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

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NO 2<br />

NO2<br />

SiX/Et 3N<br />

NO 2<br />

Me 3SiCl/DBU<br />

N OSiMe3<br />

SILYLATION OF NITRO COMPOUNDS AS A PROCESS 621<br />

O N(OSiMe 3) 2<br />

NO 2<br />

N(OSi)2<br />

Me 3SiOTf/Et 3N<br />

−30˚C<br />

336 20˚C<br />

OSiMe3 NOSiMe3 334a 337<br />

Scheme 3.198<br />

SiO<br />

a – Si = Me2But X - OTf<br />

Si ( 88%)<br />

b – Si =Me3Si +<br />

334a:337∼ 1:5<br />

338 339a,b 340b<br />

+<br />

Scheme 3.199<br />

N<br />

OSi<br />

NO 2<br />

N<br />

OSiMe3<br />

Si = Me3Si, X = Br (−30 ° )<br />

340b:339b ~ 2,5:1 (NMR)<br />

Total yield ~ 80%<br />

At the same time, the correspond<strong>in</strong>g “normal” BENAs (339a) <strong>and</strong> (339b) can<br />

be prepared us<strong>in</strong>g Me2Bu t SiOTf/Et3N orMe3SiOTf/Et3N. (467, 470)<br />

Nitroso acetal (341) was synthesized by double silylation of 1-nitro<br />

cyclohexa-1,3-diene (471) (Scheme 3.200). Nitrosal (341) can be considered as<br />

a possible equivalent of nitrosobenzene.<br />

In conclusion of this section, it should be emphasized that double silylation of<br />

AN is very regio- <strong>and</strong> stereospeciÞc. As a rule, the reaction mixtures conta<strong>in</strong> only<br />

one structural isomer of the target nitroso acetals. If the start<strong>in</strong>g AN conta<strong>in</strong>s the<br />

methyl group at the α-C atom, double silylation leads to deprotonation of this<br />

group to give term<strong>in</strong>al BENA regardless of the nature of the substituent R at the<br />

β-carbon atom (Scheme 3.201).<br />

Only the <strong>in</strong>troduction of a powerful electron-withdraw<strong>in</strong>g group (R=CO2Me)<br />

leads to a change <strong>in</strong> the deprotonation site <strong>and</strong> the regioselective generation of<br />

the correspond<strong>in</strong>g term<strong>in</strong>al BENA.

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