Highly enantioselective and diastereoselective cycloaddition of cyclopropanes with nitrones and its application in the kinetic resolution of 2-substituted cyclopropane-1,1-dicarboxylates

Article abstract of DOI:10.1002/anie.200604645

(Chemical Equation Presented) The multitalented catalyst 1/Ni(ClO ₄)₂ mediates both title processes (see scheme). Together the efficient asymmetric cycloaddition and a kinetic resolution/ cycloaddition provide access to both enantiom

Full text of DOI:10.1002/anie.200604645

Communications
DOI: 10.1002/anie.200604645
Asymmetric Catalysis
Highly Enantioselective and Diastereoselective Cycloaddition of
Cyclopropanes with Nitrones and Its Application in the Kinetic
Resolution of 2-Substituted Cyclopropane-1,1-dicarboxylates**
Yan-Biao Kang, Xiu-Li Sun, and Yong Tang*
Trisoxazolines[1] have been applied widely in asymmetric
catalysis[2] and molecular recognition.[3] In our efforts to
develop superior catalysts that are cheap,readily accessible,
air stable,and water tolerant,we designed and synthesized
the pseudo-C3-symmetric trisoxazoline 1b (Scheme 1) by the
preparation of such compounds,[10] the [3+3] cycloaddition
of donor–acceptor cyclopropanes with nitrones,pioneered by
Kerr and co-workers,[11] is a particularly elegant approach.
They found that nitrones 3 react smoothly with cyclopropanes
2 in the presence of Yb(OTf)3·xH2O to give the cis isomers 4
diastereospecifically (see Table 1). Recently,Sibi et al. devel-
oped a highly efficient asymmetric version of this reaction.[12]
They reported that nitrones react with cyclopropane-1,1-
dicarboxylates to afford tetrahydro-1,2-oxazine derivatives
with high enantioselectivity in the presence of dbfox/NiII
(10 mol%; dbfox = 4,6-dibenzofurandiyl-2,2ꢀ-bisoxazoline).
When 2-substituted cyclopropane-1,1-dicarboxylates were
used as substrates,and the quantity of the catalyst was
increased to 30 mol%,the reactions proceeded smoothly to
give the products with high ee values but with low diastereo-
selectivities (the cis/trans ratio ranged from 1.0:1.4 to 1.0:0.8).
Gratifyingly,we found that diethyl 2-phenylcyclopropane-
1,1-dicarboxylate (2a) reacted smoothly with nitrone 3a to
furnish the cycloaddition adduct in 99% yield with excellent
diastereoselectivity (d.r. > 99:1) in the presence of a catalytic
amount of 1b/Ni(ClO4)2·6H2O. Further studies showed that
ligands derived from iPr-box (box = bisoxazoline) influenced
strongly both the enantioselectivity and diastereoselectivity
of the reaction. The bisoxazoline 1a without a pendant group
gave the desired product with good diastereoselectivity (d.r.
97:3) but only 20% ee (Table 1,entry 1). However,when a
pyrid-2-yl or oxazolinyl group was introduced as a pendant
group,the enantioselectivity was improved to a moderate
level,and the high diastereoselectivity was maintained
(Table 1,entries 2,3,and 5). Both excellent enantioselectivity
and high diastereoselectivity were observed when the sub-
stituent on the nitrogen atom of the nitrone was changed from
a phenyl group to a methyl group (Table 1,entry 6). In this
case, 4b was obtained with 95% ee and d.r. 11:1 when a
catalytic amount of 1e/Ni(ClO4)2 was used. Thus,both high
enantioselectivity and high diastereoselectivity could be
attained by using 1e/Ni(ClO4)2 as the catalyst. The ester
groups of the 2-phenyl cyclopropane-1,1-dicarboxylate also
influenced the enantioselectivity. For example,when 2b was
used instead of 2a,the ee value of the product decreased from
95 to 90% (Table 1,entry 7).
Scheme 1. Trisoxazoline (tox) ligands. Py=pyridyl.
sidearm approach,and found that 1b/CuII promoted smoothly
the highly enantioselective Friedel–Crafts reaction of indoles
with arylidene malonates[4] and the Kinugasa reaction.[5] We
also described tox/CoII-catalyzed [3+2] cycloaddition reac-
tions between nitrones and alkylidene malonates in which the
endo/exo selectivity could be controlled well by the reaction
temperature. Thus,both cis and trans isoxazolidines could be
prepared enantioselectively.[6] Herein,we report that the
system tox/NiII catalyzes the [3+3] cycloaddition of racemic 2-
substituted cyclopropane-1,1-dicarboxylates with nitrones to
provide ready access to optically active tetrahydro-1,2-
oxazine derivatives with high diastereoselectivity and enan-
tioselectivity. Furthermore,this reaction can be employed for
the kinetic resolution[7] of 2-substituted cyclopropane-1,1-
dicarboxylates to furnish these compounds in optically active
form with excellent ee values.
Tetrahydro-1,2-oxazine derivatives occur frequently in
biologically active compounds[8] and are valuable synthetic
intermediates.[9] Among the methods developed for the
[*] Y.-B. Kang, Dr. X.-L. Sun, Prof. Dr. Y. Tang
The State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Lu, Shanghai, 200032 (P.R. China)
Fax: (+86)215-492-5078
E-mail: tangy@mail.sioc.ac.cn
We studied the scope of this reaction in terms of the
substrate under the optimal conditions by investigating a
variety of nitrones and cyclopropanes with different struc-
tures. As shown in Table 2,the ester groups of cyclopropanes
2 slightly influenced the enantioselectivity of the reaction.
Benzyl and ethyl diesters reacted with higher enantioselec-
tivity than the corresponding methyl diester (Table 2,
[**] We are grateful for the financial support of the Natural Sciences
Foundation of China, the Major State Basic Research Development
Program (Grant No. 2006CB806105), the Chinese Academy of
Sciences, and The Science and Technology Commission of
Shanghai Municipality.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3918
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Chemie
Table 1: Effect of the ligand on the cycloaddition.[a]
enantio- and diastereoselectivities
(Table 2,entries 10 and 11). Finally,
the reaction of an N-Bn-substituted
nitrone also gave the cycloaddition
adduct in good yield with excellent
stereoselectivity (Table 2,entry 12).
In the [3+3] cycloaddition reac-
tions described above,an excess of
Entry
R1
R2
1
t [days]
4
Yield [%][b]
d.r.[c]
ee [%][d]
the
cyclopropane
derivative
1
2
3
4
5
6
7
8
Et
Et
Et
Et
Et
Et
Me
Me
Ph
Ph
Ph
Ph
Ph
Me
Me
Me
1a
1b
1c
1d
1e
1e
1e
1c
4
4
4
4
4
4
3
3
4a
4a
4a
4a
4a
4b
4c
4c
96
99
91
99
97:3
99:1
99:1
94:6
99:1
11:1
13:1
6:1
20
55
66
13
57
95
90
88
(2.2 equiv) was used. Interestingly,
the remaining cyclopropane could
be recovered with a good ee value.
For example,in the case of entry 1
in Table 2,dimethyl 2-phenylcyclo-
99
88[e]
82[e]
77[e]
propane-1,1-dicarboxylate
was
recovered in 57% yield with
59% ee (Scheme 2). These results,
together with the importance of
cyclopropane-1,1-dicarboxylates as
intermediates as a result of their
wide application in organic synthe-
sis,[13] encouraged us to explore the
kinetic resolution of 2-substituted
cyclopropane-1,1-dicarboxylates. It
was found that under optimized
conditions the trisoxazoline–Ni-
[a] Reaction conditions: Ni(ClO4)2 (0.040 mmol), 1 (0.044 mmol), 2 (0.44 mmol), 3 (0.20 mmol), 4-Š
molecular sieves; DME=1,2-dimethoxyethane. [b] Yield of the isolated product. [c] Ratio of isomers
(cis/trans) as determined by 1H NMR spectroscopy. [d] Determined by HPLC on a chiral phase. [e] Yield
of the isolated cis isomer.
Table 2: Asymmetric cycloaddition of cyclopropanes with nitrones.[a]
(ClO4)2
catalyst
1e/Ni-
(ClO4)2·6H2O was also an excellent
catalyst for the kinetic resolution of
Entry
R1
R2
R3
t [days]
Yield [%][b]
d.r.[c]
ee [%][d]
1
2[f]
3
4
5
6
7
8
9
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Me
Bn
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Ph
Ph
Ph
3
4
4
4
4
4
4
4
3
82[e]
62[e]
88[e]
85[e]
97
13:1
10:1
11:1
12:1
11:1
12:1
13:1
13:1
4:1
90
97
95
97
97
96
90
93
92
80
80
93
2-substituted
cyclopropane-1,1-
dicarboxylates. Simply by changing
the ratio of (Æ )-2b to 3b,a variety
of 2-aryl cyclopropane-1,1-dicar-
boxylates could be resolved effi-
ciently in high yields in the pres-
ence of a catalytic amount of 1e/
Ni(ClO4)2·6H2O (Table 3). This
method provides ready access to
optically active 2-substituted cyclo-
4-BrC6H4
4-MeO2CC6H4
4-MeC6H4
4-MeOC6H4
2-furyl
styryl
80[e]
92[e]
99
Ph
76[e]
88[e]
84
10[f]
11[f]
12
vinyl
styryl
Ph
Ph
3
6:1
5:1
11:1
Ph
5[g]
5
Ph[h]
74
propane-1,1-dicarboxylates
with
[a] Reaction conditions: Ni(ClO4)2/1e=0.040 mmol/0.044 mmol, 2/3=0.44 mmol/0.20 mmol, 4-Š
molecular sieves. [b] Combined yield of the two diastereomers based on 3. [c] Ratio of isomers (cis/
trans), determined by 1H NMR spectroscopy. [d] Determined by HPLC on a chiral phase. [e] Yield of the
isolated cis isomer. [f] T=À408C. [g] t=5 h. [h] PhCH N(O)Bn was used.
excellent enantioselectivities (91–
97% ee).[14]
Interestingly,optically active
2b reacted smoothly with nitrone
3b in the presence of a catalytic
amount of Ni(ClO4)2 without the
=
entries 1–3). The electronic character of the a-aryl group on
the nitrone had a slight effect on the enantioselectivity of the
reaction (Table 2,entries 4–7). All reactions of both electron-
deficient and electron-rich a-aryl nitrones proceeded with
excellent enantio- and diastereoselectivities. A nitrone with
an a-furyl group underwent the cycloaddition smoothly with
high diastereoselectivity to afford the corresponding 1,2-
oxazine product in quantitative yield with a high ee value
(Table 2,entry 8). A styryl-substituted nitrone reacted to give
the cycloaddition product with high enantioselectivity and
good diastereoselectivity (Table 2,entry 9). Diethyl 2-vinyl-
ligand 1e to give the tetrahydro-1,2-oxazine 4c in high yield
with the same level of enantiomeric purity as that of the
starting material 2b (Scheme 3). This result further supports
the mechanism proposed by Young and Kerr for the [3+3]
and
2-styrylcyclopropane-1,1-dicarboxylates
reacted
smoothly with 3b to give the desired products with good
Scheme 2. Reaction of 3b with excess cyclopropane 2b.
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Communications
Table 3: Kinetic resolution of 2-substituted cyclopropane-1,1-dicarbox-
ylates.[a]
Experimental Section
Typical procedure for the enantioselective [3+3] cycloaddition: A
mixture of Ni(ClO4)2·6H2O (0.040 mmol) and the trisoxazoline
(0.044 mmol) in dimethoxyethane (1 mL) was stirred at 508C for
2 h under nitrogen. The mixture was then cooled to room temperature
and added to the cyclopropane diester 2 (0.44 mmol) with a syringe.
Activated 4-Š molecular sieves (100 mg) were added to the resulting
solution. The mixture was stirred at À308C for 30 min,and then the
nitrone (0.20 mmol) was added. When the reaction was complete
(monitored by TLC),the mixture was passed rapidly through a glass
funnel with a thin layer (20 mm) of silica gel (300–400 mesh),which
was then washed with CH2Cl2 (50 mL). The filtrate was concentrated
under reduced pressure,and the residue was purified by flash
chromatography to afford the product.
X
(Æ)-2
3b
[mmol]
s[b]
t
[h]
ee
Recovery
[mmol]
[%][c]
of 2 [%][d]
Ph
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.40
0.25
0.23
0.23
0.23
0.23
0.23
16
97
81
97
36
70
13
30
48
67
168
96
72
91
96
95
96
97
94
92
43
49
49
49
45
49
40
p-MeC6H4
p-BrC6H4
p-CF3C6H4
p-NO2C6H4
p-ClC6H4
p-MeOC6H4
The general procedure for the kinetic resolution of 2-substituted
cyclopropane 1,1-diesters is similar to that for the enantioselective
[3+3] cycloaddition. The reaction was carried out at the desired
temperature,and the ratio of the nitrone 3b to the 2-substituted
cyclopropane diester (Æ )-2 was changed (see Table 3). The reaction
was quenched when the conversion of (Æ )-2 was equal to or higher
than 50% (monitored by 1H NMR spectroscopy).[15]
48
[a] Reaction conditions: Ni(ClO4)2 (0.040 mmol), 1e (0.044 mmol), 4-Š
molecular sieves. For details, see the Supporting Information. [b] s=
ln[(1ÀC)(1Àee)]/ln[(1ÀC)(1+ee)]; C refers to the conversion of (Æ)-2
(1À(yield of recovered 2)). [c] Determined by HPLC on a chiral phase.
[d] Yield of isolated recovered 2.
Received: November 15,2006
Revised: January 26,2007
Published online: April 17,2007
Keywords: asymmetric catalysis · cycloaddition · cyclopropanes ·
.
kinetic resolution · trisoxazolines
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by direct cycloaddition with the nitrone in the presence of a
catalytic amount of 1e/Ni(ClO4)2 or by 1e/Ni(ClO4)2-cata-
lyzed resolution followed by cycloaddition with the nitrone
(Scheme 3).
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lective and enantioselective catalytic cycloaddition of 2-
substituted cyclopropane-1,1-dicarboxylates with nitrones in
the presence of a trisoxazoline–NiII catalyst. The diastereo-
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resolution of 2-substituted cyclopropane-1,1-dicarboxylates.
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[15] For details,see the Supporting Information.
Angew. Chem. Int. Ed. 2007, 46, 3918 –3921
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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