Convergent Total Syntheses of Oligosaccharides by One-Pot Sequential Stereoselective Glycosylations

A convergent total synthesis of F1(alpha) antigen, a member of the tumor-associated O-linked mucin glycosyl amino acid, was tried by one-pot sequential glycosylation. Highly (alpha)-selective glycosylation of amino acid 7 with thioglycoside 6 was successfully carried out by combining trityl trifluoromethanesulfonate (TrOTf) and N-iodosuccimide (NIS) which gave glycosyl amino acid 21 in high yield (97%, (alpha)/(beta) = 83/17). Next, the glycosylation of thioglycoside 4 with galactosyl phenyl carbonate 2 or fluoride 3 was tried by the promotion of trityl tetrakis(pentafluorophenyl)borate [TrB(C6F5)4] or trifluoromethanesulfonic acid (TfOH); protected F1(alpha) 25 was afforded in 80 or 89% overall yield, respectively, by the further addition of glycosyl amino acid 5 and NIS. The desired trisaccharide was obtained in high yield after removal of the protecting groups. Next, a convergent total synthesis of branched hepta-(beta)-glucoside 30 having phytoalexin-elicitor activity was efficiently performed by way of two one-pot sequential glycosylation reactions: that is, trisaccharide 34 was synthesized in high yield by TfOH-catalyzed one-pot glycosylation using three given monosaccharides (31, 35, and 36) as shown in Scheme 12 and by subsequent selective deprotection of 6ʹ-O-TBDPS group. The second one-pot glycosylation of trisaccharide 34 with three monosaccharides (31, 32, and 33d) also proceeded smoothly to afford heptaglucoside 43 stereoselectively in 48% total yield based on monosaccharide 32. Phytoalexin-elicitor active branched hepta-(beta)-glucoside 30 was afforded by the sequential deprotection.