Synthesis of pentasaccharides corresponding to the glycoform II of the outer core region of the Pseudomonas aeruginosa lipopolysaccharide Original Research Article

Cystic fibrosis (CF) is a congenital disease caused by a mutation in a gene responsible for the synthesis of a membrane protein called the cystic fibrosis transmembrane conductance regulator (CFTR). Resistance to Pseudomonas aeruginosa infection is closely related to the biological properties of CFTR; however, these properties have not been clearly linked to the known role of CFTR as a chloride and bicarbonate ion channel. Indeed, data indicate that CFTR is an epithelial cell receptor for P. aeruginosa, with CFTR binding to the oligosaccharide of the outer core region of the bacterial lipopolysaccharide (LPS), of which two distinct glycoforms have been identified. Binding leads to effective innate immunity to clear this pathogen in individuals with wild-type CFTR. To reveal the molecular basis of elimination of the bacterium through this interaction, the synthesis of pentasaccharides corresponding to both glycoforms of the outer core region of P. aeruginosa LPS was undertaken. Here we report the synthesis of the glycoform II. Like glycoform I, it was prepared as three pentasaccharides bearing naturally occurring N-alanyl and N-acetyl substituents in the galactosamine moiety as well as unnatural N-acetylalanine to reveal the role of the amino group in the alanyl substituent. Key features of the synthesis were two α-glucosylations with glucosyl donors bearing α-stereodirecting acyl groups at O-6 and/or O-3 and high-yielding reduction of the azido group followed by N-acylation and final O-debenzylation.