Sequential Visible-Light and N-Heterocyclic Carbene Catalysis: Stereoselective Synthesis of Tetrahydropyrano[2,3- b]indoles

Article abstract of DOI:10.1021/acs.orglett.0c01447

An efficiently stereoselective [4 + 2] cycloaddition of 3-alkylenyloxindoles and α-diazoketones through sequential visible-light photoactivation and N-heterocyclic carbene catalysis was achieved. A series of tetrahydropyrano[2,3-b]indoles with an all-carbon quaternary stereocenter were obtained in good yields with excellent diastereo- and enantioselectivities.

Full text of DOI:10.1021/acs.orglett.0c01447

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Letter
Sequential Visible-Light and N‑Heterocyclic Carbene Catalysis:
Stereoselective Synthesis of Tetrahydropyrano[2,3‑b]indoles
Chengyuan Wang, Zheyuan Wang, Jin Yang, Shi-Hui Shi, and Xin-Ping Hui*
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ABSTRACT: An efficiently stereoselective [4 + 2] cycloaddition
of 3-alkylenyloxindoles and α-diazoketones through sequential
visible-light photoactivation and N-heterocyclic carbene catalysis
was achieved. A series of tetrahydropyrano[2,3-b]indoles with an
all-carbon quaternary stereocenter were obtained in good yields
with excellent diastereo- and enantioselectivities.
n recent decades, N-Heterocyclic carbenes (NHCs), which
exhibit a characteristic ability to access umpolung reactivity,
herein an efficiently stereoselective synthesis of tetrahydro-
pyrano[2,3-b]indoles bearing all-carbon quaternary stereo-
centers (Scheme 1).
I
have been widely used as organocatalysts for organic
synthesis.1 Because of its own reaction mode, NHC catalysis
retains certain limitations. Along with this development,
cooperative catalysis, in which NHCs with a second activation
mode promote elusive reactivity, has also been accomplished.2
Nevertheless, the combination with NHC mainly focused on
the Lewis acid,3 Brønsted acid,4 amine,5 and transition metal.6
Because the Breslow intermediate or NHC can be oxidized by
the photosensitizer,7a which results in the destruction of
individual reactivity, few protocols of the combination of NHC
and visible-light catalysis have been developed for synthetic
transformations. In 2012, Rovis and coworkers7a developed the
catalytic asymmetric α-acylation of tertiary amines with
aldehydes by NHC/photoredox dual catalysis. Very recently,
Ye’s group reported the visible-light/NHC catalysis for the γ-
and ε-alkylation of enals7b and the ring-opening, γ-alkylation of
cyclopropane enal,7c respectively. Therefore, it is urgent to
develop new catalytic strategies involving photocatalyst-free
visible-light activation and NHC catalysis.
Ketenes are important active intermediates because of their
versatile reactivities,8 and elegant perspectives have been
developed for the synthesis, theory, and practical applications,
in particular, in the asymmetric annulation.9 Among them,
NHC as a nucleophilic catalyst has greatly contributed to the
asymmetric [2 + n] annulation of ketenes. In 2008,
enantioenriched N-Boc β-lactams were synthesized by the
NHC-catalyzed Staudinger reaction of ketenes with imines.10
Since then, NHC-catalyzed asymmetric [2 + n] annulations
involving ketenes have rapidly grown.11 The photolytic Wolff
rearrangement of α-diazoketones is an efficient method to
generate ketenes,12 which can be applied in a sequential
reaction. Recently, a few asymmetric reactions13 were reported
through visible-light photoactivation/metal or organocatalysis.
On the basis of the sequential visible-light-induced Wolff
rearrangement/NHC-catalyzed asymmetric annulation strategy
and in a continuation of our previous research,14 we report
Scheme 1. Stereoselective Synthesis of 4 by Visible-Light/
NHC Catalysis
We commenced our studies with (E)-ethyl 2-(1-benzyl-2-
oxoindolin-3-ylidene)acetate (1a) and 1-diazo-1-phenyl-prop-
an-2-one (2a) under blue light/NHC catalyst 3a (0.1 equiv)
and 0.11 equiv of NaOAc in THF at 20 °C for 6 h.
Unfortunately, a trace of product 4aa was formed (Table 1,
entry 1). When the asymmetric reaction was performed under
blue light/NHC catalyst 3b, the desired product 4aa was
obtained in 94% yield with >20:1 dr and 68% ee (entry 2).
Furthermore, a series of NHC catalysts 3c−e were explored,
and the results showed that NHC 3c was the efficient catalyst
(entry 3). For NHC catalyst 3e, a trace of product 4aa was
afforded (entry 5). After screening of solvents, ethers were
found to be favorable to this reaction, and THF proved to be
Received: April 26, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01447
© XXXX American Chemical Society
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A

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a
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Substrate Scope
temp time
b
c
entry cat.
base
solvent
(°C)
(h) yield (%) ee (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
3c
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
K2CO3
K3PO4
Cs2CO3
DBU
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
THF
THF
THF
THF
THF
DCM
MTBE
Et2O
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
20
20
20
20
20
20
20
20
20
20
20
20
0
−15
−30
−35
−40
−35
6
6
6
6
6
12
6
6
6
6
trace
94
92
68
76
71
84
trace
trace
90
87
92
93
90
84
96
92
82
82
trace
85
74
74
76
78
63
77
80
82
88
91
9
10
11
12
13
14
15
16
17
18
6
6
12
12
24
24
48
24
d
91
a
Reaction conditions: 1a (0.10 mmol), 2a (0.20 mmol, 2 equiv), cat.
3 (0.01 mmol, 0.1 equiv), base (0.011 mmol, 0.11 equiv), solvent (1
mL). The dr values (>20:1) were determined by 1H NMR (400
b
c
MHz) of the crude product. Isolated yields. Determined by chiral
d
HPLC analysis. Compound 2a (0.22 mmol) was used.
the best choice (entries 3 and 6−8). Dichloromethane gave a
trace of the product, even after 12 h (entry 6). Further
optimization of bases revealed that inorganic bases such as
NaOAc, K2CO3, K3PO4, and Cs2CO3 gave better yields than
DBU (entries 3 and 9−12), and K3PO4 was chosen as the
optimal base (entry 10). Subsequently, the reaction temper-
ature was investigated. When the reaction temperature was
reduced from 0 to −35 °C, lower yields and higher ee values
were observed (entries 13−16). Upon further reduction to
−40 °C, a trace of product 4aa was afforded (entry 17). To
improve the yield of this asymmetric reaction, the amount of
α-diazoketone 2a was increased. The product 4aa was afforded
in 85% yield with >20:1 dr and 91% ee when 2.2 equiv of α-
diazoketone 2a was used (entry 18). Finally, the optimal
reaction conditions were established to be 1a/2a/K3PO4 in a
1/2.2/0.11 molar ratio under blue light/NHC 3c (0.1 equiv)
in THF at −35 °C for 24 h.
To demonstrate the generality of the visible-light/NHC
catalyst 3c-promoted stereoselective reaction, a variety of 3-
alkylenyloxindoles and α-diazoketones were investigated
(Scheme 2). The asymmetric reaction of 3-alkylenyloxindoles
1a−c bearing different ester groups with 1-diazo-1-phenyl-
propan-2-one (2a) was first explored, and the results showed
that the ester groups were well tolerated, and the desired
products 4aa−ac were afforded in moderate to good yields
with high diastereo- and enantioselectivities. It should be noted
a
Reaction conditions: 1a (0.10 mmol), 2 (0.22 mmol, 2.2 equiv), cat.
3c (0.01 mmol, 0.1 equiv), K3PO4 (0.011 mmol, 0.11 equiv), and in
THF (1.0 mL) at −35 °C under the irradiation of blue LED for 24 h.
Isolated yields of products 4. The diastereomeric ratios were
determined by H NMR spectroscopic analysis. The ee values were
determined by HPLC. Reaction temperature: −20 °C, reaction time:
16 h.
1
b
that the 3-alkylenyloxindole 1c with a bulky hindrance ester
group worked well to give product 4ac in 78% yield with >20:1
dr and 90% ee. In addition, different N-protecting groups of 3-
alkylenyloxindoles were investigated, and the results showed
that the protective groups have a crucial effect on the
asymmetric reaction. For the N-benzoyl-group-substituted
product 4ae, moderate diastereo- and enantioselectivities
were obtained. However, N-Ts-substituted 3-alkylenyloxin-
dole, a complex reaction system, was observed under the
standard conditions. When N-unprotected 3-alkylenyloxindole
was used, a trace product was afforded at −35 °C for 36 h. On
the basis of the results, N-benzyl group was the best choice.
When the asymmetric reaction of 3-alkylenyloxindoles that
contain electron-withdrawing or -donating groups was carried
out, the products 4af−ak were obtained in good yields with
B
https://dx.doi.org/10.1021/acs.orglett.0c01447
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excellent diastereo- and enantioselectivities. Steric hindrance at
the phenyl ring had been proved to be obvious, and 8-chloro-
substituted product 4al was afforded in 80% yield with 8.2:1 dr
and 89% ee.
Scheme 4. In Situ Generation of Ketene from Acyl Chloride
Next, we turned our attention to the scope with respect to
the α-diazoketones 2. When the asymmetric reactions of 3-
alkylenyloxindole 1a and α-diazoketones 2b−h, bearing a
substituent on the phenyl ring, were conducted under the
optimized conditions, the desired products were obtained in
low yield. Thus the reaction of 3-alkylenyloxindole 1a with α-
diazoketones 2 was carried out at −20 °C for 16 h. α-
Diazoketones 2b−h with both electron-donating and -with-
drawing groups on the phenyl ring gave high yields (4am−as,
69−85%) and diastereo- and enantioselectivities (>20:1 dr,
88−92% ee). The results showed that steric hindrance had a
great influence on the yields and enantioselectivities (4ao−aq).
2-Naphthyl-substituted α-diazoketone 2i could also undergo
the cascade reaction, and product 4at was afforded in 76%
yield with >20:1 dr and 91% ee. It is worth noting that the α-
diazoketones with ethyl or but-3-en-1-yl groups gave higher
yields and enantioselectivities than those with a methyl group
(4au−av versus 4aa, 4aw−ay, 4ba versus 4af−ah, 4ao)
because ethyl- or but-3-en-1-yl-group-substituted α-diazoke-
tones 2j−m were more stable than α-diazoketone 2a in the
reaction conditions. Because part of the compound 4az
decomposed in the silica gel column, 62% yield was afforded.
The absolute configuration of the product was determined
to be (3R,4R)-4au by using X-ray crystal analysis (Figure 1).
A plausible catalytic cycle for the visible-light/NHC-
catalyzed cycloaddition is proposed, as shown in Scheme 5.
Scheme 5. Proposed Catalytic Cycle
Initially, ketene I is formed from α-diazoketone 2 through a
Wolff rearrangement reaction under blue light.13 Subsequently,
the addition of NHC 3c′ to ketene I yields enolate II, followed
by the [4 + 2] annulation of enolate II with 3-alkylenyl-
oxindole 1 to give intermediate III. Finally, intermediate III is
transformed into tetrahydropyrano[2,3-b]indole 4 with the
release of the NHC catalyst 3c′.
In conclusion, we have developed an efficiently stereo-
selective [4 + 2] cycloaddition of 3-alkylenyloxindoes and α-di-
azoketones for the synthesis of tetrahydropyrano[2,3-b]-
indoles through sequential Wolff rearrangement/NHC catal-
ysis. This cascade approach involves the combination of the
visible-light-induced generation of the ketenes from α-diazo-
ketones with NHC-catalyzed asymmetric annulation. The
products with an all-carbon quaternary stereocenter were
obtained in good yield with excellent diastereo- and
enantioselectivities. Further studies of visible-light/organo-
catalyzed reactions are ongoing.
Figure 1. X-ray crystallography of compound 4au.
The absolute configuration of the other products was
tentatively assigned by analogy. The synthetic potential of
the asymmetric reaction was disclosed by a scale-up synthesis
(Scheme 3), the annulation proceeded smoothly, and the
adduct 4av was afforded in 91% yield with >20:1 dr and 92%
ee. The method to generate ketenes has an important effect on
this reaction. It is regretful that ketene generated in situ from 2-
phenylbutanoyl chloride under triethylamine did not occur for
this reaction (Scheme 4).
ASSOCIATED CONTENT
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01447.
Experimental procedures and spectroscopic data (PDF)
Scheme 3. Scale-Up Synthesis of Adduct 4av
Accession Codes
CCDC 1985950 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
■
Xin-Ping Hui − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, P. R. China;
orcid.org/0000-0001-9108-7209; Email: huixp@
lzu.edu.cn
Authors
Chengyuan Wang − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, P. R. China
Zheyuan Wang − State Key Laboratory of Applied Organic
Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, P. R. China
Jin Yang − State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou
University, Lanzhou 730000, P. R. China
Shi-Hui Shi − Shaanxi Key Laboratory of Chemical Reaction
Engineering, College of Chemistry and Chemical Engineering,
Yan’an University, Yan’an 716000, Shaanxi, P. R. China
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01447
Notes
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support of the National
Natural Science Foundation (21971094) and the Natural
Science Basic Research Plan in Shaanxi Province
(2018JQ2063).
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