Hydrogen-bond-directed formal [5 + 1] annulations of oxindoles with ester-linked bisenones: Facile access to chiral spirooxindole δ-lactones

Article abstract of DOI:10.1021/ol500547e

A novel bifunctional thiourea catalyzed formal [5 + 1] cycloaddition of oxindoles and ester-linked bisenones was successfully developed. This strategy involves two sequential Michael additions, leading to spirooxindole δ-lactones with three contiguous ste

Full text of DOI:10.1021/ol500547e

Letter
pubs.acs.org/OrgLett
Hydrogen-Bond-Directed Formal [5 + 1] Annulations of Oxindoles
with Ester-Linked Bisenones: Facile Access to Chiral Spirooxindole
δ‑Lactones
Shuai Zhao, Jun-Bing Lin, Yuan-Yuan Zhao, Yong-Min Liang,* and Peng-Fei Xu*
State Key Laboratory of Applied Organic Chemistry, and College of Chemistry and Chemical Engineering, Lanzhou University,
Lanzhou 730000, P. R. China
S
* Supporting Information
ABSTRACT: A novel bifunctional thiourea catalyzed formal
[5 + 1] cycloaddition of oxindoles and ester-linked bisenones
was successfully developed. This strategy involves two
sequential Michael additions, leading to spirooxindole δ-
lactones with three contiguous stereocenters including an all-
carbon quaternary center with high diastereo- and enantiose-
lectivites. In addition, a remarkable N-substituent effect was
observed on the reactivity and selectivity.
actones are unique structural units frequently found in a
carbon quaternary chiral centers. To our knowledge, there is no
Lwide variety of naturally occurring molecules and precedent for the enantioselective synthesis of spirocyclic δ-
lactone with all-carbon quaternary centers so far.7
pharmaceuticals. Multisubstituted lactones are of particular
Construction of complex chiral molecular architectures from
simple achiral materials is a main focus of research in organic
chemistry. Significant progress in this area has been made
through organocatalytic asymmetric cascade/tandem reactions
due to their ability toward facile assembly of complex and
diverse molecules as well as their operational simplicity and
environmental friendliness.8 With our strong interest in
developing novel and practical organocatalytic cascade
reactions,9 and also inspired by the previous achievements in
the hydrogen-bonding activation of various Michael acceptors
toward nucleophilic attack of oxindole,10,11 we reasoned that
ester-linked bisenones could serve as powerful reaction partners
in a tandem Michael/Michael process promoted by a suitable
bifunctional hydrogen-bonding donor catalyst. More impor-
tantly, this formal [5 + 1] cycloaddition process will lead to a
spirooxindole δ-lactone with three continuous stereocenters,12
which is an important structural motif in many natural products
and biological active molecules (Scheme 1).
synthetic significance due to their capacity as synthetic
intermediates toward complex molecules.1 Consequently,
great efforts have been directed recently toward the develop-
ment of the catalytic asymmetric synthesis of lactones and their
derivatives. As a result, a multitude of elegant methods for the
construction of five-membered lactones have been reported,2
such as well-documented NHC-catalyzed [3 + 2] cyclo-
addition3 and carboxylic acid involved halocyclizations.4 By
contrast, reliable and flexible methods for the construction of
six-membered lactones are very rare despite the prevalence of
these unique architectures in various natural products and
bioactive molecules (Figure 1).5,6 Furthermore, it is still a great
challenge to stereoselectively construct δ-lactones bearing all-
We initiated the study by investigating the reaction of
substrate 2 and 3 under the catalysis of 1a in toluene at room
temperature. The desired product was obtained in good yield
and ee with only one diastereomer formed, although a
prolonged reaction time was needed when the N-substituent
was Bn group (Table 1, entry 1). It seemed that the steric
hindrance of the Bn substituent remarkably reduced the
reactivity of the substrate. When we changed the EWG group
of 3 to an Ac or formyl group, the desired products were
obtained with a very low yield (Table 1, entries 2 and 3). The
Bz was observed to be a better EWG group of 3. To enhance
Figure 1. Represented δ-lactone-containing natural products and
drugs.
Received: February 19, 2014
Published: March 12, 2014
© 2014 American Chemical Society
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Organic Letters
Letter
a
Scheme 1. Hydrogen-Bond-Directed Double Michael
Reactions
Table 2. Optimization of the Reaction Parameters
b
c
d
entry
1
catalyst loading (mol %) t (h) yield (%) ee (%)
1
1a
1b
1c
1d
1e
1f
20
20
20
20
20
20
20
20
15
10
5
72
72
72
96
96
96
72
72
72
72
72
72
72
67
60
67
61
NR
44
94
69
91
82
40
87
50
92
91
−84
91
ND
88
93
82
94
96
97
92
96
2
3
4
5
6
7
1g
1h
1g
1g
1g
1g
1g
8
9
10
11
a
Table 1. Effects of the N-Substituent of Oxindole
e
12
10
5
e
13
a
Unless otherwise noted, the reaction was carried out with 2a (0.15
mmol), 3a (0.1 mmol), and 1 in 0.5 mL of toluene at room
b
c
temperature. For all entries, the dr > 25:1. The isolated yield.
d
e
Determined by HPLC. The reactions were carried out with 2a (0.3
mmol), 3a (0.2 mmol), and 1 in 0.5 mL of toluene.
b
c
entry
R
EWG
t (h)
yield (%)
ee (%)
1
2
3
Bn
Bz
168
72
72
12
5
73
19
19
90
99
30
22
56
27
67
92
Bn
Ac
ND
ND
73
hydrogen-bond donor other than thiourea, gave disappointing
results (Table 2, entries 5 and 6). Then we changed 1d to its
analogs 1g and 1h bearing bulky substituents. The results
demonstrated that 1g was the best catalyst for this lactone-
forming reaction, which might be due to the dihedral and
relative crowding of the two groups of the catalyst. Now with
the best catalyst found, the solvent of the reaction was further
optimized. Toluene was determined to be the best solvent for
this reaction (see Supporting Information). Considering that
the loading of catalyst (20 mol %) was a little too high, we tried
to investigate the influence of the catalyst loading on the
reaction. Despite a positive correlation between catalyst loading
and reactivity, we found that reducing the catalyst loading
slightly enhanced the enantioinduction (Table 2, entries 9−11).
To increase the reaction yield, a higher concentration (0.4 M)
of the catalyst was tested. The yield increased as expected but
the ee decreased (Table 2, entries 12 and 13). Considering the
yield and ee, 10 mol % catalyst in 0.5 mL of toluene was found
to meet the requirement (Table 2, entry 10). Given the above
results, the optimal conditions included using catalyst 1g (10
mol %) in toluene (0.5 mL) at room temperature for 72 h.
Under the optimal conditions, the substrate scope of the
lactone-forming reaction was investigated and the results are
outlined in Table 3. A variety of substituents on oxindoles and
dienones were well tolerated in this catalytic system, providing
the desired products in moderate to high yields (up to 94%)
with excellent enantioselectivities (up to 97% ee) and
diastereoselectivities (>25:1). The position of the substituents
on the phenyl ring of oxindole has no significant impact on the
yield and enantioselectivity (Table 3, entry 2 vs 3 and 4 vs 5).
In contrast, the substituents with different electronic features
gave quite different results. Those substrates with electron-
withdrawing substituents gave higher yields but a lower ee
(Table 3, entries 2−5), while the yield was very low but the ee
was higher when the substrates with electron-donating
Bn
formyl
Bz
d
4
Boc
CO2Et
Ac
5
Bz
20
6
Bz
72
72
72
72
72
5
7
H
Bz
ND
20
8
Ph
Bz
9
Et
Bz
ND
92
10
Me
Bz
a
Unless otherwise noted, the reaction was carried out with 2 (0.15
mmol), 3 (0.1 mmol), and 1a (20 mol %) in 0.5 mL of toluene at
room temperature. The isolated yield. Determined by HPLC. The
b
c
d
reaction was carried out at −78 °C.
the reactivity of the oxindoles, the N-substituent was replaced
by the more electron-withdrawing Boc group. The reaction was
accelerated a few times even at −78 °C albeit with a poor ee
(Table 1, entry 2). It seemed that the N-substituent had a great
influence on the reaction in terms of the yield and ee. Then we
examined a series of N-substituents, both the electron-
withdrawing and -donating ones. The results are outlined in
Table 1. Considering the efficiency and selectivity, the reaction
with the methyl substituted oxindole 2a gave the best results
(entry 8: yield 67%, ee 92%).
With the selection of the appropriate N-substituent, we
started to optimize the reaction conditions with 2a and 3a as
the model substrates. First, a series of bifunctional thiourea
catalysts were examined with other conditions unchanged
(Table 2).13 As better results were produced by the quinine
thiourea 1a, other bifunctional hydrogen-bond catalysts derived
from the cinchona alkaloid were tested. Unfortunately, they all
failed to give a satisfactory result (Table 2, entries 2, 5, and 6).
The catalysts 1c and 1d derived from diaminocyclohexane and
L-valine resulted in a slight decrease in the selectivity or yield
(Table 2, entries 3 and 4). And catalysts 1e and 1f, with a
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Organic Letters
Letter
controlled pathway could not be ruled out. As shown in
Scheme 2, initially, the oxindole and enone are synergistically
Table 3. Substrate Scope of the Organocatalytic Tandem
Reaction
a
Scheme 2. Proposed Mechanism of the Spirooxindole δ-
Lactone-Forming Tandem Reaction
b
c
d
entry
R1
R2
R3
yield (%)
ee (%)
1
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
82 (4a)
94 (4b)
90 (4c)
86 (4d)
92 (4e)
55 (4f)
81 (4g)
90 (4h)
82 (4i)
88 (4j)
87 (4k)
84 (4l)
80 (4m)
84 (4n)
88 (4o)
87 (4p)
80 (4q)
71 (4r)
78 (4s)
93 (4t)
58 (4u)
86 (4v)
82 (4w)
96
88
90
88
90
97
88
93
93
95
90
92
95
94
93
93
90
88
90
94
97
93
96
2
4-Cl
6-Cl
5-Br
6-Br
5-Me
H
3
4
5
6
7
2-F-Ph
4-F-Ph
4-Br-Ph
2-Cl-Ph
3-Cl-Ph
4-Cl-Ph
2-Me-Ph
3-Me-Ph
4-Me-Ph
2-naphthyl
2-thienyl
4-NO2-Ph
4-CF3-Ph
Ph
8
H
9
H
10
11
12
13
14
15
16
17
18
19
20
21
22
23
H
activated by the bifunctional thiourea via the formation of
intermediate A, which immediately undergoes the intra-
molecular Michael addition to generate the enone-tethered
intermediate B. Subsequently an intermolecular Michael
reaction of the oxindole with the Si-face of the tethered
enone delivers the final product and regenerates the catalyst.
The excellent enantio- and diastereoselectivities of the product
may be ascribed to the stable six-membered transition state of
intermediate B. The catalyst 1g gave better results than other
bifunctional thiourea catalysts probably because its dihedral and
the relative crowding of the two groups generated a more stable
six-membered transition state.13c Furthermore, the remarkable
N-substituted effect was observed, probably because of not only
the different C3 reactivity with a different substituent but also
the nonbonding interaction between the substituent and the
phenyl ring of the enone. This may explain why the methyl
substituted oxindole has a faster reaction rate than the benzyl
substituted one, which has more steric hindrance and may have
a π−π interaction with the phenyl ring of enone.
In conclusion, we have developed a novel formal [5 + 1]
cyclization of oxindoles and ester-linked bisenones catalyzed by
a simple bifunctional thiourea. The trisubstituted spirooxindole
δ-lactones with three contiguous stereocenters including an all-
carbon quaternary center were obtained with high diaster-
eoselectivities (>25:1) and enantioselectivities (up to 97%). In
addition, dramatic effects of N-substituents of the oxindole on
this reaction with respect to reactivity and selectivity were
observed. Further investigation of this chemistry toward
mechanism rationale and synthetic applications is underway.
H
H
H
H
H
H
H
H
H
H
4-Me-Ph
2-Cl-Ph
4-Cl-Ph
2-furyl
H
Ph
H
Ph
H
Ph
a
Unless otherwise noted, the reaction was carried out with 2 (0.15
mmol), 3 (0.1 mmol), and 1g (10 mol %) in 0.5 mL of toluene at
b
c
room temperature for 72 h. For all entries, the dr > 25:1. The
isolated yield. Determined by HPLC.
d
substituents were used due to the lower reactivity at the C3
position (Table 3, entry 6). Subsequently, an array of dienones
3 were examined using the double Michael reaction. We were
happy to find that substrates with various substituents on both
of the phenyl rings or with other aromatic systems were well
tolerated in our reaction. The position and electronic nature of
the substituents on aromatic rings had a slight influence on the
results probably because of the long distance between the
aromatic rings and the reaction sites. The stereochemistry of 4i
was determined based on the X-ray crystallography (Figure
2).14
With regard to the reaction mechanism, a catalyst-controlled
pathway of this lactone-forming reaction was proposed based
on our experimental results, although at this stage a substrate-
ASSOCIATED CONTENT
* Supporting Information
Additional optimization of reaction parameters, experimental
procedure and characterization data for all new compounds, X-
ray crystal structure data (CIF) for compound 4i. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: xupf@lzu.edu.cn.
*E-mail: liangym@lzu.edu.cn.
Notes
Figure 2. X-ray crystal structure of compound 4i.
The authors declare no competing financial interest.
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Organic Letters
Letter
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ACKNOWLEDGMENTS
■
We are grateful for the NSFC (21372105, 21032005,
21172097), PCSIRT (IRT1138), the International S&T
Cooperation Program of China (2013DFR70580), the National
Basic Research Program of China (No. 2010CB833203), and
the“111” program from MOE of P. R. China.
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