A one-pot synthesis of [1,2,3]triazolo[1,5-a]quinoxalines from 1-azido-2-isocyanoarenes with high bond-forming efficiency

Article abstract of DOI:10.1039/c6cc08543a

An efficient approach to prepare 1,2,3-triazolo[1,5-a]quinoxaline scaffolds, starting from 1-azido-2-isocyanoarenes and terminal acetylenes or substituted acetaldehydes, has been developed. In the case of trifluoromethylation triggered cyclization, four chemical bonds, including two C-C and two C-N bonds, were formed consecutively without isolating the triazole intermediate. In addition, these triazo-fused products were readily transformed into diversified quinoxaline derivatives via rhodium-catalyzed carbenoid insertion reactions.

Full text of DOI:10.1039/c6cc08543a

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Reactivity differences of Sc
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N@C2n (2n
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A one-pot synthesis of [1,2,3]triazolo[1,5-a]quinoxalines
from 1-azido-2-isocyanoarenes with high bond-forming
efficiency
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Dengke Li, Tingting Mao, Jinbo Huang,* and Qiang Zhu*
www.rsc.org/
8
An efficient approach to prepare the 1,2,3-triazolo[1,5- could be easily prepared by Jiao’s method from anilines. However,
9
a]quinoxaline scaffold, starting from 1-azido-2-isocyanoarenes their synthetic application is underdeveloped.
and terminal acetylenes or substituted acetaldehydes, has been
developed. In the case of the trifluoromethylation triggered
cyclization, four chemical bonds, including two C-C and two C-
N bonds were formed consecutively without isolating the triazole
intermediate. In addition, these triazo-fused products were
readily transformed into diversified quinoxaline derivatives via
rhodium-catalyzed carbenoid insertion reactions.
The 1,2,3ꢀtriazoleꢀcontaining heterocycles have been widely applied
in medicinal chemistry due to their diverse pharmacological
1
properties, including anticancer, antituberculosis, and antibacterial.
For example, the scaffold of [1,2,3]triazolo[1,5ꢀa]ꢀquinoxaline exists
2
a
in Gꢀproteinꢀcoupled Niacin receptor 109A as well as inhibitors
2
bꢀc
binding to benzodiazepine and adenosine receptors.
Classical
methods for the construction of this tricyclic system require multiple
2
steps through triazole intermediates followed by cycloamidation or
3
Pictet–Spengler cyclization starting from 2ꢀnitrophenylazide or 1ꢀ
Scheme 1. Approaches to 1,2,3ꢀtriazolo[1,5ꢀa]quinoxalines.
fluoroꢀ2ꢀnitrobenzene, respectively (a and b, Scheme 1). In 2010,
Gong and Yu developed a oneꢀstep cascade cycloaddition of
substituted 1,1,1ꢀtrifluoroꢀNꢀ(2ꢀiodophenyl)butꢀ3ꢀynꢀ2ꢀimines with
sodium azide followed by copperꢀcatalyzed Ullmann coupling to
We designed this study with 1ꢀazidoꢀ2ꢀisocyanoꢀ3,5ꢀ
10
dimethylbenzene 1a and ethynylbenzene 2a through CuAAC.
Under the optimized reaction conditions involving Cu(OAc) as a
4
a
2
construct two heterocyclic rings. Later on, Cai and Ding also
applied the strategy for the synthesis of [1,2,3]ꢀtriazolo[1,5ꢀ
a]quinoxalinꢀ4(5H)ꢀones from Nꢀ(2ꢀiodophenyl) propiolamides (c,
catalyst in THF/DMSO (see table S1, ESI), [1,2,3]triazolo[1,5ꢀ
a]quinoxaline 3a was obtained in 73% yield (Scheme 2). It was
notable that no triazole intermediate was detected during the reaction,
probably due to the expeditiousness of intramolecular isocyanide
insertion into the cuprate intermediate after CuAAC. Various
aromatic, heteroaromatic as well as aliphatic substituted terminal
alkynes underwent the tandem cyclization smoothly to generate the
corresponding products 3a−3j in moderate to good yields.
Derivatives of propargyl alcohol and propargyl amine and even
unprotected butꢀ3ꢀynꢀ1ꢀol could be served as coupling partners (3kꢀ
4b
Scheme 1). However, these linear approaches are not convenient
for product diversification.
Isocyanide has been recognized as a powerful C1 synthon in the
synthesis of heterocycles. Besides acting as a C1 synthon, the inbuilt
functional group in isocyanide can also participate in the ring
5
construction together with the isocyano moiety (NC). For example,
biaryl isocyanides have been extensively studied in the synthesis of
5
ꢀ6
various C6 substituted phenanthridines. Continuing our interest on
3
o). Benzimidazole and amino acid derivatives bearing a terminal
alkyne moiety were also compatible with the reaction and produced
p and 3q in 73% and 72% yield, respectively.
7
heterocycle synthesis using functionalized isocyanide, we
anticipated that the title compounds could be accessed convergently
from 1ꢀazidoꢀ2ꢀisocyanoarenes and terminal alkynes through copperꢀ
catalyzed azideꢀalkyne [3+2] cycloaddition (CuAAC) followed by
isocyanide insertion (d, Scheme 1). The 1ꢀazidoꢀ2ꢀisocyanoarenes
3
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Journal Name
Cu(OAc)2 (10 mol%)
Na2CO3 (1.2 equiv)
N
N
bearing various substituents such as Me, OMe, OPh, F, Cl, and Br at
different positions of the benzene ring underwent the tandem
N3
N
R
+
R
NC
THF/DMSO = 4/1
annulation
smoothly
to
generate
the
corresponding
N
6
5 oC, Ar, 12-20 h
1
a
2
3
trifluoromethylated products (6a-6c, 6e-6i, and 6k-6o) in 49ꢀ78%
yields. The 1ꢀazidoꢀ2ꢀisocyanoꢀ4ꢀmethylbenzene and 2ꢀazidoꢀ1ꢀ
isocyanonaphthalene were less compatible with the reaction, giving
6d and 6j in 41% and 43% yield, respectively.
N
N
N
N
N
N
R
N
N
N
S
N
N
N
N
3
a, R = H, 73%
3
3
3
b, R = p-Me, 69%
3
e, 57%
3f, 40%
c, R = m-Me, 66%
t-
d, R = p- Bu, 70%
N
N
N
N
R
N
R
N
3k, R = OMe, 83%b
3l, R = OPh, 53%
3m, R = OBz, 84%
3n, R = TsNH, 42%
3
g, R = ethylbenzenyl, 56%b
3h, R = n-butanyl, 59%b
N
N
3
3
i, R = cyclohexanyl, 71%b
j, R = cyclopropanyl, 67%b
3
o, R = CH2OH, 58%b
N
N
N
N
N
N
N
N
O
O
N
N
BocHN
3
p, 73%
3q, 72%
a
Scheme 2. Synthesis of [1,2,3]triazolo[1,5ꢀa]quinoxalines
[
a
Reaction conditions: 1a (0.1 mmol, 1.0 equiv), 2 (1.5 equiv),
Cu(OAc) (10 mol%), Na CO (1.2 equiv) in THF/DMSO (v/v = 4/1,
.5 mL), 65 C, 12 h, in Ar. 2 (3.0 equiv), 20 h.]
2
2
3
o
b
1
Considering the versatility of bi(hetero)aryl isocyanide in
11
heterocycle synthesis, we hope that the isocyano group could
survive the triazoleꢀforming step, so that the introduction of different
substituents at the C4 position of the cyclized product would be
easier. Then, an alternative triazole synthesis without using transition
12
metal catalyst was tested. As expected, the isocyano group
remained unchanged after cycloaddition of 1a with 2ꢀ
phenylacetaldehyde 4a in the presence of DBU (10 mol%) in DMSO
o
at room temperature (25 C), furnishing 1ꢀ(2ꢀisocyanoꢀ3,5ꢀ
dimethylphenyl)ꢀ4ꢀphenylꢀ1Hꢀ1,2,3ꢀtriazole 5a in excellent isolating
yield (92%) (see table S2, ESI). Next, trifluoromethylation of 5a was
investigated preferentially due to the great importance of CF
3
ꢀ
containing molecules in agrochemicals, pharmaceuticals, and
1
3ꢀ14
specialty materials.
II) (1.2 equiv) in the presence of TBAI (10 mol%) in DMSO gave
C4 trifluoromethylated [1,2,3]triazolo[1,5ꢀa]quinoxaline 6a in good
Satisfyingly, treating 5a with Togni reagent
(
1
5
Scheme
3
Oneꢀpot
synthesis
of
4ꢀ(trifluoromethyl)ꢀ
yield (85%). Next, direct trifluoromethylation of 5a without its
purification was investigated, because both the triazoleꢀforming and
cyclotrifluoromethylation steps were highꢀyield and clean in the
a
[
[
1,2,3]triazolo[1,5ꢀa]quinoxalines
Reaction conditions: 1a (0.1 mmol, 1.0 equiv), 4 (0.15 mmol 1.5
a
o
equiv), DBU (10 mol%), DMSO (2.0 mL), 25 C, 3 h, in Ar; then
same solvent (DMSO). After stirring a mixture of 1a, 4a, and DBU
o
o
TBAI (10 mol%) and Togni (II) (1.2 equiv) were added, 85 C, 10 h,
(
10 mol%) in DMSO at 25 C for 3 h, Togni reagent (II) (1.2 equiv)
b
in Ar. 1a (0.15 mmol) and 4 (2.0 equiv), pyrrolidine (2.0 equiv),
and TBAI (10 mol%) were added to the reaction mixture which was
o
o
toluene (2.0 mL), 50 C, 20 h; then mꢀCPBA (1.5 equiv for 6w, 3.0
further stirred at elevated temperature (85 C) for 10 h. The desired
equiv for 6x), 0 to 25 °C, 5 h; then, TBAI (10 mol%), Togni (II) (1.2
equiv), in DMSO (2.0 mL), 85 C, 10 h.]
product 6a was isolated in 78% total yield for this oneꢀpot twoꢀstep
protocol in which four chemical bonds, including two CꢀC and two
CꢀN bonds, were formed consecutively (Scheme 3). It was
noteworthy that 6a could not be accessed by reacting 3a with Togni
reagent (II).
o
Next, a variety of substituted acetaldehydes were tested in
reactions with 1a. Phenylacetaldehydes substituted with electronꢀ
withdrawing 4ꢀF, 4ꢀCl, 4ꢀBr, 3ꢀBr, and 2ꢀBr or electronꢀdonating
Then, the feasibility of the current oneꢀpot tandem reaction was
4
3
ꢀMe and 4ꢀOCH had little influence on product formation,
studied with
a range of 1ꢀazidoꢀ2ꢀisocyanoarenes 1 and 2ꢀ
delivering the corresponding products 6p-6v in 45ꢀ66% yields.
phenylacetaldehyde 4a (Scheme 3). The 1ꢀazidoꢀ2ꢀisocyanoarenes
2
| J. Name., 2012, 00, 1-3
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Unfortunately, aliphatic aldehydes such as nꢀpropaldehyde and 3ꢀ
phenylpropionaldehyde were not successful in producing the
desired products under the standard conditions. To our delight,
1
2i
by following a modified condition for triazole synthesis, C3
methyl and benzyl substituted 4ꢀ(trifluoromethyl)ꢀ
1,2,3]triazolo[1,5ꢀa]quinoxalines 6w and 6x were also generated,
albeit in 43% and 26% yields.
[
Besides cascade trifluoromethylationꢀannulation, arylationꢀ and
alkylationꢀannulation of 5a also took place smoothly by following
literature methods, resulting arylated and alkylated 6y and 6z in
1
1a, 11e
excellent yields (Scheme 4).
It is reasonable to anticipate that
triazolo arylisocyanide like 5a can serve as valuable intermediate for
the synthesis of C4 diversified [1,2,3]triazolo[1,5ꢀa]quinoxalines.
a
Scheme 5 Diversification through Rh(II) catalysis
a
[
Reaction conditions: 3a or 6a (0.1 mmol, 1.0 equiv), NuH (2.0
Scheme 4 Synthesis of aryl and alkyl substituted [1,2,3]triazolo[1,5ꢀ equiv), [Rh] (2.5 mol%), toluene (2.0 mL), 120 C, 24 h, in Ar.]
a]quinoxalines from 5a
o
In summary, we have developed a cascade annulation approach to
As we know, CꢀH and XꢀH insertion into carbenoid intermediate
has been well established as a powerful method for CꢀC and CꢀX
diversely substituted 1,2,3ꢀtriazolo[1,5ꢀa]quinoxalines convergently
from 1ꢀazidoꢀ2ꢀisocyanoarenes and terminal alkynes or substituted
acetaldehydes. The reaction occurred in one pot under mild
conditions with high bondꢀforming efficiency and good functional
16a
bond formation starting from diazo compounds.
Recently,
electronꢀdeficient 1,2,3ꢀtriazoles, such as Nꢀsulfonylꢀ1,2,3ꢀtriazoles
and pyridotriazoles, were used as alternative progenitors of carbene group tolerance. Furthermore, densely functionalized quinoxalines
16bꢀc
in organic synthesis.
aforementioned triazoleꢀcontaining products as carbene precursors,
a was studied using Rh (esp) as a catalyst in the presence of
various NꢀH nucleophiles. Satisfyingly, denitrogenative ringꢀ
To test the possibility of using the
were also obtained from the triazoleꢀcontaining products through
rhodiumꢀcatalyzed NꢀH or OꢀH insertion to the carbenoid
intermediates. These reactions demonstrate that 1ꢀazidoꢀ2ꢀ
isocyanoarenes are a valuable class of functionalized isocyanides in
6
2
2
opening of the triazole moiety and subsequent carbene insertion into the synthesis of Nꢀheterocycles.
NꢀH bond of Nꢀbutyl amide indeed happened, offering
1
7a
trifluoromethylated quinoxaline derivative 7a in 67% yield. Other
amides, such as benzamide and 4ꢀmethylbenzenesulfonamide were
also suitable substrates in this reaction (7b-7c). In addition to amides,
alcohols and carboxylic acids could also be used as coupling partners
Acknowledgement
We are grateful to the National Science Foundation of China (NSFC)
(
21272233, 21472190 and 21532009) and the Science Foundation of
Guangdong Province, China (2014A030313796) for financial
17b
via OꢀH insertion,
giving the corresponding ether or ester support.
derivatives 7d-7g in moderate yields (Scheme 5). Importantly, less
electronꢀdeficient 3a could also be transformed to 7h in 65% yield.
Notes and references
State Key Laboratory of Respiratory Disease, Guangzhou
Institutes of Biomedicine and Health, Chinese Academy of
Sciences, 190 Kaiyuan Avenue, Guangzhou 510530 (China),
Fax: (+86) 20ꢀ3201ꢀ5299.
Eꢀmail: zhu_qiang@gibh.ac.cn; huang_jinbo@gibh.ac.cn.
Electronic Supplementary Information (ESI) available: See DOI:
1
0.1039/c000000x/
This journal is © The Royal Society of Chemistry 20xx
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