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

Ring transformation of 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]furan-8(7H),10(9H)-dionylium tetrafluoroborate 4(+) (.) BF4- to 7,9-dimethylcyclohepta[b]pyrimido[5,4-d]pyrrrole-8(7H),10(9H)-dionylium tetrafluoroborate 6a-1(+ .) BF4- consists of the reaction of 4(+ .) BF4- with amines and subsequent exchange of the counter-ion using aq. HBF4. Reactions of 4(+) (.) BF4- with aniline and 4-substituted anilines afforded the corresponding pyrrole derivatives 6a-c(+) (.) BF4- directly in good yields. On the other hand, reaction of 4(+) (.) BF4- with benzylamine gave the troponeimine intermediate 9, which was not converted to 6d(+) (.) BF4- and reverted to 4(+) (.) BF4- by adding HBF4; however, it was converted to 6d(+) (.) BF4- upon treatment with (COCl)(2) or SOCl2, 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(+) (.) BF4-, 5(+) (.) BF4-, and 6a,e(+) (.) BF4- 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. (C) 2004 Elsevier Ltd. All rights reserved.