Identification of a tartrate-based modular guanidine towards highly asymmetric Michael addition of 3-aminooxindoles to nitroolefins

Article abstract of DOI:10.1016/j.tetlet.2020.152741

A novel tartrate-derived guanidine accessed by a modular approach was identified to be an efficient catalyst for the Michael addition of 3-aminooxindoles to nitroolefins. A range of quaternary 3-aminooxindoles bearing adjacent quaternary–tertiary stereocenters were obtained in good to excellent yields (up to 95%) with good to excellent diastereo- and enantioselectivities (up to >20:1 dr and 98% ee).

Full text of DOI:10.1016/j.tetlet.2020.152741

Tetrahedron Letters 64 (2021) 152741
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Tetrahedron Letters
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Identification of a tartrate-based modular guanidine towards highly
asymmetric Michael addition of 3-aminooxindoles to nitroolefins
⇑
Shiqiang Wei, Xiaoze Bao, Shah Nawaz, Jingping Qu, Baomin Wang
State Key Laboratory of Fine Chemicals, Department of Pharmaceutical Sciences, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel tartrate-derived guanidine accessed by a modular approach was identified to be an efficient cat-
alyst for the Michael addition of 3-aminooxindoles to nitroolefins. A range of quaternary 3-aminooxin-
doles bearing adjacent quaternary–tertiary stereocenters were obtained in good to excellent yields (up
to 95%) with good to excellent diastereo- and enantioselectivities (up to >20:1 dr and 98% ee).
Ó 2020 Elsevier Ltd. All rights reserved.
Received 30 October 2020
Revised 1 December 2020
Accepted 3 December 2020
Available online 30 December 2020
Keywords:
Asymmetric catalysis
Guanidine
Aminooxindoles
Enantioselectivity
Michael addition
Introduction
identification of individual guanidine members that can effectively
catalyze asymmetric hydroxylation [4a], fluorination [4b], and CAC
The modular nature of a synthetic route often has the advantage
of providing targeted products with readily tunable steric and elec-
tronic properties simply by varying the participating building
blocks, thus affording ample space for function exploitation and
optimization of the synthetic targets [1]. Particularly, in the field
of asymmetric catalysis the development of privileged chiral cata-
lysts/ligands has benefited considerably from modular synthetic
approaches, because fine tuning of the steric and electronic factors
of the chiral promoters is usually required and modular modifica-
tion of the catalyst/ligand scaffold, especially at a late stage, can
arguably facilitate the optimization process [2].
In this context, in an effort to develop new and efficient chiral
guanidine catalysts for asymmetric organic transformations [3],
we recently reported the construction of a tartrate-based chiral
guanidine library by a practical, modular synthetic route [4]. Of
particular note of this route is the final stage guanidine formation
between the cyclic thiourea and amine partners in a modular man-
ner (Fig. 1). The huge variability of the amines that can be incorpo-
rated together with the facile decagram scale preparation of the
cyclic thiourea precursor renders the guanidine library large
enough, thus securing the successful search of competent guani-
dine catalysts for different asymmetric reactions. Indeed, explo-
ration into the asymmetric catalytic activities led to
bond forming Michael addition reactions (Scheme 1a, b) [4c].
Owing to the significant medicinal relevance of chiral quater-
nary 3-aminooxindole compounds [5], asymmetric synthesis of
this structural motif has become a topic of intense research [6].
Among various strategies toward this goal, catalytic asymmetric
Michael addition reactions of 3-aminooxindoles represent
a
straightforward approach [7]. Given our success in the identifica-
tion of TG-1 for highly enantio- and diastereoselective Michael
addition of 3-alkyl/aryl substituted oxindoles to nitroolefins
(Scheme 1b) [4c], we turned our attention to 3-aminooxindole sub-
strate. Unfortunately, however, TG-1 showed reduced reactivity
and stereoselectivity in this case. Given the flexible tunability of
the tartrate-derived guanidine, we think that an optimal guanidine
member might be identified (Scheme 1c). Herein, we report our
effort toward this goal.
Results and discussion
To validate this hypothesis, the Michael addition of 3-
aminooxindole 1a to nitroolefin 2a was investigated as the model
reaction for optimization (Table 1). As mentioned above, the guani-
dine catalyst TG-1, the efficient catalyst for Michael addition of 3-
alkyl/aryl oxindoles to nitroolenfins [4c], showed significantly
decreased reactivity in Et2O (entry 1). But in toluene the reaction
can occur smoothly in 72 h to afford the Michael adduct in 95%
yield with moderate levels of stereocontrol (entry 2). Thus, with
⇑
Corresponding author.
E-mail address: bmwang@dlut.edu.cn (B. Wang).
https://doi.org/10.1016/j.tetlet.2020.152741
0040-4039/Ó 2020 Elsevier Ltd. All rights reserved.

S. Wei, X. Bao, S. Nawaz et al.
Tetrahedron Letters 64 (2021) 152741
Table 1
Ar
NH
Ar
Ar
N
Ar
Ar
Optimization of the reaction conditions.
O
O
O
O
H
N
Ph
NHBoc
S
H2N
R
R
BocHN
NO2
NH
Ar
NH
Ar
TG
(10 mol %)
NO2
+
O
O
Ph
Ar
solvent, T
N
N
Tunable amines
TG
(yield: 60-90%)
Tartrate-based thiourea
on decagram scale
1a
2a
3aa
TG-1
TG-2
TG-3
TG-4
TG-5
TG-6
TG-7
TG-8
TG-9
: Ar = Ph, R = 3,5-di-tert-butylphenyl;
: Ar = Ph, R = 2,6-di-iso-propylphenyl;
: Ar = Ph, R = Bn;
: Ar = Ph, R = 4-methylbenzyl;
: Ar = p-bisphenyl, R = 4-methylbenzyl;
: Ar = Ph, R = 4-chlorobenzyl;
: Ar = Ph, R = (S)-phenylethy;
: Ar = Ph, R = (R)-phenylethy;
: Ar = p-bisphenyl, R = (S)-phenylethy;
Entrya
TG
Solvent
t (h)
Yieldb (%)
Drc
Eed (%)
__
__
1
2
3
4
5
6
7
8
TG-1
TG-1
TG-2
TG-3
TG-4
TG-6
TG-7
TG-8
TG-9
TG-9
TG-9
TG-9
TG-9
TG-9
Et2O
72
72
12
trace
95
trace
95
95
95
95
95
95
95
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCM
5:1
75
__
__
1.5
0.5
1.0
1.5
3.0
2.0
3.0
1.0
4.0
48
8:1
7:1
7:1
7:1
6:1
12:1
4:1
11:1
10:1
>20:1
>20:1
70
69
64
75
57
86
57
86
82
94
94
Fig. 1. Modular synthesis of tartrate-based guanidines.
9
10
11
12
13e
14e
mesitylene
Et2O
toluene
95
95
95
95
a) Asymmetric hydryoxylation and fluorination of dicarbonyl compounds.
Ts
N
mesitylene
54
O
O
a
O
Unless otherwise noted, reactions were conducted with 1a (0.1 mmol), 2a (0.15
mmol), and TG (0.01 mmol) in toluene (2.0 mL) at 25 °C.
TG-5
R1
R2
O
n
b
OH
O
O
Isolated yield.
Determined by 1H NMR of the crude reaction mixture.
c
R1
R2
O
n
d
Determined by chiral HPLC analysis.
Run at À20 °C.
TG-2
NFSI,
n = 1, 2
e
R1
R2
n
F
b) Michael addition of 3-alkyl substituted oxindoles to nitroolefins.
R3
R3
R1
NO2
R2
R2
TG-1
NO2
+
R1
O
O
N
N
R4
R4
c) Michael addition of 3-aminooxindoles to nitroolefins (This work).
Ar
Ar
O
O
N
N
O
O
H
N
R
NH
N
H
R1
*
NHR3
O
Ar
R3HN
Ar
NO2
R2
R2
N
H
via modification
*
NO2
H
+
R1
O
Ph
N
N
O
R4
R4
N
High efficiency
O
O
Excellent stereocontrol
H
BocN
N
Re-face attack
Scheme 1. The application of tartrate-based guanidines in asymmetric reactions.
toluene as the solvent, guanidine catalysts were initially screened.
Interestingly, the more bulky 2,6-diisopropylaniline-derived
guanidine TG-2 could not promote this reaction (entry 3). That fur-
ther compelled us to investigate our guanidine library to find a
more efficient catalyst. To our delight, the guanidine TG-3 with R
being a benzyl group, promoted this reaction to completion in
1.5 h, affording the product in 8:1 dr with a slight decrease of
the enantioselectivity to 70% ee (entry 4). These results indicated
that the steric hindrance of the guanidine dramatically influenced
the reactivity of this transformation. Then we performed a survey
of the modification at the 4-position of the benzyl moiety of the
guanidine. Both electron-donating and electron-withdrawing sub-
stituents at the 4-position of the benzyl group gave moderate ee
values (entries 5–6). To further improve the enantioselectivity, L-
1-phenylethylamine was incorporated into the guanidine catalyst,
and to our delight, it worked efficiently to give 75% ee within 1.5 h,
but D-1-phenylethylamine-based TG-8 gave poor ee (entries 7–8).
Notably, the catalyst displaying a p-bisphenyl moiety on the guani-
dine backbone markedly raised the enantioselectivity to 86% ee
and diastereoselectivity to 12:1 dr (entry 9). This effect was prob-
Fig. 2. The transition state working model of the tartrate-derived guanidine
promoted Michael addition.
ably due to the weak interaction of phenyl group between guani-
dine and substrates (Fig. 2). Based on the above results, we chose
guanidine TG-9 to further screen other conditions. Examination
of different solvents indicated that toluene and mesitylene had
better performance among others (entries 9–12). Dropping the
temperature to À20 °C, toluene and mesitylene showed the same
excellent enantio- and diastereoselectivity (94% ee, >20:1 dr), but
the latter needed longer reaction time (entries 13–14).
With the optimal conditions being identified, the generality of
the substrate scope with respect to nitroolefins and 3-aminooxin-
doles was investigated (Scheme 2). The nitroolefin component was
evaluated first. In general, a variety of substituents on the nitroole-
fins were well accommodated in this reaction, delivering Michael
adducts in uniformly high yields with excellent diastereo- and
enantioselectivities (3aa-3an). The investigation of the electronic
properties of substituents at different positions on the benzene
2

S. Wei, X. Bao, S. Nawaz et al.
Tetrahedron Letters 64 (2021) 152741
R1
NHR3
R3HN
NO2
R2
R2
TG-9
Toluene, -20 o
(10 mol %)
NO2
+
R1
O
O
C
N
N
BocHN
NO2
BocHN
NH2
NaBH4, NiCl2
R4
R4
1
2
3
CH3OH, 0 oC, 2 h
O
O
N
N
NO2
OMe
NO2
Cl
NO2
3aa
4
NO2
NO2
BocHN
BocHN
H2N
N
BocHN
dr >20:1, 94% ee
95% yield, dr >20:1, 94% ee
O
O
O
O
N
N
N
3aa
3ab
3acb
3ad
48 h, 95% yield,
dr >20:1, 94% ee
72 h, 95% yield,
dr = 20:1, 95% ee
48 h, 95% yield,
dr >20:1, 93% ee
48 h, 90% yield,
dr >20:1, 88% ee
BocHN
COOH
O
BocHN
NO2
NaNO2, AcOH
MeO
F
Cl
DMSO, 40 oC, 4 h
O
N
N
NO2
NO2
NO2
NO2
BocHN
BocHN
BocHN
BocHN
Bn
Bn
3ea
dr = 10:1, 98% ee
O
O
O
O
N
N
N
N
5
3ae
3af
3ag
3ah
80% yield, dr = 10:1, 98% ee
72 h, 95% yield,
dr >20:1, 93% ee
48 h, 90% yield,
dr >20:1, 94% ee
48 h, 95% yield,
dr >20:1, 86% ee
48 h, 95% yield,
dr >20:1,93% ee
Br
NC
F3C
Scheme 3. Transformations of the Michael addition products.
O
NO2
NO2
NO2
NO2
BocHN
BocHN
BocHN
BocHN
O
O
O
O
N
N
N
N
and 5-F groups afforded the adducts in high yields and excellent
diastereoselectivities with 87% and 92% ee, respectively (3ba,
3ca). In addition, the ethyl 1-methy-2-oxoindole-3-ylcarbamate
1d was also subjected to this reaction, affording the product with
92% ee and 10:1 dr (3da). The N-benzyl substitution of the oxindole
nitrogen also matched the process, affording the products in 10:1
dr and improved the enantioselectivity to 98% ee (3ea). Further-
more, the absolute structure of 3ai was confirmed by X-ray crystal-
lographic analysis (Scheme 2), and those of other products were
assigned by analogy.
3ai
3aj
3ak
3al
48 h, 85% yield,
dr >20:1, 91% ee
48 h, 87% yield,
dr >20:1, 97% ee
72 h, 85% yield,
dr >20:1, 87% ee
48 h, 95% yield,
dr >20:1, 93% ee
S
NO2
NO2
H2N
NO2
NO2
H2N
BocHN
BocHN
F
O
O
N
O
O
N
N
N
3amb
3anb
72 h, 80% yield,
dr = 20:1, 73% ee
3ba
3ca
48 h, 95% yield,
48 h, 95% yield,
dr = 10:1, 87% ee
48 h, 90% yield,
dr = 20:1, 91% ee
dr = 20:1, 92% ee
To show the synthetic utility of the asymmetric Michael addi-
tion process, derivatization of the quaternary 3-aminooxindoles
was implemented. To this end, in the presence of NaBH4/NiCl2,
NO2
NO2
EtOOCHN
BocHN
O
O
N
Bn
the nitro group of 3aa could be reduced easily, affording chiral
diamine derivative 4 with maintained enantio- and diastereoselec-
tivity in 95% yield. Moreover, the -carbon of the nitro group of 3ea
c-
N
3da
3ea
72 h, 95% yield,
dr = 10:1, 92% ee
72 h, 90% yield,
dr =10:1, 98% ee
a
3ai (CCDC: 2012506)
X-ray structure of compound
was oxidized to acid, which gave a novel b-amino acid derivative
bearing oxindole structure in 80% yield with 98% ee (Scheme 3).
Based on the stereochemical outcome of the Michael adduct
together with the activation mode by guanidine catalysis [3b,d],
a plausible transition state working model was proposed (Fig. 2).
The guanidine as a strong base could abstract proton from 3-
aminooxindole and activate the corresponding enolate anion via
hydrogen bonding. Meanwhile, the protonated tartrate-derived
guanidinium as a chiral hydrogen bond donor activates the nitroo-
lefin by double hydrogen bonds. Moreover, the phenyl group of
Scheme 2. Substrate scope of the asymmetric Michael addition of 3-aminooxin-
doles 1 to nitroolefins 2. aThe reactions were conducted with 1 (0.2 mmol) and TG-9
(10 mol %) in toluene (2.0 mL) at À20 °C for 10 min. Then nitroolefin 2 (0.3 mmol)
was added into the reaction mixture. Yields of the isolated products are given. The
dr was determined by 1H NMR spectroscopy of the crude products. The ee was
determined by chiral HPLC. bThe free amine products 3 were obtained for chiral
HPLC analysis.
ring of the nitroolefins showed that both the electron-donating and
electron-withdrawing groups could afford excellent results (3aa-
3ak). Notably, the chlorine and bromine substitutions on the phe-
nyl ring of the nitroolefins endowed the Michael addition products
with useful synthetic handles for further diversifications (3ah, 3ai).
Furthermore, heteroaryl nitroolefins were also accommodated, as
exemplified by the addition of 3-aminooxindoles to 2-furanyl
and 2-thienyl nitroolefins, which furnished the desired products
in high yields and excellent diastereoselectivities with 93% ee
and 91% ee, respectively (3al, 3am). But the bulky 2-naphthyl sub-
stituted nitroolefin underwent the asymmetric Michael addition
process smoothly with erosion of the enantioselectivity (3an).
After the wide generality of nitroolefins has been established,
the variations with respect to 3-aminooxindoles were next investi-
gated. With the N-methyl protection on 1-position, both electron-
donating and withdrawing substitutions on 5-position underwent
the process smoothly. For example, 3-aminooxindoles with 5-Me
guanidine probably has p-p interaction with substrates. These
stereo-arrangements facilitate the result in the stereodetermining
addition of 3-aminooxindole (as its enol tautomer) to the Re face
of the nitroolefin, delivering the quaternary 3-aminooxindole pro-
duct with observed stereochemistry.
In conclusion, we have identified a novel efficient tartrate-
derived guanidine catalyst for Michael addition of 3-aminooxin-
doles to nitroolefins. Through the reasonable modification of a chi-
ral tartrate-derived guanidine catalysts, a series of diversified
quaternary 3-aminooxindoles containing adjacent quaternary–ter-
tiary stereocenters were achieved in high yield (up to 95%) with
good to excellent diastereo- and enantioselectivities (up to >20:1
dr and 98% ee). A possible transition state working model involving
multiple hydrogen bonding interactions for this reaction was pro-
posed. Further applications of the chiral tartrate-derived guanidine
catalysts in other asymmetric transformations are ongoing in our
laboratory.
3

S. Wei, X. Bao, S. Nawaz et al.
Tetrahedron Letters 64 (2021) 152741
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Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
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We thank the Fundamental Research Funds for the Central
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