One-pot synthesis of 3-hydroxy-2-oxindoles: Via acyloin rearrangements of 2-hydroxy-indolin-3-ones generated in situ from 2-alkynyl arylazides

Article abstract of DOI:10.1039/d0nj04588h

A novel one-pot method to prepare 3-hydroxy-2-oxindoles via acyloin rearrangements of 2-hydroxy-indolin-3-ones generated in situ from 2-alkynyl arylazides has been described. The reaction was accomplished to afford a variety of 3-hydroxy-2-oxindole derivatives in moderate to good yields under mild conditions. This journal is

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One-pot synthesis of 3-hydroxy-2-oxindoles via
acyloin rearrangements of 2-hydroxy-indolin-3-
ones generated in situ from 2-alkynyl arylazides†
Cite this:DOI: 10.1039/d0nj04588h
Zhiqiang Zhou, Yao Xu, Boyu Zhu, Ping Li, Guiwen Hu, Fan Yang, Shijie Xu and
Xiaoxiang Zhang
*
Received 14th September 2020,
Accepted 9th November 2020
A novel one-pot method to prepare 3-hydroxy-2-oxindoles via acyloin rearrangements of 2-hydroxy-
indolin-3-ones generated in situ from 2-alkynyl arylazides has been described. The reaction was
accomplished to afford a variety of 3-hydroxy-2-oxindole derivatives in moderate to good yields under
mild conditions.
DOI: 10.1039/d0nj04588h
rsc.li/njc
Introduction
3-Hydroxy-2-oxindole skeletons widely exist in many natural
products and biologically active compounds.1 For example, SM-
130686 (ghrelin receptor agonist),1k Maxi K-channel opener,1k
Maremycins A/B (active against k562 leukemia cells),1k Con-
volutamydine A and B (inhibitors of leukemia cells HL-60),1k–m
and (R)-3-(cyclopropylethynyl)-3-hydroxy-1,5-dimethylindolin-2-
one (HIV-1 reverse transcriptase inhibitor)1n have been well
studied (Fig. 1). In addition, 3-hydroxy-2-oxindoles can also be
used as important synthetic precursors for the construction of
various oxindoles.2 Thus, many methods have been established
for the synthesis of 3-hydroxy-2-oxindoles. Among those
reported works, the synthetic strategies were mainly focused
on three types of starting materials. Isatins could be attacked by
organic metals such as arylmagnesium or aryllithium halides
to give 3-hydroxy-2-oxindoles.3 Furthermore, selective metal-
reduction4 and Friedel–Crafts reactions of isatins5 were also
valuable synthetic methods. Other strategies such as C–H
oxidation of 2-oxindoles6 and annulation of a-ketoamides7 were
also reported frequently. Considering their importance in
pharmaceutical chemistry, the development of novel synthetic
protocols for the formation of 3-hydroxy-2-oxindoles is still
highly desired.
Fig. 1 Natural products and biologically active compounds containing
3-hydroxy-2-oxindole skeleton.
reactions of azide–alkynes, which could be further trapped by
diverse nucleophiles. In our previous work, we reported that
a-imino palladium carbenes generated in situ from 2-alkynyl
arylazides could be attacked by TsOHÁH2O, which was followed
by a rearrangement process under basic conditions to give
3H-indole-3-one G intermediates.9 However, direct nucleophilic
addition10 or N-fused cycloaddition11 of 3H-indole-3-ones G to
various indolines was the main transformation (Scheme 1).
Based on previous reports and our interest in the metal-
catalyzed synthesis of N-heterocycles from azide–alkynes,9 we
envisioned that hydroxylation and acyloin rearrangement12 of
3H-indole-3-ones G generated in situ from 2-alkynyl arylazides
with water would be a novel method to prepare 3-hydroxy-2-
oxindole derivatives.
Recently, metal-catalyzed amination reactions of azide–alkynes
were regarded as efficient methods to form N-heterocycles.8
In those reported works, a-imino metal carbenes as important
intermediates were generated in situ from metal-catalyzed
Results and discussion
Co-Innovation Center for Efficient Processing and Utilization of Forest Products,
College of Chemical Engineering, Nanjing Forestry University, Nanjing 210037,
China. E-mail: zhangxiaoxiang@njfu.edu.cn
Initially, we chose to focus our attention on the reaction of
1-azido-2-(phenylethynyl)benzene 1a and TsOHÁH2O 2 with
Pd(OAc)2 in the presence of diverse bases (Table 1). This
revealed that treating a solution of 1a (1 equiv.) and 2 (3 equiv.)
† Electronic supplementary information (ESI) available. CCDC 1942444. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d0nj04588h
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t-BuOK was found to give the desired product 3a in yields of
17–43%, respectively (entries 2–4). However, organic base such
as DBU or Et3N could not provide the desired product 3a.
During those reactions, a mixture of unknown compounds
was formed, which could not be isolated (entries 5 and 6).
Later, the reaction temperature was also checked under similar
reaction conditions. When the reaction was conducted at room
temperature, the desired product 3a could not be detected.
In this reaction, the intermediate 2-phenyl-1H-indol-3-yl 4-methyl-
benzenesulfonate E was generated as the major compound
checked by TLC and 1H NMR (entry 7). Furthermore, a trace
amount of product 3a was formed when the reaction temperature
Scheme 1 Synthetic methods of 3-hydroxy-2-oxindoles.
Table 1 Optimization of the reaction conditionsa
Table 2 Substrate scope of 2-alkynyl arylazides 1ab
Yieldb (%)
Entry
Base
T (1C)
Time (h)
1
2
3
4
5
6
7
8
9
Cs2CO3
NaOH
K2CO3
t-BuOK
DBU
90
90
90
90
90
90
r.t.
60
90
6
6
24
6
6
6
6
6
6
81
43
17
24
Complex
Complex
n.r.
Trace
n.r.
Et3N
Cs2CO3
Cs2CO3
c
—
a
Reaction conditions: 1a (0.1 mmol), Pd(OAc)2 (5 mmol%), 2 (3 equiv.),
b
c
1,4-dioxane (1 mL), base (4.5 equiv.). Isolated yield. Without base.
with Pd(OAc)2 (5 mol%) at r.t. for 10 min and then adding
Cs2CO3 (4.5 equiv.) at 90 1C for 6 h gave the best result (entry 1).
Under these reaction conditions, 3-hydroxy-3-phenylindolin-2-
one 3a was obtained in 81% yield, which was confirmed by
1H NMR analysis and X-ray crystal structure determination
(Fig. 2). In contrast, other inorganic and organic bases were
also examined. Repeating the reaction with NaOH, K2CO3 or
a
Reaction conditions: 1a (0.1 mmol), Pd(OAc)2 (5 mol%), 2 (3.0 equiv.),
b
Fig. 2 ORTEP drawing of 3a with thermal ellipsoids at 50% probability.
1,4-dioxane (1.0 mL), Cs2CO3 (4.5 equiv.), and 90 1C. Isolated yield.
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was increased to 60 1C. The rest of the reaction was the mixture of
In order to better understand the reaction process, the
intermediate E and 3H-indol-3-one G (entry 8). Repeating this intermediate G was isolated, which was employed as the starting
reaction at 90 1C in the absence of a base resulted in no desired material in the following two control experiments (Scheme 3).
product 3a. The intermediate 2-phenyl-1H-indol-3-yl 4-methyl- It was noted that a mixture of products O18-3a and O16-3a was
benzenesulfonate E was formed as the major compound in the obtained in 60% yield in the presence of 0.1 mL of H2O18. In the
reaction (entry 9).
product mixture, O18-3a was observed by HRMS. A trace amount
With the best reaction conditions in hand, we next exam- of H2O16 in dry 1,4-dioxane might result in the formation of O16-
ined the effects of various 2-alkynyl arylazides 1 (Table 2). 3a. After FTIR spectroscopy analysis of O18-3a and O16-3a, we
Under the standard reaction conditions, electron-withdrawing found that the O18-labeling atom was present in the amide group
or electron-donating groups on the aromatic ring of 2-alkynyl of O18-3a.13 However, only a trace amount of product 3a was
arylazides 1 gave the desired products 3b–3l in moderate to good formed under dry conditions. The above results show that water
yields (R1 = CF3, halide, ester and Me). In addition, various was an indispensable reagent for this one-pot reaction.
substituted aryl groups on the alkyne carbon of substrates 1 were
also tested and the desired products 3m–3s were obtained in
Conclusions
59–93% yields. Switching the substituted aromatic group R2 to
the hetero aryl or alkyl group was found to afford the corresponding
product 3t and 3u in yields of 51 and 60%, respectively.
In summary, a novel Pd-catalyzed one-pot strategy for the
preparation of 3-hydroxy-2-oxindoles from 2-hydroxyl-indoline-3-
ones generated in situ from 2-alkynyl arylazides has been reported.
The desired products were provided in moderate to good yields
under mild reaction conditions. During the reactions, an acyloin
rearrangement process was involved. Further studies on the
application of acyloin rearrangements are underway.
Based on the above experimental results and our previous
work,9 we tentatively propose a possible mechanism in Scheme 2
for converting 1a to 3-hydroxy-2-oxindole 3a, although it is highly
speculative (Scheme 2). Pd-Catalyzed activation of 2-alkynylaryl
azide 1a to generate a-imino palladium carbene C, which could
be trapped by TsOHÁH2O to give E. After 1,3-Ts shift of E and
followed by a reductive desulfonation process, the key inter-
mediate 2-phenyl-3H-indol-3-one G was formed. Then, hydroxyla-
tion of TsOHÁH2O crystal water or water produced by acid–base
neutralization to CQN double bonds of G resulted in the for-
mation of 2-hydroxy-2-phenylindolin-3-one H. Finally, an acyloin
rearrangement of H provided the desired product 3-hydroxy-3-
phenylindolin-2-one 3a.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China Grants (21302096), the Natural Science
Foundation of Jiangsu Province-Grants (BK20171449), and the
Qing Lan Project.
Notes and references
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