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

178 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
Light irradiation of styrenes in the presence of aromatic and heteroaromatic
compounds leads to the formation of α,N-diarylnitrones in good yields (331, 332).
2.2.2.4. Synthesis from Nitroso Compounds Aryl nitroso compounds (175)
react easily with dimethyl bromomalonate in the presence of alkali to give the corresponding
N -aryl-C,C -dimethoxycarbonyl-nitrones (177) (Scheme 2.62) (333).
A general method for nitrone formation is based on the interaction of nitro
compounds with carbanions. Interaction between nitroso compounds (175) and
anions of aliphatic nitro compounds (178) leads to nitrones (179). The source of
anions are metal salts of nitro compounds, triethylamines, and trimethylsilylnitronates
(Scheme 2.63) (334, 335).
Formation of nitrones can be achieved in the ÞrststageofaKröhnke type reaction
in which p-nitrosodimethylaniline reacts with 2-ω-bromoacetylphenoxathiin
in alkaline medium (336). The synthesis of a series of cyclic nitrones of structure
(182) has been achieved by regioselective, and by an unusual [3 + 2] cycloaddition
of α-nitrosostyrenes (181) to 1,3-diazabuta-1,3-dienes (180) (Scheme 2.64)
(337a). Theoretical studies of the substitution effect at the imine nitrogen on the
competitive [3 + 2] and [4 + 2] mechanisms of cycloaddition of simple acyclic
imines with nitrosoalkenes have been reported (337b).
Chiral cyclic nitrones (185) were synthesized in the reaction of isonitrosoderivatives
of Meldrum’s acid (183) with ketones in boiling toluene (338–344).
The reaction is likely to proceed, as in the case of the cycloaddition of α-nitrosostyrenes,
by [3 + 2] cycloaddition of ketones to nitrosoketone (184), resulting
from thermolysis of (183) (Scheme 2.65) (345).
The reaction of [3 + 2] cycloaddition of nitrosoalkenes (181) to the imino
group of bicyclic 1,2-oxazines (186) gives tricyclic nitrones (187) (Scheme 2.66)
(346).
A theoretical analysis of the reactions of acyclic imines with nitrosoalkenes
has been presented (347).
Ar-NO
+
BrCH(CO 2Me) 2
175 176
NaOH
THF
Ar R 1 R 2
1-naphthyl
2-naphthyl
Ph
O
a: CO2Me CO2Me
b: Me CO 2Me
c: Ph CO 2Et
Scheme 2.62
−
+
N
Ar
CO 2Me
177
CO 2Me