
2883
4. Tomita, K.; Tsuzuki, Y.; Shibamori, K.; Tashima, M.;
Kajikawa, F.; Sato, Y.; Kashimoto, S.; Chiba, K.; Hino, K.
J. Med. Chem. 2002, 45, 5564.
5. Tsuzuki, Y.; Tomita, K.; Shibamori, K.; Sato, Y.; Kashi-
moto, S.; Chiba, K. J. Med. Chem. 2004, 47, 2097.
6. Kumar, A. R.; Reddy, J. S.; Rao, B. V. Tetrahedron Lett.
2003, 44, 5687.
Acknowledgements
The authors (A.A.S., M.S., S.A. and M.S.M.) are thankful
to the CSIR, New Delhi, for the award of research
fellowship.
7. Madhan, A.; Rao, B. V. Tetrahedron Lett. 2005, 46, 323.
8. Kamal, A.; Shaik, A. A.; Sandbhor, M.; Malik, M. S.; Kaga,
H. Tetrahedron Lett. 2004, 45, 8057.
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