threo-N-Alkoxycarbonylamino epoxides 5a–d, containing the oxazolidine moiety, were converted into trans-4,5-disubstituted-2-oxazolidin-2-ones 2 with total regio- and stereoselection by means of nucleophilic intramolecular attack of the carbamate moiety to the protonated oxirane ring. Theoretical calculations confirmed both the regioselection and the preference of the cyclocarbamation reaction vs. the intermolecular attack by the solvent, arising from different behaviour in comparison with the analogous iodonium ions.