Metal-Free Addition of Boronic Acids to Silylnitronates

Article abstract of DOI:10.1055/s-0039-1690845

We report for the first time a metal-free addition of boronic acids to silylnitronates to afford oxime derivatives through aryl transfer on the carbon nitrogen double bond. A reaction mechanism has been proposed in relation with a DFT study on the key aryl transfer. This arylation process is effective for cycloalkenyl nitro derivatives leading to oximes that may be oxidatively converted into 3-Arylisoxazole derivatives.

Full text of DOI:10.1055/s-0039-1690845

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© Georg Thieme Verlag Stuttgart · New York
2020, 31, A–E
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M. D. Kerim et al.
Letter
Synlett
Metal-Free Addition of Boronic Acids to Silylnitronates
Mansour Dolé Kerima
Pakoupati Boyodeb
Julian Garrecc
R23SiO
O
Ar
–
OH
N
R23SiO
O
N
+
B
NO2
R23SiCl
base
N
ArB(OH)2
+
O
R1
R1
R1
Ar
–
MeCN
80 °C
Ar
B
R1
OH
Laurent El Kaïm*a
0
0
0
0-0
0
0
1-5
7
2
9-
8
0
1
0
OH
DFT calculations on
the aryl-transfer step
R1 = cycloalkenyl
8 examples, yields: 24–68%
a Laboratoire de Synthèse Organique, CNRS, Ecole Polytechnique,
ENSTA Paris - UMR 7652, Institut Polytechnique de Paris, 828 Bd
des Maréchaux, 91128 Palaiseau, France
laurent.elkaim@ensta-paristech.fr
b Laboratoire de Chimie Organique et des Substances Naturelles,
Département de Chimie, Faculté des Sciences, Université de Lomé,
BP 1515, Lomé, Togo
c Unité Chimie et Procédés, ENSTA Paris – UMR 7652, Institut Poly-
technique de Paris, 828 Bd des Maréchaux, 91128 Palaiseau,
France
Received: 29.12.2019
Accepted after revision: 10.02.2020
Published online: 10.03.2020
Petassis reaction:
R1
R2
N +
O
N
DOI: 10.1055/s-0039-1690845; Art ID: st-2019-d0703-l
R
R1
R3
OH
R3
ArB(OH)2
NH
R2
+
Ar
–
B
Ar
O
Abstract We report for the first time a metal-free addition of boronic
acids to silylnitronates to afford oxime derivatives through aryl transfer
on the carbon nitrogen double bond. A reaction mechanism has been
proposed in relation with a DFT study on the key aryl transfer. This ary-
lation process is effective for cycloalkenyl nitro derivatives leading to ox-
imes that may be oxidatively converted into 3-arylisoxazole derivatives.
OH
HO OH
Our project
OH
O
B
–
–
R23SiO
O
R23SiO
O
+
R23SiCl
base
NO2
N
N
ArB(OH)2
+
Ar
R1
R1
R1
Key words nitro, silyl nitronate, boronic acid, oxime, isoxazole
OH
Ar
With the development of the Suzuki–Miyaura reaction,
boronic acids have been recognized among the most useful
arylation reagents.1 Whereas such reaction requires an acti-
vation of the aryl group through a transmetalation process
with various transition metals, in the absence of such met-
al, the boronic acid may still transfer its aryl moiety after
prior conversion into a boronate intermediate. Highlighted
with the discovery of the Petassis reaction,2 this seminal
Mannich-type behavior of boronic acids has given the im-
petus for the discovery of many other metal-free arylations
and vinylation over the last ten years.3 However, boronic ac-
ids remain moderately nucleophilic species, which often re-
quire the installation of an intramolecular aryl transfer
through ate-complex formation for efficient couplings.4
This is the case of the Petassis reaction where the lack of
electrophilicity of simple imine derivatives may be over-
come by neighboring hydroxy groups (Scheme 1). Follow-
ing our interest in nitro chemistry and considering known
nucleophilic additions on nitronic acid derivatives (Nef-
type reactions),5 we surmised that nitronic acid derivatives
might further interact with boronic acids to afford transient
ate complexes prone to undergo an intramolecular aryl
transfer onto the C–N unsaturated bond (Scheme 1).
N
R1
Scheme 1 Metal-free boronic acid addition to C–N double bonds
Due to the instability of nitronic esters (leading to oxim-
es and aldehydes)6 or acylnitronates (leading to nitrile oxide
at ambient temperature),7 we decided to focus our study on
silylnitronic esters, a relatively stable family of compounds,
readily prepared by silylation of nitro derivatives with trial-
kylsilyl chlorides, whose chemistry has been largely popu-
larized by the studies of Seebach at the beginning of the
1980s.8
Due to poor hydrolytic stability of trimethylsilyl nitro-
nates, a bulkier tert-butyl-dimethylsilyl group was selected
for this study. The tert-butyl-dimethylsilylnitronate 2a was
thus prepared in quantitative yield by treatment of ni-
tromethylcyclohexene 1a with 1.1 equiv of tert-butyl-di-
methylsilylchloride in the presence triethylamine (1.1
equiv) in dry acetonitrile. Nitronate 2a could be filtered
rapidly through a short silica gel column without signifi-
cant degradation. When 2a was heated in acetonitrile to-
gether with 4-ethylphenylboronic acid (3a) under argon,
we were delighted to observe the formation of oxime 4a re-
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
M. D. Kerim et al.
Letter
Synlett
sulting from the expected aryl transfer (Scheme 2). The low
17% isolated yield could be slightly increased by selecting
toluene as solvent at 80 °C. Finally, the best conditions were
obtained using a twofold excess of boronic acid 3a in aceto-
nitrile at 70 °C (Scheme 2).
The coupling is only efficient for 1-nitromethylcy-
cloalkenyl derivatives as shown by the various cyclohexen-
yl, cycloheptenyl, and cyclooctenyl oximes 4 formed in
moderate isolated yields from the corresponding silylnitro-
nates. Electron-poor aryl boronic acids did not appear to be
effective as shown by the unproductive addition of 4-cy-
anophenyl boronic acid. Under prolonged heating, only
degradation of 2a was observed, without any formation of
4k. All oximes were obtained as single diastereomers, how-
ever, we failed to determine their Z- or E-stereochemistry.
Surprisingly, the silylnitronate of nitromethylbenzene did
not form the expected adduct 4i with 4-ethylboronic acid.
This lack of reactivity was also observed for 1-nitropro-
pane, showing the high sensitivity of this reaction to elec-
tronic factors. Though these results limit the scope of this
reaction, it should be noted that 1-nitromethylcycloalkenyl
derivatives are easily prepared on large scale from their re-
lated cycloalkanone derivatives using a Knoevenagel-type
condensation with nitromethane.9 Indeed, this strategy
could be readily adapted to access more complex substitut-
ed oximes choosing among the relatively wide range of
commercially available cycloalkanones. Thus oxime 4h
(Scheme 3) was prepared starting from 4-phenylcyclohexa-
none, which was efficiently converted into nitro intermedi-
ate 1d on heating in a toluene/nitromethane mixture in the
presence of a catalytic amount of N,N dimethylethylenedi-
amine (Scheme 4).
–
R1R22SiO
O
+
OH
B(OH)2
N
N
+
solvent
(0.5 M)
2a
3a
4a
Et
Et
17%
0%
R
R
R
R
R
R
R
1 = t-Bu, R2 = Me, 3a (1 equiv), acetonitrile (50 °C, 90 min)
1 = t-Bu, R2 = Me, 3a (1 equiv), DMF (50 °C, 90 min)
1 = t-Bu, R2 = Me, 3a (1 equiv), toluene (80 °C, 60 min)
1 = t-Bu, R2 = Me, 3a (2 equiv), toluene (80 °C, 60 min)
1 = t-Bu, R2 = Me, 3a (2 equiv), acetonitrile (50 °C, 60 min)
1 = t-Bu, R2 = Me, 3a (2 equiv), acetonitrile (70 °C, 30 min)
1 = R2 = Ph, 3a (2 equiv), acetonitrile (70 °C, 30 min)
31%
61%
31%
68%
0%
Scheme 2 Optimized conditions for the formation of 4a
Changing the tert-butyldimethyl silyl group by a triphe-
nylsilyl group (Scheme 2) did not result in the formation of
4a under the same conditions.
To evaluate the scope of this reaction, a set of represen-
tative tert-butyldimethylsilyl nitronates 2a–c was prepared
in quantitative yields from the corresponding nitro deriva-
tives using the initial procedure, followed by rapid filtration
through a short silica plug. The results of their reaction
with different arylboronic acids are displayed in Scheme 3.
NO2
Me2N
NH2
O
OH
–
NO2
(5 mol%)
toluene, Δ
N
t-BuMe2SiO
O
+
4h
t-BuMe2SiCl
+ MeNO2
10 equiv
N
ArB(OH)2
+
Ph
Ph
R
R
Ar
Et3N
MeCN
70 °C, 30 min
3
R
75% 1d
1
2
4
1a: R = 1-cyclohexenyl
1b: R = 1-cycloheptenyl
1c: R = 1-cyclooctenyl
Scheme 4 4-Phenylcyclohexanone conversion into 1d
OH
OH
OH
N
A plausible reaction mechanism for the conversion of
the silylnitronates into oximes is presented in Scheme 5. Af-
ter complexation of the silylnitronate with the boronic acid,
aryl transfer affords an unstable intermediate B, whose
fragmentation leads to chelated nitroso intermediate C. Fur-
N
N
Et
X
X = OMe: 4b 52%
X = CN: 4k 0%
4c 57%
OH
4d 53%
OH
Me
Me
OH
N
N
N
HO
Me Me
Si
OH
Si
O
B
OH
Me
t-Bu
Me
O
t-Bu
O
–
ArB(OH)2
O
N
B
Si
O
O
+
O
–
N
N
OMe
Et
+
t-Bu
Ar
OMe
R1
R1
Ar
R1
4g 49%
4e 47%
4f 42%
A
B
2
OH
OH
OH
N
N
N
Me
OH
Me
OH
Ar
Et
O
OH
Si
O
B
N
N
–
+
t-Bu
O
Et
Et
Ph
Me
4h 66%
R1
N
R1
Ar
Me
OH
Me
4i 0%
4j 0%
R1
Ar
D
4
Si
O
B
OH
C
Scheme 3 Scope of the boronic acid addition to silylnitronate
t-Bu
Scheme 5 Possible mechanism for the conversion of 2 into 4
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

C
M. D. Kerim et al.
Letter
Synlett
ther fragmentation and nitroso–oxime tautomerism give
the final compound 4. An alternative concerted migra-
tion/fragmentation of intermediate B directly to D could
also be proposed. The formation of O-silyl oximes, which
might also have been expected, was not observed.
Me
Me
N
Ar
B
Si
Ar
t-Bu
O
O
B
–
2
Δ
O
B
O
B
+
ArB(OH)2
O
– H2O
H2O
OH
O
R1
B
Ar
O
Ar
Ar
OH
A'
To gain further insight into the mechanism of this reac-
tion, we performed density functional theory (DFT) calcu-
lations to model the aryl transfer from the boric acid to the
nitronate moiety. Despite all our efforts to model the aryl
transfer from intermediate A to intermediate B (Scheme 5),
we were not able to identify a transition state with an ener-
gy barrier lower than 30 kcal mol–1, which is probably asso-
ciated with a mechanistic pathway unlikely to occur under
our reaction conditions. In the Petassis reaction of -hy-
droxy aldehydes, the aryl transfer is usually explained via
the formation of an intermediate boronate followed by an
intramolecular 1,5-shift of the aryl moiety from the boron
to the carbon of the imine function.2 As shown by the re-
sults of our models (Figure 1, path A), a similar mechanism
as disclosed in Scheme 5 requires that the C,N,O atoms of
the nitronate adopt an out-of-plane geometry, which is
highly energy demanding. In an attempt to reduce this out-
of-plane deformation, we envisaged that a second boron
atom might be involved through the intermediacy of boron-
ic anhydrides formed during the reaction (Scheme 6).
Figure 1 summarizes the DFT calculations performed on
the crucial aryl transfer steps following the two proposed
reaction mechanisms. In pathway A (Scheme 5), the silylni-
tronate molecule initially forms a noncovalent molecular
complex with one single boric acid molecule (state RA in
Figure 1, A). Then the aryl group is transferred from the bo-
ric acid moiety to the nitronate moiety in a single step,
passing through transition state TSA and leading to the ad-
HO
HO
Ar
B
N
HO
Me
OSiMe2t-Bu
Ar
B
HO
O
B
–
B
Me
4
O
+
Si
O
O
N
t-Bu
HO
B
Ar
O B
OSiMe2t-Bu
R1
Ar
R1
Ar
HO
C'
B'
Scheme 6 Alternative mechanism for the conversion of 2 into 4 with
two boron atoms
duct PA. As mentioned above, pathway A is associated with
a very high free-energy barrier of 34 kcal mol–1. On the oth-
er hand, the alternative pathway B (Scheme 6), exhibits a
smaller energy barrier of 22 kcal mol–1, corresponding to a
reduction in activation free energy of more than 10 kcal
mol–1 with respect to mechanism A. Most importantly, the
transition state of pathway B, namely TSB, differs substan-
tially from TSA. Though the formation of a 7-membered ring
transition state TSB might be considered less favorable than
the 5-membered ring in TSA, TSB is stabilized by an internal
H-bond between the terminal boric acid OH group and the
oxygen atom that links the silyl to the nitronate moieties.
Another key difference between the two transition states
lies in the local structure around the nitrogen atom of the
nitronate group, as shown in the insets of Figure 1. Indeed,
both transition states require a partial disruption of the pla-
narity of the nitronate group, but the geometry distortion is
Figure 1 Computed free-energy pathways of the aryl transfer reaction. Insets A and B correspond to pathways A (Scheme 5) and B (Scheme 6), respec-
tively. Optimized molecular structures for each stationary state are represented for each energy level. Most H atoms are hidden for the sake of clarity.
Only polar hydrogens and the CH group directly attached to the nitronate moiety are represented. Hydrogen bonds are represented with light-blue
dotted lines while forming/breaking bonds at the transition states are represented with green-dotted lines. For each transition state, the out-of-plane
deformation of the nitronate group is shown in an inset: The O–C–O plane is shown in black, and the distance in Å between this plane and the nitrogen
atom is shown in green.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

D
M. D. Kerim et al.
Letter
Synlett
stronger in TSA than in TSB. The computed out-of-plane dis-
tance between the nitrogen atom and the O–C–O plane of
the nitronate moiety are 0.38 Å and 0.25 Å for TSA and TSB,
respectively. It should be stressed that even a small differ-
ence in planarity may affect the transition states energy
substantially, because the out-of-plane distortion tends to
disrupt the local delocalization of  electrons, which plays a
key role in the stability of the group.
In order to support this mechanistic proposition, we de-
cided to examine the behavior of silyl nitronate 2a towards
phenyl boroxine instead of phenyl boronic acid. Using the
former anhydride directly, the key synthetic intermediate
displaying two boron atoms complexed to the silyl nitro-
nate should be formed more easily, leading to a potential in-
crease in yield. While heating 2a with 2.5 equivalents of
phenyl boronic acid led to phenyl-cyclohexenyl oxime 4l in
a poor 24% yield after 30 min, heating the same reaction
with 1 equivalent of triphenylboroxine led to 4l in 32%
yield. Though not large, this increase in yield supports a po-
tential effect of boronic anhydrides on the mechanism of
this reaction.
ric quantities or in conjunction with oxygen. Indeed, when
oxime 4a was heated in acetonitrile in the presence of cop-
per chloride we observed the formation of isoxazole 5a in a
moderate 50% isolated yield (Scheme 7). Attempts to per-
form this reaction in other solvents such as DMF or THF re-
sulted in lower yields. Further optimization for this known
transformation was not pursued, and Scheme 7 displays the
results of the cyclizations under these conditions. All oxim-
es gave fused isoxazoles 5 in good to moderate yields.
Funding Information
M. D. K. thanks the Islamic Development Bank for a PhD fellowship.
We thank the ENSTA Paris for financial support.()
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1690845.
S
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References and Notes
With these unsaturated oximes in hand, we next exam-
ined their conversion into oxazole derivatives. Several con-
ditions for oxidative transformation of ,-unsaturated ox-
imes into oxazoles have been reported in the literature.10
The reaction may be observed using various oxidants such
as manganese dioxide and copper salts used in stoichiomet-
(1) For reviews on boronic acids, see: (a) Boronic Acids: Preparation
and Applications in Organic Synthesis, Medicine and Materials;
Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2011. (b) Boron Reagents
in Synthesis, ACS Symposium Series 1236; Coca, A., Ed.; American
Chemical Society: Washington DC, 2016.
(2) For reviews on the Petassis reaction, see: (a) Candeias N. R.:
Montalbano, F.; Cal, P. M. S. D.; Gois, P. M. P. Chem. Rev. 2010,
110, 6169. (b) Ramadhar, T. R.; Batey, R. A. Recent Advances in
Nucleophilic Addition Reactions of Organoboronic Acids and their
Derivatives to Unsaturated C–N Functionalities, In Boronic Acids:
Preparation and Applications in Organic Synthesis, Medicine and
Materials; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2011, Chap. 9,
427–477. (c) Guerrera, C. A.; Ryder, T. R. The Petassis Borono-
Mannich Multicomponent Reaction, In Boron Reagents in Synthe-
sis, ACS Symposium Series 1236; Coca, A., Ed.; American Chemi-
cal Society: Washington DC, 2016, Chap. 9, 275–311.
OH
N
N
O
CuCl2·2H2O (2 equiv)
Ar
Ar
MeCN/H2O (4:1)
0.1 M, 75 °C, 2 h
4
5
n
n
O
O
N
N
O
N
(3) For a review, see: (a) Roscales, S.; Csakÿ, A. G. Chem. Soc. Rev.
2014, 43, 8215. For some recent examples, see: (b) Ortega, V.;
del Castillo, E.; Csakÿ, A. G. Org. Lett. 2017, 19, 6236. (c) He, Z.;
Song, F.; Sun, H.; Huang, Y. J. Am. Chem. Soc. 2018, 140, 2693.
(d) Bomio, C.; Kabeshov, M. A.; Lit, A. R.; Lau, S.-H.; Ehlert, J.;
Battilocchio, C.; Ley, S. V. Chem. Sci. 2017, 8, 6071.
Et
OMe
Et
5c 58%
5b 54%
5a 50%
O
N
O
Me
O
N
N
(4) Candeias, N. R.; Veiros, L. F.; Afonso, C. A. M.; Gois, P. M. P. Eur. J.
Org. Chem. 2009, 1859.
Ph
OMe
OMe
(5) For a review on the Nef reaction, see: (a) Ballini, R.; Petrini, M.
Adv. Synth. Catal. 2015, 357, 2371. For examples of nucleophilic
additions leading to oximes, see: (b) Fanté, B.; Soro, Y.; Siaka, S.;
Marrot, J.; Coustard, J.-M. Synlett 2014, 25, 969. (c) Fujisawa, T.;
Kurita, Y.; Sato, T. Chem. Lett. 1983, 1537. (d) Kim, J. N.; Song, J.
H.; Ryu, E. K. Synth. Commun. 1995, 25, 1711. (e) Dumestre, P.;
El Kaïm, L.; Grégoire, A. Chem. Commun. 1999, 775.
(6) (a) Arndt, F.; Rose, J. D. J. Chem. Soc. 1935, 1. (b) Kornblum, N. R.;
Brown, A. J. Am. Chem. Soc. 1964, 86, 2681.
(7) Ono, N. Alkylation, Acylation, and Halogenation of Nitro Com-
pounds, In The Nitro Group in Organic Synthesis; Wiley-VCH:
Weinheim, 2001, Chap. 5, 126–158.
5f 63%a
5e 50%
5d 67%
Scheme 7 Formation of isoxazoles 5 from oximes 4. a The reaction was
left for 12 h for completion.In conclusion, we have disclosed a new in-
teraction of boronic acids with silyl nitronates.11 This aryl transfer from
a boron atom onto the electrophilic C=N double bond of a nitronate ex-
tends the previously disclosed oxime synthesis through aryl magnesium5c
and aryl lithium12 additions onto silyl nitronates. When performed on ni-
tromethylcycloalkenyl derivatives, the resulting oximes may be easily
converted into fused isoxazoles under oxidative conditions.13 We are
currently exploring the potential of various catalysts to trigger the addi-
tion of boronic acids onto less reactive alkyl silylnitronate species.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E

E
M. D. Kerim et al.
Letter
Synlett
(8) For reviews, see: (a) Nitrile Oxides, Nitrones, and Nitronates in
Organic Synthesis, 2nd ed; Feuer, H., Ed.; John Wiley & Sons:
New York, 2007. (b) Weber, W. P. Silyl Nitronates, In Silicon
Reagents for Organic Synthesis. Reactivity and Structure Concepts
in Organic Chemistry, Vol. 14; Springer: Berlin/Heidelberg, 1983.
(9) (a) Barton, D. H. R.; Motherwell, W. B.; Zard, S. Z. J. Chem. Soc.,
Chem. Commun. 1982, 551. (b) Barton, D. H. R.; Motherwell, W.
B.; Zard, S. Z. Bull. Soc. Chim. Fr. 1983, 61. (c) Tamura, R.; Sato,
M.; Oda, D. J. Org. Chem. 1986, 51, 4368.
(10) (a) Maeda, K.; Hosokawa, T.; Murahashi, S. I.; Moritani, I. Tetra-
hedron Lett. 1973, 14, 5075. (b) Alberola, A.; Banez, J. M.; Calvo,
L.; Rodriguez, M. T. R.; Sanudo, M. C. J. Heterocycl. Chem. 1993,
30, 467. (c) Zhu, X.; Wang, Y. F.; Ren, W.; Zhang, F. L.; Chiba, S.
Org. Lett. 2013, 15, 3214. (d) Desai, V. G.; Naik, S. R.; Dhumaskar,
K. L. Synth. Commun. 2014, 44, 1453. (e) Sun, Y.; Abdukader, A.;
Zhang, H.; Yang, W.; Liu, C. RSC Adv. 2017, 7, 55786.
titative yield (256 mg, 1 mmol).
4-Ethylphenyl boronic acid (3a; 2 equiv, 1 mmol, 150 mg) was
added to a solution of 2a (1 equiv, 0.5 mmol, 128 mg) in acetoni-
trile (1 mL). After degassing with argon, the reaction mixture
was heated at 70 °C under argon for 30 min. Evaporation of the
solvent under reduced pressure followed by purification by
flash chromatography on silica gel (PE/Et2O, 100:0 to 80:20)
afforded oxime 4a as an orange solid (78 mg, 0.34 mmol, 68%);
mp 161 °C; Rf = 0.35 (PE/Et2O, 80:20). 1H NMR (400 MHz,
CDCl3):  = 8.97 (br, 1 H), 7.27 (d, J = 8.1 Hz, 2 H), 7.15 (d, J = 8.1
Hz, 2 H), 5.71–5.61 (m, 1 H), 2.70 (q, J = 7.6 Hz, 2 H), 2.39–2.29
(m, 2 H), 2.10–2.04 (m, 2 H), 1.71–1.54 (m, 4 H), 1.28 (t, J = 7.6
Hz, 3 H). 13C NMR (101 MHz, CDCl3):  = 160.3, 144.4, 135.3,
135.0, 130.1, 128.7, 127.7, 28.8, 26.2, 24.7, 22.5, 22.2, 15.4.
HRMS: m/z calcd for C15H19NO: 229.1467; found: 229.1464. IR
(thin film):  = 3214, 3038, 2932, 2866, 1551, 1447, 1262, 997,
(11) Typical Procedure: Preparation of 4a from 1a
968, 823, 737, 722 cm–1
.
Et3N (1.1 mmol, 0.15 ml) was added to a stirred solution of
allylic nitro compound 1a (1 mmol, 141 mg) and tert-butyldi-
methylsilyl chloride (1.1 mmol, 166 mg) in acetonitrile (2 mL).
The reaction mixture was stirred for 30 min at room tempera-
ture and quickly filtered through a short column of silica gel,
eluting with dichloromethane (100 mL) to give, after evapora-
tion of the solvent, nitronate 2a as a yellow oil formed in quan-
(12) Colvin, E. W.; Robertson, A. D.; Seebach, D.; Beck, A. K. J. Chem.
Soc., Chem. Commun. 1981, 952.
(13) For more classical cycloaddition-based conversions of nitronate
derivatives into isoxazoles, see: Namboothiri, I. N. N.; Rastogi, N.
Isoxazolines from Nitro Compounds: Synthesis and Applications,
In Synthesis of Heterocycles via Cycloadditions, Vol. 12; Gupta, R.
R.; Hassner, A., Ed.; Springer: Berlin/Heidelberg, 2008, 1–44.
© 2020. Thieme. All rights reserved. Synlett 2020, 31, A–E