Synthesis of Bradyrhizose and its Disaccharides

Abstract

Bradyrhizose, a bicyclic monosaccharide, was reported in 2011 by Molinaro and coworkers at the University of Naples in Italy. This unusual carbohydrate was isolated from the lipopolysaccharide (LPS) of Bradyrhizobium sp. BTAi1 and sp. ORS278 as the only component of the O-antigen. These nitrogen-fixing bacteria live in symbiosis with legumes Aeschynomene sensitiva and indica. It was found that neither the LPS or the pure O-antigen from Bradyrhizobium sp. BTAi1 are recognized by plants as a microbe-associated molecular pattern (MAMP). This is the first example of an LPS not activating a defense response and indicates that the LPS and the O-antigen from these bacteria are non-immunogenic. Synthesizing this molecule, and disaccharides containing it, will provide compounds that can be used to probe the role of this monosaccharide in bacterial symbiosis with the legume. Two different approaches for the synthesis of bradyrhizose are discussed in this thesis. For the first route, the monosaccharide was envisioned to be obtained from a furan derivative via the Achmatowicz reaction. For the second route, two different strategies were investigated starting with myo-inositol as the basis of the cyclohexanol moeity of bradyrhizose. The racemic synthesis of bradyrhizose was achieved using the second route. The enantiomers were separated at a late stage of the synthesis to afford D- and L-bradyrhizose. The synthesis of bradyrhizose donors and acceptors (D- and L-) was accomplished starting with an intermediate used in the synthesis of the monosaccharide. The reactivity of this unusual monosaccharide in glycosylations was studied using a trichloroacetimidate donor and various types of acceptors. Disaccharides containing different enantiomeric forms of the monosaccharide (D,D; L,L; D,L; L,D) were synthesized and will be tested for their ability to induce reactive oxygen species (ROS) in different plants and legumes.