Synthesis of a C-linked disaccharide analogue of the Thomsen Friedenreich (TF)-epitope a-O-conjugated to L-serine and formation of a cluster as potential anticancer vaccine

Cell surface glycoproteins and glycolipids are responsible for cellular recognition processes. Vaccination is the procedure whereby the immune system is induced to create antibodies against a foreign molecule involved in disease or viral infection. In many disease states, the oligosaccharide chains presented on cell surface glycoprotein are altered. In some tumors, the glycan chains of glycoproteins are attenuated to only a few sugar residues. In the case of the TF-antigen, the polysaccharide chains have been shortened to a galactose-β-(1–>3)-N-acetyl-galactosamine disaccharide structure α-linked to a serine or threonine. Immunogenicity of this epitope in synthetic vaccines has been demonstrated. However, this disaccharide conjugate is relatively short-lived in the blood stream because of its hydrolysis catalysed by ubiquitous glycosidases in vivo. C-disaccharides are sugar mimetics whose interglycosidic linkage is non-hydrolysable as required for a disaccharide-based vaccine. In the first part of the work, we report the first synthesis of TF-antigen analogues applying the methodology developed by our group for the synthesis of C(1–>3)-disaccharides. Conjugate addition of diethylaluminium iodide (Et2AlI) to isolevoglucosenone leads to an aluminium enolate, which reacts with the sugar derived carbaldehyde, 2,6-anhydro-3,4,5,7-tetrakis-O-[(tert-butyldimethyl)silyl]-D-glycero-L-manno-heptose, to give an aldol. The convergent and stereoselective synthesis of this adduct allows us to obtain C(1–>3)-disaccharides. Reduction of the moiety derived from isolevoglucosenone with lithium borohydride, followed by cleavage of the 1,6-anhydro bridge produces C-disaccharides with D-galacto configuration. Königs-Knorr glycosidation of N-Fmoc-serine tert-butylester, followed by reduction of azide moiety to the corresponding acetamido group allows us to obtain TF-antigen analogues linked either by hydroxymethano (-CH(OH)-) or methano (-CH2-) group. In a second part we report the synthesis of a fully deprotected thio-glycotripeptide based on the TFantigen -C-analogues linked by hydroxymethano - N-acetyl-O-{α-D-glyco}-D-seryl-O-{α-D-glyco}-D-seryl-rac-N-(3-[(acetylthio)amino]propyl)-O-{α-D-glyco}-D-serinamide, which was covalently conjugated to the KLH protein carrier via Michael addition reaction. Biological trials with the TF (analogues)-KLH are in progress. We report also the synthesis of fully deprotected TF-antigen -C-analogues linked by hydroxymethano (-CH(OH) and methano (-CH2-) group. their conformational analysis is currently in progress.