4,6-di-O-benzoyl-3-O-benzyl-α-d-arabino-hexopyranos-2-ulosyl bromide: A conveniently accessible glycosyl donor for the expedient construction of diantennary β-d-mannosides branched at O-3 and O-6

A concise practical, large scale-adaptable six-step sequence has been developed for the transformation of diacetone-glucose into 4,6-di-O-benzoyl-3-O-benzyl-α-d-arabino-hexopyranos-2-ulosyl bromide (7), a most useful indirect β-d-mannosyl donor as its blocking group pattern allows the construction of biologically relevant β-d-mannosides branched at O-3 and O-6. The broad utility of this new ulosyl bromide 7 resides in its high anomeric reactivity, and in the ease and uniformity with which β-stereocontrol can be achieved over both, glycosidations and carbonyl reduction of the β-ulosides formed: Koenigs-Knorr conditions exclusively provide β-glycosiduloses, hydride reduction of their carbonyl functions proceeds with high stereoselectivities (> 20:1) in favor of the β-d-mannosides. These preparatively auspicious properties are materialized in an efficient, straightforward synthesis of α-d-Manp-(1 → 6)-[α-d-Manp-(1 → 3)]-β-d-Manp-(1 → O)-Octyl, the 3,6-O-branched core-mannotrioside carrying an octyl spacer instead of the chitobiosyl unit.