Pyridones, pyrones, thiinones, azinones, etc. general pattern of reactivity

2 Pyridones, pyrones, thiinones, azinones, etc. general pattern of reactivity 

These compounds are usually written in the uncharged form (788, 789 Z = NH, NR, O, S), but canonical forms of types (790) or (791) are of comparable importance, i.e. the compounds can also be considered as betaines derived from pyridinium, pyrylium and thiinium cations. They possess considerable stability and aromaticity in that in many of their reactions they revert to type . 

The reactivity pattern of these compounds considered briefly in Section 3.2.1.1.4 will now be summarized. The system of heteroatoms in these molecules can act either as an electron source or an electron sink. This, together with the possibility of readily forming cationic (792) and anionic (793) species, increases considerably the possibilities for reaction in these compounds. 

These can attack ring carbon atoms (3 to the cyclic heteroatom as shown in (794) and (795) the intermediates (e.g. 796) usually revert to type by proton loss (796 — 797). These electrophilic substitution reactions are considered in Section 3.2.1.4. 

Electrophilic reagents can also attack the carbonyl oxygen atom (e.g. 798) reactions of this type are considered in Section 3.2.3.7.3. 

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