Synthesis of C-linked Disaccharides and Analogues as Biomimetics of the TF Epitope

Glycoproteins and glycolipids are major components of the outer surface of mammalian cells. Their carbohydrate moieties, which are directed into the outer environment, serve as ligands for receptor proteins, such as lectins, pathogen proteins and antibodies. Recognition processes between the receptor proteins and the carbohydrates have a significant impact on different aspects of cell biology as they turn structure into biological response in a selective way. Cellular carbohydrate structures are found to change dramatically during embryonic development or under pathological conditions including malignant transformation. One example is the Thomsen-Friedenreich (TF) epitope, a disaccharide consisting of galactose-β-(1→3)-N-acetyl-galactosamine α-linked to a serine or threonine of a carrier protein. The TF epitope is considered as a cancer-specific carbohydrate antigen that is found on various epithelial carcinomas of human adults and can be recognized by the anti-TF antibody. It has been demonstrated that synthetic glycoconjugates of the TF epitope can act as anti-cancer vaccines by stimulating the immune system against tumor growth and proliferation. Native carbohydrates are liable to chemical and enzymatic hydrolysis of their glycosidic bonds and therefore short-lived in the blood. Their stable C-analogues in which the glycosidic oxygens have been replaced by methylene or substituted methylene units are particularly interesting as therapeutic agents. We report herein our efforts to synthesize non-hydrolysable methylene and fluorinated methylene linked disaccharides analogues of the TF epitope. Our synthesis follows a methodology previously developed in our laboratory for the preparation of C(1→3)-disaccharides. The key step is an Oshima-Nozaki coupling between a β-C-galactosyl carbaldehyde and an enone, isolevoglucosenone. The hydroxyl methylene linker that is created by this coupling can be deoxygenated or can be substituted by one fluorine atom (or two fluorine atoms via the corresponding ketone). 1,4-Addition of an amine to the enone derived from the Oshima-Nozaki coupling, ketone reduction and some protecting group manipulations gave an hydroxyl methylene linked disaccharide that could smoothly undergo Barton-McCombie deoxygenation. On the contrary, the attempts of nucleophilic fluorination of this substrate failed to give the desired outcome probably due to an intramolecular rearrangement. With the deoxygenated product in hand and after some protecting group transformations, we attempted C-allylation of the anomeric centre and at the same time cleavage of the 1,6-anhydro bridge. Some preliminary results are presented. An allyl-C-disaccharide can lead to a C-glycosyl amino acid according to the literature and therefore it can be used for preparing a C,C-analogue of the TF epitope. In our search for biomimetics of the TF epitope with less synthetic requirements, we attempted to replace the second saccharidic unit by a phenyl ring. Simple phenyl CH2- and CF2-galactosides were prepared starting from α- and β-C-galactosyl carbaldehydes. NMR studies in combination with theoretical calculations revealed that the conformational behaviour of these galactosides in solution depends on the substitution at pseudoanomeric center. Similarities and differences to their O-analogues are discussed. Additionally, NMR spectroscopy was used to detect binding to a lectin protein. The β-galactosyl carbaldehyde –common starting material for the synthesis of C-linked disaccharides and phenyl C-galactosides mentioned above – was also used to prepare a small library of CH2-galactosyl aryl ethers bearing different aromatic amides. The affinities of these derivatives towards some galactoside-specific lectins, the galectins, were measured. None of the synthesized C-galactosyl analogues appeared to be high-affinity ligand of galectins. However, such studies are useful for the future design of sugar analogues with the minimum of functionalities that assure strong binding to carbohydrate recognition proteins.