Total synthesis of the tetrasaccharide repeating units of Pseudomonas chlororaphis O-polysaccharide.

Herein we present the first total synthesis of the tetrasaccharide repeating units from Pseudomonas chlororaphis subsp. aureofaciens UCM B-111, achieved through a stepwise assembly of the core tetrasaccharide structure. The tetrasaccharide units consist of two D-glucosamine units at the terminals, with D-galactosamine and D-quinovosamine rare sugar unit linked by two α- and two β-glycosidic bonds. The total synthesis of 1 and 2 was accomplished via the longest linear sequences of 23 and 22 steps with an overall yield of 2.9% and 3.8%, respectively.