Mannosazide Methyl Uronate Donors. Glycosylating Properties and Use in the Construction of beta-ManNAcA-Containing Oligosaccharides

Mannosazide methyl uronate donors equipped with a variety of anomeric leaving groups (beta- and a-S-phenyl, beta- and alpha-N-phenyltrifluoroacetimidates, hydroxyl, P-sulfoxide, and (R-s)- and (S-s)-alpha-sulfoxides) were subjected to activating conditions, and the results were monitored by H-1 NMR. While the S-phenyl and imidate donors all gave a conformational mixture of anomeric alpha-triflates, the hemiacetal and beta- and alpha-sulfoxides produced an oxosulfonium triflate and beta- and alpha-sulfonium bistriflates, respectively. The beta-S-phenyl mannosazide methyl uronate performed best in both activation experiments and glycosylation studies and provided the 1,2-cis mannosidic linkage with excellent selectivity. Consequently, an alpha-Glc-(l -> 4)-,beta-ManN(3)A-SPh disaccharide, constructed by the stereo-selective glycosylation of a 6-O-Fmoc-protected glucoside and beta-S-phenyl mannosazide methyl uronate, was used as the repetitive donor building block in the synthesis of tri-, penta-, and heptasaccharide fragments corresponding to the Micrococcus luteus teichuronic acid.