TRIDENTATE COMPLEXES OF PERIODATE AND SOME FURANOSE DERIVATIVES

1,2-O-Isopropylidene-α-D-glucofuranose (I) is highly resistant to glycol scission by periodate in alkaline solution. This atypical effect is caused by preferential formation of a stable periodate complex. Proton magnetic resonance spectroscopy shows that the 3-, 5-, and 6-protons of I (and the 3- and 5-protons of I-6,6′-d2) become strongly deshielded during this process, which, together with accompanying changes in proton–proton coupling, establishes that the complex (II) has a 3,5,6-tridentate structure. Variations in the stability of II with pH suggest that it can exist either as a monoanion or dianion. A complex analogous to II is formed by ethyl α-D-thioglucofuranoside, whereas methyl β-D-galactofuranoside forms a complex that probably has the 2,5,6-structure. The characteristics of these tridentate complexing reactions support the general concept that a five-membered cyclic complex is a transient intermediate of α-glycol oxidation by periodate.