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

R 1
432
R 1
434
E OR 3
NO2
R 2
N(OSi)2
EX
R 2
TfOSi
E +
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 669
E +
+
E R 1
R 2
X −
E +
+ OTf− SiOR3 +
N(OSi)2
A
SiX
SiX
R 1
NO2
433
R 2
(X = AcO, Me3SiO, Xal)
Si - Me 3Si or Bu t Me 2Si
R 2
E
E R 1
N
SiO
E - ArylCH(OMe); CH(OMe) 2; Me2C(OMe); trans-MeCH=CHCH(OMe); Ar2CH +
(Ar– p-[CF3CH2(R4
O N
)N]-C6H4
AiS - (Ar - p-Tol or p-Cl-C H
S OMe
+ EX- OTf
6 4
X = Cl, Ar - ;
O
Ar2CH - Fe/Ph (Fe - ferrocenyl);
Ar
Scheme 3.232
435
AcOH/Et 3N
O
R 3 - alkyl
(R 4 – Me, Ph));
cations E + . In this case, Lewis acids are not required for generation of cations.
If acetals or related compounds are used as precursors, catalysis by silyl Lewis
acids is necessary for generating cations E + (512). The main problem is that
the coupling reactions produce during the Þrst stage reactive iminium cations A
(for the chemistry of these compounds, see Section 3.5.2.), and it is necessary
to eliminate the trialkylsilyl groups from A as rapidly as possible to prepare
nitrones (435) and then the functionalized nitro compounds (433) as target products.
The smooth transformation (435→433) also does not seem to be a easy
problem. In other words, the aimed interaction mentioned above is to perform
reaction sequences involving cationic intermediates. Secondary AN containing
the methyl group at the α-C atom and, consequently, giving terminal BENAs
are most readily involved in this process. The easy selective elimination of the
trialkylsilyl fragment from the cationic intermediate A is favored by the absence
of the proton at the α-C atom; this makes it possible to prevent side processes.