Nitrile Oxides, Nitrones, and Nitronates in Organic Synthesis : Novel ...

H O +
N
DMPO
H O
N
N
228-232
MeOH
PbO2
OMe
H
O
N
MeOH
H
OMe
N
PbO2 N
O
MeO + O
N
MN
MeO O
+
N
N
NITRONE REACTIONS 217
OMe
MeO
O
N
OMe
MeO
O
N
R R R R
H
−
O
+
N
MeOH
H
OMe
O
N
MeO O
N
MeO
OMe
O
N
R + N PbO2 R + N
R N
R N
−
O − O O O
226
H
−
O
+
N
MeOH
H
OMe
O
N
MeO
− O
+
N
MeO
OMe
O
N
R N PbO2 R N
R N
R N
225
−
−
H O
N
H But +
R = H, CH 3, OCH 3,NO,NO 2
Scheme 2.103
MeOH
PbO 2
−
−
MeO But OMe
MeO
O
N
215 277
Scheme 2.104
It is relevant to note that only cyclic aldo-nitrones tend to react in oxidative
alkoxylations to give α,α-dialkoxy-substituted nitroxyl radicals. However, the
only exception is methylene nitrone (215), which on oxidative methoxylation
gives the α,α,α-trimethoxy-substituted nitroxyl radical (277). This is due to the
proton in methylene nitrone (215), which, as in the case of cyclic nitrones, exists
in the cis-position to the N -oxide oxygen (Scheme 2.104) (517).
N