Ring-Opening of Carbamate-Protected Aziridines and Azetidines in Liquid Sulfur Dioxide

The reactions of halides or pseudohalides of group I and II metals with carbamate-protected aziridines and azetidines in liquid sulfur dioxide as solvent resulted in the efficient ring-opening of these heterocycles. Sulfur dioxide, as a highly polar solvent, solubilizes the inorganic salts and acts as a mild Lewis acid to accelerate the ring-opening. For this reason, carbamate-protected small heterocycles can be used instead of the corresponding sulfonamides, the activities of which are well-established in ring-opening reactions. Thiols also performed well in the SO2-assisted ring-opening of aziridines. This procedure occurred without racemization of the stereogenic centers.