Towards the complete synthetic O-antigen of O1, serotype inaba: improved synthesis of the conjugation-ready upstream terminal hexasaccharide determinant

Synthesis of the upstream terminal hexasaccharide part of the lipopolysaccharides (LPS) of Vibrio cholerae O1, serotype Inaba has been improved. The key improvements include but are not limited to optimized conditions for the stereoselectivity of glycosylation reactions involved and fewer number of synthetic steps, compared to previous approaches. Particularly noteworthy is conducting the glycosylation of the very reactive glycosyl acceptor 8-azido-3,6-dioxaoctanol with the fully assembled hexasaccharide trichloroacetimidate under thermodynamic control. It produced the desired α glycoside with an α : β ratio of 7 : 1, compared with the ratio of 1.1 : 1, observed when the coupling was conducted conventionally. Several substances, which were previously obtained in purity acceptable only for synthetic intermediates, were now obtained in the analytically pure state and were fully characterized. The structure of the key trisaccharide glycosyl acceptor was confirmed by single-crystal X-ray structure determination. Improved synthesis of the conjugation-ready upstream terminal hexasaccharide determinant has been described with special reference to the stereoselectivity refinement.