Mesoionic compounds nomenclature

Ollis and Ramsden state that A compound may be appropriately called mesoionic if it is a five-membered heterocycle which cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of electrons in association with the five atoms comprising the ring . From the point of view of systematic nomenclature, compounds of this type are difficult to deal with, since most available nomenclature systems are designed so as to name one particular bond- and charge-localized canonical form. 

Heterocycle (85) may be systematically described as 3-substituted anhydro-5-hydroxy-1,2,3,4-oxatriazolium hydroxide. However, this nomenclature is somewhat cumbersome and the terminology mesoionic has been used throughout this review. Thus compounds (85)-(87) and (91) are named mesoionic l,2,3,4-oxatriazol-5-ones (85), -5-thiones (86), -5-imines (87) and mesoionic l,2,3,4-thiatriazole-5-alkenes (91), respectively. Using IUPAC Rule. C-87 (85) is named 3-substituted l,2,3,4-oxatriazol-5-ylio oxide or 3-substituted l,2,3,4-oxathiazolylium-5-olate and (87 R = R1 = Ph) is named iV-[3-phenyl-5-(l,2,3,4-oxatriazolio)]anilide. 

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