Synthesis of an S‐Linked α(2→8) GD3 Antigen and Evaluation of the Immunogenicity of Its Glycoconjugate

Replacing the interglycosidic oxygen atom of oligosaccharides with a nonhydrolyzable sulfur atom has attracted significant interest because it provides opportunities for developing new glycoconjugate vaccines. Herein, a stereocontrolled and highly convergent method to synthesize a non‐reducing‐end inter‐S‐glycosidic variant of the GD3 antigen (S‐linked α(2→8) GD3) that is resistant to enzymatic hydrolysis is reported. The key steps in the synthesis are a regio‐ and stereoselective α(2→3) sialylation of a lactoside acceptor with a C8‐iodide‐derivatized sialyl donor and an anomeric S‐alkylation, which enable stereoselective construction of a terminal S‐linked α(2→8) disialyl residue. The sulfhydryl‐reactive maleimide group was used as the linker for the well‐defined conjugation of these antigens to the immunogenic protein keyhole limpet hemocyanin (KLH). Groups of mice were immunized with the GD3–KLH and S‐linked GD3–KLH glycoconjugates in the presence of complete Freund's adjuvant. Microarray analysis of the sera showed the promise of the S‐linked GD3–KLH vaccine: it stimulated a high immunoglobulin G response against S‐linked GD3 and cross‐reactivity with the O‐linked GD3 antigen was low. The activity of the S‐linked GD3–KLH vaccine was comparable to that of the O‐linked GD3–KLH vaccine, which highlighted the effectiveness of generating glycoconjugate vaccines and immunotherapies by relatively simple means. Linked in: A highly α‐stereoselective anomeric S‐alkylation was implemented for an S‐linked α(2→8) GD3 tetrasaccharide synthesis in the search for non‐natural and new thio‐sialoglycoconjugate‐based immunogens (see figure).