Synthesis of a disaccharide repeating unit of the O-antigen from Burkholderia ambifaria and its oligomers.

A disaccharide repeating unit of the O-antigen from Burkholderia ambifaria, 6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-O(CH2)3NH2 (1), and its dimer and trimer, 6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-(1 → 3)-6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-O(CH2)3NH2 (2) and 6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-(1 → 3)-6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-(1 → 3)-6-deoxy-β-d-Alt-(1 → 4)-α-d-Rha-O(CH2)3NH2 (3), were synthesized via a convergent strategy. The key disaccharyl thioglycoside 4 as a glycosyl donor was stereoselectively assembled by glycosylation of rhammnosyl acceptor 5 with 6-deoxy-altrosyl trichloroacetimidate donor 6b. The glycosidation of 4 with 3-azidopropanol followed by global deprotection afforded the target disaccharide 1. Further elongation of the carbohydrate chain of this glycosidation product with the disaccharyl donor 4 followed by global deprotection generated rapidly the dimeric tetrasaccharide 2 and the trimeric hexasaccharide 3 in a convergent [2 + 2] and [2 + 2 + 2] manner, respectively.