Glycosidation-anomerisation reactions of 6,1-anhydroglucopyranuronic acid and anomerisation of β-D-glucopyranosiduronic acids promoted by SnCl ₄

The reaction of silylated nucleophiles with 6,1-anhydroglucopyranuronic acid (glucuronic acid 6,1-lactones) catalysed by tin(IV) chloride provides 1,2-trans or 1,2-cis (deoxy)glycosides in a manner dependent on the donor structure. The α-glycoside was obtained for reactions of the donor with the 2-acyl group and 2-deoxydonors, whereas the 2-deoxy-2-iodo donor gave the β-glycoside. Experimental evi dence shows that when 1,2-cis-glycoside formation occurs, the anomerisation of initially formed 1,2-trans-glycosides catalysed by SnCl₄ is possible. The anomerisation of β-D-glucopyranosiduronic acids was found to be faster,in some cases, than anomerisation of related β-D-glucopyranosiduronic acid esters and β-D-glucopyranoside derivatives and the rates are dependent on the structure of the aglycon. Moreover, the rates of anomerisation of β-D-glucopyranuronic acid derivatives can be qualitatively correlated with rates of hydrolysis of β-D-glucopyranosiduronic acids. Mechanistic possibilities for the reactions are considered.