Novel C-2 epimerization of aldoses promoted by nickel(II) diamine complexes, involving a stereospecific pinacol-type 1,2-carbon shift

Abstract

The newly discovered C-2 epimerization of aldoses promoted by nickel(II) diamine complexes has been investigated in detail by using /sup 13/C-enriched D-glucose, /sup 13/C NMR spectroscopy, and EXAFS (extended x-ray absorption fine structure) analysis. Aldoses treated with nickel(II) diamine complexes (diamine = N,N,N'-trimethylethylenediamine (N,N,N'-Me/sub 3/en), N,N,N',N'-tetramethylethylenediamine (N,N,N',N'-Me/sub 4/en), etc.) in methanolic solutions were rapidly (60/degree/C, 3-5 min) epimerized at C-2 to give equilibrium mixtures where the ratio of C-2 epimers shifts to the side of the naturally rare mannose-type aldoses (having the cis arrangement of C-2 and C-3 hydroxyl groups) compared with those in the thermodynamic equilibrium states. The epimerization product of D-(1-/sup 13/C)glucose was exclusively D-(2-/sup 13/C)mannose, demonstrating that the reaction involves a stereospecific 1,2-shift of the carbon skeleton resulting in inversion of configuration at C-2. Furthermore, the absorption and circular dichroism spectra of the reaction solutions indicated the presence of an intermediate nickel(II) complex containing both diamine and sugar components, which was directly revealed by EXAFS analysis to be a mononuclear nickel(II) complex having octahedral coordination geometry. All these observations strongly suggest that the C-2 epimerization proceeds through an intermediate mononuclear nickel(II) complex, where the carbinolamine-like adduct of aldose with diamine in an open-chain form is epimerized atmore » C-2 by a stereospecific rearrangement of the carbon skeleton or a pinacol-type rearrangement involving a cyclic transition state. 44 refs., 5 figs., 4 tabs.« less