A concise route to access C-glycosidic tetrazolyl analogues of Kdo as bioisosteres

Ulosonic acids are an important class of carbohydrates, including well-known representatives as 3-deoxy-d-manno-oct-2-ulosonic acid (Kdo), 3-deoxy-d-glycero-d-galacto-non-2-ulosonic acid (Kdn) and N-Acetylneuraminic acid (Neu5Ac). As part of the lipopolysaccharides (LPS) and capsular polysaccharides (CPS, K-antigen), these carbohydrates can be found on the surfaces of, both, Gram-positive and Gram-negative bacteria. We developed a synthetic approach to access tetrazole-derivatives of these compounds. Tetrazoles have shown to be ideal analogues for the bioisosteric replacement of carboxylic acids in several drugs, improving biological activity and pharmacokinetic properties. The presented route features indium-mediated allylation with subsequent [2 + 3]-cycloaddition as key steps, leading to tetrazole-derivative of Kdo and its corresponding C-4 epimer. The route is flexible enough to be adapted to the preparation of further tetrazole ulosonic acids.