Preparation of N-Glycosylhydroxylamines and Their Oxidation to Nitrones for the Enantioselective Synthesis of Isoxazolidines

N-benzyl- and N-methyl-N-glycosylhydroxylamines 3a-i were conveniently obtained by reaction of sugars with N-substituted hydroxylamines according to a novel procedure. Subsequent oxidation occurred at the alkyl group, selectively affording the corresponding C-phenyl- and C-unsubstituted N-glycosylnitrones. C-phenyl-N-glycosylnitrones 10 and 13 under-went highly stereoselective 1,3-dipolar cycloaddition with dimethyl maleate, with the sugar moiety acting as a chiral auxiliary. Final removal of the glycosyl moiety afforded enantiopure enantiomeric isoxazolidines 17 and ent-17 which are oxa-analogues of proline diester derivatives.