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

138 NITRONES: NOVEL STRATEGIES IN SYNTHESIS
N
H H
32
O
H
O
H
N
m-CPBA
CH 2Cl 2, rt, 5 min
m-CPBA
CHCl3, 40˚, 5 h
m-CPBA
CHCl3 or CH2Cl2, reflux
Scheme 2.12
N
O
N
33
N
O
H
O
OH
O
O
H O
H
H
H
N
N O
− H
O
O OR
Urea complex with hydrogen peroxide (UHP) seems to be a very useful and
convenient oxidation system in nonaqueous medium. The UHP reaction proceeds
in methanol and is catalyzed by Mo (VI), W (VI) salts, or SeO2 (83). Catalytic
oxidation in a water-free medium can be carried out with alkylhydroxyperoxides,
the catalysts being titanium alkoxides (84) or selenium compounds (85). The
reaction appears to proceed quickly and with good selectivity.
The most effective and frequently used oxidant of secondary amines in organic
solvents (CH2Cl2, CH3CN, MeOH) is m-chloroperbenzoic acid (m-CPBA). Oxydation
with m-CPBA of seco-curane type indoline alkaloids of strychnobrasiline
(32), in deacetylated form, gives the corresponding nitrones (33), (34), and (35)
(Scheme 2.12) (86).
The synthesis of camphor-derived nitrones (87), of spin traps containing alkylphenylphosphoryl
(36a,b) (88) and diethoxyphosphoryl substituents (37–39)
requires m-CPBA as an oxidant (Fig. 2.3) (89, 90).
m-CPBA oxidation of cyanomethylamines (41) into nitrones (42) proceeds
regioselectively, which is obviously determined by the inductive effect of the
cyano group (91). A three-stage protocol has been proposed for converting
+
+
+
−
−
34
35
H
N