Direct synthesis of N2-unprotected five-membered cyclic guanidines by regioselective [3 + 2] annulation of aziridines and cyanamides

A novel and efficient [3 + 2] annulation of 2-substituted aziridines and N-tosyl cyanamides via a domino regioselective ring-opening/5-exo-dig cyclization procedure has been developed, allowing the direct preparation of N2-unprotected five-membered cyclic guanidines in good to excellent yields under mild conditions without metals and strong bases. Moreover, the highly biologically interesting urea analogues could also be conveniently obtained via hydrolysis of the produced guanidines.