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

Ph
H3CO
H3CO
Ph
N
O
276
N
O
281
O
•
O
N
Ar
OCH3
[H]
H3CO
NITRONE REACTIONS 219
Ph
+ +
•
−CH 3OH
[H]
−CH3OH
Scheme 2.106
Ph
+
−
N
O
N
O
−
O
N
In some cases, α-alkoxy-substituted nitroxyl radicals (276) and (281) turn out
to be convenient starting compounds in the synthesis of α-alkoxynitrones. On
reduction, they eliminate methanol affording α-alkoxy nitrones (Scheme 2.106).
This method, leading to α-alkoxy nitrones makes it possible to generate these
compounds when other methods are unsuccessful (514, 519).
2.6.3.3. Oxidative Amination of Nitrones to α-Amino-Substituted Nitroxyl
Radicals Similar to the oxidative methoxylation reaction, oxidative amination of
4H -imidazole N -oxides, in amine saturated alcohol solutions, give stable nitroxyl
(282), nitronyl nitroxyl (283), imino nitroxyl (284) and (285) radicals with the
amino group at the α-carbon atom of the nitroxyl group (Scheme 2.107) (520,
521). The observed inßuence of substituents on the ratio of amination products
at C2 and C5 atom is close to the ratio observed in the previously mentioned
oxidative methoxylation reaction. It allows us to draw conclusions about the
preference of the radical cation reaction route.
In contrast to this, it has been concluded that the formation of spin adducts on
ultraviolet irradiation or mild oxidation of indole nitrones, in the presence of a
N -heteroaromatic base, proceeds according to the Forrester-Hepburn mechanism
(522).
2.6.3.4. Oxidative Fluorination of Nitrones to α-Fluorosubstituted Nitroxyl
Radicals Formation of nitroxyl radicals by the radical cation route was observed
in reactions of various nitrones with xenon dißuoride in dry methylene chloride
(520, 523). In this reaction, more than 40 nitrones, including 4H -imidazole
N,N -dioxides (219), 4H -imidazole N -oxides (223) and (224), 2H -imidazole
N -oxides (225), 2H -imidazole N,N -dioxides (226), 3,3,5,5-tetramethylpyrroline
N -oxide (TMPO), derivatives of 3-imidazoline-3-oxides (231) and (232), have
been examined. ESR spectra of nitroxyl radicals containing one or two ßuorine
atoms at α-C have been registered (Scheme 2.108) (523). In the case of
+
−
O
Ar