Novel synthesis of benzofuran- and indol-2-yl-methanamine derivatives

Article abstract of DOI:10.1016/j.tetlet.2014.11.015

We report on a novel synthesis towards benzofuran-2-yl-methanamine and indol-2-yl-methanamine derivatives by using ortho-methoxy and ortho-nitro substituted phenylacetic acids as starting material, respectively. For each compound series, a key intermediate bearing the oxazole-4-carboxylic acid methylester moiety was produced. Refluxing the ortho-methoxy series in HBr/HAc produced the desired benzofuran-2-yl-methanamines. Accordingly, for the synthesis of indoles the nitro-group was first reduced and refluxing these intermediates in HCl gave the corresponding indol-2-yl-methanamines. The method worked well with electron donating substituents. Limitations regarding electron withdrawing substituents are discussed. This straightforward synthetic procedure can be a useful approach to generate a variety of substituted benzofuran-2-yl-methanamine and indol-2-yl-methanamine compounds by starting from readily available phenylacetic acid derivatives.

Full text of DOI:10.1016/j.tetlet.2014.11.015

Tetrahedron Letters xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Novel synthesis of benzofuran- and indol-2-yl-methanamine
derivatives
Joachim Schlosser, Eugen Johannes, Melanie Zindler, Jan Lemmerhirt, Benjamin Sommer, Martin Schütt,
⇑
Christian Peifer
Institute of Pharmacy, University of Kiel, Gutenbergstr. 76, D-24118 Kiel, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
We report on a novel synthesis towards benzofuran-2-yl-methanamine and indol-2-yl-methanamine
derivatives by using ortho-methoxy and ortho-nitro substituted phenylacetic acids as starting material,
respectively. For each compound series, a key intermediate bearing the oxazole-4-carboxylic acid meth-
ylester moiety was produced. Refluxing the ortho-methoxy series in HBr/HAc produced the desired ben-
zofuran-2-yl-methanamines. Accordingly, for the synthesis of indoles the nitro-group was first reduced
and refluxing these intermediates in HCl gave the corresponding indol-2-yl-methanamines. The method
worked well with electron donating substituents. Limitations regarding electron withdrawing substitu-
ents are discussed. This straightforward synthetic procedure can be a useful approach to generate a vari-
ety of substituted benzofuran-2-yl-methanamine and indol-2-yl-methanamine compounds by starting
from readily available phenylacetic acid derivatives.
Received 20 May 2014
Revised 30 October 2014
Accepted 4 November 2014
Available online xxxx
Keywords:
Benzofuran-2-yl-methanamines
Indol-2-yl-methanamines
Phenyl acetic acid derivatives
Methyl-2-isocyanoacetate
Oxazole-4-carboxylates
Ó 2014 Elsevier Ltd. All rights reserved.
Benzofurane and indole scaffolds are present in a huge number
of chemical entities including compounds showing interesting bio-
logical activities, among them are compounds of natural origin.1–3
In line with this notion, chemical components or building blocks
bearing the benzofurane and indole moiety are of significant
importance in synthetic strategies leading to the desired
structures.
In the present Letter we report on the discovery of a novel
straightforward synthesis towards benzofuran-2-yl-methanam-
ines and indol-2-yl-methanamines, respectively. These products
were coincidentally obtained in our recent medicinal chemistry
project towards the preparation of (1H)-pyrazin-2-ones.4 Herein,
the original synthetic approach involved 1-amino-3-(aryl)propan-
purity instead of the expected 1-amino-3-(o-methoxyphenyl)
propan-2-one (Scheme 1 B).6 In line with this notion, comparable
procedures involving HBr via cleavage of ortho-methoxy groups
were employed for the synthesis of other benzofurane deriva-
tives.7–12
Consequently, the observation that 2 could be produced from
2a prompted us to also investigate the method for the synthesis
of some analogue indole derivatives. In fact, when o-nitrophenyl-
acetic acid 7a was employed as starting material, indol-2-yl-meth-
anamine 7 could be produced accordingly (upon reduction of the
nitro to the aniline moiety, Scheme 1C).13
A large variety of methods is available for the synthesis of ben-
zofurane- and indole derivatives in general (for recent reviews see
literature14,15). However, only a few methods have been reported
for the preparation of heteroaryl-2-yl-methanamines.3,16,17 While
most of the relevant publications regarding the preparation and
applications of heteroaryl-2-yl-methanamines (mostly for drug
discovery projects) can be found in patent literature from commer-
cial entities,18 some have been published by academic researchers.
For instance, Russo et al.16 developed a Sonogashira–Linstrumelle
reaction starting from 2-iodophenol leading to benzofuran-2-yl-
methanamines. Recently, a chemoenzymatic preparation of benzo-
furan-2-yl-ethanamines was published.3 Likewise, only a few
methods are available for the synthesis of indol-2-yl-methanam-
ines.16 However, these methods often involve drawbacks such as
2-ones (a-aminoketones, e.g., 1, Scheme 1 A) as key intermediates.
These -aminoketones were typically produced from phenylacetic
a
acid derivatives (1a) by reaction with methyl-2-isocyanoacetate to
build their corresponding oxazole-4-carboxylic acid methylesters
1b.5 Subsequently, the acidic cleavage of the oxazole moiety
was used as a suitable method to avoid dimerization of the (non-
protonated)
a
-aminoketones.4 However, by refluxing methyl
5[(2-methoxyphenyl)methyl]oxazole-4-carboxylate 2b in HBr we
obtained benzofuran-2-yl-methanamine 2 in good yield and high
⇑
Corresponding author. Tel.: +49 431 880 1137; fax: +49 431 880 1352.
E-mail address: cpeifer@pharmazie.uni-kiel.de (C. Peifer).
http://dx.doi.org/10.1016/j.tetlet.2014.11.015
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
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2
J. Schlosser et al. / Tetrahedron Letters xxx (2014) xxx–xxx
(A)
Cl–
H3 N
O
O
O
I
II
+
N
O
HO
1a
O
1b
1
(B)
O
O
N
O
I
III
HO
Br –
H3N
H3C
O
O
+
O
CH3
O
CH3
2a
2b
2
(C)
O
O
O
N
O
N
O
I
H3C
IV
V
HO
NH2 Cl–
NO
2
H3C
N
H
+
NO
2
O
O
H3N
7a
7b
7c
7
Scheme 1. (A) Synthesis of
a-aminoketones as key intermediates in our recent project towards the synthesis of (1H)-pyrazin-2-ones; (B) preparation of benzofuran-2-yl-
methanamine by using o-methoxyphenylacetic acid as starting material; (C) preparation of indol-2-yl-methanamine by using o-nitrophenylacetic acid as starting material.
Reactants and conditions I: 1. CDI, 2. methyl-2-isocyanoacetate; II: HCl, reflux; III: HBr, reflux; IV: Pd/C, cyclohexene, microwave heating; V: HCl, reflux.
expensive amounts of catalysts, prolonged reaction times, tedious
work-up procedures or limited availability of starting material
with the desired decoration.
In line with this notion, when HCl was employed (instead of HBr)
the reaction produces 1-amino-3-(2-methoxyphenyl)propan-2-
one-HCl (17, Scheme 2). This indicates that the methoxy moiety
Regarding the present method, the formation of heteroaryl-
2-yl-methanamines from the oxazole moiety is discussed in
Scheme 2 exemplified by the synthesis of compound 2 starting
from 2b (the mechanism for building the oxazole moiety from
arylacetic acid 2a by reaction with methyl-2-isocyanoacetate has
been published elsewhere5). We propose that the reaction towards
the formation of benzofuran-2-yl-methanamine starts by acid
mediated degradation of the oxazole ring in 2b (Scheme 2). Ester
cleavage and subsequent decarboxylation produce the protonated
remains stable under these conditions. In turn, refluxing a-ami-
noketone 17 in HBr yielded the respective benzofuran-2-yl-meth-
anamine 2 hence providing further evidence for the proposed
reaction mechanism.
Thus, we became interested in the question if this procedure
could lead to a general synthetic method towards substituted
benzofuran-2-yl-methanamines and indol-2-yl-methanamines,
respectively. Therefore, in order to characterize this reaction in
more detail for each series, we investigated the application for a
set of substituted phenylacetic acids as starting material. Accord-
ingly, the method was successfully applied to the synthesis of
the benzofuran-2-yl-methanamines 2–6 and indol-2-yl-methan-
a-aminoketone. When HBr is used a cleavage of the methoxy-ether
occurs to form the free phenol-function. Subsequently, the
benzofurane ring in 2 is build by means of a Knoevenagel-reaction.
O
O
O
(I)
(II)
+
+
N
H3N
H3N
- CO2
- HCOOH
- MeOH
2b
O
O
O
17
O
O
O
O
H
(III)
O
- MeBr
+
+
H3N
+
H3N
H3N
- H2O
O
HO
O
O
H
2
Scheme 2. Proposed reaction mechanism exemplified for the formation of benzofuran-2-yl-methanamine 2 starting from key intermediate methyl-5-[(2-methoxy-
phenyl)methyl]oxazole-4-carboxylate (compound 2b). Reactants and conditions I/II: reflux, HCl, MeOH; III: reflux, HBr, HAc. A comparable mechanism is discussed for the
formation of the indol-2-yl-methanamines, for example, 7 by using the corresponding methyl-5-[(2-aminophenyl)methyl]oxazole-4-carboxylate 7c and HCl, not shown).
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J. Schlosser et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 1
Synthesis of benzofuran-2-yl-methanamine derivatives by using substituted o-methoxyphenylacetic acids as starting material via oxazole intermediates
Starting material
Oxazole
Product
Yielda (%)
Br –
O
O
N
+
H3N
HO
O
90
O
CH3
CH3
O
O
O
2a
2b
2
Br–
H3C
H3C
CH3
O
O
O
O
Cl–
+
H3N
O
O
O
+
H3N
61
HO
O
O
O
CH3
CH3
3
4
3b b
3a
Br–
H3C
H3C
O
O
O
+
OH
H3N
O
N
O
23
HO
O
CH3
O
CH3
4b
3a
5a
Br –
Cl
Cl
+
Cl
O
O
H3N
N
HO
HO
88
O
O
O
CH3
O
O
CH3
5
5b
Br–
CH3
CH3
+
CH3
O
H3N
O
N
O
60
O
O
CH3
O
O
CH3
6
6b
6a
bCompound 3b was synthesized from oxazole 4b by refluxing in HCl. Compound 3 was prepared by using BBr3 and workup with MeOH gave the final compound.
a
Yield is given for final step (cleavage of the oxazole).
amines 7–13 (Tables 1 and 2). The structure of compound 3 could
be determined by X-ray analysis (Fig. 1).
demonstrating a limitation of this method regarding the substitu-
tion pattern of the aryl system. Interestingly, when compound 14
Interestingly, the nature of these substituents is not signifi-
cantly influencing the yield of the terminal benzofurane/indole
ring closure. In contrast to these efficient examples (Tables 1 and
2) the p-acetyl moiety in 14 was found to strongly prevent the gen-
eration of the benzofurane product. This can be explained by the
present phenyloge carbonic acid: following the concurrent cleav-
age of the oxazole and the methylether in this intermediate a
phenyloge carbonic acid is produced in which the tautomerization
is killing the nucleophilicity of the phenol moiety (Scheme 3). This
restriction of the method should be also true for further electron
withdrawing moieties such as nitro and cyano substituents. As a
consequence the ring closure towards the benzofurane is inhibited
was reacted in HCl we obtained the a-aminoketone 16 (also see
Scheme 1). Again, by refluxing 16 in HBr no product was produced.
As the presented method requires o-nitro/methoxy-substituted
phenylacetic acids as starting material we also focused on the
availability and preparation of those compounds. The o-nitrophe-
nylacetic acids 7a, 8a and 12a and the o-methoxyphenylacetic
acids 2a, 3a and 5a are commercially available. The synthesis of
o-methoxy-phenylacetic acid 6a was performed in five steps start-
ing from 2-methoxy-5-methylbenzaldehyde which was treated
with KCN and Na2S2O5 to achieve the cyanohydrin.19 The chlorina-
tion20 of cyanohydrin by SOCl2 was followed by reduction with Zn
and AcOH to build the 2-methoxy-5-methylphenylacetonitril.21
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J. Schlosser et al. / Tetrahedron Letters xxx (2014) xxx–xxx
Table 2
Synthesis of indol-2-yl-methanamine derivatives by using substituted o-nitrophenylacetic acids as starting material via oxazole intermediates
Starting material
Oxazole
Product
Yielda (%)
Cl–
O
O
N
+
H3N
HO
NH2
NO2
63
N
H
O
O
7a
7c
7
Cl–
F
F
+
F
O
O
H3N
N
O
HO
98
33
53
N
H
NO2
NH2
Cl
O
8
8a
9a
8c
Cl –
Cl
+
O
Cl
Br
O
H3N
N
O
HO
HO
N
H
NH2
Br
NO2
Br
O
9
9c
Cl–
O
+
O
H3N
N
O
N
H
NH2
NO2
O
10a
10
10c
Cl–
CH3
O
H3C
H3C
O
O
+
H3N
O
O
N
O
99
97
81
HO
N
H
NO2
O
NH2
O
O
11a
11
12
11c
Cl–
CH3
H3C
H3C
CH3
O
H3C
O
+
O
H3N
O
O
N
O
CH3
N
O
H
HO
HO
NH2
NO2
O
12a
12c
Cl–
O
O
N
O
+
H3N
NH2
NO2
O
N
H
13a
13c
13
a
Yield is given for the final step (cleavage of the oxazole moiety). For clarity, the corresponding methyl-5-(2-nitrobenzyl)oxazole-4-carboxylates 7b–13b are not presented
in Table 2. Analytical data for all compounds can be found in SI.
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J. Schlosser et al. / Tetrahedron Letters xxx (2014) xxx–xxx
5
or ortho-nitro phenylacetic acids as starting material. The reaction
provides the products in high yield and proceeds without tedious
workup. The method may also be useful for the preparation of
further substituted benzofuran-2-yl-methanamines and indol-2-
yl-methanamines. Moreover, it may also be interesting for the syn-
thesis of benzothiophen-2-yl-methanamines and for corresponding
heteroaryl-2-yl-methanamine derivatives.
Acknowledgments
The authors thank Dr. Ulrich Girreser for excellent technical
assistance during synthesis and analytical characterization at the
Institute of Pharmacy, Kiel. The help of Dr. Dieter Schollmeyer,
Institute of Organic Chemistry, University of Mainz, Germany for
X-ray structure determination is gratefully acknowledged. We also
acknowledge financial support by DFG grant PE 1605/2-1.
Figure 1. Molecular structure of compound 3 determined by X-ray crystallography
showing the atom numbering scheme (CCDC number 1002571 via http://
www.ccdc.cam.ac.uk).
Supplementary data
Supplementary data (supplementary data including relevant
preparation procedures, 1H/13C NMR, and MS for all compounds)
associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/j.tetlet.2014.11.015.
The preparation of 6a was then achieved by hydrolysing the ace-
tonitrile moiety under basic conditions.
The synthesis of o-nitro-phenylacetic acids 9a, 10a, 11a and 13a
was performed in two steps starting from the respective aromatic
nitro compound via formation of the o-nitrile by vicarious nucleo-
philic substitution (VNS) reported by Makosza et al.22,23 The prep-
aration of the desired phenylacetic acids was then achieved by
hydrolysis of the nitrile moiety under acidic conditions24 (for syn-
thetic and analytical details also see SI).
References and notes
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to the respective chloro (9c) and bromo (10c) derivatives.
In summary we report on a novel method for the synthesis of
benzofuran-2-yl-methanamines and indol-2-yl-methanamines,
respectively, by using readily available or accessible ortho-methoxy
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General procedure for the synthesis of benzofuran-2-yl-methanamines 2–6:
1 equiv of the oxazole was dissolved in a mixture of AcOH/HBr 40% (1:1) and
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precipitate was filtered off, washed by diethylether and dried under vacuum.
O
CH3
O
CH3
O
CH3
H
Br–
Br –
HBr
O
O
O
+
N
O
+
H3N
H3N
O
14
OH
O
15
CH3
O
HCl
Cl–
O
CH3
HBr
O
+
H3N
O
16
CH3
Scheme 3. Phenyloge carbonic ester moiety in compound 14. We assume that after HBr-mediated oxazole and methoxyether cleavage the tautomerization of the potentially
resulting phenyloge carbonic acid in 15 (not isolated) strongly reduces the nucleophilic attack of the phenol to the ketone moiety thus preventing a Knoevenagel ring closure
towards the benzofurane. In contrast, by using HCl as reagent
of black tar.
a-aminoketone 16 was obtained from 14. However, refluxing 16 in HBr gave no product but a complex mixture
Please cite this article in press as: Schlosser, J.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.11.015

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