Organic & Biomolecular Chemistry
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5
DOI: 10.1039/C3OB41393D
5 6 See the ESI for more details.
7
For reviews, see: (a) Z. Cekovic, Tetrahdron 2003, 59, 8090. (b) G.
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8
For application of iminoxyl radicals to organic synthesis, see: (a) B.
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10
15
20
25
30
9
For generation of aminoxyl radicals from hydroxylamine derivatives
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10 The aerobic reaction of 1a in the absence of inorganic bases resulted
in no reaction and oxime 1a was recovered in 85% yield.
11 For a review on synthesis, reactivity, and application of 1,2,4-
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13 The reaction with N-(3-phenylpropyl)amidoxime 1t resulted in the
formation of compex mixtures with unidentified compounds (see the
ESI for more details).
35 14 To investigate another mechanistic possibilty that the present process
is initiated by autoxidation of the benzylic moiety, we tested the
aerobic reaction of O-methyl amidoxime 1u. Treatment of 1u with
Cs2CO3 (2 equiv) in DMSO resulted in formation of de-benzylated
amidoxime 1u’ in 40% yield with 59% recovert of 1u even after
40
heating at 100 °C for 24 h and at 120 °C for 4 h. This result indicated
that benzylic autoxidation might be possible but the reaction rate is
very slow under the present reaction conditions. Based on these
observations, the benzylic oxidation is likely induced by 1,5-H
radical shift with iminoxyl radical I as shown in Scheme 2.
OMe
OMe
Cs2CO3 (2 equiv)
N
N
Ph
N
H
Ph
Ph
NH2
DMSO, 100 °C, 24 h
then 120 °C, 4 h
under O2 (1 atm)
1u' 40%
1u
(59% recovery of 1u)
45
50
15 Y.-F. Wang, F.-L. Zhang, S. Chiba, S. Org. Lett. 2013, 15, 2842.
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55 18 For recent reports on transition-metal-catalyzed synthesis of
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