Construction of 1,2-cis rhamnosidic linkages and synthesis of core tetrasaccharide repeating unit of Streptococcus pneumoniae serotype 23F

Abstract

The core repeating tetrasaccharide unit of the Streptococcus pneumoniae 23F capsule polysaccharide, one of the most prevalent serotypes, was synthesized. The challenging 1,2-cis rhamnosidic linkages were assembled effectively and in a stereocontrolled manner via β-rhamnosylation with 2,3-O-(o-xylylene)-rhamnopyranosyl trichloroacetimidate mediated by ZnI₂. An S_N2-like pathway through zinc cations acting as the directing agent was proposed supported by the controlled model experiments and density functional theory calculations.