Catalytic enantioselective synthesis of α-nitroepoxides via aminolytic kinetic resolution

Article abstract of DOI:10.1039/c4cy01157k

The first enantioselective synthesis of β-aryl-substituted α-nitroepoxides, exploiting an organocatalyzed aminolytic kinetic resolution (AKR), has been developed. Ring-opening reaction of racemic α-nitroepoxides with aniline in the presence of a readily available Cinchona alkaloid-derived thiourea affords unreacted epoxides in up to 95% ee.

Full text of DOI:10.1039/c4cy01157k

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DOI: 10.1039/C4CY01157K
S
Ar
HN
N
H
N
N
(20 mol %)
O
O
O
NO2
Ar = 3,5-(CF3)2C6H3 R2
R2
NO2
R2
+
R1
+
PhNH2
R1
(-)
toluene, rt
R1
( )
NHPh
up to 95% ee
The first enantioselective synthesis of β-aryl-substituted α-nitroepoxides has been
developed and their potential as valuable intermediates demonstrated.

Catalysis Science & Technology
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DOI: 10.1039/C4CY01157K
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Catalytic enantioselective synthesis of -
nitroepoxides via aminolytic kinetic resolution
Cite this: DOI: 10.1039/x0xx00000x
Sara Meninno, Loris Napolitano and Alessandra Lattanzi*
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first enantioselective synthesis of

-aryl-substituted

-
processes to access enantioenriched -nitroepoxides is of great
nitroepoxides, exploiting an organocatalyzed aminolytic importance, especially in view of the totally unexplored
kinetic resolution (AKR), has been developed. Ring-opening potential of this class of epoxides in asymmetric synthesis.
reaction of racemic
-nitroepoxides with aniline in the
presence of a readily available Cinchona alkaloid-derived
thiourea affords unreacted epoxides in up to 95% ee.
The importance of optically active epoxides is widely
recognised by their presence as structural motives in several
natural products and mostly as highly versatile intermediates,
frequently involved in total synthesis.1 Indeed, a great variety
of functionalised products is achievable via regio- and
stereoselective opening of the oxirane ring.2
Figure 1 Ring‐opening reactions of ‐nitroepoxides.
The asymmetric epoxidation of alkenes is the most appealing
and straightforward approach to prepare epoxides.3 Amongst
the several methodologies developed so far, a steadily growing
number of efficient asymmetric nucleophilic epoxidation
reactions have been reported for differently substituted enones4
and enals.5 In contrast, according to the paucity of examples
illustrated in the literature,6 the asymmetric epoxidation of
nitroalkenes, a class of popular electron-poor alkenes,7 appears
to be a challenging process.
Racemic-nitroepoxides, readily available by epoxidation of
the corresponding nitroalkenes with H2O2/NaOH system, are
synthetically useful compounds, due to the fact that they
undergo complete regioselective ring-opening reactions at the
The kinetic resolution of racemic compounds represents a
highly valuable strategy and sometimes the unique tool in the
synthesis of a chiral non racemic compound.11 Specifically,
since the seminal work by Jacobsen and co-authors, the
hydrolytic kinetic resolution (HKR) of terminal epoxides
catalysed by a salen-Co-(III)-based complex12 became widely
applied also at an industrial scale,11b,d,13 as powerful and
exclusive approach for the preparation of these challenging
compounds in excellent enantioselectivity. In the area of
organocatalysis, hydrogen-bonding donors such as hexafluoro-
2-propanol,14 achiral thioureas15 and ureas16 have been shown
to assist the regioselective ring-opening reaction of epoxides to
give differently 1,2-functionalized alcohols. Very recently, a
few examples on the desymmetrization of meso-epoxides
catalyzed by chiral phosphoric acids17 or sulphonamides18 have

-position by a variety of nucleophiles, to give

-substituted
carbonyl compounds,8 functionalised
heterocycles9 (Figure 1).

-aminoacids,6a,b and
Chiral non racemic amines have been recently employed in the
ring-opening reaction of racemic -nitroepoxides to afford,
been reported.

Driven by our long-standing interest in asymmetric synthesis of
epoxides19 and inspired by these reports, we envisaged that an
aminolytic kinetic resolution catalyzed by chiral bifunctional
after reduction of imine intermediate, vicinal diamines in
stereoselective fashion (Figure 1).10 Hence, the development of
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1‐3 | 1

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organic promoters such as a thiourea amine could be a
We commenced our study reacting trans-2-methyl-2-nitro-3-
phenyloxirane 1a-(±) and aniline20 in the presence of 15 mol %
successful strategy to obtain enantioenriched
(Figure 2).
-nitroepoxides
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of chiral H-bonding donors as (R
)-BIDNOOI:L10.1I02139/aCn4dCY0e1a1s5i7lyK
accessible amino alcohol derived thiourea II22 at room
temperature in toluene (Table 1). In the presence of catalyst
the conversion to the expected ketone 3a was poor after 48 h
(entry 1), whereas in the presence of the thiourea alcohol II
I
,
a
slight improvement of the conversion was observed although
the epoxide was isolated almost as racemic compound (entry 2).
A similar outcome was observed when using the Brønsted base
catalyst III (entry 3). We were pleased to observe that
Takemoto’s catalyst IV provided the unreacted epoxide in 50%
yield and 23% ee (entry 4). This result showed the requirement
of a bifunctional organocatalyst for the reaction to proceed in a
more effective way, likely via a cooperative activation of
reagents as suggested in Figure 2. Among the Cinchona
alkaloid derived-thioureas, a significant improvement was
achieved when using catalyst VI, since the epoxide was isolated
Figure 2 Proposed activation model in the AKR of ‐nitroepoxide catalyzed by an
amino thiourea.
Activation of the reagents through general acid-base catalysis
provided by the bifunctional organocatalyst would have likely
given rise to the preferential opening of one enantiomer of the

-nitroepoxide in a highly regioselective manner.
Table 1 Screening of catalysts in model AKR reactiona
NH2
catalyst
(15 mol %)
O
O
O
NO2
NO2
Ph
+
Ph
Ph
+
in 63% yield and 30% ee (entry 6). Compound VII, the pseudo
-
toluene, rt
NHPh
enantiomer of VI, catalysed the process less effectively (entry
7). Finally, amine-squaramide VIII afforded a similar result as
3a
1a
1a-(±)
2a
S
NHAr
Ph
the one achieved with pseudo-enantiomeric amine thiourea
V
HN
Ph
OH
OH
(DHQD)2PHAL
(entry 8). Catalyst VI was selected for further optimization of
the AKR of epoxide 1a-(±) (Table 2).
III
OH
I
II
S
Table 2 . Optimization of the reaction conditionsa
S
S
Ar
N
Ar
N
HN
Ar
HN
N
H
H
N
H
N
N
N
H
N
Entry
RNH2
Solvent
Yield
ee
3
VI
N
V
(%)b
1a(%)c
1a (%)d
IV
1
2
toluene
toluene
toluene
toluene
CHCl3
1-naphthyl
34(b)
32(c)
38(d)
nd
64
61
47
34
65
43
59
28
38
28
62
24
20
40
6
MeO
O
O
S
2-naphthyl
4-MeOC6H4
Ar
N
N
NH
3
ArHN
NH
H
4
N
benzyl
Ph
N
5
30(a)
52(a)
36(a)
65(a)
55(a)
70(a)
35(a)
18
22
30
59
50
72
-
VII
VIII
N
6
7
8e
9f
10g
11h
tBuOMe
p-xylene
toluene
toluene
toluene
toluene
Ph
Ar = 3,5-(CF3)2C6H3
Ph
OMe
Ph
Ph
Entry
Catalyst
Time (h)
Yield
ee
3a
(%)b
12
1a(%)c
1a (%)d
Ph
1
2
3
4
5
6
7
8
48
48
48
65
69
48
48
48
I
II
nd
nd
Ph
24
21
40
37
33
40
35
68
74
50
51
63
47
59
2
6
All reactions were carried out using 1a-(±) (0.15 mmol), 2 (0.18 mmol), VI
(15 mol %) solvent (1.5 mL) for 48-113 h. Determined by 1H NMR
b
III
IV
V
c
d
e
analysis. Isolated yield. Determined by chiral HPLC analysis. Reaction
carried out with 20 mol% of VI, 1.5 equiv of aniline at C1a (0.2 M).
Reaction carried out with 20 mol% of VI, 3.0 equiv of aniline at C1a (0.05
M). Reaction carried out with 20 mol% of VI, 3.0 equiv of aniline at C1a
(0.1 M). Reaction carried out without catalyst, with 3.0 equiv of aniline at
1a (0.1 M).
-23
-11
-30
28
6
f
g
VI
VII
VIII
h
C
a All reactions were carried out using 1a-(±) (0.15 mmol), 2a (0.18 mmol),
catalyst (15 mol %) in toluene (1.5 mL). b Determined by 1H NMR analysis. c
Isolated yield. d Determined by chiral HPLC analysis. Negative ee indicates
the formation of the opposite enantiomer.
Sterically demanding 1-, 2-naphthyl amines and 4-methoxy
aniline, when reacting with epoxide 1a-(±), proved to be
inferior reagents compared to aniline (entries 1-3). More
nucleophilic benzyl amine reacted faster and less selectively
(entry 4).23 A solvent screening, on model reaction with aniline,
confirmed non polar aromatic solvents as the most suitable
(entries 5-7). The reaction was further optimized in toluene
varying reagents ratio and concentration. In order to increase
the conversion to ketone, the reactions were carried out using
Herein, we illustrate the first synthesis of enantiomerically
enriched aromatic
racemic -nitroepoxides proceeds via a highly regioselective
-nitroepoxides. The kinetic resolution of

ring-opening reaction with aniline organocatalysed by a
Cinchona alkaloid-derived thiourea.
2 | J. Name., 2012, 00, 1‐3
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20 mol% of catalyst VI, increasing the equivalents of aniline at reduced in fairly good overall yield and highly diastereoselective
different concentrations (entries 8-10). The best result was manner to anti -amino alcohol 4a.28
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achieved using 3 equivalents of aniline working at C 0.1 M,
since the epoxide was isolated in 28% yield and 72% ee (entry
10). Finally, a control experiment, performed without the
organocatalyst, showed that the background epoxide ring-
opening reaction is not a negligible process (entry 11).
DOI: 10.1039/C4CY01157K
O
N
H
O
N
Ph
NO2
NaBH4/CeCl3
MeOH, 0 °C, 5.5 h
(2 equiv)
Ph
Ph
N
CHCl3, 0 °C, 6.0 h
OH
4a-(-)
78%, anti/syn >20:<1, 67% ee
1a-(-)
73% ee
Under the optimized conditions a variety of -nitroepoxides was
reacted to study the scope and limitation of the process (Table 3).
Scheme 1 One‐pot stereoselective ring‐opening/reduction sequence to anti‐
‐
amino alcohol 4a
.
Table 3 . AKR of -nitro epoxides with aniline catalysed by VIa
Pleasingly, a slight erosion of the enantioselectivity was
observed,26,29 as assessed by chiral HPLC analysis on 1-phenyl-
1-(pyrrolidin-1-yl)propan-2-ol
4a
,
whose
absolute
configuration was determined to be (1R,2S) by comparison of
the optical rotation to literature reported data.30 Consequently,
absolute configuration of 2-methyl-2-nitro-3-phenyloxirane 1a-
(-) was assigned to be (2R,3S). Finally, other nucleophiles such
as thiols and 1,2-diamines are applicable and pharmaceutically
important targets such as quinoxalines31 are accessible
Entry
R1
R2
Ph
3(%)b Yield 1a(%)b ee 1a (%)c
1
2
Me
Me
Me
Me
Me
Et
65
79
67
48
72
59
66
64
47
28
28 (a)
17 (e)
29 (f)
36 (g)
26 (h)
21 (i)
31 (j)
33 (k)
35 (l)
35 (m)
72
84
86
77
95
92
92
90
61
16
4-MeC6H4
4-ClC6H4
3
4
4-CF3C6H4
2-naphthyl
Ph
3-MeC6H4
4-BrC6H4
3,4-Cl2C6H3
Ph(CH2)2
exploiting the organocatalysed kinetic resolution of
nitroepoxides (Scheme 2).
-
5
6
7
Et
8
Et
9
Et
10
Me
a
All reactions were carried out using 1-(±) (0.2 mmol), 2a (0.6 mmol), VI
(20 mol %), toluene (2.0 mL) for 84-158 h. b Isolated yield. c Determined by
chiral HPLC analysis.
Electron-donating and withdrawing groups are tolerated in the
aromatic ring of compounds 1 and unreacted epoxides were
recovered in satisfactory yield and good to high enantioselectivity
(entries 1-9). The present system appears to be unsuitable for the
kinetic resolution of aliphatic -nitroepoxides. The ring-opening
reaction of -nitroepoxide 1m proceeded at lower rate and unreacted
epoxide was recovered in 35% yield and 16% ee (entry 10).23 The ee
values of -amino ketones 3 were not determined as they are
supposed to be nearly racemic compounds on the basis of the
following considerations: i) the estimated stereoselectivity factors of
the AKR illustrated in Table 3 are moderate (3< S <7)24 and low ee
values25 are expected for the -amino ketone products 3;11a,b ii)
enantiomerically enriched -amino ketones were reported to be
sensitive compounds suffering partial racemization under basic
conditions.26
Scheme 2 Kinetic resolution of epoxide 1a‐(±) with thiol 5 and diamine 7.
Conclusions
In conclusion, we developed the first enantioselective synthesis
of -aryl-substituted -nitroepoxides taking advantage of an
aminolytic kinetic resolution with aniline catalysed by an easily
accessible Cinchona alkaloid-derived thiourea. The epoxides are
recovered in acceptable yield and good to high enantioselectivity.
Enantioenriched -nitroepoxides can serve as synthetically useful
intermediates, as demonstrated in the one-pot stereoselective
approach to highly valuable anti--amino alcohols.32 Despite
improvements are required for the present AKR to be of practical
value, this work highlights the great potential of bifunctional
organocatalysts in epoxides ring-opening reactions. Efforts to
improve and expand the scope of the organocatalytic kinetic
resolution of racemic epoxides are currently underway.
Jackson’s6a,b and Aggarwal’s27 groups demonstrated that ring-
opening reactions of diastereoisomerically pure arylthio nitrooxirane
or spirocyclic bis-sulfinyl oxiranes with amines proceeded
stereospecifically with inversion of configuration to give
enantioenriched -amino thioesters and amides, respectively.
In order to determine either the absolute configuration of
optically enriched epoxides and to show the utility of these
compounds in asymmetric synthesis, we set up a one-pot two-step
diastereoselective route to produce 1,2-amino alcohols (Scheme 1).
Enantiomerically enriched 1a-(-), treated with pyrrolidine at 0 °C,
afforded the corresponding -amino ketone, which was in situ
Acknowledgements
This work was supported financially by MIUR. We thank Dr. P. Iannece
for assistance with MS spectra and elemental analyses. A.L. thanks the
European COST Action CM0905-Organocatalysis.
Notes and references
This journal is © The Royal Society of Chemistry 2012
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a Dipartimento di Chimica e Biologia, Università di Salerno, Via Giovanni
Paolo II, 84084, Fisciano, Italy. Fax:0039-089-969603; E-mail:
lattanzi@unisa.it
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DOI: 10.1039/C4CY01157K
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HPLC traces for all the new compounds. See DOI: 10.1039/c000000x/
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