Synthesis of the tetrasaccharide repeating unit of the cryoprotectant capsular polysaccharide from Colwellia psychrerythraea 34H

Colwellia psychrerythraea 34H is a psychrophilic Gram-negative bacterium, able to survive at subzero temperatures by producing a unique capsular polysaccharide (CPS) with anti-freeze properties similar to those of the well-known anti-freeze (glyco)proteins. The tetrasaccharide repeating unit of the CPS – constituted of alternating amino sugars and uronic acid moieties in a glycosaminoglycan-like fashion with an amide-linked threonine (Thr) decoration – was synthesized as an O-n-propyl glycoside. The synthesis faced some challenging features such as building up a crowded [→2)α-D-Galp(1→] moiety as well as differentiating the two uronic units for the regioselective insertion of the Thr amide only on one of them. NMR data for the obtained tetrasaccharide confirmed the structure proposed for the C. psychrerythraea polysaccharide