An unprecedented route for the synthesis of 3,3′-biindoles by reductive cyclization of 3-[2-Nitro-1-(2-nitrophenyl)ethyl]-1H-indoles mediated by iron/ acetic acid

Article abstract of DOI:10.1002/ejoc.201000276

An unprecedented route for the synthesis of 3,3′-biindoles is developed. Both symmetrical and unsymmetrical 3,3′-biindoles could be generated by this method. Mild conditions, high yields of the products, and environmentally acceptable reagent are the meri

Full text of DOI:10.1002/ejoc.201000276

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000276
An Unprecedented Route for the Synthesis of 3,3Ј-Biindoles by Reductive
Cyclization of 3-[2-Nitro-1-(2-nitrophenyl)ethyl]-1H-indoles Mediated by Iron/
Acetic Acid
Chintakunta Ramesh,[a] Veerababurao Kavala,[a] Chun-Wei Kuo,[a] B. Rama Raju,[a] and
Ching-Fa Yao*[a]
Keywords: Nitrogen heterocycles / Iron / Michael addition / Nitroalkenes / Cyclization
An unprecedented route for the synthesis of 3,3Ј-biindoles is
developed. Both symmetrical and unsymmetrical 3,3Ј-bi-
indoles could be generated by this method. Mild conditions,
high yields of the products, and environmentally acceptable
reagent are the merits of the procedure.
Introduction
Indoles, a major class of nitrogen heterocycles, are em-
bedded in a number of natural and pharmaceutically im-
portant compounds and they have been the focus of numer-
ous biological studies during the last several years.[1] Deriv-
atives of indole are known to possess various biological
properties including antibacterial, cytotoxic, antioxidative,
and insecticidal activities.[2] Among the indole derivatives,
biindoles are most important because of their wide range
of applications in pharmaceuticals and functional materi-
als.[3] In addition, biindoles are known to be very good
diene systems for Diels–Alder reactions for the construction
of indolo carbazoles, which are the core skeleton of struc-
turally unique natural products possessing a wide range of
biological activities.[4] In particular, 3,3Ј-biindoles are found
in many natural products and several biologically active
compounds.[5] Moreover, 3,3Ј-biindole derivatives have been
used for treating a protein folding disorder such as Alzhei-
mer’s disease, dementia, Parkinson’s disease, Huntington’s
disease, and prion-based spongiform encephalopathy.[6]
Some of the synthetic utilities of the 3,3Ј-biindoles are
shown in the Scheme 1.
Scheme 1. Synthetic utilities of 3,3Ј-biindole derivatives.
frequently used method involves the reaction of isatin with
indole in the presence of a basic catalyst to obtain an inter-
mediate hydroxy compound, which upon treatment with a
Lewis acid generates the 3,3Ј-biindole.[9] The other two
methods are based on palladium-catalyzed cross-coupling
reactions (Scheme 2).[10] A noticeable limitation with the
former method is that it requires longer times and the over-
all yields of the products are moderate. Whereas in the lat-
ter two cases, prefunctionalization of the indoles to their
halides and the use of complex starting materials are re-
quired. The above-described facts drove us to search for a
better alternative.
Owing to the importance of biindoles, numerous meth-
ods have been reported for the synthesis of biindole deriva-
tives. However, the majority of the currently available pro-
tocols describe the synthesis of 2,3Ј-biindoles,[7] where few
of them report on the construction of 2,2Ј- and other biin-
doles.[8] As far as the synthesis of 3,3Ј-biindoles is con-
cerned, to the best of our knowledge only three multistep
synthetic routes are known to reach the target. The most
On the other hand, the reductive cyclization of nitro
compounds is the predominantly employed method for the
construction of indoles and related N-heterocompounds.[11]
Leimgruber-Batcho, Bartoli, and Reissert reactions, the
[a] Department of Chemistry, National Taiwan Normal University
88, Sec. 4, Tingchow Road, Taipei, Taiwan 116, R. O. C
E-mail: cheyaocf@ntnu.edu.tw
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000276.
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Synthesis of 3,3Ј-Biindoles by Reductive Cyclization
Scheme 2. Previously reported methods for the synthesis of 3,3Ј-
biindole derivatives.
Scheme 3. Route to the synthesis of 3,3Ј-biindolyl derivatives.
transition-metal-catalyzed reductive N-heteroannulation of
o-nitrostyrenes, and the Cadogan cyclization are all well-
known methods in this category.[12] Recently, we have
adopted Fe/acetic acid as a reductive cyclizing agent for the
synthesis of N-heterocompounds,[13] as it is an inexpensive,
environmental friendly, readily available reagent. A plethora
of literature cited this reagent for the construction of several
heterocyclic compounds including quinolines, 2H-pyrrolo-
[3,4-c]quinolines, thieno[2,3-b][1,4]thiazin-2-(3H)-ones, 3,6-
dimethyl-9H-4,5,9-triazaphenanthren-10-ones, pyrrolo[3,2-
b]indole, chiral 3-substituted benzodiazepinone aziridines,
pyrrolo[2,1-c][1,4]benzodiazepines derivatives, and α-meth-
ylene-γ-butyrolactams.[14] Furthermore, this reagent system
has successfully been applied to the synthesis of indole de-
rivatives.[15] Herein, we wish to introduce an unprecedented
approach towards the synthesis of 3,3Ј-biindole derivatives
through the reductive cyclization of 3-[2-nitro-1-(2-nitro-
phenyl)ethyl]-1H-indoles.
Previously, we reported that the reagent Fe/AcOH is an
excellent reductive cyclizing agent for the synthesis of indol-
ylquinoline derivatives.[13] Taking cues from these experi-
ments, we anticipated that the precursor compounds, 3-[2-
nitro-1-(2-nitrophenyl)ethyl]-1H-indoles, could be easily
converted into their corresponding 3,3Ј-biindoles by re-
ductive cyclization. In this regard, we first carried out the
reaction of 3-[2-nitro-1-(2-nitrophenyl)ethyl]-1H-indoles
with Fe/acetic acid. The best results could be obtained when
1a (1 mmol) was treated with Fe powder (6 mmol) in acetic
acid (5 mL) at reflux for 2.5 h to furnish desired 3,3Ј-bi-
indole 2a in excellent yield.
Encouraged by this result, we applied this methodology
for Michael adducts 1a–r, which were obtained from dif-
ferent substituted 1-nitro-2-(2-nitrovinyl)benzenes and vari-
ous substituted indoles, to synthesize substituted 3,3Ј-bi-
indoles 2a–o in excellent yields. Product yields were not in-
fluenced by substituents present on either the indole or
benzene components in the precursor compounds. It is
interesting to note that methoxy (Table 1, Entries 5, 6, and
16), chloride and bromide (Table 1, Entries 14, 15, and 18),
and fluoride (Table 1, Entries 13, 16, and 17) substituents
Results and Discussion
Our two-step synthetic strategy of 3,3Ј-biindoles involved are well tolerated under the present reaction conditions.
the Michael addition of indole to a 1-nitro-2-(2-nitrovinyl)- It is known that usually, under Fe/AcOH conditions, ni-
benzene to obtain a precursor compound, which upon fur- tro compounds give N-acetylated products as side prod-
ther treatment with iron/acetic acid produces the corre- ucts,[19] But under our conditions, we did not find any such
1
sponding 3,3Ј-biindole derivative (Scheme 3). Initially, we products, as evidenced from analysis of the H NMR spec-
focused our attention on the synthesis of precursor com- tra of the crude product.
pounds such as 3-[2-nitro-1-(2-nitrophenyl)ethyl]-1H-
1
All the compounds were characterized by H and 13C
indoles. A wide range of methods are available for the NMR spectroscopy and HRMS. Further, the structures of
Michael addition of indoles to nitroalkenes.[16] However, the compounds were confirmed by single-crystal X-ray
very few methods have described the Michael addition of analysis of representative examples of symmetrical and un-
indoles to 1-nitro-2-(2-nitrovinyl)benzenes.[17] At this junc- symmetrical biindoles 2a and 2d. The crystal structures
ture, we found the sulfamic acid was the suitable catalyst in both compounds are depicted in Figure 1.
terms of reaction time and yield. The use of this catalytic
Three plausible mechanistic routes can be envisaged for
system for the Michael addition of indoles to nitroalkenes the formation of 3,3Ј-biindole derivatives from 3-[2-nitro-1-
has been described by Zuo et al.[18] However, they did not (2-nitrophenyl)ethyl]-1H-indole. In the first route, the nitro
show any example with 1-nitro-2-(2-nitrovinyl)benzenes. group on the aromatic ring is reduced in the presence of
Hence, by modifying the reaction conditions, we prepared Fe/AcOH to obtain the intermediate 2-[1-(1H-indol-3-yl)-2-
structurally diverse 3-[2-nitro-1-(2-nitrophenyl)ethyl]-1H-in- nitroethyl]aniline, which further undergoes tautomerization
doles in good yields.
followed by cyclization to form intermediate B. Upon losing
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C. Ramesh, V. Kavala, C.-W. Kuo, B. R. Raju, C.-F. Yao
SHORT COMMUNICATION
Table 1. Reductive cyclization of 3-[2-nitro-1-(2-nitrophenyl)ethyl]-1H-indoles using Fe/acetic acid (1a–r, 2a–o).
[a] All the reactions were performed on a 1-mmol scale under reflux for 2.5 h. [b] Isolated yields.
a water molecule from B, C is formed. Intermediate C un- tionalities of the precursor compound in the presence of Fe/
dergoes aromatization by loss of HNO to biindole. In an AcOH to form intermediate D has been considered. Pro-
alternative assumption, the reduction of both nitro func- tonation of D is followed by elimination of ammonia to
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Synthesis of 3,3Ј-Biindoles by Reductive Cyclization
Oxime F undergoes cyclization to give intermediate G.
Compound G, upon loss of a hydroxylamine molecule, re-
sults in the formation of biindole.[20] All these plausible
mechanisms are depicted in Scheme 4. Although we do not
have concrete evidence for any one of the proposed mecha-
nisms, the first and the third mechanisms seem to be more
reasonable than the second.
Conclusions
In conclusion, we have successfully developed an unprec-
edented route for the synthesis of 3,3Ј-biindoles. The pro-
cedure is operationally simple, convenient, and can be ap-
plicable to synthesize a wide range of 3,3Ј-biindoles. This
methodology is useful for the synthesis of both symmetrical
and unsymmetrical 3,3Ј-biindoles. Devoid of side products,
clean reactions, and high yields of the products constitute
the additional attractive features of this methodology. To
the best of our knowledge, this procedure represents the
simplest method for the construction of 3,3Ј-biindoles from
simple and readily available starting materials.
Experimental Section
Representative Procedure for the Synthesis of 3,3Ј-Biindoles: To a
stirred solution of 1a (1 mmol) in acetic acid (5 mL) was added
powdered Fe (6 mmol), and the reaction mixture was then heated
at reflux for 2.5 h. The mixture was cooled to room temperature,
and acetic acid was removed under reduced pressure. EtOAc
(10 mL) was added; the mixture was stirred for 2 min and then
filtered to remove any iron impurities. The insoluble iron residue
Figure 1. Crystal structures of compounds 2a and 2d.
form intermediate E. Intermediate E is later aromatized to
give the biindole. A third approach towards the rationaliza-
tion of the conversion deals with the partial reduction of
the aliphatic nitro group to the corresponding oxime F.[20] was washed with EtOAc (10 mL). The filtrate and washings were
Scheme 4. Plausible mechanistic routes for the formation of 3,3Ј-biindole.
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SHORT COMMUNICATION
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yield the expected product.
1H,1ЈH-3,3Ј-Biindole (2a): White solid, m.p. 274–276 °C. 1H NMR
(400 MHz, [D6]DMSO): δ = 11.14 (br. s, 2 H), 7.77 (d, J = 7.8 Hz,
2 H), 7.63 (d, J = 2.0 Hz, 2 H), 7.44 (d, J = 8.0 Hz, 2 H), 7.14 (t,
J = 7.2 Hz, 2 H), 7.05 (t, J = 7.1 Hz, 2 H) ppm. 13C NMR
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118.8, 111.5, 109.7 ppm. MS (EI): m/z (%) = 232 (100) [M]+.
HRMS: calcd. for C16H12N2 [M]+ 232.0995; found 232.0996.
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126.9, 126.7, 124.2, 121.7, 120.9, 119.6, 119.3, 119.0, 118.5, 111.6,
111.2, 108.6, 106.5 ppm. MS (EI): m/z (%) = 308 (100) [M]+.
HRMS: calcd. for C22H16N2 [M]+ 308.1308; found 308.1313.
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CCDC-777828 (2a) and -777829 (2d) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Supporting Information (see footnote on the first page of this arti-
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