Carbon benzyl nitrones

Other nitrones (21-23) having the chiral moiety located at the carbon atom have been applied in reactions with various alkenes (Scheme 12.10) (33-35). Nitrone 21 offered poor discrimination in 1,3-dipolar cycloadditions with benzyl crotonate, as all four diastereomers were obtained in both reactions (33). The fluorinated nitrone... 

Intramolecular 1,3-dipolar additions of nitrones and nitrile oxides to carbohydrate alkene groups have met with success. Thus, treatment of the unsaturated heptose ether 68 (Scheme 17), which can be made following 1,3-dithianyl anion addition to C-l of 2,3,4-tri-0-benzyl-5,6-dideoxy-D-xy/o-hex-5-enose, with IV-methylhydroxylamine in refluxing methanol, affords the nitrone 69 that cyclizes to give the bicyclic isoxazolidine 70 (60% isolated) together with the epimer at the new asymmetric center carrying the methylene carbon atom (16% isolated) [35]. 

Steric and electronic effects on the rate and regiochemistry of the reaction between p-nitrobenzyl substrates and tertiary carbanions were also studied71. Thus, increasing the size of the alkyl groups attached to the benzylic or anionic carbons of the substrates causes substantial decrease in the proportions of C-alkylation product. In contrast with the previous reaction with nitronate anions, formation of reduction products is observed instead of a significant O-alkylation. 

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