5a-Carba-glycopyranoside primers: potential building blocks for biocombinatorial synthesis of glycosphingolipid analogues.

Three ether-linked alkyl 5a-carba-glycopyranosides 1b,d, and 5b, and 5a'-carba-lactoside 7b were examined as potent primers in mouse B16 melanoma cells for their feasibility as building blocks for oligosaccharide biosynthesis. Uptake by B16 cells was first observed for all carba-glycoside primers, and, especially, the 5a-carba-sugar analogues of N-acetyl-beta-d-glucosaminide 1b and beta-d-glucoside 1d were shown to produce two-to-four-fold larger amounts of glycosylated products than the corresponding true sugar primers 1a and 1c. The carba glycoside uptake by cells resulted in beta-galactosylation and subsequent sialylation of the incorporated galactose residues, giving rise to glycosylated products 3b and 3d having similar glycan structures as the ganglioside GM3. According to efficient uptake in cells, in addition to stability of the ether-linked pseudo-reducing ends of the oligosaccharides that formed, the carba glycoside primers have been demonstrated to be versatile building blocks for these biocombinatorial syntheses of glycolipid oligosaccharide mimetics. On the other hand, uptake for 5a-carba-galactopyranoside residue was found to be decreased by one-third for dodecyl 5a-carba-beta-d-galactopyranoside 5b. Observation of similar levels for 5a'-carba-beta-lactoside 7b under both cellular and cell-free conditions suggested that enzymes are likely to recognize the pyranose oxygen atom.