Facile synthesis of 3-aryl-3-hydroxy-2-oxindoles from 2-arylindoles

Full text of DOI:10.1002/bkcs.10807

Note
DOI: 10.1002/bkcs.10807
BULLETIN OF THE
H. R. Moon et al.
KOREAN CHEMICAL SOCIETY
Facile Synthesis of 3-Aryl-3-hydroxy-2-oxindoles from 2-Arylindoles
†
‡
†
†,*
Hye Ran Moon, Sangku Lee, Hwa Jung Roh, and Jae Nyoung Kim
†Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju
5
00-757, Korea. *E-mail: kimjn@chonnam.ac.kr
‡
Targeted Medicine Research Center, Korea Research Institute of Bioscience and Biotechnology,
Chungbuk 363-883, Korea
Received February 4, 2016, Accepted April 2, 2016, Published online June 26, 2016
Keywords: 3-Aryl-3-hydroxy-2-oxindoles, 2-Arylindoles, Semipinacol rearrangement, Aerobic oxidation
3
-Aryl-3-hydroxy-2-oxindoles have been found in many
The use of 3.0 equiv of KOH (entry 4) does not increase
the yield, although reaction time was shortened. DMF
(entry 5) or THF (entry 6) was found to be less efficient
solvent. The use of NaOH (entry 7) or tetrabutylammonium
hydroxide (entry 9) showed a similar result to that of KOH,
while LiOH (entry 8) or DBU (entry 10) was ineffective.
1
biologically important compounds, as shown in Figure 1.
As some examples, orally active potent growth hormone
secretagogue SM-130686, potent antimicrobial lead drug
1
a
1b,c
ECi8,
and its lead compound
-aryl-3-hydroxy-2-oxindoles have been used as intermedi-
ates for the synthesis of many oxindole derivatives such as
and potent opener of maxi-K channels MaxiPost
1
d,2a
have been reported. In addition,
t
3
KO Bu (entry 11) or potassium trimethylsilanolate (entry
12) showed a similar reactivity to that of KOH. The
3
-halo-, 3-alkoxy-, 3-thioalkoxy-, 3-aryl, and 3-acetamido-
reaction under N balloon atmosphere (entry 13) did not
produce 2a as expected.
2
2
2-oxindole derivatives. Thus, various synthetic approaches
3–6
for these valuable compounds have been reported.
reactions of isatin with aryllithium or arylmagnesium
halides have been reported. The reaction of arylboronic
acids and isatin in the presence of transition metal catalyst
has also been used frequently.
of α-ketoamides
tin with electron-rich arenes also provided these valuable
compounds.
The
Various 2-substituted indoles 1b–1n were prepared
9
according to the reported methods. The reactions of 1b–1n
3
were carried out under the optimized condition (KOH,
ꢀ
DMSO, 65 C, O balloon), and the results are summarized
2
1
e,4
In addition, cyclization
in Table 2. The reactions of 5-chloro-2-phenylindole (1b)
and 5-methyl-2-phenylindole (1c) afforded the correspond-
ing products 2b (79%) and 2c (86%) in good yields. How-
ever, the reaction of 5-nitro derivative 1d did not produce
2d, presumably due to delocalization of the indole anion to
the nitro group (vide infra, Scheme 3). The reactions of 2-
(4-chlorophenyl)indole (1e), 2-(4-methoxyphenyl)indole
(1f), 2-(2-naphthyl)indole (1g), and 2-(2-thienyl)indole (1h)
afforded the corresponding products 2e–2h in good yields
(77–87%). It is interesting to note that the reaction of 1f
was completed in short time (10 h) than other entries. The
result implied that a facile migration of electron-rich 4-
methoxyphenyl moiety would reduce the reaction time
(vide infra). The reaction of 2-(2-benzofuranyl)indole (1i)
was very slow, and 2i was isolated in low yield (39%) after
48 h along with recovered 1i (8%). The reaction of 2-(4-
nitrophenyl)indole (1j) was also ineffective, and the reason
might be attributed to difficult migratory aptitude of
electron-deficient 4-nitrophenyl moiety (vide infra, Scheme
3). In addition, the reactions of 2-tert-butylindoles 1k–1m
afforded 2k–2m in good yields (84–90%) in short time
(10 h). However, the reaction of 2-benzylindole (1n)
showed the formation of many intractable side products.
1b,5
and Friedel–Crafts type reaction of isa-
6
Recently, we reported an aerobic transition metal-free
synthesis of 2,3-diarylindoles via an oxidative nucleophilic
substitution of hydrogen (ONSH) pathway from 2-
7
arylindoles and nitroarenes. As an example, the reaction of
2-phenylindole (1a) and 1,3-dinitrobenzene in DMSO in
the presence of Cs CO under O2 balloon atmosphere
2
3
afforded 2,3-diarylindole in good yield (78%) in short time,
as shown in Scheme 1. During the study, we found the for-
mation of 3-phenyl-3-hydroxy-2-oxindole (2a), albeit in
moderate yield (41%), under the same reaction condition
for a long time (8 h) without 1,3-dinitrobenzene.
Encouraged by the unusual formation of synthetically
useful 3-phenyl-3-hydroxy-2-oxindole, we examined the
optimum condition for the conversion of 1a to 2a, as sum-
marized in Table 1. The yield of 2a was improved to 73%
when we replaced Cs CO to KOH under the same reaction
2
3
ꢀ
condition (90 C, 8 h), as shown in entry 1. The yield of 2a
was further improved to 84% at lower temperature (65 C)
with lesser amount of KOH (1.5 equiv), although a long
reaction time (20 h) was required (entry 2). When we used
ꢀ
8
10
The only isolable compound was 2-benzoylindole (2n).
lesser amount of KOH (0.5 equiv), 2a was formed in
low yield (12%) and most of 1a was recovered (entry 3).
Based on the experimental results, the reaction mechan-
ism for the conversion of 1a to 2a is proposed, as shown in
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Scheme 2. The indole anion I was converted to 3-
hydroxyindolenine intermediate III via the hydroperoxide
F3C
HO
HN
11
Cl
HO
intermediate II. Subsequently, III was changed to an
epoxide intermediate IV under basic condition, and a fol-
lowing semipinacol type rearrangement of IV would pro-
O
Cl
H N
2
N
O
O
N
1
2
NEt HCl
duce 3-phenyloxindole VI.
A
subsequent aerobic
2
SM-130686
ECi8
13
oxidation of VI would produce 2a.
2
As noted above, the reactions of 1d and 1j did not pro-
duce the corresponding 3-aryl-3-hydroxy-2-oxindoles. As
shown in Scheme 3, an electron delocalization of the indole
R O
R1
Cl
O
0
anion I renders the formation of a hydroperoxide interme-
N
H
F3C
0
diate II difficult. For the reaction of 1j, a rearrangement of
MaxiPost (BMS-204352): R1 = F, R2 = Me
Lead compound for MaxiPost: R1 = OH, R2 = H
4-nitrophenyl group might be difficult during the conver-
0
0
sion of epoxide intermediate IV to V .
Figure 1. Biologically important 3-aryl-3-hydroxy-2-oxindoles.
As a synthetic application, the reduction of 2a to
3
-phenylindole (3a) was examined. Actually we could
obtain 3a in good yield (74%) by the reaction of 2a under
.
3
14
the reported condition using BH THF (3.0 equiv), as
shown in Scheme 4. Thus, our protocol could provide
an easy way of two-step synthesis of 3-arylindole from
NO2
1
,3-dinitrobenzene
2
-arylindole.
(
Previous work7)
O N
2
In summary, an efficient synthetic route of 3-aryl-3-
Cs CO3 (2.0 equiv), DMSO
0 oC, O2 balloon, 2 h
2
hydroxy-2-oxindoles has been developed from 2-arylin-
doles. The procedure provides a brand-new synthetic
method of 3-aryl-3-hydroxy-2-oxindoles from readily avail-
able starting materials in high yields.
9
Ph
Ph
N
H
HO
N
H
(78%)
no 1,3-dinitrobenzene
Ph
1
a
(
This work)
O + 1a
(37%)
Cs CO3 (2.0 equiv), DMSO
0 oC, O2 balloon, 8 h
2
N
H
Experimental
9
(
initial observation)
2a (41%)
Typical procedure for the synthesis of 2a. A stirred solu-
Scheme 1. Formation of 3-phenyl-3-hydroxy-2-oxindole (2a).
tion of 1a (97 mg, 0.5 mmol) and KOH (85%, 50 mg,
ꢀ
0
2
.75 mmol) in DMSO (1.0 mL) was heated to 65 C for
0 h under O balloon atmosphere. After the usual aqueous
2
extractive workup and column chromatographic purification
process (CH Cl /EtOAc, 3:1), 2a was isolated as a white
Table 1. Optimization for the conversion of 1a to 2a.
2
2
a
b
solid, 95 mg (84%). Other compounds were synthesized
Entry
Conditions
2a (%)
similarly, and the selected spectroscopic data of 2a and 2k
ꢀ
1
2
3
4
5
6
7
8
9
KOH (2.0 equiv), DMSO, 90 C, 8 h
KOH (1.5 equiv), DMSO, 65 C, 20 h
73
84
12
2e,15
are as follows.
ꢀ
2
e
ꢀ
−1 1
ꢀ
c
Compound 2a . 84%; white solid, mp 210–212 C (Ref. 2e
2
KOH (0.5 equiv), DMSO, 65 C, 20 h
ꢀ
KOH (3.0 equiv), DMSO, 65 C, 10 h
ꢀ
ꢀ
07–209 C); IR (KBr) 3414, 3176, 1697 cm ; H NMR
77
25
<5
c
(DMSO-d , 300 MHz) δ 6.62 (s, 1H), 6.90 (d, J = 7.8 Hz,
6
KOH (1.5 equiv), DMF, 65 C, 20 h
c,d
1H), 6.96 (t, J = 7.8 Hz, 1H), 7.09 (d, J = 7.8 Hz, 1H),
KOH (1.5 equiv), THF, reflux, 20 h
NaOH (1.5 equiv), DMSO, 65 C, 20 h
13
ꢀ
7
.21–7.35 (m, 6H), 10.40 (br s, 1H); C NMR (DMSO-d6,
75 MHz) δ 77.32, 109.88, 122.08, 124.79, 125.43, 127.44,
28.11, 129.26, 133.76, 141.55, 141.96, 178.50; ESIMS m/
76
11
ꢀ
c
LiOH (1.5 equiv), DMSO, 65 C, 20 h
ꢀ
TBAOH (1.5 equiv), DMSO, 65 C, 20 h
ꢀ
1
83
+
c
z 226 [M +H].
1
1
1
0
1
2
3
DBU (1.5 equiv), DMSO, 65 C, 20 h
0
ꢀ
t
15
ꢀ
KO Bu (1.5 equiv), DMSO, 65 C, 10 h
SiOK (1.5 equiv), DMSO, 65 C, 20 h
KOH (1.5 equiv), DMSO, 65 C, 20 h
84
82
<5
Compound 2k . 90%; white solid, mp 222–224 C (Ref.
ꢀ
ꢀ
−1
1
Me
3
15 224–225 C); IR (KBr) 3358, 1716 cm ; H NMR
ꢀ
e
c,d
1
a
b
c
(DMSO-d , 400 MHz) δ 0.94 (s, 9H), 5.65 (s, 1H), 6.76 (d,
6
J = 7.6 Hz, 1H), 6.92 (t, J = 7.6 Hz, 1H), 7.18 (t,
The reaction was carried out under O
2
balloon atmosphere.
Isolated yield.
1
J = 7.6 Hz, 1H), 7.24 (d, J = 7.6 Hz, 1H), 10.11 (br s,
13
a remained.
1H); C NMR (DMSO-d , 100 MHz) δ 23.95, 36.74,
6
d
e
Not isolated.
balloon atmosphere.
7
1
9.84, 108.98, 120.71, 125.67, 128.54, 131.50, 142.34,
79.50; ESIMS m/z 206 [M +H].
N
2
+
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a
Table 2. Synthesis of 3-substituted-3-hydroxy-2-oxindoles.
Entry
Indoles 1
Products 2 (%)
Entry
8
Indoles 1
Products 2 (%)
HO Ar4
HO
Ph
S
O
c
1
Ph
O
N
N
H
N
N
H
2h (77)
2
a (84)
H
H
1h
1
a
Ar5
HO
HO
Ph
Cl
Cl
O
2
3
4
5
Ph
O
9
O
N
N
H
c,e
N
H
2b (79)
N
H
2i (39)
Ar6
H
1
b
1i
HO
Ph
HO
Me
Me
Ph
O
1
1
0
1
O
NO2
N
H
N
N
c,f
2
c (86)
N
H
2j (0)
H
H
1
c
1j
HO
Ph
HO
O N
2
O N
2
Me
Me
Me
1k
Me
Me
Me
1l
Ph
O
O
N
H
N
H
d (0)b
2
N
H
N
d
2k (90)
H
1
d
e
f
Ar1
HO
HO
HO
HO
Me
Cl
Me
Cl
Cl
O
N
H
e (81)c
12
13
O
N
H
2
d
N
H
N
2l (87)
1
1
H
Ar2
HO
6
7
OMe
O
Me
Me
Me
N
H
N
H
f (87)c,d
O
2
m (84)d
N
H
N
H
2
Ar3
1
m
O
Ph
O
1
4
N
H
Ph
N
H
2g (78)c
N
H
N
H
d
1
g
1n
2n (26)
a
b
c
d
e
f
ꢀ
Conditions: Indole 1 (0.5 mmol), KOH (1.5 equiv), DMSO, 65 C, O
2
balloon, 20 h.
Indole 1d was recovered in 91%, and some polar side products were formed.
Ar = 4-chlorophenyl, Ar = 4-methoxyphenyl, Ar = 2-naphthyl, Ar = 2-thienyl, Ar = 2-benzofuranyl, Ar = 4-nitrophenyl.
Reaction time was 10 h.
1
2
3
4
5
6
Reaction time was 48 h, and 1i was recovered in 8%.
Indole 1j was recovered in 52%, and some polar side products were formed.
H
H
O
N
Ph
KOH
O
2
1
a
Ph
O
2a
O
O
Ph
Ph
N
N
H
OOH
Ph
N
N
O N
2
I
VI
O2
H
1
d
O
N
O2
H
N
H
Ph
O OH
Ph
II'
O
N
I'
N
NO
2
V
II
semipinacol type
rearrangement
H
O
H
H
OH
Ph
H
1
j
O
N
O
KOH
Ph
N
NO2
N
N
IV'
V'
III
IV
Scheme 3. Plausible reason for the failure of 1d and 1j.
Scheme 2. Proposed mechanism.
Acknowledgments. This work was supported by the
National Research Foundation of Korea (NRF) grant
funded by the Korea government (MSIP) (NRF-
Ph
Entry 1
Table 2)
BH THF (3.0 equiv.)
3
2a THF, rt, 24 h
(
2015R1A4A1041036). Spectroscopic data were obtained
1a
(84%)
N
H
from the Korea Basic Science Institute, Gwangju branch.
3a (74%)
Supporting Information. Additional supporting informa-
tion is available in the online version of this article.
Scheme 4. Two-step conversion of 1a to 3a.
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