Highly Stereoselective Synthesis of α,β‐Linked, Nonreducing Disaccharides Related to Tunicamycin.

Abstract 3,4,6-Tri- O -benzoyl-2-(benzoyloxyimino)-2-deoxy-α- d - arabino -hexopyranosyl bromide ( 2 ) reacts with the O -protected 2-deoxy-2-phthalimido-β- d -galactosamines 3 and 4 in the presence of silver triflate and sym -collidine at −78°C, to give α,β-(1 → 1)-linked disaccharides 6a and 7a with an excellent selectivity. The 2-oxyimino function was stereospecifically converted into a 2-acetamido group by use of the LiBH 4 -Me 3 SiCl-THF reductive species, furnishing, after acetylation, the α- d -GlcNAc-(1 → 1)-β- d -GalNPhth nonsymmetrical, trehalose type disaccharides 13 and 14 related to tunicamycin ( 1 , part A). Similarly, α- d -GlcNAc-(1 → 1)-β- d -GlcNPhth ( 15 ) was prepared, starting from 2 and 5 . The factors governing the stereoselectivity of the glycosylation reactions were determined.