Acid-Base Chemistry and Tautomerism

In fact, pyridoxal-P coenzyme catalyzes at least seven very different reactions where acid-base chemistry and tautomerism is fully exploited. 

Tautomerizations involve the shift of a hydrogen atom across a tt system. The most typical tautomerization is a 1,3-shift, and the focus of this section is the interconversion of a ketone (or aldehyde) and an enol, often termed keto-enol tautomerization. The reaction can be catalyzed by acid or base, and it is technically an isomerization, a class of reactions we will cover later in this chapter. However, knowledge of the mechanism of keto-enol tautomerizations is crucial to understanding enol and enolate chemistry, and therefore we cover it here. 

Deamination of amino acids by carbonyl compounds has long been known in organic chemistry. Classical instances are the Strecker reaction of amino acids with alloxan and rimilar reactions with o-quinones, isatin, ninhydrin, methylglyoxal, and the like. In such cases deamination is usually associated with decarboxylation of the amino acid. Condensation of the amino and carbonyl groups to yield SchiiTs bases, and tautomeric transformation of these, are generally assumed as intermediary steps. 

The chemistry of 1,3,5-triazinols has been reviewed.10,14,18,21 Cyanuric acid (1,3,5-tri-azine-2,4,6-triol) where the oxo form of the oxo-enol tautomerism predominates, is a thermally very stable compound (up to 500 JC). Thus, the chemistry is mostly based on the cyclic imide portion and the replacement reactions of the imido hydrogen atoms (halogenation, alkylation). 

---