Stereoselective synthesis of a model α-glycoside of the β-d-ManNAcp-(1→4)-d-Glc disaccharide starting from lactose, avoiding the β-mannosaminylation step

A model isopropyl α-glycoside of the β-d-ManNAc-(1→4)-d-Glc disaccharide has been prepared from lactose, avoiding the β-mannosaminylation step. Three complementary approaches involving first the preparation and then the glycosidation of β-thiophenyl donors of the protected disaccharides, (a) β-d-ManNAc-(1→4)-d-Glc, (b) β-d-TalNAc-(1→4)-d-Glc and (c) lactose, were compared. The best results were obtained employing a suitably protected lactose donor, and submitting its α-isopropyl glycoside to an amination with inversion in position 2′ followed by an epimerization at C-4′.

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