Br?nsted acids of anionic chiral Co(III) complexes as catalysts for the stereoselective synthesis of cis-4-aminofuranobenzopyrans

Article abstract of DOI:10.1039/c7ob02452e

A highly enantioselective interrupted Povarov reaction of salicylaldimines and 2,3-dihydrofuran was developed, through the elegant Br?nsted acid catalysis of anionic chiral Co(iii) complexes. This reaction affords the cis-4-aminofuranobenzopyran derivativ

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DOI: 10.1039/C7OB02452E
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Brønsted Acids of Anionic Chiral Co(III) Complexes as Catalysts for
the Stereoselective Synthesis of cis-4-Aminofuranobenzopyrans
Hua-Jie Jiang,b, † Kun Liu,a, † Jing Wang,a Na Lia and Jie Yua,*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A
highly enantioselective interrupted Povarov reaction of activate substrates directly but just offers the environment of
salicylaldimines and 2,3-dihydrofuran was developed, through the centrochirality, hold unique privilege in asymmetric catalysis.5
elegant Brønsted acid catalysis of anionic chiral Co(III) complexes. Very recently, we demonstrated that Brønsted acids of anionic
This reaction affords the cis-4-aminofuranobenzopyran derivatives chiral Co(III) complexes, as bifunctional phase-transfer
with up to 95% yield, >20:1 dr and 96:4 er. Moreover, a one-pot catalysts, enabled a highly enantioselective bromoamino-
three-component procedure of salicylaldehydes, anilines, and 2,3- cyclization of olefins.4b Thus, we anticipated to expand this
dihydrofuran proves to be successful, with higher reaction privileged concept to direct synthesis of valuable 4-
efficiency.
aminobenzopyran derivatives, which are one of the common
subunits of natural products with interesting biological
activities,6 including anti-hypertensive and anti-ischemic
properties, and by that are of high synthetic importance.7
Chiral anion catalysis has seen increasing application as a
general strategy in the field of asymmetric catalysis.1 Chiral
anions (or anionic complexes) generated from organic small
molecules, i.e. the conjugate bases of chiral phosphoric acids2a-
e
and anion-binding (thio)ureas,2f,g are common in this area.
Metal-templated chiral anion catalysis is conceptually
challenging, thus remains underdeveloped.3 Our group have
been focusing on the discovery of easy accesses to octahedral
anionic chiral Co(III) complexes with tunable substituents and
steric centers. A catalyst library is established through one-
step assembly of commercially available amino acids,
salicylaldehydes and earth-abundant cobalt salts, which
facilitates rapid synthesis of optically pure heterocycles.4 It is
noteworthy that the counter cations of the anionic chiral Co(III)
complexes are also crucial in terms of reactivity and the
stereocontrol. In 2015, we reported a sodium salt of anionic
chiral Co(III) complex (Λ-4f)-catalyzed Povarov reaction,
wherein the alkali cation served as a Lewis acid to activate
imines and the weakly coordinated chiral anion delivered
excellent diastereoselectivity and enantioselectivity (Scheme
1a).4a Distinct from the traditional chiral metal complexes and
organocatalysts, this feature should be able to make anionic
chiral Co(III) complexes, in which the metal center does not
Scheme 1 Povarov reaction and interrupted Povarov reaction catalyzed by anionic
chiral CoIII complexes
Catalytic Mannich-acetalization reaction of salicylaldimines
and dienophiles, also referred as interrupted Povarov
reaction,8 was pioneered by Rueping and co-workers.9a Several
variants using different dienophiles, such as vinyl ethers and
enamides, were subsequently established, under the catalysis
of chiral phosphoric acids9b or chiral N,N’-dioxide-Sc(OTf)3
complexes9c. Thus, the development of new chiral catalytic
systems that are capable of achieving the stereocontrol of
a.State Key Laboratory of Tea Plant Biology and Utilization and Department of
Applied Chemistry, Anhui Agricultural University, Hefei, 230036, China.
b.Hefei National Laboratory for Physical Sciences at the Microscale and Department
of Chemistry, University of Science and Technology of China, Hefei, 230026, China.
† H.-J. Jiang and K. Liu contributed equally to this work.
E-mail: jieyu@ahau.edu.cn; Tel: +86-0551-65786906
Electronic Supplementary Information (ESI) available: Experimental details,
characterization and analytical data. See DOI: 10.1039/x0xx00000x
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asymmetric versions is still highly desirable. Herein, we performance of the reaction in n-hexane gave higher
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present a Brønsted acid of anionic chiral Co(III) complex- stereoselectivity and 5 Å M.S. (10.0 mg)DtOuIr:n10e.d103o9u/tC7tOoBb0e24t5h2eE
catalyzed enantioselective interrupted Povarov reaction of optimal additive for this transformation (entry 2 vs. entries 9-
salicylaldimines (1) with 2,3-dihydrofuran (2), affording cis-4- 13). Lowering the temperature resulted in a diminished
aminofuranobenzopyrans derivatives with up to 96:4 er stereoselectivity (entry 14). To our delight, variation of the
(Scheme 1b).
ratio of 1a
/2
improved both the diastereo- and
enantioselectivities (entries 15-16 vs. entry 2) and reducing the
solvent volume led to a further enhancement of the yield (95%)
and stereoselectivity (>20:1 dr and 96:4 er) (entry 17).
Table 1 Optimization of the reaction conditions.a
Under the optimized conditions, we next started to explore
the scope of the asymmetric interrupted Povarov reaction with
respect to the salicylaldimines
1. Unfortunately, the poor
solubility of imines derived from substituted aldehydes or
anilines in n-hexane may lead to significantly reduced
enantioselectivities. Thus, we turned back to examined the
reaction of substrate 1b with a 4-Br substituent on the N-aryl
Entry
1
2
3
4
5
6
7
1
4
yield [%]b
16
89
43
9
drc
5:1
10:1
8:1
9:1
4:1
5.5:1
4:1
5:1
9:1
8:1
5:1
7:1
7:1
4.5:1
11:1
18:1
>20:1
7:1
erd
49:51
84.5:15.5
64:26
45.5:54.5
58.5:41.5
57:43
56.5:43.5
61.5:48.5
84:16
84.5:15.5
72.5:27.5
76.5:23.5
67:33
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
Λ-4a
Λ-4b
Λ-4c
Λ-4d
Λ-4e
Λ-4f
Λ-4g
Λ-4h
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
Λ-4b
ring (Table 1, entry 18). The excess of 2,3-dihydrofuran
2 could
slightly improve the yield of the product 3b with maintained
stereoselectivity (entry 19). Replacement of n-hexane with
16
44
9
toluene and the use of a three times excess of
2 resulted in an
enhancement of both the diastereo- and enantioselectivity
(12:1 dr and 82:18 er) (entries 20-22). These conditions were
therefore selected as optimal for further investigations on the
substrate scope.
8
34
87
80
54
57
29
53
74
50
95
33
64
58
67
68
9e
10f
11g
12h
13i
14j
15k
16l
17l,m
18l,m
19k,m
20n
21o
22p
Table 2 Scope of the substrates.a
77:23
92:8
94.5:5.5
96:4
73.5:26.5
73.5:26.5
80.5:19.5
80:20
R1
4-Br
4-Cl
4-F
3-Cl
H
H
H
H
H
R2
H
H
H
3
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
yield(%)b
68
drc
er d
82:18
81:19
80.5:19.5
75:25
74.5:25.5
82:18
82:18
7:1
8:1
9:1
12:1
Entry
1
2
3
4
5
6
7
8
12:1
11:1
10:1
10:1
7:1
7:1
14:1
8:1
12:1
8:1
10:1
11:1
15:1
75
62
51
46
56
51
43
44
55
44
52
31
82:18
a
H
Unless otherwise noted, the reaction was performed with 1a or 1b (0.2 mmol),
2 (0.4 mmol), catalyst Λ-4 (0.02 mmol), and 5 Ǻ M.S. (10.0 mg) in n-hexane (2 mL)
4-NO2
5-Br
5-Me
5-Cl
5-OMe
5-Br
5-Me
5-Cl
5-Cl
b
c
under nitrogen at room temperature for 24 h. Isolated yield of product.
Determined by 1H NMR spectroscopy of the crude product, the relative
configuration of the major product was confirmed by 1H NMR spectroscopy.
d
81:19
Determined by HPLC analysis on a chiral stationary phase and the er values refer
9
76.5:23.5
80.5:19.5
75:25
81.5:18.5
81.5:18.5
to the major product. e 3 Ǻ M.S. were used. f 4 Ǻ M.S. were used. g CCl4 was used.
10
11
12
13
4-Br
4-Br
4-Br
4-Me
h
i
j
Toluene was used. t-Butyl methyl ether was used. The reaction was
performed at 0°C for 48 h. k The 1a/2 ratio was 1:1.2. l The 1a/2 ratio was 1.5:1. m
n
n-hexane (1 mL) was used. The 1b/2 ratio was 1:1.2 and toluene (1 mL) was
used. o The 1b/2 ratio was 1:2 and toluene (1 mL) was used. p The 1b/2 ratio was
a
1:3 and toluene (1 mL) was used. M. S. = molecular sieves.
Unless otherwise noted, the reaction was performed with 1 (0.2 mmol), 2 (0.6
mmol), catalyst Λ-4b (0.02 mmol), and 5 Ǻ M.S. (10.0 mg) in toluene (1 mL) under
b
c
Initially, the reaction of salicylaldimine 1a with 2,3-
nitrogen at room temperature for 48 h. Isolated yield of major product.
Determined by 1H NMR spectroscopy of the crude product, the relative
dihydrofuran
acids or alkali salts of various anionic chiral Co(III) complexes
(Λ-4a 4h; 10 mol%), which are derived from 3,5-disubstitued
2 was performed by screening either Brønsted
configuration of the major product was confirmed by 1H NMR spectroscopy.
d
Determined by HPLC analysis on a chiral stationary phase and the er values refer
to the major product.
-
salicylaldehydes and L-amino acids (Table 1, entries 1-8).
Among them, Brønsted acid Λ-4b, which bears 3,5-di-tert-
butyl-salicylaldehyde and L-tert-Leucine, delivered the highest
yield of 89%, diastereomeric ratio of 10:1 and enantiomeric
ratio of 84.5:15.5 (entry 2). The screening of the reaction
parameters including additives and solvents suggested that the
As shown in Table 2, various salicylaldimines 1b
substituent on the N-aryl ring (R1) were examined, giving cis-4-
aminofuranobenzopyrans 3b 3e in up to 75% yield with 12:1 dr
-1e with
-
and 82:18 er (Table 2, entries 1-4). Salicylaldimines with the
2 | J. Name., 2017, 00, 1-3
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substituent (R1) on the para-position of N-aryl ring afforded anilines and salicylaldehydes could undergo VietwhAerticltehOrneliene-
DOI: 10.1039/C7OB02452E
better results than that with the substituent on the meta- component interrupted Povarov reaction smoothly to afford
position (entries 1-3 vs. entry 4). However, the introduction of the desired heterocycles in good to excellent
4-nitro group on the salicylaldehyde moiety led to
a
diastereoselectivities and with up to 87:13 er (entries 8-11).
diminished enantioselectivity (74.5:25.5 er, entry 5). The The absolute configuration of the major product 3k was
desired 4-aminofuranobenzopyran derivatives 3g-3j with the determined to be (3aR, 4S, 9aS) by a single crystal X-ray
substituent (R2) on the aromatic ring of the salicylaldehyde diffraction analysis (Figure 1),10 which was identical with the
moiety were obtained in good diastereoselectivities (up to known structure in the chiral phosphoric acids catalyzed
14:1 dr) and moderate enatioselectivities ranging from reaction.9b
76.5:23.5 er to 82:18 er (entries 6-9). Moreover, various
salicylaldimines bearing either a 4-bromo or 4-methyl group on
N-aryl ring, were subjected to the interrupted Povarov
reaction, delivering the corresponding products 3k-3n in up to
55% yield with 15:1 dr and 81.5:18.5 er (entries 10-13).
a
Table 3 Scope of three-component asymmetric interrupted Povarov reaction.
Figure 1 X-ray Structure of the Product 3k.
In compared to the reported works,9a,9c the difference in
stereoselectivity might be due to the activation model of the
catalysts. Based on previous experimental data4 and the crystal
Entry
1
2
3
4
5
6
7
8
R1
4-Br
4-Cl
4-F
3-Cl
H
H
H
4-Br
4-Br
4-Br
4-Me
R2
H
H
H
3
3b
3c
3d
3e
3g
3h
3i
3k
3l
3m
3n
yield(%)b
72
dr c
er d
80:20
81:19
81.5:18.5
80:20
87.5:12.5
84.5:15.5
90:10
>20:1
>20:1
>20:1
>20:1
16:1
>20:1
>20:1
10:1
structure of 3k
,
transition states to explain the
85
64
74
57
60
69
59
64
stereochemistry were proposed (Figure 2). The imine could be
protonated by the chiral Co(III)-templated Brønsted acid to
form iminium ion and the nucleophilicity of the phenol group
of the salicylaldimine would be enhanced via hydrogen
bonding interaction with anion complex as shown in TS-I and
TS-II, respectively. The Mannich reaction could favorably occur
H
5-Br
5-Me
5-Cl
5-Br
5-Me
5-Cl
5-Cl
86:14
81.5:18.5
87:13
on the Si-face of the imine with 2,3-dihydrofuran in TS-I
,
9
10
11
>20:1
14:1
17:1
followed by a fast ketalization to afford the (3aR, 4S, 9aS)-
product, as the Re-face might be disfavored due to the steric
repulsion between the tert-butyl substituent on the Schiff base
and aryl of the salicylaldehyde (TS-II).
58
59
86:14
a
Unless otherwise noted, the reaction was performed with 5 (0.2 mmol), 6 (0.2
mmol), 2 (0.6 mmol), catalyst Λ-4b (0.02 mmol), and 5 Ǻ M.S. (10.0 mg) in
b
toluene (1 mL) under nitrogen at room temperature for 48 h. Isolated yield of
major product. Determined by 1H NMR spectroscopy of the crude product, the
c
relative configuration of the major product was confirmed by 1H NMR
spectroscopy. d Determined by HPLC analysis on a chiral stationary phase and the
er values refer to the major product.
The one-pot three-component asymmetric interrupted
Povarov reaction catalyzed by Brønsted acid Λ-4b was next
investigated. To our delight, the reaction of various anilines
and salicylaldehydes with 2,3-dihydrofuran was successful,
furnishing the cis-4-aminofuranobenzopyran with higher
5
6
2
3
reaction efficiency (up to >20:1 dr, 90:10 er) compared to
those observed for the corresponding reactions with
performed salicylaldimines (Table 3). The reactions between
various 4-halo substituted anilines with salicylaldehyde
Figure 2 Proposed Transition States.
provided the products 3b
with maintained enantioseletivities (Table 3, entries 1-3 vs.
Table entries 1-3). Pleasingly, higher diastereo- and
enantioselectivity (> 20:1 dr, 80:20 er) were observed for m-
chloroaniline (Table 3, entry 4 vs. Table 2, entry 4). The
salicylaldehydes bearing substituent at C5 were nicely
tolerated, and the highest enantiomeric ratio of 90:10 were
obtained for product 3i (entries 5-7). A variety of substituted
-3d in excellent diastereoselectivities
Conclusions
In summary, Brønsted acids of anionic chiral Co(III)
complexes have been revealed to be able to catalyzed an
enantioselective
salicylaldimines and 2,3-dihydrofuran with up to 95%
yield, >20:1 dr and 96:4 er. The three-component protocol also
enables efficient access to
2
interrupted
Povarov
reaction
of
a
variety of chiral cis-4-
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Moskalenko, T. F. Saveleva, T. V. Skrupskaya, K. VAie.wLyArstsicelenOknoli,neI.
A. Godovikov and M. North, Tetrahedron:Asymmetry, 2008,
19, 822; (b) Y. N. Belokon, V. I. MaleDeOv,I:D10..1A0.39K/aCt7aOeBv0,2T4.52FE.
Saveleva, T. V. Skrupskaya, Y. V. Nelyubina and M. North,
Tetrahedron: Asymmetry, 2009, 20, 1746; (c) V. I. Maleev, T.
V. Skrupskaya, L. V. Yashkina, A. F. Mkrtchyan, A. S. Saghyan,
M. M. Il’in and D. A. Chusov, Tetrahedron: Asymmetry, 2013,
24, 178.
aminofuranobenzopyrans with higher outcomes. These
findings not only showed the potential of the anionic chiral
Co(III) complexes in asymmetric catalysis, but also will be able
to inspire the future development of Brønsted acids of anionic
chiral metal complexes. Further application of the catalyst in
other reactions is currently underway.
6
7
(a) E. E. Schweizer and D. Meeder-Nycz, Chem. Heterocycl.
Compd., 1977, 31, 11; (b) J. D. Hepworth, Comprehensive
Heterocyclic Chemistry, Vol. 3 (Eds.: A. R. Katrizky and C.W.
Rees), Pergamon, Oxford, 1984, p. 737.
Conflicts of interest
There are no conflicts to declare.
(a) K. C. Nicolaou, J. A. Pfefferkorn, A. J. Roecker, G.-Q. Cao, S.
Barluenga and H. J. Mitchell, J. Am. Chem. Soc., 2000, 122
,
9939; (b) R. Bergmann and R. Gericke, J. Med. Chem., 1990,
33, 492.
Acknowledgments
8
9
For a review on the interrupted Povarov reaction, see: M.
Fochi, L. Caruana and L. Bernardi, Synthesis, 2014, 46, 135.
(a) M. Rueping and M. Y. Lin, Chem. Eur. J., 2010, 16, 4169; (b)
L. Bernardi, M. Comes-Franchini, M. Fochi, V. Leo, A.
Mazzanti and A. Ricci, Adv. Synth. Catal., 2010, 352, 3399. (c)
Y. Zhang, S. Dong, X. Liu, M. Xie, Y. Zhu, L. Lin and X. Feng,
Chem. Eur. J., 2011, 17, 13684.
We are grateful for financial support from NSFC (Grants
21202155, 21672002), Anhui Provincial Natural Science
Foundation (1408085QB24), Young Talent Program in Anhui
Provincal University (gxyqZD2017017), Opening Project of
State Key Laboratory of Tea Plant Biology and Utilization
(SKLTOF20150108) and the Key Project of Anhui Tabaco
Company (20160551015). We also sincerely thank Prof. Liu-
Zhu Gong (USTC) for his support.
10 CCDC 1470802
(
3k
)
contains the supplementary
crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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4 | J. Name., 2017, 00, 1-3
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DOI: 10.1039/C7OB02452E
A highly enantioselective interrupted Povarov reaction of salicylaldimines was
developed, through the elegant Brønsted acid catalysis of anionic chiral Co(III)
complexes.