Ring transformation of cyclohepta[ b]pyrimido[5,4- d]furan-8(7 H),10(9 H)-dionylium ion to the corresponding pyrrole derivatives via troponeimine intermediates: photo-induced autorecycling oxidizing reactions of some amines

Abstract

Ring transformation of 7,9-dimethylcyclohepta[ b]pyrimido[5,4- d]furan- 8(7 H),10(9 H)-dionylium tetrafluoroborate 4 +· BF 4 − to 7,9-dimethylcyclohepta[ b]pyrimido[5,4- d]pyrrrole-8(7 H),10(9 H)-dionylium tetrafluoroborate 6a– d +· BF 4 − consists of the reaction of 4 +· BF 4 − with amines and subsequent exchange of the counter-ion using aq. HBF 4. Reactions of 4 +· BF 4 − with aniline and 4-substituted anilines afforded the corresponding pyrrole derivatives 6a– c +· BF 4 − directly in good yields. On the other hand, reaction of 4 +· BF 4 − with benzylamine gave the troponeimine intermediate 9 , which was not converted to 6d +· BF 4 − and reverted to 4 +· BF 4 − by adding HBF 4; however, it was converted to 6d +· BF 4 − upon treatment with (COCl) 2 or SOCl 2, followed by exchange of the counter-ion. In a search for the characteristics of 9 , inspection and comparison of the X-ray crystal analyses, NMR and UV–vis spectra, and CV measurement of 9 and N,N-disubstituted troponeimine derivatives 12 were carried out to suggest the remarkable structure of 12 having ionic C–O bonding between the imine–carbon atom and the oxygen atom of the barbituric acid moiety in the solid state. Thus, characteristics of 9 were ascribed to the sterically hindered and favorable conformation of N-protonated troponeimine intermediates. Furthermore, novel photo-induced oxidation reactions of a series of 4 +· BF 4 − , 5 +· BF 4 − , and 6a, e +· BF 4 − towards some amines under aerobic conditions were carried out to give the corresponding imines in 455–8362% yields [based on compounds 4 + , 5 + , and 6a, e + ], suggesting the oxidation reaction occurs in an autorecycling process. Mechanistic aspects of the amine-oxidation reaction are also postulated.