Metal-free C3-alkoxycarbonylation of quinoxalin-2(1H)-ones with carbazates as ecofriendly ester sources

Article abstract of DOI:10.1007/s11426-018-9446-1

Quinoxaline-3-carboxylates and analogues are prevalent key structural motifs in bioactive natural products and synthetic drugs. However, the practical protocol for preparation of these motifs from simple raw materials under mild conditions remains rare. In this article, we report a facile protocol for the efficient preparation of various quinoxaline-3-carbonyl compounds (30 examples, 63%–92%) through oxidation coupling of quinoxalin-2(1H)-ones with readily available carbazates (or acyl hydrazines) in the presence of K₂S₂O₈ as an oxidant in metal- and base-free conditions. When tert-butyl carbazate was used as the coupling reagent, the decarboxylation product 3-(tert-butyl)-1-methylquinoxalin-2(1H)-one was obtained. The application of this process into a gram-scale synthesis can be easily accomplished. Mechanistic investigations reveal that the functionalization of quinoxalin-2 (1H)-ones via a free-radical pathway.

Full text of DOI:10.1007/s11426-018-9446-1

SCIENCE CHINA
Chemistry
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ARTICLES•
https://doi.org/10.1007/s11426-018-9446-1
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Metal-free C3-alkoxycarbonylation of quinoxalin-2(1H)-ones with
carbazates as ecofriendly ester sources
1
1
1
1
2
1
Long-Yong Xie , Sha Peng , Tai-Gang Fan , Yan-Fang Liu , Meng Sun , Li-Lin Jiang ,
3
3
1*
Xing-Xing Wang , Zhong Cao & Wei-Min He
1
Department of Chemistry, Hunan University of Science and Engineering, Yongzhou 425100, China;
2
Key Laboratory of Theoretical Organic Chemistry and Functional Molecule of Ministry of Education, Hunan University of Science and
Technology, Xiangtan 411201, China;
3
Hunan Provincial Key Laboratory of Materials Protection for Electric Power and Transportation, Changsha University of Science and
Technology, Changsha 410114, China
Received December 16, 2018; accepted February 25, 2019; published online March 7, 2019
Quinoxaline-3-carboxylates and analogues are prevalent key structural motifs in bioactive natural products and synthetic drugs.
However, the practical protocol for preparation of these motifs from simple raw materials under mild conditions remains rare. In
this article, we report a facile protocol for the efficient preparation of various quinoxaline-3-carbonyl compounds (30 examples,
63%–92%) through oxidation coupling of quinoxalin-2(1H)-ones with readily available carbazates (or acyl hydrazines) in the
presence of K S O as an oxidant in metal- and base-free conditions. When tert-butyl carbazate was used as the coupling reagent,
2
2
8
the decarboxylation product 3-(tert-butyl)-1-methylquinoxalin-2(1H)-one was obtained. The application of this process into a
gram-scale synthesis can be easily accomplished. Mechanistic investigations reveal that the functionalization of quinoxalin-2
(1H)-ones via a free-radical pathway.
quinoxalin-2(1H)-ones, alkoxycarbonylation, carbazates, synthetic methods, metal-free
Citation:
Xie LY, Peng S, Fan TG, Liu YF, Sun M, Jiang LL, Wang XX, Cao Z, He WM. Metal-free C3-alkoxycarbonylation of quinoxalin-2(1H)-ones with
carbazates as ecofriendly ester sources. Sci China Chem, 2019, 62, https://doi.org/10.1007/s11426-018-9446-1
1
Introduction
achieved on the C3-functionalization of quinoxalin-2(1H)-
ones, which include the C–H bond alkylation [2], arylation
[3], acylation [4], amination [5], azidation [6], benzylation
[7], trifluoromethylation [8] and phosphonation [9] of qui-
noxalin-2(1H)-ones.
Quinoxaline-3-carboxylates not only exhibit excellent
biological and pharmaceutical activities, but also act as va-
luable synthetic precursors for the construction of diverse
functionalized quinoxaline derivatives [1a, 10]. Due to the
importance of quinoxaline-3-carboxylates, various efforts
have been devoted to construct such motifs over the past
decades. Yoneda et al. [11] have pioneered a cyclo-
condensation between N-phenylbenzene-1,2-diamine with
diethyl 2-oxomalonate in xylene under reflux (Scheme 1(a)).
Quinoxalin-2(1H)-one and its derivatives are important ni-
trogen-containing heteroaromatics and are prevalent key
structural motifs in bioactive natural products and synthetic
drugs [1]. Indeed, the functional-groups appended to the
quinoxalinone and the quinoxalinone core together affect the
pharmaceutical and biological activities of the molecule, not
just the quinoxalinone core itself. Therefore, the research of
the structure-activity relationships calls for these derivatives
containing diverse substituents on the quinoxalinone units.
In the past few years, considerable progress has been
*Corresponding author (email: weiminhe2016@yeah.net)
©
Science China Press and Springer-Verlag GmbH Germany, part of Springer Nature 2019
chem.scichina.com link.springer.com

2
Xie et al. Sci China Chem
In 2013, Wróbel et al. [12] reported a method for the efficient
synthesis of various N-aryl-substituted quinoxaline-3-car-
boxylates through base-promoted cyclocondensation of N-
aryl-substituted 2-nitrosoanilines with 1,3-dialkyl malonates
2 Experimental
2
.1 General information
Unless otherwise specified, all reagents and solvents were
obtained from commercial suppliers and used without further
purification. All reagents were weighed and handled in air at
(Scheme 1(b)). In 2016, Zhang et al. [13] described acetic
acid-mediated cyclocondensation between o-benzenedia-
mine with functionalized 1,3-dicarbonyl compounds for the
preparation of various quinoxaline-3-carbonyl compounds in
good yields (Scheme 1(c)). Recently, Yu and co-workers [14]
first reported the one-step synthesis of N-substituted
quinoxaline-3-carboxylates via radical cyclization of alkyl
1
room temperature. H NMR spectra were recorded at
1
3
400 MHz, C NMR spectra were recorded at 100 MHz and
1
9
F NMR spectra were recorded at 376 MHz by using a
Bruker Avance 400 spectrometer (Germany). Chemical
shifts were calibrated using residual undeuterated solvent as
an internal reference. High resolution mass spectrometry
2
1
-(N-substituted carbamoyl)-2-chloroiminoacetates (Scheme
(d)) [15]. Although the above mentioned protocols are
(
(
HRMS) data were obtained on a Bruker micrOTOF Focus II
ESI Mode, Germany).
useful for the preparation of a series of quinoxaline-3-car-
boxylates, more general and eco-friendly methods are still in
great demand. Considering the abundant and readily avail-
able quinoxalin-2(1H)-ones, the synthesis of N-substituted
quinoxaline-3-carboxylates through the direct introduction
of an ester group doubtlessly represents a facile strategy.
Recently, carbazates are widely used in alkoxycarbonylation
reaction, given that carbazates are stable and readily avail-
able and they provides innocuous nitrogen as the side-
product [16]. However, the alkoxycarbonylation of quinox-
alin-2(1H)-ones with carbazates as the green ester source
2
.2 General procedure for the synthesis of 3
N-substituted quinoxalin-2(1H)-one 1 (0.2 mmol), alkyl
carbazate 2 (0.4 mmol), and K S O (0.6 mmol) in MeCN
2
2
8
(1.0 mL) were added to a 10 mL-Schlenk tube. The mixture
was stirred at 90 °C, the progress of the reaction was mon-
itored by thin layer chromatography (TLC). The reaction
typically took 8–12 h. Upon completion, the reaction was
cooled to room temperature, water (5 mL) was then added to
the reaction mixture. It was extracted with CH Cl (5 mL×3)
[
17], to the best of our knowledge, has not been disclosed
yet.
As part of our continuous efforts in the ecofriendly organic
2
2
and the organic extracts were dried over anhydrous Na SO ,
2
4
filtered and concentrated under reduced pressure. The crude
product was purified by column chromatography on silica
gel (eluent: petroleum ether/EtOAc) to obtain quinoxaline-3-
carboxylates 3.
synthesis [18], we have reported the green construction of
functionalized N-heteroaromatic compounds [19]. Herein,
we disclose an efficient and facile methodology for the in-
troduction of an ester group into quinoxalin-2(1H)-ones
through metal-free alkoxycarbonylation with carbazates as
an eco-friendly ester source (Scheme 1(e)).
3 Results and discussion
In our initial investigation, the alkoxycarbonylation of N-
methylquinoxalin-2(1H)-one (1a) with methyl carbazate
(2a) was chosen as a model reaction to identify the optimal
reaction conditions (Table 1). The treatment of 1a and 2a (2
equiv.) with K S O (3 equiv.) as the oxidant at 90 °C for 12 h
2
2
8
in MeCN afforded the expected product 3aa in 86% yield
Table 1, entry 1). Screening of a series of oxidants, such as
Na S O , (NH ) S O , TBHP, H O and PhI(OAc) (entries
(
2
2
8
4 2
2
8
2
2
2
2
–6), revealed that K S O was the best oxidant for this al-
2 2 8
koxycarbonylation reaction. The reaction medium also
played a critical role. Among the solvents (DCE, PhCF3,
THF, DMF, DMSO and EtOH) examined, MeCN was
proved to be the suitable reaction medium for this reaction
(entries 1, 7–12). The influence of oxidant loading and re-
action temperature was then investigated (entries 13–16).
The study on the optimal amount of K S O (entries 13, 14)
2
2
8
suggested that 3 equiv. (entry 1) was a suitable amount for
the reaction. Both increasing and lowering the reaction
temperature resulted in slightly lower yields (entries 15, 16).
Scheme 1 Preparation of quinoxaline-3-carboxylates (color online).

Xie et al. Sci China Chem
Table 2 Reaction scope
3
a)
a)
Table 1 Optimization of the reaction conditions
b)
Entry
Base (equiv.)
K S O (3)
Solvent
MeCN
MeCN
Temp. (°C) Yield (%)
1
2
90
90
86
2
2
8
Na S O (3)
trace
2
2
8
(
NH ) S O
4
2
2
8
3
MeCN
90
trace
(3)
4
5
6
7
8
9
TBHP (3)
H O (3)
MeCN
MeCN
MeCN
DCE
90
90
90
90
90
90
90
90
90
90
90
100
80
11
N. R.
N. R.
18
2
2
PhI(OAc) (3)
2
K S O (3)
2
2
8
K S O (3)
PhCF3
THF
13
2
2
8
K S O (3)
trace
N. R.
N. R.
N. R.
79
2
2
8
1
0
1
K S O (3)
DMF
2
2
8
1
K S O (3)
DMSO
EtOH
MeCN
MeCN
MeCN
MeCN
2
2
8
12
13
14
15
16
K S O (3)
2 2 8
K S O (2.5)
2
2
8
K S O (4)
86
2
2
8
K S O (3)
82
2
2
8
K S O (3)
63
2
2
8
a) Unless otherwise specified, the reactions were conducted in a vial in
the presence of 1a (0.1 mmol), 2a (0.2 mmol), solvent (0.5 mL),12 h. b)
1
Estimated by H NMR spectrum using diethyl phthalate as an internal
reference.
With the optimal reaction conditions in hand (Table 1,
entry 1), the generality of this developed alkoxycarbonyla-
tion reaction was then explored. As shown in Table 2, the
reaction is insensitive to the substituents at the N1-position.
Quinoxalin-2(1H)-ones with various alkyl chains and diverse
isomeric structure of substituents reacted well with 2a to
generate the expected esterification products in good to ex-
cellent yields (3aa–3ka). It is important to note that the
unsaturated C–C bonds (alkenyl and alkyl), ester and ben-
zoyl groups on the N1-substituent of quinoxalin-2(1H)-ones
(3da–3fa and 3ha–3ka) were well compatible, enabling an
opportunity for further transformations. N-phenylquinox-
alin-2(1H)-one (1l) is also a suitable substrate for the current
transformation, delivering the desired product (3la) in 85%
yield. No matter whether the phenyl ring of quinoxalin-2
(1H)-ones is substituted with electron-rich or electron-poor
groups, all of them afford the corresponding products in good
to excellent yields (3ma–3wa). Importantly, a series of
functional groups, such as fluoro (3ma and 3na), chloro (3oa
and 3pa), bromo (3qa and 3ra), trifluoromethyl (3sa), ester
a) Conditions: 1 (0.2 mmol), 2 (0. 3 mmol), K S O (0.6 mmol), MeCN
(1 mL), 90 °C, 8–12 h; isolated yields are reported.
2
2
8
an ester source, the corresponding product (3ab) was formed
in 78% yield. With tert-butyl carbazate (2c) as the ester
source, the decarboxylative coupling product 3-(tert-butyl)-
(3ta), benzoyl (3ua), nitro (3va) and methyl (3wa) groups,
were well tolerated. When ethyl carbazate (2b) was used as

4
Xie et al. Sci China Chem
1
-methylquinoxalin-2(1H)-one (3ac) was formed in 85%
yield. The present method is also applicable for the con-
struction of a series of 3-acyl quinoxalin-2(1H)-ones in the
presence of acyl hydrazines as acylating reagents, and the
desired products (3ad–3ah) were generated in good yields.
However, when 4-phenylsemicarbazide, 4,4-diphenylsemi-
carbazide acethydrazide, or trifluoroacetic acid hydrazide
was used as the coupling partner, no reaction was observed.
To demonstrate the practicality of the present protocol, a
gram-scale reaction was performed under the standard con-
ditions. Delightedly, this scale-up reaction provided the
corresponding product 3aa in 81% isolated yield (1.41 g)
(Scheme 2(a)). Furthermore, the important synthetic pre-
cursor 4a was easily obtained in excellent yield through the
hydrolysis of 3aa (Scheme 2(b)). The compound 3xa is a
pivotal precursor for the preparation of a commercialized
c-Met kinase inhibitor 5 [20], which could be synthesized
through the developed metal-free alkoxycarbonylation re-
action (Scheme 2(c)).
To elucidate the reaction mechanism, some control ex-
periments were carried out as shown in Scheme 3. The re-
action of 1a with 2a under the standard reaction conditions in
the presence of radical scavengers (TEMPO or BHT) led to
no conversion of substrate 1a (Scheme 3(a, b)). When 2
equiv. of 1,1-diphenylethylene was added to the reaction
mixture, the radical adduct 6 was formed in 32% yield. Ta-
ken together, these results suggested that this developed re-
action might involve an ester free-radical as the reaction
intermediate (Scheme 3(c)).
Scheme 2 Gram-scale synthesis and subsequent transformation (color
online).
On the basis of the above-mentioned experiment results
and the related reports [2a,16a], a possible reaction me-
chanism was illustrated in Scheme 4. Initially, the methyl
carbazate (2a) reacted with sulfate radical anion (in situ
generated by the hemolytic-bond-cleavage of persulfate) to
form the ester free-radical with the concomitant release of
HSO anion and molecular nitrogen. Next, the addition of
4
Scheme 3 Control experiments (color online).
ester free-radical to N-methylquinoxalin-2(1H)-one (1a)
formed the free-radical intermediate A, which subsequently
underwent hydrogen abstraction/oxidation by sulfate radical
anion to produce the desired product 3aa [9a].
4
Conclusions
In conclusion, we have developed a simple and practical
protocol for the efficient synthesis of a series of quinoxaline-
Scheme 4 Proposed Mechanism (color online).
3
(
-carboxylates through alkoxycarbonylation of quinoxalin-2
1H)-ones with carbazates as eco-friendly ester sources. The
main advantages of this method are summarized as follows:
1) the accessibility of the raw materials; (2) metal-free and
attractive both in academic and industrial perspectives.
(
Acknowledgements This work was supported by the Hunan Provincial
Natural Science Foundation of China (2019JJ20008) and the Construct
Program of Applied Characteristic Discipline in Hunan University of Sci-
ence and Engineering.
base-free conditions; (3) good functional-group tolerance
and the feasibility of scale-up; (4) good region- and che-
moselectivity. These features make our protocol potentially

Xie et al. Sci China Chem
Conflict of interest The authors declare that they have no conflict of
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