Scandium triflate-catalyzed selective ring opening and rearrangement reaction of spiro-epoxyoxindole and carbonyl compounds

Article abstract of DOI:10.1039/c6ra21264f

Scandium triflate-catalyzed reactions between spiro-epoxyoxindole and carbonyl compounds have been disclosed. The reaction with ketones can be used to synthesize spiro[[1,3]dioxolane-4,3′-indolin] species. Furthermore, an unprecedented rearrangement takes

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Scandium triflate-catalyzed selective ring opening and
rearrangement reaction of spiro-epoxyoxindole and carbonyl
compound
Received 00th January 20xx,
Accepted 00th January 20xx
Hui Jiang,a,b Haohua Jie,c and Jian Li*,c
DOI: 10.1039/x0xx00000x
www.rsc.org/
Scandium
triflate-catalyzed
reactions
between
spiro- of isatin derivatives continues to be desirable.
Owing to their easy preparation and versatile reactivity,
epoxyoxindole and carbonyl compound has been disclosed. The
reaction with ketones can be used to synthesize epoxides have been widely used in a variety of synthetic
spiro[[1,3]dioxolane-4,3'-indolin] species. Furthermore, an reactions. As such, the ring opening chemistry of epoxides
unprecedented rearrangement takes place to yield alkylidene involving regiospecific C-O9 and C-C10 cleavage has been well
oxindole when aromatic aldehydes are used as carbonyl established in the past decades. In this light, we become
component.
interested in the structural characters and properties of the
spiro-epoxyoxindole, which has an epoxy group at 3-position
of oxindole. Firstly, these compounds are important antifungal
and antitubercular agents.11 Moreover, one of the benzoyl-
substituted analogs exhibit biological activities against both
melanoma and leukemia.12 Up to now, considerable efforts
have been developed to their preparation.13 In contrast, the
ring-opening reactions of spiro-epoxyoxindole are less
common, and only a few examples have been disclosed. In
2011, Nair disclosed the aminolysis of spiro-epoxyoxindole to
Isatins are highly valuable building blocks in organic synthesis
since they can be easily prepared and work both as
electrophile and nucleophile.1 It is therefore not surprising that
much progress in this research field has been achieved in the
past decades.2 In particular, isatin and its derivatives are
considered as ideal synthetic precursors for the preparation of
significant synthetic frameworks spirocyclic oxindoles, which
are frequently found in a large family of natural products and
biologically active molecules.3 Of note is the isatin-based
methyleneindolinone (also known as alkylidene oxindole),
which has enjoyed considerable attention from organic
community.4 In this context, the organocatalyzed-asymmetric
transformations have been well documented and become a
synthesize
3-hydroxy-3-aminomethylindolin-2-ones
regiospecifically (Scheme 1, eq 1).14 In 2012, Zhang reported
that spiro-epoxyoxindole underwent selective transformation
under photosensitization conditions or Lewis acid catalysis,
respectively.15 Furthermore, this structure has also found
application in the stereocontrolled synthesis of potent
proteasome inhibitor TMC-95A.16 Of late, Hajra and co-
workers reported that the reactions between spiro-
epoxyoxindoles and indoles afforded structurally complex 3,3’-
mixed bisindoles (Scheme 1, eq 2).17a This strategy was further
applied to gram-scale formal total synthesis of (±)-gliocladin C.
Very recently, Friedel-Crafts reaction of spiro-epoxyoxindole
and other electron-rich arenes were also described for the
selective arylation.17b,17c As a consequence, exploring the
potential synthetic application of spiro-epoxyoxindole
continues to be a valuable target.
powerful tool, thus providing
a vast potential for the
construction of various spirooxindole derivatives and
corresponding biological evaluation.5 Furthermore, many
metal-catalyzed processes as well as other new strategies
were also widely investigated in an efficient manner.6
Additionally, much efforts have also been devoted to the
development of new reactions and strategies involving isatin-
based keteimine,7 and other derivatives8. In spite of these
significant advances, to further explore the synthetic potential
In the past several years, we have paid much attention to
the syntheses of functionalized carbocycles and heterocycles.
18,19
Among them, many successful synthetic strategies have
been developed using isatin-based derivatives as starting
materials. For example, we developed a fast protocol to
approach structurally unusual tricyclic spirooxindole using
multicomponent reaction strategy.19a Moreover, we disclosed
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an indium-catalyzed multiple isocyanide insertion reaction reaction. A variety of substituted spiro-epoxyoxindoles 1 were
with methyleneindolinone.19b Most recently, we disclosed that used to undergo the optimal conditions and the representative
another cycloaddition reaction between isocyanides and
results were summarized in Table 2. All new compounds were
Table 1. Optimization of the reaction conditionsa
Entry
Catalyst
Time (h)
Yield (%)b
1
2
-
8.0
8.0
0
0
CuI
3
BiCl3
8.0
trace
93
91
80
82
90
96
0
4
RuCl3
0.5
5
InCl3
0.5
6
AgOTf
Zn(OTf)2
Cu(OTf)2
Sc(OTf)3
AgOAc
La(NO3)2
Sc(OTf)3
1.5
7
1.0
Scheme
1. Representative transformations involving spiro-
epoxyoxindole
8
1.0
9
0.5
methyleneindolinones to furnish indole-fused polycyclic
skeletons with good chemoselectivity.19c As a continuation of
our previous research, herein we wish to report that
scandium-catalyzed reaction of spiro-epoxyoxindole and
ketone can offer a new pathway to spirooxindole structure.
Furthermore, an unprecedented rearrangement was observed
when aromatic aldehyde was used as carbonyl component
(Scheme 1, eq 3). To the best of our knowledge, no such
examples have been reported yet.
10
11
12c
8.0
8.0
0
10 min
99
a
Reaction conditions: spiro-epoxyoxindole 1a (1.0 mmol), acetone
2a (5 mL), catalyst (15 mol %), 40 oC. b Yield of product after silica
gel chromatography. c In such case, reaction temperature is 60 oC.
characterized by 1H NMR, 13C NMR (see the Supporting
To probe the feasibility of proposed transformation, we
selected spiro-epoxyoxindole 1a and acetone 2a as model
Information for details), and HRMS. Pleasingly, substrates
containing electron-deficient and electron-rich substituents on
3g). In
1
substrate. As shown in Table 1, in the absence of catalyst, no the aromatic ring gave excellent yields in short time (3a
-
addition, the structure of compound 3c was unambiguously
confirmed by single-crystal X-ray analysis (Figure 1).20
Furthermore, substrate 1 having substituents at position 4 also
worked well to give the desired cycloadducts 3e-3g, which
indicated that present reaction was not sensitive to sterical
hindrance. The changing of nitrogen protecting group from
methyl to benzyl group was subsequently conducted. In such
reaction occurred even the mixture was warmed up to 40 °C
(entry 1). Then a variety of Lewis acids were screened to
improve the reaction performance. For example, the
employment of several metal halides such as CuI and BiCl3 only
led to negative results (entries 2-3). To our delight, 93% yield of
product 3a was isolated with catalytic amount of RuCl3 (entry
4). Encouraged by this result, we subsequently tested several
other metal salts to promote this reaction. The experimental
result showed that metal triflates including AgOTf and Cu(OTf)2
were all effective to catalyze this transformation (entries 6-8),
whereas Sc(OTf)3 exhibited the best performance (entry 9).
Upon heating the mixture of 1a and 2a, compound 3a was
isolated in 96% yield within half an hour. During our
optimization, the influence of reaction temperature was also
examined. The experimental results showed that the
conversion rate was significantly increased together with
shorter reaction time under higher temperature. Finally, when
the reaction mixture was warmed up to 60 °C,
spiro[[1,3]dioxolane-4,3'-indolin] 3a was produced in 99%
within 10 minutes (entry 12).
case, a little decreased yield was observed (3h
broad spiro-epoxyoxindole scope was established, reactions
with substituted ketones were subsequently conducted.
-3i). After a
2
Cycloketones such as cyclohexanone and cyclopentanone were
With the optimized conditions in hand, we turned our
attention to establish the scope and limitation of present
2 | J. Name., 2012, 00, 1-3
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reaction (Scheme 2). Firstly, the coordination of
R2
O
2
Sc(III)
O
R3
O
O
ring
opening
Sc(III)
Sc (III)
O
R1
R1
O
R1
O
N
Pg
N
N
B
Pg
Pg
1
A
R2
O
R3
R3
R2
O
Sc(III)
O
O
O
3
Sc(III)
R1
O
R1
-Sc (III)
N
Pg
N
Pg
D
C
Figure 1. X-Ray structure of compound 3c.
Scheme 2. Mechanistic proposal.
Table 2. Scope of the reaction with respect to spiro-
epoxyoxindole 1 and ketone 2a
Sc(III) species to the oxygen atom in spiro-epoxyoxindole
leads to the formation of intermediate , which undergoes the
ring opening to generate indole-2-one intermediate . Then
intermediate is then attacked by the oxygen in ketone
followed by cyclization to produce key intermediate . Finally,
the elimination of Sc(III) species from cation essentially
affords product
1
A
R3
B
R2
O
O
B
2
O
Sc(OTf)3
(15 mol %)
O
D
R1
O
+
R1
O
R2
R3
D
60 oC
N
N
Pg
Pg
3.
2
1
3
To further demonstrate the versatility of this reaction,
benzaldehyde 4a was subsequently examined to react with
spiro-epoxyoxindole 1a (Table 3). To our surprise, an
interesting rearrangement occurred to yield alkylideneoxindole
5a as the product instead of the expected compound
spiro[[1,3]dioxolane-4,3'-indolin] species. Although we were
not sure about the exact mechanism for the present unusual
result, we sought to briefly investigate the scope and limitation
of this rearrangement reaction. As shown in Table 3,
substituted aromatic aldehydes
with 1a to produce compounds 5b
substituent of the aromatic ring of substrate
The experimental outcome indicated that fluoride, chloride,
bromide, methyl, and methoxy groups were well tolerated (5d
4
were firstly used to react
5c. Then, changing the
was performed.
-
1
-
5i). During this investigation, aromatic aldehydes having
methoxy, methyl, and furyl groups were also found to be
compatible (5g
-5i). Surprisingly, no reaction occurred for
unknown reason when benzaldehyde 4a and substrate
1
containing methoxy group was examined. Finally, the
replacement of methyl with Bn and Boc as the protecting
group of the nitrogen atom was also proven to be compatible
(5j-5k). It was also worthy to note that the present
a Reaction conditions: spiro-epoxyoxindole 1 (1.0 mmol), acetone 2a
o
b
Table 3 Unexpected rearrangement with spiro-epoxyoxindole 1
and aromatic aldehyde 4a,b
(5 mL), Sc(OTf)3 (15 mol %), 60 C. Yield of product after silica
gel chromatography. c In such case, reactions were conducted with
spiro-epoxyoxindole 1 (1.0 mmol), ketone 2 (3 equiv), toluene (5
mL) as solvent under 60 oC.
Ar
O
H
Sc(OTf)3
(15 mol %)
O
R1
firstly examined to produce the desired adducts 3j-3m.
R1
O
O
+
N
Pg
Ar
H
Furthermore, ketone 2d having bulky substituent was also
proven to be compatible partners to yield 3o in satisfying
behavior. Setting a limitation, the reaction did not work for
acetophenone and benzophenone probably due to the
decreased reactivity of the carbonyl group.
100 o
C
N
Pg
4
1
5
Based on the aforementioned experimental results,
plausible mechanism was proposed to explain this ring opening
a
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a
Reaction conditions: spiro-epoxyoxindole
1 (1.0 mmol),
aromatic aldehyde 4 (1.0 mmol), Sc(OTf)3 (15 mol %), toluene (5
mL), 100 oC. b Yield of product after silica gel chromatography.
strategy demonstrated excellent stereoselectivity and only E-
isomers were observed, which was quite unusual.
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In conclusion, we have disclosed the Sc(OTf)3-catalyzed
selective reaction of spiro-epoxyoxindole and carbonyl
compound. The reaction with ketone provided a rapid access
for the construction of spiro[[1,3]dioxolane-4,3'-indolin]
species. Remarkably, an unprecedented rearrangement took
place to yield alkylidene oxindole as the product when
aromatic aldehyde was used as reaction component. This
method is also distinguished by its broad substrate scope and
convenient experimental set-up. Further extension of present
strategy to build up its versatility is currently underway in our
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Acknolwledgements
We thank the Fundamental Research Funds for the Central
Universities (ZYGX2015J026) and the National Natural Science
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ABSTRACT: Scandium triflate-catalyzed reactions between spiro-epoxyoxindole and
carbonyl compound has been disclosed. The reaction with ketones can be used to
synthesize spiro[[1,3]dioxolane-4,3'-indolin] species. Furthermore, an unprecedented
rearrangement takes place to yield alkylidene oxindoles when aromatic aldehydes are
used as carbonyl component.