Straightforward Synthesis of (1→2)-Linked Pseudo Aza-C-disaccharides by the Novel Cycloaddition of Enantiopure Cyclic Nitrones to Glycals

The novel, highly stereoselective, intermolecular cycloaddition reaction of enantiopure cyclic nitrones 8 to 1,2-glycals 9 opens the way to a straightforward synthesis of a broad class of new (1-->2)-linked pseudo aza-C-disaccharides 6, suitable substrates for selective inhibition of glycosidase enzymes. The cycloadditions occur with high stereocontrol, displaying preferential interaction between the bottom face of the glycal and the face of the nitrone anti to the substituent on C-3, with the reagents approaching in an exo fashion. The cycloadditions produced tricyclic isoxazolidines 7 that represent nonreducing pseudo aza-C-disaccharides stable to hydrolytic conditions. The target pseudo aza-disaccharides 6 were obtained by sequential deprotection of the hydroxyl groups and isoxazolidine ring-opening.