Novel Synthesis, Properties, and Oxidizing Ability of 11,13-Disubstituted 3,8-Methanocycloundeca[8,9-<i>b</i>]pyrimido[5,4-<i>d</i>]-furan-12(11<i>H</i>),14(13<i>H</i>)-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.