Electrochemical synthesis of 3a-bromofuranoindolines and 3a-bromopyrroloindolines mediated by MgBr2

Article abstract of DOI:10.1039/c9cc09276e

We report an efficient and environmentally friendly electrochemical approach to perform the bromo cyclization of tryptophol, tryptamine and tryptophan derivatives. The 3a-bromofuranoindolines and 3a-bromopyrroloindolines obtained are of interest in the to

Full text of DOI:10.1039/c9cc09276e

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Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/C9CC09276E
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Electrochemical Synthesis of 3a-Bromofuranoindolines and 3a-
Bromopyrroloindolines Mediated by MgBr2.
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Ju Wu,a Hussein Abou-Hamdan,a Régis Guillot,a Cyrille Kouklovsky a and Guillaume Vincent a*
We report an efficient and environmentally friendly tryptamine and tryptophan derivatives 6 which avoids the use
electrochemical approach to perform the bromo cyclization of of an external oxidant and therefore the formation of organic
tryptophol, tryptamine and tryptophan derivatives. The 3a- wastes to form the corresponding 3a-bromofuranoindolines 7
bromofuranoindolines and 3a-bromopyrroloindolines obtained are and 3a-bromopyrroloindolines 8 (Scheme 2).
of interest in the total synthesis of natural products. This
Scheme 1. Classical synthesis of 3a-bromopyrroloindolines and their importance
in natural product synthesis.
dearomative procedure relies on the generation of an electrophilic
bromine reagent by the electrochemical oxidation of MgBr2. No
organic byproducts are generated with this protocol which avoids
the use of an additional electrolyte.
Br
or
N
COXR
N
COXR
N
N
N
COXR
N
R
R
R
R
R
B
A
R
1
R
3a-Bromopyrroloindolines 1 are versatile building blocks in
which the carbon-bromide bond could be involved in a
substitution reaction with retention of configuration in order to
form stereospecifically a new C-C, C-N or C-O bond yielding
functionalized pyrroloindolines 2 via carbocation A or radical
intermediate B.1 Such approaches are widely deployed to
access tryptophan-derived natural products as exemplified by
the total syntheses of WIN 64821, pestalazine A, drimentine G,
O
N
COXR
N
Br
N
R
2
R
4
COXR
O
O
HN
N
O
O
NH
NH
N
R
N
H
NMe
O
H
O
N
H
H
N
3
H
H
R
NH
gliocladin C
novoamaurimonine
Me
Me
H
gliocladin
C
or novoamaurimonine among others.2 3a-
O
O
NH
N
NH
NH
H
NH
H
H
O
Bromopyrroloindolines 1 are usually synthesized by the
treatment of tryptophan derivatives 3 with a stoichiometric
electrophilic bromine reagent such as NBS 4.1,2 Reaction with
the nucleophilic C3 position of the indole nucleus allows to form
3a-bromopyrroloindolines 1.
H
N
Me
O
H
H
H
N
O
NH
O
H
N
NH
i-Pr
N
H
NH
O
O
NH
H
H
O
H
H
HN
WIN 64821
N
drimentine G
H
O
More recently, bromides and others halides salts in
combination with stoichiometric oxidants were also employed
via the in-situ generation of electrophilic halogen reagents to
achieve such transformations.3,4 In particular, the group of
Tong, reported the combination of KBr and oxone to reduce the
amount of organic wastes produced.4
pestalazine A
Scheme 2. Anodic oxidation of bromide salts for the bromocyclization of indoles.
R3
In this context, we would like to report an electrochemical
protocol for the dearomative halocyclization of tryptophol 5,
XH
R2
2 e-
R3
XH
Br
Br
N
R1
R2
"
"
Br
X
R3
N
N
R1
R1
C
a. Institut de Chimie Moléculaire et des Matériaux d’Orsay (ICMMO)
Univ. Paris Sud, Université Paris-Saclay, CNRS
15 rue Georges Clémenceau, 91405 Orsay, Cedex, France
*guillaume.vincent@u-psud.fr
5 X = O
,
7 X = O
8 X = NR4
,
6 X = NR4
,
Br
,
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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Electrochemistry is emerging as a powerful and sustainable benzene part is tolerated as demonstrated with 6-bromo
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tool5 to achieve dearomatisation reactions of (hetero)arenes6 indolines 8g-i and 5-methyl indoline 8j.
DOI: 10.1039/C9CC09276E
and in particular indoles.7
Subsequently, we decided to evaluate the bromo-cyclization of
In line with our interest in the development of oxidative diversely protected (L) and (D)-trytophan derivatives leading
dearomatization reactions of indoles,8,9 we recently reported a respectively to 8k-m and 8n,o. The cyclization proceeded with
general electrochemical dearomative dialkoxylation and an exo selectivity as demonstrated by the X-ray crystal structure
diazidation of indoles.10 As in previous works,7b,c,e this reaction of 8n.
proceeds via the direct oxidation at the anode of the indole
nucleus into a radical cation. In contrast, to achieve an
Scheme 3. Electrochemical dearomative halocyclization of tryptophol derivatives.
electrochemical dearomative bromocyclization of 3-substituted
Pt(-)
C(+)
indoles into 3a-bromopyrroloindolines, we planned to rely on
the anodic oxidation of a bromide salt into an electrophilic
specie that would be intercepted by the nucleophilic indole of 5
or 6 to form bromonium ion C which would be converted into 7
or 8 while protons would be reduced into H2 at the cathode.
Indeed, the electrochemical oxidation of halides is known and is
undivided cell
X
constant current 4.5 mA/cm2
or constant cell potential 5V
R2
R2
OH
O
N
N
R1
MgX2 (1.0 equiv.)
CH3CN/H2O (4:1), rta,b
R1
7
5
a
clean halogenation method for alkenes, alkynes or
(hetero)arenes11 including indoles.12 However, in this study, we
would like to deploy electrogenerated electrophilic halogen
intermediates in dearomatization reactions (Scheme 2).
2
Br
7a R1 = Boc
,
, 80% (4.5 mA/cm ), 75% (5 V)
H
7b R1 = Ac
2
,
, 80% (4.5 mA/cm ), 69% (5 V)
O
7c, R1 = Ts
2
N
, 90% (4.5 mA/cm )
R1
93% (5 V), 93% (3.5 V vs Ag/AgCl)c
In addition to be the source of the electrophilic reagent, we
expect that the bromide salt would also act as an electrolyte
which would avoid the use of an additional electrolyte and
therefore reduce the wastes of this process.
Br
R2
Cl
H
O
N
O
N
Ts
Ts
7d R2 = Cl
2
,
, 72% (4.5 mA/cm ), 62% (5 V)
After an initial evaluation of different parameters (see SI), we
identified that we could achieve the expected electrochemical
transformation either in a galvanostatic mode with a constant
current of 4.5 mA/cm2 or in a potentiostatic mode with a
constant cell potential of 5V. Both these set of conditions
involved to perform the electrolysis in an undivided cell with a
graphite anode and platinum-plated cathode in a 4:1 mixture of
acetonitrile and water at room temperature. Among the
bromide salts tested, we found out that it was optimal to use 1
equivalent of MgBr2 as source of bromide and as electrolyte.
We first studied the scope of the bromocyclisation of tryptophol
derivatives 5 into bromofuranoindolines 7 (Scheme 3). Various
protecting groups of the indole nitrogen such as Boc, acetyl and
tosyl were successfully evaluated leading to 7a-c. It was also
possible to operate at a constant potential of 3.5 V at the anode
vs an Ag/AgCl reference electrode instead of a constant
potential between the anode and the cathode for 7c.
Tryptophol derivatives bearing substituents on the benzene
part of the indole led to the formation of 3a-
bromofuranoindolines 7d-f whether a chloro or a donating
group (methyl and methoxy) is present. Replacing MgBr2 by
MgCl2 delivered the corresponding 3a-chlorofuranoindoline 7g
albeit in a lower yield.
2 d
7g
, 34% (4.5 mA/cm )
37% (5 V), 67% (8V)e
7e R2 = Me
2
,
, 63% (4.5 mA/cm ), 59% (5 V)
7f R2 = OMe
2
,
, 73% (4.5 mA/cm ), 83% (5 V)
a) Undivided cell, graphite-SK50 anode (1.4 cm x 0.8 cm x 0.2 cm submerged), platinum
plated cathode (1.4 cm x 0.8 cm x 0.2 cm submerged), constant current of 5 mA or
constant potential of 5 V between the electrodes, 5 (0.2 mmol), MgBr2 (0.2 mmol),
CH3CN/H2O (4 mL/1 mL), room temperature; b) isolated yields are indicated (dr > 95:5);
c) constant potential of 3.5 V at the anode vs a Ag/AgCl (3M aq. KCl) ref electrode; d)
MgCl2 was used instead of MgBr2; d) constant cell potential of 8V.
We were then eager to demonstrate the synthetic utility of the
products obtained by this electrochemical halocyclization
(Scheme 5). Indeed, it is well known that the bromide atom of
3a-bromoindolines such as 8 could be substituted by various
functional groups.1,2 Therefore, allylfuranoindoline 9 was
obtained by treating 3a-bromofuranoindoline 7c with allylsilane
and AgOTf.1g As described by Ye, exo-3a-bromopyrroloindoline
8o obtained from D-tryptophan could be converted by the
addition of indole in presence of t-BuOK and triethylborane into
endo-3a-indolyl-pyrroloindoline 10 which is an intermediate of
the synthesis of gliocladins B and C.2l
Application of the electrochemical bromocyclization to (L)-
tryptophan-derived diketopiperazine 11 with 2 equivalents of
MgBr2 resulted in a double bromo-cyclisation to yield exo-endo
compound 12 and endo-endo compound 13 which were
respectively converted by Tokuyama into (–)-epi-amauromine
and (+)-novoamauromine.2m This procedure is competitive with
the use of NBS by Tokuyama which required to perform 2 steps
the double bromocyclisation.2m
We then turned our attention towards the synthesis of 3a-
bromopyrroloindolines from protected tryptamines (Scheme
4). In addition to N,N’-diBoc bromopyrroloindoline 8a, different
combinations of orthogonal protecting groups of the indole and
amine nitrogens were assessed. N-Boc,N’-Cbz and N-Boc,N’-
CO2Me bromopyrroloindolines 8b,c were thus obtained. A tosyl
group could also be present on the indole nitrogen in
combination with Boc, Cbz and CO2Me protecting groups of the
primary amine leading to compounds 8d-f. Substitution of the
2 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C9CC09276E
Scheme 4. Electrochemical dearomative bromocyclization of tryptamine and
tryptophan derivatives.
Scheme 5. Synthetic applications.
Br
H
H
SiMe3
Pt(-)
C(+)
O
N
R3
O
AgOTf
CH2Cl2, 0°C
N
Ts
Ts
undivided cell
constant current 4.5 mA/cm2
9
Br
, 56%
7c
NR4
H
R2
R2
N
R3
H
or constant cell potential 5V
CO2Me
N
ref 2l
t-BuOK, THF
then
N
R4
R1
R1
MgBr2 (1.0 equiv.)
CH3CN/H2O (4:1), rta,b
MeO2C
H
ref 2l
N
Boc
Br
Boc
8
6
N
N
H
Br
Br
Br
SO2Ph
N
N
NH
H
8o
SO2Ph
10
BEt3
, 79%
N
N
Cbz
N
N
N
N
CO Me
Boc
Boc
2
Boc
Boc
2
2
2
8c
, 89% (4.5 mA/cm )
87% (5 V)
8b
, 70% (4.5 mA/cm )
8a
, 61% (4.5 mA/cm )
46% (5 V)
Me
O
O
O
S
63% (5 V)
Me
NH
NH
Me
S
NMe
O
N
Br
Br
Br
N
N
and
H
H
O
H
N
N
Cbz
N
N
H
N
Boc
N
N
N
H
Ts
CO2Et
Ts
(+)-gliocladin B
(+)-gliocladin C
Ts
2
2
2
8e
, 69% (4.5 mA/cm )
42% (5 V)
8f
, 78% (4.5 mA/cm )
82% (5 V)
8d
, 91% (4.5 mA/cm )
81% (5 V)
C(+)
Pt(-)
Boc
8g R1 = Boc R4 = Boc
Br
constant
potential
5.0 V
O
O
Br
,
,
N
H
Me
88% (4.5 mA/cm2)
NH
H
HN
8h R1 = Boc R4 = CO2Me
N
Boc
N
,
,
N
N
Br
N
R4
undivided cell
Boc
R1
81% (4.5 mA/cm2), 82% (5 V)
Boc
MgBr2 (2.0 equiv.)
CH2Cl2/H2O (4:1), rta
2
8i R1 = Ts R4 = Boc
8j
11
, 77% (4.5 mA/cm )
,
,
88% (4.5 mA/cm2), 83% (5 V)
Boc
O
Boc
Boc
O
O
N
Br
Br
Boc
H
Br
H
H
N
H
Br
H
H
H
Br
N
+
N
NH
H
CO2Me
N
CO2Me
CO2Me
N
N
N
+
N
Ts
N
N
Ac
N
N
Br
N
N
Cbz
N
H
H
O
Boc
O
BocH
Boc
H
H
H
N
Boc
Br
Br
Boc
H
O
H
13
, 14%
14
, 26% (dr 1:1)
12
, 18%
8k
, 91% (90:10)
8l
8m
, 69% (90:10)
(4.5 mA/cm2)
, 94% (90:10)
(4.5 mA/cm2)
(4.5 mA/cm2)c
87% (90:10) (5 V)c
ref 2m
72% (90:10) (5 V)
96% (90:10) (5 V)
O
O
O
H
HN
N
HN
N
H
H
NH
H
H
CO2Me
CO2Me
+
N
N
H
N
N
Boc
Br
H
H
Ts
H
O
NH
H
Br
N
Ts
N
H
H
(-)-epi-amauromine
(+)-novoamaurimonine
SO2Ph
8n
, 51% (95:5)
8o
, 81% (95:5)
a) Undivided cell, graphite-SK50 anode (1.4 cm x 0.8 cm x 0.2 cm submerged), platinum
plated cathode (1.4 cm x 0.8 cm x 0.2 cm submerged), constant potential of 5 V between
the electrodes, 1 (0.1 mmol), MgBr2 (0.2 mmol), CH2Cl2/H2O (4 mL/1 mL), room
temperature.
(4.5 mA/cm2)c
55% (95:5) (5 V)c
(1.8 mA/cm2)d
84% (95:5) (5 V)d
8n
X-ray crystal structure of
a) Undivided cell, graphite-SK50 anode (1.4 cm x 0.8 cm x 0.2 cm submerged), platinum
plated cathode (1.4 cm x 0.8 cm x 0.2 cm submerged), constant current of 5 mA or
constant potential of 5 V between the electrodes, 6 (0.2 mmol), MgBr2 (0.2 mmol),
CH3CN/H2O (4 mL/1 mL), room temperature; b) isolated yields are indicated (dr > 95:5
unless otherwise noted in parenthesis); c) on 0.1 mmol of 6 and MgBr2 in CH3CN/H2O (4
mL/1 mL); d) on 0.6 mmol of 6 and MgBr2 (0.6 mmol) in CH3CN/H2O (12 mL/3 mL), anode
(3.4 cm x 0.8 cm x 0.2 cm submerged), cathode (3.4 cm x 0.8 cm x 0.2 cm submerged).
Conclusion
In conclusion we developed
a general electrochemical
bromocyclization of tryptophol, tryptamine and tryptophan
derivatives via the anodic oxidation of MgBr2 into an
electrophilic bromine reagent which could oxidized the indole
nucleus into bromofuranoindolines and bromopyranoindolines.
This protocol avoids the use of an additional electrolyte and of
an external oxidant.
Cyclic voltammetry (CV, see SI) of the reactants in acetonitrile
and water showed oxidation of MgBr2 at a lower potential than
NTs-tryptophol 7a. The oxidation peak observed (1.3 V vs SCE)
during the CV of MgBr2 seems to correspond to the formation
of Br2.11j,13 In presence of N-Ts tryptophol 7a, a net increase of
the intensity of this oxidation wave is observed. This
observation might be in favour of the oxidation of MgBr2 at the
anode into an electrophilic bromine reagent that would react
with the C2=C3 bond of indoles to form the bromonium bridge
C (Scheme 1).14
Conflicts of interest
“There are no conflicts to declare”.
Acknowledgment
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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H. Wang, J. Shi, J. Tan, W. Xu, S. Zhang and K. Xu, Org. Lett.,
We thank Mrs Sakna Bazzi and Dr. Mohamed Mellah (ICMMO,
Univ. Paris Sud) for their assistance in the recording of the CV.
JW thanks the China Scholarship Council (CSC) for his PhD
fellowship. We also gratefully acknowledge the ANR (ANR-17-
CE07-0050; “ArDCo”), the Université Paris Sud and the CNRS for
financial support.
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2019, 21, 9430–9433.
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‡ Electrochemical reactions were performed with ElectraSyn 2.0
package (IKA). CCDC 1910836 (8n) contains the supplementary
crystallographic data for this paper.
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4 | J. Name., 2012, 00, 1-3
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difference of potential may be attributed to the high
resistance of the reaction mixture.
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DOI: 10.1039/C9CC09276E
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