Substrate-dependent chemoselective aldose–aldose and aldose–ketose isomerizations of carbohydrates promoted by a combination of calcium ion and monoamines Original Research Article

Epimerization of aldoses at C-2 has been extensively investigated by using various metal ions in conjunction with diamines, monoamines, and aminoalcohols. Aldoses are epimerized at C-2 by a combination of alkaline-earth or rare-earth metal ions (Ca2+, Sr2+, Pr3+, or Ce3+) and such monoamines as triethylamine. In particular, the Ca2+–triethylamine system proved effective in promoting aldose–ketose isomerization as well as C-2 epimerization of aldoses. 13C NMR studies using d-(1-13C)glucose and d-(1-13C)galactose with the CaCl2 system in CD3OD revealed that the C-2 epimerization proceeds via stereospecific rearrangement of the carbon skeleton, or 1,2-carbon shift, and ketose formation proceeds partially through an intramolecular hydrogen migration or 1,2-hydride shift and, in part, via an enediol intermediate. These simultaneous aldose–aldose and aldose–ketose isomerizations showed interesting substrate-dependent chemoselectivity. Whereas the mannose-type aldoses having 2,3-erythro configuration (d-mannose, d-lyxose, and d-ribose) showed considerable resistance to both the C-2 epimerization and the aldose–ketose isomerization, the glucose-type sugars having 2,3-threo and 3,4-threo configurations, d-glucose and d-xylose, are mainly epimerized at C-2 and those having the 2,3-threo and 3,4-erythro configurations, d-galactose and d-arabinose, were mostly isomerized into 2-ketoses. These features are of potential interest in relevance to biomimic sugar transformations by metal ions.