Synthesis and properties of novel 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-triones: methodology for synthesizing cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates

Novel condensation reaction of tropone with N-substituted and N,N′-disubstitued barbituric acids in Ac2O afforded 5-(cyclohepta-2′,4′,6′-trienylidene)pyrimidine-2(1H),4(3H),6(5H)-trione derivatives () in moderate to good yields. The 13C NMR spectral study of revealed that the contribution of zwitterionic resonance structures is less important as compared with that of 8,8-dicyanoheptafulvene. The rotational barriers (ΔG‡) around the exocyclic double bond of mono-substituted derivatives were obtained to be 14.51–15.03 kcal mol−1 by the variable temperature 1H NMR measurements. The electrochemical properties of were also studied by CV measurement. Upon treatment with DDQ, underwent oxidative cyclization to give two products, 7 and 9-substituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborates ( and ) in various ratios, while that of disubstituted derivatives afforded 7,9-disubstituted cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborate () in good yields. Similarly, preparation of known 5-(1′-oxocycloheptatrien-2′-yl)-pyrimidine-2(1H),4(3H),6(5H)-trione derivatives () and novel derivatives was carried out. Treatment of with aq. HBF4/Ac2O afforded two kinds of novel products and in various ratios, respectively, while that of afforded . The product ratios of and observed in two kinds of cyclization reactions were rationalized on the basis of MO calculations of model compounds . The spectroscopic and electrochemical properties of and were studied, and structural characterization of based on the X-ray crystal analysis and MO calculation was also performed.