Stereoselective Epimerizations of Glycosyl Thiols

Glycosyl thiols are widely used in stereoselective S-glycoside synthesis. Their epimerization from 1,2-trans to 1,2-cis thiols (e.g., equatorial to axial epimerization in thioglucopyranose) was attained using TiCl₄, while SnCl₄ promoted their axial-to-equatorial epimerization. The method included application for stereoselective β-d-manno- and β-l-rhamnopyranosyl thiol formation. Complex formation explains the equatorial preference when using SnCl₄, whereas TiCl₄ can shift the equilibrium toward the 1,2-cis thiol via 1,3-oxathiolane formation.