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

Abstract

Zn^II-activated cyanamides NCNR₂ (R₂ = Me₂, Et₂, C₅H₁₀, (CH₂)₂O(CH₂)₂, Ph₂) react with the acyclic N-alkyl ketonitrones Ph₂CN⁺(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.