An Efficient Synthesis of the Pentasaccharide Repeating Unit of Pseudomonas aeruginosa Psl Exopolysaccharide

Pseudomonas aeruginosa is a biofilm-forming Gram-negative bacterium and a leading cause of life-threatening nosocomial infections. The polysaccharide synthesis locus (Psl) exopolysaccharide of P. aeruginosa is a key constituent of the defending bacterial biofilm layer and is a promising therapeutic target for resistant species. The Psl exopolysaccharide is built up from repeating pentasaccharide units which contain one alpha- and two beta-mannosidic linkages, and one l -rhamnose and one d -glucose moieties. The preparation of this pentasaccharide was first described by Boons et al. in a 34-step synthesis. Based on their work, we have developed a new and effective pathway for the synthesis of the repeating pentasaccharide unit of the Psl exopolysaccharide. We have succeeded in simplifying the synthesis of the l -rhamnose and the alpha-selective d -mannose building blocks. Furthermore, taking advantage of a chemoselective pre-activation-based beta-mannosylation, we directly prepare a thioglycoside disaccharide donor and use it in the next coupling reaction without further transformation. The pentasaccharide, in the form of a p -methoxyphenyl glycoside, is prepared in 26 steps, which is suitable for biological testing.