A new method is described for building up anomeric glycosyl sulfoxides, via the formation of transient glycosulfenic acids and their addition to unsaturated acceptors. Thermolysis of R- and â-3-[(2,3,4,6-tetra-O-acetyl-D-glucopyranosyl)sulfinyl]propanenitriles affords 1-glucosulfenic acids, which are reacted in situ with common substituted alkynes. The obtained (RS,E)-2-[(2,3,4,6-tetra- O-acetyl-R-D-glucopyranosyl)sulfinyl]-2-butendioates are involved as enantiopure sulfinyl dienophiles in Diels-Alder reactions with 2,3-dimethyl-1,3-butadiene to evaluate the role that the sugar moiety plays in the steric control of the cycloaddition. This chemistry provides a direct synthetic strategy for the stereocontrolled connection between thioglycon and aglycon moieties, thus offering the basis for an easy elaboration of new molecules incorporating thiosugar residues.