Synthesis of Pyrido[2,3-b]indole Derivatives via Rhodium-Catalyzed Cyclization of Indoles and 1-Sulfonyl-1,2,3-triazoles

Article abstract of DOI:10.1002/adsc.201901599

Acyloxy-substituted α,β-unsaturated imines generated in situ from triazoles can act as aza-[4 C] synthons and be trapped by indoles in a stepwise [4 + 2] cycloaddition reaction, thus providing rapid access to valuable pyrido[2,3-b]indoles in high yields. Attractive features of this reaction system include operational simplicity, readily available substrates, construction of sterically demanding quaternary centers, and convenient derivatization using triflate. (Figure presented.).

Full text of DOI:10.1002/adsc.201901599

Title: Synthesis of Pyrido[2,3-b]indole Derivatives via Rhodium-
Catalyzed Cyclization of Indoles and 1-Sulfonyl-1,2,3-Triazoles
Authors: Shengguo Duan, Yuehui An, Bing Xue, Yidian Chen, Wan
Zhang, Zefeng Xu, and Chuan-Ying Li
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201901599
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Advanced Synthesis & Catalysis
10.1002/adsc.201901599
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Synthesis of Pyrido[2,3-b]indole Derivatives via Rhodium-
Catalyzed Cyclization of Indoles and 1-Sulfonyl-1,2,3-triazoles
a
a
a
a
a
a
Shengguo Duan, Yuehui An, Bing Xue, Yidian Chen, Wan Zhang, Ze-Feng Xu,
a,
and Chuan-Ying Li *
a
Department of Chemistry, Zhejiang Sci-Tech University, Xiasha West Higher Education District, Hangzhou 310018, P.
R. of China
E-mail: licy@zstu.edu.cn
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract: Acyloxy-substituted α,β-unsaturated imines synthesis of pyrido[2,3-b]indoles involving the direct
generated in situ from triazoles can act as aza-[4C] cyclization of the indoles and aza-[4C] units.
synthons and be trapped by indoles in a stepwise [4 + 2]
cycloaddition reaction, thus providing rapid access to
valuable pyrido[2,3-b]indoles in high yields. Attractive
features of this reaction system include operational
simplicity, readily available substrates, construction of
sterically demanding quaternary centers, and convenient
derivatization using triflate.
Keywords: Pyrido[2,3-b]indole; [4 + 2] Annulation; Aza-
[
4C] synthon; Carbenes; 1,2,3-Triazole
Figure 1. Representative natural compounds.
In 2008, Fokin and Gevorgyan demonstrated that
Dimroth-type equilibrium between 1-sulfonyl-1,2,3-
triazoles and α-diazo imines can be interrupted by a
rhodium(II) catalyst, leading to the efficient
production of α-imino rhodium carbenes (Scheme
Indole alkaloids containing a pyrido[2,3-b]indole
motif are present in a variety of natural products,
[
1]
[2]
such as communesin F,
chaetominine,
and
[3]
neoxaline (Figure 1). Because these compounds and
[
1–3]
[9]
[10]
their derivatives are biologically active,
many
1a). Because of the easy access to triazoles, the
simple reaction procedure, and the unique reactivity
of the α-imino carbenes in the synthesis of nitrogen-
containing compounds, numerous studies have been
strategies for the construction of the pyrido[2,3-
b]indole scaffold have been developed in the past few
decades.[
4–8]
With the goal of preparing the well-
[
11,12]
known natural product communesin F, many groups
conducted in this area in recent years.
In general,
have developed methods to synthesize tetracyclic
1-sulfonyl-1,2,3-triazoles serve as aza-[3C] units in
indolo[2,3-b]quinolines.[
4]
Similarly,
many
[13]
the reaction with indoles (Scheme 1b).
For
[13a]
approaches to synthesize the 6,5,6,5-tetracyclic
skeleton have also been established in the synthesis
of chaetominine and its analogs.[ However, few
synthetic methods for the preparation of tricyclic
example, in 2013, Davies
synthesize pyrroloindoline
reported a reaction to
via
a
catalytic
5]
enantioselective formal [3 + 2] cycloaddition of C3-
substituted indoles and 1-sulfonyl-1,2,3-triazoles.
[
13b]
pyrido[2,3-b]indoles that do not bear other rings have
Subsequently, Shi
reported an intramolecular
been reported,[
6–8]
and the formation of a C–N bond
version of a similar formal [3 + 2] cycloaddition. In
[6]
[13d]
[13e]
[13f]
on the piperidine ring is the most common method.
2015, Murakami,
Anbarasan,
and Chen
method to
However, these methods[6a–d] are usually complicated
and inefficient because of their multistep nature. Garg
developed a strategy to construct pyrido[2,3-
b]indoles using an interrupted Fisher indolization
almost simultaneously reported
a
synthesize tryptamine derivatives employing C3-
nonsubstituted indoles and 1-sulfonyl-1,2,3-triazoles.
[
14]
[15]
In addition to this, Shi,
Anbarasan,
and our
[7]
[12k,12m,12o]
sequence, and Ghorai reported a [4 + 2] cyclization
reaction to construct pyrido[2,3-b]indoles using
indoles and azetidines.[ Nevertheless, these authors
only reported one example, and, thus, the
applicability of these methods is not clear. Herein, we
report an efficient and practical method for the
group
found that 1-sulfonyl-1,2,3-triazoles
can also serve as aza-[4C] units in some special cases
(Scheme 1c). In these cases, the 1,2-sulfur (or OAc)
migration of an α-imino rhodium carbene delivers an
α,β-unsaturated imine, which can participate in the
ring formation reaction as an aza-[4C] synthon. Thus,
8]
1
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Advanced Synthesis & Catalysis
10.1002/adsc.201901599
we envisaged that a compound containing a
pyrido[2,3-b]indole core could be easily obtained
when employing indoles as a [2C] synthon and the in
situ generated α,β-unsaturated imines as an aza-[4C]
synthon.
also reacted with 1,3-dimethyl-1H-indole (1a) in the
presence of Rh
2
(OAc) , and the products 3ab and 3ac
4
were obtained in 85% and 78% yields, respectively.
Remarkably, the sterically hindered trityl group had
no obvious impact on the reaction, and compound
3
ad was obtained in 87% yield. Moreover, other
functional groups such as chloride, t-
butyloxycarbonyl (Boc), and morpholine were all
well tolerated in this reaction (3ae–ag). When
changing the protecting group on the nitrogen to
methylsulfonyl, product 3ah was obtained in 73%
yield. Notably, sulfur-tethered triazole 2i could also
be used in this reaction, affording the corresponding
3ai in 68% yield.
Table 1. Optimization of Reaction Conditions.[a]
Entry Catalyst
Solvent
T/°C Yield/%[b]
Scheme 1. Previously reported reactions.
1[c]
2
3
Rh
Rh
Rh
Rh
Rh
(OAc)
(OAc)
(OAc)
(OAc)
(OAc)
1,2-DCE
1,2-DCE
toluene
1,1,2-TCE 80
DCM
80
80
80
42
62
54
37
30
42
62
63
29
81
80
85
82
74
76
2
2
2
2
2
4
4
4
4
4
We initiated our study by the treatment of readily
accessible triazole 2a (0.2 mmol) and 1,3-dimethyl-
4
[
d]
1
1
H-indole (1a, 0.2 mmol) with 2 mol% Rh
2
(OAc) in
4
5
6
7
8
9
1
1
80
90
70
60
50
60
60
60
60
60
60
,2-dichloroethane (1,2-DCE, 2 mL) at 80 °C for 4 h.
2
Rh (OAc)4 1,2-DCE
Fortunately, the expected product (3aa) bearing a cis-
pyrido[2,3-b]indole skeleton was obtained in 42%
yield (Table 1, entry 1), and the structure was
Rh
Rh
Rh
Rh
Rh
2
2
2
2
2
(OAc)
(OAc)
(OAc)
(OAc)
(OAc)
4
4
4
4
4
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
[
[
[
[
[
[
e]
unambiguously confirmed by X-ray crystallographic
0
1
analysis.[
16]
Encouraged by this exciting result,
f]
e,g]
e,g]
e,g]
e,g]
further experiments were conducted to improve the
reaction efficiency. By monitoring the reaction
process, we found that the α,β-unsaturated imine
intermediate was generated quickly through
denitrogenation and acetoxy migration of triazole 2a.
In contrast, the indole and α,β-unsaturated imine
reacted more slowly. Furthermore, the dimer
12
2
Rh (OAc)4 1,2-DCE
1
1
1
3
4
5
Rh
Rh
Rh
2
2
2
(piv)
(oct)
(adc)
4
1,2-DCE
1,2-DCE
1,2-DCE
4
4
[a]
General reaction conditions: 1a (0.2 mmol), 2a (0.4
mmol), [Rh] (2 mol%), solvent (2 mL), N
2
atmosphere, 4 h.
[b]
Isolated yield.
1
2
(
=
Scheme 1c, aza-Diels–Alder product, R = OAc, R
[c]
2
a (0.2 mmol).
tolyl) of the α,β-unsaturated imine also appeared
[d]
[e]
[f]
Carried out in a sealed tube.
,2-DCE (1 mL).
,2-DCE (0.67 mL).
when the reaction time was increased. To inhibit this
side reaction, the quantity of triazole 2a was
increased to two equivalents, and the reaction time
was controlled at 4 h (entry 2). Solvent screening
revealed that 1,2-DCE was the best solvent (entries
1
1
[g]
[
Rh] (1 mol%).
1
,1,2-TCE
=
1,1,2-trichloroethane,
DCM
=
dichloromethane, piv = pivalate, oct = octanoate, adc = 1-
adamantanecarboxylate.
2
−5). The reaction temperature was also evaluated,
and 60 °C was found to be optimal. Crucially, higher
or lower temperatures resulted in reduced yields
(
entries 2, 6−9). To our surprise, the yield of 3aa
Subsequently, the reaction scope in relation to
indole 1 was also investigated under the optimized
conditions (Scheme 3). Initially, the effect of the
protecting group on the indole nitrogen was examined.
Electron-donating substituents on the nitrogen were
tolerated in the reactions, giving related products
increased to 81% when we reduced the amount of
solvent (entries 8, 10, and 11). In addition, the yield
further increased to 85% when using 1 mol%
Rh
were tested, but Rh
entries 13−15).
Using the optimized reaction conditions (Table 1,
2
(OAc)
4
(entry 12). Various rhodium(II) catalysts
2
(OAc) gave the best yield
4
(
3
ba–da in moderate yields. In contrast, substrates
with electron-withdrawing groups such as Boc and
phenyl or without protecting groups on the nitrogen
did not afford the expected products. A methyl group
at the 4-, 5-, or 6-position of the indole was well-
entry 12), the substrate scope of triazole 2 was
investigated (Scheme 2). In addition to triazole
acetate (2a), the benzoate (2b) and phenylacetate (2c)
2
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Advanced Synthesis & Catalysis
10.1002/adsc.201901599
tolerated, and the resulting cyclization products were
produced in 72−81% yield (3ea–ga). In addition,
several products containing valuable halogens and
ethers were synthesized conveniently (3ha–la,
be obtained, and the starting material dienophiles
were recovered. The above results indicate that the
electron density of the dienophile has a significant
effect on the success of the [4 + 2] cyclization.
5
1−78%). Excitingly, compound 3ma, which has a
Subsequently, several derivatization reactions were
performed to illustrate the utility of the product.
Ketone 4 was obtained when 3aa was hydrolyzed
under weakly basic conditions, and this ketone was
converted into compound 5 when treated with a
phosphonium ylide (Scheme 4a). Additionally, in the
complicated tetracyclic skeleton and two adjacent
quaternary carbons, was obtained in 92% yield.
presence of MeLi and PhNTf , the OAc group of 3aa
2
could be easily transformed into OTf, which is one of
the most useful functional groups in metal-catalyzed
cross-coupling reactions. For example, compound 8
was obtained in 56% yield when triflate 6 was reacted
with (triisopropylsilyl)acetylene under Sonogashira
coupling reaction conditions (Scheme 4b).
Scheme 4. Synthetic transformation of 3aa.
Scheme 2. Substrate Scope of Triazole 2.
A mechanism for this reaction is proposed in
Scheme 5 on the basis of the present results and
[12,14,15]
previous reports.
In the presence of the
rhodium(II) catalyst, the denitrogenation of triazole 2
leads to the formation of α-imino rhodium carbene A.
Subsequently, the carbonyl oxygen attacks the
electrophilic carbene carbon, and the subsequent
elimination of the rhodium catalyst delivers α,β-
unsaturated imine C rapidly. In path a, compound 3 is
obtained directly through the aza-Diels–Alder
reaction of indole 1 and intermediate C. Alternatively,
the C3 of indole could attack the α,β-unsaturated
imine, delivering Michael addition intermediate D.
The subsequent intramolecular nucleophilic addition
produces the final product: 3 (path b). Notably, when
indole 1n was employed in the transformation, no
pyrido[2,3-b]indole structure was formed, and,
instead, compound 9 was obtained in 60% yield [Eq.
3
(
1)]. We speculate that when the R group of
intermediate D is hydrogen, the intramolecular proton
transfer is faster than the cyclization. Thus, the
stepwise pathway may be the most likely route.
Scheme 3. Substrate Scope of Indole 1.
In addition to indoles, other dienophiles such as 3-
methylbenzofuran, 5-methoxy-3-methylbenzofuran,
3
-methylbenzo[b]thiophene, 1H-indene, and 1,2-
dihydronaphthalene were also reacted with the
triazole 1a under the optimized reaction conditions.
Unfortunately the corresponding products could not
3
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Advanced Synthesis & Catalysis
10.1002/adsc.201901599
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[
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synthon, as used in previous reports. This
transformation is particularly attractive because of its
operational simplicity, readily available substrates,
construction of sterically demanding quaternary
centers, and convenient derivatization using a triflate
derivative. Further studies of the construction of
complex polycyclic compounds using triazoles and
their applications in natural product synthesis are
under investigation.
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̅
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Experimental Section
General procedure for the synthesis of 3
To a mixture of indole 1 (0.20 mmol) and triazole 2 (0.40
mmol) in anhydrous DCE (1 mL) under N , Rh (OAc)
2 2 4
(
0.002 mmol) was added. The reaction mixture was stirred
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with NaOH, pH 8.0–9.0) to afford product 3.
4
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Acknowledgements
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We are grateful to the National Natural Science Foundation of
China (21871235, 21801224, 21901230), Natural Science
Foundation of Zhejiang Province (LQ18B020009), Fundamental
Research Funds of Zhejiang Sci-Tech University (2019Y003), and
Science Foundation of Zhejiang Sci-Tech University (ZSTU)
under Grant No. 18062301-Y for supporting this work.
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