Cyclicalkoxyguanidines such as 2-methoxyiminoimidazolidine, 2-benzyloxyiminoimidazolidine and 6',7'-dihydro-5' H-spiro(cyclohexane-1,3'-imidazo(2,1-c)(1,2,4)oxadiazole react with isocyanates and isothiocyanates to give the corresponding urea and thiourea derivatives, which under basic reaction conditions undergo an intramolecular SN2σ nucleophilic substitution reaction at the sp 2 nitrogen atom of