Reagent-Controlled Synthesis of the Branched Trisaccharide Fragment of the Antibiotic Saccharomicin B

A concise synthesis of a branched trisaccharide, alpha-L-Dig-(1 -> 3)-[alpha-L-Eva-(1 -> 4)]-beta-n-Fuc, corresponding to saccharomicin B, has been developed via reagent-controlled a-selective glycosylations. Starting from the D-fucose acceptor, L-epivancosamine was selectively installed using 2,3-bis(2,3,4-trimethoxyphenyl)cyclopropene-1-thione/oxalyl bromide mediated dehydrative glycosylation. Following deprotection, L-digitoxose was installed using the AgPF6/TTBP thioether-activation method to produce the trisaccharide as a single a-anomer. This highly functionalized trisaccharide can potentially serve as both a donor and an acceptor for the total synthesis of the antibiotic saccharomicin B.