Tungstate complexes of aldoses and ketoses of the lyxo series. Multinuclear NMR evidence for chelation by one or two oxygen atoms borne by the side chain of the furanose ring

The formation of tungstate complexes of aldoses and ketoses with lyxo configuration was studied in aqueous solution by 13C and 183W NMR spectroscopy. Two series of complexes were structurally characterized, in which the sugars adopt the furanose form and chelate a ditungstate group at a tetradentate site. All sugars form a major complex (type L for lyxo) similar to the molybdate species, in which the sites of chelation, O-1,2,3,5 for aldoses or O-2,3,4,6 for ketoses, involve the anomeric oxygen atom and the side chain atom O-5 or O-6. A second type of complex was identified (type M for manno), in which the sites of chelation are O-2,3,5,6 for d-mannose (tungstate species only) and O-3,4,6,7 for d-manno-heptulose (tungstate and molybdate complexes). The overall equilibrium constants for the formation of the complexes are reported and show that ketoses form stronger complexes than aldoses.