TUNGSTATE COMPLEXES OF ALDOSES AND KETOSES OF THE LYXO SERIES - MULTINUCLEAR NMR EVIDENCE FOR CHELATION BY ONE OR 2 OXYGEN-ATOMS BORNE BY THE SIDE-CHAIN OF THE FURANOSE RING

The formation of tungstate complexes of aldoses and ketoses with lyre configuration was studied in aqueous solution by C-13 and W-183 NMR sp ectroscopy. Two series of complexes were structurally characterized, i n which the sugars adopt the furanose form and chelate a ditungstate g roup at a tetradentate site. All sugars form a major complex (type L f or lyxo) similar to the molybdate species, in which the sites of chela tion, O-1,2,3,5 for aldoses or O-2,3,4,6 for ketoses, involve the anom eric 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 chela tion 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 overa ll equilibrium constants for the formation of the complexes are report ed and show that ketoses form stronger complexes than aldoses.