Structure of the O-polysaccharide from the lipopolysaccharide of Providencia alcalifaciens O31 containing an ether of d-mannose with (2R,4R)-2,4-dihydroxypentanoic acid

Recently, ether-linked diastereomeric 2,4-dihydroxypentanoic acids have been reported as new components of bacterial glycans [Shashkov, A. S. et al. Nat. Prod. Commun. 2008, 3, 1625–1630]. In this work, an ether of (2R,4R)-2,4-dihydroxypentanoic acid (Dhpa) with d-mannose was identified in the O-polysaccharide of Providencia alcalifaciens O31, and the polysaccharide structure was elucidated. Studies by NMR spectroscopy confirmed the ether linkage between O-2 of Dhpa and O-4 of Man, and the absolute configuration of Man was determined after ether cleavage with boron trichloride. In the polysaccharide, Dhpa was found to exist partially in the form of 1,4-lactone. Using sugar and methylation analyses along with 1H and 13C NMR spectroscopy, including 2D 1H,1H COSY, TOCSY, ROESY, H-detected 1H,13C HSQC, and gHMBC experiments, the following structure of the tetrasaccharide repeating unit of the polysaccharide was established: