Nitrogen, ring oxygen replacement

Nitrogen, ring oxygen replacement by, 140-141 Nomenclature, 44-177, see also... 

Nitrogen and oxygen are found in level 2 of the Periodic Table, and a further alteration in ring topology may arise when the heteroatom is replaced by an element from a lower level. Here, apart from an increase in atomic diameter, the replacement element may use a hybridization state different than that of the earlier elements. Not only can this affect the shape of the molecule, it can also modify the chemical properties. 

Important reactions of this type include the replacement of a ring oxygen by a ring sulfur or nitrogen. Table 8 gives some examples and references to the section where they are discussed. 

Pyrylium ions are six-membered heterocycles in which a positively charged sp2-hybridized oxygen replaces the nitrogen in pyridine. The pyrylium ring appears in many naturally occurring flower pigments. 

The indolic nitrogen is the active redox center of indoles, due to its lone pair of electrons [101,102]. Indeed, if the oxygen replaces the nitrogen in the indolic ring, the antioxidant activity of the resulting benzofurane is much lower [47]. Delocalization of this electron pair over the aromatic system seems to be of great importance for antioxidant activity of indole derivatives. 

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