Cyclic guanidines were obtained in excellent yield using the cobalt catalyst C6, Me3NFPY•OTf and (Me2SiH)2O in toluene. (Table ). Investigation of the electronic effects using substrates containing aniline structures produced high yields of five-membered cyclic guanidines, including electron-withdrawing, electron-donating, and unsubstituted forms (6a-6c). Benzylamine was also applied (6d). Moreover, alkenyl guanidines with different carbon chains were used to form sixmembered ring 6e and seven-membered ring guanidine 6f. Next, the scope of the seven-membered ring guanidine synthesis was examined. For example, we successfully synthesized benzene-ringfused seven-membered cyclic guanidines in good yields, for example, 6g and 6h by introducing substituents on the benzene ring; 6i, by changing the benzene ring to an alkyl group; and 6k, by introducing a benzene ring fused to the benzene ring. The Cbz group was replaced with a Boc group, and cyclic guanidines 6l and 6m were obtained in good yields. Further transformations of the seven-membered cyclic guanidines enabled the formation of polycyclic compounds. Table Azetidine synthesis was also possible using homoallyl sulfonylamide as a starting material; using a tosyl group on the nitrogen is crucial in this method. Dichloromethane and toluene were the optimal solvents, different from that of oxetane synthesis (4-MTHP). This method is not applicable to monosubstituted alkenes. Various reductive conditions enabled detosylation reactions for several products. For example, bis-azetidine 6o is reductively deprotected by magnesium metal and the estrone derivative 6p by lithium aluminum hydride.