Synthesis of glycodendrimers and glycocalix[4]arenes.

Abstract

In order to further understand fundamental carbohydrate-protein interactions as well as to supply carbohydrate clusters of high affinity, glycoconjugates with differing carbohydrate densities, conformations, and interglycosidic spacings were prepared. A series of multivalent alpha-sialosides were scaffolded onto gallic acid-based dendrimers; and calix[4]arenes. Carbohydrate residues involved in the syntheses of glycoconjugates were all prepared stereoselectively in high yields from phase transfer catalysis (PTC) reaction. These carbohydrate ligands were coupled to the dendritic and calixarene cores through nucleophilic displacement of chlorides by thioglycosides. The conjugation strategy employed herein lead to the study of different chemoselective deprotection of thioacetate glycosides. The methods studied involved, (i) hydrazinium acetate in DMF (one- & two-pot reactions) and (ii) sodium methoxide in MeOH at low temperature (Zemplen conditions). Solid phase synthesis on Wang resin was used to construct the first dendritic athiosialosides. The design of these hyperbranched clusters was based on L-lysine core structures using established Fmoc protecting group and benzotriazolyl activated ester coupling procedures. Chain extension of the lysyl amino groups with chloroacetylglycylglycine active ester allowed introduction of the required functionality necessary for the coupling to an alpha-thiosialoside derivative. The synthesis of hyperbranched glycodendrimers containing sialic acid residues having 3n in valency, is also described. Gallic acid as trivalent core and oligoethylene glycol derivatives as hydrophilic spacers were used to scaffold the dendritic backbones. alpha-Thiosialoside derivative was conjugated onto N-chloroacetylated dendritic precursors by nucleophilic substitution to afford tri- and nona-valent sialodendrimers. The synthesis of glycocalix[4]arenes was then effected since these glycoconjugates have an extra advantage over chemically well-defined glycodendrimers. They possess the ability to form drug inclusion complexes. p-Tert-butylcalix[4]arene was transformed into its tetraethyl ester. This opened the way to the formation of the acid chloride which was treated with excess mono-Boc-1,4-butanediamine or mono-Boc-1,6-hexanedianiine used as spacer arms. alpha-Thiosialoside derivative was covalently attached to the calix[4]arenes by nucleophilic substitution on the cone-shaped tetra-N-chloroacetylated calix[4]arenes. The glyco-calix[4]arenes were liberated from their protecting groups to afford biologically active clusters where sialic acid is the ligand. A different tetravalent sialocalixarene was synthesized following a convergent approach. Tetraacyl chloride calix[4]arene derivative was reacted by peptide coupling with an aminosialoside having a long spacer arm. Finally, octavalent dendritic sialocalix[4]arene was then synthesized from a tetraamino, calix[4]arene and N-bromoacetylated sialoside derivatives. Both reagents have built-in spacer arms which allowed for an efficient double N-alkylation reaction. Binding studies via turbidimetric analysis confirmed the ability of the glycodendrimers and glycocalix[4]arenes to cross-link and precipitate appropriate lectins (WGA and LFA).