Enantioselective dearomative [3+2] annulation of 5-amino-isoxazoles with quinone monoimines

Article abstract of DOI:10.1039/d0cc05807f

The first enantioselective dearomative [3+2] annulation of 5-amino-isoxazoles with quinone monoimines was realized using a chiral phosphoric acid as catalyst. Various novel (bridged) isoxazoline fused dihydrobenzofurans bearing two continuous quaternary stereocenters were achieved in moderate to good yields (up to 94%) with moderate to good enantioselectivities (up to 98% ee). The absolute configurations of two products were assigned by X-ray crystal structural analyses and a plausible reaction mechanism was proposed. This journal is

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ARTICLE TYPE
Enantioselective Dearomative [3+2] Annulation of 5-Amino-isoxazoles
with Quinone Monoimines
Hui Liu, a,b,c Yingkun Yan, a,b,c Jiayan Zhang,a,b,c Min Liu, a,b,c Shaobing Cheng, a,b,c Zhouyu Wang*a and
Xiaomei Zhang*a,b
5
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
various (bridged) isoxazoline fused dihydrobenzofurans with
moderate to good yields in moderate to good enantioselectivities.
The first enantioselective dearomative [3+2] annulation of 5-
amino-isoxazoles with quinone monoimines was realized using
a chiral phosphoric acid as catalyst. Various novel (bridged)
50 Results and Discussion
10 isoxazoline fused dihydrobenzofurans bearing two continuous
quaternary stereocenters were achieved in moderate to good
yields (up to 94 %) with moderate to good enantioselectivities
(up to 98 % ee). The absolute configurations of two products
were assigned by X-ray crystal structural analyses and a
15 plausible reaction mechanism was proposed.
First, various chiral phosphoric acids were evaluated in the
dearomative [3+2] coupling of 3,4-dimethylisoxazol-5-amine 1a
with
4-methyl-N-(4-oxocyclohexa-2,5-dienylidene)benzene-
sulfonamide 2a in dichloromethane at 0 oC. As can be seen in Table
55 1, all of the chiral phosphoric acids promoted the reaction smoothly
to provide isoxazoline fused dihydrobenzofuran 3a in moderate
yields (Table 1, entries 1-6) and PA3 which bears a 2-naphthyl
group in the 3,3’ position of BINOL, gave the product 3a in
quantitative yield with the highest ee value of 79% (Table 1, entry
60 3). Thus PA3 was determined as the optimal catalyst and used in
the following investigations.
Introduction
Isoxazolines are important building blocks of a large number of
biologically active compounds.1 Isoxazolines are also versatile
intermediates for the synthesis of important organic molecules.2
20 Consequently, many efficient strategies have been developed to
construct structurally diverse isoxazolines.3,4 However, catalytic
asymmetric synthesis of enantioenriched isoxazolines has been
rarely reported.4a-4f
Table 1 Evaluation of the chiral phosphoric acids and optimization of the
conditions.a
2,3-Dihydrobenzofurans are important building blocks found in
25 a wide range of natural products and pharmaceutical substances.5
Therefore, the construction of 2,3-dihydrobenzofurans is of great
interest to the researchers.6
TsHN
NTs
PA
Solvent
N
O
O
NH2
Asymmetric dearomative reactions7 are efficient strategy to
construct chiral heterocyclic compounds. In the last decade, many
30 types of aromatic compounds have been employed in asymmetric
dearomative reactions. However, dearomative reactions of
isoxazoles have been rarely researched. Very recently, Qi and co-
workers reported a three-component tandem reaction involving
dearomative Michael addition of 5-amino-pyrazoles or 5-amino-
35 isoxazole with nitroolephins followed by cyclization with cyclic
ketones to provide a series of spiral fused polycyclic compounds
with moderate to good yields in good diastereoselectivities.8
In recent years, great progress has been made in catalytic
asymmetric reactions of quinone derivatives for the construction
40 of novel chiral heterocyclic compounds.6c,6e,6f,6h,6j,6m,6n,9,10 Our
group has also been engaged in development of new asymmetric
transformations of quinone derivatives.6m,6n,10d In continuation of
our research on asymmetric reaction of quinone derivatives, herein
we reported the first highly enantioselective dearomative [3+2]
45 annulation of 5-amino-isoxazoles with quinone monoimines
promoted by chiral phosphoric acids. In the presence of a chiral
phosphoric acid, the reactions proceeded efficiently to provide
N
1a
O
O
NH2
2a
3a
65
R1
R2
O
O
O
O
O
P
P
O
OH
O
OH
Ph
Ph
O
O
R2
P
OH
R1
PA4: R2 = 9-anthryl
PA5: R2
PA1: R1 = 9-anthryl
PA2: R1 = 2,4,6-iPr3C6H2
PA3: R1 = 2-naphthyl
=
PA6
Entry PA (mol%) Solvent
Yield ee (%)c
(%)b
T (℃)
t (h)
1
2
3
4
5
6
7
8
9
PA 1 (10)
PA 2 (10)
PA 3 (10)
PA 4 (10)
PA 5 (10)
PA 6 (10)
PA 3 (10)
PA 3 (10)
PA 3 (10)
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
DCE
0
0
0
0
0
0
0
0
0
0.3
0.3
4
0.3
4
94
95
99
99
85
67
99
96
86
59
41
79
9
13
8
79
75
58
4
10
10
20
CHCl3
toluene
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Page 2 of 4
10
11
12
13
14
15
16
PA 3 (10)
PA 3 (10)
PA 3 (10)
PA 3 (10)
PA 3 (10)
PA 3 (10)
PA 3 (5)
CH3CN
Et2O
MTBE
THF
DME
DME
DME
0
0
0
0
0
5
89
73
92
83
87
89
81
77
79
89
92
94
93
92
R4HN
MeOC6H4SO2
45
36
10
12
12
12
O
N
-10
0
O
NH2
a Unless otherwise specified, the reaction was carried out with 0.10 mmol
of 1, 0.12 mmol of 2, and 0.01 mmol of the chiral phosphoric acid PA3 in
1 mL of 1,2-dimethoxyethane at 0 ℃ Isolated yield based on 1.
30 Enantiomeric excess was determined by HPLC analysis on a chiral
stationary phase. d The absolute configuration of 3fa was determined by an
X-ray crystal structural analysis and the other products were assigned
absolute configurations by analogy. Ts = 4-toluenesulfonyl, Ns = 4-
nitrobenzenesulfonyl.
a Unless otherwise specified, the reaction was carried out with 0.10 mmol
of 1a, 0.12 mmol of 2a, and 0.01 mmol of the chiral phosphoric acid in 1
mL of solvent. Isolated yield based on 1a. Enantiomeric excess was
determined by HPLC analysis on a chiral stationary phase. Ts = 4-
5 toluenesulfonyl.
b
c
b
c
Afterwards various solvents were screened in the reaction.
Reactions in 1,2-dichloroethane, chloroform, acetonitrile and ether
gave similar enantioselectivities to dichloromethane (Table 1,
entries 7,8,10 and 11). Toluene delivered much lower
10 enantioselection (Table 1, entry 9). To our delight, reaction in some
ethereal solvents provided the product with obviously higher
enantioselectivities (Table 1, entries 12-14), in which
dimethoxyethane afforded the best result (Table 1, entry 14).
Therefore dimethoxyethane was determined as the optimal solvent
15 and used in the following investigations. When the reaction was
35
Generally,
5-amino-4-methylisoxazoles
with
different
substituents on 3-position were tolerable in the reaction with
quinone monoimine 2a to afford the corresponding isoxazoline
fused dihydrobenzofurans (Table 2, entries 1-10) in moderate to
good yields with high ee values. However, lower
40 enantioselectivities were observed with some 5-amino-3-methyl-
isoxazoles with different substituents on 4-position (Table 2,
entries 11-13), whereas 4-(4-chlorobenzyl)-3-methylisoxazol-5-
amine 1n exhibited good enantioselectivity in reaction with 2a
(Table 2, entry 14). When 3-methyl-4-phenylisoxazol-5-amine 1o
45 was subjected in reaction with 2a, no reaction was observed (Table
2, entry 15). Furthermore, various quinone monoimines with
different substituents on 3-position and quinone monoimines with
different N-substituents were also tested in reaction with 3,4-
dimethyl-isoxazol-5-amine 1a. Generally the corresponding
50 products were obtained with good ee values (Table 2, entries 16-
20), although lower yields were observed with 3ad and 3ae (Table
2, entries 18 and 19). We also tried to use N-Boc-isoxazol-5-amine
in this reaction. However, no reaction was observed, perhaps due
to the decrease of electron density of the isoxazole ring.
o
performed at lower temperature (-10 C), no better resukt was
obtained (Table 1, entry 15). Furthermore, we also tried to lower
the catalyst loading to 5 mol%, but both the yield and the
enantioselectivity decreased (Table 1, entry 16).
With the optimized reaction conditions in hand, the scope of the
reaction was investigated. The results were summarized in Table
2.
20
Table 2 Substrate scope of the enantioselective dearomative [3+2]
25 annulation of 5-amino-isoxazoles 1 with quinone monoimines 2.a
R4HN
R4
R1
R3
N
55
Bridged polycyclic frameworks widely exist in natural products
and pharmaceuticals. Construction of bridged polycyclic
compounds has always been an attractive and challenging task for
chemists.11 Herein we presented the enantioselective synthesis of
novel bridged isoxazoline fused dihydrobenzofurans 3pa-3xa via
PA3
R2
R2
(10 mol%)
R1
N
+
R3
O
O
DME, 0 o
C
NH2
N
O
NH2
1
O
2
3
Entry
3
t
Yield
ee
60 tandem dearomative [3+2] annulation-cyclization of some ethyl 4-
acetate-isoxazol-5-amines with quinone imine 2a.
(%)b (%)c,d
3aa: R1 = Me
3ba: R1= t-Bu
3ca: R1= Ph
12 h
12 h
12 h
60 h
40 h
48 h
7 d
40 h
7 d
48 h
4 d
87
63
79
73
64
82
58
79
55
60
75
49
36
60
NR
94
87
94
94
93
94
93
94
93
94
81
85
81
94
-
TsHN
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
TsHN
Ts
N
R1
R1
3da: R1 = 4-FC6H4
3ea: R1 = 4-ClC6H4
3fa: R1 = 4-BrC6H4
3ga: R1= 4-MeOC6H4
3ha: R1= 3-ClC6H4
3ia: R1 = 2-thienyl
3ja: R1 = 2-furanyl
3ka: R2 = allyl
PA3
O
(10 mol%)
CO2Et
R1
O
N
N
+
O
NH2
DME, 0oC
O
O
NH2
N
O
O
N
1
H
2a
3
TsHN
TsHN
TsHN
TsHN
3la: R2 = Bn
4 d
Cl
F
3ma: R2 = 4-FC6H4CH2 4 d
3na: R2 = 4-ClC6H4CH2 48 h
Ph
R2
O
O
O
O
O
O
O
N
3oa: R2 = Ph
48 h
N
N
O
3pa
O
N
O
N
N
N
H
O
NH2
H
H
3qa
3ra
3ab: R3 = Me
3ac: R3 = Br
16 h
16 h
16 h
94
90
35
92
91
85
TsHN
16
17
18
4.5d, 63% yield
96% ee
3d, 84% yield
90% ee
7d, 52% yield
97% ee
3ad: R3 = OMe
R3
O
N
O
NH2
19
20
3ae: R4 = Ns
3af: R4 = 4-
12 h
12 h
68
90
86
91
2 | Journal Name, [year], [vol], 00–00
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TsHN
TsHN
TsHN
reaction mechanism was proposed. As outlined in Scheme 2, first,
bifunctional phophoric acid activated both isoxazole 1f and
quinone monoimine 2a. Isoxazole 1f attacked quinone monoimine
2a to give the intermediate (4R)-A which underwent aromatization
30 immediately to give phenol intermediate (4R)-B. Finally,
intramolecular acetalization generated isooxazoline fused
dihydrobenzofuran (3aR,8aR)-3fa.
Cl
MeO
Br
O
O
O
O
O
O
N
N
N
O
O
N
O
N
N
H
H
H
3ta
3ua
3sa
60h, 59% yield
94% ee
6d, 76% yield
98% ee
7d, 55% yield
98% ee
TsHN
TsHN
Br
TsHN
O
O
O
O
Ts
O
O
S
N
H
S
HN
O
O
O
N
Br
N
N
N
O
N
O
O
O
N
O
N
NH
H
1,4-addition
H
H
H
Ar
O
P
3va
3wa
3xa
O
O
7d, 67% yield
92% ee
36h, 68% yield
89% ee
4.5d, 73% yield
95% ee
O
N
O
NH
O
Scheme
1
Construction of bridged isoxazoline fused
Ar
(4R)-A
dihydrobenzofurans. Reaction conditions: Unless otherwise
specified, the reactions were carried out with 0.10 mmol of 1, 0.12
mmol of 2a, and 0.01 mmol of PA3 in 1 mL of 1,2-
dimethoxyethane at 0 oC. Yields shown are of the isolated products
and based on 1. Enantiomeric excesses were determined by HPLC
analysis on a chiral stationary phase. The absolute configuration of
10 3xa was determined by an X-ray crystal structural analysis and the
other products were assigned absolute configurations by analogy.
As can be seen in Scheme 1, under the optimized conditions, the
reactions proceeded for prolonged time to provide the bridged
products in moderate yields with good enantioselectivities. The
15 electron nature of 3-substituent has little effect on the result. Thus
a facile strategy has been established to construct such kind of
bridged polycyclic compounds with good enantioselection.
The absolute configurations of 3fa and 3xa were determined as
(3aR,8aS) by X-ray crystal structural analyses12 (Figure 1).
20 Consequently, the other products can be assigned absolute
configurations by analogy.
aromitization
5
Ts
TsHN
HN
Br
Br
acetalization
O
OH
N
N
O
NH2
NH
O
(3aR,8aR)-3fa
(4R)-B
Scheme 2. Plausible reaction mechanism.
35 Conclusions
In conclusion, we have developed an enantioselective
dearomative [3+2] annulation of 5-amino-isoxazoles with quinone
monoimines promoted by a chiral phosphoric acid. Through this
transformation, various isoxazoline fused dihydrobenzofurans
40 were approached with moderate to good yields in moderate to good
enantioselectivities. Moreover, via tandem dearomative [3+2]
annulation-cyclization, some bridged isoxazoline fused
dihydrobenzofurans were also prepared with moderate yields in
good enantioselectivities. The absolute configurations of a fused
45 product and a bridged product were determined by X-ray crystal
structural analyses. Accordingly, a plausible reaction mechanism
was proposed.
TsHN
Br
O
N
O
NH2
(3aR,8aR)-3fa
Acknowledgments
We are grateful for the financial support from National Natural
50 Science Foundation of China (21672208 and 21372218), the fund
of Sichuan Education Department (18TD0023) and the start up
fund for recruited talent of Xihua University (Z201098).
TsHN
Notes and references
a Department of Chemistry, Xihua University
O
O
N
O
N
55 b Asymmetric Synthesis and Chiraltechnology Key Laboratory of Sichuan
Province, Chengdu Institute of Organic Chemistry, Chinese Academy of
Sciences, Chengdu, China, Fax: (+86)28-85257883; Tel: (+86)28-
85257883; E-mail: xmzhang@cioc.ac.cn
H
(3aR,8aR)-3xa
c University of Chinese Academy of Sciences, Beijing,China.
60 † Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Figure 1. X-ray crystal structures of (3aR,8aR)-3fa and (3aR,8aR)-3xa.
Based on the absolute configuration of product 3fa, a plausible
1
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12 CCDC 2012876-2012877 (3fa) and CCDC 2012879-2012880 (3xa)
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
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