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

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