Synthesis of benzofuro[3,2-b] indoline amines via deamination-interrupted Fischer indolization and their unexpected reactivity towards nucleophiles

Article abstract of DOI:10.1039/c6cc00365f

We report the access to the benzofuro[3,2-b]indoline framework of phalarine from benzofuran-2-ones via the Fischer indolization reaction which was interrupted at the deamination step. Unexpectedly, allyl nucleophiles did not add to the aminal position of these benzofuro[3,2-b]indoline amines but to the adjacent ring junction center in the presence of trifluoroborane to deliver 3,3-disubstituted indolines containing a difluoroboron-containing six-membered ring with fluorescence properties.

Full text of DOI:10.1039/c6cc00365f

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DOI: 10.1039/C6CC00365F
Journal Name
COMMUNICATION
Synthesis of Benzofuro[3,2-b]indoline Amines by a Deamination-
Interrupted Fischer Indolization and their Unexpected Reactivity
Towards Nucleophiles
Received 00th January 20xx,
Accepted 00th January 20xx
Terry Tomakinian, Régis Guillot, Cyrille Kouklovsky and Guillaume Vincent*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We report the access to the benzofuro[3,2-b]indoline framework of
phalarine from benzofuran-2-ones via the Fischer indolization
reaction which was interrupted at the deamination step.
Unexpectedly, allyl nucleophiles did not add to the aminal position
of these benzofuro[3,2-b]indoline amines but to the adjacent ring
junction center in presence of trifluoroborane to deliver 3,3-
disubstituted indolines containing a difluoroboron-containing six
membered-ring with fluorescent properties.
Figure 1 Benzofuroindoline motifs.
The C3-nucleophilicity is the predominant character of the
1
indole nucleus. In contrast, the addition of nucleophiles at this
2
C3 position is less common (Scheme 1). Usually this Umpolung
of indoles involves an oxidation step to make the C3-position
3
electrophilic. We have also reported a FeCl
3
-mediated C3-
4
regioselective hydroarylation of N-Ac indoles. During the
course of synthetic studies towards the benzofuro[3,2-
b]indoline motif, we discovered that this scaffold could display Scheme
an electrophilic character at the C3-position of the indole ring
which we would like to report in this communication.
2 Planned Fischer indolization strategy towards disubstituted
benzofuro[3,2-b]indolines.
The biogenesis of both these benzofuroindolines is thought to
imply an oxidative coupling between an indole and a phenol.7
Several investigations towards the biomimetic merging of
indoles and phenols have been overtaken with success in the
The benzofuro[3,2-b]indoline motif is a rare substructure which
is found in only one natural product: phalarine (Figure 1). Its
5
regioisomer, the benzofuro[2,3-b]indoline skeleton is more
6
present in nature
7
,8
including ours.9 The
case of benzofuro[2,3-b]indolines
bioinspired access to the benzofuro[3,2-b]indoline motif is
7
more difficult. We recently described an access to this scaffold
1
0
via a FeCl
3
/DDQ promoted oxidative coupling which is limited
to 3-substituted indoles and therefore unsuited to the synthesis
1
1
of phalarine. To overcome the issue of regioselectivity of the
bioinspired process, we decided to consider a non-biomimetic
strategy based on the Fischer indolization from arylhydrazines
Scheme 1. C3-nucleophilicity and electrophilicity of indoles.
1
2
1
and benzofuran-3-ones 2 (Scheme 2).
The Fischer synthesis of indoles involves the condensation of a
ketone or an aldehyde with an arylhydrazine which is followed
by a [3,3]-sigmatropic rearrangement of the resulting ene-
hydrazine. Over the years, the Fischer indolization has been
employed to the efficient synthesis of functionalized 3,3-
Institut de Chimie Moléculaire et des Matériaux d’Orsay (ICMMO)
Equipe Méthodologie, Synthèse et Molécules Thérapeutiques (MS&MT)
Univ Paris Sud, CNRS, Université Paris-Saclay
9
1405, Orsay (France)
E-mail : guillaume.vincent@u-psud.fr
Electronic Supplementary Information (ESI) available: detailed procedure and
analytical data. See DOI: 10.1039/x0xx00000x
13
disubstituted indolines. If the keto-partner is disubstituted in
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alpha of the carbonyl, the sigmatropic rearrangement of 3 lead Usually, arylhydrazines are commercially availaVbielweAraticsle tOhnelinier
to a tetrasubstitued carbon at the C3-position which prevent hydrochloride salts. Therefore, we sDtOudI:i1e0d.103t9h/eC6CrCe0a0c3ti6o5nF
the aromatization into an indole after the elimination of between the hydrochloride salt 9a of phenylhydrazine and 2-
ammonia from 4. While the formation of 3-alkoxyindoles is methylbenzofuran-3-one 2a in our optimized conditions and we
1
4
known,
the synthesis of 3,3-disubstituted indolines observed an important drop of the yield (Scheme 3). We think
incorporating an oxygen substituent at the 3-position from an that the use of the hydrochloride salt probably modify the pH of
1
5,16
alpha-oxygenated ketone, as we plan, is more challenging.
the reaction medium which seems to be important for the
In fact, computational investigations have demonstrated that outcome of the reaction. Neutralization of the commercial
excessive electron-donation from the substituents of the ene- arylhydrazine hydrochloride salts was thus effected before to
hydrazine intermediate weaken the N-N bond and could divert engage them in the synthesis of benzofuro[3,2-b]indolines. In
the reaction pathway by inducing the dissociation of the N-N certain cases we observed that it is better to run the reaction at
1
7
bond before the aza-sigmatropic rearrangement. Finally, room temperature to avoid the formation of compounds 7 by
1
3l
addition of a nucleophile on the resulting imine of 5 would hydrolysis. The substitution of the phenyl ring of the
deliver a benzofuro[3,2-b]indoline 6 with disubstitution across arylhydrazine partner was first investigated (Scheme 4). p-
the ring junction.
Halogenated hydrazines as well as p-trifluoromethylhydrazine
Our investigation started with the reaction between phenyl furnished the desired benzofuroindolines 4b-e in 68%, 53% 40%
hydrazine 1a (1 equiv.) and 2-methylbenzofuran-3-one 2a (1.2 and 47% yields. More electrondonating substituents were then
1
3f,g
equiv.). Based on recent reports,
of acetic acid and water as solvent (Table 1). The reaction at 68% yield while benzofuroindolines 4g and 4h from p-methyl
0°C during 4 hours delivered mainly 2-aryl indole 8a which and p-methoxyhydrazines were obtained less efficiently. We
we evaluated a 1:1 mixture evaluated; p-iso-propylhydrazine delivered 4f in a satisfactory
6
results from the Fischer indolization followed by aromatization then studied the substitution of the benzofuran-3-one.
via a reduction event (entry 1). The reaction at 40°C for 1 hour
delivered mainly the 2-arylated-2-methylbenzofuranone 7a
which resulted from the hydrolysis of the Fischer product 5 and
was therefore an encouraging result (entry 2). We then
modified the ratio of acetic acid and water. A 0.5:1 mixture
pleasantly delivered the benzofuro[3,2-b]indoline amine 4a
along with 7a (entry 3). A longer reaction time increased the
ratio of undesired 7a (entry 4). Finally, a 0.7:1 ratio of acetic acid
and water was optimal and 88% of the desired Fischer product
4a was obtained (entry 5). The use of a slight excess of the
phenylhydrazine 1a was detrimental to the yield of 4a (entry 6).
Table 1 Optimization of the Fischer indolization from phenylhydrazine 1a and 2-
methylbenzofuran-3-one 2a.
Scheme 4 Scope of the Fischer indolization for the synthesis of benzofuro[3,2-b]indoline
amines from benzofuran-3-ones.
Replacing the methyl group at the 2 position by an ethyl or allyl
group delivered the desired benzofuroindolines 4i-l in yields
ranging from 33% to 76%. The presence of a bromine, a methyl
or methoxy group on the aromatic nucleus of the
benzofuranone was also compatible with reaction conditions
and benzofuroindolines 4m-q were formed in 36-67%. It is
important to note that we expected to obtain the
benzofuro[3,2-b]indoline scaffold as benzofuroindolenine 5 or
benzofuroindolinol 6 (Nu = OH, Scheme 2) via addition of water,
the co-solvent of the reaction, to the imine of 5 which should be
formed after elimination of ammonia from the
Entry
AcOH/
Temp.
(°C)
60
40
40
40
40
40
Time
(h)
4
1
1
2
1
1
Yield
4a (%)
Yield
7a (%)
-
maj.b
16
75
9
10
Yield
8a (%)
a
a
a
H
2
O
1
2
3
4
5
1/1
1/1
0.5/1
0.5/1
0.7/1
0.7/1
-
-
48
25
88
50
80
-
-
-
-
c
6
6
(
a) isolated yield. (b) crude NMR ratio. (c) 1.3 equiv. of 1a and 1 equiv. of 2a.
1
3l
benzofuroindoline amine 4. In fact, the benzofuroindoline
amine 4 did not undergo elimination of ammonia and was
isolated which is an unusual case of interrupted Fischer
1
8
indolization. The formation of imine 5 is unfavorable because
it will hold a double bond at the junction of two fused 5-
membered rings and will indeed be highly strained.1 The
Scheme 3 Fischer indolization from the hydrochloride salt 9a of phenylhydrazine.
8a
2
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structures of the benzofuroindoline amine 4e was confirmed by phenoxy substituent. The generation of an electrophilic
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1
9
DOI: 10.1039/C6CC00365F
X-ray crystallography (Figure 2). Moreover, the reaction of character at the C3-position of the indole part represent an
benzofuroindoline amine 4a with methanesulfonyl chloride and Umpolung of this nucleus. As a result, we synthesized 3,3-
triethylamine
methanesulfonamide 10
unusually
led
to
methylsulfonyl disubstituted indolenines which are related to organic
2
0
which structure has been fluorescent materials due to the presence of a difluoroborane-
1
9
determined by X-Ray crystallography (Figure 2).
Having in hand a straightforward Fischer synthesis of the
substituted benzofuroindoline skeleton, we examined the
addition of a carbon nucleophile to the aminal position in order
to obtain benzofuro[3,2-b]indolines 6 disubstituted at the ring
containing six-membered ring.
2
1
2
junctions (Scheme 2, Nu = CH R). Surprisingly, the reaction of 4a
with allyltrimethylsilane in presence of trifluoroborane etherate
at room temperature did not delivered the expected compound
but the 3,3-disubstituted indoline 11a in 70% yield and 83%
yield with allylmagnesium bromide (Scheme 5). The structure of
Figure 2 X-Ray structures of benzofuro[3,2-b]indoline amines 4e and 10.
1
9
11a was determined by X-Ray analysis. Very interestingly, this
compound display a 6 membered-ring containing the imine part
of the indolenine and the phenoxy group complexed with a
difluoroborane. We can postulate that the first event is the
formation of amine borane complex A. The formation of an
1
8a
imine is disfavoured at this point. After elimination of HF from
A, the aminofluoroborane moiety probably activates the oxygen
of the phenoxy group in B which triggers the addition of the allyl
nucleophile onto the electrophilic tetrasubstituted carbon
center adjacent to the aminal. The nucleophile could attack
intermediate C before the formation of the imine from D.
Alternatively, the indolenium intermediate E could be formed
before the addition of the nucleophile (Scheme 6). In summary,
an electrophilic character was generated at the C3-position of
the indole nucleus.
Scheme 5 Addition of allyl nucleophiles to the Fischer indolization products.
The Fischer indolization products 4 were engaged in the
allylation reaction and transformed into the corresponding
2
2
boron containing 3,3-disubstituted indolenines 11 (Scheme 5).
On the indoline part, bromo (11b, 74%), chloro (11c, 75%),
fluoro (11d, 62%) trifluoro (11e, 51%), iso-propyl (11f, 40%),
methyl (11g, 37%) and methoxy (11h, 46%) substituents were
tolerated while a bromide could be present on the benzofurane
part (11i, 68%).
Organodifluoroboron compounds such as Bodipy are widely
2
3
used as organic fluorescent materials. We therefore wished to
2
4
evaluate the fluorescent properties of compounds 11a-i. Low
concentration solutions of 11a-i in dichloromethane display UV
absorption maxima around 380 nm and upon photoexcitation
at 390 nm, 11a-i emit fluorescence with maxima around a
Scheme 6 Postulated activation of the benzofuro[3,2-b]indoline amines.
2
5
wavelength of 440 nm.
In conclusion, the Fischer reaction between benzofuran-3-ones
and arylhydrazines in mild conditions allowed us to obtain
benzofuro[3,2-b]indoline amines. This is a very rare synthesis of
3-oxyindoline derivatives via the challenging Fischer
2 2
Figure 3 Absorption and fluorescence spectra of 11a in CH Cl .
indolization in presence of an electrondonating group. During
the Fischer reaction, the elimination of ammonia was arrested
to avoid the formation of a highly strained imine at the ring
junction of the benzofurane and indoline parts. The unfavorable
formation of the imine, probably prevents the addition of
nucleophiles to the aminal position. In fact, allyl nucleophiles in
presence of trifluoroborane could be added to the adjacent
tetrasubstituted carbon at the ring junction by activation of the
We gratefully acknowledge the ANR (ANR-12-JS07-0002), the
Université Paris Sud and the CNRS for financial support. The X-
Ray diffractometer was purchased with funds from Région Ile
de France (SESAME 2012, 12018501), IUF, LabEx CHARM3AT,
Univ. Paris Sud and CNRS. We thank Dr. Anne Léaustic and the
LCI (ICMMO) for the use of their spectrophotometer.
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4
| J. Name., 2012, 00, 1-3
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