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.