Synthesis of the biological repeating unit of Streptococcus pneumoniae serotype 23F capsular polysaccharide.

An efficient synthesis of the 3-aminopropyl glycoside of the biological repeating unit of Streptococcus pneumoniae serotype 23F capsular polysaccharide (CPS) was accomplished. The synthetic target contained a tetrasaccharide with a phosphoglycerol branch. Its oligosaccharide backbone was assembled via linear glycosylation, and the challenging β-rhamnosyl linkage in its structure was achieved stereospecifically through naphthylmethyl-assisted intramolecular aglycon delivery (IAD). The remaining 1,2-trans glycosylation reactions were executed in excellent yields and stereoselectivity based on neighboring group participation. The phosphoglycerol branch was installed by the phosphoramidite method with benzylidene-protected glycerol 2-phosphoramidite as the substrate. Eventually, the target molecule was synthesized from monosaccharide building blocks in 17 longest linear steps and a 3.85% overall yield. In addition, the synthetic target also contained a free amino group at its reducing end, facilitating its conjugation with other molecules for various biological studies and applications.