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

R 2
226a–c
R 1
N
O
+ O
N
−
+
−
hv
R 1 = Ph (a, c), Me (b)
R 2 = Ph (a), Me (b), H (c)
R 2
Δ
R 1
N
O
N
+
−
R 2
240a–c
R 2
R 1
R 1
O
O
N
N
O
O
N
N
242a–c
Scheme 2.84
hv
+
−
R 2
R 1
O
R 1 = Ph (a), Me (b)
R 2 = H (a), p-O2NC6H4 (b)
225a, b 244a, b
Scheme 2.85
NITRONE REACTIONS 205
N
hv
or
+
N
R 2
R 2
R 1
O
R 1
O
N
+ O
N
O
N
N
243a–c
−
241a–c
photolysis affords a mixture of cis- andtrans-3,7-dioxa-1,4-diazatricyclo[4.1.0.0
2,4]heptanes, and similarly to (242), only cis-isomers are able to thermally isomerize
to the starting 4H -imidazole -1,3-oxides (219) (Scheme 2.86) (454).
Stereoselective isomerization of oxaziridines into the corresponding nitrones
can be carried out in a photosensitive electron transfer (PET) reaction (455). Upon
studying photophysical and photochemical properties of a series of α,N -diarylnitrones
in polar and proton solutions, the electronic inßuence of substituents
at α-C and N atom, on the possible routes of cyclic oxaziridine transformation,
was revealed (a) with homolytic C–O bond opening and (b) with N–O bond
opening (Scheme 2.87) (456). The presence of electron donor groups at carbon
and/or nitrogen atoms facilitates the reaction (according to path A) leading to
nitrones, whereas electron-withdrawing group on the nitrogen atom stabilize the
biradical structure as a result of delocalization of three electrons at the nitrogen
atom (path B) (456).
Oxaziridines, the products of photochemical isomerization of polymeric
nitrones, are characterized by high stability. The formation of an intramolecular
hydrogen bond stabilizes the nitrone group with respect to UV-irradiation