摘要Most bacterial cell surface glycans are structurally unique,and have been considered as ideal target molecules for the developments of detection and diagnosis techniques,as well as vaccines.Chemical synthesis has been a promising approach to prepare well-defined oligosaccharides,facilitating the structure-activity relationship exploration and biomedical applications of bacterial glycans.L-Galactosaminuronic acid is a rare sugar that has been only found in cell surface glycans of gram-negative bacteria.Here,an orthogonally protected L-galactosaminuronic acid building block was designed and chemically synthesized.A synthetic strategy based on glycal addition and TEMPO/BAIB-mediated C6 oxidation served well for the transformation of commercial L-galactose to the cor-responding L-galactosaminuronic acid.Notably,the C6 oxidation of the allyl glycoside was more efficient than that of the selenoglyc-oside.In addition,a balance between the formation of allyl glycoside and the recovery of selenoglycoside was essential to improve ef-ficiency of the NIS/TfOH-catalyzed allylation.This synthetically useful L-galactosaminuronic acid building block will provide a basis for the syntheses of complex bacterial glycans.