Organocatalytic Asymmetric Annulation between Hydroxymaleimides and Nitrosoarenes: Stereoselective Preparation of Chiral Quaternary N-Hydroxyindolines

Article abstract of DOI:10.1021/acs.orglett.7b00893

An unusual and highly effective asymmetric annulation of nitrosoarenes with hydroxymaleimides catalyzed by a chiral bifunctional amine squaramide catalyst has been disclosed. A wide range of highly fused chiral N-hydroxyindolines with two consecutive quaternary stereocenters and multifunctional groups were directly and effectively prepared in excellent yields (up to >99%) with complete regioselective cyclization and excellent stereoselectivities (up to >99:1 dr and >99% ee). The efficiency and potentials of the new reaction and the target chiral entities were well demonstrated by delicate transformations into a series of new chiral indolines.

Full text of DOI:10.1021/acs.orglett.7b00893

Letter
pubs.acs.org/OrgLett
Organocatalytic Asymmetric Annulation between
Hydroxymaleimides and Nitrosoarenes: Stereoselective Preparation
of Chiral Quaternary N‑Hydroxyindolines
Yu Yang,†,‡ Hong-Xia Ren,†,‡ Feng Chen,†,‡ Zheng-Bing Zhang,†,‡ Ying Zou,†,‡ Chao Chen,†,‡
Xiang-Jia Song,†,‡ Fang Tian,† Lin Peng,† and Li-Xin Wang*,†
†Key Laboratory of Asymmetric Synthesis and Chirotechnology of Sichuan Province, Chengdu Institute of Organic Chemistry,
Chinese Academy of Sciences, Chengdu 610041, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
S
* Supporting Information
ABSTRACT: An unusual and highly effective asymmetric
annulation of nitrosoarenes with hydroxymaleimides catalyzed
by a chiral bifunctional amine squaramide catalyst has been
disclosed. A wide range of highly fused chiral N-hydroxyindo-
lines with two consecutive quaternary stereocenters and
multifunctional groups were directly and effectively prepared
in excellent yields (up to >99%) with complete regioselective
cyclization and excellent stereoselectivities (up to >99:1 dr and
>99% ee). The efficiency and potentials of the new reaction
and the target chiral entities were well demonstrated by
delicate transformations into a series of new chiral indolines.
Scheme 1. Potential Reaction Protocols of Hydroxymaleimide
as a New Synthon
he indoline skeleton, presented as a common structural
Tmotif in numerous natural products, has aroused extensive
attention from several research groups for its important role in
biological and pharmaceutical activities.1 As a result, many
efficient strategies2 for their preparation, such as asymmetric
hydrogenations of indoles,3 dearomatic cycloadditions of
indoles,4 intramolecular couplings,5 and kinetic resolutions,6
have been developed in recent years. Specifically, N-hydrox-
yindolines are key intermediates for the preparation of
neoxaline,7 bioactive in inhibiting cell proliferation and arresting
the cell cycle during M phase.1c However, there are still only
limited preparations7,8 for N-hydroxyindolines. In 2015, the
Zhou8a and Chang8b groups independently disclosed a rhodium-
(III)-catalyzed C−H cyclization of nitrones with diazoesters. To
date, no direct asymmetric approach for the preparation of N-
hydroxyindolines has been reported, especially for those
possessing quaternary stereocenters and highly functional
groups.
Nitrosoarenes,9 generally unstable with a highly reactive
nitroso group, showed N- and O-selectivities in Diels−Alder
reactions,10 oxyaminations,11 aminoxylations,12 cycloadditions,13
and other reactions.14 Among those reactions, the control of the
regioselectivity constitutes a great challenge.
hydroxyl group to the maleimide skeleton and wished it to work
as a new multifunctional synthon, which possessed ketoamide
and β-acidic C−H active groups (Scheme 1). Thus, the reactivity
and the reaction modes were totally broadened, and some new
domino reactions may be expected. As the beginning of a series of
new reactions, a possible oxyamination or an aminoxylation of
On the other hand, maleimides are also important building
blocks15 as both excellent Michael acceptors16 and good
dienophiles.17 Based on our related work in asymmetric reactions
of maleimides,16a,17c and considering the inert reactivity of
maleimide in a domino reaction after an initial nucleophilic
addition17b,d (Scheme 1), herein we devised and introduced a
Received: March 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00893
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
hydroxymaleimide with nitrosoarenes was also expected as we
have reported for such a similar kind of reaction.11c However, the
totally unexpected annulation products of N-hydroxyindolines 3
with two quaternary stereocenters rather than the expected
adduct 5 or 6 were exclusively obtained in excellent diastereo-
and enantioselectivities. Considering the scaffold and functional
groups in the obtained chiral N-hydroxyindolines, they may be
devised or transformed as chiral building blocks and new
chemical entities with potential applications in pharmaceutical
preparations. Herein, we presented the first organocatalytic
unprecedented asymmetric regioselective cyclization of nitro-
soarenes with hydroxymaleimides for a direct and highly effective
preparation of a series of new chiral dihydropyrrolo[3,4-
b]indole-1,3-diones in excellent yields with excellent stereo-
selectivities.
result was observed in toluene (67% yield, 87% ee; entry 10).
Lower catalyst loading to 5 mol % still gave an excellent result
within 12 h (95% yield, >99:1 dr, 97% ee; entry 11), which
demonstrated the efficiency of the new reaction and the catalyst.
Positively, enantioselectivity was slightly improved when low-
ering the reaction concentration (>99% yield, >99:1 dr, 98% ee;
entry 12; for more details, see the Supporting Information, SI).
Under the optimized conditions, the generality of hydrox-
ymaleimides 1 was studied. As shown in Table 2, a wide range of
Table 2. Substrate Scope of Hydroxymaleimides 1 with
a
Nitrosoarenes 2
We began our study by testing the model reaction of 3-ethyl-4-
hydroxy-1-(p-tolyl)-1H-pyrrole-2,5-dione (1a) and nitrosoben-
zene (2a) in the presence of chiral bifunctional organocatalyst 4a
(20 mol %) in dichloromethane (DCM) at 35 °C (Table 1). The
b
3/yield
c
entry
1/R1/ R2
2/R
2a/H
(%)
ee (%)
1
2
1a/4-MeC6H4/Et
1b/4-MeOC6H4/Et
1c/Ph/Et
3aa/99
3ba/92
3ca/>99
3da/88
3ea/93
3fa/94
3ga/70
3ha/94
3ia/84
3ja/84
3ka/78
3la/86
3ma/96
3na/65
3oa/93
3pa/98
3qa/81
3ra/NR
3ab/83
3ac/80
3ad/80
3ae/84
3af/99
3ag/99
3ah/87
98
95
99
96
98
97
98
97
96
96
e
a
Table 1. Optimizations of Reaction Condition
2a/H
3
2a/H
4
1d/4-FC6H4/Et
1e/4-ClC6H4/Et
1f/4-BrC6H4/Et
1g/3-MeOC6H4/Et
1h/3-MeC6H4/Et
1i/3-ClC6H4/Et
1j/3-BrC6H4/Et
1k/2-MeOC6H4/Et
1l/2-ClC6H4/Et
1m/3,5-(Me)2C6H3/Et
1n/nBu/Et
2a/H
5
2a/H
6
2a/H
7
2a/H
8
2a/H
9
2a/H
10
2a/H
d
11
f
2a/H
g
12
2a/H
90/91
13
14
15
16
17
18
19
20
21
22
23
24
25
2a/H
96
2a/H
96
b
c
entry
cat.
solvent
DCM
time (h)
yield (%)
ee (%)
1o/4-MeC6H4/Me
1p/4-MeC6H4/nPr
1q/4-MeC6H4/Bn
1r/4-MeC6H4/Ph
1a/4-MeC6H4/Et
1a/4-MeC6H4/Et
1a/4-MeC6H4/Et
1a/4-MeC6H4/Et
1a/4-MeC6H4/Et
1a/4-MeC6H4/Et
1a/4-MeC6H4/Et
2a/H
>99
96
1
2
4a
4b
4c
4d
4e
4f
48
48
12
12
12
12
12
12
12
12
12
12
65
73
85
79
92
92
89
88
95
96
97
87
97
98
2a/H
DCM
2a/H
91
3
DCM
77
2a/H
4
DCM
70
2b/4-Me
2c/3-Me
2d/3-Cl
2e/2-Me
2f/2-Cl
2g/2-Br
2h/2,4-(Me)2
92
99
97
95
>99
96
84
5
DCM
85
6
DCM
79
7
4c
4c
4c
4c
4c
4c
CHCl3
THF
81
8
72
9
1,4-dioxane
toluene
1,4-dioxane
1,4-dioxane
99
10
67
d
11
95
a
Reactions were performed with 1 (0.2 mmol), 2 (0.24 mmol), and 4c
(5 mol %) in 1,4-dioxane (0.1 M) at 35 °C for 3−36 h; >99:1 dr,
de
,
12
>99
a
b
c
determined by 1H NMR. Isolated yield. Determined by chiral
Unless otherwise noted, the reaction was performed with 1a (0.1
d
e
f
g
mmol), 2a (0.12 mmol), and catalyst 4 (20 mol %) in solvent (0.2 M)
HPLC. 2:1 dr. Enantiomers failed to be separated. 1:1 dr. Values of
at 35 °C for indicated time; >99:1 dr, determined by 1H NMR.
ee were determined from fully reduced product 12 (SI).
b
c
d
Isolated yield. Determined by chiral HPLC. 5 mol % cat. 4c used.
e
Performed at 0.1 M.
1 was tolerated. The aromatic R1 bearing para- or meta-
substituents all gave good to excellent yields with excellent
diastereo- and enantioselectivities (70 to >99% yields, >99:1 dr,
95−99% ee, entries 1−10), whereas ortho-substituents, regard-
less of the electronic characteristics, gave good yields and
significantly decreased diastereoselectivities (78−86% yields,
1:1−2:1 dr, entries 11 and 12). The possible reason for the low
diastereoselectivities may be the atropisomerism of the N−Ar
single bond of maleimide.17d 3,5-Dimethyl phenyl group also
smoothly gave excellent yield and enantioselectivity (96% yield,
>99:1 dr, 96% ee, entry 13). For an alkyl-substituted group (R1 =
nBu), 3na was efficiently obtained in moderate yield with
reaction proceeded smoothly to afford the unexpected product
3aa in 65% yield with complete regioselectivity and good
stereoselectivities (>99:1 dr, 85% ee; entry 1), whereas the
expected adduct 5 or 6 was not observed. To improve the
enantioselectivity, chiral cinchona alkaloid squaramides 4c,d and
thioureas 4e,f were evaluated, and the readily available 4c gave
the best results (77% yield, >99:1 dr, 92% ee; entry 3). Ethers and
chloro solvents gave good to excellent yields (72−99%) and
excellent stereoselectivities (>99:1 dr, 95−97% ee; entries 7−9),
and 1,4-dioxane afforded the best yield and stereoselectivities
(99% yield, >99:1 dr, 97% ee; entry 9). However, an inferior
B
DOI: 10.1021/acs.orglett.7b00893
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
excellent diastereoselectivity and enantioselectivity (65% yield,
>99:1 dr, 96% ee, entry 14). When R2 was replaced by a methyl,
n-propyl, or benzyl group, target products were smoothly
obtained in excellent yields without loss of stereoselectivities
(81−98% yields, >99:1 dr, 91 to >99% ee, entries 15−17).
Unfortunately, phenyl-substituted substrate 1r did not work with
2a, probably due to the steric hindrance and the highly
delocalized double bond enolized from the ketoaminde 1r
(Scheme 1). Further, the scope of nitrosoarenes 2 was broadened
under optimized conditions. Substituted positions had a
remarkable effect on the reaction. 4-Methylnitrosobenzene 2b
with 1a gave a good outcome (83% yield, >99:1 dr, 92% ee, entry
19). However, other para-substituted nitrosobenzenes cannot
work smoothly (SI). When 3-substituted nitrosobenzenes were
used, annulations took place in the para position exclusively
(80% yields, >99:1 dr, 97−99% ee, entries 20 and 21).
Additionally, ortho-substituents (Me, Cl, Br) also worked well
in excellent yields with excellent diastereo- and enantioselecti-
vites (84−99% yields, >99:1 dr, 95 to >99% ee, Table 2, entries
22−24). In particular, 2-chloronitrosobenzene and 2-bromoni-
trosobenzene effectively furnished the products completely
within 3 h. Dimethyl-substituted 2h also efficiently gave 3ah in
87% yield, >99:1 dr, and 84% ee. The absolute configuration of
the product 3fa was established to be (3aR,8bR) by X-ray
crystallographic analysis.18 Based on the experiments and related
literature,9a,14d,19 a plausible mechanism was suggested (SI).
Considering the great potentials of the multifunctional chiral
products and for diversity of chiral indoline preparations, we
further tried to perform the reaction on a larger scale of 1a under
the optimal conditions and furnished 3aa with equally excellent
results (Scheme 2). The efficiency and potentials of the new
atmospheric hydrogen pressure with exclusive N−OH bond
cleavage.11c Additionally, the selective reduction of the C−OH
bond was studied. However, we failed to get the desired product
7 directly, and the unexpected N-methylated product 10 was
obtained in good results (65% yield, >99:1 dr, 97% ee) under
higher hydrogen pressure (6 MPa) in methanol. When excessive
LiAlH4 used, completely reduced product 11 was obtained in
excellent results (94% yield, >99:1 dr, 96% ee). The reduction
was also found in alternative preparations for similar racemic
indolines.21 Product 8 was easily formed from 9 in methanol in
the presence of catalytic concentrated sulfuric acid.21a To the
best of our knowledges, the total preparations of those special
chiral indolines were not found in the literature, and those
transformations may provide a great potential for pharmaceuti-
cally active molecules.
In summary, hydroxymaleimide has been successfully devised
as a novel synthon. An unconventional asymmetric regioselective
cyclization of nitrosoarenes with hydroxymaleimides was first
disclosed in the presence of a chiral bifunctional amine
squaramide catalyst. A series of highly fused chiral N-
hydroxyindolines with two adjacent quaternary stereocenters
and multifunctional groups were directly and effectively prepared
in excellent yields (up to >99%) with complete regioselective
annulation and excellent stereoselectivities (up to >99:1 dr and
>99% ee). The efficiency and potentials of the new reaction and
the target chiral entities were well demonstrated by delicate
transformations to a variety of new chiral indolines.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00893.
Scheme 2. Scale-up Experiment and Representative
Transformations of the Product 3aa
X-ray data for compound 3fa (CIF)
Experimental procedures and detailed characterization
data for the new compounds (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wlxioc@cioc.ac.cn.
ORCID
Li-Xin Wang: 0000-0003-0830-7421
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.7b00893
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