Tautomerism enol-enaminone

The enol form 10 of pyrrolam A 10 could not be detected despite its aromatic character <2004JOC6105>. Also, its pyrrolinone analogues 34 exist mainly as /3-enaminones <1985TL833> the tautomeric hydroxypyrrole 34 was only observed when R = C02Et (Scheme 1) <1988JOC5680>. 

On the other hand, unsubstituted ethyl (2-pyridyl)pyruvate 113 (R = COOEt) was reported to exist predominantly in the enaminone form 113c in methanol, but as enol form in cyclohexane (94JCS(P2)2461). The tautomeric equilibrium between 113b... 

Enol imine-enaminone and phenol—quinone tautomerism in (arylazo) naphthols and in analogous Schiff bases were studied by Fabian et al. [92, 93]. In all these molecules there is a favorable N- -H- -O intramolecular hydrogen bond. Depending on the X-H sigma bond (X = N, O), there are two possible tautomers in solution. The solvent effect was calculated on the equilibrium [92], and a combined effect of the solvent and the benzene substituent was studied in [93]. While the FEP/MC simulations provided consistent organic solvent effects in accord with the experimental results [92], the wide spectrum of the solvent-effect calculation methods could predict rather diverse results for several groups of systems in [93]. 

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