Structure of Five-membered Rings with Two or More Heteroatoms

Structure of Five-membered Rings with Two or More Heteroatoms 

We classify compounds as aromatic [(4n + 2) ir-electron systems] or antiaromatic (2n TT-electron systems), if there is continuous conjugation around the ring, and as non-aromatic. Aromatic compounds are further subdivided into those without exocyclic double bonds and those in which canonical forms containing exocyclic double bonds contribute. 

Triazolium, Oxadiazolium, Thiadiazolium, Dioxazolium, Oxathiazolium, Dithiazolium 

Of the mesoionic systems, (40) and its aza derivatives (43), (44) and (49) have been designated as Class B by Ollis (76AHC(l9)l), including compounds with X = CRR there are 88 total systems. Class A mesoionic compounds include (41), (42) and their aza derivatives (45)-(48), (50) and (51), giving a total of 144 systems. Members of the latter group contain 1,3-dipoles, often reflected in their pronounced ability to undergo cycloaddition reactions. 

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