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 underwent 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
confirmed as a powerful tool for the oxidation of hydroxylamines. The synthetic route is demonstrated as efficient and user friendly, and is exploited on various carbohydrate-derived N,N-disubstituted hydroxylamines (cyclic, acyclic, and functionalized ones), affording the corresponding nitrones in good yields and regioselectivity. N-Monosubstituted hydroxylamines revealed an interesting divergent behavior
C1 and C1′ of N-glycosylnitrones has been developed. Addition of Grignardreagents to N-erythrosyl- N-benzylhydroxylamine 1 affords hydroxylamines 6 almost quantitatively with high diastereoselectivity. These compounds undergo regioselective oxidation to the corresponding C-phenyl nitrones 7, which in turn add a second Grignardreagent to give hydroxylamines 5. Synthetic usefulness of mixed bisadduct
A practical synthesis of N-benzyl-N-glycosylhydroxylamines 3 (R = CH2Ph) is reported. The ability of these compounds to act as versatile synthetic intermediates is demonstrated by their oxidation followed by cycloaddition to N-glcosyl isoxazolidines and by the novel direct cycloaddition as masked acyclic. highly functionalized chiral nitrones. (C) 2002 Elsevier Science Ltd. All rights reserved.
Manganese dioxide oxidation of hydroxylamines to nitrones
Several structurally differentiated N,N-dialkylhydroxylamines were oxidised to the corresponding nitrones using MnO2. Manganese dioxide revealed an efficient and mild reagent for oxidation of hydroxylamines, showing a level of regioselectivity comparable to HgO. Its non-toxicity makes MnO2 the reagent of choice for replacing HgO in this oxidation. (C) 2001 Elsevier Science Ltd. All rights reserved.