Asymmetric Synthesis of Fused Bicyclic N,O- and O,O-Acetals via Cascade Reaction by Gold(I)/N,N′-Dioxide-Nickel(II) Bimetallic Relay Catalysis

Article abstract of DOI:10.1002/adsc.201800576

An efficient catalytic asymmetric cyclization/inverse-electron-demand hetero-Diels-Alder cascade reaction of β,γ-unsaturated α-ketoesters with alkyl amides or alcohols were accomplished. The process was based on the utilization bimetallic catalyst system with achiral gold(I) catalyst and chiral N,N′-dioxide/Ni(II) catalyst, delivering a variety of fused bicyclic N,O-acetals or O,O-acetals in up to 99% yield and 99% ee with >19:1 dr under mild reaction conditions. Based on the control experiments and previous studies, a possible reaction pathway of bimetallic relay catalysis cascade reactions was presented. (Figure presented.).

Full text of DOI:10.1002/adsc.201800576

Title: Asymmetric Synthesis of Fused Bicyclic N,O- and O,O-Acetals
via Cascade Reaction by Gold(I)/N,N'-Dioxide-Nickel(II)
Bimetallic Relay Catalysis
Authors: Xiaoming Feng, Bowen Hu, Jun Li, Weidi Cao, Qianchi Lin,
Jian Yang, Lili Lin, and Xiaohua Liu
This manuscript has been accepted after peer review and appears as an
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201800576
Link to VoR: http://dx.doi.org/10.1002/adsc.201800576

10.1002/adsc.201800576
Advanced Synthesis & Catalysis
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Asymmetric Synthesis of Fused Bicyclic N,O- and O,O-Acetals
via Cascade Reaction by Gold(I)/N,N'-Dioxide-Nickel(II)
Bimetallic Relay Catalysis
Bowen Hu,a Jun Li,a Weidi Cao,a Qianchi Lin,a Jian Yang,a Lili Lin,a Xiaohua Liu,a and
Xiaoming Feng*a
a
Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University,
Chengdu 610064, China. E-mail: xmfeng@scu.edu.cn; Fax: +86 28 85418249; Tel: +86 28 85418249
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract:
An
efficient
catalytic
asymmetric
cyclization/inverse-electron-demand
hetero-Diels-Alder
cascade reaction of β,γ-unsaturated α-ketoesters with alkyl
amides or alcohols were accomplished. The process was
based on the utilization bimetallic catalyst system with
achiral gold(I) catalyst and chiral N,N'-dioxide/Ni(II)
catalyst, delivering a variety of fused bicyclic N,O-acetals
or O,O-acetals in up to 99% yield and 99% ee with >19:1
dr under mild reaction conditions. Based on the control
experiments and previous studies, a possible reaction
pathway of bimetallic relay catalysis cascade reactions was
presented.
Figure 1 Representative natural products contain fused
bicyclic N,O-acetals or O,O-acetals.
reached one target skeleton and still suffered from
limited substrate scopes due to the limited sources of
hetero-cycloalkenes. Xu’s group reported a bimetallic
Keywords: asymmetric relay catalysis; gold; nickel;
cycloaddition; fused bicyclic heterocycles
Au(I)/Ga(III)
intramolecular
relay
catalysis
for
cascade
hydroamination/HDA
reactions
between alkynamine and β,γ-unsaturated α-ketoesters,
giving racemic bicyclic aminals in moderate yields
(Scheme 1b).10
Fused bicyclic N,O-acetals and O,O-acetals are
important motifs in many natural products and
bioactive compounds.1,2 For examples, Graciline3a
and Macrodasine A, B3b containing tetrasubstituted
carbon centre at the ketal position are bioactive
alkaloids (Figure 1). Several effective methods for
synthesizing N,O-acetals or O,O-acetals have been
developed,4 however, the enantioselective examples
are still rare, especially for the construction of chiral
tetrasubstituted carbon centres. Wang’s group
developed BINOL−Ti(OiPr)4 complex to promote
tandem reaction of tertiary enamides with
salicylaldehydes, affording chiral fused bicyclic N,O-
acetals.5 Chiral Lewis acid catalyzed inverse-electron-
demand hetero-Diels-Alder (IED-HDA) reactions6
between hetero-cycloalkenes and β,γ-unsaturated α-
ketoesters are alternatively common methods to
access these two skeletons. Tang and co-authors
realized the asymmetric construction of bicyclic N,O-
acetals through Cu(II)/SaBOX catalyzed IED-HDA
reaction of cyclic enamines.7 Our group reported
N,N'-dioxide8/Er(OTf)3 catalyzed IED-HDA reaction
of cycloalkene ether to afford chiral bicyclic O,O-
acetals.9 However, most of these methods only
Recently, our group developed a bimetallic
gold(I)/chiral N,N’-dioxide-nickel(II) relay catalysis
for the reaction between β,γ-unsaturated α-ketoesters
and alkynamines or alkynyl alcohols (Scheme 1a).11
In this case, exocyclic alkene M1 was obtained via a
5-exo-dig process from alkynyl alcohols or amides in
the presence of π-acid Au(I), subsequently σ-metal
Lewis acid chiral N,N’-dioxide-nickel(II) catalyzed-
IED-HDA reaction with β,γ-unsaturated α-ketoesters
occurred, resulting in the corresponding spiroketals
and spiroaminals. This one-pot bimetallic relay
catalytic strategy allowed the synthesis of complex
molecular structures from simple starting materials to
be more economic and handy.12 With the aim of
synthesizing fused bicyclic N,O- and O,O-acetals, we
conceived that if choosing the compounds 1 as the
starting materials, based on Baldwin’s rules, a 5-
endo-dig way would be favored to give cycloalkene
M2, which could react with β,γ-unsaturated α-
ketoesters to yield the desired fused bicyclic products
in the following IED-HDA reaction (Scheme 1c). On
the other hand, the powerful methodology could
1
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10.1002/adsc.201800576
Advanced Synthesis & Catalysis
provide fused bicyclic N,O- and O,O-acetals
containing three continuous chiral carbon centers,
including a tetrasubstituted carbon centre13 which has
not been realized.
Ni(OTf)2/L-PiPr2 (1:1, 10 mol%) as dual catalysts in
CHCl2CHCl2 at 35 C for 48 h.
Table 1 Optimization of the reaction conditions.a
Yield Ee
Entry Au/Ag
Ligand
(%)b
Trace
21
(%)c,d
-
1
PPh3AuCl/-
L-PiPr2
L-PiPr2
L-PrPr2
L-RaPr2
L-PiEt2
L-PiMe2
L-PiPr2
L-PiPr2
L-PiPr2
L-PiPr2
L-PiPr2
L-PiPr2
L-PiPr2
-
2
3
4
5
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
PPh3AuCl/AgOTf
Me2SAuCl/AgOTf
IMesAuCl/AgOTf
IPrAuCl/AgOTf
IPrAuCl/AgNTf2
IPrAuCl/AgNTf2
-
93
0
26
20
21
21
24
Trace
46
60
86
94
66
87
41
97
-
97
97
97
97
-
6
7e
Scheme 1 a, b) The reactions of heterosubstituted hex-1-
yne with β,γ-unsaturated α-ketoesters. c) Cyclization/IED-
HDA reactions of heterosubstituted pent-1-yne with β,γ-
unsaturated α-ketoesters.
8e
9e
10e
11e
12e,f
13e,f,g
n.r.
-
Initially, alkynamine 1a and β,γ-unsaturated α-
ketoester 2a were selected as the model substrates to
identify the optimal reaction conditions. No reaction
took place when PPh3AuCl and Ni(OTf)2/L-PiPr2
were used as the bimetallic catalysts (Table 1, entry
1). Considering that counter ions of Au(I) may have
an effect on the reactivity,14 AgOTf was used to
exchange chloride ion. As expected, the desired
product 3a could be obtained in 21% yield with 93%
ee (Table 1, entry 2). The structure of N,N'-dioxides
ligands have a strong effect on the enantioselectivity,
neither the ligand L-PrPr2 derived from L-proline nor
the ligand L-RaPr2 derived from L-ramipril were
superior to an L-pipecolic acid derived L-PiPr2
(Table 1, entry 2 vs. entries 3 and 4). More sterically
hindered ortho-substituents on aniline of L-PiPr2
were beneficial to achieve higher enantioselectivity
(Table 1, entry 2 vs. entries 5 and 6). The ee value of
3a could be improved to 97% with CHCl2CHCl2 as
the solvent (Table 1, entry 7). In order to increase the
yields, different gold(I) salts were examined. It was
found that IPrAuCl gave the best result (Table 1,
entries 7-10). To our delight, the yield of the product
3a could be further increased to 86% if AgNTf2 was
used (Table 1, entry 11). When changing the ratio of
1a and 2a from 1:1 to 1.5:1, the isolated yield could
be further improved to 94% (Table 1, entry 12). The
cascade reactions could not give the desired product
3a with the absence of IPrAuCl/AgNTf2 or
Ni(OTf)2/L-PiPr2 (Table 1, entries 13-14), suggesting
that the bimetallic relay catalysis is crucial to this
reaction. Thus, the optimized conditions were
established, IPrAuCl/AgNTf2 (1:1, 5 mol%) and
14e,f,h IPrAuCl/AgNTf2
-
a
Unless otherwise noted, all reactions were performed
with 1a (0.1 mmol), 2a (0.1 mmol), Au(I)/Ag(I) (1:1, 5
mol%), Ni(OTf)2/ligand (1:1, 10 mol%), CH2Cl2 (0.2 mL),
b
c
35 C, 48 h. Isolated yields. Determined by HPLC or
d
SFC analysis on a chiral stationary phase. Up to 19:1 dr
was obtained in all cases determined by 1H NMR analysis.
e CHCl2CHCl2 (0.2 mL) was used as the solvent. f 1a (0.15
g
h
mmol) was used. Without IPrAuCl/AgNTf2. Without
Ni(OTf)2/L-PiPr2. n.r. = No reaction. The intermediate
could be generated and quickly transformed into other
unknown compounds. No IED-HDA product was observed.
With the optimized reaction conditions established,
we then tested the substrate scope. As shown in Table
2, changing the ester group from methyl to a bulkier
aliphatic ethyl or benzyl group, the yields and ee
values could still remain (Table 2, entries 2-3). The
electronic properties and steric hindrance of
substituents on the γ-phenyl group of the β,γ-
unsaturated α-ketoesters had little effect on the
cascade reactions. The corresponding products were
obtained in good yields (77-99% yield) and excellent
ee values (96-99% ee) (Table 2, entries 4-17).15 2-
Naphthyl and heteroaromatic substrates were also
tolerated in the cascade reactions (Table 2, entries 18-
20). Notably, the reaction could also perform well for
bearing γ-cyclohexyl substituted β,γ-unsaturated α-
ketoester, providing 3u in 81% yield and 96% ee
(Table 2, entry 21). It was noteworthy that only one
of the diastereoisomers was detected in these cases of
alkynamine 1a.
2
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10.1002/adsc.201800576
Advanced Synthesis & Catalysis
Table 2 Substrate scope of the β,γ-unsaturated α-
enantiomer of 4j was determined by X-ray diffraction
ketoesters.a
crystal analysis.15
Table 3 Substrate scope of alkynyl alcohols and alkynyl
amides 1a
Yield
(%)b
Ee
Entry R1
R2
3
(%)c
1
2
3
4
5
6
7
8
C6H5
C6H5
C6H5
Me
Et
94
92
96
88
91
84
94
81
83
87
90
91
95
98
99
87
97
98
99
97
98
96
99
98
98
98
97
98
99
97
99
97
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
Bn
Me
Me
Me
Me
Me
2-FC6H4
3-ClC6H4
4-FC6H4
4-ClC6H4
4-BrC6H4
2-MeOC6H4 Et
3-MeOC6H4 Me
4-MeOC6H4 Me
3-MeC6H4
4-MeC6H4
4-PhC6H4
2,6-Me2C6H3 Me
3,4-Cl2C6H3
9
10
11
12
13
14
15
16
Me
Me
Me
Me
17
Me
77
97
3q
18
19
20
2-naphthyl
2-thienyl
3-thienyl
c-hexyl
Me
Me
Me
Me
88
66
76
81
98
96
98
96
3r
3s
3t
21
3u
a
a
Unless otherwise noted, reactions were performed with
Unless otherwise noted, reactions were performed with
IPrAuCl/AgNTf2 (1:1, 5 mol%), Ni(OTf)2/L-PiPr2 (1:1, 10
mol%), 1a (0.15 mmol), 2 (0.1 mmol) in CHCl2CHCl2 (0.2
IPrAuCl/AgNTf2 (1:1, 5 mol%), Ni(OTf)2/L-PiPr2 (1:1, 10
mol%), 1 (0.15 mmol), 2a (0.1 mmol) in CHCl2CHCl2 (0.2
mL) at 35 C for 24 h. Yield is that of isolated product.
1
mL) at 35 C for 48 h. Dr > 19:1 was determined by H
1
The dr values were determined by H NMR analysis and
NMR analysis in all cases. The absolute configuration of
3p (4S,3aS,7aR) was determined by using single-crystal X-
ray diffraction analysis and others were determined by
the ee values were determined by HPLC or SFC analysis
on a chiral stationary phase. The absolute configuration of
4j (3aS,4S,7aS) was determined by using single-crystal X-
ray diffraction analysis and others (4e-l, 4n) were
determined by comparing the circular-dichroism spectra
with that of 4j. b The reaction was carried out for 48 h.
b
comparing the circular-dichroism spectra with that of 3p.
Isolated yields. c Determined by HPLC or SFC analysis on
a chiral stationary phase.
Subsequently, we turned our attention to
broadening the substrate scope to various alkynyl
alcohols and alkynyl amines. The pent-3-yn-1-amine
1a' could give the desired fused bicyclic N,O-acetals
4a in excellent yield and ee value with moderate
diastereoselectivity,
which
contained
a
tetrasubstituted carbon atom at the ketal position.
However, the reaction of but-3-yn-1-ol showed poor
reactivity at the cyclization step and only trace
amount of the product 4b was obtained under the
standard reaction conditions.16 Nevertheless, the
internal alkynyl alcohols could smoothly transform
into the corresponding fused bicyclic O,O-acetals
with good results no matter what R3 was alkyl or aryl
(4c-4n). The absolute configuration of the major
Scheme 2 a) Gram-scale version of the reaction. b)
Reduction of ester 3a to alcohol 5a.
3
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10.1002/adsc.201800576
Advanced Synthesis & Catalysis
To explore the efficiency of bimetallic gold(I) and
Figure 2 Proposed catalytic cycle.
Ni(OTf)2/L-PiPr2 relay catalysis, the gram scale
experiment was performed. Alkynyl amide 1a (4.5
mmol) reacted with β,γ-unsaturated α-ketoester 2p (3
mmol), providing the desired product 3p in 84% yield
(1.21 g) and 97% ee (Scheme 2a). Reduction of
bicyclic N,O-acetal 3a afforded the product 5a in
88% yield without loss of ee value (Scheme 2b).
To gain insight into the reaction mechanism, some
control experiments were carried out. When phenyl
alkynyl amide 1o was used in the cascade reactions,
the corresponding product 4o was obtained in 70%
yield, 2.5:1 dr with 97%/80% ee, meanwhile 29%
the fused bicyclic N,O-acetal 3a.
In summary, we realized bimetallic gold(I) and
chiral N,N'-dioxide/Ni(II) catalyzed asymmetric
cascade cyclization/inverse-electron-demand hetero-
Diels-Alder reactions. A range of fused bicyclic N,O-
acetals
and
O,O-acetals
containing
chiral
tetrasubstituted carbon center at fused ring position
were obtained in moderate to excellent yields (up to
99% yield) with excellent diastereo- and
enantioselectivities (up to > 19:1 dr, up to 99% ee).
yield of the intermediate 6o was isolated (Scheme 3a). Further studies on applying this catalyst system to
The IED-HDA reaction between 6o and β,γ-
unsaturated α-ketoesters 2a can proceed smoothly in
the presence of Ni(OTf)2/L-PiPr2 complex, providing
the desired fused bicyclic N,O-acetal 4o in 62% yield
(dr = 2.5:1, 97%/80% ee) (Scheme 3b). These results
indicated that the reaction pathway is a cascade relay
catalysis of gold-catalyzed cyclization and
Ni(OTf)2/L-PiPr2 catalyzed IED-HDA reaction.
other related reactions are underway.
Experimental Section
To a dry reaction tube was charged with L-PiPr2-Ni(OTf)
(1:1, 10 mol%), β,γ-unsaturated α-ketoester 2a (0.1 mmol)2,
IPrAuCl (5 mol%) and AgNTf2 (5 mol%). Next,
CHCl2CHCl2 (0.2 mL) was added and the mixture was
stirred at 35 C for 30 minutes. Then, alkynamine 1a (0.15
mmol) was added. The reaction mixture was stirred at 35
C for indicated time. The residue was purified by flash
chromatography on silica gel column (Eluent: petroleum
ether/ethyl acetate = 4/1) to afford the product 3a. The
enantiomeric excess was determined by chiral HPLC and
the dr of product was determined by NMR analysis.
Acknowledgments
The study was funded by the National Natural Science
Foundation of China (Nos. 21432006 and 21572136).
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Advanced Synthesis & Catalysis
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5
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10.1002/adsc.201800576
Advanced Synthesis & Catalysis
COMMUNICATION
Asymmetric Synthesis of Fused Bicyclic N,O- and
O,O-Acetals via Cascade Reaction by Gold(I)/N,N'-
Dioxide-Nickel(II) Bimetallic Relay Catalysis
Adv. Synth. Catal. Year, Volume, Page – Page
Bowen Hu, Jun Li, Weidi Cao, Qianchi Lin, Jian
Yang, Lili Lin, Xiaohua Liu, and Xiaoming Feng*
6
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