Chemical Synthesis of a Key Precursor Relevant to the Tetrasaccharide Repeating Unit from Treponema medium ATCC 700293

Comprehensive Summary Treponema is a Gram‐negative anaerobic bacterium, among which the pathogenic Treponema can cause various diseases, such as venereal syphilis (Treponema pallidum), yaws (Treponema carateum), and oral diseases (Treponema denticola and Treponema medium). Although different from conventional lipopolysaccharides, the extracellular glycoconjugate of Treponema may still be a potential antigen and provide a candidate for vaccine development. Hence, we completed the first chemical synthesis of Treponema medium ATCC 700293 tetrasaccharide precursor containing L‐ornithine (L‐Orn) and D‐aspartic acid (D‐Asp) derivatives. The efficiency of non‐reducing end disaccharide formation has been improved by optimizing the assembly of the protecting groups in the donors and acceptors. Our [3+1] glycosylation strategy attempted to reduce the length of the acceptor to increase the nucleophilicity of the hydroxyl group, thereby improving the efficiency of synthesizing the target tetrasaccharide. The L‐Orn derivative was introduced at the final stage due to its influence on the glycosylation stereospecificity and efficiency. Therefore, the successful introduction of two amino acid derivatives and the synthesis of a tetrasaccharide precursor with complex functional‐group modifications have provided valuable insights for synthesizing other complex bacterial glycans. Herein, we report on the chemical synthesis of a tetrasaccharide precursor of the Treponema medium ATCC 700293. When the [2+2] glycosylation strategy could not be carried out due to the mismatch of donor‐acceptor reactivities, we successfully completed the synthesis of the target tetrasaccharide precursor using a [3+1] glycosylation strategy.