Protonated Phenanthroline Catalyzed Stereoselective Glycosylation: Synthesis and Evaluation of Saponin Adjuvant Lablaboside F and Variants Leading to Simplified Vaccine Adjuvants

Vaccine adjuvants are substances that are coadministered with antigens to enhance the immune response. Lablaboside F, a triterpene oleanolic acid β-linked to two trisaccharide chains on its eastern and western sides, emerged as a promising adjuvant candidate due to exhibiting robust adjuvanticity and low toxicity . We developed a β-stereoselective glycosylation method using a [PhenH]⁺[BF₄]^- catalyst to glycosylate oleanolic acid with C2-branched oligosaccharides, synthesizing lablaboside F and structural derivatives (<b>S1-S8</b>) as a result of constructing the target oligosaccharide. This approach simplifies the synthetic process when varying sugars are used and provides a platform for structure-activity relationship studies. Lablaboside F and <b>S1-S8</b> were tested <i>in vivo</i> and <i>in vitro</i> for adjuvant activity, toxicity, and cytokine production. The entire branched trisaccharide on the western side of lablaboside F is not essential for adjuvant activity. Six derivatives showed low toxicity, while four increased pro-inflammatory cytokine levels. Notably, compound <b>S5</b>, composed of oleanolic acid and the eastern trisaccharide, demonstrated a favorable safety profile <i>in vitro</i> and higher antigen-specific total IgG levels than lablaboside F <i>in vivo</i>, without eliciting IgE production that can lead to allergic reactions. These findings advance our understanding of saponin structure-function relationships and provide a pathway for developing nontoxic adjuvants.