Chemical and enzymatic synthesis of 2-(2-carbamoylethyl)- and 2-(2-carboxyethyl)aziridines and their conversion into δ-lactams and γ-lactones

Article abstract of DOI:10.1021/ol202888j

Treatment of 1-arylmethyl-2-(2-cyanoethyl)aziridines with a nitrile hydratase afforded the corresponding 2-(2-carbamoylethyl)aziridines, which underwent rearrangement into 5-hydroxypiperidin-2-ones upon heating under microwave irradiation. In addition, tr

Full text of DOI:10.1021/ol202888j

ORGANIC
LETTERS
2012
Vol. 14, No. 1
106–109
Chemical and Enzymatic Synthesis of
2-(2-Carbamoylethyl)- and
2-(2-Carboxyethyl)aziridines and
Their Conversion into δ-Lactams and
γ-Lactones
Karel Vervisch,† Matthias D’hooghe,*,† Floris P. J. T. Rutjes,‡ and Norbert De Kimpe*,†
Department of Sustainable Organic Chemistry and Technology, Faculty of Bioscience
Engineering, Ghent University, Coupure Links 653, B-9000 Ghent, Belgium, and
Institute for Molecules and Materials, Radboud University Nijmegen, Heyendaalseweg
135, NL-6525 AJ Nijmegen, The Netherlands
matthias.dhooghe@UGent.be; norbert.dekimpe@UGent.be
Received October 27, 2011
ABSTRACT
Treatment of 1-arylmethyl-2-(2-cyanoethyl)aziridines with a nitrile hydratase afforded the corresponding 2-(2-carbamoylethyl)aziridines, which
underwent rearrangement into 5-hydroxypiperidin-2-ones upon heating under microwave irradiation. In addition, treatment of 2-(2-
cyanoethyl)aziridines with a nitrilase selectively afforded 5-hydroxypiperidin-2-ones in good yields. On the other hand, chemical hydrolysis of
2-(2-cyanoethyl)aziridines using KOH in EtOH/H2O furnished the corresponding potassium 3-(aziridin-2-yl)propanoates, which, upon acidification
with acetic acid, smoothly rearranged into 4-(aminomethyl)butyrolactones.
. Amino nitriles comprise an interesting class of com-
pounds, from both a chemical and a biological point of
view. These compounds possess important biological
properties, and several drugs containing an amino nitrile
moiety are currently on the market or under study in
advanced clinical trials.1
However, more importantly, amino nitriles are known
to be excellent precursors for the synthesis of the corre-
sponding amino acids and their derivatives, and there-
fore, they are mostly used to this end. In the past,
extensive research has been conducted on the hydrolysis
of amino nitriles applying both chemical and enzymatic
methods.2 Enzymes play a key role in the synthesis of
amino acids, allowing the development of clean chemo-,
regio-, and stereoselective processes under mild condi-
tions in aqueous medium and neutral pH.3 The enzyme-
catalyzed hydrolysis of nitriles is known to occur via
two different pathways to afford either amides or acids
depending on the type of enzyme used. Nitrilases con-
vert nitriles directly to the corresponding carboxylic
acids without the formation of intermediate amides,
(1) Fleming, F. F.; Yao, L.; Ravikumar, P. C.; Funk, L.; Shook, B. C.
J. Med. Chem. 2010, 53, 7902.
(2) For a few examples, see: (a) Klempier, N.; Winkler, M. In Modern
Biocatalysis: Stereoselective and Environmentally Friendly Reactions;
Fessner, W.-D., Anthonsen, T., Eds.; Wiley-VCH: Weinheim, Germany,
2009; p 247. (b) Sugai, T.; Yamazaki, T.; Yokoyama, M.; Ohta, H.
Biosci. Biotechnol. Biochem. 1997, 61, 1419. (c) Fitz, M.; Lundell, K.;
Lindroos, M.; Fulop, F.; Kanerva, L. T. Tetrahedron: Asymmetry 2005,
16, 3690. (d) Winkler, M.; Martinkova, L.; Knall, A. C.; Krahulec, S.;
Klempier, N. Tetrahedron 2005, 61, 4249. (e) Weber, K.; Kuklinski, S.;
Gmeiner, P. Org. Lett. 2000, 2, 647. (f) Thomas, C.; Orecher, F.;
Gmeiner, P. Synthesis 1998, 1491. (g) Gmeiner, P. Tetrahedron Lett.
1990, 31, 5717.
(3) Busto, E.; Gotor-Fernandez, V.; Gotor, V. Chem. Rev. 2011,111, 3998.
r
10.1021/ol202888j
Published on Web 12/09/2011
2011 American Chemical Society

whereas the combination of nitrile hydratases (NHase)
with amidases first transforms the nitriles toward amides,
followed by the hydrolysis of these amides into carboxylic
acids.8a,b,9
In this paper, both the chemical and enzymatic hydro-
lysis of 1-arylmethyl-2-(2-cyanoethyl)aziridines is inves-
tigated, providing the first synthesis of 3-(aziridin-2-
yl)propionamides by using nitrile hydratases and the
formation of potassium 3-(aziridin-2-yl)propanoates us-
ing potassium hydroxide in EtOH/H2O. These functio-
nalized aziridines were further elaborated into heterocy-
clic systems such as δ-lactams and γ-lactones.
reaction mixtures, in which the target compounds 3-
(aziridin-2-yl)propionamides 2 were present in low yields
(15À45%), due to their further hydrolysis toward 3-
(aziridin-2-yl)propanoates 3 (15À70%), 5-hydroxypiper
idin-2-ones 4 (0À33%), and 4-(aminomethyl)butyrolac-
tones 5 (0À14%) (Table 1). To conclude, treatment of 2-(2-
cyanoethyl)aziridines 1 with KOH in EtOH/H2O for 3 or
6 h results in the formation of a set of reaction products,
rendering this an inefficient route for the synthesis of 2.
Table 1. Hydrolysis of 2-(2-Cyanoethyl)aziridines 1
In continuation of our interest in 2-(ω-cyanoalkyl)-
aziridines as building blocks for the preparation of a wide
variety of amino nitriles4 and amino acid deriva-
tives,5 a new strategy was explored in this work. All
previously described methodologies were based on the
initial ring opening of the aziridine moiety, followed by
transformation of the nitrile functionality into amides,
acids, or amines. In this paper, the reverse strategy is
contemplated, involving initial hydrolysis of the cyano
group without affecting the strained three-membered ring
system. This approach would thus provide a useful entry
into functionalized aziridines such as 2-(ω-carbamoylal-
kyl)- and 2-(ω-carboxyalkyl)aziridines as suitable sub-
strates for further elaboration.
Since aziridines are sensitive to ring opening upon
treatment with acids,6,10 no acid-catalyzed hydrolysis of
the nitrile moiety in 2-(ω-cyanoalkyl)aziridines can be
employed to provide an entry into the corresponding
amides, leaving basic or enzymatic hydrolysis as the only
options. At first, 1-arylmethyl-2-(2-cyanoethyl)aziridines
1, prepared from 2-(bromomethyl)aziridines7 by treat-
ment with R-lithiated trimethylsilylacetonitrile in THF
according to a literature protocol,10 were treated with 5
equiv of KOH in EtOH/H2O (3/1) and heated under
reflux for 3À6 h. These experiments resulted in complex
1 (Ar)
1a (C6H5)
time (h)
2 (%)
3 (%)a
4 (%)a
5 (%)a
6
6
3
6
33
16
45
42
56
70
15
50
12
0
0
14
7
1b (4-MeC6H4)
1c (2-ClC6H4)
1d (2-MeOC6H4)
33
0
8
a Based on 1H NMR and/or LC of the crude reaction mixture.
Nitrile-hydrolyzing enzymes have proven to be excel-
lent tools in organic synthesis and industrial processes
and have been reviewed extensively in the past.8 The main
advantages of enzymatic approaches over chemical trans-
formations are related to the fact that no harsh conditions,
such as the use of concentrated base or acid and elevated
temperatures, are required and the fact that enzymes often
display high levels of chemo- and enantioselectivity. For
instance, nitrile hydratases can selectively transform nitriles
to the corresponding primary amides at neutral pH and
ambient temperature, whereas chemical approaches often
rely on the use of acid conditions and heating under reflux.
Only a few studies have been conducted on the enzymatic
hydrolysis of nitriles containing an aziridine moiety, all
related to the use of 2-cyanoaziridines.9 These reports
showed a good nitrile hydratase activity, and due to the
presence of a highly enantioselective amidase in the whole
cells, one enantiomer was further converted to the corre-
sponding acid, thus lowering the yield of the primary amide.
Up to now, no reports are available dealing with the
selective hydrolysis of aziridinyl nitriles toward the
(4) (a) D’hooghe, M.; Vervisch, K.; Van Nieuwenhove, A.; De
Kimpe, N. TetrahedronLett. 2007, 48, 1771. (b) D’hooghe, M.; Vervisch,
K.; De Kimpe, N. J. Org. Chem. 2007, 72, 7329. (c) Catak, S.; D’hooghe,
M.; Verstraelen, T.; Hemelsoet, K.; Van Nieuwenhove, A.; Ha, H. J.;
Waroquier, M.; De Kimpe, N.; Van Speybroeck, V. J. Org. Chem. 2010,
75, 4530. (d) D’hooghe, M.; Van Nieuwenhove, A.; Van Brabandt, W.;
Rottiers, M.; De Kimpe, N. Tetrahedron 2008, 64, 1064.
€
(5) (a) Vervisch, K.; D’hooghe, M.; Tornroos, K. W.; De Kimpe, N.
€
J. Org. Chem. 2010, 75, 7734. (b) Vervisch, K.; D’hooghe, M.; Tornroos,
K. W.; De Kimpe, N. Org. Biomol. Chem. 2009, 7, 3271.
(6) (a) D’hooghe, M.; Kenis, S.; Vervisch, K.; Lategan, C.; Smith,
P. J.; Chibale, K.; De Kimpe, N. Eur. J. Med. Chem. 2011, 46, 579. (b)
Katagiri, T.; Takahashi, M.; Fujiwara, Y.; Ihara, H.; Uneyama, K. J.
Org. Chem. 1999, 64, 7323. (c) Disadee, W.; Ishikawa, T. J. Org. Chem.
2005, 70, 9399.
(7) (a) De Kimpe, N.; Jolie, R.; De Smaele, D. J. Chem. Soc., Chem
Commun. 1994, 1221. (b) D’hooghe, M.; Waterinckx, A.; De Kimpe, N.
J. Org. Chem. 2005, 70, 227. (c) De Kimpe, N.; De Smaele, D.; Szakonyi,
Z. J. Org. Chem. 1997, 62, 2448.
(8) (a) Banerjee, A.; Sharma, R.; Banerjee, U. C. Appl. Microbiol.
Biotechnol. 2002, 60, 33. (b) Davis, B. G.; Borer, V. Nat. Prod. Rep. 2001,
18, 618. (c) Martinkova, L.; Kren, V. Biocatal. Biotransform. 2002, 20,
73. (d) O’Reilly, C.; Turner, P. D. J. Appl. Microbiol. 2003, 95, 1161. (e)
DiCosimo, R. Nitrilases and Nitrile Hydratases. Biocatalysis in the
Pharmaceutical and Biotechnology Industries; CRC Press: Boca Raton,
FL, 2007; p 1. (f) Kobayashi, M.; Nagasawa, T.; Yamada, H. Trends
Biotechnol. 1992, 10, 402. (g) Shaw, N. M.; Robins, K. T.; Kiener, A.
Adv. Synth. Catal. 2003, 345, 425. (h) Thomas, S. M.; DiCosimo, R.;
Nagarajan, A. Trends Biotechnol. 2002, 20, 238.
(9) (a) Moran-Ramallal, R.; Liz, R.; Gotor, V. Org. Lett. 2007, 9,
521. (b) Moran-Ramallal, R.; Liz, R.; Gotor, V. J. Org. Chem. 2010, 75,
6614. (c) Wang, J.-Y.; Wang, D.-X.; Pan, J.; Huang, Z.-T.; Wang, M.-X.
J. Org. Chem. 2007, 72, 9391. (d) Wang, J.-Y.; Wang, D.-X.; Zheng, Q.-
Y.; Huang, Z.-T.; Wang, M.-X. J. Org. Chem. 2007, 72, 2040.
(10) Vink, M. K. S.; Wijtmans, R.; Reisinger, C.; Van den Berg,
R. J. F.; Schortinghuis, C. A.; Schwab, H.; Schoemaker, H. E.; Rutjes, F.
P. J. T. Biotechnol. J. 2006, 1, 569.
Org. Lett., Vol. 14, No. 1, 2012
107

corresponding amides, and also the enzymatic hydrolysis
of 2-(ω-cyanoalkyl)aziridines has not been described in
the literature so far.
the solvent. The synthesis of 4 proceeds via 5-exo-trig ring
closure of the aziridine nitrogen atom across the amido
group, thus creating a bicyclic aziridinium intermediate
6 which subsequently undergoes a regiospecific ring
opening at the more substituted carbon atom by means
of water. The spectral data of piperidinone 4a were
compared to those reported in the literature,12 which
unambiguously assigned 1-arylmethyl-5-hydroxypiper-
idin-2-ones 4 as the sole reaction products in this trans-
formation. In order to provide a synthetically useful
process, the applicability of microwave irradiation was
investigated. Thus, aziridinyl amides 2b,d,e were sub-
jected to microwave irradiation in acetonitrile for 3 h
to 5 h 30 min at 130 °C, resulting in the formation of
piperidinones 4b,d,e in 71À94% yield.
The bacterial strain of Rhodococcus erythropolis
NCIMB 11540 has already been shown to possess a good
NHase activity for the synthesis of a variety of aliphatic
primary amides.10 In this paper, the cell-free extract of R.
erythropolis NCIMB 11540 nitrile hydratase was used for
the first synthesis of stable 2 starting from 1. The latter
amino nitriles 1 were dissolved in DMSO and added to a
KH2PO4/K2HPO4 buffered NHase solution, after which
the reaction mixture was incubated at 30 °C and 450 rpm
for 2 h, affording 2 in excellent yields as the sole reaction
products (Scheme 1). The only report describing primary
3-(aziridin-2-yl)propionamides comprised the synthesis
of an unstable N-Boc-protected aziridine which was not
characterized due to insufficient data.11 In this paper, the
first and highly efficient conversion of aziridinyl nitriles
toward the corresponding aziridinyl amides using a
NHase is presented. Surprisingly, 2 slowly underwent a
spontaneous rearrangement into 4 upon standing at room
temperature or at À20 °C over a period of several months
in a solution of deuterated chloroform (Scheme 2).
In addition, the enzymatic hydrolysis of 2-(2-cyano-
ethyl)aziridines 1 was investigated using nitrilases as the
hydrolyzing tools. Thus, 1 was dissolved in DMSO and
added to a KH2PO4/K2HPO4 buffered nitrilase solution,
after which the reaction mixture was incubated at 30 °C
and 450 rpm for 24À48 h, affording 4 in 80À93% yield as
the sole reaction products (Table 2). A nitrilase testing kit
from Biocatalytics containing 12 nitrilases was used, and
the best results are summarized in Table 2. Compounds
4a (Ar = 2-MeC6H4) and 4b (Ar = 2-ClC6H4) were
synthesized on a larger scale (1.5 mmol), allowing their
isolation in analytically pure form by means of column
chromatography on silica gel. The fact that no 3-(aziridin-
2-yl)propanoates 3 were obtained might be explained by
the mechanism by which these enzymes operate. Nitri-
lases possess a conserved Glu-Lys-Cys sequence in the
active site,13 where the cysteine forms a covalent bond
with the nitrile to form a thioimidate intermediate 7.14
Normally, water hydrolyzes the thiolimidate toward
a thioester, which is then further hydrolyzed to the
acid.14 In this case, however, an intramolecular attack of
the nucleophilic aziridine nitrogen atom takes place,
forming a bicyclic aziridinium intermediate 6, which is
Scheme 1. Synthesis of 3-(Aziridin-2-yl)propionamides 2
Scheme 2. Rearrangement of Aziridines 2 into Piperidinones 4
Table 2. Synthesis of 5-Hydroxypiperidin-2-ones 4
The substituent on the aromatic ring could have
an influence on the rate of this transformation, as for
example the benzyl (Ar = C6H5) and 2-chlorobenzyl
(Ar = 2-ClC6H4) derivatives 2a and 2c were completely
converted into the ring-expanded piperidinones 4a and 4c
over a period of 6 months, whereas for the other deriva-
tives only 10À15% of piperidinones 4 were formed over
the same period of time. However, this rearrangement is
probably catalyzed by traces of acid formed from the slow
decomposition of CDCl3, and the rearrangement rate can
be ascribed to different concentrations of acid present in
4 (Ar)
nitrilase
time (h)
conversion (%)a
4a (4-MeC6H4)
4b (2-ClC6H4)
4c (4-MeOC6H4)
4d (4-FC6H4)
N108
N107
N107
N101
48
24
24
48
88
93
92
80
a Based on LC of the crude reaction mixture.
(11) Ho, M. F.; Chung, J. K. K.; Tang, N. Tetrahedron Lett. 1993, 34,
6513.
108
Org. Lett., Vol. 14, No. 1, 2012

regiospecifically ring opened at the more substituted
carbon atom and further hydrolyzed toward 4 (Table 2).
A second objective of this study involved the hydrolysis of
aziridinyl nitriles toward the corresponding carboxylic acids
as valuable templates in organic and medicinal chemistry.
Thus, 1-arylmethyl-2-(2-cyanoethyl)aziridines 1 were
treated with 5 equiv of KOH in a solvent mixture of Et
OH/H2O (3/1) and heated under reflux for 16 h, affording
the corresponding potassium 3-(aziridin-2-yl)propano-
ates 3 in 80À90% yield (Scheme 3). 3-(Aziridin-2-yl)-
propionamides 2 were observed as minor constituents
(10À20%), as well, and could be easily removed from the
potassium carboxylates via extraction with CH2Cl2.
These novel 3-(aziridin-2-yl)propanoates 3 can be con-
sidered as interesting constrained γ-amino acid building
blocks for the synthesis of new heterocyclic compounds,
β-amino alcohols, and γ- and δ-amino acids,15 the latter
being important for their applications in peptidomime-
tics,16 PNA chains,17 and as GABA analogues.18
Scheme 4. Rearrangement of Propanoates 3 into γ-Lactones 5
then furnishes γ-lactones 5 as ring-expanded products.
No δ-lactone formation was observed, probably due to a
disfavored 6-endo-tet ring closure according to Baldwin’s
rules as compared to a favored 5-exo-tet cyclization.
γ-Lactones are a widespread entity in natural products,
and many of them display important biological activities
such as germination stimulation, anticancer, and anti-HIV
activity, while others are often used as food additives
(flavorants) and represent valuable building blocks in
organic synthesis.20
In conclusion, enzymatic and chemical hydrolysis have
been used in a complementary way for the synthesis of
novel 3-(aziridin-2-yl)propionamides and potassium 3-
(aziridin-2-yl)propanoates, respectively, starting from
1-arylmethyl-2-(2-cyanoethyl)aziridines without affect-
ing the sensitive aziridine ring. Furthermore, 3-(aziridin-
2-yl)propionamides were rearranged into δ-lactams, and
3-(aziridin-2-yl)propanoates were shown to be good pre-
cursors for the synthesis of γ-lactones.
Scheme 3. Synthesis of 3-(Aziridin-2-yl)propanoates 3
In order to assess their aptitude toward ring rearrange-
ments, 3 was treated with 1.1 equiv of acetic acid in water
for 15 min at room temperature, affording 5 in almost
quantitative yields (Scheme 4). Neutralization of the
alkaline medium transformed potassium salts 3 into the
corresponding amino acids, which exist as zwitterionic
structures 8 at neutral pH.
The latter intermediates 8 are highly susceptible to ring
transformation because of the combination of an electro-
philic aziridinium moiety and the nucleophilic carboxy-
late at a remote position.19 Intramolecular ring opening
Acknowledgment. The authors are indebted to the In-
stitute for the Promotion of Innovation through Science
and TechnologyÀFlanders (IWT-Vlaanderen), to the Re-
search FoundationÀFlanders (FWO-Vlaanderen), and to
Ghent University (GOA) for financial support. DSM re-
search (Geleen, The Netherlands) is kindly acknowledged
for providing the R. erythropolis NCIMB 11540 NHase.
Supporting Information Available. Spectroscopic data
of compounds 2aÀe, 3aÀd, 4a,b,e, and 5aÀc. This mate-
rial is available free of charge via the Internet at http://
pubs.acs.org.
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2009, 74, 8261. (b) Herdeis, C. Arch. Pharm. 1983, 316, 719.
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ꢀ
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16382.
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69, 6214.
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