Automated solid-phase synthesis of protected oligosaccharides containing β-mannosidic linkages

For automated oligosaccharide synthesis to impact glycobiology, synthetic access to most carbohydrates has to become efficient and routine. Methods to install "difficult" glycosidic linkages have to be established and incorporated into the overall synthetic concept. Described here is the first automated solid-phase synthesis of oligosaccharides containing the challenging beta-mannosidic linkage. Carboxybenzyl mannoside building blocks proved effective beta-mannosylation agents and resulted in excellent conversion and good to moderate selectivities. [(Triisopropylsilyl)oxy]-methyl ether (Tom), served as an orthogonal, minimally intrusive, and readily cleavable protecting group for the elongation of the C3 position of mannose. The desired oligosaccharide products were readily separated from by-products containing unwanted stereoisomers using reverse-phase HPLC. The methods described here expand the scope of carbohydrates currently accessible by automation as many oligosaccharides of biological interest contain beta-mannosidic linkages.