Novel synthesis, properties, and oxidizing ability of 11,13-disubstituted 3,8-methanocycloundeca[8,9-b]pyrimido[5,4-d]-furan-12(11H),14(13H)-dionylium tetrafluoroborates

Synthesis of novel 4,9-methanoundecafulvene [5-(4,9-methanocycloundeca-2',4',6',8',10'-pentaenylidene)pyrimidine-2(1H),4(3H),6(5H)-trione] derivatives 10a-c was accomplished. Their structural characteristics were investigated on the basis of the 1H and 13C NMR and UV-vis spectra. Upon treatment with DDQ, 10a-c underwent oxidative cyclization to give novel 11,13-disubstituted 3,8-methanocycloundeca[8,9-b]pyrimido[5,4-d]furan-12(11H),14(13H)-dionylium tetrafluoroborates 11a-c*BF4- in good yields. The spectroscopic properties of 11a-c*BF4- were studied, and the structural characterization of 11b*BF4- was performed by the X-ray crystal analysis. Cations 11a-c were very stable, and their pKR+ values were determined spectrophotometrically to be 8.3-8.9. The electrochemical reduction of 11a-c exhibited low reduction potentials at -0.43 to -0.45 (V vs Ag/AgNO3) upon cyclic voltammetry (CV). In a search for reactivity, reactions of 11a*BF4- with some nucleophiles, hydride and diethylamine, were carried out to clarify that the methano-bridge controls the nucleophilic attacks to occur with endo-selectivity. The photoinduced oxidation reactions of 11a*BF4- toward some amines under aerobic conditions were carried out to give the corresponding carbonyl compounds in more than 100% yield.