Meso-ionic compounds

This review provides a general survey of the chemistry of known type A and type B mesoionic heterocycles up to September 1974 with an appendix extending the literature coverage to the end of 1975. The extensive chemistry of sydnones and sydnone imines is not included as this is reviewed elsewhere. The difference between type A and type B mesoionic rings is demonstrated and the number of possible mesoionic rings of each type is discussed. The preparation and reactions of known systems are then systematically reviewed. Additional sections cover general properties of mesoionic rings including valence tautomerism, photochemistry, X-ray crystallography, molecular orbital calculations, pharmacological activity and the interconversion of mesoionic isomers. In a final section, the case for restricting the term mesoionic to five-membered heterocycles of type A and type B is emphasised. Examples of other dipolar heterocycles that have previously been described as mesoionic are discussed and it is recommended that these be described as mesomeric betaines.