A Convergent Route to Enantiomers of the Bicyclic Monosaccharide Bradyrhizose Leads to Insight into the Bioactivity of an Immunologically Silent Lipopolysaccharide

The synthesis of bradyrhizose, the monosaccharide component of the lipopolysaccharide O-antigen of the nitrogen-fixing bacteria Bradyrhizobium sp. BTAi1 and sp. ORS278, has been achieved in 25 steps in an overall yield of 6% using myo-inositol and ethyl propiolate as the starting materials. The route involved the late-stage resolution of a racemic intermediate to provide both enantiomers of this unusual bicyclic monosaccharide. Both the natural d-enantiomer, and the unnatural and heretofore unknown l-enantiomer, were converted to disaccharide derivatives containing different forms of the monosaccharide (d,d; l,l; d,l; l,d). Evaluation of the synthetic compounds for their ability to act as microbe-associated molecular patterns in plants, through induction of reactive oxygen species, was investigated. These experiments suggest that the immunologically silent nature of the natural glycans is due to specific structural features.