Nitronate moiety, Michael addition with

The reactions of the lithium enolates of substituted 2-cyclohexenones and 2-cyclopentenones with ( )-l-nitropropene give a mixture of syn- and ami-products3. The lithium enolate of 3,5,5-trimethyl-2-cyclohexenone gives a mixture of the syn- and //-3.5,5-trimethyl-6-(l-methyl-2-nitroethyl)-2-cyclohexcnoncs in modest diastereoselection when the reaction mixture is quenched with acetic acid after. 30 minutes at —78 =C. When the reaction mixture is heated to reflux, tricyclic products are obtained resulting from intramolecular Michael addition of the intermediate nitronate ion to the enone moiety. 

It is assumed that, after the initial formation of the oxime 2-634, a Michael addition occurs to give 2-635 with formation of a nitrone moiety which then can undergo a 1,3-dipolar cycloaddition to give 2-636. 

The nucleophilic addition of lithiated allyl phenyl sulfone to nitrones at 0 °C afforded 4-(phenylsulfonyl)isoxazol-idines as major products. The process probably involves the isomerization of the allylsulfonyl moiety of the initially formed hydroxylamine anion to vinylsulfone which then undergoes intramolecular Michael addition. For example, the chiral nitrone 536 afforded isoxazolidine 537 with high diastereoselectivity (Equation 88) <2005T3335>. When the same reaction was carried out in the presence of hexamethylphosphoramide (HMPA) at —80°C, the anti-a-sulfonyl homoallyl hydroxylamine was obtained. 

The chiral moiety of the nitrone can also be located at the carbon atom. Yokoyama et al. [50] used this approach in the asymmetric synthesis of spiro isoxazohdines (Scheme 10.21). The ribose-derived nitrone 62 is obtained from the corresponding oxime by Michael addition to methyl acrylate. With a second equivalent of methyl acrylate, the sugar nitrone 62 gave a single cycloadduct 63, which was converted into the corresponding pyrrolidine by reduction. 

---