Accumulation and Transformation of Saccharides by the Cooperative Effects of Metal Ions and Amines

Abstract

A series of nickel(II) complexes containing N-glycosides derived from the reaction of saccharides and polyamines have been prepared and characterized; the amine ligands act as anchors for the accumulation of sugar molecules on metal complexes. The real existence of the C-N bond in the N-glycoside ligand on cobalt (III) complexes was unambiguously proven by the application of the isotopic multiplets in the ¹³C-NMR spectra. The structures of the sugar units were analyzed by means of the semiempirical AM 1 calculations coupled with the conversion of the vicinal ¹H-¹H spin-spin coupling constants in the ¹H-NMR spectra into torsion angles of the corresponding H-C-C-H fragments. Aldoses are rapidly epimerized at C-2 by the cooperative effects of nickel(II) ion and N-alkylated diamines, involving a stereospecific 1, 2-carbon shift, and nickel(II) ions form complexes with only the mannosetype epimers stereoselectively. In the light of the results of NMR and EXAFS studies as well as electronic absorption and circular dichroism spetral data, we proposed a possible mechanism for the epimerization of _D-Glc to _D-Man with the nickel(II) -N, N, N', N' -tetramethylethylendiamine system in which the C-2 epimerization proceeds via a mononuclear nickel(II) intermediate complexes, where the carbinolamine-like adduct of aldose with diamine in an open-chain form is epimerized at C-2 by a stereospecific rearrangement of the carbon skeleton involving a cyclic transition state. The newly discovered sugar transformation reaction using nickel(II) -N-alkylated diamine system has been applied to the synthesis of the (1-6)-linked disaccharides having the mannose reducing terminal from their C-2 reducing terminal epimers which are abundant in nature. Aldoses are also epimerized at C-2 by combinations of certain metal ions (Co²⁺, Ca²⁺, Sr²⁺, and Pr²⁺) and N-alkylated diamines. In the system of Ca²⁺ and monoamine a novel C-2 epimerization of aldoses proceeds via the stereospecific rearrangement of the carbon skeleton and ketose forms without 1, 2-carbon shift.