Highly efficient and enantioselective biotransformations of racemic azetidine-2-carbonitriles and their synthetic applications

Article abstract of DOI:10.1021/jo9011656

(Chemical Equation Presented) Catalyzed by the Rhodococcus erythropolis AJ270 whole cell catalyst in neutral aqueous buffer at 30 °C, a number of racemic 1-benzylazetidine-2-carbonitriles, trans-1-benzyl-4-methylazetidine-2- carbonitrile, and 1-benzyl-2-m

Full text of DOI:10.1021/jo9011656

pubs.acs.org/joc
Highly Efficient and Enantioselective Biotransformations of Racemic
Azetidine-2-carbonitriles and Their Synthetic Applications
Dong-Hui Leng,† De-Xiang Wang,† Jie Pan,† Zhi-Tang Huang,† and Mei-Xiang Wang*,†,‡
†Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and
Function, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China, and ‡The Key
Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department
of Chemistry, Tsinghua University, Beijing 100084, China
mxwang@iccas.ac.cn
Received June 2, 2009
Catalyzed by the Rhodococcus erythropolis AJ270 whole cell catalyst in neutral aqueous buffer at
30 °C, a number of racemic 1-benzylazetidine-2-carbonitriles, trans-1-benzyl-4-methylazetidine-
2-carbonitrile, and 1-benzyl-2-methylazetidine-2-carbonitrile and their amide substrates underwent
efficient and enantioselective biotransformations to afford the corresponding azetidine-2-carboxylic
acids and their amide derivatives in excellent yields with ee up to >99.5%. The overall excellent
enantioselectivity of the biocatalytic reactions stemmed from a combined effect of a very active but
virtually nonenantioselective nitrile hydratase and a high R-enantioselective amidase involved in
microbial whole cells. The synthetic applications have been demonstrated by the nucleophilic ring-
opening reactions of azetidiniums of the resulting chiral azetidine-2-carbox amide derivatives for the
preparation of R,γ-diamino, R-phenoxy-γ-amino, and R-cyano-γ-amino carboxamides. The intra-
molecular CuI-catalyzed cross-coupling reaction for the synthesis of azetidine-fused 1,4-benzodia-
zepin-2-one derivative was also presented.
Introduction
derivatives have been shown to have numerous potential
applications in pharmaceutical and agrochemical fields.1
Although it has not been fully explored, azetidine-2-carboxylic
acid derivatives also provide valuable intermediates in organic
synthesis.2 Despite the importance, asymmetric synthesis
of optically active azetidine-2-caroxylic acid derivatives has
remained largely unexplored until recently. Most of the
enantiopure azetidine-2-carboxylic acid derivatives, for in-
stance, are obtained from multistep synthesis by using chiral
starting materials and chiral auxiliaries,3a-e whereas the
enantioenriched azetidine-2-carboxylic acid is obtained by
Azetidine-2-carboxylic acids and their derivatives are im-
portant entities in organic chemistry.1 As unique amino
acids, for example, azetidine-2-carboxylic acid structures
are found in natural products1 such as mugineic acid,
medicanine, and nocotianamine. Azetidine-2-carboxylic acid
(1) (a) For an overview, see: De Kimpe, N. Azetidines, Azetines, and Azetes:
Monocyclic in Comprehensive Heterocyclic Chemistry II; Katritzky, A. R.,
Rees, C. W., Scriven, E. F. V., Eds. (Padawa, A., volume Ed.); Pergamon:
New York, 1996; Vol. 1B, pp 507-536. (b) Shioiri, T.; Hamada, Y.; Matsuura,
F. Tetrahedron 1995, 14, 3939. (c) Miyakoshi, K.; Oshita, J.; Kitahara, T.
Tetrahderon 2001, 57, 3355 and references cited therein.
(2) (a) Vargas-Sanchez, M.; Lakhdar, S.; Couty, F.; Evano, G. Org. Lett.
2006, 8, 5501. (b) Couty, F.; Durrat, F.; Evano, G. Synlett 2005, 1666. (c)
Couty, F.; David, O.; Durrat, F.; Evano, G.; Lakhdar, S.; Marrot, J.; Vargas-
Sanchez, M. Eur. J. Org. Chem. 2006, 3479. (d) Couty, F.; David, O.;
Larmanjat, B.; Marrot, J. J. Org. Chem. 2007, 72, 1058. (e) Bott, T. M.;
Vanecko, J. A.; West, F. G. J. Org. Chem. 2009, 74, 2832.
(3) (a) Agami, C.; Couty, F.; Evano, G. Tetrahedron: Asymmetry 2002,
13, 297. (b) Couty, F.; Prim, D. Tetrahedron: Asymmetry 2002, 13, 2619. (c)
Couty, F.; Evano, G.; Rabasso, N. Tetrahedron: Asymmetry 2003, 14, 2407.
(d) Couty, F.; Evano, G.; Vargas-Sanchez, M.; Bouzas, G. J. Org. Chem.
2005, 70, 9028. (e) Futamura, Y.; Kurokawa, M.; Obata, R.; Nishyama, S.; Sugai,
T. Biosci. Biotechnol. Biochem. 2005, 69, 1892 and references cited therein.
DOI: 10.1021/jo9011656
r
Published on Web 07/23/2009
J. Org. Chem. 2009, 74, 6077–6082 6077
2009 American Chemical Society

Leng et al.
JOCArticle
optical resolution.4 Using the lipase (Novozym 453) from
Candida antarctica as a biocatalyst, Zwanenburg and co-
workers have reported that racemic methyl N-alkylazetidine-
2-carboxylates are resolved in an ammoniolysis reaction with
good enantioselectivity in tert-butyl alcohol at 35 °C.5 Ester-
ase-catalyzed hydrolysis of racemic N-substituted azetidine-2-
carboxylic acid esters has also been used to prepare optically
active N-substituted azetidine-2-carboxylic acid compounds
with moderate to good enantioselectivity.6
enantioselectively transform a variety of racemic nitriles into
highly enantiopure carboxylic acids and amides. Using the
highly enantioselective nitrile biotransformation approach,
many structurally diverse acids and amides that contain a
three-membered ring such as cyclopropane,12 epoxide,13 and
aziridine14 have been synthesized. Our interests in small ring
compounds12-14 and in exploration of nitrile biotransforma-
tions in organic synthesis led us to undertake the current
study. Herein, we report biotransformations of racemic
azetidine-2-carbonitriles, a highly efficient method for the
preparation of enantiopure azetidine-2-carboxylix acids and
amides, and their applications in synthesis.
Biotransformations of nitriles,7 either through a direct
conversion 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 production of carboxylic acids and
their amide derivatives. One of the well-known examples is
the industrial production of acrylamide from biocatalytic
hydration of acrylonitrile.8 Recent studies have demon-
strated that biotransformations of nitriles complement the
existing asymmetric chemical and enzymatic methods for the
synthesis of chiral carboxylic acids and their derivatives.9,10
One of the distinct features of enzymatic transformations of
nitriles is the straightforward generation of enantiopure
amides, valuable organo-nitrogen compounds in synthetic
chemistry, in addition to the formation of enantiopure
carboxylic acids. For example, we9c have shown that Rho-
dococcus erythropolis AJ270,11 a nitrile hydratase/amidase-
containing whole cell catalyst, is able to efficiently and
Results and Discussion
We initially examined the biotransformation of racemic
1-benzylazetidine-2-carbonitrile 1a. To facilitate the isola-
tion of product, acid 3a was converted into its methyl ester
with CH2N2 (Table 1). It was found that, catalyzed by
Rhodococcus erythropolis AJ270 whole cell catalyst in neu-
tral aqueous potassium phosphate buffer at 30 °C, hydrolysis
of 1 proceeded very efficiently and enantioselectively. Within
about 3.25 h, for example, enantiopure 1-benzylazetidien-2-
carboxamide S-2a and methyl 1-benzylazetidien-2-carbox-
ylate R-4a were obtained in good yield (entry 1, Table 1). To
study the effect of the substituent of benzyl on the reaction,
racemic nitrile analogues 1b-f were prepared according to a
literature precedure3a and subjected to biotransformations.
As indicated by Table 1, irrespective of the electronic nature
of the substituent on the benzyl, substrates 1b-e tested
underwent very rapid biotransformations to afford the
corresponding amides S-2b-e and esters R-4b-e in high
yields (entries 3-6, Table 1). Only when the substrate con-
tains a 1-(2-bromobenzyl) group did the biotransformation
of 1f proceed very sluggishly. Excellent yields of S-2f and
R-3f were achieved after 5 days of incubation of 1f with
microbial whole cell catalyst (entry 7, Table 1). It is note-
worthy that all nitrile biotransformations produced highly
enantiomerically enriched amide and acid products with an
enantiomeric ratio E15 being higher than 89, indicating
excellent enantioselectivity of nitrile biotransformations.
The biocatalytic reaction was also readily scaled up. This
has been demonstrated by a gram scale preparation of
enantiopure 1-benzylazetidien-2-carboxamide S-2a and
methyl 1-benzylazetidien-2-carboxylate R-4a in good yield
(entry 3, Table 1).
(4) Barth, P.; Pfenninger, A. U.S. Patent 6143903, 2000.
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J. M. M.; Zwanenburg, B. Tetrahedron: Asymmetry 1998, 9, 429.
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0974670A2, 2000.
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Rand, C. L.; Weiner, D. P.; Robertson, D. E.; Burk, M. J. J. Am. Chem. Soc.
2002, 124, 9024. (b) Effenberger, F.; Osswald, S. Tetrahedron: Asymmetry
2001, 12, 279. (c) Hann, E. C.; Sigmund, A. E.; Fager, S. K.; Cooling, F. B.;
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Hillmayer, K.; Klempier, N. Tetrahedron Lett. 2003, 44, 5057. (e) Yokoya-
ma, M.; Kashiwagi, M.; Iwasaki, M.; Fushuku, K.; Ohta, H.; Sugai, T.
Tetrahedron: Asymmetry 2004, 15, 2817. (f) Wang, M.-X.; Lu, G.; Ji, G.-J.;
Huang, Z.-T.; Meth-Cohn, O.; Colby, J. Tetrahedron: Asymmetry 2000, 11,
1123. (g) Wang, M.-X.; Li, J.-J.; Ji, G.-J.; Li, J.-S. J. Mol. Catal. B: Enzym.
2001, 14, 77. (h) Wang, M.-X.; Zhao, S.-M. Tetrahedron Lett. 2002, 43, 6617.
(i) Wang, M.-X.; Zhao, S.-M. Tetrahedron: Asymmetry 2002, 13, 1695. ( j)
Wang, M.-X.; Lin, S.-J. J. Org. Chem. 2002, 67, 6542. (k) Wang, M.-X.; Lin,
S.-J.; Liu, J.; Zheng, Q.-Y. Adv. Synth. Catal. 2004, 346, 439. (l) Wang, M.-
X.; Liu, J.; Wang, D.-X.; Zheng, Q.-Y. Tetrahedron: Asymmetry 2005, 16,
2409. (m) Gao, M.; Wang, D.-X.; Zheng, Q.-Y.; Huang, Z.-T.; Wang, M.-X.
J. Org. Chem. 2007, 72, 6060. (n) Wang, M.-X.; Wu, Y. Org. Biomol. Chem.
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Chem. 2006, 71, 9532. (p) Ma, D.-Y.; Zheng, Q.-Y.; Wang, D.-X.; Wang, M.-
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Wang, M.-X. Tetrahedron: Asymmetry 2006, 17, 2366. (r) Ma, D.-Y.; Wang,
D.-X.; Pan, J.; Huang, Z.-T.; Wang, M.-X. Tetrahedron: Asymmetry 2008,
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Org. Chem. 2008, 73, 4087.
To understand the catalytic efficiency and stereochemistry
of the nitrile hydratase and the amidase involved in Rhodo-
coccus erythropolis AJ270, kinetic resolution of nitrile
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Wang, M.-X.; Feng, G.-Q. New J. Chem. 2002, 1575. (c) Wang, M.-X.; Feng,
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Adv. Synth. Catal. 2003, 345, 695. (f) Wang, M.-X.; Feng, G.-Q.; Zheng, Q.-
Y. Tetrahedron: Asymmetry 2004, 15, 347. (g) Feng, G.-Q.; Wang, D.-X.;
Zheng, Q.-Y.; Wang, M.-X. Tetrahedron: Asymmetry 2006, 17, 2775.
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Chem. 2003, 68, 4570. (b) Wang, M.-X.; Deng, G.; Wang, D.-X.; Zheng, Q.-
Y. J. Org. Chem. 2005, 70, 2439.
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M.-X. J. Org. Chem. 2007, 72, 2040. (b) Wang, J.-Y.; Wang, D.-X.; Pan, J.;
Huang, Z.-T.; Wang, M.-X. J. Org. Chem. 2007, 72, 9391.
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Soc. 1982, 104, 7294. (b) Program “Selectivity” by: Faber, K.; Hoenig, H. http://
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Black, G. W.; O’Reilly, C. Antonie van Leeuwenhoek 2005, 87, 221.
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TABLE 1. Enantioselective Biotransformations of Racemic Azetidine-2-carbonitrilesa
S-2
R-4
entry
1
Ar
time (h)
yield (%)b
ee (%)c
yield (%)b
ee (%)c
Ed
1
2e
3f
4
5
6
1a
1a
1a
1b
1c
1d
1e
1f
C6H5
C6H5
C6H5
4-Me-C6H4
4-MeO-C6H4
4-Br-C6H4
3-Br-C6H4
2-Br-C6H4
3.25
9.5
12.25
4.7
43
46
42
45
45
43
42
42
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
>99.5
96.6
41
49
40
39
37
43
42
45
>99.5
>99.5
>99.5
>99.5
90.8
89.0
>99.5
>99.5
>200
>200
>200
>200
111
89
>200
>200
3
4.25
4.75
5d
7
8
aNitrile (1 mmol) was incubated with Rhodococcus erythropolis AJ270 cells (2 g wet weight) in potassium phosphate buffer (0.1 M, pH 7.0, 50 mL) at
30 °C. bIsolated yield. c Determined by chiral HPLC analysis. dCalculated following a literature method.15 eNitrile (2 mmol) was used. f Nitrile (13 mmol),
cells (6 g wet weight), and buffer (150 mL) were used.
TABLE 2. Biocatalytic Kinetic Resolution of Nitrilesa
hydration and of amide hydrolysis was studied by control-
ling the reaction time. As summarized in Table 2, biocatalytic
hydration of racemic nitriles 1a, 1d, and 1e was extremely
rapid, with ca. 50% conversion being achieved within
30 min. S-Nitrile and R-amide were obtained with very low
ee values. This indicated that the nitrile hydratase involved in
S-1
R-2
Rhodococcus erythropolis AJ270 is of very low R-enantio-
selectivity against azetidine-2-carbonitrile substrates. The
outcomes are consistent with previous observations9 that
the nitrile hydratases are a type of highly active but less
enantioselective enzyme against various nitrile substrates.
These properties of the nitrile hydratase, such as having a
broad substrate spectrum and possessing low or no enantio-
selectivity, are intrinsically determined by its enzyme struc-
ture in which there is a spacious pocket near the active
site.9,16 In other words, a pair of enantiomers of 1-arylmethy-
lazetidine-2-carbonitriles are hardly differentiated by the
nitrile hydratase, and almost identical biocatalytic hydration
reactions were effected.
In contrast to the nitrile hydratase-catalyzed nitrile hydra-
tion reaction, amidase-catalyzed kinetic resolution exhibited
excellent enantioselectivity. Biotransformation of racemic
amides 2a, 2e, and 2f afforded excellent yields of the corre-
sponding amide S-2 and ester R-4 of high enantiopurity
(Table 3). These results are also in agreement with the
conclusion that the amidase within Rhodococcus erythropolis
AJ270 microbial cell is highly enantioselective.9,12-14 It can
be concluded that the overall excellent enantioselectivity of
the nitrile biotransformations summarized in Table 1 origi-
nates from the combined effect of a virtually nonenantio-
selective nitrile hydratase and a high R-enantioselective
amidase, with the latter being the dominant one.
entry
1
Ar
time yield (%)b ee (%)c yield (%)b ee (%)c
1
2
3
1a C6H5
1d 4-Br-C6H4 3 min
1e 3-Br-C6H4 30 min
20 s
43
57
42
13.0
2.6
3.4
55
42
45
4.9
6.4
1.2
aNitrile (1 mmol) was incubated with Rhodococcus erythropolis AJ270
cells (2 g wet weight) in potassium phosphate buffer (pH 7.0) at 30 °C.
bIsolated yield. cDetermined by chiral HPLC analysis.
products was obtained by comparing the optical rotation of
R-4b ([R]25 þ88 (c 0.5, CHCl3)) with that of S-4b ([R]25
D
D
-84 (c 0.5, CHCl3), which was obtained from the chemical
transfromation of S-2b (Scheme 1).
Encouraged by the successful enantioselective biotrans-
formations of 1-benzylazetidine-2-carbonitrile substrates,
we attempted the hydrolysis of trans- and cis-1-benzyl-4-
methylazetidine-2-carbonitriles.17 Under the identical bio-
catalytic conditions, the racemic trans-1-benzyl-4-methyl-
azetidine-2-carbonitrile (()-5 underwent efficient nitrile
hydratase-catalyzed hydration reaction to afford amide
(()-6. Around 50% conversion, both nitrile recovered and
amide product isolated were nearly racemic (Scheme 2),
indicating again that the nitrile hydratase is almost non-
enantioselective. As expected, the amidase involved in Rho-
dococcus erythropolis AJ270 catalyzed enantioselective hy-
drolysis of racemic amide (()-6 to produce very good yields
of highly enantioenriched (-)-1-benzyl-4-methylazetidine-
2-carboxamide (-)-6 and (þ)-1-benzyl-4-methylazetidine-
2-carboxylic acid. The latter was converted into the ester
The structures of all products were established on the basis
of spectroscopic data and microanalysis (see the Supporting
Information). The absolute configuration of amide products
was determined unambiguously by the single crystal X-ray
diffraction analysis of enantiopure S-2d (see the Supporting
Information), while the absolute configuration of the ester
(17) The assignment of configurations of trans- and cis-1-benzyl-4-
methylazetidine-2-carbonitriles was based on the comparison of their 13C
NMR spectroscopic data with that of the methyl ester of trans- and cis-1-
benzyl-4-methylazetidine-2-carboxylic acids. Kingsbury, C. A.; Soriano, D.
S.; Podraza, K. F.; Cromwell, N. H. J. Heterocycl. Chem. 1982, 19, 889.
(16) Song, L.; Wang, M; Shi, J.; Xue, Z.; Wang, M.-X.; Qian, S. Biochem.
Biophys. Res. Commun. 2007, 362, 319.
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TABLE 3. Biocatalytic Kinetic Resolution of Amidesa
S-2
R-4
entry
2
Ar
C6H5
3-Br-C6H4
2-Br-C6H4
time
yield (%)b
ee (%)c
yield (%)b
ee (%)c
Ed
1
2
3
2a
2e
2f
3.7 h
6.3 h
5 d
49
47
46
>99.5
93.8
96.6
40
49
49
>99.5
>99.5
>99.5
>200
>200
>200
aAmide (1 mmol) was incubated with Rhodococcus erythropolis AJ270 cells (2 g wet weight) in potassium phosphate buffer (pH 7.0) at 30 °C. bIsolated
yield. cDetermined by chiral HPLC analysis. dCalculated following a literature method.15
SCHEME 1. Chemical Transformation of Amide S-2b into
Ester S-4b
SCHEME 2. Enantioselective Biotransformations of Racemic
trans-1-Benzyl-4-methylazetidine-2-carbonitrile 5 and Amide 6a
product (þ)-7. The highly enantiopure amide (-)-6 and ester
(þ)-7 were prepared similarly from the biotransformations
of racemic nitrile (()-5 (Scheme 2). In contrast to trans-
1-benzyl-4-methylazetidine-2-carbonitrile (()-5, cis-1-ben-
zyl-4-methylazetidine-2-carbonitrile was not a good sub-
strate at all for the nitrile hydratase within Rhodococcus
erythropolis AJ270. For example, the starting nitrile was
recovered almost quantitatively after 7 days of interaction
with microbial cells. The dramatic difference in the nitrile
hydration reaction between trans- and cis-1-benzyl-4-methyl-
azatidine-2-carbonitriles suggested that the pocket of active
site of the nitrile hydratase is also size- or shape-selective
against nitrile substrate. It is capable of recognizing and
transforming trans-1-benzyl-4-methylazetidine-2-carbonitrile
rather than the cis-isomer.
When a methyl group was introduced into the 2-position
of 1-benzylazetidine-2-carbonitirle, the resulting quatern-
ary-carbon-containing nitrile (()-8 was also accepted by
the biocatalyst as a fairly good substrate. Interaction of
(()-8 with Rhodococcus erythropolis AJ270 for 31 h led to
the isolation of enantioenriched (-)-1-benzyl-2-methylaza-
tidine-2-carboxamide (-)-9 (ee 80%) and methyl (þ)-1-
benzyl-2-methylazetidine-2-carboxylate (þ)-10 (ee 91%) in
44% and 30% yields, respectively. Optically inactive starting
nitrile was also recovered in 16% yield (Scheme 3). To
prepare highly enantiopure (-)-1-benzyl-2-methylazati-
dine-2-carboxamide (-)-9 and methyl (þ)-1-benzyl-2-
methylazetidine-2-carboxylate (þ)-10, kinetic resolution of
racemic amide (()-9 was carried out. By controlling the
conversion, both (-)-9 and (þ)-10 were obtained with ee up
to 92% (Scheme 3). It is noteworthy that the amidase-
catalyzed hydrolysis of racemic amides (()-6 and (()-9
proceeded much slower than the reaction of racemic amide
(()-2a. This implies that the amidase is very sensitive to the
steric effect of the substrate. One more methyl group intro-
duced into the azetidine ring increased the bulkiness of the
substrate, and led therefore to a diminished reaction rate.
aSubstrate (2 mmol) was incubated with Rhodococcus erythropolis AJ270
cells (2 g wet weight) in potassium phosphate buffer (pH 7.0) at 30 °C. The
2
acid product was converted into methyl ester with CH N2.
SCHEME 3. Enantioselective Biotransformations of Racemic
1-Benzyl-2-methylazetidine-2-carbonitrile 8 and Amide 9a
aSubstrate (2 mmol) was incubated with Rhodococcus erythropolis AJ270
cells (2 g wet weight) in potassium phosphate buffer (pH 7.0) at 30 °C. The
2
acid product was converted into methyl ester with CH N2. For the biotrans-
formation of (()-8, 16% of starting nitrile (ee 0%) was recovered.
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Leng et al.
JOCArticle
SCHEME 4. Nucleophilic Ring-Opening Reactions of Azetidi-
nium Salts 11
Conclusion
In summary, we have developed an efficient nitrile bio-
transformation method for the synthesis of highly enantio-
pure azetidine-2-carboxylic acids and amide derivatives. The
overall excellent enantioselectivity of the biocatalytic reac-
tions stemmed from an active but virtually nonenantioselec-
tive nitrile hydratase and a high R-enantioselective amidase
involved in Rhodococcus erythropolis AJ270 whole cell cat-
alyst. We have shown that the resulting azetidine-2-car-
boxylic acid derivatives are useful chiral compounds for
the synthesis of polyfunctionalized organic compounds such
as R,γ-diamino, R-phenoxy-γ-amino, and R-cyano-γ-amino
carboxamides via the regioselective nucleophilic ring-open-
ing reactions of azetidinium intermediates. Synthetic appli-
cation was also demonstrated by the preparation of an
azetidine-fused 1,4-benzodiazepin-2-one derivative from
CuI-catalyzed intramolecular cross-coupling reaction.
Further study of enantioselective biotransformations of
small heterocyclic nitriles and their synthetic applications is
actively pursued in this laboratory.
SCHEME 5. Synthesis of S-1,2,8,10a-Hetrahydroazeto[1,2-
a]benzo[e][1,4]diazepin-10(4H)-one 15
Experimental Section
General Procedure for the Biotransformation of Nitriles and
Amides. To an Erlenmeyer flask (150 mL) with a screw cap were
added Rhodococcus erythropolis AJ270 cells11 (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 amide (see Tables 1-3) were added
in one portion to the flask, and the mixture was incubated at
30 °C, using an orbital shaker (200 rpm). The reaction, mon-
itored by TLC and HPLC, was quenched after a specified period
of time (see Tables 1-3) by removing the biomass through a
Celite pad filtration. The resulting aqueous solution was
extracted with ethyl acetate. After drying (Na2SO4) and remov-
ing the solvent under a vacuum, the residue of the organic phase
was chromatographed on a silica gel column with a mixture of
petroleum ether and ethyl acetate as the mobile phase to give
pure amide product 2 and recovered nitrile 1. The aqueous phase
was freeze-dried (-50 to -60 °C), and the residue was treated
with CH2N2 in ether below -15 °C. When finished, the reaction
was quenched by some water and then extracted with ethyl
acetate. After drying (Na2SO4) and removing the solvent under
a vacuum, the pure methyl ester 4 was left. In a multigram scale
reaction, racemic nitrile 1a (2.24 g, 13 mmol) was converted into
S-2a and R-4a.
(S)-(-)-1-Benzylazetidine-2-carboxamide (S-2a): 1.03 g, 42%;
white solid; mp 93.0-94.0 °C; IR (KBr) ν 3379, 3265, 3158,
1682, 1652 cm-1; [R]25D -104 (c 0.5, CHCl3); ee >99.5% (chiral
HPLC analysis); 1H NMR (300 MHz, CDCl3) δ 7.25-7.35 (m,
5H), 7.00 (s, 1H), 5.43 (s, 1H), 3.72 (d, J=12.7 Hz, 1H), 3.68 (t,
J=8.6 Hz, 1H), 3.57 (d, J=12.8 Hz, 1H), 3.37 (m, 1H), 3.01 (q,
J=16.3 Hz, 1H), 2.37-2.47 (m, 1H), 2.11-2.23 (m, 1H); 13C
NMR(75 MHz, CDCl3) δ 176.2, 137.3, 128.6, 128.6, 127.4, 66.1,
62.3, 50.8, 23.0; MS (ESI) m/z (%) 191 [M þ H]þ. Anal. Calcd
for C11H14N2O: C, 69.45; H, 7.42; N, 14.73. Found: C, 69.10; H,
7.45; N, 14.55.
Enantiopure azetidine-2-carboxylic acids and their deri-
vatives obtained from nitrile biotransformations are useful
chiral intermediates in organic synthesis. To explore their
synthetic applications, we studied the ring-opening reaction
of 1-benzylazetidine-2-carboxamide S-2a and 1-benzyl-2-
methylazetidine-2-carboxamide (-)-9. Being different from
aziridine-2-carboxylic acid derivatives which undergo ring-
opening reactions readily, azetidine-2-carboxamides are
stable. To increase the reactivity of ring-opening reactions,
azetidine-2-carboxamides were first converted into azetidi-
nium species 11 simply by treatment with methyl trifluoro-
methanesulfonate. The azetidinium intermediates 11 are
reactive toward nucleophiles under mild conditions. For
example, sodium azide attacked regioselectively at the
2-position of 11a in THF at room temperature to afford
the ring-opening reaction product in excellent yield. The
subsequent catalytic hydrogenation of the azido group led to
the quantitative formation of R,γ-diamino butyramide deri-
vative R-12. In refluxing THF, azetidinium 11b underwent
similar regionselective nucleophilic ring-opening reactions
with potassium cyanide and sodium phenoxide, respectively,
to yield quaternary carbon-containing polyfunctionalized
carboxamides R-13 and R-14 (Scheme 4). It is worth
noting that all reactions gave configuration inversion pro-
ducts without racemerization. Further synthetic application
of chiral azetidine-2-carboxamides was demonstrated by
the preparation of azetidine-fused heterocyclic product.
S-1,2,8,10a-Hetrahydroazeto[1,2-a]benzo[e][1,4]diazepin-10-
(4H)-one 15 (94.1% ee), a unique 1,4-benzodiazepin-2-one
analogue, was obtained in good yield from intramolecular
cross-coupling reaction of S-2f (ee 92.6%) catalyzed by
CuI/N,N-dimethylglycine (DMGC) under basic conditions
(Scheme 5).
(R)-(þ)-Methyl 1-benzylazetidine-2-carboxylate (R-4a): 1.07 g,
40%; oil; IR (KBr) ν 1743 cm-1; [R]25D þ96 (c 0.5, CHCl3), þ118
(c 1.0 CH2Cl2) [lit.3a þ125 (c 1.0 CH2Cl2)]; ee >99.5% (chiral
HPLC analysis); 1H NMR (300 MHz, CDCl3) δ 7.23-7.33 (m,
5H), 3.80 (d, J=12.6 Hz, 1H), 3.74 (t, J = 8.4 Hz, 1H), 3.64 (s,
3H), 3.58 (d, J=12.6 Hz, 1H), 3.29-3.35 (m, 1H), 2.91-2.98 (m,
1H), 2.31-2.43 (m, 1H), 2.17-2.25 (m, 1H); 13C NMR (75 MHz,
CDCl3) δ 173.1,137.0,129.0, 128.3, 127.2, 64.3, 62.3, 51.7,
50.8, 21.6; MS (ESI) m/z (%) 206 [MþH]þ . Anal. Calcd for
J. Org. Chem. Vol. 74, No. 16, 2009 6081

Leng et al.
JOCArticle
C11H15NO2: C, 70.22; H, 7.37; N, 6.82. Found: C, 70.46; H, 7.58;
N, 6.85.
silica gel (100-200 mesh) pad. The filtrate was concentrated and
the residue was filtered with a silica gel column (100-200 mesh)
and washed by ethyl acetate to give a crude product of 15. The
second silica gel (200-300 mesh) column chromatography
eluted with a mixture of petroleum ether and ethyl acetate
(1:1) to give pure product 15 (152 mg, 81%) as white solids:
General Procedure for the Formation of Azetidinium Trifluor-
omethanesulfonate 11. Methyl trifluoromethanesulfonate
(0.45 mL, 4 mmol) was added dropwise to a solution of chiral
azetidine (2 mmol) in DCM (10 mL) at 0 °C. The reaction mixture
was stirred at room temperature for 1 h. After the solvent was
evaporated, the resulting residue was washed with a small amount
of dry diethyl ether and dried under vacuum to afford pure
azetidinium trifluoromethanesulfonate 11.
General Procedure for the Ring-Opening Reactions of Azeti-
dinium Salts. The nucleophilic reagent (5 mmol) was added to a
solution of azetidinium trifluoromethanesulfonate 11 (1 mmol)
in THF (5 mL). The suspension was stirred at room temperature
for 12 h or refluxed overnight. Water (5 mL) was added and the
resulting aqueous solution was extracted with ethyl acetate.
After the solution was dried (Na2SO4) and the solvent was
removed under vacuum, the residue of the organic phase was
chromatographed on a silica gel column, using a mixture of
petroleum ether and ethyl acetate (1:1) as the mobile phase, to
give pure product 12-14.
mp 156.0-157.0 °C; IR (KBr) ν 3220, 1674, 1635 cm-1; [R]25
D
þ28 (c 0.5, CHCl3); ee 94.1% (chiral HPLC analysis); 1H NMR
(300 MHz, CDCl3) δ 8.14 (s, 1H), 7.27-7.32 (m, 2H), 7.16 (t,
J=7.3 Hz, 1H), 7.00 (d, J=7.6 Hz, 1H), 4.06 (dd, J = 2.6 Hz, 7.8
Hz, 1H), 3.74 (d, J=10.9 Hz, 1H), 3.67 (d, J=10.9 Hz, 1H),
3.44-3.52 (m, 2H), 2.61-2.67 (m, 1H), 2.36-2.46 (m, 1H); 13
C
NMR (75 MHz, CDCl3) δ 172.8, 136.8, 130.3, 129.9, 128.7,
125.7, 121.5, 64.2, 56.7, 54.7, 30.8, 19.5; MS (ESI) m/z (%) 189
[M þ H]þ. Anal. Calcd for C11H12N2O: C, 70.19; H, 6.43; N,
14.88. Found: C, 69.86; H, 6.41; N, 15.02.
Acknowledgment. We thank the Natural Science Founda-
tion of China (20772132), Ministry of Science and Technol-
ogy (2009CB724704, 2006CB806106), and Chinese
Academy of Sciences for financial support.
Procedure for the Synthesis of (S)-1,2,8,10a-Hetrahydroazeto-
[1,2-a]benzo[e][1,4]diazepin-10(4H)-one 15. Under argon protec-
tion, a mixture of substrate S-2f (1 mmol), CuI (0.4 mmol,
76 mg), N,N-dimethylglycine hydrochloride (0.8 mmol, 112
mg), Cs2CO3 (2 mmol, 650 mg), and dry 1,4-dioxane (34 mL)
was refluxed for 3 h. After cooling, ethyl acetate (100 mL) was
added and the resulting mixture was filtrated through a short
Supporting Information Available: Characterization of
products, 1H and 13C NMR spectra of products, and X-ray
structure of S-2d (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
6082 J. Org. Chem. Vol. 74, No. 16, 2009