Asymmetric Conjugate Addition of Ethylene Sulfonyl Fluorides to 3-Amido-2-oxindoles: Synthesis of Chiral Spirocyclic Oxindole Sultams

Article abstract of DOI:10.1021/acs.orglett.9b03911

An enantioselective conjugate addition of ethylene sulfonyl fluorides to 3-amido-2-oxindoles has been developed. Quinine-derived squaramides were identified as efficient catalysts. A series of spirocyclic oxindole sultams were prepared with excellent yields and enantioselectivities. A reaction mechanism via bifunctional activation was proposed.

Full text of DOI:10.1021/acs.orglett.9b03911

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Asymmetric Conjugate Addition of Ethylene Sulfonyl Fluorides to
3‑Amido-2-oxindoles: Synthesis of Chiral Spirocyclic Oxindole
Sultams
Jie Chen, Bao-qin Huang, Zeng-qing Wang, Xue-jing Zhang, and Ming Yan*
Guangdong Provincial Key Laboratory of Chiral Molecules and Drug Discovery, School of Pharmaceutical Sciences, Sun Yat-sen
University, Guangzhou 510006, China
S
* Supporting Information
ABSTRACT: An enantioselective conjugate addition of
ethylene sulfonyl fluorides to 3-amido-2-oxindoles has been
developed. Quinine-derived squaramides were identified as
efficient catalysts. A series of spirocyclic oxindole sultams were
prepared with excellent yields and enantioselectivities. A
reaction mechanism via bifunctional activation was proposed.
n recent years, sulfonyl fluorides have received great
(NHC) catalyzed addition of arylethenesulfonyl fluorides to
1
Iattention in organic synthesis and medicinal chemistry. trimethylsilylated 1,3-dicarbonyl compounds. A series of
Sharpless and co-workers have developed several sulfur(VI)
fluoride exchange (SuFEx) reactions with excellent chemo- and
regioselectivities. Unlike conventional sulfonyl chlorides,
sulfonyl fluorides have unique physicochemical properties,
such as the resistance to the reduction, high thermodynamic
stability, controllable reactivity, and strong interaction of
hydrogen bonding. Therefore, sulfonyl fluorides were widely
explored as irreversible inhibitors or probes against protein and
nucleic acid targets (Scheme 1).2
unsaturated δ-sultones were prepared in good yields.6b
Recently, Qin and co-workers utilized ESF to synthesize a
variety of fused δ-sultone heterocycles.6c,d Despite this
progress, catalytic asymmetric reaction of ESFs had rarely
been explored before. Recently, Leung and co-workers
reported an enantioselective hydrophosphination reaction of
β-arylethenesulfonyl fluorides.7a Qin and co-workers also
developed an enantioselective conjugate addition of arylbor-
onic acid to β-arylethenesulfonyl fluorides.7b As a continuous
effort to develop organocatalytic asymmetric reactions, we
envisioned that ESFs are suitable Michael acceptors.8 The new
reactions will provide chiral sulfonyl fluorides, which are useful
candidates as covalent inhibitors and protein probes.
Scheme 1. Representative Drugs and Probes Containing a
Sulfonyl Fluoride Group
3-Amino-2-oxindoles are a class of attracting nucleophiles in
the organocatalytic conjugate additions.9 The resulting chiral
3,3-disubstituted 2-oxindoles are valuable for the synthesis of
oxindole-derived natural products and the screening of new
drug candidates.10 Herein, we would like to report an
asymmetric conjugate addition of 3-amide oxindoles to ESFs.
A series of chiral sulfonyl fluorides with 3,3-disubstituted 2-
oxindole scaffolds were prepared with excellent yields and
enantioselectivities. Furthermore, the products were readily
transformed to chiral spirocyclic oxindole sultams without loss
of the optical purity.
Among the various methods for the introduction of a
sulfonyl fluoride group, the reactions with ethylene sulfonyl
fluorides (ESFs) are extremely valuable due to their excellent
reactivity.1a,3,4 ESFs could be used as dienophiles in Diels−
Alder reactions.5 They were also used as excellent Michael
acceptors in a variety of transformations.6 During the reactions,
the SO2F group generally remains unaffected. The further
elaborations of the SO2F group were achieved under more
forced conditions or in the presence of appropriate catalysts.
Lupton and co-workers reported an N-heterocyclic carbene
Initially, the reaction of 3-benzamido oxindole 1a with ESF
2a was examined in the presence of chiral organocatalysts 4a−
4j, and the results are summarized in Table 1. Quinine 4a
Received: November 1, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03911
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Organic Letters
Letter
a
enantioselectivities were kept well at the loading of 15 mol %
and 10 mol % 4d, the yields were gradually reduced (Table 1,
entries 19−20).
With the optimal reaction conditions in hand, a series of 3-
benzamido-2-oxindoles were examined, and the results are
summarized in Table 2. The N1 substituent exerted a slight
Table 1. Optimization of the Reaction Conditions
Table 2. Asymmetric Addition of ESF to 3-Benzamido-2-
a
oxindoles 1a−1q
b
c
entry
1
R1/R2
Bn/H
Ph/H
Me/H
yield (%)
ee (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
98
88
85
97
99
98
98
99
91
92
93
99
99
93
93
86
83
99
99
99
98
98
99
99
98
99
97
99
98
98
99
98
98
99
b
c
entry
catalyst
solvent
DCM
yield (%)
ee (%)
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4f
4g
4h
4i
4d
4d
4d
4d
4d
4d
4d
4d
4d
4d
4d
86
71
97
98
97
93
55
32
72
95
91
87
71
67
78
88
69
66
83
76
-
91
94
99
−91
97
−88
−85
−91
98
98
99
96
89
96
98
88
72
Bn/5-F
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
CHCl3
DCE
toluene
THF
Et2O
EtOAc
1,4-dioxane
MeCN
MeOH
DCM
DCM
Bn/5-Cl
Bn/5-Br
Bn/5-Me
Bn/5-MeO
Bn/6-F
Bn/6-Cl
Bn/6-Br
Bn/6-MeO
Bn/7-F
Bn/7-Cl
Bn/7-Br
Bn/7-CF3
Bn/5,7-diMe
9
10
11
12
13
14
15
16
17
18
a
The reactions were conducted with 1a−1q (0.10 mmol), 2a (0.12
mmol), and 4d (0.02 mmol) in DCM (1.0 mL) at room temperature
b
c
for 12 h. Isolated yields. Determined by chiral HPLC analysis.
d
19
e
99
99
20
influence on the yield, but the enantioselectivity was not
disturbed (Table 2, entries 1−3). 5-Substitution on oxindole
was tolerated very well. 5-Halogenated (F, Cl, Br), 5-methyl,
and 5-methoxyl substrates provided excellent yields and
enantioselectivities (Table 2, entries 4−8). 6-Substituted
substrates 1i−1l also afforded excellent enantioselectivities;
however, slightly low yields were obtained (Table 2, entries 9−
12). 7-Substituted substrates were also examined. Excellent
yields and enantioselectivities were obtained for 1m−1o
(Table 2, entries 13−15). Lower yields were observed for 7-
CF3- and 5,7-dimethyl-substituted substrates 1p−1q (Table 2,
entries 16−17).
Further exploration of the substrate scope was focused on
different 3-amido-2-oxindoles 1r−1y, and the results are
summarized in Scheme 2. Excellent yields and enantioselectiv-
ities were generally obtained for 2-oxindoles with the 3-
arylamido group (such as 2-naphthamido, 3-indolecarboxami-
do, 2-thiophencarboxamido, and 2-furancarboxamido). 3-
Cyclopropanecarboxamido-2-oxindole 1v and cyclopentane-
carboxamido-2-oxindole 1w are also suitable substrates. Good
yields and excellent enantioselectivities were obtained. For 4-
tetrahydropyrancarboxamido-2-oxindole 1x, although excellent
enantioselectivity was achieved, low yield was obtained. 3-
Ethoxycarboxamido-2-oxindole 1y was also applicable. Good
yield and enantioselectivity were obtained. A single crystal of
a
The reactions were conducted with 1a (0.10 mmol), 2a (0.12
mmol), and catalysts 4a−4i (0.02 mmol) in 1.0 mL of solvent at room
b
c
temperature for 12 h. Isolated yields. Determined by chiral HPLC
analysis. 15 mol % of 4d was used. 10 mol % of 4d was used.
d
e
could catalyze the reaction, but only racemic product 3a was
obtained (Table 1, entry 1). To our delight, quinine-derived
squaramides 4b−4d provided 3a in good yields and with
excellent enantioselectivities (Table 1, entries 2−4). The
quinidine-derived squaramide 4e gave the opposite enantiomer
of 3a with excellent enantioselectivity (Table 1, entry 5). The
quinine-derived thiourea 4f is also efficient. The excellent yield
and enantioselectivity were obtained (Table 1, entry 6). The
chiral cyclohexane-1,2-diamine-derived thioureas 4g−4i also
catalyzed the transformation, however with inferior yields and
enantioselectivities (Table 1, entries 7−9).
Furthermore, a series of reaction solvents were screened
using 4d as the catalyst. Generally, excellent yields and
enantioselectivities were achieved in less polar solvent such as
chloroform, dichloroethane, toluene, THF, ether, ethyl acetate,
and 1,4-dioxane (Table 1, entries 10−16). Lower yields and
enantioselectivities were obtained in polar solvents including
acetonitrile and methanol (Table 1, entries 17−18). The effect
of the loading of catalyst 4d was also examined. Although the
B
DOI: 10.1021/acs.orglett.9b03911
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Asymmetric Addition of ESF to 3-Amido-2-
oxindoles 1r−1y
Scheme 3. Synthesis of Chiral Spirocyclic Oxindole
Sultams
a
a
a
The reactions were conducted with 3 (0.10 mmol) and DBU (0.02
b
mmol) in DCM (1.0 mL) at room temperature for 12 h. Scale-up
reaction of 3a (1.5 mmol).
Scheme 4. Gram-Scale Preparation and SuFEx Reactions of
Product 3a
a
The reactions were conducted with 1r−1y (0.10 mmol), 2a (0.12
mmol), and catalyst 4d (0.02 mmol) in DCM (1.0 mL) at room
temperature for 12 h.
product 3v was obtained, and the absolute configuration was
confirmed as S by X-ray diffraction analysis. Other products
were supposed to have S configuration analogously. We also
examined the reaction of ESF with 3-benzoxy-2-oxindole, 3-
hydroxy-2-oxindole, 3-phenylthio-2-oxindole, and 3-phenyl-2-
oxindole; however, no expected addition products were
obtained.11
While the products 3a, 3m, and 3r were treated with DBU,
the intramolecular cyclization occurred to give spirocyclic
oxindole sultams 5a, 5m, and 5r with excellent yields and
enantioselectivities (Scheme 3). The similar spirocyclic
oxindoles were reported to possess a variety of interesting
biological activities.12 We expect that spirocyclic oxindole
sultams are also highly valuable for the studies of medicinal
chemistry.
A reaction of 3-benzamido-2-oxindole 1a and ESF in 3
mmol scale was also examined. The product 3a was obtained
with excellent yield and enantioselectivity (1.3 g, 96% yield,
99% ee, Scheme 4). To demonstrate the applications of the
products in SuFEx chemistry, the reactions of 3a with
pyrrolidine and p-methoxyphenyl TBS ether were examined,
respectively. The corresponding sulfamide 7a and sulfonate 8a
were obtained in good yields and with excellent enantiose-
lectivities (Scheme 4). In both cases, a small amount of 5a was
observed due to the competitive intramolecular cyclization.
To further demonstrate the generality of this reaction, a
series of β-arylethenesulfonyl fluorides were examined, and the
results are outlined in Scheme 5. The reaction afforded
optically active spirocyclic oxindole sultams via a conjugate
addition and spontaneous cyclization. β-Phenylethenesulfonyl
fluoride and various substituted β-phenylethenesulfonyl
fluorides were applied successfully. Good yields, enantiose-
lectivities, and diastereoselectivities were achieved. The steric
configuration of the major diastereoisomer of product 6ab was
determined via X-ray diffraction analysis. The inferior
enantioselectivities in comparison with ESF may be caused
by the unfavorable steric interaction of β-aryl with 3-
benzamido-2-oxindole 1a. The β-heteroarylethenesulfonyl
fluorides 2g−2i are also applicable. Moderate yields and
good enantioselectivities were obtained; however, the diaster-
eoselectivities were lower in comparison with β-phenyl-
ethenesulfonyl fluorides.
A tentative reaction mechanism is illustrated in Scheme 6. 3-
Benzamido-2-oxindole 1a is deprotonated by catalysts 4d to
generate an enolate anion. The electrostatic attraction and
hydrogen bonding are expected between the ammonium cation
C
DOI: 10.1021/acs.orglett.9b03911
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 5. Asymmetric Addition of β-Arylethenesulfonyl
Fluorides to 1a
ASSOCIATED CONTENT
* Supporting Information
■
a c
−
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03911.
Complete experimental procedures and characterizations
of new products; NMR spectra; and HPLC chromato-
grams (PDF)
Accession Codes
CCDC 1952196−1952197 contain the supplementary crys-
tallographic 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
*E-mail: yanming@mail.sysu.edu.cn.
ORCID
Xue-jing Zhang: 0000-0003-1429-9472
Ming Yan: 0000-0001-9888-0744
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
a
The reactions were conducted with 1a (0.10 mmol), 2b−2j (0.12
mmol), and 4d (0.02 mmol) in DCM (1 mL) at room temperature
We thank the National Natural Science Foundation of China
(21772240), Guangzhou Science Technology and Innovation
Commission (201707010210), and Guangdong Provincial Key
Laboratory of Chiral Molecules and Drug Discovery
(2019B030301005) for the financial support.
b
for 24 h. Only structural drawings of major diastereoisomers were
c
provided. The dr values were determined by 1H NMR analysis.
d
Scale-up reaction of 1a (1.5 mmol).
Scheme 6. Proposed Reaction Mechanism
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unsuccessful. No expected product was obtained.
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E
DOI: 10.1021/acs.orglett.9b03911
Org. Lett. XXXX, XXX, XXX−XXX