Synthetic Studies on Tumor-Associated Antigens: Efficient Syntheses of Leaand Sialyl-LeaOligosaccharides, and Their Deaminated Analogs1

Chemical and chemoenzymatic syntheses of sialyl Lewisa tetrasaccharide antigen

Sialyl Lewisa (sLea), also known as cancer antigen 19-9, is a tumor-associated carbohydrate antigen. In this article, chemical and chemoenzymatic syntheses of a tetrasaccharide glycan 1 structurally derived from sLea are reported. Challenges involved in the chemical synthesis include the highly stereoselective construction of 1,2-cis-α-L-fucoside and α-D-sialoside, as well as the assembly of the 3,4-disubstituted N-acetylglucosamine subunit. Perbenzylated thiofucoside and N-acetyl-5-N,4-O-oxazolidinone protected sialic acid thioglycoside were employed as glycosyl donors, respectively, for the efficient preparation of the desired α-fucoside and α-sialoside. The 3,4-branched glucosamine backbone was established through a 3-O and then 4-O glycosylation sequence in which the 3-hydroxyl group of the glucosamine moiety was glycosylated first and then the 4-hydroxyl. A facile chemoenzymatic approach was also exploited to synthesize the target molecule. The chemically obtained free disaccharide 30 was sequentially sialylated and fucosylated in an enzyme-catalyzed regio- and stereospecific manner to form 1 in high yields. The linker appended 1 can be covalently attached to a carrier protein for further immunological studies.

Spacer-separated sialyl LewisX cyclopeptide conjugates as potential E-selectin ligands

Completely protected sialyl LewisX azide was synthesized from a neolactosamine azide precursor carrying a 3-O-allyloxycarbonyl group as the temporary protecting group. After its Pd(0)-catalyzed deprotection and stereoselective alpha-fucosylation, the obtained LewisX azide was subjected to O-deacetylation in the galactose unit and subsequent regio- and stereoselective sialylation. Reduction of the anomeric azido group afforded the sialyl LewisX amine building block. Two molecules of this tetrasaccharide ligand were conjugated to a preformed cyclooctapeptide containing two equidistant l-asparagine units equipped with carboxy-terminated tetraethyleneglycol side chains to give, after deprotection, the target glycopeptide conjugate. Preliminary biological evaluation of the synthesized bivalent sialyl LewisX cyclopeptide conjugate showed only slightly enhanced inhibition of E-selectin binding in spite of the given flexibility of the two linked saccharide determinants.