Stability silyl nitronates

Here two facts can be mentioned. For example, cycloaddition of nitronate (Me02C)CH=N(0)0SiMe3 to ethylene was observed (203), whereas its O -methyl analog does not react with ethylene. It is hardly probable that this fact is due to the high reactivity of the silyl nitronate. More likely, the negative result for alkyl nitronate is attributed to low stability of this derivative. 

As a result, diastereo- and enantiomerically pure functionalized nitro derivative (205) was synthesized in satisfactory yield. This compound was used in the ISOC procedure, which gave diastereo- and enantiomerically pure isoxazolidine (206) in good yield. The latter compound can be considered as a possible reagent for asymmetric synthesis. It should be noted that silylation in this procedure, like that described above (Scheme 3.145), was performed with the use of BSA as the silylating agent in the presence of a small amount of Hunig s base, the latter being evidently added for acceleration of silylation and stabilization of intermediate silyl nitronate. 

The wide variety of methods for the preparation of alkyl nitronates, gives rise to a broader diversity of structures compared to silyl nitronates. Alkyl nitronates can be grouped into two subclasses, acyclic and cyclic. Both subclasses participate in dipolar cycloadditions with similar reactivity, however, minor differences are manifest in their stability and stereoselectivity. Additionally, the ability to prepare cyclic nitronates allows access to a wide variety of novel, multicyclic ring stractures. 

The first attempted silylation of nitromethane resulted in the production of a silylated dimeric nitronate due to self-condensation (17). Subsequent investigations, however, have shown that silyl nitronates are stable compounds that can be isolated and distilled at reduced pressures, Table 2.1 (18-21). Bulkier silyl groups help increase the stability of the resulting nitronate (21). 

Nitronates, particularly silyl nitronates, are often superior to nitrile oxides in their 13DC with olefins in terms of their ease of generation from nitroalkanes, stability, and the observed selectivity during cycloaddition. Cycloaddition of alkyl or silyl nitronates with olefins generates N-alkoxy- or N-silyloxy-substituted isoxazolidines which then undergo spontaneous or acid catalyzed elimination of alcohol (or silanol) to produce isoxazolines (see Scheme 1, Sect. 2). 

I.2. Stability of Silyl- and Boryl Nitronates SENAs cannot decompose through pathways characteristic of alkyl nitronates (see Scheme 3.72). 

The same tautomeric equilibrium can be obtained in the silylation of six-membered cyclic nitronates (97) containing the siloxy or alkoxy group at C-6, which stabilizes the cationic center at the C-6 atom (293) (for more details, see Section 3.5). 

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