One-Pot Relay Gold(I) and Br?nsted Acid Catalysis: Cyclopenta[b]annulation of Indoles via Hydroamination/Nazarov-Type Cyclization Cascade of Enynols

Article abstract of DOI:10.1021/acs.orglett.5b02632

An expedient relay gold(I) and Br?nsted acid catalyzed hydroamination/Nazarov cyclization of 1-(2-aminophenyl)pent-4-en-2-ynols for the synthesis of various polyfunctionalized cyclopenta[b]indoles is described. The synthetic utility of this method has bee

Full text of DOI:10.1021/acs.orglett.5b02632

Letter
pubs.acs.org/OrgLett
One-Pot Relay Gold(I) and Brønsted Acid Catalysis:
Cyclopenta[b]annulation of Indoles via Hydroamination/Nazarov-
Type Cyclization Cascade of Enynols
Seema Dhiman and S. S. V. Ramasastry*
Organic Synthesis and Catalysis Lab, Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER)
Mohali, Sector 81, S. A. S. Nagar, Manuali PO, Punjab 140306, India
S
* Supporting Information
ABSTRACT: An expedient relay gold(I) and Brønsted acid
catalyzed hydroamination/Nazarov cyclization of 1-(2-
aminophenyl)pent-4-en-2-ynols for the synthesis of various
polyfunctionalized cyclopenta[b]indoles is described. The
synthetic utility of this method has been demonstrated by
the synthesis of a few unprecedented pentacyclic indoles and
indole−steroidal hybrids. Further, the new methodology has been successfully applied to the enantioselective synthesis of core
carbon structure of the polyveoline family of natural products.
Scheme 1. Our Strategy for the Synthesis of
Cyclopenta[b]indoles via Nazarov-Type Cyclization
old-catalyzed transformations have attracted tremendous
Gattention in recent years owing to their exceptional ability
to activate π-systems of alkynes, allenes, and alkenes toward
nucleophilic attacks.1 Most of these methods provide access to
several complex structural motifs under extremely mild
conditions which were previously inaccessible through tradi-
tional synthetic transformations. In particular, relay gold
catalytic processes were demonstrated to exhibit great potential
in rapidly assembling intricate chemical structures often
associated with pot, step, and atom economy.2
Cyclopenta[b]indoles undoubtedly are privileged substruc-
tures owing to the occurrence of several bioactive natural
products and discovery of many medicinally significant
compounds.2i,3 Among several elegant approaches for the
construction of this skeleton, the Nazarov reaction4 is
undoubtedly one of most versatile and efficient methods.
However, most of the Nazarov cyclizations leading to
cyclopentenes exploit the 4π-electrocyclizations of dienone
systems extensively (Scheme 1, eq 1). Surprisingly, Nazarov-
type cyclizations of pentadienyl cations (originating from the
respective divinyl carbinols) for the synthesis of cyclo-
pentannulated aryls and heteroaryls are rather scarce (Scheme
1, eq 2).5 Toward this, we have initiated a program for the rapid
assemblage of pentadienyl cationic systems through gold-
mediated cascade processes from easily accessible starting
compounds which should eventually lead to the cyclo-
pentannulation of indoles.
It was envisaged that 2-aminophenyl enynols A could
undergo Au(I)-catalyzed 5-exo-dig cyclization (hydroamina-
tion) to form 2-allylidene indolinols B. Further, under acidic
conditions, indolinols B could generate the pentadienyl cations
C, which potentially undergo a 4π-electrocyclization leading to
the formation of cyclopenta[b]annulated indoles D (Scheme 1,
eq 3).
Since 1-(2-aminophenyl)pent-4-en-2-ynols A are underex-
plored substrates, especially never being employed in a gold-
catalyzed process, and since modular access to this class can be
achieved from readily available 2-amino or 2-nitro benzalde-
hydes and enynes (Scheme 1, eq 3), this method thus can be a
short and efficient strategy for the synthesis of cyclo-
pentannulated indoles. Herein, we delineate our efforts toward
the development of a novel synthetic strategy aimed at the
construction of 1,2-disubstituted cyclopenta[b]indoles from
Received: September 11, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02632
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Organic Letters
Letter
enynols A facilitated by the 4π-electrocyclization of rather
uncommon pentadienyl cations with Au(I) and Brønsted acid
as promoters of the two cyclization events.6
improvements (Table 1, entries 6−8). After realizing the
inefficiency of the Brønsted acids other than TfOH in step II
(Table 1, entry 9), we opted to investigate the influence of
Lewis acids. However, among several Lewis acids screened,
except TMSOTf and BF3·OEt2, the others provided 2a in poor
yields (Table 1, entries 11−15). Other attempts provided only
moderate success (Table 1, entries 16−18).
With the optimal conditions in hand, we next focused on
investigating the substrate scope. Toward this, a diverse range
of 1-(2-aminophenyl)pent-4-en-2-ynols 1b−q were prepared
and subjected to the optimized conditions. The results are
summarized in Scheme 2. Overall, the relay catalytic process
In order to validate the mechanistic hypothesis proposed in
Scheme 1, eq 1,7 the enynol 1a was chosen as the model
substrate.8 In light of our previous success with a Au(I)−TfOH
relay catalytic system for the cyclopentannulation of hetero-
aryls,2i we chose this catalyst system in the preliminary
evaluation. Gratifyingly, reaction of 1a under the prototypical
conditions furnished the desired product 2a in 74% yield along
with an easily separable quinoline 3a9 (in 8% yield) (Table 1,
a,b
Table 1. Optimization of Reaction Parameters
a,b
Scheme 2. Substrate Scope
c
entry
catalyst
AuCl
co-catalyst
acid
time (h) 2a, yield (%)
1
2
3
4
5
K2CO3
Na2CO3
Et3N
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
−
TfOH
HClO4
TfOH
TMSOTf
Bi(OTf)3
In(OTf)3
BF3.OEt2
AgSbF6
TfOH
TfOH
TfOH
13
13
13
48
48
15
48
13
16
26
16
60
60
13
60
30
30
14
74
60
38
−
−
57
AuCl
AuCl
AuCl
AgOTf
AgSbF6
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
−
PPh3AuCl
AuCl
d
6
e
7
f
AuCl
−
−
8
AuCl
9
AuCl
64
56
70
43
49
67
36
36
59
68
10
11
12
13
14
15
PPh3AuCl
AuCl
AuCl
AuCl
AuCl
AuCl
g
16
AuCl3
Ipr AuCl
AgOAc
17
18
a
A 5 mL glass vial was filled with 1a (0.1 mmol), Au(I) catalyst (0.002
mmol), co-catalyst (0.002 mmol), and solvent (1 mL), and stirred at
60 °C. Upon disappearance of 1a (by TLC), an acid (0.01 mmol) was
added at room temperature (rt) and stirring continued at rt until 4a
disappeared (by TLC). 6−10% of the quinoline 3a also formed in
cases where step I worked. Isolated yield after column chromatog-
b
c
d
e
raphy. Toluene as solvent. When THF was used as solvent, even
f
g
a
step I did not work. Step II at 60 °C. 6 mol % of base.
A 5 mL glass vial was filled with 1 (0.1 mmol), AuCl (0.002 mmol),
K2CO3 (0.002 mmol), and 1,2-dichloroethane (1 mL) and stirred at
60 °C. Upon disappearance of 1 (by TLC), TfOH (0.01 mmol) was
added at room temperature (rt) and stirring continued at rt until 4
entry 1). With the intention of further improving the efficiency
of the reaction, various base, solvent, gold precatalysts, and acid
combinations were investigated, and the results are compiled in
Table 1.
b
disappeared (by TLC). Isolated yield after column chromatography.
Although initial success was obtained from a Au(I)/base
combination, subsequent attempts to improve the yield with
different inorganic or organic bases were discouraging (Table 1,
entries 2 and 3). Commonly employed Au(I)/Ag(I) combina-
tions10 were surprisingly unproductive (Table 1, entries 4 and
5), and the addition of a catalytic amount of TfOH generated
only a complex mixture. A brief solvent screening or performing
step II at an elevated temperature delivered no promising
was realized to be very general and efficient, and a wide range of
cyclopentannulated indoles 2b−q could be rapidly assembled in
good to excellent yields. Varied amounts (<10%) of the
respective quinolines (3) were also usually isolated. In general,
consistent reaction times were observed irrespective of the
electronic or steric factors involved. Scheme 2 outlines the
tolerable substituents across the aryl ring (R1 and R2), which
can be electron-donating as well as electron-withdrawing (such
B
DOI: 10.1021/acs.orglett.5b02632
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Letter
as −OMe, −Cl, and −OTs). Regarding the olefin, a variety of
alkyl, aryl, and heteroaryl substituents (R3 and R4) were well-
tolerated under the reaction conditions.
Scheme 3. Enantioselective Synthesis of the Core Carbon
Scaffolds of 8-Nor-polyveoline 8 and 8-Nor-polyveolinone 9
An attractive feature of this method lies in its ability to
readily assemble natural product like complex pentacyclic
indoles 2m−o. Among them, the structure of 2o was confirmed
by single-crystal X-ray diffraction analysis (see the Supporting
Information for details).11 The present strategy thus could pave
the way for the easy synthesis of their complex analogues which
may find suitable applications in medicinal chemistry and also
in materials science.
Next, we targeted the synthesis of indole-steroidal conjugates
since indoles and steroids are well-established privileged
scaffolds, and therefore, synthesis of indole−steroidal hybrids
can offer potential opportunities to validate their biological
properties.12 Toward this, the cholesteryl enynol 1p13 was
synthesized and was subjected to the optimized conditions.
Along with the desired product 2p in 52% yield, the respective
quinoline 3p (in 30% yield) was also isolated. Nevertheless, a
novel entry for the previously unknown indole steroidal hybrids
has been established.
In an attempt to synthesize the bis-indole fragment present
in the polybrominated spiro-trisindole natural product
similisine A (5)14 employing this method, enynol 1q was
prepared. Reaction of 1q under the optimized conditions
furnished the bis-indole derivative 2q, which represents the
partial structure of the tris-indole natural product similisine A
(5).
In order to further demonstrate the synthetic utility of our
method, we have undertaken the enantioselective synthesis of
the core carbon skeleton of the polyveoline family of natural
products. The first member of this family, polyveoline 6, was
isolated in 1978,15a and the latest member, 8α-polyveolinone 7,
was isolated in 2014.15b Despite the presence of complex
molecular architectures associated with interesting biological
activity profiles,15b,16 surprisingly, no total synthesis of any
member of polyveoline family has been reported thus far.17
Retrosynthetic analysis depicted in Scheme 3 readily
identified the enynol 10 as a suitable precursor for the
synthesis of the complete carbon framework of either 6 or 7.
However, since the preliminary results pertaining to Au(I)-
mediated reaction of enynols such as 10 where the olefin is
tetrasubstituted generated only the respective quinolines, we
targeted the synthesis of the carbon skeleton present in 8-nor-
polyveoline 8 and 8-nor-polyveolinone 9. For this purpose,
enynol 11 serves as a suitable retrosynthetic precursor which
can be readily obtained from the enyne 12 and the amino
benzaldehyde 13. The enyne 12 can be obtained from the
ketone 14, which in turn can be easily derived from the
Wieland−Miescher ketone analogue 15.
To begin with, commercially available 2-methylcyclohexane-
1,3-dione 16 was converted via the Wieland−Miescher
analogue 1518 into the strategic intermediate 14.19 Treatment
of 14 with TMS-acetylene followed by POCl3-mediated
dehydration generated the enyne 21 in 60% yield over two
steps. Selective deprotection of TMS group in 21 was
accomplished by TBAF in excellent yield. With the key
intermediate 12 in hand, we next attempted the synthesis of the
desired enynol 11. Thus, addition of lithiated 12 to the
aldehyde 13 generated the enynol 11 in 87% yield as a 3:1
diastereomeric mixture, an inconsequential aspect in the
succeeding steps. Ag(I)-catalyzed hydroamination followed by
TfOH-catalyzed Nazarov-type cyclization furnished the TBS-
deprotected pentacyclic indole 22 in 43% yield along with 51%
of the respective quinoline 23.20 Dess−Martin oxidation of the
alcohol 22 resulted in the formation of ketone 24. Indoles 22
and 24 possess the complete carbon framework present in 8-
nor-polyveoline 8 and 8-nor-polyveolinone 9.
In conclusion, we have successfully demonstrated a practical
and efficient relay catalytic system for indole cyclopentannu-
lations of 1-(2-aminophenyl)pent-4-en-2-ynols which involves a
cascade Au(I)-catalyzed hydroamination and Brønsted acid
catalyzed 4π-electrocyclization sequence. We have validated the
generality of this method toward the synthesis of complex
natural product like structures and indole steroidal conjugates.
Further studies to extend the scope of this method for the
buildup of scaffold diversity and elaboration toward the
enantioselective total synthesis of bioactive natural products
are currently underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b02632.
C
DOI: 10.1021/acs.orglett.5b02632
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
N. Angew. Chem., Int. Ed. 2011, 50, 10570. (b) Singh, R.; Panda, G.
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Experimental details and characterization data for all new
compounds (1H NMR, 13C NMR) (PDF)
X-ray crystal structure of 2o (CIF)
AUTHOR INFORMATION
Corresponding Author
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■
*E-mail: ramsastry@iisermohali.ac.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the DST, Government of India, for financial
support through the Fast Track Scheme (SR/FT/CS-156/
2011). We thank IISER Mohali for funding and for NMR, mass,
and X-ray facilities. S.D. thanks IISER Mohali for a research
fellowship.
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D
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