C-2 Epimerization of aldonolactones promoted by magnesium iodide: a new way towards non-enzymatic epimerization.

The first example of a non-enzymatic C-2 epimerization of aldonolactones is reported. The reaction of 2,3,4,6-tetra-O-benzyl-d-gluconolactone or 2,3,4,6-tetra-O-benzyl-d-mannonolactone with MgI(2) in EtOH afforded their respective C-2 epimer. Studies conducted in EtOD showing the incorporation of a deuterium atom only at the C-2 position of the epimerized product reveal an epimerization rather than a racemization reaction. A mechanism involving a chelation with a magnesium species is proposed to explain this C-2 inversion reaction.