Au-Catalyzed Synthesis of Thiopyrano[2,3-b]indoles Featuring Tandem Rearrangement and Hydroarylation

Article abstract of DOI:10.1021/acs.orglett.7b00617

Gold(III)-catalyzed synthesis of 14-π electron heteroaromatic thiopyrano[2,3-b]indole is reported using conjugated enyne tethered indole sulfides, featuring skeletal rearrangement conjoined with intramolecular hydroarylation (via C3-H functionalization of the indole core) and oxidative aromatization. Subsequent Pd-catalyzed C-C coupling resulted in a 16-π electron heteroaromatic isothiochromeno[1,8,7-bcd]indole.

Full text of DOI:10.1021/acs.orglett.7b00617

Letter
pubs.acs.org/OrgLett
Au-Catalyzed Synthesis of Thiopyrano[2,3‑b]indoles Featuring
Tandem Rearrangement and Hydroarylation
Mukund Jha,*,† Shiv Dhiman,†,‡ T. Stanley Cameron,§ Dalip Kumar,‡ and Anil Kumar‡
†Department of Biology and Chemistry, Nipissing University, North Bay, ON P1B 8L7, Canada
‡Department of Chemistry, Birla Institute of Technology and Science, Pilani, Pilani 333031, India
§Department of Chemistry, Dalhousie University, Halifax, NS B3H 4J3, Canada
S
* Supporting Information
ABSTRACT: Gold(III)-catalyzed synthesis of 14-π electron heteroaromatic thiopyrano[2,3-b]indole is reported using
conjugated enyne tethered indole sulfides, featuring skeletal rearrangement conjoined with intramolecular hydroarylation (via
C3−H functionalization of the indole core) and oxidative aromatization. Subsequent Pd-catalyzed C−C coupling resulted in a
16-π electron heteroaromatic isothiochromeno[1,8,7-bcd]indole.
f late, transition-metal-catalyzed reactions have played a
Ocrucial role in propelling research in the area of modern
synthetic chemistry.1 Among them gold catalysis has emerged as
one of the most important means for the synthesis of
pharmaceutically interesting heterocycles and complex organic
skeletons using alkynes, 1,n-enynes, and allenes as popular
substrates.2
Many heterocycles possessing an indolic core are among
privileged structures because they display a diverse range of
biological activities.3 Particularly, indole derivatives function-
alized at C-2 and C-3 positions are contemplated to be the most
promising to exhibit biological properties.3a Similarly, thiopyrans
and fused thiopyrans are of considerable interest due to their
activities such as analgesic,4 anticancer,5 antihyperplasia,6
antiinflamatory,7 antibacterial,8 and antipsychotic.9 In addition,
polycyclic indole moieties are another group of highly sought-
after molecules, as they are often found to be a part of bioactive
natural/synthetic compounds.10 Hence, developing efficient
syntheses of molecules having these structural motifs are in
great demand in the current literature.
There have been several reports in the literature on the
synthesis of polycyclic indole-annulated thiopyrans. Approaches,
such as a cascade reaction involving aryl radicals and 2-(2-
phenylethynyl)phenyl isothiocyanates,11 Cu-catalyzed tandem
arylation−cyclization of 2-alkynylaryl isothiocyanates with
diaryliodonium salts,12 [3 + 3]-cyclization of indoline-2-thione
(1) and benzylidenemalononitrile in the presence of a thiourea
derived catalyst,13 domino reaction of 1 with aldehyde and
malononitrile,14 and domino Knoevenagel-hetero-Diels−Alder
reaction involving 1,15 have been used to access indole-fused
thiopyrans. Furthermore, the preparation of heteroaromatic
thiochromeno[2,3-b]indole has been reported from 1 via
Vilsmeier formylation.16 The condensation reactions of 1 with
1,3-diketones have been shown to produce a heteroaromatic
thiopyrano[2,3-b]indole core involving thiopyrylium ion inter-
mediates.17 Our group has an ongoing interest in the
development of synthetic chemistry around the indole nucleus.18
Recently, we reported the synthesis of thiopyrano[2,3-b:6,5-
b′]diindole,19 thiopyrano[2,3-b]indol-2-ones,20 and isothio-
chromeno[3,4-b]indole21 ring systems, and, most recently, a
gold(III) mediated two-step synthesis of dihydrothiopyrano[2,3-
b]indoles 4 using 1 and enyne substituted aliphatic tert-alcohols
(2).22 In continuation of our efforts, we now wish to report an
efficient synthesis of 14-π and 16-π electron heteroaromatic
thiopyrano[2,3-b]indole (6) and isothiochromeno[1,8,7-bcd]-
indole (8) frameworks, respectively, utilizing gold chemistry
(Scheme 1). Although the synthesis of the heterocyclic core of 6
has been reported previously,17 the approach is limited to 1,3-
diketones with a potential for regioselectivity complications.
The cyclic core of the newly synthesized ring systems 6 and 8 is
analogous to that of biologically active indoles and natural
Scheme 1. Au(III)-Catalyzed Synthesis of 4; Thiopyrano[2,3-
b]indole 6 and Isothiochromeno[1,8,7-bcd]indole 8 Cores
Received: February 28, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00617
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Organic Letters
Letter
products such as carbazoles,23 carbolines,24 salviadione,25
morphine, and morphine-like opioids26 (Figure 1).
Table 1. Optimization of Metal-Catalyzed Hydroarylation
Reactions of Compound 5
a
yield (%)
entry
catalyst (mol %)
no catalyst
time (h)
solvent
6a 7a
Figure 1. Chemical structure of selected biologically active indoles and
natural products.
1
4
4
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCE
0
0
0
2
AuCl3 (10)
58
82
3
AuCl3 (5)
6
0
Our earlier work suggests the conjugated enyne tethered
indole sulfides 3 are particularly interesting because of their
potential to undergo intramolecular annulations (Scheme 1).22
However, the currently known synthesis of 3 from 1 using
aliphatic alcohols and BF3-etherate produces sulfides in poor
yields (30−40%, based on recovery of starting material).22
Therefore, our investigation began with the intent of improving
the S-alkylation reaction, and subsequently exploring the
hydroarylation carbocyclization of sulfides 5. The indole sulfides
5 selected in the current study possess an internal alkyne, which
were produced from sec enyne alcohols.
4
AuCl (5)
5
51
0
5
AuClPPh3 (5)
AuCl3 (5)/AgOTf (10)
AuCl3 (5)/AgSbF6 (10)
CuCl (10)
6
27
0
6
2
24
0
7
4
27
25
0
8
9
>10
17
9
PdCl2 (10)
9
0
10
11
12
13
14
AgSbF6 (10)
24
6
44
0
AuCl3 (10)
20
60
68
71
0
AuCl3 (5)
0.5
8
toluene
toluene
toluene
>10
>10
80
AuCl3 (10)/under N2 atm
AuCl3 (5)/under O2 atm
4
The desired enyne alcohols27 and indoline-2-thiones16b were
synthesized using the literature procedure. Although our
attempts to synthesize sulfides 5 replicating the Lewis acid
mediated direct alkylation22 of thiones using aliphatic alcohols
were successful, the product yields were poor (∼20−30%).
Hence, we adopted an alkylation28 of 1 with milder conditions in
the presence of benzyltriethylammonium chloride (BTEAC) as a
phase transfer catalyst. However, this required first converting
eneyne alcohols to corresponding chlorides, which were then
reacted with indoline-2-thiones under basic conditions to
produce 5a−t in good to excellent yields (see Supporting
Information (SI)). The stereochemical orientation of most of the
conjugated enyne side chain (R2 = Me) present in the
a
Isolated yields.
Figure 2. ORTEP representation of 14-π electron heteroaromatic
compound 6d.
1
synthesized sulfides could not be established using H NMR
data except for 5t (R2 = H), which was found to be E.
44% yield. However, mixtures, such as AuCl3/AgOTf and
AuCl3/AgSbF6, provided somewhat interesting results. Partic-
ularly, the use of AuCl3/AgSbF6 in toluene resulted in a mixture
of products 6a and 7a in 27% and 25% yields respectively (Table
1, entry 7), where compound 7a was found to be a
nonaromatized indole-fused dihydrothiopyran framework.
These results prompted us to focus on AuCl3 as the catalyst of
choice for a few follow-up control reactions. The change of
solvent from toluene to DCE had an impact on the reaction
outcome; the reaction resulted in a mixture of 6a (20%) and 7a
(60%) after 6 h of heating (Table 1, entry 11). We were quite
delighted to observe that a controlled cyclization of 6a can be
performed using AuCl3 to produce 7a exclusively after stirring
the reaction mixture for 0.5 h (Table 1, entry 12). It was found
that compound 7a has considerable air stability and could be
isolated and stored without undergoing further oxidation. The
isolated yield of 7a was marginally better when the reaction was
performed under a N2 atmosphere (Table 1, entry 13). The
oxidation of 7a to 6a could separately be carried out in near-
quantitative yields by simply heating 7a in toluene at 80 °C for 24
h or in the presence of AuCl3 for 4 h. As predicted, a faster
oxidative aromatization of 5a was observed when the reaction
was performed under an O2 atmosphere (Table 1, entry 14).
Overall, after thorough examination, we identified the use of 5
mol % AuCl3 at 80 °C in toluene for 6 and 0.5 h as the optimal
conditions for the synthesis of compounds 6 and 7, respectively,
from 5.
Among the synthesized sulfides 5a−t compound 5a was
selected as a model reactant for exploring the intramolecular
hydroarylation reactions. Based on our previous experience,22 we
initially subjected 5a to a series of gold-based potential catalysts
(Table 1, entries 2−5) in toluene at 80 °C. As illustrated in Table
1, in comparison to 5a, a more polar red colored product was
isolated from the reaction in all the cases. Spectroscopic analysis
revealed the formation of an indole-annulated C-3 functionalized
product, which was established to be a 14-π electron
heteroaromatic thiopyrano[2,3-b]indole (6a). Furthermore, it
was evident from the skeletal substitutions the rearrangement of
conjugated enyne side chain present on sulfide 5a has taken place
en route in the formation of the fused thiopyran ring in 6a. The
structure of the product was subsequently confirmed unambig-
uously using X-ray crystallography (CCDC No.: 1529611)
(Figure 2).29 The aromatic nature of the tricyclic core in 6d is
further substantiated with the experimentally determined C−C
(∼1.40 Å) and C−S (1.729 and 1.720 Å) bond lengths (see SI),
indicating their multiple bond character in resonance structures
6d1 and 6d2 (Figure 2). The thiopyriyium resonance form of 6,
similar to 6d2, has been described previously.17
Encouraged by these results we decided to screen a few other
potential transition metal catalysts as well as a mixture of Au and
Ag salts for the reaction under investigation. The use of CuCl and
PdCl2 resulted in 6a in poor yields (Table 1, entries 8 and 9),
while using AgSbF6 (10 mol %) compound 6a was obtained in
B
DOI: 10.1021/acs.orglett.7b00617
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Organic Letters
Letter
Next, in order to demonstrate the substrate scope, sulfides 5a−
t were subjected to hydroarylation reactions under the optimized
conditions. Most of the sulfides (5a−n) having aryl substitution
on the alkyne reacted smoothly to produce diversely substituted
indole-annulated thiopyrans 6 in excellent yields (Table 2).
appears to aid the process (Table 2). On the other hand, the N-
protected sulfides, 5r and 5s, resulted in nonaromatic products
7h and 7i (entries 7 and 8, Table 3). The sulfides (5o−q) having
alkyl substitution on the alkyne functionality yielded slightly
different results than aryl-substituted alkynes (Table 3). In these
situations, the predominant products were found to be
unaromatized compounds 7e−g and the aromatized analogues
were formed in traces except in the case of 6p, which was
obtained in 23% yield after 20 h of heating (entries 4−6, Table
3). However, the longer reaction time also caused thermal
decomposition of the starting material. Hence, we chose to
perform the reaction workup after 0.5−1 h to maximize the yields
of compounds 7e−g. Overall, these observations imply the
extended conjugation provided by the aryl substitutions on the
alkyne in sulfides 5 might be facilitating the oxidative
aromatization to yield fully aromatic fused tricyclic thiopyran
products 6a−k (Table 2).
Table 2. Synthesis of 14-π Electron Heteroaromatic
a b
,
Thiopyrano[2,3-b]indoles 6
Mechanistically, an initial activation of the alkyne functionality
with cationic gold22 appears to induce the skeletal rearrangement
of the enyne manifold present in sulfide 5 (Scheme 2). The Z
Scheme 2. Proposed Mechanism for the AuCl3 Mediated
Formation of Thiopyrano[2,3-b]indoles 6
a
Reaction conditions: 5 (0.098 mmol), AuCl3 (5 mol %) in toluene at
b
80 °C for 5−7 h. Isolated yields.
Although the reaction appears to be amenable to a variety of
substitutions, present on either the indole core or the aryl group
of the aliphatic side chain, we observed a slightly compromised
yield in cases of 6c and 6i having −F and −OMe substitutions,
respectively.
It was surprising to note that sulfides with 4-Br substitution on
the indole ring (5h, 5j, and 5n) resulted mainly in the
nonaromatic products 7b−d (Table 3) despite prolonged
orientation of CC might be important in assisting the
rearrangement step, as compound 5t (E isomer) did not
undergo any cyclization. Subsequently, the nucleophilic attack of
indole via the C-3 position effects 6-endo trig carbocyclization to
produce 7a, which ultimately undergoes oxidative aromatization
yielding compound 6a.
In order to demonstrate the strategically located vinyl group in
7 could be used for further carbocyclization via Heck-type
coupling,30 we next focused on compound 7b, having bromo
substitution aptly positioned (position 5) on the dihydrothio-
pyrano[2,3-b]indole skeleton, for a possible cross-coupling
reaction. It was anticipated that the reaction would be
challenging since the vinyl group in 7b is unactivated. After a
couple of failed attempts we were delighted to successfully
convert 7b into an 16-π electron heteroaromatic isothio-
chromeno[1,8,7-bcd]indole 8 via oxidative aromatization in the
presence of Pd(OAc)2 in 40% yield (Scheme 3).31 The newly
created heterotetracycle has a structural framework similar to
that of naturally occurring alkaloids salviadione and morphine
(Figure 1).
a
Table 3. Synthesis of Dihydrothiopyrano[2,3-b]indoles 7
b
product 7 (yield, %), reaction
entry
R
R1
4-Br
R2
time
1
2
3
4
5
6
7
8
H
H
H
H
H
H
Ph
Ph
Ph
7b (66), 1 h
7c (60), 1 h
7d (66), 1 h
7e (66), 0.5 h
7f (67), 0.5 h
7g (60), 0.5 h
7h (67), 0.5 h
7i (70), 0.5 h
4-Br
4-Br
6-Cl
6-CO2Me
H
4-EtC6H4
4-OMeC6H4
Me
Me
Me
Me
Ph
H
H
a
Reaction conditions: 5 (0.078 mmol), AuCl3 (5 mol %) in toluene at
b
80 °C. Isolated yields.
heating of the reaction (up to 24 h). Interestingly, compound
5t having unsubstituted CC in the E orientation did not
undergo cyclization under these conditions. Only the decom-
position of the starting material 5t was observed with extended
reaction time. This suggests the Z orientation in 5 might be vital
for the hydroarylation reactions to take place to produce 6 or 7,
and formation of these products could be indicative of the
stereochemistry of CC in various sulfides under investigation.
The unsubstituted indolic nitrogen caused no impedance in the
oxidative cyclization of 5 in the presence of AuCl3; rather, it
In conclusion, we have developed an efficient general
methodology for the synthesis of 14-π and 16-π electron
heteroaromatic thiopyrano[2,3-b]indole and isothiochromeno-
[1,8,7-bcd]indole frameworks starting from indoline-2-thiones. A
Au(III)-catalyzed tandem skeletal rearrangement accompanied
by hydroarylation and subsequent oxidative aromatization
resulted in tricyclic indole-fused thiopyrans. Indole-annulated
C
DOI: 10.1021/acs.orglett.7b00617
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Organic Letters
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Isothiochromeno[1,8,7-bcd]indole (8)
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ASSOCIATED CONTENT
* Supporting Information
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1152.
■
S
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(b) Jha, M.; Enaohwo, O.; Marcellus, A. Tetrahedron Lett. 2009, 50,
7184−7187. (c) Jha, M.; Chou, T. Y.; Blunt, B. Tetrahedron 2011, 67,
982−989. (d) Jha, M.; Enaohwo, O.; Guy, S. Tetrahedron Lett. 2011, 52,
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Diversity 2013, 17, 827−834.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00617.
Experimental procedure, physical and spectral (1H NMR,
13C NMR, and HRMS) data and copy of 1H and 13C NMR
of the synthesized compounds 2a−f, 5a−t, 6a−l, 7a−i, 8,
9, and the X-ray crystallographic data and crystal structure
of compound 6d (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: mukundj@nipissingu.ca.
ORCID
(19) Jha, M.; Edmunds, M.; Lund, K.; Ryan, A. Tetrahedron Lett. 2014,
55, 5691−5694.
(20) Jha, M.; Davis, C.; Fazzari, J.; Vitali, M. Tetrahedron Lett. 2014, 55,
7043−7046.
Mukund Jha: 0000-0002-0540-6355
Anil Kumar: 0000-0003-4699-3178
Notes
(21) Shelke, G. M.; Jha, M.; Kumar, A. Org. Biomol. Chem. 2016, 14,
3450−3458.
(22) Jha, M.; Shelke, G. M.; Cameron, T. S.; Kumar, A. J. Org. Chem.
2015, 80, 5272−5278.
The authors declare no competing financial interest.
(23) James, M. J.; Clubley, R. E.; Palate, K. Y.; Procter, T. J.; Wyton, A.
C.; O’Brien, P.; Taylor, R. J. K.; Unsworth, W. P. Org. Lett. 2015, 17,
4372−4375.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge the financial support
provided by the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the Canada Foundation for
Innovation (CFI) to conduct this research.
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A.; Akhmedov, N. G.; Soederberg, B. C. G. Tetrahedron 2011, 67, 4753−
4757.
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