Nitrile and amide biotransformations for the synthesis of enantiomerically pure 3-arylaziridine-2-carboxamide derivatives and their stereospecific ring-opening reactions

Article abstract of DOI:10.1021/jo701732h

(Chemical Equation Presented) Catalyzed by Rhodococcus erythropolis AJ270 (whole cell catalyst) under very mild conditions, a number of racemic trans-3-arylaziridine-2-carbonitriles and amides were efficiently transformed into enantiopure 2R,3S-3-arylazir

Full text of DOI:10.1021/jo701732h

contrast to oxirane-2-carboxylic acid derivatives,4 however, the
synthesis of optically active aziridine-2-carboxylates still remains
challenging to synthetic chemists.5 Most chiral aziridine-2-
carboxylates were prepared using either chiral starting
materials1b,2b,5 or chiral auxiliaries,1b,2b,5 and most of them are
generally time-consuming or not cost-effective. Although
catalytic asymmetric syntheses have been reported,2b,5 however,
they are limited to a narrow substrate spectrum. A few advances
in this field have been achieved very recently. For example,
Cu(I)-catalyzed aziridination of cinnamates using a chiral 2,2′-
bifuranyl-3,3′-diamine-derived C2-diimide ligand has been
reported to yield trans-3-aryl-1-tosylaziridine-2-carboxylates
with 80.1-99% ee.6 Using boron catalysts derived from vaulted
chiral binaphthol and biphenanthrol ligands, aziridination of
N-dianisylmethylimines (N-DAM-imines) with ethyl diazoac-
etate afforded cis-3-aryl(alkyl)-1-DAM-aziridine-2-carboxylates
with ee’s ranging from 63% to 97%.7 While a catalytic
aziridination of chalcones using a (+)-Tro¨ger’s base-derived
aminimide as an NH-transfer reagent gave moderate enantiose-
lectivity,8 a chiral pyrrolidine-catalyzed aziridination of unsatur-
ated aldehydes with acetyl hydroxycarbamate produced 1-Cbz-
2-formylaziridines with dr 4:1-19:1 and 84-99% ee.9 The
lipase-mediated kinetic resolution of racemic 1-arylaziridie-2-
carboxylates has been shown to give, in most cases, disappoint-
ingly low chemical yield and enantioselectivity.10
Nitrile and Amide Biotransformations for the
Synthesis of Enantiomerically Pure
3-Arylaziridine-2-carboxamide Derivatives and
Their Stereospecific Ring-Opening Reactions
Jin-Yuan Wang, De-Xian Wang, Jie Pan,
Zhi-Tang Huang, and Mei-Xiang Wang*
Beijing National Laboratory for Molecular Sciences, Laboratory
of Chemical Biology, Institute of Chemistry, Chinese Academy
of Sciences, Beijing 100080, China
mxwang@iccas.ac.cn
ReceiVed August 7, 2007
Biotransformations of nitriles, either through a direct conver-
sion from a nitrile to a carboxylic acid catalyzed by a nitrilase
or through the nitrile hydratase-catalyzed hydration of a nitrile
followed by the amide hydrolysis catalyzed by the amidase, have
become the effective and environmentally benign methods for
the synthesis of not only the chemical commodities such as
acrylamide and nicotinic acid but also chiral carboxylic acids
and their amide derivatives.11,12 The distinct features of enzy-
matic transformations of nitriles are the straightforward genera-
tion of enantiopure amides, valuable organo-nitrogen compounds
in synthetic chemistry, in addition to the formation of enan-
tiopure carboxylic acids. For example, we have shown that
Catalyzed by Rhodococcus erythropolis AJ270 (whole cell
catalyst) under very mild conditions, a number of racemic
trans-3-arylaziridine-2-carbonitriles and amides were ef-
ficiently transformed into enantiopure 2R,3S-3-arylaziridine-
2-carboxamides. While the nitrile hydratase exhibits low
selectivity against nitrile substrates, the amidase is highly
enantioselective toward 2S,3R-3-arylaziridine-2-carboxam-
ides. Upon the treatment with catalytic hydrogenation, amine,
or water in the presence of one equivalent of TFA, the
resulting aziridine-2-carboxamides underwent highly efficient
and stereospecific ring-opening reactions to produce enan-
tiopure R-amino-, R,â-diamino-, and R-amino-â-hydroxy-
propanamide derivatives in high yields.
(3) For recent examples, see: (a) Denolf, B.; Mangelinkx, S.; To¨rnroos,
K. W.; De Kimpe, N. Org. Lett. 2006, 8, 3129. (b) Lee, B. K.; Kim, M. S.;
Hahm, H. S.; Kim, D. S.; Lee, W. K.; Ha, H.-J. Tetrahedron 2006, 62,
8393 and references therein. (c) Kokotos, C. G.; Aggarwal, V. K. Org.
Lett. 2007, 9, 2099 and references therein.
(4) For a useful overview of the synthesis of chiral oxirane-2-carboxylic
acid derivatives, see: Wang, M.-X.; Lin, S.-J.; Liu, C.-S.; Zheng, Q.-Y.;
Li, J.-S. J. Org. Chem. 2003, 68, 4570 and references therein.
(5) For a useful overview of the synthesis of chiral aziridine-2-carboxylate
derivatives, see: Wang, J.-Y.; Wang, D.-X.; Zheng, Q.-Y.; Huang, Z.-T.;
Wang, M.-X. J. Org. Chem. 2007, 72, 2040.
(6) Wang, X.; Ding, K. Chem. Eur. J. 2006, 12, 4568.
(7) Lu, Z.; Zhang, Y.; Wulff, W. D. J. Am. Chem. Soc. 2007, 129, 7185
and references cited therein.
(8) Shen, Y.-M.; Zhao, M.-X.; Xu, J.; Shi, Y. Angew. Chem., Int. Ed.
2006, 45, 8005.
Chiral aziridine-2-carboxylic acid derivatives, C-activated
aziridine compounds, and a type of special amino acid deriva-
tives, are of special importance owing to their occurrence in
natural products and in synthetic pharmaceuticals and to their
versatility in the preparation of diverse chiral molecules.1-3 In
(1) (a) For a recent monograph, see: Yudin, A. K., Ed. Aziridines and
Epoxides in Organic Synthesis; Wiley-VCH: New York, 2006. For recent
reviews, see: (b) Osborn, H. M.; Sweeney, J. Tetrahedron: Asymmetry
1997, 8, 1693. (c) Xu, X. E. Tetrahedron 2004, 60, 2701. (d) McCoull,
W.; Davis, F. A. Synthesis 2000, 1347.
(2) (a) For aziridine natural products, see: Lowden, P. P. Aziridine natural
products: discovery, biological activity and biosynthesis. In Aziridines and
Epoxides in Organic Synthesis; Yudin, A. K., Ed.; Wiley-VCH: New York,
2006; Chapter 11. (b) For aziridinecarboxylates, see: Zhou, P.; Chen, B.-
C.; Davis, F. A. Asymmetric syntheses with aziridinecarboxylate and
aziridinephosphonate building blocks. In Aziridines and Epoxides in Organic
Synthesis; Yudin, A. K., Ed.; Wiley-VCH: New York, 2006; Chapter 3.
(9) Vesely, J.; Ibrahem, I.; Zhao, G.-L.; Rios, R.; Cordova, A. Angew.
Chem., Int. Ed. 2007, 46, 778.
(10) Kumar, H. M. S.; Rao, M. S.; Chakravarthy, P. P.; Yadav, J. S.
Tetrahedron: Asymmetry 2004, 15, 127.
(11) For reviews, see: (a) Sugai, T.; Yamazaki, T.; Yokoyama, M.; Ohta,
H. Biosci. Biotechnol. Biochem. 1997, 61, 1419 and references therein. (b)
Martinkova, L.; Kren, V. Biocatal. Biotransform. 2002, 20, 73. (c) Wang,
M.-X. Top. Catal. 2005, 35, 117.
(12) For recent examples, see: (a) Kamila, S.; Zhu, D.; Biehl, E. R.;
Hua, L. Org. Lett. 2006, 8, 4429. (b) Bergeron, S.; Chaplin, D. A.; Edwards,
J. H.; Ellis, B. S. W.; Hill, C. L.; Holt-Tiffin, K.; Knight, J. R.; Mahoney,
T.; Osborne, A. P.; Ruecroft, G. Org. Process Res. DeV. 2006, 10, 661.
10.1021/jo701732h CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/25/2007
J. Org. Chem. 2007, 72, 9391-9394
9391

Rhodococcus erythropolis AJ270,13 a nitrile hydratase/amidase-
containing whole cell catalyst, is able to efficiently and
enantioselectively transform a variety of racemic nitriles bearing
an R-14 or a â-stereocenter15 and prochiral dinitriles16 into highly
enantiopure carboxylic acids and amides. Recently, we have
found that biotransformations of nitriles bearing a cyclopro-
pane17 or an epoxide ring4,18 proceeded in a highly predictable
manner in terms of reaction efficiency and enantioselectivity
based on the substituents and configurations of the substrates.
Biotransformations of racemic 1-arylaziridine-2-carbonitriles
have been found to follow the same reaction model, producing
the corresponding enantiopure amide and acid products.5
Interestingly, the same reaction using a nitrile-hydrolyzing
Rhodococcus IFO15564 cell catalyst did not allow the isolation
of acid product.19 In order to explore the full scope of nitrile
and amide biotransformations in the synthesis of C-activated
enantiopure aziridine derivatives, and also to gain deep insight
into the chiral recognition of the nitrile hydratase and the
amidase involved in Rhodococcus erythropolis AJ270, we
undertook the current study. Herein we report the highly efficient
biocatalytic preparation of enantiopure trans-3-aryl-1-methy-
laziridine-2-carboxamides. Their stereospecific aziridine ring-
opening reactions in the synthesis of R-amino-, R,â-diamino-,
and R-amino-â-hydroxy acid derivatives will also be discussed.
We began our study by testing the biotransformation of
racemic trans-1-methyl-3-phenylaziridine-2-carbonitrile 1a. Prior
to biocatalytic transformation, the configuration of 1-methyl
group relative to trans-orientated phenyl and carboxamide
groups was examined. The NOESY experiment (see Supporting
Information) showed interaction between methyl protons and
both protons of the aziridine ring, suggesting a very fast flipping
of the methyl group between two sides of the three-membered
ring, or a rapid inversion of the lone-pair elelctrons on the
nitrogen in the NMR time scale. It was expected, therefore, that
the steric feature of trans-3-aryl-1-methylaziridine-2-carboni-
triles should be more or less similar to that of trans-2,2-
dimethyl-3-phenylcyclopropanecarbonitrile, a type of good
substrate for nitrile-hydrolyzing cells.17b
TABLE 1. Biotransformations of Racemic trans-3-Aryl-1-methyl-
aziridine-2-carbonitriles 1a
time
(h)
entry (()-1
Ar
2 (%)b/ee (%)c 1 (%)b/ee (%)c
1
2
3
4
5
6
7
1a
1b
1c
1d
1d
1e
1f
C6H5
4-Me-C6H4
0.7
1
45/ >99.5
48/ >99.5
47/ <5
-/ -
-/ -
-/ -
4-MeO-C6H4 1.5
-/ -
4-Br-C6H4
4-Br-C6H4
4-F-C6H4
4-Cl-C6H4
5
1
1
1.5
trace/ <5
46/ 26e
39/ 9.8
42/ <5
26/ 24d
-/ -
-/ -
a The racemic nitrile 1 (1 mmol) was incubated with Rhodococcus
erythropolis AJ270 (2 g wet weight) in phosphate buffer (0.1 M, pH 7.0,
50 mL) at 30 °C. Reaction time was optimized to the completion of nitrile
conversion and roughly 50% conversion of the amide using HPLC analysis.
b Isolated yield. c Determined by chiral HPLC analysis. d Configurations of
amide obtained are 2R,3S. e Configurations of nitrile recovered are 2R,3S.
phosphate buffer with pH 7.0 at 30 °C, racemic trans-1-methyl-
3-phenylaziridine-2-carbonitrile 1a underwent a highly efficient
and enantioselective hydrolysis. As shown in Table 1, nitrile
1a was completely hydrated within minutes, and the 50%
conversion of amide was also achieved around 40 min to give
enantiopure 2a in 45% yield (entry 1, Table 1). The biotrans-
formations of 4-tolyl-substituted analogue 1b proceeded equally
well to produce enantiopure 2b in an almost quantitative yield
(entry 2, Table 1). Surprisingly, when a 4-methoxy group was
introduced, substrate 1c gave an almost optically inactive amide
product 2c (entry 3, Table 1). When substrates were bearing a
halogen substituent on the benzene ring, the amidase-catalyzed
amide hydrolysis exceeded the nitrile hydratase-catalyzed nitrile
hydration reaction. For example, complete nitrile hydration of
racemic 1d did not allow the accumulation of amide product
2d as the latter was rapidly hydrolyzed under the biocatalytic
reaction conditions (entry 4, Table 1). To understand the
enantioselectivity of the nitrile hydratase against trans-3-aryl-
1-methylaziridine-2-carbonitriles, we quenched the hydration
reaction of 1d-f at its ca. 50% conversion (entries 5-7, Table
1). On the basis of the ee values (<5%-26%) of the recovered
nitriles 2R,3S-1d-f, it is concluded that the nitrile hydratase
involved in Rhodococcus erythropolis AJ270 is less 2S,3R-
enantioselective against aziridine-2-carbonitriles 1. It is worth
noting that, albeit in low ee (24%), the amide 2d (26% yield)
isolated from the same reaction turned out to be 2R,3S-
configured. The outcomes indicate clearly that combination of
a less 2S,3R-enantioselective nitrile hydratase and a highly
2S,3R-enantioselective amidase is responsible for the overall
enantioselective nitrile biotransformations.
Catalyzed by Rhodococcus erythropolis AJ270 whole cell
catalyst under very mild conditions, such as in an aqueous
(13) (a) Blakey, A. J.; Colby, J.; Williams, E.; O’Reilly, C. FEMS
Microbiol. Lett. 1995, 129, 57. (b) O’Mahony, R.; Doran, J.; Coffey, L.;
Cahill, O. J.; Black, G. W.; O’Reilly, C. Antonie Van Leeuwenhoek 2005,
87, 221.
(14) (a) Wang, M.-X.; Lu, G.; Ji, G.-J.; Huang, Z.-T.; Meth-Cohn, O.;
Colby, J. Tetrahedron: Asymmetry 2000, 11, 1123. (b) Wang, M.-X.; Li,
J.-J., Ji, G.-J.; Li, J.-S. J. Mol. Catal. B: Enzym. 2001, 14, 77. (c) Wang,
M.-X.; Zhao, S.-M. Tetrahedron: Asymmetry 2002, 13, 1695. (d) Wang,
M.-X.; Lin, S.-J. J. Org. Chem. 2002, 67, 6542. (e) Wang, M.-X.; Lin,
S.-J.; Liu, J.; Zheng, Q.-Y. AdV. Synth. Catal. 2004, 346, 439. (f) Wang,
M.-X.; Liu, J.; Wang, D.-X. Zheng, Q.-Y. Tetrahedron: Asymmetry 2005,
16, 2409. (g) Liu, J.; Wang, D.-X.; Zheng, Q.-Y.; Wang, M.-X. Chin. J.
Chem. 2006, 24, 1665. (h) Ma, D.-Y.; Wang, D.-X.; Zheng, Q.-Y.; Wang,
M.-X. Tetrahedron: Asymmetry 2006, 17, 2366.
(15) (a) Wang, M.-X.; Wu, Y. Org. Biomol. Chem. 2003, 1, 535. (b)
Zhao, S.-M.; Wang, M.-X. Chin. J. Chem. 2002, 20, 1291. (c) Ma, D.-Y.;
Zheng, Q.-Y.; Wang, D.-X.; Wang, M.-X. Org. Lett. 2006, 8, 3231. (d)
Gao, M.; Wang, D.-X.; Zheng, Q.-Y.; Wang, M.-X. J. Org. Chem. 2006,
71, 9532.
(16) Wang, M.-X.; Liu, C.-S.; Li, J.-S. Tetrahedron: Asymmetry 2001,
12, 3367.
In the nitrile biotransformations depicted in Table 1, no
corresponding trans-3-aryl-1-methylaziridine-2-carboxylic acids
3 were isolated because they were not stable under the reaction
conditions. Interestingly, acids 3b and 3c, which contain an
electron-donating group such as a methoxy or a methyl group
on the benzene ring, underwent spontaneous decomposition to
give mainly aromatic aldehyde compounds. Decomposition of
other acids, however, gave rise to an inseparable mixture.
Lyophilization of the reaction media followed by the treatment
with CH2N2 and column chromatography led to the isolation
(17) (a) Wang, M.-X.; Feng, G.-Q. New J. Chem. 2002, 1575. (b) Wang,
M.-X.; Feng, G.-Q. J. Org. Chem. 2003, 68, 621. (c) Wang, M.-X.; Feng,
G. Q. J. Mol. Catal. B: Enzym. 2002, 18, 267. (d) Wang, M.-X.; Feng,
G.-Q.; Zheng, Q.-Y. Tetrahedron: Asymmetry 2004, 15, 347.
(18) Wang, M.-X.; Deng, G.; Wang, D.-X. Zheng, Q.-Y. J. Org. Chem.
2005, 70, 2439.
(19) Moran-Ramallal, R.; Liz, R.; Gotor, V. Org. Lett. 2007, 9, 521.
9392 J. Org. Chem., Vol. 72, No. 24, 2007

SCHEME 1. Dimerization of Aziridine-2-carboxylic Acid 3
SCHEME 2. Biotransformations of Racemic cis-3-Phenyl-
aziridine-2-carbonitriles
of compound 4. The formation of 4 is most likely due to the
dimerization of 3a (Scheme 1).
SCHEME 3. Enantioselection of the Amidase Involved in
Rhodococcus erythropolis AJ270
Since the amidase appeared more efficient and enantioselec-
tive than the nitrile hydratase in overall nitrile biotransforma-
tions, we then investigated the Rhodococcus erythropolis AJ270-
catalyzed kinetic resolution of racemic amides 2. As illustrated
in Table 2, except for the reaction of 4-methoxy-substituted
substrate 2c which gave virtually no enantiocontrol (entry 2,
Table 2), racemic amide 2a (entry 1, Table 2) and its analogues
2d-f (entries 3-5, Table 2) bearing a 4-substituent other than
methoxy on the benzene ring were efficiently resolved into
enantiopure compounds in almost quantitative yields. An amide
having a meta substituent on the benzene moiety was also
accepted as an excellent substrate by the amidase. This has been
exemplified by the highly efficient and enantioselective reaction
of 2h (entry 7, Table 2). The further move of the substituent to
the ortho position on the benzene ring resulted in a significant
drop of enantioselectivity (entry 6, Table 2). The racemic trans-
3-phenylaziridine-2-carboxamide, 2i, the substrate that is devoid
of a methyl group on the nitrogen of aziridine, acted as an
excellent substrate to afford enantiomerically pure 2R,3S-3-
phenylaziridine-2-carboxamide, 2i, in 47% yield (entry 8, Table
2). It is worth addressing that the biocatalytic reaction can be
scaled up; a gram-scale biotransformation of 2a, for example,
produced enantiopure 2R,3S-1-methyl-3-phenylaziridine-2-car-
boxamide in an excellent yield (entry 9, Table 2).
displayed low enantioselectivity. Being different from racemic
trans-3-phenylaziridine-2-carboxamides, the cis-configured amide
analogues (()-6a and (()-6b were not accepted by the
biocatalyst, giving no conversion of amide into acid in a lengthy
incubation time (4 days).
The outcomes of the current study are consistent with previous
observations that nitrile hydratases are a type of highly active
and less selective enzymes against a wide variety of nitrile
substrates.11c These properties of the nitrile hydratase, such as
having a broad substrate spectrum and possessing no or very
little enantioselectivity, are intrinsically determined by its
enzyme structure in which there is a spacious pocket near the
active site.17,18,20 In other words, a pair of enantiomers of
3-arylaziridine-2-carbonitriles are not recognized or differenti-
ated by the nitrile hydratase, and almost identical biocatalytic
hydration reactions were therefore effected.
Rhodococcus erythropolis AJ270 cells were also able to
catalyze the hydration reaction of racemic cis-3-phenylaziridine-
2-carbonitriles. For example, the ca. 50% conversion of nitriles
5a and 5b into the corresponding amides 6a and 6b was effected
readily within 1-2 h. On the basis of the enantiomeric purity
of the amide 6 and the nitrile 5 recovered, we found that the
nitrile hydratase involved in Rhodococcus erythropolis AJ270
The formation of highly enantiopure 2R,3S-3-arylaziridine-
2-carboxamides from the overall biotransformations of racemic
nitriles and from the kinetic resolution of racemic amide
substrates indicated again that the amidase involved in Rhodo-
coccus erythropolis AJ270 is an enantioselective enzyme.
Moreover, careful scrutiny of the stereochemistries of all amide
and acid products from the kinetic resolution of the correspond-
ing amides containing a three-membered ring such as cyclo-
propane,17 oxirane4,18 or aziridine5 reveals that the amidase is
able to recognize all carboxamides with a trans-arylated three-
membered ring in the same steric sense. In other words,
irrespective of the nature of the three-membered ring, all racemic
amides are kinetically resolved into the optically active amides
and acids by the amidase following the same chiral selection
model (Scheme 3). It fits well with the proposal4,5,17,18 that the
amidase in Rhodococcus erythropolis AJ270 might comprise a
deeply buried and highly steric-demanding active site.
TABLE 2. Biotransformations of Racemic trans-3-Arylaziridine-
2-carboxamides 2a
time
(h)
2R,3S-2
2R,3S-2
entry (()-2
R
Ar
yield (%)b
ee (%)c
1
2
3
4
5
6
7
8
9d
2a
2c
2d
2e
2f
2g
2h
2i
Me C6H5
0.33
1
2
0.42
1
3.5
3
48
40
48
49
48
50
46
47
48
>99.5
<5
>99.5
>99.5
>99.5
12
>99.5
>99.5
>99.5
Me 4-MeO-C6H4
Me 4-Br-C6H4
Me 4-F-C6H4
Me 4-Cl-C6H4
Me 2-Cl-C6H4
Me 3-Cl-C6H4
As a unique type of intermediate in organic synthesis, the
ring-opening reactions of aziridines have attracted much atten-
tion. Compared to the nonactivated and N-activated aziridine
compounds, however, the studies on the ring-opening reactions
of C-activated aziridine derivatives are very limited. Previously,
we5 and others19 have demonstrated that 1-arylaziridine-2-
H
C6H5
3
4.8
2a
Me C6H5
a The racemic amide 2 (1 mmol) was incubated with Rhodococcus
erythropolis AJ270 (2 g wet weight) in phosphate buffer (0.1 M, pH 7.0,
50 mL) at 30 °C. Reaction time was optimized to roughly 50% conversion
of the amide using HPLC analysis. b Isolated yield. c Determined by chiral
HPLC analysis. d Racemic amide 2a (12 mmol) was incubated with 6 g
wet weight of microbial cells.
(20) (a) Huang, W. J.; Jia, J.; Cummings, J.; Nelson, M.; Schneider, G.;
Lindqvist, Y. Structure 1997, 5, 691. (b) Nagashima, S.; Nakasako, M.;
Dohmae, N.; Tsujimura, M.; Takio, K.; Odaka, M.; Yohda, M.; Kamiya,
N.; Endo, I. Nat. Struct. Biol. 1998, 5, 347. (c) Song, L.; Wang, M.; Shi,
J.; Xue, Z.; Wang, M.-X.; Qian, S. Biochem. Biophys. Res. Commun. 2007,
362, 319.
J. Org. Chem, Vol. 72, No. 24, 2007 9393

SCHEME 4. Stereospecific Ring-Opening Reactions of
Enantiopure Amides
In summary, we have provided a highly efficient and
enantioselective synthesis of enantiopure 2R,3S-3-arylaziridine-
2-carboxamides from the biotransformations of racemic nitriles
and amides. While the nitrile hydratase exhibits low selectivity
against nitrile substrates, the amidase is highly enantioselective
toward 2S,3R-3-arylaziridine-2-carboxamides. The results have
expanded further application of nitrile and amide biotransfor-
mations in the synthesis of highly enantiopure carboxamides
bearing a three-membered ring. The resulting chiral aziridine-
2-carboxamides act as reactive and versatile intermediates to
undergo stereospecific ring-opening reactions to produce enan-
tiomerically pure R-amino, R,â-diamino, and R-amino-â-hy-
droxypropanamide derivatives in excellent yields.
Experimental Section
carboxamides underwent highly regioselective ring-opening
reactions at the 2-position with a number of N-nucleophiles.
Having had enantiopure trans-3-arylaziridine-2-carboxamides
(the typical C-activated aziridine derivatives) in hand, we
explored their ring-opening reactions with emphasis on regio-
and enantioselectivities. In the presence of Pd/C, 2a (ee >
99.5%) and 2i (ee > 99.5%) were hydrogenated readily to afford
almost quantitatively R-2-(methyl)amino-2-phenylpropanamide
7a (ee >99.5%) and 7i (ee 90%), respectively. The absolute
configuration was assigned on the basis of its optical rotation
in comparison with that of authentic sample.21 This has also
allowed us to assign the absolute configuration of the aziridine
product obtained from biotransformations. Being different from
1-arylaziridine-2-carboxamide in which the ring-opening reac-
tion occurred at the 2-position,5 2R,3S-3-phenyl-1-methylaziri-
dine-2-carboxamide, 2a, underwent a Brønsted acid-promoted
highly efficient and stereospecific ring-opening reaction at the
3-position with benzylamine and N-methyl(benzyl)amine to
afford exclusively 3-benzylamino- and 3-benzyl(methyl)amino-
2-methylamino-3-phenylpropanamides 9 and 10 in the yield of
89% and 84%, respectively (Scheme 4). Analogously, upon the
treatment with water in the presence of one equivalent of
trifluoroacetic acid (TFA), 2a was transformed into 2-methy-
lamino-3-hydroxy-3-phenylpropanamide 8 as the sole product
in good yield. The clear-cut change of stereospecificity of the
nucleophilic ring-opening reaction of aziridine-2-carboxamides
from the 2- to the 3-position after the introduction of a phenyl
into the 3-position originates most probably from the phenyl
substituent which renders the 3-carbon (benzylic carbon) of the
aziridine ring more electron-deficient than the 2-carbon. It is
noteworthy that, except for the hydrogenation of 2i, no
racemization of either starting aziridine or products takes place
in all of the ring-opening reactions (Scheme 4).
General Procedure for the Biotransformations of Nitriles or
Amides. To an Erlenmeyer flask (150 mL) with a screw cap was
added Rhodococcus erythropolis AJ270 cells13,14a (2 g wet weight)
and potassium phosphate buffer (0.1 M, pH 7.0, 50 mL), and the
resting cells were activated at 30 °C for 0.5 h with orbital shaking.
Racemic nitriles or amides (1 mmol) were added in one portion to
the flask, and the mixture was incubated at 30 °C using an orbital
shaker (200 rpm). The reaction, monitored by TLC and HPLC, was
quenched after a specified period of time by removing the biomass
through a Celite pad filtration. The resulting aqueous solution was
extracted with ether. After drying (MgSO4) and removing solvent
under vacuum, the residue of the organic phase was chromato-
graphed on a silica gel column using a mixture of petroleum ether
and ethyl acetate as the mobile phase to give pure amide product.
All products were fully characterized by spectroscopic data and
microanalyses. Enantiomeric excess values were obtained from
chiral HPLC analysis (see Supporting Information).
General Procedure for the Ring-Opening Reaction of (2R,-
3S)-(-)-1-Methyl-3-phenylaziridine-2-carboxamide 2a. To a
mixture of (2R,3S)-(-)-2a (0.5 mmol, ee > 99.5%) and CF3COOH
(0.5 mmol) in dry acetonitrile (10 mL) was added a nucleophilic
reagent (0.6 mmol), and the resulting mixture was refluxed. After
the starting material was consumed, which was monitored by TLC,
the mixture was cooled to room temperature. The solvent was then
removed under vacuum, and the residue was subjected to the basic
alumina column chromatography, eluting with a mixture of
methanol and ethyl acetate to give pure 8-10 (see Supporting
Information).
Acknowledgment. We thank the Ministry of Science and
Technology (2003CB716005, 2006CB806106), the National
Natural Science Foundation of China and Chinese Academy of
Sciences for financial support.
Supporting Information Available: Preparation of starting
nitriles and their spectroscopic data, full characterization of products,
1H and 13C NMR spectra of products, and HPLC analytic data for
all chiral products. This material is available free of charge via the
Internet at http://pubs.acs.org.
(21) Dallavalle, F.; Fisicaro, E.; Corradini, R.; Marchelli, R. HelV. Chim.
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(22) Ayi, A. I.; Guedj, R. J. Chem. Soc., Perkin Trans. 1 1983, 2045.
(23) Tack, J. W.; Lehmann, J.; Zymalkowski, F. Arch. Pharm. (Weinheim,
Ger.) 1979, 138.
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