Modular 2,3-diaryl-2: H -azirine synthesis from ketoxime acetates via Cs2CO3-mediated cyclization

Article abstract of DOI:10.1039/c8ob00923f

A modular 2H-azirine synthesis from ketoxime acetates via Cs₂CO₃-mediated cyclization has been developed. The reaction utilizes easily available starting materials and provides a general synthetic route to 2,3-diaryl-2H-azirines in good to excellent yields under mild conditions, which is complementary to the conventional approaches for the synthesis of 2H-azirines. A gram-scale reaction was performed to demonstrate the scale-up applicability of this synthetic method. Importantly, 2H-azirines can be efficiently converted to various azaheterocycles.

Full text of DOI:10.1039/c8ob00923f

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
benzene
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PAPER
Modular 2,3-diaryl-2H-azirines synthesis from ketoxime acetates
via Cs2CO3-mediated cyclization
Mi-Na Zhao,a,b Wei Zhang,a Xu-Cai Wang,a Ying Zhang,a De-Suo Yang,b and Zheng-Hui Guan*,a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
(a) Rhodium-catalyzed synthesis of 2H-azirines from -diazo oxime ethers
A modular 2H-azirines synthesis from ketoxime acetates via
Cs2CO3-mediated cyclization has been developed. The reaction
utilizes easily available starting materials, and provides a general
synthetic route to 2,3-diaryl-2H-azirines in good to excellent yields
under mild conditions, which is complementary to the
conventional approaches for the synthesis of 2H-azirines. A gram-
scale reaction was performed to demonstrate the scaled-up
applicability of this synthetic method. Importantly, the 2H-azirines
can be efficiently converted to various azaheterocycles.
OMe
O
N
O
N
Rh2(OAc)4 (2 mol%)
DCE, 60 o
R1
R2
R1
OMe
R2
C
N2
(b) Iridium-catalyzed synthesis of 2H-azirines from isoxazol-5(4H)-ones
N
O
N
[{IrCl(coe)2}2] (5 mol%)
CPME, 100 o
Ar
O
R1
Ar
R2
R1
C
R2
(c) This work: Cs2CO3-mediated synthesis of 2H-azirines from ketoxime acetates
Introduction
OAc
N
N
Cs2CO3 (20~120 mol%)
DMF, 80 o
R3
R2
R3
2H-Azirines, the smallest nitrogen-containing heterocycles, are
important skeletal units that are prevalent in many natural
products, medicines, and antibiotic molecules (Fig. 1).1 2H-
Azirines are also important building blocks for the synthesis of
heterocyclic compounds due to their high reactivity derived
from a high ring-strain energy.2 Various types of ring-opening
reactions of 2H-azirines, such as nucleophilic addition
reactions of nucleophilic reagents to the C=N bond,3 the metal
carbenoid mediated C-N bond ring opening process,4 and
visible-light-induced C-C bond cleavage of 2H-azirines,5 have
R1
R1
C
R2
readily accessible starting materials
mild conditions
up to 93% yield
gram-scale synthesis
Scheme 1 Various approaches for the synthesis of 2H-azirines.
been developed for the synthesis of different azaheterocycles
in the past decades. Conventionally, 2H-azirines was
synthesized
by
either
the
thermal/photochemical
rearrangement of vinyl azides or the nucleophilic cyclization of
ketoxime tosylates.2c,d However, the former method typically
suffers from using explosive azides under harsh conditions. For
the latter method, the Beckmann rearrangement is often a
side reaction in the presence of the strong acidic tosyl chloride,
thus resulting in low reaction efficiency.6 Recently, rhodium-
catalyzed synthesis of 2H-azirines via Wolff rearrangement of
α-diazo oxime ethers has been developed by the group of Park
(Scheme 1a).4a Iridium-catalyzed decarboxylative ring
contraction of isoxazol-5(4H)-ones for the synthesis of 2H-
azirines has also been developed by Ohe and co-workers
(Scheme 1b).7a However, these reactions require rare and
noble transition metal catalysts. Therefore, a novel and
effective method for the synthesis of 2H-azirines, especially
from readily available starting materials, is still highly desirable.
Fig. 1 Representative biologically active molecules containing 2H-azirine fragment.
a.Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of
Ministry of Education, Department of Chemistry & Materials Science, Northwest
University, Xi’an 710127, P. R. China
b.Shaanxi Key Laboratory of Phytochemistry, College of Chemistry and Chemical
Engineering, Baoji University of Arts and Sciences, Baoji, 721013, P. R. China
*E-mail: guanzhh@nwu.edu.cn
Electronic Supplementary Information (ESI) available: Detailed experimental
procedures, characterization data and copies of NMR spectra for all products. See
DOI: 10.1039/x0xx00000x
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Table 2 Screening of different 1,2-diphenylethanone oxime carboxylatesa
Ketoximes and their derivatives are versatile building
blocks in organic synthesis due to their readily accessible,
stable to air and moisture, and nontoxic characters.8 In recent
years, transition-metal (Pd-, Rh-, Ru-, Co-) catalyzed
transformation of ketoxime carboxylates via N-O bond
cleavage for the construction of azaheterocycles is an active
area of research.9 Recently, Cu-, Fe-catalyzed coupling
reactions of ketoxime acetates have been developed by our10
as well as other groups.11 In connection with our interest in
oximes and synthesis of 2H-azirines, we envisioned that an
intramolecular cyclization of ketoxime acetates might be a
straightforward route for the synthesis of 2H-azirines. In this
paper, we describe a simple and practical Cs2CO3-mediated
cyclization of ketoxime acetates for the synthesis of 2,3-diaryl-
2H-azirines (Scheme 1c). This method provides an efficient and
operationally simple protocol which is complementary to the
conventional approaches for the synthesis of 2H-azirines.
Entry
1
Substrate 1
Product 2a
Yield (%)
93
O
N
O
1a
2
3
78
75
O
N
O
Results and discussion
Initially, we investigated our reaction by employing 1,2-
1c
diphenylethanone oxime acetate 1a as the model substrate in
o
the presence of CuI catalyst and K3PO4 base in DMF at 80 C
O
Ph
N
O
under argon atmosphere. Encouragingly, the desired 2,3-
4
5
6
73
64
85
diphenyl-2H-azirine 2a was obtained in 50% yield (Table 1,
1d
Table 1 Optimization of reaction conditionsa
Entry
Base
Solvent
T (oC)
Yield (%)
1b
2
K3PO4
K3PO4
KOH
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMA
DMSO
NMP
DMF
DMF
DMF
DMF
80
80
80
80
80
80
80
80
80
80
80
80
80
80
100
60
80
80
50
55
10
30
12
26
48
93
57
<5
<5
39
46
47
44
81
87
68
3
aReaction conditions: 1,2-diphenylethanone oxime carboxylates 1 (0.2 mmol),
Cs2CO3 (0.24 mmol), DMF (1 mL), 80 oC, Ar; isolated yield.
4
K2CO3
Na2CO3
CsOAc
CsOH
5
6
entry 1). Notably, the 1,2-diphenylethanone was observed as
the main byproduct in the reaction. Further investigation
demonstrated that 2a could be obtained in 55% yield without
CuI catalyst (Table 1, entry 2). We speculated that the
formation of 2,3-diphenyl-2H-azirine 2a involved a base-
mediated intramolecular nucleophilic cyclization of 1a. Then,
various bases were screened to examine if they could improve
the reaction efficiency. Inorganic bases, such as KOH, K2CO3,
Na2CO3, CsOAc, CsOH, and Cs2CO3 were screened (Table 1,
entries 3-8). Cs2CO3 proved to be the most effective to give the
desired 2H-azirine 2a in 93% yield (Table 1, entry 8). Organic
bases were less effective and gave 2a in lower yields (Table 1,
entries 9-11). Optimization of different solvents, such as DMA,
DMSO, and NMP suggested that DMF was still the most
suitable solvent for the reaction (Table 1, entries 12-14).
Finally, the reaction temperature was also varied, and 80 °C
7
8
Cs2CO3
DBU
9
10
11
12
13
14
15
16
17c
18d
DABCO
Et3N
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
aReaction condition: 1,2-diphenylethanone oxime acetate 1a (0.2 mmol), base
(0.24 mmol), solvent (1 mL), 80 oC, Ar; isolated yield. bCuI (10 mol%) was added as
the catalyst. cCs2CO3 (0.2 mmol).. dCs2CO3 (0.1 mmol).
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gave the best result (Table 1, entries 15-16). Notably, acetates with electron-withdrawing substituents on the Ar1,
decreasing the amount of the Cs2CO3 resulted in a long such as fluoro, chloro, bromo, and sensitive functional groups
reaction time and lower yield (Table 1, entries 17-18).
such as iodo were well tolerated and afforded the
2s in good yields. In addition, 2-
Subsequently, different 1,2-diphenylethanone oxime corresponding 2H-azirines 2p
-
carboxylates were investigated to test their reactivity (Table 2). naphthyl substituted ketoxime acetate 1t also reacted well to
Propionate, tert-butyrate, benzoate, and pentafluorobenzoate give the desired 2H-azirine 2t in 92% yield.
1b-1e showed relatively lower reactivity to afford 2,3-
diphenyl-2H-azirine 2a in 64-78% yields (Table 2, entries 2-5).
Additionally, when 1,2-diphenylethanone oxime tert-butyl
carbonate 1f was used as the substrate, 2,3-diphenyl-2H-
azirine 2a was obtained in 85% yield (Table 2, entry 6). These
results indicate that the carboxylate group at the 1,2-
diphenylethanone oxime has slight influence on the reaction.
Table 3 Substrate scope with respect to the Ar1a
Scheme 2 Cs2CO3-Mediated practical synthesis of 3-methyl-2-aryl-2H-azirine. Reaction
conditions: 1 (0.2 mmol), Cs2CO3 (0.24 mmol), DMF (1 mL), 60 oC, Ar; isolated yield.
To expand the applicability of the present method, α-aryl-
substituted aliphatic ketoxime acetates were investigated
under slightly modified conditions (Scheme 2). Gratifyingly, the
1-phenylpropan-2-one oxime acetate 1u showed good
reactivity and gave the desired 2H-azirine 2u in 70% yield.
Para-chloro substituted ketoxime acetates 1v participated in
the reaction smoothly and afforded the corresponding product
2v in good yield. Notably, when 1,1-diphenylpropan-2-one
oxime acetate 1w was employed as the substrate, the 3-
methyl-2,2-diphenyl-2H-azirine 2w was obtained in 84% yield.
N
N
N
2a
, 93%
2g
, 92%
N
2h
, 81%
N
N
2i, 89%
2j
2k
, 87%
N
, 87%
N
N
Table 4 Substrate scope with respect to the Ar2a
Et
tBu
MeO
2l, 90%
2n
, 85%
2m, 86%
OMe
OMe
N
N
N
F
Cl
b
b
2o, 85%
2p
2q
, 75%
, 70%
N
N
N
Br
I
b
2s, 61%b
2r
, 72%
2t
, 92%
aReaction condition: ketoxime acetates 1 (0.2 mmol), Cs2CO3 (0.24 mmol), DMF (1
mL), 80 oC, Ar; isolated yield. bCs2CO3 (20 mol%).
With the optimized reaction conditions established, a
series of ketoxime acetates were investigated for extending
the substrate scope. The reaction showed good functional
group tolerance and proved to be a general method for the
facile construction of 2H-azirines. First, the effect of
substituents on Ar1 adjacent to oxime acetate was examined
(Table 3). Ketoxime acetates with electron-donating groups on
the Ar1, such as methyl, cyclohexyl, ethyl, tert-butyl, and
aReaction conditions: ketoxime acetates 1 (0.2 mmol), Cs2CO3 (20 mol%), DMF (1
mL), 80 oC, Ar; isolated yield. bCs2CO3 (0.24 mmol).
methoxyl afforded the corresponding 2H-azirines 2g
-
2o in 81-
Next, substrates bearing various functional groups on the
92% yields. ortho-Methyl- or methoxyl-substituted ketoxime Ar2 were investigated to extend the substrate scope, and the
acetates proceeded smoothly to give the desired products 2h
2i 2o in 81-89% yields, suggesting that the transformation was methyl, tert-butyl, or halogen groups such as fluoro, chloro,
insensitive to steric hindrance of the substrates. Ketoxime and bromo on the Ar2 reacted smoothly to give the
-
results are summarized in Table 4. Ketoxime acetates with
,
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corresponding 2H-azirines 2x-2ad in 73-88% yields. These
Acknowledgements
results indicated that the electronic effects of the groups on
the Ar2 have little influence on the reaction. Furthermore,
when 2,3-dichloro substituted ketoxime acetate 1ae was
employed as disubstituted substrate, the reaction reacted well
to give the desired 2H-azirine 2ae in 81% yield.
This work was supported by generous grants from the National
Natural Science Foundation of China (21622203 and
21472147), China Postdoctoral Science Foundation Funded
Project (2017M620466), and the PhD Scientific Research
Project of Baoji University of Arts and Sciences (ZK2018057).
To demonstrate the synthetic utility of this reaction, a
gram-scale (5 mmol, 1.27 g) reaction was performed under the
standard conditions. The desired 2,3-diphenyl-2H-azirine 2a
was obtained in 82% yield (Scheme 3). The reaction is
particularly useful, because the resulting product 2,3-diaryl-
Notes and references
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2 have been widely applied in direct and efficient
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oxazine , dibenzocarbazole , and pyridine 7 could be easily
5
6
synthesized according to the reported procedures.4b,5c,12 These
representative transformations clearly demonstrate the
versatilities of 2H-azirines in organic chemistry.
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5
Scheme 3 A gram-scale preparation and further transformations of 2,3-diphenyl-2H-
azirine 2.
6
7
Conclusions
In summary, we have successfully developed an efficient
Cs2CO3-mediated cyclization of ketoxime acetates for the
synthesis of 2,3-diaryl-2H-azirines. The reaction proceeded
through an intramolecular eliminatve cyclization of ketoxime
acetates under mild conditions. The reaction provides a facile
and straightforward method for the synthesis of valuable 2,3-
diaryl-2H-azirines in good to excellent yields. Further studies
on the scope of the reaction and applications are in progress in
our laboratory.
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Conflicts of interest
The authors declare no competing financial interest.
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