Organocatalytic Aza-Michael/Michael Cyclization Cascade Reaction: Enantioselective Synthesis of Spiro-oxindole Piperidin-2-one Derivatives

Article abstract of DOI:10.1021/acs.orglett.0c00779

A simple, direct, and highly enantioselective synthesis of spiro-oxindole piperidin-2-one derivatives was achieved through an aza-Michael/Michael cyclization cascade sequence using a squaramide catalyst. The desired products were obtained in excellent yields (up to 99%) and good to high stereoselectivities (up to >20:1 dr and up to 99% ee) under mild conditions.

Full text of DOI:10.1021/acs.orglett.0c00779

pubs.acs.org/OrgLett
Letter
Organocatalytic Aza-Michael/Michael Cyclization Cascade Reaction:
Enantioselective Synthesis of Spiro-oxindole Piperidin-2-one
Derivatives
Qing-Gang Tang, Sen-Lin Cai, Chuan-Chuan Wang, Guo-Qiang Lin, and Xing-Wen Sun*
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ABSTRACT: A simple, direct, and highly enantioselective synthesis of spiro-oxindole piperidin-2-one derivatives was achieved
through an aza-Michael/Michael cyclization cascade sequence using a squaramide catalyst. The desired products were obtained in
excellent yields (up to 99%) and good to high stereoselectivities (up to >20:1 dr and up to 99% ee) under mild conditions.
he aza-Michael reaction, a well-known straightforward
Treaction to construct a carbon−nitrogen bond, is
frequently applied as the shortest method to synthesize β-
aminocarbonyl compounds, such as β-amino aldehydes1 and
ketones,2 β-amino acids,3 β-lactams,4 and their derivatives.5
During the past decades, numerous asymmetric aza-Michael
reactions have been reported6 in which primary amines, N-
protected secondary amines (Boc, Cbz, or Ts group),
succinimides, and hydrazines were used as nitrogen nucleo-
philes. To the best of our knowledge, N-alkoxyl acylamides are
rarely utilized as nitrogen nucleophiles to react with
unsaturated ketones or esters. Therefore, great efforts are still
being contributed to the exploration of new nitrogen
nucleophiles for catalytic asymmetric aza-Michael reactions.
A spiro-oxindole scaffold bearing a quaternary carbon
stereocenter at the 3-position, especially with an N-heterocycle,
is a ubiquitous structural moiety in many bioactive natural
products and synthetic compounds. For example, horsfiline
Figure 1. Bioactive natural and synthetic compounds containing a
spiro-oxindole scaffold.
(Figure 1),7 isolated from the traditional herbal medicine plant
were reported for these N-heterocycle spirooxindoles,11
Horsfieldia superba, displays analgesic effects. Cipargamin
concise and elegant strategies are still in great demand,
(NITD609),8 a novel antimalarial lead compound in clinic
particularly for the enantioselective synthesis of spiro-oxindole
trials, has demonstrated very effective capability in reducing
piperidin-2-one scaffolds.12
transmission to the Anopheles stephensi mosquito vector.
(+)-Alantrypinone,9 an insecticidal alkaloid isolated from the
Received: February 29, 2020
micro fungi of the genera Aspergillus and Penicillium, is
currently a lead compound for the development of a safer
insecticide based on its selective antagonist effect for housefly
g-aminobutyric acid (GABA) receptors. Compound V,10
reported by Roche Holding, shows potent inhibitory activity
against p53 protein. Although a number of catalytic syntheses
https://dx.doi.org/10.1021/acs.orglett.0c00779
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
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Letter
Currently, organocatalytic asymmetric domino or cascade
reactions provide an alternative powerful strategy, in addition
to transition-metal catalysis, for the synthesis of complex
compounds with contiguous multiple stereocenters.13 In the
past several years, our group has focused on the development
of asymmetric organocatalytic cascade reactions, resulting in
series of novel methods for highly stereoselective synthesis of
all-carbon membered spiro-oxindoles.14
We conceived that the reactions of 3-methyleneindolinones
and α,β-disubstituted acylamides could be activated through
anchoring to a bifunctional organocatalyst by hydrogen bonds
to undergo an aza-Michael/Michael cyclization cascade
sequence, leading to the formation of spiro-oxindole
piperidin-2-ones. To examine the possibility of this hypothesis,
N-Boc-protected 3-methyleneindolinone 1a and N-benzyloxy
acrylamide 2a were used in a model reaction to optimize the
suitable catalytic system and reaction conditions. The
representative results are summarized in Table 1.
(Table 1, entry 1). Thiourea catalyst 3b could improve
enantioselectivity (93% ee), albeit with slightly lower
diastereoselectivity (6:1 dr) (Table 1, entry 2). Squaramides
3c−3h offered interesting results, with 3d being identified as
the best catalyst for reaction yield (88%) and stereoselectivity
(>20:1 dr and 97% ee) (Table 1, entry 4). Squaramides 3c and
3f afforded the desired product with opposite configuration,
but in low yields (Table 1, entries 3 and 6). Squaramides 3e
and 3g proved to be much less effective, and only a trace
amount of product was detected (Table 1, entries 5 and 7).
Next, a variety of solvents were investigated. CH2Cl2 (Table
1, entry 4) was regarded as the best solvent for this cascade
reaction although other halogenated solvents, including CHCl3
(Table 1, entry 9) and DCE (Table 1, entry 13), could
generate the desired product in excellent yields and stereo-
selectivities. When the reaction was conducted in a polar
solvent like THF (Table 1, entry 11) or MTBE (Table 1, entry
14), 4a was obtained in low yield. Nonpolar solvent CCl4
(Table 1, entry 10) or toluene (Table 1, entry 12) could
produce 4a in medium yields. It is worth mentioning that the
reaction proceeded smoothly in brine, affording 4a as the
major diastereomer (76:24 dr) in moderate yield (66%) with
good enantioselectivity (73% ee, Table 1, entry 15).
Our study began with screening a set of organocatalysts 3a−
3h. To our delight, the cascade reaction proceeded readily
when thiourea catalyst 3a was used, providing the desired
product 4a in moderate yield (59%), along with good
diastereoselectivity (8:1 dr) and enantioselectivity (89% ee)
To further optimize this cascade transformation, catalyst
loading amount, reaction temperature, and concentration were
also examined. As a result of lower catalyst loading amount (2
mol % or 1 mol %), only a slightly decreased yield was
observed when 2 mol % catalyst loading was used (Table 1,
entries 16 and 17). The reactions were insensitive to the
reaction temperature in the range between 10 and 40 °C.
Higher reaction concentration could keep the enantioselectiv-
ity, albeit with lower yields. Thus, the optimal conditions were
to conduct the reaction in CH2Cl2 with 2 mol % of squaramide
3d at room temperature (see SI Table 1 for details).
a
Table 1. Screening of Organocatalysts and Solvents
With the optimized conditions in hand, the reaction scope of
various 3-methyleneindolinones (Scheme 1) was studied. As
for the R1 substitutions, 3-acetyl methyleneindolinone could
afford 4b in moderate yield (49%) and good enantioselectivity
(95% ee). The benzyl ester substrate 4e gave slightly higher
yield (91%) than Me-, Et- and t-Bu-substituted esters (4a, 4c,
and 4d). As for the R3 substitutions, both 4-Cl and 4-Br led to
poor enantioselectivities, probably due to the steric effect (4f
and 4g). Substituents at the C-5, C-6, and C-7 positions of the
oxindole ring were well tolerated, with the electron-with-
drawing groups offering slightly higher yields than the electron-
donating groups (4h, 4m vs 4i−l and 4n).
The scope and limitations of various N-protected
acrylamides were also examined. Both N-methoxy and N-tert-
butoxy acylamides could provide the desired products, with the
former giving 4o in moderate yield (58%) and good
enantioselectivity (96% ee) and the latter causing a slight
decrease in both yield (34%) and enantioselectivity (92% ee)
for 4p. The substitutions at α-position of acylamide showed
significant impact on its reactivity. The phenyl substituent gave
higher yield and enantioselectivity (4r, 53% yield, 96% ee),
while the methyl substituted acylamide was almost ineffective
(4q, trace). The substituents at β-position, regardless of their
electronic properties, were well tolerated (4s-4z). Obviously,
the steric hindrance at both α and β positions of acylamides led
to decrease in yields (23−65%), albeit with excellent
stereoselectivities (89−96% ee). It was noteworthy that the
diastereomeric ratios were excellent in all cases (>20:1 dr), and
the absolute configurations of 4r and 4u were unambiguously
b
c
d
entry
catalyst
solvent
yield (%)
dr
ee (%)
1
2
3
4
5
6
7
8
3a
3b
3c
3d
3e
3f
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
CCl4
59
36
33
88
trace
37
trace
89
86
57
27
66
83
42
66
84
64
8:1
6:1
3:1
89
93
−63
97
n.d.
−56
n.d.
−84
95
90
92
93
95
89
73
97
97
>20:1
nd
>20:1
nd
3g
3h
3d
3d
3d
3d
3d
3d
3d
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
76:24
>20:1
>20:1
9
10
11
12
13
14
15
16
17
THF
toluene
e
DCE
MTBE
brine
CH2Cl2
CH2Cl2
f
3d
g
3d
a
Typical reaction conditions: 1a (0.10 mmol), 2a (0.12 mmol), cat. 3
b
(5 mol %) in solvent (0.5 mL), at 25 °C for 48 h. Yield of isolated
c
product. Determined by 1H NMR analysis of the crude reaction
d
mixture before purification. The ee values were determined by
HPLC analysis on Chiral AD-H column with 70:30 n-hexane/i-PrOH
e
f
g
as eluant. DCE = 1,2-dichloroethane. 3d: 2 mol %. 3d: 1 mol %.
B
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Letter
a
removal of N-Boc or N-OBn group from lactam moieties was
achieved through treatment with trifluoroacetic acid or
hydrogenation over Raney Ni to give indolone 5 (95% yield,
95% ee) or lactam 8 (87% yield, 95% ee), respectively. On the
other hand, SmI2 could effectively cleave both N-Boc or N-
OBn groups, leading to bis- free lactam 6 (82% yield, 95% ee).
Notably, hydrogenation over palladium−carbon could afford
the N-hydroxy lactam 7 (92% yield, 96% ee).
Scheme 1. Synthesis of Spirooxindoles 4
Control experiments were performed in order to investigate
the stereochemical outcome of this cascade reaction (Table 2).
Table 2. Control Experiments
entry
R2
R4
R7
yield (%)
a
1
2
3
4
5
6
7
8
Boc
Cbz
Ac
OBn
OBn
OBn
OBn
Ts
Boc
PMP
OBn
H
H
H
H
H
H
H
Ph
0
91, >20:1 dr, 96% ee
90, 1:1 dr, 0% ee
Bn
0
0
0
0
0
Boc
Boc
Boc
Boc
a
Without cat. 3d.
The N-Cbz-protected 3-methyleneindolinone provided 4a′
with an excellent yield and enantioselectivity (90% yield, >
20:1 dr, 96% ee), but the N-acetyl-protected 3-methylenein-
dolinone could not afford any stereocontrol (4b′, 90% yield,
1:1 dr, 0% ee). Furthermore, the N-Bn-protected 3-
methyleneindolinone only gave a trace amount of 4c′. These
results indicated that the carbonyl group (Boc, Cbz, and Ac)
was essential for reaction yields through a hydrogen-bond
interaction with the catalyst and the good stereocontrol came
from the steric hindrance of Boc and Cbz groups. When the N-
OBn in acrylamide was replaced with N-Ts, N-Boc, or N-
PMP, the cascade reaction did not take place at all.
Additionally, (Z)-N-OBn-3-phenylacrylamide (2q) could not
deliver any desired product.
On the basis of the absolute configuration of the adducts and
the results of control experiments, the transition state and
mechanism were proposed to rationalize the outcome of
stereoselectivities (Scheme 3). The squaramide part in the
catalyst orients and activates methyleneindolinones via hydro-
gen bonding. Simultaneously, the α,β-substituted acylamides
were activated by the tertiary amine of the quinine part in the
catalyst, leading to N-Michael Re face addition of α,β-
substituted acylamides to activated 3-methyleneindolinone
succeeded by the intermolecular Si face Michael addition
resulted in an irreversible cyclization, furnishing the desired
enantioenriched and less steric hindrance product.
a
Typical reaction conditions: 1 (0.20 mmol), 2 (0.24 mmol), and cat.
3d (2 mol %) in CH2Cl2 (1.0 mL) at 25 °C for 48 h. Yields for the
isolated products were shown. Diastereomeric excesses were
determined by 1H NMR analysis of the crude products before
purification, and the ratio was >20:1 if not noted otherwise.
b
Enantiomeric ratios were determined by chiral HPLC analysis. X-
ray diffraction analysis.
determined according to the single-crystal X-ray diffraction
analysis.
To demonstrate the practical utility and efficiency of this
method, a gram-scale reaction was carried out under the
standard conditions, providing the desired product 4a in good
yield (80%) and stereoselectivity (>20:1 dr, 95% ee). Further
transformations of 4a were shown in Scheme 2. Selective
Scheme 2. Gram-Scale Synthesis of 4a Followed by
Transformations
In summary, we have developed an asymmetric aza-
Michael/Michael cyclization cascade reaction of 3-methyl-
eneindolinones with α,β-substituted acylamides, providing an
efficient access to spiro-oxindole piperidin-2-one derivatives
using squaramide as the catalyst. The reaction generates three
stereogenic centers and a quaternary carbon center in such
spiro-oxindole piperidin-2-one derivatives in high yield (up to
99%) and excellent stereoselectivities (>20:1 dr, 99% ee). To
C
https://dx.doi.org/10.1021/acs.orglett.0c00779
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Letter
Scheme 3. Proposed mechanism for the cascade reaction
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge financial support from the
National Natural Science Foundation of China (21472019).
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the best of our knowledge, our cascade reaction is one of the
first few cases to use bifunctional squaramide for catalytic
asymmetric aza-Michael/Michael cyclization involving α,β-
substituted acylamide. Further studies on this tandem reaction
and their application in synthesizing biologically active
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ASSOCIATED CONTENT
* Supporting Information
■
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The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00779.
Experimental details, characterization data, and spectral
data (PDF)
Accession Codes
CCDC 1841563 and 1879638 contain 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
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Xing-Wen Sun − Department of Chemistry, Fudan University,
Shanghai 200433, China; orcid.org/0000-0003-0432-
7935; Email: sunxingwen@fudan.edu.cn
Authors
Qing-Gang Tang − Department of Chemistry, Fudan
University, Shanghai 200433, China
Sen-Lin Cai − Department of Chemistry, Fudan University,
Shanghai 200433, China
Chuan-Chuan Wang − Department of Chemistry, Fudan
University, Shanghai 200433, China
Guo-Qiang Lin − Department of Chemistry, Fudan University,
Shanghai 200433, China; CAS Key Laboratory of Synthetic
Chemistry of Natural Substances, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032,
China
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Complete contact information is available at:
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Notes
́
́
Cohen, S. B.; Spencer, K. R.; Gonzalez-Paez, G. E.; Lakshminarayana,
S. B.; Goh, A.; Suwanarusk, R.; Jegla, T.; Schmitt, E.; Beck, H.-P.;
The authors declare no competing financial interest.
D
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