Construction of highly enantioenriched spirocyclopentaneoxindoles containing four consecutive stereocenters via thiourea-catalyzed asymmetric Michael–Henry cascade reactions

Article abstract of DOI:10.3762/bjoc.13.131

The thiourea-catalyzed asymmetric synthesis of highly enantioenriched spirocyclopentaneoxindoles containing chiral amide functional groups using simple 3-substituted oxindoles and nitrovinylacetamide as starting materials was achieved successfully. This protocol features operational simplicity, high atom economy, and high catalytic asymmetry, thus representing a versatile approach to the synthesis of highly enantioenriched spirocyclopentaneoxindoles.

Full text of DOI:10.3762/bjoc.13.131

Construction of highly enantioenriched spirocyclo-
pentaneoxindoles containing four consecutive
stereocenters via thiourea-catalyzed asymmetric
Michael–Henry cascade reactions
Yonglei Du1, Jian Li2, Kerong Chen2, Chenglin Wu2, Yu Zhou*2 and Hong Liu*2,3
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 1342–1349.
1Nano Science and Technology Institute, University of Science and
Technology of China, 166 Ren Ai Road, Suzhou 215123, China,
2CAS Key Laboratory of Receptor Research, Shanghai Institute of
Materia Medica, Chinese Academy of Sciences, 555, Zu Chong Zhi
Road, Shanghai 201203, China and 3University of Chinese Academy
of Sciences, NO.19A Yuquan Road, Beijing 100049, China
doi:10.3762/bjoc.13.131
Received: 17 March 2017
Accepted: 21 June 2017
Published: 07 July 2017
Associate Editor: T. P. Yoon
Email:
Yu Zhou* - zhouyu@simm.ac.cn; Hong Liu* - hliu@simm.ac.cn
© 2017 Du et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
asymmetric synthesis; four consecutive stereocenters; Michael–Henry
cascade reactions; spirocyclopentaneoxindoles; thioureas
Abstract
The thiourea-catalyzed asymmetric synthesis of highly enantioenriched spirocyclopentaneoxindoles containing chiral amide func-
tional groups using simple 3-substituted oxindoles and nitrovinylacetamide as starting materials was achieved successfully. This
protocol features operational simplicity, high atom economy, and high catalytic asymmetry, thus representing a versatile approach
to the synthesis of highly enantioenriched spirocyclopentaneoxindoles.
Introduction
The spirocyclic oxindole core represents an important scaffold contiguous stereocenters remains challenging [23-26]. The me-
that is encountered frequently in many biologically active mole- dicinal properties of these frameworks mean that fast enrich-
cules and natural products (Figure 1) [1-19]. Despite many ment of spirooxindoles bearing diverse functional groups is of
advances in asymmetric synthesis in the construction of hetero- considerable importance.
cyclic spirooxindoles in the past decade [2,4,11,20-22], the de-
velopment of general and practical strategies to obtain Recently, an increasing number of asymmetric catalysis strate-
saturated spirocyclopentaneoxindoles containing multiple gies with chiral transition metals [27-33], organocatalysts such
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Beilstein J. Org. Chem. 2017, 13, 1342–1349.
Figure 1: Representative spirooxindole natural products.
as secondary amines [34-36], nucleophilic phosphines [26,37- to develop effective strategies to construct biologically active
44], tertiary amines [45], N-heterocyclic carbenes (NHCs) [46- spirocyclic oxindoles [65-68], we have built successfully inter-
48], and cinchona alkaloid derivatives [25,28,49,50] have been esting spirooxindoles via an NHC-catalyzed [4 + 2] annulation
used to construct successfully spirooxindole privileged scaf- involving an oxidative γ-carbon activation of common α,β-
folds. However, most of them were devoted to exploring new unsaturated aldehydes [68]. Herein, we report another effective
catalyst systems to improve the reaction efficiency and selec- asymmetric catalytic synthesis of saturated spirocyclopenta-
tivity [51-64], and studies extending the reaction scope using neoxindoles containing four consecutive stereocenters with
functionalized nitrovinylacetamides are rare. Chiral thioureas 3-substituted oxindoles and nitrovinylacetamide using a bifunc-
[28,38,39] have evolved as powerful hydrogen bonding cata- tional thiourea catalyst in good yields (up to 95%) with excel-
lysts for the asymmetric synthesis of spirocyclopentaneoxin- lent diastereoselectivity (up to 3:97) and enantioselectivity (up
doles, which have been demonstrated as acceptable but still to 94%) (Scheme 1, reaction 1D).
considerably limited. Organocatalytic iminium–enamine
cascade reactions [35] (Scheme 1, reaction 1A) and Results and Discussion
Michael–Henry cascade reactions [25] (Scheme 1, reaction 1B) To establish the optimal experimental conditions for the synthe-
reported by Barbas III's group involve the cyclization between sis of spirocyclopentaneoxindoles, we chose 3-substituted oxin-
α,β-unsaturated aldehydes and nitrostyrenes with 3-substituted dole 1a and nitrovinylacetamide (2a) as the model substrates,
oxindoles to generate the corresponding CHO- or NO2-substi- and the results are summarized in Table 1. Initially, a variety of
tuted spirooxindole derivatives with good enantiomeric excess organocatalysts (a–f) were investigated in CH2Cl2 at −20 °C for
(ee) values, respectively. However, the utility of the reaction is 12 h to evaluate their ability to promote the transformation
limited to α,β-unsaturated aldehydes with aromatic/alkane sub- (Table 1, entries 1–6). When cinchona alkaloid-derived catalyst
stitutions and nitroolefins with aromatic substitutions. Addition- a and quinine-derived amine catalyst b were tested, however,
ally, Shao’s group (Scheme 1, reaction 1C) developed a one-pot poor yields or ee values were obtained, respectively (Table 1,
thiourea-catalyzed Michael addition/intramolecular silyl entries 1 and 2). Further experiments showed that a bifunc-
nitronate-olefin cycloaddition (ISOC)/fragmentation sequence tional thiourea catalyst d was the most efficient for the synthe-
to produce highly enantioenriched spirocyclopentaneoxindoles sis of spirocyclic oxindole derivatives in good yields (80%)
containing an oxime functional group from easily accessible with excellent diastereoselectivity (8:92 dr), and moderate enan-
3-allyl-substituted oxindoles and nitroolefins, which has tioselectivity (83% ee, Table 1, entries 3–6). Therefore, catalyst
received wide attention because of its high efficiency in con- d was chosen as the optimal catalyst for further investigation
structing functionalized spirocyclopentaneoxindoles. However, (Table 1, entry 4). The reaction temperature was investigated,
lower temperatures are required (−30 °C) [50]. Therefore, it is however elevated temperatures are detrimental (Table 1, entries
highly desirable to develop novel and efficient methods to 4, 12, 13). Different solvents, such as methanol, toluene, ace-
access directly various spirocycles. In our continuous endeavor tone, ether and chloroform were further screened. The results
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Beilstein J. Org. Chem. 2017, 13, 1342–1349.
Scheme 1: Construction of spirocyclopentaneoxindole scaffolds.
suggested that changing the solvent had an adverse effect on the selectivity (85–94% ee) for substrates containing 5-CH3 or
ee value, and CH2Cl2 remained the best choice for this transfor- 5-OCH3 groups (3b and 3d). Substrates carrying 5-F, 7-F, 5-Cl
mation (Table 1, entries 4, 7–11). Subsequently, we investigat- and 6-Cl afforded the corresponding products 3f–i in high
ed some additives, for example, p-MeC6H4SO3H, trimethyl- yields (85–95%) with excellent diastereoselectivity (17:83–3:97
silyl chloride (TMSCl) and trifluoroacetic acid (TFA) for this dr) and good enantioselectivity (76–93% ee). Surprisingly,
catalytic system to increase the ee value of the target product; introducing a bromo group into 6-position of the oxindole scaf-
however, the use of the additives proved ineffective (Table 1, fold resulted in a low enantioselectivity (52% ee for 3j), al-
entries 15–17).
though at a high yield (79%) and excellent diastereoselectivity
(3:97 dr). To further extend the reaction scope, we attempted to
With the optimized reaction conditions established, we next in- exchange the N-Boc group of the 3-substituted oxindoles with
vestigated the substrate scope of 3-substituted oxindoles in this other protecting groups, such as Bn, CH3 or an acetyl group.
transformation with nitroolefins (Scheme 2). In general, the The results demonstrated that only an acetyl protecting group
diverse 3-substituted oxindoles 1a–j with electron-donating, proved to be well tolerated, providing for the efficient synthesis
electron-withdrawing, or halide groups could undergo a smooth of spirocyclopentaneoxindoles 3k–o with moderate yields
reaction with nitrovinylacetamide (2a) in moderate yields, good (47–74%) and excellent diastereoselectivity (9:91–2:98 dr).
diastereoselectivity, and general enantioselectivity (3a–j). For However, replacing the N-Boc group with an N-acetyl group on
example, the protocol showed moderate yields (75–76%), spirooxindoles had a negative effect on the stereoselectivities of
excellent diastereoselectivity (9:91–3:97 dr) and good enantio- the target products, probably because the N-acetyl group
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Beilstein J. Org. Chem. 2017, 13, 1342–1349.
Table 1: Optimization for the reaction conditionsa.
entry
catalyst
solvent
additive
yield(%)b
drc
ee (%)d
1
2
a
b
c
d
e
f
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH3OH
toluene
acetone
Et2O
–
37
57
6
5:95
99
10
6
–
20:80
3
–
16:84
4
–
80
21
40
–
8:92
83
29
–
5
–
4:96
–
6
–
7
d
d
d
d
d
d
d
d
d
d
d
–
–
16
47
28
41
2
8
–
–
–
9
–
–
–
10
11
12e
13f
14g
15h
16h
17h
–
–
–
CHCl3
–
–
–
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
–
56
trace
72
–
–
4
–
–
–
–
p-MeC6H4SO3H
TMSCl
–
82
25
13
11
–
–
–
TFA
–
–
aReaction conditions: Unless noted, the reaction was carried out with 1a (0.11 mmol), 2a (0.1 mmol), catalyst a–f (0.01 mmol), solvent (2–3 mL), 12 h.
bIsolated yield. cDetermined by 1H NMR spectroscopy of the crude mixture. dDetermined by chiral HPLC analysis of the major diastereomer.
eTemperature (0 °C). fTemperature (rt). gSolvent (1 mL). hp-MeC6H4SO3H (0.01 mmol), TMSCl (0.01 mmol), TFA (0.01 mmol) as the additive.
“–” represents: not determined.
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Beilstein J. Org. Chem. 2017, 13, 1342–1349.
Scheme 2: Scope of enantioselective synthesis of spirooxindoles. Reaction conditions: catalyst d (0.01 mmol), oxindoles 1a–o (0.11 mmol) and
nitroolefins 2a–c (0.1 mmol) in CH2Cl2 (3 mL), method A or method B. The ee values were determined by chiral HPLC analysis of major diastereo-
mer. The dr values were determined by 1H NMR analysis. Method A: with 10 mol % of d as a catalyst, −20 °C, 12 h. Method B: with 10 mol % of d as
a catalyst, −10 °C, 24 h.
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Beilstein J. Org. Chem. 2017, 13, 1342–1349.
decreases steric hindrance. In addition, substituents (CH3, Boc) doles containing four consecutive stereocenters and a unique
at R′ and the free N-H substituted oxindole were investigated quaternary stereogenic center, with good yields, and excellent
that did not give the corresponding target products (3p, 3q and diastereoselectivity and enantioselectivity using thiourea-cata-
3r).
lyzed Michael–Henry cascade reactions. We anticipate that this
reaction will simplify the synthesis of complex spirooxindoles
On the basis of the dual activation model proposed by Take- containing multiple chiral centers with potential pharmacologi-
moto et al. [69], a plausible reaction mechanism was proposed cal properties.
in Scheme 3. The multifunctional organocatalyst d has a chiral
scaffold including a thiourea moiety and an amino group. Both Experimental
the 3-substituted oxindoles 1 and nitrovinylacetamide (2a) that General procedure for the synthesis of products (3a–j): To a
participate in this reaction are activated simultaneously via mixture of 1a–j (0.11 mmol) and 2a (0.1 mmol) in CH2Cl2
multiple hydrogen bonds. In addition, the electrophilicity of the (3 mL) was added catalyst d (0.01 mmol). Then the mixture
reacting carbon center of nitrovinylacetamide is likely en- was stirred at −20 °C for 12 h. After completion of the reaction,
hanced by H-bonding, thereby enabling the Michael addition to the solvent was removed by evaporation. The crude product was
construct a unique quaternary stereogenic center complex A purified by column chromatography on silica gel to afford the
which would cyclize concurrently via Henry reaction to give the desired products 3a–j.
product 3 and regenerates the catalyst d. The absolute configu-
ration of 3g was determined by X-ray analysis (see Supporting General procedure for the synthesis of products (3k–o):
Information File 1, Figure S1).
3-Substituted oxindoles 1k–o (0.11 mmol) and nitrovinylac-
etamide (2a, 0.1 mmol) were dissolved in 3 mL CH2Cl2, the
catalyst d (0.01 mmol) was added at −10 °C for 24 h. After
Conclusion
We have developed a highly efficient and practical strategy for nitrovinylacetamide (2a) was consumed completely, the sol-
a single step construction of saturated spirocyclopentaneoxin- vent was removed under vacuum. The crude product was puri-
Scheme 3: A plausible mechanism.
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Beilstein J. Org. Chem. 2017, 13, 1342–1349.
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Supporting Information
Supporting Information File 1
General information, experimental details, characterization
data and copies of 1H and 13C NMR spectra, and HPLC
experimental data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-131-S1.pdf]
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