Meso-ionic—Definition and Delineation

In the definition of the term meso-ionic as originally proposed by Baker and Ollis, it was stated that a compound may be appropriately called meso-ionic if it is a five- or possibly a six-membered heterocyclic compound which cannot be represent satisfactorily by any one covrdent or polar structure and possesses a sextet of electrons in association with the atoms comprising the ring. This definition has formed the basis of this review, but we now propose a modification. 

One hundred and forty-four meso-ionic heterocycles of type A (13, 19) and 84 meso-ionic heterocycles of type B (14, 20) are possible. The numbers of these two types which are now known (Table I type A, 44 representatives) and (Table II type B, 8 representatives) encourage us to put forward the proposal that the term meso-ionic should in future be restricted to five-membered heterocycles belonging to type A (13, 19) and type B (14,20). This clear restriction upon the use of the term meso-ionic has obvious advantages. It still embraces 228 different classes of heterocycles with a common structural characteristic, and the many types of meso-ionic compounds included in the authoritative review by Ohta and Kato are included. Needless to say, the restriction upon the definition of the term meso-ionic to five-mem red heterocycles of type A and type B still includes, for example, benz derivatives such as the compounds 67, 71, 110, 123, 133, 151, 206, 209, 226, 255, and 258. 

It is very important to emphasize that the terms meso-ionic compound and sydnone are not interchangeable. The term sydnone should be used only for the heterocyclic system (1). It just causes confusion if the word sydnone is used to describe other heterocyclic systems, although they are meso-ionic. The term sydnone should not be used to describe derivatives of l,3,2-oxathiazol-5-one (169) and 1,2,3,4-tetrazole-5-thione (413).  

If heterocyclic chemists can be persuaded to restrict the use of the term meso-ionic only to five-membered heterocycles of type A and type B, then this wilt necessarily exclude some compounds that have 

Other polyaza derivatives of aromatic systems, such as the blue tria-zaphentalene derivative (478), are also adequately described as meso-meric betaines or inner salts. Ohta and Kato, in a section entitled Bridged Heterocyclic Compounds (ref. 9b, page 241), have referred to seven types of compounds and the stereochemically unacceptable structures (479-485) which had been allocated to them. The possibility was considered by Ohta and Kato that some of these structures might have to be replaced by their meso-ionic equivalents. However, we have now established that this is not the case, and the relation between the original structures (479-485) and the correct structures (486-492) is given in Fig. 10.  

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D. Molecular Orbital Calculations. Pharmacological Activity of Meso-ionic Compounds Meso-ionic— Definition and Delineation... 

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