Asymmetric aziridination with chiral allyl aminosulfoxonium ylides: synthesis of alkenyl aziridine carboxylates and palladium-catalyzed E,trans/E,cis-isomerization of an alkenyl aziridine

Article abstract of DOI:10.1016/j.tetlet.2007.07.216

Chiral cyclic and acyclic allyl aminosulfoxonium ylides have been generated from aminosulfoxonium-substituted β,γ-unsaturated α-amino acids (method A) and 1-alkenyl aminosulfoxonium salts (method B) upon treatment with DBU. Their application to the asymme

Full text of DOI:10.1016/j.tetlet.2007.07.216

Tetrahedron Letters 48 (2007) 7102–7107
Asymmetric aziridination with chiral allyl aminosulfoxonium
ylides: synthesis of alkenyl aziridine carboxylates
and palladium-catalyzed E,trans/E,cis-isomerization
of an alkenyl aziridine
Vijaya Bhaskara Reddy Iska, Hans-Joachim Gais,* Shashi Kant Tiwari,
Gadamsetti Surendra Babu and Adeline Adrien
Institut fu¨r Organische Chemie der Rheinisch-Westfa¨lischen Technischen Hochschule (RWTH) Aachen,
Landoltweg 1, 52074 Aachen, Germany
Received 18 July 2007; accepted 31 July 2007
Available online 6 August 2007
Abstract—Chiral cyclic and acyclic allyl aminosulfoxonium ylides have been generated from aminosulfoxonium-substituted
b,c-unsaturated a-amino acids (method A) and 1-alkenyl aminosulfoxonium salts (method B) upon treatment with DBU. Their
application to the asymmetric aziridination of N-tert-butyl-sulfonyl imino ester, generated either in situ (method A) or externally
added (method B), gave the corresponding alkenyl aziridine carboxylates with medium to high diastereoselectivity and enantio-
selectivity. A highly stereoselective Pd(0)-catalyzed isomerization of an E,trans-configured alkenyl aziridine methanol derivative
to its E-cis-isomer is described, which proceeded with retention of the double bond configuration.
Ó 2007 Elsevier Ltd. All rights reserved.
We had recently described the synthesis, structure, and
reactivity of chiral N-titanium allyl aminosulfoxonium
ylides of type I, R2 = H, R3 = Ti(NEt2)3 (Scheme 1).1
The investigations of I were supplemented by ab initio
calculations of allyl aminosulfoxonium ylide I,
R1 = R3 = Me, R2 = H and parent ylide II. The ylides
exhibit a polar S–O single bond and are stabilized by
electrostatic and nC–rÃSO interaction. A further impor-
tant mode of stabilization of allyl ylide I is nC–p*
interaction.
The hitherto unknown chiral allyl S-ylides of type I,
R3 = Me, and III should be of considerable synthetic
interest since a number of useful transformations of
these ylides can be envisioned based on the chemistry
of II.2,3 Although chiral alkylidene S-ylides are well
established reagents,3 only a few examples of chiral con-
jugated allyl S-ylides had been described.4 Thus, we
have developed an interest in the synthesis and reactivity
of ylides of type I and III, which are expected to be con-
figurationally stable.2 Here, we describe the generation
of the conjugated cyclic and acyclic allyl aminosulfoxo-
nium ylides I and III and their application to the asym-
metric aziridination of N-tert-butylsulfonyl imino ester,
which gave enantioenriched alkenyl and cycloalkenyl
aziridine carboxylates of type IV and V, respectively,
for which only a few asymmetric syntheses had been
described.5 In particular methods for the asymmetric
synthesis of cycloalkenyl aziridine carboxylates of type
V are scarce.5b Because of the rich chemistry of alkenyl
N(Me)R3
Ph
NMe2
Ph
O
NMe2
Ph
O
O
n
R1
S
S
S
H2C
R2
II
III
I
Bus
Bus
N
Bus = SO2tBu
N
EtO2C
EtO2C
R2
IV
V
n
Scheme 1. Aminosulfoxonium ylides and alkenyl aziridine
carboxylates.
*
Corresponding author. Tel.: +49 2418094685; fax: +49 2418092665;
e-mail: gais@rwth-aachen.de
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.07.216

V. B. R. Iska et al. / Tetrahedron Letters 48 (2007) 7102–7107
7103
aziridines and aziridine carboxylates, alkenyl aziridine
carboxylates IV and V should make valuable synthetic
building blocks.3 Although the parent methylene amino-
sulfoxonium ylide II and its C-alkyl derivatives have
been much studied in asymmetric epoxidation and
cyclopropanation, nothing is known about their reactiv-
ity in the aziridination of imines.2,3
VIII upon treatment with KF in H2O/CH2Cl2 (Scheme
2).6 This transformation most likely involves allyl
aminosulfoxonium salts VII as intermediates, which
are generated in the two-phasic system through an FÀ-
catalyzed isomerization of VI. In order to generate the
corresponding allyl ylides of type I, R2 = H, R3 = Me,
from VII 1-alkenyl aminosulfoxonium salts 1a–d
(Scheme 3) were treated with DBU. Surprisingly, salts
1a–d afforded the alkenyl aziridine carboxylates cis-5a–
d and trans-5a–d in high yields with good diastereoselec-
tivities and medium to high enantioselectivities (Table
1). The enantioselectivity of the synthesis of the cis-con-
figured aziridines is significantly higher than that of the
trans-configured isomers. Preparative HPLC of the cis/
trans-mixtures afforded the pure cis- and trans-aziridines
except in the case of 5d.7 In addition to aziridines cis-5a–
d and trans-5a–d, S-configured sulfinamide 4 of P98%
ee was isolated in high yields. It is proposed that salts
1a–d reacted with DBU under deprotonation at the N-
atom followed by a fragmentation of the corresponding
anions to give conjugated allyl aminosulfoxonium ylides
2a–d and imino ester 3.8 Then ylides 2a–d combined
with 3 under aziridination to afford aziridines and the
sulfinamide. The opposite reactivity of 1a–d toward FÀ
and DBU may be due to the different reaction condi-
tions and the differences in basicity and size of the bases.
We had previously reported the facile migratory cycliza-
tion of the aminosulfoxonium-substituted unsaturated
amino acids VI with formation of 3,4-dihydro prolines
Bus
H
O
NMe2
Ph
N
N
KF, H2O, CH2Cl2
S
EtO2C
R
VI
BF4
O
NMe2
Bus
H
Bus
S
N
Ph
EtO2C
EtO2C
BF4
R
VIII
R
VII
Bus = SO2tBu
Scheme 2. Migratory cyclization of aminosulfoxonium-substituted
unsaturated amino acids.6
The synthesis of amino acids 8a–d, used as the starting
material in the synthesis of salts 1a–d, was carried out
as previously described (Scheme 4).9 Isomerization of
enantiopure 1-alkenyl sulfoximines 6a–d with DBU gave
allyl sulfoximines 7a–d. Their successive treatment with
n-BuLi, ClTi(OiPr)3 and 3 afforded the unsaturated ami-
no acid derivatives 8a–d with high regio- and diastereo-
selectivities in high yields, the methylation of which
furnished salts 1a–d in practically quantitative yields.
H
Bus
O
NMe2
Ph
N
S
EtO2C
R
1a−d
BF4
DBU, CH2Cl2
NMe2
−
•
DBU H BF4
O
Bus
N
S
R
+
The aziridination of 3 with cyclic allyl aminosulfoxo-
nium ylides of type II generated through deprotonation
of the corresponding cyclic aminosulfoxonium-substi-
tuted unsaturated amino acids has not yet been investi-
gated in detail. Its feasibility is demonstrated, however,
by the following result. Treatment of aminosulfoxonium
salt 91 with DBU afforded ylide 10c and imine 3, which
reacted with each other and gave cycloalkenyl aziridine
carboxylates cis-11c and trans-11c in a ratio of 2:1 in
82% yield (Scheme 5).
Ph
EtO2C
3
2a−d
O
S
−
Ph
NMe2
4
Bus
N
Bus
N
+
EtO2C
trans-
EtO2C
R
R
5a−d
−
cis-5a d
The formation of aziridines cis-5a–d and trans-5a–d in
the reaction of aminosulfoxonium salts 1a–d with
DBU suggested an alternative entry to the aziridines
starting from 1-alkenyl aminosulfoxonium salts 12b,
a: R = Ph; b: R = iPr; c:cC6H11, d: R =tBu; Bus = SO2tBu
Scheme 3. Generation of acyclic allyl aminosulfoxonium ylides from
aminosulfoxonium-substituted amino acids and aziridination of 3.
Table 1. Synthesis of alkenyl aziridine carboxylates from aminosulfoxonium-substituted unsaturated amino acids
1
R
5 Cis:trans
Yield (%)
Cis
Trans
4 Yield (%)
eea (%)
Yield (%)
eea (%)
Yield (%)
a
b
c
Ph
iPr
cC6H11
tBu
93:7
91:9
90:10
91:9b
94
91
93
94
P98
92
71
82
76
75
—
30
26
48
5
5
6
9
—
79
71
70
71
d
50
a Determined by HPLC. Compound 5a: Chiralpack-AD; 5b and 5d: Chiracel OD-H; 5c: Chiralpack-IA.
b Inseparable mixture.

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V. B. R. Iska et al. / Tetrahedron Letters 48 (2007) 7102–7107
NMe
O
NMe
Ph
NMe
Ph
O
O
DBU, MeCN
R
S
R
S
R
S
Ph
−
7a d
6a−d
a: R = Ph; b: R = iPr;
c: C6H11, d: R = tBu;
Bus = SO2tBu
6b 6c, 6e
,
1. n-BuLi
c
2. ClTi(OiPr)3
Me3OBF4, CH2Cl2
3
3.
NMe2
O
H
Bus
R
NMe
Ph
N
O
R
S
BF4
Me3OBF4
CH2Cl2, rt,
S
Ph
−
1a d
EtO2C
12b, 12c, 12e
−
8a d
DBU, CH2Cl2
NMe2
−
−
•
DBU H BF4
Scheme 4. Asymmetric synthesis of sulfoximine-substituted unsatu-
rated amino acids.9
O
Bus
N
R
S
+
Ph
EtO2C
2b, 2c, 2e
O
NMe2
Ph
3
O
NMe2
Ph
4
S
Bus
H
S
Bus
N
Bus
N
N
Bus
N
DBU, CH2Cl2
EtO2C
+
+
−
•
DBU H BF4
EtO2C
EtO2C
3
BF4
EtO2C
R
s-5e
10c
R
9
cis-5b, cis-5c, cis-5e
trans-5b, trans-5c, tran
− 4
b: R = Pr; c:
i
cC6H11, e: R = Me; Bus = SO2tBu
Bus
N
Bus
N
Scheme 6. Generation of acyclic allyl aminosulfoxonium ylides from
1-alkenyl aminosulfoxonium salts and aziridination of 3.
+
EtO2C
EtO2C
N,S-dimethyl-S-phenylsulfoximine, the starting material
for the synthesis of 6a–d and 8a–d, had already been
described.11
11c
trans-11c
cis-
Scheme 5. Generation of a cyclic allyl aminosulfoxonium ylide from
the aminosulfoxonium-substituted amino acid and aziridination of 3.
The diastereo- and enantioselectivities of the formation
of aziridines from 1-alkenyl salts 12 differ significantly
from those starting from the amino acid derivatives 1.
The major difference between the two pathways is the
concentration of 3. While in the case of 1 imine 3 is gen-
erated in situ and thus present only in low concentra-
tion, it is there in excess in the case of 12. However, a
rationalization of the different selectivity is difficult at
present because of the lack of further mechanistic infor-
mation. For example, it is not known whether the addi-
tion of 2 to 3 is reversible or not.
12c, and 12e and 3 (Scheme 6). Methylation of 1-alkenyl
sulfoximines 6b, 6c, and 6e, which are readily available
from (S)-N,S-dimethyl-S-phenylsulfoximine and the
corresponding aldehydes following a one-pot proce-
dure,10 gave aminosulfoxonium salts 12b, 12c, and 12e
(98% yield), respectively. The successive treatment of
salts 12b, 12c, and 12e with imino ester 3 and DBU
indeed furnished aziridines cis-5b, cis-5c, and cis-5e to-
gether with trans-5b, trans-5c, and trans-5e, respectively,
in good yields (Table 2). Thus, DBU presumably caused
a deprotonation of 1-alkenyl salts 12b, 12c, and 12e at
the c-position with formation of trans-configured allyl
ylides 2b, 2c, and 2e, respectively, which reacted with 3
to give aziridines. In addition to aziridines, sulfinamide
4 of P98% ee was isolated in high yields. It should be
noted that the stereoselective conversion of 4 to (S)-
Because of the high reactivity of a-unsubstituted ylides
2a–d toward 3, it was of interest to see whether a substi-
tuted allyl ylide of type II, R2 = alkyl, can be generated
and what its reactivity would be. Thus, methyl-substi-
tuted 1-alkenyl sulfoximine 14 was prepared starting
from 1-alkenyl sulfoximine 6b via lithiation at the a-po-
sition12 with formation of alkenyllithium derivative 13,
Table 2. Synthesis of alkenyl aziridine carboxylates from 1-alkenyl aminosulfoxonium salts and the imino ester
6
R
5 Cis:trans
Yield (%)
Cis ee (%)a
Trans ee (%)a
4 Yield (%)
b
c
e
iPr
cC6H11
Me
64:36
70:30
60:40b
70
68
65
76
47
65
49
45
—
92
90
90
c
a Determined by HPLC, Chiralpack-IA.
b Inseparable mixture.
c Could not be determined.

V. B. R. Iska et al. / Tetrahedron Letters 48 (2007) 7102–7107
7105
which upon treatment with MeI gave sulfoximine 14 in
NMe
Ph
Me3OBF4
CH2Cl2
NMe2
Ph
O
O
NMe2
Ph
O
95% overall yield based on 6b (Scheme 7). Methylation
of sulfoximine 14 afforded aminosulfoxonim salt 15
(98% yield). The treatment of salt 15 with DBU in the
presence of 3 furnished methyl-substituted allyl amino-
sulfoxonium ylide 16, which readily reacted with 3 and
gave aziridines cis-17 and trans-17 in a ratio of 60:40
in 81% yield. Interestingly, aziridine cis-17, which was
separated by HPLC, had only 28% ee. Thus, the substi-
tuted ylide 16 reacted with 3 with a much lower enantio-
selectivity than the corresponding unsubstituted ylide
2b.
S
S
S
DBU
BF4
BF4
n
n
n
−
n = 1 4
−
19a d
−
−
10a d
18a d
Bus
Bus
N
Bus
N
N
EtO2C
+
3
EtO2C
EtO2C
trans-
−
4
Having recorded a facile synthesis of acyclic aziridines
from the corresponding acyclic 1-alkenyl aminosulfoxo-
nium salts 12 and 3, it was of interest to see whether this
method could also be applied to the generation of cyclic
allyl aminosulfoxonium ylides and thus to the synthesis
of cycloalkenyl aziridine carboxylates. Methylation of
cyclic 1-alkenyl sulfoximines 18a–d, which are readily
available in high yield from (S)-N,S-dimethyl-S-phenyl-
sulfoximine and the corresponding cycloalkanones,10
furnished the cyclic 1-alkenyl aminosulfoxonium salts
Bus = SO2t Bu
n
n
−
cis-11a d
11a−d
Scheme 8. Generation of cyclic allyl aminosulfoxonium ylides cyclic
1-alkenyl aminosulfoxonium salts and aziridination of 3.
19a–d (98% yield) (Scheme 8). The successive treatment
of salts 19a–d with DBU and 3 afforded cycloalkenyl
aziridine carboxylates cis-11a–d and trans-11a–d with
low diastereoselectivities and medium to high enantio-
selectivities in good yields (Table 3).
NMe
Ph
O
S
iPr
Because of the attainment of mixtures of cis- and trans-
aziridines, a convergent conversion of the cis/trans-
mixtures of the corresponding aziridine methanol
derivatives to their cis-configured isomers was thought,
which should be interesting substrates, for example,
for an aza-Payne rearrangement.13 Thus, reduction of
ester trans-5b gave alcohol trans-20 (90%), which was
protected as silyl ether trans-21 (89%) (Scheme 9). The
Pd-catalyzed isomerization14 of E,trans-configured
alkenyl aziridine trans-21 proceeded with high dia-
stereo-selectivities and afforded E,cis-configured isomer
cis-21 with de’s P98% in 84% yield. Similarly, a mixture
of cis-5b/trans-5b was converted via cis-20/trans-20 and
cis-21/trans-21 to cis-21 with de’s P98% in 81% overall
yield.
R
6b: R = H
13: R = Li
14: R = Me
n-BuLi, THF
MeI
Me3OBF4, CH2Cl2
NMe2
O
iPr
S
Ph
Me
BF4
15
DBU, CH2Cl2
NMe2
−
•
DBU H BF4
O
Bus
iPr
S
N
+
Ph
EtO2C
Me
16
Surprisingly, the Pd(0)-catalyzed isomerization of trans-
21 to cis-21 proceeded with complete retention of the
configuration of the double bond.14b The reaction of
aziridine trans-21 with Pd(PPh3)4 is expected to afford
p-allyl–Pd(II) complex 22 (Scheme 10), which has to
undergo an inversion of configuration at Cb en route
to cis-21. A typical p–r–p-isomerization of 22 would
furnish complex 23,15 the intramolecular substitution
of which should give, however, Z-configured aziridine
cis-24 and not E-configured aziridine cis-21. This shows,
3
−
4
Bus
N
Bus
N
+
Me
Me
EtO2C
EtO2C
iPr
iPr
cis-17
Bus = SO2tBu
trans-17
Scheme 7. Generation of a a-methyl-substituted allyl aminosulfoxo-
nium ylide and aziridination of 3.
Table 3. Synthesis of cycloalkenyl aziridine carboxylates from cyclic 1-alkenyl aminosulfoxonium salts and the imino ester
18
n
11 Cis:trans
Yield (%)
Cis
Trans
4 Yield (%)
eea (%)
Yield (%)
eea (%)
Yield (%)
a
b
c
1
2
3
4
60:40b
60:40
60:40
50:50
70
73
71
66
79
76
78
70
—
42
41
29
90
56
57
25
—
28
26
30
80
81
76
70
d
a Determined by HPLC, Chiralpack-IA.
b Inseparable mixture.

7106
V. B. R. Iska et al. / Tetrahedron Letters 48 (2007) 7102–7107
Bus
N
a nucleophilic substitution of 22 by Pd(PPh3)4 via an
anti-attack of the Pd(0)-complex at the allyl moiety.16,17
Thus, the reaction of Pd(PPh3)4 with trans-21 and cis-21
perhaps resulted in the establishment of an equilibrium
in which E,cis-configured aziridine cis-21 dominated
because of its higher stability.14
Bus
N
EtO2C
EtO2C
cis-5b+trans-5b
i
Pr
iPr
trans-5b
1. DIBALH, CH2Cl2
2. ClSitBuMe2,ImH, CH2Cl2
Bus
N
Bus
N
Acknowledgments
Financial support of this work by the Deutsche For-
schungsgemeinschaft (SFB 380 and GK 440) is gratefully
acknowledged. We thank Cornelia Vermeeren for
HPLC and GC analyses, and Dr. Jan Runsink for
NOE experiments.
RO
iPr
RO
iPr
trans-20: R = H
cis-20+trans-20: R = H
trans-21: R = SitBuMe2
cis-21+trans-21: R = SitBuMe2
Pd(PPh3)4, 0 ˚C, THF
Bus
N
Bus
Bus = SO2tBu
N
References and notes
t
BuMe2SiO
t
BuMe2SiO
iPr
iPr
21
cis-
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β
N
Bus
N
+ Pd(PPh3)4
iPr
PdL2
RO
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iPr
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22
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`
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iPr
−
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PdL2
RO
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23
cis-24
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N
β
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N
+ Pd(PPh3)4
iPr
PdL2
RO
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i
Pr
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¨
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was determined by chemical correlation (1) H2, Pd/C,
EtOH, 91%; (2) CF3SO3H, anisole, CH2Cl2, 94%) with
(R)-ethyl 2-amino-5-phenylpentanoate Davis, F. A.; Qu,
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(Pd(PPh3)4)
Bus
N
Bus
N
iPr
−
Pd(PPh3)4
PdL2
RO
RO
iPr
cis-21
25
Scheme 10. Mechanistic rationalization of the Pd(0)-catalyzed isom-
erization of trans-21 to cis-21.
8. Schleusner, M.; Koep, S.; Gunter, M.; Tiwari, S. K.; Gais,
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cis-21. Such an isomerization could be accomplished by
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7107
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