Hydride Transfer Involved Redox-Neutral Cascade Cyclizations for Construction of Spirocyclic Bisoxindoles Featuring a [3,4]-Fused Oxindole Moiety

Article abstract of DOI:10.1021/acs.orglett.8b04100

The hydride transfer involved redox-neutral cascade cyclization has been developed to construct the spirocyclic bisoxindoles featuring a [3,4]-fused oxindole moiety from rationally designed C4-amine-substituted isatins, affording the diverse tricyclic [3,

Full text of DOI:10.1021/acs.orglett.8b04100

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Hydride Transfer Involved Redox-Neutral Cascade Cyclizations for
Construction of Spirocyclic Bisoxindoles Featuring a [3,4]-Fused
Oxindole Moiety
Shuai-Shuai Li,†,∥ Shuai Zhu,†,∥ Chunqi Chen,†,∥ Kang Duan,† Qing Liu,§ and Jian Xiao*,†,‡
†College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University, Qingdao 266109, China
‡College of Marine Science and Engineering, Qingdao Agricultural University, Qingdao 266109, China
§College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China
S
* Supporting Information
ABSTRACT: The hydride transfer involved redox-neutral cascade cyclization has been developed to construct the spirocyclic
bisoxindoles featuring a [3,4]-fused oxindole moiety from rationally designed C4-amine-substituted isatins, affording the diverse
tricyclic [3,4]-fused oxindoles with three consecutive chiral centers in good yields and excellent diastereoselectivities (>20:1).
pharyngitis, and pulmonary tuberculosis.2b However, the
olycyclic molecules containing oxindole moieties are
strategies for direct access to [3,4]-fused oxindoles from
Phighly valuable synthetic targets due to their ubiquity in
simple precursors remain scarce,3 especially for 6/5/6 [3,4]-
an array of natural products, pharmaceuticals, and agro-
fused oxindoles.4 In 2012, the Jia group described a [4 + 2]
cycloaddition of methyleneindolinones with arynes to
construct [3,4]-fused oxindoles.5 The subsequent break-
through was made by the Zhu group, who developed a novel
palladium(0)-catalyzed carbopalladation/1,4-palladium shift
via a sequential C−H bond activation/C−C bond-forming
process from acrylamides.6 Very recently, the Lautens group
explored a palladium(0)-catalyzed C(sp2)−H bond activation/
alkyne insertion cascade reaction, affording rapid access to 6/
5/6 [3,4]-fused oxindoles.7 Obviously, these few pioneering
studies largely relied on the transition-metal-catalyzed cross-
coupling reactions to achieve the [3,4]-fused oxindole cores.
On the other hand, spirooxindoles represent one of the most
attractive heterocyclic motifs which widely exist in alkaloids
and pharmacologically interesting compounds such as
cipargamin and satavaptan (Figure 1, IV−V).1,8 In recent
decades, significant progress have been made for the synthesis
of various functionalized spirooxindoles, owing to their diverse
bioactivities and structural complexity.9,10 Presumably, the
combination of these two significant pharmaphores, i.e., [3,4]-
chemicals.1 Among the naturally occurring oxindole alkaloids,
the tricyclic [3,4]-fused oxindoles have distinguished them-
selves with unique biological activities (Figure 1, I−III).2 For
instance, ammosamide B exhibited high activity in modulating
tubulin and actin dynamics,2a and prioline has been used in
Chinese folk medicine for the treatment of tonsillitis,
Figure 1. Natural products and drug molecules containing tricyclic
[3,4]-fused oxindoles and spirooxindoles.
Received: December 23, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b04100
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
fused oxindole and spirooxindole moieties, might produce a
privileged structure possessing promising bioactivity. However,
the synthesis of spirocyclic bisoxindoles featuring a [3,4]-fused
oxindole moiety (Figure 1, VI) poses a formidable challenge,
and there has been no example available until now. Therefore,
there is an urgent need to develop a new strategy for the direct
synthesis of spirocyclic bisoxindoles featuring a [3,4]-fused
oxindole moiety. To achieve this goal, the selection of a
mechanistically unique reaction and the rational design of a
new type of substrate constitute the key factors.
Inspired by the redox- and step-economy, we intend to
develop a strategically distinct route to access spirocyclic
bisoxindoles. In this context, we notice that the cascade
hydride transfer/cyclization has recently emerged as a powerful
tool for construction of various heterocyclic rings owing to its
high efficiency and environmental sustainability.11−13 How-
ever, the inherent drawbacks of these established reactions,
such as the essential dependence of phenyl skeleton as a linker
between hydride donors and acceptors (Scheme 1A), largely
hydride transfer rather than being captured by another
nucleophile; (3) the iminium ion III should be attacked by
an intramolecular nucleophile instead of an intermolecular one.
Moreover, the selective construction of 6/5/6 or 6/5/5 [3,4]-
fused oxindoles raised another problem. As a continuation of
our ongoing efforts toward one-step assembly of molecular
complexity,14 herein we report a cascade hydride transfer/
cyclization for the selective access to spirocyclic bisoxindoles
featuring a [3,4]-fused oxindole moiety.
To validate the feasibility of our hypothesis, the benzyl-
protected 2-indolinone 2a15 was selected as the model
substrate to react with 1-benzyl-4-(pyrrolidin-1-yl)indoline-
2,3-dione 1a (Table 1). At the outset, 20 mol % of Sc(OTf)3
a
Table 1. Optimization of the Reaction Condition
Scheme 1. Efficient Construction of Spirocyclic
Bisoxindoles Featuring a [3,4]-Fused Oxindole Moiety
b
c
entry
1
catalyst
additive
solvent
yield (%)
dr
Sc(OTf)3
Mg(OTf)2
Zn(OTf)2
AgOTf
InBr3
ZnBr2
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
CHCl3
toluene
hexane
DCE
DCE
76
trace
nr
>20:1
d
2
d
3
d
4
nr
5
6
7
8
20
trace
74
66
92
77
51
31
38
36
nr
>20:1
3 Å MS
4 Å MS
5 Å MS
5 Å MS
5 Å MS
5 Å MS
5 Å MS
5 Å MS
5 Å MS
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
9
10
11
12
13
e
14
15
a
Reaction conditions: 1a (0.1 mmol), 2a (0.15 mmol), catalyst (20
mol %), and additive (50 mg) in 1.0 mL of distilled solvent at 80 °C
b
c
for 12 h. Isolated yield after column chromatography. dr was
determined by H NMR spectroscopy. At 80−120 °C for 12 h. 10
mol % of Sc(OTf)3. nr = no reaction.
d
e
1
limited their applications in the synthesis of diverse natural
products and drug intermediates.13 To address this challenge,
we rationally design a new type of substrate decorated with an
amine at the C4 position of isatin, which could react with the
external nucleophiles to achieve the redox-neutral cascade
hydride transfer/cyclization for rapid construction of privileged
[3,4]-fused oxindoles. As depicted in Scheme 1B, initially, the
nucleophilic 2-indolinone could attack the C3 carbonyl group
of isatin to form an alcohol I under Lewis acid, then
dehydration of I occurred to generate intermediate II, which
would induce the hydride transfer to furnish iminium ion III.
Subsequently, the C3 or C3′ position of bisoxindoles would
attack the iminium ion, affording the 6/5/5 or 6/5/6 [3,4]-
fused oxindole. Remarkbly, harnessing the cascade hydride
transfer/cyclization from readily available isatins to synthesize
spirocyclic bisoxindoles remains an unknown chemistry. To
realize this reaction cascade, several challenges need to be
addressed: (1) the nucleophile should be active enough to
attack the C3 carbonyl group of isatin, since the electrophilicity
of the carbonyl group is largely reduced by the electron-
donating nitrogen atom at C4 position; (2) the electrophilic
carbon center of intermediate II should prefer triggering the
was used as a catalyst to examine this reaction at 80 °C.
Satisfyingly, this reaction proceeded very well, affording the
desired 6/5/6 [3,4]-fused oxindole 3a in 76% yield (Table 1,
entry 1). Intriguingly, this reaction exhibited excellent
stereocontrol (>20:1 dr). Encouraged by this result, a variety
of triflate salts were further evaluated and they were all
ineffective, even when the temperature was increased to 120
°C (Table 1, entries 2−4). Moreover, the use of InBr3 and
ZnBr2 was also unsuccessful (Table 1, entries 5 and 6). Then
Sc(OTf)3 was chosen as the most suitable catalyst for further
screening. Afterward, the molecular sieves were employed as
additives aiming to improve the efficiency. To our delight, the
5 Å molecular sieves turned out to be the most effective
additive, furnishing 3a in 92% yield (Table 1, entries 7−9).
The subsequent solvent screening indicated that the employ-
ment of other solvents gave inferior results (Table 1, entries
10−13). As expected, decreasing the catalyst loading was
detrimental for this reaction, albeit without influence on
diastereoselectivity (Table 1, entry 14). Finally, the control
B
DOI: 10.1021/acs.orglett.8b04100
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Letter
experiment implied that this reaction did not occur in the
absence of catalyst (Table 1, entry 15).
Afterward, the generality of 2-indolinones was also examined
(Scheme 3). Pleasingly, either electron-donating or electron-
Having identified the optimal reaction conditions, the
substrate scope with regard to isatins and 2-indolinones were
examined to check the generality of this strategy (Scheme 2). It
a
Scheme 3. Substrate Scope of 2-Indolinones
Scheme 2. Substrate Scope of C4-Amine-Substituted
a
Isatins
a
Reaction conditions: 1a (0.1 mmol), 2 (0.15 mmol), Sc(OTf)3 (0.02
mmol), and 5 Å MS (50 mg) in 1.0 mL of distilled DCE at 80 °C for
12 h; isolated yield after column chromatography; dr >20:1, dr was
1
determined by H NMR spectroscopy.
withdrawing groups on the aromatic ring of 2-indolinones were
totally compatible, giving rise to the corresponding products
3p−s in good yields. Nevertheless, the yields of electron-
donating groups were comparably lower than those of the
electron-withdrawing groups. Further investigation manifested
that 2-indolinones with free N-H and N-Ph groups could also
participate in this reaction, yielding 3t−u in high yields. In
addition, the large scale synthesis of 3u to demonstrate the
scalability of this protocol had been performed (see Supporting
Information). The structure and relative configuration of 3u
have been unambiguously confirmed by X-ray crystallographic
analysis (Figure 2). Remarkably, in all cases, this reaction
exhibited high stereocontrol (>20:1 dr). Hence, this method-
ology provides an efficient synthetic route to these privileged
structures, which are of great significance for natural products
and medicinal chemistry.
a
Reaction conditions: 1 (0.1 mmol), 2a (0.15 mmol), Sc(OTf)3 (0.02
mmol), and 5 Å MS (50 mg) in 1.0 mL of distilled DCE at 80 °C for
12 h; isolated yield after column chromatography; dr >20:1, dr was
1
determined by H NMR spectroscopy.
seemed that the protecting groups on nitrogen of isatins had
trivial effect on the efficiency. For instance, the benzyl, methyl
and ethyl carbamate were well tolerated in this cascade process,
delivering the corresponding products 3a−h in good to
excellent yields. Gratifyingly, apart from the pyrrolidine ring,
the piperidine, morpholine, azepane, and octahydroisoindole
rings also worked very well, affording the desired products 3i−
m in satisfying yields without loss in diastereoselectivities
(>20:1 dr). Besides, the isatins bearing substituents on the
phenyl ring showed slightly lower reactivity, affording 3n−o in
61% and 69% yields, respectively.
Figure 2. Single-crystal structure of 3u.
C
DOI: 10.1021/acs.orglett.8b04100
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Letter
ORCID
In order to probe the mechanism of this transformation, the
deuterated substrate [D]-1a was prepared and subjected to the
optimal conditions (Scheme 4). The deuteration (29% D) at
Shuai-Shuai Li: 0000-0001-7279-2885
Jian Xiao: 0000-0003-4272-6865
Author Contributions
∥S.-S. Li, S. Zhu, and C. Chen contributed equally.
Notes
Scheme 4. Deuterium-Labeling Experiments
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to NSFC (21702117, 21878167) and the
Natural Science Foundation of Shandong Province
(JQ201604, ZR2017BB005) as well as the Key Research and
Development Program of Shandong Province
(2017GSF218073). We thank Dr. Fengying Dong and the
Central Laboratory of Qingdao Agricultural University for
NMR determination.
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might be involved in the rate-determining step (eq 2).
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complex molecules.
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b04100.
■
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Accession Codes
CCDC 1815971 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.
AUTHOR INFORMATION
Corresponding Author
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