A study of the chemistry of some polycyclic mesoionic compounds
The attempted synthesis of three different types of polycyclic mesoionic systems containing the sydnone nucleus are reported. These systems comprise the fused-aromatic ring mesoionic, the bismesoionic with direct connection and indirect connection between the mesoionic nuclei. The preparation of o-phenylene bissydnone has been achieved and the characteristic 1,3-dipolar cycloaddition exploited using dimethylacetylene dicarboxylate to prepare the corresponding bis~pyrazole. The di-bromo derivative of the o-phenylene bissydnone has also been prepared and characterised. Several attempts to prepare a tetracyclic system by the coupling of two mesoionic rings through an atom, such as sulphur, or a group, such as carbonyl or thiocarbonyl have not been successful, but the coupling of two sydnone rings together directly via the di-bromo derivative was achieved. The resulting tetracyclic aromatic ring system represents both bismesoionic (with direct and indirect connection) and fused-aromatic ring systems. The synthesis of both sydno[3,4-a]quinoxalin-4-one and -4-thione, fused-aromatic ring compounds with the 5-niembered mesoionic sydnone ring incorporated into a conjugated f used-ring heteroaromatic system, have been achieved. Methyl, benzyl and amino derivatives of the sulphur analogue of the quinoxaline derivatives have been prepared and characterised. The success of the preparation of the quinoxaline derivatives has been confirmed by comparison of the oxygen analogue with an authentic sample of sydno [3,4-a]quinoxalin-4-one prepared by an alternative route. Unsuccessful attempts to find further new routes to the synthesis of sydno[3,4-a]quinoxalin-4-one are reported. Several attempts to prepare bismesoionic systems containing different mesoionic nuclei by building up mesoionic triazoles or thiadiazoles on a preformed derivative of the sydnone or vice versa, are discussed in the latter sections. Although the synthesis of this type of bismesoionic system was not successful, potential precursors to these systems were made, and a new method in the preparation of the mesoionic l,2,4-triazolium-3-thiolate was developed.