Convenient Synthesis of Hexa- and Pentasaccharide Repeating Units Corresponding to the O-Polysaccharides of Acinetobacter baumannii O7 and Acinetobacter baumannii O10 Strains

The synthesis of the hexa- and pentasaccharide repeating units of the cell wall O-polysaccharides of Acinetobacter baumannii O7 and O10 strains, respectively, has been achieved in very good yield using [4+2] and [4+1] block glycosylation strategies. The p-methoxybenzyl (PMB) group was used as an in situ removable protecting group, which was removed after glycosylation in the same pot by tuning the reaction condition. A challenging β-l-rhamnopyranosyl linkage was constructed using ‘armed-disarmed glycosylation’ conditions by the influence of a remotely located, H-bond mediating, picolinoyl group in the glycosyl donor. A d-mannosamine intermediate was prepared from d-glucose using minimum reaction steps. The hexa- and pentasaccharide were achieved as their p-methoxyphenyl (PMP) glycosides.