Versatile pyrolitic synthesis of tetracyclic heteroaromatic compounds

Abstract

The scope of a new synthetic method for tetracyclic heteroaromatic compounds involving flash vacuum pyrolysis of suitably functionalised phosphorus ylides has been examined in detail. The reaction involves generation of a C=C triple bond and an adjacent aryloxy or arylthio radical which cyclises to form a furan or thiophene ring and this is followed by a second cyclisation to form the tetracyclic product. A total of over sixty new stabilised ylides have been prepared and fully characterised and their pyrolysis has in most cases afforded the target heterocyclic products, many of which are the first examples of new ring systems. For the initial cyclisation, benzene, pyridine and naphthalene-containing systems worked well while for the second cyclisation benzene, thiophene, furan and naphthalene groups could be used. Use of thiophene systems in the initial cyclisation proved to be less successful but in the course of preparation of the necessary starting materials a new fundamental reaction of electron-rich thiophenemethanols was discovered. Preparation of specifically substituted tetracyclic products was also possible by starting from appropriate substituted precursors. The multitude of possible combinations of groups suggested a combinatorial approach and this was successfully achieved with three phosphonium salts and four acid chlorides being used to obtain a mixture of twelve ylides which was then pyrolysed to produce a "library" of twelve heterocyclic products. Ylides giving rise to two aryloxy or arylthio radical sites were also examined and these led to tetracyclic products with a different ring topology. In the course of this work an unexpected eight-step cascade reaction leading to the first benzothienylbenzofuran was discovered. Finally, the extension of the approach to bis(ylides) was examined in one case and this led to direct formation of a heptacyclic heterocyclic product.