Tautomeric forms interconversion and modes of reaction

As discussed in Section 2.4.5.2, hydroxy derivatives of azoles fe.g. 622, 624, 626) are tautomeric with either or both of (i) aromatic carbonyl forms (e.g. 623, 627) (as in pyridones), and (ii) alternative non-aromatic carbonyl forms (e.g. 625, 628). In the hydroxy enolic form (e.g. 622, 624, 626) the reactivity of these compounds toward electrophilic reagents is greater than that of the parent heterocycles these are analogues of phenol. 

Interconversion of the hydroxy and carbonyl forms of these heterocycles proceeds through an anion (as 630) or a cation (as 631), just as the enol (633) and keto forms (636) of acetone are interconverted 

2-Hydroxy-imidazoles, -oxazoles and -thiazoles (643 Z = NR, O, S) can isomerize to 2-azolinones (644a). These compounds all exist predominantly in the azolinone form and show many reactions similar to those of the pyridones. They are mesomeric with zwitterionic and carbonyl canonical forms 

2-Azolinones are protonated on oxygen in strongly acidic media. 0-Alkylation of 2-azolinones can be effected with diazomethane thiazolinone (645) forms (646). Frequently 0- and A-alkylation occur together, especially in basic media where proton loss gives an ambident anion (see also Section 3.4.1.3.10). 

Reactivity of Five-membered Rings with Two or More Heteroatoms 

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