4,6-di-O-benzoyl-3-O-benzyl-alpha-D-arabino-hexo-pyranos-2-ulosyl bromide: a conveniently accessible glycosyl donor for the expedient construction of diantennary beta-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-alpha-D-arabino-hexopyranos-2-ulosy l bromide (7), a most useful indirect beta-D-mannosyl donor as its blocking group pattern allows the construction of biologically relevant beta-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 beta-stereocontrol can be achieved over both, glycosidations and carbonyl reduction of the beta-ulosides formed: Koenigs-Knorr conditions exclusively provide beta-glycosiduloses, hydride reduction of their carbonyl functions proceeds with high stereoselectivities (> 20:1) in favor of the beta-D-mannosides. These preparatively auspicious properties are materialized in an efficient, straightforward synthesis of alpha-D-Manp-(1-->6)-[alpha-D-Manp-(1-->3)]-beta-D-Manp++ +-(1-->O)-Octyl, the 3,6-O-branched core-mannotrioside carrying an octyl spacer instead of the chitobiosyl unit.