Synthesis of orthogonally protected bacterial, rare-sugar and d-glycosamine building blocks

Bacterial glycoconjugates comprise atypical deoxy amino sugars that are not present on the human cell surface, making them good targets for drug discovery and carbohydrate-based vaccine development. Unfortunately, they cannot be isolated with sufficient purity in acceptable amounts, and therefore chemical synthesis is a crucial step toward the development of these products. Here we describe a detailed protocol for the synthesis of orthogonally protected bacterial deoxy amino hexopyranoside (2,4-diacetamido-2,4,6-trideoxyhexose (DATATDH), d-bacillosamine, d-fucosamine, and 2-acetamido-4-amino-2,4,6-trideoxy-dgalactose (AATAATAAT)), d-glucosamine and d-galactosamine building blocks starting from beta-d-thiophenylmannoside. Readily available beta-d-thiophenylmannoside was first converted into the corresponding 2,4-diols via deoxygenation or silylation at C6, followed by O3 acylation. The 2,4-diols were converted into 2,4-bis-trifluoromethanesulfonates, which underwent highly regioselective, one-pot, double-serial and double-parallel displacements by azide, phthalimide, acetate and nitrite ions as nucleophiles. Thus, d-rhamnosyl-and d-mannosyl 2,4-diols can be efficiently transformed into various rare sugars and d-galactosamine, respectively, as orthogonally protected thioglycoside building blocks on a gram scale in 1-2 d, in 54-85% overall yields, after a single chromatographic purification. This would otherwise take 1-2 weeks. d-Glucosamine building blocks can be prepared from beta-d-thiophenylmannoside in four steps via C2 displacement of triflates by azide in 2 d and in 66-70% overall yields. These procedures have been applied to the synthesis of l-serine-linked trisaccharide of Neisseria meningitidis and a rare disaccharide fragment of the zwitterionic polysaccharide (ZPSPS) A1 (ZPSPS A1) of Bacteroides fragilis.