Total Synthesis Of Cell Surface Carbohydrate Antigens Of Two Pathogenic Bacteria

Bacterial cell surface polysaccharides are often structurally unique and immunologically active,have been targets for developing vaccines and pathogen detection strategies.Synthesizing pure and well-characterized oligosaccharides,particularly the repeating units of carbohydrate antigens,are attractive options to identify the glycan epitope structures that are recognized by the immune system.Even for structurally diverse glycans,a family of highly functionalized aminoglycosides from pathogenic bacteria still offers a challenging synthetic task.Here,total synthesis of two representative aminoglycosides from Plesiomonas shigelloides O51 and Staphylococcus aureus type 8 was achieved.Significantly,the synthesis of these two important carbohydrate antigens will form the basis for development of carbohydrate-based vaccines and diagnostics.1.Total synthesis of Plesiomonas shigelloides O51 O-antigen trisaccharideBased on P.shigelloides serotype O51 O-antigen trisaccharide repeating unit [?4)-?-D-GlcpNAc3NHbA-(1?4)-?-L-FucpAm3OAc-(1?3)-?-D-Quip NAc-(1?],target trisaccharide equipped with an anomeric linker and its acetamido derivative were designed.Diamino-D-glucuronate building block was synthesized starting from common D-glucosamine in a total of 15 steps and 6% yield.The C3 azide was successfully introduced via stereocenter inversion twice at C3 including Lattrell-Dax inversion and nucleophilic substitution.L-Fucose served as the starting material to access L-fucosamine building block in a total of 5 steps and 55% yield.Glucal easily obtained from D-glucose served as starting material for the synthesis of D-quinovosamine in a total of 16 steps and 6% yield.The 1,2-cis-?-selectivity for glycosylation between L-fucosamine building block and D-quinovosamine building block relied on a nonparticipating C2 azide group and solvent effects of diethyl ether/thiophene.C3 azide donor was designed and utilized in assembly of trisaccharide,thus avoiding the formation of bicyclic dihydro-1,3-oxazine byproduct that formed when a C3 amide donor was utilized.In contrast to an O3?-acylated,disarmed disaccharide,an armed disaccharide equipped with an O3?-Nap was highly reactive toward diamino-D-glucuronate donor and the formation of desired trisaccharide.The orthogonally protected trisaccharide allows for selective unmasking of the C2? amine,C3? hydroxyl group,and C3?? azide,thus providing an efficient route to P.shigelloides serotype O51 O-antigen trisaccharide equipped with N-acetyl,acetamidino,and D-3-hydroxybutyryl groups and its acetamido derivative in a total of 52 steps.The rare acetamidino group in trisaccharide was successfully introduced under mild reaction condition with S-benzyl thioacetimidate hydrochloride.2.Total synthesis of Staphylococcus aureus type 8 capsular polysaccharide trisaccharideBased on S.aureus type 8 capsular polysaccharide(CP8)trisaccharide repeating unit ?3)-4-O-Ac-?-D-ManpNAcA-(1?3)-?-L-Fucp NAc-(1?3)-?-D-FucpNAc-(1?,target trisaccharide equipped with an anomeric linker was designed.D-Galactose served as the starting material to access D-fucosamine building block in a total of 13 steps and 12% yield.The C2 azide was introduced by azidoselenation of D-galactal,followed by C6 reduction.L-fucosamine building block was synthesized starting from common L-fucose via azidoselenation of fucal and regioselectively C4 benzylation.A ?-glycosylation-epimerization strategy was employed to introduce the ?-mannosyl into disaccharide intermediate.Two 1,2-cis-?-glycosidic linkages were stereoselectively formed relied on a nonparticipating C2 azide group,solvent effects of diethyl ether/thiophene and the remote participation of 3-O-acetyl group.Three azides in trisaccharide were successfully converted to amines with 1,3-propanedithiol in aqueous pyridine,followed by acetylation and subsequent TEMPO/BAIB oxidation.The S.aureus CP8 trisaccharide equipped with an aminopropyl linker at the reducing end was obtained after global deprotection in a total of 49 steps.Notably,the strategy for the differentiation of amino groups in P.shigelloides serotype O51 trisaccharide will serve well for the syntheses of other complex glycans.Furthermore,three synthetic trisaccharides equipped with aminopropyl linkers enable the further biological evaluation,particularly identification of the the carbohydrate epitopes of P.shigelloides serotype O51 and S.aureus type 8.