Zinc(II)-mediated generation of 5-amino substituted 2,3-dihydro-1,2,4-oxadiazoles and their further ZnII-catalyzed and O2-involving transformations

Zn-II-activated cyanamides NCNR2 (R-2 = Me-2, Et-2, C5H10, (CH2)(2)O(CH2)(2), Ph-2) react with the acyclic N-alkyl ketonitrones Ph2C=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 Zn-II-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.