A mild procedure for the oxidative cleavage of substituted indoles catalyzed by plant cell cultures

Article abstract of DOI:10.1016/j.tetlet.2004.08.175

We have developed a mild procedure for the oxidative cleavage of indole carbon double bonds using plant cell cultures as a catalytic system in the presence of H₂O₂. We have developed a novel procedure for the oxidative cleavage of indole carbon double bonds in the presence of H ₂O₂ using plant cell cultures as a catalytic system. The oxidative procedure has some advantages, such as mild reactions, good yields, easy work-up and safety.

Full text of DOI:10.1016/j.tetlet.2004.08.175

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 8061–8064
A mild procedure for the oxidative cleavage of substituted indoles
catalyzed by plant cell cultures
Masumi Takemoto,* Yasutaka Iwakiri, Yuki Suzuki and Kiyoshi Tanaka
School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Shizuoka 422-8526, Japan
Received 16 July 2004; revised 16 August 2004; accepted 27 August 2004
Available online 18 September 2004
Abstract—We have developed a novel procedure for the oxidative cleavage of indole carbon double bonds in the presence of H2O2
using plant cell cultures as a catalytic system. The oxidative procedure has some advantages, such as mild reactions, good yields,
easy work-up and safety.
Ó 2004 Elsevier Ltd. All rights reserved.
Oxidative cleavage of a carbon–carbon double bond is a
widely used method for the preparation of carbonyl
compounds in organic synthesis.1 A number of different
reagents have been developed for the oxidative cleavage
of the enamine carbon double bonds in the past such as
sodium periodate,2 sodium dichromate in acid,3 nitrous
acid,4 potassium permanganate5 and molecular oxygen
withcopperionsystems.6,7Ontheotherhand,thedevelop-
ment of the usage of enzymes for oxidation reactions
aimed at green chemistry is significant these days. The
biocatalytic oxidative cleavage of a carbon–carbon dou-
ble bond is preferable, because it takes advantage of the
mild reaction conditions. Three dioxygenases are known
so far: (1) tryptophan 2,3-dioxygenase,8 (2) indole 2,3-
dioxygenase,9 (3) protocatechuate 3,4-dioxygenase.10
Tryptophan, at the first step in one of its metabolic
pathways, binds molecular oxygen by the catalytic
action of tryptophan 2,3-dioxygenase leading to oxidative
cleavage of the heterocyclic ring to give formylkynure-
nin. Indole 2,3-dioxygenase catalyzes the conversion of
indole to anthranilic acid. Formylaminobenzaldehyde
and o-aminobenzaldehyde were detected as intermedi-
ates during the overall conversion.
However, only a few attempts have been reported for
the oxidative cleavage by enzymes in organic synthesis.
Electron-rich alkenes such as enamines and enol ethers
are known to react readily with singlet oxygen to yield
unstable dioxetanes, which can subsequently cleave to
two carbonyl fragments.11 The coupled oxygenation
with peroxidase in the presence of H2O2 is analogous
to the reaction of singlet oxygen.12 We have found that
plant cell cultures is an efficient source of peroxidase
enzymes as ÔreagentsÕ in organic synthesis and a huge
amount of H2O2 is produced in plant cell cultures by
the addition of foreign substrates.13 Then, our attention
turned to the subject of whether plant cell cultures have
oxidative ability as catalysts to cleave electron-rich car-
bon double bonds in the presence of H2O2. Very
recently, we found 2,3-dimethylindole (1) was converted
into o-acetylaminoacetophenone (2) with oxidative
cleavage of the carbon–carbon double bonds. In this
O
COOH
NH2
COOH
O2
NH2
N
H
tryptophan 2,3-dioxygenase
O2
NHCHO
CHO
COOH
CHO
NH2
indole 2,3-dioxygenase
N
,
NH2
NHCHO,
H
Keywords: Oxidative cleavage reactions; Enamines; Indoles; Carbazoles; Catalysts; Enzymes and enzyme reaction.
*
Corresponding author. Tel.: +81 54 264 5741; fax: +81 54 264 5740; e-mail: takemoto@ys2.u-shizuoka-ken.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.08.175

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M. Takemoto et al. / Tetrahedron Letters 45 (2004) 8061–8064
letter, we would like to present our results of the biocat-
alytic oxidative cleavage of substituted indoles and
enamines with a plant cell culture–H2O2 system.
and these results are shown in Table 2. The carbon–
carbon double bonds of all indoles (4, 7, 9, 13, 15, 17,
19,21 ) were cleaved to give the corresponding amides
(entries 1–3) and cyclic lactam (entries 5–9) in the pres-
ence of H2O2. A huge amount of H2O2 was produced in
the plant cell culture by the addition of foreign sub-
strates.13 In the absence of foreign H2O2, 11, 23 and
25 were cleaved to give 12, 24 and 26 in the plant cell
culture with produced H2O2 (entry 4, 10, 11). For com-
parison with such plant cell cultures–H2O2 system,
sodium periodate20 was also examined. The reaction of
4, 7, 9, 11, 13, 15, 17, 19, 21,23 and 25 with NaIO4 in
50% aqueous MeOH afforded 5 (Y. 47%), 8 (Y. 16%),
10 (Y. 68%), 12 (Y. 28%), 14 (Y. 55%), 16 (Y. 99%),
18 (Y. 19%), 20 (Y. 40%), 22 (Y. 8%), 24 (Y. 70%)
and 26 (Y. 15%). The yields with NaIO4 decreased in
the case of highly reactive indoles having an electron-
donating substituent into the indole ring (e.g., com-
pounds 7, 9, 11, 17 and 21). But, the yields with the plant
cell culture were much higher than that with NaIO4
(entries 2, 3, 4, 7 and 9).
The oxidative cleavage reactions of 1 were performed at
25°C with H2O2 in the medium indicated. These results
are shown in Table 1. The suspension-cultured cells,
which had originally been isolated from Camellia sinen-
sis,13,14 Nicotiana tabacum,15,16 Catharanthus roseus17,18
and Daucus carota19 were used. Sodium periodate has
been used to cleave the indolic double bond of several
2,3-disubstituted indoles to give the corresponding keto-
amides.20 For comparison with such plant cell systems,
chemical reagents21 were also examined.
Among the procedures examined, C. roseus in the pres-
ence of H2O2 showed the highest yield of 2 (entry 7). The
reaction proceeded much faster with C. sinensis, N. taba-
cum or C. roseus in the presence of H2O2 than with
FeCl3, K3Fe(CN)6 or D. carota. In the absence of plant
cell cultures, longer reaction time was required with
H2O2 to complete the reaction (entry 3). In B5 or MS
medium, reaction does not proceed (entries 9 and 10).
Though reaction with FeCl3or K3Fe(CN)6 also gave 2
and 3, the plant cell culture and sodium periodate gave
no other products than compound 2 (entries 4–8). Thus,
C. roseus, C. sinensis and N. tabacum were proven to be
effective catalysts for oxidative cleavage of 1 in satisfac-
tory yields without any side products in short reaction
time.
The representative experimental procedure is as follows:
the suspension-cultured cells, which had originally been
derived from C. roseus, N. tabacum, C. sinensis or D.
carota as described in our previous papers13–19 were
used. A substrate (1mmol) was added to the freely sus-
pended plant cell cultures (42g cells and 200mL broth).
After the mixture was shaken at 25°C on a rotary shaker
(110rpm) in the dark for 10min, 30% H2O2 (1.0mL) was
added to the mixture. After 0.5h of incubation, the incu-
bation mixture was filtered, the filtered cells were
washed with AcOEt and the filtrates were combined.
The combined mixture was extracted with AcOEt and
the organic layer was dried over anhydrous MgSO4. A
substrate (10mmol) was added to the freely suspended
plant cell culture (420g cells and 2000mL broth) with
30% H2O2 (10mL) in a bioreactor.
Next, to explore the catalytic abilities of the plant cell
cultures–H2O2 system, the oxidative cleavages of substi-
tuted indoles and enamines such as 3-methylindole (4),
2-methylindole (7), 2,3-dimethyl-5-methoxyindole (9),
2,3-cyclopentenoindole
(11),
2,3-cyclopenteno-5-
methoxyindole (13), 1,2,3,4-tetrahydrocarbazole (15),
6-methoxy-1,2,3,4-tetrahydrocarbazole (17), 2,3-cyclo-
heptenoindole (19), N-methyl-2,3-cyclopentenoindole
(21), 1-(4-morpholino)-2,2-diphenylethene (23) and 1-
(4-morpholino)-2-phenylpropene (25) using C. roseus,
C. sinensis, N. tabacum or D. carota were investigated,
The structures of 2,21 3,21 5,24 6,25 10,26 12,27 1627 and
2027 were confirmed by a comparison of the proton
nuclear magnetic resonance (1H NMR) data with those
Table 1. Oxidative cleavage of 1 with plant cell cultures or chemical reagents
O
CH3
CH3
CH3
CH3
+
CH3
N
N
NHCOCH3
H
2
3
1
2
Entry
Method
Medium
Time (h)
2a Y. (%)
3a Y. (%)
1
2
3
4
5
6
7
8
9
10
FeCl3
EtOH
24
24
8
19
16
85
49
53
95
53
0
4
2
0
0
0
0
0
0
0
0
K3Fe(CN)6
H2O2
NalO4
H2O–acetone
MS medium22
H2O–CH3OH
B5 medium23
MS medium
B5 medium
MS medium
B5 medium
MS medium
168
0.5
0.5
0.5
0.5
168
168
168
C. sinensis, H2O2
N. tabacum, H2O2
C. roseus, H2O2
D. carota, H2O2
0
a All are isolated yields.

M. Takemoto et al. / Tetrahedron Letters 45 (2004) 8061–8064
Table 2. Oxidative cleavage of indoles and enamines with plant cell cultures
8063
Entry
Indole, enamine
CH
Planta
Time (h)
Product (Y. %)b
O
CH
3
3
1
C. sinensis (H2O2)
CH
24
(37)
(14)
3
O
N
N
NHCHO
COOH
6
4
5
H
H
CH
3
2
3
C. roseus (H2O2)
2
4
(51)
N
H
NHCOCH
O
3
7
8
CH
3
MeO
MeO
CH
N. tabacum (H2O2)
2
4
3
(86)
CH
3
N
NHCOCH
9
3
10
H
O
4
5
6
C. roseus
12
12
72
(46)
N
11
N
H
H
12
MeO
O
O
MeO
N. tabacum (H2O2)
N. tabacum (H2O2)
(41)
N
N
H
13
14
H
O
O
(44)
N
H
N
H
15
MeO
16
MeO
O
O
O
7
8
N. tabacum (H2O2)
C. sinensis (H2O2)
12
96
(58)
N
H
N
H
17
18
O
(15)
N
H
N
H
19
O
O
20
9
C. sinensis H2O2
24
(28)
N
21
N
Me
22
O
Me
O
H
10
D. carota
0.2
(76)
N
24
O
O
23
O
H
CH
3
(15)
11
C. sinensis
0.2
H C
3
N
25
26
a 30% H2O2 was added to the freely suspended plant cell cultures (entries 1–3, 5–9).
b All are isolated yields.
reported. The products 8, 24 and 26 were confirmed by a
comparison of 1H NMR data, mp and TLC with a com-
mercially available sample. The structures of 14,28 1828
and 2228 were supported by MS spectra data and con-
firmed by analysis of their 1H NMR data. Indole 9
and cyclic indoles 13,17 and 21 were synthesized by
the synthetic route of Rogerts and Corson.29
In summary, a novel oxidative cleavage of substituted
indoles and enamines was developed by using plant cell

8064
M. Takemoto et al. / Tetrahedron Letters 45 (2004) 8061–8064
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advantageous features such as mild reactions, good
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1
28. Compound 14: H NMR (CDCl3): d, ppm 2.18 (2H, m),
2.50 (2H, m), 3.06 (2H, m), 3.86 (3H, s), 7.06 (2H, m), 7.65
(1H, d, J = 2.3Hz), 8.26 (1H, s). MS m/e: 219 (M+).
1
Compound 18: H NMR (CDCl3): d, ppm 1.84 (4H, m),
2.25 (2H, m), 2.91 (2H, m), 3.86 (3H, s), 7.04 (2H, m),
7.17 (1H, d, J = 8.5Hz), 7.38 (1H, s). MS m/e: 2 34
(M+). Compound 22: 1H NMR (CDCl3): d, ppm 2.40
(2H, m), 2.61 (2H, m), 2.90 (2H, m), 3.40 (3H, s), 7.20–
7.65 (3H, m), 8.05 (1H, d, J = 5.8Hz). MS m/e: 2 03
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