Synthesis of 4-Isoxazolines via Visible-Light Photoredox-Catalyzed [3 + 2] Cycloaddition of Oxaziridines with Alkynes

Article abstract of DOI:10.1021/acs.orglett.7b03369

A method for [3 + 2] cycloaddition of oxaziridines with alkynes to form 4-isoxazolines via visible-light photoredox catalysis is described. This method is a greener, atom-economical reaction that tolerates various functional groups and provides good to excellent yield. Moreover, the cyclization products can be conveniently converted into tetrasubstituted allylic alcohols and enamines. A mechanistic study suggests that the reaction involves photoredox-catalyzed in situ generation of a nitrone from the oxaziridine by SET.

Full text of DOI:10.1021/acs.orglett.7b03369

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
Synthesis of 4‑Isoxazolines via Visible-Light Photoredox-Catalyzed
[3 + 2] Cycloaddition of Oxaziridines with Alkynes
Gwang Seok Jang, Junggeun Lee, Jungseok Seo, and Sang Kook Woo*
Department of Chemistry, University of Ulsan, 93 Daehak-Ro, Nam-Gu, Ulsan 44610, Korea
S
* Supporting Information
ABSTRACT: A method for [3 + 2] cycloaddition of oxaziridines
with alkynes to form 4-isoxazolines via visible-light photoredox
catalysis is described. This method is a greener, atom-economical
reaction that tolerates various functional groups and provides good
to excellent yield. Moreover, the cyclization products can be
conveniently converted into tetrasubstituted allylic alcohols and
enamines. A mechanistic study suggests that the reaction involves photoredox-catalyzed in situ generation of a nitrone from the
oxaziridine by SET.
1,3-Dipolar cycloadditions have long been powerful synthetic
transformations in organic chemistry because they provide
efficient and atom-economical methods for construction of
various five-membered heterocycles.1 Nitrones are widely used
1,3-dipoles in [3 + 2] cycloadditions as they generate useful
products, isoxazolidines or 4-isoxazolines, which are not only
important scaffolds for various biologically active compounds but
are also precursors of broadly useful building blocks. Tradition-
ally, nitrones have been prepared by condensation of aldehydes
or ketones with N-hydroxylamines2 and via oxidation of amines,
N-hydroxylamines, and imines.3 However, traditional methods
often give poor yields and require harsh reaction conditions, such
as high temperature or a strong oxidant, and also display
problems in the handling of unstable nitrones.4 Recently, in situ
generation of nitrones from N-hydroxy amines or N-hydroxyl-α-
amido sulfones has been developed to avoid an isolation step for
the nitrone.5 Despite advances with these synthetic methods,
improvement is still desirable for a mild, efficient synthetic
protocol.
Three-membered heterocycles, which have high ring strain,
are reactive intermediates in organic synthesis. In particular, they
have been developed as precursors of 1,3-dipoles or radical ions
in [3 + 2] cycloaddition reactions through generation by thermal,
photo, and transition-metal catalysis. The ring-opening of 2H-
azirine has been reported through visible-light-mediated photo-
redox-catalyzed single-electron transfer (SET) (Scheme 1,a).
Scheme 1. Cycloadditions with Three-Membered
Heterocycles Using Visible Light Photoredox Catalysis
Visible-light-mediated photoredox catalysis is an emerging
research area in organic synthesis since visible light provides an
inexpensive, infinite, and clean energy source. Many researchers
have focused on applying visible photoredox catalysis to organic
transformations.6 Recently, [3 + 2] cycloaddition reactions using
in situ generated 1,3-dipoles via visible-light photoredox catalysis
have been reported for the synthesis of five-membered
heterocycles.7 In particular, Rueping and co-workers developed
an in situ generation of nitrones from hydroxyl amines leading to
[3 + 2] cycloadditions with alkenes to deliver isoxazolidines.7d
4-Isoxazolines are important five-membered heterocycles
found in many biologically active natural products and
pharmaceutical compounds.8 Through cleavage of the weak
N−O bond, 4-isoxazolines serve as precursors to 1,3-amino
alcohols, β-amino acids, β-lactams, and a variety of N-
heterocycles. A common synthetic method for the preparation
of 4-isoxazolines is [3 + 2] cycloaddition of nitrones with alkynes
or allenes.5a,d,9 Recently, an intramolecular cyclization of
propargylic N-hydroxylamines has been developed.10
Xiao and co-workers have developed the synthesis of pyrroles
and oxazoles from 2H-azirine by a visible photoredox-catalyzed
formal [3 + 2] cycloaddition reaction.7e,f Wang and co-workers
reported the three-component cyclization of 2H-azirines, alkynyl
bromides, and oxygen via visible light-mediated photoredox
catalysis.7g Based on these previous reports on photoredox-
catalyzed functionalization of three-membered heterocycles, we
envisioned generation of nitrones from oxaziridines by visible-
light photoredox catalysis for use as 1,3-dipoles in [3 + 2]
cycloadditions. The organic sensitized photolysis of oxaziridines
has been studied;11 however, to the best of our knowledge,
Received: October 29, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03369
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
visible-light-mediated photolysis of oxaziridines and the
subsequent cyclization reaction have not been described. Herein,
we present a highly efficient visible light-mediated photoredox-
catalyzed [3 + 2] cycloaddition of oxaziridines with alkynes to
form 4-isoxazolines in a greener and step atom-economical
method (Scheme 1,b).
Scheme 2. Substrate Scope of Various Alkynes
Initially, we explored the [3 + 2] cycloaddition reaction of
oxaziridine 1a with dimethyl acetylenedicarboxylate (DMAD) 2a
in ethyl acetate using 5 mol % visible-light photoredox catalysts
under irradiation with 6 W blue LEDs. In a photoredox catalyst
screening, the cycloaddition of oxaziridine 1a with DMAD 2a in
the presence of Fukuzumi acridinium salt (Acr+-Mes) for 24 h
gave the desired 4-isoxazoline 3a in 55% yield (Table 1, entry 5),
a
Table 1. Optimization of the Reaction Conditions
a
Reaction conditions as given in Table 1, entry 16; reported yields are
b
for isolated material. 2a (1.0 mmol), Acr+-Mes (2 mol %), EtOAc
(0.5 M) was used as solvent, without H2O, 60 °C. See the Supporting
Information for details.
b
2a
additive
(equiv)
yield
(%)
entry (equiv)
cat. (mol %)
solvent (M)
EtOAc (0.2)
was increased from 0.1 to 0.5 M (Table 1, entries 8−10). Using
water as an additive further increased the yield to 77%. To create
a greener reaction, the catalyst loading and amount of DMAD
were decreased. As a result, the yield slightly increased to 78%
with 1 mol % of catalyst loading and 2 equiv of DMAD (Table 1,
entry 16). Control experiments revealed that both light and
Fukuzumi acridinium salt were necessary for the cycloaddition to
occur (Table 1, entries 17 and 18). After detailed screening of the
reaction conditions, we found the best result was obtained with 2
equiv of DMAD and 1 mol % of Fukuzumi acridinium salt (Acr+-
Mes) in acetonitrile (0.5 M) at room temperature to afford our
desired product 3a in 78% yield.
After optimization of the reaction conditions, we next
examined the [3 + 2] cycloaddition of oxaziridine 1a with
different electron deficient alkynes 2 (Scheme 2). A variety of
alkynes (2b−e) was tolerated and gave the corresponding 4-
isoxazolines (3b−e) in moderate to good yields. For alkynes with
larger substituents such as diethyl acetylenedicarboxylate 2b and
di-tert-butyl acetylenedicarboxylate 2c, the cycloaddition was
inhibited by steric hindrance of the bulky ester groups. In
particular, the cycloaddition of oxaziridine 1a with di-tert-butyl
acetylenedicarboxylate 2c gave the corresponding adduct 3c in
only 29% yield. Because of the low solubility of dibenzoylace-
tylene 2d in MeCN, the corresponding 4-isoxazoline 3d was
formed in a moderate 43% yield. The monoactivated methyl
phenylpropiolate 2e gave the corresponding adduct 3e in 35%
yield under modified reaction conditions.
1
5.0
fac-Ir(ppy)3
(5)
nd
2
5.0
Ru(bpy)3Cl2
(5)
EtOAc (0.2)
trace
3
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
3.0
2.0
2.0
2.0
rose bengal (5)
eosin Y (5)
EtOAc (0.2)
EtOAc (0.2)
EtOAc (0.2)
CH2Cl2 (0.2)
toluene (0.2)
CH3NO2 (0.2)
CH3CN (0.2)
CH3CN (0.5)
CH3CN (0.1)
CH3CN (0.5)
CH3CN (0.5)
CH3CN (0.5)
CH3CN (0.5)
CH3CN (0.5)
CH3CN (0.5)
CH3CN (0.5)
nd
nd
55
52
13
71
72
74
65
77
76
75
76
78
nd
nd
4
5
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (5)
Acr+-Mes (3)
Acr+-Mes (1)
Acr+-Mes (1)
Acr+-Mes (1)
6
7
8
9
10
11
12
13
14
15
16
17
H2O (2.5)
H2O (2.5)
H2O (2.5)
H2O (2.5)
H2O (2.5)
H2O (2.5)
H2O (2.5)
c
18
Acr+-Mes (1)
a
Reaction conditions: 1a (0.2 mmol), 2a (quantity noted), catalyst
(quantity noted), additive (250 mol %), solvent (quantity noted) with
6 W blue LEDs irradiation at room temperature for 24 h under argon
in pressure tubes. Isolated yield by flash column chromatography. In
b
c
the absence of light source. nd = not detected.
and the structure was confirmed by single-crystal X-ray analysis
(Scheme 2 and Supporting Information). Other catalysts did not
work in the designed reaction with the exception of Ru(bpy)3Cl2,
which gave the product in a trace amount (Table 1, entries 1−4).
To generate the active radical cation species, oxidation of the
oxaziridine is essential. However, oxaziridines have a higher
oxidation potential than other amines because of the oxaziridine
oxygen. We speculate that the high oxidization ability of
Fukuzumi acridinium salt in the excited state (+ 2.06 V) is
sufficient to oxidize the oxaziridine.12 The influence of solvent on
the cyclization efficiency was also significant; when acetonitrile
and nitromethane were chosen as the solvent, the yield was
enhanced to 72% and 71%, respectively (Table 1, entries 6−9).
We chose acetonitrile as the reaction solvent because it is a more
commonly used solvent than nitromethane. The yield was
slightly increased to 74% when the concentration of the solution
Next, we turned our attention to the [3 + 2] cycloaddition of a
broad range of substituted oxaziridines 1 with DMAD 2a
(Scheme 3). Under the optimized reaction conditions, various
oxaziridines 1 containing electron-neutral, electron-rich, or
electron-deficient functional groups such as alkyl, alkoxy, and
halogen were reacted with acetylenedicarboxylate 2a to afford the
corresponding 4-isoxazolines (3f−ah) in good to excellent
yields.
Para-substituted alkylaryl oxaziridines (1f−l) gave the desired
products (3f−l) in 85−88% yield. Slightly lower yields were
observed for electron-deficient halogen-substituted aryl oxazir-
idines (1j−l). Notably, para- and meta-substituted methyl- or
chloroaryl oxaziridines (1f,l,m,n) did not influence steric
hindrance in the [3 + 2] cycloaddition. However, ortho-
substituted methyl- or chloroaryl oxaziridines (1o,p) gave the
corresponding 4-isoxazolines 3o and 3p in 37% and 19% yield,
B
DOI: 10.1021/acs.orglett.7b03369
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Substrate Scope of Various Oxaziridines
Scheme 4. (a) Gram-Scale Reaction. (b) Hydrogenation of 4-
Isoxazolines. (c) Thermal Rearrangement of 4-Isoxazolines
1a, which indicates involvement of an SET mechanism (detailed
in the SI). Next, radical-trapping experiments were undertaken.
The addition of TEMPO or γ-terpinene to the cycloaddition
completely suppressed the reaction, indicating that the reaction
involves a radical process (detailed in the SI).
a
Reaction conditions as given in Table 1, entry 16; reported yields are
b
c
for isolated material. 48 h. EtOAc (0.5 M) was used as solvent,
without H2O, 48 h. See the Supporting Information for details.
On the basis of the reported literature11,12 and the above
control experiments, a plausible mechanism for the photo-
induced [3 + 2] cycloaddition is depicted in Scheme 5. Initially,
respectively, which shows that nitrone production and the
subsequent cycloaddition with ortho-substituted aryl oxaziridines
were influenced by steric hindrance. Remarkably, 3,4-disub-
stituted aryl oxaziridine 1q and 4,4′-disubstituted aryl oxazir-
idines (1r−ab) worked well, providing the corresponding
products 3r−ab in 41−90% yield. In the case of electron-
donating methoxy-substituted aryl oxaziridines (1i,w,z), the
methoxy group accelerated oxaziridine oxidation as well as
decomposition of oxaziridines to ketones. Therefore, we
observed ketones as byproducts while generating the corre-
sponding 4-isoxazolines 3i,w,z in 41−62% yield. In addition, the
3,3-diaryl oxaziridines can be successfully extended to 3-alkyl-3-
aryl, 3-aryl, and 3-vinyl oxaziridines (2ac−ah) for visible-light-
mediated cycloaddition. These oxaziridines gave the correspond-
ing 4-isoxazolines (3ac−ah) in 48−65% yields.
Scheme 5. Proposed Reaction Mechanism
To demonstrate the efficiency of this method, we conducted
the gram-scale synthesis of 4-isoxazoline 3a. The [3 + 2]
cycloaddition of oxaziridine 1a (1 g, 3.36 mmol) with DMAD 2a
(0.84 mL, 6.72 mmol) afforded the 4-isoxazoline 3a in 70% yield
using modified reaction conditions for the larger scale (Scheme
4, a). 4-Isoxazolines (3a,t) were easily converted in good yield to
the tetrasubstituted allylic alcohols (4a,t) by N−O bond cleavage
followed by deamination and reduction of ketone in the present
of zinc powder and NH4Cl as hydrogen source (Scheme 4, b).13
In addition, thermal rearrangement of the 4-isoxazolines (3a,t)
led directly to enamines (5a,t) (Scheme 4, c).14
To gain a better understanding of the reaction mechanism,
additional experiments were carried out. First, electrochemical
analysis and Stern−Volmer luminescence quenching studies
were performed. The oxidation potential of oxaziridine 1a was
measured by cyclic voltammetry to be +2.06 V vs SCE, indicating
that the oxaziridine can be oxidized by Fukuzumi acridinium salt
in the excited state (+2.06 V vs SCE).15 In Stern−Volmer
luminescence quenching experiments, the excited state of the
photoredox catalyst (Acr+-Mes*) was quenched by oxaziridine
the acridinium catalyst (Acr+-Mes) is excited (Acr+-Mes*) under
visible-light irradiation. Single-electron oxidation of oxaziridine
1a by the photocatalyst excited state (Acr+-Mes*) generates
radical cation I along with reduced photocatalyst (Acr•-Mes).
Radical cation I is then converted to the nitrone radical II in a
ring-opening process. Single-electron reduction of nitrone
radical II with Acr•-Mes regenerates the photoredox catalyst
(Acr+-Mes) and forms nitrone III, which can be isolated by
column chromatography under standard reaction conditions.
Subsequent [3 + 2] cycloaddition of nitrone III with DMAD 2a
provides 4-isoxazoline 3a.
Next, the quantum yield of the photoinduced [3 + 2]
cycloaddition of 1a with 2a was determined to be Φ = 0.149
0.039.16 We conducted on−off experiments and reaction-
monitoring experiments by 1H NMR and HPLC. These results
showed that continuous irradiation of visible light is essential for
the cycloaddition, a radical chain propagation mechanism is not
C
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Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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of 4-isoxazolines in a visible-light photoredox-catalyzed [3 + 2]
cycloaddition of oxaziridines with alkynes. This method involves
in situ generation of nitrones from oxaziridines by SET. The [3 +
2] cycloaddition tolerates various functional groups and provides
good to excellent yields. The cyclization products can be
conveniently converted into tetrasubstituted allylic alcohols and
enamines. Moreover, a plausible reaction mechanism was
proposed based on electrochemical analysis, control experi-
ments, and reaction monitoring. Development of an asymmetric
synthesis of 4-isoxazolines is currently underway.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b03369.
Electrochemical and photophysical measurements and
mechanism studies; experimental procedures; spectro-
scopic data for all new compounds (1H NMR, 13C NMR,
19F NMR, IR, HRMS), including images of NMR spectra
and crystallographic data for 3a (PDF)
Accession Codes
CCDC 1563016 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: woosk@ulsan.ac.kr.
ORCID
Sang Kook Woo: 0000-0002-8808-037X
Notes
(11) (a) Iwano, Y.; Kawamura, Y.; Miyoshi, H.; Yoshinari, T.; Horie, T.
Bull. Chem. Soc. Jpn. 1994, 67, 2348. (b) Iwano, Y.; Kawamura, Y.; Horie,
T. Chem. Lett. 1995, 24, 67. (c) Ohba, Y.; Kubo, K.; Sakurai, T. J.
Photochem. Photobiol., A 1998, 113, 45.
(12) (a) Fukuzumi, S.; Kotani, H.; Ohkubo, K.; Ogo, S.; Tkachenko, N.
V.; Lemmetyinen, H. J. Am. Chem. Soc. 2004, 126, 1600. (b) Ohkubo, K.;
Mizushima, K.; Iwata, R.; Souma, K.; Suzuki, N.; Fukuzumi, S. Chem.
Commun. 2010, 46, 601.
(13) (a) Khan, F. A.; Dash, J.; Sudheer, C.; Gupta, R. K. Tetrahedron
Lett. 2003, 44, 7783. (b) Aschwanden, P.; Geisser, R. W.; Kleinbeck, F.;
Carreira, E. M. Org. Lett. 2005, 7, 5741. (c) Kelly, S. M.; Lipshutz, B. H.
Org. Lett. 2014, 16, 98.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
(NRF-2016R1D1A1B03934062) and the Priority Research
Centers Program (NRF-2009-0093818) funded by the Ministry
of Education of the Republic of Korea, and this work has been
performed with the financial assistance of the Ulsan Green
Environment Center (UGEC), Korea.
(14) (a) Chukanov, N. V.; Popov, S. A.; Romanenko, G. V.; Reznikov,
V. A. Tetrahedron 2008, 64, 7432. (b) Chukanov, N. V.; Popov, S. A.;
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D
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