Improved methods for the stereoselective synthesis of mannoheptosyl donors and their glycosides: toward the synthesis of the trisaccharide repeating unit of the Campylobacter jejuni RM1221 capsular polysaccharide

In view of the importance of 6-deoxymannoheptosides as structural units in the Campylobacter jejuni RM1221 capsular polysaccharide, the development of effective synthetic protocols for 4-O-6-S-α-cyanobenzylidene thio-d-mannoheptapyranoside donors carrying either 3-O-naphthylmethyl or 3-O-acetyl groups is described starting from d-mannose. In particular, tris(phenylthio)methyllithium is found to undergo highly stereoselective addition to a mannose-6-aldehyde in sharp contrast to the vinyl Grignard reagent whose reactions were essentially devoid of selectivity. A brief survey of coupling reactions with the two donors indicted the 3-O-acetyl system to be highly α-selective whereas the 3-O-naphthylmethyl congener was highly β-selective indicating that the presence of the seventh carbon atom in these donors is not detrimental to coupling selectivity in either instance.