Type A mesoionic compounds (1980–2020)

Abstract

Type A mesoionic compounds are of particular interest due to their participation in synthetically useful 1,3-dipolar cycloaddition reactions. This review covers the chemistry of type A mesoionic rings published in the period 1980–2020 together with some work published in the first half of 2021. Type A mesoionic rings are conjugated heterocyclic mesomeric betaines (HMBs). Their relationship to other heterocyclic HMBs, including cross-conjugated HMBs, semi-conjugated HMBs and type B mesoionic rings, is discussed together with general aspects of their structure and properties. This is followed by a discussion of the chemistry of all the known type A ring systems that has been published since 1980.