Ring-chain tautomerism enolates

Section 4.2.1 will be devoted to heterocycles, section 4.2.2 will cover other kinds of protomeric tautomeric equilibria (e.g., enol/ketone, formic acid, formamidine, etc.), and section 4.2.3 will discuss an example of a ring/chain tautomeric equilibrium. The order of presentation will be approximately by increasing molecular weight within each section. A review by Kwiatkowski et al. [267] covers work on formamide, pyridines, pyrimidines, purines, and nucleic... 

Simultaneous ring-chain and keto-enol tautomeric equilibria were observed (90ZOR2489) in solutions of 5-hydroxy-5-phenylpentane-2,4-dione 29. Increase of the solvent polarity and proton-accepting ability in the series... 

Solvent effects similar to those described for the keto/enol equilibria can also be found for other tautomerisms, e.g. lactim/lactam, azo/hydrazone, ring/chain equilibria, etc. [62-64], The pecularities arising here can only be illustrated by means of a few representative examples. 

The keto-enol and thione-thiol tautomerisms in pyridine and pyrimidine derivatives were already discussed in Chapter 6. Azo-hydrazo tautomerisril, CH—N=N C=N—NH, has been studied by Burawoy and co-workers34. Another type of tautomerism has been identified in the case of carbohydrates. The proportions of the ring and open structures of carbohydrates have been determined by measuring the intensity of the 280 mp carbonyl band. Pacsu and Hiller have shown that D-glucose exists almost completely in the ring form in neutral solution and to the extent of 0T per cent in the open chain form in acid solution. 

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