Abstract
Chemical studies reveal that the structure of OPS are linear or branched among P. syringae strains. The linear backbone is composed of L-, D-, or both L-and D-rhamnose. The branch OPS is composed of linear backbone with homogenous or hetrogenous saccharide substitution. Above all, the pivot issue of this thesis is to synthesize the linear backbone motif of P. syringae, a trisaccharide structure of Rha(1→3) Rha(1→2)Rhap. Utilizing the anomeric center activation manner of imidate and manipulating all factors and condition during glycosylation, we accomplish the synthesis of rhamnose linear backbone, unique biological feature of OPS, in the P. syringae LPS. Our synthesis strategy is to prepare disaccharide which is composed of electron-donating group protected donor 14 and anomeric center thio protected acceptor 19. The reducing end of the disaccharide is follewed by deprotection and activation of anomeric center, and then perform glycosylation with acceptor 18 for further synthesis of trisaccharide structure 23. As for donor, having synthesized Ac, Bz and Bn protected donors, we choose electron-donating Bn type donor 14 as buiding block of disaccharide synthesis for the sake of reactivity and synthesis strategy consideration. As for acceptor, through a acetal protection on C-2 and C-3 follewed by selective deprotection of C-2 or C-3, we could easily create precursor of aceeptor 19 and 18. It makes the synthesis of Rha(1→3) Rha(1→2)Rhap feature trisaccharide to be more facile.