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

( ) 3
H
Ph
O N
OMe
391a
H
N
Ph
O N
O
O
Me3SiX/Et3N
SILYLATION OF NITRO COMPOUNDS AS A PROCESS 651
Ph
( ) 3
O N
OSiMe3 OTf− −78°C ( ) 3
OMe
MeO O
OTf
A′
−
+
+
Me 3SiX/Et 3N
A 1
H
Ph
Ph
H
Me H
Me Me
−78°C
O N
OTf− +
N
OSiMe 3
O 391b
A2 O O
Scheme 3.219
OTf −
+
H
N
Ph
O
N
N
A 2′
OSiMe3
OSiMe3
This is indirect evidence for the cationic ring-chain tautomerism shown in
Scheme 3.218.
However, a ßexible equilibrium A⇄A ′ was not detected.
3.5.2.6. Other Procedures for Generation of Bis-N,N-Oxyiminium Cations
Chlenov was the Þrst to consider bis-N,N-oxyiminium cations as kinetically
independent species (337, 371, 494). He also developed a simple and original
procedure for generating intermediates of this type by elimination of the nitro
group from bicyclic systems (396) (Scheme 3.220).
To generate the cation A, it is necessary to invert the predominant conformation
of the starting nitroso acetal (396) into conformation (396 ′ )(IN), in
which the nitrogen lone pair and the nitro group are trans-antiparallel. As mentioned
above in other isoxazolidines (417), similar inversion can be facilitated
by the presence of nitro groups bonded to neighbouring C-atom. Owing to the
anomeric effect of the nitrogen lone pair, the nitro group can reversibly eliminate
from conformation (396 ′ ) to give the stabilized cation A. The ambident
anion, NO2 − can recombine with the cation A by C,O-coupling to form nitrite
(397), which cannot be isolated in its individual state but can be detected by
1 H NMR spectroscopy. Nitrite (397) does not eliminate the NO2 − anion. It
was hypothesized (337) that compound (397) is stable because of the hindered
IN-process.
Cationic intermediates A can react with active nucleophiles Y − to give coupling
products (399) in high yields, with the trans conÞguration of the nucleophile
and the substituent R predominating. (As in the case of the [3 + 2]-cycloaddition
reactions with six-membered cyclic nitronates (162), this stereoselectivity can
be attributed to the favorable approach of the nucleophile to the plane of the
cation which is distal with respect to the C-6 atom. It should be noted that this