A.S. Smirnov, E.S. Yandanova, N.A. Bokach, G.L. Starova, V.V. Gurzhiy, M.S. Avdontceva, A.A. Zolotarev, V.Yu. Kukushkin
“Zinc(II)-mediated generation of 5-amino substituted 2,3-dihydro-1,2,4-oxadiazoles and their further ZnII-catalyzed and O2-involving transformations”
New J. Chem., 2015, 12, 9330-9344
ZnII-activated cyanamides NCNR2 (R2 = Me2, Et2, C5H10, (CH2)2O(CH2)2, Ph2) react with the acyclic N-alkyl ketonitrones Ph2C[double bond]N+(O−)R′ (R′ = Me, CH2Ph) and N-aryl ketonitrones (R′ = Ph, p-BrC6H4, p-EtC6H4) under mild conditions. Uncomplexed 5-aminosubstituted 2,3-dihydro-1,2,4-oxadiazoles (6 examples; 49–82%) were obtained in zinc(II)-involving cycloaddition of the N-alkyl ketonitrones to the cyanamide substrates; these 2,3-dihydro-1,2,4-oxadiazoles undergo ring-opening giving carbamoylamidines and methylidenureas. The N-aryl ketonitrones react with ZnII-activated cyanamides giving the open-chain systems, viz. carbamoylamidines, N′-(2-(diphenylmethylidene)amino)-phenyl-N,N-carbamimidic acids, and methylidenureas, which are presumably formed via the cycloaddition route followed by the N–O cleavage induced by the acceptor character of the aryl groups.