The Ni (II) or Ca (II) complex-catalyzed C-2 epimerization of aldose and ketose have been investigated using
13C-enriched substrates. During our studies on this reaction, it became apparent that (1-
13C) -D-glucose changed into (2-
13C) -D-mannose or vice versa and (2-
13C) -fructose converted into 2-
13C- (hydroxymethyl) -D-ribose (hamamelose). These results said that the product sugars were prepared not
via the ene-diol rearrangement but
via stereospecific one. It was also elucidated that the coordination state of the intermediate complex consisting of metal, ligand, and sugar played an important role during the reaction.
With more hydrophobic ligands such as
N, N, -dimethyl-
N'-higher alkylethylenediamine and
N-higher alkyltrimethylenediamine, the epimerization were smoothly proceeded not only in methanolic solution also in aqueous solution, where ordinary hydrophilic complex could not show effective epimerization ability. The excellently organized reaction system due to their self aggregation ability like micelles would present a proper environment for the sugar epimerization.
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