Abstract The reactions of 2-amino- and 2-alkylamino-2-deoxyglycopyranoses with isothiocyanates gave thioureas and heterocyclic derivatives. Moreover, cis-fused glycopyrano[2,1- d]imidazolidin-2-thiones, which have a close structural analogy with some naturally-occurring glycosidase inhibitors, were synthesised for the first time. The mechanism of formation of glycofurano and glycopyrano[2,1- d]imidazolidin-2-thiones occurs via the intermediacy of monocyclic 5-hydroxyimidazolidin-2-thiones. These substances are generated by intramolecular nucleophilic addition of an NH group to the aldehyde function of the sugar. The monocyclic intermediates have been isolated and their participation in the formation of bicyclic imidazolidin-2-thiones and/or monocyclic imidazolin-2-thiones proved. In stark contrast, cis-fused glycopyranothiazolidines were prepared by nucleophilic displacements.