Enzymatic resolution of N-arylaziridine carboxylates

Article abstract of DOI:10.1016/j.tetasy.2003.10.027

N-Arylaziridine-2-carboxylates have been enzymatically resolved using the lipase from Candida rugosa to afford optically active aziridine carboxylates in moderate to high enantiomeric purity. The absolute configuration of the unknown aziridine carboxylates was assigned as S by chemical correlation.

Full text of DOI:10.1016/j.tetasy.2003.10.027

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 127–130
Enzymatic resolution of N-arylaziridine carboxylatesq
H. M. Sampath Kumar,* M. Shesha Rao, P. Pawan Chakravarthy and J. S. Yadav
Organic Chemistry Division-I, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 10 September 2003; accepted 27 October 2003
Abstract—N-Arylaziridine-2-carboxylates have been enzymatically resolved using the lipase from Candida rugosa to afford optically
active aziridine carboxylates in moderate to high enantiomeric purity. The absolute configuration of the unknown aziridine carb-
oxylates was assigned as S by chemical correlation.
ꢀ 2003 Elsevier Ltd. All rights reserved.
1. Introduction
moderate to low ee.5 Furthermore, N-arylaziridine-2-
carboxylates have been synthesized in low to moderate
enantiomeric purity by asymmetric aziridination in-
volving in-built chiral auxiliaries attached to either the
olefin or a hydroxamic acid (S, 17–49% ee) or even using
chiral phase transfer agents derived from cinchona alka-
loids (R, 45–62% ee). Lipase from Candida rugosa
(CRL) has been extensively utilized for the kinetic res-
olution of various racemic substrates.6
Aziridine carboxylates are important chiral synthons,
which can generate a plethora of useful organic inter-
mediates such as a- and b-amino acids and b-lactams.1
Furthermore, enantiomerically pure aziridine carboxyl-
ates may be used as precursors for the generation of
chiral building blocks and enantioselective routes to
unusual amino acids can be envisaged through regiose-
lective ring opening reactions involving aziridinoesters.2
Regio- and stereoselective ring opening with many types
of nucleophiles provide access to a great variety of
useful synthetic intermediates. In addition to being
attractive substrates or building blocks for organic
chemists, aziridine carboxylates can also act as chiral
auxiliaries, chiral reagents and chiral ligands for asym-
metric synthesis.3 Various chiral aziridines are available
in enantiomerically pure form either through asymmet-
ric synthesis4 or kinetic resolution of racemates.5
2. Results and discussion
To our knowledge there have not been any reports on
the enzymatic enantioselective hydrolysis of N-arylazir-
idine-2-carboxylates and in this context we herein report
the kinetic resolution of racemic N-arylaziridine-2-
carboxylates via enantioselective hydrolysis by C. rugosa
lipase (CRL). Racemic aziridines were prepared by the
ring contraction of precursor triazoline carboxylates,
which were readily obtained through 1,3-dipolar cyclo-
additions of various aryl azides with methyl acrylates.
Hydrolysis was conducted in pH 7.5 phosphate buffer
(0.1 M) and due to the low solubility of the substrates in
buffer solution, dioxane was used as the co-solvent. The
hydrolysis was terminated at around 45–50% conversion
by extraction with EtOAc. The crude optically active
esters obtained after evaporation of the solvent were
flash chromatographed on silica gel (EtOAc/n-hexane
gradient) to afford pure N-arylaziridine carboxylates.
The enantiomeric purity was determined by chiral
As the reactivity of three-membered heterocycles can be
modified by a suitable choice of substitution on the
nitrogen atom, the preparation of enantiomerically pure
N-substituted aziridine carboxylates is an attractive
option. Our literature survey reveals that only a few
reports concerning the enzymatic enantiospecific
hydrolysis of dimethyl aziridine 2,3-carboxylates and
N-acylaziridine carboxylates have been reported with
1
q IICT communication no. 030917.
* Corresponding author. Tel.: +91-40-717-3874; fax: +91-40-2716-
0512; e-mail: hmskumar@yahoo.com
HPLC and H NMR-shift reagent methods. Moderate
to high enantiomeric purity (70–99%) was observed
among the substrates (see Table 1) studied. The absolute
0957-4166/$ - see front matter ꢀ 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2003.10.027

128
H. M. Sampath Kumar et al. / Tetrahedron: Asymmetry 15 (2004) 127–130
Table 1. Enantioselective hydrolysis of aziridine-2-carboxylates by C. rugosa
Entry
Substrate (Ar)
Reaction conditions
Unchanged ester
25
E/Sa
Conversion (%) ½aŠ CHCl3
Ee (%)c
84
Absolute configuration
b
t (h)
D
4
1/2
48
50
45
48
44
46
)173.2
)156.7
)40.3
S
1
2
5
1/2
1/2
1/2
1/4
1/4
99
12
70
7
S
S
S
S
S
Br
3
4
5
6
3
O2N
H3C
F
4.5
5.5
5.5
)195.5
)18.4
Br
)15.9
15
H3C
7
5
1/2
46
)184.3
79
S
MeO
a E/S refers to enzyme, substrate (wt/wt) ratio.
b + or ) refers to the sign of the specific rotation.4
c Determined by chiral HPLC conditions: Chiracel ODTM, Daicel, Japan; 5 · 250mm, k 330 nm; 10% isopropanol in hexane; flow rate 0.6 mL/min and
by 1H NMR with Eu(hfc)3 in CDCl3.
configuration of all the unhydrolyzed N-arylaziridine
carboxylates was determined as S by comparing the
sign of the specific rotation, based on a literature pre-
cedent.4
4. Experimental
4.1. General procedure for the preparation of
N-arylaziridine-2-carboxylates7
Aryl azide (1 mmol) dissolved in 20mL of dry cyclo-
hexane was heated at 60–65 ꢁC with methyl acrylate
(1.1 mmol) for 8–10h at the end of which the solvent
was evaporated under reduced pressure. The residue was
chromatographed (silica gel finer than 200 mesh, eluent;
EtOAc/n-hexane gradient mixture) to afford pure N-
arylaziridine-2-carboxylate, which was characterized by
IR, 1H NMR, mass spectroscopy and elemental analysis
(Scheme 1).
3. Conclusion
We have successfully demonstrated herein, the kinetic
resolution of synthetically important N-arylaziridine-2-
carboxylates in moderate to high enantiomeric purity
using C. rugosa lipase. All the unhydrolyzed esters were
found to be of an (S)-configuration.
+
N
N
Cyclohexane
N
N
N
N
R
R
CO2Me
+
0
60-65 C
CO2Me
1
3
2
MeO2C
MeO2C
HO2C
Candida rugosa
pH 7.5 phosphate buffer
N
N
N
+
R
R
R
4
S
R
Scheme 1.

H. M. Sampath Kumar et al. / Tetrahedron: Asymmetry 15 (2004) 127–130
129
4.2. General procedure for C. rugosa-catalyzed hydro-
lysis of the N-arylaziridine carboxylates
4.7. Methyl N-(p-fluorophenyl)aziridine-2-carboxylate
25
D
½aŠ )18.4 (c 1.5, CHCl3); IR (KBr): 1756 cmÀ1
;
1H
Racemic N-arylaziridine carboxylate (25 mg) was added
to a phosphate buffer (pH 7.5, 25 mL, 0.1 M) and treated
with C. rugosa lipase (Sigma 0.890 units/mg) with vig-
orous stirring at room temperature (25 ꢁC). The ratio of
enzyme/aziridine carboxylate (E/S) employed is reported
in Table 1. The pH was adjusted at 7.5 and kept con-
stant by the intermittent addition of 0.1 M NaOH. Di-
oxane (1–2 mL, 4–8% v/v) was used as the co-solvent to
improve the solubility of N-arylaziridine carboxylates.
The progress of hydrolysis was monitored by TLC and
the reaction terminated at 45–50% conversion via simple
extraction with ethyl acetate (2 · 20mL) after which the
organic extract was dried using sodium sulfate and
concentrated. The residue was purified by flash chro-
matography (ethyl acetate/n-hexane gradient mixture) to
afford the pure N-arylaziridine carboxylate.
NMR: d 2.24 (d, 1H), 2.62 (s, 1H), 2.76 (dd, 1H), 3.80(s,
3H), 6.85 (d, 2H), 6.92 (d, 2H). MS: m=z (%) 196 (Mþ+1,
100), 180 (12), 136 (30), 122 (15), 83 (14), 69 (14), 55
(28). Anal. Calcd for C10H10FNO2: C, 61.53; H, 5.16; F,
9.73; N, 7.18. Found: C, 61.60; H, 5.11; F, 9.78; N,
7.26%.
4.8. Methyl N-(2-bromo-4-methylphenyl)aziridine-2-
carboxylate
25
D
½aŠ )15.9 (c 0.025, CHCl3); IR (KBr): 1755 cmÀ1; H
1
NMR: d 2.24 (s, 3H), 2.38 (dd, 1H), 3.81 (s, 3H), 6.79 (d,
1H), 6.95 (d, 1H), 7.32 (s, 1H); MS: m=z (%) 269 (Mþ,
30), 198 (36), 107 (100), 79 (62). Anal. Calcd for
C11H12BrNO2: C, 48.91; H, 4.48; Br, 29.58; N, 5.19.
Found: C, 48.83; H, 4.57; Br, 29.68; N, 5.22%.
4.3. Methyl N-(phenyl)aziridine-2-carboxylate4
4.9. Methyl N-(p-methoxyphenyl)aziridine-2-carboxylate
25
D
25
D
1
½aŠ )173.2 (c 0.25, CHCl3); IR (KBr): 1750cm À1; H
1
½aŠ )184.3 (c 0.5, CHCl3); IR (KBr): 1754 cmÀ1; H
NMR: d 2.30(dd, 1H), 2.66 (dd, 1H), 2.81 (dd, 1H), 3.80
(s, 3H), 7.02–7.05 (m, 3H), 7.22–7.26 (m, 2H); MS: m=z
(%) 177 (Mþ, 32), 163 (26), 119 (18), 104 (100), 90 (55),
77 (44). Anal. Calcd for C10H11NO2: C, 67.78; H, 6.62;
N, 7.90. Found: C, 67.69; H, 6.57; N, 7.97%.
NMR: d 2.23 (d, 1H), 2.59 (s, 1H), 2.69 (dd, 1H), 3.75 (s,
3H), 3.81 (s, 3H), 6.75 (d, 2H), 6.84 (d, 2H); MS: m=z
(%) 207 (Mþ, 95), 192 (26), 134 (100), 121 (62), 77 (24).
Anal. Calcd for C11H13NO3: C, 63.76; H, 6.32; N, 6.76.
Found: C, 63.68; H, 6.40; N, 6.89%.
Acknowledgement
4.4. Methyl N-(p-bromophenyl)aziridine-2-carboxylate
25
D
M. S. Rao thanks CSIR––New Delhi for the award of
fellowship.
1
½aŠ )156.3 (c 0.25, CHCl3); IR (KBr): 1754 cmÀ1; H
NMR: d 2.26 (d, 1H), 2.63 (s, 1H), 2.75 (dd, 1H), 3.79 (s,
1H), 6.84 (d, 2H), 7.29 (d, 2H); MS: m=z (%) 255 (Mþ,
91), 182 (96), 169 (53), 117 (100), 90 (76), 63 (43). Anal.
Calcd for C10H10BrNO2: C, 46.90; H, 3.94; Br, 31.20; N,
5.47. Found: C, 46.98; H, 3.86; Br, 31.32; N, 5.37%.
References and Notes
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4.5. Methyl N-(p-nitrophenyl)aziridine-2-carboxylate4
25
D
½aŠ )40.3 (c 0.5, CHCl3); IR (KBr): 1758 cmÀ1
;
1H
NMR: d 2.24 (d, 1H), 2.78 (s, 1H), 2.95 (dd, 1H), 3.82 (s,
3H), 7.06 (d, 2H), 8.18 (d, 2H); MS: m=z (%) 222 (Mþ,
82), 206 (70), 163 (28), 149 (100), 117 (54), 90 (43), 45
(30). Anal. Calcd for C10H10N2O4: C, 54.06; H, 4.54; N,
12.61; Found: C, 54.15; H, 4.42; N, 12.73%.
4.6. Methyl N-(p-methylphenyl)aziridine-2-carboxylate4
25
D
½aŠ )195.5 (c 0.25, CHCl3); IR (KBr): 1756 cmÀ1; H
1
NMR: d 2.25–2.28 (m, 4H), 2.59 (dd, 1H), 2.71 (dd, 1H),
3.79 (s, 3H), 6.85 (d, 2H), 7.03 (d, 2H); MS: m=z (%) 191
(Mþ, 66), 177 (30), 133 (35), 119 (100), 106 (65), 92 (60).
Anal. Calcd for C11H13NO2: C, 69.09; H, 6.85; N, 7.32.
Found: C, 69.15; H, 6.80; N, 7.4%.

130
H. M. Sampath Kumar et al. / Tetrahedron: Asymmetry 15 (2004) 127–130
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