Synthesis of Oligosaccharides Related to the O-Antigen of Shigella Flexneri Serotype 6 for Glycoconjugate Vaccine Development

Shigella flexneri is a Gram-negative bacterium, causing shigellosis (or bacillary dysentery), an invasive intestinal infection that remains a major health concern in children living in developing countries [1]. Shigella flexneri 6 (SF6) is one of the most common circulating serotypes, and one to be included in a Shigella vaccine [2]. As an alternative to Shigella subunit vaccines involving polysaccharides from biological origin [3] , our group has been investigating the use of synthetic oligosaccharides as surrogates of the O-antigen (O-Ag), the polysaccharide component of the Shigella lipopolysaccharide. Such a vaccine candidate targeting S. flexneri 2a [4] was recently evaluated in human [5]. The repeating unit of the SF6 O-Ag is the linear tetrasaccharide ABCD [6]. Following early work on the identification of this O-Ag O-acetylation pattern and synthesis of short segments thereof [7], here we report a robust and scalable chemical synthesis of fully 3A/4A-O-acetylated or not tetra- and octasaccharides, featuring one and two SF6 O-Ag repeating units, respectively. These oligosaccharides were synthesized in a form compatible with their conjugation to a carrier and use for in vivo studies.

A robust [4+4] synthesis involving the imidate glycosylation chemistry was developed. In particular, tetrasaccharide building blocks enabling site-selective O-acetylation at rhamnose A and providing good stereocontrol at all glycosidic linkages were identified. Strategic choices and improvements compatible with scale up will be discussed.