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

295a
Br
I
NaOH, MeOH
NaOH, MeOH
NaOH, MeOH
H
O
H
O
H
O
Br
I
Scheme 2.119
NITRONE REACTIONS 229
O −
O −
O −
+
N
N
+
+
N
N
N
N
OH(O • )
OH(O • )
of three tautomeric forms (A–C), with the prevalence of enol nitrone (C) and
enhydroxyamino nitrone (B). The sulfur-containing analog in solutions of CDCl3
and DMSO exists only in the enhydroxy-aminothioxo structure (306B) (Scheme
2.121) (546, 547).
2.6.5.2. Nitration of α-Aryl Nitrones The reactions of electrophilic substitution
in α-aryl nitrones have not yet been studied in depth, and at present, the
only reported reaction is the nitration of derivatives of 4-aryl-(307a–c) and
4-(2-hetaryl)-3-imidazoline-3-oxides (308a,b) (548, 549). Nitration takes place
in concentrated sulphuric acid upon treatment with HNO3. The nitrone group
in 4-phenyl-3-imidazoline-3-oxides is known to be ortho-para-directing in the
phenyl ring. The direction of α-aryl- and α-hetaryl nitrone nitrations, in more
active aromatic systems, is determined by the orientation properties of the aromatic
system per se. Thus, the p-tolyl group (307b) is nitrated in the m-position to
the nitrone group, and the p-ßuoroderivative (307c) is nitrated in the o-position.
Hetaryl derivatives (308a,b) are nitrated only in the α-position of the hetero atom
O or S (Scheme 2.122).
2.6.5.3. Electrophilic Substitution at α-Carbon Atom
2.6.5.3.1. Metalation of Nitrones Metalation of cyclic aldonitrones in the
absence of an “external” electrophile gives dimeric compounds, the nonmetalated
aldonitrone group being the electrophile. At the same time, depending on
OH