Total Synthesis of a Structurally Complex Tetrasaccharide Repeating Unit of O43.

Herein we report the first total synthesis of a densely functionalized tetrasaccharide repeating unit of O43, which contains rare deoxy amino sugars d-quinovosamine and d-viosamine attached with the rare amino acid -acetyl-l-allothreonine. Synthesis of orthogonally protected rare sugars and unnatural amino acid building blocks, stereoselective construction of three consecutive 1,2- glycosidic linkages, amide coupling, and the presence of five nitrogen atoms dispersed over four sugar units as well as the carboxylic acid functionality make the total synthesis a formidable task.