One-pot relay catalysis: Divergent synthesis of furo[3,4-: B] indoles and cyclopenta [b] indoles from 3-(2-aminophenyl)-1,4-enynols

Article abstract of DOI:10.1039/c6ob00319b

Described herein is an efficient divergent strategy for the synthesis of furo[3,4-b]indoles via a sequential Ag(i)/Bi(iii)/Pd(ii) catalysis and cyclopenta[b]indoles via a one-pot Ag(i)/Br?nsted acid relay catalysis from 3-(2-aminophenyl)-4-pentenyn-3-ols, accessible in three simple steps from 2-aminobenzaldehydes.

Full text of DOI:10.1039/c6ob00319b

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One-pot relay catalysis: Divergent synthesis of furo[3,4-b]indoles
and cyclopenta[b]indoles from 3-(2-aminophenyl)-1,4-enynols†
Received 00th January 20xx,
Accepted 00th January 20xx
Manisha, Seema Dhiman, Jopaul Mathew and S. S. V. Ramasastry*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Described herein an efficient divergent strategy for the synthesis promoted by three orthogonal metal relay catalytic systems
of furo[3,4-b]indoles via a sequential Ag(I)/Bi(III)/Pd(II) catalysis are not reported thus far.8 Herein, we report our efforts
and cyclopenta[b]indoles via a one-pot Ag(I)/Brønsted acid relay towards developing a new synthetic approach for furo[3,4-
catalysis from 3-(2-aminophenyl)-4-pentenyn-3-ols, accessible in b]indoles via Ag(I)/Bi(III)/Pd(II)-promoted relay catalytic system
three simple steps from 2-aminobenzaldehydes.
that integrates an intramolecular hydroamination, 1,3-allylic
alcohol isomerization (1,3-AAI),8d and an unprecedented
Indoles and indolines are considered privileged scaffolds from
drug discovery standpoint.1 Among several indole derivatives,
cyclopentannulated indoles have attracted great attention
from synthetic chemists owing to their occurrence in several
isofuran annulation of δ,
addition, we also report a Ag(I)/Br
facilitates the synthesis of cyclopenta[b]indoles via
intramolecular hydroamination and Nazarov-type cyclization
cascade, Scheme 1d.

-unsaturated alcohols, Scheme 1d. In
ø
nsted acid relay system that
natural
products
and
pharmaceutically
interesting
compounds.2 Consequently, several synthetic strategies were
developed for cyclopenta[b]annulated indoles.3 Among them,
Nazarov reaction-based approaches are popular (see for
example, Schemes 1a and 1b). An organized generation of a
4-electron system is the key to accomplishing the Nazarov
cyclization effectively.4 In this context, we have recently
reported a one-pot relay gold(I)/Brønsted acid relay catalytic
methodology toward the synthesis of a variety of 1,2-
disubstituted cyclopenta[b]indoles, Scheme 1c.5a
Relay catalysis has emerged as a powerful synthetic tool to
assemble complex molecular architectures in a short and
efficient manner.6 Multi-catalyst-promoted processes can be
atom-economic and thus can minimize the difficulties involved
in isolation and purification of the intermediates. But the
development of such processes is not always straightforward.
Compatibility issues between the catalytic systems and control
over the selectivity aspects complicate the advancement of
such methods. Among the several existing categories, novel
relay processes facilitated by two distinct metal catalysts or
metal/organocatalyst binary systems have been well-
explored.7 However, to the best of our knowledge, reactions
aOrganic Synthesis and Catalysis Lab, Department of Chemical Sciences, Indian
Institute of Science Education and Research (IISER) Mohali, Knowledge City, Sector
81, S. A. S. Nagar, Manuali PO, Punjab 140306, India.
E-mail: ramsastry@iisermohali.ac.in (or) ramsastrys@gmail.com
Homepage: http://14.139.227.202/faculty/sastry/
†Electronic Supplementary Information (ESI) available: Experimental details and
characterisation data for new compounds. For ESI or other electronic format see
DOI: 10.1039/x0xx00000x
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Scheme 1 Divergent approach for the synthesis of furo[3,4-b]indoles
and cyclopenta[b]indoles.
Table 1 Optimization of the reaction parametersa
To begin with, the Au(I)-catalyzed intramolecular
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DOI: 10.1039/C6OB00319B
hydroamination conditions reported earlier by our group were
tried for 2a 5a
However, no desired product was observed.
.
Among few other variations attempted, to our delight, AgOAc
successfully delivered the 5-exo-dig product 2a in excellent
chemo- and regioselectivity.11 Stage is now set for identifying
suitable conditions to secure the formation of 6a
.
The reaction of 2a catalysed by Lewis acids such as the
triflates of Bi, La, and Yb generated only the indolyl alcohol 3a
in good yields (Table 1, entries 1-3). Subsequent evaluation of
Lewis acids such as TMSOTf, BF3OEt2, and FeCl3 gave only a
complex mixture. Interestingly, under the influence of p-
toluenesulfonic acid (PTSA) and camphorsulfonic acid (CSA),
formation of the dihydrofuro[3,4-b]indole 4a was observed,
which presumably formed via the 5-exo-trig cyclization of 3a
(Table 1, entries 4 and 5). Despite realizing 4a in moderate
yields, we were pleased to establish a new one-pot approach
for the synthesis of 1,3-disubstituted 3,4-dihydro-1H-furo[3,4-
b]indoles.12 Several subsequent efforts directed to improve the
yield of 4a were met with no considerable success (Table 1,
entries 6-11), except that a marginal yield increment in case
when the PTSA reaction was performed with the purified
sample of 3a (Table 1, entry 12).
Entry
Conditions
Yield (%)b
1
2
3
4
5c
6
Bi(OTf)3 (10 mol%), DCE, 2 h, 0 oC-RT
La(OTf)3 (10 mol%), DCE, 24 h, 0 oC-RT
Yb(OTf)3 (10 mol%), DCE, 24 h, 0 oC-RT
PTSA (10 mol%), DCE, 12 h, 0 oC
CSA (10 mol%), DCE, 48 h, 0 oC
3a (85)
3a (68)
3a (64)
4a (40)
4a (35)
4a ()
4a (35)
TfOH (10 mol%), DCE, 4 h, 0 oC
7
PTSA (10 mol%), DCM, 13 h, RT
8
9
10
11
12d
4a (12)
PTSA (10 mol%), toluene, 15 h, RT
PTSA (10 mol%), MeNO2, 48 h, RT
PTSA (10 mol%), DCE, 3 h, 60 oC
PTSA (10 mol%), DCE, 72 h,
PTSA (10 mol%), DCE, 11 h, RT
4a
4a (35)
4a
(
)

20 oC
(
)
4a (45)
As part of the attempts to improve the efficiency of the
formation of 4a from 2a, we made yet another significant
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii) Pd(OAc)2
(10 mol%), DCE, 20 h, RT
13
14
15e
16
5a (45)
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii) Pd(OAc)2
observation. The reaction of 3a
, obtained via Bi(OTf)3
5a (43)
(10 mol%), DCE, 30 h, 0 oC
promoted reaction of 2a, in the presence of a catalytic amount
of Pd(OAc)2 delivered the furo[3,4-b]indole 5a in 45% yield
(Table 1, entry 13). Furo[3,4-b]indoles serve as indole-2,3-
quinodimethane synthetic analogue in diverse inter- and
intramolecular Diels-Alder reactions. These intermediates have
been applied for the synthesis of novel classes of heterocycles
such as benzocarbazoles, pyridocarbazoles, including the
antitumor alkaloid ellipticine.13
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii) Pd(OAc)2
(10 mol%), DCE, 10 h, 60 oC
5a (50)
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii) PdCl2 (10
5a
5a
(
(

)
)
mol%), DCE, 10 h, 60 oC
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii)
17
18

Pd(dppf)Cl2 (10 mol%), DCE, 10 h, 60 oC
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii) CuI (10
mol%), DCE, 4 h, RT
5a
5a
(
(

)
)
i) Bi(OTf)3 (5 mol%), DCE, 3 h, RT; ii) AgNO3
(10 mol%), DCE, 4 h, RT
19

a
b
Reaction conditions: See Supporting Information. Isolated yields
c
d
after column chromatography. Reaction did not go to completion.
Yield based on starting material recovery. Reaction performed with
e
the isolated sample of 3a
.
It was envisioned that the designer substrates
A could
generate by undergoing Ag(I)-catalyzed 5-exo-dig
B
a
cyclization. Subsequent acid promoted 1,3-allylic alcohol
isomerization (1,3-AAI) and intramolecular etherification
through a 5-exo-trig cyclization of
conditions, could afford the furo[3,4-b]indoles
On the other hand, the 5-exo-dig product
conditions could generate a 4 -electron system
undergoing a Nazarov-type cyclization could provide access to
cyclopenta[b]indoles , Scheme 1d.
C
, especially under oxidative
, Scheme 1d.
under acidic
, which by
D
E

F
G
With this background and in continuation of our efforts
towards the construction of cyclopentannulated arenes and
heteroarenes,5a,9 we started evaluating the generality of the
hypothesis presented in Scheme 1d. Accordingly, we have
initiated screening of various reagents and solvent
combinations with 1a as the model substrate, with the
intention to obtain the pentannulated indole 6a, Table 1.10
Scheme
enantioselective synthesis of 3a
Among few other variations undertaken to increase the
efficiency of the reaction, only the reaction of 3a at an
elevated temperature provided 5a with a slight increase in the
yield (Table 1, entry 15). Employing a few other Pd(II), Cu(II) or
2 Brief screening of chiral Brønsted acids for the
.
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Ag(I) salts proved to be unsuccessful (Table 1, entries 16-19). trimetallic relay mode [Ag(I)/Bi(III)/Pd(II)], better yields were
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Low yields of 4a or 5a, in general, are attributed to their observed by performing the reaction iniDtiOaIl:ly10u.1n0d39e/rCa6OoBn0e0-3p19oBt
inherent instability, which is well-documented in the bimetallic Ag(I)/Bi(III) system followed by subjecting the
literature.13
purified indolyl alcohols 3 to Pd(II) catalysis. Nevertheless, this
A brief screening of chiral Brønsted acids was undertaken method can serve as a potential alternative to the existing
for the one-pot enantioselective synthesis of 3a, Scheme 2.14 approaches describing the synthesis of the interesting and
Towards this, chiral phosphoric acids CBA-1 to CBA-4 were short-lived furo[3,4-b]indoles.
evaluated for the transformation of 2a to 3a. With the (R)-
VAPOL hydrogen phosphate CBA-2, 3a was realized in 92% ee,
however, the reaction was found to be stalling after a certain
extent of conversion. On the other hand, 3a was obtained in
87% yield with (R)-BINOL phosphoric acid, but almost as a
racemic mixture. Several of our attempts to achieve a better
result were futile. Nevertheless,
a new one-pot relay
Ag(I)/chiral Brønsted acid system was established for the
enantioselective synthesis of 3a.
Table 2 Substrate scope for furo[3,4-b]indolesa,b
Scheme 3 Plausible explanation for the non-formation of 6a from 1a
.
After successfully establishing a new method for the
synthesis of furo[3,4-b]indoles, we turned our attention to
rationalise the non-formation of 6a from 1a. A mechanistic
hypothesis for this observation is proposed in Scheme 3. The
indoline 2a under the influence of an acid generates the 4
electron system , which during the process of undergoing a
Nazarov-type cyclization builds up the potentially destabilising
2o-carbocationic intermediate
. So, an additional substitution
-
H
I
at this position creates a more stabilized 3o-carbocationic
intermediate that can pave the way for the formation of the
desired Nazarov cyclised product. Based on this hypothesis, we
have prepared the enynol 1’k which now possesses a phenyl
group at the vinylic position.15 The phenyl group at this
position can also play a pivotal role in shifting the system from
s-trans to s-cis in the pentadienyl cation intermediate H.
Among few Lewis and Brønsted acids evaluated for the
conversion of indoline 2’k to the Nazarov product 6k, PTSA,
CSA and TfOH were found to be optimum. So, the initial
reactions were performed with all the three Brønsted acids to
assess their consistency. It can be realised from Table 3 that
a
b
Reaction conditions: See Supporting Information. Isolated yields
after column chromatography.
With the optimised conditions for furo[3,4-b]indoles in
hand, we proceeded to evaluate the substrate scope, Table
2.15 Evidently, a wide range of structurally and electronically
diverse substituents across the alkyne (R1) and the aryl ring
(R2) were well-tolerated and generated the furoindoles 5a-5j in
moderate to good yields. A narrow yield range (50-56%)
indicates the robustness of the method which in turn
highlights the least dependence of the relay catalytic system
on the electron-donating/withdrawing contributions of the
the pentannulated indoles 6k, 6l and 6m formed consistently
in excellent yields under the CSA catalysis. So, CSA was chosen
as the catalyst of choice for further deliberations. Subsequent
evaluation of the substrate scope under the optimised
conditions furnished
a
variety of 1,3-disubstituted
cyclopenta[b]indoles 6n-6r in very good yields.16 This
methodology thus can provide an efficient and robust
synthetic access to several bioactive natural products and
medicinally
important
compounds
possessing
substituents. Interestingly, the furoindoles
poor yields when the reaction was carried out in a one-pot
5 were isolated in
cyclopenta[b]indole scaffold.
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Table 3 Substrate scope for cyclopenta[b]indolesa,b
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DOI: 10.1039/C6OB00319B
Scheme 5 Formation of the carbazole 9c via the Diels-Alder reaction of
the furoindole 5c and maleimide 7.
Conclusions
In summary, we have presented for the first time, a divergent
one-pot relay catalytic approach for the synthesis of 1,3-
disubstituted furo[3,4-b]indoles and cyclopenta[b]indoles
starting from easily accessible 3-(2-aminophenyl)-4-pentenyn-
3-ols. Interesting substitution dependence on the product
distribution was realised. Based on the mechanistic
considerations, this phenomenon was efficiently crafted to
yield the product of choice. The methodologies described
herein have demonstrated great potential and will stimulate
further research in the synthesis of new heterocycles. We are
currently involved in extending these strategies towards the
total synthesis of complex indole-based natural products and
the results will be communicated in due course.
a
b
Reaction conditions: See Supporting Information. Isolated yields
after column chromatography.
Having established new one-pot relay catalytic approaches
for the furo[3,4-b]indoles and cyclopenta[b]indoles, we
planned an elaboration with the former class. Earlier, Gribble
and co-workers demonstrated that furo[3,4-b]indoles behave
very well as dienes in the Diels-Alder reaction, and this
strategy was widely employed in the construction of several
complex molecular architectures.13a,b,e,f Towards this, the
Acknowledgements
furoindole 5a and N-ethylmaleimide
7 were refluxed in
This work was supported financially by the Department of Science
and Technology (DST), Govt. of India (SR/FT/CS-156/2011) and IISER
Mohali. We thank the NMR and mass facilities at IISER Mohali.
Manisha and J.M. thank DST for the INSPIRE fellowship and S.D.
thanks IISER Mohali for a research fellowship.
toluene to produce a mixture of endo/exo Diels-Alder adducts
(
8a and 8b) in 78% yield, Scheme 4. The isomeric structures
were assigned from the 1H and 13C-NMR data and further
verified from the reported data of similar structures.13b This
result indirectly confirms the structure of 5a and other
analogues as well.
Notes and references
1
Some selected reviews: (a) M. Somei and F. Yamada, Nat.
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2
For spiroindimicins, see: (a) W. Zhang, Z. Liu, S. Li, T. Yang, Q.
Zhang, L. Ma, X. Tian, H. Zhang, C. Huang, S. Zhang, J. Ju, Y.
Scheme 4 Diels-Alder reaction of the furoindole 5a and maleimide 7.
Shen and C. Zhang, Org. Lett., 2012, 14
, 3364; for
Interestingly, the reaction of furoindole 5c with N-
ethylmaleimide under toluene reflux conditions furnished,
fischerindoles, see: (b) J. M. Richter, Y. Ishihara, T. Masuda,
B. W. Whitefield, T. Llamas, A. Pohjakallio and P. S. Baran, J.
Am. Chem. Soc., 2008, 130, 17938; for yeuhchukene, see: (c)
Y. –C. Kong, K. –F. Cheng, R. C. Cambie and P. G. Waterman,
J. Chem. Soc., Chem. Commun., 1986, 47; for polyveolines,
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Hannaert and B. Schneider, Bioorg. Med. Chem. Lett., 2010,
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Lachance, M. Boyd, C. Berthelette, M. Labelle, L. Li, B. Roy, J.
Scheigetz, N. Tsou, Y. Aubin, K. P. Bateman, N. Chauret, S. H.
Day, J. F. L. vesque, C. Seto, J. H. Silva, L. A. Trimble, M. C.
Carriere, D. Denis, G. Greig, S. Kargman, S. Lamontagne, M.
C. Mathieu, N. Sawyer, D. Slipetz, W. M. Abraham, T. Jones,
7
via the [4+2] adduct 8c, the carbazole 9c in good yield, Scheme
5.13 Carbazoles are considered privileged structures from
medicinal chemistry standpoint. Carbazole scaffold is present
in several bioactive natural products, various pharmaceuticals
and functional materials.17 This method therefore can provide
an access to highly functionalized carbazoles as well.
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M. McAuliffe, H. Piechuta, D. A. N. Griffith, Z. Wang, R.
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.
3
Some recent references about cyclopenta[b]indoles, see: (a)
9
(a) B. Satpathi, S. Dhiman and S. S. V. Ramasastry, Eur. J. Org.
Chem., 2014, 2022; (b) S. Dhiman and S. S. V. Ramasastry,
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,
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8
,
Peruncheralathan, H. Ila and H. Junjappa, J. Org. Chem., 10 See Supporting Information for the synthesis of the enynol
2007, 72, 1388; (d) C. Ferrer, C. H. M. Amijs and A. M. 1a and other analogous enynols employed in this study.
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Chem. Soc., 2011, 133, 689; (h) B. Chen, W. Fan, G. Chai and
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of the type 4a
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S. Ma, Org. Lett., 2012, 14, 3616; (i) B. Xu, Z. –L. Guo, W. –Y. 13 (a) G. W
.
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4
Few selected articles on Nazarov cyclizations, see: (a) I. N.
Nazarov and I. I. Zaretskaya, Izv. Akad. Nauk. SSSR, Ser.
Khim., 1941, 211; (b) H. Pellissier, Tetrahedron, 2005, 61
6479; (c) A. J. Frontier and C. Collison, Tetrahedron, 2005, 61
,
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and S. Wang, J. Am. Chem. Soc., 2006, 128, 1442; (f) W. 15 For a detailed discussion on the substitution effects in
Nakanishi and F. G. West, Curr. Opin. Drug Discovery Dev.,
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Frontier, Eur. J. Org. Chem., 2013, 3621; (h) M. A. Tius, Chem.
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Nazarov reactions, see: (a) A. P. Marcus, A. S. Lee, R. L. Davis,
D. J. Tantillo and R. Sarpong, Angew. Chem. Int. Ed., 2008, 47
,
6379; (b) C. D. Smith, G. Rosocha, L. Mui and R. A. Batey, J.
Org. Chem., 2010, 75, 4716 and references cited therein. For
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A. J. P. White, Org. Bimol. Chem., 2011, 9, 7904.
16 For our earlier work on the synthesis of 1,2,3-trisubstituted
cyclopenta[b]indoles, see: Ref-9b. For our earlier work on
the synthesis of 1,2-disubstituted cyclopenta[b]indoles, see:
Ref-5a.
5
6
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