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

R 2
R O
C N
1
R2 R3 +
R 1 , R 2 , R 3 = Alk or Ar
R 1
N
O
−
hv
R 2
Scheme 2.87
N
O
Path A
Path B
R 1
NITRONE REACTIONS 207
O
C N
R2 R3 R 1
O
C N
R2 R3 249 250
Scheme 2.88
hv
R 1
R 2
O
N
R 1
R 1 = H, CO2Et, (CH2)2CO2Et
R 2 = H, AcO, PhO-mPh-CH2OCO-
PhCH 2-OCO-NH(CH2)3-NHCO
(457). Photochemical isomerization of nitrones to oxaziridines can occur in the
crystal phase (458).
To illustrate the synthetic use of photochemical rearrangement, the photolysis
of nitrones (249) leading to the formation of bicyclic lactams (250) is an example
(Scheme 2.88) (459).
An alternative method in the synthesis of alkaloids, photochemical rearrangement
of endocyclic nitrones into bicyclic lactams has drawn special attention.
Analyses of photochemical rearrangement and application of modiÞed conditions
of the Barton reaction testify to the comparability of results obtained in
these approaches (Scheme 2.89) (460).
Other examples of photochemical transformations of nitrones into alkaloids
have been described (461). According to the view of the authors, some transoxaziridines
give products derived from the trapping of aminyl radicals (AR) by
the pendant alkenes (Scheme 2.90).
A detailed analysis of photoisomerization of the nitrone group in the nitroxyl
radical 4-phenyl-2,2,5,5-tetramethyl-3-imidazoline-3-oxide-1-oxyl, based on double
electron-nuclear resonance methods, has shown that in the absence of oxygen
the photochemical reaction occurs without affecting the radical center. The