Novel Synthesis and Oxidizing Ability of Tropylium Ions Annulated with Two 2,4-Dimethylfuro[2,3-<i>d</i>]pyrimidine-1(2<i>H</i>),3(4<i>H</i>)-diones

Convenient preparation of novel tropylium ions annulated with two 2,4-dimethylfuro[2,3-d]pyrimidine-1(2H),3(4H)-diones, 12a+·BF4- and 12b+·BF4-, consists of a reaction of 2-methoxytropone with dimethylbarbituric acid to give 7,9-dimethyl-3-[1‘,3‘-dimethyl-2‘(1‘H),4‘(3‘H),6‘(5‘H)-trioxopyrimidin-5‘-ylidene]cyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dione 8 and the following oxidative cyclization by using DDQ or photoirradiation under aerobic conditions. On the basis of the MO calculations, the selectivity of two types of oxidative cyclization reactions of 8 was rationalized. X-ray crystal analyses and MO calculations were carried out to clarify the structural characteristics of 12a+·BF4- and 12b+·BF4-. The stability of cations 12a+ and 12b+ is expressed by the pKR+ values which were determined spectrophotometrically as 8.8 and 8.6. The electrochemical reduction of 12a+ and 12b+ exhibited reduction potential at −0.63 and −0.62 (V vs Ag/AgNO3), respectively. Reactions of 12a+·BF4- and 12b+·BF4- with some nucleophiles, hydride and diethylamine, were carried out to clarify that the reactivity of 12a+·BF4- and 12b+·BF4- was substantially dependent on the annulating position. The oxidizing ability of 12a+·BF4- and 12b+·BF4- toward alcohols and amines in the autorecycling process was demonstrated as well.