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

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**<br />

NO2<br />

O<br />

O<br />

N<br />

O<br />

*<br />

N SiMe3<br />

O<br />

PRINCIPAL PHYSICOCHEMICAL DATA AND CHARACTERISTICS 513<br />

**<br />

NO2<br />

O<br />

NO2<br />

*<br />

N<br />

OSiMe3<br />

Scheme 3.78<br />

**<br />

NO2<br />

Me3SiO<br />

NO2<br />

*<br />

N<br />

O<br />

NO2<br />

*<br />

NO2<br />

NO 2<br />

*<br />

NO2<br />

O<br />

**<br />

N<br />

Me 3Si<br />

OSiMe3<br />

O<br />

N**<br />

O<br />

*<br />

NO2<br />

N<br />

O<br />

Me3Si<br />

*<br />

NO2 O<br />

N<br />

NO2 OSiMe3<br />

*<br />

N<br />

NO2<br />

*<br />

N<br />

O<br />

Me OSiMe3<br />

E-isomer<br />

Me O<br />

Z-isomer<br />

Me O<br />

Z-isomer<br />

Me OSiMe<br />

3<br />

E-isomer<br />

Scheme 3.79<br />

3.3.4.2. R<strong>in</strong>g-cha<strong>in</strong> Tautomerism of <strong>Nitronates</strong> <strong>and</strong> Derived Im<strong>in</strong>ium Cations<br />

In these studies (291, 292), the r<strong>in</strong>g-cha<strong>in</strong> tautomerism of silylated cations A,<br />

prepared by reversible silylation of the above mentioned nitronates (96) was suggested<br />

as an explanation for the observed transformations of γ-keto-functionalized<br />

SENAs 96 (Scheme 3.80).<br />

The same tautomeric equilibrium can be obta<strong>in</strong>ed <strong>in</strong> the silylation of sixmembered<br />

cyclic nitronates (97) conta<strong>in</strong><strong>in</strong>g the siloxy or alkoxy group at C-6,<br />

which stabilizes the cationic center at the C-6 atom (293) (for more details, see<br />

Section 3.5).<br />

Si-X +<br />

O<br />

O<br />

96<br />

N OSi<br />

SiO<br />

X<br />

N<br />

O OSi<br />

−<br />

+ SiO<br />

O<br />

X<br />

N<br />

+ OSi<br />

A'<br />

−<br />

A<br />

Scheme 3.80<br />

Si-X +<br />

SiO<br />

O N O<br />

97

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