Nitroso acetals silyl nitronates

Evidently, this approach is not limited to the formation of nitronates, nitroso acetals or enoximes. The rearrangements of these compounds by elimination reactions, the trapping of intermediates and finally their reactions with various reagents are of equal importance. It should be emphasized that silylation of AN as a process in organic chemistry is characterized by an unrivalled completeness and diversity of transformations. Hence, the silylation can be considered as a separate field of application of AN in organic synthesis. 

In the second silylation step, the situation is opposite because the basicity of SENA is substantially higher than that of the starting AN. Hence, the second silylation should start with the electrophilic attack of nitronate (320) giving rise to the cationic intermediate B, whose deprotonation affords nitroso acetal... 

First, it should be noted that the starting nitronates (342) or target nitroso acetals (343) can undergo stereoisomerization under the action of electrophiles, which can be accompanied by reversible cleavage of the endocyclic N-0 bond reactions (342=342 ) or (343=343 ). To suppress these processes, it is necessary to perform the transformation (342=343) under conditions as mild as possible and to use a silylating agent containing a small excess of a base. 

Silylation of 3-alkyl-substituted five-membered cyclic nitronates remains virtually unknown, although one example of the successful synthesis of the corresponding nitroso acetal was documented (Scheme 3.203) 475. 

Table 3.19 The silylation of six-membered cyclic nitronates. The preparation of ene-nitroso acetals...

However, C,C-coupling reactions of sterically less hindered alkyl nitronates derived from secondary AN with silyl ketene acetal were successfully performed (Eq. 3). These reactions produced the target mixed nitroso acetals in moderate yields. 

The C,C-coupling reactions of six-membered cyclic nitronates were studied in most detail (274, 478). Here silyl ketene acetal was also used as the test nucleophile. The configurations of most of the starting nitronates and the resulting nitroso acetals were determined by NMR spectroscopy and X-ray diffraction, and also a conformational analysis was performed (see Tables 3.24 and 3.25). 

Elimination Reactions with Silyl Nitronates and Ene Nitroso Acetals... 

Silylation of AN and cyclic nitronates affords SENA and BENA or cyclic N-siloxy-ene nitroso acetals as the major primary products (see Sections 3.2.1.3 and 3.5.1). All these relatively unstable derivatives can undergo various elimination reactions, which will be considered in separately. 

The nitroso acetals that result from the [3 + 2] cycloaddition of alkyl nitronates with dipolarophiles typically provide several characteristic spectroscopic signals for identification. As in the silyl counterparts, the O—N—O stretch in the alkyl nitroso acetals is observed in the IR ranges between 1000 and 1030 cm (Tables 2.22 and 2.23) (57,75-77). However, this resonance is usually not strong, and can be obscured by other functional group resonances with more substituted nitroso acetals. 

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