• Group B streptococcus
• regioselective glycosylation

In the last decade glycoimmunology, a new research field dealing with the specific interactions of carbohydrates with the immune system, acquired a growing interest. The oligo- and polysaccharides that cover the cell surface of bacterial pathogens are crucial virulence factors. They are capable of interaction with the immune system introducing the production of carbohydrate specific antibodies. For this reason they represent an attractive target for vaccine design.
Among the pathogens against which a vaccine is not yet available, Streptococcus agalactiae (also called Group B Streptococcus or GBS) represents one of the leading cause of neonatal infections, despite substantial progresses in the prevention thanks to the recent introduction of intrapartum antibiotic prophylaxis. Clinical signs of the diseases caused by this bacterium results in pneumonia, sepsis and meningitis. Ten serotypes of GBS have been identified on the basis of the residues composition of the capsular polysaccharide (Ia, Ib, II, III, IV, V, VI, VII, VIII, IX) but only 5 of them are responsible for the majority of the diseases (Ia, Ib, II, III, IV).
GBS serotypes Ia, Ib and III show similarities in the sugar composition of the repeating unit, such as the GlcNAcβ(13)Gal motif, which is also present in a number of biologically relevant oligosaccharide structures. The regioselective glycosylation of galactose 3-OH to form this motif is a key step for the synthesis of fragments deriving from the three serotypes, since it reduces the number of synthetic steps connected with the classic protection/deprotection (orthogonal) strategy.
Recently, in the laboratory where I carried out my thesis project, a synthetic approach exploiting the well-recognized higher reactivity of Gal 3-OH compared to the 4-OH was optimized in order to achieve a regioselective glycosylation. The use of several glucosamine donors and galactose acceptors highlighted the importance of the protecting group pattern of the galactose acceptor with the benzoyl group proving to be the best for achieving a regioselective glycosylation. The target GlcNAcβ(13)Gal disaccharide, obtained in high yield and with a reduced number of steps, represent a key intermediate to the preparation of branched fragments from GBS PSIa capsular polysaccharide.
Considering these results, we envisaged the use of the regioselective glycosylation of galactose also for the preparation of linear oligosaccharide structures. In particular, after a short screening of a few glucosamine donors and different reaction conditions, the regioselective glycosylation of a lactose acceptor, protected with benzoyl groups with a glucosamine trichloroacetoimidate donor led to the formation of a linear trisaccharide.
The trisaccharide was then elongated in order to synthesize the linear penthasaccaride repeating unit of GBS Ia and III as well as to prepare the desialylated tetrasaccharide. Our innovative synthetic design to the target oligosaccharide structures resulted in an expeditous synthesis, which occurs with overall good yield to previously published syntheses of the same compounds. ,
Furthermore, with our approach all the final protected structures present the 4-hydroxy group of galactose unit unprotected and available for further elongation in order to obtain branched fragments of the polysaccharide structures.
These fragments will be used for supporting epitope mapping studies of GBS PSIa, taking advantage of the presence of the reducing-end amminopropylic linker for the future glycoarray screening.