Chemical Synthesis Of The Trisaccharide Repeating Units Of Lipopolysaccharide O-antigens Of Pseudomonas Aeruginosa O10

Pseudomonas aeruginosa,a gram-negative bacterium,is one of the most common opportunistic pathogens worldwide.It is a dangerous human pathogen,especially for patients with mechanical ventilation and suppressed immunity,causing severe sepsis,respiratory,and urinary tract infections,especially in hospital environment.P.aeruginosa infection is usually treated with antibiotics.However,it antibiotics resistance rate has been increasing in recent decades,resulting in the rise of healthcare costs and mortality rate.Therefore,a safe and effective vaccine for P.aeruginosa are highly sought after.Lipopolysaccharide(LPS)is one of the important virulent factors of P.aeruginosa.Its O-antigens consist of structurally unique saccharides and are excellent targets for developing relevant vaccines.The type O10 P.aeruginosa is associated with about 10%of the invasive infections,ranking fourth among all 20 serotypes.and it includes two O10a,b and O10a,c.Based on the research background,we designed and chemically synthesized two trisaccharide haptens 2-1a,b and 2-1a,c related to the repeating units of LPS O-antigens of P.aeruginosa subtypes O10a,b and O10a,c with an aim to facilitate the subsequent development of related vaccine and novel diagnostic reagents.Their structures are[→4)-α-L-GalpNAcA(1→3)-a-D-QuipNAc-(1→3)-a-L-Rhap2Ac/H-(1→]n.Assembly of the target trisaccharides 21a,b and 2-1a,c represent a synthetic challenge.First,both of the D-QuiNAc and L-GalNAcA are rare amino and commercially unavailable monosaccharides.Second,two 1,2-cis-glycosidic linkages are difficult to be constructed with high stereoselectivity.Third,the target 2-1a,b and 2-1a,c contain a set of functionalities,such as the O-acetyl,N-acetyl,free amino,and carboxyl groups.Since the L-GalNAcA as a glycosyl donor typically possess low reactivity,a postglycosidation oxidation strategy was employed to synthesize the target molecules.During the synthesis process,we explored the glycosylation methods for stereoselective construction of two 1,2-cis-glycosidic bonds in trisaccharides 2-1a,b and 2-1a,c.It was revealed that glycosylation of the 3-O-Lev protected D-QuiN selenoglycoside donor with the linker equipped L-Rha receptor under the asisstance of the α-directing additive DMF via a TfOHpromoted preactivated method to afford the α-D-QuiN glycosidic bond in high yield and stereoselectivity.Thereafter,systematic experiments were conducted to achieve the stereoselective construction of the α-glycosidic bond of L-GalN.Initially,we utilized the structurally simple 3,4-O-benzyl-6-O-TBS-L-GalN3 selenoglycoside donor and 2-azido-DGlcN acceptor as template substrates to screen the best glycosylation methods.Besides,this method showed broad substrate scope.The L-GalN3 donors 2-8,2-9,and 2-10 containing different protecting groups at the 3,4-O-position were used to couple with the monosaccharide 2-34,disaccharide 2-41,and trisaccharide 2-42 receptors under the optimized reaction conditions.Delightfully,all the corresponding products were obtained in high yields and αselectivity.