Application and limitations of the methyl imidate protection strategy of N-acetylglucosamine for glycosylations at O-4: synthesis of Lewis A and Lewis X trisaccharide analogues.

We describe here the synthesis of the allyl Le(a) trisaccharide antigen as well as that of an analogue of the Le(x) trisaccharide antigen, in which the galactose residue has been replaced by a glucose unit. Although successful fucosylations at O-4 of N-acetylglucosamine acceptors have been reported using perbenzylated thioethyl fucosyl donors under MeOTf activation, such conditions led in our case to the conversion of our acceptor to the corresponding alkyl imidates. Indeed, in this synthesis of the Le(a) analogue, we demonstrate that the temporary protection of the N-acetyl group as a methyl imidate is advantageous to fucosylate at O-4. In contrast, we report here that glucosylation at O-4 of an N-acetylglucosamine monosaccharide acceptor using the alpha-trichloroacetimidate of peracetylated glucopyranose as a donor proceeded in better yields under activation with excess BF(3) x OEt(2) than that of the corresponding methyl imidate. Therefore, we conclude that activation of thioglycoside donors by MeOTf to glycosylate at O-4 of a glucosamine acceptor is best accomplished following the temporary protection of the N-acetyl group as a methyl imidate, especially when the donors are highly reactive and prone to degradation. In contrast, if donor and acceptor can withstand multiple equivalents of BF(3) x OEt(2), glycosylations at O-4 of a glucosamine acceptor with a trichloroacetimidate donor does not benefit from the temporary protection of the N-acetyl group as a methyl imidate.