One method of achieving stereoselective glycoside synthesis is via the Lewis acid promoted anomerisation of a glycoside, which often means converting 1,2-trans glycosidic linkage to the 1,2-cis anomer. This reaction has been shown to be successful in some cases, but applications are somewhat limited due to lack of a full understanding of factors which influence the glycoside reactivity. In order to gain greater insight to the mechanism of anomerisation, a series of benzoylated butyl glycosides 1, as well as those of the corresponding glucuronic and galacturonic acid methyl esters have been prepared, where X (H, F, Me, Cl, Me, CF3, F, OMe, Br) has been varied at C-2 as shown in Fig. 1. The rates of anomerisation have been determined by 1H NMR spectroscopy. The data has been used to obtain both Hammett and Swain-Lupton plots, which can be interpreted as the 2-OBz group influencing reactivity through a combination of resonance and inductive effects. It is proposed that the carbonyl group at C-2 can contribute to cation stabilisation in a rate influencing step but does not involve significant positive charge development in the benzoate

Study of the influence of a 2-O-benzoate group on the SnCl4 promoted anomerisation of a glucopyranoside, glucuronic & galacturonic acid

ACS Spring Meeting 2021

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